Polyphase W-Sn mineralization and rare metal magmatism in relation to the late-Variscan tectono-metamorphic evolution of the southeastern French Massif Central.

Autor: Harlaux, Matthieu, Marignac, Christian, Carr, Patrick A., Mercadier, Julien, Ballouard, Christophe, Jegal, Yujin, Kouzmanov, Kalin, Foucaud, Yann, Camacho, Alfredo, Cauzid, Jean, Cuney, Michel
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Zdroj: Mineralium Deposita; Jan2024, Vol. 59 Issue 1, p47-68, 22p
Abstrakt: The southeastern French Massif Central represents an ideal area to study the linkage between regional metamorphism, crustal partial melting, emplacement of granitic magmas, and hydrothermal Sn-W mineralization in a polyphase tectono-metamorphic setting related to the late-Variscan orogeny. Here, we describe the mineralogical, structural, geochemical, and geochronological characteristics of cassiterite–wolframite-bearing quartz veins at St-Mélany, a small uneconomic Sn-W occurrence located in the North Cévennes area. The veins show evidence of ductile deformation (boudinage, asymmetric folding, dynamic recrystallization) consistent with a synkinematic emplacement during the regional low-pressure–medium-temperature metamorphism at ca. 320–315 Ma. This dominantly water-fluxed melting event reaching muscovite breakdown conditions (T < 750 °C, P ≈ 0.6 GPa) was synchronous to the emplacement of the syntectonic Rocles peraluminous granite, which is interpreted as a proximal source for the mineralizing fluids at St-Mélany. The U–Pb LA-ICP-MS dating of coexisting wolframite and cassiterite from a mineralized quartz vein yielded lower-intercept ages of 318.4 ± 2.2 Ma and 311.4 ± 1.0 Ma (2σ), respectively. These results suggest a temporal decoupling of W and Sn mineralization with a time gap of 4–10 Myr, but additional work is needed to confirm this interpretation. A weighted mean 40Ar/39Ar date of 304.5 ± 4.8 Ma (2σ) was obtained for muscovite from the selvage of a mineralized vein, interpreted as a recrystallization age related to metamorphic re-equilibration or hydrothermal overprinting. Dikes of aplites and pegmatites cut the Sn-W-mineralized veins and were emplaced at 305.9 ± 3.9 Ma (2σ) based on U–Pb LA-ICP-MS dating of magmatic cassiterite. The dikes have highly evolved compositions typical of peraluminous high-phosphorus rare metal granites with Li-F-Ta > Nb-Sn-Be enrichments. Emplacement of the granitic dikes was coeval with the regional low-pressure–high-temperature metamorphism at ca. 305–300 Ma, reaching biotite dehydration melting conditions (T > 800 °C, P ≈ 0.4 GPa), which led to the formation of the Velay anatectic dome possibly linked to lower crust granulitization. We conclude that polyphase emplacement of W-Sn-mineralized veins at ca. 320–310 Ma and rare metal granitic dikes at ca. 305 Ma results from contrasting crustal melting conditions, in relation to the late-Carboniferous orogenic evolution of the southeastern French Massif Central, and possibly related to delamination of the subcontinental lithospheric mantle. [ABSTRACT FROM AUTHOR]
Databáze: Complementary Index
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