The San Francisco Mine, also known as Cerro Prieto, is located about 65 km (40 miles) east of Magdalena, Sonora Mexico in classic Sonoran Desert terrain. The closest municipality is Cucurpe, and the mine is located on Ranch Cerro Prieto property, owned by the Pedro Trelles family (Fig.1). While the mine is only a little over 100 km south of the USA-Mexico border town of Nogales, the trip can take a half day or more to get to the mine from Nogales due to customs, immigration, and poor roads. The San Francisco Mine is a world famous classic locality for being one of the most prolific producers of wulfenite-yellow to orange, clear window panes of wulfenite to 18 cm, some with attached balls of red-orange to yellow mimetite. Many collectors consider their collections incomplete unless at least one San Francisco wulfenite graces their shelves.
On or around the end of the 1800s, the unexplored veins on Cerro Prieto were found to be enriched in gold. The Chenowith family began working the mining property, and about 1900, a 20-stamp mill equipped for amalgamation of the ore was erected to treat the ores coming from workings near the top of Cerro Prieto. Today, large red open stopes testify to the size and amount of material worked during these early days. The rumor persists of a crew of miners buried forever within Cerro Prieto when a massive cave-in took place. Mining continued intermittently until the late 1960s although no specimens are known to have been produced. Specimen mining for wulfenite started in the early 1970s and continued through a series of intriguing episodes up to 1994.
Rocks consist of Lower Cretaceous-age Bisbee Group shales, limestone and arkosic sand-stones and Lower Tertiary-age andesite-dacite flows, and silicic ash flow tuffs. The walls of a prominent north-northwest striking shear zone up to 30 m wide form what is called the East & West Veins. The dip is vertical to 80–90o easterly. The shear zone is traceable for a couple thou-sand meters on the surface. The shear zone shows movement, thus creating a complex series of parallel and horse-tailing faults within the brecciated interior underground.
Mineralization is very simple, consisting of wulfenite, mimetite, cerussite, anglesite, quartz, barite, hematite & calcite, with remnant galena and sphalerite. Rarely, visible gold can be seen without a hand lens. Mineralization (wulfenite) is restricted to the altered and brecciated Mn-rich oxides within the shear zone. Flexures (steepening/flattening) in the veins created open space to form the most productive & mineralized areas for pockets of crystallized wulfenite-mi-metite.
Today, the San Francisco Mine is held by the Goldgroup, a Vancouver mining company, who has successfully conducted leach tests for Au and Ag in 2014. No specimens are known to have been produced since 1994.
