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

• First report of greenockite from Baula-Nuasahi mafic–ultramafic complex
• Second ever worldwide report of greenockite from mafic–ultramafic association
• Unusual greenockite–siderite association without any precursor Cd-bearing phase
• Chalcopyrite containing Cd (up to 144 ppm) served as Cd-source for greenockite.
• Latest CO2-rich alteration precipitated greenockite–siderite postdating PGE mineralization.

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 (1.01–1.61 wt.%), Zn (1.89–4.54 wt.%), 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 144 ppm, 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.