New research by Depiné et al. published in Mineralium Deposita provides critical evidence to the origin of gold and uranium in the Archean Witwatersrand Supergroup in South Africa. The Witwaterstrand Basin in South Africa accounts for 40% of the world’s gold and is a critical source of uranium for the the planet (Frimmell, 2008). There are two competing models for the origin of the gold and uranium in the Witwatersrand. The first model, the paleoplacer model, suggests that the gold and uranium were derived from weathering of Archean basement rocks, and subsequently deposited as detrital grains in conglomerates (i.e., a paleoplacer model; Minter, 1999). The second model suggests that the uranium and gold were deposited after conglomerate deposition and were introduced by hydrothermal fluids (i.e., hydrothermal model; Phillips and Powell, 2011).
Depiné et al. utilize a combination of petrography, scanning electron microscope (SEM), electron microprobe, and laser ablation inductively coupled plasma mass spectrometry of uraninite grains that are associated with gold to provide support for a detrital origin for both the uraninite and gold in the Witwatersrand Basin. Depiné et al. utilize these techniques to show that the gold and uranium were deposited together, and they also obtain mineral chemical data, including rare earth element (REE) data, on uraninite grains associated with gold. Their work builds on the work of Mercadier et al., who recently illustrated that uraninite derived from different sources have distinct rare earth element signatures. In particular, Mercadier’s work illustrates that deposits formed from igneous sources have higher REE contents than those from hydrothermal sources.
Depiné et al.’s work illustrate that REE in uraninites from Witwatersrand have very high REE concentrations consistent with formation from high temperature, igneous sources, and have values that are much higher than hydrothermally derived uraninite (Figure 1). Furthermore, the uraninite have enrichments in elements commonly associated with magmatic activity (e.g., Bi, W; Figure 2). These features suggest that the uraninite are unlikely of hydrothermal origin, but were derived from igneous sources.
Figure 1. Concentration of rare earth elements versus the light rare earth element(LREE) to heavy rare earth element (HREE) ratio. Notably the Witwatersrand uraninites have very high REE and are indicative of a high temperature origin. From Depiné et al. (2013).
Figure 2. Coloured element maps of various elements for grains of uraninite from Witwatersrand. Note that the grains have enrichments in Bi and W, two elements commonly associated with igneous activity. From Depiné et al. (2013).
The occurrence of uraninite in the conglomerate units with igneous sources is consistent with the uraninite, and by association gold, being derived from weathering of Archean igneous basement rocks and subsequent deposition in conglomerate beds. Assuming that the work of Mercadier et al. holds up to further tests, it implies that the paleoplacer model for the Witwatersrand is the best model to explain gold and uranium enrichment in this important mining district.