FORTY percent of all the gold ever mined comes from one place - the Witwatersrand - and geologists still can't agree on why.
Researchers from the University of Arizona reported in the journal Science in September 2002 that the gold welled up from the earth's core some 3 billion years ago, making it more ancient than the Witwatersrand's rocks, which are 300 million years younger. The suggestion is that the gold actually surfaced somewhere else, and was gradually washed into the Witwatersrand Basin by ancient rivers.
The orthodox theory is that the Witwatersrand Basin, some 350 km long and 200 km wide, was the site of an ancient sea around 2,75 billion years ago. Rivers running through gold-bearing hills - which may have been common in the ancient earth - washed sediments of sand and gravel - and specks of gold - on to the bed of this sea. Gradually they built up a 7 km thick layer of ore-bearing conglomerate rocks. There was little oxygen in the earth's toxic atmosphere at the time, life was restricted to bacteria and primitive algae in the seas, and volcanic eruptions and meteor impacts were common.
It was volcanoes and meteors that extinguished the ancient seas and rivers - but ensured the survival of the gold. The sea appears to have died some 2,7 billion years ago, when massive volcanic activity in the area caused a layer of lava to cover the basin.
Then another catastrophe occurred 2 billion years ago - a ten kilometre wide meteor struck the earth in the region where the small town of Vredefort is today, some 100 kilometres south of Johannesburg. The impact vaporised 70 cubic kilometres of rock, and created a crater 250 to 300 kilometres in diameter. The debris thrown up by the impact covered the Witwatersrand. The blanket of debris was so thick that it preserved the gold from a further 2 billion years of erosion - which is why gold is still found here, and is rarely found elsewhere.
Today the gold-bearing "reef" forms a swathe of parallel ridges some 100 kilometres long from east to west, roughly centred on Johannesburg. The gold is buried far below the surface, and is mined at depths of up to 3,6 km. This means, ironically, that although the Witwatersrand is as high as 1,5 to1,8 km above sea level, the deepest mines run as much as 1,8 km below sea level.
Over 40 000 tons of gold have been mined on the Witwatersrand, and a similar amount is believed to still lie underground. But extracting that gold requires ever-deeper mining of ever-poorer ores, in a time of declining gold prices. In 1999, for example, it was estimated that it cost US$222 to mine an ounce of gold in South Africa, against $189 in the US and $169 in Canada. Gold mining in South Africa hit its peak in 1970, when over 1 000 metric tonnes were mined, generating US$1,16 billion in sales.
