Gold Provenance of the Black Reef Conglomerate, West and East Rand, South Africa Christoph D K Gauert Department of Geology, University of the Free State, Bloemfontein, South Africa, email: Gauertcdk@ufs.ac.za Michael Brauns Curt-Engelhorn-Centre for Archaeometry at Mannheim, University of Tübingen, Germany David Batchelor, Rolf Simon Karlsruhe Institute for Technology, Institute for Synchrotron radiation, Germany Abstract. The origin of the gold, the uranium and the PGEs in the basal conglomerate of the palaeoproterozoic Black Reef Formation of the Transvaal Supergroup in South Africa is currently debated due to its economic significance as a gold ore body. Both the heavy mineral spectra and the geochemical trace element fingerprint of the Gold are used to unravel the origin of the Black Reef Gold. Based on EMP, LA-ICP-MS and SR-μ-XRF measurements Black Reef and Witwatersrand gold can be distinguished by means of their different degree of true fineness, the Hg and Cu concentrations, and the S, and Ni trace element content. Among the elements which are correlatible with gold, Sn, Sb, Pd, and Pt, possibly in combination with Mn, Se, Pb, and Ir appear to be the most effective element distinction of gold sources. The Black Reef at Consolidated Modderfontein on the East Rand appears to have a different gold source than the underlying Witwatersrand reefs. Keywords: Black Reef conglomerate, gold fingerprint, sediment provenance, South Africa 1 Introduction The Palaeoproterozoic (2642-2584 Ma) Black Reef Formation (BR) is a widespread thin small pebble unit at the base of the Transvaal Supergroup in South Africa, locally as well–defined, deeply scoured channels and gullies eroded into the footwall (Eriksson et al., 2006). In the East Rand, Randfontein and Klerksdorp areas the unit contains considerable gold, less U, as well as minor associated PGE mineralization, exploited since the 1930s and recently since 2002 by Gold One International Ltd. quoting a probable reserve of 5.4 Mt of ore @ 6.1 g/t Au for Modder East Mine (2009; indicated resource: 28.8 Mt @ 2.84g/t Au and 15 Mt @ 2.19 g/t Au, information from company websites). Henry and Master (2008) identified shortcomings in our understanding of the genesis of Au mineralization, and the occurrence of Au, U and PGEs hosted by the Black Reef (BR), and their provenance. The depositional age for the Black Reef is constrained between around 2718 Ma (Armstrong et al., 1991), the age of the underlying Ventersdorp Supergroup, and around 2588 Ma, the age of the overlying Oaktree Formation stromatolitic limestones (Martin et al., 1998). The source of gold in these reefs has long been a matter of speculation. Figure 1. Locality map of the Black Reef sections under investigation in the Transvaal Supergroup of NE-South Africa. Boxes indicate areas of investigation being sampled (modified after Eriksson et al., 2006)