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

Most 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