Geochemistry and U–Pb Zircon Geochronology of Granite at the Pilok Sn–W Deposit, Thailand: Implications for Late Triassic Tin Metallogenesis

The Pilok Sn–W deposit in western Thailand is hosted by peraluminous S-type granites that intruded Silurian–Devonian metasedimentary rocks. This study presents whole-rock geochemistry, zircon U–Pb geochronology, and trace element data to constrain the timing of emplacement, petrogenesis, and regional metallogenic processes. Two granite types occur in the deposit: biotite–muscovite granite and tourmaline–biotite–muscovite granite, both showing high SiO₂ (72.6–75.2 wt%) and Sn (12.8–27.4 ppm) contents, strongly peraluminous characteristics (A/CNK = 1.20–1.38), pronounced negative Eu anomalies (Eu/Eu* = 0.13–0.27), and elevated Rb/Sr ratios (14.1–22.6), indicating extensive fractional crystallization. Zircon textures and chemistry of the host granites reveal four distinct types: Types I and II zircons displaying magmatic oscillatory zoning and heavy REE-enriched patterns, and Types III and IV zircons showing hydrothermal characteristics with LREE enrichment in chondrite-normalized REE patterns. U–Pb dating of magmatic zircons from four granite samples yielded ²⁰⁶Pb/²³⁸U ages of 208 ± 5.3 to 212 ± 1.5 Ma, synchronous with the granites in the Late Triassic tin metallogenic belt in Southeast Asia. Ti-in-zircon thermometry indicates crystallization temperatures of 698°C–829°C (average 754°C), and trace elements of zircon indicate strongly reduced conditions (ΔFMQ: −0.8 to −4.5, average −1.9), which were favorable conditions to retain Sn²⁺ in a residual melt. REE patterns of zircon suggest a magmatic to hydrothermal transition, recording late-stage fluid exsolution that mobilized Sn and W. The combination of Late Triassic ages, S-type peraluminous geochemistry, extreme fractional crystallization, and reduced magmatic conditions can be used as criteria for identifying tin-fertile granites in the Sibumasu Terrane and other terranes with similar tectonic settings.