RESEARCH ARTICLE The magmatic- and hydrothermal-dominated fumarolic system at the Active Crater of Lascar volcano, northern Chile F. Tassi & F. Aguilera & O. Vaselli & E. Medina & D. Tedesco & A. Delgado Huertas & R. Poreda & S. Kojima Received: 12 March 2008 / Accepted: 9 May 2008 / Published online: 3 June 2008 # Springer-Verlag 2008 Abstract Low-to-high temperature fumaroles discharging from the Active Crater of Lascar volcano (northern Chile) have been collected in November 2002, May 2005 and October 2006 for chemical and isotopic analysis to provide the first geochemical survey on the magmatic-hydrothermal system of this active volcano. Chemical and isotopic gas composition shows direct addition of high-temperature fluids from magmatic degassing, mainly testified by the very high contents of SO 2 , HCl and HF (up to 87,800, 29,500 and 2,900 μmol/mol) and the high R/Ra values (up to 7.29). Contributions from a hydrothermal source, mainly in gas discharges of the Active Crater rim, has also been detected. Significant variations in fluid chemistry, mainly consisting of a general decrease of magmatic-related com- pounds, i.e. SO 2 , have affected the fumarolic system during the period of observation, indicating an increase of the influence of the hydrothermal system surrounding the ascending deep fluids. The chemical composition of Active Crater fumaroles has been used to build up a geochemical model describing the main processes that regulate the fluid circulation system of Lascar volcano to be utilized in volcanic surveillance. Keywords Lascar volcano . Fumarolic gases . Fluid geochemistry . Isotope geochemistry . Northern Chile . Geochemical monitoring . Volcanic hazard Bull Volcanol (2009) 71:171–183 DOI 10.1007/s00445-008-0216-z Editorial responsibility: H Delgado F. Tassi (*) : O. Vaselli Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121 Florence, Italy e-mail: franco.tassi@geo.unifi.it F. Aguilera Programa de Doctorado en Ciencias mención Geología, Universidad Católica del Norte, Av. Angamos 0610, 1280 Antofagasta, Chile e-mail: faguilera@ucn.cl F. Aguilera : E. Medina : S. Kojima Departamento de Ciencias Geológicas, Universidad Católica del Norte, Av. Angamos 0610, 1280 Antofagasta, Chile O. Vaselli CNR-IGG Institute of Geosciences and Earth Resources, Via G. La Pira 4, 50121 Florence, Italy D. Tedesco Department of Environmental Sciences, 2nd University of Naples, Via Vivaldi 43, 81100 Caserta, Italy D. Tedesco Institute of Environmental Geology and Geo-Engineering, CNR-IGAG National Research Council, Pzz.e A. Moro, 00100 Roma, Italy A. Delgado Huertas CSIS Estacion Experimental de Zaidin, Prof. Albareda 1, 18008 Granada, Spain R. Poreda Department of Earth and Environmental Sciences, 227 Hutchinson Hall, Rochester, NY 14627, USA