It might be that we will have to use some kind of detection device (metal detector or whatever) to figure out where ore veins are underground.
No more giant above-ground-rock-type-ore-blocks" that sream "HERE I AM" (which I never liked anyways), but a skill that you have to learn (perk) and build (device).
Then you have to equip your detector and walk around, and when you are near/above an ore vein it is displayed on your "radar/UI/compass-thingy", similar to the already implemented "animal tracker" perk (and even more logic/realistic in case for the metal detector)