Metallogenesis and depositional environment of the Archean-Proterozoic carbonaceous phyllites from the Dharwar Craton, India

Organic carbon rich meta-sedimentary rocks are important repositories of gold, PGE and vestiges of the biogeochemical cycles that prevailed during the Precambrian time. In this study, we report gold, PGE and 834S concentrations of carbonaceous phyllites from Neoarchean Chitradurga, Gadag greenstone belts of western Dharwar Craton and Mangampeta of Proterozoic Cuddapah basin, eastern Dharwar Craton to understand their metallogenesis and depositional environments. Chitradurga phyllites (avg. Au = 180 ppb) show elevated concentrations of refractory gold compared to Gadag (avg. Au = 38 ppb) and Mangampeta shales (avg. Au = 23 ppb). The Archean carbonaceous phyllites display three generations of pyrites and their mineral chemistry suggests hydrothermal imprints while Proterozoic shales have two generations of diagenetic pyrites. Gadag has high PGE concentration (?PGE = 55 ppb) compared to Chitradurga (?PGE = 37 ppb) and Mangampeta (?PGE = 27 ppb). The PGE geochemistry is characterized by enrichment of PPGE over IPGE with Au > Pt > Pd and the Au/Pd, Pd/Pt, Ru/Ir, Ir/Pd ratios coupled with negative Pt (Pt/Pt*) anomalies indicate hydrothermal source. The Chitradurga and Gadag samples yielded low sulphur isotopic signatures (834S = 1.8?2 and 1.3?2%o respectively) whereas the Proterozoic Mangampeta samples have comparatively higher values (834S = 8.4?34.3%o) suggesting hydrothermal and sea water sulphate sources respectively. Sulphate present in Archean and Proterozoic sea water was converted to sulphur by bacterial sulphate reduction which led to the formation of pyrites. In the Archean phyllites, the metal bearing bisulphide complexes were adsorbed into the pyrites while the Proterozoic shales are characterized by the partitioning of organo-metallic complexes into the pyrite during the diagenesis. Strong positive correlations of transition metals, redox sensitive elements with PGE indicate hydrothermal source and V/(V + Ni), U/Th, Th/U, authigenic U attest euxinic paleo-redox conditions. Thus, biogenically mediated redox depositional environments which prevailed during Archean and Proterozoic times have enhanced the metal sequestration processes that subsequently resulted in the ore formation in these carbonaceous phyllites.