Platinum-Group Elements in
Sedimentary Environments in theConterminous United States

U.S. GEOLOGICAL SURVEY BULLETIN 2049-A

Platinum-Group Elements in
Sedimentary Environments in theConterminous United States
By Jocelyn A. Peterson
With a section on Platinum-Group Elements in Coal
By Robert B. Finkelman and Susan J. Tewalt
GEOLOGY AND GEOLOGIC ENVIRONMENTS PERMISSIVE FOR PLATINUM-GROUPELEMENTS IN THE CONTERMINOUS UNITED STATESMichael L. Zientek and Norman J Page, Scientific Editors
U.S. GEOLOGICAL SURVEY BULLETIN 2049-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994

U.S. DEPARTMENT OF THE INTERIORBRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY
Gordon P. Eaton, Director

For sale by
U.S. Geological Survey, Map Distribution
Box 25286, MS 306, Federal Center
Denver, CO 80225
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does
not imply endorsement by the U.S. Government.

Library of Congress Cataloging-in-Publication Data
Peterson, Jocelyn A.
Platinum-group elements in sedimentary environments in the conterminous United States / by Jocelyn A. Peterson ; with asection on platinum-group elements in coal by Robert B. Finkelman and Susan J. Tewalt.
p. cm.
(U.S. Geological Survey bulletin ; 2049) (Geology and geologic environments permissive for platinum-groupelements in the conterminous United States ; ch. A)
Includes bibliographical references.
Supt. of Docs. no. : I 19.3:2049-A
1. Platinum ores United States. 2. Placer deposits United States. 3. Sedimentary basins United States. I. Title. H
Series, m. Series: Geology and geologic environments permissive for platinum-group elements in the conterminous UnitedStates; ch. A.
QE75.B9 no. 2049
[Tn490.P7]
557.3 s dc20
[553.4'22'0973]
93-18208
CIP

PREFACE
Advances in the understanding of the geochemistry of theplatinum-group elements (PGE), the recognition of the diversity ofenvironments in which they are found, and unsettled economic andpolitical conditions throughout the world indicating a need forreliable sources of PGE have together resulted in this assessmentof environments favorable for the occurrence of deposits within
the United States that may contain PGE resources. A preliminary
assessment delineated possible conventional and unconventional
environments for the occurrence of PGE-bearing deposits (Zientek
and others, 1988). This series of publications expands, refines, andprovides documentation for the material presented in that preliminary report. In these reports we present descriptive deposit models
that include PGE-related data, tables to augment mapped information, detailed maps of conventional and unconventional environments where PGE may be present in the conterminous UnitedStates, and a substantial bibliography documenting the backgroundmaterial used to produce these maps.
This compilation delineates environments in which PGE
may be concentrated. To spatially define and to illustrate thoseenvironments on a series of maps, deposits and occurrences thatare known to have concentrated PGE were identified. These include deposits known to have produced PGE, either as the principal commodity or as a byproduct of mining other metals, based ona review of worldwide information. Also, deposits with analytical
data indicating the presence of PGE, but for which there is norecorded production, were identified. Some of the latter deposits
may contain relatively high concentrations of PGE (for example,
in unconformity uranium-gold or Olympic Peninsula-type volcanogenic manganese deposit types), whereas others have lower PGE
levels that could only be exploited as a byproduct. A review of thegeologic characteristics and geochemical associations of additional deposit types allowed them to be identified as permissive for theoccurrence of PGE even though no production or analytical information is available.
In some cases, these permissive environments corresponded
to descriptive models such as those presented in Cox and Singer(1986). These descriptive models generally have several parts; thefirst outlines the environments in which the deposit types are foundand the second identifies specific characteristics of the deposit type.
If enough data are available, grade-tonnage distributions, which
provide quantitative information on the sizes and grades for deposits of a given type, are provided. We have augmented these published models with production and analytical information on PGE,
if these data are not available in the published model, or we presented the factors that led us to believe PGE may be present in those
types of deposits. For some deposit types for which there are PGE
data that have not been included in the original model and which weview as being important, the models in Cox and Singer (1986) weregreatly expanded and revised. If no descriptive model was available
for a deposit type we recognized, we attempted to develop one.

Deposit types permissive for PGE classified by the geologic environment in which they are found[Models numbers from Cox and Singer (1986), Bliss (1992), and Mosier and Page (1988) are shown in parentheses. Entries preceded by asterisk have recorded production of
PGE; those that are italicized have concentrations that may exceed 1 ppm PGE]

Deposits associated with convergent plate margins
Deposits associated with allochthonous mafic and ultramafic rocks:*Acoje ophiolitic nickel-copper
Limassol Forest cobalt-nickel (8c)Podiform chromite (8a)Other magmatic deposits:*Alaska PGE (9)* Syenite-hosted copper-silver-PGE*Synorogenic-synvolcanic nickel-copper (7a)Deposits hosted in sedimentary rocks:
Southeast Missouri lead-zinc (32a)
Carbonate-hosted gold-silver (26a)Porphyry-related deposits:*Porphyry copper (17)*Volcanic-hosted copper-arsenic-antimony (22a)Gold-bearing skarns (?)
Epithermal quartz-alunite gold (25e)Other hydrothermal deposits:*New Rambler copper-gold-PGE
Olympic Peninsula-type volcanogenic manganese (24c.3)
Hot-spring gold-silver (25a)Hot-spring mercury (27a)Kuroko massive sulfide (28a)Low-sulfide gold-quartz veins (36a)Polymetallic veins (22c)Residual or placer deposits:*Placer gold-PGE (39a) and *PGE-gold (39b)Lateritic nickel (38a)Deposits associated with intracontinental rifts, anorogenic settings,
and Proterozoic to Archean greenstone belts
Residual or placer deposits:*Quartz-pebble conglomerate uranium-gold (29a)Magmatic deposits:*Bushveld chromium (2a)*Dunitic nickel-copper (6b)*Komatiitic nickel-copper (6a)*Merensky Reef PGE (2b)^Norilsk copper-nickel-PGE (5b)Duluth copper-nickel-PGE (5a)Anorthosite titanium (32)
Bushveld iron-titanium-vanadium (3)Stillwater nickel-copper (1)Deposits hosted in sedimentary rocks:*Sediment-hosted copper (30b)Blackbird cobalt-copper (24d)Solution-collapse breccia pipe uranium (32e)
Deposits associated with alkaline magmatism:*Carbonatite deposits (10)Syenite/gabbro-hosted copper-silver-PGE
Gold-silver-tellurium veins (22b)
Diamond pipes (12)Other deposit types:
Basaltic copper (23)
Unconformity uranium-gold (37a)Homestake gold (36b)

Deposits not restricted to any particular setting
Metalliferous black shales
Coal
III

IV

PREFACEFor certain ore deposit types, we found that the descriptivemodels were based on a single deposit or district and therefore
lacked grade-tonnage distributions. Use of these very depositspecific models ignored the underlying geologic processes that
could result in the formation of similar deposits in very different
geologic settings from those in the descriptive model. This isparticularly true for magmatic ore deposits. Although it is difficult
to group enough deposits together with enough shared characteristics to construct descriptive models with grade-tonnage distributions, it was possible to describe the general environments andprocesses that may lead to the concentration of PGE, which wewill refer to as lithotectonic terrane models. Descriptive and process-oriented summaries of magmatic sulfide and magmatic oxide
deposits were used in conjunction with the lithotectonic terranemodels to indicate permissive settings for deposits. Examples ofdeposits are summarized for each lithotectonic terrane model. For
magmatic deposit types that have enough shared characteristics andmany examples, conventional deposit models and grade-tonnage
information are also presented.
The general geologic environments presented in the models
were used to define the tracts depicted on the maps. The data andlithologic map boundaries were compiled from many sources atvarious levels of detail. Most of the information came from published literature but some came from conversations with experts on
specific areas; only a few areas have been investigated in detail in
the field by one or more of us. Considerable personal interpretationand decision making was necessary to resolve inconsistencies,
contradictions, and controversies.
Maps showing permissive areas for the various deposit
types were prepared at scales of 1:2.5 million, 1:7.5 million, and1:17 million, depending upon the degree of detail needed toshow the information. The general approach was to compile as
many environments as possible in as much detail as possible on
the large-scale map. Compilations for some of the environments
were subsequently reduced to smaller scales. Most of the sedimentary environments are shown at the intermediate scale. This
does not necessarily mean that they are less important, but theavailable information was easily shown at the smaller scale. For
some environments, particularly those for which large areas are
permissive for deposits of those types and little detailed informa-tion about abundance or distribution of PGE in the deposits isavailable, it was impractical to try to define tracts except as very
generalized areas on page-sized illustrations.
Crocket (1981) provided some general ranges of concentrations of PGE in unmineralized rocks, which indicates a range of 1
to the low tens of parts per billion in ultramafic rocks, tenths to
the low tens of parts per billion in mafic rocks with more variability than in ultramafic rocks, and thousandths to less than 10 parts
per billion (ppb) in intermediate and felsic rocks. To form an ore
body that could be exploited principally for its PGE content, PGE
must be concentrated many times over crustal abundances to concentrations well in excess of 5 parts per million (ppm); however,when present with other valuable metals that are mined and processed in such a way that the PGE are concentrated, PGE can berecovered from rocks with concentrations of 1 ppb or less.
The text in this series contains deposit models; explanatory
information concerning tracts shown on the maps; tables listing thespecific tracts depicted on the maps and information about thosetracts, including specific references used in defining the tracts; and areference list. The reference list includes specific references used inconstructing the maps and in developing the deposit models contained in this series. This includes state and topical geologic maps
used in drawing specific tracts. In addition, other literature which is
not specifically cited, but which provided insight into concepts forcertain deposit types, is included. However, this list is not meant tobe a comprehensive list for all deposit types discussed here, so insome cases, significant papers on a given topic may not be included.
Identifying permissive areas for a given deposit type on
the basis of favorable geologic characteristics is only the firststep in conducting a quantitative mineral resource assessment. In
addition to being able to establish favorable areas, grade-tonnage
models are required, along with an estimate of the number ofdeposits that could be present. Unfortunately, it is currently only
possible to show permissive areas and known PGE occurrences
within those areas because, for most deposits and occurrencesthat have enough shared characteristics to be considered together
as a deposit type, there simply is not enough information, particularly about PGE, to construct grade-tonnage models. In addition, without grade-tonnage models, estimates of the number ofdeposits in a given geologic environment cannot be made.

CONTENTS

CONTENTS
Abstract........................................................................................................................
Introduction.................................................................................................................
Acknowledgments.......................................................................................................
Deposit models ...........................................................................................................
Placer deposits.......................................................................!.........................
Regional setting............................................................!...........................
Local setting of ore deposits........................................|...........................
Description of deposits.................................................!...........................
Economic considerations..............................................I...........................
Sediment-hosted (redbed) copper deposits........................................................
Regional setting.......................................................................................
Local setting of ore deposits ...................................................................
Description of copper deposits................................................................
Ore genesis..............................................................................................
Economic considerations.........................................................................
PGE mineralization..................................................................................
Metalliferous black shales ......................................................|...........................
Regional setting...........................................................!..........................
Metal enrichment..........................................................!...........................
PGE...............................................................................J...........................
Unconformity uranium-gold deposits.....................................|...........................
Quartz-pebble conglomerate gold-uranium deposits.........................................
Regional setting.......................................................................................
Local setting of ore deposits...................................................................
Description of deposits............................................................................
Economic considerations.........................................................................
PGE mineralization..................................................................................
Blackbird cobalt-copper deposits.......................................................................
Southeast Missouri lead-zinc deposits....................................]...........................
Solution-collapse breccia pipe uranium deposits...............................................
Platinum-group elements in coal, by R.B. Finkelman and S.J. Tewalt......................
Introduction............................................................................^..........................
PGEincoal.............................................................................|...........................
Rationale for PGE-coal basins map (fig. 1).............................,...........................
Conclusions........................................................................................................
Discussion of tract designations for PGE in sedimentary environments ...................
Placer areas (map l,pl. 1).......................................................,...........................
Sediment-hosted copper tracts (map l,pl. 1)......................................................
Midcontinent region (tract 5, map 1)........................... r ..........................
Nonesuch Shale (tract 6, map 1)..................................]...........................
New Mexico region (tract 7, map 1)............................ i...........................
Belt Supergroup (tract 8, map 1) .................................J...........................
Lisbon Valley (tract 9, map 1)............................... ......\..... ......................
Mesozoic basins (tract 10, map 1)...............................j...........................
Kona Dolomite (tract 11, map 1) .................................J...........................
West Texas (tract 12, map 1)........................................,...........................
Catskill Formation (tract 13, map 1)........................................................
Northwestern New York (tract 14, map 1)...............................................

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VI

CONTENTS

Discussion of tract designations for PGE in sedimentary environments Continued
Metalliferous black shale tracts (map 2, pi. 1).....................................................
Pennsylvanian shales of the Midwest (tract 4, map 2)............................
Shales of the eastern plains states (tract 6, map 2)..................................
Northern plains shales (tract 7, map 2)....................................................
Phosphoria Formation (tract 10, map 2)..................................................
Green River Formation (tract 11, map 2)................................................
Kishenehn Formation (tract 12, map 2)...................................................
Central Texas (tract 13, map 2)................................................................
Idaho shale belt (tract 14, map 2)............................................................
California Coast Ranges (tract 17, map 2)..............................................
Klamath Mountains (tract 18, map 2)......................................................
Apache Group (tract 19, map 2)..............................................................
Atlantic and Gulf coastal plains (tract 20, map 2)..................................
Unconformity uranium-gold tracts (fig. 2)..........................................................
Quartz-pebble conglomerate gold-uranium tracts (map 1, pi. 1).........................
Wyoming (tract 15, map 1)......................................................................
Black Hills (tract 16, map 1)....................................................................
Michigan (tract 17, map 1).......................................................................
Minnesota (tract 18, map 1).....................................................................
Montana (tract 19, map 1)........................................................................
Southern Idaho (tract 20, map 1).............................................................
Salt Lake region (tract 21, map 1)...........................................................
Uinta Mountains (tract 22, map 1)...........................................................
Blackbird cobalt-copper tract (tract 23, map 1)...................................................
Southeast Missouri lead-zinc tract (tract 22, map 2)...........................................
Solution-collapse breccia pipe uranium tract (tract 23, map 2)...........................
Summary and conclusions...........................................................................................
Selected references......................................................................................................
Appendix. List of PGE-bearing minerals and their chemical compositions ...........

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PLATE
[Plate is in pocket]
1.

Permissive areas for platinum-group elements in the conterminous United States.

FIGURES1.
2.

Coal basins in the conterminous United States and their favorability for platinum-group element enrichment...... A13
Permissive tracts for unconformity uranium-gold deposits within the conterminous United States.........................
25

TABLE
1.

List of and information about tracts for deposit types in the conterminous United States shown on maps
1 and 2 (pi. 1) and figure 2.......................................................................................................................................... A16

Platinum-Group Elements in Sedimentary Environments in theConterminous United States
By Jocelyn A. Peterson

ABSTRACT
Platinum-group elements (PGE) have long been produced from placer deposits, particularly those in the Ural
Mountains of Russia and in Colombia, and more recently as
byproducts from mining of sediment-hosted copper deposits
in the Kupferschiefer of Europe and the Copperbelt of central Africa and mining of quartz-pebble conglomerate golduranium deposits in the Witwatersrand of South Africa.
Geochemical analyses of samples from other sedimentary
environments including metalliferous black shales, coal,and unconformity uranium-gold terranes indicate the presence of small amounts of PGE. In addition, the geologic
settings and geochemical suites within Blackbird cobaltcopper, southeast Missouri lead-zinc, and solution-collapse
breccia pipe uranium deposits suggest that PGE may bepresent, but limited analyses to date have not found significant quantities. To gain a better understanding of the occurrence of PGE in these sedimentary environments, I present
descriptive deposit models for each deposit type, based on
those found in Cox and Singer (U.S. Geological Survey
Bulletin 1693), that emphasize the characteristics of theoccurrence of PGE or indicate that PGE might be present.
The section on coal by R.B. Finkelman and SJ. Tewalt uses
a factor analysis to determine the favorability of the coalbasins in the conterminous United States for containing
PGE and assigns a high favorability to parts of two basins in
the midcontinent region, a medium favorability to parts ofthree basins in the Midwest and South and smaller basins
scattered throughout the U.S., and a low favorability to theremaining basins. Discussions of the other sedimentary
environments focus on the areas within the conterminous
U.S. that are permissive for the occurrence of those environments and provide rationale for delineating each permissive
tract. I have identified 4 main areas of placer occurrences;
10 areas for sediment-hosted copper deposits; 21 areas containing black shales, most of which are metalliferous; 7areas permissive for unconformity uranium-gold deposits,
although none have been identified; 8 areas of quartz-pebble
conglomerates; 1 area of Blackbird cobalt-copper deposits
and suggest that another major region is permissive for suchdeposits; 1 area of southeast Missouri lead-zinc deposits;
and 1 region of solution-collapse breccia pipes. Accompanying plate and figures show these permissive areas.

INTRODUCTION
Platinum-group elements (PGE) are found withinmany geologic environments, most of which are present
within the conterminous U.S. Zientek and others (1988)
made a preliminary assessment of PGE in the U.S. This
report provides a more comprehensive overview of PGE
in sedimentary environments within the conterminous U.S.
by presenting mineral deposit models that emphasize PGE
and by discussing specific formations or regions of thecountry permissive for the various types of deposits inwhich PGE may be found. Throughout this report these
permissive formations or regions will be referred to as"tracts." I present information on PGE analyses for specific deposits or areas when such data are available. Manyof the tracts shown on plate 1 and figures 1 and 2 are also
permissive for other commodities, but because this reportemphasizes PGE, information pertaining to other commodities is incidental and not meant to be comprehensive.
PGE are generally thought of as being associated withmafic and ultramafic rocks rather than with sedimentaryrocks; however, an important source of PGE has been placer
deposits, such as those in the Ural Mountains, Russia (Mertie,
1969). Placers are a rather specialized case because the PGE
in many placer deposits were derived from terranes where
there are large exposures of mafic and ultramafic rocks shedding PGE into stream drainages. Placer deposits are discussed
first because they provide a logical transition from mafic andultramafic environments into sedimentary environments. In
addition to placer deposits, there are other sedimentary environments where geochemical, mineralogical, and mineraldeposit data indicate that PGE are or may be present. PGE
are well documented in the Kupferschiefer sediment-hosted
copper deposits of northern Europe (Kucha, 1982, 1983).
Closely allied with sediment-hosted copper deposits in termsof mechanisms of formation (that is, deposition of metals at
Al

A2

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATES

a reducing horizon) are metalliferous black shales and unconformity uranium-gold deposits, both of which contain PGE(see references in appropriate sections below). Quartz-pebble
conglomerates in the Witwatersrand of South Africa haveyielded PGE as byproducts from several gold deposits (Pretorius, 1981). Because it has been long known that small
amounts of PGE are present in some coals, one section of
the report discusses the favorability of coal basins for containing PGE deposits. Several sedimentary environments
have either not been analyzed or have not yet been found tocontain significant concentrations of PGE, but various characteristics of the environments indicate that they are permissive for the occurrence of PGE. These environments contain
Blackbird cobalt-copper deposits, southeast Missouri leadzinc deposits, and solution-collapse breccia pipe uranium
deposits. The inclusion of Blackbird cobalt-copper deposits
within a sedimentary environment poses some problems. The
deposits are hosted by rocks having a very large mafic volcanic component (Nash and Hahn, 1989). However, because
the overall environment is sedimentary (including turbidites)
and because the deposits are stratabound, the Blackbird
deposits are included in this discussion.

ACKNOWLEDGMENTS
Discussions during the preliminary phase of thisassessment with A.K. Armstrong, R.I. Grauch, T.S. Hayes,
A.V. Heyl, B.R. Lipin, J.T. Nash, KJ. Schultz, J.F. Slack,
E.W. Tooker, and others, whom I may have inadvertently
omitted, provided a starting point for some of the material
contained in this report. I thank KJ. Wenrich, G.A. Desborough, Jim Tucker, H.L.O. Huyck, and R.B. Schultz forproviding samples for PGE analysis of solution-collapse
breccia pipes, black shales, the Monterey Formation, strata
near the Creta deposit, and black shales of Kansas, respectively, and to M.P Foose for sharing PGE data for samples
collected at the Blackbird Mine. Special thanks go to Jeff
Mauk for giving me a tour of the White Pine Mine in 1989
and for allowing collection of samples of the Nonesuch
Shale for PGE analysis.

possible discovery of economic quantities of PGE in theU.S.
PLACER DEPOSITSThis model expands upon those for placer gold-PGE(Yeend, 1986) and placer PGE-gold (Yeend and Page,1986) deposits of Cenozoic age. These two types of placers differ primarily in grade and tonnage characteristics, areflection of source terranes rather than a function of
mechanisms of concentration.
Descriptive Summary: PGE and gold minerals arepresent in flakes, grains, and rarely nuggets in gravel- to
clay-sized fractions of alluvial, beach, eolian, and glacial
deposits and lithified equivalents.
General References: Yeend (1986); Yeend and Page
(1986); Boyle (1979); Mertie (1969).

REGIONAL SETTING

Tectonic Setting: Placers commonly form in shieldareas that have produced multicycle sediments, areas ofaccreted terranes and convergent plate margins, areas containing high-level terrace gravels, and conglomerate near
major fault zones.
Regional Lithologic Characteristics: To generate placer
deposits of any kind, suitable source rocks must be exposed in
the drainage basins. For accumulation of PGE in alluvial
placer deposits, mafic and ultramafic rocks exposed within
the basins, such as Alaskan-type ultramafic complexes, suggest a greater likelihood that PGE might accumulate in placers; however, because PGE can be derived from othersources, a lack of these rocks does not necessarily preclude
the presence of PGE in placers.
Age: Most placer deposits are Quaternary; some are
Tertiary. Older deposits, especially those from more rugged
terranes, are not commonly preserved. An exception to thisis the quartz-pebble conglomerates of Proterozoic age thatare host to gold-uranium deposits (see model below).

LOCAL SETTING OF ORE DEPOSITSDEPOSIT MODELS
Some of the deposit models presented below roughly
follow the presentations of Cox and Singer (1986) in format and content. Others provide information about PGE
only or rationale for inclusion in this compilation, if PGE
data are lacking. They are presented in approximate orderof decreasing knowledge of PGE for each deposit type.
This does not necessarily correlate with what might be theorder of importance of the deposit types relative to theRock Types: Unconsolidated gravel and conglomerateof fluvial, glaciofluvial, and beach origin, particularly, but
not limited to, those containing heavy minerals indicative of
ultramafic source terranes (PGE) and low-grade metamorphic source terranes (gold), are the main hosts of placer
deposits. Sand or sandstone are less important. If bedrock ishighly fractured or shattered, placer material may be present
in the upper meter or so.
Depositional Environment: Placers are deposited instreams where gradients flatten and velocities lessen, inPLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

beaches where winnowing of the surf causes heavy minerals to concentrate, in eolian environments, and as residual
products of in situ weathering.

DESCRIPTION OF DEPOSITS

Size and Shape: Placers vary in size from small
uneconomic accumulations a few centimeters thick todeposits tens of meters thick and tens of kilometers longyielding several million tonnes of ore, as in large PGE
placers in Russia. Deposits may be tens of meters tonearly 5 km wide. Placers commonly accumulate near the
bedrock or directly above a finer grained or more compacted layer, thus causing many deposits to have a tabularform, but other ore configurations such as small pockets
are also found.
Mineralogy: Source terrane plays a significant rolein determining what minerals may accumulate in placer
deposits. Common heavy minerals found in placer accumulations include gold and gold-silver alloys, PGE and platinum-iron alloys, magnetite, ilmenite, hematite, zircon,
chromite, pyrite, garnet, rutile, corundum, monazite, cassiterite, wolframite, scheelite, cinnabar, and sulfide minerals.
Less commonly, other minerals including several gemstones
may accumulate in placers. High (and low) specific gravity
minerals that accumulate in placers have these features incommon: (1) physical resistance to abrasion and comminution, (2) chemical resistance to solution in surficial waters,and (3) general equidimensional shape (Boyle, 1979). The
dominant PGE mineral is platinum-iron alloy with inclusionsof osmium-iridium alloys. Numerous minor and rare minerals may be present, generally as inclusions or intergrowths
with the platinum-iron alloys. PGE minerals in some placers
are included in other detrital grains such as magnetite or
chromite. Cabri and Harris (1975) determined that PGE inplacers derived from alpine deposits are dominated byosmium-iridium alloys and low Pt/(Pt+Os+Ir) ratios whereas
placers derived from Alaskan-type zoned intrusions (see Page
and Gray, 1986) have high Pt/(Pt+Os+Ir) ratios.
Ore Controls: Source rock is the most important control of the minerals that accumulate in any given placer.
Weathering can also be important; for example, gold may bedissolved and reprecipitated on gold nuclei in residuum or
alluvium. In streams with favorable source terranes, trapping mechanisms are required to concentrate the heavy minerals. These include clayey or other less permeable beds
which prevent the particles from migrating downward, riffles in the streambed, fractured bedrock transverse to stream
flow, or other structures such as bedding or schistosity perpendicular to stream flow.
Ore Texture and Size: PGE and gold particles are
generally flakes or flattened particles with rounded edges;
they are rarely equidimensional nuggets. Grains are typi-

A3cally small clay to sand sized, usually less (to far less) than
1 mm. Generally, the farther detrital minerals have traveledfrom their source the smaller and more rounded they become(Boyle, 1979; Raicevic and Cabri, 1976). PGE nuggets arerare and generally are found close to their source rocks. The
largest nugget, found in the Ural Mountains, Russia, reportedly weighed more than 11 kg (Mertie, 1969).
Geochemical Indicators: The best indicator of placer
deposits are the minerals themselves. Stream sedimentsfrom placers may contain anomalous Ag, As, Hg, Sb, Cu,
Fe, S, or Cr.

ECONOMIC CONSIDERATIONS

Grade and Tonnage Models: Grade-tonnage models
are found in Orris and Bliss (1986) and Singer and Page
(1986) for placer gold-PGE and placer PGE-gold, respectively. Subsequently, Bliss and others (1987) showed thatgold grade and placer volume, in some cases, depend moreon mining method than on traditional placer classifications.
They presented revised grade-tonnage models for gold constructed on the basis of mining method. Gold-PGE deposits
tend to be larger (about 0.002 to 200 million t) than PGEgold deposits (about 0.0016 to about 7 million t). The PGEgold placer grade-tonnage models were constructed using
placer data from the Ural Mountains, Russia, where theplacers are particularly large; thus, the grade-tonnage models may be inappropriate for application to the U.S. Gold
grades are typically less in PGE-gold placers than in goldPGE placers, but high gold grades in PGE-gold deposits
overlap with low gold grades in gold-PGE deposits (seebelow). Platinum grades are not given for gold-PGE placers;
in PGE-gold placers they range from about 0.07 to 15 ppm.
Gold is present in about 30 percent of the PGE-gold deposits
and osmium, iridium, and palladium are found in about
10-20 percent of deposits in concentrations ranging fromabout 1.5 ppb (the low for Pd) to about 500 ppb (the highfor Os). The individual large alluvial-fan gold placers of
the Sierra Nevada have produced from 15.6 to about 208.4
kg PGE at grades ranging from 0.32 to 59 ppb PGE (datafrom Mineral Resources Data System). Recorded production
from other conterminous U.S. placers has been minor.
Other Distinctions Between gold-PGE and PGE-gold
Placers: In PGE-gold placers, the gold may be incidental.
The PGE lodes from which they are derived have essentiallyno gold, and the Au:PGE ratio increases away from the PGE
lode source suggesting dilution by gold supplied from othersources and dropping out of PGE close to sources. In contrast to gold-PGE deposits, in which Au:PGE=500:l to
1,000:1, PGE-gold placers have very low Au:PGE ratios(0:1 to 50:1). Sources of PGE in some gold-PGE placers
may include unconventional deposit types such as low-sulfide gold-quartz veins.

A4

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATESThis model expands upon that of Cox (1986b).
Descriptive Summary: Deposits are characterized bydisseminated, stratabound copper-sulfide minerals, with orwithout lesser concentrations of other metals, in reduced
rocks overlying a redbed sequence. The reduced rocks represent a major marine transgression in an area of intracontinental rifting or in a basin undergoing rapid subsidence.
General References: Cox (1986b); Gustafson and Williams, (1981); Hayes (1985); Boyle and others (1989) including a summary paper by Kirkham; Kucha (1982, 1983).

basins and are generally most abundant near the basin floor,
below and interbedded with the redbeds. Rocks of themarine transgression commonly contain thin organic-carbon-rich reduced shales or sandstones that host ores bodies;carbonate assemblages may also be present and, in some
deposits, host the ores (Hayes, 1985). Commonly, evaporite
deposits are present in the middle of the basins (Copperbelt
deposits, Kupferschiefer, midcontinental U.S.).
Basement Rocks: Surrounding the basins is continental
crust commonly composed of granitoids and metamorphosed
rocks.
Age: Ages range from Proterozoic to Holocene,
although Proterozoic and Carboniferous to Permian deposits are most common (Kirkham, 1989).
Metamorphism: Regional greenschist-facies metamorphism is characteristic of several of the basins (for example,
metasedimentary rocks of the Belt Supergroup (Hayes andEinaudi, 1986)), whereas rocks in other basins (midcontinental U.S.) are not metamorphosed. The metamorphism,
although post mineralization, may have, in some cases, remobilized and concentrated metals into deposits that may nototherwise have been economic (J.M. Whipple, written commun., 1991). Metamorphism resulted primarily in in situ
recrystallization of sulfide minerals and conversion of primary gangue minerals to new minerals, which makes identification of primary mineral zonation more difficult.

REGIONAL SETTING

LOCAL SETTING OF ORE DEPOSITS

Tectonic Setting: Deposits formed in depressions
thought to be associated with failed continental rifts andpassively but rapidly subsiding basins (Borg and others,
1988; Gustafson and Williams, 1981). Examples of theformer include Kupferschiefer and Zaire-Zambian Copperbelt and of the latter, Creta, Oklahoma, where a rift association is less obvious. In a very general sense, deposits
associated with rift valleys seem to have more extensive andmore complex mineralization than do those in passively
subsiding basins.
Basin Characteristics: Basins are typically deep(greater than 1,000 m) and cover hundreds of square kilometers (Jowett and others, 1987; Borg and others, 1988).
However, a large proportion of the rocks within the basins
formed from shallow-water deposits derived from deltaic
and other near-shore sediments (Hayes and Einaudi, 1986;
LaPoint, 1979; Johnson, 1976) that were shed into a subsiding basin. The basin is typically filled with a sequence ofbimodal (Gregor Borg, unpub. data; Hayes, 1985) or
predominantly basaltic (Gustafson and Williams, 1981;
Kirkham, 1989) volcanic rocks and sedimentary rocks characterized by a prominent continental redbed sequence overlain by rocks representing a major marine transgression(Gustafson and Williams, 1981; Hayes, 1985). Individual
basins differ, but volcanic rocks are more common in rifted

Stratigraphic and Lithologic Setting: The common
feature to most deposits is that ores have been deposited inrocks of the first major marine transgression over continentally derived redbeds. Hayes (1985) indicated three major
local sedimentary environments that host these copper
deposits: carbonaceous algal dolomite (White Pine, Michigan), sandstone (Spar Lake, Montana), and shale (Creta,
Oklahoma). The ores typically, but not always, are found in
reduced rocks stratigraphically above the redbed sequences,where organic compounds or reduced mineral assemblages
are present or thought to have been present.
Basinal Setting: Most deposits are located near basin
margins or basement highs rather than in the centers of thebasins (Borg and others, 1988; Jowett and others, 1987).

Exploration Guides: Presence of other placer material, particularly gold, and suitable source terrane for PGE
are the primary exploration guides. Other geochemical
indications are probably less useful than finding the placer
materials themselves.
Possible Associated Deposits: Various PGE lodedeposits, particularly those in zoned Alaskan-type plutons,
upstream from placer accumulations may be spatiallyassociated with PGE-bearing placers (Ural Mountains,
Russia), although in the U.S., many of the associated lodedeposits, if they remain, have eluded discovery.

SEDIMENT-HOSTED (REDBED)
COPPER DEPOSITSDESCRIPTION OF COPPER DEPOSITS

Size and Shape: Most of these deposits are laterally
extensive and may cover many square kilometers, but they
are also usually very thin, commonly only centimeters thick.
Those in sandstones are tongue shaped. This deposit morphology corresponds with the commonly thin beds of manyof the host rocks and, in some cases, the ore minerals arefound only in specific horizons of the host rocks (for exam-

PLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

pie, PGE in the Kupferschiefer lie within a few centimetersof the bottom of the host black shale (Kucha, 1982)).
Mineralogy and Zoning: Chalcocite is the dominanteconomic sulfide in most deposits, although in some of theZambian and Zaire deposits chalcopyrite or bornite areimportant and minor amounts of covellite, digenite, native
copper, and cuprite are present (Hayes, 1985; Sweeney andothers, 1986; Bartholome and others, 1973). Uranium andcobalt minerals (carrollite) are present in economic amounts
in some deposits (particularly in the Zaire-Zambian Copperbelt). Galena, sphalerite, PGE minerals (see below), andmetal-rich organic compounds are associated with some
deposits (Kucha, 1982; Hayes, 1985; Gustafson and Williams, 1981). A distinct mineralogic zoning has been noted atmany deposits, the notable exceptions being those deposits
in the midcontinent U.S. and some in New Mexico, wherezoning is not as pronounced (Gustafson and Williams, 1981;
Hayes, 1985). Zoning typically cuts across bedding at low
angles. Toward the center of deposits from laterally outside
and generally also from above, the zones are as follows (seeHayes, 1985; Hayes and others, 1989): pyrite, leucoxene;
trace galena, trace sphalerite, chalcopyrite, pyrite, leucoxene; chalcopyrite, trace pyrite, possible carrollite, leucoxene; bornite, digenite, hematite; digenite, chalcocite,hematite. In deposits where distinctive zoning is lacking (forexample, Creta), chalcocite disappears into weakly pyritic
fringes as carbon-rich facies pinch out. Zoning covers kilometers laterally and meters or less vertically. The characteristic zones of these deposits suggest a paragenetic sequence
resulting from a single influx of ore fluids into reactive hostrocks in which pyrite is replaced by galena and sphalerite;
pyrite is replaced by chalcopyrite and carrollite; and remnant
pyrite and chalcopyrite are replaced by bornite, digenite, andchalcocite (see Hayes, 1985). This sequence suggests continuous addition of copper-rich solutions from sources
nearer the chalcocite-rich base of the zoned assemblage.
Form of Mineralization: Ore minerals ordinarily are
disseminated. In some deposits they replace organic matteror pyrite (Gustafson and Williams, 1981), whereas in otherdeposits, like Spar Lake (Hayes and Einaudi, 1986), theore minerals occupy former pore spaces between detritalgrains. Locally, small veinlets are present, and, in the caseof deposits in the Lisbon Valley area, Utah, redbed-derived
fluids deposited vein material along faults when reduced
aquifers higher in the stratigraphic section were encountered (Morrison and Parry, 1986).
Alteration: Diagenetically related changes in mineralogy and chemistry have accompanied ore formation. Suchchanges are best described for Spar Lake, which has several
zones associated with the sulfide mineralization (see above);similar diagenetic changes can be expected in other deposits.
Development of diagenetic-related gangue minerals fromtop (outside) to bottom (center) include calcite, chlorite,
and ankerite (Hayes and Einaudi, 1986). The paragenetic
sequence of cementing materials consists of (1) rims ofA5oxides, clays, or pyrite developed around detrital grains,(2) overgrowth of potassium-feldspar and quartz, and (3)pore-space filling by carbonate and sulfide minerals (Hayes
and Einaudi, 1986).
Structures: Some ores are associated with minor
faulting that was contemporaneous with diagenesis, but
most ores are not generally associated with major structures
except in the case of Lisbon Valley, where major faults
allowed the ore fluids to migrate upward into reduced rockabove the redbed source of the ore fluids (Morrison andParry, 1986). The most important, albeit indirect, structuralcontrol is the location of basement highs and lithologic
control of more permeable versus less permeable beds.
Post Depositional Changes: Some deposits haveundergone regional metamorphism. Weathering of the copper-sulfide minerals primarily forms azurite and malachite,
but in the Copperbelt of Zambia and Zaire, copper-phosphate and -silicate minerals also formed (Clemmy, 1974;
Annels and others, 1983).

ORE GENESISTiming: Theories range from syngenetic to epigenetic; however, the most recent view is that these deposits
form during late diagenesis along with cementation andlithification (see Jowett and others, 1987, for discussion of
the Kupferschiefer; also see Hayes, 1985; Hoeve andQuirt, 1989) and are termed syndiagenetic.
Ore Fluid Characteristics: Characteristics of ore fluids
are poorly known. Temperatures were probably quite low(less than 75°C to a maximum of possibly 250°C but almost
always less than 150°C) (see Hayes, 1985; Gustafson andWilliams, 1981; Rose, 1989; Bartholome and others, 1973).
pH was acid to neutral. Intermediate fo2 is expected. No
quantitative fluid-inclusion data are available, but ore solutions must have been very saline, as dilute low-temperature
fluids cannot transport large quantities of copper. Several
chloride-bearing crystals (containing magnesium, calcium,
sodium, and potassium) have been identified in fluid inclusions at the Shinkolobwe deposit (De Magnee and Francois,1988). This suggests a possible connection to marine evaporite brines.
Mechanisms of Ore Deposition: Ores are deposited at aredox boundary usually immediately above the redbed
sequence (Jowett and others, 1987; Hayes, 1985; Gustafson
and Williams, 1981). Sulfur isotope studies indicate that
sulfate-reducing bacteria were associated with the redox
reactions, even where organic matter is no longer present in
the host rocks. There have been several models proposed forthe generation and migration of fluids. The two most often
called upon mechanisms are: (1) convection cells that form
during late diagenesis, when waters move up from the middle, deeper, parts of the basin and are then recycled back into
the basins (Borg and others, 1988) and (2) sabkha processes

A6

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATES

in which, again, fluids move up from the deeper parts of thebasins but are evaporated rather than recycled (for example inZambia (Annels, 1974)). In both cases, however, when thefluids pass into the more reduced rocks, the sulfide minerals
are deposited.
Sources of the Metals: There are three possible sources
for the metals (see Hayes, 1985; Gustafson and Williams,
1981): (1) the underlying mafic volcanic rocks within thebasin, which should have relatively high concentrations of thecopper and cobalt that could be leached by migrating fluids(see Annels, 1974); (2) the sediments themselves, from which
metals could be leached as the fluids migrate through them;and (3) rocks outside the basin, which could be leached ofmetals as waters move through them into the basins.

ECONOMIC CONSIDERATIONS

Grade and Tonnage Models: See Mosier and others(1986) for copper, silver, and cobalt grade and tonnage models. Median tonnage is 22 million t and median grade is 2.1
percent Cu. Ranges of averages for the deposits used toconstruct the model are 0.2 to 1,600 million t and 0.7 to 10
percent Cu. Average silver and average cobalt are economic
as byproducts in about 2 percent of deposits. Average silver
grades are 1 to 200 ppm and average cobalt grades are 0.16
to 0.5 ppm. PGE, uranium, and gold are byproducts of production in some Kupferschiefer and Zambian deposits (N.J
Page, oral and written commun., 1988, 1989).
Geochemical Signatures: Copper is the best geochemical indicator for these deposits. Some deposits haveassociated silver, lead, zinc, or cobalt anomalies and someare weakly radioactive.
Exploration Guides: Geochemistry and location nearbasin margins or basement highs in reduced sedimentaryrocks above redbed sequences are best guides to deposits.
Possible Associated Deposits: There may be spatiallyassociated evaporites and basaltic copper deposits and possibly a distal association with sandstone-hosted uranium
and lead and perhaps Mississippi Valley or carbonatehosted lead-zinc deposits (Kirkham, 1989). A genetic association of these deposits is not necessarily implied.

than with chalcocite. They are in the same zone as copper
mineralization in zoned deposits. In the Kupferschiefer,
PGE are within a few centimeters of the bottom of thehost shale. Factors such as clay mineralogy and organic
chemistry may be important for localizing PGE.
Mineralogy: PGE are present in many different compounds such as organic matter, base-metal sulfide structures, selenides and arsenides, and PGE minerals. Some ofthe documented occurrences in the Kupferschiefer include
platinum-rich kerogen (an organic compound), alloys of
PGE, alloys with gold, native palladium, admixtures inmooihoekite and haycockite, sobolevskite, palladiumarsenides, nickel and cobalt arsenides, disseminated inunnamed copper-iron-silver sulfide compounds, and a variety of PGE-bearing arsenides and sulfarsenides that haveno formal mineral names. At the Musonoi deposit in Zaire,
two palladium-bearing selenide minerals have been identified, oosterboschite ((Pd, Cu)7Se5) and cuproan palladian
trogtalite ((Coo.64Cuo.3iPdo.io)l=i.05Sei.95) (Cabri» 1981 ^Cabri and Laflamme, 1981).
Associated Metals: Au, Ag, Hg, and Bi; also Pb, Se,
and numerous transition metals may accompany the PGE.
Geochemical Aspects of PGE Deposition: This discussion is based on Kupferschiefer. There seems to be apositive correlation of precious metals with phosphate
content, with clay-rich shale in which illite is the mainclay, and with bituminous shale. The concentration of metals (including PGE) at the base of the black shale suggeststhat the metals acted as catalysts of oxidation and desulfurization of the organic material. Kucha (1982, 1983) has
suggested that gamma radiation from uranium-bearingorganic material may have been important in stimulatingchemical reactions that resulted in concentration of PGE
and gold.
PGE Concentrations: Concentrations vary dependingon specific host rock. Kucha (1982, 1983) analyzed several types of rock and organic compounds from theKupferschiefer. His data are presented below.
Element

Concentration

Medium analyzed

Pt+Pd

1 ppm (max.)Base of black shale

Pt+Pd

10-20 ppm

Dilution of pitchy shale in HC1 and HF

PGE MINERALIZATIONPt
Pd

700 ppm
400 ppm

Kerogen (an organic compound)

Much of the following information has come from
Kucha (1982, 1983), who has studied the PGE occurrences in the Kupferschiefer. PGE information for otherregions is limited, although PGE are known to be present
and even produced as a byproduct in some Copperbelt(Zambian and Zaire) deposits, as at Musonoi.
Host Rocks and Lithologic Controls: PGE are foundin reduced layers above redbeds but more laterally andstratigraphically restricted than copper mineralization.
PGE generally are found with digenite and covellite rather

Pt
Pd

30 ppm30 ppm

Phosphatic shalePt
Pd

30 ppm60 ppmLimestone

Pd

20 ppm

Pb-Ag-Hg-Cu-rich shalePt
Pd

10-370 ppm
10-120 ppm

Precious-metal-bearing shale

Kucha (1982, 1983) also mentioned the presence of iridium
but did not give concentrations. The Musonoi deposit inPLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

Zaire contains 6 ppm Pd and 750 ppb Pt in ore-grade rocks(N.J Page, written commun., 1988). Analyses of eight samples collected from Zambian deposits show a maximumof 18 ppb Pt and 38 ppb Pd.
METALLIFEROUS BLACK SHALES
Descriptive Summary: Black shales are fissile, exceptionally rich in organic material and iron-sulfide minerals,
and only rarely contain fossils (Pettijohn, 1975). They generally form layers millimeters to tens of meters thick withinthicker sedimentary sequences and typically have a highorganic-carbon content. The elements that are anomalously
concentrated in many black shales include Ag, Mo, Zn, Ni,
Cu, Cr, V, and more. Recent analytical work has shown that
PGE are present in some mineralized shales.
General References: Huyck (1990); Pettijohn (1975);
Vine and Tourtelot (1970).
Definitions: Black shales are typically dark-colored(though not necessarily black), fine-grained, laminated sedimentary rocks that are typically argillaceous and containgreater than 0.5 wt percent carbon (Huyck, 1990). Some
contain particularly carbon-rich or phosphatic layers that
may be only millimeters thick. Thin laminations are common, whereas evidence of bioturbation is rare. Most fossil
fauna present were derived by settling. Metalliferous blackshales are those that are enriched in a given metal by afactor of 2x (with some exceptions) relative to the U.S.
Geological Survey standard SDO-1 (Huyck, 1990). Metalliferous black shales, as used in this report, exclude thosethat host sediment-hosted copper deposits associated withredbed sequences, although they fit Huyck's definition.
Composition: As much as 15 percent organic carbon
can be present as solid carbon compounds, spores, plantfragments, or waxy-bituminous or oily material. The claysized fraction consists largely of illite or sericite, authigenic feldspar, quartz, and carbonate minerals. Pyrite or
marcasite usually is present, commonly to several percent(Bell, 1978). Micas, apatite, and barite are also common inblack shales (Vine and Tourtelot, 1970).
Sources of Mineral Constituents: Minerals may be
detrital (quartz, feldspar, micas, clay), precipitated in situ
(calcite, dolomite, biogenic silica, apatite), authigenic ordiagenetic (clay minerals, feldspar, barite, pyrite), or weathering products (iron oxides, sulfates, carbonates) (Vine andTourtelot, 1970).

REGIONAL SETTING

Depositional Environment: Black shales were deposited under anaerobic conditions. In what environment
these conditions were achieved is less certain. It appears
that black shales can form in virtually any sedimentary
environment except desert-eolian. They are associated

A7with restricted marine, or less commonly lacustrine, basinsin which there is a sharp salinity increase with depth orthey are found along continental margins where high biological productivity and organic decay reduce the bottom
to an anoxic state (Bell, 1978). Such environments include
areas of low-energy transgressions, intersection of oxygenminimum zones with the sedimentary surface, areas of
upwelling, silled basins, coastal swamps, and alluvial
plains (Meissner and others, 1984). Requirements for oilshale (organic-carbon-rich shale) deposition include ahigh-energy oxygenated environment that encouraged highorganic productivity, a low-energy anoxic environment inwhich the organic matter was preserved, and a lack ofdilution by other constituents (Meissner and others, 1984).
Preservation is accomplished by separating the site oforganic productivity from that of deposition throughmechanisms such as stratified layers of water.
Basin Characteristics: Basins are generally less than
1,500 m deep and commonly contain cool to cold water(Bell, 1978). Sediment input to the basins is very low
either because of a low sedimentation rate or because thesediment is trapped before it reaches the basins, but the
input of organic matter into these basins is high. The watermass is poorly circulated because of factors such as a
sharp salinity increase with depth or low tides. The bottoms of the basins are generally smooth and featureless,which reduces turbulence generated by storms and tides.
Age: Black shales may be any age. Though uncommon, Archean black shales are known (Ripley and Nicol,
1981; Strauss, 1986). There is a preponderance of Paleozoic black shales in the eastern U.S.
METAL ENRICHMENT

Several definitions have been proposed for metalliferous black shales, none of which are completely satisfactoryfor various reasons (see discussion in Huyck, 1990). The
definition provided above (from Huyck, 1990) allows comparison of black shales with a well-characterized standard
for easy determination of whether the black shale in questionis metalliferous. Metal enrichment can occur in black shales
deposited in any environment. In their study of metalliferous
black shales, Vine and Tourtelot (1970) provided insights
into the characteristics of and mechanisms of enrichment toproduce metalliferous black shales. The following discussion is based on their work.
Metals in Shale Components: Certain elements tend
to be associated with various components of the shale. Al,
Ti, Ga, Zr, Sc, and, in some cases, Be, B, Ba, Na, K, Mg,
and Fe are found with detrital minerals. Ca, Mg, Mn, and Sr
are enriched in the carbonate fraction. The organic-carbon
fraction locally can contain Ag, Mo, Zn, Ni, Cu, Cr, and V,
and less commonly Co, Pb, La, Y, Se, U, and Tl. This is also
the likely locus of PGE enrichment, which is discussed in
more detail below.

A8

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATESComparison to Unenriched Shales: Often, the only
way to distinguish an enriched from an unenriched shaleis through chemical analyses, as they are otherwise
indistinguishable.
Variables That May Affect Metal Content: Sea-watercomposition, metal content of the rocks supplying detritalmaterial, sedimentary facies and depositional environment,
the types of organic matter and organisms present, andpaleotectonic setting all influence metal content. These
factors, together, help determine whether metals are being
supplied to the system and, if present, whether conditions
are favorable for their extraction and concentration. Unfortunately, these factors are nearly impossible to measure.
Mechanisms of Enrichment: Metals may be derived
syngenetically by (1) extraction from seawater by organisms,(2) metabolism of organisms, (3) adsorption onto decayingorganisms, and (4) decaying organisms supplying a sulfur
source for sulfide precipitation. All syngenetic mechanisms
have problems generating the quantity and variety of metals
present in many shales. Metals may also be introduced during
diagenesis (see sediment-hosted copper model) or later epigenetically; as waters pass through the rocks, metals aredeposited in the reducing environment of the shale. It is
usually difficult to distinguish diagenetic from epigenetic
enrichment because the only significant difference is thederivation of the fluids from either inside or outside of thesedimentary system, respectively.
Uraniferous Black Shale: (See Bell, 1978; McKelvey,
1955; Nininger, 1954; Durham, 1987.) These are a special
subset of metalliferous black shales. They form large-volume
low-grade uranium deposits, which are currently uneconomic. Uranium is typically most concentrated in carbonaceous shales containing abundant plant material, sulfide
minerals (pyrite or marcasite), and less commonly phosphateminerals, and no, or very little, calcium carbonate. Nearly all
uraniferous shales are Paleozoic or older, a noted exception
being the Miocene Monterey Formation of California(Durham, 1987). Uranium is finely disseminated in theorganic-carbon-rich or clay fraction and may be present asorganic complexes and is thought to be syngenetic. Thereseems to be an inverse relation between petroleum and uranium in black shales although small amounts of petroleum arepresent in all uranium-bearing black shales (Nininger, 1954).
Examples: Mecca Quarry Shale Member of the Linton Formation, Indiana (Rainer and Richardson, 1963);
Phosphoria Formation, western U.S. There are many,
many other black shales.

rich deposits that contain an average (for seven samples) of
570 ppb Pt, 660 ppb Pd, and 550 ppb Au in a single layer
generally between 2 and 15 cm thick (R.I. Grauch, written
commun., 1991). Although only the deposits near Songlin,
Zunyi County, Guizhow Province are being mined presently, the mineralized area extends discontinuously for
1,600 km (Coveny and Chen, 1990). Thus far, PGE havenot been recovered from these deposits but stockpiles may
be mined for them in the future (Coveny and Chen, 1990).
In the Yukon Territory of Canada, Devonian nickel-richsulfide layers lithologically similar to, but less complex
than, those in China contain an average (for three samples)of 430 ppb Pt and 230 ppb Pd (R.I. Grauch, written commun., 1991). Analyses of eight representative black shale
samples from the western U.S. ranging in age from Middle
Proterozoic to Permian yielded a maximum value of 17
ppb Pt, 19 ppb Pd, and 1 ppb Ru (G.A. Desboroughthrough M.L. Zientek, written commun., 1989). PGE were
not detected in six core samples from the Monterey Formation (J.A. Peterson, unpub. data, 1990). In a retrieval of
PGE analyses from the PLUTO geochemical data base of
the U.S. Geological Survey, five samples of the Meade
Peak Phosphatic Shale Member of the Phosphoria Formation are reported to contain 1.6 to 2.0 ppm Pd; the accuracy of this information has not been verified. Orth andothers (1988) reported PGE anomalies in a 3-m section ofLower Mississippian black shale in Oklahoma. Their maximum values for PGE are 0.56 ppb Ir, 150 ppb Pt, and 7.5
ppb Os. They attributed the anomalies to precipitation fromsea water, bacterial enrichment, and detritus derived fromultramafic sources, depending on the host rock analyzed.
Analogy of metalliferous black shales, as used in thisreport, to some sediment-hosted copper environments(Kucha, 1982, 1983) and inferences made from geochemical suites that are associated with black shales (Vine andTourtelot, 1970) further suggest that PGE should be present
in this environment. If Kupferschiefer is a valid analogy,
concentrations of PGE might be in the tens of parts per
million in the highly enriched areas, which are likely to be
the very organic-carbon-rich parts of the shales where PGE
may be contained in organic compounds as well as minerals. Palladium and iridium contents of some deep-sea sediments of silt and clay size average 3.7 and 0.35 ppb,
respectively, and the data suggest that the palladium content of sediments of terrestrial origin is similar to or slightly
enriched over that of the source region (Crocket and others,1973). If this is also true for black shales, concentrations of
PGE in the low parts per billion range might also be
expected in most shales.

PGE

There are few PGE studies of black shales. Recently,
however, Grauch and others (1991) reported the presenceof PGE in black shales in China and Canada. Lower Cambrian black shales in China host molybdenum- and nickel-

UNCONFORMITY URANIUM-GOLD DEPOSITSUranium mineralization in unconformity-type deposits consists of fracture and breccia filling in metapelites,

PLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

metapsamites, and quartz arenites below, above, or acrossan unconformity between Early and Middle Proterozoic
rocks (Grauch and Mosier, 1986). Nutt and others (1986)
suggested that structural controls may, in some cases, bemore important than the unconformity itself in localizingthese deposits. Recently, a model has been proposed forthese deposits in which the mineral deposits formed at aredox boundary below redbed sequences during "intense"
diagenesis (Hoeve and Quirt, 1989), making them somewhat similar to sediment-hosted copper deposits exceptthat the reducing environment lies below rather than above
the redbeds.
Occurrences of PGE in unconformity uranium-gold
deposits have been reported at duff Lake and in the Beaverlodge area, Athabasca basin, Saskatchewan, Canada,
and at Coronation Hill and Jabiluka in Australia. In the D
Zone at Cluff Lake, uranium is associated with Co, Ni,
Cu, Pb, S, Se, Te, As, Bi, Au, and PGE (Hoeve and Sibbald, 1978). The mineralized rocks contain uraninite;
native platinum-group metals; and silver, gold, and copper
minerals (Morton, 1976). Although no grades or analysesof PGE at Cluff Lake have been published, "significant
amounts of gold and platinum" have been reported (Robertson and Lattanzi, 1972). PGE in the Beaverlodge area
have been reported at the Nicholson #2 Zone. Here, uranium-gold-PGE mineralization is present in steeply dipping, carbonate-filled shear and fracture zones at thecontact between white dolomitic quartzite and red ferruginous quartzite breccia of the Murmac Bay Formation. The
highest PGE values are from a pitchblende-bearing shear
zone juxtaposed with peridotite of the Nicholson Bay
Ultramafic Complex. The mineralized zone has a minimum strike length of 50 m and a width of 5 to 50 cm.
Average grades are 136 ppm Au, 17.3 ppm Pd, 13.8 ppm
Pt, and 0.4 ppm Rh; there are also high concentrations ofsilver and bismuth (Hulbert, 1987). Other work in the
Nicholson area has reported Pt values of 6.51 to 12 ppm
and Pd of 14.06 ppm for three intercepts 0.9 to 1.6 m long
(Kasner Group, 1987).
The Coronation Hill deposit in the South Alligator
River area of Australia has platinum and palladium gradesof 187 and 845 ppb, respectively (Wilde and others, 1989)
with announced inferred resources, in 1986, of 34 t Au andabout 2.5 t Pt and 5.3 t Pd (Hoatson and Glaser, 1989).
Other deposits in the Coronation Hill area also contain PGE(Hoatson and Glaser, 1989). The Jabiluka deposit of thePine Creek geosyncline, Australia, contains PGE and gold.
Of 101 samples analyzed for PGE, 41 contain measurable
platinum and 43 contain measurable palladium. The average
Pt and Pd values for those 41 and 43 samples are 6.0 and
682 ppb, respectively. In the same suite, 53 samples containmeasurable gold and have an average value of 15.4 ppm(R.I. Grauch, written commun., 1991). The PGE and gold
are present together in paragenetically late Si-U-Te-Re-richveins (Grauch and others, 1985). Spatial and textural rela-

A9

tions suggested to Wilde and others (1989) that uranium,
gold, and PGE were deposited during the same mineralizing
event, transported in oxidized, slightly acidic, chloride-richbrines derived from Middle Proterozoic hematitic clastic
rocks overlying the deposits; deposition occurred as these
fluids were reduced in graphitic zones in the host rocks.
Grauch and others (1985), however, have indicated that theage of the PGE mineralization is less well constrained andcould be, but is not necessarily, much younger than theuranium mineralization.
QUARTZ-PEBBLE CONGLOMERATE
GOLD-URANIUM DEPOSITSThis model expands upon that of Cox (1986a).
Descriptive Summary: Archean to Early Proterozoic
placer deposits of gold, uranium, and PGE formed prior tooxidation of the atmosphere, which allowed the preservation and concentration of detrital pyrite and uranium minerals. Detrital minerals were locally remobilized or
texturally modified during diagenesis and metamorphism.
The famous Witwatersrand in South Africa will be used inmost of the discussion below.
General References: Cox (1986a); Pretorius (1981);
Tankard and others (1982).

REGIONAL SETTING

Tectonic Setting: Deposits of this type were formedon slowly subsiding cratons within epicontinental basinsor half grabens. In some cases, such as in the stratigraphically high deposits in the Witwatersrand, multiple sedimentation cycles are evident (Tankard and others, 1982).
Regional Lithologic Characteristics: Clastic sedimentary rocks including sandstones, shales, and conglomerates
comprise the sedimentary sequence within the basins. The
sands are fluvial and lacustrine. Deposition occurred inalluvial fans within complex braided-stream systems rather
than in a marine environment. Cross bedding is commonand evidence of current or wave winnowing of the sediments is prominent. Volcanic units may be interbeddedwith the typically thick sedimentary units. Basement rocks
are generally Archean granites or greenstones.
Age: Most deposits formed in the Late Archean to
Early Proterozoic, before free oxygen became a major
component of the Earth's atmosphere (Pretorius, 1981;
Houston and Karlstrom, 1979).
LOCAL SETTING OF ORE DEPOSITSRock Types: Quartz-pebble conglomerate, and lesscommonly sandstone, host the deposits. These conglomerates
are typically oligomictic and mature within a thick sequence

A10

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATESof less mature conglomerate and sandstone. Conglomerates
have a bimodal clast size of pebbles and matrix, where thematrix is composed of quartz, micas, chlorite, pyrite, and, insome conglomerates, fuchsite. Granitic clasts are absent.
Depositional Environment: Generally large alluvialfan systems, where stream gradients and bed load lessen,
and ancient beaches that were winnowed by wave action
are where deposits formed. Within the fan systems, gold
and uranium ores are generally found in four sites (Guilbert
and Park, 1985): (1) fan head or mid fan in sandy (pluspebbly) matrix at and near the base of fluvial conglomeratein channel fill; (2) midfan in pyritic trough cross-bedded
sandstones, in erosion-deposition channels with ore minerals on forsets, in bottom-set spoons and scours, and in basal
lag deposits; (3) midfan or upper-fan base as lag deposits insheets of cross-bedded sandstones formed by winnowing ofquartz to leave thin layers of heavy minerals, also along
unconformities at the base of sedimentary units; and (4) fan
base in carbonaceous layers on unconformities, in scour
pools, and in algal mats.

DESCRIPTION OF DEPOSITS

Size: As with modern placers, the sizes of ancientplacers vary. To be economic, however, the deposits generally must be large.
Mineralogy: Feather (1976) and Feather and Koen
(1975) stated that about 70 ore minerals have been found in
the Witwatersrand, 40 of which are of detrital origin. The
most common and important minerals include gold, pyrite,uraninite, zircon, chromite, monazite, leucoxene, osmiumiridium alloys, isoferroplatinum, and sperrylite. Many mineral grains exhibit post-depositional alteration and someminerals are of secondary origin.
Ore Textures: The ore minerals are commonly disseminated throughout the matrix of the host conglomerate but
rarely are also found along parting planes between minor
unconformities and in quartz veins and dikes. Gold particles
are typically 0.005 to 0.5 mm in diameter (Feather, 1976).
They have been largely recrystallized during diagenesis andmetamorphism.
Zoning: On a large scale at Witwatersrand, uraninite isfound basinward of the gold distribution, but there is considerable overlap in location of the two commodities (Tankard
and others, 1982). More locally within fans, the amount ofconglomerate decreases and the amount of sandstoneincreases down gradient (Guilbert and Park, 1985). Alongwith this lithologic change, pyrite concentrations and grain
size decrease, as does that of gold; net uranium and U:Au
ratios increase; and algal material increases. The copperuranium-gold-rich layers may be only a few millimeters
thick but are high grade (Guilbert and Park, 1985).
Ore Controls: Initial deposition of ore minerals wasdetermined largely by characteristics of stream flow thatgovern the deposition of heavy minerals, as in modern
placers. Carbonaceous layers, which may have been algalmats, at the bases of fans have uranium and fine gold
(Cox, 1986a) but a genetic link between the carbon andthe ore has not been substantiated. Many of the minerals
are authigenic appearing and are not always concentrated
in the conglomerate where expected on the basis of sedimentation hydraulics (Boyle, 1979), which suggests thatminerals underwent later mobilization during diagenesis
and metamorphism and were concentrated after relatively
local migration into favorable porous sites within the hostconglomerates.

ECONOMIC CONSIDERATIONS

Grade and Tonnage Models: Grade and tonnage
models for these deposits have not yet been developed.
The Witwatersrand represents uniquely enormous depositsof this type. Since its discovery in 1886, the six majorgold fields with about 150 individual mines have produced
about 55 percent of all the gold ever mined (Pretorius,
1976). About 700 t of gold is produced annually alongwith significant amounts of uranium and byproduct PGE
and silver. Average grades are 10 ppm Au, 280 ppm U,
and 3.5 ppb PGE.
Exploration Guides: Late Archean quartz-pebble
conglomerates, especially if they represent multicycle
deposits or if they are radioactive, are likely host rocks.
Provenance may be important in determining whether
gold, PGE, or uranium were available during sedimentation (Boyle, 1979).
Possible Associated Deposits: Homestake gold or
low-sulfide gold-quartz veins in basement rocks may provide a source area for the gold. Some deposits are overlain
by Proterozoic iron formation. Modern placers may haveformed from erosion of fossil placer ores.

PGE MINERALIZATIONThis information comes from the studies of Feather
and Koen (1975) and Feather (1976) on the Witwatersranddeposits, where PGE are extracted as a byproduct of gold
mining.
Economics: PGE were discovered in Witwatersrandin 1892 but they are extremely rare. They have an estimated combined grade of 3.5 ppb. Cousins (1973) indicated grades for PGE of 1.1 to 212 ppb from deposits
within the Witwaterstrand, but only 4 of the 57 deposits
cited have grades greater than 15 ppb. Where PGE areabundant, Au:PGE is 100:1 and elsewhere, Au:PGE may
be as high as 8,000:1.
Origin and Association: Ore minerals were originallydetrital but subsequent alteration and remobilization

PLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

resulted in the formation of secondary PGE minerals. PGE
are associated with uraninite, chromite, and other heavy
minerals.
Mineralogy: The most common PGE minerals are
osmium and indium alloys and native metals. Less common are ruthenosmiridium, ruthenian iridium, and tracesof iridian ruthenium. Zoning and intergrowth textures arecommon, with osmium-rich cores and iridium- and, lesscommonly, platinum-rich outer parts where platinum-ironalloys are found (usually Pt3Fe). Some altered PGE minerals contain unaltered inclusions of rhodium-platinum alloy,
michenerite, moncheite, laurite, RhS2, palladium telluride,
pentlandite, geversite, and ultramafic rocks. Sperrylite
commonly surrounds alloy grains. Less commonly, hollingworthite, iridarsenite, and rutharsenite coat alloys andare themselves coated with sperrylite. Braggite and cooperite have been reported but not verified.
Geochemistry: Osmium and iridium are the mostprevalent PGE. It is postulated that paladium, platinum,
rhodium, and ruthenium were mostly lost during weathering and transportation to give a very mature osmium+iridium-rich sediment. Eighty percent of the PGE resides in
iridium-osmium-ruthenium alloys and 15 to 20 percent issperrylite and isoferroplatinum.
Ore Textures: Minerals are irregular to well rounded
and rare crystal faces can be seen. The average grain size
is 80 micrometers. The rounded grains are likely due tochemical leaching or accretion after deposition (in situ formation) rather than to abrasion during transport because ofobserved concentric phase zoning in some particles andthe small sizes of some well rounded grains that are too
small to have formed by rounding during transport (Cousins, 1973).

BLACKBIRD COBALT-COPPER DEPOSITS
Middle Proterozoic Blackbird cobalt-copper deposits
contain massive and disseminated pyrite, pyrrhotite, arsenopyrite, cobaltite, chalcopyrite, magnetite, and minor
amounts of other sulfide minerals in stratabound lenses andstringers and in quartz-tourmaline hydrothermal breccias
hosted by metasedimentary rocks having a large mafic volcanic component (Earhart, 1986; Nash and Hahn, 1989).
Several other papers provide background information about
the Blackbird type of deposit, including Bennett (1977),
Hahn and Hughes (1984), Hughes (1983), and Nash and
Hahn (1986, 1989). Modreski (1985) described these
deposits as being somewhat midway between volcanogenic
massive sulfide and sediment-hosted copper deposits intheir genesis, with the unusual characteristic of being
highly enriched in cobalt. The presence of PGE was suggested by the copper and cobalt, with which PGE are commonly associated in other ore-forming environments. To
date, however, PGE have not been found in substantial con-All

centrations in Blackbird cobalt-copper deposits. Analysesof seven samples from the area (M.P. Foose, oral commun.
to M.L. Zientek, 1988) indicated 11 ppb Pt in one sample
and 1 ppb Pt and 1 ppb Pd in another. The same set ofsamples has 2.3 to 9.4 percent Co, 0.53 to 6.6 percent Cu,
and as much as 5 ppm Au. The Sheep Creek deposit within
the Belt Basin (Himes and Peterson, 1990) has a similargeochemical suite to the Blackbird deposits and opens up
the possibility for the presence of Blackbird type deposits
in rocks of the Belt Supergroup.

SOUTHEAST MISSOURI LEAD-ZINC DEPOSITS
These stratabound, carbonate-hosted deposits containmostly galena, sphalerite, and lesser amounts of chalcopyrite in porous rocks located in reefs on paleotopographic
highs (Snyder and Gerdeman, 1968; Briskey, 1986).
Although Mississippi Valley-type lead-zinc deposits arewidespread throughout the midcontinent of the U.S., thedeposits in the Viburnum Trend and around Fredericktown
in southeastern Missouri are of particular interest withrespect to PGE because of their unusually high cobalt andnickel contents relative to Mississippi Valley-type deposits
elsewhere. Jessey (1983) and Pignolet and Hagni (1983)
provided details about cobalt-nickel mineralization. Siegenite is the most common cobalt-nickel mineral, but carrollite, nickelian carrollite, fletcherite, polydymite, vaesite,and bravoite are also present. The cobalt minerals commonly coat earlier-formed sulfide minerals such as chalcopyrite, marcasite, and pyrite. Spinels at lower stratigraphic
levels within any given ore body contain relatively more
copper and cobalt, whereas those higher in the sequence
are more nickel rich. Grades have been reported as 0.2
percent Cu, 0.01 to low tenths of a percent Co, and 0.02 to
0.7 percent Ni (Heyl, 1982). A single sample collectedfrom the Bonneterre Formation at the Sweetwater Mine in
Madison County, Missouri, contains 16 ppb Pd with nodetection of other PGE; the same sample contains greaterthan 2 percent Cu, 5,000 ppm Ni, 500 ppm Co, andgreater than 1 percent As (M.L. Zientek, oral commun.,1991).

SOLUTION-COLLAPSE BRECCIA PIPE
URANIUM DEPOSITS
Sollution-collapse breccia pipes on the Colorado Plateau contain uraninite and associated sulfide, arsenide, sulfate, and arsenic-sulfosalt minerals disseminated or as
replacements along fractures within the pipes (Wenrich,
1985; Wenrich and others, 1988, 1990; Finch, 1992). Gradeand tonnage models have been developed based on data
gathered from eight of the mineralized breccia pipes (Finchand others, 1992). PGE data for these deposits have not

A12

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATES

previously been reported; however, 25 samples submitted foranalysis yielded some PGE values from the parts per billion
to low tens of parts per billion range for Pt, Pd, Rh, Ru, andIr; Os was not analyzed (K.J. Wenrich, written commun. to
M.L. Zientek, 1989). The geochemical suite, particularly thecobalt and nickel in these deposits, and similarities in mineralogy and in possible fluid compositions to unconformity
uranium-gold deposits suggested that PGE might be present.
Mountain and Wood (1988a, b) and Wood and others (1989)
have demonstrated that PGE may be transported and deposited from various hydrothermal fluids, including the kind of
fluids that may have deposited minerals in the breccia pipes.

PLATINUM-GROUP ELEMENTS IN COAL
By Robert B. Finkelman and Susan J. Tewalt
INTRODUCTION
The map of U.S. coal basins (fig. 1) depicts the theoretical likelihood of PGE mineralization within each basin.
The map is intended to portray areas in which PGE concentrations may be above background level for coal. Because
there are an insignificant number of PGE analyses for coal,
the map was constructed essentially from geological andgeochemical principles believed to govern PGE behavior.
Although the map is not intended to convey any economic
significance, it may indicate areas where coal could be a hostto PGE or, more likely, an indicator of PGE mineralization.
Coal may present an ideal environment for trapping
and accumulating available PGE. Several reasons support
this contention; among them are:
1. The abundance of sulfide minerals in coal and thestrong siderophile nature of PGE (Goldschmidt, 1954). Sulfide minerals constitute about 2 wt percent of all U.S. coals.
Some coals, however, contain as much as 10 wt percent
sulfide minerals. Almost 30 sulfide minerals have beenreported from coal, pyrite and marcasite being, by far, themost common. Sphalerite and galena are present in mostcoals and are major components of some midcontinentcoals. Commonly, sulfide minerals are found in the cleat or
fractures in coal and are, therefore, readily separated. Some
waste products of coal-cleaning plants contain in excess of
40 wt percent sulfide minerals (Wewerka and others, 1978).
2. The abundance of chalcophile elements in coal.
Buchanan (1988) noted that PGE readily combine with antimony, bismuth, selenium, and tellurium. These are among
the most highly concentrated elements in coal with respect
to crustal abundances (unpub. data from the U.S. Geological
Survey's National Coal Resources Data System).
3. The propensity for organic matter to form PGE
complexes. Ong and Swanson (1969) speculated that colloidal platinum in contact with organic acid would form colloidal precipitates. Also, plants themselves can take up PGE.

Dunn (1986) demonstrated the relative enrichment of platinum and palladium in twigs and trunks of several types oftrees. He suggested that biogeochemical prospecting could
be a useful tool for locating concealed PGE deposits. It is
logical to assume that coal (fossil plants) can be used for thesame purpose.
4. The natural filtering and baffling action of plants
and peat. Fine-grained water-transported PGE-bearing particles could be physically trapped either by plants or bypeat. This process may have contributed to the inclusionof detrital gold (Finkelman, 1981) and even detrital diamonds (Finkelman and Brown, 1988) in coal.
5. Interaction of carbonaceous material with PGEbearing solutions. Macdonald (1987) implied that organic
matter may have played a key role in the formation of manylate-diagenetic/epithermal PGE deposits. Kucha (1982) suggested that PGE in the Kupferschiefer shales were concentrated during late-diagenetic fluid flow by auto-oxidation anddesulfurization of the organic matter. Even in metamorphosed areas, carbonaceous material in schists may haveacted as a reductant to precipitate PGE (Needham and StuartSmith, 1987).
The association of platinum with organic matter isdiscussed by Parnell (1988), who found submicrometer particles of native platinum in a pyrobitumen from New York.

PGE IN COAL
Despite these compelling reasons to look for PGE
associated with coal, few data exist. Older literature, based
primarily on spectrographic analyses of coal ash by Goldschmidt and Peters (1933), reported several hundred to
1,000 ppb Pt in the ash. Values for osmium, palladium,and rhodium generally were between 20 and 500 ppb in
the ash. Because many coals have about 10 wt percent
ash, these data indicate concentrations of PGE in coal of 2
to 100 ppb. Block and Dams (1975) found 7 to 9 ppb iridium in Belgian coals. Ruch and others (1979) analyzed 18
U.S. coals and their residues from coal liquefaction processes. In all their samples, platinum values were below the
5 ppb detection limit. Palladium values were less than 3
ppb from most samples, with one coal having 47 ppb.
Finkelman and Aruscavage (1981) found platinum tobe less than 2 ppb in six of the eight U.S. coal samples they
analyzed. The other two coals, from North Carolina andIndiana, had 6 ppb Pt. The coals from North Carolina also
had 3 ppb Pd. Palladium and rhodium were below the limit
of detection (1.0 and 0.5 ppb, respectively) in all the othersamples. Chyi (1982) analyzed for platinum in 65 low-temperature ash samples from the Kentucky No. 9 coal. He
found platinum above the detection limit of 1 ppb in 25
samples with a maximum of 210 ppb, calculated to a coalbasis. Chyi concluded that platinum was enriched in coalby a factor of 18 compared to crustal abundances (2 ppb).

125°

Regionu'ahcoalSouthwestern

120°

115°

110°
105

Figure 1. Coal basins in the conterminous United States and their favorability for platinum-group element enrichment. Line pattern indicates high favorability; stipple patternindicates moderate favorability; unpatterned areas have low favorability. Named tract outlines are generalized.

25

30

130°

OT3

i

i
6

A14

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATESVan Der Flier-Keller (1990) summarized results (Van
Der Flier-Keller and Fyfe, 1987; Van Der Flier-Keller andGoodarzi, 1988; Van Der Flier-Keller, 1991) on 97 coalsamples from seven Cretaceous and Tertiary coal deposits in
Canada. She concluded that most of the PGE concentrations
were below the limits of detection (less than or equal to 5
ppb). Anomalously high concentrations were found in samples from the Quinette deposit in British Columbia (B.C.)
and from the Blackburn Mine, Tulameen, B.C., near theplatinum-rich Tulameen ultramafic complex.
The high PGE concentrations (1,700 ppb Pt and 180
ppb Rh) in the Quinette samples were found in the uppermost coal, in contact with overlying conglomerates. The
high concentrations (greater than 10,000 ppb Pt and 6,100
ppb Pd) in the Blackburn Mine were found in fault-related
coal samples. Van Der Flier-Keller (1990) suggested that theporous conglomerates and the permeable fault zones
enhanced groundwater flow, resulting in greater contact
between the coal and PGE in the water.
The high PGE concentrations in the Blackburn Minesamples were determined by lead fire-assay direct-current
spectroscopy. Splits of these samples analyzed by nickelsulfide fire-assay neutron activation analysis showed no
PGE enrichment. Van Der Flier-Keller (1990) suggested
that these differences may be related to within-sample
variability and (or) variations in sample preparation andanalytical procedures.
During the past few years the controversy concerning
the Cretaceous-Tertiary (K-T) boundary has focused on theindium anomaly in the boundary clay. Several terrestrial
occurrences of the boundary clay have been found in western North America. In some of these locations, the boundary
clay is associated with coal. Analyses of these coals indicate
iridium background levels of 0.005 to 0.1 ppb; near theboundary clay the iridium levels are as high as 0.3 ppb (C.L.
Pillmore and C.J. Orth, oral commun., Feb. 1989). Gilmore
and others (1984) noted that, at all of the five nonmarine K-Tboundary areas they studied, the highest iridium values were
found in the coal above the boundary clay. They attributed
this displacement to a physical mixing of iridium-bearing
particles with plant matter and to chemical migration of theiridium in the acid environment associated with the transformation of plant matter to coal.
The low (less than 0.1 ppb) values for PGE in coalassociated with (but not adjacent to) the boundary clay may
be typical of low-sulfur coal having no marine influence.
Coals having higher sulfur (greater than 1 wt percent) valuesand (or) marine influence may have PGE values in the 1 to
100 ppb range.
RATIONALE FOR PGE-COAL BASINS MAP(FIG. 1)
The absence of more complete analytical data on PGE
in coal required that we base our map on fundamental geo-

logic and geochemical principles. Using information in the
National Coal Resource Data System, we determined foreach coal basin the average values for the following factors:(1) organic sulfur content (dry basis); (2) pyritic sulfur content (dry basis); (3) the sum of nickel, cobalt, copper, andvanadium content; (4) the sum of lead and zinc content; (5)the silver content; (6) proximity to areas of known PGE
mineralization (based on data from Blair and others, 1977).
We identified the basins having the highest values
for each of these factors, which were given equal weight.
The Western Interior coal basin and the southern part of
the Southwestern Interior basin have high values for fourof the factors and are, therefore, considered to have a highlikelihood of PGE enrichment. The Illinois Basin, easternpart of the Gulf Coast Province, Pacific Coast Coal Province, Tertiary lake beds in Montana, Valley Coal Fields,
Atlantic Coast Region, Michigan Basin, Hanna Field, andSouthwestern Utah Coal Region all have high values forone or two of the factors. These basins are considered tohave a moderate likelihood for PGE mineralization. All
the remaining coal basins have a low likelihood of PGE
mineralization.

CONCLUSIONS
The association of PGE with coal presents several
opportunities. The remote possibility exists for ore-gradeconcentrations of PGE in coal. Alternatively, processes
could be developed to extract PGE and other byproducts
from coal or coal wastes. Unfortunately, there is little
information on the PGE content of coal wastes. As the ash
yield of coal is commonly 5 to 15 percent, it is likely that
PGE would be relatively enriched in the waste products.
Although current PGE data are too sparse for firm conclusions concerning PGE coal geochemistry, it appears likelythat coal would be more useful as a biogeochemical indicator of PGE mineralization than as a host. Although coalhas the potential for complexing and concentrating PGE, a
key factor influencing PGE abundances in coal would betheir availability (mobility) during the development of thecoal and its peat precursor. Even if none of the possibilities discussed above occur, the association of PGE withcoal should help us to better understand the low-temperature geochemistry of these elements.

DISCUSSION OF TRACT DESIGNATIONSFOR PGE IN SEDIMENTARYENVIRONMENTS
The sections that follow provide a brief statement of
the rationale for delineating particular tracts (shown onplate 1, maps 1 and 2) as permissive for PGE in specified

PLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

deposit types in sedimentary rocks. The tracts shown on
the plate and figure 2 do not extrapolate into the subsurface
except in the case of the solution-collapse breccia pipes,
because doing so would have greatly complicated the compilation and its presentation. References are not generally
cited in the discussion below; however, table 1 provides
references for the geologic maps and geologic literature. It
also gives mineral deposit information and locations usedin constructing the tracts. The table is organized according
to deposit type in the same order as presented in the models above, and entries under each deposit type roughlypresent the more important environments first, where sucha distinction can be made. Additional explanations, to supplement table 1, for specific tracts are included in the following text. This includes information about PGE analyses
for localities within the tracts where such data are available. Samples collected from deposits within many of the
tracts have not been analyzed for PGE; unless specific data
are presented, assume that no PGE data are available for agiven tract. In some cases, comparisons of a particulardeposit type or specific tract are made with foreign deposits known to contain PGE to emphasize important differences or similarities; in other cases, unique features ofcertain tracts are noted.
PLACER AREAS (MAP 1, PL. 1)

Known PGE-bearing gold placer deposits (table 1)
are abundant in the Sierra Nevada of California (tract 1,
map 1) and the Klamath Mountains of California and Oregon (tract 2, map 1). Two other areas that have numerous
PGE-bearing placer accumulations are Pacific coastal
sands (tract 3, map 1) and northeastern Oregon (tract 4,
map 1). Scattered localities elsewhere throughout the U.S.
are reported to have small PGE-bearing placer accumulations. Of particular note are the PGE occurrences in North
Carolina and Georgia that have been found in a region offormer gold placering and the single occurrence in New
York. Because the North Carolina and Georgia occurrences are also in a region of mafic and ultramafic rocks,
further investigation of stream sediments may reveal amore widespread distribution of PGE. Likewise, the occurrence in New York came from a glacial erratic, suggestingthat till deposited in the state may have been picked up ina suitable source region and may contain other occurrences of PGE. Many PGE minerals in placers are nottraceable to specific lode deposits or source areas. An
exception is reported PGE in placers associated with the
New Rambler and Centennial Ridge districts in southeastern Wyoming.
The main placer areas in the western U.S. have beenvariously delineated, and specific placer localities havebeen marked with a dot (map 1). These placers have been
derived from both modern stream gravel and, particularly

A15

in the Sierra Nevada, from Tertiary gravels remaining
from older drainage systems. To delineate areas permissive
for modern placer gravels in the Klamath Mountains,
Sierra Nevada, and northeastern Oregon, the rivers shownon the compilation base map are highlighted along theirdrainage from exposed ultramafic rocks within the drainage basin to the base of the range (or to the ocean), where
gradients significantly decrease. This does not necessarily
indicate that all the PGE in placers are derived from ultramafic rocks, because it is likely, particularly in the Sierra
Nevada, that PGE came from other sources such as lowsulfide gold-quartz veins. Some placers in scattered localities are described in the literature as if gold does notaccompany the PGE. However, the geologic settings formost of these placers are dissimilar to settings from which
important PGE-gold placers are derived, and those few
that do have appropriate source terranes do not fit thegrade and tonnage characteristics of the PGE-gold placers
as defined by Singer and Page (1986).
SEDIMENT-HOSTED COPPER TRACTS(MAP 1, PL. 1)

Using descriptive characteristics of sediment-hosted
copper deposits (see model above) and locations of known
sediment-hosted copper occurrences within the U.S., 10
permissive regions were defined (map 1; table 1). Some of
the regions have had productive copper deposits; others
contain uneconomic copper concentrations; one has no
known copper occurrences, but the sedimentary succession
suggests an appropriate environment for such deposits.
None of these areas are presently known to contain morethan a few parts per billion PGE, but well-known environments of this type in Poland and Germany (Kupferschiefer)and Zambia and Zaire (Copperbelt) contain sufficient PGE
to constitute byproducts of some mining operations. Thus,
it is important to recognize and examine similar environments in the U.S. However, the known sediment-hosted
copper deposits in the U.S., based on descriptions in theliterature, appear to have simpler ore mineralogy than thelarge, well-known foreign deposits, and they lack significant concentrations of associated commodities such ascobalt. Without detailed chemical analyses of samples from
U.S. deposits, it cannot be determined whether the seemingly simpler mineralogy is also associated with a geochemical suite less favorable for the presence of PGE.
MIDCONTINENT REGION (TRACT 5, MAP 1)

Permian redbed sequences in the midcontinent
extend from central Texas into southern Nebraska andinclude several formations known to contain copper concentrations, the most important being the Flowerpot Shale,

Western foothills of the Sierra
Nevada, California.

Klamath Mountains, California and Oregon, includes
part of California Coast
Ranges.

Along the Pacific Coast fromabout Eureka, California,
to the Canadian border.

Area around Baker, eastern
Oregon.

Scattered placers throughout
conterminous U.S.

Broad belt extending from
central Texas to Nebraska.

West Side of Keweenaw Peninsula and south, near Lake
Superior in Upper Peninsula of Michigan.

Mostly northeastern and central New Mexico, extending into adjacent states.

Northwestern Montana, northern Idaho, extreme northeastern Washington.

2; map 1

3; map 1

4; map 1

Unnumbered
dots; map 1

5; map 1

6; map 1

7; map 1

8; map 1

Locationl;map 1

Map No.
(pi. 1, maps
1 or 2, fig.2)

17 placers (1, 2)

28 placers (1, 2)

Proterozoic Belt Supergroup (excluding
Prichard Formation).

Cretaceous Purgatoire Formation.
Triassic Dockum, Chupadera, Chinle
Formations.
Permian Glorieta Sandstone, Abo andYeso Formations, Artesia Group.
Pennsylvania Magdelena Formation.

Proterozoic Nonesuch Shale, Copper Harbor Conglomerate.

Permian Redbed sequences from the
Flowerpot Shale of El Reno Group to
Wichita and Council Grove Groups.

Spar Lake (1)31 small mines and prospects (1)(1) Hayes andEinaudi, 1986
Harrison, 1972(l)LaPoint, 1976
(2)LaPoint, 1979
(3) Soule 1956
La Point, 1989

(1) Brown, 1971
(2) Ensign and others, 1968
(3) Hamilton, 1967
(4) Seasor and Brown, 1989

Whitp Pinp d 4.^

Copper Cities (Nacimiento) (1,3)
Pintada Lode (1,2)
Scholle district (1,3)
23 small mines and prospect (1,3)(1) Ham and Johnson, 1964
(2) Johnson, 1978
(3) Smith, 1974
(4)Tooker, 1980
Johnson and Croy, 1976
Ripley and others, 1980
Waugh and Brady, 1976

Hn

Hn

(1) Blair and others, 1977
(2) Mineral Resources Data
System (MRDS)

Mineral deposit references

Crete (1,2, 3)
Mangum (2, 3)
17 prospects and occurrences in
Nebraska (3, 4)
6 prospects in Oklahoma (2, 3, 4)
3 1 prospects in Texas (2, 3, 4)

--

-----

54 placers (1, 2)

Sediment-hosted copper (map 1, plate 1)

Shoreline sands and ancient marine terraces.

Modern stream gravels and Tertiary bench
gravels.

------

24 placers (1, 2)--Name, number, and (or) type of mineraloccurrence

Placer deposits (map 1, plate 1)
Modern stream gravels and Tertiary gravels
from several formations.

Geologic unit(s) and age[Numbers in parentheses in column four correspond to numbered references in column five]

Table 1. List of and information about tracts for deposit types in the conterminous United States shown on maps 1 and 2 (pi. 1) and figure 2.

Winston, 1986

Dane and Bachman, 1965
Darton and others, 1937
Miser and others, 1954
Moore andLandes, 1937
Tweto, 1979

Morey and others, 1982

Burchett, 1986
Darton and others, 1937
Miser and others, 1954
Moore andLandes, 1937

None used

Walker, 1977

Huntting and others, 1961
Peck, 1961
Strand, 1962, 1963

Jennings and Strand, 1960
Koenig, 1963
Peck, 1961
Strand, 1962, 1963

Burnett and Jennings, 1962
Lydon and others, 1960
Matthews and Burnett, 1965
Rogers, 1966
Strand, 1967
Strand and Koenig, 1965

Geology references

CEPIN
THE
UNIOLSTATESANETINMURMET-INGEDROTUSPMarquette, Upper Peninsula ofMichigan.

El Paso area, Texas (south of
New Mexico).

11; map 1

12; map 1

Central Indiana, central Ohio,
northern Kentucky.

Encircles Lower Peninsula ofMichigan.

3; map 2

into Kentucky, Alabama,
Georgia.

Northwestern New York southof Lake Ontario.

2; map 2

14; map 1

Mesozoic basins, a narrow,discontinuous zone from
Massachusetts to northernmost South Carolina.

10; map 1

southern New York, West
Virginia, Virginia,
Maryland.

Lisbon Valley area, eastcentral Utah, west-central
Colorado.

9; map 1

do

doDevonian New Albany and Ohio Shales.

Devonian Antrim Shale

Metalliferous black shales (map 2, plate 1)

Silurian Clinton Formation and Albion
Group.

equivalent rocks called Hamsphire
Formation in southern states and part ofWest Falls Group in New York).

Hematitic iron deposits and naturalgas.

(1) Price and others, 1985
Page, N.J, written commun.
1988

7 small deposits (1)-Cretaceous Trinity Group.
Permian Undivided sedimentary rocks.
Proterozoic Millican Formation.

(1) Brown, 1986
Gair and Thaden, 1968

1 prospect(1)Proterozoic Kona Dolomite-

Lipin, B.R., written commun.
1987

ShafferandLeninger, 1981

Gillette, 1947
Newland, 1932

(2)Tooker, 1980

(1)Bateman, 1923
(2) Gray, 1988
(3) Smoot and Robinson, 1988
Froelich and Leavy, 1982
Lewis, 1907
Smith and others, 1988
Wherry, 1908

7 deposits in Connecticut (1,2, 3)

Triassic, Jurassic Newark Supergroup andlateral equivalents.

(1) Morrison and Parry, 1986
Breit and others, 1987

5 small mines (1)Cretaceous Dakota Sandstone.
Jurassic Morrison Formation.

Alien and others, 1916
Bowknocker, 1947
Johnson and Keller, 1972
Schneider and Keller, 1970

Bowknocker, 1947
Burger and others, 1971
Gray, 1972
Gray and others, 1972
McDowell and others, 1981
SchneiderandKeller, 1970
Wayne and others, 1966
Willman and others, 1967

Fisher and others, 1970, 1979
Isachsen and Fisher, 1970
Rickard and Fisher, 1970a,b

Calver and others, 1963
Cardwell and others, 1968
Cleaves and others, 1968
Fisher and others, 1970, 1979
Isachsen and Fisher, 1970

Darton and others, 1937

Morey and others, 1982

Berg and others, 1980
Calver and others, 1963
Cleaves and others, 1968
Fisher and others, 1970,1979
Isachsen and Fisher, 1970
King andBeikman, 1974
Lewis and Kiimmel, 1950
North Carolina Division of Mineral Resources, 1958
Rickard and Fisher, 1970a,b
Rodgers, 1985
Zen,1983

Hintze, 1980
Tweto, 1979

tfl

Mostly Ozark Mountains area,
central Arkansas andeastern Oklahoma.

Southern Iowa, northern
Missouri, eastern Kansas,
northeastern Oklahoma.

South Dakota, North Dakota,
southeastern Montana,northeastern Wyoming,
eastern Nebraska, westcentral Kansas, northern
New Mexico.

Southern New Mexico

Western Colorado, eastern
Utah.

Western Wyoming, eastern
Idaho, northeastern Utah,eastern Colorado.

Northwestern Colorado,
northeastern Utah, southwestern Wyoming.

--

Most of Illinois, western
Indiana, northwestern
Kentucky.

Northwestern Montana

Central Texas

Idaho shale belt, central
Idaho.

5; map 2

6; map 2

7; map 2

8; map 2

9; map 2

10; map 2

11; map 2

12; map 2

13; map 2

14; map 2

Location4; map 2

Map No.
(pi. 1, maps
1 or 2, fig.2)

Vine and Tourtelot, 1970
Desborough and Poole, 1983
Maughan, 1984
Meissner and others, 1984
Vine and Tourtelot, 1970
Meissner and others, 1984
Vine and Tourtelot, 1970
Constenius andDyni, 1983
Vine and Tourtelot, 1970

-do
-do

-do

-do
-do

Pennsylvanian Paradox Member ofHermosa Formation.
Permian Phosphoria Formation (particularly Meade Peak Phosphatic Shale
Member), Park City Formation.
Tertiary Green River Formation

Tertiary Kishenehn Formation

Ordovician, Silurian Trail Creek and Phi
Kappa Formations.
Devonian Milligen Formation.
Mississippian Copper Basin Formation.
Pennsylvanian, Permian Wood River,
Dollarhide, and Grand Prize Formations.

Cretaceous Eagle Ford Shale, Buda
Limestone.
Devonian, Mississippian Houy
Formation.

Hall, 1985
Hall and Hobbs, 1987
Link and Mahoney, 1989

A.K. Armstrong, oral cornmun.
1988

-do

Devonian Ready Pay Member of Percha
Shale.

Numerous polymetallic vein deposits containing base and precious
metals; syngenetic deposits containing primarily barite, V, andZn.

Fix, 1958
Meissner and others, 1984
Vine and Tourtelot, 1970

-doCretaceous Carlile Shale, Niobrara
Formation, Sharon Springs Member of
Pierre Shale.

Rember and Bennett, 1979a-d

Barnes, 1966,1967,1970,1972,
1974a-c, 1975,1976a-b, 1977,
1979a-b, 1981a-b, 1982,1983
Miser and others, 1954

Ross and others, 1955

Hintze, 1980
Tweto, 1979

Hintze, 1980
Love and Christiansen, 1985
Ross and Forrester, 1947
Tweto, 1979

Hintze, 1980
Tweto, 1979

Dane andBachman, 1965

Burchett, 1986
Clayton, 1980
Darton, 1951
Love and Christiansen, 1985
Moore and Landes, 1937
Ross and others, 1955

Iowa Geological Survey, 1969
Miser and others, 1954
Missouri Geological Survey,
1979
Moore and Landes, 1937

Desborough and others, 1990
Schultz and Maynard, 1990
Vine and Tourtelot, 1970

Pennsylvanian Strata containing blackshales.

doGray and others, 1970, 1979
Wayne and others, 1966
Willman and others, 1967

Geology references

Haley and others, 1976
Miser and others, 1954

Coveny and Glascock, 1989
Coveny and others, 1987
Desborough and others, 1990
Vine and Tourtelot, 1970

Mineral deposit references

Vine and Tourtelot, 1970
Strata containing black shales. Includes(but not limited to) Stanley Shale, Atoka
Formation, Arkansas Novaculite,
Jackfork Sandstone.

throughout Pennsylvanian section, particularly Mecca Quarry Shale Member.

Continued

Name, number, and (or) type of mineraloccurrence

Metalliferous black shales (map 2, plate 1)

, .
... .
,
Geologic umt(s) and age^°

Table 1. List of and information about tracts for deposit types in the conterminous United States shown on maps 1 and 2 (pi. 1) and figure 2 Continued.

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5; fig. 2

Prntprnrnir

Pf>H Prppt Oiiartyitp

Hn

-

_____

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TTinta Mnnntains TTtah

99- man 1- __Hintze, 1980
Houston and Karlstrom, 1979__ - do____ _ ______________

fin

Antelope Island and Wasatch
Range, Utah.

21; map 1

Archean, Proterozoic Farmington Canyon
Complex, highly deformed.

Houston and Karlstrom, 1979

Houston and Karlstrom, 1979
Hn

Southern Idaho and northern
Utah in Albion-Raft River
Range area.

20; map 1

Do.

Morey and others, 1982

Darton, 195 1
DeWitt and others, 1 986

Love and Christiansen, 1985

Houston and Karlstrom, 1979
Ross and others, 1955

Anderson and others, 1987
Hills, 1987
Houston and Karlstrom, 1979

Anderson and others, 1987
DeWitt and others, 1986
Hills, 1979
Houston and Karlstrom, 1979
Redden, 1987

Anderson and others, 1987
Graf, 1979
Houston and Karlstrom, 1979,1987
Houston and others, 1979

Anderson and others, 1987
Houston and Karlstrom, 1979

Hn

Hn

i-

Southwestern Montana,extending into Idaho.

Proterozoic Cherry Creek and PonyGroups, LaHood Formation, Dillon
Granite Gneiss.

vr

TVi

Hn

19; map 1

Upper Peninsula of Michigan
near Wisconsin border.

17;maplProterozoic Boxelder Creek Formation at
Nemo and similar strata at Bear
Mountain.

Phantom Lake Metamorphic Suite.

TVi

Do.

Do.

Hn

_ do

Kalliokoski and others, 1978

Kalliokoski and others,
1 978

Geology references

Hills, 1987
Houston and Karlstrom, 1979

Nemo area and Bear Mountain
Dome, Black Hills, South
Dakota.

16; map 1

One occurrence, association withmodel uncertain.

Quartz-pebble conglomerate gold-uranium (map 1, plate 1)ment), Grandfather Mountain Formation.

Proterozoic Grenville basement of gneisses and granites, Swift Run and Metchum
River Formations.

Occurrences, association withmodel uncertain.

Occurrence known in Colorado

Kalliokoski and others, 1978
Nutt and others, 1 986

Mineral deposit references

18; map 1

Sierra Madre and Medicine
Bow Mountains, southcentral Wyoming.

15; map 1

7; fig. 2

6; fig. 2

Archean Red Creek Quartzite (basement).
Proterozoic Uinta Mountains Group.

Uinta Mountains, northeastern
Utah and northwestern
Colorado.

4; fig. 2ment), Van Horn Formation.

Archean, Proterozoic Various basement
rocks including gneiss and granite.
Proterozoic Sioux Quartzite (mostly insubsurface).

Continued

Name, number, and (or) type of mineraloccurrence

Unconformity uranium-gold (figure 2)

Geologic unit(s) and age

Southwestern Minnesota andsoutheastern South Dakota.

Location

3; fig. 2

Map No.
(pi. 1, maps
1 or 2, fig.2)

Table 1. List of and information about tracts for deposit types in the conterminous United States shown on maps 1 and 2 (pi. 1) and figure 2 Continued.

ETHE
IN
CPSTATES
UNILOANETIMNUREMT-NIGEDROTUSPotoPLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

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The Proterozoic Nonesuch Shale in the Keweenaw
Peninsula of the Upper Peninsula of Michigan contains theWhite Pine deposit (map 1, deposit c). Plate 1 depicts only
the Nonesuch Shale, but the uppermost part of the underlying Copper Harbor Conglomerate is also mineralized(Hamilton, 1967; Seasor and Brown, 1989). Other deposits
have not yet been found in the Nonesuch Shale. The shalepossibly underwent two stages of mineralization, an earlier
chalcocite stage and a later native-copper stage (Jeff Mauk,oral commun., 1989). The native-copper mineralization
may have been part of the basaltic copper mineralization of
the underlying Portage Lake Volcanics. Of nine samples
analyzed from the Nonesuch Shale, platinum values are 1.1
to 3.3 ppb and palladium values are 1.4 to 4.0 ppb.

m

i
1
'=CQr

which is host to the formerly productive Creta and Mangum deposits (map 1, deposits a and b). There are also
several other organic-carbon-rich black shales within theredbed section. Most of the mines and prospects are in
Texas and Oklahoma, and although small prospects andoccurrences are also present in southern Kansas, the stratigraphic interval over which known copper occurrences arepresent is greater in Oklahoma and Texas. Analyses of 13
samples from near Creta indicate that PGE are present insmall quantities. Nine samples have 0.7 to 1.2 ppb Pt andtwo have 0.8 ppb Pd. Additionally, one mill concentratecontained 11 ppb Pt, 8.5 ppb Pd, and 2 ppb Rh.
NONESUCH SHALE (TRACT 6, MAP 1)

U

w .SP

X~N X~N X'N

A21

Si
"o
O

SI
<N

NEW MEXICO REGION (TRACT 7, MAP 1)

This area includes numerous redbed formations
ranging in age from Pennsylvanian through Cretaceousthat crop out mostly in central and northeastern New Mexico and extend into adjacent parts of Texas, Colorado,
Oklahoma, and Kansas. Because of the complex geology
of New Mexico, parts of the units shown on plate 1 havebeen generalized and simplified to retain usability. The
region includes several important copper deposits and districts, including Copper Cities (Nacimiento), Pintada Lode,and Scholle (map 1, deposits d, e, and f)» hosted by Permian and Triassic strata. Mineralization in sequences ofother ages (Pennsylvanian and Cretaceous) is less welldeveloped and generally uneconomic.
BELT SUPERGROUP (TRACT 8, MAP 1)

The Proterozoic Belt Supergroup of Montana, Idaho,
and Washington is composed of a thick sedimentary
sequence known to contain numerous copper occurrencesand deposits. The Prichard Formation, lowermost unit of
the Belt, is not known to contain copper mineralizationand does not appear to be favorable for sediment-hosted

A22

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATES

copper deposits; therefore, it is not shown for this deposit
type. The Revett Formation within the Ravalli Group contains the known economic deposits (Harrison, 1972). Belt
strata overlying the Ravalli Group contain copper concentrations which are currently uneconomic. The Spar Lake
deposit (map 1, deposit g), an important copper deposit in
the Belt, has received recent careful study of the diagenetic nature of the formation of the deposit (Hayes andEinaudi, 1986).
LISBON VALLEY (TRACT 9, MAP 1)

The area surrounding Lisbon Valley in eastern Utah
and western Colorado presents a unique type of sedimenthosted copper deposit in which the mineralizing fluids
migrated along major faults out of the redbed sequence andinto younger host rocks. In this case, the fluids are thought
to have been derived from the Pennsylvanian Hermosa Formation and moved into the Jurassic Morrison Formation and
the Cretaceous Dakota Sandstone (Morrison and Parry, 1986;
Breit and others, 1987). Both host formations are extensive
throughout the western U.S. but are shown as sedimenthosted copper tracts only in the vicinity of Lisbon Valley
near areas of known mineralization, where the units areprobably underlain by the Hermosa Formation.

and other sulfide minerals, and mineral deposition in asabkha environment.
WEST TEXAS (TRACT 12, MAP 1)

The westernmost part of Texas south of New Mexico contains several redbed sequences (see table 1).
Although the known copper deposits in this area are veins,
geologic characteristics of the redbed sequences led a teamof U.S. Geological Survey scientists in a preliminary
unpublished mineral assessment of the areas to delineate
these units as being permissive for sediment-hosted copper
deposits (N.J Page, written commun., 1988).
CATSKILL FORMATION (TRACT 13, MAP 1)

The Catskill Formation, although relatively unmineralized when compared to the tracts discussed above, has
permissive lithologic characteristics such as redbeds andreduced horizons and small copper-uranium prospects(McCauley, 1961), which merit its inclusion as an areapermissive for sediment-hosted copper deposits.
NORTHWESTERN NEW YORK (TRACT 14, MAP 1)

MESOZOIC BASINS (TRACT 10, MAP 1)

The Mesozoic Basins in the eastern U.S. extend
from Massachusetts to North Carolina subparallel to the
Atlantic coastline. These basins contain several types ofmineral deposits including some that have been classified
as sediment hosted (Smith and others, 1988; Smoot andRobinson, 1988). However, the geologic environment of
the basins differs in one important aspect from the classic
sediment-hosted copper environment; sediment-hosted
deposits are most commonly deposited in the first major
marine transgression overlying the redbed sequence (seemodel), whereas the Mesozoic basins are filled with lacustrine sequences. It is not known what, if any, effect thismight have had on associated mineral deposits.
KONA DOLOMITE (TRACT 11, MAP 1)

The Kona Dolomite, part of the Proterozoic Chocolay Group, is located in the Upper Peninsula of Michigan
near Marquette. The presence of one copper prospect andsimilarities to the dolomitic host rocks in the Copperbelt
of Zambia and Zaire suggest that this unit should be considered with the sediment-hosted copper environment.
Similarities of the Kona Dolomite with the Copperbelt
rocks include stromatolytic dolomite suggesting an intertidal environment, interbeds of slate, presence of pyrite

The Silurian Clinton Formation and Albion Groupsouth of Lake Ontario have been included as permissive
for sediment-hosted copper deposits because they containminor redbed sequences and black shales. However, theshales do not directly overlie the redbeds but are commonly interbedded with other sandstones and calcareous
rocks above the redbed sequence. Above these units theSalina Group consists of evaporite deposits. No copper
occurrences are currently known in these units but hematitic iron deposits are common in the Clinton Formationand natural gas has been produced from rocks of the
Albion Group (Newland, 1932; Gillette, 1947).
METALLIFEROUS BLACK SHALE TRACTS(MAP 2, PL. 1)
Innumerable black-shale units are present throughout
the U.S. that range in age from Ordovician to Tertiary. All
are thin units impossible to show at the scale of map 2.
Therefore, black-shale tracts are shown as the major stratigraphic units within which the shales are found. In somecases, particularly for Pennsylvanian strata, which containmany black shales, the entire stratigraphic sequence wasoutlined to show the general area in which the shales arepresent. Probably some areas underlain by black shales
have inadvertently been omitted, because a comprehensive
knowledge of the geology of the entire country would bePLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

needed to delineate all black shales. I also did not include
metamorphosed shales such as the Ledbetter Slate in the
Metaline district of Washington or the slates of the Carolina
slate belt even though these and similar slates may be metamorphosed equivalents of organic-carbon-rich shales. However, I did include most of those shales studied by Vine andTourtelot (1970) and by Desborough and Poole (1983),
other shales that were noted as organic-carbon rich in theliterature, and units on state geologic maps that could possibly fit the model for shales permissive for PGE.
I have delineated 21 areas of black shales. One
black shale known to contain PGE is a Mississippian unit
in southeastern Oklahoma (in tract 5, map 2) (Orth andothers, 1988). Few black shales in the U.S. have been analyzed for PGE. The areas outlined on plate 2 do not repeat
black shales associated with redbed sequences that areincluded in tracts for sediment-hosted copper deposits.
Only selected tracts are discussed below; see table 1 forthe general information about each tract.
PENNSYLVANIAN SHALES OF THE MIDWEST (TRACT 4, MAP2)

The Pennsylvanian section of Illinois, Indiana, andnorthwestern Kentucky contains many thin black shaleunits, at least one of which (the Mecca Quarry Shale
Member of the Linton Formation of Rainer and Richardson, 1963) is known to be enriched in metals (Coveny andGlascock, 1989). Because the shales are thin and because
they are found throughout the entire Pennsylvanian section, the complete section was delineated on the map. The
black shales of interest comprise only a small part of thearea shown.
SHALES OF THE EASTERN PLAINS STATES (TRACT 6, MAP 2)Southern Iowa, northern Missouri, and eastern Kansas
have extensive outcrops of Pennsylvanian strata containing
black shale. Equivalent strata extend into northeastern Oklahoma and cross the state. The delineated area was rather
arbitrarily cut off in northeastern Oklahoma because it wasdifficult to determine from the state geologic map of Oklahoma whether the black shales within the Pennsylvanianstrata there continued to the south. Thus, the area of blackshales may extend farther south than shown. Twelve analyses of shales from Kansas yielded low parts per billion (lessthan 10 ppb) values for platinum and palladium in most
samples and low parts per billion values for ruthenium inthree samples.

A23

Dakota, where glacial cover is present nearly everywhere
except in major stream drainages, and into Minnesota,
where all Paleozoic and younger units are very generalized
on geologic maps.
PHOSPHORIA FORMATION (TRACT 10, MAP 2)

The Permian Phosphoria Formation presented somewhat of a dilemma in classifying it with a particular deposit
type. There are some red sandstones within it; yet, the unit
is not commonly included in discussions of sediment-hosted
copper environments. The large phosphate content sets itapart from most sequences in which black shales are found.
However, its Meade Peak Phosphatic Shale Member has ahigh organic-carbon content and was considered a black shale
by Vine and Tourtelot (1970). Therefore, I have included itwith black shales. As noted in an earlier section, five samplesof this member contain 1.6 to 2.0 ppm Pd. Map 2 shows
generalized locations of the outcrops of the Phosphoria Formation; the organic-carbon-rich member is only a small partof it.
GREEN RIVER FORMATION (TRACT 11, MAP 2)

The Tertiary Green River Formation of lacustrine
rather than marine origin is known not only as a blackshale but also as an oil shale (Meissner and others, 1984).

KISHENEHN FORMATION (TRACT 12, MAP 2)

Oil shale within the Oligocene Kishenehn Formation(Constenius and Dyni, 1983) has been compared to that in
the Green River Formation. There is no data on the trace
metal content of the shale.
CENTRAL TEXAS (TRACT 13, MAP 2)

Vine and Tourtelot (1970) studied three black shales
in Texas, the Devonian and Mississippian Houy Formationand the Cretaceous Eagle Ford Shale and Buda Limestone.
West of the Eagle Ford Shale, the Buda Limestone, in largepart, has a very sinuous and sometimes sporadically distributed outcrop pattern. The Houy Formation crops out only ina small area in central Texas. Parts of the Eagle Ford Shaleand most of the Buda Limestone are limestone according togeologic map descriptions; the extent of the black shale
within the units is likely much smaller than the areas shown.

NORTHERN PLAINS SHALES (TRACT 7, MAP 2)IDAHO SHALE BELT (TRACT 14, MAP 2)

The Cretaceous Carlile and Niobrara Formations andPierre Shale are extensive throughout the northern plainsstates. It was difficult to trace these units into North

The black-shale mineral belt of central Idaho encompasses nearly the entire Paleozoic section. Numerous

A24

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATES

formations contain thin carbonaceous shale beds, many ofwhich contain mineral deposits. Some of the mineral deposits formed syngenetically, whereas others are epigenetic. These strata include both unmetamorphosed shaleand phyllitic shale. It was not practical to distinguish thetwo at the scale of map 2.

CALIFORNIA COAST RANGES (TRACT 17, MAP 2)

Numerous marine shales of Cretaceous through Tertiary age are found in the California Coast Ranges alongwith ophiolite sequences, melange, and other rock types.
One of the more extensive marine shales is in the Miocene
Monterey Formation, which has been investigated for uranium resources. Analysis of six samples from the Monterey Formation failed to detect PGE above the limits ofanalytical determination (2 ppb for Pt, Pd, and Rh; 1 ppb
for Ru and Ir). On map 2, the Monterey Formation is
included in the Miocene marine beds.

UNCONFORMITY URANIUM-GOLD TRACTS(FIG. 2)The U.S. does not have any known unconformity
uranium-gold deposits of the type found in Saskatchewan
and Australia. However, there are numerous areas scattered throughout the U.S. of Proterozoic rocks at unconformities. Kalliokoski and others (1978) developed amodel for known deposits, which is similar to that ofGrauch and Mosier (1986). Kalliokoski and others (1978)
compared areas of Proterozoic rocks within the conterminous U.S. to their model and categorized them as having ahigh, medium, or low degree of similarity to the model.
Figure 2 shows those areas which obtained a "high" rank
(table 1) in that study. The unconformities themselves aremuch more restricted than the areas depicted on the figure.
The interpretations of Nutt and others (1986) that structural controls are perhaps more important than unconformities and the importance of redox reactions below redbed
sequences (Hoeve and Quirt, 1989) might lessen the favorability of some of these areas.

KLAMATH MOUNTAINS (TRACT 18, MAP 2)

In the Klamath Mountains of southwestern Oregon is
the Jurassic Galice Formation composed primarily of shale,some of which contains organic material. The Galice Formation is part of one of the several accreted terranes thatmake up the Klamath Mountains, which suggests that otheraccreted terranes might have organic-carbon-rich shales
permissive for the accumulation of PGE.

APACHE GROUP (TRACT 19, MAP 2)

The upper part of the Dripping Spring Quartzite
within the Apache Group contains uraniferous carbonaceous siltstone known to host chalcopyrite, pyrrhotite,
pyrite, molybdenite, and galena as well as uranium-bearing minerals (Nutt, 1981). Although remobilized duringmetamorphism associated with widespread diabase intrusion, the uranium is thought to have been concentrated anddeposited initially during diagenesis.

ATLANTIC AND GULF COASTAL PLAINS (TRACT 20, MAP 2)Upper Cretaceous rocks along the Atlantic and Gulf
coastal plains locally contain black shale units in a similar
stratigraphic position as the Eagle Ford Shale of Texas.
These black shales have not been discussed in the context
of metal concentrations but were included here to provide
continuity with the black shales of Texas and to indicatethat it might be worthwhile to analyze these shales to see
whether or not they have been mineralized.

QUARTZ-PEBBLE CONGLOMERATE
GOLD-URANIUM TRACTS (MAP 1, PL. 1)
Proterozoic quartz-pebble conglomerates that could
host detrital concentrations of uraninite, pyrite, gold, andPGE are widespread in the U.S. but relatively small; mostof the eight areas outlined on map 1 show the larger sedimentary units within which the generally thin and fairly
limited quartz-pebble conglomerates are found. These conglomerates have received the greatest scrutiny in terms of
their uranium potential (Houston and Karlstrom, 1979) andmost have not been studied in detail for gold or PGE. Gold
was analyzed for but not found in the Middle Proterozoic
conglomerates of the Apache Group in Arizona (C.T. Wrucke,
oral commun., 1991). However, the analyses have high detection limits, which would not have detected small gold concentrations. Nevertheless, the Apache Group rocks have notbeen given the status of a quartz-pebble conglomerate tract
because of their young age. Fossil placers have not been
mined in any of the areas except for minor amounts in theBlack Hills, and geochemical data are generally lacking. The
tracts are delineated because of the presence and recovery
as a byproduct of PGE in the Witwatersrand of South Africa(Feather, 1976). However, comparison of possible U.S. fossil
placer areas to the Witwatersrand may not be warranted
because of the enormous size of the Witwatersrand comparedto the known quartz-pebble conglomerates in the U.S. andbecause provenance is critical to the formation of placer
PGE, and in some cases the potential source terranes are
unknown. Fossil placers of other ages may have been preserved, but I did not undertake a comprehensive search toPLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

A25

A26

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATES

find all possible fossil placers because it is likely that mostwould be small and of little economic significance, except
for the Tertiary deposits of California discussed in the placer
section above.

because the units are possibly too young to contain detritaluranium minerals and pyrite (Anderson and others, 1987).
SOUTHERN IDAHO (TRACT 20, MAP 1)

WYOMING (TRACT 15, MAP 1)

The Sierra Madre and Medicine Bow Mountains ofsoutheastern Wyoming are perhaps the best known of thequartz-pebble conglomerate areas within the U.S. The tracts
show the metasedimentary Deep Lake Group and Phantom
Lake Metamorphic Suite (Karlstrom and Houston, 1979),
both of which contain quartz-pebble conglomerate. Some of
the conglomerates have above-background radioactivity.
BLACK HILLS (TRACT 16, MAP 1)

The Nemo and the Bear Mountain Dome areas in theBlack Hills of South Dakota have quartz-pebble conglomerates in the Boxelder Creek Formation, Estes Formation, andother similar rocks (Hills, 1979; DeWitt and others, 1986).
Uranium has not been found in conglomerates of the Estes
Formation as originally thought. Although these conglomerates are in a productive gold region (Homestake Mine),
only small amounts of gold have been mined from them.
MICHIGAN (TRACT 17, MAP 1)

The tract outlines the latest Archean Dickinson
Group, near the Wisconsin border in the Upper Peninsula ofMichigan, which contains quartz-pebble conglomerates.
These rocks are older than those generally known to contain uranium mineralization (Houston and Karlstrom,1979), but they are permissive for other detrital material
such as gold and PGE.
MINNESOTA (TRACT 18, MAP 1)

The lower units of the Proterozoic Animikie Groupof east-central Minnesota contain quartz-pebble conglomerates below iron formation-bearing strata. The conglomerates may be too young for uranium mineralization but
may contain other detrital material.

A small part of southern Idaho and northern Utah in
the Raft River Range area contains exposures of the Elba
Quartzite, which may be too young to host uranium deposits (Houston and Karlstrom, 1979) but remains permissive
for gold and PGE.
SALT LAKE REGION (TRACT 21, MAP 1)

Highly deformed Archean and Early Proterozoic
metasedimentary rocks of the Farmington Canyon Complex crop out in the Wasatch Range and on Antelope
Island in the Great Salt Lake. Quartz-pebble conglomerates are present in the complex.
UINTA MOUNTAINS (TRACT 22, MAP 1)

The Late Archean Red Creek Quartzite, exposed in
the Uinta Mountains, contains beds of quartz-pebble
conglomerate.
BLACKBIRD COBALT-COPPER TRACT(TRACT 23, MAP 1)
The Yellowjacket Formation of Proterozoic age, whichcorrelates with the older formations within the Belt Supergroup, is widespread in central Idaho and southwestern Montana. Only a small part of the formation in the area of theBlackbird Mine is known to contain mineral deposits likethat at the Blackbird Mine (map 1, deposit h). Most of themineral occurrences in this area are in the middle unit (Unit
B of Hughes, 1983) of the Yellowjacket Formation, and thearea has been designated as permissive for Blackbird cobaltcopper deposits. This may be a unique area because of thepresence of volcaniclastic rocks and gabbroic sills, and similar deposits are probably not present elsewhere in the Yellowjacket Formation. However, the similar Sheep Creek
deposit in the Belt Basin (tract 8, map 1) implies that theBelt Supergroup (part of which is shown as a sediment-hosted
copper tract) is also permissive for Blackbird-type deposits.

MONTANA (TRACT 19, MAP 1)

The southwest corner of Montana and small areas
extending into Idaho have several Proterozoic units (table 1)that contain minor quartz-pebble conglomerate beds or metamorphosed equivalents. These areas may be more favorable for gold and PGE than for uranium mineralization

SOUTHEAST MISSOURI LEAD-ZINC TRACT(TRACT 22, MAP 2)

The small areas encompassing the Viburnum Trend
and Fredericktown in southeastern Missouri are designatedas permissive for PGE mineralization because of the com-

PLATINUM-GROUP ELEMENTS IN SEDIMENTARY ENVIRONMENTS

mon presence of cobalt-nickel minerals. A small amountof platinum was detected in a sample from the region (seemodel above). Other areas of Mississippi Valley-type
deposits are also reported to contain cobalt and nickel incobalt-nickel and nickel minerals but in quantities too
small for any economic interest.

SOLUTION-COLLAPSE BRECCIA PIPE
URANIUM TRACT (TRACT 23, MAP 2)
The area outlined for solution-collapse breccia pipes
includes all exposures of Mississippian Redwall Limestone through Triassic Chinle Formation where the Redwall Limestone is exposed or present in the subsurface(McKee and Gutschick, 1969). The major mineralized
pipes are also shown on map 2. In well explored, sparsely
vegetated areas the density of breccia pipes can exceed
one per 13 km2, but most of the pipes are not mineralized.
Pipes have not yet been found in all parts of the tracts,
either because they are absent or because factors such as
forest cover have prevented their identification.

SUMMARY AND CONCLUSIONSThis paper has provided a substantial amount ofbackground material about the occurrence of PGE in a variety of sedimentary environments with which certain deposit
types are associated. I used that material to outline tracts
within the conterminous U.S. where such deposits havebeen or may be found. I identified tracts permissive foreight deposit types (also see coal section above), some ofwhich contain deposits of the type in question and some ofwhich do not but which have permissive characteristics.
Thus far, there have been only a few analyses for PGE
within these environments, most of which indicate the presence of PGE in the low parts per billion range. Although
this may sound discouraging for future PGE exploration,
most of the analyses have been for samples collected forother purposes and do not represent a thorough search for
PGE. A more systematic approach to the search for PGE inthese environments and sampling at what may be morefavorable horizons within a sedimentary package could
lead to the discovery of PGE in greater concentrations thanhave been found.
It was not my purpose to rank the favorability ofgiven deposit types or of tracts within each deposit type,but some generalizations can be made as to PGE favorability. In light of current research and exploration interest
and of knowledge about PGE in similar environments
throughout the world, areas of metalliferous black shales
and sediment-hosted copper deposits should be a major
focus of PGE studies. Although placer deposits have been
important historically, those in the U.S. are small by com-

A27

parison and the productive ones have generally had very
low concentrations of PGE relative to gold. The geochemical characteristics of Blackbird cobalt-copper, southeast
Missouri lead-zinc, and sollution-collapse breccia pipeuranium deposits make these environments interesting, but
the known permissive areas are rather small relative toblack shales and sediment-hosted copper tracts and the
few PGE data available are not overly encouraging.
Because unconformity uranium-gold and quartz-pebble
conglomerate gold-uranium tracts are not even known tocontain deposits of those types and because they are sosmall, these areas are of little interest currently, but futuregeochemical data or mineral deposit information may alter
this conclusion, and the areas should not yet be ignored.

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A38

PLATINUM-GROUP ELEMENTS IN THE CONTERMINOUS UNITED STATESAPPENDIX LIST OF PGE-BEARING AND SOME LESS COMMON MINERALS USED INTEXT AND THEIR IDEAL CHEMICAL COMPOSITIONS WHERE KNOWN
Braggite
Bravoite
Carrollite
Cooperite
Fletcherite
Geversite
Haycockite
Hollingworthite
Iridarsenite
Indian ruthenium
Isoferroplatinum
LauriteMichenerite
Moncheite
Mooihoekite
Oosterboschite
Polydymite
Ruthenarsenite
Ruthenian indium
Ruthenosmiridium
Siegenite
Sobolevskite
Sperrylite
Trogtalite
Vaesite(Pt,Pd)S(Ni,Fe)S2
Cu(Co,Ni)2S4
PtS(Cu,Ni,Co)3S4
PtSb2
Cu4Fe5S8
RhAsS
IrAs2
(Ru,Ir)
Pt3Fe
RuS2
PdBiTePtTe2
Cu9Fe9 Si6(Pd,Cu)7Se5
Ni3S4
RuAs2
(Ir,Ru)(Ir,Os,Ru)(Co,Ni)3S4
Pdi.oi-i.ioBi
PtAs2
CoSe2
NiS2

*U.S. GOVERNMENT PRINTING OFFICE'.

1994-683-526/39023