Cantonnier passage
Coordinates: 45 ° 31 ′ 37 ″ N , 0 ° 41 ′ 11 ″ E
The Cantonnier Vein is an ore vein in the northwestern Massif Central . It is important for its management of very rare minerals .
geography
The corridor is just under two kilometers east of the city center of Nontron , northern sub-prefecture of the Dordogne department . It can be reached via a left side valley of the Bandiat that descends from the hamlet of Brégout to the northwest and flows into the river above Moulin de Bord . Immediately above the Ganges, the D 707 runs from Nontron to Saint-Pardoux-la-Rivière . The corridor that ends on the left slope of the valley, previously about 70 meters long (known minimum length), is 185 meters above sea level.
geology
The east-north-east (N 075) trending , steep ( angle of fall saiger up to 70 ° to the north) ore has formed in the Piégut-Pluviers-Granodiorite . It belongs to the southern edge of the granodiorite body , only about 1000 meters south-east paragneiss has already been encountered. The bedrock is formed here in the relatively dark, fine-grained hornblende facies and contains amphibole as well as biotite . This marginal facies is much more basic than the bulk of the granodiorite intrusion.
As it approaches the corridor, the granodiorite becomes more and more mylonite and traversed by small pyromorphite veins. The actual vein, which is no more than 60 centimeters wide, is asymmetrical. The approximately 25 centimeters wide contact area on the south side consists of solid, non-mineralized quartz . Traces of pyromorphite and crocoite can only be found along transverse fractures . To the north then follows a white, powdery, 10 centimeter wide mylonite zone, which is very rich in pyromorphite and crocoite. This is followed by a 5 centimeter wide quartz band in boxwork facies , which is also very rich in the secondary minerals pyromorphite and crocoite. The crocodile is formed like a skeleton, the pyromorphite is needle-shaped. The passage closes to the north with a 20 centimeter wide quartz band, which is mineralized with galena ( lead luster ) and amorphous sphalerite ( zinc blende ); beautiful crocoite and pyromorphite are found in transverse fractures. Also Cerussite is occasionally encountered here.
The Piégut-Pluviers-Granodiorite was formed at the end of the Variscan Orogeny in the Pennsylvania , more precisely in the Serpukhovian around 325 million years ago. It is believed that the vein took place during the distensive cooling phase of the granodiorite 300 to 250 million years ago BP. It does not stand alone, but is accompanied by numerous other passages that have also been mineralized. For example, the side valley mentioned above follows a south-east trending, approximately 3 km long, mineralized dike that has been mined in places (so-called Filon des Anciens ). But dikes in the neighboring paragneiss also experienced a comparable mineralization at the same time (e.g. the dikes of the Le Puy mine or the Neuil mine ).
mineralogy
Barite , calcite , chalcedony , finely divided chalcopyrite , quartz and nickel- rich pyrite ( bravoite ) occur in more common minerals . The vein mineralized primarily in the minerals galena (containing silver) and sphalerite; it also has traces of solid silver . However, its secondary minerals, some of which are very rare, are of great interest . In addition to cerussite, crocoite and pyromorphite, the minerals angelsite , embreyite , hisingerite , mimetesite , and very probably also dundasite , leadhillite , vauquelinite and wulfenite (unconfirmed finds) are to be mentioned here.
history
Mining in Nontronnais goes back to the end of the 17th century . Secure mining took place in the last quarter of the 18th century at the neighboring Filon des Anciens . Major work began on the cantonnier corridor in the 1890s. A 70 meter long tunnel was driven into the mountain side. This dismantling phase did not last too long, because in 1916 the plant was reportedly buried again. In 1965 the BRGM (French Mining Authority) had the tunnel reopened. The very rare mineral crocoite was discovered for the first time. Beyond that, however, there was no further dismantling. Today the corridor is filled again. Only minimal remains of ore can now be found in the entrance area.
meaning
The corridor consists primarily of a typical lead - zinc mineralization (association of sphalerite-pyrite-galena-chalcopyrite), which is also enriched in silver. However, the many lead secondary minerals are important, all of which are likely to have arisen from the primary galena through alteration. These include chromates (embreyite, crocoite and vauquelinite), arsenates (mimetesite) and molybdates (wulfenite). Vauquelinite also indicates the presence of copper .
The arsenates and molybdates are not unusual in the Massif Central, which experienced mineralization with these two groups of elements accompanied by gold at the end of the Variscan orogeny .
Atypical, however, is the mineralization of chromates and copper, which is quite difficult to interpret. Chromates are usually bound to oceanic or ultramafic rocks, but they are not present here (granodiorite is an acidic rock). The closest occurrences of serpentinites or metagabbros , for example, are located over 25 kilometers further east-south-east (in the Sarrazac massif of the communities of Saint-Paul-la-Roche , Jumilhac-le-Grand and Sarrazac ) and also belong to a completely different geological context. An answer can possibly be found in the marginal facies of the granodiorite, which in places produces smaller occurrences of very dark rocks ( microdiorites ) that almost border on amphibolites .
Educational conditions
On the basis of the found parageneses , the prevailing conditions during mineral formation can be roughly estimated. According to this, the primary mineralization took place hydrothermally in the middle temperature range (at temperatures from 300 to 150 ° C), the secondary minerals formed in the somewhat cooler, near-surface area.
literature
- S. Cuchet, S. Ansermet, M. Meisser, N .: L'embreyite et l'hisingerite du filon du Cantonnier, Nontron, Dordogne . In: Le Règne Minéral . tape 84 , 2008, p. 23-24 .
- Y. Laurent et al .: La crocoïte de Nontron (Dordogne) et les minéraux associés . In: Bulletin de la Société Française de Minéralogie et Cristallographie . XC, no. 3 , 1967, p. 377-382 .
- N. Legrand, E. Faure, J. Lebocey: Minéralogie des Mines du Nontronnais, Dordogne . In: Le Règne Minéral . tape 84 , 2008, p. 5-22 .