The St. Regis-Superior area occupies about 300 square miles in northwestern Montana and includes parts of the Squaw Peak Range and Coeur d'Alerie Mountains of the northern Rocky Mountains physiographic province. Nearly 50,000 feet of metasedimentary rocks of the Precambrian Belt series, chiefly varieties of quartzite and argillite, underlies most of the area. The Belt series is informally subdivided with reference to the top of the Wallace formation into lower and upper parts. In this area, the lower part of the Belt series is divided into the Prichard, Burke and Revett, St. Regis, and Wallace formations, in order of decreasing age, and the upper part of the Belt series or the Missoula group is divided »into the Spruce, Lupine, Sloway, and Bouchard formations, and an unnamed feldspathic quartzite at Rock Rabbit Ridge, also from oldest to youngest. Formations in the lower part of the Belt series are correlated with formations of the same names in the Coeur d'Alene district, and formations in the upper part of the Belt series are tentatively correlated in part with formations of the Missoula group in the vicinity of Missoula, Mont. Paleozoic quartzite, shale, limestone, and dolomite crop out in several localities in the southeastern part of the area. The limestone unit contains fragments of a single species of Glossopleura of early Middle Cambrian age which, together with lithologic similarities, has been used to correlate at least the quartzite, shale, and limestone part of this Paleozoic sequence with the Flathead sandstone, Gordon shale, and Damnation limestone sequence known elsewhere in northwestern Montana. Several small diabasic dikes and sills are present in the area, generally associated with northwestward-trending faults.

The major faults generally trend northwestward and are considered to be part of the Lewis and Clark structural line. The Osburn fault, the major element of the Lewis and Clark line through the Coeur d'Alene district and western Mineral County, has been traced to the east edge of the St. Regis- Superior area, thus extending the mapped length of the fault to about 100 miles. Evidence indicates that this major fault has diminished in intensity in this area and that most of the stress has been relieved along the Boyd Mountain fault that apparently splits from the Osburn fault a few miles west of St. Regis. Stratigraphic and structural evidence indicates a strike-slip right-lateral movement of about 3 miles along the Osburn fault. Horizontal stratigraphic separation along the Boyd Mountain fault indicates a right-lateral movement of about 13 miles.

Low-grade regional metamorphism of the sedimentary rocks in the area has caused the recrystallization of quartz grains and the formation of sericite. Argillite and quartzite have been converted to phyllite and foliated quartzite by dynamic metamorphism in a large area north of the central part of the Osburn fault. Some of the shear zones contain a large amount of introduced carbonate minerals.

From 1901 through 1953 this area has produced 8,086,827 pounds of zinc, 7,932,958 pounds of lead, 2,053,715 pounds of copper, 584,168 fine ounces of silver, and 588 fine ounces of gold. The lead, sine, and silver have come chiefly from veins in highly foliated rocks near the Osburn fault zone. The attitudes of these veins are controlled in large part by the cleavage. The principal ore minerals are galena, sphalerite, tetrahedrite, and boulangerite, and the gangue minerals are quartz, carbonate minerals, and barite. Most of the copper has come from the Amador vein where chalcopyrite and bornite are the principal ore minerals, and the gangue minerals are pyrite, quartz, and carbonate minerals. The Amador vein occurs in a belt of copper deposits that extends westward into the Coeur d'Alene district. These copper deposits may be genetically associated with diorite dikes and sills lying within the same belt.

Fluorspar has been found in three closely spaced prospects along a northward- trending zone of brecciation and small-scale folding in Dry Creek valley. Incomplete production records show that 781 tons of fluorspar has been shipped from 2 of these prospects.