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Abstract: This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr2O3 contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF. [ABSTRACT FROM AUTHOR]

Abstract: 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 (δ13Cpdb) and oxygen (δ18Osmow) isotope compositions 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 δ13C and δ18O 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 δ13Cpdb 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 dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO2 with δ13C values similar to or more enriched than parent rock, the carbonate or fluid δ13C 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. 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. [Copyright &y& Elsevier]