A terrestrial-marine continuum model for diaspore-dominated karstic bauxite deposits

Abstract Global karstic bauxite has long been thought to form in continental weathering environments. Recent micromineralogical studies have also suggested that it may precipitate on karstic surfaces in terrestrial and marine environments. However, this hypothesis lacks robust evidence. This study presents a new terrestrial-marine continuum model for diaspore-dominated karstic bauxite deposits. The model's development involved detailed geological, mineralogical, and mercury (Hg) and isotopic (e.g., Δ199Hg) analyses of the world's two largest karstic bauxite belts, which are found in the North China Craton and Youjiang Basin, China. The mineral assemblages in these deposits indicate that karstic bauxites formed across a broad spectrum of Eh-pH conditions, ranging from alkaline and strongly reducing to alkaline and suboxidizing. Volcanism was a key driving force for the formation of large-scale bauxites across both the North China Craton and Youjiang Basin. This is evidenced by distinct Hg-enriched peaks and corresponding near-zero Δ199Hg values in the upper bauxite and overlying claystone of the North China Craton, and by the extreme Hg enrichment (up to 2170 ppb) and near-zero Δ199Hg values throughout the bauxite and claystone layers of the Youjiang Basin. Bauxite deposits in the northern and central North China Craton (Δ199Hg: −0.48‰ to 0‰), southern North China Craton (Δ199Hg: −0.24‰ to 0.06‰), southern Youjiang Basin (Δ199Hg: −0.11‰ to 0.26‰), and northern Youjiang Basin (Δ199Hg: 0.09‰ to 0.24‰) were deposited in the terrestrial, terrestrial-marine transition zone, exposed isolated platform, and marine environments, respectively. The new terrestrial-marine continuum model for diaspore-dominated karstic bauxite deposits driven by volcanism suggests that high-quality bauxite typically formed in terrestrial environments characterized by intense continental weathering and efficient drainage, thereby promoting the leaching of iron and silica ions. This study indicates that paleo-karstic depressions within terrestrial depositional environments are promising targets for high-quality karstic bauxite prospecting.