By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Salt Lake County, in north-central Utah, is bounded on the northwest by the Great Salt Lake, on the east by the Wasatch Mountains, on the south by the Traverse Mountains, and on the west by the northern part of the Oquirrh Range. It is distinguished politically by Salt Lake City, the State capital, and economically by Bingham, the most productive mining district in the State.
Most of the 10,651,000 ounces of gold produced through 1959 came from this district; about 30,000 ounces came from the Cottonwood district.
Soldiers, attached to the California Volunteers under Gen. P. E. Connor, arrived in Utah in 1862 and established Camp Douglas, overlooking Salt Lake City. Many of the men, who were prospectors and had experienced the gold fever in California, began prospecting the ranges near Salt Lake City. The first mineral locations were made in Bingham Canyon in September 1863; other discoveries followed rapidly.
Gold placers also were mined in Bingham Canyon in 1865 and within a few years had yielded about $1 million in gold (48,379 ounces). They were the only important placer-gold producers in the State (Butler and others, 1920, p. 118, 340).
Ore deposits were discovered in the Little Cottonwood camp near Alta in 1865 and in the Big Cottonwood camp in 1868 or 1869 (V. C. Heikes, in Calkins and Butler, 1943, p. 71-72, 77).
The Bingham (West Mountain) district, about 20 miles southwest of Salt Lake City on the east slope of the Oquirrh Range, is the leading mining district in Utah and is one of the major copper districts in the United States. It is also the fourth largest gold producer in the United States; its total production through 1959 was about 10,610,000 ounces. In addition to copper its chief commodity - and gold, large amounts of lead, zinc, silver, and molybdenum have been produced from the district.
One of the major stimuli to early prospecting in this region was Gen. P. E. Connor, who was in charge of the Third California Infantry stationed at Camp Douglas near Salt Lake City in 1862. Many of the troops had previous mining experience in California and were encouraged by General Connor to search for mineral deposits. By September 1863, they found outcrops of lead carbonate in Bingham Canyon. This was formally located as the West Jordan claim, and in December 1863 the West Mountain district was formed.
Handicapped by lack of transportation facilities and suitable reduction plants, the miners made little progress. In 1864, however, placers were found in Bingham Canyon, and these yielded $1 million in gold by 1871 and an additional $500,000 in later years. The completion of rail facilities to Bingham by 1873 removed the major obstacle to exploiting the lodes (Boutwell, 1905, p. 81-85).
Lead and silver were the principal products of the district until the financial depression of 1893, when the price of silver dropped. Interest turned to copper deposits that previously were considered too low-grade to be mined. Experiments for treating low-grade copper ore were undertaken at the Highland Boy mine in 1896, and exploration revealed large pyritic copper ore bodies. Elsewhere in the district several mills were built to treat disseminated copper deposits in monzonite.
After several years of experimentation, exploration, and consolidation of properties, large-scale mining of the disseminated copper deposits was begun in 1907 by the Utah Copper Co. (Boutwell, 1935, p. 349). Mining continued at an increased rate. In 1936 the Utah Copper Co. was absorbed by Kennecott Copper Corp., and Kennecott became the major producer in the district. The large-scale exploitation of the disseminated copper deposits, which continued through 1959, is responsible for Bingham's status in the mining industry.
Most of the gold is recovered as a byproduct of the copper ore; thus, the gold output, which was mined from vein and replacement deposits in sedimentary rocks before 1907 and which was less than 10,000 ounces annually, jumped significantly with the inauguration of large-scale copper mining and reached a high of about 450,000 ounces in 1953.
The predominant bedrock in the Bingham district is the Oquirrh Group of Pennsylvanian age, which consists of about 16,000 feet of limestone and sandstone units. Four formations are recognized in the Oquirrh Group. In ascending order they are the Maple Formation, White Pine Formation, Butter-field Formation, and Bingham Mine Formation (J. E. Welsh and A. H. James, in Cook, 1961, p. 7-11). Many limestone and sandstone units within these formations are mappable units; some are formally named, others are not, especially in the mine areas.
In the northern part of the district, a considerable area is underlain by the Curry, Clinker, and Park City Formations of Permian age. These rocks are intruded by several granite, granite porphyry, and monzonite stocks of Tertiary age - notably the Bingham and Last Chance stocks - and by sills and dikes of monzonite. Along the eastern edge of the district, the foothills of the Oquirrh Range are capped with volcanic flows, breccias, agglomerates, and ash deposits of Tertiary age (W. H. Smith, in Cook, 1961, p. 101-116).
The structure of the Bingham district is complex and apparently exerted a strong influence on the emplacement of the stocks. A series of large northwest-trending folds known as the Bingham syncline, the Middle Canyon syncline, and the Copperton anticline are cut by several large west- to northwest-trending thrust faults and high-angle normal and reverse faults, and by a system of northeast-trending high-angle reverse faults. The stocks and mineral deposits appear to be controlled by the intersections of northeast faults with the trends of the fold axes and northwest-trending faults (A. H. James and others, in Cook, 1961, p. 49-66).
The major ore deposit of the district is that of the Utah Copper mine, an open pit 1.5 square miles in area excavated in the Bingham stock, a body of altered monzonite porphyry. Surrounding the stock is a roughly circular zone that contains most of the lead-zinc veins and replacement deposits (A. H. James and others, in Cook, 1961, p. 81-97).
The disseminated copper ore body is in a vertical cylinder of shattered and altered monzonite. Grains of copper sulfides coat the walls of fissures and are disseminated throughout the monzonite. In addition, the ore body is interlaced with veins and veinlets of quartz and orthoclase containing chalcopyrite, pyrite, molybdenite, galena, and sphalerite. Oxidized ore contains copper in the form of azurite and malachite. The zone of supergene enrichment, which constitutes the largest part of the ore body, contains the copper minerals chalcopyrite, chalcocite, covellite, and bornite. The copper mineral in the primary sulfide ore is chalcopyrite (Bout-well, 1935, p. 355-356).
The replacement deposits, which occur in limestone beds near fissures, are an important source of lead, silver, copper, and zinc. The copper ores contain pyrite, chalcopyrite, and minor amounts of arsenopyrite, bornite, and tetrahedrite and local specularite. Gangue minerals are quartz, carbonate, and garnet.
Fissure veins have been the least productive of the various deposits in the district. They occur in quartzite and limestone beds and also in some of the intrusive rocks. Most of the fissure veins have been mined for lead and silver; a few have yielded gold, copper, and zinc. The common vein minerals are pyrite, galena, sphalerite, and tetrahedrite, and gangue minerals are calcite, quartz, rhodochrosite, and barite (Boutwell, 1905, p. 126-154).
The Cottonwood district, which includes Big and Little Cottonwood camps, is on the western slope of the Wasatch Range in the southeastern corner of Salt Lake County about 20 miles southeast of Salt Lake City; it is immediately north of the American Fork district in Utah County and immediately southwest of the Park City district in Summit and Wasatch Counties.
The Cottonwood, American Fork, and Park City districts are clustered in an irregular area whose center lies at the intersection of the north-south folds and thrusts of the Wasatch Range with the westward projection of the anticlinal axis of the Uinta Mountains (Calkins and Butler, 1943, p. 3-4). At the intersection of these structures, some fairly large intrusive bodies or stocks occur and, other than dikes or sills, are the only intrusive rocks known in the Wasatch Mountains.
According to Calkins (Calkins and Butler, 1943, p. 3-4), "This segment also contains the only ore deposits in the Wasatch Range from which any considerable production has been won, the presence of these deposits having doubtless resulted from the combination of intricate structure with large intrusive bodies that are not too deeply eroded."
The total gold production of the Cottonwood district through 1959 was about 30,275 ounces, of which approximately 16,650 ounces was produced before 1900 (V. C. Heikes, in Calkins and Butler, 1943, p. 81). The chief metals recovered from the ores are, in order of value, silver, lead, copper, gold, and zinc.
The earliest recorded mining claims in the district were located in August 1865 in the Little Cottonwood camp, and in 1866 several veins of argentiferous galena were found (V. C. Heikes, in Calkins and Butler, 1943, p. 72-73). The discovery in 1868 of the Emma ore body, one of the most productive in the district, greatly stimulated prospecting in the region. The earliest locations in the Big Cottonwood camp probably were made in 1868 and 1869, but little development work was done until 1871.
By the end of 1880 more than 1,000 locations were made but most of them were relinquished; most of the remaining were productive only on a small scale (V. C. Heikes, in Calkins and Butler, 1943, p. 77). Very few properties have produced any ore in recent years.
The growth of the Emma mine into a major producer stimulated mining throughout Utah. Several rich ore bodies were discovered in rapid succession in the Little Cottonwood area in the early 1870's, and the following decade proved to be the most productive in its history (V. C. Heikes, in Calkins and Butler, 1943, p. 77-82). The Cottonwood district was active from 1867 through 1954, but production was relatively small after 1927.
The rocks of the Cottonwood district are dominantly sedimentary, ranging in age from Precambrian to Jurassic and having an aggregate thickness of about 12,000 feet. They are cut by intrusive stocks and dikes of probable Late Cretaceous or Tertiary age. The Precambrian rocks consist of several thousand feet of quartzite and shale overlain by tillite, formerly regarded as Cambrian in age but now thought to be Precambrian (M. D. Crittenden and others, in Marsell, 1952, p. 4-6). These are overlain by rocks of Cambrian age, consisting of the Tintic Quartzite, Ophir Shale, and Maxfield Limestone.
An unconformity separates the Maxfield Limestone from the overlying unnamed basal Mississippian dolomite (M. D. Crittenden and others, in Marsell, 1952, p. 9). Above this formation are Mississippian, Pennsylvanian, and Permian strata, which, in ascending order, are the Madison Limestone, Deseret Limestone, Humbug Formation, Morgan (?) Formation, Weber Quartzite, and Park City Formation. Triassic rocks are represented by the Woodside, Thaynes, and Ankareh Formations. The Nugget Formation, of Jurassic age, is the youngest sedimentary formation exposed in the district (Calkins and Butler, 1943, p. 7-33).
In Late Cretaceous and early Tertiary time the rocks were folded and dislocated by thrusts and high-angle normal and reverse faults. The thrusts have very low angles; many are along bedding planes in the sedimentary rocks. Stocks and dikes of porphyritic quartz monzonite, granodiorite, and diorite cut the sedimentary rocks and are apparently younger than the thrust faults, though some high-angle faults are younger than the intrusives (Calkins and Butler, 1943, p. 52, 61).
The most important ore deposits of the district are replacements of limestone beds adjacent to fissures and deposits in brecciated limestone along thrust faults. Other deposits, less productive, are contact metamorphic deposits and fissure veins in quartzite and siliceous shale of Precambrian and Cambrian age. The replacement and breccia deposits are similar in mineralogy. Pyrite is most abundant and is accompanied by chalcopyrite, bornite, tetrahedrite, enargite, galena, and sphalerite. Tungstenite occurs in the Old Emma mine, and bismuth-inite, in the Sells mine.
Variable amounts of quartz, barite, and iron and manganese carbonates make up the gangue. The fissure-vein deposits contain pyrite and tetrahedrite and some galena, in a quartz gangue. The contact metamorphic deposits were mined chiefly for magnetite. They contain mainly lime silicate minerals, magnetite, ludwigite, chalcocite, bornite, chalcopyrite, periclase, and spinel (Calkins and Butler, 1943, p. 91-95).