| Abstract |
Geothermometry of the Breitenbush Hot Springs area, located in the central Oregon Cascades, was studied to clarify previously reported reservoir temperatures of 129 to 202°C. Water samples collected from the Brietenbush Hot Springs area included springs (5), wells (8) and stream water samples (2) from the South Fork of the Breitenbush River. Geothermometry was interpreted using the Reservoir Temperature Estimater (RTEst) software package (Palmer, 2014) and constrained by reported mineralogy. Optimized parameters were reservoir temperature and CO2 fugacity. Adding water to the system to account for steam formation was not done as initial runs indicated that including water as an optimization parameter lead to large parameter uncertainties. Using RTEst with a chalcedony, mordenite-K, calcite, and heulandite mineral assemblage and deriving Al concentrations from assumed equilibrium with K-feldspar resulted in an average temperature estimate of 137 ±2 °C. This calculated reservoir temperature is significantly less than the 174 to 180 °C values reported in other multicomponent geothermometric studies (Forcella, 1982; Ingebritsen et al., 1992; Pang and Reed, 1998; Spycher et al., 2016). However, our new estimate shows promise in its consistency in estimated reservoir temperature, CO2 fugacity (log(fCO2) = -0.06 ±0.07) and pH (6.04 ±0.06). In addition, calculated Al concentrations (9.8 ±0.7 ug/L) are consistent with the measured concentrations (8.9 ± 4.6 ug/L). Most of the reported secondary minerals can be explained by equilibrium or supersaturated states obtained with this model. The estimated reservoir temperature is close to the reported maximum downhole temperature of 141 °C in a deep well located 3 km southeast of the hot springs. Thus, the proposed geothermometric model is consistent with mineralogical, aqueous geochemical, and temperature observations near the site. |