The Hot Springs Bay Valley (HSBV) geothermal resource area on Akutan Island, Alaska, has been explored since 2009. Geological, geochemical, geophysical surveys and the drilling of two thermal gradient wells suggest a mature neutral-chloride reservoir between 240 and 300 degrees C, with outflow temperatures 180 degrees C. A network of regional and local structures control near-surface permeability. Alteration, mineralization and geophysical patterns, including high-temperature hydrothermal minerals at unexpectedly shallow depths of formation, indicate a poorly to moderately developed clay cap, likely the result of erosion of the upper portion of an older, well developed system. (C) 2015 Elsevier Ltd. All rights reserved.