Ulrich Hein

Three major mineralization events are recorded at the Rožná uranium deposit (total mine production of 23,000 t U, average grade of 0.24% U): (1) pre-uranium quartz-sulfide and carbonate-sulfide mineralization, (2) uranium, and (3) post-uranium quartz-carbonate-sulfide mineralization. (1) K–Ar ages for white mica from wall rock alteration of the pre-uranium mineralization style range from 304.5 ± 5.8 to 307.6 ± 6.0 Ma coinciding with the post-orogenic exhumation of the Moldanubian orogenic root and retrograde-metamorphic equilibration of the high-grade metamorphic host rocks. The fluid inclusion record consists of low-salinity aqueous inclusions, together with H2O-CO2-CH4, CO2-CH4, and pure CH4 inclusions. The fluid inclusion, paragenetic, and isotope data suggest that the pre-uranium mineralization formed from a reduced low-salinity aqueous fluid at temperatures close to 300°C. (2) The uraniferous hydrothermal event is subdivided into the pre-ore, ore, and post-ore substages. K–Ar ages of pre-ore authigenic K-feldspar range from 296.3 ± 7.5 to 281.0 ± 5.4 Ma and coincide with the transcurrent reorganization of crustal blocks of the Bohemian Massif and with Late Stephanian to Early Permian rifting. Massive hematitization, albitization, and desilicification of the pre-ore altered rocks indicate an influx of oxidized basinal fluids to the crystalline rocks of the Moldanubian domain. The wide range of salinities of fluid inclusions is interpreted as a result of the large-scale mixing of basinal brines with meteoric water. The cationic composition of these fluids indicates extensive interaction with crystalline rocks. Chlorite thermometry yielded temperatures of 260°C to 310°C. During this substage, uranium was probably leached from the Moldanubian crystalline rocks. The hydrothermal alteration of the ore substage followed, or partly overlapped in time, the pre-ore substage alteration. K–Ar ages of illite from ore substage alteration range from 277.2 ± 5.5 to 264.0 ± 4.3 Ma and roughly correspond with the results of chemical U–Pb dating of authigenic monazite (268 ± 50 Ma). The uranium ore deposition was accompanied by large-scale decomposition of biotite and pre-ore chlorite to Fe-rich illite and iron hydrooxides. Therefore, it is proposed that the deposition of uranium ore was mostly in response to the reduction of the ore-bearing fluid by interaction with ferrous iron-bearing silicates (biotite and pre-ore chlorite). The Th data on primary, mostly aqueous, inclusions trapped in carbonates of the ore substage range between 152°C and 174°C and total salinity ranges over a relatively wide interval of 3.1 to 23.1 wt% NaCl eq. Gradual reduction of the fluid system during the post-ore substage is manifested by the appearance of a new generation of authigenic chlorite and pyrite. Chlorite thermometry yielded temperatures of 150°C to 170°C. Solid bitumens that post-date uranium mineralization indicate radiolytic polymerization of gaseous and liquid hydrocarbons and their derivatives. The origin of the organic compounds can be related to the diagenetic and catagenetic transformation of organic matter in Upper Stephanian and Permian sediments. (3) K–Ar ages on illite from post-uranium quartz-carbonate-sulfide mineralization range from 233.7 ± 4.7 to 227.5 ± 4.6 Ma and are consistent with the early Tethys-Central Atlantic rifting and tectonic reactivation of the Variscan structures of the Bohemian Massif. A minor part of the late Variscan uranium mineralization was remobilized during this hydrothermal event.

The 587 Ma-old hydrothermal bastnaesite mineralization of Gakara in the western branch of the East African Rift in Burundi has geological and chemical features typical of carbonatite-related REE ore deposits, but Nd, Ce, and Sr isotope data on the bastnaesite indicate a time-integrated LREE-enriched source with ɛNd( T)-11.9, ɛCe( T) 1.6, and 87Sr /86Sr ≤0.7075 . The mid-Proterozoic Nd model age of this source coincides with the formation of the intracratonic Kibaran basin of central Africa and may indicate a mid-Proterozoic rift-related event of small-scale, but locally very effective, mantle metasomatism in this region. The formation of REE ore deposits in carbonatite environments is possibly linked to enriched mantle domains.