Skarn and epithermal vein mineralization in the San Carlos Caldera region, northeastern Chihuahua, Mexico




Immitt, James Peter

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The San Carlos Pb-Zn-Ag skarn deposit and several Pb-Zn-Cu-fluorite epithermal vein prospects are located along the boundary of the San Carlos caldera near Manuel Benavides (San Carlos) in northeastern Chihuahua, Mexico. The skarn deposit is at the crest of a Laramide anticlinal fold in the Sierra Azul. The anticline was intruded and uplifted by a granitic pluton related to caldera development at 31 mybp. Detailed mapping and petrography reveal that Lower Cretaceous dolomitic limestone and calcareous siltstone have been metamorphosed and metasomatized to form a well-defined zonal alteration pattern around the intrusive core of the fold. Prograde calc-silicate metamorphism resulted in a sequence, with increasing distance from the heat source, of forsterite or diopside, tremolite, and recrystallized carbonate zones. Within the forsterite zone the pre-mineralization calc-silicate assemblage has been replaced by a metasomatic assemblage consisting mainly of magnetite, calcite, galena, and sphalerite. Precipitation of ore minerals took place when acidic magmatic hydrothermal fluids interacted with the dominantly carbonate country rocks. Retrograde serpentine alteration of forsterite and diopside is the only indication of the pre-mineralization metamorphosed lithologies. The epithermal vein prospects occur near the northwestern margin of the caldera mainly along normal faults near the apex of the La Consolacion dome. This dome was probably formed during emplacement of another peripheral intrusion believed to be present at depth. Veins occur in marbleized Lower Cretaceous shale and argillaceous limestone and in a hydrothermally altered caldera-related Tertiary rhyodacite flow. Ore deposition resulted from circulation and cooling of metal- and fluorine-bearing hydrothermal fluids. Fluid inclusion data indicate a range in temperature from 317-142°C and in salinity from 5.7-2.8 equivalent weight percent NaCl , with both generally decreasing in younger minerals. Mineralization in the San Carlos area is spatially, and may have been temporally associated with development of the San Carlos caldera. It may have been controlled by the availability of heat and hydrothermal fluid from caldera-related igneous activity, ease of fluid movement along caldera-related structures, and the presence of reactive host rocks