[throwaway5752]

It is because of geological processes like https://en.wikipedia.org/wiki/Massive_sulfide_deposits. The vast majority of heavy metals on Earth are in the core, because of gravity. Convection moves elements from inner layers of the Earth to the outer ones, and sometimes they dissolve in the ocean or get stuck in a buoyant piece like a continent before falling back down the gravity well.

It is also because chemistry, periodic table stuff related to valence electron interaction. You actually don't find veins of iron, for example you find bands of iron oxide ore like hematite. The reason those are present is one-off because iron dissolved in water precipitated out when plants evolved to product oxygen gas via photosynthesis. Gold is just relatively nonreactive so it is found in relatively pure forms, like nuggets, that get eroded out of igneous formations.