i was thinking about self sustaining worlds and all that and metal came to mind and the first thing i thought of were rock golems.
monsters you can find underground like trolls. except stronger. when dead you can mine them for ore. the ore you get depends on the level you're on.
if you have a better idea for infinite metal, lets hear it. "prayer for metal" isnt in the game yet, and was a painfully slow way of getting it anyway.