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

Abstract

Nitrogen is the dominant constituent of the atmosphere and a key component in the biosphere. High-density CO 2 -N 2 inclusions have been detected in high-grade metamorphic rocks from the upper mantle and lower crust (i.e., granulites and eclogites) 1-3 . In silicates, under conditions of elevated temperatures, low water activity, and high oxygen fugacity, N 2 is favorably released from minerals containing ammonium (e.g., feldspar and mica) 4,5 . Scheelite containing nitrogen is most commonly recognized to be traced from sedimentary rocks 6 . In sedimentary-type deposits, tungsten (W) is liberated as a result of exhalative processes, forming stratiform scheelite 6 , in which mineralizing fluids containing CH 4 and N 2 are generally attributed to in situ organic matter 6 . Several examples of scheelite deposits related to metamorphic hydrothermal fluids are reported to contain small amounts of N 2 7,8 . These types of scheelite deposits are of interest to the geologist, because they are related to Au deposits 9 . Furthermore, Gibert 8 found that the addition of only 5% N 2 would decrease the solubility of scheelite in micaschists from 40 ppm to less than 3 ppm W, indicating that N 2 may drive scheelite participation.Nearly pure N 2 (>88 mol%) was identified in fluid inclusions in W-mineralized quartz veins, which is absent of hydrocarbons. Previous studies have shown that NaCl-H 2 O and CO 2 fluid inclusions are common in ore-forming fluids; however, the presence of N 2 -rich fluids in ore deposits is rarely reported. This study aims to identify the origin of the W-mineralizing fluid and the formation of the N 2 -rich fluid inclusions by combining petrography, Raman microspectroscopy, H, O and N isotope geochemistry, and microthermometry on fluid inclusions. Results from this study will improve our understanding of how N 2 modulate scheelite mineralization. Geology of the Yangjingou scheelite depositThe Yangjingou scheelite is the largest identified scheelite deposit in the Yanbian belt, NE China. It is located between the Cuihongshan and Sanjiazi scheelite deposits in the Heilongjiang-Jilin metallogenic belt, 8 km south of the Xiaoxinancha Au deposit, approximately 25 km east of the Russian border (Fig. 1c). In this deposit, the main economic mineral is scheelite, accompanied by minor Au, Cu, Mo, Fe, and P mineralization. The area is dominated by low-grade metamorphic rocks of the Wudaogou Group, which formed during a regional episode of low-grade epidote to low-grade amphibolite metamorphism. Regional stratigraphic correlations, metamorphic studies, and U-Pb geochronology from metamorphic detrital zircons indicate that sedimentation and regional metamorphism dated 323 ± 23 Ma 10 and 266-249 Ma, respectively 11 . The Wudaogou Group is subdivided into

The ore‐forming fluids characteristics of quartz‐vein type scheelite deposits in eastern Yanbian, NE China: Evidence from in situ LA‐ICP‐MS rare earth elements of Yangjingou and Sidaogou deposits

“…Previous work of fluid inclusions of quartz and scheelite from the mineralization stage show that the oreforming fluid of the Yangjingou deposit is medium temperature (190-300 C, major scheelite mineralization stages), low salinity (3-9.6 wt% NaCl equivalent) and low density (0.88-0.97 gÁcm À3 ) NaCl H 2 O CO 2 N 2 ± CH 4 system (Ren et al, 2010a;Wang et al, 2018). The ore-forming fluid of the Sidaogou deposit is the NaCl H 2 O CO 2 N 2 ± CH 4 system with medium-high temperature (200-423 C, major scheelite mineralization stages), high-medium salinity (2-50 wt% NaCl equivalent) and low density (0.57-1.1 gÁcm À3 ) (Chen et al, 2021).…”

“…Some studies show that during the fluid migration process of WO 4À forming complexes with Na + and H + (Heinrich, 1990;Wood and Samson, 2000), the changes of Ca 2+ activity and physicochemical conditions such as temperature, pressure, and pH would result in the precipitation of scheelite. The study of fluid inclusions in Yangjingou and Sidaogou deposits shows that fluid immiscibility plays an important role in mineralization (Ren et al, 2010a;Chen, 2017;Wang et al, 2018). The escape of volatile gases such as CO 2 destroys the physical-chemical balance of the fluid, resulting in scheelite precipitation.…”

“…In this study, the Eu anomalies of the two deposits show positive Eu anomalies characteristics different from the negative Eu anomalies of mineralized granites, indicating a continuous water-rock reaction process. The appearance of N 2 rich inclusions also indicates the occurrence of water-rock reaction (Wang et al, 2018). In addition, water-rock reaction would also provide enough Ca 2+ for scheelite mineralization, which is very important for scheelite precipitation.…”

“…Yangjingou deposit is the only large-scale scheelite deposit in the study area. Some scholars have carried out detailed research on fluid inclusions (Ren et al, 2010a;Wang et al, 2018), but there is a relative lack of research on reflecting the ore-forming process through ore mineral composition. Sidaogou deposit is the latest scheelite deposit found in the metallogenic belt.…”

Evaporitic brines and copper-sulphide ore genesis at Jbel Haïmer (Central Jebilet, Morocco)

“…Aqueous-carbonic fluids play a significant role in metal transport and deposit in a great variety of Variscan deposits (Wilkinson et al, 1990;Cathelineau et al, 2020). N2-rich fluids may be the main ore fluids in scheelite deposits (Wang et al, 2018) and CH4-rich fluids in C-rich formations close to Au-As deposits (Guedes et al, 2002;Boiron et al, 2003). P-T evolution: the pressure and temperature of trapping were estimated using isochores for most fluid types (Fig.…”