SULFATE-SULFIDE ISOTOPIC FRACTIONATION REFLECTING VARIABLE METABOLIC STRATEGIES FOR SULFATE REDUCING BACTERIA IN THE DEEP GOLD MINES OF THE WITWATERSRAND BASIN, SOUTH AFRICA

Bacterial sulfate reduction (BSR) has been the focus of numerous studies in near surface environments but has been investigated only rarely in the deep subsurface due to issues of accessibility, cost, and risk. Water intersected as a result of exploration in deep (below 1 km) and ultra-deep (below 2.5 km) gold mines in the Witwatersrand Basin of South Africa allows direct sampling of waters and gases at temperatures ranging from 24 to 59 oC and following both ascending and descending fluid-flow paths. These basinal fluids have been influenced by mining operations but contamination was minimized by collecting flowing intersections and using bore-hole packers to prevent back flow of mine air and service water. At 12 sites located in four different mines, water was drawn into 50 ml plastic syringes that were pre-loaded with 5 ml of 40% zinc acetate solution. Sulfur isotopic compositions for dissolved sulfide range from -14.6 to 19 per mil VCDT compared to compositions for dissolved sulfate that range from 16.0 to 21.8 per mil VCDT. Eight samples representing four mines contain sufficient amounts of dissolved sulfide and sulfate to determine the isotopic difference between sulfur species (sulfate minus sulfide). The Kloof, Mponing, and Evander mines are located in the north/northeastern part of the Witwatersrand Basin and show isotopic differences of 11.5 to 20.6 per mil VCDT. The Beatrix mine, by contrast, is located in the southern part of the basin and shows isotopic differences of 30.3 to 32.8 per mil VCDT. The modest isotopic differences in the northern part of the basin suggest a strictly anaerobic environment. The relatively large fractionations observed at Beatrix suggest opposing oxygen-sulfide gradients with bacterial disproportionation of sulfur intermediates such as thiosulfate or sulfite. Results of this study are important in demonstrating regional presence and local variation in sulfate-reducing bacterial communities in the deep subsurface of the Witwatersrand Basin.