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

Abstract

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.

Figure 1

Geological map showing the main units in the Jilin and Heilongjiang metallogenic belt as well as ore deposit locations in the Yanbian area. (A) Location of the Central Asian Orogenic Belt and NE China; (B) Location of the Jilin and Heilongjiang metallogenic belt; (C) Location of the major tectonic units in the Jilin and Heilongjiang metallogenic belt (After Wu; https://www.researchgate.net/publication/229317465_Geochronology_of_the_Phanerozoic_granitoids_in_Northeastern_China).

Figure 2

Geological map of the Yangjingou scheelite deposit (After Ren^,; http://www.ysxb.ac.cn/ysxb/ch/reader/create_pdf.aspx?file_no=20101222&flag=1&journal_id=ysxb&year_id=2010).

Figure 3

(a) Cross section of the southern ore block in the Yangjingou scheelite deposit; (b) Cross section of the northern ore block in the Yangjingou scheelite deposit (After Shan; http://kckc.org.cn/ch/reader/create_pdf.aspx?file_no=2010Z106&year_id=2010&quarter_id=Z1&falg=1).

Figure 4

(a and b) Scheelite paragenetic quartz veins (scheelite showing blue-fluorescence under the ultraviolet irradiation); (c) arsenopyrite replaced early scheelite minerals and existed in early disseminated W-mineralizationquartz veins; (d) sphalerite and copper pyrites replacing pyrrhotite; (e) pyrrhotites replaced early scheelite; (f) subhedral scheelite; (g and h) actinolitization, epidotization; (i) muscovite quartz schist.

Figure 5

Photomicrographs of representative fluid inclusion types at room temperature in the quartz–scheelite stage. Type I: pure N₂ fluid inclusions (a–c,g,u); Type II: CO₂–N₂ inclusions (o,m,n), occasionally solid CO₂ phase below −100 °C (n,r) and three phases under the −160 °C (o); Type IIIa: inclusions containing 10%–30% gas (e,f); Type IIIb, inclusions in which the proportion of the gaseous phase ranges from 70% to 95% (d,j,i), and the liquid phase of N₂ partially exists at temperatures below −150 °C (p). One phase (k) at room temperature and two phases below −160 °C (l); Type IV: NaCl–H₂O–CO₂ fluid inclusions (e). Representative Raman spectra of fluid inclusions related to the mineralizing fluid of the Yangjingou scheelite. Panel A shows that the vapour bubbles are mainly composed of N₂; panel B shows that the bubbles contain mainly N₂ and some CO₂; and panel C shows that the fluid inclusions are mainly composed of N₂ and H₂O.

Figure 6

(a) The homogenization temperature of the fluid inclusions (Type I, II, III and IV). (b) Salinity of type IIIa and IV fluid inclusions. (c) The P–T conditions of the fluid inclusions in the scheelite quartz veins in the Yangjingou scheelite deposit (molar volums between 30 and 500 cm³/mole) (After Kerkhof amd Thiéry and Winter). Numbers denote molar volumes in cm³/mole; numbers in brackets are the homogenization temperature of N₂.

Figure 7

vX diagrams calculated for the CO₂-N₂ system of the Yangjingou deposit showing low molar volumes (v < 100 cm³/mole). The thick red line G (SL) defines the difference between S-and H-typeII inclusions. Inclusions plotting below the L = G homogenize to gas, above the critical curve to liquid. Thin read solid line = homogenization (T_hCO2). Dashed dotted red line = partial homogenization (T_hN2). Thin black line = initial melting of CO₂ (T_mCO2) (After Kerkhof and Thiéry; Thiéry et al.).

Figure 8

(a) Plot of δD values versus δ¹⁸O values for the ore-forming fluids related to the Yangjingou scheelite deposit (modified after Taylor). The data are from the following sources: Naozhi Au deposit (Huang et al.), Xiaoxinancha Au deposit (Wang), Baishilazi W deposit (Zhao) in Yanbian area and Nyakabingo W deposit, central Africa (Dewaele et al.). (b) Nitrogen isotopic compositions of the fluid inclusions in the Yangjingou scheelite deposit and (After Bebout et al.).

Figure 9

The model of the mineralization of the Yangjingou scheelite deposit under the influence of both magmatism and metamorphism. Magmatic–hydrothermal CO₂-H₂O-rich fluid interacted with the host rock and mixed with N₂–rich fluids during migration, which caused scheelite participation.