A MODEL FOR THE ORIGIN OF THERMAL SPRINGS IN ANIMAS RIVER VALLEY, LA PLATA COUNTY, SOUTHWESTERN COLORADO

Thermal springs discharge at several locations in the Animas River valley about 10 miles north of Durango, Colorado. There has been a great deal of speculation on the origin of these springs and the processes that create them. McCarthy et al. (1982) hypothesized that these springs discharge meteoric water that is heated by deep circulation through cracks and fissures within the 65 Ma intrusive complex of the La Plata Mountains. No diagnostic or robust data were available, however, to assess the sources of the water in these springs.

Oxygen (d¹⁸O values of –13.9 to -14.4) and hydrogen (dD values of –96 to –105) isotope data provide evidence that the thermal springs in the Animas River valley discharge water from meteoric sources. Using these isotope data and recent 1:24,000 regional geologic mapping, we propose that these thermal springs emit water from rain and snow that is heated by circulation along east-trending faults and fractures to depths of 1 to 2 kilometers under an assumed regional geothermal gradient of 25º C per kilometer. This is the first model for thermal springs in the Animas River valley that incorporates a diagnostic tracer with regional field data, and provides more definitive evidence for the origin of these springs.