Earthquakes may produce giant gold nuggets

Earthquakes may produce giant gold nuggets

Kate Graham Show

The pure gold bars stacked in a bank vault, the plating on this summer’s Olympic medals, or even your own gold jewelry could have been revealed by an earthquake. Stresses and strains caused by the movement of tectonic plates during an earthquake can cause them to move. Cause a chemical reaction According to the new study, this causes the tiny gold particles to clump together into larger clumps.

“The biggest discovery is that it points to a new gold-formation process and explains how very large gold nuggets form,” said study co-author Chris Voisey, a geologist at Monash University in Australia. “This has always been a bit of a conundrum, especially when there’s no field evidence to support an alternative gold-formation process.”

It’s estimated that about 75% of all gold mined comes from deposits buried in cracks inside chunks of quartz, one of the most abundant minerals in the Earth’s crust. Geochemists knew that dissolved gold could exist in fluids in the mid-to-lower layers of the Earth’s crust, and that the fluids could seep into the cracks in the quartz. But the amount of fluid involved seemed to limit how much gold could be dissolved, limiting the size of the gold clumps that formed. The larger clumps were harder to explain. Experts theorized that gold nanoparticles in the fluid could agglomerate within the quartz into larger clumps, but how that worked was unclear. Unlike dissolved gold, the nanoparticles typically don’t have enough chemical energy to start the reactions necessary to accumulate on the surfaces of the cracks and form clumps.

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The new study, published Monday, Nature Geoscience, They suggest that geological stress caused by earthquakes may activate a unique geochemical property in quartz called “piezoelectricity,” which could enable the formation of larger gold nuggets.

The piezoelectric effect has been known since the 1880s. Essentially, it is the ability of a material to generate an electric charge when subjected to mechanical stress. Many everyday items make use of this effect, including microphones, musical greeting cards, and inkjet printers, and it occurs naturally in all kinds of materials, from sugar cane to bone.

Quartz is able to experience this effect because its structure consists of a repeating pattern of positively charged silicon atoms and negatively charged oxygen atoms. When it is stretched or compressed, these atoms rearrange, distributing their charge asymmetrically. Negative and positive charges build up in different areas of the quartz, creating an electric field and changing the electrical state of the material.

Voysey and his colleagues at Monash University, in a historically gold-rich area of ​​Melbourne, reasoned that this altered electrical state could lower the energy required for gold nanoparticles in the liquid to interact with the quartz surface, setting off chemical reactions that previously couldn’t take place, allowing the nanoparticles to attach and accumulate.

To test this idea, the researchers modeled the electric field that quartz creates when subjected to an earthquake-like force. They then placed quartz mineral crystals in a liquid containing dissolved gold nanoparticles and other gold compounds and found that when subjected to a force like a seismic wave, the quartz could generate enough voltage to activate the accumulation of nanoparticles.

The findings suggest an intriguing mechanism that may have been responsible for forming at least some of the gold nuggets found in Earth’s crust, particularly “orogenic” deposits — those found where two crustal plates have collided and folded over each other to form mountain ranges.

“It certainly seems like intermittent earthquakes play a key role in the formation of these significant ‘orogenic’ gold nugget deposits,” says James Sanders, a consulting geologist who was not involved in the study. Sanders says he would like to see future studies delve deeper into the details of this process, such as how long the piezoelectric-inducing seismic forces must persist to produce such deposits, and why large gold nugget deposits might occur in only some of the fractures in quartz minerals in one area, even though a given earthquake would theoretically cause the same stresses and strains in all the fractures. “I think this is a great idea/hypothesis,” Sanders says. “I’d be interested to see if it holds up on further evaluation.”

Studying piezoelectricity on a very large scale can be difficult, says geologist Aubreyia Adams of Colgate University, who was also involved in the study. “Geologists are currently working hard to quantify how stress (or pressure) varies in 3D with time and place in the crust,” Adams says. “This is easy to measure in the lab, but it’s much harder to quantify in the crust.”

Voysey and his team plan to explore the theory further by expanding the experimental parameters, including testing different pressures and temperatures. “This is really a ‘preliminary study’ of the technology, so I’m excited to see where it goes,” he says.