Neoproterozoic diamictite in the Eastern Desert of Egypt and Northern Saudi Arabia: evidence of ~750 Ma glaciation in the Arabian–Nubian Shield?

The Neoproterozoic Atud diamictite in Wadi Kareim and Wadi Mobarak in the Eastern Desert of Egypt and the Nuwaybah formation in NW Saudi Arabia consist of poorly sorted, polymictic breccia, with clasts up to 1 m of granitoid, quartz porphyry, quartzite, basalt, greywacke, marble, arkose, and microconglomerate in fine-grained matrix. Stratigraphic relations indicate that the diamictite was deposited in a marine environment. Integrated field investigation, petrographic study and U–Pb SHRIMP zircon ages demonstrate that the Atud and Nuwaybah are correlative. The distribution of zircon ages indicate that ~750 Ma ages are dominant with a significant component of older materials, characterized by minor Mesoproterozoic and more abundant Paleoproterozoic and Neoarchean ages. Some matrix and metasedimentary clast zircons yield ages that are a few 10s of Ma younger than the age of the youngest clast (754 ± 15 Ma), suggesting Atud/Nuwaybah diamictite deposition ~750 Ma or slightly later, broadly consistent with being deposited during the Sturtian glaciation (740–660 Ma). The Paleoproterozoic and Neoarchean clasts have no source within the ensimatic Arabian–Nubian Shield. The distribution of the pre-Neoproterozoic ages are similar to the distribution of the pre-Neoproterozoic ages in Yemen and Saharan Metacraton, suggesting that these clasts have been transported hundreds of kilometers, maybe by ice-rafting. The Atud diamictite may represent important evidence for Cryogenian “Snowball Earth” in the Arabian–Nubian Shield.     

A late Precambrian (∼ 710 Ma) high volcanicity rift in the southern Eastern Desert of Egypt

The late Precambrian Shadli Metavolcanics of SE Egypt constitute a slightly metamorphosed bimodal sequence that has been previously interpreted as manifesting volcanic activity at an island arc. We report the first Rb-Sr geochronologic, trace element (including REE), and Nd isotopic data for these rocks. Two types of basalt are recognized, the stratigraphically lower suite having compositions like N-MORB ferrobasalt while the overlying basalt is similar to slightly fractionated E-MORB. The two basalt types were derived from melting of a strongly depleted source, most likely within the upper 60–75 km of the upper mantle. The origin of the felsic melts is problematic, and these could either have fractionated from a mafic melt or resulted from melting of juvenile crust. The mafic and felsic lavas yield a Rb-Sr isochron age of 712±24 Ma that probably represents the time of volcanic eruption. The trace element characteristics of both mafic and felsic members of the Shadli Metavolcanics show few of the hallmarks of subduction-related melts, and we reject the hypothesis that these formed at an island arc. Instead, the field and geochemical data are most consistent with the hypothesis that these rocks originated in a magmatic rift, where the eruption of large volumes of lava accompanied large-scale lithospheric extension. This inference suggests that the tectonic setting of the important 700–715 Ma crust-forming event in NE Africa and Arabia needs to be critically reexamined.

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Zusammenfassung Die spätpräkambrischen Shadli-Metavulkanite in der südöstlichen Eastern Desert von Ägypten sind eine schwach metamorphe bimodale Basalt-Rhyodazit-Abfolge, die bisher als Ausdruck eines Inselbogen-Vulkanismus gedeutet wurde. Zwei Basalt-Typen können aufgrund ihrer unterschiedlichen chemischen Zusammensetzung unterschieden werden: der stratigraphisch untere Typ ist ein N-MORB ähnlicher Ferrobasalt, während der überlagernde Typ Charakteristika eines leicht fraktionierten E-MORB aufweist. Die beiden Basaltvarietäten werden von stark an inkompatiblen Elementen verarmten Schmelzen aus den oberen 60–75 km des Mantels abgeleitet. Die Entstehung der sauren Metavulkanite ist nicht eindeutig geklärt; sowohl Fraktionierung aus einer mafischen Schmelze als auch Aufschmelzung juveniler Kruste vom Inselbogen-Typ sind denkbar. Die mafischen und felsischen Gesteine definieren zusammen ein Rb-Sr-Isochronenalter von 712 ± 24 Ma, das wir als den Zeitraum der Eruption deuten. Die Spurenelement-Verteilung der Shadli-Metavulkanite weist keine der charakteristischen Merkmale von Subduktionsmagmatismus auf, und wir sehen daher keinen direkten Zusammenhang mit einer Inselbogen-Entwicklung. Wir interpretieren den Shadli-Vulkanismus als Resultat eines Riftprozesses in junger kontinentaler Kruste, ähnlich dem Rio Grande-Rift oder dem Afar-Dreieck, wo starke Lithosphärendehnung die Förderung großer Lavamengen ermöglichte. Diese Interpretation stellt das einfache Schema einer panafrikanischen Krustenbildung durch Inselbogen-Addition im arabisch-nubischen Schild in Frage und erfordert eine Neubewertung bisheriger Modellvorstellungen.

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Résumé Les roches volcaniques faiblement métamorphisées du Précambrien tardif de Shadli dans le Sud-Est de l'Egypte se caractérisent par une séquence bimodale qui était jusqu'ici interprétée comme résultant d'un volcanisme d'arc insulaire. Nous présentons ici les premières données isotopiques (Rb-Sr, Nd) obtenues pour ces roches ainsi que des données d'éléments en trace (y compris les Terres Rares). Deux types principaux de basaltes peuvent être distingués: les basaltes qui se trouvent dans la partie inférieure de la colonne stratigraphique, ont une composition comparable aux ferrobasaltes de type N-MORB, tandis que les roches se trouvant dans la partie supérieure de la séquence s'apparentent plutôt aux E-MORB légèrement fractionnés. Les deux types de basalte proviennent de la fusion partielle d'un manteau fortement appauvri, et ce à une profondeur probable de 60 à 75 km dans le manteau supérieur. L'origine des laves acides pose, quant à elle, quelques problèmes: elles peuvent s'être formées par cristallisation fractionnée à partir d'un liquide basique, ou alors elles sont le produit de la fusion partielle d'une croûte juvénile. La combinaison des données Rb-Sr obtenues pour les roches acides et basiques permet l'obtention d'une isochrone définissant un âge de 712 ± 24 Ma. Cet âge est interprété comme datant les éruptions volcaniques. Les données d'éléments en traces obtenues pour les laves basiques et acides ne présentent aucune des caractéristiques associées au volcanisme d'arc insulaire. Par conséquent, nous rejetons l'hypothèse selon laquelle les roches volcaniques de Shadli se seraient formées dans un contexte d'arc insulaire. Les données géochimiques et de terrain seraient plutôt en accord avec une hypothèse selon laquelle les roches volcaniques métamorphiques de Shadli se seraient formées dans une zone de rift où de très volumineux épanchements de laves auraient succédé à une extension à grande échelle de la lithosphère. Cette interprétation des données nous conduit à suggérer un réexamen critique du contexte tectonique entourant la période de formation crustale se situant entre 700 a 715 Ma, dans le nord-est de l'Afrique ainsi qu'en Arabie.

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U–Pb TIMS age constraints on the evolution of the Neoproterozoic Meatiq Gneiss Dome,Eastern Desert,Egypt

Ages are used to constrain the temporal evolution of the Meatiq Gneiss Dome, Eastern Desert, Egypt, by dating (ID-TIMS) pre-, syn-, and post-tectonic igneous rocks in and around the dome. The Um Ba’anib Orthogneiss, comprising the deepest exposed structural levels of the dome, has a crystallization age of 630.8 ± 2 Ma. The overlying mylonites are interpreted to be a thrust sheet/complex (Abu Fannani Thrust Sheet) of highly mylonitized metasediments (?), migmatitic amphibolites, and orthogneisses with large and small tectonic lenses of less-deformed intrusives. Two syn-tectonic diorite lenses in this complex have crystallization ages of 609.0 ± 1.0 and 605.8 ± 0.9 Ma, respectively. The syn-tectonic Abu Ziran diorite, cutting across the tectonic contact between mylonite gneisses of the Abu Fannani Thrust Sheet and a structurally overlying thrust sheet of eugeoclinal rocks (“Pan-African nappe”), has a magmatic emplacement age of 606.4 ± 1.0 Ma. Zircons from a gabbro (Fawakhir ophiolite) within the eugeoclinal thrust sheet yielded a crystallization age of 736.5 ± 1.2 Ma. The post-tectonic Fawakhir monzodiorite intrudes the ophiolitic rocks and has an emplacement age of 597.8 ± 2.9 Ma. Two other post-tectonic granites, the Arieki granite that intrudes the foliated Um Ba’anib Orthogneiss, and the Um Had granite that cuts the deformed Hammamat sediments, have emplacement ages of 590 ± 3.1 and 596.3 ± 1.7 Ma, respectively. We consider formation of the Meatiq Gneiss Dome to be a young structural feature (<631 Ma), and our preferred tectonic interpretation is that it formed as a result of NE–SW shortening contemporaneous with folding of the nearby Hammamat sediments around 605–600 Ma, during oblique collision of East and West Gondwana.     

Spinels,Fe–Ti oxide minerals,apatites, and carbonates hosted in the ophiolites of Eastern Desert of Egypt: mineralogy and chemical aspects

Spinels, Fe–Ti oxide minerals, apatites, and carbonates hosted in ophiolitic serpentinites and metagabbros of Gabal Garf (southern ED) and Wadi Hammariya (central ED) of Egypt are discussed. Microscopic and electron probe studies on these minerals are made to evaluate their textural and compositional variations. Alteration of chromites led to form ferritchromite and magnetite; rutile–magnetite intergrowths and martite are common in serpentinites. Fine trillis exsolution of ilmenite–magnetite and ilmenite–hematite and intergrowth of rutile–magnetite and ilmenite–sphene are recorded. Composite intergrowth grains of titanomagnetite–ilmenite trellis lamellae are common in metagabbros. The formation of ilmenite trellis and lamellae in magnetite and titanomagnetite indicate an oxidation process due to excess of oxygen contained in titanomagnetite; trapped and external oxidizing agents. This indicates the high P _H2O and oxygen fugacity of the parental magma. The sulfides minerals include pyrrhotite, pyrite and chalcopyrite. Based on the chemical characteristics, the Fe–Ti oxide from the ophiolitic metagabbros in both areas corresponds to ilmenite. The patites from the metagabbros are identified as fluor-apatite. Carbonates are represented by dolomites in serpentinites and calcite in metagabbros. Spinel crystals in serpentinites are homogenous or zoned with unaltered cores of Al-spinel to ferritchromit and Cr-magnetite toward the altered rims. Compared to cores, the metamorphic rims are enriched in Cr# (0.87–1.00 vs. 0.83–0.86 for rims and cores, respectively) and impoverished in Mg# (0.26–0.48 vs. 0.56–0.67) due to Mg–Fe and Al (Cr)–Fe³⁺ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. The Fe–Ti oxides have been formed under temperature of ~800 °C for ilmenite. Al-spinels equilibrated below 500–550 °C, while the altered spinel rims correspond to metamorphism around 500–600 °C. Geochemical evidence of the podiform Al-spinels suggest a greenschist up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the host rocks. Al-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, suggesting relic magmatic composition unaffected by metamorphism. The composition of Al-spinel grains suggest an ophiolitic origin and derivation by crystallization of boninitic magma that belonging to a supra-subduction setting could form either in forearcs during an incipient stage of subduction initiation or in back-arc basins.     

Pan-African (intraplate and subduction-related?) metasomatism in the Fawakhir ophiolitic serpentinites, Central Eastern Desert of Egypt: mineralogical and geochemical evidences

Fawakhir serpentinites are the most western ophiolitic ultramafics relative to the Pan-African collision suture at the Qift-Quseir road in the Central Eastern Desert of Egypt. Their location is the basis for their selection in examining the possible contribution of the westerly dipping subducted oceanic slab-related melt/fluid with the intraplate granitic intrusion-related melt/fluid in the metasomatism of the Neoproterozoic ophiolitic serpentinites in the Eastern Desert. Non-residual mineralogy and geochemistry of serpentinites (SF1) far from the post-collision A2-type Fawakhir granitoids and those of serpentinites (SF2) in the vicinity of the granitoid pluton were investigated. The Fawakhir serpentinites are harzburgitic in composition and the Cr# (0.66–0.80) and Mg# (0.32–0.50) of their unaltered spinel cores are indicators for their forearc setting, where they were formed in the oceanic mantle wedge. Based on the spinel Cr# and the whole rock Yb–V bivariate, the melt extraction from the primitive mantle is in excess of 18% up to 24%. The HREE pattern of the SF1 serpentinites refers to the fractional type of melting. The formation of non-residual mineral phases particularly in SF2 samples (amphibole, biotite, apatite thorite, and monazite) and the enrichment of all serpentinites in trace incompatible elements refer to these two serpentinite groups having underwent modal metasomatism. It is suggested that viscous fluid/melt related to the Fawakhir granitoid emplacement metasomatized the SF2 serpentinites, causing a strong enrichment in LREE (display concave LREE; La_N/Sm_N?=?3.32–6.25 and U-type HREE; Gd_N/Yb_N?=?1.14–2.69) and a slight enrichment in Zr (12–16.62 ppm). All serpentinites are enriched in fluid-mobile elements by aqueous fluids, but the SF2 are more enriched in these elements. The spiked B compared to the other fluid-mobile elements (16.97–24.61 and 42.94–60.66?×?PM in SF1 and SF2 samples, respectively) suggests that these elements were added to the obducted ophiolitic Fawakhir serpentinites by the percolation of subduction-related fluids at shallow depths. The contribution of B from shallow continental crust-related fluids is debated. Hosting the Fawakhir serpentinites for the gold deposit at Fawakhir Mine implies a possible genetic relation between gold mineralizations hosted in the ultramafic rocks of the ANS and the processes of recycling of the subducted oceanic slab and the interaction with the mantle. Detailed stable and radiogenic isotopic analyses of the mineralization zones are required to address this question.     

Geochronology and geochemistry of the Dabure basalts,central Qiangtang,Tibet: evidence for ~550 Ma rifting of Gondwana

Newly discovered basalts in the Dabure area (central Qiangtang block, northern Tibet) were subjected to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating, geochemical analyses, and zircon Hf isotope analyses. Dating of magmatic zircons from three basalt samples indicates that the Dabure basalts formed during the late Ediacaran (~550 Ma). Xenocrystic zircons yield ages of 700–1150 Ma, providing evidence of the Cryogenian crust in the Tibet block. The Dabure basalts are alkaline, rich in Ti and Fe, and are strongly enriched in light rare earth elements without Eu anomalies. The basalts are geochemically similar to within-plate basalts but are relatively depleted in Nb and Ta. Although the analysed zircons show differences in their Hf isotope compositions, the geochemical data suggest that the Dabure basalts were derived from enriched mantle and that the source magmas were contaminated by the continental crust. The basalts may have erupted during rifting at ~550 Ma (from the dating of magmatic zircons), and may have been a product of the initial breakup of Gondwanaland.     

Ra’s Abdah of the north Eastern Desert of Egypt: the role of granitic dykes in the formation of radioactive mineralization,evidenced by zircon morphology and chemistry

Syenogranitic dykes in the north of Egypt’s Eastern Desert are of geological and economic interest because of the presence of magmatic and supergene enrichment of radioactive mineralization. Zircon crystal morphology within the syenogranitic dykes allows precise definition of sub-alkaline series granites and crystallized at mean temperature of about 637 °C. The growth pattern of the zircons suggest magmatic and hydrothermal origins of radioactive mineralization. Hydrothermal processes are responsible for the formation of significant zircon overgrowth; high U-zircon margins might have occurred contemporaneously with the emplacement of syenogranitic dykes which show anomalous uranium (eU) and thorium (eTh) contents of up to 1386 and 7330 ppm, respectively. Zircon chemistry revealed a relative increase of Hf consistent with decreasing Zr content, suggesting the replacement of Zr by Hf during hydrothermal activity. Visible uranium mineralization is present and recognized by the presence of uranophane and autunite.     

An overview of phosphate rocks and their mining impact on groundwater,Eastern Desert,Egypt

The Eastern Desert of Egypt represents a remote arid area which is scarce in water resources. The area is characterized by many mining sites to exploit the phosphate deposits; thus, these activities can generate groundwater contaminants. The main objective of the present study is discussing the effect of the mining activities and phosphate bearing rocks on groundwater. The obtained results pointed out that the water bearing formations can be distinguished as Quaternary alluvial, Oligocene sandstone, Campanian phosphate limestone (Duwi Formation), and Precambrian basement rocks. Some of the investigated groundwater shows relatively high concentrations of trace elements compared to other samples, such as Pb, Zn, Cu, Cr, Ni, and Sr. This is consistent with the analyses of phosphate rocks which are also enriched in the same trace elements. The high groundwater salinity is due to evaporation, limited re-charge, and leaching of salts in rocks. The results of speciation modeling reveal that majority of groundwater samples are supersaturated with calcite, aragonite, and dolomite, and some samples are also at equilibrium or supersaturated with hydroxyapatite. The groundwater quality in the study area evaluated for human drinking, livestock and poultry domestic, and industrial purposes is not suitable in most wells. It is obvious that the groundwater contamination occurs when drilling wells penetrate the phosphate bearing beds and not only at mining activity sites. Therefore, it is recommended to avoid any groundwater exploration from the Duwi Formation and also select the drilling sites outside the mining areas.     

Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the Allaqi–Heiani Suture,South Eastern Desert of Egypt

Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced ∼ 600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 733 ± 7 Ma, indicating ophiolite formation at ∼ 730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt (∼ 810–780 Ma and ∼ 730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.     

Evolution of mineralizing fluids of cassiterite–wolframite and fluorite deposits from Mueilha tin mine area, Eastern Desert of Egypt, evidence from fluid inclusion

Sn–W deposit of the Mueilha mine is one of many other Sn–W deposits in the Eastern desert of Egypt that associated with albite granite. Two forms of Sn–W mineralizations are known at the Mueilha Sn-mine area, namely fissure filling quartz veins and greisen. Cassiterite and/or wolframite, sheelite, and beryl are the main ore minerals in the greisen and quartz veins. Subordinate chalcopyrite and supergene malachite and limonite are also observed in the mineralized veins. To constrain the P–T conditions of the Sn–W mineralizations, fluid inclusions trapped in quartz and cassiterite, have been investigated. The following primary fluid inclusion types are observed: CO₂-rich, two-phase (L?+?V) aqueous, and immiscible three-phase (H₂O–CO₂) inclusions. Low temperature and low salinity secondary inclusions were also detected in the studied samples. Microthermometric results revealed that Sn–W deposition seem to have taken place due to immiscibility at temperature between 260°C and 340°C, and estimated pressure between 1.2 to 2.2 kb. Microthermometric results of fluid inclusions in fluorite from fluorite veins illustrated that fluorite seems to be deposited due to mixing of two fluids at minimum temperature 140°C and 180°C, and estimated minimum pressure at 800 bars.     

Fluid Inclusions in the Gold—Bearing Quartz Veins at Um Rus Area,Eastern Desert,Egypt

Fluid inclusions in the gold-bearing quartz veins at the Um Rus area are of three types: H₂O, H₂O−CO₂ and CO₂ inclusions. H₂O inclusions are the most abundant, they include two phases which exhibit low and high homogenization temperatures ranging from 150 to 200°C and 175 to 250°C, respectively. The salinity of aqueous inclusions, based on ice melting, varies between 6.1 and 8 equiv. wt% NaCl. On the other hand, H₂O−CO₂ fluid inclusions include three phases. Their total homogenization temperatures range from 270 to 325°C, and their salinity, based on clathrate melting, ranges between 0.8 and 3.8 equiv. wt% NaCl. CO₂ fluid inclusions homogenize to a liquid phase and exhibit a low density range from 0.52 to 0.66 g/cm³. The partial mixing of H₂O−CO₂ and salt H₂O−NaCl fluid inclusions is the main source of fluids from which the other types of inclusions were derived. The gold-bearing quartz veins are believed to be of medium temperature hydrothermal convective origin.     

On the Validity and Evolution of Assilina Species during the Ypresian–Lutetian: Example from Bir Dakhl, Eastern Desert, Egypt

The genus Assilina is a taxon within the Nummulitacea that appeared early in the Ypresian (Early Eocene) and continued until the end of the Lutetian (Middle Eocene). Thus, this taxon could be useful for the chronostratigraphy of this time interval. Lower Eocene rocks in southern Galala, Egypt are exposed at Bir Dakhl. This section includes marl sediments with debris flow shallow-marine facies deposits laid down during early Eocene times and includes fossils of large foraminifera: Assilina placentula Deshayes, 1838 and Nummulites burdigalensis de la Harpe, 1926. These are systematically treated, described and illustrated. Nummulites burdigalensis belongs to the N. burdigalensis group, and Assilina placentula belongs to the group of Assilina exponens. This assumption is based on qualitative morphology and quantitative measurements. Both species, together with Operculina libyca Schwager, 1883, enable the assignment of the Bir Dakhl (D5-40 Section) to the Early Eocene, Ypresian (SBZ10 of Serra- Kiel et al., 1998) supporting an earlier opinion that Assilina placentula belongs to that zone in the calibrated larger foraminiferal biostratigraphic zonation.     

Pan-African adakitic rocks of the north Arabian–Nubian Shield: petrological and geochemical constraints on the evolution of the Dokhan volcanics in the north Eastern Desert of Egypt

The Precambrian basement of Egypt is part of the Red Sea Mountains and represents the north-western part of the Arabian–Nubian Shield (ANS). Five volcanic sections are exposed in the Egyptian basement complex, namely El Kharaza, Monqul, Abu Had, Mellaha and Abu Marwa. They are located in the north Eastern Desert (ED) of Egypt and were selected for petrological and geochemical studies as they represent the Dokhan volcanics. The volcanics divide into two main pulses, and each pulse was frequently accompanied by deposition of immature molasse type sediments, which represent a thick sequence of the Hammamat group in the north ED. Compositionally, the rocks form a continuum from basaltic andesite, andesite, dacite (lower succession) to rhyodacite and rhyolite (upper succession), with no apparent compositional gaps. These high-K calc-alkaline rocks have strong affinities to subduction-related rocks with enriched LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and negative Nb anomalies relative to NMORB. The lower succession displays geochemical characteristics of adakitic rocks with SiO₂ >53 wt%, Al₂O₃ >15 wt%, MgO >2.5 wt%, Mg# >49, Sr >650 ppm, Y <17 ppm, Yb <2 ppm, Ni >25 ppm, Cr >50 ppm and Sr/Y >42.4. They also have low Nb, Rb and Zr compared to the coexisting calc-alkaline rhyodacites and rhyolites. The highly fractionated rhyolitic rocks have strong negative Eu anomalies and possess the geochemical characteristics of A-type suites. Trace element geochemical signatures indicate a magma source consistent with post-collisional suites that retain destructive plate signatures associated with subduction zones. The adakitic rocks in the northern ANS are generated through partial melting of delaminated mafic lower crust interacting with overlying mantle-derived magma. The Dokhan volcanics were likely generated by a combination of processes, including partial melting, crystal fractionation and assimilation.     

Preliminary results of a first record of gold and uranium in marble from Central Eastern Desert, Egypt: a witness for (syn- and post-?) metamorphic mineralization in metasediments

This paper records, for the first time, the mineralization of gold (0.98–2.76 ppm) and uranium (133–640 ppm) in marbles from the Arabian-Nubian Shield of the Eastern Desert of Egypt. These auriferous and uraniferous marbles are hosted by sheared and altered ophiolitic serpentinized ultramafic rocks of Gebel El-Rukham (ER), Wadi Daghbag (DG), and Wadi Al Barramiyah (BM). They occur as massive or banded in pod-like or bedded shapes. The ER and BM-mineralized marbles are impure calcitic, whereas the DG marble is impure calcitic to impure dolomitic. Their protolith are pure limestones and dolomitic limestones with probable argillaceous components (BM marble), and their metamorphism (Pan-African) was retrograde. Peaks of metamorphism were at granulite-amphibolite facies for the ER and BM marbles, forming diopside (Al₂O₃?=?0.17–1.07 wt.%) at 600–900°C and augite (Al₂O₃?=?2.45–9.40 wt.%) at 825–975°C, and at the amphibolite facies for DG marble, recrystallising the carbonate minerals and forming tremolite. The lowest temperatures of metamorphism were at the upper subgreenschist facies as chlorite (ER and BM marbles) and kaolinite (DG marble) were formed. Metamorphic fluids were, most probably, essentially binary H₂O–CO₂ mixtures with low NaCl and HF concentrations. Gold in the studied mineralized marbles occurs as native nuggets (10–35 μm) having globule, rod, crescent, and streak shapes, in pores, vugs, and fissures. The source of gold in all marbles is mostly the country ultramafic rocks. Timing of gold mineralization relative to the marblization and metamorphism of the country source ultramafic rocks was both syn- and post-metamorphic. Concerning the ER and DG marbles, it was syn-metamorphic, where Au liberation and transportation were mostly by the metamorphic fluids. The composition and temperature of these fluids were most probably inappropriate for formation of the sulfide complexes of gold. The gold mineralization of BM marble, on the other hand, was mostly post-metamorphic. The mineralising fluid was of surficial origin under oxidizing conditions. The encountered uranium minerals are of secondary origin such as autunite, uranophane, and carnotite. These minerals occur as fine oval aggregates and irregular grains (10–50 μm) usually filling fissures and vugs. The uranium mineralization can be classified as surficial of ages <1.5 Ma. It is proposed that the U was transported from its source (might be flesite and trachyte dikes for the ER and DG marbles and granite rocks for BM marble) to the marble rocks by surface and/or underground water related to the pluvial periods in Egypt. In BM marble, U and Au have mutual mineralizing fluid but different paragenesis.     

Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite,central Eastern Desert,Egypt

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P₂O₅ anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ～830 °C/～5 kbar. The crystallization conditions of the quartz diorites were estimated at ～570 °C/～2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.     

The Sukari Gold Mine,Eastern Desert—Egypt: structural setting,mineralogy and fluid inclusion study

The Sukari gold mine (18.8 Mt @ 2.14 g/t Au) is located 15 km west of the Red Sea coast in the southern central Eastern Desert of Egypt. The vein-type deposit is hosted in Late Neoproterozoic granite that intruded island-arc and ophiolite rock assemblages. The vein-forming process is related to overall late Pan-African shear and extension tectonics. At Sukari, bulk NE–SW strike-slip deformation was accommodated by a local flower structure and extensional faults with veins that formed initially at conditions of about 300 °C and 1.5–2 kbar. Gold is associated with sulfides in quartz veins and in alteration zones. Pyrite and arsenopyrite dominate the sulfide ore beside minor sphalerite, chalcopyrite and galena. Gold occurs in three distinct positions: (1) anhedral grains (GI) at the contact between As-rich zones within the arsenian pyrite; (2) randomly distributed anhedral grains (GII) and along cracks in arsenian pyrite and arsenopyrite, and (3) large gold grains (GIII) interstitial to fine-grained pyrite and arsenopyrite. Fluid inclusion studies yield minimum vein-formation temperatures and pressures between 96 and 188 °C, 210 and 1,890 bar, respectively, which is in the range of epi- to mesothermal hydrothermal ore deposits. The structural evolution of the area suggests a long-term, cyclic process of repeated veining and leaching followed by sealing, initiated by the intrusion of granodiorite. This cyclic process explains the mineralogical features and is responsible for the predicted gold reserves of the Sukari deposits. A characteristic feature of the Sukari gold mineralization is the co-precipitation of gold and arsenic in pyrite and arsenopyrite.Editorial handling: H. Frimmel     

The Composition of the Fluids in Alkali Basalts and Mantle-Derived Xenoliths in Eastern China

The Composition of the Fluids in Alkali Basalts and Mantle-Derived Xenoliths in Eastern China     

Geochemistry of island arc dolerites and diorites along Qift–Quseir asphaltic road,central Eastern Desert,Egypt

The Precambrian basement rocks exposed along Qift–Quseir asphaltic road, central Eastern Desert of Egypt, exhibit two contrasted tectonic units, each of which has its own lithology structural style and grade of metamorphism. They are intruded by dolerite and diorite dykes. The alkali (Na₂O+K₂O) and TiO₂ contents increase whereas Al₂O₃, FeO, MgO, CaO and MnO decrease with increasing SiO₂ from dolerites to diorites. The trace elements Ti, Zr, Cr, Y and Ni indicate that the dolerites are tholeiitic with slight tendency toward calc-alkaline and formed from basaltic magma in an active continental margin, while diorites are calc-alkaline and were formed by fractional crystallisation of high-alumina basaltic magma in an island arc and active continental margin tectonic environment where they probably represent the forerunner of G1 granites. The molecular ratios Mg values (MgO×100/MgO+FeO) of dolerites range from 47 to 49 while those of diorite range from 51 to 59, indicating that the dolerite and diorite have suffered mild fractionation. Mineral chemistry for the diorites shows that the amphiboles are classified as magnesiohornblende and the plagioclase composition is An_39–42 (i.e. the narrow range indicate that the pluton has not suffered extensive fractional crystallisation). The Al content of amphibole displays significant variation with pressure and temperature, also the change of the Ca/(Ca+Na) ratio of plagioclase is dependent on temperature. The amphibole–plagioclase geothermobarometer suggested the P-T formation conditions of studied dykes as 2 kbar and 600 °C.     

Lu–Hf and O isotopic compositions on single zircons from the North Eastern Desert of Egypt,Arabian–Nubian Shield: Implications for crustal evolution

Neoproterozoic juvenile crust is exposed in the Eastern Desert of Egypt, between the Nile and the Red Sea, forming the basement to Cambrian and younger sedimentary strata in the northernmost part of the Arabian–Nubian Shield (ANS). In order to reveal how the crust of this vast region was formed, four examples of widespread Neoproterozoic (653–595 Ma) calc-alkaline and alkaline intrusive rocks in the northwestern most exposures, in the NE Desert of Egypt (NED) were studied. Single zircon Hf–O isotopic compositions of these intrusives were used to characterize the Neoproterozoic syn- and post-collisional granitoids in the NED. The ~ 653 Ma Um Taghir syn-tectonic granodiorite (I-type) displays isotopic characteristics of a depleted mantle source, such as high εHf(t) (+ 9.1 to + 11.2) and mantle δ¹⁸O (mean = + 5.12‰). In contrast, the ca. ~ 600 Ma post-collision A-type granites (Al-Missikat, Abu Harba, and Gattar) show slightly higher δ¹⁸O values (+ 5.15 to 6.70) and slightly lower εHf(t) values (+ 6.3 to + 10.6, mean = + 8.6). We interpret these isotopic data to reflect melting of a juvenile Neoproterozoic mantle source that assimilated slightly older Neoproterozoic crustal material during magma mixing. The involvement of crustal component is also supported by Hf-crustal model ages (0.67–0.96 Ga) and by the occurrence of xenocrystic zircons with U–Pb ages older than the crystallization ages, indicating melting of predominantly Late Neoproterozoic crustal protoliths.