Mottled, blotched, fine-grained, unimpressive in the hand specimen, the ore in the Ag deposits in the little-known Mineral district near the Snake River in Washington County, Idaho, when viewed under the microscope, is found to possess extraordinary textural and structural features and a mineralogy that is unique to the state. The ore occurs as fillings and replacements along complex zones of fracturing in volcanic rocks of pre-Tertiary age and was formed close to the surface at temperatures mainly below 135 degrees C. Minerals, most of which are distinguishable and identifiable microscopically, include pyrite, marcasite, sphalerite, wurtzite, tetrahedrite, chalcopyrite, galena, and, in 1 deposit, also stannite, cylindrite, bournonite, and boulangerite. The Fe and Zn sulfides show globular and botryoidal development with radial and concentric structures indicative of colloidal deposition. Because of the influence of these colloform structures on the deposition of the other, noncolloidal minerals, and also because of the more or less selective replacement of the ore minerals by late calcite, the textural and structural relations of the ore are complicated and of more than usual interest. Abundant permeation and replacement by late calcite has been responsible for the mottled and blotched appearance of the ore. The Ag probably had its source in the tetrahedrite and galena; the ore mined in the past had probably been enriched in supergene silver sulfides.