Bentonite, kaolin and zeolite occurrences of Sardinia (Italy): alteration products originated by different minerogenetic processes effecting the Cenozoic volcanic rocks

The most important Industrial Mineral (IM) deposits associated with the Cenozoic volcanics of Sardinia include kaolin, bentonite, and zeolites. Bentonite, and to a lesser extent kaolin and perlite, form economically viable deposits and zeolites appear as very important industrial mineral commodities for future economic development. All these deposit types occur as alteration products of Cenozoic volcanic rocks, which were emplaced during three main volcanic cycles: in the Eocene (EC, 55 to 51 Ma), Oligocene-Miocene (OMC, 32 to 13 Ma), and Pliocene-Pleistocene (PPC, 5 to 0.1 Ma). Kaolin, bentonite, zeolites, occur as alteration products of the OMC volcanites. All deposits have generally derived from the alteration of the most acidic members of the Cenozoic volcanic rocks (rhyolites to rhyodacites), but differences among the associated ore-forming phenomena are recognized. Hydrothermal sensu stricto, deuteric alteration and weathering are the most important processes involved in ore formation. Weathering, although of minor importance, can also contribute to enhance the alteration of the deposits formed by hydrothermal and deuteric processes. The extent and type of occurrences are mainly controlled by the local geological and structural features, and the environmental geochemical conditions.