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Most of the world's great gold deposits can be found in underground veins of quartz.
But how exactly concentrated nuggets of gold may have formed here has been a mystery and source of debate among geologists.
Sometimes it seems like nuggets float in the middle of a quartz vein without any apparent chemical or physical explanation as to why they had accumulated there, says Chris Voisey, a geologist at Monash University.
Much of the world's gold deposits are found in rocky veins of quartz, like the white layer in the image above. (Flickr: James St John, Main vein, CC BY 2.0)
Now, research by Dr Voisey and his colleagues suggests the formation of these nuggets may be due to electricity generated by the pressure of earthquakes — a phenomenon known as piezoelectricity.
Their experiments, published today in Nature Geoscience, showed seismic activity led to an accumulation of gold particles in quartz.
"We found that once gold was on the quartz's surface, it would actually become the the main focus for further gold to deposit," Dr Voisey said.
How does piezoelectricity work?
Physical stress on a solid material produces an electric voltage.
Quartz, which is made up of the elements silicon and oxygen, is the most abundant piezoelectric mineral on the planet.
Tools like barbecue lighters contain quartz crystals that generate a high voltage spark when pressure is applied, for example.
Gold which has deposited in a vein of quartz in a hydrothermal setting. (Flickr: James St John, Gold-quartz hydrothermal vein, CC BY 2.0)
Dr Voisey was intrigued by the fact that so much of the world's gold happened to have accumulated in quartz.
"It all kind of sounds a bit too convenient that one of the only abundant piezoelectric minerals on Earth [quartz] is also the thing that hosts large gold nuggets," he said.
To explore the relationship between gold and quartz Dr Voisey and his colleagues set up an experiment to replicate what they thought was happening deep underground.
The experiment
Most of the world's gold is believed to in the planetary core, where there is enough of the metal to gild Earth's surface in a layer 0.5m thick.
Over billions of years some of this gold has made it closer to the surface, catching a ride in super-hot water-rich fluids, which moved through fractures and cracks to the rocky outer crust of the planet.
Until now, scientists thought gold precipitated out of the fluids and accumulated as it cooled or underwent a chemical change.
But Dr Voisey and his colleagues were not convinced by that argument.
One problem is that quartz is chemically inert, meaning it doesn't react with other substances, Dr Voisey said.
His team wanted to test a hypothesis that piezoelectricity generated during repeated earthquakes and seismic activity over billions of year may be at play in how the gold could come together in one place as a nugget.
Gold-quartz specimens like the one named after Bernhardt Otto Holtermann in 1872 at Hill End, New South Wales, may have formed deep underground. (State Library of New South Wales: American and Australasian Photographic Company)
To replicate what may be happening deep below the surface they took six slabs of quartz crystals that were suspended into two different solutions containing gold.
The solutions were similar in composition to what would be found in super-hot fluids underground.
One solution had 75 parts per million of dissolved gold while the other had 95 ppm of gold nanoparticles, known as colloidal gold.
They shook the quartz slabs with a motor at a frequency of 20 hertz to replicate seismic waves.
Victoria has been home to some of the largest complete nuggets ever found in the world. (State Library of Victoria: J Richardson/Australian Mining Standard)
The gold started to deposit onto the quartz in both solutions.
But as the experiment continued Dr Voisey saw something else start to happen.
Instead of new grains attaching elsewhere on the quartz, it was attracted to, and accumulated on, the previously deposited gold.
"Once gold does stick to the quartz surface, it becomes like a like a lightning rod for further reactions," Dr Voisey said.
Scanning electron microscope image of gold [red] deposited onto quartz [yellow and green] via piezoelectric reactions during experiments. (Supplied: Chris Voisey)
Over time the process could explain how large gold nuggets formed as more and more particles accumulate, according to Dr Voisey.
Findings could be useful for exploration
Rob Hough, mineral resources director at CSIRO, which was involved in the study, said the research established there was some sort of electric force involved in gold accumulation.
"These things are super-useful, ultimately, to help the industry understand how to explore more efficiently," he said.
Jim and Jimmy Larcombe with the golden eagle nugget found at Larkinville, Western Australia, in 1931. (State Library of Western Australia: Illustrations Ltd)
University of Otago emeritus professor of geology Dave Craw, who was not involved in the study, has mixed views on the study.
He said the research provided a new mechanism for understanding gold deposition in quartz veins.
"Earthquakes are ubiquitous in orogenic gold-forming environments, and this novel mechanism is a good one to explain the link between gold and quartz in a lot of these deposits," Dr Craw said.
"I especially like their suggestion that this mechanism can form networks of gold in veins.
"I have seen networks like that with gold along microfractures, and this is an entirely plausible process for making that happen."
But, Dr Craw said, whether this process could actually lead to the formation of large nuggets was another issue.
"The experiments only produced very small particles of gold, and they haven't demonstrated this could scale up to make large nuggets, although I suspect they could be right for some situations," he said.
Monash University geologist Dr Chris Voisey ran experiments exploring the relationship between gold and quartz with piezoelectricity. (Supplied: Chris Voisey)
Dr Voisey is still excited to see where the research may go.
While new insights into gold formation can help exploration and mining geologists, he also hopes the finding can give insights useful for chemical engineering.
"Something I think would be interesting is if someone used piezoelectricity for the mineral processing of the gold ore," he said.
"Processing gold ore right now can be fairly expensive ... I would like to see if this research would sort of help clean that up or even just reduce the cost."