Himadri Basu

Estimation of authigenic U in the palaeoproterozoic Gulcheru Formation in the southwestern margin of the Cuddapah basin, South India

The Proterozoic Cuddapah basin is a well established U province of India. Both the sediments and the basement granitoids of this basin are characterized by higher abundances of intrinsic U. Remobilization of this intrinsic U had presumably played a role in the formation of the deposits found in this basin. While intrinsic U in the granitoids is controlled mainly by the factors of igneous crystallization, in siliciclastics it is contributed from the detrital and the authigenic (non-detrital) sources. Authigenic U in fine siliciclastics is generally determined by estimating its detrital U with reference to the Th/U ratio of a reference rock. Considering the influence of sedimentological factors like recycling, mixing, hydraulic sorting on Th and the differential chemical stability of U and Th in the near-surface depositional conditions, detrital proxies viz. Al and Ti are taken here for estimating the detrital U.Authigenic U in the mudrocks of the Gulcheru Formation is estimated at 3.33 ppm and 4.03 ppm with reference to the Th/U and the U/Ti ratios, respectively, of the average Proterozoic cratonic shale. Considering the differences in the stability of Th and U in the sedimentary domain, the former is taken as lower and the latter as higher estimates. The redox-sensitive metal ratios (Ni/Co, V/Cr) indicated that U authigenesis in the Gulcheru Formation took place mainly in oxic-dysoxic water. This study also elucidates the bearing of authigenesis on the elevated intrinsic U abundances in the mudrocks of the Gulcheru Formation.

Chloritization and its bearing on uranium mineralization in Madyalabodu area, Cuddapah district, Andhra Pradesh

Uranium mineralization in Madyalabodu area, Cuddapah district, Andhra Pradesh, is spatially related to chloritized and brecciated quartzite of the Gulcheru Formation in the immediate vicinity of E-W to ESE-WNW trending basic dyke. Chloritization transgresses the lithological boundaries. Whole-rock geochemical data indicate enrichment of MgO and Al2O3 coupled with depletion of SiO2, Na2O, K2O, CaO and TiO2 in the chlorite-rich zone. Fe2O3 and FeO do not vary significantly in the altered and the unaltered zones. Electron Probe Microanalysis (EPMA) data reveal that the chlorites in contact with uranium minerals are enriched in MgO and depleted in FeO than in the others. Considering the petrological evidence, geochemical signature and structural constraints, it appears that chlorite acted more as an adsorbent rather than as a reductant in facilitating uranium mineralization. Uraninite crystallized later from the uranium originally adsorbed on chlorites. Chloritization might also have facilitated mineralization through the generation of nascent hydrogen, H2S and lowering pH of uranium-bearing solution.