Climatic and hydrologic controls on upper Paleozoic bauxite deposits in South China

Abstract

The late Paleozoic Era was an interval of major tectonic and climatic changes, including formation of the supercontinent Pangea and the ~60-Myr-long Late Paleozoic Ice Age (LPIA). Although bauxite formation declined globally from the Carboniferous to the Permian in conjunction with global cooling, it was an interval of widespread bauxite formation in China. In South China, (1) the Jiujialu Formation bauxite deposits in central and northern Guizhou (Zunyi area) are of early-middle Visean age; (2) the Dazhuyaun Formation bauxite deposits in northern Guizhou (Wuchuan–Zheng'an–Daozheng area) to southern Chongqing (Nanchuan area) and southeastern Guizhou (Fuquan–Kaili area) are of Late Pennsylvanian–Early Permian age; and (3) the Heshan Formation bauxite deposits in western Guangxi to Yunnan are of Middle–Late Permian age. In North China, the Benxi Formation bauxite deposits are of Late Mississippian–Middle Pennsylvanian age. The contrasting trends in bauxite metallogenesis between China and the rest of the world imply different climatic patterns in the eastern Tethys (high annual humidity with seasonal dryness) and Pangea (aridification). This hypothesis is further supported by differences in the chemical index of alteration, or CIA (>80 in the eastern Tethys vs. ~50 in western Pangea), and in paleotemperatures (mean annual temperature, or MAT = ~20 °C in the eastern Tethys vs. ~4 °C in western Pangea) determined from Permo-Carboniferous siliciclastic deposits. Permo-Carboniferous bauxite deposits in South China formed in coastal plain and coastal karstic depression environments, in which the position of the groundwater table was related to sea-level changes. During the late Paleozoic, high-frequency eustatic fluctuations caused by waxing and waning of Gondwana icesheets controlled sedimentation in these coastal depositional systems, leading to cyclic accumulation of coal and bauxite deposits and cycles of vadose- and phreatic-type bauxite formation. The results of the present study show that, during interglacial stages, lateritization resulted from high pCO2, high sea-level and groundwater-table elevations, low precipitation, and limited vegetation cover, whereas during glacial stages, bauxitization of these ferralitic weathering products was promoted by low pCO2, low sea-level and groundwater-table elevations, high precipitation, and more extensive vegetation cover. Thus, a unique combination of geographic, climatic, and eustatic factors accounted for widespread formation of bauxite in South China during the LPIA.