Genetic aspects of talc-chlorite formation in central sardinia, Italy: Metamorphism, hydrothermalism and mg-metasomatism: The case of SA matta and SU venosu mines

In Central Sardinia the talc- chlorite minerals were already being mined in the 1930's and they still play a very important role in the social life and regional economics. The importance of both the mining and the exploration activities call for further geological studies and geochemical investigations of these raw materials to better define the genetic environments, thereby assisting in finding new deposits. The source of magnesium during the mineralising event is of major importance. Dissolution of quarz during Na-metasomatism of granitoid rocks by Na-rich hydrothermal fluids, implies an enrichment in Mg of the albitizing fluids. Prograde Na-metasomatism was pervasive on enormous rock volumes and huge quantities of metals were released including Mg that gradually reached sufficient activity in the flowing ore forming solution for retrograde Mg-metasomatism of both feldspar of the original granitoids and albitized rocks. At Sa Matta and Su Venosu mines in the Ottana-Orani district, continuous transgression from the hostrock mineralogy into talc-chlorite is ubiquitous. Because of the close spatial relation of the feldspar and the talc deposits, the possibly consanguineous origin of the albite and phyllosilicates mineralisations is discussed and is one of the major tasks of the current investigations. Analytical data show that the Fe/Fe+Mg ratio distinguishes the talc of metamorphic origin in the carbonate rocks from the talc originating from metasomatic and hydrothermal alteration.