Classification,,metallogenesis,and,exploration,of,silver,deposits,in,Daxing’anling,of,Inner,Mongolia,and,its,adjacent,areas

Bio Jing, Dng-hong Wng,*, Yu-hun Chn, Tong Zhng, Xiu-lng Pu, Wn-wn M, Yn Wng,Gung Wu, Li-wn Wu, Tong Zhng, Xu-jio Li, Ji Yn, Yu-shn Zuo, Hong-jun Sun, Zhi-yun Li

a MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, China Geological Survey, Ministry of Natural Resources, Beijing 100037, China

b Xi’an Institute for Innovative Earth Environment Research, Xi’an 710061, China

c School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China

d Key Laboratory of Magmatic Activity Mineralization and Prospecting of Inner Mongolia Autonomous Region, Geological Survey Institute of Inner Mongolia Autonomous Region, Hohhot 010020, China

e Chifeng Yubang Mining Co Ltd, Chifeng 024000, China

Keywords:Silver deposit Deposit type Porphyry silver deposit Supergiant silver deposit Metallogenesis Mineral exploration engineering Prospecting direction Daxing’anling Inner Mongolia

ABSTRACT By the end of 2020, 83 silver deposits (or ore occurrences), including four super-large-scale deposits, nine large-scale deposits, 33 medium-scale deposits and 37 small-scale deposits or ore occurrences, have been proved. The amount of silver metal exceeds 86000 t with average grade of 100 g/t, which makes Daxing’anling region one of the the most important silver ore belt in China. However, the metallogenic characteristics and metallogenesis need to be clarified. The silver deposits in the study area are classified into three main types, which are magmatic hydrothermal vein type, continental volcano-subvolcanic type and skarn type, respectively. The supergiant deposits include the Shuangjianzishan deposit (silver metal amount of 15214 t with average grade of 138 g/t), the Baiyinchagandongshan deposit (silver metal amount of 9446 t with average grade of 187 g/t), the Huaobaote deposit (silver metal amount of 6852 t with average grade of 170 g/t), and the Fuxingtun deposit (silver metal amount of 5240 t with average grade of 196 g/t). The silver deposits are mainly distributed in the central and south of the Daxing’anling area, and mainly formed in the Yanshanian period. The silver polymetallic deposits in the Daxinganling area are significantly controlled by regional faults and the junction zone of volcanic rock basins and their margins.The north-east trending deep faults are the most important ore-controlling structures in this area. The distribution of silver polymetallic deposits along the main faults is obvious, and the intersection area of multiple groups of faults often form important mine catchments. The Permian is the most important orebearing formation in this area, but some important silver polymetallic deposits occur in Mesozoic volcanic basins or pre-Mesozoic strata. The magmatic rocks related to mineralization are mainly intermediate acidic or acidic intrusions, intermediate acidic lavas, pyroclastic rocks, and small intrusions of ultra-shallow or shallow facies of the Yanshanian Period. The mineralization element combination is mainly determined by the elemental geochemical background of surrounding rocks or source layers. In addition, the type of deposit, the distance from the mineralization center, and the degree of differentiation of ore-forming rock mass are also important influence factors. The article analyzes the prospecting prospects of each silver deposit type in the study area, discusses the relationship between mineralization center and deep prospecting, and proposes that porphyry silver deposits should be paid attention to. In the prospecting and exploration of silver deposits, comprehensive evaluation and multi-target prospecting need to be strengthened because silver can coexist or be associated with a variety of metals.

The Daxing’anling area (also name as Great Xing’an Range) has always been an important metallogenic belt in China, especially the Daxing’anling area of Inner Mongolia.Since the 1970s, it has been an important silver-producing region in China. A large number of silver or silver-bearing deposits have been discovered successively, including the Baiyinnuoer Pb-Zn deposit (Zhang DQ et al., 1991), the Dajingzi Cu-Ag-Sn-Pb-Zn deposit (Nie FJ et al., 2016), the Wunugetushan Cu-Mo-Ag deposit (Zhao YM et al., 1997),the Jiawula-Chaganbulagen Ag-Pb-Zn deposit (Nie FJ et al.,2011), and the Meng’entaolegai Ag deposit (Zhang Q et al.,2000; Yang H et al., 2020). Since 2000, China’s silver ore prospecting made major breakthroughs, and the Daxing’anling has become the area with the largest breakthroughs in silver mine prospecting in China. Newly discovered in this area are the Shuangjianzishan supergiant Ag deposit (Sun KW et al., 2013), the Fuxingtun supergiant Ag deposit (Inner Mongolia Land and Resources Exploration and Development Institute, 2016), the Huaaobaote supergiant Ag deposit (Li ZX et al., 2008), the Baiyinchagandongshan large Cu-Pb-Zn-Ag-Sb deposit (Yao L et al., 2017), and a number of silver deposits with very considerable resource reserves. By the end of 2020, the Daxing’anling area of Inner Mongolia had accumulated 83 proven silver deposits (locations) (including co-associated types) with an amount of silver metal exceeding 86000 t, including four supergiant-size deposits, nine largesize deposits, 33 medium-size deposits and 37 small-size deposits or occurrences, and the average grade reaches 100 g/t, which quickly became the core production area of silver mines in China (Jiang B et al., 2021). Although predecessors have studied and summarized the silver deposits in this area(Lü ZC et al., 2000; Pan J et al., 2000; Niu SY et al., 2011;Shen CL et al., 2019; Jiang HY et al., 2020), with a series of major breakthroughs and new progress in the prospecting of silver mines in recent years, the silver deposits in this area have shown new characteristics of huge resource reserves,complex mineralization processes, diverse types of deposits,multiple mineralization eras, and diverse prospecting spaces and objectives. The summary of the law of silver mineralization in this area and the discussion of the direction of prospecting will help to deepen the understanding of the silver mineralization in the Daxing’anling area of Inner Mongolia and its adjacent areas, enrich the theory of silver mineralization, and promote the practice of prospecting for silver mines.

The Daxing’anling generally run in NE or NNE direction,bordering Russia in the north and Mongolia in the west,including a vast area west of the Songliao Plain, and the administrative area includes eastern Inner Mongolia and northwestern Heilongjiang Province. Tectonically, it is located in the eastern part of the Central Asian-Mongolian Trough fold zone between the Sino-Korean (North China)Plate and the Siberian Plate. It is in an area where the Paleozoic Asian tectonic-mineralization zone and the Mesozoic Pacific Rim tectonic-mineralization zone are strongly superimposed (Zhao YM et al., 1997). The strata in the Daxinganling area are relatively fully exposed, from Proterozoic to Cenozoic strata, and the Late Paleozoic and Mesozoic strata are the most developed (Wang SJ et al.,2020). The Permian is the most widely distributed Paleozoic strata in this area, and there are many non-ferrous metal deposits (Fig. 1). The Lower Permian is mainly composed of marine clastic rocks and volcanic sedimentary rocks, while the Upper Permian is composed of inland lake sedimentary rocks. The Jurassic is the most important Mesozoic strata and widely distributed in this area. It is mainly composed of continental volcanic rocks and volcanic-sedimentary rocks(Bureau of Geology and Mineral Resources of Inner Mongolia Autonomous Region, 1991). Under the influence of the Pacific Plate movement, Daxing’anling evolved into the littoral-Pacific continental margin tectonic belt in the Late Paleozoic. Subsequently, under the tectonic compression of the relative movement of the Pacific Plate and the Eurasian Plate, some early structures removed and produced some new faults, forming mainly compression uplift and partial extensional faulted basins. The uplift, depression and magmatic belt formed by the strong Mesozoic tectonic movement and magmatic activity constitute the basic framework of Daxing’anling (Zhao YM et al., 1997; Liu B,2014). The Mesozoic tectonics in this area are mainly faults,and the structural lines are generally NE trending, EW trending and NW trending faults are also relatively developed.There are several Mesozoic volcanic faulted basins with sedimentary clastic rocks and coal seams formed by the near-NE and near-EW trending tectonic assemblages. Magmatic activity was frequent in the research area, and there were strong volcanic eruptions and magmatic intrusions from Proterozoic, Paleozoic to Mesozoic, and intrusive rocks were formed, with the Late Hercynian and Yanshanian rocks being the most widely distributed. The Late Hercynian granites were mainly exposed in the fold belt on both sides of Daxing’anling, while the Yanshanian granites were mostly formed after the Late Jurassic and distributed in the middle and east of Daxing’anling in a NE direction. There are some differences in rock type and occurrence between the Paleozoic and Yanshanian Magmatic rock mass. The former belongs to the syngenetic tectonic period granitoids with large batholith production and gneissic structure. The latter mainly intruded along faults and mostly formed small batholith or intrusive stock. The non-ferrous metal and silver deposits found in the research area are mainly related to Yanshanian fault activity,volcanic eruption and magmatic intrusion (Zhao YM et al.,1997).

In order to comprehensively predict and evaluate China’s silver resources and solve the practical problems of different exploration degrees and asymmetric geological information,the national mineral resources potential evaluation project team puts forward the concept of prediction type. This classification is based on genetic types and is also combinedwith the common metallogenic geological conditions and mineral prediction elements of the deposit. It is generally determined according to different geological elements in different tectonic units and regional metallogenic belts.According to Chen YC et al. (2010) the prediction types of silver deposits in China are mainly divided into seven categories and 25 specific types, namely hydrothermal type(vein type and stratabound type), marine volcanic sedimentary type, continental volcano-subvolcanic type, skarn type,sedimentary-metamorphic type, sedimentary type and weathering leaching type. According to the type of surrounding rock and mineralization characteristics of the deposit, combined with the number of deposits and the identified silver reserves, the silver polymetallic deposits in Daxing’anling and its adjacent areas can be divided into three main types according to their importance, namely magmatic hydrothermal vein type, continental volcano-subvolcanic type and skarn type. A total of 83 silver polymetallic deposits (or ore occurrences) have been identified in this area, with proven silver reserves of about 87990 t (Jiang B et al., 2021). Among them, there are 55 magmatic-hydrothermal vein deposits,accounting for 68% of the total, and the identified silver reserves are about 65183 t, accounting for 74% of the total; 19 continental volcanic-subvolcanic deposits, accounting for 23% of the total, and the identified silver reserves are about 19208 t, accounting for 22% of the total; Seven skarn deposits, accounting for 9% of the total, and the identified silver reserves are about 3599 t, accounting for 4% of the total(Fig. 2).

Fig. 1. Distribution of Mesozoic tectonic-magmatic Ag polymetallic deposits in Daxing’anling and its southern adjacent areas (base map modified from Zhao YM et al., 1997). Fault system: F1-Taipei margin deep fault in North China; F2-Xilamulun deep fault; F3-Eerguna-Hulun deep fault; F4-Yiliekede-Elunchun deep fault; F5-North-Erlian-Dongwuqi deep fault; F6-Huanggangliang-Ulanhot fault; F7-Nenjiang deep fault;F8-Tayuan-Mohe fault; F9-Taxi-Linkou fault; F10-Yaluhe fault; F11-Tuquan fault.

3.1. Magmatic hydrothermal vein type

The Magmatic-hydrothermal vein type is the most important type of silver polymetallic deposit in the research area. A total of 55 polymetallic deposits (or ore occurrences)of this type have been found in this area, including three supergiant (Ag 37963 t), six large (Ag 14039 t), 22 medium(Ag 11019 t), and 24 small-sized deposits (Ag 2163 t) (Fig. 2).They are mainly distributed in the central and south sections of the Daxing’anling and the areas near the northern edge of the North China Plate. The Tuquan-Wengniute Pb-Zn-Ag-Fe-Sn-REE metallogenic belt is the densest, and a large number of large and supergiant silver deposits have been developed,including the Shuangjianzishan deposit and the Meng’entaolegai deposit (Dai M et al., 2022). The Dongwuzhumuqin Banner-Nenjiang Cu-Mo-Pb-Zn-W-Sn-Cr metallogenic belt and the Bainaimiao-Xilinhot Fe-Cu-Mo-Pb-Zn-Cr-(Au-Mn)-Ge-Coal-Tronao-Mirabilite metallogenic belt are also important distribution areas for such silver deposits.The former has developed the 1017gaodi (also name as Highland No.1017) and the Hua’naote large Ag deposit, while the latter includes the Huaobaote supergiant silver deposit, the Baiyinchagandongshan supergiant Sn-Ag-Zn deposit and the Bairendaba large Ag-Pb-Zn deposit.

The surrounding rocks of this type of silver polymetallic deposit are mainly Paleozoic strata, especially the Permian.The lithology is mainly clastic rocks-volcanic sedimentary rocks, mudstones, etc. For example, the Shuangjianzishan and the Baiyinchagandongshan supergiant deposit are both produced in the clastic-volcanic sedimentary rocks of the Dashizhai Formation of the Lower Permian (Fig. 3; Sun KW et al., 2013; Yao L et al., 2017). The surrounding rocks of the Huaaobaote supergiant deposit are tuff siltstone and tuff fine sandstone splint rocks of the Shoushangou Formation of the Lower Permian. The surrounding rocks of the Dajingzi deposit are mainly Upper Permian Linxi Formation, while the Bairendaba and Weilasituo deposit occurs in the Early Paleozoic Xilinhaote metamorphic complex. There are also deposits produced in Mesozoic or Precambrian strata. It should be pointed out that although the Meng’entaolegai deposit is produced in the magmatic rock mass, chronological studies have shown that whether it is the Mengntaolegai granite base as the surrounding rock or the surrounding rock mass that has been discovered, the formation time is very different from the mineralization time (Jiang SH et al., 2011),therefore, the Meng’entaolegai granite is only an ore-bearing surrounding rock, and the deposit is classified as a magmatic hydrothermal vein type in this paper. The surrounding rocks of this type of silver deposit are dominated by medium and low temperature alteration, such as silicification, carbonation,pyrite sericitization, sericitization, chloritization, dikazite,illitization, etc. They are affected by hydrothermal properties,activity intensity, scale and method. The types and scales of surrounding rock changes in different deposits have certain differences, but generally show a certain pattern of zoning and strength changes, and are closely related to silver mineralization, which is of good instructive significance. For example, the Bairendaba and the Weilasituo deposits, within a few kilometers outward from the quartz porphyry body, with the sodium-calcium-strontium alteration zone as the center, gradually transition to a strong greisenization zone,and then to a fracture-controlled vascularized greisenization zone, and then gradually transition to a fracture-controlled silicification and sericinization zone (Liu YF et al., 2016),where silver mineralization occurs around the rock body and is related to the outermost silicification and sericinization.Fracture is the main controlling factor of this type of deposit.The ore-forming fluid filled, cooled and unloaded at favorable structural sites such as faults or fractures to form vein-like,layered, layered-like, and lenticular ore bodies. The size,quantity, morphology and stability of the ore-body basically depend on the characteristics of the controlling fracture. The Shuangjianzishan deposit, the largest silver deposit in Asia, is a typical representative of magmatic hydrothermal vein type silver polymetallic deposit in the Daxing’anling area. It has the characteristics of obvious fracture control, large scale andlarge number of ore bodies.

Fig. 2. Number and the silver reserves of major silver deposit types in Daxing’anling and its adjacent areas .

Fig. 3. Profile of the Baiyinchagandongshan Sn-Ag polymetallic deposit, Xiwuzhumuqin Banner, Inner Mongolia (modified from Yao L et al., 2017) .

The ore structure of magmatic hydrothermal vein type silver polymetallic deposits in this area shows obvious characteristics of filling and metasomatism genetic. The ore structure is mainly massive, veinlet disseminated, breccia,veinlet and banded. The mineral structure includes allohedralhypidiomorphic granular structure, residual structure,metasomatism structure, exhalation structure, inclusion structure, emulsion structure, etc. The main metallic mineral assemblages of different deposits are different, and the mineral assemblages of the same deposit are varied in each metallogenic stages. In general, each deposit generally contains galena, sphalerite, pyrite, chalcopyrite and silverbearing sulfide minerals. From the mineralization center to the periphery and the evolution of the mineralization stage from early to late. The ore-forming elements generally show the evolution sequence from Cu→Zn→Pb→Ag and form spatial mineralization zones. Silver minerals are mainly formed in the middle -late stage of mineralization and develop in the outer zone of mineralization. Except for the presence of invisible silver in minerals such as galena, sphalerite, pyrite ,etc., the main independent silver minerals formed include gold and silver intermingles, silver sulfides and silver sulphates. It should be pointed out that the mineral assemblage of magmatic hydrothermal vein type silver polymetallic deposit in this area is very rich and characteristic. In addition to the above-mentioned common minerals, some deposits have developed a large number of minerals containing Sn, Sb, Mn,Au, etc., and can even form independent ore bodies, which will eventually form large comprehensive deposits together with silver ore bodies. For example, there are many tin minerals in the Shuangjianzishan deposit. In addition to a large amount of sulfur-silver-tin ore, there are also stannite,cassiterite, etc (Jiang B et al., 2018). The Baiyinchagandongshan deposit, Similar to the Shuangjianzishan deposit, is a supergiant comprehensive deposit with Sn, Ag, Zn, Pb and Cu (Jiang B et al., 2021).

3.2. Continental volcanic-subvolcanic type

The continental volcanic-subvolcanic type silver polymetallic deposits in this article refer to deposits that develop in continental volcanic-subvolcanic basins and are closely related to the continental volcanic-subvolcanic rocks in time, space and genesis. They include not only volcanic exhalation deposits and volcanic hydrothermal deposits, but also hydrothermal deposits that are closely related to epigenetic and ultra-epigenetic subvolcanic rocks. Although the number of silver polymetallic deposits of this type has been found in this area is not large, but those with a large scale or more account for a relatively large proportion (more than 30%). Nineteen silver polymetallic deposits of this type have been discovered in this area, including one supergiant(Ag 5240 t), five large (Ag 8858 t), 10 medium (Ag 4917 t),and three small deposits (Ag 193 t) (Fig. 2). They are mainly distributed in the Tuquan-e’erguna Cu-Mo-Pb-Zn-Ag-Aufluorite metallogenic belt. Although the number of deposits of the latter is not as large as the former, four of the six large or supergiant silver polymetallic deposits (the Jiawula deposit,the Chaganbulagen deposit, the E’rentaolegai deposit and the Wunugetushan deposit) and two medium-sized deposits are located in this metallogenic belt. The ore-forming element assemblage is mainly Ag-Pb-Zn, as well as Cu-Mo-Ag assemblages. The former is such as the Wunugetushan Cu-Mo(Ag) deposit and the Lianhuashan Cu-Ag deposit. The latter is typically represented by the Aonaodaba Ag-Sn deposit. It should be pointed out that although manganese has not formed an industrial ore body, the combination of silver and manganese minerals in the E’rentaolegai and Jiawula-Chaganbulagen ore fields is relatively developed, and obvious ferromanganese mineralized rhyolite can also be seen in the Fuxingtun mining area, which can be used as an important prospecting mark.

Continental volcanic-subvolcanic type silver polymetallic deposits in the study area are mainly occurs in Mesozoic continental volcanic basin. The surrounding rocks are mainly Mesozoic volcanic sedimentary rocks. mineralization is controlled by faults, volcanic mechanisms and related supporting structures, and the mineralization depth is generally shallow. The surrounding rock has strong alteration,and the range of alteration is wide, the types are complex, and it is closely related to mineralization. The material composition and structure of the ore are complex and diverse,and the metal content varies greatly. Typical deposits include the Erentaolegai deposit (Zhang Q et al., 1996; Lü ZC et al.,2002; Huang CK and Zhu YS, 2002), the Jiawula-Chaganbulagen deposit, and the Fuxingtun deposit (Inner Mongolia Land and Resources Exploration and Development Institute, 2016). Discovered in 2016 and has a proven silver metal content of more than 5000 t, making the Fuxingtun deposit the largest continental volcanic-subvolcanic silver deposit in Daxing’anling. It is produced in the Mesozoic volcanic sedimentary basin. The Upper Jurassic Manketouebo Formation is the main ore-hosting rock (Fig. 4). Industrial mining has not yet begun in the Fuxingtun mining area. The ore bodies controlled by drilling are all hidden ore bodies. The ore bodies are stratified and lenticular, and are dominated by silver (zinc-lead), lead-zinc and zinc. The ore structure is mostly disseminated, vein-like, and lump-like, while the alteration of surrounding rock is mainly pyrophyllite,chloritization, kaolinization, and carbonation.

3.3. Skarn type

Skarn silver polymetallic deposit is a relatively minor type of silver deposit in Daxing’anling. This type of silver deposits are few in number, but there are still large silver deposits,such as the Baiyinnuoer and Erdaohe deposit. The Baiyinnuoer is dominated by lead-zinc ore and associated silver resources reach a large scale. Seven silver polymetallic deposits of this type have been discovered, including two large-scale deposits (Ag 2887 t), one medium-scale deposit(Ag 379 t) and four small-scale deposits (Ag 333 t) (Fig. 2).They are mainly distributed in the Tuquan-Wengniuite Pb-Zn-Ag-Fe-Sn-REE metallogenic belt and the Dongwuzhumuqin Banner Cu-Mo-Pb-Zn-W-Sn-Cr metallogenic belt. Except for the Chaobuleng Fe-Ag deposit, the other skarn silver polymetallic deposits in this area are mostly Ag-Pb-Zn deposit. The surrounding rocks of this type of deposit are mainly skarn carbonate rocks, such as limestone (or marble),dolomitic limestone and so on, though some surrounding rocks are volcanic rocks. The surrounding rocks of the Erdaohe deposit is altered rhyolitic crystal tuff (Fig. 5). The magmatic rocks related to skarn silver polymetallic mineralization are mainly intermediate-epigenetic to ultraepigenetic intermediate-acid and acidic intrusions, and the ore bodies are distributed in or near the contact zone of the intrusive rocks and their surrounding rocks. This type of deposit occurs mostly in skarn of the outer contact zone and afew in the altered intrusions of the inner contact zone. The ore-controlling structures are generally fold superimposed faults, interlayer broken structures, and rock mass contact structures. The control of the intersection of two tectonic systems on mineralization is usually very obvious. The morphology of ore bodies is complex, normally shows irregular layered, vein-like, lenticular and lentil-like. The ore bodies are generally of medium-size, with a strike length of 100-500 m and a thickness of 3-10 m. Mineralization has undergone hydrothermal stage from high temperature to medium-low temperature, and can be divided into skarn stage,quartz-sulfide stage, quartz-carbonate stage and epigenetic mineralization stage. The composition of the ore is complex,and the ore minerals are mainly oxides and sulfides, such as magnetite, hematite, pyrrhotite, galena, sphalerite,chalcopyrite, pyrite, arsenopyrite, pyroxene, etc. Gangue minerals are mainly diopside, garnet, wollastonite and other common minerals in skarn stage. The structure of the ore is diverse, with massive, disseminated, banded, vein-like, etc.The mineral structure is mostly crystalline, metasomatism and solid solution separation. Generally speaking, no matter the type of skarn, or the mineral composition and element assemblage, there is obvious zoning phenomenon.

Fig. 4. Profile of K10 exploration line in the Fuxingtun deposit in Horqin Right Front Banner, Inner Mongolia (modifed from Inner Mongolia Land and Resources Exploration and Development Institute, 2019).

Fig. 5. Profile of No.00 exploration line of No.1 orebody in the Erdaohe mining area, Zalantun City (modified from Zalantun Guosen Mining Co., Ltd., 2017) .

The Baiyinnuoer deposit is the most representative skarn deposit in the study area and has the characteristics of large number of ore bodies, complex morphology, large variation in yield, and distribution of ore bodies in groups (Jiang SH et al.,2017). The ore bodies are mainly distributed at the top and flanks of the anticline. Most of the ore bodies are layered and plate-like, and the spatial distribution is completely controlled by the anticlinal structure. The skarn belt has a close spatial distribution relationship with the Pb-Zn-Ag orebody. Among them, the diopside skarn is the most important surrounding rock. Both the alteration and the mineralization have obvious zoning characteristics. In the lateral and horizontal zoning,there is a gradual transition from the combination of high and medium temperature elements to the combination of medium and low temperature elements. In the vertical zoning, it shows a trend of change from Zn-Cu mineralization→Zn→Zn+Pb.The Erdaohe deposit is a newly discovered large-scale independent silver deposit. At the end of 2017, the cumulative proven silver metal amount was close to 2000 t (Jiang SH et al., 2017; Jiang B et al., 2021). Its mineralization characteristics are significantly different from those of the Baiyinnuoer and the Haobugao deposit. The surrounding rocks are not carbonate rocks, but metamorphic rhyolitic cuttings and crystal cutters tuff. The orebody is mainly irregular layered and vein-like, followed by lentil-like and lenticular. The ore body has the characteristics of branching and compounding, enlargement and shrinking along the tendency and direction, and its morphology and shape are controlled by the mineralization change and fragmentation zone. The types of alteration of surrounding rocks are skarn,silicification, carbonation, epidote, chloritization, sericite and pyritization, and the boundaries of the top and bottom plates are relatively clear. The alteration zones of the surrounding rocks in the mining area are weak and the regularity is not obvious. From the inside out, they can be roughly divided into silicified alteration zone, pyrite-sericitization zone,sericitization zone, carbonization zone, chloritization zone and epidotization zone (Liu YJ, 2014). There are two types of ore bodies in the Erdaohe deposit. One is a medium- and lowtemperature contact skarn rock-type ore body with a high grade of Ag, Pb and Zn; the other is a hydrothermal filled ore body with a relatively low grade of Ag, Pb and Zn(Wang JJ et al., 2013; Liu YF, 2014; Cui XW et al., 2015; Yang FT,2018).

4.1. Spatial and temporal distribution

Although there are many silver polymetallic deposits in the Daxinganling area and they are widely distributed, the mineralization era is very concentrated, and most of them are products of Yanshanian period. Li HN et al. (1999) divided the Yanshanian silver polymetallic deposits into three metallogenic series, corresponding to three specific metallogenic periods. With the continuous progress of dating technology and the accumulation of accurate dating data, the results show that the mineralization age of silver polymetallic deposits in the study area is mostly concentrated at 150-120 Ma (Table 1), indicating that large-scale silver mineralization occurred in the Late Yanshanian period. It should be noted that there are also a few mineralizations of the Early Yanshanian period, such as the Hua’naote deposit (173 Ma)and the Wunugetushan deposit (180-177 Ma), there are also mineralizations of the Indosinian period, such as theLianhuashan deposit. The 1017gaodi deposit (301-297 Ma) is shown as mineralization of the Hercynian period. Some deposits have a long duration of mineralization, or have experienced multiple superposition of mineralization, such as the Baiyinnuoer deposit (240-129 Ma), the Shuangjianzishan deposit (165-132 Ma), and the Lianhuashan deposit (238-137 Ma), etc. The Shuangjianzishan deposit is related to long evolutionary hypabyssal medium acid intrusion, while the Baiyinnuoer and the Lianhuashan deposit may experience the superposition of multiple mineralization (Wang FX, 2017;Jiang SH et al., 2017). Most of the mineralization age data of other deposits are concentrated at 150-120 Ma, which is just in the transition period from the Late Jurassic to the Early Cretaceous, indicating that large-scale silver-forming events occurred during this period.

Table 1. (Continued)

Table 1. Brief version of main silver deposit types and metallogenic characteristics of representative deposits in Daxing’anling, Inner Mongolia and its adjacent areas.

The distribution of silver deposits in the Daxing’anling area is relatively concentrated, mainly in the central and south. In the northern part of the Daxing’anling, there are only a few silver polymetallic deposits, such as the Erdaohe deposit, the Erentaolegai deposit, the Jiawula-Chaganbulagen deposit, the Gelataolegai deposit, the Wunugetushan deposit and the Sanhe deposit, but the scale generally reaches large scale or supergiant scale. The silver polymetallic deposits in the study area are mainly distributed in the Tuquan-Wengniuite Pb-Zn-Ag-Fe-Sn-REE metallogenic belt,Dongwuzhumuqin Banner-Nenjiang Cu-Mo-Pb-Zn-W-Sn-Cr metallogenic belt, Bainaimiao-Xilinhaote Fe-Cu-Mo-Pb-Zn-Cr- (Au-Mn)-Ge-coal-tronicalkali-Grate metallogenic belt,and Xin Barag Right Banner-Genhe Cu-Mo-Pb-Zn-Ag-Aufluorite-coal-uranium metallogenic belt (Xu ZG et al., 2008),followed by the Au-Fe-Nb-Ree-Cu-Pb-Zn-Ag-Ni-Pt-Wgraphite-muscovite metallogenic belt in the western section of the northern margin of North China Block, and the Fe-Cu-Mo-Pb-Zn-Ag-Mn-U-phosphorus coal-bentonite metallogenic belt in the eastern section of the northern margin of the North China Block. The linear distribution of silver polymetallic deposits in the study area is significant. This is related to the fact that the silver polymetallic deposits in the study area are mainly magmatic hydrothermal, and this type of deposit is mainly controlled by the fracture system. The location of the silver polymetallic deposits in the central and south part of the Daxing’anling is mainly controlled by the Huanggangliang-Ulanhot deep fault, the Xilamulun deep fault and the Erlian-Dongwuzhumuqin Banner deep fault. The distribution of the Huanggangliang-Ulanhot deep fault has an excellent linear coupling relationship with the location of the deposit, which shows that there is a direct causal connection between the two. The Erlian-Dongwuzhumuqin Banner deep fault has a similar ore-controlling pattern. The silver polymetallic deposits on this fault are mainly concentrated at the northern end, mainly large-scale or supergiant-scale silver deposit,including the Aerhada deposit, the Wulantaolegaidong deposit, the Jilinbaolige deposit, the 1017gaodi deposit, and the Hua’naote deposit. The silver polymetallic deposits on the Xilamulun deep fault are concentrated in the middle of the fault. It is worth noting that the concentrated output of the deposits on this fault is located at the intersection with the southern extension line of the Huanggangliang-Ulanhot deep fault. In fact, several so-called deposits controlled by the Xilinhot complex, including the Bairendaba and Weilasituo,are also located near this location. The number of silver polymetallic deposits in the northern section of Daxing’anling is far inferior to that in the southern section of Daxing’anling.Most of them are concentrated in Xin Barag Right Banner.They are mainly continental volcanic and sub-volcanic types and are mainly controlled by volcanic basins. The deposits mostly develop at the junction between the edge of the volcanic basin and the Pre-Mesozoic, and may belong to interfacial mineralization between the two geological bodies(Wang JW and Zhu YS, 2019). This is true of the Jiawula-Chaganbulagen, the Erentaolegai, the Wunugetushan, the Gelataolegai and the Sanhe deposit in the northern section of Daxing’anling, as well as the Youfangxi, the Xiaoyingzi and the Baoshouyingzi deposit at the southern tip.

4.2. Tectonic controlling on mineralization

The silver polymetallic deposits in the study area are significantly controlled by regional faults and volcanic basins and the uplift-depression transition zone on their margins. The mineralization environment of different sections or deposit types shows certain differences. For example, the central and south sections of the Daxing’anling are dominated by magmatic hydrothermal vein type and skarn type silver polymetallic deposits, which are mainly controlled by regional NE and NNE trending faults, and secondly by volcanic basins and rock masses. The deposits are mainly located near the uplift-depression transition zone. The northern section of the Daxing’anling is dominated by continental volcanic-subvolcanic type silver polymetallic deposits, which are mainly controlled by volcanic basins,such as the Erentaolegai, the Jiawula-Chaganbulagen and the Wunugetushan deposit are all located in the fault-uplift transition zone on the edge of volcanic basin.

A series of fracture structures of different sizes and properties have developed in the Daxing’anling area, mainly in the NE-NNE direction, EW direction and NW direction.They have formed a fracture system with deep and large faults as the stem and its derived secondary faults as branches,which controls the magma activity, the distribution of mineralization zones and the placement of deposits in this area. The NE trending deep fault is the most important orecontrolling structure in this area. The distribution of silver polymetallic deposits along the main faults is obvious, and important ore deposits are often formed at the intersection of multiple groups of faults. For example, the Bairendaba-Weilastuo ore field is structurally located on the north side of the Sauron Mountain seam zone, the splicing site where the Siberian Plate and the North China Plate subducted and collided with each other, and the Baiyinnuor-Shuangjianzishan-Haobugao ore concentration area is located at the intersection of the Xilamulun Fault, the Erlian-Hegenshan Fault, and the Nenjiang Fault (Du QS et al., 2017).However, different types of ore deposit have different orecontrolling structure types and properties. Magmatic hydrothermal vein type silver orebodies are mainly controlled by NW, NE and EW trending faults, while skarn type ore bodies are controlled not only by folds or contact zones between the rock mass and the surrounding rock in addition to fracture control. For example, most of the Pb-Zn-Ag orebodies found in the Baiyinnuoer deposit occur in the interlayer slip zone and the top collapse zone of the fold on the wings of the anticline, and a few ore bodies are found in the secondary fracture zone. Volcanic basement uplift often forms a relatively closed tectonic environment, which is conducive to the differentiation of residual magma and the enrichment of mineralization elements. The junction between volcanic basement uplift and volcanic basins is mostly broken and cut by faults, which is a favorable location for magma and oreforming fluid activity (Chen L et al., 2009). Some silver polymetallic deposits in the study area, especially the continental volcanic or sub-volcanic deposits, are not only related to regional tectonics, but also closely related to volcanic activity and the volcanic institutions caused by it.The deposits are in place around the uplift of the volcanic base and at the junction with the volcanic basin, and the ringshaped and radial fractures at the edges of volcanic apparatus,especially those that overlap with the fracture structure, are the best storage space for the ore body. In addition to the nearnorth-west main faults, secondary faults and fracture zones in the Jiawula mining area, which are all good structures for rock controlling and ore controlling, the radial fracture system and volcanic dome structure also play a certain controlling role in the formation and distribution of deposits (Zhai DG et al.,2010). The Wunugetushan porphyry Cu-Mo (Ag) deposit is controlled by the sub-volcanic channel formed by the intersection of the NW and NE trending faults that occur on the western edge of the biotite granite and the ring-shaped fracture zone produced by the sub-volcanic structure. The volcanic apparatus is well developed in the Fuxingtun mining area. Early stage volcanic mechanism provided ore-forming hydrothermal migration channels, and the formed craters or sub-craters provided the main mineralization space. Volcanic activity has the characteristics of diagenesis and mineralization at the same time (Chang SQ, 2018).

4.3. Surrounding rocks controlling on mineralization

The statistical results of predecessors show that more than 80% of the copper polymetallic deposits in the central and south of Daxing’anling occur in the Permian formations,especially the Dashizhai Formation and Huanggangliang Formation. This is related to the high abundance values of the main mineralization elements Cu, Pb, Zn and Ag in the Permian formations (Sheng JF et al., 1999). Many important silver deposits in this area are also deposited in the Permian system. For example, the surrounding rocks of the Shuangjianzishan and Baiyinchagandongshan deposit are mainly clastic rocks and tuff siltstones of the Dashizhai Formation of the Lower Permian; the surrounding rocks of the Huaaobaote deposit are tuff sandstone of the Shoushangou Formation of the Lower Permian; the Dajingzi deposit are produced in the sandstone of the Linxi Formation of the Upper Permian; the surrounding rocks of the Baiyinnuoer deposit are shallow metamorphic rocks of the Huanggangliang Formation of the Lower Permian (Fig. 6a).However, some important silver polymetallic deposits are closely related to Mesozoic volcanic basin or Pre-Mesozoic strata. For example, the Fuxingtun deposit occurs in the volcanic rocks of the Manketouebo Formation of the Jurassic,and the surrounding rocks of the Erentaolegai and the Jiawula-Chaganbulagen deposit are mainly the intermediate basic volcanic rocks of the Tamulangou Formation of Upper Jurassic. The Wulantaolegaidong deposit occurs in the intermediate-acid volcanic rocks of the Baiyinggaolao Formation, although the attribution of the Baiyingaolao Formation is still disputed, some people believe that it belongs to the Late Jurassic (Shao JD et al., 2005), and some people believe that it belongs to the Early Cretaceous (Lin M,2018), but there is no doubt that it belongs to the Mesozoicvolcanic rocks. The surrounding rocks of the Bairendaba and Weilastuo deposit are Xilinhot complex and Baiyingaole quartz diorite of the Lower Proterozoic (Fig. 6b). The Erdaohe deposit occurs in the sandstone, slate and marble of Luohe Formation of the Upper Ordovician and in the acid pyroclastic rock of Manketouebo Formation of the Upper Jurassic. There are also some deposits that are directly produced in the rock mass. For example, the newly discovered 1017gaodi and the Hua’naote deposit occur in biotite monzonite granites. Similar deposits include the Meng’entaolegai deposit. Generally speaking, the surrounding rocks of silver polymetallic deposits in this area have a certain relationship with the deposit type. Magmatic hydrothermal vein type silver polymetallic deposits are mainly dominated by Paleozoic strata, especially Permian strata. The age and lithology of the surrounding rocks of skarn type deposits are relatively dispersed, and they can be Ordovician to Permian carbonate rocks or pyroclastic rocks. Continental volcanic-subvolcanic type silver polymetallic deposits are mainly produced in Mesozoic volcanic formations.

Fig. 6. Orebody characteristics of typical Ag-polymetallic deposits in Daxing’anling. a-Pb-Zn-Ag vein of the Baiyinnuoer skarn deposit; b-fine Cu-Pb-Zn-Ag vein in the Barendaba deposit; c-sulfide veins within the faults of the Dajingzi deposit; d-quartz-antimony vein in the Baiyinchagandongshan deposit (provided by Yao Lei);e-sulfide vein rich in chalcopyrite of the Fuxingtun deposit;f-quartz-sulfide vein type ore of the Shuangjianzishan deposit;g-porphyry molybdenum mineralization of the Weilasituo deposit;h-quartz vein type lepidolite mineralization of the Weilasituo deposit.

4.4. Magmatic rocks controlling on mineralization

There is intense Magmatic activity in the Daxing’anling area. Magmatic rocks account for about 75% of the total area.Among them, the Mesozoic was the peak of tectonicmagmatic activity in the area, and the outcrops of magmatic rocks accounted for more than 50% of the area. Magmatic activity is mainly concentrated in the Late Jurassic and Early Cretaceous (Chen L et al., 2009). The main silver deposits in this area are almost related to magmatism. Magmatic hydrothermal vein type and skarn type silver deposits are related to magmatic intrusion, and continental volcanic or sub-volcanic silver deposits are related to volcanic-subvolcanism. The magmatic rocks related to silver mineralization are mainly intermediate-acid and acid intrusions, intermediate-acid lava, pyroclastic rocks and ultrashallow-epifacies small intrusions, including granite, granite porphyry, granodiorite porphyry, quartz-syenite porphyry,syenite porphyry, quartz porphyry, quartz diorite, quartz monzonite porphyry and so on (Jiang B et al., 2021).

Magmatic hydrothermal vein type silver polymetallic mineralization is related to small rock masses of neutral or acidic. Alteration of surrounding rocks near ore vein are mainly intense silicification, Fe-Mn carbonation and sericitization. Silver mineralization in this area is most closely related to granitic magmatism of the Yanshanian period.Granitic intrusions of the Yanshanian period have developed extensively in this area. The rock masses are complex and of various types. They mainly intrude along the NE and NNE trending faults on both sides of the Mesozoic fault zone. They are often closely spatially accompanied by medium-acidic volcanic rocks, and the composition has an inherited evolutionary relationship. Li HN et al. (1999) divided the granites of the Yanshanian stage of Daxing’anling into four diagenetic periods: The first stage of the Early Yanshanian,the second stage of the Early Yanshanian, the first stage of the Late Yanshanian and the second stage of the Late Yanshanian. Among them, the granite of the first stage of the Late Yanshanian is widely distributed in the Huanggangliang-Ulanhot polymetallic metallogenic belt and is closely related to silver polymetallic deposits. In the area of Xin Barag Right Banner, Manzhouli, granite intrusions in the Late Yanshanian period were also relatively developed, but on a smaller scale,mostly epimagenic-ultra-epimagenic intermediate-acid small intrusions, which are closely related to silver mineralization.Typical rock masses are Bayanhushuo, Hudugeshaoleng,Haobugao, Xiaojingzi, Huitebayanwendu, Errentaolegai and so on. The main rock types are potassium felsic granite,monzogranite, hornblende quartz monzonite, granodiorite porphyry, quartz porphyry and so on. The second stage of intrusive complex in the Late Yanshanian period mainly developed in the Xin Barag Right Banner, Manzhouli in the central and south sections of the Deerbugan metallogenic belt.The tectono-magmatic activity of the Late Cretaceous was relatively strong, forming a series of small-scale but widely distributed shallow-ultra-shallow intermediate-acid intrusions,which are closely related to the mineralization of silver, lead and zinc. The main rock types are granodiorite porphyry,quartz monzonite porphyry, granite porphyry, quartz porphyry, dacite porphyry, and rhyolite porphyry. Typical representative rock masses include Jiawula and Errentaolegai.Ore bodies are generally produced in the same tectonic zone as mineralized rock bodies, and they are closely spatially related. Whether it is the scale of the ore field or the scale of the deposit, mineralization often shows obvious zones from the center to the periphery of the mineralized rock mass. For example, in the Lianhuashan area, the Chentaitun near the rock mass is a porphyry Cu-Mo deposit, which has evolved into Cu-Ag and Pb-Zn-Ag deposits in the southwest direction away from the mineralized rock mass. There is also this pattern on the scale of the deposit, such as the Dajingzi deposit, in which mineralization is Sn to Sn+Cu (Ag) and then Pb+Zn+Ag from the ore-forming rock mass to the periphery.The Aonaodaba porphyry Ag-Sn polymetallic deposit occurs at the top of granite porphyry and its contact zone. The ore bodies are leached and reticulated. Sn mineralization is mainly produced in the middle and upper part of the rock mass. The contact zone between the rock mass and the surrounding rocks on both sides is Cu, Pb, and Zn mineralization. Ag mineralization is formed in the late mineralization stage and overlaps on the early orebody.

It should be noted that although many silver polymetallic deposits have been determined to be related to magmatism,the ore-forming rock mass has not been determined. For example, the Meng’entaolegai silver polymetallic deposit is recognized as a magmatic hydrothermal vein deposit (Bureau of Geology and Mineral Resources of Inner Mongolia Autonomous Region, 1991; Zhang Q et al., 1998; Sheng JF et al., 1999; Zhan XZ et al., 1999; Zhang Q et al., 2000, 2002,2004; Zhang JF et al., 2003; Zhu XQ et al., 2004). But the question is, which rock mass is the parent rock of mineralization or is closely related to mineralization? This is not only of concern to researchers of mineralization theory,but also one of the core contents of concern to prospecting prospectors. Unfortunately, the problem has not been resolved. Zhang JF et al. (2003) measured the age of muscovite at 179±1.5 Ma, and believed that the data roughly represented the mineralization age, but the Mengntaolegai rock mass, as the surrounding rock of mineralization, is much older than the Ar-Ar age. As most scholars know, the Mengntaolegai rock mass is obviously not a mineralization parent rock (Bureau of Geology and Mineral Resources of Inner Mongolia Autonomous Region, 1991; Zhang Q et al.,1998; Sheng JF et al., 1999; Zhan XZ et al., 1999; Zhang Q et al., 2000, 2002, 2004; Zhang JF et al., 2003; Zhu XQ et al.,2004). The Duerji pluton has been considered by some scholars to be related to mineralization, but Jiang SH et al.(2011) determined that the zircon age is 154.5±0.5Ma, which is smaller than the Ar-Ar age of altered Muscovite. The LAICP-MS zircon age of the andesitic porphyrite located in the south part of the mining area is only 127.5±0.7 Ma (Jiang SH et al., 2011), and it is not a ore-forming parent rock. In short,if the Ar-Ar age of 179±1.5 Ma of the altered Muscovite related to mineralization can represent the mineralization age,then these intrusive rocks in and around the mining area cannot be the metallogenic parent rocks. So, where is the oreforming parent rock? One possible explanation is that it is still has not yet been discovered. This understanding is consistent with the conclusion drawn by Zhang Q et al. (2002) after studying the lead isotopes in the mining area. Another possibility is that the ore-forming parent rock may have been destroyed by the intrusion of the Duerji rock mass. There is evidence that the mineralization temperature gradually decreases from west to east, which shows that the ore-forming fluid is most likely to be transported from west to east, and the rock mass related to mineralization may be located in the western part of the mining area, close to the location of the Duerji rock mass. As another example, the Shuangjianzishan deposit is considered to be a magmatic hydrothermal deposit(Kuang YS et al., 2014; Wu GB et al., 2014; Ouyang HG et al., 2016; Wang FX, 2017; Jiang B et al., 2018), but the REE characteristics of the granite porphyry and rhyolite vitrinite fused tuff are obviously different from the sulfide in the oreforming period (Jiang B et al., 2018). The time of magmatic activity and mineralization, as well as the coupling relationship between diagenesis and mineralization, are also controversial (Wu GB et al., 2013; Liu YF et al., 2016; Wang FX, 2017). Therefore, is the ore-forming parent rock a volcanic rock, a sub-volcanic rock, or a concealed rock? What is the relationship between the ore-forming materials of this deposit and the discovered magmatic products? These issues have yet to be clarified.

4.5. Ore-forming element association

The elemental combination of silver polymetallic deposits in the study area shows a significant pattern. In the Deerbugan metallogenic belt, silver polymetallic deposits are mainly distributed in the south of the metallogenic belt, and the elemental combination obviously corresponds to the geochemical background of the elements in the Permian strata. For example, the Linxi geochemical block in the southwest section of the metallogenic belt is characterized by being rich in Sn, As, Ag and Cu, and the elemental assemblage of the Dajingzi deposit in this area is Cu-Ag-Sn(Fig.6c). The Baiyinuoer-Haobugao basin in the middle section of the metallogenic belt is characterized by being rich in Pb, Zn, Ag, and Sn. In this area, it is mainly Pb-Zn-Ag (Sn)polymetallic deposits. The Tuquan basin in the northern section of the metallogenic belt is rich in Pb, Zn, Ag, and the elemental combination of the deposits in this area is Pb-Zn-Ag. This shows that the basal stratigraphic components control the combination of mineralization elements in endogenous deposits to some extent (Li HN et al., 1999). In addition, the deposit type also affects the combination of oreforming elements of silver polymetallic deposit to some extent. For example, the magmatic hydrothermal vein type is mainly Pb-Zn-Ag combination. Although the continental volcanic-sub-volcanic deposit is mainly Pb-Zn-Ag combination, there is Cu-(Mo)-Ag combination, such as the Wunugetushan deposit. The skarn deposit even has Fe-Ag combination, such as the Chaobuleng deposit. In recent years,the exploration results of silver polymetallic mines in the Daxingan’ling area have revealed that the mineralization elements have become richer and richer. For example, the Baiyinchagandongshan deposit is a supergiant comprehensive deposit with Sn, Ag, Zn, Pb and Cu as the main components(Fig. 6d), and at the same time associated with beneficial components such as Sb, Cd and In. Reserves of associated Sb resources exceed 200×103t, ranking in the top ten in China.In fact, there are many factors that cause differences in the combination of mineralization elements. In addition to the composition of the basement strata and the type of deposit,multi-stage mineralization is also an important influencing factor for the diversification and complexity of the element combination, and it is also affected by the distance between the mineralization and the mineralization center. In general, it seems that from the center of the ore-forming rock mass to the periphery, the evolution law from the combination of hightemperature mineralization elements to the combination of low-temperature mineralization elements is shown. The combination of mineralization elements in some deposits may also be controlled by the degree of crystallization differentiation of magmatic rocks related to mineralization.The Weilasituo deposit was originally discovered as a medium-sized Zn-Cu-Ag deposit. In 2014, tin mineralization was found in the hidden porphyry granite body about 1.5 km northwest of the mining area. With the deepening of the exploration work, the Weilasituo deposit has gradually developed into a large-scale deposit with tin as the main source and associated Zn, Sn, Cu, Ag, Mo, Rb, Nb, Ta and Li.Studies have shown that highly heterogeneous granites are an important cause of the polymetallic mineralization combination of the Weilasituo deposit (Liu YF et al., 2014;Zhang TF et al., 2019).

5.1. Ore prospecting of each deposit type

From the point of view of the deposit type, magmatic hydrothermal vein type silver polymetallic deposit exceeds 2/3 of the total in both quantity and silver reserves. It is the most dominant type of silver polymetallic deposit in the Daxing’anling area. The prospecting results of this type of silver mine are the most prominent in the Daxinganling area in recent years. A series of large and supergiantr silver deposits, including the Shuangjianzishan and Baiyinchagandongshan, have been discovered and proved. In the future, it will still be the type of silver polymetallic deposit that needs the most attention in the study area. Fault is the most important ore-controlling factor of this type of silver deposit. The NEE and NNE trending regional faults control the distribution of deposits, and the secondary faults and fracture system control the placement of ore bodies. Judging from the distribution characteristics of deposits, the Xilamulun deep fault (F2), the Erlianbei-Dongwuzhumuqin Banner deep fault (F5) and the Huanggangliang-Ulanhot fault(F6) are the most important regional ore-controlling faults.The ore-controlling effects of the three have their own characteristics. It is manifested that there are almost no deposits in the northern section of the F6fault, while the deposits on the southern section of the fault are densely developed and have an excellent linear distribution along the fault, and this site is also the contact zone between Mesozoic volcanic rocks and the Pre-Mesozoic. Therefore, the coupling of these three geological factors may have created favorable ore-forming conditions, which in turn triggered the formation of a large number of silver deposits. The deposits controlled by F2fault are mainly concentrated in and near the middle section of the fault, which is the intersection of F6extension and F2. This area should be regarded as an important prospecting target area. The F5fault is large in scale, but the silver deposits on it have the characteristics of small in number, large in scale and concentration in the northern section. The location of deposit is similar to the F6fault and belongs to the coupling site of three geological elements.Similar favorable conditions also exist on faults such as F7and F10, and full attention should be paid to them. The continental volcano-sub-volcanic deposit is the second most important type of silver polymetallic deposit in the study area.They are relatively concentrated in the Tuquan-Wengniute Pb-Zn-Ag-Fe-Sn-REE metallogenic belt and the Eerguna Cu-Mo-Pb-Zn-Ag-Au-fluorite metallogenic belt, especially the latter, which has concentrated four large and supergiant silver deposits showing great mineralization potential. New discovered Fuxingtun supergiant silver deposit shows that there is still great prospecting potential of this type of silver deposit. According to the characteristics of this type of silver deposit, it is necessary to search for such deposits at the edges of volcanic basins and in the tectonic development areas of volcanic basins, especially at the tectonic junctions. Whether it is intermediate acid or intermediate basic volcanic rock, it can be the surrounding rock of this type of silver deposit,which prompts us to break through the limitations of the surrounding rock lithology and broaden our prospecting ideas.It should be noted that Wang DH et al. (2007) proposed that it is necessary to strengthen the research on the silver content of manganese ore areas and Mn anomalous areas, especially the comprehensive evaluation of the areas where the fault zone of the Mesozoic basin in eastern China coincides with the Mn and Ag geochemical anomalies, and pay attention to the degree of erosion. Jiang B et al. (2020) have also pointed out that ferromanganese mineralization is a very common type of surrounding rock alteration of this type of silver deposit. It is often extensively developed on the surface or shallow parts and is easy to identify with the naked eye, which can be used as an important prospecting sign. Skarn silver deposit is not an important type in China, but it can be used as an important basis for comprehensive evaluation of other deposit type(Wang DH et al., 2007). Except for the Erdaohe and Baiyinnuoer deposit, most of them are small-scale and lowgrade co-associated types in the study area. During the exploration process, paying attention to calcareous or magnesia surrounding rocks near favorable mineralized rock masses is the key to discovering such deposits, and breakthroughs in prospecting are still expected.

5.2. Mineralization center and deep prospecting

Almost all silver polymetallic deposits in the Daxinganling area are the product of magmatism and most of them are mineralized during the Yanshanian period. In fact,magmatic rocks of different eras are widely distributed in the study area. In addition to the Yanshanian period, the rock masses of the Hercynian are also very developed. However,few silver deposits outside the Yanshanian period in the study area have been discovered. Wang ZH et al. (2013) obtained the SHRIMP U-Pb age of zircon of the 1017gaodi monzonite of 296.8±4.1 Ma, and the40Ar-39Ar plateau age of 301.2±1.8 Ma, according to which it is believed that the 1017gaodi silver polymetallic deposit was formed in the Hercynian period. In addition, only a very small number of deposits, such as the Baiyinnuoer and the Lianhuashan deposit, have evidence of mineralization during the Indosinian period. In fact, in the Daxing’anling area, large-scale mineralization occurred during the Yanshanian period, while the mineralization intensity of other geological periods was low. It was not a difference between different metal types, but a general mineralization law. For example, most of the copper polymetallic deposits in the study area are products of the Yanshanian period (Zhao YM and et al., 1997). The magmatic activities of the Hercynian and Indosinian period failed to form silver deposits on a large scale? Or is only the source layer or poor ore formed? Or is it that most of the formed deposits have been eroded or transformed and cannot be preserved? Therefore, the silver mineralization of the Hercynian or Indosinian period in the study area is still worth exploring. In some silver deposits, although the main ore body has been determined to be mineralized by magma during the Yanshanian Period, the mineralized rock mass and mineralized center have not been determined, which restricts further prospecting and exploration. The Shuangjianzishan deposit is currently the largest silver deposit in China and even Asia. The source, migration and precipitation mechanism of its huge amount of metal have yet to be determined. The volcanic mechanism and mineralization center of the newly discovered Fuxingtun supergiant silver polymetallic deposit have not been determined yet. Judging from the information revealed by the current drilling, the ore body of the deposit shows dense vein-like output. The scale of the fracture-controlled sulfide veins is not large and the number is small, but the zoning and distribution of mineralized elements are clear. From shallow to deep, the combination of mineralized elements shows a distinct Ag-Pb→Pb-Zn→Zn zoning pattern, which reflects that with the increase of depth, the mineralization has a tendency to evolve towards the high temperature stage. At present, the deepest drilling hole is about 1300 m, and the orebody is still stable in the extension direction, indicating that there is still a great potential for prospecting. Taking into account the fact that there are many copper polymetallic deposit, including the Lianhuashan Cu-Ag deposit, have been developed around the Fuxingtun deposit (Wang DH et al., 2020), and that chalcopyrite in the veins is relatively enriched locally (Fig.6e), it cannot be ignored that there is a possibility of copper ore bodies being found in the deep part of the deposit, and the determination of the mineralization rock mass or mineralization center will be conducive to the search for deep ore bodies. There is also the Shuangjianzishan deposit (Fig.6f) where copper ore bodies are expected to be found in the deep part (Jiang B et al., 2018, 2019). In addition, the Hua’naote deposit also has a pattern of gradual transition of Pb-Zn to Cu-Ag ore bodies from shallow to deep (Chen GF et al.,2016). The ore-forming materials are believed mainly come from the deep (reachable mantle) (Han YD et al., 2013;Yang C, 2019). Therefore, the deep part of the deposit is also expected to continue to explore copper and silver ore bodies,and even independent copper ore bodies.

5.3. Attention should be paid to porphyry silver deposit

It should be noted that although porphyry silver deposits are not as well-known and valued by the public as porphyry copper deposits, more and more examples show that this type of deposit cannot be ignored. Typical porphyry silver polymetallic deposits in China include the Lengshuikeng supergiant Ag-Pb-Zn deposit (Meng XJ et al., 2007) in Jiangxi Province, and the newly discovered Laoliwan silver deposit (Zhang GY et al., 2016) in Henan Province. The proven amount of silver metal is 1961 t and the silver grade is 160 g/t～225 g/t. This deposit is the only large-scale porphyry independent silver deposit in Central and South China (Jiang B et al., 2021). The Zhijiadi Ag-Pb-Zn deposit located in the Taibai-Weishan volcanic basin in Shanxi Province has also recently discovered deep porphyry mineralization, revealing a“trinity” of mineralization combination of vein-blasting breccia-porphyry type (Zhang HQ, 2014). There are already a considerable number of porphyry silver polymetallic deposits in the study area, such as the Hua’naote and the 1017gaodi independent silver deposit, as well as the Wunugetushan porphyry copper-molybdenum (associated silver) deposit. In the deep parts of some deposits, especially continental volcanic-sub-volcanic silver deposits, it is possible to find porphyry-type ore bodies. The prospecting and prediction geological model of volcanic type Pb-Zn-Ag-Mo deposit in the Daxing’anling metallogenic belt established by Ye TZ et al. (2014) shows that vein-like, layered, breccia-type, and porphyry-type mineralization systems are widely present in the Daxing’anling area. Specific to each deposit, the types of ore bodies may vary, but the model reveals that the basic laws of porphyry mineralization often exist in the deep parts of volcanic type Pb-Zn-Ag ore bodies, which are worthy of reference in prospecting and exploration. Whether there are still porphyry ore bodies in the deep part of the Fuxingtun deposit is well worth exploring. In addition, skarn Cu-Pb-Zn-Ag deposits in the north-central Daxing’anling often coexist with porphyry deposits (She HQ et al., 2009). Although porphyry mineralization has not been found in deposits such as the Baiyinnuoer and the Erdaohe deposit, porphyry mineralization of this type of deposit is also worth exploring.

5.4. Strengthen comprehensive evaluation and multi-objective prospecting

The types of silver deposit include most of the types of endogenous and exogenous deposits. Silver can be associated with various elements including Pb, Zn, Cu, Au and so on to form industrial ore bodies. Due to the relatively low mineralization temperature and relatively active chemical properties, silver is often produced in the outer belt of mineralization or at the far end of magmatic hydrothermal deposits, thereby forming a metal zoning from the center of mineralization to the periphery, from the inner belt to the outer belt. Therefore, the zoning law of silver ore and its coassociated ore species can be used to guide the deep and peripheral prospecting and exploration (Jiang B et al., 2020).In the study area, silver deposits are rich in co-associated deposits types, and they are mostly composed of combinations with Cu, Pb, Zn, Au, etc. Judging from the mineralization depth, the deposits in the north and south zones of the study area are relatively shallow, and may have relatively little lift,and the deposits are sopposed to be well preserved (Lü XB et al., 2020). For example, the Dajingzi deposit shows Sn→Cu→Pb-Zn→Ag zoning characteristics from deep to shallow. The Baiyinchagandongshan Cu-Pb-Sn polymetallic deposit also has a similar zoning pattern. It is worth mentioning that not only has the Baiyinchagandongshan deposit reached a supergiant scale of silver ore, but the associated antimony resources are also extremely rich (Jiang B et al., 2021). Ag-Sb deposits have been more common in the south of China in the past, but the discovery of Baiyinchagandongshan deposit remind us that the prospecting of Ag-Sb deposits should be expanded to the north, especially in the Daxing’anling area. The “versatile” of silver ore is also reflected in the fact that it can be co-associated with rare metals such as Rb, Nb, Ta, Li and other rare metals, which has not attract much attention in the past. The Weilasituo was originally discovered as a medium-sized Zn-Cu-Ag deposit.In 2014, tin mineralization was discovered in the concealed porphyry granitic body about 1.5 km to the northwest of the former mining area, which gradually developed the Weilasituo deposit into a large-scale deposit dominated by Sn and associated Zn, Cu, Ag, Mo (Fig. 6g), Rb, Nb, Ta and Li(Fig. 6h). The discovery of porphyry Sn-W-Rb mineralization in the deep part of the Weilasituo mining area has expanded the scale of the original deposit, enriched the available minerals, and more importantly, revealed the porphyryhydrothermal mineralization system of the deposit, and pointed out the direction for deep prospecting. At the same time, it also provides an important reference for finding similar deposits in the context of similar mineralization in the Bairendaba-Weilasituo ore field and even in the Daxing’anling area, and provides a good example for the prospecting and exploration in the Daxing’anling area. The mineralization characteristics and spatial placement laws of silver deposits should be used in the prospecting and exploration work, and attention should be paid to strengthening comprehensive evaluation and multi-target prospecting.

The goal of this study is to summarize classification,metallogenesis and exploration of silver deposits in Daxing’anling of the Inner Mongolia and its adjacent areas. Main conclusions of this study are：

(i) By the end of 2020, 83 silver deposits (or occurrences)have been proved, and the amount of silver metal exceeds 86000 t with average grade of 100 g/t, which makes the Daxing’anling area the most important silver Metallogenic belt in China. The silver deposits in the study area can be divided into three main types, namely magmatic hydrothermal vein type, continental volcano-subvolcanic type and skarn type. Judging from the spatial and temporal distribution,magmatic hydrothermal vein type and skarn type silver deposits are mainly concentrated in the central and south sections of the Daxing’anling, and the continental volcanicsubvolcanic type silver deposits are relatively concentrated in the central and northern part. The mineralization era is dominated by the Yanshanian period.

(ii) The silver polymetallic deposits in the study area are significantly controlled by regional faults and the upliftdepression transition zone of volcanic basins and their margins. NE-trending deep-lying faults are the most important ore-controlling structures in this area. The silver polymetallic deposits are distributed along the main faults, and multiple sets of faults often form important ore clusters at the intersection. The Permian is the most important ore-bearing formation in this area, but some important silver polymetallic deposits occur in Mesozoic volcanic basins or pre-Mesozoic strata. The magmatic rocks related to mineralization are mainly intermediate-acid or acid intrusions, intermediate-acid lavas, pyroclastic rocks and ultra-shallow-epifacies microintrusions of Yanshanian period. The factors that determine the combination of mineralization elements are mainly the elemental geochemical background of the surrounding rock or source layer of the mineralization. In addition, the type of deposit, the distance from the mineralization center and the degree of differentiation of the mineralization rock mass are also important influencing factors.

(iii) Magmatic hydrothermal vein deposits are the most important and most prospecting potential type of silver deposits in the study area. Continental volcano-subvolcanic type silver deposits have made breakthroughs in prospecting in recent years and have great prospecting potential,especially porphyry silver deposits, which should attract full attention. Skarn silver deposits are relatively minor.

(iv) The determination of mineralization center and rock mass is the key to deep and peripheral prospecting. Using the zoning law of silver ore and its associated ore species to guide the deep and peripheral prospecting and exploration of the deposit is an important direction. There is still huge prospecting potential in the deep and periphery of many silver deposits, including the Shuangjianzishan and the Fuxingtun deposit. In view of the variety of types of associated minerals in the silver polymetallic deposits in the research area and their high economic value, comprehensive evaluation and multi-target prospecting should be strengthened in the usual prospecting and exploration work.

CRediT authorship contribution statement

Biao Jiang conceived of the presented idea. Biao Jiang developed the theory and performed the computations. Yu-Chuan Chen and Deng-Hong Wang improved the theory. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgement

This work is financially supported by the projects of China Geological Survey (DD20221695, DD20160346 and DD20190379), the Fundamental Research Funds of the Central Public Welfare Scientific Research Institutes(JYYWF20183701 and JYYWF20183704) and the Inner Mongolia Geological Exploration Fund Project (2020-YS03).Thank the colleagues who participated in the field work.Constructive comments by three anonymous reviewers have improved the manuscript.

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