Descriptive Geoenvironmental Mineral Deposit Models
Descriptive Geoenvironmental Mineral Deposit Models
Descriptive Geoenvironmental Mineral Deposit Models
Preliminary compilation of
descriptive geoenvironmental mineral deposit models
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with
the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does
not imply endorsement by the U.S. Government.
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Denver, Colorado
1995
Read Me for
Preliminary Compilation of Descriptive Geoenvironmental Mineral Deposit Models
Edward A. du Bray, editor
U.S. Geological Survey Open-File Book 95-831
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ACKNOWLEDGMENTS
This preliminary compilation of geoenvironmental models represents a monumental effort, completed within a very short
time frame, by the authors of the individual models. All of the authors are to be commended for a job well done. I wish
to express my thanks to Bill Miller and Ted Theodore who agreed to review the first version of the compilation. This
was a huge job, completed with remarkable swiftness, for which I am very grateful. I would also like to thank Alan
Wallace, Gerry Czamanske, Steve Ludington, Jeff Doebrich, Gordon Haxel, David John, Dave Lindsey, Rob Zierenberg,
Larry Drew, and George Desborough who reviewed various sections of the compilation. Doug Klein and Don Hoover
coordinated synthesis of geophysical information pertinent to each of the geoenvironmental models; their efforts are
much appreciated.
Edward A. du Bray
CONTENTS
Magmatic sulfide deposits, by M.P. Foose, M.L. Zientek, and D.P. Klein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Th-rare earth element vein deposits, by T.J. Armbrustmacher, P.J. Modreski, D.B. Hoover,
Sn and (or) W skarn and replacement deposits, by J.M. Hammarstrom, J.E. Elliott, B.B. Kotlyar,
T.G. Theodore, J.T. Nash, D.A. John, D.B. Hoover, and D.H. Knepper, Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Vein and greisen Sn and W deposits, by J.E. Elliott, R.J. Kamilli, W.R. Miller, and K.E. Livo . . . . . . . . . . . . . . . . . . 62
Climax Mo deposits, by S. Ludington, A.A. Bookstrom, R.J. Kamilli, B.M. Walker, and D.P. Klein . . . . . . . . . . . . . . 70
Porphyry Cu deposits, by L.J. Cox, M.A. Chaffee, D.P. Cox, and Douglas P. Klein . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Cu, Au, and Pb-Zn skarn deposits, by J.M. Hammarstrom, B.B. Kotlyar, T.G. Theodore, J.E. Elliott,
D.A. John, J.L. Doebrich, J.T. Nash, R.R. Carlson, G.K. Lee, K.E. Livo, and D.P. Klein . . . . . . . . . . . . . . . . 90
Fe skarn deposits, by J.M. Hammarstrom, T.G. Theodore, B.B. Kotlyar, J.L. Doebrich,
J.E. Elliott, J.T. Nash, D.A. John, and K.E. Livo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
Polymetallic vein and replacement deposits, by G.S. Plumlee, M. Montour, C.D. Taylor,
Au-Ag-Te vein deposits, by K.D. Kelley, T.J. Armbrustmacher, and D.P. Klein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
Volcanic-associated massive sulfide deposits, by C.D. Taylor, R.A. Zierenberg, R.J. Goldfarb,
Blackbird Co-Cu deposits, by K.V. Evans, J.T. Nash, W.R. Miller, M.D. Kleinkopf,
Creede, Comstock and Sado epithermal vein deposits, by G.S. Plumlee, K.S. Smith,
Epithermal quartz-alunite Au deposits, by G.S. Plumlee, K.S. Smith, J.E. Gray, and D.B. Hoover . . . . . . . . . . . . . . . 162
Rhyolite-hosted Sn deposits, by E.E. Foord, R.A. Ayuso, D.B. Hoover, and D.P. Klein . . . . . . . . . . . . . . . . . . . . . . . 174
Low-Ti iron oxide Cu-U-Au-REE deposits, by M.P. Foose and V.J.S. Grauch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
Sediment-hosted Au deposits, by A.H. Hofstra, J.S. Leventhal, D.J. Grimes, and W.D. Heran . . . . . . . . . . . . . . . . . . 184
Stibnite-quartz deposits, by R.R. Seal, II, J.D. Bliss, and D.L. Campbell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
Algoma Fe deposits, by W.F. Cannon, D.G. Hadley, and R.J. Horton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
Sediment-hosted Cu deposits, by D.A. Lindsey, L.G. Woodruff, W.F. Cannon, D.P. Cox,
Sedimentary exhalative Zn-Pb-Ag deposits, by K.D. Kelley, R.R. Seal, II, J.M. Schmidt,
Mississippi Valley-type Pb-Zn deposits, by D.L. Leach, J.B. Viets, N. Foley-Ayuso, and D.P. Klein . . . . . . . . . . . . . 234
Solution collapse breccia pipe U deposits, by K.J. Wenrich, B.S. Van Gosen, and W.I. Finch . . . . . . . . . . . . . . . . . . 244
Superior Fe deposits, by W.F. Cannon, D.G. Hadley, and R.J. Horton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
Sedimentary Mn deposits, by E.R. Force, W.F. Cannon, and D.P. Klein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
Low-sulfide Au quartz vein deposits, by R.J. Goldfarb, B.R. Berger, T.L. Klein,
Stratabound Au deposits in iron formations, by E.H. DeWitt, W.D. Heran, and M.D. Kleinkopf . . . . . . . . . . . . . . . . 268
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GEOENVIRONMENTAL MODELS OF MINERAL DEPOSITS--
FUNDAMENTALS AND APPLICATIONS
INTRODUCTION
Economic geologists recognize that mineral deposits can readily be classified according to similarities in their
geologic characteristics (ore and gangue mineralogy, major- and trace-element geochemistry, host rock lithology,
wall-rock alteration, physical aspects of ore, etc.), as well as their geologic setting (see for example, Guilbert and
Park, 1986). Early geology-based classification schemes have evolved into mineral deposit models that classify
deposits not only on the basis of geologic characteristics, but also on the basis of geophysical and geochemical
characteristics and the genetic processes by which the deposits form (Cox and Singer, 1986; Bliss, 1992). These
conceptual mineral deposit models form the basis for most modern mineral exploration methodologies, and have also
been used as tools to help assess the potential for undiscovered mineral resources in regions with known geologic
characteristics.
A next step in the process of mineral deposit modeling is development of geology-based, geoenvironmental
models for diverse mineral deposit types. Mineral deposit geology, as well as geochemical and biogeochemical
processes, fundamentally control the environmental conditions that exist in naturally mineralized areas prior to
mining, and conditions that result from mining and mineral processing. Other important natural controls, such as
climate, and anthropogenic factors (including mining and mineral processing methods) mostly modify the
environmental effects controlled by mineral deposit geology and geochemical processes. Thus, deposits of a given
type that have similar geologic characteristics should also have similar environmental signatures that can be quantified
by pertinent field and laboratory data and summarized in a geoenvironmental model for that deposit type. Similarly,
environmentally important geologic characteristics, such as the presence of an alteration type likely to produce highly
acidic drainage water or an alteration type likely to help buffer acid drainage water, should also be common to most
or all deposits of a given type, and thus can also be summarized in a geoenvironmental model. As discussed below,
the need for and use of geoenvironmental models are immediate and varied; these range from environmental
prediction and mitigation, and baseline characterization, to grass-roots mineral exploration, and assessment of
abandoned mine lands and mine-site remediation.
This compilation presents preliminary geoenvironmental models for 32 mineral deposit types (or groups
thereof) compiled by U.S. Geological Survey earth scientists and environmental geochemists using data available as
of mid 1995. The geoenvironmental models follow the classification scheme of, and are numbered according to, the
mineral deposit models presented by Cox and Singer (1986) and (Bliss, 1992), to which the reader is referred for
additional information concerning the mineral deposit models. This first iteration of geoenvironmental model
development has resulted primarily in descriptive summaries of environmentally important geologic characteristics
for a variety of mineral deposit types; however, empirical data are included in some models. The models
summarized herein should be considered as descriptive guides concerning potential environmental impact, not
numeric tools applicable to quantitative risk assessment. Nonetheless, the models provide a basis for understanding
and interpreting environmental processes related to mineral deposits in a systematic geologic context. An important
goal of future investigations will be to integrate additional empirical data or environmental signatures for diverse
deposit types so that the models become more quantitative and can be applied to predict environmental mitigation
expenses and risks associated with mineral extraction.
The purpose of this introductory chapter is to present the geologic basis for geoenvironmental models,
discuss fundamental components of the models, and describe their uses. Individual models assume that the reader
has some knowledge of the terms and concepts of economic geology, geology, and environmental geochemistry;
however, this introductory chapter is designed to provide sufficient references, terminology, and basic concepts that
readers lacking detailed training in these topics can, with some background work, begin to use the models for a
variety of purposes. As a result, this compilation should prove useful to a wide audience, including exploration and
economic geologists, environmental scientists, land managers, regulators and others.
GENERAL DEFINITIONS
Economic geology terms
"Mineral deposits", as defined by Cox and Singer (1986), are occurrences of a valuable commodity (such as gold
or copper) or mineral (such as gems or industrial minerals) that are of sufficient size and concentration (grade) that
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they might, under the most favorable circumstances, be considered to have potential for economic exploitation. An
"ore deposit" is a mineral deposit that has been tested and discovered to be of sufficient size, grade, and accessibility
to allow it to be extracted at a profit. To slightly modify the definition provided by Cox and Singer (1986), a
"mineral deposit model" is a systematic summary of information concerning the geologic characteristics, grade, size,
and genesis of a class of similar mineral deposits; the model can be empirical (based on observations or measured
data) and (or) theoretical (based on conceptual ideas concerning deposit genesis).
The terminology used to describe geologic characteristics of mineral deposits is far too extensive and diverse
to present in this report; interested individuals requiring additional background information are referred to standard
economic geology texts, such as Guilbert and Park (1986) and references contained therein, for a complete discussion
of terminology.
Environmental terms
The "environmental signatures" or "environmental behavior" of mineral deposits are both defined here to be the
suites, concentrations, residences, and availabilities of chemical elements in soil, sediment, airborne particulates, and
water at a site that result from the natural weathering of mineral deposits and from mining, mineral processing, and
smelting. For example, the environmental signature of a mine site may include metal contents and suites in mine-
drainage water, stream sediment, and soil; surface water pH; and identification of readily soluble secondary salts
associated with mine waste. The "environmental effects" of mineral deposits are considered to be spatially broader
than environmental signatures, in that they include the influence of a site on the surrounding environment, including,
for example, the environmental effects of a mine drainage on a river into which the drainage flows.
Geologic controls
For a detailed discussion, the reader is referred to papers or volumes such as Kwong (1993), Alpers and Blowes
(1994), Jambor and Blowes (1994), and Plumlee (in press).
Ore and gangue mineralogy, host rock lithology, and wall-rock alteration: Geologic factors, including mineralogy,
host rocks, and wall-rock alteration, all influence the chemical response of mineral deposits and mineral processing
by-products on environmental signatures. Many sulfide minerals, including pyrite and marcasite (FeS2), pyrrhotite
(Fe1-xS), chalcopyrite (CuFeS2), and enargite (Cu3AsS4), generate acid when they interact with oxygenated water.
Other sulfide minerals, such as sphalerite (ZnS) and galena (PbS) generally do not produce acid when oxygen is the
oxidant. However, aqueous ferric iron, which is a by-product of iron sulfide oxidation, is a very aggressive oxidant
that, when it reacts with sulfide minerals, generates significantly greater quantities of acid than those generated by
oxygen-driven oxidation alone. Thus, the amount of iron sulfide present in a mineralized assemblage plays a crucial
role in determining whether acid will be generated (Kwong, 1993; Plumlee, in press). In general, sulfide-rich mineral
assemblages with high percentages of iron sulfide or sulfide minerals having iron as a constituent (such as
chalcopyrite or iron-rich sphalerite) will generate significantly more acidic water than sphalerite- and galena-rich
assemblages that lack iron sulfide minerals. Some non-sulfide minerals such as siderite and alunite can also generate
acid during weathering if released iron or aluminum precipitate as hydrous oxide minerals. In contrast to
acid-generating sulfide minerals, carbonate minerals, whether present in ore or in host rocks, can help consume acid
generated by sulfide oxidation. Other materials that may react with acid, though less readily than carbonate minerals,
include aluminosilicate glasses or devitrified glasses (as in volcanic rocks) and magnesium-rich silicate minerals such
as olivine and serpentine.
In the case of some industrial minerals such as fibrous silicate minerals, mineralogy plays a well-known,
key role in determining adverse health effects associated with intake of these minerals (Ross, in press). For example,
chrysotile asbestos, the most common form of asbestos used in industrial applications in the United States, apparently
has negligible effects on human cancer incidence, whereas crocidolite and amosite asbestos varieties are clearly linked
to greatly increased human mortality rates from certain types of cancer.
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Major- and trace-element composition: The major- and trace-element composition of mineral deposits and their host
rocks strongly influence the suites of elements dispersed into the environment from given deposit types. Major-
element compositions (of iron, aluminum, carbon, etc.) influence, for example, types of precipitates formed in
drainage water and can therefore influence trace metal transport mechanisms such as complexing. Metal and trace
element suites in ore are commonly reflected in environmental signatures of soil, water, and smelter emissions; for
example, most copper-rich ore produces drainage water and smelter emissions with copper as the dominant trace
element.
Mineral resistance to weathering and oxidation: The relative rates at which minerals weather play a crucial role in
environmental processes, including acid-drainage generation and release of metals into the environment from solid
mine or mineral processing wastes. Although the relative weathering rates of various sulfide minerals, as determined
in the laboratory, vary considerably from study to study (Jambor and Blowes, 1994; Smith and others, 1994), a
general sequence of "weatherability" has been established (listed here in order of decreasing reactivity): pyrrhotite
(Fe1-xS) > chalcocite (Cu2S) > galena (PbS) > sphalerite (ZnS) > pyrite (FeS2) > enargite (Cu3AsS4) > marcasite
(FeS2) > cinnabar (HgS) > molybdenite (MoS2). As is well known to most field geologists, carbonate minerals are
the most reactive of the acid-consuming minerals; of these, calcium carbonate minerals (calcite, aragonite) react most
readily with acidic water, whereas iron, magnesium, or manganese carbonate minerals (dolomite, magnesite, siderite)
tend to be the least reactive with acidic water. Aluminosilicate minerals tend to react much more weakly with acid
water than carbonate minerals; volcanic glass, devitrified volcanic glass, and Fe-, Mg-silicate minerals (such as
olivine and serpentine) are the most reactive of the aluminosilicate minerals, whereas feldspars and quartz are the
least reactive.
Mineral textures and trace element contents: The rates at which mineral deposits are weathered and oxidized are also
influenced by the textures and trace element contents of contained minerals. For example, sulfide crystals that are
fine-grained, have massive or fibrous textures, or have high trace element contents typically weather more rapidly
than coarse, euhedral, and trace element-poor crystals (Kwong, 1993; Plumlee and others, 1993).
Extent of pre-mining oxidation: As weathering and erosion expose sulfide-bearing mineral deposits, associated
potential environmental impact may be reduced as a consequence of sulfide mineral oxidation; some metals contained
therein may be subsequently incorporated in relatively less soluble minerals from which metal mobility is limited.
These less soluble minerals include hydroxides of iron (such as goethite and limonite), manganese, aluminum, and
other metals; some sulfate minerals, such as anglesite, jarosite, plumbojarosite, and alunite; carbonate minerals such
as smithsonite, malachite and azurite; and phosphate minerals such as turquoise and hinsdalite. The extent and
mineralogic products of pre-mining oxidation are a complex function of deposit geology, hydrology, topography, and
climate (see Guilbert and Park, 1986, and references contained therein). For example, along highly permeable veins
or alteration zones, sulfide minerals may be oxidized to great depths, whereas sulfide minerals immediately adjacent
to low permeability rocks (such as clay altered rocks) may remain unoxidized to within several meters of the ground
surface. In regions with steep topography, elevated mechanical erosion rates can greatly exceed chemical weathering
rates such that fresh sulfide minerals in highly altered rocks are continually exposed. As another example, a
combination of deep paleowater tables and uplift of mountain blocks in the Great Basin during Tertiary time tended
to create deeply oxidized ore deposits. Associated ore was easily mined and milled in the 19th Century; today, waste
dumps at these mines pose relatively few problems because potentially hazardous elements are tightly held in iron
oxide minerals. Many current exploration targets in the pediment areas of the Great Basin are oxidized to relatively
great depths. In contrast, areas characterized by widespread mechanical erosion, including terranes that are
tectonically active or have been glaciated, tend to have thin weathered zones that may contain sulfide minerals at
or near the surface. Mechanical erosion can enhance natural generation of acidic conditions if the climate is semi
arid to humid.
Secondary mineralogy: In contrast to secondary minerals formed by pre-mining mineral deposit weathering, many
secondary minerals formed from weathered, sulfide-bearing ore and tailings wastes are quite soluble and can play
an important role in controlling metal mobility from mine sites. Of these secondary minerals, the most common and
environmentally important are metal sulfate salts of calcium (gypsum), iron (jarosite, melanterite, copiapite,
rhomboclase, and many others), copper (chalcanthite, brochantite, and others), zinc (goslarite), magnesium
(pickeringite), and other metals. These salts form efflorescent coatings on rocks, fractures, and mine workings, and
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are produced by evaporation of sulfate-rich drainage water during dry periods or in areas sheltered from water runoff.
The salts have variable compositions, and serve as solid storage reservoirs for both metals and acid. Due to their
high solubilities, the salts dissolve rapidly during rainstorms or snowmelt; metals and acid released by salt dissolution
can lead to temporary but significant degradation of surface- and ground-water quality. Water remaining after storm
or snowmelt events can itself become a highly reactive fluid that enhances sulfide mineral oxidation; eventually, these
fluids evaporate completely and reinitiate the salt precipitation-dissolution cycle. The particular secondary salts
formed depend strongly upon deposit geology, climate, and the extent of evaporation.
Structural and physical characteristics of mineral deposits: The access of weathering agents such ground water and
atmospheric oxygen are controlled by the structural and physical characteristics of mineral deposits. For example,
veins, sulfide-mineral-rich lenses, or faults can focus groundwater flow, thereby promoting water access to sulfide-
rich material and inhibiting contact with potential acid-buffering agents in wall rocks. As another example, zones
of intense clay alteration have low permeability and inhibit ground-water flow; consequently, sulfide minerals within
clay altered rock can remain unoxidized, even when they are well above the water table.
Climate
Climate affects the environmental behavior of mineral deposits, but its effects are often subordinate to those of
deposit geology. Amounts of precipitation and prevailing temperatures influence the amount of water available as
surface runoff, the level of the water table, rates of reaction, amounts of organic material, and other parameters that
affect weathering of mineralized rocks and ore. In general, water tables are shallow in wet climates and deep in
semi-arid climates. However, depths to the water table can be highly variable across short distances within a mining
district. Some mining districts in Nevada today have water tables that vary from 0 to 350 m depths within a few
kilometers. Deep weathering (oxidation) profiles tend to develop in semi-arid climates. Leaching of elements tends
to be intense in humid tropical climates and modest in arid deserts. In humid to semi-arid climates, leaching and
transport tends to be downward, whereas in arid climates upward movement of water by capillary action becomes
a significant process. Environmental signatures associated with mineral deposits may vary somewhat on a local scale
due to microclimate variations, such as exist where mountainous areas with seasonal snow and rain are adjacent to
arid valleys in which evaporation exceeds annual precipitation (for example, Nevada and Arizona).
As described below, mine-drainage water associated with sulfide-mineral-bearing deposit types, which
generate acid mine water, tends to have lower pH and higher metal contents in dry climates than in wet climates due
to evaporative concentration of acid and metals. However, dry-climate mine drainage water with low pH and high
metal content may have less environmental impact than a similar deposit in a wet climate setting because of the
relatively small volume of surface drainage water. Evaporative processes can also operate in wet climate settings
characterized by seasonal wet and dry periods. Relative shifts in pH and metal content for a given deposit type in
different climate settings are still very much less than shifts due to differences in geologic characteristics, however.
Very cold climate can have several consequences for environmental processes. First, weathering rates
decrease substantially in very cold climates; unweathered sulfide minerals may be abundant at the surface where
climate favors permafrost formation. However, during short summer seasons in areas dominated by cold climate,
weathering of exposed sulfide minerals can lead to formation of highly acidic water (again depending upon the
mineral-deposit geology). Freeze-concentration of acid water can also lead to increased acidity and metal contents.
Climate effects on environmental impact downstream from mineral deposits can be significant. For example,
downstream dilution (and therefore environmental mitigation) of acid mine water by dilute water draining
unmineralized areas is much more efficient in wet climates than in dry climates. In contrast, downstream mitigation
is enhanced in dry climate settings by the increased buffering offered by solid material in stream beds.
The major effects of climate are perhaps best known from world studies of soil (FitzPatrick, 1980) and
studies of supergene enrichment of ore deposits (see Guilbert and Park, 1986 and references cited therein), but
systematic studies of environmental geochemistry as a function of climate are in their infancy. In detail, the subject
is complex, but some generalizations can be made by considering element mobility, deposition, and adsorption in
soil and ore deposit supergene zones (Rose and others, 1979; Anderson, 1982).
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effects of mining and mineral processing method on most environmental signatures are generally subordinate to those
of mineral-deposit geology. In most cases, abundances of acid and metals in mine water draining deposits with
similar geologic characteristics progressively increase from water draining underground workings, to that draining
mine dumps, to that draining mill tailings, and finally, to that collecting in open pits. This trend reflects increasing
access to weathering agents (water and atmospheric oxygen), increased surface area of sulfide minerals exposed to
weathering, and increased opportunities for evaporative concentration. In addition, the size of particles produced by
milling and beneficiation processes can dramatically influence the extent of environmental impact. Finely milled
ore and tailings, which enhance metal adsorption while enhancing sulfide oxidation, can more rapidly generate acid
and are more likely to be distributed by wind and water than their more coarse-grained equivalents.
One important way in which mineral processing techniques are of primary importance relates to techniques
that introduce potentially problematic chemicals. For example, mercury amalgamation was widely used as a gold
extraction technique in the United States in the last century. As a result, soil and sediment may be mercury
contaminated at many sites where amalgamation was practiced historically, but would not otherwise be characterized
by elevated mercury abundances.
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Figure 1. Variations in aqueous base metal concentrations (given as the sum of base metals base metals zinc, copper, cadmium, cobalt, nickel, and
lead) as a function of pH for water draining various types of mineralized rock in diverse sites within Colorado.
6
controls on permeability, ground water flow, and oxidation; and other deposit types with similar environmental
geology characteristics. Some of the environmental models presented in this compilation also provide available
empirical data on environmental signatures that: (1) are present prior to mining in soil, stream sediment, and ground
and surface water; (2) result from mining and mineral processing (mine drainage water, mine wastes, mill tailings
and tailings water, and heap leach solutions), and (3) result from smelting (smelter slag and stack emissions).
Environmental signatures include information concerning the elemental suites and their likely concentration in water,
waste, and soil, etc., and the ease with which the elements can be liberated into the environment (their
"geoavailability"). Empirical data from well characterized sites is lacking for some deposit types; potential
environmental signatures for these deposit types can be extrapolated from similar deposits for which data are
available. The models also include information, when available, on engineering and other types of processes that
have been or likely can be used successfully to avoid, minimize, and remediate environmental signatures summarized
in the models.
The geoenvironmental model for each deposit type is organized as follows:
I. SUMMARY OF RELEVANT GEOLOGIC, ENVIRONMENTAL, AND GEOPHYSICAL INFORMATION
A summary of geoenvironmentally relevant information for each model includes:
A. Deposit type geology
B. Examples of deposits of this type
C. Spatially or genetically related deposit types, listed by model name and number. In many mining districts, more
than one deposit type may be present; each deposit type may have different associated geoenvironmental
effects or concerns
D. Potential environmental considerations: This section is designed to summarize environmental signatures that may
be associated with each deposit type as well as some of the important geologic characteristics that affect
these signatures
E. Exploration geophysics
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III. ENVIRONMENTAL SIGNATURES
This section summarizes empirical data pertaining to environmental signatures; data have been gathered through field
studies and (or) literature surveys
A. Drainage signatures both natural and mining-related are summarized. Natural drainage data are required to define
accurate pre-mining baseline conditions
B. Metal mobility from solid mine wastes: Significant quantities of metal and acid can be stored as readily-dissolved,
secondary mineral coatings on solid mine wastes
C. Soil and sediment signatures prior to mining are also required to help establish pre-mining baseline conditions
D. Potential environmental signatures associated with mineral processing
E. Smelter signatures: Where possible, data concerning metal contents and mobility from slag and soils affected by
smelter emissions are presented
F. Climate effects on environmental signatures: This section discusses how environmental signatures vary as a
function of climatic regime variation
G. Guidelines for mitigation and remediation: This section, available for only a few models, is designed to provide
insights into the types of engineering techniques that are commonly used to mitigate or remediate
environmental effects likely to be associated with particular deposit types. In addition, deposit geologic
features that might be used to develop more effective or less expensive remedial techniques are described
H. Geoenvironmental geophysics: This section contains information on geophysical techniques that are of use to help
identify, assess, or delineate environmental signatures
Exploration
Knowledge of likely environmental effects associated with development of particular deposit types can be integrated
into grass-roots exploration efforts. For example, development of deposit types with typically high acid mine
drainage generation potential, and extreme associated metal contents, will have lower environmental mitigation
expenses in arid climates or in geologic terranes with abundant carbonate rocks than in other environments.
8
Remediation
The models presented in this compilation summarize crucial geologic, geochemical, and hydrologic information (such
as geologic controls on ground water flow, ore mineralogy, and materials geology) needed by engineers to develop
effective remediation plans at mine sites. Some remedial plans currently in implementation ignore or dangerously
oversimplify important geologic information. For example, adit plugging has been used or is proposed to reduce
acid drainage from a number of mine sites. The geoenvironmental models can be used to identify deposit types in
which faults or other hydrologic conduits might be common, thereby reducing the effectiveness of adit plugging as
a remedial solution. In addition, the models can be used to help identify likely types and orientations of faults and
other hydrologic conduits present at remediation sites.
REFERENCES CITED
Alpers, C.N., and Blowes, D.W., eds., 1994, Environmental geochemistry of sulfide oxidation, ACS Symposium
Series 550: Washington, D.C., American Chemical Society, 681 p.
Anderson, J.A., 1982, Characteristics of leached capping and techniques of appraisal, in Titley, S.R., ed., Advances
in geology of the porphyry copper deposits: Tucson, University of Arizona Press, 560 p.
Bliss, J.D., 1992, Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, 168 p.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
FitzPatrick, E.A., 1980, Soils--Their formation, classification, and distribution: London, Longman, 353 p.
Gray, J.E., Coolbaugh, M.F., Plumlee, G.S., and Atkinson, W.W., 1994, Environmental geology of the Summitville
Mine, Colorado: Economic Geology, v. 89, p. 2006-2014.
Guilbert, J. and Park, C., 1986, The geology of ore deposits: W.H. Freeman and Co., New York, 985 p.
Jambor, J.L., and Blowes, D.W., eds., 1994, Short course on environmental geochemistry of sulfide-mine wastes:
Mineralogical Association of Canada, Short Course Handbook, Volume 22, 438 p.
Kwong, Y.T.J., 1993, Prediction and prevention of acid rock drainage from a geological and mineralogical
perspective: MEND Project 1.32.1, 47 p.
Plumlee, G.S., in press, The environmental geology of mineral deposits, in Plumlee, G.S., and Logsdon, M.J., eds.,
The Environmental Geochemistry of Mineral Deposits-Part A; Processes, methods, and health issues: Society
of Economic Geologists Reviews in Economic Geology, Volume 6A.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado--implications for the prediction of mine-drainage chemistry: Proceedings, 1993 Mined
Land Reclamation Symposium, Billings, Montana, v. 1, p. 176-186.
Rose, A.W., Hawkes, H.E., and Webb, J.S., 1979, Geochemistry in mineral exploration (2nd Ed.): New York,
Academic Press, 657 p.
Ross, M., in press, The health effects of mineral dusts, in Plumlee, G.S., and Logsdon, M.J., eds., The Environmental
Geochemistry of Mineral Deposits-Part A; Processes, methods, and health issues: Society of Economic
Geologists Reviews in Economic Geology, Volume 6A.
Smith, K.S., Plumlee, G.S., and Ficklin, W.H., 1994, Predicting water contamination from metal mines and mining
waste: Notes, Workshop No. 2, International Land Reclamation and Mine Drainage Conference and Third
International Conference on the Abatement of Acidic Drainage: U.S. Geological Survey Open-File Report
94-264, 112 p.
9
BIOAVAILABILITY OF METALS
INTRODUCTION
The fate of various metals, including chromium, nickel, copper, manganese, mercury, cadmium, and lead, and
metalloids, including arsenic, antimony, and selenium, in the natural environment is of great concern (Adriano, 1986;
1992), particularly near former mine sites, dumps, tailing piles, and impoundments, but also in urban areas and
industrial centers. Soil, sediment, water, and organic materials in these areas may contain higher than average
abundances of these elements, in some cases due to past mining and (or) industrial activity, which may cause the
formation of the more bioavailable forms of these elements. In order to put elemental abundances in perspective,
data from lands and watersheds adjacent to these sites must be obtained and background values, often controlled by
the bedrock geology and (or) water-rock interaction, must be defined.
In order to estimate effects and potential risks associated with elevated elemental concentrations that result
from natural weathering of mineral deposits or from mining activities, the fraction of total elemental abundances in
water, sediment, and soil that are bioavailable must be identified. Bioavailability is the proportion of total metals
that are available for incorporation into biota (bioaccumulation). Total metal concentrations do not necessarily
correspond with metal bioavailability. For example, sulfide minerals may be encapsulated in quartz or other
chemically inert minerals, and despite high total concentrations of metals in sediment and soil containing these
minerals, metals are not readily available for incorporation in the biota; associated environmental effects may be low
(Davis and others, 1994). Consequently, overall environmental impact caused by mining these rocks may be much
less than mining another type of mineral deposit that contains more reactive minerals in lower abundance.
In aquatic environments where chemically reducing conditions may prevail, metals from mining activities
may be associated with sulfide minerals. These sulfide minerals are present either in the ore deposit or formed by
bacterial reduction of the sulfate in oxidized tailings. Most metal sulfide minerals are quite immobile, as long as
they remain in a chemically reducing environment, and they may have little impact on biota despite anomalous metal
concentrations.
BIOAVAILABILITY
Metals of major interest in bioavailability studies, as listed by the U.S. Environmental Protection Agency (EPA), are
Al, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Se, and Sb (McKinney and Rogers, 1992). Other metals that are presently of
lesser interest to the EPA are Ag, Ba, Co, Mn, Mo, Na, Tl, V, and Zn. These metals were selected because of their
potential for human exposure and increased health risk. Some highlights concerning the bioavailability of As, Cd,
Cu, Hg, Mo, Pb, Se, and Zn in water and (or) sediment are discussed in the section below entitled "Specific metals
of interest."
Metals can be dispersed in soil, water, and air. Geoscientists are mainly concerned with metals dispersed
in soil and sediment, dissolved in ground and surface water, suspended as particles in surface water, and in pore fluid
in sediment (fig. 1). In addition, metals can be dispersed into the atmosphere, by natural geochemical cycling and
by other anthropogenic processes (such as smelting and burning leaded gasoline and coal) and by microbial activities;
these metal fluxes must be considered in overall metal bioavailability studies. Bioaccumulation of metals by biota
in surface water and by plants and animals in terrestrial environments can adversely affect humans. In surface and
ground water, sediment and air, bioavailability is a complex function of many factors including total concentration
and speciation (physical-chemical forms) of metals, mineralogy, pH, redox potential, temperature, total organic
content (both particulate and dissolved fractions), and suspended particulate content, as well as volume of water,
water velocity, and duration of water availability, particularly in arid and semi-arid environments. In addition, wind
transport and removal from the atmosphere by rainfall (frequency is more important than amount) must be
considered. Many of these factors vary seasonally and temporally, and most factors are interrelated. Consequently,
changing one factor may affect several others. In addition, generally poorly understood biological factors seem to
strongly influence bioaccumulation of metals and severely inhibit prediction of metal bioavailability (Luoma, 1989).
Some of the major controls on the bioavailability of metals in surface water and soil and data concerning potentially
hazardous metals are described below.
In order to understand bioavailability, plant materials and selective chemical leaches of soil must be analyzed
and the results compared. Elemental suites for which analyses are performed and the type of selective leaches
utilized must be tailored to bedrock and soil types, and to suspected anthropogenic inputs. Soil pH, organic matter,
10
Figure 1. Interrelationships of man, metals, and the environment. Modified from Salomons and Forstner (1988).
Figure 2. The chemical forms of metals in solid phases. Modified from Gunn and others (1988).
and sulfur and carbonate contents should be determined to enable accurate assessment of elemental reservoirs,
mobility, and bioavailability. Additional work on mineralogical residences of metals is also important because metals
can be associated with several sites (fig. 2).
Soil scientists involved in agriculture havebeen studying the chemistry of nutrient elements in soil for more
than 100 years. Soil testing methods are used to determine sixteen essential elements for plant growth, but nitrogen,
phosphorous, and potassium, along with pH and salinity are the most often measured properties (Peck and Soltanpour,
1990). Physical-chemical aspects of nutrient availability are discussed by Corey (1990) and soil testing correlations
are reviewed by Dahnke and Olson (1990).
11
Table 1. Relative mobility and availability of trace metals (modified from Salomons, 1995)
Exchangeable (dissolved) cations High. Changes in major cationic composition (e.g. estuarine environment)
may cause a release due to ion exchange
Metals associated with Fe-Mn oxides Medium. Changes in redox conditions may cause a release but some metals
precipitate if sulfide mineral present is insoluble
Metals associated with organic matter Medium/High. With time, decomposition/oxidation of organic matter occurs
Metals associated with sulfide minerals Strongly dependent on environmental conditions. Under oxygen-rich
conditions, oxidation of sulfide minerals leads to release of metals
Metals fixed in crystalline phase Low. Only available after weathering or decomposition
elements is transported from roots to shoots, and (4) possible mobilization, from leaves to storage tissues used as
food (seeds, tubers, and fruit), in the phloem transport system. After plant uptake, metals are available to herbivores
and humans both directly and through the food chain. The limiting step for elemental entry to the food chain is
usually from the soil to the root (Chaney, 1988). This critical step usually depends on element concentrations in soil
pore solutions, which are controlled by local soil physical and chemical conditions including water content, pH, Eh,
and other factors.
Climate strongly influences soil types; these two factors largely control element (metals and metalloids)
mobility and availability. Arid climates in the western United States often result in small soil organic matter
abundances and large salt and carbonate abundances. These phases often contain the metals of interest. In the
eastern United States, humid climates prevail and large amounts of organic matter require determination of organic
matter-associated metals and their residence times or turn-over rates, because after some time, much of the organic
matter is oxidized and associated metals may be released or available. In tropical climate conditions, accumulation
of oxide minerals of iron, manganese, and aluminum in soil profiles may limit the mobility and bioavailability of
both metals and metalloids.
Plant species and relative abundance and availability of necessary elements also control metal uptake rates.
Abundant bioavailable amounts of essential nutrients, including phosphorous and calcium, can decrease plant
uptake of non-essential but chemically similar elements, including arsenic and cadmium, respectively. More
complex interactions are also observed: bioavailability may be related to multi-element amounts or ratios. For
example, copper toxicity is related to low abundances of zinc, iron, molybdenum and (or) sulfate (Chaney, 1988).
Many agricultural studies have been completed on this subject (see Adriano, 1986; 1992).
In the scientific literature, many studies describe anthropogenic (industrial or mining) contributions to
elemental abundances, and their bioavailability controls, in the environment. Examples include: occurrence of heavy
metals in soil near and far from urban pollution (Pouyat and McDonnell, 1991); formation of acid mine drainage
(Filipek and others, 1987); uptake of heavy metals by plants in lab experiments (Brown and others, 1995); and
uptake of metals by vertebrates in the vicinity of zinc smelters (Storm and others, 1994).
1
Metals associated with carbonate minerals in sedimentary rocks and soil constitute the carbonate fraction, which
can be newly precipitated in soil. The iron-manganese oxide fraction consists of metals adsorbed to iron-manganese
oxide particles or coatings. The organic fraction consists of metals bound to various forms of organic matter. The
crystalline fraction consists of metals contained within the crystal structure of minerals and normally not available
to biota.
Hydrogen ion activity (pH) is probably the most important factor governing metal speciation, solubility from
mineral surfaces, transport, and eventual bioavailability of metals in aqueous solutions. pH affects both solubility
of metal hydroxide minerals and adsorption-desorption processes. Most metal hydroxide minerals have very low
solubilities under pH conditions in natural water. Because hydroxide ion activity is directly related to pH, the
solubility of metal hydroxide minerals increases with decreasing pH, and more dissolved metals become potentially
available for incorporation in biological processes as pH decreases. Ionic metal species also are commonly the most
toxic form to aquatic organisms (Salomons, 1995).
Adsorption, which occurs when dissolved metals are attached to surfaces of particulate matter (notably iron,
manganese, and aluminum oxide minerals, clay, and organic matter), is also strongly dependent on pH and, of course,
the availability of particulate surfaces and total dissolved metal content (Bourg, 1988; Elder, 1989). Metals tend to
be adsorbed at different pH values, and sorption capacity of oxide surfaces generally varies from near 0 percent to
near 100 percent over a range of about 2 pH units.
The adsorption edge, the pH range over which the rapid change in sorption capacity occurs, varies among
metals, which results in precipitation of different metals over a large range of pH units. Consequently, mixing
metal-rich, acidic water with higher pH, metal-poor water may result in dispersion and separation of metals as
different metals are adsorbed onto various media over a range of pH values. Cadmium and zinc tend to have
adsorption edges at higher pH than iron and copper, and consequently they are likely to be more mobile and more
widely dispersed. Adsorption edges also vary with concentration of the complexing agent; thus, increasing
concentrations of complexing agent increases pH of the adsorption edge (Bourg, 1988). Major cations such as Mg+2
and Ca+2 also compete for adsorption sites with metals and can reduce the amount of metal adsorption (Salomons,
1995).
Particulate size and resulting total surface area available for adsorption are both important factors in
adsorption processes and can affect metal bioavailability (Luoma, 1989). Small particles with large surface-area-to-
mass ratios allow more adsorption than an equivalent mass of large particles with small surface-area-to-mass ratios.
Reduced adsorption can increase metal bioavailability by increasing concentrations of dissolved metals in associated
water. The size of particles released during mining depends on mining and beneficiation methods. Finely milled
ore may release much smaller particles that can both be more widely dispersed by water and wind, and which can
also serve as sites of enhanced adsorption. Consequently, mine tailings released into fine-grained sediment such as
silty clays found in many playas can have much lower environmental impact than those released into sand or
coarse-grained sediment with lower surface area and adsorption.
Temperature exerts an important effect on metal speciation, because most chemical reaction rates are highly
sensitive to temperature changes (Elder, 1989). An increase of 10oC can double biochemical reaction rates, which
are often the driving force in earth surface conditions for reactions that are kinetically slow, and enhance the
tendency of a system to reach equilibrium. Temperature may also affect quantities of metal uptake by an organism,
because biological process rates (as noted above) typically double with every 10oC temperature increment (Luoma,
1983; Prosi, 1989). Because increased temperature may affect both influx and efflux rates of metals, net
bioaccumulation may or may not increase (Luoma, 1983).
In recent organic carbon-rich sediments, trapped interstitial fluids can commonly form a strongly reducing
(anoxic) environment. Low redox potential in this environment can promote sulfate reduction and sulfide mineral
deposition. During diagenesis, much of the non-silicate-bound fraction of potentially toxic metals such as arsenic,
cadmium, copper, mercury, lead, and zinc, can be co-precipitated with pyrite, form insoluble sulfides, and become
unavailable to biota (Morse, 1994). Seasonal variation in flow rates or storms that induce an influx of oxygenated
(sea)water can result in rapid reaction of this anoxic sediment and thereby release significant proportions of these
metals. Pyritization and (or) de-pyritization of trace metals probably can be an important process in controlling
bioavailability of many trace metals, especially in the marine environment (Morse, 1994).
13
detritus-feeding aquatic species:(1) ingestion of metal-enriched sediment and suspended particles during feeding, and
(2) uptake from solution (Luoma, 1989). Consequently, knowledge of geochemical reactions of metals in both water
and sediment is necessary to understand controls on metal bioavailability in natural water. Unfortunately, many
biological factors controlling metal bioaccumulation in aquatic organisms are not understood; this fact severely limits
our understanding of metal bioavailability (Luoma, 1989).
Bioavailability studies indicate that aquatic organisms uptake free metal ions (metal hydroxides) from
solution quite efficiently; similarly, terrestrial animals uptake metal from solutions more efficiently than via direct
particulate matter ingestion (Luoma, 1983). Consequently, geologic and (or) environmental conditions that enhance
dissolved metal abundances (for example, lower pH) result in greater metal bioavailability. Indirect controls, such
as larger particle or sediment size, also can result in greater bioavailability of metals by reducing adsorption and
increasing dissolved metal contents. Metal assimilation from ingested particulate matter is also important, however,
because metals are highly concentrated in this form (Luoma, 1989).
The efficiency of bioaccumulation via sediment ingestion is dependent on geochemical characteristics of the
sediment. Luoma (1989) describes variation of cadmium uptake from sediment by clams as a function of sediment
iron content; cadmium uptake from iron oxide-rich sediment was not detected, but a significant proportion of total
cadmium uptake was a consequence of iron-poor sediment ingestion by clams.
14
carbonate phases, and (4) a chelating (or complexing) agent such as EDTA (ethylenediaminetetraacetic acid)
(Borggaard, 1976) or DPTA (diethyenetriaminepentaacetic acid) buffered to a pH of 7 (Crock and Severson, 1980).
Other possible extractants include (5) hydroxylamine hydrochloride for the "reducible" fraction associated with iron
and manganese oxides/hydroxides, (6) strong acid (HCl, pH 1) to identify maximum mobility of most metals
(Leventhal and Taylor, 1990), (7) oxidation by hydrogen peroxide to release metals associated with organic matter
and (or) sulfide minerals, (8) a strong oxidizing acid (HNO3) to execute steps (6) and (7) simultaneously, and (9)
a mixture of strong acid and HF to dissolve residual silicate minerals. The choice of extractants and the order in
which they are used depends on the sediment/soil type, environmental conditions, and metals of interest.
However, these sequential/partial extractions are all "operational", that is they are not completely specific
to metals or chemical phases. Therefore any determination of bioavailability should be carefully calibrated, by direct
measurement, with the actual behavior of metals in soil and plants. For example, O'Connor (1988) cautions about
the use of the DPTA method and shows that it sometimes gives results comparable to plant uptake and sometimes
it does not. As a consequence, he advises direct analysis of the total plant and (or) its component parts in addition
to chemical leaches in order to determine bioavailability.
Brief summaries of some factors controlling bioavailability of several metals and two metalloids are given below.
Additional data are available in the references listed for each metal.
Arsenic: Arsenic mobility, bioavailability, and toxicity are dependent on speciation: arsenite (AsO3-3) forms are much
more toxic to biological species and are more mobile than arsenate (AsO4-3) forms (Kersten, 1988). Arsenic is
chemically similar to phosphorous. Arsenate interferes with phosphate metabolism that is widespread in the
biosphere. Metallo-organic forms of arsenic also may be much more bioavailable than inorganic forms; however,
organic-bound arsenic is excreted by most species and does not appear to be highly toxic (Luoma, 1983).
Adsorption-desorption on iron and aluminum oxide minerals is the main factor controlling arsenic behavior in soil
and sediment. Maximal adsorption occurs at different pH for As{III} (pH 9.2) and As{V} (pH 5.5) as a function
of the adsorbing mineral; As+3 mobility is enhanced under oxic conditions. Arsenic is apparently highly mobile in
anoxic sediment-water systems. Development of acidic and oxidizing conditions tends to release large amounts of
arsenic into solution due to decreased sorption capacity of both forms of arsenic (see Léonard, 1991).
Cadmium: The redox potential of sediment-water systems exerts controlling regulation on the chemical association
of particulate cadmium, whereas pH and salinity affect the stability of its various forms (Kersten, 1988). In anoxic
environments, nearly all particulate cadmium is complexed by insoluble organic matter or bound to sulfide minerals.
Greenockite (CdS) has extremely low solubility under reducing conditions thereby decreasing cadmium
bioavailability. Oxidation of reduced sediment or exposure to an acidic environment results in transformation of
insoluble sulfide-bound cadmium into more mobile and potentially bioavailable hydroxide, carbonate, and
exchangeable forms (Kersten, 1988). Studies of lake and fluvial sediment indicate that most cadmium is bound to
exchangeable site, carbonate fraction, and iron-manganese oxide minerals, which can be exposed to chemical changes
at the sediment-water interface, and are susceptible to remobilization in water (Schintu and others, 1991). In
oxidized, near neutral water, CdCO3 limits the solubility of Cd2+ (Kersten, 1988). In a river polluted by base-metal
mining, cadmium was the most mobile and potentially bioavailable metal and was primarily scavenged by
non-detrital carbonate minerals, organic matter, and iron-manganese oxide minerals (Prusty and others, 1994).
Elevated chloride contents tend to enhance chloride complex formation, which decreases the adsorption of cadmium
on sediment, thereby increasing cadmium mobility (Bourg, 1988) and decreasing the concentration of dissolved Cd+2
and bioavailability (Luoma, 1983). Also see, Stoeppler (1991) for additional data.
Copper: In a river polluted by base-metal mining, copper is most efficiently scavenged by carbonate minerals and
iron-manganese oxide minerals and coatings and is less mobile than cadmium, lead, and zinc (Prusty and others,
1994); in most other situations lead is less mobile than copper. Elevated chloride contents decrease adsorption of
copper on sediment, due to chloride complexation, which results in greater solubility and mobility (Bourg, 1988;
Gambrell and others, 1991). In systems with high total copper contents, precipitation of malachite controls dissolved
copper contents at low pH (Bourg, 1988; Salomons, 1995). Sometimes, elemental substitution is more complex; for
example, copper toxicity is related to low abundances of zinc, iron, molybdenum, and (or) sulfate (Chaney, 1988).
15
Lead: The main sources of lead in the aquatic environment are leaded gasoline and mining (Prosi, 1989). Leaded
gasoline results in introduction of organometallic lead compounds, which eventually reach surface water, into the
atmosphere. Mining releases inorganic lead compounds. Both organic and inorganic forms of lead pose serious
health risks to all forms of life (Ewers and Schlipköter, 1990). Inorganic lead compounds (sulfide, carbonate, and
sulfate minerals) are commonly abundant in sediment but have low solubilities in natural water. Naturally-occurring
lead in mineral deposits is not very mobile under normal environmental conditions, but becomes slightly more soluble
under moderately acidic conditions. Soluble lead is little affected by redox potential (Gambrell and others, 1991).
Lead is tightly bound under strongly reducing conditions by sulfide mineral precipitation and complexion with
insoluble organic matter, and is very effectively immobilized by precipitated iron oxide minerals under well-oxidized
conditions (Gambrell and others, 1991). In the aquatic environment, total dissolved lead abundances in water and
pore water control primary uptake by organisms. Lead bioaccumulation is primarily dependent on the amount of
active lead compounds (predominantly aqueous species) in the environment and the capacity of animal species to
store lead (Prosi, 1989). Particulate lead may contribute to bioaccumulation in organisms. For humans, particles
that are inhaled but not exhaled are especially important. Variations in physiological and ecological characteristics
of individual species lead to different enrichment factors and tolerances for each organism. Studies of bottom
dwelling organisms suggests that iron-rich sediment inhibits lead bioavailability (Luoma, 1989). In a study of lake
and fluvial sediment, most lead was bound to a carbonate fraction or to iron-manganese oxide minerals, both of
which respond to chemical changes at the sediment-water interface, and are susceptible to remobilization in water
(Schintu and others, 1991). In a polluted river environment, lead is most efficiently scavenged by non-detrital
carbonate and iron-manganese oxide minerals and is less mobile than cadmium (Prusty and others, 1994).
Mercury: Mercury has three valence states in natural sediment-water systems: elemental mercury (Hgo), Hg+1, and
Hg+2 (Kersten, 1988). Hgo is considerably more bioavailable for certain organisms than Hg2+ because of the solubility
of Hgo in lipid-rich tissue (Louma, 1983). However, Hg+2 is readily available to plants, but under reducing conditions
and in presence of free sulfide ligands, mercury is stabilized in the Hg+2 state as extremely insoluble sulfide mineral
precipitates or is bound as surface complexes with organic matter containing sulfur. Methylation of Hg2+ in natural
environments leads to formation of volatile organic complexes that are several times more bioavailable than inorganic
forms of mercury. Methylated mercury species are also one of the most toxic pollutants in the biosphere. Natural
production of methyl mercury occurs under anoxic conditions and is probably mediated by microbes, but methylation
of Hg+2 is inhibited by elevated sulfide contents (Kersten, 1988). Increased pH appears to increase availability of
mercury to marsh plants possibly by causing conversion of Hg+2 to Hgo. Sediment can serve as a sink for mercury
discharged to the environment; partition coefficients, between suspended matter and water, are usually in the range
of 105 (Kersten, 1988). Sorption is the main process for enrichment of mercury in sediment. Sorption can be
influenced by chloride ligand concentration due to formation of chloro-mercurial complexes; in seawater, only
organic matter retains its sorption capacity for mercury. Most particulate mercury in natural aquatic environments
is associated with humic and other organic materials as well as oxide and sulfide minerals. Several studies have
shown that mercury is less bioavailable in sediment that is rich in organic matter (Luoma, 1989). Most mercury
released from point sources during mining is bound to sulfide compounds and is relatively non-bioavailable.
However, a large percentage of mercury becomes associated with organic complexes downstream from point sources,
possibly due to mobilization of mercury from pore fluids by humic acids. In natural water, suspended matter is the
main transporting medium for mercury. In oxic sediment, most mercury is bound in unknown (complexed?)
chemical forms that are readily susceptible to transformation, thereby affecting its mobilization and bioavailability.
Molybdenum: Molybdenum is an essential element for many animals and plants as it is required in their enzyme
system. Molybdenum can be present in molybdate anions, MoO4-2, in soil where it can be mobile and bioavailable,
because it is geochemically similar to sulfate. Molybdate ion is often associated with iron oxyhydroxide minerals,
where it competes with phosphate and organic matter. Molybdenosis in animals is associated with soil that contains
large amounts of available molybdenum, especially in forage plants with low sulfur and copper contents (Neuman
and others, 1987).
Selenium: In the arid western United States, a number of environmental problems are related to high selenium
abundances. These problems are mainly due to irrigation practices that allow selenium to accumulate in drains,
reservoirs, and wetlands. Under these conditions, selenium can be bioavailable to plants and birds and accumulate
to toxic levels (Severson and Gough, 1992). Extensive work has been done by the U.S. Geological Survey to
16
determine the geochemistry of selenium, most of which is associated with (adsorbed on) oxide minerals, such as
goethite (Balistrieri and Chao, 1987), amorphous iron oxyhydroxide and manganese oxide minerals (Balistrieri and
Chao, 1990), and is relatively immobile. A chemical leach using 0.1 M KH2PO4 can be used to determine the
soluble-available selenium (mainly as Se04-2) content of soil with low oxide mineral contents and high pH (Chao and
Sanzolone, 1989).
Zinc: In slightly basic, anoxic marsh sediment environments, zinc is effectively immobilized and not bioavailable
(Gambrell and others, 1991). Substantial amounts of zinc are released to solution if this sediment is oxidized or
exposed to an acidic environment. Very high abundances of soluble zinc are present under well oxidized conditions
and at pH 5 to 6.5, whereas low abundances of soluble zinc are present at pH 8 under all redox conditions and at
pH 5 to 6.5 under moderately and strongly reducing conditions (Gambrell and others, 1991). In polluted river
environments, most zinc is scavenged by non-detrital carbonate minerals, organic matter and oxide minerals and is
less mobile than cadmium (and perhaps less mobile than lead) (Prusty and others, 1994). Elevated chloride contents
SUMMARY
Bioavailability of metals released from mineral deposits is very complex and dependent on many interrelated
chemical, biological, and environmental processes. These processes may vary over time and among micro-organisms,
plants, and animals. In soil and surface water, the mining method, presence or absence of sulfide minerals, quantity
of water, acid-buffering capacity, presence of organic matter and iron and manganese oxide minerals, element
speciation, and concentrations of other constituents in water may impact dissolved and bioavailable metal and
metalloid contents. Field and laboratory studies of particular sites using soil, plants, and selective chemical extraction
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_________1989, Can we determine the biological availability of sediment-bound trace elements?: Hydrobiologia, v.
176/177, p. 379-396.
McKinney, James, and Rogers, Ron, 1992, Metal bioavailability: Environmental Science and Technology, v. 26, p.
1298-1299.
Morse, J.W., 1994, Interactions of trace metals with authigenic sulfide minerals: Implications for their bioavailability:
Marine Chemistry, v. 46, p. 1-6.
Neuman, D.R., Shrack, J.L., and Gough, L.P., 1987, Copper and molybdenum, in Williams, R.D., and Shuman, G.E.,
eds., Reclaiming mine soils and overburden in western U.S.: Soil Conservation Society of America, Ankeny,
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18
GEOPHYSICAL METHODS IN EXPLORATION
AND MINERAL ENVIRONMENTAL INVESTIGATIONS
INTRODUCTION
In the following discussion, the applicability of geophysical methods to geoenvironmental studies of ore deposits is
reviewed. Details of geophysical techniques are not emphasized; these are covered in standard texts (Society of
Exploration Geophysicists, 1966; 1990) and have been summarized in Hoover and others (1992).
Various geophysical methods are identified in table 1 (adapted from a chart compiled by Companie General
de Geophysique, Massy, France and published with modification by Van Blaricom, 1980). The table identifies the
utility of each method in airborne, ground, or borehole applications. Borehole methods are a specialized branch of
geophysics that are not emphasized here. However, where ground disturbance is not prohibited, or where drillholes
exist, there are a variety of useful borehole, crosshole, and borehole-surface techniques that may aid geoenvironmen
tal studies (Mwenifumbo, 1993).
Table 1 also outlines physical parameters and properties, anomaly sources, and depth of investigation for
each method. If a feature of geoenvironmental concern does not have an associated, measurable, physical property
then geophysical investigations are not applicable. Depth of burial (of contaminant plumes, for instance) is extremely
important in assessing the potential applicability of geophysics in geoenvironmental hazards identification.
Geophysical responses for more deeply buried sources decrease in amplitude and increase in spatial wavelength until
they disappear into geologic noise. Physical properties of cover, host rock, and mineral waste strongly influence
responses of potentially hazardous material and are also important for evaluating the utility of a method in
geoenvironmental investigations.
Some geophysical methods, such as gamma-ray spectrometry and remote sensing, measure surface attributes;
others, such as thermal and some electrical methods are limited to detecting relatively shallow features but may help
identify features at greater depth. Secondary effects of deeper features, such as geochemical haloes, can often be
identified by these methods.
Geophysical modeling provides generalized and non-unique solutions to questions concerning the geometry
of subsurface geologic relations. The non-uniqueness of these solutions is both a mathematical problem and one
related to the multiplicity of sources that can cause geophysical anomalies. This feature is an implicit uncertainty
in the discussion that follows. Environmental geophysics, like exploration geophysics, requires complimentary
geophysical surveys integrated with geochemical and geologic insight.
This presentation first summarizes geophysical methods. Following the methods summary, geophysical
strategies (that usually employ multi-technique approaches) for specific geoenvironmental investigations are
discussed.
GRAVITY METHOD
Gravity measurements define anomalous density within the Earth; in most cases, ground-based gravimeters are used
to precisely measure variations in the gravity field at different points. Gravity anomalies are computed by subtracting
a regional field from the measured field, which result in gravitational anomalies that correlate with source body
density variations. Positive gravity anomalies are associated with shallow high density bodies, whereas gravity lows
are associated with shallow low density bodies. Thus, deposits of high-density chromite, hematite, and barite yield
gravity highs, whereas deposits of low-density halite, weathered kimberlite, and diatomaceous earth yield gravity
lows. The gravity method also enables a prediction of the total anomalous mass (ore tonnage) responsible for an
anomaly. Gravity and magnetic (discussed below) methods detect only lateral contrasts in density or magnetization,
respectively. In contrast, electrical and seismic methods can detect vertical, as well as lateral, contrasts of resistivity
and velocity or reflectivity.
Applications of gravity to mineral deposit environmental considerations includes identification of lithologies,
structures, and, at times, orebodies themselves (Wright, 1981). Small anomalous bodies, such as underground
workings, are not easily detected by gravity surveys unless they are at shallow depth.
MAGNETIC METHOD
The magnetic method exploits small variations in magnetic mineralogy (magnetic iron and iron-titanium oxide
minerals, including magnetite, titanomagnetite, titanomaghemite, and titanohematite, and some iron sulfide minerals,
including pyrrhotite and greigite) among rocks. Measurements are made using fluxgate, proton-precession,
19
Table 1. Summary of geophysical methods and their characteristics applicable to exploration and geoenvironmental studies.
[In method column: A, airborne surveys; B, borehole surveys; and G, ground surveys]
Method Physical parameter Typical units Relevant physical Typical source of Depth of investiga-
measured property anomaly tion
Gravity: A,B,G Total attraction of Milligals or gravity unit Density Rock density con- All
Earth's gravity field (0.1 mGal) trasts
(the vertical attrac-
tion of anomalous
masses)
Gradient of Earth's Eötvös unit (10-9 gal/cm)
gravity field
Magnetic: A,B,G Vector component, Nanotesla, or gammas Magnetic suscep- Magnetic suscep- Surface to Curie
or total attraction of tibility and rema- tibility and (or) rema- isotherm
Earth's magnetic nent magnetization nent magnetization
field " contrasts
Gradient of Earth's Nanotesla/m "
magnetic field
Thermal bore-hole or shallow Thermal gradient or Degrees C/m, degrees C Thermal conductiv- Thermal flux or con- Hole depth
hole: B temperature ity ductivity variations
Thermal remote sensing: A,G Surface temperature Degrees C Thermal inertia Thermal inertia con- About 5 cm
day and night trasts
Remote sensing: A Reflected radiation Recorded as optical or Spectral reflectance, Changes in spectral Surface only
intensity (UV, VIS, digital intensity image Albedo reflectance and Albe-
IR) do
20
Overhauser, and optical absorption magnetometers. In most cases, total-magnetic field data are acquired; vector
measurements are made in some instances. Magnetic rocks contain various combinations of induced and remanent
magnetization that perturb the Earth's primary field (Reynolds and others, 1990). The magnitudes of both induced
and remanent magnetization depend on the quantity, composition, and size of magnetic-mineral grains.
Magnetic anomalies may be related to primary igneous or sedimentary processes that establish the magnetic
mineralogy, or they may be related to secondary alteration that either introduces or removes magnetic minerals. In
mineral exploration and its geoenvironmental considerations, the secondary effects in rocks that host ore deposits
associated with hydrothermal systems are important (Hanna, 1969; Criss and Champion, 1984) and magnetic surveys
may outline zones of fossil hydrothermal activity. Because rock alteration can effect a change in bulk density as well
as magnetization, magnetic anomalies, when corrected for magnetization direction, sometimes coincide with gravity
anomalies.
Magnetic exploration may directly detect some iron ore deposits (magnetite or banded iron formation), and
magnetic methods often are an useful for deducing subsurface lithology and structure that may indirectly aid
identification of mineralized rock, patterns of effluent flow, and extent of permissive terranes and (or) favorable tracts
for deposits beneath surficial cover. Geoenvironmental applications may also include identification of magnetic
minerals associated with ore or waste rock from which hazardous materials may be released. Such associations
permit the indirect identification of hazardous materials such as those present in many nickel-copper or serpentine-
hosted asbestos deposits.
GAMMA-RAY METHODS
Gamma-ray methods (Durrance, 1986; Hoover and others, 1991) use scintillometry to identify the presence of the
natural radioelements potassium, uranium, and thorium; multi-channel spectrometers can provide measures of
individual radioelement abundances. Gamma-ray methods have had wide application in uranium exploration because
they provide direct detection. Thorium is generally the most immobile of the three radioelements and has
geochemical behavior similar to that of zirconium. Thorium content, like uranium content, tends to increase in felsic
rocks and generally increases with alkalinity.
Gamma-ray spectrometry, because it can provide direct quantitative measures of the natural radioelements,
provides geoenvironmental information concerning radiation dose and radon potential. Because uranium and (or)
potassium are commonly enriched in or adjacent to some deposits, their presence may often be used to indirectly
assess the potential for release of hazardous materials from ore or waste piles. Where sulfide minerals are present
their oxidation accelerates uranium mobilization.
SEISMIC METHODS
Seismic techniques have had relatively limited utilization, due to their relatively high cost and the difficulty of
acquiring and interpreting seismic data in strongly faulted and altered igneous terranes, in mineral assessments and
exploration at the deposit scale. However, shallow seismic surveys employ less expensive sources and smaller
surveys than are typical of regional surveys, and the cost of studying certain geoenvironmental problems in the near
subsurface may not be prohibitive. Reflection seismic methods provide fine structural detail and refraction methods
provide precise estimates of depth to lithologies of differing acoustic impedance. The refraction method has been
used in mineral investigations to map low-velocity alluvial deposits such as those that may contain gold, tin, or sand
and gravel. Applications in geoenvironmental work include studying the structure, thickness, and hydrology of
tailings and extent of acid mine drainage around mineral deposits (Dave and others, 1986).
THERMAL METHODS
Two distinct techniques are included under thermal methods (table 1): (a) borehole or shallow probe methods for
measuring thermal gradient, which is useful itself, and with a knowledge of the thermal conductivity provides a
measure of heat flow, and (b) airborne or satellite-based measurements, which can be used to determine the Earth's
surface temperature and thermal inertia of surficial materials, of thermal infrared radiation emitted at the Earth's
surface. Thermal noise includes topography, variations in thermal conductivity, and intrinsic endothermic and
exothermic sources.
Borehole thermal methods have been applied in geothermal exploration, but have seldom been used in
mineral exploration. However, this method has potential usefulness in exploration and in geoenvironmental
investigations (Ovnatanov and Tamrazyan, 1970; Brown and others, 1980; Zielinski and others, 1983; Houseman and
others, 1989). Causes of heat flux anomalies include oxidizing sulfide minerals and high radioelement concentrations.
21
Conditions that may focus, or disperse, heat flow are hydrologic and topographic influences, as well as anomalous
thermal conductivity. In geoenvironmental applications, oxidation of sulfide bodies in-place or on waste piles, if
sufficiently rapid, can generate measurable thermal anomalies, which can provide a measure of the amount of metal
being released to the environment. Borehole temperatures may also reflect hydrologic and hydrothermal systems that
have exploration and geoenvironmental consequences. Airborne thermal infrared measurements have applications
in geothermal exploration, and may have potential in mineral exploration and in geoenvironmental applications
whenever ground surface temperature is anomalous due to sulfide oxidation, hydrologic conditions, or heat-flow
perturbations due to structure or lithology (Strangway and Holmer, 1966).
Thermal infrared imaging methods are a specialized branch of more generalized remote sensing techniques.
Images obtained in this wavelength range may be used for photogeologic interpretation or, if day and night images
are available, to estimate the thermal inertia of the surface. Unconsolidated or glassy materials can be distinguished
by their low thermal inertia. In places, thermal infrared images can distinguish areas of anomalous silicification
(Watson and others, 1990).
ELECTRICAL METHODS
Electrical methods comprise a multiplicity of separate techniques that employ differing instruments and procedures,
have variable exploration depth and lateral resolution, and are known by a large lexicon of names and acronyms
describing techniques and their variants. Electrical methods can be described in five classes: (1) direct current
resistivity, (2) electromagnetic, (3) mise-a-la-masse, (4) induced polarization, and (5) self potential. In spite of all
the variants, measurements fundamentally are of the Earth's electrical impedance or relate to changes in impedance.
Electrical methods have broad application to mineral and geoenvironmental problems: they may be used to identify
sulfide minerals, are directly applicable to hydrologic investigations, and can be used to identify structures and
lithologies.
Electromagnetic method
Electromagnetic measurements use alternating magnetic fields to induce measurable current in the Earth. The
traditional application of electromagnetic methods in mineral exploration has been in the search for low-resistivity
(high-conductivity) massive sulfide deposits. Airborne methods may be used to screen large areas and provide a
multitude of targets for ground surveys. Electromagnetic methods, including airborne, are widely used to map
lithologic and structural features (Palacky, 1986; Hoover and others, 1991) from which various mineral exploration
and geoenvironmental inferences are possible.
Mise-a-la-masse method
The mise-a-la-masse method is a little used technique applied to conductive masses that have large resistivity
contrasts with their enclosing host rock. In exploration, application of this method is principally in mapping massive
sulfide deposits. This method is useful in geoenvironmental investigations of highly conductive targets; it has been
applied to identify a contaminant plume emanating from an abandoned mine site (Osiensky and Donaldson, 1994).
22
or self potentials. The association of a self potential anomaly with a sulfide deposit indicates a site of ongoing
oxidation and that metals are being mobilized; other self potential anomalies are due to fluxes of water or heat
through the Earth (Corwin, 1990). Geoenvironmental applications include searching for zones of oxidation and paths
of ground water movement.
OTHER METHODS
A number of other geophysical or quasi-geophysical methods have been used, or have potential application, in
mineral exploration. Application of these methods in geoenvironmental investigations has been limited, but should
not be dismissed. Some peripheral techniques that have special uses (as in archeology), whose utilization is not
widely known in mineral exploration, that may directly apply to shallow geoenvironmental investigations. Examples
of such techniques are ground-penetrating radar (used to image the shallow subsurface in electrically resistive rock;
Davis and Annan, 1992), the piezoelectric method (used in studies of quartz veins; Volarovich and Sobolev, 1969),
ultraviolet laser induced fluorescence (the Luminex method, used to identify scheelite, hydrozincite, and other
fluorescent minerals; Seigel and Robbins, 1983), airborne gas sniffing (used in mercury exploration), the Russian
CHIM (partial extraction of metals) electrogeochemical sampling technique, and radon sensing.
23
can be used to locate the rest of the installation. These methods only trace the metallic installation, so they will not
identify workings where the installation is absent or, in the mise-a-la-masse method, where workings are disconnected
from the energized installation segment. Further, the method may not map true voids; the tunnel where the metal
was installed may have collapsed.
Magnetometers have been used to trace burned-out coal seams, particularly pyrite-bearing coal. During
combustion, pyrite may oxidize to magnetite, which then can be traced using surface or airborne magnetic
measurements. If the burned seam is still hot, geothermometry and infrared sensing methods may be used.
When the above special conditions are not met, the challenge for geophysical techniques is to find large
voids, regardless of whether the voids are manmade. More reliable approaches to this challenge involve
microgravity, seismic, ground-penetrating radar, geoelectric surveys, or a combination thereof. Microgravity requires
measurements be made at a spacing about equal to the diameter of the workings being sought. Only shallow
workings, whose tops are located at a depth less than about one opening-diameter, can be traced using microgravity.
Shallow, high-resolution seismic methods and ground penetrating radar have been used to trace caves. Both
methods are applicable to identification of air- or water-filled openings. Jessop (1995) reports that ground
penetrating radar returned massive, ringing reflections from the top of an air-filled cave buried about 6 m in
sandstone. A large, but opposite polarity, ground penetrating radar response may result from water-filled caves; air
and water filled caverns may be differentiated in this way. Ground penetrating radar signals, however, are attenuated
by wet or clay soil, and are inefficient where these conditions exist in the surficial layer.
Geoelectrical work has also been successful in tracing cave systems. In the majority of reported cases, the
caves were in resistive limestone strata. Direct current resistivity is particularly useful in identification of air-filled
openings, though in some instances the caves were partly or completely filled with water. Electromagnetic methods,
particularly at very low frequencies, are also efficient where cave floors are covered by conducting clay deposits.
Guerin and Benderitter (1995) caution, however, that for caves in France that were traced, apparently successfully,
using very low frequency techniques, the electromagnetic response was mainly generated by mud-filled fractures in
the limestones rather than the caves themselves, which are localized along such fractures.
24
However, anomalies related to local sulfide mineral oxidation may be of low amplitude and difficult to resolve.
Induced polarization is inefficient as an areal reconnaissance tool, but efficiently identifies disseminated mineral
targets whose general location is already known. Advanced induced polarization equipment can discriminate between
benign electrical conductors, like clays, and environmentally threatening ones, like heavy-metal plumes or
petrochemical spills.
Other applications: Active oxidation of sulfide minerals may produce thermal anomalies that may be identified by
infrared surveys or shallow heat-flow probes. Limited data are available concerning the use of thermal signatures
in geoenvironmental investigations; thermal signatures may be low amplitude and overshadowed by topographic
effects, thermal inertia contrasts in surrounding rocks, and heat distribution effects related to ground water circulation
(Strangway and Holmer, 1966). Oxidized pyrrhotite within limited areas, for instance along fluid conduits in waste
dumps, may be coincident with magnetic lows.
Argillic alteration
Argillic alteration that surrounds some sulfide deposits may be associated with resistivity lows, induced polarization
highs, and moderate magnetic lows, whereas silicified rock and quartz lenses associated with sulfide deposits are
associated with high resistivity, low magnetization, and increased density. Altered tracts commonly have diagnostic
reflectance patterns on remote-sensing multispectral images. In addition, trace elements and alteration products may
be absorbed by plants resulting in recognizable reflectance anomalies.
SUMMARY
Many geophysical methods commonly used in exploration have potential application to geoenvironmental
investigations. Although these methods have mainly been used to identify pollutants and record their dispersion from
mine areas, their application is not limited to studies of this sort. For instance, geophysical monitoring of pollutant
activity, which requires significantly greater study, is another aspect of geoenvironmental investigations. Monitoring
differs from detection chiefly in recurrent use of geophysical techniques. The effort required to extend application
of geophysical techniques to naturally occurring pollutants related to mineralized, but unmined, rock or to other
cultural concentrations of toxic or potentially toxic substances is minimal and could be of considerable assistance
in meeting national needs for healthy environmental conditions (Henderson, 1992).
25
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27
MAGMATIC SULFIDE DEPOSITS
(MODELS 1, 2b, 5a, 5b, 6a, 6b, and 7a; Page, 1986a-g)
Deposit geology
Magmatic sulfide deposits are sulfide mineral concentrations in mafic and ultramafic rocks derived from immiscible
sulfide liquids. A number of schemes exist for subdividing these deposits. Most are based on the tectonic setting
and petrologic characteristics of the mafic and ultramafic rocks (Page and others, 1982; Naldrett, 1989), or on the
spatial association of mineralized rock with enclosing ultramafic and mafic host rocks (stratabound, discordant,
marginal, and other; Hulbert and others, 1988). Page (1986a-g) presented discussions of several different subtypes
based, in part, on both these approaches (Models 1, 2b, 5a, 5b, 6a, 6b, and 7a). However, these deposits are similar
enough that they can be treated as a group with regard to their geoenvironmental manifestations.
The similarity of these deposits result, in part, from similar genesis. Exsolution of immiscible sulfide liquids
from mafic-to-ultramafic magmas is the fundamental process that forms magmatic sulfide deposits. Once formed,
droplets of immiscible sulfide liquid settle through less dense silicate magma. The sulfide liquid acts as a "collector"
for cobalt, copper, nickel, and platinum-group elements (PGE) because these elements are preferentially concentrated
in sulfide liquids at levels 10 to 100,000 times those in silicate liquids. To a lesser degree, iron is also preferentially
partitioned into the sulfide liquid and, because of its greater abundance, most immiscible sulfide liquid is iron-rich.
The combination of physically concentrating dense sulfide liquid and chemically concentrating elements in
the sulfide liquid is responsible for forming most economically minable, magmatic-sulfide deposits. Magmatic
sulfide ore is typically associated with: (1) abrupt variations in the cumulus-mineral succession, including major
lithologic changes, reversals or changes in crystallization order, discontinuities in mineral fractionation patterns and
cyclic units, (2) rocks near the lower contact of an intrusion that may contain country rock xenoliths and may be
characterized by irregular variations in grain size, mineralogy, and texture, (3) rocks near the base of a flow, or (4)
pegmatoids and rocks enriched in minerals that crystallize late from silicate magmas. However, for the purposes of
developing a geoenvironmental model for this group of deposits, the principal variables are the composition of the
host rocks, the abundance and types of sulfide minerals, and (to a much lesser extent) sulfide mineral composition.
General characteristics for the deposit subtypes described by Page (1986a-g) are listed in table 1.
Examples
Magmatic sulfide minerals concentrated near the margins of intrusions:
Stillwater nickel-copper (Model 1)-Mouat deposit, Stillwater Complex, Mont. (Zientek, 1993); Vaaralampi
deposit, Suhanho-Konttijarvi intrusion, Finland (Alapieti and others, 1989)
Duluth Cu-Ni-PGE (Model 5a)-Dunka Road deposit, Duluth Complex, Minn.; Great Lakes nickel deposit,
Crystal Lake Gabbro, Ontario, Canada (Eckstrand and others, 1989)
Synorogenic-synvolcanic Ni-Cu (Model 7a)-Brady Glacier deposit, La Perouse Intrusion, Alaska; Big Indian
Pond, Moxie intrusion, Maine
Noril'sk Cu-Ni-PGE (Model 5b)-Medvezhy Creek deposit, Noril'sk and Oktybr'sky deposit, Talnakh; Russia
Impact-related intrusions-Sudbury Complex, Canada
Stratiform concentrations of disseminated magmatic sulfide minerals in layered intrusions:
Merensky Reef PGE (Model 2b)-Merensky Reef, Bushveld Complex, Republic of South Africa (Naldrett
and others, 1987); J-M Reef, Stillwater Complex, Mont. (Todd and others, 1982)
Magmatic sulfide mineral and PGE concentrations at or below impermeable layers:
No model-Picket Pin deposit, Stillwater Complex, Mont. (Boudreau and McCallum, 1986)
Pegmatoidal lenses, pipes, and other discordant mineralization:
No model-Vlakfontein nickel pipes, Bushveld Complex, Republic of South Africa (Vermaak, 1976); Janet
50 zone, Stillwater Complex, Mont. (Volborth and Housley, 1984)
Magmatic sulfide minerals concentrated in ultramafic volcanic rocks:
Komatiitic Ni-Cu (Model 6a)-Kambalda deposits, Australia
Dunitic Ni-Cu (Model 6b)-Mount Keith deposit, Australia
Magmatic sulfide minerals concentrated in ultramafic cumulates in ophiolite complexes:
No model-Acoje, Philippines; Kraste, Albania
28
Table 1. General features of different magmatic sulfide deposit types. (Model numbers from Page, 1986a-g)
Deposit Model General Sulfide mineral Sulfide mineral Size Host rock
Type no. description abundance composition
Meren 2b Thin (1-5 m) dissemina- Disseminated Fe-Ni-Cu Thin zones that may Mafic
sky tions of sparse (1-5 PGE extend laterally for
Reef percent) sulfide min over 100 km. Individual
erals in mafic (gabbroic mines within these zones
and troctolitic) rocks often report reserves
within the main body of greater than 100 million
large, layered intrusions tons
Duluth 5a Disseminated to massive Mostly dissem- Fe>Cu>Ni Small massive pods to Mostly
Cu-Ni- concentrations in mafic inated, some disseminated bodies con- mafic,
PGE to ultramafic rocks in the massive taining as much as several lesser
basal parts of rift-related hundred million tons of ultramafic
intrusions ore
Noril'sk 5b Disseminated sulfide in Extensive dis- Fe>Cu>Ni>PGE Disseminated deposits tens Mafic,
Cu-Ni- lower third, and massive seminated and of meters thick over en- ultramafic,
PGE sulfide near base of com massive ore tire area of intrusion; and meta
plex, subvolcanic, elon- Massive sulfide orebodies sedimentary
gate intrusions less than as much as 45 m thick and
350 m thick 2.5 km2 in area
Komati 6a Mostly massive, lesser Mostly massive, Fe>Ni Generally deposits less Ultramafic
itic Ni disseminated, sulfide minor dissemi than 2 million tons; me-
Cu deposits at the base of nated dian size is 1.6 million
Archean or Proterozoic tons
ultramafic, komatiitic
flows
Dunitic 6b Disseminated sulfide de- Disseminated Fe>Ni Large low grade deposits Ultramafic
Ni Cu posits within Archean or with median size about
Proterozoic komatiites 30 million tons
Associated deposit types (Cox and Singer, 1986) include asbestos (Model 8d); soapstone; greenstone gold (Model
36a); Bushveld chromite (Model 2a); podiform chromite (Model 8a), Bushveld iron-titanium-vanadium (Model 3),
platinum group element placer (Model 39a), nickel laterite (Model 38a).
(1) Mining exposes sulfide minerals that have significant acid generating potential.
(2) Metals associated with sulfide (particularly iron sulfide) ore may contaminate ground and surface water.
(3) Some deposits are extremely large and development may involve ground disturbance throughout large areas.
(4) Sulfur dioxide may be vented to the atmosphere during sulfide ore smelting; downwind acid and toxic metal
29
abundances may be enhanced.
(5) Significant amounts of sulfide-mineral-bearing tailings and slag are produced; these must be isolated from surface
Many magmatic sulfide deposits are quite large or form groups that define large mining districts. Past
mining of some of these deposits or districts has produced some well-known examples of severe environmental
impact. The impact associated with mining in the Sudbury district (Canada) is probably the best documented; the
severe environmental degradation associated with the Noril'sk district (Russia) is less well known. However,
implementation of new pollution-control techniques and aggressive efforts at environmental remediation have
substantially improved the environment around the Sudbury district. Recently permitted and currently ongoing
mining of the palladium-platinum deposit in the Stillwater Complex, Mont., demonstrates that operations extracting
ore from these deposits can meet the most rigorous modern environmental standards.
Exploration geophysics
Interconnected sulfide minerals produce electrically conductive zones that can be located with induced polarization,
electromagnetic, and magneto-telluric surveys. Some of these deposits may be identified by magnetic surveys
because they may contain abundant magnetite. Magnetite is also abundant in most mafic host rocks and may indicate
regional targets or be a source of "noise" in mineral exploration. Large sulfide mineral masses may be located by
seismic refraction. Sulfide-mineral concentrations and their mafic and ultramafic host rocks may be associated with
mass excesses that can be identified by gravity surveys. Remote sensing may help identify areas in which ore is
present. In particular, band ratioing can be used to identify gossans; more generally, images can be used to identify
References
Deposit size
Host rocks
Host rocks are predominantly mafic to ultramafic igneous rocks. Occasionally significant ore is in footwall country
Deposits are in diverse geologic settings, including (1) deformed greenstone belts and calc-alkaline batholiths
associated with convergent plate margins, (2) ophiolite complexes that formed at constructive plate margins, (3)
intraplate magmatic provinces associated with flood-basalt type magmatism, and (4) passively rifted, continental
margins.
Wall-rock alteration
Hydrothermal alteration related to ore-forming processes is generally not significant for magmatic sulfide deposits.
Many deposits and host rocks have experienced varying amounts of alteration either as a result of deuteric processes,
metamorphism, or weathering. Primary silicate mineralogy consists of varying proportions of calcic plagioclase,
orthopyroxene, clinopyroxene, and olivine. Brown amphibole and biotite may be minor accessory phases. Trace
amounts of quartz, apatite, and potassium feldspar may be present. In most cases, alteration involves development
of hydrous phases. Systematic alteration zoning may be developed adjacent to faults or fractures that focus fluid
flow. Olivine is the phase most likely to be altered; alteration of plagioclase and pyroxene is somewhat less likely.
Olivine is typically altered to serpentine minerals, magnetite, and minor calcite. Plagioclase is altered to epidote,
clay minerals, sericite, and calcite. Pyroxenes are altered to actinolite/tremolite, serpentine, talc, and chlorite. Biotite
is altered to chlorite.
Nature of ore
Sulfide minerals may be concentrated in structurally low areas at the base of intrusions or flows (fig. 1) or may be
in zones where silicate magma interacted with xenoliths. Sulfide mineral concentrations in layered, cumulate
30
Figure 1. A, Schematic section of a magmatic sulfide deposit showing the vertical gradation downward from disseminated to massive ore. B,
Generalized map showing massive sulfide mineral concentrations in footwall embayments of the Sudbury Complex (Canada).
sequences may be related to major lithologic features such as cyclic-unit boundaries, unconformities, chromite seams,
pegmatoids, or stratigraphic intervals characterized by major changes or discontinuities in cumulus minerals.
Deformation and alteration can remobilize sulfide minerals into breccia ore and segregate sulfide minerals
into fractures, cleavage planes, and veins. Remobilized sulfide-mineral assemblages may be copper-rich relative to
sulfide mineral assemblages that are not remobilized. Sulfide-mineral assemblages that appear to have precipitated
from fluids moving through fault zones or along joint surfaces are dominated by pyrite.
31
Table 2. Representative major element compositions from some magmatic
sulfide deposits normalized to 100 percent sulfides (after Naldrett, 1989).
[Ni, Cu, and Co weight percent; Pt and Pd, ppb. ---, no data]
Deposit/ Ni Cu Co Pt Pd
magma type
Deposit S Pd Pt Te Bi As Sb Hg Sn
J-M Reef1 <0.01 0.38 0.33 0.03 <0.01 0.18 <0.05 <0.01 0.3
1.64 182 51.6 17 20 16.2 1 0.16 1.3
MSZ2 0.36 0.088 0.26 0.35 0.62 0.42 <0.05 <0.01 0.30
2.8 3.1 6.1 4.8 5.8 2.2 0.22 0.38 0.60
1
Median values of drill core from 3800 W stope of Stillwater Mine, J-M Reef (Zientek and others,
2
Median values of two intercepts through the Main Sulfide Zone, Great Dyke, Zimbabwe (Zientek
3
Median values of all ore samples, Noril'sk Talnakh district (Zientek, unpub. data). Maximum
approximate ranges of 60 to 650 ppm zinc, 130 to 390 ppm selenium, 11 to 210 ppm silver, 8 to 35 ppm cadmium,
15 to 35 ppm tin, 19 to 390 ppm tellurium, 150 to 690 ppm lead, and as much as 60 ppm gallium and 30 ppm
Sulfide-mineral ore assemblages, dominated by pyrrhotite, pentlandite, and chalcopyrite, result from solid-state
recrystallization of high-temperature sulfide minerals. These three minerals are the principal acid generating phases
in magmatic sulfide deposits and their proportions are determined by the initial bulk composition of the immiscible
sulfide liquid. The sulfide mineral content of these ore deposits varies from less than ten to more than sixty percent.
32
Magnetite is commonly intergrown with the sulfide minerals. Minor phases include platinum-group-element
minerals (sulfide, arsenide, telluride, antimonide, and alloy minerals), nickel- and cobalt-bearing arsenide minerals
(for example gersdorffite), galena, sphalerite, and gold, silver, and lead telluride minerals.
Gangue mineralogy is the same as that of the host and consists primarily of plagioclase, orthopyroxene,
clinopyroxene, and olivine. Minor, secondary phases include serpentine minerals, talc, magnetite, calcite, epidote,
sericite, actinolite, chlorite, tremolite, and clay minerals.
Mineral characteristics
The mineralogy and textures of sulfide ore record a prolonged and complex process starting with solidification of
the sulfide liquid, either as an iron-nickel-rich or an iron-copper-rich solid solution and continuing solid-state
transformation and recrystallization; these textures can be substantially modified by alteration and weathering. Iron,
When not modified by weathering or alteration, the textures of silicate and sulfide minerals record the
distribution and abundance of the sulfide liquids and the interaction between solid silicate minerals and molten sulfide
liquid. In rocks with less than 10 volume percent sulfide minerals (disseminated ore), sulfide minerals form fine
( <1 mm)- to coarse-grained ( >5 mm) droplet-shaped aggregates that are molded around and interstitial to the
cumulus or earlier-formed silicate minerals or may be present as fine-grained, rounded aggregates enclosed in
cumulus minerals. In rocks containing 10 to 60 volume percent sulfide minerals (matrix ore), aggregates of sulfide
minerals are interstitial to earlier-formed silicate minerals but are interconnected. In rocks with more than 60 volume
percent sulfide minerals (massive ore), sulfide minerals form the matrix of the rock.
The bulk sulfur content of sulfide-mineral aggregates is between 34 and 40 weight percent; the remaining
60 to 66 weight percent is mostly iron, plus copper and nickel. Consequently, the molecular-metal-sulfur ratio of
magmatic sulfide minerals is relatively constant at about 1:1. In contrast, pyrite has a metal-sulfur ratio of 1:2.
Therefore, magmatic sulfide ore has a much more restricted acid-generating capacity than ore that contains substantial
pyrite.
Secondary mineralogy
Minerals that may form during alteration and weathering of sulfide minerals include violarite, bornite, mackinawite,
cubanite, pyrite, marcasite, troilite, vaesite, smythite, polydymite, millerite, hematite, and magnetite. In supergene
environments, chalcocite, malachite, native copper, cuprite, nickel-iron carbonate, nickel- and nickel-iron
hydroxycarbonate, and nickel-silicate minerals may form. Gossans commonly form above sulfide-rich rocks.
Topography, physiography
Hydrology
Many deposits form stratiform sheets and lenses, near the bottoms of intrusions or flows, that could localize ground
water flow. However, no known consistent relation between magmatic sulfide ore and hydrologic controls are
known.
These deposits have been and are being mined both by underground and by open-pit methods. Underground mining
is currently in progress at platinum-group-element deposits of the Stillwater, Mont., and Bushveld, South Africa,
complexes and the komatiite-hosted nickel deposits in the Kambalda, Australia, district. Deposits being mined both
by open pit and underground methods include those in the Sudbury Complex, Canada, the Manitoba, Canada, nickel
belt, and the Noril'sk-Talnakh, Russia, district. Subsequent to mining, most ore is ground and concentrated by
flotation, gravity, or magnetic methods to form either a bulk sulfide mineral concentrate or separate copper- and
nickel-rich concentrates. Pyrrhotite-rich (iron-rich) concentrates at Noril'sk are being stored for later processing.
Gravity concentrates may be sent directly to refineries, but most ore is smelted to separate iron-rich slag from nickel-
and copper-rich matte. This matte is then refined by a variety of processes to extract nickel, copper, and platinum
33
Table 4. Composition of drainage water associated with Duluth
and Stillwater Complex sulfide deposits (SCS Engineers, 1984;
Feltis and Litke, 1987; Ritchie, 1988). Data in µg/l; --, no data.
pH Cu Ni Co Zn
ENVIRONMENTAL SIGNATURES
Drainage signatures
Studies show that the aqueous concentrations of nickel, copper, iron, and cobalt are largely controlled by their
absorption on hydrous oxide minerals of iron and manganese (Richter and Theis, 1980). Estimates indicate that of
the nickel transported by major rivers, 0.5 percent is in solution, 3.1 percent is adsorbed, 47 percent forms
precipitated coatings, 14.9 percent is in organic matter, and 34.4 percent is crystalline material (Snodgrass, 1980;
Nriagu, 1980). Only about 1 percent of copper in surface water is transported in a soluble form, while about 85
percent is moved as particulate crystalline phases, 6 percent is bound to metal hydroxide coatings on particles, 5
percent is associated with organic material, and 3 percent is adsorbed onto suspended particles (Nriagu, 1979).
Some of the limited data available for water draining unmined and mined deposits associated with mafic
or ultramafic rocks are presented in table 4. The data include analyses of water that drains naturally exposed sulfide
minerals at the base of the Stillwater, Mont., Complex and that from four different mining sites in the Duluth, Minn.,
Complex. The data from table 4 are plotted on a "Ficklin" diagram (fig. 2). Despite the fact that this water was
analyzed for only a few of the metals typically reported on this plot, the analyses show quite high metal
concentrations, especially for water that has a relatively neutral pH.
Additional information indicates that water discharged from some bulk-ore sample sites in the Duluth
Complex contains nickel, copper, cobalt, and zinc abundances as much as 400 times baseline abundances (Eger and
Lapakko, 1989); one study reports as much as 700 µg/l copper and nickel in contaminated water, whereas adjacent
ground water contained less than 25 µg/l metal (Siegel and Ericson, 1980). Best- and worse-case water quality
estimates were made for emissions from test pits and ore stockpiles as part of a regional environmental study.
Estimates for the combined abundances of copper, nickel, cobalt, and zinc were between 107 and 6,610 µg/l for
Figure 2. Plot of partial water analyses from the Duluth complex (striped) and Stillwater complex (solid).
34
discharges from tailings basins, 2,534 to 46,310 µg/l for emissions from ore stock piles, and 125 to 46,840 µg/l for
mine water (Ritchie, 1988). "Mine water" used to simulate environmental effects related to the Duluth Complex in
remediation studies has a pH of 4.5 and contains 2,000 µg/l sulfate and between 50 and 1,000 µg/l nickel (Hammack
and Edenborn, 1992). Laboratory study of Duluth Complex ore (Lapakko and Antonson, 1994) has demonstrated
a direct correlation between ore sulfur content and pH of associated drainage water. Drainage water associated with
ore that contains 0.18 to 0.4 weight percent sulfur has a pH of 6.1, water associated with ore that contains 0.41 to
0.71 weight percent sulfur has a pH between 4.8 and 5.3, and water associated with ore that contains 1.12 to 1.64
weight percent sulfur has a pH between 4.3 and 4.9. Finally, a study of Sudbury ore from the Nickel Rim nickel-
copper tailings impoundment demonstrates a well-developed vertical gradient within tailings water compositions.
Water draining the uppermost part of the tailings has low pH (2.1 to 3.5), whereas that draining the basal part of
the tailings, where acid-consuming minerals are more abundant, has a pH of about 6.5. The nickel abundance of
tailings water is sensitive to pH; nickel abundances change from 250,000 µg/l to less than 10,000 µg/l as pH increases
Magmatic sulfide deposits may contain widely variable amounts of sulfide minerals. As a generalization, deposits
may be separated into two groups. Platinum-group-element-rich deposits in large, layered intrusions tend to have
low sulfide mineral abundances (1 to 5 weight percent) and low total-metal abundances. Consequently, they have
a relatively restricted capacity to generate significant amounts of acidic and (or) metal-enriched drainage. Most of
the other economically extractable magmatic-sulfide deposits contain substantial amounts of sulfide minerals (most
greater than 15 weight percent, many exceeding 40 weight percent) and large metal abundances. They have
significantly greater potential for generating acid and (or) metal-enriched drainage.
The limited data summarized in figure 2 are for deposits with high sulfide mineral concentrations. Mine
water may contain high concentrations of metals, even when pH is near neutral. The relatively high pH of this water
may reflect both the relatively restricted acid generating capacity of these sulfide minerals as compared to ore having
lower metal-sulfur ratios and the acid buffering capability of mafic and ultramafic host rocks.
Kwong (1993) shows that acid generation capacity is directly proportional to both the metal-sulfur atomic
ratio and to the proportion of ferrous iron in the sulfide phases. Pyrite, with its 1:2 metal-sulfur ratio, for example,
has a much greater acid generating capacity than most magmatic sulfide ore, which has a metal-sulfur ratio of
approximately 1. Non-iron-bearing phases with metal-sulfur ratios 1 do not generate acid. Further, calcic
plagioclase and olivine present in the host rocks of most magmatic sulfide deposits are fast-weathering and fairly
reactive minerals (Kwong, 1993). Consequently, both mafic and ultramafic igneous rocks have moderate acid
buffering capacity. Although somewhat less reactive and slower weathering, serpentine also can be an effective
buffer and field studies document that serpentine significantly increases mine water pH (Germain and others, 1994).
Soil above magmatic sulfide deposits typically has elevated metal contents, and soil geochemistry is commonly used
as an exploration tool. Background and anomalous values depend on bedrock and ore compositions. The average
(Levinson, 1980) copper content of soil is 20 ppm (range, 2 to 150 ppm), whereas its average nickel content is 30
ppm (range, 5 to 500 ppm); typical background and anomalous values for platinum-group elements are from 2-10
Mining produces large amounts of waste. It is not unusual for 100 tons of rock to be extracted to obtain 1 ton of
metal. Sulfide mineral-free waste may be used in road construction or similar uses. However, much waste contains
sufficient sulfide minerals to generate acid and must be safely stored or isolated. Storage under water to reduce
oxidation or as backfill in mining cavities is common. Surface storage requires collection of contaminated water for
acid neutralization.
Tailings produced by the separation of ore from waste also contain significant amounts of sulfide minerals
that can generate acid. Tailings are commonly used to backfill mined areas. Surface disposal of tailings often
involves revegetation to stabilize and isolate sulfide minerals. However, these tails still have significant acid
producing capability.
Smelter signatures
Magmatic sulfide ore concentrates are typically smelted and may produce SO2- and metal-rich emissions. These,
35
in turn, may cause acid rain that significantly reduces the pH of local streams and lakes and distributes metals that
contaminate adjacent soil and plants. As an example, the daily discharge in 1971 from one smelter in the Sudbury,
Canada, district was 32.6 tons of iron, 6 tons of nickel, 5 tons of copper, 0.13 tons of cobalt, 0.5 tons of lead, and
0.4 tons of zinc (Hutchinson, 1979). Areas that have been heavily impacted by past emissions of sulfur-rich gases
from smelters include Sudbury, Canada and Noril'sk, Russia.
Whereas old smelters released virtually 100 percent of the sulfur in ore as SO2, new smelting technologies
can trap all but about 10 percent of the sulfur (Crawford, 1995). Capture of sulfur emissions can result in rapid and
dramatic changes in the environment. As an example, in 1980, 45 of the 104 lakes in the Sudbury district had a pH
of less than 5.5. Implementation of new SO2-emission controls resulted in rapid changes; by 1987, 84 lakes had pH
values greater than 5.5.
The effects of various climatic regimes on the geoenvironmental signature specific to magmatic sulfide deposits is
not known. However, in most cases the intensity of environmental impact associated with sulfide-bearing mineral
deposits is greater in wet climates than in dry climates. Acidity and total metal concentrations in mine drainage in
arid environments are several orders of magnitude greater than in more temperate climates because of the
concentrating effects of mine effluent evaporation and the resulting "storage" of metals and acidity in highly soluble
metal-sulfate-salt minerals. However, minimal surface water flow in these areas inhibits generation of significant
volumes of highly acidic, metal-enriched drainage. Concentrated release of these stored contaminants to local
Geoenvironmental geophysics
Electrical geophysical techniques can be used to delineate low resistivity and polarizable minerals that may help
identify the extent of sulfide deposits. In tailings and waste dumps, geophysical techniques can also be used to
identify sulfide mineral concentrations and acidic water formed by sulfide mineral oxidation. Resistivity, shallow
seismic refraction, and ground-penetrating radar can be used to map water-flow-controlling structures, determine
thicknesses of tailing and waste dumps, and often can aid in identification of the water table. Active redox centers
can be delimited by self-potential surveys, and possibly by shallow thermal probes, but their signals are susceptible
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36
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37
SCS Engineers, 1984, Summary of damage cases from the disposal of mining wastes: Report prepared for the U.S.
Environmental Protection Agency under contract 68-02-3179, 596 p.
Siegel, E.I., and Ericson, D.W., 1980, Hydrology and water quality of the copper-nickel study region, northeastern
Minnesota: U.S. Geological Survey Water Resources Investigations Open file report 80-739, 87p.
Smith, B.H., 1984, Geochemical exploration for nickel sulphides in lateritic terrain in Western Australia, in
Buchanan, D.L. and Jones, M.J., eds, Sulphide deposits in mafic and ultramafic rocks, Institute of Mining
and Metallurgy, London, p. 35-42.
Snodgrass, W.J., 1980, Distribution and behavior of nickel in the aquatic environment, in Nriagu, J.O., ed., Nickel
in the environment, John Wiley & Sons, Inc., New York, New York, p. 203-274.
Todd, S.G., Keith, D.W., LeRoy, L.W., Shissel, D.J., Mann, E.L., and Irvine, T.N., 1982, The J-M platinum
alladium Reef of the Stillwater Complex, Montana: I. Stratigraphy and petrology: Economic Geology, v.
77, p. 1454-1480.
Vermaak, C.F., 1976, The nickel pipes of Vlakfontein and vicinity, western Transvaal: Economic Geology, v. 71,
p. 261-286.
Volborth, A., and Housley, R. M., 1984, A preliminary description of complex graphite, sulphide, arsenide, and
platinum group element mineralization in a pegmatoid pyroxenite of the Stillwater Complex, Montana, USA:
Tschermaks Mineralogische und Petrographische Mitteilungen, v. 33, p. 213-230.
Zientek, M.L., Fries, T.L., and Vian, R.W., 1990, As, Bi, Hg, S, Sb, Sn, and Te geochemistry of the J-M Reef,
Stillwater Complex, Montana: Constraints on the origin of PGE-enriched sulfides in layered intrusions:
Journal of Geochemical Exploration, v. 37, p. 51-73.
Zientek, M.L., 1993, Mineral resource appraisal for locatable minerals: the Stillwater Complex, in Hammarstrom,
J.M., Zientek, M.L., and Elliott, J.E., eds., Mineral resource assessment of the Absaroka-Beartooth study
area, Custer and Gallatin National Forests, Montana: U.S. Geological Survey Open-File Report 93-207, p.
F1-F83.
38
SERPENTINE- AND CARBONATE-HOSTED ASBESTOS DEPOSITS
(MODELS 8d, 18e; Page, 1986; Wrucke and Shride, 1986)
by Chester T. Wrucke
Examples
Serpentine-hosted: Canada- Thetford Mines, Black Lake, and Asbestos-Shipton areas, Quebec; Cassair, British
Colombia; White Bay area, Newfoundland (Riordon, 1957; Virta and Mann, 1994). United States- Belvidere
Mountain, Vt. (an extension of the Quebec deposits) (Chidester and others, 1978); Calaveras and San Benito
Counties, Calif. (Rice, 1966; Virta and Mann, 1994; Coleman, in press). Africa and Europe- Bazhenovo district,
central Ural Mountains, Russia (Virta and Mann, 1994); Balangero, Italy (Virta and Mann, 1994); Barberton area,
South Africa (Sinclair, 1959; Anhaeusser, 1986); Troodos Complex, Cyprus (Virta and Mann, 1994); Shabani,
Zimbabwe (Anhaeusser, 1986; Virta and Mann, 1994).
Carbonate-hosted: Gila County, Ariz. (Shride, 1969; 1973); Barberton-Caroline district and near Kanye, South Africa
(Sinclair, 1959; Anhaeusser, 1986); Laiynan district, Hobei Province, China (Sinclair, 1959).
39
7) Health risks to humans from exposure to small quantities of chrysotile asbestos in the environment are
controversial. The controversy results from the EPA assumption that any amount of asbestos is potentially hazardous.
Exploration geophysics
Remote sensing techniques can detect belts of ultramafic rocks, intrusive masses, residual iron oxide minerals, and
serpentinite by infra red reflectance, thermal properties, and botanical anomalies (stress and density) in areas of
serpentinite-hosted chrysotile, and can identify diabase outcrops in areas of carbonate-hosted chrysotile. Magnetic
techniques can be used to identify serpentinite derived from ultramafic rocks because of their high magnetite content.
However, the method may not be useful in identification of serpentinite developed from carbonate rocks. This
serpentinite may contain magnetite, but the protoliths are carbonate rocks that generally have a low initial iron
content, and associated serpentinite bodies are small.
References
Geology: Shride (1969, 1973), Chidester and others, (1978), Ross (1981), Anhaeusser (1986), Virta and Mann (1994).
Environmental geology: Derkies (1985), Coleman (1995).
Host rocks
Serpentine-hosted deposits: These deposits are in massive serpentinite, commonly highly sheared and widely exposed,
that has largely replaced the host ultramafic protolith. Associated ultramafic rocks locally host asbestos veins.
Carbonate-hosted deposits: Serpentinite in these deposits has replaced metalimestone or dolostone. In Arizona, the
serpentinite bodies commonly are 1 to 3 m thick, are present at a few stratigraphic intervals in the host metalimestone
section, and are structureless, except for asbestos veins.
40
amphibolite, of early Paleozoic age. The New Idria serpentinite, Calif., which hosts the KCAC asbestos deposit and
others, is completely separated from mafic oceanic crust and is surrounded by sedimentary rocks of the Cretaceous
Franciscan and Panoche Formations. These rocks are folded with unconformably overlying Tertiary marine
sedimentary rocks.
Carbonate-hosted deposits: Deposits of this type in Arizona are in Middle Proterozoic rocks that include siltstone,
arkosic arenite, cherty dolostone, argillite, quartz arenite, local basalt, and intrusions of diabase sheets, sills, and
dikes.
Wall-rock alteration
Serpentinization of host rocks and subsequent serpentinite alteration are common kinds of wall-rock alteration
associated with asbestos. In serpentine-hosted deposits, serpentinite results from hydration of igneous protoliths,
commonly harzburgite and dunite, which are unstable in the presence of water at crustal temperatures (Coleman,
1971; Coleman and Jove, 1993). For example, the mineralogical nature of the New Idria serpentinite probably is
related to the relative amounts of orthopyroxene and olivine in the primary peridotite. Reactions with water can be
written:
Reaction 1 produces weathering-resistant silica-bearing serpentinite that forms pinnacles, common in the New Idria
serpentinite, whereas reaction 2 produces predominantly highly weathered and rounded boulders and fine soil-like
material (Malcolm Ross, written commun., 1995). Serpentinization releases calcium that becomes available for
secondary minerals. Iron in the primary silicate minerals, when not incorporated into serpentinite, may form
magnetite. With increasing metamorphic temperature, lizardite of the original serpentinite is replaced by antigorite,
which at higher temperatures is converted, by dehydration, to talc, olivine, and water (Coleman and Jove, 1993).
Common alteration products in metamorphosed serpentinites of Quebec and Vermont are talc in steatite and schistose
masses, talc-carbonate rocks, and quartz-carbonate bodies (Chidester and others, 1978).
Late stage serpentinization of carbonate-hosted deposits follows dedolomitization and development of
calcium-magnesium silicate minerals during contact metamorphism. Subsequent serpentine alteration proceeds as
outlined for serpentinite-hosted deposits.
Nature of ore
Serpentine-hosted deposits: These deposits commonly consist of stockworks (networks of veins) of cross-fiber veins
that are aggregates of two or more thin parallel layers of fibers oriented about normal to vein walls. Most deposits
contain some slip-fiber veins composed of fibers in the plane of the fracture. In a few deposits, this is the principal
fiber type. In the New Idria serpentinite, ore is in sheared and pulverized serpentinite and consists of flake-like
agglomerates and sheet-like masses of finely matted chrysotile (Mumpton and Thompson, 1975). Much of this ore
may be secondary (see section below entitled "Secondary mineralogy"). Asbestos contents vary widely in the
serpentine-hosted deposit type. Typical deposits contain 5 to 15 volume percent asbestos fiber; in the New Idria
serpentinite, ore contains in excess of 50 volume percent chrysotile (Mumpton and Thompson, 1975).
Carbonate-hosted deposits: Ore zones consist of sets of cross-fiber veins subparallel to bedding in the serpentinized
host. Commonly, veins are composed of multiple parallel layers in which the fibers are oriented about at right angles
to vein walls and aggregate 1 to 20 cm in width; multiple layers indicate incremental development. Veins pinch and
swell, anastomose, and vary in dip. Productive zones may be 10 cm to a few decimeters thick and constitute as
much as 40 volume percent of a serpentinite body (Shride, 1969; Otton and others, 1981). However, asbestos veins
commonly are contained in much greater thicknesses of massive, barren serpentinite. Chrysotile veins locally are
in fine-grained calc-silicate rocks developed from cherty dolomite.
41
Ore and gangue mineralogy and zonation
In serpentine- and carbonate-hosted deposits, the only ore mineral is chrysotile. Serpentinite-hosted deposits contain
magnetite in the serpentine gangue and locally talc, as well as quartz-calcite veins (Riordon, 1957). In Canadian
deposits, magnetite commonly is concentrated in wall rock adjacent to asbestos veins, along vein walls, in partings
between layers of multiple veins, and can be disseminated parallel to chrysotile fibers (Riordon, 1957). Chrysotile,
brucite, and magnetite are common in serpentine of the New Idria body (Coleman, in press). Carbonate-hosted
deposits may include sparse calcite veins, otherwise the gangue is serpentine.
Mineral characteristics
Chrysotile is one of six mineral species called asbestos because of their fibrous habit (Skinner and others, 1988).
Of these, chrysotile is the only fibrous serpentine mineral. The other five asbestos minerals belong to the amphibole
group; these are grunerite asbestos (commonly referred to as amosite), riebeckite asbestos (commonly referred to as
crocidolite), anthophyllite asbestos, tremolite asbestos, and actinolite asbestos. Chrysotile (Mg3Si2O5(OH)4) consists
of layers of linked SiO4 tetrahedra and misfit layers of linked MgO2(OH)4 tetrahedra that together roll into sheets,
making hollow tubes having diameters of about 25 nm (Ross, 1981). Most chrysotile contains less than 2 weight
percent iron as FeO, though as much as 8 percent has been reported (Wicks and O'Hanley, 1988). Small amounts
of aluminum, manganese, magnesium, calcium, potassium, and sodium also may be present (Ross and others, 1984).
Vein fibers range in length from less than 5 µm to 10 cm or more. They can vary in flexibility, hardness, tensile
strength, and other physical properties and chemical properties of importance in determining their commercial use
(Shride, 1973).
Secondary mineralogy
Although serpentine minerals are considered to be stable in the upper crust (Coleman and Jove, 1993), studies in
California show that they are unstable in the range of pH and Mg2+ and Si(OH)4 concentrations encountered in most
soil (Mumpton and Thompson, 1966; Wildman and others, 1971). Iron-rich montmorillonite is a common product
of serpentine in this soil type (Wildman and others, 1971). Brucite (Mg(OH)2), one of the hydrothermal minerals
that develops during serpentinization and makes up 7 to 8 volume percent of the New Idria serpentinite, is destroyed
in the weathering zone, producing coalingite (an iron- and magnesium-bearing carbonate), compositionally similar
pyroaurtite (Mumpton and Thompson, 1966), hydromagnesite, and secondary chrysotile (Mumpton and Thompson,
1975). Some chrysotile is developed during tectonic milling and may be the most important process in the formation
of chrysotile at the New Idria deposit (R.G. Coleman, written commun., 1995). Deeply weathered serpentinite can
produce the nickel-bearing minerals nepouite and pecoraite (Mumpton and Thompson, 1975), but most serpentine
contains only very small amounts of nickel.
Tremolite, an amphibole, can be fibrous and is associated with chrysotile deposits, particularly at Thetford
Mines, Quebec; Troodos, Cyprus; and Balangero, Italy (R.G. Coleman, written commun., 1995). The EPA classifies
tremolite as asbestos if the particles have an aspect ratio of 3:1. The amount and distribution of tremolite asbestos
in chrysotile deposits is poorly known. It is extremely scarce in the New Idria, Calif., deposits (Coleman, in press),
and it makes up less than one volume percent of the dust in Quebec mines and mills (Mossman and others, 1990).
Topography, physiography
Serpentine-hosted deposits: These deposits are easily eroded. Serpentinite bodies commonly are well exposed and
therefore are readily eroded. The asbestos deposit at Belvidere Mountain, Vt., is on a hillside in an area having relief
of 200 m in a radius of 1.6 km (Chidester and others, 1951). The New Idria serpentinite body in San Benito County,
Calif., occupies a high ridge, as illustrated in Mumpton and Thompson (1975), that includes rock exposures barren
of vegetation. Evidence of contact dislocation shows that this serpentinite body is rising tectonically, exposing this
soft material to long term erosion (Coleman, in press).
Carbonate-hosted deposits: Chrysotile-bearing serpentinite zones in Arizona are soft and weakly resistant to
weathering. However, these zones commonly are exposed in cliffs and steep slopes, protected by the more resistant,
overlying metalimestone beds, which reduces exposure of serpentinite and chrysotile to erosion.
Hydrology
Streams draining asbestos-bearing serpentinite can pick up chrysotile fibers. Most streams draining the Belvidere
Mountain deposit, Vt. lead to the Gihon and Lamoille Rivers and Lake Champlain, but some reach Quebec via the
Missisquoi River. Stream drainages from the northeast and southeast parts of the New Idria serpentinite, Calif., lead
42
to the San Joaquin Valley. This area drains to the west via the San Benito River to the Pacific Ocean. Debris slides
along the flanks of the serpentinite body and in the main drainage channels contain huge amounts of asbestos-bearing
material available for removal and dispersion by streams (Cowan, 1979). During flood stage, streams flowing into
the San Joaquin Valley from the New Idria mass have introduced sediment into the California aqueduct (Coleman,
in press). Asbestos fibers also have been found in the water supply for San Francisco (Kanarek and others, 1980).
Drainage from carbonate-type chrysotile deposits in Arizona reaches the watershed of the Salt River, which flows
to Phoenix via several reservoirs.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Stream channels that drain chrysotile-bearing serpentinite contain asbestos as a natural erosion product. Where
serpentinite masses crop out in mountainous terrane, as in San Benito County, Calif., chrysotile-bearing debris in
landslides, debris flows, and bedrock exposures provide extensive sources of asbestos materials to local streams.
Some of these fibers have reached the California aqueduct on the west side of the San Joaquin Valley.
Smelter signatures
Not applicable to asbestos.
43
Calif., serpentinite have been identified in downstream sedimentary accumulations such as terrace deposits and
alluvial fans (Coleman, in press). Chrysotile also is preserved in the Big Blue Formation of Miocene age, which
contains abundant debris eroded from the New Idria serpentinite (Carlson, 1981).
Geoenvironmental geophysics
See exploration geophysics.
RISK ASSESSMENT
Hazards resulting from inhalation of asbestos fibers have been documented by the EPA and have been the topic of
considerable scientific inquiry (Ross, 1981, 1984; Skinner and others, 1988; Mossman and others, 1990; D'Agostino
and Wilson, 1993; Ross and Skinner, 1994; McDonald and McDonald, 1995). Although the relationship between
asbestos and lung diseases is well documented, debate continues regarding the risk from low level exposure to
asbestos fiber. In a report prepared for the California Environmental Protection Agency, risk related to asbestos was
not ranked because data on low level exposure were considered inadequate (California Comparative Risk Project,
1994). Studies show that important factors to be considered in evaluating risk associated with asbestos inhalation
include type of asbestos mineral, length and diameter of the asbestos fibrils, amount of asbestos inhaled, and the
duration of the exposure. Yet, despite conclusions that risks from chrysotile asbestos are almost certainly lower than
for other asbestos minerals (D'Agostino and Wilson, 1993), uncertainty in the degree of risk from exposure to
chrysotile remains. The uncertainty results in part from disagreement concerning whether an exposure threshold
exists and, if so, at what fiber concentration below which inhalation is safe (D'Agostino and Wilson, 1993).
The EPA has concluded that inhalation of any amount of asbestos is potentially hazardous, that a single
asbestos fiber can be lethal (Abelson, 1990). This conclusion results from belief that a linear relationship exists
between asbestos dose and health risk such that risk exists even at very low levels of exposure (D'Agostino and
Wilson, 1993). According to this theory, any exposure to asbestos poses a risk. In a nonlinear relation, risk from
exposure decreases rapidly at low levels and a threshold value can be reached below which the risk is zero. Recent
studies suggest that low-level exposure to chrysotile asbestos in the environment has generated unwarranted concern
based on speculation (D'Agostino and Wilson, 1993) and that the single-fiber view is unproved (Abelson, 1990).
Other studies suggest that a "threshold" value, below which exposure to chrysotile asbestos causes no measurable
health effects, can be identified (Ross, 1987).
Estimates of risk to human health from numerous activities, including everyday risks, have been quantified,
and a few attempts have been made to quantify risk of exposure to chrysotile asbestos under different environmental
conditions (D'Agostino and Wilson, 1993; Coleman, in press). For example, data show that risks from inhaling
asbestos during recreational activities at the chrysotile-bearing New Idria, Calif., serpentinite or from exposure to
asbestos in schools are low. Coleman (1995) concluded that "the apparent risk in making one trip by automobile
to New Idria is 300 times greater than inhaling [chrysotile] fibers during a lifetime of recreation in this area." Risks
from occupying schools containing chrysotile fibers are even lower and have been categorized as harmlessly small
44
(Abelson, 1990; Wilson and others, 1994).
Acknowledgments.--This report benefitted from helpful reviews by R.G. Coleman and Malcolm Ross.
REFERENCES CITED
Ableson, P.H., 1990, The asbestos removal fiasco: Science, v. 247, p. 1017.
Anhaeusser, C.R., 1986, The geological setting of chrysotile asbestos occurrences in southern Africa, in Anhaeusser,
C.R., and Maske, S., eds., Mineral deposits of southern Africa: Johannesburg, Geological Society of South
Africa, p. 359-375.
Bowles, Oliver, 1955, The asbestos industry: U.S. Bureau of Mines Bulletin 552, 122 p.
Brown, C.E., 1973, Talc, in Brobst, D.A., and Pratt, W.P., eds., United States mineral resources: U.S. Geological
Survey Professional Paper 820, p. 619-626.
California Comparative Risk Project, 1994, Toward the 21st century: planning for protection of California's
environment: California Environmental Protection Agency, 642 p.
Carlson, Christine, 1981, Sedimentary serpentinites of the Wilbur Springs area-a possible Early Cretaceous structural
and stratigraphic link between the Franciscan complex and the Great Valley sequence: Stanford, Stanford
University, M.S. thesis, 105 p.
Chidester, A.H., Albee, A.L., and Cady, W.M., 1978, Petrology, structure, and genesis of the asbestos-bearing
ultramafic rocks of the Belvidere Mountain area in Vermont: U.S. Geological Professional Paper 1016, 95
p.
Chidester, A.H., Billings, M.P., and Cady, W.M., 1951, Talc investigations in Vermont, Preliminary report: U.S.
Geological Survey Circular, 33 p.
Coleman, R.G., 1971, Petrologic and geophysical nature of serpentinites: Geological Society of America, v. 82, p.
897-917.
_________(in press), New Idria serpentinite, a land management dilemma: Engineering and Environmental Geology.
Coleman, R.G., and Jove, C., 1993, Geological origins of serpentinites, in The vegetation of ultramafic (serpentine)
soils, Proceedings of the First International Conference on serpentine ecology: Andover, Intercept Ltd., p.
1-17.
Cowan, D.S., 1979, Serpentinite flows on Joaquin Ridge, southern Coast Ranges, California: Geological Society of
America Bulletin, v. 81, p. 2615-2628.
D'Agostino, Ralph, Jr., and Wilson, Richard, 1993, Asbestos: The hazard, the risk, and public policy, in Foster, K.R.,
Bernstein, D.E., and Huber, P.W., eds., Phantom risk: Cambridge, MIT, p.183-210.
Derkies, Dan, 1985, Wastes from the extraction and beneficiation of metallic ores, phosphate rock, asbestos,
overburden from uranium mining, and oil shale, prepared for Environmental Protection Agency: National
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Energy, Mines, and Resources Canada, 1976, Asbestos: Energy, Mines and Resources Canada Mineral Bulletin MR
155, 26 p.
Kanarek, M.S., Conforti, P.M., Jackson, L.A., Cooper, R.C., and Murcho, J.C., 1980, Asbestos in drinking water and
cancer incidence in the San Francisco Bay area: American Journal of Epidemiology, v. 112, p. 54-72.
Mann, E.L., 1983, Asbestos, in Lefond, S.J., ed., Industrial minerals and rocks: New York, American Institute of
Mining, Metallurgical, and Petroleum Engineers, p. 435-484.
McDonald, J.C., and McDonald, A.D., 1995, Chrysotile, tremolite, and mesothelioma: Science, v. 267, p. 778-779.
Mossman, B.T., Bignon, J., Corn, M., Seaton, A., and Gee, J.B.L., 1990, Asbestos: scientific developments and
implications for public policy: Science, v. 247, p. 294-301.
Mumpton, F.A., and Thompson, C.S., 1966, The stability of brucite in the weathering zone of the New Idria
serpentinite: Clays and Clay Minerals, v. 14, p. 249-257.
_________1975, Mineralogy and origin of the Coalinga asbestos deposit: Clays and Clay Minerals, v. 23, p. 131-143.
Otton, J.K., Light, T.D., Shride, A.F., Bergquist, J.R., Wrucke, C.T., Theobald, P.K., Duval, J.S., and Wilson, D.M.,
1981, Mineral resources of the Sierra Ancha Wilderness and Salome Study Area, Gila County, Arizona: U.S.
Geological Survey Miscellaneous Field Studies Map MF-1162-H, scale 1:48,000.
Page, N.J., 1986, Descriptive model of serpentine-hosted asbestos, in Cox, D.P. and Singer, D.A., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 46.
Rice, S.J., 1966, Asbestos, in Mineral resources of California: California Division of Mines and Geology Bulletin
191, p. 86-92.
45
Riordon, P.H., 1957, Asbestos, in The geology of Canadian industrial mineral deposits: Sixth Commonwealth Mining
and Metallurgical Congress, p. 3-53.
Ross, Malcolm, 1981, The geologic occurrences and health hazards of amphibole and serpentine asbestos, in Veblen,
D.R., ed., Amphiboles and other hydrous pyriboles-mineralogy: Mineralogical Society of America Reviews
in Mineralogy, v. 9A, p. 279-323.
_________1984, A survey of asbestos-related disease in trades and mining occupations and in factory and mining
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American Society for Testing and Materials Special Publication 834, p. 51-104.
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12, 1985: Tucson, Arizona Bureau of Geology and Mineral Technology, p. 101-115.
Ross, Malcolm, Kuntze, R.A., and Clifton, R.A., 1984, A definition of asbestos: American Society of Testing and
Materials Special Technical Publication 834, p. 139-147.
Ross, Malcolm, and Skinner, C.W., 1994, Minerals and cancer: Geotimes, v. 39, p. 13-15.
Sawyer, R.N., 1987, Asbestos exposure: health effects update: National Asbestos Council Journal, v. 5, no. 2, p. 25-
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_________1973, Asbestos, in Brobst, D.A., and Pratt, W.P., eds., United States mineral resources: U.S. Geological
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Sinclair, W.E., 1959, Asbestos, its origin, production, and utilization: London, Mining Publications, 559 p.
Skinner, H.C.W., Ross, Malcolm, and Frondel, Clifford, 1988, Asbestos and other fibrous materials, mineralogy,
crystal chemistry, and health effects: New York, Oxford, 204 p.
Stewart, P., French, G., and Anthony, D., 1990, Baie Verte wet process: Industrial Minerals, no. 273, p. 51-53.
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Virta, R.L., and Mann, E.L., 1994, Asbestos, in Carr, D.D., ed., Industrial minerals and rocks, 6th edition: Littleton,
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phyllosilicates: Mineralogical Society of America Reviews in Mineralogy, v. 19, p. 91-167.
Wildman, W.E., Whittig, L.D., and Jackson, M.L., 1971, Serpentine stability in relation to formation of iron-rich
montmorillonite in some California soils: American Mineralogist, v. 56, p. 587-602.
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D.A., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 98.
46
CARBONATITE DEPOSITS
(MODEL 10; Singer, 1986a)
Examples
Oka, Quebec; Iron Hill and Gem Park, Colo.; Magnet Cove, Ark.; St. Honore, Quebec; Mountain Pass, Calif.;
Phalaborwa, South Africa; Jacupiranga, Brazil; Kovdor, Russia.
Exploration geophysics
The geophysical signature of carbonatite complexes is variable (Hoover, 1992). In many complexes, concentrations
of magnetic minerals produce positive magnetic anomalies. Similarly, concentrations of radioactive minerals cause
pronounced radiometric anomalies associated with many carbonatite complexes.
References
Heinrich (1966), Erdosh (1979), Bell (1989), Notholt and others (1989).
Host rocks
Rocks constituting carbonatite deposits may include varieties of carbonatite, including calcite-rich (sövite), dolomite-
rich (rauhaugite), iron-rich carbonatite, silico-carbonatite, etc. Carbonatite is typically associated with concentrically
zoned complexes of alkaline-igneous rocks, though some deposits may consist of dikes or thick sheets. Associated
igneous rocks typically include ijolite, melteigite, pyroxenite, and nepheline syenite. Carbonatites are typically
associated with undersaturated igneous rocks that are miaskitic (nearly peralkaline) rather than agpaitic (peralkaline).
Any rock type, often including granite and other intrusive rocks, gneiss and other metamorphic rocks, may host the
alkaline-igneous complexes. Host rocks are often fenitized (alkali metasomatized) by alkali-rich fluids evolved from
carbonatite complexes.
47
Wall-rock alteration
Carbonatite-containing complexes are typically surrounded by an aureole of metasomatized rock that is
characteristically moderately to intensely fenitized. Quartzo-feldspathic wall rocks are typically converted to quartz-
free rocks composed of alkali feldspar, aegirine and other sodic pyroxenes, and subordinate alkali amphibole. Other
types of hydrothermal alteration, such as chloritization of ferromagnesian minerals, may also be present.
Nature of ore
Ore minerals may be disseminated throughout a large volume of carbonatite, or may be banded and concentrated in
certain intrusive, alteration, or breccia zones, or in carbonatite dikes and sills.
Mineral characteristics
Elemental and mineral associations include: phosphate-iron (fluorapatite, carbonate-fluorapatite, magnetite); niobium
(pyrochlore, perovskite, niocalite); rare earth element-barium (bastnaesite, monazite, parisite, barite). Other common
major and accessory minerals include calcite, dolomite, ankerite, ilmenite, strontianite, fluorite, pyrrhotite, pyrite,
molybdenite, galena, chalcopyrite, sphalerite, biotite, phlogopite, pyroxenes, amphiboles, forsterite, hematite, zircon,
anatase, brookite, and rutile.
Secondary mineralogy
Dissolution of carbonate and residual concentration of iron oxide are the dominant secondary processes. Partial
dissolution and reprecipitation of apatite may result in near-surface enriched deposits of francolite (carbonate-apatite).
Topography, physiography
Variable. Topography is often controlled by the more resistant silicate rocks associated with carbonatite. Carbonatite
plugs have a tendency to form subdued topography due to dissolution of carbonate minerals by ground and meteoric
water. Dikes may stand out as resistant features in arid climates, but may be removed by dissolution in wet climates.
Karst processes may produce topographic lows filled by lake sediment and underlain by residual concentrations of
insoluble minerals from carbonatite.
Hydrology
Most carbonatite deposits are within concentrically zoned igneous complexes. Most rock in this type of setting is
relatively solid, unfractured, and not associated with major fault or shear zones. Consequently, features that focus
water flow are scarce in association with carbonatite complexes. Some parts of deposits may, however, contain
vuggy or breccia zones with increased permeability. Dissolution by ground water does create the potential for
genesis of permeable channels in particularly calcite-rich carbonatites.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Stream sediments downstream from carbonatite deposits commonly contain anomalous abundances of thorium,
barium, niobium, and rare earth elements, including lanthanum, cerium, neodymium, and samarium (U.S. Geological
48
Survey, 1991; Haxel, in press).
Smelter signatures
Little data available. Most mineral commodities recovered from carbonatites are those separated by mechanical
separation means or chemical solution; neither smelting nor associated emissions are relevant factors.
Geoenvironmental geophysics
Where radon hazards may be of concern, gamma-ray spectrometry can easily distinguish between the three natural
radioelements, potassium, thorium, and uranium, and provide quantitative estimates of their concentrations. Where
sulfide minerals are present, in mineralized rock or mine waste, induced polarization methods can detect their
presence. Enhanced vegetation development, typically associated with complexes in which phosphorous and
potassium abundances have been enriched, can be detected by remote sensing.
REFERENCES CITED
Bell, Keith, ed., 1989, Carbonatites, genesis and evolution: London, Unwin Hyman, 618 p.
Erdosh, G., 1979, The Ontario carbonatite province and its phosphate potential: Economic Geology, v. 74, no. 2, p.
331-338.
Haxel, G.B., in press, Ultrapotassic rocks, carbonatite, and rare earth element deposit, Mountain Pass, California, in
Theodore, T.G., ed., Evaluation of metallic mineral resources and their geologic controls in the East Mojave
National Scenic Area, San Bernadino County, California: U.S. Geological Survey Bulletin.
Heinrich, E.W., 1966, The geology of carbonatites: Chicago, Rand McNally, 555 p.
Hoover, D.B., 1992, Geophysical model of carbonatites, in Hoover, D.B., Heran, W.D., and Hill, P.L., eds., The
Geophysical Expression of Selected Mineral Deposit Models: U.S. Geological Survey Open-File Report 92-
557, p. 80-84.
Notholt, A.J.G., Sheldon, R.P., and Davidson, D.F., 1989, Phosphate deposits of the world, v. 2, phosphate rock
resources: Cambridge, Cambridge University Press, 566 p.
Singer, D.A., 1986a, Descriptive model of carbonatite deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 51.
_________1986b, Grade and tonnage model of carbonatite deposits, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 52-53.
U.S. Geological Survey, 1991, Evaluation of metallic mineral resources and their geologic controls in the East
Mojave National Scenic Area, San Bernadino County, California: U.S. Geological Survey Open-File Report
91-427, 278 p.
49
TH-RARE EARTH ELEMENT VEIN DEPOSITS
(MODEL 11d; Staatz, 1992)
Examples
Thorium veins: Wet Mountains area and Powderhorn district, Colo.; Capitan Mountain and Laughlin Peak, N. Mex.;
Lemhi Pass district, Idaho; and Bokan Mountain district, Alaska. Carbonatite dikes: Wet Mountains area (especially
Gem Park) and Powderhorn district (especially near Iron Hill), Colo.; Mineral Hill district, Idaho-Mont.; Rocky
Boy's Indian Reservation, Bearpaw Mountains, Mont.; Bear Lodge Mountains, Wyo.; Mountain Pass, Calif.; and
Magnet Cove and Potash Sulphur Springs, Ark.
Exploration geophysics
Alkaline intrusive complexes and carbonatite complexes, with which thorium-rare earth element veins and carbonatite
dikes may be associated, have highly variable geophysical expressions that depend on their mineralogy, type of
carbonatite stock, if present, intensity of host rock fenitization, and depth of weathering (Hoover, 1992). Some
complexes contain major amounts of magnetite, ilmenite, and (or) perovskite, which result in positive magnetic
anomalies. Abundant contained thorium- and uranium-bearing minerals, such as thorite, monazite, perovskite, and
apatite may result in large positive radiometric anomalies. Gamma-ray spectrometry can quantitatively measure
thorium, uranium, and potassium contents in veins and associated rocks. Detailed spectral data from remote sensing
surveys can identify CO3, ferrous iron, and rare earth elements associated with these deposits.
References
Dellwig (1951), Olson and others (1954), Kaiser (1956), Olson and Wallace (1956), Anderson (1958, 1981),
Christman and others (1959), Hedlund and Olson (1961), Sharp and Cavender (1962), Staatz (1974, 1978, 1979,
50
1985), Staatz and others (1979, 1980), Armbrustmacher (1979, 1980, 1988), Olson and Hedlund (1981), and
Thompson (1988).
Host rocks
Alkaline intrusive rocks, granitic rocks, high-grade metamorphic rocks, aureoles surrounding alkaline intrusive
complexes and fenitized carbonatite complexes, black shale, and novaculite are host rocks for thorium-rare earth
element veins and carbonatite dikes.
Wall-rock alteration
Thorium-rare earth element veins and carbonatite dikes are typically surrounded by, at most, several-m-wide aureoles
of metasomatically altered rock. The process of alteration, fenitization, is caused by peralkaline fluids emanating
from cooling alkaline silicate or carbonatite magmas. Fenitized aureoles surrounding veins and carbonatite dikes have
limited geoenvironmental impact.
Nature of ore
Thorium-rare earth element veins and carbonatite dikes are structurally controlled. Veins and dikes directly
associated with alkaline intrusive complexes and carbonatite complexes may occupy radial fractures, related to their
host complexes. At other localities, fractures parallel to, and controlled by the dominant structural lineaments of the
area, may be occupied by veins and dikes.
Mineral characteristics
Thorium-rare earth mineral veins are chiefly quartz-barite veins with irregularly distributed thorium and rare earth
minerals. In some veins, quartz and barite are coarse grained; euhedral smoky quartz lines vein walls and
paragenetically later barite forms vein interiors. Other thorium-rich veins are fine grained; red, due to ferric iron
oxide content; and have odoriferous fresh surfaces. Ore minerals are irregularly disseminated throughout carbonatite
dikes.
51
Secondary mineralogy
Weathering of thorium-rare earth mineral vein and carbonatite deposits results in localized carbonate mineral
dissolution.
Topography, physiography
These elongate, areally restricted deposits seem to have neither topographic nor physiographic expressions that are
distinct.
Hydrology
The hydrologic regime is unaffected by thorium-rare earth element vein deposits, which are commonly small.
ENVIRONMENTAL SIGNATURES
Drainage signatures
No data available. The facts that thorium-rare earth element vein deposits contain very low sulfide mineral
abundances and consist of low-solubility, resistate minerals suggests that these veins contribute insignificantly to the
geochemistry of water draining these deposits.
Smelter signatures
The absence of significant ore having been produced from thorium-rare earth element vein deposits precludes
environmental impact attributable to smelting.
Geoenvironmental geophysics
Egress of radioactive particles from mine areas and waste dumps can be identified and monitored by gamma-ray
spectrometry or total count scintillometers; detailed airborne or ground-based surveys can be employed, depending
on the type of coverage that is warranted.
52
REFERENCES CITED
Anderson, A.L., 1958, Uranium, thorium, columbium, and rare earth deposits in the Salmon region, Lemhi County,
Idaho: Idaho Bureau of Mines and Geology Pamphlet, v. 115, p.
Anderson, J.M., 1981, The origin of rare earth, thorium, and uranium mineralization in the northern Tendoy
Mountains, Beaverhead County, Montana: Bellingham, Western Washington University, M.S. thesis, 101
p.
Armbrustmacher, T.J., 1979, Replacement and primary magmatic carbonatites from the Wet Mountains area, Fremont
and Custer Counties, Colorado: Economic Geology, v. 74, p. 888-901.
_________1980, Abundance and distribution of thorium in the carbonatite stock at Iron Hill, Powderhorn district,
Gunnison County, Colorado: U.S. Geological Survey Professional Paper 1049-B, p. B1-B11.
_________1988, Geology and resources of thorium and associated elements in the Wet Mountains area, Fremont and
Custer Counties, Colorado: U.S. Geological Survey Professional Paper 1049-F, 34 p.
Bliss, J.D., 1992, Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, 168 p.
Christman, R.A., Brock, M.R., Pearson, R.C. and Singewald, Q.D., 1959, Geology and thorium deposits of the Wet
Mountains, Colorado: a progress report: U.S. Geological Survey Bulletin, 1072-H, p. 491-535.
Dellwig, L.F., 1951, Preliminary summary report on the Wet Mountains thorium area, Custer and Fremont Counties,
Colorado: U.S. Geological Survey Trace Elements Memorandum Report, v. 287, 13 p.
Hedlund, D.C. and Olson, J.C., 1961, Four environments of thorium-, niobium-, and rare-earth-bearing minerals in
the Powderhorn district of southwestern Colorado: U.S. Geological Survey Professional Paper 424-B, p.
B283-B286.
Hoover, D.B., compiler, 1992, Geophysical model of carbonatite, in Hoover, D.B., Heran, W.D., and Hill, P.L., eds.,
The geophysical expression of selected mineral deposit models: U.S. Geological Survey Open-File Report
92-557, p. 80-84.
Kaiser, E.P., 1956, Preliminary report on the geology and deposits of monazite, thorite, and niobium-bearing rutile
of the Mineral Hill district, Lemhi County, Idaho: U.S. Geological Survey Open-File Report 56-69, 41 p.
Langmuir, Donald, and Herman, J.S., 1980, The mobility of thorium in natural waters at low temperatures:
Geochimica et Cosmochimica Acta, v. 44, p. 1753-1766.
Olson, J.C. and Hedlund, D.C., 1981, Alkalic rocks and resources of thorium and associated elements in the
Powderhorn district, Gunnison County, Colorado: U.S. Geological Survey Professional Paper 1049-C, 34
p.
Olson, J.C., Shawe, D.R., Pray, L.C., and Sharp, W.N., 1954, Rare-earth deposits of the Mountain Pass district, San
Bernardino County, California: U.S. Geological Survey Professional Paper 261, 75 p.
Olson, J.C. and Wallace, S.R., 1956, Thorium and rare-earth minerals in Powderhorn district, Gunnison County,
Colorado: U.S. Geological Survey Bulletin 1027-O, p. 693-723.
Sharp, W.N. and Cavender, W.S., 1962, Geology and thorium-bearing deposits of the Lemhi Pass area, Lemhi
County, Idaho, and Beaverhead County, Montana: U.S. Geological Survey Bulletin, 1126, 76 p.
Statz, M.H., 1974, Thorium veins in the United States: Economic Geology, v. 69, p. 494-507.
_________1978, I and L uranium and thorium vein system, Bokan Mountain, southeastern Alaska: Economic
Geology, v. 73, p. 512-523.
_________1979, Geology and mineral resources of the Lemhi Pass thorium district, Idaho and Montana: U.S.
Geological Survey Professional Paper 1049-A, 90 p.
_________1985, Geology and description of the thorium and rare-earth veins in the Laughlin Peak area, Colfax
County, New Mexico: U.S. Geological Survey Professional Paper 1049-E, p. E1-E32.
_________1992, Descriptive model of thorium-rare-earth veins, in Bliss, J.D., ed., Developments in mineral deposit
modeling: U.S. Geological Survey Bulletin 2004, p. 13-15.
Staatz, M.H., Armbrustmacher, T.J., Olson, J.C., Brownfield, I.K., Brock, M.R., Lemons, J.F., Jr., Coppa, L.V., and
Clingan, B.V., 1979, Principal thorium resources in the United States: U.S. Geological Survey Circular 805,
42 p.
Staatz, M.H., Hall, R.B., Macke, D.L., Armbrustmacher, T.J., and Brownfield, I.K., 1980, Thorium resources of
selected regions in the United States: U.S. Geological Survey Circular 824, 32 p.
Thompson, T.B., 1988, Geology and uranium-thorium mineral deposits of the Bokan Mountain granite complex,
southeastern Alaska: Ore Geology Reviews, v. 3, p. 193-210.
53
SN AND (OR) W SKARN AND REPLACEMENT DEPOSITS
(MODELS 14a-c; Cox, 1986; Reed and Cox, 1986; Reed, 1986)
by Jane M. Hammar strom, Ja mes E. Ell iott, Boris B. K otlyar, Ted G. Theodore,
J. Thomas Nash, David A. John, Donald B. Hoover, and Daniel H. Knepper, Jr.
Deposit geology
These deposits consist of tin, tungsten, and beryllium minerals in skarns, veins, stockworks, stratabound
replacement deposits, an d greisens in car bonate rocks at or near gr anite contacts.
Exampl es
Tungsten skarns: Pine Creek Mine, Calif.; Mill Cit y district, Nev.; Rock Creek district, Mont.; Mactung,
Tin replacement deposits: Renison Bell, Tasmania, Australia; Dachang, Guangxi, China.
References
Geology: Hepworth and Zhang (1982), Cox (1986), Reed (1986), Reed and Cox (1986).
Host rocks
These deposits a re hosted by carbona te rock, in cluding limestone, dolomite, ma rble, an d carbonat e-bearing pelite,
argillit e, and shale. In terms of Glass and oth ers' (1982) classification of bedrock types, most host rocks for these
deposit types are type IV, that is, they are highly calcareous sedimentary rocks or metamorphosed calcareous
sedimentary r ocks th at have ex ten sive buffer ing ca pacit y.
Nature of ore
Tungsten skarn orebodies are stratiform deposits that can extend for hundreds of meters along lithologic contacts.
Stockwork and local crosscutting veins are common. The grain size and molybdenum content of scheelite in
tungsten skarns varies. Paragenetically early, anhydrous skarn generally contains relatively fine-grained, high-
molybdenum scheelite, whereas retrograde skarn contains medium to coarse grained low-molybdenum scheelite.
Coarse grained, vuggy skarn with uneven ore grades forms from impure marble; finer-grained, compact skarns
with more evenly distributed ore grades tend to form from pure marbles (Einaudi and others, 1981).
Cassiterite in tin skarns is commonly very fine grained. Tin concentrations range from 0.1 to 1 weight
percent in deposits that include 10 to 90 million tonnes of ore. Tin may contained in silicate minerals, including
garnet and horn blende (Eadington, 1983).
Sn ska rns- Sn, W, F, Be, Zn, Cu, Ag, Li, Rb, Cs, Re, B.
See tables 1-4 for examples of ranges of geochemical signatures associated with rock, stream sediment,
and soil samples from these deposit types; also see Beus and Grigorian (1977) for additional trace element data
pertaini ng to these deposits.
Table 1. Summar y ranges of ana lytical data for 20 samples of skar n from th e Lentung W sk arn dep osit, Rock
Creek district, Mont. (data from DeBoer, 1991).
[Samples represent garnet-pyroxene skarn ± scheelite and chalcopyrite. Methods include DCP, ICP, AA, INAA for W. Sn w as analyzed, but not
detected in any o f the samples. Co ncentrations rep orted as ppm , unless otherwise ind icated. N= number of samples in
which the element was detected. See DeBoer (1991) for detection limits and details of analyses; --, not detected]
Au (ppb) 1 11 11 11
Ag 18 .5 151 10.4
Al ( %) 20 .2 224 13.7
As 13 7 55 36
Ba 20 1 212 25
Be 11 0.6 6.5 2.5
Bi 16 -- 130 31
Ca (%) 2 4.32 7.44 5.88
Cd 4 1 21 9
Ce 20 7 153 91
Co 20 4 54 18
Cr 20 10 237 82.2
Cu 20 2 6,005 411 .4
F e (% ) 8 2.08 9.74 6.17
Ga 14 3 25 12
Hg (ppb) 20 15 50 22
K (%) 4 .05 0.65 0.32
La 13 2 114 28
Li 20 3 49 12
Mg (%) 19 .13 6.12 2.32
Mn 18 1,868 19,460 8,610
Mo 20 8 1,214 176
Na (%) 20 .1 2.26 0.33
Nb 20 5 44 14
Ni 20 15 59 42
Pb 15 4 11,100 798
Rb 6 33 642 286
Sb 14 6 42 18
Sc 16 26 133 70
Sr 20 8 626 86
Ta 20 28 230 114
Th 15 11 55 28
Ti (%) 19 .02 0.65 0.22
V 20 17 352 144
W 18 15 3,200 620
Y 20 4 46 19
Zn 20 16 954 183
Zr 20 13 229 106
W skarns: Prograd e- Pyroxene (diopsid e-hedenberg ite), gar net (gr ossular-a ndra dite), idocr ase, wollaston ite.
Retrograd e- Spessarti ne-rich garnet , biotite, am phibole, pl agioclase, epi dote, quar tz, chl orite,
sulfide
minerals (chalcopyrite, pyrite, pyrrhotite, sphalerite, bornite, arsenopyrite, bismuthinite),
magnetite, fluorite, native bismuth.
Ore- Scheeli te, molybdenite, wolframite, ca ssiterit e.
Table 2. Summary ranges of analytical data for samples of skarn, endoskarn, hornfels,
marbl e, an d jas peroi d ass ociate d wit h skar n in t he Tonop ah 1°x 2° qua dran gle, N ev.
[Concentrations reported as ppm; N, number of samples in which element was detected; detection limits for
each element are given in parentheses. Unpublished emission spectrographic data for rock samples from
Lodi H ills, Ellsworth, Ced ar Mo untain, Pe g Leg M ine (San A ntonio Mo untains), T imblin Ca nyon in the T oi
yabe Range, and the Victory W mine; D.A. John]
Table 3. Summary ranges of trace element contents, in parts per million, for selected stream
sediment samples from the Sn-W-Be district of the Seward Peninsula, Alaska (data from Sains
bury and others, 1968).
[Spectrograph and spectrophotometric data for 17 stream sediment samples from 13 drainages. N, number of sam
ples in which the elem ent was detected. S ee Sainsbu ry and other s (196 8) Ta ble 1 for detection lim its and Ta ble
8 for descr iptions of m ethods an d samp les]
B 11 15 700 185
Be 12 3 160 45
Cu 17 7 150 34
Li 9 15 3,000 713
Nb 3 15 15 15
Pb 11 15 150 64
Sn 13 10 1,100 148
W 6 10 70 38
Sn skarns: Prograd e- Idocrase, ga rnet (sp essartin e-rich gr andit e, tin-bear ing an dradit e), mala yite, danbur ite,
datolite, p yroxene, wollaston ite.
Retrograde- Amphibole, mica, epidote, magnetite, chlorite, tourmaline, fluorite, sulfide minerals
(sphalerite, pyrrhotite, chalcopyrite, pyrite, arsenopyrite), Be minerals (helvite, danalite).
Ore- Cassit erite, sch eelite.
Replacement Sn: Major min erals- Ca ssiterit e, pyrrhoti te, chalcopyri te, pyrite, a rsenopyrit e, ilmen ite, fluori te.
Minor m inera ls- Pyrite, sph alerit e, galena , stann ite, tetr ahedri te, magn etite.
Late veins- Sph alerit e, galena , chalcopyrit e, pyrite, fluor ite.
Table 4. Summary ran ges of trace elemen t content s for soil associat ed with the Sn-W-B e dist rict
of the Seward Peninsula in the Lost River valley, Alaska (data from Sainsbury and others, 1968).
[Selected AA, spectrophotometric, and SQS for 40 soil samples. N, number of samples in which the element
was detected. Li present, in undetermined am ount, in 37 sam ples. See Sainsbury and others (1 968) T able 9
for detailed description of samples and methods and Table 1 for detection limits. Minimum, maximum, and
mean concentrations in parts per million]
Secondary mineralogy
Cassiterite, the primary ore mineral of tin, is very stable under most near-surface conditions and its dispersion is
dependent more on ph ysical than chemical con ditions. However, cassiterite-plus-sulfid e skarns i n hum id climat es
read ily decom pose, bu t cassi teri te-qu art z-tou rma lin e assem blages form r esidua l soil ove rlyin g deposi ts. Ox idat ion
can cause the formation of varlamoffite, a soft, earthy, impure hydrated stannic oxide that is less inert than
cassiterite. Readily dissolved varlamoffite can release tin into solution.
Topography, physiography
Granitoid plutons associated with skarn tend to form positive areas of moderate to high relief. However, in semi
arid environments subjected to extended periods of weathering, some granitoid plutons may occupy topographic
lows. Silicified rocks associated with skarn s may form knobs or ridges.
Hydrology
Tungsten skarns are associated with largely unfractured plutons. Post-mineralization faults may focus ground
water flow in undergroun d workings.
These deposits have been mined by open pit and un derground meth ods.
W skarns: Mining method generally depends on the grade and form of the deposit; higher average ore grades (0.7
weight percent WO 3 or more) are generally required to warrant costs of underground mining operations.
Underground methods include room-and-pillar (Cantung, British Columbia; King Island, Australia), cut-and-fill,
stoping (Pine Creek, Calif.) or combinations of these methods (Anstett and others, 1985). Gravity, flotation, and
chemical methods are used to produce natural and artificial scheelite, and ammonium paratungstate (APT) (Smith,
1994).
Scheelite concentrate has been produced in the United States using gravity, flotation, and magnetic
separation techniques (Stager and Tingley, 1988). After crushing and grinding, sulfide minerals and scheelite are
separated by flotation, and sulfide slimes are removed to tailings piles. Scheelite concentrates are processed to
preci pita te sil ica. Molybdenu m is r emoved a s MoS 3 via a precipitati on process that releases H 2S through scrubber-
equipped stacks. An organic solvent extraction technique is used to produce APT.
Sn skarns: Cut and fill stoping methods are used at the Renison Bell, Tasmania, replacement tin deposit, the
world' s lar gest un derg roun d min e. Sul fide-m iner al-r ich a nd su lfide-min eral -poor or e are s electi vely stockp iled on
the surface and blended ore is fed into a three-stage open crushing circuit that reduces ore from 750 mm to 15 mm.
Ore is processed by flotation to remove sulfide minerals prior to gravity concentration of cassiterite (Morland,
1986). Staged grinding is used to liberate fine-grained cassiterite. Residual sulfide minerals in the gravity
concentra te are rem oved by flotation. Cassit erite concen trates a re leached wit h sulfuri c acid to remove sider ite,
magnetic material is removed using magnetic separators; refined concentrates are then shipped to smelters.
Tailings a re combined with lime to adjust pH to 8.5 before being pumped to impoun dments.
ENVIRONMENTAL SIGNATURES
Surface distu rbance
Mining these deposits may result in associated open pits, tailings piles, and subsidence in areas of underground
mining.
Smelter signatur es
The p refer red m ethod of tin or e benefic iati on (fum ing, smel tin g, or refin ing) depen ds on t in gr ade an d cont ent of
iron an d other im puriti es (Bleiwas and oth ers, 1986) . Fumin g and smel ting m ay release sulfur and ar senic.
Climat e effects on environm ental si gnatur es
The effects of various climatic regimes on the geoenvironmental signature specific to these deposits are not known.
Because most of these deposits have relatively low sulfide mineral contents and because carbonate minerals that
have abundant acid consumption potential are abundant in association with these deposits, environmental
sign atur es associ ated wi th t in a nd (or ) tun gsten skar n an d repl acemen t deposi ts ar e proba bly not m uch a ffected by
climatic regime variation. Both scheelite and cassiterite are resistant minerals and their tungsten and tin contents
are relatively immobile within a range of surficial weather ing regimes.
PERSPECTIVE
See section entitled "Perspective" in CU, AU, ZN-PB skarn deposits model (Hammarstrom and others, this
volume).
REFERENCES CITED
Anstett, T.F. , Bleiwas, D.I., and Hurdelbrink, R.J., 1985, Tungsten availability-Market economy countries: U.S.
Bureau of Mines Information Circular 9025.
Beus, A.A., and Gr igorian, S. V., 1977, Geochemical explor ation methods for min eral deposits: Wilmette, I llinois,
Applied Publishing Ltd., 287 p.
Bleiwas, Donald I., Sabin, Andrew E., and Peterson, Gary R., 1986, Tin availability-Market economy countries, A
minerals availability program appraisal: U.S. Bureau of Mines Information Circular 90-86, 50 p.
Clark, R.N., Swayze, G.A., and Gallagher, Andrea, 1993, Mapping minerals with imaging spectroscopy, in Scott,
R.W., Jr., and others, eds., Advances related to United States and international min eral resources--
Developing frameworks and exploration techniques: U.S. Geological Survey Bulletin 2039, p. 141-150.
Cox, D.P., 1986, Descriptive model of W skarn deposits, in Cox, D.P. and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 55.
Cox, D.P., and Singer, D.A., 1986, Min eral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
DeBoer, Thoma s A., 1991, Geology and min eraliz ation of the Len tung tu ngsten skarn dep osit near Brownes Lake,
Pioneer Mountains, Montana: Bellingham , Western Washington University, M.S. thesis, 200 p.
Eadington , P.J., 1983, Geochemical exploration for tin -recent research results in Smith, R.E., ed., Geochemical
exploration in deeply weathered terrain, CSIRO.
Einaudi, M.T., Meinert, L.D., and Newberry, R.J., 1981, Skarn deposits: Economic Geology 75th Anniversary
Volume, p. 317-391.
Forstner, U., and Wittmann, G.T.W., 1981, Metal pollution in the aquatic environment: New York, Springer-
Verlag, 486 p.
Glass, N.R., Arnold, D.E., Galloway, J.N., Henry, G.R., Lee, J.J., McFee, N.W., Norton, S.A., Powers, C.F.,
Rambo, D.L., and Schofield, C.L., 1982, Effects of acid precipitation: Environmental Science and
Technology, v. 15, p. 162A-169A.
Grimes, D.J., Ficklin, W.H., Meier, A.L., and McHugh, J.B., 1995, Anomalous gold, arsenic, antimony and
tungsten in ground water and al luvium around disseminated gold deposits along the Getchell trend,
Humboldt County, Nevada: Journal of Geochemical Exploration, v. 52, p. 351-371.
Hepworth, J.V., and Zhang, Y.H., Eds., 1982, Tungsten Geology, Jiangxi, China (Proceedings of a Symposium
jointly sponsored by the ESCAP/RMRDC and Ministry of Geology, People's Republic of China, 12-22
October 1981): Bandung, Indonesia, ESCAP/RMRDC, 583 p.
Hoover, D.B., and Knepper, D.H., 1992, Geophysical model of a tin skarn and related deposits, in Hoover, D.B.,
Heran, W.D., a nd Hill, P.L., eds., The geophysical expression of selected mineral deposit m odels: U.S.
Geological Survey Open-File Report 92-557, p. 89-94.
Karunakaran, C., 1977, Fluorine-bearing minerals in India-their geology, mineralogy, and geochemistry, in
Proceedings of the symposium on fluorosis, Indian Academy of Geosciences, Hyderabad, p. 3-18.
King, T.V.V., Ager, Cathy, Clark, R.N., Swayze, G.A., and Gallagher, A.J., 1994, Application of field and
laboratory spectroscopic analysis to investigate the environmental impact of mining in the southeastern
San Juan Mountains and a djacent San Luis Valley, Colorado: U.S. Geological Circular 1103-A, p. 53-54.
Kotlyar, B.B., Ludington, Steve, and Mosier, D.L., 1995, Descriptive, grade, and tonnage models for molybdenum-
tungsten greisen deposits: U.S. Geological Survey Open-File Report 95-584, 30 p.
Kwak, T. A.P., 1983, The geology and geochemistry of the zoned, Sn-W-F-Be skarns at Mt. Lindsay, Tasmania,
Australia: Economic Geology, v. 78, p. 1440-1465.
Menzie, W.D., and Reed, B.L., 1986a, Grade and tonnage model of Sn skarn deposits, in Cox, D.P. and Singer,
D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 58-60.
_________1986b, Grade and tonnage model of replacement Sn, in Cox, D.P. and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 62-63.
Menzie, W.D., Jones, G.M., and Elliott, J.E., 1992, Tungsten-Grades and tonnages of some deposits, in DeYoung,
J.H.,Jr., and Hammarstrom, J.M., eds., Contributions to commodity geology research: U.S. Geological
Survey Bulletin 1877, p. J1-J7.
Minoguchi, Gen, 1977, The correlation of chronic toxic effect in tropical and subtropical areas between fluoride
concentration in drinking water and climate, especially mean annual temperature, in Proceedings of the
symposium on fluorosis, Indian Academy of Geosciences, Hyderabad, p. 175-186.
Morland, R., compiler, 1986, Renison Bell tin mine-technical review: Unpublished report from Renison Limited,
32 p.
Nash , J.T ., 19 88, Inter pret ation of the r egion al geoch emist ry of the T onopa h 1°x 2° qua dra ngle, Nevad a, bas ed on
analytical result s for stream-sediment a nd nonma gnetic heavy-minera l concentrate samp les: U.S.
Geological Survey Bulletin 1849, 28 p.
Reed, B.L., 1986, Descriptive model of replacement Sn, in Cox, D.P. and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 61.
Reed, B.L., and Cox, D.P., 1986, Descriptive model of Sn skarn deposits, in Cox, D.P. and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 58.
Sainsbury, C.L., Hamilton, J.C., and Huffman, Claude, Jr., 1968, Geochemical cycle of selected trace elements in
the tin-tu ngsten-beryllium dist rict, western Seward Peni nsula, Alaska --a reconnaissa nce study: U.S.
Geological Survey Bulletin 1242-F, p. F1-F42.
Smit h, Ge ral d R., 1 994, Mater ials flow of tun gsten in the Uni ted St ates: U.S. Bur eau of Mi nes In forma tion
Circular 9388.
Stager, H.K., and Tingley, J., 1988, Tungsten deposits in Nevada: Nevada Bureau of Mines and Geology Bulletin
105, 256 p.
White, Donald E ., Hem, John D., an d Waring, G.A., 1963, Dat a of Geochemistry, Sixth Edition , Chapter F.
Chemical composition of subsurface waters: U.S. Geological Survey Professional Paper 440-F, 67 p.
VEIN AND GREISEN SN AND W DEPOSITS
(MODELS 15a-c; Cox and Bagby, 1986; Reed, 1986a,b)
Figure 1. Generalized longitudinal section through the Xihuashan and Piaotang tungsten deposits in the Dayu district, China. Other vein systems
are indicated by heavy lines. I, upper limits of ore zones; II, lower limits of ore zones. Patterned area is granite batholith. Unpatterned area
is sedimentary and metamorphic rocks (from Elliott, 1992).
Figure 2. Maps and sections of tungsten vein deposits illustrating mineral and alteration zoning. A, Chicote Grande deposit, Bolivia; B,
Xihuashan, China (from Cox and Bagby, 1986).
62
Figure 3. Diagrammatic cross section of a tin greisen (from Reed, 1986b).
Examples
Pasto Bueno, Peru (Landis and Rye, 1974); Panasqueira, Portugal (Kelly and Rye, 1979); Dayu, China (Tanelli, 1982;
Elliott, 1992); Dajishan, China (Elliott, 1992); Cornwall, United Kingdom (Hosking, 1969); Herberton, Australia
(Blake, 1972); Lost River, Alaska, United States (Sainsbury, 1964; Dobson, 1982); Erzgebirge, Czechoslovakia
(Janecka and Stemprok, 1967); Baid al Jimalah and Silsilah, Saudi Arabia (du Bray and others, 1988; Kamilli and
others, 1993).
63
mobilized in highly acid environments.
The majority of deposits are in terrane consisting of plutonic granitic rocks and pre-intrusive clastic
sedimentary or metasedimentary rocks that have low acid buffering capacity. A potential hazard in regions
containing these types of deposits is the presence of high abundances of radon inside buildings because most of these
deposits are hosted by rock with elevated uranium abundances. For example, in the United Kingdom, the highest
abundances of radon are found in the counties of Cornwall and Devon (Scivyer and others, 1993), a region long
famous for production of tin and tungsten from lode deposits associated with granitic plutons (Hosking, 1969).
Radon is a health hazard in some underground Cornish mines (Dungey and others, 1979).
Mining, milling, and smelting have low to moderate environmental impact. Milling generally involves
gravity, flotation, and magnetic separation; reagents used are generally benign. Concentrates are of high unit value
and can be shipped long distances to processing plants and smelters. Smelters are not particularly harmful but may
release sulfur dioxide.
Exploration geophysics
Magnetic, gravity, and radiometric surveys can be used to define areas in which prospective leucocratic granites may
be present (Hoover and others, 1992). These granites tend to be associated with gravity and magnetic lows and have
elevated radioelement (uranium, thorium, and potassium) abundances. Airborne radiometric surveys may identify
exposed parts of prospective plutons. Remote sensing techniques may help define exposed leucocratic granites or
detect altered areas. At the district or deposit scale and in the less common case of high sulfide ore, audio
magnetotelluric techniques, induced polarization, and electric-field-ratio profiling can be used to map variations in
rock resistivity due to sulfide mineral content.
References
Hosking (1969), Kelly and Rye (1979), Cox and Bagby (1986), Reed (1986a,b), du Bray and others (1988), and
Elliott (1992).
Host rocks
Tin and tungsten deposits exhibit a close spatial association with granitic plutonic rocks, especially late-stage, highly
evolved, specialized biotite and (or) muscovite (S-type or A-type) granites and leucogranites. Small to moderate-
sized cupolas of larger subsurface plutons are especially favorable hosts; deposits may be endo- or exocontact.
Exocontact deposits usually are in pelitic and arenaceous sedimentary or metamorphic rocks and within the contact
metamorphic aureole of a pluton. Most endocontact deposits, including tin greisens, and many tin and tungsten veins,
are in or near cupolas and ridges developed on the roof or along margins of granitoids.
64
Wall-rock alteration
Alteration directly associated with ore includes greisenization, albitization, and (or) tourmalinization. Greisen is a
type of phyllic alteration (including sericitic) characterized by Li-F-bearing micas, topaz, tourmaline, fluorite, and
quartz. Kaolinization, a type of argillic alteration, is widespread in parts of Cornwall, U.K. Silicification is also
important, especially in the contact aureoles of granitic plutons and cupolas. Other alteration types include
microclinization, chloritization, and hematization. Zoned alteration has been identified in some tungsten vein
systems, including the Xihuashan mine, Dayu district, China, where upper parts of veins have well developed greisen
zones; middle parts have quartz-rich greisen and silicification; and lower parts have K-feldspar-rich greisen. Higher
tungsten grades are found in the upper and middle portions of veins (Wu and Mei, 1982).
Most alteration assemblages associated with tin and tungsten vein and greisen deposits have low acid
buffering capacity. Zones of chloritic or feldspathic alteration, usually minor in extent, have low to moderate acid
buffering capacity.
Nature of ore
Most vein deposits consist of individual veins or sets of veins that are individually minable. Some mines and
districts contain hundreds of such veins. The mineralized zone at the Xihuashan mine in the Dayu district, China,
consists of more than 650 veins arranged in three sets of steeply dipping parallel veins (Elliott, 1992). The veins
have an average thickness of 0.4 m (maximum of 3.6 m), average length of 150 m (maximum of 1,075 m), and
vertical extent of about 250 m. Other deposits consist of bulk-minable vein stockworks, as do some parts of the
tungsten deposit at Baid al Jimalah in Saudi Arabia (Kamilli and others, 1993) and the Hemerdon deposit in U.K.
(Mining Magazine, 1979). Some are truly disseminated in greisenized granite cupolas, such as Silsilah tin deposit
in Saudi Arabia (du Bray and others, 1988). Less commonly, tin greisens may have the form of pipes, lenses, or
irregular breccia zones.
Tungsten: Vein mineralogy varies from simple, consisting almost entirely of quartz and wolframite, to complex as
at Pasto Bueno, Peru, and Panasqueira, Portugal. At Pasto Bueno, the principal vein minerals are wolframite,
tetrahedrite-tennanite, sphalerite, galena, and pyrite in a gangue of quartz, fluorite, sericite, and carbonate. Minor
amounts of molybdenite, chalcopyrite, bornite, arsenopyrite, enargite, stolzite, scheelite, zinnwaldite, topaz, tungstite,
and native arsenic are present (Landis and Rye, 1974). At Panasqueira, more than 50 vein-forming minerals,
including sulfide, sulfosalt, oxide, carbonate, silicate, phosphate, and tungstate minerals, have been identified (Kelly
and Rye, 1979). In general, the most common minerals in tungsten vein deposits in addition to quartz are:
wolframite, molybdenite, bismuthinite, pyrite, pyrrhotite, arsenopyrite, bornite, chalcopyrite, scheelite, cassiterite,
Studies of zoning and paragenesis in many tungsten vein deposits (Landis and Rye, 1974; Kelly and Rye,
1979; Wu and Mei, 1982) indicate that, in general, tungsten minerals form earlier, at higher temperatures, and
possibly closer to an igneous source than sulfide and carbonate minerals. A general mineral precipitation sequence
from silicate to oxide, sulfide, and finally carbonate minerals is common to many deposits.
Tin: The mineralogy of tin vein deposits is extremely varied and complex, especially where sulfide and sulfosalt
minerals are present. The most common minerals are cassiterite, wolframite, arsenopyrite, molybdenite, hematite,
scheelite, beryl, galena, chalcopyrite, sphalerite, stannite, and bismuthinite in addition to ubiquitous quartz.
The Cornwall region (U.K.) is frequently cited as one of the classic areas of ore zoning, with Sn, Cu, Pb-Zn,
Fe-(Mn, Sb) zones distributed sequentially around individual intrusive centers and arranged according to depth to
granite contacts. The zones are roughly parallel to granite-metasediment contacts and may represent paleoisogeother
mal surfaces (Guilbert and Park, 1986). The tin zone, the deepest zone, is generally found from depths of about
65
1,300 m within the granite to a short distance outside the granite contacts in metasedimentary rocks (Guilbert and Park,
1986).
In most greisen deposits, polyphase mineralization and multiple mineralizing centers control ore distribution
and render identification of zoning patterns quite difficult. Nevertheless, the idealized disseminated greisen deposit
associated with an individual cupola is zoned with respect to distributions of Sn, Mo, As, Bi, W, Be, Ag, Pb, and
Zn (Hosking, 1969; Reed, 1986b). Abundances of elements toward the beginning of this list are greatest in
hydrothermally altered rocks nearest mineralized cupolas, whereas abundances of elements toward the end of the list
are greatest in distal parts of mineralized areas. Alteration zoning consists of albitization below the ore; pervasive
greisenization with quartz-muscovite-topaz ± fluorite ± tourmaline; and chloritic alteration. Pyrite and arsenopyrite
are common in the strongly greisenized zones. In addition to cassiterite and wolframite, other minerals include
molybdenite, bismuthinite, native bismuth, pyrrhotite, bornite, chalcopyrite, scheelite, beryl, tetrahedrite-tennanite,
sphalerite, galena, enargite, hematite, stannite, sulphostannates, siderite, and calcite. Complex uranium, thorium, rare
earth element oxide and phosphate minerals are commonly present in minor amounts.
Mineral characteristics
Tin-tungsten veins are commonly coarse-grained; grain sizes as much as several cm are common and, in the Dajishan
mine, China, wolframite crystals up to 1 m in length are found in quartz veins (Elliott, 1992). Very large wolframite
crystals are also present at deposits in eastern Nevada. Ore and gangue minerals in tin greisens are finer-grained.
Quartz is the most common gangue mineral in both veins and greisen and may account for 90 percent or more of
vein fillings. Ore and gangue minerals other than quartz are commonly enclosed or capsulated by quartz thus
protecting them from oxidation and weathering by surface and ground water. Post-mineralization faults, however,
may expose ore and gangue, including sulfide minerals, to oxidation and solution in mine water.
Secondary mineralogy
Secondary tin and tungsten minerals are rare and, if present, have limited geoenvironmental impact. Varlamoffite
((Sn,Fe)(O,OH)2), a complex oxidation product of stannite, has been reported from numerous localities. The
oxidation and weathering of wolframite or scheelite deposits can produce small amounts of tungstite (WO3·H2O) and
(or) ferritungstite ((W,Fe)(O,OH)3). Where the sulfide and (or) sulfosalt mineral content is high oxidation and
weathering may result in the formation of secondary and supergene minerals, some of which, including goethite,
limonite, jarosite, chalcanthite, and others, are soluble (underlined).
Topography, physiography
Tin and tungsten vein and greisen deposits are found in areas of varied topography. Granitic plutonic rocks
associated with these deposits tend to form positive areas of moderate to high relief. Silica-rich rocks such as
greisens, stockwork vein zones, and quartz veins form knobs or linear ridges in areas of low relief.
Hydrology
Tin and tungsten veins and greisens are commonly vuggy and are zones of high permeability relative to surrounding
granitic or metasedimentary rocks. In addition, post-mineralization faulting may follow veins and large, individual
veins may contain major water courses, making control of mine drainage a potential problem.
66
Metallurgy: Tungsten--Tungsten concentrates, containing approximately 60-65 percent WO3 (scheelite), can be
combined with coke and steel in an electric furnace and reduced to ferrotungsten; alternatively, concentrates can be
treated chemically to produce intermediate products or tungsten metal (as powder). Because of tungsten's high
melting temperature (3,400°C), chemical decomposition and purification are used, instead of pyrometallurgy, to
produce tungsten metal. This process involves three steps: (1) decomposition of tungsten minerals, (2) purification
of tungstic oxide, and (3) production of metal powder (Li and Wang, 1955). Most current production, trade, and
consumption of tungsten, however, involves an intermediate product called ammonium paratungstate (APT). The
preparation of APT is a chemical process involving calcination, pressure digestion, filtration and purification, solvent
extraction, and crystallization (Lassner, 1982). Tin--Smelting tin from cassiterite concentrates generally involves
either a (1) carbo-thermic process of heating tin concentrate with carbon or (2) fuming process in which tin
concentrate is heated with sulfur or sulfide minerals to volatilize stannous sulfide. A chloride volatilization process
is also used in tin smelting (Harris, 1979).
ENVIRONMENTAL SIGNATURES
Drainage signatures
Because of the low solubility of tin and tungsten minerals, high values in water are not expected. In the unusual
case, in which the primary ore sulfide mineral content is high, highly acidic water with elevated abundances of iron,
aluminum, fluorine, and variable copper and zinc may characterize drainage from mines or mine wastes. Highly acid
water can also mobilize fluorine and uranium, whose abundances are commonly elevated in tin and tungsten ore.
Smelter signatures
Tin smelters may release sulfur dioxide and other volatile elements, including arsenic, fluorine, chlorine, and others,
67
Geoenvironmental geophysics
Audio magnetotelluric techniques, induced polarization, and electric-field-ratio profiling can be used to detect
hydrothermally altered areas and the presence of ground water. Gamma ray spectroscopy can be used to measure
uranium and thorium abundances and radon can be determined by several special collectors used in conjunction with
laboratory analysis.
REFERENCES CITED
Blake, D.H., 1972, Regional and economic geology of the Herberton-Mount Garnet area, Herberton tinfield, North
Queensland: Australia Bureau of Mineral Resources Bulletin 124, 265 p.
Carpenter, L.C., and Garrett, D.E., 1959, Tungsten in Searles Lake: Mining Engineering, v. 11, p. 301-303.
Cox, D.P., and Bagby, W.C., 1986, Descriptive model of W veins, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 64.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Dobson, D.C., 1982, Geology and alteration of the Lost River tin-tungsten-fluorine deposit, Alaska: Economic
Geology, v. 77, p. 1033-1052.
du Bray, E.A., Elliott, J.E., and Stuckless, J.S., 1988, Proterozoic peraluminous granites and associated Sn-W
deposits, Kingdom of Saudi Arabia, in Taylor, R.P., and Strong, D.F., eds., Recent advances in the geology
of granite-related mineral deposits: Canadian Institute of Mining and Metallurgy, special volume 39, p. 142
156.
Dungey, C.J., Hore, J., and Waller, M.D., 1979, An investigation into control of radon and its daughter products in
some Cornish mine atmospheres: Transactions of the Institute of Mining and Metallurgy, v. 88, p. A35-A43.
Elliott, J.E., 1992, Tungsten- geology and resources of deposits in southeastern China, in DeYoung, J.H., Jr., and
Hammarstrom, J.M., eds., Contributions to commodity geology research: U.S. Geological Survey Bulletin
1877, p. I1-I10.
Guilbert, J.M., and Park, C.F., Jr., 1986, The geology of ore deposits: W.H. Freeman and Co., New York, 985 p.
Harris, J.H., 1979, The problems of tin, in Lead, Zinc, Tin '80: The Metallurgical Society of AIME, Warrendale,
PA., p. 733-753.
Hoover, D.B., Heran, W.D., and Hill, P.L., 1992, The geophysical expression of selected mineral deposit models:
U.S. Geological Survey Open-File Report 92-557, 129 p.
Hosking, K.F.G., 1969, The nature of primary tin ores of the south-west of England, in A Second Technical
Conference on Tin: Bangkok, International Tin Council, v. 1, p. 21-83.
Janecka, J., and Stemprok, M., 1967, Endogenous tin mineralization in the Bohemian massif, in A Technical
Conference on Tin: London, International Tin Council, v. 1, p. 245-266.
Kamilli, R.J., Cole, J.C., Elliott, J.E., and Criss, R.E., 1993, Geology and genesis of the Baid al Jimalah tungsten
deposit, Kingdom of Saudi Arabia: Economic Geology, v. 88, no. 7, p. 1743-1767.
Kelly, W.C., and Rye, R.O., 1979, Geologic, fluid inclusion, and stable isotope studies of the tin-tungsten deposits
of Panasqueira, Portugal: Economic Geology, v. 74, p. 1721-1822.
Kotlyar, B.B., Ludington, Steve, and Mosier, D.L., 1995, Descriptive, grade, and tonnage models for molybdenum-
tungsten greisen deposits: U.S. Geological Survey, Open-File Report 95-584, 30 p.
Krauskopf, K.B., 1969, Tungsten (Wolfram), in Handbook of Geochemistry, Springer-Verlag, v. II/2, p. 74-B to 74
O.
Landis, G.P., and Rye, R.O., 1974, Geologic, fluid inclusion, and stable isotope studies of the Pasto Bueno tungsten-
base metal ore deposit, northern Peru: Economic Geology, v. 69, p. 1025-1059.
Lassner, Erik, 1982, Modern methods of APT processing, in Tungsten; 1982, Proceedings of the Second International
Tungsten Symposium, June 1982: Mining Journal Books, Ltd., p. 71-80.
Li, K.C., and Wang, C.Y., 1955, Tungsten: Reinhold, New York, 506 p.
Menzie, W.D., Jones, G.M., and Elliott, J.E., 1992, Tungsten- grades and tonnages of some deposits, in DeYoung,
J.H., Jr., and Hammarstrom, J.M., eds., Contributions to commodity geology research: U.S. Geological
Survey Bulletin 1877, p. J1-J7.
Menzie W.D., and Reed, B.L., 1986a, Grade and tonnage model of Sn veins, in Cox, D.P., and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 67-69.
_________1986b, Grade and tonnage model of greisen Sn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 71-72.
Mining Magazine, 1979, Hemerdon- Britain's largest tungsten deposit: Mining Magazine, October, p. 342-351.
68
Raddatz, A.E., Gomes, J.M., and Carnahan, T.G., 1988, Preparation of ammonium paratungstate from a sodium
Reed, B.L., 1986a, Descriptive model of Sn veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models:
69
CLIMAX MO DEPOSITS
(MODEL 16; Ludington, 1986)
by Steve Ludington, Arthur A. Bookstrom, Robert J. Kamilli, Bruce M. Walker, and Douglas P. Klein
Examples
Climax (Wallace and others, 1968), Henderson (Carten and others, 1988), Urad, Mount Emmons, Winfield, Middle
Mountain, Silver Creek (Rico), and Redwell Basin, all in Colo.; Questa, N. Mex.; Pine Grove, Utah (Keith and
others, 1986); Mount Hope, Nev. (Westra and Riedell, 1995).
70
immobile, because it is strongly coprecipitated with and (or) adsorbed on ferric oxyhydride at low pH. Plants
growing in soil with a pH of 5.5 or less commonly contain only trace amounts of molybdenum, whereas plants
growing in soil with a pH of 6.5 or higher are commonly enriched in molybdenum (Hansuld, 1966).
Molybdenosis is a disease that affects ruminants that graze on molybdenum-rich vegetation that grows on
alkaline soil in which the ratio of bioavailable copper to bioavailable molybdenum (as molybdate) is less than 2:1.
Thus, molybdenosis is more related to climatic factors, soil alkalinity, and the relative bioavailability of copper and
molybdenum, than to point sources of molybdenum.
High fluorine concentrations associated with Climax deposits may be beneficial. Children who grew up at
Climax, Colo., had brown-speckled, but cavity-free teeth, due to the high fluoride content of local drinking water.
Uranium concentrations are anomalously high in Climax molybdenum systems. Granitic rocks associated
with the deposits contain uranium-bearing accessory minerals, most of which are not recovered but deposited with
mill tailings; uranium abundances in Ten Mile Creek, which receives input from Climax tailings ponds, are
significantly elevated, however. Distal veins peripheral to Climax deposits, commonly several kilometers distant,
may also have anomalously high uranium contents. Thus, radon gas in the mines is a potential hazard; radon
abundances must be monitored and mitigated by proper ventilation, as necessary.
Exploration geophysics
Alteration associated with shallow or exposed deposits produce diagnostic color (reflectance) patterns on remote-
sensing images. Pyrite and hydrothermal clays in the phyllic alteration zone display reduced resistivity and high
induced potential anomalies (Fritz, 1979). Anomalous uranium, thorium, and potassium abundances can be mapped
with airborne gamma-ray spectrometry. Radon in mines or associated with mine-related ground water can be
identified using simple detectors. At Mt. Emmons, a magnetic anomaly is coincident with a layer of hydrothermal
magnetite below the molybdenite zone (Fritz, 1979; Thomas and Galey, 1982). Local gravity is variable as a
function of rock types present in the shallow subsurface; regional gravity lows, produced by multistage, high-silica
plutons and underlying granitic batholiths are nearly ubiquitous in association with these deposits. Self potential lows
have been reported over phyllic (quartz-sericite-pyrite) alteration zones associated with several deposits (Corry, 1985).
References
Wallace and others (1968), White and others (1981), Carten and others (1988), and Keith and others (1986).
Host rocks
Deposits are found in crystalline, volcanic, and sedimentary rocks of diverse ages in the western United States.
Wall-rock alteration
Wall-rock alteration includes (1) high temperature assemblages: quartz + fluorite ± molybdenite, quartz + K-feldspar
+ fluorite ± molybdenite, and quartz alone, all found near the center of the hydrothermal system; (2) moderate
temperature assemblages: quartz + K-feldspar + magnetite + brown biotite ± topaz ± fluorite, and quartz + sericite
+ green biotite ± topaz ± fluorite; and (3) low temperature assemblages: pyrite + sphalerite + garnet + rhodocrosite
+ clay, and a large propylitic zone (albite + epidote + chlorite) that may extend kilometers beyond intrusive centers.
Pyrite, a constituent of moderate- and low-temperature assemblages, is the most significant mineral with regard to
environmental concerns. Rocks from the quartz-sericite-pyrite zone at Climax, Colo., contain about 2 to 10 volume
percent (4 to 20 weight percent) pyrite.
71
Nature of ore
Orebodies are typically overlapping, inverted, and saucer-shaped, and are stacked above one another, with or without
offset. High grade parts of composite orebodies form where individual orebodies associated with discrete stocks
overlap. Assay walls of orebodies are typically quite sharp.
Molybdenite is present primarily with high-temperature alteration assemblages, both as a vein-filling phase
and as replacements in vein selvages. Pyrite is rarely present with molybdenite, but rather is found in later, lower-
temperature veins and assemblages that cut earlier molybdenite veins. Late, insignificant sphalerite- and galena-
bearing veins may cut pyrite veins in distal parts of systems.
Climax molybdenum deposits are relatively barren of other metals, except tin and tungsten, each of which
may form weakly enriched zones in the outer parts or outside molybdenite orebodies. Wolframite was recovered
for many years as a by-product of mining at Climax, Colo. Tin is present primarily as cassiterite at Climax, but is
in ilmenorutile at Henderson, Colo.
Mineral characteristics
Molybdenite grain size varies widely, from about 0.2 mm in replacement veins to >10 cm in open-space filling.
Secondary mineralogy
In exposed deposits, most pyrite weathers to limonite and other iron oxide minerals, and molybdenite may alter to
ferrimolybdite and (or) ilsemannite, Mo3O8•nH2O; other secondary minerals include jarosite and various clay
minerals. In wet areas, some pyrite is totally oxidized causing iron to be dissolved in drainage water; iron
subsequently precipitates as hydrous iron oxide.
Where pyrite and molybdenite weather together, weathering products depend on pH, and on the ratio of iron
hydroxide to acid molybdate in water draining the area. Ferrimolybdite forms in strongly acidic environments
(Hansuld, 1966), molybdenum-bearing jarosite probably forms in moderately acidic environments, molybdenum-
bearing iron hydroxide minerals form in weakly acidic to mildly alkaline environments, and geochemically mobile
molybdate ion forms in alkaline environments, where pH >6. Ilsemannite is rare and ephemeral, because conditions
for its stability are rarely encountered in the normal weathering environment (Hansuld, 1966).
Weathered deposits commonly exhibit red hematite, yellow jarosite and ferrimolybdite, brown goethite, and
peripheral black manganese oxide minerals.
Topography, physiography
Orebodies are high in silica, and may be resistant, but most known deposits are deeply buried. Many known deposits
are within or beneath peaks stained a distinctive red color by iron oxide minerals.
Hydrology
Annually variable runoff from winter snowmelt may dramatically affect influx into tailings ponds. Most host rocks
have low porosity, but the deposits exhibit high fracture permeability.
72
Mining and milling methods
These deposits are large, bulk tonnage deposits, and are typically mined by open stope, block caving, and open-pit
methods which typically further fracture the rocks, increasing permeability and exposing the deposit and surrounding
pyritic rocks to increased flow of oxidizing ground water. Molybdenite is typically concentrated on-site by flotation
of finely-ground ore.
ENVIRONMENTAL SIGNATURES
Drainage signatures
A limited amount of information is available for deposits in Colorado. Water draining pyrite-molybdenite zones has
a pH of 1 to 3 and contains elevated dissolved metal abundances, including hundreds to thousands of mg/l iron and
aluminum, hundreds of mg/l fluoride, tens of mg/l zinc and copper, and 1 to 10 mg/l uranium. Water draining
intermedia te pyrite shells has a pH of 2 to 5 and contains elevated dissolved metal abundances, including hundreds
of mg/l iron and aluminum and <1 to about 10 mg/l zinc and copper. Water draining peripheral base-metal-bearing
zones has a pH of about 5.5 and contains elevated dissolved metal abundances, including 1 to 200 mg/l zinc and
hundreds of µg/l to several mg/l iron and copper (Plumlee and others, 1995).
Smelter signatures
The lone molybdenum smelter in the United States is in western Pennsylvania; it uses an electrolytic process.
Geoenvironmental geophysics
Sulfide mineral concentrations can be detected by induced polarization surveys. Acid pore water can be identified
by low resist ivity, and usually by enhanced induced polarization, signatures. Self potential surveys may be used to
identify redox centers in tailings; heat from these centers may be identified by infrared surveys or shallow thermal
73
probes, though numerous interference factors may complicate these investigations. Thickness and structure of tailings
may be determined using shallow seismic refraction, electrical, and ground penetrating radar surveys.
REFERENCES CITED
Burt, D.M., Sheridan, M.F., Bikun, J.G., Christiansen, E.H., Correa, B.P., Murphy, B.A., and Self, S., 1982, Topaz
rhyolites—distribution, origin, and significance for exploration: Economic Geology, v. 77, p. 1818–1836.
Carten, R.B., Geraghty, E.P., Walker, B.M., and Shannon, J.R., 1988, Cyclic generation of weakly and strongly
mineralizing intrusions in the Henderson porphyry molybdenum deposit, Colorado—Correlation of igneous
features with high-temperature hydrothermal alteration: Economic Geology, v. 83, p. 266–296.
Carten, R.B., White, W.H., and Stein, H.J., 1993, High-grade granite-related molybdenum systems—classification
and origin, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M., eds., Mineral deposit modeling:
Geological Association of Canada Special Paper 40, p. 521–554.
Corry, C.E., 1985, Spontaneous polarization associated with porphyry sulfide mineralization: Geophysics, v. 50, no.6,
p. 1985.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Fritz, F.P., 1979, The geophysical signature of the Mt. Emmons porphyry molybdenum deposit, Gunnison Co.
Colorado [abs], Geophysics v. 44, no. 3, p. 410.
Hansuld, J.A., 1966, Behavior of molybdenum in secondary dispersion media—a new look at an old geochemical
puzzle: Mining Engineering, v. 18, no. 12, p. 73.
Keith, J.D., Shanks, W.C., III, Archibald, D.A., and Farrar, E., 1986, Volcanic and intrusive history of the Pine
Grove porphyry molybdenum system, southwestern Utah: Economic Geology, v. 81, p. 553–577.
Kotlyar, B.B., Ludington, Steve, and Mosier, D.L., 1995, Descriptive, grade, and tonnage models for molybdenum-
tungsten greisen deposits: U.S. Geological Survey, Open-File Report 95-584, 30 p.
Larson, P.B., 1987, Stable isotope and fluid inclusion investigations of epithermal vein and porphyry molybdenum
mineralization in the Rico mining district, Colorado: Economic Geology, v. 82, p. 2141–2157.
Larson, P.B., Cunningham, C.G., and Naeser, C.W., 1994, Hydrothermal alteration and mass exchange in the
hornblende latite porphyry, Rico, Colorado: Contributions to Mineralogy and Petrology, v. 116, p. 199–215.
LeGendre, G.R., and Runnells, D.D., 1975, Removal of dissolved molybdenum from wastewaters by precipitates of
ferric iron: Environmental Science and Technology, v. 9, p. 744.
Ludington, S.D., 1986, Descriptive model of Climax Mo deposits, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 73.
Plumlee, G.S., Streufert, R.K., Smith, K.S., Smith, S.M., Wallace, A.R., Toth, Margo, Nash, J.T., Robinson, Rob,
Ficklin, W.H., and Lee, G.K., 1995, Geology-based map of potential metal-mine drainage hazards in
Colorado: U.S. Geological Survey Open-File Report 95-26, scale 1:750,000, 9 p.
Ranta, D.E., 1974, Geology, alteration, and mineralization of the Winfield (La Plata) district, Chaffee County,
Colorado: Golden, Colorado School of Mines, Ph.D. dissertation, 261 p.
Sharp, J.E., 1978, A molybdenum mineralized breccia pipe complex, Redwell Basin, Colorado: Economic Geology,
v. 73, p. 369–382.
Theobald, P.K., and Thompson, C.E., 1959, Geochemical prospecting with heavy mineral concentrates used to locate
a tungsten deposit: U.S. Geological Survey Circular 411, 13 p.
Thomas, J.A., and Galey, J.T., Jr., 1982, Exploration and geology of the Mt. Emmons molybdenite deposits,
Gunnison County, Colorado: Economic Geology, v. 77, p. 1985-1104.
Wallace, S.R., Muncaster, N.K., Jonson, D.C., MacKenzie, W.B., Bookstrom, A.A., and Surface, V.E., 1968,
Multiple intrusion and mineralization at Climax, Colorado, in Ridge, J.D., ed., Ore Deposits of the United
States, 1933–1967, The Graton-Sales Volume: American Institute of Mining, Metallurgical, and Petroleum
Engineers, Inc., New York, NY, p. 605–640.
Westra, Gerhard, and Riedell, K.B., 1995, Geology of the Mount Hope stockwork molybdenum deposit, Eureka
County, Nevada, [abs.]: Geology and ore deposits of the American Cordillera-A symposium, Geological
Society of Nevada, U.S. Geological Survey, Sociedad Geologica de Chile, p. A78-A79.
White, W.H., Bookstrom, A.A., Kamilli, R.J., Ganster, M.W., Smith, R.P., Ranta, D.E., and Steininger, R.C., 1981,
Character and origin of Climax-type molybdenum deposits: Economic Geology, 75th Anniversary Volume,
p. 270–316.
74
PORPHYRY CU DEPOSITS
(MODEL 17; Cox, 1986)
Examples
Bingham, Utah (Lanier and others, 1978); San Manuel, Ariz. (Lowell and Guilbert, 1970); El Salvador, Chile
(Gustafson and Hunt, 1975).
Exploration geophysics
The distribution of disseminated copper sulfide minerals and pyrite can be mapped using induced polarization;
resistivity surveys may define low resistivity areas associated with altered sulfide-mineral-bearing rock (Elliot and
MacLean, 1978; Pelton and Smith, 1976). Detailed magnetic surveys may also help delineate altered rock if
contained magnetic minerals have been destroyed by alteration. Peripheral magnetite-bearing rocks, including
magnetite-rich skarns, may produce magnetic anomaly highs around porphyry copper deposits (Grant, 1985).
Regional magnetic and gravity anomalies may identify intrusions that host porphyry copper deposits; magnetic and
gravity maps, as well as remote sensing images may show lineaments, associated with large faults, and clusters of
intrusions that may host porphyry copper deposits (Carlson and Mabey, 1963; Raines, 1978; Turner and others, 1982;
Abrams and others, 1983). Large alteration halos surrounding exposed porphyry copper deposits are easily identified
on remote sensing images. Vegetation in the vicinity of buried porphyry copper deposits may be stressed by metal
uptake. Remote sensing may identify buried deposits if associated stressed vegetation produces reflectance anomalies
(Birnie and Francica, 1981; Knepper, 1989).
75
Table 1. Summary statistics for 326 samples of mostly unoxidized rocks from the hypogene zone of porphyry copper deposits,
southern Arizona, with Cu 1,000 ppm (Maurice Chaffee, unpub. data).
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
"inst" indicates specific-ion electrode (F), titrimetric (S), fluorometric (U) analysis. Leaders (--) indicate no meaningful value]
References
Lowell and Guilbert (1970), Gustafson and Hunt (1975), Lanier and others (1978), and Titley, (1982).
Host rocks
Host rocks include tonalite to monzogranite or syenitic porphyry and associated breccia pipes intruding granitic,
76
Table 2. Summary statistics for 247 samples of mostly unoxidized rocks from the hypogene zone of porphyry copper deposits,
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
"inst" indicates specific-ion electrode (F), titrimetric (S), fluorometric (U) analysis. Leaders (--) indicate no meaningful value]
Wall-rock alteration
Alteration zones (and mineral assemblages), from bottom, innermost zones outward are: sodic-calcic (oligoclase or
albite, actinolite, and sphene), potassic (potassium feldspar, biotite, rutile, and pyrite or magnetite), to propylitic
77
Table 3. Summary statistics for 75 partly to completely oxidized rock samples from the supergene zone of porphyry copper
deposits, southern Arizona, with Cu 1,000 ppm (Maurice Chaffee, unpub. data).
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
"inst" indicates specific-ion electrode (F), titrimetric (S), fluorometric (U) analysis. Leaders (--) indicate no meaningful value]
(oligoclase or albite, epidote or calcite, chlorite, rutile, and magnetite or pyrite). Phyllic (sericite, chlorite, rutile,
and pyrite) and argillic (clay, sericite,chlorite, and pyrite) alteration may overprint early potassic assemblages. High-
alumina alteration (pyrophyllite, alunite, andalusite, corundum, diaspore, clay, and sericite) may be present in upper
part of some deposits. Propylitic or phyllic alteration zones commonly lie on the flanks of deposits.
Nature of ore
Ore consists of stockwork veinlets and disseminated copper minerals.
78
Table 4. Summary statistics for 168 partly to completely oxidized rock samples from the supergene zone of porphyry copper
deposits, southern Arizona, with Cu <1,000 ppm (Maurice Chaffee, unpub. data).
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
"inst" indicates specific-ion electrode (F), titrimetric (S), fluorometric (U) analysis. Leaders (--) indicate no meaningful value]
porphyry copper deposits of the western United States are presented in tables 1-8. The summaries have been
constructed for subsets containing samples with either 1,000 or <1,000 ppm copper; samples with copper contents
1,000 ppm are assumed to come from copper deposits and those with copper values <1,000 ppm are assumed to
be from areas surrounding the deposits (outer haloes). Data summarized are from published (Chaffee, 1992; 1994)
and unpublished compilations (M.A. Chaffee, unpub. data, 1995).
A comparison of mean values for hypogene samples shows that Ag, Au, Cu, Fe, Mo, S, Sb, Te, and V have
significantly higher mean values in the "ore" zone, whereas chromium and manganese have significantly higher
values in the areas surrounding the "ore" zones (tables 1 and 2). Other elements considered are not significantly
enriched or depleted in either zone. A similar comparison of mean values for samples from areas influenced by
weathering and supergene enrichment shows a significant enrichment of Ag, As, Cu, Mn, Mo, S, Sb, Te, U, and Zn
in secondarily-enriched ( 1,000 ppm copper) copper zones, whereas barium and chromium are higher in the sur
79
Table 5. Summary statistics for 39 rock samples from the Mount Margaret, Wash., porphyry copper deposit with Cu 1,000 ppm
(Chaffee, 1994).
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
rounding areas.
Elements with significantly higher concentrations in the supergene environment (table 3) with copper 1,000
ppm, as compared to the high copper subset in the hypogene zone (table 1), include As, B, Co, Mo, Ni, S, Sb, Te,
Tl, and Zn. Those significantly higher in the hypogene zone subset include Ba, Ca, Cu, Cs, F, Li, Mg, Mn, and Na.
Elemental abundances in hypogene and supergene samples from halo areas (tables 2 and 4) are very similar;
As, B, Ba, Co, Ni, Pb, S, Sb, Te, Tl, and V have significantly higher mean values in the supergene zone and Ca,
Cu, Cs, Li, Mg, Mn, Na, U, and Zn have higher mean values in the hypogene zone.
Samples from the Mount Margaret, Wash. deposit (tables 5 and 6), were mostly unaffected by weathering.
Ag, Au, B, Cu, Mo, Pb, and W have significantly higher values in samples containing greater than 1,000 ppm
copper, whereas As, Cd, Sb, Sr, and Te are higher in samples from outside the "ore" zone. Similarly, samples from
the Kalamazoo deposit, Ariz. (tables 7 and 8), were almost unaffected by weathering. Ag, Au, B, Ce, Cu, F, Li, Mg,
Mo, S, Sb, Se, V, and CO2 abundances are significantly higher in the subset of samples that contains 1,000 ppm
copper, whereas Fe, Hg, Mn, Pb, Rb, Te, Tl, and Zn abundances are greater in samples from areas surrounding the
"ore" zone.
Ore samples from Tanamá (table 9), Puerto Rico (Cox, 1985), with more than 4,000 ppm copper, have
different metal concentrations in different mineral zones. The potassically altered chalcopyrite-magnetite zone (table
9) has higher manganese (700 ppm) and zinc (60 ppm) than the phyllic-altered chalcopyrite-pyrite zone (table 10),
which has higher sulfur (2.3 weight percent) and selenium (6 ppm).
80
Table 6. Summary statistics for 28 rock samples from the Mount Margaret, Wash., porphyry copper deposit with Cu <1,000 ppm
(Chaffee, 1994).
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
"cm" indicates colorimetric analysis. Leaders (--) indicate no meaningful value]
Mineral characteristics
Porphyry has closely spaced quartz and feldspar phenocrysts in a fine grained, aplitic quartz-feldspar groundmass.
Ore minerals are present in stockwork veinlets and as disseminated sulfide grains.
Secondary mineralogy
Weathered outcrops in high pH environments include copper carbonate, oxide, and silicate minerals. Most deposits
that include abundant pyrite, create low pH environments in which leaching is intense; outcrops are greatly depleted
81
Table 7. Summary statistics for 121 rock samples from the Kalamazoo porphyry copper deposit, Ariz., with Cu 1,000 ppm
(Chaffee, 1992).
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
xrf, indicates X-ray fluorescence analysis; inst indicates volumetric analysis (CO
2), specific-ion electrode (F), titrimetric (S), colorimetric (Se).
Leaders (--) indicate no meaningful data]
82
Table 8. Summary statistics for 162 rock samples from the Kalamazoo porphyry copper deposit, Ariz., with Cu <1,000 ppm
(Chaffee, 1992).
[Most concentrations in parts per million; % following an element indicates weight percent data. N, not detected at lower limit of determination,
in parentheses. L, detected at concentration less than lower limit of determination, in parentheses. Mean values based on unqualified values only.
Most elements determinedby semiquantitative spectroscopic analysis; except, "aa" following element symbol indicates atomic-absorption analysis;
xrf, indicates X-ray fluorescence analysis; inst indicates volumetric analysis (CO
2), specific-ion electrode (F), titrimetric (S), colorimetric (Se).
Leaders (--) indicate no meaningful data]
83
Table 9. Summary statistics for 18 samples of mostly unoxidized rocks from the chalco-
pyrite-magnetite zone of the Tanamá porphyry copper deposit, Puerto Rico. Data, in ppm
unless otherwise noted, from Cox (1985); N, not detected at indicated abundance.
Number Percent
Element Minimum Maximum Median
unqualified unqualified
Ag 1 5 1.5 18 100
Au 0.2 0.95 0.36 18 100
Ba 150 500 200 18 100
Cu 4,400 15,600 6,700 18 100
Co 7 30 15 18 100
Cr 20 1,000 50 18 100
Mn 150 >5,000 700 18 100
Mo N5 70 15 14 78
Ni 10 70 15 18 100
Pb 4 21 8 18 100
S% 0.4 8 1.5 18 100
Se 2 8 3 18 100
Te <0.1 0.7 0.12 9 50
Ti <0.2 0.76 0.3 12 67
Zn 17 2,300 60 18 100
Table 10. Summary statistics for 15 samples of mostly unoxidized rocks from the chal-
copyrite-pyrite zone of the Tanamá porphyry copper deposit, Puerto Rico. Data, in ppm
unless otherwise noted, from Cox (1985); N, not detected at indicated abundance.
Number Percent
Element Minimum Maximum Median
unqualified unqualified
Ag 0.7 2 1.5 15 100
Au 0.18 0.65 0.36 15 100
Ba 150 500 200 15 100
Cu 4,300 13,000 8,400 15 100
Co 10 30 15 15 100
Cr N20 70 30 14 93
Mn 50 700 200 15 100
Mo N5 70 5 8 57
Ni 10 30 15 15 100
Pb 2 13 7 15 100
S% 0.6 5.6 2.3 15 100
Se 4 9 6 8 53
Te <0.1 0.79 0.11 8 50
Ti <0.2 0.68 0.34 10 67
Zn 9 270 36 15 100
Table 11. Summary statistics for soil samples from the B horizon above two distinct mineral zones in the Tanamá
porphyry copper deposit, Puerto Rico. Data, in ppm, from Learned and others (1992).
Magnetite zone Pyrite zone
Number of Number of
Element Maximum Minimum Median Maximum Minimum Median
samples samples
Cu 2,000 150 510 12 400 71 160 12
Pb 19 5 10 12 23 1 4 12
Zn 17 2 8 12 11 <1 5 12
Mo 50 <5 5 12 300 <5 20 12
Cd 2.4 0.7 1.4 12 3.5 0.4 1 12
84
in copper; other metals may be similarly affected. Leached copper is transported downward and deposited as
secondary sulfide minerals at the water table or paleowater table. Deposits of secondary sulfide minerals contain
chalcocite, covellite, as well as other Cu2S and CuS minerals that replace pyrite and chalcopyrite.
Topography, physiography
Deposits are mainly in mountainous terrain, but in the southwestern United States they commonly are found partially
or completely buried under Cenozoic basin fill deposits.
Hydrology
Potassic zone is commonly weakly jointed; phyllic and argillic zones are generally highly jointed and permeable to
ground water.
ENVIRONMENTAL SIGNATURES
Drainage signatures and metal mobility from solid mine wastes
Elements in surface materials may be remobilized by chemical leaching or mechanical weathering processes; those
whose abundances in rock or soil are elevated can become similarly elevated in derivative materials. All elements
present are transported away from porphyry system exposures by mechanical erosion; in addition, elements can be
remobilized from subsurface rock by flowing ground water. However, three factors particularly influence the
chemical environment in and around porphyry copper deposits: (1) host-rock chemistry, (2) the level of erosion in
a given deposit, (3) the abundances of elements and various mineral species at a given erosion level.
85
The chemical, as well as the physical, nature of the host rocks in the vicinity of porphyry copper deposits
influence the extent of associated chemical dispersion. Chemically reactive host rocks, such as limestone and
dolomite, react with acidic solutions containing elements, such as copper and zinc, that are mobile in the low-pH
environment commonly associated with a weathering porphyry copper deposit. Most other host-rock lithologies are
relatively non-reactive to low-pH solutions and do not much affect solution-borne elements.
Porphyry copper deposit genesis typically produces zones containing concentrations of various elements and
minerals (Chaffee, 1976a, 1982). Some elements and minerals tend to be concentrated in the core of a deposit, some
in the main chalcopyrite zone, some in the pyrite halo, and some in the outermost parts of a deposit. Data in tables
1-8, for instance, indicate that the outer halo around the chalcopyrite zone may be enriched in the elements Ag, As,
Au, B, Cu, Fe, B, Mo, Pb, S, Se, Te, Tl, and (or) Zn, whereas the inner parts of the same system have different
abundances and distributions of ore-related elements. Thus, the environmental effects of a given porphyry copper
deposit depend on the level of erosion.
Porphyry copper deposits commonly contain significant pyrite abundances. Pyrite is usually concentrated
in a zone that is generally at the outer edge of the chalcopyrite zone. During weathering, pyrite decomposition
produces a low pH environment and many mineral deposit-related elements react with the resulting acidic solutions
and, in the absence of carbonate rocks, (1) co-precipitate locally with iron-oxide phases (for instance, molybdenum
and tellurium), (2) form new minerals (for instance, chalcocite, covellite, native copper, malachite), or (3) precipitate
on unweathered sulfide grains (chalcocite, covellite). All of these reactions significantly restrict dispersion of the
elements affected. This process results a zone of supergene-enriched metals. In the absence of significant pyrite,
weathering leaches Cu, Mo, and Zn, as well as B, Co, Mn, S, Se and major rock-forming elements such as Ca, Mg,
and Na, from near-surface rocks and disperses them into the surrounding environment.
The interaction between a porphyry copper deposit and the environment is illustrated by a stream in the
Globe mining district, Ariz. that was blocked by mill tailings, causing a lake to form. Water from this lake entered
an alluvial aquifer by seepage; the aquifer and a stream to the north were contaminated. The results, as reported by
Eychaner (1991) and Stollenwerk (1994), represent the chemical signature of a pyrite-rich porphyry copper deposit
in the natural environment, although the pH in this case is much lower, and the dissolved metal content much higher,
than would be expected in a natural weathering situation. The most contaminated ground water in the aquifer had
a pH of 3.3, and contained about 9,600 mg/l sulfate, 2,800 mg/l iron, 300 mg/l aluminum, and 190 mg/l copper.
As the plume traveled north through the aquifer, the concentration of constituents decreased as the plume interacted
with alluvium and was diluted by uncontaminated water (ground water flowing upwards from lower basin-fill, water
in uncontaminated streams that join the contaminated wash, and surface rain water). In all cases, mixing was not
necessarily instantaneous, and uncontaminated water could flow parallel to the plume for hundreds of meters before
mixing was complete. The large concentration of sulfate in acidic ground water has a significant effect on the
speciation of aqueous cations: about 50 percent of Ca, Mg, Fe, Mn, Cu, Co, Ni, and Zn, and more than 80 percent
of the aluminum, are transported as sulfate-ion pair complexes. The acid-water front, pH <5, has advanced through
alluvium, which contained about 0.4 weight percent carbonate minerals, at a rate of 0.2 to 0.3 km/yr during the past
several decades. One of the reactions modeled by Stollenwerk (1994) is dissolution of calcite and dolomite by acidic
water and precipitation of large amounts of calcium as gypsum. Stollenwerk (1994) concluded that aluminum and
pH exhibit the greatest potential for continued adverse effects to ground water quality. In a work plan written by
Eychaner (unpublished data, 1989), he states, "In ground water systems the movement of acidic water is retarded
by chemical reactions with the aquifer material, but the same reactions transform the aquifer and decrease its capacity
to retard future contamination."
86
those characteristic of other geologic environments; these elevated abundances identify appropriate baseline
geochemical values that may be useful in setting remediation standards for mines associated with this deposit type.
Smelter signatures
No data specific to the environmental effects of smelting ore from deposits of this type were identified. However,
copper sulfide minerals are typically smelted and may produce SO2-rich and metal-rich emissions, which may
Geoenvironmental geophysics
Detailed induced polarization and (or) resistivity or electromagnetic surveys can identify low resistivity rocks
associated with porous and permeable, clay-altered sulfide-mineral-bearing veins, fractures and faults, and breccia
pipes that may be a source of acidic, metal-bearing water that results from oxidation of iron or copper sulfide
minerals (King and Pesowski, 1993). Oxidation of sulfide minerals may create thermal anomalies identifiable by
infrared surveys, shallow heat-flow probes, or self potential (Strangway and Holmer, 1966; Corry, 1985; Corwin,
1990). Acid- or metal-bearing water has low resistivity that can be identified with electromagnetic or direct current
induced polarization and (or) resistivity surveys and ground penetrating radar (Greenfield and Stoyer, 1976; Davis
and Annan, 1992; King and Pesowski, 1993). Structural and stratigraphic features such as bedrock topography,
buried channels, permeable fault zones, and aquitards that affect water flow away from mine areas may be studied
with electromagnetic, direct current resistivity, seismic refraction, and gravity surveys (Paterson 1995). Water flow
may be monitored using self potential techniques (Corwin, 1990). Carefully designed and interpreted self potential
surveys also may be useful in defining reduction-oxidation centers in tailings (Paterson, 1995). Remote sensing
images derived from satellite or airborne surveys can be used to identify the distribution of alteration assemblages,
which in turn help identify areas that may be sources of acidic and metal-bearing water; images derived from
airborne surveys, such as AVIRIS, also may indicate the distribution of minerals that provide acid buffering capacity.
Remote sensing can also be used to evaluate effects related to large scale open pit mining.
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87
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89
CU, AU, AND ZN-PB SKARN DEPOSITS
(MODELS 18b,c; Cox and Theodore, 1986; Cox, 1986; Theodore and others, 1991)
by Jane M. Hammarstrom, Boris B. Kotlyar, Ted G. Theodore, James E. Elliott, David A. John,
Jeff L. Doebrich, J. Thomas Nash, Robert R. Carlson, Gregory K. Lee, K. Eric Livo, and Douglas P. Klein
INTRODUCTION
The general description of skarns given below is also applicable to the SN and (or) W skarn and replacement deposits
and Fe skarn deposits geoenvironmental models.
Skarns are coarse-grained metamorphic rocks composed of calcium-iron-magnesium-manganese-aluminum
silicate minerals (commonly referred to as "calcsilicate" minerals) that form by replacement of carbonate-bearing
rocks (in most cases) during contact or regional metamorphism and metasomatism. The majority of the world's
major skarn deposits are thought to be related to hydrothermal systems (Einaudi and others, 1981). Skarns can be
barren or contain metals with economic value. Skarn deposits are important sources of base and precious metals as
well as tin, tungsten, and iron. Skarns are relatively high-temperature mineral deposits related to magmatic-
hydrothermal activity associated with granitoid plutons in orogenic tectonic settings; skarns generally form where
a granitoid pluton has intruded sedimentary strata that include limestone or other carbonate-rich rocks. The processes
that lead to formation of all types of skarn deposits include: (1) isochemical contact metamorphism during pluton
emplacement, (2) prograde metasomatic skarn formation as the pluton cools and an ore fluid develops, and (3)
retrograde alteration of earlier-formed mineral assemblages. Deposition of ore minerals accompanies stages 2 and
3. Skarn deposits are typically zoned mineralogically with respect to pluton contacts, original lithology of host rocks,
and (or) fluid pathways. Later petrogenetic stages may partly or completely obliterate earlier stages of skarn
development. Skarn deposits commonly are also associated with many other types of magmatic-hydrothermal
deposits in mineral districts. In fact, distinction between skarn and other deposit types is not always apparent, and
in many districts, skarns form an intermediate "zone" between porphyry deposits in the center of mining districts and
peripheral zones of polymetallic vein and replacement and distal disseminated deposits. In many cases, geochemical
signatures in stream sediment or water may reflect mixtures of several deposit types.
Skarns are classified as calcic if the protolith was limestone and as magnesian if the protolith was dolomite;
they are also classified by the most economically important metal present (copper, iron, lead-zinc, gold, tin, tungsten,
etc.). Each class of skarn deposit has a characteristic, though not necessarily unique, size, grade, tectonic setting,
granitoid association, and mineralogy (Einaudi and Burt, 1982; Einaudi and others, 1981; Meinert, 1983). Not
surprisingly, therefore, the various classes of skarn deposits have different geochemical signatures and oxidation
sulfidation states. Most economic skarn ore is present as exoskarn, which forms in carbonate rock that hosts a
mineralizing intrusion. Endoskarn, which is variably developed on the intrusion side of intrusion-wallrock contacts,
can be important when fluid flow was directed into the intrusion or channelized along the intrusion-wall rock contact.
Separate geoenvironmental models for three groups of skarn deposits are presented below: (1) base and
precious metal, (2) tin and tungsten, and (3) iron skarns. These groupings are based on geologic setting, associated
igneous rock compositions, skarn mineralogy and sulfide mineral content, and characteristic geochemical and
geophysical signatures (fig. 1). Differences among these groups of deposits indicate different potentials for acid mine
drainage and differences in geoavailability of potentially toxic substances. For example, the principal ore minerals
in base and precious metal skarns are sulfide minerals, whereas the principal tin and tungsten ore minerals are the
oxide and tungstate minerals cassiterite and scheelite, respectively. Data tables illustrate observed ranges of
composition for skarn, stream-sediment, soil, water, and mine waste. Much of these data reflect the authors'
unpublished and published work in north-central Nevada and in southwestern Montana. Both areas contain examples
of skarn deposits from each model group. No new data were acquired for this model development effort, but some
previously unpublished geochemical data are included. These models are preliminary compilations based on existing
data from a variety of sources.
90
Figure 1. Schematic diagram showing relative positions of different classes of skarns as a function of associated igneous rock composition and
skarn sulfide mineral content. Mean silica content, in weight percent, for igneous rocks from Meinert (1983).
that contain average ore grades greater than 1 gram of gold per tonne and are usually associated with intense
retrograde alteration. Gold-bearing skarns may contain significant amounts of numerous other commodities,
including copper, lead, zinc, and iron and (or) they may have been mined in the past for these or other metals.
Examples
Copper skarns: Carr Fork, Utah; Meme, Haiti; Cananea, Sonora, Mexico.
Gold-bearing skarns: Fortitude, Nev.; McCoy, Nev.; Nickel Plate-Hedley, British Columbia, Canada; Red Dome,
Australia.
Exploration geophysics
Local magnetic highs may indicate skarn deposits with significant abundances of pyrrhotite and (or) magnetite.
Precious metal skarns that contain abundant pyrite and quartz may lack this distinctive magnetic signature. In
extremely weathered terranes, exposures of limonite, iron hydroxide, iron sulfate gossan, clay, or carbonate rock
associated with these deposits may be identified by analyzing spectral features on Landsat Thematic Mapper
reflectance imagery. Very low frequency electromagnetic and induced polarization surveys may be used to identify
magnetite and associated disseminated sulfide bodies. Resistivity surveys can be used to model the distribution of
alteration zones as indicated by ion concentrations of water saturated rocks. Because of density contrasts between
alteration mineral assemblages and deposit host rocks, microgravity surveys may be used to delineate the distribution
of skarn deposits.
References
Geology: Einaudi and others (1981), Cox (1986), Cox and Theodore (1986), and Theodore and others (1991).
Environmental signature: Ficklin and others (1992), Smith and others (1994), Apte and others (1995), and Salomons
(1995).
91
GEOLOGIC FACTORS THAT INFLUENCE POTENTIAL ENVIRONMENTAL EFFECTS
Deposit size
Mosier (1986), Cox and Theodore (1986), Jones and Menzie (1986), and Theodore and others (1991) present deposit
size ranges, expressed as percentiles of tonnages (million metric tonnes) for populations of N deposits:
Cu skarns (N=64): 0.03 (90th percentile) 0.56 (50th percentile) 9.2 (10th percentile)
Au skarns (N= 90): 0.0013 (90th percentile) 0.28 (50th percentile) 9.8 (10th percentile)
Zn-Pb skarns (N=34): 0.16 (90th percentile) 1.40 (50th percentile) 12 (10th percentile)
The 50th percentile value is the median deposit size for each population.
Host rocks
These deposits are in carbonate rocks, including limestone or marble, dolomite, and calcareous and dolomitic marble,
pelite, argillite, shale, graywacke, and other clastic rocks. In terms of the bedrock type classification of Glass and
others (1982), most host rocks for these deposit types are type IV; they are highly calcareous sedimentary rocks or
metamorphosed calcareous sedimentary rocks that have extensive buffering capacity. Less common host rocks
include chert, volcanic flows (dacite, andesite, or basalt) and volcaniclastic rocks, and metamorphic rocks such as
slate or phyllite, quartzite, and amphibolite. These less common host rocks provide low to medium buffering
capacity. At the Beal Mountain, Mont., gold deposit, gold-bearing rock is entirely in calcsilicate-bearing hornfels
in clastic, quartz-rich host rocks. This deposit is atypical of skarns and may represent a distinct, but related type of
disseminated gold deposit in contact metamorphosed rocks
(Hastings and Harrold, 1988; Meinert, 1989).
Wall-rock alteration
Wall rock is altered to hornfels (quite widespread in places, and present throughout areas as large as 15 to 20 km2),
marble, bleached limestone, and skarn zones; potassic, sericitic, argillic, propylitic alteration assemblages are
developed and plutons may contain endoskarn. Silica and marble "fronts" (sharp boundaries between unreplaced rock
and altered rock) may be present.
Nature of ore
Ore minerals may be present in massive, stratiform, vein, and (or) disseminated form; grain size is highly variable
and ranges from fine to very coarse. Ore may be present in sulfide mineral zones, oxide zones, and in supergene,
clay-rich oxidized zones. Sulfide minerals and gold generally are deposited during late, retrograde alteration within
zones characterized by hydrous calcsilicates. Retrograde alteration may be best developed along faults cutting
paragenetically earlier assemblages. Gold is commonly associated with a late pyrite + quartz assemblage (veins or
disseminated). Note that many of these skarns contain zones that have been or could be mined for magnetite (see
Fe skarn model, Hammarstrom and others, this volume). Total sulfide mineral content at the McCoy, Nev., gold
skarn is low ( <2 volume percent) and many sulfide minerals are oxidized. In contrast, at the Fortitude, Nev., gold
skarn about 30 km to the north, high grade ore is sulfide mineral rich, and includes mostly pyrrhotite as well as
lesser amounts of chalcopyrite, pyrite, arsenopyrite, and native bismuth.
92
Table 1A. Data summary for drill core samples, pro
vided by Battle Mountain Gold Company, from Forti
tude, Nev. gold skarn deposit; tabulated by J.L. Doebrich.
[Samples from border zone immediately surrounding Lower
Fortitude ore zone (hosted by Pennsylvanian and Permian
Antler Peak Limestone; Au <0.02 oz/t; N, number of samples)]
93
Table 1C. Data summary for drill core samples,
provided by Battle Mountain Gold Company, from
Fortitude, Nev. gold skarn deposit; tabulated by
J.L. Doebrich.
[Analyses from Upper Fortitude ore zone (hosted by
N, number of samples)]
The Copper Canyon area, Nev., (fig. 2) contains copper, gold, lead-zinc skarns whose geochemical signatures
are characteristic of those associated with these deposit types. Skarn deposits in the central part of the map area (fig.
2A) were unexposed prior to mining. Broad areas of outcrop had elevated abundances of copper ( >700 ppm) prior
to large-scale mining activity (fig. 2B). Concentrations of lead, mercury, arsenic, and cadmium are controlled strongly
94
Table 3. Summary ranges of geochemical data for trace elements in fresh and variably altered
[Data, in ppm unless otherwise noted, from Johnson (1991, Table F-1). Samples include drill core, outcrops,
and ore dumps. Fire assay and atomic absorption methods. Mean based on number of samples in which ele
ment was detected at concentrations above detection limits; --, not detected]
by surface distribution of pre- and syn-mineralization faults that acted as conduits for mineralizing fluids. Most rocks
with anomalous cadmium abundances are north of the outer limit of the dispersed iron sulfide halo that surrounds
almost all of the large disseminated deposits (fig. 2F). In addition, mercury, and to a lesser degree tin abundances,
appear to be sharply zoned relative to some of the most intensely mineralized skarns (B.B. Kotlyar, unpub. data,
1995). Similar relations characterize soil geochemistry around the McCoy deposit, Nev. (fig. 3). Summary statistics
for the Fortitude, Nev., gold-rich skarn in the Copper Canyon area are shown in table 1.
Summary ranges of trace element geochemical data for fresh and variably altered rocks from the northern
part of the New World, Mont., copper-gold-silver district (Johnson, 1991), which includes the historic McLaren Mine
are listed in table 3. In addition, Van Gosen (1994) analyzed 100 samples from the Homestake breccia deposit, one
of five copper-gold-silver skarn and replacement deposits recently delineated by exploration in the New World
district, and located less than 1 km southeast of the McLaren Mine. Van Gosen (1994) demonstrated that the
Homestake orebodies are best categorized as gold-bearing skarns in the upper parts of a porphyry copper system.
By comparing metal concentrations in samples of ore (Au >3,125 ppb), sub-ore (3 ppb< Au <3125 ppb), and
background rock (Au <3 ppb), Van Gosen (1994) computed enrichment-depletion ratios for 24 selected metals in
the Homestake deposits. Relative to background, the following elements are enriched in ore and sub-ore: Au, Ag,
Cu, Mg, As, Bi, Co, Cr, Ga, Mn, Nb, Ni, Pb, and Zn. Barium, lithium, strontium, and zirconium are depleted in
ore and sub-ore samples relative to background samples, whereas aluminum, potassium, sodium, titanium, and
vanadium are depleted in ore samples but enriched or unchanged relative to background in sub-ore samples.
95
bordering pyrite cubes. Mineral characteristics in skarns are likely to vary considerably within a single deposit due
to the zoned nature of skarns and replacement of early-formed minerals by later mineral assemblages. In zinc-lead
skarns, ore commonly forms massive pods of pyrrhotite, sphalerite, galena, and chalcopyrite in pyroxene zones.
Secondary mineralogy
Gossans develop over sulfide-mineral-rich parts of skarn deposits and may concentrate metals. Manganese oxide rich
gossans are associated with zinc-lead skarns. Supergene alteration (oxidation) leads to formation of clays, hematite
and goethite (after pyrite and other sulfide minerals), and secondary copper minerals (after chalcopyrite). Gold-rich
skarn at the McCoy, Nev., deposit is highly oxidized and argillized to as much as 245 m below the present surface
(Brooks and others, 1991). Minerals in these argillized zones include a variety of clays (montmorillonite, nontronite,
smectite, and illite), manganese oxide minerals, supergene copper oxide minerals, claudetite (As2O3), willemite
(Zn2SiO4), marcasite, and hydrated iron oxide minerals. At the Red Dome Au skarn deposit in North Queensland,
Australia, high-grade gold ore is concentrated in highly oxidized karst-collapse breccia which formed during post-
mineralization uplift and erosion; oxidation of primary tellurium-rich hypogene sulfide ore promoted karst formation
and concentration of free gold (Torrey and others, 1986). Mineralogy and zoning that develops during supergene
alteration of zinc-lead-silver sulfide ore in carbonate rocks depends on local Eh and pH conditions, permeability, and
relative activities of carbonate species and sulfate ions in leaching ground water (Sangameshwar and Barnes, 1983).
For example, cerussite is the first lead mineral to precipitate where dissolved carbonate is more abundant than sulfate,
but anglesite precipitates if the situation is reversed. Supergene minerals associated with copper-lead-zinc-silver ore
include azurite, malachite, smithsonite, cerargyrite as well as cerussite, anglesite, manganese minerals (pyrolusite,
groutite), goethite and sulfide mineral products from bacterial reduction.
Topography, physiography
Skarn deposits in the western conterminous United States typically are present in mountainous areas, but can be
present in a variety of settings; some are buried in fault blocks under Tertiary or Quaternary basin fill. Skarns also
may be present in roof pendants in plutons, as well as at contacts with plutons. These types of skarns are in
continental margin, syn- to late-orogenic tectonic settings such as those exposed in British Columbia, Peru, Japan,
and the western Cordillera of the United States and Mexico.
Hydrology
In some deposits, shear zones and faults are important structural controls for channeling hydrothermal fluids and
subsequent mineralization. In other deposits, lithologic contacts serve as fluid conduits. Post-mineralization faults
may also channel fluids. At Minera Bismark, a zinc-lead skarn deposit in northern Mexico, water flows continuously
along a major post-mineralization fault, as well as along other faults and fractures in granitic rock, necessitating a
pumping system to dewater the orebody (Mining Magazine, 1994). Ferricrete deposits represent iron remobilized
from weathered sulfide minerals by surface and subsurface water and indicate locations of former springs or drainage
seepages. Manganese or iron bog deposits may provide clues to paleohydrology in the vicinity of base and precious
metal skarn deposits. Faults at Copper Canyon, Nev., are known to have channeled ground water for several km
during periods of enhanced ground-water availability.
ENVIRONMENTAL SIGNATURES
Surface disturbance
Surface disturbance associated with mining these deposits is variable. Historic deposits were typically mined by hand
100
Table 4A. Summary of water analyses associated with historic mining of the McLaren sulfide mineral-rich
Cu-Au-Ag skarn deposit in the New World district, Mont. Surface water stations and ground water observa
tion wells.
[The McLaren open pit mine and mill operated from 1934 to 1953. Data compiled from reports cited in SCS Engineers,
1984; N, number of observations; flow, in cubic feet per second; SC, specific conductance in micromhos/cm; other
measurements in mg/l. Samples were collected and analyzed in 1973 to 1975 and in 1983. N.d., not determined]
pH 4.5 5.0
Cu 3.56 --
Zn 0.14 0.12
Fe 33.7 17.2
Mn 1.01 2.94
in underground workings. Multiple adits, pits, and tailings piles are commonly present at inactive and abandoned
workings because skarn deposits were often explored or worked at different sites, and at different times in the past,
along limestone-pluton contacts. Placer gold workings are downstream from many gold-bearing skarn deposits.
Some placer gold workings have affected areas of as much as 5 km2.
Drainage signatures
Analogies with mine drainage compositions for polymetallic replacement deposits characterized by pyrite
+chalcopyrite + sphalerite + galena veins or replacements in carbonate-rich host rocks (for example, Leadville,
Colo.), indicate that mine drainages are likely to exhibit near-neutral pH ranges, carry moderate concentrations of
dissolved metals (10 to 100 mg/l total Zn+Cu+Cd+Pb+Co+Ni), but may contain elevated (as much as 100,000 mg/l)
amounts of zinc. Dissolved aluminum and iron concentrations are both likely to about 100 mg/l, but can be much
less (Smith and others, 1994). Parts of deposits that are not in contact with carbonate rocks may cause associated
drainages to be more acidic and metal rich. See tables 4 and 5 for examples of compositions of surface and ground
101
Table 5. Chemical
analysis of a water
sample from the Cananea
Cu skarn deposit, 275 m
level, Sonora, Mexico;
all data in mg/l.
[Data from White and
others (1963, Table 24).
Water affected by oxida
tion of disseminated
sulfide minerals]
SiO2 56
Al 22
Fe2+ 524
Mn 153
Cu 60
Zn 252
Ca 753
Mg 86
Na + K 198
SO4 4,460
Cl 22
water associated with selected sulfide-mineral-rich copper skarn deposits. These data indicate total recoverable metals
and, therefore, provide no information concerning dissolved versus suspended metals or metal speciation. Seasonal
fluctuations affect flow and pH in some climates, especially where snow melt affects surface water during summer
months (fig. 4). Various parameters of surface and ground water from monitoring sites in the New World, Mont.,
copper-silver-gold district are plotted in figure 5. These sites are on, or immediately adjacent (within 100 m) to, an
87,000 km3 tailings pile removed from the mine site. These data (table 4A) for water samples collected in the 1970s
and early 1980s (SCS Engineers, 1984), reflect signatures that developed from mine tailings at a mill site for ore
from the McLaren mine, a skarn that was worked as an open pit mine from 1934 to 1953. At the time of the
analyses, a stream flowed directly over part of the tailings pile. The stream ran red and was impacted for about 16
km. The stream contained no fish and was characterized by reduced benthic organism populations mainly due to
high dissolved iron and relatively high aluminum concentrations. Rock geochemical data (table 3) from the New
World district include samples from in and around the historic McLaren mine. Water data from mine and mill site
drainages are cited in table 4B. Many surface and ground water samples have near-neutral pH values (6 to 8).
Specific conductance (a measure of dissolved solid content), iron, and to a lesser extent aluminum, all increase
dramatically with decreasing pH (fig. 5). Sulfate concentration increases with increasing specific conductance (fig.
5) in both surface and ground water. However, ground water has much higher sulfate concentrations and specific
conductances than surface water for the same pH ranges (note the difference in scale on fig. 5). These same data
are plotted on a modified Ficklin diagram to show base metal abundance ranges as a function of pH (fig. 6). Note
that the original Ficklin diagram (Ficklin and others, 1992) was devised to show the variations in aqueous (not total)
base metal concentrations for systematically sampled water draining diverse ore deposit types in Colorado and
included nickel and cobalt. Nevertheless, the New World water data mainly plot (fig. 6) in the near-neutral to acid,
low to high metal fields and are much less acidic than water that drains many other deposit types. As Ficklin and
others (1992) noted, the ability of carbonate-hosted deposits to generate metal-rich water appears to be a function
of sulfide mineral content. The unusually high pyrite content of skarn ore at New World, along with the unusual
situation of stream flow directly over tailings, led to some relatively high metal abundances in water on a local scale.
However, iron and aluminum, not base metals, were cited as the primary pollutants during a U.S. Environmental
Protection Agency (EPA) investigation. Many skarns in Nevada, however, are in semi-arid to arid climatic
environments where annual rates of evaporation exceed annual rainfall and commonly most stream flow is
intermittent.
A variety of samples, including stream sediment, heavy-mineral concentrates, soil, pond and spring mud,
and iron-rich stream and mine adit flocculent precipitates, were collected in September, 1993, as part of a
geochemical baseline study of the New World, Mont., district and adjacent areas (R.R. Carlson and G.K. Lee, unpub.
data, 1995). In addition, in-situ pH and conductivity readings of water in streams, ponds, springs, seeps, and mine
102
Figure 4. Seasonal variations in A, stream flow and B, pH for surface water from monitoring sites adjacent to a tailings pile at an historic mill
site along Soda Butte Creek in the New World, Mont., district. These data reflect impacts caused by a tailings pile failure rather than drainage
from a mine. The tailings were removed from the McLaren copper-gold-silver mine and placed at the mill site several km south of the mine.
Two data sets, which represent sampling in 1974 (a) and 1975 (b), are plotted for sites 3 and 5. Dashed lines on B separate pH ranges in terms
of generalized effects of pH on aquatic ecosystems (based on Potter and others, 1982). Note that the average pH value for precipitation in this
area is 5.3. Water data from SCS Engineers (1984).
adits and open pits were made. Figure 7 is a preliminary contour map of pH ranges generated from the unpublished
data and augmented by data from the National Uranium Resources Evaluation (Broxton, 1979). Areas characterized
by high acidity (pH <5) represent water in a variety of settings. pH values in the area at the southeast end of
Henderson Mtn. represent standing water in an inactive open-pit mine and low-volume seeps down hill from the pit.
pH values for the area in the headwaters of the Stillwater River represent water draining recontoured mine waste
rock. Most of the low pH values in the upper reaches of Fisher Creek represent discharge from abandoned adits;
the remainder of these low pH values represent spring water. In all cases, highly acidic water appears to be
ameliorated to near-neutral pH, probably as a consequence of buffering by water draining limestone and calc-alkaline
intrusive rocks of the New World area, within 1000 meters of source areas.
In natural water, free metal ions such as Cu2+ are the most bioavailable and most toxic aqueous species;
however, complexing with dissolved organic matter greatly reduces bioavailability.
103
Figure 5. Plots showing A, relationships between pH and iron abundances; B, pH and aluminum abundances; C, specific conductance and pH;
and D, between sulfate abundances and specific conductance for ground (squares) and surface (circles) water in the New World district, Mont.
Water data from SCS Engineers (1984). Ground water chemistry for Middle Cambrian Meagher Limestone (at Ennis, about 130 km north-
northwest of the New World district) reported by White and others (1963, table 6) is shown for reference (open square). The Meagher Limestone
is the most important host rock for skarn deposits in the New World district (Johnson, 1991).
is commonly present in skarn deposits, waste rock may include less reactive gangue minerals, including silicate
minerals, and tailings may have highly variable acid-buffering capacity. Andrews (1975) reports typical abundances
for a number of metals, including 6 to 38 ppm lead, 10 to 65 ppm zinc, and 1 to 35 ppm copper, in tailings
associated with a number of deposit types.
Metals released from weathered primary (hypogene sulfide ore) and secondary (supergene oxide and sulfide)
minerals in tailings at the Kelley zinc-lead skarn deposit in the Magdalena, New Mex., district are fixed by
precipitation of tertiary minerals and by ion exchange into phyllosilicate minerals on grain surfaces and along
104
Figure 6. Variations in base metal concentrations (Zn+Cu+Cd+Pb) versus ground and surface water pH, New World district, Mont. Water data
from SCS Engineers (1984). Classification scheme modified from Ficklin and others (1992).
fractures. Primary sulfide minerals persist as the main source of metals in sediments derived from tailings; these
sediments contain as much as 1.9 weight percent lead, 6.2 weight percent zinc, and 279 ppm cadmium (Larocque
and others, 1995).
105
by mining activities.
The contrast between arsenic abundances in soil at the McCoy (15 to 20 ppm) and West ( >160 ppm)
orebodies shows that similar deposits in similar geologic and climatic conditions can have distinctly different
geochemical signatures. Although these two deposits formed in different carbonate strata, in different tectonic blocks,
both are copper-gold-silver skarn deposits associated with 39 Ma granitoid intrusions in north-central Nevada and
both deposits are in arid-climate settings (Bailey and others, 1994) where precipitation averages 100 to 300 mm and
elevations range from 1,200 to 3,000 m.
Smelter signatures
Copper smelters release SO2 gas and represent the largest single source of arsenic emission from industrial processing
of non-ferrous metals (Loebenstein, 1994). Flue dusts from copper smelters commonly contain lead, zinc, arsenic,
and other metals. Copper concentrations in air near copper smelters may be elevated from normal abundances of
a few to 200 ng/m3 to abundances on the order of 5,000 ng/m3. Smelters (copper, zinc, and lead) in the United States
have been regulated for emissions and metal effluents since the mid 1970s.
Geoenvironmental geophysics
Multispectral remote imaging over sparsely vegetated areas can help identify oxidized mine tailings. Ongoing heat
evolution associated with sulfide mineral oxidation can be identified by airborne infrared surveys. Electromagnetic
or direct current resistivity surveys can be used to identify and track metal-contaminated ground water flowing from
tailings or mine shafts. Self potential, shallow seismic, and electrical surveys can be combined to provide
information on the thickness, structure, and possibly the presence of oxidation-reduction centers associated with
tailings and mine waste.
107
"Drainage signatures).
Ok Tedi, Papua New Guinea: A number of recent studies address behavior of metals discharged in particulate form
directly into the Fly River from mine tailings from the Ok Tedi porphyry copper and copper-gold skarn deposit
(Salomons and Eagle, 1990; Apte and others, 1995). Mine tailings enter an upper tributary of this large tropical river
because high rainfall and site instability preclude construction of adequate waste-retention structures. In water
draining this area, calcium and bicarbonate ion concentrations are elevated because of abundant carbonate rock in
the drainage area; bicarbonate ions buffer pH levels. Copper solubility is controlled by high concentrations of
dissolved organic carbon and high alkalinity of the system; copper largely remains in particulate form (Salomons and
Eagle, 1990). Copper-enriched sediment that is transported to an estuarine environment interacts with water having
a different chemistry (greater concentrations of magnesium, calcium, and sodium) that leads to cation exchange
reactions and release of copper in soluble forms.
PERSPECTIVE
The following commentary is generally applicable to all three skarn-related geoenvironmental models.
Extensive buffering capacity afforded by limestone suggests that skarn deposits should be less likely point sources
of acid mine drainage than many other deposit types. The efficacy of limestone as a natural mitigant for acid mine
drainage depends on its availability and on economic considerations. Some limestone is commonly present near most
skarn deposits, but some roof pendants in granitoids, originally composed of limestone, are completely converted to
skarn. In many skarn deposits, calcite and other minerals, such as epidote and chlorite, characteristic of propylitic
alteration are disseminated or vein gangue minerals in ore zones. These minerals also increase the ability of the host
rock to consume acid (Smith and others, 1994).
In their study of factors controlling acidity and metal concentrations in water draining mines developed in
various types of mineral deposits in Colorado, Plumlee and others (1993) showed that important factors include acid
uffering capacity of ore deposit gangue minerals and host rocks, types and abundances of sulfide minerals and their
exposure to weathering processes, and availability of dissolved oxygen. No skarn deposits were included in their
study; however, their results, with regard to variations in aqueous base-metal concentrations as a function of pH for
high-sulfide mineral, carbonate-hosted deposits and low-sulfide mineral carbonate-hosted deposits, represent the most
appropriate available analog for predicting mine drainage signatures for skarn deposits. The length of time that
sulfide-mineral-bearing deposits are exposed to oxidation may play a significant role in their eventual contribution
to local, undisturbed environments as well as environments disturbed by mining. Some originally sulfide-mineral-
earing skarns have been thoroughly oxidized as a result of having been exposed during the last 35 million years.
Many mineralized skarn deposits are point sources for metals within much more widely altered mineralized
systems, particularly porphyry systems. Some mineralized skarns may have potential for significant acid mine water
development. However, none of the 61 mining-related sites on the Environmental Protection Agency (1995) National
Priority List (Superfund) include skarn deposits. A number of skarn deposits, however, appear on lists of legal cases
108
that resulted from reported environmental damage (SCS Engineers, 1984). These include deposits where historic
mine tailings contribute to water quality degradation and, in some cases, affected aquatic life. The extent of
degradation from historic mining appears to be related to the magnitude and duration of operations as well as deposit
type and climate. Any area that was extensively mined for long periods of time is likely to have experienced some
environmental impact.
Skarn deposits that enter production today cannot be compared with skarn deposits worked a hundred, or
even twenty, years ago. Many currently active mining or exploration projects involving skarn deposits operated at
some time in the past at a smaller scale and are currently being re-evaluated for different metals (gold, silver) than
were sought in past enterprises. Many abandoned or inactive skarn deposits are the focus of recent exploration
because of economic changes or development of modern technology that allows previously uneconomic ore to be
mined. For example, skarns that were mined in the past for base metals or tungsten may now constitute exploration
targets for gold. Known or potential environmental hazards from historic mining can, in some cases, be mitigated
by reopening the site to mining activity that involves reprocessing historic tailings using modern mining practices.
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conference on heavy metals in the environment, Symposium proceedings, v. 2, Toronto, 1975, p. 645-675.
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Guinea: Journal of Geochemical Exploration 52, p. 67-79.
Agency for Toxic Substances and Disease Registry, 1989, Public Health Statements.
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Beus, A.A., and Grigorian, S.V., 1977, Geochemical exploration methods for mineral deposits: Wilmette, Illinois,
Applied Publishing Ltd., 287 p.
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Company, 140 p.
Brooks, J.W., Meinert, L.D., Kuyper, B.A., and Lane, M.L., 1991, Petrology and geochemistry of the McCoy gold
skarn, Lander County, Nevada, in Raines, G.L. and others, eds., Geology and ore deposits of the Great
Basin, Symposium Proceedings, Geological Society of Nevada, Reno, p. 419-442.
Broxton, D.E., 1979, Uranium hydrogeochemical and stream sediment reconnaissance data release for the Billings
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of Energy Open-File Report GJBX-150(79), 200 p.
Cox, D.P., 1986, Descriptive model of Zn-Pb skarn deposits, in Cox, D.P. and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 90.
_________1992, Descriptive model of distal disseminated Ag-Au deposits, in Bliss, J.D., ed., Developments in
mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 20-22.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Cox, D.P., and Theodore, T.G., 1986, Descriptive model of Cu skarn deposits, in Cox, D.P. and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86.
Doebrich, J.L., Wotruba, P.R., Theodore, T.G., McGibbon, D.H., and Felder, R.P., 1995, Field trip guidebook: Trip
H- Geology and ore deposits of the Battle Mountain Mining District, Humboldt and Lander Counties,
Nevada: Reno, Nevada, Geological Society of Nevada and U.S. Geological Survey, Geology and ore deposits
of the American Cordillera, 92 p.
Einaudi, M.T., and Burt, D.M., 1982, Introduction: terminology, classification, and composition of skarn deposits:
Economic Geology v. 77, p. 745-754.
Einaudi, M.T., Meinert, L.D., and Newberry, R.J., 1981, Skarn deposits: Economic Geology 75th Anniversary
Volume, p. 317-391.
Emmons, D.L. and Eng, T.L., 1995, Geologic map of the McCoy Mining District, Lander County, Nevada: Nevada
Bureau of Mines and Geology Map 103, scale 1:12,000, 12 p.
Environmental Protection Agency, 1995, National priorities list for uncontrolled hazardous waste sites (40 CFR Part
300): Federal Register, v. 60, no. 79, April 25, 1995, p. 20330-20353.
Ettlinger, A.D., and Ray, G.E., 1989, Precious metal enriched skarns in British Columbia: An overview and
geological study: British Columbia Ministry of Energy, Mines, and Petroleum Resources, Mineral Resources
Division, Paper 1989-3, 128 p.
109
Ficklin, W.H., Plumlee, G.S., Smith, K.S., and McHugh, J.B., 1992, Geochemical classification of mine drainages
and natural drainages in mineralized areas, in Kharaka, Y.K., and Maest, A.S., eds., Water-rock interaction:
Seventh International Symposium on Water-Rock Interaction, Park City, Utah, July 13-18, 1992,
Proceedings, v. 1, Rotterdam, A.A. Balkema, p. 381-384.
Francis, B.M., 1994, Toxic substances in the environment: New York, John Wiley and Sons, Inc., 360 p.
Glass, N.R., Arnold, D.E., Galloway, J.N., Henry, G.R., Lee, J.J., McFee, N.W., Norton, S.A., Powers, C.F., Rambo,
D.L., and Schofield, C.L., 1982, Effects of acid precipitation: Environmental Science and Technology, v.
15, p. 162A-169A.
Hastings, J.S., and Harrold, J.L., 1988, Geology of the Beal gold deposits, German Gulch, Montana, in Schafer,
R.W. and others, eds., Bulk minable precious metal deposits of the western United States: Geological Society
of Nevada, Symposium Proceedings, April 6-8, 1987, Reno, NV, p. 207-220.
Johnson, T.W., 1991, Geology, hydrothermal alteration, and Cu-Ag-Au skarn and replacement mineralization in the
northern part of the New World district, Park County, Montana: Pullman, Washington State University, M.S.
thesis, 326 p.
Jones, G.M., and Menzie, W.D., 1986, Grade and tonnage model of Cu skarn deposits, in Cox, D.P. and Singer,
D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.
Larocque, A.C.L., Laughlin, A.W., Hickmont, D., and Chapin, C.E., 1995, Metal-residence sites in weathered
sulfide-rich tailings and sediments, Kelley mining camp, Magdalena District, New Mexico: Geological
Society of America Abstracts with Programs, v. 20, no. 6, p. A-192.
Loebenstein, J.R., 1994, The materials flow of arsenic in the United States: U.S. Bureau of Mines Information
Circular 9382.
Meinert, L.D., 1983, Variability of skarn deposits-guides to exploration, in Boardman, S.J., ed., Revolution in the
earth sciences: Dubuque, Iowa, Kendall-Hunt Publishing Co, p. 301-316.
_________1989, Gold skarn deposits-geology and exploration criteria, in Groves, D., Keays, R., and Ramsay, R.,
eds., Proceedings of Gold '88: Economic Geology Monograph No. 6, p. 537-552.
Meyer, G.A., 1995, Tailings impoundment failure and floodplain sediment contamination along Soda Butte Creek,
Yellowstone National Park, MT-WY: Geological Society of America Abstracts with Programs, v. 27, no.
4, p. 47.
Mining Magazine, 1994, Minera Bismark, p. 195-201.
Mosier, D.L., 1986, Grade and tonnage model of Zn-Pb skarn deposits, in Cox, D.P. and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 90-93.
Norman, D.K., and Raforth, R.L., 1995, Cyanide heap leaching-the process, environmental problems, and regulation
in Washington: Washington Geology, v. 23, no. 1, p. 30-41.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado: implications for prediction of mine-drainage chemistry: Proceedings, 1993 Mined
Land Reclamation Symposium, Billings, Montana, v. 1, p. 176-186.
Potter, W., Chang Ben, K-Y, and Adler, D., 1982, The effects of air pollution and acid rain on fish, wildlife, and
their habitats: Rivers and streams, U.S. Department of Interior, Fish and Wildlife Service, FWS 14-16-0009-
80-085.
Ripley, E.A., Redman, R.E., and Crauder, A.A., 1995, Environmental effects of mining: Delray Beach, Florida, St.
Lucie Press, 356 p.
Salomons, W., 1995, Environmental impact of metals derived from mining activities: Processes, prediction, and
prevention: Journal of Geochemical Exploration, v. 52, p. 4-23.
Salomons, W., and Eagle, A.M., 1990, Hydrology, sedimentology and the fate and distribution of copper in mine
related discharges in the Fly River system-Papua New Guinea: Science of the Total Environment, v. 97/98,
p. 315-334.
Sangmeshwar, S.R., and Barnes, H.L., 1983, Supergene processes in zinc-lead-silver sulfide ores in carbonates:
Economic Geology, v. 78, p. 1379-1397.
SCS Engineers, 1984, Summary of damage cases from the disposal of mining waste: Unpublished report prepared
for the U.S. Environmental Protection Agency.
Smith, K.S., Plumlee, G.S., and Ficklin, W.H., 1994, Predicting water contamination from metal mines and mining
wastes: Notes, Workshop #2, International Land Reclamation and Mine Drainage Conference and Third
International Conference on the Abatement of Acidic Drainage: U.S. Geological Survey Open-File Report
94-264, 112 p.
110
Stoughton, Julie A., 1995, The spatial distribution of grasses, soils, and trace elements along the floodplains of Soda
Butte Creek, Montana and Wyoming: Geological Society of America Abstracts with Programs, v. 27, no.
4, p. 57.
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pit mine, Lander County, Nevada: U.S. Geological Survey Open-File Report, 21 sheets.
Theodore, T.G., and Blake, D.W., 1975, Geology and geochemistry of the Copper Canyon porphyry copper deposit
and surrounding area, Lander County, Nevada: U.S. Geological Survey Professional Paper 798-B, 86 p.
_________1978, Geology and geochemistry of the West ore body and associated skarns, Copper Canyon porphyry
copper deposits, Lander County, Nevada: U.S. Geological Survey Professional Paper 798-C, p. C1-C85.
Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 1991, Gold-bearing skarns: U.S. Geological Survey
Bulletin 1930, 61 p.
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Dome (Mungana) gold skarn deposit, North Queensland, Australia, in Macdonald, A.J., ed., Proceedings of
Gold '86, International Symposium on the Geology of Gold: Toronto, Geological Association of Canada,
1986, p. 3-22.
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Montana: Golden, Colorado School of Mines, M.S. thesis, 158 p.
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subsurface waters: U.S. Geological Survey Professional Paper 440-F, 67 p.
111
FE SKARN DEPOSITS
(MODEL 18d; Cox, 1986)
Deposit geology
These deposits principally consist of magnetite in calc-silicate contact metasomatic rocks. Calcic magnetite skarns
form in island-arc settings associated with relatively mafic (diorite) intrusions. Magnesian magnetite skarns form
in orogenic belts along continental margins and are associated with felsic plutons in dolomitic host rocks as
transitional zones, or in districts that contain copper-rich or tungsten skarn deposits. Most scapolite-(albite) iron
skarn deposits are localized in basinal volcanic-sedimentary strata and are genetically related to late-stage phases of
gabbroic and dioritic magmas.
Examples
Calcic iron skarn: Daquiri, Cuba; Empire, Vancouver Island, Canada; Shinyama, Japan.
Magnesian iron skarn: Fierro-Hanover, N. Mex.; Cornwall, Pa.; Eagle Mountain, Calif.
Scapolite-(albite) iron skarn: Kachar and Sarbay, U.S.S.R.; West Humboldt, Nev.
Worldwide, some of the largest iron skarn deposits are of the scapolite-(albite) variety (Zitzmann, 1977;
Sokolov and Grigorev, 1977; Einaudi and others, 1981). In the classification scheme of Einaudi and others (1981),
these deposits are categorized as the calcic-iron part of the twofold calcium-magnesium scheme. However, because
of notable differences between scapolite-(albite) iron skarns and calcic and magnesian iron skarns we herein describe
the scapolite-(albite) iron skarns separately, somewhat along the lines adopted by Zitzmann (1977). These differences
include (1) the presence of widespread albite-(scapolite) assemblages in addition to garnet-pyroxene, (2) common
development of endoskarn in contrast to calcic exoskarn, (3) extremely large tonnages, and (4) development in
basinal volcanogenic strata as opposed to platform sequences consisting of thick miogeoclinal micrites.
Exploration geophysics
Strong magnetic anomalies may be associated with these deposits; these anomalies become attenuated with increasing
depth of burial or if magnetite has been converted to hematite. Airborne magnetic surveys have been used to identify
large scapolite-(albite) iron skarn deposits in the U.S.S.R. In extremely weathered terranes, exposures of limonite,
iron hydroxide, iron sulfate gossan, clay, or carbonate rock associated with these deposits may be identified by
Landsat Thematic Mapper reflectance imagery. Very low frequency electromagnetic and induced polarization surveys
may be used to identify magnetite and associated disseminated sulfide bodies. Resistivity surveys can be used to
112
model the distribution of alteration zones as indicated by ion content of water saturated rocks. Because of density
contrasts between alteration mineral assemblages and deposit host rocks, microgravity surveys may be used to
delineate the distribution of skarn deposits.
References
Einaudi and others (1981), Zitzmann (1977), and Cox (1986).
Host rocks
Calcic iron skarn: Host rocks include limestone, sandstone, volcanic rocks, graywacke, dolomite at contacts of dioritic
intrusions, within diorite, or at contacts of carbonate-bearing rocks with diabase flows.
Magnesian iron skarn: Host rocks are typically dolomite, or limestone, quartzite, and schist associated with dolomitic
rocks.
Scapolite-(albite) iron skarn: Host rocks are andesite, tuff, limestone, argillite; endoskarn forms in gabbroic to dioritic
intrusions.
Wall-rock alteration
Wall-rock alteration associated with calcic iron skarns involves extensive endoskarn development, which is
characterized by albite, orthoclase, epidote, quartz, and scapolite, in plutons and volcanic rocks. Endoskarn associated
with magnesian iron skarns is minor; propylitic alteration assemblages are characteristic.
Nature of ore
Ore consists of massive magnetite layers or lenses; orebodies are spatially associated with garnet zones or form in
limestone beyond calc-silicate skarn zones. Calcic magnetite skarns and scapolite-(albite) iron skarns may be present
within diorite stocks as replacements of gabbro and diorite or of limestone xenoliths (Einaudi and others, 1981).
Stockwork veins may also be present in mafic volcanic rocks overlying altered gabbro. Orebodies may be lenticular,
lensoid, tabular, contact-controlled deposits extending hundreds of meters to 4 km or more along strike and having
thicknesses of tens to hundreds of meters, as well as irregular and pipelike, replacement of mafic intrusions and
stockworks. Ore may be nearly monomineralic veins, pods, layers, or lenses of magnetite or may consist of
magnetite-rich laminae or layers alternating with, or intergrown with gangue minerals. Ore mineral associations in
113
Table 1. Geochemical data for rock samples from
the Hancock magnetite skarn, McCoy district,
Lander County, Nev.
[Analyses by inductively coupled plasma and atomic ab-
sorption spectrophotometry; D. Fey, analyst. Oxides, in
weight percent; trace elements, in ppm (Au, in ppb). N.d.,
not determined; FeTO3, total iron reported as FeO
2 3]
Sample 1 2 3 4 5
114
Table 2. Geochemical data for
rock samples from magnesian
iron skarns, western Montana.
[Analyses by inductively coupled plas-
ma and atomic absorption spectropho-
tometry; B. Adrian, analyst. Oxides in
weight percent; trace elements in ppm.
FeTO3, total iron reported as FeO
2 3].
Sample 1 2 3
a 39-38 Ma granodiorite stock, currently being mined in the McCoy district. Minor amounts of pyrite and
chalcopyrite are present in the magnetite skarn. Table 2 presents three previously unpublished analyses of magnetite
115
Table 3. Summary statistics for iron
skarn analyses.
[Data from tables 1, 2, and Hammarstrom and
Gray (1993, tables 2 and 3); N, number of
analyses used to calculate means; qualified
values were omitted from calculations. Ox-
ides (in weight percent); trace elements in
ppm (Au, in ppb)]
As 2 <10 50 40
Au 9 <2 750 181
Ba 19 2 2,700 316
Be 2 <1 3 3
Cd 4 <2 6 5
Ce 7 <4 76 44
Co 19 6 120 44
Cr 14 <1 210 32
Cu 18 <2 2,100 432
Ga 16 6 28 15
La 7 <2 51 32
Li 15 <2 29 10
Mn 19 375 6,199 1,466
Mo 7 <2 22 10
Nb 3 <4 24 13
Nd 7 <4 46 21
Ni 15 <2 170 35
Pb 9 <4 40 17
Sc 10 <2 33 11
Sn 2 <10 40 40
Sr 16 <2 750 170
Th 7 <4 20 8
V 19 10 610 139
W 3 2 5 3
Y 5 <2 38 14
Yb 3 <1 4 3
Zn 19 8 310 94
skarn ore from two different deposits in western Montana--the Pomeroy Mine, a magnesian magnetite skarn along
a contact between Upper Cambrian Pilgrim Dolomite and the Cretaceous Uphill Creek Granodiorite pluton of the
Pioneer batholith. This pluton is associated with tungsten and copper-silver-gold skarns, including deposits of the
Rock Creek district (see also, SN and (or) W skarn and replacement deposits model, Hammarstrom and others, this
volume), that formed along contacts with limestone (Geach, 1972; Pearson and others, 1988). Ranges of iron skarn
compositions from tables 1 and 2, for magnetite skarns in roof pendants of Mississippian Madison Limestone in the
Sliderock Mountain area, Sweet Grass County, Mont., and data for iron-copper skarns in Cambrian Meagher
Limestone along a granodiorite sill in the Independence mining district, Park County, Mont. (Hammarstrom and
Gray, 1993), are summarized in table 3. These data represent nineteen samples from six different deposits, including
magnesian as well as calcic skarn. No systematic differences in trace element content are apparent in the data set
for dolomite- versus limestone-hosted skarn, so the data sets were combined. Table 4 summarizes previously
unpublished data for the West Humboldt, Nev., scapolite-(albite) iron skarn deposits.
116
Table 4. Summary statistics for scapolite-(al-
bite) iron skarn analyses for West Humboldt, Nev.
[Data from G.B. Sidder, M.L. Zientek, and R.A. Zieren-
berg (written commun., 1995). Analyses by DC arc emis-
sion spectrography except for As, Bi, Cd, Sb, and Zn by
inductively coupled plasma atomic emission spectrometry]
117
Mineral characteristics
Magnetite typically forms massive layers or lenses. Sulfide mineral contents of magnetite skarns are generally low,
but can be quite variable. Reported magnetite-pyrite ratios range from 100:1 to 3:1 for the Larap deposit in the
Philippines; Larap is somewhat atypical of iron skarns because it contains economic concentrations of Cu, Mo, Au,
Ag, Co, and Ni as well as Fe (Einaudi and others, 1981). At the Cornwall, Pa., magnetite mines, sulfide minerals
were separated from magnetite; copper, gold, and silver were recovered from chalcopyrite and cobalt was recovered
from pyrite.
Secondary mineralogy
Magnetite may alter to hematite, maghemite, goethite, limonite, or lepidocrocite during oxidation and weathering.
Topography, physiography
Iron skarns have little or no surface expression; however, in most cases, this depends on their resistance to weathering
relative to adjacent rocks and on climatic setting.
Hydrology
These deposits have no known control on the local hydrologic regime.
ENVIRONMENTAL SIGNATURES
Drainage signatures
No data are available for iron skarn deposits. However, chemical analyses of water from iron ore mines and milling
operations in Canada (Ripley and others, 1995) are probably reasonable indicators of mine water discharge
characteristics applicable to iron skarn deposits because Canadian iron ore has a low sulfide-mineral content and is
dominated by magnetite, hematite, and siderite. This mine water and direct-discharge tailings effluent have pH of
6.4 to 9 and relatively low dissolved metal contents, including 0.01 to 0.4 mg/l copper, 0.05 to 1.3 mg/l iron, <1 mg/l
lead, and 0.01 to 0.15 mg/l zinc. Much higher dissolved metal concentrations, including >40 mg/l copper and >6,000
mg/l iron, were reported for marine-discharge tailings effluent. Suspended solids may contribute to stream turbidity.
Although iron is an essential nutrient for plants and animals, extreme iron concentrations downstream from historic,
large-scale iron ore operations can be harmful to salmon and benthic organisms. However, residual materials from
iron ore mining are generally considered to have minor toxicity.
118
Potential environmental concerns associated with mineral processing
The relatively low sulfide mineral content of iron skarn deposits suggests that these deposits have relatively low acid-
generating potential. Carbonate mineral-bearing strata that frequently host these deposits can buffer most of the acid
generated by these deposits. Elevated abundances of metals that pose the most significant threats to the environment
are not associated with most magnetite skarn deposits.
Smelter signatures
Iron ore is converted to iron by reduction to pig iron in blast furnaces or by direct reduction. The major impurities
in iron ore result in relatively inert acidic (silica, alumina) or basic (lime, magnesia) slag during smelting.
Geoenvironmental geophysics
Low spectral resolution reflectance imagery can be used to delineate the distribution of limonite, iron hydroxide, and
iron sulfate gossan mineral assemblages in soil and water associated with iron skarn deposits. Similarly, magnetic
surveys can be used to define the distribution of magnetite and pyrrhotite. Sulfide- mineral-rich bodies, whose
oxidation may generate acid, metal-charged water, may be located using electromagnetic, induced polarization, and
self potential surveys. Zones of leached rock altered to clay, mica, epidote, chlorite, and pyroxene may be delineated
by resistivity surveys and using Landsat thematic mapper imagery and imaging spectrometer data. Remote sensing
may be used to identify acid buffering carbonate rocks and stressed vegetation indicative of contaminated soil.
PERSPECTIVE
See section entitled "Perspective" in the CU, AU, ZN-PB skarn deposits model (Hammarstrom and others, this
volume).
REFERENCES CITED
Andrews, R.D., 1975, Tailings-Environmental consequences and a review of control strategies, in International
conference on heavy metals in the environment, Symposium proceedings, v. II, Toronto, 1975, p. 645-675.
Cox, D.P., 1986, Descriptive model of Fe skarn deposits, in Cox, D.P. and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 94.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Einaudi, M.T., Meinert, L.D., and Newberry, R.J., 1981, Skarn deposits: Economic Geology 75th Anniversary
Volume, p. 317-391.
Geach, R.D., 1972, Mines and mineral deposits, Beaverhead County, Montana: Montana Bureau of Mines and
Geology Bulletin 85, 194 p.
Hammarstrom, J.M., and Gray, K.J., 1993, Geochemical data for selected rock samples from the Absaroka-Beartooth
study area, Custer and Gallatin National Forests, Montana: U.S. Geological Survey Open-File Report 93-505,
31 p.
Lapham, D.M., 1968, Triassic magnetite and diabase at Cornwall, Pennsylvania, in Ore deposits of the United States
1937-1967, The Graton-Sales volume, York, Pennsylvania, The Maple Press Co., v. I, p. 73-94.
Loeppert, R.H., 1988, Chemistry of iron in calcareous systems, in Stuckli, J.W., Goodman, B.A., and Schwertmann,
U., eds., Iron in soils and clay minerals: Boston, D. Reidel Publishing Company, Chapter 19, p. 689-713.
Lowe, N.T., Raney, R.G., and Norberg, J.R., 1985, Principal deposits of strategic and critical minerals in Nevada:
U.S. Bureau of Mines Information Circular 9035, 202 p.
Mosier, D.L. and Menzie, W.D., 1986, Grade and tonnage model of Fe skarn deposits, in Cox, D.P. and Singer,
D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 94 -97.
Pearson, R.C., Berger, B.R., Kaufmann, H.E., Hanna, W.F., and Zen, E-an, 1988, Mineral resources of the eastern
Pioneer Mountains, Beaverhead County, Montana: U.S. Geological Survey Bulletin 1766, 34 p.
119
Ray, G.E., and Webster, I.C.L., 1990, An overview of skarn deposits: Ore deposits, tectonics, and metallogeny in
the Canadian Cordillera, Geological Society of Canada short course notes, p. 7-1-7-55.
Reeves, R.G., and Kral, V.E., 1955, Geology and iron ore deposits of the Buena Vista Hills, Churchill and Pershing
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Lucie Press, 356 p.
Schwertmann, U., 1988, Occurrence and formation of iron oxides in various pedoenvironments, in Stuckli, J.W.,
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London, Pittman, v. 1, p. 7-113.
Soler, A., 1991, Role of magnetite spheres in skarn exploration, in Skarns- their genesis and metallogeny: Athens,
Theophrastus Publications S.A., p. 641-648.
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Bulletin 88, Part II, Mineral deposits, 106 p.
Zitzmann, Arnold, 1977, The iron ore deposits of the western U.S.S.R., in Zitzmann, A., ed., The iron ore deposits
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391.
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POLYMETALLIC VEIN AND REPLACEMENT DEPOSITS
(MODELS 19a and 22c; Morris, 1986; Cox, 1986)
by Geoffrey S. Plumlee, Maria Montour, Cliff D. Taylor, Alan R. Wallace, and Douglas P. Klein
Deposit geology
Deposits consist of massive lenses and (or) pipes, known as mantos or replacement orebodies, and veins of iron, lead,
zinc, and copper sulfide minerals that are hosted by and replace limestone, dolomite, or other sedimentary rocks;
most massive ore contains more than 50 percent sulfide minerals. Sediment-hosted ore commonly is intimately
associated with igneous intrusions in the sedimentary rocks. Emplacement of these intrusions triggered ore formation
and they host polymetallic veins and disseminations that contain iron, lead, zinc, and copper sulfide minerals. Some
polymetallic replacement deposits are associated with skarn deposits in which host carbonate rocks are replaced by
calc-silicate±iron oxide mineral assemblages. Most polymetallic vein and replacement deposits are zoned such that
copper-gold ore is proximal to intrusions, whereas lead-zinc-silver ore is laterally and vertically distal to intrusions.
Examples
Leadville, Gilman, and Breckenridge districts, Bandora Mine, Colo.; Park City, Utah, district; Eureka, Nev., district;
Related deposit types (Cox and Singer, 1986) include Climax molybdenum (Model 16); porphyry molybdenum, low
fluorine (Model 21b); porphyry copper (Model 17); porphyry copper-gold (Model 20c); porphyry copper
(1) Many polymetallic vein and replacement deposits are hosted in carbonate-rich sedimentary rocks that consume
acid and inhibit metal transport. However, mine water that drains deposits not hosted by carbonate-bearing rocks
tends to be acidic to extremely acidic and contain elevated abundances of iron, aluminum, zinc, and copper and
(2) Water draining pyrite-rich, tailings and waste dumps can be acidic to highly acidic, and can contain high to
extreme abundances of iron and aluminum, and very high abundances of zinc and copper.
(3) Karst, where present, can impose significant control on the local hydrologic regime because of its ability to
(4) Slag produced by smelting may contain elevated abundances of lead, zinc, and copper, and lesser amounts of
other metals; the mobility of metals from slag varies as a function of how slag cooled.
(5) Soil downwind from smelters can contain elevated abundances of Pb, Zn, Cu, As, Sb, Mo, Hg, and Au.
Mitigation and remediation strategies for potential environmental concerns presented above are described
in the section below entitled "Guidelines for mitigation and remediation."
Exploration geophysics
Most massive sulfide replacement bodies have high density, low resistivity, are electrically chargeable when excited
by induced polarization, have high magnetic susceptibility when magnetite or pyrrhotite are present, and may generate
negative self-potential voltages (Ward, 1966; Buselli, 1980; Frischknecht and others, 1991; Fallon and Busuttil, 1992;
Thomas and others, 1992). Consequently, these deposits often can be identified by detailed electrical, gravity,
magnetic, and self-potential surveys. Specifics of particular geophysical responses are controlled by sulfide
mineralogy, concentration, and continuity; hydrologic conditions; and by host and country rock characteristics and
geometry. Airborne or ground-based electromagnetic, direct current resistivity, and induced polarization surveys can
identify low-resistivity and chargeable sulfide masses, high-resistivity silicified-carbonitized zones, and pyritic
alteration assemblages (Ward, 1966; Zonge and Hughes, 1991; Shalley and Harvey, 1992; Thomas and others, 1992).
Anomalies initially identified by airborne electromagnetic and magnetic surveys can be further studied by ground-
based geophysical surveys coupled with geologic and geochemical investigations. Targets for detailed exploration
can also be selected using remote sensing data. Large (several hundred meters long) gossans can be identified on
satellite-derived Thematic Mapper remote sensing images; smaller gossans may be detected using airborne imagery,
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including aerial photography (Watson and Knepper, 1994), or AVIRIS imagery. Regional magnetic and gravity
surveys may identify anomalies related to fault zones or intrusions that may control ore distribution or indicate
prospective terrane (Fallon and Busuttil, 1992; Shalley and Harvey, 1992).
References
Geology: Lovering and others (1978), Beaty and others (1990), Titley (1993).
Environmental: Chaffee (1980; 1987), Plumlee and others (1993), Montour (1994), Smith and others (1994).
Deposit size
Deposits are small (10,000 tonnes) to very large (as much as 30-40 million tonnes).
Host rocks
These deposits are principally hosted by sedimentary rocks (dolomite, limestone, sandstone, and shale) that have been
intruded by intermediate- to felsic-composition igneous stocks, dikes, and sills (figs. 1 and 2).
These deposits are in sedimentary rock sequences that host local igneous intrusions.
Wall-rock alteration
Carbonate-hosted replacement ore: Host carbonate rocks are commonly recrystallized, dolomitized, bleached, and (or)
sanded (the process whereby carbonate cement is removed from sedimentary rocks). Alteration of carbonate rocks
to silica-rich jasperoid is common locally in some districts. In some districts, carbonate minerals have been replaced
by calc-silicate skarn minerals (epidote, amphibole, garnet, pyroxene, and iron oxide minerals) near their contacts
Igneous-hosted ore: In the central, hottest parts of mineralized systems, igneous host rocks are altered to quartz
sericite-pyrite and quartz-clay (argillic) assemblages that grade into distal propylitic (epidote, chlorite, pyrite,
Nature of ore
Ore is present in massive lenses (mantos), pipes (chimneys), and veins of iron, lead, zinc, and copper sulfide minerals
that are hosted by and replace limestone, dolomite, or other sedimentary rocks; most massive ore contains >50
percent sulfide minerals. A given district or mine may contain a single, massive orebody or a series of orebodies
aligned along structural features such as fractures, joints, fold limbs, stratigraphic features such as karst openings,
or lithologic discontinuities (such as shale pinchouts) that control fluid movement (fig. 1). Manto and chimney ore
is compact and can be relatively impermeable. Some ore replaces carbonate clasts in karst breccias, and may fill
Polymetallic vein and replacement deposits are characterized by elevated abundances of Pb-Zn±Cu±Au±Ag±Mo±As±
Bi±Sb. In some districts, ore proximal to igneous intrusions is copper and gold rich, and grades laterally (and
sometimes vertically) into lead-zinc-silver-rich ore. A distal, manganese-enriched zone is present in some districts.
Minerals listed in approximate decreasing order of abundance. Potentially acid-generating minerals are underlined;
those that are acid-generating when oxidized by aqueous ferric iron are denoted by *.
Carbonate replacement ore: Pyrite, sphalerite*, galena*, siderite, quartz, marcasite, rhodochrosite, dolomite,
chalcopyrite, pyrrhotite, tetrahedrite, digenite*, argentite, electrum, ± enargite, ± bornite, ± arsenopyrite, ±Bi-Te-Hg-
Au-Ag minerals (hessite, petzite, pyrargyrite, etc.), ± barite, ± fluorite. Most deposits are pyrite-rich; however, a
few (rare?) deposits are pyrite-poor and are dominated by sphalerite and galena.
Igneous-hosted vein ore: Quartz, pyrite, chalcopyrite, sphalerite*, galena*, sericite, acanthite, gold/electrum, ±
Zoning: In many deposits, ore grades from copper sulfide mineral-rich (chalcopyrite, enargite, bornite) within and
near igneous intrusions, to sphalerite- and galena-rich away from intrusions, to sphalerite- and manganese carbonate-
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Figure 1. Schematic vertical section through a polymetallic replacement deposit showing distribution of ore types and host rocks.
Figure 2. Ficklin plot showing pH versus the sum of dissolved base metals Zn, Cu, Cd, Co, Ni, and Pb in mine and natural water draining
polymetallic replacement ore types.
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bearing distal ore. Most skarn ore is near the intrusion-sedimentary rock contact.
Mineral characteristics
Textures: Mineral grains are typically medium to coarse grained (0.5 to >1 cm), and range in texture from euhedral
Trace element contents: In many deposits, sphalerite typically has very high iron contents (5-20 mol percent) and
cadmium (several tenths to 1 mol percent) and minor amounts of other trace elements such as silver. Galena can
General rates of weathering: Euhedral to massive, interlocking sul fides weather at very slow rates. Samples exposed
on mine dumps for >100 years are fresh and unweathered. Marcasite may weather at faster rates than pyrite,
sphalerite, and galena. Fine-grained sulfide minerals weather more rapidly than those that are coarse-grained.
Secondary mineralogy
Minerals formed by weathering prior to mining: Carbonate ore— These minerals, including anglesite, cerussite,
smithsonite, hemimorphite, hydrozincite, hemimorphite, manganese oxide minerals (psilomelane, pyrolusite, braunite),
iron oxide minerals (limonite), cerargyrite, and jarosite, tend to be relatively insoluble and indicative of deposition
from relatively high-pH water. Igneous ore—native gold, cerargyrite, and iron oxide minerals.
Minerals formed by recent weathering after the onset of mining: These minerals are primarily soluble sulfate minerals
indicative of deposition from locally highly acidic water. Zinc sulfate minerals include goslarite, which grades to
magnesium-rich epsomite. Iron sulfate minerals include copiapite, coquimbite, melanterite, szomolnokite , fibroferrite
and roemerite. Chalcanthite is the dominant copper sulfate mineral although others may also be present.
Topography, physiography
Silica-rich orebodies in sedimentary and igneous rocks may form topographic highs.
Hydrology
Natural ground water flow in the vicinity of these deposits is dominantly along fractures and faults or through karst
systems in carbonate rocks; mine workings enhance ground-water permeability. Karst, where present, can impose
significant control on the local hydrologic regime because of its a bility to channelize ground water for long distances
from mine sites. Some flow may also occur in sedimentary rock aquifers, including sandstone and fractured
carbonate rocks. Pre-mining discharge points may in some ca ses be determined by mapping surficial iron-hydroxide
(ferricrete) deposits.
Historic: Processing typically involved milling to produce zinc-, lead-, and (or) copper-rich concentrates that were
subsequently smelted.
Modern: Processing typically involves milling; resulting concentrates are smelted. Stopes are backfilled by coarse
tailings and fine t ailings are stored in surface impoundments. Some copper-rich ore is currently being processed by
ENVIRONMENTAL SIGNATURES
Drainage signatures
Mine-drainage data (figs. 2 and 3): Data for Leadville, Kokomo, Breckenridge, and Bandora, Colo., from Plumlee
and others (1993) and Smith and others (1994); data for New World, Mont., from Montana Department of State
Lands (1994); data for Eagle, Colo., mine from Engineering Science, Inc. (1985).
(1) Mine water draining orebodies in carbonate-rich sedimentary rocks tends to be slightly acidic to near-neutral, pH
5.5 to 7.5; water draining pyrite-rich orebodies tends to contain tens to low hundreds of mg/l dissolved zinc. Water
that drains copper-rich proximal ore can have several mg/l dissolved copper. Water with low dissolved oxygen
content, such as tha t draining collapsed adits, can contain several tens of mg/l dissolved iron; as this water becomes
oxygenated, oxidatio n and precipitation of iron can lead to significant pH decreases away from the drainage source.
(2) Water draining igneous-hosted ore, ore i n carbonate-poor sedimentary rock, and massive ore (in which water has
limited interaction with host carbonate rocks) tends to be acidic (pH 3.5 to 5), contain several tens of mg/l iron and
aluminum, tens to low hundreds of mg/l zinc, several to several tens of mg/l copper, and several hundred µg/l lead,
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Figure 3. Dissolved concentrations of sulfate and major metals and various trace elements in mine water draining polymetallic replacement ore types. Data from Plumlee and others (1993), Smith
and others (1994), Engineering Science, Inc. (1985), and Montana Department of State Lands (1994).
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cadmium, and arsenic.
(3) Water draining pyrite-rich, carbonate-poor tailings and waste dumps tends to be acidic to highly acidic (pH 2 to
5), contains hundreds to several thousand s of mg/l iron and aluminum, and tens to several hundreds of mg/l zinc and
copper.
Natural-drainage data: Not available. However, anecdotal accounts indica te that cerussite, a lead carbonate mineral,
was abundant in stream sediments downstream from Leadville, Colo.; the presence of cerussite indicates that
associated stream water was not acidic. Cerussite was probably transported physically as eroded fragments of
mineralized rock; however, the possibility that some lead was transported as an aqueous phase, and subsequently
precipitated in stream sediments, cannot be ruled out entirely. Mine-water data suggest that pre-mining pH values
at Leadville were near neutral (pH 6 to 7.5); this water may have contained as much as several mg/l zinc and several
Potentially economically recoverable elements: Zinc extraction may become economically viable if aqueous extraction
Metal mobility is greatest from mine dumps having the highest pyrite and lowest carbonate mineral contents. Metals
and acid are readily liberated from pyrite-rich mine wastes and intermittently wet/dry mine workings due to the rapid
dissolution of soluble secondary salts. In above-water-table settings, soluble salts form coatings on mine wastes,
fracture fillings, and coatings on mine workings. The rate of salt dissolution (and resulting acid and metal
generation) is much more rapid than acid consumption by carbonate minerals in the dumps or rocks surrounding the
mine workings. Dumps with high carbonate mineral contents and low pyrite contents produce limited amounts of
secondary salts; associated runoff has near neutral pH and low dissolved metal contents. In contrast, pyrite-rich,
carbonate-poor dumps and mi ll tailings contain abundant secondary salts and down-gradient vegetation is adversely
Storm water samples: No data available. However, extensive vegetation kill zones downhill from pyrite-rich mine
dumps indicate that highly acidic, metalliferous water can be generated in spite of the presence of carbonate-mineral-
Results of water-rock leaching experiments: Dilute sulfuric acid leaches of mine waste from Leadville, Colo.,
(Montour, 1994) using U.S. Environmental Protection Agency Method 1312 (20:1 water-rock ratio) yielded water
with pH values mostly between 2 and 3; however, the pH of two samples was about 4 and several had pH values
near 8. These leach samples also contained elevate d metal abundances, including as much as 350 mg/l iron, 16 mg/l
manganese, as much as 22 mg/l aluminum, as much as 250 mg/l zinc, and as much as 2,500 mg/l sulfate.
Dilute sulfuric acid leaches of smelter slag and soil affected by smelter particulates generally yielded much
lower ( <1 mg/l) abundances of lead and other metals than mine wastes (Montour, 1994). Mobility of metals from
slag is a function of the way in which slag was cooled; metals in slag poured onto the ground generally are less
readily liberated into the environment tha n those from slag cooled by frothing into the air (John Drexler, oral comm.
to M. Montour, 1993).
Bioavailability studies: Results of bioavailability studies at Aspen, Colo., document the importance of particle
mineralogy and size on lead bioavailability (Andy Davis, PTI Environmental, oral presentation, 1993). Lead in
galena from mine wastes may be signi ficantly less bioavailable than lead in smelter particulates and auto emissions.
126
Potential environmental concerns associated with mineral processing
Historic processing typically involved milling to produce zinc-, lead-, and (or) copper-rich concentrates that were
smelted. Pyrite-rich tailings produced as a milling by-product have high potential for generation of highly acidic,
metal-bearing water, especially if the water is evaporatively concentrated. Storm-related flushing of soluble
secondary salts may also lead to periodic degradation of downstream surface water quality.
Modern processing typically involves milling followed by concentrate smelting; stopes are backfilled with
coarse tailings materials and fine tailings are stored in surface impoundments. If pyritic tailings are backfilled,
potential detrimental environmental effects can be largely avoided. In modern operations, gold-rich ore is
increasingly recovered using a cyanide-free flotation system. Copper-rich ore may be treated using flotation. Some
copper-mining operations use sulfuric acid heap leaching, followed by solvent extraction-electrowinning, to extract
copper. The primary drawback of the acid-leach method for copper recovery is generation of potentially large
volumes of acidic, iron- and aluminum-rich waste water that must be treated.
Smelter signatures
Chaffee (1980) demonstrated that soil geochemistry data effectively discriminates samples contaminated by smelter
Leadville (Montour, 1994): Slag contains 1,500 to 3,400 ppm lead and 5 to 15 ppm cadmium; mineralogical studies
indicate that most of the lead is present in sulfide minerals that survived smelting. Residential soil downwind from
smelters contains more than 2,700 ppm lead and 5 to 37 ppm cadmium.
Eureka, Nev. (Chaffee, 1980; 1987): Soil in the vicinity of two smelters contains >1,000 ppm lead, >500 ppm arsenic
and zinc, >100 ppm antimony and copper, >200 ppm tin, and >15 ppm molybdenum.
Currently available data for mine water pertain to moderately wet, seasonally temperate climates. Evaporation of
acid water during dry periods increases metal abundances and decreases pH. No data are available concerning
evaporation of near-neutral water draining carbonate-hosted ore; evaporation of iron-poor water probably causes pH
to increase.
Potential downstream effects of mine-drainage water associated with polymetallic replacement deposits are probably
significantly less than those associated with other deposit types because of the abundance of near-neutral mine water
and carbonate sedimentary rocks surrounding these deposits. Water draining carbonate rock terranes is highly
alkaline and can effectively buffer acid generated by deposits or resulting from the formation of hydrous ferric and
aluminum oxide particulates. In these situations, zinc and manganese are the elements most likely to remain mobile
either in solution or as colloids (Kimball and others, 1995) for appreciable distances downstream.
The greatest potential for deleterious downstream environmental effects of mine-drainage water is associated
with deposits principally composed of igneous or skarn ore; copper, zinc, manganese, and cadmium, to a lesser
extent, can remain mobile for considerable distances downstream. In addition, deposits that contain abundant
massive-sulfide ore in carbonate-poor sedimentary rocks, or carbonate-hosted massive sulfide ore in which drainage
water does not interact with carbonate-rich sediments pose significant potential for environmental degradation. These
ore types generate acidic, metal-rich water (figs. 2 and 3). Deposits that drain into geologic terranes with low acid-
buffering capacity, such as those with low amounts of carbonate-bearing rocks, may also pose significant
environmental hazards.
The most deleterious historic environmental effects associated with this deposit type are related to mining
operations that released significant large volumes of fine-grained pyritic tailings into rivers or streams; these tailings
have become part of accumulated sedimentary deposits, especially in low-velocity reaches of drainages. Oxidation
of these tailings results in long-term release of metals and acid from sediments into overlying stream water resulting
in water quality degradation; these downstream effects can be extensive.
Geoenvironmental geophysics
Distributions of conductive acid water can be delineated using electromagnetic and direct current resistivity surveys,
and in some cases, ground penetrating radar (King and Pesowski, 1993; King, 1995; Paterson, 1995). Local
hydrologic conditions, including permeable fault zones, bedrock channels, shallow caves, sandy flow channels, and
clay aquitards, may be investigated using ground electrical, radar, seismic, magnetic, and gravity surveys. The same
127
methods can help define structural relations in tailing heaps (Paterson, 1995). Induced polarization can be utilized
to estimate pyrite concentrations in tailings heaps (Paterson, 1995). Remote sensing imagery (Watson and Knepper,
1994; King, 1995) can help identify tailings distributions; bedrock lithologies, including acid-buffering carbonate
(1) Acid water draining igneous and skarn ore, some massive sulfide lenses, and tailings can be remediated
successfully using lime addition and sodium-bisulfide precipitation of metals; this process yields a potentially acid-
generating sludge. Lime addition to iron-rich drainage water may generate a quantity of suspended particulates, onto
which a major fraction of dissolved arsenic, lead, and copper can sorb, sufficient to reduce or eliminate the need for
(2) Dilution of low-oxygen, near-neutral mine-drainage water by fresh water from carbonate aquifers may provide
(3) The utility of carbonate sedimentary host rocks should be considered in acid drainage mitigation. For example,
acid water could be channeled through artificially or naturally fractured carbonate rocks, away from orebodies, to
(4) Careful fracture and karst mapping is required to adequately characterize site hydrology.
(5) Isolation of pyrite-rich waste dumps from weathering and formation of soluble secondary salts is crucial to
prevent storm- and snowmelt-related pulses of acid and metals into surface water. High carbonate mineral content
of dump material is not sufficient to prevent acid pulses because secondary salt dissolution generates acid and metals
much faster than the carbonates can react with and consume acid.
(6) Backfilling the acid-generating, pyritic part of mill tailings into workings below the post-mining water table
should be considered as a mitigative measure in modern milling operations to help avoid accidental downstream
releases.
(7) Future advances in aqueous zinc extraction technologies may render mine water draining these deposits a
potentially economic zinc resource that can be used to help defray mitigation and remediation costs.
REFERENCES CITED
Beaty, D.W., Landis, G.P., and Thompson, T.B., eds., 1990, Carbonate-hosted sulfide deposits of the central
Colorado Mineral Belt: Economic Geology Monograph 7, 424 p.
Buselli, G., 1980, Interpretation of Siro TEM data from Elura: Exploration Geophysics, Bulletin of the Australian
Society of Exploration Geophysicists, v. 11, p. 264-271.
Chaffee, M.A., 1980, Interpretation of geochemical anomalies in soil samples from a smelter-contaminated area,
Eureka mining district, Nevada [abs.]: Technical Program, American Institute of Mining, Metallurgical, and
Petroleum Engineers Annual Meeting, February, 1980, Las Vegas, Nevada, p. 56-57.
_________1987, Application of R-mode factor analysis to geochemical studies in the Eureka mining district and
vicinity, Eureka and White Pine counties, Nevada, in Elliott, I.L., and Smee, B.M., eds., GEOEXPO/86,
Exploration in the North American Cordillera: Association of Exploration Geochemists, Vancouver, Canada,
p. 94-108.
Cox, D.P., 1986, Descriptive model of polymetallic veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 125.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Engineering Science, Inc., 1985, Eagle mine remedial investigation, Appendix A: prepared for Colorado Department
of Law, p. A11-A21.
Fallon, G.N., and Busuttil, S., 1992, An appraisal of the geophysical effects of the Mount Isa ore bodies: Exploration
Geophysics, Bulletin of the Australian Society of Exploration Geophysicists, v. 23, nos. 1/2, p. 133-140.
Frischknecht, F.C., Labson, V.F., Spies, B.R., and Anderson, W.L., 1991, Profiling methods using small sources,
in Nabighian, M.N., ed., Electromagnetic methods in applied geophysics-applications, Volume 2, Part A:
Tulsa, Society of Exploration Geophysicists, p. 105-270.
Kimball, B.A., Callender, E., and Axtmann, E.A., 1995, Effects of colloids on metal transport in a river receiving
acid mine drainage, upper Arkansas River, Colorado, USA: Applied Geochemistry, v. 10, p. 285-306.
King, A., Pesowski, M., 1993, Environmental applications of surface and airborne geophysics in mining: Canadian
Institute of Mining and Metallurgy Bulletin, v. 86, p. 58-67.
128
King, T.V.V., 1995, Environmental considerations of active and abandoned mine lands: U.S. Geological Survey
Bulletin 2220, 38 p.
Lovering, T.S., Tweto, O., and Lovering, T.G., 1978, Ore deposits of the Gilman district, Eagle County, Colorado:
U.S. Geological Survey Professional Paper 1017, 90 p.
Montana Department of State Lands, 1994, Abandoned priority hardrock mine priority sites: 283 p.
Montour, M., 1994, Aqueous solubility of solid forms of lead in mining and smelting wastes, Leadville, Colorado:
Boulder, University of Colorado, M.Sc. thesis, 234 p.
Morris, H.T., 1986, Descriptive model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 99-100.
Paterson, Norman, 1995, Application of geophysical methods to the detection and monitoring of acid mine drainage,
in Bell, R.S., ed., Proceedings of the symposium of the application of geophysics to engineering and
environmental problems: Orlando, Florida, April 23-26, Environmental and Engineering Geophysical Society,
p. 181-189.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado--implications for the prediction of mine-drainage chemistry: Proceedings, 1993 Mined
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Shalley, M.J., Harvey, T.V., 1992, Geophysical response of the HYC deposit: Exploration Geophysics, Bulletin of
the Australian Society of Exploration Geophysicists, v. 23, nos. 1/2, p. 209-304.
Smith, K.S., Plumlee, G.S., and Ficklin, W.H., 1994, Predicting water contamination from metal mines and mining
waste: Notes, Workshop #2, International Land Reclamation and Mine Drainage Conference and Third
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Queensland: Exploration Geophysics, Bulletin of the Australian Society of Exploration Geophysicists, v. 23,
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Titley, S.R., 1993, Characteristics of high-temperature, carbonate-hosted massive sulfide ores in the United States,
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Modeling: Geological Association of Canada Special Paper 40, p. 585-614.
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Histories: Tulsa, Society of Exploration Geophysicists, p. 117-129.
Watson, Ken, and Knepper, D.H., eds., 1994, Airborne remote sensing for geology and the environment-past and
future: U.S. Geological Survey Bulletin, 1926, 43 p.
Zonge, K.L., and Hughes, L.J., 1991, Controlled source audio-frequency magnetotellurics, in Nabighian, M.N., ed.,
Electromagnetic methods in applied geophysics-applications, Volume 2, Part B: Tulsa, Society of
Exploration Geophysicists, p. 713-809.
129
AU-AG-TE VEIN DEPOSITS
(MODEL 22b; Cox and Bagby, 1986)
Deposit geology
Deposits consist of native gold and (or) gold-silver telluride minerals in high-grade quartz veins and (or) low-grade,
near surface disseminated native gold and pyrite (with or without telluride minerals) in permeable host rocks.
Porphyritic alkaline igneous rocks, many of which display explosive-magmatic features (diatremes, breccia pipes,
or stockworks), are spatially and perhaps genetically associated with the deposits. Veins are localized along fracture
zones; some veins are concentrated at intersections of crosscutting structural features. Disseminated deposits are
commonly adjacent to major structures. Although wall-rock alteration associated with veins is restricted (fig. 1),
alteration associated with disseminated deposits may be widespread within or near igneous centers.
Examples
Vein-type deposits: Cripple Creek, La Plata, Boulder County, and Rosita, Colo.; Horn Silver, Silver Queen,
Sulphurets Camp, British Columbia, Canada; Judith Mountains (Warm Springs, Spotted Horse), Little Rocky
Mountains (Zortman-Landusky), Golden Sunlight, Mont.; northern Black Hills (Annie Creek, Foley Ridge, and
Richmond Hill), S. Dak.; White Oaks and Ortiz, N. Mex.; Emperor, Fiji; Musariu, Sacaramb, Romania.
Disseminated deposits: Cripple Creek, Colo.; Zortman-Landusky, Golden Sunlight, Mont.; Ortiz, N. Mex.; northern
Black Hills, S. Dak.; Ladolam, Lahir Island; Porgera, Papua New Guinea.
Associated deposit types (Cox and Singer, 1986) include alkaline porphyry copper (Model 17), porphyry gold-copper
(Model 20c), polymetallic veins (Model 22c), polymetallic replacement (Model 19a), placer gold (Model 39a), distal
(1) Vein deposits are mined dominantly by underground operations; vein clusters are mined in open-pits. Low-grade
disseminated deposits are mined by open-pit operations. Most of these mining activities involve rocks with relatively
low sulfur (in sulfide minerals) and high tellurium contents. Abundant carbonate minerals in alteration and gangue
assemblages (and high CO2 contents currently venting in some underground mines, for example Cripple Creek, Colo.)
(2) Water draining these deposits has low metal contents because pH is neutral to near-neutral. In areas underlain
by rocks with low acid-buffering capacity, water draining open-pit mines may have elevated concentrations of iron,
(3) Soluble secondary minerals, including sulfate minerals, could cause short-term pulses of acidic, metal-bearing
Exploration geophysics
Aeromagnetic and gravity studies (Kleinkopf and others, 1970) at Cripple Creek, Colo., show that geophysical
anomalies are correlated with alteration intensity. Distribution of pyrite and clay in altered rock can be studied using
induced polarization/resistivity. Brecciation, fracturing, and faulting associated with volcanic centers create pathways
for pore water, oxidation of magnetite, and alteration of primary minerals to clay. Rock in these areas has low
density, magnetization, and resistivity that can be identified by gravity, magnetic, and electromagnetic or direct
current resistivity surveys. Potassic alteration and uranium and thorium in alkalic igneous rocks can be mapped by
gamma-ray spectrometry. Aerial gamma-ray surveys (Pitkin and Long, 1977) identify anomalous potassium radiation
associated with deposits at Cripple Creek, Colo. Alteration assemblages may be identified with multispectral remote
sensing. Taranik (1990) successfully mapped the distribution of supergene iron oxide minerals using this technique.
In addition, Livo (1994) characterized the distribution and nature of hydrothermal alteration using remote sensing
techniques.
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Figure 1. A, Conceptual model for a Au-Te system (based on Cripple Creek, Colo.; Pontius, 1992). Native gold and pyrite-rich disseminated
deposits may be lateral or upper level equivalents to vein deposits. B, Vein-related wall-rock alteration (Thompson and others, 1985).
References
Kelly and Goddard (1969), Witkind (1973), Giles (1983), Mutschler and others (1985), Porter and Ripley (1985),
Thompson and others (1985), Kay (1986), Saunders (1986), Ahmad and others (1987), Birmingham (1987), Afifi
and others (1988), Saunders (1991), Pontius (1992), and Mutschler and Mooney (1995).
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GEOLOGIC FACTORS THAT INFLUENCE POTENTIAL ENVIRONMENTAL EFFECTS
Deposit size
Typically 1-2 Mt (0.3-0.6 oz per ton) for vein deposits. Cripple Creek, Colo., one of the World's largest gold-
telluride vein systems, has produced 41 Mt (average, 0.5 oz per ton); disseminated bulk minable deposits range from
32 Mt (0.037 oz per ton) at Cripple Creek to 66 mt (0.03 oz per ton) at Zortman-Landusky, Mont.
Host rocks
Most gold-silver-tellurium deposits are hosted in alkalic, silica-undersaturated rocks such as syenite, monzonite,
diorite, phonolite, monchiquite, or vogesite; less common host rocks include silica-undersaturated low-titanium basalt
(shoshonite). Except for calcic plagioclase, these rocks contain minerals with minimal acid buffering capacity.
Most gold-silver-tellurium deposits are associated with the late phases of orogenesis or failed rifting events; most
deposits in the United States are along the eastern edge of the area affected by Laramide deformation and are in
terranes composed of a variety of rock types, including metasedimentary, metavolcanic, carbonate, and volcanic rocks
of diverse ages.
Wall-rock alteration
Alteration results from pre-ore igneous activity, vein-type mineralization, or disseminated gold and pyrite
mineralization.
Igneous-related alteration: Argillization and dolomitization are characterized by weak to moderate clay alteration,
sericitization, and minor recementing by dolomite; this type of alteration creates broad zones of increased
Vein-type deposits (fig. 1): Alteration is most extensively developed along vein junctions. Inner zone primarily
consists of dolomite, adularia, sericite, roscoelite, and pyrite. Outer zone primarily consists of montmorillonite,
sericite, adularia, magnetite, pyrite, and carbonate; this zone is typically 1 to 5 times (Cripple Creek, Colo.) vein
width but locally extends as much as 20 times vein width (Boulder County).
sanidine, orthoclase, and adularia along with moderate to strong sericitization and fine- to medium-grained pyrite.
This alteration is most prominent in the broad vuggy permeable zones within 300 m of the surface. Strong potassium
Nature of ore
Vein deposits are usually high-grade but may be small and erratically distributed. Nearly all gold production from
these deposits is from gold- and gold-silver telluride minerals. Base metals are infrequently present and are
volumetrically minor. Most veins are structurally-controlled. Some structural zones display extensive vertical
continuity; mineralized rock, with little evidence of mineralogic zoning or grade variation, can extend to depths of
1,000 m or more (Cripple Creek, Colo.). Lower-grade disseminated deposits containing native gold and auriferous
pyrite may also be confined to structural intersections or major shear zones. Lower-grade deposits are best developed
in permeable wall rocks, mostly igneous rocks or vent breccias, within 300 m of the surface.
Trace element geochemical signatures depend on telluride mineralogy, but primary enrichments include Ag, As, Au,
Ba, Bi, Cu, F, Fe, Hg, Mo, Mn, Ni, Pb, Sb, Te, V, and Zn. Both types of deposits appear to have had abundant CO2
distributed throughout their vertical extent; deeper workings (Cripple Creek, Colo.) currently vent extensive amounts
of CO2 along major veins. These deposits are characterized by relatively low sulfur (in sulfide minerals) abundances,
Vein-type deposits: Ore minerals include gold-silver telluride minerals (calaverite, sylvanite), native gold and (or)
native tellurium, and other telluride minerals (most commonly krennerite, petzite, hessite, coloradoite, melonite,
altaite, tetradymite). Deposits also contain variable amounts of common base metal sulfide and sulfosalt minerals
(pyrite, chalcopyrite, sphalerite, galena, and tetrahedrite); some deposits contain cinnabar. Gangue minerals include
adularia, chlorite, fluorite, sericite, roscoelite, magnetite, hematite, barite, celestite, and carbonate minerals. The
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generalized paragenesis includes early base-metal sulfide minerals, followed by telluride minerals (with or without
native tellurium), hypogene native gold, and late-stage cinnabar. Quartz, fluorite, and pyrite are present throughout
the sequence.
Disseminated deposits: Ore minerals are native gold, auriferous pyrite (as much as 2-3 percent), and occasionally
telluride minerals (sylvanite, Zortman-Landusky, Mont.), although telluride minerals are often absent (Cripple Creek,
Mineral characteristics
Carbonate minerals and quartz are dominant in banded, symmetrical veins. Ore minerals form groups of blades or
small masses and locally, open-space fillings in vugs. In veins, complex intergrowths of several telluride minerals
are common. In some case, minerals are so complexly intergrown that they cannot be easily identified. Breccia-
filling textures are common in some deposits. Vein ore ranges from fine grained (0.01 to 1.5 mm) to coarse-grained,
bladed crystals (0.5 to 2 cm). Low-grade disseminated deposits consist of microcrystalline native gold and pyrite
Secondary mineralogy
Telluride minerals are easily destroyed by weathering. Some tellurium is redeposited as green oxide minerals
(emmonsite). Native gold is deposited during supergene enrichment of some deposits. Late hydrothermal activity
and oxidation of some deposits extends to depths of 30 to 180 m (Cripple Creek, Colo.; Zortman-Landusky, Mont.)
and in others to depths of only 1.5 to 15 m (Boulder, Colo.). The depth of oxidation generally depends on the
openness and permeability of vein structures. Late hydrothermal activity deposits a variety of sulfate minerals along
with jarosite, anhydrite, opal, and chalcedony. A variety of iron (limonite) and manganese oxide minerals results
from oxidation. Dolomite formed during early igneous-related activity and present within the oxidized zone may
be leached during oxidation. Gott and others (1969) found that secondary geochemical dispersion results in
anomalies of gold, silver, and tellurium, accompanied by enrichments of iron, lead, mercury, antimony, arsenic, and
vanadium. A manganese-enriched outer halo is present commonly.
Topography, physiography
Alkaline intrusive and associated volcanic rocks commonly form topographic highs. However, if igneous-activity-
related alteration, including argillization and dolomitization, is prevalent, broad high permeability zones, which are
highly susceptible to increased erosion, may result. Alteration related to vein type mineralization is restricted in
Hydrology
Since these deposits are commonly structurally controlled, ground water flow is focused along high permeability
structures. Numerous historic underground workings, typical of some districts, also control water recharge and
discharge. Pre-mining ground water oxidizes rock to significant depths, for instance, 180 m at Cripple Creek.
Contacts between igneous rocks and surrounding wall rock constitute significant ground water conduits. Since
deposits are typically located at topographic highs associated with volcanic rocks, surface water flows radially away
in all directions.
Historic: Most historic operations involved underground workings along veins. Ore was processed by stamp mills,
followed by mercury amalgamation or cyanide vat leaching. The second largest cyanide mill in the world was at
one time located in Ruby Gulch (near the current Zortman-Landusky, Mont., mine).
Modern: Most modern operations are open-pit mines that exploit vein clusters or disseminated gold along major
ENVIRONMENTAL SIGNATURES
Drainage signatures
Natural drainage water: The only data concerning the geochemistry of natural water draining gold-silver-telluride
veins are those for the Zortman-Landusky, Mont., deposit; data are part of environmental impact studies (Scott
Haight, U.S. Bureau of Land Management office, Lewistown, Mont., oral commun., 1995; Zortman-Landusky area
Environmental Impact Statement, 1995). Pre-mining (pre-1979) data from the Zortman-Landusky area indicate the
133
following:
(1) Surface water was slightly acidic to slightly alkaline (pH 6.9 to 8.4) and contained low dissolved constituent
concentrations, including 8 to 134 mg/l sulfate, 0.002 to 0.01 mg/l arsenic, 0.011 to 0.17 mg/l iron, and 0.01 mg/l
zinc.
(2) Groundwater was slightly acidic to slightly alkaline (pH 6.5 to 8.0) and contained low to moderate dissolved
constituent concentrations, including 2 to 186 mg/l sulfate, 0.005 to 0.31 mg/l arsenic, 0.021 to 11 mg/l iron, and
0.005 to 0.88 mg/l zinc. Relatively higher concentrations of sulfate and metals in some water samples are due to
the presence of shale, which has naturally high concentrations of sulfate and metals, in some drainage areas.
(3) Elevated concentrations of a rsenic in some groundwater indicates that alluvial groundwater had been affected by
Mine drainage water: Most mine water is neutral (pH 7 to 8), but slightly acidic water (pH 6.0 to 6.9) has been
reported near the Zortman-Landusky, Mont., mine (U.S. Environmental Protection Agency, 1990). Most water
draining fro m mines has low concentrations of total dissolved metals (Zn+Cu+Cd+Co+Ni+Pb <100 µg/l) (Plumlee
and others, 1993) but may have elevated concentrations of zinc (U.S. Environmental Protection Agency, 1990;
Keffelew, 1995). Other metals, including iron, arsenic, copper, and manganese rarely are present in high
concentrations but may be present at abundances that exceed Federal Secondary Drinking Water Standards (U.S.
Environmental Protection Agency, 1990). At Cripple Creek, Colo., extensive areas of dolomitized rock, mostly at
deep levels, and CO 2 , that is currently outgassing , buffer acidic solutions (Jeff Pontius, Pikes Peak Mining Company,
oral and written commun., 1995). At Zortman-Landusky, surrounding carbonate rocks quickly buffer slightly acidic
Potentially economically recoverable elements: Scandium abundances in Cripple Creek, Colo., mine drainage are
relatively high and may represent an economically recoverable commodity (Geoff Plumlee, oral commun., 1995).
Metal mobility away from gold-tellurium deposits may be l imited if abundant carbonate rock, alteration assemblages,
and (or) gangue minerals are associated with deposits. Water quality data from wells and springs in the Zortman-
Landusky, Mont., area show that ground water in the area is hard to very hard, alkaline (pH 7 to 7.5), and of the
calcium carbonate type. Surface water from a stream draining the waste and heap leach area is slightly acidic (pH
6 to 6.9) and contains as much as 297µg/l arsenic and 36 µg/l cyanide (U.S. Environmental Protection Agency,
1990). Flow and water quality vary slightly with seasonal precipitation rates. During spring runoff, one stream
Because most gold-tellurium deposits in the United States have been mined by underground and (or) placer mining
methods since the late 1800s, including for instance, Boulder County and Cripple Creek, Colo., and Zortman-
Landusky, Mont., pre-mining geochemical data for soil and sediment associated with these deposits are rare.
However, data obtained in 1978, before initiation of open-pit mining and heap leaching operations, as part of the
National Uranium Resource Evaluation program, are available for the Zortman-Landusky, Mont., area. Stream
sediment samples collected from streams wit hin 5 km of and draining mineralized areas contain <5 to 12 ppm silver,
0.04 to 0.4 ppm gold, <5ppm cadmium, 18 to 81 ppm copper, 1.4 to 4.2 weight percent iron, 200 to 2,020 ppm
manganese, 7 to 197 ppm lead, and 70 to 1,131 ppm zinc (Shannon, 1980).
Mercury amalgamation of ore during historic operations may represent a source of mercury contamination not
Historic cyanide milling operations at Zortman-Landusky, Mont., discharged waste water to watercourses
(U.S. Environmental Protection Agency, 1990). Most modern heap leach operations rinse heap-leach residues, barren
ore, remaining after the leac hing cycle with fresh water to remove residual cyanide. Residues are left on heap leach
pads. Heap leach and other cyanide processing solutions are most likely to include gold and silver cyanide
complexes with lesser arsenic, antimony, nickel, vanadium, and iron present as weak cyanide complexes.
Smelter signatures
134
Climate effects on environmental signatures
The effects of various climate regimes on the geoenvironmental signature specific to gold-silver-tellurium deposits
are not known. However, in most cases the intensity of environmental impact associated with sulfide-mineral-bearing
mineral deposits is greater in wet climates than in dry climates. Acidity and total metal concentrations in mine
drainage in arid environments are several orders of magnitude greater than in more temperate climates because of
the concentrating effects of mine effluent evaporation and the resulting "storage" of metals and acidity in highly
soluble metal-sulfate-salt minerals. However, minimal surface water flow in these areas inhibits generation of
significant volumes of highly acidic, metal-enriched drainage. Concentrated release of these stored contaminants to
Geoenvironmental geophysics
Acid- or metal-bearing water resulting from pyrite oxidation has low resistivity and consequently can be identified
with electromagnetic or direct current induced polarization/resistivity surveys and ground penetrating radar.
Structural and stratigraphic features such as faults, bedrock topography, buried channels, and aquitards that affect
water flow away from mine areas may be studied using electromagnetic or direct current resistivity, seismic refraction
or reflection, magnetic, gravity, and ground penetrating radar surveys; water flow may be monitored using self
potential methods. The distribution of supergene iron oxide and sulfate minerals, especially jarosite, as well as
REFERENCES CITED
Afifi, A.M., Kelly, W.C., and Essene, E.J., 1988, Phase relations among tellurides, sulfides, and oxides: II.
Applications to telluride-bearing ore deposits: Economic Geology, v. 83, p. 395-404.
Ahmad, M., Solomon, M., and Walshe, J.L., 1987, Mineralogical and geochemical studies of the Emperor gold
telluride deposit, Fiji: Economic Geology, v. 82, p. 345-370.
Birmingham, S.D., 1987, The Cripple Creek volcanic field, central Colorado: Austin, University of Texas, M.S.
thesis, 295 p.
Cox, D.P., 1992, Descriptive model of distal disseminated Ag-Au deposits, in Bliss, J.D., ed., Developments in
mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 20-22.
Cox, D.P., and Bagby, W.C., 1986, Descriptive model of Au-Ag-Te veins, in Cox, D.P., and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 124.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Environmental Impact Statement (draft version), 1995, Zortman and Landusky mines, v. 2.
Giles, D.L., 1983, Gold mineralization in the laccolithic complexes of central Montana, in Proceedings Denver
Region Exploration Geologists Society Symposium--The genesis of Rocky Mountain ore deposits: Changes
with time and tectonics: Denver Region Exploration Geology Society, Denver, Colorado, p. 157-162.
Gott, G.B., McCarthy, J.H., Van Sickle, G.H., and McHugh, J.B., 1969, Distribution of gold and other metals in the
Cripple Creek district, Colorado: U.S. Geological Survey Professional Paper 625-A, p. A1-A17.
Kay, B.D., 1986, Vein and breccia gold mineralization and associated igneous rocks at the Ortiz Mine, N. Mex.,
USA: Golden, Colorado School of Mines, M.Sc. thesis, 170 p.
Keffelew, Berhan, 1995, Third party review of document entitled "Amendment No. 6 to Office of Mined Land
Reclamation, Permit M-80-244, response to OMLR technical review, acid-base potential of Cresson mine
overburden, September 1994, 22 p.
Kelly, W.C., and Goddard, E.N., 1969, Telluride ores of Boulder County, Colorado: Geological Society of America
Memoir 109, 237 p.
Kleinkopf, M.D., Peterson, D.L., and Gott, G., 1970, Geophysical studies of the Cripple Creek mining district,
Colorado: Geophysics, v. 35, no. 3, p. 490-500.
Livo, K.E., 1994, Use of remote sensing to characterize hydrothermal alteration of the Cripple Creek area, Colorado:
Golden, Colorado School of Mines, M.S. thesis, 135 p.
Mutschler, F.E., Griffin, M.E., Stevens, D.S., and Shannon, S.S., Jr., 1985, Precious metal deposits related to alkaline
rocks in the North America Cordillera--An interpretive review: Transactions of the Geological Society of
Africa, v. 88, p. 355-377.
Mutschler, F.E., and Mooney, T.C., 1995, Precious metal deposits related to alkaline igneous rocks--provisional
classification, grade-tonnage data, and exploration frontiers: Geological Association of Canada Special
Paper--IAGOD 1990 Symposium on ore deposit models, 22 p.
135
Pitkin, J.A., and Long, C.L., 1977, Interpretation of data from an aerial gamma-ray survey in the Cripple Creek
district, Teller County, Colorado: U.S. Geological Survey Open-File Report 77-534, 12 p.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado: Implications for the prediction of mine-drainage chemistry: Proceedings, Mined Land
Reclamation Symposium, Billings, Montana, p. 176-186.
Pontius, J.A., 1992, Gold mineralization within the Cripple Creek diatreme/volcanic complex, Cripple Creek Mining
district, Colorado, USA: International Minexpo 92, Las Vegas, Nevada, October 18-22, 12 p.
Porter, E.W., and Ripley, E., 1985, Petrologic and stable isotope study of gold-bearing breccia pipe at the Golden
Sunlight deposit, Montana: Economic Geology, v. 80, p. 1689-1706.
Saunders, J.A., 1986, Petrology, mineralogy, and geochemistry of representative gold telluride ores from Colorado:
Golden, Colorado School of Mines, Ph.D. thesis, 171 p.
_________1991, Gold deposits of the Boulder County Gold District, in Shawe, D.R., and Ashley, R.P., eds.,
Epithermal gold deposits--part II: U.S. Geological Survey Bulletin 1857-I, p. I137-I148.
Shannon, S.S., Jr., 1980, Uranium hydrogeochemical and stream sediment reconnaissance data release for the
Lewistown NTMS quadrangle, Montana, including concentrations of forty-two additional elements: U.S.
Department of Energy Report GJBX-206 (80), 128 p.
Taranik, D.L., 1990, Remote detection and mapping of supergene iron oxides in the Cripple Creek district: Boulder,
University of Colorado, M.S. thesis, 103 p.
Thompson, T.B., Trippel, A.D., and Dwelley, P.C., 1985, Mineralized veins and breccias of the Cripple Creek
district, Colorado: Economic Geology, v. 80, no. 6, p. 1669-1688.
U.S. Environmental Protection Agency, 1990, Preliminary assessment of the Landusky mine (Zortman mine),
Landusky, Montana: Environmental Protection Agency report MTD89515498.
Witkind, I.J., 1973, Igneous rocks and related mineral deposits of the Barker quadrangle, Little Belt Mountains,
Montana: U.S. Geological Survey Professional Paper 752, 58 p.
136
VOLCANIC-ASSOCIATED MASSIVE SULFIDE DEPOSITS
(MODELS 24a-b, 28a; Singer, 1986a,b; Cox, 1986)
Figure 1. Essential characteristics of an idealized volcanogenic massive sulfide deposit (modified from Lydon, 1984). Mineral abbreviations
as follows: Sp, sphalerite; Gn, galena; Py, pyrite; Ba, barite; Cpy, chalcopyrite; Po, pyrrhotite; and Hem, hematite.
137
Figure 2. Ficklin plot (Plumlee and others, 1993) showing aqueous metal contents and pH ranges of water associated with volcanic massive
sulfide deposits. Fields A, B, and C (Plumlee and others, 1994), are those for West Shasta, Calif., district VMS deposits, sulfide-mineral-rich
vein deposits in rocks with low buffering capacity, and sulfide-mineral-rich vein deposits in carbonate host rocks, respectively. Field D is the
composite field for water draining Prince William Sound, Alaska, VMS deposits (Goldfarb and others, in press).
Examples
Cyprus-type: Skouriotissa, Cyprus; Betts Cove, Newfoundland; Turner-Albright, Oreg.; Big Mike, Nev.
Kuroko-type: Kidd Creek, Ontario; Iron King and Penn Mine, Calif.; Mokuroko district, Japan.
VMS deposits are associated with a number of other mineral deposit types (Cox and Singer, 1986). Some VMS
deposits, especially the Besshi-type deposits as broadly defined by Slack (1993), are transitional in depositional
setting with some sedex deposits (Model 31a), such as Sullivan, British Columbia. VMS deposits are commonly
associated with regionally developed iron- and (or) manganese-rich metalliferous sediment and chert developed at
the same time-stratigraphic horizon as the massive sulfide deposits. Some Archean VMS deposits may be transitional
to volcanic-associated iron formation. VMS deposits, especially in Archean terranes, tend to be spatially associated
with shear-hosted mesothermal lode gold deposits (Model 36a) and Algoma-type banded iron formation (Model 28b).
Volcanic-associated massive sulfide deposits are among the most likely of all deposit types to have associated
environmental problems, particularly acid mine drainage. Analyses of water draining VMS deposits plot in the
extreme metal-extreme acidity field (fig. 2). VMS deposits have high iron- and base-metal-sulfide mineral contents
and are hosted by rocks with low buffering capacity. These minerals are unstable under normal oxidizing near
surface conditions and represent potential sources of highly acid and metal-rich drainage, especially in areas disturbed
by surface mining or tailings disposal. Associated high abundances of potentially toxic trace metals, including
arsenic, bismuth, cadmium, mercury, lead, and antimony, are present in some deposits, particularly those associated
Exploration geophysics
Electrical properties of sulfide minerals, combined with large sulfide mineral concentrations in VMS deposits, make
this type of mineral deposit a particularly favorable target for location by a variety of geophysical techniques. Self-
potential, induced polarization, and a wide range of electromagnetic methods have been successfully used to locate
buried VMS deposits. Pyrrhotite-rich and magnetite-bearing massive sulfide deposits may be locatable by detailed
magnetic surveys. Airborne multispectral remote sensing techniques have been used to identify areas that contain
hydrothermally altered rock and stressed vegetation that may be associated with mineralized rock.
138
References
Franklin and others (1981), Ohmoto and Skinner (1983), Fox (1984), Lydon (1984, 1988), Franklin (1993), and Slack
(1993).
Deposit size
Historically, most economic deposits are in the 1-5 million tonnes range (Singer, 1986c,d; Singer and Mosier, 1986).
Deposits of this size can still be developed in areas with an existing mining infrastructure; however, development
of new deposits in frontier areas likely requires at least 10 million tonnes of reasonably high grade ore. Most
Cyprus-type deposits contain less than 15 million tonnes of ore. Most Besshi-type deposits are also fairly small;
notable exceptions include the >300 million tonne Windy Craggy, British Columbia, deposit. Kuroko-type deposits,
especially those of Precambrian age, can be very large, such as the world class Kidd Creek, Ontario, deposit.
Host rocks
Cyprus-type deposits are hosted by submarine mafic-volcanic rocks and their altered equivalents, typically in
brecciated rocks commonly associated with pillow lavas, which have good buffering capacity. Host-rocks for
Kuroko-type deposits range from basalt to rhyolite, which have high and low buffering capacities, respectively.
Many deposits are associated with subaqueous dacitic-domes that have intermediate buffering capacity. Host rocks
are commonly brecciated and are typically moderately to highly altered. Some deposits are hosted by associated
volcaniclastic or hemipelagic sedimentary rocks that overly submarine volcanic sequences. Besshi-type deposits are
Submarine volcanic activity is a defining characteristic of VMS deposits. Most VMS deposits, including many
ophiolite-hosted deposits of the Cyprus-type, are associated with arc-related volcanism. Local extensional tectonic
environments are particularly conducive to deposition of massive sulfide deposits. Many VMS deposits are in rocks
that have undergone collisional tectonism and may be in structural contact with a wide variety of rock types.
Wall-rock alteration
Footwall alteration is moderate to locally intense around most VMS deposits. Hanging-wall alteration is typically
absent, but may be weakly developed in some deposits. Many deposits that have not been tectonically disrupted are
underlain by "stringer-zone" mineralized and altered rock. Stringer-zones are characterized by anastomosing quartz-
sulfide veins. Local zones of silicification are present near and within mineralized zones. The most common
alteration is pervasive chloritization, which is less well developed with increasing depth and distance from
hydrothermal upwelling zones, in the footwall of deposits. Deposits hosted by felsic rocks typically have extensively
developed quartz sericite alteration in the footwall. Most altered rock associated with massive sulfide deposits has
low to very low acid buffering capacity. Some massive sulfide deposits are associated with pervasive carbonate
alteration in the footwall (for instance, Sturgeon Lake, Ontario; Morton and others, 1990). These carbonate alteration
zones typically have low to moderate abundances of calcitic to ankeritic carbonate minerals. Massive sulfide deposits
with associated carbonate alteration assemblages that are easily accessible to acid water are less likely to produce
acid drainage.
Nature of ore
Massive sulfide deposits, by definition, contain zones or lenses of massive sulfide minerals, many with sulfide
mineral contents exceeding 90 volume percent. Most deposits also contain extensive zones of semi-massive sulfide
rock (25 to 50 volume percent) that contain economically exploitable ore. Stringer zone ore zones typically contain
5 to 20 volume percent sulfide minerals, hosted in quartz veins and disseminated in chloritic wall rocks.
Disseminated sulfide rock is extensively developed in footwall alteration zones; sulfide mineral abundances decrease
with depth below the massive sulfide zone horizon. Lateral development of disseminated pyrite can be continuous
for large distances at and immediately below the stratigraphic horizon of the massive sulfide lens.
Iron is nearly always the predominant metal in sulfide phases. Economically exploitable VMS deposits associated
with mafic rocks are variably enriched in copper and zinc, whereas those that contain a significant component of
139
felsic volcanic or sedimentary rock are relatively enriched in zinc and lead. Deposits associated with mafic rocks
can contain anomalous concentrations of gold, silver, and cobalt. Deposits associated with felsic volcanic and
sedimentary rocks contain minor to significant concentrations of lead, silver, arsenic, antimony, cadmium, and locally
bismuth, tin, and selenium.
Mineral characteristics
Grain size is highly variable and is generally controlled by primary sulfide mineralogy and the extent of metamorphic
recrystallization. Primary sulfide minerals of most zinc-lead-copper deposits are fine grained and intergrown,
whereas those of most copper-zinc deposits are coarser grained (Franklin, 1993). The extent of grain size changes
depends upon pressure and temperature conditions attained during metamorphism, and on the ductility of sulfide
minerals. For example, cataclastic deformation significantly reduces grain-size and therefore reactivity of brittle
sulfide minerals such as chalcopyrite and pyrite, but plastically deforms ductile sulfide minerals such as galena.
Thermal metamorphism commonly causes sulfide ore to become much coarser grained and develop mosaic or
Secondary mineralogy
Both initial seafloor and later near surface oxidation of massive sulfide minerals results in the formation of iron-rich
gossan. Intermediate stages of oxidation also can result in the formation of a wide range of iron- and base-metal
sulfate and sulfate-hydrate minerals. These highly soluble minerals are potentially important reservoirs of heavy
metals that can be easily mobilized and potentially can produce high peak loads of dissolved metals if hydrologic
conditions are suddenly altered, for example, by surface mining of a deposit in a wet climate. Secondary minerals
formed in temperate climates include goethite, crystalline and amorphous silica, jarosite, a variety of metal-bearing
hinsdalite, and brochantite), scorodite, native gold, native silver, native bismuth, barite, anglesite, litharge, covellite,
Topography, physiography
Hydrology
These deposits exert no specific influence on the adjacent hydrologic regime, partly because of their relatively small
size. The extent of mineralized outcrop and (or) mine-related excavations exposed to the atmosphere or oxidized
groundwater, and their position relative to the water table, are hydrologic factors that can significantly influence the
140
intensity and scale of environmental problems related to VMS deposits. Availability of oxidizing water is a
controlling factor for acid generating potential and dissolved metal carrying capacity of water interacting with
massive sulfide deposits or their mine-related products.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Natural drainage water: Published data concerning the chemistry of natural drainage associated with VMS deposits
include: Goldfarb and others (in press), Prince William Sound, Alaska; Greens Creek Environmental Impact
Statement (1983), Admiralty Island, Alaska; and Filipek and others (1987), West Shasta, Calif., district. Temperate
rainforest climate--Water draining Cyprus- and Besshi-type VMS deposits in the Prince William Sound area is dilute
calcium-bicarbonate-type water (5.8 mg/l Ca and 15 mg/l HCO3-) that is slightly acidic to neutral (pH 6.4 to 7.6);
the type of host rock, sedimentary or volcanic, does not seem to affect these generalizations. Specific conductance
is <50 uS/cm. Metal abundances in this water include 20 µg/l iron, 15 µg/l aluminum, <2 µg/l arsenic, 1.4 µg/l zinc,
and <1 µg/l silver, copper, cobalt, chromium, lead, molybdenum, and antimony. Stream water flowing over
undisturbed mineralized rock above several deposits has slightly elevated metal abundances, including as much as
40 µg/l iron, 2 µg/l copper, and 52 µg/l zinc; pH varies from 5.6 to 7.3. Montaine, semi-arid climate--Natural water
draining unmineralized areas above the West Shasta district (Filipek and others, 1987) also is calcium bicarbonate
type water (14 mg/l calcium and 22 mg/l HCO3-) that has a pH of 6.15 and a specific conductance of 100 uS/cm.
This water contains 8 mg/l iron, 18 mg/l aluminum, <12 mg/l copper, <10 mg/l zinc, and <1 mg/l arsenic.
Mine drainage water: Published data sources concerning the chemistry of mine drainage associated with VMS
deposits include: Goldfarb and others (in press), Beatson and eight other mines in Prince William Sound, Alaska;
Kilburn and others (1994, 1995), Holden mine, Wash.; Alpers and others (1991), Penn mine, Calif.; and Alpers and
others (1991, 1994), Iron Mountain mine, Calif. Temperate rainforest climate--The most acidic and metal-rich mine
water, draining from the base of well consolidated tailings piles of Besshi-type deposits, from two locations in the
Prince William Sound, Alaska, has a pH of 2.6 to 2.7 and contains as much as 21,000 µg/l iron, 3,600 µg/l copper,
220 µg/l lead, 3,300 µg/l zinc, 30 µg/l cobalt, 10 µg/l cadmium, and 311 mg/l sulfate. The tailings are devoid of
vegetation and the drainages support a rich growth of bright green, copper-loving, bryophyte algal mats. Slightly
acidic (pH 4.3 to 6.6) mine water was identified at six other deposits. Maximum metal values are 310 µg/l iron,
1,400 µg/l copper, and 3,100 µg/l zinc. Kilburn and others (1994) sampled acidic mine drainage from within adits
and below consolidated tailings piles at the Holden Mine, a copper-zinc VMS in amphibolitic mafic volcanic rocks.
Water within the adits has a pH of about 5 and metal concentrations of 370-580 µg/l iron, 570-580 µg/l copper, and
nearly 5,000 µg/l zinc. Effluent from tailings piles has a pH of 2.8 to 2.9 and contains 23,000 to 50,000 µg/l iron,
21 to 53 µg/l copper, and 130 to 3,800 µg/l zinc. Montaine, semi-arid climate--The Iron Mountain mine contains
the most acidic and metal-rich mine water ever recorded. Alpers and Nordstrom (1991) have reported pH values
ranging between 1.5 and <-1.0. The first ever occurrence of pH values less than one were obtained for dripping mine
water that was actively precipitating melanterite and other efflorescent metal-sulfate salts. This water contains as
much as 11 weight percent iron, 2.3 weight percent zinc, 0.5 weight percent copper, and 76 weight percent sulfate,
with 340 mg/l arsenic, 211 mg/l cadmium, 12 mg/l lead, and 29 mg/l antimony.
141
Metal mobility from solid mine wastes
Soluble sulfate salt minerals derived from weathering and oxidation of sulfide minerals in mine dumps and tailings
piles represent a potential source of metal contamination and acid generation. As percolating surface and ground
water evaporates during dry periods, efflorescent metal-sulfate salt minerals form encrustations around and below
the base of the piles, which effectively stores acidity and metals released during sulfide mineral breakdown.
Subsequent rainfall or snowmelt following a dry period is likely to release a highly concentrated pulse of acid mine
water. Mine dumps associated with lead-rich VMS deposits (Kuroko-type) may be a source of lead contamination
The elemental suite and magnitude of geochemical anomalies in soil and sediment collected from undisturbed VMS
deposits depend upon a number of factors, including VMS deposit type, extent of ore outcrop or overburden, climate,
topography, etc. Stream sediment samples collected below Kuroko-type deposits in temperate rain forest on
Admiralty Island, Alaska, contain 5 to 10 weight percent iron, as much as 10,000 ppm barium, hundreds to several
thousand ppm zinc, hundreds of ppm lead, tens to hundreds of ppm arsenic, copper, and nickel, as well as 0 to 20
ppm silver, bismuth, cadmium, mercury, molybdenum, and antimony (Kelley, 1990; Rowan and others, 1990; Taylor
Stream sediment geochemical signatures associated with undisturbed to variably disturbed Cyprus and Besshi
VMS deposits in the Prince William Sound, Alaska, are similar to those just described. They contain 10 to 40
weight percent iron, several hundred ppm barium, hundreds of ppm arsenic and zinc, tens to hundreds of ppm lead,
hundreds to thousands of ppm copper, and 0 to 20 ppm silver, bismuth, mercury, molybdenum, and antimony (R.J.
Goldfarb, unpub. data, 1995).
Tailings ponds below mills are likely to contain high abundances of lead, zinc, cadmium, bismuth, antimony, and
cyanide and other reactants used in flotation and recovery circuits. Highly pyritic-pyrrhotitic orebodies that are
exposed to oxidation by air circulating through open adits, manways, and exploration drill holes may evolve SO2 gas;
in some cases, spontaneous combustion can cause sulfide ore to burn. Tailings that contain high percentages of non-
ore iron sulfide minerals have extremely high acid-generating capacity. Surficial stockpiles of high-sulfide mineral
Smelter signatures
Most base-metal rich ore concentrates are smelted. In most cases, concentrates are shipped to custom smelters, and
therefore do not contribute to the environmental impact in the immediate mine vicinity. Larger districts are often
served by a smelter co-located in the district. Data compiled by Gulson and others (1994) document the relationship
between lead in soil near smelters and blood lead in children; similar data for the Leadville, Colo., area indicate
similar trends. Additional data may be available for the Trail, British Columbia and El Paso, Tex. smelters.
142
Climate effects on environmental signatures
Great differences between geochemical mine drainage data for deposits in the West Shasta, Calif., district, eastern
Canada, and that for deposits in cooler and wetter Alaskan climates underscore the important role of climate with
regard to acid mine drainage associated with VMS deposits. Acidity and total metal concentrations in mine drainage
in arid environments are several orders of magnitude greater than in more temperate climates because of the
concentrating effects of mine effluent evaporation and the resulting "storage" of metals and acidity in highly soluble
metal-sulfate-salt minerals. However, minimal surface water flow in these areas inhibits generation of significant
volumes of highly acidic, metal-enriched drainage. Concentrated release of these stored contaminants to local
watersheds may be initiated by precipitation following a dry spell. In wet climates, high water tables may reduce
exposure of abandoned orebodies to oxidation and continually flush existing tailings and mine dumps. Although
metal-laden acid mine water does form, it is may be diluted to benign metal abundances within several hundred
meters of mixing with a higher order stream.
Geoenvironmental geophysics
Self potential detects electrical potentials produced by ongoing redox reactions. The method is suitable for locating
"hot spots" in tailings piles. Electromagnetic surveys are useful for tracing and monitoring metal-bearing ground
water. In addition, detailed magnetic data can help delineate geologic contacts, strata, and fractures that may act as
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Falconbridge Limited, Kidd Creek division metallurgical site, Timmons, Ontario, in Jambor, J.L, and
Blowes, D.W. eds., Short Course Handbook on Environmental Geochemistry of Sulfide Mine-wastes,
Mineralogical Association of Canada, v. 22, p. 333-364.
Alpers, C.N., and Nordstrom, D.K., 1991, Geochemical evolution of extremely acid mine waters at Iron Mountain,
California--Are there any lower limits to pH?, in Proceedings, Second International Conference on the
Abatement of Acidic Drainage: MEND (Mine Environmental Neutral Drainage), Ottawa, Canada, v. 2, p.
321-342.
Alpers, C.N., Nordstrom, D.K., and Thompson, J.M., 1994, Seasonal variations of Zn/Cu ratios in acid mine waters
from Iron Mountain, California, in Alpers, C.N., and Blowes, D.W., eds., Environmental geochemistry of
sulfide oxidation, ACS Symposium Series 550: Washington D.C., American Chemical Society, p. 324-344.
Blowes, D.W., and Jambor, J.L., 1990, The pore-water geochemistry and the mineralogy of the vadose zone of
sulfide tailings, Waite Amulet, Quebec, Canada: Applied Geochemistry, v. 5, p. 327-346.
Boorman, R.S., and Watson, D.M., 1976, Chemical processes in abandoned sulphide tailings dumps and
environmental implication for northeastern New Brunswick, Canadian Institute of Mining and Metallurgy
Bulletin, v. 69, no. 772, p. 86-96.
Cox, D.P., 1986, Descriptive model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 136.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Filipek, L.H., Nordstrom, D.K., and Ficklin, W.H., 1987, Interaction of acid mine drainage with waters and
sediments of West Squaw Creek in the West Shasta mining district, California: Environmental Science and
Technology, v. 21, no. 4, p. 388-396.
Fox, J.S., 1984, Besshi-type volcanogenic sulphide deposits--a review, Canadian Institute of Mining and Metallurgy
Bulletin, v. 77, no. 864, p. 57-68.
Franklin, J.M., 1993, Volcanic-associated massive sulphide deposits, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I.,
and Duke, J.M., eds., Mineral deposit modeling, Geological Association of Canada Special Paper 40, p. 315-
34.
Franklin, J.M., Sangster, D.M., and Lydon, J.W., 1981, Volcanic-associated massive sulfide deposits, in Skinner,
B.J., ed., Economic Geology 75th anniversary volume, p. 485-626.
Goldfarb, R.J., Nelson, S.W., Taylor, C.D., d'Angelo, W.M., and Meier, A.L., in press, Acid mine drainage
associated with volcanogenic massive sulfide deposits, Prince William Sound, Alaska, in Dumoulin, J.A.,
and Moore, T.E., eds., Geologic studies in Alaska by the U.S. Geological Survey, 1994: U.S. Geological
Survey Bulletin 2152.
Greens Creek Environmental Impact Statement, 1983.
143
Gulson, B.L., Mizon, K.J., Law, A.J., Korsch, M.J., Davis, J.J., and Howarth, D., 1994, Source and pathways of lead
in humans from the Broken Hill mining community--An alternative use of exploration methods: Economic
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Jambor, J.L., 1994, Mineralogy of sulfide-rich tailings and their oxidation products, in Jambor, J.L, and Blowes,
D.W., eds., Short Course Handbook on Environmental Geochemistry of Sulfide Mine-wastes: Mineralogical
Association of Canada, v. 22, p. 59-102.
Kelley, K.D., 1990, Interpretation of geochemical data from Admiralty Island, Alaska--Evidence for volcanogenic
massive sulfide mineralization, in Goldfarb, R.J., Nash, J.T., and Stoeser, J.W., eds., Geochemical studies
in Alaska by the U.S. Geological Survey, 1989, U.S. Geological Survey Bulletin 1950, p. A1-A9.
Kilburn, J.E., Sutley, S.J., and Whitney, G.C., 1995, Geochemistry and mineralogy of acid mine drainage at the
Holden mine, Chelan County, Washington:Explore, Association of Exploration Geochemists newsletter, no.
87, p. 9-14.
Kilburn, J.E., Whitney, G.C., d'Angelo, W.M., Fey, D.L., Hopkins, R.T., Meier, A.L., Motooka, J.M., Roushey,
B.H., and Sutley, S.J., 1994, Geochemical data and sample locality maps for stream sediment, heavy-
mineral-concentrate, mill tailing, water, and precipitate samples in and around the Holden mine, Chelan
County, Washington: U.S. Geological Survey Open-File Report 94-680A, 33 p.
Lydon, J.W., 1984, Volcanogenic massive sulphide deposits, Part 1--A descriptive model: Geoscience Canada, v. 11,
p. 195-202.
_________1988, Volcanogenic massive sulphide deposits, Part 2--Genetic models: Geoscience Canada, v. 15, p. 43-
65.
Morton, R.L., Hudak, G., Walker, J., and Franklin, J.M., 1990, Physical volcanology and hydrothermal alteration
of the Sturgeon Lake caldera complex, in Franklin, J.M., Schneiders, B.R., and Koopman, E.R., eds.,
Mineral Deposits in the western Superior province, Ontario: Geological Survey of Canada, Open File 2164,
p. 74-94.
Ohmoto, H., and Skinner, B.J., 1983, The Kuroko and related volcanogenic massive sulfide deposits: Economic
Geology Monograph 5, 604 p.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado: Implications for the prediction of mine-drainage chemistry: Proceedings, 1993 Mined
Land Reclamation Symposium, Billings, Montana, v. 1, p. 176-186.
Plumlee, G.S., Smith, K.S., and Ficklin, W.H., 1994, Geoenvironmental models of mineral deposits, and geology-
based mineral-environmental assessments of public lands: U.S. Geological Survey Open-File Report 94-203,
7 p.
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mineral resources in the Sitka quadrangle, southeastern Alaska, in Goldfarb, R.J., Nash, J.T., and Stoeser,
J.W., eds., Geochemical studies in Alaska by the U.S. Geological Survey, 1989, U.S. Geological Survey
Bulletin 1950, p. B1-B12.
Singer, D.A, 1986a, Descriptive model of Cyprus massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 131.
_________1986b, Descriptive model of Kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 189-190.
_________1986c, Grade and tonnage model of Cyprus massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 131-135.
_________1986d, Grade and tonnage model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 136-138.
Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage model of Kuroko massive sulfide, in Cox, D.P., and
Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190-197.
Slack, J.F., 1993, Descriptive and grade-tonnage models for Besshi-type massive sulfide deposits, in Kirkham, R.V.,
Sinclair, W.D., Thorpe, R.I., and Duke, J.M., eds., Mineral deposit modeling, Geological Association of
Canada Special Paper 40, p. 343-372.
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Taylor, C.D., Cieutat, B.A., and Miller, L.D., 1992, A followup geochemical survey of base metal anomalies in the
Ward Creek/Windfall Harbor and Gambier Bay areas, Admiralty Island S.E. Alaska, in Bradley, D., and
Dusel-Bacon, C., eds., 1991 Geologic Studies in Alaska: U.S. Geological Survey Bulletin 2041, p. 70-85.
144
BLACKBIRD CO-CU DEPOSITS
(MODEL 24d; Earhart, 1986)
by Karl V. Evans, J. Thomas Nash, William R. Miller, M. Dean Kleinkopf, and David L. Campbell
Deposit geology
These deposits are stratabound iron-, cobalt-, copper-, and arsenic-rich sulfide mineral accumulations in nearly
carbonate-free argillite/siltite couplets and quartzites with minor acid-consuming capacity; deposits have high acid-
generating capacity.
Examples
Associated deposit types (Cox and Singer, 1986) include Besshi massive sulfide deposits (Model 24b).
(1) Most ore is sulfide-mineral rich and contains almost no carbonate minerals. Host rocks are meta-clastic and have
limited acid-buffering capacity. Locally, minor buffering capacity is provided by siderite gangue.
(2) Very high potential for generation of acid (pH 2 to 3) drainage with thousands of mg/l copper and sulfate,
hundreds of mg/l cobalt and iron, tens of mg/l aluminum, hundreds of µg/l zinc, and tens of µg/l arsenic. After
mining operations have ended, sulfide minerals in waste rock and tailings piles may continue to oxidize and produce
(3) If fractured host-rock allows oxygenated water to infiltrate underground sulfide ore and old mine workings, acid
mine drainage generation may continue after mining is complete. Non-point source emission of acid water at surface
seeps is both a known (Baldwin and others, 1978; Reiser, 1986) and potential hazard.
(4) Potential downstream environmental effects of acid drainage can be significant in spatial extent, especially if
surrounding terrane is, as at Blackbird, dominantly quartzo-feldspathic meta-clastic strata and granite with low acid-
buffering capacity. Downstream water can be highly acidic and contain elevated concentrations of copper, cobalt,
(5) Environmental mitigation should focus on isolating sulfide-mineral-bearing rock, especially that in easily oxidized
waste-rock piles, from water and oxygen. Rerouting drainages around mine sites would reduce the quantity of water
Exploration geophysics
Various satellite and airborne multispectral remote sensing techniques can be used to identify alteration mineral
assemblages and stressed vegetation sites. Purdy and others (1986) made a preliminary evaluation of stressed
vegetation in the Blackbird mine area. They demonstrated that Englemann spruce and lodgepole pine growing in
soil enriched in cobalt and copper had higher spectral reflectance, as measured by a field-portable spectroradiometer,
than the same species growing in background areas. At the time of their study, they were unable to detect these
differences using Landsat Thematic Mapper data, probably because of noise from canopy density variations and
strong topographic control of tree distribution. More recently developed remote sensing technology may be able to
identify the stressed vegetation whose presence Purdy and others (1986) identified in the Blackbird area.
In the vicinity of the Idaho cobalt belt, aeromagnetic methods may be the most useful geophysical method
in regional exploration for cobalt-copper deposits. The Blackbird mine lies on the southwest flank of a prominent
magnetic trough that parallels the Idaho cobalt belt (Lund and others, 1990). This trough may indicate a structural
or lithologic zone that is relatively depleted of magnetite. However, the oxide zone (see below for discussion of this
zone) lies stratigraphically below the Blackbird mine, and is characterized by enrichments of copper, cobalt, and
magnetite. Connor (1991) used existing aeromagnetic data to suggest potential extensions of the oxide zone to the
southeast of his study area. However, Tertiary and lower Paleozoic plutons impose strong control on regional
aeromagnetic patterns (Lund and others, 1990), which indicates the need for care in making regional interpretations.
Detailed surveys tied to similarly detailed ground control are probably needed to better define a geophysical
exploration strategy for cobalt-copper deposits of the Blackbird type.
145
Figure 1. Schematic section of mafic rock-rich (biotitite-rich) sequences and associated Co-Cu lodes in the middle unit of the Yellowjacket
Formation in the Blackbird mine area. Abbreviations are for lodes and prospects (see Nash and Hahn, 1989, for key and further information).
From Nash and Hahn (1989).
References
Geology: Bennett (1977), Nash and Hahn (1989), Nash and Connor (1993), and Evans (in press).
Environmental geology, geochemistry: Baldwin and others (1978), Reiser (1986), McHugh and others (1987), and
Desborough (1994).
Deposit size
These deposits are of small to intermediate size. Blackbird mine has about 12 million tonnes of combined mining
and indicated reserves (Nash and Hahn, 1989). Available data (generally from unpublished sources) suggest that
other known deposits in the Idaho cobalt belt are smaller than Blackbird, although none of these deposits have been
Host rocks
Host rocks for these deposits are quartzo-feldspathic siltite-argillite couplets and quartzite intercalated with significant
proportions of biotitite (biotite-dominant rock) (fig. 1). Biotitite probably formed either as a volcanogenic mafic tuff
or as a chemical sediment.
The area surrounding the Idaho cobalt belt is primarily underlain by non-calcareous Middle Proterozoic quartzite,
siltite, and argillite, Middle Proterozoic granite, and Eocene Challis Volcanic Group rocks (Evans and Zartman, 1990;
146
Evans, in press). These rocks have limited acid mine drainage buffering capacity.
Wall-rock alteration
Alteration is stratabound and coextensive with ore (Nash and Hahn, 1989). The transition from altered to unaltered
rock is abrupt ( <1 m) in many places. Altered rock in the Merle ore zone (fig. 1) at Blackbird is coincident with
biotitite and intercalated rocks and contains elevated abundances of cobalt and arsenic. Alteration zoning consists
of pyrite-siderite-quartz-muscovite in the core zone and grades outward into quartz-muscovite-(with lesser) pyrite.
Potassic alteration has enhanced biotite crystallization across the entire ore zone. Acid mine drainage that encounters
siderite is somewhat buffered; finer grained siderite has greater buffering capacity. Highly acidic water draining the
Blackbird mine suggests that the buffering capacity of siderite is overwhelmed by acid-producing ore minerals.
Nature of ore
Deposits are closely associated with stratiform biotitites in the upper middle and lower upper stratigraphic units of
the Yellowjacket Formation (Evans, in press). Other related, but smaller deposits include cobaltiferous-pyrite with
variably abundant chalcopyrite that are associated with bedded magnetite in the lower Yellowjacket (oxide zone of
Nash, 1989 and Nash and Connor, 1993; Jackass zone of Evans, in press), and relatively minor cobalt-bearing
tourmaline breccias in the lower and middle units of the Yellowjacket (Modreski and Connor, 1991). Ore at
Blackbird is both massive and disseminated. Fine- to very fine grained cobaltite and coarse-grained chalcopyrite
dominate. Pyrite is erratically distributed at the deposit scale, but generally is present as coarse-grained crystals with
chalcopyrite. Fine-grained pyrrhotite is present as cores in some concentrically banded pyrite crystals, but is most
Blackbird mine: Co, Cu, Au, Fe, As, Bi, Cl, Cr, Mg, P, Sc, Ti, V, Y, Yb
Oxide zone: Fe, Cu, Sb, As, Au, Ba, Pb, Se, Zn, Co.
Complete analytical data for 372 samples of unoxidized drill core are available for several deposits in the
Blackbird mine area (Nash and others, 1988). The following are ranges defined by the 10th and 90th percentile
values and the means (in parentheses) for metals of particular interest. All values are in parts per million (ppm)
unless otherwise indicated; nd indicates mean was not calculated because of strongly skewed data.
Fe: 6.6-18 (11.0) weight percent; S: <0.1-3.8 (0.2) weight percent; CO2: <0.1-1.3 (0.2) weight percent; Ag: <1-1.5
(nd); As: 7-9,450 (130); Bi: <10-45 (nd); Cd: <1-<2 (nd); Co: 37-7,900 (180); Cr: 22-170 (46); Cu: 8-5,000 (250);
Mo: <1-5 (nd); Ni: 10-380 (33); Pb: 2-15 (2.8); Se: <0.1-19 (9); Th: 5-19 (11); Zn: 14-62 (27).
Ore in the Blackbird mine area and associated alteration zones generally have very high abundances of iron,
arsenic, cobalt, and copper; moderate to low sulfur in sulfide minerals; and relatively low to very low abundances
of carbonate, silver, cadmium, chromium, molybdenum, nickel, lead, selenium, and zinc compared to other base-
metal deposits.
Oxide zone deposits generally have abundances in the ranges suggested for deposits at Blackbird (Nash,
1989; Connor, 1991). Trace element environmental effects of oxide zone deposits are similar to those of Blackbird
mine rocks, primarily due to the presence of acid-generating chalcopyrite and cobaltiferous pyrite.
Potentially acid-generating minerals underlined. Blackbird mine (fig. 2): Major ore minerals-cobaltite, chalcopyrite,
pyrite, pyrrhotite, native gold. Less abundant ore minerals and gangue-siderite, biotite, garnet, chloritoid, tourmaline,
Distal deposits in Blackbird stratigraphic zone: Major ore minerals-pyrite, chalcopyrite, arsenopyrite, ±cobaltite.
Oxide zone: Major ore minerals-cobaltiferous pyrite, magnetite, hematite, chalcopyrite, arsenopyrite. Less abundant
Mineral characteristics
Blackbird mine: Cobaltite is present in very fine grained layers and thin stringers. Chalcopyrite is present in coarsely
crystalline stringers and aggregates commonly enveloping cobaltite. Pyrite is coarsely crystalline, has internal
Oxide zone: Pyrite is present in four forms: (1) fine-grained euhedral crystals interlaminated with argillite, (2) coarse-
grained crystals, which form beds and cross-cutting structures, that enclose euhedral pyrite, (3) anhedral pyrite, which
formed late in the paragenesis and encloses earlier crystals, and (4) corroded pyrite with a porous texture, probably
147
Figure 2. Schematic diagram of zoning in the Merle B lode, Blackbird mine. These longitudinal sections are essentially the same as plan maps
of the originally subhorizontal lode. A, Zones of thickest mafic rock (biotitite) and richest concentrations of Co, As, and Cu expressed as
feet/percent. B, Zones of gangue minerals pyrite (originally pyrrhotite), carbonate, quartz, and muscovite; biotite is present throughout the lode.
From Nash and Hahn (1989).
indicating original precipitation as pyrrhotite. Some pyrite contains 2 to 4.5 weight percent cobalt; bismuth, lead,
and zinc may also reside in the crystal lattice of pyrite. Chalcopyrite forms coarse-grained layers and cross-cutting
structures that commonly surround pyrite. Magnetite is present in trace to major amounts as small, euhedral grains
with very low cobalt content and <0.02 weight percent TiO2. Magnetite grains form beds, with no evidence of a
Secondary mineralogy
Secondary minerals include pickeringite, chalcanthite, malachite, azurite, limonite, and erythrite(?).
Topography, physiography
In the Blackbird region, topography is that of a deeply incised plateau with steep-walled canyons. Biotitite-rich strata
weather easily and even quartzo-feldspathic strata tend to produce rubble and talus rather than prominent outcrops.
Hydrology
Most precipitation near Blackbird falls during the Winter. Baldwin and others (1978) indicate a seven-year average
of about 124 cm of snow with a water content of about 38 cm. Streamflow peaks during Spring runoff (April
through June) and falls to very low levels in late Summer and early Autumn. Highly fractured Yellowjacket
Formation at Blackbird mine permits rapid correlated recharge and discharge of ground water. During the early part
of the Spring runoff, drainage through acid-producing mine workings and waste rock/tailings piles flushes large
quantities of soluble salts containing metals and acidity into surface drainages.
148
Figure 3. Ficklin plot (Plumlee and others, 1993; Smith and others, 1994) of pH versus the sum of dissolved base metals Zn, Cu, Cd, Co, Ni,
and Pb in mine and natural water draining waste rock in the Meadow Creek/Blackbird Creek and Bucktail Creek drainages at Blackbird mine.
Values shown are from water most closely associated with sources of acid mine drainage at Blackbird mine. Available data do not include
analyses for all base metals summed in the diagram; therefore data points are minimum values. Data from Baldwin and others (1978) and
McHugh and others (1987).
Underground mining at Blackbird was from drifts following lenticular and tabular orebodies using room and pillar
methods and block caving of overhead stopes. Sand recovered from the coarse fraction of the milling operation has
been used in limited backfilling. Remaining tailings have been impounded. Open-pit mining was restricted to late
ENVIRONMENTAL SIGNATURES
Drainage signatures
Mine-drainage data: Water draining the Blackbird mine and waste rock/tailings piles is highly acidic and contains
high to extreme dissolved metal concentrations (fig. 3), including thousands of mg/l copper and sulfate, hundreds
of mg/l cobalt and iron, tens of mg/l aluminum, hundreds of µg/l zinc, and tens of µg/l arsenic (Baldwin and others,
1978; Reiser, 1986; McHugh and others, 1987). In light of the arsenic-rich cobaltite (CoAsS)-bearing ore, dissolved
arsenic concentrations are less than might be expected, probably as a consequence of efficient arsenic adsorption by
hydrous iron oxide minerals in stream channels with low pH. If treatment of acid mine drainage at Blackbird
involves large-scale "liming" to raise pH, considerable amounts of arsenic may be desorbed and mobilized.
Natural drainage water: Geochemical data for the Special Mining Management Zone—Clear Creek is presented by
McHugh and others (1987); see Lund and others, 1983 for a description of the geology of this area. Available
studies of water chemistry for the Blackbird area have concentrated on streams draining mined areas and do not
provide adequate background values for anthropogenically undisturbed regions. McHugh and others (1987) collected
a few samples from drainages in mineralized but unmined areas; the results may indicate a very small copper-cobalt
anomaly. However, sampling was conducted in July, after Spring runoff, when anomalous abundances (if present)
are generally most pronounced. A study in which samples are collected throughout the annual hydrologic cycle,
including geographically distributed samples from mineralized and unmineralized parts of the Yellowjacket
Potentially economically recoverable elements: High abundances of dissolved copper and cobalt are potentially
Numerous studies indicate very high mobility of metals and acid water from solid mine waste primarily due to the
ready availability of iron sulfide minerals (Baldwin and others, 1978; Reiser, 1986; McHugh and others, 1987; and
sources referenced by these reports). Capillary action coupled with evaporation on waste piles during winter months
149
and during dry periods through the remainder of the year produce readily dissolved salts that contain metals and
produce acid. Flushing during the early part of Spring runoff and also during thunderstorms causes dissolution of
these salts; large metal load and acidity increases result from these flushing events (Baldwin and others, 1978; Farmer
and Richardson, 1980; Reiser, 1986). High metal content (especially copper) of streams draining the Blackbird mine
area have had a detrimental effect on anadromous fish populations (Reiser, 1986 and references cited therein).
A pioneering geochemical soil sample survey (Canney and others, 1953) clearly delineated areas with anomalous
cobalt and copper abundances and led to discovery of the Blacktail ore zone at Blackbird. Bennett (1977) also
conducted a geochemical soil sample survey at and adjacent to Blackbird. Geochemical data for stream sediment
samples are given by Bennett (1977) and Evans and others (1993); these data, including streams both affected and
unaffected by past mining, readily identify areas with anomalously high metal abundances, especially cobalt and
copper.
Sulfide-mineral-rich ore in nearly carbonate-free meta-clastic rock with limited acid-buffering capacity indicates very
high potential for spatially extensive acid drainage containing thousands of mg/l copper and sulfate, hundreds of mg/l
cobalt and iron, tens of mg/l aluminum, hundreds of µg/l zinc, and tens of µg/l arsenic. Fractured host-rock
facilitates interaction between oxygenated water and underground sulfide ore, thereby perpetuating acid mine
drainage.
Smelter signatures
High arsenic and sulfur ore contents are a major hazard. Current laws restrict smelting to specific sites designed to
mitigate arsenic hazards. Historic smelting involved simplistic nineteenth-century methods that relied on
volatilization of arsenic and sulfur, thus producing severe and widespread contamination. Recent proposals for
refining Blackbird-type ore have emphasized electrochemical methods; all methods are subject to stringent
environmental regulation.
Capillary action and evaporation, which lead to the formation of soluble secondary salts during the winter and dry
periods, result in early spring and thunderstorm-related runoff with high metal loads and acidity. Mineralogical work
is needed to detail information on the salts and their relation to the annual hydrologic cycle.
Geoenvironmental geophysics
Various electrical geophysical techniques can identify potential sources of acid mine generation and trace plumes of
contaminated water away from their sources; these techniques have not been systematically applied to Blackbird
deposits, however.
REFERENCES CITED
Baldwin, J.A., Ralston, D.R., and Trexler, B.D., 1978, Water resource problems related to mining in the Blackbird
mining district, Idaho: Completion Report for Supplements 35 and 48 to Cooperative Agreement 12-11-204-
11, USDA Forest Service; Moscow, College of Mines, University of Idaho, 232 p.
Bennett, E.H., 1977, Reconnaissance geology and geochemistry of the Blackbird Mountain-Panther Creek region,
Lemhi County, Idaho: Idaho Bureau of Mines and Geology Pamphlet No. 167, 108 p.
Canney, F.C., Hawkes, H.E., Richmond, G.M., and Vhay, J.S., 1953, A preliminary report of geochemical
investigations in the Blackbird district: U.S. Geological Survey Open-File Report no. 221, 20 p.
Connor, J.J., 1991, Some geochemical features of the Blackbird and Jackass zones of the Yellowjacket Formation
(Middle Proterozoic) in east-central Idaho: U.S. Geological Survey Open-File Report 91-0259, 25 p.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Desborough, G.A., 1994, Efficacy of heavy-metal capture by clinoptilolite-rich rocks from heavy-metal-polluted
water in five drainages in Colorado: U.S. Geological Survey Open-File Report 94-140, 25 p.
_________1995, Extraction of metals from raw clinoptilolite-rich rocks exposed to water in heavy-metal-polluted
drainages: U.S. Geological Survey Open-File Report 95-56, 30 p.
150
Earhart, R.L., 1986, Descriptive model of Blackbird Co-Cu sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 142.
Evans, K.V., in press, The Yellowjacket Formation of east-central Idaho, in Berg, R.B., ed., Proceedings of Belt
Symposium III: Montana Bureau of Mines and Geology Special Publication.
Evans, K.V., Lund, K., Fairfield, R., Hopkins, R.T., Roemer, T.A., and Sutley, S.J., 1993, Analyses of rocks and
stream sediment from the Special Mining Management Zone—Clear Creek, Lemhi County, Idaho: U.S.
Geological Survey Open-File Report 93-708, 48 p.
Evans, K.V., and Zartman, R.E., 1990, U-Th-Pb and Rb-Sr geochronology of Middle Proterozoic granite and augen
gneiss, Salmon River Mountains, east-central Idaho: Geological Society of America Bulletin, v. 102, p. 63-
73.
Farmer, E.F., and Richardson, B.Z., 1980, Relationship of the snowpack to acid mine drainage from a western
surface mine, in Jackson, C.L., and Schuster, M.A., eds., Proceedings: High-altitude revegetation workshop
no. 4, p. 79-100.
Lund, K., Alminas, H.V., Kleinkopf, M.D., Ehmann, W.J., and Bliss, J.D., 1990, Preliminary mineral resource
assessment of the Elk City 1°x2° quadrangle, Idaho and Montana: compilation of geologic, geochemical,
geophysical, and mineral deposits information: U.S. Geological Survey Open-File Report 89-0016, 118 p.
Lund, K., Evans, K.V., and Esparza, L.E., 1983, Mineral resource potential map of the Special Mining Management
Zone—Clear Creek, Lemhi County, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-
1576-A, scale 1:50,000.
McHugh, J.B., Tucker, R.E., and Ficklin, W.H., 1987, Analytical results for 46 water samples from a hydrogeochem
ical survey of the Blackbird mine area, Idaho: U.S. Geological Survey Open-File Report 87-260, 8 p.
Modreski, P.J., and Connor, J.J., 1991, Tourmalinite and iron-formation in the Yellowjacket Formation, Idaho Cobalt
Belt, Lemhi County, Idaho [abs.], in Good, E.E., Slack, J.F., and Kotra, R.K., eds., USGS research on
mineral resources—1991 Program and Abstracts: U.S. Geological Survey Circular 1062, p. 57.
Nash, J.T., 1989, Geology and geochemistry of synsedimentary cobaltiferous pyrite deposits, Iron Creek, Lemhi
County, Idaho: U.S. Geological Survey Bulletin 1882, 33 p.
Nash, J.T., Briggs, P., Bartel, A.J., and Brandt, E.L., 1988, Geochemical results for samples of ore and altered host
rocks, Blackbird mining district, Lemhi County, Idaho: U.S. Geological Survey Open-File Report 88-661,
71 p.
Nash, J.T., and Connor, J.J., 1993, Iron and chlorine as guides to stratiform Cu-Co-Au deposits, Idaho Cobalt Belt,
USA: Mineralium Deposita, v. 28, p. 99-106.
Nash, J.T., and Hahn, G.A., 1989, Stratabound Co-Cu deposits and mafic volcaniclastic rocks in the Blackbird
mining district, Lemhi County, Idaho, in Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C., and
Kirkham, R.V., eds., Sediment-hosted stratiform copper deposits: Geological Association of Canada Special
Paper 36, p. 339-356.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado: Implications for the prediction of mine-drainage chemistry: Proceedings, 1993 Mined
Land Reclamation Symposium, Billings, Montana, v. 1, p. 176-186.
Purdy, T.L., Milton, N.M., and Eiswerth, B.A., 1986, Spectral reflectance of vegetation in the Idaho Cobalt district-
Potential for exploration using remote sensing: U.S. Geological Survey Open-File Report 86-587, 9 p.
Reiser, D.W., 1986, Panther Creek, Idaho, habitat rehabilitation - final report: Bonneville Power Administration
Project No. 84-29, Contract No. DE-AC79-84BP17449, 479 p.
Smith, K.S., Plumlee, G.S., and Ficklin, W.H., 1994, Predicting water contamination from metal mines and mining
wastes: U.S. Geological Survey Open-File Report 94-264, 112 p.
151
CREEDE, COMSTOCK, AND SADO EPITHERMAL VEIN DEPOSITS
(MODELS 25b,c, and d; Mosier and others, 1986a,b, and c)
Examples
Creede-type: Creede, Silverton, Bonanza, Colo.; Worlds Fair, Ariz. Comstock-type: Comstock, Nev. Sado-type:
Sado, Japan.
Associated deposit types (Cox and Singer, 1986) include hot-spring Au-Ag (Model 25a); quartz alunite-epithermal
(Model 25e); and, if carbonate-bearing rocks are present, polymetallic replacement (Model 19a) deposits.
(1) Geoenvironmental signatures associated with different parts of epithermal vein deposits are highly variable on
all scales due to well developed spatial zonation within vein and alteration mineral assemblages.
(2) Pyrite contents of vein ore and carbonate contents of ore and wall rock principally determine the pH and metal
content of water draining mines, mine waste piles, and tailings associated with epithermal vein deposits. Veins that
contain abundant pyrite and base metal sulfide minerals, relative to carbonate minerals, and veins in rocks with low
acid-buffering capacity, such as those affected by silicification and argillic or advanced argillic alteration, have
enhanced potential for associated acidic drainage water that contains elevated abundances of dissolved iron,
aluminum, manganese, zinc, copper, and lead. Water draining pyrite- and carbonate-rich ore or ore hosted by
carbonate-bearing rocks tends to have near-neutral pH but elevated abundances of copper and zinc.
(3) Historically, gold-rich ore was processed by crushing and mercury amalgamation; soil and stream sediment
around historic mining and milling sites may be mercury contaminated. After the late 1800s or early 1900s,
amalgamation was less common in ore processing; instead, sulfide-mineral-rich ore was roasted and gold was
recovered by cyanidation. After milling, some of this ore was smelted. Soil in areas around roasting or smelting
sites may be contaminated by elevated abundances of lead, zinc, copper, arsenic, or antimony.
Mitigation and remediation strategies for potential environmental concerns presented above are described
in the section below entitled "Guidelines for mitigation and remediation."
Exploration geophysics
Geophysical expressions of precious-metal epithermal veins and stockworks have been reviewed by Allis (1990),
Irvine and Smith (1990), Klein and Bankey (1992), and Watson and Knepper (1994). Silicic and carbonate alteration
of volcanic rocks produces reflectance and thermal infrared contrasts and increases their resistivity and density; the
extent of associated anomalies can be delineated with detailed gravity, multispectral remote sensing (Arribas and
152
others, 1989; Collins, 1989), and direct current and electromagnetic (Nishikawa, 1992) surveys, respectively. Gravity
anomalies may be complicated by open fractures and brecciated rock, which reduce bulk density. Electrical and
gravity anomalies associated with epithermal deposits may be difficult to distinguish from those caused by small
intrusions or faults. Resistivity lows and reflectance contrasts associated with argillically altered rock can be
delineated by electrical mapping (Bisdorf, 1995) and multispectral remote sensing, respectively. The distribution of
clay zones and electrical chargeability associated with sulfide minerals can be mapped with induced polarization.
Airborne photography and side-looking radar can identify topographic features that may be related to resistant silicic
and carbonate zones and easily-weathered areas affected by argillic alteration. Areas that contain altered magnetite
can be outlined by magnetic surveys. Rock affected by potassic alteration can be identified by spectral gamma-ray
surveys. Regional gravity, aeromagnetic, and satellite or airborne remote sensing images may help identify linear,
circular, and intersecting features associated with calderas and faulted volcanic terranes in which additional detailed
References
Geology: Becker (1882), Steven and Ratté (1965), Buchanan (1981), Berger and Eimon (1983), Hayba and others
Environmental geology and geochemistry: Moran (1974), Plumlee and others (1993), Smith and others (1994).
Deposit size
The size of most deposits is small (10,000 tonnes) to moderate (several million tonnes). However, a few districts
are large to extremely large, including Casapalca, Peru, and Comstock, Nev. (10-20 million tonnes), and Pachuca-
Host rocks
Epithermal vein deposits are in intermediate to felsic volcanic rocks (andesite, dacite, quartz-latite, rhyodacite, and
rhyolite) and associated volcaniclastic and sedimentary rocks (for example, those deposited in volcanic depressions
Most epithermal vein deposits are areas of regional faulting within intermediate to felsic volcanic fields, including
volcanoes and caldera complexes from which volcanic rocks that host the deposits were erupted.
Wall-rock alteration
Wall rock alteration assemblages are characterized by strong vertical and lateral zonation between deep, central parts
of veins and shallow and (or) distal parts of veins (fig. 1). In many (but not all) districts, host volcanic rocks are
altered to propylitic assemblages, including chlorite, epidote, calcite, and pyrite, on a regional or district-wide basis;
this type of alteration is distal to most veins. In the central parts of districts alteration varies as a function of depth.
At deep levels, the alteration assemblage is characterized by quartz and chlorite ± some potassic alteration (adularia).
At intermediate levels the alteration assemblage consists of quartz; sericite; and illite, which may grade upward and
distally to lower-temperature smectite; ±zeolite minerals. At shallow levels, alteration is characterized by massive
silicification, formation of chalcedonic sinter, and pervasive acid-sulfate alteration, including alteration to kaolinite
and alunite. In laterally distal parts of districts, rock adjacent to veins may be locally silicified and (or) pyritized.
In some places, wall rock along upper parts of veins may be altered to illite, smectite, and alunite or kaolinite.
Nature of ore
Veins and stockwork veins fill fractures in intermediate to felsic volcanic rocks. Veins vary greatly in width, from
less than several cm to more than 3 m. Most veins display banded layers characterized by substantial mineralogic
differences; the veins can also be quite vuggy and include considerable open space.
The geochemistry of epithermal veins varies laterally and vertically. Altered wall rock adjacent to veins can have
Creede-type: Specific parts of these veins are characterized by elevated abundances of various elemental suites as
follows: Deep parts of central veins- Au, Cu, Pb, Zn, ±Ag, ±Te, ±Se. Intermediate parts of central veins- Pb, Zn,
153
Figure 1. Schematic cross section through an epithermal vein deposit showing distribution of vein and wall rock alteration minerals. Modeled
primarily after Creede-type veins; however, with the exception that base metal sulfide minerals are less abundant, the same general zoning patterns
are present in Comstock-type vein ore. Figure modified from Mosier and others (1986a) and Berger and Eimon (1983).
Ag, Cu, ±Mn. Intermediate to shallow parts of central and distal veins- As, Sb, Hg, ±Au, ±Mn, ±Se. Distal, deep
parts of veins- Ag, Pb, Zn, ±Cu, ±Ba, ±As, ±Sb, ±Mn, ±Se.
Sado and Comstock-type: Geochemical zonation within most Comstock-type veins is similar to that in Creede-type
veins, except that lead and zinc are less abundant throughout all parts of veins.
Minerals are listed in approximate decreasing order of abundance. Potentially acid-generating minerals are
underlined; those that are acid-generating when oxidized by aqueous ferric iron are denoted by *.
Creede-type veins are base-metal sulfide mineral rich. They contain abundant sphalerite*, galena*, chalcopyrite, and
pyrite; lesser amounts of many other sulfide and sulfosalt minerals, such as argentite*, tetrahedrite, pyrargyrite,
±marcasite, ±botryoidal pyrite; and variable but generally subordinate amounts of quartz, carbonate minerals
(including rhodochrosite, calcite, Mn-siderite), adularia, fluorite, manganese silicate minerals (such as pyroxmangite),
Comstock- and Sado-type veins are dominated by quartz and adularia and contain variable amounts of carbonate
minerals and generally subordinate amounts of sulfide minerals, including pyrite, sphalerite*, galena*, chalcopyrite,
and arsenopyrite.
All three vein types tend to be enriched in manganese; abundances of rhodochrosite or manganiferous calcite are
moderate to high, and manganese silicate minerals are also abundant. Well developed lateral and vertical vein
mineralogy zonation is typical within ore shoots, within veins, and across districts (figs. 1 and 2). This zonation
results from variations in the hydrologic and geochemical processes that prevailed during hydrothermal ore genesis.
Mineral characteristics
Textures: Mineral grains can vary from fine to coarse ( <1 mm to >10 cm), depending upon the particular district
154
N15°W S15°E
present
ground surface
250 m 10000'
3000 m
1000 ft
2750 m
9000'
Mine working
Vein Very high Botryoidal pyrite, ± sulfosalts, ± sphalerite,
intersection galena
Figure 2. Longitudinal section of the A vein, Bulldog Mountain vein system, Creede district, Colorado, showing lateral and vertical vein
mineral zoning patterns. The various mineralogic zones are ranked according to their potential to generate acid drainage water from mine
workings or mine waste. A poorly-welded ash-flow tuff occurs immediately above the botryoidal pyrite zone. Fracture permeability in
this ash flow tuff is very low, and so the botryoidal pyrite has largely escaped oxidation even though it occurs well above the present
water table, which is within the deep rhodochrosite-dominant zone. Ground water that does penetrate beneath the ash flow tuff becomes
highly acid, but evaporates, while still in the mine workings, and leaves behind melanterite and goslarite efflorescent salts. Figure modi
fied from Plumlee and Whitehouse-Veaux (1994).
and location within the district. Textures range from euhedral to botryoidal to massive. Crustification sequences
are often well developed; bands comprised of one mineral assemblage may be overgrown by one or more successive,
Trace element contents: Sphalerite can have low to high iron contents (<1 to >15 mol percent), several tenths to
1 mol percent cadmium, and minor amounts of other trace elements, including silver. Galena can contain silver; <1
mol percent in most cases. Botryoidal pyrite and marcasite can have very high concentrations (as much as 15 weight
percent, combined) of arsenic, antimony, silver, mercury, tellurium, selenium, and (or) gold. Carbonate minerals
commonly form extensive solid solutions that include manganiferous calcite, kutnahorite, and siderite.
General weathering rates: Euhedral to massive, coarse-grained, interlocking sulfide minerals weather at very slow
rates; samples of these crystals exposed on mine dumps in cool wet climates for >100 years can still appear fresh
and unweathered. Most botryoidal pyrite and marcasite, especially if enriched in arsenic, antimony, and other trace
elements, weather very rapidly, in some cases simply by atmospheric water vapor sorption. Rates at which carbonate
minerals weather are variable, but increase with decreasing grain size and increasing trace element content. For
example, iron-rich calcite and rhodochrosite weather more readily than equivalent minerals with low iron contents.
Weathering rates for minerals can be quite high in warm, humid climates.
Secondary mineralogy
Readily soluble minerals underlined. Minerals formed by weathering prior to mining include goethite, jarosite,
alunite, halloysite, anglesite, cerussite, smithsonite, manganese-oxide minerals (psilomelane, pyrolusite, braunite), and
cerargyrite. Minerals formed by weathering subsequent to mining are primarily soluble sulfate minerals indicative
of deposition from locally highly acidic water. Zinc sulfate minerals include goslarite. Iron sulfate minerals noted
in published reports include melanterite, although other sulfate minerals, such as copiapite, are also probably present
in pyrite-rich ore. Hydrous ferric oxide and iron hydroxysulfate minerals, such as ferrihydrite and schwertmanite,
precipitate from acidic to near-neutral mine- and natural- drainage water. Aluminum hydroxysulfate minerals,
including basaluminite and jurbanite, precipitate from water having pH between 4.5 and 5.
155
Figure 3. Ficklin plot showing variations in pH and sum of dissolved base metals Zn, Cu, Cd, Co, Ni, and Pb in mine and natural water draining
epithermal veins of various geologic characteristics.
Topography, physiography
Silicified zones and quartz-chalcedony veins tend to form topographic highs. Clay altered zones probably erode more
rapidly than adjacent less altered rocks, and form topographic lows.
Hydrology
Mine workings provide the most permeability for ground water flow. Faults, joints, and veins, which also focus
ground water flow, provide the greatest natural permeability. Flow along unmineralized fractures and vuggy, open-
pace veins with continuous permeability is greatest; flow along veins completely filled by ore and gangue is minimal.
Fracture permeability is also commonly reduced where fractures cross poorly welded ash flow tuffs, and in zones
of intense clay alteration; these rocks and altered zones are aquitards or barriers to ground water flow. For example,
epithermal ore at Creede is capped by poorly welded tuff that inhibited ascending hydrothermal fluids that formed
the deposit and presently inhibits descending oxidized ground water. Most of a zone of botryoidal pyrite that formed
directly beneath the cap rock is unoxidized even though it occurs well above the present-day water table; as a result,
acid drainage from the botryoidal pyrite zone is very limited, and evaporates prior to exiting mine workings
developed in the zone. Zones of hydrothermal brecciation can also be ground water conduits, provided sufficient
open space remains; if no open space is present, these breccia zones may impede ground water flow.
Historic: Most epithermal veins were mined in underground tunnels and stopes. However, open pits and glory holes
developed in some districts. Mineral processing typically involved milling, gravity separation of coarse precious
metals, mercury amalgamation to extract gold and silver, and flotation to extract lead, zinc, and copper. Sulfide
Modern: Due to the potential for o re dilution by wall rock, vein ore is still largely extracted by underground mining.
Deposits in which economic ore is disseminated through large volumes of near-surface rock, such as shallow
stockwork vein systems or mineralized sedimentary rocks exposed in caldera moats, can be mined using open pit
156
Figure 4. Plots showing concentrations of dissolved constituents in water draining epithermal veins of various geologic characteristics.
157
techniques. After the late 1800s to early 1900s, processing by amalgamation was increasingly replaced by
cyanidation; most sulfide-mineral-rich ore has been roasted prior to cyanidation. Advances in mineral processing
technology have recently led to a decline in roasting, however. Most modern processing involves milling, which is
followed by cyanidation and extraction of precious metals via carbon-in-pulp or electrowinning. Stopes are backfilled
with coarse tailings materials and fine tailings are stored in surface impoundments.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Mine-drainage data: Mine drainage data (figs. 3 and 4) pertinent to the Creede, Silverton, and Bonanza, Colo., and
Worlds Fair, Ariz., deposits are summarized from Moran (1974), Plumlee and others (1993), Smith and others
Mine water that drains underground workings in deposits that contain sphalerite, galena, and pyrite in ore
with low carbonate mineral contents tends to be acidic, pH ranges from 3 to 5, and contain elevated dissolved metal
abundances, including hundreds of mg/l iron, aluminum, and manganese; several to several tens of mg/l zinc and
copper; and as much as 1 mg/l lead. Water draining arsenic- and pyrite-rich ore can potentially contain several
hundreds of µg/l to several mg/l dissolved arsenic.
Water that drains pyrite-rich tailings and waste dumps can be quite acidic and contain elevated dissolved
metal abundances, including thousands of mg/l iron, aluminum, and manganese; tens to hundreds of mg/l zinc and
copper; and hundreds of µg/l to several mg/l lead, cadmium, arsenic, and other metals.
Mine water that drains underground workings in carbonate-rich ore, or ore in which water reacts with
propylitically altered rock, tends to be near-neutral, pH values range from 5.5 to 7, and contain as much as tens of
mg/l dissolved zinc and several mg/l dissolved copper, if ore is pyrite rich.
Mine water that drains underground workings in pyrite-rich veins hosted by propylitically altered rocks can
be highly acidic and contain high dissolved metal contents if water flows along veins and does not react with wall
rock carbonate minerals. As an example, in 1973, water in the Rawley drainage tunnel, which drains pyrite
sphalerite-galena veins in the Bonanza district, Colo., had a pH near 3.2 and contained high dissolved metal
abundances (Moran, 1974). Subsequently, the adit collapsed, which eliminated interaction between ore and
atmospheric oxygen and increased interaction between water and carbonate minerals in propylitically altered wall
rock. Consequently, the water now has pH values near 6 (Plumlee and others, 1993; Smith and others, 1994) and
contains significantly lower dissolved iron and aluminum contents; zinc and copper abundances are essentially
unchanged.
Mine water that drains a vein hosted by poorly-welded ash-flow tuff (Creede, Colo.) has a near-neutral pH
of 6.7 and contains low dissolved metal contents (Plumlee and others, 1993). The near-neutral pH may result from
interaction between water and fine-grained, devitrified glass in poorly welded tuff. Low metals contents may reflect
a lack of base metal sulfide minerals in the veins.
Manganese enrichments characteristic of epithermal vein ore cause mine drainage water to have elevated
manganese abundances relative to those of iron and aluminum.
Natural-drainage data: Water draining broad areas of propylitically altered rocks can have near-neutral pH and
relatively low dissolved abundances of aluminum, lead, arsenic, and copper. This water may have slightly elevated
dissolved abundances of some metals, including as much as several mg/l zinc, iron, and manganese. Water draining
sulfide-bearing fractures has significantly lower pH and contains correspondingly higher dissolved metal abundances.
Metals and acid are readily liberated from pyrite-rich mine waste and intermittently wet/dry mine workings due to
the rapid dissolution of soluble secondary salts. Secondary salt dissolution (and resulting acid and metal generation)
is much more rapid than acid consumption by carbonate minerals in dumps or surrounding mine workings. The
soluble salts form coatings on mine waste, and fracture fillings in rocks and coatings on mine workings above the
water table.
Storm water samples: No data available. However, vegetation kill zones downhill from pyrite-rich mine dumps
indicate that highly acidic, metal-rich water can be generated, particularly by secondary salt dissolution, in spite of
Elevated abundances of some metals, including Pb, Mn, Fe, ± Zn, Cu, As, Sb, Hg?, are probably present downslope
from vein outcroppings due to mechanical erosion of oxidized vein ore. Elevated abundances of elements
158
concentrated in oxidized vein ore, including Pb, Mn, Fe, ± Zn, Cu, As, Sb, Hg?, are probably dispersed into nearby
stream sediments.
Historically, gold-rich ore was processed using crushing and mercury amalgamation. Consequently, soil around
historic mining and milling sites may be contaminated with mercury. After the late 1800s or early 1900s, processing
by amalgamation became less common; sulfide-rich ore was roasted and treated by cyanidation. Roaster particulates
may potentially have contaminated soil and sediment in areas surrounding roasting sites with various metals,
Smelter signatures
Sulfide-rich ore was smelted in a number of historic districts. In districts with identified historic smelting activity,
soil may contain locally elevated abundances of lead, zinc, copper, ±arsenic, ±antimony, ±selenium, ±tellurium.
Currently available data mostly pertain to moderately wet, seasonally temperate climatic regimes of the Rocky
Mountains; limited data are available for the Worlds Fair, Ariz., district in a relatively hot, dry climate. Mine
drainage in the Worlds Fair district has significantly lower pH and higher metal contents than water draining
geologically similar veins in a cooler climate at Creede, Colo. Lower pH and higher metal contents may reflect
increased evaporation within mine workings, and periodic formation and flushing of soluble salts; however, more
data are needed to verify this speculation. No data are available on effects relating to evaporation of near-neutral
pH water draining carbonate-rich ore; evaporation of iron-poor water probably causes pH to increase. Mobilization
of arsenic, uranium, and possibly selenium (if present in ore) may be enhanced in dry climates if drainage water is
alkaline. Climate can strongly affect mineral weathering rates; rates are greatest in warm, humid climates.
Acid drainage from pyrite- and sulfide-rich veins may adversely affect ground water quality in either wet or dry
climates.
(1) The greatest potential for deleterious downstream environmental effects pertain to deposits that consist of pyrite-,
sphalerite-, galena-, (±chalcopyrite)-rich ore in carbonate-mineral poor veins; deposits in volcanic terranes minimally
affected by propylitic alteration, in which associated water has low acid-buffering capacity; and deposits in which
historic mining operations released significant volumes of fine-grained pyritic tailings, which have become part of
the sediment column, into rivers or streams. Oxidation of associated tailings can facilitate long-term metal and acid
releases, which result in water quality degradation. Downstream effects of acid drainage can be potentially extensive;
copper, zinc, manganese, and lesser cadmium can remain mobile for significant distances downstream.
(2) Less significant downstream effects are most likely to be associated with deposits that principally consist of
carbonate-bearing vein ore; veins in propylitically altered rock, which tend to have drainage water with near-neutral
pH values; and deposits in dry climates, where water evaporates or seeps underground. Zinc and manganese are the
principal elements that remain mobile, either in solution or as colloids (Kimball and others, 1995), for the greatest
distances downstream. Some arsenic, uranium, selenium, and molybdenum may be mobilized if drainage water is
alkaline.
(3) Zones that contain abundant botryoidal pyrite or marcasite have especially high acid drainage generation potential.
(4) Acidic, metal-rich water can develop in pyrite-rich tailings or waste dumps, even if carbonate minerals are present
because secondary salt dissolution, and resulting acid and metal generation, is much more rapid than acid
(5) Arsenic and antimony, to a lesser extent, may pose health risks in water draining ore that contains abundant
sulfosalt or sulfide minerals that contain elevated abundances of arsenic and antimony; arsenic and antimony
abundances are greatest in acid water, but can be moderately high in near-neutral water.
Careful documentation of district- and mine-scale mineral zoning patterns; rock, soil, and water chemistry; and
(1) Acid water can be remediated successfully using lime addition and sodium-bisulfide precipitation of metals; a
potentially acid-generating sludge is created. If drainage water is relatively iron-poor, such as most water having
near-neutral pH, the amount of particulates formed by liming may be insufficient to effectively sorb all zinc,
159
cadmium, and nickel. Lime addition to iron-rich drainage water may generate sufficient suspended particulates onto
which a major fraction of dissolved arsenic, lead, and copper can sorb, thereby reducing or eliminating need for
sodium bisulfide addition; the resulting particulate sludge is non-acid-generating as well. If ore is arsenic-, selenium-
or uranium-rich, liming to excessively high pH values may enhance mobility of these elements.
(2) The utility of carbonate-bearing, propylitically altered host rocks in acid drainage mitigation should be considered.
For example, acid water could be channeled, away from veins, through these rocks via artificial or natural fractures
(3) Constructed wetlands may be useful in mitigation of less acidic mine water.
(4) Careful mapping of fractures, which focus ground water flow, and poorly welded ash flow tuffs and clay
alteration zones, which restrict ground water flow, is essential for an adequate understanding of site hydrology.
(5) Isolation of pyrite-rich waste from weathering and formation of soluble secondary salts is crucial in eliminating
storm- and snowmelt-related pulses of acid and metals into surface water. High carbonate mineral content in dump
material is not sufficient to prevent acid pulses because soluble secondary salts generate acid and metals much faster
(6) In modern milling operations, pyritic parts of mill tailings should be used as underground backfill to prevent
Geoenvironmental geophysics
Geophysical applications to geoenvironmental investigations are reviewed by King and Pesowski (1993), Watson and
Knepper (1994), and Paterson (1995). An example high-resolution airborne multispectral imagery applied to a
geoenvironmental investigation is given by King (1995). Highly acidic and (or) metal-enriched ground water is
highly conductive and may produce vegetation stress; associated anomalies can be identified by electrical and
multispectral imaging methods, respectively. Surface water that contains suspended materials or has high
conductivity can also be identified on multispectral imagery. Electrically conductive clay aquitards formed during
mineralization or by weathering of ash-flow tuff can be delineated using electromagnetic and induced polarization/re
sistivity techniques. Distributions of electrically chargeable sulfide minerals can be mapped with induced
polarization. Shallow mine workings may be located by electrical, seismic refraction, and gravity surveys, and may
be identifiable on infrared thermal imagery. Low-resistivity fluids, whose flow is channeled by brecciated rock,
REFERENCES CITED
Allis, R.G., 1990, Geophysical anomalies over epithermal systems: Journal of Geochemical Exploration, v. 36, p.
339-374.
Arribas, Antonio, Jr., Rytuba, J.J., Rye, R.O., Cunningham, C.G., Podwysocki, M.H., Kelly, W.C., Arribas, Antonio,
Sr., McKee, E.H., and Smith, J.G., 1989, Preliminary study of the ore deposits and hydrothermal alteration
in the Rodalquilar caldera complex, southeastern Spain: U.S. Geological Survey Open-file Report 89-327,
39 p.
Becker, G.F., 1882, Geology of the Comstock Lode and the Washoe district: U.S. Geological Survey Monograph
3, 422 p.
Berger, B.R., and Eimon, P.I., 1983, Conceptual models of epithermal metal deposits in Shanks, W.C., ed., Cameron
Volume on Unconventional Mineral Deposits: Society of Mining Engineers, New York, p. 191-205.
Bisdorf, R.J., 1995, Correlation of electrical geophysical data with lithology and degree of alteration at the
Summitville mine site, in Summitville Forum '95: Colorado Geological Survey, Special Publication 38, p.
70-63.
Buchanan, L.J., 1981, Precious metal deposits associated with volcanic environments in the southwest, in Dickinson,
W.R., and Payne, W.D., eds., Relations of Tectonics to Ore Deposits in the Southern Cordillera: Arizona
Geological Society Digest, v. XIV, p. 237-261.
Collins, A.H., 1989, Thermal-infrared spectra of altered volcanic rocks in the Virginia Range, Nevada: Proceedings
of the 7th Thematic Conference on Remote Sensing for Exploration Geologists, October 2-6, 1989, Calgary,
Alberta, Canada, Environmental Research Institute of Michigan, Ann Arbor, Michigan, p. 1335-1340.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Foley, N.K., and Ayuso, R.A., 1993, Trace element and Pb isotopic geochemistry of Au-Ag vein assemblages and
volcanic wall rock, North Amethyst, Creede district, CO [Abs]: Abstracts with Programs, Geological Society
of America, v. 25, p. A-164.
160
Hayba, D.O., Bethke, P.M., Heald, P., and Foley, N.K., 1985, Geologic, mineralogic, and geochemical characteristics
of volcanic-hosted epithermal precious metal deposits, in Berger, B.R., and Bethke, P.M., eds., Geology and
Geochemistry of Epithermal Systems: Society of Economic Geologists Reviews in Economic Geology, v.
2, p. 129-168.
Heald, P., Foley, N.K., and Hayba, D.O., 1987, Comparative anatomy of volcanic-hosted epithermal deposits; acid-
sulfate and adularia-sericite types: Economic Geology, v. 82, p. 1-26.
Irvine, R.J. and Smith, M.J., 1990, Geophysical exploration for epithermal gold deposits: Journal of Geochemical
Exploration, v. 36, no. 1/3, p. 375-412.
Kimball, B.A., Callender, E., and Axtmann, E.A., 1995, Effects of colloids on metal transport in a river receiving
acid mine drainage, upper Arkansas River, Colorado, USA: Applied Geochemistry, v. 10, p. 285-306.
King, T.V.V., 1995, Environmental considerations of active and abandoned mine lands: U.S. Geological Survey
Bulletin 2220, 38 p.
King, A. and Pesowski, M., 1993, Environmental applications of surface airborne geophysics in mining: Bulletin of
the Canadian Institute of Mining and Metallurgy Bulletin, v. 86, no. 966, p. 58-67.
Klein, D.P., Bankey, V.L., 1992, Composite geophysical model for continental volcanic-hosted epithermal
mineralization: Cox and Singer mineral deposit models numbered 25b, 25c, 25d, and 25e: U.S. Geological
Survey Open-file Report 92-389, 15 p.
Moran, R.E., 1974, Trace element content of a stream affected by metal-mine drainage, Bonanza, Colorado: Austin,
University of Texas, Ph.D. thesis, 167 p.
Mosier, D.L., Berger, B.R., and Singer, D.A., 1986c, Descriptive model of Sado epithermal veins, in Cox, D.P., and
Singer, D.A., eds., Mineral Deposit Models: U.S. Geological Survey Bulletin 1693, p. 154.
Mosier, D.L., Sato, T., Page, N.J., Singer, D.A., and Berger, B.R., 1986a, Descriptive model of Creede epithermal
veins, in Cox, D.P., and Singer, D.A., eds., Mineral Deposit Models: U.S. Geological Survey Bulletin 1693,
p. 145-146.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986b, Descriptive model of Comstock epithermal veins, in Cox, D.P.,
and Singer, D.A., eds., Mineral Deposit Models: U.S. Geological Survey Bulletin 1693, p. 150.
Nishikawa, Nobuyasu, 1992, The use of electrical methods in recent exploration for epithermal gold deposits in
Japan: Exploration Geophysics, v. 23, no.1 and 2, p. 249-254.
Paterson, Norman, 1995, Application of geophysical methods to the detection and monitoring of acid mine drainage:
in Bell, R.S., Proceedings of the symposium on the application of geophysics to engineering and
environmental problems: Orlando, Florida, April 23-26, 1995, Environmental and Engineering Geophysical
Society, p. 181-189.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado: implications for the prediction of mine-drainage chemistry: Proceedings, 1993 Mined
Land Reclamation Symposium, Billings, Montana, v. 1, p. 176-186.
Plumlee, G.S., Smith, K.S., Montour, M., Mosier, E.L., Ficklin, W.H., in press, Geology-based models for the
prediction of mine drainage composition: Society of Economic Geology Reviews in Economic Geology, v.
6B.
Plumlee, G.S., and Whitehouse-Veaux, P.H., 1994, Mineralogy, paragenesis, and mineral zoning along the Bulldog
Mountain vein system, Creede District, Colorado: Economic Geology, Special Issue on Volcanic Centers
as Exploration Targets, v. 89, no. 8, p. 1883-1905.
Smith, K.S., Plumlee, G.S., and Ficklin, W.H., 1994, Predicting water contamination from metal mines and mining
waste: Notes, Workshop #2, International Land Reclamation and Mine Drainage Conference and Third
International Conference on the Abatement of Acidic Drainage: U.S. Geological Survey Open-file Report
94-264, 112 p.
Steven, T.A., and Ratte, J.C., 1965, Geology and structural control of ore deposition in the Creede district, San Juan
Mountains, Colorado: U.S. Geological Survey Professional Paper 487, 90 p.
Watson, Ken and Knepper, D.H., eds., 1994, Airborne remote sensing for geology and the environment - present and
future: U.S. Geological Survey Bulletin 1926, 43 p.
White, N.C., and Hedenquist, J.W., 1995, Epithermal gold deposits: styles, characteristics, and exploration: Society
of Economic Geologists Newsletter, October, 1995, n. 23.
161
EPITHERMAL QUARTZ-ALUNITE AU DEPOSITS
(MODEL 25e; Berger, 1986)
Deposit geology
Central advanced argillic zone has iron-, copper-, and arsenic-rich sulfide and sulfosalt minerals with high acid-
generating capacity; deposits consist of vuggy veins and breccias in highly acid-altered volcanic rocks with very low
acid-consuming capacity. Central advanced-argillic zone is flanked by argillic and distal propylitic zones with some
acid-buffering capacity, decreased copper and arsenic abundances, and increased zinc and lead abundances.
Examples
Summitville and Red Mountain Pass, Colo.; Goldfield, Nev.; Paradise Peak, Nev.; Julcani, Peru.
Associated deposit types (Cox and Singer, 1986) include porphyry copper (Model 17), porphyry gold-copper (Model
(1) Dominant mining activity is in sulfide-mineral-rich, strongly altered volcanic rocks with negligible acid-buffering
capacity.
(2) Very high potential for the generation of acid-mine drainage (pH 1.5 to 3) that contains thousands of mg/l iron
and aluminum; hundreds of mg/l copper and zinc (copper>zinc); hundreds of µg/l to tens of mg/l As, Co, Ni, Cr,
U, Th, rare earth elements; tens to hundreds of µg/l beryllium; and anomalous abundances of bismuth, antimony,
(3) Unoxidized sulfide minerals can persist in clay-rich alteration zones to within 10 m of ground surface. Exposure
of these sulfide minerals during mining can further enhance potential for acid-drainage generation.
(4) In temperate or seasonally wet climates, soluble secondary iron, aluminum, and copper sulfate minerals dissolve
during storm events and snowmelt, and lead to short term pulses of highly acidic, metal-bearing water from mine
sites. The sulfate salts form by evaporation of acid mine water above the water table in open pits and underground
(5) Potential downstream environmental effects of acid drainage can be significant in magnitude and spatial extent,
especially if surrounding terrane is composed primarily of volcanic rocks with low acid-buffering capacity. Dominant
downstream signatures include water having low pH, and high iron, aluminum, manganese, copper, and zinc
abundances.
(6) Amalgamation-extraction of gold carried out during historic operations may be a residual source of mercury.
(7) Smelter emissions at historic sites have elevated abundances of arsenic, copper, and zinc, and possibly other
(8) Highly oxidized deposits and (or) deposits located in arid climates probably have lower potential for acid mine
(9) Cyanide heap leach solutions are composed predominantly of copper-cyanide complexes and thiocyanate.
Mitigation and remediation strategies for potential environmental concerns presented above are described
in the section below entitled "Guidelines for mitigation and remediation."
Exploration geophysics
Resistivity studies can be used to help map features such as alteration zones. Potassium contained in alteration
alunite may be identified by gamma ray spectrometry. Alteration mineral assemblages and stressed vegetation can
also be identified using multispectral scanning remote sensing techniques such as AVIRIS.
References
Geology: Stoffregen (1987), Vikre (1989), Ashley (1990), John and others (1991), Deen and others (1994), and Gray
Environmental geology, geochemistry: Koyanagi and Panteleyev (1993), Gray and others (1994), Smith and others
162
Hot Spring
Mineralization Volcanic Dome
Reconstructed
➞
Hg Cu, As; Pb, Zn
➞
Surface
Acid Sulfate
Elev. (ft.) Alteration
12000
Clay alteration
11000
Weak propylitic
alteration
10000
Monzonite
Quartz-sericite Intrusion
9000 pyrite alteration
Figure 1. Simplified geologic cross section of an epithermal quartz-alunite Au deposit. Based on Summitville (Perkins and Neiman,
1982; Plumlee and others, 1995a) but modified to incorporate data from Julcani, Peru (Deen and others, 1994) and Paradise Peak, Nev.,
(John and 1991).
Deposit size
Deposit size is generally small (0.1 million tonnes) to intermediate (12 million tonnes).
Host rocks
These deposits are hosted by felsic volcanic rocks, generally intrusions or lava domes (fig. 1), that have low acid-
buffering capacity. Most of these volcanic rocks are part of composite stratovolcano complexes. Some ore may be
Surrounding geologic terrane is primarily volcanic but includes underlying sedimentary or crystalline rocks.
Wall-rock alteration
Wall-rock alteration reflects progressive wall-rock neutralization of highly acidic magmatic gas condensates; alteration
Intermediate-level advanced-argillic zone: Innermost vuggy silica, grading outward into quartz-alunite (±
pyrophyllite), quartz-kaolinite, and montmorillonite-illite-smectite alteration zones. Pyrite present in all zones.
Peripheral propylitic zone: Alteration of volcanic rocks to chlorite ± epidote ± pyrite ± calcite.
Nature of ore
In some deposits (e.g. Summitville) disseminated sulfide minerals are focused primarily in vuggy silica, quartz
lunite, and quartz-kaolinite zones; however, significant pyrite and other sulfide minerals also are present in clay
altered zones, as disseminations within breccia, and in altered wall-rock veinlets. See figure 2 for typical sulfide
mineral-sulfur content ranges for Summitville alteration zones. Although sulfide mineral-sulfur content is relatively
low (<5 percent), the alteration process effectively removes nearly all of the rock's capacity to buffer acid.
Ore at Paradise Peak contains elevated abundances of Au, Ag, Bi, Sb, Pb, Tl, Hg, S and Ba ± Sn, Mo, Te, and Se.
In addition to iron sulfide (mostly marcasite), sulfate (barite) and native sulfur were abundant throughout the deposit.
Metal abundances in peripheral, argillically altered rock are essentially unchanged except that oxidized parts are
163
Vuggy
Quartz- silica Quartz-
Propylitic alunite alunite Propylitic
Quartz- Quartz
kaolinite kaolinite
0
20
Argillic Argillic
40
Depth of
Approximate
oxidation 60
depth of
80 oxidation
(meters)
100
5
4
Unoxidized rock
Weight % 3
sulfide
in rock
2
Oxidized
1 vuggy silica,
qtz. alunite
rock
0
15 12 9 6 3 0 3 6 9 12 15
M f ili
Figure 2. Schematic alteration zoning away from original fractures at Summitville, showing approximate depth of oxidation (upper plot)
and range of oxidizable sulfide sulfur in sulfide minerals (lower plot). From Plumlee and others (1995a).
enriched in iron sulfide and unoxidized rocks contain gypsum and jarosite. Mineralized rock from the deepest part
of this system tended to have elevated abundances of arsenic and copper but not as elevated as those characteristic
Minerals listed in decreasing order of abundance. Potentially acid-generating minerals underlined. In epithermal
quartz-alunite gold deposits, ore deposition usually postdates development of argillic alteration.
Advanced argillic, argillic alteration zones: Pyrite, enargite, covellite, chalcocite, chalcopyrite, native sulfur,
marcasite, native gold, barite. Late barite, sphalerite, galena, ± siderite (Julcani), ± botryoidal pyrite.
Shallow, near surface: Silica sinter, cinnabar, native mercury?, native gold, pyrite, marcasite, realgar, orpiment.
Mineral characteristics
Textures: Sulfide minerals form fine- to medium-grained (<5 mm), euhedral crystals and masses of very fine-
grained, interlocking crystals. Some coarse-grained (as much as 4-5 cm) euhedral sulfide minerals also are present.
Trace element contents: Arsenic and antimony may be present in main-stage pyrite and (or) marcasite; late botryoidal
pyrite, where present, is typically strongly enriched in arsenic, antimony, and other trace elements. Abundant
stibnite-bismuthanite is the principal mineralogic site for antimony and bismuth in the Paradise Peak deposit.
General rates of weathering: Botryoidal, high trace elements > massive, fine >> coarse euhedral, low trace elements.
Secondary mineralogy
Supergene minerals: Scorodite, goethite, limonite, K- and Na-jarosite, phosphate minerals, and plumbojarosite.
Minerals formed by recent weathering: Jarosite (likely hydronium-enriched), chalcanthite, brochanthite, melanterite,
alunogen, halotrichite, and phosphate minerals?. These minerals form by evaporation of acid water during dry
periods, and then redissolve during wet periods. These minerals can also form by evaporation in overbank stream
164
Recharge through
advanced argillic
Discharge along
alteration zones
Elev. (m) dome-host rock
contact or fractures
4000
3500
3000
Figure 3. Inferred pre-mining hydrology at Summitville. Triangle marks position of water table.
Topography, physiography
Volcanic domes generally form topographic highs. Vuggy silica zones are resistant to weathering, and form
prominent knobs and pinnacles. Because vuggy silica zones are highly resistant to weathering, physical erosion
Hydrology
Pre-mining oxidation surfaces can help identify zones of high permeability within deposits. Ferricrete deposits mark
pre-mining ground water discharge points.Vuggy silica alteration zones have the highest primary permeability and
therefore focus ground water flow; most are oxidized to deep levels (100 meters) by pre-mining ground water. Clay
alteration zones have the lowest permeability and therefore inhibit ground water flow; most are oxidized to only
shallow levels (several meters to several tens of meters) by pre-mining ground water. Post-mineralization fractures
can serve as conduits for ground water flow. Rock contacts between volcanic domes and surrounding rocks can be
significant conduits for ground water flow and can also strongly influence distribution of alteration assemblages.
In the vicinity of these deposits, the water table generally conforms to topography; the highest elevations are
coincident with volcanic domes. At Summitville (fig. 3), ground water recharge is probably along vuggy silica
zones. Ground water discharge prior to underground mining (marked by extensive ferricrete deposits) was primarily
along contact between volcanic dome and host rocks, and at scarce locations where other fractures intersected the
topographic surface of the volcanic dome. In the area around the Paradise Peak deposits, the water table is locally
perched, which resulted in the presence of large blocks of unoxidized rock at shallow depths.
Historic: Underground mine workings followed vuggy silica alteration zones in most cases. Ore was processed using
Modern: Modern operations principally involve open-pit mining of vuggy silica and surrounding clay alteration zones
but include some underground mining. Ore is processed primarily using cyanide heap leach techniques.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Mine-drainage data (figs. 4 and 5): Summitville and Red Mountain Pass, Colo. (Plumlee and others, 1993; Plumlee
and others 1995a,b). Mine water draining ore hosted by advanced argillic altered rocks is highly acidic and contains
high to extreme dissolved metal abundances, including hundreds to several thousands of mg/l iron, aluminum, and
manganese; hundreds of mg/l zinc and copper; and hundreds of µg/l to several tens of mg/l As, Co, Ni, U, Th, Be,
and REE. Water draining these deposits has elevated arsenic abundances; concentrations of uranium relative to zinc
are unusual relative to those associated with many other deposit types. Preliminary data indicate that tellurium,
mercury, and tungsten, though potentially enriched in advanced argillic ore, do not appear to be enriched in mine-
165
Figure 4. Plot of pH versus the sum of dissolved base metals Zn, Cu, Cd, Co, Ni, and Pb in mine and natural water draining epithermal quartz
alunite Au deposits. Lines show inferred likely ranges of metal content and pH for specific alteration zones. Samples are water draining adits
and waste dumps, rain and snowmelt puddles, and seeps. Data from Koyanagi and Panteleyev (1993) and Plumlee and others (1995b).
Figure 5. Box plots showing ranges of selected dissolved constituents in mine water draining ore hosted by advanced argillic alteration zones
of epithermal quartz-alunite Au deposits. For each constituent, the box encloses samples falling between the 25th and 75th percentiles, the line
shows the range between the 10th and 90th percentiles, and the dots show actual concentrations for samples falling outside the 25th and 75th
percentiles.
drainage water. Limited data indicate that mine water draining argillic alteration zones has slightly higher pH and
Mine water draining shallow hot spring ore: No data available. The best currently available data are for water
draining advanced-argillic, native sulfur-rich parts of a hot spring sulfur deposit (Leviathan, Calif.), which is acidic
(pH 2-3) and has relatively low base metal contents but elevated abundances of arsenic, antimony, and thallium (Ball
Natural-drainage data: Limited data for a relatively wet climate with high dilution rates (British Columbia) suggest
low acidity (pH between 2 and 3) and elevated dissolved metal abundances, including hundreds of mg/l iron and
aluminum; abundances of copper and zinc, tens to hundreds of µg/l, are lower than those measured in mine
drainages.
Potentially economically recoverable elements: High copper abundances in drainage water could be economically
extracted.
Metals and acid are readily liberated from sulfide-mineral-bearing mine wastes due to oxidation of sulfide minerals,
mainly pyrite. During dry periods, secondary soluble salts form by evaporation. During wet periods, the soluble
166
salts are rapidly dissolved. These salts can be present as coatings on rock material. Metal and acid are probably
not liberated in significant amounts from mine wastes associated with deposits oxidized extensively prior to mining.
Storm water samples: The pH and metal contents of water in rain and snowmelt puddles, which contain dissolved
soluble salts, is generally similar to that of water draining adits and waste dumps (Plumlee and others, 1995b).
Water-rock leaching: The results of a few leaching experiments with advanced argillic waste rock material from
Summitville (50 g sample in 1 liter of distilled water, Plumlee and others, 1995b) show that metal concentrations
and pH values of leach water rapidly (within tens of minutes) approach those of water draining adits and mine
dumps.
Mercury amalgamation of ore during historic mining may provide a source of mercury contamination not directly
Cyanide geochemistry: Heap leach and other cyanide processing solutions are likely to include copper-cyanide
complexes (containing as much as several hundred mg/l copper), with lesser zinc and silver cyanide complexes
(present as weak cyanide complexes with as much as several tens of mg/l contained metals), and strong gold-cyanide
complexes. Arsenic, cobalt, nickel, and iron may be present at low mg/l abundances in cyanide heap leach solutions.
Thiocyanate (SCN-) abundances may be quite high in ore containing unoxidized sulfide minerals. At Summitville,
degradation of cyanide accidentally released into the environment may have been enhanced by mixing with acid-mine
drainage and the resulting breakdown of copper-cyanide complexes; thiocyanate likely did not degrade rapidly.
Smelter signatures
Epithermal quartz-alunite gold ore from which copper and silver were extracted during historic mining were probably
smelted. No data have been identified concerning the mineralogy or chemical composition of soil affected by
emissions from smelters that processed epithermal quartz-alunite gold ore. The closest analogue is Butte, Mont.,
where enargite-chalcocite-bornite ore from cordilleran lode deposits were smelted. There, soil proximal to smelters
Potential downstream environmental effects of acid drainage in moderately wet to moderately dry climates can be
significant in magnitude and spatial extent, especially if the surrounding geologic terrane is primarily composed of
volcanic rocks with low acid buffering capacity. Predominant downstream signatures include elevated abundances
of acid, iron, aluminum, manganese, and copper. Iron and aluminum form hydrous oxide precipitates as a result of
dilution by downstream tributaries, and help sorb some of the dissolved metals. However, if water remains
sufficiently acidic (due to limited dilution by downstream tributaries or acid generation resulting from precipitation
of hydrous oxide minerals), manganese, copper, and zinc can persist (at abundances of hundreds µg/l, or more) in
solution well downstream from mine sites (Smith and others, 1995).
In very wet climates, dilution may significantly reduce downstream effects. In dry or seasonally wet and dry
climates, off-site drainage is greatest during short-term storm events or longer-term wet periods; reactions
between this water and surrounding alkaline sediment (caliche) and soil, and with alkaline water draining the
167
sediment and soil, probably help mitigate acid drainage. Downstream storm water evaporation, however, may lead
to the formation of acid- and metal-bearing salts that can themselves generate off-site acid drainage during storms
(1) Acid drainage can be successfully remediated using lime addition and sodium-bisulfide precipitation of metals
(which produce acid-generating sludge). Lime addition to iron-rich drainage water may generate sufficient suspended
particulates onto which a major fraction of dissolved arsenic, lead, and copper can sorb, thereby reducing or
eliminating the need for sodium-bisulfide addition; in addition, wastes are non-acid-generating.
(2) Isolation of unoxidized sulfide minerals and soluble secondary salts from oxidation and dissolution is crucial to
(3) Surrounding carbonate-bearing rocks, including carbonate sedimentary rocks or carbonate-bearing propylitically
altered rock on the fringes of deposits, should be carefully considered for their utility in acid-mine drainage
mitigation. For example, acid water could be channelled through underground fracture systems in propylitic rock
(4) Cyanide heap-leach solutions should be treated by peroxide addition or other standard techniques. Heap-leach
(5) Mixing cyanide heap-leach solutions with acid drainage may effectively neutralize both. Acid in drainage water
breaks down copper-cyanide complexes, forming volatile free cyanide and copper-iron-cyanide particulates, which
degrade photolytically. Because of the alkalinity of heap-leach solutions, iron in acidic drainage precipitates as
particulates, which then effectively sorb other metals contributed from acid drainage and heap-leach solutions.
Geoenvironmental geophysics
Resistivity studies can be used to identify rocks saturated with metal-bearing ground water. Porous rocks that can
focus ground water flow can be identified by microgravity studies. Heat generated by sulfide mineral oxidation may
have an associated thermal anomaly measurable by borehole logging or shallow probes; measures of excess heat flux
can provide an approximation of total acid generation potential. Induced polarization methods can provide qualitative
estimates of sulfide mineral percentages and grain size. The position, volume and mineralogy of clay bodies can
REFERENCES CITED
Ashley, R.P., 1990, The Goldfield gold district, Esmeralda and Nye Counties, Nevada, in Shawe, D.R., Ashley, R.P.,
and Carter, L.M.H., eds., Geology and resources of gold in the United States: U.S. Geological Survey
Bulletin 1857-H, p. H1-H7.
Ball, J.W., and Nordstrom, D.K., 1989, Final revised analyses of major and trace elements from acid mine waters
in the Leviathan Mine drainage basin, California and Nevada, October 1981 to October 1982: Water-
Resources Investigations, Report No. WRI 89-4138, 46 p.
Berger, B.R., 1986, Descriptive model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 158.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Deen, J.A., Rye, R.O., Munoz, J.L., and Drexler, J.W., 1994, The magmatic hydrothermal system at Julcani, Peru
vidence from fluid inclusions and hydrogen and oxygen isotopes: Economic Geology, v. 89, p. 1924-1938.
Gray, J.E., and Coolbaugh, M.F., 1994, Summitville, Colorado-Geologic framework and geochemistry of an
epithermal acid-sulfate deposit formed in a volcanic dome: Economic Geology, v. 89, p. 1906-1923.
Gray, J.E., Coolbaugh, M.F., Plumlee, G.S., and Atkinson, W.W., 1994, Environmental geology of the Summitville
Mine, Colorado: Economic Geology, v. 89, p. 2006-2014.
John, D.A., Nash, J.T., Clark, C.W., and Wulftange, W., 1991, Geology, hydrothermal alteration, and mineralization
at the Paradise Peak gold-silver-mercury deposit, Nye County, Nevada, in Raines, G.L., Lisle, R.E., Schafer,
R.W., and Wilkinson, W.H., eds., Geology and ore deposits of the Great Basin, Symposium proceedings:
Reno, Geological Society of Nevada and U.S. Geological Survey, p. 1020-1050.
Koyanagi, V.M., and Panteleyev, Andre, 1993, Natural acid-drainage in the Mount McIntosh/Pemberton Hills area,
northern Vancouver Island (92L/12), in Grant, B., and others, eds.: Geological fieldwork 1992; a summary
of field activities and current research, Ministry of Energy, Mines and Petroleum Resources, Report No.
1993-1, p. 445-450.
168
Perkins, M., and Nieman, G.W., 1982, Epithermal gold mineralization in the South Mountain volcanic dome,
Summitville, CO: Denver Region Exploration Geologists Symposium on the genesis of Rocky Mountain
ore deposits: Changes with time and tectonics, Denver, Colorado, Nov. 4-5, 1982, Proceedings, p. 165-171.
Plumlee, G.S., Gray, J.E., Roeber, M.M., Jr., Coolbaugh, M., Flohr, M., Whitney, G., 1995a, The importance of
geology in understanding and remediating environmental problems at Summitville, in Posey, H.H.,
Pendleton, J.A., and Van Zyl, D., eds.: Proceedings, Summitville Forum '95, Colorado Geological Survey
Special Publication #38, p. 13-22.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado-Implications for the prediction of mine-drainage chemistry: Proceedings, Mined Land
Reclamation Symposium, Billings, Montana, p. 176-186.
Plumlee, G.S., Smith, K.S., Mosier, E.L., Ficklin, W.H., Montour, M., Briggs, P.H., and Meier, A.L., 1995b,
Geochemical processes controlling acid-drainage generation and cyanide degradation at Summitville, in
Posey, H.H., Pendleton, J.A., and Van Zyl, D., eds.: Proceedings, Summitville Forum '95, Colorado
Geological Survey Special Publication #38, p. 23-34.
Smith, K.S., Plumlee, G.S., and Ficklin, W.H., 1994, Predicting water contamination from metal mines and mining
wastes: Notes, Workshop #2, International Land Reclamation and Mine Drainage Conference and Third
International Conference on the Abatement of Acidic Drainage: U.S. Geological Survey Open-File Report
94-264, 112 p.
Smith, K.S., Mosier, E.L., Montour, M.R., Plumlee, G.S., Ficklin, W.H., Briggs, P.H., and Meier, A.L., 1995, Yearly
and seasonal variations in acidity and metal content of irrigation waters from the Alamosa River, Colorado,
in Posey, H.H., Pendleton, J.A., and Van Zyl, D., eds.: Proceedings, Summitville Forum '95, Colorado
Geological Survey Special Publication #38, p. 293-298.
Stoffregen, R.E., 1987, Genesis of acid-sulfate alteration and Au-Cu-Ag mineralization at Summitville, Colorado:
Economic Geology, v. 82, p. 1575-1591.
Vikre, P.G., 1989, Ledge formation at the Sandstone and Kendall gold mines, Goldfield, Nevada: Economic
Geology, v. 84, p. 2115-2138.
169
EPITHERMAL MN DEPOSITS
(MODEL 25g; Mosier, 1986a)
Deposit geology
Manganese oxide concentrations are present as small veins, stringers, nodular masses, breccia-fillings, and coatings
of drusy cavities in fractured, argillized or silicified, Tertiary volcanic rocks of continental origin that range in
Examples
Associated deposit types (Cox and Singer, 1986) include other epithermal gold-silver veins (Models 25b, 25c, 25d,
25e); hot spring gold-silver (Model 25a); barite veins, fluorite veins.
If associated with iron, elevated abundances of soluble manganese in potable water supplies may stain plumbing
fixtures and laundry, and cause a foul odor or taste in water. If present at sufficient abundances, other metals,
including tens of thousands of mg/l iron, trace to thousands of mg/l lead, trace to thousands of mg/l zinc, and trace
to thousands of mg/l copper, that may be present in manganese ore (Wilson and Rocha, 1948; Hewett, 1964), may
Exploration geophysics
Geophysical signatures for this particular deposit type have not been investigated. The lack of sulfide minerals
References
Geology: Wilson and Rocha (1948), Hewett (1964), and Hariya (1980).
Environmental geology, geochemistry: Crerar and others (1980), Wedepohl (1980), and Abukhudair and Farooq
(1989).
Deposit size
Deposits are small; known deposits can be as large as 1.4 million metric tons of ore containing 464,000 metric tons
of manganese (Red Hill, N. Mex.); the median deposit is 0.025 million metric tons at a median grade of 30 percent
Figure 1. Schematic cross-section of a typical epithermal manganese vein in faults showing extent of argillic or silicic alteration. Modified from
Wilson and Rocha (1948) showing relations at Talamantes district, Mexico.
170
Host rocks
Host rocks include rhyolitic, dacitic, andesitic, or basaltic flows, breccias, tuffs, and agglomerates. In most cases,
manganese is more concentrated in and more rapidly leached from mafic volcanic rocks; chemical weathering causes
mafic volcanic rocks to decompose more rapidly than felsic volcanic rocks.
Surrounding terrane is primarily volcanic; basement rocks may be sedimentary, metamorphic, and (or) plutonic.
Wall-rock alteration
Argillic alteration assemblages, especially those that include kaolinite, are dominant. Silicic alteration assemblages
may extend over hundreds of meters along elongate zones; long axes of these altered zones may extend as much as
Nature of ore
Ore consists of manganese oxide minerals in open-space fillings in faults, fractures, and breccias, as coatings on drusy
Data for samples from the Optimo claim, Luis Lopez district, N. Mex., indicate that copper, zinc, molybdenum,
tungsten, and thallium contents decrease outward in manganese crusts, whereas antimony and lead increase outward;
arsenic abundances are unchanged (Hewett, 1964). Iron, barium, strontium, and phosphorous may also be present.
Characteristic ore assemblages include psilomelane and (or) pyrolusite in calcite and quartz gangue. Ore assemblages
may also include braunite, wad (hydrated manganese oxide mineral intergrowths), manganite, cryptomelane,
hollandite, coronadite, ramsdellite, manganocalcite, chalcedony, opal, cristobalite, K-feldspar, barite, fluorite, gypsum,
anhydrite, hematite, limonite, or zeolite. Fluorite and barite typically are present near manganese veins. Hewett
(1964) observed that within a given district fluorite and barite usually extend to greater depths than manganese veins.
Mineral characteristics
Crusts of manganese oxide minerals in veins are persistently layered with psilomelane, hollandite, cryptomelane or
coronadite. Psilomelane is the most widespread and abundant oxide mineral and is typically massive, sooty, and
black. Hollandite, coronadite, and cryptomelane are present as distinct fibrous layers, which may be as much as 3
mm thick. Psilomelane, hollandite, and cryptomelane may be present in botryoidal forms. Less common pyrolusite
Secondary mineralogy
Some manganese oxide minerals, limonite, and kaolinite may be supergene alteration products.
Topography, physiography
Most known epithermal manganese deposits are in well drained areas of moderate to high relief. Silicified rock
Hydrology
Faults, fractures, and rock contacts are possible ground water flow channels. Underground workings down to depths
of a few hundred meters also constitute water conduits. Argillic zones in adjacent altered rocks may impede or
restrict water flow, whereas the highly permeable nature of some volcanic host rocks may focus flow. Enhanced
permeability caused by alteration-related feldspar removal may also enhance ground water flow.
Deposits are mined in open cuts, shafts, and adits. Mining is generally shallow, usually less than 200-m depths, and
individual veins are followed for less than 1,000 m in most districts. Ore is usually concentrated by hand sorting
or by jig to grades as much as 40 weight percent manganese. Concentrated ore is shipped to off-site plants.
171
ENVIRONMENTAL SIGNATURES
Drainage signatures
No data specifically pertaining to epithermal manganese deposits is available. More studies concerning geochemical
signatures around manganese epithermal veins are required. Water and mine waste sampling and analysis of metals
from many sites need to be conducted before quantified findings can be presented.
Manganese oxide minerals have low solubility and normally do not represent an environmental concern. No data
specific to metal mobility from epithermal manganese mine wastes are available.
Manganese is commonly present as a constituent of silicate and other minerals, as an adsorbed component on clays
and humates, as colloidal organic complexes, and (or) as a constituent of most soil, in which it is present as oxide
or hydroxide minerals (Crerar and others, 1980). No data specific to the geochemistry of soil near epithermal
Because low grade ore is usually concentrated by hand or jig to high grades, stockpiles and waste piles of low grade
material, potential sources of materials with elevated quantities of manganese or other environmentally deleterious
elements, may be present at mine sites or at storage facilities. Manganese-enriched dust, which may pose health
Smelter signatures
No data available.
Because most epithermal manganese deposits contain low sulfide mineral abundances and because manganese oxide
minerals are relatively insoluble, climatic variation probably does not significantly affect geoenvironmental signatures
Geoenvironmental geophysics
Metal-bearing ground water plumes may be traceable using geoelectric methods, including the ground slingram.
Plumes containing 1,000 mg/L (or more) total dissolved metals can be detected using this method. Heavy-metal
contaminated plumes are usually more conducting than ordinary ground water, particularly in the eastern and northern
United States, where most ground water contains <1,000 mg/L dissolved metals. Slingrams such as the Geonics EM-
31 or EM-34 give readings of greater than about 25 mS/m within 5 m of metal-charged plumes. In ideal
circumstances, contaminant plumes can be rapidly outlined by sequential traverses across their edges. Silicic zones
near manganese veins are characterized by very low conductivity because conductivity values decrease with
REFERENCES CITED
Abukhudair, Mohammad Y., and Farooq, Shaukat, 1989, Kenetics of ozonation of iron (II) and manganese (II) in
a pure water system: Journal of Environmental Sciences and Health, v. 24, no. 4, pt. A, p. 389-407.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Crerar, D.A., Cormick, R.K., and Barnes, H.L., 1980, Geochemistry of manganese: an overview, in Varentsov, I.M.
and Grasselly, Gy., eds., Geology and geochemistry of manganese, v. 1, Stuttgart: E. Schweizerbart'sche
Verlagsbuchhandlung, p. 293-334.
Hariya, Yu, 1980, On the geochemistry and formation of manganese dioxide deposits in Varentsov, I.M., and
Graselly, Gy., eds., Geology and geochemistry of manganese, v. 1, Stuttgart, E. Schweizerbartsche
Verlagsbuchhandlung, p. 293-334.
Hewett, D.F., 1964, Veins of hypogene manganese oxide minerals in the southwestern United States: Economic
Geology, v. 59, no. 8, p. 1429-1472.
Mosier, D.L., 1986a, Descriptive model of epithermal Mn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 165.
172
_________1986b, Grade and tonnage model of epithermal Mn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 166-167.
Wedepohl, K.H., 1980, Geochemistry behavior of manganese, in Varentsov, I.M. and Grasselly, Gy., eds., Geology
and geochemistry of manganese, v. 1, Stuttgart: E. Schweizerbart'sche Verlagsbuchhandlung, p. 335-351.
Wilson, I.F., and Rocha, V.S., 1948, Manganese deposits of the Talamantes district near Parral, Chihuahua, Mexico:
U.S. Geological Survey Bulletin 954-E, p. 181-208.
173
RHYOLITE-HOSTED SN DEPOSITS
(MODEL 25h; Reed and others, 1986)
Examples
"Mexican-type tin deposits", for example, those in the states of Durango and Zacatecas; Black Range tin district,
Sierra and Catron Counties, N. Mex.; one locality in Thomas Range, Juab County, Utah.
No associated deposit types (Cox and Singer, 1986) are known, but geochemistry suggests that rhyolite hosting these
deposits may be the extrusive equivalent of igneous rocks that host Climax-molybdenum type deposits (Model 16).
Figure 1. Diagrammatic cross section of rhyolite-hosted Sn deposit showing relationship of cassiterite concentrations to rhyolite dome (from
Reed and others, 1986).
174
Potential environmental considerations
(1) Placer mining of known natural cassiterite concentrations in the United States and Mexico is essentially complete.
Known deposits are not economically viable at the present time; in addition, small tonnages of these deposits
probably preclude future economic viability. Similarly, associated low tonnage lode deposits are even less likely to
be exploited. Environmental impact associated with past mining activities in the United States is limited. Acid-
(2) Most old placer workings have generally been stabilized following mining activities; these present little or no
environmental concern.
(3) The solubility of tin in surface water is very low; consequently it has virtually no impact on the environment.
(4) Arsenic-bearing minerals (for instance, durangite, beudandtite, and hidalgoite) associated with lode, rhyolite-
hosted tin deposits are rare and volumetrically minor; incorporated arsenic is present in relatively insoluble mineral
phases. Thus, arsenic associated with these deposits is of limited environmental concern.
Exploration geophysics
The high radioelement content of most host rhyolites can be identified using gamma-ray methods; similarly low
thermal inertia related to elevated glass content can be used to identify these rhyolites. Resistivity studies can be
used to identify argillic alteration zones. Most alteration zones associated with tin-bearing rhyolite are areas of
intense vapor-phase activity. Vapor-phase altered rocks have local argillic alteration overprints. Multispectral remote
References
Geology: Maxwell and others (1986), Duffield and others (1990), and Rye and others (1990).
Mineralogy: Foord and others (1985), Foord and others (1989), and Foord and others (1991).
Deposit size
Tin-placer deposits associated with rhyolite lava domes are very small (less than 100 tons). Associated vein-type
deposits are also small; some veins may be as much as 30 to 40 cm wide and extend 100 m or more. Zones of
reticulate veinlets may be several thousand square meters in size. In these zones, veinlets may be spaced every few
tens of centimeters to several meters and much of the intervening rock is altered and mineralized.
Host rocks
These deposits are in high-silica, metaluminous to peraluminous, rhyolite flow domes and associated pyroclastic
rocks.
Rhyolite lava domes with associated tin deposits are present in large volcanic fields, as much as several thousand
Wall-rock alteration
Host rocks to rhyolite-hosted tin deposits experience vapor-phase alteration that locally results in argillic assemblages;
Nature of ore
Cassiterite is present as well-formed crystals in high-temperature lithophysal and miarolitic cavities, and in some
small veinlets. This type of cassiterite, deposited by vapor phase processes, is characterized by anomalous contents
of iron, antimony, and titanium. Low temperature, fluid-dominated, veins and veinlets contain microcrystalline
cassiterite that pseudomorphs hematite. Cassiterite deposited at low temperatures and under fluid-dominated
conditions is wood tin that contains minor to major amounts of indium, arsenic, zinc, silicon, and lead as well as
Rhyolite-hosted tin deposits contain elevated abundances of Nb, Th, U, Sb, Li, As, Zn, Pb, and rare earth elements
(fig. 2).
175
Figure 2. Diagram showing relative ratios of depletion or enrichment of elements in Taylor Creek Rhyolite compared to concentration in an
average granite (Krauskopf, 1979). ag=average granite, n=average amount in Oligocene Taylor Creek Rhyolite.
Type 1 deposits: cassiterite, hematite, pseudobrookite, bixbyite, braunite, topaz, quartz, cristobalite, tridymite and
sanidine.
Type 2 deposits: hematite, cassiterite, quartz, fluorite, sanidine, heulandite, stilbite, chabazite, maxwellite,
squawcreekite, tilasite, gasparite-(Ce), chernovite-(Y), calcite, beryl, "chernovite-(Ce)", cerianite, and others.
Type 3 deposits: quartz, hematite, sanidine, cristobalite, tridymite, cassiterite (crystalline and wood tin), durangite,
beudantite, vanadinite, stolzite, calcite, hidalgoite, jarosite , alunite, opal, fluorite, smectite, cryptomelane, and
todorokite.
Mineral characteristics
Cassiterite forms individual and intergrown euhedral crystals to several mm across. It is also intergrown with
hematite, which it sometimes replaces. Wood tin is present in masses, as large as 4 kg, characterized by a banded
176
Secondary mineralogy
Only secondary arsenate and arsenate-phosphate minerals have potentially deleterious environmental effects. Because
sulfide minerals are very scarce in rhyolite-hosted tin deposits, secondary iron and manganese oxide minerals, usually
Topography, physiography
All known rhyolite-hosted tin deposits lie in mountainous terrane with as much as several hundred meters of relief.
Placer deposits are located on relatively gently sloping rhyolite domes and associated ash-flow tuff deposits. In a
tropical climate, rhyolite-hosted tin deposits may have negative relief because of the rapid decomposition of rhyolite
Hydrology
Known deposits are located in a desert environment, where rainfall and consequent surface water runoff are limited.
Cassiterite-bearing placers associated with tin-bearing rhyolite have been mined by hand. Primitive gravity separation
ENVIRONMENTAL SIGNATURES
Virtually nil. Low abundances of arsenic have been detected in water draining type 3 deposits. The climate in all
known areas that contain rhyolite-hosted tin deposits is arid. Thus, potentially hazardous elements are little
mobilized. Flash floods capable of significant placer deposit redistribution and widespread detrital cassiterite
Drainage signatures
Drainage basins in the principal United States and Mexican areas that contain rhyolite-hosted tin deposits contain
sediment enriched in hematite, bixbyite, pseudobrookite, and cassiterite. Although specific data are lacking, rhyolite
that hosts tin deposits contains elevated fluorine contents; accessory fluorite and topaz are the principal residences
of fluorine in these rocks. Under some conditions, fluorine may be leached from these minerals leading to elevated
Metal mobility from these deposits is limited because they contain very minor quantities of sulfide minerals whose
Smelter signatures
Smelting tin ore derived from rhyolite-hosted deposits could produce anomalous amounts of indium, arsenic,
177
Climate effects on environmental signatures
No data; however, because cassiterite is resistant to mechanical and chemical degradation and because the sulfide
mineral content of rhyolite-hosted tin deposits is low, deposits of this type located in different climate regimes
Geoenvironmental geophysics
Acid water related to jarosite- or alunite-enriched areas that may be associated with rhyolite-hosted tin deposits can
be identified by ground or airborne geophysical surveys. Surficial acid water produces color anomalies on airborne
or satellite imagery. Subsurface acid water can produce low resistivity anomalies identifiable from resistivity
surveys. Surfaces characterized by enhanced radioactivity, such as might be present in waste piles associated with
REFERENCES CITED
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Detra, D.E., Arbogast, B.F., and Duttweiler, K.A., 1986, Analytical results and sample locality map of stream-
sediment, heavy-mineral-concentrate, and rock samples from the southern Wah Wah Mountains, Utah: U.S.
Geological Survey Open-File Report 86-161, 60 p.
Duffield, W.A., Reed, B.L, and Richter, D.R., 1990, Origin of rhyolite-hosted tin-mineralization: evidence from the
Taylor Creek rhyolite, New Mexico; Economic Geology, v. 85, p. 392-398.
Duttweiler, K.A., and Griffits, W.R., 1989, Geology and geochemistry of the Broken Ridge area, southern Wah Wah
Mountains, Iron County, Utah: U.S. Geological Survey Bulletin 1843, 32 p.
Foord, E.E., Hlava, P.F., Fitzpatrick, J.J., Erd, R.C., and Hinton, R.W., 1991, Maxwellite and squawcreekite, two
new minerals from the Black Range Tin District, Catron County, New Mexico, U.S.A., Neues Jahrbuch
Miner. Mh., H.8, p. 363-384.
Foord, E.E., Maxwell, C.H., and Hlava, P.F., 1989, Mineralogy of the Black Range Tin District, Sierra and Catron
Counties, New Mexico (abs.): New Mexico Geology, v. 11, no. 2, 39-40.
Foord, E.E., Oakman, M.R., and Maxwell, C.H., 1985, Durangite from the Black Range, New Mexico, and new data
for durangite from Durango, and Cornwall: Canadian Mineralogist, v. 23, p. 241-246.
Krauskopf, K.B., 1979, Introduction to Geochemistry (2nd edition): New York, McGraw-Hill, 721 p.
Maxwell, C.H., Foord, E.E., Oakman, M.R., and Harvey, D.B., 1986, Tin deposits in the Black Range Tin District;
New Mexico Geological Society Guidebook, 37th Field Conference, Truth or Consequences, p. 273-281.
Reed, B.L., Duffield, Wendell, Ludington, S.D., Maxwell, C.H., and Richter, D.H., 1986, Descriptive model of
rhyolite-hosted Sn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey
Bulletin 1693, p. 168.
Rye, R.O., Lufkin, J.L., and Wasserman, M.D., 1990, Genesis of the rhyolite-hosted tin occurrences in the Black
Range, New Mexico, as indicated by stable isotope studies; Geological Society of America Special Paper
246, p. 233-250.
178
LOW-TI IRON OXIDE CU-U-AU-REE DEPOSITS
(MODELS 25i and 29b; Cox, 1986a,b)
Deposit geology
These deposits consist of low-titanium iron ore that contains variable amounts of copper, uranium, gold, and rare
earth elements. In the past, these deposits have been separated into two discrete subgroups. Cox (1986a,b), for
instance, refers to the subgroup consisting primarily of iron oxide minerals with minor apatite as volcanic-hosted
magnetite deposits (Model 25i), whereas the subgroup that contains significant amounts of copper-sulfide minerals
and uranium are termed Olympic Dam-type deposits (Model 29b). However, Hitzman and others (1992) have shown
that both subgroups share common genetic traits and can be merged as a distinct deposit type; the geoenvironmental
Specific characteristics of this deposit type vary among deposits, but all deposits are characterized by
magnetite and (or) hematite concentrations with distinctly low titanium contents (generally <2 weight percent TiO2);
in addition, most contain some copper sulfide minerals. Most deposits display evidence, including fluid-inclusions,
replacement textures, coherently zoned alteration patterns, and alteration-associated veins, of a hydrothermal origin.
However, the genesis of some deposits is ambiguous, and a variety of alternative origins, including magmatic
emplacement involving immiscible iron-rich melts, syngenetic exhalations, and metamorphic dehydration, have been
proposed.
The deposits range from conformable to crosscutting and may be found as parts of ore systems that extend
more than 3 to 5 km in the vertical dimension. Data show that deeper parts of some of these ore deposits formed
from hot (400 to >600° C), magmatic ( 18O ~+8‰) fluids; at more shallow levels, these deposits formed from cooler
(200-400°C), mixed magmatic-meteoritic fluids ( 18O~ +1‰) (Gow and others, 1994). Magnetite deposition and
sodium-rich alteration zones predominate in the deeper (hotter) parts of these systems, whereas hematite,
copper-sulfide minerals, and potassium-rich alteration are more prevalent in their shallower parts (fig. 1). Deposits
are in rocks of widely variable ages, but are most abundant in Proterozoic (1.8-1.1 Ga) rocks.
Examples
Examples include the giant Olympic Dam uranium-copper-gold-silver-bearing deposit (Australia), the giant Bayan
Obo rare earth element-niobium deposit (China), numerous iron deposits in the Kiruna district (Sweden), and iron
Figure 1. Generalized cross section of an idealized low-titanium iron oxide copper-uranium-gold-rare earth element deposit. Modified from
Hitzman and others (1992).
179
deposits in Proterozoic rocks of the St. Francois Mountains (southeast Mo.) and in New York and New Jersey and
in the Grenville province of New York and New Jersey. Examples in Missouri include Pea Ridge, Pilot Knob, Iron
Mountain and Kratz Spring; those in New York include the Benson mines, Clifton, and Jayville; New Jersey deposits
include Edison, Mt. Hope, Hibernia, Scrub Oaks, and the Hurd mine.
(2) Some deposits are extremely large and development may involve ground disturbance throughout large areas.
(3) Uranium-rich deposits may produce radon or create problems associated with elevated radiation levels.
(4) Elevated fluorine abundances in drainage water may cause health problems, including brittle bones and tooth
discoloration.
Exploration geophysics
Magnetite-rich systems can be readily recognized by aeromagnetic or ground magnetic surveys although surrounding
country rocks that contain significant amounts of magnetite may complicate interpretation (for example, Kiruna area).
Magnetite-dominated deposits in Missouri, for example, are associated with high-amplitude positive magnetic
anomalies (Kisvarsanyi, 1981). Hematite-rich systems may have little magnetic expression. However, because both
magnetite and hematite enriched rocks are much more dense than host rocks, they may be detectable by gravity
surveys. Large deposits may be located on major lineaments that can be detected on satellite imagery. The
unexposed Olympic Dam deposit, for example, was partly discovered because of its association with coincident
positive gravity and magnetic anomalies and location on a major photolineament (Hoover and Cordell, 1992).
Deposits that have high sulfide mineral content can be detected by electrical methods. Alteration zones associated
with many deposits have been recrystallized during metamorphism so that they are not readily identifiable by
geophysical methods.
References
Leonard and Buddington (1964), Buddington (1966), Hauck (1990), Oreskes and Einaudi (1990), Hitzman and others
Deposit size
Deposits range from small to very large, but many representatives are small (less than 1 million tons). Most
economic iron deposits range between 3.5 and 450 million tons, with a median value of 40 million tons. The giant
Olympic Dam deposit exceeds 200 million tons. Kiruna is one of the World's eight largest iron ore producers.
Host rocks
Host rocks vary widely and may include virtually any rock type; however, most host rocks are felsic volcanic or
plutonic rocks.
Most deposits are in moderate to high grade metamorphosed felsic volcanic or plutonic rocks. Typically, these rocks
are associated with either a continental volcanic arc (Andean volcanic setting) or anorogenic felsic magmatism that
accompanied post-tectonic extensional collapse of orogens. For example, Kiruna ore is typically associated with
alkalic rhyolite, trachyte, and (or) trachyandesite. Most deposits in New York are associated with granitic gneiss
principally composed of quartz and either albite or microcline. Deposits in New Jersey are associated with a series
of predominantly monzonite or quartz monzonite intrusions. Most deposits in Missouri are associated with granite
porphyry or iron-rich trachyte. Most of these rocks have very limited acid-buffering capacity. However, other host
rock types, including carbonate rocks, that have high buffering capacity may be present.
Wall-rock alteration
Wall-rock alteration is commonly intense. Spatial alteration zonation includes, from bottom up, sodic, potassic, and
180
sericitic zones (fig. 1). Specific alteration assemblages depend on host rock type but commonly consist of:
Potassic alteration- magnetite ± hematite + apatite + potassium feldspar ± sericite ± albite ± chlorite ± biotite ±
Nature of ore
Most ore consists of either magnetite, hematite, or mixtures thereof. Less commonly, ore, including that at Olympic
Dam and Bayan Obo, is rich in combinations of copper sulfide minerals, uranium, and rare earth elements. Ore may
be either stratiform or discordant. Magnetite-rich deposits are more commonly concordant, whereas hematite-rich
deposits are commonly discordant. Magnetite-rich ore is generally equigranular. Where copper sulfide minerals are
present, chalcopyrite is dominant. At Olympic Dam, however, large amounts of chalcocite and bornite are present
in addition to chalcopyrite. Pyrite, in veins and disseminated in ore, is generally present in minor amounts. Uranium
is in uraninite, pitchblende, and brannerite. Anomalous abundances of the rare earth elements are present in almost
all deposits. At Bayan Obo, they constitute as much as 6.1 weight percent of the ore and are principally in
bastnaesite, florencite, monazite, and xenotime. Other common rare earth element-bearing phases are apatite and
fluorite.
In addition to copper, uranium, and the rare earth elements, which may be important ore constituents, deposits may
also contain anomalous concentrations of Ba, P, F, Cl, Mn, B, K, and Na. Some deposits also have elevated Au,
Minerals listed in general decreasing order of abundance (potentially acid-generating minerals underlined).
Deep: Ore- magnetite, apatite, minor chalcopyrite, and pyrite. Wall rock- albite, actinolite, and chlorite.
Shallow: Ore- hematite, magnetite, copper sulfide minerals (principally chalcopyrite, possibly bornite and chalcocite),
apatite, pyrite, monazite, bastnaesite, florencite, xenotime, uraninite, brannerite, and pitchblende. Wall rock-
potassium feldspar, sericite, chlorite, actinolite, barite, carbonate, epidote, and biotite.
Mineral characteristics
Sulfide minerals are present in some magnetite-rich deposits, but are more common in hematite-bearing ore. Sulfide
minerals are usually a minor component of most deposits and are predominantly copper or iron sulfide minerals.
Secondary mineralogy
Secondary minerals are generally not significant. Some sulfide-mineral-rich deposits have associated porphyry
copper-style supergene enrichment in which secondary chalcocite has been derived from primary chalcopyrite.
Topography, physiography
Low-titanium iron oxide deposits have neither characteristic topographic nor physiographic expressions.
Hydrology
Magnetite-rich ore appears to have low porosity and permeability. Hematite-rich ore is probably more permeable
and tends to form discordant bodies. No general known association exists between deposit setting and position of
These deposits have been and are being mined both by underground methods and by open pitting. Active
underground examples include deposits in the St. Francois Mountains, Mo., in the Kiruna district (Sweden), and at
Olympic Dam (Australia). Ore at Bayan Obo (China) is open pit mined.
Subsequent to mining, most ore at Kiruna undergoes primary crushing, is processed to produce a high grade
magnetite concentrate, and then pelletized. The Olympic Dam deposit produces copper, uranium, gold, and silver
181
by first processing ore to produce copper flotation concentrates. An acid leach is used to extract uranium from both
concentrates and gangue; the concentrates are further processed by copper smelting and electrowinning to obtain
ENVIRONMENTAL SIGNATURES
Drainage signatures
Information on the composition of pre- and post-mining drainage from these deposits appears to be very limited.
Published environmental reports on the Olympic Dam deposit are principally concerned with water use and aquifer
depletion. A report by Smith (1988) notes that water downstream from the Pea Ridge and Pilot Knob, Mo., mines
has increased turbidity, that the abundance of fine-grained material in the streambed is increased, and that the
benthic-invertebrate population is reduced. However, the study also notes that data are insufficient to directly link
The general scarcity of reactive (sulfide) minerals suggests that most of these deposits have relatively limited
acid generation capacity. However, elevated concentrations of copper and slight pH reduction may result from
deposits having moderate sulfide mineral abundances; sulfide-mineral-rich systems, such as Olympic Dam, are
probably capable of generating very low pH water and mobilizing significant amounts of base metals. The
concentrations of uranium, radium, and radon in water are a concern because these elements are known human
carcinogens. Their abundances are partly correlated with their bedrock concentrations. As a result, elevated
abundances of these elements may be associated with some deposits. Elevated fluorine concentrations in water,
which are known to damage bone and cause tooth discoloration, may be present in water associated with some
low-titanium iron deposits; however, no evidence indicates that such elevated abundances are actually present.
Studies documenting the mobility of metals from solid mine waste have not been identified. Application of general,
process-related studies (Ficklin and others, 1992; Plumlee and others, 1992; Plumlee and others, 1993) suggests the
following:
(1) Because deposits are in host rocks having widely different acid buffering capacity and because deposits contain
significantly variable sulfide mineral abundances, acid generation and metal mobility may vary greatly.
(2) Metal discharge from deposits hosted by high-acid-buffering-capacity rocks (carbonate rocks) is likely to be
minimal, especially given the generally low sulfide mineral content of most ore. Acid and metal discharge from
(3) Most deposits contain sparse copper and iron sulfide minerals and thus have a relatively low acid generation
capacity. However, ferric iron in hematite, which is commonly associated with sulfide minerals, may facilitate acid
(4) Copper sulfide- and hematite-rich deposits (such as Olympic Dam) may have significant acid generation capacity.
Although no data on the chemistry of water draining from the Olympic Dam deposit has been identified, metal
concentrations may be similar to those characteristic of drainage water associated with porphyry copper deposits.
(5) Water that contains high concentrations of uranium is known to be derived, in part, from uranium-enriched
country rock. Therefore, deposits with elevated concentrations of uranium may generate uranium-rich water which,
(6) Some geologic environments generate fluorine-rich water that can damage human bones and cause tooth
discoloration. Fluorine-rich water may develop in association with some deposits of this type.
The geochemical signatures of soil associated with these deposits apparently has not been documented. These
deposits are characterized by elevated P, S, F, CO2, Cu, Mo, V, Co, Na, and K abundances. Some or all of these
elements may be enriched in associated soil. Titanium, chromium, and nickel abundances are depleted in most of
these deposits.
Iron ore is generally beneficiated (gravity, magnetic, flotation, or selective flocculation) before treatment in a blast
furnace. Rare earth elements and uranium are generally physically concentrated and then processed by a variety of
chemical techniques. At the Olympic Dam, Australia, deposit, tailings contain elevated metal abundances, including
0.15 to 0.2 weight percent copper and 250 to 300 ppm U3O8, which may be a source of drainage contamination.
182
Smelter signatures
Copper sulfide minerals are typically smelted and may produce SO2-rich and metal-rich emissions, which may
The effects of various climate regimes on the geoenvironmental signature specific to low-titanium iron oxide
copper-uranium-gold-rare earth element deposits are not known. However, in most cases the intensity of
environmental impact associated with sulfide-mineral-bearing mineral deposits is greater in wet climates than in dry
climates. Acidity and total metal concentrations in mine drainage in arid environments are several orders of
magnitude greater than in more temperate climates because of the concentrating effects of mine effluent evaporation
and the resulting "storage" of metals and acidity in highly soluble metal-sulfate-salt minerals. However, minimal
surface water flow in these areas inhibits generation of significant volumes of highly acidic, metal-enriched drainage.
Concentrated release of these stored contaminants to local watersheds may be initiated by precipitation following a
dry spell.
Geoenvironmental geophysics
Acid or metal-bearing water, fluid movement, and active sulfide mineral oxidation can all be identified using
electrical methods. Associated geochemical signatures that involve rare earth elements or radioactive minerals can
REFERENCES CITED
Buddington, A.F., 1966, The Precambrian magnetite deposits of New York and New Jersey: Economic Geology, v.
61, p. 484-510.
Cox, D.P., 1986a, Descriptive model of volcanic-hosted magnetite, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 172.
_________1986b, Descriptive model of Olympic Dam Cu-U-Au, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 200.
Ficklin, W.H., Plumlee, G.S., Smith, K.S., and McHugh, J.B., 1992, Geochemical classification of mine drainages
and natural drainages in mineralized areas: Proceedings, 7th International Water-Rock Interaction
Conference, Park City, Utah, 1992, p. 381-384.
Gow, P.A., Wall, V.J., Oliver, N.S., and Valenta, R.K., 1994, Proterozoic iron oxide (Cu-U-Au-REE) deposits:
Further evidence of hydrothermal origins: Geology, v. 22, p. 633-636.
Hauck, Steven A., 1990, Petrogenesis and tectonic setting of middle Proterozoic iron oxide-rich ore deposits-an ore
deposit model for Olympic Dam-type mineralization: U.S. Geological Survey Bulletin, 1932, p. 4-39.
Hitzman, M.W., Oreskes, N., and Einaudi, M.T., 1992, Geological characteristics and tectonic setting of Proterozoic
iron oxide (Cu-U-Au-REE) deposits: Precambrian Research, v. 58, p. 241-287.
Hoover, D.B. and Cordell, L.E., 1992, Geophysical model of Olympic Dam in Hoover, D.B., Heran, W.D., and Hill,
P.L., eds., The geophysical expression of selected mineral deposit models: U.S. Geological Survey Open
File Report 92-557, p. 112-114.
Kisvarsanyi, E.B., 1981, Kiruna-type iron-apatite-(copper) deposits, in Pratt, W.P., ed., Metallic mineral-resource
potential of the Rolla 1° x2° quadrangle, Missouri, as appraised in September 1980: U.S. Geological Survey
Open-File Report 81-518, p. 26-34.
Leonard, B.F., and Buddington, A.F., 1964, Ore deposits of the St. Lawrence County magnetite district, northwest
Adirondacks, New York: U.S. Geological Survey Professional Paper 377, 259 p.
Oreskes, N., and Einaudi, M.T., 1990, Origin of rare earth element-enriched hematite breccias at the Olympic Dam
Cu-U-Au-Ag deposit, Roxby Downs, South Australia: Economic Geology, v. 85, p. 1-28.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., and Briggs, P.H., 1992, Geological and geochemical controls on the
composition of mine drainages and natural drainages in mineralized areas: Proceedings, 7th International
Water-Rock Interaction Conference, Park City, Utah, 1992, p. 419-422.
Plumlee, G.S., Smith, K.S., Ficklin, W.H., Briggs, P.H., and McHugh, J.B., 1993, Empirical studies of diverse mine
drainages in Colorado-Implications for the prediction of mine-drainage chemistry: Proceedings, Mined Land
Reclamation Symposium, Billings, Montana, p. 176-186.
Smith B.J., 1988, Assessment of water quality in non-coal mining areas of Missouri: U.S. Geological Survey
Water-Resources Investigations Report 87-4286, 50 p.
183
SEDIMENT-HOSTED AU DEPOSITS
(MODEL 26a; Berger, 1986)
Deposit geology
Unoxidized refractory ore: Refractory ore consists of variably decalcified, dedolomitized, argillized, silicified,
sulfidized, carbonaceous sedimentary rocks that contain disseminated iron, arsenic, antimony, mercury, and thallium
sulfide minerals. Base-metal sulfide minerals are rare or absent in most deposits. Although pyrite, marcasite,
orpiment, and realgar have high acid-generating capacity, they generally are present in small amounts (much less than
5 volume percent) and are usually disseminated in, or surrounded by, carbonate rocks with high acid-consuming
capacity. Zones with 5-50 volume percent pyrite, marcasite, orpiment, or realgar are present in some deposits. Ore
is refractory because much of the gold forms sub-micron grains in pyrite and marcasite and because carbon in the
Oxide ore: Natural weathering and oxidation of refractory ore cause formation of oxide ore (with low sulfide mineral
and carbon contents) from which gold is recovered by cyanide heap leaching. Acid generating capacity of the
surrounding carbonate rocks is low or nil, and their acid consuming capacity is high.
Jasperoid ore: Jasperoid ore is similar to refractory ore but is strongly silicified and usually lacks orpiment and
realgar. Its acid-generating capacity is moderately high due to disseminated pyrite and marcasite and its acid-
consuming capacity is low due to lack of carbonate minerals; however, jasperoids are usually surrounded by
carbonate rocks with high acid-consuming capacity. Jasperoids are brittle and often highly fractured, which enhances
permeability; in many places, they are weathered and oxidized to great depths, >300 m in a few instances, whereas
surrounding rocks are generally oxidized to shallower depths. Some poorly developed jasperoid or partly silicified
rocks have abundant, well-developed porosity in and near some of the largest Carlin-type systems in Nevada. Rocks
adjacent to oxidized jasperoids are usually decalcified and argillized due to acid attack by supergene fluids.
Examples
The following deposits are in Nevada, unless otherwise noted: Carlin, Cortez, Getchell, Gold Acres, Gold Quarry,
Jerritt Canyon, Alligator Ridge, Post-Betze, Rain, Twin Creeks, Meikle, Mercur (Utah), Ratatotok (Indonesia?), and
Carlin-type deposits show no clear genetic relationship to other types of ore deposits. Locally, they may be present
in the vicinity of volcanic-hosted precious metal deposits, epizonal pluton-related porphyry, skarn, manto, or vein
Relative to other mineral deposit types, sediment-hosted gold deposits have relatively low potential for associated
environmental concerns, especially in light of their large size. Mining activity predominantly exploits oxidized ore
with negligible acid generating and high acid consuming capacities. Refractory ore is processed in mills, and its
waste is collected in closely monitored tailings ponds. Environmental mitigation commonly emphasizes isolation
of sulfide-mineral-rich rocks (refractory ore stockpiles) with low acid consuming capacity from weathering and
oxidation. Commonly, rocks with the greatest acid generation potential and highest base-metal concentrations are
unrelated to the gold deposits and coincidentally are present in the mine area. Waste rock with high acid generating
and low acid consuming capacities or high base-metal concentrations is isolated from weathering and oxidation and
184
(or) mixed with or encased in calcareous waste rock.
Because natural ground water associated with these deposits can have elevated concentrations of Fe, Mn,
As, Sb, Tl, Hg, Se, W, ± base metals, water produced from dewatering wells may require treatment to decrease
concentrations of these elements, and to decrease abundances of suspended sediment. The large size and depth of
some deposits requires dewatering large volumes of rock, on the order of 0.1 to 1.0 km3 for some deposits or clusters
of deposits. Substantial amounts of water, some of which is used to grow alfalfa in Nevada, are produced during
mining. The temperature of ground water produced from some deposits is higher than ambient temperatures, which
increases its metal transport capability.
In comparison to other deposit types, potential downstream and offsite environmental effects are of relatively
limited magnitude and spatial extent; however, surface and groundwater may include elevated concentrations of one
or more of the elements As, Sb, Tl, Hg, Se, W, ± base metals. Vegetation such as sagebrush and grasses may
accumulate arsenic and other elements.
Dust generated by open pit mining refractory ore, which contains elevated concentrations of sulfur, arsenic,
and other elements, may be transported downwind from the mine.
Exploration geophysics
Satellite and airborne multispectral data are helpful in defining major lithologic boundaries, structural zones, and
areas of hydrothermal alteration (Rowan and Wetlaufer, 1981; Kruse and others, 1988). Airborne magnetic and
electromagnetic surveys can be used to delineate intrusive contacts, rock units and faults, and detect alteration
(Grauch, 1988; Taylor, 1990; Grauch and Bankey, 1991; Hoover and others, 1991; Pierce and Hoover, 1991; Wojniak
References
Joraleman (1951), Erickson and others (1964), Erickson and others (1966), Hausen and Kerr (1968), Wells and others
(1969), Wells and Mullins (1973), Radtke and others (1980), Bagby and Berger (1985), Rye (1985), Bakken and
Einaudi (1986), Romberger (1986), Tafuri (1987), Cunningham and others (1988), Holland and others (1988),
Bakken and others (1989), Kuehn (1989), Berger and Bagby (1990), Ilchik (1990), Leventhal and Hofstra (1990),
Massinter (1990), Nelson (1990), Hofstra and others (1991), Kuehn and Rose (1992), Arehart and others (1993a,b),
Maher and others (1993), Christensen (1994), Albino (1994), Hofstra (1994), Turner and others (1994), and Doebrich
Deposit size
Deposits are small (100,000 metric tonnes) to large (200,000,000 metric tonnes). Grades range from 1 to 20 gm
Au/t. Contained gold ranges from 1,500 Kg (50,000 oz) to 930,000 Kg (30,000,000 oz; Post-Betze, Nev).
Host rocks
Calcareous or dolomitic sedimentary rocks are the dominant host rocks for this deposit type. Ore may also be hosted
Most deposits are hosted in Paleozoic and to a lesser extent Mesozoic miogeoclinal and eugeoclinal sedimentary
rocks. Jurassic to Late Eocene calc-alkalic plutons and Eocene to Recent fluvial, lacustrine and volcanic rocks are
present locally. Carlin-type deposits in Nevada and Utah are approximately coeval and cospatial with a late Eocene
volcanic field, although a one-to-one spatial correspondence between the deposits and volcanic centers or plutons is
absent.
Wall-rock alteration
Refractory ore: Alteration associated with refractory ore reflects progressive reaction of moderately acidic CO2- and
H2S-rich ore fluids (pH 4 to 5) with carbonate host rocks. Inner zone-- decalcified, ± dedolomitized, ± argillized,
± silicified, and sulfidized. Realgar and (or) orpiment are locally present. Narrow zones that consist predominantly
of iron and (or) arsenic sulfide minerals are present in some deposits. Intermediate zone-- dolomite stable, 2m1 mica
stable, partially to completely decalcified, sulfidized, ± realgar/orpiment. Outer zone-- calcite stable, sulfidized.
Jasperoid: Silicification reflects combined effects of cooling and reaction with carbonate host rocks. Boundary with
unaltered carbonate rocks is gradational but very abrupt. Jasperoids are above, below, beside, or within refractory
185
ore zones.
Typical sulfide-mineral sulfur concentrations are <5 weight percent in refractory ore. Sulfidized and
argillized intermediate to mafic igneous rocks have sulfide-mineral sulfur concentrations >10 weight percent and
therefore have high acid generating and relatively low acid consuming capacities. Some deposits contain narrow
zones with 10 to 50 volume percent realgar and (or) orpiment enclosed by decalcified rock that has high acid
generating and low to moderate acid consuming capacities. Oxide ore is a product of supergene weathering of
refractory ore and jasperoid and generally has sulfide mineral concentrations <1 volume percent.
Nature of ore
Refractory ore: Most gold resides in trace-element-rich pyrite and marcasite as sub-micron blebs. Arsenic is the
major trace element in pyrite and (or) marcasite followed in decreasing abundance by antimony, thallium, and
mercury. Gold also resides in orpiment, realgar, and cinnabar, on the surface of clay minerals, in or on organic
Oxide ore: Gold is present as free gold, resides in iron oxide minerals or quartz, and is adsorbed on clay minerals.
These deposits exhibit a characteristic suite of trace elements, including silver, arsenic, antimony, mercury, thallium,
Minerals listed in decreasing order of abundance. Acid-generating minerals underlined. Barite is present locally.
Inner refractory ore: Quartz, ± dolomite, 2m1 mica, pyrite, marcasite, orpiment, realgar, kaolinite, illite/smectite.
Outer refractory ore: Calcite, dolomite, quartz, 2m1 mica, pyrite, marcasite.
Oxide ore: Calcite, dolomite, quartz, 2m1 mica, limonite/goethite/hematite, kaolinite, illite/smectite, relict pyrite.
Mineral characteristics
Pyrite/marcasite and arsenopyrite generally replace iron-bearing minerals and form disseminations in host rocks; they
are generally fine grained and 1 mm to 1 micron in size. Late botryoidal pyrite/marcasite is present in some
deposits. Most orpiment, realgar, stibnite, cinnabar, and barite are in open space along fractures and in breccias.
Secondary mineralogy
Supergene minerals include travertine, goethite, limonite, hematite, alunite, kaolinite, stibiconite, scorodite, gypsum,
celestite, and phosphate minerals. Small amounts of melanterite precipitate where ground water has evaporated from
Topography, physiography
In the Basin and Range Province of Nevada and Utah, sediment-hosted gold deposits are present at elevations
between 3,000 and 1,200 m, from the crest of mountain ranges to valley margins; some are concealed by pediment
gravels or alluvial valley fill. Jasperoids are resistant to erosion and commonly form bold outcrops. Because of
alteration and the presence of sulfide minerals, refractory ore zones are generally more easily eroded than unaltered
rocks.
Hydrology
Jasperoids are usually fractured, and therefore highly permeable, and can focus flow of oxidized ground water to
great depth. Decalcified refractory ore is usually porous and permeable; rocks within and above these zones are
commonly fractured or brecciated due to volume losses associated with alteration. Faults and fractures also serve
as conduits for ground water flow. Brittle siliceous rocks (chert, siltite, quartz arenite) are commonly fractured,
permeable, and focus ground water flow. Karst cavities and breccias, that also focus ground water flow, are present
in some deposits; karst may have developed at several times between the Paleozoic and Tertiary.
Position of the water table: Water table elevation relative to the deposits has a dramatic effect on the acid generating
capacity of ore. For instance, the current water table at one deposit is at a depth of ~60 m. However, associated
wall rock is oxidized to a depth of ~200 m, which suggests that the paleo-water table previously extended to much
greater depth. Consequently, present-day ground water is neutral to alkaline and contains low trace metal
abundances. Mining this ore has little impact on water quality because the rocks are already oxidized. In contrast,
mining unoxidized ore associated with many sediment-hosted gold deposits entails significant potential for
186
Figure 1. Ficklin plot (Ficklin and others, 1992) showing composition of natural ground water from Twin Creeks, Jerritt Canyon, and Gold
Quarry, Nev., mines. All data plot within the "near neutral-low metal field."
environmental degradation. However, the overall potential for undesirable environmental impact associated with
mining Carlin-type deposits is small compared to effects associated with other deposit types because these deposits
have low base-metal contents and high host-rock, acid-consuming potential.
Deposits above the water table, but hosted by thick sequences of carbonaceous, pyritic, shaley eugeoclinal
rocks are relatively unoxidized compared to those hosted by less carbonaceous, less pyritic calcareous miogeoclinal
rocks. The eugeoclinal rocks apparently consume oxygen in descending ground water before the water table is
reached.
Data for the Twin Creeks, Nev., deposit (Grimes and others, 1994; 1995) show that natural ground water,
under reducing conditions below the water table, in refractory ore zones has the highest concentrations of arsenic,
antimony, tungsten, manganese, iron and possibly thallium and selenium. These elements are concentrated in iron
and manganese precipitates under oxidizing conditions at or above the water table.
Historic: Oxide ore was produced from open pit mines and processed by cyanide heap leach solutions that potentially
may have leaked into ground water. Refractory ore was generally avoided or stockpiled.
Modern: Oxide and refractory ore are produced from open pit and underground mines; oxide ore is processed by
cyanide heap leaching. Refractory ore is oxidized using one, or a combination, of the following methods: biologic
oxidation, chlorination, pressure oxidation (autoclave), or roasting. Gold is subsequently stripped from the cyanide
ENVIRONMENTAL SIGNATURES
Drainage signatures
The U.S. Environmental Protection Agency (EPA) chronic criteria freshwater standards most likely to be exceeded
are 5.2 µg/l cyanide, 190 µg/l arsenic, 5 µg/l selenium, 0.012 µg/l mercury, 30 µg/l antimony (proposed standard),
and 40 µg/l thallium (proposed standard). Tungsten abundances are likely to be anomalous, although no freshwater
standard has been defined. In some deposits, elevated base-metal concentrations may pose a problem. The standards
for these elements are as follows: 0.12 µg/l silver, 12 µg/l copper, 3.2 µg/l lead, 110 µg/l zinc, 1.1 µg/l cadmium,
Mine drainage data: More work is needed to obtain and synthesize available data.
Natural stream/spring drainage data: More work is needed to obtain and synthesize available data.
Natural ground water data: Chemical analyses of natural ground water from the Twin Creeks, Jerritt Canyon and
Gold Quarry, Nev., mines are shown on figures 1-2. Using the classification scheme of Ficklin and others (1992),
187
Figure 2. Bar and whisker plots showing percentile ranges of values for sulfate, iron, manganese, arsenic, antimony, and total base metals in
natural ground water draining sediment-hosted gold deposits, Nev.; TC-red, Twin Creeks mine, reduced ground water; TC-ox, Twin Creeks mine,
oxidized ground water; JC, Jerritt Canyon mine; GQ, Gold Quarry mine. The line in each box corresponds to the 50th percentile value. The
top and bottom of each box correspond to the 75th and 25th percentile values, respectively. The whiskers extending from the top and bottom
of each box correspond to values above and below the 90th and 10th percentile values.
all samples lie within the "near-neutral, low-metal field". Some samples have arsenic, antimony, selenium, or
mercury concentrations that exceed EPA chronic criteria for fresh water. Concentrations of copper, lead, zinc,
cadmium, nickel, and chromium are all below the EPA fresh water standard. Ground water samples from the Twin
Creeks mine indicate anomalous concentrations of arsenic, antimony, tungsten (as much as 140 µg/l), iron, and
manganese (Grimes and others, 1994, 1995). The highest concentrations are at sites where measured Eh indicates
reducing conditions. Some elements are concentrated where alluvium is adjacent to the present-day water table.
Ground water from Gold Quarry is similar to the more oxidized samples from Twin Creeks (Davis and others, in
press) and contains e levated dissolved metal abundances, including 10 µg/l thallium, 59 µg/l selenium, and 0.4 µg/l
mercury. Arsenic concentrations in drainage water associated with Jerritt Canyon are much lower than those at Twin
Creeks due to more oxidizing conditions and rocks that contain lower abundances of realgar and orpiment (Al
Hofstra, unpub. data, 1995).
188
Metal mobility from solid mine wastes
Soil: Some soil geochemical data are available for the Getchell (Erickson and others, 1964; Brooks and Berger,
1978); Dee (Bagby and others, 1985), and Preble (Lawrence, 1986), Nev., deposits.
Stream sediment: More work is needed to obtain and synthesize available data.
Plants: Data concerning the chemistry of sagebrush growing in the vicinity of Carlin-type deposits has been published
Heap leach and other cyanide processing solutions may contain copper, zinc, and silver complexes in addition to
gold. Arsenic, cobalt, nickel, and iron may be present in low mg/l abundances in cyanide heap leach solutions.
Thiocyanate (SCN-) abundances are highest in ore that contains unoxidized sulfide minerals.
Smelter signatures
Effluent from a few old smelters has contaminated down wind soil and vegetation. Modern operations do not involve
smelting. Stack emissions from autoclaves, fluid bed roasters, chlorination circuits, etc. are monitored for compliance
In the Basin and Range Province of Nevada and Utah, most sediment-hosted gold deposits are in semi-arid to arid
climates, although deposits in alpine settings may receive 75 to 100 cm of yearly precipitation. In the dry season,
evaporation leads to formation of acid salts that dissolve during storm events or the next wet season. Surrounding
Geoenvironmental geophysics
Ground magnetic and various electromagnetic methods may be used to map faults, fractures, and highly permeable
altered zones that may serve as ground water conduits (Heran and Smith, 1984; Heran and McCafferty, 1986; Hoover
and others, 1986; Hoekstra and others, 1989). Electrical resistivity methods can delineate hydrothermally altered
areas and fault zones as resistivity lows and silicified rock as resistivity highs (Hallof, 1989; Hoekstra and others,
1989; Corbett, 1990). Electrical and seismic methods can be employed to determine depth to bedrock or locations
of permeable and impermeable beds (Zohdy and others, 1974; Cooksley and Kendrick, 1990). Electrical methods
also may be used to locate the present day water table and can delineate contaminated water plumes having
significant electrical contrasts. Induced polarization surveys can be used to estimate sulfide mineral concentrations
Acknowledgments.--An early version of this report was improved by the helpful reviews of Ted Theodore,
Barney Berger, Jeff Doebrich, Steve Peters, and Alan Wallace.
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rock sampling in exploration for deep Carlin-type deposits: Journal of Geochemical Exploration, v. 51, p.
37-58.
Arehart, G.B., Eldridge, C.S., Chryssoulis, S.L., and Kesler, S.E., 1993a, Ion microprobe determination of sulfur
isotope variations in iron sulfides from the Post/Betze sediment-hosted disseminated gold deposit, Nevada,
USA: Geochimica et Cosmochimica Acta, v. 57, p. 1505-1519.
Arehart, G.B., Foland, K.A., Naeser, C.W., and Kesler, S.E., 1993b, 40Ar/39Ar, K/Ar, and fission track geochronology
of sediment-hosted disseminated gold deposits at Post/Betze, Carlin Trend, northeastern Nevada: Economic
Geology, v. 88, 622-646.
Bagby, W.C. and Berger, B.R., 1985, Chapter 8, Geologic characteristics of sediment-hosted, disseminated precious
metal deposits in the western United States: Reviews in Economic Geology, v. 2, p. 169-202.
189
Bagby, W.C., Pickthorn, W.J., and Goldfarb, R.J., 1985, Pathfinder elements in soils over the Dee disseminated gold
deposit, Elko County, Nevada: U.S. Geological Survey Circular 949, p. 1.
Bakken, B.M. and Einaudi, M.T., 1986, Spatial and temporal relations between wall rock alteration and gold
mineralization, main pit, Carlin gold mine, Nevada, U.S.A.: in Macdonald, A.J., ed., Proceedings of Gold
'86, an International Symposium on the Geology of Gold: Toronto, Canada, p. 388-403.
Bakken B.M., Hochella, M.F. Jr., Marshall, A.F., and Turner, A.M., 1989, High-resolution microscopy of gold in
unoxidized ore from the Carlin mine, Nevada: Economic Geology, v. 84, p. 171-179.
Berger, B.R., 1986, Descriptive model of carbonate-hosted Au-Ag, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 175.
Berger, B.R., and Bagby, W.C., 1990, The geology and origin of Carlin-type gold deposits, in Foster, R.P., ed., Gold
Metallogeny and Exploration: Blackie and Son Ltd, Glasgow, Scotland, p. 210-248.
Brooks, R.A. and Berger, B.R., 1978, Relationship of soil mercury values to soil type and disseminated gold
mineralization, Getchell mine area, Humboldt County, Nevada: Journal of Geochemical Exploration, v. 9,
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192
ALMADEN HG DEPOSITS
(MODEL 27b; Rytuba, 1986)
INTRODUCTION
Environmental concerns related to mining and processing Almaden-type mercury deposits consist primarily of:
mercury contamination of soil and plants from mine smelters because of mercury vapor and dust released during ore
processing, mercury degassing from contaminated soil at mine sites and from undisturbed deposits, and potential for
acid mine drainage and toxic metal release into drainage basins. These deposits typically are extremely high grade;
central parts of deposits can contain more than 20 weight percent mercury. As a result, extremely high
concentrations of mercury are present in residual soil where these deposits are exposed at the surface. Mercury
released into creeks and rivers may contaminate lakes, estuaries, and bogs; subsequently methylmercury (CH3Hg+)
may form and become highly concentrated through biomagnification in fish, and eventually in any animals that
consume fish. For humans, most toxicity concerns related to mercury pertain to consumption of contaminated fish.
However, ambient mercury vapor pressure in underground mercury mines may allow direct mercury ingestion
through breathing. Also, direct ingestion of mercury contaminated soil can pose a health threat among children.
The global atmospheric mercury cycle contributes mercury to drainage basins and lakes through both wet
and dry depositional processes that are mediated by leaf uptake of mercury by plants (Mason and others, 1994). The
potential flux of mercury to the environment from mercury deposits and mine sites must be evaluated in the context
of its flux from the global mercury atmospheric cycle. The primary natural sources of mercury to the atmosphere
include the oceans, soil degassing, volcanoes, and geothermal systems (Varekamp and Buseck, 1986; Mason and
others, 1994). Anthropogenic sources of mercury to the atmosphere are primarily from coal combustion, waste
incineration, and smelters (Nriagu and Pacyna, 1988). Since the beginning of industrialization in about 1850, the
atmospheric mercury concentration increased until about 1970, and then has decreased moderately. The mercury
increase is reflected in the sedimentary record of lakes, estuaries, and bogs; recently deposited sediment from all these
environments record an increase in mercury concentration of about two to five times over baseline mercury
concentrations established prior to industrialization (for example see, Verta and others, 1990; and Hurley and others,
1994).
Mercury concentrations in soil associated with Almaden-type deposits can be extremely high and elevated
summer temperatures can increase rock and soil degassing rates. Native mercury contained in ore also enhances
degassing. Plants on or adjacent to these deposits may play an important role in mediating mercury vapor flux.
Mercury is taken up by plants primarily through leaves, rather than through the root system, and fixed at the site of
plant uptake. Thus, high ambient-air mercury concentrations cause plants to uptake and concentrate mercury in their
leaves, and conversely, low ambient air mercury concentrations cause plants to give off mercury through their leaves
(Lindberg and others, 1992). In mine areas where ambient air concentrations of mercury are elevated, due either
to roasting mercury ore or from natural degassing of mercury from either contaminated (see Lindberg and others,
1995) or naturally anomalous soil (Lindberg and others, 1979), plant communities concentrate mercury in their
leaves. Wash off and litter fall from these plant communities may redistribute mercury into creeks and lakes.
The formation of methylmercury is favored by the presence of ionic mercury, low pH, and high dissolved
organic carbon and sulfate. Mercury methylation is a co-metabolic reaction; sulfate reducing bacteria are the most
important mediators in the biotic methylation process (Campeau and Bartha, 1985; Summers, 1986). Acid mine
water that has a high sulfate concentration may develop at a mercury mine site or at a naturally oxidizing mercury
ore deposit. Introduction of acid water to a bog or lake may enhance methylation. Nearly all methylmercury is
formed in lakes where most of it resides in fish. In lakes and estuaries, detrital particles such as clays capture Hg++
from the water by absorption; sedimentation of these particles inhibits methylation. Similarly, tailings that enter
surface water may aid methylmercury removal from the water column and enhance sequestration in sediment.
193
mafic to intermediate composition pyroclastic flows, mafic dikes, and organic-rich black shales. Cinnabar-bearing
veins and veinlets are present primarily as gash fractures developed during regional metamorphism of these deposits
(Rytuba and others, 1988). Native mercury is present in all deposits and is the primary mineral in deposits, such
as in the Las Cuevas deposit, hosted in submarine tuffs. All of these deposits are spatially associated with mafic
submarine vent complexes that consist of mafic dikes and sills, oval craters typically with dimensions of 300 m by
150 m (Hernandez, 1985), and submarine calderas, such as at the Las Cuevas deposit (Rytuba and others, 1988).
Megabreccia composed of black shale blocks, as much as 100 m in maximum dimension and contained in a tuff
matrix, are interstratified with caldera filling lithic tuffs. Tuff breccias that fill the craters consist primarily of
sedimentary wall rocks with lesser amounts of ultramafic and plutonic igneous clasts. Juvenile magma clasts within
the craters are typically alkali basalt. Replacement deposits hosted in quartzite consist of stratiform zones of cross
cutting mineralized rock composed of cinnabar, native mercury, pyrite, calcite, and quartz. Ore zone grades and
thicknesses are highest near crater margins and decrease systematically away from crater margins.
Examples
Almaden, El Entredicho, Nueva Concepcion, Las Cuevas; Spain (Hernandez, 1985; Saupe, 1990).
Exploration geophysics
No geophysical investigations of this deposit type are known, however certain inferences may be drawn from surveys
of deposits with similar geologic relations. Aeromagnetic anomalies may delineate mafic vent complexes with which
these deposits are spatially associated. Remote sensing images can de used to identify limonite and iron oxide
minerals that develop due to pyrite oxidation. Cinnabar is resistive but native mercury is highly conductive; both
have very high specific gravity, near 8 gm/cm3. Almaden type deposits can probably be identified by detailed gravity
surveys because they are large and contain elevated abundances of high density mercury minerals. Resistivity lows
and induced polarization highs are probably associated with these deposits because of their elevated native mercury
abundances. Resistivity surveys may aid identification of cinnabar deposits because of the association between
conductive carbonaceous rock (black shale) and these deposits. Regional airborne or ground electrical surveys may
help delineate the extent of conductive, shallow-marine-basin black shale and altered volcanic rocks that host these
deposits. Induced polarization traverses may further refine the distribution of rocks that contain abundant pyrite.
References
Rytuba (1986), Saupe (1990), and Mason and others (1994).
194
GEOLOGIC FACTORS THAT INFLUENCE POTENTIAL ENVIRONMENTAL EFFECTS
Deposit size
The deposits are typically large and contain more than 10 million metric tonnes. The deposits are clustered in
stratigraphic units deposited within the same submarine basin. Most deposits have been mined by underground
methods but one deposit is mined by open pit methods; the visual impact related to mining these deposits is relatively
limited except for large associated tailings (calcine) piles.
Host rocks
Most deposits are hosted in quartzite and mafic to intermediate composition pyroclastic and intrusive rocks. Organic
rich shale and siltstone are locally important host rocks, especially where shale is in contact with pyroclastic rocks.
Wall-rock alteration
Pyroclastic and intrusive rocks are argillically and carbonate altered. Ultramafic xenoliths within sills and dikes are
altered to carbonate-bearing mineral assemblages. Strontium isotopes indicate that intrusive and volcanic rocks have
interacted extensively with sea water. Quartzite displays secondary overgrowths of quartz although silicification is
not pervasive. Pyrite comprises from about 2 to 6 volume percent of pyroclastic rocks. Carbonate vein assemblages
are present primarily in gash fractures.
Nature of ore
Most orebodies are massive replacement bodies that are stratiform in quartzite and form more irregular replacement
orebodies in pyroclastic and intrusive rocks. Orebodies in mafic dikes and sills consist of tabular to irregular
replacement masses localized at the terminations of dikes or along the upper parts of sills. Very high grade ore
composed of cinnabar commonly is present at contacts between black shale and pyroclastic and intrusive rocks.
Where black shale clasts are absent, native mercury is the dominant ore mineral in pyroclastic-hosted orebodies. In
quartzite-hosted orebodies, ore is present near the top of the units. Discrete veins and breccia bodies are present,
but most veins fill gash fractures developed during low grade regional metamorphism.
Mineral characteristics
Cinnabar is generally fine grained, about 0.2 to 6 mm, in replacement orebodies but is typically coarser grained, as
much as 2 cm, in gash fractures. Mercury globules as much as several millimeters wide are present in host rocks;
during mining operations, these may become concentrated in globules as much as several centimeters wide. Pyrite
ranges from 1 to 10 mm in maximum dimension.
Secondary mineralogy
In near-surface parts of deposits, pyrite is exposed to weathering, which results in limonite and iron oxide formation.
Cinnabar and native mercury are not affected by weathering process because of their low solubilities under ambient
conditions and reducing conditions associated with the black shale.
195
Topography, physiography
Where cut by volcanic craters, orebodies are associated with resistant ledges of quartzite. Where quartzite ledges
are interrupted by craters, the area of the craters is topographically more subdued.
Hydrology
Increase in hydrologic head during wet periods results in ground water fluid flow along regional structures that
control the location of craters and dikes.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Data that characterize drainages have not been published. The presence of associated acid mine drainage has not
yet been reported.
196
Calcine was commonly used as road metal on unpaved roads surrounding mercury districts. This may be
a problem in the case of calcine that was contaminated by mercury during the recovery process. Introduction of
tailings particles into creeks is a concern because native mercury may be oxidized and along with mercury
oxychloride and sulfate minerals can be methylated in aqueous environments. The fine grained nature of calcine also
increases water turbidity.
Smelter signatures
Large retorts and more efficient rotary and Herschoff type furnaces have been used to process ore from these
deposits. Prior to the 1970s, significant amounts of elemental mercury as vapor, ionic mercury complexed as a
chloride or sulfate, and mercury adhering to particulates were vented to the atmosphere. Elevated ambient air
concentrations of mercury result in increased wet and dry deposition of mercury and leaf uptake by plants downwind
from smelter sites. These processes result in elevated mercury concentrations in soil, water, and, through wash off
and litter fall, to the forest floor. SO2 released to the atmosphere during ore roasting may increase acid and sulfate
concentrations in water, which in turn enhance mercury methylation.
Geoenvironmental geophysics
Pyrite oxidation and dissolution, which can result from weathering Almaden type mercury deposits, may produce
conductive acid water that can be detected using electromagnetic, direct current resistivity, and probably induced
polarization methods. Ground penetrating radar can detect acid ground water at depths of less than a few meters.
Information concerning the change of fluid properties, such as resistivity, with varying concentrations of mercury
ions or compounds is not available. Similarly, remote sensing reflectance variations of vegetated areas in which
mercury uptake has been variable has not been documented; however, these subjects should be investigated.
REFERENCES CITED
Compeau, G.C., and Bartha, R., 1985, Sulfate reducing bacteria: principal methylators of mercury in anoxic estuarine
sediment: Applied Environmental Microbiology, v. 50, p. 498-502.
Hernandez, A., 1985, Estructura y genesis de los yacimientos de mercurio de la zona de Almaden: Unpub. resumen
de tesis doctoral, University Salamanca, 64 p.
Hurley, J.P., Krabbenhoft, D.P., Babiarz, C.L., and Andren, A.W., 1994, Cycling of mercury across the
sediment-water interface in seepage lakes, in Baker, A., ed., Environmental chemistry of lakes and
reservoirs: American Chemical Society Series No. 237, p. 425-449.
Lindberg, S.E., Jackson, D.R., Huckabee, J.W., Janzen, S.A., Levin, M.J., and Lund, J.R., 1979, Atmospheric
emission and plant uptake of mercury from agricultural soils near the Almaden mercury mine: Journal of
Environmental Quality, v. 8, p. 572-578.
Lindberg, S.E., Kim, K., Meyers, T.P., and Owens, J.G., 1995, Micrometeorological gradient approach for
quantifying air/surface exchange of mercury vapor: tests over contaminated soils: Environmental Science
and Technology, v. 29, no. 1, p. 126-135.
Lindberg, S.E., Meyers, T.P., Taylor, G.E., Turner, R.R., and Schroeder, W.H., 1992, Atmosphere-surface exchange
of mercury in a forest: results of modeling and gradient approaches: Journal of Geophysical Research, v.
97, no. D2, p. 2519-2528.
Mason, R.P., Fitzgerald, W.F., and Morel, F.M.M., 1994, The biogeochemical cycling of elemental mercury:
Anthropogenic influences: Geochimica et Cosmochimica Acta, v. 58, no. 15, p. 3191-3198.
Nriagu, J.O., and Pacyna, J.M., 1988, Quantitative assessment of worldwide contamination of air, water and soils
by trace metals: Nature, v. 333, p. 134-139.
Rytuba, J.J., 1986, Descriptive model of Almaden Hg, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models:
U.S. Geological Survey Bulletin 1693, p. 180.
197
Rytuba, J.J., Rye, R.O., Hernandez, A.M., Deen, J.A., and Arribas, A., Sr., 1988, Genesis of Almaden-type mercury
deposits: Almaden, Spain, 28th International Geologic Congress Abstracts with Program, p. 2-741.
Saupe, Francis, 1990, Geology of the Almaden mercury deposit, Province of Ciudad Real, Spain: Economic Geology,
v. 85, p. 482-510.
Summers, A. O., 1986, Organization, expression, and evolution of genes for mercury resistance: Annual Reviews
of Microbiology, v. 40, p. 607-634.
Varekamp, J.C., and Buseck, P.R., 1986, Global mercury flux from volcanic and geothermal sources: Applied
Geochemistry, v. 1, p. 65-73.
Verta, M., Mannio, J., Iivonen, P., Hirvi, J.P., Jarvinen, O., and Piepponen, S., 1990, Trace metals in Finnish
headwater lakes-effects of acidification and airborne load, in Kauppi and others, eds., Acidification of
Finland, p. 883-908.
198
SILICA-CARBONATE HG DEPOSITS
(MODEL 27c; Rytuba, 1986)
INTRODUCTION
Environmental concerns related to mining and processing of silica-carbonate mercury deposits consist primarily of:
mercury contamination of soil and water from mine waste rock and tailings (calcine), mercury vapor released during
ore processing, and acid mine drainage and toxic metal release into drainage basins. Mercury released into lakes and
bogs may become methylated and methylmercury (CH3Hg+) may become highly concentrated through biomagnifica
tion in fish and animals that consume fish. The mercury concern for humans is primarily related to consumption
of contaminated fish.
See Rytuba and others (preceding model, this volume) for a discussion of global mercury budgets, cycling,
and processes.
Examples
New Almaden (Bailey and Everhart, 1964), Aetna (Yates and Hilpert, 1946), Knoxville (Sherlock and others, 1993),
Culver-Baer (Peabody and Einaudi, 1992); Calif.
199
as much as several kilometers beyond the outer limits of ore. Carbonate minerals in the silica-carbonate alteration
zone partially mitigate associated acid generation.
Direct introduction of mercury into water from these deposits is minimal since the primary ore mineral,
cinnabar, is relatively insoluble and is not readily oxidized under ambient, near-surface conditions. Mine water
draining deposits that contain native mercury may have elevated mercury concentrations. Introduction of mercury
into the environment is primarily from release of mercury into the atmosphere when ore is retorted. When
atmospheric mercury concentrations are elevated, plants uptake mercury through their leaves and mercury is
concentrated in plant foliage. Wash off from plants and dry deposition of leaf litter to the forest floor is the primary
route for introduction of mercury into water and soil. Methylation of mercury in bog and lake environments is
enhanced by increased mercury concentrations as well as by sulfate generated during oxidation of pyrite and
marcasite.
Exploration geophysics
Remote sensing techniques can identify serpentinite by infra red reflectance, thermal properties, and botanical
anomalies. The sparse and unusual vegetation developed on serpentinite and its silica-carbonate altered equivalent
is prominent on remote-sensing images. Aeromagnetic highs delineate serpentinite bodies and serpentinite in fault
zones, particularly small deposits located along regionally extensive fault zones. Magnetic surveys may have limited
utility in identification of serpentinite developed from carbonate rocks, most of which have low initial iron contents.
Because cinnabar and native mercury have high specific gravity, large deposits may be detected by detailed gravity
surveys. Because mercury is highly conductive, deposits that contain high concentrations of native mercury, may
be associated with resistivity lows and induced polarization highs.
References
Geology: Rytuba (1986), and Rytuba and Miller (1994).
Environmental geology, geochemistry: Varekamp and Buseck (1986), Nriagu and Pacyna (1988), Lindberg and others
(1992), and Hurley and others (1994).
Host rocks
Shale and siltstone are locally important host rocks where they form impermeable cap rocks in anticlinal structures.
Wall-rock alteration
The silica-carbonate assemblage consists of a central core of quartz + opal + chalcedony + magnesite + dolomite +
calcite + marcasite + pyrite that grades outward into a magnesite + magnetite + calcite + dolomite assemblage and
then finally into serpentinite + magnetite. Pyrite and marcasite in the sulfidized central core of the alteration
assemblage comprise from about 2 to 10 volume percent and are the only important acid generating sulfide minerals
present. The carbonate assemblage may partially buffer acid generated by sulfide mineral oxidation. In the upper
part of these deposits an advanced argillic alteration zone that consists of kaolinite + alunite + cristobalite + sulfur
may be present; most often this alteration assemblage has been removed by erosion. Pyrite and marcasite in this
alteration assemblage may be oxidized and contribute to acid mine drainage generation.
200
Nature of ore
Orebodies are variable in character. Most consist of tabular to irregular replacement masses but discrete veins and
breccia bodies are common; more rarely, stockwork veins are present. Repeated post ore movement along controlling
fault zones causes development of blocks (phacoids) of very high grade cinnabar within fault gouge. In vein and
breccia orebodies, marcasite and more rarely stibnite are present. Most replacement orebodies are monomineralic;
they consist of cinnabar and minor native mercury.
Mineral characteristics
Most cinnabar, about 0.2 mm, is very fine grained in replacement orebodies but in rare cases is coarser grained in
veins. Pyrite and marcasite are coarser grained and typically range from 0.5 to 5 mm.
Secondary mineralogy
Near surface exposures of deposits and alteration zones expose pyrite and marcasite to weathering, which results in
formation of limonite and other iron oxide minerals. Cinnabar and native mercury are not typically affected by
weathering process because of their low solubilities under ambient conditions. However, under extreme oxidizing
and acid pH conditions mercury sulfate and oxychloride minerals may form as coatings on surface exposures. These
yellow and green minerals are photosensitive and rapidly turn black when exposed to the sun. As a result, these
minerals can be easily misidentified as manganese oxide minerals. Small amounts of mercury silicate and chromate
minerals may also be present; these, along with mercury sulfate and oxychloride minerals, are more readily
solubilized than cinnabar and native mercury.
Topography, physiography
Orebodies are associated with large zones of silica-carbonate alteration that typically form resistant ridges with sparse
vegetation. Adjacent serpentinite also has sparse vegetation. Altered serpentinite has a distinctive red-brown color
due to the presence of iron oxide minerals. Deposits are generally exposed at or near the surface and are along
regional structures that may extend for tens of kilometers.
Hydrology
Increased hydrologic head during wet periods results in ground water flow along the regional structures that control
the location of silica-carbonate alteration and ore deposits. Adjacent serpentinite bodies serve as impermeable
aquitards that focus ground water flow along contacts between altered and fresh serpentinite.
201
ENVIRONMENTAL SIGNATURES
Drainage signatures
Acid mine water has a pH of 1 to 4 and precipitates amorphous iron hydroxide that selectively absorbs mercury from
the water; these precipitates may contain as much as 20 ppm mercury. Elevated concentrations of as much as 670
ppm nickel and 200 ppm chromium, may also be present in these precipitates. Acid mine water may contain 100
to 1,000 mg/l iron and 0.1 to several µg/l mercury. Acid mine water draining deposits that include stibnite may
contain 1 to 10 mg/l antimony but base-metal concentrations as well as abundances of chemical precipitates are low.
In active geothermal water associated with recently formed deposits, very high tungsten concentrations range
from 1 to 10 mg/l; in stream water as much as several kilometers downstream from deposits, elevated tungsten
concentrations range from 260 to 680 µg/l (Rytuba and Miller, 1994). Mercury and iron concentrations, 0.2 to 2.7
µg/l and 0.05 to 1.4 mg/l, respectively, in unfiltered stream water are usually higher than in filtered water. However,
these abundances return to baseline concentrations, 0.2 µg/l mercury and 500 µg/l iron, within several kilometers
of mercury deposits and (or) geothermal sites (Janik and others, 1994).
Smelter signatures
Several different types of retorts and condensing pipes have been used to recover mercury. These range from
relatively small, inefficient single and double pipe retorts at small mines to very efficient Herschoff type retorts at
larger mines. In most cases prior to the 1970s, significant amounts of elemental mercury vapor and ionic mercury
complexed as a chloride or sulfate were vented to the atmosphere. Elevated ambient air concentrations of mercury
result in increased leaf uptake of mercury by plants downwind from retort sites and in elevated mercury
concentrations in soil through wash off and litter fall to the forest floor. SO2 was also released to the atmosphere
from less efficient mine operations. These releases may increase sulfate and acid concentrations in water, which in
202
turn enhance mercury methylation.
Geoenvironmental geophysics
Geophysical methods can be used to identify contaminated water and soil around mining operations. Electromagne
ic, direct current resistivity, and induced polarization surveys can detect and monitor conductive acidic groundwater
plumes resulting from oxidation and dissolution of pyrite and marcasite in altered rocks that host mercury deposits.
Hot spots in tailing piles that result from ongoing redox reactions can be monitored using self potential methods.
Ground penetrating radar can detect near-surface acidic water.
REFERENCES CITED
Bailey, E.H., and Everhart, D.L., 1964, Geology and quicksilver deposits of the New Almaden District Santa Clara
County, California: U.S. Geological Survey Professional Paper 360, 206 p.
Donnelly-Nolan, J.M., Burns, M.G., Goff, F.E., Peters, E.K., and Thompson, J.M., 1993, The Geysers-Clear Lake
area, California: thermal waters, mineralization, volcanism, and geothermal potential: Economic Geology,
v. 88, p. 301-316.
Janik, C.J., Goff, F., and Rytuba, J.J., 1994, Mercury in waters and sediments of the Wilbur Hot Springs area,
Sulphur Creek Mining District, California: EOS Transactions American Geophysical Union, v. 75, no. 44,
p. 243.
Hurley, J.P., Krabbenhoft, D.P., Babiarz, C.L., and Andren, A.W., 1994, Cycling of mercury across the sediment-
water interface in seepage lakes, in Baker, A., ed.,: American Chemical Society Series No. 237,
Environmental chemistry of lakes and reservoirs, p. 425-449
Lindberg, S.E., Meyers, T.P., Taylor, G.E., Turner, R.R., and Schroeder, W.H., 1992, Atmosphere-surface exchange
of mercury in a forest: results of modeling and gradient approaches: Journal of Geophysical Research, v.
97, no. D2, p. 2519-2528.
Nriagu, J.O., and Pacyna, J.M., 1988. Quantitative assessment of worldwide contamination of air, water and soils
by trace metals: Nature, v. 333, p. 134-139.
Peabody, C.E., and Einaudi, M.T., 1992, Origin of petroleum and mercury in the Culver-Bear Cinnabar deposit,
Mayacmas district, California: Economic Geology, v. 87, p. 1078-1102.
Rytuba, J.J., 1986, Descriptive model of silica-carbonate Hg, in Cox, D.P., and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 181.
_________1995, Cenozoic metallogeny of California: Geological Society of Nevada Symposium on Geology and
Ore Deposits of the American Cordillera, Program and Abstracts, p. A68.
Rytuba, J.J., and Miller, W.R., 1994, Environmental geochemistry of active and extinct hot spring mercury deposits
in the California Coast Ranges, in USGS Research on Mineral Resources-1994 Part A-Program and
Abstracts: U.S. Geological Survey Circular 1103-A, p. 90-91.
Sherlock, R.L., Logan, M.A.V., and Jowett, E.C., 1993, Silica carbonate alteration of serpentinite, implications for
the association of precious metal and mercury mineralization in the Coast Ranges, in Rytuba, J.J., ed., Active
geothermal systems and gold-mercury deposits in the Sonoma-Clear Lake volcanic fields: Society of
Economic Geology Guidebook 16, p. 90-116.
Varekamp, J.C., and Buseck, P.R., 1986, Global mercury flux from volcanic and geothermal sources: Applied
Geochemistry, v. 1, p. 65-73.
Yates, R.G., and Hilpert, L.S., 1946, Quicksilver deposits of the eastern Mayacmas District, Lake and Napa Counties,
California: California Journal of Mines and Geology, v. 42, no. 3, p. 231-286.
203
STIBNITE-QUARTZ DEPOSITS
(MODELS 27d,e and 36c; Bliss and Orris, 1986a-c; Berger, 1993)
Deposit geology
Stibnite-quartz deposits are hosted by shale, calcareous shale, limestone, quartzite, or granite (or their metamorphic
equivalents). Mineralized rock, which is associated with faults that transect stratigraphy, consists of (1) massive veins
(as much as 4 m thick), dominated by quartz and stibnite; or (2) massive stratiform replacement deposits of quartz
and stibnite along shale/limestone contacts. Significant metallic zonation generally is absent from these deposits.
Examples
Lake George, New Brunswick, Canada; Thompson Falls, Mont.; Xiguanshan, China; Kadamdzhay, Russia
Associated deposit types (Cox and others, 1986) include stibnite-bearing veins, pods, and disseminations containing
base metal sulfide minerals ± cinnabar ± tungsten (Models 27d and 27e); gold-antimony deposits (Model 36c); low-
sulfide gold-quartz veins (Model 36a); stockwork tungsten-molybdenum deposits; and less commonly, polymetallic
(1) The acid-generating potential of stibnite-quartz deposits is low due to the abundance of stibnite and low
abundances of pyrite and pyrrhotite. However, rock surrounding the veins includes alteration zones that contain
minor pyrite and arsenopyrite, which slightly increase associated acid-generating potential. Shale that hosts the vein
deposits lacks significant acid-buffering capacity. However, low acid-buffering capacity may be offset by the local
presence of calcite gangue. Limestone that hosts replacement deposits has significant acid-buffering capacity.
(2) Mine drainage and water from tailings ponds may contain elevated metal abundances, including tens to thousands
of mg/l antimony, tens to hundreds of mg/l arsenic, and tens to thousands of mg/l sulfate.
Exploration geophysics
No geophysical investigations specific to this model are known. Induced polarization methods can provide qualitative
References
Geology: Morrissy and Ruitenberg (1980), Smirnov and others (1983), Seal and others (1988), and Panov and No
204
(1989).
Environmental geology, geochemistry: Shvartseva (1972) and Woessner and Shapley (1984).
Deposit size
Most deposits are of small to intermediate size, <0.1 to 2.0 million tonnes.
Host rocks
Stibnite-quartz deposits are hosted by shale (Lake George, Thompson Falls), marl (Lake George), limestone
Wall-rock alteration
Wall rock for vein deposits (Lake George, Thompson Falls) is altered to sericitic and siliceous assemblages. Rock
altered to sericitic assemblages, which surrounds veins out to distances of tens of meters, is dominated by quartz and
sericite, with minor pyrite and arsenopyrite, whereas that altered to siliceous assemblages is dominated by quartz with
lesser pyrite, arsenopyrite, and stibnite and extends only several centimeters into wall rock. Replacement deposits
(Kadamdzhay, Xiguanshan) are typified by silicification that extends tens of meters into the host limestone.
Nature of ore
Vein deposits are overwhelmingly dominated by veins of quartz cored by massive stibnite. Most replacement
deposits form lenticular bodies of quartz and stibnite within limestone, at contacts with overlying shale, near high-
angle faults.
Main ore zones contain elevated abundances of Sb > Fe > As ± Pb ± Zn ± Cu ± U ± Ba. Lead, zinc, copper,
uranium, and barium are present as minor constituents of ore minerals in local zones in some deposits. Sericitic
Minerals listed in decreasing order of abundance. Potentially acid-generating minerals underlined. Quartz, stibnite,
pyrite, calcite, native antimony, arsenopyrite, pyrrhotite, berthierite, sphalerite, tetrahedrite, lead sulfosalts, barite,
fluorite, chalcopyrite, hematite. Veins are typically zoned from outer quartz-dominated margins to stibnite-dominated
cores. At Lake George, high sulfidation assemblages (stibnite + pyrite) in the eastern part of the vein are laterally
zoned to low sulfidation assemblages (native antimony + pyrrhotite) in its western part.
Mineral characteristics
Stibnite grains range from less than 1 mm to as much as approximately 50 cm long. Grain habits range from
anhedral to euhedral. Sheared and kink-banded fabrics are common. Vug fillings locally are present within the
Secondary mineralogy
Minerals formed by supergene oxidation include: valentinite (orthorhombic Sb2O3), senarmontite (cubic Sb2O3),
cervantite (Sb2O4), kermesite (Sb2S2O), and stibioconite ((Ca,Sb)2Sb2O6(O,OH)), and limonite. None of these
Topography, physiography
Hydrology
Rock cut by vein-filled or replacement-deposit-related faults may have enhanced permeability. Otherwise, local
205
Figure 1. Plot of pH versus dissolved antimony in mine and ground water down gradient from a tailings pond. Data from Shvartseva (1972)
and Woessner and Shapley (1984).
Figure 2. Plot of antimony, arsenic, and sulfate concentration ranges associated with stibnite-quartz deposits. A, mine water data, Kadamzhay
deposit, Russia (Shvartseva, 1972); B, tailings pond and ground water data (within 50 m, down gradient, of tailings pond), Thompson Falls
deposit, Mont. (Woessner and Shapley, 1984).
These deposits have been exploited by a combination of underground and surface mining techniques. Ore is roasted
ENVIRONMENTAL SIGNATURES
Drainage signatures
Mine water draining limestone-hosted replacement ore (Kadamdzhay, Russia) is neutral (pH 7.4 to 7.8) and contains
elevated dissolved metal abundances, including 0.4 to 5.7 mg/l antimony and 750 to 7637 mg/l sulfate (figs. 1 and
2). Stream water within 3 km of the deposit is also neutral (pH 7.0 to 7.1) and contains elevated dissolved metal
abundances, including 0.06 to 0.24 mg/l antimony and 50 to 90 mg/l sulfate (Shvartseva, 1972). No data are
Water from the tailings pond of shale-hosted vein ore waste (Thompson Falls, Mont.) has elevated dissolved metal
concentrations, including 6 to 100 mg/l antimony, 6 to 380 mg/l arsenic, 0.05 to 3.4 mg/l cadmium, copper, iron,
manganese, and zinc, and 570 to 4,800 mg/l sulfate (Woessner and Shapley, 1984); pH data for tailings-pond water
206
are not available. Down-gradient ground water within 50 m of the tailings pond for shale-hosted vein ore waste is
near neutral (pH 6.7 to 7.1) and contains elevated metal concentrations, including 0.14 to 1.9 mg/l antimony, 12 to
Above the surface projection of mineralized veins in the vicinity of the Lake George (New Brunswick, Canada)
deposit, soil samples (B horizon) contain a maximum of 565 ppm antimony. Background values are less than 2 ppm
(Austria, 1971). Regional stream sediment data for the Lake George area indicate a general correlation between
antimony and arsenic contents; maximum antimony abundances are 900 ppm, whereas maximum arsenic abundances
are 170 ppm (Austria, 1971; Pronk, 1992). Antimony and arsenic concentrations are less than 20 ppm in stream
sediment from sites upstream from the known extent of mineralized rock and from drainages in areas not known to
Water associated with tailings ponds may contain high abundances of antimony, arsenic, and sulfate.
Smelter signatures
No data.
No generalizations can be made concerning the relationship between climate and environmental signatures because
of the limited amount of environmental data available for this deposit type. However, in most cases the intensity
of environmental impact associated with sulfide-mineral-bearing mineral deposits is greater in wet climates than in
dry climates. Acidity and total metal concentrations in mine drainage in arid environments are several orders of
magnitude greater than in more temperate climates because of the concentrating effects of mine effluent evaporation
and the resulting "storage" of metals and acidity in highly soluble metal-sulfate-salt minerals. However, minimal
surface water flow in these areas inhibits generation of significant volumes of highly acidic, metal-enriched drainage.
Concentrated release of these stored contaminants to local watersheds may be initiated by precipitation following a
dry spell.
Geoenvironmental geophysics
Metal-bearing ground water plumes may be traceable using geoelectric methods, including the ground slingram.
Plumes associated with stibnite-quartz deposits may contain elevated sulfate ion contents which renders them
electrically conductive. Slingrams such as the Geonics EM-31 or EM-34 give readings of greater than about 25
mS/m within about 5 m of metal-charged plumes. In ideal circumstances, contaminant plumes can be rapidly
Comments
The stibnite-quartz geoenvironmental model is probably generally applicable to ore deposit models 27d, 27e, and 36c.
The acid-buffering capacity of igneous rocks associated with some of these deposits is probably not significantly
different from that of shale-hosted stibnite-quartz deposits. Key differences between stibnite-quartz veins and those
of models 27d, 27e, and 36c that may significantly affect associated environmental impact include: (1) the presence
of minor cinnabar in the latter, (2) the presence of siderite in deposits of the Model 36c type, and (3) specific
mineral-processing and mining techniques applied to each of the different deposit types.
REFERENCES CITED
Austria, V.B., Jr., 1971, The Cu, Pb, Zn, Mn, Mo, and Sb content of stream and spring sediments, York County,
New Brunswick: New Brunswick Department of Natural Resources, Mineral Resources Branch Report of
Investigation, No. 14, 18 p.
Berger, V.I., 1993, Descriptive and grade and tonnage model for gold-antimony deposits: U.S. Geological Survey
Open-File Report 93-194, 24 p.
Bliss, J.D., and Orris, G.J., 1986a, Descriptive model of simple Sb deposits, in Cox, D.P., and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 183.
207
_________1986b, Grade and tonnage model of simple Sb deposits, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 184-186.
_________1986c, Grade and tonnage model of disseminated Sb deposits, in Cox, D.P., and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 187-188.
Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Gumiel, P., and Arribas, A, 1987, Antimony deposits in the Iberian Peninsula: Economic Geology, v. 82, p. 1453-
1463.
Morrissy, C.J., and Ruitenberg, A.A., 1980, Geology of the Lake George antimony deposit, southern New
Brunswick: Canadian Institute of Mining and Metallurgy Bulletin, v. 73, p. 79-84.
Panov, B.S., and No, L., 1989, The Xiguanshan antimony deposit, China: International Geology Review, v. 31, p.
100-110.
Pronk, A.G., 1992, The use of regional stream sediment data in assessing drinking water quality; an example from
Southwest New Brunswick: Atlantic Geology, v. 28, p. 208.
Seal, R.R., II, Clark, A.H., and Morrissy, C.J., 1988, Lake George, southwestern New Brunswick: a Silurian, multi-
stage, polymetallic (Sb-W-Mo-Au-base metal) hydrothermal centre: Canadian Institute of Mining and
Metallurgy Special Volume 39, p. 252-264.
Shvartseva, N.M., 1972, Antimony in ground water of the Kadamdzhay Deposit: Academy Science USSR, Dokl.,
Earth Science Section, v. 207, p. 224-226.
Smirnov, V.I., Ginzburg, A.I., Grigoriev, V.M., and Yakovlev, G.F., 1983, Studies of mineral deposits: Moscow,
Mir, 288 p.
Woessner, W.W., and Shapley, M., 1984, The effects of U.S. antimony's disposal ponds on an alluvial aquifer and
Prospect Creek, western Montana: Montana University Joint Water Resources Research Center Completion
Report G-853-03, 81 p.
208
ALGOMA FE DEPOSITS
(MODEL 28b; Cannon, 1986)
Examples
Archean examples: Vermillion Iron-Formation, Minn.; Sherman Mine, Temagami, Ontario; Helen Mine, Wawa,
Ontario; Early Proterozoic example: Wadi Sawawin, Saudi Arabia.
Exploration geophysics
Gravity and magnetic methods can be used to delineate greenstone belts within granite-greenstone terranes at
provincial to regional scales. Magnetic low and gravity high anomalies are usually associated with relatively
nonmagnetic, dense greenstone terranes, whereas magnetic high and gravity low anomalies are usually associated with
magnetic, low-density granitic terranes (Innes, 1960; Bhattacharya and Morely, 1965; McGrath and Hall, 1969;
Tanner, 1969; and Condie, 1981). Gravity and magnetic methods can also be used for deposit-scale iron-formation
studies. Most iron-formation is associated with positive, high-amplitude gravity anomalies because it contains
elevated abundances of high-density iron minerals, including magnetite and hematite. The magnetic signature of
209
iron-formation is usually one to two orders of magnitude greater than that of its host rock (Bath, 1962; Sims, 1972).
Remote sensing imaging spectroscopy can also be used in regional exploration (Hook, 1990) because iron ore
minerals and their alteration products have distinct spectral signatures (Clark and others, 1993).
The magnetic character of iron-formation is dependent on magnetic mineral content, alteration, structural
attitude, and remanent magnetization. Iron-formation with low magnetite content, or deposits in which magnetite
has been oxidized to non-magnetic hematite, produce low-amplitude anomalies of tens to hundreds of nanoTeslas.
Flat-lying deposits with normal magnetic polarization typically produce positive anomalies of about several thousand
nanoTesla. Steeply dipping or folded iron-formation dominated by remanent magnetic polarization can produce
anomalies with extremely high positive amplitudes of as much as tens of thousands of nanoTesla.
Electrical and electromagnetic methods are generally not applied to iron-formation exploration because the
ore is resistive owing to high silica (chert) content. However, electrical techniques could be used to locate
conductive sulfide facies or to delineate graphitic shale horizons associated with ore deposits.
References
Geology: Goodwin (1973), Gross (1973, 1980, 1988), and Davies and Grainger (1985).
Environment: Ross (1984), Gross (1988), and Myette (1991).
Host rocks
Host rocks include a variety of volcanic and volcaniclastic rocks including basalt, andesite, dacite, rhyolite,
graywacke, shale, and graphitic shale.
Wall-rock alteration
No wall rock alteration is associated with Algoma iron deposits. Although deposits are believed to be related
genetically to submarine hydrothermal vents, nearly all known deposits are distal to vents and have no obvious
related alteration.
Nature of ore
Most ore is banded rock in which iron-rich and chert bands are interlayered on a scale of one to a few centimeters.
Several depositional facies are common and may be in stratigraphic superposition or as lateral equivalents. Oxide
facies ore consists of both magnetite and hematite ore, and is economically the most important; iron carbonate ore
is less commonly mined. Sulfide facies ore is present widely but is seldom mined. Ore grade is relatively uniform,
typically about 30 to 35 weight percent iron, but may vary from 15 to 45 weight percent. Grain size varies according
to degree of metamorphism as does the nature of gangue minerals. A critical factor for environmental consideration
is the metamorphic development of iron-amphibole, which may generate fibrous particles during processing. Iron
amphibole commonly is present in middle greenschist facies or higher metamorphic grade rocks.
210
Ore and gangue mineralogy and zonation
Ore minerals are predominantly magnetite and hematite, less commonly siderite. Gangue is mostly quartz in the
form of variably metamorphosed chert beds. Other gangue minerals that might be present, depending on original
facies of deposition and degree of metamorphism, include greenalite, minnesotaite, stilpnomelane, iron-amphibole,
iron-pyroxene, garnet, and pyrite, generally present in only trace amounts. Magnetically concentrated ore may
include hematite and iron carbonate minerals as gangue.
Mineral characteristics
The most important mineral characteristic is the presence or absence of amphibole that might contribute natural
asbestos fibers or asbestos-like grains produced during processing. Amphibole is a common metamorphic mineral
in iron-formation and may be present in middle greenschist facies or higher metamorphic grade rocks. Original grain
size is also important and varies as a function of metamorphic grade. Weakly metamorphosed iron-formation is
extremely fine-grained and requires very fine grinding (as fine as 0.03 mm in some cases) to liberate iron minerals
from gangue. More highly metamorphosed ore is coarser-grained and requires less grinding. The maximum grain
size after grinding is generally about 0.1 mm, even for the most coarse-grained ore. Grain size fineness is correlated
with increased potential for problems with dust from tailings basins, colloidal and particulate suspensions of ore and
gangue minerals in released process water, and higher tailings weathering rates.
Secondary mineralogy
Because tailings are generally very-fine grained, weathering and formation of secondary minerals may proceed
quickly. Iron oxide minerals alter to iron hydroxide minerals. Iron silicate minerals alter to iron hydroxide minerals
and clay. Most alteration minerals are highly insoluble. Sulfide minerals, mainly pyrite, may also quickly alter and
generate small amounts of acid. Much ore also contains at least trace amounts of carbonate minerals, which, when
present, are probably adequate to neutralize any acid generated.
Topography, physiography
Algoma-type iron-formation deposits are found in a variety of physiographic settings. They are characteristic
deposits of Archean shields; many are in areas of low relief. They may also be present in high relief areas,
particularly where older shields have been incorporated in younger orogenic belts. The principal topographic and
physiographic concern associated with Algoma-type iron-formation deposits relates to large volumes of tailings that
are characteristically produced. In areas of high relief, it may be difficult to site tailings impoundments with
adequate volume. In settings where rapid surface water runoff can produce flash flood hazards, impoundments must
be protected from failure.
Hydrology
Hydrologic communication between ground water, waste piles, and tailings is a predictable consequence of mining.
A detailed study of a taconite tailings basin in Minnesota and its surroundings (Myette, 1991) suggests that associated
environmental problems are minimal. Abundances of components dissolved in water of a tailings test well are well
below maximum abundances permitted by state standards for drinking water, except those of fluoride, which are near
the maximum permitted abundance. Particulate abundances in discharge water are also low, except during occasional
periods of very high precipitation or snow melt.
211
ENVIRONMENTAL SIGNATURES
Drainage signatures
Pre-mining drainage signatures for Algoma-type iron-formation deposits are unknown. In virtually all weathering
regimes, primary iron minerals break down to iron hydroxide minerals and clay that are highly insoluble. With
intense weathering, silica is lost, but not in concentrations that produce a detectable geochemical signature. Some
asbestos-like particles may be released into surface water; however, the U.S. Environmental Protection Agency has
concluded that ingestion of asbestos fibers poses no significant cancer risk (U.S. Environmental Protection Agency,
1991).
Smelter signatures
No smelting is involved in production of Algoma-type iron ore.
Geoenvironmental geophysics
Electrical methods can be used to identify conductive ground water plumes produced by high abundances of dissolved
solids, colloids, and acid. The self potential method can detect leaks in tailings impoundment dikes. Remote sensing
methods can be used to quantify areas of permanent surface disturbance related to mining and ore processing.
Remote sensing methods may also be used to identify areas of stressed vegetation related to sulfur and fugitive-metal
stack emissions and contaminated surface water.
REFERENCES CITED
Bath, G.D., 1962, Magnetic anomalies and magnetization of the Biwabik iron-formation, Mesabi area, Minnesota:
Geophysics, vol. 27, p. 627-650.
212
Bhattacharya, B.K., and Morley, L.W., 1965, The delineation of deep crustal magnetic bodies from total field
aeromagnetic anomalies: Journal of Geomagnetism and Geoelectronics, v. 17, p. 237-252.
Cannon, W.F., 1986, Descriptive model of Algoma Fe, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models:
U.S. Geological Survey Bulletin 1693, p. 198.
Clark, R.N., Swayze, G.A., and Gallagher, A., 1993, Mapping minerals with imaging spectroscopy, in Scott, R.W.,
Jr., and others, eds., Advances related to United States and international mineral resources--Developing
frameworks and exploration techniques: U.S. Geological Survey Bulletin 2039, p. 141-150.
Condie, K.C., 1981, Archean greenstone belts: Elsevier Scientific Publishing Company, 434 p.
Davies, F.B., and Grainger, D.J., 1985, Geologic map of the al Muwaylih quadrangle, sheet 27A, Kingdom of Saudi
Arabia: Deputy Ministry of Mineral Resources Geologic Map GM-82A, 32 p.
Goodwin, A.M., 1973, Archean iron-formations and tectonic basins of the Canadian Shield: Economic Geology, v.
68, p. 915-933.
Gross, G.A., 1973, The depositional environment and principal types of Precambrian iron-formation, in Genesis of
Precambrian iron and manganese deposits, Proceedings of the Kiev Symposium, 1970: UNESCO Earth
Sciences 9, p. 15-21.
_________1980, A classification of iron-formations based on depositional environments: Canadian Mineralogist, v.
18, p. 215-222.
_________1988, Gold content and geochemistry of iron-formation in Canada: Geological Survey of Canada Paper
86-19, 54 p.
Hook, S.J., 1990, The combined use of multispectral remotely sensed data from the short wave infrared (SWIR) and
thermal infrared (TIR) for lithological mapping and mineral exploration: Fifth Australasian Remote Sensing
Conference, Proceedings, Oct., 1990, v. 1, p. 371-380.
Innes, M.J.S., 1960, Gravity and isostasy in northern Ontario and Manitoba: Dominion Observatory of Ottawa
Publication, v. 21, p. 261-338.
McGrath, P.H., and Hall, D.H., 1969, Crustal structure in northwestern Ontario: Regional aeromagnetic anomalies:
Canadian Journal of Earth Science, v. 6, p. 191-207.
Mosier, D.L., and Singer, 1986, Grade and tonnage model of Superior Fe and Algoma Fe deposits, in Cox, D.P.,
and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 228-230.
Myette, C.F., 1991, Hydrology, water quality, and simulation of ground-water flow at a taconite-tailings basin near
Keewatin, Minnesota: U.S. Geological Survey Water-Resources Investigations Report 88-4230, 61 p.
Ross, M., 1984, A survey of asbestos-related disease in trades and mining occupations and in factory and mining
communities as a means of predicting health risks of nonoccupational exposure to fibrous minerals:
American Society for Testing and Materials Special Technical Publication 834, p. 51-104.
Sims, P.K., 1972, Magnetic data and regional magnetic patterns, in Sims, P.K., and Morey, G.B., eds., Geology of
Minnesota, Minnesota Geological Survey, p. 585-592.
Tanner, J.G., 1969, A geophysical interpretation of structural boundaries in the Eastern Canadian Shield: Durham,
England, University of Durham, Ph.D. Dissertation.
U.S. Environmental Protection Agency, 1991, Final national primary drinking water rules: 56 Federal Register 3578
(Jan. 30, 1991).
213
SEDIMENT-HOSTED CU DEPOSITS
(MODEL 30b; Cox, 1986)
Deposit geology
Principal features of the three sediment-hosted copper models are shown diagrammatically in figure 1. These models
portray dimensions, stratigraphic settings, favorable host rocks, mineral zones, degree of oxidation, fractures, ground
water flow, and mining methods of known deposits. Ore fluids were warm (50 to 150° C), oxidizing (hematite-
buffered), and rich in sulfate and chloride ions (to complex Cu+) (Jowett, 1986; Hayes, 1990).
RF: Deposits of the reduced-facies model are present where continental clastic sedimentary rocks are overlain by
regionally extensive marine or lacustrine shale or carbonate rocks, rich in organic material, that act as traps for
mineral deposition (Ensign and others, 1968; Oszczepalski, 1989). Host rocks may be shale or adjacent limestone,
sandstone, or conglomerate. Commonly, the reduced facies overlies basaltic volcanic rocks in rift environments.
Where evidence for a rift is lacking, reduced facies overlie coarse clastic sedimentary rocks derived from older
terranes that contain mafic rocks. Evaporite deposits overlie, or are believed to have once overlain, copper deposits
RB: Deposits of the redbed model are in the same geologic setting as those of the reduced-facies model but lack
regionally extensive reduced strata. In Devonian and later strata, copper commonly replaces local accumulations of
fossil plant matter (LaPoint, 1976). Redbed copper deposits may be present in rifts or intracratonic basins.
RV: Deposits of the Revett model are in thick beds of reduced (pyritic) quartzite (properly, metasandstone) near pre-
ore redox fronts (Hayes and Einaudi, 1986; Hayes, 1990). Orebodies may be stacked, especially near faults (Balla,
1993). Copper is not associated with solid organic matter in Revett deposits, but copper may have been deposited
214
Figure 1. Simplified cross-sections of sediment-hosted copper deposits: A, reduced facies, B, redbed, and C, Revett models. Note scale variations
and vertical exaggeration.
215
Examples
RF: White Pine, Mich.; Creta, Okla.; Kupferschiefer, Germany and Poland; and African copperbelt, Zaire and
Zambia.
The three sediment-hosted deposit types are genetically related and may be present in the same terrane.
Hypothetically, all three deposit types might form from a single ore-forming fluid that invaded reduced rocks.
Deposits belonging to the three models represent ore deposition in different reducing environments: laterally extensive
reduced black shale and carbonate rock (reduced facies model, RF), local areas of reduced rock and plant matter in
redbeds (redbed model, RB), and large reduced areas in sandstone (Revett model, RV). Where sediment-hosted
copper deposits form in a rift environment, as for example in the Keweenaw peninsula, Mich., large deposits of
native copper are present in vesicular basalt flows (Model 23; Cox and Singer, 1986).
Surface disturbance: Mines, including open pits, and mineral processing facilities occupy areas ranging from a few
Water quality: Potential for acid drainage and dissolved metals associated with these deposits is minimized by low
pyrite and chalcopyrite contents and by widespread presence of carbonate minerals in ore and waste rock. Heavy
metal (including arsenic, cadmium, chromium, copper, mercury, and lead) abundances downstream from mining and
milling operations may be elevated; however, undesirable environmental effects have not been reported (U.S Forest
Service and others, 1992). Anomalous quantities of some elements may also be present in ground and surface water
downdip or downslope from undisturbed sediment-hosted copper deposits; anomalies in ground water have been
traced to small concentrations in associated rock (Mosier, in press). Lead in ore can be recovered by smelting
(E&MJ, 1986a), and lead in tailings and waste rock can be minimized during mining by avoiding lead-rich zones
near ore. Metals in surface water may be sorbed by particulate oxyhydroxide minerals, which can be removed by
filtration through soil. Acid water may drain from tailings and waste, particularly from friable or permeable pyrite-
bearing waste rock that is removed to the surface, but the presence of carbonate rock may significantly mitigate these
effects. Elevated ammonia and nitrates from blasting may affect aquatic life.
Air quality: Particulate emissions from mineral processing, including smelting, may exceed air quality standards.
Sulfur dioxide is a product of copper smelting, but emissions can be controlled by collection and production of
sulfuric acid. Copper recovery by SX-EW instead of smelting greatly reduces impact on air quality.
Exploration geophysics
Major structural features (Unrug, 1989) and bleached redbeds (Conel and Alley, 1984) associated with uranium and
copper deposits can be delineated by the Landsat Multispectral Scanner. Integrated studies of basin structure,
thickness, and lithology, which may be applicable to sediment hosted copper deposits, have been conducted using
electrical, electromagnetic, gravity, and seismic methods (Zohdy and others, 1974; Gomez-Trevino and Edwards,
References
Geology General: Kirkham (1984, 1989), and Cox (1986). RF--Ensign and others (1968), Dingess (1976),
Oszczepalski (1989), and Mauk and others (1992). RB--Woodward and others (1974) and LaPoint (1976). RV--
Environmental geology, geochemistry: RV--Balla (1992) and U.S. Forest Service and others (1992).
Deposit size
Size and grade of deposits vary by model (table 1). Median deposit size is related to the average size of the area
potentially impacted by nearby mining; maximum deposit size indicates the size of the largest, potentially impacted
area. The maximum-size reduced facies copper deposit in the United States is White Pine, Mich., which contains
560 million tonnes of 1.2 weight percent copper (Kirkham, 1989). The largest redbed copper deposit is Nacimiento,
N. Mex. (Talbott, 1974). If Lisbon Valley, Utah, is included in the redbed model, maximum size is 35 million tonnes
216
Table 1.--Estimated median and maximum ore tonnages (reserves plus production) and copper grades of each sediment-hosted
copper model type
Model Number of Median size Copper grade Maximum size Grade of maxi-
deposits in (tonnes X 10 6) (percent) (tonnes x 10 6) mum-tonnage
model deposit (percent)
of 0.49 weight percent copper (Mining Record, 1995). If Udokan, Siberia, is included in the Revett model,
maximum size is 1,200 million tonnes of 1.5 weight percent copper (Volodin and others, 1994).
Host rocks
RF: Shale, argillite, siltstone, and their calcareous variants; adjacent rocks that may be important hosts locally include
Most sediment-hosted copper deposits are in terranes principally composed of sedimentary rocks.
RF: Alluvial clastic redbeds and rift-related volcanic rocks; may contain evaporites in subsurface.
RV: Precambrian alluvial and shallow marine sedimentary rocks of low metamorphic grade.
Wall-rock alteration
All models: Zones of reduced rock predate ore deposition and range from regional features extending many
kilometers to local features (Shockey and others, 1974; Oszczepalski, 1989; and Hayes, 1990). Reduced zones in
sandstone are bleached and contain pyrite or iron oxide pseudomorphs after pyrite; possibly formed by reducing
action of fluid hydrocarbons (for example, Conel and Alley, 1984). Reduced rock is separated from oxidized rock
Nature of ore
RF: Most ore in shale is fine-grained (typically 2 to 20 microns at White Pine, Mich., and less than 50 microns in
Kupferschiefer, Poland) disseminated chalcocite accompanied by lesser amounts of native copper (White Pine) and
chalcopyrite and bornite (Kupferschiefer); disseminated ore is accompanied by coarse-grained aggregates, lenses, and
veinlets of native copper (White Pine) and chalcocite (Kupferschiefer) (Ensign and others, 1968; Haranczyk, 1972;
Oszczepalski, 1989); Kupferschiefer sandstone ore contains pore-filling cement of copper sulfide minerals, commonly
chalcocite (Tomaszewski, 1986). Weakly metamorphosed ore is impermeable; shale ore is fissile and friable.
RB: Most ore is composed of malachite, azurite, and chalcocite in sandstone pore space; some ore minerals replace
fossil plant remains; most ore is porous and friable (Woodward and others, 1974; LaPoint, 1976).
RV: Ninety percent of ore contains disseminated sulfide minerals, including chalcocite and bornite, that replace
sandstone cement or fill pore space; crowded, disseminated sulfide minerals form clots that replace sand grains,
follow bedforms, and form ore rods across stratification; minor amount of ore is present in veins; ore is hosted by
RF: Lead, silver, and zinc abundances are locally significantly elevated; cobalt is a coproduct in Zaire-Zambian
deposits; metals associated with organic matter in Kupferschiefer, Poland, include arsenic, bismuth, chromium, cobalt,
gold, molybdenum, nickel, uranium, and platinum-group elements (Przybylowicz and others, 1990).
RB: Lead, silver, uranium, and vanadium are locally abundant in individual deposits; trace elements present in
217
Table 2.--Ore and gangue mineralogy for representative deposits of RF-, RB-, and RV-models; all data expressed as volume
percent. c, common; p, present; --, not reported
Mineral White Pine Creta 1-New Mexico 2-New Mexico Spar Lake
(RF)1 (RF)2 (RB)3 (RB)3 (RV)4
2
Johnson (1976).
3
1-N. Mex., Permian host rocks; 2-N. Mex., Triassic host rocks; data from LaPoint (1979, tab. 1); includes carbonate clasts in
Permian rocks.
4
Hayes and Einaudi (1986); quartz and feldspar from author's data on unmineralized Revett Formation.
5
Minerals that weather at slow to very slow rates: mainly plagioclase, K-feldspar, muscovite, and clay minerals.
6
Minerals that weather at intermediate rates: mainly chlorite, epidote, biotite, and hornblende.
7
Also includes authigenic magnetite and leucoxene.
8
Oxidized ore confined to outcrops and fractures; sulfide minerals adjacent to ore include as much as 0.3 percent chalcopyrite.
anomalous abundances include arsenic, barium, chromium, cobalt, molybdenum, nickel, selenium, strontium, tin,
RV: Lead and silver are abundant; silver is a coproduct; trace elements present in anomalous abundances include
barium, boron, cadmium, chromium, cobalt, mercury, nickel, scandium, vanadium, and zinc (Lange, 1975).
Ore and gangue mineralogy: Gangue assemblages associated with these deposits are summarized in table 2. The
abundance of carbonate minerals and minerals that weather at intermediate rates, principally chlorite- and epidote
group minerals, define the acid-neutralizing capacity of these deposits (Kwong, 1993). Exceptional among RB
deposits, those at Nacimiento, N. Mex., contain very small amounts (0 to 1 volume percent) of carbonate minerals
(LaPoint, 1979).
Sulfide ore mostly consists of chalcocite with or without minor to locally abundant bornite, chalcopyrite,
native copper, galena, sphalerite, and silver minerals. Oxidized ore, abundant only in near-surface RB deposits, is
composed of malachite, azurite, chrysocolla, and cuprite.
Pyrite content: RF--Most ore has low pyrite content; beyond ore, reduced shale and siltstone may contain 1 volume
percent or more pyrite. The Precambrian Nonesuch Formation near White Pine, Mich., contains 0.5 to 3 volume
percent pyrite (Daniels, 1982, p. 120)). RB--Ore has low pyrite content; no reliable data. RV--Ore has low pyrite
content; beyond ore, reduced Revett Formation contains about 0.1 to 0.2 volume percent pyrite (Hayes and Einaudi,
1986).
Zoning: Three types of zoning have been identified: (1) preore reduced and oxidized zones in host formations,
described in section above entitled "Wall-rock alteration", (2) mineralogical zoning in primary ore, formed during
hypogene deposition of sulfide minerals, and (3) secondary ore zones above primary ore, formed by near surface
supergene alteration, described below in section entitled "Secondary mineralogy."
Preore redox fronts control location of some deposits; richest sulfide ore is in reduced rock near redox fronts
in RF Kupferschiefer, Poland, deposits (Oszczepalski, 1989), RB Paoli, Okla., deposit (Shockey and others, 1974),
and RV Spar Lake, Mont., deposit (Hayes, 1990). Primary ore zoning is observed mainly in RF and RV deposits;
zoning upward and outward from the bottom of the orebody is defined by increasing solubility in the sequence
chalcocite-bornite-chalcopyrite-galena-sphalerite. In some reduced facies deposits, such as White Pine, Mich., native
copper is present at the base. Primary zoning commonly is attenuated laterally along bedding and condensed
vertically across bedding; successively higher and distal zones in single mine faces are persistent for many kilometers
laterally. Primary sulfide mineral distribution zonation generally is not reported for RB deposits, perhaps because
218
it has been overprinted by secondary ore.
Mineral characteristics
Textures: Sulfide minerals, interlocked with gangue (see section above entitled "Nature of ore") range from 2 to 50
microns in shale to 2 to 4 mm in sandstone ore; shale ore must be milled to fine particle size to enable metal
Trace element contents: RV--Trace amounts of cobalt and zinc are present in pyrite and sphalerite, respectively
General rates of weathering: Weathering rates depend on overall physical characteristics of rock: RB sandstone ore
(fast) > RF shale ore > RF metamorphosed shale ore > RV quartzite ore (slow).
Secondary mineralogy
RF and RV: Outcrops include sparse stains and fracture fillings of malachite and azurite.
RB: Secondary ore, formed above water table, consists mainly of malachite, azurite, and chrysocolla. A thin zone
that contains minor amounts of cuprite, native copper, native silver, and other minerals has been reported at the
interface between oxidized and chalcocite ore at Nacimiento, N. Mex. (Woodward and others, 1974). Zones of
chalcocite enrichment below the water table have not been reported. Jarosite and natrojarosite are present along
faults at Lisbon Valley, Utah (Schmitt, 1967), but a supergene origin has not been established.
Topography, physiography
RF: Shale hosts are poorly exposed and do not form topographic features.
RB and RV: Some sandstone host rocks form cliffs and escarpments. RV deposits are in mountainous terrane.
Hydrology
RF: Ore deposits and host formations have low permeability and therefore do not channel water flow. Fracture zones
along faults and permeable aquifer beds above and below ore may allow high-volume water entry from rivers, lakes,
and tailings ponds. Examples: White Pine, Mich., mine pumps 3.8 million liters/day; Konkola, Zambia, mine
receives major inflow from fracture zones that connect with a river and a tailings lake; it is one of the world's
RB: Near-surface deposits are permeable, which allows direct recharge. Faults may focus ground water flow locally.
Permeable sandstone hosts may serve as regional aquifers (for example see, Mosier and Bullock, 1988).
RV: Ore deposits and host formations have low permeability and therefore do not focus flow; surface recharge is
limited to fractured zones along faults; water flows down fractures and out mine workings.
RF: These deposits are mined by underground room-and-pillar method to depths of 1,000 m; in a few old mines,
longwall methods were employed. Ore is processed by pulverizing and flotation; concentrates are smelted nearby
or shipped to distant smelters (E&MJ, 1979a; 1986a,b). Some Zambian ore is mined by open-pit methods and
processed by combination of flotation-smelting and SX-EW methods (E&MJ, 1979a). Chalcocite in fractured and
vesicular basalt flows (Model 23, Cox and Singer, 1986) has been successfully processed by in situ leaching (Johnson
RB: These deposits are mined in small adits, inclines, and shafts; larger deposits are mined by open-pit methods.
Historically, ore has been processed by pulverizing and flotation; concentrates were shipped to smelters (Soule, 1956;
Talbott, 1974). Ore produced in the future will probably be processed by heap leach-SX-EW recovery of copper
RV: These deposits are mined by underground room-and-pillar methods; ore has been processed by pulverizing and
flotation near mine site; concentrates have been shipped to smelters (E&MJ, 1979b; U.S. Forest Service and others,
1992).
ENVIRONMENTAL SIGNATURES
Drainage signatures
Mine and processing facilities: All models--These deposits resemble low-sulfide mineral, carbonate-hosted ore
described by Plumlee and others (1993). Accordingly, as might be predicted from the mineralogy of these deposits,
water draining mines and tailings has near-neutral to moderately alkaline pH and low dissolved metal contents (table
219
Table 3.--pH and dissolved metal content of mine and tailings water from some sediment-
hosted copper deposits.
2
Knitzschke and Kahmann (1990).
3
U.S. Forest Service and others (1992, tabs. 6-10, 6-11, 6-12, and 6-14); they also report
3). RF--Where mining, milling, and smelting are conducted on-site, water from multipurpose basins (tailings, mine,
and slag pile drainage) may contain mercury, copper, cadmium, and arsenic. RB--Visually obvious suspended iron
oxyhydroxide particulates, which usually indicate acid drainage, are present at some deposits. Natural concentrations
of arsenic, chromium, selenium, and uranium in redbed hosts of central Oklahoma aquifer are probable sources of
drinking water contamination (Mosier, in press). RV--These deposits have low dissolved metal (Al, As, Cd, Cr, Cu,
Fe, Pb, Mn, Hg, Ag, and Zn) abundances but elevated total metal (including iron, manganese, and aluminum as
particulate oxyhydroxide minerals; and particulate-sorbed copper, cadmium, lead, and zinc) abundances in adit
drainage and water from tailings and settling ponds (U.S. Forest Service and others, 1992). Blasting operations have
Natural drainage: Data from streams draining Revett and other formations of Belt Supergroup suggest near-neutral
pH and low dissolved metals (U.S. Forest Service and others, 1992).
All models: Metal mobility from these deposits is low to moderate, primarily because of acid buffering capacity
provided by associated carbonate rocks. Dissolved metals may be sorbed from acid drainage by suspended iron
oxyhydroxide particulates (Smith and others, 1993) in drainage from open pits and mill tailings. Particulates and
sorbed metals are subsequently removed from water by filtration through soil and by plant uptake (U.S. Forest
All models: Soil and stream sediment associated with some of these deposits contains anomalous abundances of
copper, lead, silver, and possibly arsenic, mercury, and zinc. Copper clearings, in which soil is copper rich and
normal vegetation is replaced by copper-resistant and copper-accumulating plants, are present in the vicinity of Zaire-
Zambian deposits (Reilly, 1967; Reilly and Stone, 1971; Malaisse and others, 1978). Copper (Cu++) is preferentially
adsorbed by organic matter and manganese in mildly acid soil (McLaren and Crawford, 1973).
RV: Soil and sediment associated with some of these deposits contain anomalous metal abundances, including >50
to as much as 2,000 ppm copper, >150 ppm lead, and >0.5 ppm silver (Cazes and others, 1981; Wells and others,
1981).
All models: Surface disturbance results from construction of facilities including conveyors, roads, transmission lines,
mills, and tailings ponds. Local surface water may be diverted by facilities or used for milling. Drainage from
processing sites may contain elevated concentrations of arsenic, cadmium, chromium, copper, mercury, and lead.
Ammonia and nitrate contributed by blasting are of less concern. Acid mine and mill tailings drainage may develop
if inadequate buffering capacity is provided by available carbonate rock. Organic compounds used as flocculents
during milling may be toxic (for example see, U.S. Forest Service and others, 1992, p. 262). Air quality in vicinity
of facilities may be affected by significant emission of suspended particulates, nitrogen oxides, sulfur dioxide, carbon
monoxide, and hydrocarbons (for example see, U.S. Forest Service and others, 1992, tab. 6-2).
RF and RV: Lake levels may be disturbed if underground workings intersect fractured rock beneath lakes (U.S.
RF and RB: Additional surface disturbance may result from open pits and leaching facilities.
220
Smelter signatures
Smelters associated with these deposits may contribute particulates, metals, and sulfur dioxide to the environment.
At White Pine, Mich., 1990 emissions were approximately 900 tonnes/year (t/yr) of particulates and about 225 t/yr
of metal. Estimated outputs include 198 t/yr copper, 25 t/yr lead, 9 t/yr arsenic, 1.8 t/yr cadmium, and <1 t/yr each
of chromium, mercury, and nickel (Anonymous, 1990). The stack plume at White Pine had 60 to 80 percent opacity
in 1990. Most smelters control sulfur dioxide emissions by recovery as sulfuric acid (E&MJ, 1986a). Some ore is
amenable to SX-EW, which avoids smelting. RB and RV ore concentrates are shipped to distant smelters.
The effects of various climate regimes on the geoenvironmental signature specific to sediment-hosted copper deposits
are not known. However, in most cases the intensity of environmental impact associated with sulfide-mineral-bearing
mineral deposits is greater in wet climates than in dry climates. Acidity and total metal concentrations in mine
drainage in arid environments are several orders of magnitude greater than in more temperate climates because of
the concentrating effects of mine effluent evaporation and the resulting "storage" of metals and acidity in highly
soluble metal-sulfate-salt minerals. However, minimal surface water flow in these areas inhibits generation of
significant volumes of highly acidic, metal-enriched drainage. Concentrated release of these stored contaminants to
local watersheds may be initiated by precipitation following a dry spell. Extreme leaching of heavy metals from soil
Geoenvironmental geophysics
Naturally heavy-metal-distressed areas (Bolviken and others, 1977) associated with uranium and copper deposits have
been delineated by the Landsat Multispectral Scanner. Airborne remote sensing (Watson and Knepper, 1994) should
be applicable to mapping environmental effects of mining and processing. A variety of methods, including gravity,
magnetics, electrical, and electromagnetics can be employed to define fluid migration pathways such as buried stream
channels or fault zones. Induced polarization methods can be used to estimate sulfide mineral content of unmined
rock. Plumes with sufficiently high metal contents can be traced using electrical or induced polarization surveys.
Acknowledgments.--We thank Rebecca Miller of the Montana Department of Lands for assistance in
obtaining Environmental Impact Statements and Timothy Hayes of the U.S. Geological Survey for comment on the
illustration of the Revett model.
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224
SEDIMENTARY EXHALATIVE ZN-PB-AG DEPOSITS
(MODEL 31a; Briskey, 1986)
Examples
Red Dog, Lik, and Drenchwater (Alaska); Balmat (N.Y.); Meggen and Rammelsberg (Germany); Faro and other
deposits in the Anvil district, Tom, Jason, and Howard's Pass (Yukon Territory); Cirque and Sullivan (British
Columbia); Lady Loretta, McArthur River, Mount Isa, HYC, and Broken Hill (Australia); Navan, Silvermines, and
Figure 1. Diagrammatic cross section showing mineral zoning in sedimentary exhalative Zn-Pb deposits (from Briskey, 1986).
225
Figure 2. Longitudinal cross section showing development of supergene zone and iron-rich gossan (based on Elura deposit, Australia; from
Taylor and others, 1984).
Associated deposit types (Cox and Singer, 1986) include bedded barite deposits (Model 31b), stratabound lead-zinc
in clastic rocks (Model 30a), Mississippi Valley type lead-zinc (Models 32a,b), sedimentary manganese (Model 34b),
sedimentary phosphate (Model 34c), Besshi massive sulfide (Model 24b), Chinese-type black shale (Mo-Ag-As-Zn-
(1) Deposits with no associated carbonate rocks or carbonate alteration minerals may have high potential for
generation of natural acid drainage that contains high metal concentrations. Potential for acid-drainage generation
may depend on (a) the amount of iron-sulfide present (which may vary from <5 to 80 volume percent among
deposits), (b) the type of iron sulfide mineral(s) in the deposit (reactivity decreases from pyrrhotite to marcasite to
pyrite) and (c) the extent to which the deposit is exposed to air and water. Water may contain thousands of mg/l
sulfate; hundreds of mg/l iron; tens of mg/l aluminum; thousands to tens of thousands of µg/l zinc; thousands of µg/l
manganese and lead; hundreds of µg/l cadmium, copper, and nickel; and tens of µg/l cobalt (Runnells and others,
(2) Potential downstream environmental effects of natural acid drainage can be significant in magnitude and spatial
extent (as far as 30 km downstream), especially if surrounding terrane is composed exclusively of fine-grained clastic
rocks and volcanic rocks and no associated carbonate rocks (EIS for Red Dog).
(3) Deposits with associated carbonate rocks or abundant carbonate alteration minerals have limited potential for acid
drainage generation (water pH usually neutral to slightly alkaline), and most metal concentrations are low; exceptions
include as much as thousands of µg/l zinc in some natural water (Kelley and Taylor, in press).
(4) Iron-rich gossans, formed by oxidation and weathering, may develop above some deposits (fig. 2). Depending
on wall-rock buffering capacity, iron content of ore, and metal-sulfur ratios of sulfide minerals, the gossan
environment can be acidic (pH <5). Some gossans may contain a number of secondary lead-manganese minerals
as well as secondary sulfates of iron, aluminum, and potassium (Taylor and others, 1984). These minerals are very
soluble, can dissolve during periodic storms, and may lead to short term pulses of highly acidic, metal-bearing water.
(5) High-sulfide ore with elevated abundances of pyrrhotite that is exposed to air and water has potential for
spontaneous combustion ("hot muck"), which generates sulfur dioxide (Brown and Miller, 1977; Good, 1977). Hot
muck ignition is rare and can be avoided using proper blasting techniques.
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Exploration geophysics
Gravity surveys can be used to identify and delineate the extent of barite-rich zones that may indicate associated
base-metal sulfide deposits (Young, 1989; Hoover and others, 1994). Induced polarization methods have proven
useful in exploration for some of these deposits (O'Brien and Romer, 1971; Cox and Curtis, 1977). Remote sensing
images can be used to help identify associated gossans (Knepper, 1989). Magnetic and electromagnetic methods may
help define the presence and extent of massive sulfide deposits, particularly those that contain abundant pyrite and
(or) pyrrhotite. Constant source-audiomagnetotelluric methods have been used to help delineate the extent of ore
in northwest Alaska (Young, 1989). Other geophysical methods may facilitate geologic mapping of these deposits
References
Geology: Large (1981), Maynard (1983), Sangster (1983), Moore and others (1986), Turner and Einaudi (1986),
Young (1989), Sangster (1990), Maynard (1991), and Schmidt (in press).
Environmental geology, geochemistry: Lee and others (1984), Runnells and others (1992), Gulson and others (1994),
Prusty and others (1994), Sahu and others (1994), Kelley (1995), and Kelley and Taylor (in press).
Deposit size
Most deposits are 1.7 (90th percentile) to 130 (10th percentile) million tons; median deposit size is 15 million tons
Host rocks
Host rocks consist of a variety of marine sedimentary rocks including: carbonaceous shale and chert (Howard's Pass,
Yukon Territory; Red Dog and Lik, Alaska; and Sullivan, British Columbia), dolomitic shale or siltstone (HYC and
Mount Isa, Australia; and Silvermines, Ireland), and micritic limestone (Meggen, Germany and Tynagh, Ireland).
Turbidites and slump breccias are present locally; minor volcanic rocks, usually mafic, may be temporally, but not
necessarily spatially, associated. Sangster (1990) indicates that tuff units are present within ore-hosting sequences
at a number of deposits, including Meggen and Rammelsberg (Germany), Jason and Faro (Yukon Territory), and
HYC (Australia). Many deposits and their host rocks have undergone metamorphism and deformation (Sullivan,
British Columbia and Mount Isa and Broken Hill, Australia). Comparisons of host rock chemistry to average
abundances in black shale (Vine and Tourtelot, 1970) show that host rocks for these deposits have relatively low
sodium-potassium ratios and are characteristically depleted in cobalt, nickel, and copper and enriched in barium and
manganese (Maynard, 1991).
The geologic terrane surrounding these deposits primarily consists of thick sequences of deep-water marine
sedimentary rocks that include fine-grained clastic and carbonate rocks. Rocks may be metamorphosed (low to high-
grades).
Wall-rock alteration
Stockwork and disseminated sulfide and alteration minerals (commonly silicified or iron-carbonate altered rocks that
rarely contain tourmaline, albite, chlorite), which possibly represent the feeder zones of these deposits, are sometimes
present beneath or adjacent to stratiform deposits (fig. 1). In some deposits, silicification is the dominant or only
alteration (Meggen, Germany and Red Dog, Alaska). In others, alteration is less extensive and (or) carbonate-rich
(Large, 1981). Large (1983) describes more subtle types of alteration near some deposits, including increased
dolomite-calcite ratios (McArthur River, Australia; deposits in Ireland) and increased potassium feldspar-albite ratios
Nature of ore
Within stratiform mineralized rocks, sulfide minerals are generally fine-grained, and commonly form nearly
monomineralic laminae several mm to cm thick that have continuity over large parts of the deposits. Some deposits
are not laminated (Meggen, Germany and Red Dog, Alaska). Coarse-grained crustiform and comb-textured sulfide
minerals may be present in feeder veins associated with stratiform ore. The overall sheet or lens-like morphology
of stratiform ore suggests that these deposits formed, in highly restricted basins, by syngenetic to early diagenetic
227
processes at or below the seawater-sediment interface. Sphalerite, galena, and iron-sulfide minerals (pyrite, marcasite,
and pyrrhotite) are the most common sulfide minerals, but chalcopyrite and sulfosalt minerals may also be present
Feeder or stockwork zone (Large, 1983; Lydon, 1983): iron, lead, and zinc (± gold, boron, and copper).
Stratiform ore (Large, 1983; Lydon, 1983): lead, zinc (cadmium), with or without barium (zoned laterally outward
from feeder zone (fig. 1). Iron may be enriched within and adjacent to the base-metal sulfide zone (Large, 1983);
anomalous concentrations of silver ± arsenic and antimony in sulfosalt minerals (Cox and Curtis, 1977; Taylor and
others, 1984) and manganese (outer halo) are also common (Maynard, 1981). Concentrations of mercury are high
in some deposits, where it is primarily in pyrite, sphalerite, or sulfosalt minerals (Ryall, 1981).
Mineral characteristics
Grain size typically ranges from 15 to 400 microns (McClay, 1983). Primary depositional features are dominated
by fine-grain size and common layering; framboidal and colloform pyrite with euhedral overgrowths are common;
granular sphalerite, galena and barite are typical. Some deposits (Red Dog, Alaska and HYC, Australia) are
characterized by very fine-grained intergrowths of silica and sphalerite (individual sphalerite grains are 0.5 to 50
microns) or sphalerite with other sulfide minerals ( <100 microns) (McClay, 1983; Moore and others, 1986).
Metamorphism partially or completely replaces primary textures and causes grain size increases. Recrystallization
causes porphyroblastic textures in pyrite and sphalerite, barite is recrystallized to an elongate habit, and galena may
Secondary mineralogy
Oxidation of deposits may result in formation of iron-rich gossan and ferruginization of wall rocks (fig. 2). Deposits
with low iron-sulfide contents have high metal-sulfur ratios, gangue or wall rocks with high buffering capacity,
associated gossans have high pH, and gossan profiles are immature. Ore with high iron content, low metal-sulfur
ratios in sulfide minerals, and wall rock with low buffering capacity is likely to be associated with low pH water
(Taylor, 1984). Goethite and hematite, with minor quartz, kaolinite, and beudantite, are the main minerals in gossan
(Taylor and others, 1984). Anglesite and cerussite are the most abundant secondary lead minerals but coronadite,
mimetite, nadorite, pyromorphite, and lanarkite have also been reported. Silver halide minerals may also be present.
Secondary zinc minerals are rare. Secondary sulfate minerals include jarosite, barite, and alunite. Native sulfur,
produced by oxidation of marcasite, is present at Red Dog, Alaska (R.A. Zierenberg, written commun., 1995). Rock
may be oxidized to 100 m below the surface (Australian examples), and may extend to 300 m adjacent to major
faults and shear zones. Oxidation depth is controlled partly by fracture density near orebodies and presence of
pyrrhotite, which is highly reactive with oxygenated ground water (Taylor and others, 1984).
Secondary zinc minerals hemimorphite, smithsonite, and wurtzite, not found in association with other
deposits, are present in lime green mosses and as a cement within talus overlying the XY deposit (Howard's Pass,
Yukon Territory) (Jonasson and others, 1983; Lee and others, 1984).
Topography, physiography
No generalizations possible. Sedimentary exhalative deposits are widely distributed geographically (from Australia
to northern Alaska, to Ireland, for example) and so have completely different physiographies and topographies due
228
Figure 3. Diagram showing the relation between pH and metal content of water draining sedimentary exhalative deposits (modified from Plumlee
and others, 1993).
Hydrology
Hydrologic conditions associated with sedimentary exhalative deposits are highly variable due to extreme differences
of their climate settings. Fine-grained clastic host rocks generally have low permeability. Pre-mineralization and
post-mineralization fractures may serve as ground water conduits. However, depth of oxidation is dependant on a
number of factors including climate, mineralogy of ore, and (or) host rocks.
Underground and open-pit mining have been used historically and in modern times to exploit these deposits. Some
sedimentary exhalative deposits contain very fine-grained and intergrown sulfide minerals (for example, Red Dog,
Alaska and HYC, Australia) that require fine grinding during ore beneficiation. Base metal sulfide minerals are
usually separated by flotation and ore concentrates are smelted. In most cases, concentrates are shipped to custom
smelters and therefore do not contribute to environmental impact in the immediate mine vicinity. Large districts,
such as Sullivan, British Columbia, are served by nearby smelters (for instance, Trail smelter, British Columbia).
ENVIRONMENTAL SIGNATURES
Drainage signatures
Natural drainage water (Lik, Red Dog, and Drenchwater, Alaska: Dames and Moore, 1983; EIS report for Red Dog;
Kelley, 1995; Kelley and Taylor, in press): Cold semi-arid climate--Without carbonate rocks present (Red Dog and
Drenchwater), streams draining mineralized areas have low pH (2.8 to 4.7) and elevated dissolved metal abundances,
including tens to hundreds of mg/l aluminum and iron and hundreds to tens of thousands of µg/l Mn, Cd, Co, Cu,
Ni, Pb, and Zn. Sulfate concentrations, hundreds to thousands of mg/l, in water draining mineralized areas are also
many times higher than background. Sodium and barium may be depleted in water draining mineralized areas. Data
for deposits with significant associated carbonate rocks in surrounding geologic terrane (Lik, Alaska) are strikingly
different (fig. 3). Water draining these deposits is near-neutral to alkaline (pH 6.2 to 8.1) and contains low metal
concentrations, except zinc, whose abundances, thousands of µg/l, are enriched in streams draining mineralized areas.
Neutral-pH springwater near the XY deposit (Howard's Pass, Yukon Territory) contains elevated dissolved metal
abundances, including tens to thousands of µg/l zinc, tens of µg/l cadmium, and tens to hundreds of mg/l sulfate.
229
Mine drainage water (Red Dog, Alaska: Beelman, 1993; unpub. data, Alaska Department of Environmental
Conservation, ADEC, 1995; Faro, Yukon Territory: Lopaschuk, 1979; Meggen, Germany: Bergmann, 1971; Broken
Hill, South Africa: Gulson and others, 1994): Cold semi-arid climate--Immediately after mining began at Red Dog
in the Fall of 1990, water draining the mine was extremely acidic (pH as low as 1.7) and contained elevated metal
abundances, including from hundreds to thousands of µg/l lead, from hundreds to tens of thousands of µg/l cadmium,
and from tens of thousands of µg/l to thousands of mg/l zinc. However, by spring of 1991, these abundances had
decreased to pre-mining natural background concentrations (ADEC, 1995). The pulse of metal-enriched mine
drainage that followed initial mining was probably the result of dissolution and flushing of a secondary sulfate
mineral accumulation that developed as a result of marcasite oxidation in a near surface environment along steep
faults (R.A. Zierenberg, written commun., 1995). Mild wet climate--The Meggen, Germany, mine, known in the
late 1960s for mine water with high metal ion content and low pH, posed a severe pollution problem (Bergmann,
1971). Elevated metal abundances included hundreds of mg/l aluminum, tens of mg/l manganese, hundreds to
Potentially economically recoverable elements: Zinc (as zinc hydroxide produced after treatment of mine water) is
Mine dumps may be a source of lead due to soluble secondary lead minerals common in oxidized material. Some
of these dumps have recently been reprocessed, resulting in an increased volume of fine-grained lead-rich dust that
contains as much as 3 weight percent lead (Gulson and others, 1994); this dust can be important in arid to semi-arid
Bioavailability studies: The most common lead species in soil and dust associated with the Broken Hill, Australia,
deposit was identified as a complex Pb, Fe, Mn, Ca, Al, Si, O material (very soluble), with high bioavailability.
Other mining areas may have less soluble, and therefore lower bioavailability forms of lead, including galena,
Mining and milling methods significantly influence potential environmental impacts associated with sedimentary
exhalative deposits. Most underground mines dispose of mine waste and mill tailings by backfilling underground
workings. Consequently, environmental concerns related to these underground mines may be limited, with exceptions
noted below.
Modern mines discharge fine-grained sulfide-mineral-rich tailings into surface tailings ponds that have
impermeable linings. Previous tailings discharge methods resulted in significant surface and shallow ground water
contamination. Very finely ground, fine-grained and intergrown sulfide minerals may result in highly reactive
tailings products.
Until 1967, mine water draining the Meggen, Germany underground mine had a pH as low as 2.5 and
contained as much as 1,300 mg/l zinc; this mine water originates by percolation of surface water through the mine.
Acidic, metal-rich water draining the mine caused severe pollution in major river systems draining the area. Modern
techniques enabled zinc recovery from mine water; mine effluent ceased to be a pollution source (Bergmann, 1971).
Mining and milling activities at the Zawar, India mine have resulted in contamination of stream sediment
and floodplain soil (Prusty and others, 1994; Sahu and others, 1994). Stream sediment samples collected as much
as 30 km from the mill site contain elevated metal abundances, including thousands of ppm zinc and iron, hundreds
of ppm lead, and tens of ppm cadmium. Floodplain soil contains similarly elevated metal abundances.
230
"Hot muck" was an environmental concern associated with processing ore from deposits with high pyrrhotite
abundances that are exposed to air and water. "Hot muck" is the spontaneous combustion of high sulfide ore in the
mine. The primary environmental concern is evolved sulfur dioxide. Fires are ignited by the build-up of heat,
caused by ore oxidation, in stock piles, or may be triggered by blasting in areas of previously broken ore.
Periodically, air emission can exceed 9.5 ppm SO2 (Brown and Miller, 1977; Good, 1977); SO2 release can acidify
water in areas downwind from release site. Because hot muck is easily avoided by proper blasting techniques, it does
not pose significant risks in modern mining operations.
In some cold climates, it may be necessary to store wet ore, such as that produced from the Faro, Yukon
Territory, mine, prior to crushing. If the ore is stockpiled too long, oxidation may ensue (Lopaschuk, 1979), which
may cause increased metal concentrations, especially lead, in drainage water.
Smelter signatures
Smelting was and is commonly used to process sedimentary exhalative ore. Several studies focus on the correlation
between lead in soil near smelters and lead in the blood of children (Gulson and others, 1994). Smelting may
produce SO2-rich and metal-rich emissions, which may increase acidity and foster accumulation of heavy metals in
downwind areas.
Geoenvironmental geophysics
Induced polarization methods can provide qualitative estimates of sulfide mineral contents and grain size, which
strongly influence potential for generation of acidic, metal-enriched water. Electrical techniques (King and Pesowski,
1993) and ground penetrating radar (Davis and Annan, 1992) can be used to delineate low resistivity anomalies
produced by acidic, metal-enriched water that may be associated with this deposit type.
Acknowledgments.--We thank R.A. Zierenberg for helping to clarify this model and also for considerable
additional input, especially concerning the Red Dog, Alaska sedimentary exhalative deposit.
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233
MISSISSIPPI VALLEY-TYPE PB-ZN DEPOSITS
(MODELS 32a, b; Briskey, 1986a,b)
Deposit geology
The most important characteristics of MVT lead-zinc deposits (Leach and Sangster, 1993) are: (1) most deposits are
in dolostone, less commonly in limestone or sandstone, (2) ore is epigenetic and stratabound, (3) deposits are not
associated with igneous activity, (4) deposits are at shallow depths at flanks of basins, (5) deposits are in platform
carbonate sequences, located either in relatively undeformed rocks bordering foredeeps or in foreland thrust belts,
(6) most deposits are in districts that cover hundreds of square kilometers; a number of districts may even form
metallogenic provinces, (7) deposits form districts that are localized by geologic features, including breccias,
depositional margins of shale units (shale edges), facies tracts, faults, and basement highs that permit upward
migration of ore fluids, (8) ore deposition temperatures are low (50oC to 200oC), but typically higher than those
attributable to local basement-controlled thermal gradients; districts are commonly in thermal equilibrium with respect
to surrounding host rocks, (9) deposits are mineralogically simple; dominant minerals are sphalerite, galena, pyrite,
marcasite, dolomite, calcite, and quartz, (10) associated alteration consists mainly of dolomitization, brecciation,
host-rock dissolution, and dissolution/crystallization of feldspar and clay, (11) evidence of carbonate host rock
dissolution, expressed as slumping, collapse, brecciation, or some combination of these, is common, (12) ore fluids
were dense basinal brines, typically containing 10 to 30 weight percent salts, (13) isotopic data indicate crustal
sources for both metal and reduced sulfur, (14) sulfide mineral textures are extremely varied; ore ranges from
coarsely crystalline to fine-grained, massive to disseminated.
Examples
MVT deposits are found throughout the world (fig. 1); the largest and most intensely studied are in North America.
Important North American districts include the Viburnum Trend and the Old Lead Belt (Southeast Missouri Lead
district), Tri-State, Upper Mississippi Valley, Central Tennessee, East Tennessee (Mascot-Jefferson and Copper Ridge
subdistricts), Austinville-Ivanhoe, Pine Point, Nanisivik, Polaris, Daniel's Harbour, and Gays River. Active MVT
deposits in the United States are in the Viburnum Trend (southeast Mo.), Central Tennessee, and East Tennessee
districts.
Deposit types (Cox and Singer, 1986) associated with MVT deposits are believed to be part of a spectrum of
sediment-hosted ore deposits (Sangster and Leach, 1995) that includes sedimentary exhalative lead-zinc-barite deposits
(Model 31a), carbonate-hosted fluorite deposits (for example, Illinois-Kentucky fluorite deposits), carbonate-platform
deposits (for example, Mt. Isa and HYC, Australia; Balmat-Edwards, N.Y.), Kipushi copper deposits (Model 32c),
and sandstone-lead deposits (Model 30a) (for example, Laisvall, Sweden; Largentiere, France).
Environmental concerns associated with mining MVT deposits are largely unrecognized due to the common assumption
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Figure 1. Location of MVT deposits and districts throughout the World: 1, Polaris; 2, Eclipse; 3, Nanisivik; 4, Gayna; 5, Bear-Twit; 6, Godlin;
7, Pine Point district; 8, Lake Monte; 9, Nancy Island; 10, Ruby Lake; 11, Robb Lake; 12, Monarch-Kicking Horse; 13, Giant; 14, Silver Basin;
15, Gays River; 16, Daniels Harbour; 17, Metaline district; 18, Upper Mississippi Valley district; 19, southeast Missouri district (Old Lead Belt,
Viburnum Trend, Indian Creek); 20, central Missouri district; 21, Tri-State district; 22, northern Arkansas district; 23, Austinville; 24,
Friedensville; 25, central Tennessee district; 26, east Tennessee district; 27, San Vicente; 28, Vazante; 29, Harberton Bridge; 30, Silesian district;
31, Alpine district; 32, Pering; 33, Sorby Hills; 34, Coxco; 35-37, Lennard Shelf district (Cadjebut, Blendvale, Twelve Mile Bore); 38, El-Abad
ekta district. Adapted from Sangster (1990).
that the acid-buffering potential of typical carbonate host rocks controls the mobility of zinc, cadmium, iron, lead,
and other metals. However, the influence of alteration halos, regional aquifers and karst, variable iron sulfide mineral
content, climatic influences, and the presence of various trace elements in ore have not been adequately evaluated.
Surface disturbance: MVT deposits characteristically are in districts that are distributed over hundreds of square
kilometers. The widespread distribution of MVT deposits within a region requires a regional approach to
environmental studies. The Southeast Missouri lead district covers more than 2,500 square kilometers, the Tri-State
district is at least 1,800 square kilometers, the Pine Point district is more than 1,600 square kilometers, the East
Alpine district is about 10,000 square kilometers, and the Upper Mississippi Valley district is about 7,800 square
kilometers. Numerous open pits are present within some districts, especially in abandoned districts such as the
Tri-State district, Okla., Mo., and Kans. Modern mining techniques return most waste rock to underground workings;
surface tailings and mineral processing facilities pose the most serious environmental concerns.
Water quality: Because ore is dominantly carbonate-rock hosted, local rock buffers potential acid mine and tailings
water. Therefore, significant mobility of heavy metals is likely to be spatially restricted. In some districts, water
from underground MVT mines serves as a local domestic water source (for example, Viburnum, Mo. and Upper
Silesia, Poland). Most MVT deposits are in carbonate aquifers that can have enormous fluid transmissivity of
regional extent. Many ore districts contain one or more sandstone aquifers that potentially allow greater contaminant
dispersion than carbonate aquifers. Low iron oxide and clay contents of carbonate aquifers could permit greater
Mineral processing: Tailings ponds and smelter activities generally pose the greatest environmental concerns.
Significant amounts of airborne lead, arsenic, cadmium, and other elements are known to be significant sources of
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Figure 2. Grade-tonnage for MVT deposits and districts and for sedimentary exhalative deposits. Diagonal lines represent total tonnage of
contained lead and zinc (adapted from Sangster, 1990).
metal contamination in many districts. In Upper Silesia, for example, soil in the vicinity of smelters contains
Exploration geophysics
Geophysical exploration has been used su ccessfully in some districts to map known geologic ore controls (Guinness
and others, 1983). For example, airborne magnetic surveys have been used in Southeast Missouri to define buried
Precambrian topography, an important control on the localization of some ore (Allingham, 1966; Cordell, 1979;
Cordell and Knepper, 1987). Galena and pyrite are consistently conduc tive and have associated induced polarization
anomalies, whereas sphalerite's resistivity is variable (Sumner, 1976). In Ireland, induced polarization and
geochemical surveys have been combined to make discoveries of carbonate-hosted sulfide deposits (Hallof, 1966);
associated resistivity data have also been used to map subsurface faults that control mineralization and to map
References
General geology: Heyl and others (1959), a series of papers in Brown (1967), Anderson and Macqueen (1982),
Kisvarsanyi and others (1983), Briskey (1986a, b), Sverjensky (1986), Leach and Sangster (1993), Leach (1994), and
Environmental geology: Smith and Schumacher (1991) and Dudka and others (1995).
Deposit size
In Figure 2, data for deposits versus districts are identified by different symbols. District totals for metals may
represent several dozen individual deposits; for example, the P ine Point and Upper Mississippi Valley districts contain
Individual deposits are generally small; most yield les s than ten million tons of ore. An analysis of the Pine
Point district, for example, showed that most deposits contained between 0.2 and 2 million tons of ore, and the
largest had nearly 18 million tons (Sangster, 1990). For the Upper Mississippi Valley district, Heyl and others
(1959) reported that the average deposit size was between 0.1 and 0.5 million tons, although a few contained as much
as 3 million tons of ore.
Combined lead plus zinc grades in MVT deposits (districts) seld om exceed 10 percent. The Polaris deposit,
in the Canadian Arctic Archipelago, is not only unusually large (22 million tonnes), but is anomalously high grade
as well (18 percent lead plus zinc). A major ity (about 85 percent) of deposits (districts) are zinc-rich relative to lead
and have Zn/(Zn+Pb) values between 0.5 and 1.0, with a distinct mode at 0.8. A smaller group, consisting of the
entire Southeast Missouri district plus a few other small deposits, has a modal value of about 0.05 and is distinctly
236
anomalous in this regard.
Host rocks
Most MVT deposits are hosted by dolostone; less important hosts are limestone and sandstone.
MVT deposits commonly are at shallow depths along basin flanks. They form in platform carbonate sequences,
located either in relatively undeformed rocks bordering foredeeps or in foreland thrust belts. Some deposits are
associated with salt diapirs (for example, Bou Grine in Tunisia). Most deposits are surrounded by carbonate rocks
Wall-rock alteration
Dissolution, recrystallization, and hydrothermal brecciation of host carbonate rocks within and peripheral to
mineralized rock is common to virtually all MVT deposits. These effects may develop in conjunction with
silicification and dolomitization and constitute the major form of wall-rock alteration in MVT districts. Ore-related
silicification of host rocks limits the buffering capacity of rocks near ore deposits and therefore influences the extent
of metal-bearing water dispersion in some districts (for example, Tri-State). Hydrothermal dolomite may be pre-,
syn-, or post-ore and has commonly replaced pre-ore carbonate host rocks to form distinctive alteration halos around
deposits (for example, the East Tennessee and Tri-State districts). Formation of authigenic clay and feldspar minerals
Nature of ore
MVT ore is extremely varied in character and form. Orebodies range from massive replacement zones to open space
fillings of fractures and breccias to disseminated clusters of crystals that occupy intergranular pore space. Crystal
size ranges to as much as a meter or more in some "crystal caverns" in the Tri-State and Central Tennessee districts.
In some districts, notably Pine Point, Silesia, Polaris, and Cadjebut, much of the ore forms extremely fine-grained,
laminated aggregates of botryoidal (colloform) sphalerite, commonly with intergrown dendritic or skeletal galena.
Most MVT deposits show clear evidence that open space deposition was accompanied by dissolution and replacement
of host carbonate rocks. Carbonate host rock replacement is, in some cases, nearly complete, as in the massive
sulfide zones at Nanisivik and Polaris. Ore-hosting structures are most commonly zones of highly brecciated
dolomite; in some instances (for example, Pine Point and Daniels Harbour) these zones are arranged in linear patterns
suggesting a tectonic control, although faults of large displacement are never present. In some districts, notably
Upper Silesia and Ireland, faults, which reflect reactivated basement structures, are the most important ore control.
Trace and minor element suites and abundances vary between districts; the trace element content of minerals from
different paragenetic stages within a district often display significant variation. Table 1 summarizes metal abundances
characteristic of some North American MVT districts. Arsenic, thallium, and cadmium contents of sphalerite from
237
Table 1. Geochemical characteristics of selected North American MVT Deposits.
The distinction between major and minor chemical characteristic is subjective because seldom are grades for minor
metals published and there is an inherent high variability in minor metals even between deposits within a single dis
trict; typically the minor components are less than 1 percent but greater than 0.1 percent. Trace elements are a tabu
lation of elements detected in various mineral phases by a variety of techniques; therefore, they may be used as
general guides for the deposits. Trace elements in bold type may be useful geochemical pathfinders. Trace element
data from (1) Hagni (1983), (2) Heyl and others (1959), (3) Sangster (1968), and unpublished data.
and have Zn/(Zn+Pb) ratios greater than 0.5; this ratio is less than 0.1 in Southeast Missouri deposits. Some ore
deposits, including many in the East Tennessee and Daniels Harbour districts, are essentially lead-free and have
Zn/(Zn+Pb) ratios approaching 1.0. Although the Appalachian MVT deposits are generally galena poor, some
contain significant amounts of galena (for example, Austinville, Va.).
Because most MVT deposits are mineralogically simple, mineralogic zonation has been described for only
a few areas (Southeast Missouri, Pine Point, and Upper Mississippi Valley). Lead, zinc, iron, copper, nickel, and
cobalt abundances are zoned in mineralogically complex deposits of the Southeast Missouri district (Grundmann,
1977; Rogers and Davis, 1977; Sweeney and others, 1977; Hagni, 1983; Mavrogenes and others, 1992). In the Upper
Mississippi Valley district, a local zinc-copper belt is flanked by barite, whereas lead is widespread (Heyl and others,
1959). At Pine Point, Fe/(Fe+Zn+Pb) increases and Pb/(Pb+Zn) decreases outward from prismatic orebodies (Kyle,
1981).
The most abundant gangue mineral is hydrothermal dolomite, which may form alteration halos around MVT
ore deposits (for example, northern Arkansas, East Tennessee, and Tri-State). Barite and fluorite are abundant
gangue minerals in some districts. Other common gangue minerals include calcite and quartz.
Mineral characteristics
Sulfide mineral textures are extremely varied; ore ranges from coarsely crystalline to fine-grained, massive to
disseminated.
Secondary mineralogy
Secondary minerals consist of smithsonite (the dominant ore mineral in some districts prior to the 1950s), calamine,
anglesite, cerussite, malachite, sulfur, goslarite, epsomite, gypsum, melanterite, szomolnokite, copiatite, carphosiderite,
Topography, physiography
Most MVT deposits are located in flat-lying carbonate sequences. However, some deposits are located in thrust and
fold belts (for example, Monarch-Kicking Horse, British Columbia; Alpine district in Europe).
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Figure 3. Range of concentrations and mean values, in parts per million, for cadmium, arsenic, and thallium in sphalerite from major Mississippi
Valley districts: CS, Cracow-Silesia; TS, Tri-State; VT, Viburnum Trend; UM, Upper Mississippi Valley; ET, East Tennessee; PP, Pine Point;
PL, Polaris; NK, Nanisivik (from Viets and others, in press).
Hydrology
The formation of MVT deposits requires enormous quantities of fluid; therefore, most districts have some spatial
connection to major aquifers, karst systems, and faulted ground with high fluid transmissivity. However, present
hydrology can be quite different from paleohydrology. Few generalizations on present hydrological environments
are possible for MVT districts of the World; permafrost conditions prevail at the Polaris mine, whereas Saharan
239
Mining and milling methods
Present mining is generally by underground room-and pillar-or by longwall methods. Ore is processed by pulverizing
and flotation, concentrates are now generally shipped to smelters outside mining districts.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Ground water: Water from a depleted part of one mine in the Viburnum Trend is used as the municipal water supply
for Viburnum, Mo. This water meets all U.S. Environmental Protection Agency water quality standards with the
exception of that for sulfate (Missouri Department of Natural Resources, 1991). Reported sulfate content is 436 mg/l,
whereas the accepted water quality standard is 250 mg/l. Other parameters are pH=7; alkalinity=260; 0.18 mg/l iron;
5.7 mg/l sodium; 72 mg/l magnesium; 0.24 mg/l fluorine; 856 mg/l total dissolved solids; and lead, zinc, and
cadmium abundances are in the µg/l range. In addition, several communities in Upper Silesia, Poland, use water
from underground lead-zinc mines as their only domestic water supply. The sulfate content of this water is variable
but is commonly in the 400 to 800 mg/l range and lead is present in the µg/l range.
Surface water: The Big River flows through the Old Lead Belt and near many waste piles. Water quality data (Smith
and Schumacher, 1991) for a site about 15 km above mining and smelting activity reflect background water
parameters: SO4 <24 mg/l, lead <10 µg/l, cadmium <2 µg/l, and zinc <27 µg/l. During its flow through the district,
carbonate buffering causes water pH (6.6-8.5) to be virtually unchanged. However, SO4, lead, and zinc abundances
increase due to inputs primarily from surface mine wastes. At a site 5 km below the district, the water contains
40-140 mg/l SO4, 10 µg/l lead, <1 µg/l cadmium, and 110-160 µg/l zinc.
Metal mobility away from MVT deposits is limited by the abundance of carbonate rock associated with these
deposits; carbonate rock consumes acid mine drainage and inhibits aqueous metal mobility. Heavy metals abundances
were determined at four seepage sites related to mine wastes in the Old Lead Belt (Smith and Schumacher, 1991).
Maximum concentrations measured were: 850 mg/l SO4, less than 10 g/l copper, 80 µg/l lead, 18,000 µg/l zinc,
and 28 µg/l cadmium. During two years of monitoring at the four sites, pH of the seeps ranged from 6.23 to 8.61.
A lead-zinc ratio of 5 for Old Lead Belt ore (Snyder and Gerdmann, 1968) indicates that zinc is clearly more mobile
than lead or cadmium in water leaching waste materials; these relative mobilities also appear to apply to Big River
water.
240
1,500 ppm lead, 1,000 ppm manganese, and 10,000 ppm zinc.
Most modern underground carbonate hosted zinc-lead mining operations return a significant amount of mine waste
and mill tailings to underground workings as fill. In the case of the Polaris mine in the Canadian Arctic, waste
slurries are used to fill mining voids in their modified room and long pillar galleries. Once frozen, waste supports
the roof and long pillars are mined so that essentially the entire deposit is mined. In Upper Silesia, Poland, surface
disposal of iron sulfide-mineral-rich-tailings presents a potentially serious environmental problem in the district.
Surface mill tailings pose the major potential environmental concern associated with MVT deposits.
Although modern processing facilities carefully monitor water draining from processing sites, some water, with
elevated concentrations of lead, zinc, cadmium, arsenic, thallium, and a variety of trace elements characteristic of
the ore deposit, may escape. Airborne dust from tailing ponds can potentially contribute ore particulates.
Smelter signatures
Studies of the effects of airborne smelter effluent in the Viburnum Trend of southeast Missouri by Bornstein and
Bolter (1991) indicate that soil pH at distances of as much as 6.4 km from the smelter source and as deep as 15 cm
are significantly reduced, ranging from 4.4 at 15 cm to 5.1 at 5 cm depths. At 9.6 km, pH ranged from 6.8 to 7.0.
The sulfur content of the soil correlates with the pattern of pH depletion. Lead, zinc, copper, cadmium, cobalt and
nickel contents of leaf litter and underlying soil decrease with distance from the point source and depth below
surface. The extent of heavy metal enrichment was about half that of the area affected by sulfur enrichment
Palmer and Kucera (1980) conducted a similar study for lead, by sampling both sycamore leaves and twigs
and soil, around four smelter sites in southeast Missouri. Their results were very similar to those of Bornstein and
Bolter (1991) and indicate that dominant wind directions and degree of lead enrichment are correlated.
Climate is extremely critical in assessing the environmental concerns associated with this deposit type. Potential
acid-mine drainage problems related to MVT deposits in relatively dry climate settings are much less than those of
deposits in warm, wet climates. Acid-rain aggravates potential environmental problems in the mid-Atlantic and
northeastern United States, especially the Appalachians. Small MVT deposits are present throughout the
Appalachians in Cambrian to Mississippian carbonate rock sequences (for example, Shady Dolomite and Knox
Dolomite).
The Austinville-Ivanhoe, Va., deposit is a prime example of how climatic conditions and soil properties
interact to increase potential environmental problems. For example, in Wythe County, Va., soil is moderately to very
strongly acidic (pH 5.6 to <4.5) although as much as 30 percent of the surface may consist of dolomitic limestone
outcrops; the soil has moderate permeability, medium to high erosion potential, and moderate water capacity. Much
of the soil is deemed suitable for cultivated crops (legumes), hay, or pasture and much is prime farmland (corn,
vegetables, small grain, and strawberries) (U.S. Department of Agriculture, 1992). Most of the area is in the
drainage basin of the New River, which has water quality problems including excessive concentrations of trace metals
such as zinc, copper, lead, iron, and locally acidification caused by mining activities (Virginia Water Control Board,
1990a,b). Prime farmlands of the valley and ridges within and adjacent to the Appalachian Mountains (near historic
lead and zinc mines) may be affected by metal contamination resulting from past (and future) releases of lead and
zinc into soil and ground water as a result of environmental conditions specific to the region.
In contrast to the Appalachian deposits, the Polaris deposit is in a region of permafrost with little
opportunity for dispersion into the environment from surface or ground water. Likewise, deposits in arid regions
such as Bou Grine in Tunisia, have little opportunity for metal mobility in water.
Geoenvironmental geophysics
In environmental and remediation studies, electrical, seismic surveys, and ground penetrating radar (for sources within
meters of the surface) can help identify shallow mine shaft locations and map geologic structure within and beneath
tailings. Induced polarization and resistivity surveys can be used to identify the source and extent of acid drainage.
Seismic surveys can help distinguish sandstone aquifers from carbonate rocks. However, freshwater sandstone
aquifers in dominantly carbonate rock, are associated with minor resistivity contrasts unless the water contains
241
Acknowledgments.--Special appreciation is extended to Don F. Sangster of the Geological Survey of Canada
for many years of cooperation with the authors in studies concerning MVT deposits. Many data compilations on
MVT deposits from Don were used in this report.
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243
SOLUTION-COLLAPSE BRECCIA PIPE U DEPOSITS
MODEL 32e; Finch, 1992)
Deposit geology
These deposits consist of pipe-shaped breccia bodies formed by solution collapse and contain uraninite, and associated
sulfide and oxide minerals of Cu, Fe, V, Zn, Pb, Ag, As, Mo, Ni, Co, and Se with high acid-generating capacity.
Ore minerals are restricted to the near-vertical breccia pipe and surrounding ring fracture zone. Host rocks include
Examples
Orphan Lode (Chenoweth, 1986; Gornitz and others, 1988); Hack 1, 2, and 3 (Chenoweth, 1988; Rasmussen and
others, 1986); Pigeon (Schafer, 1988); Kanab North (Mathisen, 1987); Canyon (Casadevall, 1989); Ridenour
(Wenrich and others, 1990; Verbeek and others, 1988; Chenoweth, 1988); these deposits are located in the northern
Arizona breccia pipe district (fig. 1), which is the largest breccia-pipe-hosted uranium province in the world. Similar
deposits located elsewhere include Apex, southwest Utah (Wenrich and others, 1987; Verbeek and others, 1987);
Temple Mountain, Utah (Hawley and others, 1965); Pryor Mountains, south-central Mont. (Hauptman, 1956;
McEldowney and others, 1977; Patterson and others, 1988); Tsumeb, Namibia (Lombaard and others, 1986).
Figure 1. Index map of northern Arizona showing locations of mineralized breccia pipes, and San Francisco volcanic field that buries terrane
with high p otential for breccia pipes (Wenrich and others, 1989). Numbers refer to the following mines: (1) Arizona 1, (2) Canyon, (3) Chapel,
(4) Copper House, (5) Copper Mountain, (6) Cunningham, (7) DB-1, (8) EZ-1, (9) EZ-2, (10) Grand Gulch, (11) Grandview, (12) Hack 1, (13)
Hack 2, (14) Hack 3, (15) Hermit, (16) Kanab North, (17) Lynx, (18) Mohawk Canyon, (19) Old Bonnie Tunnel, (20) Orphan, (21) Parashant,
(22) Pigeon, (23) Pinenut, (24) Ridenour, (25) Rim, (26) Riverview, (27) Rose, (28) Sage, (29) Savannic, (30) SBF, (31) Snyder, (32) What.
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Spatially and (or) genetically related deposit types
None of the Cox and Singer (1986) models are known to be genetically related to solution-collapse breccia pipe
deposits. Breccia pipe deposits are, however, spatially associated with uranium concentrations in paleostream channel
deposits and Shinarump ore, hosted by the Chinle Formation near Cameron, Ariz. Cameron area ore may be
genetically related; it may have been deposited from breccia pipe mineralizing fluids that moved up and laterally
outward (Wenrich and others, 1989). Breccia pipe deposits are also spatially associated with Kaibab Limestone
(1) Radon and gamma radiation associated with active and abandoned mines, dumps, and tailings; radon in caves.
(2) Contamination related to transporting radioactive ore along highways in route to processing facilities.
(4) Acid drainage and toxic metals (such as arsenic, lead, and zinc) could be a problem in the immediate vicinity
of mineralized pipes where they are dissected and exposed to flooding and catastrophic precipitation. In general,
limestone and calcareous sandstone host rocks efficiently buffer acidic runoff water, and buffer downward percolating
Exploration geophysics
Electrical conductivity and magnetic properties of the pipes are distinct relative to those of unbrecciated host rocks.
Diagnostic differences in conductivity have been identified by scalar audiomagnetotelluric and E-field telluric profile
data for at least one ore-bearing pipe (Flanigan and others, 1986). Ground magnetometer surveys show subtle
magnetic lows over several pipes, perhaps indicating alteration of detrital magnetic minerals within reduced zones
associated with uranium deposits (Van Gosen and Wenrich, 1989). Breccia pipe locations are spatially unrelated to
magnetic lineations defined by a high resolution aeromagnetic survey (Flanigan and others, 1986). High-grade
uranium ore in northern Arizona breccia pipes is generally deeply buried ( >300 m); associated gamma-radiation is
essentially undetectable at the surface (Wenrich, 1986). Detailed gamma-radiation surveys of more than 1000 breccia
pipe and collapse features indicate that few pipes have associated gamma radiation that is more than five times
background levels (Wenrich, 1985). Scarce gamma radiation anomalies detected at the surface are coincident with
ring fracture zones and are of limited areal extent ( <1 m in diameter) (Wenrich, 1985). The limited size of these
anomalies is responsible for the fact that an airborne radiometric survey, flown about 120 m above ground, over this
area at 5 (east-west) and 10 (north-south) km flight-line spacings (LKB Resources, Inc., 1979) identified no
anomalies associated with known deposits (fig. 1). In limestone karst-hosted uranium-vanadium deposits of the Pryor
Mountains, Mont., minor increases in radioactivity (twice background level) were measured directly over mineralized
fractures, but no anomalous radioactivity was identified near karst pits (Patterson and others, 1988).
References
Hauptman (1956), Wenrich and others (1989), Wenrich and Sutphin (1989), and Finch (1992), Finch and others
(1992).
Deposit size
Most deposits are of small to intermediate size. Production and reserves associated with northern Arizona pipes
include 0.1 to 0.5 million metric tonnes of ore (Finch and others, 1992). Production and reserves from the Tsumeb
deposit include 22 million metric tonnes of ore (Lombaard and others, 1986). Deposits in the Pryor Mountains are
Host rocks
Host rocks include breccias of limestone, sandstone, siltstone, and shale in a finely comminuted sand matrix cemented
by carbonate minerals.
These deposits are in alternating sequences of sandstone, siltstone, shale, and limestone strata overlying paleokarst
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Figure 2. Simplified geologic cross section of northern Arizona breccia pipe showing dimensions and stratigraphic setting of breccia pipe,
uranium-sulfide ore, and sulfide mineral "cap" (Wenrich and Aumente-Modreski, in press). Typical diameter of pipe is 100 m.
Wall-rock alteration
The principal form of wall-rock alteration associated with these deposits is bleaching (oxidation) of iron oxide
minerals in red sandstone by reducing fluids. Alteration extends 30 to 100 m outward into wall rock of Grand
Canyon, Ariz., region deposits and less than 10 m into wall rock of Pryor Mountains, Mont., deposits.
Nature of ore
Nearly all primary ore is confined to breccia pipes (fig. 2); the only major exception is the local presence of ore in
ring fracture zones in country rocks surrounding pipes. The vertical position of most primary ore in the Grand
Canyon breccia pipe is at the Coconino Sandstone, Hermit Shale, and Esplanade Sandstone (all Permian) stratigraphic
horizons.
Randomly collected breccia pipe rock samples contain elevated abundances of a number of elements, including <2
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to 2,400 ppm Ag, 0.5 to 111,000 ppm As, 4 to 100,000 ppm Ba, <2 to 2,900 ppm Cd, 0.66 to 26,000 ppm Co, <1
to 290,000 ppm Cu, <0.01 to 140 ppm Hg, <2 to 24,000 ppm Mo, <2 to 62,000 ppm Ni, <4 to 84,000 ppm Pb, 0.13
to 2,900 ppm Sb, <0.10 to 3,000 ppm Se, 4 to 5,800 ppm Sr, 0.63 to >210,000 ppm U, <4 to 50,000 ppm V, and
Ore and gangue mineralogy and zonation (see Wenrich and Sutphin, 1988, 1989)
Potentially acid-generating minerals underlined. Primary ore minerals include uraninite, sphalerite, galena, nickeline,
linnaeite, stibnite, molybdenite, enargite, chalcopyrite, lautite, tennantite, tetrahedrite, pyrite, marcasite, arsenopyrite.
Primary gangue minerals include barite, quartz, chalcedony, pyrobitumen, kaolinite, calcite, dolomite, ankerite,
Zoning: The only recognized zoning in primary orebodies involves concentration of nickel-cobalt-iron-copper
arsenide and sulfide minerals in sulfide caps above ore. Secondary and supergene minerals are present where ore
has been exposed to oxidation, including canyon dissection and fracture-controlled oxidation (Verbeek and others,
Mineral characteristics
Textures: Uraninite, other oxide minerals, and sulfide minerals are very fine grained (generally <1 mm); rarely,
primary mineral grains are as large as 1 cm. Secondary mineral grains, commonly as much as 1 cm, tend to be
Trace element contents: Primary and secondary minerals can include any of the following: Ag, As, Ba, Cd, Co, Cr,
Cu, Hg, Mo, Ni, Pb, Sb, Se, Sr, U, V, Y, and Zn.
General rates of weathering: These deposits weather rapidly. Primary ore can oxidize within six months when
exposed to surface weathering. Within six months of opening drifts in underground breccia pipe mines in the Grand
Canyon area, some uraninite has altered to hexavalent secondary minerals, primary cobalt-bearing sulfide minerals
Secondary ore and gangue minerals include tyuyamunite, metatyuyamunite, zippeite, zeunerite, metazeunerite,
cerussite, wulfenite, rhodochrosite, hewettite, vesignieite, volborthite, calciovolborthite, roscoelite, erythrite, bieberite,
ilsemannite, chalcocite, djurleite, digenite, covellite, bornite, cuprite, tenorite, chrysocolla, azurite, malachite,
olivenite, chalcanthite, brochantite, cyanotrichite, chalcoalumite, langite, antlerite, devilline, conichalcite, acanthite,
naumannite, proustite, scorodite, melanterite, limonite, hematite, goethite, siderotil, coquimbite, jarosite, celadonite,
Topography, physiography
Solution-collapse breccia pipes are distinguished by concentric-inward-dipping beds that generally surround a basin,
amphitheater-style erosion, concentric drainage, soil and vegetation patterns, breccia, and altered and mineralized rock
(Wenrich and Sutphin, 1988). Locally, breccia plugs are silicified and more resistant to erosion. These plugs form
erosional spires or pinnacles in which ore has been oxidized; most trace metals have been leached from these pipes.
Ring fractures surrounding the pipes tend to erode readily, which causes development of concentric drainage around
pipes.
Hydrology
Ring fracture zones: These zones have high permeability and therefore locally focus ground water flow. Breccia in
solution collapse pipes is also relatively permeable where not silicified. However, in the Grand Canyon area, ground
water within 600 m of the plateau surface is rare. Consequently, unless canyon dissection has altered this situation
or a perched aquifer is present, very little water moves downward into the pipes because orebodies are located, on
Aquifers: Major aquifers in the Grand Canyon region are the Mississippian Redwall Limestone and the Cambrian
Muav Formation; both lie several hundred feet below the breccia pipe orebodies. However, in the Pryor Mountains,
south-central Mont., uranium minerals are oxidized and are within paleokarst and modern karst of the Mississippian
Madison Limestone (stratigraphic equivalent of the Redwall Limestone); this karst is a major aquifer in the region,
247
and therefore plays a major role in uranium oxidation and mobility.
Historic: Typical underground workings followed ring fracture zones in oxidized breccia pipes. Historic mines (fig.
1) produced copper, lead, zinc and silver ore (Billingsley, 1974; Chenoweth, 1988; Wenrich and Sutphin, 1988).
Modern: Uranium was not recognized in the breccia pipes until 1951 (Chenoweth, 1986). All modern mines have
been underground operations. During the 1960s, ore from breccia pipes in the Grand Canyon region was shipped
to a mill in Tuba City (where copper and vanadium were also extracted); since the 1980s it has been shipped to a
mill in Blanding, Utah. Uranium ore is dissolved by either acid or alkaline solutions, and uranium is precipitated
by either ion-exchange or solvent extraction; in either case, the product, commonly ammonium diuranate, is called
"yellowcake" because of its color (Cooper, 1986).
ENVIRONMENTAL SIGNATURES
Drainage signatures
Mine-drainage data: No data available for the Grand Canyon, Ariz., area; no surface water drains the immediate
vicinity of mines. Warchola and Stockton (1982) found no significant uranium enrichment in water draining uranium
deposits in the Pryor Mountains, Mont. More water sampling adjacent to mine areas is needed in both Arizona and
Montana.
Natural-drainage data: Data are available for north Grand Canyon rim springs (Billingsley and others, 1986) and
south Grand Canyon rim springs and surface water, including Hualapai Indian Reservation (Wenrich and others,
1994). Grand Canyon area water is primarily of the calcium-magnesium-bicarbonate type, although some is of the
sulfate and chloride types, which tend to have higher natural trace metal contents. The few spring water samples
collected near mines tend to have metal contents that are similar to, though somewhat more elevated than, those of
some sulfate-type water (Billingsley and others, 1986; Wenrich and others, 1994).
Solid waste resulting from mining breccia pipe deposits is very limited. No data pertaining to quantities of metals
mobilized by limited quantities of water interacting with these wastes are available.
Mineral processing is not done on site. Ore is sorted from waste on site and shipped to mills for processing. On
site environmental concerns include radon from mines and gamma radiation from ore and waste piles. Reclamation
entails backfilling mines with waste rock and sealing mine entrances to prevent radon leakage and future entry.
Mill signatures
Tailings at mineral processing sites are a major environmental concern because of their large volume and fine-grain
size, which permits redistribution by wind. Large tailing piles emit abundant radon and gamma radiation; if water
circulates through tailings and escapes from ponds, local aquifers and drainages may be contaminated. Potential
water contamination may be a particularly serious problem during periods of severe weather or from stress failure
of dams. For example, "on July 16, 1979, the failure of an earthen dam, which held back uranium-mining and
milling wastewater and sediment, released about 94 million gallons of highly acidic liquid and 1,100 tons of uranium-
248
mine tailings to the Puerco River through Pipeline Arroyo....Three months to a few years following the spill, several
scientific studies concluded that no trace of sediment containing radium and thorium from the spill could be
identified, although the Puerco River was still receiving high amounts of dissolved uranium from mine dewatering"
(Wirt, 1994). Although these were not breccia pipe mine tailings, they exemplify the potential problem of uranium
tailings. However, although this was the largest single release of uranium mine tailings in United States history, the
long term and long distance effects of uranium contamination on the environment in the Rio Puerco, N. Mex. and
Environmental mitigation
Environmental mitigation should include complete surface reclamation, as done in the area of the Hack and Pigeon,
Ariz., mines (see photographs in Finch, 1994). Mines should be sealed, dumps and tailings should be backfilled into
In the Grand Canyon region the climate is arid, which greatly retards orebody oxidation and the movement of trace
metals in the environment. The effects of other climate regimes on the geoenvironmental signature specific to these
deposits has not been documented. However, in most cases the intensity of environmental impact associated with
sulfide-mineral-bearing deposits is greater in wet climates than in dry climates. Acidity and total metal
concentrations in mine drainage in arid environments are several orders of magnitude greater than in more temperate
climates because of the concentrating effects of mine effluent evaporation and the resulting "storage" of metals and
acidity in highly soluble metal-sulfate-salt minerals. However, minimal surface water flow in these areas inhibits
generation of significant volumes of highly acidic, metal-enriched drainage. Concentrated release of these stored
Geoenvironmental geophysics
Electromagnetic and direct current resistivity or induced polarization surveys can be used to map acidic drainage and
water with increased metal content escaping from mill tailings. Detailed ground or airborne gamma radiation surveys
can be used to detect or monitor radioactive contamination related to flooding and ore transportation and processing.
Acknowledgments.--The authors would like to thank Vicky Bankey for her contributions to the geophysics
sections of this model.
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249
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Geochemical data files (diskette).
_________in press, Strata-bound copper deposits in the Kaibab Limestone, Coconino County, northern Arizona: U.S.
Geological Survey Bulletin on Kaibab National Forest, 38 p.
Verbeek, E.R., Grout, M.A., and Van Gosen, B.S., 1988, Structural evolution of a Grand Canyon breccia pipe--The
Ridenour copper-vanadium-uranium mine, Hualapai Indian Reservation, Coconino County, Arizona: U.S.
Geological Survey Open-File Report 88-006, 75 p.
Verbeek, E.R., Grout, M.A., Van Gosen, B.S., and Sutphin, H.B., 1987, Structure of the Apex mine, southwestern
Utah [abs.]: Geological Society of America Abstracts with Programs, v. 19, no. 5, p. 340.
Wagoner, J.L., 1979, Hydrogeochemical and stream-sediment reconnaissance basic data report for Williams NTMS
Quadrangle, Arizona: Lawrence Livermore Laboratory, Livermore, Ca., 72 p.
Warchola, R.J., and Stockton, T.J., 1982, National Uranium Resource Evaluation, Billings quadrangle, Montana: U.S.
Department of Energy publication No. PGJ/F-015(82), prepared for the U.S. Department of Energy, Grand
Junction, Colo., by Bendix Field Engineering Corp., Grand Junction Operations, 33 p., 25 plates.
Wenrich, K.J., 1985, Mineralization of breccia pipes in northern Arizona: Economic Geology, v. 80, p. 1722-1735.
250
_________1986, Geochemical exploration for mineralized breccia pipes in northern Arizona, U.S.A.: Applied
Geochemistry, v. 1, no. 4, p. 469-485.
Wenrich, K.J., and Aumente-Modreski, R.M., 1994, Geochemical soil sampling for deeply-buried mineralized breccia
pipes, northwestern Arizona: Applied Geochemistry, v. 9, p. 431-454.
_________in press, Geochemical soil surveys over solution-collapse features and mineralized breccia pipes,
northwestern Arizona--Exploration and environmental applications: U.S. Geological Survey Bulletin 1683-E.
Wenrich, K.J., Boundy, S.Q., Aumente-Modreski, R.M., Schwarz, S.P., Sutphin, H.B., and Been, J.M., 1994, A
hydrogeochemical survey for mineralized breccia pipes--Data from springs, wells, and streams on the
Hualapai Indian Reservation, northwestern Arizona: U.S. Geological Survey Open-File Report 93-619, 66
p.
Wenrich, K.J., Chenoweth, W.L., Finch, W.I., and Scarborough, R.B., 1989, Uranium in Arizona, in Jenney, J.P.,
and Reynolds, S.J., eds., Geologic evolution of Arizona: Arizona Geological Society Digest 17, p. 759-794.
Wenrich, K.J., and Sutphin, H.B., 1988, Recognition of breccia pipes in northern Arizona, in Fieldnotes: Tucson,
Arizona, Arizona Bureau of Geology and Mineral Technology, v. 18, no. 1, p. 1-5, 11.
_________1989, Lithotectonic setting necessary for formation of a uranium rich, solution collapse breccia pipe
province, Grand Canyon region, Arizona, in Metallogenesis of uranium deposits, Proceedings of a technical
committee meeting on metallogenesis of uranium deposits organized by the International Atomic Energy
Agency and held in Vienna, 9-12 March 1987: Vienna, Austria, International Atomic Energy Agency, p.
307-344.
Wenrich, K.J., Verbeek, E.R., Sutphin, H.B., Modreski, P.J., Van Gosen, B.S., and Detra, D.E., 1990, Geology,
geochemistry, and mineralogy of the Ridenour mine breccia pipe, Arizona: U.S. Geological Survey Open-
File Report 90-0504, 66 p.
Wenrich, K.J., Verbeek, E.R., Sutphin, H.B., Van Gosen, B.S., and Modreski, P.J., 1987, The Apex mine, Utah--A
Colorado Plateau-type solution-collapse breccia pipe [abs.], in Sachs, J.S., ed., USGS research on mineral
resources--1987 program and abstracts: U.S. Geological Survey Circular 995, p. 73-74, 76-77.
Wirt, Laurie, 1994, Radioactivity in the environment--A case study of the Puerco and Little Colorado River basins,
Arizona and New Mexico: U.S. Geological Survey Water-Resources Investigations Report 94-4192, 23 p.
251
SUPERIOR FE DEPOSITS
(MODEL 34a; Cannon, 1986)
Examples
Mesabi Iron Range, Minn. (Biwabik Iron-Formation); Marquete Iron Range, Mich. (Negaunee Iron-Formation);
Minas Gerais area, Brazil; Wabush Lake area, Canada
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Superior-type deposits are invariably mined from open pits resulting in large, and partly permanent surface
disturbance. Reclamation after mining generally only partly restores pre-mining surface configuration.
Exploration geophysics
Gravity and magnetic methods can be used to delineate greenstone belts within granite-greenstone terranes at
provincial to regional scales. Magnetic low and gravity high anomalies are usually associated with relatively
nonmagnetic, dense greenstone terranes, whereas magnetic high and gravity low anomalies are usually associated with
magnetic, low-density granitic terranes (Innes, 1960; Bhattacharya and Morely, 1965; McGrath and Hall, 1969;
Tanner, 1969; and Condie, 1981). Gravity and magnetic methods can also be used for deposit-scale iron-formation
studies. Most iron-formation is associated with positive, high-amplitude gravity anomalies because it contains
elevated abundances of high-density iron minerals, including magnetite and hematite. The magnetic signature of
iron-formation is usually one to two orders of magnitude greater than that of its host rock (Bath, 1962; Sims, 1972).
Remote sensing imaging spectroscopy can also be used in regional exploration (Hook, 1990) because iron ore
minerals and their alteration products have distinct spectral signatures (Clark and others, 1993).
The magnetic character of iron-formation is dependent on magnetic mineral content, alteration, structural
attitude, and remanent magnetization. Iron-formation with low magnetite content, or deposits in which magnetite
has been oxidized to non-magnetic hematite, produce low-amplitude anomalies of tens to hundreds of nanoTeslas.
Flat-lying deposits with normal magnetic polarization typically produce positive anomalies of about several thousand
nanoTesla. Steeply dipping or folded iron-formation dominated by remanent magnetic polarization can produce
anomalies with extremely high positive amplitudes of as much as tens of thousands of nanoTesla.
Electrical and electromagnetic methods are generally not applied to iron-formation exploration because the
ore is resistive owing to high silica (chert) content. However, electrical techniques could be used to delineate
conductive sulfide facies associated with ore deposits.
References
Geology: James (1954), Beukes (1983), Morey (1983), and Trendall (1983).
Environment: Bartlett (1980), Ross (1984), Gross (1988), Myette (1991), and Ross and others (1993).
Host rocks
Superior-type iron-formation is interbedded with marine shale, quartzite, carbonate rocks, and in some cases mafic
to felsic volcanic rocks. Some contain extensive diabase sills.
Wall-rock alteration
No wall rock alteration is associated with Superior iron deposits. Formation of Superior-type iron-formation results
from processes involving chemical sedimentation.
Nature of ore
Most ore is banded rock in which iron-rich bands are interlayered with chert bands on a scale of less than a
millimeter to a few centimeters. Several depositional facies are common, including oxide, silicate, and carbonate
facies. The facies may be in stratigraphic superposition within iron-formation or form lateral equivalents; sulfide
facies ore is present in some cases. Oxide facies ore consists of both magnetite- and hematite-bearing rocks, and
253
is the only economically important facies. Grade is relatively uniform, typically about 30 to 35 weight percent iron
for all facies, but may vary from 15 to 45 weight percent. Grain size and the nature of gangue minerals vary
according to degree of metamorphism. A critical factor for environmental consideration is the metamorphic
development of iron-amphibole, which may be released as discrete fibrous particles during processing. Iron
amphibole commonly is present in middle greenschist facies or higher metamorphic grade rocks.
Mineral characteristics
The most important mineral characteristic is the presence or absence of amphibole that might contribute natural
asbestos fibers or asbestos-like grains produced during processing. Amphibole is a common metamorphic mineral
in iron-formation and may be present in middle greenschist or higher metamorphic grade rocks. Original grain size
is also important and varies as a function of metamorphic grade. Weakly metamorphosed iron-formation is extremely
fine grained and requires very fine grinding (as fine as 0.03 mm in some cases) to liberate iron minerals from
gangue. More intensely metamorphosed ore is coarser grained and requires less grinding. Maximum grain size after
grinding is generally about 0.1 mm, even for the most coarse grained ore. Grain size fineness is positively correlated
with increased potential for problems with dust from tailings basins, colloidal and particulate suspensions of ore and
gangue minerals in released process water, and higher tailings weathering rates.
Secondary mineralogy
Because tailings are generally very-fine grained, weathering and formation of secondary minerals may proceed
quickly. Iron oxide minerals alter to iron hydroxide minerals. Iron silicate minerals alter to iron hydroxide minerals
and clay. Most alteration minerals are highly insoluble. Sulfide minerals, mainly pyrite, may also quickly alter and
generate small amounts of acid. Much ore also contains at least trace amounts of carbonate minerals, which, when
present, are probably adequate to neutralize any acid generated.
Topography, physiography
Superior-type iron-formation can be present in a variety of physiographic settings. They are characteristic deposits
of Precambrian shields, so many are in areas of low to moderate relief. They may also be present in high relief
areas, particularly where older shields have been incorporated in younger orogenic belts. The principal topographic
and physiographic concern relates to large volumes of tailings that are characteristically produced. In areas of high
relief it may be difficult to site tailings impoundments with adequate volume. In settings where rapid runoff can
produce flash flood hazards, impoundments must be protected from failure.
Hydrology
Hydrologic communication between ground water, waste piles, and tailings is a predictable consequence of mining.
A detailed study of a taconite tailings basin in Minnesota (Myette, 1991) and its surroundings suggests that associated
environmental problems are minimal. Abundances of components dissolved in water from a tailings test well are
well below maximum abundances permitted by state standards for drinking water, except those for fluoride, which
are about at the maximum permitted abundance. Particulate abundances in discharge water are also low except
during occasional periods of very high precipitation or snow melt.
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Mining and milling methods
Superior-type iron-formation is mined in open pits, and processed to high-grade concentrates that are pelletized and
fire hardened in kilns at or near mine sites. Open pit mining produces relatively large volumes of waste rock that
must be disposed of near mines. Mine waste generally includes shale and quartzite with which iron-formation is
interlayered, and in some cases mafic rocks that intrude the iron-formation. Ore concentration generates large
volumes of tailings, mostly composed of silica and lesser iron silicate and iron carbonate minerals. Stack emissions
of mineral dust might also be a concern; scrubbing or filtering might be required to eliminate dust problems.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Pre-mining drainage signatures for Superior-type iron-formation deposits are unknown. In virtually all weathering
regimes, original iron minerals break down to iron hydroxide minerals and clay which are highly insoluble. With
intense weathering silica is lost, but not in concentrations that produce a detectable geochemical signature. Some
asbestos-like particles may be released into surface water; however, the U.S. Environmental Protection Agency has
concluded that ingestion of asbestos fibers poses no significant cancer risk (U.S. Environmental Protection Agency,
1991).
Smelter signatures
No smelting is required to process Superior-type iron ore.
Geoenvironmental geophysics
Electrical methods can be used to identify conductive ground water plumes produced by high abundances of dissolved
solids and colloidal suspensions. The self potential method can detect leaks in tailings impoundment dikes. Remote
sensing methods can be used to quantify areas of permanent surface disturbance related to mining and ore processing.
Remote sensing methods may also be used to identify areas of stressed vegetation related to sulfur and fugitive-metal
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stack emissions, and contaminated surface water.
REFERENCES CITED
Bartlett, R.W., 1980, The Reserve mining controversy--Science, technology, and environmental quality: Bloomington,
Indiana University Press, 293 p.
Bath, G.D., 1962, Magnetic anomalies and magnetization of the Biwabik iron-formation, Mesabi area, Minnesota:
Geophysics, vol. 27, p. 627-650.
Beukes, N.J., 1983, Paleoenvironmental setting of iron-formations in the depositional basin of the Transvaal
Supergroup, South Africa, in Trendall, A.F., and Morris, R.C., eds., Iron-formation--Facts and problems:
New York, Elsevier Scientific Publishing Inc., p. 131-210.
Bhattacharya, B.K., and Morley, L.W., 1965, The delineation of deep crustal magnetic bodies from total field
aeromagnetic anomalies: Journal of Geomagnetism and Geoelectronics, v. 17, p. 237-252.
Cannon, W.F., 1986, Descriptive model of Superior Fe, in Cox, D.P., and Singer, D.A., eds., Mineral deposit
models: U.S. Geological Survey Bulletin 1693, p. 228.
Clark, R.N., Swayze, G.A., and Gallagher, A., 1993, Mapping minerals with imaging spectroscopy, in Scott, R.W.,
Jr., and others, eds., Advances related to United States and international mineral resources--Developing
frameworks and exploration techniques: U.S. Geological Survey Bulletin 2039, p. 141-150.
Condie, K.C., 1981, Archean greenstone belts: Elsevier Scientific Publishing Company, 434 p.
Gross, G.A., 1988, Gold content and geochemistry of iron-formation in Canada: Geological Survey of Canada Paper
86-19, 54 p.
Hook, S.J., 1990, The combined use of multispectral remotely sensed data from the short wave infrared (SWIR) and
thermal infrared (TIR) for lithological mapping and mineral exploration: Fifth Australasian Remote Sensing
Conference, Proceedings, Oct., 1990, v. 1, p. 371-380.
Innes, M.J.S., 1960, Gravity and isostasy in northern Ontario and Manitoba: Dominion Observatory of Ottawa
Publication, v. 21, p. 261-338.
James, H.L., 1954, Sedimentary facies of iron-formation: Economic Geology, v. 49, p. 235-293.
McGrath, P.H., and Hall, D.H., 1969, Crustal structure in northwestern Ontario: Regional aeromagnetic anomalies:
Canadian Journal of Earth Science, v. 6, p. 191-207.
Morey, G.B., 1983, Animikie Basin, Lake Superior region, U.S.A., in Trendall, A.F., and Morris, R.C., eds., Iron-
formation--Facts and Problems: New York, Elsevier Scientific Publishing Inc., p. 13-68.
Mosier, D.L., and Singer, 1986, Grade and tonnage model of Superior Fe and Algoma Fe deposits, in Cox, D.P.,
and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 228-230.
Myette, C.F., 1991, Hydrology, water quality, and simulation of ground-water flow at a taconite-tailings basin near
Keewatin, Minnesota: U.S. Geological Survey Water-Resources Investigations Report 88-4230, 61 p.
Ross, M., 1984, A survey of asbestos-related disease in trades and mining occupations and in factory and mining
communities as a means of predicting health risks of nonoccupational exposure to fibrous minerals:
American Society for Testing and Materials Special Technical Publication 834, p. 51-104.
Ross, M., Nolan, R.P., Langer, A.M., and Cooper, W.C., 1993, Health effects of mineral dusts other than asbestos,
in Guthrie, G.D., and Mossman, B.T., eds., Health effects of mineral dusts: Mineralogical Society of
America, Reviews in Mineralogy, v. 28, p. 361-409.
Sims, P.K., 1972, Magnetic data and regional magnetic patterns, in Sims, P.K., and Morey, G.B., eds., Geology of
Minnesota, Minnesota Geological Survey, p. 585-592.
Tanner, J.G., 1969, A geophysical interpretation of structural boundaries in the Eastern Canadian Shield: Durham,
England, University of Durham, Ph.D. Dissertation.
Trendall, A.F., 1983, The Hamersley Basin, in Trendall, A.F., and Morris, R.C., eds., Iron-formation--Facts and
Problems: New York, Elsevier Scientific Publishing Inc., p. 69-130.
U.S. Environmental Protection Agency, 1991, Final national primary drinking water rules: 56 Federal Register 3578
(Jan. 30, 1991).
256
SEDIMENTARY MN DEPOSITS
(MODEL 34b; Cannon and Force, 1986)
Examples
Molango (Jurassic), Mexico (Cannon and Force, 1986); Nikopol (Oligocene), Ukraine (Sapozhnikov, 1970); Groote
Eylandt (Cretaceous), Australia (Pracejus and others, 1986); Imini (Cretaceous), Morocco (Force and others, 1986);
Kalahari (Precambrian), South Africa.
Exploration geophysics
Few geophysical investigations of this deposit type are known; however, deposits with similar geologic relations
provide analogies. Battery-active deposits are electrochemically active, as described above; the associated self-
potential field is distinctive. Aeromagnetic surveys can be used to define broad terranes permissive for the presence
of this deposit type because of an association between some sedimentary manganese deposits and iron formation;
most iron formation has a distinct, positive magnetic contrast with surrounding rock (U.S. Geological Survey and
Corporaciaon Venezolana, 1993; Sangmor and others, 1982). Manganese ore minerals, including manganite,
257
"psilomelane" (see section below entitled "Ore and gangue mineralogy and zonation"), and pyrolusite, are dense (3.3-
7.9 g/cc). Resulting dense ore and moderate deposit size (up 10 m thick, and covering 10 km2) indicates that detailed
gravity surveys may help identify these deposits (Rowston, 1965). However, many sedimentary manganese deposits
are either carbonate-facies, poorly consolidated, vuggy, and (or) extremely thin, any one of which may limit the
utility of gravity surveys in exploration for this deposit type (Dorr and others, 1973). In cases where strata are
minimally deformed and their seismic characteristics are well known, seismic refraction or refraction can help
delineate orebodies. All manganese minerals except psilomelane are conductive (Keller, 1989); psilomelane ranges
from conductive to resistive. Thus, massive manganese deposits may have low associated resistivity that can be
detected by electromagnetic or direct current resistivity methods; these deposits also may be identified by induced
polarization surveys. The presence of elemental carbon in surrounding rocks enhances resistivity lows (Dorr and
others, 1973), but also renders direct detection more ambiguous. Electrical surveys over manganese deposits have
not been documented.
References
Geology: Roy (1981), Cannon and Force (1986), and Force and Cannon (1988).
Environmental geochemistry: Matrone and others (1977).
Host rocks
Host rocks include shallow marine sedimentary rocks, most commonly carbonate rocks, clay, and glauconitic sand,
commonly with shellbeds, in high-stand sequences associated with anoxic basins. Most deposits include carbonate
rocks in the host sequence. Barium-, phosphorous-, and copper-enriched rocks may be spatially or stratigraphically
adjacent to manganese-enriched rock. Reduced deposits such as black shale may be similarly associated with
manganese-enriched rock and may contain enrichments of a variety of base metals.
Wall-rock alteration
Alteration associated with these deposits is minor and diagenetic rather than hydrothermal in origin.
Nature of ore
Ore consists of thin beds of manganese oxide and (or) carbonate minerals, only incidentally influenced by structural
features. Vertical zonation may record depositional regression at high sea-level stand. Lateral zonation may involve
oxide-facies to carbonate-facies transitions.
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Carbonate-facies deposits: Ore- Rhodochrosite, kutnahorite, siderite, mixed manganese and iron carbonate minerals,
pyrite, and wad. Gangue- Clay, calcium and calcium-magnesium carbonate minerals, glauconite, organic matter,
pyrite, quartz, and biogenic silica.
Mineral characteristics
Rocks that host these deposits can include various textural features, including sedimentary laminae, sedimentary
oolites and pisolites, and diagenetic botryoidal and vuggy textures, that probably do not affect the environmental
signature of these deposits. Grain-size variation among deposits is extreme.
Secondary mineralogy
Secondary minerals include lithiophorite, nsutite, and wad. Nsutite and vernadite may be associated with
electrochemical activity.
Topography, physiography
The topography and physiography of these deposits are variable, nondiagnostic, and have limited relation to
geoenvironmental signatures. Nsutite deposits tend to be perched on ridges and plateaus.
Hydrology
These deposits exert little, if any, influence on the local hydrologic regime. Locally, manganese carbonate minerals
are present only below the water table.
ENVIRONMENTAL SIGNATURES
Drainage signatures
The geochemistry of water draining sedimentary manganese deposits is directly related to and mimics the
characteristics of individual deposits.
Smelter signatures
Manganese ore is not commonly smelted near mine sites.
Geoenvironmental geophysics
Detailed gravity, seismic (Sklash and Jiwani, 1983), and electrical surveys aid tonnage estimates and delineation of
259
deposit geometry prior to mining; accordingly, estimates concerning the potential magnitude of air and water
pollution that may be associated with ore extraction can be made. Magnetic surveys augment these techniques by
enabling definition of hydrologic features, including cavities, faults, and aquitards, within and surrounding deposits.
In favorable circumstances, resistivity, seismic velocity, and seismic reflectivity surveys may enable estimates of
depth to the water table.
REFERENCES CITED
Cannon, W.F., and Force, E.R., 1986, Descriptive model of sedimentary Mn: U.S. Geological Survey Bulletin 1693,
p. 231.
Cox, D.P. and Singer, D.A., 1986, Mineral Deposit Models: U.S. Geological Survey Bulletin 1693, 379 p.
Dorr, J.V.N., II, Crittenden, M.D., Jr., and Worl, R.G., 1973, Manganese, in United States Mineral Resources: U.S.
Geological Survey Professional Paper 820, p. 885-398.
Force, E.R., and Cannon, W.F., 1988, Depositional model for shallow-marine manganese deposits around black-shale
basins: Economic Geology, v. 83, p. 93-117.
Force, E.R., Back, William, Spiker, E.C., and Knauth, L.P., 1986, A ground-water mixing model for the diagenesis
of the Imini manganese deposit (Cretaceous) of Morocco: Economic Geology, v. 81, p. 65-79.
Keller, G.V., 1989, Electrical properties, section V, in Carmichael, R.S., ed., Practical Handbook of physical
properties of rocks and minerals, Boca Raton, CRC Press, p. 357-427.
Matrone, G., Jenne, E.A., Kubota, J., Mena, I., and Newberne, P.M., 1977, Manganese, in Mertz, W., ed.,
Geochemistry and the environment, v. II, The relation of other selected trace elements to health and disease:
National Academy of Sciences, p. 29-39.
Mosier, D.L., 1986, Grade and tonnage model of sedimentary Mn: U.S. Geological Survey Bulletin 1693, p. 231-
233.
Pracejus, B., Bolton, B.R., and Frakes, L.A., 1986, Nature and development of supergene manganese deposits, Groote
Eylandt, Northern Territory, Australia: Ore Geology Reviews, v. 4, p. 71-98.
Rowston, D.L., 1965, Gravity survey of manganese deposits in the Mt. Sidney-Woodie-Woodie, Pilbara gold field:
Geological Survey of Western Australia Annual Report for 1964, p. 49-51.
Roy, Supriya, 1981, Manganese deposits: London, Academic Press, 458 p.
Sangmor, S.S., Dorbor, J.K., Hoskins, L., Mason, J.A., Jr., Murray, G., and Pshoor, P., 1982, The Mt. Dorthrow
manganese mineralization: Monrovia, Liberian Geological Survey, Project report of investigations, 1981,
53 p.
Sapozhnikov, P.G., 1970, Manganese deposits of the Soviet Union: Jerusalem, Israel Program for Scientific
Translations, 522 p.
Sklash, M.G., and Jiwani, R.N., 1983, Groundwater contamination on Walpole Island, Ontario, Canada, in Papers
of the International conference on groundwater and man, Volume 2, Groundwater and the environment:
Australian Water Resources Council Conference Series, No. 8, p. 387-394.
U.S. Geological Survey and Corporaciaon Venezolana de Guayana, Tecnica Minera, C.A., 1993, Geology and
mineral resource assessment of the Venezuelan Guayana Shield: U.S. Geological Survey Bulletin 2062, 121
p.
260
LOW SULFIDE AU QUARTZ VEINS
(MODEL 36a; Berger, 1986)
by Richard J. Goldfarb, Byron R. Berger, Terry L. Klein, William J. Pickthorn, and Douglas P. Klein
Deposit geology
These deposits consist of Archean through Tertiary quartz veins, primarily mined for their gold content, that generally
contain no more t han 2 to 3 volume percent sulfide minerals, mainly pyrite, in allochthonous terranes dominated by
greenstone and turbidite sequences that have been metamorphosed to greenschist facies. Wall rocks contain abundant
carbonate and sulf ide minerals, quartz, and sericite. Arsenic and antimony are enriched in alteration haloes (fig. 1).
These deposits are also known as mesothermal, Mother Lode-type, orogenic, metamorphic rock-hosted, greenstone
gold (Archean), turbidite-hosted (Phanerozoic), and slate belt gold (Phanerozoic) deposits.
Low sulfide gold quartz veins in the United States are presently being mined and prospected within rocks
of accreted lithotectonic terr anes along both continental margins. Development of Paleozoic lode deposits in recent
years has been restricted to the South Carolina part of the Carolina slate belt. Open pit mining of the more than 1.5
million oz of gold at the Ridgeway mine, the largest of the deposits, is continuing at a rate of about 100,000 oz/yr.
In the Mother Lode region of central California, a few old mines have been reopened in the last 10 to 15 years as
open pit operations. Each has been recovering 500,000 to 750,000 oz of gold from low-grade, Early Cretaceous
deposits. Production from the Alaska-Juneau and Kensington Eocene vein deposits in southeastern Alaska by
underground operations is scheduled to commence during the next few years and will yield about 6 million oz of
gold. In the Alaskan interior, open-pit mining will eventually yield 4 million oz of gold from the mid-Cretaceous
Fort Knox deposit.
Examples
Yilgarn Block, Western Australia; Abitibi Belt, Superior Province, Canada; Yellowknife, Northwest Territory,
Canada; Bendigo/Ballarat, Victoria, Australia; Murantau, Uzbekistan; Mother Lode, Calif.; Juneau Gold Belt, Alaska;
261
Spatially and (or) genetically related deposit types
Associated deposit types (Cox and Singer, 1986) may include silica-carbonate mercury (Model 27c), and gold-
antimony (Model 36c), which may reflect shallower exposures of similar hydrothermal systems in areas of limited
erosion. In areas of extensive uplift and erosion, gold lodes are reconcentrated in extensive placer accumulations
(Model 39a). Low sulfide gold quartz vein deposits subjected to tropical weathering can produce lateritic saprolite
(eluvial placer) deposits (Model 38g; McKelvey, 1992). Much older, pre-accretionary Cyprus, Besshi, and Kuroko
volcanogenic massive sulfide deposits (Models 24a, 24b, 28a) are spatially associated with gold veins in terranes that
(1) Moderate amounts of acid mine drainage may be present where local, relatively high sulfide mineral
concentrations are present in gold ore, where broad zones of sulfidization characterize wall rocks, and (or) where
much of the ore is hosted by greenstone that has relatively low acid-buffering capacity.
(2) Oxidation of mine tailings that contain sulfide minerals, particularly arsenopyrite, or soil formed from unmined,
yet sulfide-mineral-bearing rock can release potentially hazardous arsenate, arsenite, and methylarsenic species.
(3) Increased concentrations of arsenic, antimony, and other trace metals may be present downstream from deposits.
Cyanide used for gold extraction at many active mines is a potential additional contaminant in waste water discharge.
(4) Mercury amalgamation carried out during historic operations may be a source of mercury contamination in
aquatic life and in surface sediment. Continued use of mercury amalgamation and roasting for gold extraction in
some parts of the world is a direct and very serious health hazard.
(5) Disposal of tailings from developed deposits can cause sedimentation problems in adjacent waterways.
(6) Modern open-pit mining methods, allowing for development of previously uneconomic, low-grade gold deposits
pose quality-of-life concerns. Potential concerns include mining-related visual impacts, increased traffic and noise,
and dust generation. Open-pit mining also produces significantly greater volumes of untreated waste rock.
(7) Long term exposure to arsenic concentrated in tailings can cause cancer and kidney disease.
Exploration geophysics
Silicified rock, much of which corresponds to wall rock that contains abundant sulfide minerals, is commonly
associated with local resistivity highs. Silicified rock and carbonate minerals along veins increase density and
resistivity and may allow indirect sulfide mineral identification using detailed electromagnetic, direct current
resistivity, and micro-gravity mapping. Disseminated pyrite, arsenopyrite, and chalcopyrite distributions can be
outlined using induced polarization/resistivity surveys. Piezo-electricity may locate sulfide-mineral-bearing quartz
veins. Host rocks that contain at least moderate amounts of magnetite may have associated magnetic lows due to
References
Geology: Berger (1986), Bliss (1986, 1992), Groves and others (1989), Kerrich and Wyman (1990), Kontak and
others (1990), Nesbitt (1991), Berger (1993), Goldfarb and others (1993), Phillips and Powell (1993), and Klein and
Day (1994).
Environmental geology and geochemistry: Bowell and others (1994), Callahan and others (1994), Cieutat and others
(1994), Azcue and others (1995), and Trainor and others (in press).
Deposit size
Deposit size is extremely variable. Archean deposits-mean tonnage is 1.08 million metric tonnes; range is 0.004 to
199 million metric tonnes. The mean tonnage of Phanerozoic deposits is 0.03 million metric tonnes; range is 0.001
to 25 million metric tonnes. The mean tonnage of Chugach-type Phanerozoic deposits is 0.003 million metric tonnes;
range is 0.001 to 0.07 million metric tonnes. Because of the high-grade nature of some veins (greater than 30 to
50 g gold/t), many low tonnage deposits (for instance, Chugach-type low sulfide gold deposits; Bliss, 1992) are
developed by small, underground workings. Deposits that contain at least 0.5 to 5 million oz gold in low grade ore
(generally 2 to 10 g gold/t) have been mined in large, more modern open-pit operations. The largest example of such
a deposit is Murantau, Uzbekistan, which contains greater than 140 million oz gold (Berger and others, 1994).
262
Host rocks
Archean ore is largely hosted by metamorphosed basalt (greenstone), although ultramafic volcanic rocks (komatiite),
felsic volcanic rocks, and granitoid intrusions are locally important hosts. Most of these deposits are in preserved
cratonic blocks. Phanerozoic deposits are hosted in slate and graywacke in deformed, continental margin orogenic
belts. Where competent, pre-ore igneous bodies are present in metasedimentary sequences, they commonly
Deposits are restricted to medium-grade, generally greenschist facies, metamorphic rocks. High-tonnage deposits,
exemplified by Murantau, Uzbekistan, have a spatial association with major structural zones that are commonly
believed to be old terrane boundaries. Contemporaneous calc-alkaline dioritic to granitic plutons, sills, and batholiths
within a few tens of kilometers of ore indicate that both are products of regional, middle to lower crustal thermal
events.
Wall-rock alteration
Mineral phases vary with host rock lithology; halo width varies with size of hydrothermal system. Alteration zones
are poorly developed in metasedimentary host rocks, but are broad and distinct in both felsic and mafic igneous
rocks. Silicified rock and carbonate minerals are ubiquitous. Disseminated pyrite and (or) arsenopyrite consistently
are present in these broad haloes. Sericite is common, but only close to discrete gold-bearing veins; in some systems,
biotite is also present adjacent to veins. Sericite gives way to a chlorite-epidote propylitic zone distal to veins. Talc,
chlorite, and fuchsite are also common in alteration zones within ultramafic host rocks; albite is common in granitoid
host rocks. Wall rocks are notably enriched in H2O, CO2, S, K, Au, W, Sb, and As (Nesbitt, 1991).
Nature of ore
Ore may be present in quartz veins and (or) adjacent sulfidized wall rock. Gold is present as free grains in quartz,
as blebs attached to wall rock ribbons, and in veinlets cutting sulfide grains. Individual veins are 1 to 10-m-wide
discrete fissure fillings that have strike lengths of less than 100 m. Many veins show ribbon texture or, less
commonly, contain brecciated wall rock fragments. In some deposits, ore forms dense stockworks of cm-wide
veinlets. Vein swarms at the large deposits can attain 5-km strike lengths, 500 m widths, and extend 2 km down
dip. Carbonate minerals may form either (1) restricted alteration zones that range from a few to tens of meters away
from small shear zones or (2) may be abundant in rocks within several kilometers of major faults.
Abundances of silver, arsenic, gold, and iron are consistently anomalous; tungsten and antimony abundances are much
less consistently anomalous; bismuth, copper, mercury, lead, and zinc abundances are anomalous in many deposits;
(2) Potentially acid-generating sulfide minerals (in order of abundance) are pyrite, arsenopyrite > stibnite >
chalcopyrite, pyrrhotite, galena, sphalerite > telluride minerals, tetrahedrite > bismuthinite > molybdenite.
(4) Potentially acid-buffering carbonate minerals include siderite, ankerite, calcite, magnesite, or ferroan dolomite;
(5) Silicate gangue minerals include quartz, muscovite, chlorite, biotite, fuchsite, tourmaline, rutile, albite, and (or)
talc.
(6) Zoning is uncommon; mineral assemblages and proportions are commonly consistent over depths of greater than
1,000 m. In some systems, shallow levels may contain more abundant stibnite or sulfosalt minerals and native silver
Mineral characteristics
Sulfide minerals are usually present as finely disseminated grains in quartz and wall rocks. In some deposits, massive
clots of arsenopyrite, as large as tens of cm, may be present locally. In rare examples, gold-bearing veins may
contain massive stibnite (10 to 50 volume percent of the vein material) throughout the deposit (see Berger, 1993).
263
Secondary mineralogy
Secondary minerals are not common. Occasionally, some arsenopyrite has weathered to scorodite. Minor limonite
Topography, physiography
The deposits do not form distinct topographic features although in some mining districts (for example, Bendigo,
Australia) mineralized zones may form distinctive linear ridges. They may have been emplaced along steep and
rapidly uplifting mountain belts; many pre-Tertiary low-sulfide gold-quartz veins are present in more tectonically
Hydrology
Veins are generally hosted in permeable fracture zones and hence are also significant local ground water conduits.
Mining relatively sulfide-mineral-rich veins of this deposit type could cause discharge of minor volumes of relatively
metal-rich ground water. However, seepage from poorly consolidated tailings piles and adits at historic workings
is likely to be the most common source of metal contamination of surface water near this type of gold deposit.
Both underground and open pit mining methods are presently being used to extract ore. In the United States, except
for southeastern Alaska, open pit mining is most common. During milling, ore is usually crushed and ground in a
ball mill; subsequently, gravity concentration is used to remove the largest gold particles. Remaining ore is reground,
classified and thickened, recycled through the gravity concentrator, and then processed in cyanide vats for 48 hours.
The slurry is subsequently sent to a carbon-in-pulp circuit where dissolved gold is adsorbed on carbon. Gold is then
stripped from carbon and electroplated on steel wool before being refined into bullion in a furnace. Alternatives to
In some parts of the world, mercury amalgamation is still being used to aid gold recovery. Sulfur
compounds, which adversely impact the amalgamation process, are eliminated by first roasting ore. Mixing mercury
with gold concentrates results in amalgam that leaves gold behind when the mercury is volatilized.
ENVIRONMENTAL SIGNATURES
Drainage signatures
For both mined and unmined orebodies, low sulfide mineral contents of ore and acid-buffering capacity of
widespread carbonate alteration assemblages generally prevent significant acid-mine drainage and heavy metal
Natural drainage: Limited data (Carrick and Maurer, 1994; Cieutat and others, 1994; Trainor and others, in press)
suggest that unmined occurrences have little impact on surface water pH or trace element content. In southern
Alaska, data indicate that arsenic abundances increase from <5 to 6 µg/l, iron from <20 to 140 µg/l, and sulfate from
<2 to 3 to 5 mg/l where natural water encounters unmined low-sulfide gold quartz vein occurrences.
Mine drainage: Arsenic and iron abundances in water draining small workings may be enhanced by one to two
orders-of-magnitude relative to background abundances downstream from unmined occurrences (Cieutat and others,
1994; Trainor and others, in press). Other metals do not exhibit corresponding enrichments. Even water draining
directly from major pits and extensive underground workings can have neutral pH and low metal contents; water
draining the Alaska-Juneau pit and underground workings, Alaska's largest gold mine, contain <6 µg/l arsenic and
<100 µg/l iron at a pH of 8.0; sulfate abundances in this water are as much as 340 mg/l (Echo Bay Mines, unpub.
company data). Water flowing out the portal of the Independence, Alaska, mine in the Willow Creek district, the
fourth largest past lode gold producer in the state, has a pH of 7.8 and contains <40 µg/l iron, 37 µg/l arsenic, and
<4 µg/l cadmium, copper, lead, antimony, tungsten, and zinc (R.J. Goldfard, unpub. data, 1995).
In rare examples, locally high concentrations of sulfide minerals can lead to significant metal-rich and (or)
acid mine drainage. Contaminated water that drained from an old adit at the site of an open pit gold mine at
Macraes Flat, South Island, New Zealand, which had a pH of 2.9 and elevated dissolved metal abundances, including
as much as 77 mg/l zinc and 80 mg/l iron (BHP Gold, New Zealand, unpub. company report, 1988), may reflect this
type of situation.
Seepage from poorly consolidated tailings piles may be more acidic. Small volumes of water seeping from
tailings in the Cariboo district, British Columbia has a pH of 2.7 and contains 556 µg/l arsenic and elevated
abundances of cadmium, copper, lead, and zinc (Azcue and others, 1995). Seepage from sulfide mineral concentrates
at an abandoned mill site, of the Treadwell, southeastern Alaska, mines has a pH of 2.9 and contains 330 mg/l iron,
264
2500 µg/l zinc, 380 µg/l copper, 160 µg/l cobalt, 100 µg/l nickel, 32 µg/l cobalt, and 21 µg/l lead (R.J. Goldfarb,
unpub. data, 1995).
Mercury, which is used for gold extraction, is extremely enriched in sediment and fish tissue in drainages
downstream from many historic low-sulfide gold mines (Callahan and others, 1994). Down-river from present-day
gold mines in Brazil, mercury abundances are as much as 20 ppm in sediment, 2.7 ppm in fish, and 8.6 µg/l in water
(Pfeiffer and others, 1989). Down-river from the California Mother lode veins, dredged river sediment contains as
much as 37.5 ppm mercury and surface and ground water contains 13 to 300 µg/l mercury (Prokopovich, 1984).
Extreme mercury abundances have been documented in sediment in drainages of the Carolina slate belt and the
Dolgellau gold belt, Wales, more than 75 years after cessation of mining (Fuge and others, 1992; Callahan and
others, 1994). Unfiltered and filtered samples of water draining tailings piles in the Fairbanks, Alaska, district
contain as much as 0.58 and 0.11 µg/l mercury, respectively (R.J. Goldfarb, unpub. data, 1995).
Acid mine drainage problems are probably restricted to water that infiltrates untreated mine dump piles. In temperate
climates, initial spring snow melt draining dumps is likely to contain significant heavy metal abundances, including
arsenic, iron, and antimony, and less commonly lead and zinc, mainly due to dissolution of soluble salts accumulated
during winter. In most cases, small volume acidic effluent seeping from waste piles is diluted to background
abundances upon entering adjacent stream channels; consequently, environmental impact is restricted to surface
channels upstream from their intersections with the nearest, major surface waterway.
Arsenic is usually the trace element of greatest environmental concern in soil associated with mine tailings.
Inadvertent soil ingestion by young children and arsenic-rich household dust pose potential risks to human health.
These risks have become serious public issues in parts of the California Mother Lode belt where housing projects
have been developed on soil derived from old mine tailings (Time, September 25, 1995, p. 36). Secondary, arsenic-
bearing salt minerals, and to a lesser extent, relatively insoluble arsenic-bearing sulfide minerals, become bioavailable
primarily by adsorption from the fluid phase in the small intestine. Geochemical factors that control arsenic
bioavailability from soil include the type of arsenic-bearing mineral, the degree of encapsulation of that mineral in
an insoluble matrix, the nature of alteration rinds on mineral grains, and the rate of arsenic dissolution in the
gastrointestinal tract (Davis and others, 1992).
Arsenic concentrations in soil and sediment are about 50-1,000 ppm near unmined deposits; background abundances
elsewhere are typically 10-40 ppm. Antimony levels are commonly >5 ppm near deposits, whereas they are normally
<2 ppm in areas unaffected by hydrothermal activity (Bowell and others, 1994; R.J. Goldfarb, unpub. data, 1995).
(1) Mercury amalgamation, commonly used in historic gold extraction processes, may have deposited significant
amounts of mercury in tailings piles. Where amalgamation is still used, volatilized mercury generated by the process
may significantly affect air quality because as much as ten percent of the mercury used is lost to the atmosphere.
(2) Most cyanide used for gold extraction is recovered and recycled, but some inevitably remains in tailings liquor
and can leak into regional ground water networks. Any loss of cyanide to the environment is a major concern
because it is toxic to a wide variety of organisms. Many mills now use chemical-treatment systems to convert
hazardous cyanide compounds to insoluble compounds that are not bioavailable. This is especially critical in cold
climates where natural volatilization of cyanide from holding ponds is relatively slow.
(3) Where heap-leaching methods are employed, environmental risks are greater. Leakage and erosion risks are
greatly increased, as are risks to wildlife, because cyanide leaching is usually done outdoors. In situ leaching, if
(4) Roasting ore that contains abundant sulfide minerals emits significant amounts of arsenic trioxide and other metal
(5) Highly acidic effluent is produced during sulfide bio-oxidation. Without proper neutralization or during spills,
(6) Crushing and grinding ore may present noise and dust hazards.
Smelter signatures
265
Climate effects on environmental signatures
In dry and seasonally wet climates, the potential for generating small-volume pulses of significantly metal-enriched
acid mine drainage is enhanced by evaporation and soluble salt accumulation. In wet climates, increased surface
runoff enhances dilution and may mitigate peak acid and heavy-metal concentrations that characterize mine drainage
Geoenvironmental geophysics
Wide-band electromagnetic systems can be used to identify major shears associated with veins that may serve as
major ground water conduits. Direct current and electromagnetic resistivity and magnetic surveys can be used to
study porous rock associated with faults and shear zones that may control ground water flow. Within and downslope
from mine areas, low-resistivity acid- or metal-bearing water can be identified with electromagnetic or direct current
induced polarization/resistivity surveys and ground penetrating radar. Structural and stratigraphic features such as
bedrock topography, buried channels, and aquitards that affect water flow away from mine areas may be studied with
electromagnetic or direct current resistivity, seismic refraction, and gravity surveys. Stratigraphic details in shallow
sand and gravel water pathways can be investigated with seismic reflection and ground penetrating radar. Water flow
REFERENCES CITED
Azcue, J.M., Mudroch, A., Rosa, F., Hall, G.E.M., Jackson, T.A., and Reynoldson, T., 1995, Trace elements in
water, sediments, porewater, and biota polluted by tailings from an abandoned gold mine in British
Columbia, Canada: Journal of Geochemical Exploration, v. 52, p. 25-34.
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral
deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
Berger, B.R., Drew, L.J., Goldfarb, R.J., and Snee, L.W., 1994, An epoch of gold riches-the Late Paleozoic in
Uzbekistan, central Asia: Society of Economic Geologists Newsletter, no. 16, p. 1, 7-11.
Berger, V.I., 1993, Descriptive and grade and tonnage model for gold-antimony deposits: U.S. Geological Survey
Open-File Report 93-194, 24 p.
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds.,
Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
_________1992, Grade and tonnage model of Chugach-type low-sulfide Au-quartz veins, in Bliss, J.D., ed.,
Developments in mineral deposit modelling: U.S. Geological Survey Bulletin 2004, p. 44-46.
Bowell, R.J., Morley, N.H., and Din, V.K., 1994, Arsenic speciation in soil porewaters from the Shanti mine, Ghana:
Applied Geochemistry, v. 9, p. 15-22.
Callahan, J.E., Miller, J.W., and Craig, J.R., 1994, Mercury pollution as a result of gold extraction in North Carolina,
U.S.A.: Applied Geochemistry, v. 9, p. 235-241.
Carrick, S., and Maurer, M., 1994, Preliminary water resource assessment of the Girdwood area, Alaska: Alaska
Division of Geological and Geophysical Surveys, Public Data File 94-51, 36 p.
Cieutat, B.A., Goldfarb, R.J., Borden, J.C., McHugh, J., and Taylor, C.D., 1994, Environmental geochemistry of
mesothermal gold deposits, Kenai Fjords National Park, south-central Alaska, in Till, A.B., and Moore, T.,
eds., Geological Studies in Alaska by the U.S. Geological Survey, 1993: U.S. Geological Survey Bulletin
2107, p. 21-25.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Davis, A., Ruby, M.V., and Bergstrom, P.D., 1992, Bioavailability of arsenic and lead in soils from the Butte,
Montana mining district: Environmental Science Technology, v. 26, p. 461-468.
Fuge, R., Pearce, N.J.G., and Perkins, W.T., 1992, Mercury and gold pollution: Nature, v. 357, p. 369.
Goldfarb, R.J., Snee, L.W., and Pickthorn, W.J., 1993, Orogenesis, high-T thermal events, and gold vein formation
within metamorphic rocks of the Alaskan Cordillera: Mineralogical Magazine, v. 57, p. 375-394.
Groves, D.I., Barley, M.E., and Ho, S.E., 1989, Nature, genesis and tectonic setting of mesothermal gold
mineralization in the Yilgarn block, Western Australia, in Keays, R.R., Ramsey, W.R.H., and Groves, D.I.,
eds., The geology of gold deposits--The perspective in 1988: Economic Geology Monograph 6, p. 71-85.
Kerrich, R., and Wyman, D., 1990, Geodynamic setting of mesothermal gold deposits--An association with
accretionary regimes: Geology, v. 18, p. 882-885.
266
Klein, T.L., and Day, W.C., 1994, Descriptive and grade-tonnage models of Archean low-sulfide Au-quartz veins
and a revised grade-tonnage model of Homestake Au: U.S. Geological Survey Open-File Report 94-250,
29 p.
Kontak, D.J., Smith, P.K., Kerrich, R., and Williams, P.F., 1990, Integrated model for Meguma Group lode gold
deposits, Nova Scotia, Canada: Geology, v. 18, p. 238-242.
McKelvey, G.E., 1992, Descriptive model of laterite-saprolite Au, in Bliss, J.D., ed., Developments in mineral
deposit modelling: U.S. Geological Survey Bulletin 2004, p. 47-49.
Nesbitt, B.E., 1991, Phanerozoic gold deposits in tectonically active continental margins, in Foster, R.P., ed., Gold
metallogeny and exploration: Blackie, London, p. 104-132.
Pfeiffer, W.C., DeLacerda, L.D., Malm, O., Souza, M.M., DaSilveira, E.G., and Bastos, W.R., 1989, Mercury
concentrations in inland waters of gold-mining areas of Rondonia, Brazil: The Science of the Total
Environment, v. 87-88, p. 233-240.
Phillips, G.N., and Powell, R., 1993, A link between gold provinces: Economic Geology, v. 88, p. 1084-1098.
Prokopovich, N.P., 1984, Occurrence of mercury in dredge tailings near Folsom South Canal, California: Bulletin
of the Association of Engineering Geology, XXI, p. 531-543.
Ripley, E.A., Redmann, R.E., and Crauder, A.A., 1995, Environmental effects of mining: Delray Beach, Florida, St.
Lucie Press, 356 p.
Trainor, T.P., Fleisher, S., Wildeman, T.R., Goldfarb, R.J., and Huber, C., in press, Environmental geochemistry of
the McKinley Lake gold district, Chugach National Forest, Alaska, in Moore, T., and Dumoulin, J.D., eds.,
Geological Studies in Alaska by the U.S. Geological Survey, 1994: U.S. Geological Survey Bulletin 2152.
267
STRATABOUND AU IN IRON-FORMATIONS
(MODEL 36b; Berger, 1986)
Examples
Homestake, S. Dak.; Jardine, Mont.; Lupine, Northwest Territories; São Bento, Brazil; Cuiaba, Brazil; Champion,
India.
Exploration geophysics
Oxide-facies iron-formation has high magnetization and density; consequently, its presence can commonly be
delineated by aeromagnetic and gravity studies (Kleinkopf and Redden, 1975; Hildenbrand and Kucks, 1985).
However, carbonate-facies iron-formation that hosts these gold deposits commonly contains no magnetite and is
weakly magnetic, except where it contains abundant pyrrhotite. In carbonate-facies iron-formation, aeromagnetic
highs are best developed in association with pyrrhotite- or troilite-rich deposits. Ground magnetic surveys have been
used to delineate oxide-facies iron-formation and predict strike extensions, bed thickness, and dip of magnetic zones
within stratigraphic sequences (Lindeman, 1984). Induced polarization and a variety of electromagnetic surveys,
which can help identify concentrations of disseminated, conducting minerals, can be used to refine the location of
disseminated, sulfide-mineral bearing deposits and to project the extent of known deposits (Lindeman, 1984).
Potential host rocks that contain iron oxide and carbonate minerals can be identified with high resolution imaging
spectrometers where vegetation does not obscure rock reflectance characteristics (Clark and others, 1990).
268
References
Rye and others (1974), Rye and Rye (1974), Bidgood (1978), Hallager (1980), Ladeira (1980), Kath (1990), Caddey
and others (1991), and Kerswill (1993).
Host rocks
Most GDIF are contained within carbonate-facies iron-formation or their metamorphosed equivalents. Oxide-facies
iron-formations host very few gold deposits. Minor amounts of silicate-facies and oxide facies iron-formation are
known. Host iron-formation may or may not be laterally continuous within the shale-dominant sedimentary
sequence. Transitions within iron-formation from one facies to another are known, and range from carbonate- to
oxide-, silicate-, and sulfide-facies. Typically, the iron-formations are less than 30 meters thick but quite continuous
along strike. Minor tuffaceous rocks are associated with some iron-formation, but this component does not appear
to be a prerequisite for GDIF. Clay-rich layers in the iron-formation may have formed by submarine weathering
of basaltic material or direct hydrothermal precipitation of iron-rich material.
Wall-rock alteration
No wall-rock alteration is associated with bedded ore. Chloritization is associated with vein-related ore.
Nature of ore
Typical bedded ore contains 1 to 15 volume percent sulfide minerals, including troilite, pyrrhotite, pyrite,
arsenopyrite. Electrum is associated with various sulfide minerals. Ore may be pyrrhotite rich, pyrite rich, or
arsenopyrite rich. Typical quartz vein ore contains 0.5 to 10 volume percent sulfide minerals, including pyrrhotite,
pyrite, and arsenopyrite. Electrum is again associated with various sulfide minerals or with quartz.
Mineral characteristics
Pyrrhotite and troilite in non-metamorphosed, bed-controlled ore are fine-grained to very fine-grained.
269
Metamorphism of GDIF increases grain size to medium grained. Arsenopyrite and pyrite in non-metamorphosed,
bed-controlled ore are coarser (medium-grained) than pyrrhotite and troilite. Metamorphism of pyrite and
arsenopyrite does not increase their grain size. Electrum in bed-controlled ore may form very fine-grained inclusions
in either pyrrhotite or arsenopyrite. Elemental gold and silver are also present in solid substitution for iron in
arsenopyrite. Quartz-vein-controlled sulfide minerals are notably coarser grained than those in bed-controlled ore.
Arsenopyrite and pyrrhotite in vein-controlled ore are commonly coarse-grained. Electrum in vein-controlled ore
may be visible to fine-grained.
Secondary mineralogy
Surface weathering of all minerals is highly dependent on local climate. Weathering of siderite, ankerite, and calcite
typically produces pits filled with iron oxide minerals. Weathering of pyrrhotite and arsenopyrite typically results
in tarnished grains that are partially replaced by pyrite. Carbonate minerals in GDIF are highly capable of buffering
oxidation reactions due to surface weathering.
Topography, physiography
Deposits are in desert, steppe, temperate forest, tundra, and tropical forest areas. Carbonate-rich rocks weather
differently in each terrane, from prominent ridges in deserts to topographically low areas in tropical environments.
Hydrology
Water flow through metamorphic rocks is typically controlled by shear zones and fractures. Porosity of all host rocks
is very low to nonexistent. Hydrologic flow models probably are not relevant for GDIF. Most existing deposits are
exploited in underground mines that have complex dewatering systems and sophisticated water treatment plants.
ENVIRONMENTAL SIGNATURES
Drainage signatures
Very few or no GDIF in the United States are contained in drainage basins that have only iron-formation-type
deposits. Homestake, S. Dak., is surrounded by Tertiary, epithermal vein and replacement deposits that contribute
metals to the drainage basin of Whitewood Creek. Jardine, Mont., is in an area covered by extensive Tertiary
volcanic rocks; contributions from minor vein deposits in the Bear Creek drainage at Jardine may obscure the
geochemical signatures attributable to Jardine. Metal contributions from the Yellowstone area hot springs above
Jardine probably also overprint drainage signatures from the GDIF below Jardine.
Deposits in the Northwest Territories, Canada, are probably the best candidates for accurate definition of
drainage signatures associated with GDIF, but topography, and therefore drainage net development, is negligible near
Lupine and surrounding deposits. Soil geochemistry surrounding these deposits might accurately reflect pre-mining
conditions, as the deposits were discovered in the early 1960s.
270
Arctic environment, as these deposits were not discovered until the 1960s. Similarly, newly discovered deposits in
the Amazon region of Brazil could be utilized to define pre-mining conditions in a tropical environment.
Smelter signatures
No currently operating mines use smelters to process ore. Historic smelters may or may not have been numerous
in different districts, depending on the number of individual mines. Smelters undoubtedly serviced many types of
ore deposits, not only those from GDIF. Soil contamination from any one or group of smelters is not directly
attributable to any one GDIF deposit.
Geoenvironmental geophysics
Detailed magnetic and electromagnetic surveys can delineate fluid migration paths along geologic contacts, faults,
and fractures (Paterson, 1995) but application of these techniques in crystalline rocks can be difficult. Electrical and
electromagnetic surveys can be used to trace and monitor acidic, metal-enriched ground water plumes (Ebraheem
and others, 1990; McNeill, 1990) in horizontally stratified rocks, but with difficulty in crystalline terranes. Hot spots
in tailings piles that result from ongoing redox reactions can be located using self potential methods. Induced
polarization surveys can be used to discriminate between environmentally benign electrical conductors, such as clay
bodies, and metal-enriched ground water (Paterson, 1995).
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271
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