Radical theory suggests earthquakes create gold nuggets : ScienceAlert

New findings by scientists in Australia may challenge what we thought we knew about how gold nuggets thrive in the vast coral reefs beneath our feet.

Under pressures of hundreds of megapascals (tens of thousands of pounds per square inch) and extremely high temperatures, water flowing from deep within the Earth's crust carries dissolved gases and minerals to the surface with each earthquake or tremor of a seismic event.

As any good prospector knows, buried layers of crystalline silicon dioxide—known as quartz—are fertile ground for gold mining, as both materials precipitate out of solution under strikingly similar conditions.

Although the basic mechanisms behind the formation of the precious ore have been understood for some time, some details are not entirely clear, and new research by scientists at Monash University, the Commonwealth Scientific and Industrial Research Organisation and the Australian Nuclear Science and Technology Organisation challenges conventional views of how gold is formed.

“While this theory is widely accepted, it does not fully explain the formation of large gold nuggets, especially given that the concentration of gold in these fluids is very low,” says Chris Voysey, a geologist at Monash University.

Gold as an element does not dissolve easily in water, making it rare to find concentrations above one part per million. On the other hand, gold deposits represent an astonishing degree of enrichment, being thousands of times more concentrated than the diffuse solution that produced them.

There may be a number of geological and biological processes that could explain the accumulation of large amounts of gold ore at certain locations. Pieces of gold may also fall out of solution before they are concentrated in one place.

But none of this explains why gold particles might settle into a lump of quartz, clumping together in chunks large enough to make any frantic digger shout “Eureka!” So ​​Voysey and his team wondered whether the gold-quartz pairing might be more closely related than it first appeared.

Silicon dioxide is an incredibly unique material. While other crystals are relatively uniform, quartz forms with a bias that produces an electrical voltage when subjected to pressure – a phenomenon known as the piezoelectric effect.

With each tremor of the Earth's crust, layers of quartz will be broken by steady currents as voltages appear and electrons rebalance.

This jump in charge is unlikely to travel very far, since quartz is an insulator. Gold, on the other hand, is a good conductor of electricity, making it more likely that electrochemical reactions within the quartz layers could act as a catalyst, extracting enough gold from the solution in concentrated spots through repeated cycles of tiny vibrations.

“The frequencies of seismic waves vary greatly depending on their size and rock composition, but range from 1 Hz to more than 20 Hz,” Voisy explained to ScienceAlert.

“Each of these waves can distort the quartz crystal and cause a piezoelectric potential to build up, which may have some chance of reducing gold from the nearby solution.”

To test whether this reduction was enough to cause the gold grains to grow in size, the researchers placed ten tiny quartz slabs cut from a natural crystal in aqueous solutions of gold.

Half of the panels were then shaken 20 times per second for an hour to simulate a small earthquake, producing an electrical voltage of between 0.4 and 1.4 volts. The other half were left alone in their bathtubs to serve as controls.

Scanning electron microscope analysis showed that micrometer-sized gold particles formed on the vibrating tiles, while none appeared on the controls.

Subsequent tests using naturally occurring gold-spotted quartz slabs revealed that the tiny “seeds” thrived more in the solutions when pressure was applied. Crucially, these tiny gold bases were prioritized as ore-forming sites, their presence reducing the chance of new gold grains forming nearby.

(Left): Scanning electron microscope image of gold deposited on quartz via piezoelectric interactions during experiments. The doping is a magnified image revealing that the gold grain texture consists of multiple gold nanoparticles bonded together. (Right): Energy dispersive spectroscopic map of the sample. (Chris Voysey)

“Once some gold is deposited, the chance increases because the gold will act as a catalyst for further reaction because of its conductivity,” Voisi explained to ScienceAlert.

What is simulated in the laboratory using concentrated solutions and long periods of vibration will of course take much longer in the real world using dilute solutions and occasional vibrations.

But on a geological scale, this process may be relatively rapid. Without the additional shock of stressed quartz, it is difficult to even explain how gold could have accumulated in such rich deposits in the first place.

Piezoelectricity may also explain why veins of gold appear to “float” in layers of quartz, with no obvious cracks or geochemical differences to explain their arrangement. Nothing but the metallic hiss of lightning storms left by the trembling ground shows the gold dust where to gather.

This research was published in the journal Nature GeoScience.

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