The Wyoming Platinum-Palladium-Nickel Province

The Wyoming platinum-palladium-nickel province lies along the edge of the Wyoming Province. The Wyoming Province (Archean) forms the core of the Wyoming craton and underlies much of Wyoming and Montana (Hausel and others, 1991) ( Figure 1 ). The southeastern boundary of the Wyoming Province lies in contact with a cratonized belt of Proterozoic gneisses and schists that were accreted to the Wyoming Province about 1,770 Ma (Loucks and others, 1988). The northern edge of this cratonized belt, known as the Green Mountain terrain, is marked by a relatively wide suture zone known as the Mullen Creek-Nash Fork shear zone, or Cheyenne Belt (Karlstrom and Houston, 1984).

Layered mafic complexes, ultramafic massifs, and discrete mafic bodies occur in and along the edge of the craton (Loucks and Glasscock, 1983; Hausel, 1991). These include the Stillwater complex (2.7 Ga) in the Beartooth Mountains of Montana (McCallum, 1988).

Other mafic and ultramafic intrusives in the Wyoming Province include the Trailside complex in the Bighorn Mountains (Luth, 1960) and the Tony Ridge peridotite in the Laramie Mountains of Wyoming.

Further south, a group of mafic and ultramafic intrusives form the Wyoming platinum-palladium-nickel province in southeastern Wyoming (Figure 2 ). These parallel the Cheyenne Belt and include the Lake Owen and Mullen Creek layered mafic complexes, and the Centennial Ridge amphibolite in the Medicine Bow Mountains. The Puzzler Hill ultramafic complex, Elkhorn gabbro, and Woods Mountain peridotite occur in the Sierra Madre to the west. To the east, the Laramie anorthosite batholith (1.5 Ga) in the Laramie Range, intrudes the Cheyenne Belt. In addition, several unexplored mafic and ultramafic bodies and fragments lie near or along the Cheyenne Belt, and a geophysical anomaly along the northern edge of the Sierra Madre, north of the Cheyenne Belt, suggests the presence of a hidden, mafic complex underlying Phanerozoic rocks in that region (Paul Graff, personal communication, 1997).

The Mullen Creek-Nash Fork shear zone represents a Proterozoic subduction zone separating cratonic rocks to the north from cratonized oceanic and island arc rocks to the south. Platinum-bearing mafic and ultramafic rocks south of the shear zone in the eugeoclinal terrain, represent ophiolite remnants stacked against the subduction zone. Samples from this region have yielded high platinum and other metal anomalies compared to average crustal abundance. For example, nine mineralized samples from the New Rambler mine averaged 1,750 parts per billion (ppb) Pt+Pd. Seventeen mineralized samples from five different historic mines in the area also averaged 3,050 ppm Cu (Houston and others, 1975). Other samples collected from numerous mines in this region have yielded anomalously high Cu, Au, Ag, Pb, Zn, Ni, Pt, Pd (Hausel, 1997).

The New Rambler shaft, located along the northern edge of the Mullen Creek complex, as well as several mines and prospects in the Centennial Ridge district to the northeast, were driven in shear zone cataclastics related to this structure. However, the Lake Owen complex to the east of the Mullen Creek complex, is relatively undeformed.

Magmatic and hydrothermal platinum-group metal mineralization, and placer platinum is reported. Platinum-group mineralization in the Lake Owen complex is clearly magmatic and associated with cumulate layers. In the New Rambler district of the Mullen Creek complex, mineralization occurs in hydrothermally altered mafic cataclastics. The mineralization was either leached from discrete mafic rocks by hydrothermal solutions, or more likely was remobilized from the layered complex (McCallum and Orback, 1968). Platinum in the Centennial Ridge district is restricted to narrow zones of altered, mafic, metaigneous schist and gneiss and appears to have been leached from the mafic country rock (McCallum, 1968). Detrital platinum in the Douglas Creek district may have been derived from the Mullen Creek complex.

Lake Owen Complex

The Lake Owen layered complex intrudes the Green Mountain terrain south of the Mullen Creek-Nash Fork shear zone. Based on structural, lithologic, and isotopic evidence, the Green Mountain terrain is part of an oceanic island arc accreted to the Wyoming province at about 1775 Ma. The Lake Owen complex has not been dated, however, the Elkhorn gabbro produced a U-Pb (zircon) date of 1774±2 Ma (Pallister and Aleinikoff, 1987) and the Mullen Creek complex yielded a U-Pb (zircon) date of 1778±2 Ma (Loucks and others, 1988).

The Lake Owen complex is virtually unaffected by deformation and metamorphism. The complex crops out over a 25 mi2 area forming a funnel-shaped intrusive tilted at an average of 75° on its side exposing a cross-section with at least 18 cyclic units (Loucks and Glasscock, 1990). The cyclic units are defined by large scale repetitions of two or more units in the lithologic sequence: troctolite, olivine gabbro, gabbronorite, Fe-Ti oxide gabbronorite; and by the cyclic variation of plagioclase composition over the range An82-An52, and olivine and orthopyroxene compositions over the range of Mg#84-50 (Loucks, 1991).

The lowermost cyclic unit has a strike length of 1.8 miles and the uppermost unit has a strike length of 7.5 miles. A fine-grained non-cumulate chilled margin borders the lower half of the intrusion. The top of the layered sequence is truncated by a younger granitic pluton, and large portions of the intrusive are hidden under gravel.

Anomalous Pt was detected in labradorite-bearing gabbroic norite, and cumulus sulfides were identified in at least 12 stratigraphic horizons in the complex, with some zones containing elevated gold and Pt±Pd (Loucks and Glasscock, 1990). Four of these horizons have laterally persistent precious metal anomalies, and platinum-group metals (PGM) + Au mineralization at grades >1 ppm. The mineralization is spotty but includes zones up to 15 feet thick with strike lengths of more than 1 mile. Cumulus sulfides (pyrrhotite, pentlandite, and chalcopyrite ± PGM minerals) in olivine-bearing rocks near the base of three cyclic units are lensy and discontinuous (Loucks, 1991).

The second principal lithologic association of cumulus sulfides is with titanomagnetite cumulates near the tops of cyclic units (most of the titanomagnetite cumulates also contain cumulus sulfides) (Loucks and Glasscock, 1989). In two cyclic units, mineralization consists of bornite + chalcopyrite + carrollite (CuCo2S4) + millerite + Ti-poor magnetite ± PGM minerals [moncheite [PtTe2], merenskyite [(Pd,Pt)(Te,Bi)2], michenerite (Pd,Bi,Te), cooperite (PtS)] ± Au-Ag-Pd alloys in pink labradorite gabbronorite.

In one unit, stratiform bornite-rich mineralization, with ppm level Au+Pt+Pd is continuous for at least 1.2 miles. In another cyclic unit, the mineralization extends over a strike interval of at least 6 miles and includes Au-Ag alloys, Pt-arsenides, Pt-Pd tellurides and sulfides associated with disseminated chalcopyrite, pentlandite, pyrrhotite, pyrite, gersdorffite (NiAsS), bornite, millerite, and PGM-bearing carrollite (Loucks, 1991).

Vanadiferous titanomagnetite cumulates are persistent in gabbronorite near the tops of some cyclic units. Sixteen principal Fe-V-Ti-oxide cumulate layers with a lateral persistence of 3 to 7.5 miles have been identified in the complex. Six of 11 layers in the Lower Zone are of sufficient thickness to warrant geochemical evaluation for economic potential (Loucks, 1991). Loucks and Glasscock (1989) estimated surface mineable vanadium oxide cumulates included at least 1.4 billion tons valued at $33 billion (1988 prices)!

Mullen Creek Complex

The Mullen Creek layered complex to the west of Lake Owen forms a large, 60 mi2, deformed, gabbroic complex ( Figure 2 ). The northern edge of the complex is truncated by the Mullen Creek-Nash Fork shear zone, which contributed to its deformation ( Figure 3 ).

Loucks and others (1988) recognized more than 21 cyclic units in the complex. On the basis of petrography, the complex was subdivided into a Lower Series, the Jays Roost Series, and an Upper Series. The Jays Roost Series includes cyclic units formed of well-layered and foliated troctolite, chromite troctolite, and anorthosite that differentiate upward to olivine-gabbro, gabbronorite, and magnetite-gabbronorite. This series closely resembles the stratigraphy of Lake Owen (Loucks and Glasscock, 1989).

A copper-gold gossan discovered along the northern flank of the complex near the turn of the 19th century, resulted in the development of the New Rambler mine. A shaft sunk to a depth of 170 feet with more than 5,000 feet of drifts and crosscuts, cut hydrothermally altered metapyroxenite and metagabbro in shear-zone tectonites and mylonitic gneiss. Coarsely crystalline, sheared, epidotized granite (Rambler granite) was encountered at a relatively shallow depth, and metadiorite and metaperidotite were also intersected (McCallum and Orback, 1968; McCallum and others, 1975).

Greenschist propylitic alteration is widespread throughout the district, but intensifies at the New Rambler mine. This assemblage includes chlorite, epidote, clinozoisite, albite, and magnetite (± pyrite). In most propylitically altered samples from the mine, hornblende and calcic plagioclase are partially altered, biotite is unaltered, and magnetite and epidote veinlets are common. Calcic plagioclase and albite are pervasively replaced by the phyllic alteration assemblage of sericite, quartz, and pyrite, whereas mafic minerals are only partially affected (McCallum and others, 1975).

The New Rambler ore body is a classical supergene enriched deposit capped by a porous spongy limonite and jaspilite gossan, overlying a 75-foot thick oxidized zone. The oxidized zone ore minerals include malachite and azurite with lesser cuprite, tenorite, chalcotrichite, and chalcopyrite. Dendrites and nuggets of native copper with atacamite, chalcanthite, tetrahedrite, and bornite are sparsely distributed. These grade downward from 75 to 100 feet into the supergene enriched zone with platinum-bearing covellite and chalcocite. Orpiment, realgar, and lorandite (TlAsS2) reported by Rogers (1912) are probably products of supergene enrichment.

Below 100 feet, the supergene assemblage graded into primary mineralized rock with quartz-pyrite-chalcopyrite veins with minor sperrylite. The association of epigenetic platinum and palladium in the shear zones in the layered complex suggests remobilization from a hidden, as yet undiscovered, platinum reef at depth (McCallum and Orback,1968).

The mine sporadically operated from 1900 to 1918. Operations terminated following a fire which destroyed the mine buildings. After the fire, the mine manager estimated probable reserves at 7,000 tons of 7 to 8% Cu, 0.25 opt Pt, with some gold and silver (Needham, 1942).

Production records of the last ore shipped from the mine following the 1918 fire showed values to range from 3.24% to 61.37% Cu, 0.0007 opt to 1.4 opt Au, 1.01 opt to 7.5 opt Ag, 0.047 opt to 3.2 opt Pt, and 0.33 opt to 12.3 opt Pd (Anonymous, 1942). Total metal production was reported at 1,753,924 pounds of Cu, 171.35 ounces of Au, 7,346 ounces of Ag, 170.16 ounces of Pt, and 451.4 ounces of Pd (Needham, 1942). Silver Lake Resources (1985) estimated platinum-group metal production was more on the order of 16,870 ounces of Pd and 910 ounces of Pt.

In addition to Pt and Pd, iridium (Ir) was also detected in samples at the New Rambler. According to Knight (1902) a composite sample of dump material from the mine yielded 0.06 opt Pt, 0.04 opt Ir, 0.04 opt Pd, 0.10 opt Ag, and a trace Au.

Much of the Pt was incorporated in the copper ore. Two assays reported by Knight (1902) included: (1) an assay of copper minerals which yielded 0.10 to 0.70 opt Pt, and (2) seven carloads of covellite ore which contained 0.40 to 1.4 opt Pt.

A search of nearby mines resulted in PGM discoveries. The Blanche shaft was sunk west of the New Rambler to a depth of 160 feet and intersected copper carbonates, chalcocite, and chalcopyrite at 120 feet (Kleunder, 1982). The host rock is sheared felsic gneiss, metagabbro, and metadiorite (McCallum and Orback, 1968). Two select samples from the dump contained limonite, abundant malachite and cuprite, with traces of chalcocite and sperrylite. These samples assayed 6 and 17 ppm Pt, 30 and 20 ppm Pd, and 20 and 17% Cu (Loucks, 1976).

The Duchess mine west of the Blanche had several exploratory shafts in shear zone tectonites and in strongly sheared metagabbro and metadiorite. Traces of copper with pyrite, hematite, limonite, and gold-bearing tetrahedrite were detected (McCallum and Orback, 1968; Loucks, 1976). PGMs were not reported.

The Henry Lode was reported to be a possible extension of the New Rambler (Stoll, 1929). A shaft was sunk 88 feet and exposed an 18-inch, iron-stained quartz vein in gabbro. The vein carried values in Au and Ag, and the gabbro carried values in Cu, Au, Ag, with traces of Pt. An assay yielded 1.94% Cu, 0.16 opt Au, 0.22 opt Ag, and 0.0054 opt Pt. A qualitative analysis of 8 feet of gabbro on either side of the vein yielded detectable Ag and Au. A sample from the 18-inch vein assayed 0.54 opt Au, 0.70 opt Ag, and trace Cu.

Centennial Ridge District

North of the Lake Owen complex and northeast of Mullen Creek, platinum was reported in some gold mines in the Centennial Ridge district that were developed in mafic metaigneous rock. The mineralization was spotty.

The richest gold ores were found in sulfide-rich zones in mafic mylonites, graphitic fault gouge, and in strongly chloritized shear zones. Platinum was restricted to narrow zones of altered, mafic metaigneous schist and gneiss. The platinum appears to have been remobilized from the mafic country rock (McCallum, 1968). Only a minor amount of platinum was produced.

Puzzler Hill

Puzzler Hill lies 6.5 miles northwest of Encampment. This is a relatively new discovery, even though the property was mined for copper near the turn of the 19th century. Puzzler Hill is formed by an ultramafic massif lying in contact with Late Archean gneiss north of the Cheyenne Belt suture zone. The complex may continue under sedimentary cover.

Whole rock data and thin section petrography show the host rocks forming Puzzler Hill to include pyroxenite and actinolite-chlorite schist. Two unmineralized samples yielded 16.4 and 19.4% MgO, 45.5 and 48.8% SiO2, 8 and <5 ppb Pt, 15 and 8 ppb Pd, 29 and 28 ppm Co, and 343 and 178 ppm Ni (Hausel, 1995).

Mineralized samples taken from prospects and mines on Puzzler Hill yielded 0.01% to 4.43% Cu, 66 ppm to 3.72% Ni, 14 ppb to 0.29 opt Au, <5 ppb to 828 ppb Pt, 5 ppb to 0.12 opt Pd, <0.1 ppm to 0.19 opt Ag, 21 ppm to 831 ppm Co, 64 ppm to 294 ppm Cr, and trace Pb and Zn (Hausel, 1995). Beeler (1906a) reported that the mineralized zone was traced 2 miles on the surface and varied from 14 feet at the Charter Oak mine to 100 feet wide elsewhere. Where the mineralized structure was widest, the ore consisted of quartz stringers mixed with low-grade material.

The Charter Oak shaft along the northern edge of Puzzler Hill, was driven to a depth of 488 feet (Beeler, 1905), with more than 2,300 feet of shafts, winzes, raises, and tunnels on four levels (Saratoga Sun, 1907). Ore reserves were estimated at 680,000 tons to a depth of 300 feet.

Elkhorn Gabbro

The Elkhorn gabbro was intruded by a troctolite/olivine-gabbro pluton that was in turn intruded by smaller peridotite bodies and mafic pegmatite dikes. Collectively, these represent a reversely zoned intrusive complex (James, 1971; Snoke and others, 1982, Beard and Day, 1988).

The ultramafic portions of such complexes are the zoned “Alaskan type” ultramafic complexes that are the source of placer platinum-group metals in Alaska, as well as the Ural Mountains, Russia (Loucks and Glasscock, 1989). The Elkhorn gabbro complex lies in the southern Sierra Madre. Much of the complex lies across the border in Colorado, with a small portion in Wyoming.

Laramie Anorthosite Batholith

The Mullen Creek-Nash Fork shear zone was intruded by the Laramie anorthosite batholith in the Laramie Range east of the Medicine Bow Mountains. The 350-mi2 anorthosite batholith is dated at 1.42 Ga (Subbarayudu, 1975) to 1.53 Ga (Smithson and Hodge, 1972).

South of the batholith, the Green Mountain terrain contains fragments of amphibolite-grade mafic to intermediate metavolcanic and associated metasedimentary rocks that date about 1.8 Ga. These are intruded by 1.39 to 1.25 Ga granite (includes the Sherman Granite and related felsic phases) (Peterman and others, 1968).

The Laramie anorthosite batholith has long been considered a potential source of low-grade aluminum ore as well as for disseminated and massive Ti-Fe-V deposits (Hagner, 1968). Some pyrrhotite, chalcopyrite, pyrite, and sphalerite have been reported in norite, hypersthene syenite, and anorthosite. Near the southern edge of the batholith, the Strong mine, a polymetallic deposit located near the head of Horse Creek, produced some Cu, W, Mo, Ag, Au, and Pb. Some samples also yielded 1.0% Ni (Beeler, 1942). Anomalous nickel was also reported from nearby properties (Beeler, 1906b). The batholith is essentially unexplored for platinum-group metals.

Other Anomalies

The Broadway mine south of Encampment in the Sierra Madre was developed by four shallow shafts sunk in a mineralized zone containing massive sphalerite, minor galena, and disseminated chalcopyrite, chalcocite, and covellite, with secondary malachite and chrysocolla. Osterwald (1947) reported the ore zone dipped 50°SE to 50°NW over a surface area of 1,000 by 50 feet but continued under a heavily wooded area. Soil geochem dispersion studies by Amselco supported that the ore zone was more extensive.

The ore was localized along the contact of granite and pegmatite with a complex of gneiss, amphibolite (pyroxenite), gabbro, and diorite (Osterwald, 1947). DeNault (1967) reported the host rock was pyroxenite rather than amphibolite. The host contains diopside with minor enstatite entirely or partially replaced by spessartine. One sample contained 35% olivine.

Five character samples collected by the U. S. Bureau of Mines ranged from 0 to 12.5% Zn, and 0.5 to 1.9% Pb. Anomalous amounts of Pb and Zn, and a small amount of platinum-group metals were reported in a spectrographic analysis. Grab samples were also anomalous, but none were tested for platinum-group metals or nickel. The samples ranged from 0.003 to 0.82% Cu, 0.52 to 1.2% Pb, 0.031 to 6.9% Zn, 0.11 to 9.1 opt Ag, and 0.1 to 0.5 ppm Au (Osterwald, 1947).

Five samples collected by Hausel (1992) were also highly anomalous. These ranged from 0.05 to 1.82% Cu, 0.30 to 5.66% Pb, 0.02 to 8.17% Zn, 0.2 to 12.18 opt Ag, 104 ppb to 3.28 ppm Au. Only one ultramafic rock was tested for platinum-group metals and yielded no detectable Pt, and 2 ppb Pd. The Broadway appears to be a skarn and is not a likely target for platinum-group metals.

Several miles to the north, magnetite, pyrrhotite, and chalcopyrite were found localized along a contact between hornblende schist and a norite intrusive at the Creede property a few miles west of Puzzler Hill. The schist strikes east-west and dips to the south. Trace Ni and Co were reported by Spencer (1904).

At the Leighton-Gentry (Jack Creek) mine near the Continental Divide at the head of Jack Creek west of Puzzler Hill, a 3-inch wide gossan is structurally overlain by 4 feet of black, crumbly, mica schist. The schist is overlain by a thick norite sill.

The footwall of the gossan is formed by an 11-foot-thick limy quartzite. Pyrite, pyrrhotite, and chalcopyrite were found along bedding planes and disseminated throughout the quartzite. The rocks strike N80°W, and dip 30°S. An assay yielded 3.07% Cu, 0.67% Ni+Co, with a trace of zinc (Spencer, 1904).

Six samples collected from a dump in the SW/4 section 29, T14N, R85W by Menzer (1981) yielded 40 ppm Cu to 8.8% Cu. Two samples yielded detectable silver and ranged from 0.5 ppm and 1.5 ppm Ag. Samples tested for gold and platinum group metals yielded only 0.005 ppm to 0.010 ppm Au, 0 to 0.020 ppm Pt, and 0 to 0.070 ppm Pd.

Anomalies North of the Wyoming Platinum-Palladium-Nickel Province

Some serpentinites north of the Wyoming platinum-palladium-nickel Province are potential hosts for platinum-group metals and nickel. These include ultramafic rocks in the Elmers Rock greenstone belt, and in the Casper Mountain and Deer Creek supracrustal belts of the Laramie Range. In the Elmers Rock belt, disseminated and veinlet chromite was reported in serpentinite and talc-tremolite schist (metakomatiites) in an area about 200 ft across. Samples contained 0.87% Cr2O3, with veinlets containing 9.3 % Cr2O3 (Fields, 1963).

In the Deer Creek supracrustal belt several miles north of Elmers Rock, approximately 2,300 tons of chromitite was mined from serpentinite during the early 1900s. The ore averaged 35 to 45% Cr2O3 (Dietz, 1932; Beckwith, 1939). The host serpentinite extends several hundred feet to the east and west, and is bisected by Deer Creek Canyon. The chromitite forms a 2 to 5 foot thick layer associated with kammererite [(Mg, Cr)6(AlSi3)O10(OH)8] and wolchonskoite (Beckwith, 1955).

To the west of Deer Creek, chromite on Casper Mountain forms disseminations, pods, and lenses in tremolite-talc-chlorite schist (Burford and others, 1979). The schist averages only 2% Cr2O3, but has bands of higher grade rock with 5 to 45% Cr2O3. The U.S. Bureau of Mines identified a 4.16 million ton resource averaging 2.5% Cr2O3 to a 95 foot depth (Julian and Moon, 1945). Samples of cumulate peridotite and talc-chlorite schist yielded 16.5 to 35% MgO, 0.6 to 17.01% Cr2O3, and 225 to 1,456 ppm Ni (Hausel, 1987). It is not known if any of these rocks have been sampled for platinum-group metals.

The Kennedy dike swarm in the central Laramie Range includes a group of large-scale ultramafic and mafic intrusives. One of these, the Tony Ridge peridotite, is a zoned intrusive that is approximately 3 miles in length and reaches a maximum width of about 2,000 feet (Snyder and others, 1989a,b). This intrusive, as well as the rest of the Kennedy dike swarm, is untested for platinum-group metals and nickel.

Platinum-enriched gold placers have been identified in northwestern Wyoming in the Snake River area (US Geological Survey, 1968). The source of the platinum-group metals was suggested to be related to platinum-group metal-enriched fluids expelled from black shales of the Phosphoria Formation (Lechler, 1991). This area remains unexplored.

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