B-Na rich Palaeoproterozoic Aravalli metasediments of evaporitic association, N. W. India: a new repository of gold mineralisation

Abstract

We report field observations and preliminary petrochemical and stable isotopic compositions of the host rock and sulfide minerals for a recently discovered sulfide-hosted gold deposit from the Palaeoproterozoic Aravalli rocks in northwestern India. Gold occurs associated with Fe-As-Cu (± Co) sulfides in lithofacies of diverse mineralogical composition, dominated by amphibole-bearing dolomitic marble and albite-rich quartzo-feldspathic rocks (QFR). The presence of vugs, believed to be after dissolution of halite patches, coupled with the development of scapolite-bearing assemblages in dolomite marble, occurrence of tourmaline-rich mineralized rocks and very high Na content of carbonates and QFR indicate hypersaline depositional environment for the host rock. A narrow spread of δ ³⁴S_CDT values from +10.5 to +12.8%. for stratiform arsenopyrite and remobilized pyrrhotite, corroborates hypersaline to evaporitic depositional environment. The δ¹³C values for gold-bearing carbonates show bimodal distribution; one population (mean -0.4%._V-PDB) is close to the sea water carbonate values while the other (-3.l%.) indicates probable mantle affinity. The δ ¹⁸O values for carbonates (mean=+18.4%._V-SMOW) are not discriminative as they appear to have been modified by subsequent metamorphism and hydrothermal activity. Interpretation of mantellic source for carbon is borne out from the absence of biogenic activity and incompatibility of significantly negative δ ¹³C values in the evaporitic setting of deposition of Bhukia rocks. We suggest that Na-B rich fluids, channeled through rift-related faults, were responsible for creating hypersaline conditions that eventually evolved into an evaporitic environment. Mantle carbon, through some still deeper faults, was probably responsible for the gold transport and its subsequent deposition in an otherwise uncommon (evaporitic) repository of carbonates and QFR.