Petrographic and mineral analysis applied to the evolution of the Acoculco volcanic geothermal system in Trans-Mexican Volcanic Belt

In this contribution we present the results of a work developed in Acoculco geothermal system (currently classified as a HDR system), located at the eastern part of TMVB, in central México. We used petrographic combined with geochemistry and bulk mineral analysis on surface and core samples to characterize the alteration paragenesis in Acoculco Caldera. Hydrothermal events overprint the protolith as sharp changes in mineralogy. Inside the caldera, the alteration is widely distributed, and from petrographic analysis we propose three main events. The first one was liquid-rich and produced silicic alteration near conducts and argillic further away. The second event was more violent, forming a great number of fracture samples filled with silica polymorphs and to a lesser extent hydrothermal breccias. The last alteration stage derived from steam heated, acid-sulphate waters. Hydrothermal fluid was probably become more acidic as it reached the surface, possibly due to diverse boiling events.