if im reading the biomes.xml right, iron will not spawn at bedrock but in the layers between 7 blocks below the surface to 11 blocks above bedrock
anything below 11 blocks above bedrock will be the biome specific ore.
also tunneling can still yield ore if you aim your tunnels toward the surface rock/ gravel areas. you might need to clear out a space to find it because your tunnel might miss the vein
also if you hit gravle try to carefully mine to the top of it and then dig it out from the top down. you will need frames for support but dont dig too high if you are close to the surface unless you want it to turn into an open pit mine
I do this all the time and I have never died while mining