Trace element compositions and geochronology of cassiterite from the Bushveld Large Igneous Province, South Africa

Abstract

This study presents trace element compositions and ages of four representative cassiterite samples from the Zaaiplaats, Union and Rooiberg Sn fields, all spatially associated with the Bushveld Large Igneous Province (LIP), South Africa. The Zaaiplaats Sn field is endogranitic and entirely hosted within the variably-altered Bobbejaankop and Lease granites of the Lebowa Granite Suite. In contrast, the exogranitic Union and Rooiberg Sn fields occur within the Union Tin Member of the Rooiberg Group, and the Pretoria Group of the Transvaal Supergroup, respectively. Collectively, all the cassiterite samples display quasi-parallel chondrite-normalised rare earth element (REE) patterns characterised by relative enrichments in the heavy REE, and yielded ages varying within a relatively restricted interval between 2061 ± 40 Ma and 2054 ± 3 Ma. This age range compares well with those reported from the Bushveld LIP and, when combined with mineral compositions, suggests a common granitic source for cassiterites from all the Sn fields. The exogranitic Rooiberg and Union Sn fields are identified by cassiterites with Zr/Hf of ∼40, whereas the endogranitic Zaaiplaats Sn field is characterised by cassiterites enriched in most incompatible elements (e.g., Nb, Ta, Zr, Hf, In, F and B) and Zr/Hf of ∼10. These Zr/Hf ratios agree with the endogranitic and exogranitic Sn ores from other localities, but are different from the very low Zr/Hf ratios of ∼3 reported for endogranitic cassiterites associated with rare-metal granites. The Zr versus Hf discrimination diagram may therefore be useful for distinguishing the endogranitic and exogranitic origins of cassiterites and for identifying the extent of fractionation of the causative source.