South African Gold Reef Geology

Although there were isolated reports of alluvial gold and some quartz deposits in South Africa as early as 1853 (just after the Californian and Australian gold rushes), the real discovery took place in February 1886, when George Harrison found an outcrop of the main reef of gold-bearing conglomerate on Langlaagte Farm near Johannesburg.

Cutaway of the multi-layered gold bearing reefs of the Witwatersrand
(Credit: courtesy The Chamber of Mines of South Africa)

1 Ventersdorp Contract Reef
2 Mondeor Conglomerate formation
3 Kimberly Conglomerate formation
4 Bird Conglomerate formation
5 Livingstone Conglomerate formation
6 Johnstone Conglomerate formation
7 Main Conglomerate formation
8 Crown Formation
9 Veldschoen Reef (inner basin reef)
10 Magnetic Horizon in in lower Jeppestown shale
11 Buffelsdoorn (Outer Basin reef)
12 Boulders Reef
13 Magnetic shale in Witport Jie Formation
14 Magnetic West Rand shales
15 Rietfontein tillite

The key difference between this field and the earlier Californian and Australian discoveries was that it was not an alluvial deposit which thousands of freelance gold diggers could work, but the tip of low grade reefs which could be mined only at depth and at great initial capital cost.

What outcropped on Langlaagte Farm was one of a crescent of gold reefs around the whole Witwatersrand Basin. These reefs plunge at an angle of 25 degrees or more, sloping inwards towards the centre, to depths of at least 5,000 metres (16,400 feet). Indeed, they have not yet 'bottomed out'.

Cross section of a South African mine (Credit: courtesy The Chamber
of Mines of South Africa)

This gold-bearing conglomerate, usually grading no more than 15g/t (0.48 oz) and often much less, stretches from 65 kms (40 miles) east of Johannesburg to 145 kms (200 miles) west, then swings down south-west to the Orange Free State 320 kms (200 miles) away. Another field, Evander, was found much later 130 kms (80 miles) 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.

With production of almost one million ounces a year,
Great Noligwa is Anglogold's most productive gold mine.
(Credit: courtesy AngloGold)

The reefs, averaging 20-30 centimetres (7.8-11.8 inches) are extracted from stopes around one metre (3.3 feet) 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.

An underground pumping station - an accelerator is added to the
aggregate at this stage before it is pumped out to the work area
(Credit: courtesy AngloGold)

See also: South African Mining Introduction; South African Mining History; South African Transition 1990 to 2001;

Chamber of Mines; Rand Refinery; South African Reserve Bank