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The magmatic-hydrothermal fluid history of the Harrison Pass Pluton, Ruby Mountains, NV: implications for the Ruby Mountains-East Humboldt Range metamorphic core complex and Carlin-type Au deposits




Gates, Christopher Harry, author
Ridley, John, advisor
Sutton, Sally, committee member
Strauss, Steven, committee member

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Intrusion of the ~36 Ma, calc-alkaline, granodiorite-monzogranite Harrison Pass Pluton (HPP) occurred as magmatic fronts migrated southwest across the Great Basin during the Eocene. The HPP was locally intruded into the Ruby Mountains-East Humboldt Range (RMEHR), a classic Cordilleran metamorphic core complex that would undergo rapid tectonic exhumation during the late Cenozoic. Although the emplacement depth of the HPP provides an estimate for the magnitude and timing of subsequent uplift, disagreement exists between published mineral thermobarometry data and stratigraphic reconstructions. Synchronous with emplacement of the HPP was a regional hydrothermal fluid event responsible for deposition of >200 Moz of Au in sediment-hosted Carlin-type deposits (CTD's) along four linear trends. Magmatic, meteoric, and metamorphic models have been invoked to explain the origin of fluids and Au for CTD's, but few studies have directly examined the fluids generated by a potential source intrusion such as the HPP. Investigation of the magmatic-hydrothermal fluid history of the HPP, particularly the pressure-temperature conditions of fluid entrapment and fluid geochemistry, is an effective means of testing and improving existing models for the development of the RMEHR metamorphic core complex and for the origin of CTD's. Field and petrographic observations of pegmatites, aplites, miarolitic cavities, quartz veins, and multiple types of hydrothermal alteration, coupled with data from fluid inclusion microthermometry, LA-ICP-MS fluid inclusion geochemistry, and oxygen stable isotopes from magmatic and hydrothermal quartz, demonstrate that two-stage intrusive assembly was paralleled by a two-stage magmatic-hydrothermal fluid system. Early stage fluid activity was dominated by two aqueous, low salinity (~3 wt % eq. NaCl), B-Na-K-Rb- Sr-Cs-bearing, ore metal-poor fluids. These fluids were entrapped at ~600-700°C and ~2400-7600 bar in pegmatites, miarolitic cavities, and quartz veins within early stage units, as well as in quartz and calcite veins in base-metal skarns along the pluton margin and in the contact metamorphic aureole. Late stage fluid activity was dominated by one aquo-carbonic, low salinity (~3 wt % eq. NaCl), B-Na-K-Rb-Sr-Cs-bearing, ore metal-poor fluid. This fluid was entrapped at 570-680°C and ~4800-7200 bar in pegmatites, aplites, and quartz veins, and did not migrate out of late-stage intrusive units. Magmatic δ18O values for quartz demonstrate that this magmatic-hydrothermal fluid system evolved without significant dilution from meteoric inputs until the late influx of post-intrusion hydrothermal fluids, interpreted to be of mixed magmatic-meteoric origin. These fluids were aqueous, low-temperature (320-410°C), low salinity (<4 wt % eq. NaCl), and were entrapped at <2400 bar in fault-hosted microcrystalline quartz veins.The entrapment conditions for early stage magmatic-hydrothermal fluids determined from fluid inclusion microthermometry data indicate that the HPP was emplaced at depths of 9-18 km in the Ruby Mountains-East Humboldt Range. Brittle-ductile deformation of the HPP on the regionally-exposed Ruby Mountain Shear Zone indicate that at least 9 km of vertical exhumation has occurred since the intrusion of the HPP. Such emplacement depth estimates are consistent with published mineral thermobarometry from the HPP and from nearby metamorphic rocks. It is interpreted that the disparity between these estimates and the relatively shallow minimum emplacement depths of 4-6 km suggested by stratigraphic reconstructions is supportive of the existence of poorly preserved, Mesozoic thrust sheets that augmented the thickness of the overlying rock package during the Eocene. Although a well-accepted model for the origin of fluids and Au for CTD's remains outstanding, the model of Muntean et al. (2011) argues for the separation of a low-salinity, vapor-rich, Au-partitioning fluid from a high-salinity, base metal-partitioning fluid in mid- crustal magma chambers as a critical process in the evolution of CTD ore fluids. Although HPP emplacement depths and the existence of a robust magmatic-hydrothermal fluid system are broadly supportive of this model, no evidence of a high-salinity fluid was observed. Low concentrations of base metals (Cu, Pb) and CTD pathfinder elements (As, Tl) relative to whole-rock values are not consistent with the efficient fluid partitioning of metals invoked by Muntean et al. (2011). Also, low concentrations of CTD pathfinder elements relative to published values for CTD ore fluids indicate that the HPP was not Au-enriched. However, similar salinities and δ18O values suggest that HPP fluids may represent the minor magmatic component of ore fluids detected at some CTD's, but other fluid inputs and an external Au source would be required to produce these ore fluids. Thus, it is suggested that the magmatic-hydrothermal fluid history of the HPP is more consistent with a dominantly amagmatic fluid model for the origin of CTD's.


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