Set up a mining camp, one that contains not only the mine itself, but also all of the facilities necessary for the miners to live, i.e. boarding houses, mess halls, recreational areas, etc.
Of course, you still charge your workers for all of these; it costs rent for them to sleep in the boarding houses, money for each meal, etc. But you don't make them fork over coin for each, instead simply taking the costs out of whatever you're paying them.
The key point here is that these amenities collectively add up to
more than each worker's individual salary. So they're actually earning negative pay, placing them in debt to you. If they make seven silver pieces for each day of work, have it cost a gold piece for them to eat, sleep, and entertain themselves during that same period.
Just like that, you have indentured servants whom you don't need to pay (other than actually supplying them with meals and maintaining the facilities) while you cash in whatever it is you're having them mine. If they complain, have the local magistrate throw them in debtor's prison.
Of course, this might result in your alignment changing to Lawful Evil, but that's part of the cost of doing business.