Tolfa Mts District, Metropolitan City of Rome Capital, Lazio, Italyi

Regional Level Types
Tolfa Mts District	District
Metropolitan City of Rome Capital	Metropolitan City
Lazio	Administrative Region
Italy	- not defined -

The Tolfa Mountains district, located a few kilometres from Civitavecchia and about 60 km NW of Rome, is formed by a hilly mass that rises to 638 meters and is characterized by variable morphology. The landscape is characterised by numerous ranges, mostly covered with coppice or extensive grazing land. The towns of Tolfa and Allumiere are situated on the top of these ranges, which formed following a series of volcanic eruptions in the Pliocene period. The town of Allumiere owes its origins and its name to alum, a mineral substance of world-wide interest during past centuries, obtained from the large alunite deposits here discovered in the 15th century by Giovanni di Castro, a former textile merchant. Tolfa Mountains district is part of the Tolfa-Cerite (Cerveteri)-Manziana volcanic complex, which is considered the southernmost exposure of the Tuscan magmatic province.

The Tolfa-Cerite-Manziana volcanism makes up a multi-centre acid volcanic complex formed by lava flows and domes ranging from quartz-latites to high silica rhyolites. The age is 4.2 to 2.3 Ma (Fornaseri, 1985). Pinarelli (1987 and 1991) and Bertagnini et al. (1995) suggested that the Tolfa-Cerite-Manziana magmatism is generated by crustal melting and variable mixing with mafic potassic liquids. Mafic enclaves show incompatible element patterns similar to those of mafic rocks from Tuscany, and for this reason the Tolfa-Cerite-Manziana volcanic complex is considered the southernmost end of the Tuscan magmatic province (Peccerillo & Donati, 2003).
The three main outcrops are located in the area Tolfa-Allumiere, at Cerveteri, and Manziana, whereas smaller outcrops are present at Tolfaccia, Roccaccia, Montagnola, and Fosso Eri. The volcanic rocks occur as a series of lava domes, often fumarolized and silicified, bearing variable amounts of magmatic enclaves, and forming a NW-SE trending uplifted structure. An explosive volcanic activity, leading to the formation of ignimbritic deposits, now eroded or intensely altered, preceded the emplacement of the lava domes.

Following volcanite emplacements, hydrothermal fluids upwelling through fault pattern metasomatised both the Cretaceous-Palaeogene flysch formation and volcanics. As regards the former terranes, hydrothermal alteration produced a heavy loss of carbonate component, while volcanic rocks were severely kaolinitised, alunitised, and silicified. Mineralising processes generated abundant deposits of phanerocrystalline calcite, while sulphide-minerals were mainly disseminated in hydrothermal calcite as well as altered flysch, only locally being concentrated in veins. The most important sulphide-minerals occurring at Tolfa Mts. are pyrite, galena, sphalerite, and chalcopyrite; subordinate occurrences of other minerals, however, are also reported. Mine workings at Tolfa Mts., commenced in medieval times, have been completely abandoned in the 1950s, since they were uneconomical.

Kaolinite, dickite, and halloysite (both 7Å and 10Å), in decreasing order of abundance, are among the main constituents of the altered acidic lavas and ignimbrites of the Tolfa-Cerite volcanic groups and are often associated (Lombardi & Mattias, 1979; Lombardi, 1984). Alunite is the main component of the hydrothermally-altered volcanics in the Tolfa-Allumiere area. More than 18 million tons of alunite were extracted from the area between 1465 and the beginning of the 20th century.

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References