Gemological, Geochemical, and Geochronological Study of Apatite from the Shijiangshan, Inner Mongolia

Apatite, a ubiquitous accessory mineral, serves as a valuable recorder of magmatic processes. This study presents a systematic investigation of gem-quality apatite from the Shijiangshan Pb-Zn deposit, Inner Mongolia, integrating gemological characterization, chemical composition, vibrational spectroscopy, coloration mechanism and in-situ U-Pb geochronology. The apatite crystals exhibit a light-green color with distinct pleochroism, attributed to Ce3+-SiO3−/SO3− photochromic centers and Ce3+/Ce4+ charge-transfer transitions. Geochemically, the apatite is F-rich and Cl-poor, classifying it as fluorapatite, with a magmatic origin consistent with crystallization from a magmatic–hydrothermal system as supported by SiO2 vs. FeO/MnO discrimination diagrams. Rare earth element (REE) patterns display steep right-sloping profiles with strong light REE (LREE) enrichment, heavy REE (HREE) depletion, moderate negative Eu anomalies (δEu = 0.45–0.49), and weak negative Ce anomalies (δCe = 0.94–0.95), indicating crystallization from a moderately oxidized, evolved magmatic–hydrothermal system. Low Th/U ratios (avg. 1.82) together with elevated As concentrations (avg. 2551 ppm) suggest significant fluid activity and magmatic differentiation. In situ U-Pb dating yields a weighted mean 206Pb/238U age of 133.5 ± 3.1 Ma, constraining the timing of late-stage magmatic–hydrothermal activity associated with regional Early Cretaceous Pb-Zn mineralization.