Mineral deposits on Willoughby Island occur in a reefoid marble of Devonian or Silurian age that is part of a large carbonate package that underlies much of central Glacier Bay (Brew and others, 1978). At the mine, sulfide-rich replacement veins, as much as 1 foot thick, occur in marble at and near the intersection of lamprophyre dikes (Reed, 1938, p. 70-71). The lamprophyre dikes are relatively young, probably Tertiary in age. One vein was traced for about 100 feet. Sulfide minerals also occur as joint-fillings in the marble host rock.

The deposit was located in 1907 and leased to Treadwell Yukon Co. in 1925 (Shephard, 1926). Probably about that time, a forty-four foot long adit was driven, but it failed to hit the dike intersection that appeared to localize the ore on the surface. Buddington (1924) reported that a high-grade sample contained 1.74 ounces of gold and 42 ounces of silver per ton, and 25 percent each of lead and antimony, suggesting the possibility of jamesonite, which Buddington reported at another site on western Willoughby Island. Shephard (1926) sampled a 1-foot-thick massive sulfide vein at the property; the sample contained 0.12 ounce per ton gold, 11.9 ounces of silver per ton, 13.4 percent antimony, and 29.75 percent lead.