How Large Gold Nuggets Form in Quartz Crystals During Earthquakes

Earthquakes may help prospectors strike gold: When these seismic events stir and squeeze quartz veins deep underground, the mineral creates an electric field that gold particles clump to, forming gigantic golden nuggets.

Geologists have known that earthquakes can produce bits of gold on quartz. However, a new study published last week in Nature Geoscience details precisely how the seismic waves form the coveted nuggets, helping to solve a long-standing mystery about how gold can cluster together and grow into large, prized pieces.

“Gold forms in quartz all the time,” lead author Chris Voisey, a geologist at Monash University in Australia, tells Live Science’s Sascha Pare. “The thing that’s weird is really, really large gold nugget formation. We didn’t know how that worked—how you get a large volume of gold to mineralize in one discreet little place.”

Quartz can generate electrical charges when squeezed, stretched, compressed or vibrated and is well-known for powering quartz watches. The type of charge produced is called piezoelectricity, and quartz is the only abundant mineral on Earth that creates it, according to the study.

About 75 percent of gold mined on Earth comes from deposits found in the fissures of quartz, per the paper. The precious metal naturally forms in quartz when it is flooded with hydrothermal fluids from Earth’s crust that contain dissolved gold. In theory, the gold should cover the quartz evenly, because the hydrothermal fluids cover its surface. But in reality, the nuggets only form in concentrated areas. So, the team of Australia-based researchers wanted to figure out why.

To study how earthquakes help form the gold nuggets, scientists conducted lab experiments with quartz slabs that were submerged in gold-rich liquid. Then, they mimicked seismic waves by placing the slabs into a machine that simulated the shaky motion of an earthquake. Afterward, they checked the quartz and found gold along its surface.

Put simply: “If you shake quartz, it makes electricity. If you make electricity, gold comes out,” of the hydrothermal liquid in Earth’s crust, Voisey tells Robin George Andrews of National Geographic.

The process happens because quartz is an insulator, and gold is a conductor. Any gold that has formed on the crystal takes on the voltage from the quartz and acts like a magnet for additional gold particles in Earth’s hydrothermal liquid. In this way, more gold is drawn out from the solution in areas with pre-existing gold grains. “In essence, the quartz acts like a natural battery, with gold as the electrode slowly accumulating more gold with each seismic event,” Voisey tells Newsweek’s Tom Howarth.

With each passing earthquake, the nugget grows. The largest nuggets in orogenic gold deposits—those that form between tectonic plates that have crashed together to create mountain ranges—weigh about 130 pounds, reports Live Science. In nature, gold nuggets are most likely created not by one earthquake but by multiple.

“It appears to be a certainty that episodic earthquakes are important in helping form these important ‘orogenic’ gold nugget deposits,” James Saunders, a consultant geologist who was not involved in the study, tells Scientific American’s Kate Graham-Shaw. He adds that future research could get even more specific—exactly how long does an earthquake have to last to create a gold nugget, and why do these deposits form in only some of an area’s quartz veins?

“I think it is a great idea/hypothesis,” he adds to the publication. “I'll be interested if it stands up upon further evaluation.”

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.