Miners working at porphyry copper and gold deposits should be aware that the deeper a deposit is, the more copper there will be, while gold-rich deposits are closer to the surface. This, according to a study published in Nature Communications.
In the article, Massimo Chiaradia, a researcher from the University of Geneva, presented his findings after investigating how the metals are accumulated over the time duration of a mineralizing event in porphyry deposits and finding a correlation between the amounts of copper and gold extracted from the deposits.
Not only did Chiaradia discover that the depth of the deposits influences the quantity of metals produced, but also that over 95% of the gold is lost to the atmosphere through volcanic emissions.
“A copper deposit can contain from one to 150 million tonnes, while the quantity of gold varies from 10 tonnes to 2,500 tonnes per deposit,” the researcher said in a media statement.
Chiaradia used a range of statistical models to analyse two hypotheses: either the magmatic fluids have varying amounts of metal from the outset, or the fluids are identical but it is the effectiveness of the precipitation of the metals that influences the quantity of copper and gold.
“I quickly saw that the first hypothesis wasn’t right and that the answer lays with precipitation but with differences for gold and copper-related to the duration of mineralisation,” he said. “The longer the mineralisation time, the richer the deposit will be in copper. And for the mineralisation to be as long as possible, the deposit must be deep – 3 kilometres from the surface – to guarantee a certain degree of insulation and a long magma life.”
The geologist observed that less than 1% of the gold is captured in the ores in the deep copper-rich deposits. On the other hand, in deposits located at a depth of up to 3 kilometres, the rate climbs to 5%.
Chiaradia said that although gold escapes easily in volcanic emissions, it is retained more in shallow deposits where separation takes place between the liquid and the vapour, which helps its precipitation.
“In the deeper deposits, however, liquid and vapour form only a single fluid phase, which precipitates the copper quickly and makes the gold leak into the atmosphere as the fluid rises to the surface,” Chiaradia said.