The Zn–Pb–Ag skarns of Zacatepec, Northeastern Oaxaca, Mexico: A study of mineral assemblages and ore-forming fluids

The Zn–Pb–Ag skarns of Zacatepec are located in the eastern Sierra Madre del Sur, southern Mexico. They are hosted by the Cuicateco Terrane, a composite unit made up of deformed Jurassic and Cretaceous volcanic and sedimentary rocks deposited in an arc environment that also hosts several precious- and base-metal epithermal deposits and Cu–Mo porphyries. The mineralization occurs as irregular masses along the contact between Cretaceous limestones and Tertiary porphyritic dacites. Over widths of few decimeters, an epidote-rich (Ep44–97Czo01–55Pie00–03) endoskarn is developed. For the most part, the mineralization consists in skarns of calcic garnet (Ad62–22Gr78–38) with disseminated hematite. Calcic garnets show chemical oscillatory zonation. Skarns rich in calcic clinopyroxenes (Di28–54Hd22–55Jo07–32) locally occur, farther from the porphyry contact. As a result of a retrograde event, the above assemblages are locally silicified, and ore minerals (sphalerite, galena, pyrite, chalcopyrite and minor scheelite) and chamosite deposited. The iron contents in sphalerite range between 5.0 and 9.4 mol% FeS. An assemblage of sulfosalts, consisting of vikingite Ag5Pb8Bi13S30 and an Ag–Bi–S phase probably of the aikinite group, occurs. Locally, magnesian skarns develop and are composed of almost pure diopside (Di98–100Hd00–01Jo00), clinochlore, chrysotile, and vesuvianite notoriously rich in Ti (6.48 to 7.93 wt.% TiO2). The magnesian skarns yield the highest metal grades, up to 595 g/t of Ag, 9.94% Pb and 3.27% Zn. After weathering of the ores, botryoidal crusts of supergene minerals as smithsonite, rosasite (Cu,Zn)2(CO3)(OH)2 and hemimorphite developed. Microthermometric studies of fluid inclusions were carried out in calcite, calcic garnet, quartz and sphalerite. Homogenization to the liquid phase (Th) occurred between 160 °C and 470 °C. Salinities were calculated assuming an H2O–NaCl system and range from 10.5 to 19.8 wt.% NaCl eq. The temperature of formation of the prograde skarn, calculated from microthermometric data with an assumed pressure of 500 bar, ranges between 470 °C and 510 °C. A slight increase in ƒO2 caused the formation of garnet after clinopyroxene. The formation of the retrograde assemblage implies a decrease in temperature and, initially, in salinity, coupled with a great increase in ƒO2. These changes can be explained by mixing with cooler, oxidizing and dilute meteoric water. Subsequently, a progressive increase in fluid salinity as Th dropped from 300 °C suggests that boiling took place triggering sulfide precipitation.