Rare greenockite (CdS) within the chromite–PGE association in the Bangur Gabbro, Baula-Nuasahi Complex, Eastern India

Abstract

We report the presence of greenockite from the chromite–PGE–base metal sulfide association in the Bangur Gabbro, Baula-Nuasahi mafic–ultramafic complex. The CdS phase occurs, sans any precursor Cd-bearing phase, as minute grains within siderite micro-veins in close proximity to chlorite in the chalcopyrite+pyrrhotite+pentlandite+violarite assemblage. Using various calibrations, chlorite compositions yielded temperature ranges of 245 to 325°C. The associated siderite might have formed at or little below the above temperature. Electron probe micro-analyses (EPMA) of greenockite reveal small amounts of Fe (1.01–1.61wt.%), Zn (1.89–4.54wt.%), and substitution of Zn for Cd. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of chalcopyrite nearer to and away from siderite micro-veins that host greenockite confirms maximum Cd concentrations of 78 and 144ppm, respectively, entailing possible derivation of Cd from chalcopyrite. We propose the transport of Cd as bisulfide complex in a low temperature alkaline and reducing fluid and the simultaneous precipitation of greenockite and siderite by reaction with Fe-rich minerals, aided by decrease in pH and/or increase in fO2.