Fig. 7- 14 Schematic Diagram of Magmatic Rock Distribution in the East of Nanling
1. The existence of ancient land blocks rich in uranium
Since Grabau( 1924) put forward "Cathaysian system", the existence of Precambrian paleolandmass in South China and its temporal and spatial scope have always been a controversial issue in geosciences (Chen,1956; Zhang Wenyou,1959; Ren Jishun,1964; Huang,1977; Guo Lingzhi, 1980). Huang Xuan and De Paul (1989) believe that the Paleozoic granitoids in South China came from the ancient continental basement deep in the block, and the oldest age of this basement is about 2 billion years. Li et al. (1998) measured the zircon U-Pb age of amphiboles in southwest Zhejiang and northwest Fujian at (1766 19) Ma by SHRIMP, and thought that the Cathaysian ancient land was probably formed by remelting and crystallizing Archean crustal recycling materials in the late Proterozoic. Deng Ping (2002a) considered that the Precambrian in South China once existed in an ancient land block (Cathaysian ancient land), characterized by schist, gneiss and migmatite, with a plane distribution and an age of 65.438+800 million ~ 65.438+000 million years (according to the structural assemblage, structural characteristics and isotopic dating data of medium-deep metamorphic rocks exposed in Zhejiang, Fujian, Jiangxi and Guangdong). In 800-900 million years, the ancient land collided and stitched with the Yangtze plate along the Shaoxing-Jiangshan-Pingxiang-Guilin structural belt, and subcontinental cracking occurred at the beginning of Sinian, which was stitched again in the early and middle Devonian. According to the large strike-slip ductile shear zones on both sides of Wuyishan, the new mica Ar-Ar ages are 390Ma and 420Ma, respectively (Shu Liangshu et al., 1999). It is inferred from the crustal thickness (29 ~ 35 km) and the thickness of Sinian-Ordovician metamorphic rock series (7 ~ 13 km) in this area that the thickness of Precambrian ancient land block can reach 20km.
The eastern part of Nanling is located at the edge of the early Paleozoic fold belt of the ancient block in southern Jiangxi and northern Guangdong. The spatial distribution of uranium deposits is closely related to the early Paleozoic fold belt, and almost all uranium deposits in the whole Nanling area are distributed in or around the early Paleozoic fold belt (Figure 7- 15). The bedrock in this area is mainly composed of Neoproterozoic-Paleozoic terrigenous debris mixed with submarine eruption, which is formed by medium-low metamorphism and migmatization. The uranium content is unbalanced, and there are Neoproterozoic-Paleozoic uranium-rich layers (segments), such as early Paleozoic structural layer (Z-S), with the uranium content of (3.3 ~ 4.6) × 10-6, reaching 36× 65438 locally. The uranium content of uranium-producing granites in this area is mostly above 13× 10-6, and the highest is 24× 10-6, which is several times the Clark value of uranium in general granites (Du Letian et al.,1982; Jin Jingfu (1984) is also 2 ~ 4 times the uranium content of metamorphic rocks in this area (4× 10-6 ~ 6× 10-6). Studies show that the late Jurassic strong peraluminous granitoids in this area originated from ancient blocks or sedimentary rocks (Zhou et al., 2000). The uranium-rich granite in this area is generally peraluminous granite, and its parent rock age is1.3 ~ 2.4 billion years (Tan, 1.996), indicating that the original rock of the uranium-rich rock mass is a uranium-rich continental block, and Chen Zuyi et al. estimated that its uranium content reached 20.53× 1-6 (65438).
Figure 7- 15 Relationship between Spatial Distribution of Uranium Deposits and Early Paleozoic Fold Zone in Nanling Area (Ren Jishun, 2002)
The distribution of uranium deposits in South China is closely related to the Cathaysian block, with obvious zoning in the region (Figure 7- 16). Volcanic-type uranium deposits are dominant in the northeast of Huaxia Block, granite-type uranium deposits in the southwest, sandstone-type uranium deposits and carbonaceous-siliceous-mudstone-type uranium deposits in the northwest. There are almost no uranium deposits in the adjacent areas outside the Cathaysian block, which indicates that the Cathaysian block and its edge are favorable areas for uranium mineralization.
Fig. 7- 16 uranium distribution in Cathaysian block
2. The change of crustal thickness meets the deep structure.
Constrained by Jiu Feng EW uplift belt, Wanyang SN- uplift belt and Wanyangshan NE uplift belt, this area is the intersection of early Mesozoic NW-trending rock-controlling deep faults, middle and late Jurassic EW-trending and NE-trending rock-controlling deep faults, Cretaceous NW-trending crust-cutting faults and K2-E NE-trending basin-controlling deep faults. Regionally, the Moho depth in this area gradually increases from southeast to northwest, and within the span of 1 km, the Moho depth increases from 3 1 km to 38 km. According to the depth of Moho, this area is divided into two parts, north and south, bounded by the 25 north latitude line. The crustal thickness in the northern region changes greatly, and the deep structure of NE and NNE is obvious. The Moho surface in the southern area is slightly undulating, and the crust thickness is mostly within 34km, which is dominated by east-west deep structures (Figure 7- 17). Uranium-rich deposits in the Zhuguang-Guidong ore concentration area are mostly formed in the mantle slope belt where gravity and depression meet at the edge of the paleouplift from Late Paleozoic to Early Mesozoic, and the mantle slope section at the front of the tongue with lower gravity is the main area to control uranium-rich deposits, which indicates that the ore concentration area is obviously restricted by the paleouplift structure and deep structure, and many groups of deep faults have played a positive role in multi-stage magmatism and deep uranium source activation.
3. Multi-stage and multiple strong tectonic magmatism.
The southern part of Zhuguang rock mass and the eastern part of Guidong rock mass are located in a strong tectonic magmatic activity area, with frequent magmatic activity, from early Paleozoic to late Paleozoic-early Mesozoic and late Mesozoic (Deng Ping, 20 1 1a, 20 12). The magmatic activity is characterized by multiple periods of simultaneous and alternating deep and shallow magmatic activities. Controlled by tectonic magmatism, uranium deposits in this area are mostly distributed in multi-directional and multi-stage magmatic activity centers or intersections, that is, NW-trending early Mesozoic magmatic belt, EW-trending and NE-trending middle-late Jurassic magmatic belt, NW-trending Cretaceous magmatic belt or multi-stage magmatic activity centers, and ore bodies occur around late small rock bodies or at the top of caves. Magmatic activity centers are mostly multi-stage magmatic activity areas except for a few multi-stage magmatic activity areas at the same time. The former is only a single deposit occurrence area, while the latter may develop a large-scale uranium enrichment area. For example, the intersection of early Mesozoic NW-trending magmatic belts in southern Zhuguang and eastern Guangxi, middle and late Jurassic EW-trending and NE-trending magmatic belts and Cretaceous NW-trending basic dikes is a large granite-type uranium accumulation area. ?
Fig. 7- 17 contour map of Moho surface in South China (according to Jiangxi Geological Research Institute 1985)
4. Basic dikes and alkali metasomatism are developed in the upwelling area of mantle.
The formation of uranium-rich deposits in this area is closely related to the deep magma and alkaline fluid in this area (Du Letian,1996,2001; Li Ziying et al., 1999). NWW and NNE basic dikes are well developed in the mining area, and uranium mineralization is closely related to basic dikes and Maofeng rock mass in space (Figure 7- 18). The intrusion of basic magma and Maofeng rock mass is considered as a sign of deep magmatic activity in this area. Maofeng type rock mass is fine-grained biotite granite containing albite mica, and its diagenetic age is141.2ma (138 ~143ma, K- Ar, U-Pb, Rb-Sr method), 87 Sr/. From the aspects of formation environment, petrochemistry, characteristics of accessory minerals, stable isotopes and rare earth elements, it is shown that the diagenetic materials of this kind of rock mass are mainly mantle-derived materials and mixed with a certain amount of crust-derived materials (Liu Ruzhou et al., 1995). With the intrusion of basic magma and Maofeng rock mass, alkali metasomatism, electro-petrochemical, pegmatization and muscovization occurred, and the rock mass itself and its adjacent granite were transformed into mica granite and sodium metasomatic granite, resulting in one areal alkali metasomatism and two zonal alkali metasomatism (Zhang Yanchun, 200 1, 2002), so alkali metasomatism is very common in uranium deposits.
Systematic chronology, element and Sr-Nd isotope geochemistry of basic dikes in this area show that basic dikes are mainly formed in three stages (140Ma, 105Ma, 90Ma(K-Ar, Ar-Ar method)), which are related to uranium mineralization in this area. Deng Ping, 2002b). The chemical composition of basic dikes is dominated by tholeiite, and the high εNd(t) value (+5) indicates that its parent magma originated from the mantle source region with long-term loss of large ion affinity and light rare earth elements (Li et al., 1999). By studying the hydrogen and oxygen isotopes of ore-forming fluids in 338 and 339 deposits (Deng Ping, 2002e), it is concluded that the δ18H2O =1.4 ‰ ~ 6.6 ‰ and δ DH2O =-65 ‰ ~-34 ‰ of ore-forming fluids before and during mineralization, indicating that the ore-forming fluids are mainly composed of mantle fluids. The δ 13C of calcite in veins is -8.5 ‰ ~-3. 1 ‰, indicating that the mineralizer σ σσCO2 comes from the mantle.
Fig. 7- 18 Spatial relationship between uranium distribution and mantle-derived basic magma in Zhuguang and Guidong plutons.
Alkaline metasomatism, veins and deformation structures related to mantle-derived basic magmatic activities in Zhuguang-Guidong uranium-rich area, as well as the extensive development of "swelling" phenomenon of fault zone rocks caused by fluid activities (Li Jianwei et al., 1999), and the time of mantle-derived basic dikes and uranium mineralization (140 ~ 47 Ma, U-Pb method). Mantle fluid is a supercritical fluid rich in alkali (K, Na, li), volatile matter (CO2, S, H2O) and original gas (3He, 36Ar). This fluid is neither water nor rock slurry, but "HACONS" (Du Letian, 1996) composed of hydrogen, halogen, heat (H), alkali metal (A), carbon (C), oxygen (O), nitrogen (N) and sulfur (S), which has unique solubility and solubility. These mantle fluids not only produce metasomatism in the mantle, but also in the mantle xenoliths carried by them during their ascent. Especially when they pass through the upper mantle and Moho surface and enter the lithospheric crust, they have a strong metasomatism with crustal rocks, which leads to the large-scale activation and transfer of ore-forming elements from the crust, thus contributing to the formation of large and super-large deposits and giant ore belts.
5. Tectonic environment of transition from extrusion to extension
Affected by the subduction of the Middle Tethys, the southwest part of the Cathaysian landmass where this area is located experienced the early discrete block compression (230 ~ 150 Ma, K- Ar, Ar-Ar method) and the late extension-strike-slip-expansion (145 ~ 70 Ma, K-Ar, Ar-Ar method) (. The magma source area gradually developed from the upper crust source area-middle and lower crust source area-continental lithosphere mantle source area-asthenosphere mantle source area to 140Ma, which is an important turning point of Nanling geological tectonic stress field from compression to relaxation and extension. In this period, K-Ar, N-Ar, N-Ar, and N-Ar are common in granites.
The extensional activities in the area are characterized by NW and NE extension: the NW thermal uplift led to the intrusion of mantle-derived materials along the NWW extension fault, and two groups of intermediate-basic rock walls were formed in the area: diabase (140Ma, K-Ar and Ar-Ar methods) and lamprophyre (105Ma, K-Ar and Ar-Ar methods). The NE-trending rifting extension is characterized by the formation of NE-trending uplift structure, accompanied by the emplacement of NE-trending intermediate-basic dikes (90Ma, K- Ar method) and granite porphyry dikes. The fault basin related to extensional structure has received huge red clastic rock accumulation, accompanied by basaltic magmatism (82Ma, 67Ma, K-Ar, Ar-Ar method). In the early tertiary period, the rifting expansion in this area tended to end.
The uranium mineralization in South China is 140 ~ 47ma (U-Pb method), which coincides with the extension period (Chen et al., 1998) (Hu,1990; Jiang et al.,1990; Jill Zhang Shundeng,1992; Shu et al, 1998,1999; Li Ziying et al., 1999), uranium deposits mostly occur in the fault depression zone formed during extrusion to extension (fig. 7- 19).
6. Uranium-rich sedimentary formations and uranium-rich rock bodies
The total thickness of Proterozoic structural layer (Pt) in this area is greater than 13000m, and the uranium content is (2.6 ~ 4.8) × 10-6 (carbonaceous clastic rocks reach 19× 10-6) (Table 7-/kloc-6) The total thickness of the early Paleozoic structural layer (Z-S) is greater than 10000m, and the uranium content is (3.3 ~ 4.6) × 10-6 (the carbon-silicon slate of the Lower Cambrian Xiangnan Formation in Zhuguang surrounding rock is 36 × 10-6). The total thickness of Late Paleozoic-Early Mesozoic structural layer (D-T2) is about 8000m, and the uranium content is (2.3 ~ 4.6) × 10-6 (up to (5.5 ~ 6) × 10-6 locally). The uranium content of red clastic rocks in the upper part of Mesozoic tectonic layer (K) and Cenozoic tectonic layer (R-Q) is (3 ~ 4.0) × 10-6, while the uranium content of light clastic rocks is relatively high, reaching as high as 13. 1× 10-6. The above-mentioned sedimentary formations are rich in uranium, and the uranium-rich strata as surrounding rocks are favorable conditions for the formation of uranium-producing rock bodies. According to the statistical results of uranium and thorium chemical analysis data of granite bodies in South China (Tan et al., 1998), uranium-rich granite bodies with uranium content greater than or equal to 13× 10-6 are: Guidong, Baipu, Qingzhang, Dadongshan (East), Daping, Zhongba and Dadongshan. Rock with uranium content of18 (10 ~13) ×10-6. There are 55 uranium-bearing rocks with uranium content of (6 ~ 10) × 10-6. The above data show that the uranium content of most granite bodies in South China is more than 2 ~ 5 times of the average content of acid rocks in the crust, and the uranium-rich geochemical background provides good conditions for uranium mineralization in this area. Granite-type uranium deposits in this area generally occur in rocks with uranium content greater than 13× 10-6. ?
Fig. 7- 19 Relationship between the faulted zone and uranium deposits in Zhuguang-Guidong area.
Table 7- 10 Uranium-bearing property of Paleozoic strata in eastern Nanling (10-6)
sequential
(According to the information of 290 Institute of Nuclear Industry)
7. Multi-stage alteration superposition
Hydrothermal alteration has many effects on uranium mineralization and enrichment, which can change the physical and mechanical properties of surrounding rocks and provide necessary channels and ore-hosting space for ore-forming solution migration and mineral precipitation. Changing the existing form of uranium in surrounding rocks can increase the content of active uranium, which is beneficial to the activation and migration of uranium and provides some uranium sources for ore-forming solutions. It can also provide favorable geochemical environment and uranium fixative for the precipitation and fixation of ore-forming materials (Zhang Bangtong et al., 199 1). High, medium and low temperature alteration superimposed areas and acid-base alteration superimposed areas are usually the areas where uranium deposits are concentrated. In particular, uranium mineralization is closely related to alkali metasomatism, chloritization and muscovization controlled by ductile regional structures, with the characteristics of early alkali and late silicon, upper silicon and lower alkali (Du Letian,1992; Wang Yusheng et al., 2000) and superposition law of redox alteration (Jin Jingfu et al., 1992). Rock fragmentation, surface alkali metasomatism, muscovite and chloritization development occurred in the superimposed area of multi-stage alteration. Alkaline metasomatism and muscovite play a decisive role in uranium mineralization. Granite without alkali metasomatism and muscovite, no matter how high its uranium content, can not be mineralized. Biotite is the carrier mineral of the most widely distributed ore-forming elements in granite (Zhang Bangtong, 1994), and its average uranium content is 6.98× 10-6, while that of muscovite is 1. 14× 10-6.
8. Migration of high and low level radioactive elements
Through the processing and research on the airborne gamma-ray spectrum data of 1: 50000 in this area (Jane et al., 1993), it is shown that the migration field of radioactive elements in rock mass in this area has the phenomena of high U and high Th, low U and low Th, high U and low Th, and low U and high Th. In the case of high U, high Th or low U and low Th, the ratio of Th/U is stable in the range of 1.6 ~ 2.8, which indicates that the late transformation of the rock mass is weak, reflecting the original state of the rock mass and the rock mass is basically not mineralized. However, high U, low Th or low U and high Th reflect the separation and redistribution of U from Th after intense post-activation transformation. High u and low Th are uranium metallogenic enrichment areas; Low u and high Th are the contribution areas of uranium mineralization caused by lack of u, and they are closely adjacent to each other, forming a migration and enrichment field of radioactive elements conducive to uranium mineralization. Therefore, uranium-rich deposits in this area are mostly located in high uranium fields, high potassium fields and low thorium fields, especially (or near) ultra-low thorium fields, which are often uranium-rich deposits (Figure 7-20). For example, there are 36 1 and 20 1 uranium-rich deposits in the thorium low field in the north of Longhua Mountain in Zhuguang, and there are rich ore bodies in the thorium low field 238 in the southeast. There are 337 uranium-rich deposits in the northeast of Shaxi-Yang Gong pit ultra-high thorium field (70×10-6 ~104×10-6) in eastern Guangxi. There are 338 and 339 uranium-rich deposits in thorium low field, uranium high field and potassium high field in high thorium rocks respectively.
Fig. 7-20 U×K/Th contour map of rock mass content in eastern Guangxi
9. Good reduction conditions and closed environment
A good reduction environment is a necessary condition for uranium mineralization. The uranium-rich granite body in the area is rich in reducing gas, and the reducing gas in the late small rock body is obviously higher than that in the early main magmatic intrusive rock. According to the calculation of Chen Anfu and Ou Guangxi (1997), 1m3 granite can precipitate gases above 100L at 100 ~ 500℃, in which hydrogen and hydrocarbons account for 20% and the rest is CO2. CO2 and carbonate minerals are actually oxidized organic compounds such as CH4 and CO, which still reflect the existence of hydrocarbons. Hydrogen and hydrocarbon reducing gas contribute to the reduction and precipitation of U and ore enrichment. In addition, uranium-rich deposits in this area are mostly located in chlorite alteration zone and intermediate-basic dikes, and the rocks in these areas often contain more Fe2+, which provides rich reducing agents for uranium mineralization. The change of chemical composition in chloritization is mainly caused by the sulfidation and hydration of rocks, and the content of pyrite in its accessory minerals is obviously increased (Wang Jianfang et al., 1989). Therefore, the chloritization belt itself can act as a reduction barrier, which is undoubtedly a very favorable geochemical background field for uranium mineralization.
A closed environment is an ideal place for uranium mineralization, enrichment and preservation. The rock mass in this area has a good closed environment at the top and edge of the rock mass during the mineralization period, which is an ideal condition for uranium mineralization, such as 20 1, 238, 337, 36 1, ZTJ and other uranium-rich deposits, and the ore bodies are all produced at the top or edge of the rock mass.
To sum up, Zhuguang-xia zhuang area is a famous uranium gathering area in China, where numerous uranium resources are gathered in southern China. The metallogenic age of most uranium deposits in this area coincides with the late Mesozoic magmatic activity, which reflects the inherent and inevitable relationship between the late Mesozoic tectonic-magmatic activity, the migration of crust and mantle materials, and the thermal energy and the location of the deposits (Deng et al.,1999; Chinese people, etc.,1999; Tao et al., 1999). Late Mesozoic granites are widely developed in Nanling area, which is related to lithospheric tension (Chen Peirong, 2004) and is the main dynamic condition of uranium mineralization in this area. At the same time, there are uranium-rich basement sedimentary rocks or metamorphic rocks in Nanling area, and the uranium content in granite is generally high, which lays a rich uranium source for uranium mineralization. The essence of large-scale mineralization is the accumulation process of huge ore-forming materials (Mao Jingwen et al., 1999). The concentrated distribution of uranium-rich deposits in Zhuguang and Guidong and a series of favorable geological backgrounds for uranium mineralization show that this area and even the whole Nanling area are favorable areas for uranium mineralization, and there is great potential for finding uranium-rich deposits. In view of this, the author puts forward that the nine characteristics described in this paper should be used as the basis for finding uranium-rich deposits. A new round of uranium prospecting research in Nanling area has a good prospect and is expected to find more granite-type uranium-rich deposits.