GEOCHEMICAL CHARACTERIZATION OF THE MOUNTAIN HOME AIR FORCE BASE GEOTHERMAL SYSTEM

The Mountain Home Air Force Base geothermal system lies beneath the western Snake River Plain magmatic province. The Snake River Geothermal Drilling Project encountered 140°C artesian flowing water from a depth of 1745 meters and extensive mineralization including calcite, quartz, and laumontite in the recovered core. We report carbon and oxygen stable isotope ratios and fluid inclusion microthermometry results from hydrothermally precipitated calcite along ~100 meters of basalt core. Oxygen isotopes in combination with fluid inclusion microthermometry and petrographic analysis are used to reconstruct a detailed thermal history of mineralization in the core, whereas carbon isotope ratios track deep-seated sources of CO₂. The present day groundwater has δ¹⁸O and δD values of -3.2 ‰ and -88 ‰ (SMOW), respectively, suggesting mixing between meteoric and evolved geothermal fluids. Groundwater is currently low in dissolved CO₂ with a δ¹³C of -10.9 ‰ (PDB) similar to regional Snake River Plain aquifer values. Measured δ¹³C values in calcite range from -7.2 to -2.0 ‰ (PDB) and δ¹⁸O values range between -20.5 and -15.9 ‰ (PDB) with respective anomalies of 0.66 ‰ and -8.1 ‰ suggesting a possibility of a different water source or fractionation process at a depth of ~1722 meters. Carbon values suggest mixing of deep-seated magmatic fluids and meteoric waters. The majority of the oxygen values suggest calcite precipitation from meteoric waters over a range of temperatures. Preliminary fluid inclusion microthermometry has identified primary and secondary inclusions with homogenization temperatures of 203°C and 147°C, respectively.