Formation and fluid-driven transformation of accessory Zr–Th–U and REE minerals in an F-rich environment: the Markersbach A-type granite, Erzgebirge, Germany

The Markersbach intrusion (Erzgebirge, Germany) is a late Variscan, early post-collisional, highly evolved, autometasomatically and hydrothermally altered F-rich, P-poor, aluminous A-type granite. It ranges from coarse-grained biotite monzogranite to fine-grained leucosyenogranite. The rich accessory mineral assemblage comprises, in addition to diverse Nb–Ta–Sn–W–Ti–Fe species, abundant zircon, thorite, topaz fluorite and fluocerite-(Ce), rare xenotime-(Y) and chernovite-(Y), synchysite-(Y) enriched in thorbästnasite component, synchysite-(Ce) and extremely rare monazite-(Ce). Lithium-rich siderophyllite commonly hosts annular radiohaloes developed around Th-rich zircon and thorite. These consist of Th-rich, metamict cores surrounded by a clay-rich alteration zone and a non-stoichiometric Th–Y–Si rim that locally is rimmed by goethite (± limonite). Their textures reflect alpha-recoil damage and localised element redistribution. The dimensions of the haloes and mineral compositions indicate radiation damage controlled predominantly by the ²³²Th decay chain. Zircon, thorite and xenotime form complex Zr(Hf)–Th(U)–Y(REE) solid-solutions (with REE – rare earth elements), which commonly experienced strong metamictisation, hydration and fluorination [≤ 6.8 wt% F in thorite, ≤ 3.8 wt% F in zircon, ≤ 1.1 wt% F in xenotime-(Y)]. Zircon and thorite are the principal hosts of immobile Th, whereas U is largely sequestered in secondary pyrochlore. Interaction with late fluids gave rise to local destabilisation of monazite-(Ce) and promoted various dissolution–reprecipitation reactions, producing Y-poor fluorite II, synchysite-(Y), and sporadic Ca0.5Th0.5F3 micro-inclusions in re-equilibrated fluocerite-(Ce). Rare chernovite-(Y) and As-bearing zircon–thorite assemblages further imply the involvement of oxidising, As-rich fluids. The evolution of REE–Zr–Th–U mineral phases records both the fractionation history of the halogen-enriched, P-poor parental melt and the subsequent subsolidus fluid-mediated re-equilibration in an F-rich environment, resulting in multi-stage LILE and HFSE redistribution within the granite, postdated by widespread U removal from the granite in the Cretaceous.