Research and exploration progress on lithium deposits in China

1.   Introduction

Rare metals are the critical mineral resources of strategic emerging industries (Wang RJ et al., 2015), which play an irreplaceable role both in the high-end equipment manufacturing industry and in the field of new energy vehicles. Being one of the most concerned minerals resources among the rare metals at present, the search for lithium ore has become a global prospecting activity, which is in full swing and has achieved remarkable results (Wang DH et al., 2016, 2017a, 2017b, 2018). Supported by the “Investigation and Evaluation of Mineral Resources in shortage and Strategic emerging Industries” project of China Geological Survey, the investigation, evaluation and exploration of rare metals such as lithium (Li), beryllium (Be), niobium (Nb) and tantalum (Ta) continued throughout the country, and important progress has also been made since 2011. In this paper, the progress related to the research and exploration of lithium deposits is briefly introduced and some key issues are briefly discussed to make more progress.

2.   Prospecting progress on lithium deposits

From 2016 to 2019, through the implementation of the China National Key Research and Development Program during the “13th Five-Year Plan Period” and by the China Geological Survey project “Comprehensive investigation and Evaluation of Jiajika Large Lithium Mineral Resources Base in Western Sichuan”, seven new Li-bearing pegmatite veins are found in Keeryin, Sichuan Province. Meanwhile, more than 40 pegmatite veins have been newly discovered in Guzhai, Jiangxi Province, which is expected to be found a medium-sized or above spodumene ore area. The significant increase of resources is mainly due to the discovery of the X03 vein and some new Li-bearing veins have been drilled at the Yakeke area (Fig. 1), with the total Li₂O resources more than 2 Mt.

Figure  1.  Distribution map of lithium deposits in China, showing the major discovered deposits (modified from Li JK et al., 1995, 2015).

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2.1   Prospecting progress in Sichuan Province

The rare mineral resources including Li, Be, Nb and Ta are abundant in western Sichuan Province. In the past six years, a lot of discovered Li deposits or deposits with significantly expanded resource reserves are distributed in this area, such as Jiajika, Lijiagou, Markam (Dangba), Yelonggou, Redamen, and Waying, etc. These deposits (points) are mainly distributed in Jiajika, Markam, and Pingwu, etc. (Fig. 2). The amount of prospective resources can reach 7 Mt (Dai HZ, et al., 2019).

Figure  2.  Distribution of rare-metal deposits in the Songpan-Ganze orogenic zone. A–Jiajika Li-Be-Nb-Ta pegmatite deposit; B–Xuebaoding W-Sn pegmatite deposit; C–Keeryin Li-Be-Nb-Ta pegmatite deposit; D–Danba muscovite pegmatite deposit; E–Zhawulong Li-Be-Nb-Ta pegmatite deposit; F–Dahongliutan Li-Be-Nb-Ta pegmatite deposit (modified from Li JK et al., 2015).

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The Jiajika orefield in Sichuan Province is one of the most concentrated areas of spodumene resources in China and even in the world. In recent years, the Li₂O resources of the No.X03 pegmatite vein has reached 885500 t, which has been the largest single spodumene vein in Asia. The No.X03 pegmatite vein is 3 km north from the Majingzi granite. It is suitable for open-pit mining and has the much development value (Fig. 3) benefit from its large scale, shallow burial and slow occurrence. The main body of the No.X03 pegmatite vein is a concealed orebody. The cristobalite-andalusite-bearing two mica-schist of the second member of the Xinduqiao Formation (T₃xd₂) is sporadically exposed near the No.X03 pegmatite vein, which is largely covered by the quaternary system, with a thickness of 2–10 m (Fig. 4a). According to the constructed drilling works (Figs. 4b–c), the No.X03 pegmatite vein orebody is simple in shape and generally in layered, lenticular and simple branch compound shape. The dip angle is about 25°. It has the characteristics of branch and compound in the deep, with the strike close to the north and south, inclining to the west. The vein length is more than 1050 m, the thickest part of the orebody is 110.17 m, and the average thickness is 66.4 m. The whole vein of the No.X03 pegmatite has lithium industrial mineralization with an average Li₂O grade of 1.51%. The associated mineral resources including Rb, Ta, Nb and Sn are rich, all of which meet the industrial requirements of comprehensive utilization. It is a super large comprehensive rare metal deposit rich in medium-fine grained spodumenes (Fig. 5) as the main mineral of lithium industrial minerals.

Figure  3.  The outcrop of the No.X03 pegmatite vein in the Jiajika orefield, Sichuan Province. a–panorama of the No.X03 pegmatite vein; b–local outcrop of spodumene-bearing pegmatite of the No.X03 pegmatite vein; c–spodumene-bearing outcrop with residual formation.

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Figure  4.  Geological map (a) and profile map of exploration line 7 (b) and line 11 (c) in the No.X03 vein of the Jiajika pegmatite.

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Figure  5.  Hand specimens and microscopical photos of typical spodumene-bearing structural units in the No.X03 vein of the Jiajika orefield. a–hand specimen of microcrystalline lithium ore; b–microscopic photos of microcrystalline lithium ore; c–hand specimen of fine-grained lithium ore; d–microscopic photos of fine-grained lithium ore; e–hand specimen of medium-grained comb-like lithium ore; f–microscopic photos of medium-grained comb-like lithium ore; g–hand specimen of coarse-grained lithium ore; h–microscopic photos of coarse-grained lithium ore; i–microcrystalline lithium ore replaced medium coarse-grained lithium ore; j–no metasomatic relationship between the medium-grained and coarse-grained spodumene units; k–two stages of activity in the structural units of medium coarse-grained lithium ores. Ab–albite; Ms–muscovite; Qtz–quartz; Spd–spodumene; Tur–tourmaline.

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The Keeryin orefield is located in northwest Sichuan Province, with a lot of pegmatite-type lithium deposits distributed along the Keeryin intrusive body (Fig. 2), which is the largest Li orefield in China with its total Li₂O resources more than 2.5 Mt. There are new Li-bearing veins that continue to be discovered. For example, in 2017, seven spodumene orebodies have been discovered in Guanyinqiao area, west of the Keeryin orefield. Among them, the No.5 orebody is covered by Quaternary and only exposed in a deep ditch in Guanyin village, forming a steep wall about 15 m high, with exposure thickness about 4 m and about 30 m lengths along the strike of 165°. The ores show grayish-white coarse crystalline granular texture and massive structure, with the mineral composition dominated by feldspar (about 45%), quartz (about 35%) and spodumene (about 15%), in addition to a small amount of mica and dark minerals (about 5%). The surrounding rock is mainly sandy slate of the Triassic Xinduqiao Formation.

The No.7 orebody appears on the steep wall near the Guanyinqiao across the Dadu River. The orebody is 175° strike, about 5 m of apparent thickness, and about 30 m of recoverable length. The ores show grayish-white coarse grain structure and massive texture, and with the mineral composition of feldspar (about 55%), quartz (about 30%), spodumene (about 10%) and mica (about 5%). Spodumenes are mostly grayish and white, with short columns distributed uniformly in the orebody, with a complete crystal shape and 1–5 cm of long axis length, some are longer than 10 cm (Fig. 6). The surrounding rock is mainly metamorphosed sandstone characterized by variable residual sandy, massive structure, and dominated by quartz (about 25%) and feldspar (about 40%), with mica and Fe-bearing minerals to be the secondary minerals.

Figure  6.  The outcrop of spodumene-bearing pegmatite veins in Guanyinqiao area, Keeryin orefield, Sichuan Province and its ore microscopic characteristics. a–Li-bearing pegmatite vein; b–spodumene occurs as medium coarse comb; c–microscopic characteristics of spodumene. Ms–muscovite; Spd–spodumene.

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The No.12 orebody is exposed on the edge of Shiban village, in Guanyinqiao town. The ore body is about 49° in strike direction, the apparent thickness of the surface is about 3 m, the length of outcrop along the strike is only a few meters, and the rest of the orebody is covered by Quaternary. The ore is grayish-white, coarse granular texture and massive structure. The main mineral components are feldspar (grayish-white, crystal shape is not obvious, content is about 45%), quartz (smoke gray-colorless transparent, crystallization is not obvious, accounts for approximately 40%) and spodumene (columnar crystallization, particle size is 1–6 cm, accounts for approximately 15%). The surrounding rock of orebody is metamorphosed sandstone, the rock has a variable residual granular texture, block structure. The mineral compositions mainly are quartz (about 25%) and feldspar (about 40%), secondary minerals are mica, with a small amount of limonite.

The No.13 orebody is located on the edge of a highway in Shiban village, Guanyinqiao town. The orebody moves towards 100° and extends about 50 m along with the trend. The apparent thickness of the exposed area of the highway is about 8 m, and the rest of the area is covered by Quaternary. The ore bodies are crushed and relatively broken. The ore is grayish-white, coarse crystalline granular texture, block structure. The mineral compositions are mainly feldspar (about 55%), quartz (about 30%), spodumene (about 10%) and mica (about 5%), etc. Due to strong weathering, some feldspars are altered into kaolinite. There are no obvious zonations of the orebody. From near to far of the surrounding rock, the mineral size tends to increase. The surrounding rock of the orebody is mainly metamorphosed sandstone with variable residual sand texture and massive structure. The mineral components mainly are quartz (about 25%) and feldspar (about 45%), and the secondary minerals are mica, etc. The exposed strata are affected by the intrusion of the magmatic vein, and the boundary between the strata and orebody is clear.

2.2   Prospecting progress in Jiangxi Province

In addition to the pegmatite type lithium deposit, the altered-granite type rare metal deposit is also an important type of new progress in 2017. A series of amblygonite-type lithium deposit has been found in northwestern Jiangxi Province.

Although amblygonite is not a common lithium mineral, its theoretical content of Li₂O (9.29%) is higher than that of spodumene, reflecting that it is can be used as an industrial mineral for the lithium extraction industry. Because its visual recognition characteristics are similar to quartz, not as intuitive as spodumene and lepidolite, amblygonite may be ignored in the field geological survey. In 2017, it was identified that there is a heterogeneous distribution of amblygonite in granite bodies in Jiuling metallogenic belt, Jiangxi Province (Fig. 7). The content of amblygonite can reach 4%–5%, which can be classified as a rock-forming mineral. The content of Al₂O₃ changed from 35.90% to 39.09%, averaged at 37.59%; and the content of P₂O₅ changed from 45.34% to 50.95%, averaged at 48.81%.

Figure  7.  The amblygonite-bearing granite sample (a, b) and its micro-feature (c) of ores in Jiuling, Jiangxi Province. Qtz–quartz; Amb–amblygonite.

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The Li content of the samples varied from 0.024% to 0.770%, which was higher than that of the local pegmatite (0.015%–0.174%). Most of the samples had reached the industrial grade of granitic pegmatite lithium ore (Wang CH, et al., 2018). Even the amblygonite is widely distributed in the granite of light color, and the leucogranite. Unfortunately, huge lithium resources have been wasted, because it has been just exploited as a rock-forming minerals for a long time (Fig. 7). Besides, fine-grain granites, white (lithium) mica fine-medium alkali feldspar granites and pegmatite veins in Jiuling area, Jiangxi Province, which also have common rare metal minerals such as beryl, tantalum-rich tin, columbite-tantalite, topaz, fluorite and other minerals rich in B and P complex elements (Table 1), which can also be used as prospecting indicators.

Table  1.  The content of the major elements (%) and the trace elements (10^–6) of different type rocks from the Jiuling area.

Sample No.	Ycjc-1	Ycszl-1	Ycszl-3	Ycszl-5	Ycxz-1	Yccf-1	Yccf-2		Sample No.	Ycjc-1	Ycszl-1	Ycszl-3	Ycszl-5	Ycxz-1	Yccf-1	Yccf-2
Location	Jiaochong	Shiziling			Xiaozhuang	114°31′26″;27°37′13			Location	Jiaochong	Shiziling			Xiaozhuang	114°31′26″;27°37′13
SiO2	73.53	70.11	70.97	70.66	88.27	70.69	73.89		Ba	34.6	14.9	181	4.93	88.5	55.9	432
TiO2	0.02	0.01	0.20	0.02	0.08	0.04	0.04		Sr	986	121	85.7	112	36.4	20.0	32.8
Al2O3	15.81	18.66	15.48	17.26	6.60	18.84	16.44		In	<0.05	<0.05	<0.05	<0.05	<0.05	0.32	0.29
Fe2O3	0.05	<0.05	<0.05	<0.05	0.32	<0.05	<0.05		Bi	31.8	1.74	1.26	1.37	3.36	9.48	7.31
FeO	0.51	0.32	1.46	0.59	0.39	0.69	0.82		Mo	6.48	0.27	0.34	0.17	0.31	0.16	0.25
MgO	0.03	0.02	0.38	0.01	0.32	0.14	0.19		Tl	7.05	12.0	9.48	11.2	1.46	4.81	2.30
MnO	0.13	0.14	0.06	0.14	0.04	0.23	0.08		Cr	16.2	12.2	18.7	19.7	24.5	32.4	14.9
CaO	0.01	0.20	0.91	0.68	0.20	0.00	0.05		Co	1.13	0.43	2.53	0.34	0.96	1.89	1.12
K2O	3.93	2.76	4.63	3.44	2.33	4.12	3.97		Ni	2.70	0.47	2.94	0.76	0.57	41.4	26.9
Na2O	4.08	4.20	2.70	4.55	0.07	0.13	1.47		Cu	3.32	10.3	12.2	4.90	4.00	2.66	16.6
Li2O	0.086	1.255	0.440	0.779	0.064	0.128	0.034		Zn	69.9	76.2	63.2	119	27.8	83.5	44.7
Rb2O	0.169	0.302	0.147	0.257	0.043	0.118	0.071		Pb	12.5	7.72	15.7	3.73	17.2	29.3	16.3
Cs2O	0.015	0.055	0.142	0.059	0.009	0.004	0.002		Ga	33.0	26.8	25.9	28.6	21.2	44.7	41.5
BeO	0.031	0.071	0.017	0.046	0.003	0.0002	0.003		Cd	0.33	0.07	<0.05	<0.05	<0.05	<0.05	<0.05
Nb2O5	0.013	0.023	0.003	0.017	0.001	0.008	0.007		Bi	31.8	1.74	1.26	1.37	3.36	9.48	7.31
Ta2O5	0.013	0.019	0.001	0.008	0.0002	0.003	0.002		Sb	0.18	0.12	0.18	0.07	0.11	0.17	0.24
SnO2	0.005	0.015	0.008	0.012	0.0001	0.005	0.005		As	1.20	0.90	1.06	0.79	0.61	3.33	2.29
P2O5	0.17	1.02	0.62	0.43	<0.01	<0.01	<0.01		V	1.03	0.62	13.0	0.35	6.68	1.74	1.62
CO2	0.19	0.24	0.35	0.56	0.67	0.26	0.28		Sc	3.70	2.85	6.01	1.97	3.18	47.7	47.4
H2O+	1.31	1.83	1.62	1.13	1.30	5.14	2.89		La	1.94	1.64	25.3	1.46	7.80	22.0	13.9
LOI	1.28	2.48	1.64	1.47	1.22	5.08	3.00		Ce	1.39	3.20	54.0	2.35	20.8	39.5	40.6
Li	399	5831	2045	3621	296	595	157		Pr	0.38	0.22	6.06	0.19	1.75	6.91	5.40
Rb	1549	2768	1344	2347	397	1082	652		Nd	1.22	0.79	21.8	0.54	6.02	27.2	22.2
Cs	140	520	1340	561	84.8	39.4	20.7		Sm	0.38	0.11	4.58	0.11	1.16	9.61	10.2
Be	111	257	61.4	166	11.3	7.00	11.6		Eu	0.05	<0.05	0.50	<0.05	0.16	0.48	0.26
Nb	89.6	163	20.3	121	5.99	55.8	49.4		Gd	0.39	0.13	3.74	0.10	1.02	10.2	12.7
Ta	109	156	10.1	66.0	1.88	23.3	20.4		Tb	0.07	<0.05	0.49	<0.05	0.17	1.93	2.43
Sn	40.7	115	59.8	92.7	6.81	39.4	38.0		Dy	0.31	0.06	2.25	0.11	0.94	11.7	14.0
W	22.8	17.7	8.56	25.1	1.71	7.04	7.68		Ho	<0.05	<0.05	0.34	<0.05	0.18	2.02	2.29
Th	4.03	0.32	21.1	0.63	9.21	17.2	15.3		Er	0.10	<0.05	0.87	<0.05	0.46	5.44	5.76
U	3.58	5.68	7.07	6.77	9.38	4.64	8.06		Tm	<0.05	<0.05	0.11	<0.05	0.07	0.81	0.83
Zr	20.1	20.0	104	18.8	39.9	43.0	38.8		Yb	0.09	<0.05	0.72	<0.05	0.44	5.22	5.24
Hf	3.47	3.16	3.86	2.04	1.35	5.62	5.27		Lu	<0.05	<0.05	0.11	<0.05	0.06	0.68	0.66
Ti	92.9	95.4	1064	61.8	386	226	210		Y	1.08	0.35	10.7	0.79	5.98	89.2	95.6
Mn	1041	1024	458	1076	220	1713	928
Note: Ycjc-1–aplite; Ycszl-1–fine-grained trilithionite alkali feldspar granite; Ycszl-3–protolithionite-trilithionite alkali feldspar granite; Ycszl-5–fine grained trilithionite alkali feldspar granite; Ycxz-1–feldspar in pegmatite; Yccf-1–aplite; Yccf-2–aplite.

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In-depth research and experimental study on the effectiveness of this kind of massif-type lithium polymetallic deposits are in progress. Once it is confirmed that such deposit can be developed and utilized, it is necessary to propose the survey and evaluation techniques and industrial indicators that are different from pegmatite-type lithium mines and brine-type lithium deposits. It is worth noting that amblygonite may also exist in the small leucogranite in South China and the Himalayan leucogranite belt in the south of the world-famous Qinghai-Tibet Plateau, which may have been mistaken for quartz or other minerals by the naked eye before. In summary, this type of deposit has good development prospects.

2.3   Prospecting progress in Xinjiang Uygur Autonomous Region

Xinjiang Uygur Autonomous Region is famous for lithium not only for Li-ores but also for Li-industry. With the world-famous No.3 pegmatite vein in Koktokay, Altay, being exploited to the exhaustion of resources, it has become an urgent need for the sustainable development of the lithium industry to find new lithium deposits.

Besides the Altay Mountain, the Kunlun-Altyn Mountain located in the south is also a possible metallogenic belt for prospecting lithium deposits. Although few rare metal deposits have been found in previous years, some large to super-large scale rare metal deposits are discovered with the development of mineral exploration in recent years. Among these newly discovered deposits, the Dahongliutan is the biggest pegmatite-type lithium deposit in the West Kunlun, located on the south side of the Kangxiwa fault and the north side of the Dahongliutan-Guozha fault, where is the Triassic feldspar granite widely developed. For example, based on the systematic survey and data collection from 2012 then on, Tu QJ et al. (2019) carried out comprehensive research to determine the basic characteristics of the spodumene ore, track the latest exploration, and systematically carry out research on metallogenic conditions and metallogenic regularity. The ore-forming ages of rare metal deposits are the Late Triassic–Early Jurassic. The ore body is controlled by pegmatite veins.

2.4   Prospecting progress in Qinghai Province

Qinghai Province is one of the major producers of lithium with its brine type lithium resources to be the largest in China (Li RQ et al., 2018). However, hard rock type lithium resources cannot be compared with that in Xinjiang Uygur Autonomous Region and Sichuan Province. It was a relief that the spodumene-bearing pegmatitic dike swarm have been discovered in the Chakabeishan area on the northeastern margin of the Qinghai-Tibet Plateau. These pegmatite dikes are densely exposed and distributed in a narrow NW band along the northern side of the Southern Zongwulong Mountain fault (Fig. 8). So far, nine beryl-bearing spodumene pegmatite veins (contents of Li₂O from 1.11% to 3.13%,) have been found. Zircon U-Pb dating determined that the diagenesis and mineralization age was 217 Ma. The spodumene pegmatites have high SiO₂ (75.73%–77.34%), Al₂O₃ (15.58%–17.52%) and Na₂O (3.0%–3.16%), low K₂O (0.36%–0.79%) and total amount of REEs (5.3×10^–6 to 6.0 ×10^–6), as well as slight enrichment of light rare earth [(La/Yb)_N=3.1 to 4.6] with strong Eu negative anomalies (δEu=0.17 to 0.23). The study finished by Wang BZ et al. (2019) indicated that the Li-bearing pegmatites possibly originated from the remelting of the Paleoproterozoic crustal materials in the Quanji-block. The discovery of spodumene pegmatite in the Chakabeishan area inferred that there should be a new and important Li and Be metallogenic belt in the eastern part of Zongwulong tectonic belt, northern Qinghai-Tibet Plateau.

Figure  8.  Geological map of Chakabeishan area (a) and pegmatite distribution of Qiarangmalongwa area (b) (after Wang BZ et al., 2019). 1−gneiss rock group of Dakendaban Group; 2−schist rock group of Dakendaban Group; 3−marble rock group of Dakendaban Group; 4−Ganjia Formation; 5−Longwuhe Formation; 6−Quaternary; 7−Ordovician quartz diorite; 8−Ordovician gabbro; 9−Triassic adamellite; 10−Triassic granite porphyry; 11−pegmatite (undivided); 12−beryl and spodumene-bearing pegmatite; 13−beryl-bearing pegmatite; 14−geological boundary; 15−angular unconformity geological boundary; 16−regional fault; 17−fault.

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3.   Progress of research on lithium deposits in China

3.1   A genetic model of “multi-cycle, deep circulation, integration of internal and external”

Although the genesis of pegmatite deposits is controversial (Černý P, 2005; Linnen RL, 2012; Dill HG, 2015), it is generally believed that pegmatite is the product of crystallization differentiation of granite, which leads to the enrichment of rare metals in residual melt (London D, 2008; Deveaud S, 2015). The reason why genetic relationship between pegmatite and granite is not fully established is that: (1) Pegmatite does not coexist with peraluminous granite (Simmons WB et al., 1995, 2008); (2) Discontinuity of geochemical characteristics in granite-pegmatite system (Martins T et al., 2012); (3) There is a time gap between granite and pegmatite (Melleton J et al., 2012). The content of rare metals, different melting rates and tectonic background in the melting source region show that pegmatitic magma can be formed by the remelting of continental materials (London D, 2014).

In recent years, lithium isotopes have provided new evidence for the study of the genetic relationship between granites and pegmatites (Thomas R and Davidson P, 2014; Teng FZ, et al., 2006a, 2006b; Tomascak PB, et al., 1999). The available data show that granite has an uneven δ⁷Li value range from –3.1‰ to 6.6‰, while the δ⁷Li value of variable sedimentary rock range from –3.1‰ to 2.5‰, which is the possible source area of granite. The δ⁷Li value of pegmatite and granite (biotite, muscovite and lithium mica) range from –3.6‰ to 3.4‰, which is not affected by separation crystallization and crustal deep melting, in which the light δ⁷Li value comes from the variable sedimentary rocks (Maloney JS, et al., 2008). The biotite in granite and pegmatite has similar lithium isotopic composition, indicating that the extreme magmatic fractionation of peraluminous granite cannot form pegmatite. The lithium isotopes of salt lakes have also been studied, with the results show that the brine of Miramar Con, Canada, originated from seawater (Bottomley DJ, et al. 1999; Barnes et al., 2012; Godfrey LV, et al., 2013). Li in Hombre Muerto in the central Andes is mainly derived from nearby geothermal water and Li-rich water in volcanic sedimentary areas (Godfrey LV, et al., 2013). The lithium isotopic composition of different aquifers in the Puna area, Argentina, shows that lithium mainly comes from andesite, pegmatite and igneous clastic deposits (Orberger B, et al., 2015). These data suggest that Li in granitic pegmatite may be inherited from sedimentary rocks, and lithium-rich in sedimentary basins can also be provided by volcanism.

Lithium deposits in the world are mainly two kinds of occurrence states, the hard rock type lithium deposits and the brine type lithium deposits. The former is mainly pegmatite spodumene deposit, the latter is mainly Li-bearing brine (including surface and underground). The former is generally considered to be endogenic deposits, while the latter is generally considered to be exogenic deposits. However, the source of lithium in brine has not been solved, and the possibility of continuous supply of deep hydrothermal and thermal fluids cannot be ruled out. That is to say, “endophytic and exogenous formed outside and originated inside”. In particular, the article by Thomas RB (2017) published in Nature has aroused widespread interest in academia. Based on the comparison of lithium concentration of magma formed in different tectonic environments and the in-situ determination of trace elements in quartz melt encapsulation, it is considered that the magma with moderate to extreme enrichment of lithium is related to the participation of the crustal material of felsic continent. Li extracted from volcanic eruptions by precipitation and hot water fluids can be gradually enriched in the clay layer, deposited in the lacustrine phase of the crater. When the lithium concentration reaches a potentially economically available level, it can be used as a promising target for lithium exploration in western North America, such as the Cenozoic crater in Yellowstone Park and other inland environments where such magma is produced.

Generally, it is considered that there is no relationship between the hard rock type ore and the brine type ore, or there is no systematic work to study whether there is an inherent genetic relationship between them. Even spodumene deposits of hard rock type are often simply understood as spodumene deposits of granitic pegmatite type, which is not so simple. For example, most of the Li-bearing ores in Jiajika, Sichuan Province, are not pegmatitic, but “fine grain” (Figs. 5, 9). In other words, the fine size spodumene minerals of < 5 mm are the main minerals.

Figure  9.  Micrograph of the ore in the depth of 10 m and 219.4 m in ZK801, Jiajika Yakeke mining area, Sichuan Province. a−hemihedral spodumene with different grain sizes in granite pegmatite at 10 m depth; b−the apatite produced in the contact zone between alkali feldspar granite and pegmatite inserted into pegmatite at a depth of 219.4 m. Ab–albite; Ms–muscovite; Qtz–quartz; Spd–spodumene; Amb–amblygonite.

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High content of Li have been discovered in the Qaidam Basin and Sichuan Basin of western China, the Jianghan Basin of central China and the Jitai Basin of eastern China (with LiCl content ranging from 611 mg/L to 1136 mg/L) (Editorial Committee of Jiangxi Volume of Geology of Mineral Resources in China, 2015). Lithium also exists in Zhoutian Basin of Jiangxi Province, which is also large in scale. On the one hand, the source of lithium is the migration and enrichment of lithium-rich granite in the process of surface weathering; on the other hand, it does not rule out the possibility of deep lithium-bearing fluid replenishment. For example, the granite around Jitai Basin in Jiangxi Province is rich in lithium itself, so it is not surprising that lithium accumulated in the sedimentary basin after weathered. However, its high lithium content exceeds the general level of lithium content in the continental fault basin in the granite area, so the possibility of recharge with lithium-bearing hot brine along the deep fault could not be ruled out. The content of Li in clean water can still reach 1.24–2.56 mg/L after filtration in modern hot springs in the periphery of Jiajika area, Sichuan Province, which is higher than the national standard of lithium-bearing mineral water. In conclusion, the sources of lithium in brine are probably both epiphytic and deep.

As a general rule, the content of Li in sedimentary rocks is the highest (about 60×10^–6), even higher than that in granite (about 40×10^–6). Meanwhile, shale is the highest in sedimentary rocks (66×10^–6). The average content of Li in mudstone in eastern China is 38×10^–6 (Chi QH and Yan MC, 2007). The contents of lithium in Triassic sandstone, mudstone and slate formed by shallow metamorphism, as well as in staurolite schist, andalusite schist and cordierite schist under the action of granite thermal domes, are shown in Table 2. It can be seen that the regional enrichment of Li, in silty slate and sandstone in and around the Jiajika area in Sichuan Province can be further enriched in Li during thermal metamorphism, especially in cordierite schist. At the same time, rare metal elements including Be, Rb and Cs are also significantly enriched. This may be a mechanism of Li enrichment in the formation of thermal domes, but the abundance of Li in sedimentary rocks should be the material basis for mineralization. The following metallogenic sequence occurred: Lithium rich sedimentary rocks→Formation of lithium-rich metamorphic rocks by deep burial metamorphism→Lithium rich granite formed by deep melting of metamorphic rocks→Deep crystallization and differentiation of Li-rich granites to form Li-rich melts and fluids→Lithium rich melts and fluids intrude to form lithium ore bodies and lead to the enrichment of lithium in horned and altered surrounding rocks.

Table  2.  Contents (10^–6) of rare metal elements in sedimentary rocks in the outside of the Jiajika orefield.

Lithology	Li	Rb	Cs	Be	Nb	Ta
Regional Xikang Group Sand slate (n=4)	84.25	158.50	14.48	2.74	16.65	1.29
Schist series (min)	72.10	31.10	7.25	0.48	5.19	0.65
Schist series (max)	1673.00	3465.00	1165.00	152.00	106.00	174.00
Schist series (n=59)	608.15	310.16	186.71	8.93	17.95	5.60
Mica schist (n=16)	678.76	600.68	253.18	20.01	27.54	17.03
Andalusite schist (n=14)	402.15	186.16	122.66	4.64	15.12	1.31
Cordierite schist (n=13)	922.38	194.62	256.72	6.09	13.87	1.34
Cruciform schist (n=16)	462.48	222.00	119.40	3.93	14.14	1.37
Tourmaline hornstone in ZK1101 (n=2)	248.00	39.50	6.805	10.655	22.00	11.35
Altered surrounding rock in ZK1101 (n=7)	1412.57	893.14	307.57	29.03	15.29	2.10
Normal surrounding rock in ZK1101 (n=11)	535.00	172.27	53.88	3.31	16.82	1.45

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The sedimentary strata are generally concentrated distribution area of pegmatite deposits, especially of some large and super large spodumene deposits. These deposits at home and abroad tend to occur in argillaceous sedimentary rocks with a large proportion of shale or muddy sedimentary rocks, but rarely in carbonate rocks （Liu LJ et al., 2017; Wang DH et al., 2018; Xu ZQ et al., 2020）. It is reflected in the pegmatite deposits at home and abroad, such as the Koktokay in Xinjiang Uygur Autonomous Region, the Jiajika and Keeryin in Sichuan Province, the Kings Mountain in the United States, etc. It is also an important mineralization form in some bauxite deposits (Wang DH et al., 2013; Sun BD et al., 2019). This may be since lithium is easier adsorbed by clay minerals in the process of deposition. The components of mudstone and shale are mainly clay minerals. Lithium can be enriched again in the process of metamorphism, deep burying, remelting and granitization, and especially in the process of crystallization differentiation of granitic magma. The primary enrichment occurs in the sedimentary process, secondary enrichment in granitic process and tertiary enrichment in pegmatitization. Although it is a multi-stage metallogenic process, these three periods are not completed under the same and single tectonic background. The enrichment of lithium is the product of multicycle tectonic events, and it is formed in the metallogenic mechanism summarized as “multi-cycle, deep circulation, integration of internal and external”. The brine type deposit represented by “endogenesis to exogenesis” can be the product of weathering and denudation of granitic containing ore, i.e. In contrast, the hard rock type lithium deposit represented by “exogenesis to endogenes” can be the product of remelting and metamorphism of clay rock. This theory, which considers the multi-cycle enrichment mechanism of lithium as a whole, needs to be deeply studied, but it is of guiding significance for determining the prospecting direction.

The spodumene deposit is an important type of lithium deposit, but it’s prospecting has been discontinued because of its higher cost than the salt lake in extracting lithium. Recently, the demand for lithium has multiplied due to the rapid development of emerging industries, and the recovery of spodumene has become an important source of lithium resources. By summarizing some metallogenic characteristics of large and super-large spodumene deposits at home and abroad, it is concluded that certain particularities are always associated with large-scale spodumene mineralization. For example, spodumene ore deposit of pegmatite type can be hosted with basic rocks instead of granite, gneiss, schist and other common host rocks. The size of spodumene grain can be either coarse or fine in the pegmatite. The zonality of pegmatite veins containing spodumene can be of good or not. The shape of the pegmatite veins can be simple or extremely complex. The metallogenic epoch can be old in the Archean or be new in the Cenozoic. The metallogenic tectonic environment can be a stable platform or an active Himalay orogenic belt. It points out that prospecting can neither be confined to the periphery of the granite rock mass nor taking the Greenbushes in Western Australian or the Tanco in North American located in the old platform as the only case. It can’t only think about the complexity of Koktokay, Xinjiang Uygur Autonomous Region, and oversight other super-large deposits whose size can be ten times in large deposits but in a simple form such as the Jiajika and the Keeryin pegmatite fields in Sichuan Province. Also, it can not only consider the traditional geological method but also combine the actual situation to establish proper geophysical and geochemical prospecting models. As long as the specific problem is analyzed, the prospecting method is expanded, the exploration technique is used properly, and it is completely possible to obtain a new prospecting breakthrough.

3.2   Exploration model of “five levels + basement”

The basic concept and application scope of the “five levels + basement” exploration model in the tungsten mining area has been expounded in the literature (Sheng JF and Wang DH eds., 2017). However, it is still a new subject whether the exploration model of “five levels + basement” applies to rare metal mining areas. In the traditional metallogenic theory of pegmatite deposit, it is difficult to find the mature experience in exploration technology and method of “layer-controlled” or layered large-scale pegmatite deposit. Most of the existing literature on pegmatite always emphasizes the “zonation” of pegmatite vein itself, but seldom considers the internal relationship of pegmatite in different occurrence states and its guiding significance for exploration. According to the continuous practice in the Jiajika pegmatite field in Sichuan Province for nine years and a large number of phenomena denudated in the Keeryin pegmatite field (Fig. 10), this paper summarizes the exploration model of “five levels + basement” in the rare metal mining area (Fig. 11). The key is that in addition to looking for vertical pegmatite veins (similar to “five levels” in the Nanling tungsten mine, south China), it is also necessary to find the pegmatite veins with the near horizontal occurrence and roughly bedding occurrence. According to such an exploration model and prospecting idea (Wang DH et al., 2016; 2017c), new prospecting progress has been made in the Keeryin orefield and the Jiajika orefield, Sichuan Province. The prediction based on this model has been verified by YZK001 and YZK002 drills in the Yakeke area in the southeast of Jiajika orefield. The thickness of the lithium ore body in YZK001 is more than 63 m.

Figure  10.  The basement of the nearly horizontal pegmatite vein in the Keeryin orefield, Sichuan Province. The pegmatite vein integrated with the schist stratum with the stratum occurrence.

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Figure  11.  Jiajika-style of the “five levels+basement” model for prospecting in western Sichuan (after Wang DH, et al., 2017c). I–microcline pegmatite zone; II–microcline-albite pegmatite zone; III– albite pegmatite zone; IV–spodumene pegmatite zone; V–lapidolite-muscovite pegmatite zone. ρ_Li–Li mineralized pegmatite; ρ_Be–Be mineralized pegmatite; ρ_NbTa–Nb-Ta mineralized pegmatite; ρ_q–quartz in pegmatite.

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4.   Progress on technology for exploration lithium deposits

Although lithium deposits have been mined for more than five decades, the methods of exploration, including geological survey, geochemical exploration, geophysical exploration, remote sensing and drilling, have yet not caught up with the increasing development of the industry. The authors tried to make some improvements.

4.1   Geological survey

As mentioned in the book General Summary Volume of The Discovery History of Mineral Deposits of China (Editorial Committee of The Discovery History of Mineral Deposits of China, 1996), almost all pegmatite deposits in China were found by traditional geological survey methods before 1996. However, nearly all kinds of geochemical or geophysical or other modern methods, not only digital geological methods but also remote sensing methods and others, have been applied to the exploration field in recent years for lithium deposits.

4.2   Geochemical exploration

In the Jiajika orefield, the exposed strata are only Triassic and Quaternary series. It is more and more difficult to continue to explore the rare metals due to the Quaternary coverage accounts for more than 80% in the Jiajika orefield. Although lithium (Li), beryllium (Be), rubidium (Rb), cesium (Cs) and other rare metal elements account for a small proportion, they all have strong chemical activity. Especially, lithium is easily adsorbed by clay minerals, caused that lithium content increased in soil, and easy to form the secondary halo. In recent years, the 1∶10000 soil geochemical survey method plays a key role in the discovery of the No.X03 vein characterized by a concealed vein. The average content of Li in the soil above the No.X03 vein is 110.6×10^–6, which is twice that of strata in the Jiajika orefield and 3.4 times that of the average content of the soil around the whole country (Chi QH et al., 2007). The geochemical survey method provides a demonstration for the prospecting of the periphery and the same type deposits in the Jiajika orefield (Xiao RQ et al., 2018).

4.3   Geophysical exploration

Geophysical exploration is not a common method used for rare metal deposits, because the difference between ores and host rocks is not big enough to be identified from each other usually. However, if the orebody is big enough, the difference might be measured by the modern geophysical exploration method with the increasing precision. Based on many years of practice, this paper systematically summarizes the comprehensive geophysical characteristics of the No.X03 pegmatite vein, such as gravity, magnetic, electrical and audio magnetotelluric methods. Being the largest Li-orebody in the Jiajika orefield, the No.X03 pegmatite vein has the following comprehensive geophysical characteristics: (i) The gravity survey results show that it is located in the Bouguer gravity gradient belt and the residual low gravity anomaly area; (ii) The magnetic survey results show that it is located in the geomagnetic pole gradient belt and the magnetic source low gravity anomaly area; (iii) The electrical survey results show that the high-power induced polarization (IP) middle ladder results located in the north-south high anomaly distribution area of apparent resistivity; (iv) The sounding results show that it is located in the shallow high anomaly area of apparent resistivity, deep anomaly area with more large-scale and stronger apparent resistivity anomalies. These geophysical characteristics are consistent with the geological characteristics of the vein body such as scale, burial depth and extension length. At the same time, it is found that the known ore vein bodies are mainly distributed in the north-south direction. Therefore, the high anomaly of apparent resistivity in the north-south direction is the focus of the next exploration. It provides a direction for further prospecting in the deep part of the Jiajika orefield and lays a theoretical and methodological foundation for geophysics.

4.4   Remote sensing

Being one of the richest areas of Li resources in China and even in the world, geological mapping and ore prospecting study using remote sensing technology have been conducted in the Jiajika orefield. Spectra of typical rocks and minerals collected in the Jiajika orefield were measured by using ASD FieldSpec-4 spectroradiometer. Then, spectral library of typical rocks and minerals collected in this area was built, and spectral characteristics of rocks and minerals including biotite schist, grenatite schist, granite-cordierite schist, hornstone, granite, pegmatite containing spodumene, pegmatite without spodumene, quartz vein, feldspar, monocrystal spodumene, biotite, aquamarine were analyzed by Dai JJ et al. (2018). The results indicate that there are great differences between the spectra of rock mass and surrounding rocks. Because the reflectance of the surrounding rock is less than 0.2, causing the absorption features are not obvious. There are three different absorption features of the rock mass at 1413 nm, 1911 nm and 2197 nm due to the higher reflectance (less than 0.5). Also, granite, pegmatite containing spodumene, pegmatite without spodumene can be discriminated by their unique spectra on 1413 nm and 2197 nm. Then geological mapping and ore prospecting were studied based on image processing and interpretation of middle spatial resolution remote sensing data Landsat 8 and high spatial resolution remote sensing data Geoeye-1. The results (Fig. 12) indicate that remote sensing technology will be instructive for geological mapping and ore prediction for the Jiajiaka-type lithium ores (Dai JJ et al., 2017, 2019).

Figure  12.  The interpretation image of pegmatite boulders and ore prospecting areas in the Jiajika area, Sichuan Province.

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4.5   Untraditional methods

4.5.1   X-ray fluorescence (XRF) methods

Most of the pegmatites have a high content of K, while the rare metal-bearing pegmatites have a high content of Na. It is easy to measure the element K in the field by XRF methods. For example, by comparing the XRF spectral lines of the Jiajika ore specimen with the contents of rare metals (including Rb, Nb, Y, Li) and the contents of elements riched in hostrocks (such as Fe, Sr, Zr), it is clear that the XRF test results can be used as the basis for distinguishing the ore from non-ores. Applying this research result, a full-hole XRF spectral measurement was performed on the three boreholes in Jiajika, and the possible mineralized layers were given one by one. Core sampling analysis in the late stage in the exploration area confirmed the reliability of the XRF spectral measurement results. Statistics show that, by using XRF spectral measurement as a guide, about 30% of the sample analyze work by chemical methods can be reduced, and the economic benefits are significant (Zhou SC et al., 2018).

4.5.2   Geogas survey methods

In the conventional geophysical exploration method for pegmatite lithium mineral exploration, there exist the problems that the difference between the orebody and the surrounding rock is not significant, the application effect is poor, and the geochemical exploration method has no obvious effect on the concealed lithium deposit. Because of that, Yang JC et al. (2019) investigated the technical problems related to the application of Geogas method in the exploration of pegmatite lithium deposits. With Kalu’an in Xinjiang Uygur Autonomous Region as the study area, they deployed two geogas research sections with a total length of 6605 m, and completed the case study of pegmatite lithium deposits with 191 survey points, which was performed for the first time both in China and abroad. The contents of 39 elements such as Li, Rb, Be, Cs, Nb, and Y were determined. On such a basis, the characteristics of geogas anomalies in the Kunlun pegmatite lithium deposit were studied. The results show that significant gas anomalies containing Li, Rb, Be, Cs, Nb, Y, and other elements can be captured on the tilted side of the pegmatite lithium veins. That means, above the barren pegmatite, the abnormal amplitude of geogas in the Rb element will increase, and the anomaly amplitude of other elements will obviously decrease, with no gas anomaly, and that, in other lithologies, there is no abnormal geogas. It is confirmed that the geogas survey method is accurate and effective in the search for pegmatite lithium deposits.

4.5.3   Biological methods

The scientific investigation of biological methods of prospecting for minerals can be traced to about the 1950s and the first comprehensive English book Biogeochemical Methods of Prospecting (Brooks RR, 1986). As one of the hot subjects in the world, biochemistry can help geologists to understand and explore the geological and geochemical processes on the earth with new theories and methods (Brooks RR, 1990). This is the first time that the authors can develop the biological method in the Jiajika orefield. Is has a significant impact on the research and application in the field of resources and environment, including geobotany (visual examination of the vegetation cover), biogeochemistry (chemical analysis of vegetation), geozoology and microbial prospecting. Since 2016, observational evidence with more than 276 biological samples reflects accumulated years of experience, which show that biological methods of prospecting for minerals could be effectively applied to prospect concealed Li and Be rare metal ore deposits under covers (Yu Y et al., 2019). With the deepening and development of research, biological methods of prospecting for minerals might play a more important role in the development of theory and technology for the lithium deposits survey in China.

4.5.4   Hydrogeochemical exploration method

Hydrogeochemical exploration methods were applied widely in China from the 1950s (Chen Y, 1958), which has the characteristics of easy to handle the sampling, easy to trace the source, long spread distance of chemical anomaly. In the previous study, hydrogeochemical exploration methods were widely used to explore gold, uranium, copper, lead, zinc, nickel, molybdenum and cobalt deposit and obtained excellent consequent (Ye ZX and Wei QH, 1962; Zhang WM et al., 1996).

The authors applied hydrogeochemical exploration methods in Jiajika orefield for the first time to clarify whether it is effective for rare metal deposits. From 2016 to 2018, 88 surface water samples of Jiajika orefield were collected to investigate the enrichment rules of rare metal elements such as Li, Be, Nb, Ta, Rb, Cs, Zr and Hf. The results of rare metal concentration in surface water in Jiajika orefield revealed that the average concentration of rare metal elements such as Li, Be, Rb, Nb, Cs and Ta is much higher than the average value of rivers in western Sichuan Province, and the Sr concentration is lower than that of western Sichuan Province. The concentration of Li, Rb, Cs, and Be in the surface water of the rare-metal-vein-bearing area are significantly higher than those in the rare-metal-vein-free area, indicating that the distribution of rare metal veins has a significant effect on Li, Rb, Cs, and Be in surface water. Based on these, the authors conclude that the enrichment of Li, Be, Rb, Cs in surface water can be effectively used to prospecting mineral resources of Li, Be, Rb and Cs (Gao JQ et al., 2019).

5.   Conclusions

More and more data show that lithium mineralization is cyclic and endophytic, that is, lithium in brine can be supplemented by deep hot brine, or absorbed and enriched by clay minerals in sedimentary basins after surface Li-bearing granite or other Li-bearing rocks weathered. The sedimentary rocks rich in lithium can form internal lithium deposits through deep burying, remelting, granitic and crystallization differentiation, thus forming the metallogenic mechanism of “multi-cycle deep cycle endophytic integration”. Based on the exploration model of “five levels + basement”, in western Sichuan and Mufushan-Jiuling ore concentration area in Hunan Province, Hubei Province and Jiangxi Province, attention has been paid to the search for layered pegmatite lithium deposits in the areas widely distributed in the cut vein, and good results have been obtained.

Lithium, beryllium, niobium, tantalum and other rare metal mineral resources are often associated with each other, such as the famous Koktokay No.3 vein in Xinjiang Uygur Autonomous Region, China. However, separate mineralization often occurs, or rare metals are associated with other minerals. The most typical examples of the former, such as Qinling-Dabie emerald deposits and emerald deposits in southeastern Yunnan Province (Zhu JT, 1992), are associated with scheelite deposits, while the latter are co-associated with beryllium and uranium deposits in Baiyanghe, Xinjiang Uygur Autonomous Region. Among them, several spodumene veins have been found in Guanyinqiao area of northwest Sichuan Province, and thick orebodies have also been found in Jiajika in Sichuan Province. Furthermore, amblygonite, Ta-rich cassiterite, columbite, tantalite ore and other industrial rare metal minerals are found for the first time in fine-grain granite, alkali feldspar granite and pegmatite vein in Jiuling area. These discoveries strongly prove that the deep prospecting effect of the Renli tantalum-niobium deposit in the Mufushan area can continue to expand.

Acknowledgments

This research has been supported by the China National Key Research and Development Program during the “13th Five-year Plan Period” (2017YFC0602700), the Geological Survey Projects of China Geological Survey (DD20160056, DD20160346), the Major Project of National Social Science Fund “Research on the supply risk management mechanism for China’s strategic three-rare mineral resources” (19ZDA111). The authors are grateful for the constructive comments from the reviewers.