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Article

Whole-Rock Geochemistry and Mica Compositions in Lijiagou Pegmatite Spodumene Deposit, Western Sichuan, China

by
Xiaojie Chen
1,
Cuihua Chen
1,*,
Xiang Lai
1,
Yulong Yang
1,
Ying Gu
1 and
Yunhua Cai
2
1
College of Earth Science, Chengdu University of Technology, Chengdu 610059, China
2
Geochemistry Exploration Team, The Sichuan Bureau of Geology and Mineral Resources, Deyang 618000, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(1), 69; https://doi.org/10.3390/min14010069
Submission received: 17 October 2023 / Revised: 19 December 2023 / Accepted: 22 December 2023 / Published: 5 January 2024
(This article belongs to the Section Mineral Deposits)
Browse Figures
Versions Notes

Abstract

:
The Lijiagou pegmatite spodumene deposit, located in the middle of the Songpan–Garze Fold Belt and southeast of the Ke’eryin ore field, is a newly discovered super-large deposit. In order to reveal the metallogenic tectonic environment and evolution process of pegmatite, based on the study of the geological characteristics of pegmatite, we carried out a whole-rock geochemical analysis of Ke’eryin two-mica granite and Lijiagou pegmatite and carried out a detailed electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of mica minerals in each zonal pegmatite. The results show that the Ke’eryin two-mica granite is mainly formed in the transition period from syn-collision to post-collision. After the end of the continental collision, the crust is squeezed and thickened in the post-collision extensional transition tectonic environment. Mica from the microcline pegmatite zone (MP) to the albite spodumene pegmatite zone (ASP) in pegmatite show different compositions and structural characteristics, with the evolution trend in the direction from muscovite to Li-bearing mica. The type of mica from MP to AP is mainly muscovite, and Li-bearing mica appears in ASP, which is secondary and metasomatic at the edge of primary muscovite. From MP to ASP, there was a negative correlation between Nb/Ta, K/Rb and the Li, Rb, and Cs contents of mica, while the contents of Li, Rb, Cs, and F in the Li-bearing mica of ASP increased sharply. This evidence illustrates that the favorable tectonic environment contributed to the formation of the Lijiagou pegmatitic spodumene deposit. Lijiagou pegmatite experienced the magmatic–hydrothermal evolution process and has a high degree of differentiation and evolution from MP to ASP, which gradually increased. Combined with the change in mica type, it is considered that ASP formed from the stage of magmatic transition to hydrothermal and was a hydrothermal environment, and Li, Rb, and Cs mainly began to enrich at the stage of magmatic–hydrothermal transition.

1. Introduction

Lithium (Li) is one of the most important rare-metal elements and the lightest metal element in nature. Because of its excellent physical and chemical properties, it is used in new energy, new materials, information technology, and aerospace fields. It is an indispensable strategic metal for the development of emerging industries and has important strategic significance for China and the rest of the world [1,2,3,4]. Granitic pegmatites are generally considered to be formed from highly differentiated, volatile-rich, residual granitic magma, which is of great economic significance because it contains various rare-metal elements such as Li, Be, Nb, Ta, Rb, Cs, W, and Sn [5,6,7,8,9,10]. Due to the high cost of mining salt lake Li resources, granite pegmatite Li deposits have become the main source of Li in China [11,12,13]. The evolution of granite pegmatite is a very complex process, and its degree of evolution determines the mineralization of rare-metal elements to a large extent. The magmatic–hydrothermal transition stage in the evolution process is the beginning of the high enrichment of rare metals [14]. Granite pegmatite rare-metal deposits are the product of the magmatic–hydrothermal stage, ore-forming fluids mainly originate from highly differentiated granitic magmas, and the differentiation evolution process plays an important role in controlling diagenesis and mineralization [7]. Although pegmatite melt is mostly considered to be the residual melt produced by magma separation and the crystallization of mother granite [7,15,16,17], experimental studies have shown that fluid immiscibility is also a critical mechanism of rare-metal enrichment during magmatic evolution [18,19,20].
Mica is common in granitic pegmatite and is also an important rock-forming mineral in granitic pegmatite. They not only crystallize in the magmatic stage but are also partly involved in hydrothermal processes. As a phyllosilicate mineral, mica has a TOT layered structure, which can enable it to host many rare elements such as Li, Be, Rb, Cs, Ba, Sn, Nb, and Ta [21]. The type, texture, and chemical characteristics of mica can indicate the degree of evolution of pegmatite melt and the evolution process of pegmatite veins [22,23,24,25,26]. These characteristics make it a rare-metal carrier mineral and an indicator mineral for rare-metal mineralization. Therefore, a study of micas can provide constraints on the complex magmatic–hydrothermal processes related to rare-metal mineralization.
Lijiagou pegmatite spodumene deposit is located in central SGFB (proven resources of 0.51 Mt at 1.27% Li, 3696 t at 0.009% Nb, 1747 t at 0.004% Ta, 19,807 t at 0.049% Be, and 21,240 t at 0.052% Sn). The deposit is the third largest spodumene pegmatite deposit in Asia, after the Dangba (Li resources of 0.66 Mt) and Jiajika (Li resources of 0.92 Mt) deposits [27]. Previous studies of the Lijiagou pegmatite spodumene deposit have mainly focused on geological characteristics [28], the origin and characteristics of the fluid [19,27,29,30], the geochronology of pegmatite [31,32], the origin of pegmatite magmas, and their relationship with the Ke’eryin pluton granites [19]. In this paper, we selected representative samples of the Ke’eryin pluton and pegmatites from the Lijiagou deposit for a geochemical analysis of whole rock and mica and used these data to analyze the metallogenic tectonic environment characteristics and magmatic–hydrothermal evolution processes of pegmatites. These efforts aim to obtain a better understanding of the evolution of Lijiagou rare-metal pegmatite and facilitate the exploration of related granitic pegmatite deposits.

2. Geological Background

Lijiagou pegmatite spodumene deposit is located in Jinchuan County, western Sichuan Province (Figure 1a), situated in the southeastern part of the Ke’eryin pegmatite ore field, central Songpan–Garze Fold Belt. It is bounded by the Kunlun–Anyemaqen suture on the north, the Ganzi–Litang suture on the southwest, and the Longmenshan thrust on the southeast (Figure 1b). The area of the Lijiagou deposit covers approximately 4.35 km2. It is dominated by metasedimentary rocks of the Triassic Xikang Group, which mainly comprise gray, black meta-felsic sandstone, meta-siltstone, sericite slate, silty slate, phyllite, schist, and minor limestone [33]. The metasedimentary rocks show characteristics of high-temperature and middle–low-pressure metamorphism, forming zonal patterns with sillimanite–kyanite, garnet–staurolite, and biotite–andalusite zones from core to periphery [34]. Folds and faults are well-developed in this area, which was influenced by the late Indosinian orogeny; many NW- and WNW-trending folds developed while NE-striking reverse faults resulted from a horizontal compressive stress field in the Jurassic Yanshan orogeny [35,36]. The magmatic rocks in the region are mainly exposed to two-mica granite with a wide distribution range and a total outcrop area of 188 km2, which intruded into the Triassic Xikang Group strata from around 231 to 200 Ma [31,37] and caused different degrees of thermodynamic contact metamorphism. Some rare-metal pegmatite deposits are distributed around the Ke’eryin two-mica granite pluton, including Guanyinqiao, Lijiagou, Dangba, Yelonggou, Simancuo, Sizemuzu, and Redamen deposits (Figure 1c). The maximum width of these pegmatite dikes is 30 m, and the longest reaches up to 2200 m [38]. The pegmatite dikes have a zonal distribution in the horizontal directions from the Ke’eryin granite pluton outwards. Based on their mineral composition, the pegmatite dikes can be divided into five types (Figure 1c), with increasing distance from the Ke’eryin granite pluton, which are: (I) microcline pegmatite (MP), (II) microcline albite pegmatite (MAP), (III) albite pegmatite (AP), (IV) albite spodumene pegmatite (ASP) and (V) lepidolite pegmatite (LP) [39]. The vertical partition of pegmatite is imperfect, and some pegmatite types overlap. A single pegmatite body may contain two or three types of pegmatite.
Lijiagou deposit outcrops in the Upper Triassic Zhuwo Formation (T3zh). The joint fissures are well-developed, which are EW, WNW-ESE, and NW-SE trending joint fissures, and are mainly related to the ore-bearing granitic pegmatite. Aplite dikes have mainly been identified in and near the Lijiagou deposit from both drilling and field exploration. However, based on analyses of the zircon and monazite U-Pb ages, zircon Hf and monazite Nd isotopes, whole-rock and mineral chemical data, Fei et al. proposed that aplite does not have rare-metal mineralization potential [41]. Approximately 85 pegmatite veins are exposed in the Lijiagou deposit and 15 ore bodies have been identified (I–X, XI (XI-1, XI-2, XI-3, XI-4, XI-5), XII, XIII, XIV and XVI) (Figure 2). These are mainly veins, such as No. I and II, with tabular or lenticular shapes. The No. I pegmatite is the largest orebody. It is a vein-shaped body with a predominant NE orientation and is 10–124 m thick, 375 m deep, and 2060 m long [34].

3. Sample and Petrographic Characteristics

In the Ke’eryin ore field, pegmatites can be divided into the barren pegmatites (I-MP, II-MAP, III-AP) and the ore-bearing pegmatites (IV-ASP). In this study, samples of two-mica granite were collected from the southeastern branch of Ke’eryin pluton for whole-rock geochemical analysis. The barren pegmatite samples were collected from outcrops and ore-bearing pegmatite samples were taken from the drill core of No. I orebody in Lijiagou deposit for whole-rock and mica geochemical analysis. The two-mica granite is mainly gray, ranges from fine to medium-grained, and is composed of K-feldspar, plagioclase, quartz, biotite, and muscovite. Some characteristics of the pegmatite samples are described in Section 3.1.

3.1. Petrographic Characteristics of the Pegmatite

The MP, MAP, AP, and ASP all have outcrops in the field (Figure 3a,d,g,j), while LP has few outcrops and only appears locally. The ASP is the main type, with minor AP in the Lijiagou deposit. The MP intruded into two-mica granite and biotite moyite. The MAP, AP, and ASP mainly intruded into metasedimentary rocks. MP and MAP have a similar composition (Figure 3b,c,e,f), being mainly composed of microcline, albite, quartz, muscovite, biotite, tourmaline and a small amount of garnet, while MP has a higher content of microcline. The AP exhibit a weaker internal zonation than the MP and MAP. It is mainly composed of albite, muscovite, and quartz (Figure 3h,i). The main spodumene-bearing pegmatite is ASP. They consist of albite, spodumene, quartz, muscovite and small amounts of columbite (Figure 3k,l). The U-Pb ages of coltan and cassiterite might represent the crystallization time for pegmatite during the magmatic stage [42,43,44]. According to the columbite–tantalite U-Pb geochronology, the ASP in the Lijiagou deposit was formed at ±211.1 Ma [31].

3.2. Mineralogical Characteristics of Micas

Micas exists in each zone as a penetrative mineral and presents a change in texture from the MP to ASP zone, while muscovite–Li-bearing mica series are the main constituents of the micas from Lijiagou pegmatites. The mica types have a similar texture in the barren pegmatites (from the MP to AP zone) of the Lijiagou area, which are mainly muscovite, with a wide range of particle sizes ranging from the micrometer to millimeter level, and often occur in a semi-euhedral to euhedral flake symbiosis with albite or quartz (Figure 4a–f). The muscovite shows no zoning in BSE images (Figure 4j,k). However, the type of micas has changed in rocks of the ore-bearing pegmatite (ASP zone), where it occurs in both primary and secondary forms. Primary muscovite generally appears as a large flake with curved cleavages (Figure 4g), and does not show bright and dark domains in high-contrast BSE images (Figure 4l), indicating that it is chemically homogeneous. Li-bearing mica occurs sporadically as a secondary form around primary muscovite or along cleavage planes, and is brighter than muscovite in high-contrast BSE images (Figure 4m–o). Secondary Li-bearing mica is smaller than primary muscovite and commonly fills interstitial spaces between spodumene and primary muscovite (Figure 4h). This may be the result of the migration of ore-forming fluids along mineral gaps and the metasomatism of primary minerals (Figure 4i).

4. Analytical Methods

4.1. Whole-Rock Analysis

The whole-rock major, trace, and rare-earth elements were analyzed at the China In-stitute for Multipurpose Utilization of Mineral Resources, CAGS. The rock samples were crushed to a centimeter level, and fresh samples without alteration were selected. The sample was then crushed into a powder of less than 200 mesh for further analysis. For the analysis of the major elements, 0.5 g of the sample, 5.0 g of the cosolvent (Li2B4O7), and 0.3 g of the crucible protectant (NH4NO3) were weighed and mixed well. After adding 1~2 drops of LiBr, the mixture was melted in a heating furnace. After cooling, the molten glass was analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES, PE 5300V, Perkinelmer, Inc., Waltham, Massachusetts, USA), and the analysis error was less than 1%. For the analysis of trace elements, 50.00 mg (±0.50 mg) of samples with a particle size of less than 75 μm were accurately weighed and placed in a polytetrafluoroethylene (PTFE) tank. A total of 1.0 mL HF and 0.5 mL HNO3 were added and shaken. After that, the mixture was placed on an electric heating plate at 140 °C and evaporated to dryness. Finally, the trace elements in the samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700, Agilent Technologies Inc., Santa Clara, California, USA). The analysis error of most trace elements was less than 5%, and the analysis error of some volatile elements and very low contents of these elements was less than 10% [45,46].

4.2. EPMA Analysis

Major element analyses of muscovite were carried out at the Electron Probe Laboratory, School of Earth Science and Technology, Southwest Petroleum University. Using a JEOL-JXA-8230(Shimadzu, Inc., Kyoto City, Japan) electron microprobe equipped with EDAX-GENESIS Energy dispersive spectroscopy (Shimadzu, Inc., Kyoto City, Japan). The electron probe analysis was carried out under the conditions of an accelerating voltage of 15 kV and an accelerating current of 20 nA, and the selected beam spot diameter was 10 μm. All test data were corrected by ZAF. The measurement time of the characteristic peaks of Na, Mg, K, Ca, Fe, Ti, Al, Si, Ni, Cr, and Mn elements was 10 s, and the measurement time of the upper and lower background was half of the characteristic peak measurement time, respectively.

4.3. LA-ICPMS Analysis

In situ trace element analyses of muscovite were carried out using a laser ablation inductively coupled–plasma mass spectrometer at the FocuMS Technology Co. Ltd., Nanjing, China. An Agilent 7700× ICP-MS instrument (Agilent Technologies Inc., Santa Clara, CA, USA) was coupled with a Teledyne Cetac Technologies 193 nm ArF excimer laser ablation system. For this sample analysis, the laser beam spot size was 44 μm and the frequency was 5 Hz. Standard materials BIR-1G, BCR-2G, and BHVO-2G were used for multi-external standard calibration without internal standard correction [47]. Finally, the measured data were processed offline by the software ICPMS DataCal 12.2 [47]. The analytical uncertainties are 5%–10% for trace elements and generally better than 5% for major elements. The analytical uncertainties are 5%–10% for trace elements and generally better than 5% for major elements.

5. Results

5.1. Whole-Rock Geochemistry

The major element composition of the Ke’eryin granite pluton and Lijiagou pegmatites are shown in Table 1. The average content of Al2O3 in two-mica granite (average 15.5 wt%) is slightly higher than that in barren pegmatite (average 14.09 wt%) and ore-bearing pegmatite (average 14.57 wt%). The average content of CaO (average 1.39 wt%) in two-mica granite is higher than that in barren pegmatite (average 0.49 wt%) and ore-bearing pegmatite (average 0.32 wt%). The content of P2O5 in two-mica granite (0.11–0.33 wt%) is not very different from that in barren pegmatite (0.07–0.35 wt%), but the content of P2O5 in ore-bearing pegmatite (0.22–0.78 wt%) is significantly higher. The increase in P content in the melt often reflects the enrichment of rare-metal elements [48,49]. The Na2O/K2O ratio of two-mica granite ranges from 0.42 to 0.79, with an average of 0.62, and the K content is greater than the Na content, indicating that it is a potassium-rich granite. The Na2O + K2O values of two-mica granites range from 7.83 to 8.56 wt%, those of barren pegmatite from 8.21 to 8.83 wt%, and the values of ore-bearing pegmatite range from 5.24 to 7.40 wt%, which is significantly lower.
The differentiation index (DI) of the two-mica granite and the pegmatite, with an average value of 92.06, generally shows the characteristics of highly differentiated granitic magma. In the TAS diagram (Figure 5a), Lijiagou pegmatite and two-mica granite drop points basically fall into the granite area. In the A/NK-A/CNK diagram (Figure 5b), the two-mica granite and pegmatite samples fall into the peraluminous area. The A/CNK of the two-mica granite is 1.42–1.74, and the barren pegmatite is 1.32–2.69. The A/NK and A/CNK indexes of the ore-bearing pegmatite are generally higher than those of the two-mica granite and the barren pegmatite. In the K2O-SiO2 diagram, the two-mica granite and barren pegmatite show high-potassium calc-alkaline characteristics, and the ore-bearing pegmatite is biased towards medium-to-low-potassium calc-alkaline characteristics (Figure 5c). The Rittmann index (σ43) of granite and pegmatite ranges from 0.85 to 2.90, both of which are less than 3.3, indicating that they are calc-alkaline series rocks. In the alkalinity rate diagram (Figure 5d), both the two-mica granite and the barren pegmatite fall into the alkaline area, and a few ore-bearing pegmatites are calc-alkaline, which is different from the characteristics of the Rittman index. This may be attributed to the inherent complexity of the magma source or the assimilation and contamination of the surrounding rock during the evolutionary process. In summary, the two-mica granite is a high-potassium, alkaline–calcium, alkaline, strongly peraluminous granite, and Lijiagou pegmatite is an alkaline–calcium, alkaline, strongly peraluminous granite pegmatite.
The trace and rare-earth element compositions of the Ke’eryin granite pluton and Lijiagou pegmatites are shown in Table 2. The Li content in the two-mica granite is 93–211 ppm, the barren pegmatite of the Lijiagou deposit is 36–446 ppm, and the ore-bearing pegmatite can reach 9353–11,176 ppm. The content of Be in the two-mica granite is 4.31–6.46 ppm, the barren pegmatite of the Lijiagou deposit is 9.27–160.53 ppm, and the ore-bearing pegmatite is 184.99–399.64 ppm. The Rb content in the two-mica granite is 251–289 ppm, the barren pegmatite of the Lijiagou deposit is 320–982 ppm, and the ore-bearing pegmatite is 496–1120 ppm. In the spider diagram of trace elements (Figure 6a), large-ion lithophile elements such as Ba and Sr in pegmatite generally show negative anomalies due to loss, and Ba and Sr in two-mica granite also show negative anomalies. The LREE/HREE of the two-mica granite is 7.50–16.55, which is characterized by the enrichment of light rare-earth elements and relative loss of heavy rare-earth elements.
The LaN/YbN of the two-mica granite is 10. 63–30.29, indicating fractionation between light and heavy rare-earth elements, and the δEu is 0.41–0.97, showing a weak–medium negative Eu anomaly. The δCe of the two-mica granite is 0.82–0.98, and the cerium anomaly is slight or not obvious. The ∑REE abundance value of the two-mica granite is 87.99–244.76 ppm. Compared with the two-mica granite, the ∑REE abundance value in the pegmatite is greatly reduced, only 0.19–3.43 ppm. In the rare-earth element distribution diagram (Figure 6b), it can also be seen that the distribution curve of the pegmatite is located below the two-mica granite. Compared with the LREE/HREE and LaN/YbN ratios of the two-mica granite, the LREE/HREE in the pegmatite is 2.78–13.86, and the LaN/YbN is 2.04–11.02. The ratio is significantly reduced, indicating that the fractionation of light and heavy rare earths is relatively weak at this time, and then the ratio tends to increase.

5.2. Major and Trace Element Compositions of Muscovite

The major and trace element compositions of micas from Lijiagou pegmatite are summarized in Table 3. In terms of the composition of major elements, except for the similar K2O content (average 10.29–11.01 wt%), the changes in other elements show regularity (Figure 7). The contents of SiO2 (45.55–46.97 wt%), Al2O3 (35.72–38.15 wt%), FeO (1.66–2.11 wt%), MnO (0.02–0.11 wt%), MgO (0.29–0.51 wt%), TiO2 (0.03–0.07 wt%) and F (0.38–0.62 wt%) in muscovite from MP and MAP are similar. The contents of Al2O3 (35.83–37.64 wt%) and MgO (0.32–0.44 wt%) in muscovite of AP are lower than those in MP and MAP. The contents of SiO2 (45.66–46.97 wt%) and F (0–1.91 wt%) are higher than those in MP and MAP. Compared with the content of SiO2 (44.62–47.80 wt%), Al2O3 (33.32–37.49 wt%), FeO (0.64–3.96 wt%) MnO (0.07–0.58 wt%) and F (0.44–5.86 wt%) in Li-bearing mica (low Li content) of ASP. The Li-bearing mica (high Li content) in ASP has lower Al2O3 (21.14–23.21 wt%) and higher SiO2 (48.72–50.55 wt%), FeO (5.79–6.46 wt%), MnO (0.53–1.59 wt%) and F (8.94–10.87 wt%). According to the EPMA analysis results, the empirical chemical formula calculated based on 10 O atoms and 2 additional anions, the formula of muscovite from MP-AP is close to KAl2 (Si3AlO10) (OH)2, and the formula of Li-bearing mica(high Li content) from ASP is (K0.85Na0.01) (Al0.86Fe0.33Mn0.05Li0.63) (Si3.17Al0.83O10) F2.02, which may be Fe-rich luanshiweiite [55,56].
In terms of trace element composition, the contents of Li, Rb, and Cs in micas show regular characteristics, and their variation ranges are 273.74–25,646.55 ppm, 980.68–9303.18 ppm, and 45.44–1974.36 ppm, respectively, showing a gradual increase from MP to ASP. Among these elements, Li, Rb, and Cs reached their highest value in ASP. In addition, from MP to MAP, the content of Nb in muscovite gradually increased (164.09–288.5 ppm, average 228.8 ppm), but from AP to ASP, the content of Nb in muscovite and Li-bearing mica began to decrease (14.85–239.92 ppm, average 187.22 ppm), while the content of Ta in each type of pegmatite varies less (with an average content 23.27–71 ppm).

6. Discussion

6.1. Tectonic Environment

The formation of peraluminous granites is generally related to the continental collision environment, while strongly peraluminous granites are generally formed in the post-collision environment. In the Ta-Yb discriminant diagram (Figure 8a), the two-mica granite exhibits the characteristics of volcanic arc granite (VAG) and syn-collision granite (syn-COLG). In the Rb-(Nb + Y) discrimination diagram (Figure 8b), the two-mica granite falls into the region of post-collision granite (post-COLG). In the Rb-Hf-Ta discrimination diagram (Figure 8c), the plot point of the two-mica granite is close to the boundary transition position from the syn-COLG to post-COLG. In the R2-R1 discriminant diagram (Figure 8d), the two-mica granite transitioned from syn-collision to post-orogenic. Therefore, it is concluded that the Ke’eryin two-mica granite was mainly formed during the transition period from syn-collision to post-collision. That is, after the end of continental collision, the crustal compression thickened to the post-collision extensional transition tectonic environment.
The zircon U-Pb dating of the Ke’eryin two-mica granite shows that its crystallization age is 219.2 ± 2.3 Ma [31], and the zircon U-Pb age of the adjacent Jiajika two-mica granite is 223 ± 1 Ma [60], both of which were formed in the Late Triassic–Early Jurassic period. Based on the geochemical characteristics of the whole rock, the tectonic environment and process of granite formation in the mining area can be inferred as follows: with the closing of the Southern Paleo-Tethys Ocean Basin at the end of the Triassic period, continental collision occurred in the Yangtze block, North China Block and Qiangtang block, resulting in crustal thickening and the partial melting of crust-source materials to form granite in the region. The bidirectional contraction of the Songpan–Garze orogenic belt was caused by the collision, which resulted in large-scale slip nappe structures, accompanied by strong magmatic and metamorphic activities in the area, which formed several important compound dome structures in the eastern part of the orogenic belt [35,61], providing a good tectonic environment for the mineralization of pegmatite.

6.2. Indications of Mica for the Evolution of Pegmatite

The formation of granite pegmatite is a highly evolved magmatic–hydrothermal process [62]. The initial magma-forming pegmatites, due to the crystallization differentiation, lead to the enrichment and saturation of volatiles in the residual melt [63,64,65], while the magmatic fluid phase exsolution occurs and forms an independent water-bearing fluid phase, thus entering the magmatic–hydrothermal transition stage [66]. As the melt phase finally crystallized completely, the system entered the hydrothermal stage. During the evolution of pegmatite magma, the changes in the K/Rb ratio, Nb/Ta ratio, and Li, Rb and Cs contents of mica can indicate the trend and degree of magmatic–hydrothermal differentiation and evolution. With the gradual increase in magmatic differentiation and evolution, the contents of volatile elements Li and incompatible elements Rb and Cs also increase, while the ratios of K/Rb and Nb/Ta decrease [67,68,69,70,71,72,73]. In highly evolved magmatic systems, the K/Rb ratio is usually reduced to less than 50 [74,75].
In this study, the K/Rb ratio of MAP to ASP mica varies from 9.76 to 42.02, which generally exhibits the characteristics of high magmatic evolution. The mica composition in Lijiagou pegmatite displays evolutionary characteristics (Figure 9). The contents of Li, Rb, Cs, and F in mica gradually increase, and the K/Rb ratio gradually decreases (Figure 9a–c,f), indicating that the degree of magmatic evolution is increasing, and the type of mica also shows the evolution characteristics of muscovite to Li-bearing mica. The contents of Li, Rb, and Cs of Li-bearing mica in ASP reached the highest value, and the content of F also increased from 0.41%–5.86% to 8.94%–10.87%, indicating that the magmatic evolution reached a very high degree at this stage. In this stage, Li-bearing mica mainly appeared as the secondary form on the edges of primary muscovite, with a metasomatic structure, which may be attributed to late Li-rich fluid metasomatic muscovite, and the enrichment of Li and F combined with changes in mica types, indicating the hydrothermal environment [76]. In general, a Li-F rich environment is extremely favorable to the formation of lepidolite, while Li-bearing mica only appears as a secondary form at the edge of the primary muscovite, which may be due to the relatively low F concentration in the pegmatite system [77]. In Figure 9g–i, there is a negative correlation between Nb/Ta and the Li, Rb, and Cs contents of mica. During the evolution process from magmatic to hydrothermal, the generation of the fluid phase also promotes the mineralization of Nb. In the muscovite of MP and MAP, the content of Nb increases slightly, indicating that Nb is gradually enriched in the melt with evolution. However, in the mica of AP and ASP, the content of Nb gradually decreases, indicating that Nb begins to precipitate to form columbite after entering the magmatic–hydrothermal stage and resulting in the gradual loss of Nb (Figure 9d). The variation range of Ta content in mica is small (Figure 9e), which may be because Ta cannot easily replace Al, Fe, and Mg in micas as Nb [78,79,80]. In addition, the characteristics of fluid inclusions in ASP pegmatites reveal the fluid characteristics of pegmatite magma in the magmatic–hydrothermal transition stage. In the magmatic–hydrothermal transition stage, the fluid properties of the system are volatile-rich silicate fluids. With the decrease in temperature and pressure, the phenomenon of symbiosis between low-salinity, high-density crystal-rich inclusions and high-salinity, low-density CO2-H2O inclusions appears, indicating that fluid immiscibility occurs in the system, which is also a critical mechanism of Li enrichment and precipitation in the Lijiagou deposit [19,29]. ASP underwent a magmatic–hydrothermal transformation and post-crystallization hydrothermal process during its formation and evolution, and the high enrichment of Li also occurred in the magmatic–hydrothermal transition stage [30].
Therefore, the Lijiagou barren pegmatite (MP-MAP) transformed into ore-bearing pegmatite (ASP) through a magmatic–hydrothermal evolution process, which resulted in a gradual increase in the degree of evolution. The significant enrichment of rare metals, such as Li, Rb, and Cs, primarily occurred during the magmatic–hydrothermal transition stage.

7. Conclusions

(1) The two-mica granite was mainly formed during the transition period from syn-collision to post-collision. After the end of the continental collision, the crust was squeezed and thickened into the post-collision extensional transition tectonic environment, The favorable tectonic environment contributed to the formation of the Lijiagou pegmatite-type spodumene deposit.
(2) From MP to ASP, the micas in Lijiagou pegmatite exhibit an evolutionary trend from muscovite to Li-bearing mica. From MP to AP, muscovite is the main type. Starting from ASP, the type of mica begins to change. The Li-bearing mica appears in the secondary form, at the edge of the primary muscovite, and has a metasomatic structure. This may be the result of the subsequent Li-rich fluid metasomatism of muscovite. The mineral chemical characteristics of mica indicate that the Lijiagou pegmatite has a high degree of differentiation and evolution, and the degree of evolution gradually increases from MP to ASP.
(3) Lijiagou pegmatite experienced magmatic–hydrothermal evolution. From MP to ASP, there is a negative correlation between Nb/Ta, K/Rb and the Li, Rb and Cs contents of mica, while the contents of Li, Rb, Cs and F in Li-bearing mica of ASP increased sharply. Combined with the change in mica type, it is considered that ASP formed during the transition from magmatic to hydrothermal and was a fluid-rich environment, and Li, Rb, and Cs mainly began to enrich during the magmatic–hydrothermal transition.

Author Contributions

Conceptualization, X.C. and C.C.; methodology, X.C.; validation, X.L. and Y.Y.; investigation, X.C. and X.L.; resources, Y.C.; data curation, X.C., X.L. and Y.G.; writing—original draft preparation, X.C.; writing—review and editing, X.C. and C.C.; supervision, C.C.; project administration, C.C.; funding acquisition, C.C. All authors have read and agreed to the published version of the manuscript.

Funding

Sichuan Energy Investment Lithium Technology Co. Ltd. horizontal research project: Research on process mineralogy of spodumene ore, a key technology for development and comprehensive utilization of spodumene ore in western Sichuan, grant number AHG043.

Data Availability Statement

Data supporting the findings of this study will be made available from the corresponding author, upon reasonable request.

Acknowledgments

We would like to thank Fei Guangchun from College of Earth Science, Chengdu University of Technology for guidance and advice. We also thank to Huang Qiaoyuan from Chongqing Survey Institute and Yu Xiao from Petroleum Engineering Technology Research Institute of Sinopec Jianghan Oilfield Company for assistance in field work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Geographical location of Jinchuan County; (b) tectonic position of Ke’eryin (modified from [40]); (c) geological sketch map of Ke’eryin area (modified from [31,40]).
Figure 1. (a) Geographical location of Jinchuan County; (b) tectonic position of Ke’eryin (modified from [40]); (c) geological sketch map of Ke’eryin area (modified from [31,40]).
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Figure 2. Geological map of Ljiagou spodumene deposit (modified from [27]).
Figure 2. Geological map of Ljiagou spodumene deposit (modified from [27]).
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Figure 3. Photographs of outcrops, hand specimens and microscope images of various types of pegmatite from the Lijiagou area; (a–c) photographs from microcline pegmatite; (d–f) photographs from microcline albite pegmatite; (g–i) photographs from albite pegmatite; (j–l) photographs from albite spodumene pegmatite. MP: microcline pegmatite; MAP: microcline albite pegmatite; AP: albite pegmatite; ASP: albite spodumene pegmatite; Mc: microcline; Qtz: quartz; Ms: muscovite; Tur: tourmaline; Ab: albite; Spd: spodumene; Col: columbite.
Figure 3. Photographs of outcrops, hand specimens and microscope images of various types of pegmatite from the Lijiagou area; (a–c) photographs from microcline pegmatite; (d–f) photographs from microcline albite pegmatite; (g–i) photographs from albite pegmatite; (j–l) photographs from albite spodumene pegmatite. MP: microcline pegmatite; MAP: microcline albite pegmatite; AP: albite pegmatite; ASP: albite spodumene pegmatite; Mc: microcline; Qtz: quartz; Ms: muscovite; Tur: tourmaline; Ab: albite; Spd: spodumene; Col: columbite.
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Figure 4. BSE images and micrographs of micas in different types of pegmatite in the Lijiagou area. (a–f) Micrographs of muscovite in barren pegmatites; (g) micrographs of muscovite in ore-bearing pegmatite; (h,i) micrographs of Li-bearing mica in ore-bearing pegmatite; (j–l) BSE images of muscovite in barren pegmatites; (m–o) BSE images of Li-bearing mica in ore-bearing pegmatite. Mc—microcline; Qtz—quartz; Ms—muscovite; Grt—garnet; Ab—albite; Spd—spodumene; Li-Mica—Li-bearing mica.
Figure 4. BSE images and micrographs of micas in different types of pegmatite in the Lijiagou area. (a–f) Micrographs of muscovite in barren pegmatites; (g) micrographs of muscovite in ore-bearing pegmatite; (h,i) micrographs of Li-bearing mica in ore-bearing pegmatite; (j–l) BSE images of muscovite in barren pegmatites; (m–o) BSE images of Li-bearing mica in ore-bearing pegmatite. Mc—microcline; Qtz—quartz; Ms—muscovite; Grt—garnet; Ab—albite; Spd—spodumene; Li-Mica—Li-bearing mica.
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Figure 5. Major element diagrams of the Ke’eryin granite pluton and Lijiagou pegmatite. (a) Total alkali versus silica diagram [50]; (b) A/NK vs. A/CNK diagram after [51]; (c) SiO2 vs. K2O diagram [52]; (d) alkalinity ratio variation diagram [53].
Figure 5. Major element diagrams of the Ke’eryin granite pluton and Lijiagou pegmatite. (a) Total alkali versus silica diagram [50]; (b) A/NK vs. A/CNK diagram after [51]; (c) SiO2 vs. K2O diagram [52]; (d) alkalinity ratio variation diagram [53].
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Figure 6. (a) Primitive mantle-normalized incompatible element distribution patterns and (b) Chondrite-normalized REE distribution patterns of the Ke’eryin granite pluton and Lijiagou pegmatite. Normalization values are taken from [54].
Figure 6. (a) Primitive mantle-normalized incompatible element distribution patterns and (b) Chondrite-normalized REE distribution patterns of the Ke’eryin granite pluton and Lijiagou pegmatite. Normalization values are taken from [54].
Minerals 14 00069 g006
Minerals 14 00069 g007
Figure 7. Box plots of major element changes in micas from Lijiagou pegmatite. I: muscovite in microcline pegmatite; II: muscovite in microcline albite pegmatite; III: muscovite in albite pegmatite; IV: Li-bearing mica (low Li content) in albite spodumene pegmatite; IV *: Li-bearing mica (high Li content) in albite spodumene pegmatite. MP—microcline pegmatite; MAP—microcline albite pegmatite; AP—albite pegmatite; ASP—albite spodumene pegmatite.
Figure 7. Box plots of major element changes in micas from Lijiagou pegmatite. I: muscovite in microcline pegmatite; II: muscovite in microcline albite pegmatite; III: muscovite in albite pegmatite; IV: Li-bearing mica (low Li content) in albite spodumene pegmatite; IV *: Li-bearing mica (high Li content) in albite spodumene pegmatite. MP—microcline pegmatite; MAP—microcline albite pegmatite; AP—albite pegmatite; ASP—albite spodumene pegmatite.
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Figure 8. (a,b) Ta-Yb and Rb-(Nb + Y) discriminant diagrams [57]; (c) Rb-Hf-Ta triangular plot discriminant diagrams [58]; (d) R2 vs. R1 discriminant diagrams [59]. syn-COLG: syn-collision granites; VAG: volcanic arc granites; WPG: within-plate granites; ORG: ocean ridge granites; post-COLG: post-collision granites. R1 = 4Si − 11(Na + K) − 2(Fe + Ti); R2 = 6Ca + 2Mg + Al. Data from Table 1 and Table 2.
Figure 8. (a,b) Ta-Yb and Rb-(Nb + Y) discriminant diagrams [57]; (c) Rb-Hf-Ta triangular plot discriminant diagrams [58]; (d) R2 vs. R1 discriminant diagrams [59]. syn-COLG: syn-collision granites; VAG: volcanic arc granites; WPG: within-plate granites; ORG: ocean ridge granites; post-COLG: post-collision granites. R1 = 4Si − 11(Na + K) − 2(Fe + Ti); R2 = 6Ca + 2Mg + Al. Data from Table 1 and Table 2.
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Figure 9. Plots of K/Rb vs. Li (a), K/Rb vs. Rb (b), K/Rb vs. Cs (c), K/Rb vs. Nb (d), K/Rb vs. Ta (e), K/Rb vs. F (f), Nb/Ta vs. Li (g), Nb/Ta vs. Rb (h) and Nb/Ta vs. Cs (i) in micas of different zones from Lijiagou pegmatite. I: muscovite in microcline pegmatite; II: muscovite in microcline albite pegmatite; III: muscovite in albite pegmatite; IV: Li-bearing mica (low Li content) in albite spodumene pegmatite; IV *: Li-bearing mica (high Li content) in albite spodumene pegmatite. MP—microcline pegmatite; MAP—microcline albite pegmatite; AP—albite pegmatite; ASP—albite spodumene pegmatite.
Figure 9. Plots of K/Rb vs. Li (a), K/Rb vs. Rb (b), K/Rb vs. Cs (c), K/Rb vs. Nb (d), K/Rb vs. Ta (e), K/Rb vs. F (f), Nb/Ta vs. Li (g), Nb/Ta vs. Rb (h) and Nb/Ta vs. Cs (i) in micas of different zones from Lijiagou pegmatite. I: muscovite in microcline pegmatite; II: muscovite in microcline albite pegmatite; III: muscovite in albite pegmatite; IV: Li-bearing mica (low Li content) in albite spodumene pegmatite; IV *: Li-bearing mica (high Li content) in albite spodumene pegmatite. MP—microcline pegmatite; MAP—microcline albite pegmatite; AP—albite pegmatite; ASP—albite spodumene pegmatite.
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Table 1. Major-element compositional variations in the Ke’eryin granite pluton and Lijiagou pegmatite.
Table 1. Major-element compositional variations in the Ke’eryin granite pluton and Lijiagou pegmatite.
(Ke’eryin Pluton)
Two-Mica Granite		(Barren Pegmatite)
MP–MAP–AP		(Ore-Bearing Pegmatite)
ASP
Min–MaxAvgMin–MaxAvgMin–MaxAvg
SiO2 (wt%)68.06–73.6172.09 73.14–77.5274.868.68–75.2971.70
TiO2 (wt%)0.05–0.360.16 --0.01–0.040.02
Al2O3 (wt%)14.37–15.9715.15 12.08–15.1114.09 13.43–16.9114.57
Fe2O3 (wt%)0.16–2.590.66 0.31–0.70.45 0.27–0.620.45
MnO (wt%)0.02–0.220.06 0.02–0.370.15 0.07–0.360.15
MgO (wt%)0.03–0.700.25 0.01–0.040.03 0.01–0.20.09
CaO (wt%)0.95–2.621.39 0.34–0.60.49 0.2–0.40.32
Na2O (wt%)2.50–3.593.13 2.66–7.173.86 1.98–5.324.06
K2O (wt%)4.03–6.015.16 1.03–6.173.86 0.97–3.272.60
P2O5 (wt%)0.11–0.330.17 0.07–0.350.20 0.22–0.780.47
LOI0.32–1.170.68 0.33–1.090.74 0.5–0.690.59
Na2O + K2O7.83–8.568.30 8.21–8.838.58 5.24–7.46.66
Na2O/K2O0.42–0.790.62 0.43–6.942.78 0.61–5.192.04
DI81.32–92.6589.71 94.71–97.5295.71 88.84–94.6492.57
σ432.00–2.902.37 2.22–2.442.31 0.85–2.051.56
AR2.73–3.433.04 3.19–5.924.21 2.03–3.362.73
R12174–28172505.70 2097–27902423.30 2367–36162862.80
R2406–633466.67 277–367334.67 321–381343.17
A/NK1.71–1.961.83 1.37–1.841.65 1.8–2.882.24
A/CNK1.42–1.741.57 1.32–1.721.56 1.73–2.692.13
Min–Max: minimum value–maximum value; avg: average value; σ43 = (Na2O + K2O)2/(SiO2 − 43); AR = [Al2O3 + CaO + (Na2O + K2O)]/[Al2O3 + CaO − (Na2O + K2O)]; R1 = 4Si − 11(Na + K) − 2(Fe + Ti); R2 = 6Ca + 2Mg + Al; A/NK = (Al2O3/(Na2O + K2O); A/CNK = (Al2O3/(CaO + Na2O + K2O); MP—microcline pegmatite; MAP—microcline albite pegmatite; AP—albite pegmatite; ASP—albite spodumene pegmatite.
Table 2. Trace and rare-earth element compositional variations in the Ke’eryin granite pluton and Lijiagou pegmatite.
Table 2. Trace and rare-earth element compositional variations in the Ke’eryin granite pluton and Lijiagou pegmatite.
(Ke’eryin Pluton)
Two-Mica Granite		(Barren Pegmatite)
MP–MAP–AP		(Ore-Bearing Pegmatite)
ASP
Min–MaxAvgMin–MaxAvgMin–MaxAvg
Li (ppm)93–211149.00 36–446228.00 9353–11,17610,264.5
Be (ppm)4.31–6.465.19 9.27–160.5359.06 184.99–399.64292.32
Sn (ppm)5.21–16.59.00 12.6–295.5120.45 258.55–266.27262.41
Cs (ppm)13.5–19.416.60 40.6–83.258.70 148.8–191.1169.95
Ga (ppm)16.9–22.419.38 13.5–27.521.80 12.3–19.515.9
Se (ppm)0.03–0.660.19 0.03–0.060.04 0–0.030.03
Rb (ppm)251–289270.75 320–982738.33 496–1120808
Ba (ppm)158–16691004.25 5.92–24.5313.37 0.69–2.691.69
Th (ppm)10.2–20.817.60 0.39–1.90.94 0.08–2.311.2
U (ppm)2.74–8.015.04 0.51–31.410.85 5.53–14.8310.18
Ta (ppm)1.08–3.081.82 3.83–11.268.41 57.39–87.8272.61
Nb (ppm)10.67–20.615.97 11.73–49.6732.87 141.31–185.48163.4
Sr (ppm)60.2–335224.51 9.62–52.5225.32 0.95–1.681.32
Zr (ppm)58.3–164123.66 3.8–25.5212.94 5.76–16.1210.94
Hf (ppm)2.32–4.323.69 0.15–1.40.65 0.47–1.731.1
Yb (ppm)1.14–1.561.32 0.01–0.220.14 0.010.01
Y (ppm)17.1–18.217.77 0.1–1.490.84 0.15–0.240.2
K/Rb154.47–198.69166.9026.71–56.0839.9614.30–24.1519.27
Zr/Hf25.13–38.6832.6118.22–24.5821.969.34–12.2510.80
Nb/Ta6.69–12.319.513.06–4.413.711.61–3.232.42
La (ppm)16.9–54.938.88 0.05–0.640.42 0.11–0.220.16
Ce (ppm)34.5–10271.05 0.09–1.220.83 0.13–0.320.23
Pr (ppm)4.21–12.58.68 0.01–0.130.08 0.010.01
Nd (ppm)17.1–50.433.55 0.01–0.420.26 0.010.01
Sm (ppm)4.38–8.916.21 0–0.190.10 --
Eu (ppm)0.55–2.11.44 0.01–0.020.02 --
Gd (ppm)3.68–6.055.03 0.01–0.180.09 0.010.01
Tb (ppm)0.6–0.790.68 0–0.040.02 --
Dy (ppm)2.34–3.363.05 0.01–0.250.13 0.01–0.030.02
Ho (ppm)0.6–0.620.61 0–0.040.02 --
Er (ppm)1.49–1.781.64 0.01–0.110.07 0.01–0.020.01
Tm (ppm)0.2–0.250.23 0–0.020.02 --
Yb (ppm)1.14–1.561.32 0–0.220.14 0.010.01
Lu (ppm)0.17–0.230.21 0–0.040.02 --
∑REE87.99–244.76172.58 0.19–3.432.21 0.36–0.610.48
∑LREE77.64–230.81159.80 0.17–2.571.71 0.26–0.570.42
∑HREE10.35–13.9712.77 0.03–0.860.50 0.1–0.040.07
LREE/HREE7.5–16.5512.22 2.97–6.74.51 2.78–13.868.32
LaN/YbN10.63–30.2920.88 2.04–7.613.92 5.25–11.028.14
δEu0.41–0.970.73 0.34–6.412.48 0.18–0.570.37
δCe0.82–0.980.92 0.99–1.171.06 0.86–10.93
Min–Max: minimum value–maximum value; avg: average value.
Table 3. Major and trace element compositions of micas from the Lijiagou pegmatite.
Table 3. Major and trace element compositions of micas from the Lijiagou pegmatite.
TypeValuewt%
SiO2TiO2Al2O3FeOMnOMgONa2OK2OF
IAvg45.930.0636.571.780.060.430.3711.010.52
Min45.930.0435.981.710.030.330.2910.820.41
Max46.310.0736.831.870.10.510.4311.30.62
IIAvg46.210.0536.041.880.050.370.410.840.46
Min45.550.0335.721.660.020.290.2810.520.38
Max46.760.0736.662.110.110.490.5611.130.56
IIIAvg46.40.0236.591.570.070.380.3110.480.98
Min45.66035.831.2400.320.1610.080
Max46.970.0737.641.910.140.440.3911.221.91
IVAvg45.660.0234.72.650.280.010.3410.292.05
Min44.62033.320.640.0700.158.550.44
Max47.80.0637.493.960.580.030.5210.865.86
IV *Avg49.460.0122.436.10.880.010.1110.399.96
Min48.72021.145.790.5300.0810.248.94
Max50.550.0323.216.461.590.030.1410.6410.87
TypeValueppmK/RbNb/Ta
LiBeBRbCsNbTaSn
IAvg299.1714.64118.321163.654.72191.9823.27194.4780.058.37
Min273.7411.04109.21980.6845.44167.0920.30167.2768.746.13
Max326.0718.79126.191354.6867.11210.7927.25228.3895.649.9
IIAvg941.9922.75121.442205.1197.57265.6253.26417.9341.115.24
Min869.6121.66113.032088.6893.17246.5132.85385.9639.354.52
Max1027.7624.24131.182297.73101.57288.5063.02453.4342.027.92
IIIAvg1961.4920.2173.015673.41657.63111.0171.00510.1615.521.69
Min1603.7718.09118.073922.32325.8165.4030.26401.8113.641.12
Max2466.9823.2211.246343.63866.97176.10115.69723.6721.572.63
IVAvg4469.8024.63187.105630.59693.80141.5544.37540.2215.773.35
Min2444.1118.71132.224735.45225.9514.8510.07453.2113.081.06
Max7317.6733.71223.947051.881265.15239.92157.16635.8518.937.95
IV *Avg24,480.3416.3322.198733.761708.60121.8747.15171.0710.344.25
Min23,241.0213.0515.217780.911240.9138.375.47134.699.762.02
Max25,646.5521.6528.479303.181974.36222.22103.78212.311.646.47
Min–Max: minimum value–maximum value; Avg: average value. I: muscovite in microcline pegmatite; II: muscovite in microcline albite pegmatite; III: muscovite in albite pegmatite; IV: Li-bearing mica (low Li content) in albite spodumene pegmatite; IV *: Li-bearing mica (high Li content) in albite spodumene pegmatite.
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Chen, X.; Chen, C.; Lai, X.; Yang, Y.; Gu, Y.; Cai, Y. Whole-Rock Geochemistry and Mica Compositions in Lijiagou Pegmatite Spodumene Deposit, Western Sichuan, China. Minerals 2024, 14, 69. https://doi.org/10.3390/min14010069

AMA Style

Chen X, Chen C, Lai X, Yang Y, Gu Y, Cai Y. Whole-Rock Geochemistry and Mica Compositions in Lijiagou Pegmatite Spodumene Deposit, Western Sichuan, China. Minerals. 2024; 14(1):69. https://doi.org/10.3390/min14010069

Chicago/Turabian Style

Chen, Xiaojie, Cuihua Chen, Xiang Lai, Yulong Yang, Ying Gu, and Yunhua Cai. 2024. "Whole-Rock Geochemistry and Mica Compositions in Lijiagou Pegmatite Spodumene Deposit, Western Sichuan, China" Minerals 14, no. 1: 69. https://doi.org/10.3390/min14010069

APA Style

Chen, X., Chen, C., Lai, X., Yang, Y., Gu, Y., & Cai, Y. (2024). Whole-Rock Geochemistry and Mica Compositions in Lijiagou Pegmatite Spodumene Deposit, Western Sichuan, China. Minerals, 14(1), 69. https://doi.org/10.3390/min14010069

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