Exploring Optical and Geochemical Zoning Variation in Chromite: Metasedimentary vs. Orthomagmatic Origins in Singhbhum Craton, Eastern India

Abstract

The Banded Chromite-Quartzite (BCQ) deposits of Ghutrigaon, Singhbhum Craton, exhibit a metasedimentary geneology, diverging significantly from the world's well-established orthomagmatic deposits of Sukinda located in the southern fringe of the Singhbhum Craton, Eastern India. This study explores the zoning characteristics of chromite grains from Sukinda and Ghutrigaon in the Singhbhum Craton, revealing insights into their genetic evolution. It aims to elucidate the connection between their development and the physico-chemical changes that occurred during post-protolithic processes. Petrography and mineral chemistry analysis reveal two distinct features i.e., Ghutrigaon chromite displays a brighter core zone and is in sharp contrast with Sukinda chromite grains that show greater marginal reflectivity. The zoning of Ghutrigaon chromite follows an aluminium trend, with a core enrichment of Cr and Fe and a rim concentration of Al and Mg. The iron trend is evident in the zoned chromite of Sukinda, with concentrations of Mg, Al, and Ni dominating at the core, while Cr, Fe, Ti, and Mn are enriched at the periphery. In Ghutrigaon chromite, there is a preferential migration of Mg ions from the core to the altered rim, while trace elements such as Sc, Nb, and U move from rim to core, and Sr, Hf, and Zr increase from core to rim. In contrast, Sukinda chromite experiences leaching of Cr and Fe from the core, with subsequent precipitation in the rim. Trace elements in Sukinda chromite show a decreasing trend for Cu, Sr, Zr, Hf, and La with Mg#, indicating that Mg variations also influence these trace and rare earth elements during alteration. Ghutrigaon chromite likely originated in a Supra-Subduction Zone environment and zoning in its Cr-spinel is attributed to solid-state diffusion during metamorphism. In contrast, the Sukinda chromite is associated with boninitic magma in a back-arc rifting environment and its chromite zoning is the product of secondary changes that occurred during serpentinisation.