F-rich magmatic–hydrothermal Sn–W mineralization in the Himalaya: Constraints from wolframite U–Pb geochronology and fluorite geochemistry of the Cuonadong deposit

Deciphering how rare metals are transferred from magmatic enrichment to hydrothermal mineralization is essential for understanding granite-related rare-metal ore systems. In this study, we investigate the Cuonadong Sn–W polymetallic deposit by integrating in situ wolframite U–Pb dating, in situ fluorite trace-element analysis, fluid-inclusion microthermometry, and H–O–Sr–Nd–Pb isotope geochemistry, with the aim of reconstructing fluid evolution and its controls on Sn–W mineralization in a fluorine-rich magmatic–hydrothermal system. Wolframite U–Pb age is 14.8 ± 0.6 Ma, broadly coeval with the emplacement of highly fractionated leucogranites and pegmatites, establishing a genetic link between mineralization and late-stage, highly evolved granitic magmatism. Fluorite trace-element compositions vary markedly among the paragenetic stages: FL-I1 fluorite has ΣREE of 152.61–194.72 ppm (mean 172.64 ppm), δEu of 0.23–0.31 (mean 0.28), and δCe of 0.80–0.97 (mean 0.86); FL-I2 fluorite is characterized by lower and more variable ΣREE (4.59–79.11 ppm, mean 22.08 ppm), δEu of 0.16–0.53 (mean 0.34), and δCe of 0.33–1.04 (mean 0.65); and FL-II fluorite has intermediate ΣREE (14.23–118.92 ppm, mean 44.49 ppm), δEu of 0.08–1.33 (mean 0.71), and δCe of 0.79–1.15 (mean 0.92). Fluid inclusions in fluorite are dominated by liquid–vapor two-phase inclusions, with homogenization temperatures of 186–363 °C and salinities of 8.8–21.8 wt% NaCl eqv.; the mean homogenization temperatures and salinities are 296 °C and 17.0 wt% NaCl eqv. for FL-I1, 212 °C and 19.2 wt% NaCl eqv. for FL-I2, and 302 °C and 12.6 wt% NaCl eqv. for FL-II. These systematic variations record changes in element mobility and in the physicochemical properties of the ore fluid during hydrothermal evolution. H–O isotope data show that FL-I1 fluids (δD = − 85.7 ‰ to − 73.7 ‰; δ 18 O = 0.1 ‰ to 1.6 ‰) are derived from magmatic water, whereas FL-I2 (δD = − 98.2 ‰ to − 92.1 ‰; δ 18 O = − 2.0 ‰ to 1.0 ‰) and, most notably, FL-II (δD = − 152.9 ‰ to − 126.3 ‰; δ 18 O = − 11.6 ‰ to − 10.4 ‰) record progressively increasing involvement of meteoric water. In contrast, the Sr–Nd–Pb isotopic compositions of fluorite (initial 87 Sr/ 86 Sr = 0.7178–0.725; ε Nd (t) = − 13.5 to − 10.6; 206 Pb/ 204 Pb = 18.742–20.839; 207 Pb/ 204 Pb = 15.674–15.902; 208 Pb/ 204 Pb = 38.563–41.321) are consistent with those of the regional leucogranites, indicating that fluorine and the ore-forming metals were derived principally from highly fractionated granitic magma. Collectively, our results indicate that Sn–W precipitation was jointly controlled by cooling, fluid mixing, and reaction with carbonate wall rocks: fluid–rock interaction was critical for the skarn-type orebodies, whereas cooling and fluid mixing were more important for vein-type mineralization. Therefore, fluorite is a robust indicator mineral for fluorine-rich magmatic–hydrothermal activity.