Devonian system in west Qinling

Devonian in West Qinling is widely distributed, all of which are marine sediments, consisting of shallow-sea basin-type argillaceous rocks and carbonate rocks on the active-quasi-active continental margin, and the sedimentary center tends to migrate from south to north. Carbonate rocks in areas with weak deformation and metamorphism often contain Devonian standard fossils such as brachiopods, corals and bamboos. According to the research division, it is divided into four Devonian basins (regions): Xicheng (Xihe-Chengxian), Lixian, Fengtai and Zhashan, which basically follow the existing customary names and refer to the new division plan of Qinling orogenic belt. Due to the collision orogeny between the Yangtze block and the North China block, the Devonian system in the Middle Qinling suffered from strong east-west shear deformation shortly after deposition, and nearly east-west folds generally appeared, and even rheology and schistosity replacement between argillaceous rocks and carbonate rocks appeared. The original sedimentary surface (S0) is largely replaced by metamorphic schistose, which is difficult to identify, and the workload of restoring the sedimentary surface (S0) and stratigraphic sequence before deformation is extremely huge.

(1) Xicheng District

Xicheng area is sandwiched between Lixian-Mayanhe fault and Dangchang-Liangdang fault, and the exposed Devonian is the Western Han Water Group. According to the traditional division scheme, flysch-carbonate formation assemblage distributed in Minxian-Tanchang area is also regarded as a part of Xicheng basin, and its stratigraphic development characteristics will be described and discussed in the relevant chapters of Daijiazhuang lead-zinc mine. There are many schemes to further divide water groups in the Western Han Dynasty, such as Gansu Bureau of Geology and Mineral Resources, Shaanxi Bureau of Geology and Mineral Resources, Qin Feng (1976) and Zhu Weiyuan (1989). The recent representative schemes are: 1∶50000 geological mapping scheme and Wang Jilei (1996) scheme of Gansu Nonferrous Geological Exploration Bureau. The former divides the strata in the area into Silurian Wujiashan Formation, Hai jiusan Formation, Anxigou Formation, Anjiacha Formation, Guangjinba Formation, Wujiaba Formation, Zhugesi Formation, Dongshan Formation and Shuanglanggou Formation from bottom to top. Considering that the geological work of lead-zinc mines and gold mines in Xicheng area is basically based on the grouping scheme of Wang Jilei et al. (1996), which is simple and easy to operate, this book also uses this system to divide the Xihanshui Formation into Wujiashan Formation, Anjiacha Formation, Xihanshui Formation and Dongshan Formation from bottom to top.

1. Wujiashan Formation (D 1w)

It is mainly developed in Wujiashan area of Chengxian County, with a strip shape of 30km from east to west and 5 ~ 8 km from north to south, which constitutes the core of Wujiashan anticline and has a deep metamorphic degree, including Daliuba section in the lower part and Linkou section in the upper part. Daliuba member is dark gray carbonaceous quartz schist and calcite quartz schist, the lower part of Linkou member is extremely thick layered metamorphic sandstone or calcite metamorphic sandstone with sericite phyllite, and the top part is white medium-thick layered marble, that is, Haijiushan marble, which is distributed around the core of Wujiashan anticline, but developed from the north side to the south side. Wujiashan Formation was classified as Silurian in the 1∶50000 regional survey report, but in recent years, Zhang Chuanlin et al. (1998) and Yang Zhihua et al. (2000, 2002) classified it as Precambrian because they obtained a batch of paleoage data from it.

2. Anjiacha Formation (D2a)

It is mainly distributed outside Wujiashan Formation, and locally in Lixian-Tanchang area, where biogenic limestone, slate and sandstone are mainly developed. In the past, Anjiacha Formation was considered as one of the main ore-bearing beds of jet deposition in Xicheng Basin. In the1∶ 50,000 mineral geological map of Xicheng area completed in 1990s, Anjiacha Formation is widely distributed, including not only low metamorphic clastic rocks and crystalline limestone in Miaogou, Jiaogou and Anjiacha, but also vast areas in Huangzhuguan, Changba-Xiangyang Mountain, Tianzishan-Bijiashan-Barrow and other places. Because the metallogenic characteristics of Tianzishan-Bijiashan-Barrow deposit are similar to those in the southern belt, and the metamorphic degree of the host rocks is low, the assemblage and biological assemblage of the host rocks are similar to those of the Xihanshui Formation in Dengjiashan-Jianyagou and other places. Wang Jilei et al. (1996) divided the strata in these areas into Xihanshui Formation, but the metamorphic ore-bearing strata in Changba-Lijiagou-Xiangyangshan area are still regarded as Anjiacha Formation.

Based on the comprehensive analysis of the ore-bearing strata in Changba-Xiangyangshan area and the comparison with the lead-zinc deposits in the southern belt, it is considered that the former Anjiacha Formation in Changba-Xiangyangshan area is comparable to the Xihanshui Formation in the southern belt, and the difference between it and the Xihanshui Formation in the southern belt is more caused by the heterogeneity of thermal metamorphism in this area, so the Anjiacha Formation in this area should be merged into the Xihanshui Formation (see Chapter 4 for details). SEDEX-type (layered) lead-zinc deposits are all incorporated into the Xihanshui Formation, that is, the crystalline limestone and sericite phyllite originally used as Jiaogou and Miaogou Anjiacha Formation are all included in the Xihanshui Formation. At the same time, considering the continuity with the previous works, this book retains the division of this group, but thinks that the distribution range of Anjiacha Group is small.

3. Xihanshui Formation (D2x)

This formation is widely distributed in Xicheng basin. The lower part of the lower member is sandstone and phyllite mixed with limestone, and the upper part is biolimestone and micrite limestone. The lower part of the upper member is siltstone and argillaceous phyllite mixed with limestone, the middle part is siltstone and micrite limestone, and the upper part is siltstone, phyllite and phyllite. Xihanshui Formation is the host bed of SEDEX-type lead-zinc deposit, and the lead-zinc body mainly occurs between the lower biolimestone and the upper argillaceous phyllite. The main deposits are Changba-Lijiagou, Bijiashan, Barrow, Jianyagou and Dengjiashan.

4. Dongshan Formation (D3d)

The lower part of the lower member is sandstone, siltstone and slate, the middle part is slate and silty slate with argillaceous limestone, and the upper part is interbedded with bioclastic limestone and silty limestone. The upper member is argillaceous slate and silty slate. Location: Dongshan in Xihe County and Zhayu River in Tanchang County.

Anjiacha Formation is shallow sea facies, and Xihanshui Formation includes shallow sea sand bar facies, carbonate platform facies and shallow sea facies. Du (1997) believes that Xicheng area is a small carbonate platform with marginal reefs on both sides, which was formed on the basis of Wujiashan uplift and can be divided into five facies areas from north to south: the northern open shelf area (the area north of Dongshan-Changba); Dongshan-Changba reef belt; Social security mountain-Xiangyang mountain lagoon; Yeshuihe-Xiangyangshan Reef and Hill Zone; Nankai underground shed area (south of Shui Ye River-Bijiashan). Dongshan Formation is exposed in the west and north of Xicheng Basin, indicating that the uplift of Devonian region led to the retreat of the ocean from east to west.

(2) Lixian area

The exposed stratum in this area is Bali Group, which is a part of the former Shujiaba Group (group) after the disintegration of Qinling Group and Li Ziyuan Group (Li Jian Zhongzhong, 1996). Baliqun is located in the north of Lixian-Mayanhe fault, in the south of Niangniangba-Li Ziyuan fault, facing the North Qinling Mountains, with a melange belt in the north. In recent years, due to the discovery of early Paleozoic molecules and the obvious difference from the metallogenic assemblage of the eastern Bali Group, Mengjiagou Group was independently established in the thick clastic rocks of the original Bali Group in the west of Zhongchuan pluton. A large number of middle and late Devonian fossils have been found in Bali Group, which are covered by Carboniferous, and the age is generally defined as middle and late Devonian. According to Li Jianzhong (1996), this book defines the age of Baliqun as Upper Devonian. The main body of Ba Li Group is a set of thick deep-water and semi-deep-water turbidite deposits, which reflects the rapid sedimentary environment and typical flysch formation. Ba Li-Jinshan area is divided into four continuous sedimentary lithologic sections from bottom to top.

The first lithologic member (D2lb 1): gray-gray-green thick layered sandstone, with slate or silty slate at the top and gray-gray-green sandstone at the middle and lower parts, with a thickness of 50-60m.

The second lithologic member (D2lb2): it is composed of gray-gray-green medium-fine grained thin-medium thick layered sandstone and siltstone mixed with silty slate, with gray-gray-green fine sandstone, siltstone, rare silty slate or slate in the lower part, gray-gray-green slate interbedded in the middle part and gray-green slate mixed with sandstone in the upper part.

The third lithologic member (D2lb3): gray-gray black slate or silty slate, with silty slate mixed with fine sandstone at the bottom, with sandy components gradually decreasing upward and argillaceous and calcareous components increasing, and slate and calcareous slate mixed with thin sandstone at the upper part.

The fourth lithologic member (D2lb4): the bottom is gray-gray black marl mixed with calcareous slate, and the middle and upper parts are light gray banded marl, crystalline limestone, sandy limestone and micrite limestone.

The depositional system of Bali Group includes gravity flow deposition (turbidity current deposition) and normal deposition. Most people think that it belongs to the deep sea-semi-deep continental slope-submarine fan sedimentary system, and the paleocurrent is fan-shaped from northeast to southwest, and the direction of paleoslope is basically the same as that of paleocurrent, which is just the opposite of the Xihanshui Formation (Sun Shengli et al., 1995). Li Jianzhong (1996) identified four sedimentary facies according to the four lithologic sections of Ba Li Formation, namely, thick sandstone facies deposited by granular flow (D2lb 1), sandstone facies deposited by turbid flow (D2lb2), static cement facies (D2lb3) and static marlite facies (D2lb4).

Strictly speaking, the Dacaotan Group (D3dc) in southern Gansu does not belong to Lixian-Mayanhe Community, and is widely distributed in Min County, Lixian County and Huayangyu in Tianshui, Gansu Province, south of Wushan-Deqian Fault and north of Shujiaba-Gucheng Fault. Gravel is developed at the bottom, which is semi-circular to round, with poor sorting and sandy, mainly composed of timely sandstone and limestone. The middle and upper part is purplish red sandstone. At present, most people think that Dacaotan Group has the characteristics of molasse formation (Ning Xichun et al.,1994; Du,1995; Zhang Guowei et al., 200 1) is considered as the most direct and important evidence of Caledonian movement in North Qinling.

(3) Fengtai area

Devonian is the main ore-bearing stratum in Fengtai ore concentration area, with an exposed area of about 1550km2, sandwiched between Shangdan fault and Jiudianliang-Banyanzhen fault. It can be divided into Middle Devonian Macaogou Formation, Gudaoling Formation, Upper Devonian Xinghongpu Formation and Jiuliping Formation.

1. Machaogou Formation (D2m)

Mainly distributed in Macaogou-Lvjiashan, Lianghekou-Erlangba, Yinjiagou and other places. The lithology is mainly thin-medium thick metamorphic feldspar chronological complex, metamorphic calcareous chronological complex and metamorphic chronological complex, followed by metamorphic siltstone and thin sandy limestone, with a small amount of sandy sericite phyllite, calcareous sericite phyllite and ankerite phyllite. , is a set of shallow metamorphic clastic rocks, mainly clastic rocks, supplemented by calcareous argillaceous rocks It is in overall transitional contact with the limestone of the overlying Gudaoling Formation, and most of the bottom is not completely exposed, but it is in parallel unconformity contact with the underlying Silurian only in the eastern part of Lujiashan. The thickness is greater than 506.42m.

At present, the discovered deposits in this horizon are mainly historical gold spots, with many old caves and abnormal development of gold, silver and arsenic. The mineralization type is metal sulfide chronological pulse type, and the main gold-bearing mineral is galena, which has a special mineralization type.

2. Gudaoling Formation (D2g)

It is mainly distributed in Sujiagou-Huangbaiyuan, Shuanghezi-Guanyin Gorge, Lianghekou-Erlangba, Mogou-Daheigou and Qiandongshan-Shuibaigou, and some sporadic outcrops form the core of the anticline. The main lithology is medium-thin crystalline limestone, carbonaceous biolimestone, (containing) iron dolomite limestone, medium-thick crystalline limestone mixed with a small amount of calcareous sericite phyllite, ankerite phyllite, silty silty limestone, etc. , which is 654.25m thick. Due to the restriction of paleostructure and paleogeography, the lithology of southwest and northeast is different.

Southwest China: Limestone is pure and rich in biological fossils. Reef limestone is developed and thick, with unstable siliceous rock layer at the top. It can be divided into three parts: lower, middle and upper. The lower part is thin-medium thick crystalline limestone, containing a small amount of sea lily stems and coral fossil debris; The middle part is sandwiched with thin limestone (including biological fossils) and thin iron dolomite limestone; The upper part is interbedded with thin biological limestone and thin carbonaceous limestone, and the top part is unstable carbonaceous phyllite mixed with thin carbonaceous limestone, lead-zinc silicalite or ankerite silicalite.

Northeast China: Limestone has high sand content, local sandstones, small thickness and rich in biological fossils.

The overall change characteristics of the rock assemblage of Gudaoling Formation from southwest to northeast are as follows: the thickness of southwest is large, but the thickness of single layer is small; Rock composition is relatively simple, with less sand content and high calcium content; Fossils are rich and diverse. The overall thickness of northeast China is small, but the thickness of single layer is large; Rock composition is complex, with high argillaceous content and low calcium content. Fossils are few in content and single in species. At present, the main lead-zinc deposits (spots) found in this area are mainly produced in siliceous rocks at the transition position between the top and Xinghongpu Formation. In addition to the main elements such as lead and zinc, there are also beneficial components such as gold, silver, copper, cadmium, mercury and gallium. Lead-zinc deposits are mainly distributed in the west of Xihe River, with few in the east.

See Table 2-2 for chemical composition of limestone in Gudaoling Formation west of Xihe River, which is mainly carbonate with low sandy content.

See Table 2-3 for the element content of strata rocks related to lead-zinc mineralization. As can be seen from the table, the contents of elements related to mineralization in the area are all low, indicating that Gudaoling Formation has not provided material sources for mineralization, while the contents of ore-forming elements in the mining area are high, indicating that ore-forming materials mainly come from deep or basement, and mineralization is closely related to basement hot water jet deposition and tectonic-magmatic transformation.

3. Xinghongpu Formation (D3x)

Xinghongpu Formation is the most widely distributed and fully exposed stratum in Fengtai area. It is a set of shallow metamorphic fine clastic rocks, mainly composed of phyllite (sandstone in the northeast) and a small amount of thin limestone. According to lithologic combination, it can be divided into three lithologic sections.

(1) the first lithologic member (D3x 1)

The lower part is mainly composed of carbonaceous calcareous phyllite, carbonaceous sericite phyllite and calcareous phyllite, with a thin layer of (carbonaceous) limestone, and the two are partially interbedded; The upper part is mainly composed of light gray sericite phyllite from ankerite and silty phyllite from ankerite, with a small amount of ankerite phyllite containing green mud, and unstable marl and banded limestone with a thickness of tens of meters at the top. The transition section between the bottom and Gudaoling Formation is the lead-zinc ore-bearing horizon, and the middle-upper part is the gold ore-bearing horizon. The maximum thickness of this section is 1005m, and the minimum thickness is 120m, with a general thickness of 500~700m. It has a gradual transition relationship with the underlying Gudaoling Formation.

Table 2-2 Contents of Main Elements in the Rock of Gudaoling Formation in the West of Fengtai Area (wB/%)

Table 2-3 Contents of Trace Elements in Lead-zinc Ore-bearing Strata West of Xihe in Fengtai Area (wB/ 10-6)

(2) The second lithologic section (D3x2)

Generally, it is characterized by gray-green rocks, mainly composed of light green-gray chlorite-containing silty phyllite, chlorite sericite phyllite and silty sericite phyllite, with banded sandy limestone, ankerite phyllite and banded limestone at the top. The maximum exposed thickness is 950m, and the minimum exposed thickness is 130m, generally 300~600m.

(3) The third lithologic member (D3x3) is generally characterized by dark carbon-bearing strata. The top is carbonaceous phyllite mixed with thin sandstone, the upper part is carbonaceous phyllite, and the lower part is interbedded between carbonaceous phyllite and carbonaceous thin limestone or between carbonaceous thin limestone and thin crystalline limestone. The sand content in the north has increased. Maximum exposure thickness 1 120m, minimum exposure thickness 159m, generally 300~800m. It gradually transits with the overlying Jiuliping Formation.

Xinghongpu Formation is roughly bounded by Xiba rock mass-Kongguan-Anhesi line. The rocks in the southwest are mainly argillaceous, with relatively little silt and more calcium. Carbonate rocks are mostly thin limestone and argillaceous limestone, chlorite and ankerite are common, and there are many biological fossils. The sandy and silty components in the rocks in the northeast are obviously increased, while the calcareous and chlorite components are reduced, and the metamorphic degree is shallow, mainly sandstone slate. Biological fossils have also decreased significantly.

4. Jiuliping Formation (D3j)

It is the latest Devonian stratum in Central China, mainly distributed in Angou-Pogenjiao, Simaoling-Songshuwan, Tianchiliang-Zheliyuan, Guchahe-Shiziba and other places, and is the core stratum of syncline.

Generally speaking, it is a set of sandstone-sandy phyllite-silty carbonate rocks similar to flysch formation. Southwest China can be divided into four lithologic sections.

The first lithologic member (D3j 1): the middle and lower part is medium-fine grained feldspar sandstone mixed with sandy microcrystalline limestone, the middle and upper part is sandy phyllite mixed with microcrystalline limestone, and the top part is sandy microcrystalline limestone with a thickness of 485.5m, which is integrated on Xinghongpu Formation.

The second lithologic member (D3j2): the lower part is metamorphic sandstone, the middle part is sandy phyllite, and the upper part is sandy microcrystalline limestone. 416.4m thick.

The third lithologic member (D3j3): a rhythmic layer composed of feldspathic sandstone, silty phyllite and silty microcrystalline limestone, with a thickness of 546.4m.

The fourth lithologic member (D3j4): the lower part is metamorphic feldspar sandstone mixed with silty microcrystalline limestone, the middle part is metamorphic siltstone mixed with silty phyllite, and the upper part is silty phyllite mixed with thin layers of fine sandstone and silty microcrystalline limestone. Thickness: 156.2m (without top).

The degree of work in Northeast China is very low, and there is no detailed stratification. The characteristics of rock assemblage are similar to those in southwest China, but the sand content increases.

Devonian in the south of Fengtai area is distributed in the south of Wafangba-Jiangkou fault, and it is in contact with the underlying Silurian by detachment fault, mostly with the overlying strata by fault contact, and locally by integration contact. Affected by faults, the horizon is unstable and the deformation and metamorphism are obviously different. There are Dafenggou Formation (D2d) and Yanglinggou Formation (D2y), which are only distributed in the two wings of Yingmiaoping anticline in Jiangxi Province. Dafenggou Formation is a set of clastic rock assemblage dominated by schist, with carbonate rocks in some parts and sandstone in Macaogou Formation in the middle. Yanglinggou rock is a set of fine-grained limestone containing sand. The lithology is gray medium-thick layer-thin layer limestone, containing bioclastic, silty-microcrystalline limestone and algal limestone. It can be compared with the Gudaoling Formation in the middle. At present, no lead, zinc, gold spots and valuable anomalies have been found in the exposed areas of the two rocks, and the background value of ore-forming elements of the rocks is very low.

(4) Zuoshan area

Zhashui-Yangshan Fe-Ag-Cu-Pb-Zn-Au (Ni-Co) ore concentration area is located between Shangdan fault and Fengzhen-Yang Shan fault, which not only controls the boundary of the basin, but also controls the Devonian paleogeographic environment and its development and evolution law. The tectonic position belongs to the Hercynian fold belt in South Qinling, and it is one of the main hot water sedimentary basins in Devonian in Qinling. The exposed strata are mainly Niuerchuan Formation, Chigou Formation, Daxigou (Qingshiya) Formation, Xiadonggou Formation and Tongyusi Formation, which are a set of hydrothermal flysch strata with simple rock types and developed sedimentary rhythm. The main rocks are argillaceous rocks, clastic rocks and carbonate rocks, with few fossils, belonging to shallow-semi-deep sea turbidite deposits.

1. Niuerchuan Formation (D2n)

The upper part is mainly silty (fine) sandstone mixed with sericite and silty slate, the middle part is silty slate, sericite and calcareous sericite mixed with argillaceous limestone, and the lower part is missing due to fault dislocation. The exposed thickness is greater than 1757m, and the west of Daxigou in Zhashui is dominated by clastic rocks mixed with argillaceous rocks, with relatively many carbonate rock interlayers, and the bottom is conglomerate or gravelly calcareous sandstone. In the east of Wuwanggou, carbonate interlayer also increased obviously. Most of the bottom boundary is unclear, and some of them can be seen in parallel unconformity contact with CAMBRIAN-Ordovician. Fossils are scarce, and corals can be seen occasionally.

2. Chigou Formation (D2c)

The lower part is mainly hornfelted silty slate sandwiched with timely sandstone and siltstone, and the surrounding rocks are biotite amphibole and biotite timely amphibole; The upper part is mainly composed of calcareous silty slate, banded diopside amphibole mixed with feldspar, timely sandstone, argillaceous marble and skarn, and diorite porphyrite veins invading along east-west bedding are seen locally. On the whole, it is a set of shallow metamorphic fine clastic rocks-carbonate rocks, with a small amount of metamorphic andesite tuff in some areas, which is a copper-bearing layer.

3. Daxigou Formation (D2d)

The lower part is dominated by hornfel, hornfelted siltstone and silty slate, mixed with sandy limestone and crystalline limestone; The upper part is silty slate, silty limestone and marl. This rock series is the main host rock series of metal deposits in Zuoshan Basin. The thickness varies greatly, averaging about 1500m, and brachiopod and coral fossils are produced in the lower part.

4. Xiadonggou Formation (D3xd)

The lithology is slate, calcareous slate, marl mixed with siltstone, and the thickness is greater than1.300m.. Abundant spore fossils were found in Miaogou, east of Chefanggou, Zhashui, such as Gymnosporus and tuberose, and other late Devonian-early Carboniferous genera and species.

5. Tongyusi Formation (D3ty)

The lithologic combination is timely sandstone with sand slate and siltstone, with a thickness of 525 ~ 3000m.