Comportement des fluides dans l'évolution magmatique puis hydrothermale du granite à Ta, Nb, Li de Beauvoir d'après la géochimie isotopique de l'oxygène et l'hydrogène  [The behavior of fluids in the magmatic then hydrothermal evolution of the tantalum-, niobium- and lithium-bearing Beauvoir Granite based on oxygen and hydrogen isotopic geochemistry]

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Alternate Title:The behavior of fluids in the magmatic then hydrothermal evolution of the tantalum-, niobium- and lithium-bearing Beauvoir Granite based on oxygen and hydrogen isotopic geochemistry
Authors:Fouillac, A. M.; Kosakevitch, A.; Merceron, T.; Meunier, A.; Rossi, P.
Author Affiliations:Primary:
B.R.G.M., Orleans, France
Volume Title:Géologie de la France
Source:Géologie de la France, Vol.2-3, p.279-293. Publisher: Bureau de Recherches Géologiques et Minières, Orleans, France. ISSN: 0246-0874
Publication Date:1987
Note:In French with English summary; illus., incl. 1 table, geol. sketch map
Summary:The Beauvoir albite-topaz-lepidolite granite cupola was intersected through an interval of 700 m in a cored borehole at Echassieres. This is a late-Hercynian 'rare metals' hololeucocratic granite, bearing Ta-Nb-Li-Be mineralization. Three types of granite have been defined on the basis of petrographic and geochemical studies. Experiments indicate an early interaction of the granite with external fluids from magmatic to submagmatic stage, localized in the upper part of the cupola. Early timing of this interaction is supported by the magmatic characteristics of the samples, by the quartz-muscovite fractionation, and by the low wt.% of water of the whole-rock. These results, and the inverse correlation that exists between Ta and delta 18O values in the granites suggest the following metallogenetic hypothesis: the presence of meteoric water, in which exchanges have previously occurred with the surrounding rock, leads to variations in the physico-chemical conditions of the magma-magmatic fluid system and favours the concentration of rare elements in the crystallizing later phases. The introduction of meteoric water into the magmatic system is made possible by hydraulic fracturing that occurs during the liquidus-solidus transition in the apical granite. values of delta D H2O calculated from the delta D values of the lepidolites of the core-samples of magmatic rock from the drillhole, of the lepidolites of certain core-samples of apical granitic rock and of the pervasive newly crystallized muscovite are all similar indicating that the magmatic and meteoric fluids have identical isotopic ranges of variation in hydrogen. Two core-samples of apical granitic rock from close to the Colettes granite are distinguished by extensive muscovitization, by lack of impoverishment in delta 18O and a higher delta D for muscovite (-55%). These samples have probably undergone exchange with water external to the Beauvoir granite that has itself undergone exchanges with Colettes hot granite. The newly crystallized pervasive and vein minerals were deposited between 400° and 200°C by a meteoric fluid mixed to varying degrees with the magmatic fluid and/or undergone exchange with the host-rock. A model is proposed for the emplacement and evolution of the magma- magmatic fluid system, pointing especially to the introduction of meteoric water as early as during the liquidus-solidus course of crystallization, in the upper part of the granitic slab. [A.W.H.]
Subjects:Alkali metals; D/H; Deuterium; Emplacement; Fluid phase; Geochemistry; Granites; Hydrogen; Hydrothermal alteration; Igneous rocks; Isotope ratios; Isotopes; Lepidolite; Lithium; Magmas; Metals; Metasomatism; Mica group; Niobium; O-18/O-16; Oxygen; Plutonic rocks; Sheet silicates; Silicates; Stable isotopes; Tantalum; Allier France; Beauvoir Granite; Europe; France; Western Europe; Echassieres Massif
Coordinates:N455000 N464500 E0040000 E0023000
Abstract Numbers:88M/3934
Record ID:2005000023
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique
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