How earthquakes build solid gold nuggets

When strained by earthquakes, underground networks of quartz veins can generate enough tension to leach gold from passing fluids, researchers report Sept. 2 in Nature Geoscience. The findings explain how fluids carrying small amounts of gold can fabricate large pieces, even in chemically inert environments.

“You find a quartz vein 2 meters wide, and there’s a big gold nugget right in the middle, and nothing around [that] could have reacted,” says geologist Christopher Voisey of Monash University in Melbourne, Australia. “This is a puzzle.”

Most of the world’s gold is mined from networks of branching mineral layers – or veins – made mainly of quartz, called orogenic gold deposits. These deposits are built piecemeal by earthquakes, usually about six to 12 kilometers underground. The vibrations open fractures, which are then infiltrated by underground fluids that deposit quartz and gold, forming veins. Subsequent earthquakes create more fractures and reopen existing veins, gradually swelling and branching the deposit over time.

Geologists have shown how this process can form small, widespread amounts of gold, but not how the metal concentrates in large chunks, Voisey says. The fluids involved contain small amounts of gold and the quartz is a non-reactive medium.

Voisey and colleagues hypothesized that the key was quartz’s piezoelectricity, or its ability to develop an electrical charge when strained. They immersed quartz plates in solutions containing dissolved gold or gold nanoparticles and used actuators to shock some of the immersed plates at a frequency of 20 hertz, simulating small earthquakes (SN: 4/12/23).

The shock generated voltages of up to 1.4 volts, causing gold grains to accumulate on the plate’s surfaces. No gold deposited on the plates that were not struck.

When the experiment was repeated with part of a gold-quartz vein, the gold deposit was concentrated on the already existing gold, Voisey says. The existing gold takes on the tension of the charged, less conductive quartz, attracting more gold to itself and concentrating the mineralization, he explains. “It’s like a lightning rod for further feedback.”