A collector's guide to San Francisco Mine, Mexico: its geology, mining history and notable minerals, illustrated with the 172 specimens documented from this locality on EarthWonders.
Key facts
San Francisco Mine, better known in much of the older specimen literature as Cerro Prieto, is one of the classic wulfenite localities of northern Mexico. It lies in the Sonoran Desert near Cucurpe, in the Magdalena de Kino–Cucurpe region of Sonora, where gold- and silver-bearing structures cut a long-lived, structurally complex belt of Mesozoic and Tertiary rocks. For collectors, however, the mine’s fame rests not on bullion but on the supergene suite that formed in its oxidized, manganese-rich shear zone: transparent yellow to orange “windowpane” wulfenite, sparkling orange to yellow mimetite, and uncommon white to colorless barite perched on dark gossanous matrix.
The best San Francisco specimens have a look so distinctive that seasoned collectors can often recognize them across a room. Thin, square to rectangular wulfenite plates, commonly bevel-edged and glassy, rise from orange mimetite botryoids like tiny panes of stained glass. On the finest pieces the wulfenite is not merely translucent but genuinely gemmy, with warm golden color and sharp edges; the mimetite adds a granular, sugary surface and, on special pieces, rich red-orange spheres. Compared with Mapimí or Los Lamentos material, San Francisco wulfenite is generally more delicate, more transparent, and more architectural, with the finest thumbnails and miniatures ranking among the most desirable Mexican lead-oxidation specimens ever produced.
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The mine is also important because its specimen history is unusually well documented for a Mexican wulfenite locality. It began as a gold operation around the turn of the twentieth century, was later examined by major mining interests, and then, in the 1970s, became a collector’s locality through pockets that reached the Tucson market. Finds in 1973, 1976, 1979, and later campaigns into the early 1990s supplied many of the examples now dispersed through old American collections, museum displays, and the modern fine-mineral market.
Photo: DerHexer, Wikimedia Commons
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San Francisco Mine is the historic underground mine on Cerro Prieto, near Cucurpe in Sonora. Locality listings place it at approximately 30°18′ N, 110°41′ W, a few kilometers from Cucurpe and roughly 50–65 km from Magdalena de Kino depending on route and reference point. Older collector labels may read San Francisco mine, Cerro Prieto mine, Sierra Prieta, Magdalena district, or simply Sonora, Mexico; these labels generally refer to the same celebrated wulfenite locality, though “San Francisco” is also a name used by other Mexican mines and should always be checked against Cerro Prieto or Cucurpe wording.
Geologically the deposit belongs to the Cerro Prieto mineralized system, a structurally controlled gold-silver-lead-zinc-molybdenum occurrence developed along the Cerro Prieto shear. Published technical descriptions of the broader project classify the mineralization as a low- to intermediate-sulfidation epithermal silver-gold system in a region of northwest-trending structures. The local rock package includes sedimentary and volcanic units of the Magdalena de Kino–Cucurpe region; the mineralized structures are associated with quartz veins and veinlets, chalcedonic silica, manganese oxides, iron oxides, argillic alteration, and strongly oxidized sulfide remnants. For specimen collectors, the critical zone is the oxidized, brecciated, manganese-rich material within the shear, where open-space conditions allowed wulfenite and mimetite to crystallize.
The old ore system was mined first for gold. Near the end of the nineteenth century, gold-enriched veins at Cerro Prieto attracted development, and by about 1900 a stamp mill had been built to treat ore from workings high on the hill. Historical summaries describe the mine as operating in the early twentieth century and closing around the Mexican Revolution era, with later intermittent examinations and renewed modern exploration. Mindat’s locality summary records the mine as initially a gold mine and later a gold-silver-molybdenum mine, with Anaconda ownership or evaluation in 1934, historical production reported at 720 tons of ore per day in 1909, and workings reaching about 1,100 feet, or roughly 350 meters, deep.
The specimen-producing story is later. Although the mine had long been known as an ore property, collectible wulfenite does not seem to have entered the market in quantity until the early 1970s. The celebrated pockets were found in oxidized zones of the old underground workings, particularly where changes in dip and flexures in the veins created open space. The mineralization was simple but visually spectacular: wulfenite, mimetite, cerussite, anglesite, quartz, barite, hematite, calcite, and relicts of primary galena and sphalerite, with native gold rarely visible. Wulfenite was concentrated in the altered manganese-rich oxides, not distributed evenly through the mine.
By the mid-1970s, San Francisco wulfenites had become Tucson classics. Fine specimens from the 1973 material are especially prized for jewel-like yellow to orange wulfenite plates with red-orange mimetite balls; later 1976 and 1979 recoveries added more material, including well-preserved thumbnails and larger cabinet pieces. A few specimens are attributed to the 1000-foot level, and some modern dealer and collection records note old combinations of wulfenite, mimetite, and barite from the 1970s finds. Specimen mining continued episodically into the early 1990s, but no significant new specimen production has been reported since 1994.
Collecting access today should be considered closed unless permission is obtained from the landowner and mineral-rights holder. The Cerro Prieto project has been the subject of modern corporate exploration and open-pit heap-leach gold mining, and the old underground workings are hazardous. Loose collector access to historic stopes, dumps, and adits is not appropriate; besides the normal dangers of abandoned mines, the locality is on private and concession-controlled ground in an active mining district.
San Francisco wulfenite is the mine’s signature mineral: thin tabular to square “windowpane” crystals, usually golden yellow, honey-orange, or butterscotch, commonly transparent to translucent and edged by fine bevels. The largest crystals are reported to about 18 cm, but most collectible specimens show crystals from a few millimeters to several centimeters; truly choice thumbnails with a single perfect, glassy plate on mimetite-rich matrix are just as coveted as cabinet pieces. The finest examples came from the classic early-1970s pockets and later professional collecting episodes in 1976 and 1979, with the best pieces combining saturated color, transparency, upright presentation, minimal edge chipping, and intimate association with orange to red-orange mimetite. Ordinary San Francisco wulfenites may be colorful but crowded, damaged, or too opaque; the great ones have the rare combination of luminous glass, sharp geometry, and a dark-to-orange matrix that makes the crystals visually float.
Mimetite from San Francisco is usually the partner mineral that turns a good wulfenite into a classic. It occurs as botryoidal coatings, tiny sparkling crystals, and rounded orange, yellow, or red-orange balls on gossanous matrix, often forming the carpet from which the wulfenite plates emerge. The mimetite is typically small-scale rather than dominated by large individual prisms, but its color and texture are crucial: rich orange botryoids behind transparent yellow wulfenite give the best specimens their unmistakable glow. Superior pieces show crisp contrast, fresh luster, and mimetite placed compositionally rather than merely as a dull crust; the most admired 1973-style pieces have unusually vivid red-orange mimetite spheres nestled among glassy wulfenite panes.
Barite is much less common than wulfenite or mimetite in San Francisco specimens, but it is a prized accessory when well crystallized. Documented pieces show white to colorless, thin tabular or bladed barite crystals perched on mimetite-coated matrix with yellow wulfenite; crystals around 1–2 cm are reported in specimen descriptions, and dense white blades can give a strong three-color contrast against orange mimetite and honey wulfenite. Fine barite combinations are usually attributed to older 1970s material and are scarcer than straight wulfenite-mimetite specimens. Condition matters severely: barite blades and wulfenite plates are both fragile, so the best examples retain complete white blades, undamaged wulfenite faces, and balanced placement rather than appearing as a busy mass of broken tabs.
Other documented minerals from San Francisco include anglesite, cerussite, galena, goethite, hematite, native gold, pyrite, pyromorphite, quartz including rock crystal, and sphalerite. No accepted type-locality mineral is normally associated with San Francisco Mine in standard locality listings, but several minor species matter to specialists. Pyromorphite is the most interesting rarity for collectors: it is historically reported from the mine and is represented by scarce specimens, sometimes with wulfenite, but it is not part of the common modern San Francisco suite. Cerussite and anglesite fit the oxidized lead environment, while galena and sphalerite are remnants of the primary sulfide stage that fed the spectacular secondary mineralization.
The first authenticity issue is locality confusion. “San Francisco Mine, Mexico” is not specific enough: serious labels should mention Cerro Prieto, Cucurpe, Sonora, or the Magdalena district. The most convincing San Francisco specimens have a recognizable habit—thin yellow to orange wulfenite panes on orange or yellow mimetite over dark oxidized matrix—but other Mexican localities, especially Ojuela and Los Lamentos, can also produce wulfenite-mimetite combinations. San Francisco wulfenite is generally thinner, more glassy, and more windowpane-like than the common blockier Mapimí material, and its mimetite is often botryoidal or sugary orange rather than the more varied habits seen at Ojuela.
Condition is the central grading issue. Wulfenite has perfect cleavage and thin San Francisco plates chip easily along edges and corners. Look closely for bruised margins, repaired plates, missing crystal centers, and old glue where a large windowpane may have been reattached. Minor edge wear is common and sometimes acceptable on old 1970s pieces, especially larger cabinets, but premium thumbnails and miniatures should have at least one complete focal crystal. Mimetite can look deceptively sturdy but may shed tiny balls or grains if abraded; barite combinations are even more vulnerable because both the barite blades and wulfenite plates are fragile.
No famous, systematic fake industry is associated with San Francisco in the way that some localities have dyed, glued, or assembled material, but repairs and composite-looking restorations should be watched for on high-value pieces. Transparent wulfenite mounted on a pedestal of mimetite is exactly the natural aesthetic collectors want, so any oddly isolated large plate deserves careful inspection under magnification. Check that the crystal enters the matrix naturally, that mimetite coats around the base consistently, and that the color and luster of any adhesive-filled fracture do not differ from the crystal.
These specimens should be handled as display minerals, not as rugged teaching pieces. Avoid ultrasonic cleaning, prolonged soaking, strong acids, and aggressive mechanical cleaning. Wulfenite is a lead molybdate and mimetite is a lead arsenate chloride, so common-sense handling applies: wash hands after handling, keep dust out of the display case, and do not trim or saw specimens without appropriate controls. Fluorescence is not a major collecting attribute for San Francisco wulfenite-mimetite specimens; value is driven instead by aesthetics, transparency, crystal completeness, association, provenance, and the specific character of the old finds.
On the market, San Francisco material remains available but increasingly stratified. Small damaged pieces and old fragments still appear regularly, but fine, undamaged thumbnails with jewel-like wulfenite and vivid mimetite are much scarcer than the number of ordinary specimens might suggest. Large cabinet pieces with many transparent panes, strong color, and old collection provenance are major Mexican classics. Barite-bearing combinations and rare pyromorphite associations command specialist interest because they sit outside the standard wulfenite-mimetite formula while still belonging unmistakably to the San Francisco story.
The San Francisco story begins like a mining-camp tale rather than a mineral-show tale. Around the end of the nineteenth century, prospectors recognized that the veins on Cerro Prieto were enriched in gold. The Chenowith family began work on the property, and by about 1900 a 20-stamp mill for amalgamation had been erected to process ore from workings near the top of the hill. The old stopes were large enough that later visitors described them as red open scars in the mountain, reminders that before collectors prized the glassy wulfenite, miners had already removed a great deal of oxidized ore.
A darker story persisted around the mine: the rumor of miners buried inside Cerro Prieto during a massive cave-in. Whether the episode can be pinned to a documented accident record or not, it became part of the mine’s oral history, and it suits the place. San Francisco was not a roadside collecting cut. It was a deep, oxidized, structurally broken underground mine in desert country, and the later specimen collectors inherited the risks left by ore miners.
The modern specimen history has its own cast. Tony Potucek, who later wrote on late-twentieth-century wulfenite collecting at San Francisco, recalled time spent there in the 1970s and 1990s as both exhilarating and hair-raising. Getting to the mine from Nogales could consume half a day despite the modest straight-line distance, because the route involved border formalities, poor roads, and desert travel. In one account he remembered being stopped at night by the Mexican equivalent of the U.S. Alcohol, Tobacco and Firearms authorities, with automatic weapons pointed at the party. On other trips, monsoonal flooding turned the river crossings between Cucurpe and the mine into serious obstacles.
The May 1973 pocket became one of the legendary moments in the mine’s collecting history. The material included glassy, transparent yellow windowpane wulfenites with unusually vivid red-orange mimetite balls. Wayne Thompson and Susie Davis are credited in specimen histories with helping bring that pocket to market, and the best pieces quickly moved into important collections. One thumbnail later described from that material had a 2.2 x 2.1 x 1.1 cm size and was praised as being “as close to a jewel as a wulfenite thumbnail can get.” Its chain of custody is part of the mineral’s romance: purchased by Ernie Schlichter in 1973 from Western Minerals’ Gene Schlepp, later passing into the Shields Flynn thumbnail collection, and chosen as the cover piece for Flynn’s private collection book.
Museum pieces also came out of those early channels. According to later recollections, the Arizona-Sonora Desert Museum’s San Francisco wulfenites from the May 1973 pocket and related finds were financed by Rukin Jelks, an important financial supporter of the museum’s mineral purchases in the 1970s. Curator Bill Panczner selected specimens from Tucson dealer Suzie Davis, who acquired many of the best examples of the period. The Smithsonian’s examples are said to have come through her as well, a reminder that the San Francisco pockets were not merely dealer stock; they entered the institutional record almost immediately.
Another collector’s memory captures the 1970s market atmosphere perfectly. Hal Krueger reportedly came upon a pickup truck full of flats of San Francisco wulfenites at Tucson in 1977. The material was so delicate that he was afraid to trim it himself, so the careful work fell to his high-school-aged son. The reward was a few trimmed thumbnails and miniatures—small pieces at the time, but later recognized as fragments of a finite and historic find. That scene, flats in a truck and a teenager doing “delicate surgery,” says much about how even great localities pass into collections: not as polished museum legends, but as fragile rock in need of nerve, judgment, and a steady hand.
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