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Geology and Minerals from the Zn-Pb-Cu-Ag Mine March 2003 | |||||||||
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Picture shows the Charcas mine with view to the west. Process plant and general shaft are located at the right site of the picture. The San Bartolo shaft is seen in the left centre. In the background low mountains composed of limestone. | |||||||||
Introduction The Charcas mine is located 4 km to the NW of the small town of Charcas at an elevation of about 2175 m above sea level. It is situated 110 km to the NNW of the city San Luis Potosi, capital of the state of San Luis Potosi, Mexico. The region of Charcas is rather dry, raining occurs principally during the summer. The Charcas mines look back on a long mining history. This mine became famous for the world’s finest danburite crystals, but also for the great variety of beautiful calcite and the rare natural citrine crystals. Currently the mine is operated by the company “Industrial Minera Mexico” (IMMSA) a subsidiary of “Grupo Mexico”. | |||||||||
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Location map for the Charcas deposit in Mexico | |||||||||
History The Spaniards in the vicinity of Charcas discovered silver ore in 1572, with an average silver grade (in 1583) of about 1.5 kg per metric ton. Consequently the original village of “Santa Maria de las Charcas” was founded near to the mine, but was moved a few years later to its current location. Mining for silver ore was performed with interruptions over hundreds of years. The rich silver ore was mined from veins (San Isabel) and the “La Bufa” skarn ore body. In about 1911 commenced systematic mechanized underground mining. Since 1924 the Charcas mine is owned and operated by the mining company Industria Minera Mexico (IMMSA). Up to the present over 30 million tons of ore have been extracted.
Present Mining Currently IMMSA is extracting and processing 4500 tons of ore per day. The average grade per ton is 100 to 150 g silver, 1% lead, 5.5 to 6% Zinc, 0.4% copper and traces of gold. The mine has 18 levels, level 18 is situated at an elevation of 1550 m, this results in a mining depth of 650 m. Copper grades increase with depth, below level 16 copper may exceed 1% per ton. The mine is subdivided into three mines (see plan below): Aurora mine, Rey/Reyna mine and San Bartolo mine. | |||||||||
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Longitudinal section for the Charcas mine. Yellow zones represent mined out areas. Areas with current mining are shown with blue and bright-green color. The dark-green hatched zones show areas with indicated ore reserves (together with proposed ramps and shafts in red color). Original mine plan from IMMSA | |||||||||
Access exists via ramps, the Leones shaft, the General shaft and the San Bartolo shaft (not in use). For the future it is planned to exploit the ore bodies to a depth of 900 m. Ore reserves between level 12 and 18 are 18 million tons with less than 0.5 million tons left in the Aurora ore body. The total ore reserve to 900 m depth is over 25 million tons.
General Geology The regional Geology is dominated by folded and thrusted limestone strata of Cretaceous and Jurassic age. Principal rock units are massive limestone of the Zuloaga Formation (Jurassic) and the Cuesta del Cura Formation (Cretaceous). Several km to the west these limestone units are underlain by the Formation Zacatecas, a clastic sedimentary rock sequence composed of sandstone, siltstone, and shale (see geologic map and schematic geologic section). | |||||||||
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Simplified geologic map for the Charcas region | |||||||||
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Diagrammatic geologic cross section (not to scale, but about over 5 km length) for the Charcas deposit, showing relationship between intrusive and sedimentary rocks, faults, ore bodies and veins | |||||||||
In the mine area the limestone sequence has been intruded in an about 4 km long and one km wide WNW-ESE trending zone with irregular shaped stocks of subvolcanic dacite and numerous mainly west-east trending dikes of dacite and quartz-latite. The dacite and quartz-latite exhibit a porphyritic texture with phenocrysts of plagioclase, quartz, sanidine and abundant hornblende. The age of the intrusive rocks has been determined by Ing. Diaz de Leon (in 1999, personal communication from the mine geologist) to 33 million years. In the mine area, to the north of the intrusive rocks is located a major fault zone, striking NW-SE and dipping to the NE. Here the limestone strata has been displaced by several hundreds meters. Mineralization appears in veins and replacement bodies (also as mantos), principally near to the intrusive contacts at the north and south side of the dacitic stocks. Mineralization in veins trends principally NW-SE, replacement/skarn ore bodies trend mainly W-E. Mineralization is controlled by dikes and sills of dacite/quartz-latite, intrusive contacts, thrust zones/ bedding in limestone, and NW-SE faults (see the schematic geologic section). To a minor extend appear also veins and replacement ore bodies along N-S and NNE to NE structures.
Mine Geology The principal mineralized structure is the San Isabel (eastern zone) – La Reyna (western portion) vein system with NW-SE strike and NE dip, situated along the major fault system. The vein system is economically mineralized over a length of about 2 km. Often this vein system is strongly related to dikes (5 to 25 m wide) of dacite or quartz-latite. The host rocks for the veins are recrystallized limestone, skarn or dikes. The veins consist in open space filling, but often also in replacement ore (replacement of limestone by ore-minerals). Vein width in the San Isabel-Reyna system varies in ore bodies between 5 and 10 m, replacement ore bodies in the vein system are up to 30 m wide (see idealized cross section of San Isabel vein). | |||||||||
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Idealized geologic cross section for the San Isabel vein system in the San Bartolo mine. Original plan from IMMSA | |||||||||
Current mining (see longitudinal mine section) exploits the San Isabel vein zone in the San Bartolo mine (accessed via the General shaft) between level 14 and 18 (500 to 650 m depth) and the La Reyna vein zone, together with other replacement/skarn bodies, in the El Rey-Reyna mine (accessed by ramp and the Leones shaft) between level 10 and 18 (400 to 650 m depth). In the Rey-Reyna mine appear to the south, sub-parallel to the La Reyna vein, large steep dipping replacement/skarn ore bodies hosted by recrystallized limestone. The two principal replacement/skarn ore bodies are named La Trinidad and El Rey. They occur to the south and north of an irregular dike zone of dacite/quartz-latite and dip to the north. The replacement ore bodies strike E-W and approach a width of up to 100 m. In level 16 both replacement ore bodies intersect the La Reyna vein system. Below level 16 the La Reyna vein consists only in limestone replacement ore, it merges at this depth with the Trinidad and El Rey replacement/skarn ore bodies and therefore it is nearly impossible to distinguish below level 16 the veins from replacement ore bodies. Another separate ore body, the Aurora mine, exists to the southwest of the intrusive zone. The Aurora ore zone consists in flatly north dipping tabular ore bodies (mantos), from about 5 to 30 m thick. They appear at depth between 50 and 250 m below surface, but do not outcrop at surface. The Aurora ore manto follows the folded bedding of the limestone, but occurs also above and below an intrusive sill of dacite (see Aurora mine cross section). The Aurora ore bodies are controlled by the intrusive contact to the dacitic sills, but also by bedding and thrust zones of the limestone. The Aurora ore body is almost exhausted and exploitation might be discontinued during this year. | |||||||||
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Cross section for the Aurora mine, note the strong spatial relationship between ore bodies and a dacitic sill. Original plan from IMMSA | |||||||||
Mineralization Earliest event in the development of the Charcas deposit occurred before ore deposition near to and at the contacts of the intrusive stocks and dikes, it included recrystallization from limestone to marble and formation of exoskarn (replacement of limestone) and endoskarn (replacement of intrusive rock). Principal skarn minerals are green and red grossular garnet, wollastonite, quartz, calcite, pyroxene and as late stage epidote in the endoskarn. Fluid inclusion measurements in garnet indicate a skarn formation temperature of 400° to 450° Celsius. Mineralization with Zn-Pb-Cu-Ag ore minerals and gangue minerals followed the early skarn formation as a high temperature limestone replacement ore, in a later cooler event also as hydrothermal vein fillings. Limestone replacement ore may or may not contain calc-silicate skarn minerals, especially garnet. Ore minerals include abundant dark-brown sphalerite, pyrite, galena, chalcopyrite, tetrahedrite and traces of silver-sulfosalts. The replacement ore occurs as massive sulfide ore, breccia ore, but mostly as banded “zebra ore”, where bands of ore minerals intercalate with bands of course crystalline calcite and minor quartz. In a later stage at lower temperatures fluids rich in boron deposited in open space (vugs and veins) massive green datolite, danburite, quartz, ore minerals and finally calcite. Investigation of fluid inclusion indicated high temperature and salinity during the early ore formation in the replacement ore bodies and lower temperatures in veins and late open space fillings. Measured homogenization temperatures in sphalerite range from 220° to 356° Celsius with a measured salinity (NaCl) between 9 and 25 wt%. In calcite from the “zebra ore” homogenization temperature ranged between 300° and 320° Celsius and a salinity of 0.7 to 5.4 wt%. In quartz (from early to late stages) homogenization temperature ranged between 230° and 377° Celsius and a salinity to 9.2 wt%. It has been estimated that the parent hydrothermal solution responsible for the formation of the Charcas deposit had a temperature of 400° Celsius (all fluid inclusion data are from unpublished IMMSA data and reports). The Charcas deposit type can be included to the “high temperature-carbonate hosted Ag-Pb-Zn-Cu deposits” as proposed from Peter Megaw et. al. (1988, Economic Geology, Vol. 83, p. 1856-1887). Other deposits in Mexico of this mineralization type include for example Naica, Concepcion del Oro, Velardena, Santa Eulalia, San Martin/Sabinas and Real de Cartorce. | |||||||||
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Image shows the typically banded texture of the “zebra ore” | |||||||||
Collector Minerals Danburite: CaB2(SiO4)2 The clear crystals occur generally only above level 14, but especially and most abundant in the Aurora ore body. As in the Rey-Reina and San Bartolo mine exploitation is performed currently below level 14, the only site for clear danburite is confined to the Aurora mine. Here freely grown danburite crystals appear in vugs (some over 1 m in diameter) within the manto replacement body. Those cavities developed from dissolution of limestone during the ore formation. Danburite crystals may have inclusions of older minerals (mainly chalcopyrite) and are often overgrown by quartz (also amethyst and citrine), calcite, a thick cover of gray colored clay-minerals (not yet analyzed) and late aragonite. Occasionally danburite is altered and partly replaced by clay-minerals (?) and opal, there appear also pseudomorphs of calcite and quartz after danburite. Since reserves in the Aurora mine are almost exhausted, the recovery of clear danburite will decrease sharply in the very near future. | |||||||||
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Cover-page of the German mineral magazine “Mineralienwelt” showing a perfect twin crystal of clear danburite (4.5 cm long, and twinned parallel to c-axis), sitting on a larger danburite crystal. The purple crystal in front is amethyst. Collection from Matthias Jurgeit | |||||||||
Datolite CaBSi4(OH) | |||||||||
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Former cavity has been filled completely with massive green datolite (crystal faces visible in the center of the image) and late massive danburite | |||||||||
Quartz There appear also short and long prismatic amethyst crystals and smoky quartz crystals to 10 cm length, a few with phantom crystals. Crystals to about 2 cm of a late quartz stage with pseudo-hexagonal shape are perched often on danburite crystals. Sometimes this late stage quartz occurs as beautiful natural citrine (this is not a heat treated amethyst). A couple of years ago numerous specimens with superb doubly terminated citrine crystals of gem quality to 3 cm length were found and are today highly appreciated under collectors.
Calcite
Other Minerals In the oxide zone of the deposit, exploited decades ago, were found native gold, native copper, native silver, chrysocolla, smithonite and other oxide minerals. Good crystallized sulfide-mineral specimens like sphalerite, galena or chalcopyrite are extremely seldom. Nevertheless pyrite may occur in extraordinary beautiful groups with cubic and octahedral crystals to 3 cm, perched on large danburite crystals. There have been found tiny crystals of pyrargyrite, acanthite and sulfosalt minerals. About 4 years ago a large pocket with superb fibrous jamesonite crystals to several cm long sitting on milky quartz, was recovered. Rather abundant but generally as small crystals appear apophyllite and several zeolite minerals including mesolite and natrolite. Extremely rare are fluorite (colorless cubes and octahedrons to 2 cm) and barite. From the skarn minerals the green grossular garnet with a diameter to 2 cm is rather attractive. The other calc-silicate minerals are not freely crystallized and therefore rarely collected. Aragonite is very abundant, found as a late stage mineral in open vugs and cavities. Aragonite has been deposited from cold descending solutions, it appears as spherical botryoidal masses or as large stalactites. | |||||||||
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Specimen with several cm long acicular crystals of jamesonite in the hand of a Mexican mineral collector | |||||||||
Acknowledgements I would like to express my appreciation for the management of the IMMSA mining company for the permit to access the mine, full support, and the access to unpublished mine reports, data and drawings. I am especially grateful to the mine geologist Ing. Armando Arranda Rodriguez for his help in general and the detailed introduction to the mine geology. | |||||||||
Literature Cook, Robert B. (2003): “Danburite”. Rocks and Minerals. Vol. 78, Iss. 6; p. 400 | |||||||||
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Figure 1. Perched on matrix are two water-clear nifontovite crystals, about 3 cm high and up to 0.6 cm thick (specimen OC-01). White crusts consist of lizardite, clinochlore and calcite. Photo & collection M. Jurgeit |
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Among many mineral dealers and collectors it was believed that no additional nifontovite crystals or specimens had been retrieved after this initial find (therefore those single crystals were traded at very high prices). Nevertheless a considerable amount of matrix-specimens had been recovered from the same site a couple of days and weeks later. A great portion of those “early” specimens ended up in three private Mexican mineral collections, but a few did also enter the mineral collector market. I am very grateful that these, here unnamed, collectors gave me the opportunity to acquire the principal part of their collections. In the second half of 2009 (probably around September) the same nifontovite-bearing fault zone was encountered again during mine operation and local miners resumed with the extraction of nifontovite specimens (of yet unknown quantity and quality). Certainly the nifontovite discovery at Charcas in 2008/2009 represent the worldwide largest and most spectacular find of this rare borate mineral. To this find belong most, if not all, of the finest nifontovite specimens.
Charcas is a Zn-(Pb-Ag-Cu) skarn and CRD deposit (high-temperature-carbonate-replacement deposit) that formed in and around stocks and dikes of dacite and quartz-latite, intruding Cretaceous limestone. Several hydrothermal stages at lower temperatures took place and overprinted the earlier mineralization. Mineralization occurs as replacement of limestone and as open space fillings in veins. Typical gangue minerals are danburite, datolite, quartz and calcite, less abundant are garnet and vesuvianite. Principal metallic minerals include pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, pyrrhotite and small amounts of Ag-sulfosalts. As I had not had the opportunity to examine the nifontovite discovery site in person, it is not possible for me to give a first-hand description of the local geology or the surrounding host rock. All of the following descriptions are based on the investigation of obtained sample material and mineral specimens. The larger nifontovite specimens and samples represent a matrix-supported breccia. The often highly porous and vuggy matrix consists mainly of acicular or radially-fibrous nifontovite crystals up to several cm long, often of brownish to gray color due to abundant inclusions. Enclosed angular and subangular rock fragments range in size from less than 1 cm to about 10 cm. These clasts consist of white calcite marble and an unidentified completely altered rock (mainly to nifontovite-lizardite). The partly preserved porphyritic texture indicates an igneous rock of basic or intermediate composition (intrusive dike or stock). There are also small angular fragments, smaller than 0.5 cm, of garnet-vesuvianite skarn. The white marble clasts contain traces of metallic minerals (possibly sphalerite, Ag-sulfosalts and a bit of pyrite) and about 2-5% of mm-size aggregates composed of tiny grains and crystals of yellow garnet (andradite), vesuvianite and white clinochlore. The color of the matrix is gray, cream and pale brown. The coarsely fibrous matrix-nifontovite contains abundant inclusions and is covered with crusts of an older mineral association. Water-clear nifontovite crystals formed either in matrix cavities that opened during a late brecciation event still contemporaneously with nifontovite crystallisation, or in cavities protected from in-falling rock- and mineral debris. XRD analysis (through B. Rixen, Beindersheim, Germany) was utilized for determining the matrix mineral assemblage. Tiny crystals, but also angular grains (mostly from 0.01 to 1 mm in size) of yellow garnet (principally andradite) and angular fragments of green vesuvianite represent relict minerals and predate the low-temperature hydrothermal event responsible for the precipitation of nifontovite. Three Mg-rich minerals crystallized before nifontovite. Fe-poor clinochlore (Mg,Fe++)5Al(Si3Al)O10(OH)8, a mineral from the chlorite group, occurs as mm-size aggregates consisting of white flakes (from 0.01 to 0.5 mm). It formed early, probably coevally with yellow andradite garnet, during contact metamorphism. The second represents an unidentified, now pseudomorphosed, mineral species showing often nearly perfect octahedral shape. Individual crystals of this unidentified precursor mineral range in size between 0.1 and 1 mm, but can appear as crystal aggregates several mm across. Octahedra and distorted or skeletal octahedra are common, indicating a cubic or tetragonal crystal system. This unknown mineral most likely was magnesium-rich, as it has been replaced completely by the Mg-rich phyllosilicate lizardite having the formula Mg3Si2O5(OH)4. The most probable candidate for this unknown precursor mineral is the cubic mineral periclase MgO, which forms typically from dolomite or magnesite in a contact-metamorphic environment (at high temperatures). Lizardite usually forms at low temperatures (< 100° C); it is a well-known serpentinization mineral, occurring as a pseudomorph after olivine. Here however, white, cream or pale brown colored microcrystalline lizardite replaced an unknown precursor mineral (periclase?) before or perhaps during the formation of nifontovite. There exists no evidence (no relicts or textures) for the original presence of danburite or datolite at this place. Therefore the formation of nifontovite through hydrothermal dissolution of precursor danburite or datolite is not indicated. It is suggested that nifontovite crystallized directly from B-rich, silica-poor, very low-temperature epithermal solutions. During crystallization of the nifontovite, tiny mineral particles and crystal aggregates (andradite, vesuvianite, clinochlore and lizardite pseudomorphs) fell off the surrounding wall-rock breccia and were completely or partly incorporated into the nifontovite crystals. As the latest epithermal stage, crusts of clear calcite formed locally. The crystals are smaller than 1.0 mm. This hydrothermal calcite mostly covers marble clasts, but also overgrows clinochlore, garnet, vesuvianite and lizardite. Only rarely is it sitting on nifontovite. This late calcite stage shows strong fluorescence of dark red color under short-wave ultraviolet light. No quartz has been observed in this mineral assemblage. Another mineral detected through XRD analysis was covellite.
Nifontovite Crystal HabitsClear and transparent nifontovite crystals appear in cm to dm size cavities within the matrix. Two kinds of crystal habits can be recognized. The first is already known from the single crystals that reached the collector market in late 2008 (see Figure 1). These are columnar crystals up to 3.5 cm in length dominated by four prism faces of the form {110} exhibiting striations parallel to the pinacoid {001}. Crystal tips exhibit modified forms including the pinacoid {001}, but also {111} (with striations) and {11X} (several forms with X greater than 1, all with striations). Cleavage is almost perfect parallel to {001}, but imperfect parallel to {110}. The second habit is represented by more acicular, sometimes spear-shaped crystals (Fig. 9), occasionally flattened. Crystal aggregates composed of parallel-acicular intergrown individuals are common; sometimes these can exhibit a sheaf-like appearance (Fig. 10). Noteworthy are doubly terminated crystals, the longest undamaged crystal (with parallel-growth) approaching a length of 4.4 cm. This doubly terminated crystal-aggregate is perched atop a matrix-specimen with a great number of additional large nifontovite crystals, representing one of the world’s finest nifontovite specimens. The largest observed crystal from this find observed so far is 8.1 cm long and up to 1 cm thick; with other crystals it forms a gorgeous crystal group. Unfortunately, both crystal tips are broken as these were attached to matrix. Although damaged, it might represent the world’s largest nifontovite crystal. Crystal forms for the second habit are the same as those observed from the first, however, in addition occasionally there appear small faces belonging to the prism form {010} or {100} and additional modified forms at the crystal tips including {011} and {01X} or {101} and {10X}.
ACKNOWLEDGEMENTS Special thanks are due to my friend Boris Rixen for providing XRD analysis and Peter Modreski and John White for reviewing the manuscript.
REFERENCES Jurgeit M. (2003) - Geology and Minerals from the Zn-Pb-Cu-Ag Mine of Charcas, San Luis
Potosi, Mexico. Internet: www.matsminerals.com/html/charcas.html Internet-Link: Nifontovite mineral data: http://webmineral.com/data/Nifontovite.shtml Picture Gallery for Nifontovite Specimensall photos by the author, of specimens from his collection |
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Figure 2. Matrix supported breccia (specimen N-12) with angular fragments of white marble, 2-4 cm in diameter; pale brown matrix consists mainly of coarsely fibrous inclusion-rich nifontovite and grains and masses of cream-colored lizardite (pseudomorphous after an unknown mineral). The matrix also contains finely crystalline calcite, clinochlore, andradite and vesuvianite. Perched on matrix are countless transparent nifontovite crystals, 1-2 cm in length with partly dull crystal surfaces (due to a thin cover of a powdery unidentified mineral). The specimen is 15 cm across. |
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Figure 3. Superb nifontovite group with crystals to 5 cm in length, most are of pale cream color due to abundant inclusions of lizardite and clinochlore, but a few crystals are also water-clear. Specimen (OC-04) size is 10x8x8 cm. |
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Figure 4. Heavily included (mainly with lizardite) columnar nifontovite crystals up to 3 cm in length, freely crystallized growing from the matrix into a cavity. Within a separated vug (protected from in-falling rock debris water-clear nifontovite crystals formed (lower left portion of the specimen) approaching lengths of up to 4 cm, most crystal tips are however damaged. White wall-rock material consists mainly of clinochlore, calcite and traces of garnet. The dark spot contains fine-grained metallic minerals, probably covellite. The specimen (Z-4) is 22 cm across. |
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Figure 5. Close-up of figure 4 (specimen Z-4). Horizontal field of view is 6 cm. Shown are nifontovite crystal groups with inclusions and crusts of lizardite (pale brown) and clinochlore (white). Note the clear termination on one of the nifontovite crystals. |
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Figure 6. Nifontonite crystal group (specimen I), 5 cm across: on top of a larger (broken) nifontovite crystal smaller gemmy crystals are sitting. Several small mineral aggregates (white color) occur as inclusions or cover nifontovite as crusts; they consist of a fine-crystalline mineral assemblage of white lizardite (tiny octahedra), white clinochlore and yellow garnet (less than 0.1 mm). The clinochlore and garnet are older, but lizardite (pseudomorph after periclase?) could have formed contemporaneously with nifontovite. Late, finely crystalline calcite overgrows partly exposed lizardite and clinochlore. |
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Figure 7. Shown are gemmy, about 1 cm long nifontovite crystals (specimen N-9) on matrix. Associated are cream-colored lizardite pseudomorphs after an unknown precursor mineral (periclase?) as distorted octahedra up to 1 mm. Horizontal field of view is 4.0 cm. |
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Figure 8. Close-up of figure 7. Horizontal field of view is roughly 2 cm. Intergrown with coarsely acicular nifontovite are mm-size cream-colored pseudomorphs of lizardite after an unknown precursor mineral (periclase?) showing a distorted octahedral habit. |
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Figure 9. Close-up of specimen N-10 showing spear-shaped, transparent nifontovite crystals to 3 cm length. Horizontal field of view is 5 cm. |
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Figure 10. Nifontovite crystals exhibit partly a sheaf-like habit. The doubly terminated composite-crystal in the center is 3.2 cm long. Front of specimen (N-4) measures 6.5 cm. |
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