Geochemistry and Petrogenesis of Late Ediacaran Rare‐metal Albite Granites of the Arabian‐Nubian Shield

Abstract

The Abu Dabbab albite granite (ADAG), in the central Eastern Desert of Egypt, hosts the most significant rare metal ore deposit in the northern part of the Neoproterozoic Arabian-Nubian Shield. Here, we report detailed field, petrographic, mineralogical and geochemical investigation of the ADAG, an isolated stock-like granitic body with sharp intrusive contacts against metamorphic country rocks, probably emplaced at about 600 Ma. The fine-grained porphyritic upper unit is a preserved remnant of the shallowly-emplaced apex of the magma chamber, whereas the medium-grained lower unit crystallized at deeper levels under subvolcanic conditions. The peraluminous leucocratic ADAG shares common geochemical characteristics with post-collisional intraplate A-type magmas. In addition to the conspicuous enrichment in Na 2 O, the ADAG is remarkable for its anomalous concentrations of Ta, Nb, Li, Hf, Ga, Sn, Zn and heavy rare-earth elements. Nb-Ta minerals in the ADAG are mixed with Fe-Mn oxides, forming black patches that increase in abundance toward of the base of the intrusion. Columbite-tantalite, cassiterite and wolframite are the most important ore minerals. Pronounced negative Eu anomalies (Eu/Eu * = 0.10-0.24) reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interaction. The ADAG was most likely generated by partial melting of the juvenile middle crust of the ANS as the geotherm was elevated by erosional uplift following lithospheric delamination and it was emplaced at the intersection of lineations of structural weakness. Although formation of the ADAG and its primary enrichment in rare metals are essentially due to magmatic processes, late-stage metasomatism caused limited redistribution of rare metals. Fluid-driven subsolidus modification was limited to the apex of the magma chamber and drove development of greisen, amazonite, and quartz veins along fracture systems.

Assessment of magmatic versus post-magmatic processes in the Mueilha rare-metal granite, Eastern Desert of Egypt, Arabian-Nubian Shield

“…5f), their high Mn contents place them outside the S-type magmatic field of Dahlquist et al (2007) and outside any of the granite-type garnet fields in the compilation of Zhang et al (2012). Yet the compositions of the garnets from the present study are similar to garnet compositions reported from several post-collisional granites in the Eastern Desert of Egypt (Azer et al, 2019;Abuamarah et al, 2019). The most likely resolution of the observation that the garnet compositions in the Mueilha garnet-bearing phase are broadly magmatic and yet anomalous compared to all the standard tectonicallyinterpreted granite types is that they represent a transitional stage, somewhere on the spectrum from late-magmatic to hydrothermal, having grown from a very H 2 O-and Frich silica-rich peralkaline magma at a temperature below the F-free granite minimum.…”

“…The concentrations are well-correlated with SiO 2 for most elements. Similar variability has been described in other rare-metal bearing granites in the Eastern Desert of Egypt (Abuamarah et al, 2019;Azer et al, 2019;Heikal et al, 2019). Selected trace elements are plotted against SiO 2 in Fig.…”

“…The mode of emplacement of the Mueilha intrusion is very similar to the Abu Dabbab and Nuweibi intrusions in the Eastern Desert of Egypt (Azer et al, 2019;Abuamarah et al, 2019). Each of these intrusions is divided into hypabyssal and porphyritic varieties suggesting that a gradient in emplacement depth extending to very shallow crustal levels is preserved and exposed in each case.…”

“…A number of potential sources and models of formation have been proposed for the A-type rocks of the Eastern Desert of Egypt (e.g. Sami et al 2017Sami et al , 2018Heikal et al, 2019;Abuamarah et al, 2019. The large differences in major and trace element concentrations and isotope ratios among various A-type granitic rocks in the Eastern Desert of Egypt strongly suggest that a variety of processes and sources were involved in their genesis (Farahat et al, 2007;Ali et al, 2012).…”

“…Rare-metal bearing granites in the Eastern Desert of Egypt have been attributed to various magmatic processes (Morsy and Mohamed, 1992;Mohamed, 1993;Jahn, 1996;Helba et al, 1997;Abou El Maaty and Ali Bik, 2000), to post-magmatic hightemperature metasomatism (El-Galy et al, 2016;Abu El-Rus et al, 2017), or to a combination of magmatic and post-magmatic processes (Abuamarah et al, 2019;Heikal et al, 2019).…”