The Cordilleran Section, Butte, Montana: The Butte district, located in western Montana, is one of the most important mining camps in the western US. Butte is of importance both on account of the size and extraordinary richness of its deposits. In fact, it is one of the largest producers of copper in the world. By 1920, it had yielded over 8,000,000,000 pounds of copper, more than 400,000,000 ounces of silver, and 2,000,000 ounces of gold, also much zinc and smaller amounts of arsenic, lead and manganese. The history of this mining camp is full of interest. The camp began its mining career as a placer gold producer in 1864, but difficulties in working the gravels directed attention to the mineral vein outcrops, and unsuccessful attempts were made to work their copper and silver contents, so that it was not until 1875, following a period of quiescence, that the discovery of rich silver ore in the Travona lode revived the mining industry of Butte. In 1877 several silver mines were opened, followed by others; but this did not last many years, for with the drop in the price of silver many mines closed, although one, the Bluebird, had produced 2,000,000 ounces of silver from 1885 to 1892. The copper mines were worked to only a limited extent at first, and the industry did not assume permanence until 1879-1880, when matte smelting was introduced. In 1881 the Anaconda mine, which was first worked for silver, began to show rich bodies of copper ore, and since then the output f copper has steadily increased, there being a number of large smelting plants located at Anaconda and Great Falls. Developments extend to depths greater than 3,400 feet, where ores of good grade are found. It will always remain in the literature of mining geology as distinctively a copper camp.
Butte lies on the western border of the Boulder batholith, the latter having a width of 75 miles and a length of over 100 miles. Lying between the main range of the Rocky Mountains on the east, and the Bitterroot Mountains on the west, the batholith seems to have been intruded in the Eocene (?) after a period of folding and thrust faulting, and without causing any doming. Associated with the batholith are a number of fissure veins, one type of which is found only in the Butte district, and therefore concerns us here. The rocks of the Butte district include: (1) Granite or quartz monzonite, the Butte granite, much jointed, and hence permeable to solutions; (2) Aplite, in irregular bodies and dikes, especially in the northwestern portion of the district; (3) Quartz porphyry dikes, roughly parallel in an east-west direction, and following the earliest vein system; (4) Rhyolite, of intrusive and extrusive character, especially in west and northwest part of district, its offshoots cutting both the copper and silver veins; (5) Andesite, of pre-Tertiary age and bearing no relation to the copper ores. The granite is cut by many faults, which are hard to detect, and which are often mineralized. Fissures are common in the batholith, and there are two main series, striking east-west, and northwest-southeast, corresponding broadly to the two most important fracture zones of the district. and displacement is found along some. These systems are:
The fissuring in the district is exceedingly complex. There have been identified six distinct fissure systems, which cut the granite, aplite and quartz porphyry, but not always the rhyolite. The systems are (1) Anaconda system; (2) Blue system of fault fissures; (3) Mountain View breccia faults; (4) Steward system; (5) Rarus fault; (6) Middle faults; (7)Continental fault.
1. The Anaconda system is composed of
easterly fissures which are generally heavily mineralized and along which
there has been but little displacement. In general they dip south at high
angles. In the copper-producing area there are two notable groups of veins
belonging to the Anaconda system. On the south or Anaconda group are the
deposits of the Gagnon, Original, Parrot, Never Sweat, Anaconda, St.
Lawrence, Mountain View, Leonard, West Colusa, and other mines.
North of this group is the one which includes the Syndicate, Bell,
Speculator, and associated deposits. Some of the easterly fractures are
joined by many closely spaced smaller fractures, doubtless of the same age.
They form altogether a network having what Sales has designated "horsetail"
structure. In some mines the most valuable copper ore bodies are along these
northwesterly conjugated fractures. North of the copper-bearing area are the
great easterly
silver and zinc lodes. These also are believed to belong to
the Anaconda system of fissures.
2. The Blue system is composed of several fissures that strike northwest.
These cross and fault the veins of the Anaconda system. The lodes of the
Blue vein system carry large deposits, although they are much less valuable
and less uniformly mineralized than the easterly veins, on which the
deposits are almost continuous except where displaced by faults.
3. The Mountain View breccia faults, which are later than the Blue Vein
system, strike about N. 75 E. Near veins they contain locally enough
brecciated ore to be worked.
4. The Steward system includes fault fissures that strike about N. 65 E. and
extend across the Butte district. A few of them carry drag ore.
5. The Rarus fault is a complex fissure that is later than the mineralized
faults. It is a crushed zone 20 to 250 feet wide and contains drag ore.
6. The Middle faults are later than the Rarus. They are not metallized.
7. The Continental fault, on the cast edge of the mineralized area, is
likewise later than the metallization of the district. The rhyolite
intrusion of Butte was subsequent to the earlier vein fissures; the silver
veins are cut off by intrusive dikes of rhyolite.
Hydrothermal alteration of the quartz monzonite is extensive. Where large veins are closely spaced the entire area of quartz monzonite is hydrothermally altered; where the veins are less closely spaced fresh rock is found between them. The granite is much altered by hydrothermal metamorphism, especially in the Anaconda Hill area, so that it is now a mass of pyrite, sericite and quartz near the veins. Geologists have recognized two phases of alteration an earlier chloritic phase and a later sericitic phase. According to W. H. Weed, hot alkaline solutions leached the metals from the granite at considerable depths and wherever the fissures were open they were filled with copper ore and where the fissures were narrow their walls were replaced, so that the vein matter shades off into the country rock. In the central part of the area, the more highly heated and acid solutions deposited the copper ores, while the zinc, manganese and lead were precipitated toward the periphery where the temperature was lower, and the solutions more alkaline from reactions with the granite.
The surface material consists of a red or brown quartz. Beneath this there is a zone of oxides carrying both gold and silver. The alteration products at times reach a depth of 300 ft. or more, depending upon the susceptibility of the original material to the agencies of the weather and meteoric water. Beneath this there is a zone of unaltered sulfides which furnish the main ores of the camp.The veins are replacement deposits, and 60 to 80 per cent, of the ore is altered quartz monzonite with disseminated sulfides. The ores are of three classes copper, siliceous silver, and zinc. Chalcocite, enargite, and bornite are the most common copper minerals. Covellite occurs in large amounts in the Leonard mine, and chalcopyrite is present in workable quantities in a few properties. Pyrite is the most common sulfide. Native silver occurs in the copper ores, especially in those from the upper levels. Proustite ruby silver and black sulfantimonites and sulfarsenides occur in the siliceous silver ores. Free gold is rare. The gangue minerals include quartz, sericite, and several residual minerals of the altered country rock. Much rhodonite and rhodochrosite and some fluorite occur in the silver and zinc ores. Sales states that there is a central zone of copper ore, mainly chalcocite and enargite, which grades into an intermediate zone that contains ores with the same minerals and also sphalerite, rhodochrosite, and rhodonite, with a slight increase of silver content. In an outer or peripheral zone the ores carry sphalerite, rhodonite, rhodochrosite, tetrahedrite, tennantite, and chalcopyrite, but rarely chalcocite or bornite. Their chief metals are silver, gold, zinc, and some lead. Some silver lodes crop out conspicuously, but the outcrops of copper lodes are not prominent. The leached zone extends downward in places 300 or 400 feet below the surface. It contains silver, locally 30 ounces or more to the ton, but little copper. Below the oxidized zones of copper lodes, grading into them locally within 2 or 3 feet, are enormous masses of chalcocite, with some bornite and covellite. In the great ore bodies of the upper levels of the Anaconda vein chalcocite occurred in nearly pure masses 20 feet or more wide. In depth the mineral shows a more crystalline structure, and it is found in all the mines in greater or less abundance and purity, but as a rule it forms small grains scattered through the ores. The chalcocite ores are present in large quantities also between the 2,000- and 3,400-foot levels. Investigators who studied the copper lodes in the earlier stages of their development regarded the chalcocite ores as secondary deposits formed by descending waters. More recent investigations, have shown that the deeper chalcocite ores are primary.
The primary ore was chalcopyrite and pyrite. It is the enormous deposits of secondary chalcocite that have been the large producers of the metal. Other copper minerals appearing as ores are bornite, enargite, covellite and tetrahedrite. The veins are quite largely replacement deposits along fissures in the sheeted granite. The country rock consists of two types of granite. One is a dark hornblendic granite or quartz monzonite known as the Butte granite. The other is an acid granite or better an aplite termed the Bluebird granite. These granites are intersected by dikes of quartz porphyry. Dikes of both intrusive and extrusive rhyolite intersect the copper veins. According to some, the veins exhibit a curious uniformity of direction, most of them striking nearly east and west, and few of them departing more than 15 or 20 degrees from the vertical.
They show considerable variation in width, ranging from a few feet to 150 ft. where the altered country rock is impregnated with chalcocite. In some instances there is no distinct hanging wall and the distinction between the vein and the country rock becomes commercial. The ore deposits are fissure veins, formed by the filling of fissures and replacement of the country rock, the oldest fissures having been continuously mineralized. Within the Butte district there is: (1) A main or central copper zone, free from zinc and manganese ; (2) An intermediate zone, with copper predominant, and with some sphalerite, rhodochrosite and rhodonite; (3) An outer peripheral zone, without copper, but filled chiefly with quartz, rhodonite, rhodochrosite, sphalerite, and pyrite and which is silver bearing. In the central or copper zone, the order of relative abundance of the copper sulfides is (74); chalcocite, enargite,, bornite, chalcopyrite, tetrahedrite, tennantite and covellite. Quartz and pyrite form the gangue.
Chalcocite has supplied 60% of the Butte copper to date, occurring in veins of all ages, and at all levels down to below 3000 feet. Once regarded entirely as a downward secondary enrichment product, it is now divisible into, (a) sooty secondary chalcocite, forming a dull black coating on pyrite and other sulfides or replacing pyrite, sphalerite, enargite and chalcopyrite; (b) massive chalcocite, considered as primary because: (1) it is abundant in the deepest levels (over 3000 feet); (2) its intimate association with bornite, enargite and pyrite show it to be contemporaneous; (3) it occurs at all depths without relation to topography; (4) it is found in dry veins, at deep levels, cut by the older faults; (5) it replaces granite at deep levels; and (6) there is no evidence of present replacement except in the sooty material. Enargite is of wide vertical and lateral distribution, of comparatively old mineralization, and usually primary but sometimes secondary. Bornite is primary, of all ages, and at all levels. Chalcopyrite is unimportant and chiefly primary, so also is covellite. The vein outcrops are usually barren of copper, and while the oxidation depth is variable, it averages 250 feet. In the silver zone, quartz and manganese are the common gangue materials, the veins showing on the surface as ledges of manganese-stained quartz. The Butte ores have been derived primarily from igneous rocks, the quartz porphyry having perhaps opened up the way for the ore-bearing solutions, the elements carried by the latter having included Si02 , S, Fe, Cu, Zn, Mn, As, Pb, Ca, W, Sb, Ag, Au, Te, Bi, and K. Similar veins occur at Cerro de Pasco, and Morococha, in Peru, and at Chuquicamata in Chile
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