Kiiash Journalofafrican Earth Sciences

Home , Cassiterite, Greisen

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 to the major Nugrus Shear Zone. Structural details are from Greilling et al. (19871. The locations of the detailed geological maps of(C) and (D) are outlined by dashed lines. ICI Geological map of the Sikait area compiled and modified from El Shazly and Hassan (19721, Geological Survey Egypt (19781, Greilling et al. (1987) and Greilling 119901. lDI Detailed geological map of the Urn Kabu area modified from Zalata et al. (19731.

namely the Wadi Sikait Duplex (WSD: Ries et al., gneiss contact. A zone of emerald-bearing pockets 1983; Greiling et al., 1987). The biotite ortho- and lenses (l-20 m thick.), composed dominantly gneiss, outcroping between Wadi Sikait and Wadi of pure, medium-grained, scaly, golden-brown to Abu Rusheid, constitutes the lowermost horse of greyish mica rock (i.e. phlogopitites-see later), this duplex. The next overlying horse includes is associated with actinolite schist and carbonate different varieties of schists, of which the biotite rock. schist is dominant. Emerald mineralisation is Old mine workings for emerald were also re- frequently hosted in the biotite schist at the middle corded at Urn Kabu. The area is dominated by part of Wadi Sikait and for a distance of 3 km. schists interlayered with serpentinite bands. Horn- The mineralisation is restricted to the schist-biotite blende gneiss structurally overlies the schist.

Journal of African Earth Sciences 583 H. M. ABDALLA and F. H. MOHAMED

Garnet-muscovite-leucogranite intrudes the schists. may display a petrographical zonal pattern in Manifestations of post-magmatic alterations, such response to the magmatic evolution, as exampli- as feldspathisation and silicification, are restricted fied by the Nuweibi granitoids, with a lower zinn- to the apical part of the granite. Numerous lenticu- waldite-amazonite-albite granite zone, a middle lar pegmatitic pods and quartz veins cross-cut lithian muscovite-albite granite zone, and a roof the schist along the northwest trending shear zone. zone of white mica-albite granite. Moreover, the Emerald occurrences are confined to this zone, roof zone has a carapace of banded pegmatoidal both in the biotite schists, mica rocks (phlogopi- (stockscheider) crust (0.5-4.5 m thick) with an tites), pegmatite, and quartz veins and stringers. upper band made up of gigantic quartz crystals The emeralds occur within small pockets (- 5 cm and a lower band of K-feldspar. The quartz crystals wide), massive quartz veins ( - 30 cm thick and commonly grow inward from the granite/meta- - 60 m strike length) and quartz stringers ( - 10 cm sediment contact and show no evidence for late long). Pegmatite veins are composed of mega- emplacement of the pegmatite crust. Furthermore, crystic K-feldspar, massive quartz and a few a characteristic taxitic, very fine-grained albite emerald crystals aggregated along the pegmatite/ granite zone ( - 10 cm thick) commonly shields schist contacts. the white mica-albite granite from the pegmatite In both the Sikait and Urn Kabu occurrences, crust. Textural characteristics are dominantly mag- the emerald-bearing schists show a marked rela- matic and indicate the co-precipitation of quartz tion to the metasomatised zones of the leuco- and albite from a progressively fractionating Na- granites and its pegmatitic vein system. rich melt. In these granitoids, beryl exhibits one or more of the following mode of occurrences: Beryl-granitoid association stockwork greisen veins; greisen bodies; and The second type of beryl mineralisation can be cassiterite * wolframite quartz veins. subdivided according to the modes of specialisa- tion of their granitoids into: i) Beryl mineralisation associated with metaso- SAMPLING AND ANALYTICAL TECHNIQUES matically-specialised granitoids. These granitoids, Combined electron-microprobe (JEOL JXA-50A), most commonly called apogranites, are small (l- wet, and thermogravimetric analyses have been 3 km*) and spatially-related to the apical fine- to performed for emerald, beryl and the associated medium-grained, dome-shaped projections or micas of the host schist rocks. Purified (hand- apophyses of the post-erogenic, last intrusive picked) fractions of emerald (phlogopite inclusion- phase, which is itself generally emplaced at the free-see later), beryl and the associated micas roof zone (1.5-3 km from the surface) of the were subdivided into two portions. One portion batholith. The apogranites commonly display a was submitted as grain mounts for determination petrographical vertical zoning in response to the of the elements: Si, Ti, Al, Cr, Mg, Fe* (total Fe post-magmatic metasomatic processes, with as FeO), Mn, Ca, Na and K by EPMA techniques. lower unaltered biotite or muscovite granites and The electron probe analysis was detected as an a roof zone of bleached, grey, albite-enriched (i.e. average of 20 points for each sample. Furthermore albitised) granites. However, a smaller volume of lo-point line profiles were measured across greisenised (H+-metasomatised) granite is super- orientated hexagonal cross sections of beryl and imposed on the albitised zones and confined to emerald crystals to detect chemical zoning. Stan- the fissures and fractures. Textural characteristics dards used for EPMA analyses were synthesised are dominantly of subsolidus metasomatic replace- pure oxides and natural minerals. The conditions ments. In these environments, beryl mineralisation of an accelerating voltage of 15 kV, probe current occurs in pegmatoidal lenses and veins, greisens, of 5 nA, a beam diameter of about 1 pm and and cassiterite * wolframite quartz veins. counting time of 20 seconds were used. The ii) Beryl mineralisation associated with mag- second portion was processed for determination matically-specialised granitoids. These granitoids of Rb, Ba, Ga, Sn and Zn (XRF); Be and Li (atomic occur as small stocks (0.2-4 km*) of circular to absorption spectrometry); F (selective ion elec- dyke-like outcrop. They are characterised by the trodes following the method of Kanisawa, 1978); presence of snow-ball quartz (euhedral, porphyritic and H,O by combustion to 1 050°C (Tables 1 and quartz with albite lath inclusions arranged concen- 2). Precision of the analytical data were monitored trically along its growth zones) set in a matrix of by international rock standards and found to be fine-grained and randomly-orientated albite, K- better than 2% (for Be, Li and F) and 5% (for feldspar, Li-mica, topaz and quartz. The granitoids H,O). In addition, the obtained d-values from X-

584 Journal of African Earth Sciences Mineralogical and geochemical investigation of emerald and beryl mineralisation

Table 1. Chemical composition and cell parameters of emerald and beryl, Eastern Desert, Egypt

Analysis No. 1 2 3 4 5 6 7 8 9 10 11 12 13 Sample No.* SE-1 SE-2 KE-1R KE-1C KE-1 l-3 l-4 MB-1 MB-2 DE-1 NE-1 NE-2 HE-1 SiOl 64.80 64.29 64.63 64.39 64.48 65.90 65.76 64.86 65.00 65.30 64.88 64.69 65.80 TiO, 0.01 - 0.03 0.05 0.03 0.04 0.03

A& 14.42 12.87 13.90 13.00 13.40 18.29 18.05 17.96 16.34 18.01 18.05 18.66 17.45 Cw& 0.08 0.15 0.12 0.16 0.13 FeO” 0.38 0.96 0.73 1.40 1.20 0.30 0.33 0.41 1.40 0.31 0.36 0.32 1.10 MnO 0.02 0.02 - 0.01 0.01 0.01 0.02 0.02 0.04 0.03 0.00 0.01 0.03 MgD 2.28 2.75 2.23 2.81 2.59 0.05 0.05 0.04 0.65 0.04 0.02 0.06 0.12 cao 0.02 0.04 0.01 - 0.02 El?0 13.00 13.30 13.00 13.10 13.06 13.30 13.22 13.30 13.41 13.61 13.04 L&O 0.01 0.02 0.04 0.03 0.02 0.01 0.01 - 0.01 0.06 Na,O 1.50 1.73 1.48 1.71 1.62 0.27 0.14 0.34 0.66 0.23 0.37 0.36 0.78

KzD 0.02 0.03 0.02 0.08 0.04 0.03 0.03 - 0.01 0.01 0.08 0.02 0.14 RbzO 0.01 0.02 0.10 cs,o 0.03 Hz0 2.43 2.40 2.31 1.10 1.30 1.40 1.20 0.99 1.08 1.01 1.50 Total 98.96 98.56 98.82 99.12 98.79 98.39 98.58 98.26 98.26 98.77 100.15 Trace elements in ppm F 270 250 250 320 750 380 850 1480 1200 1300 800 aa 18 35 22 20 15 12 S” 10 12 a 28 32 35 16 20 26 55 15 Z” 20 ia 25 55 40 48 50 60 52 80 12 Ga ia 22 15 27 42 25 33 45 55 52 37 No. of cations per formula”’ Ti 0.000 0.000 0.000 0.004 0.000 0.007 0.004 0.005 0.000 0.004 0.000 Al 3.198 2.872 2.983 3.954 3.920 3.922 3.583 3.920 3.935 4.044 3.781 Cr 0.011 0.023 0.019 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Fe” 0.059 0.151 0.189 0.047 0.051 0.063 0.218 0.048 0.056 0.050 0.168 MS 0.638 0.776 0.728 0.013 0.013 0.011 0.181 0.010 0.005 0.016 0.032 M” 0.003 0.003 0.002 0.002 0.003 0.003 0.006 0.005 0.000 0.002 0.005 Sum. of cations 3.909 3.825 3.921 4.020 3.987 4.060 3.992 3.988 3.996 4.116 3.985 in the octahedral sites Be 5.868 6.049 5.900 5.770 5.783 5.925 5.905 5.896 5.959 6.010 5.758 Li 0.007 0.000 0.030 0.022 0.014 0.007 0.007 0.000 0.007 0.000 0.044 SI 12.171 12.171 12.181 12.083 12.116 12.019 12.090 12.053 11.991 i i .a90 12.090 Sum. of cations 18.046 18.220 la.111 17.875 17.913 17.951 18.002 17.949 17.957 17.900 17.892 I” the tetrahedral sites Li 0.000 0.009 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Na 0.545 0.635 0.594 0.097 0.049 0.122 0.239 0.084 0.132 0.129 0.278 K 0.005 0.007 0.009 0.007 0.007 0.000 0.002 0.002 0.018 0.005 0.032 Rb 0.000 0.000 0.000 0.001 0.002 0.000 0.000 0.000 0.000 0.000 0.012 CS 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 Ca 0.004 0.008 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.000 Sum. of cations 0.554 0.659 0.603 0.105 0.058 0.122 0.241 0.086 0.150 0.134 0.324 in the channel sites HzD 1.518 1.518 1.453 0.671 0.797 0.865 0.743 0.611 0.665 0.620 0.919 Sum of 2.072 2.177 2.056 0.776 0.855 0.987 0.984 0.697 0.815 0.754 1.243 channel sites MgIIMg + Fe) 0.915 0.837 0.793 0.216 0.203 0.150 0.453 0.172 0.082 0.242 0.160 a 9.25013) 9.289(z) 9.284(Z) 9.214I21 9.22113) 9.219(3J 9.21412) c 9.200(Z) 9.216121 9.23914) 9.19113) 9.195(61 9.19614) 9.204(61 c/a 0.994 0.992 0.995 0.997 0.997 0.997 0.999 v 681.500 688.800 689.700 675.800 677.100 677.940 676.700

*: Sample Nos SE-1 and SE-2 are Sikait emeralds in mica schist and mica rock, respectively; KE-1 is an emerald in mica rock from Urn Kabu. However, analysis Nos 3 and 4 refer to the rim and core of the emerald crystal of sample No. KE-1. Sample Nos l-3 and l-4 are lgla beryls from a quartz vein cutting the lgla albite granite and schist exogreisen, respectively; MB-l and MB- 2 are Mueilha beryls from a quartz vein and quartz greisen vein in metavolcanics, respectively; DB-1 is a beryl from a schist exogreisen, Abu Dabbab; NB-1 and and NB-2 are Nuweibi beryls from the schist exogreisen and endogreisen of Fig. 28, respectively; HB-1 is a beryl from a pegmatite vein, Homret Akarem.

???:? Total Fe determined as FeO.

?????:? No. of cations based on 36 oxygens in an anhydrous formula. a, c and v: Direct unit cell parameters in A, A, and A3, respectively. The numbers in parentheses are the standard deviation of the digits to their immediate left. -: Below detection limits.

Journal of African Earth Sciences 585 H. M. ABDALLA and F. H. MOHAMED

Table 2. Chemical composition of micas of emerald-hosting schists and beryl-bearing schist exogreisens Analysis No. 1 2 3 4 5 6 7 Sample No. * SE-l SE-2 SE-2 KE-1 l-4 DB-1 NB-1

SiO 2 42.22 41.60 43.36 41.81 40.94 38.53 36.42 Ti02 0.32 0.31 0.24 0.42 0.42 1.13 1.85 Ai203 11.76 12.01 11.25 12.06 13.34 16.75 17.51 Cr203 0.24 0.20 0.40 0.26 0.23 0.20 0.12 Fe0 + 9.26 8.10 6.92 9.67 10.04 11.07 19.20 MnO 0.28 0.12 0.05 0.24 0.18 0.12 0.12 MgC 20.50 21.53 23.00 20.02 20.38 17.60 11.20 CaO 0.02 0.02 - 0.01 0.02 0.04 0.01 Li20 Na20 0.31 0.32 0.16 0.29 0.29 0.19 0.20 K20 9.44 9.86 9.79 9.40 8.86 8.78 9.35 H20 + 4.21 4.01 3.95 3.72 3.90 4.32 F Total 98.56 98.08 95.17 98.13 98.42 98.31 100.30 * **No. of cations per formula Si 6.141 6.084 6.196 6.138 6.004 5.671 5.457 Ti 0.035 0.034 0.026 0.046 0.046 0.125 0.208 AI(W) 1.824 1.883 1.778 1.816 1.950 2.204 2.335 AI(W) 0.195 0.187 0.116 0.270 0.355 0.702 0.757 Cr 0.017 0.014 0.027 0.018 0.016 0.014 0.008 Fe* 1.126 0.991 0.827 1.187 1.231 1.362 2.405 Mn 0.034 0.015 0.006 0.030 0.022 0.015 0.015 Mg 4.445 4.694 4.900 4.381 4.455 3.862 2.502 Ca 0.003 0.003 0.000 0.001 0.003 0.006 0.002 Li 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Na 0.087 0.091 0.044 0.083 0.082 0.054 0.058 K 1.751 1.839 1.785 1.760 1.657 1.648 1.790 OH 4.087 3.915 4.000 3.870 3.641 3.832 4.321 F 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mg/(Mg + Fe1 0.798 0.825 0.855 0.787 0.783 0.739 0.510

?:? For sample numbers, see Table 1. However, analysis No. 3 is an average composition of 1 o phlogopite inclusions within the emerald crystal of sample No. SE-2.

???:? Total Fe as FeO. ***: No. of cations based on 24 oxygens except for analysis no. 3, which is based on 22 oxygens in an anhydrous formula. -: trace amounts and the blank places indicate no determinations.

ray diffractographs were used in computing the PETROGRAPHICAL AND MINERALOGICAL refined cell parameters (Table 1) using the program CHARACTERISTICS of Sakurai (1967). Also, 15 samples from different Emerald-schist association zones of the Mueilha and Nuweibi Granites were The host schist is a medium-grained quartzofeld- determined for their Be contents using atomic spathic mica rock with subordinate actinolite, absorption spectrometry. chlorite and talc. The mica is Mg-rich biotite

Ffgure 2. Textural characteristics of the investigated emeralds and beryls. (A) Photomicrograph showing the mottled pattern of clear and dull zones in a Sikait emerald. The rounded, fine-grained phlogopite inclusions are also observed. (B, CI Polished slabs showing a beryliferous endogreisen pocket within the Nuweibi albite granite along the contact with the country schist and a exogreisen in the country metavolcanics of the lgla albite granite, respectively. Em: emerald: Ph: phlogopite; Sch: schist; Mv: metavolcanic; Gg: greisenised granite; Be: beryl; MS: muscovite; Q: quartz. Mineralogical and geochemical investigation of emerald and beryl mineralisation

Journal of African Earth Sciences 587 H. M. ABDALLA and F. H. MOHAMED

(analysis Nos 1, 2 and 4; Table 2) and constitutes country rocks (Fig. 2C). The greisenisation of the 50-70% of the schist and increases up to 100% metasomatised granites (i.e. apogranites) is dis- and shifts its composition to phlogopite in the played by the introduction of fine-grained, greenish- mica rocks (i.e. phlogopitite). The mica schist yellow, muscovite at the expense of the pre- exhibits many deformational textures, including existing feldspars and, with increasing intensity, folding and crenulations of the mica bands, and a series of facies or a zonal pattern is developed boudinaged quartzofeldspathic lenses. Emerald in the sequence of: (1) unaltered granite + (2) occurs as disseminated crystals within schist in greisenised granite + (3) green muscovite quartz the close vicinity of quartz veins, where the greisen -+ (4) topaz-fluorite-beryl greisen -+ (5) crystals transect the foliation planes of the host quartz-green muscovite greisen core. In this schist. Emerald crystals (3 cm long) ranging from sequence, facies 2 occurs as patchy zones or pale green to deep green occur within schists, aureoles (5-20 m thick), which may encircle facies whereas the finer varieties (0.7 cm long) are deep 3 to 5. The latter facies commonly constitute bluish green and restricted to small pockets in lenticular to vein-like greisen bodies, which are the mica rocks. The crystals exhibit length/width steeply dipping, 1 O-50 cm thick and 3-40 m long. ratio ranges between 3:l and 2: 1. Some of the Beryl in the greisens of apogranites occurs as long crystals are transversely cracked normal to euhedral, long prismatic (length/width ratio ranges the c-axis, whereas the finer variety do not show between 4: 1 and 7: 1) pale yellowish to greyish this feature. In thin section, emerald exihibits a crystals. Some crystals are bent, transversely distinctive zoning with a commonly pale greenish, fractured and intensively altered into deep brown turbid (dusty inclusion-rich) core, and a usually and friable material. colourless, clear (inclusion-free) rim. Also, the However, the greisen types of beryl occurrences emerald crystals characteristically display a mot- in magmatically-specialised granitoids are repre- tled pattern of clear and dull zones of the same sented by pockets, lenticular bodies, veins and chemical composition (Fig. 2A). However, dense stockwork greisen veinlets. They are made up of patches of rounded, randomly-orientated, and of massive milky quartz, muscovite, beryl, topaz and highly variable size (from dust-like to ~80 pm), fluorite. The greisen pockets (20 cm in length) phlogopite inclusions are frequently encountered and lenses II m length) are intensively concen- within the emerald crystals. The inclusions show trated at the granite/country schist endocontacts neither orientation with the foliation of the host (i.e. endogreisens, Fig. 2B), as well as at the schist, nor consistency with the growth lines of exocontact zones (i.e. exogreisens, Fig. 2C) within the enclosing emerald. the schist country rocks (as distant as -400 m). The greisen veins (100 m long) cross-cut the Beryl-specialised granite association granite massif at the apical parts and the country The beryl pegmatite lenses and veins (l-70 m rocks, and most commonly selvaged by a zone of length and 5-40 cm width) contain colourless to muscovite (3 mm to 5 cm thick). The stockwork translucent, bluish-green, coarse (2-7 cm long), greisen occurs at the exocontact zones of the beryl crystals with length/width ratios in the range granite massif (best developed in the lgla and Abu of 6:l to 3:1, in addition to pink microcline, mas- Dabbab albite granites) and is represented by frag- sive quartz and fluorite. The beryl is commonly mented (3-50 cm across) metavolcanic country zoned with some crystals showing parallel, con- rocks cemented by greisen (muscovite, quartz, centric rings of different hues of green and milky topaz, fluorite, and beryl) net works (3-10 cm white. Commonly, selvage zones (0.5-3 cm thick), thick). Muscovite of the greisen net works occurs composed of yellowish-green mica, topaz and as a selvage zone. Beryl occurs as pale yellowish fluorite, are developed between the pegmatite to brownish, prismatic crystals (0.5-3 cm long) veins (lenses) and the granite wall rocks. with length/width ratios between and 5:l and The greisens, as defined by Scherba (1970), 7: 1. It is worth noting that very limited zones (I 0 are rocks composed of mica, quartz, topaz, tour- cm thick and 1 m length) of greisenised granite, maline, beryl and fluorite, developed in response associating greisen pockets and lenses were to H+ metasomatic alteration of the granite massif detected within the magmatically specialised at its apical parts and confined exclusively to the granitoids along their contact with the country linear fissures within the granite. They are classi- rocks. Field investigations revealed that the fied as endogreisens when occurring within the greisenisation of the country rocks (i.e. exo- granite massif [Fig. 2B), and exogreisens when greisens) is a prominent phenomenon associated occurring at the exocontact zones within the with the magmatically-specialised granitoids,

588 Journal of African Earth Sciences Mineralogical and geochemical investigation of emerald and beryl mineralisation whereas the greisenisation of the granite massif itself (i.e. endogreisens) is commonly encountered A in the metasomatically-specialised types. Detailed contribution to the greisenisation processes ??Emerald 1 ??e affecting the rare metal granitoids of Egypt is given 0 Beryl 0.8 in Abdalla (in prep.). 0 The beryl-bearing cassiterite + wolframite veins are abundant, cutting the granite cupolas at their apical zones as well as their exocontact zones within the country rocks. Veins occur as groups = ig , , , ,; ,; b,o, , or series of contiguous veins and veinlet zones. Each series includes up to 30 veins which are t ._* 2.5 2.8 3.1 3.4 3.7 4 32 steeply dipping, subparallel and are 1 O-50 cm thick Z Octahedral Al Content. and 30-500 m long. Numerous parallel veinlets .Z (0.2-3 cm thick and 0.1-10 m long) branch off 2 B the main vein at acute angles. The vein is com- z posed of coarse crystalline milky quartz selvaged 1 by discontinuous margins (0.3-5 cm thick) of mus- ??* covite and topaz-muscovite. In addition to beryl, 0.8 ?? cassiterite and wolframite are also encountered 0.6 standing on the selvage zone, or in greisen pockets within the central parts of the vein. However, the volatile influxing into Sn-Be veins is considerably less than into greisens, as manifested by the absence or very minor development of altered 0 0.2 0.4 0.6 0.8 aureoles of wall rock. 5 Alkali Content per Channel. G :, S C CHEMICAL CHARACTERISTICS 0 Emeralds and beryls ; 1 e@ In the ideal beryl structure, Be,AI,Si,O,,, both Be and Si are tetrahedrally co-ordinated, whereas Al 0.8 occupies a distorted octahedra between the ring 0.6 1 structure made by the Si and Be tetrahedra (Gibbs et al., 1968). According to this arrangement, channel-like cavities parallel to the c-axis are “:%1 ;, , , formed and sometimes occupied by non-stoichio- metric water molecules and alkali ions (larger than 9.175 9.225 9.275 9.325 Li). Normal beryl possesses a composition ap- a-axis (A) proaching the ideal formula, whereas ‘octahedral’ beryl has trivalent and divalent cations replacing Figure 3. Octahedralsubstitutions (i.e. S, Fe *, Mg, Mn, Cr and Al, and ‘tetrahedral’ beryl has Li mainly substi- Til versus IA) the octahedral Al content; IBl the alkali content tuting for Be (Aurisicchio et al., 1988). in the channels li. e., S, LiCh.,Na, K, Rb and Csl; and (Cl the The elevated contents of Mg, Fe, Cr and Na of lattice parameter, a (A), in the investigated emerald and beryl. emerald from Urn Kabu and Sikait characterise this emerald as ‘octahedral beryls’. The occupancy of octahedral Al is negatively correlated with the and K towards the rim, whereas Al (and slight Si) (Fe + Mg + Mn + Cr) content (Fig. 3A), confirming decreases towards the core. This type of zoning the mutual substitution of these cations in the is the reverse to that recorded by Franz et al. octahedral sites. This relation is also shown in (I 9861, which is formulated as an exchange reac- Fig. 3B, which depicts the role of alkalis (essentially tion with other minerals present during the growth Na) in maintaining the charge balance of the struc- of emerald, and attributed by Aurisicchio et a/. ture. This phenomenon is also depicted at the grain (1988) to the decreased T and P during or after scale, as displayed by the chemical zoning of the uplift from an early high pressure to a late meduim emerald crystals, with a decrease in Fe, Mg, Na pressure environment.

Journal of African Earth Sciences 589 H. M. ABDALLA and F. H. MOHAMED

Figure 4. Sketch diagram showing the fluid inclusion types encountered in the examined emeralds and beryls. I-a and b: monophase aqueous type; 2: two phase aqueous type; 3: multi-phase aqueous type; 4: two phase CO,{+ CH&-H,OINaCII type; and 5: monophase CO,{+ CH,)-rich type. Types 4 and 5 commonly show a separation of CO, phase into an inner CO, vapour and an outer CO, liquid phase.

Beryls of specialised granitoids are of normal presence of the CO, phase and daughter minerals composition with little substitution of elements at room temperature has been conveniently used. in the octahedral and tetrahedral sites. They are However, both emerald and beryl commonly show characterised by lower water and alkalis and en- a tube-shaped, negative to partially faceted crystal riched in Sn, F and Ga, especially from the greisens. inclusion forms which are most probably of pri- However, beryl of sample MB-2 (analysis No. 9, mary origin. Five types of inclusions are recognised Table 1) is highly fractured and may be affected in the investigated emerald and beryl (Fig. 4). Type by later solution activity. Pegmatite beryl is enriched 1 is frequent in greisen beryl associated with in Li, Na, Rb and Fe (analysis No. 13, Table 1). A granitoids and is rarely recorded in emeralds. How- slight variation in Fe content was detected associa- ever, type 3a is common in emerald and sometimes ted with the concentric colour variation in the encountered in greisen beryls, whereas type 3b pegmatite beryl, whereas those of the greisens was only detected in emerald. Daughter minerals and veins are typically homogeneous. of type 3a were not successfully identified due to The plot of octahedral substitution of Fe, Mg, their diffused optical characteristics and small Cr and Mn versus the cell edge a-axis (Fig. 3C) sizes. The platy form, high birefringence and refrac- indicates that cation substitution in emerald is tory nature of the mineral inclusions contained in accompanied by an increase in the length of the type 3b, coupled with their variable numbers and a-axis. Thus, the two paragenetic types of the volume per cent, may indicate that this solid phase investigated beryls can be distinguished by their is phlogopite, which was heterogeneously entrapped c/a ratio, which is 0.992-0.995 for emerald (octa- during inclusion formation. The results of the micro- hedral beryl) and 0.996-0.999 for normal beryl thermometric analysis are summarised in Table 3 of the specialised granitoid association. and will be discussed later.

Micas of the hosting schists The major element composition of mica associated DISCUSSION with emerald corresponds to phlogopitic biotite. From the aforementioned data, the two paragenetic However, the micas of the schist rocks hosting types of Egyptian beryls are clearly characterised. the beryl exogreisens in close vicinity to the Igla, Besides textural characteristics, the distinctive Abu Dabbab and Nuweibi albite granitoids range composition of emerald and the cell edge dimen- in composition between biotite and Mg-rich biotite sions have a special bearing on the environment (Table 2). of their formation. The phlogopite of the emerald-hosting schists have a high Cr content and a Mg/(Mg + FeIBtomratio FLUID INCLUSION INVESTIGATIONS similar to the hosted emerald crystals (analysis Fluid inclusion studies were conducted using 25 Nos 1,2 and 5, Table 1 and Nos 1,2 and 4, Table doubly-polished wafers. The microthermometric 2), implying either their contemporaneous deri- analyses were performed utilising a Linkham TH vation, or the epitactic nucleation of emerald on 600 heating/freezing stage. Taken into considera- phlogopite relicts as the result of an exchange tion the genetic types of inclusions (primary, reaction during the introduction of a (Be, Al, Na)- secondary and pseudosecondary), an appropriate bearing solution. However, the randomly orientated scheme based on the phase proportions and the phlogopite inclusions, their distinctive composition

590 Journal of African Earth Sciences Table 3. A summary of microthermometric analysis for the examined inclusions in emeralds of schist and beryls of granitoid associations, Eastern Desert of Egypt

1 2 3 4 5 6 7 8 Rock type Primary/secondary Size (pm) Inclusion type Number Vol.% CO;! Xcol Density of CO;! pseudosecondary phase (g cmm3) Emerald in (P, S) (5-35) (2, 3a, 3b) 40 Schists (P, PS) (5-20) 4 20 (20-70) (0. I-0.45) (0.82-0.94) (P, PS) (5-I 2) 5 7 -100 (0.88-0.96) Metasomatically Beryl in pegmatites (P, S) (5-25) (lb, 2, 3a) 20 Beryl in greisens (S, PS) (5-20) (lb, 2, 3a) 38 specialised (S, PSI (5-I 8) 4 14 (20-35) (0.03-0.2) (0.66-0.78) (S, PSI (5-l 0) 5 5 -100 (0.77-0.87) granitoids Beryl in Sn-quartz (P, s, PSI (5-30) (lb. 2) 25 veins (S, PSI (5-20) 4 5 (20-40) (0.05-0.25) (0.56-0.79) Magmatically Beryl in greisens (S, PS) (5-25) (lb, 2, 3a) 42 (S, PS) (5-20) 4 12 (20-40) (0.05-0.27) (0.67-0.75) specialised 6, PSI (5-I 0) 5 5 -100 (0.70-0.84) Beryl in Sn-quartz (P, s, PS) (5-35) (lb, 2) 27 granitoids veins (S, PS) (5-20) 4 5 (20-30) (0.03-0.22) (0.74-0.83)

I1 9 10 11 12 13 14 I

IRock type X CH4 T,(COA T,(lce) TH(CO2) T,(tot) Salinity, wt% 1 NaCl equiv. Emerald in (-5 to -19.8) (260-382) (8-22) Schists (2-26) (-56.9 to -61.5) (-2 to 16) (265-390) (4-30) (-57.5 to -62.4) (-6 to 8) Metasomatically Beryl in pegmatites (-4.5 to -12.8) (320-480) (7-I 6) Beryl in greisens (-2.8 to -4.5) (260-390) (4.8-7) speciaiised (1.8-I 5) (-56.8 to -59.5) (20 to 28) (220-400) (I-18) (-56.8 to -60.1) (9.5 to 21) granitoids Beryl in Sn-quartz (-1.2 to -2.2) (200-380) (2-3.5) veins (0.5-8.8) (-56.6 to -58.3) (18.5 to 29.8) (200-370) Magmatically Beryl in greisens (-2.5 to -4.5) (I 90-400) (4-7) (0.2-I 5) (-56.6 to -59.1) (22.5 to 27.6) (210-380) specialised (2-16) (-57.0 to -59.8) (12 to 25) c Beryl in Sn-quartz (-1 .I to -2.5) (190-360) (2-4) granitoids veins (0.6-14) (-56.8 to -59.0) (14 to 23.5)

P, S, and PS refer to primary, secondary and pseudosecondary inclusions, respectively. The inclusion types are given in Fig. 4. Vol.% CO, is a visual estimate at 40°C. Xco2 is the CO, molar fraction, Xc_ = CO,/(CO, + H,O). X,,, is the CH, molar fraction, X,_,* = CH,/(CH, + CO,); this was estimated using the method of Heyen et al. (1982). TJCO,) is the partial homogenisation temperature of the CO, phase. T,(totl is the total homogenisation temperature. Density of the CO, phase is estimated using T&CO,) and the mode of homogenisation of the CO, phase. T,(lce) is the final melting of the frozen aqueous phase. H. M. ABDALLA and F. H. MOHAMED

e-e Inclusions in emerald of Urn Kabu...... Inclusions in emerald of Sikait. --.- Inclusions in beryl of pegmatite. Homret Akarem. --- Inclusions in beryls of greisens. - Inclusions in beryls of Sn- and W-quartz veins. *-> 227 // 1 / I / I 20 ' /' , t I' I /. ,, ‘8 I ’ i I .’ , ,’ I e- -.-. .; 16 - : 1’ : \ $ \ i TffA ,: z 14 - ‘\.__-* ,. : 1 ..4 _._ _’

2 12 - .e-, .’ I I po- ‘. ‘. : /“I3,’ ‘. : I at 6- .= ;._ 6-

* 4-

150 250 350 450 550 Homogenization temperature, TH tot.

Figure 5. Salinity versus total homogenisation temperature lT,tot.l for aqueous fluid inclusions in the investigated emeralds and beryls. The arrow indicates the evolution trend of the greisenising fluids. See text for the shown fields.

(as compared to the phlogopite of the matrix of the normal composition of these beryls, imply that the phlogopitite and schists hosting the emerald), it is the chemistry of the Be-bearing fluids (rather coupled with the chemical zoning exhibited by than that of the bulk rock, as suggested by the emerald crystals, all suggest that emerald is Aurisicchio et a/., 1988) that controls the for- formed during the introduction of a (Be, Al, Na)- mation of diverse paragenetic types of emerald- bearing solution at the expense of the break down schist and beryl-schist exogreisens associated with of phlogopite. The high lattice energy difference granitoids. necessary for epitactic nucleation of the neo- Fluid inclusion data of the examined emeralds formed emerald (ring silicate) on the phlogopite (Table 3, Fig. 5) indicate homogenisation tempera- (phyllosilicate) can be compensated by the stabilis- tures ranging from 260-382OC and a salinity of ing of Mg, Fe and Na within the emerald structure. 8-22 wt% NaCl equiv. for the aqueous fluid inclu- Although the micas of the schist rocks hosting sion types. However, Fig. 5 shows a somewhat beryl exogreisens associated with granitoids, and large salinity range (8-18.5 wt% NaCl equiv.) those of the schist hosting the emerald are similar corresponding to a temperature interval of 90°C in composition (analysis Nos 1, 2, 4 and 5, Table for the Sikait emerald. This may indicate incor- 21, a remarkable discrepancy in the Mg/(Mg + Felatom poration of low salinity inclusions related to fluid of beryl of the exogreisen and micas of the hosting activity associated with a later tectonic episode. schist is clearly seen (analysis No. 7, Table 1 and Meanwhile, the overlapping areas of fields of No. 5, Table 2). This discrepancy, coupled with aqueous inclusions of emeralds from Sikait and

592 Journal of African Earth Sciences Mineralogical and geochemical investigation of emerald and beryl mineralisation

Urn Kabu (Fig. 5) can be considered as the Table 4. Average Be contents in the Nuweibi and prevailing conditions (i.e. 260-350°C and 13.6- Mueilha beryl-bearing granitoids 18.5 wt% NaCl equiv.) during emerald deposition. Rock type average Be (ppm) N However, the CO,-H,O and COP-rich inclusions NUWEIBI show final melting of the CO, solid phase (T,CO,) Greisenised albite granite 125 3 in the range of -56.9 to -62.4OC, reflecting the White mica albite granite 14 2 presence of another gas, most probably CH, (as Li-muscovite albite granite 16 2 no phase changes below -12OOC was detected Zinnwaldite amazonite albite granite a 2 to deduce the presence of NJ. The large compo- MUEILHA sitional, density and volume per cent variation of Greisenised granite 75 2 Muscovitised granite 47 2 the CO,(CH,) phase in the inclusions of the same Unaltered leucocratic muscovite granite 12 2 population suggests a heterogeneous entrapment N: number of samples analysed. of fluids that have been unmixed into H,O(NaCI)- rich fluid and CO,-rich vapour (Bowers and Hel- geson, 1983). Thus, it can be inferred that the determination of Be (Table 4). Compared to the Be-bearing solutions were moderately saline, but apical white mica-albite granite of the Nuweibi CO,(CH,)-rich. The low contents of F in emerald Pluton, Be is considerably concentrated in the and its impoverishment in the associated micas greisenised albite granite in close vicinity to the of the emerald-hosting schists and phlogopitite, greisen pockets at the endocontact with the and the scarce occurrence of F-bearing minerals, country schist (Fig. 2B). The low Be contents of either in the pockets or quartz stringers, may rule the apical albite granite contrasts with their out the promoting role of F as a mobilising medium enriched contents of Li and F, with which Be is for Be, Al and Na. Thus, it is implied that Be was known to be concentrated (Kovalenko eta/., 1977). most probably complexed by carbonate( + CH,)- However, the tendency of Be to be accumulated chloride base as suggested by Beus and Dikov in the near contact endogreisens, as well as in (1967). the exogreisens (along with the F-rich mineral The metasomatically formed granitoids and the assemblage) may reflect the role of F in com- associated rare metal mineralising processes are plexing the Be (thus inhibiting concentration of thought to result from the upward migration of Be in the magmatic stage) and its separation into late to post-magmatic, volatile-rich aqueous fluids the post-magmatic fluid. On the other hand, Be through the consolidated granite cupolas (Beus shows a high roofward enrichment in the meta- et al., 1962). The fluid phase, aided by its hydro- somatic facies of the Mueilha Apogranite, to fluorophile nature, is exsolved and accumulates especially in the albitised and greisenised granites in the apical parts of the granite massifs at grain (Table 4). The role of F during the process of Na- boundaries, and in microfractures and vugs (Pollard metasomatism (i.e. albitisation) of the Mueilha and Taylor, 1986). Subsolidus re-equilibration of granites was stressed by Morsey and Mohamed these fluids with the granites will cause zonal (1992). Moreover, the experimental work of Beus alteration assemblages. et al. (I 963) revealed that Be is transported in Meanwhile, the close consistency of the mag- the post-magmatic fluids as Na-fluoroberyllate matically-formed rare metal albite granitoids with complexes, and the albitisation of the wallrock the experimental data of the Li- and F-saturated will lead to the deposition of beryl. However, the haplogranite systems (e.g. Manning, 1981; Martin, superimposed greisenisation is mainly attributed 1983) indicates the important role of F and Li in to the sharp increased a,+ and decreased activities the generation of such highly evolved magmas. of the alkalis, as evidenced by the partial to These volatiles reduce the viscosity and solidus complete destruction of the feldspars. The stability temperature of the melt, leading to an increase in fields of the mineral paragenesis commonly diffusion rates of rare metals (including Be), thus encountered in the investigated greisens and permitting late liquid-liquid ultrafractionation of quartz veins are shown in Fig. 6. The detected rare metal granitic magma (Hannah and Stein, zonal pattern exhibited by minerals within the 1990). greisens may indicate that deposition of those Considering the geochemical distribution of Be minerals at that definite stage occured as the result in the specialised granitoids, the Mueilha Sn granite of replacement of its predecessors along the (of metasomatic origin: Morsey and Mohamed, evolution trend given in Fig. 6. This trend is inter- 1992) and Nuweibi Ta-albite granite (of magmatic preted by Burt (1981) as the result of cooling and origin: Helba et al., 1997) were chosen for neutralisation of the fluid.

Journal of African Earth Sciences 593 H. M. ABDALLA and F. H. MOHAMED

separation can be attributed in part to the boiling f Phenakite or unmixing of the fluid as the result of pressure release. The presence of vapour- and liquid-rich inclusions (type 1) may indicate that local boiling had occurred. However, the aqueous inclusions examined in the greisen beryls of metasomatised granites show a shorter range of homogenisation temperatures (260-39OOC) and salinities (4.8-7 wt% NaCl equiv.; Fig. 5, field C) as compared to those of magmatically-specialised granitoids (field D). This phenomenon can be partly attributed to the late development of the fracture system during the crystallisation history of the metasomatised granites, as suggested by Abdalla et a/. (1996). This will result in little or no contribution from pKF (Salinity) meteoric waters, pervasive alteration of the stocks L (including endogreisenisation), and the predomin- Figure 6. Phase relations in the system K,O-A,O,-SiO,-H,O- ance of the disseminated type of rare metal F,O.,, after Burt (1981). The dashed line shows the stable mineralisation. deposition of wolframite instead of scheelite in a high aHF environment. The arrow indicates the possible evolution of greisenising fluids responsible for the beryl-forming processes in the specialised granitoid associations. METALLOGENETIC AND EXPLORATION MODELS Emerald-schist association The aqueous fluid inclusions examined in beryls Different modes of origin have been attributed to associated with granitoids (Table 3, Fig. 51 show the schist-hosted emerald deposits. The exometa- a sequence of formation with decreasing tempera- morphic origin of Fersman (1929) and its adoption tures and salinities: beryl pegmatite+greisen by Shinkankas (1981) is still the most acceptable bodies+cassiterite-quartz veins. The two fields model for such deposits. The model relates the shown by the aqueous inclusions in the pegmatite deposits to the interaction between granitic peg- beryls (Fig. 5) can be related to two distinct events matites and/or their derived fluids with pre-existing of fluid evolution. Field A (homogenisation mafic to ultramafic rocks interlayered with a vol- temperatures 390-480°C and salinities 13.3-I 6 canosedimentary sequence. However, the restric- wt% NaCl equiv.) is most probably related to the tion of emerald deposits to blackwall zones devel- continuous transition from the magmatic to the oped at the contact between the tectonically supercritical hydrothermal stage. Meanwhile, juxtaposed schist and ultrabasic rocks during low- inclusions of field B may reflect an early emergence grade regional metamorphism led Grundmann and of greisenising fluid. The selvage zone of musco- Morteani (1989, 1993) to postulate a pure meta- vite and topaz developed along the beryl pegmatite morphic origin for the schist-hosted emerald veins against the host granite may support this deposits. veiw. However, the great range of temperature In the Urn Kabu and Sikait emerald deposits, the (1 90-400°C) and the accompanying decreased schist sequence, including slices of serpentinites, salinity displayed by the beryl of the greisens of is intruded by pegmatitic veins (Fig. 7A). The fre- the magmatically-specialised granites (Fig. 5, field quent occurrence of emerald mineralisation in both D) may indicate a related and inseparable hydro- the quartz and pegmatite veins, which are spatially thermal event. Besides, the mixing of the mag- related and geochemically linked to the proximal matically-emerged greisenising fluids with other leucogranite, suggest that the granites and the cooler and less-saline fluids (meteoric water?] can associated pegmatites are the source rocks for be inferred from the trend given in Fig. 5. The the Be-bearing fluids. The remarkable enrichment evolution trends given for the greisenising fluids of the pegmatitic varieties of the leucogranite in in Figs 5 and 6 are consistent. The greisenising K (K,O/Na,O = 2.5-6; Mohamed and Hassanen, fluid can be evolved initially by separation of a 1997) and the restriction of post-magmatic altera- dense brine from the exhausted granite melt tions such as feldspathisation (alkali metasoma- (Roedder, 19771, followed by the emergence of tism) and silicification to the apical parts of the an acidic, vapour-rich phase from the brine. Phase granite, are all features reflecting circulation of K-

594 Journal of African Earrh Sciences Mineralogical and geochemical investigation of emerald and beryl mineralisation

,p, Convective hydrotherm \ system

Pegmatite vein system

Hornblende gneiss

Thrust faults. \

Contribution of meteoric water. Migration of exsolved postmagmatic fluids. Stt-W-Quartz veins. Grcisens. Greisenized wallrock. Zone of Na-metasomatism (albitization). Zone of K-metasomatism (Microclinization). Zone of maximum accumulation of residual fluids. Alkali feldspar or biotite or muscovite granite cupola. Volcano-sedimentary country rocks. Fractures.

Contribution of meteoric water. Migration of exsolved posrmagmatic fluids. Sn-W-Quartz veins. Greisens. Greisenited wallrock. Stockscheider crw. Taxitic or very fine-grained albite granite crust. White mica-albite granite. Li-muscovite albite granite. Amazonite albite zinnwaldite granite. Albite-feldspar granite cupola. Volcano-sedimentary country rocks. Fractures.

Figure 7. Metallogenetic and exploratory models for (Al emerald-schist associations (based on Giuliani et al., 1990); (B) beryl metasomatically-specialised granitoids (modified from Abdalla et al., 19961; and (C) beryl magmatically-specialised granitoid associations (e.g. the Nuweibi albite granite). rich, acid fluids related to the leucogranite em- liberation of Fe, Mg, and Cr into solution. The placement. Infiltration of such fluids through the restriction of emerald deposits to the ductile major nearby permeable sheared schist sequence and Nugrus Shear Zone, along which leucogranite the intercalated serpentinite bands will cause per- intrusions are syn-tectonically emplaced, supports vasive metasomatism of serpentinites and schists such a model for the generation of emeralds at into talc schist and phlogopitite rocks, with the the Urn Kabu and Sikait areas.

Journal of African Earth Sciences 595 H. M. ABDALLA and F. H. MOHAMED

Grundmann and Morteani (1993) considered the Beryl-specialised granites association Egyptian emerald deposits to have a metamorphic Close examination of the investigated granitoids origin. However, the geological and geochemical suggested the following metallogenetic and features of the emerald deposits of Egypt all exploration criteria (as simplified in Fig. 78, C) oppose such a metamorphic origin and substan- when prospecting for rare metal, Be-bearing tiate the role of a pegmatitic-derived fluid phase granitoids: to generate emerald mineralisation. There are five il occurrence of post-erogenic, leucocratic, meta- features mentioned here: to peraluminous granitic stocks emplaced along i) The fluid flow process, as well as the minerali- intersecting magma generating faults and fractures; sation, are closely related to the infiltrational mech- ii) the granitoids occur as domal protrusions or anism (Giuliani et al., 1990) and display a clear cupolas with gentle outward dipping contacts tectonic control. The main mineralising centres which permit the retention of post-magmatic fluids are located along the major ductile Nugrus Shear and hence increased the metasomatic processes, Zone. Further, the emerald-bearing pegmatitic and especially the greisenisation; quartz veins are confined to the shear zones. iii/ post-magmatic metasomatic albite-enrich- ii) The major tectonic deformation, including ment and extensive endogreisenisation of the thrusting and folding, with which the leucogranite granite massifs along fracture zones and peripheral plutons are syn-tectonically emplaced, are well- parts of the stocks may distinguish the metasoma- known to post-date the regional metamorphic event tically Be-bearing granitoids from the magmatically in the studied area (e.g. El Biyoumi and Greiling, formed ones; and 1984). iv) the metasomatically-specialised granitoids are iii) The emerald crystals show features indicating commonly characterised by the development of static growth with randomly orientated phlogopite radiometric anomalies (e.g. Abdalla et a/., 1996). inclusions. The phlogopite inclusions have a high Cr content and a Mg/(Mg + FeIatomratio similar to the hosted emerald crystals, reflecting their meta- CONCLUSIONS somatic derivation from the same interacting fluids. Two paragenetic types of beryl mineralisation Further, the phlogopite inclusions are chemically occur in the Precambrian rocks of Egypt: (1) distinct from the micas of the host schists and emerald-schist; and (2) beryl-specialised granitoid phlogopitite rocks. associations. Geological and geochemical features iv) Inclusion-free emerald crystals commonly of the emerald deposits substantiate the role of cross-cut the biotite foliation in the schist, which syn-tectonically emplaced leucogranites as a source reflects a clearly post-metamorphic origin for the for the Be solutions. Infiltration of such solutions mineralisation. through the nearby permeable sheared schist v) The characteristic chemical zoning exhibited sequence and the intercalated serpentinite bands by the emerald crystals of the Urn Kabu and Sikait will cause pervasive metasomatism of the serpen- areas indicate a normal fractionation trend of alkali- tinites and schists into phlogopite-rich rocks and rich, acid fluids. Conversely, the emeralds derived subsequent localisation of emer,alds. from metamorphic fluids, such as those of Hab- Beryl associated with granitoids occurs in achtal, Austria, exhibit reversed zoning with a pegmatite veins, greisen bodies and cassiterite consistent increase in Mg, Fe and Cr towards the quartz veins, of which the greisens are more likely rims (Grundmann and Morteani, 1989). to have the highest potential. The greisenisation It is suggested that major ductile shear zones, of the country rocks (i.e. exogreisens) is a promin- along which a metapelite-metavolcanic sequence ent phenomenon associated with the magmatic- associated with ultramafic rocks is highly folded ally-specialised granitoids (Li-albite granites), and imbricated, should be investigated when whereas the greisenisation of the granite massif prospecting for emerald deposits in Egypt. Most itself (i.e. endogreisens) is commonly encountered important is the emplacement within this in the metasomaticaly-specialised granitoids sequence of syn-tectonic pegmatitic leucogranites (apogranites). from which K- and Be-rich fluid phases were It is suggested that the diverse chemistry of derived. This is manifested in the field by the the Be-bearing fluids (carbonatef + CH,)-chloride- development of a system of emerald-bearing based for emerald and Na-fluoride-based for beryl) pegmatitic pods and veins confined to the shear play the leading role in the formation of the differ- zones, as well as a broad zone of alkali meta- ent paragenetic types of emerald-schist and beryl- somatism. schist granitoid exogreisens.

596 Journal of African Earth Sciences Mineralogical and geochemical investigation of emerald and beryl mineralisation

ACKNOWLEDGMENTS Greiling, R.O., Kroener, A., El Ramly, M.F., Rashwan, A.A., 1987. Structural relationships between the southern and The chemical analyses and fluid inclusion investi- central parts of the Eastern Desert of Egypt: details of a gations have been performed during the leave of fold and thrust belt. In: El Gaby, S., Greiling, R. (Eds.), The the authors to the Hokkaido (H.M.A.) and Tohoku Pan-African belt of NE Africa and adjacent areas-tectonic (F.H.M.) Universities, Japan. The authors are evolution and economic aspects of a Late Proterozoic orogen. Vieweg, Wiesbaden, pp. 121-l 45. indebted to Dr H. Matsueda and Prof. S. Kanisawa Grundmann, G., Morteani, G., 1989. Emerald mineralization for their generous facilities given during the during regional metamorphism: the Habachtal (Austria) and laboratory work. The Division of XRF and XRD of Leydsdrorp (Transvaal, South Africa) deposits. Economic Geology 84, 1853-l 849. the Nuclear Materials Authority, Egypt is also Grundmann, G., Morteani, G., 1993. Emerald formation during acknowledged for their assistance in analysing regional metamorphism: the Zabara, Sikeit and Urn Kabu some additional samples. The earlier manuscript deposits (Eastern Desert, Egypt). In: Thorweihe, U., benefitted greatly from suggestions by the reviewers. Schandelmeier, H. (Eds.), Geoscientific research in Northeast Africa. Balkema, Rotterdam, pp. 495-498. Editorial Handling - J. R. Baldwin & D. C. Turner Hannah, J.L., Stein, H.J., 1990. Magmatic and hydrothermal processes in ore-bearing systems. In: Stein, H., Hannah, J. (Eds.), Ore-bearing granite systems; petrogenesis and REFERENCES mineralizing processes. Geological Society America, Special Paper 246, l-l 0. Abdalla, H.M., Ishihara, S., Matsueda, H., Abdel-Monem, A.A., Helba, H., Trumbull, R.B., Morteani, G., Khalil, SO., Arslan, 1996. 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