Genesis of the Yaguila Pb-Zn-Ag-Mo skarn deposit in Tibet: Insights from geochronology, geochemistry, and fluid inclusions

The Yaguila skarn deposit with reserves of 52 Mt at 4% Pb, 2% Zn, and 64.3 g/t Ag is located in the eastern segment of the central Lhasa subterrane, Tibet. The stratabound skarn Pb-Zn-Ag orebodies are hosted in the Upper Carboniferous-Lower Permian Laigu Formation, which consists of meta-quartz sandstone, slate, and marble. Molybdenum occurs as quartz-molybdenite +/- pyrite veinlets, veins, and partial disseminations in the Paleocene quartz porphyry, slate, and garnet skarn. Zircon U-Pb dating suggests that three major magmatic events occurred in the Yaguila area, producing Cretaceous rhyolitic rock (ca. 128 Ma), Paleocene quartz porphyry (65.6 +/- 1.2 Ma), and Miocene granite porphyry (17.8 +/- 0.4 Ma). Combined with the published data, the quartz porphyry has variable epsilon(Hf)(t) values of -13.9 to +12.1, (Sr-87/Sr-86)(i) values of 0.71718-0.72805, and epsilon(Nd)(t) values of -5.2 to -13.5, whereas the granite porphyry displays relatively restricted epsilon(Hf)(t) values of +2.3 to + 6.5, (Sr-87/Sr-86)(i) values of 0.70798-0.71678, and epsilon(Nd)(t) values of -2.8 to -5.5. Geochemical and isotopic (Sr-Nd-Pb-Hf) signatures indicate that the Cretaceous rhyolitic magma was derived from the melting of continental lower crust of the ancient Lhasa subterrane with a strong fractional crystallization. The emplacement of the Paleocene quartz porphyry probably resulted from the magma mixing process between the dominant Lhasa middle-upper crust-derived magma and varying contributions of mantle-derived materials during the collision and Neo-Tethyan subduction slab rollback. The Miocene granite porphyry was likely formed by the melting of thickened Lhasa lower crust, which was triggered by upwelling of hot asthenosphere. Our molybdenite Re-Os data from garnet skarn, together with published data from quartz porphyry and slate, display an excellent isochron age of 64.3 +/- 1.6 Ma. Fluid inclusion data from this study reveal that the ore-forming fluids associated with diverse types of molybdenum mineralization were a common magmatic-hydrothermal fluid with moderate to high temperatures (290-400 degrees C) and moderate salinities (9-22 wt% NaCl equiv.). Our study suggests that the Yaguila Pb-Zn-Ag and Mo mineralization occurred under the same ore system associated with the Paleocene quartz porphyry.