EXPLORATION HISTORY OF THE HOT SPRINGS BAY VALLEY (HSBV) GEOTHERMAL RESOURCE AREA, AKUTAN, ALASKA

The Hot Springs Bay Valley (HSBV) geothermal resource area on Akutan Island, Alaska, is one of the most promising commercially developable prospects in the State of Alaska. Statewide reconnaissance exploration began in 1981, which identified a neutral-chloride geothermal field with ~20 hot springs linearly arranged in the lower portion of HSBV and a vigorous fumarole field at the valley’s head and established base-level fluid chemistry, geothermometry and geophysics. Detailed exploration of the HSBV resource, driven by the City of Akutan, began in 2009 and included geological, geochemical and magentotelluric (MT) surveys. The results supported an upflow-outflow model with a reservoir temperature of >220 ^oC. The resulting conceptual model was used to target two thermal gradient wells (TG-2 and TG-4) drilled in 2010. Measured well discharge temperatures exceeded 180 ^oC at 178 m depth in well TG-2 (near the hot springs), and geothermometry of well and fumarole discharges indicate a neutral-chloride reservoir at temperatures ranging from 220 - 240 ^oC beneath the hot springs and approaching 300 ^oC beneath the fumaroles. In 2012 focused structural and alteration mapping identified three island-wide structural trends that overlap in the HSBV, with intersections apparently focusing near‑surface permeability. Gravity and MT data collected in 2012 were combined with existing geophysical data to reveal the presence of a dense, resistive region cresting at ~1500 m depth beneath the fumarole field and extending ~2 kilometers northeast, interpreted as one or more intrusive bodies. A relatively narrow zone of low resistivity (<50 ohm-m) at shallow depth was also identified, particularly near the hot springs and around the fumaroles. These low-resistivity zones are interpreted as a poorly- to moderately-developed clay cap, matched by the occurrence of minor smectite and illite in well cores. The presence of moderate-temperature secondary minerals (adularia, epidote, etc.) at impossibly shallow depths suggest that the existing clay cap is an eroded version of a more thoroughly developed system. A separate USGS-led investigation comparing hot springs discharge between 1981 and 2012 reveal a 20-25% increase in hot springs fluid discharge and an order of magnitude increase in heat discharge (Bergfeld et al., 2014).