Aux origines des techniques minières. L’exploitation d’un gisement filonien au Premier Âge du fer. Les mines de Silter di Campolungo et de Baita Cludona di Fondo (Val Camonica, Alpes lombardes, Italie)

Résumé En Fr

Val Camonica is well known for its large and exceptional quantity of rock carvings, but it is also an area rich in metallic ores. Two groups of mining works have been explored and are the subject of this paper. The mining areas of Silter di Campolungo and of Baita Cludona di Fondo are in Val Grigna, a small hanging valley of glacial origin. It lies in the territories of the communities of Bienno and Collio. The mining area of Val Trompia to the east of Val Camonica can be reached through Val Grigna and the Goletto di Cluodona pass. The distance between the two groups of mines of Silter di Campolungo to the north and Baita Cludona di Fondo to the south is 2 km as the crow flies. The bedrock in the area is formed by a sequence of siliceous sediments belonging to the Collio formation, lower Permian in age. They are layers of massive sandstones and conglomerates with a sandstone matrix of various thicknesses (from decimetres to metres). This formation is crossed by faults which are mineralized with 15-20 cm thick veins of chalcopyrite-bearing white epigenetic quartz. These two copper mines, which have been dated to the Early Iron Age, were the subject of archaeological exploration between 1997 and 2002. Their study has shown some mining techniques previously unknown for that age. The mining activity at Campolungo begun as an opencast mine with the excavation of deep trenches where the ore was visible on the surface. These works begin at 1535 m above sea level and reach an attitude of 1635 m. At 1535 m there is the opening of the drainage adit while the higher altitude corresponds to the last of the opencast works. In order to reach the deepest copper lodes of Campolungo the ancient miners dug an adit about 120 m long. In this way they were able to connect the deepest excavations of the mine and to create an excellent permanent drainage system. The rock was dug by fire setting and by using heavy spherical lumps of quartz and sandstone picks. To carry the ore out of the mine large wooden rectangular sleighs were used, one of which was discovered near the rock face. It points to the organized management of the underground transport. Among the tools found in the mines there are polygonal or spherical quartz mauls and sandstone picks. Many fragments of resinous wood torches were also found in the underground galleries. Underhand and overhand stoping were used to extract the ore. Special timbering techniques were used to help with the management of the working space and to reach the different working areas easily. The spoil dumps underground show that a first sorting was performed in the mine itself. Prospecting in the upper part of Val Grigna allowed us to discover a second copper ore deposit at an altitude of between 1620 and 1670 m above sea level. On the eroded surface on the left-hand side of the stream and at the foot of a rocky cliff there are three ore veins, two of which are more or less parallel. The veins were worked by underhand stoping from the surface. The pollen samples from the two peat bogs close to the mining sites show the variation of the forest cover in the area. Forest clearing began during the Early Iron Age at about 900 BC. These data confirm that the mining activities at Campolungo are contemporary with those at Baita Cludona di Fondo. The numerous charcoal pits of the area testify a rich charcoal production, which could be the reason for ancient deforestation. Possibly this charcoal was used in the ancient furnaces and forges in the nearby Val Gabbia or lower down in Val delle Forme. In both the mining areas of Silter di Campolungo and Baita Cludona di Fondo specific places were used for the mine spoil dumps and for the primary ore dressing activities : sorting, washing, crushing and grinding. One of these sites on the right bank of the Grigna stream was thoroughly excavated. On this site many stone tools were found : hammers, mortars and millstones. Some of these tools were made from imported rocks such as vulcanite from Mt. Auccia, 3 km away as the crow flies. More often vein quartz and the hard siliceous sandstone from the mine itself were used to make these tools. The fragment of a tool interpreted as a rotary grinding stone was found too. A few sherds dating from the 6th-5th century BC were also discovered on this site. A small amount of copper slag in secondary deposition was found too. Its study and analyses have shown that it is connected to copper matte production. These mining works are not the remains of “ informal” mining activities, but show the high level of technical knowledge which allowed the ancient miners to extract the ore from the deepest lodes. The mine structures discovered at Campolungo are exceptional, the underground works allowed an excellent exploitation of the mineralized veins. This means that the mining operations were coordinated and directed by some authority. The way the galleries were dug and their plans point to the knowledge of precision measuring systems. The Campolungo and Baita Cludona di Fondo mines show that during the Early Iron Age there were already infrastructures and perfectly organized technical assistance for this sort of enterprise. These infrastructures imply the existence of a central authority with economic and political power sufficiently developed to organize a mining operation on such a scale in such a remote place. Such power can only exist in a highly hierarchized society. The Campolungo and Baita Cludona mining area is one of the very few places of Val Camonica devoid of any prehistoric rock-carvings. This fact could be connected with the mining activities ; the carving of rocks may have implied some magical/ religious meanings in contrast with the ore extraction activities. Because of this the hypothesis of a land division in relationship with the exploitation of natural resources must be taken into consideration.

Les zones minières de Silter di Campolungo et de Baita Cludona di Fondo sont situées dans le Val Grigna, qui met en communication le Val Camonica avec le Val Trompia par le col de Goletto di Cludona et se situent sur les communes de Bienno et de Collio. Deux kilomètres séparent les deux sites. Dans cette région, la roche encaissante est constituée de sédiments détritiques de la formation de Collio, d’âge Permien inférieur, traversés par des failles minéralisées. Datées du Premier Âge du fer, ces deux mines de cuivre ont fait l’objet de plusieurs campagnes de recherche. À Campolungo, les travaux s’échelonnent sur une dénivelée de 100 m entre 1 535 m et 1 635 m. Pour atteindre les filons minéralisés, les mineurs ont creusé un travers-banc d’exhaure de près de 120 m de développement, mettant en relation les chantiers profonds et assurant en permanence l’exhaure. L’abattage associait la taille au feu et l’utilisation de maillets et de masses en quartz ; la présence d’une galerie d’aérage témoigne de cette méthode d’exploitation. La mine de Campolungo est ainsi dotée d’une infrastructure et d’ouvrages techniques d’assistance parfaitement planifiés et remarquables pour une époque aussi reculée. Pour transporter le minerai, les mineurs utilisaient des traîneaux en bois monoxyles dont un exemplaire a été découvert. Le mobilier retrouvé atteste l’utilisation par les mineurs de maillets de forme sphérique ou polygonale et de pics en roche. De très nombreux fragments de torches ont été également découverts. Un soutènement en bois a été installé pour favoriser l’accès aux chantiers. Les stériles stockés sous terre témoignent d’un premier tri sélectif à l’intérieur de la mine. En amont du Val Grigna, un deuxième gîte cuprifère s’étage entre 1 620 m et 1 670 m d’altitude. Trois filons, sensiblement parallèles, se développent sur la surface d’érosion qui modèle la rive gauche du torrent. L’extraction s’effectuait par dépilage descendant. Des carottages polliniques effectués dans deux tourbières proches mettent en évidence plusieurs phases de déforestation dont l’origine remonte au début de l’Âge du fer (vers 900 av. J.-C.). L’activité minière à Campolungo était sensiblement contemporaine de celle de Baita Cludona di Fondo. Les déforestations ont été provoquées par la production de charbon de bois qui alimentait les fourneaux et les forges construits en amont du Val Gabbia, et en aval dans le Val delle Forme. En surface, les mines de Silter di Campolungo et de Baita Cludona di Fondo sont jalonnés de haldes et d’espaces aménagés où s’effectuaient le tri, le concassage et le broyage du minerai. La fouille d’un atelier de minéralurgie a livré de nombreux artefacts : percuteurs, pierres à cupules, mortiers et meules en pierre. Un fragment de moulin rotatif figure parmi le mobilier ainsi que plusieurs tessons de céramique datés des VIe-Ve siècles av. J.-C. Quelques scories de cuivre ont été recueillies en position secondaire. Le processus de réduction utilisé s’apparente à la production de matte (nonslag tapping furnaces). Dans cette région couverte de gravures rupestres comme le Val Camonica, le district minier de Campolungo figure comme l’un des très rares espaces fréquentés durant la Préhistoire où les gravures sont inexistantes. L’hypothèse d’une répartition territoriale en relation avec l’exploitation des ressources minérales est une hypothèse à privilégier.