I tend to think in terms of quantity. You can find traces of Iron almost anywhere. In absolute terms, there are very few places were there is not enough iron for whatever use you might have for it. The question is, is it easy to get at, and is it concentrated? One could sift through the rocks and soil and find iron... but that takes a looong time, is inefficient, and who's going to bother spending their days doing that? Well, the answer is, a scientist might. A man (or woman) with a passion for alchemy or some such thing might get a hold of a small amount of whatever resource we're talking about. From that small sample, (s)he can perform some experiments, determine its properties, and find uses for it. (S)He can then show government officials and explain how such a material might be used. This can be done without a large deposit of Iron, or Uranium, or saltpeter, or whatever. BUT to equip troops, you'll need A LOT of whatever substance you just discovered the usefulness of. Want to equip a horseman unit? You'll need not one, or two, or three horses, but as many thousands. Want to give your soldiers iron blades instead of those old flint axes? You're going to need more than the several handfulls you used to discover iron's properties - you'll need a large deposit that can be easily mined and extracted. So it applies for all resources. Just because there's no iron in your territory doesn't mean there's NO IRON at all. What it means is that there is not enough in accesible locations to have large-scale extraction that would be necessary for wide-spread use as it's modelled in civ terms.