Enrichment of ion-exchangeable rare earth elements by felsic volcanic rock weathering in South China: Genetic mechanism and formation preference

Ore Geology Reviews 114 (2019) 103120

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Enrichment of ion-exchangeable rare earth elements by felsic volcanic rock weathering in South China: Genetic mechanism and formation preference

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Wei Fua,b, , Peng Luoa, Zuoying Hua, Yangyang Fenga, Lei Liua, Jingbao Yanga, Meng Fenga, Hongxia Yua, Yongzhang Zhoub a b

Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology, Guilin 541004, China Center for Earth Environment & Resources, Sun Yat-sen University, Guangzhou 510275, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Ion-exchangeable REEs Ion adsorption-type REE mineralization Felsic volcanic rock Rhyolite Weathering South China

Felsic volcanic rocks are an important source for generating ion adsorption-type rare earth element (REE) resources. To better understand the supergene enrichment of REEs related to felsic volcanic rock weathering, this study conducted an investigation of Indosinian felsic volcanic rocks from Guangxi, southwest China, which have been subjected to deep weathering. The weathering has formed large areas of thick regolith with the potential to host ion adsorption-type REE mineralization. The felsic volcanic rocks belong to the dacite–rhyolite series and are peraluminous, high-K, calc-alkaline, and REE-rich (325–376 ppm). Abundant REE-bearing accessory minerals are present in the felsic volcanic rocks, including titanite (average ΣREE = 15.1 wt%), allanite (average ΣREE = 14.9 wt%), and apatite (average ΣREE = 1510 ppm), and these three minerals contain an estimated 88.7% of the whole-rock REEs content. Significant REE enrichment is present in the felsic volcanic rock-derived regolith. A typical rhyolite-derived regolith profile has REE contents that increase from 376 ppm in the rhyolite to 1737 ppm in the regolith, representing an almost five-fold enrichment due to weathering. The REEs in the regolith are present in an ion-exchangeable form (iREEs), which accounts for 52%–87% of the total REEs present (TREEs). The occurrence of iREEs is closely linked to clay minerals, showing an affinity in the order halloysite > kaolinite > illite. Continuous operations of REEs by an eluviation–illuviation process from the source minerals (titanite + allanite + apatite) to sink minerals (kaolinite + halloysite + illite) results in an iREE-enriched zone in the middle and lower parts of the rhyolitic regolith. Notably, besides the studied Indosinian felsic volcanic rocks, there are multi-epochs of such lithology outcropped in South China. Their related iREEs mineralizations, however, are preferentially developed in the weathered terrains of early Yanshanian and Indosinian felsic volcanic rocks, and not in those of the Yanshanian and late Yanshanian felsic volcanic rocks. A comparison of the mineralized and barren units indicates that the iREEs mineralization hosted in felsic volcanic rocks regolith is controlled by some key endogenic and exogenic ore-forming factors. High initial REE concentrations in the unaltered felsic volcanic rocks, a suitable climate, and a relatively quiescent tectonic setting are favorable for the formation and preservation of iREEs mineralization.

1. Introduction Ion adsorption-type rare earth element (REE) deposits are of great significance in meeting the global demand for REE resources (Kynicky et al., 2012; Wang et al., 2013; Simandl, 2014). Such REE deposits have large reserves, are inexpensive to mine and extract, and supply nearly all of the heavy REEs (HREEs) used globally (Yang et al., 2013). This specific mineralization type of REE resource has been known to exist for half a century and is only exploited in South China (Xie et al., 2016; Li et al., 2017). In recent years, a small number of such deposits

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and potential prospects have been reported in other countries with comparable geological and climatic conditions to South China, including in Southeast Asia (e.g., Vietnam, Myanmar, Laos, Thailand, and the Philippines), North and South America (e.g., USA and Brazil), and Africa (e.g., Malawi and Madagascar) (e.g., Sanematsu et al., 2013; Berger et al., 2014; Foley et al., 2014; Foley and Ayuso, 2015; Sanematsu et al., 2016; Padrones et al., 2017). Therefore, research into ion adsorption-type REE mineralization is an important topic in REE ore geology. Favorable source rocks for generating ion adsorption-type REE

Corresponding author at: College of Earth Sciences, Guilin University of Technology. Jiangan Road No. 12, Guilin 541004, China. E-mail address: [email protected] (W. Fu).

https://doi.org/10.1016/j.oregeorev.2019.103120 Received 16 January 2019; Received in revised form 31 August 2019; Accepted 10 September 2019 Available online 11 September 2019 0169-1368/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Simplified distribution map of the felsic volcanic rocks in South China, with the ion-adsorption REE mineralization concentrated areas related to the felsic volcanic rock weathering (modified after Ma, 2002; Yuan et al., 2012).

Fig. 2. (A) Simplified geological map of the study area, showing the locations of sampled regolith profile (modified after Guangxi GMEB, 1985); (B) Landform of the felsic volcanic rock terrain in the study area; (C) Typical profile of the rhyolite-derived iREE-rich regolith in the study area.

rocks is the focus of this study. Previous studies have demonstrated that chemical weathering of felsic volcanic rocks can facilitate significant enrichment of ion-exchangeable REEs (iREEs) in regolith. Consequently, weathered felsic volcanic rock terrains are a potential exploration target for ion adsorption-type REE resources (e.g., Song and Shen, 1982, 1987; Zhang, 1990; Chen and Yu, 1994; Deng, 2013; Foley and Ayuso, 2013; Wang et al., 2013; Yang et al., 2015; Zhao et al., 2017). According to Zhang (1990), ion adsorption-type REE deposits related to felsic volcanic rocks account for 37.87% of the total of such type deposits in South China, and these ores are of good quality with a high percentage of iREEs (> 65%). In contrast, granite-related ion adsorption-type REE ores, despite their wider distribution, have a relative

deposits include various lithologies, but are mainly granites, felsic volcanic rocks, and some metamorphic rocks (Zhao et al., 2019). Therefore, to establish a systematic model for the formation of ion adsorption-type REE deposits, it is necessary to investigate examples formed from different source rocks. The nature and origins of graniterelated ion adsorption-type REE mineralizations have been extensively studied (e.g., Bai et al., 1989; Wu et al., 1990; Murakami and Ishihara, 2008; Zhang et al., 2013; Zhao et al., 2014; Liu et al., 2016; Wang et al., 2017; Zhao et al., 2017; Fu et al., 2019; Li et al., 2019). However, there have been few studies of such mineralization formed from non-granite source rocks. Ion adsorption-type REE mineralization related to felsic volcanic 2

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Fig. 3. Studied weathering profiles derived from the rhyolite, showing its sampling columns, division of lithostratigraphic units, macrophotographs of each lithostratigraphic unit, and mineral compositions determined by quantitative XRD method. (A) The sampling column of the studied regolith profile with four lithostratigraphic units; (B–E) Macrophotographs of the main lithostratigraphic units, including the full-weathered horizon, the highly-weathered horizon, the semiweathered horizon and the parent rock respectively; (F–I) Mineral compositions of each lithostratigraphic unit determined by quantitative XRD method. In the XRD spectrum, the Obs curve in red color represents the initial experimental spectrum, while the Calc curve in blue color refers to the calculated spectrum by treatment for mineral quantification. (Abbreviations: Ms-muscovite; Clc-Chlorite; Kln-kaolinite; Hal-Halloysite; Qtz-Quartz; Ab-Albite; Ill-Illite; Bt-biotite; Kfs-K-feldspar).

coastal and interior regions of the Cathaysia Block (Fig. 1). These volcanic rocks are associated with granite and form a regional volcanic–plutonic belt (Zhou, 2007; Faure et al., 2009; Li and Zhou, 2017). Felsic magmatism occurred mainly during the Jingningian, Indosinian, and early and late Yanshanian (Xing and Feng, 2015), and was associated with various tectono-magmatic events, such as continental rifting and subduction during different stages of the evolution of the South China Plate (e.g., Yu, 1987; Deng et al., 2004; Lu and Gu, 2007; Zhou, 2007; Lu et al., 2009; Qin et al., 2011). The main lithologies are rhyolite, dacite, and porphyroclastic lava. The volcanic rocks have I- or A-type geochemical affinities and formed in a range of tectonic settings (e.g., Lai and Wang, 1996; Lu, 1997; Zhou, 2007). The temporal and spatial distributions of the felsic volcanic rocks in South China are linked to the occurrence of Cu, Pb, Zn, Au, U, REE, and other metallic ore deposits (e.g., Hu and Zhou, 2012; Yuan et al., 2012; Mao et al., 2013). This study focused on felsic volcanic rocks in Chongzuo district, southwestern Guangxi Autonomous Region, South China. This location is close to the southwestern segment of the Qinzhou–Hangzhou tectonomagmatic belt, which marks the suture between the Yangtze and Cathaysia blocks (Liang et al., 2001). Large areas of felsic volcanic rock are exposed at the surface in this region (Fig. 2A), and it represents the

lower percentage of iREEs (> 60%) as Zhang (1990) reviewed. The most representative example of ion adsorption-type REE mineralization related to felsic volcanic rocks in South China is the Heling deposit in Jiangxi Province. This deposit occurs in regolith developed on a rhyolitic volcanic sequence. The REE concentration in these deposits is as high as 4900 ppm (e.g., Zhang, 1990; Bao, 1992; Bao and Zhao, 2008). To better understand how iREEs are enriched during the weathering of felsic volcanic rock, we conducted a study of Indosinian felsic volcanic rocks at Chongzuo, Guangxi Autonomous Region, South China. In this region, large areas of these volcanic rocks have been subjected to deep weathering, resulting in a regolith-covered terrain with ion adsorption-type REE ore exploration potential (Hu, 2016). We also undertook a comparative study with other weathered felsic volcanic rock terrains in South China to explore the key factors responsible for the formation of this type of deposit. 2. Study area and methods 2.1. Geological setting In South China, abundant felsic volcanic rocks crop out in the 3

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Fig. 4. Micro-photographs of the studied parent rock and regolith samples. (A) Vitrophyric to porphyritic texture of the rhyolite under microscope; (B) Large amount of glassy materials in the matrix of the rhyolite observed by electron microscope; (C–F) Back-scattered electron (BSE) images of representative accessory minerals in the rhyolite samples; (G–I) SEM images of clay minerals detected from the regolith samples. (Abbreviations: Qtz-Quartz, Or-Orthoclase, Pl-Plagioclase, Ap-Apatite, Ilm-Ilmenite, Mnz-monazite, Zrn-Zircon, Kln-kaolinite, Hal-halloysite, Ttn-titanite, Kfs- K-feldspar, Ill- Illite, Gla-Volcanic glass).

et al., 2002). These conditions cover the latitudes 22–29°N (Zhang, 1990; Wang et al., 2013) and favor the development of ion adsorptiontype REE ores in South China. The warm and humid climate has led to the development of abundant and diverse vegetation and, in particular, subtropical mixed broad-leaved evergreen forests. In general, the landscape of the study area (Fig. 2B) is hilly terrain with relatively low relief (100–450 m in elevation). Due to variations in the exposed lithologies, the landforms of the study area are not uniform and exhibit two different types. Areas of felsic volcanic and pyroclastic rocks are characterized by continuous massifs with rounded hilltops and gentle slopes. In contrast, areas of carbonate rocks have a typical karst geomorphology.

main site of Indosinian felsic volcanic rocks in South China. These volcanic rocks are part of the Beisi Formation and are of Early and Middle Triassic age (Yang and Chen, 1997; Liang et al., 2001). In general, the volcanic rocks comprise a succession of felsic lavas (basaltic andesite–dacite–rhyolite) interlayered with pyroclastic rocks (dominantly agglomerates, breccias, tuffs, brecciated ignimbrites, and ignimbrites). The felsic volcanic rocks can be further divided into two stages. The early stage is a complex lithological assemblage of basaltic andesite, dacite, rhyolite, and various pyroclastic rocks, whereas the late stage is a simple assemblage of rhyolite and tuff. Qin et al. (2011) reported a zircon U–Pb age of 246 ± 2 Ma that constrains the age of felsic volcanism. Geochemical signatures indicate that these felsic volcanic rocks have a subduction-related origin (Qin et al., 2011). The geology in the area of the volcanic rocks comprises a succession of Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Quaternary strata (Guangxi Zhuang autonomous region Geological and Mineral Exploration Bureau, Guangxi [GMEB], 2006), dominated by clastic and carbonate rocks. During the Cenozoic, the study area has experienced a subtropical–tropical climate due to the establishment of the Indian summer monsoon (Guangxi GMEB, 1985), which is characterized by a humid monsoonal climate with alternating dry and wet seasons. Presently, the annual mean temperature is 22.4 °C and the average annual precipitation is > 1200 mm, which occurs mainly from March to August (Gu

2.2. Description of regolith profile and sampling Due to strong chemical weathering, the outcrops of felsic volcanic rocks in the Chongzuo area are covered by regolith of variable thickness (5–15 m, Fig. 2C). In general, the regolith is thickest (> 10 m) on flat hilltops and gradually thins in the downslope direction. In situ regolith has a layered structure comprising several regolith units (Fig. 3A) that from base to top are: (1) a semi-weathered horizon (C horizon); (2) a highly weathered horizon (B horizon); and (3) a fully weathered horizon (A horizon). Each regolith unit can be identified according to color, structure, and mineralogy. Specifically, the fully weathered horizon 4

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Table 1 Chemical composition, pH and OM data of the studied rhyolite-derived profile. Note: CIA chemical index of alteration determined by the molecular proportion of [Al2O3/(Al2O3 + CaO + Na2O + K2O)] * 100 (Nesbitt and Young, 1982). Each of the oxides is a mole percentage. Horizon

Full-weathered horizon

Highly-weathered horizon

Semi-weathered horizon

Sample Depth/m

CZ-1 0.4

CZ-2 0.8

CZ-5 1.6

CZ-6 2

CZ-7 2.5

CZ-8 3

CZ-10 3.9

CZ-12 5

CZ-13 5.6

CZ-15 6.6

CZ-17 8

CZ-18 9.0

SiO2 (%) Al2O3 CaO TFe2O3 K2O MgO MnO Na2O P2O5 TiO2 LOI (%) Total (%) CIA (%) pH OM (wt%) Sc (ppm) Rb Sr Ba Th U Pb Nb Ta Zr Hf V Co Ga Ge Ti Cu Zn

57.2 22.16 0.09 8.9 0.13 0.25 0.01 0.01 0.07 0.54 9.57 98.83 99.02 4.56 4.57 10.5 17.2 11 80.9 44.6 4.1 58.9 19.9 1.78 637 18.2 52.7 3.28 32.6 1.87 3138 14.2 79.1

59.29 21.64 0.09 7.66 0.13 0.27 0.01 0.01 0.06 0.51 9.13 98.8 98.99 4.96 5.82 15 14.3 9.08 77.4 41.9 5.75 41.8 18.4 1.7 490 14.5 43.4 2.02 32.1 2.03 2951 13.5 53.4

66.03 20.25 0.11 3.1 0.31 0.42 0.01 0.02 0.06 0.48 7.97 98.76 97.87 4.80 1.83 11.4 37.2 20.3 406 37 5.98 57.7 17.5 1.59 566 16 26.9 2.17 30.9 2.03 2788 14.1 126

67.41 18.27 0.11 3.99 0.5 0.36 0.01 0.04 0.07 0.51 7.6 98.87 96.56 4.91 1.66 8.86 51.5 20.5 383 32.2 4.7 39 18.6 1.58 831 22.7 27.3 1.64 25.7 1.97 2957 8.26 43.3

65.2 18.51 0.19 4.2 2.6 0.42 0.01 0.54 0.04 0.36 6.63 98.7 84.75 5.29 1.03 12.4 161 25.8 760 33.8 6.18 111 14.9 1.44 636 17.7 28.6 5.79 31.3 2.21 1962 11.2 102

61.75 19.47 0.25 5.8 3.31 0.43 0.03 0.68 0.06 0.41 6.85 99.04 82.12 5.55 2.35 13 206 38.1 981 31.5 5.79 85.5 15.3 1.41 658 17.9 31.8 10.4 31.9 2.09 2449 13.2 96.2

64.25 17.97 0.23 5.74 3.43 0.37 0.02 0.69 0.06 0.44 6.04 99.24 80.51 5.29 1.03 12.2 206 35.2 802 33.9 6.52 50.8 16.6 1.4 766 19.9 34.8 4.69 29.9 1.82 2574 13.8 90.5

66.95 16.33 0.24 4.62 3.78 0.3 0.03 0.87 0.08 0.43 5.13 98.76 76.96 5.29 2.57 14.7 212 39.7 906 32.5 5.63 49.3 16 1.4 630 17.3 32.6 8.51 28 1.67 2579 12.9 118

61.85 18.41 0.24 6.66 3.56 0.4 0.03 0.73 0.09 0.45 6.12 98.54 80.25 5.29 1.01 14.9 233 40.8 932 35.3 7.89 49.6 15.5 1.33 624 16.8 34.9 11.2 31.2 1.86 2552 13.7 96.2

62.57 17.84 0.22 6.64 3.82 0.44 0.02 0.72 0.08 0.59 5.92 98.86 78.94 5.33 0.29 15.5 257 39.3 975 40.3 8.48 44.5 19.4 1.55 680 18.5 39.1 9.74 30.2 1.87 3447 12 101

63.42 17.72 0.27 5.76 3.78 0.53 0.04 0.79 0.07 0.6 5.86 98.84 78.55 5.16 0.80 15.3 224 38.8 1062 31.6 6.45 37.3 19.4 1.52 754 19.7 33.6 15.4 29.4 1.71 3531 10.9 127

71.56 12.6 1.26 3.73 4.71 0.61 0.03 2.22 0.16 0.53 1.29 98.7 60.61 7.66 0.06 14 234 113 1088 25.6 8.35 34.5 18.1 1.52 584 15.8 34.8 6.61 23.2 1.4 3193 16.1 220

(Fig. 3B) is dark red in color, soft, porous, fine-grained, and dominated by clay minerals. The highly weathered horizon (Fig. 3C) is yellow–red in color, porous, fine-grained, and contains a small amount of quartz along with clay minerals. Fine-grained and weathering-resistant quartz can easily be distinguished from the coexisting clay minerals in this unit. Therefore, the presence of quartz distinguishes the highly and fully weathered units. The semi-weathered unit (Fig. 3D) is pale white, porous, friable, and contains randomly distributed spheroidal residuals of unweathered bedrocks in the lower part of the unit. The unweathered felsic volcanic rocks are dark gray in color and typically have vitrophyric and porphyritic textures (Fig. 3E). To identify a representative REE-rich regolith profile for further detailed investigation, oxalic acid and ammonium sulfate processing was firstly conducted at various sites in the field. Based on this geochemical surveying, a typical regolith profile with significant REE enrichment was selected for sampling and laboratory analyses. The selected profile overlies rhyolite on a flat hilltop, and is almost 8 m thick (Figs. 2C and 3A). The profile was systematically sampled from the fresh parent rock through to the semi-weathered, highly weathered, and fully weathered units (Fig. 3A–E). Twelve samples from the regolith profile (CZ-1 to CZ-18) were collected. In addition, five samples of the unweathered felsic volcanic rocks (B1–B5) were randomly collected within the study area.

Parent rock

(ICP–MS) analyses. Prior to the XRD and whole-rock geochemical analyses, the samples were air-dried, crushed, powdered in an agate mortar, and sieved to 200 mesh. Electron microprobe (EMP) and laser inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of accessory minerals were undertaken on polished thin-sections. The regolith profile samples (CZ-1 to CZ-18) were also analyzed by scanning electron microscopy energy dispersive spectrometry (SEM–EDS), and for organic matter content, pH, and sequential elemental extraction (SEE). Most of the analytical work was conducted at the Key Laboratory of the Guangxi Hidden Metallic Ore Deposits Exploration, Guanxi, China. Except that, the SEE experiment is conducted at the laboratory of the China Nonferrous Metals (Guilin) Geology and Ming institute, Guanxi, China, and the XRF and ICP–MS analyses were carried out at the Chinese National Research Center of Geoanalysis, Beijing, China. The XRD data were obtained with a Philips X’Pert MPD diffractometer using a Cu target, and operated at 40 kV and 40 mA. The range of 2θ scanning was from 5° to 80°, and the scan step and step duration were 0.05° and 3 s, respectively. A standard sample grinding and mounting procedure was performed prior to XRD analysis. Qualitative analysis was first conducted to obtain the mineral compositions of the samples with Highscore software. The XRD-Rietveld method was then used to obtain quantitative data. The basis of this technique for quantitative analysis is full-spectrum fitting refinement (Rietveld, 2014), which was carried out with Topas software (academic V5.1). EMP analysis was conducted on polished thin-sections to determine the accessory minerals in the parent rock samples, and was carried out with a JXA-8230 EMP and a wavelength dispersive system. SEM–EDS

2.3. Analytical methods The fresh felsic volcanic rock samples were subjected to petrographic observations, and quantitative X-ray diffraction (QXRD), X-ray fluorescence (XRF), and inductively coupled plasma–mass spectrometry 5

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error was ≤5% at the ppm level. The in-run signal intensity for indicative trace elements was monitored during analysis to ensure that the laser did not ablate mineral inclusions in the target mineral. The pH analysis used a Mettler Toledo SG23 pH/Eh tester, with a precision of 0.01. For the pH measurements, 5 g of powdered sample (200 mesh) was added to 25 mL of ultrapure water and shaken intermittently for 30 min, and then the supernatant was analyzed. The pH test was undertaken at a temperature of 25.5 °C. The organic matter content was determined with the dichromate oxidation method (Nelson and Sommers, 1982). In brief, 1 g of soil was oxidized at 150 °C for 1 h with 25 mL of a mixture of potassium dichromate and sulfuric acid. A reverse titration of Cr2O72– ions was then conducted using an acidified ferrous ammonium sulfate solution. For the whole-rock geochemistry analyses, the samples were first dried at 70 °C, and then both the dried regolith and fresh parent rock samples were powdered. The powder was heated at 105 °C to remove adsorbed water prior to analysis. Major elements were measured by XRF spectrometry. The analytical precision for the major elements is better than ± 1%, and the detection limits are generally better than 30 ppm. Trace elements were measured with a X-Series ICP–MS. Total analytical errors for trace elements are ± 6% (1σ). Detailed procedures for the trace elements analysis are similar to those of Fu et al. (2019). A seven-step sequential extraction was performed to determine the REE speciation of the regolith samples. The REE extraction procedure was described by Shi et al. (2014). After each extraction step, the collected filtrate is analyzed for its REE concentration by ICP–MS. The total recovery of all seven REE fractions relative to the total REE content is 100% ± 10%.

Table 2 Major and rare earth element contents of the felsic volcanics in the study area. Note: Sample B1–B5 data are from this study. Elements

SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO Na2O P2O5 TiO2 LOI (%) Total (%) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ∑REE LREEs/HREEs Ce/Ce* Eu/Eu*

Felsic volcanic rocks B1

B2

B3

B4

B5

69.49 14.11 1.75 4.26 3.96 0.89 0.03 2.53 0.19 0.55 1.58 99.04 59.9 128 13.6 61.5 12.3 1.22 14.7 2.09 10.7 2.31 7.9 1.28 7.89 1.18 74.3 324.57 5.75 1.10 0.28

72.05 10.98 1.74 4.33 4.56 0.4 0.03 2.73 0.19 0.45 1.77 99.23 62.5 129 14.3 60.6 14.6 1.01 14.5 2.11 14.2 2.44 8.55 1.12 8.29 1.05 70.9 334.27 5.40 1.06 0.21

71.77 11.85 1.52 4.54 3.32 0.54 0.12 2.72 0.18 0.58 1.92 99.06 66.2 132 15.8 59.9 14.2 1.58 15 2.96 15.5 3.31 9.44 0.89 8.85 1.11 73.8 346.74 5.08 1.00 0.33

71.72 10.77 1.27 5.01 3.78 0.53 0.04 2.99 0.17 0.6 1.86 98.74 68.9 136 14.6 68.5 15.7 1.77 17.1 2.63 16.8 3.03 10.2 1.37 9.3 1.24 83.2 367.14 4.95 1.05 0.33

71.56 12.6 1.26 3.73 4.71 0.61 0.03 2.22 0.16 0.53 1.29 98.7 70.2 137 16.4 72.4 16 1.92 17.3 2.9 16.8 3.34 9.47 1.42 9.27 1.38 87.3 375.8 5.07 0.99 0.35

3. Results 3.1. Mineralogy and petrology

analyses were conducted to determine the clay minerals in the regolith samples, and were undertaken with a field emission–scanning electron microscope (FE–SEM; SIGMA series, Zeiss) equipped with an EDS X-ray spectrometer. The FE–SEM was operated at an accelerating voltage of 0.1–30 kV and probe current of 4 pA to 20nA. The resolution of this instrument is 1.3 nm and the maximum magnification is 1 million times. Prior to testing, fine particles of the regolith samples were selected and mounted on the surface of a glass slide with epoxy resin. In situ LA–ICP–MS analysis was used to determine the REE concentrations of accessory minerals in the felsic volcanic rocks. These analyses were conducted with an Agilent 7500 ICP–MS coupled to a 193 nm laser. The diameter of the laser ablation spot was 32 μm. The NIST 610 standard glass was used as a calibration standard for all samples, with 29Si and 43Ca used as internal standards for quantitative silicate and REE-bearing mineral analysis, respectively. The analytical

The felsic volcanic rocks have vitrophyric to porphyritic textures, containing many anhedral phenocrysts of K-feldspar, plagioclase, quartz, and biotite of various sizes (Fig. 4A). The phenocrysts are set in a fine-grained matrix that contains abundant glassy material with irregular sharp edges (Fig. 4B). Accessory minerals include F-bearing apatite, titanite, ilmenite, allanite, zircon, and minor monazite (Fig. 4C–F). Apatite and titanite are the major accessory minerals in the rhyolites. Apatite has a stumpy, elongate, or hexagonal prismatic shape, is up to 22 μm long, and occurs mostly as inclusions within biotite. Titanite and allanite are anhedral and found as inclusions in biotite and feldspar. The mineralogy of the regolith samples was determined by XRD (Fig. 3F–G). Residual rock-forming minerals (quartz, K-feldspar, and plagioclase) and clay minerals (kaolinite, halloysite, and occasional illite) are the common minerals in most samples. The total clay mineral

Fig. 5. Geochemical diagrams for the studied felsic volcanic rocks. (A) TAS taxonomy from Bas et al. (1986); (B) A/NK-A/CNK diagram from Maniar and Piccoli (1989); (C) SiO2-K2O diagram. 6

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Table 3 REE geochemistry data of the studied rhyolite-derived profile (in ppm). Note: ΣREE = Σ(La—Lu); LREEs = La—Eu; HREEs = Gd—Lu; Ce/Ce* = CeN/(LaN × PrN)1/ 2, Eu/Eu* = EuN/(SmN × GdN)1/2, where subscript N represents normalization by C1-chondrite (Sun and McDonough, 1989). Horizon

Full-weathered horizon

Highly-weathered horizon

Semi-weathered horizon

Parent rock

Sample Depth/m

CZ-1 0.4

CZ-2 0.8

CZ-5 1.6

CZ-6 2

CZ-7 2.5

CZ-8 3

CZ-10 3.9

CZ-12 5

CZ-13 5.6

CZ-15 6.6

CZ-17 8

CZ-18 9.0

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ∑REE LREEs/HREEs Ce/Ce* Eu/Eu*

121 307 26.1 110 19.1 2.85 17.8 2.79 15.7 3.04 8.52 1.15 7.5 1.13 75.2 643.7 10.17 1.34 0.47

177 218 39.9 165 29.9 4.36 27.3 4.27 24.9 4.87 13.6 1.89 12.1 1.8 124 724.9 6.99 0.64 0.47

322 157 70.7 284 57.7 9.04 58.5 9.36 51.5 9.37 25.7 3.34 21.4 3.1 250 1082.7 4.94 0.26 0.48

285 81.9 60.7 245 49.3 7.91 52.1 8.56 47.3 8.95 24.6 3.24 20.1 2.98 248 897.6 4.35 0.15 0.48

553 150 114 444 98.9 16.4 109 18.1 104 19.6 52.9 7.0 43.4 6.28 553 1736.6 3.82 0.15 0.48

397 181 86.3 350 75.1 12.1 84.3 13.6 78.6 14.5 39.9 5.26 31.6 4.58 427 1373.8 4.04 0.24 0.46

159 119 35.4 146 31.9 5.2 35.6 5.97 35 6.71 17.9 2.5 15.6 2.31 194 618.1 4.08 0.39 0.47

125 176 27.8 115 22.8 3.42 23.7 4.0 22.7 4.31 12 1.68 10.8 1.58 116 550.8 5.82 0.73 0.45

127 200 28 118 23.6 3.71 24.5 3.96 23.2 4.44 12.5 1.72 11.2 1.65 123 583.5 6.02 0.82 0.47

122 221 27.6 116 23.2 3.58 24 3.96 22.5 4.31 12 1.69 10.8 1.61 112 594.3 6.35 0.93 0.46

113 152 26.3 109 22.7 3.37 23 3.63 20.8 4.03 11.2 1.57 10.3 1.54 105 502.4 5.60 0.68 0.45

70.2 137 16.4 72.4 16 1.92 17.3 2.9 16.8 3.34 9.47 1.42 9.27 1.38 87.3 375.8 5.07 0.99 0.35

Fig. 6. Vertical variations of some important REEs indices throughout the studied regolith profile, including total REE content, LREE/HREE ratios, Ce anomaly values, Eu anomaly values and CIA values.

(2.22–2.99 wt%), and K2O (3.32–4.71 wt%). These data suggest that the felsic volcanic rocks belong to a typical peraluminous, high-K, calc-alkaline, dacite–rhyolite series (Fig. 5A–C), which is consistent with previous studies (e.g., Feng and Fang, 1986; Qin et al., 2011). Compared with the parent rocks, all the regolith samples have higher Al2O3 and Fe2O3 contents, and slightly lower SiO2 contents. In addition, Na2O, CaO, K2O, MgO, and P2O5 contents are significantly lower in the regolith profile. Rb, Sr, and Ba concentrations are depleted in the regolith samples compared with the parent rocks, whereas Zr, Nb, Hf, and Th are enriched. Some redox-sensitive trace elements, including Cr, As, Sb, and U, show no clear trends through the regolith profile. The organic matter content varies widely from 0.06 to 5.82 wt% in the analyzed samples, with the highest value for the fully weathered unit (CZ-2) and the lowest for the bedrock (CZ-18). The organic matter content displays a decreasing trend downwards in the regolith profile: 4.57–5.82 wt% (average = 5.20 wt%) in the fully weathered unit;

content in the highly weathered unit is double that in the semiweathered unit. Kaolinite was identified by the characteristic diffraction peaks at d = 7.13, 4.46, and 3.5 Å in XRD spectra and its irregular and hexagonal shape in SEM images (Fig. 4G). Halloysite was identified from the characteristic diffraction peaks at d = 3.34, 10, and 5.02 Å, and its hair- or tube-like morphology (Fig. 4H). XRD-Rietveld quantitative analysis further revealed that the kaolinite content increases upsection (16% → 25% → 58%), whereas the halloysite content decreases.

3.2. Whole-rock geochemistry and organic matter The whole-rock geochemical, pH, and organic matter content data are listed in Table 1. The fresh felsic volcanic rocks have bulk geochemical compositions dominated by Al2O3 (10.77–14.11 wt%) and SiO2 (69.49–72.05 wt%) (Table 2). Other major and minor components include Fe2O3 (3.73–5.01 wt%), CaO (1.26–1.75 wt%), Na2O 7

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Fig. 7. Chondrite-normalized REE pattern of the parent rocks and regolith samples.

have total REE concentrations that are generally < 100 ppm, in the following order: volcanic glass (average = 67 ppm) > biotite (average = 55 ppm) > plagioclase (average = 46 ppm) > K-feldspar (average = 36 ppm) > quartz (average = 7 ppm). REE data for the accessory minerals (apatite, titanite, allanite, and zircon) are listed in Tables 4 and 5. The accessory minerals have large variations in REE concentrations, from < 1000 ppm (e.g., ilmenite) to > 100,000 ppm (e.g., titanite and allanite). These concentrations are similar to those reported in several previous studies. For example, allanite from Guanxi and Qingxi has REE concentrations of 15.76 to 20.46 wt%, and titanite from Zahibei and Guanxi has REE concentrations of 7307 to 11,793 ppm (e.g., Wang et al., 2014; He et al., 2016). In general, the trend of REE concentrations in the accessory minerals is titanite (average = 15.1 wt%) > allanite (average = 14.9 wt %) > apatite (average = 1510 ppm) > zircon (average = 1376 ppm). The accessory minerals have much higher REE concentrations than the main rock-forming minerals. In chondrite-normalized REE diagrams (Fig. 8), apatite, zircon, mica, plagioclase, and K-feldspar are LREE enriched, whereas zircon is HREE enriched. The studied minerals show negative Eu/Eu* anomalies of various sizes. These REE patterns are broadly consistent with those for minerals formed in felsic magmatic systems (Henderson, 1984).

1.03–2.35 wt% (average = 1.58 wt%) in the highly weathered unit; and 0.29–2.57 wt% (average = 1.22 wt%) in the semi-weathered unit. 3.3. Whole-rock rare earth element data Table 3 presents the whole-rock REE data for the parent rock and regolith samples. The vertical variations of REE parameters are shown in Fig. 6. The total REE (TREE) concentrations of all the regolith samples range from 502 to 1737 ppm, with an average of 846 ppm, is significantly higher than that of the parent rock (376 ppm). The highest TREE concentrations are found for the highly weathered unit (CZ-7), whereas the lowest concentrations are found for the semi-weathered unit (CZ-17). For each unit, TREE concentrations vary significantly from the fully weathered unit (644–725 ppm; average = 684 ppm) to the highly weathered unit (618–1737 ppm) and semi-weathered unit (502–594 ppm). Light REE/heavy REE (LREE/HREE) ratios vary from 3.82 to 10.17 through the profile, indicating that the LREEs are enriched relative to the HREEs. The highest LREE/HREE ratio (10.17; CZ-1) was measured at the top of the profile, and this ratio shows an overall decreasing trend down-section in each unit (8.58 → 4.25 → 5.95 → 5.07). Ce/Ce* anomalies of all the weathered units (δCe/Ce* = 0.15–0.93) are uniformly negative, with the exception of sample CZ-1 (δCe/Ce* = 1.34) in the fully weathered unit. Eu/Eu* anomalies are typically negative (0.45–0.48; average = 0.47), and slightly less negative than that of the parent rock (0.35). In chondrite-normalized REE diagrams (Fig. 7), the regolith samples have LREE-enriched patterns and negative Eu/Eu* anomalies. The patterns are parallel to that of the parent felsic volcanic rock, but show pronounced negative Ce/Ce* anomalies.

3.5. Rare earth element speciation REE speciation data for the weathered rhyolite samples are summarized in Tables 6 and S1. Percentages of each extracted REE fraction relative to the total REE content of each sample are presented in Fig. 9, and the distribution pattern of each fraction by chondrite normalization is shown in Fig. 10. Most of the REEs are present in the ion-exchangeable fraction (F2), followed by the residue (F7), carbonatebound or specific adsorption (F3), Fe–Mn oxide (F5), strongly organically bound (F6), humic acid (F4), and water-soluble (F1) fractions. The fraction of ion-exchangeable REEs (iREEs) is of economic significance and, in these samples, makes up 46.6%–85.5% of the TREE

3.4. Mineral rare earth element data REE data for the main rock-forming minerals (K-feldspar, plagioclase, quartz, biotite, and glass) are listed in Table 4. These minerals 8

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Table 4 LA-ICP MS analyzed REE chemistry data (in ppm) of K-feldspar, plagioclase, quartz, glassy material, biotite, zircon and apatite from the parent rock. Elements

K-feldspar

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ∑REE LREEs/HREEs Eu/Eu* Ce/Ce*

16.98 25.30 2.64 6.87 0.72 0.08 0.33 0.06 0.27 0.04 0.13 0.02 0.13 0.02 1.61 53.58 52.71 0.42 0.83

Elements

Biotite

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ∑REE LREEs/HREEs Eu/Eu* Ce/Ce*

32.92 48.42 4.38 15.19 1.12 0.11 0.79 0.11 0.46 0.07 0.26 0.04 0.24 0.03 2.92 104.16 50.85 0.35 0.86

Plagioclase 12.32 17.31 1.78 5.01 0.53 0.04 0.22 0.03 0.14 0.03 0.07 0.02 0.11 0.01 0.93 37.63 57.67 0.28 0.80

6.42 9.55 1.03 2.29 0.31 0.05 0.13 0.03 0.10 0.02 0.06 0.01 0.06 0.01 0.63 20.06 48.77 0.65 0.82

10.72 15.35 1.54 4.45 0.41 0.07 0.26 0.03 0.20 0.02 0.08 0.01 0.08 0.01 0.85 33.23 46.72 0.61 0.82

22.27 34.56 3.27 9.06 0.81 0.23 0.43 0.06 0.35 0.04 0.17 0.03 0.20 0.03 1.81 71.70 53.66 1.07 0.88

Quartz 8.32 12.35 1.31 3.44 0.44 0.05 0.15 0.02 0.13 0.02 0.07 0.01 0.06 0.01 0.71 26.32 54.28 0.48 0.83

12.33 18.54 1.80 4.89 0.78 0.09 0.22 0.03 0.22 0.03 0.11 0.01 0.10 0.02 1.05 39.14 51.50 0.50 0.85

Zircon 5.67 8.45 0.78 2.53 0.37 0.06 0.20 0.03 0.17 0.03 0.08 0.02 0.15 0.02 1.15 18.55 25.79 0.62 0.86

9.38 13.43 1.34 3.89 0.36 0.06 0.23 0.03 0.17 0.02 0.07 0.01 0.07 0.01 0.75 29.07 46.72 0.61 0.82

21.57 31.95 3.24 8.32 0.97 0.09 0.59 0.06 0.33 0.07 0.21 0.03 0.21 0.03 1.74 67.67 43.45 0.33 0.83

Volcanic glass

2.08 4.02 0.55 1.67 0.29 0.05 0.38 0.05 0.32 0.06 0.18 0.03 0.22 0.03 1.46 9.94 6.76 0.47 0.93

1.93 3.84 0.46 1.63 0.25 0.04 0.30 0.04 0.24 0.05 0.14 0.02 0.14 0.02 1.56 9.08 8.70 0.48 1

1.02 2.66 0.32 1.12 0.20 0.02 0.19 0.02 0.14 0.03 0.08 0.02 0.10 0.02 0.85 5.92 9.13 0.38 1.15

1.04 2.01 0.21 0.60 0.08 0.02 0.15 0.02 0.13 0.03 0.09 0.01 0.10 0.02 0.66 4.50 7.28 0.45 1.06

12.4 25.0 3.3 9.6 2.3 0.8 2.6 0.5 3.0 0.7 1.9 0.3 2.0 0.3 4.4 64.7 4.7 0.99 0.95

8.9 16.9 2.4 10.0 2.4 0.9 3.0 0.6 3.8 0.8 2.4 0.4 2.5 0.4 4.4 55.3 3.0 1.01 0.9

10.4 19.0 2.7 10.8 2.8 1.0 3.4 0.6 3.7 0.9 2.6 0.4 2.9 0.4 5.0 61.8 3.1 1.03 0.88

14.2 28.3 3.7 13.6 4.0 1.5 4.9 0.8 5.4 1.2 3.4 0.5 3.6 0.5 4.1 85.6 3.2 1.05 0.96

Apatite

0.26 1.88 0.18 1.9 2.36 0.12 27.1 15.9 178 70.9 351 66.6 548 106 1980 1370.2 0 0.03 2.05

0.8 2.78 0.39 2.79 4.54 0.23 30.7 14.3 185 80.9 323 69.9 521 145 1756 1381.33 0.01 0.04 1.21

492.27 745.88 81.24 220.66 22.64 4.08 20.03 1.94 8.33 1.55 5.33 0.73 5.64 0.69 57.63 1611.04 35.4 0.57 0.83

443.43 690.17 65.33 194.61 25.3 2.38 13.43 1.97 9.05 1.38 4.71 0.7 3.4 0.72 47.95 1456.56 40.2 0.36 0.88

469.47 727.06 69.51 191.75 18.81 3.11 15.07 1.97 6.46 1.14 4.73 0.61 6.31 0.69 54.03 1516.7 40.01 0.55 0.88

446.13 692.86 72.44 224.41 23.75 2.6 8.85 1.83 9.74 2.14 5.66 0.67 4.1 0.73 59.45 1495.9 43.36 0.45 0.86

460.32 676.19 69.13 206.91 19.74 3.5 13.46 1.61 10.26 1.21 4.34 0.51 4.3 0.41 45.11 1471.89 39.77 0.62 0.83

4. Discussion

concentration. The highest values are in the highly weathered unit, in which clay minerals are abundant (Fig. 11). The fraction of residual REEs is 2.4%–29.2% of the TREE concentration, with higher values in the lower part of the profile where there has been less weathering. Only small amounts of REEs (4.98%–8.75%) are present in the carbonatebound or specific adsorption fraction (F3). Given that there are no carbonate minerals in the profile, this REE fraction can mainly be attributed to specific adsorption by secondary oxides or clay minerals (Wang et al., 1997). The Fe–Mn oxide (F5) and strongly organically bound (F6) fractions contain 1.17%–11.11% and 2.05%–22.30% of the REEs, respectively. Higher percentages are likely related to higher concentrations of organic matter and Fe–Mn oxide minerals. The REE percentages of the water-soluble (F1) and humic acid (F4) fractions are low (0.03%–0.12% and 0.42%–1.71%, respectively). In chondrite-normalized REE diagrams, all seven REE fractions show general LREE enrichment and negative Eu/Eu* anomalies (Fig. 10). However, the Ce/Ce* anomaly varies in the different REE fractions. The ion-exchangeable fraction is characterized by a negative Ce/Ce* anomaly, which ranges from 0.08 to 0.43, similar to those of the specific adsorption (F3), water-soluble (F1), and humic acid (F4) fractions. In contrast, the Fe–Mn oxide (F5) and organic-bound (F6) fractions have marked positive Ce/Ce* anomalies (Fig. 12C), with 6.91–40.9 and 14.62–78.65, respectively.

4.1. Source minerals of the iREEs The parent rock is the primary source of REEs in a regolith, if an external REE input does not exist (e.g., Alderton et al., 1980; Sawka and Chappell, 1988; Wu et al., 1990; Bao and Zhao, 2008; Fu et al., 2019). The initial REE contents of the felsic volcanic rocks in the present study area are 324–375 ppm, which is higher than the average REE content of granitic rocks (289 ppm) that generate ion adsorption-type REE deposits (Zhao et al., 2014). In the rhyolite, accessory minerals have high REE concentrations, including titanite, allanite, apatite, zircon, and ilmenite (Fig. 4D–F). The accessory minerals typically contain 10 to 10,000 times more REEs than the main rock-forming minerals, such as glass, biotite, plagioclase, K-feldspar, and quartz. The accessory minerals contain 88.7% of the REEs in these samples, compared with 11.3% in the main rock-forming minerals (Table S2). As such, the REEs in the rhyolites are related mainly to the accessory mineral assemblage (i.e., allanite + apatite + titanite + zircon), which is consistent with previous studies (e.g., Bao and Zhao, 2008; Braun et al., 2018; Fu et al., 2019; Li et al., 2019). Theoretically, only the accessory minerals that can be weathered can contribute mobile REEs to the regolith and generate iREEs. Those accessory minerals that are resistant to weathering retain the REEs in the residual form, with less links to iREEs source (e.g., Braun et al., 1998, 2018; Sanematsu and Watanabe, 2016). Apatite, allanite, and 9

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Table 5 Electron microprobe analyzed REE chemistry data (wt%) of titanite, allanite, zircon and ilmenite from the parent rock. Elements

Titanite

SiO2 Al2O3 FeO CaO MgO Na2O TiO2 MnO P2O5 F Cl Cr2O3 ThO2 UO2 Nb2O5 SrO ZrO2 HfO2 La2O3 Ce2O3 Pr2O3 Nd2O3 Sm2O3 Yb2O3 Y2O3 Total ∑REE LREEs/HREEs

31.91 15.05 13.39 12.47 0.06 0.17 6.65 0.18 0.09 0.06 0.03 0.24 b.d.l. b.d.l. b.d.l. 0.12 0.04 b.d.l. 2.91 8.36 1.22 5.21 b.d.l. b.d.l. 0.66 98.82 15.13 29.1

31.96 15.07 13.37 12.82 0.04 0.06 6.18 0.13 0.15 0.30 0.03 0.17 0.02 b.d.l. b.d.l. 0.13 0.08 b.d.l. 2.84 8.52 1.17 5.09 b.d.l. b.d.l. 0.71 98.84 15.05 26.88

Allanite

Zircon

32.49 16.04 14.13 15.05 0.04 0.01 2.15 0.13 0.11 0.21 0.02 0.29 b.d.l. b.d.l. b.d.l. 0.13 0.03 b.d.l. 2.98 7.86 1.28 4.56 b.d.l. 0.03 0.75 98.29 14.87 22.98

34.82 0.10 0.42 0.26 b.d.l. b.d.l. 1.29 0.01 b.d.l. 0.13 b.d.l. 1.13 0.06 b.d.l. 0.03 0.04 61.04 b.d.l. b.d.l. 0.05 b.d.l. b.d.l. b.d.l. 0.09 b.d.l. 99.47 0.12 0.44

Ilmenite 37.28 0.23 0.02 0.06 0.02 b.d.l. 0.02 b.d.l. b.d.l. 0.10 0.01 0.13 0.05 b.d.l. 0.05 0.08 60.12 b.d.l. b.d.l. b.d.l. 0.03 b.d.l. b.d.l. 0.02 b.d.l. 98.22 0.05 1.5

33.26 0.18 0.05 0.02 b.d.l. b.d.l. 0.09 b.d.l. b.d.l. 0.07 b.d.l. 0.31 0.09 0.02 0.05 0.06 63.14 0.82 b.d.l. 0.04 0.04 b.d.l. b.d.l. 0.03 b.d.l. 98.27 0.09 2

0.04 3.98 44.82 0.02 0.07 b.d.l. 52.06 0.38 b.d.l. 0.05 b.d.l. 0.21 b.d.l. b.d.l. 0.11 0.02 0.12 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 101.88 – –

0.09 0.16 44.91 0.02 0.05 0.01 55.15 0.38 b.d.l. b.d.l. b.d.l. 0.33 b.d.l. b.d.l. 0.10 b.d.l. 0.07 b.d.l. b.d.l. b.d.l. b.d.l. 0.03 b.d.l. b.d.l. b.d.l. 101.31 0.03 –

bb.d.l., below the detection limits.

to Dessert et al. (2001), felsic volcanic rocks generally have weathering rates that are 5–10 times higher than those of common intrusive igneous rocks (e.g., granite). Using the chemical index of alteration (CIA = [Al2O3/(Al2O3 + CaO + Na2O + K2O)] × 100 (%); Nesbitt and Young, 1982), we confirmed that the studied rhyolite has experienced strong chemical weathering, with CIA values of up to 99%. This is also consistent with the pronounced enrichment of immobile elements (i.e., Al2O3, Zr, and Hf) and depletion of mobile elements (i.e., CaO, Na2O, and K2O) in the weathered samples. At this advanced weathering stage, liberation of REEs from source minerals is promoted, and further eluviation–illuviation processing of REEs in the regolith is likely to occur. Migration of REEs in a regolith is facilitated by bicarbonate or organic matter complexes (e.g., Dupre et al., 1999; Oliva et al., 1999). This mechanism is applicable to the present study because organically bound REEs were identified in the regolith samples by the sequential extraction experiments. The organic-matter-related REE migration might be largely restricted to the upper part of the regolith where the organic matter content is high. In addition to organic matter, F can facilitate REE migration. Abundant apatite was observed in the parent rock, which is a F-bearing primary mineral (Ca5(PO4)3(F,Cl,OH)). Apatite weathering would lead to F– release and the formation of mobile REE fluoride complexes (Wu et al., 1990; Wood, 1990; Sanematsu et al., 2013). REE fixation can occur by several mechanisms, including secondary precipitation, adsorption, and ion exchange (e.g., Banfield and Eggleton, 1989; Aubert et al., 2001; Kohler et al., 2005; Laveuf and Cornu, 2009). In the present study, the mobile REEs were fixed to various weathering products by complex speciation. Most of the REEs are present as iREEs (52%–87%), and the remainders are mainly associated with organic matter (2%–22%) and Fe–Mn oxides (1%–11%). Such variations in the REE speciation depend on the distribution of minerals and other soil components in the regolith. In the zone of significant REE enrichment (i.e., the highly weathered unit; e.g., sample CZ-7), the typical REE minerals are likely to be rhabdophane, florencite, and cerianite (e.g., Banfield and Eggleton, 1989; Aubert et al., 2001; Kohler et al., 2005; Li et al., 2017), but these were not detected by our

titanite are more easily chemically weathered than monazite and zircon (e.g., Harlavan and Erel, 2002; Price et al., 2005, 2015; Chen et al., 2006; Harouiya et al., 2007; Yusoff et al., 2013; Sengupta and Van Gosen, 2016). In the present study, it is difficult to quantify which and how much of the REE-bearing accessory minerals have contributed to the iREEs by weathering. However, some geochemical features do provide insights into the differential weathering of the REE-bearing accessory minerals in the studied rhyolite. Firstly, there is a strong depletion (up to 75%) of P2O5 in the regolith profile from the parent rock to the highly weathered samples, which can be taken as evidence of apatite weathering, because this is the main phosphate mineral in the studied rhyolite. Secondly, the sequential REE extraction experiments show a substantial decreasing trend (6.09 → 4.13) in the LREE/HREE ratios of the residual fractions. This suggests that the residual materials of the weathered products contain lower contents of minerals with high LREE/HREE ratios (e.g., apatite, allanite, and titanite), whereas contents of minerals with low LREE/HREE ratios (e.g., zircon) increase with weathering. Hence, apatite + allanite + titanite with high REE concentrations, which are readily weathered, have largely contributed to the iREEs in the regolith. The liberation of REEs from these accessory minerals is necessary for ion-exchangeable REE enrichment in the rhyolite-derived regolith. 4.2. iREEs enrichment process REE enrichment in a regolith is controlled by both the parent rock and weathering processes, as well as the geochemical behavior of REEs (e.g., Nesbitt, 1979; Braun et al., 1990, 1998; Tyler, 2004; Laveuf and Cornu, 2009). The iREE enrichment documented in the present study is related to the weathering of felsic volcanic rock. This process involved REE liberation from source minerals, REE migration within the regolith, and REE fixation by secondary clay minerals. The liberation of REEs from source rocks is affected by the degree of weathering. The studied felsic volcanic rocks are sensitive to chemical weathering, due to their glassy nature, high surface area, and high porosity (e.g., Berner and Berner, 1996; Wilson et al., 2017). According 10

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Fig. 8. Chondrite normalized REE distribution patterns for K-feldspar (A), plagioclase (B), quartz (C), volcanic glass (D), biotite (E), zircon (F), apatite (G) from the Chongzuo rhyolite. Chondrite standard values are from McDonough and Sun (1995). Table 6 Results of the seven-step extraction experiments, showing the total REE concentrations of each extraction step (in ppm) and their percentages (%). Sample

CZ-1 CZ-2 CZ-5 CZ-7 CZ-8 CZ-10 CZ-12 CZ-15 CZ-17

F1

F2

F3

F4

F5

F6

ΣREE ppm

F7

ppm

%

ppm

%

ppm

%

ppm

%

ppm

%

ppm

%

ppm

%

2.14 0.35 0.69 0.98 0.45 0.30 0.54 0.36 0.63

0.40 0.04 0.07 0.06 0.03 0.05 0.12 0.07 0.15

278.70 431.70 772.92 1493.00 1079.09 474.20 262.66 333.88 275.09

51.84 54.41 77.22 86.56 77.74 79.43 57.65 64.17 65.43

47.05 66.99 64.55 125.21 116.88 39.22 23.47 26.03 20.93

8.75 8.44 6.45 7.26 8.42 6.57 5.15 5.00 4.98

7.74 13.53 4.93 7.30 7.31 3.17 3.59 2.55 2.03

1.44 1.71 0.49 0.42 0.53 0.53 0.79 0.49 0.48

59.70 55.91 17.77 20.86 28.85 7.01 8.71 24.45 18.42

11.11 7.05 1.78 1.21 2.08 1.17 1.91 4.70 4.38

103.22 181.41 29.84 35.72 61.45 26.92 23.37 60.47 37.90

19.20 22.86 2.98 2.07 4.43 4.51 5.13 11.62 9.01

39.06 43.56 110.28 41.78 93.95 46.22 133.23 72.56 65.43

7.27 5.49 11.02 2.42 6.77 7.74 29.24 13.95 15.56

537.62 793.45 1000.99 1724.85 1387.98 597.04 455.58 520.30 420.44

F1-Water soluble fraction, F2-Ion-exchangeable fraction, F3-Specific adsorption fraction. F4-Humic acid fraction, F5-Fe-Mn oxide fraction, F6-Organic-matter fraction, F7-Residue fraction.

Ma, 1998; Islam et al., 2002; Wilson, 2004; Dill, 2016). The glassy material in the rhyolite (Fig. 4B) may also have been important in clay mineral formation. Clay minerals can fix REEs in an ion-exchangeable form (e.g., Braun et al., 1990, 1993; Condie et al., 1995; Aubert et al., 2001; Chi et al., 2012; Berger et al., 2014), and such fixation is dependent on the nature of the clay minerals and the physico-chemical conditions (Laveuf and

detailed mineralogical investigations. However, clay minerals are well developed, and the assemblage of kaolinite + halloysite + illite accounts for up to 75.5% of the entire regolith mineralogy. Quantitative XRD data revealed that increasing clay mineral content corresponds to a decrease in mica and feldspar minerals (Fig. 3), suggesting the authigenic formation of secondary clay minerals by the hydrolysis of biotite, K-feldspar, and plagioclase (e.g., Violante and Wilson, 1983; 11

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Fig. 9. Percentages of sequential extracted REE concentrations relative to the whole REE contents in the regolith samples, determined by the method of seven-step extraction experiments.

Fig. 10. Chondrite-normalized patterns of each REE fraction determined by the sequential extraction experiment for the regolith samples.

alkaline pH conditions. This indicates that pH was not a strict factor for iREE enrichment in the regolith (Fig. 11). Fractionation of REEs determines whether LREEs or HREEs are more enriched in the regolith, which leads to LREE- or HREE-type ion adsorption-type REE ores (Yuan et al., 2012; Li et al., 2017). In the present study, the iREEs have LREE/HREE ratios of 3.78–5.93, indicative of LREE enrichment (Fig. 10). Given that the most weathered regolith sample (CZ-1) has the highest LREE/HREE ratio, the LREEs may be preferentially scavenged relative to HREEs by kaolinite-group clay minerals (Coppin et al., 2002; Yusoff et al., 2013). Notably, the iREEs do not exhibit the greatest LREE/HREE fractionation. Both the Fe–Mn oxide and organically bound REEs have larger LREE/HREE fractionations (Fig. 12-B), with values of 14.36–70.99 and 38.05–138.55, respectively. This indicates that Fe–Mn oxyhydroxides and organic matter play a more important role than clay minerals in generating LREE/ HREE fractionation in weathered products (e.g., Bau and Dulski, 1999; Ohta and Kawabe, 2001). The negative Ce/Ce* anomalies in the iREEs can also be explained by the redox-sensitive nature of Ce relative to other REEs (Bao and Zhao, 2008). In the upper part of the profile, mobile Ce3+ is prone to be oxidized to less mobile Ce4+ (e.g., Brookins,

Cornu, 2009; Sanematsu and Watanabe, 2016; Li et al., 2017). SEM observations indicate that the clay minerals in our samples are mainly present as micron- to nanometer-scale irregular flakes and hair- or tubelike particles (Fig. 4). The large specific surface area of such particles, combined with their negative charge (Sanematsu and Watanabe, 2016), results in a high cation exchange capacity (CEC). Therefore, positively charged REE cations in solution can be adsorbed to variable negative charge centers formed from broken Si–O and Al–O bonds at the edge and basal sites in kaolinite-group minerals (Chi et al., 1995; Chi and Tian, 2008; Li et al., 2017), forming an outer-sphere REE complex. The halloysite-dominated sample (CZ-7) has the highest iREE concentration (up to 1493 ppm), which is significantly higher than those of the kaolinite-dominated sample (CZ-2; 432 ppm) and illite-dominated sample (CZ-12; 263 ppm). This suggests that halloysite was the most important phase for iREE enrichment during rhyolite weathering, and the roles of kaolinite and illite appear to be less significant. In addition to the nature of the clay minerals, pH may also play a role in iREE enrichment (e.g., Deng, 1985; Bao and Zhao, 2008; Yuan et al., 2015; Li et al., 2017; Zhao et al., 2017). However, all the iREE-enriched samples formed under acidic pH conditions (4.56–5.33), rather than the expected 12

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Fig. 11. Corresponding relationships of clay minerals concentration, total REE concentration, ion-exchangeable REE concentration and pH value along with the studied profile.

mineralization appears to be controlled in part by the location and age of the felsic volcanism. The above opinion could get interpreted by combining parent rock REE geochemistry (i.e., endogenic factor) and regolith situated environment conditions (i.e., exogenous factors). The felsic volcanic rocks with a close relationship to ion adsorption-type REE ores (i.e., those of early Yanshanian and Indosinian age) are varied in terms of field occurrence and tectonic setting. The early Yanshanian felsic volcanic rocks (180–149 Ma) are exposed mainly in the interior of the South China Block in southern Jiangxi Province, northern Guangdong Province, and locally in Fujian Province (e.g., Lai and Wang, 1996; Lu, 1997; Zuo et al., 1999; Deng et al., 2004; Zhou, 2007). This period of felsic magmatism is represented by the Middle Jurassic Changpu Formation and the Late Jurassic to Early Cretaceous Jilongzhang Formation. These rocks were also coeval with basaltic magmatism, forming a typical bimodal volcanic sequence with a continental rift basin-related origin (Lai and Wang, 1996; Zhou, 2003). The Indosinian felsic volcanic rocks (260–228 Ma) are mainly found in the southwestern South China Block, within the Lower and Middle Triassic Beisi Formation in Guangxi Province (e.g., Feng and Fang, 1986; Qin et al., 2011). The present study area is covered by part of this formation, which is considered to have a subduction-related origin (Qin et al., 2011). Despite differences in age and tectonic setting, these two felsic volcanic units share some similarities in terms of lithology and geochemistry. Specifically, both are alkali-rich, peraluminous, and have high REE concentrations (typically > 300 ppm). REE concentrations range from 132 to 482 ppm (Wang and Ruan, 1989; Lu, 1997; Zhou, 2007). In particular, in Heling district in Jiangxi Province, REE concentrations are 391–482 ppm (Wang and Ruan, 1989). The total REE concentrations of the Indosinian felsic volcanic rocks are 325–376 ppm (Table 2), which are higher than the average value (209 ppm) of granite in South China (Shi et al., 2005). In broad terms, the high REE concentration is linked to the type of magmatism. The peraluminous nature of these volcanic rocks reflects the contribution of upper crustal materials to the magma source. Magma generated by partial melting of the upper crust may inherit the high-Al and -REE nature of the sedimentary protolith (Chappell and White, 1984). REEs are generally incompatible in magmatic systems,

1989; Prudencio et al., 1995; Patino et al., 2003; Ma et al., 2011), which caused a lesser amount of Ce relative to the other REEs to migrate downwards with percolating water. Thus, when the REEs in the percolating water were adsorbed by clay minerals and converted to iREEs, they inherited this negative Ce/Ce* anomaly (Fig. 10). In summary, iREE enrichment can be explained by a complex process of REE migration and changes in speciation as the rhyolite was transformed to regolith (Fig. 13). The enrichment was controlled by the parent rock and by REE redistribution in the regolith. Strong REE eluviation–illuviation occurred during rhyolite weathering, which drove the generation and gradual enrichment of the iREEs from source (mainly allanite + apatite + titanite) to sink (mainly kaolinite + halloysite + illite).

4.3. Factors controlling REE mineralization by felsic volcanic rocks weathering Li et al. (2017) proposed that the nature and tectonic setting of parent granites and felsic volcanic rocks do not control the formation of ion adsorption-type REE deposits. However, despite the wide distribution of felsic volcanic rocks in South China, related ion adsorption-type REE deposits are rather limited. We compared ion adsorption-type REE deposits associated with felsic volcanic rocks of various ages (Jingningian, Indosinian, early Yanshanian, and late Yanshanian) as shown in Fig. 1, and found that felsic volcanic rocks of early Yanshanian and Indosinian ages are preferentially linked to the formation of ion adsorption-type REE mineralization. In areas of early Yanshanian felsic volcanic rocks, a number of ion adsorption-type REE deposits have been found in the Anyuan–Xunwu and Longnan–Quannan regions in southern Jiangxi Province, as well as in the Heping–Longchuan and Pingyuan–Meixian regions in northern Guangdong Province (Wu, 1989; Zhang, 1990; Yuan et al., 2012). In areas of Indosinian felsic volcanic rocks, several deposits of this type (e.g., the Liutang deposit; Hu, 2016) have been discovered in the southwestern Guangxi Autonomous Region. In contrast, few deposits have been reported in areas of Jingningian and late Yanshanian felsic volcanic rocks. As such, the formation of these ion adsorption-type REE 13

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Fig. 12. Variation of the REE geochemical signatures in various speciations.

mobility of REEs in the alkaline melt (Kogarko, 1990). Thus, protracted magma fractionation and crystallization can result in felsic volcanic rocks with high concentrations of REEs, and late-stage crystallization of apatite, monazite, and titanite will incorporate large amounts of LREEs (e.g., Christiansen et al., 1986; Sanematsu and Watanabe, 2016). In

become enriched in the melt during fractionation, and are preferentially concentrated in mineral phases during late-stage crystallization (e.g., Miller and Mittlefehldt, 1982; Cameron and Cameron, 1986; Medlin et al., 2015). Moreover, an alkali-rich magma can enhance the solubility of volatiles in the melt, which increases the 14

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Fig. 13. Schematic diagram illustrating the REE migration and redistribution process from source to sink during the rhyolite weathering, with emphasize on the REEbearing minerals evolution path, the REE speciations transformation, as well as the environmental factors related to REE geochemical behaviors.

Guangxi Province, have been subjected to prolonged weathering and formed an iREE-enriched regolith. The REEs are mainly bound to clay minerals and present in an ion-exchangeable form. iREE enrichment was a result of a parent rhyolite with high REE concentrations and strong eluviation–illuviation REE processing induced by intense weathering. The source of the iREEs was traced to titanite + allanite + apatite, which together with zircon and monazite provided most of the REEs. The sink of the iREEs is mainly kaolinite + halloysite + illite, which had different capacities to fix iREEs (i.e., halloysite > kaolinite > illite). The LREEs are enriched relative to HREEs in the iREEs, which was largely inherited from the parent rhyolite REE signature. Although there are large areas of felsic volcanic rocks with different ages and tectonic settings in South China, those of early Yanshanian and Indosinian ages are more likely to be associated with ion adsorptiontype REE mineralization. This suggests that formation of these deposits is not only related to high REE concentrations in the felsic volcanic rocks, but also an ideal climate for weathering and a quiescent tectonic setting for preservation of the deposits.

particular, a moderate degree of magmatic differentiation is expected to generate LREE-enriched felsic volcanic rocks (Li et al., 2017). In some cases, the felsic volcanic rocks associated with limited ion adsorption-type REE deposits (i.e., those of Jingningian (1000–850 Ma) and late Yanshanian (116–65 Ma) ages) do have high REE concentrations. For example, REE concentrations of Jingningian felsic volcanic rocks vary from 82 to 821 ppm (e.g., Lu and Gu, 2007; Lu et al., 2009; Jowitt et al., 2017), and those of late Yanshanian age range from 132 to 482 ppm (Wang and Ruan, 1989; Lu, 1997; Zhou, 2007). Hence, the main reason for the scarcity of ion adsorption-type REE mineralization in these two felsic volcanic rock weathered terrains is likely associated with factors other than the nature of the parent rock. The Jingningian felsic volcanic rocks are distributed in the northeastern South China Block (e.g., Lu and Gu, 2007; Lu et al., 2009), with only a relatively small outcrop area (Fig. 1). This location is nearly out of the climate window (22–29°N; Zhang, 1990; Wang et al., 2013) required for the development of ion adsorption-type REE ores. Thus, in terms of both the source rock distribution and climatic conditions, this is not an optimal site for generating ion adsorption-type REE mineralization. The late Yanshanian felsic volcanic rocks are distributed mainly along the coastal region of the South China Block (e.g., Lu, 1997; Zhang et al., 1999; Li and Zhou, 2000; Zhou et al., 2006). Tectonically, this is an area of uplift due to the strong compression caused by subduction of the Pacific Plate since the late Paleozoic (Yu, 1987; Shu et al., 2004). As such, the preservation of regolith in this area is poor due to the strong erosion associated with uplift. This likely explains why there are few ion adsorption-type REE mineralizations in the late Yanshanian felsic volcanic rock weathered terrain, despite it being the largest felsic volcanic rock unit in South China.

Acknowledgments This research was financially supported by the National Natural Science Foundation of China (41462005), Guangxi Natural Science Foundation (2014GXNSFAA118304), and the project of Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi.

Appendix A. Supplementary data 5. Conclusions

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.oregeorev.2019.103120.

Indosinian felsic volcanic rocks cropping out in the Chongzuo area, 15

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