1. Overview
The Bayan Obo deposit in Inner Mongolia is a world-famous super-large iron-niobium rare earth (Fe-Nb-REE) deposit and the largest rare earth deposit discovered in the world. Its rare earth oxide reserves are approximately 8.6×107t, accounting for more than 70% of the world’s proven reserves, with a grade of 3% to 6%; its Nb2O5 reserves are approximately 2.8×106t, with a grade of 0.07% to 0.28%. It is the largest niobium deposit in my country. The iron reserves in the mining area are about 1.5×109t, with a grade of 33% to 35% (Zhu Xun et al., 1999). The iron ore body in Bayan Obo was first discovered by Mr. Ding Daoheng in 1927. In 1935, Mr. He Zuolin discovered rare earth minerals in the iron ore body and named them dolomite and Obo mine. They were later identified as bastnaesite and monazite. Mr. Huang Chunjiang discovered the East Iron Ore and West Iron Ore Group in 1944. The super-large iron-niobium rare earth deposit in Bayan Obo is rare in the world for its large scale, rich grade and complex mineralization process. The origin of the Bayan Obo deposit has not yet been determined, and examples of the occurrence of rare earth elements and iron are rare worldwide, which also increases the difficulty of research on the deposit. In recent years, some people believe that the Bayan Obo iron-niobium rare earth deposit should be classified as an iron oxide copper-gold deposit (IOCG deposit) (M.W. Hitzman et al., 1992), while others believe that it is similar to the Maoniuping and Mainland China rift zones in the Panxi rift zone in southern my country. Cao and other rare earth deposits are indeed of the same type (Xu et al., 2008). In view of the particularity of the Bayan Obo iron-niobium rare earth deposit and the importance of its status, this article summarizes its prospecting model in order to further search for this type of deposit.
II. Geological Characteristics
1. Regional geological background
The super-large iron-niobium rare earth deposit in Bayan Obo is located on the northern edge of the ancient Sino-Korean plate and the ancient Central Asian Ocean. The intersection of plates. The ancient continental plate and the ancient oceanic plate are separated by the Ulanbaolige Deep Fault. To the north is the ancient Central Asian Oceanic Plate, which is a set of oceanic deposits containing ophiolite; to the south to Baotou is the ancient Sino-Korean Plate. The northern edge of the Inner Mongolia axis is the northern part (Bai Ge et al., 1996; Hao Ziguo et al., 2002).
(1) Strata
The area is distributed from bottom to top with Archean Jining Group granulite strata, Wulashan Group feldspathic quartzite, diopside marble, and graphite flakes Neiss, amphibolite gneiss, and amphibolite strata; Paleoproterozoic Selten Mountain Group migmatized gneiss, migmatite, amphibole plagioschist, and metamorphic strata, Erdaowa Group chlorite schist, green cloud schist and amphibolite schist formation; Mesoproterozoic Baiyun Obo Group quartzite, slate and carbonate rock formation; Middle and Lower Silurian Baoerhantu Group marine medium-acidic- Intermediate volcanic formation; limestone, sandstone, slate and schist strata of the Bad Aobao Group of the Upper Silurian System; limestone, hard sandstone intercalated with tuff and acidic volcaniclastic strata of the Upper Carboniferous System; Jurassic spherical rhyolites rock and purple sandstone strata; the Tertiary series is widely distributed in various river valleys (Institute of Geochemistry, Chinese Academy of Sciences, 1988; Bai Ge et al., 1996).
(2) Structure
The structures in the area are dominated by tight folds and faults in the EW direction. From north to south, the fold structures are Beikang anticline, Wenguogeguo-Birut syncline, Kuangou anticline, Bayan Obo mining area syncline, Bayan Obo mining area south anticline, Sumutu syncline and Baiyun Zhennan anticline. About 20km NE in the mining area is the Ulanbaolige Deep Fault, which is the dividing line between the two plates. In addition, the Bayan Obo-Bayinjiakolak fault is also exposed in the Bayan Obo mining area. This fault starts from Baiyun Obo in the east and goes to Baiyinjiaolac and Ubulgong in the west. The exposed length in Bayan Obo is about 15km, which is the famous wide fault. Gou fault (Institute of Geochemistry, Chinese Academy of Sciences, 1988; Bai Ge et al., 1996).
(3) Igneous rocks
The igneous rocks are widely distributed in the region, among which intrusive rocks are the main ones, accounting for about one-third of the total area of ??the region. The lithology is medium-acidic rock. Mainly, there are few basic-ultrabasic rocks and even fewer alkaline rocks. The Archean rock mass is distributed in the southern foothills of the Zhertai Mountains and the northeastern part of Shetai, with a U-Pb isotope age of 2370-2470 Ma. Proterozoic intrusive rock bodies are mainly distributed in the Selten Mountains and Daqingshan Mountains. The Paleozoic intrusions are mainly late Paleozoic granite, mainly concentrated in the area from Guyang to Bayan Obo, and their lithology is mainly potassic feldspar granite and monzogranite.
Mesozoic rock bodies are sparsely distributed, mainly found in Guyang, Baotou, Wuchuan and other places, and are dominated by small potassic feldspar granite and alkali feldspar granite (Bai Ge et al., 1996).
2. Geological characteristics of the deposit
(1) Mining area strata and ore-bearing surrounding rocks
The mining area mainly exposes the Selten Mountain Group and the Bayan Obo Group. The former is located under the latter, and the two are in unconformable contact relationship. The Bayan Obo Group is a set of epimetamorphic rock series with shallow marine facies, large thickness, and drastic changes in lithofacies. It is mainly composed of quartzite, slate, and carbonate rock. It can be divided into 6 rock groups and 18 rock sections. From bottom to top, it consists of the Dulahala Formation (H1-H3), Jianshan Formation (H4-H5), Halahugot Formation (H6-H8), Bilut Formation (H9-H10), Baiyin Baolage Formation (H11-H13) and the Hujiertu Formation (H14-H18), the main mineralization layer is the H8 layer, and its main lithology is dolomite (Bai Ge et al., 1996), as shown in Figure 1.
Figure 1 Geological map of the Bayan Obo mining area (quoted from Wang Xibin et al., 2002, revised) 1—H1-H9 section of the Bayan Obo Group; 2—Erdaowa Group; 3—Migmatite; 4—Granite ; 5-H8 (ore-bearing carbonate rock body); 6-H9 (potassium-rich slate, local mineralization); 7-carbonate rock wall group and its number; 8-Birut ultrabasic rock; 9-gabbro Rock; 10—andesite; 11—stratal boundary; 12—inferred stratigraphic boundary; 13—fault; 14—ore body
Structurally, the ore-bearing dolomite can be divided into fine-grained dolomite and coarse-grained dolomite Dolomite, of which fine-grained dolomite is distributed in the periphery of the ore body, is mainly composed of dolomite and ankerite, and contains a certain amount of magnetite, monazite, bastnasite, barite and fluorite. Coarse-grained dolomite is mainly composed of dolomite, apatite and magnetite.
(2) Mining area structure and ore-controlling structure
The fold structure of the mining area is mainly a Kuangou complex anticline composed of Kuangou anticline and Baiyun syncline. This anticline structure The axis is EW, leaning westward. The rare earth minerals in Bayan Obo mainly occur in the transition zone between H8 dolomite and H9 slate on both sides of the Dolomite syncline on the southern flank of Kuangou anticline. The Institute of Geochemistry, Chinese Academy of Sciences (1988) divided the faults in the mining area into three groups based on the formation mechanics and combination relationship of the faults, namely, the nearly EW-trending wide-channel large faults, the strike reverse fault group, and the NE- and NW-trending ***yoke torsion groups. Fissure group. Large wide-groove faults cut through the crust and connect to the mantle, providing a channel for the migration of mineral-forming materials ( Yang et al., 2009 ).
(3) Magmatic rocks in the mining area
The intrusive rock masses in the Bayan Obo mining area are mainly granite, which is distributed in a large area in the north and south of the Bayan Obo deposit in the form of bedrock; followed by gabbro Rock types are produced in the form of small rock strains and rock walls; in addition, there are various types of basic, alkaline and acidic rock veins. Granite is widely distributed in the Bayan Obo mining area, accounting for about 2/5 of the bedrock distribution area. It is likely to be the product of the remelting of ancient continental crust in the Archaean (Bai Ge et al., 1996). The isochron age values ??of granodiorite, medium-coarse-grained biotite K-feldspar granite and fine-grained biotite K-feldspar granite are 316. 1 Ma, 257. 7 Ma and 236. 3 Ma respectively. Therefore, in the Bayan Obo mining area It mainly develops granitic rock bodies emplaced in the late Paleozoic (Yang Xueming et al., 2000). Gabbro bodies are distributed in the north and south of the Bayan Obo mining area. They were formed earlier than granite. There is no transitional relationship between them. They are not differentiated products of homologous magma (Bai Ge et al., 1996). These magmatic activities were obviously later than the mineralization time, and due to space limitations, only part of the ore body was transformed. Le Bas et al. (1992) reported dozens of carbonate dykes found in wide-channel mixed gneiss. These dykes are generally 0.5-2m wide and 10-20m long, cutting through the formation in a nearly upright shape. The trend is mainly NE and NW, and occasionally near EW veins are produced. Tao Kejie et al. (1998) divided carbonatite dykes into three types according to rock types: dolomite type, dolomite-calcite type and calcite type. In addition, alkaline rock veins such as neolite veins, soda amphibole veins, syenite veins and albite veins are also developed in the mining area.
(4) Ore body characteristics
The mining area starts from Dulahara in the east, to Abda and Ouluula in the west, and to Baiyun Village in the south. It is about 18km long from east to west and about 18km wide from north to south. 2km. The mining area is a nearly EW-trending syncline structure. The syncline axis is composed of H9 slate of the Bayan Obo Group, and the two wings are mainly distributed with H6-H8 dolomite of the Bayan Obo Group. The rare earth element ore body mainly exists in the H8 dolomite. Figure 2 shows the plane and cross-sectional distribution of ore-bearing dolomite carbonate rock bodies and iron ore bodies.
Figure 2 Schematic diagram of the plane (a) and cross-section (b) distribution of dolomite carbonate rock bodies and iron ore bodies in the Bayan Obo mining area (cited from Hao Ziguo et al., 2002, revised and quoted from China Institute of Geochemistry, Academy of Sciences, 1988)
For niobium-rare earth, the entire dolomite is an ore body. As for iron, according to the delineation of industrial grade, it is divided into four ore sections: the central ore section, the eastern ore section, the western ore section and the Sumutu ore section (Bai Ge et al., 1996). The iron ore bodies in the central ore section, eastern ore section, and western ore section are all accompanied by niobium-rare earth mineralization with industrial value.
1) Middle mining section: This mining section is located in the middle section of the syncline in the Bayan Obo mining area. The central ore section is the main ore section of the Bayan Obo deposit, consisting of the main iron ore body, the East Iron ore body, the East Jelegler iron-niobium rare earth ore body, the main iron ore body and the dolomite-type niobium rare earth in the footwall of the East Iron ore body. It consists of 5 ore bodies including the ore body and the syncline core ore body. The ore body is in the shape of a large lens and thick layers, with both ends pinching out in the dolomite. It changes greatly towards the depth and forms wedge-shaped branches pinching out in the dolomite. The axial length of the ore body is 1250m, the maximum thickness is 415m, the average thickness is 215m, and the maximum depth is 1030m. The occurrence is consistent with the surrounding rock. Rare earth reserves account for 32.1% of the total in the entire mining area, and niobium reserves account for 21% of the total in the entire area (Figure 3).
2) Eastern mining section: This mining section is composed of dolomite on the northern flank of the eastern section of the Dolomite syncline and slate and dark rock series in the core of the syncline intercalated with dolomite lenses. The strata on the southern flank of the syncline are entirely engulfed by Hercynian granite. The main types of ore bodies include contact zone skarn ore bodies, striped niobium rare earth iron ore bodies, massive niobium rare earth iron ore bodies and sodium amphibole type niobium rare earth iron ore bodies. It is in the shape of an irregular lens, narrow in the west and wide in the east. It is branch-like and pinches out in dolomite and biotitized slate, and narrows and branches in dolomite in the deep part. The axial length of the ore body is 1300m, the maximum thickness is 340m, the average thickness is 179m, and the maximum depth is 870m. The rare earth reserves in the east mining section account for 21.5% of the total reserves in the entire area, and the niobium reserves account for 10.8% of the total reserves in the entire mining area. The chassis outside the boundary of the east mining section is relatively rich, with a rare earth grade of 3.55%, and its reserves account for 16% of the total in the entire mining area.
Figure 3 Plane distribution diagram (top) and cross-sectional diagram (bottom) of ore types in the main iron ore body and the east iron ore body in the Bayan Obo mining area (cited from Hao Ziguo et al., 2002)
3) Western ore section: The western ore section is a nearly EW-trending syncline structure. Biotitized slate forms the core of the syncline, and dolomite forms the two wings of the syncline. There are alternately layered transition zones in between. The iron ore bodies are in the shape of irregular lenses and strips on the surface. The occurrence is consistent with the surrounding rocks and the trend is nearly EW. The ore bodies on the north and south wings of the syncline are connected at the lower part. The ore types are mainly dolomite type niobium rare earth iron ore and biotite sodium amphibole type niobium rare earth iron ore. The sodium and fluorine metasomatism here are weak. This ore section consists of 16 ore bodies, which are layered and lens-like distributed on both sides of the syncline and connected at depth. The ore body is 300-5300m long, with a maximum thickness of 9-110m and an average thickness of 9-27m. Among them, No. III and No. V ore bodies are the largest, with a maximum depth of 855m. The ore body strikes nearly EW direction, with an inclination angle of generally 70° to 80°. Rare earth reserves account for 8.5% of the total reserves in the region, and niobium reserves account for 43.7% of the total reserves in the region (Figure 4).
4) Sumutu Ore Section: Located about 2km south of the West Ore Section, the ore-bearing rock system is composed of 4 to 5 layers of dolomite mixed with interbedded potassium-rich slate rock layers or amphiboles. Composition of rock formations.
(5) Ore types, mineral combinations and structural characteristics
Bai Ge et al. (1996) divided ore types into layered lenticular ores and vein-like ores according to their occurrence. Two major categories; according to mineralized elements, they are divided into 3 categories: niobium rare earth iron ore, niobium rare earth ore and a small amount of niobium ore.
Niobium rare earth iron ore mainly occurs in the middle and upper parts of the dolomite layer or in the transition zone between dolomite and slate. The ore body is layered or lens-shaped. As far as iron ore is concerned, the central ore section has the best mineralization and the largest ore body size; followed by the western ore section; and the Sumutu ore section is even third. Dolomite niobium rare earth ore and dolomite niobium ore are the most widely distributed in the mining area, and the ore layers are consistent with the stratigraphy, forming the main body of the syncline and the north and south wings of the Sumutu syncline in the Bayan Obo mining area. Carbonate vein type niobium rare earth ores are mainly distributed in the gneiss belt of the Kuangou anticline axis and its flanks H1-H4 clastic rock series. The vein trend is perpendicular or oblique to the stratigraphic trend, and the occurrence is steep. The surrounding rock near the vein wall has a neonite alteration zone.
Figure 4 Schematic diagram of the interbedded structure of rock and mineral layers in the 40-line section (a) and 10-line section (b) in the western ore section of the Bayan Obo mining area (cited from Hao Ziguo et al., 2002)
The mineral composition of the Bayan Obo deposit is extremely complex, and more than 190 kinds of minerals (including variants) have been discovered, including nearly 20 kinds of niobium and tantalum minerals and nearly 30 kinds of rare earth minerals. Rare earth minerals monazite and bastnasite are the most widely distributed. Monazite is dominant in dolomite; bastnasite is dominant in the main iron ore body and Dongtie ore body, followed by monazite; Huanghe ore is mainly found in late veinlets in neolite-type iron ore body, and brown curtain Stone and bastnaite are mainly found in skarnized dolomite niobium rare earth ores in the eastern contact zone. The most widely distributed niobium minerals in deposits are mainly columbite, pyrochlore, pyrolite and niobium rutile. The iron minerals in the mining area mainly include magnetite, hematite and siderite, of which magnetite is the most widely distributed.
The most characteristic ore structure of the Bayan Obo deposit is a strip structure composed of minerals of different colors distributed in strips; massive structures are found in the main iron ore body and the middle part of the East Iron ore body and in dolomite and It is found in some feldspar slate ores; disseminated structures are mainly found in dolomite type and feldspar slate type ores. The ore structure is mainly fine-grained and unequal-grained crystalline granular structure, with a small part having granular columnar structure, scaly structure and fibrous structure.
(6) Wall rock alteration
The Institute of Geochemistry, Chinese Academy of Sciences (1988) divided the wall rock alteration in the Bayan Obo mining area into three types: contact metasomatic alteration, Fluorine-sodium metasomatic alteration and hydrothermal filling metasomatic alteration. Contact metasomatic alteration mainly develops in the contact zone between granite and dolomite in the east. Potassium metasomatism is relatively prominent, and fluorine and sodium metasomatism are also present. Various magnesia skarns are formed at this stage. Fluorine-sodium metasomatic alteration is the most widely developed metasomatism in the area, and is mainly developed in the main ore, east ore and west ore. Fluorine mainly reacts with calcareous rocks, while sodium mainly accumulates in rocks with higher siliceous content. Hydrothermal filling metasomatic alteration is characterized by the formation of veinlets with various mineral combinations. Specifically, the wall rock alterations mainly include feldsparization, fluoritization, neonization, alkaline hornblenization, biotitization, phlogopitization, apatitization, bariteization, carbonatization and skarnization. etc. (Yang Kuifeng, 2008).
(7) Age of rock formation and mineralization
So far, the metallogenic age data of Bayan Obo reported in the literature span a wide range, and most ages use U-Pb, Pb of rare earth minerals. -Pb, Sm-Nd isotope system analysis and testing, the age value is 0. 4 ~ 2. 0Ga, and the age data are mostly concentrated in 1. 2 ~ 1. 4Ga (Zhang Zongqing et al., 1994, 2003; Fan Hongrui et al., 2002; Liu Yulong et al., 2005). Ren Yingchen et al. (1994) divided the major geothermal events experienced by the Bayan Obo deposit into the following phases based on a large amount of isotope age data: ① Middle Neoproterozoic; ② Caledonian; ③ Hercynian. These age data imply that the magmatism of the Bayan Obo carbonatites occurred in the Mesoproterozoic, while the Caledonian and Hercynian age data represent the emplacement time of the granites in the Bayan Obo area. Mineralization is closely related to Mesoproterozoic carbonate activity.
3. Origin of mineral deposits and signs of ore prospecting
1. Origin of mineral deposits
The origin mechanism of this mineral deposit has always been controversial, and there are mainly 6 views: :
1) Sedimentary origin: Meng Qingrun et al. (1992) pointed out after studying the O and C isotopes of dolomite that the residual microcrystalline and micrite limestone in H8 dolomite is a sign of sedimentary origin.
2) Biological sedimentary origin: Qiao Xiufu et al. (1997) pointed out based on the study of sequence stratigraphy and event stratigraphy that the ore-bearing dolomite of the Bayan Obo deposit is neither igneous carbonate nor general layered. Sedimentary rock, but a giant microcrystalline mound.
Figure 5 Schematic diagram of the mineralization model of Bayan Obo (cited from Baige et al., 1996)
3) Origin of hydrothermal metasomatism: Chao et al. (1992) mainly based on the Caledonian formation of the deposit The dolomite is regarded as the product of Caledonian hydrothermal metasomatism based on the ore age and mineral triplet metasomatism characteristics.
4) Origin of intrusive carbonate: Liu Tiegeng (1986) relied on the study of O and C isotopes of dolomite and concluded that dolomite is not of sedimentary origin, but should be an igneous dolomitic carbonate of endogenous origin.
5) Origin of dolomite metasomatized by mantle-derived fluids based on carbonate walls: Yang et al. (2009) believed that H8 dolomite was metasomatized by mantle-derived fluids based on carbonate walls based on C-O-Nd isotope data. Metasomatic formation of ultra-large rare earth deposits.
6) Origin of volcanic eruption: Yuan Zhongxin et al. (1991) pointed out that the minerals forming the Bayan Obo deposit may come from the mantle in the form of rare earth-rich fluids. Dolomite is very similar to the world's carbonate rocks in chemical and mineral composition, but they are not ordinary intrusive carbonate rocks. They may be formed when deep carbonate magma enters the ocean basin and is deposited in the form of volcanic eruptions or fumes.
The current mainstream view in academic circles is that deep-source fluids from the earth's mantle rise along faults, enter the faulted ocean basin, mix with seawater, and deposit, thereby forming eruptive sediments-hydrothermal metasomatism type iron-niobium. - Rare earth deposits (Chen Hui et al., 1987; Zhai Yusheng et al., 1999). The northern edge of the North China Craton experienced a large-scale extension process during the Mesoproterozoic, triggering large-scale rifting in the area (Shao Jian et al., 2002). Because the Bayan Obo area has been in a rift environment for a long time, the upper mantle material in the area is in a partially molten state, forming a large-scale alkaline magma chamber. In the later stage of the Jianshan Formation, the Kuangou fault cut through the mantle, causing rare earth-rich carbonate fluids to continuously rise and overflow. Due to the low viscosity and density of carbonate fluids and the relatively high content of gas-liquid components, carbonate fluids have strong fluidity and a strong ability to transport large-ion lithophile elements and high-field-strength elements such as alkali metals and LREEs. The above-mentioned fluids are continuously ejected. Due to the high salinity and density of the fluids, they gradually gather into high-salinity hot brine at the bottom of the ocean basin. Elements such as niobium and rare earths also diffuse from the overflow center to the surrounding areas. Deposition begins when elemental concentrations reach saturation, starting with Ca-CO3, which is a mixture of mantle-derived calcium and carbon dioxide and seawater calcium and carbon dioxide. Due to the precipitation of CaCO3, the concentration of magnesium ions in the hot brine increases, and most of the calcite micrite is metasomatized by magnesium and transformed into dolomite, followed by the deposition of siderite and magnetite (Figure 5). This process probably took place over a long period of time, resulting in the formation of the famous iron-niobium-rare earth element deposits in Bayan Obo.
2. Prospecting signs
(1) Geological prospecting signs
1) The development of regional deep faults plays a role in the formation of this type of deposits It plays a very important role in cutting through the crust, communicating with the mantle, and providing an ascending channel for the mantle-derived magma fluid system. Bai Ge et al. (1996) listed 5 metallogenic belts for searching for this type of mineral deposits: ① The Yanshanian alkaline-subalkaline volcanic rock distribution area in the northern margin of the North China Platform and the Hercynian fold belt; ② The southern foothills of the Tianshan Mountains and the Tarim Massif Transition zone; ③ the north and south sides of the Qinling fold system; ④ the Luxi platform convex area on the west side of the Tanlu fault zone; ⑤ the Kangdian axis and the Ailaoshan fold belt.
2) The ore-bearing strata of the Bayan Obo Group are mainly dolomite, which should be a key target for mineral prospecting and exploration. The original material of the dolomite and iron ore layers was deposited in a trough-shaped lagoon.
3) The emergence of basic rock-alkaline rock-carbonate rock combination is an important symbol of this type of deposit. Because this type of deposit is closely related to carbonate rock, it is an important carrier of rare earth metals.
4) The appearance of rare earth minerals is a direct sign of the prospecting of this type of mineral deposit, such as monazite and bastnasite that are widely distributed in the mining area.
5) Aggregates of altered minerals such as alkaline hornblende, phlogopite, nite, albite, and disseminated or vein-like alkali appear in carbonate rocks and schists close to carbonate rocks. Sexual intercourse is a sign of rare earth prospecting.
6) The existence of fluorine metasomatism in carbonatite can also be used as a sign of ore prospecting. This metasomatism can be easily discovered based on fluoritization.
(2) Geophysical prospecting signs
Magnetic anomalies in the area are generally low, which is not only related to the deep burial of iron ore bodies, but also to the fact that some magnetite bodies in the area It is related to oxidation to hematite, so it shows weak magnetism. Therefore, when carrying out magnetic prospecting in mining areas, attention should be paid to the evaluation of weak magnetic anomalies and comprehensive analysis combined with abnormal morphology and geological conditions.
(Yang Zongxi)