How do gold nuggets form?

 Important factor behind the creation of large gold nuggets are earthquakes

Gold has always been a hot commodity. But these days, finding a nugget isn’t too tricky: Much of the world’s gold is mined from natural veins of quartz, a glassy mineral which streaks through large chunks of Earth’s squashed-up crust. But the geologic process that put gold nuggets there in the first place was a mystery. Scientists have finally solved a long-standing mystery about the geologic process behind these large pieces of gold found in quartz rock. Crack open a chunk of white quartz from a gold mine and you might see bright metal streaks inside. For more than a century, geologists looked at scenes like that and said, “Gold got here in hot water.” They meant that super‑hot fluids moved through cracks in the rock, carried dissolved gold, and then left that gold behind when conditions changed. The idea explains a lot, but it raises a tough question: those fluids usually carry only tiny amounts of gold compared with the volume of water, so how can that kind of solution leave behind large nuggets inside quartz, a mineral that hardly reacts with anything? This puzzle still bothers geologists. The nuggets owe their existence to the strange electrical properties of common quartz. When squished or jiggled, the mineral generates electricity. That drags gold particles out of fluid in Earth’s crust. The particles crystallize out as grains of gold, and, over time, with enough electrical stimulation, those grains bloom into nuggets.

“If you shake quartz, it makes electricity. If you make electricity, gold comes out,” says Christopher Voisey, a geologist at Monash University in Australia. Earthquakes are the most likely natural source of that shaking, and the team’s lab experiments show that earthquakes can make gold nuggets. To probe that idea, the scientists ran a series of controlled lab experiments. They placed pieces of quartz into solutions containing dissolved gold, similar to hydrothermal fluids deep underground. Then they mechanically stressed the quartz to imitate the sudden push and pull of an earthquake and examined the crystal surfaces with high‑resolution microscopes. Metallic gold appeared: bright specks, clusters of nanoparticles, and small pseudo‑hexagonal crystals perched on the quartz grains. These shapes match what researchers expect from electrochemical deposition, where dissolved gold ions gain electrons and turn into solid metal on a surface. Instead of always using quartz with no metal, they also started with quartz that already contained a little gold, much closer to a natural vein. In that setup, the small gold grains acted as conductors within the system. When stress created an electric field in the quartz, those metal grains concentrated the field around themselves, so new gold nanoparticles tended to grow on and around the older ones, forming halos and tight clusters. Under these conditions, the electric charges could “plate” gold out of solution, so fresh metal coated the quartz surface and thickened the deposits.

The idea that gold nuggets appear because of electricity instead of a more conventional geologic process is, at first, a peculiar thought. But “it makes complete sense,” says Thomas Gernon, a geoscientist at the University of Southampton in England. Quartz veins host a disproportionate number of gold nuggets and their environments experience plenty of earthquakes. Geologist Christopher Voisey at Monash University, together with colleagues at CSIRO and the Australian Centre for Neutron Scattering (ANSTO), tested a different twist on the story: electricity generated during earthquakes can help build gold inside quartz veins. They focused on a property of quartz called piezoelectricity. When a quartz crystal is squeezed, bent or twisted, its atomic structure shifts enough to separate positive and negative charges. One side of the crystal becomes relatively positive, the other relatively negative, so a voltage appears across it. The same effect drives quartz watches, but there it is carefully controlled by tiny electrical circuits. Fault zones that host gold deposits contain many quartz veins. In those zones, rocks break, slip and grind past each other as tectonic plates move. During a quake, stress builds up in the quartz and is released, so piezoelectric charges appear and fade. The team asked a simple, testable question: are those voltages strong enough to move electrons, pull gold out of solution, and attach gold directly to quartz surfaces?

Gold is extracted from a variety of geologic deposits, but it’s frequently found within quartz veins. From afar, alabaster sheets of quartz can look like bright cobwebs weaving through rock. Gold-bearing quartz veins are found in parts of the crust that have undergone a lot of stress and strain from events like mountain formation. These stressed, warped and fragmented areas are riddled with faults. When faults rupture during earthquakes, hot geologic fluids, sometimes containing gold particles, rush into the cracks, cool and form gold-rich quartz veins. It’s normally thousands to tens of thousands of pulses of [this] fluid that comes in during earthquake events then, over time, that builds an orogenic gold deposit. These gold particles find their way into quartz veins isn’t unexpected. But within these veins, miners tend to find large nuggets of gold sitting by themselves rather than just tiny grains all over the place. “How do you get such massive concentrations of gold in quartz veins?” says Iain Pitcairn, an ore geologist at Stockholm University in Sweden. “It’s strange and difficult to explain that.” Something must be forcing all the gold particles into specific locations, but what? In a fault zone filled with such veins and bathed in gold‑bearing fluids, each earthquake briefly turns the system into an electrochemical cell. On some quartz surfaces, electrons accumulate, and dissolved gold species pick up those electrons and become metallic gold. On other surfaces, complementary reactions occur, and charged ions in the fluid move to balance out the charges. Each “squeeze”, each earthquake, charges the quartz a little and drives a small amount of gold plating onto existing grains or onto fresh nucleation sites.

Quartz itself is also quite odd. It’s a simple mineral, made with just silicon and oxygen. But it’s also the only common mineral whose crystals lack a center of symmetry, meaning it’s structurally wonky. This means, under certain conditions, quartz’s internal electrical configuration is also imbalanced, which allows it to do something weird: create electricity. Quartz doesn’t spark up by itself. But if you apply a force to a quartz crystal, stamp on it, say, then it generates an electric field. This phenomenon is known as piezoelectricity (which derives from piezo, the Greek word for “push”). The more force you put in, the higher the piezoelectric response. If you hit a quartz crystal hard enough that it breaks, you’ll get the most voltage you could possibly get out of it. Once there is even a tiny “seed” of gold, that grain becomes the preferred place for more gold to plate out during each stress event. Quartz acts as an electrical insulator and does not let electrons move easily through its interior, which makes it hard to start nugget growth from nothing. Gold, on the other hand, conducts electricity well. As soon as a small conductive grain forms, it concentrates the electric field at its surface and lets electrons move efficiently right where they are needed. As the reactions continue, the system develops a “rich get richer” pattern: fewer, larger gold pieces rather than many tiny ones. In another set of experiments, the team immersed quartz in a liquid filled with gold nanoparticles. When they stressed the quartz, those particles no longer stayed spread out evenly in the fluid. They drifted, gathered and clumped into larger clusters directly on the quartz surface. “The results were stunning,” said study co-author Professor Andy Tomkins, from the Monash University School of Earth, Atmosphere and Environment. “The stressed quartz not only electrochemically deposited gold onto its surface, but it also formed and accumulated gold nanoparticles,” Tomkins explained. “Remarkably, the gold had a tendency to deposit on existing gold grains rather than forming new ones.” This behavior shows that electric fields around stressed quartz can gather and concentrate mobile gold particles even before they fuse into a continuous grain.

In nature, quartz veins bearing gold nuggets are probably formed not through a single earthquake event, but by a cornucopia of them. After the first few quakes sprout grains of gold, additional earthquakes cause more and more gold particles to crystallize atop those grains, eventually forming nuggets. The electrical mechanism does not replace classic models of gold formation. Hot, gold‑bearing fluids still have to move through fractures at suitable temperatures and pressures, and changes in fluid chemistry still help metal separate from solution. The new work adds an extra step: piezoelectric voltages during earthquakes focus gold growth onto particular spots in quartz, especially where some metal already exists. Most of the world’s large gold nuggets come from quartz veins in orogenic gold systems, which supply roughly three‑quarters of the gold mined in human history. In many of these deposits, miners encounter large lumps of metal in thick veins instead of a thin dusting of gold spread everywhere. In essence, the quartz acts like a natural battery, with gold as the electrode, slowly accumulating more gold with each seismic event. This study suggests that seismic activity, by charging and discharging quartz over geologic time, helps explain the tight partnership between gold and quartz and the rare cases where nature builds especially hefty nuggets. Centuries ago, the notion that you could shake run-of-the-mill quartz about and generate gold would be considered nothing short of alchemy. Study shows that, although nature is capable of acts of magic, it just takes the right team, and the right experiment, to reveal how the trick is performed. And sometimes, the secret seems to be disarmingly simple for all to understand.