Tin mineralization in the Archean Sinceni rare element pegmatite field, Kaapvaal Craton, Swaziland

Abstract

The Sinceni pegmatite field constitutes a large group of 3.0 Ga granitic pegmatites in the exocontact of the Sinceni pluton in south-central Swaziland. Field relations, age dating, and geochemical evidence indicate a genetic relationship between the pegmatites and the Sinceni pluton. The Sinceni pegmatites are moderately fractionated granitic pegmatites belonging to the beryl type of the rare element class. The pegmatite field is compositionally zoned, with the highest fractionation and strongest mineralization in a restricted area referred to as Sinceni West. Only the alluvial and eluvial tin concentrations are of economic interest, and the largest of these is the Sinceni West deposit. Primary tin mineralization at the Sinceni West deposit consists of cassiterite in late-stage pegmatite units of sugary albite, associated with mica-tourmaline metasomatic selvages in the host rock. The albitic units are interpreted as the products of late residual melt, and the metasomatic selvages reflect fluid exsolution from the melt at the latest stage of pegmatite consolidation. Several factors contributed to the development of primary tin mineralization in the Sinceni West area. First, the pegmatites in this area are the most highly fractionated, as shown by Ta, Rb, and Cs concentrations in blocky K feldspar. Second, the richest mineralization occurs in pegmatites that lack tourmaline, whereas tourmaline is abundant in the barren pegmatites. A lack of tourmaline allows the buildup of boron in residual melts, which promotes extended fractionation by increasing water solubility and depressing the solidus temperature. Third, the best tin values occur in pegmatites that exhibit boudinage and other signs of ductile deformation caused by an adjacent shear zone. Microstructures show that at least some cassiterite predates the deformation and that the albitic units and wall-rock alteration are synkinematic. The ductile deformation may have influenced the distribution of residual melt and its interaction with preexisting pegmatite and wall rocks, but it was probably of minor importance for the mineralization itself.