Specific Mines.
Pretty much a cave where you will only find one type of ore.
-Coal mines
-Iron mines
-Gold mines
-Etc. Etc.
Ofcourse this would be too OP(Overpowered) from the looks of it but we could decrease the loot rate by looking how much each ore is worth.
This will ensure you can get the resources you are looking for but with a cost of the chance that it may take somewhat longer.