Geologists in Australia have been studying the relationship between gold deposits and earthquakes. Apparently, 80% of the world's commercial gold deposits are produced during a process called flash vaporization. Earthquakes cause extreme drops in pressure and beneath the earth's crust, and this pressure change can cause fluids to rapidly expand. According to io9, "This 'flash' effect results in the rapid deposition of silica, along with gold-enriched quartz veins."
New Scientist adds:
The fluid cannot get from the surrounding rock into the hole fast enough to fill the void, Henley says, so pressure drops from 3000 times atmospheric pressure to pressures almost the same as those at Earth's surface in an instant. The nearby fluid flash-vaporises as a result – and any minerals it contains are deposited as it does.
Later, incoming fluid dissolves some of the minerals, but the less-soluble ones, including gold, accumulate as more episodes of quake-driven flash deposition occur.
"Large quantities of gold may be deposited in only a few hundred thousand years," says Weatherley – a brief interval by geological standards. "Each event drops a little more gold," adds Henley. "You can see it microscopically, tiny layer after tiny layer. It just builds up."
Over the course of thousands of years, this process can create 100-metric-ton gold deposits. That's a lot of gold, and you can read the complete study here.