Abstract
Carbon-hydrogen-oxygen isotope compositions have been measured in regional cold waters, geothermal fluids, and hydrothermally altered rocks from the Roosevelt Hot Springs geothermal area. These data have been used, in conjunction with other geological and geochemical data from this geothermal system, to place some limits on the origin of geothermal fluids and reservoir carbon, the fluid recharge area, physical-chemical environment of hydrothermal alteration, and relative permeability of the geothermal system. The similarity of hydrogen isotope compositions of local meteoric water and geothermal reservoir fluid indicate that the geothermal fluids are virtually entirely of surface derivation. An isotopically reasonable source area would be the Mineral Mountains directly to the east of the Roosevelt system. Hydrothermal calcite appears to be in isotopic equilibrium with the deep reservoir fluid. The deltaC/sup 13/ values of deep calcites and T- pH-f0/sub 2/ conditions of the reservoir defined by measured temperature, fluid chemistry, and alteration mineralogy fix the delta/sup 13/C value of the geothermal system to -5 to -6.5% (PDB). These values do not unambiguously define any one source or process, however. There is a relatively small increase in /sup 18/O of geothermal fluids relative to their cold surface water precursors and significant /sup 18/O depletion accompanyingmore » hydrothermal alteration of the granitic host rock. These isotopic shifts indicate a high ratio of geothermal fluid to altered rock for the geothermal system, implying relatively rapid (geologically) recirculation rates and significant permeability of the geothermal system.« less
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