STRUCTURE AND ORIGIN OF THE IXTACAMAXTITLÁN HYDROTHERMAL DEPOSIT, PUEBLA STATE, MEXICO

The Ixtacamaxtitlán municipality is located north of the Puebla State in Central Mexico. The local geology comprises (1) Lower to Middle Cretaceous limestone formations of the Sierra Madre Oriental, and (2) igneous intrusive and extrusive rocks of Tertiary to recent age, of the Sierra Madre Occidental and the Trans-Mexican Volcanic Belt (TMVB). The hydrothermal deposits of the area are hosted by pyroclastic deposits and the plutonic to sub-volcanic rocks that intruded them.

The Ixtacamaxtitlán hydrothermal deposit is made up by a succession, from bottom to top, of: (1) quartz veins and stockwork, with subordinate sulphides (pyrite and chalcopirite) enclosed in a porphyritic subvolcanic body with propylitic alteration haloes and a former potassic alteration event (biotitization) preserved; (2) an overlying, kaolinitized rhyolitic tuff; and (3) a layered opal deposit with preserved mud cracks and bioturbation. This vertical arrangement coupled with the distribution of the alteration assemblages lead to the interpretation of the whole as a low-sulfidation epithermal deposit, with a partially preserved silica sinter on top.

Both the fluid inclusion study carried on the veins and the stockwork and the stable isotopic analyses of the kaolinitized bodies suggest the presence of two major hydrothermal events. An early event, characterized by hot, hypersaline fluids (up to 280ºC and 36 wt% NaCl eq.) closely associated with the potassic alteration episode, with primary fluid recrystallized, showing hook-shaped morphologies, due to an uplifting period that occurred before the emplacement of the second hydrothermal stage. The late event is distinguished by cooler and dilute fluids (up to 150ºC and 4 wt% NaCl eq.) associated with propylitic and quartz-sericite alterations at depth, and acid-sulfate alteration close to the paleosurface. The calculated isotopic composition of water in equilibrium with the kaolinitic sequence, plot close and underneath to the meteoric water line, partially overlapping the Los Humeros geothermal fluids. These evidences coupled with the petrographic observations suggest that steam-heated phreatic waters altered the rhyolitic tuffs. This would have occurred when acid vapors exsolved from deeper hydrothermal fluids by boiling reached the local paleowater table and condensed.