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

Abstract

Platinum group minerals (PGMs) belonging to laurite (RuS2) - erlichmanite (OsS2) solid solution series and Os-Ir-Ru alloys generally occur as minute inclusions (<10 mu m) 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 (similar to 30 mu m) 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 similar to 43-45, Os similar to 12-15, S similar to 35, Ir similar to 2-3 and As similar to 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 degrees C. The unusual and complex oscillatory zoning (i.e. Os-rich and Os-poor bands) observed within the laurite could have developed below 1200 degrees C (but above 1000 degrees C) in a late magmatic stage due to local fluctuations in the surrounding ther-modynamic conditions, primarily the sulphur fugacity.