Iron Vein
From the Slaves Wiki
Iron veins are ore nodes in the wilds. You need a Pickaxe to mine one: stand next to it and press F or the pickaxe's slot. A vein has 60 HP and a pickaxe deals 20 per swing, so it breaks in 3 swings for 1 Iron.
Workers
Slaves assigned to a Blacksmith walk to the nearest vein and mine it with a pickaxe, much more slowly than you do, then carry the iron back to the forge.