Abstract

Reconnaissance field and geochemical studies were conducted at 26 thermal-spring sites and fumarole fields between Adak Island and Becharof Lake. Hydrothermal-reservoir temperatures exceeding 150°C are tentatively estimated for at least 13 of these sites. Thermal fields on the flanks of Great Sitkin, Kliuchef, Makushin, and Akutan Volcanoes are characterized by mild to moderate fumarilic activity that is indicative of at least a shallow vapor-dominated zone. Thermal spring waters associated with fumarole fields are low in Cl, rich in SO₄, and commonly acidic. CO₂ and H₂S are the dominant gases in nearly all fumaroles sampled. ³He/⁴He ratios in the fumarole gases average about six to seven times atmospheric, which is typical of island-arc settings. The excess ³He is thought to be of mantle origin.

On the basis of chemical and isotope geothermometry, hot-water reservoirs with temperatures exceeding 150°C were identified for Adak, Geyser Bight, Hot Springs Cove, Akutan, Emmons Lake, and Mother Goose hot-spring systems. These springs occur in valleys or along the coast near active volcanoes. The thermal waters typically have low to moderate concentrations of alkali and CL, and are rich in SiO₂. Convective-heat discharge by spring flow in these systems ranges from one to several megawatts.

Akutan Island, northern Unalaska Island, and northeastern Atka Island are particularly promising sites for the development and use of geothermal energy. High-temperature hydrothermal systems in these areas are near existing population centers on well protected, deep-water harbors. The extent of the hydrothermal reservoirs in these areas and elsewhere in the Aleutian Arc is as yet largely unknown. A major exploratory drilling program on Makushin Volcano near Unalaska is being funded by the State of Alaska and was scheduled for summer of 1982. Geophysical exploration aimed at understanding the shallow regime of Akutan Hot Springs was inaugurated in 1982.