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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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