Naica Mine, Naica, Saucillo Municipality, Chihuahua, Mexicoi

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

Mine Information: Naica began operations in 1794 and was taken over completely by Grupo Peñoles in 1955. The underground mine is serviced by a number of shafts, declines, and spiral ramps. Processes are highly automated. Ore is crushed underground and transported to the surface by conveyer.

Water is the big challenge for the mine; 60,000 l/min is pumped out to avoid flooding. The groundwater is 110 m below the adit level: production when the mine flooded in 2015 was up to 900m below that. Currently (2021) 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 Lorna 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 mantos) 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.

The area is cut by 3 main faults, and the 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



Four major caves with gypsum crystals have been found - “Cueva de las espadas” (Swords Cave), “Cueva de los cristales” (Crystal Cave), “Ojo de la Reina” (Queen’s Cave), and “Cueva de las velas” (Sails Cave).
In 2000, a cave 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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