PB AND SR ISOTOPE AND ORIGIN OF PROTEROZOIC BASE-METAL (FLUORITE) AND GOLD DEPOSITS, TRANSVAAL SEQUENCE, SOUTH-AFRICA

The early Proterozoic carbonate Chuniespoort and Ghaap Groups of the Transvaal Sequence in South Africa host a number of Mississippi Valley and vein-type Pb-Zn(F) deposits of variable size and grade. The Chuniespoort Group carbonates and overlying Pretoria Group shales in the eastern Transvaal also host a number of hydrothermal Au deposits which are peripheral to the Bushveld Complex. Field evidence indicates that there are a number of deposits intermediate between the Mississippi Valley and hydrothermal Au types. Pb in sulfides of the Pb-Zn deposits becomes more radiogenic to the north and east in the Transvaal subbasin (Pb-206/Pb-204 = 14.53-16.93) and toward the south in the Griqualand West subbasin (13.43-14.20). The spread in the Sr-87/Sr-86 ratios (at 2.0 Ga) of the gangue minerals of the Pb-Zn deposits ranges from 0.7153 to 0.7305, compared with 0.7092 for the host rocks. These data collectively reflect systematic districtwide variations in the Sr and Pb isotope signatures. The data also imply that the gangue minerals derived their Sr isotope compositions from sources exterior to or distant from the local dolomite sedimentary package in the case of the Pb-Zn deposits. The isotopic data on the ore and gangue minerals are consistent with a basinwide, tectonic and hydrothermal event at ca. 2.0 Ga which concentrated metals in structurally prepared sites in the host rocks. Migration of the mineralizing brines responsible for the Pb-Zn(F) deposits was possibly a response to tectonic overpressure resulting from the middle Proterozoic Kheis orogeny at the western margin of the Kaapvaal craton. The oxidizing ground water required for deposition of the ores was possibly provided by the red-bed sequences overlying the Transvaal Sequence rocks. On the other hand the isotopic data of the Au deposits indicate more local derivation of the mineralizing fluids. These deposits are probably associated mainly with the hydrothermal circulation generated by the Bushveld Complex magmatism, intruded during the Kheis orogeny. Therefore, these Mississippi Valley- and vein-type Pb-Zn and Au deposits may be linked in space and time.