The two zones being mined occur along a continuous 1,500-m mineralized zone that is crudely parallel to Cretaceous strata. In the larger Main Zone, ore minerals, principally pyrite, chalcopyrite, and tetrahedrite, are finely disseminated in the matrix of a volcanic breccia and are intimately associated with an aluminous and borosilicate alteration assemblage. The smaller Southern Tail Deposit contrasts with the Main Zone. Ore minerals, principally pyrite, arsenopyrite, chalcopyrite, and tetrahedrite with quartz and chlorite gangue are moderately coarse grained and fill open space in a crudely tabular brittle fractured zone. Whole-rock K-Ar dates of Southern Tail wall rock and a highly tourmalinized breccia more than 1 km from the Main Zone both correspond closely to the older and more distant of the two Eocene stocks, a 59.4-m.y.-old quartz monzonite. Suggested link between very weak sulfide development in the stock, tourmaline breccia, and the ore zones. Epithermal ore genesis model in which intrusive activity of quartz monzonite age heated acidic meteoric water to a high temperature and contributed a saline magmatic component to create an ore fluid. Movement of the ore fluid was controlled by structures and stratigraphy of the volcanic host rocks.--Modified journal abstract.