Petrographic and Thermal Evidence of High-Temperature Geothermal Activity from the MH-2B Slimhole, Western Snake River Plain, Idaho

Abstract

The Mountain Home (MH) geothermal system of the western Snake River Plain magmatic province was discovered by the Snake River Geothermal Drilling Project on January 26, 2012, when artesian flowing water with a temperature of 140 °C was encountered at a depth of 1,745 m. We present thermal and compositional variations of geothermal fluids recorded in mineralization observed in the MH-2B core and compare them to the present-day fluid in order to characterize the evolution of the MH geothermal system and the geothermal potential of the western Snake River Plain. Mineralized fracture networks of pectolite-prehnite, calcite, and laumontite were documented 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 the paragenesis of various alteration zones. Fluid inclusion microthermometry identified primary inclusions with trapping temperatures ranging from 186-368 °C consistent with a boiling geothermal system at depth.