AN ASSESSMENT AND COMPARISON OF THE GEOCHEMICAL SIGNATURES OF THERMAL SPRINGS IN THE ANIMAS RIVER VALLEY, SOUTHWESTERN COLORADO

A study of the chemical signatures (Na, K, Mg, Ca, Fe, Cu, As, Li, Al) of the thermal springs in the Animas River valley over a one-year period provides further evidence that these springs developed as near-surface circulation of meteoric water along regional fracture systems. Water that discharges from these springs is characterized by pH = 6.5, temperatures of 30 to 45 degrees Celsius, and high concentrations of dissolved salts. A comparison of geochemical trends between Trimble and Pinkerton thermal springs indicates that element trends over time are similar but concentrations are higher at the Pinkerton system, possibly due to a lower discharge.

A tritium analysis of water at Trimble springs yielded a concentration of 4.8 (TU) which indicates that water in the system is either dominantly post-1952 or it is represents a mixture of deeper and older fluids that mixed with younger, near-surface water. Sulfur isotope data obtained in this study suggests that water in these thermal springs interacted with sulfate and carbonate minerals in Pennsylvanian (Hermosa) and Permian (Cutler) rocks exposed in the area.

Collectively, these data are consistent with a shallow-geothermal system recharged by meteoric fluids that circulated along fractures, were heated at depth, and then discharged along the edges of the Animas River valley. Using are geothermal gradient of 25º C/kilometer, the water would need to circulate to depths of 0.75 to 1.5 kilometers to attain the temperatures observed at the surface. Our model contrasts with some regional studies that suggest a deep, perhaps mantle source for fluids in thermal springs in Colorado, Arizona, and New Mexico.