Uranium Mineralisation in the Neoproterozoic Bhima Basin at Gogi and near Ukinal: an Ore Petrological Study

Uranium mineralisation in the Neoproterozoic Bhima basin todate has been found near Ukinal and at Gogi. U-mineralised phosphatic horizon near Ukinal includes the lithounits of phosphatic micritic limestonelchert and siliceous, calcareous phosphorite, some of which are manganiferous and glauconite-bearing. Uranium in these phosphatic rocks, correlatable with P₂O₅content (r>0.9), occurs mostly in collophanc and is difficult to extract. This mineralisation is sedimentary-diagenetic.

Mineralisation at Gogi is more promising. This occurs within the major E-W trending Gogi-Kurlagere fault, near to its intersection with a NE-SW trending fault and is hosted by both brecciated, siliceous limestone and deformed basement rock represented by low-Ca biotite granite-granodiorite. Over 80% of the mineralisation at Gogi occurs as veins, veinlets and fracture-fills, composed of coffinite, lesser pitchblende, accessory U-Ti-Si complex (only in granitoid), associated intimately with reductants like sulphides and organic matter, and also clays (illite, smectite). Pitchblende is of two generations, both replaced by and replacing coffinite, and these two U-minerals, in turn, arc replaced by U-Ti-Si complex. The U-minerals occurring in limestone are Th-poor and are characterised by variable contents of U, Pb, Si and lanthanides (LREE>HREE), and as compared to those in granitoid are marked by higher contents of U, Pb, Fe and lower contents of Si and lanthanides. Amongst the sulphides, pyrite is predominant and occurs in three modes, viz., coarse euhedral-subhedral, framboidal and reticulate. The rest comprise marcasitc, chalcopyrite and galena in accessory to minor amount. These sulphides carry notable contents of Co, Ni, Se As, Ag and Au, with enrichment of Ag, Co, Ni in chalcopyrite followed by pyrite (more in framboidal type), Se in galena and Au more in sulphides from the granitoid. These high-value metals can be recovered as by-products during extraction of U. The organic n~atteris heterogenous and contains U, Si, Ca, Al, Pb and Fe, with the one rich in U is also high in the other elements. Radioactive clay contains U, P and REE. Both radioactive organic matter and clay are almost free of Th and Zr, with LREE>HREE. Paragenetically, the uranium minerals were formed later than the sulphides and organic matter. U-mineralisation at Gogi is poly-phase and poly-metallic, hydrothermal vein type with an Eh of -0.2 to -0.3V, pH of 7 to 8, and temperature of <200°C. The controls for this U-mineralisation are: (a) fertile granitoid as source, (b) structurally weak and permeable zones of faults, folds, brittle-ductile deformation and unconformity, along basin margin, (c) presence of strong reductants of organic matter and sulphides, (d) impervious and reactive cover rock of limestone and (e) dyke activity in the basement which may have acted as heat source and created necessary geothermal gradient for remobilisation and precipitation of U. These can be taken as guides for U-exploration in similar geologic set-up.