VARIATIONS IN THE DISTRIBUTION OF PLATINUM-GROUP ELEMENTS ASSOCIATED WITH VARYING DEGREES OF SILICATE ALTERATION WITHIN THE J-M REEF OF THE STILLWATER COMPLEX, MONTANA, USA

Twenty-five samples from three separate drill cores at the Stillwater mine with varying degrees of hydrothermal alteration, along with three relatively unaltered Reef samples from both the 3400 level of the Stillwater mine and the 6450 level of the East Boulder mine were used in this study. Less altered Reef samples are olivine gabbros, with trace amounts of chlorite and serpentine (approx. 5 vol. %) restricted to micro-fractures and cleavage planes. Base metal sulfide assemblages (pyrrhotite, chalcopyrite, pentlandite) occur interstitial to primary silicates. The severely altered Reef samples used in this study are primarily composed of chlorite, actinolite, talc, serpentine, epidote and clinozoisite (greater than 60 vol, %). Altered Reef samples show poor correlation between PGEs and base metals (Ni, Cu) while unaltered Reef samples better preserve this positive correlation. Further, altered Reef samples show poor correlation between PGEs and sulfur (R² < 0.15) where unaltered Reef samples exhibit stronger PGE-sulfur correlation with R² values of 0.75. Excellent correlations exist among the two populations when comparing individual PGEs (IPGEs to PPGEs), with an average R² value of 0.94. Platinum-group minerals such as braggite, cooperite, isoferroplatinum and Pt-Pd tellurides are present in both Reef populations. Braggite and cooperite are found within, or in close proximity to, base-metal sulfide assemblages in both unaltered and strongly altered Reef rocks. Conversely, PGE-tellurides are largely found distal to base-metal sulfide assemblages, occurring in alteration halos. Here, base-metal sulfides have been extensively replaced by secondary silicates. Both Reef populations are characterized by a tight range in δ³⁴S values, with unaltered samples showing values from -0.4 to 0.0 ‰, and altered samples characterized by δ³⁴S values ranging from -0.5 to 0.8 ‰.

The preliminary results show that hydrothermal alteration of the J-M Reef has done little to modify the PGE bearing phases or sulfur isotope systematics. Evidence including conversion of sulfide to magnetite confirms previous studies which concluded that desulfurization has occurred in the Reef. Conversion of PGE concentrations to 100% sulfide shows that Pt and Pd have been preferentially depleted in the altered Reef relative to IPGEs.