A replica of the Welcome Nugget, the second largest gold nugget ever discovered. Anatoli Lvov/CC BY-SA 3.0 –
Gold doesn’t form here on Earth – it’s thought to be forged in supernovae and collisions between neutron stars. These cataclysms then scatter it through the cosmos, so the shiny stuff then turns up in the dusty discs from which planets form. As such, trace amounts of gold have been present on Earth for billions of years. The heavy element sinks deep into the planet, but seismic activity can push it back up near the surface, while asteroid strikes can deliver more.
Most of the time, gold takes the form of small flakes, but on rare occasions it can form larger nuggets weighing up to a few dozen kilograms. Exactly how it clumps together into these nuggets remains a bit of a mystery.
“The standard explanation is that gold precipitates from hot, water-rich fluids as they flow through cracks in the Earth’s crust,” said Dr. Chris Voisey, lead author of the study. “As these fluids cool or undergo chemical changes, gold separates out and becomes trapped in quartz veins. While this theory is widely accepted, it doesn’t fully explain the formation of large gold nuggets, especially considering that the concentration of gold in these fluids is extremely low.”
For the new study, researchers at Monash University investigated a new suspect you might not expect: electricity. Quartz is piezoelectric, meaning it produces an electric charge when mechanical stress is applied, a phenomenon that’s used in things like watches and lighters. The team wondered if this effect could be causing gold to clump together into nuggets, with the mechanical stress coming from earthquakes.
To test the idea, the researchers submerged quartz crystals in a fluid that was rich in gold, then used a motor to simulate the kind of stress the mineral might experience from earthquakes. Afterwards, they used a microscope to check the growth of gold.
“The results were stunning,” said Professor Andy Tomkins, co-author of the study. “The stressed quartz not only electrochemically deposited gold onto its surface, but it also formed and accumulated gold nanoparticles. Remarkably, the gold had a tendency to deposit on existing gold grains rather than forming new ones.”
The key seems to be that quartz is an insulator, while gold is famously a great conductor of electricity. Each of these little piezoelectric zaps pulls more dissolved gold out of the surrounding fluids to settle on the grains already there.
“In essence, the quartz acts like a natural battery, with gold as the electrode, slowly accumulating more gold with each seismic event,” said Dr. Voisey. “Our discovery provides a plausible explanation for the formation of large gold nuggets in quartz veins.”
The research was published in the journal Nature Geoscience.
Source: Monash University via Scimex
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