Hydrothermal alteration of surface and subsurface volcanic rocks at the promissory geothermal area of Los Negritos, Michoacán (México)

Abstract The information on the hydrothermal alteration of surface and subsurface rocks in identifying the zones of alteration is helpful during the initial stage of geothermal exploration. The study involves mineralogical, petrological, fluid inclusion, and rock magnetic susceptibility parameters of surface rocks and rock cuttings from well EN-1 in the Los Negritos geothermal field, México. The mineral composition of the surface rocks shows that the andesite and basalt are the least altered (with anorthite as the dominant mineral, followed by traces of cristobalite), whereas the rhyolitic tuff is altered (with quartz as the dominant mineral, followed by albite). The observed variation in mineralogy and magnetic susceptibility values within a lithounit may be assigned to hydrothermal alteration. However, a combination of lithological variations and hydrothermal alteration may result from changes in the boundaries of lithounits. Moreover, fluid inclusion data, hydrothermal mineral content, and Na/K and silica geothermometry indicate a temperature inversion at depths of 700–1200 m, which may be attributed to the mixing of deep thermal fluids with shallow meteoric water. However, further down (> 2000 m), an increase in homogenization temperature and salinity, as well as the presence of high-temperature mineral epidote, may imply proximity to a zone characterized by high-enthalpy fluids. Thus, these studies provide valuable insights into the thermal history and hydrothermal alteration processes.