APPLICATION OF 3HE/4HE, CO2 FLUX MEASUREMENTS, AND HYDROCHEMICAL ANALYSES TO ASSESS THE HYDROLOGY AND MANTLE-TO-SURFACE CONNECTIONS OF THE WESTERN SAN JUAN MOUNTAINS GEOTHERMAL SYSTEM OF COLORADO

We use multiple-tracers to assess the fluid and gas pathways in hot springs of the western San Juan volcanic field. Our hypothesis is that there are connections among surface hot springs, fault networks, young volcanic and plutonic rocks (< 7 Ma), and low-velocity upper mantle domains. Noble gas analyses of regional hot springs reveal air corrected ³He/⁴He values of: at Rico (4.09-5.88 R_A), Dunton (3.11-4.54 R_A), Geyser Warm Spring (3.39 R_A), and Paradise Warm Spring (2.72 R_A). Rico Hot Springs have the highest mantle volatile component of any spring in Colorado with up to 73% mantle helium component; this near-MORB value indicates rapid transport of mantle-sourced helium into the groundwater system. CO₂ is considered the carrier gas for mantle helium and CO₂/³He values range from near-MORB values of 3 x 10⁹ at Orvis, to 7 x 10¹² at Pinkerton. This wide range of CO₂/³He reflects variable mixing with crustal gases. Distinct major and trace element signatures (Piper diagram) for each spring system reflects different water-rock interaction along flowpaths. Stable isotope analyses shows that waters fall on the global meteoric water line with 2 exceptions: Paradise and Geyser both are offset both “up” and “down” (samples have variable δD at similar δ¹⁸O) interpreted to be fractionation due to phase separation. But, spring water is interpreted to be dominantly meteoric by volume with chemical variations reflecting endogenic gas input plus water-rock interactions along different fluid circulation pathways. Surface CO₂ flux (not at springs) using the EGM-5 flux meter vary from background values of 0.17 g/m²/hr up to 36.2 g/m²/hr. Higher values are along faults and are comparable to many geothermal fields, thus showing that faults act as pathways for CO₂ flux. We conclude that water volume of western San Juan hot springs is dominated by meteoric fluids but geothermal solute concentrations reveal a component of chemically potent endogenic brines. Similar to the entire Southwest, ³He/⁴He ratios inversely correlate with underlying (~ 100 km) mantle velocity suggesting vertical and rapid transport of mantle helium and CO₂ into the groundwater system, with faults as the upper crustal conduit system. We view this fluid flux as a neotectonic continuation of the young magmatism (4.1Ma-614ka) that characterizes this region of Colorado.