N 2 -rich fluid in the vein-type Yangjingou scheelite deposit, Yanbian, NE China

Nearly pure N2 fluid inclusions (Th (L) = −151~−168 °C; Th (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. ThN2 and ThN2-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.