Earthquakes can produce large gold nuggets

Earthquakes can produce large gold nuggets

TO Kate Graham-Shaw

Bars of pure gold stacked in bank vaults, the layers of plating on this summer's Olympic medals, or even your own gold jewelry could owe their existence to earthquakes. The stress and strain caused by the movement tectonic plates during these earthquakes it may cause a chemical reaction New research suggests this causes tiny gold particles to clump together into larger nuggets.

“The biggest discovery is demonstrating a new process for gold formation and explaining how really large gold nuggets can form,” says Chris Voysey, a co-author of the study and a geologist at Monash University in Australia. “It’s always been a bit of a mystery, especially when there’s no field evidence to support alternative gold formation processes.”

It is estimated that about 75 percent of all gold mined comes from deposits located in cracks within lumps of quartzone of the most common minerals in Earth's crustGeochemists knew that dissolved gold existed in liquids in the planet’s middle and lower crust, and that the liquids could seep into cracks in the quartz. But the amount of liquid involved seemed to limit the gold that could dissolve, and therefore the size of the gold chunks that formed. The larger nuggets were difficult to explain: experts speculated that gold nanoparticles in the liquid could combine into these larger chunks inside the quartz, but it was unclear how. Unlike dissolved gold, the nanoparticles typically lacked enough chemical energy to initiate the necessary reaction to accumulate at the surface of cracks and form a nugget.

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A new study published Monday in Nature Geoscience, suggests that geological stress caused by earthquakes may activate a special geochemical property of quartz called “piezoelectricity,” and that this property makes the formation of larger gold nuggets possible.

The piezoelectric effect has been known since the 1880s. Essentially, it’s the ability of a material to generate an electric charge when subjected to mechanical stress. Many everyday objects, including microphones, musical greeting cards, and inkjet printers, use the effect, and it occurs naturally in substances from cane sugar to bones.

Quartz can experience this effect because of its structure: it is made up of a repeating pattern of positively charged silicon atoms and negatively charged oxygen atoms. When it is stretched or compressed, the arrangement of these atoms changes, and the charges are distributed asymmetrically. Negative and positive charges accumulate in different areas of the quartz, creating an electric field and changing the electrical state of the material.

Voisey and colleagues at Monash University, located in Melbourne's historically gold-rich region, suggested that this altered electrical state could reduce the energy required for gold nanoparticles in the liquid to interact with the quartz surface, triggering a previously impossible chemical reaction and allowing the nanoparticles to stick and accumulate.

To test their idea, the researchers simulated the electric field that quartz can produce when subjected to earthquake-like forces. They then placed quartz crystals in a liquid containing dissolved gold nanoparticles and other gold compounds and found that when subjected to seismic-like forces, quartz was able to produce enough voltage to start accumulating nanoparticles.

The study's findings point to an intriguing mechanism that may be responsible for the formation of at least some large gold nuggets in the Earth's crust, particularly “orogenic” deposits, or those found at the site of the collision of two tectonic plates that may have overlapped to form a mountain range.

“There seems to be no doubt that episodic earthquakes play an important role in the formation of these important ‘orogenic’ gold nugget deposits,” says James Saunders, a consultant geologist who was not involved in the study. He says he would like to see future research explore more of the specifics of the process, such as how long the earthquake forces that produce piezoelectricity must last to cause such deposits, and why large gold nugget deposits can only form in a few quartz mineral fractures in one area – despite a given earthquake theoretically causing the same stress and strain in all the fractures. “I think it’s a great idea/hypothesis,” he says. “I’ll be interested to see if it holds up with further evaluation.”

Studying piezoelectricity at very large scales can be challenging, says Colgate University geologist Aubrey Adams, who was also not involved in the study. “Geologists are currently working very hard to quantify how stress (or pressure) varies in three dimensions over time and location in the crust,” she says, “something that’s easy to measure in the lab but much harder to quantify in the crust.”

Voisey and his team intend to expand the experimental parameters, such as testing different pressures or temperatures, to further explore their theory. “This is very much a ‘pilot study’ for this technique,” ​​he says, “so I’m excited to see where it can go.”