Provenance and genesis of karstic bauxite deposits in China: Implications for the formation of super-large karstic bauxite deposits

Abstract

Since the Carboniferous, over 7.0 billion tons (Gt) of karstic bauxite deposits have formed in the North China Craton (NCC) and the South China Block (SCB), rendering them the largest karstic bauxite deposit resource bases globally. Karstic bauxite deposits in the NCC primarily occur in the Late Carboniferous (>5.0 Gt), and those in the SCB occur in the Early Carboniferous (∼0.2 Gt), Early Permian (∼0.3 Gt), Late Permian (>1.0 Gt), and the Cenozoic (∼0.5 Gt). >120 large karstic bauxite deposits have been found in China, among which several super-large karstic bauxite deposits (single deposit >0.1 Gt) formed during the Late Carboniferous of the NCC and the Late Permian of the SCB. Karstic bauxite deposits that formed during the five levels have different sources, controls, and ore-forming processes. However, the current provenances and processes of karstic bauxite deposits of all five levels, which are primarily based on detrital zircon and mercury isotope analyses, remain unclear. New detrital rutile UPb ages and geochemistry revealed intimate details of the thus-far poorly understood metamorphic source rocks. The predominance of 1950–1800 Ma rutile from karstic bauxite deposits throughout the NCC confirmed the contribution of 1950–1800 Ma metamorphic rocks, which further approved the presence of a string of the Paleoproterozoic paleo-massifs during the bauxitization period. The Al-rich rocks, primarily including the metamorphic rocks inside the NCC and the magmatic rocks in the continental arcs flanking the NCC, experienced strong weathering under the promotion of contemporaneous volcanism at the northern margin of the NCC and formed a series of large to super-large karstic bauxite deposits. In the SCB, the Early Carboniferous, Early Permian, and Cenozoic karstic bauxite deposits contain abundant 650–500 Ma detrital rutile grains that were primarily formed during metamorphism along Gondwana margin and recycled into the regional Cambrian to Silurian strata. These Cambrian to Silurian strata, together with a small quantity of 900–700 Ma magmatic and metamorphic rocks in the Jiangnan Orogenic Belt, were subsequently exposed and weathered, forming the Early Carboniferous karstic bauxite deposits in central Guizhou and the Early Permian karstic bauxite deposits in the northern part of Guizhou and central Yunnan. Al-poor recycled clastic and carbonate rocks limited the substantial formation of the Carboniferous–Permian super-large karstic bauxite deposits. Rare detrital rutile was discovered in the Late Permian karstic bauxite deposits, affirming the opinion that their intensive formation was induced by volcanic eruptions related to the Emeishan mantle plume and Pacific Plate subduction. The study of detrital rutile and zircon from the Cenozoic low-quality karstic bauxite deposits in central Guangxi showed that the Al-poor sedimentary rocks and a small amount of magmatic rocks exposed around the karstic depression underwent long-term weathering and contributed source materials. This study reveals that favorable source rocks, contemporaneous volcanism, and well-developed karstic depressions are key factors affecting the scale and quality of karstic bauxite deposits.