Britholite, monazite, REE carbonates, and calcite: Products of hydrothermal alteration of allanite and apatite in A-type granite from Stupné, Western Carpathians, Slovakia

Abstract

An uncommon assemblage of primary and secondary accessory REE minerals was identified in a Permian A-type granite clast in polymict conglomerates intercalated in the Cretaceous flysch sequence of the Pieniny Klippen Belt, Western Carpathians, northwest Slovakia. A detailed electron-microprobe study of the granite reveals extensive subsolidus alteration of primary magmatic allanite-(Ce) to ferriallanite-(Ce) and fluorapatite. The Y, Ce-rich fluorapatite was replaced by the dissolution–reprecipitation process to the britholite group mineral members: fluorbritholite-(Y), britholite-(Y), fluorcalciobritholite, and its hydroxyl-dominant analogue (“calciobritholite”). Britholite-(Y) contains up to 5.2wt.% ThO2 (0.15 apfu Th); the highest Th content yet reported in naturally occurring Y-dominant britholites. Moreover, the alteration of (ferri)allanite-(Ce) resulted to complex pseudomorphs and overgrowths, including mainly REE carbonate phases: [synchysite-(Ce) to its hydroxyl-dominant analogue “hydroxylsynchysite-(Ce)”, bastnäsite-(Ce)] and calcite, rarely monazite-(Ce), epidote, clinochlore, titanite, TiO2 phase, and pseudorutile. In some cases, secondary carbonate minerals (mainly synchysite and calcite) replaced a substantial part of former allanite crystals. Moreover, primary magmatic biotite (annite) was partly transformed to acicular stilpnomelane. Textural and compositional data indicate extensive replacement and breakdown of the primary magmatic allanite and apatite by aqueous fluids rich in fluorine and carbon, liberated during a younger post-magmatic, low-temperature hydrothermal-metamorphic overprint of the granite.