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

Abstract

The Beauvoir granite (Massif Central, France) represents an exceptional case in the European Variscan belt of a peraluminous raremetal 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 ( e < −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 (10 1 -10 4 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.

Contrasting fluid behavior during two styles of greisen alteration leading to distinct wolframite mineralizations: The Echassières district (Massif Central, France)

“…Fig.14.A), indicating dissolution/reprecipitation of the older generation, topaz I, during percolation of greisen fluids and precipitation of topaz II. The T h values for topaz I coincide with data reported inHarlaux et al (2017; they did not recognize the topaz II generation, less abundant).Lithium concentrations in the fluids responsible for the topazification and second greisen formation seem to be significantly different, as suggested by the low eutectic temperature recorded only for FI from the former (near -72°C). The nature of accompanying micas, i.e., F-Li-rich lepidolite to F-rich biotite (unpublished data) for topazification alteration and OH-rich muscovite for the Beauvoir greisen fluid(Fonteille, 1987), suggests that F and Li are enriched in the former case, but depleted in the latter.…”

“…However, at the time of their writing, it had not been recognized that the episode of topazification of the La Bosse stockwork predates the emplacement of the Beauvoir granite (Monnier et al, 2019). Consequently, we suggest that the fluid inclusions from the La Bosse stockwork studied by Harlaux et al (2017) record the orthomagmatic signature of the magma that sourced the topazification fluid, not that of the Beauvoir granite.…”

“…However, Heinrich (1990) and Yang et al (2019) argue that orthomagmatic fluids contain sufficient Fe and Mn to permit wolframite precipitation. This affirmation is verified by LA-ICP-MS analyses of magmatic-related FI, which record high amounts of Fe and Mn, in addition to W (Audétat et al, 1998;Harlaux et al, 2017;Yang et al, 2019). Yokart et al (2003), Legros et al (2019) and Liu et al (2018) propose mixing between orthomagmatic and meteoric fluids as the cause for wolframite deposition, while Pan et al (2019) suggest an input of sedimentary fluids as instrumental in precipitating wolframite.…”

“…This distinction in fluid behavior may explain the restriction of wolframite at Mazet. Indeed, during boiling of a fluid, W is known to strongly fractionate into the liquid phase (Audétat et al, 1998;Harlaux et al, 2017), suggesting that flashing triggered wolframite precipitation by causing vaporization of the totality of the liquid. In case of only limited boiling on the other hand, W could fractionate into the liquid phase without necessarily exceeding its saturation in W, which is likely what happened at Suchot, hence the absence of W mineralization.…”

Mixing of magmatic-hydrothermal and metamorphic fluids and the origin of peribatholitic Sn vein-type deposits in Rwanda

“…A direct genetic link between the mineralization and the silicic magmatism is often debated, especially in the case of peribatholitic deposits (cf. Harlaux et al, 2017;Hulsbosch et al, 2016;Kozlik et al, 2016). Although a magmatic origin of the Sn or W mineralization is experimentally predicted (e.g.…”