Earthquakes May Forge Large Gold Nuggets

Earthquakes May Forge Large Gold Nuggets

By Kate Graham-Shaw

Solid gold bars stacked in bank vaults, layers of plating on this summer’s Olympic medals or even your own pieces of golden jewelry could owe their existence to earthquakes. The stress and strain produced by moving tectonic plates during these temblors may trigger a chemical reaction that causes minuscule particles of gold to coalesce into larger nuggets, a new study proposes.

“The biggest finding is showing a new gold-forming process and providing an explanation for how really large gold nuggets might form,” says Chris Voisey, a co-author of the study and a geologist at Monash University in Australia. “This was always a bit of a conundrum, especially when there isn’t field evidence supporting the alternative gold-forming processes.”

It is estimated that around 75 percent of all mined gold comes from deposits nestled in cracks inside of hunks of quartz, one of the most abundant minerals in Earth’s crust. Geochemists have known that dissolved gold existed in fluids in the middle to lower levels of the planet’s crust and that the fluids could seep into quartz cracks. But the amount of fluid involved seemed to limit the gold that could dissolve and thus the size of the gold chunks that formed. Larger nuggets were hard to explain: experts had theorized that gold nanoparticles within the fluid might aggregate into those bigger chunks within the quartz, yet it was unclear how. Unlike dissolved gold, nanoparticles typically wouldn’t have enough chemical energy to start the necessary reaction to build up on the cracks’ surface and form a nugget.

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The new study, published on Monday in Nature Geoscience, suggests that the geological stress caused by earthquakes might activate a peculiar geochemical property of quartzes called “piezoelectricity”—and that that property makes the formation of larger gold nuggets possible.

The piezoelectric effect has been known since the 1880s. It is essentially the ability of a material to generate an electric charge when placed under mechanical stress. Many everyday items, including microphones, musical greeting cards and inkjet printers take advantage of the effect, and it occurs naturally in substances from cane sugar to bone.

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

Voisey and his colleagues at Monash University—located in the historically gold-rich area of Melbourne—thought that this changed electric state could lower the energy needed for gold nanoparticles in the fluid to interact with the quartz surface, causing a previously unviable chemical reaction to occur and allowing the nanoparticles to stick and accumulate.

To test their idea, the researchers modeled the electric field that quartz could produce when subjected to earthquakelike forces. They then placed quartz mineral crystals in a fluid containing dissolved gold nanoparticles and other gold compounds and found that, when under seismic-wave-like forces, the quartz was able to produce enough voltage to jump-start a buildup of nanoparticles.

The study findings point to an intriguing mechanism that could be responsible for forming at least some of the larger gold nuggets in Earth’s crust—especially “orogenic” deposits, or those found where two tectonic plates have collided and may have folded onto each other to create a mountain range.

“It appears to be a certainty that episodic earthquakes are important in helping form 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 look more into the specifics of this process, such as how long piezoelectricity-causing earthquake forces have to last to cause such deposits and why large gold nugget deposits might happen in only some cracks of quartz mineral in one area—despite a given earthquake theoretically inducing the same stress and strain on all of the cracks. “I think it is a great idea/hypothesis,” he says. “I’ll be interested if it stands up upon further evaluation.”

Studying piezoelectricity at a very large scale may be difficult, says Colgate University geologist Aubreya Adams, who was also not involved in the study. “Geoscientists are currently working very hard to quantify how stress (or pressure) varies in 3D with time and location in the crust,” she says, “something that is easily measured in a lab but much harder to quantify in the crust.”

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