The Stratigraphy, Sedimentology and Gold Distribution of the VSS/Beatrix Composite Reef at H.J. Joel Gold Mine, Orange Free State, Goldfield, Republic of South Africa
The zone of economic interest at H.J. Joel Gold Mine is the VSS/Beatrix Composite Reef, a quartz-pebble conglomerate of Archean age. The reef occurs at the base of the Eldorado Formation, Central Rand Group, of the Witwatersrand Supergroup. At least three stratigraphically distinct conglomerates form this economic zone. In order of deposition these are the Footwall, Beatrix, and the VSS/Beatrix reefs. The Footwall and Beatrix reefs are equated with the Aandenk Formation with transport directions toward the northwest, whereas the VSS/Beatrix Reef is of Eldorado age, with transport directions toward the south. The change in the drainage direction was caused by a major basin-wide tectonic event that occurred prior to the deposition of the Eldorado Formation. The Beatrix Reef was formed in a proximal, braided, fluvial environment by a process of aggrading gravel bars between channelways. The VSS/Beatrix Reef was formed by the desegregation of pre-existing Beatrix Reef with the introduction of new material containing much shale detritus and is seen to have been deposited in a more distal, braided, fluvial environment, similar to the Beatrix Reef.
The average gold concentrations are different for each conglomerate; the Beatrix at 32.1 ppm Au, the Footwall at 24.4 ppm Au, and the VSS/Beatrix at only 12.4 ppm Au. A sedimentary gold concentrating process related to higher energy braid channels was primarily responsible for these high gold grades. Not only the basal unconformity surface, but also, internal degradation surfaces, were prime sites for concentration. An understanding of the spatial distribution of these differing conglomerates and their braid channels forming the economic reef zone is necessary in predicting ore grades for future mining.