By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Silver Bow County is immediately west of the Continental Divide in western Montana. It is the leading mining county of Montana and is one of the most productive mining areas in the United States. In addition to copper and gold, Silver Bow County has produced large amounts of silver, lead, zinc, and some manganese. Almost all the metals have come from the immensely rich copper deposits at Butte. The Highland district, south of Butte, has yielded a moderate amount of gold. The total gold production of the county from 1864 through 1959 was about 2,800,000 ounces.
The Butte (Summit Valley) district is in northeastern Silver Bow County in the vicinity of the city of Butte. More than 90 percent of the annual mineral output of Montana comes from the Butte district, one of the great copper camps of the world. Gold has been a byproduct of the copper ores, although some gold was mined from placers before 1875. Silver, zinc, lead, and manganese have also been recovered in substantial amounts. The total gold production of Butte through 1959 was approximately 2,725,000 ounces, which includes about 363,000 ounces of estimated production from placers in the district (Winchell, 1914a, p. 102; Lyden, 1948, p. 143-144).
The first mineral discoveries were the gold-bearing gravels found in 1864 in Missoula Gulch, at a point now within the Butte city limits. Later that same year placers were found along Silver Bow Creek and in German Gulch. These deposits were fairly profitable in the 1860's and 1870's. In the first 3 years about $1^ million worth of gold was recovered (Weed, 1912, p. 18). The placers, which were not very rich, soon declined in importance, as interest was diverted to developing the manganese-stained silver lodes which were also located in 1864. Silver mining reached its peak in 1887, at which time about 500 tons of ore, averaging $25 per ton in silver and gold, were treated daily in stamp mills and smelters in the district (Weed, 1912, p. 19). The drop in the price of silver in 1893 put an end to large silver-mining operations in the district. Copper lodes were developed slowly and gained prominence in the 1880's when railroads afforded cheap transportation. A significant event in the history of the district was the organization of the Amalgamated Copper Co. in 1899, a venture that consolidated most of the large copper mines and provided sufficient capital for future expansion. Exploration and development continued through the early 1900's. The Anaconda Co., the largest of the consolidated properties, assumed increasing importance and has been the controlling interest in the district for many years.
Bedrock in the district is mainly quartz monzonite and is part of the Boulder batholith of Late Cretaceous age or Early Tertiary age. It is cut by dikes and sheets of aplite and by dikes of quartz porphyry. A mass of dacite-rhyolite, partly intrusive into the quartz monzonite and partly extrusive, is exposed in the western part of the district and forms the butte from which the city received its name (Weed, 1912, p. 26-46). The quartz monzonite, aplite, and quartz porphyry - but not the rhyolite - are cut by numerous faults and fissures, along some of which the mineralizing solutions traveled. Extensive studies in the district (Sales, 1914; Sales and Meyer, 1948) resulted in grouping these fissures into the following systems, listed chronologically and beginning with the oldest:
1. Anaconda or east-west system (contains the most extensive and persistent veins).
2. Blue system (also heavily mineralized).
3. Mountain View breccia faults (slightly mineralized).
4. Steward system (slightly mineralized).
5. Rarus fault (barren).
6. Middle faults (barren).
7. Continental fault (barren).
Sales (1914, p. 58-61) recognized a distinct concentric zoning of the ores of the Butte district. The central zone contains veins in which chalcocite and enargite are the major copper minerals and the gangue is pyrite and quartz. Veins in the intermediate zone are characterized by sphalerite, and bornite, chalcopyrite, tetrahedrite, and tennantite are more abundant than in the central zone. Rhodonite and rhodochrosite are relatively common. The peripheral zone contains veins valued for silver, gold, and zinc. Rhodonite, rhodochrosite, galena and sphalerite are abundant; quartz is the major gangue mineral, and pyrite is common. The copper minerals chalcopyrite, tetrahedrite, tennantite, chalcocite, and bornite occur in only small amounts.
Detailed studies of the wallrock alteration of the Butte district revealed that all veins are accompanied by two types of alteration - a sericite zone adjacent to the ore-bearing fracture and an argillized zone between the sericite and fresh quartz monzonite. These changes reflect mineralogical stability ranges of the wallrock in response to a new physical and chemical environment produced by the mineralizing solutions rather than in response to abrupt changes in the composition of the solutions (Sales and Meyer, 1948, p. 25).
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