I've been away from the game for a little under 24 hours and come back to find that all my lv3 mines have been exhausted. This I don't have too much of an issue with - however, what I do mind is that a) you can only top them up by using gems at the moment and b) you loose all the tools you used to level them up.
This makes getting the mines back up to level 3 a) time consuming and b) expensive.
I've spent nearly €90 on this game and looking at the prospect of having to restablish all the mines again (given the length of time it takes the geologist to do the searches, and I've got the Jolly version as well), I'm thinking - why bother.
This will drive people away from the game, where as if you left the mine there, but put an exclamation mark over it (like you do with any other building where there aren't enough resources to produce anything) and then allow the player to refill them - you won't loose players and you'd probably actually get some buying gems to top them up.
The way the simply get exhausted and you then have to pull them down, sent the geologist out and rebuild them simply takes too long.
If it happens again, I doubt I'll carry on playing, I simply don't have the time to be on here 24 hours a day.