| Abstract |
The Milford FORGE deep well site is located 5 km west of the Roosevelt Hot Springs, on alluvial fan deposits (200-600 m thick) that overlie a large volume of hot crystalline basement rock. The site occurs within a region that is geologically complex and characterized by extensional faulting, sporadic magmatism, and zones of anomalously high heat flow, inside the southeast margin of the Great Basin. The basement rocks at the FORGE deep well site are made up of Precambrian gneiss and Tertiary plutons. These crystalline units are exposed across the eastern part of the Milford Valley basin, from the Mineral Mountains in the east to the Acord-1 well in the west, and they are separated by intrusive and fault contacts. Gneiss contains biotite, hornblende, K-feldspar, plagioclase, quartz, and sillimanite, and isotopic dating indicates Proterozoic metamorphism ~1720 Ma. Plutonic rocks comprise diorite, granodiorite, quartz monzanite, syenite, and granite, containing variable amounts of biotite, clinopyroxene, hornblende, K-feldspar, magnetite-ilmentite, plagioclase, and quartz. The oldest intrusion was emplaced ~25 Ma followed by younger intrusion events at ~18 Ma and 11 to 8 Ma. Hydrothermal alteration is widespread, but weak, and it is made up of quartz, illite, chlorite, mixed-layered clays, epidote, leuxcoene, hematite, calcite, anhydrite, and K-feldspar, which partly replace precursor minerals or deposited into open spaces. Temperature-sensitive phases lack well-defined depth zonation, which suggest that most of the alteration formed during earlier periods of hydrothermal activity associated with Tertiary magmatism. Modern hydrothermal activity is responsible for steam-heated acid alteration in the vicinity of fumaroles and steaming ground north of the Negro Mag fault, and silica sinter deposition along the Opal Mound fault. High angle normal faults formed from east-west Basin and Range extension, but listric sliding and block rotation also produced low angle structures characterized by narrow zones of cataclasis in crystalline rocks. The Opal Mound fault is a prominent high-angle fault that dips east and offsets surficial deposits of alluvium and silica sinter, forming a hydrological barrier to lateral fluid flow. Additional north-south trending normal faults, which are blind to the surface, are likely to occur in basement rocks to the west beneath the alluvial cover. The Negro Mag fault is high-angle too, but it trends east-west, cutting across the Mineral Mountains. Lineament analysis and field mapping show that the crystalline rocks exposed in the Mineral Mountains host a fault-fracture mesh, characterized by relatively dense joint spacing, with fractures oriented in many different directions. Such fractures could play an important role in stimulating new permeability beneath the FORGE deep well site. The groundwater regime across the FORGE deep well site is controlled by the west sloping potentiometric surface and an unconfined aquifer hosted in alluvial gravels. Geochemical data trace shallow hydrothermal outflow to the northwest and west, consistent with temperature profiles in gradient wells. The groundwater is chemically benign, non-potable, and suitable for EGS heat transfer experiments. |