Adel A. Surour

Crystal structure analyses of four tourmaline specimens from the Cleopatra’s Mines (Egypt) and Jabal Zalm (Saudi Arabia), and the role of Al in the tourmaline group

Abstract Fe-rich “oxydravite” and dravite from the Late Proterozoic ophiolitic mélange 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 Mössbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe2+ over the Y and Z sites, and an ordering of Fe3+ at Y. The disordering can be explained by the substitution mechanisms 2YMg+ZAl+WOH = 2YAl+ZMg+WO2- and 2YFe2++ZFe3++WOH = 2YFe3++ZFe2++WO2-, which are consistent with reducing the mismatch in dimensions between YO6 and ZO6 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 YO6 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 MgO6 octahedra and the smaller Si6O18 tetrahedral rings. 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 YO6, ZO6, and TO4 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 ZAl combined with the occurrence of B at T in Al-rich tourmaline. ZAl has a greater effect than YAl as long as Al does not fully occupy the Z site.

Ophicarbonates: calichified serpentinites from Gebel Mohagara, Wadi Ghadir area, Eastern Desert, Egypt

An ophicalcite occurrence is recorded in the uppermost part of the Precambrian ophiolitic serpentinites at Gebel Mohagara (Wadi Ghadir area) in the Egyptian Eastern Desert. In this locality, the serpentinites and their ophicalcites are sometimes directly overlain by pelagic shales and calcareous sediments along thrust planes. Field relations suggest that these ophicalcites are present as serpentine- carbonate breccias that develop along conjugate shear planes and brecciation zones. Typical sedimentary features are common, such as the presence of micritic carbonate, colloform texture, geopetal-like structures and the presence of vugs. The latter are often filled by coarse calcite spars due to diagenesis and neomorphism. Another older type of less brecciated ophicarbonates (ophimagnesites) is also present and shows extensive replacement of serpentine minerals by magnesite. The ophicalcites are considered as sedimentary breccias formed in a weathered serpentinite lithology with fabrics of typical calichified rocks. It is believed that the calichified serpentinites represent a reworked oceanic calcite that have been formed after the abduction of the ophiolite nappe on the continent. The dissolution of the calcareous material in the pelagic cap furnished the needed carbonate influx to fill the brecciated serpentinite below. D 1997 Elsevier Science Limited.

Medium-to high-pressure garnet-amphibolites from Gebel Zabara and Wadi Sikait, south Eastern Desert, Egypt

Abstract Garnet-amphibolites from Gebel Zabara and Wadi Sikait in the southern Eastern Desert of Egypt occur as highly flattened metamorphosed basic volcanic bands enclosed within garnetiferous metasediments. Samples from both localities have almost the same metamorphic assemblage of garnet-amphibole-plagioclase-ilmeniterutile. An electron microprobe study indicates that garnet, amphibole and plagioclase are cryptically zoned only in samples from Wadi Sikait. The composition of amphiboles (tschermakitic hornblende to tschermakite) reflects a temperature range equivalent to that of the staurolite-kyanite zone of the metapelitic sequences. Geothermometric calculations of the pairs garnet-amphibole and amphibole-plagioclase indicate average temperatures of 550°C for samples from Wadi Sikait and Gebel Zabara, respectively. Pressures of about 6.8 kbar and 7.7 kbar are obtained using some mineral equilibria of both silicates and opaque phases. The garnet-amphibolites are considered as a part of the infrastructural suite in the Eastern Desert. A comparison with the Pan-African amphibolites from the Eastern Desert and Sinai is presented.

Yukonite-like alteration products (Ca–Fe arsenate and As-rich Fe-oxyhydroxide) formed by in situ weathering in granodiorite, Bi'r Tawilah gold prospect, Saudi Arabia

Samples from drilling of the Bi9r Tawilah gold prospect in Saudi Arabia reveal the occurrence of a Ca–Fe arsenate phase, which is similar in appearance and chemistry to yukonite. Upon weathering of a granodiorite host, oxidation of arsenopyrite (0–25 m deep) leads to the formation of a very peculiar brown, amorphous to very poorly crystalline aggregate with cellular-like texture. This mixture consists of Ca–Fe arsenate and arsenic-rich ferric oxyhydroxide resulting from the oxidation of arsenopyrite. It is intergrown with colloform ferric oxyhydroxide, the latter resulting from the oxidation of pre-existing pyrite. The EMPA analyses indicate that the Ca-rich domain contains the maximum As 2 O5 content (up to 22.3 wt%) whereas the colloform ferric oxyhydroxide contains the highest amount of Fe 2 O 3 among the sample studied (60.8–63.1 wt%) associated to higher H 2 O content (31.4–33.2 wt%) than in the case of common goethite and lepidocrocite. As far as typical yukonite, scorodite or arsenosiderite are absent in the studied weathered granodiorite, it is believed that oxidation took place at elevated pH (>7) and temperature up to ∼75 °C. The source of Ca 2+ can be derived from alteration of plagioclase in the granodiorite but its possible derivation from strongly corroded marble bands cannot be discarded. It is evident that availability of Ca 2+ and high pH buffered by the dissolution of calcite in the marble, in addition to the prevailing temperature upon weathering, played important roles in the formation of these pseudomorphs at Bi9r Tawilah.

Margarite in ultramafic alteration zones (Blackwall) A new occurrence in Barramiya Area, Egypt

Margarite is recorded here for the first time in the alteration zones (blackwall) of ultramafic rocks. The mineral occurs in metasomatized metapyroxenites (chloritites) of the Barramiya area in the central Eastern Desert of Egypt. These metapyroxenites belong to the Neoproterozoic Pan-African melange of northeast Africa. Two sub-types of chloritites are distinguished, namely margarite-free and margarite-bearing chloritites. Margarite is only found in the metapyroxenites when they are in direct contact with the melange metadiabases. Margarite is considered here as a metasomatic mineral resulting from the addition of both Ca 2+ and Al 3+ from the juxtaposing metadiabases. This is also accompanied by Fe-Mg substitution causing the conversion of the coexisting chlorite species from sheridanite to ripidolite. Also, extensive alteration of ilmenite to rutile is observed. The formation of post-kinematic margarite followed the event of chloritization but it is contemporaneous with the appearance of andalusite (chiastolite) in the matrix metapelites due to the intrusion of within-plate granites (GII). The assemblage margarite-ripidolite-rutile-ilmenite is free of quartz and plagioclase and hence its formation characterizes very low silica activity (μ SiO 2 ) in the system. It is suggested that the stability limits of the mineral in this case are within the temperature range of 355–405°C at 1–3 kbar.