All samples used in this study were systematically collected from tunnel and trench of Annage deposit. Doubly

polished thick sections (0.06 - 0.08 mm thick), representing different locations, ore types and stages, were prepared for fluid inclusion study. These sections were studied first for petrography to establish the relative timing of minerals, and then used for fluid inclusion micro-thermometry. Twelve of them were finally selected for heating and freezing measurements. These samples, with well-developed inclusions in quartz, provide information on the conditions of mineralization during the mineralization stages A and B. Fluid inclusions were carefully observed to identify their genetic and compositional types, vapor-liquid ratios and spatial clustering. Sample features and locations are shown in Table 1.

Micro-thermometric measurements were performed using the Linkam THMS 600 heating-freezing stage in the laboratory of fluid inclusions of Central South University. The stage, which was calibrated by using synthetic fluid inclusions, has a maximum temperature limit of 600˚C and a minimum temperature limit of −196˚C. The estimated precisions of the measurements are ±0.1˚C and ±1˚C for freezing and heating, respectively. The heating/freezing rate generally ranged from 0.2˚C/min to 5˚C/min but decreased to <0.2˚C/min near phase transformation.

The measurements include freezing temperatures (Tf), incipient melting temperatures of ice (Ti-ice), ice melting temperatures (Tm-ice), CO₂ melting temperatures (Tm−CO₂), clathrate melting temperatures (Tm-cla), homogenization temperatures of CO₂ (Th-CO₂), and total homogenization temperatures (Th). Salinities of NaCl-H₂O, and CO₂-bearing FIs were estimated by the final ice melting temperatures and clathrate melting temperatures of CO₂ clathrate, respectively, by using the equation of Brown and Lamb [13] in the computer program of FLINCOR [14] .

Four samples (An-2, An-6, An-8, An-14) from quartz-polymetallic-sulfide veins in stage B were used to identify the oxygen and hydrogen isotope compositions of quartz. Hydrogen and oxygen isotope analyses were measured on a MAT-253 mass spectrometer at IGGCAS. Quartz samples were hand-picked and/or separated by using a magnetic separator. Oxygen was liberated from quartz by reaction with BrF₅ [15] and converted to CO₂ on a platinum-coated carbon rod. The hydrogen isotopic compositions of the fluid inclusions were analyzed on the samples. Samples were first degassed of labile volatiles by being heated under a vacuum at 150˚C for 3 h. Water was converted to hydrogen by passage over heated zinc powder at 410˚C [16] . Stable isotope data for hydrogen and oxygen are expressed in the standard δ notation as per million (‰) relative to standard mean ocean water (SMOW). The precision of measurements for stable isotopes was ±0.2‰ for δ¹⁸O and ±2‰ for δD.

The hydrogen-oxygen isotope analysis results of four quartz samples from stage B are present in Table 3. The δ¹⁸O values of quartz and δD values of fluids are from 10.6 × 10⁻³ to 12.9 × 10⁻³ and from −83.8 × 10⁻³ to −72.3 × 10⁻³, respectively (Figure 6).

Using the equation 1000lnα = 3.38 × (10⁶/T²) − 3.40 of Clayton [19] and the corresponding fluid inclusions homogenization temperatures T of the samples, we can calculate oxygen isotopic compositions of the ore- forming fluids from quartz and the δD_V-SMOW values of the extracted waters for quartz samples. Overall average homogenization temperature of FIs in Annage deposit was measured as 330˚C, taking T = 603 K. The calculated δ¹⁸OH₂O values of fluids are from 4.2 × 10⁻³ to 6.5 × 10⁻³.

According to microthermometric data, different types of inclusions with variable vapor/total ratios coexist in the mineralized quartz veins of Annage deposit, reflecting the heterogeneous state of fluid during inclusions capture [19] [20] .

Histograms of homogenization temperatures and salinities (Figure 5) and diagram of homogenization temperatures versus salinities (Figure 7) indicate a wide range of homogenization temperatures from 140˚C to 395˚C, and medium-low salinities from 0.53% to 22.01%, respectively. Type I inclusions, along with minor type II and type III inclusions are present in stage A. Homogenization temperatures for type I inclusions range from 180˚C to 300˚C, whereas that of type II inclusions are from 268˚C to 366˚C, with calculated salinities from 0.53% to 21.44%. In addition to development of type I inclusions, type II and III inclusions increased significantly in stage B. Homogenization temperatures of type I inclusions range from 160˚C to 250˚C, and that of type II inclusions are of 260˚C - 360˚C, with calculated salinities from 1.32% to 22.01%. Based on microthermometric results, no obvious decrease of salinities is observed along with a decrease in temperature from stage A to stage B, showing a natural cooling process of the system, indicating that the mineralization occurred during the internal fluid evolution, however, the large range of salinity indicates a wide source of fluid, maybe mixed with low-salinity exotic fluid.

On the δD-δ¹⁸OH₂O diagram (Figure 6), samples of main mineralization stage in Guoluolongwa deposit mostly plot in magmatic/metamorphic water area, reflecting that ore-forming fluid in the area mainly derived from magmatic/metamorphic water [3] ; whereas most samples of Annage plot between left side of magmatic water box and metamorphic water line, and one is located in magmatic water area, indicating a primary magmatic origin for the mineralizing process and mixed with other fluids (maybe metamorphic hydrothermal). High density of SO₄²⁻, representing all phases of sulfur, including S²⁻, HS⁻, SO₄²⁻ and etc [21] , has effectively indicates the existence of magmatic water in ore-forming fluids. Abundance of SO₄²⁻ in ore-forming fluids of main mineralization stage speculated the presence of magmatic hydrothermal, corresponding to the fact that one projection point is situated at the magmatic water area on the hydrogen-oxygen isotope diagram.

Based on diagram of homogenization temperatures versus salinities (Figure 7), homogenization temperatures of most type I inclusions are from 140˚C to 300˚C, which may reflect the ore-forming temperature, whereas that of type II inclusions and some type I inclusions are slightly higher, showing characteristics of heterogeneous capture, and are of no significance. Salinities of fluid inclusions are mostly medium-low values, clustering around 7% - 22%. However, occurrence of low salinity inclusions, namely 0.53%, reflects the addition of low salinity fluid during mineralization process. Besides, a continuous decrease in temperature may indicate a natural cooling process of the fluid, inferring that the mixed water maybe high-temperature, low-salinity water (probably metamorphic hydrothermal) derived from the deep, or underground water.

To sum up, ore-forming fluids in Annage gold deposit are of CO₂-rich, H₂O-NaCl-CO₂ type with medium- high temperatures (140˚C−>300˚C) and medium-low salinities (0.53% - 22.01%) and experienced immiscibility during the mineralization process [22] . The original H₂O-NaCl-CO₂ fluid with medium-high temperature and medium salinity separated into NaCl-H₂O-rich (with few CO₂) fluid and CO₂-rich, medium-low salinity fluid, and mixed with high-temperature metamorphic hydrothermal or groundwater in the late.

Inclusion trapping pressure could be obtained from the homogeneous temperature of pure CO₂ and pure water inclusions at the P-T diagram of H₂O and CO₂ combined systems if pure CO₂ inclusions and pure water inclusions were simultaneously trapped. In this study, the two end compositions trapped by the inclusions in stage B were used to estimate pressure by the isometric intersection method. The trapping pressures of inclusions obtained from the isometric intersection in the two ends are shown in Figure 8, with the pressure ranging from 79

MPa to 130 MPa, corresponding to trapped temperatures from 185˚C to 350˚C. Sulfides are formed later than quartz, with slightly lower temperatures than that of quartz. Therefore, the range of 185˚C - 350˚C represents the upper limit of mineralization temperature, indicating that ore-forming temperature is of medium-high type.

Based on the estimated pressure, we calculated the mineralization depth. Considering that the study area is in compressive orogenic environment, metallogenic depth can be estimated via lithostatic pressure using the formula: H = P/(ρ × g) (ρ represents average density of rocks, namely 2.70 g/cm³). The maximum pressure of 130 MPa corresponds to an upper limit depth of 4.8 km.

Ore-forming fluids in the deposit area are characterized by wide range of homogenization temperatures and salinities as well as high density of CO₂ and CH₄. Wall rock alterations are dominated by silicification, sericitization and carbonation, indicating that ore-bearing hydrothermal is rich in Si, CO₂ and alkali. CO₂ in the fluids may be mainly from mantle, medium-high grade metamorphic fluid from lower crust and magmatic fluid, whereas CH₄ may be derived from deep-source fluids and organic-rich sedimentary metamorphism. On basis of micro−thermometry, ore-forming fluids are mainly deep-source magmatic hydrothermal fluid at the early stage [23] , and then mix with low-salinity metamorphic hydrothermal or underground water in the late. Evolution of fluids experienced multistages.

At the early mineralization stage, ore-forming fluids are CO₂-rich magmatic hydrothermal which may be derived from high salinity intermediate-acid magmatic hydrothermal, and form high-salinity high-temperature fluid inclusions.

At the main mineralization stage, natural cooling occurs in the ore-forming fluids. Upward intrusion of fluids to the shallow crust along EW fracture zone brings about the mineralization pressure transition from lithostatic to hydrostatic, causing repeated fluid immiscibility. Fluid immiscibility has been reported as one of the most important mechanisms for mineralization in many hydrothermal deposits [6] [24] , and the H₂O-NaCl-CO₂ fluid immiscibility is closely related to gold mineralization [25] . H₂O-NaCl-CO₂ fluid immiscibility can cause phase separation, resulting in the escape of volatiles, such as CO₂, CH₄ and N₂. The original NaCl-H₂O-CO₂ fluids of Annage deposit separated into CO₂-rich fluids and H₂O-rich fluids, resulting in capture of low-salinity CO₂-bearing inclusions and slightly higher-salinity aqueous inclusions. During phase separation, escape of CO₂ and CH₄, concentration of ore-forming fluid and decomposition of carbonate ion and bicarbonate ion lead to huge changes in fluid composition and physical and chemical conditions, leading to decrease of Au solubility in ore-forming fluids and the rapid precipitation of gold in the favorable ore-hosting structures.

At the late mineralization stage, a sudden reduction of CO₂-bearing inclusions occurs due to escape of CO₂. Ore-forming fluid system transits from a relatively closed state to an open state, which is beneficial to addition of other fluids. Based on the above discussion, deep-source low-salinity high-temperature metamorphic hydrothermal or underground water are speculated to mix with magmatic fluid, resulting in decrease of fluid salinities and capture of low-salinity inclusions..

Annage gold deposit is located at east section of eastern Kunlun belt. The eastern Kunlun orogenic belt experienced multicycle evolution, and the late Variscan-Indosinian tectonic cycle is the most closely related to gold mineralization [26] . Intensive tectonic and magmatic activities occurred in the late Variscan-Indosinian period, formed a series of deep faults, shear zone and fracture-fissure structures, as well as large-scale magmatite intrusion, and provided metallogenic dynamics and mineral sources. The metallogenic mechanism is as follows.

In Caledonian period, intracontinental subduction took place in eastern Kunlun belt, led to the ore-forming fluid activation and initial enrichment of ore-forming materials. Few intermediate-acidic magmatite of this period was developed in Annage deposit.

In Variscan-Indosinian period, strong subduction and collision, intensive tectonic deformation and metamorphism occurred, forming the EW shear zone. The dynamic metamorphism is obviously observed along the fracture zone. Besides, subduction and collision provides thermal and power for migration of CO₂-rich magmatic hydrothermal along the EW shear zone. During the migration, water-rock interaction between wall rock and hydrothermal fluid contributes to further enrichment of minerals, forming ore-bearing hydrothermal solution with higher temperature.

In Indosinian-Yanshanian period, secondary tensional faults and shear fractures are formed on both sides of the nearly EW shear zone, and ultramafic rocks are developed near the shear zone. Fluids rich in ore minerals moved upward along the shallow tensional fault system. Addition of shallow underground water destroyed balance of ore-bearing hydrothermal, resulted in fluid immiscibility and escape of large amounts of CO₂, CH₄, thus forming the residual fluids containing large amounts of Au, Ag and other minerals. Decrease of pressure and fluid temperature, addition of extraneous fluid reduced the solubility of ore-forming minerals in the fluid, bringing about precipitation of minerals in the EW shear zone and the SN tensional fault. Furthermore, metasomatism between metallogenic hydrothermal and fracture zone or wall rocks formed quartz-vein and altered rocks, and then finally formed gold ore bodies.

To sum up, Annage gold deposit is a medium-high temperature magmatic hydrothermal quartz vein type deposit formed in intracontinental orogenic environment.