The Dachang gold deposit in Qumalai County, Qinghai Province is located in Qumalai County, Yushu Prefecture, Qinghai Province. The geographical coordinates are 96°14′45″～96°18′00″ east longitude and 35°15 north latitude. ′45″～35°19′00″. Since the gold mine was discovered by the former Fourth Geological Team of Qinghai Province in 1997, major breakthroughs have been made in mineral survey and exploration, confirming that its reserves have reached a large scale, with gold resources of approximately 140 tons.

1 Mineralization geological background

The strata in the area are mainly Triassic Bayan Harshan Group and a small amount of Lower Permian Buqingshan Group. The Bayan Harshan Group is a set of shallow-marine to semi-deep-marine facies mud-sandy flysch. It is composed of sandstone-mudstone (metamorphosed into slate)-sandstone from bottom to top, reflecting the deposition from transgression to regression. Spin. The Lower Permian Buqingshan Group spreads NW and is obviously controlled by faults. Its lithology is mainly medium-basic volcanic rock, clastic rock and carbonate rock.

The regional structure is dominated by the Indosinian deformation, and fault folds are developed. The folds are generally large-scale Zhaling Lake complex anticlines. The core is composed of Permian horst-like fault blocks, and on its two wings There are complex plastic wrinkle structures developed in the Triassic strata. The northern part of the mining area is the Gander-Maduo Deep Fault. There is a series of NWW-trending secondary faults parallel to it in the Dachang area, and there is also a set of NE-trending translational faults. NWW-trending faults are the main ore-controlling structures in the area; NE-trending faults mostly cross-cut strata and NWW-trending structures.

The magmatic activity is relatively weak. The intrusive rocks are mainly intrusive during the Indosinian period, followed by the Yanshanian period. The lithology mainly includes quartz diorite, biotite quartz monzodiorite rock body and porphyritic biotite monzogranite, which are produced in the form of rock beads. There is no rock mass intrusion in the mining area. Extrusive rocks appear as marine volcanic deposits in the Early Permian strata. The volcanic activities are mostly in the form of intermittent fissure eruptions. The lithology is andesite, basalt and pyroclastic rocks.

2 Geological characteristics of the deposit

The Dachang gold deposit is located in the North Bayan Har orogenic belt of the Songpan-Ganzi Indosinian fold system. The exposed strata in the mining area are mainly sandstone and slate interbedded in the Triassic Bayan Harshan Group Middle Subgroup (TBy2), which is the ore-bearing stratum in the mining area. Sandstone and slate form rhythmic interbeds, showing typical turbidity sedimentation characteristics, among which the carbonaceous slate contains higher gold content. The Permian Buqingshan Group Malzheng Formation (P1m) strata are distributed between the Gander-Maduo fault zone in the northeast corner of the mining area. The Gander-Madou deep fault is the largest fault in the mining area. Affected by it, secondary feather-like faults and folds are relatively developed in the strata on both sides. This fault trends NW, tilts NE, and has a dip angle of about 60°. It is a brittle-ductile reverse fault with a fracture bandwidth of 20 to 200m. Silicification and pyrite develop in the fracture zone, providing a favorable channel for ore-bearing hydrothermal fluids. Affected by the Gande-Maduo Deep Fault, the sandstone and slate interbedded secondary faults and interlayer fracture zones in the Middle Triassic Bayan Harshan Group on the south side of the fault, that is, in the footwall of the fault zone, are very complex. Developed, feather-like parallel distribution, trending 110° to 130°, tilting SW, with an inclination angle of 40° to 60°, which is an ore-bearing structure in the area. The bandwidth of these ore-bearing fragmentation alterations is generally 1 to 20m, and most of the lengths are more than 1km. The fault planes show gentle waves in strike and tendency, and various structural rocks (such as porphyry, mylonite, etc.) are often developed. There are network veins, veinlets and lenticular quartz veins. No rock mass is exposed in the mining area, but the mineralization in the area is closely related to magma intrusion activity. Geophysical data shows that there are intermediate-acid hidden rock masses deep in the Dachang gold mining area.

2.1 Ore body characteristics

Up to now, 35 gold ore bodies have been delineated in the Dachang gold deposit, mainly distributed in the north of Dachang River, 3km wide and 5km long. within the range (Figure 1). The ore body occurs on the southwestern side (footwall) of the Gander-Madou main fault. The gold ore body is strictly controlled by the structural fracture and alteration zone, and its scale is related to the fracture zone. If the fracture zone is large and the alteration is strong, the gold ore body will be large in scale and high grade; otherwise, the gold ore body will be small in scale and low grade. The ore-controlling fractured alteration zone runs parallel to the main fault zone and is a secondary fault derived from the main fault. The distribution of gold ore bodies seems to be equidistant from north to south, with intervals of 400 to 600m. The ore bodies are mostly strip-like, layer-like, pod-like and lens-like in shape, with wavy bending, expansion and shrinkage, branch compounding and bifurcation along the strike. The change pattern along the trend is not clear yet.

Figure 1 Geological sketch of Dachang gold deposit

(Revised according to Qinghai Provincial Geological Survey Institute, 2002)

Q—Quaternary; TBy2 —The gray-green sandstone slate of the middle subgroup of the Triassic Bayan Harshan Group. 1—Gold ore body

The ore body is 80-3240m long, and the ore body with a length >1000m accounts for more than half of the total ore body. The ore body is lens-shaped along the strike, with wavy bends, expansion and shrinkage along the strike. and branching phenomena. The surface thickness is generally 1.4~4.57m, with a maximum of 15.64m. The gold grade ranges from 0.53×10-6 to 24.9×10-6, with an average grade of 7.5×10-6.

2.2 Ore characteristics

According to the ore’s mineral combination, production conditions and mineralization characteristics, the ore types in this area are divided into cataclastic sulfide alteration rocks. type and gold-bearing pyrite quartz vein type.

Clastic sulfide alteration rock type is the main ore type in this area and is widely distributed. The ores are subjected to varying degrees of silicification, sulfideation, sericitization, mudification and other alterations. The metallic minerals are mainly pyrite, arsenopyrite and natural gold. The pyrite is in the form of irregular, other-shaped granules with a particle size of 0.1~ 1mm, content 3% ~ 5%, arsenopyrite is needle-shaped, size 1 ~ 3mm, content 5% ~ 10%. Non-metallic minerals include feldspar, quartz, chlorite, sericite and slate fragments.

The gold-bearing pyrite quartz vein type is a secondary ore type. The ore is produced in the form of fine veins, network veins or lumps. The quartz content in the ore is 90% to 95%, and the pyrite content is generally 5% to 10%. Most of them are in the shape of other-shaped grains, some are cubic, and the particle size is 0.5 to 2mm. In the oxidation zone, pyrite has been oxidized to limonite, and gold is occasionally found in this type of ore.

2.3 Gold occurrence status

The metal minerals in the ore mainly include natural gold, pyrite, arsenopyrite, antimony, chalcopyrite, galena and sphalerite wait. The content of arsenopyrite is 5% to 15%, pyrite is 2% to 20%, stibnite is 1% to 4% (limited to the surface), and trace amounts of chalcopyrite, galena and sphalerite are present. Oxidized minerals include limonite, malachite and antimonite. Non-metallic minerals include quartz, feldspar, calcite, sandy slate debris, clay and sericite.

Multi-element chemical analysis results: Au is 0.1×10-6~110×10-6, average 6.3×10-6, Sb is 0.01%~0.68%, average 0.06%, As is 0.1% ~1.44%, with an average of 0.52%, indicating that the mineralizing elements in this mining area are characterized by high contents of Au, S, As, and Sb and low contents of Ag, Cu, Pb, and Zn.

The occurrence state of gold in ores is complicated. Among gold-bearing pyrite quartz vein-type gold ores, natural gold (particle size 0.74-2mm) accounts for about 21%, and those with particle size <0.74mm and invisible gold account for about 79%. A large amount of natural gold was found in the light film identification of the stibnite quartz veins. A small amount of natural gold has also been found in the artificial heavy sand identification of fragmented sulfide altered rock type gold ores, with particle sizes ranging from 0.01 to 0.2 mm, in dendritic, flake, granular and film forms. Analysis of gold content of single minerals shows that pyrite contains gold of 40×10-6~80×10-6, arsenopyrite contains gold of 177×10-6, and stibnite contains gold of 2×10-6~50×10- 6. The gold grade of the original sample of artificial heavy sand is 11×10-6. The natural gold content is 2.65×10-6, accounting for 21% of the total gold content, indicating that a large amount of gold in gold ores exists in the form of microscopic and ultramicroscopic (particle size <0.02mm) in the ore, mineral fissures and crystal lattice Among them, gold is closely related to pyrite and arsenopyrite.

According to this analysis, in addition to natural gold monomers (visible gold), gold also exists in microscopic inclusions (such as inclusion gold, interstitial gold, and fissure gold, etc.). Since the content of gold is closely related to pyrite, arsenopyrite, and stibnite, the possibility of the existence of lattice gold cannot be ruled out. Since there is a lot of mud and clay in the ore, it is inferred that a small amount of colloidal particles adsorb gold.

2.4 Surrounding rock alteration

Surrounding rock alteration develops in the mining area, and its scale and intensity are determined by the scale and nature of the structure and the degree of rock fragmentation. The main alterations include silicification, sericitization and sulfide formation, and locally kaolinization and carbonation. Among them, pyrite, sericitization, and silicification are most closely related to gold and antimony mineralization. Alteration is spatially manifested as silicification, sulfide-sericitization, carbonation, and kaolinization in order from the center of the ore body outward.

3 Origin of mineral deposits

3.1 Geochemical characteristics of mineral deposits

The multi-element analysis of ore chemistry is shown in Table 1.

As can be seen from Table 1, the ore-forming elements in the mining area are characterized by high contents of Au, S, As, and Sb and low contents of Ag, Cu, Pb, and Zn, w (Au)/w (Ag) ≈ 1 , contains a small amount of organic carbon and may be involved in mineralization.

Through the scanning electron microscope composition analysis of pyrite and arsenopyrite (Table 2), the Au content of vein pyrite and disseminated pyrite is 4.20% and 4.30% respectively, and the Pt content is 1.98 respectively. %, 2.24%, arsenopyrite contains Au and Pt, which are 2.43% and 1.35% respectively. Au and Pt are closely related and positively correlated. The Au and Pt content in pyrite is higher than the Au and Pt content in arsenopyrite, w (Au)/w(Pt)≈2/1, the presence of Pt indicates that deep source (mantle source) minerals may be involved.

Table 1 Ore chemical multi-element analysis results w (B)/%

Note: The data was tested by the Qinghai Provincial Rock and Mineral Testing and Application Research Institute, 2004. According to Zhao Junwei, 2007.

Table 2 Scanning electron microscope composition analysis of pyrite and arsenopyrite w (B)/%

Note: Data were tested by Qinghai Provincial Rock and Mineral Testing and Application Research Institute, 2004. According to Zhao Junwei, 2007.

3.2 Characteristics of fluid inclusions

Through microscopic observation of 5 thermometric slices (Zhao Caisheng et al., 2005), it was found that both quartz and calcite contain abundant fluid inclusions. And they are all primary inclusions related to mineralization. These inclusions appear in groups, have similar gas-liquid ratios and uniform temperatures, and have relatively consistent internal compositions, with the main components being CO2 and H2O.

3.2.1 Types and characteristics of fluid inclusions

Based on the physical phase and chemical composition of inclusions at room temperature, primary inclusions in samples can be divided into 3 types: Ⅰ Type (gas-liquid two-phase inclusions), Type II (three-phase inclusions containing CO2) and Type III (CO2-rich inclusions) (Zhao Caisheng et al., 2005). Type I is a gas-liquid two-phase inclusion, that is, NaCl-H2O type, accounting for approximately 77% of the total inclusions. It is mainly composed of two phases of gas and liquid, which is composed of (H2O + NaCl) (liquid phase) and H2O (gas phase), with the liquid phase being the main one. The gas phase is generally 5% to 30%, and most are 10% to 15%. The long axis of inclusions is generally 6 to 40 μm, and most are between 10 and 15 μm. The shapes of inclusions are elliptical, rectangular and irregular, and a few are regular negative crystalline and incomplete negative crystalline. Although laser Raman spectroscopy analysis results show that the gas phase of this type of inclusion contains CO2, the volume of the gas phase is so small that no CO2 phase is observed at room temperature and low temperature. The extremely small amount of CO2 is not enough to change the NaClH2O in the inclusion. basic characteristics. This type of inclusions is the most widely developed and is the main inclusion type in the Dachang gold deposit.

Type II is a three-phase inclusion containing CO2, that is, CO2-H2O-NaCl type, accounting for about 13% of the total inclusions. It has three phases at room temperature, consisting of CO2 (gas phase) + CO2 (liquid phase) + (H2O + NaCl) (liquid phase). Gas phase CO2 often sloshes. Although a few inclusions appear as two phases of CO2 and salt water solution at room temperature, they appear in the CO2 gas phase when cooled to about -10°C. The φ (CO2) (volume fraction) of the CO2 phase is 10% to 50%, and most are 30% to 40%. The shapes of inclusions include ellipses, irregular shapes, long strips, etc. The long axis of inclusions is generally 8 to 40 μm, and most are between 12 and 15 μm. This type of inclusions is more developed but unevenly distributed.

Type III is CO2-rich inclusions, accounting for about 10% of the total inclusions. Almost all are filled with CO2, and the inclusion shapes are negative crystal, elliptical, and irregular. The long axis is generally 7 to 15 μm, and most are <10 μm. The gas-to-liquid volume ratio of CO2-rich inclusions is generally 75% to 95%, and its distribution characteristics are very similar to those of CO2-containing three-phase inclusions, and often occur with them. CO2-rich inclusions sometimes appear in two phases at room temperature, but after cooling, they appear in three phases. The overall color of the inclusion is darker and the center is transparent. There are also a small amount of pure CO2 inclusions developed, most of which are gas-liquid two-phase at room temperature, and a small amount are a single gas phase or liquid phase, all of which are uniform to the gas phase.

3.2.2 Microscopic temperature measurement results

The temperature of 55 inclusions in 5 samples of gas-liquid two-phase fluid inclusions (Type I) was measured. The tm of phase inclusions is -6.2~-1.2℃, with an average of -3.6℃, ??concentrated at -6~-2℃. The th of gas-liquid two-phase inclusions ranges from 152.2 to 314.7°C, with an average of 211°C, concentrated at 170 to 270°C (Figure 2).

Figure 2 Histogram of temperature measurement data of Dachang gold deposit

(According to Zhao Caisheng et al., 2005)

Using the salinity calculation formula of Hall et al. (1988) : , the salinity value of the corresponding gas-liquid two-phase inclusion can be obtained. The results show that the salinity of gas-liquid two-phase inclusions in the Dachang gold mine area is 2.1% to 9.5%, with an average value of 5.83, and the main variation range is 5 to 8 (Figure 3).

Figure 3 Uniform temperature-salinity diagram of fluid inclusions

(According to Zhao Caisheng et al., 2005)

Based on the obtained uniform temperature of this type of inclusions and salinity, apply the empirical formula of Liu Bin et al. (1987): (a, b, c are all dimensionless parameters) to calculate the fluid density. The calculation results show that the fluid density in the Dachang gold mine area ranges from 0.78 to 0.95 g/cm3, with an average of 0.89 g/cm3.

According to the uniform temperature of the fluid inclusion and the fluid salinity, use the empirical formula of Shao Jielian (1988) to calculate the fluid pressure: p=p0th/t0 (in the formula, p0=219+2620 w, t0=374+920 w), Find the fluid pressure of the corresponding inclusion. The results show that the fluid pressure of the gas-liquid two-phase inclusion in the Dachang mining area is 41×106~87×106Pa, with an average of 57×106Pa, mainly concentrated between 45×106~75×106Pa.

The temperature of three-phase inclusions containing CO2 (type II) was measured on 9 inclusions in 2 samples. The tm (CO2) was -57.2~-56.9℃. The th (CO2) of this type of inclusion is 23.6~29.6℃, with an average of 26.3℃; th(cla) is 5.0~8.1℃, with an average of 6.0℃; th is 218.2~304.5℃, with an average of 254.3℃. Some inclusions of this type exploded before they were evenly homogenized, and failed to achieve a completely uniform temperature.

Collins (1979) believed that there is a certain functional relationship between the melting temperature of CO2 clathrate compounds and the salinity of the aqueous solution. By measuring the melting temperature of the clathrate compounds, the salinity of the inclusion aqueous solution can be obtained indirectly. According to the salinity calculation formula of Bozzo et al. (1973): , the salinity of the aqueous solution of this type of inclusion is calculated to be 3.8% to 9.0%, concentrated at 8.3% to 9.0% (Figure 3).

According to the completely uniform temperature of the three-phase inclusion containing CO2 and the salinity of the aqueous solution, the fluid density can be calculated by applying the empirical formula of Liu Bin et al. (1987). The fluid density in the Dachang gold mine area is mainly distributed at 0.74 ~0.89 g/cm3, with an average of 0.85 g/cm3. Using the P-t phase diagram of the H2O-CO2-NaCl system of Brown et al. (1989), the fluid pressure can be found to be 57×106~82×106Pa, with an average of 72×106Pa.

CO2-rich inclusions (type III). Zhao Caisheng et al. (2005) measured the temperature of 8 inclusions in 2 samples and found that the initial melting temperature tm (CO2) of this type of inclusion was -57.3 ~ -56.8°C, slightly lower than the triple point of CO2 - 56.6°C, indicating that the CO2 in the inclusion is relatively pure; the partial homogenization temperature th (CO2) is 19.2 to 24.6°C, with an average of 21.7°C; the disappearance temperature of the clathrate compound, th (cla), ranges from 5.5 to 9.9°C, with an average of 6.01°C. Such inclusions are uniform in the gas phase, with th ranging from 273.0 to 323.5°C, with an average of 295.4°C. Pure CO2 inclusions cannot obtain completely uniform temperatures (Figure 2).

According to the formula of Bozzo et al. (1973), the salinity of CO2-rich inclusions is calculated to be 0.2% to 8.3%, with an average of 4.5%.

The empirical formula of Liu Bin et al. (1987) was used to calculate the density of the rich (or) pure CO2 inclusion fluid in the Dachang gold mining area, ranging from 0.69 to 0.78 g/cm3, with an average of 0.73 g/cm3. This value was put into Roedder et al. ( 1980) of the H2O-CO2 system P-x phase diagram, the fluid pressure was 40×106~83×106Pa, with an average of 65×106Pa.

3.2.3 Fluid inclusion composition

The gas phase composition of type I inclusions is mainly H2O (Zhao Caisheng et al., 2005), and its relative content x (H2O) is generally 92.12% ~ 97.57%; followed by CO2, x (CO2) is generally 0.61% to 6.87%; it also contains a small amount of CH4, C2H2, H2S, CO, N2 and H2. The liquid phase component is mainly H2O, with x (H2O) ranging from 95.31% to 99.36%; x (CO2) generally ranging from 0.1% to 1.29%; it also contains a small amount of CH4 and CO, and H2S, N2, and C2H2 were detected in individual inclusions. C2H6, C3H8 and C6H6; the anionic components are mainly Cl-.

The gas phase of type II and III inclusions is dominated by CO2, with x(CO2) ranging from 39.47% to 84.3%; followed by H2O, with x(H2O) ranging from 8.29% to 29.04%; the contents of N2 and CO are relatively Higher, x (B) are 2.7% ~ 11.1% and 2.08% ~ 9.94% respectively; individual inclusions contain small amounts of CH4, C2H4, C2H6, C3H8 and C6H6, and their x (B) does not exceed 3%. The liquid phase components of type II inclusions are mainly H2O, followed by CO2 and N2, with lower contents of CO, CH4, C2H2 and C2H6.

Generally speaking, the ore-forming fluid is rich in CO2 and is a NaCl-H2O-CO2 system type. In addition, it also contains a small amount of CO, H2S, CH4, N2, H2 and trace amounts of organic components such as C2H2, C2H4, C2H6, C3H8 and C6H6, indicating that it is a saline solution containing organic matter. The existence of organic components is consistent with the relatively high carbon content of the ore-bearing surrounding rocks of the Dachang gold deposit. The presence of organic matter in hydrothermal fluids enhances the ability of hydrothermal fluids to activate and migrate metal mineralization elements in rocks (Lu Huanzhang et al., 2000), which plays an important role in the mineralization of the Dachang gold deposit.

3.3 Ore-controlling factors

3.3.1 Stratigraphic ore-controlling factors

The gold deposits and gold spots discovered in the Dachang area are all produced in the Triassic Among the sand slates of the Bayan Har Mountain Group, petrological statistics in this formation show that the mylonite has the highest gold content of 33.63×10-9, with a variation coefficient of 210%; the siltstone has a gold content of 12.07×10-9, with a variation coefficient of 210%. 400%; slate 9.46×10-9, variation coefficient 450%; sandstone 3.5×10-9, variation coefficient 180%. The variation coefficients of slate and siltstone are the largest, so it is initially believed that the minerals in the area mainly come from the widely distributed Triassic sand and slate formations.

To sum up, from the Late Triassic to the Early Jurassic, the ocean/ocean basin subducted northward and gradually closed from east to west, forming the Bayan Har orogenic belt. This subduction-collision increased geothermal heat and provided a heat source for the subsequent formation of thermal fluids. It not only led to the low greenschist phase metamorphism and folding of the Triassic flysch sedimentary rocks, but was also strongly deformed by thrusting and related to At the same time, large-scale thrusts, strike-slip faults, ductile-brittle shear zones and their supporting low-level structural systems were formed, and the interbedded production of muddy and silty slates and sandstones in the formation formed a favorable The ore-containing structure and barrier layer provide paths and places for fluid migration and deposition and mineralization driven by the increasing heat flow during the orogeny process, and also provide favorable conditions for large-scale enrichment and mineralization.

3.3.2 Structural ore-controlling factors

The Gander-Maduo deep fault in the north of the mining area is one of the NWW-trending regional faults formed during the Indosinian orogeny. The early stage showed ductile shearing, followed by strong thrusting and strike-slip, which were respectively related to the northward subduction of the Bayan Hara basin (ductile shearing) and then the oblique collision with the East Kunlun block (forming a transform compression zone). The above process is realized in the process of regional folding and uplift, that is, the structural evolution from deep-level ductile shearing to shallow-level brittle rupture, which is consistent with regional orogenic uplift.

A series of axially NW dipping folds develop in the Dachang mining area. These folds are composed of strongly schistose slate and strongly fragmented sandstone (Lower Triassic Bayan Har Group), and the bedding is strongly replaced by foliation. The thick lenticular Au (Sb) ore body occurs in the fault-fissure system near the end of the dip fold axis. This ore-containing structure of the deposit is the supporting structure of the oblique (dextral) thrust of the Gander-Madou fault in the Indosinian period. Based on this, it is believed that the Dachang gold deposit is the product of the assembly process of the Bayan Har tectonic unit and the East Kunlun block. The ore-controlling structures of the deposit were formed in the late stage of this process (oblique collision period). Zhang Dequan conducted an age determination using the sericite 40Ar-39Ar method on the altered cataclastic gold-antimony ore in the Dachang deposit, and the result was (218.6±3.2) Ma, which proved the rationality of this reasoning.

The Dachang gold deposit is controlled by the EW-NEE large-scale shear zone in the Triassic Bayan Harshan Group flysch turbidite sedimentary rock system (low greenschist phase metamorphism). Associated with specific mineralization are brittle fractures in large shear zones. The reason for the ore control in shear zones may be that gold deposits produced in shear zones usually undergo a gradual enrichment process. During the mylonitization process, shearing can reduce the gold content in the original rock to a very low level. Migration occurs, forming disseminated and veinlet-disseminated gold ores in mylonite. When the brittle deformation stage superimposed on it occurs after the mylonite stage, the porosity of the rock increases due to the development of cleavage, schistosity, and fissures, which is conducive to later hydrothermal activity and mineral precipitation. Therefore, , at this stage, vein-like and altered rock-type rich gold deposits can be formed, and the spatial production location of this rich gold deposit is controlled by shear zones. The natural type of ore is broken altered rock gold ore, and its mineral components include pyrite, arsenopyrite, antimony sand and natural gold. Non-metallic minerals include quartz, feldspar and clay minerals. The deformation fabrics in the ores are abnormally developed, all with fragmented or mylonitic structures, showing strong ductile-brittle deformation characteristics. Quartz minerals contain abundant fluid inclusions, mainly liquid-rich and gas-liquid multiphase inclusions. Mainly, a few have gas-rich inclusions.

The above characteristics reflect that the Dachang gold deposit is driven by strong tectonic forces to form a huge fluid circulation, resulting in large-scale structural deformation and hydrothermal filling mineralization alteration. This type has a concentrated zoning distribution. Characteristics, which are directly related to the activity of medium-acidic magma during orogeny. Generally, they are not selective for strata. They mostly occur in high background, stratigraphic fragmentation alteration zones or altered rock masses, and are affected by NW-trending faults and adjacent faults. There are a series of parallel arranged NW on the side, and the NWW direction ductile shear zone is strictly controlled. The ore body is characterized by parallel arrangement.

In addition, the ore body was also modified by later folds and deformed along with the fold deformation. The main ore body in the central and western part of the mining area often forms some small NW-dipping "nose-shaped" folds that match the regional folds along the strike. It can be inferred that the formation time of the ore body is after regional metamorphism and before fold deformation.

In summary, the Dachang gold deposit ore body is located in the Indosinian (Late Triassic) orogeny process. From the region, ore field to the deposit (body), it is mainly affected by the North Bayan Har. Three levels of structural control include deep faults (or collision zones), large shear zones and brittle fractures.

3.3.3 Mineralization mechanism

The Dachang deposit is the product of Au-Sb mineralization that occurred on the northern edge of the convergent plate during the late stage of the regional Indosinian orogeny. The collision and the resulting geothermal warming drive metamorphic water (formation metamorphism and dehydration) to migrate along large faults, continuously extracting minerals along the way, including carbon, sulfur, gold, antimony and arsenic. During the regional uplift process, atmospheric precipitation continues to penetrate, resulting in the formation of CO2-NaCl-H2O fluid rich in minerals. In the late stage of the orogenic process, the Gander-Madou fault thrust rightward, and the strata on its southern plate were pulled and folded, forming a fault-fissure system at the fold axis and both wings. When CO2-rich mineral-forming materials After the NaCl-H2O fluid enters these fracture-fissure systems, it first reacts with the surrounding rocks to form pyrite-sericite. As the fluid is cooled, the fluid immiscibility separates the ore-forming fluid at a temperature of 236 to 275°C. And gold and gold-antimony ore bodies were deposited in these fracture-fracture systems. The ore-forming fluid belongs to the medium-temperature, low-salinity H2O-NaCl-CO2±CH4±N2 system.

Dachang gold mineralization is closely related in time and space to the above-mentioned orogenic process, and has typical orogenic characteristics. The deposits (bodies) are all produced in the fault and fracture zones of large-scale shear zones, reflecting the late Indian The long-term tectonic activities of the branch movement and the characteristics of multi-source mineralization hydrothermal modification.

References

Bao Cunyi, Xu Guowu, Li Yuchun, etc. 2003. Analysis of the genetic types and mineralization potential of gold deposits in Dachang area. Qinghai Land Economic Strategy, (3): 17～22

Feng Chengyou, Zhang Dequan, Wang Fuchun et al. 2004. Study on the geochemistry of ore-forming fluids of the East Kunlun orogenic gold (antimony) deposit in Qinghai. Acta Petrologica Sinica, 20 (4): 949～960< /p>

Wang Weiqing, Wang Zengshou, Li Bo. 2005. Analysis of ore-controlling factors of Dachang gold deposit in Qumalai County, Qinghai Province and significance of regional prospecting. Qinghai Land Economics, (3): 32～36

Zhao Caisheng, Sun Fengyue, Mao Jingwen et al. 2005. Fluid inclusion characteristics and geological significance of Dachang gold deposit in Qinghai. Mineral Deposit Geology, 24(3): 305～316

Zhao Junwei, Sun Fengyue, Li Shijin et al. 2007. Geological characteristics of gold (antimony) mineralization in turbidite in the northern Bayan Harshan area of ??Qinghai—taking the Dachang-Jiajilongwa area as an example. Gold, 28(9): 8～13< /p>

(Written by Li Jiemei and Wang Meijuan)