U-th-rich zircon, thorite and allanite-(ce) as main carriers of radioactivity in the highly radioactive ultrapotassic melasyenite porphyry from the šumava mts., moldanubian zone, Czech Republic

Abstract

A melasyenite porphyry from the vicinity of Kašperské Hory (the Šumava Mts., Moldanubian Zone) shows a high natural radioactivity (K = 4.4-6.1 wt. %, eU = 15-24 ppm, eTh = 38-49 ppm). The most important carriers of U and Th are zircon, thorite, allanite-(Ce) and scarce monazite. Most of the zircon and thorite grains are altered by post-magmatic processes due to which especially zircon shows a large degree of chemical variability. Primary magmatic zircon is nearly stoichiometric, poor in ThO2 (up to 0.13 wt. %) and UO2 (up to 0.42 wt. %). It often encloses strongly altered U-and The-rich cores. These cores show a conspicuous zoning in the BSE. The bright zones are enriched in UO2 (0.66-1.36 wt. %) and ThO2 (0.21-0.94 wt. %) whereas the darker zones are highly enriched in CaO (up to 5.19 wt. %), with UO2 contents ranging between 0.95 and 2.15 wt. % and the ThO2 between 0.99 and 1.67 wt. %. The first percolation point of zircon was estimated at 0.4 D[dpa]. Thorite is also strongly altered, being depleted in Y,REE and Si in its more hydrated parts. A further consequence are strongly varying UO2 contents (1.79-27.60 wt. %) and locally enriched ZrO2 (up to 10.92 wt. %), P2O5 (0.22-6.39 wt. %) and Y,REE (0.73-12.81 wt. % of REE2O3 and Y2O3 combined). Elevated amounts of Fe, Ca, Pb, As, Al, S, and Ti were detected in the most hydrated parts. Allanite-(Ce) is weakly zoned with 0.20-1.22 wt. % ThO2 and up to 0.11 wt. % UO2. Scarce monazite-(Ce) has the ThO2 contents (13.57-15.85 wt. %) strongly prevailing over UO2 (1.51-1.91 wt. %).