A replica of the “Welcome Stranger,” a 100 kg gold nugget discovered in Australia in 1869.
Ian Dagnall/Alamy
Earthquakes can create electric fields that attract gold dissolved in liquids pushed up from deep within the earth, causing gold nuggets to form in the quartz.
Giant gold nuggets are often associated with quartz, a ubiquitous but chemically inert mineral. The world’s largest gold nuggets can weigh nearly 100 kilograms, but until now no one has been able to explain how such masses of precious metal formed.
“The mystery was how someone could create such a large nugget of gold in one place without any obvious chemical or physical traps,” he said. Chris Voysey At Monash University, Melbourne.
Voysey and his colleagues discovered a possible mechanism: applying pressure to the quartz creates a voltage that attracts gold dissolved in water.
The secret lies in the structure of quartz, Voysey explains. Quartz is the only abundant mineral whose crystals have no center of symmetry. This means that when these crystals are strained or stressed by seismic activity, their internal electromagnetic makeup changes, generating electricity. Electricity generated in response to mechanical stress is known as piezoelectricity.
Gold-bearing hydrothermal fluids rise up through fissures during seismic activity from the mid-to-lower crust, 15-20 km below the surface, but gold is so dilute that it would take the equivalent of five Olympic swimming pools of hydrothermal fluid to produce 10 kg of gold.
Voysey and his colleagues hypothesized that the piezoelectric properties of quartz would cause the gold to concentrate in nodules within the veins during repeated earthquakes. To test this idea, the team performed experiments in which they placed quartz crystals in a gold-containing solution and applied moderate pressure from an actuator.
Quartz samples that were not subjected to pressure did not attract gold, but samples subjected to force generated a voltage and attracted the metal. Some of the samples were coated with iridium to accentuate the piezoelectric response of the quartz and artificially mimic the expansion of seismic activity. In these samples, large gold flakes grew, over 6000 nanometers, compared to 200-300 nanometers in uncoated quartz.
Once gold starts to deposit on the quartz, it quickly attracts other gold, Voysey says. “Gold is a conductor, so gold in solution tends to deposit on top of existing gold,” he says. “It becomes like a lightning rod that attracts more gold.”
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