Bernard Sanjuan Sanjuan

Frédérick Gal

Ruth Alfaro

Frederick Gal

Chapter 3 -Developments of auxiliary chemical geothermometers applied to the Los Humeros and Acoculco high-temperature geothermal fields (México)

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The knowledge of the temperature of the deep geothermal fluids, rock permeability and water reservoir capacity are three key parameters for developing deep geothermal energy. Since 1965, one of the major applications of fluid geochemistry in the exploration of the potential geothermal reservoirs involves estimation of their temperature using classical chemical, isotope and gas geothermometers on fluids collected from geothermal wells and thermal springs, such as:

-Na-K [START_REF] Fournier | A revised equation for the Na/K geothermometer[END_REF][START_REF] Michard | Géothermomètres chimiques[END_REF][START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF], Na-K-Ca [START_REF] Fournier | An empirical Na-K-Ca geothermometer for natural waters[END_REF], K-Mg [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF], SiO2 [START_REF] Fournier | Estimation of underground temperatures from the silica content of water from hot springs and wet-steam wells[END_REF]Fournier, 1977;[START_REF] Michard | Géothermomètres chimiques[END_REF]; δ 18 OH2O-SO4 [START_REF] Lloyd | Oxygen isotope behavior in the sulfate-water system[END_REF][START_REF] Mizutani | Oxygen isotopic composition of sulphates, 3. Oxygen isotopic fractionation in the bisulphate ion-water system[END_REF][START_REF] Kusakabe | Oxygen and sulfur isotope equilibria in the system from 110 to 350°C and applications[END_REF]Sakaï, 1977;[START_REF] Seal | Stable isotope systematics of sulfate minerals[END_REF][START_REF] Zeebe | A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water[END_REF][START_REF] Boschetti | Oxygen isotope equilibrium in sulfate-water systems: A revision of geothermometric applications in low-enthalpy systems[END_REF]; -CO2-CH4-H2S-H2, CO2-CH4-H2, CO2-CH4, H2-Ar, CO2-Ar (D'Amore and [START_REF] D'amore | Evaluation of deep temperatures of hydrothermal systems by a new gasgeothermometer[END_REF][START_REF] Marini | Geochemistry of North Ghoubbat -Asal region[END_REF][START_REF] Giggenbach | Collection and analysis of geothermal and volcanic water and gas discharges[END_REF][START_REF] Giggenbach | Redox processes governing the chemistry of fumarolic gas discharge from White Island, New Zealand[END_REF][START_REF] Giggenbach | Chemical techniques in geothermal exploration[END_REF].

Most of the solute geothermometers are based on empirical or semi-empirical laws derived from known or unknown chemical equilibrium reactions between water and minerals occurring in the geothermal reservoirs. Unfortunately, these classical tools do not always yield concordant estimations of reservoir temperatures, even at very high temperatures, in acidic environments, for example. Discrepancies in temperature estimates may also be due to different processes occurring during the geothermal fluid ascent up to the surface and its cooling: mixing with surface cold waters or seawater, degassing, precipitation/dissolution processes, etc.

Since the early 1980s, numerical multicomponent geochemical models are also being developed for direct application to chemical geothermometry for geothermal exploration [START_REF] Reed | Calculation of multicomponent chemical equilibria and reaction processes in systems involving minerals, gases, and an aqueous phase[END_REF][START_REF] Michard | Modelling of the chemical composition of alkaline hot waters[END_REF][START_REF] Reed | Calculation of pH and mineral equilibria in hydrothermal waters with application to geothermometry and studies of boiling and dilution[END_REF][START_REF] Spycher | Integrated multicomponent solute geothermometry[END_REF]Peiffer et al., 2014a). These models allow numerical calculations of equilibration temperature of the geothermal water with respect to a suite of reservoir minerals, and thus the estimation of the reservoir temperature. Multicomponent geothermometry is not intended to replace classical geothermometers, but rather to supplement these geothermometers, and by doing so to increase confidence in temperature estimations. However, such approach cannot be applied carelessly and without a sound conceptual understanding of the area being studied (Al and pH poorly determined, for example). For this approach, a state of full chemical equilibrium is necessary and the conditions of this equilibrium state are not always reached.

Since the early 1980s, in parallel, several auxiliary geothermometers combining a major with a trace element, like Na-Li [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF][START_REF] Kharaka | Chemical geothermometers applied to formation waters, Gulf of Mexico and California basins[END_REF][START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF][START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF], Mg-Li [START_REF] Kharaka | Chemical geothermometers and their application to formation waters from sedimentary basins[END_REF], Na-Rb, Na-Cs, K-Sr, K-Mn, K-Fe, K-F and K-W [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF]Sanjuan et al., 2016a, b;2017), have been also developed and are available for specific types of geothermal fluids and geological environments.

BRGM aims to develop and validate this type of auxiliary chemical geothermometers and the δ 18 OH2O-SO4 isotope geothermometers in order:

to improve the geochemical methods for geothermal exploration in volcanic fields such as Los Humeros and Acoculco, with high-temperature (HT) and relatively low permeability;

to acquire a better knowledge about the circulation of HT deep fluids and their possible interaction with more superficial waters in this type of geothermal fields, from chemical and isotopic water analyses from surface thermal springs.

In the Los Humeros field, where numerous deep wells were drilled and are presently producing, the temperature values measured at the bottom-hole and estimated using classical water and gas geothermometers, will be used to test and calibrate these auxiliary geothermometers on the fluids collected from the deep geothermal wells. Some neighbouring thermal springs will be also sampled.

In the Acoculco field, where only two deep wells were drilled (EAC-1 in 1994, at a depth of 2000 m, and EAC-2 in 2008, at a depth of 1900 m, 500 m east of EAC-1; Peiffer et al., 2014b), but were not productive, these auxiliary geothermometers will be applied on fluids collected from surface thermal springs.

The temperatures estimated for the waters collected from the thermal springs during this study will be compared with those given by the classical gas and water geothermometers, with reference to the deep temperatures close to 290-330°C measured at a depth of about 2000 m in both los Humeros [START_REF] Gomez | -An updated conceptual model of the Los Humeros geotermal reservoir (México)[END_REF][START_REF] Pinti | Fluid circulation and reservoir conditions of the Los Humeros Geotermal Field (LHCF), México, as revealed by a noble gas survey[END_REF] and Acoculco fields (Peiffer et al., 2014b). These temperatures correspond to a gradient of 14°C/100 m, three times higher than the baseline gradient measured within the Trans-Mexican Volcanic Belt [START_REF] Ziagos | Heat flow and the thermal effects of subduction in Southern México[END_REF].

In order to be able to obtain relevant results, the BRGM activities have been planned as follows:

a preliminary literature review relative to the geological, geophysical and geochemical data about the Los Humeros and Acoculco geothermal fields, with the collaboration of the other partners, especially the Mexican partners (CFE, for example), in order to collect the main geological information and most of the geochemical data of the fluids sampled from Los Humeros deep wells and from thermal Acoculco springs); the participation to a campaign of fluid sampling and on site measurements in the Los Humeros and Acoculco geothermal areas (from deep wells at Los Humeros and from thermal springs at Acoculco), with the collaboration of CNR (Matteo Lelli's team) and Mexican teams (Ruth Alfaro, CFE…); chemical (major and some trace species) and isotope (δ 18 OH2O, δ 18 OSO4, δ 11 B, δ 7 Li, 87 Sr/ 86 Sr…) analyses of the waters collected during the campaign of fluid sampling, in the BRGM laboratories ; data interpretation, including the use of thermodynamic considerations, and main conclusions.

Literature review

Several interesting papers have been found during the literature review carried out by BRGM, among which the main ones are:

- [START_REF] Arzate | The Los Humeros (México) geotermal field model deduced from new geophysical and geological data[END_REF], Peiffer et al. (2018), Carrasco-Núñez et al. (2018, 2017a andb), [START_REF] Pinti | Fluid circulation and reservoir conditions of the Los Humeros Geotermal Field (LHCF), México, as revealed by a noble gas survey[END_REF], [START_REF] García-Soto | Solute geothermometry of Cerro Prieto and Los Humeros geotermal fields, Mexico: considerations on chemical characteristics of thermal water[END_REF], [START_REF] Norini | Structural analysis and thermal remote sensing of the Los Humeros volcanic complex: implication for volcano structure and geothermal exploration[END_REF], Arellano [START_REF] Gomez | -An updated conceptual model of the Los Humeros geotermal reservoir (México)[END_REF][START_REF] Gomez | Análisis de datos de producción y condiciones termodinámicas del fluido de alimentación de pozos de Los Humeros[END_REF][START_REF] Gomez | The response to exploitation of the Los Humeros (México) geothermal reservoir[END_REF], [START_REF] Bernard | Chloride and boron behaviour in fluids of Los Humeros geothermal field (Mexico): a model based on the existence of deep acid brine[END_REF]; [START_REF] García-Gutiérrez | Estado térmico inicial del campo geotérmico de Los Humeros, Puebla, México[END_REF], Barragán-Reyes et al. (2008[START_REF] Zeebe | A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water[END_REF], Gutiérrez-Negrín and [START_REF] Gutiérrez-Negrín | Review and update of the main features of the Los Humeros geothermal field, Mexico[END_REF], [START_REF] Izquierdo | Características mineralógicas y micro-termométricas del yacimiento geotérmico de Los Humeros, Pue[END_REF][START_REF] García-Gutiérrez | Estado térmico inicial del campo geotérmico de Los Humeros, Puebla, México[END_REF]), Lopez Romero (2006), [START_REF] Martínez-Serrano | Chemical variations in hydrothermal minerals of the Los Humeros geotermal system, Mexico[END_REF], [START_REF] Portugal | Hydrodynamic model of Los Humeros geotermal field, México, based on geochemical, mineralogical and isotopic data[END_REF], [START_REF] Prol-Ledesma | Pre-and post-exploration variations in hydrothermal activity in Los Humeros geothermal field, Mexico[END_REF], and [START_REF] Cortés | Studies of isotopic hydrology of the basin of Mexico and vicinity: annoted bibliography and interpretation[END_REF] for the Los Humeros geothermal field;

-Sosa-Ceballos et al. (2018), [START_REF] Avellán | Geology of the Acoculco caldera complex at the eastern Trans-Mexican Volcanic Belt (México)[END_REF], [START_REF] García-Palomo | NW-SE Pliocene-Quaternary extensión in the Apan-Acoculco región, Eastern Trans-Mexican Volcanic Belt[END_REF], [START_REF] Canet | A statistics-based method for the short-wave infrared spectral analysis of altered rocks: an example from the Acoculco Caldera, Eastern Trans-Mexican Volcanic Belt[END_REF]Canet et al. ( , 2015aCanet et al. ( , 2015b)), Peiffer et al. (2014b[START_REF] Peiffer | Numerical modelling of cold magmatic CO2 flux measurements for the exploration of hidden geothermal systems[END_REF], [START_REF] Lermo | Monitoreo sísmico en la zona geotérmica de Acoculco, Pue[END_REF], [START_REF] Lopez-Hernández | Hydrothermal activity in the Tulancigo-Acoculco Complex, central México: exploration studies[END_REF], [START_REF] Verma | Geochemical evidence for a lithospheric source for magmas from Acoculco Caldera, Eastern Mexican Volcanic Belt[END_REF], [START_REF] Lopez-Hernández | Exploratory drilling at Acoculco, Puebla, Mexico: a hydrothermal system with only non-thermal manifestations[END_REF], [START_REF] Hijonosa | Geoquímica de lodos de perforación del pozo EA-1, Acoculco, Puebla[END_REF], [START_REF] Hijonosa | Geoquímica de la zona geotérmica de la caldera de Acoculco, Puebla. Informe 34-86[END_REF], 1987[START_REF] Giggenbach | Chemical techniques in geothermal exploration[END_REF], [START_REF] Ledezma-Guerrero | Hoja Calpulalpan 14Q-h (3), con Resumen de la geología de la hoja Calpulalpan, Estados de México, Puebla, Tlaxcala e Hidalgo[END_REF] for the Acoculco geothermal field.

Los Humeros geothermal field

The main existing data of fluids from geothermal wells and neighbouring thermal springs used for this study are presented below.

Documents and Excel sheets provided by CFE in 2017 have allowed to collect geological, temperature and pressure logs from 16 wells located in the Los Humeros geothermal field (wells H-5, H-12, H-22, H-23, H-24, H-25, H-26, H-27, H-28, H-29D, H-41, H-43, H-49, H-56, H-58, H-59). Fluid and gas geochemical data collected between 1987 and 2017 are available for only 13 of these wells (absence of data for the wells H-5, H-25 and H-26). Measurements of water stable isotopes (δD and δ 18 O) have been performed by CFE, between 2013 and 2016, on fluids from 29 wells (wells H-3, H-6, H-7, H-9, H-11, H-12, H-13R, H-15, H-17, H-19, H-20, H-29, H-30, H-31, H-32, H-33, H-34, H-35, H-37, H-38R, H-39, H-40, H-41, H-42, H-43, H-44, H-45, H-48, H-49).

Chemical data for two neighbouring thermal springs (El Tesoro and Noria Nuevo Pizarro) were also found in the tables of fluid monitoring given by CFE.

Detailed fluid geochemical data from the geothermal wells of Los Humeros field and neighbouring springs are also given by:

-Prol-Ledesma (1998) during pre-and post-exploitation of the Los Humeros geothermal field (gas and water chemical and isotopic data from deep wells, including the stable isotopes of water and δ 18 O values of dissolved sulphates);

-Arellano- From the geothermal reservoir consisted of medium-to low-permeability pre-caldera andesites, the wells produce biphasic fluid, with variable but high fractions of steam and limited liquid water contents, with enthalpy values over 2400 kJ/kg (Gutiérrez-Negrín and [START_REF] Gutiérrez-Negrín | Review and update of the main features of the Los Humeros geothermal field, Mexico[END_REF], except for some wells as H-1, which has always produced water with a low enthalpy of 1500-1700 kJ/kg. Some geothermal fluids reach 400°C in the northern production area.

Acoculco geothermal area

Geochemical data for waters sampled from 39 thermal springs located in the Acoculco geothermal area, with temperature values ranging from 13 to 49°C, have been collected in the CFE report 34-86 (Tello Hinojosa, 1986). Chemical data for 10 gas samples from the Acoculco caldera are also presented in this report. All these data have been interpreted by [START_REF] Hijonosa | Geoquímica de la zona geotérmica de la caldera de Acoculco, Puebla. Informe 34-86[END_REF], and some main conclusions and recommendations were given. Some of these data were also presented and interpreted by [START_REF] Lopez-Hernández | Hydrothermal activity in the Tulancigo-Acoculco Complex, central México: exploration studies[END_REF]. Partial chemical data (pH, Na, K, SO4, B) are presented for samples of drilling fluid collected from the EAC-1 exploration well in the Acoculco area, in a CFE document [START_REF] Hijonosa | Geoquímica de lodos de perforación del pozo EA-1, Acoculco, Puebla[END_REF]. [START_REF] Hijonosa | Geoquímica de la zona geotérmica de la caldera de Acoculco, Puebla. Informe 34-86[END_REF], interpreted all the results and drew up some main conclusions, among which the estimation of deep temperatures ranging from 243 to 353°C, using gas geothermometry (CO2-Ar, CH4-CO2, CO2 and H2S). These values are in agreement with the measured well bottom hole temperatures (267 and 300°C). They might explain the intense hydrothermal alteration observed in the upper 800 m of volcanic rocks, with most abundant alteration minerals being quartz, amorphous silica, calcite, pyrite, clays (illite, smectite, kaolinite), and hematite.

The presence of a deep-water reservoir was not revealed during the EAC-1 drilling (depth of 2000 m), in 1994, in the locality of Los Azufres. However, a few permeable layers, at depths of 70 m and 300-450 m, and inflow of warm water together with significant amount of gas were observed (López-Hernández and Castillo-Hernández, 1997;López-Hernández et al., 2009).

The second well of 1900 m deep (EAC-2), drilled in 2008, 500 m east of EAC-1, showed a promising deep temperature of 267°C accompanied by low permeability similar to that of the EAC-1 well. springs and the two exploration wells (modified in Peiffer et al., 2014b, after López-Hernández et al., 2009).

According to Peiffer et al. (2014b), the high grade of alteration of the volcanic deposits induces low rock permeability and acts as a caprock probably impeding the recharge of the system by meteoric waters (López-Hernández et al., 2009), and causing the absence of thermal manifestations within the caldera complex. Instead, springs with close to ambient temperatures are reported as well as hydrothermally altered grounds, cold diffuse soil degassing and bubbling pools [START_REF] Polak | Isotopic composition and concentration of inert gases in Mexican hydrothermal systems[END_REF][START_REF] Hijonosa | Geoquímica de la zona geotérmica de la caldera de Acoculco, Puebla. Informe 34-86[END_REF][START_REF] Bernard-Romero | Nuevos datos geoquímicos de yacimientos geotermales de alta temperature del cinturón vólcanico mexicano: Isótopos de helio, boro, nitrógeno y elementos traza[END_REF].

Peiffer et al. (2014b)

Cold degassing is probably due to conductive cooling of the deep gases on their way to the surface. Los Azufres, where EAC-1 well was drilled, and Alcaparrosa are the only two regions with noticeable active degassing (H2S smell), bubbling pools and springs with temperatures of 16-25°C (fig. 3.2.2.1). The only springs with temperature significantly above the ambient temperature are located outside the caldera towards the east and south-east at Chignahuapan (49°C, inside a thermal bath resort), Quetzalapa (30°C), Jicolapa (32°C) and El Rincón (32°C;fig. 3.2.2.1). Apart from Chignahuapan spring, all these springs are characterized by bubbling.

The geological and geothermal setting are presented in Chapter 1.

Main remarks

If all the dissolved major species (Na, K, Ca, Mg, HCO3, Cl, SO4, SiO2) and stable water isotopes (δD and δ

Water sampling and analytical results

Water sampling

The campaign of water sampling was carried out by BRGM, in collaboration with CFE, University of Michoacana and CNR Lelli's team, from March 20 to 28, in the Los Humeros and Acoculco geothermal fields (fig. 3.3.1.1). Fluid samples were collected from seven geothermal wells and four thermal springs located in the Los Humeros area, from eight thermal springs located in the Acoculco area, and from three neighbouring crater lakes (lagunas), as points of surface reference. Their locations are reported in figure 3.3.1.1 and in table 3.3.1.1.

Nuevo Pizarro

Los Azufres

El Tesoro

Among the two-phase geothermal waters from Los Humeros field, very rich in steam, those which indicated the most high fractions of liquid water were selected with the valuable help of CFE (table 3.3.1.1).

Collection of the fluid samples in the field was accompanied by appropriate on-site measurements such as water temperature, conductivity, pH redox potential and alkalinity. The temperature, conductivity, pH and redox potential measurements were performed on the raw water samples, whereas alkalinity was analysed on fluid samples filtered at 0.45 µm. Absolute uncertainty concerning the pH measurements was 0.05 pH units and relative uncertainty concerning the other parameters varied from 5% to 10%, depending on the parameter and the range of measured values. All these measurements are given in table 3.3.1.1. Collection and conditioning of all the water samples followed the classical procedures recommended for each of the chemical and isotopic analyses to be performed. Thus:

for the chemical analysis of major anions and some trace elements, such as Cl, SO4, Br, F, NH4 and PO4, the water samples were filtered at 0.45 µm and collected in 100 ml polyethylene bottles; for the chemical analysis of major cations, the water samples were filtered at 0.45 µm, then acidified using Suprapur HNO3 and collected in 100 ml polyethylene bottles; in order to avoid silica precipitation, the samples of hot water for silica analysis (high contents) were collected in 50 ml polyethylene bottles and immediately diluted by a factor of 10 using Milli-Q water; for the chemical analysis of the other trace elements, such as B, Sr, Li, Ba, Mn, Fe, Al, Cs, Rb, Ge, As, Nd, Ag, Cd, Co, Cr, Cu, Ni, Pb and Zn, as well as for the isotopic Li and Sr analyses, the water samples were filtered at 0.2 µm, then acidified using Suprapur HNO3 and collected in 50 ml polyethylene bottles; for the isotopic analysis of B, the water samples were filtered at 0.45 µm, then acidified using Suprapur HNO3 and collected in 1 l polyethylene bottles; untreated fluid samples for the isotopic analysis of D and 18 O in the water and of 13 C in the carbon dioxide were collected in 100 ml and 1 l polyethylene bottles, respectively; for the isotopic analysis of 18 O in the dissolved sulphate, cadmium acetate was added to the water samples collected in 1 l polyethylene bottles.

Analytical results

All the chemical analyses for both major and trace elements in the collected water samples were done in the BRGM laboratories using standard water analysis techniques such as Ion Chromatography, Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), Flame Emission Spectrophotometry, TIC analysis and Colorimetry. The chemical analysis results, for which the analytical precision is better than ± 5% for the major elements and ± 10% for the trace elements, are given in tables 3.3. The absolute uncertainty for the analyses of δD and δ 18 O in the water samples was ± 0.8‰ and ± 0.1‰, respectively. The absolute uncertainty for the δ 18 O analyses of the dissolved sulphate was ± 0.1‰. The external reproducibility of the δ 7 Li and δ 11 B analyses was estimated at around ± 0.5‰ and ± 0.3‰, respectively, and the in-run precision of the 87 Sr/ 86 Sr ratio was generally better than ± 10 x 10 -6 (2σm).

All the geochemical data obtained during this study have been uploaded and stored in the GEMEX Open Access Database (OADB), as well as the required information. 2) and boron (among the highest ones in the world), with TDS and pH values ranging from 0.3 to 1.8 g/l and 6.67 to 7.58, respectively, traduce a high interaction process with the reservoir rocks, at high temperature.

b) Water origin

The isotopic δD and δ 18 O values for the Los Humeros wells show a wide dispersion probably related to boiling, mixing, phase separation and condensation phenomema (fig. 3.4.1.3). The high values of δ 18 OH2O of the geothermal waters towards the right of the Global Meteoric Water Line (GMWL) are not only in concordance with high-temperature values, but also suggest a low water-rock ratio of the geothermal reservoir, when compared with the lower values observed in the Krafla geothermal field, in , where the fluids are also biphasic, but the water-rock ratio is much higher. This is in agreement with a reservoir consisting of medium-to low-permeability pre-caldera andesites. According to Arrellano et al. (2003) and [START_REF] Barragán-Reyes | Geoquimica isotópica (d18O, dD) inicial de fluidos de pozos del campo geotérmico de Los Humeros, Pue[END_REF], the isotopic composition of the water molecule would be dominated by two main processes.

The first process would be a mixing of recharge meteoric water with a deep fluid-type andesitic water (δD ≈ -20‰ and δ 18 O ≈ 6‰), as defined by [START_REF] Giggenbach | Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin[END_REF], leading to a positive correlation between δD and δ 18 O, with a slope close to 3.0. In this case, from the literature existing data, the intersection of the mixture line with the GMWL would result to vary from -14.5‰ to -11‰ for δ 18 O, and from -105‰ to -78.5‰ for δD [START_REF] Portugal | Hydrodynamic model of Los Humeros geotermal field, México, based on geochemical, mineralogical and isotopic data[END_REF][START_REF] López-Romero | Actualización del modelo geoquímico de Los Humeros, Pue[END_REF][START_REF] Barragán-Reyes | Geoquimica isotópica (d18O, dD) inicial de fluidos de pozos del campo geotérmico de Los Humeros, Pue[END_REF][START_REF] Bernard | Chloride and boron behaviour in fluids of Los Humeros geothermal field (Mexico): a model based on the existence of deep acid brine[END_REF]. These values are lower than those measured for the meteoric waters in Los Humeros area (Oriental Basin), which are generally close to -10.7‰ for δ 18 O and -77.3‰ for δD [START_REF] Quijano | Algunos aspectos de la geohidrogeología isotópica de la región de Valsequillo[END_REF]. From these values, the proportion of andesitic water was estimated to be between 25% and 50% [START_REF] Portugal | Hydrodynamic model of Los Humeros geotermal field, México, based on geochemical, mineralogical and isotopic data[END_REF][START_REF] Barragán-Reyes | Geoquimica isotópica (d18O, dD) inicial de fluidos de pozos del campo geotérmico de Los Humeros, Pue[END_REF][START_REF] Bernard | Chloride and boron behaviour in fluids of Los Humeros geothermal field (Mexico): a model based on the existence of deep acid brine[END_REF].

The second process would be boiling and phase separation. At fluid temperatures higher than 220°C, 18 O is preferentially partitioned into the fluid phase, while deuterium is slightly partitioned into the vapor phase. The resulting fractionation scatters the points a few per mil perpendicular to the main mixing trend in the corresponding δDδ 18 O diagram. Another assumption for the origin of the geothermal waters could be the contribution of meteoric water with a δD value similar to that of the geothermal fluid, affected by a strong water-rock interaction process at high-temperature and low water-rock ratio, which enriches its 18 O content (up to 7-8‰). The wide dispersion observed for the isotopic values in the δDδ 18 O diagram could be explained by different water-rock interaction factors and processes such as kinetic fractionation at temperatures close to boiling temperatures [START_REF] Giggenbach | Processes controlling the isotopic composition of steam and water discharges from steam vents and steam-heated pools in geothermal areas[END_REF], with characteristic slopes of 3.0-3.5, and phase separation. In this case, the isotopic values for the meteoric water would be slightly heavier than those reported by [START_REF] Quijano | Algunos aspectos de la geohidrogeología isotópica de la región de Valsequillo[END_REF] for the Los Humeros area, but they coincide with hydrologic studies that identify the main recharge to Los Humeros area from the Sierra Madre Oriental, with groundwater flow in a NE-SW direction [START_REF] Prol-Ledesma | Pre-and post-exploration variations in hydrothermal activity in Los Humeros geothermal field, Mexico[END_REF]. According to [START_REF] Cedillo-Rodríguez | Hydrogeologic model of the geothermal reservoirs from Los Humeros , Puebla, México[END_REF], recharge might also occur locally, from rainfall infiltrating the reservoir through its fault and fracture systems.

In this study, the data obtained for the water isotopic values are in the range of the previous data and it is difficult to give a preference about the different assumptions. According to the first assumption, the δD and δ 18 O values for the meteoric water were estimated to be close to -110‰ and -15‰, respectively (fig. 3.4.1.3). For the second assumption, these values would be rather close to -65‰ and -9.5‰, respectively (fig. 3.4.1.3). Other arguments and more information about the water recharge and origin of the Los Humeros geothermal waters would have to be probably proposed in the works carried out by other teams like CNR, within the framework of the task 4.3 of this project.

The Los Humeros geothermal waters are also characterized by high Cl/Br ratios with respect to the thermal waters (fig. 3.4.1.4), much higher than that of seawater, which could be partially explained by supply of Cl from the degassing magma chamber. High B concentrations as those observed for these geothermal waters are rare in the world (fig.

3.4.1.7). Natural hydrothermal solutions have generally B concentrations from 1 to 10 mg/l in high-temperature two-phase fluids from basaltic aquifers of Iceland such as Krafla, Nesjavellir, etc. (Arnórsson and Andrésdóttir, 1995;Aggarwal et al., 2000;HITI-FP6 project, 2014).

B concentrations higher than 100 mg/l were only observed in fluids from aquifers composed of sedimentary and metamorphic rocks (Larderello, Italy; The Geysers, California; Ngahwa, New Zealand), of dacite-rhyolite volcanic rocks (Los Azufres, Mexico), and of marine carbonate and magmatic rocks, in the Yunnan-Tibet geothermal belt, in China. et al., 2000).

According to Arnórsson and Andrésdóttir (1995), the variable B and Cl concentrations and Cl/B ratios in high-temperature geothermal waters, as well as high B concentrations can be attributed to a combination of several processes. They include: i) supply of these elements from the degassing magma chamber, ii) supply from the rock with which the water interacts, and iii) phase separation in producing aquifers of wells.

Aggarwal et al. (2003) suggested a main source of B in Ngawha to be the greywacke wall rocks and the reason for such high B concentrations (up to 1000 mg/l) to be a lower water/rock ratio deduced from the high O isotope shift (+11‰) of deep water relative to the local meteoric water. [START_REF] Leeman | Boron isotopic variations in fumarolic condensates and thermal waters from Vulcano Island, Italy: Implications for evolution of volcanic fluids[END_REF] reported up to 240 mg/l in condensates of 300°C volcanic vapors from Vulcano, in Italy. They interpreted these values as the result of mixing of a magmatic endmember with about 70 mg/l of B and vapor derived from boiling of a modified seawater hot brine that was in contact with B-enriched Vulcano rhyolites and trachytes at low fluid/rock ratio.

The observed high B content and variable Cl concentrations in the Los Humeros geothermal waters could be the result of mixing of magmatic fluid from a deep magmatic chamber, the heat and fluid source for the system, leaching of wall rocks of the deep aquifer at a low fluid/rock ratio and phase separation process. [START_REF] Bernard | Chloride and boron behaviour in fluids of Los Humeros geothermal field (Mexico): a model based on the existence of deep acid brine[END_REF] proposed a model based on the existence of deep acid brine to explain the B and Cl behavior in the geothermal fluids of the Los Humeros geothermal field.

d) Geothermometry

The Na-K-Mg ternary diagram from [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF] and the main classical geothermometers such as Silica-quartz, Na-K, Na-K-Ca and K-Ca, as well as the isotopic δ 18 OH2O-SO4 geothermometric relationships established by [START_REF] Kusakabe | Oxygen and sulfur isotope equilibria in the system from 110 to 350°C and applications[END_REF] and [START_REF] Zeebe | A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water[END_REF], indicate that the full chemical equilibrium is reached for most of these waters at about 290 ± 30°C (fig. 3.4.1.10; tabl. 3.4.1.2 and 3.4.1.3). For the H-55 well, which has a very low fraction of liquid water, the temperature estimated using Silica-quartz is underestimated (163°C) because the concentration of dissolved silica is decreased by probable dilution of steam condensate. For the H-32, H-39 and H-55 water samples, the isotopic δ 18 OH2O-SO4 geothermometer also gives underestimated temperature values (112 to 228°C), indicating that the isotope equilibrium conditions are not attained for these samples. [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF].

This temperature range is concordant with the presence of an upper liquid-dominated reservoir area, located in augite andesites, between 1025 and 1600 m a.s.l., with neutral pH at 290-330°C, suggested in numerous studies [START_REF] Gomez | -An updated conceptual model of the Los Humeros geotermal reservoir (México)[END_REF][START_REF] Gutiérrez-Negrín | Review and update of the main features of the Los Humeros geothermal field, Mexico[END_REF]. The other deeper, two-phase, low-liquid saturation reservoir area, with high fractions of steam, is located in basalts and hornblende andesites, between 800 and 100 m a.s.l., with low pH fluids at temperatures of between 300 and 400°C.

Area

Sampling point Date

T 18 OH2O-SO4 (1) T 18 OH2O-SO4 (2) T 18 OH2O-SO4 (3) T 18 OH2O-SO4 (4) T 18 OH2O-BaSO4 T 18 OH2O-CaSO4 (1) T TQz: Fournier (1977); TChalced.: [START_REF] Michard | Géothermomètres chimiques[END_REF] Recommended value TNa-K (1): [START_REF] Michard | Géothermomètres chimiques[END_REF];TNa-K (2): [START_REF] Fournier | A revised equation for the Na/K geothermometer[END_REF]; TNa-K (3): [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF]. TNa-K-Ca: [START_REF] Fournier | An empirical Na-K-Ca geothermometer for natural waters[END_REF]. TNa-K-Ca-Mg : Fournier and Potter (1979). TK-Mg (1) [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF]; TK-Mg [START_REF] Seal | Stable isotope systematics of sulfate minerals[END_REF] : [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF]. TCa-K (1): [START_REF] Fournier | An empirical Na-K-Ca geothermometer for natural waters[END_REF]; TCa-K (2): [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF].

Among the different Na-Li geothermometric relationships existing in the literature [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF][START_REF] Kharaka | Chemical geothermometers applied to formation waters, Gulf of Mexico and California basins[END_REF][START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF][START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF][START_REF] Sanjuan | Final report on chemical geothermometers and tracers[END_REF]2017), only the Na-Li auxiliary geothermometer defined for North-Icelandic high-temperature geothermal dilute waters [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF]tabl. 3.4.1.4), give concordant temperature estimations (320 ± 30°C) with those estimated using the classical Silica-quartz, Na-K and Ca-K geothermometers, and the isotopic δ 18 OH2O-SO4 geothermometer, for most of the Los Humeros geothermal waters (apart H-9 and H-32 waters). The Na-Cs auxiliary geothermometer defined by Sanjuan et al. (2016a, b) also yields concordant temperature values (300 ± 30°C) for all the waters, and to a lesser extent, with lower estimated temperature values, and only for some waters, the Na-Rb, K-Sr and K-W auxiliary geothermometers defined by [START_REF] Sanjuan | Final report on chemical geothermometers and tracers[END_REF]. These estimations range from 227 to 255°C, 241 to 319°C, and 244 to 248°C, respectively. Thermodynamic binary diagrams representating log (H4SiO4) as a function of log (Na/K), log (Ca/K 2 ), log (Mg/K 2 ), log (Na/Li), log (Na/Cs), log (Na/Rb) and log (K 2 /Sr) were constructed in order to illustrate these results (figs. 3.4.1.11 and 3.1.4.12).

In the log (H4SiO4) -log (Na/K) diagram (fig. 3.4.1.11), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by Fournier (1977) for Silica-quartz and by [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF] for Na-K; the other equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by Fournier (1977) for Silica-quartz and by [START_REF] Michard | Géothermomètres chimiques[END_REF] for Na-K.

In the log (H4SiO4) -log (Ca/K 2 ) diagram (fig. 3.4.1.11), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by Fournier (1977) for Silica-quartz and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for Ca-K; the equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by [START_REF] Michard | Géothermomètres chimiques[END_REF] for Silica-chalcedony and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for Ca-K.

In the log (H4SiO4) -log (Mg/K 2 ) diagram (fig. 3.4.1.11), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by [START_REF] Michard | Géothermomètres chimiques[END_REF] for Silica-chalcedony and by [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF] for K-Mg; the equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by [START_REF] Michard | Géothermomètres chimiques[END_REF] for Silica-chalcedony and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for K-Mg. TNa-Li (1): [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF]; TNa-Li (2): [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF]; TNa-Li (3): [START_REF] Sanjuan | Thônex fluid geochemistry and geothermometric aplications. Contribution to the final IMAGE-D8.02 report[END_REF].

Area

Recommended value

TNa-Rb (1), TNa-Cs (1), TK-Fe (1), TKF, TKW: [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF].

TNa-Rb ( 2), TNa-Cs ( 2), TK-Sr, TK-Fe (2): Sanjuan et al. (2016a, b).

Figure as a function of log (Na/K), log (Ca/K 2 ) and log (Mg/K 2 ) for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas (all the concentrations are given in mol/l). In the log (H4SiO4) -log (Na/Li) diagram (fig. 3.4.1.12), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by Fournier (1977) for Silica-quartz and by [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF] for Na-Li; the equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by [START_REF] Michard | Géothermomètres chimiques[END_REF] for Silica-chacedony and by [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF] for Na-Li.

In the log (H4SiO4) -log (Na/Cs) diagram (fig. 3.4.1.12), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by Fournier (1977) for Silica-quartz and by Sanjuan et al. (2016a, b) for Na-Cs; the equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by [START_REF] Michard | Géothermomètres chimiques[END_REF] for Silica-chalcedony and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for Na-Cs.

In the log (H4SiO4) -log (Na/Rb) diagram (fig. 3.1.4.12), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by Fournier (1977) for Silica-quartz and by Sanjuan et al. (2016a, b) for Na-Cs; the equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by Fournier (1977) for Silica-chacedony and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for Na-Cs.

In the log (H4SiO4) -log (K 2 /Sr) diagram (fig. 3.1.4.12), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by Fournier (1977) for Silica-quartz and by Sanjuan et al. (2016a, b) for K-Sr; the other equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by Fournier (1977) for Silica-chalcedony and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for K-Sr. Other thermodynamic binary diagrams such as log (Na/Cs), log (Na/Rb), and log (K 2 /Sr), as a function of log (Na/Li) may also illustrate these results (fig. 3.4.1.13).

In the log (Na/Cs) -log (Na/Li) diagram (fig. 3.4.1.13), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF] for Na-Li and Sanjuan et al. (2016a, b) for Na-Cs; the other equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF] for Na-Li and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for Na-Cs.

In the log (Na/Rb) -log (Na/Li) diagram (fig. 3.4.1.13), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF] for Na-Li and Sanjuan et al. (2016a, b) for Na-Rb; the other equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF] for Na-Li and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF] for Na-Rb.

In the log (K 2 /Sr) -log Na/Li) diagram (fig. 3.4.1.13), the equilibrium reaction (equation 1) at different temperatures was determined using the thermometric relationships given by [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF] for Na-Li and by Sanjuan et al. (2016a, b) for K-Sr; the equilibrium reaction (equation 2) at different temperatures was defined using the thermometric relationships given by [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF] for Na-Li and by Sanjuan et al. (2016a, b) for K-Sr.

The geothermometric relationships used in this study are as follows (T in K):

Silica-quartz (Fournier, 1977):

T = 1309 / [0.41 -log (H4SiO4)]
Silica-chalcedony [START_REF] Michard | Géothermomètres chimiques[END_REF] where all the specie concentrations must be expressed in mol/l. The analytical results obtained in this study for the El Tesoro, Noria Nuevo Pizarro, Jicolapa, El Rincón, Quetzalapa and Chignahuapan thermal waters are close to those found in previous studies, when the analyses have been done (Tello Hinojosa, 1986;López-Hernández et al., 2009;Peiffer et al., 2014b;CFE data). For the Los Azufres thermal springs, waters like Los Azufres 3 with relatively high pH (7.94) had never been previously measured.

b) Water origin

Apart the Nuevo Pizarro, Tepeyahualco and Los Azufres waters, all the other waters show composition in δD and δ 18 O falling close to GMWL (fig. 3.4.1.3), which indicate a meteoric origin. The position of the Los Azufres 1 and 2 waters, at the right of GMLW suggest an enrichment in 18 O due to water-rock interaction at high-temperature and/or low water-rock ratio. That of Los Azufres 3 water could probably result from a boiling process taking into account its δ 11 B value and its Cl content. As for the crater lake waters (lagunas), also positioned at the right of GMLW, the Nuevo Pizarro and Tepeyahualco have probably been affected by a process of water evaporation or by a mixing with an evaporated water. Given their similar values in δD, the Chignahuapan, Jicolapa, El Rincón, Los Azufres 1 thermal waters could have the same recharge area. If we consider the second assumption for the origin of the Los Humeros geothermal waters (mainly meteoric water with high water-rock interactions) and the δD values, this area of water recharge could be also close to that of the Los Humeros geothermal waters. The relatively wide dispersion in δD and δ 18 O values observed for the Virgen del Carmen, El Tesoro, and Quetzalapa waters suggest that these waters have different recharge areas, probably located in the Sierra Madre Oriental.

The Cl/Br mass ratios of all these waters are much lower (from 70 to 700) than those of the Los Humeros geothermal waters (from 1200 to 1700 ; fig. 3.4.1.4). However, the higher Cl/Br values for the thermal waters are those associated with the Los Azufres 1 and 3, Chignahuapan and Tepeyahualco (600-700). The El Tesoro, Virgen del Carmen, Nuevo Pizarro and Quetchalapa waters indicate similar values of Cl/Br around 400 (fig. 3.4.1.4).

c) Processes of water-rock-gas interaction

Los Azufres 1 and 3 thermal waters have B concentrations (253-354 mg/l) much higher than those analysed in the other thermal waters (from 0.12 to 20 mg/l; fig. 3.4.1.5) and specific δ 11 B signatures (-5.61 and -4.42‰; fig. 3.4.1.6), which suggest mixing with low proportions of deep geothermal waters enriched in B (probably similar to those from the Los Humeros).

However, the relatively high B concentrations observed in most of the thermal waters and their δ 11 B signatures could traduce very small fluxes of deep geothermal waters in these waters, especially for the Chignahuapan thermal water (B ≈ 3 mg/l), which has also a high Cl concentration (115 mg/l). López-Hernández et al. (2009) already mentioned that the Chignahuapan thermal water, discharged from a spring located in an area of ancient system faults (Tulancingo-Tlaxco) connecting both zones, might be the farthest SE discharge of the Acoculco hydrothermal system, constituted of a mixture of deep geothermal fluid and shallow waters. Water isotopic data do not differ from meteoric values, because important dilution with shallow meteoric water could mask the deep signature.

The high B and Cl concentrations (≈ 20 mg/l and 775 mg/l, respectively) for the Tepeyahualco water, and its specific δ 11 B value (8.55‰), close to that of the Atexcac water (crater lake), suggest that this water is affected by a process of evaporation, like probably the Nuevo Pizarro water.

For most of the thermal waters, the shows that these waters interact with calcium carbonates and have high Ca concentrations, compared with the Los Humeros geothermal waters, which are depleted in Ca. Apart the Los Azufres thermal waters, the high Sr concentrations of the other thermal waters, compared with those of the Los Humeros geothermal waters (fig. 3.4.1.8), and their 86 Sr/ 87 Sr ratios, ranging from 0.706272 to 0.707262 (fig. 3.4.1.9), confirm that these waters are interacting with marine carbonate rocks formed during the Mesozoic period (probably the thick series of Jurassic and Cretaceous limestones, mentionned in Carrasco et al., 2017a).

For Los Azufres thermal waters, the Sr concentrations are high (from 133 to 2500 µg/l), but their 86 Sr/ 87 Sr signature (from 0.704778 to 0.705065; fig. 3.4.1.9) is closer to volcanic rocks (rhyolites?). These values could also traduce a mixing process between a deep water in contact with volcanic rocks and low proportions of water interacting with sedimentary rocks.

d) Geothermometry

The Na-K-Mg ternary plot developed by [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF] indicates that all the thermal waters of the Los Humeros and Acoculco areas are immature, having not reached full chemical equilibrium with the host rocks (fig. 3.4.1.10). In addition, geothermometry cannot be applied to acidic waters. However, the Na-K and Na-Li diagrams (fig. 3.4.2.2) show that the Na/K and Na/Li ratios for numerous thermal waters are close to those of the Los Humeros geothermal waters. Associated with δ 7 Li values ranging from 4.7 to 6.5‰ and relatively high B concentrations (especially for the Los Azufres waters), these ratios suggest that very low proportions of deep geothermal waters at about 300°C could be present in these thermal waters. These Na/K and Na/Li ratios, and to a lesser extent, the Na/Rb and Na/Cs ratios (fig. 3.4.2.2), could be the only witness of the presence of very small fluxes of high-temperature geothermal waters in the thermal waters, because the permeability of these areas is low, these geothermal waters are completely depleted in Ca, Mg and Sr whereas the thermal waters have relatively high concentrations in these elements, and the high silica concentrations of the geothermal waters can significantly decrease due to silica precipitation during their cooling and/or their dilution with shallow waters.

In this case, the geothermometers Na/Li and Na/K, and Na/Rb and Na/Cs sometimes, determined by [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF]2016a,b), give temperature estimations ranging from 248 to 353°C (tabl. 3.4.2.4), close to the temperature values measured in the wells and estimated using thermometric relationships.

In the Acoculco area, not only escapes of deep gases from the geothermal reservoir reach the surface (Peiffer et al., 2014b), but also small fluxes of deep waters coud be able to preserve, more or less, their original Na/K and Na/Li ratios, resulting from the temperature of their deep reservoir, even after a significant mixing with cold waters, during their ascent up to the surface.

In such a context (low-permeability environment, presence of low-salinity deep waters with very low concentrations of dissolved calcium, magnesium and strontium, precipitation of dissolved silica or dilution by shallow waters during the ascent and cooling of the deep water, dissolution of marine carbonates which provides relatively high concentrations in calcium, magnesium and strontium to the thermal waters), the application of geothermometers to thermal waters in order to estimate the temperature of deep reservoir is very difficult, but the Na/K and Na/Li ratios, and sometimes the Na/Cs and Na/Rb ratios, the boron concentrations and their isotopes, as well as analyses of associated non condensable gases (Peiffer et al., 2014b) may be useful tools for high-temperature geothermal exploration.

Classical geothermometers such as Silica-chalcedony, Na-K-Ca, K-Mg and Ca-K, based on the fluid equilibration with chalcedony, muscovite, clinochlore, K-felspar and calcite, which can re-equilibrate relatively fast, probably indicate subsurface temperature estimations for these thermal waters, ranging from 60 to 100°C (tabl. 3.4.1.2). The δ 18 OH2O-SO4 geothermometers defined by [START_REF] Kusakabe | Oxygen and sulfur isotope equilibria in the system from 110 to 350°C and applications[END_REF], Zeng (1999) and [START_REF] Zeebe | A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water[END_REF], as well as the K-Sr and K-F auxiliary geothermometers determined by [START_REF] Sanjuan | a) -Three new auxiliary chemical geothermometers for hot brines from geothermal reservoirs[END_REF] and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF], respectively, also seem to indicate concordant temperature estimations, which vary from 60 to 100°C (tabl. 3.4.1.3 and 3.4.1.4).

The relatively high concentration of dissolved silica analysed in the Atexcac water (fig. 3.4.1.2) could be explained by a thermal water input throughout the bottom of the Atexcac crater lake, according to [START_REF] Macek | Changes in protozoan assemblage structure and activity in a stratified tropical lake[END_REF].

Main conclusions

The main objectives of this study were to develop and validate auxiliary chemical geothermometers such as Na-Li, Na-Cs, Na-Rb, K-Sr, … and the δ 18 OH2O-SO4 isotope geothermometers in order:

to improve the geochemical methods for geothermal exploration in volcanic fields such as Los Humeros and Acoculco, with high-temperature and relatively low permeability;

to acquire a better knowledge about the circulation of high-temperature deep fluids and their possible interaction with more superficial waters in this type of geothermal fields, from chemical and isotopic water analyses from surface thermal springs.

In order to attain these objectives, a preliminary exhaustive literature review about the geological setting and the existing geochemical data on the geothermal and termal waters from Acoculco and the Los Humeros areas was carried by BRGM. After this review, fluid samples were collected by BRGM, between March 22 to 28, 2018, in collaboration with CFE, University of Michoacana and CNR Lelli's team, from seven geothermal wells and four thermal springs located in the Los Humeros area, from eight thermal springs located in the Acoculco area, and from three neighbouring crater lakes (lagunas), as references of surface waters. Among the twophase geothermal waters from Los Humeros field, very rich in steam, those which indicated the most high fractions of liquid water were selected with the valuable help of CFE. The chemical (major and trace species) and isotopic analyses (δD H2O and δ 18 OH2O, δ 18 OSO4, δ 11 B, δ 7 Li and 87 Sr/ 86 Sr ratios) were performed in the BRGM laboratories, and data interpretation was then carried out.

The geothermal Na-HCO3-Cl waters discharged from Los Humeros wells, completely deleted in calcium and magnesium, enriched in silica and boron (among the highest ones in the world), with TDS and pH values ranging from 0.3 to 1.8 g/l and 6.67 to 7.58, respectively, traduce a high interaction process with the reservoir rocks, at high-temperature. The high values of δ 18 OH2O of these waters towards the right of of the Global Meteoric Water Line (GMWL) are not only in concordance with these high-temperature values, but also suggest a low water-rock ratio of the geothermal reservoir, when compared with the lower values observed in the Krafla geothermal field, in North-Iceland, where the fluids are also biphasic, but the water-rock ratio is much higher. This is in agreement with a reservoir consisting of medium-to low-permeability pre-caldera andesites. The volcanic nature of the reservoir rocks was confirmed by the Sr and Li isotope signatures.

The isotopic δD and δ 18 O for the Los Humeros wells show a wide dispersion probably related to boiling, mixing, phase separation and condensation phenomema. Two main assumptions were considered in this study:

a mixing of recharge meteoric water with a deep fluid-type andesitic water (δD ≈ -20‰ and δ 18 O ≈ 6‰), as defined by [START_REF] Giggenbach | Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin[END_REF], leading to a positive correlation between δD and δ 18 O, with a slope close to 3.0, and a proportion of andesitic water estimated to be between 25% and 50%, accompanied with processes of boiling and phase separation (Arrellano et al., 2003;[START_REF] Barragán-Reyes | Geoquimica isotópica (d18O, dD) inicial de fluidos de pozos del campo geotérmico de Los Humeros, Pue[END_REF];

a contribution of meteoric water with a δD value similar to that of the geothermal fluid (average values of δD ≈ -62‰ and δ 18 O ≈ -3‰ for the total geothermal fluid, steam + water, characterized by data from [START_REF] Verma | Acidic fluid in Los Humeros geothermal reservoir. A preliminary outlook[END_REF][START_REF] Gomez | -An updated conceptual model of the Los Humeros geotermal reservoir (México)[END_REF][START_REF] Tello | Estado de equilibrio Soluto-Mineral y saturación de Minerales de alteración en fluidos geotérmicos de alta temperature[END_REF]and Bernard, 2008), affected by a strong water-rock interaction process at high-temperature and low water-rock ratio, which enriches its 18 O content (up to 7-8‰). The wide dispersion observed for the isotopic values could be explained by different waterrock interaction factors and processes such as kinetic fractionation at temperatures close to boiling temperatures [START_REF] Giggenbach | Processes controlling the isotopic composition of steam and water discharges from steam vents and steam-heated pools in geothermal areas[END_REF], with characteristic slopes of 3.0-3.5, and phase separation.

In this study, the data obtained for the water isotopic values are in the range of the previous data and it is difficult to give a preference about the different assumptions. According to the first assumption, the δD and δ 18 O values for the meteoric water were estimated to be close to -110‰ and -15‰, respectively. For the second assumption, these values would be rather close to -65‰ and -9.5‰, respectively. In this last case, the isotopic values for the meteoric water would be slightly heavier than those reported by [START_REF] Quijano | Algunos aspectos de la geohidrogeología isotópica de la región de Valsequillo[END_REF] for the Los Humeros area (Oriental Basin), close to -77.3‰ for δD and -10.7‰ for δ 18 O, but they would coincide with hydrologic studies that identify the main recharge to Los Humeros area from the Sierra Madre Oriental, with groundwater flow in a NE-SW direction [START_REF] Prol-Ledesma | Pre-and post-exploration variations in hydrothermal activity in Los Humeros geothermal field, Mexico[END_REF].

According to Arnórsson and Andrésdóttir (1995), the variable B and Cl concentrations and Cl/B ratios in high-temperature geothermal waters, as well as high B concentrations can be attributed to a combination of several processes. They include: i) supply of these elements from the degassing magma chamber, ii) supply from the rock with which the water interacts, and iii) phase separation in producing aquifers of wells. The observed high B contents and specific isotopic signatures, as well as the variable Cl concentrations in the Los Humeros geothermal waters, could be the result of mixing of magmatic fluid from a deep magmatic chamber, the heat and fluid source for the system, leaching of wall rocks of the deep aquifer at a low fluid/rock ratio and phase separation process.

The Na-K-Mg ternary diagram from [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF] and the main classical geothermometers such as Silica-quartz, Na-K, Na-K-Ca and K-Ca, as well as the isotopic δ 18 OH2O-SO4 geothermometric relationships established by [START_REF] Kusakabe | Oxygen and sulfur isotope equilibria in the system from 110 to 350°C and applications[END_REF] and [START_REF] Zeebe | A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water[END_REF], indicate that the full chemical equilibrium is reached for most of these waters at about 290 ± 30°C. This temperature range is concordant with the presence of an upper liquiddominated reservoir area, located in augite andesites, between 1025 and 1600 m a.s.l., with neutral pH at 290-330°C, suggested in numerous studies [START_REF] Gomez | -An updated conceptual model of the Los Humeros geotermal reservoir (México)[END_REF][START_REF] Gutiérrez-Negrín | Review and update of the main features of the Los Humeros geothermal field, Mexico[END_REF].

Among the different Na-Li geothermometric relationships existing in the literature [START_REF] Fouillac | Sodium/lithium ratio in water applied to geothermometry of geothermal reservoir[END_REF][START_REF] Kharaka | Chemical geothermometers applied to formation waters, Gulf of Mexico and California basins[END_REF][START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF][START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF]2017), only the Na-Li auxiliary geothermometer defined for North-Icelandic high-temperature geothermal dilute waters [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF], give concordant temperature values (320 ± 30°C) with those estimated using the classical Silica-quartz, Na-K and Ca-K geothermometers, and the isotopic δ 18 OH2O-SO4 geothermometer, for most of the Los Humeros geothermal waters (apart H-9 and H-32 waters). The Na-Cs auxiliary geothermometer defined by [START_REF] Sanjuan | a) -Three new auxiliary chemical geothermometers for hot brines from geothermal reservoirs[END_REF] also yields concordant temperature values (300 ± 30°C) for all the waters and to a lesser extent, with lower temperature estimations and only for some waters, the Na-Rb, K-Sr and K-W auxiliary geothermometers defined by [START_REF] Sanjuan | a) -Three new auxiliary chemical geothermometers for hot brines from geothermal reservoirs[END_REF]. These estimations range from 227 to 255°C, 241 to 309°C, and 244 to 246°C, respectively.

Thermodynamic binary diagrams such as log (H4SiO4) as a function of log (Na/K), log (Ca/K 2 ), log (Mg/K 2 ), log (Na/Li), log (Na/Cs), log (Na/Rb) and log (K/Sr 2 ), or representing log (Na/Cs), log (Na/Rb), and log (K 2 /Sr) as a function of log (Na/Li), were constructed in order to illustrate these results.

The El Tesoro, Noria Nuevo Pizarro, Virgen del Carmen and Pozo de Tepeyahualco waters from the Los Humeros area, sampled during this study, Na-HCO3-Cl-(SO4) type, have temperature, TDS and pH values ranging from 16 to 23.8°C, from 0.69 to 4.1 mg/l and from 6.75 to 8.82, respectively. Those sampled from the Acoculco area (Los Azufres, Jicolapa, El Rincón, Quetzalapa and Chignahuapan), HCO 3-SO4-Na-Ca and HCO3-Na-Ca type, have temperature, TDS and pH values varying from 19 to 49°C, from 0.35 to 2.1 g/l and from 3.22 to 7.94, respectively. The Los Azufres 2 water, SO4-Na-Ca type, which has the lowest values of pH (3.22) and TDS (0.35 g/l), is probably an acidic steam condensate. The Los Azufres 3 water with relatively high pH (7.94) had never been previously sampled and studied.

Apart the Nuevo Pizarro, Tepeyahualco and Los Azufres waters, all the other waters show composition in δD and δ 18 O falling close to GMWL, which indicates their meteoric origin. The position of the Los Azufres 1 and 2 waters, at the right of GMLW suggest an enrichment in 18 O due to water-rock interaction at high-temperature and/or low water-rock ratio. That of Los Azufres 3 water could result from a boiling process taking into account its δ 11 B value and its Cl content. As for the crater lake waters (lagunas), also positioned at the right of GMLW, the Nuevo Pizarro and Tepeyahualco have probably been affected by a process of water evaporation or by a mixing with an evaporated water. Given their similar values in δD and δ 18 O, the Chignahuapan, Jicolapa, El Rincón, Los Azufres 1 thermal waters (Acoculco area) could have the same recharge area. The relatively wide dispersion in δD and δ 18 O values observed for the Virgen del Carmen, El Tesoro, and Quetzalapa waters suggest that these waters have different recharge areas, probably located in the Sierra Madre Oriental. Given the similar δD values measured in the Los Humeros geothermal waters and in the Los Azufres 1 and Chignahuapan thermal waters (Acoculco area), the meteoric water recharge area could be close for these two geothermal fields, following the selected interpretation for the origin of the Los Humeros geothermal waters.

The Cl/Br mass ratios of all these waters are much lower (from 70 to 700) than those of the Los Humeros geothermal waters (from 1200 to 1700). However, the higher Cl/Br values for the thermal waters are those associated with the Los Azufres 1 and 3, Chignahuapan and Tepeyahualco (600-700). The El Tesoro, Virgen del Carmen, Nuevo Pizarro and Quetchalapa waters indicate similar values of Cl/Br around 400.

For most of the thermal waters, the Ca-HCO3 diagram shows that these waters interact with calcium carbonates and have high Ca concentrations, compared with the Los Humeros geothermal waters, which are depleted in Ca. Apart the Los Azufres thermal waters, the high Sr concentrations of the thermal waters, compared with those of the Los Humeros geothermal waters, and their 86 Sr/ 87 Sr ratios, ranging from 0.706272 to 0.707262, confirm that these waters are interacting with marine carbonate rocks formed during the Mesozoic period (probably the thick series of Jurassic and Cretaceous limestones, mentionned in Carrasco et al., 2017a). For Los Azufres thermal waters, the Sr concentrations are high, but their 86 Sr/ 87 Sr signature (from 0.704778 to 0.705065) is closer to volcanic rocks (rhyolites?). These values could also traduce a mixing process between a deep end-member water in contact with volcanic rocks and low proportions of water interacting with sedimentary rocks.

The Na-K-Mg ternary plot developed by [START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF] indicates that all the thermal waters of the Los Humeros and Acoculco areas are immature, having not reached full chemical equilibrium with the host rocks. In addition, geothermometry cannot be applied to acidic waters. However, the Na-K and Na-Li diagrams show that the Na/K and Na/Li ratios for numerous thermal waters are close to those of the Los Humeros geothermal waters. Associated with δ 7 Li values ranging from 4.7 to 6.5‰ and relatively high B concentrations (especially for Los Azufres waters), these ratios suggest that very low proportions of flux of deep geothermal waters at about 300°C could be present in these thermal waters.

Indeed, these Na/K and Na/Li ratios, and to a lesser extent, the Na/Rb and Na/Cs ratios, could be the only witness of the presence of very small flux of deep high-temperature geothermal waters in the thermal waters, because the permeability of these areas is low, these geothermal waters are completely depleted in Ca, Mg and Sr whereas the thermal waters have relatively high concentrations in these elements, and the high silica concentrations of the geothermal waters can significantly decrease due to silica precipitation during their cooling and/or their dilution with shallow waters. In this case, the geothermometers Na/Li and Na/K, and Na/Rb and Na/Cs sometimes, determined by [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF]2016a,b), give temperature estimations ranging from 248 to 353°C, close to the temperature values measured in the wells and estimated using thermometric relationships.

In the Acoculco area, not only escapes of deep gases from the geothermal reservoir reach the surface (Peiffer et al., 2014b), but also small fluxes of deep waters coud be able to preserve, more or less, their original Na/K and Na/Li ratios, resulting from the temperature of their deep reservoir, even after a significant mixing with cold waters, during their ascent up to the surface.

In such a context (low-permeability environment, presence of low-salinity deep waters with very low concentrations of dissolved calcium, magnesium and strontium, precipitation of dissolved silica or dilution by shallow waters during the ascent and cooling of the deep water, dissolution of marine carbonates which provides relatively high concentrations in calcium, magnesium and strontium to the thermal waters), the application of geothermometers to thermal waters in order to estimate the temperature of deep reservoir is very difficult, but the Na/K and Na/Li ratios, and sometimes the Na/Cs and Na/Rb ratios, the boron concentrations and their isotopes, as well as analyses of associated non condensable gases (Peiffer et al., 2014b) may be useful tools for high-temperature geothermal exploration.

Classical geothermometers such as Silica-chalcedony, Na-K-Ca, K-Mg and Ca-K, based on the fluid equilibration with chalcedony, muscovite, clinochlore, K-felspar and calcite, which can re-equilibrate relatively fast, probably indicate subsurface temperature estimations for these thermal waters, ranging from 60 to 100°C. The δ 18 OH2O-SO4 geothermometers defined by [START_REF] Kusakabe | Oxygen and sulfur isotope equilibria in the system from 110 to 350°C and applications[END_REF], Zeng (1999) and [START_REF] Zeebe | A new value for the stable oxygen isotope fractionation between dissolved sulfate ion and water[END_REF], as well as the K-Sr and K-F auxiliary geothermometers determined by [START_REF] Sanjuan | a) -Three new auxiliary chemical geothermometers for hot brines from geothermal reservoirs[END_REF] and by [START_REF] Michard | Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr[END_REF], respectively, also seem to indicate concordant temperature estimations, which vary from 60 to 100°C.

Figure 3

 3 Figure 3.2.1.1a) Simplified tectonic map of the central part of the Los Humeros caldera with the major faults and the position of the production and re-injection wells (from Pinti et al., 2017). b) Location of the main geothermal wells within Los Humeros geothermal field (from Carrasco et al., 2017a).

Figure 3

 3 Figure 3.2.2.1 -a) Location of the Acoculco caldera within the Trans-Mexican Volcanic Belt (TMVB) and schematic map of the Tulancingo-Acoculco caldera complex with the position of the main fault systems, thermalsprings and the two exploration wells (modified inPeiffer et al., 2014b, after López-Hernández et al., 2009).

Figure 3

 3 Figure 3.3.1.1 -Location of the geothermal wells and thermal springs which were sampled during the campaign of water sampling carried out by BRGM between March 22 and 28, 2018, in the Los Humeros and Acoculco areas.

  characteristics of the fluids The geothermal Na-HCO3-Cl waters discharged from Los Humeros wells (fig. 3.4.1.1), completely deleted in calcium and magnesium, enriched in silica (fig. 3.4.1.

Figure 3

 3 Figure 3.4.1.1 -Position of the geothermal and thermal waters collected in the Los Humeros and Acoculco areas in the Cl-HCO3-SO4 ternary diagram of[START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF].

Figure 3

 3 Figure 3.4.1.2 -Diagram SiO2 -Cl for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Figure 3

 3 Figure 3.4.1.3 -Diagram δDδ 18 O for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

  According to data from[START_REF] Verma | Acidic fluid in Los Humeros geothermal reservoir. A preliminary outlook[END_REF][START_REF] Gomez | -An updated conceptual model of the Los Humeros geotermal reservoir (México)[END_REF],[START_REF] Tello | Estado de equilibrio Soluto-Mineral y saturación de Minerales de alteración en fluidos geotérmicos de alta temperature[END_REF] andBernard (2008), the total geothermal fluid (steam + water) from Los Humeros is characterized by average values of δD ≈ -62‰ and δ 18 O ≈ -3‰.

Figure 3

 3 Figure 3.4.1.4 -Diagram Cl/Br -Cl for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Figure 3

 3 Figure 3.4.1.5 -Diagram B -Cl for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Figure 3

 3 Figure 3.4.1.6 -Diagram δ 11 B -B for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Figure 3

 3 Figure 3.4.1.7 -Diagram δ 11 B -B for worldwide geothermal waters (after[START_REF] Lü | Geochemical processes and origin of boron isotopes in geothermal water in the Yunnan-Tibet geothermal zone[END_REF].In contrast, similar δ 11 B isotopic values (from -3.7 to -1.5‰) were observed in the hightemperature basaltic waters from Icelandic geothermal fields(Krafla, Nesjavellir, etc.; Aggarwal et al., 2000; FP6-HITI project, 2010; tabl. 3.4.1.1) as well as in the Ngahwa(Aggarwal et al., 2003) and Lardarello[START_REF] Pennisi | Boron and helium isotopes in geothermal fluids from Larderello (Italy). Water-rock Interaction[END_REF] geothermal fields. These values are lower for the Yunnan-Tibet thermal waters (-6.0 to -6.8‰;[START_REF] Lü | Geochemical processes and origin of boron isotopes in geothermal water in the Yunnan-Tibet geothermal zone[END_REF]. Tonarini et al.(1998) suggest that δ 11 B of exsolved fluids during tourmaline crystallization from pegmatites of the Elba Island could vary between -6 and -2‰ at temperatures ranging from 300 to 600°C. As a matter of fact, the δ 11 B data related to the Tuscan magmatic Province show negligible variations. The restricted range in δ 11 B values compared to the total concentration variations suggest that the B isotope ratios reflect differences in the δ 11 B values of the rock rather than the results of secondary processes, such as phase separation or deposition of secondary minerals(Aggarwal et al., 2000).

Figure 3

 3 Figure 3.4.1.8 -Diagram Sr -Ca for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Figure 3

 3 Figure 3.4.1.9 -Diagram 87 Sr/ 86 Sr -Ca/Sr for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Figure 3

 3 Figure 3.4.1.10 -Position of the geothermal and thermal waters collected in the Los Humeros and Acoculco areas in the Na-K-Mg ternary diagram of[START_REF] Giggenbach | Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators[END_REF].

Figure 3

 3  as a function of log (Na/Li), log (Na/Cs), log(Na/Rb) and log (K 2 /Sr) for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas (all the concentrations are given in mol/l).

  / [5.84 -log (K 2 /Mg)] Na/Li (Sanjuan et al., 2014): T = 2002 / [log (Na/Li) + 1.322] Na/Li (Fouillac & Michard, 1981): T = 1000 / [log (Na/Li) + 0.38] (Cl < 0.3 M) Na/Cs (Sanjuan et al., 2016a, b): T = 2585 / [log (Na/Li) + 0

Figure 3

 3 Figure 3.4.1.13 -Diagrams log (Na/Cs), log (Na/Rb), and log (K 2 /Sr) as a function of log (Na/Li) for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas (all the concentrations are given in mol/l).

Figure 3

 3 Figure 3.4.2.1 -Diagram HCO3 -Ca for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Figure 3

 3 Figure 3.4.2.2 -Diagrams K -Na, Li -Na, Rb -Na, and Cs -Na for the geothermal and thermal waters collected in the Los Humeros and Acoculco areas.

Pinti et al. (2017) Carrasco-Nuñez et al. (2017a)

  Peiffer et al. (2014b) have reported geochemical data for waters from four Acoculco thermal springs and associated non-condensable gases (fig.3.2.2.1). They integrated some of the previous data obtained by

  18 O) were determined among most of the existing geochemical data, few trace elements (only B, Li, F, NH 4, Fe, Al and As) were studied. Very few data were also found for δ 18 OSO4 and δ 11 B values, and 87 Sr/ 86 Sr ratios. No value was found for δ 7 Li values. The campaign of fluid sampling planned by BRGM was therefore very important and necessary to test and develop our auxiliary chemical geothermometers, integrating trace elements such as Rb, Cs, Sr, Mn, F and W, and the δ 18 OH2O-SO4 geothermometer. It has also contributed to acquire new isotopic data of δ 11 B and δ 7 Li values.

The table 3.2.3.1 summarizes the existing chemical and isotopic data obtained on the scarce thermal waters of the Los Humeros and Acoculco areas.

Table 3

 3

	.2.3.1 -Existing chemical and isotopic data of the main thermal springs discharging from the Los
	Humeros and Acoculco geothermal areas, with literature references.

Table 3

 3

	.3.1.1 -Field data corresponding to the campaign of fluid sampling carried out by BRGM between March
	22 and 28, 2018, in the Los Humeros and Acoculco geothermal areas.

El Tesoro (in front of the first spring)

	Area	Sampling point	Specific enthalpy						Depth	Date	T	Cond. 25°C pH	Eh	Alk.	Alk.
			J/g	Longitude (°E WGS84) Latitude (°N WGS84)	X	Y	Z (m)	m		°C	µS/cm		mV	meq/l mg/l HCO3
	Los Humeros area	Los Humeros H-39		-97.44192672	19.64813614	663433	2173524	2901	2890	22/03/2018 12:00 61.9	591	7.14 -260	4.45
		Los Humeros Unit-11 (fluid mixing)		-97.44426546	19.64789310	663188	2173495	2896		22/03/2018 12:50 52.7	1294	7.00 -240	4.56
		Los Humeros H-56	2006	-97.45238985	19.65824117	662325	2174632	2840	2380	22/03/2018 13:25 74.1	1196	7.58 -346	5.68
		Los Humeros H-49	1640	-97.45575398	19.66414467	661967	2175282	2824	2030	22/03/2018 13:55 68.0	1051	7.70 -270	4.64
		Los Humeros H-9		-97.46771954	19.69284192	660683	2178448	2756	2752	22/03/2018 15:05 66.8	752	7.17 -295	4.63
		Los Humeros H-32		-97.44843294	19.69107099	662707	2178270	2815	2818	22/03/2018 15:35 67.8	595	6.67 -235	1.55
		Los Humeros H-55	≈ 2600	-97.44178982	19.68610194	663409	2177726	2830		22/03/2018 16:08 68.9	254	7.55 -335	0.73
		El Tesoro		-97.25200094	19.73436294	683253	2183272	2080		23/03/2018 09:30 22.5	787	7.49 220	6.05
						683259	2183302	2080		23/03/2018 10:00 21.0	610	7.50 240
		Noria Nuevo Pizarro		-97.45232003	19.49040046	662501	2156054	2342	20.18 23/03/2018 11:45 16.0	2090	8.82 130	11.21
		Pozo Hacienda San Miguel Barrientos		-97.57049400	19.50323691	650084	2157367	2352		23/03/2018 12:45 18.5	441	7.81 210
		Pozo de Pochintoc							95	23/03/2018 13:45 20.2	660	7.80 190
		Virgen del Carmen		-97.53992885	19.26677171	653514	2131220	2364		23/03/2018 15:46 19.1	2080	7.10 205	12.36
		Pozo de Tepeyahualco		-97.46732500	19.51728056	660899	2159015	2426	130	24/03/2018 10:10 23.8	4970	6.75	-50	33.70	2056
		Laguna de Atexcac		-97.45423537	19.33296456	662456	2138625	2363		24/03/2018 14:15 22.0	12640	8.63	50	2.05
		Laguna de Alchichica		-97.39758881	19.40821429	668331	2147009	2324		23/03/2018 16:45 18.5	13250	9.07 106	41.75	2547
		Laguna de Quechulac		-97.34922053	19.37386715	673448	2143255	2342		24/03/2018 16:30 20.4	894	8.73 160	6.75
	Acoculco area	Los Azufres 1		-98.14404722	19.92236110	589656	2203350.946	2840		26/03/2018 15:05 25.4	1489	6.41 -355
		Los Azufres 2		-98.14442500	19.92255830	589616	2203372.57	2840		26/03/2018 16:00 19.2	693	3.22	-51

Table 3

 3

	Area	Sampling point	Date	T	Cond. 25°C pH	Eh	Na	K	Ca	Mg	Alk.	Cl	SO4	NO3	SiO2 TDS I
		.3.2.1 -Chemical composition (major species) of the geothermal and thermal waters collected during the
		campaign carried out by BRGM between March 22 and 28, 2018, in the Los Humeros
			and Acoculco geothermal areas.

.B. °C µS/cm mV mg/l mg/l mg/l mg/l mg/l HCO3 mg/l mg/l mg/l mg/l g/l % Los Humeros area Los Humeros H-39

			22/03/2018 12:00 61.9	591	7.14 -260	119	23.8 < 0.5 < 0.5	268	46.5	2.3	< 0.5	745	1.20 0.31
		Los Humeros Unit-11 (fluid mixing) 22/03/2018 12:50 52.7	1294	7.00 -240	270	36.9	2.2	< 0.5	307	98.8	227	< 0.5	733	1.68 2.37
		Los Humeros H-56	22/03/2018 13:25 74.1	1196	7.58 -346	244	38.4	1.0	< 0.5	383	147	63.6 < 0.5	931	1.81 1.31
		Los Humeros H-49	22/03/2018 13:55 68.0	1051	7.70 -270	212	35.8	1.1	< 0.5	305	129	86.1 < 0.5	834	1.60 -0.71
		Los Humeros H-9	22/03/2018 15:05 66.8	752	7.17 -295	146	30.9 < 0.5 < 0.5	283	69.5	39.0	0.8	538	1.11 0.41
		Los Humeros H-32	22/03/2018 15:35 67.8	595	6.67 -235 82.6 12.4	2.6	< 0.5	95	86.1	6.3	< 0.5	567	0.85 -1.68
		Los Humeros H-55	22/03/2018 16:08 68.9	254	7.55 -335 34.4	5.6	< 0.5 < 0.5	51	38.4	13.8 < 0.5	155	0.30 1.51
		El Tesoro	23/03/2018 09:30 22.5	787	7.49 220	84.6 11.0	34.5	30.2	369	63.9	12.0	7.9	78.3 0.69 -0.71
		Noria Nuevo Pizarro	23/03/2018 11:45 16.0	2090	8.82 130	410	46.3	17.4	18.0	684	240	115	43.0	47.9 1.62 1.47
		Virgen del Carmen	23/03/2018 15:46 19.1	2080	7.10 205	85.3	8.2	273	107	755	99.1	557	2.3	91.0 1.98 -1.45
		Pozo de Tepeyahualco	24/03/2018 10:10 23.8	4970	6.75 -50	659	30.9	315	133	2056	775	0.6	< 0.5 91.8 4.06 0.99
		Laguna de Atexcac	24/03/2018 14:15 22.0	12640	8.63	50	2022 92.7	16.6	604	1474	3854	264	5.7	68.7 8.40 1.78
		Laguna de Alchichica	23/03/2018 16:45 18.5	13250	9.07 106 2506	218	6.8	431	2547	3259 1088 < 0.5	4.0 10.06 -3.78
		Laguna de Quechulac	24/03/2018 16:30 20.4	894	8.73 160	82.1	7.7	16.7	62.3	412	90.7	19.5	1.0	14.6 0.71 0.04
	Acoculco area	Los Azufres 1	26/03/2018 15:05 25.4	1489	6.41 -355	147	29.3	104	28.7	463	36.0	353	< 0.5 77.4 1.24 -2.87
		Los Azufres 2	26/03/2018 16:00 19.2	693	3.22 -51	12.8	8.2	23.0	6.0	< 10	0.7	225	< 0.5 75.3 0.35 -43.53
		Los Azufres 3	28/03/2018 09:45 26.6	1735	7.94 -140	210	33.7	153	46.5	463	52.0	644	< 0.5 56.1 1.66 -2.37
		Jicolapa	27/03/2018 09:45 30.7	1372	6.31 -241 31.7 15.1	278	13.4	958	7.6	4.3	< 0.5	152	1.46 4.76
		El Rincon	27/03/2018 11:00 26.6	550	5.67 110	12.3 12.0	90.7	5.2	291	2.3	48.4 < 0.5	140	0.60 -0.22
		Baños de Quetzalapa	27/03/2018 16:00 30.0	1964	6.37 135	150	16.8	291	37.4	1436	16.1	0.8	< 0.5	114	2.06 2.47
		Baños de Chignahuapan	28/03/2018 07:30 49.0	1448	6.51 -271 93.0 14.2	191	25.9	756	115	26.5 < 0.5 44.1 1.27 -0.34

Table 3

 3

.3.2.2 -Chemical composition (minor and trace species) of the geothermal and thermal waters collected during the campaign carried out by BRGM between March 22 and 28, 2018, in the Los Humeros and Acoculco geothermal areas.

Table 3 .

 3 3.2.3 -Isotopic composition of the geothermal and thermal waters collected during the campaign carried out by BRGM betweenMarch 22 and 28, 2018, in the Los Humeros and Acoculco geothermal areas.

	Area	Sampling point	Date	T	δD

δ 18 O δ 18 OSO4 δ 7 Li δ 11 B

87 Sr/ 86 Sr °C

	Area	Sampling point	Date	T	NH4	PO4	F	B	Br	Sr	Ba	Mn	Li	Rb	Cs	Ge	Al	As	Fe	W	Ag	Cu	Zn	Ni	Pb	Co	Cd	U
			°C	mg/l mg/l mg/l mg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l	µg/l µg/l µg/l	µg/l	µg/l	µg/l	µg/l
	Los Humeros area Los Humeros H-39	22/03/2018 12:00 61.9	0.11	0.67	13.4	951	32.0	1.52	0.71	7.05	397	240	305	60.1	4193 42323	38	86.5 < 0.01 < 0.1 1.96 0.35 < 0.05 < 0.05 0.06 < 0.01
		Los Humeros Unit-11 (fluid mixing) 22/03/2018 12:50 52.7	0.15	0.70	20.6	1819 73.7	41.2	5.92	25.9	619	315	330	56.0	3310 21721	90	85.4	0.02 < 0.1 0.32 0.65 < 0.05 < 0.05 < 0.01 < 0.01
		Los Humeros H-56	22/03/2018 13:25 74.1	5.51	0.84	4.2	256	90.9	11.0	1.06	18.7	871	392	705	47.6	2042 8077	110	84.5 < 0.01 0.53 0.42 0.65	0.07 < 0.05 0.01 < 0.01
		Los Humeros H-49	22/03/2018 13:55 68.0	3.59	0.33	4.0	593	94.9	16.2	1.08	8.66	676	259	370	46.3	2362 3838	38	103 < 0.01 0.16 0.46 0.48 < 0.05 < 0.05 < 0.01 < 0.01
		Los Humeros H-9	22/03/2018 15:05 66.8	5.33	1.15	7.0	1459 56.8	2.09	0.64	19.9	1182	178	122	8.0	2430 5E+05 105	22.9 < 0.01 0.77 0.69 0.65 < 0.05 < 0.05 < 0.01 < 0.01
		Los Humeros H-32	22/03/2018 15:35 67.8	0.14	0.15	9.3	1447 < 10	38.6	12.1	22.9	394	99.6	94.8	21.0	1925 54597	58	23.4 < 0.01 0.20 0.39 0.70 < 0.05 < 0.05 0.06 < 0.01
		Los Humeros H-55	22/03/2018 16:08 68.9	10.8	0.16	1.3	43.20 22.8	6.03	1.58	23.1	131	38.3	52.2	7.89	245	3379	22	9.93	0.10 < 0.1 0.47 0.33 < 0.05 < 0.05 0.10 < 0.01
		El Tesoro	23/03/2018 09:30 22.5 < 0.05 0.17	0.4	1.267 153	237	41.6 < 0.1	121	22.1 < 0.5 < 0.5 2.49	2.37	< 20	0.20 < 0.01 < 0.1 0.17 < 0.1 < 0.05 < 0.05 0.01	0.97
		Noria Nuevo Pizarro	23/03/2018 11:45 16.0 < 0.05 14.58	1.5	2.711 568	230	61.1	1.7	421	22.6 < 0.5 < 0.5 2.60	32.8	< 20	1.25 < 0.01 2.58 2.31 2.98	0.08	0.73	0.33	2.62
		Virgen del Carmen	23/03/2018 15:46 19.1	0.51 < 0.05 0.8	1.493 244	2488 91.3	132	108	13.3 < 0.5 < 0.5 1.61	3.05	< 20 < 0.05 < 0.01 0.40 1.45 0.26 < 0.05 < 0.05 0.65	0.03
		Pozo deTepeyahualco	24/03/2018 10:10 23.8	0.27 < 0.05 1.1	19.79 1139 3112 6110	311	1787 49.3	7.39 < 0.5 4.41	137	8118 0.14	0.01 < 0.1 2.24 1.93 < 0.05 0.78	0.28	0.25
		Laguna de San Luis Atexca	24/03/2018 14:15 22.0	0.27 < 0.05 0.5	65.56 5429	105	16.4	13.8	2667	104	21.9 < 0.5 6.50	93.8	< 20	0.32	0.01	0.31 0.22 0.76	0.05	0.07	0.12	0.17
		Laguna de Alchichica	23/03/2018 16:45 18.5	0.49 < 0.05 < 1	39.89 4822 31.9	14.2	5.03	2460	305	1.3	< 0.5 7.49	114	< 20	1.59	0.01	0.18 0.57 0.23 < 0.05 < 0.05 0.98	1.92
		Laguna de Quechulac	24/03/2018 16:30 20.4	0.11 < 0.05 0.5	0.561 158	89.6	21.6	8.22	2.87	7.61 < 0.5 < 0.5 5.06	3.48	< 20 < 0.05 < 0.01 1.33 0.23 < 0.1 < 0.05 < 0.05 0.42	0.92
	Acoculco area	Los Azufres 1	26/03/2018 15:05 25.4 14.04 1.13	0.2	253	52.7	1732 39.2	1219 38.6	66.8	2.39	3.99	40.1	135	42	0.45 < 0.01 < 0.1 1.12 0.17 < 0.05 < 0.05 0.02 < 0.01
		Los Azufres 2	26/03/2018 16:00 19.2 < 0.05 < 0.05 0.5	1.444 < 10	133	24.3	1160 6.47	19.3 < 0.5 < 0.5 12762 24.3	8284 < 0.05 < 0.01 0.18 105 5.81	0.18	5.31	0.04	0.01
		Los Azufres 3	28/03/2018 09:45 26.6	7.66	2.39	0.3	354	74.6	2500	114	2258 81.3	71.3	2.14	5.53	69.5	8421	70	1.09 < 0.01 1.01 1.37 2.43	0.09	0.9	0.07	0.08
		Jicolapa	27/03/2018 09:45 30.7	0.81	0.08	0.7	1.915 20.7	1242	311	179	95.3	58.3	5.99	1.07	6.08	1.37	70	< 0.05 < 0.01 0.24 0.78 0.18 < 0.05 < 0.05 1.19 < 0.01
		El Rincon	27/03/2018 11:00 26.6	0.49 < 0.05 0.5	0.124 33.3	424	159	120	13.2	33.8	1.81 < 0.5 6.54	13.3	2854 < 0.05 < 0.01 0.23 3.49 0.23 < 0.05 0.11	0.08 < 0.01
		Baños de Quetzalapa	27/03/2018 16:00 30.0	1.01 < 0.05 0.4	2.240 44.3	1070	726	132	138	31.3	11.9	2.6	1.20	1.25	< 20 < 0.05 < 0.01 0.14 0.23 < 0.1 < 0.05 < 0.05 0.15 < 0.01
		Baños de Chignahuapan	28/03/2018 07:30 49.0	0.57 < 0.05 0.8	3.076 182	691	147	27.2	374	63.9	63.1	0.98	25.8	24.1	311	0.14 < 0.01 0.17 0.89 0.18 < 0.05 < 0.05 0.03	0.05
														‰	‰	‰		‰	‰
		Los Humeros area Los Humeros H-39				22/03/2018 12:00 61.9 -63.8 -1.2	9.3
			Los Humeros Unit-11 (fluid mixing) 22/03/2018 12:50 52.7 -45.9 1.5	4.0			-2.50
			Los Humeros H-56				22/03/2018 13:25 74.1 -61.6 -1.1	1.2		7.2	-2.23 0.704310
			Los Humeros H-49				22/03/2018 13:55 68.0 -58.3 -0.3	0.9			-0.74
			Los Humeros H-9				22/03/2018 15:05 66.8 -53.1 1.2	3.4
			Los Humeros H-32				22/03/2018 15:35 67.8 -61.5 -0.9	4.8		2.3	-2.52 0.704283
		0	Los Humeros H-55 0,2			0,4	22/03/2018 16:08 68.9 -51.3 0.5 0,6 0,8	5.2	1
			El Tesoro						23/03/2018 09:30 22.5 -79.3 -11.1	3.7
			Noria Nuevo Pizarro			23/03/2018 11:45 16.0 -38.4 -5.1	4.3
			Virgen del Carmen				23/03/2018 15:46 19.1 -87.2 -11.9	3.4		12.4		0.707095
			Pozo de Tepeyahualco			24/03/2018 10:10 23.8 -73.7 -9.4			8.9	8.55 0.706862
			Laguna de Atexcac			24/03/2018 14:15 22.0 -23.4 0.0	11.8		10.1 6.49 0.706864
			Laguna de Alchichica			23/03/2018 16:45 18.5 -12.2 1.0	17.7
			Laguna de Quechulac			24/03/2018 16:30 20.4 -30.0 -1.4
		Acoculco area	Los Azufres 1					26/03/2018 15:05 25.4 -69.5 -8.1	4.7		44.8 -4.42 0.705065
			Los Azufres 2					26/03/2018 16:00 19.2 -62.1 -8.4					0.704778
			Los Azufres 3					28/03/2018 09:45 26.6 -29.8 0.1	5.6			-5.61 0.705045
			Jicolapa						27/03/2018 09:45 30.7 -67.5 -10.0	5.3		4.9	-6.79 0.707262
			El Rincon						27/03/2018 11:00 26.6 -68.3 -10.1			5.1		0.707114
			Baños de Quetzalapa			27/03/2018 16:00 30.0 -64.2 -9.1	6.5		5.5	-1.20 0.706804
			Baños de Chignahuapan		28/03/2018 07:30 49.0 -70.8 -10.1	6.0		4.7	-0.17 0.706272

Alchichica Atexcac Tepeyahualco Nuevo Pizarro Los Humeros wells Quechulac Los Azufres 1, 3 H-55 H-56 H-39 H-49 Unit-11 H-9 Chignahuapan El Rincon Virgen del Carmen Jicolapa Quetzalapa El Tesoro Cl/Br seawater mass ratio Los Humeros wells Los Humeros thermal springs Los Azufres thermal springs (Acoculco) Other thermal springs Lagunas (crater lakes) c) Processes of water-rock-gas interaction

  The high boron concentrations of these geothermal waters (from 43 to 1819 mg/l; fig.3.4.1.5) and their δ 11 B values(-2.52 to -0.74‰; fig. 3.4.1.6) are close to those previously determined by[START_REF] Bernard | Chloride and boron behaviour in fluids of Los Humeros geothermal field (Mexico): a model based on the existence of deep acid brine[END_REF], which range from 214 to 932 mg/l and from -1.7 to 0.3‰, respectively, and by[START_REF] Tello | Estado de equilibrio Soluto-Mineral y saturación de Minerales de alteración en fluidos geotérmicos de alta temperature[END_REF] and[START_REF] Gomez | Thermodynamic evolution of the Los azufres, México, geothermal reservoir from 1982 to 2002[END_REF], which vary from 118 to 3168 mg/l.

Quetzalapa Los Humeros wells Los Humeros thermal springs Los Azufres thermal springs (Acoculco) Other thermal springs Lagunas (crater lakes)

	Tepeyahualco
	Atexcac
	H-2			Los Humeros wells
	Chignahuapan H-1
	H-56	H-6	H-49	H-3	H-32	Unit-11
	Los Azufres 1
	Jicolapa

Los Humeros wells (Bernard et al., 2011)

		2000							Los Humeros wells
				Unit-11				Los Humeros thermal springs
									Los Azufres thermal springs (Acoculco)
				Los Humeros			Other thermal springs
		1500	H-9	wells			Lagunas (crater lakes)
				H-32
	B (mg/l)	1000		H-39 H-32 H-3
				H-49
		500	Los Azufres 3
		H-2		H-6
	H-55 Los Azufres 1	Nuevo Pizarro H-56 H-1	Tepeyahualco			Alchichica	Atexcac
	0 El Rincon	0		500	1000	1500	2000	2500	3000	3500	4000
		Quechulac	Chignahuapan		Cl (mg/l)
					Los Azufres 3

Los Humeros wells (Bernard et al., 2011)

Table 3

 3 The low Sr concentrations and the 87 Sr/ 86 Sr ratios of these geothermal waters, ranging from 1.52 to 41.2 µg/l (fig.3.4.1.8) and 0.704283 to 0.704310 (fig.3.4.1.9), respectively, suggest these waters are in contact with andesite rocks in the reservoir, which is in good agreement with the well observations[START_REF] Gomez | -An updated conceptual model of the Los Humeros geotermal reservoir (México)[END_REF]. Lower Sr isotopic values between 0.7032 and 0.7036 (FP6-HITI project, 2010) are observed for the high-temperature geothermal fluids from North-Iceland (Krafla, Nesjavellir, etc.) in contact with basalts in the reservoir (tabl. 3.4.1.1).The high Li concentrations and the δ 7 Li values of the Los Humeros geothermal waters, varying between 131 and 1182 mg/l, and between 2.3 ant7‰ (tabl. 3.3.2.2 et 3.3.2.3), respectively, confirm that these waters interacts with volcanic rocks at high-temperatures. If the average Li concentrations are slightly higher than those observed in the high-temperature geothermal

.4.1.1 -Comparison of water chemical and isotopic data obtained in three high-temperature (≥ 290°C) geothermal fields: Los Humeros in

Mexico, and Krafla and Nesjavellir in North-Iceland.

waters from the North-Iceland basaltic reservoirs (from 119 to 745 µg/l), the isotopic values are close (from 6.5 to 8.1‰;

FP6-HITI project, 2014; tabl. 3.4.1.1)

. Note that the Cs concentrations analysed in the Los Humeros geothermal waters (from 52 to 705 µg/l) are much higher than those observed in the North-Icelandic geothermal dilute waters (from 2 to 12 µg/l) whereas the Rb concentrations are closer

(FP6-HITI project, 2010; tabl. 3.4.1.1)

.

Los Humeros

field, Mexico (this study) Nesjavellir and Krafla fields, Iceland (FP6-HITI project; Sanjuan et al., 2014)

	Well		Unit-11 (fluid mixing)	H-39	H-49	H-56	NJ-16	NJ-10	NJ-14	NJ-16	NJ-19	KS-01	K-05	K-27	K-37
	Parameters	Unit	22/03/2018	22/03/2018 22/03/2018 22/03/2018 11/06/2008 11/06/2008 11/06/2008 11/06/2008 11/06/2008 13/06/2008 13/06/2008 13/06/2008 12/06/2008
	Cond. 25°C	µS/cm	1294	591	1051	1196	860	872	965	860	783	930	820	1087
	pH		7.00	7.14	7.70	7.58	8.42	9.03	8.79	8.42	8.11	9.36	9.21	8.72
	Eh	mV	-240	-260	-270	-346	-103	-31	-341	-103	-44	-305	-239	-284
	Na	mg/l	270	119	212	244	145	157	161	145	126	175	180	206	268
	K	mg/l	36.9	23.8	35.8	38.4	29.0	33.4	32.8	29.0	31.4	36.4	17.8	32.1	48.8
	Ca	mg/l	2.2	< 0.5	1.1	1.0	< 0.5	< 0.5	< 0.5	< 0.5	< 0.5	0.34	8.83	3.13	1.46
	Mg	mg/l	< 0.5	< 0.5	< 0.5	< 0.5	11.7	< 0.5	< 0.5	11.7	< 0.5	< 0.5	< 0.5	< 0.5	< 0.5
	Alk.	mg/l HCO3	307	268	305	383	296	257	159	296	128	400	162	237
	Cl	mg/l	98.8	46.5	129	147	69.7	129	193	69.7	162	76.1	45.0	39.2	64.7
	SO4	mg/l	227	2.3	86.1	63.6	90.5	81.6	44.0	90.5	35.9	40.6	248	296	130
	SiO2	mg/l	733	745	834	931	697	752	718	697	797	1001	361	538	1333
	TDS	g/l	1.68	1.20	1.60	1.81	1.34	1.41	1.31	1.34	1.28	1.73	1.02	1.35	1.85
	F	mg/l	20.6	13.4	4.0	4.2	1.25	1.57	1.11	1.25	1.13	1.68	0.89	0.98	2.72
	B	mg/l	1819	951	593	256	1.82	1.52	1.76	1.82	3.77	2.68	0.54	0.56	2.38
	Br	µg/l	73.7	32.0	94.9	90.9	200	500	700	200	600	200	< 500	< 500	< 500
	Sr	µg/l	41.2	1.52	16.2	11.0	3.0	2.5	2.3	3.0	2.0	2.1	29.3	15.5	14.1
	Ba	µg/l	5.92	0.71	1.08	1.06	2.5	1.0	0.8	2.5	1.0	0.1	1.5	5.2	3.0
	Mn	µg/l	25.9	7.05	8.66	18.7	0.6	1.0	0.6	0.6	0.5		0.9	1.0	23.8
	Li	µg/l	619	397	676	871	157	290	287	157	263	254	119	186	745
	Rb	µg/l	315	240	259	392	96	140	198	96	108	209	138	167	273
	Cs	µg/l	330	305	370	705	2.4	6.5	7.5	2.4	4.2	6.7	3.7	5.9	12.1
	Ge	µg/l	56.0	60.1	46.3	47.6	59.8	60.6	39.6	59.8	42.7	39	34.6	34.2	49.0
	As	µg/l	21721	42323	3838	8077	8.5	5.6	9.7	8.5	22	384	2.1	2.8	202
	Zn	µg/l	0.32	1.96	0.46	0.42	1.1	0.6	0.8	1.1	1.1	0.6			4.2
	Ni	µg/l	0.65	0.35	0.48	0.65	0.30	0.30	0.20	0.30	0.40	0.10	0.30	0.20	0.70
	B/Cl	molal ratio	60	67	15	6	0.09	0.04	0.03	0.09	0.08	0.12	0.04	0.05	0.12
	Na/K	molal ratio	12.5	8.5	10.1	10.8	8.5	8.0	8.4	8.5	6.8	8.2	17.2	10.9	9.3
	Na/Li	molal ratio	132	90	95	84	279	163	170	279	145	208	458	334	108
	Na/Rb	molal ratio	3190	1836	3047	2310	5620	4156	3029	5620	4343	3120	4860	4580	3647
	Na/Cs	molal ratio	4735	2246	3317	1998	349560	139190	124346	349560	173665	151343	281880	201592	127947
	K/Sr	molal ratio	2007	35086	4952	7822	21635	29940	31969	21635	35129	38842	1363	4642	7752
	δD	‰	-45.9	-63.8	-58.3	-61.6	-71.1	-73.1	-69.9	-71.1	-68.3	-102.2		-79.9	-58.9
	δ 18 O	‰	1.5	-1.2	-0.3	-1.1	-5.4	-5.7	-5.8	-5.4	-3.9	-6.3		-10.5	-7.3
	δ 18 OSO4	‰	4.0	9.3	0.9	1.2	-0.1	3.6		-0.1				-6.2	-5.9
	δ 7 Li	‰				7.2	8.1	7.8	8.0	8.1	6.8	6.8	8.1	6.5	7.1
	δ 11 B	‰	-2.50		-0.74	-2.23	-2.68	-2.19	-3.37	-2.68	-2.84	-5.45		-3.98
	87 Sr/ 86 Sr					0.704310 0.7036201 0.7034461 0.7035571 0.7036201 0.7035351 0.7034731 0.7032241 0.7032021

Table 3

 3

	Table 3.4.1.3 -Classical isotope geothermometers applied on waters
	from the Los Humeros and Los Azufres geothermal areas.

.4.1.2 -Classical chemical geothermometers applied on waters from the Los Humeros and Los Azufres geothermal areas.

Table 3 .

 3 4.1.4 -Auxiliary chemical geothermometers applied on waters from the Los Humeros and Los Azufres geothermal areas.

Los Humeros wells Los Humeros thermal springs Los Azufres thermal springs (Acoculco) Other thermal springs Lagunas (crater lakes)

	Log (H 4 ) 4 SiO	200°C 150°C	150°C	Tepeyahualco Quechulac Chignahuapan 50°C 25°C 25°C Atexcac Los Azufres 1, 3 50°C El Rincon Nuevo Pizarro 100°C Virgen del Carmen Jicolapa Quetzalapa El Tesoro Los Azufres 2 100°C
					Alchichica
	00	1.00	2.00	3.00	4.00	5.00	6.00
					Log (Mg/K 2 )

Acknowledgments: We want to thank the CFE staff from Los Humeros and from Morelia for permission and recommandations to sample their wells in the Los Humeros geothermal field. We also are particularly grateful to Oscar López Romero from CFE for his help in collecting fluid samples in the field. We would like to express our gratitude to Rafael Alfaro from CFE for his help in collecting waters from thermal springs located in the Los Humeros area and to Ernesto Bahena Palomares, who was our driver and guide during all our field campaign. Finally, we are very grateful to Aída López Hernández, scientific coordinator of the GEMEX project, for her constant and valuable support. 6. Dissemination activities BRGM participated to three GEMEX meetings, which occurred in Utretch (Netherlands), in March 2017 (F. Gal), in Akureyry (Iceland), in October 2017 (B. Sanjuan), and in Morelia (Mexico), in October 2018. It has also contributed to a GEMEX e-News document for WP4 -Task 4.3, in 2018.

All the geochemical data obtained during this study have been uploaded and stored in the GEMEX Open Access Database (OADB), as well as the required information, by Eugenio Trumpy from CNR, responsible for maintaining this database.