Gold nuggets reside primarily in quartz veins, and the current paradigm holds that gold precipitates from hydrothermal and carbon dioxide-rich fluids due to changes in temperature, pressure, and/or fluid chemistry. However, the widespread occurrence of large gold nuggets is inconsistent 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 the seismic activity that drives the formation of gold deposits means that quartz crystals within veins are stressed thousands of times. New research by scientists from Monash University, CSIRO Mineral Resources and the Australian Neutron Scattering Centre suggests that stress on quartz crystals may generate enough voltage to not only electrochemically precipitate gold from solution, but also accumulate gold nanoparticles.
Energy dispersive spectroscopy map of the sample studied by Voysey othersImage courtesy of Chris Voisey.
“Prized for their rarity and beauty, gold nuggets have been at the heart of gold rushes for centuries,” says Monash University geologist Chris Voisey.
“The standard explanation is that gold precipitates from hot, water-rich fluids that flow through cracks in the Earth's crust.”
“As these fluids cool and undergo chemical changes, the gold separates and becomes trapped in the quartz veins.”
“Although this theory is widely accepted, it does not fully explain the formation of large gold nuggets, especially considering the extremely low concentrations of gold in these liquids.”
Dr. Voysey and his colleagues tested a new concept called piezoelectricity.
Quartz, the mineral that typically contains these gold deposits, has a unique property called piezoelectricity, which means it generates an electric charge when subjected to pressure.
This phenomenon is already well known in everyday items such as quartz watches and barbecue lighters, where small mechanical forces generate large voltages.
What if earthquake stresses caused something similar to happen within the Earth?
To test this hypothesis, the researchers conducted experiments designed to replicate conditions that quartz might experience during an earthquake.
They submerged the crystals in a gold-rich liquid and used a motor to apply pressure to it, replicating 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 surprising,” said Professor Andy Tomkins, from Monash University.
“The stressed quartz not only electrochemically deposited gold on its surface, but also formed and accumulated gold nanoparticles.”
“Surprisingly, the gold tended to deposit on top of existing gold grains rather than forming new gold grains.”
“This is because quartz is an electrical insulator, while gold is a conductor.”
“Once the gold is deposited, it becomes a focus for further growth, effectively attaching even more gold to the gold particle.”
“Our findings provide a plausible explanation for why large gold nuggets form in quartz veins,” Dr Voysey said.
When the quartz is repeatedly compressed by an earthquake, a piezoelectric voltage is generated, causing dissolved gold to be reduced and precipitated from the surrounding liquid.
Over time, this process can accumulate large amounts of gold, eventually producing giant gold nuggets that fascinate treasure hunters and geologists.
“Essentially, the quartz acts like a natural battery, with the gold as its electrodes, slowly accumulating gold with each earthquake,” Dr Voysey said.
“This process may explain why large gold nuggets are frequently associated with quartz veins that form in earthquake-related deposits.”
“This new understanding of the formation of gold nuggets not only solves a long-standing geological mystery, but also highlights the interrelationships between Earth's physical and chemical processes.”
a paper A paper describing the results was published today in the journal Nature Chemistry.
_____
CR Voysey othersThe formation of gold nuggets due to earthquake-induced piezoelectric effect in quartz. National GeographyPublished on September 2, 2024, doi: 10.1038/s41561-024-01514-1
Source: www.sci.news
