Critical metals distribution in Tethyan karst bauxite: the Cretaceous italian example

Mineral supplies have become a global concern and, within this frame, the value of bauxite ore might be possibly increased. The latest European Union report on critical raw materials listed many elements, some of which are usually found in bauxites. Gallium is a common by-product of Al extraction, and bauxites are also considered possible resources of REEs and Sc. Consequently, more attention has been recently paid to the processes that control the distribution of several minor elements in these deposits. With this in mind we present the factors affecting the distribution of the trace metals Sc, V, Cr, Co, Ni, Ga, Y, Nb, and REEs in Cretaceous karst bauxites deposits distributed across southern Italy and Sardinia. The deposits have been extensively studied in recent years (Mameli et al., 2007; Mondillo et al., 2011; Boni et al, 2013; Mongelli et al., 2014, 2016) providing a large textural, mineralogical and chemical data-set (n=103). All bauxites share the same texture consisting of sub-circular aggregates, mainly formed by boehmite and hematite, dispersed in a fine grained matrix and are mostly composed of four major oxides: Al2O3, Fe2O3, TiO2 and SiO2. Among the trace metals Cr (median=547 ppm), ΣREEs (median=489 ppm), V (median=405 ppm), and Ni (median=200 ppm) are those with the highest content. R-mode factor analysis was performed (on the whole dataset, including the outliers) in order to evaluate interelemental relationships. Five factors explain 81.3 % of the total variance showing significant relationships including V, Sc, Ce, and LREE (F1, 25.6 Var %), TiO2 and Nb (F2, 22.3 Var %), SiO2, Al2O3, and Ga (F3, 13.5 Var %), Fe2O3 and HREE (F4, 11.6 Var %), and finally Co and Ni (F5, 8.3 Var %).