The Mountain Home geothermal system of the western Snake River Plain magmatic province was discovered by the Snake River Geothermal Drilling Project. On January 26, 2012, artesian flowing water with a temperature of 140°C was encountered at a depth of 1,745 m. In order to understand present-day geothermal characteristics, an extensive investigation was undertaken to describe the thermal and compositional history of fluids responsible for observed mineralization. This fossil system was then compared to that of the present-day to characterize the evolution of the MH geothermal system and the geothermal potential of the western Snake River Plain.
Extensive mineralized fracture networks of pectolite-prehnite, calcite, and laumontite were discovered in the recovered core. Observations of the core, thin section petrography, X-ray diffraction, and Electron Microprobe analyses were performed in order to describe mineral parageneses of various alteration zones. Carbon and oxygen stable isotope ratios, along with temperatures of homogenization from fluid inclusions in hydrothermally precipitated calcite, were measured along ~100 m of basalt core from 1709-1809 m depth. d13CPDB and d18OPDB values in hydrothermally precipitated calcite suggest a mixture of deep-seated mantle derived and meteoric fluids. An anomalous zone from 1722-1725 m depth displays a range in d13CPDB and d18OPDB suggests non-equilibrium fractionation due to boiling. Fluid inclusion microthermometry identified primary inclusions with trapping temperatures that agree with a boiling geothermal system. A calcite-water geothermometer used to calculate paleo-fluid oxygen isotopic composition and a comparison with present-day fluid oxygen isotopic composition reveals a cooling trend with mixing of meteoric waters and deeply derived fluid consistent with a cooling magma body.