By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Discussions of these two counties, which adjoin each other along the east slope of the Wasatch Range, are combined because the major mining district in the area, the Park City district, straddles the boundary. Gold production data for the counties were not found, but as the Park City district is the only mining center of any consequence in either county, the district's production of approximately 790,000 ounces through 1959 is assumed to be equivalent to that of both counties.
PARK CITY DISTRICT
The Park City district, about 25 miles southeast of Salt Lake City, encompasses the Uinta district in the southwest corner of Summit County and the Snake Creek and Blue Ledge districts in the northwest corner of Wasatch County.
Though silver and lead are the chief commodities, considerable gold, copper, and zinc are also mined. The 790,000 ounces of gold produced through 1959 makes Park City the fourth largest gold district in Utah.
After the discovery of ore in the middle 1860's in the Little Cottonwood and other districts on the west slope of the Wasatch Range, prospectors crossed to the east slope of the range and discovered lead-silver ore in the Park City district in about 1869. The first shipment of ore was made in 1870 or 1871 (Boutwell, 1912, p. 19).
Important discoveries of lead-silver ore were made in rapid succession and resulted in the organization of the Uinta district in Summit County in 1869 and in the Snake Creek and Blue Ledge districts in Wasatch County in 1870. In 1872, about 2 years after the first locations were made, the famous Ontario vein was discovered. Its bonanza ore bodies stimulated interest in the new area and gave Park City early prominence in the turbulent mining industry.
Prospecting in the western part of the district led to discovery of rich lead-silver replacement deposits on Treasure Hill and, in 1892, to the opening of extensive replacement deposits at the Silver King site.
Because of the high content of the ores, the prosperity of the district fluctuated with the price of silver; the decline in price in the 1890's, especially in 1893, caused temporary intermittent production or closure in some mines. However, the lower prices in 1893 stimulated development of more economical methods of mining and reduction and more effective recovery methods, and large concentrating mills were erected to treat larger volumes of low-grade ores.
In 1901 new discoveries of rich smelting ore were made and Park City production increased abruptly. After 1905 zinc became an important product. Recent activity in the district is toward the consolidation of the older properties and deeper exploration for bedded replacement ore bodies; the rich oxidized surface ores have long been depleted. The two major operators in the area are United Park City Mining Co. and New Park Mining Co. (C. L. Wilson, in Williams, 1959, p. 182).
Predominant bedrock in the district is a section of sedimentary formations that are folded, faulted, and intruded by igneous rocks and are covered locally by volcanic rocks.
The oldest rock unit is the Mineral Fork Tillite of Precambrian age. It is overlain by the Tintic Quartzite of Cambrian age, the lowermost formation of a thick interval of Paleozoic rocks, which includes the Ophir Shale and Maxfield Limestone of Cambrian age, the basal Mississippian dolomite, the Gardison Limestone, Deseret Limestone, Humbug Formation, and Doughnut Formation of Mississippian age, the Round Valley Limestone and Weber Quartzite of Pennsylvanian age, and the Park City Formation of Permian age.
Mesozoic sedimentary rocks are represented by the Woodside Shale, Thaynes Formation, and Ankareh Shale of Triassic age and the Nugget Sandstone of Jurassic age. During Tertiary time, folding and faulting preceded and accompanied igneous intrusions of granite, quartz monzonite, diorite, and diorite porphyry. Faulting also occurred as recently as Pliocene or Pleistocene time. The main structural feature is the north-plunging Park City anticline on which are superimposed smaller east-trending folds which only slightly modify the Park City anticline (C. L. Wilson, in Williams, 1959, p. 183).
The ore deposits are in fissure veins in both sedimentary and igneous rocks and are in bedded replacement deposits mostly in limestones near fissures. Veins mined in the early years were found in the Weber Quartzite and younger rocks; in recent years ore bodies have been found in veins in diorite porphyry, Humbug Formation, and Deseret Limestone. Replacement deposits were first found in the Park City and Thaynes Formations; they have recently been found in the Humbug Formation and Deseret Limestone (C. L. Wilson, in Williams, 1959, p. 186-188).
Oxidized lead-silver ores containing cerussite, anglesite, iron oxides, argentite, smithsonite, azurite, malachite, and chrysocolla were the bonanza ores of early operations. Lead-silver sulfide ores of somewhat lower grade were mined later. These ores contain galena, tetrahedrite-tennantite, pyrite, some sphalerite, and rarely bournonite. Some deposits contain argentite, famatinite, and ruby silver minerals. Lead-zinc sulfide ores contain galena and sphalerite with pyrite and some tetrahedrite. Nearly all types of ore in the district contain small amounts of gold, and in the New Park mine, some calcite-quartz veins are rich in gold (C. L. Wilson, in Williams, 1959, p. 188).
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