I like the core concept, but I definitely need some more decisions to make during a gameplay. On Unity2D course by GameDev.tv, Rick said that good gameplay is when you have at least three (as far as I remember:P) decisions to make (f.e. in tower defense - (1) if you want to buy defensive unit or economy unit, (2) if you want to buy cheaper but weaker or more expensive but stronger unit, (3) where to put it).
So I think here would be cool if I would be able to decide which type of tile I want to dig. So I can decide to dig faster (but the chance for good treasure will be smaller) and just focus on upper tiles and in any moment I can click button "next" and respawn all the tiles. Because now I need to dig everything.
Second thing that would be cool is to have different digging tools that I can buy for money from treasures.
Overall the main mechanic is cool because it's like taking the most addictive element of Minecraft :D dig, dig, dig! This can be very addictive! :D