Different Mn-ores in a continental arc setting: Geochemical and mineralogical evidences from Tertiary deposits of Sardinia (Italy)

This contribution examines two types of Mn deposits, which were exploited in the past in Sardinia. Both types are Tertiary in age, but not strictly coeval and they are both linked to a continental arc setting characterized by the presence of calcalkaline volcanics and shallow marine sediments. Mn-deposits formed in terrestrial or shallow marine environments are unusual compared to the most common deep oceanic ones, and therefore poorly studied. The Mn-ores of Sardinia occur either as stratabound deposits with stratiform geometry associated to rudites with littoral facies or as vein-type deposits hosted within volcanic rocks. In the stratiform deposits the Mn ores form the matrix or cement of a transgressive conglomeratic bed overlain by shallow water limestones and underlain by calc-alkaline volcanics. The vein-type deposits consist of Mn oxides hosted by latite lava domes, comenditic lava flows and rhyolitic ignimbrites. Although these geological settings are markedly different, the main Mn-bearing phases are the same, namely pyrolusite and hollandite. The deposits differ in the presence of accessory minerals, such as todorokite, manganite, coronadite, cryptomelane and gangue minerals.Rare earth element geochemistry is the most effective tool in discriminating the two types of Mn deposits. These analyses indicate that the vein-type deposits were formed from geofluid mixtures linked to the Oligocene–Miocene calcalkaline volcanic cycle. Mineralogical and geochemical data indicate that the crystallization of the Mn oxides in the stratabound deposits was diagenetic, related to sudden changes in Eh and pH in coastal, meteoric waters that came in touch with the marine coastal water after having flushed the calcalkaline volcanics.