Conceptual Model At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)

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Exploration Activity: Conceptual Model At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)

Exploration Activity Details
Location Long Valley Caldera Geothermal Area
Exploration Technique Conceptual Model
Activity Date 1985 - 1988

Usefulness useful
DOE-funding Unknown

Exploration Basis
Sorey et al. (1991) integrated information from previous scientific and private industry investigations with new data obtained from fluid sampling, test drilling, and geological and geophysical studies conducted between 1985-1988 into a comprehensive conceptual model of the present-day hydrothermal flow system at Long Valley caldera. Lithology and temperature gradient data from wells drilled prior to 1988 are summarized in detail in the compilation, which includes information from numerous wells described in previous studies, and data from many of the wells are available online through the U.S. Geological Survey (Farrar et al., 2010). Thermal conductivity, XRD, and isotopic analyses of core cuttings from several of the wells discussed have been completed in several studies, and seem to prove useful in most cases (Flexser, 1991; Goff et al., 1991; Smith and Suemnicht, 1991). Results from these studies are also summarized in Sorey et al. (1991). Relevant data from chemical and isotopic studies published during the same year are also considered in the review.
Data from drill holes, geochemical analysis of rock and fluid samples, geologic mapping, and geophysical investigations indicate that the roots of the present-day hydrothermal system at Long Valley exist beneath the caldera's west moat. Under this model, the roots of the system include the heat source, the source reservoir consisting of the deepest and hottest temperature reservoir within the system, and the principal zone(s) of fluid upflow that transmits thermal waters into the shallower outflow zones that exist within the caldera-fill volcanic rocks. Reservoir temperatures between 202-214°C encountered in volcanic fill penetrated by wells drilled in the west moat are among the highest measured at Long Valley at the time the report was published (1991), however chemical geothermometers suggest that maximum reservoir temperatures as high as 248°C exist within the hydrothermal system. Fluids from these wells were isotopically similar to those sampled from springs and wells at Casa Diablo, suggesting a common source reservoir. Interpretations of resistivity data, temperature profiles in several deep wells, geochemical isotope studies, and stratigraphic displacements encountered in wells drilled into the west moat suggest that the source reservoir is located in metamorphic basement rocks, from which thermal fluids likely ascend along steeply dipping structures. The alignment of low resistivity zones established from the magnetotelluric (MT) data with intracaldera projections of fault intersections between the Laurel-Convict fault, the NE-trending Discovery fault zone, and the westernmost faults of the resurgent dome provide likely candidates for areas hosting enhanced vertical permeability that would facilitate fluid upflow beneath the rhyolite plateau. Thermal fluids are then inferred to flow laterally at shallower depths through the volcanic sequence, as indicated by temperature reversals measured in virtually every thermal well drilled in the caldera leading up to 1988, to the west towards well 44-16 and to the southeast in the direction of well RDO-8 and Casa Diablo. The correlation between increased seismic activity and rising helium isotope ratios in fumaroles at Mammoth Mountain measured in 1989 suggest that recent magmatic intrusions contribute to the heat input of the present hydrothermal system. Although resistivity data are less complete in this area, structural intersections between north-south and northeast-trending faults within the ring fracture zone beneath Mammoth Mountain cannot be ruled out as a potential zone of upflow in the caldera's southwest corner. Recharge of the hydrothermal system is thought to occur along the western rim of the caldera (including Mammoth Mountain), as indicated by stable water isotope data. Additional drilling into the deep metamorphic basement beneath the caldera's west moat is needed to definitively test the conceptual model posed by the authors of this report.

Additional References