Iron and manganese accumulations in Sardinia: the role of supergenic and hydrothermal processes

This paper examines the compositional features of some Fe and Mn accumulations of Sardinia in order to reconstruct the palaeoenvironmental conditions of ores formation, to better characterize the processes that lead to Fe and Mn minerals formation and to compare the Fe and Mn geochemical behavior within different depositional environments. The research focused on comparison between: 1) continental Fe oxyhydroxides concentrations, formed under oxic conditions on a palaeosurfaces of the Tacchi area, and the Uppermost Ordovician oolitic ironstones of shallow anoxic marine environment of the Nurra district; 2) marine supergene Mn ores of Logudoro Basin, hosted in a littoral conglomerate laying at the base of the Miocene transgressive calcareous sediments, and continental hydrothermal Mn mineralizations of San Pietro Island and Anglona district where they are the stockwork and veins filling within Miocene volcanic sequences. Chemical analyses (XRF, INAA and ICP-MS), XRD analysis, OM and SEM-EDS observations were performed on each samples. The analyses results suggest that although the Sardinian ores formed in different environments (marine vs continental) and under contrasting climatic (sub-glacial vs tropical) and environmental (supergene vs hydrothermal) conditions, they share similar mineralogical or geochemical features. Therefore, not necessarily dramatic palaeoenvironmental differences find a counterpart in geochemical features of the chemical sediments.