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Âge et genèse de la coupole granitique à métaux rares (Sn, Li, Nb-Ta, W) de Montebras (Creuse, Massif central français)

Abstract : The Montebras cupola is a small massif of granite with rare metals (Sn,W, Li, Nb- Ta) located north of the French Massif Central which emplaced during the upper Carboniferous in an older host rock, the Chanon granite (357.2 ± 2.1 Ma). Two magmatic episodes, a microgranite (316.1 ± 4.3 Ma), and an albitic leucogranite (309.8 ± 3.9 Ma) are distinguished. The second develops contact formations at its roof, in particular a thick stockscheider pegmatite (309.7 ± 4.5 Ma), passing east to lithium greisens and tinbearing flat quartz veins, previously exploited (303.8 ± 4.8 Ma). The cassiterite deposition (associated with manganocolumbite) spreads from the magmatic phase with crystals disseminated in leucogranite, until the end of the pneumatolytic phase marked by cassiterite-bearing quartz veins. This spreading is marked by a progressive drop in the concentrations of trace elements (Nb, Ta, Fe, Mn, Mg, Ti) in cassiterite but without reaching the domain of typically hydrothermal compositions. During the pneumatolytic phase, cassiterite is accompanied by rare scheelite, the rare qitianlingite and a frequent wolframite, the composition of which (hubnerite) indicates a magmatic origin for the metal and the fluids involved during the deposition of tungsten. The deposition of sulfides rich in Cu, As and Sn (löllingite, chalcopyrite, tennantite, stannoidite, mawsonite...) marks the transition to the hydrothermal stage and suggests an origin in host rocks for copper and arsenic. The liasic fluorite-baryte event is manifested by the local appearance of veinlets with violet fluorite, baryte and manganapatite. The Montebras dome provides a representative example of the rare metal granites of the Variscan orogen. It is contemporary with the other rare elements granitic magmas of the northern French Massif Central with which it presents similarities but also differences. Its emplacement could result from a mechanism of the cauldron subsidence type.
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Submitted on : Monday, March 29, 2021 - 4:24:45 PM
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Eric Marcoux, Benjamin Barré, Michel Pichavant, Marc Poujol. Âge et genèse de la coupole granitique à métaux rares (Sn, Li, Nb-Ta, W) de Montebras (Creuse, Massif central français). Bulletin de la Société Géologique de France, Société géologique de France, 2021, Special Issue Minéralisation périgranitiques, 192, pp.16. ⟨10.1051/bsgf/2020042⟩. ⟨insu-03184611⟩

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