I'm sure that Jorbtar have heard of this before, but I couldn't find a post after searching. Anyway, I have an idea for implementing early world metal.
https://en.wikipedia.org/wiki/Bog_iron
To sum up the wikipedia article, bog iron occurs when water carries dissolved iron to swamps and springs, where it forms iron oxide. People, especially from Northern Europe, have been using it as a source of iron since before the Roman empire. Usually it's harvested by removing the top layer of material, usually peat, from the swamp to reveal the iron ore granules beneath. The bog iron naturally grows over time.
I propose the addition of bog iron to the game as a foragable ore, requiring relatively low per*exp. Naturally, it will be found in swamps and be fairly rare. It should have a very low smelting chance, to give an incentive for actual mining, around 4%, or one bar per full smelter. Also, it might be necessary to limit their quality.