## 5 Discussions
### Mechanism of Uranium Concentration in Ferruginous Sandstone  
(1) Ferruginous sandstone sedimentary period (uranium preconcentrated period)  
The ferruginous sandstone in the Wajid Group is mainly found in the Khusayyayn Formation of the Early Devonian. The lithology of the Khusayyayn Formation is brownish-yellow and light-yellow medium-coarse sandstone containing lenticular ferruginous sandstone, with large-scale cross-bedding and a small number of parallel laminae, which is a typical littoral continental beach deposit. The humid and hot paleoclimate conditions are conducive to the weathering of the rocks in the Arabian Shield and provide a sufficient source of iron [61]. Ferrous matters were transported to the shallow water environment of the relatively closed or semi-closed inland bay in the Wajid Basin in the form of colloidal and fine suspended gel oxides from the Arabian Shield. With the increased sea level, its hydrodynamic conditions were weaker, which was more favorable for the sedimentation of ferruginous sandstone. The ferrous mineral assemblages are hematite, limonite, and goethite and were mainly formed during the sedimentary and diagenetic periods. Uranium matters were also weathered and transported to the basin from the uranium-rich granite bodies in the Arabian Shield [62], then adsorbed by ferrous cements. The uranium was preconcentrated during the sedimentary and diagenetic periods. A small part of uranium was distributed in heavy minerals, most of which were free stated and adsorbed by the ferrous cement (Figure 5d), and the uranium content was generally 20-50 ppm (Figure 11a).  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Sample** & **U (\(\mu\)g/g)** & **Pb (\(\mu\)g/g)** & **238**U/**204**Pb** & **208 Pb/204**Pb** & **207 Pb/204**Pb** & **206 Pb/204**Pb** \\ \hline UA5.1/SM01 & 86.9 & 37.3 & 160.655 & 39.767 & 16.113 & 23.003 \\ \hline UA5.1/SM04 & 39 & 15.5 & 170.314 & 39.135 & 15.956 & 20.803 \\ \hline UA5.1/SM05 & 215 & 16.3 & 906.008 & 39.054 & 16.123 & 23.333 \\ \hline UA5.1/SM10 & 183 & 21.0 & 625.821 & 39.128 & 16.274 & 26.766 \\ \hline UA5.1/SM11 & 320 & 29.5 & 977.673 & 39.232 & 17.347 & 46.660 \\ \hline UA5.1/SM17 & 365 & 64.4 & 380.684 & 39.115 & 16.034 & 21.565 \\ \hline UA5.1/SM18 & 627 & 33.5 & 1309.290 & 39.014 & 16.197 & 24.807 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Isochron age of U and Pb of ferruginous sandstone in Wajid Basin.  
(2) Ferrruginous sandstone transformation period (uranium transformation period)  
With the slow uplift of the Arabian shield, the ferruginous sandstone of the Khusayyayn Formation was exposed to the surface and denuded. The ferruginous sandstone was transformed, and the uranium deposits were superimposed and transformed by the metallogenic fluid. The main type of uranium mineral was pitchblende, distributed in the fissures, solution pores, and solution fissures of quartz particles. The isochronous uranium metallogenic age given by the whole-rock U and Pb isotope assaying was 22.58 \(\pm\) 0.47 Ma in the early Neogene. Due to the hot and dry climate in the Neogene, the limonite and goethite in the ferruginous sandstone were further oxidized and dehydrated to form hematite. The uranium preconcentrated in the ferruginous sandstone during in sedimentary period was superimposed and transformed by the oxygen-containing uranium fluid and further concentrated in the cracks or dissolved pores of quartz to form uranium mineralization. The uranium content is generally 80-1000 ppm (Figure 11b).