As Earth formed, most of its heavy metals, including iron and gold, sank to its molten core. But as the planet cooled, small amounts of heavy metals remained in its rocky mantle. Sometimes, gold trapped in Earth’s mantle works its way back up to the surface by hitching a ride during a geological shakeup––like an earthquake.
North American Gold
Published August 29, 2018
Onscreen: Where does North American gold come from?
Lisa White: This is such an incredible mine for so many different reasons. Unbelievably they dug out the earliest part of this mine by hand.
Kirk Johnson: Wow look at that, you can actually see the pick marks where they carved this thing.
As Lisa leads me further into the mountain, we spot a change in the rock.
White: Ah, here we go.
Johnson: That’s something different.
White: It is, it is. It’s a quartz vein. It’s important and distinctive because when the miners would locate these quartz veins they knew they were in the area they wanted to be in for gold.
Johnson: So the gold is actually in the quartz?
White: The gold is in the quartz, yes it is…and so looking for these was the way to the gold. I’ve got a quartz sample here in my pocket with some gold flakes in it.
Johnson: Wow there are a lot of flakes there aren’t there?
White: Right, it’s really beautiful.
Johnson: Yeah, if you like gold! But the gold is in the quartz but not in the rock next to the quartz?
White: Exactly, so that’s key to understanding its formation.
Johnson: When Earth was forming, most heavy metals like iron and gold sank to the molten core. But small amounts remained in the rocky mantle as Earth cooled. Later asteroids deposited more of these metals. But for gold to work its way back up to the surface, it had to hitch a ride on some kind of geological shake up. Like an earthquake. Deep below millions of years of earthquakes and pressure from molten rock have created a network of cracks—these provide pathways for superheated water full of minerals, like gold from deep in the earth.
When that hot fluid rises up through the fractured rock, it cools down, and the minerals carried within crystalize. Over time, that builds up a vein of quartz. And anything trapped inside the quartz—are bits of gold. Over millions of years, they formed the seams found all over California.
White: So every time you look at a vein of quartz you’re really seeing an ancient earthquake in many ways.