GEOLOGIC MAP OF THE BLACK WARRIOR BLIND GEOTHERMAL AREA, TRUCKEE RANGE, NORTHWESTERN NEVADA

The 1:24,000 scale geologic map of the Black Warrior geothermal area showcases the results of detailed stratigraphic and structural analyses. The map focuses on the Truckee Range in NW Nevada and covers portions of four adjoining 7.5’ quadrangles. The detailed map was produced to evaluate the structural controls of a blind geothermal system (i.e., no surface hot springs or fumaroles present) and overall geothermal potential of the area.

Geologic mapping of surficial and bedrock lithologies identified faults and stratigraphic relationships between successive and interfingering Miocene volcanic sequences that nonconformably overlie Mesozoic plutonic and metamorphic basement. Middle Miocene basaltic andesites, dacites, tuffs, sandstones and conglomerates are well distributed throughout the study area, whereas outcrops of Mesozoic basement rocks are limited. Mesozoic basement rocks include Cretaceous granodiorite, hornfels, and skarn, as well as Upper Triassic to Middle Jurassic meta-siltstone, quartzite, carbonate (dolomite and marble), phyllite, and schist. Quaternary alluvial fans reside in low-lying areas adjacent to and within the Truckee Range.

The structural framework is dominated by gently to moderately east-tilted fault blocks bound by moderately to steeply west-dipping, north-northeast-striking normal faults. The fault blocks consist of highly deformed and metamorphosed Mesozoic rocks and faulted and tilted Miocene volcanic rocks. Major strands of the west-dipping fault system terminate and step in the vicinity of the thermal anomaly. Major extension began ~12 Ma and has continued into the present, as evidenced by Quaternary fault scarps. The fault pattern suggests two possible structural settings for this geothermal system: (1) a fault termination of the southeastern range-front fault with accompanying ~2 km wide horse-tailing producing abundant closely spaced faults and high fracture permeability; and (2) a ~3-5 km wide fault step-over between two major W-dipping normal faults, whereby many closely-spaced minor faults produce a zone of high fracture permeability. These structurally-complex characteristics are advantageous for geothermal activity and suggest that Black Warrior may host a viable geothermal system with suitable permeability at depth.