SEEKING MAGMATIC, HIGH-ENTHALPY GEOTHERMAL SOURCES IN THE EXTENSIONAL GREAT BASIN, WESTERN U.S., USING INTEGRATIVE MAGNETOTELLURICS (MT), ISOTOPE GEOCHEMISTRY AND STRUCTURAL GEOLOGY

Geothermal systems in the actively extending Great Basin province number in the hundreds, but the power potential of all but a handful lies seemingly only within a few 10’s MWe. We wish to identify systems with larger enthalpy sources at an earlier stage in assessment, as well as find hidden systems of high potential. Well-sampled MT profiling in the Great Basin is revealing interrelations between resistivity, geochemistry, structural geology, and other geothermal observations. Numerous conductive, sub-horizontal zones in the deep crust appeared to indicate magmatic underplating and hydrothermal fluid exsolution, corroborated by seismic surveying where coincident. Average depth to these zones is inversely correlated with rate of extension, with the more active subregions of northwestern Nevada and western Utah being relatively shallow. These zones commonly have steep, dike-like conductors extending surfaceward, inferred to be large fault zones, several of which appear to feed into known high-temperature geothermal systems. One such system, the flagship Dixie Valley area of NW Nevada, also shows diagnostic magma-sourced gas isotopes, in particular ³He. However, Dixie Valley until recently has been the only extensional system where adequate MT and isotope data coincide to corroborate magmatic input. Moreover, such systems generally form at 3D structural intersections that promote dilatency, which are not represented in 2D transect models. Thus, fully 3D MT surveys were carried out at Dixie Valley and also the newly recognized McGinness Hills system of central Nevada where transect MT also suggests deeper magmatic connection. At Dixie Valley, the 3D analog to the 2D crustal fault zone is a complex of NE and NNW striking planar conductors dipping steeply westward under the Stillwater Range. At McGinness Hills, the 3D structure has abrupt along-strike bounds and also is a complex of NW and NNE trends in a fault accommodation zone. Production well fluids show anomalous ³He values, and CO₂ flux anomalies exist near intersections between NNE and NW faults. This result greatly strengthens the case that the geothermal MT structures we are resolving are linked to magmatic processes. MT, isotopic sampling, and structural analysis have promise for revealing deep heat sources and their plumbing into geothermal systems.