SOIL GASES AS INDICATORS OF FLUID CIRCULATION AND PERMEABLE ZONES: SANTA MARIA- CERRO QUEMADO-ZUNIL VOLCANIC COMPLEX, GUATEMALA

Santa Maria, Cerro Quemada and Zunil volcanoes form a volcanic complex located in Eastern Guatemala, Central America. The Zunil hydrothermal system is associated to this volcanic complex. Zunil volcano located to the SE of the geothermal field has been proposed as the heat source for this field, even when the more active volcanoes Santa Maria to the SW and Cerro Quemado to the NW are also close to the system. Self-potential studies (Finizola et al., this meeting) and soil gas emissions have been investigated along a transect from Zunil volcano to Santa Maria volcano, passing trough Cerro Quemado. The purpose of these investigations was to understand fluid sources and circulation within this system. Soil gas samples were collected and CO2 efflux was measured along this transect using an accumulation chamber and a LICOR 820 CO2 analyzer. Soil gas samples were taken at 40 cm depth and analyzed in a Quadrupole Mass Spectrometer for concentrations of H2, CO2, O2, N2, He, Ar, Ne, Kr, CH4, SO2, and H2S. Radon and thoron concentrations were measured using a Pylon AB-5 monitor. Mean CO2 efflux was 21 g.m-2.day-1 with a maximum of 125 g.m-2.day-1. These values are within the range of other hydrothermal areas in Central America. Relatively higher concentrations of sulfur dioxide, probably derived from hydrogen sulfide oxidation, were found at the NW slope of Santa Maria Volcano, at an older caldera border. Statistically significant correlations (p<0.05) were found between self-potential and radon, thoron, SO2, H2, and SO2/CO2, between elevation and thoron, CO2, H2, and SO2/CO2, and between CO2 efflux and thoron, He, Ar, SO2, and N2/Ar, as well as between other gas pairs. The correlation between gas emissions, self-potential and elevation support the uprising and downward flow of fluids identified with the self-potential method (Finizola et al., this meeting) such as the uprising zone that corresponds to a hidden caldera boundary at the base of Santa Maria volcano and the fault system formed during the emplacement of Cerro Quemado dome, where fluid recharge and circulation seems to be happening. The direct transect between Santa Maria and Zunil volcanoes (to close the loop) has not been done yet. This transect is important because it intercepts the Zunil geothermal system and could give important information about the origin of fluids and heat for the hydrothermal field. The results of the self-potential and soil gas studies at this volcanic system suggest that the combination of both methods can have important information for the identification of permeable faults and fluid and heat circulation in other volcanic systems of the world, identifying zones and targets for hydrothermal exploitation.