Geochemical modeling of the hydrothermal alteration in Los Humeros geothermal field, Mexico

Abstract

The equilibrium state between present day fluids and hydrothermal mineral phases was examined at Los Humeros geothermal field. Hydrothermal minerals in Los Humeros are typical for neutral to alkaline fluids, presenting, in most areas of the field, a low intensity alteration. Geothermal fluids are sodium-chloride waters, with very high steam fractions in most wells. Fluids coming from wells H-1, H-6, H-7, and H-12 showed oversaturation for quartz, minnesotaite, alunite, kaolinite, muscovite, talc, pyrite, and clino-chlore. Important hydrothermal minerals observed in the field (such as calcite, hematite, and epidote) are undersaturated or absent in our calculations. Consequently, no chemical equilibrium exists between hydrothermal alteration and present hydrothermal fluids. Hydrothermal minerals in Los Humeros may have precipitated from a sodium-chloride geothermal fluid, with a higher chemical concentration than present fluids. Acid fluids in the field seems to be of recent occurrence, as no mineralogical evidence for acid fluids have been yet observed.