Mineralogy, geochemistry and age dating of shear zone-hosted Nb-Ta-, Zr-Hf-, Th-, U-bearing granitic rocks in the Ghadir and El-Sella areas, South Eastern Desert, Egypt

Abstract

The rare metal minerals of mineralized altered granites within the Ghadir and El-Sella shear zones, are represented by betafite, U-minerals (uraninite and uranophane), zircon, monazite, xenotime, and rutile in the Ghadir shear zone. While they are columbite-tantalite minerals as ferrocolumbite, pyrochlore, and fergusonite, Th-minerals (cheralite, uranothorite, and huttonite monazite), Hf-zircon, monazite and xenotime in the El-Sella shear zone. Hf-zircon in the El-Sella and Ghadir shear zones (increasing from the core to the rim) contains high inclusions of U-Th, and REE minerals such as cheralite, uranothorite, huttonite monazite and xenotime especially in the El Sella shear zone. The rare-metal minerals, identified from peralkminous granites of the shear zones are associated with muscovite, quartz, chlorite, fluorite, magnetite, and biotite that are restricted to the two shear zones. Uraninite (low Th content) occurring in the Ghadir shear zone indicates the hydrothermal origin, but there are thorite, uranothorite, cheralite, and Hf-zircon in the El Sella shear zone, also indicating the hyrothermal proccess after magmatic origin. Compositional variations of Ta/(Ta+Nb) and Mn/(Mn+Fe) in columbite from 0.07–0.42 and 0.04–0.33, respectively, and Hf contents in zircon are so high as to be 12%, especially in the rim in the El Sella shear zone. This feature reflects the extreme degree of magmatic fractionation. Four samples from the altered granites of the Ghadir shear zone also are very low in TiO2 (0.04 wt%–0.17 wt%), Sr [(82−121)×10−6], and Ba [(36−380)×10−6], but high in Fe2O3 T (0.46 wt%–0.68 wt%), CaO (0.64 wt%–1.23 wt%), alkalis (8.59 wt%–8.88 wt%), Rb [(11-203)×10−6], Zr [(98–121)×10−6], Nb [(9-276)×10−6], Ta [(2–139)×10−6], U [(14–63)×10−6], Th [(16–105)×10−6], Pb [(13–32)sx10−6], Zn [(7–8)×10−6], Y [(15–138)×10−6], Hf [(3–9)×10−6], and ΣREE [(81–395)×10−6, especially LREE [(70–322)×10−6]. They are very high in Zr/Hf (15.07–85.96) and Nb/Ta (7.17–21.48), and low in Rb/Sr (2.56–3.36) and Th/U (0.096–3.36). Four samples of the altered granites from the El Sella shear zone are very low in TiO2 (0.23 wt%–0.38 wt%), Sr [(47–933)×10−6], and Ba [(82–175)×10−6], with high Fe2O3 T (1.96 wt%–2.87 wt%), CaO (0.43 wt%–0.6 wt%), alkalis (4.46 wt%–10.7 wt%), Rb [(109–313)×10−6], Zr [(178–1871)×10−6], Nb [(11–404)×10−6], U [(56–182)×10−6], Th [(7-188)×10−6], Ta [(0.5–57)×10−6], Pb [(12–28)×10−6], Zn [(1–13)×10−6], Y [(62–156)×10−6], Hf [(3–124)×10−6], and ζREE [(101–184)×10−6], especially HREE [(7–139)×10−6]. This is consistent with the very fractionated, fluorine-bearing granitic rocks that were altered and sheared in the El Sella shear zone. Zr/Hf (14.23–39.79) and Nb/Ta (1.98–7.01) are very high, and Rb/Sr (0.14–1.7) and Th/U (0.25–2.5) are low in the Ghadir shear zone. Field evidence, textural relations, and the composition of ore minerals suggest that the main mineralizing event was magmatic (615+/−7 Ma, and 644+/−7 Ma CHIME monazite), especially in the El Sella shear zone, with later hydrothermal alteration and local remobilization of the high-field-strength elements.