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Rivers of gold

More than 35 percent of all the gold ever mined has come from a single source: the Witwatersrand Basin in South Africa. For over a century, geologists have debated how this basin became so enriched with gold. The answer could help prospectors isolate the highest grade ores within the basin and determine what features to look for when hunting for a new Witwatersrand-type deposit — knowledge particularly needed now because production from the basin has steadily dropped since its peak in 1970. The average grade of ore is nearly half what it once was.

Shedding light on this debate is a paper in the Sept. 13 Science that says the Witwatersrand gold first formed 3 billion years ago, when upwelling from the mantle formed a large section of the South African continental crust. The new work shows the gold is a quarter of a billion years older than its host sedimentary rock, suggesting the gold first formed outside of the basin and then washed into the basin along with sediments.

Thin layers of gold and pyrite cut through this argillite from the Witwatersrand Basin. Jason Kirk, a graduate student at the University of Arizona in Tucson, and colleagues found that gold in samples similar to this one is older than the surrounding rock. They report that their finding indicates the gold first formed outside the basin and then washed into the basin many millions of years later. Image courtesy of Lori Stiles, University of Arizona.

“Our results support a placer model: the gold is derived from source rocks north and west of the basin and transferred by streams and rivers into the basin and converted into rock,” says Jason Kirk, a graduate student at the University of Arizona in Tucson and lead author of the paper.

Rivers filled the basin with sediments between 2.7 and 2.9 billion years ago. Today the gold lies within thin sedimentary layers, or reefs, that wrap around the edge of the basin and dive to depths of 5,000 meters or more.

If the placer model is correct, prospectors looking for higher ore grades within the basin should concentrate on sediments that accumulated under calm sections of the ancient rivers. Most of the gold would have fallen out in these areas.

Furthermore, valuable remnants of the vast deposit that originally supplied the placer gold could lie near the basin. “Komatiites, magnesium-rich basalts that formed much more frequently during the Archean than they do today, are the best candidates for the initial hosts of the gold,” Kirk says.

The age of the gold Kirk’s team found weakens an alternative, hydrothermal explanation for the gold’s origin. The hydrothermal theory holds that hot water migrated up through fractures and pores in the sedimentary rock. Gold ions dissolved in the water precipitated when they brushed against hydrocarbons within the rock. This theory requires the gold to be younger or the same age as the host rock.

If the hydrothermal theory is correct, it will have dramatically different implications for gold exploration. Prospectors should look for areas with fractures that allow hot water to migrate, and for sedimentary rock riddled with cracks and crevices that could trap the gold.

A hydrothermal origin would also suggest that Witwatersrand-type deposits are not that unique and could be found in other areas. “The hydrothermal theory opens up many areas for Witwatersrand-type deposits, necessitating basins with the appropriate tectonics, but not necessarily tied to other requirements,” says Andy Barnicoat, a geologist at the University of Leeds and proponent of the hydrothermal theory.

Kirk and colleagues determined the age of the gold by measuring the amount of rhenium and osmium within the gold. These elements occur in low concentrations, on the order of a few parts per billion, but sensitive mass spectrometers are able to detect them. Rhenium-187 decays to osmium-187 with a half-life of 45 billion years. The researchers calculated the age of the gold by measuring the amount of rhenium decay.

“The rhenium-osmium system is one of the newest isotopic systems developed because of analytical advances since the mid-1980s,” Kirk says.

Previous studies dated the gold by determining the age of pyrite and other minerals thought to have formed at the same time as the gold. Some of the minerals were younger than the sedimentary rock, and some older, and it was not clear which group, if any, reflected the same process that generated the gold.

Even the rhenium-osmium system is not foolproof, Barnicoat says. The morphology of the gold within the reefs suggests that it has dissolved and recrystallized at least once during its 3 billion years within the sedimentary rock. So the gold may not have remained a closed system since its inception; it may have gained or lost rhenium and osmium atoms during recrystallization. The current concentrations of these elements in the gold may not reflect the gradual radiogenic decay required for an accurate age.

“In general, isotopic systems do not remain unaffected by recrystallization and I can’t see why the rhenium-osmium data should differ from any other radiogenic isotope system,” Barnicoat says.

The debate will continue, says Joaquin Ruiz, a geologist at the University of Arizona and co-author of the paper. “The critics of the placer model are probably going to come back to us and argue that the age dates are flawed. That is where the arguments are going to go now.”

Greg Peterson

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