Origin of Ni–Cu (Co) sulphides in the 3.5 Ga komatiites from the Gorumahishani greenstone belt, Singhbhum Craton, India

Komatiites from the Late Archean greenstone belts are a significant source of Ni–Cu–Co–(PGE)-sulphide deposits. These deposits are rare in komatiites from the early Archean greenstone belts. The 3.5 Ga metamorphosed komatiitic dunite-peridotite from the lower cumulate zone of the Gorumahishani greenstone belt contains localised disseminated sulphides (<1 modal%). Two types of assemblages are identified: (i) pentlandite + violarite + millerite and (ii) chalcopyrite + bornite. The komatiitic magma reached sulphide saturation due to the fractional crystallisation of olivine, chromite and pyroxene that lowered the FeO activity of the magma to segregate a small amount of sulphide liquid. The S-poor and metal-rich character of this sulphide liquid leads to the exsolution of pentlandite from monosulphide solid solution (MSS) and chalcopyrite from intermediate solid solution (ISS) at a relatively low temperature. Violarite was also exsolved from MSS and characterised by high Co content (8.85–13.64 wt%). Pentlandite grains were extensively replaced by millerite, whereas chalcopyrite grains were replaced by bornite during metamorphic alteration of the primary assemblages. Metamorphism and the resulting Ni–Co-rich fluid led to an increase in their concentrations in sulphides. High concentrations of Pb (51–295 ppm), Sb (1–60 ppm), Bi (37–319 ppm), and As (399–791 ppm) in violarite and millerite are due to hydrothermal alteration, as these elements are not compatible in the structures of the early crystallised MSS or ISS. In the presence of oxidised hydrothermal fluid, Fe was released from the primary sulphides and crystallised into magnetites that surround the sulphide mineral assemblages.