Cassiterite geochronology and geochemistry of the Yunling Sn deposit: implication for late cenozoic mineralization in western Yunnan, China

Abstract

Tin deposits within the Baoshan Block in western Yunnan are posited as the northern extension of the Southeast Asian Tin Belt, yet they have been relatively underexplored in terms of geochronology. This study concentrates on the Yunling tin deposit, globally recognized for its production of gemstone-quality cassiterite crystals. We applied U–Pb geochronology on cassiterite, complemented by analyses of its trace element composition and in situ oxygen isotopes in cassiterite and quartz, aiming to delineate the deposit's age and genesis. The Yunling orebodies are hosted by deformed Triassic granite, closely adjacent to the Cenozoic Nantinghe strike-slip shear zone. Three distinct hydrothermal stages have been identified: quartz-cassiterite-muscovite-tourmaline (stage I), arsenopyrite-pyrite-cassiterite-quartz (stage II), and arsenopyrite-calcite-quartz (stage III). Cassiterite grains from a quartz-cassiterite-muscovite-tourmaline vein yield a U–Pb age of 24.4 ± 1.4 Ma (2σ, n = 41, MSWD = 1.6), notably younger than the ore-hosting Triassic granite. Paired cassiterite and quartz oxygen isotopes yield δ¹⁸O_H2O values of 5.8 – 7.2 ‰, indicating a magmatic fluid source during stages I and II. The trace element compositions of the Yunling cassiterite resemble those of granite-related tin deposits, suggesting a genetic link between tin mineralization and an unexposed late Cenozoic granite intrusion. Notably, the Triassic granite of Yunling shows a lower degree of magmatic fractionation, thus presenting a limited potential for tin mineralization. The timing of mineralization is correlated with the activity of the Nantinghe fault, alongside geophysical evidence of crust-mantle decoupling and asthenosphere upwelling. Consequently, our findings imply that the Yunling tin mineralization is genetically related to hidden granites, to guide future exploration efforts in western Yunnan.