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

Abstract Carbon (δ 13 C PDB ) and oxygen (δ 18 O 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), δ 13 C pdb of carbonates of the QCVs fall in the range from − 2.2‰ to − 9.7‰ and the δ 18 O values range from 12.0‰ to 30.5‰ SMOW. The calculated fluid δ 13 C ∑ C compositions for these deposits range from − 2.1‰ to − 9.6‰ and δ 18 O H 2 O from 6.8‰ to 25.9‰, respectively. Carbonate δ 13 C and fluid δ 13 C ∑ 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‰, Burrows et al., 1986) and/or mantle (− 6 ± 2‰, Ohmoto, 1986) carbonates. As dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO 2 with δ 13 C values similar to or more enriched than parent rock, the carbonate or fluid δ 13 C ratios of the QCVs (which fall in the compositional range of mantle/magmatic derived CO 2 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, close 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.