ABSTRACT: A Hydrologic Model of the Newcastle Geothermal System, Southwestern Utah

Robert E. Blackett, Craig B. Forster

Geological, geophysical, and geochemical studies contributed to a hydrologic model of the "blind" (no surface expression), moderate-temperature Newcastle geothermal system, located in the Basin and Range-Colorado Plateau transition zone of southwestern Utah. Temperature gradient measurements define a thermal anomaly centered near the surface trace of the range-bounding Antelope Range fault with a broad dissipative plume extending into the adjacent Escalante Valley. Spontaneous potential and resistivity surveys sharply define the geometry of the upflow zone (not yet explored), indicating that fluids issue from a narrow aperture along the Antelope Range fault and discharge into a gently dipping aquifer that production wells show lies at a depth between 85 and 95 m. Major el ment, oxygen, and hydrogen isotopic analyses of water samples indicate that the thermal water is a mixture of meteoric water derived from recharge areas in the Pine Valley Mountains and cold, shallow groundwater. A northwest-southeast-trending system of faults, enclosing a zone of increased fracture permeability, collects meteoric water from the recharge area,

allows circulation to a depth of 3 to 5 km, and intersects the northeast-striking Antelope Range fault. We postulate that mineral precipitates form a seal along the Antelope Range fault, preventing the discharge of thermal fluids into basin-fill sediments at depth, and allowing heated fluid to approach the surface. Eventually, continued mineral deposition could result in the development of hot springs at the ground surface.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990