Abstract

The Tolhuaca Geothermal System (TGS) represents a potential 13 MWe geothermal reservoir located on the NW flank of Tolhuaca stratovolcano, in the Southern Volcanic Zone (SVZ) of the Andes. Despite decades of scientific exploration on the chemistry of its high-enthalpy surface geothermal system, the full understanding and connection to the underlying magmatic system has yet to be demonstrated. A novel combined approach studying gas emissions from the plume and fumarole has found that the TGS magma-gas component is dominated by CO2-rich fluids (relatively high CO2/H2S and CO2/CH4) that feed the fumaroles and exploration wells in the Amphitheater structure. Gas measurements near the summit of Tolhuaca show little atmospheric contamination (low CO2/Ar) but higher CO2/H2S ratios relative to fumaroles on the lower flanks, which may be caused by scrubbing effects on H2S within a hydrothermal reservoir at less than similar to 1.5 km beneath the surface (permeable geothermal zone). The atmosphere-corrected He-3/He-4 ratio (Rc/Ra) measured by bulk fluid inclusion (FI) analysis of olivines from the Cono Cancion minor eruptive center (6.40 +/- 0.38 Ra) is similar to other fumarole systems measured in the literature that are <400 m from the vent (6.49 +/- 0.05 Ra). The magma erupted from this center is andesitic to dacitic in composition. In addition, minor amounts of crustal-derived He-4 indicate alteration of the initial 3He-rich magmatic fluids (1.37.10(-9) CO2/He-3). In contrast, fluid inclusions from olivines in the syn-glacial Tolhuaca Fissural eruptive center give values of 8.16 +/- 0.63 Rain (typical SVZ regional MORB signature), corresponding to basaltic-to-basaltic andesite melt which has preserved the magmatic He-3 in fluid inclusions (2.64.10(-8) CO2/He-3). For the first time in this volcano, we use melt inclusions (MI) to investigate the magma-volatile component of a magmatic plumbing system in the context of geothermal exploration (parental melts contain 1.94 wt% H2O, 2496 mu g/g CO2, 142 mu g/g S, and 1095 mu g/g Cl). The volatile dataset from the naturally quenched melt inclusions agrees with decompression and degassing models, interpreted here as resulting from magmatic release of water and oxidized sulfur at depths of 1.08-1.8 km, inferred to be the primitive vapor feeding the geothermal vapor zone. This is a few tens of meters below the water boiling point identified in past exploration well studies at TGS (0.95 km). This new solubility model provides a means of combining geothermal exploration with location of magma depth, and of geochemically characterizing the narrow vertical window between the base of characteristic hydrothermal convection cells and the roof of the underlying magmatic system in other high enthalpy geothermal systems around the world.