The Guanajuato resurgent graben caldera, Sierra Madre Occidental, central México: Revised volcanic stratigraphy and geologic evolution

The Guanajuato Mining District of central Mexico is one of the main silver and gold deposits in the world. It is in the State of Guanajuato in the southern part of the Sierra Madre Occidental (SMO) volcanic province. The mining district developed within a mid-Tertiary volcano-sedimentary sequence that includes thick alluvial-fan deposits accumulated in a tectonic basin during the Eocene-Oligocene named the Guanajuato Red Conglomerate and an overlying volcanic sequence mostly pyroclastic of Oligocene age. The mid-Tertiary stratigraphy of Guanajuato is revised and reinterpreted in the light of new fieldwork and U-Pb ages, which document a close timing between all units of the volcanic succession at the top of the Guanajuato Red Conglomerate. This sequence is made of pyroclastic density current deposits formed during episodic events from the Guanajuato caldera. A new nomenclature of the caldera's units is proposed; the Guanajuato Caldera Volcanic Group, which includes the Guanajuato Pyroclastic Formation represented by the Loseros PDC deposits and the Bufa-Calderones ignimbrites emplaced around 32.8 ± 0.2 Ma, and the post-collapse lava domes of El Rodeo and Chichíndaro formations emplaced at 31-30 Ma. Apparently, a resurgent pulse of the caldera uplifted the collapsed intra-caldera blocks, so that the caldera floor is now exposed. The caldera collapse was controlled by the pre-existing normal faults inherited from the previous tectonic basin; thus, it is classified as a graben-type caldera, with a square shape and a size of 15 × 16 km. By comparison with other similar calderas of Mexico, the Guanajuato caldera is another case study of graben-type calderas of the SMO coinciding with mineral districts, such as Bolaños (Jalisco).

We thank Carlos Ortega for his assistance in zircon U-Pb dating at Instituto de Geociencias (IGC) of Universidad Nacional Autónoma de México (UNAM), and to Juan Tomás Vásquez for the elaboration of thin sections also at the IGC-UNAM. We acknowledge the careful review of this work by the anonymous referees and Prof Olivier Lacombe, Geological Magazine’s editor, whose comments improved the manuscript. This study was possible thanks to the DGAPA-PAPIIT-UNAM grants IN112312 and IN108621 to the first author, and a scholarship from the first project to Mariana Coutiño-Taboada, to the DGAPA-PAPIME-UNAM grants PE101816 and PE102419, and to grants from the International Academic Interchange Program of the Coordinación de la Investigación Científica - CIC-UNAM in 2021, 2022 and 2023. We also thank to the Ministerio de Ciencia e Innovación del Gobierno de España for the grant PID2020-114273GB-C21, MACIN Project.

Peer reviewed