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The Witwatersrand
Witwatersrand
(locally the Rand or, less commonly, the Reef) is a 56-kilometre-long (35 mi), north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which account for the name Witwatersrand, meaning "ridge of white waters" in Afrikaans.[1] This east-west-running scarp can be traced with only one short gap, from Bedfordview
Bedfordview
(about 10 km [6 mi] west of O.R. Tambo International Airport) in the east, through Johannesburg and Roodepoort, to Krugersdorp
Krugersdorp
in the west (see the diagram at left below).[2] The scarp forms the northern edge of a 7-to-10-kilometre-wide (4–6 mi) plateau (or ridge) which rises about 200 m (660 ft) 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 and 1800 metres (5600–5900 ft). The Witwatersrand
Witwatersrand
plateau forms a continental divide, with the run-off to the north draining into the Indian Ocean
Indian Ocean
via the Crocodile and Limpopo rivers, while the run-off to the south drains via the Vaal into the Orange River
Orange River
and ultimately into the Atlantic Ocean.[2][3] Because of the extraordinary quantities of gold that have been extracted from the Witwatersrand
Witwatersrand
rocks, the South African currency was named the Rand in 1961.

Contents

1 Geology of the Witwatersrand
Witwatersrand
ridge

1.1 Geological origin

2 Consequences of mining the ancient Witwatersrand
Witwatersrand
rocks 3 History 4 See also 5 Further reading 6 References 7 External links

Geology of the Witwatersrand
Witwatersrand
ridge[edit] Main article: Witwatersrand
Witwatersrand
Basin

A diagrammatic north-south cross section through the Witwatersrand ridge/plateau, below the Johannesburg
Johannesburg
city center. Only the major subgroups of rocks are shown, each of which is made up of many separate layers of varying composition. The harder layers form the hills, and the softer layers the valleys. The oldest rocks of the Witwatersrand
Witwatersrand
Supergroup form the Orange Grove Quartzite
Quartzite
layer. This forms the scarp in the north, from which the Witwatersrand
Witwatersrand
derives its name. The granite to the north of the Orange Grove Quartzite
Quartzite
scarp is around 3.2 billion years old, and is an exposed portion of the underlying Kaapvaal Craton
Kaapvaal Craton
on which a large portion of South Africa rests. (See illustration below.) To place this diagram into its broader context, see the diagrammatic geological cross section through the Vredefort crater
Vredefort crater
at the end of the article below.

The Witwatersrand
Witwatersrand
plateau consists of a 5000-to-7000-metre-thick (3.1–4.3 mi) layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2.97 billion years ago.[4] The entire series of rocks, known as the " Witwatersrand
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 2.97 billion years ago) form the northern scarp of the Witwatersrand
Witwatersrand
plateau; the youngest (laid down 2.71 billion years ago) are those that form the southern edge of the plateau. Gold
Gold
is found in the conglomerate strata of the younger members of the Supergroup, locally referred to as banket. The abundance of this gold is without equal anywhere else in the world. Over 50000 tonnes (55000 short 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.[2] 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 (0.04 in) in size, called “flyspeck carbon”, or as continuous layers about 10–20 mm (0.4–0.8 in) thick.[1][4] The gold-bearing conglomerates occur chiefly in the upper, younger layers of the Witwatersrand
Witwatersrand
Supergroup of rocks, on the southern side of the Witwatersrand
Witwatersrand
plateau. The Witwatersrand
Witwatersrand
Supergroup strata which reach the surface in Johannesburg
Johannesburg
dip downwards to the south at an angle of about 30°.[2] From there on they are almost everywhere, with very few exceptions (see below), covered by younger rocks.[5] Gold
Gold
mining in these buried portions of the Witwatersrand
Witwatersrand
Supergroup is sometimes carried out at depths of 4 kilometres (2.5 mi) below the surface.[4][6] Geological origin[edit]

A diagrammatic representation of the position and size of the Kaapvaal Craton, outlined in red, in relation to present-day Southern Africa. The blue area depicts the portion of the craton that subsided below the “ Witwatersrand
Witwatersrand
Sea”, about 3000 million years ago. It is in this sea that the sediments accumulated that would ultimately form the “ West Rand
West Rand
Group” portion of the “Witwatersrand Supergroup” of rocks. The younger “Central Rand Group” of rocks accumulated on the low, flat coastal plain (see the diagram below) after the Witwatersrand
Witwatersrand
Sea had retreated southwards as a result of uplifting of the craton, especially in the north. CT indicates Cape Town, D Durban, B Bloemfontein, J Johannesburg, and K Kimberley.

An ashtray carved out of a soft form of banded ironstone. Note the alternating red and white layers that make up this rock. The red layers were laid down during the daylight hours when Archaean photosynthesizing cyanobacteria produced oxygen that immediately reacted with the dissolved iron in the water, to form insoluble iron oxide (rust). The white layers are sediments that settled during the night when there was no oxygen in the water. The earth’s atmosphere was oxygen-free until about 2 billion years ago, when the rate of photosynthetic oxygen production began to exceed its rate of reaction with oxidizable materials (i.e. reducing agents).[7]

A diagrammatic presentation of the rivers that flowed into the Witwatersrand
Witwatersrand
sea after 4300 m (14100 ft) of sediment had already been deposited in the basin. The fast flowing rivers cascading down the mountains to the north now flowed over a wide flat coastal plain to form broad deltas of sluggish braided rivers, where the heavy materials (cobbles, gold, uranium and iron pyrite etc.) carried down from the mountains settled out, to form the gold bearing "Central Rand Group" deposits. Today's gold ore is confined to the fossil river deltas.

The position of the Kaapvaal Craton
Kaapvaal Craton
(khaki coloured area) beneath the South African landscape, and the shrunken, shallow Witwatersrand
Witwatersrand
Sea (light blue) at the time that gold was deposited in the broad, river deltas of six rivers that flowed into that sea, depositing all their heavier materials (cobbles, gold, uranium iron pyrite etc.) in the braided rivers of the deltas (see illustration on the left). Most of these gold deposits are deep under the South African surface, but form outcrops (exposures at the surface) along the length of the Witwatersrand
Witwatersrand
ridge. The six gold fields thus established are, in order from the west, moving clockwise, to the northern shore of the Witwatersrand
Witwatersrand
sea, Welkom, Klerksdorp, Carletonville, West Rand, East Rand and Evander.[1]

The Kaapvaal Craton, one of the first microcontinents to form on Earth through plate tectonics, was assembled about 3.9 billion years ago. Its size and position relative to Southern Africa today is indicated in the diagram on the left. About 3 billion years ago local cooling of the underlying asthenosphere caused subsidence of the south-eastern portion of this microcontinent below sea level.[4] The floor of this newly formed “ Witwatersrand
Witwatersrand
sea” consisted of smoothly eroded granites. Sandy sediments brought in by rivers from the north started being deposited on the granite about 2.97 billion years ago. This sandy layer eventually became compressed to form the Orange Grove Quartzite, the lowermost layer of the Witwatersrand
Witwatersrand
Supergroup. This quartzite formation forms the scarp from which the Witwatersrand
Witwatersrand
derives its name (see above). As the sea deepened finer grained and muddy sediments accumulated. There was no free oxygen in the Earth’s atmosphere till about 2 billion years ago, but there were photosynthesizing cyanobacteria.[7] 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.[7] Fluctuating sea levels resulted in the accumulation of a further variety of sediments, ranging from mud, to sand, to gravel, and banded ironstone. Tillite
Tillite
deposits, dating from 2.95 billion years ago, are indicative of the first glaciation episodes on earth.[4][8] Within 60 million years, 4300 m (14100 ft) of sediment had accumulated on the granite base, to become the “ West Rand
West Rand
Group” of rocks that contribute over 60% of the total thickness of the Witwatersrand
Witwatersrand
Supergroup.[2][4][8] Uplifting of the north of the Kaapvaal Craton, in addition to orogenesis (mountain formation), towards the end of the deposition of the “ West Rand
West Rand
Group” of sediments caused the Witwatersrand
Witwatersrand
sea to retreat. The area of the craton on top of which Johannesburg
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.[2][4][8] The kerogen that is found in association with the gold deposits almost certainly represents what remains of these Archean
Archean
photosynthesizing micro-organisms.[4]

A timeline of the earth's geological history, with an emphasis on events in Southern Africa. W indicates when the Witwatersrand supergroup was laid down, C the Cape supergroup, and K the Karoo Supergroup. The graph also indicates the period during which banded ironstone formations were formed on earth, indicative of an oxygen-free atmosphere. The earth's crust was wholly or partially molten during the Hadean
Hadean
Eon; the oldest rocks on earth are therefore less than 4 billion years old. One of the first microcontinents to form was the Kaapvaal Craton.

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 (8200 ft) thick layer of rock that is termed the “Central Rand Group”, which together with the “ West Rand
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
Witwatersrand
Supergroup is known as the Witwatersrand
Witwatersrand
Basin. The ultimate source of the gold is unknown.[2][4] But that it is restricted to the areas of the craton’s coastal plain where the rivers flowing into the Witwatersrand
Witwatersrand
sea formed braided deltas,[1] indicates that the source was in the mountains to the north. The Welkom, Klerksdorp, Carletonville, West Rand, East Rand
East Rand
and Evander gold mines are all situated over these Archean
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 2.714 billion years ago.[4] The cause of these lava outpourings is a matter of speculation. It might be related to the collision of the Kaapvaal craton
Kaapvaal craton
with the Zimbabwe craton, eventually to become knitted together to form a single continental unit.

A schematic diagram of the area surrounding the Vredefort Dome, where a massive meteor created an impact crater 300 km (190 mi) in diameter 2.02 billion years ago. The red dot represents the epicenter of the impact. The outer circle has a radius of 150 km (93 mi), and indicates the approximate location of the crater rim. The inner circle marks the 100 km (62 mi) distance from the center. Note that the outcrops (exposure of the rocks on the surface) of Witwatersrand
Witwatersrand
rocks (yellow areas) are located at 25 km (16 mi) from the epicenter of the impact and then again at about 80–120 km (50–75 mi) from the epicenter. The locations of important towns and cities in the region are indicated in the appropriate places. The red line in the detail of the Johannesburg
Johannesburg
region shows the location of the scarp/ridge that gave the "Witwatersrand" its name; the purple line the location where the main gold bearing reef is exposed at the surface, just south of Johannesburg
Johannesburg
.

A final event that had a major impact on the geology of the Witwatersrand
Witwatersrand
Supergroup and its exposure in the Johannesburg
Johannesburg
region, was a massive meteor impact 110 km (68 mi) to the southwest of Johannesburg
Johannesburg
2.02 billion years ago.[2][4] 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.[2][4] A meteor 10–15 km (6–9 mi) across created a 300 km (190 mi) diameter crater, distorting all the rock strata within that circle. Johannesburg
Johannesburg
is just within the outer edge of this impact crater.

A schematic diagram of a NE (left) to SW (right) cross-section through the 2.02 billion-year-old Vredefort impact crater and how it distorted the contemporary geological structures. The present erosion level is shown. Johannesburg
Johannesburg
is located where the Witwatersrand
Witwatersrand
Basin (yellow layer) is exposed at the "present surface" line, just inside the crater rim, on the left. Note the present exposure of the basement granite, known as the " Johannesburg
Johannesburg
Dome", belonging to the Kaapvaal craton, to the north of Johannesburg
Johannesburg
city center. Not to scale.

In the immediate vicinity of the impact all the subterranean strata were uplifted and upturned, so that Witwatersrand
Witwatersrand
rocks are exposed in an arc 25 km (16 mi) away from the impact center. There are unfortunately no gold deposits in these outcrops. The meteor impact, however, lowered the Witwatersrand
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.[4] In fact, apart from the Witwatersrand
Witwatersrand
outcrops (i.e. where the Witwatersrand
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 (50–70 mi) from the center of the impact crater, to the west, northwest, north and northeast.[5] Thus, it is possible that if it had not been for the Vredefort meteor strike 2 billion 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.[4] Consequences of mining the ancient Witwatersrand
Witwatersrand
rocks[edit] Apart from the obvious hollowing out of the rocks below southern Johannesburg, causing unpredictable sinkholes, surface instabilities and earth tremors,[9] 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.[4][9][10] Sulfuric acid
Sulfuric acid
also erodes concrete and cement structures, resulting in structural damage to buildings and bridges.[9][10] History[edit]

The street entrance of George Harrison Park

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 (3 mi) west of what would become 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.[1][2] 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.[1] Harrison had stumbled on the Main Reef conglomerate (part of the “ Johannesburg
Johannesburg
Subgroup” of rocks — see illustration above). The conglomerate was quickly traced east and westward for a total continuous distance of 50 km (31 mi) to define what became known as the “Central Rand Gold
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.[2] This heralded the historic Witwatersrand
Witwatersrand
Gold
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: seekers', or prospectors') Claim No 19 was declared a national monument in 1944, and named Harrison’s Park.[11] The park is on the busy Main Reef Road, immediately west of Nasrec Road.[2] In 1887 Cecil John Rhodes registered “The Gold
Gold
Fields of South Africa” in London, South Africa’s first mining house, with a capital of 7005250000000000000♠£250000. His brother Thomas was the first chairman.[2]

Gold
Gold
production on the Witwatersrand 1898 to 1910[12]:134

Year

No. of mines

Gold
Gold
output (fine ounces)

Value (million GB£)

Relative 2010 value (million GB£)[13]

1898 77 7006429560800000000♠4295608 £15.14 7003691000000000000♠£6910

1899 (Jan–Oct) 85 7006394654500000000♠3946545 £14.05 7003630000000000000♠£6300

1899 Nov – 1901 Apr 12 7005574043000000000♠574043 £2.02 £908

1901 (May–Dec) 12 7005238994000000000♠238994 £1.01 £441

1902 45 7006169010000000000♠1690100 £7.18 7003309000000000000♠£3090

1903 56 7006285948200000000♠2859482 £12.15 7003522000000000000♠£5220

1904 62 7006365824100000000♠3658241 £15.54 7003664000000000000♠£6640

1905 68 7006470643300000000♠4706433 £19.99 7003849000000000000♠£8490

1906 66 7006555953400000000♠5559534 £23.62 7003989000000000000♠£9890

1907 68 7006622022700000000♠6220227 £26.42 7004108000000000000♠£10800

1908 74 7006678253800000000♠6782538 £28.81 7004117000000000000♠£11700

1909 72 7006703913600000000♠7039136 £29.90 7004122000000000000♠£12200

1910 63 7006722831100000000♠7228311 £30.70 7004124000000000000♠£12400

See also[edit]

Geography of South Africa Great Escarpment, Southern Africa Vredefort crater Witwatersrand
Witwatersrand
basin List of mountain ranges of South Africa Pilanesberg Borakalalo Game Reserve

Further reading[edit]

Breckenridge, Keith Derek (1995) An Age of Consent: law, discipline, and violence on the South African gold mines, 1910–1933. Ph.D. thesis, Northwestern University, Evanston, Ill. Cammack, Diana (1990) "The Rand at War: the Witwatersrand
Witwatersrand
and the Anglo-Boer war 1899–1902. London: James Currey Herd, Norman (1966) 1922: the revolt on the Rand. Johannesburg: Blue Crane Books

References[edit]

^ a b c d e f Truswell, J.F. (1977). The Geological Evolution of South Africa. pp. 21, 27–28, 33–36. Cape Town: Purnell. ^ a b c d e f g h i j k l m Norman, N.; Whitfield, G. (2006) Geological Journeys. pp. 38–49, 60–61. Cape Town: Struik Publishers. ^ Encyclopædia Britannica (1975). Micropaedia, Vol. X. p. 720. Helen Hemingway Benton, Chicago. ^ a b c d e f g h i j k l m n o McCarthy, T., Rubridge, B. (2005). "The Story of Earth and Life." p. 89–90, 102–107, 134–136. Struik Publishers, Cape Town ^ a b Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa. ^ National Geographic ^ a b c Margulis, L., Sagan, D. (1995). "What is Life?". p. 81–83. Weidenfeld and Nicolson, London. ^ a b c Tankard, A.J., Jackson, M.P.A, Erikson, K.A., Hobday, D.K., Hunter, D.R., Minter, W.E.L. (1982). "Crustal Evolution of Southern Africa. 3.8 Billion Years of Earth History" pp. 118–139. Springer-Verlag, New York. ^ a b c Brink, A.B.A. (1996). "Engineering Geology of Southern Africa". pp. 81–160. Building Publications, Pretoria. ^ a b Behind gold's glitter, torn lands and pointed questions, New York Times, 24 October 2005. (PDF). Retrieved on 4 May 2012. ^ "Outcrop of Main Reef Group of Conglomerates Langlaagte Johannesburg-9/2/228/0196". South African Heritage Resource Agency.  ^ Yap, Melanie; Leong Man, Dainne (1996). Colour, Confusion and Concessions: The History of the Chinese in South Africa. Hong Kong: Hong Kong University Press. p. 510. ISBN 962-209-423-6.  ^ Measuring Worth, Relative Value of a UK Pound Amount – average earnings, retrieved on 27 January 2011

External links[edit]

Relationships between the Vredefort structure and the Witwatersrand basin within the tectonic framework of the Kaapvaal craton
Kaapvaal craton
as interpreted from regional gravity and aeromagnetic data Johannes

.