Does it fill with rain water or ground water?

Pumping out water is much cheaper (usually) than moving that much rock.

Another idea, would be to allow your pond to be a construction material dump for the county or some other large entity. If they were only dumping inert material like broken concrete or unusable fill dirt that contains too many rocks, that might be a viable solution.

Final idea would be to get a blasting permit. I doubt they would allow it, but blasting the remaining highwalls in the quarry would make your lake larger in surface acres, but fill in the bottom and make it shallower.