Mineralogical and geochemical changes due to hydrothermal alteration of the volcanic rocks at Acoculco geothermal system, Mexico

The Acoculco Caldera Complex (ACC), located in eastern Mexico, began its activity during the Pleistocene ~ 2.7 Ma. One of the most relevant and largest rhyolitic eruption in the complex, the Piedras Encimadas Ignimbrite (PEI), occurred during the late post-caldera phase at ~ 1.2 Ma. This ignimbrite is unique with respect to the other caldera products and other contemporaneous ignimbrites in the Trans-Mexican Volcanic Belt (TMVB) because of its ultra-high crystallinity and the absence of pumice fragments. The PEI is made almost entirely of crystals where the main constituents are k-feldspars and silica polymorphs that range from ≤ 5 µm to tens of centimeters in size. XRD on bulk rock, geochemical modeling, FTIR, Raman, and EPMA analyses were carried out in all mineral phases to assess the origin and the causes of high crystallinity within the PEI. We interpret the high crystallinity on the basis of magmatic crystallization of a magma body that was remobilized and altered by post-depositional hydrothermal alteration processes. We suggest that ACC rhyolites are geochemically influenced by at least one crystal mush established during the Pleistocene. We suggest that the PEI could be the result of an erupted crystal mush (melt + crystals), or a cumulate, or an ancient and crystallized reservoir generated after the first ACC collapse due to intrusion or underplating of mafic hot magmas. Extensional episodes within the ACC facilitated the ascent of mafic magmas. This interaction increased the liquid fraction of the mush through partial melting/crystal dissolution, generating a drop in density and viscosity in the mush, thus triggering eruption. The PEI provides evidences for an association between the geochemically-diverse ACC rhyolites with the complex interaction between mafic transitional alkaline magmas and a crustal mush system, promoted by continuous changes in the stress field during the Pleistocene.

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The influence of alteration and fractures on gas permeability and mechanical properties of the sedimentary and volcanic rocks of the Acoculco Caldera Complex (México)

Ramírez-Guzmán,

Pola,

Macias

et al. 2024

Bull Eng Geol Environ

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Petrophysical and mechanical properties of limestones and volcanic altered and unaltered rocks were estimated by a series of laboratory measurements. The aim of this work was to study the changes in matrix permeability and the influence of an induced vertical fracture. The phenomenon was studied by direct comparison of different properties (porosity, UCS, permeability) and by a multiple linear regression analysis. Porosity, UCS, and matrix permeability of limestones and non-altered volcanic rocks showed homogeneous values. However, the permeability increased from poor to moderate after fracture induction (10–17 to 10–14 m2). Altered volcanic specimens showed a more complex pore system. Thus, considerable variations in all measured properties were identified. For example, porosity ranged from 6 to 50% and Permeability ranged from 10–14 to 10–12 m2. Comparison plots including UCS, porosity, and matrix permeability showed a good fitting (R2=0.75). Multiple linear regression analysis in fractured specimens has a moderate R2 (0.55) The model that better explains the permeability evolution of fractured specimens has the fracture permeability as the response variable and the porosity values as independent variables. The usage and repeatability of these equations in other zones should include more singularities of every single specimen as pore size, pore distribution, and fractures.

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Mineralogical and geochemical changes due to hydrothermal alteration of the volcanic rocks at Acoculco geothermal system, Mexico

Cited by 12 publications

References 56 publications

GraHyAlt—A Computer Program for the Graphical Presentation of the Hydrothermal Alteration Induced Effects in Geochemical Parameters of Volcanic Rocks

GraHyAlt—A Computer Program for the Graphical Presentation of the Hydrothermal Alteration Induced Effects in Geochemical Parameters of Volcanic Rocks

Ultra-crystalline pyroclastic deposits and rhyolitic lavas controlled by crystal mushes: insights from the Acoculco Caldera Complex, México

The influence of alteration and fractures on gas permeability and mechanical properties of the sedimentary and volcanic rocks of the Acoculco Caldera Complex (México)

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