Summit and Wasatch Counties Utah Gold Production

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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