Comparison and Controls of Thermal Spring Chemistry in Cascade Range and Olympic Mountains Geothermal Provinces, Washington

摘要

Early geothermal surveys conducted by the Washington State Department of Natural Resources -Geology and Earth Resources Division from the late 1970's to the early 1980's identified significant hydrogeochemical variation among Washington thermal springs. This study characterizes five thermal springs in the Cascade Range (Carson, Bonneville, Baker, Ohanapecosh, and Goldmyer) and two in the Olympic Mountains (Olympic Complex and Sol Duc) using geochemical indicators (solute chemistry, isotopic signatures, and geothermometry estimates) commonly employed during geothermal exploration. We conclude that the variations result from differences in rock-water interactions (dissolution and low-grade alteration) as well as features of the systems including source water, heat source, reservoir connectivity, and topographic relief. The Cascade thermal springs occur in a geothermal environment that is influenced by magmatic fluids, as supported by the widespread volcanic activity in the region. Chemical enrichment (x_(TDS): 1198 mg/L) and diversity (i.e., Cl: 94-683 mg/L) may be results of magmatic fluid contribution, topographic lateral flow, and exposure to a diverse suite of igneous rocks. Inherently, Cascade geothermal fluids are also hotter (53-153 °C) and seem to undertake a more indirect path to the surface, as evidenced by signs of dilution (Carson = 24%) and degassing (Bonneville = 23% and Ohanapecosh = 2%) and significant differences (x_(difference): 53 °C) between discharge and estimated reservoir temperatures. Conversely, the Olympic thermal springs are manifestations of a cyclic geothermal system that is controlled by subvertical thrust faults. These chemically dilute (x_(TDS): 355 mg/L) and homogeneous (i.e., Cl: 6-12 mg/L) spring waters are probably salinized by salts in or connate waters of the turbidite host rocks. The cooler (53-103 °C) Olympic geothermal fluids appear to be more directly connected to their reservoir as differences (x_(difference): 27 °C) between discharge and subsurface temperatures are smaller. Oxygen (-15.12‰ to -10.52‰) and hydrogen (-116.18‰ to -73.55‰) stable isotopic signatures suggest that both geothermal provinces are likely recharged by local meteoric waters.