Release, Migration, Sorption, and (Re)Precipitation of U during Peraluminous Granite Alteration under Oxidizing Conditions in Central Portugal

In this work, in order to study the release, migration, sorption, and (re)precipitation of uranium (U) during alteration under oxidizing conditions, we carried out a systematic study using scanning electron microscopy, X-ray maps, and electron microprobe analyses on uranium minerals—such as uraninite, cofnite, saleeite, meta-saleeite, and thorite—and U-bearing minerals—such as xenotime, monazite, apatite, and zircon—from unaltered and altered Variscan peraluminous granites and related hydrothermal brecciated uranium–quartz veins. The paragenetic sequence of the granite and the mineralized quartz veins from Vale the Abrutiga is presented. Uraninite is magmatic in origin and occurs mainly in unaltered granite; it is rare in altered granite, and was not found in the mineralized quartz veins. Uraninite from the altered granite was fractured and hydrated; it had radioactive damage halos filled with late pyrite, U–S-bearing phases, and Fe oxyhydroxides; its analytical totals were also lower than in the uraninite from the unaltered granite. The alteration zones and crystal rims were poorer in U (86.7 wt. % UO2) than in the cores and unaltered zones (90.2 wt. % UO2), and some uraninite crystals were replaced by cofnite, which resulted from uraninite alteration. The U contents in the cofnite crystals ranged between 65.0 wt. % UO2 in the rims to 84.0 wt. % UO2 in the cores of the crystals. Thorite was found in all of the granite samples, and its composition was variable from 0.5 wt. % UO2 to 10.4 wt. % UO2.