Epithermal precious metal deposits of Italy - an overview

Epithermal precious metal deposits have only quite recently been added to t he metallogenic view of Italy. It was only in the late 1980s that two distr icts were recognized, in Sardinia and southern Tuscany, previously long kno wn for a large variety of other commodities. The Sardinian epithermal preci ous metal district is associated with the Oligocene-Miocene Sardinian calc- alkaline magmatic cycle. The most relevant areas include the Au deposit at Furtei, already in production, and the Osilo prospect, where extensive expl oration is under way. The deposit at Furtei contains at least six tonnes of gold metal. The mineralogy of the deposit and of the alteration assemblage s is typical of the acid-sulfate (high-sulfidation) class of volcanic-hoste d epithermal deposits. Fluids associated with alteration and mineralization have moderate temperatures (200-300 degrees C) and low salinities (less th an 6% wt. NaCl equivalent); high-temperature, high-salinity fluids similar to porphyry-style systems also circulated at Furtei. At Osilo, a number of quartz veins containing up to several ppm Au have been identified. The alte ration assemblage includes adularia and illite, i.e. is typical of low-sulf idation deposits. Fluid inclusion homogenization temperatures range from 19 8 to 270 degrees C, with salinities of less than 4% wt. NaCl equivalent. In southern Tuscany, a number of carbonate-hosted ("Carlin type") gold show ings occur at the edges of the geothermal fields of Larderello, Amiata and Latera, an area previously known for Sb mineralization. In fact, many showi ngs coincide with former Sb mines, and stibnite, along with pyrite, is the most common sulfide mineral. Gold is typically invisible. Fluids hosted in a variety of minerals from these occurrences span a relatively large temper ature range (132 to 245 degrees C), with constantly low salinities (less th an 7% wt. NaCl equivalent). The onset of (presumably meteoric) hydrothermal fluid circulation can be ascribed to the emplacement of Neogene Tuscan mag matic rocks, but the ultimate source of gold remains speculative.