another resource of course is that of time
...
though you obviously can't change that, or the whole system collapses...
as for making mines have limited resources... well, the op2 mines were in a sense limited, just that, well you'd need a few decades to exhaust a mine wouldn't you (and basically no game lasts that long in game terms) ...
ok, i know that you have ppl live and die in the space of a game, but, well, i'm sure everyone can agree that some of the time settings are not in sync (like the convecs move time against its build time for example).
well to put it really simply, if you put resource limits on mines, that's gonna cause a lot of balancing issues, and re-evaluation of map ideas. (for one thing, Eden's magma well would make them the winner of any war of attrition)
and i'd not recommend playing with the yield values (the system works well, and op3 will be running a similar system to op2, lets not overwork knux, ok), execept maybe put a single 4 bar mine in somewhere as an easteregg