Currently if every single peice of cast iron and every pick axe were deleted from the game, it would be impossible to produce any more iron, because iron can only be obtained through using a iron pick axe; an unbreakable paradox, if you are without the means of admin-spawning one or the other.
So, to fix this problem, i suggest that:
1. An alternative pick axe be made possible, from bone. This pick axe isn't strong enough to dig a mine shaft (whenever that gets put in) but it is strong enough to chip at the walls of a mine and
2. Dig for iron inside the rocks on the surface. To prevent these rocks from completely replacing mines, the stone would be perminately destroyed like turning it into a grindstone or runestone does. Once destroyed, it leaves behind 9 iron ores. [edit - the number of ores could be modified by the nature/industry slider]