Most of the world’s gold nuggets come from veins of quartz found in ancient mountain belts. Hot solutions of water and dissolved elements migrated deeply into these regions when they were active, depositing quartz along faults, fractures, and other rock boundaries. From those hydrothermal fluids, bits of gold also crystallized.
Until now, scientists have wondered how the gold could be concentrated enough to form such large (pebble- to fist-sized) nuggets. New research published in Nature Geoscience shows that ongoing earthquakes may have seeded and grown them.
Before this work, it was “almost impossible” to explain the concentration of gold located in some parts of quartz veins, said Damien Gaboury, a retired geologist from the Université du Québec à Chicoutimi in Canada who wasn’t involved with the work.
“Because the solubility of gold is low in natural fluids, it would take a huge volume of fluid to make a gold nugget in a vein,” said Chris Voisey, a geologist at Monash University in Melbourne, Australia, who led the new work. For example, if there’s 1 part per million of gold in solution, it would take five Olympic swimming pools of that fluid to form a 10-kilogram (22-pound) chunk of gold.
That’s where earthquakes come in. Earthquakes send seismic waves rippling through the earth, compressing and deforming rocks. Quartz, a mineral made of silica, has a property known as piezoelectricity. When quartz is squeezed, it produces electrons and generates voltage. The voltage caused by tectonic stress such as that from earthquakes can play a role in some chemical reactions. Voisey and his colleagues wondered whether the reactions that form gold could also be affected.
Pounding Quartz
Voisey and his colleagues tested the idea. “What I wanted to do was basically make an earthquake in the lab,” Voisey said. Using a tool akin to an automated hammer, he pounded quartz crystals in a gold-bearing solution for an hour. When he looked at the crystals under a microscope, he saw tiny bits of metallic gold stuck to the quartz. “I went nuts,” Voisey said. “I was, like, jumping around the lab, throwing chairs.” A quartz piece that hadn’t been hammered didn’t show signs of gold metal.
The researchers noticed that the gold had clustered on the mineral slabs that had formed gold, forming small particles rather than a uniform coating on the quartz. “It was basically trying to make nuggets,” Voisey said.
When the group hammered quartz pieces that naturally had gold, they saw that gold from the solution deposited onto gold already in the vein. “That was the exciting bit,” he said. “This was where we could start building large gold accumulations.”
Because this deposition of gold is an electrochemical process, the gold in solution prefers to glom onto metallic gold, an electrically conductive material, rather than onto quartz, which is an insulator and tends to trap charges rather than allow a current to flow.
“This type of mechanism is exactly what you need to get that gold out of solution,” said Iain Pitcairn, an ore geologist at Stockholm University in Sweden. “You need something that traps the gold that’s in the fluid quickly and in one place, or else it just dissipates away and it never forms a gold deposit.”
Although this experiment provides a first demonstration of how gold can form metallic chunks on environmental quartz electrochemically, the process is reminiscent of industrial processes for depositing gold, Gaboury noted.
The places where orogenic gold deposits occurred would have experienced multiple low-magnitude earthquakes every day for between 10,000 and a million years, allowing gold to build up nearly continuously, he said.
Scientists have known that gold in natural fluids can form supersmall particles in some ores. And researchers have explored how quartz’s piezoelectricity can kick off reactions to produce hydrogen. But how long, interconnected networks of gold formed in quartz was still unknown, said Pitcairn, who was not involved in the study. “It’s the first time that somebody’s really explained that kind of texture.”
The new study puts together what scientists knew about quartz veins and the material properties of quartz. “It’s probably one of the most important contributions since decades of research on gold deposits,” Gaboury said.
There’s far more to explore at the intersection of quartz piezoelectricity and geology, Voisey said. The team plans to continue investigating earthquake-enabled deposition and whether it can seed other minerals, such as those containing both gold and antimony.
The quest to understand orogenic gold deposits isn’t over yet. Researchers still don’t know what makes some veins stuffed full of gold and others not, Pitcairn said.
—Carolyn Wilke (@CarolynMWilke), Science Writer
Citation: Wilke, C. (2024), Earthquakes may lace quartz veins with gold, Eos, 105, https://doi.org/10.1029/2024EO240447. Published on 8 October 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
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