Geochemistry of lamprophyre dykes,Wadi Sikait area,South Eastern Desert,Egypt

The Wadi Sikait area lies at about 95 km southwest of Marsa Alam City along the Red Sea Coast, Eastern Desert, Egypt. It is occupied by Precambrian rocks of ophiolitic mélange, metamorphosed sandstones (MSS), gab-bros and monzogranites which were later intruded by lamprophyre dykes and quartz veins. The lamprophyre dykes were extruded in NW-SE and NE-SW trends cutting monzogranites and metamor-phosed sandstones. The lamprophyres are porphyritic and composed of clinopyroxene, olivine and amphibole phenocrysts enclosed in a fine-grained groundmass of clinopyroxene, amphibole, opaque and lithium mica. The al-teration products are represented by amphibole (tremolite-actinolite and hornblende), carbonate, epidote, chlorite, iddingsite, clay minerals, limonite and serpentine. The Sikait lamprophyre dykes can be classified as alkaline lamprophyres characterized by silica contents rang-ing from 41.65 wt% to 50.88 wt% and Na2O>K2O. They are enriched in LILE, LREE and HFSE, but strongly de-pleted in compatible elements such as Cr and Ni relative to the primitive mantle. Sikait lamprophyres have moderate Zr/Hf (35.6-52.8) and Nb/Ta (20.5-22.5) ratios. Most of these features are attributed to the origin of these dykes from the metasomatized mantle affected by subduction-related fluid. These lamprophyres are compositionally similar to Salu lamprophyres in eastern China. The Sikait lamprophyre samples have high LREE (320×10-6-419×10-6) relative to HREE (20×10-6-33×10-6) with ratios (LREE/HREE=11.6-18.7) and no negative Eu anomaly (Eu/Eu*=0.9-1.04). The relative presence of posi-tive Ce anomaly (Ce/Ce*=1.04) in lamprophyre samples suggests the oxidizing condition under which the REEs were precipitated due to the common occurrence of fluorite and apatite.     

A preliminary study of graphite-bearing rocks at Wadi Sikait area, South Eastern Desert, Egypt

The studied graphite-bearing rocks are located at Wadi Sikait in the southern part of the Eastern Desert of Egypt to the west of Marsa Alam on the Red Sea coast. They are intruded by granitic rocks and they have low radioactivity level. Mica-graphite schists are considered as a matrix of ophiolitic mélange. Graphite occurs in mica-graphite schists as disseminated grains and in talc carbonates rocks as bands or veins. Petrographically, the mica-graphite schists are mainly composed of quartz, plagioclase, muscovite, biotite, and graphite. Geochemical characteristics show that trace elements analysis reflects higher content in Cr and Ni in ash-free graphite than mica-graphite schists. Spectrometrically, the graphite-bearing rocks at Wadi Sikait are showing eU values greater than eTh values, indicating that the graphite-bearing rocks gain U from the country rocks. The U/eU ratio range from 2.7 to 11 manifesting enrichment of chemical uranium (U) may be related to recent uranium transported from the mineralized country rocks. There is a role of graphite and carbonaceous matter in the genesis of U deposits.     

Geology and geochemistry of Tertiary basalt in south Wadi Hodein area, South Eastern Desert, Egypt

Tertiary basalt is widespread in the area south of Wadi Hodein, south Eastern Desert, Egypt. It is the youngest unit in the basement rocks of the Central Eastern Desert classification of El Shazly (Proc 22nd Intl Geol Congr, New Delhi 10:88–101, 1964) and El Ramly (Ann Geol Surv Egypt II:1–17, 1972), traversed all the previous succession of the basement rocks as well as the Nubia Sandstone of Cretaceous age, forming sheets, small hills, ridges, and dikes. This Tertiary basalt is strongly associated with the opening of the Red Sea. Geologic, petrographic, and petrochemical studies as well as microprobe and X-ray analyses were performed on samples from Wadi Hodein Tertiary basalt. Field and petrographic studies classified the Tertiary basalt in south Wadi Hodein into porphyritic olivine basalt, plagiophyric basalt, and doleritic basalt. Opaque minerals (magnetite and ilmenite) constitute 6–7.5% of this basalt. Petrochemical studies and microprobe analyses reveal that they are low-TiO₂ basalt with low uranium and thorium contents, classified as being basaltic andesite to andesite, originated from calc-alkaline magma, and developed in within-plate tectonic environment. Scanning electron microscopy shows that magnetite and ilmenite are the prevalent opaque minerals in this Tertiary basalt. Field radiometric measurements of the Tertiary basalt in south Wadi Hodein reveals low uranium and thorium contents. Uranium contents range from 0.5 to 0.9 ppm, while thorium contents range from 1.2 to 3.2 ppm. Fractional crystallization and mass balance modeling indicate that the most-silica low-TiO₂ Tertiary basalt in south Wadi Hodein can be derived from the relatively less-silica low-TiO₂ Tertiary basalt of south Quseir and Gabal Qatrani through fractional crystallization of plagioclase, olivine, augite, and titanomagnetite oxides. Tertiary basalts in south Wadi Hodein and south Quseir have nearly the same age, 25 Ma (Sherif, The Fifth International Conference on the Geology of Africa, 2007), 24 Ma (Meneisy and Abdel Aal, Ain Shams Sci Bull 25(24B): 163–176, 1984), and 27 Ma (El Shazly et al., Egypt J Geol 1975), respectively. Finally, the fractionation modeling and geochemical characteristics of these basalts suggested their origination from one basaltic magma emplaced in late Oligocene.     

Mineralogy,geochemistry, and geotectonic significance of the Neoproterozoic ophiolite of Wadi Arais area,south Eastern Desert,Egypt

ABSTRACT

The dismembered ophiolites in Wadi Arais area of the south Eastern Desert of Egypt are one of a series of Neoproterozoic ophiolites found within the Arabian–Nubian Shield (ANS). We present new major, trace, and rare earth element analyses and mineral composition data from samples of the Wadi Arais ophiolitic rocks with the goal of constraining their geotectonic setting. The suite includes serpentinized ultramafics (mantle section) and greenschist facies metagabbros (crustal section). The major and trace element characteristics of the metagabbro unit show a tholeiitic to calc-alkaline affinity. The serpentinized ultramafics display a bastite, or less commonly mesh, texture of serpentine minerals reflecting harzburgite and dunite protoliths, and unaltered relics of olivine, orthopyroxene, clinopyroxene, and chrome spinel can be found. Bulk-rock chemistry confirms harzburgite as the main protolith. The high Mg# (91.93–93.15) and low Al₂O₃/SiO₂ ratios (0.01–0.02) of the serpentinized peridotite, together with the high Cr# (>0.6) of their Cr-spinels and the high NiO contents (0.39–0.49 wt.%) of their olivines, are consistent with residual mantle rocks that experienced high degrees of partial melt extraction. The high Cr# and low TiO₂ contents (0.02–0.34 wt.%) of the Cr-spinels are most consistent with modern highly refractory fore-arc peridtotites and suggest that these rocks probably developed in a supra-subduction zone environment.     

Mineralogy, geochemistry, and origin of hydrothermal manganese veins at Wadi Maliek, Southern Eastern Desert, Egypt

The present work deals with the geology, mineralogy, geochemistry, and origin of the metagabbroic-hosted manganese deposits at Wadi Maliek in the southern Eastern Desert of Egypt. The manganese veins are found in the shear zones and channel ways of the fault planes within the metagabbroic rocks pointing to those hydrothermal solutions carrying manganese and iron load penetrating along these fractures. These faults are striking N 80° E?CS 80° W with dipping 65°. These veins vary in thickness from 15?cm up to 125?cm wide; each vein may show difference in thickness from bottom to top. Microscopic examinations, X-ray diffraction, infrared spectral, differential thermal (DTA), thermogravimetric (TGA), and ESEM-EDAX analyses revealed that the manganese minerals consist mainly of pyrolusite, psilomelane, and ramsdellite. Goethite and hematite are the common iron minerals. Petrographically, the manganese deposits can be classified into three ore types based on the predominance of manganese and iron minerals: manganese, manganese?Ciron, and iron ore types. The geochemistry of Maliek deposits indicated that the total averages of some major oxides in manganese, manganese?Ciron, and iron ore types are respectively as follows: SiO₂ (15.64%, 11.52%, and 20.58%), MnO (39.9%, 17.81%, and 0.77%), FeO* (7.13%, 33.31%, and 37.08%), CaO (5.89%, 5.82%, and 5.32%), and Na₂O (1.04%, 1.61%, and 1.53%). With regard to trace elements, the Maliek manganese deposits are rich in Zn, Ba, Pb, Sr, and V. Based on the geological, mineralogical, and geochemical results, the studied manganese deposits are considered to be precipitated from hydrothermal solution.     

Mineralogy and microchemistry of the Egat gold deposit, South Eastern Desert of Egypt

Gold-bearing quartz lodes from the Egat gold mine, South Eastern Desert of Egypt, are associated with pervasively silicified, highly sheared ophiolitic metagabbro and island-arc metavolcanic rocks. The mineralized quartz veins and related alteration haloes are controlled by NNW-trending shear/fault zones. Microscopic and electron probe microanalyses (EPMA) data of the ore and gangue minerals reveal that fine-grained auriferous sulfarsenides represent early high-temperature (355–382 °C) phases, with formation conditions as (fS₂?=??10, and fO₂ around ?31). A late, low-temperature (302–333 °C) assemblage includes coarse pyrite, arsenopyrite, and free-milling gold grains (88–91 wt.% Au), with formation conditions as (fS₂?=??8 and fO₂ around ?30). Gold was impounded within early sulfarsenides, while free-milling gold blebs occur along microfractures in quartz veins and as inclusions in late sulfides. Infiltration of hydrothermal fluids under brittle–ductile shear conditions led to mobilization of refractory Au from early sulfarsenide phases and reprecipitated free gold, simultaneous with silicification of the host rocks. The positive correlation between Au and As favors and verifies the use of As as the best pathfinder for gold targets, along the NNW-trending shear zones.     

Geometry and texture of quartz veins in Wadi Atalla area,Central Eastern Desert,Egypt

Several quartz vein sets with varying orientation, geometry and internal structure were recognized in the Atalla area. The veins were associated with the deformation phases affecting the area. En echelon and extensional veins are the main geometrical types. Syn-kinematic veins associated with the major northeast-over-southwest thrust faults were later boudinaged, folded and re-folded. En echelon veins, fibrous veins, and extensional veins are associated with the NNW–SSE faults. Other veins are associated with the NW–SE, N–S, NE–SW and E–W faults. Veins are concentrated at the intersection zones between faults. The internal structure of the veins comprises syntaxial, antitaxial, and composite types and reflects a change from a compressive stress regime to an extensional one. Chocolate-tablet structures and synchronous and co-genetic vein networks indicate later multi-directional extension of the area. Interaction between cracking and sealing of fractures is a common feature in the study area indicating that it was easy for the pore pressure to open pre-existing fractures instead of creating new ones. The reopening of pre-existing fractures rather than creating new ones is also indicated by the scattering of vein data around σ₃. There is an alteration and change in characteristics of the wall rock due to increase in fluid flow rate. Fault-valving probably is also a cause of the complex geometry of some veins.     

Origin of dyke swarms in Wadi El Redi-Wadi Lahami area,southern Eastern Desert of Egypt

Dykes predominate within the Neoproterozoic rocks, especially granites, of Wadi El Redi-Wadi Lahami area in the southern Eastern Desert of Egypt. The dyke swarms form three major suites: from the oldest to the youngest, they are basaltic andesite—Suite 1 (E-W and ENE-WSW), rhyolite—Suite 2 (NE-SW), and andesite—Suite 3 (NNE-SSW, NNW-SSE, and NW-SE). Despite the wide ranges of the dyke compositions, the feldspar and amphibole are usually the essential forming minerals. The plagioclase arrays between Ab_0.9An_99.10 in the basaltic andesite and Ab_98.80An_0.70 in the rhyolite, while sanidine ranges from Or_44.60Ab_49.70 to Or_98.40Ab_1.60. Amphibole in Suite 1 and 3 (Al₂O₃, TiO₂, Na₂O, and K₂O are the lowest and those of SiO₂ and CaO are the highest) samples are usually magnesio-hornblende, whereas it is edenite and tschermakite in Suite 2 dykes. Despite all parent magmas have calc-alkaline affinity, some elements such as Ni show an erratic behavior against the progressing differentiation from one magma chamber and implying for an assimilation of the country rocks. The high contents of amphibole, the depletion in Ti, and the enrichment in large-ion lithophile elements (such as K, Rb, Ba, Sr, and Ba) compared to the primitive mantle composition are consistent with parent hydrous melts generated due to extension above the subduction zone. The estimated compositions of liquids in equilibrium with amphiboles and the pressures at which they crystallized (4.61–7.8 kbar for the Suite 2 and 1.5–2 kbar for the Suites 1 and 3) are greatly varied. These are indications for a difference in the source regions of the parent magmas of the studied dykes. It is supposed that the Suite 1 and 2 dykes are a conjugate set emplaced due to the NW-SE crustal extension in the Arabian-Nubian shield, whereas the Suite 3 dykes generated due to the rifting along the Red Sea.     

Geochemistry, petrogenesis and radioactivity of El Hudi I-type younger granites, South Eastern Desert, Egypt

The present study deals with geochemical characteristics and petrogenesis of three younger granite varieties (coarse-grained biotite-muscovite granites (CBG), garnetiferous muscovite granites (GMG) and Abu Aggag biotite granites (AAG)) in El-Hudi area, east of Aswan, southeastern desert of Egypt. Mineral chemistry and whole rock chemistry data revealed that all granites have high SiO₂ (70.8-74.7 wt.%), Al₂O₃ (12.8-14.3 wt.%), Na₂O and K₂O (>3.2 wt.%) contents with high Na₂O/K₂O ratios (～>1). Plagioclase feldspars range in composition from albite to oligoclase (An_9-27) in CBG, oligoclase (An_13-18) in GMG and albite (An_2-6) in AAG. Potash feldspars are mainly perthitic microcline and exhibit chemical formulae as (Or_93-96 Ab_7-4 An₀) in CBG, (Or_95-98 Ab_5-2 An₀) in GMG and (Or_82-98 Ab_18-2 An₀) in AAG. Biotites from CBG and GMG are enriched in (Mg and Ti) and depleted in (Al, Fe, Mn and K) compared with those of AAG. Biotites from CBG and GMG had been derived from calc-alkaline magma, whereas those from AAG had been derived from peraluminous magma. Chlorites from CBG and GMG are Mg-Fe bearing, while that from AAG is Fe-rich chlorite (chamosite). The CBG and GMG are Mg-rich monzogranites originated from high-K calc-alkaline magma with metaluminous to mildly peraluminous nature. The AAG are Fe-rich monzogranites to syenogranites generated from high-K calc-alkaline peraluminous magma. Both CBG and GMG are late- to post-orogenic granites, while the AAG are post-orogenic granites. All three granite varieties are considered as evolved I-type granites, formed under low to moderate water pressures (～ 0.5-7 kbars) and relatively high ranges of crystallization temperatures (～700-890°C). They were generated from partial melting of crustal materials at lower (CBG >30 km depth) and intermediate (GMG & AAG ～20-30 km depth) levels. The crystal fractionation was the predominant process during differentiation of parent magmas of these granites. Geochemical characteristics manifest that AAG represent the highly fractionated member of magma cycle differs from that produced CBG and GMG. The CBG are relatively enriched in both U and Th existing only within the accessory minerals such as zircon, sphene, and allanite.     

Evolution of the Pan-African Wadi Haimur metamorphic sole, Eastern Desert, Egypt

By comparison with the general features of metamorphic soles (e.g. vertical and lateral extension, metamorphic grade and diagnostic mineral parageneses, deformation and dominant rock types), it is inferred that the amphibolites, metagabbros and hornblendites of the Wadi Um Ghalaga–Wadi Haimur area in the southern part of the Eastern Desert of Egypt represent the metamorphic sole of the Wadi Haimur ophiolite belt. The overlying ultramafic rocks represent overthrusted mantle peridotite. Mineral compositions and thermobarometric studies indicate that the rocks of the metamorphic sole record metamorphic conditions typical of such an environment. The highest P – T conditions ( c . 700 °C and 6.5–8.5 kbar) are preserved in clinopyroxene amphibolites and garnet amphibolites from the top of the metamorphic sole, which is exposed in the southern part of the study area. The massive amphibolites and metagabbros further north (Wadi Haimur) represent the basal parts of the sole and show the lowest P – T  conditions (450–620 °C and 4.7–7.8 kbar). The sole is the product of dynamothermal metamorphism associated with the tectonic displacement of ultramafic rocks. Heat was derived mainly from the hot overlying mantle peridotites, and an inverted P – T  gradient was caused by dynamic shearing during ophiolite emplacement. Sm/Nd dating of whole-rock–metamorphic mineral pairs yields similar ages of c . 630 Ma for clinopyroxene and hornblende, which is interpreted as a lower age limit for ophiolite formation and an upper age limit for metamorphism. A younger Sm/Nd age for a garnet-bearing rock ( c . 590 Ma) is interpreted as reflecting a meaningful cooling age close to the metamorphic peak. Hornblende K/Ar ages in the range 570–550 Ma may reflect thermal events during late orogenic granite magmatism.     

Radioactivity and mineralogy of the altered granites of the Wadi Ghadir shear zone, South Eastern Desert, Egypt

Quartz-diorite, gneissose granodiorites, two-mica granite and perthite leucogranie are the main rock units cropping out in the Wadi Ghadir area, South Eastern Desert of Egypt. Along the NNE-SSW mega-faults, a broad brittle shear zone is developed in the Ghadir two-mica granite. Brittle deformation is manifested by severe myloniti-zation and alteration of these granites. These sheared altered granites are characterized by the presence of radioactive mineralization, associated with alteration features such as silicification, hematization and kaolinitization. Radioelement measurements revealed that the unaltered and altered two-mica granites are considered as uraniferous granites. The average uranium and thorium contents in the unaltered two-mica granites are 12.29×10-6 and 19.81×10-6, respectively, and the average Th/U ratio is 1.62. The altered granites exhibit higher concentrations of U (averaging 97.949), but have lower Th and Th/U ratios (13.83 and 0.16, respectively), which indicates uranium enrichment in the granites. Binary relations of eTh/eU against either eU or eTh and eU with eTh in the studied gran-ites suggest that the distribution of radioactive elements not only magmatic (positive correlation between eU and eTh), but also due to hydrothermal redistribution of radioelements (weak correlation between eU and eTh/eU). The magmatic U and Th are indicated by the presence of uraninite, thorite, zircon and monazite whereas the evidence of hydrothermal mineralization is the alteration of rock-forming minerals such as feldspar and the forma-tion of secondary minerals such as uranophane and pyrite. Microscopic, XRD and scanning electron microscopic studies revealed the presence of uraninite, uranophane, thorite, Ce-monazite and zircon, in addition to phlogopite-fluor mica in the studied altered granites of the Wadi Ghadir shear zone.     

Radioactive anomalies associated with the graphite-bearing metasediments,Wadi Umm Gheig area,Central Eastern Desert,Egypt

The study area is located in the central part of the Eastern Desert of Egypt and is mainly covered by different varieties of Precambrian basement rocks represented mainly by younger and older granites, metadiorite, metavolcanics, and metasediments. The analysis and interpretation of airborne gamma-ray spectrometric survey data are essentially based on the computation of the three radioelements (U, Th, and K) favorability indices, estimation of uranium migration rate percentage, variation of eU with eTh and eU/eTh ratio, and the construction of the contour map of the (eU-eTh/3.5) in the different rock units. The highest percent of uranium migration- out or leaching rate is connected with the red and pink granites of Gebel Kadabora El-hamra, metadiorite, and red and pink granites of G. Umm Rakham (??20.8%, ??18.57%, and ??8.45% respectively), which indicates that they could represent a major U-source bodies in the area. It was observed that the metasediments and associated graphite-bearing schists west and northwest of G. Kadabora El-hamra reflect more precipitation than the other locations around the pluton (the uranium migration rates varies between 2.59 and 30%) which mean that the graphite may have acted as a reducing agent for uranium carried in oxidizing fluids (surface meteoric water) and resulted in its precipitation. In the light of the availability of uranium source, its mobility, and graphite-bearing metasediments, the area has a good potential for the possible occurrence of uranium mineralization.     

Petrogenesis of the Wadi Dib alkaline ring complex, Eastern Desert of Egypt

Summary The Wadi Dib magmatic complex is the oldest known alkaline ring complex in the Egyptian part of the Pan-African orogenic belt. Rb-Sr isotope data for seven samples suggest a Vendian age of 578±16 Ma, and a⁸⁷Sr/⁸⁶Sr initial ratio of 0.7048±0.0010. The igneous complex has a diameter of 2 km and was emplaced within granodioritic Pan-African host rocks at the intersection of two faults. It shows distinct concentric compositional zoning with several syenitic outer ring sheets, a mainly trachytic intermediate ring sheet, and a quartz syenite inner ring sheet with a granitic core; relative ages decrease from margin to core. The mineralogical and chemical features are characteristic of within-plate (A-type) magmatic complexes. Major and trace element patterns underline the co-magmatic origin of the suite but indicate three stages of evolution with several pulses of emplacement. A common feature of element distribution patterns is the small systematic change in the early lithologies, but a distinct evolution trend in the late quartz-bearing rocks.We propose that an alkali-basaltic parent magma was emplaced within deep or middle levels of the juvenile Pan-African crust. Differentiation mainly occurred by fractional crystallization of olivine, clinopyroxene, plagioclase, and apatite. During the late stages of evolution, limited assimilation of island-arc magmatic rocks may have occurred. Emplacement took place along ring fractures at a subvolcanic level and was probably related with formation of a caldera during emplacement of the trachytic lithologies. The anorogenic character of the magmatic suite indicates consolidation of the Pan-African crust of NE Africa at the time of emplacement of the alkaline body.

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Petrogenese des alkalischen Wadi Dib ringkomplexes, Östiche Wüste Ägyptens

Zusammenfassung Der Wadi Dib-Komplex ist die älteste bekannte Ringstruktur im ägyptischen Teil des Panafrikanischen Orogengürtels. Rb-Sr Isotopendaten von sieben Proben ergeben ein vendisches Alter von 578±16 Ma and ein initiales⁸⁷Sr/⁸⁶Sr-Verhältnis von 0,7048±0,0010. Der magmatische Komplex besitzt einen Durchmesser von 2 km und hat am Schnittpunkt zweier Störungen innerhalb panafrikanischer Granodiorite Platz genommen. Er weist eine konzentrische Zonierung mit mehreren syenitischen äußeren Ringen, einem vorwiegend trachytischen mittleren Ring und einem quarzsyenitischen inneren Ring mit einem granitischen Kern auf; die relativen Alter der Gesteine nehmen vom Rand zurn Kern hin ab. Mineralogische and chemische Charakteristika sind die von Intraplatten- (A-Typ-) Komplexen. Haupt- and Spurenelementmuster weisen auf eine ko-magmatische Entstehung hin, zeigen aber eine Entwicklung in drei Stadien mit mehreren magmatischen Pulsen auf. Charakteristika der Elememverteilungen sind wenig systematische Änderung in den älteren Lithologien, aber ein gerichteter Entwicklungstrend in den späten, quarzführenden Lithologien.Wir schließen, daß ein alkali-basaltisches Magma in ein tiefes oder mittleres Niveau der jungen panafrikanischen Kruste intrudierte. Differentiation erfolgte im wesentlichen durch fraktionierte Kristallisation von Olivin, Klinopyroxen, Plagioklas und Apatit. Während später Entwicklungsstadien gab es vermutlich begrenzte Assimilation von Inselbogen-Kruste. Die Platznahme erfolgte entlang von Ringbrüchen in einem subvulkanischen Stockwerk und war vermutlich mit der Bildung einer Caldera während der Platznahme der trachytischen Lithologien verbunden. Der anorogene Charakter der magmatischen Folge zeigt an, daß die panafrikanische Kruste Nordost-Afrikas zur Zeit der Platznahme der alkalischen Intrusion bereits konsolidiert war.

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With 13 Figures     

Geochemistry, mineralogy, and radioactivity of Upper Cretaceous Abu Khruq Ring Complex, South Eastern Desert, Egypt

The upper Cretaceous Abu Khruq ring complex (ARC) is located in the South Eastern Desert between latitudes 24°00′10′′ and 24°03′15′′ N, and longitudes 33°54′50′′ and 33°58′ E and has a roughly circular shape with a diameter of 7 km. ARC is built up by major extrusion of alkaline volcanic rocks comprising mainly rhyolite porphyry and alkaline trachyte rocks at the center of the ring complex followed by successive intrusions of alkaline gabbro and syenitic rocks comprising quartz syenite (oversaturated), syenite (saturated), and nepheline syenite (undersaturated). Petrographical and geochemical studies were carried out for the rocks of the forming ARC. For mineralogical and radioactive investigations, samples were collected from the most promising locations representing the hematitized nepheline syenite, nepheline syenite pegmatites, and quartz syenite. The most important minerals comprise: phosphuranylite, zircon, monazite, xenotime, plumbopyrochlore, pyrite, huttonite, apatite, REE mineral, rutile, and atacamite. The hematitized nepheline syenite is the most U- and Th-rich rocks, where eU content in this rock ranges from 375 to 788 ppm with an average 502 ppm and the average eTh content is 2,345 ppm ranging from 1,918 to 3,067 ppm. The pegmatite syenite and quartz syenite contain relatively low concentrations of U and Th, where the average eU content are 11 and 16 ppm and average eTh contents are 27 and 327 ppm, respectively.     

Geochemical characterization of zoned zircon from Wadi Abu Rusheid psammitic gneiss, south Eastern Desert, Egypt

A unique zircon was studied in the gneiss samples collected from the Wadi Abu Rusheid psammitic gneiss using electron scanning microscope and electron probe microanalyses. This zircon can be categorized into two types according to the texture and trace element content: (l) magmatic zircon slightly enriched in HfO₂ with ordinary zone. (2) Overgrowths of zircon occur as two species, the first species being highly enriched in HfO₂ with irregular zoning. The second species is highly enriched in HfO₂ forming a rim around the second species with a very sharp thinner boundary. The first type shows a distinct oscillatory internal zoning pattern without change in shape of this zone and has conspicuous inclusion-free zircon overgrowths with distinct poor concentrations in Y, Hf, Th, U, Nb, and Ta in both rim and core. The second type shows two species, the first one displays distinct irregular interval zoning and irregular overgrowth with abrupt change in composition of these zones with distinct enrichment in Y, Hf, Th, U, Nb, and Ta in the rim relative to the core. The second species is forming a rim around the first species also with distinct enrichment in Y, Hf, Th, U, Nb, and Ta content. These indicate that two events (crystallization environment) have played an important role in the formation of this zircon and largely reflect differences in whole-rock trace element contents between the successive generations of this zircon. The first event is believed to be of magmatic origin giving rise to normal composition of magmatic zircon. The second event shows an intense successive process of metasomatic activity during the formation of the Abu Rusheid radioactive gneiss. Electron microprobe analysis indicates that oscillatory zoned zircon shows poor content of Y, Hf, Th, U, Nb, Ta, and rare earth elements (REE) in the rim and core, while overgrowths of zircon are slightly enriched by these elements. Also, these analyses indicate that the Abu Rusheid psammitic gneiss has been significantly enriched by the thorite mineral (Th content up to 54.72% ThO₂) and columbite-bearing minerals (Nb content up to 64.74% Nb₂O₅, Ta content up to 9.32% Ta2O₅). The poor content of REE in overgrowths of zircon indicates mobilization of REE during the metamorphism processes of gneiss.     

Distribution and environmental geochemistry of some heavy metals in stream sediments of Wadi Allaqi, South Eastern Desert of Egypt

The study focuses on the lateral distribution and the environmental geochemistry of seven heavy metals: Fe, Mn, Cu, Co, Ni, Pb, and Zn in addition to Al in the stream sediments of Wadi Allaqi. Forty-two samples were collected from the upstream, the midstream, and the downstream of this Wadi. Results of the grain size analysis show that the sediments of Wadi Allaqi are fine to medium sand. The heavy metals content reflects the weathering impact on the hinterland. The highest concentrations of Fe, Al, Mn, Cu, Pb, and Zn are recorded in the midstream that is dominated by clastics of felsic and intermediate composition. Moreover, the downstream, occupied by ultrabasic–basic rocks, shows the highest averages of Co and Ni. These results suggest that the felsic and intermediate rocks are the main source of the former metals, whereas the ultrabasic–basic rocks are the source of later two metals. All the analyzed heavy metals have average concentrations lower than their backgrounds, except for Co and Pb. The pollution level by these heavy metals has been evaluated using enrichment ratio (ER), pollution load index (PLI), and index of geoaccumulation (Igeo). The calculated values of ER, CF, PLI, and Igeo indicate that Wadi Allaqi sediments are almost pristine except occasional feeble pollution level by Co and Pb.     

Alteration Mechanisms of Chromian-Spinel during Serpentinization at Wadi Sifein Area, Eastern Desert, Egypt

Wadi Sifein podiform chromite deposits, Central Eastern Desert of Egypt, are hosted by fully serpentinized peridotite that is a part of the dismembered Pan‐African ophiolite complexes. Relics of primary minerals and the chemical characters indicate that the ophiolitic rocks were derived from depleted mantle peridotite of harzburgite and subordinate dunite compositions. The mantle rocks were initially formed at a mid‐oceanic ridge and subsequently thrust at a supra‐subduction zone. The chromite mineralization at Wadi Sifein area displays either pod‐shaped bodies with massive and lumpy chromitite appearance or dissemination of chromian‐spinel in serpentinite matrix. The podiform chromitite exhibits a very limited compositional range in terms of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]. The chromian‐spinel, however, frequently displays optical and geochemical zoning. Four zones can be identified from core to edge: inner core representing the original composition of the chromian‐spinel; narrow Cr‐rich ferritchromit zone; wide ferritchromit zone; and outer Cr‐magnetite/magnetite zone. The zonation of chromian‐spinel is interpreted to be a result of serpentinization rather than magmatic or metamorphic processes. The geochemical data obtained from the chromitite and chromian‐spinel was statistically processed using discriminant and R‐mode factor analyses. Two trends, minor and major, were achieved considering the formation of ferritchromit. The minor trend is controlled by the redistribution of trivalent cations, where Cr₂O₃ increased on the expense mainly of Al₂O₃ and to less extent Fe₂O₃ to form zone II during the peak of serpentinization. The major trend of alteration, however, is explained by the exchange between Mg‐Fe²⁺ rather than Cr, Al, and Fe³⁺ to form zone III. Kammererite formation was accompanied the formation of zones III and IV at a 314°C temperature of formation.