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

Abstract

The epithermal high-sulfidation gold deposit at Furtei (Sardinia, Italy) contains at least 2.15 million metric tons (Mt) at 2.82 g/t 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 K/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 km 2 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 degrees , N 40 degrees , 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 degrees and 280 degrees C and relatively low salinity (max 5.5 wt % NaCl 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 % NaCl equiv), high-temperature (390 degrees -500 degrees 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 % NaCl equiv), low-temperature (255 degrees -275 degrees C) inclusions may be the result of episodic mixing of deep-saline brines with low-salinity meteoric fluids. Late-stage barite contains low-salinity (