Alteration in Chrome Spinel of the Sukinda Ultramafic Rocks, Singhbhum Craton, Eastern India and Its Bearing on the Origin of Ferritchromit

Abstract

The chrome spinel in Sukinda ultramafic rocks, serpentinite-dunite-peridotite in particular, in Singhbhum Craton, Odisha, eastern India, often exhibits optical and chemical alteration. Four compositionally different zones are identified in some of the altered chromite grains; though optically they are distinguished into two such as: dull grey and white reflective. The reflectivity is related to a marked enrichment of FeO and NiO and depletion of Cr2O3, MgO, and Al2O3, resulting in the formation of spinelloid composition that lies between the end-members of FeCr2O4 (chrome-magnetite) and Fe3O4 (magnetite). Chromite replacement by ferritchromit/chrome-magnetite/magnetite usually takes place from grain boundaries or micro-cracks and fractures inwards giving rise to altered grains consisting of mostly relict chromite cores enveloped by thin ferritchromit and variably thick chrome-magnetite/magnetite rind. Progressive increase in Cr# and drastic decrease in Mg# from grey core to white reflective mantle suggests post-formation fracturing. The ferritchromit and its associated spinel phases in the Sukinda ultramafic rocks are attributed to elemental exchange and its variable precipitation during serpentinisation and represent alteration products formed through progressive, fluid-driven recrystallisation during regional tectonothermal events below 500 degrees C.