Possibilities of unconformity-related uranium mineralization in intracratonic palaeo to meso proterozoic Kolhan Basin, Eastern Indian Shield

A narrow linear belt (60 km x 3-12 km) of Palaeo to Meso-Proterozoic supracrustals occurring between Archaean Singhbhum granitoid (SG) and Koira synclinorium of Iron Ore Group (IOG) in the western margin of Singhbhum Orissa Craton is known as Kolhan Basin. The sandstone-carbonate-shale intracratonic litho-package deposited unconformably over Singhbhum granite and Iron Ore Group are mostly undeformed and unmetamorphosed. Kolhan equivalent rocks are exposed in Chamakpur-Keonjhar Basin, Mankarchuan basin in the western part and Sarapalli-Kamakshyanagar quartzite band in the southwestern part of the Singhbhum Orissa Craton (SOC). Limited geological and radiometric studies conducted by Atomic Minerals Directorate for Exploration and Research (AMD) indicated presence of uranium, predominant thorium and mixed (U+Th) anomalies associated with the basal conglomerate-sandstone sequence at several places. In Kokrajhar nala section radioactive conglomeratic sandstone boulders are found to contain uraninite and metatorbernite. Th and mix U+Th anomaly zones contain monazite, xenotime, zircon, sphene and rutile as significant accessory minerals. Favourable Palaeo and Meso-Proterozoic time hiatus, fertile basement rocks, presence of palaeosols and undeformed red bed fluviatile clastics overlain by marine evaporite, large ductile shear zones in basement, reverse fault structures transecting the unconformity and the basinal sediments, suspected alteration features viz, illitisation and kaolinisation near unconformity, reductants in the form of carbonaceous shale, basic dykes and Fe-Mg alteration zones along with the occurrence of several radioactive anomalies are the factors invoked to construe unconformity-related uranium mineralization in Kolhan basin. Probabilities and prospects are discussed in the light of recently developed concepts and alternative exploration methodology and strategy have been suggested to build up an exploration model. (author)