Boletín de la Sociedad Geológica Mexicana

Volumen 72, núm. 3, A140420, 2020


The Pliocene Ixtacamaxtitlán low sulfidation epithermal deposit (Puebla, Mexico): A case of fossil fungi consortia in a steam-heated environment


El depósito epitermal de baja sulfuración pliocénico Ixtacamaxtitlán (Puebla, Mexico): Un caso

de consorcio de hongos fósil en un ambiente de aguas calentadas por vapor


Antoni Camprubí1,*, Edith Fuentes-Guzmán1,2,3, Pilar Ortega-Larrocea1, María Colín-García1,

Janet Gabites4, Luis F. Auqué5, Vanessa Colás1, Eduardo González-Partida6


Instituto de Geología, Universidad Nacional Autónoma de México. Ciudad Universitaria, 04510 Coyoacán, CDMX, Mexico.

Laboratorio Nacional de Geoquímica y Mineralogía (LANGEM). Ciudad Universitaria, 04510 Coyoacán, CDMX, Mexico.

Programa de Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México. Ciudad Universitaria, 04510 Coyoacán, CDMX /Boulevard Juriquilla 3001, 76230 Juriquilla, Querétaro, Mexico.

Pacific Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia; Earth Sciences Building, 2207 Main Mall, Vancouver, British Columbia, V6T 1Z4, Canada.

Departamento de Ciencias de la Tierra, Universidad de Zaragoza. C/ Pedro Cerbuna 12, 50009 Zaragoza, Spain

Centro de Geociencias, Universidad Nacional Autónoma de México. Boulevard Juriquilla 3001, 76230 Juriquilla, Querétaro, Mexico.

* Corresponding author: (A. Camprubí) This email address is being protected from spambots. You need JavaScript enabled to view it.


How to cite this article:

Camprubí, A., Fuentes-Guzmán, E., Ortega-Larrocea, P., Colín-García, M., Gabites, J., Auqué, L.F., Colás, V., González-Partida, E., 2020, The Pliocene Ixtacamaxtitlán low sulfidation epithermal deposit (Puebla, Mexico): A case of fossil fungi consortia in a steam-heated environment: Boletín de la Sociedad Geológica Mexicana, 72 (3), A140420.



The Ixtacamaxtitlán area in northern Puebla (central Mexico) contains middle Miocene Cu-Mo-Au porphyry/skarn and Pliocene low-sulfidation Au-Ag epithermal deposits that are geologically associated with the evolution of the Trans-Mexican Volcanic Belt (TMVB). In this paper, a new 40Ar/39Ar age (2.87 ± 0.41 Ma) is provided for rhombohedral alunite from a kaolinite + alunite ± opal ± cristobalite ± smectite advanced argillic alteration assemblage. This age contributes to the definition of a metallogenic province that is confined to the TMVB, a relevant feature for regional exploration. A ~12 My gap is established between the formation of the Cu-Mo-Au porphyry/skarn and low-sulfidation Au-Ag epithermal deposits, which rules out the possibility that their overlapping was the result of telescoping. Advanced argillic alteration is conspicuous throughout the mineralized area. This alteration assemblage consists of a widespread kaolinite-rich blanket that underlies silica sinters, polymictic hydrothermal breccias, and an alunite-rich spongy layer that consists of vertical tubular structures that are interpreted as the result of gas venting in a subaerial environment. The above indicate a shallow hypogene origin for the advanced argillic alteration assemblage—that is, formation by the partial condensation within a phreatic paleoaquifer of acidic vapors that were boiled-off along fractures that host epithermal veins at depth. The formation of the spongy alunite layer and silica sinters is interpreted to have been synchronous.

Within the alunite-rich spongy layer, tubular structures hosted microbial consortia dominated by fungi and possible prokaryote (Bacteria or Archaea) biofilms. Such consortia were developed on previously formed alunite and kaolinite and were preserved due to their replacement by opal, kaolinite, or alunite. This means that the proliferation of fungi and prokaryotes occurred during a lull in acidic gas venting during which other organisms (i.e., algae) might have also prospered. Periodic acidic gas venting is compatible with a multi-stage hydrothermal system with several boiling episodes, a feature typical of active geothermal systems and of low-sulfidation epithermal deposits. The microstructures, typical for fungi, are mycelia, hyphae with septa, anastomoses between branches, and cord-like groupings of hyphae. Possible evidence for skeletal remains of prokaryote biofilms is constituted by cobweb-like microstructures composed of <1 µm thick interwoven filaments in close association with hyphae (about 2.5 µm thick). Bioweathering of previously precipitated minerals is shown by penetrative biobrecciation due to extensive dissolution of kaolinite by mycelia and by dissolution grooves from hyphae on alunite surfaces. Such bioweathering was possibly predated by inorganically driven partial dissolution of alunite, which suggests a lull in acidic gas venting that allowed living organisms to thrive. This interpretation is sustained by the occurrence of geometrical dissolution pits in alunite covered by hyphae. Fungal bioweathering is particularly aggressive on kaolinite due to its relatively poor nutrient potential. Such delicate microstructures are not commonly preserved in the geological record. In addition, numerous chalcopyrite microcrystals or microaggregates are found within the alunite layer, which could be related to sulfate reduction due to bacterial activity from the sulfate previously released by fungal bioweathering of alunite. Hydrogeochemical modeling constrains pH to between ~3.2 and ~3.6 and temperature to between 53 and 75 °C during the stage in which fungi and other organisms thrived. These waters were cooler and more alkaline than in earlier and later stages, which were characterized dominantly by steam-heated waters. The most likely process to account for this interlude would be mixing with meteoric water or with upwelling mature water that did not undergo boiling.

Keywords: 40Ar/39Ar dating, alunite, advanced argillic alteration, steam-heated grounds, fungi, bacteria, bioweathering, biomineralization, Trans-Mexican Volcanic Belt.