San Martin Mines

Zacatecas

Juan de Tolosa was a Spanish Basque conquistador who arrived in Mexico about 20 years after the conquest by Hernán Cortés in 1519-21. Tolosa was more of a prospector and explorer than a conquistador, and in 1556 he claimed to have discovered the rich silver mines at San Martin in the western part of Zacatecas State in central Mexico – among the richest silver deposits ever found in Mexico. But it’s fairly certain that the native population exploited the deposit and showed it to Tolosa rather than him discovering it on his own.

The location is part of a linear northwest-southeast-trending zone in southwestern Mexico that represents part of the Cretaceous margin of the continent. Before the Atlantic Ocean and Gulf of Mexico formed, when the supercontinent Pangea was still intact, and North America, Mexico, Central America, and South America were all more or less closely attached to each other, Mexico was an amalgam of semi-discrete blocks including Yucatan, Chortis (now in Honduras and Nicaragua), and less-well defined terranes (Tampico, Mesa Central, Del Sur, and others; Bird & Burke, 2006, Pangea breakup: Mexico, Gulf of Mexico, and Central Atlantic Ocean: Soc. Expl. Geoph. Technical Meeting Abstracts, 25:1). There was an oceanic spreading center to the west, floored by oceanic basalt and called the Arperos Basin.

The margin of the Arperos Basin, where San Martin is located today, saw deposition of the Cuesta del Cura limestone during Cretaceous time. The ocean spreading center ceased expansion when a large composite block called the Guerrero Superterrane collided (Martini and others, 2014, Correlating the Arperos Basin from Guanajuato, central Mexico, to Santo Tomás, southern Mexico: Implications for the paleogeography and origin of the Guerrero terrane: Geosphere 10(6): 1385-1401).

Guerrero was probably something like Indonesia today, a combination of an island arc and relatively small continental blocks, but it was enough to produce subduction and igneous intrusion along the Cretaceous margin about 46 million years ago (Eocene) when the Cerro de la Gloria quartz monzonite stock intruded (Rubin & Kyle, 1988, Mineralogy and geochemistry of the San Martin skarn deposit, Zacatecas: Economic Geology 83 (8): 1782–1801). The igneous rocks interacted with the limestones they intruded to both metamorphose (change form by heat and pressure) and metasomatize them (change the chemical make-up of the rocks). The resulting rocks are called skarns, and they are among the most prolific producers of minerals in any geological setting.

At San Martin the main production was initially the rich silver in the 1500s, but later copper, lead, and zinc were important. Silver from San Martin contributed significantly to Mexico’s wealth and supported its war of independence from Spain in 1810-1821.

From the mineral collector standpoint, San Marin is probably more famous for its non-ore minerals, although spectacular examples of native silver are known. San Martin is noted especially for its calcite, fluorite, and apophyllite, and for minor ore minerals such as jamesonite (lead-iron-antimony sulfide) and stibnite (antimony sulfide). My four San Martin specimens scattered through this post show calcite (G173, #1365), stibnite (#1351), and jamesonite (#94).

The mine operated more or less continuously from the 1500s to 2007, when a strike over safety concerns shut it down until it reopened in 2019. The mine had an estimated 36 million ounces of silver in reserves as of 2021. Today Mexico is the world leader in silver production with almost a quarter of the total (China is #2 with 14%). The United States depends on imports for about 80% of the silver we consume, and almost half of that comes from Mexico.

More than a third of the silver consumed in the US goes to coins, jewelry, and bullion for investment, but about of fifth of it is used in electronics. Volumetrically smaller but important uses also include antimicrobial bandages, clothing, pharmaceuticals, dental amalgam, and plastics; batteries; bearings; brazing and soldering; catalytic converters in automobiles; electroplating; inks; mirrors; photography; photovoltaic solar cells; water purification; and wood treatment (from USGS Minerals Commodities Summaries, 2022).

Juan de Tolosa did not manage his rich silver mine well. Even though he was a son-in-law of Hernán Cortés, he apparently died in poverty before 1594.