DOE OSTI2017
Our research group comprising personnel from University of Utah/EGI (UU/EGI), University of Nevada at Reno (UNR) and Lawrence Berkeley National Laboratory (LBNL) has been applying magnetotellurics (MT), diagnostic structural affiliations, and soil gas flux and geochemistry to assist in identifying hidden, high-enthalpy geothermal systems in extensional regimes of the U.S. Great Basin. We are specifically looking for upwelling low-resistivity and geologic structures that can carry fluids from magmatic or high-grade metamorphic conditions in the deep crust upward to exploitable depths, and to verify the nature of the deep sources through soil gas geochemical compositions. The project was motivated by prior MT transect coverage of western and central Nevada centered upon the Dixie Valley geothermal system where such favorable indicators were first recognized. The high-angle structures are taken to be fluidized fault zones connecting deep magmatic/metamorphic activity with the geothermal system but the concept requires verification by testing at other systems. The project was set up with a two-phased organization. Phase I was carried out at the McGinness Hills system, central Nevada, where Ormat’s flagship power facility is located and a considerable amount of pre-existing data was available. Transect MT data also showed a strong low-resistivity upwelling originating from interpreted deep crustal magmatic underplating. Controlling structures on production as indicated by Ormat data and new mapping by UNR were favorable to dilatency being an accommodation zone between major normal faults of opposing dip. A 3D MT survey and inversion confirmed the existence of the steep low-resistivity zone dipping ESE toward the deep crust and placed N-S bounds upon the feature. Ormat personnel cooperated with PI Wannamaker in the field in providing fluid samples from the production intervals for He isotope composition. Elevated 3He was verified through analysis at LBNL confirming that there was a magmatic connection into the producing system. High CO2 soil gas flux including possibly metamorphic 13C and 14C component was measured over the area of dilatent structures. Hence, the triad of indicators posed above was confirmed in Phase I. Subsequently, Phase II of the project proceeded in the greenfield Kumiva-Blackrock Desert district of northwestern Nevada to see if a new system could be identified. Transect MT data also showed a strong low-resistivity upwelling originating from interpreted deep crustal magmatic underplating. However, given that there was no exploitable geothermal system recognized at the outset in the region, the collection of new geoscientific data was required. An areal MT survey of 131 sites over the Kumiva-Blackrock Desert district was completed, and a 3D inversion produced using the in-house, DOE-supported finite element inversion algorithm. Particular MT low resistivity upwellings that received followup study occurred under the west flank of the Seven Troughs Range, under the playa immediately west of the Blue Wing Mountains, and under northern Granite Springs Valley. Structural assessment of the project area by Co-I J. Faulds at UNR provided numerous favorable Quaternary fault settings, which were correlated to the MT upwelling structures. Extensive profiling for CO2 soil gas flux over favorable geophysical and geological structure was performed by LBNL personnel. Soil gas flux anomalies generally were not large but did show correlation with MT upwelling structure and favorably dilatent geological structures. Isotope analyses showed presence of possible inorganic/metamorphic 13C but 14C concentrations did not exceed background values. We view the initial concept of a confluence of MT low-resistivity upwelling, favorably dilatent 3D geological structure, and elevated soil gas flux including 13C component to be supported by the further evidence of this project although the indicators in the Phase II study were more diffuse. The northern Granite Springs Valley structure is receiving followup in the UNR Play Fairway Analysis, being in the southern termination of a major normal fault with silicified soil and warm temperature probe results