Geochemistry of a Sub-Glacial Volcanic Hydrothermal System at Mount Spurr, Alaska

Abstract

Mt Spurr is an ice and snow-covered andesitic volcano located at the northern extent of the Aleutian arc in south central Alaska. Previous workers (Turner and Wescott, 1986; Martini et al., 2010) have identified a prospective geothermal system on the south side of the volcano, beneath Crater Peak. Whether or not this system can be tapped into from a location with less volcanic hazard is uncertain. This research applies mineral and aqueous geochemical techniques at Mt Spurr in order to characterize the hydothermal system, focusing on the possible extent, temperature, origin of fluids, and fluid pathways occurring in the system. Preliminary results show that spring waters are of meteoric origin and have shifted upwards of 2 permil towards the magmatic water endmember. A component of mantle/crust/igneous influence is recorded in the C and S stable isotopic signatures of some spring waters (ä34S=0 to 4.38‰, ä13C=-10.48 to -5.01‰), while biogenic processes or seawater influence dominates other samples (ä34S=8.06 to 10.2‰, ä13C=0 to -3.24‰ and -15.9 to -20.8‰). Major chemistry of spring waters indicate that the magmatically influenced fluids are affected by shallow processes and likely mix with groundwater. This is shown by a positive linear correlation between Cl and B, as well as clustering near the SO4 and HCO3 corners of a Cl-SO4-HCO3 ternary. Water-rock equilibrium has not been obtained, and initial deep rock dissolution processes as seen on a Cl-B-Li ternary suggest the influence of both Cl magmatic vapors and crustal rock. Geothermometers have not been applied because the assumptions of rapidly rising fluid, minimal mixing and no dilution are not reasonable for this system.