Genesis of uranium mineralization in Kanchankayi-Hulkul sector, Bhima basin, Yadgir district, Karnataka

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)