The geochemistry of the Bushveld granites in the Potgietersrus tin-field

An area of 324 km² was selected for a pilot-study of the geochemical characteristics of the Bushveld granites in the Potgietersrus tin-field. Two hundred and sixty nine samples were collected from a regular grid of stations. Additional samples collected from selected sample stations provide data on “about station” variance to test the validity of conclusions based on regional variation. The samples were analysed for Ba, Ce, Co, La, Li, Nb, Ni, Rb, Sc, Sn, Sr, Ti, Y, Zn and Zr together with the major elements Ca, Na and K.The stratiform Bushveld granites have mean concentrations for these elements that compare closely with those in low Ca-granites. Ce, La, Rb, Y, Zn and Zr are enriched relative to low Ca-granites while Sr is markedly impoverished. The stanniferous Bobbejaankop granite stock is distinguished from the stratiform granites with respect to Ba, Sr, Ti and Zn. The median tin concentration in the stratiform granites is 6.5 ppm and in the Bobbejaankop granite it is 9 ppm. Economically viable deposits of tin disseminated through the upper 120 m of the Bobbejaankop granite stock have been exploited for over 70 years.It is concluded that the Bobbejaankop granite crystallized at low temperatures from a hydrous residual melt that had collected beneath a roof of felsite and early crystallization products of the original granitic magma. The presence of this concentration of volatile-rich, hydrous liquid depressed the melting temperatures of the quasi-solid, granitic crystal much below the ambient temperature initiating re-mobilization. The enrichment in Sn of the stratiform granites in the Potgietersrus area is compared with the “normal” (i.e., ± 3 ppm) Sn-content of the Bushveld stratiform granites building the Sekukuni Plateau in the eastern lobe of the Bushveld Complex where no tin mineralization has been reported.