COMPOSITIONAL CHANGES IN THE COPAHUE VOLCANIC HYDROTHERMAL SYSTEM, ARGENTINA, AFTER THE 2000 ERUPTION

Copahue volcano (38 oS) has an active volcano-hydrothermal system with acid hot springs (pH~0.4), a crater lake, and an acid river draining the summit region. Chemical and isotopic data indicate that these fluids, with exit temperatures close to 100 oC, are made up of more than 50% magmatic volatiles diluted with glacial melt water. The acid river has acidified a large glacial lake (Lake Caviahue, pH~2.0). Copahue erupted in July 2000, and continued into early January 2001. The crater lake was destroyed but in the following year the lake was re-established and hot spring flow resumed. We obtained weekly river samples from January to June 2001, and have annual depth transects of Lake Caviahue, plus crater lake, spring and river water samples from spring and fall 2002. The composition of the springs and rivers shows only minor changes in the concentrations of the volcanic elements (Cl, F, SO4) after the eruptions, but a strong increase in the cation loadings. We use the Mg/Cl ratios as a proxy for the activity of the volcano, indicating the availability of new fresh magma to the acid hydrothermal fluids and/or the creation of new ‘reactive surface area’ through shallow seismic activity. Combining compositional data with river water flux measurements provides element fluxes, which give quantitative insights into water-rock interaction in the volcano-hydrothermal system. Lake Caviahue is thermally but not chemically stratified in the austral summer and has a consistent hypolimnion temperature of 6-8°C. The lake composition records the activity of Copahue over a longer period, given the large volume of the lake (~0.5 km3), and modeling indicates that the element fluxes must have occurred for at least 10 years at the current levels. The Cl concentration has been increasing since 1997 (from 82 ppm to 100 ppm) and we predict it may increase to 140 ppm if the Cl flux from the volcano remains constant. Ratios of Cl/SO4 are very different in the river input and the Lake Caviahue fluids, indicating a sulfur sink in the lake (possibly sedimentary sulfide formation). The Copahue fluid compositions and the highly acidic environment are a natural analog to lake waters acidified by mine drainage.