The Significance of Erosion Channels on Gold Metallogeny in the Witwatersrand Basin (South Africa): Evidence from the Carbon Leader Reef in the Carletonville Gold Field

Within the eastern portion of the Carletonville gold field, the gold- and uranium-rich Carbon Leader reef of the Central Rand Group (Witwatersrand Supergroup) is truncated by an erosion channel. This channel is asymmetrical and lenticular in shape, measuring 150 to 180 m in width and up to 100 m in depth. High-resolution seismic data show that the erosion channel cuts from the Carbon Leader reef into all older units of the Central Rand Group down to the Roodepoort Formation of the underlying West Rand Group. A total of seven bore-holes were drilled into the channel, revealing that it is composed of quartzite at its base (9 m thick), overlain by deformed (lower) and laminated (upper) chloritoid-bearing shale (21 m thick) and quartzite (18 m thick). The Carbon Leader reef is highly enriched in gold (5-40 g/t Au), whereas the gold tenor of the erosion channel fill is in general much lower (<1 g/t Au), although locally grades of as much as 3.8 g/t Au are reached. Detailed seismic, sedimentological, and petrographic analyses revealed that the channel was filled with locally sourced sediments from the Main Formation. A closed-system mass balance further demonstrates that gold in the erosion channel could have been entirely sourced from the Carbon Leader reef. Sediment load played a crucial role in the distribution of gold in the channel, thus supporting a stratigraphically controlled modified placer model for the origin of gold in the Carbon Leader reef.