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The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing
scarp
scarp
in
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over 60 million people, it is the world's 23rd-most populous nation and covers an area of . South Africa has three capital citie ...

South Africa
. It consists of a hard, erosion-resistant
quartzite Quartzite is a hard, non-Foliation (geology), foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usu ...

quartzite
metamorphic rock, over which several north-flowing rivers form waterfalls, which account for the name Witwatersrand, meaning "ridge of white waters" in
Afrikaans Afrikaans (, ) is a West Germanic language spoken in South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over 60 million people, it is the world's 23rd-most po ...
.Truswell, J.F. (1977). ''The Geological Evolution of South Africa''. pp. 21, 27–28, 33–36. Cape Town: Purnell. This east-west-running scarp can be traced with only one short gap, from
Bedfordview Bedfordview is an affluent suburb in western Ekurhuleni, sharing an administrative boundary with the City of Johannesburg Metropolitan Municipality, Gauteng, South Africa. Bedfordview has been part of the City of Ekurhuleni Metropolitan Municipality ...

Bedfordview
(about west of
O.R. Tambo International Airport O. R. Tambo International Airport is an international airport situated in Kempton Park, Gauteng, Kempton Park, Gauteng, South Africa. It serves as the primary airport for domestic and international travel to/from South Africa and is Africa's ...
) in the east, through
Johannesburg Johannesburg (, also ; ; Zulu language, Zulu and xh, eGoli ), informally known as Jozi, Joburg, or "The City of Gold", is the largest city in South Africa, classified as a Megacity#List of megacities, megacity, and is List of urban areas by ...

Johannesburg
and
Roodepoort Roodepoort is a town in the Gauteng Gauteng ( ; tn, Gauteng; Sotho language, Northern and Southern Sotho: ; zu, eGoli; ts, Gauteng/; Southern Ndebele language, Ndebele, xh, iRhawuti; nr, I-Gauteng; ve, Gauteng) is one of the nine province ...
, to
Krugersdorp Krugersdorp (Afrikaans Afrikaans (, ) is a West Germanic language spoken in South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over 60 million people, it is ...
in the west (see the diagram at left below).Norman, N.; Whitfield, G. (2006) ''Geological Journeys''. pp. 38–49, 60–61. Cape Town: Struik Publishers. The scarp forms the northern edge of a plateau (or ridge) which rises about above the surrounding plains of the
Highveld The Highveld (Afrikaans: ''Hoëveld'', where ''veld'' means "field") is the portion of the South Africa#Geography, South African inland plateau which has an altitude above roughly 1500 m, but below 2100 m, thus excluding the Lesotho mountain regio ...
. A number of picturesque Johannesburg suburbs, including
Observatory An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysics, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been cons ...
, Linksfield Ridge and Upper Houghton are located along the scarp, overlooking the rest of northern Johannesburg with views up to the
Magaliesburg Magaliesburg is a small town situated below the Witwatersrand mountain range in Gauteng Gauteng ( ; tn, Gauteng; Sotho language, Northern and Southern Sotho: ''Gauteng''; zu, eGoli; Tsonga: ''Gauteng/eXilungwini;'' Southern Ndebele language ...

Magaliesburg
(although locals refer to segments of the scarp using area-specific names, such as ''Linksfield ridge'', ''Parktown Ridge'' or Observatory Ridge). The entire plateau-like structure is also often called the Witwatersrand. The plateau's elevation above sea-level is between . The Witwatersrand plateau forms a
continental divide A continental divide is a drainage divide on a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as co ...

continental divide
, with the run-off to the north draining into the
Indian Ocean The Indian Ocean is the third-largest of the world's five ocean The ocean (also the or the world ocean) is the body of that covers approximately 70.8% of the surface of and contains 97% of . Another definition is "any of the large ...

Indian Ocean
via the
Crocodile Crocodiles (family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society. I ...
and
Limpopo Limpopo is the northernmost Provinces of South Africa, province of South Africa. It is named after the Limpopo River, which forms the province's western and northern borders. The capital and largest city in the province is Polokwane (formerly ...
rivers, while the run-off to the south drains via the
Vaal The Vaal River ( ; Khoemana ǃOrakobab or ''Khoemana'', also known as Korana, ǃOra, or Griqua, is a Moribund language, moribund Khoe languages, Khoe language of South Africa. Names "Khoemana" (from ''khoe'' 'person' + ''mana'' 'language') ...

Vaal
into the
Orange River The Orange River (from Afrikaans Alaric speaking Afrikaans. Afrikaans (, ) is a West Germanic language spoken in South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. ...
and ultimately into the
Atlantic Ocean
Atlantic Ocean
. Because of the extraordinary quantities of
gold Gold is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

gold
that have been extracted from the Witwatersrand rocks, the South African currency was named the ''rand'' in 1961 upon the declaration of the republic. ''Witwatersrand'' and ''the Rand'' are names for the conurbation that developed along the range, although the terms are falling into disuse and ''Witwatersrand'' was the "W" in ''PWV'' (Pretoria-Witwatersrand-Vereeniging), the initial name of
Gauteng Gauteng ( ; tn, Gauteng; Northern and Southern Sotho: ; zu, eGoli; ts, Gauteng/; Ndebele Ndebele may refer to: *Southern Ndebele people, located in South Africa *Northern Ndebele people, located in Zimbabwe and Botswana About It is a lang ...
province. In this context, it has lent its name to institutions including the
University of the Witwatersrand The University of the Witwatersrand, Johannesburg (), is a multi-campus South African Public university, public research university situated in the northern areas of central Johannesburg. It is more commonly known as Wits University or Wits ( o ...
(Wits University) and the defunct
Rand Afrikaans University The Rand Afrikaans University (RAU) was a prominent South African institution of higher education and research that served the greater Johannesburg Johannesburg ( , also ; ; Zulu language, Zulu and xh, eGoli), informally known as Jozi, ...
(RAU, now part of the
University of Johannesburg The University of Johannesburg (UJ) is a public university #REDIRECT Public university#REDIRECT Public university A public university or public college is a university or college that is in state ownership or receives significant Government s ...

University of Johannesburg
), and to towns and regions such as the
East Rand The East Rand is the urban eastern part of the Witwatersrand The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa South Africa, officially the Republic of South Africa (RSA), is ...
,
West Rand 300px, A map of Gauteng, showing the West Rand District Municipality. The West Rand is the name of the urban western part of the Witwatersrand The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing scarp in ...
and
Randburg Randburg is an upmarket area located in the Gauteng province Gauteng ( ; tn, Gauteng; Northern and Southern Sotho: ; zu, eGoli; ts, Gauteng/; Ndebele Ndebele may refer to: *Southern Ndebele people, located in South Africa *Northern Ndebele ...
.


Geology

The Witwatersrand plateau consists of a layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2.97 billion years ago. The entire series of rocks, known as the "Witwatersrand Supergroup," consists of very hard erosion resistant
quartzite Quartzite is a hard, non-Foliation (geology), foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usu ...

quartzite
s, and some marine
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
deposits, interspersed with softer, more easily eroded
tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is di ...
s,
mudstone Mudstone, a type of mudrock Mudrocks are a class of fine-grained Granularity (also called graininess), the condition of existing in granular material, granules or Grain, grains, refers to the extent to which a material or system is composed ...

mudstone
s and conglomerates. The oldest rocks (laid down 2.97 billion years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2.71 billion 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, locally referred to as banket. The abundance of this gold is without a natural equal anywhere else in the world. Over have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 22% of all the gold that is accounted for today. 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 The mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, ...
and
uraninite Uraninite, formerly pitchblende, is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus The atomic nuc ...
, as well as carbon rich materials such as
kerogen Kerogen is solid, insoluble organic matter Organic matter, organic material, or natural organic matter refers to the large source of Carbon compounds, carbon-based compounds found within natural and engineered, terrestrial, and aquatic environm ...
, or
bitumen Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous The viscosity of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or externa ...

bitumen
, which occurs in small balls less than in size, called “flyspeck carbon”, or as continuous layers about 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.Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa. Gold mining in these buried portions of the Witwatersrand Supergroup is sometimes carried out at depths of below the surface.


Witwatersrand Basin

The Witwatersrand Basin is a largely underground geological formation which surfaces in the Witwatersrand. It holds the world's largest known
gold Gold is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

gold
reserves and has produced over 1.5 billion ounces (over 40,000
metric tons The tonne ( or ; symbol: t) is a metric unit of mass equal to 1,000 kilogram The kilogram (also kilogramme) is the base unit of mass Mass is the physical quantity, quantity of ''matter'' in a physical body. It is also a measu ...
), which represents about 22% of all the gold accounted for above the surface. The basin straddles the old provinces of
TransvaalTransvaal is a historical geographic term associated with land north of (''i.e.'', beyond) the Vaal River in South Africa. A number of states and administrative divisions have carried the name Transvaal. * South African Republic (1856–1902; af, Z ...
and the
Orange Free State The Orange Free State ( nl, Oranje Vrijstaat, af, Oranje-Vrystaat, abbreviated as OVS) was an independent Boer Boers () ( af , Boere) refers to the descendants of the proto-Afrikaans-speaking colonist A settler is a person who ha ...
, and consists of a 5000–7000 m thick layer of
Archean The Archean Eon ( , also spelled Archaean or Archæan) is one of the four geology, geologic eon (geology), eons of Earth, Earth's history, occurring (4 to 2.5 Billion years, Gya). During the Archean, the Earth's Crust (geology), crust had ...

Archean
, mainly sedimentary rocks laid down over a period of about 260 million years, starting about 3000 million years ago. The entire series of rocks, known as the “Witwatersrand Supergroup’’ consists of
quartzite Quartzite is a hard, non-Foliation (geology), foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usu ...

quartzite
s, ,
mudstone Mudstone, a type of mudrock Mudrocks are a class of fine-grained Granularity (also called graininess), the condition of existing in granular material, granules or Grain, grains, refers to the extent to which a material or system is composed ...

mudstone
s,
tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is di ...
s, conglomerates and some marine
lava Lava is magma Magma () is the molten or semi-molten natural material from which all s are formed. Magma is found beneath the surface of the , and evidence of has also been discovered on other and some s. Besides molten rock, magma may al ...

lava
deposits. Most of the basin is deeply buried under younger rocks, but outcrops occur in
Gauteng Gauteng ( ; tn, Gauteng; Northern and Southern Sotho: ; zu, eGoli; ts, Gauteng/; Ndebele Ndebele may refer to: *Southern Ndebele people, located in South Africa *Northern Ndebele people, located in Zimbabwe and Botswana About It is a lang ...
, the
Free State The Free State ( af, Vrystaat; st, Freistata; xh, iFreyistata; tn, Foreistata; zu, iFuleyisitata; before 1995, the Orange Free State) is a Provinces of South Africa, province of South Africa. Its capital is Bloemfontein, which is also South A ...
, 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 Johannesburg (, also ; ; Zulu language, Zulu and xh, eGoli ), informally known as Jozi, Joburg, or "The City of Gold", is the largest city in South Africa, classified as a Megacity#List of megacities, megacity, and is List of urban areas by ...

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 , 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. 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 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 Kinross (, gd, Ceann Rois) is a burgh A burgh is an autonomous The federal subject The federal subjects of Russia, also referred to as the subjects of the Russian Federation (russian: субъекты Российской Фед ...
. 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 now at depths of 4 km below the surface. Although many of the older mines, around Johannesburg, are now nearly exhausted, the Witwatersrand Basin still produces most of South Africa's gold and much of the total world output.
Silver Silver is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical ele ...

Silver
,
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
, and
iridium Iridium is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, beh ...

iridium
are recovered as gold-refining by-products.


Geological origin

The Witwatersrand basin was created during the Archean Eon, and is therefore amongst the oldest geological structures on earth. It was laid down in two stages, over the course of 260 million years starting just short of 3000 million years ago. The first phase, lasting 60 million years, consisted of sedimentary deposits in a shallow sea, conveniently named the “Witwatersrand Sea”. The resulting 2500–4500 m thick layer of sediments is termed the “West Rand Group” of Witwatersrand rocks. The second phase, which lasted for 200 million years, followed on from the first phase, with on-land deposits, resulting from the retreat of the Witwatersrand Sea, leaving a wide almost flat coastal plain over which rivers from the north formed wide braided river deltas, into some of which rich deposits of gold were deposited. The resulting 2500 m thick layer of rock is termed the “Central Rand Group”. The “West Rand Group” and “Central Rand Group” of rocks together form the “Witwatersrand Supergroup”, the full horizontal extent of which is termed the Witwatersrand Basin. There were no continents during the early stages of the Archean Eon, but
island arcs Island arcs are long chains of active volcano A volcano is a rupture in the of a , such as , that allows hot , , and to escape from a below the surface. On Earth, volcanoes are most often found where are or , and most are found underw ...
did form. It was the coalescence of several of these island arcs that led to the formation of the
Kaapvaal Craton The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Year#SI prefix multipliers, Ga (billion years ago) crust on Earth. Simil ...
, one of the first microcontinents to form on earth about 3900 million years ago. Its size and position relative to
Southern Africa Southern Africa is the south South is one of the cardinal directions or compass points. South is the opposite of north and is perpendicular to the east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Pr ...

Southern Africa
today are indicated in the diagram on the left. About 3000 million years ago local cooling of the underlying
asthenosphere The asthenosphere ( grc, ἀσθενός 'asthenos''meaning "without strength", and thus "weak", and 'sphaira''meaning "sphere") is the highly viscous The viscosity of a fluid In physics Physics is the that studies , its ...
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 Granite () is a coarse-grained igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ...

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 layer can be seen lying on its granite base in
Johannesburg Johannesburg (, also ; ; Zulu language, Zulu and xh, eGoli ), informally known as Jozi, Joburg, or "The City of Gold", is the largest city in South Africa, classified as a Megacity#List of megacities, megacity, and is List of urban areas by ...

Johannesburg
, where it forms a 56 km long east-west ridge over which several rivers running to the north form waterfalls, giving rise to the name Witwatersrand, which in
Afrikaans Afrikaans (, ) is a West Germanic language spoken in South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over 60 million people, it is the world's 23rd-most po ...
means “Ridge of White Waters”. There was no free oxygen in earth's atmosphere until about 2000 million years ago, before the appearance and sufficient development of
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

cyanobacteria
. The oxygen produced by these microorganisms rapidly reacted with, amongst others, Fe2+ ions dissolved in water, precipitating insoluble red
iron oxide Iron oxides are chemical compounds composed of iron and oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, ...

iron oxide
(
hematite Hematite (), also spelled as haematite, is a common iron oxide Iron oxides are chemical compounds composed of iron and oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member o ...

hematite
). For various reasons, the sedimentation of the fine grain mud was affected by cyclic episodes with more or less precipitation of iron oxides. The result was alternating red and beige layers of mud which, when consolidated, became . As the sea deepened finer grained and muddy sediments accumulated. But changing geographical conditions resulted in the accumulation of a wide variety of sediments, ranging from mud, to sand, to gravel, and .
Tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is di ...
deposits, dating from 2950 million years ago, are indicative of the first glaciation episodes on earth.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. Within 60 million years, up to 4500 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 Orogeny is the primary mechanism by which mountains are formed on continents. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. This leads to both structural deformation Deformation ...
(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, in an arc extending from Evander in the east, through Johannesburg, Carletonville and then southwards to Klerksdorp and Welkom in the south-west. 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 Gold is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

gold
,
iron pyrite The mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, ...
, and
uraninite Uraninite, formerly pitchblende, is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus The atomic nuc ...
. The gold was in its free elemental form. Cyanobacteria grew in relative abundance in these mineral rich waters. The
kerogen Kerogen is solid, insoluble organic matter Organic matter, organic material, or natural organic matter refers to the large source of Carbon compounds, carbon-based compounds found within natural and engineered, terrestrial, and aquatic environm ...
, or
bitumen Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous The viscosity of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or externa ...

bitumen
, 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 Central Rand Group that contains most of the gold bearing conglomerates, locally referred to as banket, that are today of great economic importance. The “Central Rand Group” of deposits was brought to an abrupt end by massive outpourings of lava, which form the Ventersdorp lavas which erupted 2715 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 The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Year#SI prefix multipliers, Ga (billion years ago) crust on Earth. Simil ...
with the
Zimbabwe Craton The Zimbabwe Craton is an area in Southern Africa of ancient continental crust, being a part of the ancient continent of Western Gondwana Gondwana () or Gondwanaland was a supercontinent In geology Geology (from the Ancient Greek γ ...
, 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 Basin 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 impact was close to the present village of Vredefort, which has given its name to the geological remnant of this immense event: the . Not only are the remains of this impact among the oldest on earth, but it is also one of the largest meteor impacts to have 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 centre. There are unfortunately no gold deposits in these outcrops. The meteor impact, however, lowered the Witwatersrand basin inside the crater. This protected it from erosion later on; but, possibly more importantly, bringing it to the surface close to the crater rim, near Johannesburg. In fact, apart from the Witwatersrand outcrops (i.e. where these 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 centre 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 uninterrupted removal of a several kilometres thick layer of deposits from the surface of the Southern African Plateau in the relatively recent geological past: i.e. the past 150 million years, but especially during the last 20 million years.


Gold origin

The vast majority of the Earth's
gold Gold is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

gold
and other heavy metals are locked up in the
earth's core The internal structure of Earth, structure of the solid Earth, or simply structure of Earth refers to concentric spherical layers subdividing the Solid earthSolid earth refers to "the earth beneath our feet" or '' terra firma'', the planet's sol ...

earth's core
. Evidence from
tungsten Tungsten, or wolfram, is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, ch ...

tungsten
isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of ...
studies indicates that most gold in the crust is derived from gold in the
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
which resulted from a
meteorite A meteorite is a solid piece of debris from an object, such as a comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varie ...
bombardment some 3900 million years ago (i.e. at approximately the time that the
Kaapvaal Craton The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Year#SI prefix multipliers, Ga (billion years ago) crust on Earth. Simil ...
formed). The gold bearing meteorite events occurred millions of years after the segregation of the earth's core. The gold in the Witwatersrand Basin area was deposited in Archean
river deltas A river delta is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Lan ...
having been washed down from surrounding gold-rich
greenstone belts Greenstone belts are zones of variably Metamorphism, metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archean, Archaean and Proterozoic cratons between granite and gneiss bodies. The name co ...
to the north and west.
Rhenium Rhenium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical eleme ...

Rhenium
-
osmium Osmium (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximat ...

osmium
isotope studies indicate that the gold in those mineral deposits came from unusual 3000 million year old
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
-derived intrusions known as
komatiite Komatiite () is a type of ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging ...
, present in the greenstone belts.Kirk, Jason; Joakin Ruiz; John Chesley and Spencer Titley; ''The Origin of Gold in South Africa,'' American Scientist, Vol 91, Nov.-Dec 2003, pp. 534–53

/ref>


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,Brink, A.B.A. (1996). "Engineering Geology of Southern Africa". pp. 81–160. Building Publications, Pretoria. 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,
oxidise (mild reducing agent) are added to powdered potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4 and composed of potassium ion, K+ and permanganate, . It is a purplish-black crystalline salt, th ...
s to insoluble
ferric oxide Iron(III) oxide or ferric oxide is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is ...

ferric oxide
(Fe2O3) and
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
(H2SO4). Thus, when mine waste comes into contact with oxygenated rainwater, sulfuric acid is released into the ground water.
Acid mine drainage Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the outflow of acidic water from metal mining, mines or Coal mining, coal mines. Acid rock drainage occurs naturally within some environments as part of ...

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 Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
,
cadmium Cadmium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical eleme ...

cadmium
,
lead Lead is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elements ...

lead
,
zinc Zinc is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical element ...

zinc
,
copper Copper is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

copper
,
arsenic Arsenic is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

arsenic
and
mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

mercury
found in the mine dumps, facilitating their passage into surface water and ground water.Behind gold's glitter, torn lands and pointed questions
New York Times, 24 October 2005. (PDF). Retrieved on 4 May 2012.
The
tailings In mining Mining is the extraction of valuable mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal stru ...
ponds contain an average of 100 mg/kg of U3O8, and uranium is measureable in human hair. Sulfuric acid also erodes concrete and cement structures, resulting in structural damage to buildings and bridges.


History

Although gold had been discovered in various locations in South Africa, such as
BarbertonBarberton may refer to: * Barberton, Mpumalanga, South Africa * Barberton, Ohio, United States * Barberton, Washington, United States * Barberton, Western Australia * A greenstone belt in the Kaapvaal Craton, southern Africa {{d ...
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, 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. 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 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 issued a proclamation declaring nine farms public mining diggings, starting on 20 September 1886. This heralded the historic
Witwatersrand Gold Rush The Witwatersrand Gold Rush was a gold rush in 1886 that led to the establishment of Johannesburg, South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over Demographics of ...
. 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. The park is on the busy Main Reef Road, immediately west of Nasrec Road. In 1887
Cecil John Rhodes Cecil John Rhodes (5 July 1853 – 26 March 1902) was a British mining magnate A magnate, from the late Latin ''magnas'', a great man, itself from Latin ''magnus'', "great", is a noble or a man in a high social position, by birth, wealth or ot ...
registered “The Gold Fields of South Africa” in London, South Africa's first mining house, with a capital of . His brother Thomas was the first chairman.


See also

* Geography of South Africa *
Great Escarpment, Southern Africa The Great Escarpment is a major topographical feature in Africa that consists of steep slopes from the high central South Africa#Geography, Southern African plateauAtlas of Southern Africa. (1984). p. 13. Readers Digest Association, Cape Town do ...
* Borakalalo Game Reserve *Pilanesberg *List of mountain ranges of South Africa *
Witwatersrand Gold Rush The Witwatersrand Gold Rush was a gold rush in 1886 that led to the establishment of Johannesburg, South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over Demographics of ...


Further reading

* 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 and the Anglo-Boer war 1899–1902''. London: James Currey * Herd, Norman (1966) ''1922: the revolt on the Rand''. Johannesburg: Blue Crane Books


References


External links

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

{{Authority control Landforms of Gauteng Landforms of South Africa Greater Johannesburg, Geography of Johannesburg Mountain ranges of South Africa Escarpments of Africa