Genesis of the Baitoushan Cu-Polymetallic Deposit, Qimantagh Region, Eastern Kunlun Orogen: Evidence from geochemistry of garnets and geochronology

The Qimantagh region in the western segment of the Eastern Kunlun Orogenic Belt has undergone multiple phases of tectonic-magmatic activity from the Paleozoic to Early Mesozoic, resulting in the formation of numerous skarn- and porphyry-type deposits. The newly discovered Baitoushan skarn Cu-polymetallic deposit remains poorly studied. This paper focuses on two types of garnet from the deposit: dark garnet (Grt-1) and light garnet (Grt-2) that replaces the earlier-formed Grt-1 with their chemical compositions analyzed using EPMA and LA-ICP-MS. The results indicate that both Grt-1 and Grt-2 belong to the grossular-andradite solid-solution series, Grt-2 is enriched in the andradite component. Both garnets exhibit low total rare earth element (REE) contents. However, Grt-1 shows significantly higher ΣREE and LREE/HREE ratios than Grt-2. Grt-1 displays a pronounced negative Eu anomaly and a right-inclined REE pattern, whereas Grt-2 exhibits a moderate negative Eu anomaly with a gently left-inclined pattern. These compositional differences suggest that Grt-1 crystallized under relatively low-oxygen-fugacity and weakly acidic conditions, while the increased Fe and decreased U contents in Grt-2 reflect a higher oxygen fugacity and a higher pH during its formation. LA-ICP-MS U-Pb dating of garnet and apatite yielded ages of 219.4 ± 3.4 Ma and 220.8 ± 6.4 Ma, respectively, both falling within the Late Triassic (Indosinian period). These ages confirm the Indosinian mineralization event at the Baitoushan deposit. The compositional variations between the two garnet generations record a systematic increase in oxygen fugacity and pH during fluid evolution, demonstrating the utility of garnet geochemistry in tracking skarn-forming processes. The precisely determined U-Pb ages of garnet and apatite provide robust temporal constraints on the ore formation, confirming a Late Triassic magmatic-hydrothermal origin. Collectively, these results, integrated with regional magmatic and tectonic data, establish the Baitoushan deposit as a product of the large-scale Indosinian skarn metallogenic event in the Qimantagh region, which was fundamentally controlled by thickened crust, mantle-derived oxidized adakitic magmas, and trans -crustal structures