矿床地球化学国家重点实验室知识库

The Yudong bauxite deposit is located in the Kaili–Huangping bauxite-concentrated area, in south-eastern Guizhou Province, China. The ore-bearing rock series consists of the lower part (Fe-bearing member) and middle part (Al-bearing member) of the Middle Permian Liangshan Formation, covering the Upper Devonian Gaopochang Formation. The elemental content in the ore-bearing profile is variable. In general, Al, Ti, V, Cr, Zr, Nb, Hf, Th, and U increase from the Fe- to the Al-bearing member, whereas Si, Fe, Rb, Cs, Ba, and total rare earth elements (REE) decrease. These observations indicate the occurrence of bauxitisation, which corresponds to a process of elemental variation during which Si and Fe were dissolved and migrated, and Al was enriched. Compared with Ordovician–Silurian (O–S) mudrock, the ore-bearing rock series are enriched in Al, Ti, V, Zr, Hf, Nb, Ta, Th, and U, but depleted in Si, Fe, Rb, Cs, and Ba as a result of weathering. The REEs show negative correlations with Si, Fe, K, Na, Ca, Mg, and Y, indicating that the REEs may be associated with clay, Fe-bearing, and Y-bearing minerals.

The La/Y ratio, (La/Yb)_N ratio, and Ce anomalies show that the Fe-bearing member formed in an alkaline reducing environment, whereas the Al-bearing member formed in an acidic oxic environment. The immobile element ratios, Eu anomalies, REE distribution patterns, and Cr–Ni diagram, as well as the comparison with worldwide bauxite deposits, show that the ore-bearing rock series may have originated in the O–S mudrock, with contributions from the dolomite of the Gaopochang Formation. Furthermore, the position of the samples in the Al₂O₃–Fe₂O₃–SiO₂ ternary plot, the chemical index of alteration, and the strong positive correlations of some stable elements (such as Al, Ti, Zr, Hf, Nb, and Ta) indicates that the degree of weathering gradually increases from the O–S mudrock to the bauxitic mudrock, and further increases into the bauxite; the bauxitic mudrock and bauxite therefore originated from the O–S mudrock. The laterisation products of the O–S mudrock from the northern part of the ore collection area were transported to the karst depression in the early stages of the Middle Permian. The weathering products of the O–S mudrock and a small amount of the Gaopochang Formation dolomite accumulated on the karst interface, forming the original bauxite layers through cycles of compaction and concretion. These bauxite layers were then uplifted to or near the surface during the Late Cretaceous to Paleogene, eventually, forming karstic bauxite.