Spatial Characterization of Hydrothermal Alteration in a Fault-Controlled Amagmatic Geothermal System at Tungsten Mountain, Nevada USA

The Tungsten Mountain geothermal field, operated by Ormat Nevada, Inc. is located in Churchill County, Nevada. Since November 2017, the field has produced 29 MWe gross geothermal power production, with a resource temperature of 150 °C. The geothermal system is confined to a normal fault step-over along the western margin of Edwards Creek Valley. The primary fluid flow pathways for the geothermal reservoir are associated with the damage zones of faults, primarily in northeast-striking normal faults and in their intersection with west-northwest-striking normal faults in the Mesozoic meta-sedimentary basement. In this study, we investigate the characteristics of hydrothermal alteration minerals at multiple scales (surficial, borehole, drill core, thin section) and compare these data with temperature logs, known feed zones, geologic structure, geophysics, and fracture distribution, to explore the relationship between secondary mineralization and the hydrogeologic properties of the geothermal reservoir. An additional goal is to interpret past reservoir thermal conditions and fluid flow regime using the temperatures and chemical conditions implied by the hydrothermal alteration mineral assemblages. The methods employed include analysis of surficial samples, drill core and drill cuttings from 8 geothermal wells and boreholes using hyperspectral imaging in the SWIR (Short Wave InfraRed 400-2500 nm) and LWIR (Long Wave InfraRed, 8-12 μm) wavelength ranges, X-Ray Diffraction, and petrographic microscopy. Our findings show a pattern of montmorillonite or kaolinite occurrence in shallow samples that grades into illite, chlorite, and muscovite assemblages at depth. This is consistent with the progression expected in active geothermal systems with a general increase in temperature with depth. However, in some locations (near interpreted faults), reversals are observed where montmorillonite smectite is found at depths below the higher temperature illite-chlorite assemblages.