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

U. S. Geol. Surv., Menlo Park, CA, United States

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 paleodepths 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 degrees during the late Cenozoic. Fluid inclusion populations in igneous quartz also vary systematically with paleodepth; 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 degrees to 20 degrees east, separate rocks containing high-salinity fluid inclusions (above the high-paleosalinity horizon) from those lacking such fluid inclusions. Comparison of fluid-inclusion 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 of immiscible high-salinity 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 ₂ -rich fluids.

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