Multi-stage emerald formation during Pan-African regional metamorphism: The Zabara, Sikait, Umm Kabo deposits, South Eastern desert of Egypt

Grundmann, Günter; Morteani, G.

doi:10.1016/j.jafrearsci.2007.09.009

Cited by 25 publications

(21 citation statements)

References 27 publications

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“…The Egyptian emerald occurrences of Gebels Zabara, Wadi Umm Kabu, and Sikait occur in a N-W trending band circa 45 km long in the Nugrus thrust [128][129][130]. The deposits are located in a volcano-sedimentary sequence featuring an ophiolitic tectonic melange composed of metamorphosed M-UMR overlying biotite orthogneiss.…”

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Emerald Deposits: A Review and Enhanced Classification

et al. 2019

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Although emerald deposits are relatively rare, they can be formed in several different, butspecific geologic settings and the classification systems and models currently used to describeemerald precipitation and predict its occurrence are too restrictive, leading to confusion as to theexact mode of formation for some emerald deposits. Generally speaking, emerald is beryl withsufficient concentrations of the chromophores, chromium and vanadium, to result in green andsometimes bluish green or yellowish green crystals. The limiting factor in the formation of emeraldis geological conditions resulting in an environment rich in both beryllium and chromium orvanadium. Historically, emerald deposits have been classified into three broad types. The first andmost abundant deposit type, in terms of production, is the desilicated pegmatite related type thatformed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar graniticbodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks,or shales derived from those rocks. A second deposit type, accounting for most of the emerald ofgem quality, is the sedimentary type, which generally involves the interaction, along faults andfractures, of upper level crustal brines rich in Be from evaporite interaction with shales and otherCr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is themetamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faultsor shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Beand Cr (±V) may either be transported to the deposition site via the fluids or already be present inthe host metamorphic rocks intersected by the faults or shear zones. All three emerald depositmodels require some level of tectonic activity and often continued tectonic activity can result in themetamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deepercrustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks,blurring the distinction between metamorphic and magmatic deposit types. In the present paper,we propose an enhanced classification for emerald deposits based on the geological environment,i.e., magmatic or metamorphic; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, andgranitoids; degree of metamorphism; styles of minerlization, i.e., veins, pods, metasomatites, shearzone; type of fluids and their temperature, pressure, composition. The new classification accountsfor multi-stage formation of the deposits and ages of formation, as well as probable remobilizationof previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Suchnew considerations use the concept of genetic models based on studies employing chemical,geochemical, radiogenic, and stable isotope, and fluid and solid inclusion fingerprints. The emerald occurrences and deposits are classified into two main types: (Type I) Tectonic magmatic-relatedwith sub-types hosted in: (IA) Mafic-ultramafic rocks (Brazil, Zambia, Russia, and others); (IB)Sedimentary rocks (China, Canada, Norway, Kazakhstan, Australia); (IC) Granitic rocks (Nigeria).(Type II) Tectonic metamorphic-related with sub-types hosted in: (IIA) Mafic-ultramafic rocks(Brazil, Austria); (IIB) Sedimentary rocks-black shale (Colombia, Canada, USA); (IIC) Metamorphicrocks (China, Afghanistan, USA); (IID) Metamorphosed and remobilized either type I deposits orhidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassifieddeposits.

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“…Syntectonic intrusions of leucogranites and pegmatites occurred along the ductile shear-zone [43]. The study of [130] described three beryl-emerald generations that crystallized during magmatic, post-magmatic hydrothermal, and regional-metamorphic events. The genetic succession was based on chemical and microstructural studies.…”

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Emerald Deposits: A Review and Enhanced Classification

et al. 2019

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“…Its formation in the metamorphic units was controlled by syn-to post-tectonic deformation due to the upthrust of chromium-rich ophiolitic units, which resulted in metasomatic reactions. Thus, the concentration of magnesium-rich and beryllium-rich fluid favored the crystallization of beryl (Turner et al, 2005;Grundmann and Morteani, 2008;Groat et al, 2008). The host rock of this type of beryl occurrence is similar to beryl (emerald) deposits that are formed via syn-to post-tectonic reactions under low-grade regional metamorphism and within biotite schist tectonically overlain by ophiolite, such as in southern Egypt (Abdalla and Mohamed, 1999) and Habachtal, Austria, (Grundmann and Morteani, 1989) where the source of chromium is believed to be metasomatized ophiolitic rocks (Groat et al, 2008).…”

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confidence: 90%

“…The high magnesium contents of the metamorphic units may indicate a genetic relationship to beryl (Schwarz and Giuliani, 2001). The chromium (619 ppm) and nickel (681 ppm) contents in the garnet-glaucophane schist can be explained by the close association with ophiolitic rocks, which contribute chromium and nickel via leaching (Grundmann and Morteani, 2008). Furthermore, LILE (barium, strontium), transition metals (cobalt, except nickel) and HFSE (niobium, zirconium, and yttrium) element values that were low in stream sediments relative to metamorphic rocks suggest that these elements were transported during weathering of primary minerals, similar to what has been reported for LILE values in Sri Lanka .…”

Section: Discussionmentioning

confidence: 99%

Occurrence of Cr-bearing beryl in stream sediment from Eskişehir, NW Turkey

Erkoyun

Kadır

2016

Earth sci. res. j.

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Beryl crystals are found within stream sediments transecting schists in the northeast of Eskişehir, western Anatolia. This paper studied the Eskişehir beryl crystals with optical microscopy, scanning electron microscopy (SEM-EDX), infrared spectroscopy (IR) and geochemical analyses. Beryl is accompanied by garnet, glaucophane, quartz, epidote, muscovite and chlorite in the stream sediments. The crystals are euhedral emerald (green gem beryl) and light bluishgreen aquamarine, with ideal sharp IR bands. Wet chemical analysis of Eskişehir beryl yielded 61.28% SiO2, 15.13% Al2O3, 12.34% BeO, 0.18% Cr2O3, 1.49% MgO, 1.69% Na2O, 0.98% Fe2O3, and 0.008% V2O3, resulting in the formula (Al1.75Cr0.01Mg0.22Fe0.08)(Be2.90Si6.00)(Na0.32)O18. Large Ion Lithophile Elements (LILE) (barium, strontium), some transition metals (cobalt, except nickel) and High Field Strength Elements (HFSE) (niobium, zirconium, and yttrium) in stream sediments that are associated with beryl exhibited low content about metamorphic rocks. Beryl formation appears to be controlled by upthrust faults and fractures that juxtaposed them with Cr-bearing ophiolitic units and a regime of metasomatic reactions. Such beryl crystals have also been found in detrital sediments that are derived from the schists.Cristales de berilio fueron encontrados en sedimentos de corrientes que atraviesan en esquistos en el noreste de Eskisehir, al oeste de Anatolia. En este artículo se presentan resultados sobre el estudio de los cristales de berilio de Eskisehir con microscopio electrónico de barrido (SEM-EDX, del inglés Scanning Electron Microscopy), espectroscopia infrarroja y análisis geoquímicos. El berilio estaba acompañado de granate, glaucofana, cuarzo, epidota, moscovita, y clorito en las corrientes sedmientarias. Los cristales son esmeraldas de formas definidas (gema verde de berilio) y aguamarinas color verde celeste, con bandas de espectroscopia infrarroja de buena nitidez. El análisis químico húmedo del berilio de Eskisehir mostró 61.28 % de SiO2, 15.13 % de Al2O3, 12.34 % de BeO, 0.18 % de Cr2O3, 1.49 % de MgO, 1.69 % de Na2O, 0.98 % de Fe2O3, y 0.008% de V2O3, lo que resulta en la formula (Al1.75Cr0.01Mg0.22Fe0.08)(Be2.90Si6.00)(Na0.32)O18. Los elementos litófilos de ion grande (bario, estroncio), algunos metales de transición (cobalto, excepto níquel) y los elementos de gran campo de fuerza (niobio, circonio e itrio) en las corrientes de sedimentos que están con berilio mostraron un bajo contenido sobre las rocas metamórficas. Las formaciones de berilio aparecen controladas por fallas de cabalgamiento y fracturas que se yuxtaponen con las unidades ofiolíticas relacionadas con cromo y un régimen de reacciones metasomáticas. Estos cristales de berilio se han encontrado también en sedimentos detríticos que se derivan de los esquistos. EARTH SCIENCES

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“…In the Egyptian occurrences, tourmaline is found in pegmatites and metasediments associated with gem-quality beryl (emerald) at the famous, ancient Cleopatra's Mines in the Eastern Desert (Grundmann andMorteani 1993, 2008;Harrell 2004). Mineral parageneses in the Egyptian occurrences can be summarized as follows: quartz-feldspars-garnet-biotite-phlogopitetourmaline±beryl (in the case of pegmatites and pelitic schists) or tremolite-actinolite-tourmaline-apatite (in the case of metapyroxenite).…”

Section: Geological Setting Of Samples and Paragenesismentioning

confidence: 99%

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

Bosi

Balić-Žunić

Surour

2010

American Mineralogist

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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(2+) over the Y and Z sites, and an ordering of Fe(3+) at Y. The disordering can be explained by the substitution mechanisms 2(Y)Mg+(Z)Al+(W)OH = 2(Y)Al+(Z)Mg+(W)O(2-) and 2(Y)Fe(2+)+(Z)Fe(3+)+(W)OH = 2(Y)Fe(3+)+(Z)Fe(2+)+(W)O(2-), which are consistent with reducing the mismatch in dimensions between YO(6) and ZO(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(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 MgO(6), octahedra and the smaller Si(6)O(18) 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 YO(6), ZO(6), and TO(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 (Z)Al combined with the occurrence of B at Tin Al-rich tourmaline. (Z)Al has a greater effect than (Y)Al as long as Al does not fully occupy the Z site

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Multi-stage emerald formation during Pan-African regional metamorphism: The Zabara, Sikait, Umm Kabo deposits, South Eastern desert of Egypt

Cited by 25 publications

References 27 publications

Emerald Deposits: A Review and Enhanced Classification

Emerald Deposits: A Review and Enhanced Classification

Occurrence of Cr-bearing beryl in stream sediment from Eskişehir, NW Turkey

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

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