Genesis of the Carboniferous karstic bauxites in Qingzhen region, central Guizhou, southwest China

Abstract

In addition to the major source of aluminum, karstic bauxite also contains high concentrations of trace elements such as Li, Ga, V, Sc, Co, Nb, Zr, Ni, U and REE. At present, more than 33.9 million tons of early Carboniferous bauxite overlies the karstic surface of the Cambrian carbonate rocks in Qingzhen region, central Guizhou, southwest China. Here, new mineralogical and geochemical evidence of the Lindai bauxite in Qingzhen region have been provided, aiming to identify parental affinity, discuss the provenance differences between the Qingzhen and Xiuwen areas, illustrate the mineral phase transformation, analyze the factors affecting the distribution of critical elements, and reveal the ore-forming process. Mineralogically, the bauxite ores are dominated by diaspore, boehmite, illite, kaolinite and anatase, with small amount of zircon, hematite, pyrite, cassiterite and medium-low temperature hydrothermal minerals such as barite, chalcopyrite, galena, arsenopyrite. Textural characteristics are dominated by matrix-supported clastic texture, followed by pseudo-breccias, recrystallizational and ooidal textures. Clastic grains with various sizes and shapes are often mixed together without obvious sorting and directional structures, indicating that bauxite ores have undergone short-distance transportation in the form of gravity flow. Provenance analysis indicates that the protoliths of the Lindai bauxite were a mixed lithology including underlying Cambrian Shilengshui Formation dolomite, hydromica claystone and purple-red mudstone rather than single dolomite considered by previous studies. Particularly, the presence of hydromica claystone and purple-red mudstone has a significant contribution to high concentrations of trace elements and REE in the Jiujialu Formation. Studies of mineral genesis have shown that the formation of diaspore includes supergene crystallization and metamorphism. R-mode factor analysis revealed that (i) these elements including Al, Ti, Ga and HFSE are relatively stable and immobile during weathering process, and tend to be enriched in the karstic bauxite, (ii) the distributions of HREE are controlled by the REE minerals and Fe oxide-hydroxides, (iii) vanadate minerals have crucial roles in the distribution of LREE in addition to the phosphate minerals, and (iv) the enrichment of Ni is related to the adsorption of hydroxo complex of uranium. The studies reported in this paper provide new insights into the provenance and mineral phase transformation of the Carboniferous karstic bauxite in central Guizhou area.