Extractive geo-sites in the high Sessera Valley (Biella, Italy)

In the paper ruins of past extractive activities are proposed as geo-sites in the high Sessera Valley (Italian western Alps). In this relatively small area (9000 hectares), two of the principal alpine geostructural domains outcrop. On the west, the Australpine Domain is represented by the Sesia-Lanzo metamorphic rocks (gneisses and micaschists) intruded by magmatic rocks (essentially monzonites in this area) of the Biella pluton. In the eastern side of the area, gabbros and diorites of the the diorite kinzigitic zone (Ivrea Verbano) represent the Sudalpine Domain. A grandiose andesite dick directed N-S intruded in tertiary age in correspondence the Canavese Line separate the two domains. The presence of silver mineralizations formed in the terminal parts of the magmatic intrusion in the eclogitic micaschists brought to the mining exploitation since the 13th Century. The Sessera Valley mines were quite important for the Savoy State economy, essentially because of the extraction of minerals associated with silver veins. The principal mines of the zone were the mine of Costa l’Argentera, probably developed through several levels of mining tunnels, and those situated near the Alpe Piovale whose entrance is regrettably obstructed by landslide debris. In these sites is yet possible to observe rests of galleries, mining dumps, stone pipelines, mule tracks and manufactures for metal working. The cycle relative to the old mineral working can be synthesized in three steps: mineral extraction, mineral preparation an metal treatment. The transformation industry of the extracted mineral was situated down the mines, in proximity of the Sessera river. After its extraction, the mineral was prepared by a group of specialized people through the following four operations: a) crushing and selection; this operation was made just down the mines where dumps can be yet observed; b) roasting: the rests of an ancient oven can be still observed near the Alpe Rondolere about one kilometer downwards along the Sessera river; the structure is 3.15 m high and 1,70 m large; c) washing: after the firing, the material was stored in pools for separating the impurities; two washing basins complete with alimentation channels are located near the Alpe Rondolere; d) reduction in metal of minerals: this operation was made by a melting furnace in presence of coal in order to reduce the iron oxides in pure iron. This activity, whose traces are not founded in the zone, was probably made near Piana del Ponte, where ancient documents reveal the presence of furnaces in locality Fonderia (Foundry) della Piana. The obtained iron could be worked in a forge through a hammer operating by water power. The rests of such a tool can be found near the Alpe Rondolere, downwards the washing basin. It is desiderable that these sites will be protected and appreciated in order to keep alive the historical and cultural traditions of the Sessera Valley.