Uranium mineralization in Singhbhum shear zone Bihar: IV. Origin and geological time frame

On the basis of an analysis of the various characteristics of the uranium deposits of the Singhbhum Shear Zone, a new sequence of events has been postulated to explain the oriin of mineralization. The main events are: (i) The Singhbhum Granite is the geochemical source of uranium, (ii) The metavolcanic rocks closely associated with the Chaibasa Formation rocks along the shear zone are the geochemical source of base metals, (iii) Uranium was first syngenetically deposited in the sediments of the Chaibasa Formation, (iv) This uranium was mobilized during metamorphic-metasomatic process, accompanied by shearing, leading to syntectonic granitization and extensive feldspathization. During this episode a part of the geochemical stock of uranium was incorporated in the metasomatic refractory uranium minerals, and the rest in (a) deuteric uraninite, (b) tiny crystals of uraninite included in metasomatic minerals and (c) molecular and ionic desseminations occluded in crystals and inter-granular fluids in the metasomatised rocks, (v) uranium was remobilized again by tectonically heated waters during further extensive shearing and redeposited as uraninite in suitable structural traps, (vi) emplacement of basic sills at Jaduguda aided this remobilization and exerted a geochemical control in precipitating uranium along with other metals such as Cu and Ni, (vii) in some parts of western sector (eg. Narwapahar) exogenic remobilization also played a part, (viii) the sooty pitchblende, secondary uranium minerals and supergene uraniferous iron oxides are due to subsequent local remobilization and deposition. Thus, uranium mineralization in the Singhbhum Shear Zone is the result of a series of overalapping geological processes. (author)