Journal of Earth Science  2019, Vol. 30 Issue (1): 121-130   PDF    
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Exploration and Sources of Bauxite Deposit in the Boloven Plateau, Southern Laos
Yongzhen Long1,2, Xiaoyong Yang3, Yang Mu1,2, Dexian Zhang1,2    
1. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, Central South University, Changsha 410083, China;
2. School of Geosciences and Info-Physics, Central South University, Changsha 410083, China;
3. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
ABSTRACT: The Boloven bauxite deposit occurs either in the weathered basalt (alkali basalt and tholeiite), or in the Cretaceous sandstone. It is generally agreed that the bauxite deposits/laterites overlying the alkali basalt and tholeiite were derived from weathering of underlying basalt, however, the origin of bauxite deposit overlying the sandstone remains controversial. Chondrite-normalized REE patterns show that the bauxite ores/laterites overlying the sandstone exhibit quite similar chondrite-normalized REE patterns to those overlying the alkali basalt. Diagram of Al-Ti-Zr shows that the bauxite ores/laterites overlying the sandstone, tholeiite and alkali basalt are close to each other and to the calc-alkaline suite, however, significantly different from the sandstone and shales. Binary diagram of log Nb/Y vs log Zr/Ti further indicates that the parent rocks of bauxite ores/laterites overlying the sandstone belong to the suit of ultra-alkali to alkali basalt. Multivariate statistical analysis of geochemical data exhibit that the geochemical characteristics of HREE, Y, LREE and Al2O3 for bauxite ores/laterites overlying the sandstone are similar to those overlying the alkali basalt (15.7 Ma), obviously different from those overlying the tholeiite (1.2 to 0.5 Ma). Consequently, it can be inferred that the bauxite deposits/laterites overlying the sandstone were derived from the alkali basalt.
KEY WORDS: late ritic-type bauxite    XRD    datum processing    exploration    sources    Boloven Plateau    Laos    

0 INTRODUCTION

Laterites are near-surface weathering products developed on variable parent rocks (Dill, 2017; Hanilçi, 2013; Sanematsu et al., 2011a; Schellmann, 1994; Bárdossy and Aleva, 1990). They are categorized into autochthonous (in situ), parautochthonous and allochthonous types (Dill, 2017).

Lateritic-type bauxites are more strongly weathered laterites generally derived from the underlying alumosilicate rocks. They are restricted to a narrow range of wet tropical climatic conditions (Bárdossy and Aleva, 1990). In spite of apparent similarities, each lateritic profile (weathering sequences of rock-forming minerals from the crystalline parent rocks into the zone of the bauxite or laterite formation) is unique in relation to its development depending on factors of bedrock composition, climate, and hydrogeology (Dill, 2017). The in-depth understanding of the lateritic materials therefore needs detailed multidisciplinary studies, especially for the geochemical regime, and the facies-diagnostic minerals of the various types of supergene laterization (Dill, 2017; Hanilçi, 2013; Sanematsu et al., 2011b). Multivariate statistical methods are usually used to treat datum sets for a given system, which can provide results and patterns to understand the geochemical processes of the system (Levitan et al., 2015; Drew et al., 2010; Grunsky, 2010).

Lateritic-type bauxite is the dominant bauxite deposit, which is mostly concentrated in the southern part of the country in the Boloven Plateau (Fig. 1a; Vilayhack et al., 2008). It has the potential to be one of the biggest bauxite deposits in the world. Except for a few geological data obtained from a small number of mountain engineering and laterite survey in this area (Sanematsu et al., 2011b), the exploration work lacks systematic engineering control and sample analysis, and yielded no information on the spatial distribution, occurrence and ore qualities. On the other hand, the source of the bauxite deposit overlying the sandstone in Boloven Plateau bauxite remains controversial. Karinen et al. (2011) and Sanematsu et al. (2011b) suggested that the bauxite deposits overlying the sandstone were derived from the footwall rocks (Triassic to Cretaceous sandstone), those overlying the basalts were formed from weathered basaltic lavas. However, Wei et al. (2011) and Luo et al. (2011) concluded that the source rocks for bauxite deposits in the Bolovens Plateau were basalts.

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Figure 1. (a) Major geological units and fold belts of the country and the position of the study area (after Karinen et al., 2011); (b) the distribution of basalts and bauxite/laterite profiles in the Boloven Plateau, southern Laos (modified after Vilayhack et al., 2008); (c) area of exploration.

In this study, exploration by means of geological mapping, geological record compiling of exploration shallow well, sampling as well as chemical analysis and datum processing have been undertaken in the southeast of Champasak District, 45 km distance to Paksong County (Fig. 1c). Based on field exploration and geochemical datum processing, typical bauxite/laterite profiles overlying the sandstone and overlying the basalts (alkali basalt and tholeiite, respectively) are studied by means of petrologic, geochemical, and multivariate statistical methods. We aim to unravel the spatial distribution, occurrence, source rock and ore quality of the bauxite (especially for the bauxite deposits overlying the sandstone) and to discuss the ore-forming geochemical process of the Lateritic-type bauxite in the Bolovens Plateau.

1 REGIONAL AND LOCAL GEOLOGY

The Boloven Plateau, located in the central SE Asia Peninsula, southern Laos, east to the Truongson belt and west to the Khorat Plateau, is a part of Indo-China terrane (Fig. 1a; Zhang et al., 2017; Qian et al., 2016; Wan et al., 2016; Zhang et al., 2013; Karinen et al., 2011; Fan, 2000). The basement of this region is composed of Upper Triassic to Cretaceous sediments (Fig. 1b). In the Late Yanshan-Himalaya periods, the collision of India Plate and Indo-China Plate resulted in the Indo-China Block uplift constantly and basaltic volcanic eruption from the thinning continental crust (Sanematsu et al., 2011a; Tapponnier et al., 1986).

The outcrop stratum of the Boloven Plateau mainly consists of Jurassic, Cretaceous, Quaternary–Neogene and Quaternary (Sanematsu et al., 2011a). The Triassic to Cretaceous sedimentary rocks consist of sandstone interbedded by a small amount of mudstone. The Lagnao-Kang Formation consisting of sandstone interbedded by mudstone beds (thickness from 600 to 1 500 m) overlies unconformably the Jurassic Kanglo Namho Tai Formation. The Latsaluay Formation consisting of kaolin-rich clay beds in the lower part and sandstone in the upper part (thickness about 100 m) overlies unconformably the Lagnao-Kang Formation (Sanematsu et al., 2011b; Vilayhack et al., 2008).

The magmatic rocks in southern Laos are mainly conposed of Tertiary and Quaternary basalts, distributed in approximately 7 000–8 000 km2 and forming the Boloven Plateau on the basement of Jurassic–Cretaceous (Fig. 1b;). These basalts are classified into three categories based on normative compositions and groundmass 40Ar/39Ar ages: (ⅰ) small volumetric alkali basalt consisting of nepheline-olivine basalt and olivine basalt (eruption age: 15.7 Ma) (Vilayhack et al., 2008); (ⅱ) large volumetric olivine tholeiite (1.2 Ma) and (iii) quartz tholeiite (younger than 0.5±0.2 Ma). The tholeiites of (ⅱ) and (ⅲ) are plagioclase basalt and clinopyroxene-olivine basalt (Sanematsu et al., 2011b; Vilayhack et al., 2008). The basalts of these three periods, porphyritic to ophitic texture, contain landmass of plagioclase, clinopyroxene and volcanic glass as well as opaque minerals of ilmenite, magnetite and titanomagnetite. Plagioclase is partly altered to kaolin and smectite, while, clinopyroxene and olivine are partly altered to iddzingsite and talc (Sanematsu et al., 2011b).

2 FIELD WORK AND EXPERIMENTAL METHODS

Exploration work includes geological mapping, exploration shallow well digging, and sampling. (ⅰ) Geological survey and mapping were performed to block out bauxite ore bodies and to uncover the regional geology features. (ⅱ)The exploration shallow wells were designed to reveal and control the dimensions and occurrence of the bauxite ore-bodies. (ⅲ) Samples of bauxite ores were collected from ground surface and exploration shallow wells. In the wells, samples were picked by channel sampling.

Seven hundred and ninety-seven bauxite ore and laterite samples were collected from both ground surface and exploration shallow wells (sampling locations are seen in Fig. S1), washed and/or crashed to 200 meshes before analysis. Four composite samples mixed by 16 samples (X1 and X2 from the ground surface; X3 and X4 from the shallow wells which footwall rock are sandstone) were chosen for major elements, rare earth elements (REE), X-ray diffraction (XRD) analysis, and petrographic examination, the others for Al2O3, Fe2O3, SiO2, TiO2 analysis.

The mineralogical compositions of bauxite ores were investigated by petrographic examination of thin (30 µm) and thick (120–150 µm) polished sections. The XRD studies were performed with a Rigaku D/Max-2200 model instrument using a Cu Kα tube with settings of 40 kV and 20 mA; the scanning scope, step length and speed were set to 2°–60°, 0.04° and 10°/min, respectively. Semi-quantitative estimation of mineral components was conducted using the K value method. Major elements (Al2O3, Fe2O3, SiO2, TiO2) were analyzed by a wet chemistry method, the data quality controlled by duplicated samples and international standard samples (GBW07179, GBW07405) yielding precision better than 5%. The analyzing methods and quality of testing follow the standard of GB/T3257-1999, China. The analysis was performed at the Analytical Detection Central of Changsha Research Institute of Mining and Metallurgy, Hunan, China. REE were analyzed by an inductively coupled plasma mass spectrometer (VG PQ Excel ICP-MS, Thermo Electron Corporation) at the Key Laboratory of Metallogenic Prediction of Nonferrous Metals, Hunan, China, following the procedure described in Qi et al. (2000).

Statistical treatments were conducted for bauxite ores/ laterites from bauxite ore bodies and lateritic profiles. These statistical analyses were done using SPSS 19.0 software. The samples were classified based on footwall rocks. Multivariate statistical treatment (hierarchical cluster analysis) was conducted on concentration data of bauxite ores and laterites to evaluate relationships between variables and differences between the data sets. Hierarchical variable clustering was calculated by SPSS. 19.0 software, in detail see Levitan et al. (2015).

3 RESULTS 3.1 Characteristics of Bauxite Ore Bodies

The finished exploration and evaluation work for the Boloven Plateau bauxite deposit are summarized in Table S1. The bauxite ore bodies occur in the monadnock top or along the broad and gentle ridge of plateaus with a gentle occurrence. Outcrops displaying a complete bauxitic profile are rarely observed. As results of geologic mapping and exploration trenches, shallow wells and gulches carried out in the southeast area of the Boloven Plateau, the bauxite deposits were distributed predominantely on the Triassic to Cretaceous sandstones, then on the weathered basalts (Fig. S2). From top to bottom, the bauxite ore-bearing stratum generally consists of (1) laterite layer, (2) bauxite layer, (3) saprolite/lateriate layer and footwall rock.

(1) The laterite layer, with thickness from 0 to 1 m, light red to dark brown, displays a poorly developed pisolitic or nodules texture (Figs. S2a, S2d, S2e and S2g).

(2) The Bauxite layer, with thickness from 3 to 8 m, yellow ocher to dark brown (Fig. S2), and muddy, colloidal, consolidated, pisolitic and honeycomb-like structures, was marked by nodules, block and concretion textures (Fig. 2).

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Figure 2. The photos and photomicrograph showing the features of the Baoaven Plateau bauxite, Laos. (a) Cretaceous sedimentary; (b) basalt; (c) bauxite nodules; (d) gibbsite (light colored; irregular shape); (e) lath shaped gibbsite replacing feldspar, along with micro gibbsite veins (light colored) occurring with goethite (yellow to brown); (f) gibbsite with good (010) cleavage.

(3) The saprolite/lateriate layer, gray-yellowish to light-brown, overlying the footwall rock with thickness from 0 to 2 m, highly brittle but still coherent rock, remnants the basaltic textures with the similar constituent minerals of the parent basalts and altered minerals such as iddingsite, talc and kaolin (Fig. S2).

The reserves of bauxite ores are estimated around 170 million tons according to the thickness of ore bodies disclosed by wells and mapped ore bodies' acreage.

3.2 Mineral Assemblage of Bauxite Ores

Results of optical microscopy and XRD studies indicate that the bauxite ores and laterites are mainly composed of gibbsite, hematite, goethite and anatase (Fig. 3), then, kaolinite, chamosite, quartzand anatase and small amounts of maghemite, rutile, ilmenite and zircon. Gibbsite, light colored, exhibiting lath shaped (Fig. 2e), irregular shape with (010) cleavage (Figs. 2d and 2f) and microveins (Fig. 2e), intermixes intimately with goethite (yellow to brown).

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Figure 3. X-ray diffraction pattern of bauxite composite sample (X3).
3.3 Chemical Compositions

Chemical compositions of major element and REE for composite bauxite ores are listed in Table 1, statistical data of Al2O3, Fe2O3, SiO2 and TiO2 for 797 bauxite ores and laterites are listed in Table 2. Chemical compositions of alkali basalt, tholeiite and bauxite ores/laterites on this plateau were reported by Sanematsu et al. (2011b). Table 2 shows that Al2O3 contents of bauxite ores/laterites are ranging from 15.16%–66.41% (average 44.45%) for 797 samples, 19.24%–61.27% (average 36.8%, N=33) for those overlying the basalt, 15.96%–57.81% (average 44.82%, N=38) for those overlying the sandstones. The Al2O3 contents for bauxite ores/laterites overlying the sandstones are relatively higher than those overlying the tholeiite and alkali basalts. The weathering product indexes (WPrI=moles (SiO2)/moles (Al2O3+SiO2+TiO2+Fe2O3); see Pope, 2013) range from 0.048–0.069, 0.062–0.117 and 0.176–0.512 for typical bauxite/laterite profiles overlying the sandstones, alkali basalts and tholeiites, respectively (Fig. 4).

Table 1 Concentrations of major elements (wt.%) and REE (ppm) ofcomposite bauxite ores from the Boloven Plateau bauxite deposit
Table 2 Statistical data of analyzed bauxite ores or laterites (wt.%) from the Boloven Plateau
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Figure 4. WPrI (weathering product index) of bauxite or laterite profile with tholeiite footwall (1702); sandstone footwall (PKS-XV-QJ-08); alkali basalt footwall (PKS-I-QJ-03). Where, WPrI=moles(SiO2)/moles (Al2O3+SiO2+ TiO2+Fe2O3) (Pope, 2013); data of 1702 (Sanematsu et al., 2011a).

Four composite bauxite ores/laterites are characterized by high LREE/HREE ratios (11.06–11.69), unobvious Eu anomalies (Eu/Eu*=1.01–1.05) and Ce anomalies (Ce/Ce*=0.80–1.05) (Table 1). They exhibit quite similar chondrite-normalized REE patterns to the bauxite ores/laterites overlying the sandstones and alkali basalts (Fig. 5b). In contrast, the bauxite ores/laterites overlying the tholeiites have similar chondrite- normalized REE patterns to the tholeiites, which are distinguished by significantly positive Ce anomalies, relatively low ΣREE contents and low LREE/HREE ratios (Fig. 5a). Plots of Zr-Al2O3-TiO2 show that the bauxite ores/laterites overlying the sandstones are significantly different from sandstones and shales, however, get close to calc-alkaline suite (Fig. 6a). Plots of Log Nb/Y vs Log Zr/Ti further show that the bauxite ores/laterites overlying the sandstone correspond to ultra to alkali basalts (Fig. 6b).

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Figure 5. Chondrite-normalized REE patterns of laterite/bauxite ores, bauxite composite samples, alkali basalt, tholeiite and sandstone in the Boloven Plateau bauxite; the data except for bauxite composite samples are from Sanematsu et al. (2011a).
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Figure 6. (a) Diagram of Zr-Al2O3-TiO2 weathered rock type (modified after Garcia et al., 1994; data from Sanematsu et al., 2011a); (b) binary diagram of log Nb/Y vs. log Zr/Ti (modified after Pearce., 1996; data from Sanematsu et al., 2011a).
3.4 Multivariate Statistical Analysis

Table 3 presents correlation coefficients of Al2O3, HREE, LREE with other geochemical compositions for bauxite/laterite profiles. In the profile with tholeiite footwall, Al2O3 is moderately correlated with TiO2, Sc, Fe2O3, V, Cr, Ga, Y, and Hf; and negative correlated with SiO2, Y, Ba and LREE. In the profile with sandstone footwall, Al2O3 is weakly positive correlated with Co, and negative correlated with Fe2O3, V and P2O5. In the profile with alkali basaltic footwall, Al2O3 is moderately correlated with U, and highly to moderately negative correlated with TiO2, Fe2O3, V, Cr, Co, Ba, LREE and HREE.

Table 3 Correlation coefficients of Al2O3, LREE, HREE with the other geochemical compositions

Cluster dendrograms for each bauxite/laterite profile were plotted in Fig. 7. In the profile with alkali basalt footwall, TiO2, V, Th, Fe2O3, MnO, Co, Cr and Ga in a large cluster, Al2O3 and U in a cluster, as well as Y, LREE, HREE and Sc in a cluster (Fig. 7a). In the profile with sandstone footwall, Zr, Hf, Ta, Nb, Th, Zn, TiO2, Ga and U in a large cluster, and Y, LREE, HREE, Sc, Cu, Fe2O3, V and Cr in a cluster, as well as Al2O3 and Co in a cluster (Fig. 7b). While, in the profile with tholeiite footwall, Al2O3, Zr, Hf, TiO2, Ga, Y and HREE in a large cluster, and Fe2O3, V, Sc and U in a cluster; as well as Ta, Th, Nb and LREE in a cluster (Fig. 7c).

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Figure 7. Hierarchical variable clustering by average linkage from hierarchical variable clustering for bauxite/laterite profile samples (data from Sanematsu et al., 2011a). (a) Bauxite ores/laterites overlying alkali basalt; (b) bauxite ores/ laterites overlying sandstone; (c) bauxite ores/laterites overlying tholeiite.
4 Discussion 4.1 Provenance of the Bauxite Deposit Overlying Sandstone

Although it is generally agreed that lateritic-type bauxite overlying aluminosilicate rocks is inferred to be derived from weathering of the bedrocks (Mameli et al., 2006). Karinen et al. (2011) and Sanematsu et al. (2011b) had suggested that the Boloven Plateau bauxite deposits overlying the sandstone derived from the footwall Triassic–Cretaceous sandstones. However, Wei et al. (2011) and Luo et al. (2011) indicated that the Bolovens Plateau bauxite deposits overlying the sandstone were directly from the basalts.

Immobile elements, such as Al, Ti, Zr, Th Nb and Y, have been widely used to constrain the provenances of sedimentary suites (Garcia et al, 1994; Mongelli, 1997; Long et al., 2017). The Al-Ti-Zr covariations have been used as good tools for the chemical identifications of shales, sandstones andplutonic rocks (Garcia et al, 1994). In this study, the Al-Ti-Zr diagram shows that the bauxite ores/laterites overlying the sandstone, tholeiite and alkali basalt are close to each other within the calc-alkaline suite, but quite different from the sandstones, shales and the peraluminous granites (Fig. 6a). The log Nb/Y vs. log Zr/Ti binary diagram also provides similar results as the Al-Ti-Zr diagram, most bauxite ores/laterites overlying the sandstone are highly close to those overlying the alkali basalt, some to those overlying the tholeiite. We infer that origin of these bauxite ores/laterites overlying the sandstone belong to the suite of ultra-alkali to alkali basalt (Fig. 6b). Rare earth elements are regarded as an excellent instrument for tracing the sources of weathered materials (Long et al., 2017; Zhang et al., 2017; Zhao et al., 2017). The normalized REE distribution pattern of the weathering products mostly inherited certain particularly geochemical characteristics of their parent materials (Zheng et al., 2017; Mameli et al., 2007; Liu et al., 2013). Our study obviously shows that the bauxite ores/laterites overlying the alkali basalt and tholeiite are derived from their basement rocks. While the chondrite-normalized REE distribution patterns of the bauxite ores/laterites overlying the sandstone and the ground bauxite ores are similar to that of the alkali basalt (Fig. 5b). Bauxite deposits can form from almost every type of rock that contains alumina (Dill, 2017; Hanilçi, 2013), however, in most cases, bauxite is formed most readily from rocks whose contents are high in aluminum with weak resistance to weathering (Patterson et al., 1986). In the Boloven Plateau, both the alkali basalt and tholeiite have higher contents of aluminum (Sanematsu et al, 2011b) and weaker resistance to weathering than sandstone.

Consequently, we suggest that the bauxite/laterite overlying on the Cretaceous sandstone in the Boloven Plateau was directly derived from the alkali basalt (pheline-olivine basalt and olivine basalt) with eruption age of 15.7 Ma, while the bauxite/laterite overlying the tholeiite was sourced from the tholeiite (eruption age ≤1.2 Ma).

4.2 Geochemical Characteristics of REE and Al2O3

Cluster dendrograms of chemical compositions of the bauxite ores/laterite samples for both typical profiles overlying the sandstone and overlying the alkali basalt show similar characteristics, such as LREE, HREE and Y in a close cluster (Figs. 7a and 7b) and significantly positive correlation between LREE and HREE with correlation coefficient 0.93 and 0.90, respectively (Table 3). However, for bauxite/laterite profiles overlying the tholeiite, there is obvious difference, i.e., HREE, Zr, Hf, TiO2, Al2O3 and Y in a close cluster, while LREE, Ta, Nb, Th, Mn and Co in another cluster (Fig. 7c).

The fractionation of REE depends on several factors including Eh, pH, stable primary REE-bearing minerals and secondary minerals (Zheng et al., 2017; Li et al., 2013; Wang et al., 2013; Sanematsu et al, 2011b). As mentioned above, the bauxite deposits/laterites overlying the sandstone and alkali basalt derived from the alkali basalt (eruption age: 15.7 Ma) had been subjected to longer intensively chemical weathering and leaching than those from the tholeiite (eruption age≤1.2 Ma). From the moderate to strong positive correlations between HREE and Nb, Ta, Zr, Hf, P2O5, Y, as well as between LREE and Nb, Ta, Zr, Hf, P2O5, Y (Table 3), it can be inferred that the parent rocks contain relatively stable REE-bearing minerals such as tantalum, niobium, zircon and xenotime (Mongelli, 1997; Richardson, 1959). In contrast, the low correlation between the HREE and LREE, and a closer clustering with Nb, Ta and Th for LREE in the profile overlying the tholeiite can be inferred that HREE were separated from REE minerals and/or more LREE-bearing secondary assemblages were formed during the laterization. This leads to the laterites to be enriched in LREE relative to the parent rocks (Wang et al., 2013; Sanematsu et al., 2011b; Mongelli, 1997), which correspond well to the obviously positive Ce anomalies for the laterites overlying the tholeiite (Fig. 5a). It should be attributed to the difference of stable primary REE-bearing minerals between the alkali basalt and tholeiite because of the same wet tropical climatic conditions during the laterization process in this region (Sanematsu et al, 2011b).

The closer clustering of Al2O3 and stable elements (Zr, Hf, TiO2, Fe2O3), and the moderately positive correlations between Al2O3 and TiO2, Cr, Hf and Fe2O3 for the laterites overlying the tholeiite suggest that Al2O3 tends to be associated with Fe oxide/oxy-hydroxides in the ferruginous residuum, whereas, others (Zr, Hf, TiO2) are retained, since their hosts are particularly stable under supergene conditions (Dill, 2017; Levitan et al., 2015), such as Zr, Hf in zircon, Cr in chromite, and Ti in rutile or/and anatase (Jadhav et al., 2012). On the contrary, for the bauxite ores/laterites overlying the alkali basalt and sandstone, Al2O3 has moderate to high negative correlation with TiO2, Fe2O3, V, Cr and Th, quite different from the clustering of Zr, Hf, TiO2 and Fe2O3, while Al2O3 has moderate to weak positive correlation with U or Co which can be easily dissolved under chemical weathering supergene environment (Dill, 2017; Cumberland et al., 2016; Fayek., 2011). The mechanism of gibbsite formation has been extensively studied (Dill, 2017; Bogatyrev et al., 2009; Bárdossy and Aleva, 1990; Patterson et al., 1986; Wilke et al., 1978). For example, gibbsite is suggested to be formed as a result of relatively high Si mobility (Log activity of SiO2(aq) < -4) compared to Al, and minor changes in pH at the sediment-water interface under tropical conditions with alternate dry and wet (Patterson et al., 1986; Dill, 2017). Gibbsite can be formed mostly in three ways during supergene conditions (Dill, 2017; Jadhav et al., 2012; Patterson et al., 1986): (i) removal of silica from halloysite or kaolinite which is the common crystalline intermediate from feldspar to gibbsite, (ii) dehydration of Al-gel with crystalline and microcrystalline forms and (iii) precipitation from solution.

It is well known, rock porosity in sandstone, enabling easy access and free circulation of water, is more effective for desilication than in basalt. The bauxite ores overlying the sandstone have higher quality (with lower WPrI values) than those overlying the tholeiite and alkali basalt (with higher WPrI) in this region (Table 3 and Fig. 4). Gibbsite is the principal mineral which has crystalline and microcrystalline structures (Figs. 2d, 2e, 2f), and exhibits cleavage (Figs. 2d and 2f), or micro-veins (Fig. 2e). Consequently, we infer that the bauxite deposits were formed at the sediment-water interface with relatively high Si mobility and minor changes in pH conditions, and most of the dissolved aluminium was precipitated as the form of Al-gel.

In a word, the bauxite deposit overlying the Cretaceous sandstone of the Boloven Plateau were mainly derived from the alkali basalt (pheline-olivine basalt and olivine basalt; 15.7 Ma). Under alternately dry and wet tropical conditions, the alkali basalt was subjected to long intensive weathering. Such a formation process of long laterization had either transformed the entire parent rock into lateritic, or the weathering product was carried to the surface of sandstone. Rock porosities in sandstones, enabling free circulation of water in the lateriate overlying the sandstone, is more effective for desilication than in basalts, that lead to formation of the high quality of bauxite ores overlying the sandstonein this region.

5 CONCLUSIONS

(1) The Boloven Plateau bauxite occurs mainly on the surface of sandstone, then on the surface of alkali basalt and tholeiite. The bauxite ore-bodies are distributed at the monadnock top, along the broad and gentle ridge of plateaus with a gentle occurrence, which were mainly formed by weathering of alkali basalt.

(2) The principal aluminum mineral is gibbsite exhibiting crystalline or microcrystalline forms. The gibbsite was formed at the sediment-water interface with relatively high Si mobility compared to Al, and minor changes in pH conditions. The geochemical compositions between alkali basalt and tholeiite are different, especially for the contents of LREE-bearing minerals.

(3) The reserves of these bauxite ores are estimated around 170 million tons according to the thickness of ore bodies disclosed by wells and mapped ore-bodies' acreage.

ACKNOWLEDGMENTS

This study is supported by Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, Central South University (No. 2018YSJS01), the National Natural Science Foundation of China (No. 41673040), the DREAM project of MOST China (No. 2016YFC0600404) and QingzhenYunfeng Aluminum and Iron Mining Co. (No. 738010033). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0857-1.

Electronic Supplementary Materials: Supplementary materials (Table S1 and Figs. S1 and S2) are available in the online version of this article at https://doi.org/10.1007/s12583-019-0857-1.


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