Facies variations and permeability of ignimbrites in active geothermal systems; case study of the Xaltipan ignimbrite at Los Humeros Volcanic Complex

Ignimbrites' facies are key for interpreting the dynamics of explosive eruptions. Furthermore, they provide evidence of the physicochemical processes that acted during and after their deposition. Nevertheless, ignimbrites commonly present complex facies associations that require an integrative analytical methodology over a robust stratigraphic framework for their proper characterization. This work presents a facies characterization of the caldera-forming X ' altipan ignimbrite at Los Humeros Volcanic Complex, by integrating a combination of petrophysical and geochemical analyses aimed at its outflows (exposed) and intracaldera (buried) deposits. Based on the petrophysical properties, a three-facies association is identified controlled by welding and secondary mineralization effects: (1) The basal facies, characterized in the outflows by highly welded deposits (ranks V-VI), with low porosity (similar to 4-8%), and practically impermeable conditions (<0.01-0.05 mD). In the intracaldera deposits, this facies shows pervasive geothermal-related propylitic mineralization that can reduce porosity up to ca. 80%. (2) A middle facies, with moderately welded deposits (ranks III-IV) and porous conditions consisting of a combination of intershard, vesicular, and fracture spaces, partially filled by secondary minerals, which reduce the porosity (6-11.5% in the outflows; 16-28% for the intracaldera), resulting in semi-permeable conditions (6.11-46.38 mD). (3) The upper facies are defined by being not to poorly welded deposits (ranks I-II), and characterized by being highly porous (dominated by intershard spaces) and permeable (310-1557 mD), and barely affected by secondary mineralization. The geochemical results (major and trace element) indicate that the X ' altipan ignimbrite shows a vertical compositional zonation, from rhyolitic (70.9-76.2 wt% SiO2) in the lower flow unit to rhyolitic-trachydacitic (67-70.7 wt% SiO2) composition in its upper flow unit. This last unit shows a geochemical affinity with the subsequent pyroclastic deposits. Geochemical analyses of intercalated lavas from well logs also provided evidence of unprecedented breccias formed during the caldera collapse, discarding the existence of previously assumed thick lava flows. Facies correlation, among other well logs, shows that the facies distribution is directly controlled by the ignimbrite thickness. Where the ignimbrite is > 400 m thick, the entire facies association is observed, while when the thickness is < 400 m, the upper and middle facies occurs preferentially. This facies analysis may contribute to the overall model of Los Humeros subsurface stratigraphy, by defining the Xaltipan ignimbrite distribution and its porosity-permeability-based zones, which are of particular interest for geothermal purposes. Finally, this work provides the first example of a caldera-forming ignimbrite that presents analog facies associations between its outflows and intracaldera deposits that may be applied for other ignimbrites worldwide.