The genesis of Archaean chromitites from the Nuasahi and Sukinda massifs in the Singhbhum Craton, India

The chromitite deposits of the Nuasahi and Sukinda massifs are part of layered ultramafic bodies which occur within Archaean low-grade metamorphic rocks of the Iron Ore Group (IOG) in the Singhbhum Craton of the Indian Shield. The chromitite seams are interlayered with dunite, and associated with orthopyroxenite. Detailed electron microprobe study reveals very high Mg-numbers (0.66–0.82) and Cr-numbers (0.75–0.87) for chromite in massive chromitite from the seams, high Fo content (Fo92–95) for olivine in dunite and high En content (En89–94) for orthopyroxene in orthopyroxenite. Our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks. The primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the Mesoarchaean. The parental magma from which the massive chromitites crystallized was of siliceous high-Mg basaltic or boninitic compositions, similar to the compositions of spatially associated chromite-bearing siliceous high-Mg basalts of the Iron Ore Group. The parental magma may have generated due to the interaction of a depleted mantle with a fluid-enriched melt possibly derived in response to the dehydration of a subducting slab. Monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated H2O concentrations in the parent magmas. The parental magmas that produced chromitites may have intruded into the volcano-sedimentary greenstone belts of a supra-subduction zone setting.