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Taxco |
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Guerrero, Mexico |
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Ag Pb Zn Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Taxco silver-lead-zinc deposit is located in Guerrero state of southern Mexico.
Small scale silver and gold workings in the district predate the arrival of Europeans. Following the conquest of the Aztecs, iron and tin were worked in the district by the Spaniards, followed by the discovery of silver in 1534. Further important rich discoveries of silver were made in the 18th century, with a society being formed in 1778 to exploit the mineralisation. At the end of the 19th and into the early 20th century mining declined due to civil unrest during the Mexican civil war. Production during the 18th and 19th centuries totalled around 4000 t Ag. In 1942 ASARCO acquired a number of the mines and commenced production, with the Guerrero mine being the main operation. In 1986 the ore grade was 140 g/t Ag, 1.5% Pb, 3.8% Zn. Production grades in 1990 were 125 g/t Ag, 0.7% Pb, 2.2% Zn (GIMMSA, 1990).
Regional Setting
For a brief overview of the distribution and character of the deposits in the carbonate replacement and related vein Pb-Zn-Ag belt in Mexico and the western United States, and links to the deposits of that belt, see the Regional Setting section of the Fresnillo record.
Geology
The host sequence at Taxco is as follows, from the base (GIMMSA, 1990):
Jurassic,
Taxco Schist, unknown thickness - micaceous, talcose and chloritic schist, with abundant segregated quartz lenses, alternating with phyllites. Locally this is unconformably overlain by an interstratified sequence of andesitic tuffs, breccias and lavas which are also of late Jurassic age, overlain in turn by early Cretaceous limestones.
Unconformity
Cretaceous,
Morelos Limestone, up to 900 m thick - comprising a succession of light grey to dark coloured, thick limestone and dolomite strata. Interbeds of argillaceous limestone and calcarenites are also present, as are thin beds of chert. It is of Aptian to Cenomanian age and is the main host to manto replacement ores. The lower contact is a zone of mylonite and gouge, implying the presence of a Laramide thrust plane.
Unconformity
Mexcala Formation, up to 1200 m thick - occurring as thin beds of shale and sandstone with thicknesses of 1 to 30 cm. The base is calcareous with thin interstratified limestones between the shales, while the upper sections of the unit, which are more shaly, contain thicker 2 to 8 m limestone bands.
Unconformity
Tertiary,
Balsas Group, composed of a great variety of rocks, mainly red terriginous continental sediments, including fine grained sandstones, marls, sandy conglomerates and sub-angular conglomerates. Clasts are limestone, shale, schist and igneous rocks, cemented by a sandy or clay cement. Bedding varies from a few centimetres to several metres. It is of late Eocene to early Oligocene age.
Tilzapotla Rhyolite, composed of pink coloured rhyolite with fluidal, spherulitic or porphyritic textures, with accompanying tuffs and ignimbrites. The rhyolite has been dated as Oligocene, at 33.1±0.8 Ma.
Intrusives
Three intrusive phases have been mapped in the Taxco district. The first and oldest are greenish-grey intermediate dykes and sills with extreme alteration and small pyroxene phenocrysts. These are classified as dioritic and only intrude the Taxco Schist. The second occurs as stocks and dykes of acid composition, cutting all of the rocks of the district. The majority of the dykes occupy the same fractures as the ore veins, varying from trachyte, to rhyolite, with intense kaolinisation. The third and latest are dykes and plugs of basic composition, generally cutting and displacing veins.
Structure
Deformation comprises two periods of faulting, pre- and post-mineralisation, and two periods of folding. The earlier of the folding episodes accompanied metamorphism. Two systems of important faults are recognised, related to the folding. The first affected the sequence up to the base of the Eocene Balsas Formation, and has north-south and 300° strikes; while the second post dates the Tilzapotla rhyolite. The main example of the second phase of faulting strikes at 290° over a length of 6 km, and has a dip of 60°NE (GIMMSA, 1990).
Mineralisation
Mineralisation within the district is present as five different types of deposit, which are, in order of importance (GIMMSA, 1990):
Fissure Veins - which are represented by two systems distributed over a substantial area within the Taxco-Zitacuaro massif. The two systems have trends of north-south and 300° respectively, and variable dips. The extent of the veins is irregular, depending on the host rock. In schists and phyllite the veins are well defined with silicification along the vein walls. In limestone however, the vein is lost and replacement bodies with rich shoots occur at the fracture contact. Where the structure is at the contact of shales and sandstones, the fissure is very erratic, and Pb and Ag values decrease with depth, although Zn increases. The paragenetic sequence comprises a first stage of quartz, argentiferous galena, sphalerite and pyrite; and a second, much younger silver rich phase, associated with the mafic intrusives, and characterised by proustite, pyrargyrite, jamesonite and other sulpho-salts. The host rock alteration is mainly silicification. The average grade in these fissure veins is 2 to 3% Pb, 3 to 6% Zn, 12 to 15% Fe and 70 to 130 g/t Ag.
Replacement Veins - which are similar to those described immediately above, except that the mineralisation does not fill a pre-existing fissure. They are generally found within a calcareous host, usually limestone or calcareous shale, such as the Morelos Limestone, or at the base of the Mexcala Formation. Internal structure is indefinite and replacement is irregular. Mineralogy consists of galena, sphalerite and pyrite in a calcitic gangue.
Replacement Mantos - found only in the Morelos Limestone and the basal section of the Mexcala Formation. The ore zone is irregular, both with regard to width and grade. The mineralogy is basically sphalerite and galena with variable Ag values. Where the host limestone is <20 m thick, the manto is only found in the lower or upper parts, particularly at the contact between the Taxco Schist and the Mexcala Formation. Where the limestone ranges from 10 to 15 m in thickness, there are good orebodies in zones near veins, particularly near vein intersections. The mantos do not have a definite internal structure. They consist of magnetite, pyrite and specularite, with sphalerite, galena and a gangue where ilvaite predominates. Away from associated veins, manto mineralisation is almost exclusively composed of pyrite and marcasite. Grades in the Guerrero mine are of the order of 2 to 3% Pb, 5 to 7% Zn, 12 to 16% Fe and 35 to 60 g/t Ag, in a matrix of calcite, quartz and iron silicates. In the Remedios mine however, the ore contains 0.3 to 0.5% Pb, 1 to 1.5% Zn and 150 to 200 g/t Ag.
Breccia Chimneys - which are represented by a single deposit located in the north-eastern part of the district, some 10 km away. The ore occurs in a thick sequence of the Morelos Limestone, possibly controlled by fissure intersections, followed by collapse, which formed the host breccia of the chimney. The breccia is represented by limestone fragments replaced by fluorite in a quartz matrix.
Stockworks - occurring as abundant veinlets and stringers associated with veins whose widths range from 0.1 to 0.6 m. They contain galena, sphalerite, chalcopyrite and pyrite in large quantities, accompanied by quartz. One example forms a zone that is irregular, with a thickness of 7 to 20 m.
Published reserve, grade and production figures include:
8.62 Mt of ore (Reserve tonnage in 1989, AME, 1995),
125 g/t Ag, 0.7% Pb, 2.2% Zn (Production grades in 1990, GIMMSA, 1990),
6000 tonnes of Ag from ore grading 160 g/t Ag, 0.3 g/t Au, 3.6% Pb, 7.6% Zn (Production, 1942-1971, GIMMSA, 1990),
14.24 Mt of ore (Production 1942-1983, GIMMSA, 1990)
Production in 1992 comprised 0.79 Mt of ore which yielded 11 534 t Pb and 26 720 t Zn (AME, 1995)
140 g/t Ag, 1.5% Pb, 3.8% Zn (Head grade in 1986, GIMMSA, 1990).
The most recent source geological information used to prepare this decription was dated: 1997.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Pi T, Sole J and Taran Y 2005 - (U–Th)/He dating of fluorite: application to the La Azul fluorspar deposit in the Taxco mining district, Mexico: in Mineralium Deposita v39 pp 976-982
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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