A major epigenetic fluorite deposit having measured reserves in excess of 3.2 million short tons of 36 percent CaF 2 occurs at Bayhorse, Custer County, Idaho. Fluorite was emplaced into the Ordovician Bayhorse Dolomite as an open-space filling of collapse breccia features, probably in Tertiary time. Important stratabound mineralization occurs at two principle stratigraphic horizons and to a much lesser extent in a third horizon. Major fault zones, which also cut the underlying Cambro-Ordovician Garden Creek Formation, acted as conduits to the depositional horizons and are mineralized. The presence of fluorite in structures cutting the Eocene Challis Volcanic Series indicates an Eocene or post-Eocene age for mineralization. A volcanic-related fluorine source is suggested by geological and geochemical data.