Ni-Cu-PGE Mineralization of Heti Prospect, Western Bastar Craton, Central India: An Appraisal Based on Petrographic, SEM and EPMA Study

The Gondpipri mafic-ultramafic complex consists of several tectonically dismembered gabbro-websterite-troctolite peridotite bodies disposed intermittently in a 10 km long NE-SW trending linear belt within tonalite- trondhjemite-granodiorite (TTG) and charnockite-enderbite, Western Bastar Craton (WBC), Central India. Ni-Co sulphide associated PGE mineralization is recorded in gabbro variants viz. norite gabbro-anorthositic gabbro-gabbro at Heti. The mineralized gabbroic body extends for 500 m strike length with 40 m width and dips 45° to 55° southerly following regional structural trends. The host rocks contain total Cu + Ni up to 5000 ppm with a Cu: Ni ratio <1 and PGE concentration with a maximum of 660 ppb Pd, 506 ppb Pt, 84 ppb Rh, 180 ppb Ir and 75 ppb Ru. Ore microscopic study coupled with Scanning Electronic Microscopy (SEM) and Electron Microprobe Analyzer (EPMA) reveals that Ni-mineralization is mainly hosted in sulphide phases, associated with oxide-silicates. SEM and Ore petrographic studies shows that mineral paragenesis involve pyrrhotite exsolved pentlandite at high temperature followed by chalcopyrite and PGM formed at relatively low temperatures, where Pt and Pd were eventually bonded with Te, Bi in the residual liquid to form moncheite (PtPdTe 2 ). PGM occurs as dispersed grains at the interface of sulphide and silicate as well as inclusions in Cr-magnetite and silicates. Coexisting oxide and sulphides suggest that sulphide-silicate liquid immiscibility was predominantly induced by the precipitation of Fe rich mineral like magnetite and Cr-magnetite, which causes sulphur saturation melt by decreasing in sulphur solubility and helps in concentration of PGE in sulphide melt. Ni-Fe-S Phase diagram of Heti prospect furthermore shows clustering of Ni, Fe and S suggesting steady sulphur saturation condition that prevailed during the crystallization of sulphides, which probably formed the Ni mineralization associated with PGE concentration in sulphide phase.