[jhayward]

Deep in the earth matter tends to clump together by density, just as oil separates from water due to density. There is still some dynamism though so temporary mixing does occur.

Gold at earth's surface tends to be found in deposits where water with gold particles in it was flowing through a crack for an extended period of time.

Over time some of the gold is deposited on the sides of the crack, essentially concentrating it through evaporation and deposition.

These cracks then become what miners call 'veins'. Other metals and minerals have similar stories.

TLDR: water from deep in the earth carries gold particles upward. Cracks form consistent flows which concentrate deposits over time.

[pfdietz]

Materials separate by MACROSCOPIC density. However, heavy elements can be chemically bound in light materials. Uranium, for example, is highly concentrated in the Earth's crust compared to the mantle.

[jhayward]

Good point, although I believe gold is usually found in its elemental form (although sometimes alloyed with other metals).

I didn't know that fact about U, thanks.

[pfdietz]

Gold is a siderophile element, chemically favoring to bind with iron metal, and preferentially went into the Earth's iron core when the planet differentiated.

This is why mining asteroids for siderophiles (like gold, but also platinum group elements) has been a staple of space advocacy.