Continental weathering led to the accumulation of Early Carboniferous bauxite deposits in the SW South China Craton

The Carboniferous karstic bauxite-bearing rock series within the Jiujialu Formation were widely distributed in central Guizhou, South China, with a prominent enrichment of ferrous clay materials at the lower part and fluctuations of Al contents at the upper part of those deposits. Yet, the evolution, distribution and formation mechanism of those bauxite ore-bearing rock series require further investigation. We used thickness data from similar to 800 drillcores in central Guizhou to generate a high-precision paleo-geomorphological reconstruction of the study area, which reveals a series of karstic depressions from Southwest to the Northeast on a coastal karstic plain. Based on a typical drillcore of ZK47-12 located at a karstic depression, a relatively complete depositional records of bauxite deposits in the Jiujialu Formation have been yielded. The study bauxite deposit contains the lower Fe-rich layer and upper Al-rich layer. Within Al-rich deposits, bauxite ore comprises Al-bearing minerals including diaspore, boehmtie, with slight amounts of Fe-bearing and clay minerals, while bauxitic claystone yields higher contents of clay minerals. And Fe-rich layers comprise Fe-bearing minerals (mainly for chamosite). Geochemistry of the drillcore samples and mass changes reveal strong losses of most mobile elements, relative enrichment of Al within the bauxite ores and enrichment of some trace elements and REEs within claystones, caused by leaching process. Mineralogical and geochemical cycles within the upper Al-rich layer directly link with variations of the groundwater table (GWT) during the bauxitization in central Guizhou. The B content and B/Ga ratios indicate that the Jiujialu Formation in central Guizhou contains variations of paleosalinity, this evidence further supports that the fluctuations of GWT may cause by the Early Carboniferous sea-level changes. Our new data indicate that multiple factors control the formation of the Carboniferous bauxite deposits in central Guizhou. Paleo-geomorphological features of a series of karstic depressions upon the underlying carbonate rocks providing deposition space, tropical climate near the paleo-equator contributing to the weathering process, frequent fluctuation of groundwater table enhancing or releasing the intensity of leaching process, glacio-eustatic cycles promoting karstification and driving the fluctuations of GWT, and input of seawater influencing the intensity of leaching process during bauxitization.