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Since Antiquity, sustainable resources of gold and copper have been mined at two prominent prospects in the north Eastern Desert of Egypt, namely the south Gabel Um Monqul (SGUM) and Gabal Al Kharaza (GKZ). Mineralization is hosted by Neoproterozoic shield rocks represented by dacite and monzo- to syenogranite at the SGUM prospect whereas they are diorite, granodiorite, and quartz feldspar porphyry at the GKZ prospect. These hosts have been emplaced in an island arc environment from calc-alkaline magmas with a peraluminous to metaluminous signature. They are hydrothermally altered including albitization, sericitization, silicification, epidotization, and chloritization. The Au and Cu mineralizations are confined to shear zones that lately filled with auriferous quartz veins adjacent to mineralized alteration zones. In the GKZ prospect, the old trenches trend mainly in a NW–SE direction whereas it is NE–SW and NW–SE in the SGUM prospect. Evidence of shearing ranges from megascopic conjugate fractures and shear planes in the outcrops to microscopic sheared and crumbled Au-Cu ore assemblages dominated by Fe-Cu sulfides, specularite, and barite. Microscopic investigation suggests that the formation of specularite is due to the shearing of early existing magnetite. The ore textures and paragenetic sequence indicate that pyrite in the alteration zones is oxidized, leading to the liberation of gold up to 3.3 g/t. The formulae of the analyzed electrum lie in the range Au74.5-76.8 Ag22.2-24.5. Integration of the field, geochemistry, and mineral chemistry data, combined with the gold fire assay data prove the presence of sustainable amounts of disseminated Au and Cu, not only in the mineralized quartz veins, but also in the alteration zones. Data materialized in our paper show similarities in the style of mineralization at the SGUM and the GKZ prospects with iron oxide-copper-gold (IOCG) deposits elsewhere in the Arabian-Nubian Shield (ANS) and other world examples. [ABSTRACT FROM AUTHOR]

Fe-rich 'oxydravite' and dravite from the Late Proterozoic ophiolitic melange of the Arabo-Nubian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mossbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and [Fe.sup.2+] over the Y and Z sites, and an ordering of [Fe.sup.3+] at Y. The disordering can be explained by the substitution mechanisms [2.sup.Y] Mg+[sup.Z]+[sup.W]OH = [2.sup.Y]Al+[sup.Z]Mg+[sup.W][O.sup.2-] and [2.sup.Y][Fe.sup.2+]+[sup.2][Fe.sup.3+]+[sup.W]OH = [2.sup.Y][Fe.sup.3+]+[sup.2][Fe.sup.2+]+[sup.W][O.sup.2], which are consistent with reducing the mismatch in dimensions between Y[O.sub.6] and Z[O.sub.6] octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three [YO.sub.6] octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing Mg[O.sub.6] octahedra and the smaller [Si.sub.6][O.sub.18] tetrahedral tings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit between [YO.sub.6], Z[O.sub.6], and T[O.sub.4] polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of [sup.Z]Al combined with the occurrence of B at T in Al-rich tourmaline. [sup.Z]Al has a greater effect than [sup.Y]Al as long as Al does not fully occupy the Z site. Keywords: Chemical analysis, tourmaline, crystal structure, Mossbauer spectroscopy, order-disorder, XRD data DOI: 10.2138/am.2010.3357