Hydrothermal rutile as a recorder of fluid evolution and W mineralization: Insights from the Northern Baoshan Block, China

Abstract Hydrothermal rutile (TiO2) can record key metallogenic information and thus offers valuable insights into mineralization process. However, the genesis of rutile in W-Sn mineralized veins and its role in ore formation remain poorly understood. In this study, detailed petrographic observations, LA–ICPvMS U–Pb dating of cassiterite and hydrothermal rutile, as well as major and trace element analyses of rutile, were used to investigate the origin of rutile and its relationship with W mineralization in the Wuchashu and Shiganghe W-Sn deposits, located in the Lushui area in the northern Baoshan Block, China. Cassiterite from the Wuchashu deposit yields a U–Pb ages of 70.2 ± 2.1 Ma. Hydrothermal rutile in the Shiganghe deposit formed synchronously with quartz, cassiterite, and minor scheelite, indicating that it records the timing of mineralization. U–Pb dating of rutile from Shiganghe ore yields an age of 73.0 ± 5.5 Ma. Consistent U–Pb ages of cassiterite and rutile demonstrate that W-Sn mineralization in both deposits formed during the same magmatic-hydrothermal event. In both deposits, the formation of rutile is associated with magmatic-hydrothermal fluids, while two distinct types of rutile have been identified based on crystallographic and chemical characteristics. Type I rutile from the Wuchashu deposit occurs in the forms of sagenite and columnar rutile and formed via replacement of ilmenite, commonly preserving pseudocrystals habit. Type II rutile from the Shiganghe deposit crystallized directly from hydrothermal fluids and was later altered by late-stage fluid overprints. It occurs as idiomorphic to subhedral granular crystals, with some forming mineral assemblages with titanite and/or ilmenite. Both types of rutile show distinct differences in trace element concentrations, particularly in W, Nb, and V. In type I rutile, variations in Zr content are related to fluid temperature. For type II rutile, when the Zr content exceeds 100 ppm, its variations are associated with fluid temperature; however, when Zr decreases below 100 ppm, a concurrent decrease in the (Hf/Zr)N ratio is attributed to the precipitation of Zr-rich minerals. Variations in redox-sensitive elements (e.g., Fe, Cr, V, Sb), their valence states, and interelement correlations indicate that type I rutile crystallized under higher oxygen fugacity conditions (fO2) than type II. Nevertheless, during fluid evolution, the compositional zoning and trace element variation characteristics of rutile indicate that both types experienced progressive decreases in oxygen fugacity. The geochemical and textural features of rutile and its associated Ti-bearing minerals (e.g., titanite, ilmenite) reveal two distinct mechanisms for W release from rutile into the hydrothermal fluid: (1) In type I rutile, a decrease in oxygen fugacity alters the dominant coupled substitution mechanism from 2(Fe, Al, Sc)3+ + W6+ ↔ 3Ti4+ to 2V3+ + W6+ ↔ 3Ti4+, resulting in the release of W into the fluid; (2) In type II rutile, W release is primarily driven by late-stage fluid alteration, which promotes the formation of titanite and/or ilmenite, along with limited decoupling of W6+, Nb5+, and Fe3+ from the rutile lattice under decreasing oxygen fugacity. This study highlights that within W-, P-, and F-enriched mineralizing fluids associated with granitic magmatism, W preferentially partitions into early crystallized rutile rather than precipitating as scheelite. Rutile thus functions as an effective temporary reservoir for W, preventing significant W loss. Subsequently, the stored W within rutile undergoes remobilization—a process likely initiated by decreasing oxygen fugacity and alteration by late-stage fluids—thereby facilitating W mineralization.