Stable isotope composition of quartz-calcite veins in the Witwatersrand Basin, South Africa: implication for basin-scale fluid circulation

Abstract : The South African Witwatersrand Basin on the Kaapvaal Craton is the largest gold deposit in the world but the metallogenic model is still not clear; the debate revolves mostly around the role of syngenetic (placer) vs. epigenetic hydrothermal processes. However, it is clear that several fluid events have been recorded throughout the basin. Consequently, it is important to determine the scale of these fluid circulations. For that purpose, we sampled calcite-quartz veins in three goldfields in close vicinity to gold-bearing reefs. In addition, we systematically sampled their host rocks (sedimentary or mafic rocks) in order to investigate the effects of rock-fluid interactions. We performed oxygen and carbon isotope analyses on both the veins and their host rocks, as well as a fluid inclusion study. We show that the oxygen isotope composition of quartz in veins is buffered by the host sedimentary rocks. It is therefore not possible to determine the nature of the original fluid. Another result is that carbon isotopes analyses in quartz-calcite veins hosted by mafic rocks present a range of values that suggest two distinct sources, regardless of their location within the Witwatersrand Basin. One end member is represented by organic matter found locally within the basin sedimentary rocks whereas the second end-member has a marine signature. The most likely candidate for this marine signature is the overlying limestones from the Chuniespoort Group. This implies a basin-scale downward fluid circulation from the overlying rocks down to the basin sedimentary rocks after ca. 2642-2432 Ma (age of the Chuniespoort Group deposition). We propose a model of large-scale circulation that allows elements to be transported over long distances within the Witwatersrand Basin. This does not mean that U and Au must have been brought to the basin from external sources, but rather, this means that redistribution of these elements from placer sources may have occurred on a large scale.