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Willie Maartens

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South Africa: Wealth and History Engraved in Rock
by Willie Maartens   
Rated "G" by the Author.
Last edited: Saturday, February 09, 2008
Posted: Wednesday, February 06, 2008

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A superficial canter over the South African landscape from the Hadean to the early Mesozoic.


 

Stephen Jay Gould (1941-2002) made the point that the most profound contribution geology has made to human thought is the concept of Deep Time. This concept, originally coined and popularised by John McPhee (1931- ), refers to the immensity of geological time and the problem that humans have in conceptualising the several-billion-year time-span over which geological processes on the earth has been in operation.


 

Perhaps the most challenging aspect of geology is the scale of geological time, which covers some 4600 million years. If we take one millimetre as one year (a lifetime of 80 years will cover a mere 80 millimetres, or 8 centimetres), a metre will be one thousand years, a kilometre will be one million years, and a thousand kilometres will be comparable to a billion (a thousand million) years. In this analogy 4600 million years (4.6 billion years) equals 4600 km. Now, compare this with the radius of the Earth, which is about 6400 km (at the equator).

 

Rocks in themselves are uninteresting to most people, and understandably so. However, rocks are also time capsules because they capture information about their environment when they form, and preserve it.

 

*****

 

The total surface area of the Earth is roughly 510 million square kilometres. The oceans occupy approximately 70% of the surface area of the Earth, leaving ±149 million square kilometres of the Earth’s surface for the continents.

 

While Africa covers only 20% of the world’s land mass, it accounts for 88% of global platinum reserves, 78% of diamond, 60% of manganese, 60% of cobalt, and 40% of the world’s gold reserves.

 

South Africa is located at the southernmost region of Africa, with a long coastline that stretches more than 2500 kilometres and across two oceans (the Atlantic and the Indian). At 1,219,912 square kilometres, South Africa is the world’s 25th-largest country. Njesuthi in the Drakensberg (berg = mountain) at 3408 metres is the highest point in South Africa.

 

South Africa is geologically speaking, a very diverse and in many ways unique place, without equal on the globe. Several factors contribute to this uniqueness.

 

South Africa is a treasure house of valuable minerals. Despite occupying only 0.8% of the Earth’s land surface, the country is, or once was, the world’s largest producer of gold, chromium, diamonds, vanadium, manganese, and platinum group metals (platinum, palladium, ruthenium, rhodium, osmium, and iridium). It possesses huge reserves and is a first-rate producer of uranium, vermiculite, zirconium, nickel, antimony, iron, titanium, zinc, coal, fluorspar, refractory minerals, and phosphate, and also produces huge amounts of copper, zinc, and lead.

 

It has been said that hectare for hectare, the northern half of South Africa is the richest piece of real estate on Earth.

 

South Africa has a very long geological history, its oldest rocks dating back more than 3600 million years (My). Rock-forming events extend from this ancient moment in Deep Time virtually to the present, providing a long, albeit punctuated, geological history. The preservation of these ancient rocks is quite remarkable and many look little different today from their equivalents formed in very recent times.

 

These ancient rocks record events during many crucial periods in Earth’s history and provide insight into globally important changes that took place in the past, such as the changing composition of the Earth’s atmosphere and the assembly and fragmentation of a number of supercontinents (from Columbia to Rodinia, and eventually Pangaea).

 

South Africa’s rocks contain a very special and long record of life on Earth. The very earliest life forms are preserved as fossils in these rocks; the evolution of land plants and animals, and especially the origin of mammals and dinosaurs, are well preserved. South Africa also has probably the best record of the origin of hominins.

 

*****

 

The geological her-story of the Earth (Gaia is female!) is usually divided into four Eons, namely: the Hadean (4600-3800 million years ago or Mya), Archaean (3800-2500 Mya), Proterozoic (2500-545 Mya), and Phanerozoic (545-0 Mya). The Archaean and Proterozoic together is also known as the Precambrian.

 

Each Eon can be divided into different Eras. The Archaean and Proterozoic Eons in South Africa are divided into five Eras, namely: the Swazian, Randian, Vaalian, Mokolian, and Namibian Eras. The Phanerozoic is divided into three Eras, the Palaeozoic (545-300 Mya), Mesozoic (300-65 Mya), and Cenozoic Era (65-0 Mya). On this timescale, Eras are further subdivided into Periods, and periods into Epochs.

 

By careful mapping of the distribution of sedimentary and associated volcanic rocks, it is possible to identify those rocks formed within the same episode of subsidence, and to separate them from accumulations formed during other episodes of subsidence. All those rocks deposited during the same episode of subsidence belong to a single event, and for convenience are grouped together and referred to as a Supergroup. Supergroups are again divided into lesser rock layers, i.e. Groups, Subgroups, Formations, Members, and Beds.

 

*****

 

During the HADEAN EON (4600-3800 Mya), according to the best scientific guestimates at present, a cloud of very hot gas and dust in the outer reaches of the Milky Way galaxy began to collapse under the influence of gravitation. As it shrank, it began to rotate ever faster, flattening into a disc shape. Its interior also became hotter until the centre reached several million degrees centigrade.

 

Nuclear fusion of hydrogen atoms commenced, producing the Sun. Material in the disc around the Sun began to clump to produce smaller bodies circling the Sun – the planets.

 

About 4100 Mya, plate (oceanic crust plus the lithosphere) tectonics on Earth are believed to have started and the oldest known rock, a granite in Labrador, Canada, had formed.

 

About 3900 Mya, the oldest known sedimentary rocks, Armitsoq terrane, Greenland, had formed.

 

During the SWAZIAN ERA (3800-3080 Mya), the oldest known oceanic crust, consisting of Komatiite (3700-3600 Mya), from the lower Onverwacht Group, at Barberton, South Africa had formed.

 

About 3200 Mya, island arcs started to amalgamate to form the first micro-continent (Ur). Micro-continents continued to accrete to form the Earth’s oldest known continent, the Kaapvaal Craton.

 

Cratons are the kernels on which continents grow. They are much older and deeper than the neighbouring crust and mantle. The Kaapvaal Craton is some 3.7 billion years old (Byo). At 200 kilometres deep, it is well below the crust-mantle boundary.

 

The Kaapvaal Craton covers an area of approximately 1.2 million square kilometres. It is welded to the Zimbabwean Craton to the north by the Limpopo Belt. To the south and west, the Kaapvaal Craton is flanked by Proterozoic orogens (the Namaqua-Natal Belt), and to the east by the Lebombo monocline that contains Jurassic igneous rocks associated with the breakup of Gondwana. The early Achaean crust is well exposed only on the east side of the craton and comprises a collage of sub-domains or crustal blocks characterised by distinctive igneous rocks, deformation history, and tectonic style.

 

The Kaapvaal Craton was formed by the deposition of Archaean Granites and Gneisses around the Greenstone belts (e.g. the Barberton Greenstone Belt – BGB) of the island arcs.

 

Early Archaean granites and gneisses constitute the oldest preserved material on the Earth’s surface and are vital in deciphering the evolutionary history of the Earth’s crust.

 

Although present in many parts of the world, there are only two cratons known (i.e. the Pilbara of Western Australia and the Kaapvaal Cratons) which have retained large tracts of relatively pristine pre-3100 million years old (Myo) rocks.

 

This early crust, which has survived intact for thousands of millions of years, comprises largely granitic gneisses with infolded greenstone belts or greenstone belt remnants.

 

The greenstone belts are linear to irregularly shaped features, commonly attaining widths of 10-50 km and lengths of 100-300 km. they are composed largely of extrusive mafic and, to lesser extends, ultramafic and felsic rocks.

 

The oldest rock yet dated on the Kaapvaal Craton (3644 Myo) – and probably on the entire African continent – is banded tonalite gneiss from the Ancient Gneiss Complex (AGC) located south of the BGB in the Piggs Peak area of northwest Swaziland. The tonalitic protoliths of this material also exhibit mafic remnants, which might be similar or older than this age. Additional evidence for the existence of Eoarchaean (3800-3600 Mya) crust in the Kaapvaal Craton is provided by the presence of xenocrystic zircons in the Vlakplaas Granodiorite Stock north of the Steynsdorp Pluton, and in the Ngwane Gneiss of the AGC, central Swaziland. These rocks have been dated at 3702 and 3683 Myo respectively. No other Eoarchaean intrusions have yet been found in other parts of the craton.

 

The BGB, occupying an area of 120 x 50 km, is situated south and southeast of Nelspruit, South Africa, and is a strongly folded, ENE-trending, mid-Archaean (3600-3100 Mya), volcano-sedimentary remnant, entirely surrounded by a variety of granitoids of the Kaapvaal Craton.

 

During the RANDIAN ERA (3080-2650 Mya), the Dominion Group, the Pongola, and Witwatersrand Supergroups (between2800-2300 Myo) had been deposited on the Kaapvaal Craton and the Limpopo Belt (2650 and 2000 Mya) had formed.

 

The Dominion Group (3074 Myo) is a sequence of volcanic and minor clastic sedimentary rocks, now metamorphosed to greenschist-amphibolite grade. It overlies the granite-greenstones basement terrane in the central part of the Kaapvaal Craton and is, in turn, overlain by the Witwatersrand Supergroup. The Dominion Group conglomerates contain gold, and were mined from 1888 onwards.

 

The discovery of gold in quartz-pepple conglomerates (“reefs”) by George Harrison in March 1886 first focussed the world’s attention on a sequence of rocks that was to become the greatest source of gold on Earth, the Witwatersrand Supergroup.

 

Since 1886, between a quarter and half of all the gold that ever had been mined on Earth has come from the Witwatersrand Supergroup and it is still producing despite higher costs due to deep mining and the low gold price.

 

Up until 2007, South Africa has been the world’s largest gold producer, then in 2007, marking the lowest production level since 1956; China has surpassed South Africa in being the largest gold producer. In 2003 gold production fell by an estimated 6.5% to 373,074 kilograms, however gold still accounted for an estimated 37% of dollar export revenue within the country. Ninety-five per cent of South Africa’s gold mines are underground operations reaching depths of over 3.8 kilometres. Workers on the Highveld (±1600 metres above mean sea level); effectively work far below sea level everyday!

 

South Africa has enormous gold ore reserves, estimated at 40,000 tonnes, representing 40% of global reserves. South Africa’s main gold producing area is concentrated on the Achaean Witwatersrand Basin. The Witwatersrand Basin, which has been mined for more than 100 years and has produced more than 41,000 tonnes of gold, remains the greatest unmined source of gold in the world.

 

South Africa does have other smaller gold producers outside of the Witwatersrand Supergroup, in the form of Achaean Greenstone Belts. The main gold producing greenstone belts are the Barberton Greenstone Belt (BGB) and the Kraaipan Greenstone Belt (KGB).

 

Rocks on the Witwatersrand and in the Barberton area can be considered as Deep Time.

 

Subsequent to the stabilisation of the Kaapvaal Craton, a series of four basins developed on it between 3000 and 2100 Mya. The Ventersdorp Supergroup, which developed in the second last of these basins, provides a unique volcano-sedimentary supracrustal record and contains the largest and most widespread sequence of volcanic rocks on the Kaapvaal Craton. Although volcanic terranes provide favourable environments for mineralisation, no economic or even significant mineral deposits are yet known from the volcanic successions of the Ventersdorp Supergroup.

 

Both the Kaapvaal and the Zimbabwean Cratons are typical granitoids-greenstone terranes. Between these two cratons, a broad zone of gneisses, now known as the Limpopo Belt, extends from eastern Botswana through southern Zimbabwe and northern South Africa. The exposed strike length of the Limpopo Belt is close to 700 kilometres, with a width of about 200 kilometres.

 

About 2700 Mya, the Kaapvaal Craton collided with the slightly younger micro-continent, the Zimbabwean Craton, forming mountains as high as the Himalayas. These mountains have completely eroded away. This collision probably triggered the start of the volcanic eruptions of the Ventersdorp Supergroup

 

As the two cratons collided, intense metamorphism led to widespread crustal melting around 2650 Mya, followed by another clustering of metamorphic ages around 2000 Mya. These structures, metamorphism, and ages distinguish these events from the adjoining cratons. The Limpopo Belt is a highly complex geological province.

 

Like most high-grade metamorphic belts in the world, the Limpopo Belt is not rich in mineral deposits, although nickel, copper, graphite, and gold are, or was, mined in the Belt.

 

During the VAALIAN ERA (2650-2050 Mya), the Wolkberg Group, and Transvaal Supergroup had been deposited and were intruded by the Bushveld Complex (Rooiberg Group, Rustenburg Layered Suite, Rashoop Granophyre Suite, and Lebowa Granite Suite).

 

The Transvaal Supergroup is preserved within three structural basins on the Kaapvaal Craton. The Transvaal Supergroup rocks in the Transvaal Basin were intruded the Bushveld Complex (2061-2059 Mya). A number of minerals is, and was, mined from the Transvaal Supergroup; e.g. Gold, dolomite, limestone, Lead, Zinc, Fluorspar, iron, manganese, asbestos, and andalusite.

 

The mafic rocks of the Bushveld Complex constitute the most voluminous preserved mafic, layered intrusion in the world. They underlie an area of roughly 65,000 square kilometres (400 x 300 kilometres). The maximum vertical thickness of the layered rocks approaches 8 kilometres. The world’s largest ore reserves of platinum-group elements (PGE), chromium, vanadium, fluorite, andalusite, and gold are being mined in this complex. The associated felsic, volcanic rocks are also among the most voluminous known.

 

The MOKOLIAN ERA (2050-1000 Mya) can be divided into two Periods, namely: the Kheisian and Namaquan.

 

During the Kheisian Period (2050-1600 Mya), the Olifantshoek Supergroup, Waterberg and Soutpansberg Groups (the Earth’s oldest so-called red-beds), and related rocks (e.g. Blouberg Formation) had been deposited.

 

The sandstones of the Waterberg and Soutpansberg Groups are stained red by iron oxide, indicating that they were deposited under an atmosphere containing free oxygen. These rocks are the oldest so-called red-beds known on Earth, and record the first appearance of substantial quantities of free oxygen in the Earth’s atmosphere.

 

About 2023 Mya (the largest and oldest known impact structure on Earth, the Vredefort Dome, had formed. The impact crater is ±250-300 kilometres wide, and ±5 kilometres deep (the Dome is only the central uplift of the entire structure). Johannesburg and Pretoria occupy positions near the northern rim of the original crater – the crater itself no longer exists.

 

During the same period, the metamorphic rocks of the Ubendian Belt (the Kheis province) had formed.

 

During the Namaquan Period (1600-1000 Mya), the Namaqua-Natal (Kibaran) Belt had formed.

 

During the assembly of the supercontinent Rodinia (1100 Mya), the sea separating the Congo and Kaapvaal-Zimbabwe Cratons closed, as did the sea across Bushmanland. The resulting mountains have since been removed by erosion, exposing a belt of metamorphic rocks across the southern portion of the Kaapvaal Craton – called the Namaqua-Natal Belt – and between the Congo and Kaapvaal-Zimbabwe Cratons. These belts form part of a global network of metamorphic belts – the Kibaran (or Grenville) belts.

 

Rodinia began to fragment around 700 Mya.

 

The Namaqua-Natal Belt hosts several mineral deposits, most important of which are copper, lead, and zinc.

 

From 2100 to 1200 Mya, a series of alkaline complexes was emplaced on the Kaapvaal Craton. These complexes were formed through the intrusion of granite, Syenite, gabbro, pyroxene, and corbonatite bodies, and the extrusion of mafic and felsic lavas.

 

The economically important Phalaborwa Complex is situated immediately south of the town of Phalaborwa. It is the site of the largest open-pit copper mine in Africa, as well as important phosphate (apatite) and vermiculite deposits, with magnetite, sulphuric acid, zirconium (from baddeleyite), uranium, nickel, gold, silver, and platinum group elements (PGE) as by-products. It is a deeply eroded root zone of an ultramafic corbonatite-type volcano.

 

Between the time of deposition of the Waterberg Group (circa 1750 Mya) and the onset of the Dwyka Glaciation (circa 324 Mya – during the time of the Karoo Supergroup), the Kaapvaal Craton experienced an extraordinary long interval of relative tectonic stability, non-deposition, and comparatively little erosion. During this period, the most spectacular series of events was the widespread alkaline volcanic and plutonic activity that gave rise to the Pilanesberg Alkaline Province of predominantly silica-undersaturated rocks.

 

During the NAMIBIAN ERA (1000-545 Mya), Gondwana had assembled and rocks of the Pan African Belt had formed (Gariep, Malmesbury, Kango, Kaaimans Groups, and Cape Granites).

 

During the MESOZOIC ERA (300-65 Mya), the Cape and Karoo Supergroups had been deposited, the Cape Fold Belt had formed, and Gondwana had fragmented, forming the present South African coastline.

 

Karoo Dolerites (circa 190-150 Mya), including the Lebombo, and Drakensberg Groups, most Kimberlites (1900-70 Mya), had formed. The Karoo Igneous Province (dolerites, rhyolites, and basalt) is one of the world’s classic continental flood basalt provinces and its constituent extrusive and intrusive rocks occur over a very extensive area of southern Africa.

 

Kimberlite is a hybrid, volatile-rich, potassic, ultramafic igneous rock derived from deep within the Earth (deeper than 150 kilometres), and the primary source of diamonds. Kimberlites, therefore, also provide a better window on the Earth’s mantle than any other rock type.

 

Thus, as can be readily seen from this short introduction, South Africa is truly a country of unsurpassed geological splendour. The rock record in southern Africa preserves a rock record of more than 3600 My of Earth her-story!

 

 

Willie Maartens
 
 
 


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Reviewed by Leland Waldrip 7/25/2008
Wow! Nice work. I like what you've recorded here. The herstory of South Africa is a good reference I am keeping.
Best regards,
Leland
Reviewed by Keith Rowley 2/6/2008
What a fantastic journey through time! Excellent.

Keith