Lauriéras gold mine

Lauriéras gold mine
	General information about the mine
	Partial view of the Lauriéras gold mine
Mining technology	Open pit
	Information about the mining company
Start of operation	1908
End of operation	2001
	Funded raw materials
Degradation of	Gold / silver / copper / arsenic
Degradation of	silver
Degradation of	copper
Degradation of	arsenic
	Geographical location
Coordinates	45 ° 34 '23 " N , 1 ° 14' 17" E Coordinates: 45 ° 34 '23 " N , 1 ° 14' 17" E
	Location Lauriéras gold mine
Location	Lauriéras
local community	Saint-Yrieix-la-Perche
Department ( NUTS3 )	Haute-Vienne
republic	French Republic
Country	France

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The Lauriéras gold mine is a former gold mine in the French mining district of Saint-Yrieix-la-Perche in the Haute-Vienne department . It belongs to the type of a hydrothermal vein deposit that follows a significant shear zone in the metamorphic basement of the Variscikums . The mineralization occurred in the Upper Carboniferous around 300 million years ago.

etymology

Prehistoric gold prospecting is referred to in French as L′aurière . Lauriéras, sometimes also L'Auriéras , is the Occitan form .

history

Gold was mined in Lauriéras as early as the 5th century BC by the Celtic Gauls . They left a 40,000 cubic meter heap that still has a concentration of 2 to 14 grams of gold per ton. In the neighboring Clovis mine , the Gallic mining front following the mineralized fault can be seen along with overburden. In the lower area even wooden support beams were preserved. With the conquest of Gaul by the Romans under Julius Caesar , gold mining came to a standstill, as the Romans had more profitable deposits in Spain .

After the mining engineer François Ernest Mallard rediscovered Gallic mining in the second half of the 19th century, gold mining in the Limousin was resumed at the beginning of the 20th century. Lauriéras reopened in 1908. During the two world wars, there was a standstill or a sharp decline in activities. After the Second World War, the Societé des Mines du Bourneix , a subsidiary of Cogema , took over the management of the mine in 1988. In 2001, however, Lauriéras was finally closed.

The maximum annual output was 2.54 tons of gold in 1995 for the entire district.

geography

The gold mine is located 6.8 kilometers north-northeast of Saint-Yrieix-la-Perche, to whose municipal area it belongs. It is 5.5 kilometers to La Meyze in the north-northwest . The gold district named after the small town of Saint-Yrieix is located in southern Limousin and covers an area of around 350 square kilometers. It extends from Jumilhac-le-Grand ( Dordogne department ) in the southwest to Meuzac ( Corrèze department ) in the east. The landscape is a leveled hull area with altitudes between 250 and 400 meters.

geology

The gold mine, indexed as number 6.4003 in the national deposit list, is located in paragneiss of the lower gneiss cover ( Briance unit ). This is followed by a 16-kilometer-long, east-north-east trending fault (N 075) dipping at 60 to 65 ° to the north-north-west , the Lauriéras-Puy-Roux structure . This is filled with brecciated quartz over an area of 5 to 11 meters , in which small gold grains are embedded. The adjacent rock can also be completely brecciated near the fault.

The Clovis and Puy Roux mines are located in the immediate vicinity of the same fault . But Lauriéras itself also has several shafts and drives, for example the two shafts Coudert (24 and 54 meters depth), Fernand shaft (72 meters) and Pierre Pinet shaft (17 meters). The longest advance was 180 meters.

Other significant gold mineralized faults in the district include the northeast trending (N 045) Bourneix structure and the east northeast trending Cheni-Nouzilleras structure .

The gold district of Saint-Yrieix-la-Perche forms part of the east-west trending Meuzac anticlinory - a dome-like bulge of the ceiling stack in southern Limousin, which is cut further east by the north-south trending Argentat fault . As a result, the lower gneiss cover appears on the surface over a width of almost 14 kilometers instead of the upper one. The faults falling in the north-west direction not only penetrate the lower gneiss cover, but also the underlying parautochthonous mica slate unit , in which they run out at a depth of just under 10 kilometers. Gravity measurements have shown that the anticlinory is characterized by a negative gravity anomaly, which can be assigned to a granite-migmatite dome with a density of 2550 kilograms / cubic meter. This magmatic dome structure is flat at 20 kilometers and arches to a depth of 9 kilometers. It lies concordantly on a stratified lower crust that reaches down to the MOHO at a depth of 30 kilometers. It is assigned an age of 335 to 330 million years. This intrusion may have primarily caused the arching of the ceiling stack, but its magma may also have been "drawn into" the existing anticline area secondarily.

mineralogy

A total of four quartz generations grow through the fault, two of which are gold-bearing. The gold mineralization of quartz consists of millimeter-sized flakes that can be seen with the naked eye. These can be accompanied by pyrite (FeS ₂ ), arsenopyrite (FeAsS), silver-containing galena (PbS), zinc blende (ZnS) and antimonite (Sb ₂ S ₃ ), rarely also by boulangerite (Pb ₅ Sb ₄ S ₁₁ ) and argyrosis ( Silver nanoparticles). There is also some gold in the arsenopyrite and boulangerite.

In addition to calcite , dolomite , gypsum and goethite still very rare minerals such as appear Akanthit , Beudantit , Bismuthinit , Brochantite , Cosalit , Devillin , Karminit , Pharmakosiderit , Plumbogummit , Posnjakit , Pyromorphite , Skorodit and possibly even bismite , Cannizzarit , Cyrilovit and jarosite .

geochemistry

The gold content in Lauriéras is quite high with average concentrations of 5 to 25 grams per ton (5 to 25 ppm) (peak values in the district of Saint-Yrieix-la-Perche could even reach several kilograms per ton and were then in the per mille range) .

In ancient mining, concentrations of 0.03 to 1.78 ppm were found, which, however, increase to 12.64 ppm in modern open-cast mining.

In modern open-cast mining, three types of ore can be distinguished, which differ in their silver content. Type 1 has 13 weight percent silver in gold and only 4 weight percent silver in tetrahedron . The mineral assemblage of this type is associated with Pb-Sb ± Cu, Fe-thio-salts and comprises Bournonit , Boulangerit, Jamesonit , galena, sphalerite, chalcopyrite together and arsenopyrite. Type 2 is very rich in silver with 49 percent silver in gold and 17 percent silver in tetrahedron. Its sulfide-rich paragenesis consists of sphalerite, chalcopyrite, galena, pyrite and arsenopyrite. Type 3 has 25 percent silver by weight in gold and is dominated by arsenopyrite and pyrite.

Isotope ratios

The gold mineralization of Lauriéras provides the following isotope ratios :

Isotope ratio	Ancient mining	Antique / modern dismantling	Modern dismantling	Secondary rock
²⁰⁶ Pb / ²⁰⁴ Pb	18.133-18.197	18.171 - 18.848	19.045-19.416	19,432
²⁰⁷ Pb / ²⁰⁴ Pb	15.253-15.602	15.579-15.647	15.668-15.699	15.679
²⁰⁸ Pb / ²⁰⁴ Pb	38.291-38.586	38.220-38.683	38.664-38.789	39.762
ε ¹⁰⁹ Ag		- 0.240 - - 0.160
δ ⁶⁵ Cu	- 0.040-0.182	- 0.180-0.124	- 0.060-0.055	- 0.136

For comparison, the values measured in the metamorphic bedrock.

The distinction between gold ores in two groups with different model ages (early Paleozoic and Variscan) is not as clear in Lauriéras as in Bourneix, although here, too, the ores from ancient mining belong to pre-Variscan grouping 1, but mixed samples can fall into both groups.

See the Bourneix Gold Mine for a more detailed discussion of isotope ratios.

Petrology

In the district of Saint-Yrieix-la-Perche, the following quartz generations can be distinguished:

massive white milk quartz - oldest sterile generation
gray, gray-blue to blue microcrystalline quartz - gold mineralization of low to medium grade
white vitreous (hyaline) quartz - main mineralization with pure gold flakes
white quartz in geode form - the latest sterile generation, contains pyrite and amethyst .

Very late breccias can also be cemented with calcite.

In addition to arsenic and silver , small amounts of copper were also produced when the ore concentrate was melted down .

Paragenesis

In general, two levels of paragenesis can be separated in Lauriéras:

In the first stage P 1, the tectonic movements at the shear band are accompanied by the deposition of arsenopyrite and, to a lesser extent, pyrite. Gold substitutes in the accompanying minerals.
the second stage P 2 is of a much more complex nature and is characterized by a gold-lead-silver-copper-antimony sequence. Gold appears dignified in the smallest hairline cracks.

Hydrothermal fluids

These two different parageneses are also reflected in the hydrothermal fluid development that causes them .

Stage P 1 involves H ₂ O-CO ₂ liquids which have organic concentrations of CH ₄ and N ₂ . Homogenization temperatures measured in liquid inclusions range between 260 and 450 ° C. The δ ¹⁸ O values scatter between + 8.0 and + 12.5 ‰ (SMOW). The prevailing pressure is estimated at 0.3 GPa, which corresponds to a depth of around 10 kilometers.

The liquids of the gold-bearing stage P 2 are H ₂ O-NaCl solutions of moderate to low salinity . Their homogenization temperatures are between 160 and 340 ° C, with δ ¹⁸ O values between + 0.3 and 6.0 ‰ (SMOW). The pressure should have been around 0.1 GPa, corresponding to a depth of just under 3 kilometers.

The much hotter hydrothermal solutions of stage P 1 were consequently cooled down on their ascent and further diluted near the surface with cooler, meteoric groundwater . A tectonic elevation of 7 kilometers may also be documented on the basis of the two levels.

Age

A slightly different perspective on the former Lauriéras gold mine

Age determinations with the aid of the potassium-argon method on illite , which had been carried out at several deposits in the district, could prove two different hydrothermal events. The older was 317 million years, while the younger was between 307 and 301 million years. The ascent of the mineralizing fluids should therefore have been pulsating. It is not yet clear whether the lamprophyres in the Saint-Yrieix-la-Perche district , dated 290 ± 5 million years ago, influenced gold mineralization.

Formation of gold mineralization

The gold mineralization in the district of Saint-Yrieix-la-Perche is relatively deep with a depth of 7 to 2 kilometers and is tied to significant crustal faults trending northeast, which dip at 45 to 80 degrees to the northwest. In addition to a normal component, these also show a left-shifting component. Supercritical aqueous solutions with a temperature of more than 374 ° C penetrated the faults. They were enriched in silicon , chlorides , fluorides , sulfates , carbonates and various metals , such as iron , lead , zinc , silver, antimony , arsenic and gold. Gold was dissolved in complex form as AuCl ₂^- , Au (HS) ₂^- and Au (HS) S ₃^- . The solutions were under a pressure of more than 22 MPa. During their ascent along the faults, they cooled down, their pressure sank - but the pH and redox potential depended on the host rock through which the flow was flowing. In general, their solubility potential decreased , which is why the dissolved substances were precipitated. Above all, silica in the form of quartz, various types of sulphides (have a very low solubility product) and, in the case of Limousin, metallic gold.

Gold is a very rare element in the continental crust with an average content of only 1 to 4 milligrams per ton of rock (or 1 to 4 ppb ). In mafic and ultramafic rocks it occurs in a somewhat higher concentration - for example in ophiolites and also in late orogenic lamprophyren, which occur in places in the district. Above all, however, the gold-containing solutions must have undergone a significant enrichment process in order to achieve minable concentrations.

Bouchot and colleagues (2005) see the source of this enrichment in the granulitized , stratified, continental lower crust (depth range 20 to 30 kilometers), which was possibly underlain by mafic mantle magmas. The period of granulation varies between 315 and 300 million years. It is due to the collapse of the thickened Variscan orogen , which caused the elements gold, arsenic, tungsten , tin and rare metals to evaporate from the lower crust. Another possibility would be the granite-migmatite intrusive body in the middle crust, which, however, is likely to be eliminated as a reservoir due to its much older age.

literature

H. Ahmadzadeh: Le district aurifère de Saint-Yrieix (Haute-Vienne). Étude des minéralisations auro-antimonifères dans leur cadre géologique, Thèse de doctorat de troisième cycle (doctoral thesis) . Univ. Clermont II, 1984.
J.-P. Bellot et al. a .: Relationships between As-Au hydrothermal paleofields and crustal scale structures: the case study of the Saint-Yrieix gold field (Limousin, French Massif Central) . In: R. Moritz and V. Bouchot (eds.): Documents du BRGM . vol. 297, 2000, pp. 68-70 .
V. Bouchot, Y. Gros and M. Bonnemaison: Structural controls of the auriferous shear zones of the Saint-Yrieix district, Massif Central, France; evidences from the Le Bourneix and Lauriéras gold deposits . In: Economic Geology . tape 84 , 1989, pp. 1315-1327 .
M. Chenevoy et al. a .: Notice explicative de la feuille Nexon à 1/50 000 . In: Éditions du BRGM . Orléans 1990, ISBN 2-7159-1712-0 .
S. Essaraj, M.-C. Boiron, M. Cathelineau and S. Fourcade: Multistage deformation of Au-quartz veins (Lauriéras, French Massif Central): evidence for late gold introduction from microstructural, isotopic and fluid inclusion studies . In: Tectonophysics . tape 336 , 2001, pp. 79-99 .
J. Nicaud: Contrôle structural de la mise en place des minéralisations aurifères du district de Saint-Yrieix-la-Perche (Massif Central français): analyze de la fracturation, étude des altérations hydrothermales (doctoral thesis) . Université de Limoges, 2001, p. 254 .
J.-C. Touray, E. Marcoux, P. Hubert and D. Proust: Hydrothermal processes and ore-forming fluids in the Le Bourneix gold deposit, central France . In: Economic Geology . tape 84 , 1989, pp. 1328-1339 .

Individual evidence

↑ M. Chenevoy et al. a .: Notice explicative de la feuille Nexon à 1/50 000 . In: Éditions du BRGM . Orléans 1990, ISBN 2-7159-1712-0 .
↑ ^a ^b ^c ^d Vincent Bouchot u. a .: Late Variscan mineralizing systems related to orogenic processes: The French Massif Central . In: Ore Geology Reviews . tape 27 , 2005, pp. 169–197 , doi : 10.1016 / j.oregeorev.2005.07.017 .
↑ J.-P. Bellot et al. a .: Relationships between As-Au hydrothermal paleofields and crustal scale structures: the case study of the Saint-Yrieix gold field (Limousin, French Massif Central) . In: R. Moritz and V. Bouchot (eds.): Documents du BRGM . vol. 297, 2000, pp. 68-70 .
↑ M.-C. Boiron, M. Cathelineau, DA Banks, S. Fourcade and J. Vallance: Mixing of metamorphic and surficial fluids during the uplift of the Hercynian upper crust: consequences for gold deposition . In: Chemical Geology . tape 194 , 2003, p. 119-141 .
↑ J. Nicaud: contrôle structural de la mise en place of mineralization aurifères du district de Saint-Yrieix-la-Perche (Massif Central français): analyze de la fracturation, étude of altérations hydrothermal (PhD) . Université de Limoges, 2001, p. 254 .
↑ M. Chalier, D. Virlogeux and J.-L. Duthou: Les lamprophyres du district aurifère de Saint-Yrieix (Limousin, Massif Central français). Âge Rb / Sr Autunien et relations chronologiques avec le dépôt de l'or . In: CR Acad. Sci. Paris . 319, II, 1994, pp. 1511-1518 .
↑ A. Gebelin: Déformation et mise en place des granites (360 - 300 Ma) dans un segment de la Chaîne Varisque (Plateau de Millevaches, Massif Central) (unpublished doctoral thesis) . Université Montpellier II, 2004, p. 236 .

[1] M. Chenevoy et al. a .: Notice explicative de la feuille Nexon à 1/50 000 . In: Éditions du BRGM . Orléans 1990, ISBN 2-7159-1712-0 .

[Bouchot-2] Vincent Bouchot u. a .: Late Variscan mineralizing systems related to orogenic processes: The French Massif Central . In: Ore Geology Reviews . tape 27 , 2005, pp. 169–197 , doi : 10.1016 / j.oregeorev.2005.07.017 .

[3] J.-P. Bellot et al. a .: Relationships between As-Au hydrothermal paleofields and crustal scale structures: the case study of the Saint-Yrieix gold field (Limousin, French Massif Central) . In: R. Moritz and V. Bouchot (eds.): Documents du BRGM . vol. 297, 2000, pp. 68-70 .

[4] M.-C. Boiron, M. Cathelineau, DA Banks, S. Fourcade and J. Vallance: Mixing of metamorphic and surficial fluids during the uplift of the Hercynian upper crust: consequences for gold deposition . In: Chemical Geology . tape 194 , 2003, p. 119-141 .

[5] J. Nicaud: contrôle structural de la mise en place of mineralization aurifères du district de Saint-Yrieix-la-Perche (Massif Central français): analyze de la fracturation, étude of altérations hydrothermal (PhD) . Université de Limoges, 2001, p. 254 .

[6] M. Chalier, D. Virlogeux and J.-L. Duthou: Les lamprophyres du district aurifère de Saint-Yrieix (Limousin, Massif Central français). Âge Rb / Sr Autunien et relations chronologiques avec le dépôt de l'or . In: CR Acad. Sci. Paris . 319, II, 1994, pp. 1511-1518 .

[7] A. Gebelin: Déformation et mise en place des granites (360 - 300 Ma) dans un segment de la Chaîne Varisque (Plateau de Millevaches, Massif Central) (unpublished doctoral thesis) . Université Montpellier II, 2004, p. 236 .