New Mexico Mineral Symposium — Abstracts

Naica is arguably the world’s best-known mineral locality—not because of the wonderful specimens of fluorite, sulfides and anhydrite that grace many of the world’s finest collections, but because of the enormous gypsum crystals in the Cave of the Crystals. Far beyond the mineral collecting community, these giant crystals have captured the imagination of millions worldwide—making them perhaps the first global crystal celebrities. Their sheer size makes them too big to even contemplate removing from the mine, which protects them for now but ensures that when the mine eventually closes permanently and floods they will be lost to us forever. Massive inflows of water inundated the deepest reaches of the mine in 2015 and forced closure, but the Cave of the Crystals was never touched by the flooding. Exploration teams have successfully identified new resources to the west and the mine is now headed back into production!

Geologically, Naica stands with related Carbonate Replacement Deposits in northern Mexico like Santa Eulalia, Ojuela, Los Lamentos and San Pedro Corralitos—districts famous for deep oxidation, elongate flat-lying “manto” and tubular “chimney” orebodies and long mining histories. Naica’s story is different—she was not discovered until 1794 and her near-surface oxidized ores were too limited to support a sustained bonanza—so her history (and mineralogy!) is much less colorful. Only the top of the famous “Torino–Tehuacan” Chimney reached the surface—as an outcrop a mere 15 meters in diameter. This orebody produced half a million tonnes of oxide ores before the water table and sulfide ores were encountered and the mine was largely abandoned. The bulk of the deposit was not discovered until the mid-1900s, when the sulfides were followed to depth. Through the 1960s and 70s Naica became an increasingly important operation, producing over 40 million tonnes of ore by 2015. Mining has shown that Naica is composed of over 80 sulfide “chimneys” up to 800 meters tall that coalesce at depth into giant bodies of mineralized skarn (Fig. 1). It is probably the dominance of vertical orebodies that makes Naica such a superb primary species specimen locality; as the chimneys grew upward they collapsed periodically, repeatedly creating void-rich breccia columns into which large crystals of ore sulfides and gangue species could grow.

The Sierra de Naica is a broad northwest-southeast elongate dome 1.5 by 4 km developed in a thick sequence of homogenous Lower Cretaceous limestones of the Benigno, Lagrima and Finlay Formations, commonly referred to as the Aurora Group. These limestones are underlain by anhydrite-rich evaporites of the Cuchillo Formation and overlain by Upper Cretaceous shales and sandstones. This host rock sequence was folded and thrust faulted during the Laramide Orogeny (60–55 Ma) and the mine lies in a secondary flexure developed on the northeast flank of the principal dome. The dome is cut by prominent pre-mineral southwest-dipping thrust faults and high-angle N25-40E and N50W trending fault and fracture sets with pre- and post-mineral movements. Several show over 1 km of displacement and host the major gypsum caves. The mine area was invaded on its southwest side by a granitic intrusion, known from a combination of geophysics and drill holes. A series of crowded porphyry to aphanitic felsic dikes and sills, dated at 30.2 Ma, were derived from it and were intruded along the thrust fault upwards from the southwest to the northeast. The felsites also invaded the northeast and northwest faults, especially at their intersections. Early skarn-forming hydrothermal fluids followed the felsites laterally and vertically for several kilometers. In the proximal zone, the skarn-forming fluids replaced both the felsite and the surrounding limestone to massive calc-silicates, but with increasing distance from the fluid source the skarn development becomes progressively limited to the limestones, leaving a shell of skarn surrounding a felsite core. The sulfide mineralizing fluids followed, cutting and filling fractures in the skarn and developing massive sulfide replacement bodies in the surrounding limestone. Wherever the sulfide-mineralizing fluids encountered high-angle structures they followed them, especially where two sets intersected. The ultimate result was a series of orebodies that thicken and coalesce to the southwest towards the intrusive source. This includes 17 large skarn bodies that ramp upwards to the northeast and more than 75 massive sulfide chimneys that sprout off the upper surface of the skarn bodies. The skarn bodies range up to 25m thick and 500 x 500m wide and long. The massive sulfide chimneys range from 3 to over 80m in diameter and up to 800m tall— the largest being the Torino–Tehuacan chimney—the only orebody that reached the surface.

Mineralization at Naica is progressively zoned from copper-rich sulfide-mineralized skarns near the porphyry intrusive in the deep southwestern zone through intermediate zinc-lead-silver sulfide-mineralized skarns developed around felsite dike and sill offshoots from the porphyry to distal massive sulfide ores developed along the skarn margins and extending upwards as tubular chimneys. The sulfides are everywhere younger than the skarn silicates and either replace or cut them. The sulfide assemblages are dominated by pyrrhotite or pyrite (and pyrite after pyrrhotite), sphalerite, galena, chalcopyrite and arsenopyrite, with lesser amounts of complex sulfosalts, molybdenite, scheelite/powellite and cassiterite. Gangue mineralogy is dominated by massive garnet, epidote and vesuvianite skarn and void-filling quartz, calcite, dolomite, fluorite, anhydrite and gypsum.

Thirty million years of erosion exposed the uppermost sulfide ores to oxidizing surface waters, which removed most of the soluble sulfur, zinc and copper, leaving iron-oxide gossan bodies with insoluble secondary lead and silver minerals residually enriched by the removal of the soluble materials. These were the first ores encountered in the district and were largely worked out long before mineral collecting became widespread. By 1958 the various oxide producing mines in the district (including: the Descubridora, Estrella, Gibraltar, Lepanto, Maravillas, Reinas, San Francisco, San Francisquito, San Patricio, Santa Juliana, Siglo XX, and Xochitl mines) had all been consolidated into the Naica Mine and most access to them was sealed off. This means specimens of oxide minerals from the district are extremely rare outside museums and unless they are reliably attributed to a specific mine by a reliable source, it is safest to label them simply “Naica District.” The closure of the oxide mines coincides both with the beginning of exclusive sulfide mining and significant mineral collecting from the district, so the overwhelming majority of primary sulfide and related gangue minerals from the sulfide zone are properly labeled “Naica Mine.” Many dealer labels cite the “Gibraltar,” “Maravillas” and Siglo XX mines, but none are valid for sulfide-mining era specimens. Proper labeling for most non-oxide specimens should be: Naica Mine (or Mina Naica), Naica, Saucillo Municipality, Chihuahua, Mexico.

Hot water, gypsum, and caves are inextricably intertwined at Naica. From the discovery of the Cave of the Swords in 1910, to the Cave of the Crystals in 2000, large voids lined with huge gypsum crystals have given Naica a prominent mineralogical profile. New insights into crystal growth and over 35 cave minerals have been documented through detailed scientific studies stimulated by the giants of the Cave of the Crystals. This combination also gave Naica a complicated mining history because she has long been plagued by floods of hot, sulfate-laden waters that pour into the workings. These began as 1,000 gallons per minute (gpm) inflows of 45oC water in the 1920s that grew to 18,500 gpm of 54oC water by 2006. Moving the equivalent of nearly 40 tons of water per ton of ore was an expensive challenge that ended abruptly in 2015 when unstemmable inflows estimated at over 40,000 gpm poured out of an open fault at depth and forced closure. But simply keeping the pumping infrastructure functional was a thornier problem because as the waters cooled below 52oC, gypsum crystals deposited everywhere-in sumps, pipes and even valves (Fig. 2). These human activity-related crystals grow to specimen size in few months and are routinely harvested with many getting mislabeled as coming from the Cave of the Swords. This misattribution is also true for most of Naica’s natural gypsum specimens, when in fact, most natural gypsum crystals in collector’s hands came from one of the myriad smaller caves scattered throughout the mine. Again, safest is to simply label them Naica Mine. Notably, no specimens have been harvested from the Cave of the Crystals.