GEOCHEMISTRY AND GEOTHERMOMETRY OF THE BREITENBUSH HOT SPRINGS AREA

Twenty well, spring, and stream samples from Breitenbush Hot Springs (in the Cascade Range of Oregon, USA) were collected and analyzed for major, minor, and select trace anions and cations, as well as stable oxygen and hydrogen isotopes. These data indicate that the thermal waters are predominantly Na-Cl types waters with < 10% (HCO₃^- + SO₄^-2) and contain elevated levels of F, B and As. The chemical data were used to develop a new estimate of the geothermal system’s reservoir temperature and the fraction of water derived from a magmatic source. Multi-component geothermometry was applied using the Reservoir Temperature Estimator (RTEst) software package coupled with local mineralogical data to determine a reservoir temperature of 137 ±2 °C. This estimate was obtained using an equilibrium mineral assemblage consisting of chalcedony, calcite, heulandite, and mordenite-K, and calculating Al concentrations from assumed equilibrium with K-feldspar. This estimate contrasts with previous classic and multi-component geothermometry estimates, which ranged from 129 to 202 °C, but which tended to emphasize a reservoir temperature between 170 and 180 °C. In addition to temperature, our model provides consistent estimates of reservoir CO₂ fugacity (log(fCO₂) = -0.06 ±0.07), pH (6.04 ±0.06), and calculated Al concentrations (9.8 ±0.7 ug/L), the latter which closely corresponds to measured Al concentrations (8.9 ±4.6 ug/L). In addition, the majority of reported secondary minerals in proximal outcrops and boreholes can be explained by equilibrium or supersaturated states and the estimate is close to the maximum recorded borehole temperature in a nearby 2,457-meter-deep well (141 °C). Oxygen and hydrogen isotopic data indicate that thermal water is a mix of “andesitic waters” of Giggenbach and meteoric water from the crest of the Oregon Cascades Mountain Range, located approximately 15 km to the east of Breitenbush Hot Springs. A contribution of andesitic magmatic water is consistent with He isotope studies. A meteoric source close to the Cascade crest with deep circulation before discharge at Breitenbush Hot Springs is consistent with other models of fluid flow in the region.