Baguamiao Gold Mine in Fengxian County, Shaanxi Province

Baguamiao gold deposit in Fengxian County, Shaanxi Province was discovered in 1989, and it belongs to super-large gold deposit after detailed investigation. The deposit is located in the northern part of Fengxian-Taibai (hereinafter referred to as Fengtai) lead-zinc polymetallic ore field in the east of West Qinling, about 40 kilometers east of Fengxian, Shaanxi Province. Fengtai ore field is one of the areas with the strongest Devonian rift activity in Qinling (Wulieshan, 1999), and it is also an important polymetallic metallogenic belt in Qinling. In recent years, large and medium-sized gold deposits such as Wang Shuang, Pangjiahe and Baguamiao have been discovered in this area. The discovery of Baguamiao gold deposit provides an example for the study of gold prospecting and metallogenic theory in orogenic belt, which has important theoretical and practical significance.

The main minerals in this area are lead, zinc and gold. At present, nearly 30 mineral deposits have been discovered, including super-large gold deposits 1, 2 large and medium-sized lead-zinc mines and 4 small lead-zinc mines. The accumulated proven gold reserves are 106 t, and the lead-zinc mine reserves are 2.5 million t. ..

1 regional metallogenic geological environment

1. 1 geotectonic unit

The tectonic position is the Variscan fold belt in the middle Qinling Mountains, the Caledonian fold belt in the north Qinling Mountains and the Caledonian-Indosinian fold belt in the south Qinling Mountains.

1.2 regional stratum

The exposed strata in Fengtai area are mainly Devonian, Carboniferous and Permian (Figure 1). Devonian is the most widely distributed, which can be divided into three groups from bottom to top: ① Gudaoling Formation (D2g) of Middle Devonian, which is mainly composed of carbonate rocks with a small amount of clastic rocks; ② Upper Devonian Fu Xinghong Formation (D3x), mainly composed of argillaceous clastic rocks and carbonaceous clastic rocks with banded limestone; ③ Upper Devonian Jiuliping Formation (D3j) interbedded with clastic rocks and carbonate rocks. The direction of regional tectonic line is roughly NWW-SEE direction, which consists of a series of axial NWW closed linear folds and faults, followed by ne-trending and near-SN-trending faults (Wu Lieshan, 1999).

Figure 1 Regional Geological Schematic Diagram of Fengtai Ore Field

(According to Li Jianhua, 1999, modified)

K 1dh—— Donghe Group of Lower Cretaceous; P- Permian; C- carboniferous; Upper Devonian D3t—-Tongyusi Formation; D3j— Upper Devonian Jiuliping Formation; Upper Devonian D3x-Xinghongpu Formation; D2g-Gudaoling Formation of Middle Devonian; D— Devonian (non-layered); -Yanshanian granite; -Indosinian granite; -Indosinian granodiorite; 1- failure; 2- deep fault; 3- Lead-zinc deposit; 4- Cu-Pb-Zn deposit; 5- copper deposit; 6- gold deposit

1.3 regional tectonic framework

Baguamiao gold deposit is located in the west of Fengtai ore concentration area in Qinling orogenic belt, which is the Indosinian fold belt of southern Qinling of Qinling fold system and the northern edge of the western end of Fengxian-Zhen 'an fold bundle. Structurally, it is the Baguamiao secondary syncline in the northern wing of the western end of Sujiagou-Kongguan syncline. Because Changgou-Erlihe brittle-ductile shear zone passes through the mining area, the rocks in this area have strong brittle-ductile shear deformation.

1.4 metallogenic unit

The deposit is located in the west Qinling metallogenic belt of Qinling-Dabie metallogenic province in Qin-Qi-Kun metallogenic domain.

2 Geological characteristics of mining area

2. 1 mining stratum

The strata in the mining area are middle and upper Devonian shallow metamorphic argillaceous clastic rocks and carbonate rocks, which can be divided into four groups from top to bottom: ① Upper Devonian Jiuliping Formation (D3j) is timely sandstone; ② Upper Devonian Xinghongpu Formation (D3x) can be divided into three lithologic sections from top to bottom-D3x3 is carbonaceous phyllite, the top of D3x2 is ankerite silty phyllite (secondary gold-bearing horizon), the middle and lower part is chlorite silty phyllite, the top of D3x 1 is ankerite silty phyllite (main gold-bearing horizon), and the lower part is carbon fiber. ③ Middle Devonian Gudaoling Formation (D2g) is limestone, and the transition position with D3x 1 phyllite is the main ore-bearing horizon of lead and zinc (copper); ④ Middle Devonian Macaogou Formation (D2m), lithology is feldspathic sandstone (secondary gold-bearing horizon).

2.2 magmatic rocks in mining area

Magmatic rocks in the mining area are not developed, and there are only albite fine-grained dikes and diorite porphyrite dikes in the north, and there are Xiba rock mass and Shidigou rock mass in the southeast of the periphery about 15km, which are obviously controlled by the structure. Xiba rock mass emplaced from east to west from the deep, and distributed nearly east-west. According to the data of Zhang (1996), the U-Pb and K-Ar ages of the rock mass are148.1~ 213.5 Ma, which are the products of the early Indosinian-Yanshan movement, and are related to the formation age and gossip of the dikes in the mining area. At the same time, the gold content in the vein of the mining area is relatively high (0.24× 10-6), which indicates that the magmatic hydrothermal solution provides thermal power for the activation, migration and enrichment of ore-forming materials, and may also provide some ore-forming materials (Zhang En, 200 1).

2.3 ore-controlling structure (ore-hosting structure)

The north and south of the ore field are bounded by two large faults with nearly east-west strike, Fengxian-Yangshan deep fault in the north and Liangdang-Zhen 'an deep fault in the south, which has long-term activity. In addition, many secondary faults are derived from the two deep faults, which divide Fengyang-Taibai ore field into many contemporaneous fault basins. It is these faults and their fault basins that provide possible channels and ore-forming spaces for deep provenance (Figure 2).

2.4 surrounding rock alteration

Wall rock alteration mainly includes silicification, sericitization and iron carbonation; Followed by pyritization, chloritization and so on. , is a set of medium-low temperature mineral symbiosis and wall rock alteration. The near vein altered rocks are a set of faded altered rocks, mainly pyrite sericitization; Weak altered rocks far away from gold-bearing quartz veins are transitional rocks between strong altered rocks near veins and surrounding rocks, mainly pyrrhotite, pyrite and sericite. Seasonal veins are usually accompanied by alteration and fading. The closer to the gold-bearing quartz vein, the stronger the alteration and fading degree, and the contents of Au, SiO2 _ 2, Fe2O3, K2O and Na2O increase obviously, while the contents of As, FeO, MgO, CaO and Al2O3 decrease obviously.

3 Geological characteristics of ore bodies

3. 1 deposit (entity) characteristics

Gold ore bodies generally appear in groups and are divided into three ore belts: north, middle and south. The occurrence of ore bodies is generally consistent with surrounding rocks, and is controlled by brittle-ductile shear zone, ne-trending fracture and joint-intensive zones on both sides. The strong shear compression position and schist zone are the best zones for ore body development and ore-bearing property. According to the open-pit mining index, the length of the four gold mines is between 375 ~ 1 195 m, the thickness is between 0.75 ~ 70.50 m, the inclined depth is about 705m, and the average grade of a single ore body is between 3.60×10-6 ~ 8.38×1.

Fig. 2 Geological map of Baguamiao super-large gold deposit in Fengxian County, Shaanxi Province

Lower member of Upper Devonian Xinghongpu Formation:-the fourth layer,-the third layer,-the second layer,-the first layer; D2g2— Upper part of Gudaoling Formation of Middle Devonian. 1- gold ore body; 2- timely pulse; 3- Fault fracture zone; 4- Fault

The gold ore body is layered lenticular, "crab" in plane and lentil in section. The center of the ore body is the thickest and the gold grade is the richest. The superposition of NW brittle ductile shear ore-hosting structure and NE extensional shear fracture (joint) structure leads to a crab-like gold deposit as a whole, with a crab belly near the exploration line 67.

3.2 Ore composition

There are three types of gold deposits (Yu Xueyuan et al., 1996), namely, altered rock type in fracture zone, gold-bearing quartz vein type and gold-bearing iron dolomite chronological vein type. The ore structure is irregular granular and irregular gelatinous. Ore structures are mainly disseminated, dotted, veined, reticulated, breccia and banded. Ore minerals are relatively simple, and the content of metal minerals is generally less than 5%, mainly pyrrhotite and pyrite, with a small amount of chalcopyrite, sphalerite, galena and magnetite, as well as a small amount of tellurite, tellurite, natural gold, nickel sulfide-arsenic-cobalt-nickel ore, tungsten-ruthenium ore and so on. Gangue minerals mainly include quartz, sericite, ankerite, chlorite, biotite, albite and calcite (Zhang En, 200 1).

The useful components in the ore are single, only one element is Au, and the contents of other elements such as copper, lead and zinc are very low, which can not meet the requirements of comprehensive recovery. The content of silver is extremely low, so it has no value of separate recovery. Because it exists in the form of intermetallic compound with gold in natural gold, silver can be recovered at the same time of smelting and recovering gold. The content of harmful elements arsenic and carbon in ore is extremely low, which has no effect on mineral processing and smelting process. Generally, gold ore contains arsenic < 0.00 1%, lead 0.01/5%, zinc 0.022%, copper 0.0074%, silver 1.3075% and sulfur 0.1272.

Gold minerals are mainly silver-bearing natural gold, and the particle size is mostly bright gold-micro gold, mainly in the form of intergranular gold and fissure gold, and inclusion gold is only a minority. The main gold-bearing minerals are Yingshi, pyrrhotite and pyrite, and common monomer gold. See table 1 for the fineness of natural gold at different depths in Baguamiao gold deposit.

3.3 Ore fabric and metallogenic stage division

Most fresh ores are light gray-gray, and most of them are yellowish brown after oxidation. The ore structure is mainly micro-scale crystallization, mostly millstone and mylonite structure with point, block and strip structure.

The deposit has experienced three metallogenic stages. Devonian hydrothermal gold deposits were pre-enriched to form ore embryos (Yingshi-sericite-sulfide stage), ductile shear in Indosinian collision orogeny stage made the gold deposits initially located (Yingshi-chlorite-sericite-polymetallic sulfide stage), and brittle deformation-magmatic hydrothermal action in late Indosinian-Yanshan stage enriched the gold deposits to form industrial deposits (Yingshi-ankerite-sulfide stage) (feudal medium, 2003; Qian Daiyi, 2003).

Table 1 natural gold fineness at different depths in Baguamiao gold deposit

3.4 Geochemical characteristics of elements

Through the analysis of trace elements in more than 200 samples of the deposit, it is found that the main ore-forming elements of the deposit have obvious zonation. From the thick part in the center of the ore body to the pinch-out part at the edge of the ore body, the ore-forming elements are Au → Au+Ag → Au+Ag+Pb+(Bi) (Wu Lieshan, 1999).

4 genetic analysis of the deposit

4. 1 Characteristics of mineral inclusions

Inclusions are mostly developed in gangue minerals and iron-bearing dolomite, with a large number and diverse and complex shapes. Their shapes are mainly negative crystals, polygons, eyeball shapes and irregular shapes. The size of inclusions is generally between 5 and 25 microns, and they are mainly primary.

By SEM/EDS and EPMA analysis, the fluid inclusions in Baguamiao Gold Mine contain pyrite minerals, halite minerals and complex secondary minerals. LRM analysis of fluid inclusions shows that CO2 accounts for a large proportion in gas and liquid components, CH4 is detected in most samples, and H2S, N2 and C2H4 are detected in some samples, which reflects the relative reduction environment of Baguamiao Gold Mine and accords with the mineral symbiosis characteristics with pyrrhotite as the main metal sulfide (Table 2) (Qian Dayi et al., 2003).

Table 2 Laser Raman Probe Analysis Results of Fluid Inclusions in Baguamiao Gold Mine

4.2 Physical and chemical conditions

The homogenization temperature of inclusions in Baguamiao gold mine is between 130 ~ 467℃, with an average of 300℃, while that in lead-zinc mine is 200℃. The mineralization temperature of Baguamiao gold deposit is obviously high, which may be related to the participation of tectonic fluid-magmatic hydrothermal solution.

Cao Yuangui determined the salinity of timely inclusions in Baguamiao gold mine during the main metallogenic stage by freezing method. The salinity NaCl is between 3% ~ 10.7%, which belongs to medium-low salinity fluid inclusions. The salinity (NaCl) measured by Zheng Zuoping is 19.6% ~ 2 1.2%, which is a high salinity fluid inclusion. Zhang Changnian's analysis of the composition of the corresponding inclusions shows that the early average salinity (NaCl) is 6.65%, the middle average salinity is 14.3%, the late salinity is higher than the previous two periods, and the solution salinity is in the middle range. Qian Dayi and others observed under microscope and scanning electron microscope that the salinity of inclusions containing NaCl minerals should be > 26%, indicating that the salinity of ore-forming fluids is quite different.

Generally speaking, the inclusions in Baguamiao gold deposit are in a low-temperature epithermal environment with weak acidity, weak reduction and low salinity.

4.3 Isotopic geochemical markers

4.3. 1 sulfur isotope

The δ34S of pyrite in the main metallogenic period is 7.4 ‰ ~ 15.4 ‰, with an average value of10.7 ‰. The δ34S of pyrrhotite is 6.8 ‰ ~ 15.4 ‰, with an average value of 10.5‰. The sulfur isotopic composition of the ore is similar to that of the strata, indicating that the ore-forming materials mainly come from the surrounding rocks.

4.3.2 Timely hydrogen and oxygen isotopes

The timely δD is -53.38 ‰ ~- 1 17.90 ‰ (average-81.42 ‰); δ 18O is 5.69 ‰ ~ 19.84 ‰ (average 15.36‰). The δ 18O value of hydrothermal solution is 3.07 ‰ ~ 13.3 ‰ (average 5.6‰), indicating that the source of ore-forming hydrothermal solution is multi-source, which is influenced by both atmospheric precipitation and magmatic water.

4.3.3 Carbon and oxygen isotopes of iron dolomite in ore-bearing rocks and veins

δ 18O of iron dolomite in host rocks and veins is 16.64 ‰ ~ 19.73 ‰ (average19.5438+04 ‰); δδ 13C is -4.87 ‰ ~- 1.85 ‰ (average -2.43‰), indicating that the ore-bearing rocks have the same genesis as the intra-vein ankerite, and both are products of hydrothermal activity. From the carbon isotope value, it is close to that of kimberlite [(-4.7 1. 1.2)%], indicating that the carbon isotope of this ore comes from the deep (Zhang En, 2000).

4.4 Rare Earth Elements

According to the characteristics of rare earth elements in altered rocks (Table 3), altered rocks can be divided into two types. The first type (weakly altered rocks) is rich in light rare earths and loses Eu, which is similar to surrounding rocks. σσREE is198.83×10-6 ~ 225.08×10-6, and the average δEu is 0.77. There is little difference between light and heavy rare earths in the second type (strongly altered rocks), σσREE is 27.50×10-6 ~ 78.78×10-6, δEu is 0.74, and W (σ Ce)/W (σ Y) = 0.36 ~ 2.43. It shows that the REE fractionation characteristics of altered rocks are between surrounding rocks and gold-bearing quartz veins, and their REE characteristics are closer to gold-bearing quartz veins when the alteration intensity is high (Wu Lieshan et al., 1999).

Table 3 Composition characteristics of rare earth elements in Baguamiao gold deposit

The study of rare earth elements in the ore of Baguamiao Gold Mine shows that the total amount of rare earth elements in the ore-bearing construction is relatively high, and the negative Eu anomaly is obvious (δ EU = 0.68 ~ 0.73 in Baguamiao Gold Mine), while the Ce anomaly in Baguamiao Gold Mine is not obvious (δ Ce = 0.62 ~ 1.02), which may be caused by the addition of deep components in the ore-bearing construction of Baguamiao Gold Mine or diagenesis.

4.5 metallogenic age

The samples are taken from the NW-trending pulse and the NE-trending pulse. The selected samples are fresh and transparent, and the purity is over 99%. The samples were tested in 40Ar/39Ar dating laboratory of Institute of Geology, Chinese Academy of Sciences. Argon was extracted from the sample by step heating method and analyzed by mass spectrometry. A stable saddle-shaped platform is formed at 530 ~ 850℃. The NW-trending pulse Ar-Ar age is 222.14 3.45 Ma, belonging to Indosinian period. The Ar-Ar age of the NE-trending pulse is129.45 0.35 Ma, which belongs to Yanshanian period.

4.6 deposit type

At present, there are three main views on the genetic types of ore deposits: Carlin type-Carlin-like type (Wei Longming et al.,1994; Kerrich et al., 2000; Chen Yanjing et al., 2004); Sedimentary-hydrothermal reformation type (Wei Longming et al.,1996; Wang Xueming et al, 2001); Moderate temperature vein type (orogenic type) controlled by shear zone (Zhong Jianhua et al.,1997; Feng Jianzhong et al., 2002).

refer to

, Shao, Wang et al. 2002. Ore-controlling characteristics of brittle-ductile shear zone and dynamic mechanism of metallogenic structure in Baguamiao gold deposit, Shaanxi Province. Geology of China, 29 (1): 50 ~ 58.

Feng Jianzhong, Wang Dongpo, Wang Xueming and others, 2003. Geological characteristics and mineralization of Baguamiao superlarge gold deposit in Fengxian County, Shaanxi Province. Journal of Geology, 77 (3): 387 ~ 399.

Qian Dayi, Xie Yuling, Xu Jiuhua. 2003. Composition characteristics of fluid inclusions in Baguamiao gold deposit, Fengxian County, Shaanxi Province. Journal of University of Science and Technology Beijing, 25 (1): 1 ~ 4.

Wu Lieshan, Wei. 1999. Geochemical characteristics and provenance analysis of Baguamiao superlarge gold deposit. Geological exploration series, 14 (4): 62 ~ 68.

(Author Zhang Yanchun)