Uranium may occur as primary vein deposits in igneous and metamorphic rocks or as ‘flat lying deposits’ in sedimentary rocks.
Primary uranium minerals are converted into secondary minerals in the surface oxidation zone to become hexavalent. In this form uranium is easily soluble in water and is carried away by rivers and temporary torrents into seas and lakes. These mazy either disseminate through the sedimentary rocks deposited on the floor of seas or lakes or under favorable conditions, concentrate in bedded deposits which are economically important.
Uranium ores often detected not only by characteristic physical properties of the ore minerals seen at the outcrop of primary rocks but also by their aureoles or haloes of dispersion. Rocks with primary ores break up into fragments and detritus which remain either in the overburden alone or creep downhill to produce mechanical haloes. The formation of aureoles largely depends on the climate and behaviour of ground water. In the arid climate, the haloes are well preserved. The delineation of such haloes is an important aspect of geochemical prospecting for uranium.
A large quantity of uranium, concentrated in the hydrothermal deposits is economically very important.
Surface and subsurface uranium deposits are developed by pits or shafts and for large and deep ore bodies, room and pillar or the cut-and-fill shrinkage methods are undertaken.
Uranium mines in India:
In India the Uranium mines are found in :
The shear zone around Dongargarh region in Madhya Pradesh.
Surguja-Palamau area of Madhya Pradesh-Bihar.
Khetri-Khandala region of Rajasthan.
The earliest reference to a uranium mineral in India appeared in a German publication in 1860 in which Emil Stoehr recorded its occurrence at Lapso hill in Singhbhum district, Bihar. The recorded mineral was copper uraninite [Cu (UO2)2, (PO4)2, 8H20], an old name of torbernite, a hydrated phosphate of uranium and copper, also known as ‘uranium mica’.
The most important mines of uranium has been located around Jadugora, Singhbhum district, Bihar, in the Singbhum thrust belt. According to a report, there are about 72500 tons of U3O8 in a total reserve of about 210 million tons of uranium ore, which is of hydrothermal origin. While the good-quality uranium ores of Australia, USA, Canada and a few other countries, contain U3O8 ranging from 0.1 to 0.2 per cent, the uranium ores located in India are much inferior in grade.
In the mica-fields of Gaya, Munger and Hazaribagh districts in Bihar, about 30 uranium-bearing pegmatite have been located. Out of these, only three (Abraki hill, Pichli and Sarada of Gaya district) pegmatite contain a little amount of uraninite, while secondary uranium minerals have been found in a few others. Some other pegmatite contains uranium-bearing columbite-tantalite minerals.
A few pegmatites of the Rajasthan mica-fields contain traces of uranium. The pegmatite of many districts of Rajasthan contain uraninite.
In Udaipur district, uranium and copper mineralization occur in the chloritic and carbonaceous phyllites and shales. The richest concentratiuon of uranium is at Umra about 14 km south-east of Udaipur city, but the reserve is small.
In North-Eastern Region, uranium occurrences of significance have so far been discovered in the Jaintia Hills district, and in the Garo Hills district of Meghalaya.
The recent discovery of uranium mineral mines in parts of Andhra Pradesh, Himachal Pradesh, Madhya Pradesh and Uttar Pradesh (Sarai, Lalitpur district) is quite important, though a detailed account of the discoveries is not yet available. It has been recently reported that a big deposit of uranium has been found around Bodal in Surguja district, Madhya Pradesh. In the Surguja district, 4 episodes of deformation are believed to be responsible for uranium and thorium mineralization.
Uranium Reserves in India:
During the last four decades, various ore reserves totaling over 80000 tons of U3O8 in grades ranging from 0.015 to 0.07 per cent of U3O8 have been identified. These comprise both the ‘inferred’ and ‘indicated’ reserves, though not all of them are commercially exploitable. The above reserves also include some by-product uranium recoverable from copper tailing. In addition, the phosphoric acid from phosphate-based fertilizer plants, containing 0.01 to 0.03 per cent of U3O8, is also considered a potential source.
Of the various ore reserves, about 49000 tons of U3O8 are considered commercially exploitable and are located mostly in the Singhbhum district of Bihar and in Madhya Pradesh, Meghalaya and Karnataka. The copper tailings from Roam-Rakha, Surda and Mosaboni copper mines are expected to yield another 7800 tons of U3O8. Production of uranium from phosphoric acid also yields about 1750 tons of U3O8.