Abstract

The Roosevelt Hot Springs thermal area is a newly discovered geothermal power prospect in Utah. Seven production wells have been drilled with a maximum per well flow capability averaging 4.5 X 10 5 kg of combined vapor and liquid per hour at a shut-in bottom hole temperature near 260 degrees C.The thermal area is located on the western margin of the Mineral Mountains, which consist dominantly of a Tertiary granitic pluton 32 km long by 8 km wide. Rhyolitic tuffs, flows, and domes cover about 25 km 2 of the crest and west side of the Mineral Mountains within 5 km of the thermal area. The rhyolitic volcanism occurred between 0.8 and 0.5 m.y. ago and constitutes a major Pleistocene thermal event believed to be significant to the evaluation of the Roosevelt Hot Springs thermal area. Thermal waters of the (now) dry spring, a seep, and the deep reservoir are dilute (ionic strength 0.1 to 0.2) sodium chloride brines.Spring deposits consist of siliceous sinter and minor sulfur. Alluvium is cemented by sinter and altered in varying degrees by hot, acid-sulfate water to opal and alunite at the surface, grading successively to alunite-kaolinite, alunite-kaolinite-montmorillonite, and muscovite-pyrite within 60 m of the surface. Observed alteration and water chemistry are consistent with a model in which hot aqueous solutions containing H 2 S and sulfate convectively rise along major fractures. Hydrogen sulfide oxidizes to sulfate near the surface decreasing the pH and causes alunite to form. Opal precipitates as the solutions cool. Kaolinite, muscovite, and K-feldspar are formed in sequence, as the thermal water percolates downward and hydrogen ion and sulfate are consumed.Major swarms of earthquakes occur 30 km to the east-northeast near Cove Fort, Utah, but only minor earthquake activity occurs near the Roosevelt Hot Springs thermal area. Delayed P-wave traveltimes generated from the Cove Fort microearthquakes, and observed west of the northern Mineral Mountains, are suggestive of a low velocity zone beneath the Mineral Mountains; the vertical and lateral resolution of the data is inadequate to delineate the zone. Gravity and magnetic surveys are useful in determining the structure and depth of valley fill of the area of the northern Mineral Mountains, but neither one has detected an igneous intrusive source of heat. Thermal gradient measurements that range up to 960 degrees C/km in 30 to 60 m deep holes outline a 6 by 12 km thermal field. Heat flow and resistivity data both outline anomalous zones along a system of faults that controls the near-surface fluid flow. The source of heat is interpreted to be the convective circulation of thermal water. The lowered resistivity is due to the hot brine and the associated hydrothermal alteration. Magnetotelluric data are highly anomalous over the field but means for their quantitative interpretation are unavailable at present; the anomalous data could as readily be interpreted as due to surface conductors as deep conductors which one might like to associate with a source of heat.Any current model of the subsurface is highly speculative but can be expected to improve once existing seismic refraction and magnetotelluric data are fully interpreted. Then multiple-data-set modeling, combined with subsurface control from existing wells, should result in a reasonable model of the geothermal system. This modeling will be aided also by hydrologic, isotopic, structural, and additional P-wave delay studies currently in progress. Based upon this case history, an exploration sequence appropriate to the eastern Basin and Range province should consist of phase 0, a digest and synthesis of available data; phase 1, a regional airphoto accumulation and analysis; phase 2, regional geologic mapping, regional radiometric dating of all intrusive and extrusive rocks, regional isotopic and chemical analysis of waters, regional aeromagnetic and gravity surveys, and regional collection of thermal gradients in available holes; phase 3, heat flow measurements in strategically located holes; phase 4, dipole-dipole resistivity surveys; phase 5, petrological, mineralogical, and geochemical studies on cuttings and cores from heat flow drill holes; phase 6, model test drilling accompanied by petrological, chemical, and isotopic analyses of cuttings and cores plus chemical and isotopic analyses of fluids; phase 7, detailed seismic refraction and reflection surveys; and phase 8, modeling and synthesis of all available data.

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