Mineralogy, geochemistry, geochronology, and fluid inclusion constraints of the Concepción del Oro-Mazapil Fe - Cu skarns, central Mexico

The Concepción del Oro – Mazapil mining District, Zacatecas, is considered one of the most important in Mexico because it contains massive Fe – Cu skarns mined for almost 500 years. Their origin has been related to post-orogenic granodioritic – monzodioritic intrusion (40–45 Ma) in the Mesozoic Zuloaga (Oxfordian) and Tamaulipas Inferior (Valanginian-Albian) limestones. In the present study, an updated paragenetic model is proposed for the Sol y Luna, Aranzazú and Cata Arroyo deposits. The hydrothermal system evolution has been explained applying a methodological scheme that included: (a) petrography, (b) electron microprobe analysis, (c) scanning electron microscopy, (d) whole-rock and zircon geochemistry, (e) U-Pb in zircon and K-Ar in annite geochronology, and (f) fluid inclusion analysis. A mineral assemblage (andradite > grossular, diopside, wollastonite, vesuvianite, and quartz) and fluid inclusion analysis suggest a prograde stage with a general homogenization temperature = 373–514 °C and salinity = 10.5–61.7% NaCl eq. The Fe-Cu mineralization occurred during the retrograde stage (homogenization temperature = 201–321 °C, salinity = 4.0–10.9% NaCl eq.). It was constituted by magnetite > hematite > pyrite ≈ chalcopyrite > bornite, accompanied by minor amounts of sphalerite, tetrahedrite, hessite, Bi-sulfosalts, covelline, goethite, and Mn oxides. An annite K-Ar age indicates that the iron deposit probably started at ∼41.3 Ma. Chemical composition of iron oxides from skarn deposits shows the interaction of fluid-rock during its precipitation. Retrograde alteration petrographic and chemical composition of hydrothermal chlorites from the Concepción del Oro-Mazapil skarns show the evolution of hydrothermal fluids and represent an exploration guide that can be used as a vector towards orebodies. Chlorite and sphalerite geothermometers indicate a retrograde stage for polymetallic deposit at 208–263 °C for Sol y Luna, 208–234 °C for Aranzazú, and at 181–215 °C for Cata Arroyo. The Concepción del Oro-Mazapil ore system shutdown was marked by low temperature hydrothermal event (homogenization temperature = 120–242 °C) and low salinity (1.7–6.2% NaCl eq.), represented by As-Sb and Au-Zn jasperoids. The proposed paragenetic model provides a better understanding of the Concepción del Oro-Mazapil skarn, being valuable information for future exploration/exploitation activities. Finally, a mineralogical and geochemical comparison of Fe-Cu (Sol y Luna) and Cu-Fe (Aranzazú and Cata Arroyo) skarns has been conducted.