concept

Siderophile element

The "metal-loving" elements - platinum, gold, iridium, nickel and their relatives - which dissolve into liquid iron rather than staying in molten rock, by preference factors of ten thousand or more. So when metal sinks to form a core it strips them from the crust: most of Earth's gold is in the core, unreachable. That is why a heavy orebody in a crust is something that needs explaining.