Borehole Temperature Analysis and Interpretation of the Marysville Montana Geothermal Area

Abstract

The Marysville Geothermal Area in western Montana is a unique geothermal study region due to its history of high-precision temperature-depth data dating back to the 1970s and the unusual characteristics of the geothermal system: lack of known surface manifestations, shallow depth, and the location in a structural and topographic high with no evidence of recent volcanism, hot springs, or fumaroles. Previous NSF/ERDA sponsored exploration of the anomaly comprises a variety of detailed geological, geophysical, and geochemical studies. These are briefly summarized and updated here. Since the Marysville anomaly is, as far as is presently known, a blind system, a detailed stream and spring ?18O and ?2H isotopic analysis of the area was carried out to provide insight into any possible surface manifestations which are common in other shallow geothermal areas and might be so subtle as to have been unrecognized in the past. This study provides a denser coverage of stream and spring sampling focusing on probable outflow regions based on analysis of previous geophysical results. A review of the gravity and magnetics surveys with the addition of new temperature-depth and surface water geochemical data is presented. The conclusion is that the Marysville Geothermal Area as a blind system with the circulating geothermal fluids having no identified surface discharge.