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Jordi Tritlla, Universidad Nacional Autónoma de México (Mexico)
Gilles Levresse, Universidad Nacional Autónoma de México (Mexico)
Eduardo Mascuñano, Universidad Nacional Autónoma de México (Mexico)
David Banks, University of Leeds (United Kingdom)
Rodolfo Corona-Esquivel, Universidad Nacional Autónoma de México (Mexico)
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The Sb deposit of Wadley is located at the west flank of the Sierra de Catorce Ranges (San Luis Potosí, Mexico), an anticline of Laramide age made up mainly by a Jurassic to Cretaceous sediment (clays, sandstones and platform carbonates) coverture with a Triassic core. This anticline is intruded by Tertiary granodiorite bodies that generated some Cu-Zn-Ag skarns and associated low sulfidation Ag-rich epithermal deposits.
At Sierra de Catorce, several Sb deposits appear arranged as an aureola always enclosed within the upper part of the Oxfordian-Kimmeridgian limestones of the Zuloaga Fm, near the contact with La Caja Fm. This unit is characterized by the abundance of sedimentary evaporites, both as local anhydrite horizons and as calcite pseudomorfs after gypsum.
Wadley deposit was discovered in 1898 and since then was mined almost continuously until 2002. At the origin, only the supergene concentrations of Sb oxide minerals where mined; lately, the deposits were mined for the whole recovery of hypogene and supergene ores.
Nine mineralized horizons are presently known, whose disposition was physically and chemically controlled by the presence of 9 folded evaporite (anhydrite) horizons interstratified within the upper limestones of the Zuloaga Fm. When mineralized, these anhydrite beds are replaced by seldom mineralized, heavily banded rhythmites made up by the growth and concatenation of botryoids, composed by radiated calcite crystals; locally, a silicification of the evaporitic horizon is also detected. The resulting rock mimics the former tabular disposition of the evaporitic horizon resulting in mineralized blankets or "mantos". Sb mineralization is represented almost exclusively by hypogene antimonite. It appears in euhedral crystals up to 15 cm long, radiating from the walls and precipitated prior and during evaporite the evaporite replacement episode. Fetid calcite, locally intergrown with selenitic gypsum, filled up the remaining porosity. First microthermometric data indicates the presence of hot (≈170°C), saline (≈14wt% eq. NaCl) brines in primary fluid inclusions within gangue calcite and quartz.
Late N-NW striking faults crosscut the mineralized blanket resulting in the accumulation of high-grade, supergene Sb-oxides. Karst cavities lined up with insoluble stibiconite pseudomorfs after antimonite and, locally, valentinite and powder-like cinnabar are also found.
All the ore controls, textures and fluid data suggest an origin related with the upflow of Sb and Hg-bearing basinal brines that interacted with evaporitic horizons. Dissolution coupled with TSR-like reactions would account for the rhythmite formation and ore precipitation in an scenario similar to cap-rock ore deposits.
We thank Lic. J. Cerrillo-Chowell of Negociación Minera Santa María de La Paz y Anexas, S.A. de C.V. for the support given to this study. This research is supported by projects PAPIIT IN100707, IN114106 and Conacyt 49234-F.
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