Gibbsite as low-temperature hydrothermal overprint on lateritised Cenozoic sediments, Quadrilátero Ferrífero of Minas Gerais, Brazil

ABSTRACT Gibbsite, typically a clay-sized mineral in lateritic profiles, occurs as crystals between 50 and 150 µm across in druse-like cavities that characterise veins hosted in lateritised Cenozoic sediments, exposed along a road cut in the Quadrilátero Ferrífero of Minas Gerais, Brazil. Both the veins and the host rock, a red diamictite, contain gibbsite, but the drusy gibbsite in the former is remarkably larger than the matrix gibbsite in the latter. It is suggested that the coarser gibbsite is a low-temperature hydrothermal mineral. The coarse grain size of gibbsite and its occurrence in veins comprise important criteria to recognise hydrothermal overprint of low temperature on lateritic profiles.