Why is there gold in the earth's crust?

Question

I would naively assume that when the earth was formed and still molten, all the heavy stuff such as gold would have sunk to the centre, so almost nothing would remain in the earth's crust where humans could access it. Is it that "almost nothing" is not nothing? Or was it added to the surface by later impact events as suggested on Wikipedia?

The answers to Why is there Uranium in Earth's Crust? and Why is uranium only in the crust, really? seem to be that uranium chemistry is such that it was carried along with lighter elements to the surface, but isn't gold chemistry much less favourable for such processes? Is this why gold is 500 times less common than uranium in the earth's crust?

Answer 1

Is it that "almost nothing" is not nothing?

It's obviously "almost nothing" because there still is gold in the Earth's crust. That said, while gold is 20 times more abundant in the solar system than is uranium, it is 600 times less abundant than uranium in the Earth's crust. That's a close to a perfect differentiation. No chemical process is perfect. After all, there is still a good amount of iron and nickel in the Earth's crust and mantle. Not as much as in the core, but still a good amount.

Whether the gold that remains in the crust is primordial or came from late bombardment asteroids remains highly debated.

Answer 2

Assuming gold, a siderophile, is more abundant in the metallic core, this gold may still be dredged to the surface.

On Earth, there is more to tectonic activity than meets the eye. Hoggard et al. 1, with a summary in Ref. 2, describe mixing processes in the Earth that extends throughout the mantle and into the core (Picture from [2]):

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Thus elements from the core may be brought up to the upper mantle and crust via the rising black plumes pictured above. The amount of material rising from the core in this manner is, of course, small, so only small amounts of siderophile elements such as gold emerge at the surface/crust in this way. Siderophiles that are more reactive to oxygen, such as iron, can partition directly into the mantle and thus may be convected into the crust more efficiently.

Reference

M.J. Hoggard et al. (2016) "Global dynamic topography observations reveal limited influence of large-scale mantle flow." Nature Geoscience 9, 456–463. https:// doi.org/10.1038/ngeo2709

https://www.geologyin.com/2016/05/map-of-flow-within-earths-mantle-finds.html