The north-western sector of Caviahue caldera (Argentina), close to the active volcanic system of Copahue, is characterized by the presence of several hydrothermal sites that host numerous fumarolic emissions, anomalous soil diffuse degassing of CO 2 and hot soils. In March 2014, measurements of soil CO 2 fluxes in 5 of these sites (namely, Las Máquinas, Las Maquinitas I, Las Maquinitas II, Anfiteatro, and Termas de Copahue) allowed an estimation that 165 t of deeply derived CO 2 is daily released. The gas source is likely related to a relatively shallow geothermal reservoir containing a single vapor phase as also suggested by both the geochemical data from the 3 deep wells drilled in the 1980s and gas geoindicators applied to the fumarolic discharges. Gas equilibria within the H-C-O gas system indicate the presence of a large, probably unique, single phase vapor zone at 200-210°C feeding the hydrothermal manifestations of Las Máquinas, Las Maquinitas I and II and Termas de Copahue. A natural thermal release of 107 MW was computed by using CO 2 as a tracer of the original vapor phase. The magmatic signature of the incondensable fumarolic gases, the wide expanse of the hydrothermal areas and the remarkable high amount of gas and heat released by fluid expulsion seem to be compatible with an active magmatic intrusion beneath this portion of the Caviahue caldera.

This study presents the chemical and isotopic compositions of hydrothermal gases from fumaroles discharging in the surroundings of Copahue volcano (Argentina). Gas samples, including those from two fumaroles from the summit of the active crater, were collected during several surveys carried out by different researcher teams from 1976 to February 2016. The time-series of H2, CO and light hydrocarbons showed episodic increases in correspondence of the main events of the last eruptive cycle that started on the 19 th of July 2012. Concentration peaks were likely caused by enhanced inputs of hot magmatic fluids affecting the hydrothermal reservoir. These evidences were apparently in contrast with the temporal variations shown by Rc/Ra and  13 C-CO2 values in 2012-2014, which indicated increasing inputs from a crustal fluid source. In 2015-2016, these isotopic parameters showed opposite trends since they were approaching the compositional features of the two summit fumaroles, which were possibly corresponding to those of the deep magmaticrelated end-member. The delayed and smoothed compositional changes of the peripheral hydrothermal fluid discharges in response to the 2012-2016 eruptive events suggest that geochemical surveys of these emissions can unlikely provide premonitory signals of Manuscript Click here to download Manuscript Evolution Copahue gases final.docx Click here to view linked References volcanic unrest if the volcanic activity will keep centered in the main crater. At Copahue volcano, seismic activity and ground deformation seem to represent the most reliable approaches for monitoring purposes. However, the relative compositional stability of the hydrothermal reservoir is a great advantage in terms of geothermal resource exploitation, encouraging new investments in the Copahue geothermal project abandoned in the 1990's.

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