Journal of Earth Science  2019, Vol. 30 Issue (4): 707-727   PDF    
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Geochronology and Geochemistry of Li(Be)-Bearing Granitic Pegmatites from the Jiajika Superlarge Li-Polymetallic Deposit in Western Sichuan, China
Hongzhang Dai 1, Denghong Wang 1, Lijun Liu 1,2, Yang Yu 1, Jingjing Dai 1     
1. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China;
2. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
ABSTRACT: Strategic emerging minerals such as lithium, beryllium, niobium and tantalum are the most important rare metals currently, especially with the increasing demand of emerging industries on rare metals in China. The Jiajika deposit with a complete Li-Be-Nb-Ta metallogenic series is the largest pegmatite type rare metal deposit in China at present. In this paper, systematic researches of geochronology and petrogeochemistry were carried out to understand the genetic relationships between mineralization and magma evolution in the Jiajika deposit, which might be helpful to further rare-element prospecting in Songpan-Garze area. Zircon LA-ICP-MS U-Pb dating yields a concordia age of 217±1.1 Ma and a weighted mean 206Pb/238U age of 217±0.84 Ma for the aplite from the No. 308 pegmatite. Cassiterite LA-MC-ICPMS dating yields concordant ages of 211±4.6 Ma for the No. 308 pegmatite vein and 198±4.4 Ma for the No. 133 pegmatite vein, indicating that the rare metal mineralization mainly occurred in the Late Indosinian Period, further suggesting that the granites, aplites and pegmatites in Jiajika formed during a relatively stable stage after the intense orogeny of the Indosinian cycle. The rare metal-bearing granitic rocks and pegmatites show a clear linear relationship between A/CNK and A/NK and are enriched in total alkalis and depleted in CaO, FeO, MnO, MgO, Ba and Sr. All barren rocks and mineralized rocks feature similar rare earth element and trace element geochemical patterns. Thus, these characteristics indicate that the aplites and pegmatites represent the highly differentiated products of the two-mica granite (MaG) in this area, which is the most likely parent magma. During the evolution of magma, strong alkali metasomatism occurred between the melt phase and the volatile-rich fluid phase; as a result, large-scale rare metal mineralization occurred in certain structural zones of the pegmatite veins in the Jiajika deposit.
KEY WORDS: granitic pegmatites    rare metals    metallogenic epoch    geochemical characterization    Jiajika    
0 INTRODUCTION

Li, Be and Ta are the most important rare metals currently, especially with the increasing demand of emerging industries on rare metals. In China, although Li reserves associated with salt lakes are huge, because of their high Mg/Li ratios (Zhang et al., 2016), the exploitation and processing technologies for Li in salt lakes are challenging, whereas hard-rock type Li deposits are more efficient to exploit and contain other important rare elements such as Be, Nb, Ta, Rb, Cs and so on, which cannot be matched by any salt lake type Li deposit. Granitic pegmatite-type Li deposits are the most important types of hard- rock type Li deposits, which are dominant in China (Wang et al., 2017a). In China, granitic pegmatite-type Li deposits are mainly distributed in the Altay region, Xinjiang Autonomous Region, and Jiajika and Ke'eryin in Ganzi, Sichuan Province. In the past five years, lots of Li deposits like the Jiajika, Lijiagou, Ma'erkang (Dangba), Yelonggou, Redamen and Waying, etc. are newly discovered or significantly expanded in Sichuan Province, China. The amount of prospective resources can reach 7 Mt, forming a world-class ore concentration area. The major Li- and Be-bearing minerals are spodumene and beryl, respectively. These deposits are Mesozoic in age and characteristically formed in an orogenic environment (Chen et al., 2013; Liu et al., 2012; Li et al., 2007a; Zou and Li, 2006; Wang et al., 2005, 2002; Li and Chen, 2004; Yuan and Bai, 2001).

Although it is still a controversial issue on the existence of parent granite pluton in pegmatite deposits (Martin and Vito, 2005; Černý, 1982). It is generally believed that granitic pegmatite veins are products of extreme differentiation of granitic melts (Breaks and Moore, 1992; Černý, 1991a, b), so finding the parent rock of pegmatite dikes is of great significance for the prospecting of rare metals in pegmatite and the establishment of a complete granite granitic pegmatite system evolution model. Meanwhile, the zonal distribution of pegmatite group is one of the most complicated and perplexing problems in pegmatite research (London, 2005). For the LCT (lithium-cesium- tantalum) pegmatite association, it commonly displays gradual transitions among the individual pegmatite types, and regional zoning of these pegmatites around parent granites is observed in many localities (Černý, 1991c). Different types and subtypes of pegmatite zoning are distributed from the granitic intrusions inward to outward, showing as follows: barren pegmatites, Be-rich pegmatites, Be(Nb, Ta)-rich pegmatites, Li(Be, Nb, Ta) pegmatites and Li(Cs, Ta) pegmatites.

In recent years, with the governmental support in policy and the further development in technique, exploration for rare metals is coming into another rush time, reinflaming the study of pegmatite type rare metal deposits. A series of prospecting progresses have been achieved in western Sichuan, represented by No. X03 pegmatite vein in the Jiajika deposit (Wang et al., 2017a, b) and No. Ⅷ pegmatite vein in the Dangba deposit (Wang et al., 2018). The Jiajika deposit, which is located in the Songpan-Garze Fold Belt (SGFB) and with a complete Li-Be-Nb-Ta metallogenic series, contains more than 2 million tons of identified Li2O resources (Wang et al., 2017a) and a large scale of BeO resources, and has become the largest hard-rock type Li (Be, Nb, Ta) deposit in China. The Jiajika deposit is generally associated with a granite pluton and a series of different mineralization types related to granitic pegmatite veins. Previous studies mainly focused on the geological characteristics (Liu et al., 2017a, 2015; Qin et al., 2015; Zhao et al., 2015; Tang and Wu, 1984), diagenetic and metallogenic epoch (Dai et al., 2018; Hao et al., 2015; Wang et al., 2005), source of ore-forming material and fluid (Hou et al., 2018a; Li and Chou, 2017, 2016; Liu et al., 2017b; Su et al., 2011; Li et al., 2013a, b, 2008, 2007b, 2006a, b) and metallogenic mechanism (Hou et al., 2018a, b; Pan et al., 2016) of a single granite pluton or pegmatite vein. There are more than 500 pegmatite veins with a certain scale and most of them are poorly studied. High precision data on geochronology and component analysis of pegmatite veins are still lacking. Meanwhile, it has been found that the aplite type is the important ore type in many pegmatite veins (for example, No. 308, No. 134 and X03, etc.) which has been neglected in the past (Wang et al., 2017a) and no precise chronological data are available about the aplite in the Jiajika deposit. Therefore, additional work is required to characterize the diagenetic and metallogenic epoch and geochemical compositions of granites, aplites and pegmatites related to Li (Be, Nb, Ta) mineralization.

The aims of this study are to discuss the genetic relationship among aplite, pegmatite and granite in the Jiajika deposit and to further establish the geochemical parameters for the exploration of Li- and Be-bearing pegmatite-type deposits. The geochemical signature of the granites, granitic rocks, aplites and pegmatites enriched in Li and Be and those without mineralization (barren rocks) from the Jiajika deposit are compared in this paper. To distinguish the Li(Be)-bearing granites, aplites and pegmatites, only the genetic and spatial relationships of the granitic rocks, aplites and pegmatites are considered. The size and grade of the mineralization are not taken into account. This paper presents new geochronological and geochemical data for representative pegmatite veins in the Jiajika deposit, which may contribute to a better understanding of the genesis of granite pegmatite-related Li, Be and other related rare metals mineralization in the area.

1 GEOLOGICAL SETTING

SGFB covers an area of more than 2×105 km2 and occupies a triangular area between the Qiangtang, Qiadam, North China and Yangtze blocks, bounded by the East Kunlun to the north, Qinling to the northeast, Longmen Shan to the southeast and the Yijun Block to the southwest (Yuan et al., 2010). The Jiajika Li-polymetallic deposit is concentrated in domes in the uplift zones between the Eurasian and Indian plates and is located in the Songpan-Garze orogenic zone (Fig. 1). The eastern SGFB was an earlier fore-arc basin developed above the passive margin of the Yangtze Block. Recent geophysical and geochemical studies have revealed continental basement beneath the eastern SGFB, most likely a continuation of the Yangtze Block (Wang et al., 2007; Zhang et al., 2007, 2006; Roger et al., 2004). The SGFB is characterized by a thick Triassic flysch complex known as the Xikang Group (BGMRSP, 1991). Flysch sediments of the Xikang Group were deposited in several separate basins or depocenters (Li et al., 2018; Weislogel, 2008). The outcropping strata are mainly Triassic mudstones, siltstones and sandstones. Overall, the sedimentary facies of the Triassic strata indicate that the environment changed from shallow- to deep-water, and back to shallow- water again, clearly reflecting the development of the SGFB (Yuan et al., 2010). More than one hundred silicic plutons occur in the SGFB with ages ranging mainly from Permian to Cenozoic (BGMRSP, 1991). These plutons show a wide range in composition and include calc-alkaline, alkaline, peralkaline and peraluminous rocks that formed at different stages during evolution of the SGFB. The Mesozoic acid plutons, which are closely related to Li-polymetallic mineralization, are mostly irregular rounded or strip shaped ones and mainly intruded into the Triassic metamorphic rocks, and a few of them are located in the Paleozoic metamorphic rocks (Fig. 1). The NW-SE-trending dome-like Jiajika anticline, accompanied by a series of NW-SE-trending faults, constitutes the main tectonic system in this area. The emplacement of most pegmatites was controlled by the pre-existing NE-SW- and NW-SE-trending shear fractures or contemporaneous conjugate joint and fissure sets (Tang and Wu, 1984). The rare mineral resources such as Li, Be, Nb and Ta in the region are abundant. The deposits (points) found at present are mainly distributed in Pingwu, Ma'erkang and Jiajika, etc. (Fig. 1).

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Figure 1. Sketch of regional tectonics and distribution of rare metal deposits in western Sichuan, China (after Li et al. 2014; Yuan et al., 2010; Roger et al., 2004).
2 GEOLOGY OF THE JIAJIKA DEPOSIT

A two-mica granite (also named the Majingzi granite, simplified as MaG), which comprises over 5.3 km2 of outcrops, intruded into the down-plunge end of the Jiajika anticline and is only exposed as a marginal facies (Fig. 3a), which is characterized by a fine-grained texture and may contain rare spodumene (Hao et al., 2015; Li et al. 2007b; Tang and Wu, 1984). During magmatism, the rocks hosting the Majingzi granite experienced multi-stage metamorphism and developed five distinct metamorphic zones surrounding the granite: from the inner to outer regions, including the diopside, staurolite, andalusite-staurolite, andalusite and biotite zones. In the Jiajika deposit, the rare metal mineralizations of pegmatite veins are closely related to the metamorphic belts, which are closely associated with the Majingzi granite pluton and all kinds of pegmatite veins (Tang and Wu, 1984).

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Figure 3. Representative field photographs, hand specimen and photomicrographs of the Majingzi granite pluton and granitic pegmatites from the Jiajika deposit. (a) Majingzi granite; (b) albite-spodumene zone of the No. 308 pegmatite vein; (c) overall perspective of the No. 133 pegmatite vein; (d) aplite and microcline pegmatite from the No. 308 pegmatite vein; (e) aplite and albite-spodumene pegmatite from the No. 308 pegmatite vein; (f) part zoning of the No. 133 pegmatite vein; (g) albite-spodumene pegmatite in the No. 133 pegmatite vein; (h) beryl-bearing microcline pegmatite in the No. 308 pegmatite vein; (i) tourmaline pegmatite in the No. 308 pegmatite vein, locally visible fluorite output; (j) spodumene, cassiterite and Nb-Ta minerals-bearing pegmatite in the No. 133 pegmatite vien; (k) characteristics of aplite under a polarizing microscope; (l) characteristics of microcline pegmatite under a polarizing microscope; (m) characteristics of albite and spodumene pegmatite in the No. 308 pegmatite vein under a reflecting optical microscopy; (n) characteristics of albite and spodumene pegmatite in the No. 133 pegmatite vein under a reflecting optical microscopy. Abbreviation: Ab. albite; Brl. beryl; Bt. biotite; Cas. cassiterite; Fl. fluorite; Qtz. quartz; Mrc. microcline; Ms. muscovite; Spd. spodumene; Tur. tourmaline.

Thus far, over five hundred pegmatite veins have been found in the Jiajika deposit, which are prolific in the top and outer contact zones of the Majingzi granite (Fig. 2). Most of them have intruded metamorphic rocks, and only a few have intruded the inner contact zone. Unlike that observed in the Koktokay No. 3 pegmatite vein, the vertical zonation of pegmatite veins is not obvious in either the entire ore field or a single vein (Wang et al., 2017b; Tang and Wu, 1984). Based on their distance from the rock mass (MaG), the pegmatite types (considering only their rock-forming minerals) can be classified into five types (Fig. 2, Wang et al., 2017a; Tang and Wu, 1984); from proximal to distal regions, and these types include (Ⅰ) the microcline pegmatite type (Figs. 3d, 3f, 3l), (Ⅱ) the microcline- albite pegmatite type, (Ⅲ) the albite pegmatite type, (Ⅳ) the spodumene pegmatite type (Figs. 3e, 3f), and (Ⅴ) the muscovite (lepidolite) pegmatite type, which is further divided into the muscovite subtype (Ⅴa) and the lepidolite subtype (Ⅴb). Although the occurrences of pegmatite are rather variable along the main steep slope, there is a regular change in the pegmatite types in different output locations: (1) pegmatites are produced in the steeply dipping areas of the longitudinal and horizontal cracks in the MaG, and most of them belong to type Ⅰ. (2) Pegmatites in the interface and proximal contact zones near the MaG mainly occur in gently dipping tension-stripping fissures and layered fissures, and most of them belong to types Ⅱ and Ⅲ. (3) Pegmatites in the distal contact zone are mainly produced in the steeply dipping modified shear cracks, and most of them belong to types Ⅳ and Ⅴ. Among these pegmatites, barren pegmatites mainly belong to type Ⅰ, Be-bearing pegmatites mainly belong to type Ⅱ(Ⅲ), and Li-bearing pegmatites mainly belong to type Ⅳ(Ⅲ). From the inside to the outside of the MaG, the following enrichment order was observed: Barren→ Be(Nb)→Li(Be, Nb, Ta)→Ta(Nb, Cs, Sn) (Tang and Wu, 1984). The composition of this material is complex; more than 40 kinds of minerals have been found (Tang and Wu, 1984), and their rare and rare earth minerals mainly include beryl (Fig. 3h), spodumene (Figs. 3g, 3m, 3n), niobite (Fig. 3j), cyrtolite, sicklerite, allanite, and hamartite.

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Figure 2. Simplified geological map showing the distribution of pegmatite in the Jiajika rare mining area, western Sichuan, China (after Wang et al., 2017b; Li et al. 2013; Tang and Wu, 1984). 1. Nb-Tb mineralized pegmatites; 2. Nb-Tb industrial mineralized pegmatites; 3. Li-Nb-Ta mineralized pegmatites; 4. Li mineralized pegmatites; 5. Li industrial mineralized pegmatites; 6. Li-Be mineralized pegmatites; 7. Be mineralized pegmatites; 8. Be industrial mineralized pegmatites; 9. Be-Nb-Ta mineralized pegmatites; 10. non mineralized pegmatites; 11. granite aplite; 12. quartz vein; 13. pegmatite number; 14. two-mica granite; 15. the second of member of the Xinduqiao Formation (T3xd2); 16. the first of member of the Xinduqiao Formation (T3xd1); 17. the second of member of the Zhuwo Formation (T3zh2); 18. the first of member of the Zhuwo Formation (T3zh1); 19. measured faults; 20. inferred faults; 21. boundary and number of pegmatite zoning; 22. mineralization type; 23. microcline pegmatite zone; 24. microcline-albite pegmatite zone; 25. albite pegmatite zone; 26. spodumene pegmatite zone; 27. lapidolite-muscovite pegmatite zone. Notes: (1) X03 pegmatite vein is the projection of the concealed orebody in the plane. (2) Some samples are not shown in the figure because of the same sampling location. (3) The age of barren pegmatite vein is 204±3.7 Ma (ziron U/Pb, unpublished data). The age of the Majingzi granite pluton is 223±1 Ma (zircon U-Pb, Hao et al., 2015). The ages of the ore-bearing granitic pegmatite veins are 195.7±0.1 Ma (plateau age) and 195.4±2.2 Ma (isochron age) for the No. 134 pegmatite vein; 198.9±0.4 Ma (plateau age) and 199.4±2.3 Ma (isochron age) for the No. 104 pegmatite vein (Muscovite Ar-Ar, Wang et al., 2005); and 216±2 Ma (zircon U/Pb) and 214±2 Ma (Nb-Ta oxide U/Pb) for the No. X03 pegmatite vein (Hao et al., 2015).

The sizes of these pegmatites vary widely. Most of them are small and medium-sized, but a few are large in size. Veins are generally 100 to 500 m long, with a maximum length of 1 450 m; they are generally 1 to 10 m thick, with a maximum thickness of 630 m; and they are 50 to 300 m deep, with a maximum depth of more than 500 m. The No. 308 pegmatite vein has the largest surface exposure area of approximately 0.5 km2 (Fig. 2). The pegmatite types (Figs. 3b, 3d, 3e, 3h, 3i) and ore-bearing properties of No. 308 pegmatite vein regularly change along strike from south to north (Figs. 4a, 4b). Types Ⅲ and Ⅳ are mainly produced in the north with a relatively small size (Fig. 3b), while only a small amount of aplite is exposed (Figs. 3d, 3e, 3k), most of them mainly occurred in the deep by drilling (Fu et al., 2017). The largest opencast lithium ore vein is the No. 134 pegmatite vein (51.22 Mt, Wang et al., 2005), which is characterized by the presence of typical pegmatite- type ores (Figs. 4a, 4c), and the strength of Li mineralization gradually increases from deeper to shallower depths (Fig. 4d). A relatively complete pegmatite zoning (Figs. 3c, 3f, 3g, 3j) in the vertical direction of the No. 133 pegmatite vein is located in the south of the No. 134 pegmatite vein (Fig. 2). The largest concealed lithium ore vein (64.31 Mt) is the newly discovered No. X03 pegmatite vein, which is characterized by aplite-type ores and no obvious zonation (Wang et al., 2017b).

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Figure 4. Geological and geochemical characteristics in vertical and strike of the No. 308 pegmatite vein and the No. 134 pegmatite vein in the Jiajika deposit (adapted from Tang and Wu, 1984).
3 SAMPLING AND ANALYTICAL METHODS

For this study, 77 samples were collected in the Jiajika deposit, including granites and different types of aplites and pegmatites from different pegmatite veins within each pegmatite zones of the Jiajika deposit. The location of each sampling is marked in Fig. 2. One aplite sample in the No. 308 pegmatite vein was prepared for the Zircon U-Pb dating. Two samples of spodumene-bearing pegmatite No. 308 and No. 133 pegmatite vein are prepared for the cassiterite U-Pb dating and in-situ composition analysis. All collected samples were analyzed by elemental geochemical analysis.

Zircons for dating were separated by heavy liquid and magnetic separation methods, before handpicking under a binocular microscope. Transparent euhedral zircons were mounted in epoxy resin and polished until grain interiors were exposed. Zircons were then U-Pb dated by laser ablation- inductively coupled plasma-mass spectrometry (LA-ICP-MS) using an Agilent 7500a ICP-MS equipped with a UP193SS laser ablation system at the MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China. The analytical methods are detailed in Qin et al., 2019.

The in-situ U-Pb dating of cassiterites was performed using an MC-ICPMS (Neptune, ThermoFisher Scientific) coupled to an (ESI) UP193FX ArF Excimer laser ablation system with a 193 nm wavelength, which are both housed at the Tianjing Institute of Geology and Mineral Resources. The MC-ICP-MS instrument is equipped with nine Faraday cups, including one axial Faraday cup and eight off-axis Faraday cups, as well as four ion counters. The spot size is 35 μm, the laser density is 10-13 J/cm2 and the laser pulse frequency is 8-10 Hz. Data processing was completed using ICPMSDatacal (Liu et al., 2010) and Isoplot (Ludwig, 2003). The analytical methods are detailed in Yuan et al. (2011).

The in-situ elemental analysis of cassiterite was carried out in the Laboratory of the National Geological Experiment Center, Beijing. The instrument is a laser plasma mass spectrometer (fs-LA-ICPMS) consisting of a femtosecond laser ablation system (ASI J200) and a four-stage rod plasma mass spectrometer (X series). The laser operating frequency is 8 Hz, the laser beam wavelength is 343 nm, the test spot beam diameter is 30 μm, the single pulse energy is ~49 mJ/cm2, the single measurement time is 50 s, the background measurement time is 40 s, the eroded material carrier gas is helium, and the flow rate is 0.87 L/min. The standard samples used here are synthetic silicate glass (SRM610 and SRM612).

Whole-rock major, trace and rare earth element concentrations were analyzed at the National Geological Experiment Test Center, Beijing. Whole-rock major elements were analyzed using a plasma spectrometer (PE8300). All results were normalized against the Chinese rock reference standard JY/T015- 1996; among them, H2O+ is analyzed on the basis of GB/T 14506.2-2010, FeO contents are normalized to GB/T 14506.14- 2010, LOI contents are normalized to LY/T 1253-1999 and CO2 contents are normalized to GB 9835-1988. The analytical uncertainties are less than ±2%.

Whole-rock trace element and rare earth element contents were analyzed using a coupled plasma mass spectrometer (PE300D). All results were normalized against the Chinese rock reference standard DZ/T0223-2001. The analytical uncertainties are less than ±3%.

4 RESULTS 4.1 Ziron and Cassiterite U-Pb Ages

The zircons in the aplite from the No. 308 pegmatite vein are euhedral and range in size up to 200 mm, are mostly transparent, and exhibit magmatic oscillatory zoning (Fig. 5a). All Th/U ratios in spite of JJKYG157-2-20 in the zircons are greater than 0.1 (Table 1), indicating that they are mainly magmatic zircons. Twenty-five grains were analyzed from the aplite (sample JJKYG157-2) from the No. 308 pegmatite vein, and of these analyses can be pooled to yield a concordia age of 217±1.1 Ma (Fig. 5b) and a weighted mean 206Pb/238U age of 217±0.84 Ma (Fig. 5c).

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Figure 5. Cathodoluminescence images and ages of zircons in the aplite from the No. 308 pegmatite in the Jiajika deposit.
Table 1 Results of zircon U-Pb dating of aplite from the No. 308 pegmatite vein in the Jiajika deposit

In the Jiajika deposit, the main Sn-bearing independent minerals in pegmatites are cassiterites. Microscope observations of two different pegmatite veins reveal that there are two types of cassiterite in the Li-bearing pegmatites. Most of the cassiterites are idiomorphic or hypautomorphic granular crystals with grain sizes ranging from 100 to 300 μm. Cassiterites in the No. 308 vein (Cas-1) contain abundant Nb-Ta minerals (Figs. 6a-6i); in contrast, cassiterites in the No. 133 vein (Cas-1) contain few Nb-Ta minerals, and their surfaces are relatively smooth (Fig. 6b), indicating that the formation times of these two types of cassiterites may be different. To explain these chronologic data more objectively, particles that are relatively smooth and contain fewer Nb-Ta minerals on their surfaces (Figs. 6a-ii, 6b) were selected for in-situ elemental and isotopic analyses.

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Figure 6. Cassiterite in spodumene-bearing pegmatites from the No. 308 pegmatite vein and No. 133 pegmatite vein in the Jiajika deposit. (a-i)-(a-ii) Reflected images of cassiterite from the No. 308 pegmatite; (b) reflected images of cassiterite from the No. 133 pegmatite; (c) the U-Pb corresponding concordia diagrams of cassiterite in spodumene-bearing pegmatite from the No. 308 pegmatite vein in the Jiajika deposit; (d) the U-Pb corresponding concordia diagrams of cassiterite in spodumene-bearing pegmatite from the No. 133 pegmatite vein in the Jiajika deposit. (e) (f) In-situ La-ICP-MS trace elements analysis of cassiterite in the No. 308 (Cas-1) and No. 133 (Cas-2) pegmatite veins in the Jiajika deposit. Abbreviations: Cas. cassiterite.

Two types of cassiterite were analyzed by LA-MC-ICPMS, and their U-Pb data are listed in Table 2. The 238U/206Pb ratios of Cas-1 and Cas-2 vary from 22.45 to 30.94 and 4.81 to 29.66, respectively; their 238U/207Pb ratios vary from 50.47 to 628.12 and 7.11 to 345.37, respectively; and their 206Pb/207Pb ratios vary from 2.69 to 21.90 and 1.52 to 12.29, respectively. The characteristics of their 206Pb/207Pb ratios indicate that the common lead contents in these two samples are relatively low; thus, more accurate ages can be obtained using the T-W diagram (Cui et al., 2017). The LA-MC-ICPMS dating of cassiterite yields concordant ages of 211±4.6 Ma for Cas-1 (Fig. 6c) and 198±4.4 Ma for Cas-2 (Fig. 6d).

Table 2 LA-MC-ICPMS U-Pb isotopic data for cassiterite in the spodumene-bearing pegmatites from the Jiajika deposit
4.2 Major and Trace Elements

Major and trace element compositions of the 77 samples from the Jiajika deposit are given in Table S1.

The results indicate that samples of the Majingzi granite pluton (MaG) contain high concentrations of SiO2 (71.13 wt.%-74.66 wt.%), Al2O3 concentrations of 14.61 wt.%-15.46 wt.% and Na2O+K2O concentrations of 7.26 wt.%-9.26 wt.% (4.07 wt.%-5.46 wt.% K2O > 3.1 wt.%-3.8 wt.% Na2O) and low concentrations of MgO (0.16 wt.%-0.32 wt.%). The components of barren aplites are very similar to the Majingzi granite pluton, containing 73.38 wt.%-74.59 wt.% SiO2, 14.37 wt.%- 14.92 wt.% Al2O3, 7.82 wt.%-8.33 wt.% Na2O+K2O (4.62 wt.%-5.01 wt.% K2O > 3.20 wt.%-3.35 wt.% Na2O) and 0.18 wt.%-0.25 wt.% MgO. Compared with samples of the Majingzi granite pluton and barren aplites, the mineralized aplites are rich in Al2O3 (13.73 wt.%-18.35 wt.%) and Na2O (3.89 wt.%-8.07 wt.%) and depleted in K2O (0.85 wt.%-3.4 wt.%, mostly < 2 wt.%) and MgO (0.01 wt.%-0.27 wt.%, mostly < 0.05 wt.%). For samples of pegmatites, the content of major elements varied greatly depending on pegmatite types in different structural zones (Fig. 2) and characterized by SiO2 concentrations of (58.05 wt.%-82.45 wt.%, mostly > 70 wt.%), Al2O3 (12.35 wt.%-27.37 wt.%, mostly > 15 wt.%), Na2O (0.16 wt.%-8.07 wt.%, mostly > 3.5 wt.%) and K2O (0.24 wt.%-5.59 wt.%, mostly < 4 wt.%).

Rocks of the Majingzi granite pluton contain high Li (177 ppm-443 ppm), Be (5.43 ppm-66.4 ppm), Nb (6.76 ppm-26.2 ppm), Ta (1.23 ppm-13.3 ppm), Sn (15.6 ppm-53 ppm), Rb (291 ppm-442 ppm) and Cs (32.4 ppm-168 ppm), and low Ba (37.2 ppm-64.4 ppm), Sr (21.2 ppm-35.9 ppm), Sc (1.07 ppm-2.8 ppm), Hf (1.32 ppm-2.23 ppm), Zr (40.5 ppm-45.2 ppm) and Th (2.82 ppm-6.51 ppm). The contents of trace elements in the barren aplites are pretty much the same with the characteristics of high Li (292.48 ppm-561.63 ppm), Be (7.32 ppm-30.29 ppm), Nb (13.62 ppm-33.43 ppm), Ta (2.42 ppm- 14.71 ppm), Sn (24.72 ppm-60.69 ppm), Rb (293.43 ppm- 504.42 ppm) and Cs (30.23 ppm-156.37 ppm), and low Ba (28.73 ppm-54.37 ppm), Sr (16.84 ppm-53.8 ppm), Sc (1.45 ppm-2.24 ppm), Hf (1.18 ppm-1.7 ppm), Zr (28.69 ppm-35.19 ppm) and Th (2.51 ppm-3.95 ppm). Compared with samples of the Majingzi granite pluton and barren aplites, the mineralized aplites are more enriched in Li (18.85 ppm-11 730.4 ppm), Sn (6.39 ppm-116.58 ppm, mostly > 30 ppm), Zr (7.66 ppm-52.64 ppm), Hf (0.64 ppm-9.01 ppm), especially Be (131.36 ppm-338.39 ppm), Nb (53.34 ppm-261.39 ppm), Ta (13.74 ppm-145.09 ppm) and Rb (32.31 ppm-1 330.19 ppm), and more depleted in Ba (0.91 ppm-8.44 ppm), Sr (0.51 ppm-22.39 ppm), Th (0.12 ppm-2.2 ppm, mostly < 1 ppm) and Sc (0.14 ppm-2.09 ppm, mostly < 1 ppm). The composition characteristics of various types of pegmatites are similar to those of mineralized aplites. At the same time, the content of rare metals changed regularly in the different types of pegmatite with the distance increasing by the Majingzi granite pluton.

5 DISCUSSION 5.1 Geochronology

The intrusion age of the Majingzi two-mica granite pluton is 223±1 Ma (Hao et al., 2015), while the diagenetic age of the aplite from the No. 308 pegmatite vein in this study is 217±0.84 Ma. Li-mineralized age of the No. 308 and No. 133 pegmatite veins is 211±4.6 and 198±4.4 Ma, respectively, reflecting that the rare metal mineralization mainly formed in the late stage of the Indosinian. The same conclusion is reflected by the results of the geochronologic analyses of the other granites and pegmatite in the Jiajika deposit. The age of barren pegmatite vein is 203.5±3.7 Ma (ziron U/Pb, unpublished data). The ages of the ore-bearing granitic pegmatite veins are 195.7±0.1 Ma (plateau age) and 195.4±2.2 Ma (isochron age) for the No. 134 pegmatite vein; 198.9±0.4 Ma (plateau age) and 199.4±2.3 Ma (isochron age) for the No. 104 pegmatite vein (Muscovite Ar-Ar, Wang et al., 2005); and 216±2 Ma (zircon U/Pb) and 214±2 Ma (Nb-Ta oxide U/Pb) for the No. X03 pegmatite vein (Hao et al., 2015). These various results show that the rare metal mineralized pegmatite veins formed later than the two-mica granite (MaG) in the Jiajika deposit. At present, predecessors generally believe that this period (223 to 195.7 Ma, choosing more precise age data representing each geological body) coincides with the turning point after the peak period of the Indosinian Movement (230 Ma) (Liu et al., 2015; Sigoyer et al., 2014; Roger et al., 2010; Wang et al., 2005). Rare metal elements such as Li and Be are active elements and are difficult to aggregate in a strong orogenic environment. Instead, this large-scale rare metal accumulation occurred during a relatively stable stage of post-orogenic evolution (Wang et al., 2005, 2002). Therefore, these ages also indicate that the mineralized pegmatite veins in the Jiajika deposit formed during a relatively stable stage after the intense Indosinian Movement. The evolution process of granitic magma in the Jiajika deposit had undergone nearly 28 Ma from granite to various types of pegmatite veins. Such a long stable tectonic environment created a favorable condition for the accumulation and mineralization of rare metals.

In the Jiajika deposit, the No. 308 pegmatite vein has the maximum exposed area; the aplite with strong Be mineralization in its marginal zone is only partially exposed, further indicating that the Be-bearing aplite likely extends to greater depths. Various types of pegmatite have been completely exposed to the surface, and many spodumene pegmatite boulders occur in the eastern and northeastern regions of the No. 308 pegmatite vein; additionally, the scale of the albite-spodumene belt is much smaller than that of the No. 134 pegmatite vein in the eastern region of the MaG (Fig. 4a), indicating that the Li-bearing ore bodies in the No. 308 pegmatite experienced a strong denudation (Dai et al., 2018). In contrast, the No. X03 and No. 134 pegmatite veins are relatively well-preserved. The No. X03 pegmatite vein is a concealed ore body, which formed earlier (~215 Ma, Hao et al., 2015) than the No. 308 pegmatite vein. The No. 134 pegmatite vein formed later (~195 Ma, Wang et al., 2005) than the No. X03 and No. 308 veins, and it was relatively weakly affected by weathering and denudation.

5.2 Petrogenesis 5.2.1 Pegmatite classification

Based on the geochemical characteristics of the secondary minerals and their internal structures, as well as the temperature and pressure conditions of crystallization, the initial classification of the pegmatites can be divided into three categories, namely, lithium-cesium-tantalum (LCT)-type, niobium-yttrium- fluorine (NYF)-type and mixed-type pegmatites (Černý, 1991a, 1982). The LCT pegmatite is the crystallization product of magma enriched in Li, Cs and Ta; schist, gneiss and early granite are the most important wall rocks of LCT-type rare metal mineralized pegmatites (Černý, 1991a). LCT-type pegmatite has a high economic potential (compared to the NYF-type pegmatite), particularly the albite-spodumene type (Černý and Ercit, 2005), which is present in a variety of shapes (i.e., as lenticular, mushroom-shaped, and dickitic bodies, which also can be found in the Jiajika deposit). In recent years, several classification schemes for this pegmatite have been proposed (Dill, 2015; Tkachev, 2011; Ercit, 2004; Pezzotta, 2001). The main problem is that all rare pegmatites are attributable to these two types of pegmatites (LCT, NYF), but many examples in the world do not fit this classification scheme (Novák et al., 2012). Dill (2015) proposed a new rare element pegmatite classification, called "CMS", as a descriptive classification scheme for pegmatites and fine-grained rocks. In this rare element pegmatite system, the chemical composition, mineral assemblage and structural geology of pegmatitic rocks represent the three most important ore-controlling factors. Although this taxonomy is advantageous due to its descriptive nature and avoidance of genetic interpretations, most pegmatite studies still use the LCT and NYF classification of Černý (1991a).

In sum, based on their geological and compositional features, the Majingzi granite in the Jiajika deposit belongs to the typical Li-F rich granite, and the pegmatites mainly belong to the Li-rich rare metal subclass (REL-Li) of the LCT family related to the orogenic belt (Černý, 1991a), despite the fact that Nb > Ta in all types of granites and pegmatites. Based on the rock types and geochemical test results, the samples in this study area can be further divided into two categories and eleven subcategories. These categories include barren rocks, mainly including the Majingzi two-mica granites (MaG), barren granitic aplites (B-A), and barren pegmatites (B-P); the other category is mineralized rocks, which are further divided into Be (Nb, Ta)-bearing granitic aplites (Be-A), Be(Nb, Ta)-bearing pegmatites (Be-P), Li(Be, Ta, Nb)-bearing pegmatites (Li-P), and Nb, Ta-bearing pegmatites (Nb, Ta-P).

5.2.2 Source of magma

Except for a small number of pegmatites, most samples fall into the granite field (Fig. 7a). Comparing the granites, aplites and pegmatites reveals that these rocks become gradually enriched in Al (Fig. 7b) and exhibit a trend from calc- alkaline to calcic compositions (Fig. 7c). All types of rocks are classified as peraluminous and subalkaline, and they show a clear linear relationship between A/CNK and A/NK (Fig. 7b), similar to the pegmatites in Altai, Xinjiang (Leng et al. 2007), Ke'eryin, Sichuan (Fei et al., 2014), Chuanziyuan and the southern margin of the Mufushan area, Hunan (Wen et al., 2016). In the meantime, contents of Rb are negatively correlated with Th and Y (Fig. 7d), reflecting that both MaG and B-A are characterized as S-type granites.

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Figure 7. Selected geochemical diagrams of the granites, aplites and pegmatites. (a) TAS diagram (after Middlemost, 1994); (b) A/CNK-A/NK diagram (after Peccerillo and Taylor, 1976); (c) log σ-log τ diagram (after Yang, 2007), where =(Na2O+K2O)2/(SiO2-43) (after Rittmann, 1957) and =(Al2O3-Na2O)/TiO2 (Grasso, 1968), units in wt.%. Field A represents lavas of volcanoes situated in an orogenic regions, field B volcanoes in orogenic belts and island arcs, and field C enter the alkaline derivatives of both (i.e., trachyte, phonolite, tephrite), among which the sodic types are generally linked to A, and the potassic ones to B; (d) Rb-Y and (e) Rb-Th diagrams (after Chappell, 1999).

The S-type granitoid is generally considered as the peraluminous granitoid, which is mainly formed by the partial melting of pure crustal materials (Castro et al., 1999; Deng et al., 1994). Meanwhile, Nizamoff et al. (1999) proposed that a peraluminous LCT family marked by the prominent accumulation of Li, Cs and Ta (in addition to Rb, Be, Sn, B, P and F) was mainly derived from S-type granites. The above features indicate that all granitic rocks in the Jiajika deposit are strongly peraluminous subalkaline S-type granitoids produced by the anatexis of the crust.

The Hake diagrams of selected elements vs. Li (Fig. 8) and the Al2O3-Na2O×10-K2O×10 (ANK) triangular diagram (Fig. 9) indicate that the Li(Be)-bearing granitic pegmatites are enriched in total alkaloids and depleted in CaO, FeO, MnO and MgO. The changes in the contents of these mineral elements suggest that the mineralization and enrichment of rare elements such as Li, Be, Nb and Ta in aplite and pegmatite are closely related to the metasomatism of alkali metals. The early Be(Nb) mineralization may be related to the separation of sodium-rich fluid from the granitic magma due to the crystallization of a large amount of microcline. Compared to the Be-mineralized aplites and pegmatites, the Li-P are characterized by poor Na contents. The negative correlation between Na and Li reflects that the albite is not acting as an ore bearing mineral but playing an important role in this process of reducing the viscosity of the melts. The self-purification in ablites (Hu, 1980) of alkali metasomatism is beneficial to the complexation and stable migration of the metallogenic elements (Du, 1986; Hu, 1980). The reduction of the sodium concentration caused by the mass crystallization of ablates in the fluid leads to the decrease of the stability of the complex. At the same time, the rare metals especially for Li are often closely associated with albites because of the relatively closed metallogenic space. The transformation of magmatic hydrothermal fluid into a sodium-poor fluid indicates that Li mineralization is closely related to sodium fluid during the evolution of a highly differentiated magma.

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Figure 8. Harker diagrams of the granites, aplites and pegamtites in the Jiajika deposit. Symbols are as in Fig. 7.
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Figure 9. Al2O3-Na2O×10-K2O×10 triangular diagram of the granites, aplites and pegamtites in the Jiajika deposit. Symbols are as in Fig. 7.

Trace element contents are very useful for assessing the genetic type, degree of fractionation and potential for Li, Be, Nb, Ta, Cs, W and Sn mineralization in individual granitic rocks and granitic pegmatites (Saleh, 2007; Srivastava and Sinha, 1997; London, 1990). Based on the mean values, the various rock types in the Jiajika deposit, especially MaG and B-A, exhibit similar REE patterns after normalization to the chondrite (Sun and McDonough, 1989), with right deviation Ⅰ-shaped light REE enrichment patterns, negative Eu anomalies and very low ∑REE contents (Fig. 10a). The normalized REE patterns of B-A are generally similar to those of MaG (Fig. 10a), suggesting that they are derived from the same source and that they are most likely derived from the upper crust (Fig. 10a). Meanwhile, all B-P and mineralized rocks (aplites and pegmatites) show another set of similar REE patterns, and the ∑REEs (especially HREE) has a decreasing trend with the mineralization sequence of barren→Be→Li→Ta, reflecting that all minerzlized aplites and pegmatites have obviously experienced a high degree of crystallization differentiation of the parent magma of the Majingzi granite pluton in the Jiajika deposit. The same features are reflected in the primitive manlte- normalized trace element spider diagrams (Fig. 10b). The pattern of MaG is similar to those of the aplites and pegmatites, and all rocks are significantly enriched in Li, Ta, Sn and Cs and depleted in Ba, Nb and Sr, indicating that the mineralization process of rare metals in the Jiajika deposit was closely related to the evolution process of the magmatic melt and fluid.

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Figure 10. The chondrite-normalized REE distribution patterns and primitive manlte-normalized race element spider diagrams of BR and MR in the Jiajika deposit. The data of the chondrite, primitive mantle and four crustal units computed from Sun and McDonough (1989).
5.2.3 Evolution process

The Nb/Ta ratio can be used to effectively identify the characteristics of a magmatic source area (Nguyen et al., 2019; Ballouard et al., 2016; Eby et al., 1998). The Nb/Ta ratios in the MaG are 1.97-5.22 (Tang and Wu, 1984). The Nb/Ta ratios of various types of granitic rocks, aplites and pegmatites range from 0.59 to 8.69, which further indicates the homology of these rocks. Petford and Atherton (1996) found that the Nb/Ta ratios of mantle-derived magmatic rocks can reach up to 15.5, while crustal magmatic rocks have relatively lower ratios (approximately 11-12). The lower Nb/Ta ratios of the granitoids and pegmatites indicate that their source material is more likely to have been derived from the crust. Petrographic observations show that the MaG and B-A rocks produce primary Al-rich minerals such as muscovite, which are consistent with their higher A/CNK ratios and lower Nb/Ta ratios; these factors jointly reflect the S-type granite properties of the MaG and B-A. Meanwhile, the complexation ability of F with Ta is stronger than that with Nb, which caused the fractionation of Ta and Nb and resulted in the decrease of Nb/Ta ratios (Badanina et al., 2010; Thomas et al., 2005). The same evolution process is also reflected by the microscopic observations and in-situ elemental analyses of cassiterite (Table 3) in spodumene-bearing pegmatites from the Jiajika deposit. The mineralization ages of these two types of cassiterite were verified by in-situ LA-MC-ICPMS U-Pb dating (Figs. 6c, 6d); compared with Cas-1 of the No. 308 pegmatite vein, which formed earlier, the contents of Fe and Ta and associated Nb-Ta minerals in Cas-2 of the No. 133 pegmatite vein are significantly lower (Table S1; Figs. 6e, 6f), reflecting that the ore-forming fluid may have been derived from a highly differentiated granitic magma. In addition, tourmaline is commonly found in various types of pegmatite (Figs. 3e, 3f, 3h, 3i), and fluorite usually occurs at the late stage of mineralization (Fig. 3i), further indicating that the pegmatites may have been formed by magmatic differentiation due to the F-rich magmatic fluid (Bezmen and Gorbachey, 2014). Previous studies and this paper have shown that the magmatism of granites and pegmatites occurs in a Li-F-rich magma system (Tang and Wu, 1984), showing that volatiles such as F play a key role in the migration and mineralization of rare metals.

Table 3 In-situ LA-ICP-MS trace elements analysis results of cassiterite in the No. 308 (Cas-1) and No. 133 (Cas-2) pegmatite veins in the Jiajika deposit

Černý et al. (2005) proposed that crystallization differentiation is an important mechanism leading to the separation and enrichment of rare metals from rare metal-rich magma. However, this mechanism is undeveloped in the low-differentiation facies of the granite system (Černý et al., 1998; Černý, 1982). The granites enriched in rare metals (e.g., Li, Rb, Cs, Be, Sn, Zr, Th, U, Nb, Ta, W, Au) and REEs are highly differentiated rocks that typically exhibit disseminated mineralization (Černý et al., 2005; Černý, 1991a; Clark and Černý, 1987; London, 1987; Norton, 1983; Jahns, 1982). All samples fall in the field of strongly differentiated granites on a Rb-Ba-Sr ternary diagram (Fig. 11a). The same conclusion is reflected by their compositional characteristics; compared to the barren rocks, the mineralized rocks have lower K/Rb ratios (Fig. 11b), which suggests that they represent the strongly differentiated products of the parent magma (Srivastava and Sinha, 1997). Tischendorf (1977) proposed that the mineralized granites, which are enriched in Li, Be, W, Sn, Nb and Ta and depleted in Ba, Sr and Zr, are characterized by K/Rb ratios that are greater than 100 and Rb contents that are less than 500 ppm. The K/Rb ratios in the MaG, B-A and mineralized rocks are far less than 250 (Fig. 11b), suggesting that the granitic magmas derived from the melted crust, and especially those of the MaG and B-A, originated from the same source region.

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Figure 11. (a) Rb-Ba-Sr (ppm) ternary diagram (after Bouseily and Sokkary, 1975) and (b) SiO2 (wt.%) vs. K/Rb diagram (after Blevin, 2004) for granites, aplites and pegmatites in the Jiajika deposit. Symbols are as in Fig. 7.

Additionally, the Zr/Hf, La/Ta and Nb/Ta ratios of all samples in the Jiajika deposit obviously deviate from their corresponding chondritic values (Bau, 1996), reflecting that strong interactions occurred between the melt phase and fluid phase of a highly fractionated S-type granite. Meanwhile, interactions with volatile-rich (F, Cl) fluid are the major factors leading to the tetrad effects of REE, which can be used as important indicators to distinguish mineralized granites from barren ones. Although the ranges of discrimination are different from those used to distinguish tungsten granites (Srivastava and Sinha, 1997) and rare element-class pegmatites (Saleh, 2007), the barren rocks and mineralized rocks in the Jiajika can be well distinguished by using the ratios of K/Rb (Fig. 11b), which further indicates that the MaG is the main metallogenic mother rock and the later aplite and pegmatite types resulted from the continuous differentiation of the same magma.

The degree of relative differentiation gradually increases from granites and aplites to pegmatites (Fig. 11a). Various types of barren rocks and mineralized rocks in the Jiajika deposit have lower oxygen fugacity values, exhibiting the characteristics of ilmenite-series granite (also known as reduced-type granite, ) (Lehmann, 1990), thus showing that the barren rocks and mineralized rocks in the Jiajika deposit formed in a reducing environment. The granitic magma ascended and intruded at medium-shallow depths. During the evolution of the magma, strong alkali metasomatism occurred between the melt phase and the volatile-rich fluid phase, causing large-scale rare metal mineralization to occur in certain structural zones of the pegmatite veins in the Jiajika deposit.

In this study, we correlate the rare element deposits with the evolution processes of aplites and pegmatites. Beryllium deposition occurs in a wide range of zones and is concentrated in the melt stage. Niobium and tantalum deposition also occurs in a wide range, but tantalum is concentrated in the melt-fluid stage and fluid stage. Lithium concentrated in the melt-fluid stage after the system adjusted. The coupling relationship of evolution stages showing different degrees of fractionation and rare element deposition is related to the geochemical properties of elements, the physical and chemical conditions of the system and the roles of fluxes. Therefore, we suggest that the different types of aplites and pegmatites formed from the same parent magma after continuous and pulsatile intrusion, and the crystallization differentiation was more important during the evolution process.

5.3 Geodynamic Setting of Rare Metal Mineralization

The granitic rocks exhibiting the tetrad effects of REE are not suitable for identifying the tectonic setting by using trace elements (Jahn et al., 2004). In this paper, trace elements are only used to discuss the tectonic setting of the MaG and B-A, both of which are located in the syn-COLG field (Fig. 12), indicating that the formations of the MaG and B-A are related to the orogenic process. In addition, all sample points, except for one sample of NbTa-P, plot in field B, which represents lavas erupted from volcanoes in orogenic belts and island arcs (Fig. 6c).

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Figure 12. Discriminant diagram of tectonic setting for the granitites and barren aplites (after Pearce et al., 1984). VAG. Volcanic arc granites; ORG. ocean ridge granites; WPG. within plate granites; Syn-COLG. syn-collision granites. Symbols are as in Fig. 7.

The Indosinian Movement has a great influence on the development of palaeogeographic environment in China. It changed environments of Mesozoic sea in south and continent in north before the middle of the Triassic Period, leading to folding and orogeny in western Sichuan, Gansu and southern Qinghai area. The Jiajika deposit formed under the above regional tectonic setting.

The above analysis indicates that the Bayan Har-Songpan- Garze area experienced an intense compressional orogeny and the strong fold occurred during the period of Indosinian Movement. In the process of collisional orogeny-intra-continent orogeny in the Songpan-Garze area, the Yajiang dome metamorphic group, taking the Jiajika, Rongxuka, Changzheng, Waduo, and Murong as the center, developed at the core of SGFB caused by the N-S- and E-W-trending coaxial compression, and then the Triassic clastic rock strata remelted to the granitic magma, emplacement mechanism of which was diapiric uprising. In the process of magmatic crystallization differentiation, the aplite and pegmatite veins formed along the fold collapse and tensile fractures around the dome by filling or by metasomatism. Different types of pegmatite and rare metal mineralization have been formed around the Indosinian granite pluton along with the increasing degree of crystallization differentiation.

5.4 Ore-Forming Potential Indicators

Numerous geological and mineralogical studies of granitic rocks and pegmatites associated with various types of mineralization have been undertaken in recent years to establish useful methods for detailed prospecting (Saleh, 2007; Koester et al. 2002; Kalsbeek et al. 2001; Sylvester, 1998; Srivastava and Sinha, 1997; Peccerillo and Taylor, 1976).

Granite pegmatite usually occurs in groups, and within a few kilometers scale, the types of pegmatite vary with distance around a granite body (Černý, 1991a). The small granite bodies in China and abroad (Srivastava and Sinha, 1997; Liu and Ma, 1993; Imeokparia, 1983; Ayres et al. 1982) that intrude metasediments represent the most productive granites in multi-stage intrusive terrains, whereas large batholiths are barren. It is also observed that the granitic intrusions (MaG) at Jiajika have an outcrop area of less than 6 km2 (Hao et al., 2015) and that their predicted depth determined by audio-frequency magnetotellurics (AMT) may be less than 5 kilometers, suggesting that the MaG are characterized by small and shallow emplacement (Wang et al., 2017a; Stemprok, 1979; Tischendorf, 1977) and that they may be the parent magma providing the ore-forming materials for pegmatite mineralization (Stemprok, 1979; Tischendorf, 1977).

Generally, because of the mineral zonation characteristics of the evolution of granitic pegmatite and the huge sizes of its single mineral crystals, it is difficult to analyze whole-rock pegmatite samples to determine the whole-rock elemental composition of pegmatite. According to field observations in the Jiajika deposit, most mineral crystals of various types of pegmatites are smaller than 5 cm; thus, samples that were collected by manual acquisition are basically representative of their compositional characteristics. It should be noted that a considerable amount of Be(Nb) mineralized rocks are aplite- type rocks, so it is necessary to use geochemical analysis to determine their degree of mineralization. As long as we pay attention to the sampling methods, we can basically guarantee the homogeneity of samples with statistical significance, which is supported by the analysis results in our study. At the same time, the rare metal minerals in the B-A, Be-A and a considerable number of pegmatites are relatively small, which are not characteristics of the typic pegmatites. Dill (2015) proposed that aplitic rocks are compositionally similar to pegmatites but are strikingly different due to their fine-grained textures. Therefore, analyzing the major and trace element compositions of granites, aplites and pegmatites is still an effective way to study the evolutions of magmas and their potential for mineralization.

In China, numerous elemental geochemical studies of rare metal pegmatite-type deposits have shown that rare metal pegmatites are enriched in SiO2, Al2O3, Na2O and depleted in K2O, ∑REEs and HREE (Wen et al. 2016; Zhou et al. 2012; Zhang and Chen, 2010; Leng et al., 2007). However, these geochemical characteristics are not sufficient to distinguish the orebearing potential and indicate the evolution law of rare metal. This study shows that the above three diagrams can be used to distinguish rare metal mineralized rocks from barren rocks (Fig. 9, Fig. 11b). Srivastava and Sinha (1997) proposed the effective quantitative parameter of the Geochemical Characterization Index (GCI)=lg (Rb3×Li×104)/(Mg×K×Ba×Sr) (all values are in ppm) to distinguish W-bearing granites from barren granites, and they indicated that positive GCI values for any granite will suggest its W potentiality in an area. For pegmatite type rare metal deposits, especially for which contains a lot of aplite type ores, our study has shown that regular changes in elements such as Rb, Mg, Ba, and Sr occur in all types of rocks. Taking into account that Li is the main type of mineralization in the Jiajika deposit, the Li in the above parameter must be eliminated. Otherwise, it will be difficult to distinguish other types of mineralization like Be, Nb and Ta, etc. At the same time, the trend of different mineralization types cannot be directly observed. Consequently, we use a modified version of the GCI=lg (Rb3×104)/ (Mg×K×Ba×Sr) (all values are in ppm). The results are listed in Table 3 and Fig. 13. The GCI values of the MaG and BGA in Jiajika range from 0.299 to 1.334 (n=10), which are similar to the granite plutons (-1.52- -0.53) in the Ke'eryin mining area, and significantly different from those of the other types of two-mica granite in China, which range from -4.236 to -3.201 (n=11, different tectonic settings and diagenetic ages, the data are cited from Shi, 2008), and they are also similar to the characteristics of W-bearing granites (excluding Li, Srivastava and Sinha, 1997), further indicating the metallogenic potential of the MaG in the Jiajika deposit. The GCI has an increasing trend with the mineralization sequence of Barren→Be→Li→Ta in the Jiajika deposit, which is the same as in the Ke'eryin mining area (Fig. 13), reflecting that all rare metals-bearing aplites and pegmatites have obviously experienced a high degree of crystallization differentiation of the parent magma. Although more data are needed to validate whether this parameter is applicable to other spodumene-bearing pegmatite type deposits, aplite type deposits in particular, the preliminary results indicate that this parameter can act as the qutitative evaluation criteria of ore potentiality and used as a useful exploration tool of lithium- polymetallic pegmatite type deposits.

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Figure 13. Geochemical characterization index (GCI) for different lithium granitic rocks and barren granitic rocks (data of the other two-mica granites in China computed from Shi (2008); data of the other peraluminous granite in western Sichuan computed from Wang et al. (2008); data of the Garze muscovite natriuretic granite and Ke'eryin two-mica granite computed from Li (2006); data of pegmatite in the Ke'eryin mining area are unpublished data of our project).
6 CONCLUSIONS

(1) LA-ICP-MS zircon U-Pb dating yields a concordia age of 217±1.1 Ma and a weighted mean 206Pb/238U age of 217± 0.84 Ma for the aplite from the No. 308 pegmatite. LA-MC- ICPMS cassiterite U-Pb dating yields ages of 211±4.6 Ma for the No. 308 pegmatite vein and 198±4.4 Ma for the No. 133 pegmatite vein in the Jiajika deposit. The formation of pegmatite veins and rare metal mineralization occurred during the late Indosinian period. Similar to other rare metal mineralized pegmatite veins, these veins represent the products of the relatively stable stage after the intense orogeny of the Indosinian cycle.

(2) The MaG and B-A are characterized as peraluminous S-type granites that exhibit homology. The differentiation of the magmatic fluid gradually increased from granitic aplites to pegmatites. The granitic magma ascended and intruded at medium-shallow depths and in a partial reducing environment. During the evolution of magma, strong alkali metasomatism occurred between the melt phase and the volatile-rich fluid phase; as a result, large-scale rare metal mineralization occurred in certain structural zones of the pegmatite veins in the Jiajika deposit.

ACKNOWLEDGMENTS

This research was jointly supported by the National Key Research and Develpoment Program of China (No. 2017YFC0602701), the China Postdoctoral Science Foundation (No. 2017M610960), the China Geological Survey's projects (Nos. DD20190173 and DD20190379). The manuscript has been significantly improved by critical reviews of Prof. Stefano Albanese and two anonymous reviewers. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1011-9.

Electronic Supplementary Material: A supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-019-1011-9.


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