Geochemical distribution and mineralogical control of scandium in karst bauxite, NW Sardinia, Italy

In the last years, karst bauxite have attracted considerable attention as secondary sources of critical metals, including scandium and, in this respect, we investigated the geochemical pathways of scandium in karst bauxites from the Nurra district, north-west Sardinia (Italy). The Nurra karst bauxite mainly consist of boehmite, goethite, hematite, kaolinite, and anatase. In the whole-rock the average Sc concentration is 51 mg/kg. Microanalysis show that Sc is, by far, hosted in Fe-rich domains where the average Sc content is 171 mg/kg. An unusual content of Zr is measured all along the micro analyses, dispersed ubiquitously in both Fe-rich domains and Al-rich domains. The distribution of Sc and Zr in neoformed structures, such as Fe –“agate” and Fe-veins, suggests that these elements were mobile during the formation of these textural components. The Nurra bauxite hosts three main zircon populations of different age that are disrupted, fractured and corroded. Only one zircon population has been identified as the main Sc host (from 1184 to 4164 Sc mg/kg). We suggest that the source of Sc in the Nurra bauxite comes from the partial dissolution of the Sc-rich zircon since: (i) no other Sc-bearing primary mineral has been detected; (ii) the UCC-like parent material and (iii) the Zr content dispersed in all the bauxite micro domains suggesting that Zr may experience a significant degree of mobility during bauxitization. On these basis, further studies are needed to investigate the pH oscillations during karst bauxite formation and their implications on the geochemical mobility of critical elements.