Abstract

Laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) trace element maps of pyrite and gold from the Carbon Leader Reef in the Witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal, derived from the West Rand Group or equivalent shales stratigraphically below this reef. Rounded detrital pyrite grains in the Carbon Leader Reef vary from compact to porous to sooty, with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the West Rand Group. Detrital porous and sooty pyrites contain 0.4 to 11.3 ppm solid-solution gold, sulfur isotope δ34S values from −17 to +16%, and high levels of As and Te, plus a wide range of other trace elements (in decreasing order: Ni, Co, Sb, Cr, Cu, U, Pb, Bi, Mo, Zn and Ag). The sooty pyrite is the most Au, Te, and Mo rich and is intergrown with alumino-silicates and organic matter. The detrital pyrite textures, S-isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions. The LA-ICP-MS maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro-inclusions of gold, brannerite, and Ni-As sulfides. The pyrite overgrowths are also enriched in solid-solution gold, tellurium, and other trace elements including, in decreasing order, As, Co, Ni, Cr, Cu, Mn, Pb, Bi, and Ag. Fractured and brecciated pyrite associated with brittle bedding-parallel fracture zones in the Carbon Leader Reef are also enriched in these elements due to alteration of pyrite surrounding the fractures. Laser ICP-MS Pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 Ma with hydrothermal overgrowths originating between 2100 and 2020 Ma incorporating highly radiogenic Pb.

This study demonstrates that both of the two competing theories for the origin of the Witwatersrand gold reefs are likely to be correct. We suggest that the hydrothermal event was widespread (kilometer scale) and involved basinal fluids that scavenged gold, tellurium, arsenic, and trace elements (Co, Ni, Cr, Cu, Mn, Pb, Bi, Ag) from gold-bearing sedimentary units in the Central Rand Group.

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