A team of German scientists has proven for the first time that the concentration of gold in the notorious ‘fool’s gold’ directly depends on the level of arsenic in the pyrite where the gold is encrusted.
To the untrained eye, pyrite is commonly mistaken for gold and is hence referred to as ‘fool’s gold’, but it isn’t entirely useless.
Pyrite often leads to discovering real gold given they both form together under similar conditions, making the discovery all the more important.
The Carlin-type gold deposits in Nevada, United States is the origin of five per cent of global production, however, in these deposits, gold doesn’t appear in nuggets or veins, but is hidden, stuck with arsenic in pyrite.
The potential to unlock the gold from other deceiving minerals was recognised in Tasmania by Newcrest Mining in 2012, when they provided a $2.5 million grant to the University of Tasmania to develop a mineral research facility.
The gold research facility used micro-analytical techniques to help solve ore definition issues, including traces of arsenic which affected the extraction and recovery of gold.
Fast-forward seven years and a team of scientists from the Helmholtz Centre Potsdam can now show experimentally that the higher the concentration of arsenic, the more frequently gold will chemically bind with pyrite.
In the Earth’s crust, gold generally occurs in concentrations of 2.5 parts per billion but in order to mine it economically, gold concentration must be thousands of times higher and found in a focused area close to the surface.
When gold is ‘invisible’ it is because it occurs in tiny pyrite rims that grow on older ‘fool’s gold’ grains originating from sedimentary rocks.
In the laboratory experiments, researchers led by Christof Kusebauch showed that arsenic plays the crucial role in extracting gold from hot solutions, likely magmatic systems, that pass through the rock.
Similar to the natural ore system, authors used iron-rich carbonates and sulphur-rich solutions to synthesise the ‘fool’s gold’ crystals.
“Only then we were able to show that the partition coefficient which controls how much gold is incorporated into pyrite depends on the amount of arsenic,” Kusebauch said.
“The major challenge was to experimentally grow gold and arsenic bearing pyrite crystals that were big enough to analyse.”
Ultimately, the findings show that if solutions containing gold and arsenic from magmatic sources pass through sedimentary rocks with large amounts of smaller ‘fool’s gold’ grains, bigger gold deposits can be formed.
The researchers believe the new discovery may help in the future discovery of gold deposits.