Platinum-group minerals (PGM) in the chromitite from the Nuasahi massif, eastern India: further findings and implications for their origin

Abstract

We report the first find of Al-rich chromitites in the Nuasahi massif located in the Singhbhum Craton, eastern India. Electron microprobe analyses yielded Al 2 O 3 contents between 19.86 and 25.86 wt%. The Cr 2 O 3 and Fe 2 O 3 concentrations vary (in wt %) from 34.8 to 40.67 and 5.38 to 7.05, respectively. TiO 2 content ranges from 0.04 to 0.71 wt%. The silicates interstitial to chromite consist of enstatite, magnesiohornblende and clinochlore. One sample contains 855 ppb of total platinum-group elements (PGE) and shows a chondrite-normalized PGE pattern with a slightly positive slope, with enrichment in Pt and Pd relative to Os and Ir. Ruthenium and Rh show positive and negative peaks, respectively. In agreement with the PGE distribution, the following platinum-group minerals (PGM) have been found: laurite ((Ru,Os)S 2 ), erlichmanite ((Os,Ru)S 2 ), irarsite (IrAsS), vysotskite ((Pd,Ni)S), braggite or cooperite ((Pt,Pd,Ni)S) and Pd-telluride. The PGM form small grains, less than 10 μm across, and occur associated with base metal sulphides (BMS) and rutile included in magmatic minerals, such as chromite and enstatite. The BMS occur exclusively included in chromite. Mineralogical observations suggest that the (Os, Ir, Ru), Pt- and Pd-bearing phases crystallized in the magmatic stage, before or during the precipitation of the host chromite, under relatively high sulphur fugacity. Part of the Pd, together with Te, was initially enclosed at high temperature as small clusters in the sulphide liquid and then incorporated in the crystallizing BMS. Finally, these elements precipitated as Pd-telluride blebs, as an exsolution product at lower temperatures. Rutile occurs as a minor phase in association with PGM, included in chromite or in the contact chromite–silicate. The mode of occurrence of rutile in the Nuasahi chromitite is similar to that reported from the stratiform chromitites emplaced in stable cratonic settings.