Determining multiple fluid pulse and evolution using zoned garnet in Mengya’a skarn Pb-Zn-polymetallic deposit, Tibet

Abstract

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.