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A comprehensive study on the chemical considerations of thermal waters (springs and geothermal wells) on the performance of solute geothermometers in predicting the reservoir temperatures of Cerro Prieto and los Humeros geothermal fields of Mexico has been carried out. The reservoir temperatures from these thermal waters were calculated by applying all the available 28 solute geothermometers and the obtained temperatures were compared with the bottom hole temperatures (BHT) of the geothermal wells. Thermal waters of springs and geothermal wells of Cerro Prieto geothermal field (CPGF) are Cl type and majority of them are partially equilibrated. In the case of Los Humeros geothermal field (LHGF), spring waters are HCO 3 type and all of them have indicated non-equilibrium conditions. Majority of the predicted reservoir temperatures by Na/K geothermometers from spring waters of CPGF and LHGF are within ±20 % differences compared to average BHTs, whereas estimated temperatures by all other solute geothermometers (non-Na/K) from spring waters of both the geothermal fields have shown wide differences (> ±20 %). In the case of geothermal well waters, majority of the reservoir temperatures estimated by only Na/K geothermometers from well waters of CPGF are within ±20 % differences and in the case of wells of LHGF, majority of the reservoir temperatures estimates by all solute geothermometers are having differences > ±20 %. The present study indicates the following: (1) only 3 geothermometers (out of 28) from spring waters of CPGF have predicted reservoir temperatures similar (statistically no significant differences) to the average of BHTs, although all springs are of Cl type; (2) different chemical characteristics of spring and well waters of CPGF and LHGF may be the result of the wide differences in the lithology of their reservoirs; (3) more number of solute geothermometers from non-equilibrated spring waters of CPGF and LHGF have predicted reliable reservoir temperatures than partially equilibrated waters; (4) In general, Na/K geothermometers have predicted more reliable reservoir temperatures from spring and well waters of both geothermal fields, when compared to other solute geothermometers; (5) the prediction of non-reliable reservoir temperatures by other than Na/K geothermometers may be the result of dilution/ mixing of deep thermal waters with the meteoric surface waters; and (6) more number of geothermometers have predicted reliable reservoir temperatures from well waters in comparison to spring waters of both the studied geothermal fields.

Most of the volcanic rocks in the Acoculco geothermal system (AGS), near to the surface manifestations, are intensively altered. Thirty‐four surface volcanic rocks and five rocks from outcrop sections are considered in this work. Silica polymorphs (quartz, cristobalite and tridymite) are the dominant hydrothermal minerals in almost all the rock samples. The following other minerals are present in minor concentrations: alkali‐feldspars (sanidine potassic, anorthoclase and microcline), plagioclase feldspars (albite and anorthite), clay minerals (illite, kaolinite and montmorillonite), anhydrite, buddingtonite, titanoaugite, magnetite and anatase. This type of mineral assemblage of the rocks indicates an intensive silicic and argillic alteration. Comparison of the elemental concentrations of the surface acid rocks of the AGS with that of the precursor rock has indicated alteration‐induced changes in (a) chemical mobility in the concentrations of major element oxides, with an increase in the concentrations of SiO2, and TiO2, loss in those of Al2O3, Fe2O3, MnO, MgO, CaO, Na2O and K2O, and no statistically significant changes in those of P2O5, (b) no significant changes in the concentrations of LREE and MREE (except Dy and Ho), however, less but significant loss in the concentrations of Dy and Ho, and HREE, and (c) there is an enrichment in the concentrations of Nd, Sr, Sc, V, Nb, Ag, Sn, Pb, Tl, Ta, U and Th; and depletion in Be, Co, Zn, Ga, Ge, Rb, Y, Zr, Mo, Cs, Ba and Hf compared to those of the precursor rock. This type of distribution of the elements may indicate (a) an intense silicic alteration; and (b) REEs might have been leached from the original rocks because of hydrothermal alteration processes, but only LREEs, and MREEs (except Dy and Ho) can be redistributed and accommodated in the hydrothermal minerals, providing them with somewhat inert (immobile) behaviour. The mass change calculations have indicated that MnO and Sr have experienced the greatest and smallest mass changes, respectively. The identification of outliers in chemical data has revealed the presence of two distinctive groups of rocks. Ten out of the total 27 acid volcanic rocks with several elements as outlier data are negligibly to moderately altered (Group‐1), and the remaining 17 acid rocks without outlier data are intensively altered rocks (Group‐2).

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