A Review of the Witwatersrand Basin - The World’s Greatest Goldfield

Abstract

The world's greatest deposit of gold is hosted by the Archaean Witwatersrand sedimentary basin, situated in the central portion of the Kaapvaal Craton of South Africa. The geological setting of this remarkable clastic sedimentary deposit, which has yielded more than one third of all the gold ever produced on the planet, is discussed. The stratigraphy and structure of the Witwatersrand Supergroup is reviewed together with the sedimentology, mineralogy and geochronology of the more important auriferous conglomerate (reef) horizons. Geophysical methods of exploration are discussed briefly. The Witwatersrand Basin was formed in response to a series of crustal plate movements from the north and west in a foreland basin setting. Deposition commenced in a shallow marine environment with quartzite and shale, including prominent iron rich shale horizons forming the West Rand Group (WRG). The overlying dominantly arenaceous Central Rand Group (CRG) was deposited in a basin which regressed as it filled. Important gold deposits formed on braided fluvial plains which sometimes terminated in a shoreline. Gold reefs at the base of the CRG show evidence of later marine reworking. Higher in the sequence, fluvial deposits become dominant. The focus of this paper is the Central Rand Group where most of the gold mineralisation occurs. Detrital gold (largely within sulphides) and uranium (as uraninite) were originally concentrated along with placer minerals such as chromite and zircon, at the base of fluvial cycles. In addition to the placer mineralisation, prokaryotic bacteria flourished and provided carbon-rich material that was highly effective in the concentration of non-detrital gold. Although most of the lithologies, including reef horizons, have great continuity and can be correlated across much of the Witwatersrand Basin, discrete entry points of reef material, with distributary channels and associated ore shoots, can generally be clearly defined. Three major events reshaped and metamorphosed the Witwatersrand. Lateral crustal plate movements caused severe faulting and folding, especially along the western and northern sides of the Basin. The second event was the catastrophic Vredefort meteorite impact which struck the centre of the basin. The resulting fluid movement which pervaded the Witwatersrand strata caused widespread metasomatic alteration. Localised alteration and remobilisation of the gold mineralisation is thought to have been in partial response to this event. The third major event was the emplacement of the Bushveld Complex, which is the largest layered igneous intrusion of its kind on Earth. The heat generated by this intrusion certainly exceeded that of the Vredefort event, also making it an important agent of mineral alteration and remobilisation, especially along the northern flank. The Great Debate on the origin of the gold has been the subject of heated discussions for decades. The three main points of view are a placer source, a hydrothermal source and a combination of the two called the modified placer theory. Recognition of the problem was realised soon after the discovery of the gold and continues to this day. Evidence has been presented in favour of all arguments, lending more credence for the modified placer process. Although the Witwatersrand is a mature goldfield with declining production, it is estimated that it still contains six times more gold than the world's second largest goldfield. Much of the remaining resource occurs at considerable depths; however, there are still opportunities for extracting some of this resource, as well as generally somewhat lower grade mineralisation, at moderate to shallow depth. The Basin thus remains a major exploration target. Gold mining in the Witwatersrand Basin has been responsible for the creation of the metropolis of Johannesburg and many thriving mining towns and has had a major influence in the shaping of South Africa. It still plays an important role in the country's economy.