I would stay out of abandoned mines - there are too many opporitunities to get killed. Most of the gold you will find in the walls are likely to be very small samples, and beyond the abilities of many metal detectors. The Falcon MD 20 will help you find small gold - in that sense it can be used as a pin pointer for gold (not many pin pointers work well for gold prospecting).
Keep in mind that many gold and silver mines started out with easy to identify ore at the surface, and then as they tunneled deeper, they ran into sulfides of gold and silver (in many cases, your metal detector is not likely to detect anything there, even if there is good mineral content).
You might do a lot better working thru the tailings piles.