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Sandro Fadda, Consiglio Nazionale Ricerche (Italy)
Maddalena Fiori, Consiglio Nazionale Ricerche, CNR (Italy)
Silvana Maria Grillo, Cagliari University (Italy)
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The island of Sardinia has been for centuries the most important mining region of Italy and one of the most important in Europe largely owing to deposits associated with Paleozoic rocks. In the late 80's the potential of the Tertiary volcanics as a host for base- and precious metal mineralizations has been fully appreciated. During the waning phases of Oligocene-Miocene volcanism, hydrothermal ore-forming activity occurred at different times and places producing alteration as well as ore bodies of various types and sizes, including epithermal mineralizations of low- and high-sulfidation type. In the Furtei area a high-sulfidation gold-copper epithermal system occurs that is the first and only modern economically viable gold deposit discovered in the island. During the development of prospecting studies on epithermal phenomena associated with Tertiary volcanism, a group of mineralized bodies was discovered in calcalkaline volcanics in the Osilo area, northwestern Sardinia. The general characteristics of the mineralised bodies, which have an overall vein shape and the distinctive occurrence of adularia and illite in the alteration assemblage assigns the Osilo mineralization to the adularia-sericite, low-sulfidation class of epithermal deposits. Exploration to date has identified five principal vein areas. Economic mineralisation is known to occur over a 250-300 m vertical range. The current resource for the district stands at 1.66 Mt grading 7.1 g/t Au and 39 g/t Ag, for a resource of 376.000 ounces of gold and 1.58 million ounces of silver. Precious metal mineralization at Osilo occurs as electrum and argentite group minerals which are the main gold and silver minerals deposited in epithermal veins. The mineralization associated with the major Au-Ag bearing veins, have been broadly divided into two stages. The first is represented principally by massive infill of microcrystalline quartz, with local occurrences of crustiform colloform banded material containing alternating layers of chalcedony, quartz and sulfides. The second stage is formed by pockets of hydrothermal breccia, clasts are angular to round and comprise, in places hydrothermal quartz from the first stage. The ore zone opaque minerals consist mainly of disseminated pyrite, with lesser amounts of As-rich pyrite, marcasite with minor chalcopyrite, stibnite, sphalerite, tetrahedrite, covellite, bornite, argentite group minerals, galena, electrum and native silver. Tetrahedrite is mainly represented by Ag-rich tetrahedrite where silver, substituting for Cu, shows a broad compositional range from low silver bearing terms 13 wt.% up to values of 20.75 wt.% (freibergite). The Osilo area appears to be the most promising among the low-sulfidation occurrences. Gold responds to conventional gold ore processing methods for this type of mineralisation, as practiced at the Furtei mine with the oxide ores, and may yield gold recovery in the range of 90%.
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