Mercury anomalies and transport mechanisms in a hydrothermal system associated with normal faults in Araró, Michoacán (western Mexico)

Abstract

In this paper, we infer a mechanism for Hg transport and mobility in the Araró-Simirao geothermal system, which is associated with a normal fault system of general NE-SW orientation, through the hydrogeochemical modeling of Hg in water and soils and their relationship with the Hg concentration in the air. A sampling of thermal water and soils was carried out, and it was compared with gaseous elemental Hg emissions. The predominant Hg species in water are Hg⁰_(aq), HgCl₂ and HgCl₃^-, although the Hg⁰_(aq) abundance is lower with respect to Hg²⁺. Hydrothermal alteration minerals such as barite and particles with Hg contents were found in the soil. In general, are inferred the geochemical processes involved in the Hg transport between the environmental compartments, water, soil, and atmosphere are volatilization, oxidation, complexation, and deposition. The deep hydrothermal system favors the transport of elemental gaseous mercury, of probable magmatic origin, to the surface through the areas with the highest permeability, such as the Araró-Simirao fault. In its ascent, the hydrothermal fluids interact with the subsurface aquifer, enriching the aquifer in Hg and forming chlorinated complexes that are transported in the aqueous medium. When thermal water enriched in Hg is discharged to the surface, the mercury volatilizes as Hg⁰_(g) and remains in the atmosphere or can be deposited on the soils surrounding the springs, adhering to particles of organic matter or clays, and forming aureoles of Hg anomalies on the surface. According to the spatial distribution of the anomalies, the area closest to geothermal fluids (vertical rise of gases) is the southern part of the system.