Introduction

The Witwatersrand Basin is a large underground geological formation which lies on the Kaapvaal Cratonwhich surfaces in the Witwatersrand, South Africa. The name “Witwatersrand” is Afrikaans for “White Waters Ridge”. The name was derived from the white quartzite ridge which strikes parallel to the edge of the basin in which the sediment was deposited.

The Witwatersrand Basin is approximately 400km long and 200 km wide traversing across the Free State, North West and Gauteng provinces in South Africa. The Basin holds the world’s largest known gold reserves and has produced over 2 billion ounces over its existence (Source: Chamber of Mines of South Africa).

Witwatersrand Basin Geology

The entire series of rocks, known as the “Witwatersrand Supergroup’’ consists of quartzites, banded ironstones, mudstones, tillites, conglomerates and some marine lava deposits. Most of the basin is deeply buried under younger rocks, but outcrops occur in Gauteng, the Free State, as well as in some of the surrounding Provinces. The outcrop in Gauteng forms the Witwatersrand ridge, from which the basin and its rocks derive their name. It was on the southern portion of this ridge that gold was first discovered on the farm Langlaagte in 1886, 5 km west of Johannesburg.

Since this gold was embedded in a conglomerate, it was first assumed that this was alluvial gold in an old riverbed that had been tilted as a result of earth movements. However, when it was found that, traced down-dip, the conglomerate was not merely developed for the narrow width of a river, but continued in depth, there came the realization that this conglomeratic zone was part of a sedimentary succession. The conglomerate was quickly traced east and westward for a total continuous distance of 50 km to define what became known as the – Central Rand Gold Field.

It has since been established that the rocks that make up the Witwatersrand Ridge dip downwards and southwards to form the largely underground “Witwatersrand Basin” which covers an elliptical area with a 300 km long major axis from Evander in the north-east to Theunissen in the south-west, and 150 km wide stretching from Steynsrus in the south-east to Coligny in the north-west, with a small subsidiary basin at Kinross (Source: The Chamber of Mines of South Africa).

Witwatersrand Basin
Figure 1: Witwatersrand Basin

 

Gold Mineralisation at Witwatersrand 

Gold occurs only along the northern and western margins of this basin, but not in a continuous band. The gold bearing rocks are limited to 6 sites where Archean rivers from the north and west formed fan deltas, with many braided channels, before flowing into the “Witwatersrand Sea” to the south, where the earlier sediments that form the older rocks of the Witwatersrand Supergroup had been deposited.

Some of these gold bearing fan deltas are possibly the deepest mines in the world and after 120 years of mining on the Kaapvaal Craton, operations in this area have now reached depths of up to 4,000 meters below the surface.

The gold mines in this area are situated around an ancient sea (over 2700 million years old) where rivers deposited their sediments in the form of sand and gravel which became the conglomerate containing the gold.

This gold-bearing conglomerate, usually grading no more than 15g/t (0.48 oz) and often much less, stretches from 65 kilometers east of Johannesburg to 145 kilometers west, then swings down south-west to the Orange Free State 320 kilometers away. Another field, Evander, was found much later 130 kilometers south-east of Johannesburg, outside the main Witwatersrand system.

The Witwatersrand reefs were deposited between 3,000 and 2,700 million years ago. The reefs vary widely, but the majority are conglomerate, with pebbles of quartz and chert in a matrix of quartz grains, silicate and various sulphides, mainly pyrite. They range from thin, small-pebble reefs, often with great lateral extent, to thick conglomerate.

Many reefs are the product of reworking by fluvial agencies, while others have features compatible with formation on beach-like surfaces in the environment of lakes and seas. They were all formed under shallow water. A large number of different reefs are mined, including the Main Ventersdorp Contact, Kimberley, Carbon Leader and Basal reefs.

The reefs, averaging 20-30 centimeters are extracted from stopes around one meter high at planned depths – rather like extracting a thin slice of meat from a sandwich. The narrow stoping widths and mining hard rock reefs at extreme depths have given the South African industry its unique character。

Source: www.info.goldavenue.com

List of mining companies operating within the Witwatersrand Basin

Today there are many exploration companies prospecting in South Africa, such as:

South Africa Gold Production History

South Africa Gold Production History
Figure 2: South Africa Gold Production History (Source: United States Geological Survey)

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