Figs. 1-2: Left: disseminated sulphide mineralization in sericitized intrusive rock (sparsely feldsparphyric granodiorite or quartz monzonite?). Visible ore minerals, grain size 0.5 to 4 mm, are pyrite plus galena, sphalerite and chalcopyrite. The local sphalerite is rich chocolate brown, and so probably iron-rich ("marmatite"). The galena shows classic cubic cleavage. The pyrite is often striated, the chalcopyrite tawny, tarnished. Some white calcite in abundant silica. Sample 051, loose, near Stope 4-156. Right: sphalerite and galena (respective maximum grain sizes 8 mm and 20 mm), plus finer-grained chalcopyrite and pyrite in quartz-calcite matrix. Sample 052, in situ, from the Queen Stope.
Samples shown here, 049-052, were all collected on an underground tour on Monday, 28 September 1981. The mine visit was part of a University of Toronto graduate student field trip organized by A. James Macdonald.
"Rock of the Month # 251, posted for May 2022" ---
The Charcas mine is situated 4 km northwest of the town of the same name, in the north of the state of San Luis Potosi. Charcas is about 120 km north of the state capital, the eponymous San Luis Potosi. The town of Charcas was founded in 1574. The first discovery of veins in the area followed in 1583. The region lies on a plateau at an elevation of approximately 2,000 m, between the Sierra Madre Occidental and Sierra Madre Oriental. Historically, initial finds in Mexico were often oxidized manto and chimney deposits, exploited as compact bodies of Ag-rich secondary minerals of Pb (e.g., cerussite and anglesite). At the time, associated Zn -rich oxide deposits, with >20% Zn in Fe-Mn-Zn oxides, were ignored as of no value (Megaw, 2009). Oxide ores were depleted by 1870, and underlying sulphide ores became the principal source of metals.
The basement includes diverse Paleozoic sediments, overlain by Mesozoic limestones. The sediments are intruded by a granitic stock of mid-Eocene age, varying from granite porphyry to granodiorite or quartz monzonite. Contact metamorphism around the stock has formed a thin carapace of diopside- garnet skarn and banded marble.
Mineralization occurs along veins and in manto (blanket-like) replacements of the carbonate host rocks (see, e.g., Gonzalez Reyna, 1950; Casteneda, 1991). Such limestone replacements are an important style of deposit (Megaw et al., 1988; Titley, 1993), with examples in Arizona and Nevada, Utah, Colorado and elsewhere. The veins cut Triassic and mid-Cretaceous carbonate rocks, while the mantos lie within Cretaceous sediments. Ore minerals include pyrite, sphalerite (marmatite) as well as argentiferous galena and chalcopyrite. Other minerals include tetrahedrite, arsenopyrite, bornite, covellite, digenite, chalcocite, native silver, hematite and goethite (Castenada, 1991). Chalcopyrite and galena are associated with the Ag minerals jalpaite (Ag3CuS2) and argentite (acanthite, Ag2S), while late diaphorite (Ag3Pb2Sb3S8) occurs exclusively in skarn (Payan, 1987). There are two generations of chalcopyrite, the second accompanied by galena. Ag-Cu-S phases like jalpaite occur as inclusions in the galena. The gangue includes actinolite, garnet, chlorite, calcite and borosilicates. Faulting, associated with emplacement of the felsic intrusion, controls some of the ore. Veins, where oxidized in the uppermost 100 metres, may carry secondary native silver.
Figs. 3-4: Left: coarse sphalerite with minor tawny chalcopyrite and pale pyrite, on finely granular quartz with calcite. Sample 049. Right: at least two stages of veining cutting an enigmatic granular grey skarn, an altered (?) calcarenite. The later stage of veining is rich in sulphides (striated pyrite and minor sphalerite) and quartz. Sample 050. Two samples collected loose in the King Stope.
There are (or were, in 1981) two principal ore zones, the El Rey (King) Stope and the La Reina (Queen) Stope. El Rey is reported to carry more Zn overall, La Reina more Pb and Ag. The skarn-type alteration is more extensive than the mineralization, and thus is something of a guide to the ore. By the early 1990s, the Charcas mine produced some 85,000 tonnes of Zn concentrates yearly from 3 sections, one of which was La Aurora. According to Walker (1992), reserves at that time were 17 MT grading some 60 ppm Ag, 0.41% Pb, 0.22% Cu and 5.41% Zn. Programmed grades for 1991 were (Pb concentrate) 34.3% Pb, 4700 ppm Ag, 13.8% Cu and 11.7% Zn (7,000 T/year) and (Zn concentrate) 58% Zn, 0.98% Cu, 0.44% Pb and 147 ppm Ag (85,000 T/year).
Minerals from Charcas have been used in surveys of chalcophile elements in sulphides (Burnham, 1959). Soil sampling may help detect more carbonate-hosted replacement deposits: high Zn, Pb, Cu and Ag values occur in local soils (e.g., the E1 anomalies, 15 km southeast of the Charcas mine: Anon, 2001). Polymetallic skarn, manto and vein targets occur in a wide area NNE of Charcas (Martinez Ramos, 1975), including oxide and sulphide Sb mineralization in fracture zones, with a gangue of calcite plus quartz, gypsum, cinnabar, Fe oxides and occasionally native S.
The mine and area have also yielded high-quality, cm-scale crystal specimens in the past, including the zeolite natrolite and, in particular, boron minerals (borates and borosilicates), e.g., danburite (orthorhombic CaB2(SiO4)2: Barlow et al., 1996; Dyar et al., 2001), datolite (monoclinic CaBSiO4(OH): Wanda Zyla and Irwin Kennedy, pers.commun., April 2022) and nifontovite (monoclinic Ca3B6O6(OH)12.2H2O: Hawthorne et al., 2005; Bernard and Hyrsl, 2015). A brief review of articles in ten additional issues of Mineralogical Record, concerning either recent mineral shows (Tucson, Denver) or profiles of mineral collections, notes the following range of high-end specimens from Charcas:
Lastly, a literature search for "Charcas" will also dig up a lot of research on a well-known, historic, 1.4-tonne meteorite found in the area in 1804. This particular Charcas is an osmium-rich, IIIAB iron meteorite and has featured in many important studies (e.g., Goldstein and Short, 1967; Horan et al., 1992; Morgan et al., 1992; Smoliar et al., 1996). Formerly known as Descubridora, the Charcas iron is represented in many museum collections.
Anon (2001) San Miguel yields high zinc values in soils. Northern Miner 87 no.12, 16, 14 May.
Barlow,FJ, Jones,RW and LaBerge,GL (editors) (1996) The F. John Barlow Mineral Collection. Sanco Publishing, Appleton, WI, 408pp.
Bernard,JH and Hyrsl,J (2015) Minerals and their Localities. Granite, Prague, Czech Republic / Mineralogical Record Bookstore, Tucson, 3rd edition, 920pp.
Burnham,CW (1959) Metallogenic provinces of the southwestern United States and Northern Mexico. New Mexico State Bureau of Mines and Mineral Resources Bull. 65, 73pp. plus 7 maps.
Castaneda A,F (1991) Economic geology of the Charcas mining district, San Luis Potosi. In `Economic Geology, Mexico' (Salas,GP editor), GSA DNAG volume P-3, 438pp., 279-286.
Dyar,MD et al. (2001) Reference minerals for the microanalysis of light elements. Geostandards Newsletter 25, 441-463.
Goldstein,JI and Short,JM (1967) The iron meteorites, their thermal history and parent bodies. Geochim.Cosmochim.Acta 31, 1733-1770.
Gonzalez Reyna,J (1950) Geologia, paragenesis y reservas de los yacimientos de plomo y zinc de Mexico. IGC 18, Great Britain, 1948, Part VII, `The Geology, Paragenesis, and Reserves of the Ores of Lead and Zinc' (Dunham,KC editor), 121-142 (in Sp.).
Hawthorne,FC, Pinch,WW and Pough,FH (2005) Nifontovite from Charcas, San Luis Potosi, Mexico. Mineral.Record 36, 375-376.
Horan,MF, Morgan,JW, Walker,RJ and Grossman,JN (1992) Rhenium-osmium isotope constraints on the age of iron meteorites. Science 255, 1118-1121, 28 February.
Huizing,T and Wilson,WE (2015) Mineral Collections in the American Midwest. Mineral.Record 46 no.4, supplement, 240pp.
Martinez Ramos,CJ (1975) Exploracion geologico-minera por antimonio en la Sierra de Catorce, estado de San Luis Potosi. Consejo de Recursos Minerales Seminario Interno 5 sobre Exploracion Geologico-Minera, 813pp., 23-71 (in Sp.).
Megaw,PKM (2009) Evaluation of oxidized Pb-Zn-Ag carbonate replacement deposits of Mexico in light of supergene zinc and residual lead enrichment processes. In `Supergene Environments, Processes, and Products' (Titley,SR editor), SEG Spec.Publ. 14, 149pp., 51-58.
Megaw,PKM, Ruiz,J and Titley,SR (1988) High-temperature, carbonate-hosted Ag-Pb-Zn(Cu) deposits of northern Mexico. Econ.Geol. 83 no.8 (Mexico special issue), 1856-1885.
Morgan,JW, Walker,RJ and Grossman,JN (1992) Rhenium-osmium isotope systematics in meteorites I: magmatic iron meteorite groups IIAB and IIIAB. Earth Planet.Sci.Letts. 108, 191-202.
Payan F,M (1987) Charcas Mine. Mining Mag. 156 no.1, 20-27, January.
Smoliar,MI, Walker,RJ and Morgan,JW (1996) Re-Os ages of group IIA, IIIA, IVA, and IVB iron meteorites. Science 271, 1099-1102, 23 February.
Titley,SR (1993) Characteristics of high-temperature, carbonate-hosted massive sulphide ores in the United States, Mexico and Peru. In `Mineral Deposit Modeling' (Kirkham,RV, Sinclair,WD, Thorpe,RI and Duke,JM editors), GAC Spec.Pap. 40, 770pp., 585-614.
Walker,S (1992) La Aurora means dawn of new mining era at Charcas. Eng.Min.J. 193 no.5, 16HH-16KK, May.
Wilson,WE (2005) The Mineralogical Record Label Archive. Mineral.Record 36, 451-457.
Graham Wilson, posted 30 April-02,04 May 2022.
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