Summary:
Mineralisation style: Palaeo-placer and braided stream-type sedimentary deposit scavenged gold from the hinterland and underlying deposits. Mineralisation is concentrated in robust conglomeratic and carbonaceous units.
Mineralisation characteristics: Gold is associated with disseminated sulphides in the form of pyrite. High grades occur where the reef is characterised by carbon specks or bands.
Evander Mines exploits the Kimberley Reef in the Evander Basin, the eastern-most extremity of the Witwatersrand Supergroup. The Kimberley Reef is mined throughout the major gold mining districts within the Witwatersrand Supergroup, including the East Rand, Central Rand, West Rand, Far West Rand and Free State goldfields. Deposition models for gold within the conglomeratic horizons follow a palaeoplacer-type sedimentological deposition along with winnowing, erosion and concentration of gold-bearing footwall lithologies. Various studies have highlighted the importance of hydrothermal activity for deposition, remobilisation and enrichment within certain packages of the Witwatersrand Supergroup.
The Kimberley Reef is an oligomictic, pebbly conglomerate and comprises a composite sequence of channel sediments that define longitudinal gravel bars and sand bars with pebbly veneers. The reef in the area strikes in an east-west direction and dips to the north at about 10°. The area is also divided by two major normal faults, striking in an east-north-east to west-south-west direction. The reef thickness varies from a waste on contact up to a 50cm well-developed oligomictic conglomerate. Average reef thickness is 35cm within the 8 Shaft vicinity. High gold values in the Kimberley Reef are mostly located at the base of the unit and are associated with the presence of carbon and some visible gold on the footwall contact.
Evander Mines’ 7 Shaft – Egoli project
The Kimberley Reef in the area strikes in an east-west direction and dips to the north at about 10°. The reef thickness varies from a waste on contact up to a 50cm well-developed oligomictic conglomerate. Average reef thickness is 30cm within the 7 Shaft vicinity. High gold values in the Kimberley Reef are mostly located at the base of the unit and are associated with the presence of carbon and some visible gold on the footwall contact.
The Egoli project orebody is a defined high-grade fluvial channel (payshoot) and is a large orebody of world-class proportions. This payshoot represents a divergent fluvial channel which forms part of the basin-wide Kimberley Reef deposition system. The Kimberley Reef has been mined from the Evander goldfield in the east as well as at operations in the Welkom goldfield in the west (an extent of over 400km). Locally, the Egoli project payshoot is comparable with the currently mined Kinross payshoot (at the 8 Shaft) in geology, mineralisation as well as time and style of deposition. The successfully mined Kinross payshoot is therefore analogous to the Egoli project payshoot and has been deposited by the same fluvial system with the same source areas of sediment.
Rolspruit project
The Kimberley Reef strikes in an east-west direction and dips at 28° to the north and has been intersected at an average depth of 2,300m below surface. The footwall sill break is an intrusive sill that is associated with a fault, which resulted in a 90m displacement of the reef horizon. The Kimberley Reef at Rolspruit is a well-developed oligomictic conglomerate up to 1m thick, averaging about 37cm. In this area, the Kimberley Reef is very similar to that of the 8 Shaft with high gold grade values mostly located at the base of the unit and associated with the presence of carbon and some visible gold on the footwall contact.
Poplar project
The Kimberley Reef occurs at a depth below surface of between 500m in the west and 1,200m in the east. The reef strikes north-south and dips 9° to 24° to the east. The Kimberley Reef comprises a sequence of fluvial channel sediments that were deposited in a braided stream environment. Deposition of the reef was influenced by the footwall lithologies. The Kimberley Reef horizon has a channel width of approximately 30cm, generally a thin reef hosting high gold grades. The reef has north-east to south-west trending channels or payshoots which are evident in other parts of the Evander Basin.
A series of seven major, subparallel and evenly spaced faults traverse the property. These are all orientated in a roughly north-north-east to south-south-west direction. Throws on these faults vary between 50m and 400m.
Evander South project
The Kimberley Reef at Evander South occurs at a depth of between 300m in the west and 1,200m in the east, below surface, with a north-south strike and dips between 6° and 19°. It comprises a sequence of fluvial channel sediments that were deposited in a braided stream environment with final deposition influenced by the footwall lithologies. The high-grade Kimberley Reef is associated with carbon and is a narrow, small pebble, clast-supported and well-packed oligomictic conglomerate. Carbon was observed in several of the borehole intercepts drilled at the Evander South project.