Exploration of the Hot Springs Bay Valley (HSBV) geothermal resource area, Akutan, Alaska

•Five years of exploration has revealed a viable geothermal resource.•Fluid and gas chemistry indicates a 240–300°C reservoir with ∼180°C outflow.•Near-surface permeability is focused at intersections of overlapping structures.•The clay cap is moderately developed, with chlorite abundant throughout core.•Geophysics, alteration, mineralization indicate partial erosion of relatively mature system. 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°C, with outflow temperatures ∼180°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.