Platinum-group element geochemistry of boninite-derived Mesoarchean chromitites and ultramafic-mafic cumulate rocks from the Sukinda Massif (Orissa, India)

The Mesoarchean Sukinda Massif in the Singhbhum craton is the largest chromite ore deposit in India that contains approximate to 95% of Indian Cr resources. The Sukinda Massif consists of an elongated layered ultramafic unit (approximate to 25 km x 400 m) that occurs within the 3.5 Ga supracrustal sequences of the Tomka-Daitari-Mahagiri greenstone belt in eastern India. The ultramafic unit comprises serpentinized dunite, orthopyroxenite and chromitite. There are six chromitite seams which are present within the serpentinized dunite. All ultramafic rocks are extensively weathered and capped by laterites (approximate to 30 m thick). In the southwestern part of the Sukinda Massif, in the Katpal area, chromitite seams and the host ultramafic rocks are fragmented, forming breccias cemented by the gabbroic to granodioritic rocks. Whole-rock major, trace and platinum-group elements (PGE) geochemistry indicates that the ultramafic unit and the gabbro, including the cementing matrix materials of the breccias, are cogenetic. The ultramafic rocks formed by fractional crystallization of a parental boninitic or high-Mg siliceous magma. The cementing gabbroic rocks formed from an evolved boninitic magma generated from fractionated boninite or high-Mg siliceous basalt. The positive correlation between MgO and Ni, and MgO and Cr is due to fractionation of early cumulus like olivine and chromite from the high-Mg parental magma. The correlation between Zr and Cu suggests that the parental boninitic magma was S-undersaturated. However, the matrix gabbro shows a flatter trend with increasing Zr and Cu, indicating sulfide saturation occurred later in the evolved boninitic magma. In Sukinda, the concentrations of PGE in the massive chromitites (approximate to 176-875 ppb) from the main ultramafic unit and in chromitite fragments (approximate to 61-279 ppb) from the breccias are higher than in serpentinite (approximate to 19-71 ppb), orthopyroxenite (approximate to 14-19 ppb) and gabbro (approximate to 3-11 ppb). Three samples of the massive chromitites from the main ultramafic unit have significant PGE concentrations (PGE(total) approximate to 651-875 ppb) with IPGE (Ir, Os, Ru) approximate to 528-634 ppb, much higher than PPGE (Pd, Pt, Rh) approximate to 93-332 ppb. Detailed PGE mineralogical studies of these samples revealed presence of IPGE-bearing platinum-group minerals (PGM) dominated by Os-Ir-Ru alloys (containing minor Pt) enclosed in chromite grains. Other PGMs are laurite present as composite grain within chromite plus irarsite which are associated with cracks in chromite grains and a small sperrylite grain attached to an Os-Ir-Ru alloy. All the As-bearing PGM are associated with cracks in the chromite suggesting introduction of As during alteration of primary PGM to PGM-arsenides. The positive correlation between IPGE (Os, Ir, Ru) and MgO or Cr indicates they are fractionated during the early stage of magmatic differentiation from a S-undersaturated boninitic magma. The low Ir of the silicate rocks indicate prior removal of Ir-bearing alloys from the parental boninitic magma which would account for the occurrence of Os-Ir-Ru alloys in massive chromitites. The results suggest that Os-Ir-Ru alloys from the Sukinda chromitites crystallized at relatively high temperature and low fS(2) condition within a S-undersaturated boninitic magma ascending from the upper mantle where they entrapped by the growing chromite crystals. Overall, the initial PGE analysis from the Sukinda breccias, presented herein, indicate similarities with the Nuasahi breccias in the Singhbhum craton, and highlight the potential for mineralization in this area.