The Comarca Minera Geopark is located in the center-south of the Mexican state of Hidalgo. It extends over nine municipalities: Atotonilco el Grande, Epazoyucan, Huasca de Ocampo, Mineral del Chico, Mineral de la Reforma, Mineral del Monte, Omitlán de Juárez, Pachuca de Soto and Singuilucan. These municipalities are articulated by a network of 31 geosites, elected because they express the geological history of the territory and highlight the relationship between mining and metallurgy, and the historical spaces and cultural development of the region. In addition, they are important for science and education and, mostly, have a significant landscape value.
Geology
The Pachuca and Las Navajas mountain ranges constitute the most important physiographic element of the Comarca Minera, with maximum altitudes in the Peña de las Ventanas (3090 m) and in the Cerro de las Navajas (3212 m), respectively. This orographic set has a northwest–southeast general orientation, and separates the area of the Sierra Baja and Barranca de Metztitlán (at north of the mountain ranges), with altitudes of ~ 1300–2000 m, from the northern end of the Mexico Basin (at south of the mountain ranges), with an altitude of ~ 2200–2400 m. The huge geodiversity of the park, appreciable in its geosites, is due to the fact that two physiographic provinces converge in the area, the Sierra Madre Oriental and the Transmexican Volcanic Belt, as well as two magmatic provinces, one related to the Transmexican Volcanic Belt and the other related to the volcanism of the Sierra Madre Occidental.
The Comarca Minera is characterized by an outstanding geopatrimony, which includes at least five geological (and cultural elements related to geology) that can be considered of international relevance:
The epithermal system (Ag-Au) of Pachuca-Mineral del Monte, one of the largest in the world; its historical production represents 6% of the silver produced globally during the last five centuries. The type locality, in the Cerro de San Cristóbal de Pachuca geosite, of tridymite and cristobalite, minerals discovered by Gerhard vom Rath in the years 1868 and 1887, respectively. The basalts with columnar jointing of Santa María Regla, Huasca de Ocampo, popularly known as the "prismas basálticos" (basaltic prisms), described and drawn by Alexander von Humboldt in 1803; they are generally considered the longest in the world (they reach 40 m in length). The mining industrial heritage, which includes haciendas de beneficio from the colonial era, as well as various mining works, among which those from the English mining period (mid-19th century), with splendid examples of Cornish-type machine houses, stand out. The Cerro de las Navajas, main point of exploitation and supply of obsidian for Mesoamerica, and exceptional site where obsidian in its golden and green varieties can be found.
Cretaceous
The oldest rocks in the geopark are limestones and shales of marine origin, of Cretaceous age, which are found in the northeastern portion of the territory and outcrop at the bottom of the Barranca de Metztitlán. It can be seen in the geosites:
Barranca de Aguacatitla
Aguas termales de Amajac
Paleogene
During this period, the belt of folds and ridges was formed, geographically known as Sierra Madre Oriental, through a compressive process that thickened and shortened the crust and raised the previously formed rocks in the sea to their current position. Towards the end of this period began the volcanism related to the subduction of the western Mexico. In the northwest sector of the geopark there are deformation structures, mainly asymmetric or even lying folds, of NNO-SSE direction, which can be seen in the geosites:
Barranca de Aguacatitla
Aguas termales de Amajac
The volcanic rocks from the Oligocene are found in:
Depósitos tobáceos de Cubitos
Mirador Cerro del Lobo
Miocene
The magmatism initiated at the end of the Paleogene until the Middle Miocene, and it was responsible for large volumes of silicic volcanic rocks that constitute one of the most southeastern manifestations of the igneous province of the Sierra Madre Occidental. Associated with these rocks the epithermal mineral deposits of silver were formed, with an early Miocene age in the case of Pachuca-Real del Monte deposits, and a somewhat more recent (late Miocene) for those of Mineral del Chico.
Rhyolitic tuffs of the Tezoantla and Cerezo formations as well as rhyolitic and dacitic porphyry can be seen in the geosites:
Peña Las Monjas
Peña del Cuervo
Las Ventanas
Depósitos Volcánicos de El Chico
The mineralization of late Miocene can be seen in: