Investigation of Large-scale Brine Circulation as Mechanism of Ore Formation in the Kalahari and Postmasburg Fe-Mn Fields, South Africa

The Kalahari and Postmasburg ore fields are host to giant, economically significant and texturally diverse iron and manganese ores. While hydrothermal metasomatism has been well-demonstrated in the Kalahari district to have locally upgraded the sedimentary manganese orebodies, ore-genesis in the Postmasburg field has been attributed primarily to surficial lateritic processes. However, many ore occurrences in the latter show comparable alkali metasomatic signals to that seen in the Kalahari field. Initial results from our ongoing mineralogical, geochemical and isotopic study, document the regional similarities, the metasomatic nature of the alkali-rich assemblages (replacements, vug-fills, veins) and the remarkable enrichment of the mineralizing fluids in Na, K, Ba, Sr, Ca, Mg, Li, S, Pb, As and V. The widespread occurrence of barite, aegirine, banalsite, natrolite, witherite, phlogopite and other Ba- or Na-bearing silicates observed across Postmasburg ores and neighboring lithologies correlate well with alteration mineralogy described from the Kalahari field. Sulfur isotopes from barite suggest a model invoking fluids of at least partly evaporitic origin, being responsible for the observed alkali metasomatism in both fields. Hydrothermal brine migration is further buttressed by the enrichment in radiogenic Sr-87 in barite, consistent with Ba in the involved fluid(s) being derived from basement.