Trace-element and UPb isotope compositions of pyrite types in the Proterozoic Black Reef, Transvaal Sequence, South Africa: Implications on genesis and age

The Proterozoic Black Reef is, in most cases, indistinguishable from the gold-, uranium- and pyrite-bearing Archaean Witwatersrand reefs. However, the prevailing opinion that the Black Reef gold mineralization was largely derived from reworking of the underlying Kimberley Reef is not supported by the morphology, trace-element geochemistry and UPb isotope data on detrital pyrite in these reefs. The grade of metamorphism recorded in the Black Reef Quartzite Formation is significantly lower than the 300–400°C that has been proposed for the sediments of the Witwatersrand Supergroup. The UPb isotope and trace-element geochemical signatures in pyrite have, therefore, been preserved in the Black Reef. The morphological and compositional differences retained by the detrital and synsedimentary pyrite types, both occurring together in the Black Reef, imply that they were formed by different processes and in contrasting geochemical environments. In the Black Reef, the data for the detrital pyrite are consistent with their derivation from a ∼ 3000-2800-Ma granitic, probably hydrothermally altered, source region. By contrast, the geochemical and Pb-isotopic composition of the concretionary pyrite is interpreted to reflect its synsedimentary formation in a lagoonal or tidal flat environment prior to ∼ 2500 Ma. The morphology, primary mineral inclusions and the unradiogenic Pb-isotopic compositions preserved in the Black Reef detrital pyrite are irreconcilable with models proposed for the formation of Witwatersrand pyrite by a process of pyritization of Fe-bearing detrital minerals. Fluids present in the Black Reef depositional/diagenetic environment were anomalously enriched in radiogenic Pb and any transformation process would not be able to escape acquiring this enriched Pb-isotopic signature.