Fluid, Gas, and Isotopic Variation of Thermal Springs in the Southern Rocky Mountains: Colorado

Abstract

New helium isotope data measured for geothermal springs in the State of Colorado range from crustal helium isotope values (0.02 R/Ra) to ratios that overlap with subcontinental mantle lithosphere (3 ? 7 R/Ra). The geothermal resource potential for Colorado has generally been overlooked by many industry developers however new geochemical data warrant further investigation of several key resources that have helium isotope signatures that exceed many successfully producing blind Basin and Range resources, such as Dixie Valley. Helium isotope ratios up to 5.88 Rc/Ra measured on gases in carbonic geothermal springs near Rico are the highest yet to be measured in the state, and isotope ratios for gases represent a mantle 3He isotope component of 73 %. Quartz geothermometer temperatures of 155 °C for Rico indicate minimum equilibration temperatures when considering dilution with shallow groundwater, and geothermal gradients ranging from 60 to 80 °C/ km predict potential resources between 1.8 and 2.2 km depth within permeable Precambrian basement rocks. Fluid-mineral equilibration temperatures, geothermal gradients, and basin depth for many other resources in Colorado largely imply that equilibrated geothermal fluids originate at depths well within Precambrian basement rocks, and suggest a similar deep origin that is not obvious from the geochemically diverse hot springs that emanate at the surface. The helium isotope distribution for Colorado geothermal springs appears to be strongly associated with faults, extended crust, and Pliocene magmatic intrusions and more discretely associated with diffuse lower to middle crustal permeability and low-velocity mantle anomalies. This research investigates the potential of using helium isotope ratios as an exploration tool in low-temperature systems to vector toward high crustal permeability zones in regions of anomalous crustal heat flow.