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.
Highlights
Some of the most important ore deposits in the world form due to successive magmatic/hydrothermal episodes, each playing a role in concentrating the metals (e.g., Cu, Au, REE [1,2,3])
In this contribution, based on new geochronological data and on a review of the literature, we show that wolframite [(Fe,Mn)WO4] mineralization in the Echassières district of the French Central Massif (FMC) is linked to three different magmatic bodies that were emplaced at different stages during the evolution of the Variscan orogen, to the polyphased deposits of the Nanling range
We suggest that Visean peraluminous granitic plutons of the French Massif Central (FMC), were emplaced in a similar tectonic context marked by growth of an orogenic plateau associated with the lateral expansion of the zone of thickened crust accommodated by strike-slip crustal faults and the horizontal flow of the partially molten ductile crust
Summary
Some of the most important ore deposits in the world form due to successive magmatic/hydrothermal episodes, each playing a role in concentrating the metals (e.g., Cu, Au, REE [1,2,3]). W mineralization occurs in a wide diversity of magmatic/hydrothermal environments [8,9] and contrasting tectonic settings [10,11], both favoring multiple ore generations In this contribution, based on new geochronological data and on a review of the literature, we show that wolframite [(Fe,Mn)WO4] mineralization in the Echassières district of the French Central Massif (FMC) is linked to three different magmatic bodies that were emplaced at different stages during the evolution of the Variscan orogen, to the polyphased deposits of the Nanling range. Combining these chronological data with mineralogical observations, we compare the mineralization of the Echassières district with analogous deposits elsewhere in the world in order to discuss how tectonic events marking major orogenic transitions control the occurrence and form of W deposits
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