I had one iron ore mine that I exhausted. It was previously owned by Greece, but I stole it. I then made 5 iron ore units and it was finished. I literally sat there from classical age to industrial not able to make any iron units. Furthermore, trading for iron with AI/city states did not re-enable iron ore usage. I'm thinking this is a glitch, so I posted it in the bug forums. If you only have one exhausted mines, it seems like trading for more iron does not re-enable building iron units. So What's the deal with iron? Does it run out permanently and never regenerate? If so this seems just plain stupid, as I might as well restart an emperor game each time I do not have at least a couple of sources of iron. My last game was going quite well until I had to sit around for 100 turns waiting for musketmen/cannons as I had no iron to build/upgrade my units to keep up with the AI.