Bourneix gold mine
| Bourneix gold mine | |||
|---|---|---|---|
| General information about the mine | |||
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| Mining technology | Open pit | ||
| Information about the mining company | |||
| Start of operation | 1982 | ||
| End of operation | 2002 | ||
| Funded raw materials | |||
| Degradation of | Gold / silver / arsenic | ||
| Degradation of | silver | ||
| Degradation of | arsenic | ||
| Geographical location | |||
| Coordinates | 45 ° 32 '17.9 " N , 1 ° 5' 5.7" E | ||
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| Location | Cros Gallet | ||
| local community | Le Chalard | ||
| Department ( NUTS3 ) | Haute-Vienne | ||
| republic | French Republic | ||
| Country | France | ||
The Bourneix 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.
geography
The Bourneix gold mine was named after the hamlet of Bourneix that belongs to the municipality of Le Chalard . It is located 2.5 kilometers west-southwest of the center of Le Chalard in the Cros Gallet (also Cro-Galet ) district. It is approximately midway between the town of Ladignac-le-Long 4.4 km to the north-northeast and already the Dordogne belonging Jumilhac-le-Grand 6.2 km further south-southwest. It is around 10 kilometers to Saint-Yrieix-la-Perche in the east-southeast.
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 almost 500 meters. It is cut by the upper reaches of the Isle .
history
Gold mining in Bourneix goes back to pre-Christian times, as the Celtic Gauls were already digging for gold here in the 6th to 1st centuries BC. This has been clearly demonstrated in two of Bourneix's modern open-cast mines. In Les Fouilloux, for example, remains of the wooden support beams have been preserved and in Cros Gallet the old mining front has been exposed. With the conquest of Gaul by the Romans , ancient mining came to a standstill.
The Massif Central was the most productive metal province in the Variszikum of France with sometimes very different deposit types. Gold, silver, lead and tin have been mined since ancient times . In the Limousin, more than 250 prehistoric gold deposits were identified, which were exploited over days in more than 1100 individual drives by the tribe of the Celtic Lemovics . Their cumulative gold production between the 6th and 1st centuries is estimated at 74 tons of gold. This estimate is based on archaeological experiments which suggest an average gold concentration of 20 grams per ton with a 90 percent extraction rate.
In the modern age (19th and 20th centuries), mainly uranium , tungsten , antimony and gold were mined in the Limousin . In total, modern gold mining has produced 45 tons of gold so far. Other industrially important minerals such as barite and fluorite were also of great interest .
In 1910 the first exploration work was carried out on the structure of Cros Callet , which increased significantly in 1969. In 1982, industrial mining began using the open pit method, carried out by Société le Bourneix, which was transferred to Société des Mines du Bourneix - a subsidiary of Cogema - from 1988 .
Bourneix was the last existing gold mine in the gold district of Saint-Yrieix-la-Perche. It was closed in 2002.
geology
introduction
Four main stages of mineralization can be distinguished in the variscosity of France:
- Gold mineralization around 400 million years ago. This Eovariszian stage can be demonstrated primarily in Brittany (in the Saint-Georges-sur-Loire unit in the Armorican massif ) and is characterized by base metal sulfides and electrum .
- Gold mineralization around 360 million years ago. Mesovariate stage. It originated in the basin of the Viseums massive zinc - copper -Blei- barium - sulphide deposits , the locally with precious metals are associated ( Chessy series in Brevenne ).
- Mineralization without gold between 350 and 325 million years. Early Neo-Varisian stage . Best documented in the Cevennes .
- Gold mineralization around 300 million years ago. Late Neo-Varisian stage. The main mass of the French gold deposits fall into this stage, as does the gold district of Saint-Yrieix-la-Perche, which can be characterized geochemically as a gold- antimony mineralization. This was preceded by tungsten mineralization around 320 million years ago.
In the Massif Central, gold deposits are concentrated in three districts - around Saint-Yrieix-la-Perche, around Salsigne and along the fault zone Marche - Combrailles .
description
The Cros Gallet deposit of the Bourneix gold mine crosses mica-rich paragneiss of the lower gneiss cover ( Dronne group ) with flat foliation . It follows a 13 kilometer long northeast trending fault (N 048) dipping 55 ° to the northwest , the Bourneix structure . In addition to the main Cros Gallet deposit, the Les Fouilloux mines in the south-west and Roche Froide, Les Renardières and Mas Vieux further in the north-east are linked to this fault . A small intrusion of the 300 million year old Bourneix leuco granite is occurring on the southeast slope side of the fault towards Bourneix . On the northwest Lying side are leptinites on.
Other significant faults in the district, also mineralized with gold, appear further east - such as the east-northeast trending (N 075) Cheni-Nouzilleras structure and the Lauriéras-Puy-Roux structure with the Lauriéras gold mine running parallel to it .
The mica-rich paragneiss are completely disrupted, brecciated and sericified along the fault . The disturbed zone can be up to 10 meters wide. The principal fault is accompanied on the lying side by a storey of white quartz passages , some of which can be very massive. Late secondary breccias are cemented with calcite . Even now mylonitized , dark, former fault lines run through the fault zone, whereby the mylonitization is older than the cataclase . Due to the late orogenic expansion along the fault zone, quartz lenses formed - the actual carriers of gold mineralization.
The hydrothermal solutions also penetrated the host rock and changed it. A phengite - chlorite association followed by a propylitic chlorite- carbonate association was formed in the mica-rich paragneiss .
In general, the following generations of quartz veins can be distinguished in the district of Saint-Yrieix-la-Perche:
- massive white milk quartz - oldest sterile generation. In Bourneix formation of arsenopyrite and pyrite.
- gray, gray-blue to blue microcrystalline, microsaccharoid quartz - gold mineralization of low to medium grade. In Bourneix formation of arsenopyrite, gold, pyrite, galena and lead sulfosalts.
- white vitreous (hyaline) quartz - main mineralization with pure gold flakes. In Bourneix, in addition to silver-rich gold, formation of sphalerite, galena, tetrahedrite and sulfosalts.
- white quartz in geode form - the latest sterile generation, contains pyrite and amethyst .
mineralogy
The quartz veins are predominantly mineralized with arsenopyrite (FeAsS), followed by pyrite (FeS 2 ) and other sulfides such as galena (PbS), sphalerite (ZnS) and chalcopyrite (CuFeS 2 ) as well as lead-antimony compounds. Solid gold (Au) is associated with arsenopyrite and silver (Ag) is hidden in gold and in the lead-antimony compounds.
More encountered minerals are Boulangerit (Pb 5 Sb 4 S 11 ), Bournonit (PbCuSbS 3 ), ferrihydrite (Fe 5 3+ O 3 (OH) 9 ) leading to the sulfosalts counting minerals Aramayoit (Ag (Sb, Bi) S 2 ), Lindströmmit (Pb 3 Cu 3 Bi 7 S 15 ), Miargyrite (AgSbS 2 ), Owyheeit (Pb 10 Ag 3 Sb 11 S 28 ) and tetrahedrite ((Cu, Fe) 12 Sb 4 S 13 ) as well as solid bismuth (Bi ).
geochemistry
The gold content is regular and of medium concentration, but in rare cases of modern underground mining it can exceed the exceptionally high concentration of 1 kg gold per ton of rock (1005.85 ppm). However, the modern mean concentration in the open pit is very low, fluctuating between 0.15 and 0.66 ppm. In ancient ores, the concentration reached 306.89 ppm.
The silver content in ancient gold is 11 to 19 and reaches 28 percent in modern underground mining. There is also some silver in the tetrahedron. The silver concentration of the ancient mining was 16.3 ppm and that of the modern underground mining was 45 ppm. The modern opencast mine produced only low silver concentrations below 0.5 ppm.
The modern lead concentration in opencast mines fluctuates between 6 and 18 ppm and in underground mines 624 ppm are found. Ancient mining yielded 104 ppm lead.
Isotope ratios
The gold mineralization of Cros Gallet provided the following isotope ratios :
| Isotope ratio | Ancient mining | Modern dismantling | Secondary rock |
|---|---|---|---|
| 206 Pb / 204 Pb | 18.058 | 18.076-19.229 | 18.593-18.752 |
| 207 Pb / 204 Pb | 15,583 | 15,600-15,628 | 15.597-15.608 |
| 208 Pb / 204 Pb | 38.166 | 38.208-38.867 | 38.342-38.478 |
| ε 109 Ag | −0.150 | −0.246 | |
| δ 65 Cu | −0.620 | −0.549 - 0.062 | −0.574 - −0.063 |
For comparison, the values measured in the metamorphic bedrock.
All lead isotope ratios are generally lowest in ancient mining. The values of modern mining are generally higher, show a wide range of variation and include the values measured in the adjacent rock. On the basis of the uranium / lead ratio and the 206 Pb / 204 Pb ratio, two groups can be distinguished:
- Grouping 1 has a U / Pb <1 and very low 206 Pb / 204 Pb ratios of 18.0 to 18.2. Found in ancient and modern underground mining.
- Grouping 2 has a U / Pb> 1 and a 206 Pb / 204 Pb ratio greater than 18.2. Found in modern quarrying and in the bedrock.
The two groups also differ in their model ages - group 1 delivers Cambrian - Ordovician model ages (500 to 430 million years), group 2, however, Variscan model ages.
The gold ores in Cros Gallet thus document a paleozoic , pre-Variscan gold mineralization event. Strangely enough, the ancient mining had clearly preferred this grouping 1, whereas the modern opencast mining followed the Variscan grouping 2.
A coherent mineralogical explanation of these different model ages is the partial reprocessing of the gold-silver ores belonging to group 1 in the Limousin by later hydrothermal fluids as well as their integration into newly formed ore bodies in the course of the Veariscian orogeny. Even if all ores were ultimately formed due to the Variscan tectonics, some still recrystallized from an already existing early Phanerozoic fund.
The gold mineralization of grouping 1 was quite pure and low in silver (including low-silver tetrahedrite), whereas the gold of grouping 2 was enriched in silver (with up to 50 percent silver by weight) and the ores also contain silver-rich tetrahedrite. As a result, the reprocessing process removed significantly more silver than gold from the fund - and, conversely, explains the relative gold enrichment in group 1 (with only 10 percent silver by weight). Compared to group 1, group 2 is enriched in addition to silver in Al, Na, K, rare earths, Y, Zr, Hf, Th, Ga and Nb and in addition to gold in Pb, Zn and Bi - which indicates an albitization process.
Hydrothermal formation
Gold mineralization was carried out using gold-enriched hydrothermal fluids. A two-phase development is generally assumed. In the first phase P 1, carbonic acid-rich solutions with organic enrichments of CH 4 and N 2 (COHN-NaCl system with CO 2 >> CH 4 > N 2 ) percolated the basement. These fluids, known as pseudometamorphic , were not only bound to crustal faults, but also circulated in the crevices of the surrounding metamorphic rocks. They were about 450 to 400 ° C and were initially under a lithostatic pressure of about 0.3 GPa, which corresponds to a depth of 10 to 15 kilometers. Due to the slow uplift of the southern Limousin with the accompanying expansion tectonics, there was a pressure drop of up to 0.08 GPa. At a depth of 5 to 6 kilometers, hydrostatic conditions were reached, allowing relatively cold meteoric surface waters to penetrate, mix with the deep waters and cause cooling to 300 to 350 ° C. The primary mineral deposit was arsenopyrite and pyrite.
The second phase P 2 was characterized by deep, very salty (with more than 15 percent by weight NaCl equivalent) liquids at temperatures above 400 ° C, which document a renewed introduction of heat into the hydrothermal system. These solutions were also cooled by meteoric surface waters at a pressure of 0.05 GPa to 150 to 250 ° C, which initiated the gold mineralization. Deposited paragenesis gold-galena-sphalerite under the guidance of sulfosalts .
See also
- Mining in the Limousin
- Geology of the Limousin
- Geology of the Massif Central
- Lauriéras gold mine
- Salsigne gold mine
- Gold quartz passage
- Lower gneiss cover
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.
- Sandrine Baron et al .: Geochemistry of gold ores mined during Celtic times from the northwestern French Massif Central . In: Scientific Reports, Nature Research . 2019, p. 1-15 , doi : 10.1038 / s41598-019-54222-x .
- 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: Chem. Geol. Volume 194 , 2003, p. 119-141 , doi : 10.1016 / S00092541 (02) 00274-7 .
- V. Bouchot, Y. Gros and M. Bonnemaison: Structural controls on the auriferous shear zones of the Saint Yrieix district, Massif Central, France: evidence from the Le Bourneix and Lauriéras gold deposits . In: Econ. Geol. Band 84 , 1989, pp. 1315-1327 , doi : 10.2113 / gsecongeo.84.5.1315 .
- M. Calli: La mine d'or de Cros Gallet-Le Bourneix (Limousin, France). Géologie, structure, mineralogie et géochimie des concentrations aurifères à As, Pb, (Ag, Sb, Cu). Thèse (doctoral thesis) . Univ. Paul Sabatier, Toulouse 1988, p. 237 .
- J. Gautier, C. Grosbois, A. Courtin-Nomade, JP Floc'h and F. Martin: Transformation of natural As-associated ferrihydrite downstream of a remediated mining site . In: European Journal of Mineralogy . tape 18 , 2006, p. 187-195 .
- PC Guiollard: Les Mines d'or du district de Saint-Yrieix-La-Perche (Haute-Vienne) . PC Guiollard Ed., 1991, pp. 140 .
- 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
- ↑ V. Bouchot, J.-P. Milési, J.-L. Lescuyer and P. Ledru: Les minéralisations aurifères de la France dans leur cadre géologique around 300 Ma . In: Chron. Calc. Min. Volume 528 , 1997, pp. 13-62 .
- ↑ B. Cauuet, C. Tamas, M. and M. Munoz Boussicault: quantites et contrôle de l'or produit à l'âge du Fer de Gaule du Center-Ouest . In: C. Rico and A. Orejas (eds.): Los metales preciosos: de la extracción a la acuñación (Antigüedad - Edad Media) (= Dossier des Mélanges de la Casa de Velazquez, Nouvelle série . Volume 48 , no. 1 ). 2018, ISSN 0076-230X , p. 13-42 .
- ↑ a b M. Souhassou: Les circulations fluid dans le Sud bati Limousin à la fin du Carbonifère: relations entre les systèmes de la faille hydrothermaux d'Argentat et Saint-Yriex; conséquences pour la métallogénie de l'or. Dissertation (doctoral thesis) . Université de Lorraine, 2011.
- ↑ JP Milesi and JP Lescuyer: The Chessy Zn-Cu-Ba massive sulfide deposit and the devonian Brevenne volcano-sedimentary belt (Eastern Massif Central, France) . In: Document du BRGM . tape 224 , 1993, pp. 1-250 .
- ↑ F. Arnaud: Analysis structurale et thermo-barométrique d'un système de chevauchements varisque: les Cévennes centrales (massif central français). Microstructures et mécanismes deformation in the zones de cisaillement schisteuses. Thèse Univ. INPL (doctoral thesis) . Nancy 1997, p. 351 .
- ^ C. Marignac and M. Cuney: Ore deposits of the French Massif Central: insight into the metallogenesis of the Variscan collision belt . In: Mineralium Deposita . tape 34 , 1999, pp. 472-504 .
- ↑ 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: Chem. Geol. Volume 194 , 2003, p. 119-141 , doi : 10.1016 / S00092541 (02) 00274-7 .
- ^ V. Bouchot, Y. Gros and M. Bonnemaison: Structural controls on the auriferous shear zones of the Saint Yrieix district, Massif Central, France: evidence from the Le Bourneix and Lauriéras gold deposits . In: Econ. Geol. Band 84 , 1989, pp. 1315-1327 , doi : 10.2113 / gsecongeo.84.5.1315 .
- ↑ 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 .
- ^ A b Sandrine Baron et al .: Geochemistry of gold ores mined during Celtic times from the northwestern French Massif Central . In: Scientific Reports, Nature Research . 2019, p. 1-15 , doi : 10.1038 / s41598-019-54222-x .