0016-7622/2010-75-5-709/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.75, May 2010, pp.709-714 Occurrence of Proterozoic Black Shale-hosted Uranium Mineralisation in Tal Group, Sirmour District, Himachal Pradesh T. P. S. RAWAT, G. B. JOSHI, BHASKAR BASU and NURUL ABSAR Atomic Minerals Directorate for Exploration & Research, R.K. Puram, New Delhi -110 066 Abstract: Black shale type uranium deposits, though of low grade (<0.001 to 0.05% U 3 O 8 ), contain large uranium resources because of their immense volume. The Neoproterozoic-Cambrian Krol-Tal Himalayan sequence covering a cumulative area of about 1000 sq km in five different synclinal basins from Sirmour district, Himachal Pradesh in the west to Nainital district, Uttar Pradesh in east contain such black shale horizons. The uranium mineralisation found in black shale in the Nigalidhar syncline of Himachal Pradesh and its implication of being an indicator for search of such uranium deposits in Himalayas is discussed. Keywords: Uranium, Black shale, Proterozoic, Tal Group, Himachal Pradesh. shale of Early Cambrian age (Bhargava et al. 1984, 1998). These metasedimentary rocks have been folded into a number of synclines e.g. Pachmunda and Kamlidhar syncline (Solan district, H. P.), Nigali Dhar and Korgai syncline (Sirmour district, H. P.); Mussoorie and Garhwal syncline in Uttaranchal (Fig.1b). The Krol Belt (Auden, 1943) in Nigali Dhar syncline (Fig.1a), Sirmour area is divided in to Krol Group and Tal Group. The Krol is essentially carbonate sequence, whereas the Tal constitutes an arenite-argillaceous sequence with chert-phosphorite in its basal part and limestone in upper part. The Tal Group in Nigali Dhar and Korgai syncline is divided in to Shaliyan, Sankholi and Koti Dhaman Formations (Srikantia, 1982; Bhargava, 1984). The Shaliyan Formation consists of shale/siltstone, reminiscent of tuffites/ ash beds, massive chert, phosphorite and carbonaceous shale. The Sankholi Formation is an argillo-arenaceous unit in general devoid of carbonaceous matter, the Koti Dhaman Formation is predominantly a coarse grained arenaceous sequence. The Krol-Tal basin also received some volcanoclastic/ash material. Volcanicity is well known during the terminal Proterozoic near Precambrian-Cambrian Boundary in many parts of the world. In Indian plate, it is known from Kinnaur (Magnetite tuffs, Bassi and Chpora, 1984) and Salt Range (Khewra Trap, Bhargava et al. 1998). Previous Studies on Uranium Mineralisation Uranium mineralisation was earlier recorded in the INTRODUCTION Ranstad, Sweden (Precambrian – Cambrian) one of the best known, black shale-hosted uranium deposits contain recoverable resource of 0.30 million tones U 3 O 8 of 0.035% av. grade (Young, 1984). Besides, Chattanooga shale, USA (Devonian) and Kolm shale, Norway (carboniferous) are some other important deposits. Most of these deposits occur in black shales of Paleozoic age but deposits do occur in sediments of Proterozoic age such as the uraniferous black/ graphitic shale of Northern Territory, Australia (Fraser, 1975, 1980). All black shales contain more uranium than the average sedimentary rock, but to be classified as uraniferous black shale it must contain more than 50 ppm U 3 O 8 (Swanson, 1961). Uraniferous black shales, which are of marine origin, have high (>2%) sapropelic organic carbon (Bell, 1979). Uranium content increases with increase in organic matter. Uranium occurs as adsorbed material in organic or phosphatic molecules or absorbed by clays. No uranium minerals have been identified from these sources. Besides uranium, they contain small quantities of other metals such as Mn, Ti, V, Cu, Cr, Mo, P and REE. GEOLOGICAL SETTING In India Neoproterozoic-Cambrian Krol–Tal meta- sedimentary sequence of Lesser Himalayan range, extending from Mussoorie (Uttaranchal) in the east to Solan (Himanchal Pradesh) in the west, exposes carbonaceous