Researchers have sharpened our understanding of gold inside pyrite, which could pave the way for a greener and cheaper method to mine the precious metal.
Australia has the largest gold reserves in the world, according to 2020 data from the US Geological Survey.
With estimated reserves of 10,000 tonnes, new brownfield and greenfield gold deposits have become a hot topic among Tier 1 companies and junior explorers in Australia.
At the other end of the spectrum, gold exports earned Australia a record $27 billion last year thanks to its existing mining projects, including Kirkland Lake Gold’s Fosterville mine in Victoria and Newcrest Mining’s Cadia operation in New South Wales.
While Fosterville and Cadia are mined using more traditional methods, a more sustainable alternative to an underground or open pit site may be looming.
Curtin University researcher Denis Fougerouse led a team of seven researchers, as part of a collaboration with the University of Western Australia and China University of Geoscience, to develop an understanding of gold trapped in pyrite and its potential contribution to mining in the future.
The study in free access, A new kind of invisible gold in pyrite hosted in deformation-related dislocations, took pyrite samples from a Chinese gold mine, finding a new way gold atoms are distributed that was never seen before by exploring the mineral at the nanoscale.
Pyrite is a common mineral colloquially known as fool’s gold as it appears to share similar characteristics to gold.
Fougerouse and his team observed gold in pyrite at a scale 100,000 times smaller than the width of a human hair using a high-tech instrument known as an atom probe.
By discovering this so-called “invisible gold”, Fougerouse believes it may hold the key to more sustainable mining practices.
“We’ve known that gold in pyrite is a thing for quite a long time” Fougerouse tells Australian Mining.
“What we found was that this new type of gold may be prominent in minerals which have been deformed over the course of time – minerals that have been twisted and kinked in shear zones are probably very likely to have gold hosted in what we call crystal defects.”
The study identified that when more defects are seen in a pyrite crystal, more gold will be hosted within the defects.
“The thing that’s really interesting about the defect is that they are only a few nanometres in size – if you take the average COVID-19 virus particle size, it’s 100 times smaller,” Fougerouse says.
While atom probe tomography has been used in the creation of computer chips for decades, it is a relatively new technology for geology.
“The power of atom probe technology is to make measurements at incredibly small scales” Fougerouse says.
“It has been used in many different materials and objects particularly in the semiconductor industry to analyse computer chips.
“Only in the last five to six years have we started to use atom probes for geological studies. This instrumentation has made possible to ‘see’ invisible gold in pyrite.”
According to Fougerouse, the discovery rate for gold deposits, as well as the grades of the gold ore that is being found, is declining.
This was reflected in an S&P Global Market Intelligence report from September 2020, which revealed that 16 of the 20 largest gold producers worldwide have experienced a decrease in their overall years of remaining production since 2010.
S&P believes that major gold producers are now more focused on acquisitions rather than exploration to increase their gold output.
Fougerouse says this decline could pave the way for selective leaching to be a sustainable and environmentally friendly extraction method for gold miners.
His study suggests that if gold could be extracted from pyrite using this targeted method it could also replace energy-intensive pressure oxidising techniques.
Selective leaching instead uses a fluid to dissolve gold from the pyrite, which is enabled by the pyrite defects acting as pathways to wash out the gold without having to dig out the pyrite.
With pyrite being the most abundant sulphide mineral on Earth, there are plenty of places to seek out the gold trapped inside.
Fougerouse says Australia is likely to have deformed pyrite similar to what was is used in the study.
“Pyrite in Australian deposits could host a lot of these new types of gold microstructure,” Fougerouse says.
“The Australian gold deposits are very often hosted in shear zones and are quite old, particularly in Western Australia. It is likely these types of deposits underwent several episodes of deformation and quite a long history.
“Our observations can be reproduced anywhere that has a minimum amount of deformation in pyrite, which is rather common in gold systems.”
Mining companies are also being pressured to meet net-zero carbon commitments and they need to find new methods to achieve this. A gold extraction technique such as selective leaching from pyrite could materialise into an alternative.
“If the mine of the future can use selective or in-situ leaching, that would be fantastic,” Fougerouse says. “The environmental footprint would be a fraction of a current.
Fougerouse says planning for the next stage of research of gold in pyrite is progressing and includes the prospect of collaborating with more researchers.
“We would really like to collaborate with other researchers who are more legible in mineral processing to make some tests, experiments to measure selective leaching and characterise it in details,” Fougerouse says.
“Once we have done that, we hope to provide tangible end-user outcomes. The research is still in an intermittent state.”
While the study itself was completed in weeks, Fougerouse says more needs to be done to build on the understanding of gold trapped in pyrite.
“The maths is pretty simple: the more energetically costly a mine becomes the more impact it has on climate change,” he says.
“People have talked about selective leaching before but the new ideas we’re putting forward have not been tested yet and may be a tipping point for its application in specific content.”
This story also appears in the September edition of Australian Mining