Naica Mine, Naica, Saucillo Municipality, Chihuahua, Mexicoi

Regional Level Types
Naica Mine	Mine (Planned Future Operation)
Naica	- not defined -
Saucillo Municipality	Municipality
Chihuahua	State
Mexico	Country

Mine Information: Naica began small scale hand-mining operations in 1794. These expanded modestly through the 19th and early 20th centuries but the district was hampered by divided property ownership and long-term legal battles. The district was taken over completely by Grupo Peñoles in 1955, and grew to being Mexico's largest lead producer by 1985. The orebodies are nearly vertical and extend to considerable depth, so the underground mine is serviced by a number of shafts, declines, and spiral ramps. Currently (2024) development is focused on new orebodies lying to the west of the historic mine and production is expected to resume in another year or two,

Water is the big challenge for the mine; 60,000 l/min is pumped out to avoid flooding. The mine geologists talked of "mining" 40 tonnes of water for every tonne of ore! The groundwater is 110 m below the adit level: production when the mine flooded in 2015 was up to 900m below that. Importantly, the flooding never got within 100m of the Cave of the Crystals and they remain safe. Currently (2024) the mine appears to be moving slowly back towards production, but the ramp-up is expected to take several years.

Geology: The mine is situated on a structural dome measuring about 12 km by 7 km. Country rock is a thickly bedded Cretaceous limestone. The ore deposits occur in limestones of the Finlay, Lagrima and Benigno Formations (Aurora Group, which is over 1000m thick). The limestone sequence may be overthickened by thrust faulting. Alteration of the limestone is typically limited to a few centimeters to 8m of recrystallization or marbelization. Overlying the Finlay limestones are calcareous shales of the Benavides Formation and limestones of the Loma de Plata Formation. Virtually no alteration or mineralization extended through the Benavides, suggesting that it formed an effective seal on the system that caused the orefluids to spread out laterally below it-creating near-surface orebodies with a pattern like the roots of an inverted tree. Hydrothermally altered Tertiary (30.2 Ma) felsites occur as thin discontinuous dikes and sills that localize the ore and cut the limestone. These coalesce to the west and ultimately merge into a granitic stock west of the historic part of the mine.

The deposit consists of mantos and chimneys, but unlike most replacement deposits in Mexico where mantos and chimneys are distinguished by their geometry, the usage at Naica is confusingly based on composition. Mineralized skarns, consisting of calc-silicates with disseminated sulfides (originally found as flat-lying geometric bodies) are referred to as mantos, whereas the massive sulfide bodies are referred to as chimneys. [This confusing terminology is explained at the beginning of every publication on Naica...a clear indication of its misuse!). Massive sulfide bodies (of many geometries) occur along the sides and tops of the skarn bodies and as true chimneys and mantos extending away from the skarns. ALL massive sulfides can be traced to skarn, and many of the skarns are cored by felsite dikes which were replaced by first calc-silicates and later sulfide mineralization. These felsites thicken and coalesce to the west and lead ultimately to the granitic stock that drove the system. The granitic stock is weakly mineralized and has increased copper values in the skarns that flank it. Recently large skarn orebodies have been found surrounding the stock and these are the focus of future production.

The area is cut by 3 main faults, and most ore is found between these structures. Generally, the sulfides are controlled by the fractures and the skarns seem to be localized by the dikes. While mineralization is largely structurally controlled, the geometry of certain sulfide bodies indicates that some limestone horizons are more favorable for mineralization. Post-mineralization faults are sometimes lined with large (up to a few meters) gypsum crystals.

Sulfide minerals are mostly pyrite, galena and sphalerite with lesser amounts of chalcopyrite, arsenopyrite, pyrrhotite, matildite, kobellite-tintinaite and molybdenite. Scheelite occurs in economic concentrations. Fluorite, quartz, adularia and calcite seem to be in part contemporaneous with the sulfides but also occur as crystals in vugs in the sulfide and silicate ore. The ores show the kind of complex repeated paragenesis typical of large, long-lived skarn-replacement systems, The ores are gold, silver, lead, zinc, and much smaller amounts of copper and tungsten. Mercury was historically recovered from some of the near-surface gossans



Five major caves with gypsum crystals have been found - “Cueva de las espadas” (Cave of the Swords), "Cueva Xochitl" (Xochitl Cave, “Cueva de los cristales” (Cave of the Crystals), “Ojo de la Reina” (Queen’s Eye Cave), and “Cueva de las velas” (Cave of the Sails). The Cave of the Swords was the first cave found in 1912, and its discovery caused quite a stir. The Xochitl Cave was found next...and despite labeling attributing many gypsum crystals to the Cave of the Swords, most came from the Xochitl Cave. (Including those in the Denver Museum). Many smaller caves and crystal lined fractures have been found and had their crystals mined and sold.
In 2000, the Cave of the Crystals was encountered while driving a communication tunnel. The cave hosts selenite crystals up to 14m x 2m in length and width, respectively. These crystals formed underwater where the hot and calcium carbonate and sulfides saturated fluids transported through the Naica fault, mixed with colder fluids originating from the surface. It is estimated that the crystals have been growing for about one million years. There is an enormous volume of literature available on these crystals and their growth.

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Wikipedia:	https://en.wikipedia.org/wiki/Naica_Mine
Wikidata ID:	Q1639253

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