Ore genesis and its relationship to volcanism, tectonism, granitic activity, and metasomatism along the Singhbhum shear zone, eastern India

Abstract

The Singhbhum shear zone is a 200-km-long arcuate mineralized belt in the eastern part of the Indian Precambrian Shield, well known for copper, uranium, apatite-magnetite, and kyanite deposits. This zone evolved from a preexisting zone of deep-seated fracture and volcanism that separated an Archean granitic platform to the south from a Proterozoic mobile belt to the north. It progressively evolved in time, between 2,000 to 850 m.y. ago, as rocks of the northern mobile belt were deformed, metamorphosed, and thrust southward against the platform in four successive phases. Higher temperature and pressure conditions prevailing within this zone, during the course of its evolution, set up geochemical gradients which caused granitic activity in the western and southeastern sectors during the first two phases of deformation, and albite metasomatism, biotitization, chloritization, and sericitization, mainly in the central sector, during the third phase. The ore materials, which appear to have been introduced into this zone during the pre-deformational phase of volcanism, were subsequently mobilized by the chemically active fluids, permeating through the crushed and sheared rocks of this zone, to give rise to the copper, uranium, and apatite-magnetite deposits at shallower levels. The central sector of this zone, which was affected by intense deformation and metasomatism during the third phase, was more extensively mineralized than the western or southeastern sectors where intensity of deformation and metasomatism was lesser. Kyanite deposits were localized along the northern boundary of the shear zone along which somewhat higher pressure prevailed. Ore genesis thus appears to be related to the total evolutionary history of this zone and cannot adequately be described merely as epigenetic hydrothermal or syngenetic metamorphosed as proposed by other workers. These ore deposits may be better described as remobilized syngenetic.