Mineralogical and geochemical investigations on the iron-rich gibbsitic bauxite in Yongjiang basin, SW China

Iron-rich gibbsitic bauxite, located in the Yongjiang basin, southwestern part of South China Block, characterized by nodules mixed with loose clayey soil. Bauxite ores consist of gibbsite, kaolinite, hematite and goethite and few zircon and quartz. The nodules have higher ratios of Al/Si and stronger ferrallitization compared to the enclosing soil. The nodules have a lower content of Rb and Cs that were easily depleted during the weathering process, and a higher content of Ga, Sc, V, Cr, Ni, Co, Pb and Th that enriches due to the occurrence of Fe-Mn minerals and zircon grains than the clayey soil. Chondrite-normalized REE patterns of the nodules reflect enrichment in LREE compared to HREE and negative Eu anomalies. Bauxite samples mostly show positive Ce anomalies, indicating an oxidizing condition during deposition. Ultra-high Ce anomalies in nodules are associated with Mn oxides, suggesting an enrichment in Mn oxide during the process of nodule formation. The facts including lower content of aluminium hydroxide in iron-rich gibbsitic bauxite than karst bauxite, similarity in ore structure and distribution of trace elements and REE between iron-rich gibbsitic bauxite and karst bauxite suggest the bauxite in Yongjiang basin is the precursor of karst bauxite. Based on the mineralogical and geochemical (especially for the plots of Ni vs Cr and Eu/Eu* vs TiO2/Al2O3) analyses, it indicates that source material for the bauxite includes the weathering products of surrounding granites, clastic rocks and substrate carbonate rocks. It is proposed that the iron-rich gibbsitic bauxite experienced the deposition of a primary layer of a mixture of clay and gibbsite which later become mixed with the ferrallitic soil to form the nodules and clayey soil. After that, a breakup process followed concomitant with the thorough mixing with the clayey soil matrix.