The Iron Mine counts as a building.
upgrade => Medium Mine (not yet confirmed)
You need to prospect the underground (preferrably hills) outside the capital.
You can only build a Iron Mine on a square where you discovered a deposit of iron.
The construction of the building requires 1 political point, 2,000 gold and 25 wood.
The Iron Mine can employ up to 100 workers.
It produces 1 iron per worker per year.
Even if the storage (for iron) in the city is full, the workers continue to empty the reservoir.
Once the deposit is exhausted, the mine is destroyed.
You can have not more than 4 subterranean buildings (mines or quarries) in a province.