F.H. Mohamed

Geochemical constraints on the tectonomagmatic evolution of the late Precambrian Fawakhir ophiolite, Central Eastern Desert, Egypt

Abstract The Fawakhir ophiolitic sequence is composed mainly of serpentinites, metagabbros, boninitic metagabbros and metavolcanics. The serpentinites have geochemical features similar to those of depleted-mantle peridotites. Major and trace element characteristics show that the ophiolitic metagabbro and metavolcanic rocks are of tholeftic to calc-alkaline affinity. REE abundances in the metagabbros and metavolcanics are characterised by flat to fractionated patterns. The Fawakhir ophiolitic sequence shows a spectrum of compositions ranging from arc-type lavas, P-type MORB and boninitic rocks, suggesting that these rock units developed in a back-arc environment. The evolution of the Fawakhir ophiolite sequence from early tholeftes to later boninitic-type magmas may suggests that these rocks could be generated by fractional fusion and melt extraction during ascent of the magma source within a mantle melting column. The boninites were generated from a more refractory mantle source relative to the other rocks.

MINERALOGICAL AND GEOCHEMICAL INVESTIGATION OF EMERALD AND BERYL MINERALISATION, PAN-AFRICAN BELT OF EGYPT: GENETIC AND EXPLORATION ASPECTS

Abstract Mineralogical, geochemical and fluid inclusion studies reveal two favorable environments for the localisation of beryl mineralisations in the Precambrian rocks of Egypt: (1) emerald-schist; and (2) beryl-specialised granitoid associations. Emerald occurs within the mica schists and is typically confined to the Nugrus major shear zone. However, beryl associated with granitoids occurs in pegmatite veins, greisen bodies, and cassiterite quartz veins cutting the granites and the exocontacts of the volcanosedimentary country rocks. Compositionally, emerald is of octahedral type and its cell edge is lengthened along the a-axis, while beryl associated with granitoids is normal in composition and structural constants. Emerald is thought to be formed as the result of epitactic nucleation of Be, Al and alkali-rich solutions on the mica of the schist country rocks. Fluid inclusion studies show that the solutions are saline (8–22 wt% NaCl equiv.) and the reactions proceeded in the temperature range 260–382°C. On the other hand, aqueous inclusions in beryl associated with granitoids show the following sequence of formation with decreasing temperatures and salinities: beryl pegmatite (320–480°C and 7–16 wt% NaCl equiv.)→greisen bodies (190–400°C and 4–7 wt% NaCl equiv.)→cassiterite-quartz veins (190–380°C and 2–4 wt% NaCk equiv.). This study suggests that factors such as the chemistry of the Be-bearing fluids (rather than that of the bulk host schists) and syn-tectonic intrusions of leucogranites and pegmatites (Bederiving sources) along major ductile shear zones are the important factors controlling emerald formation. However, the endogreisens and exogreisens are the most important targets characterising the metasomatically- and magmatically-specialised, Be-granitoids, respectively. The aqueous inclusions examined in greisen beryls of metasomatised granites show a shorter range of homogenisation temperatures (260–390°C) and salinities(4.8-7 wt% NaCl equiv.) as compared to those of magmatically-specialised granitoids (190–400°C and 4–7 wt% NaCl equiv.). This phenomenon can be partly attributed to the late development of the fracture system during the crystallisation history of the metasomatised granites, where little or no contribution from meteoric waters occurred.

Geochemistry and Petrogenesis of the Neoproterozoic Granitoids in the Central Eastern Desert, Egypt

Abstract Neoproterozoic rocks in the Umm Gheig Province (UGP), central Eastern Desert, comprise five plutonic rock suites, i.e. a gabbro-diorite suite (GDS), an older granitoid suite (OGS), an I-type younger granitoid suite (I-YGS), an A-type younger granitoid suite (A-YGS) and a garnet-bearing granitoid suite (GBGS). The GDS exhibits a tholeiitic affinity generated in a volcanic arc, subduction-related environment. The unfractionated REEpattern of the gabbro {(La/Lu) N =0.6}differs significantly from the fractionated pattern of the diorite {(La/Lu) N =5.07}. They are, however, characterized by flat HREE{(Gd/Lu) N =1.24 and 1.45, respectively}and the absence of a Eu anomaly (Eu/Eu*=1.03 for both). The rocks of the GDSdo not represent a primary magma, and the gabbro could be generated by high degree melting of a garnet-free source followed by crystallization of clinopyroxene and olivine. The dioritic melt could be produced by melting of gabbroic rocks. The granitoids can be divided into four general suites emplaced late- (OGS, I-YGS and GBGS) and post-tectonically (A-YGS), each of which shows some variations in their bulk chemical compositions. All the granitoid suites display ferriferous and metaluminous affinities and belong to the non-alkaline series:OGS and I-YGS represent calc-alkaline magmatism while A-YGSand GBGS encompasses highly fractionated calc-alkaline magmatism. There is substantial uniformity in REEpatterns between the OGS, I-YGS and A-YGS, with no real change in light to heavy REEratios. Chondrite-normalized REEpatterns of the GBGSis characterized by strong enrichment in HREE and pronounced negative Eu anomalies (Eu/Eu*=0.095) compared to the other granitoid suites. The geochemistry and petrological characteristics of the granitoid suites suggest close genetic relationships and it is suggested that they were formed from a single parent magma, over a relatively short period of time. Major and trace element variations in the different granitoid suites indicate that they can be modelled by fractional crystallization of plagioclase, K-feldspar, biotite and hornblende. Variation in REEpatterns are governed by fractionation of accessory phases such as apatite, monazite and zircon which occur as inclusions in phenocryst phases.

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization and metallogenetic aspects

Three “younger granite” plutons from the Eastern Desert of Egypt are studied: petrographic and geochemical characteristics of the barren pink granites at Wadi Sikait and Wadi Nugrus are similar, of alkaline, mildly peraluminous nature and are enriched in LIL-elements and LREE with moderate negative Eu anomalies. In contrast, the Sn-Ta-W-bearing albite granite of Abu Dabbab is alkaline, peraluminous muscovite granite; its chemical specialization is manifested by the pronounced enrichment in Ta, Sn, W, F, Rb and Li coupled with marked depletion in Ca, Ti, Mg, Sr and Ba. Elemental ratios (e.g., K/Rb, Rb/Sr, Ba/Rb) discriminate the albite granite and the pink granites into “mineralized and barren granites”, respectively. The albite granite is derived from Na-rich magma of within-plate characteristics. Fluorine was an important complexing anion during magmatic evolution history. The albite granite is emplaced at shallow depth (<100 MPa) and at the intersection of structural weaknesses. The pink granites might have a crustal and/or LIL-element enriched mantle sources, in which the subduction-related fingerprints are partly obliterated. For both types, reactivation of regional structures played a significant role in magma generation. Acid metasomatism is mainly manifested by the development of thin greisen veins along fracture systems in the albite granite. The chemistry of greisenization using mass balance approach reveals that the process is accompanied by dramatic increase in SiO2, Fe2O3, MnO, F, Sn and Li as well as significant loss in Na2O, K2O, Ba, Nb and Zn. The process causes a significant increase in volume (30%). Changes in chemical components are consistent with the observed mineralogical changes. Microprobe results reveal that the wolframite crystals are typically huebnerite with Fe-rich cores and Mn-rich rims. Compositional variations in wolframite crystals are attributed to the physicochemical conditions (pH, T, etc.) and chemistry of the ore-bearing fluids.

Geochemical and petrological evidence of calc-alkaline and A-type magmatism in the Homrit Waggat and El-Yatima areas of eastern Egypt

Abstract The Neoproterozoic plutonic complex in the Homrit Waggat and El-Yatima areas, central Eastern Desert of Egypt, comprises a deformed calc-alkaline I-type metagabbro-diorite complex and tonalite-granodiorite suite invaded by felsic high level intrusions of A-type characteristics. The metagabbro-dionte complex exhibits petrological and geochemical characteristics of mantle-derived island-arc basalts, and its magma was derived possibly from partial melting of a mantle wedge above an early Pan-African subduction zone. The rocks of the ton alite-granodiorite suite have a wide range of SiO 2 (62–71%), and K, Rb and Ba enrichment relative to Nb and Y. Their chemical variations suggest that they are not related to the gabbro-diorite complex, but most probably derived by partial melting of the amphibolitic lower crust in a subduction zone. The A-type granites are mainly syeno- and alkali-feldspar granites characterised by sub- and hyper-solvus textures, late magmatic interstitial biotite and interstitial or vein-fluorite. They are geochemically evolved (SiO 2 =74–78%), metaluminous to mildly peraluminous, enriched in Fe, Y, Nb, Rb, Zr and F, and depleted in CaO, MgO, Ba and Sr. Although they can be classified as A-type and within-plate granites, the least differentiated samples have F, Nb, Y and Rb contents similar to those in the surrounding I-type ton alite-granodiorite suite. This similarity suggests that high concentrations of these elements in the A-type granite are mostly related to unusual fractionation processes rather than to source rock (A-type source). This simply indicates that these granites are I-type and their classification as A-type reflects the process of evolution. A petrogenetic model of dehydration partial melting of an early Pan-African lower crust along major shear zones in a post-collisional environment to produce granodioritic melt seems likely. Fractional crystallisation of this granodioritic melt gave silicic granites, during which a F-rich fluid phase was evolved. Late magmatic F-rich fluid-rock interaction and F complexing played an important role in the evolution and chemical characterisation of the A-type granites.

Garnet-bearing leucogranite in the El-Hudi area, southern Egypt: evidence of crustal anatexis during Pan-African low pressure regional metamorphism

Abstract The Wadi El-Hudi area, in the south Eastern Desert of Egypt, comprises a high-grade metamorphic complex of migmatite and biotite gneiss that host a garnet-bearing leucogranite body. This complex has been intruded later by post-orogenic pink granites. Gneisses and migmatites show obvious irregular layering, which lies concordently with the main foliation. The garnet-bearing leucogranite is a texturally heterogeneous small body (about 3 km 2 ) that possesses a similar mineralogical composition to that of gneisses and migmatites. This mineralogical similarity, beside the presence of ubiquitous metasedimentary xenoliths of the same mineralogical composition as the migmatite, indicates a cogenetic relationship. Geochemically, the garnet-bearing leucogranite is strongly peraluminous (A/CNK > 1.1) with normative corundum ranging between 1.3% and 4.0%. Major and trace element contents show considerable variations related to textural and mineralogical heterogeneity. Most of the samples have low CaO (0.23–1.15%), Sr (26–183 ppm), Y (11–35 ppm) and Zr (10–48 ppm) and high Na 2 O (2.78–4.02%), Rb (67–118 ppm) and Rb/Zr (2–12). These data, together with the field and mineralogical observations, imply that the garnet-bearing leucogranites were formed by dehydration partial fusion of chemically immature pelitic materials. The high contents of Zr (154–766 ppm), Cr (60–70 ppm), Y (116–177 ppm) and LREE (La + Ce + Nd = 290–335 ppm) are explained, in some samples, by their being retained from refractory mineral phases where the solid residue did not completely escape. The garnet-bearing leucogranite was emplaced as a consequence of high-temperature metamorphism during a major collisional event in eastern Egypt, when a Pan-African terrane assembly was attached to the East Saharan Craton. The high-temperature metamorphism was induced by anomalously high heat influx to shallow crustal levels subsequent to collision and crustal thickening.

Chemistry of Micas in Rare-Metal Granitoids and Associated Rocks, Eastern Desert, Egypt

Paragenetic, textural, and chemical characteristics of micas from 10 rare-metal granitic stocks and the associated greisens were examined in order to identify the metallogenetic processes of the host granitoids. The investigated granitoids and type occurrences can be categorized as: (1) metaluminous, Nb + Zr + Y-enriched alkali granite (e.g., Hawashia, Ineigi, and a stock northwest of Um Naggat); (2) peraluminous, Ta > Nb + Sn ± W + Be-enriched Li-albite granites (e.g., Nuweibi, Igla, and Abu Dabbab); and (3) metasomatized, Nb » Ta + Sn + Zr + Y + U ± Be ± W-enriched apogranites (e.g., Um Ara, Abu Rusheid, Mueilha, and Homr Akarem). Mica of the alkali granite is of the annite-siderophyllite series, and is characterized by an average FeO∗ of 28.14, low MgO of 0.05, a mean Fe∗/(Fe∗ + Mg)atom. value of 0.996, TiO2 of 0.69, enhanced Al2O3 of 14.91, MnO of 0.58, Li2O of 0.26, and moderate to low F of 0.86. These characteristics are representative of the relatively highly evolved nature of the annite-siderophyl...

Geochemical evolution of arc-related mafic plutonism in the Umm Naggat district, Eastern Desert of Egypt

Abstract The Umm Naggat metagabbro-diorite suite (UNGD) forms a part of the mafic intrusive province in the Arabian-Nubian Shield. The rocks range in composition from gabbro through diorite to quartz diorite. The suite forms a compositional continuum with a wide major element variation, particularly in terms of SiO 2 (49–61%), CaO (5–11 %) and MgO (3–10%) content. Geochemically, the suite has a tholeiitic/calc-alkaline affinity typical of subduction-related rocks. The compatible behaviour of Mg, Ca, Fe, Cr, Ni and Sc, together with the incompatibility of Na, K, Ba, Rb, Zr and Y, suggest that the evolution of the suite is largely controlled by crystal melt fractionation. In the Egyptian Shield, three chemically different groups of the orogenic-related mahc magmatism are discriminated: the I-type gabbro (group I), to which the UNGD complex belongs, is dominantly a calc-alkaline, arc-related suite characterized by a rather more fractionated trend with evolution towards more silica-rich compositions; the O-type gabbro (group II) includes the ophiolitic-related metagabbros with typical tholeiitic affinity and closely resembling the mid-ocean ridge basalts (MORB); the Y-type gabbro (group III) encompasses calc-alkaline/ tholeiitic fresh gabbros that have a more primitive and less fractionated character compared to group I. The UNGD suite is characterized by low abundances of HFS elements such as Nb, Ti, Zr and Y, which reflects a depleted (relative to MORB) mantle wedge overlying the subducted slab. The addition of a slab-derived fluid to such a depleted source is responsible for the relatively high abundances of LIL elements, particularly K, Rb, Ba, Th and Sr. Subsequently, the primary magma was differentiated largely by fractional crystallization of plagioclase, clinopyroxene, amphibole and magnetite to generate the more evolved rocks in the UNGD suite.

Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

ABSTRACT Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and 87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.

Geochemistry of the Wadi Hawashia Granite Complex, northern Egyptian Shield

Abstract The Hawashia Complex is comprised of calc-alkaline granitic rocks of peraluminous character. The bulk of the complex is composed of biotite granite, while the other intrusive phases, namely the “leucocratic-and alkali feldspar” granites, only constitute a minor proportion of the exposed surface area of the complex. Fractional crystallization is the dominant mechanism, which is necessary to explain the chemical attributes within the complex. Geochemical modelling reveals that an early stage of crystallization, which was controlled by plagioclase and amphibole separation, is necessary to generate the biotite granite. Later stages are dominated by K-feldspar crystallization, which evolves into the alkali feldspar granite. In contrast with the leucocratic granite, the alkali feldspar granite displays a significant enrichment in HFS elements (Nb, Y, Th, Zr) and a strong depletion in Ba and Sr. The small volume and the restriction of the alkali feldspar granite to the roof of the pluton, together with the specialized chemical signature, all substantiate the role of the volatile enhancement in the upper portion of the magma chamber. This volatile fraction was efficient in removing highly charged cations from the leucocratic granite and re-enriching them in the alkali feldspar granite.