PLATINUM-GROUP METALS 



539 



Table 110. — Principal minerals and domestic occurrences of 

 the platinum-group metals identified by electron micro- 

 probe or X-ray diffraction analysis 



consisting of layers formed by seciimentary, meta- 

 morphic, an(i igneous processes, that are intruded 

 into metamorphic rocks of eugeosynclinal erogenic 

 belts; and (3) Alpine — irregularly shaped discordant 

 complexes consisting of discordant lenticular layers 

 of rocks with tectonite fabrics, that are tectonically 

 emplaced in eugeosynclinal orogenic belts and island 

 arcs. 



Layered deposits or contact-metamorphic deposits 

 associated with stratiform and concentric complexes 

 are the major sites of platinum-lode deposits. Ex- 

 amples are the Merensky Reef in the stratiform 

 Bushveld Complex, South Africa (Cousins, 1969), 

 and the deposit of the Noril'sk district, Siberia 



(Mertie, 1969), which could be classified as a con- 

 centric mafic complex. 



Platinum placers consist of unconsolidated alluvial 

 deposits in present or ancient stream valleys, ter- 

 races, beaches, deltas, and glaciofluvial outwash, and 

 in lithified placers of similar origin. The placer 

 metals are commonly derived from mafic and ultra- 

 mafic rocks, but may come from nearby sources in 

 sedimentary rocks, having ultimately been derived 

 from mafic and ultramafic rocks during a previous 

 erosion cycle. As the densities and sizes of platinum 

 alloys are generally similar to those of alluvial gold, 

 some platinum metals, when present, are recovered 

 from gold placers. 



A domestic source of platinum-group metals is as 

 a byproduct of refining copper from porphyry and 

 other copper deposits and from lode and placer gold 

 deposits, although the grade is extremely low. Other 

 minor lode sources are in copper-gold contact-meta- 

 morphic deposits, copper deposits associated with 

 syenitic rocks, and gold-quartz veins. Relatively 

 small potential deposits exist domestically ; such ores 

 represent only a minor platinum resource, but serve 

 to indicate the range of geologic environments. 



STRATIFORM COMPLEXES 



The Stillwater Complex (Hess, 1960; Jones and 

 others, 1960; Jackson, 1961; Page and Nokleberg, 

 1970) of Precambrian age in southwestern Montana 

 contains several potential platinum lodes. Platinum, 

 palladium, and rhodium are concentrated in the 

 chromitite zones, but individual samples vary widely 

 in contents and ratios of these three platinum metals 

 (Page and others, 1969; 1971). The lowermost 

 chromitite, the A zone, averages 0.10 oz/ton and 

 attains a maximum known value of 0.60 oz/ton of 

 Pt + Pd + Rh. Stibiopalladinite, sperrylite, cooperite, 

 platinum-iron alloy, and laurite are the only platinum 

 minerals identified (Page, 1971) ; they occur as in- 

 clusions (average diameter 5-10^) in chromite and 

 in interstitial sulfides. Chromium shows the strong- 

 est correlation with platinum metals, V a moderate 

 correlation, and Ni, Co, and Cu a lower correlation. 

 Platinum metals occur in the greatest amounts 

 where the oxidation ratio of chromite is lowest. This 

 relation suggests that less oxidizing conditions 

 favoring crystallization of chromite also favored the 

 concentration of platinum metals from the magma. 

 Platinum metals, though in lesser amounts, also oc- 

 cur below the A zone in the basal zone and are 

 associated with massive and disseminated copper- 

 nickel sulfides. Small amounts of platinum metals 

 have also been reported from disseminated sulfide 



