[frostix]

Most likely do to some process resulting in separation by density. You’d probably also need a geologist to contribute who understands previous states of earth. I’d hazard a guess that most of it occurred over time which much of earth was in a sort of fluid hot state and things clumped together. Structures you see today might be due to the surrounding materials that also tended to clump and how things cooled over time (slow, fast, etc.) There’s also the fact earth still has active convection going on heating things up, spinning them around, letting gravity pull, rising up, cooling, and then more complex motions like plate tectonic movement, fracture, etc. I suspect it would be pretty difficult to say exactly why any specific mineral deposits tend to follow the structures we find them in due to how complex the process is but someone may know.