A mountain may yield those, but may also yield gold or gems, sometimes in association with the commoner minerals. They earn money.
A miner's first visit costs $100. He sets up the mine as a going concern producing the first level of each discovered product (two of each common metal but only one of each precious metal) each turn, then he can move on to develop another.
After the appropriate technology is available and purchased, the miner can return for further development, costing $1000 and doubling the output.