Stable isotope (C-O-S) and geochemical studies of auriferous quartz carbonate veins, Neoarchaean orogenic Ajjanahalli and Gadag Gold Field, Chitradurga schist belt, Dharwar Craton, southern India: Implication for the source of gold mineralizing fluids

Abstract

Carbon (δ13CPDB) and oxygen (δ18OSMOW) isotopic compositions of carbonates of auriferous quartz-carbonate veins (QCVs), S-isotope (δ34SCDT) composition of gold bearing sulphide minerals and REE geochemical characteristics of the auriferous QCVs of Ajjanahalli and Gadag Gold Fields in the Neoarchaean Chitradurga-Gadag greenstone belt, Dharwar Craton, southern India have been studied in detail to constrain the source of auriferous vein fluids. The carbonate δ13C values of the auriferous QCVs of Ajjanahalli fall in the range −2.09 to −8.59‰ (average: −4.5 ± 1.5‰); δ18O are in the range 11.27–26.12‰ (average: 15.69 ± 4.12‰). The δ13C and δ18O values of carbonates of the BIF that host the auriferous QCVs respectively are −1.54 to −2.00‰ (average: −1.76 ± 0.19‰) and 14.12–26.36 (average: 21.9 ± 5.6‰). The carbonates from the carbonated metabasalts, also host for QCVs show δ13C values between −1.39 and −1.92‰ (average: −1.59 ± 0.24‰) and δ18O between 11.44 and 12.91‰ (average: 11.88 ± 0.6‰). δ13C and δ18O of QCVs are clearly distinct from those of meta-sedimentary carbonates in BIF and carbonates in carbonated metabasalts.The calculated isotope composition of the original fluid for the auriferous QCVs δ13C∑c are in the range −2.97‰ to −9.45‰ (average: −5.2 ± 1.4‰) and δ18OH2O fall in the range between 6.46 and 20.58‰ (average: 7.8 ± 0.95‰). The δ13C and corresponding δ13C∑c values of the QCVs are comparable to those of mantle derived fluids or those emanating from juvenile magmas (−6 ± 2‰). Though many of the δ18OH2O values are similar to those of mantle or juvenile magmatogenic fluids (−8 ± 2‰), some are heavier. These heavier δ18O and δ18OH2O values are shown to have resulted from post-vein emplacement water–rock interaction at lower temperatures.Mantle/juvenile magmatic source for fluids is also indicated by the S-isotope (δ34SCDT) composition of gold bearing sulphide minerals. Pyrite δ34Spy of Ajjanahalli and GGF fall in the range −0.21 to 5.8‰ (average: 3.08 ± 1.82‰) and 0.93 and 3.36‰ (average: 1.98 ± 0.82‰) respectively. The arsenopyrite δ34Sasp values range between 1.09‰ and 3.8‰ (average: 2.95 ± 1.08‰) for Ajjanahalli and 2.84‰ for Gadag. The corresponding sulphur isotope composition of the original fluids δ34SH2S (calculated from δ34SCDT values) for Ajjanahalli gold deposit is in range −0.03 to 4.6‰ (average of 1.89 ± 1.82‰). For the GGF, the values range from −0.17 to 2.11‰ (average: 0.88 ± 0.82%). All these δ34SCDT and their corresponding δ34SH2S are very close to juvenile δ34S values (i.e. 0 ± 2%) confirming the mantle/juvenile magmatic origin of the mineralizing fluids.Mantle/juvenile origin of auriferous fluids for auriferous QCVs from both the areas is also supported by initial Sr isotope ratios as low as 0.702 and positive ∈Nd values up to +10 or higher.LILE, HFSE, REE geochemistry of the auriferous QCVs are also consistent with the above conclusions based on isotope data. The QCVs of both these deposits show +ve Eu anomaly which is typical for hydrothermal fluids of mantle origin. The LILE and HFSE data of the QCVs show that fluids required for them have not been generated by melting of subducted slabs.It is therefore concluded that the new isotope and geochemical data provide convincing evidence for direct involvement of auriferous fluids of mantle origin for these orogenic gold deposits.