INVESTIGATION OF MULTIPLE GENERATIONS OF FLUID FLOW IN THE GEOTHERMAL SYSTEM AT AKUTAN ISLAND USING FLUID INCLUSION ANALYSES IN EPIDOTE AND CALCITE

Core recovered from thermal gradient wells (TG-2 and TG-4) drilled into the Akutan geothermal resource area in the Aleutian Islands, AK reveals the thermal history of the hydrothermal system hosted by interlayered basaltic lava flows and mass wasting deposits up to 0.05 Ma and has broad propylitic to argillic alteration. 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, forming at temperatures > 200 °C, in vugs and as replacement minerals in the rock groundmass indicates recent cooling in the hydrothermal system. Variations in fluid temperature, composition, and flow direction relative to the system have been determined from fluid inclusions hosted in epidote and calcite. Epidote is compositionally variable due to the temperature and pressure of the system, permeability, and compositions of hydrothermal fluids and the country rock; however, epidote consistently shows prograde solubility and therefore representing discharge from the system (outflow). Calcite typically exhibits retrograde solubility despite CO₂ fugacity, temperature, pH, and calcium ion activity influencing formation, and thus represents recharge into the system (inflow). Epidote existing with calcite in vugs represents the use of a single fluid pathway for multiple fluid generations moving into and away from the system. Paragenetic analysis of vug-filling secondary mineralization establishes a relative history of secondary mineralization in a sequence where chlorite is formed first, followed by two generations of clear, equant, euhedral calcite crystals, then small (~0.2 mm), radial acicular epidote crystals, followed by anhedral, strongly twinned and fractured calcite crystals, the latter filling the vug. Epidote also occurs as larger (~1 mm) subhedral to euhedral crystals occurring in the groundmass, although the relationship of the euhedral epidote to vug-filling mineralization is difficult to determine. Fluid inclusions are hosted in each crystal type, although inclusions in the small, acicular epidote are typically too small to be useful for thermometric studies. We present the results of a fluid inclusion investigation of thermal and compositional variability of the Akutan geothermal system and detail the systems thermal and compositional history.