Mineralogical and geochemical investigations on the iron-rich gibbsitic bauxite in Yongjiang basin, SW China

Mineralogical and geochemical investigations on the iron-rich gibbsitic bauxite in Yongjiang basin, SW China

Jiahao Chen

^a

, Qingfei Wang

^a,⁎

, Qizuan Zhang

^b

, Emmanuel John M. Carranza

^c,d

, Jiaqi Wang

^a

aState Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China

bThe Bureau of Geo-exploration Guangxi and Mineral Development, Nanning 530023, China

cEconomic Geology Research Centre (EGRU), James Cook University, Townsville, Queensland, Australia

dInstitute of Geosciences, State University of Campinas (Unicamp), Campinas, São Paulo, Brazil

A R T I C L E I N F O

Keywords:

Yongjiang basin Iron-rich gibbsitic bauxite Geochemistry and mineralogy Metallogenic process

A B S T R A C T

Iron-rich gibbsitic bauxite, located in the Yongjiang basin, southwestern part of South China Block, characterized by nodules mixed with loose clayey soil. Bauxite ores consist of gibbsite, kaolinite, hematite and goethite and few zircon and quartz. The nodules have higher ratios of Al/Si and stronger ferrallitization compared to the enclosing soil. The nodules have a lower content of Rb and Cs that were easily depleted during the weathering process, and a higher content of Ga, Sc, V, Cr, Ni, Co, Pb and Th that enriches due to the occurrence of Fe-Mn minerals and zircon grains than the clayey soil. Chondrite-normalized REE patterns of the nodules reflect en- richment in LREE compared to HREE and negative Eu anomalies. Bauxite samples mostly show positive Ce anomalies, indicating an oxidizing condition during deposition. Ultra-high Ce anomalies in nodules are asso- ciated with Mn oxides, suggesting an enrichment in Mn oxide during the process of nodule formation. The facts including lower content of aluminium hydroxide in iron-rich gibbsitic bauxite than karst bauxite, similarity in ore structure and distribution of trace elements and REE between iron-rich gibbsitic bauxite and karst bauxite suggest the bauxite in Yongjiang basin is the precursor of karst bauxite. Based on the mineralogical and geo- chemical (especially for the plots of Ni vs Cr and Eu/Eu* vs TiO2/Al2O3) analyses, it indicates that source material for the bauxite includes the weathering products of surrounding granites, clastic rocks and substrate carbonate rocks. It is proposed that the iron-rich gibbsitic bauxite experienced the deposition of a primary layer of a mixture of clay and gibbsite which later become mixed with the ferrallitic soil to form the nodules and clayey soil. After that, a breakup process followed concomitant with the thorough mixing with the clayey soil matrix.

1. Introduction

Bauxite deposits, containing high content of aluminium hydroxide, can be classified into two main categories according to the bedrock.

Bauxite lying on aluminosilicate rocks is termed laterite bauxite, and that lying on carbonate rocks is karst bauxite (Bárdossy, 1982;Bárdossy and Aleva, 1990;Mameli et al., 2007;Zarasvandi et al., 2008, 2012).

Karst bauxites can take Al from the insoluble residue of limestones and aluminosilicate material (volcanic ash and clayey interlayers among limestones) or aluminosilicate rock (Gow and Lozej, 1993;Bogatyrev et al., 2009).

Iron-rich gibbsitic bauxite was explored in Yongjiang basin with prospective reserves of nearly 1 billion tons (Wang R.H. et al., 2011;

Wang and Li, 2011). The bauxite is stratiform and stratoid, complying with the shape of karstic topography. The bauxite mineral in Yongjiang

basin is mainly gibbsite, clay mineral is mainly kaolinite, iron minerals are mainly hematite and goethite (Wang, 2013;Wei et al., 2014;Zhang et al., 2015;Qiao, 2016). The iron-rich gibbsitic bauxite contains high iron and low aluminium (Wei et al., 2014; Zhang et al., 2015), and many researchers studied the distribution of several minor elements in such deposits (Boni et al., 2013; Hanilçi, 2013; Peh and Kovačević Galović, 2014;Buccione et al., 2016;Mongelli et al., 2014, 2016, 2017;

Yuste et al., 2017). The provenance of the iron-rich gibbsitic bauxite in Yongjiang basin has been debated for a long time.Li and Liu (2008) suggested that, based on the study of stable elements, the source of bauxite comes from the weathered carbonate rocks. However, the suggested provenance is debatable as the source of gibbsite is not clear.

Moreover, detailed understanding of the bauxitization process is still ambiguous, especially the formation process of the nodules in Yong- jiang basin.

https://doi.org/10.1016/j.gexplo.2018.02.007

Received 8 October 2017; Received in revised form 4 January 2018; Accepted 17 February 2018

⁎Corresponding author at: State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, No. 29, Xueyuan Road, Beijing 100083, China.

E-mail address:wqf@cugb.edu.cn(Q. Wang).

Available online 22 February 2018

0375-6742/ © 2018 Elsevier B.V. All rights reserved.

T

In this paper, the geochemical and mineralogical characteristics of bauxite samples from Yongjiang basin, with different sizes and from different layers, were studied. Such characteristics can represent an important knowledge for the exploitation of Fe-Al minerals and REE.

This study shows the iron-rich gibbsitic bauxite can represent the pre- cursor of karst bauxite and its genesis is implicative to karst baux- itization process.

2. Geological setting

The study area is in Yongjiang basin, southwestern South China Block (Fig. 1a). And the study area is near the tropic of cancer (23° 26′

21.488″N). Since the Proterozoic era, Yongjiang basin has experienced complex geological evolution (BGMRGR, 1985). The late Precambrian continental collision and rifting, and the Early Paleozoic and Mesozoic orogenic reworking and widespread magmatism in the South China Block (Li et al., 2013) have exposed Yongjiang basin to active periods of tectonic movements. Since Paleozoic, Yongjiang basin has been affected by the subduction of the paleo-Pacific plate, by late Paleozoic mantle plume, by extension of the South China Sea basin during the Cenozoic, and by the closure of the Songma ocean and the southeastward escape

of the Indochina plate (Lepvrier et al., 2004, 2008;Li et al., 2006;Li and Li, 2007;Wang et al., 2007;Lin et al., 2008;Liu et al., 2012;Zheng et al., 2013).

The stratigraphic succession of the study area is relatively complete, since the early Paleozoic to Quaternary, except Ordovician and Silurian, are all exposed. However, the Paleozoic strata are the most developed.

Among them, the Middle Devonian to the lower Permian strata are dominated by carbonate rocks whereas the other strata are dominated by clastic rocks. It is noteworthy that the Cretaceous, Jurassic and Devonian systems developed a large number of iron-rich red sandstone.

The Cambrian strata are exposed in the central region of Zhenlongshan, Lianhuashan and Kunlunguan, whereas the Devonian and Carboniferous strata are exposed around. To the north is the Laibin fold belt, comprising Devonian to Triassic strata. The Shiwandashan graben basin is comprised of Jurassic and Cretaceous strata, and the Guiping basin of Cretaceous strata (Fig. 1b;BGMRGR, 1985).

The bauxite profiles are distributed in Nanning, Guigang and Laibin city. We have carried out research on six typical ore districts in Yongjiang basin, namely Maling, Yunbiao, Menggong, Renzhu, Luxin, and Shiya. The main sedimentary sequence, from bottom to top, is basement carbonate rocks, underlying clayey soil, nodule layer, Fig. 1.Geological map of Yongjiang basin (modified from the regional geological map of 1:500000, Guangxi) showing the sampling locations. Ages in Fig. 1b for Darongshan granitoid belt are fromJiao et al., 2015, age for Kunlunguan granitoid are fromTan et al., 2008, age for Pingtianshan granitoid are fromDuan et al., 2011.

overlying clayey soil (Figs. 2, 3a). However, because of later geological activities, other different sequences exist, although bauxite orebodies are mainly overlying the Devonian and Carboniferous carbonate rocks (Wang, 2013;Qiao, 2016).

3. Samples and analytical methods

Fig. 1shows the location of the samples and cross-section (Fig. 2).

Thefifty samples were collected from different layers (Fig. 3a, b). Small amounts of ore-bearing nodules and pisolites or ooids got mixed with the overlying clayey soil and underlying clayey soil, we separated them from the overlying clayey soil and the underlying clayey soil samples before the analyses. Nodule layer samples were refined and classified according to their diameter (Fig. 3c–f). There are four groups of samples with regard to their diameters, namely < 0.1 cm, 0.1–1 cm, 1–3 cm

and > 3 cm. We regarded samples with diameters < 0.1 cm as clayey soil samples from the nodule layer. We removed the ferric crust from the large size nodules to analyze the internal mineral composition be- fore XRD. In addition, we took three samples containing abundant kaolinite from the clayey soil layer in the Shiya ore district.

Whole-rock XRD was carried out at the Petroleum Geology Research and Laboratory Center, Beijing, using the Japanese Rigaku D/Mac-RC and CuKα1 radiation with the following operating conditions: voltage 40 kV, beam current 80 mA, graphite monochromator, continuous scanning, scanning speed 8°/min, slit DS = SS = 1°, ambient tempera- ture 18 °C, humidity 30%.

For whole-rock geochemical analyses, all samples were crushed to 200-mesh using an agate mill. The analyses were conducted at the Geological Survey and Laboratory Center of Langfang, China. Whole rock abundances of major oxides (except FeO, H2O⁺ and CO2) and Fig. 2.Typical cross-section of the ore district (modified from thefield exploration line profile mapping by Guangxi Sixth Geological Brigade). Qh-Holocene, Qp²-Upper Pleistocene, Qp¹- Lower Pleistocene.

Fig. 3.(a, b) Sampling profile of Renzhu and (c, d, e, f) photographs of hand specimen.

some trace elements (Ba, Cr, Rb, Sr, V, S, Zn, and Zr) were determined by XRF using a Philips Model 1480 spectrometer. FeO contents were analyzed by a volumetric method; H2O⁺was determined by a gravi- metric method and CO2 was determined using potentiometry. Trace elements (Be, Bi, Cs, Cu, Ga, Li, Hf, Nb, Ni, Sc, Th, Ta, U, and W) and rare earth elements were analyzed using inductively coupled plasma mass spectrometry. The detection limit was≤0.1 wt% for major ele- ments (as oxides) and≤2 ppm for trace elements and rare earth ele- ments (except that detection limit was 5 ppm for Ba, Cr, Rb, Sr and V and 50 ppm for S).

4. Analytical results

4.1. Ore structure and mineral composition

The nodules are mainly characterized by lump, mottled or oolitic structures. Ferric crust wrap around pisolites and ooids. The diameter of nodules varies from a few millimeters to several centimeters (Fig. 3c–f).

Under the microscope, microstructures of nodules exhibit small dia- meter pisolites or ooids (Fig. 4a, b, c) and clay minerals cemented by clay and ferric materials. Furthermore, there are many white clay ma- terials in ooids or pisolites (Fig. 4b, c). We can see a narrow dark iron- rich crust to occur at the surface layer of nodules (Fig. 4a). And there are many small size nodules inside the nodules with the ferric crust.

Large size nodules also contain a certain amount of quartz grains (Fig. 4d).

Three large size nodules (diameter > 3 cm) from Maling, Shiya and Renzhu have similar mineralogical characteristics with each other (Fig. 5a, b, c), and the clayey soil sample from the nodule layer show a similar feature with them (Fig. 5d). The XRD and microscopic analyses revealed that the main alumina-bearing minerals in bauxite are gibbsite with clay minerals dominated by kaolinite and iron minerals dominated by hematite and goethite. Gibbsite, kaolinite, hematite and goethite are

the major minerals in nodules regardless of size. The nodules with diameters ranging from millimeters to centimeters consist of kaolinite- gibbsite minerals in the core and ferric minerals on the surface layer.

Small amounts of zircon and quartz are also present (Fig. 4d,Wei et al., 2014). The samples from the Shiya clayey soil layer mainly contain kaolinite and gibbsite (Fig. 5e). The weathered materials of Kunlunguan granite mainly contain quartz, orthoclase and nacrite (Fig. 5f).

4.2. Geochemistry

The major and trace elements data and REE data for 50 samples, which come from Maling, Yunbiao, Menggong, Renzhu, Luxin, Shiya, are given in Supplementary Table 1.

4.2.1. Major elements

The main components of the samples from the iron-rich gibbsitic bauxite include Al2O3, Fe2O3, SiO2, TiO2and LOI (Table 1). We used box and whisker plots (Fig. 6) to study the change of major elements in nodules of different size. In addition to minimum, maximum and median, the box and whisker plots also depict the first and third quartiles and the outliers for dataset. These data show that, for the nodules of different sizes, the variation of Al2O3 content (18.50–33.07 wt%) was very small. The mass percentage of Fe2O3in clayey soil samples (9.65–14.74 wt%) from the nodule layer is low but high in the nodules (31.87–53.33 wt%). The SiO2content in clayey soil samples (26.85–34.45 wt%) from the nodule layer is higher than no- dules (6.60–14.77 wt%). The variation of MnO content (0.04–4.83 wt

%) in samples was small, and individual large size nodules are rela- tively enriched in MnO (Fig. 6). The content of active elements such as Mg, Ca, Na, K, P were low in the bauxite section, and the content of TiO2 (0.62–2.85 wt%) and other inert elements in the samples with different sizes is relatively stable (Supplementary Table 1). There are several outliers associated with ML1-3-H, ML1-4-H, ML1-4-K-2, MG14- Fig. 4.Photomicrographs of nodules: (a) nodules with ferric crust; (b) white clay minerals in ooids or pisolites; (c) large size samples containing small size ooids or pisolites; (d) a sample containing quartz grains.

3-K-1, LX6-2-K-1 and YB6-1-K among the major oxides (Supplementary Table 1,Fig. 6). ML1-3-H and ML1-4-H have a high content of Al2O3

(41.45 wt%, 41.25 wt%) and a low content of Fe2O3(9.74 wt%, 9.80 wt

%) and SiO2(26.88 wt%, 26.85 wt%). We suggest that both samples are contained clay-rich materials derived from the inside of nodules. ML1- 4-K-2 have a high content of Al2O3(32.43 wt%). MG14-3-K-1 and LX6- 2-K-1 have a high content of MnO (2.42 wt%, 2.23 wt%). However, the outliers of these samples are normal for large size nodules. These samples may have been broken from large size nodules. YB6-1-K have a low content of TiO2(0.62 wt%). We suggest that this sample have a weak weathering with a relativity high content of SiO2(14.77 wt%) in nodules.

Samples from different layers were analyzed, and the content of SiO2 in either the underlying clayey soil (25.67–49.14 wt%) or the overlying clayey soil (37.38–38.26 wt%) is higher than in the nodule layer (Table 1;Fig. 7). The ranges of Al2O3content in nodule layer from Maling, Renzhu, Menggong and Yunbiao mine are 26.85–41.45 wt%, 24.97–33.99 wt%, 23.00–34.03 wt%, 18.50–32.80 wt%, respectively.

The Fe2O3content in nodule layer (9.65–53.33 wt%) is generally higher than in the underlying clayey soil (8.08–13.72 wt%) and overlying clayey soil (12.83–14.05 wt%). The content of K2O in the underlying clayey soil (0.66–5.96 wt%) is generally higher than in the upper no- dule layer (0.12–2.33 wt%). The content of MnO (0.03–4.14 wt%) in the nodule layer is generally higher than in other layers. The content of Fig. 5.Results of XRD analysis of samples from Yongjiang basin.

Table 1

Main components of samples from the iron-rich gibbsitic bauxite in Yongjiang basin.

Samples SiO2(wt%) Al2O3(wt%) Fe2O3(wt%) TiO2(wt%) LOI (wt%)

range avg range avg range avg range avg range avg

Overlying clayey soil 37.4–38.3 37.8 30.5–30.5 30.5 12.8–14.1 13.4 1.4–1.5 1.5 13.6–13.9 13.7

Nodule layer

< 0.1 cm 26.9–34.5 31.6 31.9–41.5 35.0 9.7–14.7 13.3 1.2–1.9 1.6 14.3–19.0 15.9

> 0.1 cm 6.6–14.8 11.0 18.5–33.1 26.5 31.9–53.3 41.3 0.6–2.9 1.7 11.1–20.0 17.1

Underlying clayey soil 25.7–49.1 39.4 26.0–41.7 31.1 8.1–13.7 10.6 0.7–1.6 1.3 7.0–19.0 11.1

Ferric crust 17.7 15.9 53.5 0.8 10.0

TiO2and other inert elements in each layer is relatively stable (Fig. 7).

Due to the strong weathering, small amounts of MgO, CaO, Na2O and P2O5exist in bauxite horizons. In the Renzhu ore district, the white weathered material (Fig. 3b) with high content of CaO above the limestone can be judged as weathered limestone based on the major element content (Fig. 7b). The major element characteristics of the overlying clayey soil, underlying clayey soil and clayey soil samples from the nodule layer exhibit a certain similarity (Fig. 7).

FollowingSchellman (1986)andAleva (1994), triangular diagram of SiO2-Al2O3-Fe2O3was used to classify the bauxite and to determine the intensity of laterization, and we used to better observe our data. In this diagram, Al2O3-rich samples are indicative of higher intensities of

lateritization, while SiO2-rich composition implies weak lateritization (Meyer et al., 2002). Nodules with diameter > 0.1 cm in the nodule layer fall within the laterite, ferrite and bauxitic ferritefields (Fig. 8a), implying a strong ferrallitization stage (Fig. 8b). Nodules have high Fe2O3and high degree of ferrallitization. The Shiya clayey soil samples, underlying clayey soil, clayey soil samples from the nodule layer, and overlying clayey soil containing higher SiO2, mostly fall within the bauxite, laterite, kaolinite, and bauxitic kaolinitefields (Fig. 8a), in a moderate ferrallitization stage (Fig. 8b).

As weathering intensity is higher, Al2O3 and TiO2 contents are higher and the SiO2 content is lower. Variation diagrams show the correlations among major oxides in the studied iron-rich gibbsitic bauxite (Fig. 9). The data are divided into two groups, thefirst group is about nodules with diameter > 0.1 cm, the second group associated with the samples from underlying clayey soil, clayey soil from the no- dule layer, and overlying clayey soil. We found negative correlations between SiO2 and TiO2 and between Al2O3 and SiO2 (Fig. 9a, c).

Especially for the samples from the clayey soil between Al2O3 and SiO2(r =−0.93). With the variation of Al2O3, the TiO2content is stable (Fig. 9b). We found that nodules containing ferric crust have higher Al/

Si ratios compared to other samples (Fig. 9d).

4.2.2. Trace elements

The upper continental crust (UCC) normalized spidergram pattern of trace elements reveals that the ore samples from nodule layer show enrichment in B, F, Li, Cs, Be, Cu, Zn, Bi, V, W, U, Mo, Sb and As (Fig. 10a). The underlying limestone is characterized by low values of trace elements (Fig. 10). Large ion lithophile elements (LILE) Rb, Sr and Ba are relatively depleted in samples from the nodule layer. The Rb and Cs contents of clayey soil samples from the nodule layer are higher than in the nodules with diameter > 0.1 cm. The contents of Ga, Bi, Sc, V, Cr, Th, Co and Pb in the clayey soil samples from the nodule layer are lower than those in nodules with diameter > 0.1 cm. The contents of Nb, Ta, Zr and Hf in nodules with different sizes are stable (Fig. 10a).

Fig. 6.Box and whisker plots for selected major elements of samples of different grain sizes from the nodule layer. The horizontal bar in the box refers to the median value; the ends of the whiskers are the maximum and minimum values of variables; the top and bottom of the boxes are the values offirst and third quartiles; circles represent the outlier values of the dataset.

Fig. 7.Distribution pattern of major elements in samples from the different layers.

The pattern of trace elements in Shiya clayey soil reveals similar characteristics with clayey soil samples from nodule layer.

The vertical distributions of trace elements in the different ore districts are different. The underlying clayey soil layer and the nodule layer are significantly different from each other in Mongong (Fig. 10b;

Supplementary Table 1). The contents of F, Li, Rb, Cs, in the underlying clayey soil are generally higher than those in the nodule layer. The contents of S, Bi, Sc, V, Co and highfield strength element (Zr, Hf, Th, U, Pb) in the underlying clayey soil are generally lower than those in the nodule layer. The characteristics of trace elements in the overlying clayey soil from Yunbiao are similar to those in the underlying clayey soil and the clayey soil samples from the nodule layer in other ore districts (Fig. 10b; Supplementary Table 1).

4.2.3. Rare earth elements

The underlying limestone is characterized by low content of∑REE (39.27–48.69 ppm), negative LREE slope compared to HREE (LREE/

HREE = 12.04–23.65) with (La/Yb) N ranging from 26.49 to 30.95, negative Eu anomalies (Eu/Eu* = 0.61–0.65), and distinct Ce anoma- lies (Ce/Ce* = 0.43–1.62). Chondrite-normalized REE patterns of the nodules with different sizes show similar features such as enrichment of LREE, depletion in HREE, steep downward LREE pattern compared to HREE (LREE/HREE = 5.3–80.4) with (La/Yb)Nranging from 6.64 to 21.96, and negative Eu anomalies (Eu/Eu* = 0.24–0.68). Ce anomalies in different samples are different (Fig. 11), and show a wide range of Ce/Ce* (Ce/Ce* = 0.44–41.72). The REE patterns of the limestone are similar to those of the nodules. The REE patterns of iron-rich gibbsitic bauxite samples show that REE contents (∑REEs) are strongly enriched in the bauxite profile (average ∑REEs = 763 ppm) relative to the Fig. 8.Triangular diagrams of SiO2-Al2O3-Fe2O3: (a) afterAleva (1994); (b) afterSchellman (1986).

Fig. 9.Variation diagrams of major oxides in the studied bauxite profile, showing correlations between: (a) SiO2vs TiO2; (b) Al2O3vs TiO2; (c) Al2O3vs SiO2; (d) Fe2O3vs Al/Si. The dashed line represents clayey soil samples (overlying clayey soil, clayey soil from the nodule layer, and the underlying clayey soil), and the solid line represents nodules (dia- meter > 0.1 cm).

limestone. Samples of clayey soil from Shiya have high LREE content (average ∑LREE = 498 ppm), relatively constant LREE/HREE ratios (10.42–11.48), negative Eu anomalies (Eu/Eu* = 0.63–0.64). The samples from the Shiya clayey soil layer show Ce negative anomalies (Ce/Ce* = 0.67–0.70), while the most nodules show different degrees of positive Ce anomalies. Chondrite-normalized REE patterns of the samples from underlying clayey soil and overlying clayey soil show a similar feature, and part of samples from underlying clayey soil consist with Shiya clayey soil (Fig. 11). Chondrite-normalized REE patterns of

the overlying clayey soil enrichment of LREE, depletion in HREE, steep downward LREE pattern compared to HREE (LREE/

HREE = 9.85–10.35) with (La/Yb)Nranging from 8.49 to 8.88, nega- tive Eu anomalies (Eu/Eu* = 0.62–0.64), and positive Ce anomalies (Ce/Ce* =1.21–1.29). The underlying clayey soil is characterized by negative LREE slope compared to HREE (LREE/HREE = 5.27–15.10) with (La/Yb)Nranging from 8.12 to 23.49, negative Eu anomalies (Eu/

Eu* = 0.58–0.67), and distinct Ce anomalies (Ce/Ce* = 0.14–2.26).

Fig. 10.(a) Distribution patterns of trace ele- ments normalized to upper continental crust (UCC) for nodules with different sizes, Shiya clayey soil samples and limestone samples. (b) Distribution patterns of trace elements normal- ized to upper continental crust (UCC) for samples from different layers in typical ore districts (standard value according toRudnick and Gao, 2003).

Fig. 11.Chondrite-normalized REE patterns of the samples from Yongjiang basin (Sun and McDonough, 1989). Data for Tianyang karst bauxite are from Liu et al., 20,016. P = Permian bauxite; Q = Quaternary bauxite.

But the content of aluminium hydroxide in iron-rich gibbsitic bauxite is lower than that in karst bauxite (Wang Q.F. et al., 2011,Wang et al., 2012a, 2012b;Liu et al., 2017). The nodules are mainly characterized by lump, mottled or oolitic structures, which is similar with some karst bauxite (e.g.,Liu et al., 2010;Ahmadnejad et al., 2017).

The distribution of Al2O3 content in the nodules is relatively re- stricted, and the content of Al2O3in each layer varies little, mostly between 20% and 35%. Nodules have high Fe2O3content mainly be- cause the cement material is iron-based. Iron is easily moved and either leached out or reprecipitated in alternately moist and dry conditions, so that the nodules may have formed in a relatively humid environment.

Nodules with ferric crust have a low percentage of Al2O3content due to the high content of Fe2O3, but their Al/Si ratios are higher than those of other samples (Fig. 9d, Supplementary Table 1). Compared to karst bauxite, the iron-rich gibbsitic bauxite contains high iron and low aluminium. The underlying clayey soil concentrates clay minerals (kaolinite, etc.), and its SiO2content is higher than the nodule layer because of weaker weathering. Due to the addition of continuously forming new weathering materials, the overlying clayey soil is in the transitional stage of ferrallitization (Fig. 8) and the SiO2content is high.

The mixing of clayey soil samples from the nodule layer with the overlying clayey soil and the underlying clayey soil materials, and the continuous conversion of clay minerals into gibbsite resulted in Si loss in nodules. Both facts or processes results in the SiO2content in the clayey soil samples from the nodule layer to be higher than those in nodules (diameter > 0.1 cm). The content of MnO in the deposit is low, but the individual large size nodules are relatively rich in MnO (Fig. 6). In the nodules of various size, the contents of Mg, Ca, Na, K, P and other active elements are lower than that inert elements, such as Ti, etc. This is because during the strong weathering process, most of MgO, CaO, Na2O and P2O5leached out from the bauxite horizons.

5.2. Migration of trace elements and REE

During bauxite formation, the bauxite ores are generally enriched in trace elements, including REE, Ga, Ti, Cr, Zr, etc. (Mordberg, 1993;

Mordberg et al., 2001;Liu X.F. et al., 2013,Liu et al., 2016;Mongelli et al., 2014, 2016, 2017;Ahmadnejad et al., 2017;Yuste et al., 2017).

As illite and feldspar accumulate and evolve, new clay minerals con- tinue to generate, which, together with the enhancement of oxidation intensity or chemical weathering, altogether influence the distribution of elements in the weathering profile (Bárdossy, 1982;Ji et al., 2004a, 2004b;Feng, 2011;Yuste et al., 2017).

The distribution pattern of trace elements normalized to UCC in the bauxite horizons reveals similar characteristics (Fig. 10). The ore dis- trict experienced strong weathering, similar with karst bauxite, so that the nodules are depleted in LILEs such as Rb, Sr and Ba, compared to UCC (e.g.,Calagari and Abedini, 2007;Zamanian et al., 2016).Fig. 10a shows the difference between the nodules with different sizes. The Rb and Cs contents of clayey soil samples from the nodule layer are higher than in the nodules, probably because the latter have a higher degree of ferrallitization and the elements were more depleted during the baux- itization process (Calagari and Abedini, 2007;Zamanian et al., 2016).

The contents of Ga, Sc, V, Cr, Ni, Th, Co and Pb are higher in nodules than in the clayey soil samples from nodule layer. Low solubility ele- ments, like Ga, were possibly concentrated in boehmite and hematite during the later stages of bauxitisation as suggested byMongelli et al.

generally lacks V (Mongelli, 1993), but nodules from Yongjiang basin are V-rich (Fig. 10a). Similarly, the concentration of Ni and Co within nodules may be affected by adsorption mechanisms related to the presence of iron oxyhydroxides (Mongelli et al., 2014), so that Ni and Co is relatively enriched in the nodules.Calagari and Abedini (2007) suggested that Cr has a similar surface geochemical behavior to that of Fe, so that the content of Cr is higher in nodules. The nodules have a higher degree of ferrallitization, thus the highfield strength element Pb was more enriched in nodules.

The distribution and fractionation of trace elements and REEs in bauxite are mainly controlled by the presence of REE-bearing minerals and thefluctuations in soil solution pH, REE ionization potential and the presence of bicarbonates or organic matter (Mutakyahwa et al., 2003;Pokrovsky et al., 2006;Wang et al., 2010;Liu X.F. et al., 2013;

Mongelli et al., 2014;Zamanian et al., 2016;Ahmadnejad et al., 2017;

Yuste et al., 2017), which is of significant influence from the point of view of the migration and distribution of elements in the different layers. FromFig. 10b, we have observed that the contents of S, Bi, Sc, V, Co and highfield strength elements (Zr, Hf, Th, U, Pb) are higher in the nodule layer whereas the contents of active elements (F, Li, Rb, Cs, etc.) are higher in other layers (Fig. 10b). So that the features characterizing the samples from the different layers are mainly controlled by the in- tensity of the weathering process. The geochemical characteristics of the samples from the Shiya clayey soil layer are consistent with those of the nodules, and both sets of samples possess similar mineralogical compositions (Fig. 5), which indicates that there was a direct trans- formation relationship between them. The bauxite horizons have a high content of Be, Cu, Zn, W, Mo, Sb, Sn and As (Fig. 10b), and we suggest that these elements include additions from external sources (e.g., soil pollution by human activities).

The chondrite-normalized REE patterns of the nodules with dif- ferent sizes show similar feature (Fig. 11). As the erosion is sufficiently slow and chemical weathering becomes sufficiently rapid, REEs become enriched in the weathering profiles (Maksimovic and Pantó, 1991;Ji et al., 2004a, 2004b). To scrutinize the behavior of Ce and the rest of the REEs, Ce anomalies and∑REE were plotted against different layers (Fig. 12a and b). The REE patterns of bauxite samples in Yongjiang basin, similarly with other karst bauxite, show that REE contents (∑REEs) are strongly enriched in the bauxite profile (Fig. 12a), espe- cially in the nodule layer. The differences of Ce and Eu anomalies in the different samples show that during the ore-forming process redox conditions might have changed, and the above anomalies reflect the complexity of the ore-forming process. This means that Ce anomalies can be a valuable tool in terms of identifying different geological con- ditions and processes (Dia et al., 2004;Class and Le Roex, 2008;Mameli et al., 2008; Chetty and Gutzmer, 2012; Mongelli et al., 2014). For positive Ce anomaly,Nyakairu et al. (2001)andCompton et al. (2003) reported that Ce is usually retained in the upper parts of the weathering profiles due to the oxidation of Ce³⁺into the less mobile Ce⁴⁺. The negative Ce anomaly recorded in some samples would be related to variable oxidation conditions favoring Ce³⁺over Ce⁴⁺, probably re- lated to the acidic and reducing solutions, which promoted late kaoli- nization stages (Yuste et al., 2017). Most of samples show positive Ce anomalies in bauxite horizons while some samples from the nodule layer and underlying clayey soil become negative (Figs. 11, 12b). This is mainly because the mixing process between different parts of weath- ered profile underfluvial environment. There are two outlier values in

Fig. 12a and four outlier values inFig. 12b associated with the Mn-rich samples (Supplementary Table 1). Fe and Mn-oxides and hydroxides (e.g., goethite), phosphate minerals (e.g., apatite), clay minerals (e.g., kaolinite), along with some rock-forming minerals (e.g., zircon) are the most important reservoirs of REEs (Banfield and Eggleton, 1989;Braun et al., 1993;Mutakyahwa et al., 2003;Pokrovsky et al., 2006; Wang et al., 2010;Ahmadnejad et al., 2017;Yuste et al., 2017). These samples have a significant adsorption effect on REE and Ce. By scanning elec- tron microscope (SEM) and electron probe micro analysis (EPMA) coupled with energy diffraction spectrum (EDS), Wei et al. (2014) found that the monazite exists in iron-rich gibbsitic bauxite, and the monazite rich in La, Ce, Nd and Sm. In general, the behavior of Ce in the iron-rich gibbsitic bauxite is related to oxidation intensity, Fe-Mn oxides and Ce-bearing minerals (monazite). The nodule layer in Yong- jiang basin has a wide range of Eu anomalies (Eu/Eu* = 0.24–0.68), but most of them are similar to UCC (Eu/Eu* = 0.66). According to the analyses above, the characteristics of the chondrite-normalized REE patterns of the underlying clayey soil samples and the overlying clayey soil samples are similar with the nodules, and they have the same mi- neralogical characteristics, indicating that there is a transitional re- lationship between them.

5.3. The relationship between iron-rich gibbsitic bauxite and karst bauxite By contrast, we found that the iron-rich gibbsitic bauxite lying on carbonate rocks contains high iron and low aluminium and have lower ratios of Al/Si (0.5–4.3) relative to karst bauxite. Furthermore, the content of aluminium hydroxide in iron-rich gibbsitic bauxite is lower than that in karst bauxite (Wang Q.F. et al., 2011,Wang et al., 2012a, 2012b). And the lump, mottled or oolitic structures in nodules is similar with some karst bauxite (e.g., Liu et al., 2010; Ahmadnejad et al., 2017). The iron-rich gibbsitic bauxite and karst bauxite have similar characteristics of trace elements and REE (Fig. 11, Liu et al., 2016).

Their REE contents (∑REE) are lower than the Tianyang Permian bauxite (Fig. 11). We suggest that the iron-rich gibbsitic bauxite is an

intermediate product during bauxitization, which eventually converted to karst bauxite by further weathering or intense submarine chemical leaching.Bárdossy and Aleva (1990)andBogatyrev et al. (2009)sug- gested that bauxites are formed on the continental surface and con- trolled by several factors (e.g., climatic, tectonic, petrochemical, pet- rophysical, geomorphological, hydrogeological, biogenic, and temporal features). And absence of even one of these factors rules out the pos- sibility of bauxite formation. The vast iron-rich gibbsitic bauxite in Yongjiang basin has not been converted to karst bauxite mainly because the less weathering induced by high latitude (near the tropic of cancer) and absence of submerge process, and/or the addition of iron-rich materials later. Thus, we suggest that the iron-rich gibbsitic bauxite in Yongjiang basin is the precursor of karst bauxite.

5.4. Provenance

During the weathering of limestone, the Al content is continuously enriched (e.g.,Zamanian et al., 2016;Ahmadnejad et al., 2017), which indicates that the weathered limestone provides some of the ore- forming materials for the iron-rich gibbsitic bauxite (Fig. 7b). Chon- drite-normalized REE patterns can also be used to identify parent ma- terials (e.g., Zamanian et al., 2016; Ahmadnejad et al., 2017). The underlying limestone and bauxite horizon show similar chondrite-nor- malized REE plots and negative Eu anomalies (Fig. 11). And both them have similar LREE/HREE ratio values (avg. 17.85 and avg. 19.15; re- spectively) in Renzhu ore district (Supplementary Table 1), which also indicate that the limestone provides for at least a certain amount of the ore-forming materials (Fig. 11). Large size nodules also contain a cer- tain number of quartz grains (Fig. 4d). Because the weathered materials surrounding the granitic plutons contain orthoclase, quartz and nacrite (formed by long-term weathering of aluminosilicate minerals like feldspar and mica) (Fig. 5f), these quartz grains possibly come from the granitic plutons, and the other weathered minerals (like feldspar and mica) may have provided a certain amount of ore-forming substances, too. (Bárdossy and Aleva, 1990;Ji et al., 2004a;Herrmann et al., 2007).

Fig. 12.(a) Variations of the total concentration values of REE; (b) Variation of the Ce-anomalies in the samples from different layers. Data from Maling, Renzhu, Menggong and Yunbiao.

Bivariate plots of Cr and Ni concentrations (e.g.,Schroll and Sauer, 1968;Valeton et al., 1987;Calagari and Abedini, 2007) were employed to infer the precursor rock(s) of the iron-rich gibbsitic bauxite in Yongjiang basin (Fig. 13a). Although the geochemical data suggest basalt as the main composition of their parent material, there is no direct field evidence for basalt in the Yongjiang basin and in the sur- rounding areas. The plots stretch from the karst bauxite region to the high iron lateritic bauxite (Fig. 13a), meaning that it is a mixture of weathering products of a wide range of rocks. Geochemical data show that the nodule layer in Yongjiang basin has a wide range of Eu anomalies (Eu/Eu* = 0.29–0.69), but most of them are like those of the UCC (Eu/Eu* = 0.66;Taylor and McLennan, 1985), and some samples close to Kunlunguan and Pingtianshan granite. As illustrated inFig. 13b (e.g.,Mameli et al., 2007), most TiO2/Al2O3and Eu/Eu* values plot close to the composition of Triassic turbidites in Youjiang basin. Iron- rich gibbsitic bauxite are widely distributed in the Yongjiang basin (Fig. 1). In addition, the Cretaceous, Permian, Devonian and Cambrian clastic rocks are well developed within the basin and along its margins.

These rocks can provide most primitive minerals (feldspar, mica and clay minerals), from which the gibbsite can be directly transformed in the condition of free drainage (Bárdossy and Aleva, 1990;Herrmann et al., 2007). This suggests a part of the argillaceous sediments from a UCC source. We observed that the value of TiO2/Al2O3is mostly high.

This is resulted from that the large size nodules contain more iron minerals and relatively small content of Al2O3(Fig. 6), which elevates the value of TiO2/Al2O3as the TiO2content is stable. The concentration of Ni within nodules may be affected by adsorption mechanisms related to the presence of iron oxyhydroxides (Mongelli et al., 2014). Thus, we can explain the inconsistency of the precursor rock(s) inFig. 13a and b.

The iron-rich gibbsitic bauxite in Yongjiang basin contain high iron (Figs. 6, 7), so the nodules is relatively rich in Ni. And this causes the plots to fall into the basalt area inFig. 13a.

According to the mineralogical and geochemical analyses above, we deduce that the source material for the iron-rich gibbsitic bauxite was derived from the weathering products of surrounding granites, clastic rocks and basement carbonate rocks.

5.5. Metallogenic process

Clay minerals are usually changing from smectite to kaolinite and gibbsite (Liu W.J. et al., 2013). Gibbsite is derived from gibbsite itself or similar minerals (e.g., chamosite, kaolinite, etc.) that are present in the

parent material, where percolation is sufficiently intense and fast due to low evapo-transpiration to rainfall ratios and where parent materials are coarse-textured (Herrmann et al., 2007). The original deposit of gibbsite is absent in Yongjiang basin and vicinity, and based on the combination of the mineralogical and geochemical characteristics of the samples, we believe that the gibbsite in the bauxite was derived from kaolinite. Based on the geochemical characteristics of the over- lying clayey soil, the clayey soil samples from the nodule layer, and the underlying clayey soil, the gibbsite-rich nodule layer may have evolved during ore deposit formation. Supergene Mn-oxides are widely devel- oped and distributed in Guangxi, Guangdong, Yunnan, and Hunan Provinces (Li et al., 2007), as accumulations of Mn oxides are facilitated by intense weathering under wet and warm climates (Vasconcelos, 1999), which are similar conditions required to explain the presence of high iron gibbsite-rich nodules in Yongjiang basin.

In the early stage of deposit evolution, pre-weathered material rich in clay minerals or bauxite-minerals (inherited from older weathering profiles) accumulated on the surface of the carbonate terrain (Fig. 14a).

After depositions of this weathered blanket, elements such as K, Na, Ca and Mg were leached and removed from it, by soil solution, whereas Al, Fe and Ti remained (Wei et al., 2014). Clay minerals as nuclei generated ooids. During the next stage, the monsoon climate intensified and the environment of the deposit became episodically characterized byfluvial environment and strongly humid climate (Ren and Beug, 2002;Li et al., 2007;Wei et al., 2014). Iron-rich crusts wrapped around pisolites and ooids, became cemented by clay and ferruginous materials (Fig. 14b). It is noteworthy that in the early stage of this evolution, only a small amount of iron-rich materials is involved. The reason is that the un- derlying clayey soil, especially for Shiya clayey soil, have a lower content of iron (Table 1). The weathering of the surrounding strata is not sufficient to form the primary clay-gibbsite layer. And we suggest that, based on the geochemical characters, the sedimentation of the primary clay-gibbsite layer consisted of allochthonous materials. In the middle stage, ferrallitic soil mixed with clay materials to form the no- dules and iron-rich clayey soil. Subsequently, the nodules spent a long time in the upper part of the section during the strong weathering en- vironment, and do not plot on the same development line of weathering with the bottom clayey soil (Fig. 9). In thefinal stage, Yongjiang basin experienced subtropical monsoon climate, relatively hot and rainy in summer, warm in winter and little rain, but hot and humid essentially throughout the year (Ren and Beug, 2002;Li et al., 2007;Wei et al., 2014), which made the upper ferruginous nodules to become more Fig. 13.(a) Plots of Ni vs. Cr for various types of bauxites in relation to various possible precursor rocks (afterSchroll and Sauer, 1968); (b) Eu/Eu* and TiO2/Al2O3binary plot (after Mameli et al., 2007). Data for Darongshan granitoid belt are fromDeng, 2003andQi et al., 2007, Data for Kunlunguan granitoid are fromTan et al., 2008, Data for Pingtianshan granitoid are fromDuan et al., 2011, Data for Triassic turbidites are fromYang et al., 2012. Most samples plot close to the average composition of upper continental crust rocks.

weathered and mixed with the continuous generation of newly weathered materials in the surface to form the overlying clayey soil (Fig. 14c). This can explain that the overlying clayey soil reached only the stage of moderate ferrallitization (Fig. 8b). In the meantime, the clay minerals in ferruginous nodules continually transformed into gibbsite.

6. Conclusions

(1) The bauxite ores are dominated by gibbsite-bearing nodules with diameter > 0.1 cm in the Yongjiang basin. The nodules have high iron content, high degree of ferrallitization, and relatively low ratio of Al/Si compared to karst bauxite. The iron-rich gibbsitic bauxite in Yongjiang basin is the precursor of karst bauxite.

(2) The distribution and fractionation of trace elements and REEs in the Yongjiang basin bauxite are mainly controlled by the presence of Fe-Mn minerals, clay minerals, REE-bearing minerals (e.g., zircon), and the intensity of weathering process.

(3) The provenance of the iron-rich gibbsitic bauxite includes the sur- rounding granites, clastic rocks and the underlying carbonate rocks.

The formation of the iron-rich gibbsitic bauxite was introduced by the sedimentation of a primary clay-gibbsite layer which subse- quently became mixed with iron-rich ferrallitic soil to form the nodules and clayey soil, and then a breakup process concomitant with the mixing with the clayey soil.

Supplementary data to this article can be found online athttps://

doi.org/10.1016/j.gexplo.2018.02.007.

Acknowledgements

This research was jointly supported by the Key Project of the Resource Exploration Bureau in Guangxi Province (No. 201649), National Basic Research Programs of China (Nos. 2015CB452602, 2015CB452606) and the National Natural Science Foundation of China (No. 41202061).

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