GENETIC ASPECTS OF "SA PERRIMA" OCCURENCE AT FURTEI GOLD DEPOSIT, SARDINIA, ITALY: MINERALOGICAL FEATURES AND ORE FORMING CONDITIONS

In 1985, the first potentially economic gold occurrences were discovered in Sardinia. The Au mineralisation near Furtei shows features typical of high-sulfidation epithermal systems with the mineralization mostly contained in vuggy silica within breccias. The primary sulfide zone is largely hosted by diatreme breccia and is characterized by a vertical zoning of the mineral assemblage, dominated by pyrite-enargite-luzonite-gold at higher levels, whereas in the deep zone tetrahedrite and Te-rich minerals are present, native Au always appears as blebs in enargite. The Sa Perrima occurrence is characterized by some distinctive differences: It consists of a tabular body of volcanoclastites which locally contains organic material, associated with marls and surge deposits, andesitic blocks and ash-flow tuff. The mineralization is associated with massive silica veins. Sulfides are mainly represented by ubiquitous disseminated pyrite and veins and nodules of pyrite-sphalerite-wurtzite and minor galena. A characteristic of this locality is the local association of gold with small horizons of carbonaceous materials and fossilized plants within volcano-sedimentary units. The genetic relationship between gold deposition and carbonaceous matter, if any, is unknown. The solutions which introduced gold also carried significant amounts of wurtzite, a sulfur-deficient phase relative to sphalerite. At low temperatures, wurtzite may be deposited in highly reducing environments like those locally acting at Sa Perrima. Probably gold, pyrite and quartz were deposited simultaneously, in bed containing small amounts of organic carbon and the presence of the various type of carbonaceous materials became a dominant influence, at least in the removal of gold from hydrothermal solutions.