15 Undergraduate Research Session (Posters)

Tuesday, 15 May 2018
8:30 AM - 6:30 PM
Outdoor Tents

A COMPARISON OF GEOCHEMICAL AND HELIUM ISOTOPE SIGNATURE OF THERMAL FLUIDS, RICO AND DUNTON SPRINGS, SAN JUAN MOUNTAINS, COLORADO

Paper number: 15-22
Program: Theme Sessions
Time: 8:30 AM – 6:30 PM

Authors

Abstract

The western San Juan Mountains are home to a regional geothermal system, distinguished by numerous thermal springs. Previous studies revealed that thermal waters at Rico and Dunton had distinct geochemical signatures, despite their close proximity and similar host-rock plumbing. In this study, water geochemistry and helium-gas isotopes at the two sites were compared to identify spatial trends, and further understand source variations and fluid circulation. These data reveal distinctions in fluid sources and mantle-gas contributions, and offer insight into the complex regional “plumbing” system of the thermal springs.

Average pH and temperature values vary across the study area: Rico (6.58, 46.5˚ C), Dunton (6.82, 42.7˚ C), and Paradise Spring (6.42, 37.0˚ C). Total dissolved solids (TDS) are highest at Paradise Spring (6,756 ppm) compared to Rico (3,426 ppm) and Dunton (2,669 ppm). R/Ra values range from 5.43 to 4.09 at Rico, 2.70 to 2.52 at Dunton, and 2.33 at Paradise Spring. These data hint that either that: 1) different proportions of mantle gases are contributed to the emissions; 2) there are different degrees of near-surface mixing; or 3) both mechanisms are involved.

Thermal spring samples at Rico and Dunton are characterized by high concentrations of HCO3. Rico samples have relatively higher concentrations of SO4, Ca, Na, K, and Mg compared to Dunton sites; the latter is distinguished by slightly elevated Cl. Samples taken at Paradise Spring are characterized by high concentrations of Na (1947 ppm), K (327 ppm), and Cl (3,407 ppm), and elevated Li (11 ppm).

Our results indicate different mantle contributions to gases in the Rico-Dunton thermal springs with an overall decrease to the west. Distinctive geochemical signatures in fluid geochemistry reveal that sources and fluid-rock interactions vary between Rico and Dunton springs though the causes of these variations are not uniquely constrained by major-element chemistry. Water-rock interactions coupled with mixing are evaluated as processes to explain the differences.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-314156

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