Neuil mine
Encyclopedia
The Neuil Mine , in French
Mine de Neuil, is a mine
emplaced in paragneisses of the northwestern Massif Central
. The mine is part of the commune of Saint-Pardoux-la-Rivière
in the Dordogne
. Extracted from the mine was galena
, slightly enriched in silver
.
. The main production shaft, at an elevation of 250 meters above sea level, can be reached by following a small valley on the left side of the D 707 Nontron
- Thiviers
just before arriving at Neuil. The shaft is located on the right-hand side of the small stream; it is fenced in and closed to the public because of its hazardous nature.
The ore was taken to Nègrecombe, washed and then transported by train (on the now redundant railway line Angoulême
-Thiviers) to a smeltering plant.
were mined. The reserves are estimated at several thousand tons.
paragneisses of the Variscan
basement
of the northwestern Massif Central
. The in places tightly folded
paragneisses strike mainly N 130 and dip 35° to the northeast. The ore-bearing lode
s crosscut the strike of the paragneisses, their orientation varying between N 045 and N 065. Their dip
is generally 65° to the north. The lodes are therefore roughly parallel with the boundary fault of the Massif Central north of Saint-Pardoux-la-Rivière.
The lodes of the Neuil Mine are no single occurrence. To the northwest (towards Nontron) several other lodes follow, which were also mined (Le Puy Mine
, Cantonnier Lode
, Tabataud Quarry
among others).
matrix enriched more or less in baryte. The baryte can become very abundant so that it completely replaces the quartz. Lots of fractures cut through the quartz-baryte core. The sulfide
s galena
, sphalerite
and marcasite
are enclosed within the quartz/baryte. The galena forms mats of octahedral crystals, the sphalerite is dense and anhedral and the marcasite powdery or displays its cock's comb habit. The lodes are mylonitized
at the edges; the sulfide mineral aggregates within this zone are sheared. Secondary mineralisations cover the central fracture surfaces and the mylonitic edges. In the core region geode
s of quartz and of chalcedony
can occur. In the quartz geodes the habit of quartz is hexagonal-pyramidal but can show occasional bipyramids. Druse
s in baryte can be clad with gypsum
. Worth mentioning is also the appearance of apple green pyromorphite
forming hexagonal prisms.
The mineralisation is due to hydrothermalism
in the temperature
range 150 – 300°C. The ores were deposited in two stages, each with a characteristic paragenesis
: a higher temperature stage with galena and quartz and a lower temperature stage with baryte and marcasite. These minerals are accompanied by sphalerite, pyrite
and chalcopyrite
. The ore therefore belongs to the mineralisation type sphalerite-pyrite-galena-chalcopyrite.
lead isotope study on the basement lodes yielded an age of 300 to 250 million years BP
(Lougnon, J. et al. 1974). The lodes belong clearly to a late Variscan phase and were defintely formed after the intrusion of the Piégut-Pluviers Granodiorite
into the paragneisses during the late Pennsylvanian
(elsewhere lodes can be found that also formed within the granodiorite
). Yet the metal impregnation is most likely genetically related to the granodiorite.
within the basement rocks. By that time the paragneisses had cooled down to below 300° C. Additionally the lodes experienced shearing motions as shown be the mylonitized edges. This fact combined with the variations in strike possibly hints at a shear zone
oriented southeast: the lodes following N 065 can then be interpreted as antithetic surfaces, whereas the lodes following N 045 are probably R'- Riedel shears. Furthermore the fairly
steep northwesterly dip of the lodes hints at a top to the southeast sense of motion (“bookshelfing”).
The mineralizing hydrothermal solutions most likely came from the Piégut-Pluviers Granodiorite.
French language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...
Mine de Neuil, is a mine
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
emplaced in paragneisses of the northwestern Massif Central
Massif Central (geology)
The Massif Central forms together with the Armorican Massif one of the two big basement massifs in France. Its geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has...
. The mine is part of the commune of Saint-Pardoux-la-Rivière
Saint-Pardoux-la-Rivière
Saint-Pardoux-la-Rivière , in occitan Sent Pardol la Ribiera, is a commune in the Dordogne department in Aquitaine in southwestern France. It forms part of the Parc naturel régional Périgord Limousin.- Etymology :...
in the Dordogne
Dordogne
Dordogne is a départment in south-west France. The départment is located in the region of Aquitaine, between the Loire valley and the High Pyrénées named after the great river Dordogne that runs through it...
. Extracted from the mine was galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
, slightly enriched in silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
.
Geography
The mine is situated about 1 kilometer north of Neuil, a small hamlet in the commune of Saint-Pardoux-la-RivièreSaint-Pardoux-la-Rivière
Saint-Pardoux-la-Rivière , in occitan Sent Pardol la Ribiera, is a commune in the Dordogne department in Aquitaine in southwestern France. It forms part of the Parc naturel régional Périgord Limousin.- Etymology :...
. The main production shaft, at an elevation of 250 meters above sea level, can be reached by following a small valley on the left side of the D 707 Nontron
Nontron
Nontron is a commune in the Dordogne department in Aquitaine in southwestern France.-History:According to the historian Ribault de Laugardière, the name Nontron derives from the Tyrian language, from Nata and Dun...
- Thiviers
Thiviers
Thiviers is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-Personalities:It is notable as being the town in which Jean-Paul Sartre lived as a child. Painter Pierre Bouillon was born there.-References:*...
just before arriving at Neuil. The shaft is located on the right-hand side of the small stream; it is fenced in and closed to the public because of its hazardous nature.
Mine works
The partially collapsed main shaft was once 47.50 meters deep and led to three exploitation levels. An additional horizontal access is now flooded.The ore was taken to Nègrecombe, washed and then transported by train (on the now redundant railway line Angoulême
Angoulême
-Main sights:In place of its ancient fortifications, Angoulême is encircled by boulevards above the old city walls, known as the Remparts, from which fine views may be obtained in all directions. Within the town the streets are often narrow. Apart from the cathedral and the hôtel de ville, the...
-Thiviers) to a smeltering plant.
History
The mine was in operation mainly between 1921 and 1928, between 1956 and 1959 and once more between 1976 and 1984. Several hundred tons of oreOre
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
were mined. The reserves are estimated at several thousand tons.
Geology
The Neuil Mine is emplaced in NeoproterozoicNeoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
paragneisses of the Variscan
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
basement
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...
of the northwestern Massif Central
Massif Central (geology)
The Massif Central forms together with the Armorican Massif one of the two big basement massifs in France. Its geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has...
. The in places tightly folded
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
paragneisses strike mainly N 130 and dip 35° to the northeast. The ore-bearing lode
Lode
In geology, a lode is a deposit of metalliferous ore that fills or is embedded in a fissure in a rock formation or a vein of ore that is deposited or embedded between layers of rock....
s crosscut the strike of the paragneisses, their orientation varying between N 045 and N 065. Their dip
Strike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
is generally 65° to the north. The lodes are therefore roughly parallel with the boundary fault of the Massif Central north of Saint-Pardoux-la-Rivière.
The lodes of the Neuil Mine are no single occurrence. To the northwest (towards Nontron) several other lodes follow, which were also mined (Le Puy Mine
Le Puy Mine
The le Puy Mine is an ancient lead mine in the northwestern Massif Central, France. The mine produced mainly silver-bearing galena.- Geography :The Le Puy Mine, in French Les Mines du Puy, was named after the hamlet Le Puy...
, Cantonnier Lode
Cantonnier Lode
The Cantonnier lode formed in the Piégut-Pluviers Granodiorite, part of the basement rocks of the northwestern French Massif Central. Its importance is highlighted by very rare mineral parageneses.- Geography :...
, Tabataud Quarry
Tabataud Quarry
The Tabataud Quarry, situated in the northwestern French Massif Central, is a very interesting site for minerals. The quarry used to be mined for its granodiorite..- Geography :...
among others).
Mineralogy
The central part of the lode consists of a quartzQuartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
matrix enriched more or less in baryte. The baryte can become very abundant so that it completely replaces the quartz. Lots of fractures cut through the quartz-baryte core. The sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
s galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
, sphalerite
Sphalerite
Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...
and marcasite
Marcasite
The mineral marcasite, sometimes called white iron pyrite, is iron sulfide with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide...
are enclosed within the quartz/baryte. The galena forms mats of octahedral crystals, the sphalerite is dense and anhedral and the marcasite powdery or displays its cock's comb habit. The lodes are mylonitized
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
at the edges; the sulfide mineral aggregates within this zone are sheared. Secondary mineralisations cover the central fracture surfaces and the mylonitic edges. In the core region geode
Geode
Geodes are geological secondary sedimentary structures which occur in sedimentary and certain volcanic rocks. Geodes are essentially spherical masses of mineral matter that were deposited sygenetically within the rock formations they are found in. Geodes have a Chalcedony shell containing...
s of quartz and of chalcedony
Chalcedony
Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic...
can occur. In the quartz geodes the habit of quartz is hexagonal-pyramidal but can show occasional bipyramids. Druse
Druse
Druse can refer to:* Druze, a Middle Eastern religious community* Druse , an aggregation of calcium oxalate crystals found in certain plants* Druse , an incrustation of small crystals on the surface of a rock or mineral....
s in baryte can be clad with gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
. Worth mentioning is also the appearance of apple green pyromorphite
Pyromorphite
Pyromorphite is a mineral species composed of lead chlorophosphate: Pb53Cl, sometimes occurring in sufficient abundance to be mined as an ore of lead. Crystals are common, and have the form of a hexagonal prism terminated by the basal planes, sometimes combined with narrow faces of a hexagonal...
forming hexagonal prisms.
The mineralisation is due to hydrothermalism
Hydrothermal circulation
Hydrothermal circulation in its most general sense is the circulation of hot water; 'hydros' in the Greek meaning water and 'thermos' meaning heat. Hydrothermal circulation occurs most often in the vicinity of sources of heat within the Earth's crust...
in the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
range 150 – 300°C. The ores were deposited in two stages, each with a characteristic paragenesis
Paragenesis
Paragenesis is a petrologic concept meaning an equilibrium assemblage of mineral phases. It is used in studies of igneous and metamorphic rock genesis and importantly in studies of the hydrothermal deposition of ore minerals and the rock alteration associated with ore mineral deposits...
: a higher temperature stage with galena and quartz and a lower temperature stage with baryte and marcasite. These minerals are accompanied by sphalerite, pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
and chalcopyrite
Chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...
. The ore therefore belongs to the mineralisation type sphalerite-pyrite-galena-chalcopyrite.
Age
A radiometricRadiometry
In optics, radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye...
lead isotope study on the basement lodes yielded an age of 300 to 250 million years BP
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
(Lougnon, J. et al. 1974). The lodes belong clearly to a late Variscan phase and were defintely formed after the intrusion of the Piégut-Pluviers Granodiorite
Piégut-Pluviers Granodiorite
The Piégut-Pluviers Granodiorite is situated at the northwestern edge of the Variscan Massif Central in France. Its cooling age has been determined as 325 ± 14 million years BP .- Geographical situation :...
into the paragneisses during the late Pennsylvanian
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
(elsewhere lodes can be found that also formed within the granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
). Yet the metal impregnation is most likely genetically related to the granodiorite.
Conclusion
The spatial arrangement of the lodes points towards local southeasterly extensionExtension
Extension may refer to:* A cheerleading stunt* The building of community capacity by outsiders, for instance agricultural extension* Extension , relating to the pulling apart of the Earth's crust and lithosphere...
within the basement rocks. By that time the paragneisses had cooled down to below 300° C. Additionally the lodes experienced shearing motions as shown be the mylonitized edges. This fact combined with the variations in strike possibly hints at a shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
oriented southeast: the lodes following N 065 can then be interpreted as antithetic surfaces, whereas the lodes following N 045 are probably R'- Riedel shears. Furthermore the fairly
steep northwesterly dip of the lodes hints at a top to the southeast sense of motion (“bookshelfing”).
The mineralizing hydrothermal solutions most likely came from the Piégut-Pluviers Granodiorite.
Literature
- Guillot, P. - L. et al. Feuille Thiviers. Carte géologique de la France à 1/50000. BRGM
- Legrand, N. et al. (2008). Minéralogie des Mines du Nontronnais, Dordogne. Le Règne Minéral, vol. 84, Munich.