Cantonnier Lode

45.5269444444440.68638888888889Koordinaten: 45 ° 31 ' 37 "N, 0 ° 41 ' 11 " O

The Cantonnier -course is a vein in the northwestern Massif Central. He is significant for his leadership of very rare minerals.

Geography

The walk is less than 2 kilometers east from the city center Nontrons removed, northern sub-prefecture of the department of Dordogne. He can be reached via a left, pulling down from the hamlet Brégout to the northwest tributary of the Bandiat which opens above Moulin de board into the river. Immediately above the Ganges passes the D 707 of Nontron to Saint -Pardoux- la -Rivière. The ending on the valley slopes, previously about 70 meters long corridor ( known minimum length ), located at 185 meters altitude.

Geology

The east-northeast (N 075) trending, steeply dipping ( dip angle Saiger to 70 ° to the north) vein has formed in Piégut -Pluviers granodiorite. He is on the southern edge of the Granodioritkörpers, paragneiss are only about 1000 meters southeast already encountered. The parent rock is here in the relatively dark, fine-grained Hornblendefazies trained and performs alongside biotite and amphibole. These Marginal Facies is much more basic than the bulk of the Granodioritintrusion.

In approaching the transition body of granodiorite is increasingly mylonitisert and crossed by Pyromorphitäderchen. The actual, not more than 60 cm wide vein is an asymmetrical structure. The approximately 25 -cm-wide contact area on the south side is made of solid, unmineralized quartz. Only along transverse fractures can be found in it traces of pyromorphite and Krokoit. To the north is then followed by a white, powdery, 10 centimeters wide Mylonitzone, which is very rich in pyromorphite and Krokoit. This is followed by a 5 centimeter wide quartz band in Boxwork facies connects to, which is also very rich in secondary minerals pyromorphite and Krokoit. The Krokoit is formed skeletal, the Pyromorphit acicular. The course closes to the north with a 20 centimeter wide quartz band centering on galena ( galena ) and amorphous sphalerite ( zinc blende ) mineralized; is found in the transverse fractures beautiful Krokoit and pyromorphite. Also Cerussite is found here occasionally.

The Piégut -Pluviers granodiorite emerged at the end of the Variscan orogeny in the Pennsylvanian, more precisely in Serpukhovian before about 325 million years ago. It is assumed that the transition during the cooling phase of the granodiorite distensiven in front of 300 million to 250 million years BP took place. He does not stand alone but is accompanied by numerous other passages, which were also mineralized. Thus, for example, follows the above-mentioned side valley a southeast - trending, about 3 km long mineralized lode, which was mined in places (so-called Filon des Anciens ). But transitions in the neighboring paragneisses learned at the same time a similar mineralization ( eg the transitions of Le Puy- mine or the Neuil mine ).

Mineralogy

At more ordinary minerals occur barite, calcite, chalcedony, finely divided chalcopyrite, quartz and nickel -rich pyrite on ( Bravoit ). The transition mineralized primarily to the minerals galena ( argentiferous ) and sphalerite; He also leads traces of native silver. However, are of great interest especially his part very rare secondary minerals. These include, in addition to cerussite, pyromorphite Krokoit and the minerals anglesite, Embreyit, Hisingerite, Mimetite, and very likely also Dundasit, Leadhillit, Vauquelinite and Wulfenite (still unconfirmed finds).

History

The mining industry goes back in Nontronnais to the late 17th century. Assured degradation took place in the last quarter of the 18th century on the adjacent Filon des Anciens. On Kantonnier -course was started in the 1890s with larger works. A 70-meter tunnel was driven into the mountain side. However, this reduction phase did not last very long, because in 1916 the plant was reportedly already spilled again. 1965 was the BRGM ( French mining authority ) open the new tunnel. Here, the very rare mineral Krokoit was discovered for the first time. In addition, however, there was no further reduction more. Today, the gear is filled again. In its entrance are now only remains to find minimum of ore.

Importance

The course consists primarily of a typical lead-zinc mineralization ( socialization sphalerite -pyrite - galena, chalcopyrite ), which is also enriched in silver. Of importance, however, are the many lead- secondary minerals that are likely to be all emerged through alteration of the primary galena. These include chromates ( Embreyit, Krokoit and Vauquelinite ), arsenates ( Mimetite ) and molybdates are ( Wulfenite ). By Vauquelinite the presence of copper is also displayed.

The arsenates and molybdates are not uncommon in the Massif Central, the learned mineralization with these two groups of elements in the company of gold to the Variscan orogeny output.

Unusually, however, the mineralization of chromates and copper, which is quite a difficulty of interpretation. Chromates are usually bound to oceanic ultramafic rocks or, although not present in this case ( the granodiorite is one of the acidic rocks ). The nearest deposits of example serpentinites or Metagabbros are over 25 kilometers further ostsüdostwärts ( in Sarrazac massif of the communes of Saint -Paul -la -Roche, Jumilhac -le- Grand and Sarrazac ) and also belong to a completely different type of geological context. One answer may be found in the Marginal Facies of the granodiorite, which places smaller deposits of very dark, bordering almost on amphibolite rocks ( Mikrodiorite ) produces.

Formation conditions

On the basis of the found parageneses can be the ruling in the mineral formation conditions to estimate roughly. Accordingly, the primary mineralization was hydrothermally in the medium temperature range (at a temperatures of 300 to 150 ° C), the secondary minerals formed in the cooler, near-surface region.