Origin of the Okrouhla RadouA episyenite-hosted uranium deposit, Bohemian Massif, Czech Republic: fluid inclusion and stable isotope constraints

The Okrouhla RadouA shear zone hosted uranium deposit is developed along the contact of Variscan granites and high-grade metasedimentary rocks of the Moldanubian Zone of the Bohemian Massif. The pre-ore pervasive alteration of wall rocks is characterized by chloritization of mafic minerals, followed by albitization of feldspars and dissolution of quartz giving rise to episyenites. The subsequent fluid circulation led to precipitation of disseminated uraninite and coffinite, and later on, post-ore quartz and carbonate mineralization containing base metal sulfides. The fluid inclusion and stable isotope data suggest low homogenization temperatures (similar to 50-140 A degrees C during pre-ore albitization and post-ore carbonatization, up to 230 A degrees C during pre-ore chloritization), variable fluid salinities (0-25 wt.% NaCl eq.), low fluid delta O-18 values (-10 to +2 aEuro degrees V-SMOW), low fluid delta C-13 values (-9 to -15 aEuro degrees V-PDB), and highly variable ionic composition of the aqueous fluids (especially Na/Ca, Br/Cl, I/Cl, SO4/Cl, NO3/Cl ratios). The available data suggest participation of three fluid endmembers of primarily surficial origin during alteration and mineralization at the deposit: (1) local meteoric water, (2) Na-Ca-Cl basinal brines or shield brines, (3) SO4-NO3-Cl-(H)CO3 playa-like fluids. Pre-ore albitization was caused by circulation of alkaline, oxidized, and Na-rich playa fluids, whereas basinal/shield brines and meteoric water were more important during the post-ore stage of alteration.