We present a study of the serpentinized peridotites of the Ghadir-Mohagar-Ambaut area, Egypt. They represent the mantle section of a dismembered ophiolite, tectonically emplaced over a volcanosedimentary succession of island arc assemblages. The serpentinites are variably metamorphosed from greenschist to lower-amphibolite facies, metasomatized, and altered, including development of talc-carbonate and quartz-carbonate rocks, especially along shear zones and fault planes. Nevertheless, some samples contain relics of primary chromian spinel, olivine, and pyroxenes. Relict textures and whole-rock compositions (Mg# ½molar Mg=(Mg 1 Fe 21 ) p 0:92-0:93, with low Al 2 O 3 and CaO contents) both suggest harzburgite protoliths. The high Mg# and Ni contents of relict olivine and the high Cr# (molar Cr=(Cr 1 Al)) of fresh chromian spinel cores indicate that the protoliths experienced high degrees of partial melt extraction (∼34%-39%), well beyond the limit for exhaustion of clinopyroxene from the residue and consistent with formation in a forearc suprasubduction zone environment. The serpentinized ultramafic rocks in the study area are divided into massive serpentinite, serpentinitehosted magnesite masses, and magnesite-filled veins. The carbonation and formation of magnesite ores took place through two metasomatic stages; the first is represented by the magnesite masses and associated with deep-seated metasomatism and alteration during serpentinization, whereas the second, vein-forming stage postdates serpentinization and occurred during obduction of the ophiolite. The differences in chemical composition between massive serpentinite and serpentinitehosted magnesite masses suggest leaching of some elements and enrichment of others during carbonation; MgO, Cr, and Ni are depleted, whereas Fe 2 O 3 , CaO, MnO, Nb, Ba, Cu, Pb, Sr, and Zn are enriched in the serpentinite-hosted magnesite masses, relative to the host massive serpentinite.

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.

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This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.