By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Gold production in Cascade County has been almost entirely from lode deposits in the Montana (Neihart) district.
The Montana (Neihart) district in the southeast corner of Cascade County, in the central Little Belt Mountains just east of Neihart, has produced chiefly silver and lead, some copper and zinc, and gold as a byproduct. Rich silver ore was discovered in 1881, and the district had an estimated gold production of $800,000 (38,703 ounces) from 1881 to 1898 (Weed, 1900, p. 404). The gold production from 1902 to 1948 inclusive (Robertson, 1951, table 3) was 28,010 ounces, and from 1949 through 1952, about 188 ounces. There was no recorded production from 1952 through 1959. Total recorded gold production through 1959 was about 67,000 ounces.
The oldest rocks of the Montana district are contorted Precambrian gneiss and schist and a Pre-cambrian diorite phacolith. They are overlain by rocks of the Precambrian Belt Series of which the basal unit, the Neihart Quartzite, forms the country rock in the southeastern part of the area (Schafer, 1935, p. 6-12, pi. 2; Weed, 1900, p. 371-381, pi. 41). All the rocks are intruded by stocks, laccoliths, and dikes of granite porphyry and quartz porphyries and black mafic dikes, all probably of early Tertiary age.
The economically important deposits occur in veins in the gneisses and schists and in the diorite, along contacts of these rocks with the Tertiary intrusive rocks, and locally as disseminated low-grade deposits in the brecciated Tertiary intrusive bodies (Robertson, 1951, p. 18-19). In the Snow Creek area gold is an important constituent of veins that cut the Neihart quartzite and Tertiary porphyry (Schafer, 1935, p. 15, 26-29).
The common ore minerals are galena, pyrite, sphalerite, polybasite, argentite, pyrargyrite, native silver, gold, and chalcopyrite; the gangue minerals are quartz, barite, and ankerite (Weed, 1900, p. 405-410).
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