Source of Ore Fluid in Lode Gold Deposits of Eastern Dharwar Craton: An Intricate Issue

Abstract

The source of auriferous fluid in late Archean lode gold deposits has long been debated. Among a number of propositions, most of the workers agree upon a metamorphic or magmatic ancestry of ore fluid. With regard to gold deposits in Eastern Dharwar Craton (EDC), a metamorphogenic parentage of ore fluid has been almost exclusively advocated. The generation of ore fluid is attributed to devolatilization reactions during greenschist-amphibolite metamorphism of greenstone rocks. However, at least in Indian context, such a hypothesis warrants further investigation in terms of retention and release of devolatilized fluid from rocks of amphibolite facies. Metamorphism and subsequent fluid production is often tied-up with fault-valve type process in lode gold systems. Yet, we need to ascertain the operation of such a cyclic process and its compatibility with metamorphogenic fluid production in the mineralized domains of EDC. As an alternate to metamorphism, a magmatic model is often postulated, whereby felsic magma is believed to be the source of gold-bearing fluid during late-stage crystallization. Most of the support for this model has come from isotopic composition of gangue minerals in ore zones worldwide. However, the isotope ratios of various elements are not always suggestive of any particular source. This discrepancy in various isotopic datasets possibly manifests fluid-rock interaction or/and varied source for different elemental constituents within ore fluid in question. Recently, in an attempt to testify the magmatic model in parts of EDC, late Archean granitoids proper were considered for detailed investigations. Such studies show that late-magmatic fluid can not only retain precious metal (gold) in appreciable amounts, rather can also carry it for considerable distance along trans-crustal shear zones. However, as of yet, temporal records on ore enrichment processes in EDC are quite poor, which is crucial to link up metallogeny with crustal evolution, and to locate the source of ore fluid as well. Nevertheless, it is challenging to place precise temporal constraints on magmatic-hydrothermal activity, metamorphism, deformation and ore-fluid overprinting processes.