Transition from porphyry to epithermal Cu-Au environments in Sardinia - Italy

Abstract. In Sardinia acidic fluids form argillic alteration halos and/or silicic lithocaps over Tertiary porphyry systems which may host subsequent mineralization. The spatial, geochemical and geophysical evidences suggest that the two systems may be genetically related and may help to determine the transition of a magmatic hydrothermal system from porphyry to epithermal environments. The tetrahedrite-tennantite series minerals occur extensively in hydrothermal deposits although in minor quantity as compared with other sulfides. At Furtei Au deposit available chemical data of these minerals and of the telluride-rich parts of drillcore samples, indicate direct magmatic inputs to the mineralizing solutions thus a transition of this system from porphyry to epithermal environments is envisaged and the relationship between mineral assemblages and sulfidation states may be assessed as well as evolution of ore solutions outlined. The Tertiary bodies occurring in the Siliqua sector display alteration and mineralisation patterns which can be referred to as a porphyry copper system consistent with a diorite model with the presence of epithermal silica-pyrite alteration. Field observations and time and space relationships seem to suggest development of porphyry-style veining followed by several epithermal stage mineralised breccias which may be related to the adjacently outcropping porphyry system or telescoped to the periphery of some other stocks still buried at depth.