Hydrothermal alteration of the surface volcanic rocks at the Acoculco geothermal field, Mexico: a multi-parametric approach

The studies on hydrothermal alteration-induced effects in surface and subsurface rocks provide useful information in the characterization and exploitation of a geothermal reservoir. Generally, these studies are based on traditional, and reliable methods like petrography (primary and secondary minerals, and grade of alteration), and geochemistry (mobility of elements, changes in mass and concentration of elements, and fluid inclusions). Recently, apart from these established methods, some methods based on the geochemical (Chemical Index of Alteration, CIA; Weathering Index of Parkar, WIP; Loss on Ignition, LOI; and Sulfur, S) and rock magnetic properties (magnetic susceptibility, χlf; and percentage frequency-dependent susceptibility, χfd%) are also being applied in the identification of whether a rock is an altered or a fresh one. The Acoculco Geothermal Field (AGF), Mexico, is characterized by high temperature and very low permeability, and it is considered a promissory Enhanced Geothermal System. The following changes are observed in the rocks as a result of an increase in hydrothermal alteration: (1) an increase in CIA, LOI, and S values, and a decrease in WIP; (2) an increase in quartz and quartz polymorph minerals (silicification), and clay minerals (argillization); and (3) decrease in χlf values. At AGF, the most altered surface acid rocks are characterized by entirely quartz and its polymorphs, and clay minerals. The present study also indicates the applicability of the binary plots of major elements (felsic vs mafic component) and rock magnetic parameters (χlf vs. χfd%). The rock with χfd% value of 2–10 and χlf value < 0.5 × 10^–6 m³ kg⁻¹ indicate the presence of single domain and stable single domain grains, which in turn suggests that it is an altered rock. These methods are simple to apply, rapid, reliable, and have the potential to become effective tools for the identification of hydrothermally altered rocks during the initial stage of geothermal exploration.