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Kiiash Journalofafrican Earth Sciences kiiash JournalofAfrican Earth Sciences. Vol. 28. No. 3. DD. 581-598. 1999 Pergamon a 1999 Eisevier Science Ltd Pll:SO899-5382(99)00033-O All rights reserved. Printed in Great Britain 0699.5362199 $- see front matter Mineralogical and geochemical investigation of emerald and beryl mineralisation, Pan-African Belt of Egypt: genetic and exploration aspects H. M. ABDALLA’ and F. H. MOHAMED2 ‘Nuclear Materials Authority, Cairo, Egypt *Geology Department, Faculty of Science, Alexandria University, Egypt ABSTRACT-Mineralogical, geochemical and fluid inclusion studies reveal two favourable environments for the localisation of beryl mineralisations in the Precambrian rocks of Egypt: (1)emerald-schist; and (2) beryl-specialised granitoid associations. Emerald occurs within the mica schists and is typically confined to the Nugrus major shear zone. However, beryl associated with granitoids occurs in pegmatite veins, greisen bodies, and cassiterite quartz veins cutting the granites and the exocontacts of the volcanosedimentary country rocks. Compositionally, emerald is of octahedral type and its cell edge is lengthened along the a-axis, while beryl associated with granitoids is normal in composition and structural constants. Emerald is thought to be formed as the result of epitactic nucleation of Be, Al and alkali-rich solutions on the mica of the schist country rocks. Fluid inclusion studies show that the solutions are saline (8-22 wt% NaCl equiv.) and the reactions proceeded in the temperature range 260-382OC. On the other hand, aqueous inclusions in beryl associated with granitoids show the following sequence of formation with decreasing temperatures and salinities: beryl pegmatite (320-480°C and 7-16 wt% NaCl equiv.)+greisen bodies (190-400°C and 4-7 wt% NaCl equiv.)+cassiterite-quartz veins (1 90-380°C and 2-4 wt% NaCl equiv.). This study suggests that factors such as the chemistry of the Be-bearing fluids (rather than that of the bulk host schists) and syn-tectonic intrusions of leucogranites and pegmatites (Be- deriving sources) along major ductile shear zones are the important factors controlling emerald formation. However, the endogreisens and exogreisens are the most important targets characterising the metasomatically- and magmatically-specialised, Be-granitoids, respectively. The aqueous inclusions examined in greisen beryls of metasomatised granites show a shorter range of homogenisation temperatures (260-39OOC) and salinities (4.8-7 wt% NaCl equiv.) as compared to those of magmatically-specialised granitoids (1 90-400°C and 4-7 wt% NaCl equiv.). This phenomenon can be partly attributed to the late development of the fracture system during the crystallisation history of the metasomatised granites, where little or no contribution from meteoric waters occurred. @1999 Elsevier Science Limited. All rights reserved. RESUME-L’btude mineralogique, chimique et d’inclusions fluides met en evidence deux environnements favorables pour la localisation des mineralisations en beryl des roches precambriennes d’Egypte: (1) les schistes a Bmeraude, (2) les associations granitiques specialisee a beryl. L’emeraude se rencontre dans les micaschistes et se concentre prdferentiellement dans la zone de cisaillement du Nugrus. Le beryl des associations granitiques se trouve dans des veines pegmatitiques, des corps de greisen, des veines a cassiterite-quartz recoupant les granites et dans les exocontacts des roches encaissantes volcano-sedimentaires. L’emeraude est de type octaedrique et sa maille est allongee le long de I’axe-a. Elle a dO se former par nucleation epitaxique de solutions riches en Be, Al et alcalins sur le mica des schistes encaissants. Les inclusions fluides dans l’emeraude sont salines (8-22% en masse equivalent NaCI) et les reactions se sont produits dans un intervalle de temperatures de 260 a 38OOC. Par contre, le beryl granitique a une composition et une structure normales. Les inclusions aqueuses dans le beryl montrent la sequence 8 temperature et salinite decroissantes: pegmatite Journal of African Earth Sciences 58 1 H. M. ABDALLA and F. H. MOHAMED B beryl (320-480°C et 7-16% equivalent NaCI)-+greisens (IgO-400°C et 4-7% equivalent NaCl)+veine a cassiterite-quartz (IgO-380°C et 2-4% equivalent NaCI). Cette etude suggere que des facteurs comme la composition chimique des fluides a Be (plutot que celle des schistes h8te.s) et I’intrusion syn-tectonique de leucogranites et de pegmatites (d’oir derivent les sources de Be) le long de zones majeures de cisaillement contrdlent la formation d’emeraude. Cependant, les endogreisens et les exogreisens constituent les meilleures cibles pour caracteriser les granites metasomatises et magmatiques specialises en Be, respectivement. Les inclusions fluides des beryls de greisen de granite metasomatise ont des intervalles de temperatures d’homogeneisation (260-39OOC) et de salinites (4.8-7% equivalent NaCI) plus Btroits que dans les granites magmatiques (190-4OO’C et 4-7% equivalent NaCI). Ce phenomene peut etre attribue en partie au developpement tardif du systeme de fractures au tours de la cristallisation des granites m6tasomatis6s avec une contribution faible B nulle des eaux m6t6oriques. 0 1999 Elsevier Science Limited. All rights reserved. (Received 219197: revised version recevied 1414198: accepted 213198) INTRODUCTION The emerald-schist and beryl-specialised granitoid and Mohamed (I 9921, Mohamed (I 993) and associations constitute the two modes of beryl Helba et al. (1997). occurrences recognised in the Central and South- eastern Desert of Egypt. Emerald deposits are located in a geological setting characteristic of GEOLOGICAL SETTING emerald-schist mineralisations elsewhere in the Emerald-schist association world (e.g. Giuliani et al., 1990). The second type The Precambrian emerald deposits of southern of beryl mineralisation is associated with the post- Egypt are confined to a regional ductile shear zone, erogenic, geochemically-specialised granitoids. namely the Nugrus Thrust (Fig. 1 B, Cl, which marks Speciallisation of these granitoids is reflected in the boundary between two different lithological their enrichment in some of the rare elements, domains-the Central and Southern Eastern such as Li, Rb, Cs, Be, Nb, Ta, REE, Sn, U, Th, Zr, Desert (Stern and Hedge, 1985). The major Nugrus and Y. The granitoids were formed either by auto- Thrust separates the medium-grade association, metasomatic, postmagmatic alteration processes dominantly metapelites and gneisses, in the (i.e. the so-called apogranites of Beus et al., 1962; footwall from the low-grade ophiolitic melange Abdalla et a/., 1996) or as ultimate differentiates assemblage with subordinate metasediments in from haplogranitic melt by simultaneous crystalli- the hanging wall (Greiling et al., 1987). The sation of minerals from melt and fluid under emerald-schist zone extends for some 45 km in a conditions of high F and Li activities (e.g. Li-albite northwest trend along the Nugrus Thrust with granites, as characterised by Pollard, 1983; three main mineralised centres, namely Zabara, Schwartz, 1992). In the present study the mineral- Sikait, and Urn Kabu. The emeralds are commonly ogical, geochemical and fluid inclusion character- restricted to volcanosedimentary series, domin- istics of emerald and beryls of the two associations antly biotite schists, in which subordinate slices are examined to elucidate the factors responsible of amphibolites and serpentinites are imbricated. for localising different paragenetic types of beryl The schist rocks structurally overlie a unit of biotite mineralisation, even with similar Be-deriving sources. orthogneiss. The whole sequence occurs as imbri- Besides, metallogenetic and exploratory schemes cated structures affected by complex folding and which may aid in further discovery of Be occur- deformation. Leucogranitic bodies and spatially rences in Egypt are also discussed. Seven localities related aureoles of a pegmatitic vein system in- were selected for the present investigations (Fig. truded this sequence along the Nugrus Shear Zone 1A). These areas are Sikait and Urn Kabu (for the (Fig. IC, D). The granites are alkali-rich, with emerald-schist association); Nuweibi, Abu Dabbab, aluminous mineral assemblages such as garnet and lgla (for the magmatic-specialised granitoid and muscovite, and exhibit characteristics of syn- association); and Mueilha and Homret Akarem (for collisional granites. These leucogranites are the metasomatic-specialised granitoid associa- derived by dehydration melting of metapelitic tion). The discrimination between the magmatic schists (Mohamed and Hassanen, 1997). and metasomatic types was based on the criteria In the Sikait area, the different lithological units cited by Pollard (19831, Schwartz (I 9921, Morsey are imbricated to form a typical duplex structure, 582 Journal of African Earth Sciences Mineralogical and geochemical investigation of emerald and beryl mineralisation 24’ . 1” ZSk ? EXPLANATION Wadi Deposits. Leucogranite: l- Sikait; 2- Urn Kkeran # :: Biotite Gneiss. /‘/‘/ Metagabbro-Diorite Complex. \’ ,’ Hornblende Gneiss. .::j:c: Schist. I!!! 7 Nugrus Thrust. - Minor Fault. - Pegmatite Veins. $?? Emerald Mines. m Minor Thrust. -4f- Fold Trace: antiform, synform. r.s Quartz Veins. WST Wadi Sikait Thrust. Figure 7. (A) index map showing the location of the investigated beryl and emerald occurrences. 1: Nuweibi; 2: Abu Dabbab; 3: lgla; 4: Mueilha; 5: Sikait; 6: Urn Kabu; 7: Homret Akarem. @I Location of the emerald deposits of Egypt in relation
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