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

Abstract

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, coffinite, 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 coffinite, which resulted from uraninite alteration. The U contents in the coffinite 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. Some thorite seemed to be primary, whereas the other thorite was related to the granite alteration, replaced apatite and monazite, was associated with xenotime, and filled the fractures of several minerals. In the altered granite, thorite had low UO2 contents (0.46 wt. %) in the fractured crystal zones. Monazite from the altered granite had a pervasive porosity; some crystals were formed by the alteration of apatite, and were frequently replaced by thorite. Monazite and xenotime from the altered granite and hydrothermal veins had lower U contents than these minerals from the unaltered granite. In the altered granite, xenotime crystals were zoned, and their cores were richer in U than the rims. Apatite from the altered granite was fractured, showed dissolution, and had lower U and P contents than the apatite from the unaltered granite. In the quartz veins, apatite crystals were replaced by uranium phosphates and had high U contents (~1.1 wt. % UO2). In the quartz veins, zircon rims had an extraordinary U enrichment (up to 18 wt. % UO2). The most altered rims of chlorite and anatase from the quartz veins were partially replaced by U-bearing Fe oxyhydroxides containing up to 5.7 wt. % UO2. Meteoric water warmed by deep circulation through granite faults, shear zones, and quartz veins became enriched in U, P, and Mg due to the solubilization of mainly uraninite, coffinite, thorite, monazite, apatite, and chlorite. Uranium from these solutions was later adsorbed on Fe oxyhydroxides and the weathered surfaces of anatase, zircon, and apatite, or precipitated as saleeite and meta-saleeite on the surface of Fe minerals and the apatite-weathered surface due to local saturation.