Coexistence of porphyry and epithermal environments within volcanic complexes in Sardinia, Italy: A case study

The andesitic and dioritic bodies occurring in the Siliqua sector, formed during the Tertiary calc-alkaline magmatic cycle, display alteration and mineralisation patterns which can be referred to as a porphyry copper system consistent with a diorite model. In the northern part of this sector an exploration drilling campaign showed high gold contents, strong silicification and the presence of epithermal silica-pyrite alteration. The field observations and time and space relationships between the porphyry and epithermal styles of mineralisation and host rocks seem to suggest successive development of alternating porphyry-style veining and igneous and/or hydrothermal breccias, followed by several epithermal stage mineralised breccias within previously advanced argillic-altered subvolcanic and volcanic rocks. The epithermal occurrences may be considered to be related to the adjacently outcropping porphyry system or telescoped to the periphery of some other stocks still buried at depth. Detailed petrographic studies for the reconstruction of such a complex scenario are in progress.