Mineralogical and geochemical evolution of the Fusui

Journal of Geochemical Exploration 146 (2014) 75–88

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Mineralogical and geochemical evolution of the Fusui bauxite deposit in Guangxi, South China: From the original Permian orebody to a Quarternary Salento-type deposit Wenchao Yu a, Ruihu Wang b, Qilian Zhang b, Yuansheng Du a,⁎, Yue Chen b, Yuping Liang b a b

State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan 430074, China General Academy of Geological Survey of Guangxi, Nanning 530023, China

a r t i c l e

i n f o

Article history: Received 27 December 2013 Accepted 29 July 2014 Available online 8 August 2014 Keywords: Karstic bauxite Metallogenic process Permian Quaternary Guangxi South China

a b s t r a c t Two types of bauxite deposits have been discovered in the Fusui area, Guangxi Province, South China. The original Permian bauxite occurs on the unconformity between the Middle Permian Maokou Formation and the Upper Permian Heshan Formation. The Salento-type (or karstic accumulational) bauxite is composed of the Quaternary incompact sediments that transformed from the original Permian bauxite. Samples were collected from two drilling cores and two profiles in study area, and field observations and mineralogical and geochemical analyses are integrated to reconstruct the metallogenic process from the original Permian bauxite to the Quaternary Salentotype bauxite. In the Permian lateritization and bauxitization, alkali metals and alkali earth metals are preferentially dissolved and removed from the parent rocks due to the intense chemical weathering; other major elements from the original profile are also depleted, as shown by the mass-change calculation result. Elements with a low mobility (e.g., Ti, Zr, Cr, Nb and V) are positively correlated with Al. When the bauxite deposit is affected by the modern groundwater system in the near-surface environment, the elements in the deposit display varied mobility due to the leaching intensity and the drainage conditions. After the bauxite ore horizons are exposed in the surface environment, impurities (primarily Fe and Si) are removed from the profiles, and Al is enriched; the clay minerals (kaolinite and chlorite) in the ores convert into aluminum minerals (boehmite and diaspore). When the exposed original bauxite orebodies break down and accumulate as bauxite gravel in the modern karstic depressions, the Salento-type bauxite deposits appear, and the ore quality is further improved. The intensity of the leaching process and the drainage conditions are the most important factors that control the ore quality during the ore-forming process that converts the original bauxite into the Salento-type bauxite. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Based on the different basements present beneath bauxite deposits, the current classification of bauxite divides it into three types: karstic, lateritic and sedimentary. In contrast to the lateritic and sedimentary bauxite deposits that form on aluminosilicate rocks, the karstic bauxite deposits form on the paleokarstic surface of carbonates (Bárdossy, 1982; Bárdossy and Aleva, 1990; Bogatyrev et al., 2009). Salento-type bauxite, which is a subtype of karstic bauxite, is characterized by erosional and redepositing processes of the original orebody, and it is considered to be a type of supergene bauxite deposit (Bárdossy, 1982). In a Salento-type bauxite profile, bauxite gravels inlay into the lateritic matrix, and the mélange distributes within the Quarternary karstic depressions. Although the Salento-type bauxite only constitutes 2% of the worldwide bauxite reserves, it is the most important type of proven bauxite reserves in the Guangxi and the Yunnan provinces, South China. ⁎ Corresponding author. Tel.: +86 18986127299; fax: +86 27 87481365. E-mail address: [email protected] (Y. Du).

http://dx.doi.org/10.1016/j.gexplo.2014.07.020 0375-6742/© 2014 Elsevier B.V. All rights reserved.

The West Guangxi bauxite metallogenic belt, which is the fourthlargest bauxite resource area in China, hosts ~16% of the Chinese bauxite reserves. There are two types of bauxite in this metallogenic belt; aside from the Salento-type bauxite, the Permian original karstic bauxite is also minable. Elaborate classification and profile descriptions of the Salento-type bauxite in West Guangxi have been proposed by recent researchers (Liu, 1987; Liu et al., 2010, 2012b). Particularly, mineralogical and geochemical investigation of Salento-type bauxite in the Debao, Pingguo and Jingxi countries have made remarkable progress (Liu et al., 2010, 2012a,b; Wang et al., 2010, 2011; Wei et al., 2013). It has been proven that the bauxite gravels in the Quarternary bauxite orebodies were formed by the breakup and the weathering of a preexisting Permian bauxite horizon. Although some researchers believe that the Permian bauxite may have a pyroclatic origin (Deng et al., 2010; Liu et al., 2010), further evidence indicates that the underlying Permian Maokou Formation carbonate significantly contributes to the material source of the bauxite (Wang et al., 2010, 2011; Wei et al., 2013). However, there has been no attempt to reconstruct a complete evolutional model from the original bauxite orebody to the Salentotype bauxite. During the transformation, the original low-grade ores

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W. Yu et al. / Journal of Geochemical Exploration 146 (2014) 75–88

become high-quality ores through mineralogical alterations and elemental migrations. This paper reports on the mineralogical and geochemical investigation of a newly discovered bauxite deposit in the Fusui area on the southeastern border of the West Guangxi bauxite metallogenic belt. Both the Permian karstic orebody and the Quarternary Salento-type deposit were found in the orefield. An integrated study of two drilling wells and two profiles samples quantitatively reveals the geochemical and mineralogical changes that occurred during the transformation from the original Permian orebody to the supergene Salento-type bauxite. We will also discuss the possible reasons for these changes and their controlling factors. Finally, we rebuild the formation process of the Salento-type bauxite. 2. Geological background Guangxi Province is located in the southeastern part of the Youjiang Basin (or Nanpanjiang Basin, according to several articles) in the South China Craton (Fig. 1). The original Permian bauxite hosts within the carbonate platforms in the Youjiang Basin. During the Permian, the Youjiang Basin was located at the continental margin of the craton, and isolated carbonate platforms were separated by the interplatform troughs in this basin. The Middle-Upper Permian transition on the carbonate platfroms is marked by a widespread unconformity between the Middle Permian Maokou Formation and the Upper Permian Heshan Formation (Lehrmann et al., 2005; Mei et al., 2004; Qiu et al., 2013) (Fig. 2). The unconformity is related to an end-Middle Permian tectonic uplift which lasted for 3 Myr, and the rise height may have reached 50–450 m (He et al., 2003, 2010; Sun et al., 2010). The underlying Maokou Formation is composed of massive bioclastic and siliceous

carbonates, while the overlying Heshan Formation consists of black calcareous and carbonaceous mudstone at the bottom and transforms into marlstone with several coal-bearing layers in the upper part (Dai et al., 2013; Zeng et al., 2005). Under climatic conditions favorable to chemical weathering (Chen et al., 2013; Enkin et al., 1992; Retallack, 2013), denudation of several-hundred-meter Maokou Formation carbonates provided abundant materials for bauxite formation and left a fluctuant paleokarst surface on the top of the Maokou Formation. An appropriate paleoclimate and abundant laterite as metallogenic materials in the Permian Youjiang Basin contributed to the formation of bauxite deposits on the isolated carbonate platforms. The bauxitization was terminated by a Late Permian transgression, and the carbonate platforms submerged into the sea again. Cenozoic tectonic events caused a crust uplift (BGMRGR, 1985), in which bauxitebearing layers were exposed and broken ores accumulated in the karstic depressions, transforming the structure into the Quaternary Salento-type bauxite. The Fusui area is located in Southwest Guangxi Province. In the study area, a Carboniferious to Quaternary sequence outcrop, strata are controlled by a series of NE trend faults. The bauxite orebodies appear in both the Permian and the Quaternary sequences. The original Permian bauxite deposit has a varied thickness (0.3–20 m) due to the undulating karstified carbonate basement. From the observation of the drilling cores, the deposit can be divided into two parts: a lower, red, compact layer with occasional clastics, ooliths and pisoliths and an upper, grayish layer with a dominant compact texture. On the outcrops, however, the Permian orebodies show a red color due to iron oxidation, and the gossans also exist in some profiles. The Salento-type bauxite developed in the Quaternary lateritic profiles. The thickness of the lateritic profiles varies. Numerous subangular to subrounded bauxite gravels

Fig. 1. (A) Location of the Youjiang Basin in South China (gray shadow area). (B) Location of the study area in West Guangxi. (C) Sketched geology map of the Fusui area, A′–B′ is a cross section across the study area.


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Fig. 2. Simplified paleogeographic map of West Guangxi and adjacent area in Middle Permian Maokou Period, modified from Mei et al.(2004) and Lehrmann et al.(2005).

with sizes ranging from 1 to 50 cm are embedded in the lateritic matrix. The ores are red, brownish-red on the surface and light gray inside, with a high hardness. Compact massive texture is the predominant texture, and clastic, pisolitic or ooidic textures are occasionally observed in the bauxite gravels.

3. Sampling Sample sites are distributed around the town of Shanxu, Fusui County (see Fig. 1). Two sample sites (ZK4353, ZK15108) are drilling cores and the other two (MY, KL) are profiles on the ground.

Fig. 3. Field photographs of (A) original Permian bauxite deposit MY and (B) Quaternary Salento type deposit KL. Sampling positions are marked on the profiles.

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Fig. 4. Microphotographs (in plane polarized light and crossed polarizer) of (A) oolite in the lower part of ZK15108; (B) stratified structure in the upper part of ZK15108; (C) broken oolites in the lower part of ZK4353, red dot circles indicate recovered forms of oolites; (D) original structures have been totally transformed by epigenetic aluminum minerals in MY-2. (E) and (F) quartz veins on the boundary between the Maokou Formation and the bauxite layer.

Table 1 XRD semi-quantitative mineralogical analysis results of bauxite and bauxite clay samples. Samples

Chlorite (%)

Kaolinite (%)

Boehmite (%)

Diaspore (%)

Quartz (%)

Hematite (%)

ZK15108-1 ZK15108-2 ZK15108-3 ZK15108-4 ZK15108-5 ZK15108-6 ZK15108-7 ZK4353-3 ZK4353-4 ZK4353-7 ZK4353-8 ZK4353-10 ZK4353-11 MY-2 MY-3 MY-4 KL-1 KL-2 KL-3 KL-4

29 26 16 25 15 15 18 0 0 12 13 14 0 20 17 58 30 17 23 14

16 16 29 24 76 85 82 48 38 47 48 46 73 6 5 0 22 5 7 0

45 50 35 38 0 0 0 35 46 21 25 22 12 3 0 6 0 0 3 0

0 0 5 3 0 0 0 8 10 7 5 6 0 66 65 36 36 68 59 71

0 0 0 0 0 0 0 2 1 0 0 0 0 0 0 0 0 0 0 0

10 8 15 10 9 0 0 7 5 13 9 12 15 5 13 0 12 10 8 15


ZK15108 is a deep drilling well (180 m underground) located in the southeast of Shanxu. The lower portion is a 7.5-m thick red bauxite layer, and the upper portion is a 1.5-m thick grayish black bauxitic clay layer; the transition between these two portions is sharp. Five samples (from ZK15108-1 to ZK15108-5) were taken from the lower red and reddish part and two other samples (ZK15108-6 and ZK15108-7) came from the upper gray part of the drilling well. ZK4353 is a 20-meter-deep drilling well. Three-meter-thick bauxite deposit can be divided into two parts by its colors: the lower is red to reddish-brown bauxite (0.5 m) and the upper is black to gray bauxitic clay (2.5 m). Seven samples were collected from this drilling well, including a carbonate sample (ZK4353-1) from the underlying Maokou Formation. Two samples (ZK4353-3, -4) are taken from the lower red layer while four were taken from the upper gray layer. A fresh Maokou Formation limestone L1 from the outcrop near the ZK4353 was also collected. Profile MY represents the original Permian bauxite deposit under the surface conditions. The overlying layers have been removed by tectonic activities, and the 4 m thick orebody is covered by a 30-cm thick Quaternary soil horizon. The bauxite deposit has various surface colors, including red, brown and gray. Three samples were taken from this profile in 1 m interval (Fig. 3A). Profile KL, 3 km away from the southeastern side of Shanxu, is a typical Salento-type bauxite profile. Quaternary lateritic mélanges are the result of the erosion and weathering of the nearby Permian bauxite-bearing layers. The thickness of the lateritic layer reaches ~3 m. For there is no bedding texture in this profile, four samples were randomly collected (Fig. 3B). The maximum vadose depth of the groundwater in Guangxi depends on the seasonal and the topographic factors; it changes from

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Fig. 6. Plots of samples in a Fe–Al+ Ti–clay minerals ternary diagram after Bárdossy (1982).

−10 to −80 m in the plain and the foothill area and reaches −100 m in the peak-cluster depressions during a rainy season (Qian, 2001). The 20 m deep drilling well ZK4353 is beyond the groundwater table, and therefore, it suffers from groundwater leaching. In contrast, ZK15108 is 180 m underground, at a depth that is far deeper than the vadose zone, so it is barely affected by the groundwater system. Profiles KL and MY are on the surface and undergo reworking by surface leaching.

Fig. 5. Typical XRD patterns of bauxite and bauxitic clay samples. B, boehmite; C, chlorite; D, diaspore; H, hematite; K, kaolinite.

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Table 2 Major, trace and rare earth elements composition of bauxite and carbonate rock samples and CIA of bauxite and bauxitic clay samples in Fusui area. Elements

Samples

ZK15108-1

ZK15108-2

ZK15108-3

ZK15108-4

ZK15108-5

ZK15108-6

ZK15108-7

L1

ZK3453-1

ZK3453-3

Major oxides (%)

SiO2 Al2O3 TFe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO LOI Total CIA* S Li Rb Be Sr Ba Cu Zn Ga Bi Ni Cr V Sc Zr Hf Nb Ta W Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y LREE HREE ∑REE L/H Eu/Eu* Ce/Ce* (Ce/Yb)N (La/Sm)N (Gd/Lu)N

20.05 41.01 21.75 0.15 0.20 0.02 0.19 4.06 0.07 0.08 12.26 99.84 98.55 0.03 161 1.2 2.23 23 30 55.6 37 48.3 1.55 92.1 116 148 18.6 674 18.6 90.1 6.34 4.8 18.5 6 144 328 31.3 112 22.2 3.91 22.1 3.51 20.5 4.01 11.3 1.61 10.7 1.51 95.7 663.51 148.84 812.35 4.46 0.53 1.10 7.94 4.08 1.82

19.33 41.74 22.06 0.17 0.20 0.03 0.20 4.13 0.07 0.10 11.84 99.87 98.50 0.03 151 4 8.47 51 50 80.6 246 50.9 2.18 323 306 186 17.1 985 24.4 155.5 10.35 10.3 22 5.3 60 152 13.6 51.2 15.9 3.32 18.5 3.2 20.3 3.98 11.9 1.9 11.7 1.7 100 314.52 154.68 469.20 2.03 0.59 1.21 3.37 2.38 1.35

25.24 38.62 20.05 0.12 0.21 0.03 0.09 3.65 0.12 0.12 11.98 100.23 98.64 0.01 141 4.1 8.22 47.6 50 77.5 237 50.4 2.15 301 314 183 15.8 843 21.4 149.5 10.3 8.1 20.9 5.2 45.2 167 7.8 25 4.83 0.821 5.1 0.91 6.21 1.21 3.81 0.6 4.51 0.6 29.2 255.75 47.05 302.81 5.44 0.50 1.94 9.60 5.89 1.06

21.90 40.63 21.13 0.13 0.20 0.04 0.07 3.71 0.08 0.15 11.96 99.99 98.79 0.01 177.5 1.2 5.07 80.4 50 91.9 118 40.4 2.08 260 219 163 16 818 21.6 129.5 8.87 4.4 18.9 4.3 234 283 48.2 162 30.3 5.21 20.4 3.41 18.3 3.39 10.1 1.6 11.6 1.61 88.1 783.11 138.11 921.22 5.67 0.61 0.60 6.32 4.86 1.58

36.19 31.95 16.04 0.08 0.13 0.03 0.02 3.17 0.02 0.03 12.62 100.27 99.08 0.08 159.5 1 4.78 47.6 40 99.1 114 43.4 2.05 265 216 176 16.8 910 22.4 132.5 8.65 5.7 17 4.3 431 437 65.6 180 26.8 4.41 17.6 2.8 15.3 2.91 9.21 1.59 11.6 1.7 78.3 1162.41 123.41 1285.82 9.42 0.58 0.55 9.76 10.12 1.29

38.76 33.84 6.58 0.38 0.38 0.03 0.03 2.83 0.02 0.37 16.70 99.92 97.78 0.10 146 0.2 1.86 22.1 30 232 46 37.1 1.36 111 174 147 15.4 765 19.8 107.5 7.17 2.5 15 5.2 416 461 93.2 342 66.2 11.1 41.2 5.71 24.9 4.41 12.4 2.01 13.6 2.01 101 1430.70 166.04 1596.74 8.62 0.61 0.53 8.78 3.96 2.55

40.47 34.35 5.20 0.37 0.34 0.04 0.12 2.42 0.02 0.32 16.38 100.03 97.70 0.08 172.5 0.3 2.44 17.9 20 63.5 37 39.2 1.58 106 163 167 18.2 835 22.2 113 7.65 3.6 16.1 6.1 413 362 73.9 243 45.7 7.71 29.6 4.32 20.9 3.8 11.4 1.91 13 1.8 87.4 1174.91 144.53 1319.44 8.13 0.60 0.45 7.22 5.69 2.05

6.86 0.20 0.13 0.41 51.53 0.01 0.02 0.01 0.01 0.03 40.70 99.91 0.01 0.8 1.4 0.15 662 10 1.5 3 0.41 0.03 5 16 15 0.8 6.3 0.1 0.5 0.05 0.1 0.4 2.7 9.01 5.4 1.7 6.9 1.4 0.3 1.5 0.2 1.1 0.2 0.6 0.1 0.5 0.1 10.6 17.20 13.40 30.60 1.28 0.63 0.31 2.80 4.05 1.87

0.52 0.70 0.60 0.34 54.40 0.01 0.03 0.04 0.01 0.02 43.00 99.67 0.02 5 0.9 0.08 132.5 10 4.4 5 1.02 0.04 1.1 36 7 2 9 0.3 1.1 0.08 23.4 1 0.8 13 34.9 2.8 11.3 2.5 0.41 3.01 0.51 3.09 0.6 1.81 0.4 1.7 0.3 27 67.92 35.42 103.34 1.92 0.46 1.31 5.32 3.27 1.25

31.40 44.54 7.54 0.08 0.11 0.04 0.10 1.74 0.01 0.01 14.30 99.87 99.18 0.02 293 1.2 0.9 14.8 10 17.9 37 45.7 1.09 84.6 253 139 19.5 661 19.1 68.5 4.82 4.1 22.2 5.4 37.7 180 7.4 26.9 6.2 1.02 6.21 1.1 7.38 1.59 4.88 0.8 5.41 0.8 42.1 265.43 64.06 329.49 4.14 0.50 2.40 8.62 3.83 0.97

Sulfur (%) trace elements (ppm)

Rare earth elements (ppm)

4. Methodology To investigate the mineralogical composition of the 20 bauxite and bauxitic clay samples, a lithological examination of thin sections and X-ray diffraction (XRD) were performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. The XRD studies were performed with a PANalytical X'Pert Pro model instrument using a Cu–Ni tube at 40 kV and 40 mA, under continuous scanning with a speed of 8°/min. The main mineral phases were identified by comparing the results with the standard mineral card in the laboratory. The mass percentage (mass %) of the main mineral phases identified was semi-quantified with an analytical error of ±10%.

For whole-rock abundances in the major element analyses, 22 samples were analyzed by SHIMADZU 1800X X-Ray fluorescence (XRF), using the national bauxite XRF analysis standard GSB 04-2606-2010 as the standard sample to construct a calibration curve; the detection limit was b3%. The total sulfur content was obtained by the chemical titration method. The trace elements and the rare earth elements (REE) were analyzed by an inductively coupled plasma-mass spectrometer (ICP-MS) Varian 820-MS. The detection limit for trace elements and REE ranges from 2 to 8 ppm for various elements. To estimate the enrichment and the loss ratio of the elements compared to the parent rocks during the bauxitization (Bárdossy and Aleva, 1990), a masschange calculation was introduced into the study of the Fusui bauxite orefield. Numerous cases have demonstrated that no element is


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Table 2 Major, trace and rare earth elements composition of bauxite and carbonate rock samples and CIA of bauxite and bauxitic clay samples in Fusui area. ZK3453-4

ZK3453-7

ZK3453-8

ZK3453-10

ZK3453-11

KL-1

KL-2

KL-3

KL-4

MY-2

MY-3

MY-4

25.03 49.71 8.90 0.08 0.10 0.04 0.10 1.76 0.01 0.00 14.18 99.91 99.31 0.03 238 1.3 1.11 17.4 160 20 45 47.8 1.25 44.5 250 121 15.2 682 18.7 66.5 4.97 5.1 20.5 5.9 33.9 179 6.5 23.6 5.3 0.91 5.42 0.1 6.41 1.41 4.3 0.8 5.01 0.7 39.3 254.63 58.93 313.56 4.32 0.52 2.68 9.26 4.03 0.96

24.31 25.20 26.80 0.07 0.08 0.03 0.04 0.91 0.01 0.02 22.80 100.28 99.02 4.06 206 0.9 0.51 11.8 10 18.3 17 22.5 0.72 72.7 154 63 15.8 375 10.8 34.3 2.33 7.5 21 10.8 15.2 112 3.4 12.6 2.71 0.42 2.81 0.41 3.12 0.6 1.79 0.3 2.02 0.3 21.1 149.14 29.65 178.79 5.03 0.46 3.53 14.37 3.53 1.16

28.32 30.93 20.00 0.12 0.15 0.03 0.07 1.21 0.01 0.06 19.42 100.32 98.71 5.65 278 1.1 0.79 19.4 20 8.4 17 38.6 1.09 45.4 178 95 25 559 16.1 49.8 3.52 5.6 34.4 15.2 15.5 129 3.9 15 3.72 0.61 4.1 0.71 4.9 1.01 3.01 0.501 3.31 0.4 29.4 171.83 43.24 215.07 3.97 0.48 3.81 10.10 2.62 1.28

33.46 36.41 10.70 0.10 0.08 0.04 0.07 1.45 0.01 0.02 17.42 99.76 99.21 5.09 359 0.6 0.89 17.3 60 6.6 27 40.4 1.15 30.3 214 93 17.8 631 17.2 59.1 4.2 6.6 27.2 14.7 12.5 89.2 3 11.5 3.11 0.62 3.81 0.61 4.41 0.912 2.81 0.4 3.1 0.4 25.5 123.74 38.15 161.89 3.24 0.55 3.33 7.46 2.53 1.17

37.43 34.32 8.44 0.10 0.13 0.04 0.12 1.56 0.01 0.01 17.42 99.58 98.75 5.62 287 1.7 1.42 15.7 20 9.3 12 42 1 36.4 195 98 13.1 494 15.5 55.4 4.05 7.3 18.6 7.7 34.1 161 7.1 26.3 6.15 1.12 6.52 1.11 7.21 1.41 4.02 0.6 4.21 0.6 37.1 242.29 56.26 298.55 4.31 0.54 2.32 9.91 3.49 1.36

19.79 33.23 31.00 0.05 0.04 0.00 0.01 3.23 0.07 0.03 12.24 99.69 99.73 0.02 23 5.1 1.22 38.1 40 13.1 6 55.9 0.77 56.1 276 170 25.2 1065 28.4 113.5 8.2 7.1 31.4 12.4 67 1350 10.9 34.2 9.32 1.9 10.6 1.91 11.6 2.19 6.41 1.02 6.91 1.01 55 1483.92 86.05 1569.97 17.24 0.58 10.76 50.63 4.52 1.31

5.35 55.44 24.01 0.02 0.03 0.00 0.00 3.17 0.02 0.01 12.18 100.23 99.90 0.03 4.1 0.2 1.66 12.5 10 3.1 3 38.3 1.52 33.2 334 146 22.2 1220 30.7 123 8.26 64.2 27.8 17.1 21.5 129 3.3 13.4 7.14 1.41 7.92 1.52 9.81 1.81 5.32 0.8 6.11 0.901 41.7 183.67 67.97 251.64 2.70 0.57 3.26 5.47 1.90 1.09

8.65 58.69 14.80 0.03 0.01 0.00 0.00 2.75 0.04 0.00 14.66 99.64 99.95 0.02 8.9 0.2 1.42 16.1 9 11 6 39.8 0.98 23.9 310 109 27.2 1085 27.5 103 7.03 28.7 30.8 10.5 32.2 231 5.4 19.9 6.19 1.01 5.81 1.01 6.91 1.29 4.09 0.7 5.01 0.6 31.6 301.51 51.21 352.72 5.89 0.51 3.80 11.95 3.27 1.20

5.92 56.01 22.58 0.02 0.03 0.00 0.00 2.83 0.03 0.01 12.86 100.29 99.90 0.02 3.7 0.4 2.77 9.6 10 4.6 11 39 1.74 46.4 290 128 25.7 1090 27 113.5 7.59 80.9 24.9 9.9 25.6 116 3.8 13.3 6.72 1.61 10.7 2.51 15.9 2.7 7.11 1.1 8.41 1.21 43.5 177.73 82.44 260.17 2.16 0.58 2.49 3.57 2.40 1.10

7.87 62.71 10.36 0.07 0.06 0.00 0.00 3.65 0.03 0.00 15.40 100.15 99.83 0.03 36.6 0.2 2.28 20.2 10 53.8 46 40.8 2.36 102 455 126 32.1 1765 44.3 190.5 11.55 33.5 37.9 15.7 61.9 126 8.6 34.8 17.5 4.12 24.7 5.13 29.2 5.21 13.7 2.11 14.2 2.02 101 277.62 172.57 450.19 1.61 0.61 1.14 2.30 2.23 1.52

6.08 52.65 25.88 0.02 0.07 0.00 0.00 3.04 0.04 0.01 12.58 100.37 99.74 0.02 26.4 0.4 1.98 39.5 20 9.8 30 32.2 1.69 53.1 266 468 17.3 1115 27.4 137 8.86 53.3 22 25.1 89.3 224 12.1 40.1 12.2 2.7 14.9 2.71 14.7 2.51 6.5 1.01 7.01 1.01 51.6 395.30 87.05 482.35 4.54 0.61 1.41 8.28 4.61 1.84

17.55 41.64 24.90 0.43 0.03 0.02 0.10 3.44 0.12 0.01 12.10 100.33 99.56 0.03 271 0.3 2.05 404 10 4.4 87 37.5 1.63 188.5 253 95 17.5 1120 28.1 139 9.38 29.7 15.9 8.9 179 360 23.1 76.3 9.81 1.53 7.12 1.19 7.71 1.41 4.21 0.6 4.81 0.6 30.8 656.86 51.33 708.19 12.80 0.54 1.15 19.40 11.48 1.47

completely immobile during the bauxitization; moreover, it is difficult to confirm all of the sources of the bauxite due to intense mechanical and chemical weathering (Bárdossy, 1982; MacLean et al., 1997). However, there are still several elements that can be considered as nearly immobile, such as Ti, Zr and Cr, and these immobile elements can be utilized as the monitor elements in the altered system (MacLean and Barrett, 1993; MacLean et al., 1997). As only one highly correlated trend is present for each immobile element pair (Figs. 8 and 9), the concentrations of the immobile elements in the Maokou formation carbonate and the Heshan Formation bauxite can be utilized to estimate the chemical modifications and the mass changes that have taken place during the bauxitization (Hanilçi, 2013; MacLean and Barrett, 1993; Mameli et al., 2007; Oliveira et al., 2013; Zarasvandi et al.,

2010). The mass change (ΔM) was calculated following (MacLean and Barrett, 1993): ΔM ¼ wid

wjp −wip wjd

ð1Þ

In this equation, wip is the content of the monitor element in the precursor rock, wij is the content of the monitor element in the altered rock, wjp is the content of the mobile element in the precursor rock and wjd is the content of the mobile element in the altered rock. A positive result corresponds to an increase in the abundance of an element, and a negative result corresponds to a decrease in the abundance of the element.

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Fig. 7. Binary diagrams showing the correlations between Al2O3 and SiO2, TFe2O3, K2O, Na2O, MgO, CaO for samples.

5. Results 5.1. Ore texture and mineralogy The textures of the original Permian bauxite deposit are preserved in the deep drilling well ZK15108 without the modern groundwater reworking. In the lower part of ZK15108, the red bauxite has a compact texture and contains gray to brown ooliths, pisoliths and clastics. These ooliths and pisoliths are present in the final structure (Fig. 4A). Stratified structures are observed in the upper, gray part of ZK15018 (Fig. 4B). In the shallow drilling well ZK4353, the lower, red, compact bauxite layer and the upper, gray, compact part can also be easily distinguished. Ooliths and pisoliths are found in the red layer of ZK4353; however, most of them are broken by epigenetic aluminum minerals (Fig. 4C), and new formed quartz veins are observed on the boundary of the bauxite layer and underlying carbonate (Fig. 4E and F). The surfaces of the samples collected from the outcrops are covered by a red iron crust and commonly have a compact texture; the ooliths and the pisoliths have almost completely disappeared due to the extensive weathering process (Fig. 4D). The XRD results are given in Table 1, and some XRD patterns of bauxite and bauxitic clay are shown in Fig. 5. Statistics from five of

seven XRD analyses in lower part of ZK15108 indicate that the minerals are chiefly boehmite (on average 33.6%) with substantial amounts of chlorite (on average 22.2%), kaolinite (on average 32.2%) and hematite (Fig. 5 and Table 1). Other two samples from ZK15108 are mainly composed of clay minerals such as kaolinite (on average 83.5%) and chlorite (on average 16.5%), whereas hematite and aluminum minerals disappear. In the drilling well ZK4353, the lower, red part and the upper, gray part contain clay minerals (kaolinite, +/− chlorite), hematite, boehmite and diaspore, but the contents of these minerals change from the bottom to the top: the clay minerals increase from ~ 40% to ~ 70%, the aluminum minerals decrease from ~ 40% to ~ 10%, hematite increases from 7% to 15%, and at the bottom of the deposit, a small quantity of quartz (1–2%) is detected. Furthermore, the content of the ooliths and pisoliths decreases upward. Profile MY and Profile KL have similar mineral compositions: diaspore is dominant (N 57%), chlorite and hematite are widespread, and boehmite and kaolinite are also found. According to the mineralogical classification of Bárdossy (1982), seven drilling well samples and two profile samples fall into the bauxitic clay category, six drilling well samples and one profile sample fall into the clayey bauxite category and four profile samples fall into the bauxite to lowiron bauxite category (Fig. 6). From drilling well samples to profile samples, the qualities of the ores improve.

Fig. 8. Binary diagrams showing high positive correlations (R N 0.9) between Al2O3 and TiO2 in each sample site.


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Fig. 9. Binary diagrams showing the correlations between Al2O3 (wt.%) and trace elements Zr, Cr, Nb, V, Ni and Y (ppm).

5.2. Geochemistry 5.2.1. Major geochemistry Geochemical analyses show that the compositions of the 20 bauxite and bauxitic clay samples contain 5.35–40.47% SiO2, 25.2–62.71% Al2O3, 5.2–31% Fe2O3 and 0.91–4.13% TiO2 as the main oxides; S contents are very low (0.1–0.01%) in most samples while in some samples from ZK4353, they are unusually high (5.65–4.06%) (Table 2). The concentrations of the alkali and the alkali earth elements are very low because of their extreme mobility during the weathering process. SiO2 and Al2O3 display a negative correlation. The plots of K 2 O, Na 2 O, MgO and CaO against Al2O3 show large-scale mobility and ultimate depletion with increasing Al2 O 3. The complicated behavior of Fe2 O 3 caused scatter (Fig. 7). Additionally, all of the bauxite and bauxitic clay samples show high CIA values (≥ 97.70) which were calculated by CIA = 100 * [Al2O3 / (Al2O3 + CaO + Na2O + K2O)], indicating an intense chemical-weathering process during bauxitization (see Table 2) (Nesbitt and Young, 1982). The plots of TiO2 against Al2O3 show high positive correlations (R N 0.9), and the correlation lines pass through the Maokou Formation carbonate plots (samples ZK4353-1 and L1), which indicate that the Maokou Formation carbonate can be the standard for the mass-change calculation (Fig. 8).

5.2.2. Trace and REE geochemistry Nb, Zr, Cr and V are concentrated in the bauxite ores; the correlation lines also pass through the carbonate plots and show a positive correlation with a high correlation coefficient when plotted against Al2O3 (Table 2 and Fig. 9). The contents of Y, Ni, Rb and Ni increase to the maximum value and then decrease with increasing Al. For the REE composition (Table 2), ∑REE in the 20 bauxite samples ranges from 161 to 1597 ppm, with more concentrated LREEs (523 ppm on average) than HREEs (91 ppm on average). Chondrite-normalized REE distributions are LREE-enriched in the bauxite samples, with dramatic LREE–HREE fractionations evidenced by (Ce/Yb)N, which varies from 2.3 to 50.63. The (La/Sm)N range of 1.9–11.4 and the (Gd/Lu)N range of 0.96–2.55 indicate a larger fractionation in LREE. Negative Eu anomalies are observed in all of the samples, and positive Ce anomalies are also found in most of the samples, excluding the Maokou Formation sample L1 and four samples (ZK15108-4 to ZK15108-7) from the ZK15108 (Fig. 10). 5.2.3. Mass-change calculation The mass changes were calculated based on Eq. (1), and sample ZK4353-1 was utilized as the precursor rock. As the most immobile element, TiO2 was utilized as the monitor element, and the results are

Fig. 10. The chondrite-normalized REE patterns of the bauxite, bauxitic clay and the Maokou Formation carbonate (ZK4353-1, L1).

12MY-4 12MY-3

−19.87 0.38 −9.43 −12.77 −0.34 −0.98 −0.24 −0.86 −19.62 0.17 −18.18 −12.75 −0.34 −0.98 −0.25 −0.86

12MY-2 12KL-4

−19.72 4.19 −10.26 −12.77 −0.34 −0.98 0.00 −0.25 −18.10 6.58 −14.26 −12.76 −0.34 −0.98 0.00 −0.24

12KL-3 12KL-2

−20.35 0.64 −10.88 −12.77 −0.34 −0.98 0.00 −0.25 −13.50 −10.46 −7.76 −12.76 −0.34 −0.71 0.00 −0.23

12KL-1 ZK3453-11

14.04 7.59 −14.22 −12.68 −0.30 −0.86 −0.25 −0.86 12.62 12.39 −11.18 −12.68 −0.30 −0.91 −0.25 −0.84

ZK3453-10 ZK3453-8

13.13 13.09 2.92 −12.63 −0.29 −0.89 −0.25 −0.79 18.23 16.37 22.84 −12.67 −0.29 −0.90 −0.24 −0.83

ZK3453-7 ZK3453-4

−1.03 17.22 −14.76 −12.71 −0.31 −0.89 −0.25 −0.86 4.87 13.14 −15.88 −12.71 −0.30 −0.89 −0.25 −0.86

ZK3453-3 ZK15108-7

2.82 −4.44 −19.25 −12.55 −0.32 −0.90 0.00 −0.25 −1.84 −7.89 −18.97 −12.58 −0.32 −0.96 0.00 −0.25

ZK15108-6 ZK15108-5 ZK15108-4

−15.74 −10.74 −14.33 −12.72 −0.33 −0.90 0.00 −0.24 −15.34 −10.75 −14.30 −12.73 −0.33 −0.91 0.00 −0.24

ZK15108-3 ZK15108-2 ZK15108-1

Table 3 Mass changes in different samples from Fusui area. TiO2 is considered as the immobile component.

A bauxite deposit is a complicated and sensitive mineralogical system that is in balance with the environment from the time of its formation, and its evolution reflects all of the chemical effects during the entire process of mineralization. Varied, even contradictory, mineralogical transformations can be found in different deposits (Bárdossy, 1982). In this study, the diversity in the mineral assemblages from different samples is observed (Fig. 12). Generally speaking, red color in the bauxite ore indicates an oxidized freshwater environment and/or a good drainage over the geological history, and gray-to-black color indicates the syngenetic-to-eodiagenetic reduced environment in a brackish water basin (Bárdossy, 1982; Sheldon, 2005). ZK15108 is a representative of the original Permian bauxite deposit. In ZK15108, chlorite and kaolinite are widely distributed. The chlorites come from the transformation of the iron minerals and kaolinite in the syngenetic-to-eodiagenetic reduced environment, because the chlorites can reach mineral phase equilibrium with the aluminum minerals at a settled temperature and pressure (Ahn and Peacor, 1985; de Caritat et al., 1993). Hematite and aluminum minerals in the lower red part of ZK15108 are the representative mineral assemblages related to the strong chemical weathering (D'Argenio and Mindszenty, 1995; Tardy and Nahon, 1985). In the upper, grayish-black part of ZK15108, the aluminum minerals and hematite disappear, the clay minerals such as kaolinite and chlorite dominate and the bauxitic clay layer gradually transitions into the overlying black carbonaceous mudstone at the top of the orebody. From the bottom to the top of ZK15108, elements with a weak mobility (Ti, Nb, Zr, Cr and V) display a positive correlation with the Al content, indicating that these elements remain during the chemical weathering. Enrichment of LREEs in the samples is considered to be the result of preferential adsorption onto secondary minerals and amorphous surface coatings under the acidic condition related to laterization (Braun et al., 1998; Condie et al., 1995; Muzaffer Karadağ et al., 2009). The positive Ce anomalies in the lower part of ZK15108 are also evidence of an oxidized acidic condition because Ce3+ will transform into Ce4+ and be preserved as REE minerals, so it is common to find positive Ce anomalies in the weathering profiles (Braun et al., 1990; Ji et al., 2004; Mameli et al., 2008; Mongelli, 1997; Mongelli et al., 2014; Wang et al., 2013). Nevertheless, accompanying the color changes in ZK15108, negative Ce anomalies are observed in the upper, grayish-black part of ZK15108, which are direct consequences of the transgression, in which formerly exposed platforms submerged and transformed into restricted, reduced environments. From the results of the mass-change calculation, the depletion of Si and Al in ZK15108 decrease with decreasing depth. These facts show an important similarity to the results from studies of other global bauxite deposits: most elements in the weathering profile are depleted, whereas the depletion of aluminum is less than that of the other elements, so it becomes relatively enriched (Hanilçi, 2013; MacLean

−5.36 −10.78 −14.76 −12.74 −0.33 −0.97 0.00 −0.25

6.1. Minerals transformation and elements behavior during the bauxitization process

−5.36 −10.78 −14.76 −12.74 −0.33 −0.97 0.00 −0.25

6. Discussion

−13.85 −9.44 −13.78 −12.73 −0.32 −0.95 0.00 −0.23

displayed in Table 3. In ZK15108, except for ZK15108-7 at the top of the deposit reveals the enrichment of SiO2, all the elements in the other samples show depletion, with the main losses coming from SiO2 and Fe2O3, reaching −15.74% and −19.25%, respectively. The loss of Al2O3 reaches − 10.78%. From the bottom to the top, the depletion of SiO2 shows a progressive decrease and converts into enrichment at the top of the bauxite deposit; Al2O3 shows larger depletions in the lower part of the deposit, whereas Fe2O3 shows a larger depletion in the upper part. In ZK4353, the enrichment of SiO2 and Al2O3 is obvious, the mobility of iron is unstable and the other major elements show depletion. In the samples taken from Profile MY and Profile KL, most of the elements except for Al2O3 show depletion, and the Al2O3 enrichments are larger in Profile KL (Table 3 and Fig. 11).

−15.08 −7.66 −11.40 −12.59 −0.33 −0.93 −0.21 −0.86


SiO2 (%) Al2O3 (%) TFe2O3 (%) MgO (%) Na2O (%) K2O (%) P2O5 (%) MnO (%)

84


85

Fig. 11. Mass changes relative to the Maokou Fm. carbonates. The changes are considered as loss and gain in wt.%, red bar means element loss and black bar means element gain.

Fig. 12. Integrated compilation profile of minerals content against SiO2, Al2O3 and Fe2O3 mass changes in wt.%. Red dots indicate losses of elements and black dots indicate gains of elements.

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et al., 1997; Mameli et al., 2007; Oliveira et al., 2013). Furthermore, for there is no evidence of resilicification in ZK15108\, and combined with the obtained mineralogical and geochemical data, we believe that the depletion of elements decreasing with the changing depth in ZK15108 is the result of sedimentary environment changing, the exposed and oxidized environment that was needed for the formation of the vadose bauxite becomes restricted and reduced, and then phreatic bauxite forms (Bárdossy, 1982; D'Argenio and Mindszenty, 1995). The 20-meter-deep drilling well ZK4353 is in the range of modern groundwater activity. Although ZK4353 can be divided into two parts according to its red and gray colors, the mineral content in ZK4353 indicates an oxidized and well-drained condition: all the samples in ZK4353 contain boehmite (+/− diaspore), kaolinite, chlorite, and hematite, bits of quartz (1–2%) are found at the bottom of the drilling well. Field observations and laboratorial experiments demonstrate that the kaolinite can hydrolyze to boehmite and orthosilicic acid (Hanilçi, 2013; Huang, 1993; MacLean et al., 1997). Orthosilicic acid is stable in an acid environment, and it can be transported in the colloidal condition. When the geochemical condition turns to neutralization, orthosilicic acid will precipitate, and the accompanying dehydration leads to precipitation of amorphous quartz (Exley et al., 2002). In the case of ZK4353, the underlying Maokou Formation carbonate can form the geochemical barrier to block the downward transportation of liquid acid and orthosilicic acid colloids, and the neutralization leads to the precipitation of amorphous quartz; quartz veins that occur on the boundary between the bauxite layer and the underlying carbonate are the evidence of desilicification (Fig. 4E and F). Meanwhile, positive Ce anomalies in all ZK4353 samples demonstrate that the deposit suffers from the modern near-surface leaching process. Additionally, unusually high sulfur content (~4–6%) is detected in the upper part of ZK4353. This can be related to the vertical leaching of the overlying black swamp sediments with abundant sulfide and carbon above the bauxite deposit in the Heshan Formation (Zeng et al., 2005); oxidation of the sulfide leads to the formation of acidic and sulfate-rich solutions, which percolate and move downward through the deposit. This results in leaching and bleaching of the upper part of the bauxite layer; a similar metallogenic mechanism has been observed in other “coal-bauxite-iron” structured bauxite deposits (Kalaitzidis et al., 2010; Zhang et al., 2013). However, the efficiency of this mechanism is related to changes in the groundwater table; when the groundwater table is too high to remove the harmful elements in time for bauxitization, these dissolved elements may enter into the bauxite deposit, just as Si and S did in ZK4353. Samples from profiles MY and KL in the surface environment differ from the drilling well samples. The exposed Profile MY and the Salento Profile KL both have a better drainage condition than the buried bauxite deposit. The mineral composition is mainly composed of diaspore and, sometimes, boehmite; chlorite is the dominant clay mineral, kaolinite is sparsely distributed and the hematite content ranges from 0 to 12%. Either metamorphism (Hanilçi, 2013) or supergene crystallization

(Hatipoglu et al., 2010; Liu et al., 2012b; Mordberg, 1999) may generate diaspore in a bauxite deposit. So far, no metamorphosis evidence has been reported in the rock of the Fushui bauxite deposit. Additionally, simulated thermodynamic calculations prove that the surface condition can fully meet the requirements of the transformation between boehmite and diaspore (Chen and Zeng, 1996; Peryea and Kittrick, 1988). Therefore, we proposed that the diaspore in the Fushui bauxite deposit was epigenetic and formed in the near-surface weathering environment. Corresponding with the mineral content changes, Si and Fe are depleted in the profiles, and Al is enriched in bauxite samples. In this process, the quality of the ores is improved, resulting in the formation of high-Al and low-Si content bauxite ores. The Cenozoic epigenetic weathering and leaching transform the original bauxite ores. 6.2. Evolution from the original Permian orebody to a Quarternary Salentotype bauxite deposit Based on the lithological, mineralogical and geochemical investigations, the evolution of the bauxite deposit in the Fusui area can be described as follows: at the end of the Middle Permian, the Maokou Formation carbonates became exposed and suffered weathering due to the tectonic uplift of the platform. The advantages of the parent rock, the weather and the landform led to the formation of lateritic materials. Bauxitization began after the materials accumulated in the karstic depressions on the surface of the Maokou Formation. Continuous, intense chemical weathering and good drainage conditions resulted in the removal of alkali and alkali earth elements and desilicification. Finally, the relative enrichment of Al led to the formation of the original karstic bauxite (Fig. 13A). In the early Late Permian, the groundwater table rose, causing the red vadose bauxite to transform into gray or black phreatic bauxite and the mineral assemblage to be dominated by clay minerals. Weakening weathering processes caused a slowing in the removal of the elements with lower activities. When the platform subsided into the water again, the bauxite deposits were capped by the transgression sequences that were representative of the mudstone and marlstone (Fig. 13B). The original bauxite deposits evolved to the epigenetic stage, and one of the most important reactions in the deposit was the transformation of kaolinite to chlorite (de Caritat et al., 1993). The original bauxite ores were dominated by a compact texture with a high hardness. Due to Cenozoic tectonic activities, some bauxite deposits were buried deeply and became closed systems to preserve their original components, such as ZK15108; others, such as ZK4353, were uplifted to the modern leaching zone, and the deposit became open system which was reworked by the groundwater activities (Fig. 13C). Depending on the intensity of the leaching and the drainage conditions, the vertical movement of the groundwater may carry the detrimental elements (e.g., Si and S) into the bauxite deposit from the overlying strata and reduce the quality of the ore, or it may remove the impurities and transform the clay minerals into aluminum minerals.

Fig. 13. Evolutional model from Permian original bauxite to Quaternary Salento-type bauxite in Fusui area.


After the overlying strata eroded away, the bauxite deposits disintegrated into gravel of various sizes and accumulated in the karstic depressions within a lateritic matrix (Fig. 13D), resulting in the formation of the Salento-type bauxite. Because most bauxite gravels are massive and compact structures with a high hardness, they did not decompose and suffer argillization during the supergene weathering. The boehmite in the ores transformed into diaspore, and the floating flow on the surface removed the dissolved elements from ores to further improve the qualities of the bauxite ores. 7. Conclusions Field observation and mineralogical and geochemical analyses are performed on bauxite deposit from the Fusui area, and we rebuilt the evolution from the original Permian bauxite deposit to the Quaternary Salento-type bauxite deposit. The following can be concluded: (1) Bauxite ores are composed of aluminum minerals (boehmite and diaspore), clay minerals (kaolinite and chlorite), hematite and a small quantity of quartz. The leaching intensity and the groundwater drainage control the mineral assemblage in the bauxite deposit. (2) Laterization and bauxitization lead to elements with low mobility (e.g., Ti, Zr, Nb, Cr and V) showing high positive correlations with Al. Most alkalis and alkali earth elements are removed from the bauxite. Chondrite-normalized REE distributions show that the LREEs are enriched and the HREEs have a flat shape. Most bauxite samples also display negative Eu anomalies and positive Ce anomalies. (3) Results from a mass-change calculation reveal that all of the elements are depleted in the original weathering profile, but Al is relatively enriched compared to the other elements. When the bauxite deposit is transformed by a modern groundwater system, the quality of the ores is controlled by vertical leaching and the drainage system. In the surface environment, both the exposed bauxite deposit and the Salento-type deposit suffer from intense leaching in a good drainage condition, and the quality of the bauxite ores improves. Acknowledgement This work was supported by China Geological Survey “Formation and Enrichment Regularities of Bauxite Deposit in Integrated Exploration Area of West Guangxi” Program. Two anonymous reviewers are thanked for their valuable and constructive comments and suggestions which greatly improved the quality of the paper. References Ahn, J.H., Peacor, D.R., 1985. Transmission electron microscopic study of diagenetic chlorite in Gulf Coast argillaceous sediments. Clay Clay Miner. 33, 228–236. Bárdossy, G., 1982. Karst Bauxites. Bauxite Deposits on Carbonate Rock. Elsevier Scientific Publishing Company, Amsterdam. Bárdossy, G., Aleva, G.J.J., 1990. Lateritic bauxites. Developments in Economic GeologyElsevier, Amsterdam, p. 27. BGMRGR (Bureau of Geology and Mineral Resource of Guangxi Zhuangzu Autonomous Region), 1985. Regional Geology of Guangxi Zhuangzu Autonomous Region. Geological Publishing House, Beijing, pp. 1–853, (in Chinese with English abstract). Bogatyrev, B., Zhukov, V., Tsekhovsky, Y.G., 2009. Formation conditions and regularities of the distribution of large and superlarge bauxite deposits. Lithol. Miner. Resour. 44, 135–151. Braun, J.-J.,Pagel, M.,Muller, J.-P.,Bilong, P.,Michard, A.,Guillet, B., 1990. Cerium anomalies in lateritic profiles. Geochim. Cosmochim. Acta 54, 781–795. Braun, J.-J., Viers, J., Dupré, B., Polve, M., Ndam, J., Muller, J.-P., 1998. Solid/liquid REE fractionation in the lateritic system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions. Geochim. Cosmochim. Acta 62, 273–299. Chen, Q.,Zeng, W., 1996. Calorimetric determination of the standard enthalpies of formation of gibbsite, Al(OH)3(cr), and boehmite, AlOOH(cr). Geochim. Cosmochim. Acta 60, 1–5.

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