|Elevation 1,913 m (6,276 ft)|
Width 10 km (6.2 mi) NNE/SSW
|Length 56 km (35 mi) ESE/WNW|
Country South Africa
|Provinces North West, Gauteng and Mpumalanga|
Orogeny Vredefort meteor impact crater
The Witwatersrand aka The Reef, is a 56-kilometre-long north-facing scarp in the Gauteng Province of South Africa. It consists of a hard, erosion-resistant quartzite sedimentary rock, over which several north-flowing rivers form waterfalls, which account for the name “Witwatersrand”, meaning the “ridge of white waters” in Afrikaans. This east–west-running scarp can be traced with only one short gap from Bedfordview (about 10 km west of O.R. Tambo International Airport) in the east, through Johannesburg and Roodepoort, to Krugersdorp in the west (see the diagram bottom-left below).
- Geology of the Witwatersrand ridge
- Geological origin
- Consequences of mining the ancient Witwatersrand rocks
The scarp forms the northern edge of a 7- to 10-kilometre-wide plateau (or ridge) which rises about 200 m above the surrounding plains of the Highveld. The entire plateau-like structure is also often called the “Witwatersrand”. The plateau’s elevation above sea-level is between 1700–1800 m.
The Witwatersrand plateau forms a continental divide with the run-off to the north draining into the Indian Ocean via the Crocodile and Limpopo Rivers, while the run-off to the south drains via the Vaal into the Orange River and ultimately into the Atlantic Ocean.
The Witwatersrand lies within the province of Gauteng, formerly called the “PWV”, an abbreviation for Pretoria, "Witwatersrand" and Vereeniging. When used in this latter context, the term Witwatersrand refers to the entire Greater Johannesburg Metropolitan Area. This conurbation is oblong in shape and runs from Springs in the east to Randfontein and Carletonville in the west, as well as Soweto to the south. This area is also often colloquially referred to as “the Rand” or “Wits”.
Because of the extraordinary quantities of gold that have been extracted from the Witwatersrand rocks, the South African currency was named the “Rand” in 1961.
Geology of the Witwatersrand ridge
The Witwatersrand plateau consists of a 5000- to 7000-metre-thick layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2970 million years ago. The entire series of rocks, known as the “Witwatersrand Supergroup’’ consists of very hard erosion resistant quartzites, banded ironstones and some marine lava deposits, interspersed with softer, more easily eroded tillites, mudstones and conglomerates. The oldest rocks (laid down 2970 million years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2710 million years ago) are those that form the southern edge of the plateau.
Gold is found in the conglomerate strata of the younger members of the Supergroup. The abundance of this gold is without equal anywhere else in the world. Over 50 000 tons have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 50% of all the gold ever mined on earth.
Not all the conglomerates contain gold, and of those that do (known as “reefs” by the miners), the gold is not uniformly distributed throughout the layer, but tends to occur in streaks, where the pebbles that make up the conglomerate are larger than elsewhere. Here the gold is associated with other minerals, especially iron pyrite and uraninite, as well as carbon rich materials such as kerogen, or bitumen, which occurs in small balls less than 1 mm in size, called “flyspeck carbon”, or as continuous layers about 10–20 mm thick. The gold-bearing conglomerates occur chiefly in the upper, younger layers of the Witwatersrand Supergroup of rocks, on the southern side of the Witwatersrand plateau.
The Witwatersrand Supergroup strata which reach the surface in Johannesburg dip downwards to the south at an angle of about 30°. From there on they are almost everywhere, with very few exceptions (see below), covered by younger rocks. Gold mining in these buried portions of the Witwatersrand Supergroup is sometimes carried out at depths of 4 kilometres (2.5 mi) below the surface.
The Kaapvaal Craton, one of the first microcontinents to form on Earth through plate tectonics, was assembled about 3900 million years ago. Its size and position relative to Southern Africa today is indicated in the diagram on the left. About 3000 million years ago local cooling of the underlying asthenosphere caused subsidence of the south-eastern portion of this microcontinent below sea level. The floor of this newly formed “Witwatersrand sea” consisted of smoothly eroded granites. Sandy sediments brought in by rivers from the north started being deposited on the granite about 2970 million years ago. This sandy layer eventually became compressed to form the Orange Grove Quartzite, the lowermost layer of the Witwatersrand Supergroup. This quartzite formation forms the scarp from which the Witwatersrand derives its name (see above).
As the sea deepened finer grained and muddy sediments accumulated.
There was no free oxygen in earth’s atmosphere till about 2000 million years ago, but there were photosynthesizing cyanobacteria. The oxygen these microorganisms produced rapidly reacted with, amongst others, any dissolved iron compounds in the water, producing insoluble red iron oxide (rust), which precipitated out during the daylight hours. At night these reactions stopped. The result was alternating red and gray layers of mud which, when consolidated, became banded ironstones.
Fluctuating sea levels resulted in the accumulation of a further variety of sediments, ranging from mud, to sand, to gravel, and banded ironstone. Tillite deposits, dating from 2950 million years ago, are indicative of the first glaciation episodes on earth. Within 60 million years, 4300 m of sediment had accumulated on the granite base, to become the “West Rand Group” of rocks that contribute over 60% of the total thickness of the Witwatersrand Supergroup.
Uplifting of the north of the Kaapvaal Craton, in addition to orogenesis (mountain formation), towards the end of the deposition of the “West Rand Group” of sediments caused the Witwatersrand sea to retreat. The area of the craton on top of which Johannesburg is now situated, became a vast riverine plain, which extended along the entire northern and western shoreline of the shrunken sea. The rivers formed braided deltas with many interlacing, slow flowing channels where all the heavy materials brought down from the mountains were deposited: large pebbles, and heavy minerals, such as gold, iron pyrite, and uraninite. The gold was in its elemental metal form. Cyanobacteria grew in relative abundance in these mineral rich waters. The kerogen that is found in association with the gold deposits almost certainly represents what remains of these Archean photosynthesizing micro-organisms.
It is clear that for the next 200 million years the flood plain was repeatedly inundated, sometimes eroded, and sediments re-deposited. The result was a 2500 m thick layer of rock that is termed the “Central Rand Group”, which together with the “West Rand Group”, forms the “Witwatersrand Supergroup”. It is the younger Group of rocks that contains the gold bearing conglomerates that are today of great economic importance. The largely underground horizontal extent of the Witwatersrand Supergroup is known as the Witwatersrand Basin.
The ultimate source of the gold is unknown. But that it is restricted to the areas of the craton’s coastal plain where the rivers flowing into the Witwatersrand sea formed braided deltas, indicates that the source was in the mountains to the north. The Welkom, Klerksdorp, Carletonville, West Rand, East Rand and Evander gold mines are all situated over these Archean fan deltas.
The “Central Rand Group” of deposits was brought to an abrupt end by massive outpourings of lava, which form the Ventersdorp lavas which erupted 2714 million years ago. The cause of these lava outpourings is a matter of speculation. It might be related to the collision of the Kaapvaal craton with the Zimbabwe craton, eventually to become knitted together to form a single continental unit.
A final event that had a major impact on the geology of the Witwatersrand Supergroup and its exposure in the Johannesburg region, was a massive meteor impact 110 km to the south-west of Johannesburg 2020 million years ago. The epicenter of the impact was close to the present village of Vredefort, which has given its name to the geological remnant of this immense event: the Vredefort Dome. Not only are the remains of this impact the second oldest on earth, but it is also the largest meteor impact that has left its imprint on the earth’s geology of today. A meteor 10–15 km across created a 300 km diameter crater, distorting all the rock strata within that circle. Johannesburg is just within the outer edge of this impact crater.
In the immediate vicinity of the impact all the subterranean strata were uplifted and upturned, so that Witwatersrand rocks are exposed in an arc 25 km away from the impact center. There are unfortunately no gold deposits in these outcrops. The meteor impact, however, lowered the Witwatersrand rocks within the crater. This protected them from erosion later on; but, possibly more importantly, bringing them to the surface close to the crater rim, near Johannesburg. In fact, apart from the Witwatersrand outcrops (i.e. where the Witwatersrand rocks are exposed at the surface) in the immediate vicinity of the Vredefort Dome, virtually all the other outcrops occur in an arc approximately 80–120 km from the center of the impact crater, to the west, north-west, north and north-east. Thus, it is possible that if it had not been for the Vredefort meteor strike 2000 million years ago, we would either have never discovered the rich gold deposits beneath the Southern African surface, or they would have been eroded away during the extensive removal of the surface of the Southern African Plateau during the past 150 million, and more especially during the past 20 million years.
Consequences of mining the ancient Witwatersrand rocks
Apart from the obvious hollowing out of the rocks below southern Johannesburg, causing unpredictable sinkholes, surface instabilities and earth tremors, the bringing to the surface of rocks that had been laid down in oxygen-free conditions had unforeseen effects. Iron pyrite (FeS2), which is relatively plentiful in the gold ores of the Witwatersrand, oxidises to insoluble ferric oxide (Fe2O3) and sulfuric acid (H2SO4). Thus, when mine waste comes into contact with oxygenated rainwater, sulfuric acid is released into the ground water. Acid mine drainage, as the phenomenon is called, has become a major ecological problem, because it dissolves many of the heavy elements, such as the uranium, cadmium, lead, zinc, copper, arsenic and mercury found in the mine dumps, facilitating their passage into surface water and ground water. Sulfuric acid also erodes concrete and cement structures, resulting in structural damage to buildings and bridges.
Although gold had been discovered in various locations in South Africa, such as Barberton and Pilgrim's Rest, as well as at several sites near the Witwatersrand, these were alluvial concentrates in contemporary rivers, or in quartz veins, in the form that gold had always been found elsewhere on earth. When George Harrison, probably accompanied by George Walker, found gold on the farm Langlaagte, 5 km west of the city of Johannesburg, in an outcrop of conglomerate rocks, in February 1886, they 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 downdip, the conglomerate was not merely developed for the narrow width of a river, but continued in depth, there came the realisation that this conglomeratic zone was part of a sedimentary succession. Harrison had stumbled on the Main Reef conglomerate (part of the “Johannesburg Subgroup” of rocks — see illustration above). 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”.
Harrison declared his claim with the then-government of the Zuid Afrikaanse Republiek (ZAR), and in September 1886 President Paul Kruger issued a proclamation declaring nine farms public mining diggings, starting on 20 September 1886. This heralded the historic Witwatersrand Gold Rush. Harrison is believed to have sold his claim for less than £10 before leaving the area, and he was never heard from again. Harrison’s original “Zoekers” (in English: seeker’s, or prospector’s) Claim No 19 was declared a national monument in 1944, and named Harrison’s Park. The park is on the busy Main Reef Road, immediately west of Nasrec Road. In 1887 Cecil John Rhodes registered “The Gold Fields of South Africa” in London, South Africa’s first mining house, with a capital of £250 000. His brother Thomas was the first chairman.