Auriferous Quartz Pebble Conglomerates from the Singhbhum Craton, Eastern India: Implications for Gold Occurrence and Source Rock Characteristics

The Singhbhum craton in Eastern India, hosts complex rock assemblages of Archean to Proterozoic age. Unlike other cratonic blocks of India, the Singhbhum cratonic successions exhibit continuous depositional records from the Palaeoarchaean to Mesoproterozoic. Radioactive quartz pebble conglomerates exhibiting high incidences of gold occur at the basal unconformity overlying the basement Singhbhum Granite Complex (SBG). These Archean-Proterozoic conglomerates have been delineated within the Gorumahisani-Badampahar Iron Ore Group and basal succession of Dhanjori volcano-sedimentary belt. We present the first report of refractory gold occurrences within QPC's of Eastern India. This work also explains source rock characteristics of the conglomerates based on whole rock geochemistry and rutile thermometry. The QPC's are mineralogically immature and texturally submature. Sedimentological studies indicate a debris flow mode of origin for the conglomerates. EPMA data and X-ray compositional maps reveal high incidences of refractory gold within euhederal pyrites. Radioactive minerals include rare detrital uraninite, monazite and brannerite. Uraninite is rare because of its susceptibility to oxygenic environment. Presence of detrital chromite as well as zircon within the same sample confirms a mixed provenance (felsic and mafic/ultramafic). Low values of Cr/Th and Sc/Th in most of the sample analysed suggest a significant granitic input in the source. However, some samples show higher ratios which indicate a mafic source. A strong negative Eu anomaly shows that the source is felsic but higher concentration of the Cr suggest a mafic/ultramafic source. Hence, a mixed provenance of felsic as well as mafic rocks can be inferred. Various discrimination diagrams show that the most of the samples fall within the continental margin setting with some samples pointing towards continental island arc region. Rutile thermometry reveals a temperature range of 946o to 1200oC for its source which indicate granulite to amphibolite grade metamorphism. Rutiles appear to have been derived from a metamafic source, most likely the older metamorphic group rocks. Hence, the basement SBG along with enclaves of the mafic older metamorphic group are the most probable source rocks for the studied conglomerates.