Principles of Geochemical Prospecting-Hawkes
Principles of Geochemical Prospecting-Hawkes
Principles of Geochemical Prospecting-Hawkes
Geochemical
Prospecting
GEOLOGICAL SURVEY BULLETIN 1000-F
CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING
FOR MINERALS
By H. E. HAWKES
ABSTRACT
« Shaw (1952).
TABLE 2. Source data for abundance of elements in fresh water
Fresh water
Number
Element of Water (micrograms per liter) Residue (ppm) ' Reference
samples Average Median 2 Range * Average Median
(1) (2) (3) (4) (5)
Aluminum 700 4,800 Polynov (1937).
Arsenic - 37 1 <3 7 Kehoe and others (1944).
Barium. _ 27 160 120 50-300 1,100 820 Braidech and Emery (1935); Wilska (1952).
Boron. . . 51 110 80 20-lcO 750 550 Braidech and Emery (1935); Huberty and
others (1945).
O
ii
Calcium ... _ 29 , 600 202 , 700 Conway (1942).
Chromium. 24 5.3 2 1-10 36 14 Braidech and Emery (1935). 3H
Cobalt. ------- 16 4.9 2.3 .82-10 33 16 . Maliuga (1945).
CO
Copper 4 _,--_ 61 70 30 20-200 480 200 Braidech and Emery (1935); Kehoe and others
(1944). O
*1
Copper 5,. 67 21 5 <2-33 140 34 Huff (1948) ; Vogt and Rosenqvist (1943).
404 250 200 < 100-400 1,700 1,400 U. S. Geological Survey (1952). o
M
Gold----.----- 23 .0033 .0014 .0001-. 0073 .023 .0096 Haber and Jaenicke (1925). O
172 510 220 40-1 , 500 3 , 500 1,£00 F. W. Clarke (1924b). : O
38 3.4 2 <l-9 23 14 Braidech and Emery (1935); Grushko and 8
Shipitsvn (1948); Huff (1948); Kehoe and
others (1940).
4,400 30,300 Conway (1942).
Manganese. 74 52 10 2-50 360 70 Braidech and Emery (1935); Harvey (1949); i
Kehoe and others (1944); Wilska (1952).
3 .05 .05 .Ol-.l .34 .34 Stock and Cucuel (1934).
NickeL ------ 44 15 4 1-13 100 27 Braidech and Emery (1935); Maliuga (1945). §
CO
2,900 20,200 Conway (1942).
69 8 1 <1-20 55 7 de Salas (1947). o
8,000 54,500 Conway (1942). H
i i
Silver-. ------- 23 .0071 .0034 <.0001-.01 .048 .023 Haber and Jaenicke (1925). 25
3,800 26,300 Conway (1942). O
Sulfur.--. -,._- 5,500 37 , 900 Do.
3 25 ~20 170 ~140 Kaminskaia (1944).
Zinc-____ --- ~10 ~70 Huff (1948).
1 Figures for composition of mineral residue of river water are computed from data in columns 1 and 2 on the basis of 146 ppm average salinity for river water (Conway,
1942).
1 The median (or mode) is statistically .the most common value; if all the values quoted are listed in ascending order, the median is in the center of the list. to
* The figures listed represent the range after the highest and lowest 16 percent of the reported values have been omitted; they are thus analogous to a standard deviation. to
4 Determinations by spectrographic analysis. CO
5 Determinations by wet chemical analysis.
230 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
The second, and for the purposes of this report the most sig-
nificant, phase of the science of geochemistry is the study of the
laws of equilibrium governing the distribution and migration of
elements in the earth. As Clarke (1924a, p. 10) has stated:
Each rock may be regarded, for present purposes, as a chemical system,
in which, by various agencies, chemical changes can be brought about.
Every such change implies a disturbance of equilibrium, with the ultimate
formation of a new system, which, under the new conditions, is itself stable
in turn.
Although Clarke is considering only petrogenesis, his comments
apply equally well to processes of weathering, transportation, and
sedimentation at the earth's surface.
THE GEOCHEMICAL CYCLE
The disturbance of equilibrium necessary in Clarke's concept
of changing chemical systems can be caused by a static change in
the temperature and pressure within a closed system or by a dy-
namic movement of material into new chemical and physical envi-
ronments. In nature, closed systems are rarely if ever found;
virtually all geologic processes involve a certain amount of move-
ment of material.
The movement of earth materials from one environment to an-
other can be conveniently visualized in terms of a partly closed
cycle, as illustrated in figure 32. Starting on the right-hand side
of the dkgram ano! moving clockwise, sedimentary rocks are pro-
gressively metamorphosed as they are subjected to increasing tem-
perature, pressure, and availability of juvenile increments from
depths, They may eventually attain a state of fluidity such that on
recrystallization they can differentiate into several kinds of igneous
rocks and hydrothernial extracts. When erosion brings the result-
ing suite or rocKs into me surnclal environment again, the com-
ponent elements are re-sorted by weathering agencies in accordance
with their relative solubility in water. A new series of sedimentary
rocks is then deposited and the cycle is closed. The diagram pre-
PRINCIPLES OF GEOCHEMICAL PROSPECTING 281
WEATHERING SEDIMENTATION
Principal metals:
200 250,000 1 , 2CO
Cobalt________. _______ 23 5,000 200
70 10,000 140
Gold................. 0.005 10 2.000
Iron... ..._-_-_____--- 50.000 300,000 (i
Lead ... . ... 10 50,000 3,000
1,000 250,000 250
Mercury _____ _____._. 0.077-0.5 3,000 10,000
Molybdenum. _____ ... 2.5-15 5,000 TOO
Nickel. ------..-.. .. 80 15.000 190
O.J 500 5,000
Tin. ___.-. -_ ___.__. 40 10,000 2.;o
Tungsten.... __ .__ . 1 . 5-69 5,000 200
Vanadium. ___- _ . ... 150 25,000 160
Zinc 132 SO, 000 620
Byproduct metals:
Antimony. __.. . ... 1 ro.ooo 50,000
Arsenic- .____ . _ 1 5 , 000 5,000
0.2 3 . 500 17.500
Cadmium.... . _ .1.1 1.000 7,000
Selonium. .._._.__ .... .09 200 2.000
Tellurium _ __ . .0018 2-1.000 1 3 , 000 , 000
-Ks for Or, Cu, An, Fc, I'b, Mn, Ni, AB, Sn, ami Zn from Floisclier
PRIMARY DISPERSION
Although the same basic principles of physical chemistry apply
under all conditions, the processes that dominate the migration
of elements in the depths of the earth are very different from the
principal processes that are operative under surficial conditions.
In a deep-seated environment, geoehemical processes are controlled
by the relative stability of minerals under any given set of condi-
tions of pressure, temperature, and availability of material. Ele-
ments that can enter into the crystal lattice of a stable mineral are
immobilized; those that cannot remain in a mobile state until they
reach an environment where they can be accommodated in a stable
mineral. In a general way, this rule holds true regardless of whether
the rock-forming process is regarded as primarily metamorphism,
metasomatism, or crystallization from a magma.
The effect of availability of material is such that most minerals
are stable only in the presence of an abundant supply of the com-
ponent elements. The common rock-forming minerals, therefore,
are those containing only the nine most common elements as major
constituents. These elements are, in order of decreasing abundance,
oxygen, silicon, aluminum, iron, calcium, sodium, potassium, mag-
nesium, and titanium. In terms of atomic abundance, hydrogen is
also a major constituent of rocks, although its relative abundance
is difficult to estimate in quantitative terms. Rare elements occur
as major constituents only in a few accessory minerals of igneous
and metamorphic rocks, principally zircon (zirconium), and mona-
zite (rare earths, phosphorus). Thus out of the 88 naturally occur-
ring elements, only 12 plus the rare earths occur as major constitu-
ents in common rock-forming minerals.
. Many of thQ remaining elements m to p1^ incorporated into
One Or more Of the common minerals iby occupying a position in the
orystai lattice that is ordinarily filled by OYiQ Of the mSJOF COM-
ponent dements, Tiiie can take place omy wnen ^e Puy»u*a ^OP-
erties of the substituting ion are within certain limits 01 mOSG 01
the ion for which it is proxying. For example, the ionic radius and
ionization potential of lead are close to those of potassium, and
therefore lead can substitute for potassium in potash feldspai'S.
similarly, zinc, copper, and cobalt can proxy for iron and mag-
nesium in amphiboly M flWOffi, aflfl Mourn m jiw *~ T
droxyl in micas. It is generally assumed that the abundance of
rare elements in minerals of this kind is proportional to their
PRINCIPLES OF GEOGHEMICAL. PROSPECTING 237
GASEOUS DISPERSION
A few of the components of the earth's crust move at low tem-
peratures as gases through the pore spaces of rocks and soils from
which they escape directly into the atmosphere. Movement of this
kind depends on the existence of continuous open channels through
which the gas can move without hindrance. Near the surface the
flow of gas may be modified by changes in barometric pressure,
causing alternating inward and outward movement or "breathing."
Gaseous dispersion patterns may be detected either by direct
analysis of the gas itself, or under some conditions by analyzing
the material through which the gas has passed for products of
condensation.
It is claimed that volatile hydrocarbons may escape from oil
reservoirs through the overlying rocks and pass by way of the
soil into the air. This possibility is the basis for many of the
geochemical methods of exploration for petroleum, where soil or
soil air is systematically sampled and analyzed for traces of certain
diagnostic hydrocarbon compounds. Hydrocarbon anomalies may
be thought of as very weak oil seeps, so weak that the deposition of
material at the surface cannot be recognized without chemical
analysis. Although geochemical methods of locating petroleum are
the subject of a very extensive literature, their general effective-
ness is a matter of considerable difference of opinion among oil
men. A full discussion of these problems is beyond the scope of
this report.
The nuclear decay of certain radioactive elements results in the
generation of noble gases. Radon and helium are produced from the
disintegration of uranium, and thoron and helium from thorium.
Similarly, argon is produced by the decay of the K40 isotope of
potassium. Under favorable conditions these may travel substan-
tial distances from the source to form patterns that are helpful in
prospecting for the parent elements. Arndt and Kuroda (1953)
found a significant radon content of stream waters draining the
uraniferous black shale beds of Arkansas. They also report a high
radon content of spring water emanating from a uranium-bearing
syenite. Inasmuch as the noble gases are chemically unreactive,
they cannot have any chemical effect on the matrix through which
they travel and tend to escape into the atmosphere by the shortest
possible route. It has been estimated that radioactive spring water
loses 41 percent of its radon to the atmosphere in the first 4 feet
of surface flow. Radon and thoron, however, decay through a series
of intermediate radioactive products including isotopes of po-
lonium, lead, and bismuth. Conceivably dispersion patterns formed
246 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
cial conditions; for a few others, such as silicon, the reverse rela-
tionship holds, In general, there is no immediately obvious correia
tion between the mobility of elements in the primary and in the
secondary geochemical cycles.
The mobility Of element in the surficial environment and the
factor* governing it are of particular importance in geochemical
prospecting. The characteristic behavior of elements released by
247
" Except as noted, the data of table 5 are for solubility in water
at a pH of 7, The solubilities of the salts of weak acids, such as
fluorides, carbonates, and phosphates, commonly increase with de-
creasing pH. Also, the amount of carbon dioxide dissolved in the*
water will modify the solubility of carbonates. Because of the;
wide variations in the pH and carbon dioxide content of natural
waters, absolute values for solubility cannot be assigned. The data
of table 5 are intended only to give a qualitative idea of the general
order of magnitude of mineral solubilities.
Composition of
Salt Equivalent mineral saturated aqueous Temperature
solution ' (ppm) (°C)
1 Unless otherwise specified, figures are for solubility in pure neutral water taken from data assembled
by SeideU (1940).
2 In presence of 0.29 g COz per liter.
»In presence of 0.34 g COz per liter.
«Computed from solubility product value of IQ-"' 115 determined by Jowett and Price (Iyo2). ^
pH 5.0, solubility of pyromorphite is O.G ppm.
In this series, lithium has the weakest bond and hydrogen tlie
strongest. Thus, ions to the left in the series will tend to give up
their exchange positions to ions on the right. The pH of a soil is
effectively a measure of the abundance of exchangeable hydrogen
ions. Inasmuch as exchange reactions obey the law of mass action,
increasing the concentration of an ion in the solution can effect
replacement of substantial amounts of more tightly held ions. As
Grim (1953, p. 144) has pointed out, there is "no single universal
replaceability series. The series vary depending on the experimen-
tal conditions, on the cations involved, and on the kind of clay ma-
terial."
Most field studies of adsorption and exchange capacity have
been carried out by agricultural scientists, who were interested
primarily in the nutrient status of soils. Ions held by adsorptive
forces are as available for plant nutrition as ions in solution, and
hence the exchange capacity of a soil becomes an extremely im-
portant factor in fertility. Exchange capacity is commonly ex-
pressed in terms of milliequivalents per 100 cc of oven-dry soil,
and commonly ranges from 1 or less for sandy soils to 150 for
some clay soils. Cation exchange capacity is generally determined
by saturating the soil with ammonium or barium ions and deter-
mining the amount held at pH 7 (Grim, 1953, p. 155; Lepper, 1950,
P. 41).
Absorption. Under certain conditions, ions can enter the open
lattice structure of some minerals and either occupy a void of the
appropriate size or replace an ion already present. This phe-
nomenon is observed most frequently with clay minerals of the
montmorillonite group. These minerals are characterized by a
relatively open lattice structure that contains passageways
through wnich ions can enter both the spaces between the lattice
layers and the layered structures themselves. The slow modification
of montmorillonite to "illite" under surficial conditions is visualized
as the absorption of potassium, magnesium, calcium, iron, and
other cations into the relatively open spaces between the layers
where they take up stable positions. In one laboratory experi-
ment, montmorillonite and kaolinite were dried with K2Si03.
X-ray studies of the product showed "illite" crystal structures had
been formed in the montmorillonite sample, whereas the kaoli-
nite was unaltered (Mortland and Gieseking, 1951). Experi-
ments in which montmorillonite is treated with aqueous zinc
chloride solution show that a substantial part of the zinc ions is
absorbed in a nonexchangeable form. This is interpreted as the
absorption of Zn+2 ions into voids within the oxygen tetrahedra
256 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
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PRINCIPLES OF GEOCHEMICAL PROSPECTING 267
SECONDARY DISPERSION
The factors that govern the partition of elements between the
mobile aqueous phase and the immobile solid phase of the sur-
face environment have been reviewed. The dispersion patterns
that form as a result of this partition depend on the many dif-
ferent agencies that influence the patterns of movement of ma-
terials on the earth's surface.
Weathering, in the restricted sense of its definition, is only
the first phase of the secondary geochemical cycle illustrated in
figure 32. It is the phase dominated by the physical and chemical
changes necessary to bring rocks into equilibrium with the new
conditions at the surface of the earth. Movement of material in
the weathering process is a minor consideration. In the later
phase of the cycle, however, the physical and chemical changes
that the products of weathering undergo as they are transported
from their source to the site of sedimentation are influenced to
a very considerable degree by the various transporting or "pro-
cessing" agents. Here the patterns of flow of ice and particularly
of water as it moves through soils, down drainage channels, and
through the circulatory system of plants determine the dispersion
patterns of the products of weathering. In presenting the prin-
ciples responsible for the development of geochemical anomalies,
therefore, it is more convenient to consider weathering processes
first and to review the various kinds of dispersion patterns result-
ing from the transportation of weathering products in separate
sections.
WEATHERING
Weathering has been defined by Polynov (1937, p. 12) as "the
change of rocks from the massive to the clastic state." It in-
cludes the processes by which the parent rock is fragmented,
and by which minerals stable in the subsurface environment
are reconstituted to form new minerals that are stable in the
surface environment.
The physics and chemistry of weathering have been reviewed
by Reiche (1950) and for a more detailed treatment of the sub-
ject and for further bibliography the reader is referred to this
excellent work.
Processes of iveathering. The dominant physical process of
weathering is the disintegration of massive rocks into succes-
sively smaller fragments. Expansion of the rock due to unload-
ing may cause cracks and joints to form. Differential expansion
and contraction resulting from extremes of heat and cold at the
268 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
lead, tin, arsenic and antimony are immobile and are held in
the residual soil; zinc and cobalt tend to be impoverished; cop-
per, tungsten, and molybdenum appear to be intermediate.
Although the major part of the mobile metals dissolved from
the weathering products of a vein is entirely removed, a signi-
ficant part may be transported in solution only for short dis-
tances and then reprecipitated locally. The effect is a lateral
and downhill spreading of the dispersion pattern outward from
the bedrock source. Zinc anomalies hundreds of feet wide have A
been observed in residual soil associated with veins measuring
only tens of feet in width. The lead anomalies over the same
veins were not only much narrower but corresponded much
more closely with the location of the vein. Over such a vein,
samples spaced at 100- or 200-foot intervals would have been ade-
quate for locating the zinc anomaly, whereas a 50-foot spacing ^
would have been necessary for the lead anomaly. It has been the
writer's experience, therefore, that where complex ores contain- <
ing two or more metals are sought, greatest economy is achieved
when reconnaissance geochemical soil surveying is based on the
more mobile metal and detailed work is based on the less mo-
bile one.
The homogeneity of a metal pattern in residual soils is appar-
ently also related to mobility. The characteristically homogene-
ous patterns formed by mobile metals such as zinc indicate that
original local and erratic variations in the distribution in soils
of the mobile metals tend to be smoothed by solution in rich
spots and local redeposition in lean spots. With immobile metals,
solution is inhibited and the original spottiness of distribution is
preserved. Thus Tikhomirov and Miller (1946) report that the
molybdenum pattern over the Kounrad molybdenite deposit is
less erratic than the tin patters commonly found over Cassiter-
ite veins. Ttie relative homogeneity of a g-eochemicai anomaly is
an important consideration in determining the most efficient
spacing of samples in practical survey work. An anomaly de-
fined by a mobile metal witk its more homogeneous pattern can
be located and mapped with fewer sampi^ «.»» ~« anomaly de-
fined by an Jrn^o^1- z»et«*l.
Geochemical anomalies in residual cover may be distorted by
downslope movement of the soil. The result is an asymmetri-
cal curve in which the metal content fails off less rapidly OH the
downhill side than in the Uphill side, as illustrated in the dia-
grams presented by Huff (1952). Asymmetry of this kind can
also be caused by the action of metal-rich solutions depositing
PRINCIPLES OF GEOCHEMICAL PROSPECTING 275
metal in the soil on the downslope side, although the net effect
is the same whatever the cause. In extreme cases of asymmetry
owing to downslope movement, geochemical anomalies may be
detected by sampling along the foot of a slope, hundreds of feet
below the outcrop of the vein. Riddell (1954) describes recon-
naissance exploration work in an area of high relief by sys-
tematic sampling of soils barely above the modern stream ter-
races.
Patterns in. residual soil may also be distorted by compaction
slumping. At the Friend's Station deposit in eastern Tennessee,
an average of 50 feet of residual clay overlies a gently dipping
limestone sequence. The clay overburden was estimated to
represent the weathering product of three times its volume of
parent rock. The resulting flattening of the dip of the gently
dipping ore caused the geochemical soil anomaly to come to the
surface several hundred feet on the footwall side of the subout-
crop of the ore (Hawkes and Lakin, 1949).
Gossans. A gossan is the mass of residual limonitic material
that remains behind after removal of the soluble products of
weathering of a sulfide-bearing deposit. Being residual, gossans
can be traced downward through successively less weathered
zones into the unweathered primary sulfide minerals. Historical-
ly, gossans have been one of the best guides to the prospector in
areas of residual cover and deep weathering.
At the surface, blocks of gossan material may be dislodged
from the main mass and scattered over the immediate vicinity
by frost action, slumping, and slope creep. If the fragments are
sufficiently durable, they may on occasion be transported for
substantial distances by glaciation or stream action. Studies of
the dispersion pattern of limonitic fragments, particularly in
alluvial material, have led many prospectors to the parent gossan
mass and the hidden sulfide deposit beneath.
The necessary prerequisite for the formation of a gossan, in
addition to deep weathering and an oxidizing environment, is
the presence in the bedrock of iron as sulfide or carbonate to
provide the raw material for the formation of limonite. The
common gossan-making minerals are pyrite, marcasite, pyrrho-
tite, copper-iron sulfides, arsenopyrite, siderite, and ankerite. In
general, the more iron is present in the primary ore, the more
pronounced will be the gossan. The occurrence of economically
valuable metals in the ore is only indirectly related to gossan
formation, in that ore minerals are commonly associated with
the gossan-making iron sulfides and carbonates. Thus not all
276 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
GLACIATION
The grand scale of movement of the continental ice sheet is
attested by the identification of glacially transported erratics
that have been dropped hundreds of miles from their SOUTC8.
Fragments of native copper derived from the Keweenaw
PRINCIPLES OF GEOCHEMICAL PROSPECTING 277
by the water that forms the ice and is released when the ice melts.
Experiments have shown that a growing frost crystal is fed by
soil moisture moving upward from below, where temperatures
are not sufficiently low to freeze the films of water on the surface
of the clay minerals. The growing crystals thus act as collectors
and concentrators of moisture gathered from a considerable
volume of material. Any salts dissolved in the soil moisture are
enriched in the aqueous phase as the water is removed to feed
the frost crystals. As the crystals grow they exert pressure on
the surrounding solid fragments, causing minor readjustments
in the relative position of the fragments and keeping them in
open packing. When the ice melts, the pressure is released and
the water concentrated by the frost-forming process forms a
mobile slurry with the silt and clay from the surrounding
material. The total effect of this process repeated over a period
of many years is a turbulent mixing of the surficial material to
the maximum depth of the frost action. If the angle of slope is
sufficient, a downslope movement is combined with the turbulent
mixing, and the finer grained materials may be removed com-
pletely in the muddy water that drains out after the melting
of the frost each spring. Under proper conditions the meltwater
from the frost may mobilize a large mass of unconsolidated
material, which then can move down the slope as a mudflow
or landslide.
In geochemical prospecting, the turbulent mixing and down-
slope creep of surficial material owing to frost action is a factor
deserving special consideration, at least in some areas. The
Blackbird cobalt district of Idaho, where extensive experiments in
geochemical prospecting methods were conducted by the Geological
Survey, is in an unglaciated area of deep weathering where the
zone of frost mixing extends commonly to a depth of about 4 feet.5
The maximum intensity of frost activity probably coincided in
time with periods of active glaciation elsewhere. Mineral explora-
tion by conventional methods in this area was hampered by the
scarcity of outcrops and especially by the virtual absence of
fragments of oxidized ore or gossan in the surficial material over-
lying the cobalt deposits. Well-developed gossan, however, could
be observed in the undisturbed weathered rock immediately below
the zone of frost action. Very probably one of the effects of frost
action was to disintegrate the soft limonitic fragments derived
from the weathered rock into fragments too fine grained to be
identified in the zone of frost mixing. Soil analysis for copper and
5 See footnote 1 on p. 234.
280 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
with copper and zinc, and that 0.03 to 0.1 percent nickel could
be regarded as an indication of nickel-bearing ore. The back-
ground copper, zinc, and nickel content of peat is, respectively,
0.03, <0.03, and 0.01 percent.
Vadose ivater. The processes already described whereby dis-
solved salts move upward against the force of gravity can result
in a pattern of dispersed metal in the surficial material directly
over the source in the bedrock. Although ionic diffusion is
probably only one of the mechanisms responsible for upward
movement, it is convenient to refer to the resulting dispersion
patterns as "diffusion patterns."
Diffusion patterns are best developed in a matrix containing
a substantial proportion of clay-sized particles. The fine-grained
matrix has both a higher retentivity for water and a higher
exchange capacity, so that the dispersion patterns can be readily
fixed as superimposed halos. Fine-grained alluvial and glacial
deposits are relatively good hosts, whereas sand and loess are
poor hosts. Diffusion halos undoubtedly form also in residual
soils but are masked by the inherited patterns of residual metals.
In the Austinville zinc district of Virginia, Fulton (1950)
reports a distribution of zinc in Tertiary river sediments that
closely reflects the zinc content of the underlying mineralized
bedrock. Patterns of this kind were lacking in the most recent
river sediments, where presumably the shortness of time had
not permitted the maturing of the diffusion pattern.
A well-defined example of a diffusion pattern was mapped in
detail in the Abakaliki lead-zinc district, Nigeria, where relatively
unweathered bedrock was overlain unconformably by a 6-foot
deposit of alluvial silt (Hawkes, 1954), Sampling on a vertical
section through the silt as exposed in an exploration trench showed
an easily contoured distribution of zinc, falling off uniformly with
distance from the underlying source from a maximum of several
thousand parts per million directly over the ore to several hundred
at the surface. Historical evidence indicates that the silt in which
this pattern was formed was deposited about 400 years ago, so
that an estimate of the time required for the evolution of a
diffusion pattern can be made.
Somewhat similar patterns, although not as clearly developed,
have been" found in the course of experimental work in glaciated
terrane inithe United States and Canada, White and Alien (1954)
report copper in glacial till over -what is probably a leakage halo
TABLE 7. Average copper and zinc content, in parts per million of dry .weight, of leaves
or needles, and stems bearing them
[Compiled from data of Warren, Delavault, and Irish, 1952b]
Copper Zinc
Species
Leaves or Leaves or
needles Stem needles Stem
(dry weight) for zinc, and outside the limits 0.07 to 0.23 for the
copper-zinc ratio. They have applied these techniques on a com-
mercial scale throughout Canada and have reportedly uncovered
a number of mineralized areas. Warren and his colleagues have
PRINCIPLES OF GEOCHEMICAL PROSPECTING 297
Physiographic provinces
Upstream to downstream
Minerals
Valley and
Ridge Blue Ridge Piedmont Coastal Plain
Limonite. ____.______..__. F A C
Ilmenite and magnetite. C C C A
Zircon. __________ _______ R R A C
Tourmaline. ______________ R R R R
Rutile __ ________________ R R R R
Leucoxene. ______-___---_. C C R
Hornblende.. . ____________ R A C
Epidote.________ _______ R C C
Garnet. __________________ R R C
Muscovite.--.. . ------- C C R
Chlorite. _________________ C C R
Hypersthene. _-________-_. R R R
Staurolite. .--_-__.__--_. R C
Andalusite.-..- __________ R R
Kyanite____ ____ _______ R C
Sillimanite. ___-_____--____ R C
Sphene. __________________ R R
Actinolite. ._-_____._--__. R
Apatite. ________ ______ R
Biotite ___________________ R
Diopside .-_.._________-_ R
Monazite_.______ __.__.-_ R
Trernolite.-_.__ ___ R
Distance from mouth Heavy metal Distance from mo'st'-i Heavy metal
of tunnel (miles) pH (ppm) of tunnel (miles) ,,H (ppin)
stream, will lose much of their copper content. This effect may be
seen in the data on the copper content of the drainage below the
mines at Butte, Mont., collected by Huff (written communication,
1947) as shown in table 10. Here, if it is assumed that the zinc
content falls off by a simple factor of dilution, the copper-zinc
ratio will be a measure of the tendency of the copper to remain in
solution. It is notable that the ratio remains constant until the
pH exceeds 5.0 and then falls off rapidly, indicating that copper
joins the solid load of the stream in some form.
TABLE 10. Metal content of water in drainat/e front copper mines at Butte, Mont.
[Samples collected by L. C. Huff; analyses by Norman Davidson and L. C. Huff]
streams and rivers, where laminar rather than turbulent flow pre-
dominates, many miles may be required for complete mixing of
PRINCIPLES OF GEOCHEMICAL PROSPECTING 311
shovel has been found satisfactory for shallow holes. In .clay soils,
a soil auger is usually easier and faster than the pick-and-shovel
technique. A suitable soil auger may be assembled by brazing or
welding a large wood-auger bit to a section of light iron pipe, and
attaching additional lengths of pipe as needed for greater depth;
a T-shaped handle may then be fitted to the top section of pipe. It
is found that the auger bit is more effective in cutting through soils
if the leading screw point is cut out. If samples are to be taken at
a predetermined 'depth, it may be easier to drive a crowbar into
the soil for the required distance, remove the bar, and then extract
the sample from the hole with an auger. Where deep holes are
required, and where the terrane permits, the use of mobile power
augers should be investigated.
The size fraction of soil used for analysis may make some dif-
ference in the significance of the data. In some problems it has
been found that the metal content of the fine fraction was some-
what, but not greatly, higher than that of the coarse fraction. In
other experiments, no significant variation with size was apparent.
The standard procedure adopted for Geological Survey work has
been to collect the fines (minus 80 mesh) for analysis and discard
the coarse fraction. This avoids the need of grinding the sample
before analysis and may cut down erratic data resulting from
possible coarse fragments of oxidized ore minerals such as lead
carbonate or malachite. An alternative procedure is to collect all
sizes less than 2 mm in diameter and grind before analysis. This
procedure may be preferable where the soil lacks an appreciable,
proportion of fines or where the ore metal is concentrated in the
coarser sizes.
It is sometimes possible in dry climates to sieve samples as they
are collected. Usually, however, fresh samples are so damp that
the fines cannot be separated by sieving until the samples have had
a chance to dry. Most samples may either be collected without
sieving or, if desirable, sieved to minus 2 mm on the spot. If
paper containers are used, such samples should be dry enough for
sieving to the desired size in a day or two of storage in a dry room
or in the sun without opening the containers.
Harold Bloom of the Geological Survey has conducted compara-
tive experiments on sieves composed of brass, stainless steel, and
silk, and has found that contamination of the sample by copper
and zinc from brass screening under extreme conditions is less
than 10 ppm. Thus, except for very precise work, the use of stand-
ard brass sieves is permissible. The advantage of stainless steel
besides being noncontammating is its strength and durability. It
326 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
more economical than the chemical methods. With the recent con-
struction of a mobile spectrographic unit these methods may again
have a wide application in geochemical prospecting, especially in
reconnaissance work.
Methods employing paper chromatography have recently been
developed for geochemical prospecting by chemists at the Chem-
ical Research Laboratory of the National Physical Laboratory in
Teddington, England. In preliminary experiments these methods
show great promise, and it is possible that some of them will
replace the rapid colorimetric methods now in common use.
Reliability. Accuracy and precision of analytical data are not
so important as the reliability with which the figures reflect the <
geochemical pattern. Most of the colorimetric procedures com-
monly used in geochemical prospecting were developed from the
standard colorimetric methods of trace analysis by omitting or
simplifying many of the steps needed for precise, quantitative work.
As a result, the data contain errors that would.not be tolerated in
the average analytical laboratory. With the relatively weak ex- >
tractants used in the rapid tests, rarely more than 80 percent
of the total metal present in the sample will be measured. Further-
more, the reproducibility may be within a factor of only 1.5 or
even 2.0 of the mean of several replicate determinations. Most
geochemical anomalies, however, show a contrast of anomalous
values against a background equal to many times the errors in
the analytical tests. Thus, even with relatively inaccurate methods, ,
as judged by usual chemical standards, the pattern should still
appear.
One of the unavoidab]..- consequences of the many shortcuts used
in the procedures for the sake of speed is that seriously erratic
analytical results will appear in a certain (usually small) pro-
portion of the reported figures. The analyst has no way of know-
ing which of his figures may be questionable without running the
entire lot in duplicate. The geologist familiar with his problem,
however, is much better equipped to choose samples where the
reported analysis does not seem to fit the geologic picture and to
ask for confirmatory repeat analyses. This should be done by the
geologist as a routine matter as soon as he receives an analytical
report.
Cost. With a properly equipped laboratory, one man can make
30 determinations per working day with most of the procedures
developed by the Geological Survey. With a few of the procedures,
the rate may be as high as 100 per man-day. The cost of materials
and glassware is negligible, so that the bulk of the cost is in labor.
PRINCIPLES OF GEOCHEMICAL PROSPECTING 333
GLOSSARY
Accumulator plant. A plant that takes up a particular element in quantities
considerably in excess of that in "normal" plants (Robinson and Edging-
ton, 1945).
Alluvium. Detrital material deposited by running water.
Background. The abundance of an element or any chemical property of a
naturally occurring material in areas where the chemical pattern has not
been affected by the presence of a mineral deposit.
Biogeochemical prospecting. Synonymous with geochemical plant survey.
Blind deposit. A deposit that does not extend to the surface of bedrock.
Botanical anomaly. A geochemical anomaly in which the chemical composition,
ecological assemblage, or morphology of plants indicates the presence of a
mineral deposit.
Contrast. A ratio expressing the geochemical relief, computed as the ratio
either of maximum value to threshold, of maximum to background, or of
threshold to background values.
Converter plant. A plant that takes up an insoluble element from the soil,
builds it into its living structure, and at death returns it to the soil in
soluble £orm.
Diffusion pattern. A dispersion pattern resulting from upward movement
of ions in vadose water,
Dispersion pattern. A pattern of distribution of chemical elements resulting
from the movement of natural materials.
Ecology. The study of relationships between organisms and their environ-
ment.
Extractable metal. A metal that can be extracted from a sample by any
given chemical treatment.
Fan. A dispersion pattern that spreads predominantly to one side of the
source of material.
Genetic Halo. A. g-eochemical anomaly resulting from primary dispersion.
Geochemical anomaly. Area where the chemical properties of a naturally OC-
curring material indicate the presence of a mineral deposit in the vicinity,
Geochemical prospecting. Mineral exploration based on systematic measure-
ment of the chemical properties of a naturally occurring material.
PRINCIPLES OF GEOCHEMICAL PROSPECTING 337
UNITS OF MEASUREMENT
The units of measurement commonly found in the literature
of geochemical prospecting are denned in tables 11, 12, and 13.
The common unit of weight is the microgram, equal to a mil-
lionth of a gram and represented by the Greek letter gamma (y).
For solid material the common unit of concentration is the part
per million (ppm), equal to 0.0001 percent. In expressing data
on the composition of plants, the basis of measurement, whether
ash or dry weight of the plant, should be specified. For aqueous
solutions, concentrations may be expressed as parts per million
(micrograms per milliliter) or micrograms per liter. Parts per
billion should be avoided because of the difference in the Amer-
ican and British usage of the term billion.
Metric equivalent
Unit Symbol (grams)
t 1,000,000
Kilosr&tn --_- _ kg 1,000
g 1
*"S .001
.000001
Microgram, gamma.--.---. .-.._..__..- 7
Short ton...... ...-- ' 907,180
A-voiT-dvifiois ipOxind ib 453.6
Avoirdupois ounce...... .-.--.--.- -- - oz 28. 3S
1 An equivalent (eq.) IB the weight in grams which in its reaction corresponds to a grain-atom of hydro-
gen. For our purposes, it is taken as being numerically equal to the atomic or molecular weight divided
hy the valence. One thousand milliequivalents (men.) is equal to one equivalent.
1 To express concentration in terms ol "B," multiply concentration expressed us "A" by factor shown.
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348 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
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INDEX
Page Page
A Chromium :
Absorption ..............1.......... 255 256 distribution, in detrital minerals.. 802
Abundance of elements in in plants .................. 296
earth materials .................. 227-230 in rocks ................... 228
Acknowledgments .................. 227 in water .................. 229
Adsorption ............ 254-255, 283, 306, 809 mobility ....................... 250, 264
Aluminum: Cobalt:
distribution, in plants .......... 228 distribution, in plants .......... 298
in rocks ................... 228, 236 in precipitates ............. 312
in water ................... 229 in residual
mobility ........... 250, 251, 260, 263, 270 cover ........ 234, 241, 273, 274, 280
Analytical techniques, in rocks ............... 228, 236, 241
chromatographic .................. 332 in sediments ............... 306
colorimetric ................... 330-331 in water .................. 229, 307
spectrographic ......... 314, 315, 331-332 mobility ........... 250, 253, 259, 265, 274
Animal activity .................... 280 Colloidal solutions .................. 260
Antimony: Complex ions, inorganic ............ 268
distribution, in rocks ........... 228 Contamination ..................... 335 336
in residual cover ........... 274 Contrast of geochemical anomalies .. 234-235
mobility ....................... 274 Copper:
Area! dispersion patterns ....... 240-241,244 distribution, in detrital minerals.. 304
Arsenic: in plants ...... 228, 234, 292, 294-296,
distribution, in plants .......... 292 298, 299
in precipitates ............. 287 in precipitates ..... 285, 287, 302, 305
in residual cover ....... 234, 273, 274 in residual
in rocks ................... 228, 244 cover ............... 265, 266, 273,
in water ................... 229 274, 276, 279, 281
mobility ............... 253,259,265,274 in rocks ........... 228, 236, 241, 244
in sediments ............... 306, 312
B in transported
Background ................ 227-230, 233, 335 cover ........ 243, 278, 286, 287, 288
Barium: in water .............. 229,309-311
distribution, in detrital mobility .......... 250-253, 256, 258, 269,
minerals ..................... 304-305 261, 262, 266, 269, 270, 274,
in plants .................. 228 305, 309
in residual cover ........... 276 Coprecipitatlon ........ 253-264, 287, 805, 811
in rocks ................... 228
in water ................... 229
mobility ............... 262-254,263,269
Definition of terms ................ 336-338
Beryllium, distribution, in rocks ..... 228, 244
Dithizone method of analysis ........ 265, 311
mobility ................... 250, 259, 269
Boron, distribution, in
detrital minerals .................. 302 E
in plants ...................... 228,292 Exchange reactions.. 254-255, 283-289, 290, 330
in rocks ....................... 228, 239
in sediments ................... 239
F
in water ...................... 229,239
Fluorine:
distribution, in rocks .......... 228,236
Cadmium: in water .................. 229,306
distribution, in plants .......... 298 mobility ....................... 262
in rocks ................... 228 Frost action ....................... 278-280
mobility ....................... 250
Calcium:
distribution, in plants .......... 228, 292 Gallium:
in rocks ................... 228, 236 distribution, in precipitates 305
in water ................... 229 in rocks 228
mobility ............... 262, 254, 265, 263 mobility ... 269
353
354 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS
Page Page
Gaseous dispersion ................. 245-246 in rocks ................... 228, 246
Geochemical cycle .................. 230 232 in water ................... 229
anomalies ............. 227, 233-236, 273 mobility ....................... 246, 250
provinces ...................... 237 239 Microbiological factors
Glacial dispersion .................. 276-279 affecting mobility ............ 260-262, 312
Gold: Mobility ........................... 246-266
distribution, in detrital j Molybdenum:
minerals .......... ....' ...... 302, 30 i distribution, in plants ...... 278, 292, 297
in plants .................. 297, 299 in precipitates ............. 305
in rocks ..................... 228, 239 in residual cover ........... 274
in water .............. 229, 305, 311 in rocks ................... 228
mobility ..............: 259, 264, 265, 269 mobility ........... 261, 264, 265, 274, 290
Gossans ....................;... 275-276,287
H N
Heavy metals. See Zinc. Nickel:
Homogeneity of geochemical distribution, in plants .......... 296
anomalies ........................ 235-236 in residual cover ........... 273
Hydrogen-ion concentration. See pH. in rocks ................... 228,239
in transported cover ........ 288
in water .................. 229, 312
Iron: mobility ....................... 250, 259
distribution, in plants .......... 298,299 Niobium:
in rocks ............... 228,236, 242 distribution, in detrital
in transported cover ........ 278 minerals ..................... 304
in water .......... 229, 284, 310, 311 in rocks ................... 228, 239
mobility . . . 250, 255-258, 260-263, 270, 310 mobility ....................... 269
Page Page
in water ................... 305
mobility ................... 250, 269, 274
Sampling: instructions .............. 321-328
Tungsten:
Secondary dispersion ........... 232, 246-313
distribution, in
Secondary enrichment of ores ...... 284
detrital minerals ............. 302-304
Selenium:
in plants .................. 296
distribution, in plants .......... 291, 299
in residual cover ........... 272, 274
in rocks ................... 228
in rocks ................... 288
in water ................... 229
mobility ....................... 269, 274
mobility ....................... 291
Silicon: U
distribution, in plants .......... 228 Units of measurement .............. 388-339
in rocks ............... 228, 236, 241
Uranium :
in water ................... 229 distribution, in
mobility ....................... 260, 263 detrital minerals ............. 303
Silver:
in plants .......... 234,291,293,297
distribution, in plants .......... 297, 299 in rocks ................... 241
in rocks ............... 228, 241, 243
mobility ....................... 250,256
in water ................... 229
mobility ....... 250, 252, 258, 259, 264, 265
Sodium: Vanadium:
distribution, in plants .......... 228 distribution, in plants ...... 228,291,297
in rocks ................... 228, 236 in precipitates ............. 305
in water ................... 229 in rocks ................... 228
mobility ....................... 254, 263 mobility .................. 256, 259, 264
Soil sampling: instructions .......... 321-326 Vegetation ................. 228,289-300,326
Soils and soil formation .... 230, 266, 280-282
Solubility of salts .............. 248, 251-253 W
Streams and rivers ................ 300-311 Wall-rock dispersion ........... 240, 243, 244
Stream sediments, heavy Water, geochemical dispersion
minerals in ...................... 301-304 in ............... 227-229,282-289,300-312
sampling instructions .. 321-324, 327-328 Water sampling instructions. . 321-324, 326-327
unstable components in ........ 304-306 Weathering ................ 230-232, 246-282
Sulfur, sulfate, distribution,
in plants .................... 228, 292, 299
in rocks ....................... 228 Zinc:
in water .................. 284, 311 distribution, in detrital minerals.. 302
Superimposed halos ............. 284-289 in plants .............. 228, 292-299
in precipitates ............. 305,312
in residual
Thorium: cover ....... 242-243, 265, 266, 273,
distribution, in detrital minerals.. 303 274, 276, 281
in rocks ................... 239,241 in rocks ........... 228. 236, 242-244
in sediments ............... 306 in transported
mobility ....................... 250 cover ............ 278, 285,287, 288
Threshold of geochemical in water ...... 229,284,285,307-312
anomalies ............... 233, 241, 243, 335 mobility ...... 250-253,255,258,269,261,
Tin : 262, 264, 265, 270, 274, 290,
distribution, in 309, 310
detrital mineraJs ............. 302,304 Zirconium :
in plants .................. 302, 304 distribution, in
in residual cover ........... 272, 274 detrital minerals ............ 302,303
in rocks ............... 228,237-239 in rocks ............... 228,236,241
in sediments ............... 304 mobility ....................... 250