Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

Abstract

Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS). The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite “a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration.

Le gisement à étain–cuivre–indium de Charrier (Allier) : un skarnoïde viséen (340 Ma) traceur de la métallogenèse varisque du nord Massif central

“…6) Il est très vraisemblable que tous ces filons appartiennent, comme les filons tardifs de Charrier, à l'épisode fluo-barytique liasique (vers 200 Ma) ouest-européen bien représenté dans le Massif central (Marcoux et al, 1990 ;Sizaret et al, 2004). La présence de l'indium dans ces filons illustre le concept de permanence métallogénique (Routhier, 1980) : ce métal arriverait dans la croûte via les magmas au Mississippien pour former des gisements de type Charrier, avant d'être ensuite remobilisé et redistribué par les évènements hydrothermaux postérieurs Les dépôts épisodiques de wolframite sur plus de 40 millions d'années à Échassières (Harlaux et al, 2018 ;Monnier et al, 2021), pourrait être une autre illustration de cette permanence, pour le tungstène.…”