The area known as the BlackSheep District began to be referred to BlackSheep in the 1960s, presumably when the prospects were staked as mercury targets. During this period, three shallow holes (~50 to 70 feet (15 to 21 m) were drilled, presumably to test for mercury mineralization. In the late 1980s, Johns Manville, an international industrial minerals producer, mounted a gold exploration project on the Owyhee Plateau of eastern Oregon and western Idaho. Sampling of the BlackSheep prospect by Johns Manville geologists yielded anomalous gold and silver values in both the hot spring deposit cap and the underlying volcanic rocks. These geologists recognized the geology at BlackSheep as that typical of a hot springs precious metal deposit. Johns Manville, and other companies that followed, were unable to make a deal with the owner for the BlackSheep area. Coincident with this work, geologists from the Delamar Mine (which operated from the mid-1970s to the late 1990s) discovered a small low-grade gold-silver deposit on their claims to the southeast of the old mercury prospects at BlackSheep and named this deposit Milestone.