Ⅲ-Dabie-Sulu Neoproterozoic phosphorus metallogenic belt (Dabie-Sulu Neoproterozoic continental margin sedimentary phosphorite metallogenic series)

Dabie Mountain is adjacent to Qinling Mountains in the west and Tanlu Fault in the east, which is a composite orogenic belt. The research shows that the Sulu area on the east side of the Tanlu fault was originally an eastward extension of Dabie Mountain. Dabie-Sulu area is the paleometamorphic block with the largest exposed area in China (Xu Zhiqin et al., 1997). Susong Group, Hong 'an Group, Feidong Group and Haizhou Group in Sulu area in the south of Dabie orogenic belt belong to phosphorus-bearing rock series.

Susong Group in Dabie orogenic belt refers to the phosphorus-bearing schist series distributed in Erlang River in the northwest of Susong and extending in the northwest direction. Susong Group can be roughly divided into two lithologic combinations: the first rock combination is bimodal, but mainly acidic metamorphic volcanic rocks, and the main lithology is dolomite albite schist, albite granulite, quartzite, amphibole and greenschist; The second rock assemblage is sedimentary metamorphic phosphorus-bearing rock series, the main lithology is dolomite schist, dolomite marble and phospholimestone, and there are graphite schist, limonite and talc schist in marble. Therefore, Susong Group is actually a set of volcanic sedimentary metamorphic formations composed of volcanic rocks, marble, clastic rocks and phosphorus-bearing rocks. The protolith of the basic rock component in the first rock combination is continental alkaline basalt, which may be the product of bimodal volcanic rocks formed by early rift. The structural background of the second rock assemblage is shallow platform facies and phosphorus-containing terrigenous clastic deposits, which are passive continental margin deposits in a stable environment (Xu Shutong et al., 2002).

Among the horizons corresponding to Susong Group, from west to east are Hongan Group (Hubei), Susong Group (Anhui), Feidong Group (Anhui) and Haizhou Group (northern Jiangsu), all of which contain phosphate deposits. Among them, there are Songyang Phosphate Mine, Huangmailing Phosphate Mine and Sifangshan Phosphate Mine in Hubei, Susong Phosphate Mine, Gaojian Phosphate Mine, Nanchong Phosphate Mine and Dahengshan Phosphate Mine in Anhui, and Xinpu Phosphate Mine and Jinping Phosphate Mine in northern Jiangsu, which constitute the concentrated area of Dabie Sulu Phosphate Mine and the metallogenic series of continental margin sedimentary phosphate deposits.

Geological characteristics of (1) deposit

See Table 5-9 for the deposit characteristics of this metallogenic belt.

Table 5-9 Geological Characteristics of Neoproterozoic Phosphorus Metallogenic Belt in Dabie-Sulu

(2) Examples of mineral deposits

1. Jinping (Haizhou style) phosphate rock

Jinping Phosphate Mine is located in the north of Jiangsu, south of Lianyungang 14km.

The mining area is located in the metamorphic rock area on the southern margin of Sulu orogenic belt, east of Tanlu fault and northwest of Huaiyin Xiangshuikou fault. This area is the eastward extension of Qinling-Dabie orogenic belt. The phosphorus-bearing rock series of Haizhou Group is unconformity covered on the East China Sea complex and distributed between Jinping-Hong Lan-Yanghe line and Xiangshui-Huaiyin fault. It is a set of clastic chemical sedimentary rocks formed in the marine environment of continental shelf.

In this area, NE-trending folds and faults are developed, and magmatism is intense, and it has experienced amphibolite facies-greenschist facies metamorphism and granitization. Stratigraphic division and phosphorus sequence are as follows (Table 5- 10):

Table 5- 10 Stratigraphic Division and Lithological Characteristics of Haizhou Group

Phosphate deposits occur in the phosphorus-bearing rock series of Jinping Formation in Haizhou Group (Figure 5-3 1), also known as "Haizhou-style" phosphate deposits. The ore-bearing layer is divided into upper and lower sections, and the lower section is the main ore-bearing layer with a thickness of150 ~ 400 m.

Figure 5-3 1 Geological Map of Jinping Phosphate Mine

(According to Zhang Qiusheng et al., 1984)

1- potash granite; 2- Xili potash granite; 3 eyeball gneiss; 4- Phosphorus-bearing rock series; 5- muscovite gneiss

Jinping Phosphate Mine is located in Nanping Mountain Area, Haizhou, Lianyungang City. Yishan potash granite gneiss is a semi-complete dome structure in the area, which constitutes the axis of the inverted anticline, and the dip end of the axis is the phosphorus-containing rock series of Jinping Formation of Haizhou Group. The phosphorite area is located on the two wings of the anticline and is divided into two parts: Dongshan and Xishan mining areas. The ore-bearing strata exposed in Dongshan mining area are mainly the upper member of Jinping phosphorus-bearing rock series, and the sequence is: (amphibole-bearing) dolomite granulite, garnet-bearing dolomite granulite, (phosphorus-bearing) marble with phosphorus limestone mixed with gray-green argillaceous calcareous mica schist, and the bottom is gray-green calcareous mica schist with white spots, which are in structural contact with each other. The ore body is more than 0/000 meters long/kloc-0, with a strike of 75 ~ 45 to the northeast and a dip of 50 to the southeast. The ore body is lenticular and multi-layered, and the coal seam is 30m thick and extends to-350m. The ore is fine-grained phospholimestone, which belongs to medium and low grade.

The ore-bearing strata exposed in Xishan mining area are mainly the lower member of Jinping phosphorite series, and the sequence is: dark green chlorite (mica) schist mixed with dolomite marble, phospholimestone, quartzite, graphite schist and manganese phosphate lens, with gravel dolomite schist in contact with Yishan granite gneiss structure at the bottom. Ore bodies generally extend hundreds of meters, and the thickness of the middle part is 10~20m ~ 20m, extending to -400m, which is not indicated. The ore body is inclined to the northeast with an inclination of 30 ~ 50. The ore is mainly composed of fine-grained phosphorite, manganese phosphorite and mica phosphorite with low grade.

* * * The mineral is manganese ore, which mainly occurs between mica phospholimestone and manganese phosphate rock in the shallow or middle part of phosphate rock layer. It is a small lens with a thickness of 0.5 ~ 6m. The mineral composition is mainly pyrolusite and a small amount of pyrolusite. Most of them are concentrated in the oxidation zone above 50 meters above sea level. Uranium in Jinping area is rich in associated components, mainly occurring in mica phospholimestone and phosphorus-containing marble, followed by marble, granite gneiss, mica schist and muscovite plagioclase gneiss.

Fan Jintao 200 1 suggested that the Rb-Sr isochron age of the Niu Shan granite gneiss in western Haizhou was 797 million years, which was formed in the extensional environment after the collision and orogeny of Sulu orogenic belt. It is revealed that the Sulu orogenic belt began to transform into cracking in the middle Neoproterozoic. This can be compared with 850 ~ 800 ma from convergence to pyrolysis in North Qinling (Jiang Changyi, 1998). They also show the convergence-splitting process of Neoproterozoic North China and Yangtze landmasses.

It can be inferred that in the early Neoproterozoic (1000 ~ 800 Ma), the Yangtze block collided with the North China block, forming a large-scale high-pressure metamorphic belt and orogenic and post-orogenic magma intrusion. Volcano-magmatism of granite gneiss series in Yi Shan cracked the supercontinent or active continental margin. A long arc-shaped narrow subsidence zone in northeast China was opened up, and the original rock was deposited as intermediate-acid volcanic-clastic rock series.

Haizhou rock series is a set of shallow marine pyroclastic rocks and carbonate rocks alternately deposited, and phosphate rocks are deposited at the same time. Due to multi-stage sedimentary cycle, the phosphorus-containing layer in this area is multi-layered. The deposit of phosphate rock is closely related to marble strata, especially impure marble. As far as thickness is concerned, the relationship between marble and phosphate rock is anti-fluctuating. According to the fact that phosphorite layer contains a lot of pyrite and carbonaceous mica schist containing apatite, it shows that phosphorite was formed under good reducing conditions in still water medium. Because of the similar sedimentary conditions of magnesium, manganese and phosphorus, dolomite and calcite bands often appear in phospholimestone, and there are black and white manganese phosphate rocks.

2. Huangmailing Phosphate Mine

The mining area is located in Dawu County, Hubei Province. The strata in the area are mainly Huangmailing Formation, Tiantaishan Formation, Qijiaoshan Formation and Tagang Formation of Neoproterozoic Hongan Group. At the bottom is the Paleoproterozoic Dabie rock group. Its upper part is covered by Cretaceous, Tertiary and Quaternary.

Huangmailing Phosphate Mine is located in the southwest of Daleishan dome. The core of the dome of Daleishan is composed of acidic volcanic rocks of Dabie rock group. The surrounding strata are Neoproterozoic Hongan Group, and the main lithology is dolomite schist, gneiss, garnet amphibole, dolomite schist, epidote amphibole, graphite-bearing schist, marble, granulite, quartzite, graphite schist, phosphorite and manganese soil layer. Phosphate rocks occur in Huangmailing Formation of Hongan Group. The core acid volcanic rock is the floor of phosphate rock, and there are many phosphate rocks around the core.

The phosphorus-bearing stratum of Huangmailing Formation is a set of accessory metamorphic rock series mixed with basic volcanic rocks, with several small sedimentary rhythms. Generally, there are two layers of phosphorite with a total thickness of 250-850m, which are mainly distributed around the dome and on the two wings of the anticline (Figure 5-32).

Figure 5-32 Geological Schematic Diagram of Huangmailing Phosphate Mine Area

(According to the Sixth Geological Brigade of Hubei Geological Bureau 1976)

The ore body is divided into two layers: the lower layer is a small-scale, large-change, discontinuous seam, about 1500m long and 2-6m thick, and the general content of P2O5 is11.72%; The upper coal seam is dominated by continuous layers, with a length of 2980m, a total thickness of 5 ~ 30m and a P2O5 content of 8.27 ~ 9.26%. The floor of coal seam is phosphorus-bearing granulite, and the interlayer is mainly marble, followed by mica schist, phosphorus-bearing granulite, phosphorus-bearing granulite and graphite schist. There are four natural ore types: manganese phospholimestone, shallow phospholimestone, banded granulite phospholimestone and granulite phospholimestone. The first two ores have high phosphorus content, and the mineral composition is mainly fluorapatite. The deposit type belongs to regional metamorphic sedimentary metamorphic phospholimestone deposit.

The phosphorite layer is layered and quasi-layered, which is closely related to carbonate rock and quartzite in space. Its assemblage consists of phosphorite layer, carbonate layer, quartzite, granulite (granulite) and mica quartz schist, often containing carbon and manganese. Locally interbedded with phospholimestone and quartzite, or transformed into carbonate rocks along strike dip zone. Most ore bodies are layered or quasi-layered, and a few are lenticular or lentil. It is mainly low-grade crystalline phospholimestone, and pure phospholimestone can be formed in some areas, with P2O5 content above 40%.

Phosphate deposit belongs to shallow marine sedimentary metamorphic phospholimestone deposit. It often contains manganese, which is high in some areas and can form manganese ore deposits or manganese phosphate deposits. Manganese has nothing to do with phosphorus, which is mainly determined by the original sedimentary environment.

Due to weathering, exposed iron-bearing manganese black-brown rocks, irregular tabular quartzite and sporadic exposed carbonate rocks, as well as the surrounding acidic volcanic rocks (granite gneiss) are the prospecting indicators in this area.

3. Discussion on metallogenic age of Dabie-Sulu phosphorite belt.

For a long time, Hong 'an Group, Susong Group and Feidong Group were not integrated in Dabie Group, while Haizhou Group was not integrated in Donghai Group, belonging to Paleoproterozoic. Therefore, the sedimentary metamorphic phosphate deposits in these strata have also been identified as Mesoproterozoic deposits.

In recent years, with the deepening of regional geological mapping and special research, a new understanding of the age of phosphate deposits in this area has been put forward.

Phosphate deposits in this area are concentrated in Susong Group and the same horizon, including Hongan Group (Hubei), Feidong Group (Anhui) and Haizhou Group (Jiangsu). Susong Group and its equivalent horizons are intermittently exposed from west to east in Lengpeng and Huangmailing in Dawu County, Hubei Province, Sifang Mountain and Tuanshangou in Huangpi County, Malong in Xishui County, Songyang Bridge in Wuxue City, Tarfan in Huangmei County, Liu Ping and Gaojian, and Nanchong in susong county, Anhui Province. Feidong to the east of the Tanlu fault and Jinping to the north of Jiangsu are also exposed.

According to the recent regional geological survey report of1:250,000 Taihu Lake sheet (2002) of Anhui Regional Geological Survey, Susong Group was in angular unconformity contact with Dabie Complex in the early stage, and then superimposed by tectonic action. Possible stromatolites have also been found in Susong Group in Wu Weiping, indicating that their age is newer than their original ownership (1:250,000 Taihu Lake Area Survey Report, 2002). The zircon U-Pb age obtained by Tong Jiafu et al. (1999) in the metamorphic volcanic rocks of Susong Group is 866±48Ma, indicating that the metamorphic-acidic volcanic rocks of the original Susong Group were formed in Neoproterozoic, and the phosphorite in them belongs to Doushantuo period (Tong Jiafu et al., 2000, 2002). Hou et al. (1995) obtained many microfossils in marble of Susong Group. According to Yan Yongkui of Nanjing Institute of Geology and Mineral Resources, it is considered that this assemblage is similar to the Sinian microfossil assemblage in Yangtze Stratigraphic Area, and its geological age is defined as Sinian.

In the regional geological survey of Hubei Province, it was found that the "Dabie Group" or "Tongbai Group" under the Huangmailing Formation (phosphorus-bearing layer) of Hong 'an Group, which was originally considered unconformity, were gneiss granite rocks. Although these granitic rocks have undergone strong ductile shear deformation, they still retain a good porphyritic granite structure and are actually deformed and metamorphic intrusions. The zircon U-Pb isochron age obtained by Wang et al. (2002) is 823Ma, and the zircon U-Pb age obtained by China Geo University is 858Ma, indicating that it belongs to Jinning rock mass.

Recently, in the survey of1:250,000 Macheng Sheet (2006,5438+0) conducted by Hubei Geological Survey Institute, blue-green algae fossils (identified by Professor Yu Suyu of China Geological University) were found in the marble of Qijiaoshan Formation of the former Hong 'an Group, which are common fossils in Sinian. The microfossil assemblage in some samples was determined as Late Sinian.

In northern Jiangsu, the Geological Survey Institute of Jiangsu Province disintegrated the former Haizhou Group and determined the deformed and metamorphic intrusions in the East China Sea complex. Xu Huifen et al. (200 1) determined the magmatic zircon U-Pb method of Yishan granitic gneiss. Pb-Pb isotopic age is about 820Ma, and it is considered that the protolith of Yishan gneiss was formed in Neoproterozoic Jinning period.

According to the above data, the Dabie-Sulu phosphorite belt was formed in the Neoproterozoic passive continental margin deposits.