HYDROTHERMAL ALTERATION ASSOCIATED WITH SEDIMENT-HOSTED ROOIBERG TIN DEPOSITS, SOUTH-AFRICA

The Rooiberg tin field, also known as the Rooiberg Fragment, is located in the western lobe of the Bushveld Complex, South Africa. The Fragment is triangular in shape, consists of Palaeoproterozoic Transvaal Supergroup volcano-sedimentary Lithotypes and is surrounded by granitoid intrusives of the 2050-m.y.-old Lebowa Granite Suite. Almost all of the significant tin deposits are hosted by arkoses at the top of the Boschoffsberg Quartzite Member. On a regional scale the individual tin deposits are broadly similar and are collectively classified as replacement and open space-filling types. Particular styles of mineralization, such as tourmaline pockets/orbicules, disseminated cassiterite, steep- and flat-dipping sulphidic fractures and hydrothermal carbonate breccias, appear to dominate at each mine. Petrological and geochemical investigations of four mineralized centres, based mainly on exploratory borehole cores, were conducted. The results indicate the presence of a stratified zone, 500-600 m thick and continuous over at least 20 km along strike, consisting of pervasively altered elastic arenitic sediments. The alteration displays a distinct zonal distribution that comprises a grey-green sodic (albitized) footwall, an approximately 80m thick sericitized-tourmalinized sulphidic tin zone and a red, hematitic, potassic hanging-wall. The observed zonal pattern and localization of three successive phases of cassiterite mineralization are consistent with the tectonic history and with the evolutionary development of multiple pulses of stanniferous magmatic hydrothermal fluids that were derived from late differentiates of the surrounding Bushveld granites over a prolonged period. Areas of high fracture density (tectonic focal points) acted as conduits for the initially rapidly ascending reactive fluids. Factors such as a decreasing fluid-lithostatic pressure contrast, limited vertical development of fracturing and/or the presence of impermeable shaly arkose retarded their ascent. This caused the accumulation, cooling and lateral spreading of fluids and pervasive alteration over a broad, stratified, possibly isobaric, front. Lateral overlapping of the local fronts generated at various centres creates the impression of a continuous zone. An abundance of petrological and geochemical similarities between the various focal points, which are marked by former tin mines, points to a shared composition and source for these contemporaneous exogranitic fluids.