Abstract:
The Mesoarchean Boula-Nuasahi ultramafic complex is situated in the south eastern flank of the Singhbhum Craton, India and consists mainly of gabbro-anorthosite, peridotite and pyroxenite litho-units that host a series of chromitite bodies. A contemporaneous but slightly younger magmatic event has structurally disturbed the area with intrusion of a coarse-grained gabbro unit known as the Bangur gabbro (similar to 3.1 Ga). This has caused extensive hydrothermal alteration of the original host rocks and is associated with hydrothermal sulphide-and Pt-Pd mineralizations. Petrographic study shows that well-crystallized and euhedral arsenopyrite crystals are associated with quartz and carbonates in veins aligned along the foliation planes/fractures in the host rocks. This vein assemblage (arsenopyrite + quartz + calcite) has formed during the late stage of hydrothermal mineralization. Electron probe micro-analysis and LA-ICP-MS analysis of arsenopyrite demonstrates that its chemical composition varies within narrow limits and that the concentrations of trace elements are below 900 ppm. The only trace element present in significant concentrations is Sb with an average value of 490 ppm. The As/S atomic ratio varies between 0.93 and 1.01 and As and S show a clear negative correlation according to the substitution FeAs1-xS1+x. The average composition is very close to the stoichiometric value. Because the arsenopyrite crystals are vein minerals, have very low concentrations of any impurities and do not show any compositional zoning, they were used for geothermometric calculations. Matching the measured As atom% with one of the experimentally calibrated isopleths in the established Fe-As-S phase diagram, calculated temperatures are in the range of 335-425 degrees C. This temperature range is in very good agreement with temperature estimates of 300-500 degrees C (based on cobaltite-gersdorffite compositions) for late-stage hydrothermal mineralization in the Boula-Nuasahi ultramafic complex available in the literature.