Sub-solidus devolatilization of metasomatized mantle lithosphere produces siliceous fluids for quartz-vein gold deposits

Quartz-vein gold deposits dominate Earth’s gold endowment. However, the origin of their ore-forming fluids remains contested. While the traditional model invokes crustal metamorphism of silica-rich rocks, fluids derived from metasomatized mantle have emerged as a viable alternative. Yet how siliceous ore-forming fluids are released from this mantle source remains unclear. Here we propose that sub-solidus devolatilization of metasomatized mantle lithosphere generates siliceous hydrothermal fluids without partial melting, based on samples from the post-collisional Yangtze and North China craton margins. Silicon and oxygen isotope systematics show that both hypozonal (δ 30 Si = −0.30‰ to −0.20‰) and mesozonal gold deposits yield δ 30 Si values consistent with potassic mafic rocks, including lamprophyres, and metasomatized mantle sources. Integrated with a marked increase in mantle xenolith equilibration temperatures from pre- to post-mineralization in the Jiaodong gold province, phase-equilibrium calculations​ for the phlogopite-lherzolite system reveal a critical thermal window (700–800°C, 1–2 GPa) where​​ devolatilization and silicon release are maximized below the wet solidus. We demonstrate that silica-poor metasomatized mantle at 700–950°C releases fluids with Si solubility comparable to that of silica-rich lower crust at 400–700°C. This model incorporates solid-state fluid generation, a key feature of the crustal metamorphic model, and clarifies ​the origin and capacity of siliceous fluid production​ within the mantle-derived paradigm. It supports a scenario in which spatiotemporally associated quartz-vein gold ores and potassic mafic rocks are successive but independent products of mantle devolatilization and melting, respectively, as the thermal profile crosses the solidus. We conclude that this previously overlooked sub-solidus mantle devolatilization applies broadly to post-collisional and other orogenic gold systems.