Karst Bauxite Research Articles

Origin and geochemical evolution from ferrallitized clays to karst bauxite: An example from the Lower Cretaceous of NE Spain

Four outcrops of Lower Cretaceous (Barremian) karst bauxites located in Teruel (NE Spain) were analysed. The deposits show a heterogeneous-chaotic lithostructure consisting of pisolitic bauxite blocks embedded in lateritic red clays filling karst cavities. The research has focused on the geochemical study of major, minor, and trace elements (including some critical to industry) of both the bauxites and clays. The objective was to investigate the bauxite precursor material and to characterize the system’s geochemical evolution. Geochemical analyses were carried out by inductively-coupled plasma optical emission and mass spectroscopy. An absolute weathering index has been calculated to estimate element mobility, assuming Ti as an immobile element and the Upper Continental Crust (UCC) as parent material. Further, selected samples were observed by field emission scanning electron microscopy. The data indicate that both the bauxites and red clays originated by intense chemical weathering from more mafic argillaceous sediments than the UCC. Ongoing weathering caused the bauxitization of the upper parts of the original profile, preventing the lower parts from being bauxitized, thus producing the ferrallitized clays underlying the pisolitic bauxites. Subsequent karst reactivation gave rise to the current lithostructure. Ferrallitization is related to Fe, Sc, and V enrichment. On the other hand, although bauxites are relatively enriched in some elements compared to clays, the more intense chemical weathering associated with bauxitization led to chemical homogenization and widespread element depletion. During the bauxitization, Al, Ti, Zr, Cr, and probably Hf and the critical element Nb behaved as more immobile elements in the system. Bauxitization also enhanced homogenization and depletion of the REE, which is more pronounced for the LREE. HREE trends seem to be partly related to the concentration of Ti oxides in the bauxites, whereas P-bearing phases, more frequent in the clays, control the LREE. Subsequent to bauxitization, partial kaolinization of the bauxite took place related to the circulation of acid solutions that also caused the karst reactivation. These late processes caused some Al depletion in the bauxites and enhanced Fe loss together with V and, to a lesser extent, Ge.

Importance of hydrogeological conditions during formation of the karstic bauxite deposits, Central Guizhou Province, Southwest China: A case study at Lindai deposit

Karstic bauxite deposits are widespread in Central Guizhou Province, SW China, and high-grade ores are frequently sandwiched with overlying coal and underlying iron-rich layers and form a special “coal–bauxite–iron” structure. The Lindai deposit, which is one of the most representative karstic bauxite deposits in Central Guizhou Province, was selected as a case study. Based on textural features and iron abundances, bauxite ores in the Lindai deposit are divided into three types of ores, i.e., clastic, compact, and high-iron. The bauxite ores primarily comprise diaspore, boehmite, kaolinite, illite, and hematite with minor quartz, smectite, pyrite, zircon, rutile, anatase, and feldspar. The Al2O3 (53–76.8wt.%) is the main chemical contents of the bauxite ore samples in the Lindai district, followed by SiO2, Fe2O3, TiO2, CaO, MgO, S, and P etc. Our geological data on the Lindai deposit indicated that the ore-bearing rock series and its underlying stratum have similar rare earth elements distribution pattern and similar Y/Ho, Zr/Hf, and Eu/Eu∗ values; additionally, all ore-bearing rock samples are rich in MgO (range from 0.16wt.% to 0.68wt.%), and the plots of the dolomites and laterites lie almost on or close to the weathering line fit by the Al-bearing rocks in Zr vs. Hf and Nb vs. Ta diagrams; suggesting that the underlying Middle Cambrian Shilengshui Formation dolomite is the parent rock of bauxite resources in the Lindai district.Simulated weathering experiments on the modern laterite from the Shilengshui Formation dolomite in the Lindai bauxite deposit show that hydrogeological conditions are important for karstic bauxite formation: Si is most likely to migrate, its migration rate is several magnitudes higher than those of Al and Fe under natural conditions; the reducing inorganic acid condition is the most conducive to Al enrichment and Si removal; Fe does not migrate easily in groundwater, Al enrichment and Fe removal can occur only in acidic and reducing conditions with the presence of organic matter.The geological and experimental studies show that “coal–bauxite–iron” structure in Lindai deposit is formed under certain hydrogeological conditions, i.e., since lateritic bauxite or Al-rich laterite deposited upon the semi-closed karst depressions, Si can be continuously removed out under neutral/acidic groundwater conditions; the coal/carbonaceous rock overlying the bauxitic materials were easily oxidized to produce acidic (H2S, H2SO4, etc.) and reductant groundwater with organic materials that percolated downward, resulting in enrichment of Al in underlying bauxite; it also reduced Fe3+ to its easily migrating form Fe2+, moving downward to near the basal carbonate culminated in precipitating of ferruginous (FeS2, FeCO3, etc.) strata of the “coal–bauxite–iron” structure. Thus, the bauxitic materials experienced Al enrichment and Si and Fe removal under above certain hydrogeological conditions forming the high-quality bauxite.

Relationship between geometric parameters and compositional data: A new approach to karst bauxites exploration

Southern Italy karst bauxites occur in Apulia and Campania regions. In Apulia there are the autochthonous concentrations of Spinazzola (Murge) and San Giovanni Rotondo (Gargano), as well as the allochthonous Salento-type deposits. Typical Campania bauxites are those of the Matese Mts and in the Caserta area. During Cretaceous, both Apulia and Campania experienced sub-aerial carbonate exposures, which promoted bauxite accumulation.Southern Italy bauxite deposits share the same ooidic texture consisting of sub- circular aggregates dispersed in a fine-grained matrix. The ooids are mainly formed by a large core of Al-hematite surrounded by alternating layers of boehmite and Al-hematite reflecting dry and wet climate periods. The ooids of the allochthonous Salento-type bauxite consist of a large boehmite core surrounded by a thinner rim of Al-hematite.Image analysis, performed on these karst bauxites provided geometric data such as ooids circularity and aspect ratio, number of ooids and fractal dimension D. In the Southern Italy bauxite deposits significant correlations exist between the results of image analysis and compositional data. In the allochthonous Salento-type bauxites significant correlations between geometric features (number of ooids and aspect ratio) and some critical elements (Sc, Ni and REE) have been observed. In the autochthonous Apulian bauxites a significant correlation between Ni and circularity has been observed instead.The arrangement and the growth of the ooids can be described with a molecular diffusion pattern, and their growth can be described in terms of fractal geometry. The values of fractal dimension are different between Apulia and Campania bauxite deposits, this suggesting that during long-lasting sub-aerial events, the diffusion-limited cluster aggregation process may prevail over the simpler diffusion-limited one, which is responsible for ooids formation during time-limited exposure events.Image analysis and its integration with other analytical techniques provide interesting information about the genesis of bauxite deposits, and can therefore represent an innovative and useful tool for the exploration of bauxite ores.

Content and mode of occurrences of rare earth elements in the Zagrad karstic bauxite deposit (Nikšić area, Montenegro)

Three vertical sections through the Zagrad deposit of Jurassic karst bauxite in central Montenegro have provided knowledge of the vertical distribution of major and some selected trace elements, including rare earth elements (REE). Variations in the mineralogy, particularly those hosting REE, have been studied. This has revealed the presence of authigenic mineral phases such as xenotime, mottramite and monazite (best proved using Raman microprobe analysis) as well as residual phases such as zircon, titanite and monazite. The mobility of the elements during bauxitization processes has been studied to show that the REE minerals ensure progressive concentration of these elements during removal of major elements through weathering. The similarity of normalized REE in the bauxite to the typical Post-Archean Australian Shales (PAAS) and Upper Continental Crust (UCC) profile, and the preserved Eu anomaly, are evidence that the bauxite was not derived from carbonates and represents alteration of shale, marly limestone and volcanogenic or proximal igneous sourced detritus that accumulated in the original karst landform. Mass change during bauxitization, using Ti as “index” element and compared to PAAS composition, revealed almost 100% depletion of Si and weak enrichment in Al. Deeper parts of the deposit with authigenic minerals exhibit very strong enrichment in all REE. The bauxite ores have high ΣREE contents (693.5–6953.4ppm), especially ΣLREE contents (582.8–4882.9ppm), while ΣHREE contents (106.6–2070.5ppm) are much lower.

Selective recovery of rare earths from bauxite residue by combination of sulfation, roasting and leaching

Bauxite residue (red mud) that is generated from karst bauxite ores is rich in rare-earth elements (REEs). The REEs can be recovered from bauxite residue by direct acid leaching but the extraction yields are generally low. The extraction yields can be increased by increasing the acid concentration but this will increase the dissolution of iron as well. Large amounts of iron in the leach solution create problems in the further recovery processes. Therefore, a combined sulfation–roasting–leaching process was developed to selectively leach the REEs while leaving iron undissolved in the residue. In this process bauxite residue was mixed with water and concentrated H2SO4 followed by drying, roasting and then leaching of the roasted product with water. Most of the oxides were converted to their respective sulfates during the sulfation process. During subsequent roasting, unstable sulfates (mainly iron(III) sulfate) decompose to their respective oxides. Rare-earth sulfates, on the other hand, are stable during roasting and dissolve during water leaching, leaving the iron oxides in the residue. The effect of the roasting temperature, roasting time and amount of acid on leaching of the different elements was studied. Decreasing the roasting temperature increased the dissolution of the REEs, but also that of iron and aluminum. Increasing the amount of acid led to higher REEs extraction. Acid to bauxite residue mass ratio beyond 0.75 at 650°C increases the iron and aluminum dissolution due to increase in the iron(III) and aluminum sulfate amounts. The extraction of REEs slightly increased (<5%) with roasting time up to 2h at 675°C, but a further increase of the roasting time has a negative effect on the REEs extraction as the low amount of iron sulfate in the roasted mass increases the pH of the leach solution. About 60% of scandium and more than 90% of the other REEs can be dissolved at optimum conditions, while only a very small amount of iron (<1% of total iron) is solubilized. The residue after leaching was rich in Fe2O3, Al2O3, SiO2 and CaSO4·0.5H2O.

Geochemistry of the apulian allochthonous karst bauxite, Southern Italy: Distribution of critical elements and constraints on Late Cretaceous Peri-Tethyan palaeogeography

A comprehensive study of pebbles from the 'Salento-type' allochthnous bauxite deposit (Otranto, southern Italy), originally derived from a pristine Campanian bauxite, has been performed for evaluating: 1) the chemical fractionation and inter-elemental relationships, especially for critical elements, 2) the climatic conditions that promoted bauxite formation, and 3) the provenance of the protolith(s) using zircon age data and conservative elemental proxies. The study confirms the capability of bauxite to concentrate many elements defined as critical by the European Union report on critical raw materials. Sc, Co, Ga, and especially Cr, are enriched when compared with the UCC composition and assuming Nb is immobile. Other critical elements such as the REEs, with the exception of La, are moderately depleted. R-mode factor analysis suggests that most of the variance in our chemical dataset is explained by a factor with significant weightings for TiO2, Al2O3, Fe2O3, Sc, V, Nb, REEs, Pb and Th. This arises from climate effects affecting the distribution of the more abundant oxides and some trace elements, including the critical metals Nb and REEs. The texture of the pebbles is typical for Apulian karst bauxites and consists of sub-spheroidal ooids composed of boehmite and dispersed in a fine-grained matrix. The growth of the ooids, which formed under dry climate, was described in terms of fractal geometry. The average fractal dimension value of the ooids in the pebbles is close to that of the diffusion-limited aggregation models suggesting the ooid growth can be modelled using a molecular diffusion pattern, based on Fick's first law. The calculated time required for growth of the boehmite concretions is ~45÷310ka. This finding is consistent with an intra-Campanian emersion event (74–76Ma) that occurred during a dry and warm climatic stage. Since most of the karst bauxites worldwide have an ooidic texture, evaluation of the composition of concretions and the time required for their growth represents a powerful tool in reconstructing the palaeoenvironment. The zircon grains collected from the pebbles of the Salento-type karst bauxite define several concordant age populations. The youngest cluster, Early Cretaceous in age (99÷127.5Ma), suggests that windborne particles from Cretaceous volcanics, possibly originating in the Carpatho-Balkan orogenic belts, provided material for further bauxitisation. The largest cluster (623÷689Ma) is of Neoproterozoic age, predominately from the Late Ediacaran and Cryogenian p.p.. The 900–540Ma Pan-African orogenic cycle was followed by continental-scale uplift and erosion, leading to the deposition of thick Cambrian–Ordovician siliciclastic sequences that represent the most widespread detrital sequence ever deposited on continental crust and that now cover large parts of North Africa. These Cambrian–Ordovician sandstones contain a large population of Neoproterozoic zircons of Cryogenian age. Neoproterozoic zircons also occur in the youngest (Silurian–Mesozoic) sandstones of the Saharan Metacraton. These sandstones also contain 1.0Ga detrital zircons, suggesting as the oldest zircons found in the Salento-type bauxite pebbles (866Ma and 941Ma in age) are younger representatives of the zircon cluster present in this sandstone unit.These zircon age determinations suggest that the source material for the Salento-type bauxite pebbles was a combination of magmatic material from a distant source and clastic material derived from a continental margin (North Africa). This result concurs with the Eu/Eu* vs. Sm/Nd binary diagram, on which bauxite pebbles fall close to a mixing curve with andesite and cratonic sandstone end-members. As our results indicate that material was sourced from the North African continental margin, we suggest that a continental bridge separated oceanic domains in the Late Cretaceous of the Peri-Tethyan domain.

A review of the potential for rare-earth element resources from European red muds: examples from Seydişehir, Turkey and Parnassus-Giona, Greece

AbstractRare-earth elements (REE) are viewed as 'critical metals' due to a complex array of production and political issues, most notably a near monopoly in supply from China. Red mud, the waste product of the Bayer process that produces alumina from bauxite, represents a potential secondary resource ofREE. Karst bauxite deposits represent the ideal source material forREE-enriched red mud as the conditions during formation of the bauxite allow for the retention ofREE. TheREEpass through the Bayer Process and are concentrated in the waste material. Millions of tonnes of red mud are currently stockpiled in onshore storage facilities across Europe, representing a potentialREEresource. Red mud from two case study sites, one in Greece and the other in Turkey, has been found to contain an average of ∼1000 ppm totalREE, with an enrichment of light over heavyREE. Although this is relatively low grade when compared with typical primaryREEdeposits (Mountain Pass and Mount Weld up to 80,000 ppm), it is of interest because of the large volumes available, the cost benefits of reprocessing waste, and the low proportion of contained radioactive elements. This work shows that ∼12,000 tonnes ofREEexist in red mud at the two case study areas alone, with much larger resources existing across Europe as a whole.

Genesis of REE minerals in the karstic bauxite in western Guangxi, China, and its constraints on the deposit formation conditions

Karstic bauxites in western Guangxi, China, comprise two subtypes: Permian bauxite and Quaternary bauxite. The Quaternary bauxite originated from the breaking up, rolling, and accumulating of Permian bauxite in karstic depressions in Quaternary. Various types of rare earth element (REE) minerals were discovered during the formation of the Permian and Quaternary bauxites from the Xinxu, Longhe, and Tianyang bauxite deposits in this study. Five types of REE minerals, including bastnäsite, parisite, cerianite, rhabdophane, and churchite, were identified. Bastnäsite and parisite are the most abundant, and they are widely developed in the Permian ore and also present in the Quaternary ore. Obvious variations in bastnäsite and parisite REE compositions were observed, which is ascribed to distinctions in the source materials in the primary weathering profile from different areas. The mode of occurrence of bastnäsite and parisite suggests they were mainly precipitated under alkaline and reducing conditions during the Permian bauxite-forming stage and underwent intensive corrosion in the Quaternary. Churchite was formed during the Permian weathering stage under acidic condition. Both cerianite and rhabdophane occur in fractures within the Permian bauxite ore, indicating that both formed during the Quaternary weathering stage. It is considered that the rhabdophane enriched in Ce have formed locally, in the process of that the Ce3+, released from bastnäsite rapidly, entered the rhabdophane lattice before being oxidized to Ce4+. Cerianite was mainly found in association with Mn–Al hydroxides, suggesting that the released Ce3+ was oxidized into Ce4+ and precipitated cerianite in fractures within the Permian bauxite ore. Mass balance equations reveal a depletion in nearly all REEs during the transformation from the Permian to the Quaternary bauxite ore, mainly caused by the dissolution of bastnäsite and parisite. The genesis of the REE minerals, together with the occurrence of other minerals, indicates that intensively acidic and oxidizing conditions developed before the formation of the Permian bauxite ore. Towards the end of the Permian, the conditions became reducing and alkaline, favorable for the large-scale bauxitization. The Quaternary bauxite-forming stage was characterized by variable pH and Eh conditions, with acidic (pH=4–6) and oxidizing (Eh>2) conditions at the surface of the exposed Permian bauxite ore.

Bryophyte communities as biomonitors of environmental factors in the Goujiang karst bauxite, southwestern China

Bauxite mining on karst results in several ecological and environmental issues including heavy metal pollution, soil erosion and the destruction of vegetation. In turn, these may affect the distribution of plant communities and endanger human health. In general, bryophytes (mosses, liverworts and hornworts) are pioneer plants, lacking roots, vascular systems and well-developed cuticles. Due to their high sensitivity to the environment, they are often used to monitor air and soil pollution. A total of 25 bryophyte taxa from 19 genera and 9 families were recorded on Goujiang karst bauxite near the city of Zunyi in the Guizhou Province of southwestern China. Eleven principal bryophyte communities were identified, most of which consisted of only one species (monospecific assemblage), although the proportion of these single-species communities differed at the six locations. The levels of heavy metals also differed in soil from the six locations: iron, 8748.9–10,023μg/g; zinc, 146.7–240.9μg/g; copper, 24.6–60.4μg/g; and nickel, 35.6–95.1μg/g. A canonical correspondence analysis (CCA) of the bryophyte communities and environmental variables revealed the effect of gradient (slope), altitude and heavy metals in the soil on the distribution of the principal bryophyte communities. More than 36% of bryophyte taxa identified reproduced asexually by gemmae, as gemmiferous bryophyte communities tolerate substrates with high levels of heavy metals more readily than non-gemmiferous communities do. The distribution of heavy metals in the soil is reflected in the distribution of the bryophyte communities. The distribution characteristics of the principal bryophyte communities and of the gemmiferous bryophyte communities are useful in monitoring heavy metal pollution in karst bauxite.

Genesis of autochthonous and allochthonous Apulian karst bauxites (Southern Italy): Climate constraints

The Apulian Carbonate Platform (ACP) in southern Italy has experienced several episodes of subaerial exposure, which were in some cases associated with the formation of karst bauxite deposits. The ACP contains both autochthonous canyon-like bauxite and allochthonous Salento-type bauxite, with the latter having been derived from a weathered and eroded pristine bauxite deposit. The remnants of this pristine bauxite are preserved as transported pebbles embedded in a clayey matrix.The autochthonous bauxite and the pebbles of the allochthonous bauxite have the same texture of sub-spheroidal components (ooids) dispersed in a fine-grained matrix. The fractal dimension of ooids from both deposits is very similar and corresponds to the growth of "aggregates" under a diffusion-controlled process.The ooids of the autochthonous bauxite have a different composition to those in the pebbles of the allochthonous bauxite, because they formed under different climatic conditions. During the Turonian, autochthonous bauxite ooids formed in alternating wet tropical conditions (promoting Al-hematite formation) and drier conditions (favouring boehmite stability). In the allochthonous pebbles, ooids formed mainly in a dry climate, promoting the formation of large boehmite cores. The ooids/matrix ratio and the geometrical features of the ooids reflect these climate differences.The differences in composition and age (post-Turonian) of the Salento-type bauxite bedrock suggest that the pristine bauxite that produced the Salento-type pebbles was different in composition to and younger in age than the Canyon-like bauxite. The latter probably formed during a middle Campanian emersion event (evidenced by large karstic cavities), which is correlated with the subaerial exposure of karst recorded on the Adriatic island of Brač.

Mineralogical characteristics of the karstic bauxite deposits in the Xiuwen ore belt, Central Guizhou Province, Southwest China

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

Genesis and mineral transformations in Lower Cretaceous karst bauxites (NE Spain): climatic influence and superimposed processes

Four outcrops of Lower Cretaceous (Barremian) karst bauxites located in Teruel (NE Spain) were analysed to determine their mineral associations and genesis related to climatic palaeoweathering events and late superimposed kaolinization processes. The materials comprise metric-sized pisolitic blocks embedded in a clay-rich red groundmass. Fourteen samples were examined by X-ray diffraction, optical microscopy, scanning and transmission electron microscopy and the major elements were analysed by inductively-coupled plasma mass spectroscopy (ICPMS). The samples are composed of kaolinite, gibbsite, goethite, and hematite as the main phases, with diaspore, boehmite, anatase, and rutile as accessory minerals. The results show a complex sequence of mineralogical and geochemical processes that transformed the parent rock into the current bauxite materials. The clay-rich groundmass constitutes the lateritic parent material of the pisolitic bauxites. In the parent material authigenic kaolinite (e.g. vermicular kaolinite and kaolinite between cleavage sheets of pre-existing mica) has been observed; Fe oxides formed subsequent to kaolinite. In the pisolitic bauxites, mineralogical and textural evidence indicates that bauxitization took place at the expense of previous kaolinite, with gibbsite post-dating the other Al hydroxides. The pisolitic bauxites also show a more homogeneous chemical composition and a relative Ti, Al and Zr enrichment. The data are consistent with an intense palaeoweathering event during the Lower Cretaceous (Barremian) under tropical climatic conditions (warm and humid). Several stages probably took place during the bauxitization process, suggesting variations in water saturation conditions. Subsequent karst reactivation stages and related collapses were responsible for the present lithostructure of the deposits and allowed late kaolinization not related to climate to take place. Copyright © 2014 John Wiley & Sons, Ltd.

Red calcite: an indicator of paleo‐karst systems associated with bauxitic unconformities

AbstractA unique red calcite generation, which fills fractures/cavities, is hosted by Mesozoic carbonates in the Transdanubian Range, Hungary. Solid inclusions are located along growth zones of calcite. Hematite, the most abundant solid inclusion, gives the red colour of it. Outcrop‐scale geometry, mineralogical features and detrital mineral assemblage (hematite, gibbsite, goethite, kaolinite, smectite, illite, Cr‐spinel, monazite, xenotime, zircon, apatite and Ti‐oxide) of calcite precipitates suggest strong correlation between the calcite and nearby karst bauxite deposits. Fluid inclusion petrography and microthermometry (T &lt; 50°C; salinity from 0 to 0.17 NaCl eq. w%) of primary fluid inclusions, and the stable isotope trend of the calcite, following the meteoric water line, clearly indicate vadose and phreatic meteoric origin in a near‐surface karst system. The late Cretaceous to mid‐Eocene unconformity‐related cavity‐filling deposits occur close to the surface; indicating that the most recent Quaternary exhumation re‐exposed those surfaces that existed at the time of calcite mineralization. Thus, red calcite precipitates are interpreted as being speleothems, vestiges of the subterranean part of the pre‐Middle Eocene karst. The infiltrated, fine bauxite particles enclosed by the calcite are the witnesses of the once areally extensive pre‐Middle Eocene bauxitic blanket that became partially eroded by the time of the deposition of the cover beds. Red calcite when found in core samples may provide good evidence on bauxite formation associated with the overlying unconformity, even if it was later removed by erosion. Therefore, presence or absence of red calcite may be used as distinguishing criteria between karst episodes with or without bauxite formation.

Geochemistry of the Apulian karst bauxites (southern Italy): Chemical fractionation and parental affinities

This study focuses on a late Cretaceous karst bauxite deposit in the Murge area of the Apulia district, southern Italy. The first analysis of the vertical distributions of a wide range of elements (including REEs and selected trace elements) throughout the deposit was shown in order to identify the processes determining element fractionation during the evolution of the bauxite.The studied karst bauxite deposit exhibits an ooidal texture, is mineralogically homogeneous, and contains higher abundances of boehmite than of hematite, kaolinite, and anatase. The major element composition of the bauxite is dominated by elevated concentrations of SiO2, TiO2, Al2O3, and Fe2O3, and analyses of element mobility within the bauxite indicate that all elements except for Nb and Ni, and to a lesser extent Cr, are depleted relative to the immobile element Ti. R-mode factor analysis revealed a number of key findings: (i) some low-solubility elements (e.g., Zr, Th, Ti, V, Ga) were concentrated in detrital zircon and monazite (Zr, Th), in anatase (Ti, V), and possibly in boehmite and hematite (Ga) during the later stages of bauxitisation; (ii) Fe and Cr were concentrated during wet conditions, whereas Al and Co accumulated during dry conditions; (iii) distributions of the light REEs (LREEs) and the heavy REEs (HREEs) are controlled by the same processes, suggesting that little LREE/HREE fractionation occurred during bauxite formation; and (iv) the behaviour of cerium is different from that of the other REEs, and highly variable cerium anomalies are observed across the deposit, with three characteristic Ce/Ce* maxima with values of >2. Parisite was the only authigenic cerium mineral detected during this study. Thus, we propose a three-step model to explain the distribution of Ce: cerianite is dissolved by cerium reduction, is transported by downward-moving meteoric water (per descensum), and finally parisite is precipitated. This cycle was repeated several times in the Apulian karst bauxite in response to Eh decreases under alkaline conditions, promoted by fluctuations in the groundwater level. Finally, we used the value of the Eu anomaly to discuss the parental affinity or protolith of the bauxite. The value of the Eu anomaly (min. Eu/Eu*=0.76, max. Eu/Eu*=0.89) indicates that the bauxite was not derived from carbonates, but rather, that the majority of the bauxite was influenced by intermediate to mafic magmatic sources. Eu/Eu* vs Sm/Nd diagram suggests that the parental material for the bauxite was derived from a combination of a distant magmatic source and clastic material derived from a continental margin (northern Africa) to the south.

REE geochemical characteristics of titanium-rich bauxites: the Permian Kanigorgeh horizon, NW Iran

The Permian Kanigorgeh bauxite horizon is one of the typical titanium-rich bauxite deposits in the northeast of Bukan, West Azerbaijan Province, NW Iran. It is a part of the Irano-Himalayan karst bauxite belt and was developed as discontinuous stratified layers and lenses in Upper Permian carbonates. Mineralogically, diaspore and kaolinite are 2 major mineral phases accompanied by boehmite, smectite, illite, rutile, anatase, hematite, goethite, chlorite, quartz, and plagioclase as minor phases. Geochemical considerations for a selected profile indicate that the concentration values of rare earth elements (REEs) in the bauxite ores range from 8.9 to 200.4 ppm. Eu and Ce anomalies show varying values within the range 0.42-0.65 and 0.12-3.23, respectively. On the basis of Ce behavior, the studied profile can be divided into 2 parts by redox boundary. The most pronounced geochemical characteristics of the studied profile are the occurrences of positive Ce anomalies in the ores of low REE content in the upper part of the profile and negative Ce anomalies in the ores of high REE content in the lower part of the profile. The occurrence of positive and negative Ce anomalies are due to the change of oxidation state of Ce (from Ce3+ to Ce4+) in the upper part and the complexing of Ce4+ with carbonate ligands in the lower part of the profile. The concordance of the analytical values of Na and Ca with those of Eu revealed that the degree of plagioclase alteration is the most important controlling factor for Eu anomalies in the ores. The obtained results revealed that the variations in the chemistry of weathering solutions (e.g., pH), function of carbonate bedrocks as a geochemical barrier, and discrepancy in degree of stability of lanthanide-bearing primary minerals are the major factors influencing the mobilization, distribution, and fractionation of REEs. The correlation coefficients among elements suggest that Mn-oxides are the principal hosts for REEs in the ores at Kanigorgeh.

Discovery of the REE minerals in the Wulong–Nanchuan bauxite deposits, Chongqing, China: Insights on conditions of formation and processes

The karstic bauxite deposits in the south Chongqing–north Guizhou Province bauxite belt are enriched in rare earth elements (REEs). REEs in this belt have been considered to be adsorbed on the surface of clay or diaspore minerals in the ion state. However, extracting tests have demonstrated that the amount of REE adsorption in bauxite ore is very low. In order to comprehensively understand the occurrence of REE in bauxite ores, the chemical and mineralogical compositions of Wulong–Nanchuan karst bauxites and their textural components have been studied. The bulk samples are composed of diaspore, boehmite, kaolinite, illite, hematite, anatase, and pyrite. The ∑REE concentrations vary widely, from 17 to 3610ppm REE, in the bauxite ores and most of the ores show an obvious positive Ce anomaly.Many kinds of REE minerals were identified in these bauxite ores including synchysite, bastnaesite, cerianite, and churchite. These minerals seldom occur in the same sample (with the exception of synchysite and bastnaesite). Synchysite occurs rarely as homogeneously single crystals, but typically contains a microfine phase of bastnaesite. Cerianite occurs mainly as framboidal clusters among chamosite or as fracture or micro-crack fillings. The existence of REE mineral phases is responsible for the high REE content and positive Ce anomaly in the ores. The compositions of the REE minerals show that the Eu anomaly can change subtly during the weathering. The REE compositions of the bauxite ores suggest that the REE mainly derives from the weathered remnants of the underlying Hanjiadian Formation shales and probably, a minor fraction derives from the Huanglong carbonates.The formation processes of these REE minerals can be explained assuming: (1) Ce oxidation and cerianite precipitation in the uppermost part of the deposits; (2) the mobile phosphates combined with the Y-enriched sediments and the HREE-enriched solutions under an acidic environment allow the precipitation of churchite-(Y); and (3) remobilization of cerium as fluoride complex, as a consequence of more acidic conditions in the uppermost part of the deposit, and precipitation of Ce3+ as fluorocarbonate mineral toward the carbonate bedrock barrier, at alkaline pH.

Study of Trace and some Rare Earth Elements of Hussainiyat Karst Bauxite, Iraq: Leaching Efficiency

Study of Trace and some Rare Earth Elements of Hussainiyat Karst Bauxite, Iraq: Leaching Efficiency

Genesis of the Guangou karstic bauxite deposit in western Henan, China

The Guangou bauxite deposit in western Henan, China, is located in the North China Block and to the north of the North Qinling orogenic belt. The orebody is hosted within the lower member of the Carboniferous Benxi Formation, which overlies unconformably upon the Ordovician Majiagou Formation. In the lower member of the Benxi Formation, the bauxite orebody (with a diaspore–illite–anatase mineral assemblage) is sandwiched between underlying ferric clay (illite–hematite–goethite) and overlying top clay (kaolinite–quartz–goethite). According to field observations and geochemical evidence including trace- and rare-earth-element (REE) compositions, especially Zr/Hf ratios, the ferric clays were weathered from the underlying argillaceous limestones in the Majiagou Formation. During this weathering process, trace elements S, Zn, Ni, Cr, V, Sc, F, Ba, and Be are depleted; Li, Rb, and B are enriched; and Zr, Hf, Bi, Ta, Ga, Nb, and Th are relatively immobile. REEs exhibit evident differentiation with enrichment of La, Ce, Pr, and Nd and depletion of Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The bauxite formed in weak reducing, slightly basic water conditions according to the geneses of coexisting diaspore and anatase, and it was altered during the epigenetic and second exposure periods, forming kaolinite and goethite, respectively. Interelemental relationship analyses of the ores suggest that the elemental behaviors of trace elements and REEs in the bauxitization process are mainly controlled by the mineral compositions in bauxite ore and chemical properties of the elements. For instance, zircon, rutile, and anatase contain Zr, Hf, Nb, Ta, W, and U. Based on geochemical evidence, together with age data and chemical compositions of the detrital minerals, both distant igneous rocks in the plate margin and Paleozoic carbonates and Precambrian rocks inside North China Block provide the miscellaneous material for the bauxite ore and the top clay.

Quantitative Mineralogical Characterization of Karst Bauxite Deposits in the Southern Apennines, Italy

The southern Italian karst bauxite deposits occur at a Late Cretaceous hiatus on a Bahamian-type carbonate platform succession cropping out in the southern Apennines, and are mainly located in the Abruzzi Mountains, in Apulia (Gargano peninsula and Murge area), and in Campania. The bauxites of southern Italy are presently uneconomic, but can be considered as a model analogue for economic karst bauxite ores. The texture of the bauxite deposits is mainly oolitic to pisolitic but can also be arenitic-conglomeratic, suggesting reworking of evolved lateritic soils. The minerals boehmite and hematite are more abundant in the ooids, whereas kaolinite is found at high levels in the groundmass, and titanium oxides are also present. Furthermore, small amounts of rare earth elements have been detected in most of the bauxite occurrences. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and geochemical analyses on several of the deposits confirmed the diagenetic texture of the bauxite, and provided an estimate of the mineralogy of the different components and their chemical composition. The mineralogical and petrographic characteristics of bauxite are crucial for the Bayer process and can influence its economic evaluation: both the particle size distribution in the ore and the presence of reactive silica phases associated with Ti and Fe oxides can reduce the efficiency of processing. Quantitative analyses combining different methods (X-ray diffraction [XRD]-Rietveld quantitative phase analysis [QPA] and QEMSCAN ® ) have been carried out on a limited number of samples from two of the bauxite districts of the Campania region: the Matese Mountains and the Caserta province. QEMSCAN ® analysis allowed a more rapid quantification of mineralogy (including trace detrital phases) and assessment of the individual textural characteristics of the bauxite lithotypes, showing a detailed image of the distribution of economic and noneconomic minerals and their intergrowths. This methodology can augment or replace other time-consuming quantitative phase analyses for mineralogical studies of bauxites, provided that the species identification protocol (SIP) database has been carefully validated by preliminary use of XRD and SEM-EDS. Both XRD (QPA) and QEMSCAN ® analytical techniques can be complementary for bauxite ore evaluation, and a very powerful tool for exploitation and mineral processing.

Geochemical characteristics of Kanigorgeh ferruginous bauxite horizon, West-Azarbaidjan province, NW Iran

Ferruginous bauxite horizon of Kanigorgeh is located ~ 20 km northeast of Bukan, West- Azarbaidjan province, northwest Iran. The horizon is a part of the Irano-Himalayan karst bauxite belt that was developed in a form of 8 discontinuous stratified layers and lenses varying in thicknesses (5-17 m) with extending over 3.2 km along the contact of Permian carbonates, Triassic dolomites. In this study, parental affinity and controlling factors of elements distribution in the bauxite ores are surveyed by applying analytical mineralogy, mass and volume changes calculations (method of isocon), elemental ratios, and correlation coefficients. Mineralogical analyses reveal that diaspore, hematite, and kaolinite are the major minerals in the ores with lesser and variable amounts of boehmite, goethite, muscovite-illite, rutile, and montmorillonite. In contrast to the presence of diasporic-boehmitic mineralogical composition in ores, the geochemical data (i.e., ratios of Pb/Y, Ga/Pb, Zr/Pb, and Cr/Ni), testify to gibbsitic composition for the original aluminum hydroxides. Microscopic studies and geochemical characteristics of major and trace elements indicate that the ores were formed authigenically by the alteration and weathering of basaltic parent rocks. Mass change calculations suggests that enrichment of many elements in the ores resulted by losing Si, Ca, K, Na, Mg, and P during weathering of plagioclase, K-feldspar, ferromagnesians, and apatite. In addition, variable amounts of Co, Sr, and Ba were lost during bauxitization. Geochemical considerations prove that distribution of major, minor, trace, and rare earth elements in the studied ores were principally controlled by factors such as cation exchanges, adsorption, increasing of pH in weathering solutions due to buffering by carbonate bedrocks, scavenging by Fe-oxides and hydroxides, isomorphic substitutions, co-precipitation, and differences in stability of primary minerals.