Crucial factors for the generation of magmatic-hydrothermal Sn mineralization in the tengchong block, SW China: Evidence from geochemistry zircon characteristics

• Crucial factors controlling magmatic-hydrothermal Sn mineralization. • High degree fractionation causes the first-order Sn enrichment in magmas. • Depressurization-induced rapid cooling facilitates Sn mineralization. Magmatic-hydrothermal Sn deposits are commonly associated with high silica magmas, but why most global high silica granites do not bear economical Sn ore grades remains unclear. Two crucial factors controlling magmatic-hydrothermal Sn mineralization were revealed in the case study on Guyong granitic pluton linked with Xiaolonghe Sn deposit, in the Tengchong block, SW China. The Guyong granitic pluton comprises three petrological facies: less evolved biotite syenogranite, evolved alkali granite and leucogranite, and highly evolved facies (the protolith of greisenized granite). Similar crystallization ages (∼77 Ma) and gradual contact between different petrological facies indicate the Guyong granitic pluton recorded a continuous fractionation process. Monte Carlo-revised Rayleigh fractionation model suggests the fractionation degree of the Guyong pluton is markedly high (>87 wt.%) that can only be achieved by high initial water (≥4 wt.%) contents of the parent granitic magma revealed by rhyolite-MELTS estimation. Advanced degree fractionation causes the first-order Sn enrichment but it also significantly increases the viscosity of evolved magmas, suppressing the transport of hydrothermal fluid. Hence, it must be compensated by the second critical factor- depressurization-induced rapid cooling. This is reflected by the occurrence of highly metamict zircons in the greisenized granite. The highly metamict feature, indicated by the large full width at half maximum (FWHM) values of zircon ν 3 (SiO 4 ) peak (>19.5 cm –1 ), suggests these zircons do not experience thermal annealing but rapidly ascend into a shallow cooling environment. Depressurization-induced rapid cooling facilitated exsolution and transport of hydrothermal fluids, which interacted with wall rocks and resulted in Sn mineralization.