Geochemistry of U-bearing minerals from the Vale de Abrutiga uranium mine area, Central Portugal
Cabral Pinto, Marina M.S.; Silva, Maria M.V.G; Neiva, Ana M.R
published: Nov 28, 2008
ArtNo. ESP154018502005, Price: 29.00 €
The mineralized quartz veins contain muscovite, chlorite, zircon, monazite, ferberite, hübnerite, xenotime, pyrite, anatase, Fe oxides-hydroxides, saleeite, meta-saleeite, other dehydration U-phosphate phases and other P-U-bearing phases. Uraninite is rare and was only found in the Variscan granite. Some uraninite analyses show low totals and high values in Fe and Si, indicating some alteration. Ce-monazite in the granite is richer in U, P, Ca and poorer in LREE than Ce-monazite in the phyllite and quartz veins. In the mineralized quartz veins, zircon crystals are dissolved and vacuolated. The grains have cores chemically close to the endmember [(Hf,Zr)SiO4], but the rims have very low totals (∼ 82 wt.%), low Si and Zr and very high Fe and U (∼18 wt.% UO2) contents. In the mineralized quartz veins, anatase grains are vacuolated and they often contain or are rimmed by U-bearing Fe oxyhydroxides. In the mineralized veins, the chlorite which is in contact with the U-phosphates has a higher Mg/(Mg+Fe) ratio and higher U contents than the chlorite which occurs dispersed in these veins. The rims of chlorite crystals have higher U and Fe and less Mg contents than the cores, and some crystals are altered to Fe oxyhydroxides. Fe oxyhydroxide phases contain high values of Si and Al, but also P, U and sometimes both. The high contents of U and P found in the Fe oxyhydroxides may be due to nano-inclusions of U-phosphates. The precipitation of saleeite and meta-saleeite at the surface of Fe minerals was responsible for U retention within the quartz veins, leading to the uranium phosphate mineralization. Saleeite and meta-saleeite are associated with pyrite, chlorite and Fe oxyhydroxides. Other U-phosphate phases with similar compositions to saleeite but with different H2O content also occur. Also a ferrous-substituted-saleeite was found in the adjacent phyllite.