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Geothermal resources of the western Black Rock Desert, northwestern Nevada: Hydrology and aqueous geochemistry
AuthorWelch, Alan Herbert
AdvisorJacobson, Roger L. L.
Geological Sciences and Engineering
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The western Black Rock Desert includes several systems, some of which exceed 150 and may reach 200 degrees Celsius at depth based on chemical geothermometry. Geochemical and isotopic data, used in conjunction with hydrologic and geophysical information, indicate that several hydrologically distinct systems are present and are all recharged by local meteoric water. The cation composition of the thermal water appears to be controlled by alumino-silicate minerals that are commonly found in other active hydrothermal systems. Estimates of the equilibrium temperatures at which some mineral pairs are stable, when compared with the more commonly applied geothermometer estimates, indicate that thermodynamic data may be useful for estimating deep aquifer temperatures. The use of a chemical geothermometer with a thermodynamic foundation (rather than an empirical formula) allows the incorporation of geologic and mineralogic information into the evaluation of a hydrothermal system. Disadvantages of this approach can be inadequate thermodynamic data and a lack of mineralogic data. The thermal water at Great Boiling and Mud Springs, which has a chloride concentration of about 200 milligrams per liter and 4,500 milligrams per liter total dissolved solids, appears to have been affected by shallow evapotranspiration in an adjacent playa prior to deep circulation. This model of recharge within the basin floor is distinctly different from models proposed for most other hydrothermal systems in the northern Great Basin. The conclusion that shallow evapotranspiration is occurring is based on the isotopically evolved character of the water and the correspondence between the stable isotope composition and the chloride concentrations in the thermal and local nonthermal water. Systems at Trego Hot Springs and Fly Ranch may also be recharged by water that has been affected by shallow evapotranspiration prior to deep circulation. A consequence of this conclusion is that the primary control on the geochemistry is evaporative concentration prior to deep circulation with perhaps only relatively minor net mass transfer from the aquifer matrix to the aqueous phase, except for an obvious increase in silica concentrations within the hotter parts of the system. A small amount of net mass transfer is consistent with a system that has a large water to rock-surface-area ratio, such as a fault-controlled or fractured system. The small or nonexistent oxygen shift is consistent with this type of system.
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SubjectBlack Rock Desert
deep aquiger temperatures