Mineralogical characteristics of the karstic bauxite ...

Ore Geology Reviews 65 (2015) 84–96

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Mineralogical characteristics of the karstic bauxite deposits in the Xiuwen ore belt, Central Guizhou Province, Southwest China Kun-Yue Ling a,b, Xiao-Qing Zhu a,⁎, Hao-Shu Tang a, Zhong-Gang Wang a, Hui-Wen Yan c, Tao Han a, Wen-Yi Chen d a

State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China Guiyang Medical University, Guiyang 550004, PR China d Guizhou Bureau of Geology and Minerals, Guiyang 550004, PR China b c

a r t i c l e

i n f o

Article history: Received 28 April 2014 Received in revised form 1 September 2014 Accepted 2 September 2014 Available online 16 September 2014 Keywords: Karstic bauxite “Coal–bauxite–iron” structure Diaspore Mineral assemblage Organic matter

a b s t r a c t The karstic bauxite deposits in the Xiuwen ore belt of Central Guizhou Province are hosted by the Lower Carboniferous Jiujialu Formation and show parallel unconformity with the overlying and underlying strata. The orebearing rock series that exhibits a typical “coal–bauxite–iron” structure is divided into three segments from the bottom upward, namely, an iron layer, a bauxite layer, and a coal layer. Based on the textural features and iron abundances, the bauxite ores occurring in the middle segments are divided into three types of ores—clastic, compact, and high-iron. The bauxite ores primarily comprise diaspore, boehmite, kaolinite, illite, and hematite with minor zircon, pyrite, rutile, and feldspar. The existence of rutile within diaspore lumps, which were inherited from laterization, suggests that diaspore was transformed from gibbsite and that karstic bauxite was transformed from laterite bauxite. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalyses (EPMA) suggest that diaspore and boehmite differ with regard to appearance and element abundance: diaspore (2–10 μm) displays short prismatic or platy shapes and contains low SiO2 abundances (ranges from 0.2 to 0.51 wt.%), whereas boehmite (0.2–5 μm) exhibits granular or irregular shapes and contains high contents of SiO2 that vary between 2.71 and 10.4 wt.%. The Fe–Cu–Pb–Zn–Ba sulfide and sulfate mineral assemblages associated with cryptocrystalline kaolinites are discovered in the Xiaoshanba deposit from the Xiuwen ore belt. Mineralogical and geochemical studies reveal that the genesis of these assemblages is controlled by the decomposition processes of organic matter and the activities of microorganisms within the ore-bearing rock series. Geochemical data suggest that elements rich in high-organic cryptocrystalline kaolinite are produced from the adsorption effect of organic matter, whereas the low concentration of these elements in pure kaolinite can be attributed to the scavenging effect during kaolinite crystallization. In addition, coexisting organic matter, cryptocrystalline kaolinite, and sulfide and sulfate minerals (see sample XSB-14) indicate that the forming environment of the bauxite was hot, humid tropical or subtropical climates. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Bauxite resources consist of economic concentrations of aluminum that can be obtained worldwide, especially in Africa (32%), Oceania (23%), South America and the Caribbean (21%), Asia (18%), and other countries (6%) (Boni et al., 2013; Bray, 2011). Bauxite resources in China are abundant and rank fifth in the world; New Guinea, Australia, Brazil, and Jamaica are also large producers of bauxite ore. China is the second largest producer of bauxite ore (15.61%) after Australia (30.73%) (Boni et al., 2013; Bray, 2011; USGS, 2009). The bauxite deposits in China are primarily found in the Shanxi, Guizhou, Guangxi, and Henan provinces. Guizhou's reserves and basic reserves ⁎ Corresponding author. Tel.: +86 0851 5891701. E-mail address: [email protected] (X.-Q. Zhu).

http://dx.doi.org/10.1016/j.oregeorev.2014.09.003 0169-1368/© 2014 Elsevier B.V. All rights reserved.

consisted of 155.23 million tons and 216.25 million tons, respectively, by the end of 2008, which rank first among the Shanxi, Guizhou, Guangxi, and Henan provinces; Henan, Guangxi and Shanxi provinces are also large producers of bauxite (Gu et al., 2013a, b; Ling et al., 2013; USGS, 2009; Wu et al., 2006). Bauxite deposits in Guizhou are divided into five ore belts from south to north, namely, Xiuwen, Xifeng, Zunyi, Zhengan, and Daozhen ore belts. The Xiuwen ore belt was selected as the study area for this paper. Bauxite deposits are generally divided into two categories according to their occurrence status and genesis: (1) lateritic bauxite contains gibbsite as the main aluminum-rich mineral, is formed in lateritic profiles, and is the predominant global source of bauxite (N90%); (2) karstic bauxite, is present in carbonate rocks, contains diaspore and/or boehmite as the main aluminum-rich mineral and is transformed from lateritic bauxite or Al-rich laterite after deposition on karst depressions (Bárdossy, 1982).

K.-Y. Ling et al. / Ore Geology Reviews 65 (2015) 84–96

The mineralogy and texture of ore deposits are important for the aluminum extraction industry because they comprise the main factors that determine whether a metallurgical process will succeed or fail, which renders a deposit economic or uneconomic (Boni et al., 2013). Therefore, the evaluation of bauxite ore from petrography and mineralogy is important to understand its economic potential. However, the formation of karstic bauxite involves four stages: weathering, transport and deposition, diagenesis, and supergene leaching (Ling et al., 2013); this long-term evolution results in complex mineral compositions. Ore bodies can also be folded, faulted, and altered by hydrothermal fluids after formation (Liu et al., 2012; Wang et al., 2011; Zarasvandi et al., 2008), which change the ore texture and the shape of some minerals. Furthermore, the mineral assemblages in karstic bauxite reveal their mineralization condition and genetic mechanism (D'Argenio and Mindszenty, 1995; Mongelli and Acquafredda, 1999; Mongelli, 2002; Temur and Kansun, 2006). For example, the coexistence of ferrous iron mineral (chamoisite) and ferric iron mineral (hematite) indicates that the Quaternary Dajia salento-type bauxite in West Guangxi, China was transformed after the division, weathering, and oxidation of Permian karstic bauxite ore bodies (Liu et al., 2010). The existence of framboidal pyrite in bauxite ores indicates that the bauxite was formed under a reducing condition (Butler and Rickard, 2000; Zarasvandi et al., 2012). Some special textures are also useful because they provide information about the formation of the karstic bauxite. For instances, diaspore was enwrapped by anatase aggregates suggesting that diaspore and anatase formed synchronously; the existence of pisolites and lumps indicate that bauxite was transformed from laterite (Liu et al., 2013). Therefore, mineralogical and micro-mineralogical studies can provide information about the bauxite formation process and the denudation history of the source rocks (D'Argenio and Mindszenty, 1995; Dunkl, 1992; Kiss, 1955; Sinkovec, 1973; Susnjara and Scavnicar, 1978). However, although the minerals in bauxite have been investigated for decades (Boni et al., 2013; Gu et al., 2013a; Laskou and Economou-Eliopoulos, 2007; Liu et al., 2012; Liu et al., 2013; Temur and Kansun, 2006; Wang et al., 2010, 2012), the genesis of major minerals remains obscure. The identification of diaspore from boehmite due to their small particle sizes and identical chemical formulas (Al2O3·H2O) is challenging. Recent studies have focused on the Zhengan, and Daozhen ore belts (Northern Guizhou bauxite) (Gu et al., 2013a, b; Li et al., 2013; Wang et al., 2013) but have not given attention to the Xiuwen, Xifeng, and Zunyi ore belts (Central Guizhou bauxite) in Guizhou Province in Southwest China. Therefore, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalyses (EPMA) were performed to analyze the mineralogical characteristics of bauxite ores from Xiaoshanba (XSB) and Lindai (LD) karstic bauxites to gain a better understanding of the evolutionary relationships among aluminum minerals (gibbsite, boehmite, and diaspore), and define the genesis of karstic bauxite. In addition, high-organic kaolinites accompanied with a sulfide and sulfate mineral assemblage (Fe–Cu–Pb–Zn–Ba) succeeding on the Xiaoshanba bauxite ore body was examined, because it is an uncommon association in bauxite deposits (covering above the bauxitic clay, see Fig. 1, sample XSB-6, 14) and may provide some enlightenment on the forming environment and climate of the bauxite and the metal sulfide deposit with regard to mineral assemblage, appearance, and genesis aspects. 2. Geological background and geological features 2.1. Geological background The Xiuwen bauxite ore belt is located in Central Guizhou Province in Southwest China (Fig. 1), Xiaoshanba bauxite is deposited in Xiuwen County, and Lindai bauxite is located at Qingzhen County; both deposits are karst-type bauxites. The study area is located in the Guiyang Complex Tectonic Deformation Zone in the North Guizhou Anticline of the

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Yangtze Paraplatform in China (Guizhou Bureau of Geology and Minerals, 1987). Throughout the early Paleozoic era, the Guizhou area was covered by ocean until central and northern Guizhou were uplifted by the Caledonian movement. This uplift formed the central Guizhou Doming, which belongs to the ancient Upper Yangtze Land. Subsequently, Central Guizhou became a peneplain in the Devonian period after it experienced nearly 100 million years of weathering and erosion. The Xiuwen and Lindai areas became karst depressions or shallow lake facies. The south to north transgression of the South China Sea during the early Carboniferous resulted in sedimentation and the formation of the Jiujialu Formation Al-bearing rock series (bauxites and argillites) (Gao et al., 1992; Guizhou Bureau of Geology and Minerals, 1987).

2.2. Geological features The Xiuwen bauxite ore belt is a famous bauxite production base in China; its recent annual production of bauxite exceeded 2 million tons. Since the Yunwushan deposit was discovered in 1941, more than 29 bauxite deposits, such as the Xiaoshanba, Lindai, Maochang, Yanlong, and Changconghe deposits in the Xiuwen ore belt, have been discovered (Gao et al., 1992). Rock units include the Lower Cambrian Jingdingshan Formation; the Middle Cambrian Shilengshui Formation; the Middle and Upper Cambrian Loushanguan Group; the Lower Carboniferous Jiujialu Formation Al-bearing rock series and the Baizuo Formation; the Lower Permian Qixia Formation; and the Lower Triassic Maocaopu Formation (Fig. 1). Xiaoshanba and Lindai deposits are hosted by the Lower Carboniferous Jiujialu Formation which displays unconformable contacts with overlying and underlying lithologies (Fig. 1c and 2). The overlying rocks of the Xiaoshanba and Lindai deposits include Lower Carboniferous Baizuo Formation limestone and dolomite, respectively; the underlying strata comprise the Middle and Upper Cambrian Loushanguan Group and Middle Cambrian Shilengshui Formation dolomite, respectively (Bárdossy, 1982; Ling et al., 2013; Ye et al., 2007). The Xiaoshanba bauxite deposit located northwest of Guiyang City was discovered in 1957 and has mineral reserves in excess of 20 million tons and an average grade (Al2O3) of 67.91% (Ye et al., 2007). It extends for an approximate length of 7 km and width of 1 km; the thickness of the ore-bearing rock series ranges from approximately 2–20 m with an average thickness of 8 m; the thickness of the ore body ranges from approximately 2–3 m (Ye et al., 2007). The orebearing rock series Jiujialu Formation in the Xiaoshanba deposit dips 5° to 10° towards the northeast (Fig. 2a). It exhibits a typical “coal– bauxite–iron” layering that is divided into three segments: (1) the lower segment consists of iron layers that are primarily composed of ferruginous clay and clay stone and iron rock composed of red, steel gray, or dark iron ferruginous clay with thicknesses ranging from 0.5–5 m; (2) the middle segment consists of bauxite layers with thicknesses ranging from 1–10 m (general 3–5 m) that are composed of compact bauxite, fragmented bauxite, and high-iron bauxite (Fig. 3); and (3) the upper segment consists of coal layers comprising clay stone and black carboniferous shale with thicknesses ranging from 2–6 m (Figs. 1c and 2) (Ling et al., 2013). The ore body of the Xiaoshanba deposit, which is conformable with the underlying and overlying strata, is commonly stratified (Fig. 2a and e) or lenticular in shape (Fig. 2b). Generally, clastic particles show an upward-fining sequence in the ore-bearing rock series, with the exception of the lowest ferruginous clay. The lenticular ore body (clastic bauxite) is similar to the distributary channel deposit. Former studies show that the Xiuwen area is associated with shallow lake facies during the bauxite mineralization (Gao et al., 1992; Ling et al., 2013). Geological characteristics of the ore-bearing rock series of the Lindai bauxite are similar to the Xiaoshanba deposit described by Ling et al. (2013).

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Fig. 1. (a) Inset map of South China Plate showing the location of the Xiuwen bauxite ore belt, Guizhou Province, China; (b) Geologic map illustrating the geological features of the Xiaoshanba and Lindai bauxite deposits in Xiuwen ore belt (after Ling et al., 2013); (c) location of collected samples and stratigraphic column illustrating the ore body features of the Xiaoshanba bauxite deposit.

3. Analytical methods Mineral components were analyzed by X-ray diffraction (XRD) (Dmax/2200; Japan) operating under the following conditions: at 40 kV and 20 mA; scanning scope, step length and speed were set to 2°–60°, 0.04° and 10°/min, respectively. The XRD measurement was monitored by the instrument standard Cu Kα target, and semiquantitatively calculated by the K value method and the results are summarized in Table 1 (Mordberg et al., 2000). Electron probe X-ray microanalysis (EPMA) was performed using a Shimadzu EPMA-1600 Scanning Electron Microscope (SEM) equipped with a wavelength dispersive spectrometer (WDS) and EDAX Genesis energy dispersive spectrometer (EDS). Silicates, oxides, and pure elements were used as standards (analytical errors are 1% for major elements and 3% for minor elements), and the results of the EPMA analysis are summarized in Table 2. Back scattered electron images were obtained at an accelerating voltage of 120 kV and a beam current of ~ 10 μA. The XRD and EPMA analyses were conducted at the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS). Total organic carbon (TOC %) contents were analyzed by Leco CS344 performed at the Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese

Academy of Sciences and the relative standard deviation is better than 5%. The rare element abundances were analyzed in a quadrupole inductively coupled plasma mass spectrometer (PerkinElmer, ELAN DRC-e). The sample (50 mg, 200 mesh) was dissolved in high-pressure Teflon beaker for 24 h at 190 °C using HF + HNO3 mixture, and then desiccated by the electric boiling plate. This process was repeated with 0.5 ml HNO3. Next, 0.5 mg Rh as the internal standard was added to the desiccated sample, together with 2 ml HNO3 and an appropriate volume of ultrapure water, followed by heating for 5 h at 140 °C, to ensure that all substances had completely dissolved. Then 0.4 ml of this solution was transferred to a centrifuge tube, and ultrapure water was added to yield a final volume of 10 ml (Qi et al., 2000; Ling et al., 2013). This tube was retained for inductively coupled plasma mass spectrometry (ICP-MS) analysis. The ICP-MS measurement was monitored by the International Standard samples OU-6, AMH-1 and GBPG-1 (Potts et al., 2000; Thompson et al., 1999), and the trace element concentrations were calibrated by the recovery rate of the standard samples. The relative standard deviation (RSD) in the rare earth element analysis was below 10%. ICP-MS analysis was conducted at the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS). The analytical procedures


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Fig. 2. Field photographs illustrating the paleokarstic terrain and ore body features of the Xiaoshanba bauxite deposit, Guizhou Province, China. (a) Opencast works on the Xiaoshanba bauxite deposit; (b) vertical section showing the clay stone and ferruginous rock overlying and underlying the ore body of Xiaoshanba pit, respectively; (c) paleokarstic basement of the ore body in Xiaoshanba pit; (d) the carbonaceous shale above the ore body in Xiaoshanba pit; (e) vertical section showing the ferruginous rock underlying the ore body in Xiaoshanba pit.

are detailed in Franzini et al. (1972) and Qi et al. (2000), and the results are summarized in Table 3. All samples were collected from open pits and profile outcrops. 4. Results 4.1. Mineralogy and texture The textures of the bauxite ores in the Xiuwen area are clastic and compact (Figs. 3 and 4). In clastic bauxite ores, the diaspore particles exhibit middle to well-rounded shapes, which indicates that it was transported over extensive distances by the surface water (Gao et al., 1992). Because the fine-grained and disordered mineral components in bauxite ores cannot be identified under an optimal microscope, scanning electron microscope with energy-dispersive spectroscopy (SEMEDS) and EPMA analyses were performed to analyze the mineralogical characteristics of these ores. The analyses can confirm the mineral category of the bauxite and provide estimates of the mineralogical

composition of the different components. Fig. 4 shows the types of minerals in bauxite. The complete analysis of the bauxite samples performed by XRD, SEM-EDS, and EPMA shows that the Xiaoshanba and Lindai bauxite ores comprise diaspore, boehmite, kaolinite, illite, smectite, hematite, and a small amount of detrital minerals such as rutile and zircon. As with many other bauxite deposits throughout the world, diaspore and boehmite are the main economic aluminum-rich minerals in these two deposits. Wall rocks of bauxite (clay stone and ferruginous clay) primarily comprise kaolinite, illite, smectite, and hematite and have similar detrital minerals as in bauxite ores (Figs. 4, 5, and 7). In the majority of cases, diaspore with a hypautomorphic crystal structure exhibits short prismatic or platy shapes (Fig. 4a and b), whereas boehmite with a xenomorphic structure exhibits irregular granular shapes (Fig. 4c and d). Clay minerals such as kaolinite, illite, and smectite with a subidiomorphic or xenomorphic structure are scattered within the matrix; detrital minerals such as rutile and zircon occur in all bauxite ores (Fig. 4e, g, and h). Some iron minerals also occur in bauxite ores: e.g., pyrite and diaspore coexist in ore (Fig. 4b), a core of diaspore

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Fig. 3. Sample structures of the Xiaoshanba and Lindai bauxite deposits, Guizhou Province, China. (a) Clastic bauxite; (b) semi-clastic bauxite; (c) compact bauxite; (d) high-iron bauxite; (e) iron ore; (f) coexisting hematite and clay stone.

surrounded by a cortex of hematite (Fig. 4f), and hematite and goethite coexist (Fig. 4f). Mineral components of the matrix in the bauxite ores are mainly cryptocrystalline diaspore, kaolinite, and hematite. A large amount of boehmite and diaspore minerals have been analyzed by SEM-EDS: e.g., boehmite is the only aluminum-rich mineral in LD-8 through XRD analysis (27.8 wt.%), and the result of SEM-EDS analysis shows that boehmite (Spot 6) contains 3.63 wt.% of SiO2; conversely, SiO2 was not detected in a diaspore (Spot 9) from LD-7, which contains 98 wt.% of diaspore and no boehmite (Fig. 5c, d, e, and f). These results suggest that boehmite contains a large concentration of SiO2 with the exception of diaspore. In addition, the EPMA method that was performed to verify these results supports the suggestion that boehmite contains a large concentration of SiO2, with the exception of diaspore (Table 2). The results of the SEM-EDS and EPMA analyses reveal different element compositions and shapes between boehmite and diaspore: (a) boehmite contains large concentrations of SiO2, which vary between 2.71 and 10.4 wt.%, whereas diaspore contains low SiO2 abundances (ranging from 0.2 to 0.5 wt.%); (b) diaspore contains larger concentrations of FeO (range from 1.02 to 1.33 wt.%, average 1.22 wt.%) compared with boehmite (average of 0.95 wt.%) (Table 2); (c) in addition, diaspore exhibits a deeper color depth compared with boehmite in back-scattered electron (BSE) pictures from the SEM (Fig. 5a, b, c, and d). An argillaceous rock layer (XSB-14) with a thickness of 0.5 m, with cryptocrystalline kaolinite, overlies the about 0.8 m thick bauxitic clay

layer (Fig. 1c). These cryptocrystalline kaolinites (Fig. 6a, b, c, and d), which exhibit different sizes (2–10 mm), and are semitransparent and complete extinction under crossed nicols, are divided into two categories: pure kaolinite with regular shapes (Fig. 6a and b) and black highorganic kaolinite with irregular shapes (Fig. 6c and d). In addition, the sulfide and sulfate mineral assemblages (Fe–Cu–Pb–Zn–Ba) that are associated with organic matter are located within high-organic cryptocrystalline kaolinite (Fig. 6e, f, g, and h). 4.2. Geochemical composition of cryptocrystalline kaolinite Trace and rare earth element (REE) concentrations of sample XSB-14 (whole-rock sample), XSB-14-1 (pure kaolinite), and XSB-14-2 (highorganic kaolinite) have been measured (Table 3). The XSB-14 wholerock sample shows medium trace and REE concentrations, whereas the pure kaolinite shows low concentrations of trace and REEs (Table 3). Compared with the whole-rock sample of XSB-14, most element concentrations are lower in the analyzed pure kaolinite, with the exception of Li, Cr, Mo, Sb, and As. However, the high-organic kaolinite sample is enriched in REE with the exception of Li, V, Zr, Nb, Ag, Hf, Ta, W, and U. The element concentrations in the high-organic kaolinite are more than three times higher than those of the whole-rock sample and are highlighted with a boldface font in Table 3. The high-organic kaolinite contains abundant Sr and Ce, with concentrations of 7630 and 2680 ppm, respectively. However, the opposite situation is evident in

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pure kaolinite (58.6 and 11.2 ppm). The concentrations of the LREEs (La–Pm) and the middle rare earth elements (MREEs: Sm–Dy) in the high-organic kaolinite (5.18–2680 ppm) are substantially higher than those of the whole-rock sample (0.34–55.6 ppm), whereas the HREEs (Ho–Lu) show relative similar concentrations to both the whole-rock sample and the high-organic kaolinite (0.23–1.54 and 0.41–5.92 ppm, respectively) (Table 3). These three samples present similar REE patterns which are characterized by a regular decrease from La to Lu (Fig. 8a). The abundances of REE in high-organic kaolinite are high, especially the LREE elements, whereas REE concentrations in pure kaolinite are low, especially the HREE (close to chondrite values). A spider diagram of the trace elements show similar relationships among these three samples with REE patterns (Fig. 8b); however, U, Nb, Zr, and Hf do not adhere to this rule because their concentrations in the highorganic kaolinite are lower than in the whole-rock sample. Notes: “–” means not determined; “y” represents mineral phases that have been identified by XRD, but their normative values cannot be calculated due to their very low abundances.

1.77 – – – – – – – 2.4 – – –

2.61 1.62 1.02 – 2.16 1.65 – 1.21 – – – 1.76 – – – – – – – – y – 5.42

Hematite

– – – – – – – – – 36.2 – – – – – 6.13 18.2 29.2

iron oxide Dolomite Calcite

– – – – – – – – 4.8 13.5 – – – – – – – – – 1.62 – – – 3.72 6.85 4 1.06 3.86 1.85 3.54 3.22 – 4.02 1 13.5 19.2

Quartz Illite

0.87 2.86 2.06 1.01 2.75 1.78 1.24 1.86 1.87 5.86 2.76 3.26 32.0 – – – 21.6 35.2 4.83 4.08 1.74 1.24 4.35 4.36 2.06 3.72 6.74 10.3 2.96 5.2 5 – – 2.66 4.75 6.24 89.9 88.1 37.3 9.26 80.6 88.5 87.2 87.6 81.8 39.6 42.2 83.5 30.2 – 64.8 – 3.46 5.16 – – – – 1.72 – – – – – – – – – – – – –

Smectite Kaolinite Gibbsite Boehmite

– 1.75 28.2 – – – 0.92 – – 1.01 48.4 1.24 26.2 – 27.8 – 2.02 3.15 – – 29.8 88.5 – – – – – – – – – 98.2 – 88.8 – –

Diaspore Fe2O3

30.0 8.07 1.34 – 0.98 0.63 0.41 0.40 – – 1.12 18.4 1.36 2.18 2.20 6.29 14.0 15.7 16.5 39.0 26.4 – 43.4 43.3 43.8 41.9 – – 27.4 35.0 34.1 1.20 34.5 1.53 36.3 40.3

SiO2 Al2O3

14.0 33.2 55.7 – 39.3 39.2 38. 5 40.6 – – 53.0 31.8 46.2 76.8 44. 9 70.9 21.5 21.6

Lithology

Ferruginous clay Clay stone Clastic bauxite Clastic bauxite Clay stone Clay stone Carbonaceous rock Carbonaceous rock Clay stone Ferruginous rock Compact bauxite Ferruginous clay Bauxite clay Clastic bauxite Bauxite clay High-iron bauxite Loose laterite Massive laterite

Sample no.

XSB-2 XSB-3 XSB-4 XSB-5 XSB-6 XSB-14 XSB-15 XSB-20 XSB-26-1 XSB-26-2 LD-2 LD-3 LD-4 LD-7 LD-8 LD-10 LD-11 LD-12

Table 1 Abundance (wt.%) of elements and minerals in selected bauxite samples (XRD, %; calculated by the K value method).

– – – – – – – – – 1.25 0.86 – 1.65 0.75 – – 36.12 –

– y y y 3 y – 1.62 1.75 – y y 1.72 1.02 1 – – –

Feldspar

Amphibole


5. Discussion Previously it has been shown that diaspore and boehmite are the main economic aluminum-rich minerals and the textures of the bauxite ores in the Xiuwen ore belt are clastic and compact. The results of the SEM-EDS and EPMA analyses reveal different element compositions and shapes between diaspore and boehmite. However, the genesis and evolution relationship between gibbsite, boehmite, and diaspore are unknown. In this section, we sort out the distinctions between diaspore and boehmite and then discuss the evolution relationships between those three aluminum-rich minerals, thereby leading to an interpretation of the genesis of the Xiuwen karstic bauxite. An argillaceous rock layer with pure and high-organic cryptocrystalline kaolinites occurs on top of the bauxite ore-bearing rock series (Figs. 1c and 6a, b, c, and d). The sulfide and sulfate mineral assemblages (Fe–Cu–Pb–Zn–Ba) that are associated with organic material are located within highorganic cryptocrystalline kaolinite (Fig. 6e, f, g, and h). Additionally, pure kaolinite (XSB-14-1), high-organic kaolinite (XSB-14-2), and the whole-rock sample (XSB-14) exhibit completely different geochemical characteristics. In this discussion, we investigate the mineralogical, petrological, and geochemical characteristics of these three samples, and interpret the genesis relationships between cryptocrystalline kaolinites, organic maters, sulfide and sulfate mineral assemblages, leading to the re-creation of the sedimentary environment of the bauxite ores. 5.1. Distinctive structures within bauxite ores A core of diaspore surrounded by a cortex of hematite occurs in LD-7 (Fig. 4f); this structure was reported in the Wulong–Nanchuan bauxite deposit in Chongqing, China (Li et al., 2013). Iron hydroxide colloids can transform to goethite and hematite in alkaline and acidic conditions, respectively (Bárdossy, 1982). Therefore, the hematite in this structure may be transformed from iron hydroxide colloids, which transported downward from the upper layer and precipitated to gradually form the goethite assemblages in an alkaline condition in the Lindai bauxite deposit. Goethite dehydrated and formed the hematite cortex, which wrapped around the diaspore under overburden formation pressure. Low formation pressure in void spaces may cause the existence of goethite (Fig. 4f). Diaspore lumps with different shapes are scattered within the kaolinite matrix in LD-9 (Fig. 4g and h). Detrital minerals (e.g., rutile) inside the diaspore lumps may suggest that the diaspore lumps and rutile formed synchronously. The differences in texture between the kaolinite matrix and the diaspore lump indicate that the precursor material of the lump was most likely gibbsite, which was inherited from the laterite lump that formed in the process of chemical weathering. Additional evidence that supports this conclusion is that gibbsite was the main aluminum-rich mineral in laterite bauxite and diaspore and boehmite rarely occurred (Bárdossy, 1982; Beneslavsky, 1974; Grubb, 1979; Hanilçi, 2013; Meyer et al., 2002). Gibbsite lumps within laterite were

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Table 2 Electron probe X-ray microanalysis (wt.%) of minerals in bauxite sample. Spot no.

Minerals

MgO

Al2O3

TiO2

MnO

FeO

CaO

Na2O

SiO2

K2O

P2O5

Total

1 2 3 4 5 6 7 8 9 10 11 12

Boehmite Boehmite Boehmite Boehmite Boehmite Boehmite Diaspore Diaspore Diaspore Kaolinite Kaolinite Kaolinite

0.09 0.06 0.57 0.31 0.15 0.24 0.02 0.04 0.06 4.05 0.61 0.2

84.5 90.4 73.9 66.8 83.4 80.3 88.1 84.6 87.4 48.6 36.5 46.5

0.84 0.62 0.75 14.2 0.47 0.71 0.12 0.88 0.16 0.67 0.18 0.57

– 0.02 – – – – – – 0.01 0.07 – 0.02

0.63 0.44 0.63 0.95 1.75 1.29 1.02 1.31 1.32 4.21 0.63 0.26

0.04 0.01 0.05 0.07 0.04 0.06 0.03 0.04 0.02 5.99 1.08 0.03

0.03 0.03 0.06 0.14 0.03 0.03 0.01 0.05 0.02 0.39 0.28 –

3.52 2.71 11.8 6.98 8.38 10.4 0.2 0.39 0.51 28.3 22.3 46.4

0.03 0.01 1.16 0.49 0.39 0.19 – 0.04 0.04 1.07 1.72 0.32

0.01 – 0.03 0.05 0.01 0.03 – 0.25 0.2 0.1 1.18 0.03

89.7 94.3 89 90 94.6 93.3 88.5 87.6 89.7 93.5 64.5 94.3

Note: “–” means not determined.

deposited on the karst depression after an extensive diagenetic process and transformed to boehmite and diaspore in suitable conditions. 5.2. Differences between diaspore and boehmite The differences between diaspore and boehmite have been recognized (Bárdossy, 1982; Beneslavsky, 1974): (a) diaspore shows a column or flake shape, boehmite displays an irregular granular texture, and boehmite (0.1–1 μm) usually is small in size compared with diaspore (0.1–20 μm) (Beneslavsky, 1974); (b) diaspore contained large concentrations of FeO (7%) and MnO (4%), whereas pure boehmite minerals contained abundant SiO2 of 1.5–5.0% (Beneslavsky, 1974); (c) large-grained boehmite (0.2–0.3 mm), which contained 2.1% of SiO2, has been discovered in the Ural mountain area (Beneslavsky, 1974); (d) Li (1989) examined bauxite deposits in China and discovered that Si can be detected in addition to other common elements (e.g., Fe and Mn) in boehmite through energy-dispersive spectroscopy (EDS). Although the identification of diaspore and boehmite in bauxite is challenging due to identical chemical formulas (Al2O3·H2O or AlO(OH)), SEM and EPMA can be used to distinguish diaspore from boehmite (Fig. 5 and Table 2). Although it is possible to distinguish diaspore from boehmite based on differences in their physical and chemical characteristics, their genetic relationship remains controversial. Gibbsite is the original aluminum-rich mineral of karst bauxite; the majority of gibbsite was transformed from feldspar and clay minerals during lateralization

processes (Bárdossy and Aleva, 1990; Horbe and Anand, 2011; Liu et al., 2012). Boehmite is likely to be the original bauxite mineral that forms in surface environments. It can be transformed from gibbsite within the temperature range of 35–50 °C for a specific formation pressure from the overlying strata (Al2O3·3H2O = Al2O3·H2O + 2H2O) (Bárdossy, 1982; Valeton, 1972). Experimental studies and field observations indicate that diaspore can be transformed from boehmite not only in metamorphic conditions but also in a near-surface environment (Bárdossy, 1982; D'Argenio and Mindszenty, 1995; Ervin and Osborn, 1951; Gorecky et al., 1949; Keller, 1962; Keller and Stevens, 1983; Liu et al., 2010; Nia, 1971; Özlü, 1983; Valeton, 1964). The XRD analysis results show that diaspore occurs only in the clastic bauxite samples, such as XSB-4, XSB-5, LD-7, and LD-10 (high-iron bauxite with clastic texture), whereas boehmite occurs in the compact and clastic bauxite or bauxite clay samples (Table 1). The reasonable interpretation for this phenomenon is that boehmite (0.1–1 μm) usually exhibits smaller sizes than diaspore (0.1–20 μm); therefore, the bauxite ore that contains boehmite shows only a compact texture, whereas the bauxite that contains large amounts of diaspore shows a clastic texture (Beneslavsky, 1974). 5.3. The “coal–bauxite–iron” structure As mentioned in the previous section, the bauxite ores and host rocks in this study show a “coal–bauxite–iron” series, this concept was proposed by Zhang et al. (2013), in which the Al-bearing rock series

Table 3 Trace and REE (×10−6) element composition of sample XSB-14 in different sampling methods. Sample no.

XSB-14

XSB-14-1

XSB-14-2

Sample no.

XSB-14

XSB-14-1

XSB-14-2

Sample no.

XSB-14

XSB-14-1

XSB-14-2

Lithology

Whole rock sample

Pure kaolinite

High organic kaolinite

Lithology

Whole rock sample

Pure kaolinite


Lithology

Whole rock sample

Pure kaolinite


Li Be Sc V Cr Co Ni Cu Zn Ga Ge As Rb Sr Y Zr Nb Mo

613 5.34 11.1 126 41.7 10.2 55.2 21.9 77.6 15.5 0.49 10.2 4.19 188 13.5 471 28.6 0.51

684 4.53 7.82 47.6 56.2 8.07 49.5 0.38 63.4 11.6 0.34 16.8 3.3 58.6 3.11 124 4.47 1.78

512 9.8 27.8 75.9 62 29 92 68.5 80.7 54 4.21 24.5 5.12 7630 38.5 167 16.5 0.87

Ag Cd In Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er

1.77 0.14 0.17 5.52 0.76 1.74 10.6 27.6 55.6 3.79 12.5 2.06 0.41 1.78 0.34 2.29 0.52 1.54

0.15 0.13 0.08 2.91 1.51 1.51 7.42 3.97 11.3 0.58 1.84 0.26 0.08 0.34 0.06 0.46 0.1 0.31

0.36 0.37 0.34 13.5 1.38 2.46 109 608 2680 147 548 131 18.9 66.4 5.18 11.7 1.67 5.92

Tm Yb Lu Hf Ta W Tl Pb Bi Th U ∑REE LREE HREE LR/HR (La/Yb)N Ce/Ce* Eu/Eu*

0.24 1.63 0.23 12.4 2.28 4.62 0.04 12.7 0.26 12.1 10.6 111 102 8.57 11.9 0.72 1.33 0.65

0.04 0.31 0.04 4.15 0.32 0.91 0.01 1.86 0.04 2.89 1.23 19.7 18.0 1.67 10.8 0.15 1.83 0.83

0.46 2.98 0.41 4.70 1.25 2.47 0.06 42.3 1.38 67.9 4.42 4228 4133 94.8 43.6 2.17 2.2 0.62

Notes: Ce/Ce⁎ = CeN / (LaN·PrN)1/2;Eu/Eu⁎ = EuN / (SmN·GdN)1/2; bold-font indicates that its abundance is three times higher than that in whole-rock sample.


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Fig. 4. Backscattered electron (BSE) images of bauxite ores in Xiuwen ore belt, Guizhou Province, China. (a) An automorphic granular diaspore with longitudinal striation; (b) coexisting pyrite and diaspore; (c) flake diaspore and granular boehmite showing different appearances; (d) boehmite scattered within kaolinite matrix; (e) coexisting smectite, kaolinite, and rutile; (f) A core of diaspore surrounded by a cortex of hematite occurring in LD-7; (g) diaspore lumps with different shapes are surrounded by kaolinite matrix; (h) debris mineral (rutile) occurs in diaspore lumps suggesting that diaspore and rutile formed synchronously.

consists of three segments, iron argillite, bauxite, and coal layers ordered from the lower segments to the upper segments of the profile. After investigation of the Al-bearing rock series in Guizhou, Guangxi, Henan, and Yunnan provinces, we determined that bauxite deposits are frequently associated with the iron-rich stratum and coal seams that formed a “coal–bauxite–iron” special structure, which were also prevalent in karst deposits in other countries, such as the Ghiona bauxite deposit in Greece (Kalaitzidis et al., 2010), the Nurra bauxite deposit in Italy (Mameli et al., 2007), and the Kanisheeteh bauxite deposit in Iran (Calagari and Abedini, 2007) (Zhang et al., 2013). In these deposits, organic-rich sediments deposited on top of the bauxite layer, pyrite and other sulfides in the “coal” layer can be easily oxidized, consequently,

the acidic and reducing groundwater (H2S, H2SO4, etc.) percolated downwards, resulting in bleaching and Al enrichment of the underlying bauxite (Kalaitzidis et al., 2010). These fluids can also reduce Fe3+ to Fe2 + and transport iron along the basal carbonate layer, resulting in the local precipitation of iron-rich minerals (FeS2 and FeCO3). 5.4. Genesis of Fe–Cu–Pb–Zn–Ba sulfide and sulfate mineral assemblages The observations by optical microscope and SEM suggest that sphalerite is the main mineral of this sulfide and sulfate mineral assemblage, followed by galena, chalcopyrite, and barite (Fig. 6). The ore-bearing rock series in the Xiaoshanba bauxite deposit comprises a set of

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Fig. 5. Backscattered electron (BSE) images of bauxite ores from the Xiuwen ore belt, Guizhou Province China. (a) Flake diaspore and granular boehmite showing different appearances; (b) coexisting diaspore and boehmite, sample XSB-4; (c) boehmites scattered within kaolinite matrix; (d) diaspore with column or flake shapes; (e) SEM-EDS image of spot 6 showing the existence of silicon contents within boehmite; (f) SEM-EDS image of spot 9 indicate that diaspore does not contain silicon. (The numbers from 1 to 12 indicate the locations of spots by EPMA analysis (using 1 μm electron beams).)

sedimentary strata that is unaffected by hydrothermal fluid. The total organic carbon content (TOC %) of XSB-14 (whole-rock sample), XSB14-1 (pure kaolinite), and XSB-14-2 (high-organic kaolinite) are 0.08%, 0.11%, and 0.55%, respectively, which demonstrates that black (high-organic) cryptocrystalline kaolinite (XSB-14-2) with chalcopyrite–galena–sphalerite–barite sulfide and sulfate minerals contain more organic matter compared with the other two samples. It suggests that the occurrence of sulfide and sulfate mineral assemblages in the ore-bearing rock series from the Xiaoshanba bauxite is related to organic matter. After decades of studies of karstic bauxite ore samples around the world, most researchers have suggested that bauxites are mainly formed in humid, tropical to subtropical climates (e.g., Bárdossy and Aleva, 1990; Butler and Rickard, 2000; Gu et al., 2013a; Zarasvandi et al., 2012). The sulfide and sulfate mineral assemblages that are associated with high-organic kaolinite at the top of the bauxite provide a good method to research the bauxite. For instance, the occurrences of coal seams and abundant organic materials in the Parnassus-Ghiona Unit of Central Greece indicate that the depositional environment was the hot, humid tropical to subtropical climates (Kalaitzidis et al., 2010). Abundant organic matter occurred during formation of the high-organic kaolinite, as well as the sulfide and sulfate minerals, indicating that the forming environment of the Xiaoshanba Al-bearing

rocks including the XSB-14 was hot, humid tropical or subtropical climate. Previous studies have shown that the occurrence of organic matter significantly promoted the formation of sulfide deposits during its mineralization, especially in the Pb-Zn deposit metallogenic system (Saxby, 1973; Trudinger, 1976; Kesler et al., 1994; Southgate et al., 2006). Organic matter can absorb metal ions and trace elements, such as As, Sb, V, Mo, Ni, Re, Hg, S, Se, U, Cd, Ba, B, F, and W, which are abundant in organic-rich rocks (Chen and Wang, 2004). In oilfield brine, some elements were greatly enriched, such as the D-1 oil well in the Caddis Farms oil field: it demonstrated a high abundance of Ca (36,400 ppm), Cl− (158,200 ppm), Fe (298 ppm), Zn (300 ppm), Sr (1100 ppm), Ba (61 ppm), Mg (1730 ppm), Pb (80 ppm), and SO24 − (310 ppm) (Carpenter et al., 1974). Another contribution of organic matter to sulfide and sulfate deposit mineralization is primarily derived from organic decomposition processes and microorganism activities in organic matter. Since organic matter was deposited, decomposition processes and the release of ac2− − − tive groups (\COO−, \NH2, \PO− 4 , \S , \HS , and \SO4 ) which easily form a complex or chelate with metal ions or trace elements derived from surrounding sediments, were encountered. In addition, organic matter is abundant in some metal ions such as Mn2 +, Cu2 +, Zn2+, Fe2+, and Mg2+ because these ions are the essential substances


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Fig. 6. Photographs, photomicrographs, and backscattered electron (BSE) images showing the existence of sulfide and sulfate mineral assemblages associated with cryptocrystalline kaolinite in sample XSB-14 from the Xiaoshanba deposit, Guizhou Province, China. (a) Pure cryptocrystalline kaolinite (XSB-14-1); (b) pure cryptocrystalline kaolinite shows an appearance similar to that of the rhizome of a plant; (c) cryptocrystalline kaolinite clay matrix and sulfide and sulfate mineral assemblages in high-organic (black) cryptocrystalline kaolinite; (d) photograph showing sulfide and sulfate mineral assemblages; (e) photomicrograph showing sulfide mineral assemblages scattered within kaolinite matrix; (f) photomicrograph showing sphalerite + galena + chalcopyrite + barite assemblages. (g) BSE image showing coexisting sphalerite, galena, chalcopyrite, and barite in high-organic cryptocrystalline kaolinite; (h) fine-grained chalcopyrite within sphalerite suggests that chalcopyrite could be exsolved from the solid solutions.

for the metabolism of organisms (Chen and Wang, 2004). Sulfate materials can be transformed to H2S and HS− through bacterial sulfate reduction (BSR) and natural reduction effects, which provided the negative ions required by metal sulfide deposits (Saxby, 1973; Trudinger, 1976). Sulfide and sulfate mineral assemblages (chalcopyrite–galena– sphalerite–barite) rapidly precipitated due to reactions between these metals and negative ions. Saxby (1973) investigated the diagenetic mechanism between metal and cystine by thermal simulation experiments and discovered that the Fe-, Cu-, Pb-, Zn-, and Ni-organic complexes change immediately to corresponding sulfide minerals with released 35% gas and 25% soluble oil when the experiment system attained a temperature of 200 °C.

These discussions reveal the genesis of the sulfide and sulfate mineral assemblages in XSB-14. The hot and humid weather in early Carboniferous enabled plants to flourish on land and in shallow water. Therefore, plant debris that absorbed large amounts of metal and trace elements occurred and were combined with clastic materials followed by deposits in the depression that formed this special stratum (XSB14). Subsequently, plant debris broke down and formed a large amount of organic matter, which was consistent with the proliferation of microorganisms. Although these organics continuously absorbed metal and trace elements throughout the entire process of diagenesis, the majority of the organics would decompose and depart; only 0.01–10% of organic matter could be preserved in the strata during the evolution of organic

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matter (Durand, 1980). Simultaneously, the vacancies that resulted from the migration of organic matter were filled with kaolinite colloids, which were derived from the surrounding sediments. Over millions of years, these kaolinite colloids and metals crystallized to form the cryptocrystalline kaolinite within sulfide mineral assemblages (Fig. 6c and d). The contact relationships between these sulfide minerals and the highest concentrations of zinc suggest that sphalerite formed first, followed by galena, chalcopyrite, and barite (Fig. 6e, f, and g). The fine-grained chalcopyrite that existed in sphalerite suggests that chalcopyrite may be exsolved from solid solution (Fig. 6h). Therefore, the sulfide mineral assemblages in XSB-14 may have initially formed as a solid solution mixture and subsequently exsolved to form these sulfide minerals with a reduction in temperature and pressure. This phenomenon has been observed in experimental research. For instance, experimental studies by Hutchison and Scott (1981) revealed that the chalcopyrite blebs could be exsolved from sphalerite in the Fu–Fe–Zn–S solid solution system. Kinetic studies indicated that pentlandite was exsolved from pyrrhotite under a low temperature range from 473 to 573 K (199.85 to 299.85 °C) (Wang et al., 2005). 5.5. Geochemical characteristics of cryptocrystalline kaolinites The rock/chondrite values of REE in the high-organic kaolinite progressively decreases from Ce to Lu (La does not comply with this rule due to varying environments of sedimentation) attributed to the different absorption energy of REE3+, which increases from La to Lu with a reduction in the ionic radius (Ling et al., 2013; Ye et al., 2007). The highorganic kaolinite (XSB-14-2) is characterized by large concentrations of REE and trace elements, which are greater than that of the wholerock sample (XSB-14) with the exception of U, Nb, Zr, and Hf (Fig. 8a and b). These trends suggest that this phenomenon is related to the absorption effect of organic matter and microorganism activities. However, the differences among U, Nb, Zr, and Hf, as shown in the spider diagram, are attributed to the notion that these four elements are highfield-strength elements (HFSEs), which are elements that remained relatively stable during the entire process of diagenesis (Fig. 8b). Conversely, pure kaolinite (XSB-14-1) is characterized by low contents of these elements, similar to chondrite values in MREEs and HREEs (Fig. 8a and b). This finding is related to the scavenging effect during kaolinite crystallization (Boni et al., 2013). This effect also explains the substantially lower concentration of some elements in pure kaolinite compared with that in the whole-rock sample (Table 3).

Fig. 8. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace elements spider diagram of XSB-14, XSB-14-1 and XSB-14-2 from the Xiaoshanba deposit, Guizhou Province, China. Normalizing values from Sun and McDonough (1989), Masuda et al. (1973) and McDonough et al. (1992).

6. Conclusions The main conclusions from this study are as follows: (1) Existence of rutile within diaspore lumps that are inherited from laterization offers strong evidence that diaspore was transformed from gibbsite and karstic bauxite was transformed from laterite bauxite. (2) As demonstrated in previous studies, diaspore and boehmite differ with respect to appearance and element abundance: diaspore

Fig. 7. XRD analyses of the bauxite and clay stone in Xiuwen ore belt, Guizhou Province, China. Here A denotes anatase, D denotes diaspore, K denotes kaolinite, R denotes rutile, B denotes boehmite, Q denotes quartz, I denotes illite, H denotes hematite, and S denotes smectite.


(2–10 μm) displays short prismatic or platy shapes and contains a slight abundance of SiO2 (ranges from 0.2 to 0.51 wt.%),whereas boehmite (0.2–5 μm) shows granular or irregular shapes and contains large concentrations of SiO2 between 2.71 and 10.4 wt.%. (3) The chalcopyrite–galena–sphalerite–barite sulfide and sulfate mineral assemblages on top of the ore-bearing rock series are controlled by the decomposition processes of organic matter and the activities of microorganism within ore-bearing rocks. (4) Our studies revealed that the abundances of trace and rare earth elements in high-organic cryptocrystalline kaolinite are produced by the adsorption effect of organic matter, whereas the low concentration of these elements in pure kaolinite can be attributed to the scavenging effect during kaolinite crystallization processes. (5) Coexisting organic matter, cryptocrystalline kaolinite, and sulfide and sulfate minerals in clay stone (XSB-14) on the adjacent bauxitic clay indicate that the forming environment of the bauxite was hot, humid tropical or subtropical climates.

Acknowledgments The constructive reviews from the editor of Ore Geology Reviews and two anonymous reviewers are greatly appreciated. We are grateful to Professor Zhengwei Zhang and Liu Tiegeng for their guidance and assistance. We also appreciate Jing Hu for her help with the sample treatment and Yan Huang for her help with the REE analysis. This study was funded by the Major Project of Chinese National Programs for Fundamental Research and Development (973 Program) (No. 2014CB440906 and 2012CB416602) and the Key Foundation of the State Key Laboratory of Ore Deposit Geochemistry (SKLODG-ZY125-01).

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