Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Grande granite (Armorican Massif, France)

International audienceMost of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310 Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande leucogranitesuggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment inthe apical zone of the granite likely occurred during both fractional crystallization and the interactionwith magmatic fluids. The low Th/U values (< 2) measured on the Guérande leucogranite likelyfavored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of theGuérande leucogranite (δ18Owhole rock = 9.7 – 11.6 ‰ for deformed samples and δ18Owhole rock = 12.2 - 13.6 ‰ for other samples) indicate that the deformed facies of the apical zone underwent subsolidusalteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz combfrom a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1-6wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trappingtemperatures in the range of 250-350°C suggest an elevated geothermal gradient, probably relatedto regional extension and the occurrence of magmatic activity in the environment close to thedeposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran andMétairie-Neuve deposits reveal three different mineralizing events. The first event at 296.6 ± 2.6 Ma(Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of lateleucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6 ±1.0 Ma (Métairie-Neuve) and 274.6 ± 0.9 Ma (Pen Ar Ran), respectively. Backscattered uranium oxideimaging combined with major elements and REE geochemistry suggest similar conditions ofmineralization during the two Pen Ar Ran mineralizing events at ca. 300 Ma and ca. 275 Ma, arguingfor different hydrothermal circulation phases in the granite and deposits. Apatite fission track datingreveals that the Guérande granite was still at depth and above 120°C when these mineralizing eventsoccurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande leucogranite is the mainsource for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration viasurface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmaticuranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. Assimilar mineralizing events occurred subsequently until ca. 275 Ma, meteoric oxidizing fluids likelypercolated during the time when the Guérande leucogranite was still at depth. The age of the Umineralizing events in the Guérande region (300 – 275 Ma) are consistent with those obtained onother U deposits in the EHB and could suggest a similar mineralization condition, with long-termupper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertileperaluminous leucogranites during the Late Carboniferous to Permian crustal extension events