Geochemical characteristics and genetic type of a lithium ore (mineralized) body in the central Yunnan Province, China

Bi-dong Sun, Jun-ping Liu, Xio-hu Wng, Yn Do, Gui-xing Xu, Xio-zhung Cui,Xue-qing Gun, Wei Wng, Dong-hu Song

a Yunnan Institute of Geological Survey, Kunming 650216, China

b Kunming University of Science and Technology, Kunming 650093, China

c Chengdu Center, China Geological Survey, Ministry of Natural Resources, Chengdu 610081, China

Keywords:

Sedimentary-type lithium deposit

Stratified deposit

Geochemical characteristics

Metallogenic mechanism

Mineral resources exploration engineering

Yunnan Province

China

A B S T R A C T

Lithium ore (mineralized) bodies in the area A of central Yunnan Province belong to a sedimentary-type,which are controlled by stratum.The studied ore (mineralized) body mainly occurs in the Middle Permian Liangshan Formation.This work described the morphology, structures, main ore types and geochemical characteristics of this ore body in detail, and discussed the ore-forming material source, occurrence state of lithium and the formation mechanism of lithium ores to clarify the prospecting marks.In the further exploration, comprehensive evaluation of the lithium resources of known bauxite ore bodies in central Yunnan Province should be strengthened, and the exploration of hidden lithium ore bodies should be intensified in order to discover more large and super-large lithium orebodies, which will fill the gap of the national demand for lithium resources, and promote the national defense construction and new energy industry development.

1.Introduction

Lithium, as a new and important energy strategic metal,has played a significant role in the field of lithium batteries,new energy vehicles, controllable nuclear fusion, etc.(Liu LJ et al., 2017).It is known as the energy metal of the 21stcentury, and is also an important strategic resource of our country (Peng AP, 2012; Li JK et al., 2014; Zheng RR et al.,2016; Zhou SF et al., 2017; Xu ZQ et al., 2018).China is rich in lithium ore reserves, ranking the third in the world.China is suggested to contain proven lithium resources amounting to about 7 million tons, accounting for 17.2% of the world’s total resources.However, China is also the largest lithium consumption country, accounting for 47.3% of the world’s total consumption, and the external dependence reaches up to 84.5% (Wang QS et al., 2015; Fan J, 2016; Zhang C et al.,2017; Wang DH et al., 2017).With the rapid development of lithium-related emerging industries such as new energy vehicles, the gap of lithium resources in China is growing,which has caused great potential safety hazards in the supply of lithium (Deng ZH et al., 2016).Therefore, it is urgent to strengthen theoretical research on different types of lithium deposits and to search for large and super-large lithium deposits.

This work comprehensively analyzed the mineralogy and lithogeochemistry of the newly discovered sedimentary lithium ore (mineralized) body in the Middle Permian Liangshan Formation of central Yunnan Province.It is considered that this ore body belongs to a sedimentary-type.The initial constituents are mainly weathering crust materials from basic volcanic rocks, with small amounts of calcium lateritic weathering crust material from bottom carbonate rocks.The mineralization process mainly includes two stages:sedimentary stage and reformation stage.These new understandings are of great significance to supplement and improve the metallogenic theory of sedimentary lithium deposits and to guide regional prospecting.

2.Metallogenic geological background

The study area is located in the coastal Pacific metallogenic domain-Upper Yangtze (continental block)metallogenic Province-the south-central Yunnan metallogenic belt (Fig.1).The regional basement is a double basement: the Paleoproterozoic basic volcanic rocks + epimetamorphic rocks, and the fold basement of Mesoproterozoic epimetamorphic rocks.The fold beds of Mesoproterozoic epimetamorphic rocks mostly cover above the Paleoproterozoic basic volcanic rocks + epimetamorphic rocks series in the form of thrusting nappe, and most of the Paleoproterozoic basic volcanic rocks+epimetamorphic rocks series in the local area are exposed in the form of structural windows (Geological Survey of Yunnan, 2018).The Lüzhijiang fault, Qujing-Zhaotong fault and Maile-Shizong fault control the western, eastern and southern boundaries of the Xikang-Yunnan axis, respectively, while the Pudu River fault and the Lüzhijiang fault control the eastern and western boundaries of the Wuding-Yimen secondary uplift area.Before the mid-Mesozoic, the central Yunnan region had experienced many large-scale transgression-regression processes, and developed most sedimentary strata from the Late Proterozoic to Early Mesozoic.Lithium ore bodies in this area occur in the Permian Liangshan Formation (P2l)(Geological Survey of Yunnan, 2013).The outcropped basic and acid intrusive rocks in the study area are controlled by several major deep fracture zones in the area (Yunnan Bureau of Geology and Mineral Resources, 1990).The basic intrusive rocks are dominated by Paleoproterozoic-Mesoproterozoic diabase, with a small amount of Indosinian diabase, mostly occurring in dikes and apophysis.The acidic intrusive rocks are dominated by Neoproterozoic granite, and occur in batholith and rock pillar.A small amount of Mesoproterozoic and Paleoproterozoic granites occur mostly in dikes and apophysis.

Fig.1.Map showing regional geology and sampling location of basic volcanic rocks in central Yunnan.1-Quaternary; 2-Neogene; 3-Triassic-Cretaceous/Jurassic/ Paleogene; 4-Permian; 5-Emeishan Basalt Formation; 6-Yangxin Formation; 7- Liangshan Formation; 8-Sinian-Carboniferous; 9-Huanglong Formation; 10-Zaige Formation; 11-Haikou Formation; 12-Qiongzhusi Formation; 13-Yuhucun Formation;14-Dengying Formation; 15-Kunyang Group; 16- Dongchuan Group; 17-Yimen Group; 18-basalt; 19-diabase; 20-granite; 21-lithium-rich aluminum ore; 22-fault; 23-stratigraphic boundary; 24-parallel unconformity boundary; 25-angle unconformity boundary; 26-sampling location; 27-cities; 28-counties.

The outcropped strata in this area are the Upper Sinian Dengying Formation (Z2dy), Lower Cambrian Yuhucun Formation (1y), Lower Cambrian Qiangzhusi Formation(1q), Middle Devonian Haikou Formation (D2h), Upper Devonian Zaige Formation (D3z), Carboniferous Huanglong Formation (Ch), and the Middle Permian Liangshan Formation (P2l) and Yangxin Formation (P2y).The regional structure is relatively simple.The strata are generally monoclinic, and the fault structure is NW-SE-trending, which is composed of two Himalayanian northeast-trending thrust faults.The lithium ore body occurs in the Middle Permian Liangshan Formation (P2l) (Geological Survey of Yunnan,2013; Fig.2 and Fig.3), which is strictly controlled by stratum and has a gentle dip angle of 10°-20°.

Fig.2.Stratigraphic correlation of the Permian Liangshan Formation in the study area.

Fig.3.Structural characteristics of the lithium ore body and the sampling location in the area A of central Yunnan.1-carbonaceous mudstone;2-aluminous clay rock; 3-brecciated bauxite; 4-cardamom bauxite; 5-dense bauxite; 6-powder-fine crystalline dolomite; 7-dolomite limestone; 8-powder-mud crystal limestone; 9-limonite; 10-Huanglong Formation; 11-Liangshang Formation; 12-Yangxin Formation;13-fault boundary;14-sampling location; 15-bio-chamber limestone.

3.Features of lithium ore (mineralized) body

3.1.Morphology, occurrence and scale of the ore(mineralized) body

The lithium ore body mainly displays layered, lenticular and depression-like shapes.Layered ores are formed in relatively gentle paleokarst depression areas, funnel-like and depression-like ores are formed in funnel and depression-like paleogeomorphic areas, and lenticular ores occur in the transitional areas of the above two paleogeomorphologies.The thickness of the Liangshan Formation is controlled by paleo-weathering denudation surface, being larger in karst depression and smaller in other positions (Fig.2 and Fig.3).Lithium ore (mineralized) body mainly occurs in karst depression, and the thickness of ore (mineralized) body is positively correlated with the thickness of Liangshan Formation.In the paleokarst depression, both the Liangshan Formation and ore (mineralized) body have a large thickness,with the highest average lithium grade.The Liangshan Formation becomes thinner at the paleotopographical uplift,and the ore (mineralized) body thins and the average lithium grade decreases.

The lithium ore body (Fig.3) has a relatively simple structure, showing a “three-storey” pattern.The bottom is iron clay rock (locally developed lenticular limonite), with a generally very low Li2O grade; the middle is lithium-rich bauxite, with an average high grade of Li2O at 0.74%, which is the main ore-bearing layer of lithium ores; the upper is aluminous claystone, with a medium average grade of Li2O at 0.59%.Horizontally, with the thinning of orebody thickness,the vertical ore-bearing strata will gradually thin, and the average grade of Li2O in the orebody will also decrease until the whole orebody pinches out.

The lithium ore (mineralized) body extends more than 6 km along the strike of the stratum, and its thickness is mostly 0.7-2.0 m.The thickness of the local area can reach up to 9-11 m, with an average thickness of about 1.5 m.

3.2.Ore types

The main ore types of the lithium ore body in the area A are light gray lithium-rich oolitic bauxite, light gray lithiumrich brecciated bauxite, gray-green lithium-aluminum-rich clay rock, gray-green iron clay rock, yellow-brown basalt(iron-alumina) clay rock, etc.

Light gray oolitic bauxite (Fig.4a-Fig.4c) has oolitic and massive structures, consisting of round, elliptical aluminum soybean grains (15%), ooid (33%) and aggregates (15%) of varying sizes (from 0.2 mm to 7.0 mm), with a small amount of aluminous breccia (10%) and iron (5%), argillaceous lenses(5%) and chlorite oolitic grains (5%).A small amount of diaspore and kaolinite clay filled the fissures, showing a porous cementation.Some altered feldspar debris is commonly observed, mostly in an irregular angular shape,few in subhedral plate shape, mostly leached into voids.The size of oolitic and soybean grains varies greatly.Concentric lamellar structure is shown due to the difference of color composition.The lamellar structure with high iron content is brown yellow, and that with high aluminum content is light yellow.

Fig.4.Petrographic characteristics of major rocks and ores from the lithium ore body.a-c-characteristics of oolitic bauxite hand specimen and oolitic and soybean grain structure (showing concentric lamellar structure, being brown black for iron-rich lamellar layer and light yellow for aluminium-rich layer); d, e-hand specimen and textural characteristics of brecciated bauxite; f-textural characteristics of aluminous clay rocks mainly composed of cryptocrystalline illite minerals; g-limonite occurs as spotted and raspberry-like aggregates in ferrous clay rocks; h,i-residual subhedral granular plagioclase phenocrysts in ferric aluminum (basaltic) clay rocks and the characteristics of residual strongly altered micro-slab plagioclase microcrystals with a disorderly distribution to form a frame; j-tourmaline minerals exist in oolitic bauxite; k-strongly altered euhedral plagioclase phenocrysts in oolitic bauxite; l-angular basaltic volcanic clasts or basaltic vitreous clasts in brecciated ores;m-strongly altered euhedral plagioclase phenocrysts in brecciated bauxite; n-strongly altered medium- to fine-grained plagioclase phenocrysts in basaltic (iron-alumina) clay rocks; o-residual strongly altered micro-slab plagioclase microcrystals in basaltic (iron-alumina) in clay rocks,with a discord distribution to form a frame; Chm-Chamosite; Dsp-Diaspore; Bhm-Boehmite; Lm-Limonite; Ill-Illite; Pl-Plagioclase;Tur-Tourmaline.

Light gray brecciated (slump breccia) bauxite displays brecciated and massive structures.The breccia shows irregular fragmentation, with different particle sizes, single composition.It is mainly composed of bauxite and illite(80%), part transformed into boehmite, and with disorderly distribution, iron cementation and porous cementation, which may result from weak consolidation of aluminum strata by collapse.

Light gray brecciated bauxite (Fig.4d, Fig.4e) has brecciated and massive structures.The breccias (30%-40%)are irregular, and have greatly varying size.The composition of the breccias is complex, mainly diaspore breccias, with a small amount of aluminous mudstone and basalt claystone breccias, etc.The cements are aluminous-iron clay with a certain amount of feldspar debris by residual fragmentation and strong alteration.

Gray-green aluminous clay rock (Fig.4f) has cryptocrystalline texture and massive structure.It is mainly composed of cryptocrystalline clay minerals (illite) (95%),mixed with a certain amount of cryptocrystalline aluminous(boehmite) and a small amount of cryptocrystalline star-like,fine disseminated iron oxides, with a small amount of strong kaolinization feldspar debris (5%).

Gray-green iron clay rock has cryptocrystalline texture and massive structure.It is mainly composed of cryptocrystalline illite (hydromica) clay minerals (75%),mixed with a certain amount of aluminum and iron (20%) and a small amount of strong clavization feldspar clasts (5%).Some displays subhedral platy structure, and has undergone weathering and abscission process.The iron in the rock mostly occurs in spotted and strawberry-like aggregates(Fig.4g).

Yellowish-brown basaltic (ferric aluminum) claystone(Fig.4h, Fig.4i) has residual porphyry texture and massive structure, mainly composed of intermixed brown-yellow cryptocrystalline iron, aluminum and argillaceous content(75%), with a certain amount of fine-grained strongly altered plagioclase porphyry (15%) and strong clavization.Locally,residual strongly altered plagioclase microcrystals (10%) are distributed in disorder to form a framework, and the original rock is basalt.

4.Samples collected and analytical methods

This study systematically collected samples from the orehosted Liangshan Formation, underlying Huanglong Formation and the eastern adjacent basic volcanic rocks (Fig.1 and Fig.3).Among them, 20 lithium ore samples were collected from the middle and upper part of lithium ore(mineralized) body in the study area A; nine clay rock samples were collected from the upper roof; five ferroaluminous (basaltic) claystone samples were collected from the lower Liangshan Formation and the lower floor of the lithium ore body in the study area; three limonite samples were collected from the lower part of the lithium ore body;and 46 basic volcanic rock samples were collected from basalts of different periods exposed in the form of structural window in Kangdian ancient land in the eastern adjacent area.Therefore, the samples collected in this study are representative and can meet the requirements of this study.

Table 1.Main element content (%) andimportant characteristic value of test samples.

Table 2. Testing rare earth element content (×10-6) in samples.

Table 2.(Continued)

Table 3.Content (×10-6) of trace elements in samples.

Fig.5.Correlation diagram of Al2O3-SiO2 of ores (lithium-rich breccia bauxite, lithium-rich dense bauxite, soybean meal bauxite,lithium-rich soybean meal bauxite, lithium-rich aluminum clay rock).

6.1.2.Geochemical evidence

Aluminum and titanium are very stable and difficult to dissolve under supergene conditions, which can be accumulated as residues (Li QJ et al., 1996).The titanium ratios of ores (rocks) with the same ore-forming source are generally very close, which can reflect the provenance characteristics (Mameli P et al., 2007).In this study, the titanium ratios of various lithium-rich ores in the lithium ore(mineralized) body in the area A are distributed in a small range of 17.96-23.26 (Table 1), indicating that they may have the same or similar provenance.

Ti, V, Sc and Fe are typical element symbiotic assemblages of basic rocks (Jiang JY et al., 2006).The strong enrichment of these elements in ore-bearing beds may inherit the high content characteristics of corresponding elements in basalt, indicating that the basalt in adjacent areas may contribute greatly to lithium mineralization.The elements of Hf, Zr, Th, Nb, Cr and Ta have relatively stable geochemical properties (Panahi A et al., 2000), and their oxides are very stable and insoluble under supergene conditions.The ratios of these stable elements are similar to those of the original rocks,and can be used to trace provenance (Calagari AA and Abedini A, 2007).Therefore, the ratios between them can be used to explore the material sources and geological environment in the process of diagenesis and mineralization(Jin ZG et al., 2009).Compared with the continental upper crust, the lithium-rich ores in the lithium ore body of the area A are more enriched in V, Zr, Hf and Th, which is positively correlated with the content of Al2O3(Fig.6).This indicates that these elements have not migrated during the formation of lithium deposits.These elements have very similar geochemical activities, belonging to a group of stable elements, and their ratio in lithium ores is similar to that in parent rock (Wang ZZ, 1997).

Fig.6.Correlation diagrams between Al2O3 vs.V, Zr, Hf, Th in different types of lithium ore samples

Some scholars believe that the source of lithium ore(mineralized) body in the Permian Liangshan Formation of central Yunnan is the product of calc-lateritization of underlying carbonate rocks.With the development of mantle plume tectonic theory and the new understanding of the tectonic framework and stratigraphic assemblage of Kangdian ancient land fold basement (Li J et al., 2018) and the continuous enrichment and development of metallogenic theory, we believe that the lateritization of Carboniferous carbonate rocks in central Yunnan is not enough to be the only mineralizing material source of lithium in the paleo weathering crust at the C/P interface.It is because that the erosion time of strata in the study area is relatively short, the amount of eroded-leached carbonate rock is limited, and the concentration of Li in eroded-leached carbonate rock is low(1.69×10-6-67.8×10-6).Calcium lateritization of carbonate rock with limited volume and low Li concentration is difficult to form thick weathering crust bauxite material and high concentration of lithium.Therefore, whether the calclateritization of the underlying carbonate rocks of Liangshan Formation in central Yunnan can provide sufficient sources of aluminum and lithium is worth further discussion.

The stable element ratio (such as Ti/Zr) in sedimentary deposits can be used to determine parent rock (Maclean WH,1990).When these ratios fall on the focus, the numerical points show a linear array highly correlated with the parent rock (Nesbitt HW, 1979).In Zr/Hf and Nb/Ta diagrams (Fig.7),the corresponding data ratios of the lithium-rich ore samples are linearly related to those of basalt and bottom carbonate rock samples in the adjacent area, which all basically fall in the weathering line (WL) fitted by the sample points of lithium-rich bauxite.However, the sample points of the bottom carbonate rocks and the Mesoproterozoic basic volcanic rocks in eastern adjacent areas are near the origin of coordinate system, far from the points of lithium-rich bauxite samples, while those of the Paleoproterozoic basic volcanic rocks in eastern adjacent areas is closest to lithium-rich bauxite samples (Fig.7).It is indicated that the formation of the lithium ore body in the area A is related to basalts in the upper adjacent areas and underlying carbonate rocks reflected by lithofacies and palaeogeography.The ore-forming materials are multi-source, and the Paleoproterozoic basic volcanic rocks in the adjacent area are more closely related to the ore body.

Fig.7.Binary diagram of Zr vs.Hf and Nb vs.Ta.

The occurrence, enrichment and distribution of REEs are mainly controlled by the stability of primary minerals(especially by REE-rich accessory minerals), mineral composition, occurrence state of mineral elements, chemical weathering degree and pH and Eh values in parent rocks(Nesbitt HW, 1979).During the supergene and late diagenesis of sedimentary rocks, LREEs show similar activity, except for individual elements such as Eu and Ce which were affected by Eh value; HREEs show similar activity, and display some enrichment during supergene process due to the weak activity.Therefore, it is of great reference value to determine the provenance of sedimentary lithium deposits by REE assemblage and distribution curves.

The REE-La/Yb correlation diagram(Fig.8) can roughly distinguish the sources of different aluminum ores (Li PG et al., 2012).Fig.8 indicates that the different types of ores in the lithium ore body mainly come from basalts.

Fig.8.REE-La/Yb diagram of lithium ore sample.

The chondrite-normalized REE curves of lithium-rich ores and bottom carbonate rocks in the area A and basalt samples in the eastern adjacent area (Fig.9) show the following characteristics.(1) the distribution curves of the rocks and ores samples are enriched in LREE; (2) lithium-rich ores in lithium ore (mineralized) body are more enriched in HREE than other samples; (3) compared with bottom carbonate rock samples, the assemblage of LREE (except Eu and Ce) in ores of the lithium ore (mineralized) body is closer to that of different periods of basalt samples in eastern adjacent areas.Among them, the LREE assemblage of lithium-rich oolitic bauxite samples is most similar to that of Paleoproterozoic basalts in the eastern adjacent area, and that of lithium-rich brecciated bauxite and lithium-rich compact aluminous clay samples are most similar to those of Paleoproterozoic basic volcanic rocks in the eastern adjacent area.

Fig.9.Chondrite-normalized REE distribution patterns of different types of lithium ore, bottom carbonate rock and basic volcanic rock samples in adjacent areas.

In summary, the composition of this lithium ore body mainly comes from the Paleoproterozoic basic volcanic rocks in the eastern adjacent area.

This conclusion is well confirmed by the binary diagrams of stable elements Zr-Hf, Nb-Ta and the primitive mantle and upper crust normalized distribution curves of trace elements.The binary diagrams of stable elements (Zr-Hf, Nb-Ta)further show that the Paleoproterozoic basic volcanic rocks in the study area provide more ore-forming materials compared with the Mesoproterozoic basic volcanic rocks and the bottom carbonate rocks.

Based on the geochemical analysis of different types of lithium-rich ores, bottom carbonate rocks and basic volcanic rocks in the main period of the eastern adjacent area, the following conclusions were drawn.(1) The ore-forming material source of the lithium ore body is not single rather than multi-sources; and (2) the main source should be the Paleoproterozoic basic volcanic rocks outcropped on the Kangdian ancient land in the upper reaches of the region inferred from lithofacies palaeogeography in the eastern part of the lithium ore body, with a small amount from the Mesoproterozoic basic volcanic rocks and the bottom carbonate rocks.

6.2.A preliminary study on the types of ore (mineralized)bodies

In order to analyze the occurrence state of lithium element, we quantitatively analyzed the minerals of five lithium-rich bauxite-type ore samples from the ore(mineralized) body by means of whole-rock X-ray powder diffraction (XRD) as shown in Fig.10.The test results show that the lithium-rich ores in the lithium ore body are dominated by diaspore, boehmite, oolitic chlorite, kaolinite,plagiochlorite, mica/illite and anatase.The powder X-ray diffraction patterns of each sample were fitted and analyzed quantitatively to obtain the mineral composition and content shown in Table 4.It can be seen that kaolinite and clinochlore are the main clay minerals of brecciated and compact lithiumrich bauxite, and oolitic chlorite is the main clay mineral of oolitic lithium-rich bauxite and the most important siliconbearing mineral.

Fig.10.X-ray powder diffraction spectra of main types of lithium ore samples in the study area.

Table 4.Mineral composition and content (%) of main types of lithium ores in the area A of the study area.

From the mineral composition and corresponding chemical analysis of lithium-rich bauxite samples, it can be seen that: (1) the Li2O grade in oolitic lithium-rich bauxite is weakly negatively correlated with Al content in a certain range, while there is no obvious correlation between Li2O grade and Al content in clastic and brecciated lithium-rich bauxite; and (2) for these samples with different types of chlorite-bearing minerals, the grade of Li2O was positively correlated with the total amount of chlorite in the samples,and (3) the grade of Li2O in all samples was positively correlated with the total amount of clay minerals.

In addition, the leaching rate experiments of Li-rich oolitic bauxite samples were preliminarily carried out jointly by the staff of the testing center.The lithium mineralized samples (at Li2O grade of 0.6%) were ground to 200 mesh, and the oxalic acid solution and NH4Cl solution were selected as leaching reagents to obtain the leaching rate of Li as X% and Y%,respectively.

Based on the above experimental results, it is preliminarily inferred that the Li element in lithium ore(mineralized) body in the study area A occurs in the adsorption state of clay minerals, and the object of study should be ion-adsorbed lithium ores (Sun BD et al., 2019).

6.3.Analysis of mineralization process

6.3.1.Sedimentary stage

Based on the analysis of geological characteristics,mineralogical characteristics of rock/ore, geochemical characteristics of ore (mineralized) body and regional tectonic background of lithium ore body in the area A, especially the reconsideration of the tectonic framework of folded basement in Kangdian ancient land, and combined with the latest achievements of 1:50000 regional geological mapping in the adjacent area, we predicted the mineralization and evolution of lithium ores in the area A as follows.In the Early Permian,the crust of central Yunnan was uplifted, which exposed the basic volcanic rocks and Carboniferous carbonate rocks of different periods on the Kangdian ancient land and the eastern margin of the ancient land to the surface for long, and suffered intense weathering and denudation.At that time, the central Yunnan area was located near the equator with abundant vegetation and rainfall (Yu WC et al., 2014).Under the influence of different types of weathering, the basic volcanic rocks exposed to the surface of the Kangdian ancient land in different periods, especially the exposed Paleoproterozoic basic volcanic rocks originated from mantle plume, were subjected to clavization to form silica-aluminum and aluminous weathering crust.The widely distributed Carboniferous carbonate rocks in low topography of central Yunnan underwent calc-lateritization to form a lateritic weathering crust.Driven by various external forces such as surface runoff, different types of weathering crust materials accumulated in karst depressions, karst funnel and other negative relief from higher terrain.Humus-rich acidic solutions brought the soluble elements Li, K, Na, Ca and Mg in basalt (highly developed fissures) and paleo-weathering crust to lower carbonate karst depressions continuously by surface runoff and groundwater at a high water level along surface water systems and groundwater channels, and mix weathering in low-lying areas.Under the leaching effect of atmospheric precipitation, soluble elements such as Na, Ca,Mg and Si were leached to varying degrees, while inactive elements such as Al, Ti and Fe are relatively enriched, and lithium ions in ionic state and lithium-bearing minerals migrated and transported in mechanical suspension or colloidal form were carried away by solutions or colloidal solutions rich in humic acid.When acid or colloidal solution rich in humic acid seeped through mixed weathering crust in alkaline environment of carbonate karst solution depression,the Eh and pH values changed significantly and formed a geochemical barrier, which directly lead to mineral differentiation, metasomatism and precipitation of colloidal solution.In a relatively long geological period, this mechanism has been repeated numerous times, providing a rich material basis for the formation of lithium ore bodies.

In the early Middle Permian, with the westward transgression in the eastern margin of the ancient land, the groundwater level in central Yunnan increased gradually, and the leaching mechanism of ore-bearing materials in negative terrain gradually weakened until stagnation.As a result,swamp zones gradually formed on negative terrain, and a set of carbonaceous clay rocks with coal seams and coal lines were deposited.During the later transgression, the carbonaceous clay strata well protected the ore (mineralized)body.The relatively stable tectonic setting is a prerequisite for the whole metallogenic process.

6.3.2.Late reformation stage

The morphology of the ore (mineralized) body (Fig.3)and microscopic photographs of some lithium-rich ores (Fig.11)suggest that the lithium ore (mineralized) body develops slump structures.This indicates that more than one strong karstification may have occurred on the unconformity of Carboniferous carbonate rocks beneath the ore body after the diagenesis of Liangshan Formation.The Liangshan Formation and lithium ore (mineralized) body slumped due to the dissolution of bottom carbonate rock, resulting in a large number of fractures and faults with small fault throw in the ore body.Accordingly, the vertical leaching effect of groundwater on the ore body increased.The clay minerals related to lithium in the upper part of the ore body migrated downward, which directly results in the decrease of the lithium content in clay minerals and ores and the further enrichment of lithium in the middle part of the ore body.Of course, this kind of leaching mechanism has limitation.If the vertical leaching of groundwater is too strong, the horizontal leaching will be correspondingly enhanced, and lithium will gradually be migrated out of the ore body.The migrated lithium elements may be enriched in a relatively lower favorable area or be lost and diffused into the groundwater system.

Fig.11.Microstructures of different types of reformed lithium ores.a-Non-uniform limonite mineralization characteristics of chloritefree smectite distributed in soybean-like bauxite; b-microscopic characteristics of slump structure in compact bauxite; Chm-Chamosite; Lm-Limonite; Ant-Anatase; Bhm-Boehmite; Dsp-Diaspore.

In summary, it is inferred that the low-quality bauxite orebodies and aluminous clay ore bodies in the eastern margin of the Wuding-Yimen secondary uplift area in central Yunnan are the targeted lithium ore bodies.The upper and middle parts of the Liangshan Formation develop Carboniferous clay rocks or coal seams, and the lower part has thick strata, and the area weakly affected by the later tectonism is favorable for searching for lithium ore bodies.

7.Conclusions

(i) The main ore types of the lithium ore body in the area A are light gray lithium-rich oolitic bauxite, light gray lithium-rich brecciated bauxite, light gray lithium-rich compact bauxite and gray green lithium-rich aluminous clay rock.The lithology of the direct floor of the ore body is greygreen ferrous clay rock, and that of the direct roof is carbonaceous clay rock.The structure of the ore body is relatively simple, showing a “three-storey” model: the bottom is a ferrous clay ore horizon (locally developed lenticular limonite), with a generally very low average of Li2O; the middle is a lithium-rich low-quality bauxite horizon, with a high average grade of Li2O, which is the main lithium-bearing layer of lithium ores; the upper is a lithium-rich bauxite layer,with a medium average grade of Li2O.

(ii) Comprehensive mineralogical and petrogeochemical study shows that the ore-forming materials of the lithium ore body in the area A are not single source, but should be multisource.The main source should be the Paleoproterozoic basic volcanic rocks exposed on the Kangdian ancient land, with a small amount of ore-forming materials from the Mesoproterozoic basic volcanic rocks and bottom carbonate rocks from the Kangdian ancient land.

(iii) Based on the quantitative analysis of the minerals of different types of lithium-rich ores in the lithium ore body and the comprehensive analysis of the regularity of Li2O grade, it is preliminarily inferred that the lithium element in the lithium ore body occurs in the adsorption state of clay minerals, and the research object should be ion-adsorbed lithium ores.

(iv) There are two stages of mineralization of the lithium ore body in the study area: sedimentary stage and late reformation stage.Different types of paleo-weathering crusts in the sedimentary stage provide the initial material for lithium ore body; in the later reformation stage, the lithium concentration in different parts of lithium ore body has changed, being enriched in favorable area and depleted in disadvantageous position.

Acknowledgement

Field work was completed with other members of“1:50000 Yimen County frame, Erjie frame, Pubei frame and Mingyihe frame Area Geological Survey” Project Team (DD20160017), and the work was financially supported by Summary and Service Product Development of Regional geological survey area (DD20160345-02), Yunnan Science and Technology Leading Talents Training Program(2013HA001), and China Mineral Geological Records Project(DD20160346, DD20190379).The relevant sample testing work was mainly conducted by the Kunming Mineral Resources Supervision of Ministry of Land and Resources.During the course of the writing, the authers received the guidance of Prof.Jing Li and Prof.Xiao-min Cao from the Yunnan Geological Survey, and Prof.Jing Wu from Kunming University of Science and Technology.We would like to express our gratitude.