Genesis of Sulfides and Platinum-group Minerals (PGMs) from the Shankaraghatta Ultramafic-Mafic Belt, Western Dharwar Craton (southern India)

The Shankaraghatta ultramafic-mafic belt within the Shimoga greenstone belt (Western Dharwar Craton) is constituted by sulfide-bearing serpentinized dunites that are compositionally representative of komatiites. The dunite mesocumulates flanking the S-poor dunite adcumulates host 5-8 modal% disseminated sulfides represented by millerite-pentlandite-pyrite-chalcopyrite that are associated with zoned chromites and secondary magnetites. Scanning Electron Microscope (SEM) study reveals the accessory alloys and platinum-group minerals (PGMs; 2 25 m) to occur as inclusions or at the edges of the sulfide and oxide grains. Michenerite (PdBiTe) and Ni-bearing michenerite (Ni-PdBiTe) are hosted predominantly by millerite followed by pyrite, which also hosts an unnamed Pd-Pb alloy. Variable phases like Ni-Au-bearing bismuthide (NiAuTeBi), tsumoite (BiTe), galena (PbS), malsovite (PtBiTe), and kotulskite (PdTe) are also hosted within millerite, whereas native Au and nisbite (NiSb2) occur as inclusions within Fe-oxides. The sulfide-bearing serpentinites were derived from a plume of komatiitic magma that left majorite garnet in the source, as understood from the subchondritic Al2O3/TiO2 ratios of the serpentinites (~9.9) and the negative Zr-Hf anomalies. Sulfide saturation was triggered by Assimilation Fractional Crystallization (AFC) processes (modelled at 12% for serpentinites), and was achieved late, after emplacement of the komatiitic magma and crystallization of the olivine adcumulates, followed by contamination with the ambient TTGs. Although the sulfides are magmatic in origin, they are later overprinted by low-T hydrothermal processes, as is evident from the significant enrichments in serpentinites for Au, As, Bi, and Te, which occurred perhaps during the replacement of pentlandites by millerite. The hydrothermal processes may have leached out the PGEs and Ni from pyrrhotite and pentlandite during desulfurization, as is evident from the common occurrences of PGMs at the edge of the sulfide grains, and the predominance of phases like millerites, pyrites, and secondary Fe-oxides that may have formed at the expense of now exhausted pyrrhotite.