ARTÍCULO

Hydrogeochemical reconnaissance of the Atotonilco el Alto-Santa Rita geothermal system in the northeastern Chapala graben in, Mexico

Pérez-Martínez I., Villanueva-Estrada R.E., Cardona-Benavides, A., Rodríguez-Díaz, A.A., Rodríguez-Salazar M.T., Guadalupe-Rojas J.
Geothermics,83, 101733. ISSN: 0375-6505. https://doi.org/10.1016/j.geothermics.2019.101733, 2020.

ABSTRACT:

The objective of this study was preliminary reconnaissance of the Atotonilco el Alto-Santa Ritageothermal system in the northeastern Chapala graben in the state of Jalisco, Mexico. This wasaccomplished by evaluating the hydrogeochemical characteristics of the aquifer and thegeological context. The study area is located in the western sector of the Neogene-Quaternarycontinental arc of the Trans-Mexican Volcanic Belt (TMVB). Hydrothermal deposits in the areaconsist of crusts, veinlets, stockwork, and replacements. Deposits and hydrothermal alterationsare associated with minerals of montmorillonite, zeolites, opal, quartz, calcite, pyrite,romanechite, nontronite, and celadonite. Thirty water samples were collected from wells,springs, and a spa containing thermal- and cold-water discharges. The thermal water had aneutral to basic pH (7–8.6) with high electrical conductivity (EC) and total dissolved solids(TDSs) at 216–870 ?S/cm and 142–575 mg/L. The cold water had a slightly acid to neutral pH (6–7) and higher ranges of EC and TDS than the thermal water (419–943 ?S/cm and 277–623 mg/L,respectively), indicating a greater concentration of ions in solution. According to the waterchemistry of the system, three water groups are present: shallow water with a predominantmeteoric component, a mixture of diff erent types of water, and thermal water. These watergroups form a single system containing distinct hydrochemical facies. The Chebotarevsequence provides evidence of a groundwater evolution process where water passes through thelithological profi le of the zone (alluvial deposits, conglomerates, and volcanic rock). Processestaking place within the aquifer include ion exchange, silicate weathering, mixing. Ananomalous ? O enrichment characteristic of the geothermal fl uids is present in some thermalspring and well waters. The mineralogical results suggest the presence of a hydrothermalsystem with semi-neutral to alkaline characteristics of thermal fl uids. The reservoirtemperature was estimated using chemical geothermometry, which indicated that thehydrothermal system can be categorized as an intermediate-temperature system (between 80°and 160 °C). The Atotonilco el Alto-Santa Rita hydrothermal system has characteristics of anon-magmatic convective system of extensional domain, this extensional domain is due to thediff erent rollback rates of the Cocos and Rivera plates, provoked by the northeastward directedtoroidal mantle fl ow through the gap between the Rivera and Cocos slabs. The distribution ofthermal manifestations is controlled by a NW SE system of faults and the crossing of ENE–WSW and E–W faults in which the Chapala graben originates. Finally, the heat source likelyoriginates in the thinning of the Earth’s crust due to extensional tectonics.