Geochemical Signature of Magmatic-Hydrothermal Fluids Exsolved from the Beauvoir Rare-Metal Granite (Massif Central, France): Insights from LA-ICPMS Analysis of Primary Fluid Inclusions

Matthieu Harlaux,Christian Marignac,Wilédio Marc-Emile Bonzi,Julien Mercadier,Valentin Kremer,Michel Cuney

doi:10.1155/2017/1925817

Matthieu Harlaux, Christian Marignac + Show 4 more

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https://doi.org/10.1155/2017/1925817

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Abstract

The Beauvoir granite (Massif Central, France) represents an exceptional case in the European Variscan belt of a peraluminous rare-metal granite crosscutting an early W stockwork. The latter was strongly overprinted by rare-metal magmatic-hydrothermal fluids derived from the Beauvoir granite, resulting in a massive topazification of the quartz-ferberite vein system. This work presents a complete study of primary fluid inclusions hosted in quartz and topaz from the Beauvoir granite and the metasomatized stockwork, in order to characterize the geochemical composition of the magmatic fluids exsolved during the crystallization of this evolved rare-metal peraluminous granite. Microthermometric and Raman spectrometry data show that the earliest fluid (L1) is of high temperature (500 to >600°C), high salinity (17–28 wt.% NaCl eq), and Li-rich (Te<−70°C) with Na/Li ratios ~5. LA-ICPMS analyses of L1-type fluid inclusions reveal that the chemical composition of this magmatic-hydrothermal fluid is dominated by Na, K, Cs, and Rb, with significant concentrations (101–104 ppm) in rare-metals (W, Nb, Ta, Sn, and Li). This study demonstrates that primary fluid inclusions preserved the pristine signature of the magmatic-hydrothermal fluids in the Beauvoir granite but also in the metasomatized W stockwork, despite the distance from the granitic cupola (>100 m) and interaction with external fluids.

Highlights

Rare-metal deposits are mainly associated with peraluminous to peralkaline granites, granitic pegmatites, and carbonatite complexes, which represent major resources for economically strategic metals such as Li, Be, Ta, Nb, Sn, W, and REE [1, 2]
The aims of this work are (i) to determine the trace element composition of the magmatic fluid inclusions trapped in primary quartz and topaz crystals from the Beauvoir granite and to characterize the geochemical signature of these rare-metal-rich magmatic fluids; (ii) to study the partitioning of the minor and trace elements between the coexisting vapour and brine phases resulting from the phase separation during the boiling of the early primary magmatic fluid; (iii) to determine the chemical evolution of the magmatic fluids exsolved from the Beauvoir granite during their interaction with the enclosing micaschists with distance from the granitic cupola
A discrepancy can be observed between the Na/Li ratios calculated from the microthermometric data and the LA-ICPMS data

Summary

Introduction

Rare-metal deposits are mainly associated with peraluminous to peralkaline granites, granitic pegmatites, and carbonatite complexes, which represent major resources for economically strategic metals such as Li, Be, Ta, Nb, Sn, W, and REE [1, 2]. In Europe, the Variscan belt represents an important metallogenic province for these two types of rare-metal deposits, which are located mainly in the Bohemian Massif, the Iberian Massif, Cornwall, the Armorican Massif, and the French Massif Central (FMC). These deposits formed during Carboniferous to Permian times between 330 Ma and 280 Ma in relation to the lateVariscan orogenic evolution [6,7,8,9,10]. These studies led to a complete petrological, geochemical, geophysical, structural, and geochronological description of the Beauvoir granite and of its host-environment and to the elaboration of integrated genetic model for this type of rare-metal deposit [12, 13]

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