Isotope (C and O) composition of auriferous quartz carbonate veins, central lode system, Gadag Gold Field, Dharwar Craton, India: Implications to source of ore fluids

Carbon (delta C-13(PDB)) and oxygen (delta O-13(SMOW)) isotopic compositions of auriferous quartz-carbonate veins (QCVs) of gold deposits from Sangli, Kabuliyatkatti, Nagavi, Nabapur and Mysore mining areas developed on the Central Lode system of the Gadag Gold Field (GGF) in the Neoarchaean Gadag schist belt of the Dharwar Craton, southern India have been examined for the first time to understand the origin of the mineralising fluids. In majority of the samples (46 out of 49), delta C-13(PDB) of carbonates of the QCVs fall in the range from -2.2 parts per thousand to -9.7 parts per thousand and the delta O-18 values range from 12.0 parts per thousand to 30.5 parts per thousand SMOW. The calculated fluid delta C-13(Sigma C) compositions for these deposits range from -2.1 parts per thousand to -9.6 parts per thousand and delta O-18(H2O) from 6.8 parts per thousand to 25.9 parts per thousand, respectively. Carbonate delta C-13 and fluid delta C-13(Sigma C) compositions of the carbonates of the QCVs of the GGF are not only distinct from the carbon isotope range of marine carbonates or meta-sedimentary carbonates of the Chitradurga schist belt, but are consistent with C-isotope values of magmatic (-5 +/- 3 parts per thousand, Burrows et al., 1986) and/or mantle (-6 +/- 2 parts per thousand, Ohmoto, 1986) carbonates. As dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO2 with delta C-13 values similar to or more enriched than parent rock, the carbonate or fluid delta C-13 ratios of the QCVs (which fall in the compositional range of mantle/magmatic derived CO2 or carbonates) obtained in this work cannot be the result of metamorphism. The present study corroborates our previous reports from Ajjanahalli and G.R. Halli gold deposits (Sarangi et al., 2012) occurring in the vicinity of the southern extension of the same crustal scale shear zone on which all the GGF deposits are located. The age of gold mineralisation in this area has been reported to be 2522 +/- 6 Ma by Sarma et al., 2011. Chardon et al. (2011) have proposed large-scale remobilization of the older gneissic basement, as well as, emplacement of juvenile granites between 2559 Ma and 2507 Ma, dose to the crustal scale shear zone along the eastern margin of the Chitradurga schist belt. Based on these observations and our isotope studies, it is proposed that gold mineralising fluids were derived from mantle/juvenile magmatic melts and were channelled through crustal scale shear zones to give rise to the gold deposits in the GGF. (C) 2015 Elsevier B.V. All rights reserved