Age and mineralization mechanisms of the Babanqiao tin polymetallic deposit in the Dupangling area, Nanling region, South China

The Nanling region in South China contains widespread granites of variable ages. Large-scale W and Sn mineralization were closely associated with Yanshan granites, but the mineralization related to granites of other ages has been little studied. Understanding the metallogenic significance of Indosinian granites is crucial for refining exploration models, as recent discoveries suggest they may have played a greater role than previously recognized. The weighted mean zircon U Pb age of the biotite granite from the eastern part of the Dupangling pluton is 224.7 ± 4.8 Ma, while the cassiterite U Pb age from the Babanqiao Sn-polymetallic deposit is 223.6 ± 4.0 Ma. These results indicate that the Babanqiao deposit formed contemporaneously with late Indosinian granitic magmatism, suggesting that the deposit was likely associated with granitic activity during the late Indosinian period. Fluid inclusion data reveal the initial ore-forming fluids were part of a NaCl–H₂O system with high to moderate temperatures and salinities, which contained CH₄ ± N₂. Homogenization temperatures ranged from 182.1 °C to 381.5 °C in the early mineralization stage and decreased to 145.4 °C to 279.8 °C in the main mineralization stage, with corresponding salinities of 13.62–25.15 wt% NaCl equiv. and 7.45–19.76 wt% NaCl equiv., respectively. These values indicate that ore deposition occurred under reducing conditions, with a progressive decrease in temperature and salinity as meteoric water mixed with the magmatic fluids. δ 18 O and δD values show that the ore-forming fluids were magmatic waters that gradually transitioned to meteoric waters. This progressive dilution likely influenced metal transport. Integrated analysis indicates that during the Indosinian period, Sn-bearing magmatic–hydrothermal fluids migrated into closely spaced microfractures and joints, where localized boiling may have occurred, producing limited precipitation. The fluids then ascended along these structures and mixed with infiltrating meteoric water, destabilizing Sn Cl complexes and precipitating Sn in fault zones to form quartz-vein–type Sn ore. Thus, fluid mixing was the primary mineralization mechanism at Babanqiao. These findings also highlight the exploration potential of Indosinian granites in South China as hosts of previously unrecognized W Sn systems. • Zircon and cassiterite U Pb ages indicate Babanqiao tin formed at ~223 Ma. • Ore-forming fluids were NaCl-H 2 O ± CH 4 ± N 2 , with high–moderate T and salinity. • The initial ore-forming fluid originated from magmatic sources. • The ore-forming materials were sourced from deep-seated magma. • Fluid mixing is the primary mechanism for metallogenesis.