This suggestion should be simple enough.
While the romans developed pretty good mining techniques to get metals back in the day, large parts of northern Europe was mining its iron out of bog deposits, to the extent of "most Viking era iron was smelted from bog iron". (http://en.wikipedia.org/wiki/Bog_ore)
With Haven and Hearth being placed in a pre-civilization / borderlands-type / central to northern-Europe setting, I feel adding bog iron would fit in great. In addition, it's a different source of metal than the current cave-exclusive one, which removes some of the dependancy on caves, torches (or night vision clients) and mineholes to get the early tools, which could help make the startup phase more varied than it currently is now.
What I'm suggesting, is adding nuggets of iron (cast?) foragable in swamp areas. They'd spawn in swamps and marshland like grey clay does at shallows, on harvest yielding two nuggets at maxed Industry and none at maxed Nature, with one nugget at the base. It's quality could be attatched to the soil quality of the swamp / marsh rather than an individual or rock-based quality, as I would believe its interaction with the swampland would naturally affect the metal. Their spawn rates and such would be like most foragable items, and not linked to any sort of spawn node - unlike minebased metal, foraged metal would not be "mined out".
Considering that finding a good-quality swamp is harder than digging down deep and finding good quality ore in the ground, plus that a foraging-based spawn system would limit the ammount available within a short time frame, I don't feel that this type of metal would make regular mining-based metal redundant. It would simply be a potential method to get started with the metal tools in the earlier stages of gameplay without having the full industry set up yet.