I somehow come across somewhere with the same idea but of course there's a difference from theirs and yours. Anyway, where will you get the money, the coins or token to reward the users. To elaborate, they mine in your in game but it doesn't mean that they actually mined though still they should have real and equivalent value which if they decided to cash out then they can and all the time that they had spent playing this game would not go to waste. It is not "just" a game after all but you're trying to say that you can also "earn."
Do you have some hash power to back this up? Also, I think you should use the in game currency as the thing to sell in your ICO but of course you need to turn it into a token which means it is also tradable rather than just spend it in game.
Maybe it could work to add a small mining power like pos to the game so you automatically stake coins while you are playing. I think that would be a good thing to help support the coin and make it easier...