I think we may all be talking about different things here. When I research iron working, all iron tiles are revealed everywhere I have explored. Fine.
Now my question is why the mine says that it has a small chance of discovering iron. Hasn't iron already been discovered in all the pre-arranged places on the map? It's always been there, but I couldn't see it before I researched iron working.
Now what Renata said makes sense to me. If you have a mine that's not mining a known resource, there's a small possibility that one may show up there even though it wasn't a pre-arranged location, and therefore didn't show up after researching iron working. In other words, there's a small possibility that the mine will create a resource in the tile it occupies. Is my last statement correct?