Characteristics and evolution of sedimentary facies of bauxite-bearing rocks series of Permian bauxite deposits in western Guangxi

The study on sedimentary facies of Permian bauxite−bearing rock series in western Guangxi has been rare so that a sediment sequence has not been set up yet. Based on line survey and section study, adding microscopic observation and elemental analysis of rocks and ores, this paper has determined four sedimentary facies and eight subfacies of ore−bearing rock series, the four faces are swamp carbonaceous mud facies on the top, the coastal plain mudstone facies in the upper, the peneplain bauxite facies in the middle and the lakeshore−lake ferric−aluminous rocks facies in the lower, and the peneplain bauxite facies can be divided into subfacies such as valley bauxite laminated mudstone, gentle−slope residual bauxite, valley sedimentary bauxite and slope bauxite subfacies, and the lakeshore−lake ferric−aluminous rocks can be divided into the aluminous mudstone−ferro mudstone−ferroaluminite of the shore−shallow lake and the paleosoil of lakeshore plain. The continental volcanic ash remains in all facies. In bauxite−bearing rock series, the composition variations (ICV) of mudstone fluctuates around 1, indicating the existence of recycling sediments, and chemical index alteration (CIA) of mudstone is greater than 97, indicating a high degree of weathering. The contents of sulfurophilic elements (Cu, Zn, Co, Ni) and ferriophilic elements (V) are generally higher in the lower ferric−aluminous rocks than ones in the bauxite and mudstone, while oxygenophilic elements (Th, U, Nb, Ta, Sn, W) are on the contrary. The values of REE in bauxite phases are the highest, and the δCe and LREE/HREE of bauxite and mudstone are significantly higher than those of ferro−aluminous rocks. The Fe₂O₃−Al₂O₃−SiO₂ projection shows that monsoon climate persist all the time when the ore−bearing rock series developed, and the special element values of the mudstones in bauxite facies are the extreme ones in all mudstones that mean a period of extreme laterization produced by peneplain, and the gravel and element geochemistry show that the ore−bearing rock series are of inheritance. Corresponding to the four facies the sequence of sedimentary facies can be derived into three stages: the early laterite aggregation, then the peneplain−forming (bauxite mineralization) and the last plain−forming; and volcanic ash kept settling during the development of ore−bearing rock series. The bauxite is a product of secondary weathering crust.