Geochemical and Geochronological Constraints on the Genesis of Ion-Adsorption-Type REE Mineralization in the Lincang Pluton, SW China

: Granites are assumed to be the main source of heavy rare-earth elements (HREEs), which have important applications in modern society. However, the geochemical and petrographic characteristics of such granites need to be further constrained, especially as most granitic HREE deposits have undergone heavy weathering. The LC batholith comprises both fresh granite and ion-adsorption-type HREE deposits, and contains four main iRee (ion-adsorption-type REE) deposits: the Quannei (QN), Shangyun (SY), Mengwang (MW), and Menghai (MH) is also a decisive factor for Ree 3+ enrichment. Stable geology within a narrow altitudinal range of 300–600 m enhances Ree 3+ retention.


Geological Background
The LC batholith and the Triassic volcanic belt constitute the southern Lancangjiang zone of SW China. The southern Lancangjiang tectonic zone in the central part of the Sanjiang orogenic belt includes the Jinsha, Lancang, and Nu rivers. The Lanping-Simao Block and Jinshajiang-Ailaoshan suture lie east of the Sanjiang orogenic belt, which represents a Paleo-Tethyan block-arc oceanic basin. The Changning-Menglian suture belt is a remnant of the main Paleo-Tethyan ocean crust in SW China and extends to the south of the Nan and Sr Kaeo sutures.

The Lincang Granitic Batholith
The LC batholith comprises different types of granitic rock with an area of over 7400 km 2 , and measures 350 km long by 10-48 km wide. It extends from Yunxian county in the north to Menghai county in the south, with intermittent southward connection to Thailand, the Malay Peninsula, and Sumatra granites (Figure 1a,b). The exposed area represents the largest batholith in Yunnan and is separated into three segments by the Xiaojie-Nadong and Nanling-Chengzi faults (Figure 1c). The altitude of the LC batholith is in the range of 1000-2500 m, with the topography of the region dominated by low hills of 300-600 m in height. Denudation rates are fairly low and weathering profiles are well preserved. The region has a temperate subtropical monsoon climate with an annualaverage temperature, rainfall, and humidity of 17.3 °C, 1504.5 mm, and 72.54%, respectively [33].   [33,36]). 1-granite; 2-porphyritic biotite granite; 3-granodiorite; 4-metamorphic rock; 5-fault; 6-national boundaries; 7-river; 8-iRee deposit; 9-Sampled location.

Sampling
As shown in Figure 1c, four QN granite samples (lc2-j2, lc2-j3, lc3-j1, and lc4-j1) were taken from the QN iRee deposit in the central LC batholith, and one MH granite sample (mh-j1) from the MH iRee deposit in the southern batholith. The corresponding granite weathering profile was separated to three horizons, A, B, and C. Horizon A, the upper weathering profile, comprises mainly soil and organic matter with some detrital quartz. Horizon B comprises weathered granite, with most minerals being altered to secondary minerals, such as clay, oxides, and phosphates. This sample contains K-feldspar and detrital quartz, and was crushed easily by hand. Horizon C represents a weathering zone where the sample was sandier and stonier than horizon B, and could be crushed by a hammer. Sample numbers in each weathering profile followed Lu et al. (2019Lu et al. ( , 2020 [28,29].

Methods
Whole rock major and trace element compositions were determined at the National research center of Geoanalysis, Beijing, China (NRCG, CAGS). Major elements of the analyses were determined by X-ray fluorescence spectrometry (XRF) using fused glass discs, with precisions of 1-2%. Trace element analyses were determined by inductively coupled plasma-mass spectrometry (ICP-MS) (PE-300D).
Chemical compositions of minerals in the granites from the LC granite were determined by a JEOL-8230 electronmicroprobe at the Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Trace elements of minerals were carried out by fs-LA-ICP-MS, using a femto-second laser ablation system (ASI J200) coupled to an inductively-coupled mass spectrometer (Thermo X series II).
Meanwhile, the zircon U-Pb age and Sr-Nd-Pb isotope of LC granite were examined. Measurements of U, Th, and Pb isotopes of zircon grains were conducted using a Camera IMS-1280HR SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. Sr-Nd and Pb isotope analyses were performed on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Dreieich, Germany) at the Wuhan Sample Solution Analytical Technology Co., Ltd, Hubei, China. For the details on the analysis methods, we refer to the supplementary materials.
The REE concentrations for each REE mineral are listed in Table 1. The accessory minerals allanite, monazite, and xenotime have relatively high REE contents (>5 wt.%). Based on their LREE/HREE ratios, REE minerals can be divided into two categories: LREE-type and HREE-type. For example, allanite and monazite are relatively enriched in LREE (LREE/HREE > 1) and are therefore 'LREE minerals,' whereas xenotime and fluorite(-Y) are relatively enriched in HREE, and are therefore 'HREE minerals. ' Allanite, monazite, apatite, and zircon are the main accessory minerals in the LC granite, with minor cerite and xenotime associated with feldspar and quartz. In medium-fine-grained biotite granite (lc3-j1), allanite (

Weathering Profile of the LC Granite
It is generally considered that weathering crusts are formed by the weathering of granite in a humid environment, with the crust comprising detrital and secondary minerals. Detrital minerals can be subdivided into two groups: (1) quartz, feldspar, and biotite; and (2) heavy minerals that are relatively stable during weathering, such as zircon, monazite, and xenotime. The REE minerals such as allanite, fluorapatite, fluorite, and fluorcarbonate may occur in host rocks, but not in the weathering crust.
Secondary minerals carrying exchangeable REE (Ree 3+ ) include clay minerals and Fe-Mn oxides. Here, the secondary-mineral content of the LC weathering profile ranges from 21% to 33%, from top to bottom. The clay mineral content is highest in horizon A, with most clay minerals finally altering to gibbsite (~7%) in this horizon [28,29]. Horizon B is the most important for Ree 3+ enrichment. Kaolinite, montmorillonite, and illite are the main clay minerals in the LC granite weathering profile. The proportion of clay minerals varies between horizons based on the intensity of weathering. The horizon A kaolinite content is up to 85%, with 4% illite and 5% vermiculite. Horizon B contains 74-81% kaolinite, 9% illite, and 8-17% montmorillonite (mixed with illite). Horizon C contains 36% kaolinite, 12% illite, and 13% chlorite [29].

Zircon U-Pb Dating
Zircons from the QN porphyritic biotite granite sample (lc4-j1) are predominantly yellow to colorless euhedral crystals, 50-150 μm long with aspect ratios of 1-3, and exhibit oscillatory magmatic zoning in cathodoluminescence (CL) images. Their U and Th contents are 117-21,152.8 and 69-28,873 ppm, respectively, with Th/U ratios of 0.19-1.36, which is typical of magmatic zircons. Twenty analyses yielded concordant 206 Pb/ 238 U ages of 231.4-215.2 Ma, with a weighted-mean age of 220.3 ± 1.2 Ma (mean squared weighted deviation, MSWD = 0.71), which is interpreted as the crystallization age of the central part of the LC batholith (Table 3; Figure 8a [48,49]. These ages suggest that the petrogenesis of the LC granite was associated with anatexis of crustal materials. The 19 youngest analyses are concordant, with a weighted-mean age of 232.2 ± 1.7 Ma (MSWD = 0.58; Table  3; Figure 8e,f).

Sr-Nd-Pb Isotopic Compositions
Results of whole-rock Sr-Nd isotope analyses of the QN and MH granites are shown in Table 4 and    Figure 9. Diagrams of ( 87 Sr/ 86 Sr)i-εNd(t) (a) and Pb-isotopic composition (b) of biotite granite obtained from the LC granite (the LC batholith data after e.g., [41,50]).

The Timing of REE Mineralization in the LC Granite
Previously published ages for the LC granite have generally been in the range of 252-199 Ma (e.g., [17][18][19]33,[40][41][42][43][44][45]50,51]; Figure 10). Our zircon U-Pb dating in the central LC batholith, which contains HREE mineralization, yielded an age of ca. 220.3 Ma, and zircons from the southern area yielded an age of ca. 233.2 Ma, both of which are within this range, suggesting that the main LC granitic body intruded during the middle-late Triassic.
Zircon usually crystallizes during the early stages of rock formation and can survive temperatures as high as 3000 °C. Zircon U-Pb ages may, therefore, reflect those of their host granite [52]. However, REE minerals such as monazite, allanite, and xenotime crystallize later in granite than zircon. Furthermore, the types and amounts of REE mineral (including HREE minerals) increase with hydrothermal alteration after crystallization being [30,53]. Hence, zircon U-Pb ages should be considered upper limits of the REE-mineralization age; i.e., 220.3 Ma in the QN granite, and 233.2 Ma in the MH granite.

Petrogenesis and Tectonic Setting of the LC Pluton
The decrease in FeOt, MgO, TiO2, CaO, MnO, and P2O5 contents of the LC granites with increasing SiO2 content suggests that the magmas underwent continuous crystallization during magmatic evolution, and their peraluminous nature (A/CNK > 1.1) indicates S-type affinity (Section 4.1; Figures 4, 5c, and 6; [54][55][56][57][58]). Geochemical, isotopic, and petrological constraints confirm that most large-volume peraluminous granites originate from the melting of crustal rocks (e.g., [54]). Here, the strong negative Eu, Ba, Nb, Sr, P, and Ti anomalies, and positive Rb, Th, and U anomalies of the LC granite are similar to those of magma derived from the partial melting of crustal rocks [56,59].
S-type granites are produced mainly from the melting of metasediments such as psammite and pelite [60], and their CaO/Na2O ratios are influenced by their magma source. The CaO/Na2O ratios in psammite-derived melt are generally >0.3 (average 0.8), whereas in pelite-derived melt the ratios are <0.5 [54]. Here, most LC granite samples have CaO/Na2O ratios of 0.06-2.67 (n = 147), with only 21 samples having ratios of <0.5, and Al2O3/TiO2 ratios of 9.29-150.9, with only four samples having ratios of >79. In the (CaO/Na2O)-(Al2O3/TiO2) diagram (Figure 10a), most samples plot in the field of psammite-derived melt. The Rb/Ba and Rb/Sr ratios in psammitederived melt are lower than those in pelite-derived melt [56]. In the (Rb/Ba)-(Rb/Sr) diagram ( [61]; Figure 10b), most LC granite samples plot close to the psammite-derived melt field, with such melt having a clay-poor source, including minor shale. Some samples plot near the clayrich field with a mixed source dominated by quartz-feldspathic psammites. In the (Al2O3/(MgO + FeOt))-(CaO/(MgO + FeO T )) diagram (Figure 10c), most samples plot in the field of psammitederived melt. Furthermore, Sr and Eu are enriched mainly in plagioclase, whereas K-feldspar is enriched in Ba. The strong negative Eu, Ba, and Sr anomalies in LC granite samples indicate that plagioclase and K-feldspar are the main residual phase in partial melting. In the (Rb/Sr)-Ba diagram (Figure 10d), most LC granite samples plot with 20%-60% plagioclase, suggesting that at least 20%-60% plagioclase was present in the magma source [54,55]. It follows that the parental magma source for the LC granites involved mainly quartz-feldspathic materials such as psammite-dominated clastic metasediments, with minor shales.
The high initial 87 Sr/ 86 Sr ratios, low εNd(231 Ma) values and Pb isotopic compositions, consistent with the isotopic composition of the field of remelting granites of SCB (South China Block; [62]; Figure 9), indicates that the LC granitic magma was derived from partial melting of ancient crystallized basement.
The compositions of the Changning-Menglian suture belt in the Lancangjiang zone of SW China, the Chiang Mai-Inthanon suture belt in northern Thailand, and the Bentong-Raub suture belt in the Malay Peninsula indicate that the Paleo-Tethys Ocean opened in the early Devonian and closed in the Middle-Late Triassic [33,34,[40][41][42][43][63][64][65]. The LC batholith, exposed to the east of the Changning-Menglian suture zone, represents a tectono-magmatic zone that records the evolution of the Paleo-Tethys Ocean from subduction to post-collisional regimes [41,64].
The LC granite exhibits peraluminous and S-type characteristics and, with psammite-derived upper-crustal melt being the main source of S-type granites, some studies have concluded that the LC batholith formed in a syn-collisional setting [33,51,[55][56][57]. This is supported by most LC granites being peraluminous (Figure 5c) and plotting in the volcanic arc and syn-collision fields in the Nb-Y diagram (Figure 11a). Peralkaline and alkaline granites are commonly associated with post-tectonic within-plate extension [55]. In the SiO2-K2O diagram (Figure 5a), most LC granite samples plot in the peralkaline and alkaline fields, while in the Rb-(Y+Nb) diagram (Figure 11c), all but one plot in the post-collision field, consistent with a post-collision tectonic setting. Our zircon U-Pb ages of 233.2-217.8 Ma from the LC granite are consistent with those found in most other studies (252-199 Ma), and the common absence of Early Triassic strata in the Lancangjiang zone reflects pronounced uplift and erosion at that time [33,40]. Moreover, in the Rb-(Y+Ta) diagram (Figure 11b), all LC granites plot in the within-plate granite field, indicating a within-plate extensional environment. Experimental studies have demonstrated that bimodal igneous suites comprising mafic members of tholeiitic gabbros and basalts, and felsic members displaying A-type characteristics, are common in such a setting [37,40,66]. We, therefore, conclude that closure of the Paleo-Tethys Ocean at ca. 295 Ma was followed by syn-collisional extension accompanied by crustal melting and a degree of mantlematerial ascent, with magmatism occurring mainly at ca. 199 Ma within a post-collision extensional environment. The LC granite, thus, originated from the partial melting of early Paleozoic crustal basement.
REE minerals are the main source of iRee deposits in the LC granite. Although feldspar, biotite, sphene, and apatite have low REE contents, weathering cycles may result in these minerals contributing to the deposit. Minerals in granites enriched in LREEs contribute to iLRee deposits when they are dissolved in the weathering crust. Allanite(-Y), fluorite(-Y), and REE fluorcarbonate are relatively enriched in HREEs, and are the main HREE source for iRee deposits. However, HREE minerals are not common in the LC granite. The QN granites, for example, are mainly enriched in LREEs, with relatively few exhibiting HREE enrichment, leading to QN deposits being rich in LREEs, but with a degree of HREE enrichment. The HREE minerals are scarce in MH granite, so the MH deposit is enriched with LREEs only.

Secondary Minerals
Rock-forming minerals such as feldspar and mica (biotite and muscovite) are altered to clay minerals during weathering and become carriers of Ree 3+ [7,[24][25][26][27][28][29]69]. Clay minerals such as those of the kaolin-group (kaolinite, dickite, nacrite, and halloysite), illite, and montmorillonite, which adsorb Ree 3+ , can also be considered as iRee ores. This study found that kaolinite, illite, and montmorillonite are the main clay minerals in the LC granite weathering profile, while halloysite also occurs in the NL weathering profile.
The REE adsorption capacity is influenced by surface structure, composition, and surface charge of clay minerals [31]. Illite and montmorillite occur in 2:1 clays, with higher adsorption capacities than kaolin-group minerals in 1:1 clays. The high adsorption capacity of montmorillonite is due to negative charges generated by isomorphism in the lattice, whereas kaolinite and halloysite have negative charges where -OH in the crystal lattice releases H + . The higher the pH, the higher the adsorption capacity [31,68,69]. The pH points of zero charge (pHpzc) of kaolinite, illite, and montmorillonite are <3.7, ~2.5, and 7-9, respectively [70], so under natural pH conditions (4-7), kaolin-group minerals (especially kaolinite and halloysite) and illite are more capable of surface complexation of Ree 3+ than montmorillonite [32,69]. In both the LC and NL areas, the kaolin-group represents the predominant clay minerals in the weathering profile, providing enrichment in Ree 3+ . In the appropriate pH range, the deposit's Ree 3+ content, thus, depends on the amount of kaolin-group minerals in the weathering profile.
Horizon B of the granite weathering profile has the highest Ree 3+ content, owing to its pH range of 4-6.8 and higher kaolinite and/or halloysite content. In the LC area, horizon B contains an average of 21%-26% clay minerals [29], whereas in the NL area the clay-mineral content is up to 50% [24][25][26]32]. In the NL weathering profile, the content of kaolinite decreases but that of halloysite increases with depth, with both clay minerals contributing to the accumulation and fractionation of REEs at pH 5.5-6.3 [32]. The higher Ree 3+ content of the NL weathering profile than that of the LC profile is, therefore, likely related to its higher kaolinite and halloysite contents.

Intensity of Granite Weathering
The Ree 3+ content of the weathering profile is influenced by the intensity of granite weathering; the stronger the weathering, the more REE minerals are dissolved, and the more Ree 3+ is released to the profile. The LC weathering profile has a lower REE content than the NL profile: 134-1111 ppm (mean 447 ppm; n = 77) and 168-2347 ppm (mean 572 ppm; n = 70), respectively, and the difference may be due to the relative intensities of weathering. In the NL area, the chemical index of alteration (CIA; 100 × Al2O3/(Al2O3 + CaO + Na2O + K2O)) of granite and the weathering profile are in the ranges of 45%-60% and 65%-95%, respectively, with the REE content increasing with CIA in the range 45%-60%, but decreasing in the CIA range of 65%-95% [27]. The CIA of LC granite and weathering profile are similar to the NL profile (Figure 12a,b). The intensity of weathering can also be reflected in the amounts and types of secondary minerals in the horizon. In natural chemical weathering, feldspar and mica (muscovite and biotite) are altered to clay through several stages, with each stage corresponding to a different clay mineral : Halloysite is in the kaolin group, and normally coexists with kaolinite in modern soils and sediments. The higher the intensity of weathering, the higher the content of clay minerals, such as those of the kaolin group (and possibly gibbsite). In NL granite weathering profile, halloysie is in common and play an important role in the carrier of Ree 3+ [32,69]. But in the most of LC granite weathering profile, there is kaolinite only [28,29], which could be the reason why the concentration of Ree 3+ in NL area are higher than LC area.
Weathering cycles are key to the secondary mineralization of Ree 3+ in the weathering profile, with re-enrichment and re-fractionation and with Ree 3+ being re-migrated and re-accumulated. As soil pH changes from acidic to alkalescent with increasing depth in the weathering profile, the Ree 3+ concentration increases with the increasing adsorption capacity of clays. Previous studies have shown that a pH range of 5.4-6.8 is optimum for the accumulation of Ree 3+ [32,70,71]. The HREEs of higher atomic mass are more soluble than LREEs in water, and accumulate at higher pH [32,70,71], with the depth of HREE enrichment, thus, being greater than that of LREE enrichment.

Conclusions
(1) REE minerals, including allanite, fluorapatite, and monazite, and HREE minerals such as xenotime, thorite, allanite(-Y), fluorite(-Y), and REE fluorcarbonate, make the greatest contribution to the REE content of the LC granite.The QN and MH granites are host rocks for Ree deposits with zircon U-Pb ages of ca. 217 Ma and 232 Ma, respectively, with these being the earliest times of REE mineral formation.
(2) The host rocks of the Ree deposits are strongly peraluminous with S-type granite affinities, suggesting they originated from partly melted crustal basement. Sr-Nd-Pb isotopic systematics and geochemical signatures of the LC granites indicate that they were derived from partial melting of the upper crust. Paleo-Tethyan continent-continent collision occurred during the early Indosinian, followed by post-collisional extension. Middle Indosinian magmatism was generated in a postcollisional tectonic setting related to early-middle Indosinian slab break-off.
(3) REE minerals such as allanite(-Y), fluorapatite, fluorite(-Y), and REE fluorcarbonate dissolved in the weathering crust are the main sources of Ree deposits. Dissolution of LREE and HREE minerals lead to LREE and HREE enrichment, respectively. Secondary minerals, such as kaolin-group minerals and illite (from altered feldspars and micas), are carriers of Ree 3+ , and its enrichment is influenced by the intensity of weathering; the stronger the weathering, the more REE minerals are dissolved in the weathering profile. Weathering cycles are key to the secondary mineralization of Ree 3+ , and stable geological conditions over a narrow altitudinal range promote retention of Ree 3+ in the weathering profile.
Author Contributions: Writing-original draft preparation, L.L.; writing-review and editing, Y.L. and H.L.; methodology, Z.Z.; investigation, C.W.; resources, C.W. and X.X.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.