Geology and geothermal potential of the Roosevelt Hot Springs area, Beaver County, Utah

The Roosevelt area contains Roosevelt Hot Springs, one of two known Geothermal Resource Areas in Utah. The Roosevelt area is located on the western flank of the Mineral Range in Beaver County. Precambrian(?) metamorphic rocks and Tertiary igneous rocks crop out in the eastern part of the Roosevelt area. Unconsolidated deposits of Tertiary and Quaternary age cover most of the Roosevelt area. The Precambrian(?) metamorphic rocks, principally biotite gneiss, are present both as isolated outcrops and as blocks within the granite of the Mineral Range pluton. The pluton is primarily composed of coarse- to medium-grained granite. Radiometric age determinations show that parts of the pluton range in age from late Miocene to early Pliocene. Silicic volcanic rocks crop out in Negro Mag Wash and in Wildhorse Canyon. In both areas, the volcanic rocks were extruded onto an eroded surface of the granite and are thought to be Pliocene in age. No pre-Tertiary sedimentary rocks crop out within the Roosevelt area. Most of the area is covered by alluvial fan deposits. Most of the alluvium was derived from the granite, but some was derived from the silicic volcanic rocks. Some of the alluvium was worked into V-shaped embankments at different stages of Lake Bonneville. North-trending faults are present within the foothills of the Mineral Range in the Roosevelt area. Another conspicuous north- trending fault, the Dome Fault, offsets Units A and C and also Pleistocene(?) hot-spring deposits. A well that yielded steam may have penetrated the Dome Fault. The west block of the Dome Fault is dis¬placed upward relative to the east block; movement on the fault occurred during the Pleistocene. North- and northeast-trending faults that produced small displacements in the alluvial fan surface were mapped in the central and western parts of the Roosevelt area. An east-trending fault may be present beneath Negro Mag Wash. Roosevelt Hot Springs are no longer flowing, but a patch of soil near the main orifice is 204°F. Other patches of warm ground and small deposits of siliceous sinter are present in Negro Mag Wash. Three rock units of silica-cemented alluvium, called Unit A, Unit B, and Unit C, and differentiated on the bases of lithology, outcrop pattern, and degree of sorting, were mapped in the Roosevelt area. Approximately 50,000 square feet of siliceous sinter is exposed at the south end of the Dome Fault. Several holes were drilled in and around this siliceous sinter deposit. One hole reached a depth of 275 feet, where steam blew the drilling equipment out of the hole; the temperature of the steam was 270°F. The silica geothermometer and the sodium-potassium-calcium geothermometer were both applied to published analyses of Roosevelt Hot Springs water. The silica geothermometer indicated reservoir temperatures of 210° and 195°C; the Na-K-Ca geothermometer gave temperatures of 298° and 292°C. The presence of relatively young intrusive and extrusive igneous rocks, the steam well, hot springs, and the favorable geochemical data all indicate that a hot subsurface igneous body exists in this area. Primary porosity in the granite and the metamorphic rocks is probably very low; secondary porosity along faults may be very important. There are not sufficient data to evaluate the hydro- logic conditions that might prevail in a geothermal system in the Roosevelt area.