Bentonite occurrences in the Tertiary volcanic rocks in Central Sardinia, Italy

Investigations carried out during the last decade suggest that the main Italian reserves of bentonite are hosted in the Tertiary calc-alkaline volcanics of Sardinia. Several of the most important occurrences of Sardinia are in the Sarcidano-Barigadu area (Central Sardinia). Here, the mineralized bodies occur either as interlayered beds in the Tertiary ignimbrite sequence, which consists of several units, or as originally cineritic single lenses overlaying Jurassic limestones. The petrology and mineralogy of the volanics and their associated mineralization allow an evaluation of potential exploitation of the deposits. Two types of deposits (I and II), distinguished on the basis of the geologic and petrologic evidences, were investigated. The deposits of type I are localized in the upper part of an ignimbritic body, often along faults and/or joints. The deposits of type II are developed in the single volcaniccineritic lenses, interbedded within fluvial-deltaic Miocene sedimentary rocks. From an economic point-ofview, the most important deposit are located in the Neoneli area. Physico-chemical properties also confirm the economic importance of Busachi, Su Lau and Neoneli deposits. The others are of lesser importance because of their low reserves (Samugheo) or argillization (Is Porcilis), excess of sand and low CEC (Ponte Nuovo).