THE INVOLVEMENT OF F, CO2, AND As IN THE ALTERATION OF Zr-Th-REE-BEARING ACCESSORY MINERALS IN THE HORA SVATE KATERINY A-TYPE GRANITE, CZECH REPUBLIC

The Zr(Hf)-Th(U)-REE minerals from a subvolcanic A-type granite from Hora Svate Kateriny, in the Czech Republic, were studied in an effort to distinguish how both magmatic and hydrothermal processes affected the rock system. The primary accessory phases include fluorite, magnetite and zircon, with rare xenotime-(Y), monazite-(Ce), and thorite. Complex solid-solutions with nearly equal contents of (Th+U), (Zr+Hf) and REE components usually form irregular domains within zircon and thorite crystals and are supposedly also primary. The granite was affected by strong reactions with late-to postmagmatic fluids. Pyrite and arsenopyrite crystallized from the first phase of hydrothermal activity, with low fugacity of oxygen. Later, fluid with a higher fugacity of oxygen altered magnetite to hematite, sulfides to Fe hydroxides, and dissolved xenotime. At this stage, Th- and U-bearing zircon and thorite became partially metamict. Arsenic in the fluid phase was oxidized to As5+, and intensively metasomatized xenotime, Y-rich zircon and thorite, forming chernovite. Even later, fluid rich in CO2 dissolved metamict thorite and partially dissolved monazite; after only a short distance of transport, Th-rich bastnasite deposited in contact with fluorite.