Nature | 'SEARCH TERMS'

“(Left): Scanning electron microscope image of gold deposited onto quartz via piezoelectric reactions during experiments. Inlay is a zoomed in image revealing that the texture of the gold grain is composed of multiple gold nanoparticles stuck together. (Right): Energy dispersive spectroscopic map of the sample. Credit: Dr Chris Voisey.”

DISSOLVED GOLD
https://nature.com/articles/s41561-024-01514-1
https://nationalgeographic.com/science/article/gold-nugget-earthquake-electricity
How do gold nuggets form? Earthquakes may be the key
by Robin George Andrews / September 2, 2024

“Gold has always been a commodity. But these days, finding a nugget isn’t too tricky: Much of the world’s gold is mined from natural veins of quartz, a glassy mineral that streaks through large chunks of Earth’s squashed-up crust. But the geologic process that put gold nuggets there in the first place was a mystery. Now, a new study published today in Nature Geoscience has come up with a convincing, and surprising, answer: electricity, and earthquakes—lots of them. Those nuggets owe their existence to the strange electrical properties of common quartz. When squished or jiggled, the mineral generates electricity. That drags gold particles out of fluid in Earth’s crust. The particles crystallize out as grains of gold—and, over time, with enough electrical stimulation, those grains bloom into nuggets.

“If you shake quartz, it makes electricity. If you make electricity, gold comes out,” says Christopher Voisey, a geologist at Monash University in Australia and the lead author of the new paper. Earthquakes are the most likely natural source of that shaking, and the team’s lab experiments show that earthquakes can make gold nuggets. The idea that gold nuggets appear because of electricity instead of a more conventional geologic process is, at first, a peculiar thought. But “it makes complete sense,” says Thomas Gernon, a geoscientist at the University of Southampton in England and who was not involved with the new work. Quartz veins host a disproportionate number of gold nuggets and their environments experience plenty of earthquakes.

Gold is extracted from a variety of geologic deposits, but it’s frequently found within quartz veins. From afar, alabaster sheets of quartz can look like bright cobwebs weaving through rock. Gold-bearing quartz veins are found in parts of the crust that have undergone a lot of stress and strain from events like mountain formation. These stressed, warped and fragmented areas are riddled with faults. When faults rupture during earthquakes, hot geologic fluids—sometimes containing gold particles—rush into the cracks, cool, and form gold-rich quartz veins. “It’s normally thousands to tens of thousands of pulses of [this] fluid that comes in during earthquake events then, over time, that builds an orogenic gold deposit,” says Voisey.

That gold particles find their way into quartz veins isn’t unexpected. But within these veins, miners tend to find large nuggets of gold sitting by themselves rather than just tiny grains all over the place. “How do you get such massive concentrations of gold in quartz veins?” says Iain Pitcairn, an ore geologist at Stockholm University in Sweden and who was not involved with the new work. “It’s strange and difficult to explain that.” Something must be forcing all the gold particles into specific locations, but what? Quartz itself is also quite odd. It’s a simple mineral, made with just silicon and oxygen.

But it’s also the only common mineral whose crystals lack a center of symmetry, meaning it’s structurally wonky. That means, under certain conditions, quartz’s internal electrical configuration is also imbalanced, which allows it to do something weird: create electricity. Quartz doesn’t spark up by itself. But if you apply a force to a quartz crystal—stamp on it, say—then it generates an electric field. This phenomenon is known as piezoelectricity (which derives from piezo, the Greek word for “push”). “The more force you put in, the higher the piezoelectric response,” says Voisey. “If you hit a quartz crystal hard enough that it breaks, you’ll get the most voltage you could possibly get out of it.”

The presence of gold nuggets in veins of a piezoelectric mineral seemed suspicious to Voisey. “It’s just such a coincidence,” he says. Or was it? To find out if earthquakes were the mysterious force at play, Voisey and his team placed slabs of pure natural quartz in sealed chambers containing gold-bearing liquid solutions. Some slabs were jostled about by a machine that replicated seismic waves from an earthquake. Others were left unshaken as control experiments. The hope was that gold would start appearing in a solid form atop the quivering slabs. After the fake quakes subsided, Voisey took the quartz and looked at it under a powerful microscope. There they were: myriad gold particles, glittering across the surface.

“false color 3D X-ray tomography reconstruction of high-grade gold in quartz by Chris Voisey”

Voisey originally had this hunch that quakes and electrical fields might forge gold several years earlier as a Ph.D. student; to see it realized was an exhilarating moment. “I went nuts when that worked,” he says. “I kicked back on my chair, screaming out and stuff. Dude, I exploded.” As expected, the wobbling quartz had created an electrical field. The gold particles dissolved in the solution were dragged out of it and deposited as solid grains onto the quartz’s surface. But that wasn’t the end of the story. “The big conundrum is how you make really, really large gold nuggets,” says Voisey, as opposed to assorted flecks of the element.

To address this, Voisey subjected another chunk of quartz to his artificial quake machine — but this time, the quartz already a little nugget of gold in it. And when that sparkly slab was shaken up, the gold nugget began to grow. Since gold conducts electricity, if there are gold grains already there, they act as electrodes to which gold particles migrating out of the solution are preferentially drawn. Preexisting grains and nuggets “become a lightning rod” to the rest of the gold, Voisey says. In nature, quartz veins bearing gold nuggets are probably formed not through a single earthquake event, but by a cornucopia of them. After the first few quakes sprout grains of gold, additional earthquakes cause more and more gold particles to crystallize atop those grains, eventually forming nuggets. “It is elegant, and it’s also really novel,” says Pitcairn.

Despite knowing exactly where to look to find gold nuggets, geologists have been baffled by their very existence. “I think this goes a long way to explaining it,” says Pitcairn. Centuries ago, the notion that you could shake run-of-the-mill quartz about and generate gold would be considered nothing short of alchemy. This study shows that, although nature is capable of acts of magic, it just takes the right team, and the right experiment, to reveal how the trick is performed. And sometimes, the secret seems to be disarmingly simple. “It’s almost too neat, too tidy,” says Voisey. “But I’m happy for that.”

“Gold nuggets form inside quartz veins, which are cracks
in the rock infilled with mineral-rich hydrothermal fluids”

PIEZOELECTRIC MINERALS
https://science.org/shake-rattle-and-gold-earthquakes-may-spark-gold-formation
https://livescience.com/earthquakes-can-trigger-quartz-into-forming-giant-gold-nuggets
by Sascha Pare / 9/2/24

“Scientists have discovered exactly how earthquakes trigger quartz into forming large gold nuggets — finally solving a mystery that’s puzzled researchers for decades. Gold naturally forms in quartz — the second-most abundant mineral in Earth’s crust after feldspar. But unlike other types of gold deposits, those found in quartz often cluster into giant nuggets. These nuggets float in the middle of what geologists call quartz veins, which are cracks in quartz-rich rocks that periodically get pumped full of hydrothermal fluids from deep within the crust.

“Much of the world’s gold deposits are found in rocky veins of quartz”

“Gold forms in quartz all the time,” said Chris Voisey, a geologist at Monash University in Australia and the lead author of a new study published Monday (Sept. 2) in the journal Nature Geoscience. “The thing that’s weird is really, really large gold nugget formation. We didn’t know how that worked — how you get a large volume of gold to mineralize in one discreet little place,” Voisey told Live Science. Hydrothermal fluids carry gold atoms up from the deep and flush them through quartz veins, meaning gold should theoretically become evenly spread in the cracks rather than concentrated into nuggets, Voisey said. These nuggets are exceptionally valuable and represent up to 75% of all the gold ever mined, according to the study.

“Gold which has deposited in a vein of quartz in a hydrothermal setting”

Two separate clues helped Voisey and his colleagues solve the gold nugget mystery, he said. The first was that the largest nuggets occur in orogenic gold deposits, which are deposits that form during earthquakes. The second was that quartz is a piezoelectric mineral, meaning it creates its own electric charge in response to geologic stress, such as the stress generated by earthquakes. “When you actually put it together, it almost works out a bit too neat,” Voisey said. The researchers found that earthquakes fracture rocks and force hydrothermal fluids up into the quartz veins, filling them with dissolved gold.

In response to the stress of the earthquake, quartz veins simultaneously generate an electric charge that reacts with the gold, causing it to precipitate and solidify. Gold concentrates in specific spots because “gold dissolved in solution will preferentially deposit onto pre-existing gold grains,” Voisey said. “Gold is essentially acting as an electrode for further reactions by adopting the voltage generated by the nearby quartz crystals.” This means that in quartz veins, gold solidifies into clusters that grow bigger with each earthquake. The largest orogenic gold nuggets found to date weigh around 130 pounds (60 kilograms), Voisey said.

“Gold-quartz specimens like the one named after Bernhardt Otto Holtermann
in 1872 at Hill End, New South Wales, may have formed deep underground”

To test this idea, the researchers simulated the effect of an earthquake on quartz crystals in the laboratory. They submerged the crystals in a liquid containing gold and replicated seismic waves to generate a piezoelectric charge. The experiment confirmed that under geologic stress, quartz can produce a large enough voltage to precipitate gold out of solution. The simulation also confirmed that gold preferentially solidifies on top of existing gold deposits in quartz veins, which helps explain the formation of large gold nuggets. “Having pre-existing gold and having it become basically the catalyst or the lightning rod that other gold would attach to was very, very exciting,” Voisey said.

“Scanning electron microscope image of gold [red] deposited onto quartz
[yellow and green] via piezoelectric reactions during experiments” (Chris Voisey)

One of the implications of the study is that scientists can now make large gold nuggets in the lab, “but it’s not alchemy,” Voisey said. “You’d have to have gold in a solution and then you just move it from basically being in a liquid to sticking to something else.” However, the results don’t give geologists and exploration companies new clues as to where to mine for gold nuggets. The best science can offer for now is a device that detects piezoelectric signals from quartz at depth, Voisey said. “This can tell you where quartz veins are — but not tell you if there is gold in those quartz veins.”

ELECTRIFYING EARTHQUAKES
https://abc.net.au/piezoelectricity-could-be-behind-gold-nugget-formation
https://cosmosmagazine.com/science/chemistry/earthquake-ekectric-gold-nuggets/
Electrifying earthquakes produce giant gold nuggets
by Evrim Yazgin / September 3, 2024

“According to the US Geological Survey, 244,000 tons of gold have been discovered to date – about 57,000 of this is still in underground reserves. The total amount of gold found would fit into a cube with 23-metre sides. It’s no wonder, then, that humans have spent so much time trying to figure out how to produce gold. Alchemists for centuries believed that they could turn lead – atomic number 82 – into the precious element gold – atomic number 79. Scientists more recently have tried to explain how gold is produced in nature. Some have even been able to produce gold nanoparticles in the lab.

“The standard explanation is that gold precipitates from hot, water-rich fluids as they flow through cracks in the Earth’s crust,” says Monash researcher Chris Voisey, lead author of the new study. “As these fluids cool or undergo chemical changes, gold separates out and becomes trapped in quartz veins. While this theory is widely accepted, it doesn’t fully explain the formation of large gold nuggets, especially considering that the concentration of gold in these fluids is extremely low.” The researchers sought to understand better how large gold nuggets form through testing a new theory: piezoelectricity. The piezoelectric effect was first discovered in 1880 by French brothers and physicists Jacques and Pierre Curie – the latter being the husband of physicist Marie Curie.

Piezoelectricity occurs when certain solid materials can convert mechanical energy into electrical energy, or vice-versa. Some crystals, like quartz, can generate a piezoelectric voltage when a mechanical stress is applied. This effect is commonly found in quartz clocks and BBQ lighters. Voisey’s team wondered if the mechanical stresses produced during earthquakes could see piezoelectricity produced in quartz, leading to the electrical and chemical changes required to make large gold nuggets. To test their idea, the team tried to replicate the conditions quartz might experience during an earthquake. The quartz crystals were submerged in a gold-rich fluid and stress applied using a motor. The quartz samples were then studied under a microscope.

“The results were stunning,” says co-author Andy Tomkins. “The stressed quartz not only electrochemically deposited gold onto its surface, but it also formed and accumulated gold nanoparticles. Remarkably, the gold had a tendency to deposit on existing gold grains rather than forming new ones.” This is because quartz is an electrical insulator while gold is a conductor. The result is that gold deposited on the quartz due to piezoelectric voltages become a pole of attraction for other dissolved gold particles. “Our discovery provides a plausible explanation for the formation of large gold nuggets in quartz veins,” Voisey says. “In essence, the quartz acts like a natural battery, with gold as the electrode, slowly accumulating more gold with each seismic event.”

PREVIOUSLY