Geologic setting, depths of emplacement, and regional distribution of fluid inclusions in intrusions of the central Wasatch Mountains, Utah

Abstract

Nine mid-Tertiary calc-alkaline stocks, a subvolcanic porphyry system, and coeval volcanic rocks are exposed in a 45-km-long east-trending belt across the central Wasatch Mountains, Utah. The intrusions vary systematically from west to east in texture, style of emplacement, extent of contact metamorphism, hydrothermal alteration, and mineralization. Pressure-depth estimates based on metamorphic mineral assemblages, stratigraphic reconstructions, and fluid inclusion data indicate a regular variation in palcodepths ranging from about 11 km on the west to less than 1 km on the east. These data indicate that the central Wasatch Mountains have been tilted down to the east about 20 ø during the late Cenozoic. Fluid inclusion populations in igneous quartz also vary systematically with palcodepth; high-salinity (halite-saturated) fluid inclusions are present in the eastern porphyry stocks and in the upper parts of the Alta and Clayton Peak stocks in the center of the belt but are absent in the deeper parts of the Alta and Clayton Peak stocks and in the Little Cottonwood stock on the west side of the belt. In the Alta and Clayton Peak stocks, nearly planar high-paleosalinity horizons, presently dipping 15 ø to 20 ø east, separate rocks containing high-salinity fluid inclusions (above the high-paleosalinity horizon) from those lacking such fluid inclusions. Comparison of fluidinclusion populations in igneous and vein quartz in the Alta and Clayton Peak stocks indicates that high-salinity fluids predated most of the vein-forming hydrothermal activity and provide the earliest record of fluids to circulate in these stocks. High-salinity fluids probably formed either by boiling of fluids released during the late stages of crystallization in the parts of the intrusions where pressure was less than about 1,300 bars or by exsolution ofimmiscible highsalinity brines from the crystallizing magmas. Most hydrothermal mineralization associated with the intrusions, including Ag-Pb-Zn ores in the Park City district, are associated spatially with parts of the intrusions where high-salinity fluids were present. The major exception is the porphyry molybdenum system in the eastern part of the Little Cottonwood stock, which probably was at too great a depth (approximately 7 km) to form high-salinity brines and is dominated by low-salinity CO•-rich fluids. Introduction IN THE central Wasatch Mountains, Utah, a mid-Tertiary series of nine eale-alkaline granitoid stocks, a subvolcanic porphyry system, and coeval volcanic rocks are exposed in an east-trending belt that is the westward extension of the Uinta arch (Figs. 1 and 2). Important Ag-Pb-Zn replacement and fissure ores in the Park City district are spatially and temporally related to porphyry stocks (Barnes and Simos, 1968; Bromfield et al., 1977), and scattered base and precious metal mineralization is associated with several other stocks (Calkins and Butler, 1943; James, 1978; Bromfield and Patten, 1981). Variations in texture, style of emplaeement, hydrothermal alteration, and minerali...

Origin and emplacement of igneous rocks in the central Wasatch Mountains, Utah

“…The stocks are grouped based on their location and texture (John, 1989a;John et al, 1998). The western type consists of the Little Cottonwood, Alta, and Clayton Peak stocks; they are coarse-grained and mostly equigranular (Wilson, 1961;Atkin, 1982;John, 1989a;Marsh, 1992;Hanson, 1995). Each of these three stocks is compositionally distinct (Figs.…”

“…To the east of the eastern stocks is the Keetley volcanic field, a sequence of lahars, debris avalanche deposits, volcanic conglomerates, air-fall tuffs, ash-flow tuffs, and minor lava flows which represent the eroded r e m n a n t of a stratavolcano (Woodfill, 1972;Leveinen, 1994;Feher, 1997 I I I I I I I I I l l I I I (after Gill, 1981) and occur in the calcalkaline field (lower diagram). and may be its intrusive equivalents (John, 1989a(John, , 1989bJohn et al, 1998). The deposits in Keetley volcanic field extend over a 330-km2 area between the eastern stocks and the west end of the Uinta Mountains (Bryant, 1990;Leveinen, 1994).…”

“…They formed between 36-30 Ma. Subsequently, middle to late Cenozoic flexural-isostatic uplift associated with the Wasatch normal fault produced a regional tilt to the east (Parry and Bruhn, 1987;John, 1989a). The Little Cottonwood stock at the western end of this belt is the most deeply exposed, having been emplaced at a depth of about 11-5.6 km (Parry and Bruhn, 1987;John, 1989a).…”

“…Sample sites designated with open circles. Tkiangles denote sample sites and paleodepth determinations in kilometers (John, 1989a Mountains (Figs. 1 and 2).…”

Significance of the precambrian basement and late Cretaceous thrust nappes on the location of tertiary ore deposits in the Oquirrh Mountains, Utah

“…There are several lines of evidence for the Uinta trend: (1) A number of west-trending linearly disposed intrusive and extrusive rocks of Tertiary Miocene age in the Wasatch Mountains and in mountain ranges of the Great Basin west of the Wasatch Fault (Armstrong and others, 1969;Moore and others,1979;McKee,1983 andJohn (1989). (2) Evidence of local uplift and erosion of sedimentary rocks overlying the Bingham plutons during Oligocene igneous activity (Slentz, 1955).…”