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Madero, Francisco I. Madero |
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Zacatecas, Mexico |
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Zn Ag Pb Cu
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Super Porphyry Cu and Au
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The Madero, formerly Francisco I. Madero, sediment hosted Zn-Ag-Pb-Cu deposit is located ~20 km west of the city of Zacatecas, in central Mexico (#Location: 22° 49' 28"N, 102° 43' 39"W).
Prospecting and surface exploration had been undertaken in the Madero area during the 1940s and 50s, before being explored by the Mexican Geological Survey (formerly Consejo de Recursos Minerales) between 1976 and 1994, using geophysics to target drilling. The project was inactive between 1983 and 1990, during the period of low metal prices. It was revived between 1990 and 1992, when a detailed interpretation of the available data indicated potential for >30 Mt of ore with 5% lead+zinc. The project was acquired by Servicios Industriales Peñoles in 1994, who then drilled 130 000 m of exploration and delineation diamond holes. Mining commenced in 2001 with a 7000 t/day processing plant. In the period to 2005, 9.6 Mt of ore containing 147 tonnes of silver, 30 280 t of lead and 309 700 t of zinc was produced (González and López-Soto, 2010). During its life, exploration continued to expand the reserve to keep pace with mining. The mine continued operation until May 2020 when an indefinite, temporary suspension of operations was declared as it had ceased to be profitable as a result of to high overheads, low grade of the ore, hardness and depth, coupled with low zinc prices.
Regional Setting
The Madero deposit is located within the Mesa Central (or Altiplano) physiographic province of central Mexico. It lies within the Guerrero Composite Terrane, which is interpreted to underlie much of western Mexico. However, the Mesozoic metavolcano-sedimentary assemblages that define the Terrane, are only exposed over <5% of the surface area, occurring as scattered erosional windows through the extensive Tertiary and Quaternary volcanic and sedimentary strata of the Sierra Madre Occidental Province and Trans-Mexican Volcanic Belt. The Guerrero Terrane is the composite product of complex subduction-related processes, major translation and periods of rifting during the Mesozoic along the western margin of Mexico (Campa and Coney, 1983; Centeno-García et al., 2008).
The Guerrero Composite Terrane embraces many other Cretaceous and Tertiary ore deposits (Miranda-Gasca, 2000). Cretaceous deposits are predominantly volcanic hosted massive sulphides (VHMS e.g., the Tizapa and Capela deposits), whilst those of Tertiary age include precious- and base-metal epithermal/carbonate replacement (e.g., Real de Angeles and Cozamin), as well as porphyry-copper and skarn deposits (Clark, 1999; Miranda-Gasca, 2000).
Geology
The host sequence to the Madero deposit commences with a Mesozoic metapelitic unit composed of shales, meta-siltstones and meta-subarkoses. According to Canet et al. (2009), it is probably of Upper Jurassic to Lower Cretaceous age (González and López-Soto, 2010), although no radiometric or paleontological dating was available at that date. This lower unit is conformably overlain by an up to 600 m thick series of Lower Cretaceous calcareous-pelitic rocks (González and López-Soto, 2010). These are fine-grained and change progressively from black shales at the base to micritic limestones at the top. The top of the latter host the Madero ores. These are, in turn, overlain by an up to ~300 m thick volcano-sedimentary sequence which contains basaltic pillow lavas and submarine tuffs, interlayered with sandstones and shales. K-Ar dating of the volcanic rocks in the latter sequence yielded an Upper Cretaceous age of 94 Ma (González and López-Soto, 2010). A small gabbroic stock, of probably Cretaceous age, intruded the Mesozoic sequence.
The whole of this Mesozoic sequence was deformed during the Late Cretaceous to Paleocene Laramide Orogeny and metamorphosed to lower greenschist facies (e.g., Salinas-Prieto et al., 2000). This compressive event is interpreted to have resulted in the collision of several volcanic arcs, including those that formed the Guerrero Composite Terrane (Tardy et al., 1994; Centeno-García et al., 2008). In the district surrounding the Madero deposit, this compression led to the formation of large, NW trending open folds, both limbs of which dip at ~20°. Subsequently, ENE–WSW directed post-Laramide extension produced normal faults, and horst and graben networks (González and López-Soto, 2010).
The Mesozoic succession is unconformably overlain by a >1000 m thick Tertiary felsic volcanic sequence of the Sierra Madre Occidental Volcanic Province. These comprise a lower andesite sequence, dated at 42 Ma (K-Ar; González and López-Soto, 2010), and rhyolitic ignimbrites at the top, with intercalated beds of continental volcanic conglomerates. The Mesozoic sequence is cut by a suite of metre-thick porphyritic dykes associated with these Tertiary volcanics. These dykes, which were emplaced along post-Laramide normal faults, range from diorite to tonalite and granite in composition, and produced an intense marmorisation (marble formation) of Mesozoic limestone beds. Additionally, minor granitoid stocks crop out locally in the district.
Mineralisation
The Madero deposit covers an area of ~6 km2, and is concealed at depths of between 60 and 550 m below the present surface. It comprises several roughly stratabound orebodies hosted by Mesozoic calcareous-pelitic sedimentary rocks. The mineralised zone, including interleaved non-economic intervals, changes abruptly from a few tens of metres, up to as much as ~170 m in thickness. Individual ore lenses that are 3 to 60 m thick are found within calcareous beds. As a generalisation, the mineralised unit forms a large dome-shaped structure, the flanks of which are cut by NW trending normal faults, across which the marginal portions of the mineralisation are downthrown. Many of these faults host porphyritic dykes. Interpretation of magnetic and gravimetric data implies deep magmatic intrusions are present below the deposit area (Paterson 1995), although none had been encountered in outcrop or drilling.
The base of the mineralised zone comprises 60 to 75 modal.% sulphides, occurring as tabular 'mantos', composed of major pyrite and pyrrhotite, with minor sphalerite, galena and chalcopyrite, accompanied by quartz and pyrite. Cu-(Ag-) and Pb- and Zn-rich lenses that are up to tens of centimetres thick occur locally, as either banded and/or massive sulphides. These sulphide are interleaved with moderately silicified black shale beds which contain disseminated pyrite. The banded ores in this lower interval are mostly fine grained (up to 0.5 mm) and are very rich in pyrrhotite (~50 modal.%) and pyrite (~20 modal.%), with minor sphalerite and chalcopyrite. Towards the bottom of the banded ores, the pyrite content and grain size increase, and accessory arsenopyrite is found. The banded ore contains mm-scale layering, defined by alternating sulphide- and chlorite-rich layers. Massive ores are coarse grained (up to 5 mm) and form irregular, replacive bodies composed of sphalerite and pyrrhotite (total ~50 modal.%), with minor galena, chalcopyrite, calcite, chlorite, sericite, quartz and epidote.
The upper sections of the mineralised zone mainly comprises calc-silicate rich assemblages with strong variations in texture, composition and grade. It contains, in order of abundance: i). micro-crystalline epidote-chlorite; ii). almost monominerallic macro-crystalline calcic-clinopyroxene; and; iii). banded calcic garnet-rich assemblages. Calcite, quartz, adularia, sericite, Ca-amphibole, stilpnomelane, titanite, rutile and apatite are also found in varying amounts. The ore minerals are, in order of abundance, sphalerite, galena, marcasite, pyrite, chalcopyrite, arsenopyrite and magnetite, reaching up to 25 modal.%. Calc-silicate rich assemblages locally develop banded and fold-like patterns.
The contact relationships between calc-silicate- and sulphide-rich units are complex, including both sharp-replacive or gradual transitions. Un-deformed crustiform-banded veins up to tens of centimetres thick, and rarely breccias, crosscut these same assemblages. These veins and breccias are found around Tertiary felsic porphyritic dykes, and are essentially lined by calcite and quartz. Calcite mostly occurs as bladed crystals and quartz as chalcedony. Such veins also contain fluorite, chlorite and dolomite, and are locally enriched in Au and Ag (González and López-Soto, 2010). These veins have also been reported to contain a complex assemblage of Ag-, Bi-, Pb- and Cu-tellurides, sulphides and sulphosalts, including hessite, matildite, tetradymite, bismuthinite, tsumoite, aikinite and wittichenite, although they do not contribute significantly to the economic resource (Yta et al., 2003).
Underlying the base of the orebody, cm-scale, irregular, metamorphic quartz veins, often with pyrite crystals but devoid of other metallic minerals, are hosted in the basal metapelites, which in the vicinities of orebodies are altered and contain chlorite and epidote.
Lead isotope studies of the Madero ores suggest that the deposit is Cenozoic in age (Camprubi et al., 2017). This has been interpreted to suggest, that at least part of the deposit is epigenetic, and related to continental arc magmatism (Mortensen et al., 2008). Based on paragenetic relationships, three main stages of development can be inferred, from early to late: i). sedimentation and regional metamorphism; ii). epigenetic hydrothermal stage; and iii). supergene alteration, which produced a supergene assemblage, consisting of locally developed goethite and
carbonates (Canet et al., 2009).
Replacement textures are interpreted to suggest that the mineralisation was predominantly formed at the expense of the carbonate beds of the Lower Cretaceous sequence, in the epigenetic, hydrothermal metasomatic stage (Canet et al., 2009). This stage formed a complex sequential replacive deposition of minerals, characterised by three sub-stages:
i). prograde, when Ca-clinopyroxene and garnet, along with minor magnetite and titanite were formed (Canet et al., 2009).
ii). retrograde, forming hydrous silicates (mainly epidote and chlorite), sulphides and quartz which occur interstitially with respect to clinopyroxene crystals, and pseudomorph garnet. Whilst the early banded ores and a later replacive massive ores of this sub-stage can be macroscopically differentiated, microscopic and compositional differences between them are only slight. Intergrowths and textures common to both indicate an overlap in the crystallisation of sulphides (Canet et al., 2009); and
iii). vein filling (Canet et al., 2009).
Reserves and Production
As detailed in the introductory paragraph, prior to 2005, the mine had produced 147 tonnes of silver, 30 280 t of lead and 309 700 t of zinc, from 9.6 Mt of ore milled (Canet et al., 2009). This corresponds to:
- 9.6 Mt @ 3.23% Zn, 0.32% Pb, 15.3 g/t Ag.
Remaining Reserves at Francisco I. Madero as at 31 December 2008 were (Industrias Peñoles Annual Report, 2008)
- 33.997 Mt @ 2.46% Zn, 0.72% Pb, 0.10% Cu, 22.68 g/t Ag.
Production during financial year 2019, the last full year of operation (Industrias Peñoles Annual Report, 2019) totalled:
2.348 Mt of ore milled for 23.23 t of silver, 8905 t of lead, 41 541 t of zinc, 895 t of copper, equating to a head grade of,
9.89 g/t Ag, 0.38% Pb, 1.77% Zn, 0.04% Cu.
Remaining Reserves at Madero as at 31 December 2019 were (Industrias Peñoles Annual Report, 2019):
- 26.509 Mt @ 2.02% Zn, 0.68% Pb, 0.07% Cu, 18.62 g/t Ag.
The most recent source geological information used to prepare this decription was dated: 2017.
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.
Madero
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Selected References:
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Camprubi, A., Gonzalez-Partida, E., Torro, L., Alfonso, P., Canete, C., Miranda-Gasca, M.A., Martini, M. and Gonzalez-Sanchez, F., 2017 - Mesozoic volcanogenic massive sulfide (VMS) deposits in Mexico: in Ore Geology Reviews v.81, pp. 1066-1083. doi.org/10.1016/j.oregeorev.2015.07.027.
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Canet, C., Camprubi, A., Gonzalez-Partida, E., Linares, C., Alfonso, P., Pineiro-Fernandez, F. and Prol-Ledesma, R.M., 2009 - Mineral assemblages of the Francisco I. Madero Zn-Cu-Pb-(Ag) deposit, Zacatecas, Mexico: Implications for ore deposit genesis: in Ore Geology Reviews v.35, pp. 423-435. doi:10.1016/j.oregeorev.2009.02.004.
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Miranda-Gasca, M.A., 2000 - The metallic ore-deposits of the Guerrero Terrane, western Mexico: an overview: in J. of South American Earth Sciences v.13, pp. 403-413.
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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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