Space-Time Controls in Precambrian Uranium Mineralisation in India

A review of the geochronological and stratigraphic data of the Indian Precambrian in relation to the major uranium mineralised belts in India brings out the time as we11 as space-bound character of the uranium mineralisation. An early period of quartz-pebble conglomerate type mineralisation met with in parts of South Karnataka, Eastern India and parts of Madhya Pradesh, Uttar Pradesh and Rajasthan falls in the timeband of 2900-2600 milion years. The hydrothermal shear-controlled mineralisation occurring along the Singhbhum shear zone of Bihar and in the shears dissecting the Dongargarh system of Madhya Pradesh have a time-range of 1400-1600 million years. The next inferred period is between 1200 and 700 million years and includes the mineralisation episodes in the granulite mobile belt of the Southern Peninsular Shield, the Sarguja-Palamau area of the Chotanagpur migmatite-granulite belt and the Sikar-Dariba-Mahendragarh region in Rajasthan. Uranium mineralisation along the MCT region of Himalaya is of two periods, an early shear controlled disseminated mineralisation in the time range of 1200-1400 million years and a subsequent vein-type around 750 million years.

The distribution of geochemically enriched zones with special reference to uranium in space is controlled by many geological and regional factors arising from the styles or evolution of the Precambrian crust. These include-(i) The restriction of quartz-pebble conglomerates to the late Archaean crust and specific sedimentary environments of fluvial regimes, (ii) The paucity of uranium in the tonalite crust and its enrichment in K-rich granitoids wit11 higher Rb, (iii) depletion of uraniumin the granub ite crust and enrichment ia lower grade metamorphic belts and (iv) remobilisation and localisation of uranium deposits in major zones of crustal dislocation, especially along cratonic boundaries with mobile belts and within mobile belts. Recognition of these criteria helps in understanding the distribution of known belts of U enrichment and choosing areas for further exploration.

A comparison of the periods of uranium mineralisation with the suggested chelogenic cycles brings out a broad correlation, but such a relationship may prove to be rather limited when new discoveries of uranium are made especially of low grade ores, since chelogenic events are episodic, while crustal processes are continuous.