Evea play Red Alert 2? there are these outposts you can capture doted over the map. once captured you get a little bit of income from them.
this is kinda similar. the idea is there is a mine way out from your base so you cant tube to it very soon but its say in your land that your defending so you could get the mine up and runing by building a small mineing colony on it.
on big maps this would work realy well. your defended land could have lots of mineing colonys on it and much less work because you dont have to build the tubes.
its one strucutre, the mine, the smelter, the storage etc.
but is it realistic? as the metals cant get to your factories because there is no tube. maybe you have to collect it with a cargo truck or cargo train and take it to base. but then its realy just a mine again..