Gold nuggets occur predominantly in quartz veins, and the current paradigm posits that gold precipitates from hot water and carbon dioxide-rich fluids owing to changes in temperature, pressure and/or fluid chemistry. However, the widespread occurrence of large gold nuggets is at odds with the dilute nature of these fluids and the chemical inertness of quartz. Quartz is the only abundant piezoelectric mineral on Earth, and the cyclical nature of earthquake activity that drives the formation of gold deposits means that quartz crystals in veins will experience thousands of episodes of stress. New research by scientists from Monash University, CSIRO Mineral Resources and the Australian Centre for Neutron Scattering suggests that stress on quartz crystals can generate enough voltage to electrochemically deposit gold from solution as well as accumulate gold nanoparticles.
Energy dispersive spectroscopic map of the sample studied by Voisey et al. Image credit: Chris Voisey.
“Gold nuggets, prized for their rarity and beauty, have been at the heart of gold rushes for centuries,” said Monash University geologist Chris Voisey.
“The standard explanation is that gold precipitates from hot, water-rich fluids as they flow through cracks in the Earth’s crust.”
“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.”
Dr. Voisey and his colleagues tested a new concept — piezoelectricity.
Quartz, the mineral that typically hosts these gold deposits, has a unique property called piezoelectricity — it generates an electric charge when subjected to stress.
This phenomenon is already familiar to us in everyday items like quartz watches and BBQ lighters, where a small mechanical force creates a significant voltage.
What if the stress from earthquakes could do something similar within the Earth?
To test this hypothesis, the researchers conducted an experiment designed to replicate the conditions quartz might experience during an earthquake.
They submerged quartz crystals in a gold-rich fluid and applied stress using a motor to simulate the shaking of an earthquake.
After the experiment, the quartz samples were examined under a microscope to see if any gold had been deposited.
“The results were stunning,” said Monash University’s Professor Andy Tomkins.
“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.”
“This is because, while quartz is an electrical insulator, gold is a conductor.”
“Once some gold is deposited, it becomes a focal point for further growth, effectively plating the gold grains with more gold.”
“Our discovery provides a plausible explanation for the formation of large gold nuggets in quartz veins,” Dr. Voisey said.
As the quartz is repeatedly stressed by earthquakes, it generates piezoelectric voltages that can reduce dissolved gold from the surrounding fluid, causing it to deposit.
Over time, this process could lead to the formation of significant gold accumulations, ultimately producing the massive nuggets that have captivated treasure hunters and geologists alike.
“In essence, the quartz acts like a natural battery, with gold as the electrode, slowly accumulating more gold with each seismic event,” Dr. Voisey said.
“This process could explain why large gold nuggets are so often associated with quartz veins formed in earthquake related deposits.”
“This new understanding of gold nugget formation not only sheds light on a longstanding geological mystery but also highlights the interrelationship between Earth’s physical and chemical processes.”
A paper describing the results was published today in the journal Nature Geoscience.
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C.R. Voisey et al. Gold nugget formation from earthquake-induced piezoelectricity in quartz. Nat. Geosci, published September 2, 2024; doi: 10.1038/s41561-024-01514-1