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| Metallogeny |
| Source of the text : Euromin Projet www.cri.ensmp.fr/euromin/ |
| with the agreement of Lydie Touret from the ENSMPP |
From the geological point of view, the characteristic of various sites of the district is related to the structural geology of the massif and the relative distribution of various gneisses dated from the infra-Cambrian era. Those gneisses are generally divided in two group, the biotite and sillimanite gneisses and the complex gneisses which overlap the first.
The seams were constituted starting from pre-existent fractures in the geological layers. The opening of the fractures, caused by some movement inside the Earth's crust, was supported by the relative rigidity of the rocks: it is the case with the complex gneisses, which contain the most interesting seams for the mining activity (Neuenberg-Altenberg). The more flexible sillimanite and biotite gneisses, did not let appear large faults: thus the seams here are irregular and sinuous (Bluttenberg). Other metamorphic rocks such as leptynites were fractured during the tectonic shortening of the region and many faults were regularly disseminated inside: the seams are not very wide there, but can be very rich (Lorrain side of the district).
Hydrothermal solutions circulated in the fractures and dissolved metal elements contained in the old gneisses. While approaching surface they were cooling and released the dissolved substances which ones concentrated in the faults and became seams. Generally, seams are not made of a single planar structure but are composed of several veins at the origin of seams field. The chronological succession of the deposits was divided into six formation steps:
 step 1: calcite with haematite and chalcedony. This formation step is illustrated by the paragenesis observed in the area of Brézouard
- step 2: essentially cupriferous: carbonates and quartz with "grey coppers" (tetrahedrite-tennantite), chalcopyrite, cubanite and bismuth minerals;
- step 3: native arsenic, arsenides. The transition between formations steps 1 and 2 would be marked by lautite deposition, followed by rammelsbergite
- step 4: galena, sphalerite, "grey coppers" (tetrahedrite-tennantite)
- step 5: fluorite and barite, which settles generally in the upper part of the seams
- step 6: noble carbonates with silver bearing minerals, this formation step is at the origin of the richness and celebrity of the Sainte-Marie-aux-Mines deposits.
The various seams have different directions and paragenesis. They can be gathered in four sectors.
- The Altenberg type seams generally infilled North-South or North-East / South-West faults. They are constituted by silver-bearing galena in a siderite matrix. Several characteristics of the Altenberg sector render possible to establish correlation between this layer and those of La-Croix-aux-Mines as well as with deposits of Black-Forest (Prinzbach and Hauserbach in the Kinzig valley). The age of those deposit could be related to the tertiary era and be contemporary to the Rhine graben collapse.
- The Neuenberg type seams, the mean direction of which range from East-West with east-south-east / west-north-west direction, present the richest minerals paragenesis with 6 successive ore precipitation phases. They occupy the valleys of Rauenthal and Small-Lièpvre.
- The seams of the Lorraine side of the district (Saint-Pierremont, Musloch) are very resembling to the Neuenberg type, but their antimoniferous "grey coppers" (tetrahedrite) are particularly rich in silver.
- The Bluttenberg seams present a mixed and simplified paragenesis, low in silver content, which is characterised by an important oxidation upper zone (iron cap).
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| Last update : 04.01.2002 |
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