Origin of granodiorite hosted Neoarchaean orogenic gold ore deposits: Stable isotopic and geochemical constraints with example from the Dharwar craton, southern India

Abstract

The source of auriferous ore fluids for the orogenic gold deposits of all ages world-wide is considered to be metamorphic except those of largest granitoid hosted gold deposit of Jiadong Peninsula, North China craton. Such conclusions need to be geochemically and isotopically verified preferably in some other granite hosted gold deposit of some other craton of the world. This work presents isotope and geochemical studies of auriferous quartz carbonate veins associated with the granodiorite hosted Jonnagiri gold deposit, Dharwar Craton to find any geochemical and isotopic evidence in support or against the metamorphic origin for such kind of deposits and discuss their implication to the origin of orogenic gold deposits world-wide in general.The Jonnagiri gold deposit, Dharwar Craton is the only granodiorite hosted orogenic gold deposit reported so far from India. The gold mostly occurs in the form of sulphides mineralisation in quartz carbonate veins (QCVs) emplaced in sheared granodiorites. In the present work, C and O isotope, HFSE, LILE and REE compositions have been studied in these auriferous QCVs to constrain the source of auriferous ore fluids for this deposit. The δ13CPDB and the corresponding δ18OSMOW values range between −1.9‰ to −12.6‰ (average −5.2 ± 1.9‰) and 1.66‰ to 20.56‰ (average 8.5 ± 3.6‰) respectively. Though there is a large spread in the both δ13C and δ18O values, maximum number of data cluster within δ13C values of −5‰ to −6‰ and δ18O between 8‰ to 9‰. A mathematical procedure, “fluid rock interaction modelling” was employed to find out the reason behind this spread and to determine the extent of post depositional alteration effect on the original isotope data. This model shows that the pristine isotope data has been very partially altered by different types of fluids both in the endogenetic and exogenetic environment. In the former case, they have interacted with later hydrothermal fluids of same isotopic composition, magmatic and metamorphic fluids. In the exogenetic environment i.e. at the present position of their occurrence, they have been subjected to interaction by meteoric and ground water circulated both at low and high temperature. However, maximum of these alterations has found to have taken place at the Fc/Rc ratio < 1, so that except two, all other samples have retained > 75% of their primary δ13C and δ18O values enabling us for constraining the source of ore fluids.The δ13C values like −5‰ to −6‰ (average-5.2 ± 1.9‰) and δ18O values like 8‰ to 9‰ (average 8.5 ± 3.6‰) are neither consistent with any marine carbonates nor with the meta-carbonates of Dharwar Craton. Therefore, these indicate a non-metamorphic source of ore bearing fluids. However, such C and O isotope compositions are consistent with those of mantle and/or magma derived carbonates. The auriferous fluids could be either of mantle or juvenile magmatic origin or both with a major contribution from mantle derived fluids as the fluid rock interaction modelling shows a minor interaction of the auriferous QCVs with later magmatic fluids.LILE, HFSE, REE geochemistry of the auriferous QCVs are also consistent with the above conclusions based on isotope data. The lack of correlation between La/Sm (n) and Th/Nb, Ba/Th and Th/Nb, La/Sm(n) and Th/La, La/Sm (n) and U/Nb, La/Sm(n) and Th/Nd (Fig. 6e) and lack of mirror image pattern of REE between the QCVs and host/associated rocks of Jonnagiri gold deposit do not indicate that the fluids necessary for their emplacement are of metamorphic origin. The REE patterns are also characterised by positive Eu/Eu* anomaly which is also typical for hydrothermal fluids of mantle/juvenile magmatic origin.This result is also consistent with the recent reports from Ajjanahalli and Gadag gold deposit located in the same craton. Hence it has a regional significance. These Neoarchaean gold deposits from Dharwar craton do not show a metamorphic origin of the auriferous fluids. This contradicts the general suggestion that all the orogenic gold deposits world-wide are of metamorphic origin except those of Jiadong Peninsula. These deposits are rather second set of deposits after those of Jiadong Peninsula to have a major mantle and/or minor juvenile magmatic source and we suggest to further explore the geochemical and isotopic evidence in favour of or against the metamorphic origin in many other orogenic gold deposits of the world.