On the relationship between auriferous talc deposits hosted in volcanic rocks and massive sulphide deposits in Egypt     

Nagy Shawky Botros 《Ore Geology Reviews》2003,23(3-4):223-257

The volcanic-hosted massive sulphide (VHMS) deposits in the Eastern Desert of Egypt (e.g., Um Saki deposit) are associated with Precambrian coarse acid pyroclastic rocks. The upper contacts of the massive sulphide body are sharp and well-defined; while the keel zone to the mineralization is always associated with pervasive alteration, characterized by the presence of septechlorite and talc, associated with variable amounts of carbonate and tremolite. On the other hand, the economic talc deposits in Egypt are hosted intensively altered volcanic rocks. Besides talc, chlorite, carbonates and tremolite that occur in variable amounts in these deposits, anomalously high concentrations of gold are also present.The present study showed that alterations in the talc deposits of Darhib, El Atshan, Abu Gurdi, Egat, Um Selimat and Nikhira are similar to those occurring in the keel zone underlying the VHMS of Um Samuki and that the chemical modifications due to alteration processes (additions of Mg, Fe, Mn and Ca coupled with depletions in silica, alkalies, alumina and titanium) are comparable, even the host rocks are different, thus reflecting a genetic relationship. It is suggested that, the examined localities of talc deposits are hosted in the intensively altered volcanics in the keel zones of volcanogenic massive sulphide deposits. Recently, detailed geophysical prospecting program, including electric (resistivity, self-potential and induced polarization), electromagnetic and magnetic methods, was carried out at Darhib, Abu Gurdi and Um Selimat talc deposits. The quantitative interpretation of these geophysical measurements revealed the presence of subsurface bodies of sulphides. The present distribution of talc and allied minerals in Darhib, El Atshan, Abu Gurdi, Egat, Um Selimat and Nikhira could be explained by a tectonic process in which the coarse acid pyroclastic rocks with massive sulphides have tilted in such way that the footwall rock alterations (talc and allied minerals) are exposed on the present-day surface at these localities. Structural studies are currently under way in an attempt to explain the deformation regime that led to the present situation of talc deposits.Two distinct spatial and mineralogical associations of gold mineralization could be identified in the volcanogenic massive sulphide deposits and their footwall alterations (the keel zone) in the Eastern Desert of Egypt. These are (1) gold–silver–zinc association, and (2) gold–copper association. In the former, gold grades are very low and silver is anomalous. This association occurs typically in the upper levels of the VHMS deposit where low-temperature sulphides are abundant. Gold was deposited because of the mixing between the ascending hot solutions and the sulphate-rich seawater. The upper levels of Um Samuki sulphide body represent this association. Gold–copper association, on the other hand, typically occurs in the footwall altered rocks (the keel zone) and the lowest parts of the massive sulphide body. Gold grades reach up to 5.54 ppm, but the average is 1 ppm. Silver is very low, usually in the range of 4–10 ppm. Lead usually, but not always, accompanies gold in this association. Deposition of gold probably took place due to decreasing of temperature and/or increasing pH of the ascending hot brines. The keel zones at Darhib, Abu Gurdi, El Atshan, Um Selimat, Nikhira and Egat talc mines better represent this association.  相似文献   

Uranium isotopic evidence for the origin of the Bahariya iron deposits, Egypt     

Adel A. Dabous   《Ore Geology Reviews》2002,19(3-4)

The economic iron ore deposits of Egypt are located at Bahariya Oasis in the Lower Middle Eocene limestone. The main iron minerals are goethite, hematite, siderite, pyrite, and jarosite. Manganese minerals are pyrolusite and manganite. Gangue minerals are barite, glauconite, gibbsite, alunite, quartz, halite, kaolinite, illite, smectite, palygorskite, and halloysite. Geochemical comparison between the ore and the Nubia sandstone showed that the ore is depleted in the residual elements (Al, Ti, V, and Ni) and enriched in the mobile elements (Fe, Mn, Zn, Ba, and U) which indicates that the Bahariya iron ore is not a lateritic deposit despite the deep weathering in this area. On the other hand, the Nubia sandstone showed depletion in the mobile elements, which demonstrates the leaching process in the Nubia Aquifer. The presence of such indicator minerals as jarosite, alunite, glauconite, gibbsite, palygorskite, and halloysite indicate that the ore was deposited under strong acidic conditions in fresh water.Isotopic analyses of the uranium in the amorphous and crystalline phases of the ore, in the country rocks, and dissolved in the Nubia Aquifer water, all support the conclusion that U and Fe were precipitated together from warm ascending groundwater. U and Fe display strong co-variation in the ore, and the ²³⁴U/²³⁸U activity ratio of the newly precipitated U in the country rock and the leached component of U in the groundwater are identical. There is only slightly more uranium in the amorphous phase than in the crystalline and only a slightly lower ²³⁴U/²³⁸U activity ratio, suggesting that the iron in the two phases have a similar origin. Comparison of the excess ²³⁴U in the water and in the total ore leads to the conclusion that the precipitation of the U, and by inference the iron, occurred within the last million years. However, that both precipitation and leaching of U have occurred over the last 300,000 years is evidenced by the extreme ²³⁰Th/²³⁴U disequilibria observed in some of the samples. Some of the amorphous depositional events have been very recent, perhaps within the last 10,000 years.  相似文献   

The upper Cenomanian (Cretaceous) ammonite Neolobites vibrayeanus () in the Middle East: taxonomic and palaeoecologic remarks     

Frank Wiese  Frauke Schulze 《Cretaceous Research》2005,26(6):930-946

In the Middle East and Africa, the Cenomanian ammonite genus Neolobites occurs, partly in great abundance, in shallow marine shelf settings. The genus includes several species but it has remained uncertain to what extent these species reflect biospecies, chronospecies or morphological variants. Based on material from Jordan and Egypt, the morphological variation of Neolobites vibrayeanus is described and discussed. From the several species of Neolobites described in the literature, only N. vibrayeanus, N. fourtaui and N. peroni appear to deserve specific separation. Sedimentological and lithological data, notably the common occurrence in strata that were possibly deposited above storm wave base, suggest that Neolobites was able to inhabit shallow marine settings. In these settings, it seems to form a distinct assemblage with the nautiloid Angulithes. Although N. vibrayeanus shows morphological variability between distinct populations, local N. vibrayeanus assemblages seem to develop morphologically stable populations that show only a little morphological overlap with those of other areas. This may be the reason for the earlier taxonomic splitting. The occurrence in habitats in which other stratigraphically significant ammonites are often missing may be the reason for the previous poor dating of Neolobites assemblages.  相似文献   

GLOBALIZATION AND EXTERRITORIALITY IN METROPOLITAN CAIRO     

PETRA KUPPINGER 《Geographical review》2005,95(3):348-372

ABSTRACT. Rapid construction of new spaces like hotels, malls, private clubs, and gated communities in Greater Cairo, Egypt produces structures disconnected spatially and conceptually from most of the existing urban fabric. Their spatial concepts and practices, as well as architectural forms and expertise, are based largely on globally available models. Planning and construction are guided by the search for security in the face of real or imagined fear of the urban masses and political upheaval. Concrete walls, guarded entrances, and high‐tech security technology bear witness to these fears. Analysis of the Mena House Hotel, the Grand Egyptian Museum project, and the First Mall in Giza shows how these projects globalize Cairo and localize the global. Often these globalized spaces are remade by creating local and regional ties and design features that were not anticipated by the planners. Such changes shed light on underlying dynamics and contribute to a better understanding of in situ globalization. Whereas their physical features tend to accentuate their globalized nature, these spaces do not exist in isolation from their geographical and cultural contexts. Their everyday realities tell tales of reterritorialization that are frequently overlooked in scholarly debates.  相似文献   

Electrical structure of the tectonically active Kalabsha Fault, Aswan, Egypt     

Mahmoud Mekkawi  Pierre-Andr Schnegg  Tarek Arafa-Hamed  Essam Elathy 《Earth and Planetary Science Letters》2005,240(3-4):764-773

In this work, we use the magnetotelluric (MT) method to detect geoelectrical conductivity anomalies in the Earth's crust and link them to local seismic activity. This application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity. MT measurements were carried out in the period range 0.0046–420 s at nine sites along a 15 km-long North–South profile crossing the Kalabsha Fault, on the western bank of Lake Aswan. Data were analysed by 2D simultaneous inversion of both polarisations. The resulting model is compared with the local seismicity map and reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in the Aswan area. Our MT investigations show the following features:

The measured MT strike aligns with the seismic epicentre axis corresponding to the Kalabsha Fault.

While crossing the Fault, enhanced conductivity is found down to depths of 5 km on a 1–2 km profile segment.

At mid-crustal depths (20 km), a very high conductive body is found to coincide with the main seismic cluster in the Aswan area.

These observations indicate that seismic activity and high electrical conductivity are related. The link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed. Their presence is also required to trigger the observed seismicity. In addition, we explain the lower conductivity of the local upper crust in terms of stress-modulated rock porosity. We believe that these results are of general significance, as they could explain the mid-crustal seismicity of tectonically active zones.  相似文献   

Sustainable development and international cooperation in the Eastern Nile Basin     

Salah El-Din Amer  Yacob Arsano  Atta El-Battahani  Osman El-Tom Hamad  Magdy Abd El-Moenim Hefny  Imeru Tamrat  Simon A. Mason 《Aquatic Sciences - Research Across Boundaries》2005,67(1):3-14

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Metal pollution loading,Manzalah lagoon,Nile delta,Egypt: Implications for aquaculture   总被引：1，自引：0，他引：1  

F. R. Siegel  M. L. Slaboda  D. J. Stanley 《Environmental Geology》1994,23(2):89-98

High cultural enrichment factors are found for Hg (13×), Pb (22.1×), and other potentially toxic metals (e.g., Sn, Zn, Cu, Ag) in the upper 20 cm of sediment cores from the southeastern Ginka subbasin of Manzalah lagoon, Nile delta, Egypt. Cores from other areas of the lagoon show little metal loading. Metal loading followed the closure of the Aswan High Dam, the availability of abundant cheap electricity, and the development of major power-based industries. Industrial wastes containing potentially toxic metals are dumped into the Nile delta drain system. The load carried by Bahr El-Baqar drain discharges into the Ginka subbasin, which acts as a sink and results in metal loading of the sediment deposited there. Further development of aquaculture in this subbasin, of food-stuff agriculture on recently reclaimed lagoon bottom, or where irrigation waters come from Bahr El-Baqar drain or its discharge should be halted or strictly limited until potentially toxic metals in the drain waters and sediment are removed and polluted input drastically reduced. This environmental assessment of heavy metals in aquaculture or agriculture development should extend to other waterbodies in the northern Nile delta, particularly Idku lagoon and lake Mariut, where industrial metal-bearing wastes discharge into the waterbodies.  相似文献   

An Overview on the REE-Mineralized Alkaline Granites in Egypt, with Special Reference to the Abu Khruq and Gara El Hamra Bodies     

Hamdy M. Abdalla 《Resource Geology》2006,56(3):365-370

Abstract. The REE-mineralized alkaline granites in Egypt are divided into the following three classes: (1) Mesozoic, anorogenic nepheline syenite ring complexes with REE amounting up to 1.3 %, particularly in their fenitized parts (e.g. Abu Khruq), (2) an orogenic peralkaline syenite-granite, composed of i) Zr, Nb, - REE, and Th-enriched peralkaline granite-syenite complexes with REE amounting up to 0.5 % (e.g. Um Hibal, Tarbite North and South, Gharib, and Zarget Naam) and ii) Y, Th, HREE, and P-enriched post-Cretaceous peralkaline complexes that intrude the Phanerozoic rocks of the Southwestern Desert with REE amounting up to 2 % (e.g. Gara El Hamra), and (3) upper Proterozoic, post-orogenic siderophyllite alkali feldspar granite with REE amounting up to 0.8 %, particularly in their apical miarolitic pegmatites and albitized zones (e.g. Kadabora-Abu Dob and Um Naggat). Special attention is given to the Abu Khruq and Gara El Hamra granitic bodies.  相似文献   

Geologic relationships and mineralization of peralkaline/alkaline granite-syenite of the Zargat Na''am ring complex, Southeastern Desert, Egypt     

Gehad M. Saleh 《中国地球化学学报》2006,25(2)

The Zargat Na'am ring complex crops out 90 km NW of Shalatin City in the Southeastern Desert of Egypt. The ring complex forms a prominent ridge standing high above the surrounding mafic-ultramafic hills. It is cut by two sets of joints and faults which strike predominantly NNW-SSE and E-W,and is injected by dikes, porphyritic alkaline syenites, and felsite porphyries. It consists of alkali syenites, alkali quartz syenites, and peralkaline arfvedsonite-bearing granitic and pegmatitic dikes and sills.The complex is characterized locally by extreme enrichments in REEs, wolframite and rare, high field strength metals (HFSM), such as Zr and Nb. The highest concentrations ( 1.5 wt% Zr, 0.25 wt% Nb,0.6 wt% ∑REEs) occur in aegirine-albite aplites that formed around arfvedsonite pegmatites. Quartzhosted melt inclusions in arfvedsonite granite and pegmatite provide unequivocal evidence that the peralkaline compositions and rare metal enrichments are primary magmatic features. Glass inclusions in quartz crystals also have high concentrations of incompatible trace elements including Nb (750 × 10-6), Zr (2500 × 10-6) and REEs (1450 × 10-6). The REEs, Nb and Zr compositions of the aegirine-albite aplites plot along the same linear enrichment trends as the melt inclusions, and Y/Ho ratios mostly display unfractionated, near-chondritic values. The chemical and textural features of the aegirine-albite aplites are apparently resultant from rapid crystallization after volatile loss from a residual peralkaline granitic melt similar in composition to the melt inclusions.  相似文献   

Joint inversion of gravity and geoelectrical data for groundwater and structural investigation: application to the northwestern part of Sinai, Egypt   总被引：2，自引：0，他引：2  

F. A. Monteiro Santos  S. A. Sultan  Patrícia Represas  A. L. El Sorady 《Geophysical Journal International》2006,165(3):705-718

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