A new understanding of how gold-rich magma forms in the presence of sulfur may help in the hunt for buried treasure — or at least in finding Earth's deep stashes of the rare metal.
Humans have prized and searched for gold for thousands of years. Gold is produced in mines found on every continent except Antarctica and to date, in all of human history about 165,000 tonnes of the shiny stuff has been mined, according to the World Gold Council, an industry group.
New exploration turns up new mines, but not all of Earth's gold is easy to find.
"Almost every gold mine has been found from rocks on the surface of the earth," said Robert Linnen, an economic geologist at The University of Western Ontario, in Canada, highlighting California's gold rush as one easy example.
"If you want to find gold now, you have to look for the more difficult stuff," Linnen told OurAmazingPlanet.
But how do you find gold that is, say, 100 meters or more below ground? You can't just dig everywhere. The answer, Linnen suggests, is to begin with a deep knowledge of how gold deposits form.
Sulfur may be a key ingredient in that recipe, according to a study by Linnen and his international colleagues that was recently published in the journal Nature Geoscience.
While still hotly debated among some geologists, one of the origins for gold mines is thought to be within melts deep in the Earth's mantle that then erupt at the upper crust, where they form deposits.
"If that's the case, then we want to understand how the gold comes out of the mantle and gets transported to the surface," Linnen said. "Not all magmas form gold deposits, so we need to know what unique circumstances have to be in place."
It had been previously assumed that as long as there were sulfide minerals in the mantle, gold would remain in those minerals. This meant that the only way for gold to be extracted from the mantle by magma was for the sulfide minerals to first be melted out, or destroyed in some other way.
"The solubility of gold in sulfur-rich magmas was thought to be very, very low," said Linnen, describing the tendency of a substance to dissolve in another medium.
Based on a series of experiments, the researchers now suggest that this may not be the case. In fact, sulfur's presence actually increased the amount of gold that they found dissolved in magma by up to eightfold, depending on certain chemical conditions.
The new finding could help future gold explorations focus on areas that are more likely to contain deposits, such as rock formations high in sulfur. Beneath extinct volcanoes in the western United States might be one good place to start looking, Linnen said.
"The demand for gold is continuing to rise," he added. "We need to find more resources to replace what we're using up."
