Gold ions are found in dilute concentration in both seawater and the magma. Seawater slowly seeps through fractures in rock and ends up underneath land. Sometimes seawater gets into magma, like at plate boundaries where ocean crust subducts beneath the land, and takes some water down with it, like the Andes. The incredible heat and pressure of the magma makes the now superheated magmatic water really, really want to escape. When it finds a crack in the crust it starts flowing up into it. The seawater underneath the land also gets heated up by the magma underneath it and gets driven upwards by sort of the same idea. But the crust is a lot cooler, and in the process the gold-laden water cools down too. The colder it gets, the less gold it’s able to hold onto, so it starts dropping gold atoms all over its subterranean path until it slowly builds up into a layer.
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