Mineralogical, micromorphological and geochemical evolution of the facies from the bauxite deposit of Barro Alto, Central Brazil

The hydrolytic alteration of anorthosite from the Barro Alto Stratiform Mafic–Ultramafic Complex (Central Brazil) caused the formation of an isalteritic bauxite. Petrological studies using X-ray diffraction (XRD), X-ray fluorescence (XRF), mass balance calculation and micromorphological description via optical microscopy were undertaken to understand the evolution of the bauxitic massif. The results suggest the transformation of the isalteritic bauxite (F1) into varied alteration facies and their filiations as follows: the formation of semi-compact massive bauxite (F2) and compact massive bauxite (F3) by the recrystallisation of gibbsite filing voids, resulting in enriched absolute aluminium (9 and 25%, respectively); the formation of laminar bauxite (F4) and fragmentary bauxite (F5), which is associated with different types of fragmented massive duricrust, such as those formed by mechanical (root activity, tectonics, etc.) and geochemical (dissolution of gibbsite, kaolinite neoformation, etc.) processes; the formation of palaeopediment bauxite (F6); the formation of degraded clay with gibbsitic nodules (F7), which is associated with the resilicification process by the oscillation of the water table caused by changes in the shapes of slopes and the cycling of silica by vegetation. Isalteritic clay (F8) facies directly derived from the anorthosite are also found. The relationship between the facies reflects the polygenetic evolution of the bauxitic massif.

► The different alteration facies identified showed the polygenetic evolution of Barro Alto bauxite.
► The facies formed from anorthosite were porous bauxite and isalteritic clay.
► All other facies were derived from transformation of the bauxite.
► The main processes of transformation were transfer and accumulation, fragmentation and resilicification.