Tectonic evolution of the Cevennes para-autochthonous domain of the Hercynian French Massif Central and its bearing on ore deposits formation

2001 ◽  
Vol 172 (6) ◽  
pp. 687-696 ◽  
Author(s):  
Michel Faure ◽  
Xavier Charonnat ◽  
Alain Chauvet ◽  
Yan Chen ◽  
Jean-Yves Talbot ◽  
...  

Abstract The Cevennes area belongs to the para-autochthonous domain of the Hercynian Belt of the French Massif Central. Three lithological series, namely: sandstone-pelite, black micaschist and gneiss-micaschist, are identified. They form an imbrication of five tectonic units which overthrust the unmetamorphosed Viganais Paleozoic units to the south and the gneissic Mamejean Unit to the north. The structural, metamorphic and magmatic evolution of the Cevennes area is characterized by three events, namely: (1) southward shearing coeval to a MP/MT metamorphism dated around 340 Ma; (2) post nappe anatexis (T<750 degrees C, P>5 kb); (3) Namurian (ca 315 Ma) E-W extensional tectonics and plutonism. The structure of the Mt-Lozere-Borne granitic complex is constrained by new AMS and gravimetric data. The plutons are the driving power of the hydrothermal convective circulations responsible for an early deposition of diffuse arsenopyrite in the thermal aureole. Gold bearing sulfides are afterwards concentrated in quartz veins along brittle normal and wrench faults around the granite. Lastly, ore bearing quartz pebbles are sedimented in the Stephanian Ales coal basin.

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Sandrine Baron ◽  
Călin G. Tămaș ◽  
Marion Rivoal ◽  
Béatrice Cauuet ◽  
Philippe Télouk ◽  
...  

AbstractThe Celtic culture of Western Europe left magnificent gold objects, such as jewellery and weapons from nobility graves and hoarded coins, as well as field evidence of pre-Roman gold mining and metallurgical workshops that attest to the mining of local ores. This is the case of Central France where many precious metallic ores have been mined throughout the ages from the Prehistoric times onwards. One of the lingering problems in assessing the provenance of gold artefacts and coins is the lack of relevant data on the isotope geochemistry and mineralogy of ore sources. Forty gold ores samples were collected and studied from Limousin (French Massif Central), a very significant gold mining district from the Celtic times. Their Pb isotope compositions clearly show a local dichotomy i.e. two distinct groups of ores, one of Late Proterozoic to Early Paleozoic Pb model age and another associated to Variscan ages and consistent with field relationships, mineralogy and elemental analyses. The use of Cu and Ag isotopes, and their coupling with Pb isotopes, will refine the tracing of future metal provenance studies, but also highlight some metallurgical practices like deliberate metal additions to gold artefact or debasement of gold coins. The newly acquired Pb, Ag, and Cu isotopic data on gold ores improves our understanding of ore deposits geology and provide clarifications on the provenance of Celtic gold from this area and its economic importance.


2019 ◽  
Author(s):  
Oswald Malcles ◽  
Philippe Vernant ◽  
Jean Chéry ◽  
Pierre Camps ◽  
Gaël Cazes ◽  
...  

Abstract. The evolution of intra-plate orogens is still poorly understood. Yet, this is of major importance for understanding the Earth and plate dynamic, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m, with the highest peak elevations ranging from 1500 m to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geophysical (i.e. geodesy and seismology) data. Because the Cévennes margin allows the use of karst sediments geochronology and morphometrical analysis, we study the vertical displacements of that region: the southern part of the French Massif-Central. Geochronology and morphometrical results, helped with lithospheric-scale numerical modelling, allow, then, a better understanding of this intraplate-orogen evolution and dynamic. Using the ability of the karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age (10Be/26Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83.4 +17.3/−5.4 m Ma−1 during the last ca 4 Myrs (Plio-Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the north and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from dating technics and are consistent with the southward tilting obtain from morphological analysis. We presume that the remaining part is related to dynamic topography or thermal isostasy due to the Massif Central plio-quaternary magmatism.


2002 ◽  
Vol 173 (3) ◽  
pp. 207-218 ◽  
Author(s):  
Philippe Olivier ◽  
Laurent Améglio

Abstract Introduction. – The Variscan basement of the French Massif Central is considered [Faure, 1995] to have suffered (i) a southwestward nappe stacking during the early Carboniferous ; (ii) a NW-SE trending extension during the late Visean and granite emplacement during Namurian and Westphalian time ; (iii) a NE-SW trending extension during late Carboniferous-early Permian. The structure of the Veinazès and Marcolès monzogranitic plutons, located in the Châtaigneraie (southern French Massif Central), is studied in this paper through the anisotropy of magnetic susceptibility technique (AMS) to better understand the effects of the Namurian-Westphalian phase which is poorly documented in this region. Magnetic and structural study. – The long axis K1 of the AMS ellipsoid represents the magnetic lineation and the short axis K3 is normal to the magnetic foliation. This magnetic fabric may be equated with the mineral fabric. Km, the mean magnetic susceptibility, corresponds to the arithmetic mean (K1+K2+K3)/3. Pp %, the magnetic anisotropy ratio, corresponds to ((K1 – D/K3 – D) – 1) × 100, D being the diamagnetic component. Magnetic susceptibility (K) values (fig. 3 ; table I) range from 3.5 to 18.9 × 10−5 SI (mean 12.3 × 10−5 SI) for Veinazès, and from 0.7 to 13.1 x 10−5 SI (mean 8.1 × 10−5 SI) for Marcolès. Such values are typical of dominantly paramagnetic rocks. This is confirmed by the very good correspondence between these measured values and the magnetic susceptibilities calculated from sites for which chemical analyses are available (table II). For Veinazès the highest K values are mainly located in the western part of the pluton, whereas the weakest are in the northeastern part (fig. 3). This corresponds roughly to the distribution of the main petrographic facies determined by Joubert [1978] and Bogdanoff et al. [1989a]. The anisotropy of the magnetic susceptibility ratio (Pp %) (fig. 4 ; table I) ranges from 0.9 % to 5.3 % (mean 2.2 %) for Veinazès and from 0.8 % to 4 % (mean 2.2 %) for Marcolès. The part of the Veinazès pluton located to the south-west of the Sansac-Montsalvy line displays a mean anisotropy (2.7 %) clearly higher than the northeastern part of the pluton (1.6 %). For Marcolès, the map of Pp % does not display significant distribution heterogeneities. Three types of microstructures were determined in thin-sections in the Veinazès and Marcolès granites (fig. 5) : 1–undeformed magmatic textures ; 2– late magmatic weakly deformed textures ; 3– late magmatic deformed textures. There is a good correspondence between the mapped distribution of these microstructures (fig. 6) and the Pp % values (fig. 4). Magnetic foliations (fig. 7 & 8 ; table I). Veinazès : to the north-east of the Sansac-Montsalvy line, the foliations are roughly organized in a dome. To the south-west of this line, the foliations are mainly WNW-ESE trending with a medium to steep northward dip. Near the borders of the pluton the foliations become WSW-ENE. Marcolès : the foliations are concentrated around a NW-SE trending mean direction, with generally steep (≥ 60o) and northeast dips. Near the extremities of the pluton the foliations are closer to an E-W trend. In both plutons the pattern of foliations tends to form Z-sigmoids. Magnetic lineations (fig. 7 & 9 ; table I). Veinazès : to the north-east of the pluton, the lineations display generally shallow plunges and N-S trending directions. In the central part of the pluton they display very shallow plunges and are mainly E-W trending. To the west, the lineations display shallow to steep plunges mainly trending either E-W or NW-SE. Marcolès : most lineations are NW-SE to E-W trending, with middle SE or eastward plunges. These lineations form a Z-sigmoid pattern in plan view. Discussion and interpretation. – The shallow plunging lineations carried by rather steep foliations and the sigmoidal features observed in plan view, show that most magmatic fabrics of both plutons may correspond to strike-slip movements. Moreover, the organization of the main cleavage in the country rocks around the Veinazès pluton could correspond to an E-W-trending left-lateral transcurrent zone (fig. 1). In the western part of the Châtaigneraie, the mean direction of the cleavage becomes N150oE-165oE, with eastward middle to steep dips, indicating a clockwise rotation with respect to the regional trend. Dextral shearings are linked to this NNW-SSE band. Gravimetric data indicate that the Veinazès pluton is characterized by a rather flat floor and a single zone of relative deepening (2.5 km) between Ladinhac and Sansac-Veinazès (fig. 10), interpreted as a possible root. For the Marcolès pluton the maximum depth (1.25 km) is located beneath the village of Marcolès. We propose the following model (fig. 11) for the emplacement of the Veinazès and Marcolès plutons during the late Namurian-early Stephanian time-span: initial opening of the crust corresponding to the present deepest parts of the plutons floor, led by ENE-WSW-trending left-lateral strike-slip faults (fig. 11a) ; magmatic ascent and accretion of the plutons (fig. 11b) ; about 40o clockwise rotation of the Marcolès pluton, the SW end of the Veinazès pluton and their country rocks, linked to a NNW-SSE right-lateral strike-slip band (fig. 11c). We consider that these events predate the low temperature sinistral movements along the NNE-SSW-trending Sillon Houiller during the Stephanian time. Conclusion. – The magmatic fabrics measured in the Veinazès and Marcolès plutons, combined with previous data from the country rocks, show the probable importance of transcurrent movements for the emplacement of these plutons. The rotation suffered by the western part of this region after granites were emplaced would need to be better contrained by palaeomagnetic measurements on the Marcolès pluton.


2010 ◽  
Vol 2 (4) ◽  
Author(s):  
Ye Cao ◽  
Shengrong Li ◽  
Meijuan Yao ◽  
Huafeng Zhang

AbstractThermoluminescence (TL) of monomineralic separates have been widely used in various geosciences fields in order to trace the thermal history and aid in prospecting for gold deposits. Quartz is a ubiquitous mineral in the Shihu gold deposit, which is situated in the northern part of the Taihang orogenic belt in the North China craton (NCC). The deposit is hosted by ductile-brittle faults within an Archean metamorphic core complex of the Fuping Group. This deposit is characterized by gold-bearing quartz-polymetallic sulfides and quartz veins. New TL results have been obtained for quartz, in which four type-TL glow curves were identified. The gold-bearing quartz present type III glow curves that consist of two peak glow curves at the middle and high peak temperatures with the similar TL intensity. In addition, the cross-sections of peak temperatures and TL intensity highlight the valuable area where the Au-bearing quartz present weak TL intensity and low-middle peak temperatures. Our results significantly enhance the usefulness of quartz in metallogenic studies of the North China craton and as an indicator mineral in mineral exploration of the Taihang Mountain region.


2006 ◽  
Vol 96 (2) ◽  
pp. 271-287 ◽  
Author(s):  
C. Cartannaz ◽  
P. Rolin ◽  
A. Cocherie ◽  
D. Marquer ◽  
O. Legendre ◽  
...  

Author(s):  
Michel Faure ◽  
Eugène Bé Mézème ◽  
Manuel Duguet ◽  
Carine Cartier ◽  
Jean-Yves Talbot

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