Geological, mineralogical, geochemical features and genesis of the albitite deposits of central Sardinia (Italy)

The largest albitite occurrences in Europe, mined for albite, are located in Central Sardinia, at the Southern margin of the Tirso river rift valley, where the Hercynian granitoid massif extensively crops out. The parent rocks of the albitites are the Hercynian granitoids, mainly granodiorites, which consist of quartz, plagioclase and K-feldspar. Biotite and allanite are the main accessory minerals. The main mineralogical association of albitites consists of rarely zoned plagioclase and quartz. Subordinate minerals are K-feldspar, biotite, chlorite, epidote, titanite, and muscovite. Compared to the unaltered rock, the albitites generally show higher Na2O and Al2O3 contents, while Fe2O3, K2O, Rb e Ba, are generally depleted. This suggests that plagioclase, K-feldspar and biotite were affected by remarkable chemical-structural transformations, involving their dissolution and recrystallization in newly formed minerals. The mobilization of Rare Earth Elements (REE) caused a re-distribution of these elements in the albitites. The REE are mainly accommodated in epidotes (LREE) and in titanites (HREE). Geological, geochemical, mineralogical and petrographic evidences suggest that the most probable origin of the albitites can be related to the action of metasomatic processes by circulation of later fluids through the metasedimentary basement and the Hercynian granitoids. Recent studies carried out on Sr/Nd-isotope ratios provide information about the possible ages of albitization and the nature of the albitizing fluids and the rocks involved in the metasomatic process. These studies suggest that the age of albitization likely took place at about 271 Ma, and the fluids were not magmatic in origin. The field trip (stops 2÷4) evidenced the geological assessment of the area, the relationships between unaltered granitoids and mineralization, through a transitional lithotype, and the gradual mineralogical and textural transformation of the granitoid into albitite.