N₂-rich fluid in the vein-type Yangjingou scheelite deposit, Yanbian, NE China

Nearly pure N₂ fluid inclusions (T_h (L) = -151~-168 °C; T_h (V) = ~150.3 °C) were identified in W-mineralized quartz veins from the Yangjingou scheelite deposit, in the eastern Yanbian area, NE China. Other fluid inclusion populations include N₂-CO₂, NaCl-H₂O ± N₂ and CO₂ ± N₂-NaCl-H₂O, but no hydrocarbons were detected. The host rocks are part of the Wudaogou Group metamorphic series, which mainly consist of Ca-rich mica schist. Subhedral sulfide minerals occur in early disseminated W-mineralized quartz veins, or have partially replaced early scheelite. T_hN2 and T_hN2-H2O indicate N₂ fluid-trapping from 315 °C to 410 °C and from 80 MPa to 350 MPa. Oxygen and hydrogen isotopic data (δD = -74.9‰~-77‰, δ¹⁸O = 9.6‰~12‰, V-SMOW) suggest that the mineralizing fluids were composed of mixed magmatic and metamorphic water, N₂-rich inclusions (δ¹⁵N = -0.5‰ to 1.4‰) indicate fluid-rock interaction with metamorphic rocks. The N₂-rich fluid was closely associated with scheelite precipitation. During thermal decomposition under high oxygen fugacity conditions, which occurred synchronously with metamorphism and magmatic activity, large amounts of N₂ were liberated from NH₄⁺-micas, which then accumulated in the parent fluid of the quartz scheelite veins.