• The U-Pb ages of garnets are 158.0 ± 4.7 Ma, 158.5 ± 8.9 Ma, and 159.2 ± 4.4 Ma. • Garnet in the northeastern section was formed in slightly reducing neutral hydrothermal system. • Garnet in western section crystallized under a relatively open, weakly oxidized, and acidic condition. • Eastward decreases in andradite and Sn contents reveal a concealed mineralization center beneath the western section. In a distal skarn system, identifying and establishing zoning patterns of the typical skarn minerals in their colors, grain sizes, and compositions is of great significance for locating potential proximal skarn and concealed causative intrusions. The redox gradient formed between oxidized magmatic intrusions and reduced wall rocks in skarn systems serves as a key tracer for fluid migration pathways and can be effectively recorded by changes in garnet color, grain size, and composition. Based on the skarn zoning model, this study investigates horizontal garnet zoning in the northeastern and western ore sections of the Xingjiashan distal skarn Mo-W deposit. The results are used to reconstruct fluid migration pathways and infer the possible location of the mineralization center. The garnets in the northeastern section are subhedral to euhedral, light flesh red, medium-fine-grained (0.5–2 mm), with relatively uniform composition (And 6.04-23.4 Gro 68.0-89.2 ) and no oscillatory zoning. In contrast, the garnets in the western section are mostly euhedral, dark red-brown to red-brown, medium-coarse-grained (1–25 mm), and developing obvious oscillatory zones (And 9.64-47.2 Gro 41.6-83.6 ). In situ LA-ICP-MS U-Pb dating of garnets from the northeastern and western sections has yielded three T⁃W lower intercept 206 Pb/ 238 U ages of 158.0 ± 4.7 Ma, 158.5 ± 8.9 Ma, and 159.2 ± 4.4 Ma. These are consistent with the previously reported molybdenite Re-Os ages, directly constraining the timing of skarn formation. Garnets from both sections are significantly depleted in large ion lithophile elements (LILEs) such as Rb, Sr, and Cs, and relatively enriched in high field strength elements (HFSEs) such as Nb, U, Ta, and Hf. Garnets from the western section are characterized by higher andradite contents, Sn contents, Eu N /Eu N *, and Y/Ho ratios, and depleted light rare earth element (LREE) with enriched heavy rare earth element (HREE), indicating that they were crystallized from a relatively open, weakly oxidized, and acidic hydrothermal system. Despite the fact that the garnets in the northeastern ore section also show LREE depletion and HREE enrichment, their lower andradite and Sn contents, Eu N /Eu N *, and Y/Ho ratios imply that they were formed in a relatively closed and slightly reducing, neutral hydrothermal system. The systematic decrease in oxygen fugacity from the western section to the northeastern section indicates that the magmatic-hydrothermal fluid exsolved from the concealed weakly oxidized ore-forming intrusion might migrate from west to east. The more euhedral, larger, and darker garnet crystals in the western ore section further prove that the western ore section is closer to the mineralization center, and there may be concealed causative intrusions underneath the western section. This study emphasizes that the horizontal zoning of garnet textural and compositional features could serve as a reliable proxy for locating the mineralization center of a certain skarn Mo-W deposit and provides scientific guidance for deep and peripheral ore exploration.
Li et al. (2026) studied this question.