Geological and geochemical characteristics of gold deposits in Xicheng-Lixian area and their relationship with lead-zinc mineralization

(1) Geological and Geochemical Characteristics of Gold Mineralization in the Surrounding Area of Zhongchuan Rock Mass

The Devonian basins in Xicheng, Limin and Fengtai, which are closely related to gold deposits, are both related and different. Among them, the similarities between the eastern part of Xicheng and Fengtai are more obvious in structural background, metallogenic assemblage and metamorphic characteristics, while the differences between Li and Fujian are even greater.

In recent 20 years, large-scale gold mineralization has been discovered in Bali Group around Zhongchuan rock mass in Lixian County (Figure 4-32), among which the main gold deposits are Ba Li, Jinshan, Yejiping, Sanrengou, Zhenggouli, Hotel, Shidonggou, Xiecaoshan, Chenjiagou, Dazhuangli, Ma Quan and Miaoshan, which are distributed in different strata, including mud layers. The geological characteristics of these gold deposits are very similar, and argillaceous clastic rocks are the host rocks. The distribution and shape of ore bodies are controlled by ductile shear zone structure. The ore minerals are simple, and a set of medium-low temperature hydrothermal alteration is developed, and the metallogenic age is slightly later than the diagenetic age of granite. Granite rock mass mainly plays the role of heat source in the process of mineralization, and the deposit type belongs to epithermal deposits (feudal medium, 2003). Previous studies on the isotopic chronology of gold deposits in Ba Li show that the metallogenic age is 17 1.6Ma (Cheng Yu et al., 2005, Rb-Sr isochron age of fluid inclusions) and 2 10.6Ma (middle feudal age, 2003, corresponding to the Ar-Ar plateau age), which is similar to that of middle feudal gold deposits.

1. Characteristics of regional strata and Zhongchuan rock mass

Ba Li Group is mainly composed of a set of sandstone, siltstone, silty slate and spotted slate formed by deep-sea turbidites, with thick marl at the top. Li Jianzhong (1996) divided it into four groups, among which Hejiamo Formation (D2lb2), the second lithologic member, is the host bed of Jinshan, Ba Li and other gold deposits. The Bali Group and the Western Han Water Group were deposited in different periods at the same time, and they underwent at least three deformation transformations, including early vertical structural replacement characterized by solid plastic rheology, Hercynian regional wide and gentle opening fold, Indosinian thrust nappe, shearing and post-orogenic extension (Cheng Yao et al., 200 1).

There are many periods of magmatic activity in the area, among which three main intrusions are Indosinian medium coarse porphyritic biotite granite, medium coarse porphyritic biotite granite and Yanshanian medium fine biotite granite, and the three main intrusions are distributed from the edge to the center in time series. The acidity of Zhongchuan complex granite is relatively high, with the content of SiO2 _ 2 generally higher than 70%, a/nck = 1.42 ~ 1.47, K2O/(K2O+Na2O) = 0.54 ~ 0.57, and the aluminum saturation further increases towards the later stage (Lu Zhe et al., 2003). The rock mass intruded three times from morning till night, forming medium-coarse grained porphyritic biotite adamellite (2 19Ma), coarse grained porphyritic biotite adamellite (196Ma) and medium-fine grained biotite adamellite (18 1.5Ma) respectively (Lu et al.

Thermal contact metamorphism occurs between rock mass and stratum, which makes sedimentary formation become amphibole, biotite and speckled. From the outside to the inside of the rock mass, it can be divided into three metamorphic zones (Figure 4-32), namely, chlorite sericite zone-mainly regional metamorphic chlorite sericite phyllite; Biotite belt-composed of biotite amphibole, contact biotite phyllite and schist; Andalusite cordierite amphibole zone-composed of andalusite amphibole and cordierite amphibole. Most gold deposits occur in biotite porphyry belt.

Figure 4-32 Contact metamorphic belt and gold deposit distribution map of Zhongchuan granite complex in Lixian County, Gansu Province (according to Liu Miao 1994).

2. Geological characteristics of gold deposits

As the largest gold deposit in the region, Ba Li Gold Mine has carried out the most in-depth research work (Sun Shengli et al., 2001; Wang, 1999). This project focuses on Jinshan, Yejiping and Ma Quan gold mines.

The gold deposits around Zhongchuan rock mass occur in different strata. Most of the gold deposits, including Ba Li and Jinshan, occur in the second lithologic member (D2lb2) of Bali Group, with calcareous silty sericite phyllite and chlorite sericite phyllite mixed with porphyritic phyllite in the lower part, silty sericite phyllite in the upper part and calcareous sericite phyllite at the top. There were a few gold deposits in Carboniferous, among which Ma Quan Gold Mine is located about 2 kilometers east of Jinshan Gold Mine. In the past, its surrounding rock was considered to be Carboniferous, and its rock and mineralization characteristics were similar to those of Jinshan Gold Mine. Yejiping gold deposit is located in the east of Zhongchuan rock mass, which occurs in Carboniferous siltstone, slate and marl, and the ore body is controlled by ductile shear structure. These gold deposits have similar metallogenic geological characteristics in different horizons.

The direct surrounding rocks or mineralized rocks of gold deposits are porphyritic phyllite or porphyritic slate, and the spots are generally composed of chlorite, biotite and carbonate minerals. The characteristics of porphyry and porphyry are similar to those of Fengtai Baguamiao and Xicheng Xiaogouli gold deposits. However, in contrast, the metamorphic mineral biotite content in porphyritic slate of gold deposits in Zhongchuan area is higher (Figure 4-33A) and chlorite content is lower. The content of chlorite in porphyritic slate in Xiaogouli Gold Mine is high, and the content of biotite is low, and Baguamiao Gold Mine is between them. Speckle slate is mostly light grayish brown. In the porphyry phyllite of Ma Quan gold deposit, there are a large number of transverse time-dependent pulses besides the longitudinal coarse time-dependent pulse (5 ~ 15 cm wide), the pulse width is less than 2 cm, and a few of them are more than 5 cm, with strong fading and alteration along both sides of the pulse (Figure 4-33B) and the width is 3 ~ 15 cm.

Fig. 4-33 Speckle Characteristics of Gold Deposits and Fading Alteration of Speckle Phyllite

The wall rock alteration of gold deposits ranges from several meters to more than ten meters, from the inner zone to the outer zone, which is generally pyrite sericitization zone-sericitization zone-chloritization zone-biotite zone (Sun Shengli et al., 200 1), in which biotite is the product of regional metamorphism (or thermal metamorphism), and this alteration zoning is similar to most gold deposits.

The relationship between ore body and surrounding rock is transitional, and the boundary is delineated by grade. There are strong silicification, sericitization and chloritization in the ore bodies. The total amount of sulfide in ore bodies is generally less than 3%, mainly pyrite, arsenopyrite and pyrrhotite, which are distributed in a fine disseminated form, and a small amount of galena, sphalerite and stibnite. , with medium-fine particles and authigenic crystals, gold minerals have fine particle size.

3. Geochemical characteristics of the deposit

The geochemical profile from Zhongchuan rock mass to Ma Quan gold deposit shows that the contents of major elements and trace elements in clastic rocks change regularly from the rock mass to the outside. In addition to Au, As, Zn and other elements closely related to gold mineralization, the contents of Al2O3, Fe2O3, CaO, V, Ti, Cr, Co, Ni and Bi also increased obviously. It is slightly increased, and its content is the highest in Ma Quan gold ore body. However, the Na2O content has decreased, while the K2O content has remained basically unchanged (Figure 4-34).

Figure 4-34 Geochemical Profile of Zhongchuan Rock Mass-Ma Quan Gold Deposit

The values of Al2O3 and A/NKC are highly consistent with the changing laws of Ti, V and other elements, and increase with the values of Au, As, Zn, Co, Ni, Bi and B near the gold mineralization location, with good correlation. The lithology of Bali Group has gradually changed from sandstone and fine sandstone to argillaceous siltstone, slate and spotted slate, and gold mineralization occurs in argillaceous spotted slate. Except for Au and As, the contents of most elements change widely and slowly, which reflects that this change may be influenced by rock properties to a greater extent except for Zhongchuan rock body. Sr/Ba ratio decreases outward from the rock mass, and there is no abnormal change near the gold ore body, which shows that the lithology of the whole rock series changes gradually and there is no obvious sign of jet deposition. The total amount of rare earth elements in the whole section clastic rocks is equivalent, and the distribution characteristics of rare earth elements are very similar. There seems to be no large-scale and extensive hydrothermal metasomatism in the process of mineralization, and this hydrothermal activity related to mineralization is mainly confined to the vicinity of ore bodies. The geochemical characteristics of small sections of ore bodies passing through Jinshan Gold Mine are generally similar. With the increase of Au, the contents of As, Sb, W and other elements in the ore body increase, but the change of Sr/Ba ratio is not obvious, and the distribution of rare earth elements in the ore body is very similar to that in the footwall and surrounding rock (Table 4-13; Figure 4-35), all these indicate that the ore-forming hydrothermal solution of the gold deposit occurred locally, but these did not bring enough rare earth components. The characteristics of rare earth elements in altered ore are basically the same as those in original rock.

Table 4- 13 Rare Earth Element Composition of Ore Surrounding Rock in Jinshan Gold Mine

Note: The analysis unit is Northwest Nonferrous Metals Mineral Geology Test Center, 2005; The analytical method is ICP-ms. Unit: Al2O3%, Au 10-9, others 10-6.

Figure 4-35 Distribution of Rare Earth Elements in V Orebody Profile Samples of Jinshan Gold Mine

Sulfur isotope: δ34S of Ba Li gold deposit varies from 4.0 ~ 10.6 (Table 4- 14), and the average value of 18 sample is 8. 139, which is obviously lower than that of Xicheng SEDEX lead-zinc mine and He Xiaoyu hydrothermal lead-zinc mine of Xihanshui Formation. These sulfur isotopes can better reflect the characteristics of formation sulfur. The gold ore δ34S is not only similar to disseminated pyrite in surrounding rocks nearby, but also comparable to disseminated pyrite δ34S in sedimentary rocks in Xicheng ore field.

Table 4- 14 Sulfur Isotope Analysis Results of Ba Li-Xiaogouli Gold Mine

Note: The data of Ba Li gold mine are quoted from the internal data of Tianshui Corps of Gansu Nonferrous Geological Exploration Bureau, and other data are analyzed in this book.

Hydrogen, oxygen and silicon isotopes: See Table 4- 15 for the hydrogen, oxygen and silicon isotopes of Ba Li gold mine. The isochrones of different occurrences in the ore body and its surrounding rocks have similar oxygen, hydrogen and silicon isotopic compositions, indicating that the isochrones in the ore body and its surrounding rocks have similar genesis and are all formed by ore-bearing hydrothermal metasomatism. Its hydrogen and oxygen isotopic composition has the characteristics of magmatic water (or metamorphic water)-atmospheric precipitation, which is mainly magmatic water (or metamorphic water) in the early stage and a large amount of atmospheric precipitation is added in the later stage.

Table 4- 15 Isotopic Composition of Hydrogen, Oxygen and Silicon in Ore Body and Surrounding Rock of Ba Li Gold Mine

(2) Geological characteristics of gold mineralization in Xicheng area.

1. Xiaogouli-Sanyeba gold deposit

In Xicheng basin, gold mineralization is also widely distributed, mainly occurring in clastic phyllite of the Western Han Shuizu Formation. The discovered deposit is small in scale, and its representative is Xiaogouli-Sanyaoba gold deposit.

The deposit is located at the core of Xiaogouli anticline and is an extension of the east side of Jianyagou deposit (Figure 4-36). The exposed stratum is the second lithologic stratum (D2x) of the upper member of the Middle Devonian Xihanshui Formation, which is equivalent to the upper ore-bearing stratum of the Jianyagou deposit. The overall occurrence is SW 180 ~ 2 10 ∠ 45 ~. The lower part is timely sericite phyllite, sericite phyllite, chlorite sericite phyllite, dolomite sericite phyllite, a small amount of dolomite timely sodalite, argillaceous limestone, etc. , distributed in the south of the mining area. The middle part is thin silty limestone with a small amount of calcareous sericite phyllite, which is distributed in the middle part of the mining area. The upper part is mainly calcareous sericite phyllite mixed with thin limestone, sandy phyllite and metamorphic sandstone, distributed in the northern part of the mining area.

Figure 4-36 Regional Geological Map of Sanyeba-Xiaogouli

Layered lead-zinc deposits are distributed in limestone in the core of the anticline on the north side of the gold mine or on the interface between limestone and phyllite (Figure 4-37), and the mineralization characteristics are completely consistent with those of Jianyagou lead-zinc deposit. The southeast of the mining area 1 ~ 2 km is Dashan porphyry granite, and there are a large number of Indosinian porphyry granite veins in the mining area, which are distributed along phyllite bedding and parallel to gold-bearing quartz veins. The petrochemical characteristics of these dikes are basically consistent with Dashan granite and similar to most Indosinian intrusive granites in Xicheng area (Table 4- 1). The distribution characteristics of rare earth elements in Sanyeba gold vein rock are obviously different from those in regional granite and from those in fine-grained albite and coarse-grained albite vein in Xiaogouli gold mine. Heavy rare earths have suffered serious losses, and there is no Eu abnormality (Figure 4-38).

In the Xiaogouli-Sanyaoba gold deposit, there are many albites distributed near the ore body, among which coarse albite veins contain a small amount of chronological. Coarse-grained albite is veined, mixed with fine-grained albite (Figure 4-39A), which is abnormal, develops flaky twins, and basically contains no impurities. Fine-grained albite has a particle size of < < 0. 1mm, which is irregular, sometimes flaky twin, containing a large amount of hairy rutile and sometimes a small amount of fine-grained tourmaline (Figure 4-39B). There are some differences in chemical composition between the two albites (Table 4- 1), and the purity of coarse-grained albite is higher. The distribution of rare earth elements in the two albites is different. In Figure 4-38, their curves are almost parallel, but the total rare earth element content of coarse albite is very low. Compared with the surrounding rocks rich in albite (XC-20, XC-2 1) in the lead-zinc mine of Lijiagou Gold Mine, the characteristics of fine albite are closer. On the other hand, there are a large number of hairy rutile and a small amount of tourmaline in fine-grained albite, and it is further said that fine-grained albite has the origin of jet deposition, while coarse-grained albite is the result of hydrothermal transformation in the later stage and is widely used.

Figure 4-37 Geological Schematic Diagram of Sanyeba-Xiaogouli Gold Mine (According to Lin Guofang, 20065438+0; Zhang, 2001; There are modifications)

Fig. 4-38 REE distribution of albite and dike in Xiaogouli-Sanyeba gold deposit.

The ore-bearing porphyritic phyllite spots are 0.5 mm in diameter and mostly round, and the main components are chlorite and calcite (maybe some dolomite). There are no biotite spots in Sanyeba-Xiaogouli mining area, which indicates that the thermal metamorphism of the deposit is lower than that of Jinshan, Ba Li and Baguamiao gold mines.

Figure 4-39 Characteristics of albite veins and spots in Xiaogouli Gold Mine

There are more than 20 gold mineralization veins, including gold-bearing quartz veins and veinlets disseminated type, with gold-bearing quartz veins as the main ones and controlled by ductile shear zones. Vein disseminated gold and gold-bearing sulfide are distributed in altered mottled phyllite as fine particles. Gold-bearing quartz veins are milky white with a width of 1 ~ 3m. The gold grade changes greatly, and disseminated gold mineralization often occurs on both sides of veins. There are few sulfides in the ore body, and the main metal minerals are pyrite and arsenopyrite, with a small amount of galena, sphalerite, chalcopyrite and pyrrhotite. Gangue minerals include quartz, sericite, chlorite, calcite, dolomite and albite.

The sulfur isotope δ34S = 8.3 ~14.9 of the ore in Xiaogouli gold deposit, and its distribution range is narrow, which is equivalent to δ 34s of disseminated pyrite in the surrounding rock of lead-zinc deposit, which is similar to that of gold mineralization in Lixian and Fengtai areas.

2. The distribution of gold deposits in Xicheng basin and the degree of Devonian metamorphism.

In recent years, a number of gold deposits and mineralization points have been discovered in Xicheng basin, mainly distributed in the west of Anjiacha-Huaqiaozi and the east of Dujiaying-Dagouli. In Anjiacha-Jianyagou-Dengjiashan area, besides Xiaogouli-Sanyaoba gold deposit, there are small gold deposits such as Anjiacha and Sanhuazui, and their geological characteristics are similar to those of Xiaogouli deposit. These gold mineralization are distributed in phyllite in the upper layer of lead-zinc mine, and the host rocks are dotted with different degrees. The speck composition is characterized by chlorite, and the mineralization is mainly ivory gold-bearing quartz veins. In more western areas, such as the clastic rocks in the upper part of the Xihanshui Formation in the west of Caoguan-Shangxiang, there are also many gold mineralization, such as Dagouli, Chengjiadanshan and Jianganshan. Compared with the Xiaogouli-Sanyeba gold deposit, the mineralization is dominated by gold-bearing ivory chronological veins, with fine chronological particles, and sometimes aphanitic silicification. There are few or no spots in phyllite surrounding rocks, indicating that the hydrothermal metamorphism is low.

The distribution of gold deposits with different characteristics in Xicheng basin is closely related to the degree of thermal metamorphism. The metamorphic degree in Changba-Lijiagou area is the highest, followed by the core area of Wujiashan uplift, and the lowest in Caoguan-Yeshuihe area in the west. The distribution of gold mineralization is also closely related to the degree of regional hydrothermal metamorphism (see Figure 4-5). There is almost no independent gold mineralization in the highly metamorphic staurolite-garnet belt. In the biotite metamorphic belt, gold mineralization is also rare; The main gold mineralization is distributed in chlorite-sericite belt.

(3) Classification of gold mineralization types and comparison of gold mineralization characteristics in different regions.

1. Main types of gold mineralization

According to the characteristics of gold mineralization in Xicheng and Lixian areas, gold mineralization is preliminarily divided into time-dependent pulse type, fault fracture zone type, altered rock type, altered rock-fracture zone type and time-dependent pulse-silicified zone type.

Time-dependent vein type: the ore body is a gold-bearing quartz vein with disseminated gold mineralization on both sides of the vein. Gold-bearing quartz veins are controlled by east-west fault zones, close to related intrusions, and generally occur along layers; The surrounding rock is mottled phyllite, with chlorite, carbonate and biotite in the mottled structure. The representative gold deposits are Xiaogouli-Sanyeba and Sanhuazui.

Fracture zone type: the ore body is a strong compressive ductile fracture zone with strong thermal metamorphism. The speckled components in the surrounding rock of argillaceous rocks are mainly biotite and calcite, and andalusite appears, with strong silicification in the ore body. The representative deposit is Ba Li mine.

Altered rock type: Mineralization is related to many longitudinal and transverse time veins developed in phyllite, and there are obvious discoloration and alteration on both sides of the veins. The spots in phyllite are mainly chlorite and calcite, which are widely distributed and have low thermal metamorphism. The representative deposits are Baguamiao and Ma Quan.

Altered rock-fracture zone type: the degree of thermal metamorphism is low, the surrounding rock is slate, and locally phyllite. The distribution of spots in phyllite is small, only near the ore body, and the spots include biotite and pyrite. , containing a small amount of chlorite. Gold mineralization is related to silicification, but the scope of silicification is small. The representative gold deposit is Jinshan Gold Mine.

Time-related pulse silicification zone type: the degree of thermal metamorphism is very low, generally it has not suffered obvious thermal metamorphism, and there is no spot or spot is not obvious. Gold mineralization is related to timely veinlets and silicification at relatively low temperature. At present, the discovered gold mineralization is mainly distributed in Dagouli-Chengjiadanshan area in the west of Xicheng ore field.

Time-related pulse type, fracture zone type and altered rock type are more manifestations of mineralization, which may be more influenced by structural occurrence. If the degree of thermal metamorphism is distinguished, it is manifested as metamorphic minerals in argillaceous rocks, mainly the characteristic components in spots, that is, Ba Li gold mine has a high degree of metamorphism, and the spots are mainly biotite with a small amount of andalusite; Followed by Bagua Temple, porphyritic minerals include biotite and chlorite; Thirdly, in Xiaogouli-Sanyaoba, the spotted characteristic minerals are chlorite and dolomite; The gold deposits with the lowest degree of thermal metamorphism are Dagouli and Chengjiadanshan in the west of Xicheng ore field, and there are basically no metamorphic spots in the ore and surrounding rock. Based on this thermal metamorphism, a set of gold mineralization series in this area can be established.

2. Comparison of mineralization differences in different Devonian basins.

The Devonian basins in Xicheng, Limin and Fengtai, which are closely related to gold deposits, are both related and different. Among them, the structural background, metallogenic assemblage and metamorphic characteristics of the eastern part of Xicheng Basin are more similar to Fengtai Basin, but more different from Lixian Basin.

There are great differences in sedimentary facies and metamorphic characteristics between Bali Group and Western Han Water Group. Ba Li Group is mainly composed of extremely thick medium-fine clastic rocks, with few fossils, showing the characteristics of deep-sea-semi-deep sea turbidite, few carbonate rocks and low degree of regional metamorphism. There are no siliceous rocks and albite rocks in Bali Group. It directly reflects the hydrothermal sedimentation on the seabed, and even gold deposits contain almost no albite and tourmaline. This is an important feature of the sedimentary rocks of the Western Han Water Group contained in Fengtai Basin and Xicheng Basin (especially in the eastern part of Xicheng Basin). A large number of albite and siliceous rocks (microcrystalline quartzite) can be seen in many gold mines and lead-zinc mines. The Western Han Water Group or the Western Han Water Group is mainly carbonate rocks and clastic rocks deposited on the platform, which are rich in shallow sea fossils.

In Xicheng and Fengtai basins, where hydrothermal sedimentary rocks are widely developed, there are not only a large number of SEDEX-type lead-zinc deposits, but also a considerable amount of gold mineralization in clastic rocks above lead-zinc ore-bearing strata, and some of them have formed a huge scale. Their metallogenic characteristics are similar to those of gold deposits in Lixian basin, and most of them are fine disseminated. However, in the Bali Group in Lixian area, except for a large number of gold mineralization, SEDEX-type lead-zinc mineralization has not been found so far, and even typical jet sedimentary rocks have not been found. It can be seen that Lixian basin is obviously different from Xicheng and Fengtai basins, with different sedimentary assemblages and different metallogenic assemblages.

(d) The difference of sulfur and lead isotopic evolution between gold mines and lead-zinc mines

The sulfur isotopic composition of gold deposits distributed in different regions has some characteristics of * * * *, and δ34S is low and positive, which is obviously richer in light sulfur than lead-zinc deposits and has a narrow distribution range. For example, δ 34s = 8 ~ 14 of pyrite in the surrounding rock of Changba represents the background value of sulfur isotopes in regional strata, and δ 34s = 8.3 ~ 14.6 of sulfide in Xiaogouli gold mine in Xicheng ore field, which are completely consistent. In the process of sulfur isotope study in Ba Li-Jinshan gold mine, a similar situation was found, that is, the δ34S of ore mainly distributed in 4 ~ 10, while the δ 34s of pyrite in peripheral metamorphic siltstone and phyllite were 5.6 ~ 6.7, which were similar. The predecessors interpreted this low positive δ34S composition as a mixture of deep magmatic sulfur and crustal sulfur, reflecting the characteristics of magmatic hydrothermal type (feudal medium, 2002, 2004).

Even compared with the stratabound lead-zinc ore body near the gold mine, the δ34S of the gold mine is still obviously low. For example, δ34S= 16.9 of sphalerite in the layered lead-zinc mine on the east side of Xiaogouli Gold Mine and δ34S=20.7 of the layered lead-zinc mine on the west side of Sanyeba Gold Mine. The sampling points of these samples are less than 500 m away from the gold ore body (Figure 4-36 and Figure 4-37), which are equivalent to the sulfur isotope composition of lead-zinc mine in the southern ore belt of the region, but

The evolution characteristics of lead isotopes in gold mines are also obviously different from those in lead-zinc mines. Compared with lead-zinc deposits, lead isotopes in gold deposits are not only rich in radioactive lead, but also linearly distributed, showing the characteristics of multi-stage evolution. This is similar to the lead isotope characteristics of most gold deposits in China. The lead isotopic composition of layered lead-zinc ore in the equivalent horizon outside Sanyeba-Xiaogouli mine is similar to that in the southern part of this area, but it is obviously different from the adjacent gold mines. Combined with the research results of fluid inclusions, the geochemical characteristics of fluid inclusions in gold deposits are obviously different from those in lead-zinc deposits. Gold deposits not only have high gas-liquid ratio of fluid inclusions, but also pyrite microcrystals are found in some deposits. The gas phase composition of fluid inclusions is rich in CO2 and CH4, while N2 is poor, and the Na/K ratio of liquid phase composition is obviously higher than that of lead-zinc mine, indicating that sulfur mainly comes from surrounding rocks rather than nearby layered sulfide ore bodies during gold mineralization.

The above characteristics show that although the gold mineralization period is similar to that of the lead-zinc mine in the southern belt of Xicheng area, and it is adjacent in some areas, the gold mineralization process is obviously independent of the geological characteristics of lead-zinc mines and the evolution process of sulfur and lead isotopes. Sulfur mainly comes from the surrounding rocks in the area, and a large amount of radioactive lead is added to the lead isotope composition. That is, the adjacent lead-zinc mines and gold mines still have obviously different sulfur and lead isotopic compositions, and the fluid properties are also different.

(5) Study on the relationship between gold mineralization and lead-zinc mineralization.

In recent ten years, with the discovery of large-scale gold mineralization in some SEDEX-type lead-zinc mineralization concentrated areas, such as Xiaotongjiapuzi (Liu Guoping, 1999) and Baguamiao (feudal medium, 2005) in the periphery of Qingchengzi lead mine in Liaoning Province, the idea of searching for gold deposits in the periphery of SEDEX-type lead-zinc mineralization has been widely paid attention to, and some fine-grained leaching has also been carried out. SEDEX-type lead-zinc mineralization was also discovered. For example, SEDEX-type lead-zinc-gold mineralization was found in Devonian in Kalinkin mining area, Nevada (Hutchinson, 2002). The internal relationship between gold mineralization and lead-zinc mineralization has been widely discussed (Wang et al., 2001; Zhang Fuxin, 1996). At present, it is believed that there is an internal relationship between lead-zinc deposits and gold deposits. It is mainly in the process of jet flow, that is, the mineralization of layered lead-zinc deposits, or in a delayed period, mainly at a relatively far position from the nozzle, that gold is enriched for the first time to form source beds, and even layered gold bodies are formed locally. In the late orogeny, gold was activated and enriched, forming a gold deposit.

There are obvious differences in the occurrence space between gold deposits and lead-zinc deposits. Gold deposits are widely distributed, and the distribution range is much larger than that of lead-zinc mines. Almost all exposed strata in Qinling area have different degrees of gold mineralization. Gold deposits not only occur in large quantities in Devonian system in the northern belt of South Qinling, but also exist in large-scale gold mineralization in the southern belt of South Qinling and Mianlvening area. Gold mineralization has been found in Devonian in Fengtai, Xicheng and Lixian basins, with similar types, but obvious differences. Except Fengtai Baguamiao and Wang Shuang, the main mineral deposits are distributed in Lixian area, such as Jinshan and Ba Li. In Xicheng and Fengtai basins, gold deposits mainly occur in clastic rocks in the upper part of lead-zinc deposits, that is, timely spotted sericite phyllite. In contrast, the Bali Group (D2lb) is mainly clastic rocks of turbidite facies, in which neither lead-zinc mineralization nor jet sedimentary rocks have been found.

According to the metallogenic environment, ore-hosting structure and metallogenic geochemical conditions, SEDEX lead-zinc deposits can be roughly divided into two types (see Chapter 6 for details). The first type of lead-zinc deposits, represented by Changba-Lijiagou, are poor in gold and silver, and the associated gold in lead-zinc ores of this type of deposits is generally less than 0. 1g/t, while the first type of lead-zinc deposits, represented by Xitieshan, are rich in gold and silver, and the associated gold in ores is generally greater than 1g/t.

Due to the in-depth understanding of the relationship between gold mineralization and SEDEX lead-zinc mineralization, a lot of geological prospecting work has been carried out in Xicheng, Fengtai, Limin, Langshan, Liaoning Liaohe Group, Ji 'an-Laoling, the northern margin of Qaidam Basin, Maizi Devonian Basin in Xinjiang, the periphery of Fankou, Guangdong and Liao Yan subsidence zone. Except for a few areas, the degree of gold mineralization in most areas is similar to other local strata.

The geological and geochemical study of Xiaogouli-Sanyeba gold deposit and its surrounding lead-zinc deposits in Xicheng Basin shows that although the gold deposits were formed in the late Indosinian period, the lead-zinc deposits, especially those in the southern belt, were also strongly reformed or rebuilt in the late Indosinian period, with the participation of large-scale hydrothermal activities. At least it can be considered that even the adjacent lead-zinc deposits and gold deposits still had relatively independent evolution processes in the late Indosinian period. Of course, this does not rule out that some important factors affecting gold mineralization are closely related to the evolution of SEDEX-type lead-zinc deposits in this area, which leads to the close spatial distribution of Devonian SEDEX-type lead-zinc deposits and gold deposits in Qinling Mountains.

The relationship between gold mine and lead-zinc mine is indirect, and its compactness is fragile. In Xicheng and Fengtai basins with lead-zinc deposits, most of the gold deposits are concentrated in the Middle Devonian phyllite in the upper part of the lead-zinc deposits, while the sedimentary environment and structural characteristics of the foreland sediments of Bali Group in Lixian basin are different from those of the argillaceous phyllite in the upper part of Xihanshui Formation. In the past, there was a strong view that Xicheng and Fengtai lead-zinc mine concentrated areas should pay attention to the search for gold deposits, and there was a potential for searching for SEDEX-type lead-zinc mines in Li (Min) county, where gold deposits were widely distributed (Wang,1998; Zhang Fuxin, 1996). Many years of prospecting practice and regional mineral geology research show that the geological characteristics of the two basins are very different, and the gold deposits and lead-zinc deposits may not be necessarily related in time, space or genesis. Xicheng SEDEX-type lead-zinc mineralization concentration area has the metallogenic conditions of fine disseminated gold deposits, but the SEDEX-type lead-zinc deposits around Zhongchuan rock mass in Lixian County do not have the metallogenic conditions.