Maybe have an option in deposits to research new mining Techniques which will restore x amount of resources to a particular mine ie
Iron Coal Gold. but it takes a long time to produce.
i know alot of people are frustrated with the game in regards to mines and how you have to spend either way too many resources to build the mine sometimes for what you get. or spend money on the game. this option would kinda even the playing field abit more for those who are unable to buy gems.
this is just an idea and comments suggestions and critics are all welcome