Genesis of and uranium mineralization in leucogranite, Rossing, Namibi

The multiphase leucogranite of Rossing can be subdivided into six types, types A F, but only types D and E form leucogranite uranium deposits. Trace element and Pb isotopic analyses show that the Rossing leucogranite has the characteristics of mixed crustal source, and the two-stage Nd model ages suggest the highly radioactive pre-Damara basement is the source area. The biotite electron probe data show that biotite in type D leucogranite has higher fluorine content compared to types A C and F, and type D obviously has higher Nb/Ta contents. Types A C and F leucogranite show a negative correlation between Ba content and Rb/Sr ratio, consistent with muscovite dehydration melting; whilst types D and E show a more complex relationship consistent with biotite dehydration melting. The Rossing area experiences four deformation stages (D1 D4), where the emplacement of types A C leucogranites occurs no later than D3 while types D and E coincide with D4. The stress transformation during D4 changes the mode of anatexis in the ancient basement from muscovite to biotite melting, and the biotite melt provides fluoride ion, a uranium mineralizer. Therefore, the heterogeneous melting of the ancient basement is the cause of differential uranium enrichment in leucogranite of different phases. The Rossing leucogranite experiences strong crystallization differentiation, where the resulting crystalline minerals are mainly potassium feldspar, biotite, apatite, ilmenite and monazite. The fractional crystallization of biotite may cause uranium depletion, which is unfavorable to the mineralization of residual magma; whereas the fractional crystallization of K-feldspar, ilmenite and monazite is beneficial to the enrichment of uranium and formation of uraninite. © 2023 Science Frontiers editorial department. All rights reserved.