Combining ammonium mapping and short-wave infrared (SWIR) reflectance spectroscopy to constrain a model of hydrothermal alteration for the Acoculco geothermal zone, Eastern Mexico

Abstract

The Acoculco geothermal system is hosted by a caldera complex located at the eastern portion of the Trans-Mexican Volcanic Belt. Surface manifestations are scarce and consist of low temperature, bubbling, acid–sulfate springs that are concentrated in two zones separated from each other by ~1750 m. In the northernmost one, there are conspicuous features suggesting recent, explosive, hydrothermal activity. Most of the rocks that crop out are tuffs and breccias that show pervasive hydrothermal alteration. Six SWIR-active minerals have been revealed by reflectance spectroscopy in the surface altered rocks: Opal, kaolinite, alunite, ammoniojarosite, buddingtonite and interstratified illite–smectite; they are indicative of alteration assemblages comparable to those reported in other geothermal systems and in epithermal deposits. Opal is the most widespread alteration mineral and occurs in association with tridimite and anatase. Kaolinite is also widely distributed but has a greater presence in the area with active surface manifestations, which, moreover, is the only area with alunite and ammoniojarosite occurrences. Buddingtonite occurs in the same zones as sulfates. As a complement to the alteration maps, we are proposing a simple method for mapping ammonium anomalies, based on total N analyses in altered rocks. Total N data show a discontinuous distribution within a wide range of variation (0.01–3.28 wt.%). All medium and high N values (>0.26 wt.%) plot in three clusters, two of which coincide with zones of gas emission. A deep circulating, active hydrothermal system cannot be ruled out in Acoculco although it may be confined to the deep caldera structure by an impermeable cap-rock of silicic and argillic alteration. In this scenario, hydrothermal fluids would reach the surface periodically, when explosive hydrothermal fracturing allowed it. The occurrence of hydrothermal explosion features and of acid–sulfate alteration supports this scenario. Therefore, further exploration by drilling would be advisable in Acoculco.