Ore deposits of the French Massif Central: insight into the metallogenesis of the Variscan collision belt |
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Authors: | C Marignac M Cuney |
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Institution: | (1) CRPG-CNRS, BP 20 54 501 Vandoeuvre Cedex France, and EMN-INPL, 54 042 Nancy Cedex France e-mail: marignac@mines.u-nancy.fr, FR;(2) CREGU-UMR G2R 7566, BP23, 54 501 Vandoeuvre Cedex France, FR |
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Abstract: | The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution
and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the
mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation
does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic
stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important;
the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during
the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic
because of the thermal reactivation linked with the Alpine orogenies.
The Eo-Variscan stage (Cambrian-Silurian) corresponded to the pre-collision history, marked at the WEV belt scale by a fragmentation of the northern
Gondwana (immature crust evolved from the Late Proterozoic Cadomian orogeny), up to the break-up of the crust and the formation
of oceanic basins (Cambrian-Ordovician), followed by their resorption by subduction during the Silurian. In the FMC, no subduction-related
magmatism is known (being rare at the WEV belt scale), and consequently subduction-related mineralisation, e.g. porphyry copper,
is unknown in the WEV belt. Although some ophiolitic remnants are known, they never display Cyprus-type VMS deposits, nor
massive podiform chromitites. Beside platformal sedimentary deposits on passive margins, the only deposits formed during the
Eo-Variscan stage were of the SEDEX type, linked with the early rifting of the Gondwanian crust.
The Meso-Variscan stage (Devonian-Early Carboniferous) corresponded to the collision proper, with the formation of crustal-scale nappe structures
and the intrusion of collision-related peraluminous granites. Although these granites were enriched in rare metals they did
not yield significant hydrothermal mineralisation, due to the great depth of their emplacement, as the similar granites in
the Himalayas. However, they were a source of rare metals (in particular, uranium) for later mineralisation events. At the
WEV belt scale Devonian distensive events are coeval with the collision. They were recorded by the formation of sedimentary
basins of limited time and space extent, corresponding to the splitting of the continental crust (up to formation of oceanic
domains in many cases), and were characterised by a bi-modal (“spilite-keratophyre”) volcanism. These basins formed in transtensional
(or pull-apart) settings along major strike-slip faults, a peculiarity of the Variscan collision belt (which may conveniently
be described as a “strike-slip orogen”). In such basins, many deposits linked with the volcanic thermal energy were formed:
SEDEX deposits of the Meggen-type, iron deposits of the Lahn-Dill-type and VMS base metal deposits, the latter being the only
ones known in the FMC (Brévenne deposits).
The Neo-Variscan stage corresponded to the “hypercollision” and was characterised by a shift from compressional tectonics (late thickening of the
crust during the Sudetian event and long-lasting dextral strike-slip tectonics along NW-SE to NE-SW fault zones) towards extensional
tectonics (“basin and range” of the Late Stephanian-Early Permian), as well as by high heat flows, recorded by LP-HT metamorphism,
extensive granitisation and granulitisation of the lower crust. These characteristics record the development of a lithospheric
delamination process. In response to the energetic input released by this process, numerous hydrothermal deposits were formed
in the FMC, as well as in the whole WEV belt, during the Neo-Variscan stage. These are mainly: (1) high-temperature granite-centered
tungsten deposits, mainly associated with cordierite-bearing high level intrusions of Namurian-Westphalian age; (2) rare metal
granites (and the associated hydrothermal tin mineralisations), resulting from fluid-induced low-degree partial melting of
the middle crust in relation with the devolatilisation of the granulitised lower crust; (3) shear-zone hosted gold and antimony
deposits, related to crustal-scale hydrothermal circulation, triggered by the transition to extensional tectonics at about
300 Ma; and (4) uranium deposition in extensional settings related to the Early Permian distension.
The Post-Variscan mineralising events recorded the renewal of thermal flows in the lithosphere linked with early Alpine events
(mainly the Trias-Lias distension in the Tethyan realm and the middle Cretaceous opening of the Bay of Biscay in the Pyrenean
realm). They resulted in low-enthalpy geothermal systems, leading to a variety of deposits, mainly: (1) F-Ba districts, reworking
F and Ba from Late Variscan granites and ignimbrites; (2) a major uranium deposit (Lodève), reworking uranium from the Permian
Lodève basin; and (3) Zn-Pb districts of the MVT-type.
Finally, the mineralisation of the Variscan collision belt is mainly the consequence of the Neo-Variscan lithospheric delamination
process. By contrast, the absence of such a process in collision belts like the Himalayas or the Alps is the key of them being
devoid of mineralisation. It appears that the mechanical energy released by the collision itself is not sufficient to mobilise
and concentrate the trace elements involved in the metallogenic processes.
Received: 1 September 1998 / Accepted: 3 February 1999 |
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