Uranium mineralisation in some Phanerozoic sandstones in India

Peneconcordant sandstone type uranium mineralisation occurs in (i) the Lower Gondwana Permian beds in the Satpuras of Madhya Pradesh; (ii) the Cretaceous Mahadek beds along the southern slopes of the Shillong Plateau; and (iii) the Siwalik formations along the northwest Sub-Himalayan foothills. Each of these has evolved in a specific tectono-sedimentological regime - the Gondwanas in graben-like basins, the Mahadeks along the fringes of a horsted-up block of the Peninsular Shield, and the Siwaliks in the southern foredeep of the rising Himalayas. The Gondwanas and the Siwaliks are fluviatile and the Mahadeks fluvial to marginal marine. The evolution of these basins is not marked by any acid or alkalic magmatism but the Gondwanas are associated with post- and the Mahadeks with pre-sedimentation basic volcanism. The sediments in all the three regimes have a uranium-rich provenance, the speedy tectonic uplift which gave rise to accumulation of uranium-bearing immature sediments. The genetic model of enrichment envisages activation of hydro-dynamic gradients due to tectonic uplift and mobilisation of uranium from oxidised zones through groundwater media and its increment in the reduced zones below the water table aided by organic carbon and Fe2+. In the Mahadeks of Gomaghat area, however, initial pene-syngenetic enrichment seems likely followed by epigenetic additions from the overlying oxidised upper Mahadeks. Uranium mineralisation is mostly due to uraninite, pitchblende, coffinite and many secondaries. The three sedimentary sequences signify the vast possibilities of locating workable uranium ore bodies in each of them. Some of the exploration problems are briefly discussed. (author)