The bauxite of North-Western Sardinia

Abstract. Bauxite outcrops of North-Western Sardinia are described here, taking into account literature data and new SEM-EDS evidences on the development of the mineral textures in relationship with the genesis. The Nurra bauxite deposits are divided into three main types related both to the substratum nature and the Mid-Cretaceous tectonics. Textural analyses have highlighted the importance of iron-hydroxides in the formation of accretive structures, such as ooliths and pisoliths, as well as the role of early microfractures in the bleaching of the mineral. Latest fractures and faults conversely favoured the circulation of silica rich solutions which caused the bohemite risilicization, with consequent production of epigenetic kaolinite. At SEM observations, the neoformed kaolinite appears more crystalline than kaolinite occurring in the bohemitic clays at the base of the bauxite horizon. Two stops were able to illustrate the main features of the Nurra Bauxite: the first one at Olmedo mine where the different role of fracturation is strikely evident in a bauxite which lies on a mainly marly foot-wall; the second one at Monte Murone where a strongly iron-rich oolitic bauxite developed on a dolomitic foot-wall