N2-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.

Several generations of magmatic intrusions, including late Hercynian granodiorite (267 ± 1 Ma) 14 and early Indosinian Yangjingou granodiorite (249.4 ± 2.7 Ma) 15 have locally deformed rocks of the Wudaogou Group. Based on the Yangjingou granodiorite chemical signature, a transition from I-to S-type magmatism is observed, interpreted as a result of subduction 16 . Yanshanian intrusives, including monzonitic granodiorite (178.   17 and dioritic porphyry dykes (120.73-157. 27 Ma) 17 were later emplaced within zones of pre-existing structural weakness. Previous studies have revealed a close association between Indosinian magmatism and W mineralization in the Yangjingou deposit 16,18 . Furthermore, Shan 19 and Hu 17 suggest that the dikes present in our study area are genetically related to an unexposed granite intrusion.
The scheelite ore bodies are mainly hosted in Ca-rich mica schists. The deposit can be subdivided into a southern and a northern ore block, separated by a N-W-trending fault (Fig. 2). In the northern ore block (Fig. 3b), mineralization is structurally controlled by normal faults striking 295-315° and dipping 65-75°. Mineralization occurs in quartz stringers and large quartz veins that have been emplaced along intraformational faults, which partially crosscut the upper Hercynian granodiorite. Individual veins are typically sub-parallel, appear as lenticular or spindle-shaped bodies, exhibit pinching and swelling, and measure approximately 10 cm in width. The ore bodies exhibit a total length of more than 900 m, a width from 60 to 425 m, and a stacking thickness of approximately 95 m.
Mineralization in the southern ore block is structurally controlled by normal faults striking 300-350° and dipping 60-75° (Fig. 3a). The width of individual veins ranges from 2 to 10 cm (locally reaching up to 65 cm). The total length of the ore bodies is approximately 800-1400 m, with a maximum width of 450 m. The scheelite-bearing veins in both the northern and southern ore blocks share similar characteristics in terms of their orientation and geometry.

Analysis and Methods
Fluid inclusions. Twenty-six highly transparent 100-300-μm doubly polished thick quartz plates of paragenetic scheelite quartz veins from the southern and northern ore blocks of the Yangjingou deposit were analysed. Fluid petrography and microthermometry analyses were performed at the Geological Fluid Analysis Center of Jilin University, Changchun, China. The microthermometric analysis of fluid inclusions was conducted with a Linkam THMSG 600 stage mounted on a Carl Zeiss Axiolab microscope (10 × 50) capable of temperature measurements from −196 to 600 °C. The accuracy between 31 °C and −100 °C is better than ±0.2 °C and better than 5 °C at >300 °C. The heating/freezing rate is generally 0.2-1 °C/min but is reduced to 0.1 °C/min close to the phase transformation point. The FLUIDS 20,21 software package used for the calculation of the isochors and the phase transition temperature of N 2 -CO 2 were plotted in the diagram of Thiery 22 and Van den Kerkhof & Thiery 23 in order to determine vX properties of the fluid.

Raman microspectroscopy analysis. Raman analysis was performed at the Geological Fluid Analysis
Center of Jilin University. The Raman instrument is a Renishaw RM-1000 and was mounted on an Olympus BX40 microscope. A 532 nm green diode-pumped solid state laser was used as an excitation source. The spectrum counting time was 30-60 s, the spectral resolution 1-2 cm −1 , and the laser beam diameter 1-2 μm. A detailed description of the measurement procedure was provided by Wopenka 24 . The measurements reveal that the major gaseous fluid inclusion components are N 2 and CO 2 . A large proportion of the inclusions show only a single strong band at 2328.2-2328.3 cm −1 , indicating pure N 2 , which is the first time pure N 2 inclusions have been observed in the W deposit (  Stable isotope analysis. The oxygen and hydrogen isotopic compositions of primary fluid inclusions in the main stage (oxide stage) of the disseminated scheelite quartz veins were analysed at the analytical laboratory of Beijing Research Institute of Uranium Geology (BRIUG), China, using a MAT253-type mass spectrometer. Grains (40-60 nm) were handpicked to ensure the absence of mineral impurities. Oxygen was liberated from quartz for isotopic analyses via quantitative reaction with BrF 5 using a CO 2 laser as a heat source.  Seven quartz samples from scheelite quartz veins were selected for N isotopic analysis. In order to exclude atmospheric interference, samples were degassed for 15 minutes under vacuum (<10 −3 Pa). N 2 was then directly released from fluid inclusions by heating the quartz samples in a high-purity quartz cell to temperatures above 600 °C. The released N 2 was then separated using a chromatographic column cold trap 26 . Nitrogen isotopic compositions of the samples are reported using δ notation, where The results are reported in per mil (‰) relative to atmospheric N 2 27 , and the precision is ±1‰ at the 1σ level.

Results
Fluid inclusions. The criteria by Roedder 28 for defining primary, secondary and pesudosecondary inclusions were applied. However, microtheromology measurements were conducted on primary fluid inclusions, because only primary inclusions represent W-mineralized fluid. Primary inclusions occur in isolation or as random clusters within intragranular quartz crystals 28 . Our study focuses on the main W-mineralizing stage, i.e., the scheelite-quartz stage. Based on composition, phase proportion at room temperature (21 °C) and phase transition during total homogenization, four types of fluid inclusions have been identified in quartz crystals from the scheelite paragenetic quartz veins: pure N 2 fluid inclusions (Type I), CO 2 -N 2 fluid inclusions (Type II), CO 2 ± N 2 -NCl-H 2 O inclusions (Type III) and aqueous two-phase VL inclusions (Type IV; Table 1).
The Type I (pure N 2 ) fluid inclusions ( Fig. 5a,b,c,g,u) occur as a single phase at room temperature and are 6-30 μm in size. The inclusions are mostly oval in shape or exhibit a negative crystal shape and are randomly distributed in quartz grains. Type I inclusions are identified as N 2 homogenizes to liquid or gas (L + V-> L(G)) and are closely related to scheelite deposits, but have been rarely reported. At low temperatures, bubbles nucleate at −162 °C~−182 °C. The final homogenization of the liquid phase T h (L) occurs between −151 °C~−168 °C,   although a few Type I fluid inclusions homogenize to the gas phase at −150.3 °C (Fig. 6a,c). Raman analysis of the Type I inclusions yields compositions of >88 mol% N 2 , with minor amounts of CO 2 exhibiting molar volumes of 42-150 cm 3 29,30 . Bubbles of nitrogen nucleate between −178 and −159 °C (Fig. 5o); solid CO 2 is infrequently observed below −100 °C (Fig. 5n,r). In S-type inclusions the partial homogenization (T hN2 ) to a liquid (gas) of is between −163.2 and −147.5 °C. Critical homogenization can be observed around −147 °C and the total homogenization (T hCO2 ) is characterized by sublimation to liquid (gas) phase from −86~−69 °C. The T hN2 of H-type inclusions to the liquid (gas) phase coincided with critical homogenization and occurred between −149 and −148 °C. The total homogenization (T hCO2 ) to liquid (gas) varied from −60.8 to −45.6 °C with the T mCO2 between −60.9 and −60.7 °C. Raman spectroscopy shows that the inclusions are composed of 42-79 mol% N 2 , and 21-58 mol% CO 2 with molar volumes between 30 and 65 cm 3 /mole, in agreement with microthermometry results (Figs 6a and 7).
The Type III (NaCl-H 2 O) inclusions contain liquid and gas at room temperature and have been subdivided into Type IIIa and Type IIIb populations. Type IIIa inclusions contain gas volume fractions of <50%, where most are between 10 and 30% (Fig. 5f). N 2 less than 10% were infrequently detected in these inclusions. Fluid inclusions are generally 4-16 μm in size, clustering between approximately 6 and 10 μm and appear as elongated and spindle shapes, which are randomly distributed in quartz grain. These inclusions yield Tm ice of −6.7 °C to −3.5 °C and homogenize to liquid between 250 and 400 °C, predominantly between 300 and 375 °C (Fig. 6a). Therefore, the salinity of these inclusions is 6-10 equiv. wt% NaCl 20 (Fig. 6b). In Type IIIb inclusions, the gas volume fractions are >50%, where most are between 70-95% (Fig. 5d,j,i). The total homogenization temperature to the gas phase occurs between 314 and 409 °C, predominantly between 334 and 381 °C (Fig. 6a). The liquid phase of N 2 can be locally observed at temperatures below −150 °C (Fig. 5p).
The Type IV (NaCl-H 2 O-CO 2 ± N 2 ) inclusions contain CO 2 gas, CO 2 liquid, and a saline solution at room temperature (Fig. 5e). The volume fraction of CO 2 bubbles in the inclusions exceed 40%, with the majority of the inclusions featuring volume fractions of 50-80%. The gas phase comprises 75-90% of this volume fraction. These primary inclusions show euhedral and irregular polygonal shapes. They occur in randomly distributed clusters in quartz grains and individual inclusions are small, measuring 4-20 μm. Solid CO 2 melts at −60.8~−58.1 °C, indicate the presence of minor N 2 . CO 2 clathrate melting occurs at 5.3-8.1 °C and the salinity of the inclusions ranges from 4 to 9 equiv. wt% NaCl (Fig. 6b). Partial homogenization of CO 2 to liquid occurs at 9.8-20.5 °C, and total homogenization to liquid occurs at 283-423 °C, predominantly at 320-402 °C (Fig. 6a).  Table 2). Wood and Samson 31 demonstrated that scheelite precipitation in W deposits hosted by siliceous rocks occurs at temperatures of 200-500 °C. Consequently, the mode  (Table 2; Fig. 8b). In the δ 15 N graph, all the samples from the W-mineralized quartz veins fall in the range characterized by sub-greenschist-facies metamorphism.

Discussion
Nitrogen in ore-forming fluids. The presence of N in W-bearing mineralizing fluids has previously been reported 7,8,32 ; however, fluids with >50 mol% N 2 are rare. Lin 33 assumed that the N 2 -rich fluid inclusions identified at the Dongchuan Cu deposit resulted from either mantle devolatization during the break-up of Rodinia or the trapping of gas derived from decomposing organic materials. In contrast, the Yangjingou deposit formed in a compressional environment during the closure of the Paleo-Asian Ocean. There is no evidence for mantle degassing during regional orogenic activity in this area. Therefore, the presence of N 2 in the deposit may be associated with plate collision, ocean closure, regional Barrovian metamorphism, and eclogite-facies metapelites 34   Under alkaline conditions, W is transported in the form of H 2 WO 4 , M 2 WO 4 (M = Na, K), and WO 4 −2 complexes in W-mineralizing fluid. The presence of NH 4 + in the solution stabilizes the W polyacid, thereby increasing its compatibility in migrating fluids 36 . The stability of NH 4 + (substituting for K + ) widely depends on the metamorphic and oxygen fugacity conditions 27,37 . Nevertheless, under metamorphic conditions, NH 4 + -bearing minerals may release N 2 (350-600 °C) or NH 3 (650-700 °C) at higher oxygen fugacity and concentrate in inclusions. The following equation illustrates the production of N 2 under oxidizing conditions 38 : The main cause for scheelite precipitation is an increase in the activity of Ca 2+ , which results in the chemical equilibration of the mineralizing fluid with the host rock and has been recorded at a range of pressures and temperatures 32 . The addition of the non-polar volatile N 2 via water-rock interactions in mica may decrease scheelite solubility in common metamorphic assemblages. N 2 has been demonstrated to dominate changes in the distribution of W aqueous species and increase coefficient activity 8 . In other words, an increase in the N 2 content in the mineralizing fluid may promote scheelite precipitation. N 2 -rich fluids in the Yangjingou scheelite deposit. Nature of the ore-forming fluids. The ore must have precipitated from H 2 O-CO 2 -N 2 -NaCl-bearing fluids with low to moderate salinity at moderate to high temperatures. The only volatile components in this fluid are CO 2 and N 2 ; hydrocarbons (i.e., CH 4 ) have not been detected. From the vx plot (Figs 6, 7c) we can observe that the fluid inclusions richer in CO 2 (30-65 cm 3 /mol) have lower molar volumes than those richer in N 2 (42-150 cm 3 /mol). This has been observed in many metamorphic areas 3,29 indicating that CO 2 and N 2 may not have the same origin 38 . The homogenization temperatures of N 2 (T hN2 ) and H 2 O ± N 2 -NaCl (T hH2O ) constrain the pressure-temperature (P-T) conditions of trapping to 315-410 °C at 80-350 MPa (Fig. 6c), suggesting that the W mineralization occurred during late stage greenschist-facies metamorphism at temperature of ~300-600 °C 39 .
Stable isotopes. The stable isotopic composition of a vein provides direct information regarding the transport processes during vein formation 25 . Several sources have been suggested for mineralizing fluids in hydrothermal systems. These include magmatic origin, in which fluids evolve in relatively closed systems. Such mineralizing fluids are only observed in association with vein-type wolframite deposits 40 . Several models that result in mineral precipitation from magmatic fluids of mixed origin are as follows (Fig. 8a): 1) a dominantly magmatic fluid mixing with meteoric waters (e.g., the Xiaoxinancha porphyry Cu-Au deposit, Naozhi alteration Au deposit and Bashilazi skarn-type scheelite deposits) 16,41,42 and 2) the interaction of magmatic fluids with NH 4 + -rich host rocks, and/or mixing with metamorphic fluids in equilibrium with metasedimentary lithologies (e.g., the vein-type Nyakabingo W deposit) 7 .
Low δ 18 O and δD of the Yangjingou scheelite deposit indicate that the magmatic component of the fluid played a more dominant role in this deposit compared to the Nyakabingo deposit, in which the mineralizing fluid was metamorphic (Fig. 8a). Earlier C and S isotopic studies yielded δ 13 C V-PDB values of −3.4‰ to −6.5‰ for CO 2 and δ 34 S V-CDT values of −0.4‰ to 4.7‰ for pyrrhotite, pyrite, and arsenopyrite in scheelite-mineralized veins 15 , consistent with mantle values (δ 13 C V-PDB = −5‰ and δ 34 S V-CDT = ±5‰), demonstrating that the fluid was derived from the mantle. However, compared with magmatic hydrothermal deposits in this area (Fig. 8a), the Yangjingou scheelite deposit shows different δD values which may be interpreted in one of two ways: 1) the original mineralizing magmatic hydrothermal fluid interacted with NH 4 + -rich metamorphic minerals and/or mixed with metamorphic fluids during host rock equilibration or 2) the mineralizing fluid of magmatic origin, mixed with low-δD meteoric waters. Because W-mineralized in the presence of Ca-mica schist host rock coupled with N 2 -rich fluid inclusions in the Yangjingou deposit, the first explanation is preferred. The N isotopic system may potentially provide a detailed record of fluid-rock interaction characteristics and other mixing processes in the crust and mantle 37 . N 2 has been shown to be released during metamorphism and the breakdown of NH 4 + -bearing minerals, such as biotite, cordierite, and white mica 43 . During metamorphism, isotopically "light" N is preferentially fractionated into metamorphic fluids 39,[44][45][46] , and the δ 15 N values of fluid inclusions in quartz veins in low-grade metamorphic rocks yield values ranging from −3 to 5‰ 31 . Similarly, the W-mineralized quartz veins in the Yangjingou deposit feature low δ 15 N values (Fig. 8b), which may be attributed to fluid-rock interaction and the continuous liberation of N 2 in a metamorphic environment. The "light" N is preferentially fractioned into the fluid and is then trapped in the fluid inclusions.
The origin of the Yangjingou N 2 -rich fluid. The Wudaogou Group host rock was deposited at 323 ± 23 Ma 10 . From 269-228 Ma, the tectonic regime of the region was dominated by the subduction of the Paleo-Asian oceanic plate beneath the North China plate 47 . Regional greenschist-to epidote-amphibolite-facies metamorphism occurred between 269 ± 4 Ma and 249 ± 4 Ma 13 . The age of muscovite crystallization in the W-mineralized quartz veins is 230.79 ± 1.19 Ma 16 , indicating that W mineralization occurred in the earliest Late Triassic and during late-stage Wudaogou Group metamorphism. Therefore, the fluid system containing N 2 ± CO 2 -H 2 O-NaCl was intimately associated with complex multiphase tectonics and magmatic activity.
There are three mechanisms by which N 2 ± CO 2 may have entered the mineralizing system: 1) trapping of a volatile phase separated from its parent H 2 O-NaCl-CO 2 -N 2 -rich fluid, 2) mixing of a magmatic-hydrothermal CO 2 -H 2 O-rich fluid with N 2 released via the thermal decomposition of NH 4 + -bearing minerals, or 3) CO 2 -N 2 degassing of the mantle.
The stable isotope data in this study indicate that the mineralizing fluid was of a mixed magmatic and metamorphic origin. Magmatic melt inclusion studies have shown that CO 2 is released during magmatic differentiation 48,49 , producing a distinct δ 13 C signature 17 . Meanwhile, the Wudaogou Group contributed Ca 2+ to the system, facilitating the precipitation of Ca(WO) 4 . Within this metamorphic environment, N occurs as NH 4 + , which substitutes for K + in detrital phases such as K-feldspar, authigenic clays and illite. With increasing pressure and temperature, both feldspar and clays participate in low-grade metamorphic reactions, and illite gradually transforms into micas 50 . During this transformation, a large amount of carbon is released in the form of CO 2 due to decarbonization during progressive metamorphism. Moreover, the reported content of CO 2 in schist (<0.25%) is notably lower than that in clay and shale (2.5-5%) 36 . Thus, the contribution of organic matter to the mineralizing fluid was negligible. No CH 4 was identified in the W-mineralizing fluid, which may be due to the high oxygen fugacity 51 . Subsequently, Ca 2+ from the Ca-rich mica schist country rocks was incorporated into the mineralizing fluid. The P-T conditions of the Yangjingou deposit (315-410 °C at 110-350 MPa) are lower than greenschist-facies P-T conditions (350-550 °C at 150-1100 MPa) 52 , indicating that scheelite mineralization likely occurred in the later stages of Wudaogou Group metamorphism. Additionally, the δ 15 N values of the W-mineralized quartz veins show that water-rock interaction occurred under sub-greenschist-facies conditions. However, minor mantle N 2 participation cannot be completely ruled out.
Yangjingou scheelite mineralization is related to the intrusion of granodiorites at 249 ± 2.7 Ma 16 , synchronous with Wudaogou Group metamorphism. A/CNK (1.1-1.18) and Na 2 O/K 2 O (2.45-7.06) 16 major element values are characteristic of alkali series I-to S-type transient magmatism. The volatile component of the melts was influenced by pressure, which affected the volatile solubility in the magma during ascent. With decreasing pressure, gas was liberated from the magma 53 , resulting in an increasing volume of CO 2 . CO 2 dissolves relatively readily in intermediate-alkaline magmas, leading to more widespread and more rapid emplacement of mineralizing fluids 46 . Under these conditions, the diffusion of the W-rich mineralizing fluid was accelerated along interformational structures, causing the fluid to migrate to and rapidly expand into the host rocks of the Wudaogou Group. Only minor N 2 (<0.25 mol%) 54 was identified in fluid inclusions in the Xiaoxinancha Cu-Au porphyry deposit (123.35 ± 0.8 Ma) 42 , which is younger than the deposits in the Wudaogou Group and other stratigraphic units. The high concentration of N 2 observed in the scheelite deposits cannot simply be attributed to organic-rich sedimentary rocks. Findings from this study demonstrate that N 2 -rich fluids are closely related to the degree of metamorphism and magmatic activity.
Thus, we assume that the N 2 ± CO 2 fluid was a product of late-stage Wudaogou Group metamorphism, which caused H 2 O-NaCl-CO 2 -bearing magmatic-hydrothermal fluids containing HWO 4 − and MWO 4 − to migrate through fractures and pre-existing structural weaknesses via fluid circulation, thereby promoting interaction with metamorphic wall rocks (Fig. 9). An initial increase in P-T conditions at ~269 Ma resulted in early low grade metamorphic reactions that transformed feldspar and clays within illite into micas. During this process, NH 4 + substituted for K + , resulting in an increase in NH 4 + within the constituent minerals of the Ca-rich mica schists 50 . During late-stage metamorphism (~249 Ma), particularly under greenschist-facies conditions, NH 4 + was released via continuous metamorphism or thermal decomposition (i.e., complete breakdown of host minerals, such as mica) 27,50 . Simultaneously (~249 Ma), the circulation of a magmatic-hydrothermal W-mineralizing fluid under high oxygen fugacity conditions transferred heat (~400 °C) to the surrounding metamorphic rock and broke down micas in the host rock 37,51 . Within an oxidizing environment at this temperature range, large amounts of N 2 were released from the NH 4 + -bearing host rocks and stabilized into fluid 43 . The addition of sufficient amounts of N 2 to the mineralizing fluid increased activity coefficients and catalysed the saturation of Ca 2+ and WO 4 2-, greatly decreasing the scheelite solubility and accelerating scheelite precipitation. Moreover, saturated fluids, driven by advection, ascended into the surrounding country rock, resulting in alteration (albitization, chloritization, epidotization, and sericitization) and the incorporation of H 2 O into new minerals, leaving N 2 ± CO 2 trapped in quartz inclusions. Thus, the N 2 -rich fluid represents W-mineralizing fluid evolution and is indicative of W fluid saturation and scheelite precipitation. 3. The fluid inclusions were trapped at 315-410 °C and 80-350 MPa. The NaCl-H 2 O-CO 2 fluids were magmatic-hydrothermal in origin, and the N 2 was derived from the breakdown of NH 4 + -bearing minerals during alteration. 4. Hydrogen and oxygen isotopic data indicate that the mineralizing fluids were of a mixed magmatic and metamorphic origin, and the nitrogen isotopic data from the W-mineralized quartz veins suggest that water-rock interaction occurred under low greenschist-facies conditions. 5. The N 2 -rich fluid was intimately associated with the scheelite precipitation. During alteration under high oxygen fugacity conditions, N 2 was released from NH 4 + -bearing minerals. The N 2 -rich fluid represents the W-mineralizing fluid evolution, following W saturation and scheelite precipitation. Therefore, areas that have experienced regional metamorphism (orogenesis) and exhibit similar geologic histories to the Yanbian area in NE China are likely candidates for scheelite precipitation as observed in the Yangjingou deposit. In this study, we demonstrate that scheelite precipitation can be genetically related to almost pure N 2 fluid inclusions. The presence of the almost pure N 2 fluid inclusions in these types of localities therefore can be used as a proxy for scheelite precipitation and thus, the presence of W ± Au deposits.