The relationship between stratabound Pb-Zn-Ag and porphyry-skarn Mo mineralization in the Laochang deposit, southwestern China: Constraints from pyrite Re-Os isotope, sulfur isotope, and trace element data

The Laochang polymetallic deposit is located in the Sanjiang Tethyan metallogenic province of southwestern China. This deposit contains stratabound (and locally vein-type) Pb-Zn-Ag mineralization and underlying porphyry-skarn Mo mineralization. The stratabound Pb-Zn-Ag mineralization is characterized by massive sulfide ores composed mainly of pyrite, sphalerite, galena, and minor chalcopyrite, whereas the underlying porphyry-skarn Mo mineralization is characterized by quartz-sulfide veins comprising mainly pyrite, chalcopyrite, molybdenite, and minor sphalerite and galena. The age and origin of the stratabound Pb-Zn-Ag mineralization has long been debated, and its relationship to the porphyry-skarn Mo mineralization remains unknown. Integrated pyrite Re-Os isotope, sulfur isotope, and in situ trace element data are used here to constrain the age and origin of both types of mineralization and their genetic relationships. Pyrite related to porphyry-skarn Mo mineralization yields a Re-Os isochron age of 47.3 +/- 4.8 Ma, which is consistent with a previous molybdenite Re-Os age of the quartz-sulfide ores and zircon U-Pb ages of the granite porphyry (-44-50 Ma). Pyrite from the stratabound Pb-Zn-Ag mineralization failed to yield a geologically meaningful age, but associated sphalerite and galena have a Re-Os isochron age of 308 +/- 25 Ma (Liu a al., 2015), which is consistent with the Carboniferous age of the host basalts. Pyrite, sphalerite, galena, and chalcopyrite from the stratabound Pb-Zn-Ag mineralization have 8 34 S values (-2.1 to 0.5; average 0.48 parts per thousand) indistinguishable from those of sulfides from the porphyry-skarn Mo mineralization (-4 to 1.9 parts per thousand; average 0.52 parts per thousand). Initial Os isotope ratios indicate a mainly crustal origin for both types of mineralization. The calculated (Se/S) flurd and Co/Ni values of pyrite from the stratabound Pb-Zn-Ag mineralization indicate a mixed sedimentary and hydrothermal origin, with a significant magmatic contribution. The identification of a magmatic component, Eocene sulfide mineralization, and phyllic and propylitization alteration in stratabound Pb-Zn-Ag ores indicates that the magmatic-hydrothermal component reflects the overprinting of Carboniferous volcanogenic massive sulfide mineralization by magmatic-hydrothermal fluids derived from an Eocene granite porphyry.