AGE AND PROVINCES OF PRECAMBRIAN MVT MINERALIZATION, TRANSVAAL SUPERGROUP, SOUTH AFRICA

The Transvaal Supergroup of South Africa hosts the oldest known MVT deposits in the world, including the Bushy Park and Pering sphalerite districts, which lack fluorite, and the Zeerust fluorite district, with locally important galena and sphalerite. Geologic relations indicate that the MVT mineralization is pre-2.0 Ga. We have attempted to date sphalerite from Bushy Park using the Rb-Sr method and fluorite from Zeerust using the Sm-Nd method, and to determine whether the districts were part a single large MVT province.

Rb and Sr concentrations in sphalerite from Bushy Park are less than 10 ppb, two to three orders of magnitude below those in sphalerite that has been dated from other MVT deposits, and the isotopic compositions do not generate an isochron. 87Sr/86Sri values calculated for 2.0 Ga are about 0.716, suggesting that strontium in the mineralizing fluid came from an older source with a high Rb/Sr ratio, probably paleoaquifers in the underlying Ventersdorp Group. Nd concentrations in fluorite at Zeerust range from about 0.50 to 50 ppm. All samples have small negative Eu anomalies and no Ce anomalies, and coarse-grained fluorites show LREE depletion relative to fine-grained and disseminated fluorites. The Sm-Nd isotopic data generate a 4-point isochron of 2057±42Ma (MSWD=1.6) and 143Nd/144Ndi=0.509721±0.000059, which is essentially identical to the 2.05-2.06 Ga Bushveld Complex that produced a contact metamorphic aureole in the district. Although it is possible that hydrothermal activity related to the Bushveld Complex remobilized earlier fluorite, similar initial ratios for Zeerust fluorite and the F-rich, felsic (Rooiberg) part of the Bushveld Complex suggest that Zeerust REE and F are of magmatic origin.

Sulfur isotope compositions for Zeerust pyrite and pyrrhotite (1.1 to 9.2‰) and Bushy Park sphalerite and galena (15.2 to 19.7‰ and one outlier of 24.1‰) indicate diagenetic and seawater sources, respectively, for sulfur in the two districts. Combined with the close relation of Zeerust to the Bushveld Complex, these data suggest that the Bushy Park-Pering and Zeerust districts were separate Paleoproterozoic MVT systems.