Uranium and U-bearing minerals in the Husab uranium deposit in Namibia: Occurrence, composition, age, and implications for uranium mineralization process

Abstract

The Husab uranium deposit in Namibia is one of the largest alaskite-type uranium deposits in the world. However, few studies have been carried out on this deposit, especially on uranium and U-bearing minerals, greatly limiting the understanding of the deposit’s genesis. Accordingly, based on the α-track etching experiment, this study carried out fine mineralogical analysis of uranium ore using a polarizing microscope (PM), scanning electron microscope (SEM), and electron probe X-ray microanalyzer (EPMA). The types of uranium and U-bearing minerals were determined, and the mineralization stages were divided. Three mineralization stages are believed to exist in the Husab uranium deposit: magmatic, hydrothermal alteration, and supergene leaching. Among them, the magmatic mineralization stage is the main stage, and uraninite is the most important uranium mineral. The uranium minerals formed in this stage include monazite, zircon, and rutile. The uranium minerals in the hydrothermal alteration stage primarily comprise pitchblende and coffinite, while those in the supergene leaching stage include uranothorite, coffinite, boltwoodite, sodium boltwoodite, and uranophane. Based on the mineralogical analysis and mineralization stage division described above, the EPMA chemical U–Th–Pb dating of uraninite was carried out by selecting uraninite and its associated monazite as the dating minerals. The weighted average age of uraninite was 500.4 ± 1.8 Ma (mean squared weighted deviation (MSWD) = 1.12, n = 46/49). Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in situ U-Pb isotope dating was carried out at certain probe test points, and the ²⁰⁶Pb/²³⁸U weighted average age of uraninite was 498.2 ± 2.4 Ma (MSWD = 0.41, n = 24). The monazite associated with uraninite was selected for LA-ICP-MS in situ U-Pb isotope dating, and the weighted average ²⁰⁶Pb/²³⁸U age of monazite was 499.7 ± 2.4 Ma (MSWD = 2.1, n = 23). The ages of the three groups are highly consistent, indicating that the main metallogenic age of the Husab uranium deposit is approximately 500 Ma. The deposit originated in a decompressive and tensional environment following the collision of the Kalahari and Congo cratons. The in situ trace element test results for uraninite revealed a low U/Th ratio (8.49–15.08, mean: 9.99) and high REE content. The ∑REE is between 12,852 × 10⁻⁶ and 25,019 × 10⁻⁶, with an average of 18,041 × 10⁻⁶, indicating that uraninite has a magmatic origin (consistent with the magmatic mineralization stage identified earlier). Moreover, the abundant secondary uranium minerals in the ores may contribute to the formation of high-grade uranium ores in the Husab uranium deposit.