[en] This note presents the first account of uranium occurrence (250 m x 2-3 m ) in conglomerate at the base of Satyavedu Formation of Upper Gondwana sediments of Palar Basin, near Allapakonta and Vembakam, Chittoor district, Andhra Pradesh. Grab samples (n=7) have analysed 38 ppm to 218 ppm eU₃O₈, 32 ppm to 232 ppm U₃O₈ with negligible thorium. Radioactivity in conglomerate is associated with chlorite, zircon and ferruginous material (limonite). (author)

[en] 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. (author)

[en] The presence of a palaeoweathering profile (palaeosol), at the top of a crystalline basement, buried under clastic sedimentary succession in Mesoproterozoic intracratonic basins has long been recognized as one of the exploration guides for unconformityassociated uranium mineralization. However, by virtue of low preservation potentiality and stratigraphic position, direct use of the palaeoweathering profile in exploration is often constrained. The chemical index of alteration of clastic sedimentary rocks covering the basement crystallines can therefore be used as a proxy for assessing palaeoweathering of the basement and mineralization potentiality. The intracratonic Vindhyan Basin in Central India, formed during a protracted period of quiescence (1.8–1.4 Ga) after Hudson Orogeny, is globally considered potential for hosting unconformity-associated uranium mineralization. The chemical index of alteration (CIA) as a proxy to intensity of palaeoweathering has been used in this study to assess the potentiality of this basin for uranium mineralization. The temporal and spatial variation in palaeoweathering of the basement has been studied and correlated with relevant palaeomagnetic data. On the basis of corrected (pre-metasomatized) CIA, a slight change in palaeoweathering from intense or moderately intense during lower Vindhyan times to moderately intense or moderate during upper Vindhyan times has been inferred. This also indicates marginal shift of palaeoclimate from semi-humid to semi-arid/arid. Spatial variation of palaeoweathering of the provenance is contrasting as indicated by gradual lowering of pre-metasomatized CIA from 82–99 in the north-western segment to 73–83 in the extreme eastern segment of the basin. Correlation with palaeomagnetic data indicates position of the north-western/western part of the Vindhyan Basin at low latitudes during deposition of the Semri Group and relatively closer to the equator during the entire span of deposition of upper Vindhyans, thereby corroborating the observed variation in CIA. Considering chemical weathering of the crystalline basement as an important factor for the formation of unconformity-associated uranium deposits in Mesoproterozoic intracratonic basins, the north-western part of the Vindhyan Basin appears to have greater potentiality. (author)

[en] Significant uranium occurrences are discovered recently for the first time in the Gulcheru Quartzite of Papaghni Group of Cuddapah Supergroup near Gandi, east of Papaghni river, Cuddapah District, A.P.. Mineralisation is associated with an E-W fault. Friable whitish to light pink quartzite is the main host rock of uranium. Mineralisation occurs in the form of lensoidal bodies having dimensions of 20-200 m x 2- 10 m in an E-W fault scarp section, over 2 km in length. Dip section is also mineralised. Samples have revealed anomalous concentrations of uranium, up to 1.441% U₃O₈ with negligible thorium. Concentration of Ni (<25 to 1033 ppm), Mo (<25 to 643 ppm), Pb (33 to 1515 ppm) and Cu (<25 to 170 ppm) points to the economic potentiality of the area. Petrological studies have identified uranium minerals such as pitchblende, uraninite and coffinite as fracture fillings and disseminations in the chlorite-sericite matrix of quartzite. The associated ore minerals are chalcopyrite, arsenopyrite, pyrite and limonite. These findings have important implications in the selection of new exploration targets for uranium in the basal beds of Cuddapah Supergroup unconformably overlying the fertile granitoid basement in the Eastern Dharwar Craton occurring in the southwestern part of Cuddapah basin. (author)

[en] The petrographic characteristics of framework clastics coupled with mineralogy of heavy mineral suite have been studied to constrain the source rocks and tectonic setting of deposition of the sandstones of the Palaeoproterozoic (~2.0 Ga) Gulcheru Formation exposed along the southwestern part of the Cuddapah basin, Andhra Pradesh. Petrographic studies reveal the dominance of fine- to medium-grained (0.12-0.45 mm) monocrystalline quartz, and extreme textural and mineralogical maturity of the sandstones of the Gulcheru Formation. The content of polycrystalline quartz is significantly high in some areas. Feldspar and rock fragments form a minor component. The predominance of considerably well rounded, well-sorted monocrystalline quartz coupled with a suite of well-rounded ultra stable heavy minerals, paucity of feldspars and rock fragments, and presence of well-rounded monocrystalline quartz grains with abraded overgrowth cement indicate that the sediments of the Gulcheru Formation were derived from a low-lying, cratonic interior provenance comprising dominantly granite-gneiss and subordinate older metasedimentary rocks presumably of the greenstone belts occurring as enclaves. This is further substantiated by plots of the quantified mineral composition of the Gulcheru Formation in QFL and Q_mFL_t ternary diagrams in the ‘craton interior’ and ‘recycled orogen (quartzose recycled)’ fields. The distinct quartz-rich nature of the sandstones of the Gulcheru formation is indicative of passive continental margin setting. High quartz contents (mineralogical maturity due to aeolian abrasion), high K-feldspar to plagioclase ratios, and the presence of well-rounded polycrystalline quartz are indicative of derivation of the sediments from a terrane without significant chemical weathering. High degree of sorting, bimodal roundness and grain size, and coexistence of well-rounded quartz and feldspar grains are typical of intense aeolian activity, and therefore arid or semiarid climate. This study supports warm and dry global palaeoclimate during the Palaeoproterozoic. (author)

[en] The discovery of uranium mineralization in the sediments of the Badami Group, especially at Deshnur, the Kaladgi basin has attracted geologists as one of the most favorable targets for uranium exploration. Three broad fining-upward sedimentary sequences were delineated within the Badami Group of sediments. Mesoscopically, sixteen different sedimentary facies have been identified and grouped into five genetically related facies associations (FA-1 to FA- 5). The lateral variation of the identified facies and their vertical stacking pattern indicates that the lower part of the succession is dominated by an alluvial fan to channel-fill depositional regime with periodic deposition of over bank sediments. The middle part of the succession is characterised by deposition in tide-dominated estuarine conditions, whereas, the facies motifs indicate deposition of the upper part of the succession in storm-dominated shallow shelf environments. Uranium mineralisation is mainly hosted by matrix-supported conglomerate (F1), clast- to matrix-supported conglomerate grading to mudstone (F2), crudely laminated mudstone (F9) that are parts of alluvial fan (FA-1) and fluvial channel (FA-2) facies associations. Mineralization is confined mainly along bedding planes, and the mineralized zones occur as 0.2- 6.0 m thick lenses. Uraninite and coffinite commonly occur along the grain boundaries of quartz and feldspar and within the interstitial pore spaces, invariably associated with pyrite, thus indicating epigenetic introduction. Geochemically, mineralized sandstone is characterized by relatively low SiO₂ and CaO, moderate Na₂O and high Al₂O₃, K₂O, whereas non-uraniferous sandstone contains high SiO₂, moderate Al₂O₃, K₂O and low Na₂O and CaO compared to average Proterozoic sandstone. Uraniferous facies has higher concentration of Cu, Ni, Zn and Pb contained in sulphides. In comparison to non-mineralised sandstone, mineralised sandstone show the lower values of the ratios SiO₂/Al₂O₃ and K₂O/Al₂O₃ (avg. 0.32) are related to silica dissolution and formation of clay minerals during diagenesis as well as alteration by mineralizing fluids. Petrographic features coupled with geochemical data indicate modification of primary porosity and permeability of immature fluviatile sediments during diagenesis. This played a significant role in facilitating the migration of uranium bearing solution and subsequent uranium mineralization. (author)

[en] Provenance studies on the basis of U, Th and K reveal that the sediments of the Gulcheru formation were derived from a minor heterogeneous but dominantly granitoid provenance, which often underwent moderate chemical weathering. Mixing of reworked and heavy mineral bearing matured sediments with newly generated detritus took place at different times. Sedimentation took place in three different phases. Initially the sediments were deposited in fluvioaeolian environment. Later, subsidence of the depo-centre along basinal growth faults led to the transgression of seawater over earlier sediments. The last phase of the sedimentation history continued in the marginal marine domain. The depocentre most possibly experienced semi-arid climate. (author)

[en] Uranium mineralization at Umra, Udaipur district, Rajasthan is hosted by carbon and siliceous phyllite (with minor calcareous components) of the Paleoproterozoic Aravalli Supergroup. The mineralized zone trends NE-SW to N-S, dipping 45-55° towards SE to E. Uraninite, coffinite and brannerite are the main uranium phases observed that occur in association with sulphides (pyrrhotite, pentlandite, gersdorffite, chalcopyrite and pyrite), magnetite and ilmenite. Uraninite is found to occur as veins and disseminated ultrafine grains. Colloform and botryoidal texture with shrinkage/ synaeresis cracks indicate the presence of pitchblende also, formed at relatively low-temperature. Textural features suggest remobilization of disseminated syngenetic and / or diagenetic uranium minerals during low-grade metamorphism and deformation. Textural features suggest multiple phases of sulphide mineralization also. Mineral chemical data indicate that uraninite and pitchblende are compositionally similar containing 80.09-87.17 wt.% UO₂ (av. 84.45 wt.%) and 11.37-11.82 wt.% PbO (av. 11.58 wt.%) with negligible SiO₂, CaO, TiO₂, FeO, ThO₂ and RE2O₃. High U/Th ratio (>1000) in uraninite/ pitchblende indicates limited cationic substitution and therefore low temperature (<350°C) hydrothermal origin of mineralization. Chemical age, calculated on the basis of UO₂, ThO₂ and PbO concentration in uraninite, indicates the minimum age of mineralization is 956±12 Ma. S isotopic data of pyrite suggest syn-sedimentary or diagenetic origin by bacterial reduction of seawater or pore water sulphate under anoxic conditions (-24.7‰ to -16.2‰), as well as hydrothermal (-3.7‰ to +8.8‰) origin. Similarity of S isotopic composition of hydrothermal pyrite with that of pyrite (+5.2‰) from meta-basic rock indicates that a component of S has been derived from the associated basic/meta-basic rocks in the area. Signatures of multiple generations of uranium mineralization, and multiple episodes of deformation, metamorphism and hydrothermal alteration reveal a complex metallogenic history in Umra. Compositional details of uraninite and their occurrence in the form of veinlets in association with hydrothermal pyrite suggest dominantly epigenetic hydrothermal mineralization events coeval with deformations of the Aravalli sediments. (author)

[en] 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. (author)

[en] The Kanchankayi-Hulkul sector located along the Kurlagere-Gundanahalli (KG) fault zone in the Bhima basin, hosts some of the richest grade uranium deposits of India (e.g. Gogi, Kanchankayi, Hulkul). The uranium mineralization is structurally controlled and is hosted by brecciated limestone of the Shahabad Formation and fractured and brecciated basement granite of the Eastern Dharwar Craton. The uranium ore body of the Kanchanakyi-Hulkul sector is linear, pencil type (length (~3km) width (~100m)) and is parallel to sub-parallel to the fault zone. Uraninite and coffinite are identified as the primary uranium minerals with pyrite and/or carbonaceous matter as the main reductant. Coffinites in general is enriched in Si, Y, Fe, Al, ΣREE and depleted in U, Pb with respect to uraninite. The coffinite is subjected to a variable degree of alteration as indicated by the variation in their mineral chemistry. A very low intrinsic uranium content of the undeformed Shahabad Limestone (< 1 ppm) suggests that the mineralizing fluids have been generated from the deformation of the fertile (av. 25 ppm intrinsic U) basement granite. Further, the enrichment of organic carbonaceous matter (δC13: -26.2‰ to -13.6‰) within the fractured granite and limestone indicate their translocation by the basinal fluids. The δ13C (V-PDB) (-2.09 to 2.92%o) and δ18O (V-SMOW): +19.03‰ to +23.81‰) values of the calcite veins (-2.09 to 2.92‰) from the deformed horizons within the Shahabad limestone suggest the expulsion of basinal fluids from the Shahabad limestone and other clastic litho-units of Rabanapalli Formation due to deformation and induced pressure-dissolution. The present study proposes that the mineralizing system of the Kanchanakayi-Hulkul sector has witnessed fluids of two different origins viz. (1) basinal fluids derived from sediments (relatively enriched in Fe and P) and (2) mineralizing hydrothermal fluids derived from the basement granitoids (relatively enriched in V, Si and REE). The uraninite was precipitated at temperature ~250°C and subjected to (1) radioactive decay of U to Pb, (2) incorporation of cations and concomitant Pb loss (3) oxidation of U4⁺ to U6⁺ and (4) coffinitisation. (author)