| Abstract |
Mapping of statewide heat flow and geothermal gradients indicates an area of both high heat flow and increased geothermal gradient in southwest Colorado centered on the Town of Rico. Seven flowing thermal springs were sampled in this area: four springs near Rico in the Dolores River valley; and Dunton Spring, Paradise Spring, and Geyser Warm Spring in the West Fork Dolores River valley. All of these springs have surface temperatures in a narrow range between 40 and 44°C, excepting Geyser Warm Spring at 28.5°C. The springs were analyzed for a suite of 44 constituents including major ions, silica, CO2, deuterium, and 18O. Geothermometer calculations applied to these data, and to previous 1970’s water chemistry data, variously estimate subsurface reservoir temperatures ranging from 60°C to 150°C. Water samples from the Rico area springs exhibit less agreement, with silica, Na-Ca and isotopic geothermometers giving subsurface temperature estimates of ~120-140°C, whereas K-Mg and Na-KCa geothermometers estimate ~60°C. Springs in the West Fork Dolores River valley have relative agreement between silica and K-Mg geothermometers, with subsurface temperatures at Dunton and Geyser Warm Springs of ~60-70°C, and at Paradise Spring of ~150°C. All of the springs in this area are strongly effervescent with CO2, some of which obtain a minor geyser action as a result of CO2 out gassing. Isotopic analyses indicate that all the samples are very similar to meteoric waters. Temperature data from mineral exploration drill holes within a 0.75 km radius of Rico yield geothermal gradients of 91 and 92 °C km-1. Therefore, meteoric water circulation depths ranging from about 0.6 to 1.9 km are necessary to meet the range of subsurface reservoir temperatures predicted by the geothermometers. |