N2-RICH FLUID IN THE VEIN-TYPE YANGJINGOU SCHEELITE DEPOSIT, YANBIAN, NE CHINA

High nitrogen contents in geological environments are generally observed in nitrate deposits. However, in this study, nearly pure N₂ (T_N2 (L) =-151.2～-167.8℃, T(V)-~150.3℃) fluid inclusions were identified in W-mineralized quartz veins at the Yangjingou scheelite deposit, east of Yanbian area in NE China. Other fluid inclusion populations identified include N₂–CO₂, NaCl–H₂O±N₂ and CO₂±N₂-NaCl-H₂O fluid inclusions; no hydrocarbon components were detected. The host rocks consist of the Wudaogou group metamorphic series and the lithology is mainly Ca-rich mica. Subhedral sulfides occur in early disseminated W-mineralization quartz veins, or have partially replaced early scheelite. T_hN2 and T_hN2-H2O values indicate that N₂ fluid trapping P-T ranges from 315℃ to 410℃ and 80 MPa to 350 MPa. O and H isotope data suggest that mineralizing fluids were composed of mixed magmatic and metamorphic water and the δ¹⁵N values of N₂-rich inclusions indicate that fluid-rock interaction occurred within a metamorphic environment. The nitrogen-rich fluid is closely associated with scheelite precipitation. During thermal decomposition and fo₂-increase, which occurred synchronously with retrograde metamorphism and magmatic activity, large amounts of N₂ has liberated from NH₄⁺ bearing micas, which accumulated in quartz scheelite veins.