Origin of zoned laurite – a platinum group mineral from the Mesoarchaean Bangur chromite deposit, Boula–Nuasahi ultramafic complex, Odisha, India

Platinum group minerals (PGMs) belonging to laurite (RuS2) – erlichmanite (OsS2) solid solution series and Os–Ir–Ru alloys generally occur as minute inclusions (<10 mm) in both stratiform and podiform chromitite deposits. In a recent study of chromitite samples from Bangur chromite deposit in the Boula–Nuasahi ultramafic complex, Odisha, India, we found a relatively coarse-grained (~30 mm) laurite crystal in addition to other PGMs. This laurite occurs as an included crystal within a ferritchromite grain and has a chemical composition (wt%): Ru ~ 43–45, Os ~ 12–15, S ~ 35, Ir ~ 2–3 and As ~ 0.2, as measured by electron probe micro-analyzer. Due to the high osmium content, it is named as Os-laurite. The mode of occurrence, texture, structure and chemical composition of laurite support a magma­tic origin for this mineral that might have crystallized at a temperature slightly lower than 1200°C. The unusual and complex oscillatory zoning (i.e. Os-rich and Os-poor bands) observed within the laurite could have developed below 1200°C (but above 1000°C) in a late magmatic stage due to local fluctuations in the surrounding thermodynamic conditions, primarily the sulphur fugacity