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

摘要

The objective of this study was preliminary reconnaissance of the Atotonilco el Alto-Santa Rita geothermal system in the northeastern Chapala graben in the state of Jalisco, Mexico. This was accomplished by evaluating the hydrogeochemical characteristics of the aquifer and the geological context. The study area is located in the western sector of the Neogene-Quaternary continental arc of the Trans-Mexican Volcanic Belt (TMVB). Hydrothermal deposits in the area consist of crusts, veinlets, stockwork, and replacements. Deposits and hydrothermal alterations are 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 a neutral to basic pH (7-8.6) with high electrical conductivity (EC) and total dissolved solids (TDSs) at 216-870 mu 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 mu S/cm and 277-623 mg/L, respectively), indicating a greater concentration of ions in solution. According to the water chemistry of the system, three water groups are present: shallow water with a predominant meteoric component, a mixture of different types of water, and thermal water. These water groups form a single system containing distinct hydrochemical facies. The Chebotarev sequence provides evidence of a groundwater evolution process where water passes through the lithological profile of the zone (alluvial deposits, conglomerates, and volcanic rock). Processes taking place within the aquifer include ion exchange, silicate weathering, mixing. An anomalous delta O-18 enrichment characteristic of the geothermal fluids is present in some thermal spring and well waters. The mineralogical results suggest the presence of a hydrothermal system with semi-neutral to alkaline characteristics of thermal fluids. The reservoir temperature was estimated using chemical geothermometry, which indicated that the hydrothermal system can be categorized as an intermediate-temperature system (between 80 degrees and 160 degrees C). The Atotonilco el Alto-Santa Rita hydrothermal system has characteristics of a non-magmatic convective system of extensional domain, this extensional domain is due to the different rollback rates of the Cocos and Rivera plates, provoked by the northeastward directed toroidal mantle flow through the gap between the Rivera and Cocos slabs. The distribution of thermal 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 likely originates in the thinning of the Earth's crust due to extensional tectonics.