STRATEGIC AND CRITICAL MINERALS IN THE MIDCONTINENT REGION, UNITED STATES

Geology and Mineral Paragenesis of the Pea Ridge Iron

Ore Mine, Washington County, Missouri Origin of the
Rare-Earth-Element- and Gold-Bearing Breccia Pipes
By Laurence M. Nuelle1 , Warren C. Day, Gary B. Sidder, andCheryl M. Seeger1

Abstract thermal fluids underwent second boiling and decompression

and caused crystallization and release of a volatile phase.
Breccia pipes containing rare-earth elements (REE) and Fluid-inclusion evidence of boiling includes a mixed popu-
gold are a potentially significant economic target in the Pea lation of vapor-rich fluid inclusions coexisting with liquid-rich
Ridge iron ore mine, Washington County, Missouri. The Pea inclusions in quartz from the REE-bearing breccia pipes. The
Ridge deposit is one of eight known volcanic-hosted iron ore concomitant volume increase associated with boiling released
deposits in the Middle Proterozoic St. Francois terrane that are significant mechanical energy and caused fracturing and brec-
similar to the Olympic Dam type of deposits of Australia. The ciation of the wallrock. The REE-bearing fluid and crystal
iron orebody is steeply dipping and tabular shaped, and it cuts mixture streamed upward along zones of weakness (faults
across the host rhyolite. The footwall of the deposit is zoned and contacts), entrained and milled wallrock fragments, and
from the massive apatite-bearing magnetite core outward to formed the breccia pipes.
heterolithic breccia, pseudobreccia, a specular hematite zone, The REE- and, locally, gold-bearing breccia pipes associ-
and an extensive silicified zone. The hanging wall is zoned ated with the Pea Ridge iron deposit are attractive targets in the
from the massive magnetite core outward to heterolithic midcontinent of the United States for exploration of large
breccia, pseudobreccia, and an amphibole skarn zone. The deposits of iron, copper, REE, gold, and uranium similar to the
REE-rich breccia pipes cut through the various rock types along Olympic Dam deposit in Australia.
the footwall at or near contacts. They are typically about 60 m
long and 15 m wide, and they extend at least 120 m vertically.
The pipes consist of fragments of rhyolite host rock and zones INTRODUCTION
of massive magnetite, specular hematite, and silicified rocks
supported in a matrix of rock flour, barite, feldspar, monazite, This study of the Pea Ridge mine is part of a 5-year
apatite, chlorite, xenotime, and, locally, gold. Total REE oxide cooperative effort between the Missouri Department of
content of samples of the groundmass material, which are not Natural Resources-Geology and Land Survey and the U.S.
diluted with lithic fragments, average about 20 weight percent. Geological Survey (USGS) under the USGS Midcontinent
Grades from working faces in the mine are lower, averaging Strategic and Critical Minerals Project. One of the overall
about 12 weight percent. Gold distribution is erratic, but goals of the cooperative effort is to provide a genetic and
concentrations are as high as 371 parts per million. predictive model for the possible occurrence of iron, copper,
The Pea Ridge iron deposit represents an intrusion-type REE, and gold deposits in the midcontinent region that may
deposit in which the magnetite orebody sloped upward into
be similar to the Olympic Dam deposit of Australia
the host rhyolite and created the various breccia zones around
(Oreskes and Einaudi, 1990). Initial work centered on the
the margins of the deposit. The REE-bearing breccia pipes
probably formed from late-stage magmatic-hydrothermal Pea Ridge deposit inasmuch as it offers the most complete
fluids that evolved from the main magnetite orebody. Sanidine lateral and vertical view of this deposit type in the midcon-
phenocrysts (or xenocrysts) in the breccia pipes confirm a tinent region. This report presents observations and hypoth-
magmatic component for their origin. The magmatic-hydro- eses generated from our mine mapping program. Traverses
that cross lithologic contacts and structural features in the
mine were selected in order to establish paragenetic
Manuscript approved for publication August 5, 1991. relationships and to serve as a base for geochemical and
'Missouri Department of Natural Resources, Rolla, MO 65401. petrographic studies.

Pea Ridge Iron Ore Mine, Missouri A1

GEOLOGIC SETTING

Middle Proterozoic rocks of the St. Francois terrane,

which includes rhyolitic ash-flow tuffs, lava flows, and
coeval granitic plutons, host Missouri's Precambrian iron
ore deposits. Zircon crystals from the granite bodies have
yielded U-Pb isotopic ages of 1,480-1,450 Ma (Bickford,
1988). Kisvarsanyi (1980, 1981) recognized three types of
granitic rocks in the St. Francois terrane: subvolcanic
massifs, ring intrusions, and central plutons (fig. Al). The
subvolcanic massifs, which are intrusive equivalents of
coeval rhyolitic rocks, are epizonal biotite granite having
granophyric and rapakivi textures and containing perthitic
alkali feldspar; biotite is the characteristic mafic mineral.
Magnetite is a ubiquitous accessory mineral. Ring intru-
sions, which include intermediate- to high-silica amphibole
granite, biotite-hornblende granite, adamellite, and syenite,
were emplaced in ring fractures related to caldera collapse
and cauldron subsidence. In contrast, central plutons of
high-silica, two-mica granite were emplaced in resurgent
cauldrons. The central plutons have distinct accessory
minerals, such as fluorite, topaz, allanite, monazite, garnet,
and cassiterite, and a characteristic trace-element suite that 0 10 20 KILOMETERS
includes elevated abundances of Sn, W, Nb, Y, Be, Li, Rb,
Ba, and F. The central plutons have a unique negative EXPLANATION
magnetic anomaly signature (Kisvarsanyi, 1984; Kisvar- Granite central pluton
sanyi and Kisvarsanyi, 1989). Most authors agree that the
Trachyte porphyry ring intrusion
magmas were generated from melting of previously
accreted crustal material (Nelson and DePaolo, 1985). Granite porphyry ring intrusion
Kisvarsanyi (1975) proposed that the St. Francois terrane
formed in an anorogenic extensional tectonic setting (failed Subvolcanic granite massif

cratonic rift environment), whereas Patchett and Ruiz Rhyolitic volcanic rocks
(1989) suggested that these rocks are not anorogenic, but
formed during orogenic-accretionary processes associated Active underground mine
with the early stages of the Grenville Orogeny. Inactive mine
The St. Francois terrane hosts eight known magnetite
and hematite deposits (fig. Al), which together constitute an A Undeveloped deposit

iron metallogenic province (Kisvarsanyi and Proctor, 1967; o Town

Snyder, 1969). The iron deposits occur as both intrusive and Contact
replacement bodies within volcanic rocks of the terrane. The
Figure A1. Geologic map of the Middle Proterozoic St.
deposits may be genetically related to the host anorogenic Francois terrane, southeastern Missouri, showing locations of
rhyolite rocks, as suggested by Day and others (1989) for eight known magnetite and hematite deposits (modified after
the Pea Ridge deposit. Kisvarsanyi, 1981).
The southeastern Missouri iron metallogenic province
contains reserves estimated at nearly 1 billion tonnes of iron only iron ore producer, and it is the only remaining
ore (Arundale and Martin, 1970). Iron ore has been con- underground iron mine in the nation. To date, about 41.6
tinuously produced from the province since 1815, except for million tonnes of usable iron ore have been produced from
one year during the Great Depression. Until 1963, Pre- the Pea Ridge mine.
cambrian hematite deposits were the major source of iron
ore in Missouri; since the opening of the Pea Ridge mine in
1964, all iron ore production has been from subsurface GEOLOGY OF THE DEPOSIT
Precambrian magnetite deposits. The Pilot Knob under-
ground mine opened in 1967 and produced slightly more The Pea Ridge magnetite-apatite deposit is a tabular
than 9.8 million tonnes of usable iron ore before closing in body that is discordant to bedding of the rhyolitic host
1980. Since 1980, the Pea Ridge mine has been the State's rocks. The orebody strikes roughly N. 55° E. and dips

A2 Strategic and Critical Minerals, Midcontinent Region, U.S.

75°-90° SE. (Husman, 1989). Xenotime from a quartz vein bearing breccias were contemporaneous and that it is useful
that cuts the iron ore yielded a U-Pb age of 1.46 Ga (W.R. to treat the magnetite orebody as a zone composed of
Van Schmus, University of Kansas, written commun., subzones of massive magnetite, magnetite-cemented hetero-
1988), which is a minimum age for the deposit; this age is lithic breccia, and pseudobreccia.
within the age range of 1.45-1.48 Ga for the St. Francois
terrane as reported by Bickford and Mose (1975) and
Massive Magnetite
Bickford (1988).
Rocks of the Pea Ridge deposit are divided into four Ore faces in the massive magnetite contain as much
zones: (1) the amphibole-quartz zone; (2) the magnetite as 65 volume percent magnetite, with average grades
zone, which is made up of massive magnetite, magnetite- ranging from 47 to 55 percent magnetic iron (Emery, 1968).
cemented heterolithic breccia, and pseudobreccia; (3) the The texture varies from massive and shiny ore having
specular hematite zone; and (4) the silicified zone. Other subconchoidal fracture to finely crystalline and granular.
rock types include magnetite veins, quartz veins, aplite Some ore has a porphyritic texture in which magnetite (or
dikes, mafic dikes, and a unique banded rock. REE-bearing martite after magnetite) and hematite megacrysts are in a
breccia pipes cut rocks of the footwall. Figures A2 and A4 massive, fine-grained magnetite groundmass. Gangue
present the paragenetic relationships in the deposit, and minerals in the magnetite ore are predominantly apatite,
figure A3 is a geologic map of the 2,275-ft level. quartz, pyrite, and monazite, and minor ferroactinolite,
biotite, chlorite (after biotite), fluorite, barite, grunerite, and
talc (fig. A2). The gangue forms interstitial intergrowths,
Amphibole-Quartz Zone net-textured veinlets, and pods within the massive mag-
netite ore.
The amphibole-quartz zone occurs in both the
hanging wall and footwall of the deposit. It consists of Magnetite-Cemented Heterolithic Breccia
massive, coarse-grained actinolite (blades as much as 5 cm
long) and interstitial apatite, magnetite, pyrite, chalcopyrite, This breccia is characterized by fragments of host
and calcite. Quartz is present as both interstitial grains and rhyolite, chloritized rhyolite, and rock from the amphibole-
as massive pods 1-50 cm in diameter. From the eastern edge quartz zone in a matrix of massive magnetite and (or)
of the deposit westward, the amphibole-quartz zone thins in hematite. It occurs discontinuously along the margins of the
the hanging wall from massive amphibole-quartz rock into orebody, but is particularly well developed along the
host rock in which fractures are filled with amphibole and hanging wall (fig. A3).
have silicified walls (Emery, 1968). Locally, contacts grade The breccia was formed in a manner similar to an
into host rhyolite wallrock, which exhibits incipient stages intrusion breccia as described by Laznicka (1988). In situ
of amphibole replacement. Farther from the contact, veins exfoliation of wallrock fragments, filling of the planes of the
of magnetite cut massive actinolite. The footwall zone is exfoliated sheets with magnetite, and the presence of
less brecciated and contains fewer quartz pods than the wallrock schlieren suggest that the magnetite ore fluid had
amphibole-quartz zone in the hanging wall. Theological properties similar to an intrusive magma.
This zone represents a skarn alteration front that
preceded the emplacement of the magnetite orebody. The
protolith of the amphibole-quartz zone is the host rhyolite. Pseudobreccia
Metasomatic replacement of the protolith has partially Laznicka (1988) described one variety of pseudo-
destroyed obvious textural or chemical evidence of the breccia as being formed by replacement of host rock, which
original rock type. Relict textures of rhyolitic wallrock have results in a breccialike appearance; the rock fragments are
been noted in amphibole-quartz fragments along the not produced by physical abrasion, nor are they displaced or
hanging wall (J.R. Husman, The Doe Run Company, oral rotated. The term "pseudobreccia," as used here, is defined
commun., 1990). as host rhyolite that has been partially to totally replaced by
magnetite and (or) hematite along fractures and has a
breccialike appearance. The fragments do not appear to
Magnetite Zone have been rotated or mechanically fragmented.
The pseudobreccia has a sharp contact with the
In previous reports, the magnetite and rhyolite magnetite orebody. Along its outer margins, the pseudo-
porphyry breccia zones (Emery, 1968) and the magnetite breccia grades inward from iron oxide-cemented crackle
and brecciated wallrock-magnetite zones (Nuelle and breccia to iron oxide-cemented mosaic breccia, and then
others, 1989) were treated separately. However, further into rubble breccia near the magnetite orebody. Roundness
mapping and documentation of temporal relations have and metasomatic alteration of the rhyolite fragments
shown that the massive magnetite orebody and magnetite- increases towards the orebody.

Pea Ridge Iron Ore Mine, Missouri A3

 TIME
Old Young
Specular
Amphibole- Magnetite Magnetite Silicified Quartz REE-bearing Quartz
hematite
quartz zone zone veins zone veins breccia pipes veins
zone
Quartz

Actinolite

Magnetite

Hematite

Pyrite

Chalcopyrite

Apatite

Monazite ?

Xenotime

Biotite

Chlorite

Epidote

Muscovite/sericite

Potassium feldspar

Fluorite - - __

Barite

Tourmaline _ _ _

Rutile

Calcite

Grunerite

Talc

Anhydrite - -

Figure A2. Paragenesis of major and minor minerals in the Pea Ridge mine, Washington County, Mo. Solid line, major
deposition; dashed line, minor deposition.

Specular Hematite Zone orebody are commonly gradational, and the hematite con-
tains irregularly distributed patches and areas of magnetite.
The specular hematite (specularite) zone separates the However, the contacts are sharp locally. The specularite is
silicified zone from the magnetite orebody along the foot- finely to coarsely crystalline, generally platy, compact, and
wall, and rhyolite host rock from the orebody along the massive. Most of the specularite is an alteration product of
eastern edge of the deposit (fig. A3). The width of the zone magnetite.
varies, and it thins with depth (Husman, 1989). Contacts Mapping documents that the specular hematite zone
between the specular hematite zone and the magnetite in part formed along fault zones and that the width of the

A4 Strategic and Critical Minerals, Midcontinent Region, U.S.

 E95000______________ E96000 E97000

EXPLANATION

REE-bearing breccia pipe

Silicified zone

Specular hematite zone

Magnetite - cemented heterolithic breccia

Magnetite zone
1675 porphyry
Amphibole-quartz zone

Rhyolitic volcanic rocks

1825 porphyry

8t°f Contact Dashed where inferred

Observed fault Dashed where inferred;
bar and ball on downthrown side, 2275 porphyry
arrow indicates dip direction
Aplite dike
X-11 fault
Mafic dike
V-12pipe

N148000

0 1000 2000 FEET

N147000
0 25 50 METERS
Unnamed
porphyry

Figure A3. Geologic map of the 2,275-ft level, Pea Ridge mine, Washington County, Mo. (modified after Hussman, 1989).

zone varies proportionally with the width of the fault zones. TIME-
The specularite is locally sheared and foliated parallel to the Amphibole-quartz zone «
orientation of post-ore faults. Magnetite zone
Specular-hematite zone
Silicified zone
Silicified Zone Quartz veins
REE-bearing breccia pipes
Silicified wallrock is extensively developed in the Mafic dikes
footwall (fig. A3). Horizontal underground drilling of the Aplite dikes
footwall northward away from the orebody on the 2,275-ft
level penetrated 120 m of Silicified rock without exiting the Figure A4. Paragenetic sequence of the rock types in the
zone, and surface drill holes more than 400 m north of the Pea Ridge mine, Washington County, Mo.

Pea Ridge Iron Ore Mine, Missouri A5

orebody intersected silicified volcanic rock. The silicified Aplite Dikes
zone is characterized by massive, white to light-gray quartz
that replaced the host rhyolite wallrock to varying extents; Aplite dikes cut the host rhyolite and the magnetite
areas that are greater than 75 percent quartz are not orebody. Contacts with the REE-bearing breccia pipes are
uncommon. Potassic alteration associated with siliciflcation not exposed, and their relative ages are therefore unknown.
included the addition of potassium feldspar to the wallrock. The mutually crosscutting relation between the aplite dikes
Potassium feldspar flooding converted the grayish or and rocks of the silicified zone indicates that their emplace-
reddish-brown volcanic rocks to bright moderate-reddish- ment was coeval (fig. A4).
The dikes have variable dips, and they fill fractures
orange rock. Locally, quartz and potassium feldspar form
that strike N. 25° W. (Emery, 1968). They consist of
pegmatitic pods and veins. Accessory minerals in the potassium feldspar and quartz with minor to trace amounts
silicified zone include fluorite, muscovite, biotite, tour- of biotite, and they have a fine-grained, equigranular (apli-
maline, chlorite after biotite, epidote, calcite, barite, rutile, tic) to pegmatitic texture. Some dikes grade into quartz
pyrite, and chalcopyrite. In addition, Husman (1989) noted veins near their terminations. Some dikes have traces of
monazite, apatite, and topaz. disseminated molybdenite.
The silicified zone is a product of both open-space The aplite dikes are alaskitic and are high in SiO2
filling and replacement. An increase in the number of (72.0-74.7 weight percent and K2O (6.7-9.8 weight per-
fractures from the wallrock into the silicified zone suggests cent). Geochemical modelling of the trace-element abun-
that stockwork fracture systems may have controlled dances demonstrates that the aplite dikes and the host
siliciflcation. On the 2,275-ft level, along the X-ll drift, rhyolite are cogenetic (Day and others, 1989). The cross-
cutting relations of the aplite dikes with the host rhyolite,
siliciflcation is associated with a high-angle fault. Sericite
the magnetite orebody, and the silicified zone indicate that
along fractures and fragment surfaces shows that sericitiza-
the magnetite ore was emplaced during the regional Middle
tion accompanied silicification. The silicified zone grades Proterozoic igneous activity in the St. Francois terrane.
into areas of fracture-fill quartz veins. Silicification also
extends several meters into the specular hematite zone.
Younger sets of quartz veins cut both the zone and the Banded Rock
adjacent wallrock. Relict breccia textures in the silicified
zone suggest that brecciated zones were conduits that were Banded rock is a relatively rare rock type in the Pea
later rehealed by silica (Seeger and others, 1989). Ridge deposit. It consists of alternating bands of light-gray
and dark-gray minerals. The light-gray bands consist of
igneous rock fragments, quartz, and potassium feldspar; the
dark bands are martite and chlorite. The bands may repre-
Mafic Dikes sent altered horizons in a bedded volcaniclastic sediment.
Beds range in thickness from 1 or 2 mm to several
centimeters and exhibit ripple marks and graded beds
Mafic dikes cut across the host rhyolite, the magnetite
(Marikos and others, 1989a). The banded rock occurs along
orebody, and the silicified zone. Contact with the REE- the footwall near the contact between the specularite and
bearing breccia pipes was not observed. The temporal silicified wallrock. Locally, it has a penetrative fabric, as
relationship between the mafic and aplite dikes is ambig- defined by mineral lineations, and has been deformed by a
uous (fig. A4). Emery (1968) observed mafic dikes that are drag fold along a fault zone (ADE drift, 2,275-ft level;
cut by aplite dikes. However, our mapping indicates that a fig. A3). The rock grades into specularite, which suggests
mafic dike also cuts an aplite dike. that it was part of the host rock sequence that was replaced
The dikes are black to dark greenish gray and have a by magnetite (converted later to martite) during orebody
greasy luster. They range in thickness from less than 1 cm emplacement. The protolith was probably a water-laid
to more than 3 m and occur in two fracture systems. One pyroclastic airfall tuff deposited as a volcaniclastic intra-
fracture system strikes N. 60° E. and dips 60°-80° SE., flow sediment.
whereas the other strikes N. 85° W. and dips 60°-80° SW.
(Emery, 1968).
Pervasive chloride alteration partially obscures the REE-Bearing Breccia Pipes
original composition of the dikes. However, major and
trace-element data indicate that the dikes are tholeiitic basalt Four REE-bearing breccia pipes have been delineated
(W.C. Day, unpub. data, 1990). These dikes are similar to along the footwall and eastern edge of the 2,275-ft level of
those emplaced throughout the St. Francois terrane during the orebody (fig. A3). The pipes are at or near the contacts
the waning stages of rhyolitic volcanism (Bickford, 1988). between the major Hthologic zones. The contacts with

A6 Strategic and Critical Minerals, Midcontinent Region, U.S.

rhyolite and rocks of the silicified zone are abrupt and Mineralogy and Chemistry
commonly sheeted; those with the specular hematite zone
are irregular and embayed. Locally, apophyses of breccia Barite, potassium feldspar, chlorite, monazite, apatite,
pipes intrude adjacent rocks along fractures. quartz, and calcite are the most abundant minerals in the
The pipes dip steeply (>60°) and are elongate to ovoid pipes; biotite, fluorite, tourmaline, chalcopyrite, and pyrite
in plan view. They range in length from several meters to as are accessory minerals. Marikos and others (1989b)
much as 60 m and have widths as much as 15 rn. Their reported anhydrite in some pipes. Barite occurs as massive
maximum vertical extents are not known, but the X-13 pipe cement and open-space fill; crystals range from 1 mm to as
(fig. A3) extends from the 2,675-ft sublevel to at least the much as 50 cm long. Sanidine and orthoclase are present as
2,275-ft level, a distance of about 120 m. fractured and broken euhedral phenocrysts (or xenocrysts)
The breccia pipes contain fragments of rhyolite, that are as much as 2 cm in diameter. Apatite forms
magnetite-hematite ore, and rock of the silicified zone in a subhedral to euhedral crystals that range in size from 0.5 to
groundmass predominantly composed of rock flour, feld- 1.0 cm. Quartz forms both rounded and embayed pheno-
spar, chlorite, barite, apatite, monazite, quartz, and calcite crysts (or xenocrysts) and secondary overgrowths as cement
(fig. A2). The rock-flour consists of milled volcanic wall- within the groundmass. Areas of broken barite and potas-
rock and disaggregated specularite grains. The volcanic sium feldspar record a post-cementation brecciation event.
rock fragments grade in size from rock-flour to about 0.5 m REE-bearing minerals in the breccia pipes include
in diameter. They are reddened, due to potassium feldspar monazite, xenotime, and rare bastnaesite and britholite(?).
alteration, and have subrounded to angular edges and Monazite and xenotime occur as 0.5- to 1.0-mm-long
moderate to high sphericity. The specularite fragments crystals in aggregates having a granular texture, as radial
reach several meters in length, are angular, and have low crystal aggregates, as acicular crystals that replaced wall-
sphericity. Where specularite fragments are prevalent, they rock microfragments, and less commonly as irregular
form a tight-fitting, fragment-supported breccia having crystals that fill cracks in barite and potassium feldspar.
angular voids between fragments. Where wallrock frag-
Swarms of barite and calcite veins and radioactive
ments predominate, they are irregularly distributed and are
matrix supported. zones serve as exploration guides to blind pipes. Barite and
Most of the breccia pipe material may have been well calcite vein swarms and crackle breccias adjacent to pipes
indurated. For example, the X-ll pipe (fig. A3) has a represent extensions of pipe mineralization. The radio-
particularly hard, fine-grained matrix. Other pipes are now activity results from thorium and uranium associated with
friable because of the coarseness and excellent cleavage of the REE-bearing minerals. The breccia groundmass con-
potassium feldspar and barite and post-emplacement brec- tains notable amounts of thorium and uranium, averaging
ciation. 3,321 ppm (parts per million) and 189 ppm, respectively
Geopetal structures within void spaces in one of the (table Al). The thorium-uranium ratio is about 19, whereas
REE-bearing breccia pipes indicate that the magnetite the normal crustal ratio is about 4 (Rose and others, 1979).
deposit has not been tilted significantly since emplacement The breccia pipe groundmass has variable concen-
of the breccia pipes. The geopetal structure on the 2,440-ft trations of the REE. Data listed in table Al represent grab
level is characterized by horizontally bedded, granulated samples of groundmass material, and as such do not
rock particles that fill the bottom part of the vug and calcite represent average ore grades of the breccia pipes. Total REE
that fills the upper part of the vug. oxide concentrations of our samples range from 4.9 to 37.8
The relative age of the REE-bearing breccia pipes is weight percent, averaging 20.3 weight percent. In contrast,
poorly constrained. Rhyolite wallrocks, magnetite ore, and ore grades determined from bulk samples range from 7 to 25
rocks of the silicified zone occur as fragments within the weight percent and average about 12 weight percent (C.W.
pipes. Thus, the breccia pipes were emplaced after for- Whitten, U.S. Bureau of Mines, oral commun., 1990). The
mation of the silicified zone. Aplite dikes cut across rocks of REE-bearing breccias are enriched in the light REE, having
the silicified zone; however, relations between the aplite a pronounced negative europium anomaly (fig. A5).
dikes and the breccia pipes have not been observed. Gold is erratically distributed in the breccia pipes.
The absolute age of the REE-bearing breccia pipes Husman (1989) reported that gold occurs as electrum and
can be estimated from a U-Pb date on a xenotime crystal in sylvanite. Gold concentrations uncommonly exceed 1 ppm,
a quartz vein that is cut by a breccia pipe. The xenotime but assays of drill core and chip samples are as high as
yielded a date of 1.46 Ga (W.R. Van Schmus, written 371 ppm (Husman, 1989). Our samples have yielded gold
commun., 1988). It occurs both in quartz veins cut by the values of less than 6 ppb (parts per billion) (table Al). The
REE-bearing breccia pipes and in the pipes themselves extremely high concentrations may be due to the "nugget
(fig. A2). Therefore, formation of the REE- bearing breccia effect," where atypically large grains of gold give a higher
pipes was about 1.46 Ga. assay value than the average for the pipe.

Pea Ridge Iron Ore Mine, Missouri A7

 Table A1 . Analyses for rare-earth elements (REE), uranium, thorium, and gold in REE-bearing breccia pipes, Pea Ridge
mine, Washington County, Mo.

[The REE were determined by the ICP-MS method as outlined by Lichte and others (1987). Uranium and thorium were determined by
delayed neutron activation analysis (McKown and Millard, 1987) and gold by graphite furnace (Meier, 1980). <, less than; -, insufficient
number of samples above detection limit to calculate meaningful value]

Element Sample No.

(mine level)
PR-21 PR-71 PR-72 PR-168 PR-33C PR-33D PR-32 PR-126 PR-127 Average Standard
(2,370 ft) (2,370 ft) (2,370 ft) (2,370 ft) (2,440 ft) (2,440 ft) (2,475 ft) (2,675 ft) (2,675 ft) deviation
Parts per million
La 34,000 35,000 22,000 35,000 52,000 18,000 5,700 14,000 12,000 25300 13,863
Ce 34,000 56,000 33,000 58,000 60,000 29,000 9,500 22,000 20,000 35,722 17,242
Pr 5,400 5,500 3,200 5,800 8,900 2,800 850 2,100 1,900 4,050 2391
Nd 18,000 19,000 12,000 20,000 31,000 9,700 3,100 7,600 6,700 14,122 8,175
Sm 2,800 2,900 2,100 2,900 4,300 1,500 450 1,200 1,000 2,128 1,140
Eu 420 380 270 360 580 230 51 150 130 286 156
Gd 1,900 1,600 1,700 1,800 2,500 1,200 460 860 680 1,411 622
Tb 360 320 270 350 340 240 68 150 110 245 104
Dy 2,100 1,700 1,500 2,200 1,400 1,700 410 870 630 1,390 594
Ho 400 300 270 450 170 370 86 170 110 258 124
Er 1,200 830 760 1,400 310 1,200 240 530 340 757 409
Tm 190 140 110 240 32 200 33 91 53 121 72
Yb 1,500 990 760 1,800 160 1,600 210 670 390 898 580
U 297 135 192 94 354 284 23 199 121 189 101
Th 4,400 1,940 804 6,160 10,100 2,710 227 1,950 1,600 3,321 2,948
Weight percent
RE2O3t* 23.9 29.2 18.2 30.5 37.8 15.8 4.9 11.8 103 203 10.2
Parts per billion
Au 6 <2 <2 2 <4 <4 <4 <2 2 - -
*Sum of rare earth oxides.

Structure GENESIS OF THE REE-BEARING

BRECCIA PIPES
Faults are a common feature in the mine and appear to
have occurred throughout the history of the deposit. Two
The REE-bearing breccia pipes are similar to
styles of deformation are present in the deposit. One magmatic-hydrothermal breccias related to porphyry-type
occurred at the transition between the ductile and brittle deposits (Sillitoe, 1985). However, they are not entirely
deformation regimes, whereas the other was under condi- analogous due to the disparity between the two types of ore
tions of brittle deformation. In the former, penetrative fabric systems (one is a high-grade magnetite body, whereas the
defined by mineral lineations, mullions, foliations, and other is a porphyry copper deposit having a low metal
elongation of pseudobreccia fragments is parallel to and content). Nonetheless, the geochemical character and the
within the fault planes. These faults commonly dip at low intimate spatial relation of the REE-bearing breccia pipes
angles (45° to 60°). Also, on the 2,675-ft sublevel pseudo- with the magnetite ore system implies a genetic link
breccia fragments are elongated within a fault plane. The between the two.
ductile-brittle deformation resulted from a transpressional Magmatic-hydrothermal breccias are commonly asso-
event that affected the host rhyolite, magnetite ore, banded ciated with subvolcanic ore deposits. The breccias are a
rocks, and mafic dikes. result of fluids that exsolve from water-saturated magmas
Brittle deformation is recorded by high-angle faults in subvolcanic or plutonic environments. The exsolved
that produced angular fault breccias and clayey fault gouge fluids undergo second boiling and decompression as they
along fault planes. An example is the fault on the X-l 1 drift cool. Burnham (1979, 1985) quantified the process of
of the 2,275-ft level (fig. A3), where a high-angle reverse second boiling as the exsolution of a vapor phase from a
fault roughly parallels the iron ore-wallrock contact. Spec- water-saturated melt, with the reaction: H2O-saturated melt
ular hematite was drag folded and developed foliation = crystals + vapor. The violent rapid expulsion of fluid from
parallel to the fault plane. magma and the increase of volume due to the expansion and

A8 Strategic and Critical Minerals, Midcontinent Region, U.S.

 1,000,000
EXPLANATION
Sample No. See table 1
O PR-21
PR-71
D PR-72
PR-168
100,000
A PR-33C
A PR-33D
LLJ
V PR-32
T PR-126
cr
Q + PR-127
o
§
o
10,000

1,000

100
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb

Figure AS. Plot of abundances of rare-earth elements in breccia pipes in the Pea Ridge mine, Washington County, Mo.
Abundances normalized to Cl chondrite values of Evensen and others (1978).

subsequent decompression of the vapor phase release hydrothermal fluids, which exsolved from the iron ore
sufficient mechanical energy to generate steep tensile system, were enriched in K, Ba, REE, U, Th, P, SO2, F, Cl,
fractures in the wallrocks, or reopen existing faults and and Au. The volatile phase released during second boiling
fractures, and further widen them by hydraulic fracturing of provided the mechanical energy for brecciation and frac-
their walls (Burnham, 1985; Sillitoe, 1985). Upward turing of the wallrocks. The REE-bearing fluids streamed
streaming of the magmatic-hydrothermal fluid and vapor upward into the footwall of the magnetite orebody along
results in mixing and milling of fragments, production of fractures, faults, and zones of weakness at lithologic
rock-flour matrix, and varying degrees of upward transport contacts.
of material (Sillitoe, 1985). The fluids entrained fragments of wallrock, magnetite
The REE-bearing breccia pipes were forcefully ore, and rocks of the silicified zone and abraded them during
emplaced into the Pea Ridge magnetite deposit. Rock transport. Extreme abrasion resulted in the milling of
fragments rounded by abrasion as well as swirl textures of fragments into rock flour. Some of the pipe-fill minerals
intermixed hematite and rock flour are textural evidence of may have crystallized during second boiling, including
fluidization during pipe emplacement. Emplacement of sanidine, orthoclase, barite, monazite, apatite, quartz, and
breccia pipes appears to have been contolled by lithologic other accessory minerals in the groundmass of the breccia
contacts in the footwall along which faults and fractures pipes. Evidence for boiling includes populations of both
formed. vapor-rich and liquid-rich fluid inclusions coexisting in
According to our proposed magmatic-hydrothermal quartz within the groundmass. Some of the fluids may have
breccia model, the REE-bearing breccia pipes of the Pea circulated in the pipes after boiling and replaced microfrag-
Ridge deposit were emplaced during the waning stages of ments, cemented rock flour and fragments, and formed
the magnetite ore system. The magnetite orebody was crystal-lined vugs.
emplaced as an iron-rich magmatic-hydrothermal fluid. The During formation of the breccia pipes, quartz veins
presence of sanidine phenocrysts (or xenocrysts) confirms a adjacent to the pipes were reopened, and breccia pipe
magmatic constituent for the origin of the breccia pipes, and minerals of the same suite were deposited in the reopened
possibly the entire ore system. Late-stage magmatic- veins. In addition, crackle breccias and vein swarms of

Pea Ridge Iron Ore Mine, Missouri A9

barite and calcite formed adjacent to the pipes. Rebrecci- REE-bearing breccia pipes. Euhedral crystals of barite and
ated, recemented fragments record more than one breccia- other accessory minerals may have formed during second
tion event during the evolution of the magmatic- boiling. The sanidine phenocrysts (or xenocrysts) indicate
hydrothermal system. that the breccia pipes had a magmatic component. Release
of the vapor phase during boiling resulted in fracturing at
lithologic contacts. Fluidization, coupled with the upward
SUMMARY streaming of magmatic-hydrothermal fluids, resulted in the
The Pea Ridge deposit is a tabular body of magnetite widening of the fractures and faults and in the brecciation of
that sloped upward into the host rhyolitic wallrocks. the wallrock. After boiling, some fluids may have circulated
Development of an amphibole-quartz skarn preceded in the pipes and replaced rock microfragments and rock
magnetite deposition. The deposit is crudely zoned suc- flour, cemented the pipes, and formed crystal-lined vugs.
cessively outward from a massive magnetite core to
magnetite-cemented heterolithic breccia, to pseudobreccia,
and to distal amphibole-quartz skarn. Other rock types ACKNOWLEDGMENTS
include a specular hematite zone along the footwall and
eastern edge of the orebody, massively silicified rock of the
The study was funded jointly by the U.S. Geological
footwall, banded volcaniclastic rock, aplite dikes, and mafic
Survey Strategic and Critical Minerals Program (contract
dikes.
number 14-08-0001-A0486) and the Missouri Department
The specular hematite is in part an alteration of
of Natural Resources, Division of Geology and Land Survey
magnetite ore and seems to have developed along fault
general revenue funds.
zones. During and after specularite development, the
The authors are indebted to the personnel of the Pea
massive silicified zone formed by open-space filling and
Ridge mine; without their cooperation, this report would not
wallrock replacement; silicification extends into the hema-
be possible. We particularly thank Robert Z. Reed, past
tite zone. Fracture-fill veins of quartz continue from the
President of the Pea Ridge Iron Ore Company, and John
silicified zone into the adjacent altered rhyolite. Potassium
Wright, Chief Executive Officer of Big River Minerals. We
feldspar flooding and serialization accompanied silici-
also thank Gene R. Koebbe, General Manager, and John
fication.
Schoolcraft, Plant Superintendent, for their support. We are
Faults occur throughout the orebody. Some faults
grateful for the assistance of Don Roberts, Assistant
developed a penetrative fabric that formed in the ductile-
Manager, and Larry J. Tucker, Head Mine Engineer. Their
brittle transitional regime. A fault zone along the footwall
open-door policy to both the mine and their data files is
contains two, and possibly three, REE-bearing breccia
almost unprecedented in the mining industry and reflects
pipes; another pipe is along a high-angle reverse fault at the
their philosophy that an understanding of this deposit type
eastern edge of the orebody.
will ultimately lead to economic benefits for the mining
Breccia pipes, containing potentially economic con-
industry.
centrations of REE in monazite and xenotime, cut all rock
We also wish to thank Marco T. Einaudi and Naomi
types. The pipes include fragments of volcanic wallrock,
Oreskes for their help during the early stages of our
silicified rock, and iron ore. Feldspar, quartz, and barite
mapping program. Mark Marikos, Richard Eisenberg, and
occur as euhedral phenocrysts in chloritic groundmass.
Jim Husman provided stimulating discussions and crucial
Barite also occurs as massive replacement cement and
observations during the course of the study.
open-space fill. Monazite and xenotime are present as
granular crystals in the groundmass, as replacement of
microfragments, as radial aggregates of acicular crystals in
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Pea Ridge Iron Ore Mine, Missouri A11