Lower Zone-Hosted Chromitite PGE-Ni-Cu Mineralization at Zwartfontein, Northern Bushveld Complex, South Africa: The Role of Chromite and Volatiles

Abstract The Zwartfontein Lower zone body, in the northern limb of the Bushveld Complex, has a footwall of Archaean granite-gneiss and wall and roof rocks of Malmani dolomite. It hosts platinum-group element (PGE) mineralization within two chromitites. The upper chromitite, CrB2, is associated with low-grade PGE + sulfide mineralization; and the lower chromitite, CrB1, is associated with higher grades of PGEs and more abundant sulfide. In places, an intersection of disseminated base metal sulfides and PGEs with no significant chromite (BM1) is present beneath CrB1. Within CrB1 and BM1, sulfides dominantly comprise a primary magmatic assemblage of pentlandite with lesser pyrrhotite + chalcopyrite ± pyrite (primary). Platinum group minerals (PGMs) are characterized by a Pt-Pd-Bi-Te–dominant PGM assemblage (74–87% of PGM by area), with lesser PGE sulfide and Au-Ag minerals, along with minor Os, Ir, Ru (iridium-group PGE [IPGE]) sulfides, native Pt, and PGE-arsenides. Palladium (up to 131 ppm), Rh (up to 11 ppm), and Ru (up to 11 ppm) are also hosted within solid solution in pentlandite and pyrrhotite, whereas Pt (up to 26 ppm) is hosted within late-stage primary magmatic pyrite as well as secondary pyrite. CrB2 has been overprinted by secondary hydrothermal alteration and consists of millerite ± pentlandite, with dominantly PGE arsenosulfide (54% of PGM by area), IPGE sulfide (23% of PGM by area), and Pt-Pd-Bi-Te (13% of PGM by area) as PGM species. Evidence of volatiles (H2O) ± carbonate melts present at the point of chromite nucleation and sulfide saturation comes from (1) chromite-hosted inclusions of carbonate, sulfide, and hydrous minerals and (2) carbonate inclusions within primary magmatic sulfides. We propose introduction of volatiles (H2O) ± carbonate to the system, possibly from assimilation of the Malmani dolomite, as well as a reduction in pressure related to magma ascent produced a chromite(+ sulfide)-only saturated melt. Chromite nucleated in situ on the intrusion floor, and sulfide saturation also occurred due to the addition of carbonate and volatiles, resulting in formation of CrB2 and CrB1. The underlying BM1 was formed through sulfide percolation from CrB1, down through the crystal mush. The style of mineralization at Zwartfontein is formed through the localized in situ involvement of fluids ± carbonate melts. Therefore, it is unlikely to be correlatable stratigraphically with other occurrences of Lower zone mineralization.