Abstract Base- and precious-metal deposits in the central Wasatch Mountains southeast of Salt Lake City were mined for more than 100 years beginning in 1868. Deposits present in the Park City, Little Cottonwood, and Big Cottonwood mining districts include Ag-Pb-Zn ± Cu ± Au replacements and veins, a low-grade porphyry Cu-Au deposit, Cu-bearing skarns, a quartz monzonite-type (low F) porphyry Mo deposit, and high sulfidation (quartz-alunite) Au deposits. Most production came from polymetallic replacement and vein deposits in the Park City mining district, which has a recorded production of more than 1.4 million oz Au, 253 million oz Ag, 2.7 billion lbs Pb, 1.5 billion lbs Zn, and 129 million lbs Cu from 1872 to 1978. Production in the Little and Big Cottonwood districts, mostly from Pb-Ag replacement deposits, was much smaller. Most mineral deposits in the central Wasatch Mountains are genetically related to the Wasatch igneous belt, a series of high-K calc-alkaline stocks and cogenetic volcanic rocks that formed about 41(?) to 30 Ma. The mineral deposits mostly formed near the end of magmatic activity between about 36 to 31.4 Ma. A subeconomic porphyry Mo deposit in the Little Cottonwood stock is notably younger having formed about 26 to 23.5 Ma. The intrusive rocks were emplaced mostly along the westward extension of the west-trending Uinta arch during a period of NW-SE-directed extension, and much of the mineralization in the Park City district was controlled by ENE-striking normal faults. About 15 degrees of eastward tilting of the central Wasatch Mountains during Late Cenozoic Basin and Range extension has resulted in progressively deeper levels of exposure from <1 km on the east to about 11 km on the west and in profound variations in the types of mineral deposits exposed in different parts of the range. Most deposits formed at paleodepths ≤5 km, and the most productive deposits in the Park City district formed at depths of 1 to 2 km. The porphyry Mo deposit in the Little Cottonwood stock formed at greater depths of about 6 km.

Abstract Twenty-one new K-Ar and 10 new 40 Ar/ 39 Ar ages are reported for igneous and hydrothermal minerals from intrusive rocks of the Wasatch igneous belt in the central Wasatch Mountains. Interpretation of our new data combined with previously published K-Ar ages and with new 40 Ar/ 39 Ar and U-Pb ages reported by Vogel et al. (1997) suggests that the Clayton Peak stock was emplaced at about 36 to 35 Ma, the Alta stock at about 35 to 33 Ma, and the Little Cottonwood stock at about 31 to 30 Ma. Biotite K-Ar ages progressively increase from west to east in the Little Cottonwood stock, which is consistent with more rapid cooling of the eastern part of the stock and with other evidence suggesting about 15 degrees of eastward tilting of the central Wasatch Mountains following emplacement of the Wasatch igneous belt. Most porphyry stocks in the Park City mining district were emplaced at about 41 to 40 Ma; the Ontario stock was emplaced at about 36 Ma. Vein deposits in the Park City mining district formed at about 36 to 33 Ma. The Park Premier stock was emplaced in several pulses between about 35 to 32 Ma. Alteration related to porphyry copper mineralization in the Park Premier stock formed at 33.5 Ma and advanced argillic alteration and gold mineralization formed at 31.4 Ma. Molybdenum mineralization in the eastern part of the Little Cottonwood stock formed between 26 to 23.5 Ma.

Abstract Today's field trip examines late Eocene and Oligocene granitoid intrusions, cogenetic volcanic rocks (Keetley Volcanics), and associated hydrothermally altered and mineralized rocks in the central Wasatch Mountains. Because of late Cenozoic tilting related to Basin and Range extension, a continuum of mid-Tertiary paleodepths is exposed that ranges from about 11 km on the west side of the Little Cottonwood stock to the actual paleosurface on the east side of the range (Fig. 1; John, 1989a). Consequently, we will see a wide variety of textures and styles of emplacement in the intrusive rocks, and a correspondingly wide variety of hydrothermal alteration types and mineral deposits (Lawton et al., 1980; John, 1989a). Mid-Tertiary igneous rocks in the central Wasatch Mountains consist of three phaneritic stocks exposed in the western and central parts of the range (Little Cottonwood, Alta, and Clayton Peak stocks); six porphyry to fine-grained phaneritic stocks exposed in the middle and eastern parts of the range, primarily in the Park City mining district (Flagstaff, Glencoe, Mayflower, Ontario, Pine Creek, and Valeo stocks); a subvolcanic porphyry complex (Park Premier stock) exposed near the Park Premier mine which is now partly covered by water filling the Jordanelle Reservoir; and coeval volcanic rocks (Keetley Volcanics), subvolcanic intrusions, and a volcanic neck (Indian Hollow plug) are exposed on the east side of the range (Figs. 2 and 3). The intrusive rocks range from coarse-grained, coarsely porphyritic on the west to fine-grained, porphyroaphanitic on the east (John, 1989a). They form a high-K, calc-alkaline series (Vogel

Abstract The Oquirrh and Wasatch Mountains lie at the eastern edge of the Basin and Range province in north-central Utah. North-central Utah has had a long and complex deformational history, including two compressional events and two extensional events in the Mesozoic and Cenozoic, respectively. The ranges host three major mining districts largely containing pluton-related mineralization. The districts are aligned along the east-west trending Uinta-Cortez axis that is a manifestation of an Archean-Proterozoic suture. The axis is the fundamental control on pluton emplacement and related metallogeny. The major intrusions in the districts are Paleogene in age and were emplaced in an extensional stress regime with a NW-SE least principal stress direction. Within the mining districts mineralization is controlled by Mesozoic compressional structures and NE-striking faults and fractures. The Wasatch and Oquirrh Mountains lie in north-central Utah at the eastern edge of the Great Basin. The mountain ranges host three major mining districts, including the world-class Bingham mining district (Fig. 1 ). Because of the location of the districts, this discussion will only focus on the Oquirrh and the central Wasatch Mountains. North-central-Btah has had a long and complex structural history that has direct bearing on the distributionof plutonic rocks and the metallogenic evolution of the two mountain ranges. This paper will briefly review the structural history of the region followed by a discussion of the structural controls on plutonism and metallogeny. The structural control of plutonism and metallogeny in north-central Utah has been recognized for more than 7 5 years. Most of the previous work has focused on the importance of the Uinta trend, an east-west trend of intrusions and structures in the eastern Great Basin, which was first noted by Butler et al. (1920). Stokes (1968) statistically examined the relationship between fault trends and mineralization in the eastern Great Basin, and noted that mineralization is dominated by east and northeast trends. Erickson (1974) summarized the evidence for the Uinta trend and emphasized its economic importance. Within the Wasatch and Oquirrh Mountains, several studies have addressed the structural control in the respective mining districts and will be cited below. The southern margin of the Archean Wyoming Province strikes east-west through north-central Utah. The edge of the Archean craton is represented by thesouthernmost exposure of the Farmington Canyon Complex in the central Wasatch Mountains (Bryant, 1988). The suture between the Archean Wyoming Province to the north and the Proterozoic Yavapai Province

Apatite fission-track ages from basement rocks in the Wasatch Mountains between Ogden and Bountiful, Utah, range from 5 m.y. near the Wasatch fault along the west margin of the mountains to 94 m.y. on the crest of the range and show a correlation with altitude within individual fault blocks. Analysis of these ages yields an uplift rate of 0.4 mm/yr for the last 10 m.y. for the fault blocks having the most sustained rapid uplift. A variety of intermediate ages of apatite from the crest of the range and at lower altitudes is interpreted as representing fault blocks with rocks having mixed ages due to various degrees of tilting and varying but lesser amounts of uplift.

High-grade metamorphic rocks and migmatites in the Farmington Canyon Complex were derived from igneous and sedimentary rocks possibly, as old as 3000 m.y. They were probably metamorphosed about 2600 m.y. ago, and they were severely metamorphosed, migmatized, and intruded by quartz monzonite 1790 m.y. ago. A very high initial Sr 87 /Sr 86 ratio of 0.769 indicates that the quartz monzonite magma was derived by melting of the more leucocratic parts of the layered gneisses. On and just west of Antelope Island, plutons of granite were emplaced 2020 m.y. ago.

Abstract The Oquirrh and Wasatch Mountains lie at the eastern edge of the Basin and Range province in north-central Utah. North-central Utah has had a long and complex deformational history, including two compressional events and two extensional events in the Mesozoic and Cenozoic, respectively. The ranges host three major mining districts largely containing pluton-related mineralization. The districts are aligned along the east-west trending Uinta-Cortez axis that is a manifestation of an Archean-Proterozoic suture. The axis is the fundamental control on pluton emplacement and related metallogeny. The major intrusions in the districts are Paleogene in age and were emplaced in an extensional stress regime with a NW-SE least principal stress direction. Within the mining districts mineralization is controlled by Mesozoic compressional structures and NE-striking faults and fractures.