The Witwatersrand Basin formed as a vast ancient inland lake around 3 billion years ago. Over about 300 million years, successive strata settled down in the lake, and when the water dried up the rocks compacted to form hard quartzites.
Some seams or “reefs” contain the richest gold field the world has ever known. Mining activities have yielded over 1.5 billion gold ounces since the late 19th century – a third of all the gold that has ever been produced.
Where did the gold come from originally? It was formed, like other elements, in the burning of the universe’s first-generaion stars – specifically, in the superheated explosions of supernovae. When the resulting space dust coalesced into second-generation stars and planets, gold found its way into the crust of the Earth where humankind would later hunt for and treasure it.
Why exactly so much gold was concentrated in the Witwatersrand remains a mystery. We do know that it was brought down into the basin as placer gold (in powder form) from very high mountains. High-energy rivers dropped the heavy gold along with pebbles in deltas on the edge of the lake, forming conglomerates known as “banket” (nut pudding). This is the prized gold ore.
Australian prospector George Harrison was one of many who picked over the landscape for years in a frustrating hunt for the elusive yellow metal. When he found signs of gold outcropping along the surface of a ridge of hills, he did not know that the reefs plunged down deeply as a result of the mighty Vredefort asteroid impact.
The thin gold-bearing strata dip steeply towards the centre of the crater near Vredefort, some 140km to the south, disappearing under later geological deposits.
The impact happened about 700 million years after the Witwatersrand Basin had been fully formed as a stratigraphic series of rock and mineral layers. Outcrops of the gold-bearing reefs were traced downwards by miners who developed large-scale engineering technologies to descend into the hard quartzite. Johannesburg, the city of gold, flourished on the surface while a labour force of black miners (recruited from all over Southern Africa) sweated deep underground.
The country’s wealth and its apartheid system both developed out of this harsh mining environment.
The economic boom continued for well over a century but today the gold deposits appear to be reaching depletion. Meanwhile the people of South Africa have reached a political settlement giving equal rights to all and setting their sights on growth beyond mining.
None of this – good and bad – would have happened had the Vredefort impact not occurred, capsizing the reefs and thrusting them deep down. Had the Witwatersrand strata continued to lie flat, as they were intially formed, it is most likely that over the succeeding span of two billion years all the gold would have eroded away into the oceans.
- “Why the gold came to the Reef”: http://www.joburg.org.za/index.php?option=com_content&task=view&id=274&Itemid=51
- “Mangalisa Geology” : http://superiormining.com/properties/south_africa/mangalisa/geology/