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).
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