Ages of altered granites adjoining the Witwatersrand Basin with implications for the origin of gold and uranium

Abstract

ONE of the main problems in understanding the origin of the Witwatersrand Basin mineralization concerns the source of the prodigious concentrations of gold and uranium in the basin; another problem is the age and nature of the rocks in the source area. Recent data1,2 indicate that the source area was largely 3,200–2,900 Myr old, becoming progressively younger as the basin evolved. Granites adjoining the basin are commonly overprinted by pervasive and vein-related hydrothermal alteration, which resulted in an enrichment of both gold and uranium3,4. Here we report uranium-lead data demonstrating that these hydrothermally altered granites were intruded sporadically, both before and during the 360-Myr period5 constraining Witwatersrand sediment deposition, and that hydrothermal alteration was associated with granitoid intrusion and subsequent cooling. Thus, 'fertilization' of the crust occurred during basin evolution, a feature that supports the idea19 that gold and uranium mineralization occurred simultaneously with sediment deposition.