Gabbro-diorite-tonalite¡granodiorite complexes, derived from subduction-related, calc alkaline magmas represent a distinct unit in the Precambrian basement of Egypt. These intrusive complexes are commonly intruded into ophiolitic mé lange, schistose or gneissic rock assemblages. Well-constrained geological mapping of the Um Eleiga complex, combined with geochemical studies, should enable a better understanding of its genesis, especially its potential as a host for gold mineralisation. This complex represents a composite pluton intruded into the allochthonous, ophiolitic Abu Dahr massif and its associated undifferentiated metavolcanosedimentary mé lange matrix in the southern Eastern Desert of Egypt. Field and petrographic investigations have revealed a compositional continuum from gabbro to granodiorite through diorite and subordinate tonalite. Petrography and bulk rock geochemical data have been used to determine the magma type and tectonic setting of the Um Eleiga complex, whereas the electron microprobe data were valuable for the purpose of determining the pressure-temperature and oxygen fugacity conditions. The compositional continuum, pattern of zonation, shallow magma origin and marked common negative Nb anomalies suggest that a close petrogenetic relationship between the gabbro, diorite, tonalite and granodiorite units of the Um Eleiga complex. Subducted upper mantle wedge is proposed as being the source region for the gabbroic magma, whereas zoning of the complex is attributed to interplay of fractional crystallisation and assimilation. Pressure estimates based on the Al (iv) -in-hornblende geobarometry indicate that the gabbroic and dioritic rocks crystallised under 4-5 kbar, whereas tonalite and granodiorite were formed at 3-4 kbar. The hornblende-plagioclase thermometer indicates a formation temperature range of about 720 to 880uC for the complex. Varied oxidation states are inferred from the mineral assemblages, including magnetite, titanite and ilmenite and checked by Fe/(FezMg) ratios of biotite in the granitoid rocks. The tectonic setting, mineralogical, geochemical characteristics and varied redox state of the Um Eleiga intrusive complex are typical for arc-related intrusives potential for hosting intrusion-related gold in a collisional environment.
Although there is relatively minor gold production from the Arabian-Nubian shield at present, extensive alluvial and lode fields along the western side of the Red Sea in Upper Egypt and northern Sudan were worked out by the ancient Egyptians for thousands of years. In the Eastern Desert of Egypt, numerous but small gold deposits are generally related to auriferous quartz veins commonly associated with brittle-ductile shear zones, generally cutting through the Neoproterozoic crystalline basement rocks and trending in different directions. Gold mineralisation at the Betam mine area, south Eastern Desert, is related to a series of milky quartz veins along a NNW-trending brittle-ductile shear zone cutting through successions of pelitic schists, next to a small granite intrusion. Gold-sulphide mineralisation (pyrite, arsenopyrite, galena, subordinate chalcopyrite and gold) is closely associated with a conspicuous hydrothermal alteration halo. Textural relationships, including replacement and crosscutting of mineral phases and quartz veins record a post-foliation alteration assemblage of quartzzsericitezchloritezcalcite¡ albite¡epidote. The hydrothermal alteration halo alongside the auriferous quartz veins comprises three distinct zones, namely distal chlorite-calcite zone (chloritezcalcite¡biotite¡pyrite¡ sericite¡epidote), an intermediate sericite-chlorite zone (sericitezchloritezpyrite¡biotite), and a proximal pyrite-sericite zone (quartzzpyritezsericite¡albite). These zones merge to each other gradually, ending outwards into the unaltered metasediments. The pyrite-sericite zone contains the highest gold grades, especially in zones thickly seamed with sulphide-rich quartz veinlets. Mass balance calculations have revealed that the pyrite-sericite zone experienced significant metasomatic changes relative to limited mass and volume changes for the chloritecalcite zone. The overall picture of chemical gains and losses with increasing the intensity of hydrothermal alteration is indicative of addition of SiO 2 , K 2 O, Na 2 O, and volatile elements (S, CO 2 ), removal of MgO, and relatively inert behaviour of Al 2 O 3 , TiO 2 , MnO, Fe 2 O 3 . CaO is variably mobile; slightly enriched in the chlorite-calcite and sericite-chlorite zones but depleted in the pyrite-sericite zone. Concentrations of the trace elements are variable in the different alteration types, but a notable increase in Au, As, Ba, Sr, Rb, V, and Ni in the intensively altered rocks is apparent. Investigation of the rare earth element (REE) behaviour reveals a little modification; heavy REE are more or less unchanged, whereas light REE are significantly mobile in all alteration types. New geochemical data provide evidence for progressive silicification, sericitisation, and sulphidation as a function of gold mineralisation. A proposed model for the hydrothermal alteration system for the Betam deposit includes fluctuation in pH and redox state (fO 2 ), mainly during the wallrock sulphidation. This might have destabilised gold complexes and lowered gold solubi...
Remote sensing data are used to discriminate between the different lithologies covering the Um Had area, Central Eastern Desert of Egypt. Image processing techniques applied to the Enhanced Thematic Mapper (ETM+) data are used for mapping and discriminating the different basement lithologies of Um Had area. Principal component analysis (PCA), minimum noise fraction (MNF) transform and band rationing techniques provide efficient data for lithological mapping. The study area is underlain by gneisses, ophiolitic melange assemblage (talc-serpentinite, metagabbro, metabasalt), granitic rocks, Dokhan volcanics, Hammamat sediments and felsites. The resulting gray-scale PC2, PC3 and PC4 images are best to clearly discriminate the Hammamat sediments, amphibolites and talc-serpentinites, respectively. The gray-scale MNF3 and MNF4 images easily discriminate the felsites and talc-serpentinites, respectively. The band ratio 5/7 and 4/5 images are able to delineate the talc-serpentinites and Hammamat sediments, respectively. Information collected from gray-scale and false color composite images led to generation of detailed lithologic map of Um Had area.
Remote sensing and geochemical data are used to discriminate between the different varieties of island arc assemblage covering Marahiq area, South Eastern Desert of Egypt. The Precambrian basement rocks exposed in the study area comprise ophiolite and island arc assemblages. The ophiolitic rocks include serpentinites and talc-carbonate, while the metavolcanics and metavolcanosedimentary rocks represent the island arc assemblage. Petrographically, the island arc units are differentiated in to metabasalt, meta-andesite, metadacite, meta-andesitic tuffs, chlorite talc schist and biotite chlorite schist. The geochemical data revealed that these metavolcanics and metavolcanosedimentary rocks are calcalkaline to tholeiite in nature with low to medium-K, and likely represent an immature island arc. Image processing techniques applicable to the Enhanced Thematic Mapper (ETM+) data are used for mapping and delineating exposures of the different island arc units at the Marahiq area. Principal component analysis, minimum noise fraction and band rationing techniques provide efficient data for lithological mapping. New detailed map was generated as a result of the processed ETM+ images.
Geochemistry of gabbroid and granitoid plutonites from the Um Had area indicates island arc subalkaline basic magma with tholeiitic affinity and calc-alkaline, metaluminous and slightly peraluminous magma, respectively. Although different in age both plutonite types were emplaced under compressional regime, where subduction-related environment was dominant. They were formed under relatively low to moderate water-vapour pressure (1-5 k-bars) at moderate depths (20-30 km). Biotite granites were formed at a relatively high temperature range (800-840 C), while biotite-muscovite granites were formed under relatively moderate temperature conditions (760-800 C). These two units may represent evolution from island arc to active continental margin. It is suggested that island arc gabbros might have sourced the late subduction-related calc-alkaline granitoids during the waning stages of the pan-African orogeny. The I-type nature of the investigated plutonites in the study area and elsewhere suggests the juvenile character of the basement complex of the Eastern Desert of Egypt.In this study, the geochemical characteristics of the Um Had-Um Shegila plutonites are addressed to clarify the tectonic environment that the Central Eastern Desert represents. The Um Had GranitoidsThe Um Had area contains a large elliptical structure trending northwest outlining a core of medium-to high-grade partly gneissic rocks enclosed by a domed thick mylonitic shear zone (e.g., Fowler, 2001; Figure 1). The core is up-heaved by the Um Had granitoid intrusion. The latter and its equivalents scattered throughout the Egyptian Eastern Desert was emplaced during Pan-African orogeny. These granitoids are referred to as late-to post-orogenic calc-alkaline to transitional A-type granitoids, interpreted by Black and Liegeois (1993) as being the result of delamination of the continental lithospheric mantle following the tectonic collisions of East and West Gondwana. These granitoids are characterised by unfoliated, small, nearly circular to elongate epizonal plutons with sharp intrusive contacts.
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