ALBERT

Description: The Puy-les-Vignes deposit, located in the northwestern part of the French Massif Central, is a remarkable occurrence in the West European Variscan belt of hydrothermal tungsten mineralization associated with a breccia pipe structure. The late stage of the mineralization consists of a mineral paragenesis composed of zircon, xenotime, monazite, Nb-Fe-W rutile, and Nb-Ti-Y-HREE-W-U oxide minerals (hereafter referred to as NTox) within an adularia-tourmaline-chlorite matrix. This study is focused on these rare-metal oxides, which display complex internal textures and uncommon chemical compositions with variable concentrations of Nb, Ti, Y, HREE, and W, not described until now. They are characterized by low microprobe totals (76 to 95%), together with the presence of OH – groups within the crystallographic structure as detected by FTIR spectroscopy, which is interpreted as the result of alteration, such as hydration and/or metamictization. The crystallochemical study shows that these crystals appear as a complex multi-polar solid solution, involving chemical mixing between two groups of binary solid solutions: a first group of anatase-columbite solid solution and a second group of euxenite-(Y)-columbite solid solution. Interpretation of their internal texture and their chemistry suggest that the NTox were formed during multi-phase crystallization in an open system by the mixing of two different hydrothermal fluids: a first fluid (L 1 ) enriched in Ti〉Nb, Fe, and W, with the same geochemical signature as the main mineralization, and a second fluid (L 2 ) enriched in Nb〉Ti, Fe, Y, REE, and W, with a geochemical signature clearly contrasting with the former and coeval with the crystallization of adularia, xenotime, monazite, zircon, and rutile. This mineral paragenesis is characterized by a P, Y, HREE, Nb 〉 Ta, Ti, Zr, and U geochemical signature, typical of rare-metal peralkaline magmatism, thus suggesting rare-metal mobilization by late hydrothermal fluids with a peralkaline signature, likely derived from an unknown source at depth ( e.g. , NYF pegmatites or related granite), during the late metallogenic stages at Puy-les-Vignes.

Description: The West European Variscan chain is a remarkable illustration of how partial melting marks out the geodynamic evolution of mountain belt through time. Here, we focus on the Late Carboniferous melting events reported in the southeastern French Massif Central (Velay dome), with emphasis on the modes of partial melting, relationships between partial melting and magma emplacement, transition between the melting episodes and related P-T-t path. Following nappe stacking events under medium pressure/temperature conditions (M1 and M2 events), three melting events are identified in the southern envelope of the Velay dome. A first melting episode (M3 event) occurred within the biotite stability field at 325–315 Ma (T 720°C and P = 0.5–0.6 GPa). It led to the complete disappearance of muscovite and to the formation of migmatites consisting of biotite ± sillimanite melanosome and of granitic/tonalitic leucosomes depending on protolith composition. It is interpreted as the result of internal heating mainly linked to decay of heat producing elements accumulated in a thickened crust. It resulted in the formation of a partially molten middle crust with decoupling between the lower and upper crust, late-collisional extension and crustal thinning. The second episode of melting (M4 event) occurred at ca. 304 Ma (T 800°C and P 0.4 GPa), synchronously with emplacement of the Velay granites and growth of the dome. It led to the breakdown of biotite and growth of cordierite (locally garnet or tourmaline), with formation of diatexites and heterogeneous granites. This high-T event synchronous with crustal extension is considered to result from intrusion of hot mantle-derived and lower crustal magmas triggering catastrophic melting in the middle crust. This event ends with local retrograde hydrous melting within the stability field of biotite close to the solidus in response to local input of water during temperature drop in the late stage of emplacement of the Velay dome. The last evidence of melting in this area (M5 event) corresponds to emplacement of late granites generated under conditions estimated at 850°C and 0.4–0.6 GPa. They may have been generated from melting of specific lithologies triggered by injection of mafic magmas. These granites emplaced in a partly cooled crust (medium-grade conditions). The emplacement age of these granites is not well constrained (305–295 Ma) though they clearly post-date the Velay granites. The melting episodes in the Velay area and generation of granites appear to correspond to the conjunction between (i) the effects of collision-related crust thickening and (ii) those related to slab break off and asthenospheric mantle decompression melting. The driving process is mainly the internal radiogenic heat in a first stage, relayed by the propagation of a thermal anomaly initially located in the lower crust (M 3 event), but which subsequently rose to the middle and upper crustal levels through magma transfer (M 4 event). Overall, the Velay example is a remarkable illustration of the progressive dehydration and sterilisation of a thickened crustal segment. It documents how large amounts of granitic magmas can be produced at shallow crustal levels in relation to the injection of mantle-derived magmas.

Description: 〈span〉〈div〉Abstract〈/div〉In rare-metal granites, niobium and tantalum are generally hosted by Nb–Ta oxides. However, in SE China, the Nb-specialized Huangshan granites are a unique occurrence in which Nb is essentially hosted by Li–Fe micas. The Huangshan granites are part of the Early Cretaceous (Late Yanshanian) Lingshan granite complex and belong to the A-type granite series, with two facies differing by their mica compositions: medium-grained “protolithionite” granite and medium-grained lithian (lithium-rich) annite granite. The granites are characterized by elevated whole-rock Nb contents (average 144 ppm in “protolithionite” granite and 158 ppm in annite granite), quite low Ta contents (average 9 and 4 ppm, respectively), leading to very high Nb/Ta ratios (average 15.3 and 31.2). Niobium is mainly hosted in the micas, with an average Nb content of 1347 ppm in the lithian annite and 884 ppm in the “protolithionite,” which is the highest ever reported in granitic mica. With an estimated endowment of ∼80 kt Nb, the Huangshan granites represent a new style of potential Nb resource. Contrasting with the great rarity of columbite, there is abundant Hf-rich zircon, Y-rich fluorite, and Th-rich fluocerite included in the Huangshan micas. Such accessory minerals being typical of alkaline rhyolitic magmas and niobium enrichment in the Huangshan granites results from A-type melt. The extreme Nb enrichment in the micas results from the highly compatible behavior of Nb in this melt, combined with the high magma temperature (estimated at 790–800 °C) and possibly enhanced magma oxidation.〈/span〉

Description: We present U-Pb thermal ionization mass spectrometer (TIMS) ages of wolframite from several granite-related hydrothermal W±Sn deposits in the French Massif Central (FMC) located in the internal zone of the Variscan belt. The studied wolframite samples are characterized by variable U and Pb contents (typically 〈10 ppm) and show significant variations in their radiogenic Pb isotopic compositions. The obtained U-Pb ages define three distinct geochronological groups related to three contrasting geodynamic settings: (i) Visean to Namurian mineralization (333–327 Ma) coeval with syn-orogenic compression and emplacement of large peraluminous leucogranites (ca. 335–325 Ma), (ii) Namurian to Westphalian mineralization (317–315 Ma) synchronous with the onset of late-orogenic extension and emplacement of syn-tectonic granites (ca. 315–310 Ma) and (iii) Stephanian to Permian mineralization (298–274 Ma) formed during post-orogenic extension contemporaneous with the Permian volcanism in the entire Variscan belt. The youngest ages (276–274 Ma) likely reflect the reopening of the U-Pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration (e.g. ferberitization). Our results demonstrate that W(±Sn) mineralization in the FMC formed during at least three distinct hydrothermal events in different tectono-metamorphic settings over a time range of 40 Ma.