Cantonnier Lode
Encyclopedia
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
.
lies about 2 kilometers east of the town of Nontron
, the northern subpréfecture of the Dordogne. It can be reached via a small left side stream of the Bandiat
River; the stream starts at the hamlet of Brégout, flows to the Northwest past the lode and merges just upstream from Moulin de Bord with the Bandiat. Higher up on its left hillslope passes the D 707 from Nontron to Saint-Pardoux-la-Rivière
. The lode is known to be at least 70 meters long ending at an elevation of 185 meters near the left bank of the stream.
. Only a mere 1000 meters farther to the southeast country rocks (plagioclase-bearing paragneisses) are already encountered. The host granodiorite
appears in its darker, finer-grained hornblende facies with amphibole
and biotite
. The rim facies has a much more mafic
chemistry than the ordinary, coarser-grained facies to the North.
As one approaches the lode the granodiorite becomes mylonitized
and is traversed by veinlets covered in pyromorphite
. The lode itself is not wider than 60 centimeters and shows an asymmetric arrangement. The 25 centimeter wide contact zone on the south flank is composed of massive, nonmineralized quartz
. Only some joint
s are covered in pyromorphite and crocoite
. To the north follows a 10 centimeter wide band of white, powdery mylonite very rich in pyromorphite and crocoite. Next comes a 5 centimeter wide quartz band in boxwork facies, that is also very rich in the secondary minerals pyromorphite and crocoite. The crocoite appears skeletal and the pyromorphite acicular. The lode terminates on its north side with a 20 centimeter wide quartz band strongly mineralized in galena
and sphalerite
. Cross-joints show beautiful pyromorphite and crocoite, occasionally also cerussite
.
The Piégut-Pluviers granodiorite formed at the end of the Variscan orogeny in the Massif Central and is dated as Pennsylvanian
( Serpukhovian
) at 325 million years BP. It is assumed that the lode was emplaced during the distensive cooling phase of the granodiorite, supported by a lead isotope date of 300 to 250 million years BP. The Cantonnier lode is not a singular occurrence but rather is accompanied by several other lodes, which were mineralized at the same time. For example the little side stream valley is traversed by a 3 kilometer long, southeast striking lode which was also mined (the so-called Filon des Anciens). The lodes are not restricted to the granodiorite, but can also be found in its hosting paragneisses (lodes of the Le Puy mine
and the Neuil mine
).
, chalcedony
, finely disseminated chalcopyrite
and nickel
-bearing pyrite
(variety bravoite). Primary ore minerals are silver
-bearing galena and sphalerite; traces of native silver have also been found. Of great importance are the numerous secondary ore minerals, amongst them some very rare species. Besides cerussite, crocoite and pyromorphite appear anglesite
, embreyite, hisingerite
and mimetesite. Most likely dundasite
, leadhillite
, vauquelinite
and wulfenite
are also present (need confirming).
. The primary lead-zinc-silver association indicates hydrothermal deposits of a medium temperature
range (300 to 150°C). The secondary minerals must have formed in a cooler environment closer to the surface (oxidation zone).
The mylonites within and near the lode implicate tectonic movements during the emplacement of the lode, most likely shearing due to bookshelving (if one assumes an overall dextral shear zone oriented Northwest-Southeast, which not only affected the country rocks but also the granodiorite)
s (crocoite, embreyite and vauquelinite), arsenate
s (mimetesite) and molybdate
s (wulfenite). Vauquelinite indicates the presence of copper
.
Arsenates and molybdates are not foreign to the Massif Central, which was mineralized in those two element groups at the end of the Variscan orogeny. In certain places the arsenates and molybdates were also accompanied by gold
.
The chromates and copper are rather unusual and pose a problem. Normally chromates are associated with oceanic and ultramafic rocks like serpentinite
s or metagabbros
. The closest occurrence of such rocks can only be found in the Sarrazac massif 25 kilometers farther to the eastsoutheast (near Saint-Paul-la-Roche
, Jumilhac-le-Grand
and Sarrazac
), their geological setting being very different to that of the local granodiorite. Maybe an answer to this dilemma can be found within the dark border facies of the granodiorite itself, which locally can produce very fine, dark, microdioritic
rocks that almost resemble amphibolite
s in their appearance.
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 :...
, part of the basement rocks of the northwestern French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
Massif Central
Massif Central
The Massif Central is an elevated region in south-central France, consisting of mountains and plateaux....
. Its importance is highlighted by very rare mineral parageneses
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...
.
Geography
The lodeLode
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....
lies about 2 kilometers east of the town of 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...
, the northern subpréfecture of the Dordogne. It can be reached via a small left side stream of the Bandiat
Bandiat
The Bandiat is a small river that is 91 km long, a left tributary of the Tardoire. It flows through the Haute-Vienne, Dordogne and Charente departments in France.-Geography:...
River; the stream starts at the hamlet of Brégout, flows to the Northwest past the lode and merges just upstream from Moulin de Bord with the Bandiat. Higher up on its left hillslope passes the D 707 from Nontron to 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 :...
. The lode is known to be at least 70 meters long ending at an elevation of 185 meters near the left bank of the stream.
Geology
The nearly vertical lode strikes eastnortheast (N 065) and is enclosed in the southern rim facies of the Piégut-Pluviers GranodioritePiégut-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 :...
. Only a mere 1000 meters farther to the southeast country rocks (plagioclase-bearing paragneisses) are already encountered. The host 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...
appears in its darker, finer-grained hornblende facies with amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
and biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
. The rim facies has a much more mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
chemistry than the ordinary, coarser-grained facies to the North.
As one approaches the lode the granodiorite becomes 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...
and is traversed by veinlets covered in 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...
. The lode itself is not wider than 60 centimeters and shows an asymmetric arrangement. The 25 centimeter wide contact zone on the south flank is composed of massive, nonmineralized quartz
Quartz
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,...
. Only some joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
s are covered in pyromorphite and crocoite
Crocoite
Crocoite is a mineral consisting of lead chromate, PbCrO4, and crystallizing in the monoclinic crystal system. It is sometimes used as a paint, being identical in composition with the artificial product chrome yellow. It was discovered at Berezovsky deposit near Ekaterinburg in the Urals in 1766;...
. To the north follows a 10 centimeter wide band of white, powdery mylonite very rich in pyromorphite and crocoite. Next comes a 5 centimeter wide quartz band in boxwork facies, that is also very rich in the secondary minerals pyromorphite and crocoite. The crocoite appears skeletal and the pyromorphite acicular. The lode terminates on its north side with a 20 centimeter wide quartz band strongly mineralized in 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...
and 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...
. Cross-joints show beautiful pyromorphite and crocoite, occasionally also cerussite
Cerussite
Cerussite is a mineral consisting of lead carbonate , and an important ore of lead. The name is from the Latin cerussa, white lead. Cerussa nativa was mentioned by Conrad Gessner in 1565, and in 1832 F. S. Beudant applied the name cruse to the mineral, whilst the present form, cerussite, is due to...
.
The Piégut-Pluviers granodiorite formed at the end of the Variscan orogeny in the Massif Central and is dated as 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...
( Serpukhovian
Serpukhovian
The Serpukhovian is in the ICS geologic timescale the uppermost stage or youngest age of the Mississippian, the lower subsystem of the Carboniferous. The Serpukhovian age lasted from 328.3 Ma tot 318.1 Ma...
) at 325 million years BP. It is assumed that the lode was emplaced during the distensive cooling phase of the granodiorite, supported by a lead isotope date of 300 to 250 million years BP. The Cantonnier lode is not a singular occurrence but rather is accompanied by several other lodes, which were mineralized at the same time. For example the little side stream valley is traversed by a 3 kilometer long, southeast striking lode which was also mined (the so-called Filon des Anciens). The lodes are not restricted to the granodiorite, but can also be found in its hosting paragneisses (lodes of the 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...
and the Neuil mine
Neuil mine
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...
).
Mineralogy
Besides the nearly ubiquitous quartz more common minerals in the lode are baryte, calciteCalcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, 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...
, finely disseminated 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...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-bearing 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...
(variety bravoite). Primary ore minerals are 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...
-bearing galena and sphalerite; traces of native silver have also been found. Of great importance are the numerous secondary ore minerals, amongst them some very rare species. Besides cerussite, crocoite and pyromorphite appear anglesite
Anglesite
Anglesite is a lead sulfate mineral with the chemical formula PbSO4. It occurs as an oxidation product of primary lead sulfide ore, galena. Anglesite occurs as prismatic orthorhombic crystals and earthy masses, and is isomorphous with barite and celestine. It contains 74% of lead by mass and...
, embreyite, hisingerite
Hisingerite
Hisingerite is an iron phyllosilicate mineral with formula . A black or dark brown, lustrous secondary mineral, it is formed by the weathering or hydrothermal alteration of other iron silicate and sulfide minerals....
and mimetesite. Most likely dundasite
Dundasite
Dundasite is a rare lead aluminium carbonate mineral. The mineral is named after the type locality, Dundas, Tasmania, Australia. The mineral was first discovered in the Adelaide Proprietary Mine. Dundasite was first described by William Frederick Petterd in 1893.Dundasite is an uncommon secondary...
, leadhillite
Leadhillite
Leadhillite is a lead sulfate carbonate hydroxide mineral, often associated with anglesite. It has the formula Pb4SO422.Leadhillite crystallizes in the monoclinic system, but forms pseudo-hexagonal forms due to crystal twinning. It forms transparent to translucent variably colored adamantine crystals...
, vauquelinite
Vauquelinite
Vauquelinite is a complex mineral with the formula CuPb2CrO4PO4OH making it a combined chromate and phosphate of copper and lead. Discovered in 1818 in the Beryozovskoye deposit, Urals, Russia, it is named after Louis Vauquelin , a French chemist....
and wulfenite
Wulfenite
Wulfenite is a lead molybdate mineral with the formula PbMoO4. It can be most often found as thin tabular crystals with a bright orange-red to yellow-orange color, sometimes brown, although the color can be highly variable. In its yellow form it is sometimes called "yellow lead ore".It crystallizes...
are also present (need confirming).
History
Mining in the Nontronnais goes back to the end of the 17th century. The exploitation of the neighbouring lode Filon des Anciens is documented for the end of the 18th century. At the Cantonnier lode work started during the 1890s. A 75 meter long shaft was excavated in the hillside. But these activities must have been of rather short duration, because already in 1916 it was reported that the shaft had filled in again. In 1965 the BRGM (French mining authority) reopened the shaft and it was at that occasion that the mineral crocoite was discovered. Besides this reconnaissance no further works were undertaken and the mine filled in once more. Today hardly any ore material is left.Formation
The physical conditions reigning during the formation of the lode can be judged by the existing mineral paragenesesParagenesis
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...
. The primary lead-zinc-silver association indicates hydrothermal deposits of a medium 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 (300 to 150°C). The secondary minerals must have formed in a cooler environment closer to the surface (oxidation zone).
The mylonites within and near the lode implicate tectonic movements during the emplacement of the lode, most likely shearing due to bookshelving (if one assumes an overall dextral shear zone oriented Northwest-Southeast, which not only affected the country rocks but also the granodiorite)
Conclusions
The lode clearly consists of a magmatic lead-zinc-association (more precisely a sphalerite-pyrite-galena-chalcopyrite association) enriched in silver. The importance of the Cantonnier-lode lies in the diversity of its secondary alteration minerals, some very rare species being present. Amongst these galena-derived lead minerals we find chromateChromate
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...
s (crocoite, embreyite and vauquelinite), arsenate
Arsenate
The arsenate ion is AsO43−.An arsenate is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As[V]....
s (mimetesite) and molybdate
Molybdate
In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state.The...
s (wulfenite). Vauquelinite indicates the presence of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
.
Arsenates and molybdates are not foreign to the Massif Central, which was mineralized in those two element groups at the end of the Variscan orogeny. In certain places the arsenates and molybdates were also accompanied by gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
.
The chromates and copper are rather unusual and pose a problem. Normally chromates are associated with oceanic and ultramafic rocks like serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
s or metagabbros
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
. The closest occurrence of such rocks can only be found in the Sarrazac massif 25 kilometers farther to the eastsoutheast (near Saint-Paul-la-Roche
Saint-Paul-la-Roche
Saint-Paul-la-Roche, in Occitan Sent Pau la Ròcha, is a commune in the northeast of the Dordogne department in the Aquitaine region in southwestern France...
, Jumilhac-le-Grand
Jumilhac-le-Grand
Jumilhac-le-Grand is a commune in the Dordogne department in Aquitaine in southwestern France.The village lies on the road followed by Richard the Lionheart and on one of the many branches of the Camino de Santiago pilgrim route....
and Sarrazac
Sarrazac, Dordogne
Sarrazac is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-External links:* *...
), their geological setting being very different to that of the local granodiorite. Maybe an answer to this dilemma can be found within the dark border facies of the granodiorite itself, which locally can produce very fine, dark, microdioritic
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
rocks that almost resemble amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s in their appearance.
Literature
- Cuchet, S., Ansermet, S. & Meisser, N. (2008). L'embreyite et l'hisingerite du filon du Cantonnier, Nontron, Dordogne. Le Règne Minéral, vol. 84, p. 23 – 24.
- Laurent, Y. et al.(1967). La crocoïte de Nontron (Dordogne) et les minéraux associés. Bulletin de la Société Française de Minéralogie et Cristallographie, vol. XC, n° 3, p. 377 – 382.
- Legrand, N., Faure, E. & Lebocey, J. (2008). Minéralogie des Mines du Nontronnais, Dordogne. Le Règne Minéral, vol. 84, p. 5 – 22.