Geology, mineralogy, and fluid inclusion data of the Furtei high-sulfidation gold deposit, Sardinia, Italy

The epithermal high-sulfidation gold deposit at Furtei (Sardinia, Italy) contains at least 2.15 million metric tons (Mt) at 2.82 glt Au (725/,000 t oxidized ore, 1,375,000 t sulfide ore, and ,50,000 t mixed ore). Potential by-products are silver and copper. The geology of the area features an Oligocene-Miocene volcano-sedimentary sequence, including andesite porphyry domes and associated block and ash flows, pyroclastic flows, fall and surge deposits, and interlayered epiclastic and sedimentary sequences. A phreatic and/or phreatomagmatic diatreme breccia also crops out in the mineralized area. The volcanic rocks belong to the calc-alkaline Sardinian magmatism, their KI Ar ages in the area are 23.6 to 25.5 Ma. Hydrothermal circulation affected most of the volcano-sedimentary sequence over an area of about 5 km2 during or immediately after volcanic activity. Early hydrothermal activity was responsible for the acid alteration typical of high-sulfidation deposits, with formation of four hypogene alteration types of decreasing intensity: silicification, advanced argillic alteration, argillic alteration, and propylitization. These alteration styles show a rough spatial zonation. Silicification consists of massive and vuggy silica veins and bodies. These are controlled by preferential structural directions (N 140°, N 40°, and N-S) and by the contacts between the andesite porphyry domes and the diatreme breccias, or between domes and epiclastic-pyroclastic sequences. The advanced argillic alteration borders vuggy and massive silica alteration zones and is characterized by the widespread presence of kaolinite-dickite minerals. This alteration pervasively affects the diatreme breccia and epiclastic-pyroclastic units. Advanced argilliC alteration grades outward into argillic alteration (marked by the presence of montmorillonite), which passes to a districtwide propylitic alteration halo. The ore stage clearly postdates hydrothermal alteration, as indicated by the occurrence of ore minerals in vuggy cavities and fractures in silica bodies. The Furtei mineralization comprises a surficial oxidized zone of supergene nature and a deeper primary sulfide zone and is largely hosted by the diatreme breccia. The sulfide zone (up to 300 m of vertical extension) is characterized by a vertical zoning of the mineral assemblage: at higher levels pyrite-enargite-luzonite-gold are present, whereas in the deep zone the above minerals are accompanied by tennantite and tellurides. Native gold (with 0.7- 7.8 wt % Ag) occurs as blebs in enargite and luzonite. Fluid inclusions indicate that the fluid present at the time of alteration and epithermal mineralization had temperatures generally between 190° and 280°C and relatively low salinity (max 5.5 wt % NaCI equiv). The local coexistence of liquid- and vapor-rich inclusions suggests boiling conditions. The inclusions were trapped at depths of between 60 and 680 m below the water table. The occurrence of high-salinity (31..5-44.8 wt % NaCI equiv), high-temperature (390°-500°C) inclusions in cataclastic hydrothermal quartz suggests that a dense brine of probable magmatic derivation circulated in the Furtei system. This is consistent with geophysical data suggesting the presence of an intrusion at approXimately a 1- to 1.5-km depth. A few relatively high-salinity (around 22 wt % NaCI equiv), low-temperature (255°-275°C) inclusions may be the result of episodic mixing of deep-saline brines with low-salinity meteoric fluids. Late-stage barite contains low-salinity « 1.6 wt % NaCI equiv), low-temperature (around 100°C) fluid inclusions