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  • 1
    Publication Date: 1992-01-01
    Print ISSN: 0263-4929
    Electronic ISSN: 1525-1314
    Topics: Geosciences
    Published by Wiley
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  • 2
    Publication Date: 2016-07-09
    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.
    Print ISSN: 0008-4476
    Topics: Geosciences
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  • 3
    Publication Date: 1998-08-01
    Print ISSN: 1001-6538
    Electronic ISSN: 1861-9541
    Topics: Natural Sciences in General
    Published by Springer
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  • 4
    Publication Date: 2015-04-02
    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.
    Print ISSN: 0037-9409
    Electronic ISSN: 0037-9409
    Topics: Geosciences
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  • 5
    Publication Date: 2018
    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〉
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences , Physics
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  • 6
    Publication Date: 2013-08-14
    Description: The upper Miocene Oued Belif ring-shaped breccia is located in the Nefza mining district of northern Tunisia, within the internal zone of the collisional Alpine Maghrebide belt. It encloses chaotic Triassic material (evaporites, altered siltites, and pelites) within an extrusive diapiric structure reactivated in a late Miocene nappe emplacement episode. This deep-rooted structure drove the hydrothermal activity that caused the brecciation of the Oued Belif breccia and the emplacement of two generations of shallow felsic magmatism, which, most notably, include a Serravallian granodiorite stock and a Tortonian rhyodacite dome. The inverted cone-shaped matrix-supported Oued Belif breccia comprises clasts of Triassic material and others deriving from the regional substrate, as well as a very small amount of volcanic-related material that suggests a possible phreatomagmatic origin. Fine-grained, low-Ti, Si-Al–rich hematite (≥20 vol %) is the main matrix mineral, along with rare earth element (REE) minerals (bastnaesite, parisite, and subordinate monazite) and U minerals (uraninite). The Oued Belif breccia is not an economic Cu-Au deposit, although it does possess a distinct geochemical anomaly in mafic (Cu-Co-Au) elements, with local Cu enrichment at depth (from drill core material), as well as in felsic (Bi-W-Sn-U) elements. A multiphase alteration episode (K-Fe-(Mg) metasomatism) responsible for the formation of K-feldspar, Fe phlogopite, Fe muscovite, and tourmaline, with fluorite and barite, predates the Fe-light REE (LREE)-U mineralization. The major brecciation event occurred slightly after this alteration and is coeval with the main iron oxide-LREE-U mineralization event and the emplacement of the Oued Belif rhyodacite intrusion. Hydrothermal mineralizing fluids were hot (≥540°C), saline and F-CO 2 –rich brines, possibly with a mixed, basinal (salt-related) and magmatic (alkaline-related) origin. Except for its lack of economic Cu and Au content, the Oued Belif breccia shares most typical attributes of the hematite group of iron oxide copper-gold (IOCG) deposits. One may therefore classify it either as an "iron oxide-associated–alkali-altered" ( Porter, 2010 ) or as an "iron oxide uranium" ( Skirrow, 2010 ) mineral system. K-Ar dating of K-feldspar sets the Oued Belif breccia formation at 9.2 ± 0.25 Ma, which could make it the youngest presently known representative of the iron oxide-associated–alkali-altered–IOCG class of deposits. From a geodynamic viewpoint, the Oued Belif structure fits within a still active collisional belt, even though the Oued Belif breccia was formed in a postcollisional stage. In this respect, Oued Belif differs from most other IOCG deposits, especially from older Proterozoic ones. It nevertheless shares two essential characteristics considered necessary to yield IOCG deposits ( Skirrow, 2010 ), namely a metasomatized subcontinental lithospheric mantle and a lithospheric delamination process. Identifying the Oued Belif breccia as a member of the iron oxide-associated–alkali-altered–IOCG family opens new frontiers within the study of IOCG deposits within the geodynamic environment of collisional belts. Future research in this area should focus particularly on the circum-Mediterranean segment of the Alpine belt.
    Print ISSN: 0361-0128
    Topics: Geosciences
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  • 7
    Publication Date: 1862-12-01
    Print ISSN: 1618-2642
    Electronic ISSN: 1618-2650
    Topics: Chemistry and Pharmacology
    Published by Springer
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  • 8
    Publication Date: 1874-12-01
    Print ISSN: 1618-2642
    Electronic ISSN: 1618-2650
    Topics: Chemistry and Pharmacology
    Published by Springer
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  • 9
    Publication Date: 1865-12-01
    Print ISSN: 0016-1152
    Topics: Chemistry and Pharmacology
    Published by Springer
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  • 10
    Publication Date: 1868-12-01
    Print ISSN: 0016-1152
    Topics: Chemistry and Pharmacology
    Published by Springer
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