Fluid Geochemistry of Hot Springs Bay Valley, Akutan Island, Alaska

Abstract

Several thermal springs and a fumarole field occur in Hot Springs Bay Valley on Akutan Island. The thermal springs are low in Mg, SiO2 rich and moderately concentrated with hot water hydrothermal systems. B/C1 ratios indicate the several sets of springs are derived from a common parent hot water reservoir. Application of silica, cation, and sulfate water oxygen isotope geothermometry gives a range of 160-`90°C for the parent reservoir. Recent geophysical investigations performed in cooperation with the Geophysical Institute, U of Alaska, identified a broad zone of apparent low resistivity underlying the lower part of the valley in the vicinity of the thermal springs. Relatively high CH4 content and low H2/CH4 ratios occur in gases from both the thermal springs and form the fumerole field located at the head of the valley about 4 km southeast of the hot springs. The similarity in H2/CH4 suggests underlying reservoirs are similar in both areas. Both sies have a ratio of He3/He4 WRT atmospheric levels of about 7.0.