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Long-lasting, intense volcanic activity took place in Sardinia towards the end of the Oligocene. During its waning phases a significant hydrothermal activity produced local alteration and both precious- and base-metal mineralisation, as epithermal high- and low-sulphidation veins. At the Furtei gold deposit, the main orebody in the Santu Miali sector was examined using deep sampling of the hill to help define the orebody's pipe-like shape, the spatial evolution of the acid-sulphate base and precious metal ore-forming system, and the 200-300 m or more vertical extent of mineralisation and hydrothermal alteration. The drilling also showed the continuity of the system below the oxidised zone, suggesting potential for additional reserves and discoveries., Long-lasting, intense volcanic activity took place in Sardinia towards the end of the Oligocene. During its waning phases a significant hydrothermal activity produced local alteration and both precious- and base-metal mineralisation, as epithermal high- and low-sulphidation veins. At the Furtei gold deposit, the main orebody in the Santu Miali sector was examined using deep sampling of the hill to help define the orebody's pipe-like shape, the spatial evolution of the acid-sulphate base and precious metal ore-forming system, and the 200-300 m or more vertical extent of mineralisation and hydrothermal alteration. The drilling also showed the continuity of the system below the oxidised zone, suggesting potential for additional reserves and discoveries.

The Ottana-Orani district has Palaeozoic metasediments, Hercynian granitoids and Tertiary volcanic rocks: mica schists, marbles and gneisses are intruded by syn-tectonic leucogranites and late- or post-tectonic mica granites. Talc-chlorite and Na-feldspar deposits are hosted by the Palaeozoic series, economic deposits having formed as a result of intense hydrothermal activity. It is thought that the metasomatic events post-dated intrusion of the granites. First results of fluid inclusion analysis revealed high concentrations of Br and depletion in Na, possibly reflecting albitisation and indicating that talc-chlorite mineralisation occurred at the same time as the ubiquitous albitisation., The Ottana-Orani district has Palaeozoic metasediments, Hercynian granitoids and Tertiary volcanic rocks: mica schists, marbles and gneisses are intruded by syn-tectonic leucogranites and late- or post-tectonic mica granites. Talc-chlorite and Na-feldspar deposits are hosted by the Palaeozoic series, economic deposits having formed as a result of intense hydrothermal activity. It is thought that the metasomatic events post-dated intrusion of the granites. First results of fluid inclusion analysis revealed high concentrations of Br and depletion in Na, possibly reflecting albitisation and indicating that talc-chlorite mineralisation occurred at the same time as the ubiquitous albitisation.

In the studied districts, the association of hessite, stutzite, sylvanite, petzite, goldfieldite, coloradoite and altaite with native tellurium indicate direct magmatic inputs to the mineralising solutions. The sulphidation states of the telluride-bearing ore fluids fluctuated between intermediate and high even within the same deposit: in the NE Greek deposits, tellurides are present in porphyry-related base-metal veins as well as in intermediate- and high-sulphidation assemblages in the epithermal environment, while in the Furtei mine, the only Te-rich Sardinian system, the tellurides are also related to both high- sulphidation and intermediate-sulphidation assemblages. A relationship between the sulphidation state and the timing of bulk ore introduction seems probable., In the studied districts, the association of hessite, stutzite, sylvanite, petzite, goldfieldite, coloradoite and altaite with native tellurium indicate direct magmatic inputs to the mineralising solutions. The sulphidation states of the telluride-bearing ore fluids fluctuated between intermediate and high even within the same deposit: in the NE Greek deposits, tellurides are present in porphyry-related base-metal veins as well as in intermediate- and high-sulphidation assemblages in the epithermal environment, while in the Furtei mine, the only Te-rich Sardinian system, the tellurides are also related to both high- sulphidation and intermediate-sulphidation assemblages. A relationship between the sulphidation state and the timing of bulk ore introduction seems probable.

An Oligocene-Miocene calcalkaline volcanic cycle, related with the drift of the Sardinian-Corsican massif, covered wide areas of western Sardinia with its products. Both low sulphidation and high sulphidation type precious metal occurrences have been found. The former are characterised by vein-like quartz-chalcedony bodies, with mineralisations including gold and silver where the bodies are related to potassic alteration, and only base metals where only propylitic alteration is present. The latter, whose mineralisation mostly includes gold, copper and locally tellurium minerals, are characterised by propylitic and advanced argillic alteration. The overall characteristics of the known orebodies and in particular the geochemical patterns and isolated evolution of Sardinian volcanism and, consequently, of epithermalism, seem to delineate an individual, homogeneous province., An Oligocene-Miocene calcalkaline volcanic cycle, related with the drift of the Sardinian-Corsican massif, covered wide areas of western Sardinia with its products. Both low sulphidation and high sulphidation type precious metal occurrences have been found. The former are characterised by vein-like quartz-chalcedony bodies, with mineralisations including gold and silver where the bodies are related to potassic alteration, and only base metals where only propylitic alteration is present. The latter, whose mineralisation mostly includes gold, copper and locally tellurium minerals, are characterised by propylitic and advanced argillic alteration. The overall characteristics of the known orebodies and in particular the geochemical patterns and isolated evolution of Sardinian volcanism and, consequently, of epithermalism, seem to delineate an individual, homogeneous province.

The volcanic rocks of the Tertiary calc-alkaline cycle have undergone late-stage hydrothermal alteration which has given rise to a variety of mineralisation, in a zone bounded to the east by the Miocene cover and to the west by the Campidano graben. Two main areas of kaolin deposits, consisting mainly of kaolinite with some montmorillonite and dickite, are centred on Monte Porceddu and Coronas Arrubias, but the silica and pyrite content limits current extraction to the production of aluminous refractories. Metalliferous mineralisation is concentrated almost exclusively in the area of maximum argillic alteration, intense silicification and pyritisation. Exploration is in progress of two gold deposits at Monte Santu Miali and others at Is Concas and Sa Perrina. Mineralisation at Santu Miali and Is Concas is rich in As, with the main minerals pyrite, enargite-luzonite, tennantite, covellite, chalcopyrite, digenite and sphalerite; the native gold is almost pure (1-2% Ag), in micrometre-scale granules in enargite and luzonite. At Sa Perrina the visible mineralisation is disseminated pyrite with some chalcopyrite and local, rare wurtzite, sphalerite, galena, covellite and digenite. The gold, though not visible, must be in a sufficiently native and free state to have responded satisfactorily to processing. Other resources are rock products including inert materials, construction stone, dimension stone and industrial materials such as cement., The volcanic rocks of the Tertiary calc-alkaline cycle have undergone late-stage hydrothermal alteration which has given rise to a variety of mineralisation, in a zone bounded to the east by the Miocene cover and to the west by the Campidano graben. Two main areas of kaolin deposits, consisting mainly of kaolinite with some montmorillonite and dickite, are centred on Monte Porceddu and Coronas Arrubias, but the silica and pyrite content limits current extraction to the production of aluminous refractories. Metalliferous mineralisation is concentrated almost exclusively in the area of maximum argillic alteration, intense silicification and pyritisation. Exploration is in progress of two gold deposits at Monte Santu Miali and others at Is Concas and Sa Perrina. Mineralisation at Santu Miali and Is Concas is rich in As, with the main minerals pyrite, enargite-luzonite, tennantite, covellite, chalcopyrite, digenite and sphalerite; the native gold is almost pure (1-2% Ag), in micrometre-scale granules in enargite and luzonite. At Sa Perrina the visible mineralisation is disseminated pyrite with some chalcopyrite and local, rare wurtzite, sphalerite, galena, covellite and digenite. The gold, though not visible, must be in a sufficiently native and free state to have responded satisfactorily to processing. Other resources are rock products including inert materials, construction stone, dimension stone and industrial materials such as cement.

A high-sulphidation epithermal gold deposit is currently being mined in the Serrenti-Furtei area and important porphyry-Cu indications have been found at Siliqua-Decimoputzu, 25 km away. Hydrothermal breccias have been observed in both areas, thought to be associated with epithermal mineralisation. Breccias from Furtei can be divided into monomictic and polymictic breccias and can be either mineralised or non-mineralised. The polymictic breccias are interpreted as diatremes and contain Palaeozoic basement shale and quartzite and Tertiary andesite and pyroclastic fragments. The monomict breccias are composed almost entirely of andesitic clasts. The mineralised breccias have abundant sparry cement containing calcite, pyrite and enargite. The Siliqua breccias are intruded dyke breccias and are indications of explosive hydrothermal activity in the area., A high-sulphidation epithermal gold deposit is currently being mined in the Serrenti-Furtei area and important porphyry-Cu indications have been found at Siliqua-Decimoputzu, 25 km away. Hydrothermal breccias have been observed in both areas, thought to be associated with epithermal mineralisation. Breccias from Furtei can be divided into monomictic and polymictic breccias and can be either mineralised or non-mineralised. The polymictic breccias are interpreted as diatremes and contain Palaeozoic basement shale and quartzite and Tertiary andesite and pyroclastic fragments. The monomict breccias are composed almost entirely of andesitic clasts. The mineralised breccias have abundant sparry cement containing calcite, pyrite and enargite. The Siliqua breccias are intruded dyke breccias and are indications of explosive hydrothermal activity in the area.

The discovery of important albitite deposits near the already known talc-chlorite deposits of central Sardinia reopened discussion about the genesis of these deposits. It was commonly agreed that Mg had been supplied by limestones, even though these rocks are not abundant in the area. Taking into account the volume of albitised rocks, their mean Mg content and the distribution of the newly formed silicates, it appears that the Mg displaced by Na-metasomatism could produce the assessed quantity of magnesium silicates., The discovery of important albitite deposits near the already known talc-chlorite deposits of central Sardinia reopened discussion about the genesis of these deposits. It was commonly agreed that Mg had been supplied by limestones, even though these rocks are not abundant in the area. Taking into account the volume of albitised rocks, their mean Mg content and the distribution of the newly formed silicates, it appears that the Mg displaced by Na-metasomatism could produce the assessed quantity of magnesium silicates.

The Flumendosa-Campidano system of artificial lakes is the major water resource for southern Sardinia. The main basins of the system are situated in ore-rich terrains; the ore occurrences are responsible for the supply of different heavy metals to the bottom sediments forming in the lakes. The chemical and mineralogical composition of these sediments reflects the lithology of the country rocks and is strongly influenced by the surrounding mineral occurrences. Seasonal oscillations occur and consequently the bottom sediments are periodically prone to the action of meteoric waters and the atmosphere. This causes oxidation and remobilisation of most metals present in the sediments, causing them to precipitate and accumulate in the deepest parts of the sedimentary piles., The Flumendosa-Campidano system of artificial lakes is the major water resource for southern Sardinia. The main basins of the system are situated in ore-rich terrains; the ore occurrences are responsible for the supply of different heavy metals to the bottom sediments forming in the lakes. The chemical and mineralogical composition of these sediments reflects the lithology of the country rocks and is strongly influenced by the surrounding mineral occurrences. Seasonal oscillations occur and consequently the bottom sediments are periodically prone to the action of meteoric waters and the atmosphere. This causes oxidation and remobilisation of most metals present in the sediments, causing them to precipitate and accumulate in the deepest parts of the sedimentary piles.

Tetrahedrite-tennantite series minerals occur extensively in hydrothermal deposits, although in minor quantities compared with other sulphides. Quantitative electron microprobe analysis results showed that these minerals exhibit compositional variations in which base metals substitute for Cu, Sb for As, and there is substitution of Te4+ for (Sb, As)3+. Goldfieldite, tellurian tetrahedrite, tennantite and Te-free tetrahedrite coexist in a polymetallic assemblage highly enriched in telluride minerals. The association of hessite, stutzite, sylvanite, petzite, coloradoite and altaite with native Te indicates direct magmatic inputs to the mineralising solutions. A transition of this zoned magmatic hydrothermal system from porphyry to high-sulphidation epithermal mineralisation environments is proposed., Tetrahedrite-tennantite series minerals occur extensively in hydrothermal deposits, although in minor quantities compared with other sulphides. Quantitative electron microprobe analysis results showed that these minerals exhibit compositional variations in which base metals substitute for Cu, Sb for As, and there is substitution of Te4+ for (Sb, As)3+. Goldfieldite, tellurian tetrahedrite, tennantite and Te-free tetrahedrite coexist in a polymetallic assemblage highly enriched in telluride minerals. The association of hessite, stutzite, sylvanite, petzite, coloradoite and altaite with native Te indicates direct magmatic inputs to the mineralising solutions. A transition of this zoned magmatic hydrothermal system from porphyry to high-sulphidation epithermal mineralisation environments is proposed.

At the northern border of the Narcao Rift (SW Sardinia), Oligocene-Miocene calc-alkaline volcanics, both effusive and subvolcanic, are in contact with Cambrian sandstones, shales, and, mainly, carbonate rocks and Eocene continental clastics. The volcanics of andesitic type are accompanied by thermal metamorphism at the contact with limestones. A small Pb mine operates in the Cambrian limestones, along the contact with some of the andesitic dykes. The study of Pb, Zn and Cu contents along the contacts between andesitic dykes and Cambrian limestones showed that Cu behaved differently from Pb and Zn. Cu is proposed to be supplied by the skarn formation phenomena related with the volcanic activity, while Pb and Zn are believed to be already present in the Cambrian limestones. The skarns themselves show character intermediate between the Cu-skarns and the Au-skarns., At the northern border of the Narcao Rift (SW Sardinia), Oligocene-Miocene calc-alkaline volcanics, both effusive and subvolcanic, are in contact with Cambrian sandstones, shales, and, mainly, carbonate rocks and Eocene continental clastics. The volcanics of andesitic type are accompanied by thermal metamorphism at the contact with limestones. A small Pb mine operates in the Cambrian limestones, along the contact with some of the andesitic dykes. The study of Pb, Zn and Cu contents along the contacts between andesitic dykes and Cambrian limestones showed that Cu behaved differently from Pb and Zn. Cu is proposed to be supplied by the skarn formation phenomena related with the volcanic activity, while Pb and Zn are believed to be already present in the Cambrian limestones. The skarns themselves show character intermediate between the Cu-skarns and the Au-skarns.

These bodies, which are related to an ESE-WNW to ENE-WSW fracture system, consist of wide continuous bands of argillic alteration, which contain zones of silicified rock and veins and stockworks of quartz and chalcedony. Pyritisation is often associated with the silicified materials. A few analyses on mineralised samples showed interesting contents of gold, which is microscopically observed as electrum, and silver, which occurs, besides in freibergite and electrum, in various sulphides of the argentite group., These bodies, which are related to an ESE-WNW to ENE-WSW fracture system, consist of wide continuous bands of argillic alteration, which contain zones of silicified rock and veins and stockworks of quartz and chalcedony. Pyritisation is often associated with the silicified materials. A few analyses on mineralised samples showed interesting contents of gold, which is microscopically observed as electrum, and silver, which occurs, besides in freibergite and electrum, in various sulphides of the argentite group.

Sampling of an area of Tertiary volcanic rocks 1500 by 2500m at 100m intervals revealed anomalies in lead, zinc and copper distributions in soils and rocks., Sampling of an area of Tertiary volcanic rocks 1500 by 2500m at 100m intervals revealed anomalies in lead, zinc and copper distributions in soils and rocks.