> Without that impact, it is assumed that almost no platinum-group metals and gold would have remained in the crust.

Wow, its wild to think of a counterfactual world without gold. Would those metals have emerged to the crust from volcanism or is that material not sourced deeply enough?

Volcanism at most brings material from the upper mantle, but usually such material becomes mixed with material from the crust, while ascending.

The mantle has slightly bigger concentrations of precious metals than the crust, but the concentrations remain many times smaller than in the core.

The reason is that both the mantle and the crust are made mostly of silicate rocks. The mantle is made of heavy silicate rocks and the crust is made of light silicate rocks, which float on the denser mantle.

The metals that are resistant to oxidation do not mix well with silicates, so they tend to segregate from them, and then, being heavier than rocks, they tend to descend towards the core. If they reach the core, then they dissolve into the melted iron.

When lava is expelled by volcanism, the precious metals contained in it usually separate from the silicates together with metal sulfides and arsenides, which makes them easier to find than if they were dispersed uniformly in the rocks. Other elements that ere much more abundant, for instance germanium and gallium, are harder to mine than the precious metals because they are not concentrated in distinct minerals but they are uniformly distributed in many rocks.