Detailed assessment of platinum-group minerals associated with chromitite stringers in the merensky reef of the eastern bushveld complex, South Africa

The minerals of the platinum-group elements (PGM) and their associated host minerals from the Merensky Reef at the Two Rivers platinum mine, South Africa, were investigated using an FEI 600F field emission Mineral Liberation Analyzer. Polished sections were prepared from intersections of upper and basal chromitite stringers collected from 12 boreholes drilled into a variety of Merensky Reef facies types. Primary minerals associated with the chromitite stringers, on the scale of a thin section, include orthopyroxene (30-60%), clinopyroxene (~2%), plagioclase (20-40%), and base-metal sulfides (up to 2%), whereas secondary silicates include tremolite (~4%), phlogopite (~1%), chlorite (<1%) and serpentine (<1%). The most abundant base-metal sulfides (BMS) occurring in the Merensky Reef in this area of the Bushveld Complex are pyrrhotite (46% of all BMS), pentlandite (32% of all BMS) and chalcopyrite (22% of all BMS). Abundances of the PGM (in area %, total PGM) are dominated by maslovite (PtBiTe, 29.7%), isoferroplatinum (Pt3Fe, 26.9%), braggite [(Pt,Pd,Ni)S, 13.3%], cooperite [(Pt,Pd,Ni)S, 11.9%] and laurite (RuS2, 8.3%), with michenerite (PdBiTe), sperrylite (PtAs2), stibiopalladinite (Pd5Sb2), geversite (PtSb), platarsite (PtAsS), and a variety of unnamed phases accounting for the remainder. There is a strong association of most PGM with pentlandite and chalcopyrite, but less so with pyrrhotite. Observed PGM-silicate mineral associations include a marked tendency for PGE antimonides, (sulf)arsenides, bismuthotellurides and sulfides to be associated with secondary silicates (in addition to BMS), which is taken as evidence for the redistribution of these PGMs by late fluids. The data presented here are consistent with existing models for mineralization within the Merensky Reef, which involve the separation of a sulfide melt from a silicate liquid, that then scavenged PGE while percolating downward until arrested by impermeable chromitite.