JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.79, February 2012, pp.117-134 0016-7622/2012-79-2-117/$ 1.00 © GEOL. SOC. INDIA Clay Mineralogical Studies on Bijawars of the Sonrai Basin: Palaeoenvironmental Implications and Inferences on the Uranium Mineralization SURENDRA KUMAR JHA 1 , J. P. SHRIVASTAVA 1 and C. L. BHAIRAM 2 1 Department of Geology, University of Delhi, Delhi - 110 007 2 Atomic Mineral Directorate for Exploration and Research, Northern Region, Delhi - 110 066 Email: jpshrivastava.du@gmail.com Abstract: Clays associated with the Precambrian unconformity-related (sensu lato) uranium mineralization that occur along fractures of Rohini carbonate, Bandai sandstone and clay-organic rich black carbonaceous Gorakalan shale of the Sonrai Formation from Bijawar Group is significant. Nature and structural complexity of these clays have been studied to understand depositional mechanism and palaeoenvironmental conditions responsible for the restricted enrichment of uranium in the Sonrai basin. Clays (<2 μm fraction) separated from indurate sedimentary rocks by disaggregation, chemical treatment and centrifugation were examined using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Presence of tv-1M type illite is inferred from the Rohini and Bandai Members of the Sonrai Formation, indicative of high fluid/rock interaction and super-saturation state of the fluids available in proximity with the uranium mineralization. It is observed that the Sonrai Formation is characterized by kaolinite > chlorite > illite > smectite mineral assemblages, whereas, Solda Formation contains kaolinite > illite > chlorite clays. It has been found that the former mineral assemblage resulted from the alteration process is associated with the uranium mineralization and follow progressive reaction series, indicating palaeoenvironmental (cycles of tropical humid to semi-arid/arid) changes prevailed during maturation of the Sonrai basin. The hydrothermal activity possibly associated with Kurrat volcanics is accountable for the clay mineral alterations. Keywords: Clay mineral assemblages, Illite crystallinity, Uranium mineralization, XRD patterns, Bijawar Group, Sonrai basin. INTRODUCTION In Sonrai basin (Latitpur district of U.P.), uranium mineralization is associated with shale hosted bitumen, sandstone and carbonate rocks of the Sonrai Formation and chloritic shale of the Solda Formation of the Bijawar Group (Prakash et al. 1975; Singh and Bagchi, 1994; Mishra, 1996 and Roy et al., 2004). Fertile K- feldspar rich granitic basement, large number of fractures zones, high degree of illitization and Proterozoic age with unconformable sedimentary contacts represent an ideal set-up for hosting unconformity related (sensu lato) mineralization in this area as almost similar observations were reported by Roy et al. (2008) in the Gwalior basin. Roy et al. (2004) have reported considerable grade (0.037-0.066 % eU 3 O 8 at the cut off grade of 0.01) of uranium mineralization in bituminous shale of the Rohini Member of the Sonrai Formation, suggesting different phases of mineralization that initiated high degree of remobilization. The organic matter and inorganic collides interact with each other and forms clay organic complexes. In this context the presence of organic carbon rich black carbonaceous shale and Precambrian unconformity-related uranium mineralization in Sonrai area is significant. The major clay minerals associated with sedimentary rocks are detrital in origin. They strongly reflect characters of their source material and slightly modified by their depositional environments (Weaver, 1958b, 1958c). Most of the ancient sedimentary rocks composed ~ 70% of mudstones containing ~50% clay-size fraction (Blatt et al. 1980). Clay minerals form one of the major constituents of alteration halos in the Precambrian Unconformity-Type of Uranium deposits (Hoeve and Quirt, 1987; Percival and Kodama, 1989; Iida, 1993 and Pacquet and Weber, 1993). They are significant in the understanding of hydrothermally altered halos and also provide ideas about mysterious conditions of uranium deposition. Crystal structures and crystal chemistry of clay minerals were studied (Velde, 1985;