A New Type of Li Deposit: Hydrothermal Crypto-Explosive Breccia Pipe Type

0. INTRODUCTION

Lithium (Li) and its compounds are widely used in many industries such as glass, ceramics, lubricants and light alloys. In recent years, due to the increasing demand for Li-batteries, many western countries have listed Li as a key strategic metal (Chakhmouradian et al., 2015), making it the best choice for energy storage in industries such as portable electronic equipment, power grid and new energy vehicles (Kesler et al., 2012).

Lithium deposits in the world are mainly divided into three types, including (1) brine and salt lake type; (2) highly differentiated granite or pegmatite type; (3) carbonate-clay type (Fig. 1). Among them, brine and salt lake type lithium resources are the most abundant, and its proved reserves account for 65% of the total proved lithium resources (Shaw, 2021). Formation of this type Li deposits includes leaching of a large amount of lithium from various minerals in the crust, then the brine flows into a closed basin and forms a lithium rich salt lake through evaporation and concentration (Chen et al., 2020; Godfrey et al., 2013; Hofstra, et al., 2013), and lithium mainly exists in the form of soluble ions. Brine and salt lake type Li deposits occur mainly in the Quaternary and modern salt lakes. They are distributed in three major Plateaus in the world: the Tibet Plateau in China, Andes Plateau in western South America and Plateau in western North America, forming three major brine type lithium mining areas (Fig. 1; Liu et al., 2021; Bradley, 2013). Highly differentiated granite or pegmatite type Li deposits are the product during the late crystallization of felsic magma rich in volatiles (London, 2008). Highly differentiated granites have obvious lithofacies zoning, and the degree of evolution gradually increases from bottom to top of the pluton, followed by biotite granite, two mica granite, muscovite granite, albite granite and topaz-lepidolite-albite granite (Jiang et al., 2020; Yang et al., 2014). The Li ore body is mainly distributed in the uppermost part (roofing zone) of the highly fractionated granite body. Pegmatite Li mineralization is mainly LCT (Li-Cs-Ta) type pegmatite, and the pegmatite in most cases is usually genetically related to peraluminous granite, with characteristic enrichment of Li, Rb, Cs, Be, Sn, Ta and other elements, and these ore-bearing pegmatites usually develop regional and internal compositional zonations (Černý and Ercit, 2005). The main ore minerals of highly differentiated granite or pegmatite type Li deposits include spodumene, lepidolite, and petalite. In the carbonate-clay type Li deposits, the hydrothermal fluid leaches Li from rhyolitic lava, volcanic ash or sedimentary carbonate and clastic rocks and incorporates them mainly in clay minerals in the form of adsorption or isomorphism. These clays were deposited in volcanic ash rich basins in many cases. The ore minerals are mainly lithium montmorillonite and Jadarite [LiNaSiB₃O₇(OH)]. The world's famous carbonate-clay type Li deposits include McDermitt (King Valley) lithium deposit in northern Nevada, Sonora lithium deposit in the central basin of Mexico, Jadar lithium deposit in Serbia (Benson et al., 2017; Zhao et al., 2015). In China, many of this type Li deposit have also been reported, such as those hosted in Lower Carboniferous and Lower Permian strata in southwestern China (Wen et al., 2020). Other types of Li deposits include oil field brine type, geothermal brines, seawater and hydrothermal ferromanganese crust and nodules (Tabelin et al., 2021). Although the global lithium resources are abundant at present, the exploration and study of new types of Li deposits always draws considerable attention for economic geology society. In this paper, we summarize the latest research results of the Weilasituo Sn-Li-Rb polymetallic deposit in Inner Mongolia, which occurs in the middle-southern section of the Great Xing'an Range metallogenic belt, and we propose that it is a new type of rare metal deposit―hydrothermal crypto- explosive breccia pipe Li deposit. This new deposit type deserves further study. This paper systematically summarizes the geological characteristics, geochemistry, and geochronology of the Weilasituo deposit, and proposes a metallogenic model.

Figure  1.  Distribution of major types of Li deposits in the world (modified after Shaw, 2021; Liu L J et al., 2019).

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1. HYDROTHERMAL CRYPTO-EXPLOSIVE BRECCIA PIPE TYPE DEPOSITS

Since the 1980s, geologists have paid more and more attention to the mineral prospecting of the crypto-explosive breccia type deposits. In 1983, a symposium on "Brecciation and Mineralization: Geological Occurrence and Genesis" was held internationally, and its results were published in the special collection of "A Special Devoted to Ore-Hosted Breccias" in the journal of Economic Geology. In recent decades, with the deepening of prospecting, exploration and theoretical research, many large and super-large crypto-explosive breccia deposits have been found, and great progress has been made in the study of their genetic mechanism and metallogenic model (e.g., Cui et al., 2021; Xiong et al., 2019; Jiang et al., 2015; Chen et al., 2012; Tornos et al., 2006; Baker and Andrew, 1991). Crypto-explosive breccia deposits are usually large-scale, mineralized intensively and easy to be exploited, and have high economic value, including mainly the Au, Cu, Mo, W, Sn, Pb, Zn, U, REE and other metals. They can form super-large deposits alone, such as the Golden Sunlight deposit in the United States and the Kidston deposit in Australia, and the Qiyugou deposit in China, which are all super-large scale gold deposits with reserves of more than 50 t Au (Xiong et al., 2019; Spry et al., 1996; Baker and Andrew, 1991). This type of ore deposits can also occur as an associated deposit in the porphyry type, hydrothermal vein type and other magmatic hydrothermal type deposits, such as the crypto-explosive breccia type REE deposit in the Pea Ridge IOA deposit in the United States and the crypto-explosive breccia type Cu-Mo deposit in the Dahutang W mining district in China (Aleinikoff et al., 2016; Jiang et al., 2015). Harlaux et al. (2021) reported the only known breccia pipe type W-(Nb-Ta-Sn) deposit (Puy-les-Vignes, Massif Central) in France, that formed by multistage development of magmatic and hydrothermal processes over a period of 24 myr (324 to 300 Ma). Breccia pipes are also reported in the large Zinnwald-Cinovec Sn-W-Li deposit in the Eastern Erzgebirge/Krušné Hory, German and Czech Republic (Breiter et al., 2019).

Crypto-explosive breccia refers to a set of breccia like rock assemblages formed by concealed eruption of magma in hypabyssal environments. In terms of its genesis, it is generally considered to be related to intermediate-felsic magmatism and generally developed on the top (roof zone) of the intrusive rock body (e.g., Yu et al., 2017; Solomovich et al., 2012; Qing and Han, 2002; Liu, 1982). The study of the Chinese Dalucao REE deposit and the Spanish Aguablance Ni-Cu-PEG deposit indicates that carbonatite and mafic rock intrusions can also form a certain scale of crypto-explosive breccia type deposit (Liu et al., 2017, 2015; Tornos et al., 2006). Crypto-explosive breccia body is also the ore body, whose occurrence is obviously controlled by structure and usually presents a tubular or funnel shape (e.g., Wang et al., 2020; Tornos et al., 2006; Spry et al., 1996; Baker and Andrew, 1991).

The cross section of the crypto-explosive breccia pipe is nearly circular or oval, and the plane diameter is generally between tens of meters and hundreds of meters, and a few can reach thousands of meters. The vertical extension is usually within 1 500 m, and a few can reach 2 000 m (Jiang et al., 2019). The mineralization occurs usually in the whole breccia pipe, mainly as disseminated, vein and veinlets mineralization. However, a few crypto-explosive breccia type deposits show irregular mineralization characteristics, such as the Mannoth breccia pipe in Copper Creek deposit in the United States, where the Cu mineralization is mainly concentrated at either the bottom or top of the pipe (Anderson et al., 2009). The alteration assemblage of the crypto-explosive breccia type deposit is very diverse, and there occur often multi-stage alterations. However, some deposits show a similar alteration zoning pattern to porphyry deposits, that is, from the mineralization center outward, the alteration minerals basically show the evolution from high-temperature assemblage to medium low- temperature assemblage (e.g., Anderson et al., 2009; Qing and Han, 2002). In general, the ore-forming temperature of the crypto-explosive breccia type deposits of different metals is slightly different, for example, the temperature of Pb-Zn deposits is relatively low, generally concentrated at 130–400 ℃ (e.g., Xie et al., 2021; Wang F X et al., 2017; Ruan et al., 2015); whereas the temperature of the Cu-Mo-Au deposits is relatively high, which can generally reach 450–500 ℃ (e.g., Fan et al., 2011; Baker and Andrew, 1991; Norman and Sawkins, 1985).

The direct controlling factor for the formation of the concealed magmatic explosion is the quick release of heated fluid or gas. This heated fluid or gas may have multiple sources, such as magmatic gas-liquid, groundwater heated by magma, and the mixing of magmatic water and groundwater (Sillitoe, 1985). Among them, the concealed explosion of magmatic gas-liquid is the main form. The evolution process of the crypto- explosive breccia can be summarized as follows (Jiang et al., 2019; Qing and Han, 2002; Song et al., 2002; Zhang, 1991; Liu, 1982): when magma intruded from deep to near surface, decompression and exhaust occurred due to the sudden drop of pressure and temperature, resulting in a large amount of volatile releases. More importantly, a large amount of volatile rich fluids will be exsolved in the late stage of magma crystallization differentiation. These volatiles and fluids accumulate continuously in the trap environment. When the pressure exceeds the sum of the tensile strength of surrounding rock and the static pressure of the rock, the hidden explosion will take place, and the surrounding rock will be broken by shock, resulting in a large number of blasting cracks. The fluid exsolved from the magma is injected along the fracture to replace the breccia with different components and make it fluidized. Fluid injection will cause instantaneous decompression at the top of the magmatic rock body and promote more fluid to be exsolved from the magma. The huge pressure gradient will lead to the decompression and boiling of the fluid, the volatiles take away a lot of heat, and the temperature decreases, the ore-forming materials are then precipitated, and the breccia type ore body is formed. In addition, the instantaneous release of pressure may cause the broken surrounding rock to condense and heal again, so as to form a new trap space. The fluid and volatile gather again to form a high-pressure body and produce crypto-explosion, and finally form a crypto-explosion breccia type deposit that has experienced multi-stage blasting and mineralization. In addition, due to the effect of gravity, collapsed breccia with large rock blocks is often formed at the top of the breccia pipe. To sum up, an ideal crypto-explosive breccia pipe can be roughly divided into magmatic gas-liquid accumulation zone, metasomatic/fluidized breccia zone, crypto-explosive breccia zone, collapsed breccia zone and blasting fracture zone near the surface/surrounding rock from bottom to top (Fig. 2).

Figure  2.  An ideal model for the crypto-explosive breccia pipe (modified after Jiang et al., 2019; Wang, 2001).

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2. THE WEILASITUO Sn-Li POLYMETALLIC DEPOSIT

2.1 Geological Characteristics of the Deposit

The Weilasituo Sn-Li polymetallic mining area is located in the Keshketeng Banner, Inner Mongolia, NE China (Fig. 3a). There are three deposits in the study area, namely the Weilasituo Sn-Li-Rb deposit, the Weilasituo Cu-Zn deposit and the Bairendaba Ag-Pb-Zn deposit (Fig. 3b). Many mineralization types occur, including granite type Rb, granite type Sn-Zn, crypto-explosive breccia pipe type Li-Rb (Fig. 3c), quartz vein type Sn-Zn (Fig. 3d) and sulfide vein type Ag-Pb-Zn orebodies. Previous studies have shown that the three deposits are genetically close related and should belong to the same metallogenic system, while the Weilasituo Sn-Li-Rb polymeta-llic deposit corresponds to the high-temperature end member of the metallogenic system (Liu et al., 2016). The total metal reserves of the Weilasituo Sn-Li-Rb polymetallic deposit include 90 000 t of Sn (Fan et al., 2017), 688 300 t of Li₂O (Li et al., 2018), 369 700 t of Rb₂O (Sheng et al., 2020), 12 700 t of WO₃ (Li et al., 2018), and 200 million tons of Pb+Zn+Cu (Fan et al., 2017), of which lithium and tin meet the standards of large deposits, with average grade of 1.25% Li₂O and 0.85% Sn (Li et al., 2018; Fan et al., 2017).

Figure  3.  (a) The location of the Weilasituo deposit (modified from GS(2016)1594); (b) geological map of the mining area; (c) plan of the Weilasituo deposit; (d) profile of the Weilasituo deposit (modified after Wang Z G et al., 2017; Liu et al., 2016).

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In the mining area, the Quaternary sediments, the Xilingole metamorphic complex, the crypto-explosive breccia and the Carboniferous quartz diorite are exposed (Fig. 3c). Alkali feldspar granite is developed in the deep part of the mining area as a concealed body, which occurs directly below the Weilasituo Sn-Li-Rb polymetallic mineralization zones and ca. 400 m below the surface (Fig. 3d). At present, the drilling hole does not fully reveal the depth and shape of the granite body. Previous studies show that the causative granite related to mineralization is this hidden alkali feldspar granite (Wang F X et al., 2017; Liu et al., 2016; Zhu et al., 2016). This granite shows a porphyritic structure, and is light gray color, but due to the occurrence of Rb-rich amazonite at the top zone of the granite, the color can be in light blue (Figs. 4a–4d). The main minerals include albite, K-feldspar, amazonite, quartz, topaz, and lepidolite-zinnwaldite, and no dark minerals (such as biotite, amphibole) are developed. The phenocrysts include quartz, K-feldspar and albite, which show a great crystal size difference. The matrix shows fine-grained crystalline structure (Figs. 4e–4g). The ore minerals occur mainly as cassiterite-sphalerite droplets, and pyrite can be seen occasionally (Figs. 4h–4i).

Figure  4.  Petrography of alkali-feldspar granite. Ab. albite; Ama. amazonite; CGM. Columbite. group mineral; Cst. cassiterite; Py. pyrite; Qz. quartz; Sp. sphalerite; Stn. stannite; Tpz. topaz; Znw. zinnwaldite.

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The ore body mainly occurs in the Xilinhot metamorphic complex and Carboniferous quartz diorite intruded into the complex. The Xilingole complex is the most important surrounding rock in the mining area. It is a set of rock assemblage that has experienced strong metamorphism. The lithology is mainly biotite-plagioclase gneiss, biotite granulite and plagioclase-amphibole gneiss of amphibolite facies, which is mainly composed of plagioclase, quartz, biotite and amphibole. Zircon U-Pb dating shows that it is the product of Early Paleozoic magmatism, sedimentation and metamorphism. There are two major age cycles, one at 400–443 Ma and the other at 323–350 Ma, representing the age of protolith and the metamorphic age respectively (Liu, 2009; Xue et al., 2009). The quartz diorite intruded into the Xilingole complex. The rock is grayish white, subhedral fine-grained structure, with slight gneissic structure. The gneissic foliation is consistent with that of the Xilingole complex. The main minerals are composed of plagioclase, quartz and biotite, with subordinate amphibole. The weighted average age of zircon U-Pb dating is 308.3 ± 4.2 Ma (Wang et al., 2013). The Sn-ore minerals in the deposit are mainly cassiterite in quartz vein, and the Li-Rb ore minerals include mainly lepidolite-zinnwaldite in the crypto-explosive breccias.

2.2 Mineralization and Alteration Characteristics of the Crypto-Explosive Breccia Pipe

The Li ore body is composed of the whole crypto-explosive breccia pipe in the Weilasituo with Li₂O grade of 0.8%–3.6% (Wu et al., 2021; Li et al., 2018). The surface exposure shows an elliptical shape, with a long axis of about 200 m and a short axis of about 80 m, and the cross section shows this pipe is thin and small at the top and thick and large at the bottom (Fig. 5). From inside to outside, the breccia pipe can be divided into blasting breccia zone, shattering breccia zone and cracking fracture zone (Li et al., 2018). There is no obvious boundary between each zone, showing a transitional relationship. In the blasting breccia zone, the rock is relatively broken, the breccia is small with local dissolution, the cement content is high, and the Li₂O grade is 1.5%–2%. In the shattering breccia zone, the breccia is large, the relative movement distance of the breccia is very short-distance, and it can be basically restored. The Li₂O grade is between 1% and 1.5%. In the cracking fracture zone, only micro fractures are developed in the rock, with weak hydrothermal activity in the later stage, and the grade of lithium and rubidium generally cannot reach the industrial grade (Fu et al., 2020; Li et al., 2018).

Figure  5.  Detailed profile of the Weilasituo Sn-Li deposit (modified after Gao et al., 2019).

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The granite in contact with the crypto-explosive breccia has experienced strong greisenization, often containing a large amount of lepidolite-zinnwaldite (Figs. 6a–6h). In the crypto-explosive breccia pipe, the breccia is mainly composed of surrounding rock of gneiss, which also has strong greisenization, and the compositions of cement show some changes from the bottom to the top. At the contact zone between the crypto- explosive breccia and the granite, the cement contains magmatic components, which is consistent with the composition at the top of alkali feldspar granite. It can be seen that the granite may contain pieces of gneiss breccia that have partially altered to zinnwaldite (Fig. 6c), which indicates that the magma was not fully consolidated when crypto-explosion took place. Towards the upper part, the cement changes into fine zinnwaldite/lepidolite and quartz (Figs. 6d–6f). At the position far away from alkali feldspar granite, the cement contains mainly quartz (Figs. 6g–6i). Miarolitic structure is common. Ore minerals such as sphalerite, cassiterite and molybdenite and a small amount of rare earth minerals (Fig. 7) appear locally, and beryl also occurs (Fig. 6i).

Figure  6.  Hand specimen of rock or ore in the cryptoexplosive breccia pipe. Ab. albite, Brl. beryl, FI. Fluorite, Qz. quartz, Znw. zinnwaldite.

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Figure  7.  Petrographic characteristics of lithium ores based on polarizing microscope and SEM. FI. fluorite; Mnz. monazite; Mol. molybdenite; Qz. quartz; Sp. sphalerite; Tpz. topaz; Znw. zinnwaldite; Zrn. zircon.

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2.3 Geochemical Characteristics of Ore Minerals

The main ore minerals of the deposit include cassiterite, zinnwaldite/lepidolite and sphalerite, and niobium-tantalum oxides are also found locally in some ore bodies. The following is a summary of the geochemical characteristics of relevant minerals.

2.3.1   Cassiterite

Cassiterite can be divided into two types (magmatic and hydrothermal), and the hydrothermal cassiterite occurs in quartz veins and also in greisen. Cassiterite from quartz veins is the main economic mineral.

(1) Cassiterite in alkali feldspar granite (Cst-Ⅰ). This type of cassiterite shows subhedral granular crystals, with size less than 200 μm. It is sporadically distributed in the granite (Figs. 4a, 4g, 4h), with a close association with stannite, sphalerite, wolframite, Nb-Ta oxides and U-rich minerals, and its composition is generally complex.

(2) Cassiterite in greisen (Cst-Ⅱ). This type of cassiterite is anhedral, sparsely disseminated in greisen, sometimes occurring at the edge of sphalerite, with little content and crystal size less than 500 μm.

(3) Casserite in quartz veins (Cst-Ⅲ). This type of cassiterite is euhedral, clean and transparent, with complex zonation and different grain sizes and colors (Fig. 8). It often coexists with sphalerite, and the contents of major and trace elements of dark and light colored cassiterite vary greatly (Table 1).

Figure  8.  Petrography of quartz vein type tin ore based on hand specimens, polarizing microscope and CL(Cst-Ⅲ). Cst. cassiterite, Sp. sphalerite.

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Table  1.  Average content of trace elements of three types of cassiterite in the Weilasituo deposit

Element	MnO (wt.%)	FeO (wt.%)	TiO2 (wt.%)	Nb (ppm)	Ta (ppm)	W (ppm)	In (ppm)
Cst-Ⅰ	0.022 6	1.31	0.076	13 714	16 067	725	-
Cst-Ⅱ	0.000 4	0.041	1.38	1 023	499	835	10
Cst-Ⅲ (dark)	0.001 9	0.149	1.16	3 515	1 075	1 792	59.2
Cst-Ⅲ (light)	0.000 1	0.053	1.07	394	76	68.5	62.7

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In general, the contents of Fe and W in cassiterite related to granite magmatic hydrothermal fluids are generally much higher than those in SEDEX/VHMS deposits, indicating that Fe and W are two key discriminant factors (Guo J et al., 2018; Hennigh and Hutchinson, 1999). The enrichment of tungsten in cassiterite in granite-related deposits can be attributed to the high-concentrations of W, Nb, Ta and Zr in the highly fractionated granitic melts. These elements can incorporate in the cassiterite lattice or in the form of mineral/fluid inclusions. The Fe and W contents of various cassiterites in the Weilasituo deposit are very high (Table 1), indicating that cassiterite was formed in the magmatic hydrothermal system. The Mn contents in all cassiterites are very low. The contents of Nb and Ta show significant difference in different types with the highest values in the magmatic cassiterite, and the contents of Nb and Ta in the dark part of greisen and quartz vein cassiterites are also at a high level (Table 1). The comprehensive utilization of Nb and Ta can be considered in the process of tin ore extraction.

2.3.2   Micas

Mica is common in the Weilasituo deposit, occurs in different parts and has complex occurrence (Fig. 9). Mica of different stages can be superimposed at the same location, which can be roughly divided into four categories according to the mineral morphology and occurrence of mica:

Figure  9.  Photograph and photomicrograph of different types of mica. (a), (b) Zinnwaldite in alkali feldspar granite (Mica-Ⅰ); (c) large flake zinnwaldite and fine flake zinnwaldite (Mica-Ⅱ); (d) Fine flake zinnwaldite (Mica-Ⅱ); (e) large flake zinnwaldite (Mica-Ⅱ) and Later muscovite (Mica-Ⅲ); (f) biotite in surrounding rock (Mica-Ⅳ). Ab. albite, Bi. biotite, Ms. muscovite, Qz. quartz, Tpz. topaz, Znw. zinnwaldite.

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(1) Primary magmatic zinnwaldite in granite pluton and dyke (Mica-Ⅰ); (2) Zinnwaldite/Lepidolite in the crypto-explosive breccia (Mica-Ⅱ); (3) Muscovite as rims that metasomatizing previous mica (Mica-Ⅲ); (4) Biotite in host metamorphic rocks (Mica-Ⅳ).

The classification diagram of the four types of mica is shown in Fig. 10. Among them, the Mica-Ⅱ in the crypto- explosive breccia is the main ore mineral, which occurs as large flake or fine-grained aggregates in cements or veins. The average content of major elements of the four types of mica in the Weilasituo deposit is listed in Table 2.

Figure  10.  Classification map of mica (modified after Tischendorf et al., 2004).

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Table  2.  Average major elements contents of the mica in the Weilasituo deposit

Element	F (wt.%)	Na2O (wt.%)	MgO (wt.%)	Al2O3 (wt.%)	Rb2O (wt.%)	SiO2 (wt.%)	K2O (wt.%)	CaO (wt.%)	TiO2 (wt.%)	FeO (wt.%)	MnO (wt.%)	ZnO (wt.%)
Mica-Ⅰ	7.99	0.21	0.659	20.2	0.985	46.9	10.2	0.000 6	0.114	9.81	0.647	0.293
Mica-Ⅱ (Fine flake)	8.1	0.189	2.03	20.7	0.812	47.6	10.5	0.006 7	0.195	7.68	0.665	0.096
Mica-Ⅱ (Large flake)	8.02	0.205	1.44	20.6	0.89	47.2	10.3	0.004 9	0.138	8.67	0.619	0.329
Mica-Ⅲ	1.92	0.05	1.41	30	0.331	50.1	10.7	0.059 3	0.025	2.15	0.088	0.011
Mica-Ⅳ	0.51	0.09	10.11	15.6	0.119	37.8	9.61	0.043 3	2.219	19.6	0.15	0.063

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Based on the electron microprobe data of mica, the content of Rb₂O in the Li-rich mica is close to 1%, therefore the lithium ore is also an important rubidium ore. The Fe and Rb contents of mica in the granite are higher than those of mica formed in the later stages. The Mg content of mica in the crypto-explosive breccia is higher than that in the granite. Combined with the content of Mg in biotite in surrounding metamorphic rock, it indicates that biotite may provide some Mg and therefore contribute to the formation of Li-rich micas in the breccia type lithium ore.

2.3.3   Sphalerite

The types of sphalerite in the Weilasituo deposit are complex. According to the occurrence and morphological characteristics, we divided the sphalerite into five types (Fig. 11, Zhu et al., 2021).

Figure  11.  Photomicrograph of different types of sphalerite. (a) Sphalerite in magmatic rocks (Sp-Ⅰ); (b) sphalerite in greisen (Sp-Ⅱ); (c) black sphalerite in quartz veins (Sp-Ⅲ); (d) brown sphalerite in quartz veins (Sp-Ⅳ); (e) brown-green sphalerite in quartz veins (Sp-Ⅴ); (f) sphalerite of sulfide stage in the periphery of the deposit (Bairendaba deposit). Ab. albite; Ccp. Chalcopyrite; Cst. cassiterite; Gn. galena; Qz. quartz; Po. pyrrhotite; Sp. sphalerite; Stn. stannite; Tpz. topaz; Znw. zinnwaldite.

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(1) Sphalerite in the granite (Sp-Ⅰ). Sphalerite in the granite is black, anhedral and disseminated. It is often associated with chalcopyrite and tetrahedrite, and some of the sphalerite rims (outer zones) are associated with cassiterite in hydrothermal period.

(2) Sphalerite in the greisen (Sp-Ⅱ). Sphalerite in the greisen is brownish red and disseminated, in paragenetic association with mica and topaz, and also with cassiterite at a few rims (outer zones) of sphalerite crystals.

(3) Dark color sphalerite in the quartz veins (Sp-Ⅲ). Dark color sphalerite is paragenetically association with arsenopyrite and galena, and also with wolframite in some places. This kind of sphalerite is relatively rare. According to the mineral paragenetic relationship, its formation temperature is high and it is the product of late magmatism.

(4) Brownish red sphalerite in the quartz veins (Sp-Ⅳ). Brownish red sphalerite is closely associated with cassiterite and widely distributed in the quartz veins. It is an important ore mineral in the main metallogenic stage and the main source of zinc ore in the deposit.

(5) Brownish green sphalerite in the quartz veins (Sp-Ⅴ). The brownish green sphalerite is closely related to cassiterite and widely distributed in the quartz veins. It is an important ore mineral in the main metallogenic stage, but its abundance is less than the brownish red sphalerite. This type of sphalerite closely occurs with the brownish red sphalerite, but there is no obvious crosscutting relationship, but it can crosscut the cassiterite zonation.

According to our previous study, the formation sequence of sphalerite is as follows: magmatic sphalerite-quartz vein phase Ⅰ sphalerite-greisen sphalerite-quartz vein phase Ⅱ sphalerite-quartz vein phase Ⅲ sphalerite (Zhu et al., 2021). The average content of major and trace elements of the five types of sphalerite in the Weilasituo deposit is summarized in Table 3.

Table  3.  Average element contents of sphalerite in the Weilasituo deposit

Element	Mn (ppm)	Cu (ppm)	Ga (ppm)	Ge (ppm)	Se (ppm)	Ag (ppm)	Cd (ppm)	In (ppm)	Sn (ppm)	Bi (ppm)	Pb (ppm)	Zn (wt.%)	Fe (wt.%)
Sp-Ⅰ	1 295	451	12.8	0.381	82.3	4.56	1 599	256	31.7	0.072	0.067	59.2	6.68
Sp-Ⅱ	848	481	3.95	0.435	258	1.67	1 523	857	33	0.075	0.183	63.8	3.11
Sp-Ⅲ	928	1 501	13.4	0.469	8.6	21.5	1 444	222	1 378	4.22	149	62.1	4.42
Sp-Ⅳ	586	522	4.71	0.366	91.2	3.35	1 448	959	29.3	0.112	0.19	63.7	2.92
Sp-Ⅴ	484	1 012	3.08	0.326	239	5.46	1 648	1 940	36.3	4.25	0.542	64.7	1.85

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The average Fe and Mn contents of sphalerite in granite are 6.68% and 1 295 ppm respectively, which is the highest of all types of sphalerite. The average Fe and Mn contents of dark color sphalerite (SP-Ⅲ) in the quartz veins are 4.42% and 928 ppm respectively, second only to magmatic sphalerite. The average content of In is 222 ppm in the dark color sphalerite (SP-Ⅲ) is similar to that in magmatic sphalerite (256 ppm), while the content of In in other sphalerites is also generally very high, which may indicate the continuous formation of these sphalerites from magmatic to hydrothermal stages. In addition, the average contents of Ag, Cu and Sn in sphalerite in the phase I quartz veins are the highest among all types of sphalerite, with 21.5 ppm Ag, 1 501 ppm Cu and 1 378 ppm Sn respectively. The extremely high contents of Sn and Pb may be due to the mechanical mixture of some solid mineral inclusions (tetrahedrite and other Pb-bearing minerals) in the sphalerite of this type (Zhu et al., 2021).

It is generally suggested that the higher the formation temperature of sphalerite, the higher the Fe content. However, according to previous studies on sphalerite in the Weilasituo mining area (Tao, 2017), the Fe content of sphalerite in the Weilasituo Cu-Zn deposit is high (9.5%–23%, Tao, 2017). In this study, even the highest Fe content of sphalerite in the granite is no more than 8%. The Fe content of sphalerite in the quartz veins in the Weilasituo Sn-Li-Rb polymetallic deposit is generally 1%–2%, which is far lower than that of the nearby Weilasituo Cu-Zn deposit. In the metallogenic system, the Fe content of sphalerite in the Sn-Li-Rb polymetallic deposit with high-temperature is significantly lower than that of sphalerite in the Cu-Zn polymetallic deposit with low-temperature, which indicates addition of an external Fe-containing fluid, and this fluid is possibly the meteoric water circulating in the host metamorphic rocks that may leach Fe and other elements (such as Mg for mica) from the country rocks during the later fluid evolution. Previous studies on lead isotopes in the Weilasituo Cu-Zn deposit show that the Pb in the ores comes from surrounding rock strata and deep magma (Jiang et al., 2010), which proves that the metallogenic material of the Weilasituo Cu-Zn deposit does contribute from surrounding rock strata. It is worth noting that there are also a large number of surrounding rocks of gneiss in the Weilasituo Sn-Li-Rb polymetallic deposit, but Fe rich sphalerite is not formed. The possible reason is that the mineralization of large-scale zinnwaldite/lepidolite in the greisen stage before the formation of sphalerite in the hydrothermal stage consumes Fe in the fluids that derived from the surrounding rock strata, which is consistent with the geochemical result of mica, that is, biotite in the surrounding rock provides a certain amount of Fe and Mg.

According to the temperature estimation from sphalerite chemistry, the formation temperature of sphalerite is between 250–320 ℃ (Fig. 12), and the temperature decreases along with the mineralization evolution (Zhu et al., 2021). In addition, most sphalerite in the Weilasituo Sn-Li-Rb polymetallic deposit is rich in In and Cd. The maximum content of In in sphalerite that formed in the late stage of the quartz veins can reach 2 343 ppm. The average In content of sphalerite in the last stage is 1 940 ppm, which meets the cut-off grade for economic usage of In as a by-product. The content of Cd is also high and suitable for economic usage as by-product, which reaches an average content of 1 648 ppm for sphalerite that formed in the late stage of the quartz veins.

Figure  12.  Scatter diagram of major elements in sphalerite and estimated metallogenic temperature of sphalerite (According to Zhu et al., 2021).

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2.3.4   Rare metal minerals

The alkali feldspar granite body in the Weilasituo Sn-Li-Rb polymetallic deposit contains various amount of accessory minerals, mainly Nb-Ta oxides and cassiterite, as well as some U-rich minerals. It is common that these minerals may form an aggregate with the main body being cassiterite or wolframite, and the middle part is Nb-rich columbite that surrounding by Nb-Ta oxides with higher tantalum content as the rims. In addition, some minerals show the transition between wolframite and columbite, which have higher values of niobium, tantalum and tungsten (Fig. 13).

Figure  13.  SEM image of accessory minerals in granite.

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In the alkali feldspar granite, columbite crystallized in the early stage and tantalite in the late stage. Magmatic cassiterite, wolframite and zircon crystallize in a certain order from Nb-rich to Ta-rich. The first is likely the magmatic cassiterite, followed by wolframite, but both may crystallize simultaneously. Then zircon crystallizes since it cuts cross the cassiterite and wolframite. In the later stage of magma evolution, the content of calcium and fluorine increases, and Ta-rich microlite starts to form, following by the crystallization of petscheckite, uraninite and thorite.

Previous studies suggest that the different solubility of columbite and tantalite is the driving force controlling the Ta and Nb fractionation during the process of magma evolution. They indicate that Nb is easier to enter the columbite-group minerals than Ta in the early stage of granitic magma evolution. Therefore, the Ta/Nb ratio in the melt gradually increases, and Ta and Nb are gradually fractionated (London, 2008; Linnen, 1998). Chevychelov et al. (2010) found that in the peraluminous granitic magma, Nb is easier to enter the columbite-group minerals than Ta (the partition coefficient D_Nb/D_Ta > 1), thus increasing the Ta/Nb ratio in the residual magma.

2.4 Characteristics of the Ore-Forming Fluid

The mineralization of most Sn-polymetallic deposits worldwide has experienced continuous magmatic to hydrothermal evolution, and particularly the Sn-ore minerals precipitate during the magmatic-hydrothermal transitional stage (Zhu et al., 2016). Previous studies on the petrography of fluid inclusions in the Weilasituo Sn-Li-Rb polymetallic deposit show that the type of fluid inclusions has changed significantly from alkali feldspar granite, greisen and quartz veins, from mainly melt inclusions and daughter mineral-bearing multiphase inclusions to mainly gas-liquid two-phase inclusions (Zhang, 2020; Guo L X et al., 2018; Liu et al., 2018b; Sun et al., 2017; Mei et al., 2015a). The Weilasituo Sn-Li-Rb polymetallic deposit is rich in inclusion types, and a large number of fluid inclusions can be seen in different types of minerals (Fig. 14). The ore-forming fluid belongs to the H₂O-NaCl ± CO₂ ± CH₄ system with moderate-to-high temperature and large salinity variation. The temperature measurement results from previous fluid inclusions studies are shown in Table 4.

Figure  14.  Morphology of fluid inclusions in the Weilasituo Sn-Li polymetallic deposit.

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Table  4.  A summary of fluid inclusion temperature and salinity in the Weilasituo deposit

Metallogenic stage	Mineral	Inclusion type	Ice point (℃)	Homogenization temperature (℃)	Salinity (wt.% NaCl equiv.)	Reference
Late magmatic period	Quartz	Liquid-rich two-phase	-5.2– -2.9	313–383	2.9–10.4	Zhang (2020)
Late magmatic period	Quartz	Multiphase inclusions containing daughter minerals	-	-	46.4–50.9	Liu et al. (2018b)
Late magmatic period	Quartz	Gas-rich two-phase	-7.1– -3.2	388–473	5.3–9.5	Liu et al. (2018b)
Late magmatic period	Quartz	Liquid-rich two-phase	-5.2– -4.9	372–408	8.1–18.6	Liu et al. (2018b
Late magmatic period	Quartz	Liquid-rich two-phase	-4.7– -4.1	324–333	6.6–7.5	Guo L X et al. (2018)
Greisenization	Quartz	Liquid-rich two-phase	-8– -1.6	210–390	2.7–11.7	Sun et al. (2017)
Greisenization	Quartz	Melt-fluid inclusion	-	205–387	-	Sun et al. (2017)
Greisenization	Quartz	CO2-rich three-phase	-	283–359	6.4–9.2	Sun et al. (2017)
Quartz vein stage	Quartz	Liquid-rich two-phase	-5.2– -2.2	210–353 (mostly 260–323)	4.5–7.2	Zhang (2020)
Quartz vein stage	Quartz	Multiphase inclusions containing daughter minerals	-	-	46.4–48.5	Liu et al. (2018b)
Quartz vein stage	Quartz	Gas-rich two-phase	-7.4– -2.6	328–401	4.3–11	Liu et al. (2018b)
Quartz vein stage	Wolframite	Liquid-rich two-phase	-11.1– -5.2	282–345	8.17–15.07	Mei et al. (2015a)
Quartz vein stage	Quartz	Liquid-rich two-phase	-8.2– -3.4	242–314	5.56–11.93	Mei et al. (2015a)
Quartz vein stage	Quartz	Liquid-rich two-phase	-14.5– -8.9	243–395	4.3–11	Liu et al. (2018b)
Quartz vein stage	Quartz	Liquid-rich two-phase	-5.0– -2.1	201–278	3.6–7.9	Guo L X et al. (2018)

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The quartz phenocryst core of the alkali feldspar granite crystallizes early. The inclusions captured in these quartz phenocrysts should represent the fluid characteristics of the early magmatic evolution, including albite formed in the early stage. The inclusions are mainly melt inclusions, up to more than 90%. The gas-liquid composition is very small, and the solid-liquid ratio is more than 90% (Zhu et al., 2016). In the outer part of the quartz phenocrysts and those quartz formed in the late stage, it occurs mainly melt-fluid inclusions and gas-liquid two-phase inclusions. Liu et al. (2018b) measured the gas-rich two-phase fluid inclusions and liquid-rich two-phase fluid inclusions in quartz, and the homogenization temperature shows a range of 388–473 and 372–408 ℃, and the salinities are 5.3 wt.%–9.5 wt.% and 8.1 wt.%–18.6% wt.% NaClequiv..

In the process from the magmatic stage to greisen stage, the magma gradually differentiates and more hydrothermal fluid exsolves, and the volatiles become increasingly enriched. At this time, the melt phase and gas-liquid phase coexist, and the fluid characteristics are between magmatic and hydrothermal fluid. This is called as the transitional stage in the evolution process from magma to hydrothermal fluid (e.g., Zhu et al., 2016). It is the superposition stage of the two fluid environments, with complex composition and fluid properties. During this period, the number of melt inclusions in the minerals decreases sharply, and the gas-liquid two-phase inclusions begin to appear. The partial homogenization temperature of the gas-liquid multiphase inclusions containing daughter minerals in greisen is 205–387 ℃, peaked at 250–280 ℃. The homogenization temperature of gas-liquid two-phase fluid inclusions in greisen is 210–390 ℃, peaked at 240–280 ℃, with the salinity range of 2.7 wt.%–11.7 wt.%, and peaked at 4 wt.%–9 wt.% NaClequiv. (Sun et al., 2017).

The complete homogenization temperature of daughter mineral bearing multiphase fluid inclusions in greisen is basically the same as that of gas-liquid two-phase fluid inclusions. Laser Raman analysis shows that the solid phase composition is consistent with the composition of melt inclusions, indicating that the solid phase composition shares the characteristics of melt inclusions (Sun et al., 2017). Previous studies have shown that the inclusions formed in the greisen stage are transitional inclusions between melt inclusions and fluid inclusions. Yu et al. (2015) conducted heating and melting experiments on the melt inclusions in the isolated greisen in the Yaogangxian tungsten deposit (Hunan Province), the results show that the existence of melt inclusions actually reflects the differentiation and heterogeneous capture process of melt and fluid in the late magmatic period.

In the quartz vein stage of the Weilasituo Sn-Li-Rb polymetallic deposit, the quartz-sulfide vein stage of the Weilasituo Cu-Zn deposit and the Bairendaba Pb-Zn-Ag deposit, the responsible ore fluids are all dominated by hydrothermal fluid. At this stage, the fluids are rich in volatiles and ore metals. Therefore, the gas-liquid two-phase fluid inclusions in the quartz are abundant with various gas-liquid ratios (Guo, 2016; Mei et al., 2015a). Sun (2018) measured that the homogenization temperature of gas-liquid two-phase fluid inclusions in the cassiterite-sphalerite-quartz veins from the Weilasituo Sn-Li-Rb polymetallic deposit is 166–320 ℃, peaked at 200–240 ℃, with salinity of 1.6 wt.%–10.7 wt.%, peaked at 2 wt.%–6 wt.% NaClequiv. Liu et al. (2018b) reported that the homogenization temperature is 328–401 ℃ and salinity is 4.3 wt.%–11 wt.% NaClequiv. It proves that the span of temperature and salinity in this stage is very large. In the quartz-sulfide veins of the Weilasituo Cu-Zn deposit, the homogenization temperature of gas-liquid two-phase fluid inclusions is 208–294 ℃, and the salinity is 4.65 wt.%–12.39 wt.% NaClequiv. (Mei et al., 2015a), whereas in the Bairendaba Pb-Zn-Ag deposit, the homogenization temperature of gas-liquid two-phase fluid inclusions is in the similar range of 150–284 ℃ and the salinity is 3.06 wt.%–9.60 wt.% NaClequiv. (Mei et al., 2015a).

From the binary diagram of carbon and oxygen isotopes (Fig. 15a), the Weilasituo Sn-Li-Rb polymetallic deposit shows δ¹³C values of the fluid inclusions from -14.9‰ to -15.5‰, with little difference from magmatic stage to quartz vein stage (Liu et al., 2018b). Liu et al. (2018b) suggested that the carbon in the fluid in the Weilasituo deposit mainly comes from the magma, but they interpret the very negative carbon isotopes as strong fractionation in the metallogenic process, for example, ¹³CO₂ + H¹²CO₃^- = ¹²CO₂ + H¹³CO₃^-, resulting in the carbon isotope composition in the hydrothermal solution being significantly lower than that of the magmatic system (around -7‰). However, it may be more likely that the lower carbon isotopes are due to the light carbon addition from the country rocks of organic carbon rich meta-sedimentary rocks leached by circulated meteoric water. Quan (2017) also reported the similar and even lower δ¹³C values of -19.7‰ to -19.1‰ of fluid inclusions, which is attributed to the strong water-rock reaction between ore-forming fluid and surrounding rock. From the binary diagram of hydrogen and oxygen isotopes (Fig. 15b), the data of the Sn-Li-Rb polymetallic deposit samples plot near the magmatic water field, indicating that the ore-forming fluid is mainly magmatic water mixed with increasing amounts of meteoric water from magmatic stage to Sn-polymetallic stage. In contrast, the data for the peripheral Cu-Zn and Pb-Zn-Ag deposits plot away from the magmatic water field with slightly lower δ¹⁸O and significantly lower δD values, indicating that the precipitation of these minerals during sulfide stages is late with higher amounts of meteoric water (Guo L X et al., 2018).

Figure  15.  C-O isotopes (a) and H-O isotopes (b) characteristics of fluids in the Weilasituo Sn-Li polymetallic deposit (data from Guo L X et al., 2018; Liu et al., 2018b; Quan, 2017; Mei et al., 2015a; Ouyang et al., 2014; Wang, 2009).

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2.5 Petrogenetic and Metallogenic Age

Several studies have carried out U-Pb dating on the granite related to mineralization in the mining area, with age in the range of 135–140 Ma (Liu et al., 2018a; Zhu et al., 2016; Zhai et al., 2014), which is consistent with the age of the regional Beidashan granites (140 ± 2 Ma, Wu et al., 2021; Liu et al., 2018a) adjacent to the mining area. Previous studies suggested that both granites may have derived from the same magma source at depth, and then experienced different degrees of differentiation and evolution; and the Weilasituo granite is likely the highly fractionated part of the Beidashan granites in the deep (Zhang et al., 2019). Cassiterite is the most common ore mineral in tin deposits. Cassiterite lattice can contain high content of U and low content of Pb, which makes it a potential good mineral for isotopic dating (Gulson and Jones, 1992). The U-Pb dating of cassiterite can directly constrain the metallogenic age of tin deposits and help to further explore the temporal and spatial relationship between the tin deposits and causative metallogenic granites. The metallogenic age of the Weilasituo cassiterite obtained by LA-ICP-MS U-Pb dating method in this study is 135.6 ± 2.1 Ma (MSWD = 2) (Fig. 16), which is consistent with the previous cassiterite dating results (Liu et al., 2018a; Wang F X et al., 2017). The mineralization age of the deposit is also consistent with the peak period of Sn-polymetallic and rare metal deposits in the south section of Great Xing'an Range (Fig. 17, 140–130 Ma). The whole area experienced large-scale magmatic activity and Sn-polymetallic mineralization events in the Early Cretaceous. Therefore, the Sn-Li-Rb polymetallic mineralization in the Weilasituo is also a part of the Yanshanian mineralization events in this region. Meanwhile, Zhang (2020) also report a Re-Os isotopic isochron age of molybdenite from the quartz veins of 137.3 ± 2.5 Ma (MSWD = 5.7), in agreement with the cassiterite age. However, younger Re-Os ages (125.7 ± 3.8 Ma, 116.6 ± 1.8 Ma) of molybdenite were also reported by Zhai et al. (2016) and Guo (2016), indicating that there may be multi-stage mineralization in the region (Zhang et al., 2019).

Figure  16.  Cassiterite U-Pb dating in the Weilasituo Sn-Li polymetallic deposit.

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Figure  17.  The formation ages of tin-polymetallic and rare metal deposits in the southern section of the Great Xing'an Range (data after Zhang and Jiang, 2021; Zhang et al., 2019; Guo S et al., 2019; Guo W K et al., 2018; Liu R L et al., 2018a; Wang Z G et al., 2017; Yao et al., 2017; Liu Y F et al., 2016; Zhu et al., 2016; Mei et al., 2015b; Liu and Jiang, 2014; Zhai et al., 2014; Shu et al., 2013; Liao et al., 2012; Pan et al., 2009; Ishihara et al., 2008).

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2.6 Metallogenic Model

The metallogenic model of the Weilasituo Sn-Li-Rb polymetallic deposit is shown in Fig. 18. The Weilasituo granite pluton has experienced a high degree of differentiation and superimposed greisenization and hydrothermal metasomatism in the late stage. There are disseminated Rb and Sn-Zn mineralization in the roof part of the causative granite, and the breccia pipe type Li-Rb mineralization extending to the metamorphic country rocks, and vein type Sn-Zn and Pb-Zn-Ag mineralization in the country rocks away from the granite pluton.

Figure  18.  Metallogenic model of the Weilasituo Sn-Li polymetallic deposit.

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Therefore, the mineralization at Weilasituo mining district should be the comprehensive result of high-degree differentiation of magma and late fluid-melt interaction and hydrothermal fluid activity. The former processes should be the control factor for lithium, rubidium, and the latter is the main factor controlling the mineralization of tin, zinc, lead and silver.

The crypto-explosion has a great impact on the environment of the mineralization, changing the temperature, pressure and composition of the fluid in a certain period of time. It is an important geological process for the mineralization of many different ore metals as we mentioned above, but the Li mineralization of this type is rare. There is no major tin-tungsten mineralization in the breccia pipe, indicating that the degassing and decompression process of ore-forming fluid is not the main reason for the massive unloading of Sn and W-forming minerals, while a large number of lepidolite and zinnwaldite cemented breccia prove that this stage is the main precipitation stage of Li-bearing minerals, and the main economic mineralization stage of Sn is after this breccia stage, namely at the hydrothermal vein stage.

The metallogenic materials in the whole Weilasituo- Bairendaba mining area are multi-source, and the highly differentiated magmatic rock and biotite plagioclase gneiss in the surrounding rock are two important components. The ore-forming fluid mainly comes from the granitic magma, and the meteoric water is also involved in the late stage. The granite rock itself is Fe-poor, and Fe mainly comes from the surrounding rock. The surrounding rock on both sides of the quartz veins has undergone strong greisenization and alteration, causing the decomposition of biotite in the gneiss, releasing Fe and Mg in biotite and entering the fluid.

3. CONCLUSIONS

(1) The Weilasituo Sn-Li-Rb polymetallic deposit in Inner Mongolia is a unique deposit with multiple mineralization types, including granite type Rb and Sn-Zn, hydrothermal crypto-explosive breccia pipe type Li-Rb, quartz vein type Sn-Zn and sulfide vein type Pb-Zn-Ag mineralization. Among them, hydrothermal breccia pipe type Li-Rb ore is a newly identified rare metal mineralization type in this study.

(2) The major lithium ore body in the Weilasituo includes the whole crypto-explosive breccia pipe, with a cylinder shape of thin at the top and thick at the bottom. There is no major tin-tungsten mineralization in the breccia, but a large number of lepidolite and zinnwaldite cemented the breccia, indicating that this stage is the main precipitation stage of Li-forming minerals, followed by the main mineralization stage of Sn-Zn, and then the Pb-Zn-Ag veins.

(3) The study of mineral geochemistry and fluid inclusions shows that the crypto-explosion has a great impact on the mineralization environment, changes the temperature, pressure and composition of the fluid, and causes the precipitation of related Li-Rb ore minerals.

(4) The Weilasituo Sn-Li-Rb polymetallic mineralization should be the comprehensive result of high-level magmatic differentiation and late fluid-melt interaction and hydrothermal fluid activity. The magmatic process controls the mineralization of lithium, rubidium, niobium and tantalum, while the hydrothermal process controls the mineralization of tin, zinc, lead and silver.