Abstract

The Roosevelt Hot Springs KGRA is located in the Basin and Range province along the western flank of the Mineral Mountains. It is within the Wah Wah–Tusher mineral belt which has been the locus for rhyolitic intrusive and extrusive activity through Tertiary and into Quaternary time. The area is just east of the Sevier (Cretaceous) thrust belt and is near the margin of the Intermountain seismic belt. Geologic mapping has identified three metamorphic and plutonic units of Precambrian age, nine intrusive phases of the Tertiary Mineral Mountains pluton, three Pleistocene rhyolitic extrusive phases, and siliceous hot spring deposits of relatively recent age. Cuttings from exploration holes indicate that the Precambrian and Tertiary crystalline rocks host the present geothermal system.

The structure of the area is dominated by low-angle normal faults (denudation faults) which dip to the west. The hanging wall of the principal denudation fault was intensely brecciated during the fault episode forming steep fault zones which generally strike northwest. Both these low- and high-angle fault zones show the development of intense, silicified mylonites. Adjacent to these mylonite zones, the crystalline rocks are highly fractured. East-west and northeast-trending high-angle faults cut the denudation faults and channel much of the recent hot-spring activity.

The geothermal system is a high-temperature, water-dominated resource which is probably related to an igneous heat source. The low primary permeability of the reservoir rocks and the location of the geothermal field indicate structural control of the system. The reservoir geometries and permeabilities result from the intersections of the principal fault systems.

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