1. Geological characteristics of Lengjiaxi Group
Lengjiaxi Group is the oldest stratum discovered so far in this area, which can be compared with Fanjingshan Group in eastern Guizhou, Bosi Group in northern Guangxi, Shuangqiaoshan Group in northeastern Jiangxi, Shuangxiwu Group in northern Zhejiang and Shangxi Group in southern Anhui. Lengjiaxi Group in this area is mainly distributed in Anhua and Taoyuan, with sporadic distribution in Changde, Shimen, Zhijiang and Guzhang, and the rest of the vast areas are not exposed.
Lengjiaxi Group can be divided into five rock groups from bottom to top, and all rock groups are in integrated contact.
The first rock group is mainly light gray and light gray green banded silty slate, mixed with a lot of slate, or interbedded with light metamorphic fine sandstone, siltstone and lenticular siliceous dolomite, limestone, tuffaceous dolomite or banded, and occasionally lenticular glutenite. The maximum thickness of this group is greater than 24 10m.
The second rock group is mainly composed of light gray, light gray green, light gray yellow and cyan silty slate, banded silty slate and chlorite sericite mother rock, with light metamorphic quartzite siltstone, fine sandstone and a small amount of siliceous sericite silty slate, light metamorphic tuffaceous fine sandstone and feldspar quartzite fine sandstone. The maximum thickness of this group is 5100m.
The third rock group is dominated by light gray, light gray green and blue gray banded siliceous sericite and banded siliceous sericite, with sericite, light metamorphic fine sandstone, siltstone and tuffaceous light metamorphic fine sandstone, siltstone and silty slate. The maximum thickness of this group is 6161m.
The fourth rock group is light gray, blue gray, gray green sericite, banded silty sericite, banded slate, chlorite sericite, sandy slate, slightly metamorphic fine sandstone, siltstone, tuffaceous siltstone and sandy slate. The maximum thickness of this group is 8447 meters.
The fifth rock group is gray-green, light gray and light gray metamorphic fine sandstone, mudstone and sand slate with a small amount of spotted slate. Sandstone mixed with shallow metamorphic feldspar timely sandstone block, calcareous sandstone block, and occasionally slate mixed with rhyolitic dacite tuff block. The maximum thickness of this group is more than 5257 meters.
Wang Ziqiang et al. (1986) divided Lengjiaxi Group into two sedimentary types. ① Island arc-turbidite sedimentary type. The distribution of this type is limited, and it is only found in Shizuitang area of Yiyang. The obvious sedimentary characteristics are the development of Ma Bao sequence, the development of basic volcanic rocks (Koma upgraded basalt) on the horizon, and the clastic materials all contain a lot of tuff. Its main body is muddy turbidite deposit. This sedimentary type was basically the product of island arc zone in Mesoproterozoic. ② Sedimentary types of turbidite in back-arc basin. Most areas in western Hunan belong to this sedimentary type. Ma Bao sequence in this kind of deposit is well developed, and the lithology is sandy muddy turbidite rich in tuff, and no associated volcanic rocks are found. This type shows relatively stable sedimentary characteristics of back-arc basin.
To sum up, Lengjiaxi Group is a set of flysch formation composed of shallow metamorphic fine clastic rocks, argillaceous rocks and tuffaceous fine clastic rocks, and it is an important part of Proterozoic gold-antimony-tungsten ore-bearing formation in western Hunan.
2. Geological characteristics of Neoproterozoic Banxi Group
(1) The research history and present situation of Banxi Group.
For many years, there has been controversy about the age and stratigraphic correlation of Banxi Group. Because its meaning has changed several times, people's understanding of its meaning has been confused. Since the 1980s, Professor Xu Jinghua and others have put forward the concept of "Banxi Mixed", which makes Banxi Mixed more noticeable, and the arguments and differences become more and more intense.
"Banxi System" was initiated by Xiaoqing Wang in 1936 in the exploration of antimony deposits in Banxi Village (now Taojiang County), about 60 kilometers southwest of Yiyang County, Hunan Province. In the geological record of Changchang area written in the same year, the age of Banxi Group was designated as CAMBRIAN. In fact, it includes the upper part of the present Wuqiangxi Formation and the Sinian Nantuo Formation, while the lower part of the present Wuqiangxi Formation and Madi Formation are called the Sinian Majitang System in this paper. Later, the age was determined several times, either before or during the Sinian period. 1958, the 4 13 geological team of Hunan Bureau of Geology and Mineral Resources first discovered the angle unconformity in Banxi system in Yuanling and Taoyuan areas, and named it "Wuling Movement". From this, the strata below the unconformity surface are called Lengjiaxi Group, while the strata above the unconformity surface are still called Banxi Group. In the same year, Wang Henian of Nanjing University also discovered this kind of angular unconformity in Taoyuan and Anhua, named it Dongan Movement, and named the stratum above the unconformity as Dongan Group. 1962, the Precambrian Stratigraphic Working Group of Hunan, Guangxi and Guizhou called Banxi Group an upper subgroup on the unconformity surface. According to the characteristics of structural lithofacies belt, the Banxi Group was divided into strata zones for the first time, with Madiyi Formation and Wuqiangxi Formation in the north, Lalan Formation, Qingshuijiang Formation and Jiangkou Formation in the south, and unified into Cangbang Formation and Lalan Formation. In the same year, the compilation of the stratigraphic table of Central South China was also called Zhongbanxi Group, and the Jiangkou Formation was classified as Shangbanxi Group. 1965, Nanjing University Zuo proposed the viewpoint of the southern part of the Sinian system, taking the unconformity surface as the bottom boundary of the Sinian system, so the Banxi Group was classified as the lower part of the Sinian system. 1979, Banxi Group was divided into Madiyi Formation and Wuqiangxi Formation in Hunan Strata by Hunan Institute of Geological Sciences, belonging to Proterozoic. 1986, the regional investigation team of Hunan Bureau of Geology and Mineral Resources divided Banxi Group into Madiyi Formation (or Gaojian Formation) and Wuqiangxi Formation (or Mo Bin Formation) in Hunan stratum, belonging to Neoproterozoic.
Because of the controversy over the division and age attribution of Banxi Group, people have different understandings of the meaning of Banxi Group. The meanings of Banxi Group used in previous literatures can be summarized into three categories: the first category refers to the "Dabanxi Group" distributed in a strip shape, including Danzhou Group, Chujingshan Group (Guizhou), Banxi Group, Lengjiaxi Group (Hunan), Shuangqiaoshan Group (Jiangxi), Shangxi Group (Anhui), Shuangxiwu Group, Baisha Group (Zhejiang) and Lou Group. The second category refers to Banxi Group, Xiajiang Group and Danzhou Group in Hunan, Guizhou and Guangxi, which were recognized by Precambrian stratigraphic workers in Hunan, Guizhou and Guangxi in the 1970s. The third type is a Neoproterozoic lithostratigraphic unit in Xuefeng-Wulingshan area, which consists of Madiyi Formation and Wuqiangxi Formation. This is the Banxi Group redefined in recent years according to the principle of multiple stratigraphic division, and it has been accepted by most scholars.
With the proposition of Professor Xu Jinghua's geotectonic view of South China, the question of whether Banxi Group is Precambrian lithostratigraphic unit or ophiolite melange in Mesozoic collision orogenic belt appears. This is the most controversial issue in the study of Banxi Group in recent ten years. At present, there are three main understandings. The first view is that Banxi Group is the pre-Sinian basement, which is the traditional view and the opinion of most people at present. The Banxi Group here still refers to the banded Dabanxi Group. The second view is that Banxi Group is a ophiolite melange formed by Mesozoic collision orogeny. This was put forward by Professor Xu Jinghua when he studied the tectonics in South China since 1980s. Recently, he reinterpreted it as a pre-Devonian proliferative complex. The Banxi Group here also refers to the original banded Dabanxi Group. The third view holds that the Banxi Group redefined according to the principle of multiple stratigraphic division is a set of normal sedimentary strata, not a mixed rock. The next set of metamorphic rock series containing ophiolite exotic rocks in Banxi Group is a set of ophiolite melange, which needs further demonstration. However, they believe that even if this is a set of ophiolite melange, it is the product of pre-Sinian, not the result of Mesozoic collision orogeny.
Due to the different understanding of the meaning and stratigraphic nature of Banxi Group, there are also great differences in the understanding of the age attribution of Banxi Group at present, which can be roughly summarized into the following three viewpoints.
1) "Dabanxi" and "Xiaobanxi" before Sinian are both representative views. The most powerful evidence is that Banxi Group is unconformity under Sinian.
2) The Mesozoic melange series was put forward by Professor Xu Jinghua when he was studying the tectonics of South China, and it is also the most controversial point at present. They believe that: ① the reason why the Banxi Group melange belongs to the pre-Sinian system is entirely due to two wrong assumptions, that is, when the melange contacts the Sinian system, it is assumed to be the unconformity overburden of the Sinian system, and when the melange contacts the younger strata, it is assumed to be the fault contact; ② Strata older than Sinian do exist, but they are pre-Sinian deposits of Yangtze block, not Banxi melange; ③ Banxi melange may contain pre-Sinian exotic rocks, but the final gathering age of Yangtze and South China is Mesozoic. For the last point, they put forward the following evidences: ① Structurally, a series of flying peaks and structural windows are defined in the distribution area of Banxi Group, which are considered to be the result of Mesozoic collision orogeny; ② Chronologically, "the isotopic age of the melange ranges from Precambrian to Mesozoic"; ③ Stratigraphically, "Banxi melange is in contact with all rocks in Yangtze block from Precambrian to Mesozoic"; ④ Exogenous ophiolite blocks exist in Banxi melange. Recently, they interpreted "Banxi complex" as a complex wedge that proliferated before Devonian. After Devonian, accretionary complex was no longer formed due to the existence of sedimentary pad, but the final collision between Yangtze and South China occurred in Mesozoic.
3) It is considered that the original banded Dabanxi Group is not the same stratum, and its parts should be analyzed in detail. Rodgers thinks that there are two sets of "Banxi Group" as the basement of Yangtze block and tectonic melange, the former belongs to pre-Sinian and the latter belongs to Mesozoic. The core idea of this view is that the original banded Dabanxi Group is not the same stratigraphic body, which has been accepted by most scholars in China.
In recent years, with the rapid development of isotope testing technology, a number of isotopic age data about equivalent horizons of Banxi Group and Lengjiaxi Group in South China have been obtained (table 1). Judging from the results of these data, their age belongs to the pre-Sinian period.
Table 1 Correlation between Banxi Group and Lengjiaxi Group and corresponding strata in South China
Liu Haichen and Zhu Bingquan (1994) systematically collected lead isotope samples of tuffaceous slate of Wuqiangxi Formation, purplish red slate of Madiyi Formation, slate of Lengjiaxi Formation and phyllite of Banxi Group during their field investigation in Jishou, Guzhang, Yuanling, Taoyuan and Yiyang in western Hunan, and obtained a set of well-fitted lead-lead isotope isochron ages. Their results show that the sample data of Madiyi group are scattered and cannot form Pb-Pb isochron age. Wuqiangxi Formation has formed a good isochron with the age of (1335124) Ma (r = 0.9945), and Lengjiaxi Formation has also formed a good isochron with the age of (6544). They believe that due to the low degree of metamorphism of the samples, the two groups of ages represent the formation ages of Banxi Group and Lengjiaxi Group respectively, both of which belong to Mesoproterozoic. These are also the two oldest strata measured so far in this area. By comparison, they think that the ages of Banxi Group and Lengjiaxi Group are about 65438±0.400Ma and 65438±0.700Ma, which may represent two important crustal formation events in South China respectively, and have certain universal significance. They also pointed out that the Lengjiaxi Group has a wide range of lead isotopic composition and high radioactive lead content, while the Banxi Group has a small range of lead isotopic composition and low radioactive lead content. Based on this, it is considered that the lead isotopic composition does not support the view that Banxi Group was uplifted from South China block to Yangtze block, but Lengjiaxi Group may come from South China block.
There has been a long-standing debate about the semantics, nature and age attribution of Banxi Group. In recent ten years, most discussions have focused on Professor Xu Jinghua's view of South China's tectonics, which is also the key to understand the tectonic background and evolution of South China. The research results will also have a great influence on the understanding of metallogenic model, metallogenic age and deposit formation mechanism in large areas of South China. The debate continues, and the understanding will eventually be unified. The final settlement of the dispute seems to require long-term, solid and comprehensive basic geological work.
Looking at the academic thoughts of most domestic scholars, whether it is "Dabanxi" or "Xiaobanxi", the era belongs to pre-Sinian, and the Banxi Group redefined according to the principle of multiple stratigraphic division has been accepted by most domestic scholars. In view of the present research situation of Banxi Group and the author's work, it is considered that this attribution and division is reasonable.
(2) Geological characteristics of ore-bearing formation in Neoproterozoic Banxi Group.
According to the principle of multiple stratigraphic division, Banxi Group refers to a lithostratigraphic unit in Xuefeng-Wuling mountain area. This narrow sense of Banxi Group is only used in Hunan, including Madiyi Formation (or Gaojian Formation) and Wuqiangxi Formation (or Mo Bin Formation). Banxi Group can be compared with Xiajiang Group in eastern Guizhou and Danzhou Group in northern Guangxi.
Banxi Group, widely distributed in western Hunan, is the main body of ore-bearing formation in Xuefeng Mountain Au-Sb-W metallogenic belt. It is a set of thick-bedded assemblage composed of clastic rock, argillaceous rock, tuffaceous siltstone (slate), tuff and lava. The rocks are generally metamorphic, but the metamorphic degree is lower than that of greenschist facies. Taking the Qianyang-Xupu-Anhua deep fault as the boundary, the lithology between the north and the south is very different. To the south of the fault is a set of thick gray-green flysch formation, commonly known as "Green Banxi", and to the north of the fault is a set of red molasse-like rocks and flysch-like rocks, commonly known as "Red Banxi".
Hongbanxi is mainly distributed in Xinhuang-Zhijiang-Guzhang-Yuanling-Anhua area. Banxi Group in this area is divided into two groups: Xiamadiyi Group and Shangwuqiangxi Group, which are in integrated contact. Madiyi Formation of Banxi Group is in angular unconformity contact with the underlying Lengjiaxi Group.
Madiyi Formation is divided into three lithologic sections from bottom to top.
The first section is mainly gray-green massive or extremely thick shallow metamorphic conglomerate, glutenite and sandy slate. In some areas, the content of conglomerate decreased, and the phase changed into shallow metamorphic gravelly sandstone, gravelly siltstone or coarse sandstone. Near Guzhang Daxi, there is a layer of hematite in the rock of Madiyi Formation, but its thickness is extremely unstable, and it only extinguishes itself within a few kilometers. The sedimentary thickness of this section changes greatly, and the general trend is from thin to thick from north to south. The thickness of Siduping in Dayong, Shirongxi and Liaoyexi in Taoyuan is only tens of meters to several hundred meters, and Xupu and Anhua can be increased to several hundred meters.
The second section is mainly purplish-red light metamorphic silty slate, sandy slate and calcareous slate, with limestone and marl lens locally, and shallow slate in some sections is copper mineralization.
The third member is tuff, sandy slate, shallow metamorphic sandstone and tuffaceous siltstone. The sedimentary thickness of this section increases gradually from north to south and from east to west, and the content of volcanic materials increases gradually from north to south.
The thickness of Madiyi Formation is 1000~3000m, generally 1 100m, and some sections are less than 1000 ~ 3000m.
Wuqiangxi Formation is the most developed in Yuxiankou-Xihuangshan area of Zhijiang, with a thickness of 3170m ... It can be divided into four sections (from bottom to top): the first section is gray-green thick metamorphic feldspar timely sandstone, shallow metamorphic conglomerate and glutenite; The second section is gray and gray-green residual tuff mixed with metamorphic sandstone and slate; The third member is mainly gray-green slate and banded slate, with a small amount of metamorphic sandstone; The fourth member is interbedded with grayish green light metamorphic sandstone and slate, and locally metamorphic sandstone is mixed with metamorphic conglomerate.
Except for Wuqiangxi Formation in Yuxi Kou-Xihuangshan area, which is well developed, only the first and second members are generally preserved in other areas. Changde Taiyangshan, Cili Xikou, Dayong Siduping, Huayuan Minle Line, the first section is purplish red medium-thick layered medium-coarse metamorphic chronological sandstone, with a small amount of gray-green metamorphic feldspar chronological sandstone, sandy slate and metamorphic tuffaceous rock; The second section is fine-medium grained metamorphic feldspar timely sandstone and sandy slate, containing tuffaceous rocks, with a thickness of 600 ~1000m. In the south, it reaches Liaoyexi in Taoyuan, Lushuipu in Yiyang, Hunan Slope in Anhua, Songxi in Yuanling and Wuqiangxi, and its thickness has increased to more than 2,000 meters. A large number of gray-green and gray-yellow variegated rocks are mixed in the purple medium-thick metamorphic fine-grained feldspar timely sandstone in the lower part, and the fine clastic rocks such as sandy slate and slate are also obviously increased, with more tuffaceous rocks in the upper part. Especially in Liaoyexi, Xiangpo and Xi Kui, there are a large number of residual tuffs and residual tuffs, accounting for 265,438+0% ~ 365,438+0% of the whole rock assemblage.
In a word, Wuqiangxi Formation in this area is rich in coarse clastic rocks, with simple lithology and abundant pyroclastic materials in some areas. Zhijiang-Xinhuang area has a large sedimentary thickness, while other areas range from several hundred meters to 2000 meters from north to south.
"Green Banxi" is mainly distributed in Qianyang, Huitong and Tongdao. From bottom to top, it is divided into Gaojian Group and Mo Bin Group.
The horizon of Gaojian Formation is equivalent to Madiyi Formation in Hongbanxi area. Due to the difference of sedimentary characteristics, the Gaojian Formation of 1980 was newly established in Hunan. The formation can be divided into four lithologic sections from bottom to top: the first section is mainly gray and gray-green light metamorphic sandstone and fine sandstone, mixed with light metamorphic siltstone, sandy slate and slate; The second section is dominated by gray, gray-green, purple-gray slate and silty slate, with limestone and dolomite thin layers or lenses, and the copper mineralization is weak; The third section is mainly gray-black and dark gray carbonaceous slate, with gray-green, gray-white, gray-yellow sandy slate and slate; The fourth section is grayish green and grayish yellow light metamorphic sandstone, tuffaceous sandstone, sandy slate and slate interbedded.
The thickness of this group increases gradually from north to south, from 2000 meters to 4000 meters, and the volcanic material also increases gradually from north to south.
Mo Bin Formation is similar to Wuqiangxi Formation in horizon and was discovered by Hunan Dispatching Team 1980. This formation is well developed in Tongmobin and Tuokou areas, with a thickness of 4,757 meters, and can be divided into three lithologic sections: the first section is gray-green and gray thick metamorphic feldspar timely sandstone, metamorphic miscellaneous sandstone, sandstone and metamorphic conglomerate; The second section is gray-green, gray and light purple residual tuff and residual tuff, mixed with metamorphic sandstone and slate; The third member is gray, light gray green sandy slate and banded slate, with a small amount of metamorphic sandstone.
According to the sedimentary geotectonic background and stratigraphic sedimentary characteristics, Banxi Group in this area can be roughly divided into two sedimentary types (Wang Ziqiang et al., 1986). (1) Coastal shallow water type and turbid heavy type. This sedimentary type is mainly distributed in the south of Shimen-Dayong line and north of Qianyang-Xupu-Anhua fault, which actually includes most areas where Hongbanxi is distributed. The Madiyi Formation in this area has a great phase change, and the northern part of Changde is dominated by purple sand argillaceous deposits in coastal and shallow seas, with argillaceous dolomite in the lower part and increasing argillaceous upward. Turbidite appears in the upper part of the southern stratum, and then the level of turbidite gradually decreases to the south. In Madiyi area of Yuanling, there is a set of debris flow-induced conglomerate at the bottom of Madiyi Formation, which is turbidite deposit upward, which may represent the root channel facies deposit in the early stage of turbidite fan, and then turbidite deposit appears with the retreat of overlying fan. Gray-green tuffaceous turbidite only appears at the top of Wuqiangxi Formation in Changde area, and the level of turbidite gradually decreases to the south, similar to Madiyi Formation. ② Turbidity sedimentary types. It is mainly distributed in the south of Qianyang-Xupu-Anhua fault, and the distribution area of "Qingbanxi" mostly belongs to this sedimentary type. Both Gaojian Formation and Huang Bin Formation are gray-green tuffaceous turbidites. The thickness of "Green Banxi" in this area suddenly increased to 4000 ~ 5000 m, which represents the transitional type of back-arc basin deposition.
Generally speaking, "Hongbanxi" is a near-source turbidite with low maturity, and "Lvbanxi" has the characteristics of distant turbidite deposition. Banxi Group has good lithology, lithofacies and formation combination, and has become an important part of Proterozoic ore-bearing formation in western Hunan.