ISOTOPIC EVIDENCE FOR MICROBIAL SULFATE REDUCTION AND METHANOTROPHY DURING THE LATE ARCHEAN, WITWATERSRAND BASIN, SOUTH AFRICA

Stable isotope ratios were determined for carbon (d ¹³C) in organic matter and carbonate and for sulfur (d ³⁴S) in pyrite for a stratigraphic profile from a core containing 60 meters of the late Archean Booysens/Kimberly (B/K) Shale Formation and a portion of the underlying Krugersdorp Quarzite Formation. The studied core was collected at a subsurface depth of 1.8 km in Evander shaft #8 operated by Harmony Gold Inc. The Evander sub-basin is located on the northeast rim of the Witwatersrand Basin. Silty shales are the dominant lithofacies in the core but interbedded siltstones and several meters of underlying gray quartzites are present. Carbonate (C_carb) and Organic carbon (C_org) contents in core samples range from <0.05 to 2.26 wt% and <0.05 to 0.42 wt%, respectively. Values of d ¹³C for organic matter (d ¹³C_org) and carbonate minerals (d ¹³C_carb) range from -42‰ to -35‰ and —14.6‰ to -10.1‰, respectively. Distinct negative shifts in d ¹³C_org values occur in laminated clay-rich intervals while positive shifts occur in carbonate—rich intervals. Values of d ³⁴S for Cr-reducible disulfide minerals (predominantly pyrite) range from -3.2‰ to +2.3‰. The most negative d ³⁴S values occur in the laminated intervals that contain the most negative d ¹³C_org values. d ¹³C_carb values lower than -10‰ are inferred to reflect bicarbonate derived from methane. Microprobe imaging shows partial to complete chlorite replacement of original rhombohedral carbonate grains. The combined carbon and sulfur isotopic evidence suggest a thriving community of methanotrophic and sulfate-reducing bacteria in laminated, muddy sediment deposited during maximum marine flooding of the Witwatersrand Basin.