Sub-glacial Origin of the Hot Springs Bay Valley hydrothermal System, Akutan, Alaska

Abstract:

Exploration for geothermal energy in Hot Springs Bay Valley (HSBV) on Akutan Island, Alaska, has revealed a rich hydrothermal history, including what appears to be a stage of peak activity during a significant glacial period. Alteration mineralogy observed in 754 m of drill core recovered from the outflow zone is dominated by chlorite and includes minor smectite clays, a suite of zeolite species and several moderately high-temperature hydrothermal minerals (epidote/clinozoisite, prehnite, adularia and wairakite). The latter minerals each have minimum formation temperatures exceeding 200 ^oC, and fluid inclusion results in related calcite crystals indicate temperatures of formation to be as high as 275 ^oC, some 100 ^oC hotter than the modern boiling point with depth (BPD) curve at that depth (>62 m). In order to maintain liquid temperatures this high, the pressure during mineralization must have been substantially greater (~680 bar), a pressure change equivalent to erosion of ~280 m of rock (ρ=2.5 g/cm³). Although glacial erosion rates are too low (0.034 mm/yr; Bekele et al., 2003) for this amount of erosion to occur in a single glaciation, glacial melting and ablation are substantially more rapid (~100 mm/yr; Bekele et al., 2003; Person et al., 2012). Thus, a more probable scenario than pure erosion is that peak hydrothermal conditions occurred during a large glacial event, with the added pressure from the overlying ice allowing the high temperature minerals to form closer to the ground surface. Subsequent melting of the ice eroded upper tributary valleys and upper levels of the originally smectite-rich alteration assemblage, explaining the paucity of swelling clays in the region. We present mineralogical, fluid inclusion and geochronologic evidence to support these conclusions, and discuss the general implications of sub-glacial hydrothermal system formation and geothermal resource potential.

References:

Bekele, E., Rostron, B. and Person, M. (2003) Fluid pressure implications of erosional unloading, basin hydrodynamics and glaciation in the Alberta Basin, Western Canada. J. of Geochem. Exploration, 78–79, 143–7.

Person, M., Bense, V., Cohen, D. and Banerjee, A, (2012). Models of ice-sheet hydrogeologic interactions: a review. Geofluids, 12, 58-78