CHARACTERIZING HYDROTHERMAL ALTERATION AT AKUTAN, AK USING FLUID INCLUSIONS AND LA-ICP-MS ANALYSES

Core recovered from two thermal gradient wells drilled into the Akutan geothermal area in the Aleutian Islands, AK reveals the thermal history of the hydrothermal system hosted by interlayered basaltic and andesitic lava flows and mass wasting deposits. Although modern fluid flow at Akutan reaches a maximum temperature of 182°C at 178-179 m depth at TG-2, the presence of epidote (forms above 200°C) in vugs and as replacements in the rock groundmass indicates a recent period of cooling in the hydrothermal system. Hydrothermal mineralization in vugs and fractures records variations in fluid temperature and fluid-rock interaction. Paragenetic analysis of vug-filling secondary mineralization establishes a relative history of secondary mineralization in a sequence where chlorite is formed first, followed in some cases by a generation of radial acicular epidote and/or euhedral clinozoisite crystals. LA-ICP-MS analyses of clinozoisite reveal its homogeneity, suggesting relatively constant mineral growth conditions. The remaining void space is most commonly filled by calcite with occasional zeolites. Calcite growth patterns vary with local permeability, where calcite formed in low permeability zones tends to be homogenous, and calcite in high permeable zones tend to form banded growth rings, suggesting subtle variations in conditions. Microanalysis of calcite-hosted liquid-dominated fluid inclusions reveal that salinity of ancient fluids range from 0.18 to 4.65 wt%, possibly reflecting variable input from meteoric and oceanic waters. Fluid inclusion temperatures range from 122.3°C to 315.4°C in a complex pattern throughout the core.