A Modified Placer Origin for Refractory Gold Mineralization Within the Archean Radioactive Quartz-Pebble Conglomerates from the Eastern Part of the Singhbhum Craton, India

摘要

Archean quartz-pebble conglomerates are one of the most extensively studied stratigraphic horizons and are globally well known for their gold and uranium prospects of economic grades. Similar quartz-pebble conglomerate horizons have been delineated within the Neoarchean-Paleoproterozoic Gorumahisani-Badampahar schist belt and Dhanjori volcano-sedimentary basin lying along the northeastern fringe of the Singhbhum-Orissa craton in eastern India. Gold and uranium occurrences have been reported previously from the quartz-pebble conglomerates of the Singhbhum craton; however, no detailed report on gold occurrences and genesis is available. This work focuses on the nature and genesis of gold mineralization hosted by radioactive quartz-pebble conglomerates from the eastern Iron Ore Group and the Dhanjori basin within the Singhbhum-Orissa craton. The quartz-pebble conglomerates reported in this work are predominantly matrix supported and oligomictic in nature. Petrography, microprobe data, and scanning electron microscopy studies reveal the presence of chronrite, zircon, rutile, magnetite, monazite, pyrite, chalcopyrite, pyrrhotite, stibnite, and brannerite. Laser ablation-inductively coupled plasma-mass spectrometry analyses on various generations of arsenian pyrites reveal significant concentrations of Au in solid solution. The gold was initially incorporated in its detrital form onto growing pyrite surfaces, possibly due to microbial mediation. The trace element data coupled with backscattered electron images and X-ray compositional maps suggest remobilization of detrital Au and associated trace elements from the diagenetic pyrites and their precipitation onto the younger hydrothermal pyrites. The sulfide-hosted gold mineralization, trace element signatures, and zoning patterns studied in this work mimic the mineralization style reported from sediment-hosted gold deposits such as Spanish Mountain and Carlin. However, in broad terms, the origin of Au within these quartz-pebble conglomerates resembles the modified placer model of gold deposition within the Witwatersrand conglomerates. This is the first report of refractory gold mineralization within arsenian pyrites from the radioactive quartz-pebble conglomerates of eastern India and has important implications for gold exploration in this region and analogous areas.