GRAVITY AND HYDROTHERMAL MODELING OF THE ROOSEVELT HOT-SPRINGS AREA, SOUTHWESTERN UTAH

The geothermal field at Roosevelt Hot Springs, Utah, produces water at temperatures exceeding 230-degrees-C. The source of the heat for the geothermal field and the nature of the hydrothermal system have been investigated by analyses of gravity, thermal, and hydrogeologic data. A deep, cylindrically shaped, anomalous mass approximately 10-15 km in diameter is inferred to exist approximately 5 km beneath the geothermal field based on inverse modeling of a - 17 mGal gravity anomaly isolated by strike filtering. This body is assumed to be a young intrusion and the heat source for the geothermal field. The configuration of the anomalous body was used in a vertical cross-sectional model of the hydrothermal system. The results of the modeling indicate (1) the source of the fluid recharge for the field is the Mineral Mountains; (2) the age of the source of the geothermal field may be younger than 500,000 years, the age of nearby rhyolites; (3) water at high temperature may be found beneath the sedimentary basin west of the geothermal field if sufficient permeability can be found; (4) the deep body beneath the field has significant permeability, in excess of 5 x 10(-18) m2; and (5) the fault zone which acts as a conduit for upwelling thermal waters and provides the production at the field is probably bounded by low-permeability material.