Geothermal Copahue volcano system, Argentina. New stable isotope and geochemical data.

- CASELLI, A.T., DAPEÑA, C., AGUSTO, M. Y DELGADO HUERTAS, A.

Uruguay

Simposio; V South American Symposium on Isotope Geology, Uruguay.; 2006

Universidad

The Geothermal Effusive Copahue-Agrio-Caviahue Complex, is composed by the Copahue polygenic fissural-stratovolcano and the Agrio square caldera (Fig. 1). It is located on the west border of Argentina, in the Neuquén province, on the Andes Range nearby the boundary with Chile. This area belongs to the Southern Andean Volcanic Zone (SVZ: 33.3º- 46ºS). The volcanotectonic activity begun since the Pliocene with a relatively frequent eruptive activity in recent times. The Copahue volcano (37º45’S-71º10.2’W, 2977 m a.s.l.) is an active andesitic to basaltic-andesitic stratovolcano, nested on the western rim of the Late Pliocene Agrio caldera (Linares et al., 1999). It is elongate to N40º direction and has an average height of 1350 m above the surrounding basement. The volcano peak has nine craters that are aligned in a N60º direction and modified by glacier action. A modern small glacier provides meltwater to the crater lake. The eastern crater is the active one and is filled with a lake of nearly 125 m in diameter with acid waters (pH 0.3- 0.8) and molten sulfur floating on its surface (Mange, 1978; Varekamp et al., 2001; Caselli et al., 2005). Other surface expressions of an extensive volcanic-magmatic hydrothermal system geothermal field are two acid hot springs. The Copahue volcano has had 12 low-magnitude phreatic and phreatomagmatic eruptions during the past 250 years (Martini et al., 1997; Naranjo y Polanco, 2004). A new eruptive cycle started in July 1992, with explosions continuing in 1993, and major eruptions during 1994 and September 1995 (Delpino and Bermúdez, 1993). The Copahue's last eruption took place at July-October 2000, being at present in a fumarolic stage (Global Volcanism Network, 2000 a, b; Delpino y Bermúdez, 2002). Volcanic lakes are the surface expressions of high level geothermal systems and act as condenser for fluids and gases released by shallow magma bodies (Giggenbach, 1974). The compositional characteristics of lake waters will respond to chemical and physical changes in the subsurface hydrothermal/magmatic system. For that reason, volcanic lakes are considered excellent environments for monitoring and in some cases, forecasting future volcanic activity (Varekamp et al.2000; between others). These changes reflect water–rock (WR) interactions that occur in and beneath the lake and variations in the flux of heat and volatiles derived from subsurface hydrothermal/magmatic systems (Varekamp et al. 2000). In this work, we present new isotopic and chemical water data in order to show changes in the composition of the Crater Lake and hot springs of Copahue volcano with the aim to use like an indicator of future volcanic activity.