Earth Science Frontiers ›› 2022, Vol. 29 ›› Issue (1): 377-402.DOI: 10.13745/j.esf.sf.2020.10.31

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Assimilation-fractional crystallization (AFC) of pegmatitic magma and its implications for uranium mineralization: A case study of the Husab uranium deposit, Namibia

HUANG Ranxiao1,2(), WANG Guosheng1, YUAN Guoli1,*(), QIU Kunfeng1, Hounkpe Jechonias BIDOSSESSI3   

  1. 1. School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China
    2. China National Nuclear Geology and Mining Technology Group Co., Ltd, Beijing 100013, China3. Department of Earth Sciences, Faculty of Sciences and Engineering, University of Abomey-Calavi, Cotonou 999105, Benin
  • Received:2020-06-19 Revised:2020-09-15 Online:2022-01-25 Published:2022-02-22
  • Contact: YUAN Guoli


The Husab uranium deposit in the southern Central Zone of the Damara Orogenic Belt, Namibia, is a typical pegmatite-hosted ore deposit with uraninite as the main industrial uranium mineral. The mechanisms of uranium enrichment and precipitation in different ores are not clear. In order to reveal the relationship between pegmatitic magma evolution and uranium mineralization, we carried out whole-rock and mineral geochemical studies on pegmatite ores with different mineral compositions. Field and microscopic investigations showed the mineralized pegmatites can be divided into ‘simple’ and ‘complex’ types. The ‘simple’ pegmatite has normal granitic pegmatite texture, with a uniform distribution of uraninites among rock-forming minerals and low to medium level mineralization. The ‘complex’ pegmatite is characterized by heterogeneous texture with extremely high level mineralization and spatial association of uraninite with biotite agglomerates. Geochemical analysis showed that, in ‘simple’ pegmatite, uranium enrichment is mainly achieved through fractional crystallization of pegmatitic magma; while in ‘complex’ pegmatite, magmatic evolution is clearly controlled by assimilation and contamination, and uranium mineralization is the result of assimilation-fractional crystallization (AFC) of pegmatitic magma. The mixing of external mafic components (FeO, MgO, TiO2, MnO), specifically, led to change in the mineral crystallization sequence in ‘complex’ pegmatite melt, where “delayed” crystallization of feldspar minerals provided biotite with more favorable crystallization space/condition and led to massive biotite aggregation. Abundant biotite precipitation induced hydrolysis of UFm4-m complex in residual magama, which resulted in the precipitation of crystalline uraninites in and around biotite agglomerates. Thus, by studying the two types of uraniferous pegmatites one can better understand the magmatic evolutionary process and uranium mineralization mechanism, so as to enrich our knowledge about the pegmatite-type uranium deposit and provide a scientific basis for its future exploration and development.

Key words: pegmatite-type uranium deposit, mineralization process, assimilation-fractional crystallization (AFC) process, Damara orogenic belt, Namibia

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