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[Display omitted] • First- and second-order pulsed fluids of Mengya'a deposit are documented in garnet crystal. • The multiple fluid flow has more potential for large-size skarn metallogenesis. • The pulse frequency and pulse intensity of Sn in garnet are potential indicators of skarn Sn mineralization. Skarn deposits, which serve as a significant source of Pb, Zn, Fe, W, Cu, and Au, are typically formed through interactions between fluids and carbonate rocks. However, the discovery of skarn deposits faces obstacles due to a limited understanding of the factors influencing their size. Many studies increasingly assume that the presence of large skarn deposits may be closely associated with multi-stage fluid pulses. Nonetheless, this assumption has not undergone explicit testing. In this study, zoned garnet grains from the large Mengya'a Pb-Zn-polymetallic deposits were used to unravel the multi-stage fluid pulse and evolution within a magmatic-hydrothermal system. In-situ major and trace elemental analysis reveals that the garnets exhibit two distinct zones, namely Ti-Al-enriched (G1) and Ti-Al-poor (G2) zones. The G1 domains are characterized by high HFSEs, slight enrichment in heavy rare earth elements (HREE), and minor variable Eu anomalies, as well as a linear relationship between REE and Y concentrations. These observations suggest that their formation reflects the fluid flux in a nearly closed system. The G2 domains exhibit oscillatory zoning in Al, Fe, Sn, HFSE, and various other elements, reflecting the intermittent flow of fluids originating from a degassing magma reservoir. The Al-, Zn-, and HFSE-rich characteristics of G2, along with their low δEu anomalies in comparison to G1, are associated with fluids that have undergone multiple recharge events from a magma chamber containing a significant amount of meteoric water in an open system. The presence of Sn-rich and In-rich zones within G2, accompanied by high δEu anomalies, indicates a gradual cooling and evolution of the pulsed fluids, resulting in the enrichment of Sn and volatile elements in the remaining fluid. Consequently, it can be concluded that the contribution of multiple fluid flows from a recharged magma chamber, undergoing extended cooling and evolution, plays a pivotal role in the formation of large skarn deposits. An additional implication of this study is that the zoning of Sn in garnet serves as a crucial indicator of Sn mineralization within skarn deposits. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Beimulang is a newly discovered porphyry Cu deposit in the Gangdese belt. • There are at least two episodes of fluid exsolution events of granites at Beimulang. • The highly oxidized and hydrous magmas are crucial to form porphyry Cu deposits. The porphyry Cu deposits in the Gangdese metallogenic belt were mainly formed in the eastern portion. The Beimulang is a newly discovered deposit formed in post-collisional setting in the western part of the belt and adjacent to the super-large Zhunuo porphyry Cu deposit. There are three epochs of magmatism at Beimulang, including pre-mineralization quartz porphyry (PQP; 49.7 ± 0.4 Ma), inter-mineralization monzogranite porphyry (IMGP; 14.8 ± 0.2 Ma), monzogranite (IMG; 14.1 ± 0.2 Ma), and late-mineralization granite porphyry (LGP; 11.7 ± 0.1 Ma). The various degrees of alteration and sulfide occurred at the IMG(P) and LGP indicate that there are at least two episodes of fluid exsolution events associated with different intrusions spanning ∼ 3 m.y. at Beimulang, which are unlike other Gangdese porphyry Cu deposits showing single fluid exsolution around only one porphyry at Miocene (e.g., Qulong, Zhunuo, Dabu). The PQP is characterized by high SiO 2 (75.6–81.4 wt%) and K 2 O (2.1–4.3 wt%) concentrations, high Nb/Ta (8.4–11.3) ratios, and low Mg# (22–43), Ni (1.5–2.7 ppm) contents. They have high (87Sr/86Sr) i (0.705098–0.707767) ratios and low ε Nd (t) (-4.9 to −1.9) values. These geochemical characteristics indicate that they were probably derived from a juvenile basaltic lower crust during the ongoing Indian-Asian collision. The IMGP and IMG show similar petrography and geochemical characteristics, such as high SiO 2 (65.0–69.8 wt%), K 2 O (3.6–4.3 wt%), Sr (550–809 ppm) contents, high Sr/Y (79–89) ratios, and low Mg# (43–48), Th/Yb, and Nb/Yb values, combined with high (87Sr/86Sr) i (0.707254–0.708017) and low ε Nd (t) (-7.9 to −6.0), implying a similar source probably originated in the subduction-modified lower crust metasomatized by Neo-Tethyan oceanic slab-derived fluids. The LGP displays trends produced by fractionation of magma with a composition similar to the IMG(P), but wall rock assimilation plays a significant role during magmatic evolution. Zircons from the IMG(P) show higher Eu/Eu* (>0.3), 10000*(Eu/Eu*)/Y (>4), and (Ce/Nd)/Y (>0.04) ratios than those of the PQP, indicating the fertile magmas were more oxidized and hydrous. The high magmatic oxidation state for the IMG(P) is also supported by using the zircon (ΔFMQ = +4.1 to + 6.1), and amphibole (ΔFMQ =+2.6 to + 3.4) oxybarometers. Their high magmatic water contents are supported by evaluations using amphibole (∼3.5 wt%) and plagioclase (∼6.7 wt%) compositions, respectively. However, the LGP has higher magmatic oxygen fugacity (Eu/Eu* >0.3, ΔFMQ = +4.2 to + 7.3) caused probably by auto-oxidation of the magmas, but lower magmatic water contents (Sr/Y = 31 ± 15; V/Sc = 6 ± 4) caused presumably by previous fluid exsolution and/or fractionation of hydrous silicate minerals during magma ascent than those of the IMG(P). Therefore, the highly hydrous and oxidized magmas with juvenile material are favorable to form porphyry Cu deposits in Tibetan orogen. On the contrary, magmatic evolution can probably reduce the water content of magma chamber, which is not beneficial to porphyry Cu mineralization. [ABSTRACT FROM AUTHOR]

The apatite chemical compositions show that the parental magma of the Pulang pluton is more fertile for Cu mineralization than those of the Cuojiaoma, Dongcuo, and Daocheng plutons, because it is more oxidized and richer in Cu, Cl, and S. • Apatite is an efficient probe for magmatic fertility. • The parental magma of the Cu-mineralized Pulang pluton is more fertile than that of the non-mineralized plutons. • The difference in magmatic fertility results from magma sources. The Paleo-Tethyan granitoids in eastern Tibet are generally Cu-barren, and only a few can generate porphyry Cu deposits. It is still unclear what magmatic differences cause these coeval granitoids to have different mineralization potentials. In this study, the major, minor, and trace element abundances and Nd isotope in the apatites from four Triassic granitoids from eastern Tibet have been determined using EPMA, LA-ICP-MS, and LA-MC-ICP-MS. The following are the selected plutons: the Pulang pluton, which is associated with the porphyry-type Cu deposit, and the coeval Cuojiaoma, Dongcuo, and Daocheng plutons, three of which do not generate any Cu mineralization. Our study reveals that Ga, Ce, S, Cl, and Cu in apatite are effective indicators for both magmatic oxidation states and the abundances of volatiles and ore-forming metals. The Nd isotope of apatite can be used to trace the magma source. According to the above chemical compositions of apatite and Ce4+/Ce3+ ratio of zircon, the parental magma of the Pulang pluton is identified to be more fertile for Cu mineralization than those of the other three plutons, i.e., Cuojiaoma, Dongcuo, and Daocheng, because it is more oxidized and richer in Cu, Cl, and S. The aforementioned differences in magmatic physical and chemical conditions among the plutons investigated are ascribed to the different sources and natures of magmas, as revealed by both the Nd isotope and δEu, Sr/Y, and La/Yb values of apatite. Generally, the results indicate that magmatic fertility mainly governs the Cu-mineralization potential of Paleo-Tethyan granitoids in eastern Tibet, and that apatite is a robust probe for magmatic fertility. [ABSTRACT FROM AUTHOR]