The metal deposits of the southern Cascades are in stockworks or breccia zones and narrow fissure veins, containing only moderate amounts of copper and small amounts of lead, zinc, molybdenum, gold, and silver. Predominant ore minerals are pyrite, magnetite, chalcopyrite, bornite, galena, and sphalerite. At some occurrences these minerals are accompanied by free gold, especially in the oxidized parts of the veins. Common secondary copper minerals associated with chalcopyrite and bornite consist of azurite, malachite, and chrysocolla. Tourmaline is a common accessory mineral at several copper mineralized breccia deposits.
Hillsides are steep and the summits of most mountains are rounded.
Although deposits of gold, silver, copper, lead, and zinc are present in the St. Helens district , the deposits consist chiefly of narrow quartz-sulfide veins that are not sufficiently mineralized to be of economic value. Exploration and development work at the Sweden mine has established inferred ore reserves of about 20,000 tons of 2.5 percent copper. However, parts of the vein contain as much as 10 percent copper over 5-foot vein widths. The rocks of the St. Helens mining district consist chiefly of thick sequences of andesitic flow rocks, breccias, tuffs, and volcanic sandstones and siltstones, which range in age from Eocene to early Miocene. On the northeastern edge of the district, rocks of the Fifes Peak Formation have been intruded by small stocks, dikes, and sills of andesitic porphyry and dacite.