The Origin of Gold in South Africa
Ancient rivers filled with gold, a spectacular upwelling of magma and a colossal meteor impact combined to make the Witwatersrand basin a very special place
Crammed shoulder to shoulder with a couple dozen burly miners in an all-too-small elevator, we are about to descend into one of South Africa's largest gold mines. Three men, who must weigh well over 100 kilograms each, lock elbows and heave backwards in unison, forcing us visiting geologists off our feet and farther into the lift. The gate is forced shut and, as we plummet down the billion-dollar shaft, our ears pop and swirling debris finds its way into our eyes and noses. Two minutes and two vertical kilometers later, we tumble out into oppressive heat and humidity. Walking through enormous tunnels, dodging ore carts and monstrous mining trucks, we reach a series of ski lifts that take us a kilometer deeper. After a long, muddy hike, we near a passage that slopes up at a 20-degree angle and is less than a meter high. Hunched over, we make our way toward the sound of pneumatic drills and water jets, where workers are blasting away at the working face of the mine. More than three kilometers of rock, weighing thousands of tons, lies above our heads. The sedimentary rocks being mined are uninspiring to look at from arm's length—they are what geologists call conglomerates, being composed mostly of rounded pebbles (here consisting predominantly of quartz) cemented together. But under a small magnifying lens, hundreds of small specks of gold appear. The rocks are chock-full of it. Nearly half the gold ever mined in human history has come from these conglomerates in South Africa's Witwatersrand basin.
Small-scale gold prospecting and mining began in this area in the early 1850s, but the first mother lode wasn't discovered until 1885. Two itinerant prospectors, George Walker and George Harrison, stumbled on surface outcrops of gold-rich conglomerate on an old farm—land that is now near the center of Johannesburg. In what must be one of the biggest financial blunders in history, both men quickly sold their claims for the equivalent of a few hundred dollars. Today, the gold fields in the region are worth many billions.
The Witwatersrand basin, which covers an area about the size of West Virginia, contains almost as much gold as the rest of the Earth's surface combined. There is, of course, enormous interest in the origin of these deposits, and more than a hundred years of mining and scientific research have revealed a complex history for these gold-bearing conglomerates. But the chronology of geologic events is imperfectly understood and the ideas about how all this gold came to be here are quite controversial.
Our research group at the University of Arizona has recently taken significant steps toward answering questions about the origin of the Witwatersrand gold. Here we discuss the research and the implications of our results for the geologic history of South Africa.