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[Display omitted] • The pH of bottom bauxite-ore from alkaline to acidic along the profile upwards. • The δ15N and redox proxies revealed that bauxite formed in the suboxic facies. • The denitrification was the predominant process at nitrogen cycling in bauxites. • The δ15N and salinity proxies revealed that higher saline favor low-oxygen. Over 5 billion tons of karstic bauxites are hosted in the Late Carboniferous Benxi Formation in the North China Craton (NCC). However, the bauxite formation environment in the central NCC remains unknown. The geological, mineralogical, geochemical, nitrogen concentrations, and δ15N isotope evidence of the Huilong and Shangwutou bauxite deposits in the central NCC was conducted, aiming to clarify the formation environments. Typically, bauxites in the central NCC contains three layers from bottom to top: Fe-bearing claystone/clayey ironstone, bauxite ore, and bauxitic claystone. The Fe-bearing claystone/clayey ironstone is composed of kaolinite, goethite, and hematite; the bauxite ore is composed of diaspore, kaolinite, but also minor hematite, ilmenite, and svanbergite; and the bauxitic claystone consists of kaolinite, but also some diaspore and goethite. Diaspores coexisted extensively with hematite, anatase, and svanbergite, forming cryptocrystalline matrix of bauxites, suggesting that they may be formed during early diagenesis. Geochemical discriminant diagrams also indicated that two deposits were primarily affected by the hydrogenous deposition and early diagenesis, and no sedimentary recirculation. The ∑REEs concentrations and fractionation (Pr n /Yb n) features along the profiles mainly concentrated in the bottom bauxite ore, which reflected that the pH values could be alkaline in bottom bauxite ore, and decreased weakly acidic for the central bauxite ore to bauxitic claystone along the profiles upwards. Combining the δ15N data with two geochemical redox proxies, and the widely coexistence of diaspore and hematite, indicated that the two deposits mainly formed under suboxic conditions, and thus denitrification was the predominant process in nitrogen cycling. The δ15N values are positively correlated with paleosalinity proxies, indicating that the higher saline bottom water may lead to a strong vertical density differential and water-column stratification, conducing to forming suboxic restricted pore-water environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mineral composition of bauxite is affected by karstic topography. • Bauxite formation conditions range from oxidation to reduction. • Minerals deposited under different conditions are rich in microorganisms. • Microorganisms may promote the deposition of diaspore and boehmite. The metallogenic process of karstic bauxite in Central Yunnan Basin (CYB) remain obscure. Mineralogical, geochemical, and isotopic analyses of two profiles (profile A and B) in the Yuxi karstic bauxite deposit within CYB were conducted to explore the formation environment and metallogenic process of Early Permian bauxite. Profile A in the karstic uplifted area comprises clayey bauxite, claystone, and oolitic bauxite from the bottom to the top, while profile B in the karstic depression area contains oolitic bauxite, claystone, oolitic bauxite, and claystone. The oolitic bauxite mainly contains diaspore and chamosite, and a small amount of kaolinite and anatase, while the clayey bauxite primarily includes boehmite and kaolinite, and a small amount of goethite and anatase. The claystone is dominated by kaolinite and illite. The δ15N value of the oolitic bauxite in the top of profile B is close to zero (−0.8 ‰), indicating a reducing deposition environment. The δ13C org value of profile A is negatively shifted between clayey bauxite and oolitic bauxite, while that of profile B is not significantly changed between the lower oolitic bauxite and the upper oolitic bauxite. This indicates that REDOX changes in the formation of clayey bauxite and oolitic bauxite in profile A are obvious, while REDOX changes in profile B are not evident. Mineral assemblages and isotopic results suggest that the mineralization conditions of profile A experienced a transition from oxidizing and weakly alkaline to reducing and alkaline, while the mineralization conditions of profile B were always reducing and alkaline. The presence of large-scale microorganisms, negative δ34S values (−42.2 to −10.1 ‰), and framboidal and cuboidal goethite provide evidence for microbial involvement in the entire mineralization process. The above results confirm two metallogenetic events of Yuxi karstic bauxite deposit. In the early metallogenetic event, the water table was low and most of the karstic terrains were exposed except some karstic depressions. Abundant boehmite and kaolinite thus formed in the karstic uplifted areas forming the clayey bauxite, and diaspore and chamosite in the karstic depressions forming the oolitic bauxite. In the late metallogenetic event, with the rise of water table, all karstic terrains were submerged, and large-scale diaspore and chamosite were widely deposited, thus forming oolitic bauxite. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mineral paragenesis denote an alkaline and reducing metallogenic environment. • Microorganisms are widely involved in the formation of bauxite. • Contemporaneous volcanic activities are key for the bauxite formation. • Variations in pH and Eh along weathering profile lead to large-scale deposition. More than 1 billion tons of bauxite was formed on the karstic surface of the Permian Maokou Formation limestone in the western Guangxi area of Youjiang Basin (YB). However, its formation mechanism is still a matter of debate. In this study, the Taiping bauxite deposit, a representative large deposit in this area, was selected to carry out detailed mineral microanalysis and pyrite sulfur isotope analysis to further understand the metallogenic process of bauxite in this region. Taiping bauxite contains two layers: the lower bauxite layer and the upper claystone layer. Bauxite ore was mainly composed of diaspore, pyrite, anatase, kaolinite, minor pyrophyllite, parisite, and bastnäsite. Small amounts of detrital moissanite, native boron, and zircon were also found in the ore. The claystone was dominated by kaolinite, diaspore, pyrite, and chlorite. The occurrence of detrital zircon, moissanite, and native boron suggests that they were most likely derived from volcanic ash or lava, further confirming the contribution of volcanoes. The mineral assemblages of diaspore, pyrite, anatase, parisite, and bastnäsite in the bauxite ore indicate alkaline and reducing depositional environments. The extensive development of ovoid microorganisms and the significant negative δ34S values of pyrite (−42.80 ‰ to −12.54 ‰) indicate that microorganisms were involved in the formation of bauxite. This view was further confirmed by the wide development of fine pyrite (∼1 μm) cemented by cryptocrystalline diaspore in the ore. Extensive volcanism promotes the weathering of parent rocks, releasing Al3+, Fe3+, Ti4+, and REE3+ ions into the weathering system. Under strongly acidic and oxidizing conditions in the upper part of the weathering profile, these ions can migrate downward until they reach an alkaline and reducing environment on the carbonate surface. A large amount of diaspore, pyrite, anatase, parisite, and bastnäsite was formed near the karstic surface with the participation of microorganisms. Epigenetic modifications were also evident in the widespread development of kaolinite and pyrophyllite formed by the silicification of diaspore. [ABSTRACT FROM AUTHOR]