This paper deals with the products of wathering of alkaline rocks of the Lages district, state of Santa Catarina, the southernmost known occurence of bauxite in South America. The alteration of these rocks under suitable conditions of temperature and availability of water has led to the formation of a bauxite deposit of 5.10(6) tons of Al2O3 >; 50%. The chemical and mineralogical characteristics of the different horizons of the alteration profile from the fresh rock to the uppermost leveis are here described. There is a transformation of the primary mineral assemblage of K-feldspar, nepheline and aegirine to a secondary one of gibbsite and goethite or halloysite/kaolinite and goethite. It is a lateratisation process involving leaching of silica and alkalies and concentration of alumina, as demonstrated by isovolumetric calculations. The analysis of these data suggests that texture and structure of the rock and topographic site are the principal factors controlling the process of bauxitisation. The bauxite deposit of Lages is compared with other similar deposits developed upon alkaline rocks like Poços de Caldas and Arkansas
