Mineralogical, geochemical and Sr-Nd isotopes characteristics of fluorite-bearing granites in the Northern Arabian-Nubian Shield, Egypt: Constraints on petrogenesis and evolution of their associated rare metal mineralization

Abstract

The Central Eastern Desert (CED) of Egypt, a part of Neoproterozoic Arabian Nubian Shield (ANS), embraces a multiplicity of rare metal bearing granitoids. Gabal El-Ineigi represents one of these granitic plutons and is a good example of the fluorite-bearing rare metal granites in the ANS. It is a composite pluton consisting of a porphyritic syenogranite (SG; normal granite) and coarse- to medium-grained highly evolved alkali-feldspar granite (AFG; fluorite and rare metal bearing granite) intruded into older granodiorite and metagabbro-diorite rocks. The rock-forming minerals are quartz, K-feldspar (Or94-99), plagioclase (An0-6) and biotite (protolithonite-siderophyllite) in both granitic types, with subordinate muscovite (Li-phengite) and fluorite in the AFG. Columbite-(Fe), fergusonite-(Y), rutile, zircon and thorite are the main accessory phases in the AFG while allanite-(Ce) and epidote are exclusively encountered in the SG. Texture and chemistry of minerals, especially fluorite, columbite and fergusonite, support their magmatic origin. Both granitic types are metaluminous to weakly peraluminous (A/CNK=0.95–1.01) and belong to the post-collisional A2-type granites, indicating melting of underplated mafic lower crust. The late phase AFG has distinctive geochemical features typical of rare metal bearing granites; it is highly fractionated calc-alkaline characterized by high Rb, Nb, Y, U and many other HFSE and HREE contents, and by extremely low Sr and Ba. Moreover, the REE patterns show pronounced negative Eu anomalies (Eu/Eu∗=0.03 and 0.06) and tetrad effect (TE1,3=1.13 and 1.27), implying extensive open system fractionation via fluid–rock interactions that characterize the late magmatic stage differentiation. The SG is remarkably enriched in Sr, Ba and invariably shows a relative enrichment in light rare-earth elements (LREEs). The SG rocks (569±15Ma) are characterized by relatively low initial 87Sr/86Sr ratios (0.7034–0.7035) that suggest their derivation from the mantle, with little contamination from the older continental crust. By contrast, the AFG has very high 87Rb/86Sr and 87Sr/86Sr ratios that reflect the disturbance of the Rb-Sr isotopic system and may give an indication for the high temperature magma-fluid interaction. The positive εNd(t) values of AFG (+7.40) and SG (+5.17), corresponding to young Nd-TDM2 ages ranging from 707 to 893 Ma, clearly reflect the juvenile crustal nature of Gabal El-Ineigi granitoids and preclude the occurrence of pre-Neoproterozoic continental crust in the ANS. The field relationships, chemical, petrological and isotopic characteristics of El-Ineigi SG and AFG prove that they are genetically not associated to each other and indicate a complex origin involving two compositionally distinct parental magmas that were both modified during magmatic fractionation processes. We argue that the SG was formed by partial melting of a mid-crustal source with subsequent fractional crystallization. In contrast, the AFG was generated by partial melting and fractionation of Nb- and Ta-rich amphibole (or biotite) of the lower crust. The appreciable amounts of fluorine in the magma appears to be responsible for the formation of rare metal element complexes (e.g., Nb, Ta, Sn and REEs), and could account for the rare metal mineralization in the El-Ineigi AFG.