To get to the precious ores, you need to go deep. Very deep. In the Eos ateroid belt I have a station at 420km depth, another one at 850km depth, and finally a station at the Elysium moon belt, for storage and refueling while mining for those ores. There's no holes, but different depth regions which define what ores you can find there, and each region is a gradient where the deeper you go, the more valuable ores become more common over more basic ores. In Eos belt, it is most obvious by the asteroid shell materials corresponding to which region you are currently in.
The map of the belt regions is a common knowledge in Starbase Discord community.