Formation and fluid-driven transformation of accessory Zr–Th–U and REE minerals from the Markersbach highly evolved aluminous A-type granite, Erzgebirge, Germany: The mobilisation and reprecipitation of rare lithophile elements in an F-rich environment

The Markersbach granite (Erzgebirge/Krušné Hory, Germany) is a late Variscan, post-collisional, highly evolved, autometasomatically altered high-F, low-P aluminous A-type granite. It ranges from fine-grained leucosyenogranite to coarse-grained biotite monzogranite, characterised by low deformation and a typical magmatic quartz–feldspar–mica assemblage. Accessory minerals include abundant zircon, thorite and topaz, rare xenotime-(Y) and chernovite-(Y), extremely rare monazite-(Ce), synchysite-(Y) enriched in thorbästnasite component, synchysite-(Ce), and fluocerite-(Ce). Biotite 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 and may locally include goethite (± limonite) rim. 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 and thorite form complex Zr(Hf)–Th(U)–Y(REE) solid-solution series and commonly exhibit 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, with variable U removal from the granite suggesting post-magmatic U mobility and partial loss. Yttrium-rich fluorite I and fluocerite-(Ce) are major Ca–F carriers. Interaction with late fluids promoted dissolution–reprecipitation, producing Y-poor fluorite II, synchysite-(Y), sporadic Ca _0.5 Th _0.5 F ₃ micro-inclusions in re-equilibrated fluocerite-(Ce), and locally destabilising monazite-(Ce). Rare chernovite-(Y) and As-bearing zircon–thorite assemblages indicate involvement of oxidising, As-rich fluids. The evolution of REE–Y–Zr–Th–U mineral phases records both the highly fractionated, halogen-enriched, P-poor parental melt and subsequent subsolidus fluid-mediated re-equilibration, resulting in multi-stage HFSE redistribution within the granite.