The Ixtacamaxtitlán kaolinite deposit and sinter (Puebla State, Mexico): a magmatic-hydrothermal system telescoped by a shallow paleoaquifer

The Ixtacamaxtitlán hydrothermal deposit is made up of a succession, from bottom to top, of: (1) a porphyritic subvolcanic body, crosscut by quartz veins, and a stockwork with subordinate sulfides (pyrite and chalcopyrite), showing propylitic alteration haloes overprinting a previous potassic alteration event (biotitization); (2) an overlying, kaolinized lithic‐rich rhyolitic tuff; and (3) a layered opal deposit with preserved sedimentary structures. This vertical zonation, coupled with the distribution of the alteration assemblages, lead us to the interpretation of the whole as a porphyry‐type deposit grading upwards to a barren, steam‐heated, acid‐leached, kaolinite blanket with a partially preserved silica sinter on top. Both the fluid inclusion study carried out on the veins and stockwork, and the stable isotopic analyses of the kaolinized bodies, suggest the presence of two major hydrothermal events. The older event is characterized by the occurrence of hot hypersaline fluids (up to 320°C and 36 wt% NaCl equivalents), likely of magmatic origin, closely associated with the emplacement of the underlying early Miocene porphyry‐type deposit. The later event is characterized by the presence of cooler and dilute fluids (up to 140°C and 4 wt% NaCl equivalents) and by advanced argillic alteration close to the paleosurface. The calculated isotopic composition of water in equilibrium with the kaolinitic sequence plots close to and underneath the meteoric water line, partially overlapping the Los Humeros present‐day geothermal fluids. This evidence coupled with the petrographic observations suggests that steam‐heated phreatic waters altered the lithic‐rich rhyolitic tuff. This would have occurred when acid vapors, exsolved from deeper hydrothermal fluids by boiling, reached the local paleowater table and condensed, after a sector collapse that changed the system from lithostatic to hydrostatic conditions.