Mesozoic Hydrothermal Overprint on Carboniferous Bauxite in China

Abstract Bauxite is the world’s main source of aluminum and typically consists of gibbsite, boehmite, and minor amounts of diaspore. However, bauxite deposits from the North and South China blocks consist mostly of diaspore and associated minerals, including anatase and illite. Much of this illite is authigenic and occurs as three polytypes (1M, 1Md, and 2M1), with Kübler indices ranging from 0.23 to 0.47 indicating precipitation temperatures of 175° to 300°C. The Raman spectra of anatase show an intensity ratio of <1.5 for G (~1,600 cm–1) and D bands (~1,350 cm–1) diagnostic of organic matter, suggesting its presence during bauxite sedimentation followed by heating (165° to 270°C). The K-Ar ages of authigenic illite from the South China block (178–137 Ma) and the North China block (214–203 Ma) are synchronous with known regional Mesozoic tectono-thermal events generating hydrothermal overprints resulting in (1) the formation of illite, (2) the conversion of some diaspores from thermal of gibbsite and boehmite, and (3) the heating of anatase postsedimentation of the Carboniferous bauxites.