Intrusion-related mineralization in the central sector of the Sierra Madre Oriental, Mexico

Abstract

The Concepcion del Oro, Providencia, Noche Buena and Santa Rosa igneous centers are located in the central sector of the Sierra Madre Oriental, to the southwest from the city of Saltillo, in the state of Zacatecas, Mexico. Their associated mineral deposits constitute a coherent igneous and metallogenic area. It is characterized by a sequence of magmatic bodies intruding a predominantly carbonate sedimentary sequence of Mesozoic age. Field reconnaissance, whole rock major, trace and isotope geochemistry, and mineral chemistry define three magmatic pulses. The first pulse is characterized by a intermediate to mafic, relatively oxidized magma with low contents in halogens that becomes more contaminated with crustal material, enriched in fluorine relative to chlorine, and reduced as it evolves. The second pulse is at first more oxidized than the previous, but, like its later phases, it shows also significant crustal contamination and relatively high halogen contents. It becomes significantly more reduced, its overall halogen contents decreases and becomes strongly dominated by fluorine towards the late stages. A late magmatic pulse consisting of mafic dikes with major and trace element characteristics suggestive of a crustal, rather than mantle source, does not appear to be related to significant alteration and mineralization in the area. Trace element and radiogenic isotope geochemical data suggest similar sources for the different intrusive phases in the area with a significant component from either a mafic crust and/or the mantle. Amphibole geobarometry suggests final emplacement of the different intrusives at less than or around 1 kbar pressure, in agreement with the regionalGeology. Values obtained from relict inclusions, show a cluster of values between 3 and 4 kbars that suggest possible mid-crustal residence for the magmas in the area. Halogen contents and oxidation state of the different intrusives appear to describe similar evolutionary trends. Available field evidence, geochemical data and documented metal suites associated with the intrusives, indicate that mineralization is dominated by copper in the early phases, and lead-zinc-silver in the later. While the lead-zinc-silver mineralization has many parallels to other mineralized systems in the Sierra Madre Oriental, the significant copper mineralization is uncommon for the region.