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The Los Humeros volcanic complex, a 21 × 15 km diameter caldera edifice nesting volcanic domes and a complex faulting structure, is located at the eastern edge of the Trans-mexican volcanic belt (TMVB). It is a young edifice (<0.5 Ma) that hosts one of the five main geothermal fields of Mexico with still an important energy production (∼65 kW installed capacity). Being one of the more studied producing fields, the geothermal system of the caldera is largely unknown at depths greater than ∼2.4 km, which is the approximate penetration range of the available geothermal wells. Here we present the results of a geophysical survey in Los Humeros caldera and surroundings with the aim to provide further insight on the physical characteristics of the geothermal system at depths greater than 2.4 km. The survey comprised 70 broadband magnetotelluric (MT) soundings distributed within and in the periphery of the caldera edifice of which we present here three EW profiles. We also accomplished a mesh of 718 accurately leveled gravity stations. These data sets were complemented with 13 TDEM soundings for static shift control, as well as with the aero-magnetic digital chart of the area (#E14-3, SGM, 2004). The MT data analysis yielded an average electric azimuth of N23W for the central profile where the field production is concentrated, which is quite consistent with the mapped NW-SE geological structure. However, at individual frequency ranges the strike follows the local faulting structures, most of which are apparently controlled by the deeper crater structure. At the production zone, the conductivity model reveals the existence of an eastward dipping resistive body, which follows the isotherms registered at the wells. The inclined conductivity interface above it seems to play an important role in controlling the heat and fluid flow towards shallow depths. Petrographic studies of well samples provide evidence of mineralogical assemblages that suggest magnetite-metasomatose hydrothermal alteration. The production zone coincides with maximum gravity and magnetic gradients, at the western edge of the well-defined circular crater anomaly. At this point the central MT section (profile 1) shows the shallower depth (2–3 km) to the relatively high resistivity (∼400–500 ohm-m) and magnetized intrusive-like body. The interpreted geophysical and surface geological data backed by well data support a reservoir and plume model structure consisting of a resistive propylitic core that feds the geothermal field through fractures and deep- seated faults. Surface conductors associated with stratified mineralization produced from lixiviation of geothermal fluids are well differentiated from deeper conductors. The better preserved northern sector of the Humeros caldera yields an anomalous deep conductor at depths of 6–7 km below sea level (profile 5) as well as at the central production sector (profile 1) whereas the anomalous conductivity zone at the southern profile (profile 6) could be as shallow as 5 km below sea level. These anomalous conductivity zones are expected to be associated to the primary energy source of the Los Humeros geothermal system in the form of partial fusion or hypercritical trapped fluids within the upper crust, conditions that have been concluded to prevail along cordilleras of the American continent (e.g. Hyndman, 2017). [ABSTRACT FROM AUTHOR]

• The Juchipila graben is an unconventional geothermal system located in central Mexico. • Thermal water has anomalous concentrations of F, B, Li, and As. • δ2H and δ18O indicate a common meteoric source but different evaporation processes. • Mantle helium is transported into the system through deep-rooted faults. This work presents hydrochemical results for groundwater and dissolved gas samples collected from a thermal and cold aquifer in the Juchipila Basin, in southern Sierra Madre Occidental, central Mexico. Thermal springs in the Juchipila Basin reach temperatures of 60 °C, these manifestations are not related to recent or active volcanism as are all the known geothermal fields in Mexico. The thermal waters (>32 °C) are Na-HCO 3 and Na-SO 4 type, with an anomalous concentration of F, B, Li, and As. Their chemistry likely results from water-rock interaction processes. The cold waters (<32 °C) have a Ca-HCO 3 composition typical of recent infiltration and shallow flow, but they have an anomalous concentration of NO 3. The δ2H and δ18O indicate a common meteoric source for the warm and cold water plotting along an evaporation line. The waters have higher CO 2 and He concentrations than the air-saturated water. The helium composition is mainly atmospheric and terrigenous with a mantle helium contribution of up to 14%. This suggests that faults affecting the region are deeply rooted, permitting mantle helium uprise. Geothermometry gives mean reservoir temperatures of 58–102 °C. Based on these results, we propose a model of hydrothermal circulation in the Juchipila Basin, in which rainwater infiltrates deeply through the graben edges fault system, dissolves ions and crustal helium, incorporates mantle helium, while heated by the geothermal gradient, and eventually surges and mixes with the cold, shallow aquifer along faults cutting the whole succession within the graben. [ABSTRACT FROM AUTHOR]