The Union Tin Member of the Rooiberg Group: Geodynamic implications for the Bushveld Large Igneous Province, South Africa

Abstract

The Union Tin Member, comprising a succession of pyroclastic and shale-dominated sedimentary rocks associated with the 2.05 billion-year-old Bushveld Large Igneous Province (LIP), is well-exposed particularly in the Union Sn field located 150 km north of Pretoria, South Africa. The Union Tin Member is a laterally extensive (>200 km-wide) marker horizon situated between the rhyolitic Kwaggasnek and Schrikkloof formations of the Rooiberg Group. The shale unit exhibits low total organic carbon (TOC) contents (168 ± 70 ppm) along with a narrow range of δ¹³C values (−27.7 ± 1.7 ‰) and δ¹⁵N values mostly around 0 ‰, resembling characteristics typical of marine shales. These siliciclastic sedimentary rocks were deposited in a shallow-marine sedimentary environment subjected to sea-level fluctuations, suggesting widespread submergence of the Kaapvaal Craton despite ongoing LIP magmatism. The whole-rock compositions, particularly rare earth elements (REE), Th, Sc, Zr, Ni, V and TiO₂, indicate a provenance dominated by the underlying Rooiberg Group, which may also include the Paleoproterozoic Transvaal Supergroup, Archean tonalite-trondhjemite-granodiorite (TTG) suites and greenstones. Variable enrichments of the shale in Sn, Li, Cs, W and U are primarily associated with chlorite, sericite, illite and hematite. Notably, the rhyolites, immediately underlying and overlying the Union Tin Member, are similarly altered to a mineral assemblage dominated by the Fe-Al-Mg-bearing phases with relative enrichments especially in Sn. Alteration is linked to large-scale Sn-bearing hydrothermal fluids derived from the Lebowa Granite Suite. The periodic emplacement of extensive volcanic rocks of the Rooiberg Group is likely to have resulted in a downsagged >200 km-wide basin underlain by the Transvaal Supergroup. The deposition of the Union Tin Member within this basin could potentially represent the surface expression of a deep-seated, exceptionally large magma reservoir suggested to have contributed to the formation of the entire Bushveld LIP.