Evidencias de evolución de un fluido básico a ácido a partir del análisis de la alteración hidrotermal del campo geotérmico de Los Azufres, Michoacán

Abstract

Hydrothermal alteration at the Los Azufres geothermal field is mostly composed of calc-silicate minerals that define a propylitic type alteration zone, which shows progressive dehydration with depth and temperature increase. A generalized zoning of the calc-silicate zone zone can be observed, with zeolites in the upper part and epidote-clinozoisite at the deepest levels. An argillic alteration zone overlies the calc-silicate zone and is the dominant surface manifestation of the hydrothermal alteration. In some parts, there is a mineral assemblage composed of kaolinite-alunite-native sulfur-quartz (advanced argillic zone) formed by the interaction of vapor and shallow groudwater.
The proto-fluid at the Los Azufres geothermal system is related to a neutral sodium chlorine brine, which favors deep propyllitic alteration (productive zone). This zone is characterized by secondary permeability due to fracturing. At depth the geothermal field is dominated by a pressurized liquid, yielding to vapor at more shallow zones. The gradual change from a liquid to a vapor phase occurs through boiling at depths between 1,200 and 1,500 m, and is accompanied by changes in the hydrothermal alteration mineralogy. The type of alteration passes from propylitic to argillic by means of an oxidation-acidification process, which includes the participation of a gas, particularly CO2.
Considering the physicochemical characteristics of the brine and the evolution of the paragenetic sequence, the Los Azufres geothermal field could de considered a model for hidrothermal behavior at ore deposits which develop by boliling and oxidation of low sulfidation fossil hydrothermal fluids.