The middle and lower reaches of the Yangtze River and its adjacent areas have experienced long-term geological structural evolution. Generally speaking, the tectonic evolution of the study area has experienced three main periods. The first stage is the independent development period of the Yangtze (North China and Cathaysia) landmass before Sinian, which formed its own distinctive basement and crustal structure. The northern and southern margins of the eastern part of the Mesoproterozoic Yangtze block experienced trench, island arc and basin collision, orogeny and continental proliferation. In the second stage, from the Sinian, the Yangtze block changed from an active period of caprock deposition to a stable period of caprock deposition, and extremely thick marine carbonate rocks and clastic rocks were widely accepted. The third stage is Mesozoic-Cenozoic intracontinental activation and transformation. The collision orogeny between Yangtze block and North China block began in Mesozoic Indosinian period, and the study area entered a period of intense activation, characterized by material exchange, adjustment and transformation at different levels of crust and mantle, forming a complex nappe structural system, extensive granite intrusion and volcanic eruption. During the evolution of tectonic environment in the above period, mineralization has many characteristics, but the most important metallogenic tectonic environment in this area is the collision between Mesozoic Yangtze block and North China block, and the intraplate subduction tectonic environment restricted by the late paleo-Pacific plate and Indo-Australian plate in eastern China.
1. Mineralization in the tectonic environment of the northern and southern margins of the Mesoproterozoic East Yangtze Block.
According to Wang and (1990), the Mesoproterozoic Wudang Group in the northern margin of the Yangtze block is equivalent to the island arc environment of the active continental margin, and the Shennongjia Group in the south is equivalent to the tectonic environment of the back-arc basin. During this period, the ancient Qinling ocean plate subducted southward under the Yangtze plate, causing strong volcanic activity, forming island arc volcanic rocks, and calc-alkaline volcanic rocks were widely distributed. In the middle of volcanic activity, a large number of acidic volcanic rocks (metamorphic amphibole tuff, Shi Ying amphibole, etc. The upper subgroup of Dangyuhe Formation in Wudang Group erupted, accompanied by silver and gold mineralization, forming a large Yindonggou volcanic hydrothermal deposit of silver, gold, lead and zinc. After that, the area continued to decline, and a set of flysch formations rich in sulfur, phosphorus and carbon were deposited in Yangping Formation. Volcanic eruption and mineralization of gold, silver and tellurium occurred at the same time, forming Xujiapo gold, silver and tellurium medium-sized volcanic deposit (Figure 2-26A). In Neoproterozoic (about 850Ma), there was a strong crustal movement in the northern margin of the Yangtze block, and the tectonic stress reversed, from compression to extension, forming a marginal rift trough, and volcanic rocks such as Yaolinghe Group and Zhangbaling Group erupted in this environment. Since then, the northern margin of the Yangtze block has entered a period of passive continental margin evolution.
The Mesoproterozoic Jiangnan ancient island arc developed in the southern and southeastern margins of the Yangtze block, and the formation and development of the ancient island arc were unbalanced. The western segment developed earlier, starting from the end of Proterozoic, that is, the formation of the flysch formation of the Bos Group and the formation of basic and ultrabasic rocks (the Sm-Nd isochron age of the volcanic rocks of Wentong Formation is 22 19Ma, Mao Jingwen, 1990). In the middle part of Lengjiaxi Group, Shuangjiaoshan Group and Shangxi Group, a set of argillaceous and tuff flysch formations is deposited, containing a small amount of spilite porphyry, and island arc volcanic rock assemblages are developed on the south side of the back tectonic environment, such as Tongchang Group in northeast Jiangxi and Shuangxiwu Group in northwest Zhejiang. The copper-bearing massive sulfide deposit of spilite porphyry system in Qiu Xi, Shaoxing occurs in Shuangxiwu Group. The following focuses on the ore source and enrichment mechanism of the metallogenic environment in the southeastern margin of the Yangtze block at that time:
1) The εNd(t)(t= 1000Ma) values of argillaceous rocks and tuffs in the Shuangjiaoshan Group and Shangxi Group in the back arc basin are -2.57 ~ 4.2 1, and their throwing points are close to chondrites on the εNd(t)-t diagram (Figure 2-27).
Fig. 2-26A Schematic diagram of mineral distribution in the tectonic environment of the active continental margin on the north and south sides of the Mesoproterozoic Yangtze block.
1-oceanic crust; 2- Ophiolite; 3- Shuangjiao Mountain and Shangxi Group; 4- Shennongjia Group; 5- Wudang Mountain Group; 6- Double Happiness Group 5; 7- basement of Yangtze block; 8— transition shell; 9-Volcanic hydrothermal arsenic-gold deposit; 10- marine volcanic Cu-Zn deposit; 1 1- mantle upwelling direction
Fig. 2-26B Schematic diagram of mineral distribution in passive continental margin tectonic environment from Sinian to Early Triassic in East Yangtze Block.
1-shallow platform facies carbonate rocks and clastic rocks; Double slump breccia; Sanpo marl; 4- Deep-water basin facies carbonaceous mudstone, siliceous rock and thin limestone; 5- continental basement; 6— Element migration direction; 7- sea level
Fig. 2-26C Schematic diagram of mineral distribution in the eastern part of Yangtze block during the period of intracontinental activation and transformation in Mesozoic and Cenozoic.
1- caprock; 2- Normal metamorphic basement; 3- Metamorphic rock basement; 4- Metamorphic intermediate basic igneous gneiss; 5-Metamorphic argillaceous gneiss; 6- Archean basement; 7- mantle-derived magma; 8- basic rocks; 9- Intermediate rock; 10-acid rock; 1 1-w.sn reservoir; 12- deep fault
2) The analysis of the content characteristics of Cu and other ore-forming elements in the Mesoproterozoic Jiangnan basement shows that Cu in argillaceous rocks and tuffaceous rocks tends to be dispersed and depleted, with Cu in argillaceous rocks being (23.8 ~ 32.7) × 10-6 and K.K.TureKian being (196 1). Although the spilite keratophyre series contains a high content of Cu and other elements, the content of Cu and other elements tends to decrease during the evolution from basic to neutral and then to acidic, while the Cu and Zn deposits in Zheyaoshan and Huoyanshan in Baiyinchang, Gansu Province occur in moderately acidic and basic-acidic sodium volcanic rocks with good differentiation.
Fig. 2-27 Isotope Evolution of Shallow Metamorphic Sedimentary Rocks in Northwest Jiangxi and Southern Anhui.
(According to Ling, 1990)
DM evolution line of depleted mantle; Evolution line of chondrite
3) The formation environment of marine volcanic massive sulfide deposits is related to the scale and intensity of submarine volcanic eruption or submarine ore-bearing hydrothermal solution. According to the rock characteristics of Shangxi Group and Shuangjiaoshan Group, a layer of volcanic lava with a thickness of nine meters only appears at intervals of tens to hundreds of meters or even longer, so it is difficult to enrich and enrich Cu and other ore-forming elements with such volcanic eruption intensity and form an industrial deposit of a certain scale.
4) Shuangxiwu Group (Tongchang Group) located in the island arc environment has a strong volcanic eruption intensity and a short time interval. A porphyry copper-bearing massive sulfide deposit with intermediate-basic lava and strong underwater eruption was formed in Qiu Xi, Shaoxing, Zhejiang.
5) Since Neoproterozoic (970-800 Ma), Cathaysian landmasses collided and spliced along Yangtze landmasses and Yangtze River, forming Fuchuan ophiolite suite (containing a small amount of chromite deposits) and northeastern Jiangxi ophiolite suite (Figure 2-26A). At the same time, a large-scale collisional cordierite plagiogranite base in Jiuling, Jiangxi, three cordierite granodiorites in Xiuning, Xucun and Shexian in southern Anhui, as well as the Daode Formation in the piedmont of Deng Jia Formation and the continental volcanic rocks of Puling Formation and Jingtan Formation in orogenic period were formed. By the end of Jinning, after the collision between Cathaysian block and Yangtze block was completed, Sinian began to accept caprock deposition.
2. Mineralization in the passive continental margin tectonic environment of the East Yangtze block during the sedimentary period of caprock.
The northern and southern margins of the East Yangtze block evolved into passive continental margins from Sinian, and entered a relatively stable period of 600-700 million years. During this period, thick marine carbonate rocks and clastic rock series were deposited. Its thickness is rare in other standard platforms in the world. This feature fully shows that the sedimentary process of the Yangtze block is in a tensile and relaxed stress environment, with the basement thinning and extensive and intense settlement. The thinning of basement crust mainly depends on stretching, which is the result of weathering and denudation at the bottom of crust and mantle denudation. The general characteristics of mineralization during the period of stable tectonic environment of passive continental margins on both sides are as follows:
1) has two relatively stable and unstable sub-environments and sedimentary rock series in temporal and spatial distribution. One is stable shelf slope deposits, mainly shallow thick limestone, dolomite and clastic rocks; The other is the rift valley deposit on the continental margin, which is mainly composed of carbonaceous mudstone, siliceous rocks and thin limestone, with submarine volcano or hot brine activity, showing a relatively active environment (Figure 2-26B).
2) Sedimentation is dominant in these environments. There are three main ways to supply metal elements: ① bringing them from land; (2) replenishment at sea; ③ submarine hot brine or volcanic source. Only when ore-forming elements are continuously supplemented and precipitated can a large-scale mineralized layer be formed in a favorable sedimentary environment. Through the above systematic study of sedimentary geochemistry of caprock, it shows that: ① the information of the latter two sources is very weak; (2) Even though terrigenous belts were dominant, in the early stage of sedimentary basin formation (Sinian, Cambrian and Ordovician), the basin was separated, and the supply of terrigenous belts was limited. The northern part is mainly from the North China landmass, while the southern part is mainly from the South China landmass.
3) The provenance of sedimentary caprocks in this area mainly comes from the weathering and denudation products of the adjacent continental crust, and the ore-forming elements such as Cu, Pb, Zn, Ag, Au, U, V, P, Mo and Sb migrate through weathering and denudation under supergene conditions and re-precipitate in the relatively reduced water body in the ocean basin. Part of the sulfur-tolerant heavy metal elements are trapped in the reduced sedimentary layer, and its enrichment mechanism is as follows: ① organic matter is reduced to form sulfur-tolerant precipitant S2-, which promotes its precipitation; ② The organism absorbs metal elements; ③ Organic matter or organic carbon adsorbs metal ions after death. Judging from the occurrence state of ore-forming elements in the caprock in this area, the first mechanism is dominant. Therefore, rich layers of heavy metals and other elements are formed in Z2, O3, P2 and other horizons (Figure 2-26B).
4) The analysis of provenance and enrichment mechanism of tectonic environment from early Sinian to late Triassic shows that it is unlikely that large-scale heavy metal sulfide deposits will be formed by sediments, submarine hot brine and volcanic hydrothermal solution in this area. However, their positive significance can not be ignored, that is, many colorful derived ore source beds are reserved for later mineralization.
In a word, the sedimentary period of the eastern Yangtze block from Sinian to Late Triassic is low tide period, sluggish period and dispersed period of mineralization of heavy metal elements such as Cu. However, everything in the universe is positive and negative, and so is the mineralization of the regional lithosphere. This period is the accumulation period of mantle energy, the adjustment and reserve period of ore-forming materials and the construction period of ore-bearing space in this area. This is the contradictory transformation of things and the law of their development.
3. Mineralization in the period of intraplate activation and transformation in the eastern Yangtze block in Mesozoic and Cenozoic.
After a long period of energy accumulation and stable crustal evolution, the lithosphere in the eastern part of the Yangtze block finally began to subduct into the North China block in the Late Triassic (Li Shuguang, 1992), and soon transformed into block collision, followed by a strong tectonic activation stage of large-scale intracontinental detachment subduction and nappe. At the same time, the tectonic stress fields of the ancient Pacific plate and the Indo-Australian plate have influenced the East Asian continent. It led to large-scale magmatic intrusion and volcanic eruption, and finally the two continents, North China and Yangtze, were welded together (paleomagnetic research shows that the south and north poles in southeastern China of Upper Jurassic are exactly the same). Since then, eastern China has entered a new pattern of circum-Pacific tectonic magmatic belt.
Due to the differences in geochemical composition of lithosphere in each structural unit, different magma sources in different lithosphere, different diagenetic modes, and different properties and distributions of metal-rich basement and caprock, rich and colorful minerals with different shapes have been formed in different structural parts:
1) is the upper arch of asthenosphere near the suture line where the North China (Dabie) block and the Yangtze block collide with the orogenic belt, where the crust is the thinnest. Copper-rich magma differentiated by intense mantle metasomatism melted the lower crust of diorite-amphibole-granodiorite-igneous gneiss, forming basaltic andesite magma chamber. Differentiated copper-rich magma intruded into the favorable part of shallow crust along the strong enrichment mechanism of crust and mantle under diagenesis, and formed a series of intermediate-acid magma closely related to copper and iron in time and space (Figure
2) On the north and south sides of the collision orogenic belt (Dabie Uplift and Tailong in the south of the Yangtze River), large-scale acidic granite magma intrusion of molten crust was induced by deep continental detachment and subduction. Because of the difference in geochemical composition of molten crust, they are marked with obvious "genetic genes". Tailong granite in the south of the Yangtze River melted the basement crust rich in W and Sn reservoirs, which led to the common mineralization of W, Sn, Nb, REE and U. It is just a "mountain root" basement with high differentiation and good maturity, and the remelted granite is often powerless in the mineralization of oxygen-loving elements.
3) Mesozoic magmatic upwelling not only brought deep ore-forming materials, but also was the thermal field source that drove the activation, migration and re-enrichment and precipitation of metal elements in various source beds, and formed rich and colorful sedimentary superimposed and reformed deposits in basement and caprock.
This period is the heyday of mineralization in the middle and lower reaches of the Yangtze River and its adjacent areas, and even in eastern China.
4) Mineralization in Mesozoic-Cenozoic tectonic environment of East Qinling. With the end of the late Paleozoic collision orogeny between the Yangtze block and the North China block, the overall tectonic framework of Qinba area was established, and the main part of Chinese mainland, namely the north-south joint block, was formed. However, the tectonic activity of this block has not subsided. Under the background of Indian plate subduction and collision to Eurasian plate and Pacific plate subduction to China plate, Qinba area inherited and continued the deep subduction from south to north and the large-scale contraction tectonic movement of the upper crust. Qin Deyu et al. (1988) proposed that the second Mesozoic intracontinental subduction tectonic movement occurred in Qinba area, which affected the whole area. In the deep part, the basement of the Yangtze block slides northward along the Shangdan Rift and subducts under the basement of the North block and the Caledonian block, forcing the fold cover in the South Qinling-North Dam area to thrust and slide southward. However, the thrust block north of the main interface of intracontinental subduction (Shangdan boundary fault) is strongly uplifted, and a ductile shear zone characterized by mylonite appears in the deep part. Therefore, the deep and shallow tectonic activities in this area are obviously different. The deep part has the nature of continental plate subduction, and the shallow part has the characteristics of detachment, nappe, extrusion and uplift. The former controls the formation of deep lava magma through subduction and subduction interface, and produces in groups and belts from basement to caprock in the southern margin of North China platform. The latter partially melted the materials in the earth's crust and formed the granite base derived from the earth's crust. In these processes, ore-forming elements were activated and migrated in their respective evolutionary history, which enriched the ore and finally formed the most distinctive dominant minerals of Au(Ag) and Mo(W) in the East Qinling region. The mineralization of Mo(W) is obviously controlled by small intermediate-acid rocks in the deep Mesozoic, and the mineralization of Au(Ag) is related to the activation and migration of ore-forming elements in the ancient crystalline rock series of Mesozoic molten crust granite. In addition, the Mesozoic-Cenozoic magmatism in South Qinling is much weaker than that in the southern margin of North China Platform, so the mineralization of Hg and Sb is mainly related to the deep fault tectonic activities. It can be seen that magmatism or fault activity caused by Mesozoic-Cenozoic tectonic environment in East Qinling is the dominant factor of Au(Ag), Mo(W), Hg and Sb mineralization.