(A) regional metallogenic geological characteristics
The Nanhua-Sinian system in southern Anhui is covered by regional unconformities of different horizons, such as Likou Group or Xikou Group in Neoproterozoic Qingbaikou period. Nanhua Formation, including Xiuning Formation and Leigongwu Formation, is 3,000m thick, and its main lithology is conglomerate, glutenite, sandstone, pyroclastic rock and moraine mud conglomerate. The Sinian strata include Lantian Formation (equivalent to Doushantuo Formation in the north) and Piyuancun Formation, with a thickness of 400 meters, and are mainly composed of carbonate rocks, black shale and siliceous rocks.
There are two main groups of fault structures in this area: NE and nearly EW. In time, southern Anhui and its southeast and south China belong to the marginal sea area after Jinning period, which has the characteristics of complex continental margin with multiple rifts, arcs, basins and small plots. Indosinian movement caused large-scale folds and fractures of sedimentary caprocks before Triassic, forming a series of fold fault zones extending to northeast and northeast. During the Yanshan movement, the area turned into an active belt on the continental margin, characterized by strong differential block movements.
The Sinian system in this area is an extensional fault basin on the continental margin-southern Anhui fault basin. Its northern boundary is Jiangnan deep fault, and the fault reaches Chaohu and Chuzhou in the north, which is a large area of seawater carbonate rock (Figure 5-39). Dexing-Wuzhen fault is the southern boundary of the basin, and the south is Hua Kai-Lin 'an carbonate platform in Zhejiang. Faults developed in the basin, such as Qimen-Sanyangkeng Wanjiaqiao fault, are all distributed in the east-west direction. The water depth of the basin is not large, ranging from tens of meters to hundreds of meters. Lantian Formation belongs to the continental shelf marginal basin facies (Yingdeng, 1998). Lantian Formation stratum is the most developed stratum in this metallogenic belt, and it is also a representative ore-bearing stratum in this metallogenic belt.
Figure 5-39 Paleogeographic Map of Lantian Formation in Southern Anhui
(According to Li, 200 1)
Some faults have obvious inheritance and long-term activity characteristics. For example, the Qimen-Sanyangkeng Wanjiaqiao fault is nearly east-west. There were activities from Jinning to Himalayan, involving Yuanguyu to Mesozoic strata, which not only controlled the distribution of magma, but also controlled the development of strata, lithofacies and the distribution of minerals.
The magmatic rocks in the area are mainly granitoids, and the intrusive ages are Jinning and Yanshan. No deposits related to Lianhuashan and Baijishan plutons in the late Jinning period have been found. There are Yixian rock mass, Taiping rock mass and Xiaoyao rock mass in the early Yanshan period, and Huangshan rock mass and Fuling rock mass in the late Yanshan period. Magmatic activity was the strongest in this period, forming a multi-stage and multi-stage complex rock mass. Only mineralization is found in these rocks or contact zones, and no industrial ore bodies are found. In other words, the relationship between the deposit and the rock mass is not close.
The Pb-Zn-Ag polymetallic deposit occurs in the continental margin fault depression zone and belongs to continental crust mineralization. Linear fault structure controls the distribution of ore deposits, volcanic activity is not developed, and the ore-bearing rock series is clastic rock-carbonate rock. The deposit is characterized by simple sulfide, and the main ore-forming elements are silver, gold, lead, zinc, tungsten, iron and manganese. The horizontal zoning of the deposit is obvious, Cu, W→Pb, Zn, Ag→Fe, Mn. These deposits can be compared with submarine jet lead-zinc deposits (large, 198 1) and massive sulfide deposits in South China (Gu Lianxing et al., 1997), and have similar metallogenic background and mechanism as submarine jet syngenetic deposits. The lead-zinc polymetallic deposit in Lantian Formation (silver) contains late Yanshanian intrusions and layered ore bodies reconstructed by hydrothermal superposition.
The strata of Lantian Formation are in pseudo-conformity contact with Leigongwu Formation of Nanhua System, and in conformity contact with the overlying Wuyuan Formation. The total thickness of Lantian Formation is 70 ~ 208m. Generally speaking, the east is thick and the west is thin.
Bottom: gray and grayish brown manganese-bearing dolomite and dolomitic limestone with a thickness of 32 ~ 120m (Table 5- 12). Fe-Mn deposits and siliceous rocks are locally developed. The relative thickness of manganese-containing carbonate rocks is relatively large. The combination of ferromanganese deposits with siliceous rocks and manganese-containing carbonate rocks is distributed in the first line of Qimen, yi county, Xiuning, Jixi and Ningguo, that is, along the Qimen-Sanyangkeng-Wanjiaqiao fault, with a length of more than 200 km, a width of 20-30 km and an area of about 500km2, while in Dongzhi and Shitai, it is transformed into dolomitic limestone and silty slate (see Table 5-5 for the main deposit characteristics)
Table 5- 12 Distribution of Manganese Carbonate in Sinian System (Lantian Formation) in South Anhui
(According to Li, 200 1, modified)
Figure 5-40 Distribution Map of Sinian Deposits and Manganese-containing Carbonates in Southern Anhui
(According to Li, 200 1)
1-dolomite limestone; 2- Manganese-bearing carbonate rocks; 3- Lithofacies boundary; 4- provincial boundaries; 5- all kinds of mineral deposits (points); 6— Fracture and number; 7-probably a short layer. ① Qimen Sanbao (copper) Ag-Pb-Zn polymetallic deposit; ② Xikeng Ag-Pb-Zn polymetallic deposit in Yixian County; ③ Guiyuan Fe-Mn deposit in Xiuning; ④ Dabeiling limonite deposit in Xiuning; ⑤ Rukeng Fe-Mn deposit in Jixi; ⑥ Jixi Xiaoyao silver polymetallic deposit; ⑦ Jixi Xia (tungsten) silver deposit; ⑧ Yuanchuan Bikeng Fe-Mn deposit in Ningguo; ⑨ Xiaoyinshan ferromanganese deposit in Ningguo: ⑩ Xiaocao gold-bearing ferromanganese deposit in Ningguo; Huangshan Taiping Youzhukeng Fe-Mn deposit; Niaoqueping pyrite deposit in Yixian County: Mulikeng Fe-Mn deposit in Ningguo
Table 5- 13 Geological Characteristics of Sinian Major Deposits in Southern Anhui
(According to Li, 200 1, modified)
Central part: Lantian is dominated by black carbonaceous shale, rich in layered, disseminated, nodular pyrite, calcareous mudstone and calcareous slate. The upper part of this layer is sandwiched with several layers of gray dolomite limestone.
Upper part: gray banded marl, containing pyrite nodules, lumps and bands, with a thickness of 18 ~ 22m. Top: taupe to black sandy mudstone containing manganese powder, locally containing carbon, with a thickness of about 18 ~ 40m, found in Shiyan section.
(2) Qimen Sanbao Pb-Zn deposit
The deposit is developed in Lantian Formation. There are three types of ore bodies (spots) of copper, silver, lead and zinc, tungsten and molybdenum, and iron and manganese from bottom to top, which are regularly distributed in three intervals of Lantian Formation, namely, manganese-bearing carbonate rocks (copper, silver, lead and zinc) at the bottom, shale rocks (tungsten and molybdenum) at the middle and manganese-bearing clastic rocks (iron and manganese) at the top. This mineralization is not only in Sambo, southern Anhui, but also in southwest Zhejiang. Along the fixed horizon of Lantian Formation, the mineralization continues to deepen intermittently, forming many sulfide polymetallic and nonferrous metal deposits (Gu Lianxing et al., 1987). Ore bodies are mainly composed of sulfide plates and lenses. The thickness of ore bodies is mostly 0.3~20m, and the lateral scale is mostly over 500m. Nickel, iron and manganese are small in scale. Netted or veined Fe-Pb-Zn ore bodies are often seen at the bottom of layered ore bodies. The lead reserves of the deposit are 6.5438+0.9 million tons, and the zinc reserves are 48.9 million tons.
Ore bodies are integrated in manganese-bearing carbonate rocks, marls and carbonaceous shale, and basically fold synchronously with surrounding rocks. In space, ore bodies often transition to manganese-bearing siliceous rocks and manganese-bearing mudstone. Manganese-bearing carbonate rocks formed by hot water deposition are widely distributed, covering thousands of square meters. The ore deposits (bodies) are distributed intermittently along the Qimen-Sanyangkeng Wanjiaqiao fault, and the vertical distance from the fault is almost less than 20 kilometers, mainly distributed on the northwest side of the fault (the upper wall of the fault).
Ore type, texture and mineralization stage: mainly pyrite-sphalerite, pyrite-sphalerite-galena, chalcopyrite-pyrite-sphalerite-galena and other simple primary sulfides. Metal minerals include sphalerite, galena, chalcopyrite and pyrite, followed by hematite, magnetite and white iron ore. There are pyrite, hematite and hydromagnesite in manganese deposits. Gangue minerals include barite, quartz, fluorite, siliceous rocks and calcite. The main structures are semi-self-shaped-shaped fine-grained structure, self-shaped granular structure, fracture structure, gap structure, colloid structure and so on. The main structures are massive, angular, banded, reticulate, disseminated and layered. The metallogenic stage is divided into three stages, namely, the first stage is dominated by sedimentation, the second stage is syngenetic sedimentary diagenesis and mineralization, and the third stage is late magmatic re-enrichment.
Wall rock alteration: mainly silicification, followed by carbonation and tremolite.
Genesis of the deposit: According to regional geology and deposit geology, the deposit was formed by hot water deposition and controlled by fault structure. Hematite is a common component in metal-bearing sediments at the bottom of ocean basins, and is usually regarded as a sign of hydrothermal activity. Polymetallic deposits in the Sinian system in southern Anhui can be well compared with large-scale (198 1), Tuguangchi (2000) and other hydrothermal sedimentary deposits, that is, lead-zinc deposits are often concentrated in hydrothermal vents and their vicinity (such as Sambo), accompanied by copper and silver. However, with the increase of the distance from the nozzle, some iron-manganese deposits (such as Xiuning) are formed, accompanied by barite, fluorite and siliceous rocks, which are typical products of hot water overflow. Outward, with the dilution of hot water solution, a transition zone of alternating hot water deposition and normal seawater deposition appears, forming manganese halo, that is, silver, lead, zinc (copper is only found in Sambo) → iron, manganese → manganese halo.
From the geochemical characteristics, the major elements of different rock types in the ore body are in the Fe/Ti-Al/(Al+Fe+Mn) diagram. Siliceous rocks belong to the hot water deposition area near the two wings of the Pacific Uplift, while black shale and micrite limestone belong to basin deposition, which is close to the end members of terrestrial materials and biological materials. However, manganese-containing carbonate rocks are located in the transitional zone between basin deposition and hot water deposition, indicating the superposition characteristics of hot water deposition. The enrichment coefficients of trace elements such As Ba, Ag, As and Hg, which are used to judge the genesis of hot water deposition, are all above 8, showing obvious enrichment. The highest content coefficient of Ba is 3068× 10-6, which exceeds the enrichment value of Ba under the action of submarine hot water in the East Pacific Ocean. The highest manganese content in manganese-bearing carbonate rocks is 8.49× 10-6, which is higher than that in Pacific ridge and red sea hot water sediments, indicating the signs of hot water deposition and mineralization (equal to 200 1).
The sulfur isotope of Sanmao pyrite in Qimen has changed greatly, which may reveal two sources: seawater and underground hot water. The ore-forming temperatures are 250℃ and 300℃, respectively, indicating that they are mixed fluids of ore-forming hot water and seawater. It can also be compared with Changba lead-zinc mine formed by hot water deposition in Qinling Mountains.
In a word, the deposit (and most of the deposits in the metallogenic belt) is a hydrothermal deposit (MVT type) occurring at the edge of the Sinian manganese-bearing carbonate rift system.