Structural control of the Norris Hot Springs and associated geothermal system

Abstract

Many active and productive geothermal systems are structurally controlled, suggesting that certain structural configurations have higher potentials for geothermal energy. Analyzing potential productive geothermal wells is known as geothermal play fairway analysis (PFA). Identifying these high potential structural configurations is another way to locate geothermal systems without surface expressions, which represent an unquantified natural resource for geothermal energy. By studying geothermal systems with surface expressions, I move closer to being able to locate additional natural resources. The Norris Hot Springs, located in the foothills of the Tobacco Root Mountains, Southwestern Montana, presents a novel study area to test structural control on geothermal system development. I combine geologic mapping, UAV-based photogrammetry, zircon U-Pb geochronology, seismic data, and 3D geologic modeling to date and interpret the multistage development of local structures and their influence on the geothermal system beneath the Norris Hot Springs. A comprehensive structural model suggests the geothermal system associated with the Norris Hot Springs is hosted by a deep network of faults and fractures in Precambrian gneiss basement. A nearby fault system primarily comprised of high angle NNW-SSE trending normal faults, which often display pervasive alteration and polymetallic vein mineralization, likely connects surficial features with deeper reactivated Laramide-era structures. A primary goal of this study was to investigate the benefits of supplementing traditional field work with novel methods. Results from this study demonstrate the utility of combining geologic mapping, 3D modeling, structural analysis, 3D modeling and UAV surveys for geothermal exploration and constrain the timing and influence of local faulting on geothermal activity at the Norris Hot Springs.