Giant bauxite deposits of South China: Multistage formation linked to Late Paleozoic Ice Age (LPIA) eustatic fluctuations

The Lower Carboniferous Jiujialu Formation bauxite deposits at Zunyi, northern Guizhou are distinguished by their great thickness (to 110 m) and cyclic alternation of bauxite ore and bauxitic claystone layers. These features are typical of the Kazakhstan subtype of karst bauxite deposits, although the genesis of this type of deposit is poorly understood. Here, we undertook a petrological, mineralogical, and geochemical study of the Zunyi deposit in drillcore ZK5600 to gain insights into its formation history. The Zunyi profile contains seven bauxite cycles, as recorded by petrological, mineralogical and geochemical features, indicating a multistage formation history. Each cycle is composed of a lower bauxitic claystone layer and an upper bauxite ore layer. The bauxite ore is characterized by high Al2O3, high Al-mineral (diaspore), and low SiO2 content. Positive Ce anomalies, high CIA values ( > 90), and strong depletion of major elements indicate that the bauxite ore formed through intense chemical weathering and leaching in a well-drained soil environment. The bauxitic claystone is characterized by relatively lower Al2O3 and higher SiO2 and clay mineral (mostly illite) content. This lithology exhibits both negative and positive Ce anomalies, moderate CIA values (70-80), and enrichment of some elements (e.g., Fe, K), indicating formation through moderate chemical weathering in a poorly drained soil environment. Secular variations in chemical weathering intensity and soil drainage conditions were linked to regional hydrologic changes. During the early Carboniferous, the Zunyi area was located in a karstified coastal upland setting, and its hydrologic system was affected by glacio-eustatic fluctuations. High groundwater table elevations during eustatic highstands led to formation of bauxitic claystone layers, whereas low groundwater table elevations during eustatic lowstands led to formation of bauxite ore layers.