Super-large tungsten polymetallic metallogenic belt in southern Hunan

Shizhuyuan deposit is a super-large tungsten polymetallic deposit, in which tungsten, tin, bismuth, beryllium, fluorine, lead, zinc and silver all have considerable scale. In recent years, with the general survey and exploration of the periphery of the mining area, a number of large and medium-sized lead, zinc and silver deposits have been discovered one after another. Why is there so much metal accumulation in such a small area of Shizhuyuan? This is a significant metallogenic problem. Tu Guangchi (1989), Yang Chaoqun (1989), Pei et al. (1990), et al. (1993) and et al. (1994) have made preliminary discussions in this respect. This paper discusses the geological background and special factors of Shizhuyuan super-large tungsten deposit.

Tungsten, tin, molybdenum and bismuth are a group of elements friendly to the earth's crust, which tend to be enriched in the upper crust during the formation and evolution of the earth. After accumulation, mantle differentiation, mantle differentiation and crust differentiation, the abundances of tungsten, tin and molybdenum in the mantle, lower crust and upper crust are 0.0 16 μg/g, 0. 13 μg/g and 0.059μg/g；/g, respectively. /g。 0.7 μ g/g, 1.5 μ g/g, 0.8 μ g/g and 2.0 μ g/g, 2.5~5.5 μ g/g, 1.5 μ g/g (Lyman, 1994).

Metal elements not only have different affinities for different earth layers, but also are unevenly distributed in the earth. In a metallogenic belt, one or several elements are relatively concentrated, which often leads to multi-cycle mineralization. Crustal rocks and mantle rocks of different origins in this zone are rich in ore-forming elements. For example, in the Jiuwandashan-Yuanbaoshan tin polymetallic metallogenic area, the tin contents of ultramafic rocks, mafic rocks, granodiorite, biotite granite, metamorphic siltstone and metamorphic mudstone are 7.0 μg/g, 15.3 μg/g, 22.4 μg/g, 40.3 μg/g and/kloc-0, respectively. Nanling area is a huge tungsten-tin polymetallic deposit, which itself reflects the abnormal enrichment of these metal elements in Nanling area. Cheng Xianyao (1984) and Xu Keqin et al. (1987) demonstrated that the Proterozoic stratum is a source bed rich in tungsten and tin. Mao Jingwen (199 1) thinks that mantle degassing, submarine volcanic eruption and ancient sand mineralization are the three forms of initial enrichment of tungsten-tin deposits. This is because the super-large tungsten-tin deposit formed in Nanling area of tungsten-tin rich mining area has rich material basis.

The tungsten-tin deposits in Nanling metallogenic province have obvious zonation (Chen Yuchuan et al., 1998), that is, the eastern part (southern Jiangxi, northern Guangdong and western Fujian) is rich in tungsten, the western part (northern Guangxi and southeastern Yunnan) is rich in both tungsten and tin, and the central part (southern Hunan) is rich in molybdenum, bismuth and beryllium. In addition, there are tungsten-rich tin deposits in the north (Zengjialong tin field in northern Jiangxi, Taizibi tin deposit, Wuyuan tungsten deposit and Yangchuling tungsten deposit), and there are many tin deposits in the south (eastern Guangdong, western Guangdong, Qinjia in Guangxi and Dulong tin deposit in Yunnan). Tungsten-tin mineralization in Nanling area has a multi-cycle spiral upward trend. The tin-tungsten deposits in Nanling area began in Proterozoic (Mao Jingwen et al., 1990) and formed in Baotan and Jiumao deposits in Jiuwandashan-Yuanbaoshan area of Neoproterozoic. In Caledonian, there were Qinjia tin-copper mine and Niutangjie tungsten mine, and in Indosinian, there were Limu and other tungsten-tin-niobium-tantalum deposits. In Yanshan, tungsten-tin mineralization reached its peak, forming hundreds of tungsten-tin deposits, with metal reserves accounting for 95% ~ 99% of all metallogenic areas.

Shizhuyuan super-large tungsten polymetallic deposit is located in the middle of Nanling in space, and its metallogenic age is the middle Yanshan period. Therefore, the massive accumulation of polymetallic (W, Sn, Mo, Bi, Be, Pb, Zn, Ag) is the inevitable result of geological historical evolution to some extent.

Deep faults and tungsten-tin mineralization are very developed in Nanling area, and deep faults in northeast and east-west directions play a very important role in controlling the formation of tungsten-tin deposits and related granites. According to the statistics of Tang Jifang et al. (1989), there are 12 tungsten-tin polymetallic metallogenic belts and more than 80 large and medium-sized deposits in this area, all of which are not affected by deep faults, and the mineralization is relatively concentrated at the intersection of deep faults in the northeast, northwest and east-west directions. Influence of deep faults on mineralization: ① Granite (Mo Zhusun, 1989) was formed by deep melting of the earth's crust. Because the crust is rich in tungsten and tin, these granites are mineralized after differentiation and evolution, and the secondary faults of deep and large faults are often the space for rock mass positioning and the passage of mineral liquid. (2) Lamprophyre dikes widely distributed along faults indicate not only the cutting depth of large faults, but also the upward conduction of mantle fluid and heat energy, and the regional high temperature field caused by them plays an important role in the formation, differential evolution and mineralization of granite; ③ Mantle fluid and mantle gas mainly migrate upward in the form of H+, K+, OH-, CO2, CH4, Ar and he. It plays a positive role in mineralization.

Shizhuyuan tungsten polymetallic deposit is located in the east of Chaling-Chenzhou-Linwu deep fault (Figure 7-4). Jiufengshan-Huichang-Xianyou basement is fractured in the east-west direction, extending to the west, and passing through Shizhuyuan area in a concealed state. The NW-trending Shaoyang-Chenzhou deep fault also passes through the south of Shizhuyuan. The intersection of these three deep faults has formed a series of hot spots or columns, which make the mantle airflow and deep magma chamber continuously provide ore-forming materials and energy for this area.

There is obvious gravity gradient zoning along the Chaling-Chenzhou-Linwu fault. The northwest of the fault is the Indosinian depression and fold area, and thick carbonate rocks are distributed in the Upper Paleozoic, forming a regional high gravity value area. The southeast of the fault is Caledonian uplift area, which is composed of clastic rocks of Lower Paleozoic. A low gravity anomaly zone is formed along the fault, which consists of Xianghualing, Qitianling, Baofeng County, Qianlishan and Baoshan.

Metallogenic anomaly of Qianlishan granite body Qianlishan granite body is a compound rock body, which consists of three stages. From morning till night, it is composed of porphyry biotite granite (5.9km2), equigranular biotite granite (4. 1km2) and granite porphyry group. Although the rock mass is only 65,438+00 km2, geophysical data show that there is a trapped rock mass in its depth, which may also be connected with Wang Xianling rock mass. Although the exposed area of Qianlishan rock mass is small, mineralization is accompanied by diagenesis in various stages, and lithology shows a series of metallogenic particularities.

Figure 7-4 Structural characteristics and aeromagnetic anomalies of typical ore fields in southern Hunan.

Shizhuyuan mining area has gathered multi-stage and multi-source ore-forming materials, and skarns are mainly composed of calc-skarns. However, in the late stage of skarn formation and evolution, a large number of fine-grained manganese skarns appeared. In tungsten polymetallic deposits, the appearance of manganese skarn seems to indicate the directionality of skarn formation and evolution and the periodicity of diagenesis and mineralization. Manganese in skarn comes from rock mass or manganese-rich carbonate rock. In many skarn deposits, primary skarns are mostly degenerated and altered, forming amphibole, chlorite and mica hydrous mineral assemblages. In Shizhuyuan mining area, the sub-diopside magnetite-fluorite assemblage was formed by metamorphism and alteration, and then it experienced multi-stage metamorphism or hydration, namely actinolite (Jiujiaoamphibole)+magnetite+fluorite, chlorite+magnetite+fluorite to alkali feldspar+fluorite+chlorite+magnetite (+biotite+timely). This obvious alteration zoning phenomenon can also be seen in space. The whole degradation and alteration process gradually changed to the direction of hydration enhancement, fluorine fugacity increase and iron element precipitation. Finally, alkaline and siliceous components are enriched to make the final product. In the process of metamorphism and alteration, tin and tungsten originally dispersed in garnet and other minerals are activated to form fine cassiterite, scheelite, biscite and molybdenite. The rhenium and osmium isotopic ages of molybdenite in this stage are 15 1Ma (Li Hongyan et al., 1996). In a word, the mineralization in this period occurred in a relatively closed system, mainly characterized by skarn and altered rock. In addition, at the top of the uplift of porphyritic biotite granite, there are also mineralized biotite syndacite blocks or muscovite syndacite blocks. Most of these greisens appear as residues at the edge of equigranular biotite granite.

Mineralization related to equigranular biotite granite, although a small amount of calcareous skarn veins rich in W-Mo-Bi-Sn have also been formed, and even the phenomenon of gradually becoming skarn veins at the tip of granite branches can be seen (Wang Changlie et al., 1987), but the most prominent manifestation is the mineralization of greisen. Generally speaking, greisenization occurs within several hundred meters of the inner and outer contact zone of the equal-grained biotite granite uplift, but it extends for several thousand meters along the Nanxi fault. The space announcement is superimposed with primary massive skarn, metamorphic altered rock, amphibole (including marble) and porphyritic biotite granite (such as Yejiwei rock branch). Greisen can be divided into biotite greisen, muscovite greisen, topaz greisen and alkali feldspar greisen.

In the process of greisenization, the occurrence state and ore-forming elements of greisen are banded in space, that is, massive greisen (W-Sn-Mo-Bi) with uplifted rock mass from bottom to top, dense and sparse veined greisen (W-Sn-Mo-Bi) superimposed on skarn, and veined greisen (Sn-Be-Bi) superimposed on marble. This mineralization zoning is essentially restricted by a fault system, three kinds of ore-forming surrounding rocks and two stratigraphic structural interfaces. In the process of emplacement and condensation of equigranular biotite granite, an arc joint and a group of * * * yoke joints were formed at the uplift of the rock mass, and the * * * yoke joints extended and developed in the upper massive skarn. Because of the brittleness of skarn, a joint width trap is formed. When the same stress acts on marble or porphyritic biotite granite other than skarn, it becomes a fine network crack, which may be caused by the ductile trap and weak stress of the latter two.

The mineralization of greisen is controlled by the fault system, which indicates that the mineralization is in an open environment. However, this openness is very limited, because the fracture network is pointed out hundreds of meters upward from the contact zone of rock mass uplift. The muddy marble in the upper part may play a shielding role, so that ore-forming fluids and minerals can be effectively preserved. Although water and other materials are not excluded from mineralization, the isotopic composition of hydrogen and oxygen changes in space (Wang Changlie et al.,1987; Zhang Ligang, 1989) and the orderly distribution of ore-forming materials, such as biotite → albite → brittle mica → wolframite → scheelite, Fe-Al garnet-Mn-Al garnet solid solution → Mn-Al garnet, all indicate that there is a close metallogenic relationship between greisen and equigranular biotite granite.

In the third stage, Pb-Zn-Ag mineralization is vein-like and coexists with granite porphyry in space. The NE-trending granite porphyry vein group is 6 kilometers wide and 30 kilometers long. Only in Shizhuyuan mining area and its periphery, a large number of mineral deposits such as Yejiwei, Caishan, Qixingping, Hengshan, Nanfen 'ao and Zongshuban have been discovered. The mineralization in this period took place in a relatively open environment, with a set of vein and reticular manganese skarns (Mao Jingwen et al., 1994) and a large number of water-bearing minerals such as phlogopite, muscovite, chlorite, brittle mica and epidote. Although it is accidental that the mineralization of this period overlaps with the previous two periods in space to some extent, it also has the characteristics of a lot of fluorine and more tourmaline.

The two types of ore-forming materials are intertwined: the above three ore-forming systems are all related to granitic rocks, that is, the ore-forming elements are differentiated and enriched by remelting granite slurry. This is the main theme of Shizhuyuan tungsten polymetallic deposit and the main source of metallogenic materials. On the other hand, some ore-forming materials may come from strata. Ji Kejian et al. (1989) carried out stratigraphic geochemical work in the periphery of Shizhuyuan mining area, and the results confirmed that there was a negative anomaly area of tungsten polymetallic elements in the periphery of the ore body between 800 and 5000 m. They believe that in the process of granite emplacement, the rock mass provides the heat source, and the atmospheric precipitation is the water source, forming an underground hot water circulation system, quenching the tungsten polymetallic elements in the stratum and transporting them to the contact zone for accumulation and mineralization. The work of Wang Changlie et al. (1987) and Liu et al. (1994) confirmed this understanding. Liu et al. (1994) also proved that the Sinian clastic rocks and Devonian carbonate rocks in Shizhuyuan area are rich in W, Sn, Mo, Bi, Zn, F and other elements, indicating that in Jurassic period, southern Hunan was a water storage basin rich in water resources. Quasi-water seeping along faults, interlayer structures and crevices is heated into hot water by granite body, and then leached into strata to extract organic matter into weakly acidic solution, so that ore-forming elements such as tungsten, tin, molybdenum and bismuth can be effectively extracted into metallogenic convection circulation system. In addition, taking the positive anomaly area of ore-forming elements surrounded by negative anomaly area as the prospecting indicator, it is pointed out that Lanjia in the east of Shizhuyuan mining area is a favorable target area for large-scale tungsten polymetallic deposits.

To sum up, Shizhuyuan area is located at the eastern edge of Haixi depression trough on the west side of Xiangdong uplift in southern Jiangxi. Due to the weathering erosion and sedimentation of the ancient land, the Devonian strata were initially rich in elements such as lead and zinc, and there was a zoning law of Fe→Mn→Pb→Zn from east to west (Wang Changlie et al., 1987). It is precisely because the surrounding rocks are rich in manganese (Mao Jingwen, 1994, 1996) that the formation and evolution of skarn in each stage of the mining area have the characteristics that garnet and pyroxene tend to be rich in manganese in the later stage. In the lead-zinc-silver mineralization stage, a large number of vein manganese skarns, manganese-rich phlogopite (containing MnO2, 1% ~ 5.5%) and pyrolusite were formed. In addition, the black shale at the bottom of Cambrian in Hunan contains a large number of rare, radioactive and iron-loving elements. During the Yanshanian granite emplacement mineralization, driven by high thermal energy field and volatile matter, these elements may also participate in the metallogenic system. This may also be one of the reasons why there are so many ore-forming elements in Shizhuyuan mining area.

South China is a tungsten polymetallic anomaly area, which laid a material foundation for mineralization. With the evolution of geological history, the strength of multi-cycle spiral of tungsten-tin mineralization increased and reached its peak in Yanshan period. Deep faults in northeast and east-west directions control the formation and upwelling position of regional granite, and are the channels for the rising of mantle gas and liquid. More importantly, it leads to the upwelling of mantle heat energy, which makes the vicinity of faults, especially the intersection of two sets of faults, in a state of high heat for a long time.

The exposed area of Qianlishan granite is only 10 km2, but it is connected with Wang Xianling and other rock masses in the deep, even with Baofengxian, Qianlishan, Qitianling and Xianghualing granite bodies in the deep along the Chaling-Chenzhou-Linwu fault. The granites in the three periods have experienced highly differentiated evolution, which is a typical Bayliff granite and also a kind of high-heat granite. Based on this, three metallogenic systems related to granite series were formed. The first metallogenic system, represented by skarnization, has been degraded and altered many times after skarnization, so that the metallogenic elements are enriched to the maximum extent. The second metallogenic system, represented by greisenization, is controlled by the joint system produced by the solidification process of rock mass, forming the zoning of mineralized elements in Shizhuyuan mining area. Lead-zinc-silver mineralization and manganese skarn represent the third metallogenic system, which are widely distributed in the area together with granite porphyry veins. The superposition of these three metallogenic systems forms a relatively closed metallogenic system, which effectively preserves and enriches ore-forming materials.

The interweaving of two groups of deep faults in Chenzhou area led to the rise of mantle hydrothermal solution, and the successive three emplacements of Qianlishan high-temperature granite made Shizhuyuan area in a high-heat environment of 70 ~ 80 ma. This high heat environment not only delays the crystallization differentiation and evolution of magma, but also makes the ore-forming materials in the rock mass converge to the maximum extent, forming a series of convection and circulation systems outside the rock mass, making W, Sn, Bi, Mo, Be, Zn, Ag and so on. These elements originally enriched in the stratum are activated, migrated and aggregated.