Carbon isotope studies of auriferous quartz carbonate veins from two orogenic gold deposits from the Neoarchean Chitradurga schist belt, Dharwar craton, India: Evidence for mantle/magmatic source of auriferous fluid

摘要

Carbon and oxygen isotopic compositions of carbonates from auriferous quartz carbonate veins (QCVs) of two orogenic gold deposits - Ajjanahalli and Guddadarangavvana Halli (G.R. Halli) - from the Neoarchean Chitradurga schist belt of the Dharwar craton, southern India are examined to understand the origin of the mineralizing fluids. The average carbonate carbon (δ~(13)C(pdb)) and oxygen (δ~(18)O_(smow)) isotope composi-tions of QCVs of Ajjanahalli are -5.5 ± 1.3‰ and 14.1 ± 2.7‰, respectively. The same ratios for the QCVs of G.R. Halli are -6.2 ± 1.9‰ and 14.1 ± 0.5‰. The corresponding average fluid δ~(13)C and δ~(18)O compositions are -5.81 ± 1.14‰, 13.78 ± 5.1‰, for Ajjanahalli and 4.64 ± 0.7‰, -6.50 ± 0.6‰ for G.R. Halli. The δ~(13)C_(pdb) of syn-sedimentary carbonates of BIF of Ajjanahalli (-1.8 ±0.1‰), carbonated metabasalts of Ajjanahalli (-1.4‰) and G.R. Halli (-1.3‰) fall in the compositional range of marine carbonates (0 ± 2‰). As disso-lution/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. It is proposed that gold mineralizing fluids were derived from juvenile magmatic melts and were channeled through crustal scale shear zones to give rise to the gold deposits.