Titanite-recorded tungsten migration and fluid boiling-triggered precipitation: Insights from the Xiaoyao deposit, South China

Mechanisms governing hydrothermal tungsten precipitation, including cooling, boiling, fluid mixing and fluid-rock interaction, represent longstanding controversial issues in economic geology. Herein, six genetically distinct titanite types from the Xiaoyao skarn-type tungsten deposit were investigated to reconstruct the complete magmatic-hydrothermal evolutionary history of the deposit. These types include primary magmatic titanite (Ttn-M) in fresh granodiorite; altered granitic titanite (Ttn-A) in altered granodiorite; as well as prograde skarn titanite (Ttn-P), early oxide-stage titanite (Ttn-O1), late oxide-stage titanite (Ttn-O2), and carbonate–sulfide-stage titanite (Ttn-C) hosted in skarn. In situ EPMA and LA-ICP-MS analyses reveal systematic compositional variations across all titanite populations. Ttn-M exhibits elevated W concentrations compared with that of W-barren granites, indicative of early pre-enrichment in the parental magma before hydrothermal exsolution. Ttn-A, Ttn-P and Ttn-O1 feature extremely high W, reflecting robust tungsten migration from residual magma into hydrothermal fluids and subsequent transport toward skarn. Abrupt W depletion in Ttn-O2 corresponds to large-scale scheelite precipitation. Consistent Y/Ho and Sr compositions, together with variable REE patterns, strongly favor fluid boiling—rather than fluid mixing, simple cooling, or fluid-rock reaction—as the dominant ore-forming trigger. This process elevates fluid pH and promotes the depolymerization of aqueous polytungstate species, thereby facilitating intensive scheelite precipitation. Titanite geochemical data also record a low-F ore-forming system, which inhibits magmatic differentiation and restricts the total tungsten endowment of the deposit. These findings demonstrate that multistage titanite geochemistry profiling can effectively trace tungsten migration pathways and precisely pinpoint the key trigger mechanisms for tungsten precipitation in skarn systems.