The geyser bight geothermal area, Umnak Island, Alaska

Abstract The Geyser Bight geothermal area contains one of the hottest and most extensive areas of thermal springs in Alaska, and is the only site in the state with geysers. Heat for the geothermal system is derived from crustal magma associated with Mt. Recheshnoi volcano. Successive injections of magma have probably heated the crust to near its minimum melting point and produced the only high-SiO 2 rhyolites in the oceanic part of the Aleutian arc. At least two hydrothermal reservoirs are postulated to underlie the geothermal area and have temperatures of 165° and 200°C, respectively, as estimated by geothermometry. Sulfate-water isotope geothermometers suggest a deeper reservoir with a temperature of 265°C. The thermal spring waters have relatively low concentrations of Cl (600 ppm) but are rich in B (60 ppm) and As (6 ppm). The As/Cl ratio is among the highest reported for geothermal waters.