Early uranium mobilization in late Variscan strike-slip shear zones affecting leucogranites of central western Spain

Abstract

Mineralization related to the post-collisional reorganization in Iberia occurs along strike-slip shear zones (Sn, Au, ± U), in extremely differentiated leucogranites and pegmatites (Sn, Nb, Ta, Be, Li), in skarns (W, ± Au, ± U), and in quartz-veins with greisen (W, Sn, ± Au, ± U). Episyenites/albitites with uraninite mineralization in strike-slip shear zones yield U–Pb ages of 304 ± 6 Ma (2σ) that are identical within error with the 40Ar/39Ar age of micas grown during fabric development (306 ± 3 Ma, 2σ), suggesting shearing and formation of episyenite/albitite and uraninite mineralization are directly related. Mineral chemistry of uraninite from episyenite and deformed muscovite-rich leucogranite host are indistinguishable and has immobile element signatures typical for uraninite that formed from a hydrothermal or metamorphic fluid. The mineral chemistry of uraninite hosted in the deformed muscovite-rich leucogranite, together with the large time span between granite emplacement (347.6 ± 1.9 Ma, U–Pb zircon) and uraninite (chemical age 300 ± 3 Ma) excludes a magmatic origin of uraninite. Mass balance indicates that leaching of rock-forming minerals (especially muscovite and monazite) of the muscovite-rich leucogranite during extensive metasomatism (e.g. albitite or episyenite formation) could provide most uraninite-hosted U. The precipitation of the uraninite-pyrite association with carbonaceous matter in episyenite/albitite was related to local reducing conditions and/or fluid unmixing processes, analogue to hydrothermal uraninite from retrograde scheelite skarn and W ± Au quartz veins in the Iberian Massif. Chemical ages of uraninite from episyenite and muscovite-rich deformed granite demonstrate that episyenitization is not a late-stage Variscan process, but a post-orogenic one. Metal mobilization is structurally controlled. Chemical dating of uraninite may provide a first-order age constraint in unravelling the post-Variscan tectonic history.