With underground mines, everything changes: depth, the continuity and mining widths of the ore and the vertical or horizontal plane of the ore all comes into play as well as many other factors. Generally, to be on the safe side, if you can find gold grades of 10 grams (about a third of an oz.) or more per tonne across mineralized sections averaging 3-4 meters or more in width, then you are looking at good potential. Lower grades across wider widths also work (i.e. 6-7 grams across 10 meters) Keep in mind these are rough guidelines and subject to many other factors, like depth, vein continuity, overall tonnage and much more. But the sweet spot in this industry is high grades across wide zones of mineralization.