ELEMENT AND HEAT FLUXES FROM THE VOLCANO-HYDROTHERMAL SYSTEM OF COPAHUE VOLCANO, ARGENTINA

The active volcano Copahue in the Argentine Andes (37.5 S) has an acid crater lake, and chloride-sulfate hot springs that feed the Upper Rio Agrio that discharges into glacial Lake Caviahue. We determined the chemistry and element fluxes in this acid system from 1997 to 2006, with data from before, during and after the 2000 Copahue eruptions. The crater lake and hot springs had pH<0.5 prior to the eruption with up to 6.5 % sulfate and 1 % chloride. The fluids were saturated in gypsum/anhydrite and copper sulfides. Sulfur isotope studies suggest that the sulfate and associated acidity of the water formed from the disproportionation of magmatic SO2 at about 300 oC. Just prior to and during the 2000 eruption, the fluids became saturated with additional minerals, including jarosite and hematite, and concentrations of the rock forming elements (RFE) increased, presumably reflecting interaction of a small magma intrusion with the hot acid fluids. The post-eruptive fluids were depleted in K, but enriched in Mg. Mixing with glacial meltwater increases the pH of the Upper Rio Agrio to values of 1-2, and this river acidified Lake Caviahue to a pH of ~ 2.4. Measured element fluxes through the Upper Rio Agrio were compared with the concentration time trends in Lake Caviahue, from which mean annual input flux values were calculated. Integrated annual element fluxes before the 2000 eruption were on the order of 15,000 tonnes Cl, 25,000 tonnes SO4 and 1500 tonnes F, with RFE fluxes of ~4000 tonnes each of Mg, Fe, and Al. Mean annual volcanogenic trace element fluxes through the river include 7.5 tonnes As, 5 tonnes B and 0.6 tonnes Li. The hydrothermal heat flux was on the order of 15-25 MW. Following the eruption, the system became more dilute, the element fluxes decreased, and as a result, Lake Caviahue became less acidic (pH~2.8). The hot springs had pH values < 0.5 in 2006, suggesting that the system is becoming more active again. The overall chemical dynamics of the system are driven by volcanic inputs of SO2, HCl, HF, water and heat, and this hot, acid mixture dissolves surrounding volcanic rocks with subsequent precipitation of hydrothermal minerals, including liquid sulfur. The surface fluids become gradually diluted and neutralized through mixing with meteoric waters with mineral precipitation (mainly hydrous Fe- and Al-oxides).