Influence of geomorphology and leaching on the formation of Permian bauxite in northern Guizhou Province, South China

Journal Pre-proof Influence of geomorphology and leaching on the formation of Permian bauxite in northern Guizhou Province, South China

Peigang Li, Wenchao Yu, Yuansheng Du, Xulong Lai, Shenfu Weng, Pang Dawei, Xiong Guoling, Zhiyuan Lei, Shuang Zhao, Shiqiang Yang PII:

S0375-6742(18)30749-0

DOI:

https://doi.org/10.1016/j.gexplo.2019.106446

Reference:

GEXPLO 106446

To appear in:

Journal of Geochemical Exploration

Received date:

24 December 2018

Revised date:

13 July 2019

Accepted date:

14 December 2019

Please cite this article as: P. Li, W. Yu, Y. Du, et al., Influence of geomorphology and leaching on the formation of Permian bauxite in northern Guizhou Province, South China, Journal of Geochemical Exploration (2018), https://doi.org/10.1016/ j.gexplo.2019.106446

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© 2018 Published by Elsevier.

Journal Pre-proof

Influence of geomorphology and leaching on the formation of Permian bauxite in northern Guizhou Province, South China

Peigang Lia,b, c, Wenchao Yua, c*, Yuansheng Dua, c, Xulong Laia, Shenfu Wengb, Pang Daweia, Xiong Guolinga, Zhiyuan Leic, Shuang Zhaob, Shiqiang Yangb

a

State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences,

106 Geological Brigade, Guizhou Bureau of Geology and Mineral Exploration and Development,

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b

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China University of Geosciences, Wuhan 430074, China

c

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Zunyi 563099, China

Innovation Center of Ore Resources Exploration Technology in the Region of Bedrock, Ministry

Guizhou Bureau of Geology and Mineral Exploration and Development, Guiyang 550004, China

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c

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of Natural Resources of People's Republic of China, Guiyang 550004, China

Abstract

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*Corresponding author: [email protected]

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Geomorphology is an important control on the formation of bauxite. However, geomorphological research on bauxite deposit formation is usually hampered by the lack of systematic detailed data collection. This study focuses on the Wuchuan– Zheng’an–Daozheng (Wu–Zheng–Dao) area of northern Guizhou Province, South China, an area containing bauxite deposits of the Dazhuyuan Formation. These deposits formed during the early Permian in a period associated with the peak timing of the late Paleozoic Ice Age (LPIA). This study used data from 329 drillcores within the Dazhuyuan mining district (DMD) to generate high-precision geomorphological reconstructions for the period of bauxite formation. This includes a series of contour maps showing the total thickness of the bauxite deposit, ore grades, the thickness of Lower (bauxitic claystone) and Upper (bauxite ore) members of the bauxite formation, and the distribution of different types of bauxite ore. These maps indicate that thicker

Journal Pre-proof deposits of bauxite are concentrated in deep depressions, whereas no bauxite is found in highly elevated erosional regions. High-grade bauxite ores, including clastic, oolitic, and porous types of mineralization, are located within depressions or in gently sloping areas, whereas low-grade bauxite deposits, including massive bauxite and bauxitic claystone mineralization, are located at the bottom of depressions or are present as thin units within uplifted areas. The geochemistry of the drillcore samples from the study area indicates that leaching is the main control on bauxite ore quality in the study area. Finally, a low groundwater table and water-permeable underlying units

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provide good drainage, which also contributes to the generation of high-grade bauxite

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ores.

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Keywords: Paleosurface, karstification; lateritization; chemical weathering; soil

1. Introduction

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drainage; leaching.

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Pre-Cenozoic bauxite deposits in China formed mainly during the Carboniferous and Permian (Gao et al., 2015; Yu et al., 2018), contrasting with other parts of the

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world that record a contemporaneous decrease in or lack of bauxite formation as a result of global cooling and drying during the Late Paleozoic Ice Age (LPIA; Bárdossy and Combes, 1999; Bogatyrev et al., 2009). This suggests that the prevailing environment in China was conducive for bauxite formation during the LPIA (Yu et al., 2018). Bauxite deposits form as a result of the interplay of a number of different and often complex processes, such as tectonics, geomorphology, climate, hydrology, the presence or absence of ideal protoliths, and vegetation (Bárdossy, 1982; D’Argenio and Mindszenty, 1995; Combes and Bárdossy, 1996; Valeton, 2009; Retallack, 2010; Yu et al., 2018). However, geomorphological controls on bauxite formation are often not considered, partly because the detailed data required to assess these controls are difficult to acquire over entire mining districts (Bárdossy, 1982). Geomorphological (or paleosurface or paleomorphology; e.g., Bárdossy and

Journal Pre-proof Combes, 1999; Valeton, 2009) research into the formation of bauxite deposits aims to reconstruct the landforms that hosted laterite or bauxite formation. Lateral changes in bauxite deposit thickness can reflect changes in topographic elevations, whereby high-elevation regions undergo intense weathering and become source areas for erosional materials, whereas low-elevation regions or negative landforms in high-elevation regions provide depositional space that allows for the accumulation of erosional materials (D’Argenio and Mindszenty, 1995; Valeton, 2009). A case study of the upper Permian bauxite deposits in western Guangxi Province, South China,

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demonstrates the relationship between paleokarst geomorphology and the distribution

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of orebodies. Here, high-elevation karstic depressions contain bauxite orebodies with greater average thicknesses but lower grades than bauxites developed in low-elevation

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areas dominated by karstic plains (Yang et al., 2017). Research into Mediterranean

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bauxite deposits has provided insights into the relationship between geomorphology

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and hydrology (D’Argenio and Mindszenty, 1995; Combes and Bárdossy, 1996; Bárdossy and Combes, 1999; Valeton, 2009). Here, bauxite deposits that formed

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above or below the groundwater table (GWT) are divided into vadose (above the GWT) and phreatic (below the GWT) types (D’Argenio and Mindszenty, 1995).

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Variations in drainage condition determine the type of alteration during bauxitization and therefore the type of bauxite deposit that eventually forms (Samama, 1986; D’Argenio and Mindszenty, 1995). High-elevation regions above or around the level of the GWT have oxic and well-drained conditions that promote the formation of vadose-type bauxite deposits dominated by well-oxidized Al and Fe oxide minerals. In comparison, low-elevation regions below the GWT are dominated by water-saturated conditions that generate suboxic–anoxic conditions and stagnant drainage. These conditions form phreatic-type bauxite deposits that are dominated by poorly oxidized Al and Fe oxide minerals, are dark gray in appearance, and are massive. The bauxite deposits in northern Guizhou Province, South China, are located mainly within the Wuchuan–Zheng’an–Daozheng (Wu–Zheng–Dao) area (Fig. 1). The palynological records preserved within these deposits indicate they formed during

Journal Pre-proof the early Permian (Shi et al., 2014). Total reserves equal ~600 Mt of bauxite, indicating the importance of the orefields in this area. Previous mineralogical and geochronological research into these bauxite deposits indicate that the Al was derived mainly from clastic rocks of the underlying lower Silurian Hanchiatien Formation (Gu et al., 2013b; Jin et al., 2013; Zhao et al., 2013; Yu et al., 2014b). The bauxite mineralization within the Dazhuyuan Formation is dominated by Al-hydroxide minerals (diaspore and boehmite), hematite, and clay minerals (kaolinite, illite, smectite, and chlorite), in addition to accessory gibbsite, goethite, pyrite, anatase, and

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zircon (Gu et al., 2013a; Yu et al., 2014a). No geomorphological reconstruction of the

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location of these bauxite deposits during their formation has been attempted to date. This study uses data from 329 drillcores covering the Dazhuyuan mining district

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(DMD) within the Wu–Zheng–Dao area and uses a statistical and mapping approach

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that enables the identification of large-scale geomorphological features (e.g., uplifted

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areas and depressions) within the mining district. Combining these geomorphological reconstructions with the mineralogical and geochemical profiles for these drillholes

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allows the identification of mineral transitions and element transportation during the

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weathering process that formed the bauxites.

2. Geological background

2.1. General Geology of bauxite deposits in Guizhou The bauxite deposits in Guizhou Province are divided into two types: (1) lower Carboniferous Jiujialu Formation bauxite deposits that formed on Cambrian carbonates within the central part of the province, indicating these represent carbonate-type bauxite deposits (Fig. 1B); and (2) bauxite in northern Guizhou (Fig. 1B) hosted by the lower Permian Dazhuyuan Formation bauxite deposits that developed on clastic rocks of the lower Silurian Hanchiatien Formation and carbonates of the upper Huanglong Formation. Weathered lithic fragments of carbonate and shale are present at the base of some bauxite deposits in this area, indicating that eroded material has been transported within the region (Yu, 2017). This

Journal Pre-proof indicates that the Dazhuyuan Formation bauxite deposits are sedimentary-type, according to the classification of Bárdossy (1982). The northern part of Guizhou Province contains Cambrian to Cretaceous geological units with distributions that are controlled by a series of NNE–SSW trending Jura-type platform folds generated during the Cenozoic Himalayan orogeny. Two regional unconformities are present within the Paleozoic sequence of northern Guizhou. The first occurs between the upper Carboniferous Huanglong Formation and the underlying lower Silurian Hanchiatien Formation (Fig. 1C). Here, middle Silurian to early Carboniferous units

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are absent due to the far-field effect of the amalgamation of the Yangtze and Cathaysia

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blocks (the Guangxi orogeny) during the early Silurian. This caused the long-term (middle Silurian–late Carboniferous, >110 Myr) subaerial exposure of this region in

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northern Guizhou (Rong et al., 2011; Chen et al., 2012; Yu et al., 2015). A late

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Carboniferous transgression flooded the northern Guizhou area with seawater, causing

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the deposition of the Huanglong Formation limestones (Huang et al., 2012, 2013). The second unconformity is located between the bauxite deposits of the lower

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Permian Dazhuyuan Formation and the Huanglong or Hanchiatien formations. The presence of fusulinid fossils (Pseudostaffella antiqua and Profusulinella) in the

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Huanglong Formation indicates these sediments were deposited between the Bashkirian and early Moscovian stages of the late Carboniferous (Huang et al., 2012). The Dazhuyuan Formation also contains a Calamospora and Florinites sporopollen assemblage that is indicative of deposition between the Asselian and Artinskian stages of the early Permian (Shi et al., 2014). This subaerial exposure in northern Guizhou lasted more than 10 Myr and was linked to a global regression caused by an early Permian glacial event (Yu et al., 2018). Paleogeographical research in northern Guizhou indicates that the early Permian in this region was characterized by a northward-opening bay environment (Fig. 2; Huang et al., 2012, 2013). Three main paleogeographical units have been identified in this region (Lei et al., 2013a, b; Du et al., 2014), as follows. (1) Shallow marine deposits occur within the central Wu–Zheng–Dao area. This area contains an ~80 m thick section of the upper Carboniferous Huanglong Formation and sporadically

Journal Pre-proof exposed Dazhuyuan Formation deposits that are dominated by black or gray massive bauxitic claystones, indicating that these are phreatic-type bauxite deposits. (2) Coastal wetland deposits surround these marine deposits and contain high-quality bauxite deposits. These deposits contain Dazhuyuan Formation units consisting of clastic, oolitic, and porous bauxite ores that formed in this wetland environment and contain fragments of plant fossils. (3) The last paleogeographical unit is the coastal plain environment that does not contain bauxite deposits and is dominated by clastic rocks of the Hanchiatien Formation, indicating that this area was being eroded at this

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time and might have provided materials that were potentially eroded, transported and

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deposited as bauxites.

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2.2. Geology of the Dazhuyuan Mining District (DMD)

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This study focuses on the Dazhuyuan Mining District (DMD), which is located ~20 km to the north of Daozhen county in northern Guizhou and covers an area of

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~50 km2 (Fig. 1C). This area is located within the northern hinge of the Liyuan

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syncline (Figs 1C, 3), and contains lower Silurian, upper Carboniferous, Permian, and Triassic units. The Dazhuyuan Formation is underlain by units that include the lower

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Silurian Hanchiatien and upper Carboniferous Huanglong formations. The Hanchiatien Formation contains red, purple, and grayish–green shales and siltstones and is >200 m thick, whereas the Huanglong Formation consists of a 0.2–14.1 m thick gray or pink limestone unit that is commonly absent within the northwestern part of the DMD (Fig. 3). The bauxite deposits within the lower Permian Dazhuyuan Formation of the DMD range in thickness from 0.8 to 12.7 m (mean of 6.1 m; Fig. 4). These bauxite deposits are divided into Lower and Upper members, where the Lower Member is a gray or grayish green bauxitic claystone containing 20%–30% clay minerals (chlorite, illite or kaolinite). The basal part of this Lower Member is often dominated by hematite or pyrite/siderite (15%–20%) and contains elevated concentrations of Fe (15%–40%; Gu et al., 2013a; Yu et al., 2014a). The Upper Member is a light gray or white bauxite layer with clastic, oolitic, massive, and porous textures. A thin layer (0.2–0.5 m) of bauxitic claystone is often interbedded

Journal Pre-proof with or covers the Upper Member (Fig. 4). The Dazhuyuan Formation is covered by carbonaceous mudstones of the lower Permian Liangshan Formation or limestones of the lower Permian Qixia Formation.

3. Samples Data were collected from 329 drillholes and additional outcrops within the DMD. The majority of the DMD has a drillhole density of 7 drillholes/km2, increasing to 100

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drillholes/km2 in key exploration regions (e.g., the central and eastern DMD) (Fig. 3). Nine drillcores were used for stratigraphic correlation, generating a 3 km long WNW–

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ESE cross-section (Fig. 3B) through the study area.

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Systematic sampling of drillcore ZK8208 was undertaken during this study. This drillcore is located within the central DMD (28°52′42″ N, 107°50′42″ E) and contains

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an 8.5 m thick section of Dazhuyuan Formation bauxite, of which 5.6 m is Lower

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Member bauxitic claystones and the remaining 2.9 m is Upper Member bauxitic ore. The Dazhuyuan Formation in ZK8208 directly overlies shales of the lower Silurian

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Hanchiatien Formation and is in turn overlain by limestones of the lower Permian Qixia Formation, with the Huanglong and Liangshan formations absent from this

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drillhole (Fig. 4). A total of 17 samples of Dazhuyuan Formation bauxite were taken from this drillhole as well as a single sample from the underlying Hanchiatien Formation shale (Fig. 4). The 17 samples of the Dazhuyuan Formation are split into 10 samples (ZK8208-1 to -10) from the Upper Member with a sampling interval of ~0.1 m and seven samples (ZK8208-11 to 17) from the Lower Member at a sampling interval of ~0.8 m. These samples were used for thin section and geochemical analyses to determine their mineralogy and major element compositions. Further data used in this study include major element compositional data for bauxite/bauxitic claystone samples obtained during previous research (Cui et al., 2013; Gu et al., 2013a; Yu et al., 2014a). These data were used to study the relationship between different types of bauxite and associated variations in ore grade. An additional seven samples from drillcore ZK5604, which were used for major element analysis by Cui

Journal Pre-proof et al. (2013), are also used in this study.

4. Methods The statistical analysis of this study considered the following: (1) the total thickness of the Dazhuyuan Formation, (2) the thickness of the Upper Member bauxite, (3) the thickness of the Lower Member bauxitic claystone, (4) the thickness of clastic bauxite, (5) the thickness of oolitic bauxite, (6) the thickness of porous

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bauxite, (7) the thickness of massive bauxite, and (8) the average ore grade (Al content in wt.%) of bauxite mineralization intercepted during drilling. Contouring was

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undertaken using Golden Software’s Surfer® software (version 8.0). The fact that

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bauxite deposits formed within negative landforms means that the total thickness of bauxite deposited represents the depth of the depression in question, whereas regions

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free of bauxite deposits represent areas of high elevation that underwent erosion

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(Bárdossy, 1982; D’Argenio and Mindszenty, 1995; Valeton, 2009). This assumption allowed the construction of a three-dimensional (3D) map in Surfer® using the inverse

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value of the total thickness of the Dazhuyuan Formation. Textures and mineral assemblages were determined by thin section observation.

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The whole-rock major element compositions of 18 samples from drillhole ZK8208 were determined at the State Key Laboratory of Geological Process and Mineral Resource (GRMR), China University of Geosciences, Wuhan, China. Prior to analysis, any surface contamination was removed before samples were crushed in a corundum jaw crusher to 60 mesh before being powdered in a tungsten carbide ring mill to 200 mesh. Major element concentrations were determined by X-ray fluorescence using a Shimadzu 1800X instrument, with a GSB 04-2606-2010 Chinese national bauxite standard used for calibration. Detection limits vary between 0.1 and 1.0 ppm, and analytical precision is better than ±3% of the reported values. Loss on ignition (LOI) values were used to determine total volatile contents and were measured using the approaches outlined in the GB/T14506.2-1993, GB 9835-1998, and LY/T1253-1999 standards.

Journal Pre-proof Chemical weathering intensities were determined using the chemical index of alteration (CIA) approach (Nesbitt and Young, 1982), where CIA values were determined as follows: CIA = 100 × Al2O3/(Al2O3 + CaO* + Na2O + K2O)

(Eq. 1)

where all oxides are on a molecular proportion basis and CaO* denotes total CaO corrected for the presence of silicate minerals as follows: if CaOmolar < Na2Omolar,

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CaO* = CaOmolar; if CaOmolar > Na2Omolar, CaO* = Na2Omolar (Bock et al., 1998).

5. Results

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5.1. Macro- and microscopic observations of bauxite deposit samples

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All of the bauxite deposits within the Dazhuyuan Formation of the DMD have similar sedimentary structures. All drillcores within the DMD intercept Lower

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Member bauxitic claystones that in some areas form the main part of the bauxite

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deposits (e.g., drillcores ZK5604 and ZK8208 in Fig. 4). High-grade bauxite ore layers containing clastic, oolitic, or porous type mineralization are present within the

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middle or upper parts of deposits (Fig. 4). Bauxite ore layers are also often sandwiched between two layers of bauxitic claystone (e.g., in drillcores ZK1602 and

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ZK8208 in Fig. 4) and are locally covered by units of the Liangshan or Qixia formations (e.g., drillcores ZK5801, ZK5408, ZK5604, ZK5412, ZK1209, and ZK1404 in Fig. 4). Finally, some drillcores do not contain any bauxite ore but instead only contain bauxitic claystones (e.g., drillcore ZK10616 in Fig. 4). The Upper Member of the Dazhuyuan Formation is varitextured and contains clastic, oolitic, massive, and porous types of bauxite. Clastic and oolitic bauxites are the most common, and the clastic bauxite ore is white or light gray in color and contains pebble- to sand-sized rounded to sub-angular grains (Fig. 5A). These bauxites are grain-supported and contain bright microcrystalline diaspore (Fig. 5F) within a dark gray matrix that is dominated by very fine-grained clay minerals (e.g., kaolinite, illite, and chlorite) and Al-oxyhydroxides (mostly diaspore). The oolitic bauxite ore contains significant amounts of 0.05–2.00 mm diameter elliptical or

Journal Pre-proof spherical ooids (Fig. 5B). The majority of these ooids contain concentric rings that reflect multistage precipitation from Fe–Al colloids during weathering (Mongelli and Acquafredda, 1999). They commonly contain radial micro-fractures (Fig. 5G) caused by the shrinkage and dehydration of the colloidal ooids (Bárdossy, 1982). The massive bauxite ore is dominated by a homogenous and massive pelitomorphic texture free of macroscopic bedding and clasts (Fig. 5C). The best-quality ore within the Dazhuyuan Formation is the porous bauxite ore, which is white, friable, is not compact (Fig. 5D), and was classified as earthy using the Valeton (1972) classification

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approach during previous research (e.g., Gu et al., 2013a; Wang et al., 2013a, b).

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However, Bárdossy (1982) indicated that hardness (hard, soft, or friable) terms should not be used in classification, suggesting that this type of bauxite ore should not be

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classified as earthy. Here, we use the term ‘porous’ to classify this bauxite, as these

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units are highly porous (20%–40% porosity). The bauxitic claystones are dark gray or

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greenish gray and are predominantly massive. The bauxite ores and bauxitic claystones contain distinct mineralogical assemblages (Table 1) that can be classified

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using Al-oxyhydroxide (e.g., diaspore) and clay mineral (e.g., kaolinite, illite, and chlorite) mineralogical end-members. Bauxite ore samples contain 70%–85%

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diaspore and only 15%–32% clay minerals, whereas bauxitic claystone samples contain 42%–85% clay minerals and 13%–50% diaspore. Both types of bauxite also contain accessory diagenetic pyrite, quartz, hematite, zircon, and rutile. Different bauxite ore types generally appear in a particular sequence within the layers of bauxite ore. Porous sublayers are generally located below clastic ore sublayers, whereas oolitic sublayers are located below the clastic or massive sublayers (Fig. 4). Thin sublayers (0.2–0.4 m) of bauxitic claystone are occasionally interbedded with the main bauxite ore layers.

5.2. Major element compositions Bauxite ore samples in drillcore ZK8208 contain >50% Al2O3 (60.9%–70.1%) and show Al/Si values of >1.5 (4.1–10.7), with these lower limits representing the Chinese standard limits for industrial bauxite (Liu, 2013). The bauxite ore samples

Journal Pre-proof contain consistent concentrations of TiO2 (2.8%–3.6% with a mean of 3.2%) and Al2O3 (60.9%–70.1%), but low concentrations of SiO2 (6.2%–14.7% with a mean of 10.1%) and Fe2O3 (1.9%–6.0% with a mean of 3.3%). They also contain low concentrations of mobile elements such as K, Na, and Ca (>0.5%), but higher concentrations of Mg (1.6%–4.1% with a mean of 2.76%). The bauxitic claystones contain lower and more variable concentrations of TiO2 (0.6%–3.3% with a mean of 1.9%) than the bauxite ore samples, as well as lower concentrations of Al2O3 (21.6%– 49.3%) and higher concentrations of SiO2 (24.5%–44.2% with a mean of 32.9%). The

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basal Lower Member samples (ZK8208-16 and -17) contain elevated concentrations

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of Fe2O3 (35%–36%) that rapidly decrease upwards to 1.3%–7.0% (mean of 4.2%). The bauxitic claystone samples contain low concentrations of Na, K, and Ca (<0.5%),

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barring K2O in the topmost sample (ZK8208-1; ~5%). These bauxitic claystones also

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contain 1.2%–7.0% Mg (mean of 4.0%). All of the bauxite samples from drillcore

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ZK8208 have CIA values >90 barring the topmost bauxitic claystone sample (ZK8208-1; CIA = 84), indicating that the bauxite deposit formed as a result of strong

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chemical weathering. The Hanchiatien Formation shale sample (ZK8208-18H) has a CIA value of 79, indicating moderate chemical weathering.

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The textures of the bauxite ores and bauxitic claystones, and their major element compositions can be linked in box plots (Fig. 6). Clastic and oolitic bauxite ore types contain high concentrations of Al2O3 (55%–70%) but low concentrations of SiO2 and Fe2O3 (both 5%–20%). Massive bauxite ores contain 50%–60% Al2O3 and moderate concentrations of SiO2 (10%–50%) and Fe2O3 (5%–30%). The porous bauxite ores have the highest Al2O3 concentrations (70%–80%) and the lowest SiO2 and Fe2O3 concentrations (both <10%). The bauxitic claystones contain 25%–40% Al2O3 and 25%–50% SiO2 and Fe2O3. All of the samples contain relatively uniform concentrations of TiO2 (1%–4%), low concentrations of K, Na, and Ca (<1%), and variable concentrations of Mg (1%–8%).

5.3. Contour mapping The geomorphological character of the basement of the bauxite deposits within

Journal Pre-proof the DMD can be examined using variations in the total thickness of the Dazhuyuan Formation. Total thickness contour and three-dimensional reconstruction maps for the area (Fig. 7A, C) indicate that the northwestern part of the DMD is free of bauxite deposits, indicating that this was a high-elevation area undergoing erosion during bauxite formation. The thickest areas of bauxite (12–13 m) are located within several depressions in the northern and central parts of the DMD, each of which covers an area of 0.25–0.4 km2. The western part of the DMD contains a shallower but larger depression that hosts a belt of bauxite with thicknesses of 6–9 m. The southern DMD

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is generally flat and contains bauxite deposits with thicknesses of 3–5 m. A total of 13

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uplifted areas are present in the southern and central DMD that range in area from 0.1 to 0.5 km2 and contain thin bauxite deposit profiles (0.5–3.0 m).

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Bauxite thickness variations are also associated with changes in mean ore grades.

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This is clearly shown in an ore grade contour map (produced using mean bauxite

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Al2O3 contents; Fig. 7B) where high-grade ores (Al2O3 > 64%) are generally located in depressed areas containing greater thicknesses of bauxite. In comparison,

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low-grade bauxites (Al2O3 < 50%) are located in uplifted areas with thinner bauxite profiles. This transformation from high- to low-grade bauxite is gradual and reflects

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variations in topography (Fig. 7A, B). The series of contour maps reflecting the location of different types of bauxite deposit also indicates links between the distributions of different types of bauxite and geomorphology (Fig. 8). The thickness of the Lower Member of the Dazhuyuan Formation ranges from 0.0 to 3.4 m, with the thickest areas located in the deepest negative landforms and thinnest areas in uplifted regions (Figs 4, 7A, 8A). The distribution of the Upper Member of the Dazhuyuan Formation is similar to the overall distribution of the formation (Figs 7A, 8B). Depocenters for clastic and oolitic type bauxite ores are concentrated within the central and eastern parts of the DMD where these deposits reach thicknesses of 4.4 m (Fig. 8C). The porous bauxite ores are concentrated within the northern and eastern DMD, and they are only weakly linked within the locations of deep depressions, but are instead concentrated in transitional areas between uplifts and depressions that are underlain by limestone

Journal Pre-proof units (Fig. 8D). Massive bauxite ores are widely distributed in the DMD and generally accumulated within a NE–SW trending belt as well as deep depressions elsewhere within the region (Fig. 8E).

6. Discussion 6.1. Mining-district-scale geomorphological reconstructions and geomorphological controls on bauxite formation

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The contour maps generated during this study indicate geomorphological controls on the thickness and type of Dazhuyuan Formation bauxite developed within

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the DMD. Areas with thick bauxites (>8 m) are associated with deep depressions, as

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these negative landform provided the space necessary for the accumulation of the products of early stage weathering (in the majority of karstic or laterite type bauxites)

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or re-deposited bauxite (for sedimentary type bauxites; Bárdossy, 1982; Bárdossy and

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Aleva, 1990; Fig. 7). The Lower (claystone) and Upper (bauxite ore) members of the Dazhuyuan Formation also have a coupled distribution (Fig. 8A, B). This reflects the

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fact that depression formation is usually associated with the evolution of terrestrial landforms, causing the deepening and broadening of these depressions during

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weathering (Bárdossy, 1982). The upper Carboniferous Huanglong Formation limestone in the DMD initially underwent erosion and weathering to generate early stage weathering products including red residue (“terra rossa” in earlier studies; e.g., Merino et al., 2006; Muhs and Budahn, 2009), which contains high concentrations of Fe (8%–20%) and moderate concentrations of Si and Al (20%–40%; Ji et al., 2004a, b). These red residues were concentrated in depressions and formed the basal Fe-rich layer within the Lower Member of the Dazhuyuan Formation. Alternatively, the vertical mobility of elements during weathering generated Fe enrichments at the base of the weathering profile. The fact that Fe is relatively mobile but precipitated as Fe-oxyhydroxides in oxidized conditions (Valeton, 2009) means that thicker weathering profiles accumulated more Fe in their basal sections. The contour maps produced for different types of bauxite indicate that

Journal Pre-proof geomorphological changes influenced the distribution of different bauxite ore types. Clastic and oolitic bauxite ores are concentrates in depressions (Figs 7A, 8B, C). Distributions of porous and massive bauxite ores are inverse (Fig. 8D and E), the thickest porous bauxite ore (4.0-4.8 m) concentrate in the northwestern part of the DMD, where the massive bauxite ore is only 0-1.2 m. In comparison, the northeastern DMD contains 3–4 m thick massive bauxite but no porous bauxite. This reflects the fact that the porous and massive bauxites represent two end-members that formed under

different

weathering

conditions.

The

high-porosity

(~30%),

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Al-oxyhydroxide-dominated (>70%), and low-hardness (soft–very soft) porous

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bauxite within the DMD provides evidence of intense and thorough leaching. Bárdossy (1982) explained that the porosity of these types of ore reflects the

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dissolution of certain textural elements (e.g., clasts, ooids, or pisoids). The majority of

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the massive bauxites within the DMD are light gray or gray and contain more Si

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(10%–50%) and Fe (5%–30%) than the porous bauxites, indicating they represent phreatic-type bauxite deposits. Porous bauxite deposits within the DMD are

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associated with two geomorphological features. The first is areas underlain by the Huanglong Formation within the northwestern part of the DMD, an area that also

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contains the porous bauxite deposits (Figs 3, 8D). Fractured limestones represent permeable layers that allow the passage of groundwater, increasing the intensity of leaching relative to areas underlain by shales. Second, landforms between uplifts and depressions (Figs 3A, C, 8D) generate longitudinal slopes that allow for good drainage, yielding conditions ideal for the formation of high-grade bauxite deposits. The ore grade contour map for the DMD (Fig. 7B) indicates that high-grade ores (Al2O3 > 66%) are concentrated within the central and eastern DMD, coincident with the distribution of clastic, oolitic, and porous bauxites (Fig. 8C, D). This is reflected by the statistical analysis of the quality of the different bauxites within the study area (Fig. 6), confirming that the clastic, oolitic, and porous bauxites within the DMD represent the highest-grade bauxite ores in this region.

6.2. Relationship between geomorphology, groundwater regime, and leaching during

Journal Pre-proof weathering The linkages between bauxite deposits, the genesis of different types of bauxite ore, and geomorphology within the DMD were outlined in section 6.1. We build on these interpretations by examining the geochemical profiles of drillcores from the study area. There are differences in variations in major element concentrations within the Upper and Lower members of the Dazhuyuan Formation in drillcores ZK5604 and ZK8208 (Fig. 9). The Lower Member has Ti concentrations that are either uniform (ZK5604) or increase upwards (ZK8208), with Si and Al both also increasing

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upwards within these drillholes. The Lower Member also has Fe concentrations that

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decrease by 10%–20% upwards from the basal Fe-rich layer to the overlying bauxitic claystone layer. The Upper Member has Ti concentrations that increase toward the

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surface, from 1% to 2% in ZK5604 and from 2% to 3% in ZK8208, but the bauxitic

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claystones in ZK8208 that are interbedded with or overlie the Upper Member record

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significant decreases in Ti concentrations. The Upper Member also contains decoupled Si and Al concentrations, where Si concentrations decrease and Al

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concentrations increase. Finally, the Upper Member contains uniformly low concentrations of Fe (2%–3%).

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The textures and geochemical variations within bauxite profiles provide evidence of the weathering that occurred in this area (Liu et al., 2010, 2017; Mameli et al., 2007). The coupled variations in Si and Al concentrations within the dark gray bauxitic claystones of the Lower Member of the Dazhuyuan Formation indicate that they formed in pockets where weathering products developed and infilled poorly drained depressions (Combes and Bárdossy, 1996; Mameli et al., 2007). The relatively uniform concentrations of Ti within the Lower Member also provide further evidence of weak leaching. In contrast, the decoupling of Si and Al concentrations within the Upper Member provides evidence of autochthonous (or in situ) weathering that generated upwardly increasing Al and decreasing Si concentrations within the bauxite profiles (Combes and Bárdossy, 1996; Mameli et al., 2007). The Upper Member contains oolitic bauxites that are located beneath clastic bauxites in a pattern that is similar to that expected for typical autochthonous weathering (Valeton, 1995; Weng et

Journal Pre-proof al., 2019). The dramatic increases in Ti within the Upper Member indicate a leaching environment where mobile elements (Si, K, Ca, Mg, and Na) were leached from the profile but immobile elements (Al and Ti) were retained and increased in concentration as a result. The bauxitic claystone layer within the Upper Member was the result of an increase in the GWT level, causing a transition from vadose- to phreatic-type bauxite (Weng et al., 2019). The formation of bauxite within the DMD was the result of a combination of geomorphological, groundwater regime, and leaching factors. Geomorphology

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determines the distribution of bauxites, where high-elevation areas cannot retain

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weathering products, meaning these areas became erosional and do not contain bauxite. Depressions were filled by in situ detrital material and the weathering

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products derived from erosional areas, generating the initial materials involved in the

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subsequent leaching. The quality of the bauxite generated during this process was

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controlled mainly by the prevailing leaching conditions. For example, low-GWT conditions combined with appropriate landforms (e.g., depression or gentle slope) and

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underlying units (e.g., permeable limestones) generated good drainage conditions that favored the formation of high-grade clastic, oolitic, and porous bauxite ores. Areas

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with poor drainage as a result of high-GWT conditions, unsuitable landforms (e.g., regional uplifts), and underlying units (e.g., impermeable shales) led to the generation of massive bauxite ores or even bauxitic claystone deposits. The early Permian variations in GWT within the DMD were probably associated with glacial and eustatic changes during the LPIA (Yu et al., 2018). The coastal environment that prevailed in northern Guizhou at this time underwent sea level fluctuations that would have directly affected regional GWT levels. This high-frequency oscillation in the regional GWT levels in the DMD was also beneficial for bauxite formation because it generated vertical and lateral drainage (Valeton, 1995).

6.3. Formation of lower Permian bauxite deposits in the Wu–Zheng–Dao area, northern Guizhou The Wu–Zheng–Dao area of northern Guizhou Province, China, was exposed to

Journal Pre-proof weathering after the cessation of a late Carboniferous transgression. The prevailing Intertropical

Convergence

Zone

(high

mean

annual

temperature

and

precipitation)-type paleoclimate that prevailed in this area caused intense karstification (Zhu et al., 1998; Yu et al., 2018; Fig. 10A), resulting in intense weathering of the upper Carboniferous Huanglong Formation limestones, generating red residues that accumulated within depressions. High-elevation areas became limestone free, exposing shales of the underlying Hanchiatien Formation that were initially weathered before being eroded and transported to be deposited in surrounding

of

depressions (Fig. 10B). The relationship between depression depth and the level of the

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GWT caused weathered materials to be deposited in vadose zones above the GWT and saturated zones below the GWT. The presence of Fe-rich dark gray bauxitic

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claystone layers at the bottom of the Dazhuyuan Formation suggests that the majority

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of the depressions within the Wu–Zheng–Dao area were located below the GWT.

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Variations in sea level and associated changes in GWT level led to two different scenarios. Lowstand conditions associated with falling sea levels caused a lowering of

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the GWT, enabling the vertical leaching of the weathering materials in depressions and generating high-grade bauxite layers (Fig. 10C). In comparison, highstand

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conditions were associated with increases in both sea level and GWT levels, thereby submerging weathering profiles and generating phreatic-type bauxite layers (Fig. 10D).

7. Conclusions (1) The distribution of the lower Permian Dazhuyuan Formation bauxites within the DMD was controlled by topography. Contour maps of the study area indicate that thick bauxite deposits were concentrated in deep depressions whereas high-elevation areas and a region of high elevation in the northwest of the DMD are free of bauxites. High-grade clastic, oolitic, and porous bauxites are associated with particular landforms, including the upper parts of depressions or gentle slopes, whereas low-grade bauxite deposits (including massive bauxite and bauxitic claystone units)

Journal Pre-proof are usually located within the lower parts of depressions and in uplifted areas. (2) The grades of the bauxites within the DMD were controlled mainly by leaching. Areas with low GWT levels containing low landforms and water-permeable underlying units (i.e., limestones) were well-drained, forming ideal conditions for weathering and the generation of high-grade bauxite ores. In contrast, high GWT levels, high-elevation landforms, and areas underlain by impermeable units all led to poor drainage and conditions conducive for the formation of low-grade bauxite

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deposits.

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Acknowledgements

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The editor and three anonymous reviewers are thanked for their constructive suggestions which have greatly improved the quality of this paper. This study was

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supported by the Natural Science Foundation of China (No. U1812402 and No.

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41802116), Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) No. CUG170684 and CUGQY1908, State Key

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Laboratory of Biogeology and Environmental Geology, China University of Geosciences (No. GKZ18Y660) and the Research Project of Guizhou Bureau of

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Guizhou Province: Evidence from detrital zircon chronology. Journal of Palaeogeography, 16 (1): 19-29 (in Chinese with English abstract). Yu, W., Du, Y., Zhou, Q., Jin, Z., Wang, X., Qin, Y., 2012. Biomarkers of bauxite-bearing

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Figure and table captions

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Fig. 1: (A) Simplified tectonic map for China and location of study area (NCC = North China Craton, SCC = South China Craton); (B) Bauxite deposits in Guizhou Province, South China; (C) Geological map for Wuchuan-Zheng’an-Daozheng (Wu-Zheng-Dao) area, northern Guizhou and location for the Dazhuyuan Mining District (DMD). I: Daozhen Syncline, II: Datang Syncline, III: Taoyuan Syncline, IV: Luchi Syncline, V: Liyuan Syncline, VI: Huanxi Syncline, VII: An’chang Syncline,

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VIII: Xinmo Syncline, IX: Zhangjiayuan Syncline.

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Fig. 2: (A) Paleogeographic map for the Upper Yangtze area, (B) Paleogeographic

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map for the Wu-Zheng-Dao area, northern Guizhou, South China. I: Daozhen Syncline, II: Datang Syncline, III: Taoyuan Syncline, IV: Luchi Syncline, V: Liyuan

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Syncline, VI: Huanxi Syncline, VII: An’chang Syncline, VIII: Xinmo Syncline, IX:

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Zhangjiayuan Syncline.

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Fig. 3: (A) Geological map for the Dazhuyuan Mining District (DMD) and locations

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for drillcore and outcrops. (B) Cross section X-X’.

Fig. 4: Lithologic columns for the Lower Permian Dazhuyuan Formation bauxite deposit in the Wu-Zheng-Dao area, northern Guizhou. Locations of drillcores are shown inFig. 3A–B. Sampling sites in drillcore ZK8208 are marked on the right side of the column.

Fig. 5: Hand samples of (A) clastic bauxite ore ZK8208-3, (B) oolitic bauxite ore ZK8208-10, (C) massive bauxite ore ZK8208-6, (D) porous bauxite ore ZK8208-8, (E) bauxitic claystone ZK8208-12 and thin section micrographs of (F) clastic bauxite ore ZK8208-3 (10X, cross-polarized), (G) ooid in oolitic bauxite ore ZK8208-10 (4X, plane-polarized), (H) massive bauxite ore ZK8208-6 (20X, plane-polarized). Dia = diaspore, CM = clay matrix; Hem = hematite.

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Fig. 6: Box plots for SiO2, Al2O3, TiO2, Fe2O3, Na2O, K2O, CaO, and MgO of (A) clastic and oolitic bauxite ore, (B) massive bauxite ore, (C) porous bauxite ore and (D) bauxitic claystone. Data were collected from Yu et al. (2012), Gu et al. (2013a), Cui et al. (2013) and this study.

Fig. 7: (A) Contour map for the total thickness of the Dazhuyuan Formation bauxite deposit in the Dazhuyuan Mining District (DMD), (B) contour map for bauxite ore

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grade of the Dazhuyuan Formation in the DMD, (C) geomorphological reconstruction

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map for early Permian in the DMD.

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Fig. 8: Contour map for (A) the Lower (claystone) Member of the Dazhuyuan

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Formation, (B) the Upper (bauxite ore) Member of the Dazhuyuan Formation, (C)

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bauxite ore layers.

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clastic and oolitic bauxite ore layers, (D) porous bauxite ore layers, (E) massive

Fig. 9: Lithological and geochemical profiles for (A) drillcore ZK5604 and (B)

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drillcore ZK8208 in the Dazhuyuan Mining District (DMD), northern Guizhou. Geochemical data of ZK5604 are from Cui et al. (2013).

Fig. 10: Formation process for the lower Permian Dazhuyuan Formation bauxite deposit in the Wu-Zheng-Dao area, northern Guizhou. Caution: Materials being transported from uphill is deposited (Fig. 10B) and THEN weathered (Fig. 10C, D).

Table 1: Mineralogical composition for samples in drillcore ZK8208 in the Dazhuyuan Mining District (DMD), northern Guizhou, South China.

Table 2: Major element composition for samples in drillcore ZK8208 in the Dazhuyuan Mining District (DMD), northern Guizhou, South China.

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Tabel 1: Mineralogical composition for samples in drillcore ZK8208 in the Dazhuyuan Mining District (DMD), northern Guizhou, South China Mineral composition (%) Depth Sample No. Clay (m) Diaspore minerals Quartz Hematite Pyrite ZK8208-1 560.5 13 85 0 0 2 ZK8208-2 561 50 0 0 8 42 ZK8208-3 561.5 72 26 0 0 2 ZK8208-4 561.7 65 32 0 0 3 ZK8208-5 561.9 73 25 0 0 2 ZK8208-6 562.1 23 75 0 0 2 ZK8208-7 562.3 75 25 0 0 0 ZK8208-8 562.5 85 15 0 0 0 ZK8208-9 562.7 75 20 0 0 5 ZK8208-10 562.9 72 28 0 0 3 ZK8208-11 563 23 75 0 0 2 ZK8208-12 564 18 80 0 0 2 ZK8208-13 565 12 85 0 0 3 ZK8208-14 565.5 20 77 0 1 2 ZK8208-15 566 15 82 1 1 1 ZK8208-16 567 16 80 1 1 3 ZK8208-17 568 16 80 1 1 3 ZK8208-18H 569 0 85 10 5 0

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Table 2: Major element composition for samples in drillcore ZK8208 in the Dazhuyuan Mining District (DMD), northern Guizhou, South China. Sample

Depth (m)

TiO2

SiO2

Al2O3

Al/Si

Fe2O3

MnO

MgO

ZK5604-2 ZK5604-3 ZK5604-5 ZK5604-6 ZK5604-7 ZK5604-8 ZK5604-9

521.3 523.5 525.3 526 528.4 529.9 531.3

1.9 2.2 1.4 1.4 1.5 1.4 1.2

29.2 43.9 42.2 44.5 30.8 41.4 34.9

50.8 37.6 35.3 36.6 30.3 35.4 28.5

1.7 0.9 0.8 0.8 1.0 0.9 0.8

2.5 1.3 4.8 2.0 23.5 6.1 19.6

0.0 0.0 0.0 0.0 0.1 0.0 0.0

0.8 0.3 0.3 0.3 2.6 0.9 2.8

ZK8208-1 ZK8208-2 ZK8208-3 ZK8208-4 ZK8208-5 ZK8208-6 ZK8208-7 ZK8208-8 ZK8208-9 ZK8208-10 ZK8208-11

560.5 561 561.5 561.7 561.9 562.1 562.3 562.5 562.7 562.9 563

1.7 3.3 3.2 3.2 2.8 2.6 3.5 3.6 3.2 3.0 2.3

41.8 26.1 10.8 14.7 9.7 26.8 12.3 8.2 6.2 8.8 25.9

33.3 48.3 65.9 60.9 68.6 47.4 63.7 70.1 65.7 63.4 49.3

0.8 1.8 6.1 4.1 7.1 1.8 5.2 8.5 10.7 7.2 1.9

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

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2.7 3.1 2.6 2.6 2.0 3.4 2.6 1.9 6.0 5.6 3.2

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f o

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K2 O

P2O5

LOI TOTAL

CIA

e

0.0 0.0 0.1 0.1 0.1 0.1 0.2

0.1 0.1 0.1 0.4 0.1 0.1 0.3

0.2 0.1 0.1 0.6 0.0 0.1 3.9

0.1 0.1 0.1 0.1 0.1 0.1 0.2

14.5 14.4 15.7 14.1 10.9 14.5 8.3

100.0 99.9 100.0 100.0 99.9 99.9 100.0

99 99 99 96 99 99 85

3.9 4.2 3.1 4.1 2.7 5.0 3.4 2.3 1.6 2.3 4.7

0.4 0.2 0.1 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1

0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1

5.1 0.4 0.1 0.1 0.1 0.4 0.1 0.0 0.0 0.1 0.2

0.0 0.0 0.1 0.0 0.1 0.0 0.1 0.1 0.0 0.0 0.0

11.6 17.1 17.1 17.5 16.3 16.2 17.0 17.3 20.0 18.9 16.4

99.1 99.5 99.8 100.2 99.7 99.5 99.4 100.0 99.6 99.2 100.0

84 98 100 99 100 98 100 100 100 100 99

r P

Journal Pre-proof

ZK8208-12 564 ZK8208-13 565 ZK8208-14 565.5 ZK8208-15 566 ZK8208-16 567 ZK8208-17 568 ZK8208-18H 569

2.4 2.1 1.7 1.6 1.0 0.6 0.75

35.8 38.2 44.2 41.4 24.5 24.5 61.57

38.2 37.8 38.1 36.4 21.9 21.6 17.41

1.1 1.0 0.9 0.9 0.9 0.9 0.28

6.8 5.9 1.3 7.0 35.4 36.3 7.97

l a

o J

n r u

0.0 0.0 0.0 0.0 0.1 0.1 0.04

3.3 2.7 1.2 1.6 6.7 7.0 3.15

0.1 0.1 0.1 0.2 0.1 0.0 0.12

0.1 0.1 0.1 0.7 0.2 0.2 3.85

f o

o r p

e

r P

0.1 0.1 0.2 0.2 0.2 0.1 0.38

0.1 0.1 0.0 0.0 0.0 0.0 0.10

15.7 15.0 14.7 12.1 10.9 9.8 4.56

100.3 100.1 99.8 99.6 100.0 99.5 99.15

99 99 99 96 98 99 79

Journal Pre-proof

Highlights 1. Geomorphological reconstruction in the Dazhuyuan Mining District, South China. 2. Leaching process rebuilding based on geochemical profiles.

Jo ur

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ro

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3. Formation model for the early Permian bauxite deposit of the Dazhuyuan Formation.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10