Saline waters in basement rocks of the Kaapvaal Craton, South Africa

The chemical composition and geochemical evolution of saline waters found in deep (1.9–3.5 km) gold mines in the Kaapvaal Craton, South Africa, are reported and discussed. Twenty eight samples, representing the most saline waters available were studied. The salinity of the samples ranges between 2.5 and 99 g·TDS·L− 1 with chloride being the dominating anion. Magnesium is practically absent from most samples. apvaal waters were sampled in the Carletonville region (Driefontein, Kloof, Mponeng and Tau Tona mines) and at Evander. The samples, defined as fracture waters, were retrieved from the Witwatersrand Basin Proterozoic quartzite rocks and from metamorphosed basalts and basaltic andesites. on water salinity, Na/Cl–Br/Cl ratios and Ca–Mg–SO4 relationship, we conclude that the chemical composition of the waters stems from ancient seawater that experienced three major processes by the following order: (1) concentration of seawater by freezing; (2) density-driven infiltration into the underlying crystalline basement, accompanied by water–rock interaction (WRI); and (3) dilution of the brine by freshwater. Because the last glaciation in South Africa occurred in Carboniferous–Permian (Gastaldo et al., 1996) we attribute this age to the Kaapvaal brines, rather than to a much earlier period proposed previously. their cryogenic concentration in a marginal trough around the Gondwanan ice sheet, the brines infiltrated and migrated into the underlying crystalline rocks through fissures and shear zones, displacing less dense resident freshwater. Therein, the brines were further modified via water–rock interaction, which included chloritization and albitization of Ca-plagioclase. Chloritization was responsible for the almost total removal of Mg and SO4 from solution and for the significant Ca enrichment observed. Albitization, which affected only a part of the samples, caused a loss of Na and an additional, equivalent gain in Ca. In view of similar findings, limited so far to crystalline shields in the Northern Hemisphere, our interpretation of the South African data may have a much farther-reaching meaning. We argue that similar saline waters should be expected at depth in all rocks, crystalline and sedimentary alike, in regions that were covered by major glaciers, such as eastern South America, large parts of Australia, India and Antarctica during the Carboniferous–Permian.