Hm nope I thought it might be Boulder Dash also, Ark, but it isn't it. The game I am thinking of you controlled a little man that started on the surface of the level. You then rode a small elevator down into the ground (any amount you wanted on the screen) and then started digging through the dirt picking up stuff (like gold?). You could at any time though re-enter the elevator, go back to the surface, and there was either a bank and/or a store on top that you could deposit your findings, or buy tools. When in the cave I think you died because random sections of dirt were filled with water and it could cause a flood, thus drowning the ambitious worker.
Using the listing site mytymaus posted I started just checking all of the 'Miner' games. From memory (mind you this is what, 25 years ago?), the game looked similar to the image below, except I remember it having more refined colours/graphics (although still Commodore capable). On the note matsadona mentioned, maybe someone else just re-wrote their own version and named it Miner 649'er, or perhaps we simply labeled the floppy that way for some strange reason.
After seeing the two 'Miner' games for C64 that look to be similar, I'm even more sure I was playing it for the ol'Commodore. Unfortunately I've still been unable to actually find a visual of it or what/who the title/author was. I'm gonna spend a bit more time looking through some real disks and also through some romsets online, but I may have to settle for only memories lol
Thanks for the help.