Re-Os geochronology of gold mineralization in the Fawakhir area,Eastern Desert,Egypt

We report the first Re-Os data on gold-associated arsenopyrite from mesothermal gold-quartz veins in the ancient Egyptian Fawakhir–El Sid gold mining district in the central Eastern Desert. This mining district has an ~5000-year-old history and is displayed in the Turin Papyrus Map (about 1150 BC), which is widely acclaimed as the world’s oldest geographic map, as well as the oldest geologic and mine map. The Fawakhir–El Sid district is part of a regional NNW-trending shear corridor (15 km wide) that hosts several other historic gold mines associated with left-lateral wrench structures and related granite intrusions. Vein-style gold mineralization is hosted within and at the margin of an I-type and magnetite-series monzogranite, the Fawakhir granite intrusion, and a Pan-African (~740 Ma) ophiolite sequence. The ore mineralogy of the mineralized quartz veins includes pyrite-arsenopyrite-pyrrhotite-sphalerite-galena-chalcopyrite-electrum plus a number of tellurides of Ag, Au, and Bi. The ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os regression on 5 points of arsenopyrite gives an age of 601 ± 17 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.24 ± 0.07 (2 σ; MSWD = 17). This age coincides within error with the U-Pb age on zircon from the Fawakhir monzogranite (598 ± 3 Ma). The age coincidence and the hydrothermal Te and Bi metal signature suggest a foremost role of granite-related fluids in the quartz-vein system.     

Talc mineralization of ultramafic affinity in the Eastern Desert of Egypt

Petrographical and petrochemical studies of the talc host rocks of Rod Umm El-Farag and Wadi Thamil in the Eastern Desert of Egypt reveal that they consist mainly of metavolcanic rocks, whilst the geology, petrography, mineralogy, chemistry and quality of the enclosed talc lenses reveal that the ore has ultramafic affinity. The setting of the talc ore is similar to that hosted by metavolcanic rocks in terms of the type of host rocks, but it differs in its ultramafic affinity, resembling the talc ore hosted by ultramafic rocks. The parent ultramafic rocks occur in the form of small bodies obducted later along a tectonized fault plane within metavolcanic host rocks (Precambrian) and their tuffaceous equivalents. The metavolcanic host rocks consist mainly of metabasalts, meta-andesites and metatuffs with a smaller amount of dacite, rhyolite and tuffaceous lava. The metamorphic grade is low corresponding to greenschist facies. The calc-alkaline and tholeiitic characters of the volcanic rocks are determined by the behaviour of trace elements on some chemical discrimination diagrams. After the emplacement of the ultramafic bodies, they underwent regional metamorphism which was accompanied by further serpentinization. Metasomatic changes, related to regional metamorphism (corresponding to the emplacement of granitic plutons at a distance) include talc, carbonate, tremolite and chlorite formation. SiO₂, H₂O and CO₂ have been supplied from hydrothermal solutions but all other constituents are considered indigenous to the ultramafic bodies, and none of the metavolcanic components have been added during talc formation. Mineralogically, the talc ore is relatively simple, including talc, tremolite, actinolite, chlorite and chromite. On the basis of mineral abundances, pure talc (>90% talc), chlorite-rich and tremolite-actinolite-rich (50–70% talc) ore types have been recognized. Chromite is largely zoned and occurs as disseminated grains within the talc matrix. Cr, Al and Mg were released during the formation of ferrite chromite and accommodated in the talc and chlorite structures. The chemical data show that there is very little variation in the contents of MgO, Fe₂O₃, FeO, NiO, Cr₂O₃, and Co between the parent ultramafic rocks and talc ore. Al₂O₃, CaO, Fe₂O₃ and FeO are the main impurity oxides in the talc ore. They decrease the whiteness of the ore and consequently limit the use of talc. Received: 26 March 1999 / Accepted: 10 October 1999     

Application of kinematic vorticity and gold mineralization for the wall rock alterations of shear zone at Dungash gold mining,Central Eastern Desert,Egypt

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in wall rocks alterations of shear zone at Dungash gold mine. Furthermore, it shows the relationship between the gold mineralization and deformation and also detects the orientation of rigid objects during progressive deformation. The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Neoproterozoic basement rocks. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. The kinematic vorticity number for the metavolcanic and metasedimentary samples in the Dungash area range from 0.80 to 0.92, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. Furthermore, we conclude that disseminated mineralization, chloritization, carbonatization and silicification of the wall rocks are associated with fluids migrating along shearing, fracturing and foliation of the metamorphosed wall rocks.     

The nature of gold-bearing fluids in Atud gold deposit,Central Eastern Desert,Egypt

Electron microprobe analyses of gold and associated ore minerals as well as stable isotope analyses of sulphide and carbonate minerals were performed in order to determine the metal and fluid sources and temperature of the mineralizing systems to better understand the genesis of the Atud gold deposit hosted in the metagabbro–diorite complex of Gabal Atud (Central Eastern Desert, Egypt). The gold can be classified as electrum (63.6–74.3 wt.% Au and 24.6–26.6 wt.% Ag) and is associated with arsenopyrite and As-bearing pyrite in the main mineralization (gold-sulphides) phase within the main mineralized quartz veins and altered host rocks. Based on the arsenopyrite geothermometer, As-contents (29.3–32.7 atom%) in arsenopyrite point to deposition in the Log ?_S2 and T ranges of ?10.5 to ?5.5 and 305–450°C, respectively, during the main mineralizing phase. Based on the δ³⁴S isotopic compositions of the sulphides, they are originated from magmatic fluids in which the sulphur is either sourced directly from magma or remobilized from the magmatic rocks (gabbroic rocks). On the other hand, calcite formed from fluids having mainly magmatic mixed with variable metamorphic signatures based on its δ¹³C and δ¹⁸O values. This work concluded that the gold-bearing ores at Atud deposit have magmatic sources leaching from the country intrusive rocks during water/rock interactions then remobilized during a metamorphic event. Therefore, the Atud gold deposit is classified as an intrusion-related gold deposit, in which the gabbro–diorite host intrusion acted as the source of metals which were mobilized and deposited as a result of the effects of NW–SE shearing.     

Exploration of new gold occurrences in the alteration zones at the Barramiya District,Central Eastern Desert of Egypt using ASTER data and geological studies

The use of ASTER data and fieldwork supported by mineralogical and geochemical analyses enabled exploration of new gold occurrences in the alteration zones in the ultramafic–mafic successions at the Barramiya district. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) band ratios (4/8, 4/2 and 8/9 in R, G and B channels, respectively) helped in recognising of two listwaenite alteration zones (areas 1 and 2) promising for gold in the north-east and south-east of the Barramiya gold mine. Mineralogical studies and X-ray diffraction analysis revealed that areas 1 and 2 are characterized by variable concentrations of talc, ankerite, magnesite, quartz and calcite. Ore microscope studies revealed the sulphide minerals carry gold within these alteration zones; moreover, goethite and malachite are also observed. Fire assay results show Au contents in the range of 5.04 ppm in the graphite schist, 4.02 ppm in the quartz veins and 3.76 ppm in the listwaenite alteration. The atomic absorption analysis (AAS) of samples from area 1 yields an average Au content in the quartz-veins of 2.4 ppm, Ag content is 8.0 ppm and Cu content is 2.4 wt%. The listwaenite alteration gives an average Au content of 4.4 ppm and a Cu content of 2.8 wt%. In area 2, the AAS of the quartz-veins revealed an average Au content of 2.6 ppm, 6.2 ppm Ag and 1.9 wt% Cu. The listwaenite alterations of area 2 grade 3.5 ppm Au and 2.4 wt% of Cu. The Barramiya District is made up of ophiolitic ultramafic belts of serpentinites, talc carbonates and talc graphite schists, mainly thrust over the metavolcanic sequences. They include highly strained and tectonised parts enriched in sulphides, iron oxides and carbonates, with developed listwaenite alterations along the thrust contacts. Gabbro and granitic intrusions were intruded in the ultramafics and metavolcanic rocks. ASTER data are an accurate and helpful tool for detecting and mapping alteration zones for gold exploration.     

Two orogenies in the Meatiq area of the Central Eastern Desert,Egypt

The Pan-African basement exposed in the Meatiq area west of Quseir, Egypt, consists of an infracrustal basement overthrusted by a supracrustal cover. The infracrustal rocks were developed as a result of an old orogeny referred to as the Meatiqian orogeny where granite—gneiss, migmatitic gneisses and migmatized amphibolites were formed. The granite—gneiss represents a deformed granite pluton emplaced at 626±2 Ma, whereas the migmatitic gneisses and amphibolites are of mixed igneous and sedimentary parentage. In view of the data so far available, the nature of the Meatiqian orogeny could not be deciphered. In spite of the young isotopic ages, it is suggested that at least the metasedimentary gneisses represent older rocks in the stratigraphic sequence of the infracrustal basement.The supracrustal cover represents a part of an extensive ophiolitic mélange obducted onto the infracrustal basement during the next orogeny (Abu Ziran orogeny) which culminated at 613±2 Ma. An active continental margin-type regime can adequately explain the evolution of such a supracrustal cover. During obduction, the ophiolitic mélange and the upper 2 km thick part of the infracrustal basement were intensely deformed and metamorphosed under PT conditions of the greenschist—epidote amphibolite facies. The deformed infracrustal basement was converted into mylonitic—blastomylonitic rocks and schists composing five thrust sheets, and subsequently intruded by synkinematic granitoid sheets. Later, both the infracrustal basement and the overlying supracrustal cover were isostatically uplifted, subjected to complex shallow folding giving rise to the major Meatiq domal structure, and were intruded by a postkinematic adamellite pluton at 579±6 Ma.     

Cryogenian ophiolite tectonics and metallogeny of the Central Eastern Desert of Egypt

The Central Eastern Desert (CED) is characterized by the widespread distribution of Neoproterozoic intra-oceanic island arc ophiolitic assemblages. The ophiolitic units have both back-arc and forearc geochemical signatures. The forearc ophiolitic units lie to the west of the back-arc related ones, indicating formation of an intra-oceanic island arc system above an east-dipping subducted slab (present coordinates). Following final accretion of the Neoproterozoic island arc into the western Saharan Metacraton, cordilleran margin magmatism started above a new W-dipping subduction zone due to a plate polarity reversal. We identify two belts in the CED representing ancient arc–forearc and arc–back-arc assemblages. The western arc–forearc belt is delineated by major serpentinite bodies running ～NNW–SSE, marking a suture zone. Ophiolitic units in the back-arc belt to the east show an increase in the subduction geochemical signature from north to south, culminating in the occurrence of bimodal volcanic rocks farther south. This progression in subduction magmatism resulted from diachronous opening of a back-arc basin from north to south, with a bimodal volcanic arc evolving farther to the south. The intra-oceanic island arc units in the CED include coeval Algoma-type banded iron formations (BIFs) and volcanogenic massive sulphide (VMS) deposits. Formation of the BIFs was related to opening of an ocean basin to the north, whereas development of the VMS was related to rifting of the island arc in the south. Gold occurs as vein-type mineral deposits, concentrated along the NNW–SSE arc–forearc belt. The formation of these vein-type gold ore bodies was controlled by the circulation of hydrothermal fluids through serpentinites that resulted in Au mobilization, as constrained by the close spatial association of auriferous quartz veins with serpentinites along the western arc–forearc belt.     

Remobilization of gold from a chalcopyrite-pyrite mineralization Hamash gold mine,Southeastern Desert,Egypt

Pyrite, chalcopyrite, and gold occur in quartz veins in granitic rocks and as scattered and disseminated impregnations in shear zones of the highly altered metavolcanics in the Hamash area, Southeastern Desert, Egypt. The minerals are associated in part with pyrrhotite, digenite, tetrahedrite, chalcocite, bornite, and covellite. Pyrite occurs in two forms: (1) idio- to hypidiomorphic coarse crystals with inclusions of preexisting sulfides, and (2) fine-crystalline aggregates. Chalcopyrite occurs in three forms: (1) idiomorphic coarse crystals, (2) fine-crystalline microinclusions, and (3) xenomorphic relicts. Three genetic phases of sulfide mineralization were identified. They are related to the successive cooling of the crystallizing solutions. Gold was hosted in the older sulfide minerals during a high-temperature disorder phase. Native gold was formed during the latest, decreasing-temperature phase through remobilization of auriferous pyrite. Microprobe analysis confirmed that gold and copper are relatively enriched in the late pyrite. Identified surface-alteration products include goethite, limonite, gold, carbonates, and sulfates of iron and copper.     

Stream sediment geochemical exploration for gold in Central Eastern Desert,Egypt: Application of the concentration-number fractal model,factor analysis,and geochemical mineralization probability index

Geochemical exploration by stream sediment sampling using bulk leach extractable gold (BLEG) technique and applying concentration-number (C-N) fractal model, factor analysis (FA), and geochemical mineralization probability index (GMPI) resulted in the recognition of new Au occurrences around the Sukari gold mine in the central Eastern Desert of Egypt. The geochemical data of 128 stream sediment samples collected from the study area was used for delineating the geochemical anomalies and characterizing the dispersion trains of ore and associated elements (Au, Ag, As, Sb, Cu, Pb, Zn, Mo). Statistical analysis of the geochemical data applying the C-N fractal modeling enabled us to identify significant anomaly and background populations of the investigated elements and to construct reliable geochemical anomaly maps. Factor analysis using centered log-ratios (CLR), to address the problem of closed compositional data, revealed significant element associations for mineralization (Au, As, Mo, Zn, Ba), country rock compositions (Rb, Li, Be, Sn, Bi for granite, and Co, Cr, Ni for mafic rocks), and element mobility (e.g. Sb, Zr, and Ag). Weak and moderate Au anomalies that cannot be detected by factor score maps can be delineated clearly by using the C-N fractal method and GMPI distribution map. Our study revealed that Ag, As, and Sb are the main pathfinder elements for gold mineralization in arid to semiarid regions exemplified by the Sukari gold district. Silver can be used as a “direct” pathfinder, whereas As and Sb are “indirect” pathfinders for Au in such regions. The spatial distribution of Au and Ag anomalies indicate that gold mineralization in the Sukari district is structurally controlled. However, the spatial distribution of Cu, Pb, Zn, and Mo is controlled by mineralogical and lithological factors and is not related to any significant base metal deposits.     

3-D magnetic inversion and satellite imagery for the Um Salatit gold occurrence,Central Eastern Desert,Egypt

The Um Salatit is a gold occurrence situated in the Central Eastern Desert of Egypt. The studied area is characterized by an intensive imbrication zone of serpentinites and metavolcanic rocks. The integrated use of aeromagnetic and spectral reflectance remote sensing data has proved effective for mapping geology related to ore deposits. Spectral reflectance maps have been produced to discriminate various rocks, such as serpentinites, gabbros, and metavolcanic rocks. 3-D inversion of aeromagnetic data acquired over the Um Salatit study area produced a 3-D magnetic susceptibility model showing magnetic bodies surrounded by less magnetic host rock. The magnetic features are elongated ENE-WSW parallel to the major thrust fault and mapped the Um Salatit serpentinized rocks. Remote sensing data allow investigating surficial geological features and mapping the mineralized areas. Remote results, in conjunction with 3-D inversion of aeromagnetic data, demonstrate that gold occurrence in the study area was effectively restricted to the highly magnetic zone interpreted as host rocks.     

Orogenic gold in the Egyptian Eastern Desert: Widespread gold mineralization in the late stages of Neoproterozoic orogeny

The Arabian-Nubian Shield (ANS) is a rapidly emerging world-class province for gold resources mainly in structurally-controlled quartz ± carbonate veins that are best classified as late Neoproterozoic orogenic gold deposits. Gold has been mined in the Eastern Desert of Egypt, in the northwestern part of the ANS, for >6000 years, that is since the times of the Pharaohs, but production prior to the 1900s was likely only about 25 t and mainly from alluvial workings. In the first half of the 1900s, about 7 t Au was recovered from the El Sid orogenic gold deposit. Today, Sukari is the single major producing mine in the Eastern Desert but many other significant gold occurrences are being actively explored.Formation of the ANS took place during closure of the Mozambique Ocean between the East and West Gondwana continental blocks. Ocean closure led to amalgamation of numerous ca. 870–625 Ma juvenile arc and back-arc igneous and sedimentary rock sequences, with many resulting terrane sutures marked by mafic-ultramafic ophiolitic assemblages and fragments. The 100 m.y. of orogeny beginning at ca. 650 Ma included crustal shortening, lithospheric reworking, escape tectonics, and eventual orogenic collapse. Peak metamorphism was reached in different parts and depths of the orogen diachronously between 620 and 585 Ma, magmatism was widespread during 650–580 Ma, and rapid exhumation of the metamorphosed rocks and mid-crustal intrusions took place from ca. 600 to 580 Ma. Regional fault sets that controlled much of the gold occurrences were related to initial transpression by oblique convergence between the arcs and associated with subsequent sinistral shearing reported as overlapping the exhumation. Because existing geological data are not adequate to fully evaluate the overall terrane history, we have subdivided the Eastern Desert into nine structural blocks, rather than arc terranes, based commonly on bounding shear zones and major faults.The greatest abundance of gold deposits is associated with the NW-trending Najd Fault System that comprises many splays throughout the blocks in the Central Eastern Desert that underwent episodes of shearing at ca. 640–570 Ma. Important deposits are also notably widespread along reactivated east-west thrust faults in the Allaqi-Sol Hamed block of the South Eastern Desert, with significant shearing at 610–580 Ma. Sulfide mineralogy of the Eastern Desert gold-bearing veins is dominated by pyrite, arsenopyrite, and (or) pyrrhotite, in addition to subordinate chalcopyrite, sphalerite, galena and tetrahedrite as well as alteration minerals that include white mica, chlorite, and carbonate, are those typical of orogenic gold deposits. Many gold occurrences are located along sheared margins to granitic intrusions or along contacts between different lithologies; sheared silica- and carbonate-altered ultramafic rocks along many fault zones are particularly widely associated with many of the gold occurrences. Ore-forming fluids were generally low-salinity aqueous-carbonic with most measured δ¹⁸O of mineralized quartz ranging from 8‰ to 15‰, δ¹³C for associated carbonate from −10‰ to −1‰, and δ³⁴S for ore-related sulfides varying between −5 and +10‰, but much more consistent for individual occurrences or deposits. Gold was deposited at temperatures of generally between 250° and 370 °C, depending on location, and largely variable pressures. The few obtained absolute dates on ore formation, from the Fawakhir and Atalla deposits in the Central Eastern Desert, indicate that at least some of the mineralization was formed at ca. 600 Ma.Orogenic gold formed after the cessation of arc-terrane accretion in the ANS and during a period in which most of the shield became established with a 30–40 km-thick continental crust and underwent a transition from compressional/transpressional terrane accretion to post-amalgamation transtensional shearing. This also was marked by a petrogenetic transition from pre- to syn-accretionary, arc-related calc-alkaline I-type magmatism to late- or post-tectonic A-type magmatism within the newly formed shield. Concurrently, the Eastern Desert was affected by widespread crustal flow of aqueous-carbonic fluids, particularly on the through-going, extensive strike-slip shears of the Najd Fault System. Gold and sulfide minerals were deposited in the Eastern Desert shallow and middle crust coevally with rapid terrane exhumation, during changes in fluid chemistry associated with pressure cycling and multiple vein quartz precipitation.     

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.     

Geophysical signature of the vien-type uranium mineralization of Wadi Eishimbai, Southern Eastern Desert, Egypt

The application of various geophysical tools with different responses succeeded in fixing U-mineralization in Wadi Eishimbai area. The area was studied using detailed ground spectrometric, magnetic, and filtered very low-frequency electromagnetic (VLF-EM) surveys. The interpretation of the obtained spectrometric maps clearly reflects the sharp increase of equivalent uranium (eU) content. Meanwhile, K and Th contents show sharp decreases. The eU/equivalent thorium (eTh) ratio correlates positively with eU concentrations and negatively with eTh concentrations, indicating an increase in U potentiality than the surrounding granite. The N?CS shear zone displays an eU content ranging from 20 to 140?ppm. The ENE-trending lamprophyre is characterized by elongated uranium anomalies trending in the E?CW direction, with values >90?ppm. Equivalent uranium content of the brecciated granite attains values up to 700?ppm. The ground magnetic and VLF-EM surveys played important roles in providing structural information which are proven useful in geological mapping and mineral exploration for the discovery of uranium mineralization in the study area. This study follows the expected subsurface extension of the Sela shear zone under Wadi sediments. The ground total magnetic intensity map shows a relatively narrow and an elongated shape for the lamprophyre anomaly extending for about 600?m in the Wadi toward the western direction. VLF-EM contour maps of the two used frequencies (17.1 and 28.5?kHz) show excellent agreement, indicating that the shear zone is distinguished with slightly strong conductivity westwards as an extension of the main shear zone. It is elongated in an ENE?CWSW trend and extends in the western direction, referring to the existence of conductive materials. Most of the NW/SE-trending faults cause sudden changes in the magnetic and VLF-EM contour spacing over an appreciable distance, which suggests a discontinuity in depth due to their left-lateral strike-slip displacements. The interpreted faults, with an ENE?CWSW trend representing the main trend of Sela shear zone through which hydrothermal solutions flowed, cause high alteration and uranium mineralization.     

Geochemical sampling techniques in the Eastern Desert of Egypt

This paper describes various problems in carrying out geochemical prospecting under the conditions that prevail in the Eastern Desert of Egypt. Conclusions are based on four years experience in geochemical exploration in the Eastern Desert, including studies of over 25,000 geochemical samples. In prospecting for ore deposits in stony and sandy deserts, different kinds of geochemical survey techniques can be successfully applied. In carrying out regional geochemical surveys, stream-sediment sampling in combination with panning of heavy concentrates may be used with a considerable degree of success. The method of the cold extraction of metals produces best results when the alluvial silt and argillaceous fraction (minus 0.075 mm) is sampled. Spectrographic analysis, on the other hand, provides dependable data when the minus 1-plus 0.25-mm fractions are sampled. It is established as a fact that sieving-out of the minus 0.25-mm fraction helps to get rid of aeolian sands which otherwise “dilute” anomalies.In the localities where colluvium is mixed with aeolian sand, sampling of the minus 1-plus 0.25-mm fraction considerably increases the contrast of anomalies (by 3.7 times for Sn, 3.8 for Mo, and 1.5 for Cu) and enlarges their size (4 times for Sn, 2.6 for Mo, and 2.5 for Cu). A possibility of forming of “false” anomalies in colluvium, resulting from the secondary concentration of certain heavy minerals resistant to weathering (e.g. Sn, W, Nb, Ta, Zr, etc.) was proven. This should be taken into consideration while carrying out geochemical prospecting in similar conditions.     

Determining heterogeneous deformation for granitic rocks in the northern thrust in Wadi Mubarak belt,Eastern Desert,Egypt

Finite-strain was studied in the mylonitic granitic and metasedimentary rocks in the northern thrust in Wadi Mubarak belt to show a relationship to nappe contacts between the old granitic and metavolcano-sedimentary rocks and to shed light on the heterogeneous deformation for the northern thrust in Wadi Mubarak belt. We used the R_f/ϕ and Fry methods on feldspar porphyroclasts, quartz and mafic grains from 7 old granitic and 7 metasedimentary samples in the northern thrust in Wadi Mubarak belt. The finite-strain data shows that old granitic rocks were moderate to highly deformed and axial ratios in the XZ section range from 3.05 to 7.10 for granitic and metasedimentary rocks. The long axes (X) of the finite-strain ellipsoids trend W/WNW and E/ENE in the northern thrust in Wadi Mubarak belt. Furthermore, the short axes (Z) are subvertical associated with a subhorizontal foliation. The value of strain magnitudes mainly constants towards the tectonic contacts between the mylonitic granite and metavolcano-sedimentary rocks. The data indicate oblate strain symmetry (flattening strain) in the mylonitic granite rocks. It is suggested that the accumulation of finite strain was formed before or/and during nappe contacts. The penetrative subhorizontal foliation is subparallel to the tectonic contacts with the overlying nappes and foliation was formed during nappe thrusting.     

Geochemical and petrographic studies of Ta mineralization in the Nuweibi albite granite complex, Eastern Desert, Egypt

The Nuweibi albite granite is one of 14 known Sn-Ta-Nb bearing granitoids in the Eastern Desert region of Egypt. The granite is a highly leucocratic, albite-rich rock with accessory columbite-tantalite, cassiterite, microlite and ixiolite as well as topaz, garnet and white mica. Ages of 450–600 Ma were obtained from zircons by the ²⁰⁷Pb/²⁰⁶Pb evaporation method. Great uncertainty is caused by the small size and poor quality of the grains, but the precision is sufficient to indicate that the granite is late- or postorogenic with respect to the Panafrican orogeny. The Nuweibi granite is divided into a western and an eastern part by a regional fault. Both parts of the granite are compositionally similar but there are important differences and a clear compositional gap between them, so they are considered separate facies of an intrusive complex. The eastern part of the granite is more highly mineralized, has higher modal albite contents and higher Ta/Nb ratios, both in the whole rock and in the ore minerals. It is suggested that the two parts of the granite evolved from a common source and were emplaced sequentially, the eastern part representing a later, more fractionated magma. Textural evidence strongly suggests that the granite has a magmatic origin overall, but disturbance of geochemical trends at the whole-rock scale and at the scale of zoning profiles in individual grains of columbite-tantalite indicate post-magmatic overprinting. By analogy with other Ta-bearing albite granites, the sodic bulk composition of the Nuweibi granite can be explained by fluorine enrichment in the magma. Fluorine contents in the magma were high enough to stabilize topaz, and muscovites contain 2–4 wt.%. F. However, whole-rock F contents are low. We speculate that the low Ca, Al and P contents of the magma prevented abundant F-bearing minerals to form and led to loss of fluorine to now-eroded roof rocks. Received: 8 November 1995 / Accepted: 10 June 1996     

Structures controlling U and Th mineralisation in the Gebel Felat area of the south Eastern Desert of Egypt

In the Eastern Desert of Egypt, younger granites host U and Th mineralisations which are concentrated along faults and joints. In particular, the Gebel Felat Pluton is characterised by a high level of radioactivity as shown by an aeroradiometric survey. The U content is 82 ppm and the Th content is 15 ppm in areas of high radioactivity. The rocks are cross-cut by two main sets of fractures trending east-west and northwest-southeast. The contour maps of these two trends can be correlated with the aeroradiometric map of the same area.     

Geochemistry and Radioactive Potentiality of Um Naggat Apogranite, Central Eastern Desert, Egypt

Abstract: The northern part of Um Naggat granite massif (UNGM) has suffered extensive post-magmatic metasomatic reworking which results into the development of (Zr, Hf, Nb, Ta, U, Th, F)– and albite-enriched and greisenized apogranite body (UNAP) of 600 m thick, and more than 3 km in the strike length.
Albitization produced an enrichment in Zr (av. 2384 ppm), Hf (61), Nb (419), and U (43). The Th/U ratio ranges between 1. 33 and 1. 90. Extreme albitization (i. e. the albitite rock) is characterized by sharp decrease in the rare metals contents. However, extreme greisenization (i. e. endogreisen bodies) is characterized by a considerable enrichment in Zr (av. 5464 ppm), Hf (143), Nb (2329), Ta (152), U (66) and Th (178). The Th/U ratio ranges between 1. 57 and 3. 60. In contrast to extreme greisenization, it seems that extreme albitization does not apparently change the fluid pH and therefore poor amounts of rare metals are localized in the albitites.
It is suggested that the presence of Na⁺, H⁺ and F^- in the ore fluids was essential to stablize complexes of Zr, Hf, Nb, Ta, U, Th, and HREE during extraction and transportation. In contrast, contemporaneous decrease of temperature and increasing pH due to decreasing pressure are considered the essential factors for localization of disseminated mineralization of Zr and Nb in the apical parts of the UNAP. The enhanced uranium content in the alteration facies of UNAP coupled with the absence of significant uranium mineralization may indicate the metalliferous rather than mineralized nature for the UNAP. The high uranium contents are stabilized in refractory accessory minerals. However, with repect to Zr and Nb, the UNAP especially the albitized and greisen facies, can be categorized as a mineralized productive granite.