A stratigraphic and sedimentological study of the depositional environment of the A reef in the Ventersburg-Henneman district, Free State Goldfields

Abstract

The Ventersburg Project is located in the Free State Goldfields south of the town of Hennenman. The A Reef particularly its stratigraphic and sedimentological character is the subject of study in the project area. The reef occurs stratigraphically in the Aandenk Formation of the Central Rand Group, Witwatersrand Supergroup, South Africa. The A Reef occurs a few meters below the angular unconformity underlying the Eldorado Basal Conglomerate and above an unconformity surface between the Spes Bona and Aandenk Formations. The fact that most Witwatersrand placers occur on unconformity surfaces is well known throughout the Witwatersrand Basin (Phillips and Law, 2000). The A Reef in the project area is a low grade, shallow ore body (less than 1000 meters below surface). Intersections of the A Reef from borehole core were used to determine the facies and sub-environments. The A Reef placer is distinctly different from the underlying polymictic Big Pebble Marker comprising mature, oligomictic, small to medium pebble conglomerates and siliceous quartzites showing upwards fining cycles, typical of braided alluvial plain environments. Sediment transport directions towards the east-south-east are similar to those in the Free State Goldfields. Channels of the A Reef are highly complex, occurring in large channel complexes of more than 500 meters wide with individual smaller channels of 200 to 500 meters wide. The A Reef in the northern part of the project area is separated from the underlying Big Pebble Marker by between 1 to 3 meters of argillaceous quartzwacke. To the south, the A Reef is directly on top of the Big Pebble Marker. Mineralisation in the A Reef decreases to the south where it sub-outcrops against the Eldorado Basal Conglomerate and only localised higher gold grades occur within the Big Pebble Marker. At increased high gold prices, previously uneconomic ore bodies can become renewed targets for exploration in the near future.