The silver mines of Guanajuato Mexico: bringing riches to the region

The city of Guanajuato was founded in 1559 due to the region’s rich silver and gold deposits. The conquistador Juan de Tolosa observed the native people with shiny rocks. He started looking in Zacatecas (a state located farther north in Mexico) and in 1546 found silver mines that made him very rich. Other conquistadors started looking too, and in 1548 Juan de Rayas discovered the first mine in Guanajuato, which became one of the richest places in New Spain almost 300 years before Mexico’s independence from Spain. The extraordinary wealth obtained from the mines funded the construction of opulent buildings that led to Guanajuato’s designation as a UNESCO World Heritage Site.

To thank Guanajuato for the wealth it provided to Spain, in 1557 King Carlos and his son Felipe II gave to “Mineral de Guanajuato” an ancient image of the Virgin Mary that supposedly had been hidden in Andalucian caves for about 800 years during the Muslim conquest. The virgen “Patrona de Guanajuato” is now housed in the Basilica de Nuestra Señora de Guanajuato (pictured above) which was completed in 1696 and is considered one of the premier examples of 17th century Baroque architecture in the Americas.

During our visit to Guanajuato in October 2018, we of course had to visit some mines! Because of the importance of geology, whose processes are responsible for the formation of the economically-valuable metals, the University of Guanajuato has one of the best geology departments in the country. Our guide (with red hat) was a geology student who took us through “El Nopal” mine, a shaft that is no longer used commercially but that provides a place for the students to learn about mining procedures. Here our guide is with the three geologists in our group: me, Tim Lane and Jay Ach (photo by Gabriela Cardenas Vazquez).

Here another member of our group, David Frevert, tries out mining in the old-fashioned way (photo by Jay Ach). Conditions were extremely poor during the first centuries of the mines’ operation—the indigenous population was exploited in near-slave conditions and had very short life spans. Continued worker exploitation was one reason this region was where the war for independence from Spain began in the early 1800s. Today, conditions have improved, although this is still not a job most of us would want. Fortunately for the geologists, they provide information about mineralization locations but are not involved in the actual mining.

We also visited the Valenciana mine, the crown jewel of the mining district that is being restored and has been converted into a museum for the general public. This main shaft goes down about 600 meters (2000 feet)—imagine that commute each day—but visitors can only go down about 60 meters (200 feet). In the photo (taken by Jay Ach) are, from right to left, Juan Carlos Delgado (our host for the day), me, our guide (a mining engineering student), David Frevert, Lisa Lane, Lynne Frevert, Tim Lane, the guide’s brother, Fabián (Juan Carlos’ son).

This chart shows the amount of silver (toneladas de plata = tons of silver) produced by the mine, which was discovered in 1750 (chart shows years up to 1940). At its height (late 18th century) Guanajuato was the world’s leading silver district, with the Valenciana mine producing about 30% of the world’s total supply.

The mines provided so much wealth to Spain that King Carlos decorated the owners with titles of Counts and Viscounts. The silver baron of Valenciana, Conde de Rui, to give thanks for his wealth (or maybe to atone for his mistreatment of the workers?), built the extravagant Templo La Valenciana across from the mine entrance. The interior (above) is in Churriqueresque style and dazzles with gold. It is also known as San Cayentano which was the saint the church was dedicated to.

Geologically, the Guanajuato Mining District (GMD) is located at the southern end of the Sierra Madre/Mesa Central, which is the southern end of the Basin and Range extensional province that extends far north into the United States (city locations are L=Leon; G=Guanajuato; Q=Querétero). The Volcanic Belt to the south is the current location of the active volcanoes associated with subduction of the Cocos (oceanic) plate along the west coast of Mexico (figure from Aranda-Gómez et al., 2012, Geology and tectonics of the southeastern portion of the Sierra de Guanajuato). According to an article by Nieto-Samaniego et al. (2015, New stratigraphic, geochronological, and structural date from the southern GMD), the events that produced the GMD world-class ore deposit occurred around 30 million years ago when volcanism associated with normal (extensional) faulting was followed by hydrothermal activity and mineralization. The Veta Madre (mother vein) fault is an extensive 25-km-long zone that is the locus of most of the economically-valuable mineralization. In addition to silver, gold, copper, lead and zinc have also been recovered. Unique minerals such as Valencianite, a form of potassium feldspar, have been discovered.

The gorgeous hills surrounding the city of Guanajuato are volcanic rocks that were formed around the time of the mineralization (Jay is enjoying the view from the terrace of the Hotelito Casa Dionisio). Today, Guanajuato is still a principal silver-producing state and more mines continue to be discovered. According to a 2018 article by INNSPIRED Investing News (https://investingnews.com/daily/resource-investing/precious-metals-investing/silver-investing/mexico-mining-industry-fresnillo-guanajuato-silver-district/), Mexico has long led the world in silver production, taking the top spot again in 2017. Because of continued investment, especially by Canadian resource companies, the mining industry is expected to grow by 3.3% annually to reach a value of $17.8 billion in 2020. The economic viability of Guanajuato is therefore likely to continue for some time!

You may wonder about the environmental effects of centuries of mining. There have been plenty. In the days before petroleum was a fuel, lots of wood was needed to create heat for processing the ore; as a result, large areas were deforested. The waste that is left after valuable metals are removed have been stored in tailings piles that are scattered in a 100 square-kilometer region around the city of Guanajuato. Although a 2002 study by Carrillo-Chávez et al. (Environmental geochemistry of the Guanajuato Mining District) showed that natural buffering has prevented large-scale acid leaching, there remains concern about contamination of the local aquifers.