Summary:
IMMSA unit includes Charcas, Santa Barbara, San Martin, Santa Eulalia and Taxco.
Charcas
The Charcas mining district is in the east-central part of the central mesa of Mexico, which is part of the larger metallogenic province of Sierra Madre. The mineral deposits found within the Charcas mining district are tertiary polymetallic skarn (silver, lead, zinc and copper) deposits hosted in carbonate rocks of the Jurassic-Cretaceous periods and in shales and sandstones of the Late Triassic. In the carbonate rocks, veins and mantos form the predominant mineralization, while less mineralized fractures tend to occur within the shales and sandstones. The varied style of mineralization largely corresponds to the lithological variety of units that serve as host rocks.
The Charcas intrusive complex (“CIC”) was emplaced in Triassic to upper Cretaceous sedimentary rocks. Some dikes from the CIC have developed metamorphic halos with related polymetallic mineralization. There are two recognized stages of mineralization. In the first stage, the mineralization is enriched in silver, lead, and zinc and characterized with calcite and small quantities of quartz and chalcopyrite (CuFeS) present. In the second stage, the mineralization is copper and silver rich with lesser amounts of chalcopyrite. The mineralization also includes lead ore with associated silver, plus pyrite and only minor amounts of sphalerite (ZnS). The mineralization occurs as replacement sulfides in carbonate rocks and as filling fracture veins. The typical sulfides found at the Charcas include chalcopyrite, sphalerite, galena (PbS), and silver minerals.
Santa Barbara
The pre-mineral rock types found at Santa Barbara consist of a thick calcareous shale formation and andesite flows. The postmineral rock types consist of dikes and sills of rhyolite and diabase, a thin conglomerate formation, basalt flows, and unconsolidated stream sediments. Pre-mineral faulting took place in two stages, forming four fault systems. All faults within each system have similar strike and dip. Movement along these faults, vertical in the first-stage faults and horizontal in the second-stage faults, formed openings and breccia zones.
Hydrothermal solutions, emanating from depth, were introduced into the faults. The walls and breccia fragments within the faults were silicified, and the high-temperature silicates, garnet, pyroxene, and epidote were formed. Accompanying and following the formation of the silicates, the sulfides, such as sphalerite, galena, chalcopyrite, pyrite, and arsenopyrite, with associated gold and a silver mineral, were introduced with quartz, calcite, and fluorite. Most of these minerals replaced the silicates and altered shale. The parts of the faults where wide pre-mineral openings were located filled with quartz and a higher ratio of sulfides than in the narrow portions of the faults. Quartz, calcite, fluorite, and barite were among the last minerals deposited.
San Martin
San Martín mine is located in the Central Mesa of Mexico, between Sierra Madre Occidental and Sierra Madre Oriental. The Cuesta del Cura (Upper Cretaceous) limestone is the main sedimentary formation in the district. This is a sequence of shallow marine limestone and black chert which is overlain by Indura Formation that consists of alternating shales and fine-grained clayey limestones.
The mineral deposits in this district are associated with replacement veins and bodies formed in the skarn in close proximity to the Cerro de la Gloria granodiorite intrusion. The main mineralized veins are San Marcial, Ibarra and Gallo-Gallina which are oriented parallel to the intrusive contact and have thicknesses varying from 0.4 m to 4 m and horizontal extents of up to 1,000 m to the east/northeast from the granodiorite contact. The mineralization is associated with massive and disseminated sulfides occurring in replacement ore bodies between the main veins and in the skarn and include chalcopyrite (CuFeS), sphalerite (ZnS), galena (PbS), bornite (CuFeS), tetrahedrite (CuFe Sb S), native Silver (Ag), Pyrite (FeS), arsenopyrite (FeAsS) and stibnite (SbS).
Santa Eulalia
Santa Eulalia mineralization corresponds in its majority to ore skarns — silicoaluminates of calcium, iron and manganese with variable quantities of lead, zinc, copper and iron sulfides, located in the planes of crossings in the interstices of the silicates. Economic ore is found as replacement in the Limestone Glen Rose in the contact with dikes and sills and replacements in diabase sills. The mineralogy is comprised predominantly of sphalerite (ZnS), galena (PbS) and small quantities of pyrargyrite (Ag SbS ).
Taxco
There are four types of ore deposits found in Taxco district. In order of importance they are as follows: fissure-filling veins, replacement veins, blanket-like replacement bodies (so called “mantos”), stock works and brecciate chimneys. Economic ore is found in the deposit in veins. Ore mineral include argentiferous galena (PbS), sphalerite (ZnS), pyrargyrite (Ag3SbS3), and other sulfosalts, and replacement “mantos.” The most mineralized zones are in the vicinity of the veins with the limestone.