The oldest hard-rock mountains are 1.5 billion to 3.5 billion years ago. Those include some heavily mined areas, such as the Appalachians, If a previous civilization had been into serious mining, we'd probably have seen evidence of that by now. There may be nothing left of the mine, but an area without the good stuff and the remains of a big rubble dump would be interesting.

Conversely, a concentration of rusted metal is not likely to occur naturally.