Huge nuggets of gold are often associated with quartz, a ubiquitous but chemically inert mineral. The world’s largest gold nuggets can weigh almost 100 kilograms, but until now no one has been able to explain how such pieces of the precious metal are formed.
“The conundrum is how do you create a large gold nugget in one place without any obvious chemical or physical traps?” says Chris Voisey of Monash University in Australia.
He and his colleagues have now discovered a possible mechanism. When quartz is subjected to pressure, it develops a voltage that attracts the gold dissolved in the water.
The secret lies in the structure of quartz, explains Ch. Voisey. Quartz is the only abundant mineral whose crystals do not have a center of symmetry. This means that when these crystals are distorted or stressed by seismic activity, their internal electromagnetic configuration changes and they begin to produce electricity. Electricity produced by mechanical stress is called piezoelectricity.
Gold-bearing hydrothermal fluids from the middle and lower part of the Earth’s crust, located 15-20 km below the surface, are pushed up through cracks during seismic activity. However, gold is so “diluted” that it would take five Olympic swimming pools of this hydrothermal fluid to produce a 10-kilogram gold nugget.
Ch. Voisey and his colleagues hypothesized that the gold concentrates in the nuggets in the gold veins due to the piezoelectric properties of the quartz that occur during repeated earthquakes. To test this idea, the research team conducted experiments with quartz crystals placed in a solution containing gold and subjected to moderate pressure.
The quartz samples that were not subjected to pressure did not attract gold – but those that were pressurized created stress and attracted the metal. Some specimens were coated with iridium, which enhances the piezoelectric response of quartz, artificially simulating greater seismic activity. In these samples, larger pieces of gold “grew” – up to 6,000 nanometers, compared to 200-300 nanometers.
When gold began to form on the quartz, it quickly attracted more gold, according to Ch. Voisey. “Because gold is a conductor, the gold in the solution has a preference for depositing on top of the gold that’s already there,” he says. “It becomes like a lightning rod that attracts more gold.”
The study is published in Nature Geoscience.
Parengta pagal „New Scientist“.
2024-09-03 12:22:31