By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Juab County, in western Utah along the Nevada boundary, is in the eastern part of the Great Basin. The most important ore deposits are in the Tintic district, in the northeastern corner of the county, where large amounts of silver, gold, lead, copper, and zinc were mined. Several other districts produced gold, but only the Tintic had an output of more than 10,000 ounces. It produced approximately 2,648,000 ounces through 1959; however, this total included production of the East Tintic district, which is contiguous with the main Tintic district but is in Utah County.
The Tintic district, which includes the East Tintic district in Utah County, is about 60 miles south of Salt Lake City in the central part of the East Tintic Mountains. Eureka, the principal town, is near the center of the area.
One of the three most important mining districts in Utah, the Tintic yielded about $428,000,000 worth of silver, lead, gold, copper, and zinc through 1959. Of the 2,648,000 ounces of gold produced through 1959, about 65 percent came from the main Tintic district and about 35 percent from the East Tintic. The principal mines were the Chief, Centennial Eureka, Mammoth, Gemini, Eureka Hill, Iron Blossom, Tintic Standard, North Lilly, and Eureka Lilly.
The first claim was located in December 1869, and the district was organized the following year. In 1870, many deposits of silver-bearing lead-carbonate ore were found but were not developed on a large scale because of poor transportation facilities. The small amounts of rich silver and gold ores that were mined in the early 1870's were shipped to smelters at San Francisco, Calif., Baltimore, Md., and Swansea, Wales. Somewhat later most of the ores were sent to Argo and Pueblo, Colo., and the Salt Lake valley, Utah, for treatment (Lindgren and Loughlin, 1919, p. 105).
Railroad transportation, which became available in the late 1870's and early 1880's, had an immediate impact on the district. In 1879 the output of the district nearly doubled; production for 1885 was valued at $1 million and for 1890, at $5 million (Lindgren and Loughlin, 1919, p. 106).
The success of the district was established by important discoveries of several concealed ore bodies. In 1881 and 1882 the Iron Blossom and Godiva ore bodies were found at depths of a few hundred feet below the surface, and the subsurface extension of the Gemini ore zone was located. In 1909, drifts on the Chief 1 mine intercepted rich silver-lead ore from the northward continuation of the Mammoth ore zone (Cook, 1957, p. 76, 77). In 1916, exploration in the East Tintic area reached its climax when the large silver-lead ore body of the Tintic Standard mine was found at a depth of more than 1,000 feet (M. B. Kildale, in Cook, 1957, p. 104).
The annual gold production in the Tintic district was relatively small through most of the 1880's, but it increased sharply in 1889, and in 1907 reached its peak of 113,000 ounces valued at $2,335,000 (V. C. Heikes, in Lindgren and Loughlin, 1919, p. 108). Except for declines during World Wars I and II, the district was active until 1957. No mining was reported in 1958 and 1959.
According to H. T. Morris (in Cook, 1957, p. 4-26), the East Tintic Mountains are composed of a section of sedimentary rocks more than 32,000 feet thick, which are complexly faulted and folded, intruded by quartz monzonite and monzonite stocks, plugs, and dikes, and covered by quartz latite and latite flows and pyroclastics. The oldest rocks are late Precambrian in age and are phyllitic shales and quartzites tentatively correlated with the Big Cottonwood Formation of the Wasatch Range.
Rocks of Cambrian age are the Tintic Quartzite and the super jacent carbonate and shale formations consisting of the Ophir Formation, Teutonic Limestone, Dagmar Limestone, Herkimer Limestone, Bluebird Dolomite, Cole Canyon Dolomite, Opex Formation, and Ajax Limestone. The carbonate lithology is persistent throughout the remainder of the Paleozoic section, which consists of the following formations: Opohonga Limestone and Fish Haven Dolomite of Ordovician age, the Bluebell Dolomite of Late Ordovician, Silurian, and Devonian age, the Victoria Formation of Devonian age, the Pinyon Peak Limestone of Late Devonian and Mississippian (?) age, the Madison Limestone, Deseret Limestone, Humbug Formation, and Great Blue Limestone of Mississippian age, the Manning Canyon Shale of Mississippian and Pennsylvanian age, the Oquirrh Formation of Pennsylvanian age, and the Diamond Creek (?) Sandstone and Park City(?) Formation of Permian age.
This entire sedimentary section was folded into a series of north-trending anticlines and synclines, the most prominent of which are, from west to east, the North Tintic anticline, the Tintic syncline, and the East Tintic anticline. The rocks were also complexly faulted several times and were intruded by quartz monzonite and monzonite porphyry stocks and latite, andesite, and diabase plugs, dikes, and sills. Tertiary extrusive rocks, consisting of the Packard and Fernow Quartz Latites and younger latite and basalt, cover large areas in the southern and eastern part of the East Tintic Mountains (H. T. Morris, in Cook, 1957, p. 30-51).
The ore deposits occur in limestone replacement bodies and in fissure veins, which have a spatial relationship with each other and with the intrusive rocks. Many of the replacement bodies, which were by far the most productive deposits in the district, are on the northward projection of fissure veins. Four principal replacement ore zones are recognized - the Gemini, Chief, Godiva, and Iron Blossom. They are elongate bodies, continuous in strike but discontinuous vertically, occupying a strati-graphic interval of 6,000 feet. The ore zones are continuous across faults; at certain fault intersections, chimneys of ore as much as 2,400 feet in vertical dimension are present (Cook, 1957, p. 63-70).
The common ore minerals of the replacement deposits are galena, sphalerite, argentite, enargite, and tetrahedrite. Oxidation of these deposits extends to depths of 2,000 feet and is marked by accumulations of malachite, azurite, chrysocolla, covellite, anglesite, cerussite, smithsonite, calamine, hydrozincite, cerargyrite, native silver, and plumbo-jarosite. Ore minerals of the fissure veins are chiefly enargite, argentite, and galena, and minor amounts of sphalerite, chalcopyrite, arsenopyrite, and tetrahedrite are present. Gangue minerals are pyrite, quartz, calcite, and barite (Cook, 1957, p. 70-71).
Both types of ore deposits are associated with bands of hydrothermal alteration. Near fissure veins, the rocks are impregnated with pyrite, jas-peroid, barite, and sericite; the replacement deposits are surrounded by zones of jasperoid, clay minerals, dolomite, pyrite, and sericite (F. H. Howd, in Cook, 1957, p. 124-134).
Native gold is a rare constituent of the Tintic ores, though some oxidized ore shoots of the Mammoth mine contained flakes of native gold associated with jasperoid and quartz. Most of the gold is recovered from ore containing abundant enargite (Lindgren and Loughlin, 1919, p. 142).