I am big fan of mining mini-games in MMORPG like EVE Online, and standalone ones like Motherload, Boom Town and Utopian Mining. However, mining games are usually designed as a huge grind. You keep spending time mining and eventually you will accumulate more and more money, regardless of how good or bad you are at it.
I was thinking: how about a mining game where you can actually lose money on a bad run? E.g. start out with free tools to mine cheap materials like copper and bronze etc. Then, when you have enough money, you can buy more powerful equipment to go after more valuable silver and gold. But, the equipment runs on expensive fuel and needs repair, so it is possible to run out of money to buy fuel or repair the machine before finding/mining significant silver or gold to be profitable.
The main trouble with this is that casual players will find it miserable as they accumulate wealth through the free tools and get really excited with getting that shiny new mining equipment, only to lose it all on their first disastrous expedition.