You first have access to an iron mine when the HQ is at level 10. The iron mine produces iron. You can use iron to build and upgrade other buildings. As you upgrade the mine you increase both the amount of iron the mine can hold and the amount of iron you produce per hour. You also gain Xp points to boost your city level. There is a total of 2 iron mines to gain during the game. You will be able to access the second mine when the HQ is at a high enough level. In order to upgrade the iron mine, the HQ needs to be at a certain level.