Original paper

Chemical composition of radioactive accessory minerals: implications for the evolution, alteration, age, and uranium fertility of the Fichtelgebirge granites (NE Bavaria, Germany)

Förster, H.J.; Rhede, D.; Hecht, L.

Abstract

The late Variscan granites of the Fichtelgebirge form an older, Namurian intrusive complex (OIC-p and OIC-e) and a younger, post-Westphalian intrusive complex (YIC-1 and YIC-2). Both complexes have distinct radioactive accessory-mineral assemblages and compositions. The OIC-p biotite monzogranites contain monazite-(Ce) and minor thorite, but apparently lack magmatic xenotime-(Y) and uraninite. The more evolved OIC-e two-mica granites bear monazite-(Ce) occasionally rich in Th (up to 21 wt% ThO2) and U (8 wt% UO2), xenotime-(Y) of moderate U content (< 3.3 wt% uo2), and uraninite poor in Th and the REE. The most fractionated YIC Li-mica granites (YIC-2) may contain cheralite extremely high in Th (40.5 wt% ThO2) and U (8.6 wt% UO2), xenotime-(Y) rich in U (6.3 wt% UO2), and also a Th-REE-poor uraninite. Dissolution-reprecipitation processes account for the formation of huttonite-rich domains in monazite-(Ce), uraninite in U-depleted areas in zircon, and Mn-Fe-rich phosphorus minerals in fluorapatite. The accommodation of U in Th-poor uraninite made the OIC-e and YIC granites highly susceptible to U leaching. However, currently known U mineralization spatially associated with these granites is economically negligible.Electron-microprobe analyses yield a mean U-Th-total Pb monazite+xenotime age of 321 ± 8 Ma (2σ) for the OIC, which is further supported by the average age of uraninite in one of the studied OIC-e granites (323 ± 6 Ma). These results are in harmony with previous age data and imply a penecontemporaneous emplacement of the OIC-p and OIC-e granites, between about 326 and 321 Ma. The YIC-1 and YIC-2 granites display statistically indistinguishable mean ages of 294 ± 6 Ma and 296 ± 6 Ma. These age data imply formation of the YIC granites during one intrusion period and question recent models postulating their generation during two discrete magmatic episodes separated by ∼15 Ma. The mean monazite+xenotime age of 296 ± 4 Ma obtained for the emplacement of the Younger Intrusive Complex, together with the age of uraninite in one of the YIC-1 granite samples (294 ± 3 Ma), suggest intrusion of this granite assemblage at the Carboniferous/Permian boundary. The YIC postdates compositionally similar granites in the neighbouring Erzgebirge-Vogtland Zone by 20-25 Ma, which emplaced penecontemporaneously with the OIC.

Keywords

monazitehuttonitexenotimeuraninitedissolution-reprecipitationlanthanide tetrad effecturanium mineralizationth-u-pb age