BASE-METAL AND PGE DISTRIBUTION IN THE IRON MOUNTAIN AREA OF THE STILLWATER COMPLEX, MONTANA

Rocks in the Basal series (BS) and Ultramafic series, Peridotite zone (PZ) of the Stillwater Complex are known to have elevated concentrations of Ni, Cu, Co, Cr, and PGE. High concentrations of PGE and Au are associated with chromitite layers in the PZ, particularly the “A” and “B” chromitite seams. However, elevated concentrations of PGE are also noted in drill core transecting chromitite-poor portions of the BS and PZ, including orthopyroxenite. The geochemistry and mineralogy of these zones were examined with emphasis on Co and PGE distribution, from past exploration drilling projects in the Iron Mountain area. Although pentlandite of the J-M Reef may contain up to ~2 wt.% Pd in solid solution and account for ~42-73% of the total Pd budget (Godel and Barnes, 2008), Pd was below detection (<0.015%) in our electron microprobe analyses of pentlandite from the BS and PZ. This suggests a higher proportion of Pd may occur as discrete Pd minerals. SEM-BSE imaging revealed the following exsolution sequence in massive sulfides of the Iron Mountain area: pyrrhotite, pentlandite, cobaltite, altaite+Pd-Bi telluride. Pd appears to have partitioned into discrete Pd minerals upon cooling, much like Pd and Pt minerals of the J-M Reef. The Pd-Bi telluride (michenerite or merenskyite) most commonly occurs with altaite (PbTe) as outgrowths on cobaltite inclusions at contacts between base-metal sulfides (BMS) and between BMS and gangue minerals. Nickeline, hollingworthite (RhAsS), Pd-Pb alloy, and a Ni-Pd-Bi telluride were also identified. Similar Pd-Bi-Pb-Ag-Te phases and arsenides are found in the basal sulfides of the Mouat and Cresent Creek areas. Cobalt and Ni concentrations in Iron Mountain pentlandite are 0.41-3.89 wt.% and 21.65-38.54 wt.%, respectively. These ranges are consistent with microprobe results for pentlandite in cumulate rocks of the Mouat area. Violarite, an alteration product of pentlandite, is present in four samples of olivine cumulate with oikocrystic bronzite and massive sulfide, each showing partial serpentinization or evidence of sulfide remobilization. Talnakhite [Cu₉(Fe,Ni)₈S₁₆] and sugakiite [Cu(Fe,Ni)₈S₈] are present in four different, similarly altered samples of olivine cumulate. Investigation of Co and PGE distribution up-section and in less massive sulfide accumulations are ongoing.