Genesis of karst bauxite-bearing sequences in Baofeng, Henan (China), and the distribution of critical metals

Ore Geology Reviews 115 (2019) 103161

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Genesis of karst bauxite-bearing sequences in Baofeng, Henan (China), and the distribution of critical metals

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Shujuan Yanga, Qingfei Wanga, , Jun Denga, Yizhe Wanga, Wei Kanga, Xuefei Liua, Zhongming Lib ⁎

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State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China Henan Institute of Geology Survey, Zhengzhou 450001, China

ARTICLE INFO

ABSTRACT

Keywords: Karst bauxite Geological model Critical metals Distribution Resource

Bauxite-bearing sequences are not only the main economic sources of Aluminum, but also of many critical metals. The distribution of critical metals was determined from six bauxite profiles in Baofeng, and an evolution model was proposed for the formation of bauxite-bearing sequences. Following a terrestrial deposition of the lower pisolitic bauxite horizon and the underlying clayey layer, transportation and redeposition of bauxitic materials during submergence led to the formation of the upper bauxite horizon presenting finely-clastic and oolitic textures, as well as the intercalated clayey layers. Both bauxite ores and clay samples contain highly elevated contents of Li, B, Sc, V, Ga, Zr, Nb, W and total REEs. Li contents are extremely low in high-grade bauxite ores (10.92–44.77 ppm), compared to highly elevated values in low-grade bauxite ores (35.81–1601 ppm) and clay samples (38.54–1965 ppm), enlightening a new potential resource for Li, the bauxite-bearing sequences resting on karstified carbonate. The distribution pattern of Ga is similar to that of Li, and the correlation analysis indicates that Ga is primarily associated with silicates, i.e. clay minerals. The vertical distribution of Li, B, V, Cr and Ga shares the same pattern, similar to that of K2O and SiO2, being highly enriched in clayey layers relative to bauxite horizons. Both Sc and total REEs are also highly enriched in Baofeng bauxite profiles, highlighting a potential Sc and REEs resource.

1. Introduction Bauxites are products of intense continental subaerial weathering and alteration, generally under humid tropical to subtropical climates (Bárdossy, 1982). Three main genetic types were proposed based on host-rock lithology: 1) lateritic bauxites (88%), derived by in situ lateritization of underlying aluminosilicate rocks; 2) tikhvin-type bauxites (0.5%), being the erosional products of pre-existing lateritic bauxites and overlying the eroded surface of aluminosilicate rocks; and 3) karst bauxites (11.5%), developed on eroded surface of carbonate bedrocks (Bárdossy, 1982; Bárdossy and Aleva, 1990; Bárdossy and Combes, 1999). By contrast, over 90% of China’s bauxites belong to karst type, which are divided spatially and temporally into five groups: 1) Early Carboniferous deposits in central Guizhou, South China; 2) Late Carboniferous deposits widely distributed in North China; 3) Early Permian deposits in the northern Guizhou; 4) Middle Permian deposits in Shandong, North China; and 5) Late Permian deposits in Guangxi and Yunnan, South China (Wang et al., 2005; Wang et al., 2012; Liu et al., 2017; Wang et al., 2018; Yang et al., 2019). Over 1 billion tons of bauxite ores have been exploited in western

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Henan Province, which were resting on Cambrian to Middle Ordovician carbonates and thus categorized into karst type. Previous studies indicated that the formation of Late Carboniferous bauxites in North China is primarily associated with a long-term stratigraphic gap enhanced by regional tectonism in the North China Craton (NCC) since Middle Ordovician. The occurrences of karst bauxites are usually controlled by paleokarst geomorphology. High-grade bauxites usually occur in the lower part, as lenticular, sinkhole-filling or stratiform deposits, and pass into low-grade bauxites and bauxitic clays to the bottom, edge and top of the bauxite profiles. Although, the mineralogy, geochemistry, sedimentary features, paleo-geographical and paleoclimatic factors of karst bauxites have been extensively researched (Bárdossy and Combes, 1999; Bogatyrev et al., 2009; Liu et al., 2013; Yang et al., 2019; and references therein), the nature of origins and the formation mechanism of the characteristic concentric structure of karst bauxite deposits still remain unresolved. Some investigators interpret this phenomenon by assuming a resilification of the bauxites from above, the sides and below (Bárdossy, 1982; Keller and Clarke, 1984). Others suggest that ground water solutions saturating the deposits started to seep outward during dry seasons, and carried away the silica

Corresponding author. E-mail address: [email protected] (Q. Wang).

https://doi.org/10.1016/j.oregeorev.2019.103161 Received 27 June 2019; Received in revised form 26 September 2019; Accepted 4 October 2019 Available online 04 October 2019 0169-1368/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Geological map of the western Henan showing distribution of bauxite deposits and clay deposits. NCC: North China Craton; QOB: Qinling Orogenic Belt; QB: Qaidam Block; YB: Yangtze Block; CB: Cathaysia Block; TB: Tarim Block.

between 8 and 800 ppm, with an average of ca. 58 ppm, in the world’s bauxite deposits (Burton et al., 1959; Schulte and Foley, 2013). REEs are usually enriched in bauxite ore as ions adsorbed on the surface of the minerals, replacing similar ions in some minerals, and in REE minerals (Li et al., 2013; Ahmadnejad et al., 2017), and tend to end up in bauxite residues during the Bayer process (Borra et al., 2016; Deady et al., 2016). Scandium (Sc) is the most valuable element among the REEs that are present in the bauxites and bauxite residues. It has been estimated that 70% of the world’s Sc resources might be found in bauxites and bauxite residues (Lavrenchuk et al., 2004; Klauber et al., 2011; Nguyen et al., 2016). Furthermore, some bauxites also contain abundant amounts of other critical metals, such as V, Cr, Co, Ni, and Nb (Mongelli et al., 2017). With this in mind, we report a new geochemical data set obtained from six new bauxite profiles in Baofeng, propose a geological model for the formation of the bauxite-bearing sequences, and focus to reevaluate the critical metals distribution in this type of sedimentary rocks.

Fig. 2. Local geological map of Baofeng showing the location of the considered bauxite profiles.

2. Geological setting

dissolved in them (Balkay, 1973). Bauxites were considered as the main economic source not only of Aluminum, but also of many critical metal elements (Klauber et al., 2011; Borra et al., 2016; Mongelli et al., 2017). Lithium (Li), for instance, which is classified as an energy-critical element for green technology, reaches up to 2715 ppm in the bauxite-bearing sequences in Guizhou, China (Wang et al., 2013). According to geochemical analysis and data compilation, we found Li is highly enriched in most karst bauxite-bearing rocks of China, indicating great potential of Li resources. Gallium (Ga) is usually enriched in clays and bauxites and extracted as a common by-product of aluminum, and ranges in contents

Western Henan is located within the Huaxiong depression in the southwest of North China Craton (NCC) (Fig. 1). The Archean to Proterozoic complex basement and meta-sedimentary strata mainly outcrop on the Mountains to the northwest, and sporadically on the hills in the middle of the western Henan, overlain by dominantly carbonate rocks of Cambrian to Middle Ordovician age. Caledonian orogeny caused the tectonic uplift and exhumation of NCC, presenting a longterm hiatus until Later Carboniferous transgression. Bauxites developed at the base of Late Carboniferous Benxi Formation as a result of extensive chemical weathering on the surface of karstified carbonates under the tropical to subtropical paleoclimate, covered by coal-bearing

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3. Ore deposit geology Baofeng bauxite deposits are hosted in the Lower Member of the Upper Carboniferous Benxi Formation, and unconformably overlie the paleokarst surface of Upper Cambrian carbonate bedrocks (Fig. 2). The occurrence and distribution of bauxite are controlled by fractures and folds, as well as by paleogeomorphology (Fig. 3). At places of higher relief, bauxite orebodies usually occur as a single horizon filling in sinkhole or as lenticular forms. At lower depressions, orebodies usually occur as two or more horizons, and take the form of lenticular deposit at the bottom, but pass into stratiform deposit upward (Fig. 3). A thin layer of Fe-rich rust generally lies immediately on top of the underlying karstified carbonate, occasionally along with solution-collapsed breccia cemented by bauxitic or clayey materials. Based on mineralogy, facies and textural changes, five distinct layers or horizons, from bottom to top, are recognized for the typical bauxite profiles (Figs. 3 and 4): (1) The lower clayey layer: grayish green colored, rich in iron, dominated by pelitomorphic and clastic textures, and with a thickness of approximately 0.3–3 m. (2) The lower bauxite horizon (referred to as the pisolitic bauxite): gray to grayish yellow colored, with the presence of nodules and pisolites with varying sizes and shapes in pelitomorphic matrix, with a varying thickness from 0.5 to 3 m. (3) The middle clayey layer: dominated by mottled clay and bauxitic clay, with intercalations of bauxite, with a varying thickness from 0 to 3 m. (4) The upper bauxite horizon (referred to as the oolitic bauxite): gray colored, characterized by clastic textures or the presence of ooids in pelitomorphic matrix, occurring as stratiform deposit and usually has a stable thickness around 2 m. (5) The upper clayey layer: gray colored, dominated by bauxitic clay, with a thickness of about 1.5 m. 4. Sampling and analytical methods Two bauxite profiles in Guangling deposit (GL1 and 2), four in Bianzhuang deposit (BZ3, 5, 7, and 12) were chosen in Baofeng area, samples were collected from each of the layers, and 56 samples in total were subjected to mineralogical and geochemical analysis (Supplementary Table 1). Mineralogical compositions were determined by X-ray diffraction (XRD) in the Laboratory of Petroleum Geology Research Center (Beijing). XRD analyses were conducted with a graphite monochromator according to the methods and procedures described in details by Yang et al. (2019). Mineralogical and geochemical analysis were carried out on polished thin sections using a Hitachi S-3400N scanning electron microscope equipped with a Link Analytical Oxford IE 350 ED X-ray spectrometer (SEM-EDX) at China University of Geosciences (Beijing). All the 56 samples were pulverized to −200 mesh for whole-rock chemical analysis using inductively coupled plasma-mass spectrometry (ICP-MS), emission spectrography (ES), and X-ray fluorescence (XRF) at the Laboratory of the Geological Survey of China in Langfang, Hebei. Major and minor oxides were analyzed on fused beads following a LiBO2 fusion and acid digestions, while Ba, Cr, Rb, S, V, and Zr were analyzed on pressed power pellets. The FeO contents were determined using the volumetric-method; the loss on ignition (LOI) was determined by weight difference after ignition at 1000 °C. Other elements were analyzed by ICP-MS, except for B, which was determined by ES. The

Fig. 3. Field photographs of the considered bauxite profiles in Baofeng: (a) a worked-out bauxite deposit in a collapsed doline controlled by faults in Zhaogou; (b) bauxite profile BZ5 containing two bauxite horizons; (c) the cross section of Guanling bauxite deposit showing the two sampled profiles GL1 and GL2.

clastic and carbonate sequences of Upper Carboniferous and thick terrestrial clastic sedimentations of Permian-Cenozoic age. At a regional scale, the thickness of Upper Carboniferous deposition ranges from 20 to 200 m and increases northward (Wu et al., 1996). Conversely, bauxite deposits occur in the vicinity of ancient high-reliefs or encircling paleo-islands, with increasing distance from which both the thickness of bauxite deposits and the grade of bauxite ores decrease. Bauxite horizons tend to thin out northward, while clay deposits gradually appear and increase at the expense of bauxite deposits (Fig. 1) (Wang et al., 2012). Paleozoic intrusive rocks with U-Pb ages of ca. 450 Ma occur widely in the adjoining North Qinling Orogenic Belt (NQOB) to the southwest, while Mesozoic and Cenozoic igneous rocks are exposed sporadically in the study area of western Henan.

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Fig. 4. Field photographs of a typical bauxite profile (BZ3) showing the vertical change of textures: (a) geomorphology of the bauxite profile; (b) nodular and pisolitic bauxite in the lower part; (c) oolitic bauxite in the upper layer; (d) finely-clastic bauxite in the upper part.

analysis was validated by the measurements of standard reference materials, and the detection limits for major elements were 0.05 and 0.1 wt%, for most trace elements ≤2 ppm, for Ba, Cr, Rb, and V ~5 ppm, for F 100 ppm, for S 50 ppm, and for most REEs < 0.1 ppm (except for La and Y, the detection limits of which are 1 ppm). 5. Results 5.1. Texture and mineralogy According to microphotographs, the underlying clayey layer is characterized by pelitomorphic and clastic textures (Fig. 5a and b), the lower bauxite horizon displays nodules and complex pisoids with varying sizes and shapes embedded in pelitomorphic matrix (Fig. 5c and d), while the upper bauxite layer exhibits finely-clastic and oolitic textures (Fig. 5e and f). The clasts in the upper bauxite layer are notably subangular or rounded in shapes, and the grain sizes range from 40 to 250 μm (Fig. 5e). XRD patterns and electron probe analysis results reveal the mineralogical components (Figs. 6 and 7). The constitutes of the underlying carbonate strata are dominated by dolomite and limestone, but contaminated by clay minerals such as kaolinite and illite at the top (Fig. 6, samples BZ12-1 and 2). The lowermost clayey layer is mainly composed of illite, kaolinite, hematite, and chamosite, with a small quantity of diaspore, anatase and goethite, while the bauxite horizon is dominantly consisting of diaspore with minor amount of anatase, illite and chamosite (Fig. 6, samples BZ12-3, 4, 5, and 6). Hematite disseminates or occurs as veins penetrating clayey matrix (Fig. 7a and b). Diaspore coexists with illite and kaolinite, while anatase and zircon of micrometer scale are occasionally dispersed either in the matrix, or in nodules, pisoid and ooids (Fig. 7c–f).

Fig. 5. Microphotographs of the Baofeng bauxite and clay samples: (a–b) the underlying clays showing pelitomorphic and clastic textures; (c–d) the lower bauxite layer showing nodules and complex pisoids in pelitomorphic matrix; (e–f) the upper bauxite layer showing clastic and oolitic textures.

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Fig. 6. XRD patterns of a typical bauxite profile (BZ12). (Do: dolomite; D: diaspore; H: hematite; Go: goethite; K: kaolinite; An: anatase; I: illite, Ch: chamosite.)

5.2. Major elements geochemistry

highly enriched in both bauxite ores and clay samples, while S, Ni and Ba are uniformly depleted (Fig. 8). Tungsten (W), most notably, is extremely elevated in the middle part of the two profiles of Guangling (GL1 and GL2) (Fig. 8, Supplementary Table 1). Bauxite and clay samples contain elevated Li (10.92–1965 ppm), B (22.8–219 ppm), Sc (13.74–108.47 ppm), V (54.85–563.95 ppm), Ga (7.21–211.7 ppm), Zr (138.1–2970.3 ppm), Nb (22–212.76 ppm), and W (4.54–4136.41 ppm) (Supplementary Table 1). Nineteen among the 47 bauxite and clay samples have Li contents greater than 230 ppm (equal to Li2O content greater than the cut-off grade of 0.05% when mined as associated element), with the highest value of 1965 ppm. Li contents are particularly low in high-grade bauxite ores (10.92–44.77 ppm, with an average of 26.38 ppm), compared to highly elevated values in low-grade bauxite ores (35.81–1601 ppm, average 476.14 ppm) and clay samples (38.54–1965 ppm, average 567.53 ppm) (Fig. 9a), indicating Li occurrence is primarily associated with clay minerals. Ga contents in Baofeng bauxites mostly range from 7 to 63 ppm (with an outlier of 211.7 ppm), with a relatively low average of 29 ppm, compared to the average of 58 ppm for karst bauxite deposits worldwide (Schulte and Foley, 2013). Ga displays a similar distribution

Combining the bauxite ore samples from all profiles, Al2O3 strongly and negatively correlates with SiO2 (R2 = 0.74), while it positively correlates with TiO2 (R2 = 0.67). Bauxite ores are composed of Al2O3 (40.84–72.39 wt%) and SiO2 (8.06–23.41 wt%), with erratic Fe2O3 (0.07–26.62 wt%), small amounts of TiO2 (2.37–3.77 wt%), trace amounts of P2O5 (0.03–0.48 wt%) and CaO (0.06–11.12 wt%), as well as negligible amounts of other components. We classify the bauxitebearing rocks into high-grade bauxite ores (Al2O3 ≥ 55 wt%, Al2O3/ SiO2 ≥ 3.8), low-grade bauxite ores (with Al2O3 ranging 40–55 wt% and Al2O3/SiO2 1.8–3.8), and clay samples (Al2O3 ≤ 40 wt%, Al2O3/ SiO2 ≤ 1.8). 5.3. Trace elements geochemistry The trace element contents in the underlying carbonate strata are always much lower than those of the bauxites and clayey layers, but increase at the immediate contact with the clayey layer as a result of dissemination (Supplementary Table 1). When normalized to the upper continental crust (UCC), Li, B, Sc, Ti, V, Cr, Ga, Zr, Nb, Hf, Ta and W are

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Fig. 7. Backscattered electron micrographs of Baofeng bauxite and clay samples: (a-b) hematite-vein penetrating illitic matrix in the underlying clay; (c-f) diaspore coexisting with illite, kaolinite, hematite, zircon and anatase in bauxite ores.

pattern to that of Li, with contents being rather low in high-grade bauxite ores (7.21–17.59 ppm, with an average of 13.37 ppm), instead relatively elevated in low-grade bauxite ores (17.08–31.75 ppm, average 23.52 ppm) and clay samples (20.65–63 ppm, average 37.8 ppm) (Fig. 9b). Ga has a highly positive correlation with SiO2 contents (R2 = 0.73, Fig. 10), indicating Ga is mainly hosted in clay minerals.

clayey layers (94.29–1330 ppm, with an average of 569.71 ppm, excluding the three outliers), with LREE/HREE ratios ranging from 1 to 26 (average ~8). When normalized to post archean australian shale (PAAS), the bauxite ores and clayey samples uniformly display a flat and wedge-shaped pattern (Fig. 11), indicating the LREEs are more fractionated between different layers, compared to HREEs. Neither of the samples show any Eu anomalies, but display variable Ce anomalies when normalized to PAAS. ∑REE content is positively correlated with ∑LREE/∑HREE ratio and La/Y ratio, with correlation coefficients of 0.57 and 0.55, respectively (Fig. 12), indicating that LREE/HREE differentiation increases with the total REE content.

5.4. Rare earth elements geochemistry The carbonate rocks are characterized by extremely low contents of ∑REE (12.35–91.10 ppm). Total REEs are enriched in bauxite ores and

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Fig. 8. UCC-normalized trace elements spider diagrams of the analyzed bauxite profiles. UCC data are from Condie (1993).

5.5. Vertical distribution patterns of major, trace and rare earth elements Al2O3 contents are highest in the lower bauxite horizon and tend to decrease towards both the top and the bottom (Figs. 13-16). They also increase with the thickness of bauxite horizon, which in turn, correlates with the total thickness of the Benxi Formation hosting the bauxite sequence. Fe2O3 content usually increases with depth in all four BZ bauxite profiles, and reaches up to 41% in the underlying clayey layer. In contrast, the two GL profiles uniformly display highly elevated Fe2O3 contents near top of the deposit. SiO2 content is highly elevated in clayey layers, indicating this component is mainly hosted in clay minerals. K2O, Li, V, Cr, and Ga share a similar vertical distribution pattern, being highly enriched in clayey layers relative to bauxite horizons. By contrast, TiO2, MnO, P2O5, Sc, Nb, Hf, and Ta show irregular distribution pattern along the vertical section (Figs. 13 and 14). ∑REE contents usually reach maximum near the lower bauxite horizon, and decrease both upward and downward, corresponding to the patterns of LREE/HREE ratio and La/Y ratio. La/Y ratio is usually > in the lower part of the profiles, but decreases to < 1 in the underlying carbonate (Figs. 15 and 16). The vertical distribution of both Ce/Ce* and Eu/Eu* fluctuate irregularly along all the profiles. Ce anomalies (Ce/Ce*) display a range of 0.58–7.1 in bauxite horizons, most of them being positive, while clayey layers show Ce anomalies ranging from 0.31 to 4.68, most of them being negative. The Eu anomalies (Eu/Eu*) are between 0.51–0.70, and there is no significant difference between the Eu anomalies of bauxite, clay and carbonate samples (Figs. 15 and 16; Supplementary Table 1).

Fig. 9. Box and whisker plots for Li (a) and Ga (b) contents in high-grade bauxite ores (Al2O3 ≥ 55 wt%, Al2O3/SiO2 ≥ 3.8), low-grade bauxite ores (with Al2O3 ranging from 40 to 55 wt% and Al2O3/SiO2 1.8–3.8), and clays (Al2O3 < 40 wt%, Al2O3/SiO2 < 1.8).

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bauxite with the silica content of ground water under reducing conditions (Komlóssy, 1976; Bárdossy, 1982; Liu et al., 2016). The abundant presence of these two minerals indicate diagenetic reworking on the phase of clay minerals, as well as other aluminum and iron-bearing minerals (Temur and Kansun, 2006; Ahmadnejad et al., 2017). These transformations definitely play an important role in controlling the element behaviors, distributions and contents. 6.2. Geological model for the formation of Late Carboniferous bauxitebearing sequences in Baofeng Numerous U-Pb ages of detrital zircons were obtained for bauxites in western Henan and other Late Carboniferous bauxite deposits in North China (Liu, 2011; Wang et al., 2016; Cao et al., 2018), and they ubiquitously revealed a major peak at ca. 450 Ma, indicating a dominant contribution from the felsic igneous rocks of NQOB along the southwestern margin of the NCC. In addition, much of the sediments of the bauxite-bearing sequences – the lower member of Late Carboniferous Benxi Formation in NCC – could come from the NQOB, and the travel distance probably reached up to 4000 km northward, which is well supported by the U-Pb age patterns (Wang et al., 2016). Other minor contributions might be made by reworked old crustal materials from highlands in the vicinity and autochthonous argillaceous components of the underlying carbonates (Yang et al., 2019). Yang et al. (2019) have discussed the paleogeographic and tectonic background of the formation of Late Carboniferous bauxite in NCC, and argued that NCC migrated northward across the equator, and had a humid and hot tropical climate during Ordovician to Carboniferous. The accretion with North Qinling arc system caused the topographic

Fig. 10. Binary diagram showing the correlation between SiO2 and Ga.

6. Discussion 6.1. Mineral genesis Mineral phases reflect parental affinities and record all the chemical effects during the ore-forming processes, and many researchers have discussed the mineral genesis of karst bauxite (Bárdossy, 1982; Wang et al., 2012; Liu et al., 2013; Ahmadnejad et al., 2017; Vind et al., 2018; Yang et al., 2019). In this study, illite and chamosite were found to be very common in bauxite and clayey layers. Illite can not only be derived from terrestrial detrital residues but it might have been formed also at temperatures ranging from 200 to 300 °C during burial diagenesis (Yang et al., 2019). Chamosite, on the other hand, does not form during supergene processes, instead, it may be a reaction product of iron-rich

Fig. 11. PAAS-normalized REE patterns of the analyzed bauxite profiles. PAAS data are from Taylor and McLennan (1985).

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Fig. 12. Binary diagrams of ∑REE against ∑LREE/∑HREE ratio (a) and La/Y ratio (b).

Fig. 13. Vertical distribution pattern of major and trace elements in bauxite profiles GL1 and GL2.

uplift by the Late Paleozoic, and led to the large stratigraphic gap, the long-lasting subaerial exposure and the development of karst bauxite at the base of Benxi Formation. The total thickness of Benxi Formation in western Henan ranges from 20 to 200 m, and increases northeastward. The lithology is dominated by ferric clay, bauxitic clay and bauxite deposition in the south and west, replaced by a few marine fossilbearing limestone horizons in the north and east, indicating a marine transgression from the northeast (Wu et al., 1996). Based on the textures of the bauxite-bearing rocks, as well as mineralogical, geochemical, geochronological and paleo-geographical evidences, we propose the following evolution model for the formation of Late Carboniferous bauxites in western Henan (Fig. 17). The lower

bauxite horizon usually displays the lenticular or irregular morphologies, and passes into stratiform upward, indicating changes in the depositional environments (Fig. 3c). We infer that the lower clayey layer and the lower bauxite horizon were formed during terrestrial phases as a result of parautochthonous or allochthonous deposition (Fig. 17a), which is well supported by the presence of nodules of varying sizes and shapes comparable to the Quaternary gibbsite nodules in Guangxi (Yang et al., 2018) and to many other karst bauxites (Fig. 4b and 5c, d). These deposits filled paleokarst depressions, such as dolines, sinkholes and karst valleys. During the following submergence, the area received new sedimentation of re-elaborated weathering products transported from unsubmerged higher-reliefs in the vicinity. At places of lower 9

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clayey layer in the profiles GL1 and GL2 (Fig. 3c). The upper bauxite horizon is characterized by finely-clastic and oolitic textures, as well as relatively constant thickness, indicating that bauxitic materials were probably transported and redeposited in submerged basins (Figs. 4c, d and 5e, f). The depositional faces of karst bauxite have remained controversial for decades, and still unresolved. However, Mongelli and Acquafredda (1999) discussed that the ooids in karst bauxites form in a pedogenic environment with fluctuating groundwater, and Liu et al. (2016) also considered karst bauxites primarily form at reducing and alkaline conditions due to the rising groundwater, based on mineral phases respectively. However, we agree that most of the karst deposits of Mediterranean type tend to be freshwater at the base, and lagoon or neritic near the top (Bárdossy, 1982). As we have mentioned above, the nodules and complex pisoids with varying sizes and irregular shapes in the lower bauxite horizon were preferentially formed in such a pedogenic environment. By contrast, the concentric ooids widespread in the upper bauxite horizon feature the typical characteristics of oolites, which are normally formed in shallow water close to the paleo-equator (Opdyke and Wilkinson,1990; Li et al., 2015), indicating that these ooids should be deposited in a warm shallow marine environment. Typical laterite profiles are characterized by three distinct horizons: a loose topsoil horizon, a hard ferric crust and a lower clay layer passing into the underlying parent rock. Allochthonous laterites lying on a carbonate basement usually have the same three horizons and are well exemplified by Quaternary laterites in central Guangxi (Yang et al., 2018) (Fig. 17a). The topsoil horizon is characterized by acidic and oxidizing conditions, covered with a humic zone with pH values usually between 4 and 5 at most laterite profiles (Kumada, 1987). This is favorable for leaching of both Fe and Al and will result in the formation, of Si-enriched podzol soils (Robb, 2005; Bardy et al., 2007). The topsoil is loose and readily to be eroded, exposing the Fe-Al nodules-rich crust. The eroded topsoil material could have been transported to submerged basins and redeposited as intermittent clayey layers (Fig. 17b). And the bauxite materials produced by ongoing chemical weathering on the unsubmerged highlands, especially NQOB, could be transported by colloid into coastal basins forming the upper bauxite horizon, presenting finely clastic and oolitic textures (Fig. 17c). 6.3. Critical metals distribution in karst bauxite-bearing sequences Bauxite deposits are economic sources not only of Al, but also of many critical metal elements, such as Li, Sc, Ti, V, Ga, Zr, Nb, Hf, Ta, and REEs, etc. The distribution of most critical metals is associated with Al-, Fe-, Ti-oxyhydroxides and clay minerals, and to a lesser extent, with detrital phases (Simandl, 2014; Mongelli et al., 2017; Vind et al., 2018). These elements can be adsorbed to Fe-, Al-, Ti-minerals and clays, incorporated within crystal lattice, or exist as independent minerals. Although Li, B, V, Cr, and Ga were considered as the “bauxitophile” elements (Mongelli et al., 2017; Putzolu et al., 2018), we found that Li, B, V, Cr, and Ga share a distribution pattern, similar to that of K2O and SiO2 (Figs. 13 and 14), being highly enriched in clayey layers relative to bauxite horizons, indicating they are strongly associated with clay minerals. K2O should be mainly hosted in clay minerals, i.e. illite, which is abundantly present in Baofeng bauxites. Lithium has been listed as one of the critical elements due to increasing demand for Li-ion batteries (Chakhmouradian et al., 2015). Currently, more than three-quarters of the world’s Li are produced in Australia and Chile (Jaskula, 2016), highlighting the strategic necessity for other countries to reduce import dependence by exploring domestic Li resources (Benson et al., 2017). The present and potential resources

Fig. 14. Vertical distribution pattern of major and trace elements in bauxite profiles BZ3, 5, 7 and 12.

relief, more than one bauxite horizons are intercalated with clayey layers, reflecting the base level fluctuations (Fig. 17b and c). This inference is supported by previous investigations, which revealed that many major bauxite deposits in western Henan lie within 5–10 km distance of unsubmerged highlands, both the thickness and ore grade decreasing with greater distance from these highlands, pinching out and passing into clayey layer (Fig. 1) (Wu et al., 1996). Some of these highlands could likely be subjected to erosion, whereas submerged only during maximum transgression, hence producing one single bauxite horizon occasionally intercalated with a very thin clayey layer. This is confirmed very well by the lateral changes in thickness of the middle

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Fig. 15. Vertical distribution of major and rare earth elements in bauxite profiles GL1 and GL2.

of Li occur primarily in closed-basin deposits (58%), pegmatites and Lienriched granites (26%), clays (hectorite, 7%), and rarely in brines and zeolites (Benson et al., 2017; Bradley et al., 2017). However, we discovered that Li is ubiquitously enriched in bauxite-bearing sequences resting on karstified carbonate in western Henan, especially in the lowgrade bauxite ores and clays (Fig. 9a, Supplementary Table 1). According to our data compilation, we found Li is also highly enriched in many other karst bauxite-bearing sequences in China, such as those of Lower Carboniferous age in Guizhou (Wang et al., 2013), and Late Permian in Guangxi (Liu et al., 2017). Recently a major Li deposit was reported in Yunnan Province, China, which is found to be also hosted in bauxite-bearing sequences resting on Late Permian karstified carbonate. In view of this, the bauxite-bearing sequences have great potential to host high-tonnage Li resources. Explorations of this type of sedimentary rocks could secure additional strategic Li resources for countries, e.g. Europe, Russia, and China (e.g. over 90% of China’s bauxites belong to karst type), to reduce import dependence. All types of Li deposits are thought to be associated with felsic rocks as a result of enrichment in residual melts formed during extreme fractional crystallization due to incompatibility (Benson et al., 2017). The Li deposit associated with karst bauxite is likely no exception, because all the karst bauxites in China are found to be related to felsic

igneous rocks, which could provide abundant aluminum materials, according to provenance analyses (Liu, 2011; Wang et al., 2016; Yu et al., 2016; Cao et al., 2018; Wang et al., 2018). As mentioned previously, the Late Carboniferous bauxites in NCC are closely related to felsic igneous rocks of NQOB and/or Bainaimiao arc terrane, to the south and north of NCC, respectively (Wang et al., 2016; Yang et al., 2019). Both the Lower Carboniferous and Lower Permian bauxites in Guizhou Province are likely derived from the Proterozoic felsic volcanic and intrusive rocks (Panxi-Hannan arc) occurring along the western margin of the South China Block (SCB) (Wang et al., 2018). The Late Permian bauxite in Guangxi was probably derived from felsic volcanic rocks of the Emeishan Large Igneous Province in the middle of SCB and Truong Son volcanic arc located between the SCB and Indochina craton (Yu et al., 2016). Despite of the lack of geochronological data on contemporaneous bauxite-bearing sequences in Yunnan Province, we infer that most of the sediments came from the same source as those of the Permian bauxite in Guangxi. Most Li resides in silicate minerals of felsic origin, and tends to be easily mobilized by meteoric water and incorporated by clay minerals within closed basins on karstified carbonate depressions (Lechler et al., 2015; Benson et al., 2017). The present world’s supply of newly mined Ga metal mainly comes from bauxite, and Ga is currently derived as a byproduct of the

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Fig. 16. Vertical distribution of major and rare earth elements in bauxite profiles BZ3, 5, 7 and 12.

processing of bauxite ore for aluminum, with lesser amounts produced from sediment-hosted lead-zinc deposits (Foley et al., 2017). Ga, in the form of GaO(OH), was thought to be most likely enriched in diaspore, and to a less extent, in other aluminum hydroxides, such as gibbsite and boehmite (Foley et al., 2017). However, we found that Ga tends to be more enriched in the low-grade bauxite ores and clay samples than in high-grade bauxite ores, and displays a strongly positive correlation with SiO2 component (Fig. 10). Ga incorporation into Al-bearing minerals, in which it can be substituted for Al3+ due to similar ionic radius, are thought to be likely dominated by silicates (Brandt and Kydd, 1998; Murshed and Gesing, 2008). However, chemical analysis of discrete minerals, rather than bulk samples, need to be performed to determine the host structures of Gallium (Jonsson and Högdahl, 2019). Sc is highly enriched in Baofeng bauxite profiles, ranging from 13.74 to 108.47 ppm, averaging 43.49 ppm. According to Xu and Li (1996), materials with a Sc content between 20 and 50 ppm can be considered as an ore. Lavrenchuk et al. (2004) estimated that 70% of the world’s Sc resources might be found in bauxites and bauxite residues, a by-product of alumina production, which enriches approximately 98% of scandium in the bauxite ore (Klauber et al., 2011; Nguyen et al., 2016). Borra et al. (2016) found that bauxite residues generally contain more than 50 ppm of Sc, and thus considered bauxite residue as a potential Sc resource. Sc occurs mainly in hematite, goethite and zircon (Vind et al., 2018), and is often associated with other rare earth elements, Ti (Krishnamurthy and Gupta, 2015; Li et al., 2018), V and U (Wang et al., 2011). Many researchers suggest that REEs are variably mobile and readily fractionate during pedogenesis due to several factors, including mineral phases, redox and acidic conditions, etc., during pedogenesis (Braun et al., 1990, 1993; Laveuf and Cornu, 2009; Babechuk et al., 2014). La/

Y ratios in bauxite are usually considered as indicators of pH, with La/ Y < 1 indicating acidic conditions, while La/Y > 1 indicating alkaline conditions (Maksimovic and Panto, 1991; Zarasvandi et al., 2012). The positive correlation between total REE contents and La/Y ratios indicates that pH plays a critical role in the precipitation of REEs, and that Y’s behavior likely mirrors that of the REEs. It is well known that the leaching of Fe and Si, as well as precipitation of Al oxyhydroxides is controlled primarily by pH and Eh, which also affects the accumulation of total REEs (Inguaggiato et al., 2015). Many mineral phases, such as phosphate minerals and alunite, are thought to play an important role in controlling REE content and LREE/HREE fractionation (Ma et al., 2002; Picard et al., 2002; Tyler, 2004; Köhler et al., 2005; Stille et al., 2009; Berger et al., 2014). 7. Conclusion Bauxite-bearing sequences resting on karstified carbonates typically consist of either one single or more than one bauxite horizon, sandwiched between an underlying and an overlying clayey layer. The lower bauxite horizon usually takes the lenticular or unregular morphologies, and displays pisolitic textures, while the upper bauxite horizon is uniformly stratiform, and displays oolitic and finely-clastic textures. Based on the textures, as well as on mineralogical, stratigraphical, geochronological and paleogeographical evidences, an evolution model was proposed for the formation of Late Carboniferous bauxites in western Henan. We infer that the lower pisolitic bauxite horizon was formed as terrestrial deposition, while transportation and redeposition of bauxitic materials during submergence led to the formation of the upper bauxite horizon. Bauxite-bearing sequences are not only the main economic sources

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.oregeorev.2019.103161. References Ahmadnejad, F., Zamanian, H., Taghipour, B., Zarasvandi, A., Buccione, R., 2017. Mineralogical and geochemical evolution of the Bidgol bauxite deposit, Zagros Mountain Belt, Iran: implications for ore genesis, rare earth elements fractionation and parental affinity. Ore Geol. Rev. 86, 755–783. Babechuk, M.G., Widdowsonc, M., Kamber, B.S., 2014. Quantifying chemical weathering intensity and trace element release from two contrasting basalt profiles, Deccan Traps, India. Chem. Geol. 363, 56–75. Balkay, B., 1973. Bauxitization and Underground Drainage. Travaux. ICSOBA. 9, pp. 151–161. Bárdossy, G., 1982. Karst bauxites: bauxite deposits on carbonate rocks. In: Dev. Econ. Geol. 14. Elsevier, Amsterdam, pp. 441. Bárdossy, G., Aleva, G.J.J., 1990. Lateritic Bauxites: Developments in Economic Geology. Elsevier, Amsterdam, pp. 624 27. Bárdossy, G., Combes, P.J., 1999. Karst bauxites: interfingering of deposition and palaeoweathering. In: In: Thiry, M., Simon-Coincon, R. (Eds.), Paleoweathering, paleosurfaces and Related Continental Deposits. International Association of Sedimentologists Spec. Publ. 27. Blackwell Science, pp. 189–206. Bardy, M., Bonhomme, C., Fritsch, E., Maquet, J., Hajjar, R., Allard, T., Derenne, S., Calas, G., 2007. Al speciation in tropical podzols of the upper Amazon Basin: a solid-state 27Al MAS and MQMAS NMR study. Geochim. Cosmochim. Acta 71, 3211–3222. Benson, T.R., Coble, M.A., Rytuba, J.J., Mahood, G.A., 2017. Lithium enrichment in intracontinental rhyolite magmas leads to Li deposits in caldera basins. Nat. Commun. 270, 1–9. Berger, A., Janots, E., Gnos, E., Frei, R., Bernier, F., 2014. Rare earth element mineralogy and geochemistry in a laterite profile from Madagascar. Appl. Geochem. 41, 218–228. Bogatyrev, B.A., Zhukov, V.V., Tsekhovsky, Y.G., 2009. Formation conditions and regularities of the distribution of large and superlarge bauxite deposits. Lithol. Miner. Resour. 44, 135–151. Borra, C.R., Blanpain, B., Pontikes, Y., Van Binnemans, K., Gerven, T., 2016. Recovery of rare earths and other valuable metals from bauxite residue (red mud): a review. J. Sustain. Met. 2, 365–386. Bradley, D.C., Stillings, L.L., Jaskula, B.W., Munk, LeeAnn, McCauley, A.D., 2017, Lithium, chap. K of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, Bradley, D.C., (Eds. ), Critical mineral resources of the United States—Economic and environmental geology and prospects for future supply. U.S. Geological Survey Professional Paper 1802, pp. K1–K21. Brandt, K.B., Kydd, R.A., 1998. Gallium and chromium substitution for aluminum in synthesized beidellite. Clays Clay Miner. 46, 139–144. Braun, J.J., Pagel, M., Herbillon, A., Rosin, C., 1993. Mobilization and redistribution of REEs and thorium in a syenitic lateritic profile: a mass balance study. Geochim. 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Fig. 17. A geological model for the formation of bauxite-bearing sequences in Baofeng.

of aluminum, but also of many critical metals. Bauxite ores and clay samples contain highly elevated contents of Li, B, Sc, V, Ga, Zr, Nb, W and total REEs. Li and Ga, as well as V and Cr, share a similar distribution pattern, with contents being rather low in high-grade bauxite ores, compared to relatively elevated values in low-grade bauxite ores and clay samples. The distribution pattern and correlation analysis indicate that Li primarily occurs with clay minerals, enlightening a new potential resource for Li, the bauxite-bearing sequences resting on karstified carbonate. Both Sc and total REEs are also enriched in karst bauxite-bearing sequences, highlighting a potential Sc and REEs resource. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research is jointly supported by the National Natural Science Foundation of China (No. 41672089), the National Basic Research Program of China (No. 2015CB452600), and the Project of the Ministry of Science and Technology of China (BP0719021).

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