Summary:
All data is sourced from the EIS Report 2012 and has been translated from Spanish with the assistance of AI.
The deposits in the region where the project is located are located on the shores of a Cretaceous platform made up of calcareous rocks mixed with volcanogenic sulfide deposits. The surface geology is constituted by the presence of metavolcanic materials, shales-sandstones, shales-phyllites, tuffs, deformed breccias, agglomerated tuffs, breccias and lavas.
The Capela deposit lies within the Guerrero Terrane, which is interpreted to underlie much of western Mexico. However, the Mesozoic 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. Capela falls within the Teloloapan Sub-terrane of the Guerrero Terrane.
The Capela deposit is developed near the top of the volcanic arc succession of the Teloloapan Sub-terrane. Two main lithologic assemblages have been recognised in the deposit area: i). an ~3500 m thick basal calc-alkaline basaltic to andesitic pile, and ii). an overlying, ~1500 m thick package of sedimentary rocks. Sulphide mineralisation at Capela is hosted within a 400 m thick sequence of rhyolitic/felsic volcanic and volcaniclastic rocks with interlayered carbonaceous siliciclastic rocks and andesitic volcanic rocks that locally predominate in the upper portion of the volcanic pile.
The mineralized deposits of the region are volcanogenic deposits of massive zinc-lead-copper-silver-gold sulfides, located approximately 150 kilometers southwest of Mexico City. The presence of oxides and sulfides of zinc, lead and copper, with values of gold and silver in the mantle area, known specifically as Rey de Plata (since the early 1900's). In fact, there are vestiges of what was once an oxide foundry in this area; However, it was not until the end of the seventies, when mineral bodies began to be found, in the form of lentiform sulphides, projecting towards the southwest, the dip of the Rey de Plata outcrops. At that time, around two million tons of ore were estimated based on 52 diamond drill holes.
In the area that houses the project's deposits, four main lithological units are located. The oldest one is composed of intensely chloritized andesitic volcanic and volcanoclastic rocks. Overlying the andesitic units in a concordant manner are felsic pyroclastic rocks, interbedded with black shales. Andesitic volcanic rocks are also present as interbedded layers at the top of the felsic unit. Pressing against the andesitic and felsic units are calcareous carboniferous limestones, interbedded with clays and siltstones.
The mineralization at the bottom of the Tehuixtla body consists mainly of pyrite and chalcopyrite. At the top there are sphalerite, sulfosalts and galena. Gold is located mainly in the lower silicified zone. In this body, three mineralized zones called “yellow ore” (massive sulfides) and “mixed ore” (disseminated sulfides with a higher silver grade) have been determined.
The lower andesitic unit is likely several miles thick and comprises the main part of the volcanic package where the mineral deposits are located. The upper part of the andesitic unit consists of several pyroclastic, epiclastic, and lava bodies tens of meters thick. The lower andesitic units, as well as the higher ones, are heavily chloritized. The andesite is cut by abundant veins of calcite and ankerite. The mineralization beneath the Tehuixtla body consists mainly of pyrite and chalcopyrite. Above, there are sphalerite, sulfosalts, and galena. Gold is mainly located in the silicified zone at the bottom. In this body, three mineralized zones have been determined, named "yellow ore" (massive sulfides) and "mixed ore" (disseminated sulfides with a higher silver grade).
Sulphide mineralisation occurs as a group of bodies, that may be either of both stockworks and conformable lenses or mantos developed in schists and meta-volcanics of felsic composition. The lens-shaped bodies range in size from 3 to 60 m in thickness, from 300 to 1200 m length, and from 100 to 500 m width (Monter-Ramírez and Zavala-Esquivel, 2011). The ore is composed of pyrite, sphalerite, galena, bornite, chalcopyrite, pyrargyrite, tetrahedrite-tennantite, native gold, arsenopyrite and jamesonite, along with gypsum, barite and calcite as gangue phases (Miranda-Gasca et al., 2001). The main orebody, Tehuixtla, is strongly zoned from: i). a lower zone of mainly pyrite, including framboidal aggregates; ii). chalcopyrite and bornite; and iii). an upper interval of mainly of pyrite, sphalerite, galena, barite and, sporadically, silver sulphosalts (Monter-Ramírez and Zavala-Esquivel, 2011). The sulphide assemblages are predominantly banded, with a weakly developed tectonic foliation. Gold is found in the upper part of the deposit, and is associated with ankerite-rich silicification.