‘C’ and ‘O’ isotope studies on micritic limestone from Shahabad formation: Evidence of inorganic marine carbonate deposition in Bhima Basin, Karnataka, India and its implication on Uranium exploration Pandey U. K.1,*, Asha K.1, Bhattacharya D.2, Roy Debasish2, Sharma A. K.1, Banerjee Rahul1, Bhatt A. K.1, Verma M.B.1 1Atomic Minerals Directorate for Exploration and Research, Hyderabad, India 2Atomic Minerals Directorate for Exploration and Research, Bengaluru, India *E-mail: ukpandey.amd@gov.in
Online published on 13 February, 2019. Abstract The Yalwar area in Yadgir district, Karnataka, lies in the central part of Bhima Basin, having surface exposures of purple shale of Harwal Formation and Deccan traps. Boreholes drilled in the purple shale in this area intercepted limestones of Shahabad Formation (Sedam Subgroup, Bhima Group), which lies unconformably over the Precambrian granite-greenstone terrain of Eastern Dharwar Craton. Petrographically these limestones have been identified as micritic limestone, pyritiferous-micritic limestone and laminated argillaceous-micritic limestone. They are mostly massive and fine grained. Pyrite is of diagenetic origin and occurs as coarse euhedral to fine grained irregular dissemination. Micritic limestone samples, from different depths (109.0m to 277.5m) retrieved from 10 boreholes drilled in Yalwar area, were analyzed for carbon and oxygen isotopes using Nu Carb-Isotope Ratio Mass Spectrometer. The ‘C’ isotope analyses of the above samples (n=23) gave δ13C values varying from 0.26 to 3.32 ‰ (V-PDB). A majority of the samples (n=19) have yielded δ13C values within a narrow range between 1.8 to 2.1‰ (V-PDB). The average δ13C value in these samples is 1.73 ‰ (V-PDB). The δ18O values of these samples display a narrow range from -7.65 to -6.22 ‰ (V-PDB) except for one sample with δ18O value of -9.74 ‰ (V-PDB), the average δ18O value being-6.82 ‰ (V-PDB). The δ13C and δ18O isotopic results of study area are similar to those of earlier reported values on Shahabad carbonates from Malkhaid and Sedam areas (13C = 0 to 3.7 ‰ (V-PDB) and δ18O = -7.61 to -6.04 ‰ (V-PDB) respectively. Studies from other parts of the world have also reported δ13C and δ18O values in Precambrian sedimentary carbonate rocks as 0.4 ± 2.7 ‰ (V-PDB) and 10.58 ± 4.2‰ (V-PDB) respectively. Based on the present carbon and oxygen isotope results it is inferred that Shahabad limestones were deposited in a pure marine environment in the Bhima basin and are similar to Precambrian sedimentary carbonates. Near zero to slight positive δ13C values, of these carbonates ((0.26 to 3.32 ‰ (V-PDB)), suggest that these are chemical precipitate of inorganic origin. Such carbonates are poor in intrinsic uranium content hence in Bhima basin the uranium might have been derived mostly from external sources. Top Keywords Isotope studies Purple shale, Precambrian granites, Eastern Dharwar Craton, Limestone, Mass Spectrometer. Top |