Did the majority of Earth's precious metals sink below the crust during Earth's formation?
This is in part marketing hype by wanna-be asteroid mining companies. That said, some asteroids are suspected to be richer in precious metals than is the Earth's crust. For example, the Earth's crust is significantly depleted in gold compared to the solar system as a whole. I wrote about the reasons why this is the case at physics.stackexchange.com.
Gold and related precious metals are siderophiles, which means "iron-loving". When the Earth differentiated, the iron and nickel that sank to the center of the Earth took other siderophiles with them. In a sense, the precious metals are more siderophilic than is iron itself. Gold et al. easily dissolve in molten iron. Precious metals are so chemically inert that they do not readily combine form compounds with other elements.
There is a lot more gold and other precious metals in the Earth's core than there is in all of the asteroids combined.
I have no knowledge of the quantified specifics, but would like to point out two effects that may be relevant:
- We already exploited the easiest precious metal deposits on the top of Earth's crust to the extent that we could find and access them. This goes back millenia and intensified in recent centuries. Modern Earth is not natural.
- Asteroids can be cherry-picked. You want to mine the freak asteroid, not the normal asteroid.