Very very cool. Kinda reminded me of the old flash game Motherlode, and had a similar gameplay loop (between having to come up to the surface to refuel, avoid taking damage from mining, a number of upgrades to each part of the miner, etc). Some small thoughts/tweaks that could further improve the game --
1) I think other commenters suggested implementing an elevator or something like that to the game. I think just reducing the fuel penalty for falling (or make it zero) and then an "auto-fall" feature might be nice.
2) Animations for bombs, rockets, etc. I think I saw some quick animations both, but would be nice to have a better indicator that a bomb was used, etc.
3) Maybe instead of using upper-case letters for the rock/ore types, those could just be filled tiles of a solid color? (like 1-bit graphics) Then the enemies can be upper-case ASCII characters and it might be a bit more readable.
I'll add more if I think of anything else. I really enjoyed this entry though. It just needs some catchy music and I'd be hooked :)