kiiash JournalofAfrican Earth Sciences. Vol. …rjstern/egypt/PDFs/SE...RESUME-L’ btude mineralogique, chimique et d’ inclusions fluides met en evidence deux environnements favorables

kiiash JournalofAfrican Earth Sciences. Vol. 28. No. 3. DD. 581-598. 1999

Pergamon Pll:SO899-5382(99)00033-O a 1999 Eisevier Science Ltd

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 associations constitute the two modes of beryl occurrences recognised in the Central and South- eastern Desert of Egypt. Emerald deposits are located in a geological setting characteristic of emerald-schist mineralisations elsewhere in the world (e.g. Giuliani et al., 1990). The second type of beryl mineralisation is associated with the post- erogenic, geochemically-specialised granitoids. Speciallisation of these granitoids is reflected in their enrichment in some of the rare elements, such as Li, Rb, Cs, Be, Nb, Ta, REE, Sn, U, Th, Zr, and Y. The granitoids were formed either by auto- metasomatic, postmagmatic alteration processes (i.e. the so-called apogranites of Beus et al., 1962; Abdalla et a/., 1996) or as ultimate differentiates from haplogranitic melt by simultaneous crystallisation of minerals from melt and fluid under conditions of high F and Li activities (e.g. Li-albite granites, as characterised by Pollard, 1983; Schwartz, 1992). In the present study the mineralogical, geochemical and fluid inclusion characteristics of emerald and beryls of the two associations are examined to elucidate the factors responsible for localising different paragenetic types of beryl mineralisation, even with similar Be-deriving sources. Besides, metallogenetic and exploratory schemes which may aid in further discovery of Be occurrences in Egypt are also discussed. Seven localities were selected for the present investigations (Fig. 1A). These areas are Sikait and Urn Kabu (for the emerald-schist association); Nuweibi, Abu Dabbab, and lgla (for the magmatic-specialised granitoid association); and Mueilha and Homret Akarem (for the metasomatic-specialised granitoid association). The discrimination between the magmatic and metasomatic types was based on the criteria cited by Pollard (19831, Schwartz (I 9921, Morsey

582 Journal of African Earth Sciences

and Mohamed (I 9921, Mohamed (I 993) and Helba et al. (1997).

GEOLOGICAL SETTING Emerald-schist association The Precambrian emerald deposits of southern Egypt are confined to a regional ductile shear zone, namely the Nugrus Thrust (Fig. 1 B, Cl, which marks the boundary between two different lithological domains-the Central and Southern Eastern Desert (Stern and Hedge, 1985). The major Nugrus Thrust separates the medium-grade association, dominantly metapelites and gneisses, in the footwall from the low-grade ophiolitic melange assemblage with subordinate metasediments in the hanging wall (Greiling et al., 1987). The emerald-schist zone extends for some 45 km in a northwest trend along the Nugrus Thrust with three main mineralised centres, namely Zabara, Sikait, and Urn Kabu. The emeralds are commonly restricted to volcanosedimentary series, dominantly biotite schists, in which subordinate slices of amphibolites and serpentinites are imbricated. The schist rocks structurally overlie a unit of biotite orthogneiss. The whole sequence occurs as imbricated structures affected by complex folding and deformation. Leucogranitic bodies and spatially related aureoles of a pegmatitic vein system intruded this sequence along the Nugrus Shear Zone (Fig. IC, D). The granites are alkali-rich, with aluminous mineral assemblages such as garnet and muscovite, and exhibit characteristics of syn- collisional granites. These leucogranites are derived by dehydration melting of metapelitic schists (Mohamed and Hassanen, 1997).

In the Sikait area, the different lithological units are imbricated to form a typical duplex structure,

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.

I!!!

\’ ,’ Hornblende Gneiss. .::j:c: Schist.

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., 1983; Greiling et al., 1987). The biotite orthogneiss, outcroping between Wadi Sikait and Wadi Abu Rusheid, constitutes the lowermost horse of this duplex. The next overlying horse includes different varieties of schists, of which the biotite schist is dominant. Emerald mineralisation is frequently hosted in the biotite schist at the middle part of Wadi Sikait and for a distance of 3 km. The mineralisation is restricted to the schist-biotite

gneiss contact. A zone of emerald-bearing pockets and lenses (l-20 m thick.), composed dominantly of pure, medium-grained, scaly, golden-brown to greyish mica rock (i.e. phlogopitites-see later), is associated with actinolite schist and carbonate rock.

Old mine workings for emerald were also recorded at Urn Kabu. The area is dominated by schists interlayered with serpentinite bands. Horn- blende gneiss structurally overlies the schist.

Journal of African Earth Sciences 583


Garnet-muscovite-leucogranite intrudes the schists. Manifestations of post-magmatic alterations, such as feldspathisation and silicification, are restricted to the apical part of the granite. Numerous lenticular pegmatitic pods and quartz veins cross-cut the schist along the northwest trending shear zone. Emerald occurrences are confined to this zone, both in the biotite schists, mica rocks (phlogopitites), pegmatite, and quartz veins and stringers. The emeralds occur within small pockets (- 5 cm wide), massive quartz veins ( - 30 cm thick and - 60 m strike length) and quartz stringers ( - 10 cm long). Pegmatite veins are composed of mega- crystic K-feldspar, massive quartz and a few emerald crystals aggregated along the pegmatite/ schist contacts.

In both the Sikait and Urn Kabu occurrences, the emerald-bearing schists show a marked relation to the metasomatised zones of the leucogranites and its pegmatitic vein system.

Beryl-granitoid association The second type of beryl mineralisation can be subdivided according to the modes of specialisa- tion of their granitoids into:

i) Beryl mineralisation associated with metasomatically-specialised granitoids. These granitoids, most commonly called apogranites, are small (l- 3 km*) and spatially-related to the apical fine- to medium-grained, dome-shaped projections or apophyses of the post-erogenic, last intrusive phase, which is itself generally emplaced at the roof zone (1.5-3 km from the surface) of the batholith. The apogranites commonly display a petrographical vertical zoning in response to the post-magmatic metasomatic processes, with lower unaltered biotite or muscovite granites and a roof zone of bleached, grey, albite-enriched (i.e. albitised) granites. However, a smaller volume of greisenised (H+-metasomatised) granite is superimposed on the albitised zones and confined to the fissures and fractures. Textural characteristics are dominantly of subsolidus metasomatic replace- ments. In these environments, beryl mineralisation occurs in pegmatoidal lenses and veins, greisens, and cassiterite * wolframite quartz veins.

ii) Beryl mineralisation associated with magmatically-specialised granitoids. These granitoids occur as small stocks (0.2-4 km*) of circular to dyke-like outcrop. They are characterised by the presence of snow-ball quartz (euhedral, porphyritic quartz with albite lath inclusions arranged concen- trically along its growth zones) set in a matrix of fine-grained and randomly-orientated albite, K- feldspar, Li-mica, topaz and quartz. The granitoids


may display a petrographical zonal pattern in response to the magmatic evolution, as exampli- fied by the Nuweibi granitoids, with a lower zinnwaldite-amazonite-albite granite zone, a middle lithian muscovite-albite granite zone, and a roof zone of white mica-albite granite. Moreover, the roof zone has a carapace of banded pegmatoidal (stockscheider) crust (0.5-4.5 m thick) with an upper band made up of gigantic quartz crystals and a lower band of K-feldspar. The quartz crystals commonly grow inward from the granite/meta- sediment contact and show no evidence for late emplacement of the pegmatite crust. Furthermore, a characteristic taxitic, very fine-grained albite granite zone ( - 10 cm thick) commonly shields the white mica-albite granite from the pegmatite crust. Textural characteristics are dominantly magmatic and indicate the co-precipitation of quartz and albite from a progressively fractionating Na- rich melt. In these granitoids, beryl exhibits one or more of the following mode of occurrences: stockwork greisen veins; greisen bodies; and cassiterite * wolframite quartz veins.

SAMPLING AND ANALYTICAL TECHNIQUES

Combined electron-microprobe (JEOL JXA-50A), wet, and thermogravimetric analyses have been performed for emerald, beryl and the associated micas of the host schist rocks. Purified (hand- picked) fractions of emerald (phlogopite inclusion- free-see later), beryl and the associated micas were subdivided into two portions. One portion was submitted as grain mounts for determination of the elements: Si, Ti, Al, Cr, Mg, Fe* (total Fe as FeO), Mn, Ca, Na and K by EPMA techniques. The electron probe analysis was detected as an average of 20 points for each sample. Furthermore lo-point line profiles were measured across orientated hexagonal cross sections of beryl and emerald crystals to detect chemical zoning. Stan- dards used for EPMA analyses were synthesised pure oxides and natural minerals. The conditions of an accelerating voltage of 15 kV, probe current of 5 nA, a beam diameter of about 1 pm and counting time of 20 seconds were used. The second portion was processed for determination of Rb, Ba, Ga, Sn and Zn (XRF); Be and Li (atomic absorption spectrometry); F (selective ion elec- trodes following the method of Kanisawa, 1978); and H,O by combustion to 1 050°C (Tables 1 and 2). Precision of the analytical data were monitored by international rock standards and found to be better than 2% (for Be, Li and F) and 5% (for H,O). In addition, the obtained d-values from X-


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

aa

S” 10

Z” 20

Ga ia

250 250 320 750 380 850 1480 1200 1300 800

18 35 22 20 15 12

12 a 28 32 35 16 20 26 55 15

ia 25 55 40 48 50 60 52 80 12

22 15 27 42 25 33 45 55 52 37

Ti 0.000 0.000

No. of cations per formula”’

0.000 0.004 0.000 0.007 0.004 0.005 0.000 0.004 0.000

Al 3.198

Cr 0.011

Fe” 0.059

MS 0.638

M” 0.003

Sum. of cations 3.909

in the octahedral sites

Be 5.868

Li 0.007

SI 12.171


I” the tetrahedral sites

Li 0.000

Na 0.545

K 0.005

Rb 0.000

CS 0.000

Ca 0.004


in the channel sites

HzD 1.518

Sum of 2.072

channel sites

MgIIMg + Fe) 0.915

a 9.25013)

c 9.200(Z)

c/a 0.994

2.872 2.983 3.954 3.920 3.922 3.583 3.920 3.935 4.044 3.781

0.023 0.019 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.151 0.189 0.047 0.051 0.063 0.218 0.048 0.056 0.050 0.168

0.776 0.728 0.013 0.013 0.011 0.181 0.010 0.005 0.016 0.032

0.003 0.002 0.002 0.003 0.003 0.006 0.005 0.000 0.002 0.005

3.825 3.921 4.020 3.987 4.060 3.992 3.988 3.996 4.116 3.985

6.049 5.900 5.770 5.783 5.925 5.905

0.000 0.030 0.022 0.014 0.007 0.007

12.171 12.181 12.083 12.116 12.019 12.090

18.220 la.111 17.875 17.913 17.951 18.002

5.896 5.959 6.010 5.758

0.000 0.007 0.000 0.044

12.053 11.991 i i .a90 12.090

17.949 17.957 17.900 17.892

0.009 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.635 0.594 0.097 0.049 0.122 0.239 0.084 0.132 0.129 0.278

0.007 0.009 0.007 0.007 0.000 0.002 0.002 0.018 0.005 0.032

0.000 0.000 0.001 0.002 0.000 0.000 0.000 0.000 0.000 0.012

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002

0.008 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.000

0.659 0.603 0.105 0.058 0.122 0.241 0.086 0.150 0.134 0.324

1.518 1.453 0.671 0.797 0.865 0.743 0.611 0.665 0.620 0.919

2.177 2.056 0.776 0.855 0.987 0.984 0.697 0.815 0.754 1.243

0.837 0.793 0.216 0.203 0.150 0.453 0.172 0.082 0.242 0.160

9.289(z) 9.284(Z) 9.214I21 9.22113) 9.219(3J 9.21412) 9.216121 9.23914) 9.19113) 9.195(61 9.19614) 9.204(61

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.



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

Si

98.56

6.141

98.08 95.17 98.13 98.42 98.31 100.30 * **No. of cations per formula

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.




(analysis Nos 1, 2 and 4; Table 2) and constitutes 50-70% of the schist and increases up to 100% and shifts its composition to phlogopite in the mica rocks (i.e. phlogopitite). The mica schist exhibits many deformational textures, including folding and crenulations of the mica bands, and boudinaged quartzofeldspathic lenses. Emerald occurs as disseminated crystals within schist in the close vicinity of quartz veins, where the crystals transect the foliation planes of the host schist. Emerald crystals (3 cm long) ranging from pale green to deep green occur within schists, whereas the finer varieties (0.7 cm long) are deep bluish green and restricted to small pockets in the mica rocks. The crystals exhibit length/width ratio ranges between 3:l and 2: 1. Some of the long crystals are transversely cracked normal to the c-axis, whereas the finer variety do not show this feature. In thin section, emerald exihibits a distinctive zoning with a commonly pale greenish, turbid (dusty inclusion-rich) core, and a usually colourless, clear (inclusion-free) rim. Also, the emerald crystals characteristically display a mottled pattern of clear and dull zones of the same chemical composition (Fig. 2A). However, dense patches of rounded, randomly-orientated, and of highly variable size (from dust-like to ~80 pm), phlogopite inclusions are frequently encountered within the emerald crystals. The inclusions show neither orientation with the foliation of the host schist, nor consistency with the growth lines of the enclosing emerald.

Beryl-specialised granite association The beryl pegmatite lenses and veins (l-70 m length and 5-40 cm width) contain colourless to translucent, bluish-green, coarse (2-7 cm long), beryl crystals with length/width ratios in the range of 6:l to 3:1, in addition to pink microcline, massive quartz and fluorite. The beryl is commonly zoned with some crystals showing parallel, concentric rings of different hues of green and milky white. Commonly, selvage zones (0.5-3 cm thick), composed of yellowish-green mica, topaz and fluorite, are developed between the pegmatite veins (lenses) and the granite wall rocks.

The greisens, as defined by Scherba (1970), are rocks composed of mica, quartz, topaz, tour- maline, beryl and fluorite, developed in response to H+ metasomatic alteration of the granite massif at its apical parts and confined exclusively to the linear fissures within the granite. They are classi- fied as endogreisens when occurring within the granite massif [Fig. 2B), and exogreisens when occurring at the exocontact zones within the


country rocks (Fig. 2C). The greisenisation of the metasomatised granites (i.e. apogranites) is displayed by the introduction of fine-grained, greenish- yellow, muscovite at the expense of the pre- existing feldspars and, with increasing intensity, a series of facies or a zonal pattern is developed in the sequence of: (1) unaltered granite + (2) greisenised granite + (3) green muscovite quartz greisen -+ (4) topaz-fluorite-beryl greisen -+ (5) quartz-green muscovite greisen core. In this sequence, facies 2 occurs as patchy zones or aureoles (5-20 m thick), which may encircle facies 3 to 5. The latter facies commonly constitute lenticular to vein-like greisen bodies, which are steeply dipping, 1 O-50 cm thick and 3-40 m long. Beryl in the greisens of apogranites occurs as euhedral, long prismatic (length/width ratio ranges between 4: 1 and 7: 1) pale yellowish to greyish crystals. Some crystals are bent, transversely fractured and intensively altered into deep brown and friable material.

However, the greisen types of beryl occurrences in magmatically-specialised granitoids are represented by pockets, lenticular bodies, veins and stockwork greisen veinlets. They are made up of massive milky quartz, muscovite, beryl, topaz and fluorite. The greisen pockets (20 cm in length) and lenses II m length) are intensively concentrated at the granite/country schist endocontacts (i.e. endogreisens, Fig. 2B), as well as at the exocontact zones (i.e. exogreisens, Fig. 2C) within the schist country rocks (as distant as -400 m). The greisen veins (100 m long) cross-cut the granite massif at the apical parts and the country rocks, and most commonly selvaged by a zone of muscovite (3 mm to 5 cm thick). The stockwork greisen occurs at the exocontact zones of the granite massif (best developed in the lgla and Abu Dabbab albite granites) and is represented by frag- mented (3-50 cm across) metavolcanic country rocks cemented by greisen (muscovite, quartz, topaz, fluorite, and beryl) net works (3-10 cm thick). Muscovite of the greisen net works occurs as a selvage zone. Beryl occurs as pale yellowish to brownish, prismatic crystals (0.5-3 cm long) with length/width ratios between and 5:l and 7: 1. It is worth noting that very limited zones (I 0 cm thick and 1 m length) of greisenised granite, associating greisen pockets and lenses were detected within the magmatically specialised granitoids along their contact with the country rocks. Field investigations revealed that the greisenisation of the country rocks (i.e. exogreisens) is a prominent phenomenon associated with the magmatically-specialised granitoids,


whereas the greisenisation of the granite massif itself (i.e. endogreisens) is commonly encountered in the metasomatically-specialised types. Detailed contribution to the greisenisation processes affecting the rare metal granitoids of Egypt is given in Abdalla (in prep.).

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 or series of contiguous veins and veinlet zones. Each series includes up to 30 veins which are steeply dipping, subparallel and are 1 O-50 cm thick and 30-500 m long. Numerous parallel veinlets (0.2-3 cm thick and 0.1-10 m long) branch off the main vein at acute angles. The vein is composed of coarse crystalline milky quartz selvaged by discontinuous margins (0.3-5 cm thick) of muscovite and topaz-muscovite. In addition to beryl, cassiterite and wolframite are also encountered 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 aureoles of wall rock.

CHEMICAL CHARACTERISTICS Emeralds and beryls In the ideal beryl structure, Be,AI,Si,O,,, both Be and Si are tetrahedrally co-ordinated, whereas Al occupies a distorted octahedra between the ring 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 formed and sometimes occupied by non-stoichio- metric water molecules and alkali ions (larger than Li). Normal beryl possesses a composition ap- proaching the ideal formula, whereas ‘octahedral’ beryl has trivalent and divalent cations replacing Al, and ‘tetrahedral’ beryl has Li mainly substi- tuting for Be (Aurisicchio et al., 1988).

The elevated contents of Mg, Fe, Cr and Na of emerald from Urn Kabu and Sikait characterise this emerald as ‘octahedral beryls’. The occupancy of octahedral Al is negatively correlated with the (Fe + Mg + Mn + Cr) content (Fig. 3A), confirming the mutual substitution of these cations in the octahedral sites. This relation is also shown in Fig. 3B, which depicts the role of alkalis (essentially Na) in maintaining the charge balance of the structure. This phenomenon is also depicted at the grain scale, as displayed by the chemical zoning of the emerald crystals, with a decrease in Fe, Mg, Na

A

1

0.8

??Emerald ??e

0 Beryl 0

=

t

ig , , , ,; ,; b,o, ,

._ * 2.5 2.8 3.1 3.4 3.7 4 32 Z

.Z Octahedral Al Content.

2

z B

1 ??*

0.8 ??

0.6

0 0.2 0.4 0.6 0.8

5

G Alkali Content per Channel.

:, S C 0

; 1 e@

0.8

0.6 1

“:% 1 ;, , , 9.175 9.225 9.275 9.325

a-axis (A)

Figure 3. Octahedralsubstitutions (i.e. S, Fe *, Mg, Mn, Cr and Til versus IA) the octahedral Al content; IBl the alkali content in the channels li. e., S, LiCh., Na, K, Rb and Csl; and (Cl the lattice parameter, a (A), in the investigated emerald and beryl.

and K towards the rim, whereas Al (and slight Si) decreases towards the core. This type of zoning is the reverse to that recorded by Franz et al. (I 9861, which is formulated as an exchange reaction with other minerals present during the growth of emerald, and attributed by Aurisicchio et a/. (1988) to the decreased T and P during or after uplift from an early high pressure to a late meduim pressure environment.



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 composition with little substitution of elements in the octahedral and tetrahedral sites. They are characterised by lower water and alkalis and enriched in Sn, F and Ga, especially from the greisens. However, beryl of sample MB-2 (analysis No. 9, Table 1) is highly fractured and may be affected by later solution activity. Pegmatite beryl is enriched in Li, Na, Rb and Fe (analysis No. 13, Table 1). A slight variation in Fe content was detected associated with the concentric colour variation in the pegmatite beryl, whereas those of the greisens and veins are typically homogeneous.

The plot of octahedral substitution of Fe, Mg, Cr and Mn versus the cell edge a-axis (Fig. 3C) indicates that cation substitution in emerald is accompanied by an increase in the length of the a-axis. Thus, the two paragenetic types of the investigated beryls can be distinguished by their c/a ratio, which is 0.992-0.995 for emerald (octahedral beryl) and 0.996-0.999 for normal beryl of the specialised granitoid association.

Micas of the hosting schists The major element composition of mica associated with emerald corresponds to phlogopitic biotite. However, the micas of the schist rocks hosting the beryl exogreisens in close vicinity to the Igla, Abu Dabbab and Nuweibi albite granitoids range in composition between biotite and Mg-rich biotite (Table 2).

FLUID INCLUSION INVESTIGATIONS

Fluid inclusion studies were conducted using 25 doubly-polished wafers. The microthermometric analyses were performed utilising a Linkham TH 600 heating/freezing stage. Taken into considera- tion the genetic types of inclusions (primary, secondary and pseudosecondary), an appropriate scheme based on the phase proportions and the


presence of the CO, phase and daughter minerals at room temperature has been conveniently used. However, both emerald and beryl commonly show a tube-shaped, negative to partially faceted crystal inclusion forms which are most probably of primary origin. Five types of inclusions are recognised in the investigated emerald and beryl (Fig. 4). Type 1 is frequent in greisen beryl associated with granitoids and is rarely recorded in emeralds. How- ever, type 3a is common in emerald and sometimes encountered in greisen beryls, whereas type 3b was only detected in emerald. Daughter minerals of type 3a were not successfully identified due to their diffused optical characteristics and small sizes. The platy form, high birefringence and refrac- tory nature of the mineral inclusions contained in type 3b, coupled with their variable numbers and volume per cent, may indicate that this solid phase is phlogopite, which was heterogeneously entrapped during inclusion formation. The results of the microthermometric analysis are summarised in Table 3 and will be discussed later.

DISCUSSION

From the aforementioned data, the two paragenetic types of Egyptian beryls are clearly characterised. Besides textural characteristics, the distinctive composition of emerald and the cell edge dimen- sions have a special bearing on the environment of their formation.

The phlogopite of the emerald-hosting schists have a high Cr content and a Mg/(Mg + FeIBtom ratio similar to the hosted emerald crystals (analysis Nos 1,2 and 5, Table 1 and Nos 1,2 and 4, Table 2), implying either their contemporaneous derivation, or the epitactic nucleation of emerald on phlogopite relicts as the result of an exchange reaction during the introduction of a (Be, Al, Na)- bearing solution. However, the randomly orientated phlogopite inclusions, their distinctive composition

Tabl

e 3.

A

su

mm

ary

of

mic

roth

erm

omet

ric

anal

ysis

fo

r th

e ex

amin

ed

incl

usio

ns

in

emer

alds

of

sc

hist

an

d be

ryls

of

gr

anito

id

asso

ciat

ions

, E

aste

rn

Des

ert

of

Egy

pt

Met

asom

atic

ally

spec

ialis

ed

gran

itoid

s

Mag

mat

ical

ly

spec

ialis

ed

gran

itoid

s

1 2

3 4

5 6

7 8

Roc

k ty

pe

Prim

ary/

seco

ndar

y S

ize

(pm

) In

clus

ion

type

N

umbe

r V

ol.%

C

O;!

Xco

l D

ensi

ty

of

CO

;!

pseu

dose

cond

ary

phas

e (g

cm

m3)

E

mer

ald

in

(P,

S)

(5-3

5)

(2,

3a,

3b)

40

Sch

ists

(P

, P

S)

(5-2

0)

4 20

(2

0-70

) (0

. I-0

.45)

(0

.82-

0.94

) (P

, P

S)

(5-I

2)

5 7

-100

(0

.88-

0.96

) B

eryl

in

pe

gmat

ites

(P,

S)

(5-2

5)

(lb,

2,

3a)

20

Ber

yl

in g

reis

ens

(S,

PS

) (5

-20)

(lb

, 2,

3a

) 38

(S

, P

SI

(5-I

8)

4 14

(2

0-35

) (0

.03-

0.2)

(0

.66-

0.78

) (S

, P

SI

(5-l

0)

5 5

-100

(0

.77-

0.87

) B

eryl

in

S

n-qu

artz

(P

, s,

P

SI

(5-3

0)

(lb.

2)

25

vein

s (S

, P

SI

(5-2

0)

4 5

(20-

40)

(0.0

5-0.

25)

(0.5

6-0.

79)

Ber

yl

in

grei

sens

(S

, P

S)

(5-2

5)

(lb,

2,

3a)

42

(S,

PS

) (5

-20)

4

12

(20-

40)

(0.0

5-0.

27)

(0.6

7-0.

75)

Ber

yl

in

Sn-

quar

tz

6,

PS

I (5

-I 0)

(P

, s,

P

S)

(5-3

5)

5 (lb

, 2)

5

-100

(0

.70-

0.84

) 27

ve

ins

(S,

PS

) (5

-20)

4

5 (2

0-30

) (0

.03-

0.22

) (0

.74-

0.83

)

I1

9 10

11

12

13

14

I

IRoc

k ty

pe

Em

eral

d in

S

chis

ts

X C

H4

T,(C

OA

T,

(lce)

TH

(CO

2)

T,(to

t) S

alin

ity,

wt%

1

NaC

l eq

uiv.

(-5

to

-19.

8)

(260

-382

) (8

-22)

(2

-26)

(-

56.9

to

-6

1.5)

(-

2 to

16

) (2

65-3

90)

(4-3

0)

(-57

.5

to

-62.

4)

(-6

to

8)

Met

asom

atic

ally

B

eryl

in

pe

gmat

ites

(-4.

5 to

-1

2.8)

(3

20-4

80)

(7-I

6)

Ber

yl

in

grei

sens

(-

2.8

to

-4.5

) (2

60-3

90)

(4.8

-7)

spec

iaiis

ed

gran

itoid

s c Mag

mat

ical

ly

(1.8

-I 5)

(-

56.8

to

-5

9.5)

(2

0 to

28

) (2

20-4

00)

(I-18

) (-

56.8

to

-6

0.1)

(9

.5

to

21)

Ber

yl

in

Sn-

quar

tz

(-1.

2 to

-2

.2)

(200

-380

) (2

-3.5

) ve

ins

(0.5

-8.8

) (-

56.6

to

-5

8.3)

(1

8.5

to

29.8

) (2

00-3

70)

Ber

yl

in g

reis

ens

(-2.

5 to

-4

.5)

(I 90

-400

) (4

-7)

(0.2

-I 5)

(-

56.6

to

-5

9.1)

(2

2.5

to

27.6

) (2

10-3

80)

spec

ialis

ed

Ber

yl

in

Sn-

quar

tz

(2-1

6)

(-57

.0

to

-59.

8)

(12

to

25)

(-1

.I to

-2

.5)

(190

-360

) (2

-4)

gran

itoid

s ve

ins

(0.6

-14)

(-

56.8

to

-5

9.0)

(1

4 to

23

.5)

P, S

, and

PS

refe

r to

pri

mar

y,

seco

ndar

y an

d ps

eudo

seco

ndar

y in

clus

ions

, re

spec

tivel

y.

The

incl

usio

n ty

pes

are

give

n in

Fig

. 4.

V

ol.%

C

O,

is a

vis

ual

estim

ate

at

40°C

. X

co2

is t

he

CO

, m

olar

fr

actio

n,

Xc_

=

CO

,/(C

O,

+ H

,O).

X,,,

is

the

C

H,

mol

ar

frac

tion,

X

,_,*

=

CH

,/(C

H,

+ C

O,)

; th

is

was

es

timat

ed

usin

g th

e m

etho

d of

H

eyen

et

al

. (1

982)

. TJ

CO

,) is

the

pa

rtia

l ho

mog

enis

atio

n te

mpe

ratu

re

of

the

CO

, ph

ase.

T,

(tot

l is

the

to

tal

hom

ogen

isat

ion

tem

pera

ture

. D

ensi

ty

of t

he

CO

, ph

ase

is e

stim

ated

us

ing

T&C

O,)

an

d th

e m

ode

of

hom

ogen

isat

ion

of t

he

CO

, ph

ase.

T,

(lce)

is

the

fi

nal

mel

ting

of

th

e fr

ozen

aq

ueou

s ph

ase.


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 *->

20 ' // 1 / I / I

t /' ,

I' I

‘8 I .; 16 - $ z 14 -

2 12 -

po-

at 6- .= ._ ; 6-

* 4-

/. ,, ’

I .’ i

, ,’ I

: 1’ e- -.-.

: \ \ i

,. 1 ‘\.__-* : TffA ,:

..4 _._ _’

.’ I .e-,

I ‘.

‘. :

‘. : I /“I3 ,’

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 phlogopitite and schists hosting the emerald), coupled with the chemical zoning exhibited by the emerald crystals, all suggest that emerald is formed during the introduction of a (Be, Al, Na)- bearing solution at the expense of the break down of phlogopite. The high lattice energy difference necessary for epitactic nucleation of the neo- formed emerald (ring silicate) on the phlogopite (phyllosilicate) can be compensated by the stabilis- ing of Mg, Fe and Na within the emerald structure.

Although the micas of the schist rocks hosting beryl exogreisens associated with granitoids, and those of the schist hosting the emerald are similar in composition (analysis Nos 1, 2, 4 and 5, Table 21, a remarkable discrepancy in the Mg/(Mg + Felatom of beryl of the exogreisen and micas of the hosting schist is clearly seen (analysis No. 7, Table 1 and No. 5, Table 2). This discrepancy, coupled with

the normal composition of these beryls, imply that it is the chemistry of the Be-bearing fluids (rather than that of the bulk rock, as suggested by Aurisicchio et a/., 1988) that controls the formation of diverse paragenetic types of emerald- schist and beryl-schist exogreisens associated with granitoids.

Fluid inclusion data of the examined emeralds (Table 3, Fig. 5) indicate homogenisation temperatures ranging from 260-382OC and a salinity of 8-22 wt% NaCl equiv. for the aqueous fluid inclusion types. However, Fig. 5 shows a somewhat large salinity range (8-18.5 wt% NaCl equiv.) corresponding to a temperature interval of 90°C for the Sikait emerald. This may indicate incor- poration of low salinity inclusions related to fluid activity associated with a later tectonic episode. Meanwhile, the overlapping areas of fields of aqueous inclusions of emeralds from Sikait and



Urn Kabu (Fig. 5) can be considered as the prevailing conditions (i.e. 260-350°C and 13.6- 18.5 wt% NaCl equiv.) during emerald deposition. However, the CO,-H,O and COP-rich inclusions show final melting of the CO, solid phase (T,CO,) in the range of -56.9 to -62.4OC, reflecting the presence of another gas, most probably CH, (as no phase changes below -12OOC was detected to deduce the presence of NJ. The large compo- sitional, density and volume per cent variation of the CO,(CH,) phase in the inclusions of the same population suggests a heterogeneous entrapment 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 Be-bearing solutions were moderately saline, but CO,(CH,)-rich. The low contents of F in emerald and its impoverishment in the associated micas of the emerald-hosting schists and phlogopitite, and the scarce occurrence of F-bearing minerals, either in the pockets or quartz stringers, may rule out the promoting role of F as a mobilising medium for Be, Al and Na. Thus, it is implied that Be was most probably complexed by carbonate( + CH,)- chloride base as suggested by Beus and Dikov (1967).

The metasomatically formed granitoids and the associated rare metal mineralising processes are thought to result from the upward migration of late to post-magmatic, volatile-rich aqueous fluids through the consolidated granite cupolas (Beus et al., 1962). The fluid phase, aided by its hydro- to fluorophile nature, is exsolved and accumulates in the apical parts of the granite massifs at grain boundaries, and in microfractures and vugs (Pollard and Taylor, 1986). Subsolidus re-equilibration of these fluids with the granites will cause zonal alteration assemblages.

Meanwhile, the close consistency of the magmatically-formed rare metal albite granitoids with the experimental data of the Li- and F-saturated haplogranite systems (e.g. Manning, 1981; Martin, 1983) indicates the important role of F and Li in the generation of such highly evolved magmas. These volatiles reduce the viscosity and solidus temperature of the melt, leading to an increase in diffusion rates of rare metals (including Be), thus permitting late liquid-liquid ultrafractionation of rare metal granitic magma (Hannah and Stein, 1990).

Considering the geochemical distribution of Be in the specialised granitoids, the Mueilha Sn granite (of metasomatic origin: Morsey and Mohamed, 1992) and Nuweibi Ta-albite granite (of magmatic origin: Helba et al., 1997) were chosen for

Table 4. Average Be contents in the Nuweibi and Mueilha beryl-bearing granitoids

Rock type NUWEIBI

average Be (ppm) N

Greisenised albite granite 125 3

White mica albite granite 14 2

Li-muscovite albite granite 16 2 Zinnwaldite amazonite albite granite a 2 MUEILHA

Greisenised granite 75 2

Muscovitised granite 47 2

Unaltered leucocratic muscovite granite 12 2

N: number of samples analysed.

determination of Be (Table 4). Compared to the apical white mica-albite granite of the Nuweibi Pluton, Be is considerably concentrated in the greisenised albite granite in close vicinity to the greisen pockets at the endocontact with the country schist (Fig. 2B). The low Be contents of the apical albite granite contrasts with their enriched contents of Li and F, with which Be is known to be concentrated (Kovalenko eta/., 1977). However, the tendency of Be to be accumulated in the near contact endogreisens, as well as in the exogreisens (along with the F-rich mineral assemblage) may reflect the role of F in com- plexing the Be (thus inhibiting concentration of Be in the magmatic stage) and its separation into the post-magmatic fluid. On the other hand, Be shows a high roofward enrichment in the metasomatic facies of the Mueilha Apogranite, especially in the albitised and greisenised granites (Table 4). The role of F during the process of Na- metasomatism (i.e. albitisation) of the Mueilha granites was stressed by Morsey and Mohamed (1992). Moreover, the experimental work of Beus et al. (I 963) revealed that Be is transported in the post-magmatic fluids as Na-fluoroberyllate complexes, and the albitisation of the wallrock will lead to the deposition of beryl. However, the superimposed greisenisation is mainly attributed to the sharp increased a,+ and decreased activities of the alkalis, as evidenced by the partial to complete destruction of the feldspars. The stability fields of the mineral paragenesis commonly encountered in the investigated greisens and quartz veins are shown in Fig. 6. The detected zonal pattern exhibited by minerals within the greisens may indicate that deposition of those minerals at that definite stage occured as the result of replacement of its predecessors along the evolution trend given in Fig. 6. This trend is inter- preted by Burt (1981) as the result of cooling and neutralisation of the fluid.



f Phenakite

pKF (Salinity) L

Figure 6. Phase relations in the system K,O-A,O,-SiO,-H,O- F,O.,, after Burt (1981). The dashed line shows the stable 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.

The aqueous fluid inclusions examined in beryls associated with granitoids (Table 3, Fig. 51 show a sequence of formation with decreasing temperatures and salinities: beryl pegmatite+greisen bodies+cassiterite-quartz veins. The two fields shown by the aqueous inclusions in the pegmatite beryls (Fig. 5) can be related to two distinct events of fluid evolution. Field A (homogenisation temperatures 390-480°C and salinities 13.3-I 6 wt% NaCl equiv.) is most probably related to the continuous transition from the magmatic to the supercritical hydrothermal stage. Meanwhile, inclusions of field B may reflect an early emergence of greisenising fluid. The selvage zone of muscovite and topaz developed along the beryl pegmatite veins against the host granite may support this veiw. However, the great range of temperature (1 90-400°C) and the accompanying decreased salinity displayed by the beryl of the greisens of the magmatically-specialised granites (Fig. 5, field D) may indicate a related and inseparable hydrothermal event. Besides, the mixing of the magmatically-emerged greisenising fluids with other cooler and less-saline fluids (meteoric water?] can be inferred from the trend given in Fig. 5. The evolution trends given for the greisenising fluids in Figs 5 and 6 are consistent. The greisenising fluid can be evolved initially by separation of a dense brine from the exhausted granite melt (Roedder, 19771, followed by the emergence of an acidic, vapour-rich phase from the brine. Phase

separation can be attributed in part to the boiling 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 meteoric waters, pervasive alteration of the stocks (including endogreisenisation), and the predomin- ance of the disseminated type of rare metal mineralisation.

METALLOGENETIC AND EXPLORATION MODELS

Emerald-schist association Different modes of origin have been attributed to the schist-hosted emerald deposits. The exometa- morphic origin of Fersman (1929) and its adoption by Shinkankas (1981) is still the most acceptable model for such deposits. The model relates the deposits to the interaction between granitic pegmatites and/or their derived fluids with pre-existing mafic to ultramafic rocks interlayered with a volcanosedimentary sequence. However, the restriction of emerald deposits to blackwall zones developed at the contact between the tectonically juxtaposed schist and ultrabasic rocks during low- grade regional metamorphism led Grundmann and Morteani (1989, 1993) to postulate a pure metamorphic origin for the schist-hosted emerald deposits.

In the Urn Kabu and Sikait emerald deposits, the schist sequence, including slices of serpentinites, is intruded by pegmatitic veins (Fig. 7A). The frequent occurrence of emerald mineralisation in both the quartz and pegmatite veins, which are spatially related and geochemically linked to the proximal leucogranite, suggest that the granites and the associated pegmatites are the source rocks for the Be-bearing fluids. The remarkable enrichment of the pegmatitic varieties of the leucogranite in K (K,O/Na,O = 2.5-6; Mohamed and Hassanen, 1997) and the restriction of post-magmatic alterations such as feldspathisation (alkali metasomatism) and silicification to the apical parts of the granite, are all features reflecting circulation of K-

594 Journal of African Earrh Sciences


,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.



Grundmann and Morteani (1993) considered the Egyptian emerald deposits to have a metamorphic origin. However, the geological and geochemical features of the emerald deposits of Egypt all oppose such a metamorphic origin and substantiate the role of a pegmatitic-derived fluid phase to generate emerald mineralisation. There are five features mentioned here:

i) The fluid flow process, as well as the mineralisation, are closely related to the infiltrational mech- anism (Giuliani et al., 1990) and display a clear tectonic control. The main mineralising centres are located along the major ductile Nugrus Shear Zone. Further, the emerald-bearing pegmatitic and quartz veins are confined to the shear zones.

ii) The major tectonic deformation, including thrusting and folding, with which the leucogranite plutons are syn-tectonically emplaced, are well- known to post-date the regional metamorphic event in the studied area (e.g. El Biyoumi and Greiling, 1984).

iii) The emerald crystals show features indicating static growth with randomly orientated phlogopite inclusions. The phlogopite inclusions have a high Cr content and a Mg/(Mg + FeIatom ratio similar to the hosted emerald crystals, reflecting their metasomatic derivation from the same interacting fluids. Further, the phlogopite inclusions are chemically distinct from the micas of the host schists and phlogopitite rocks.

iv) Inclusion-free emerald crystals commonly cross-cut the biotite foliation in the schist, which reflects a clearly post-metamorphic origin for the mineralisation.

v) The characteristic chemical zoning exhibited by the emerald crystals of the Urn Kabu and Sikait areas indicate a normal fractionation trend of alkali- rich, acid fluids. Conversely, the emeralds derived from metamorphic fluids, such as those of Hab- achtal, Austria, exhibit reversed zoning with a consistent increase in Mg, Fe and Cr towards the rims (Grundmann and Morteani, 1989).

It is suggested that major ductile shear zones, along which a metapelite-metavolcanic sequence associated with ultramafic rocks is highly folded and imbricated, should be investigated when prospecting for emerald deposits in Egypt. Most important is the emplacement within this sequence of syn-tectonic pegmatitic leucogranites from which K- and Be-rich fluid phases were derived. This is manifested in the field by the development of a system of emerald-bearing pegmatitic pods and veins confined to the shear zones, as well as a broad zone of alkali metasomatism.


Beryl-specialised granites association Close examination of the investigated granitoids suggested the following metallogenetic and exploration criteria (as simplified in Fig. 78, C) when prospecting for rare metal, Be-bearing granitoids:

il occurrence of post-erogenic, leucocratic, meta- to peraluminous granitic stocks emplaced along intersecting magma generating faults and fractures;

ii) the granitoids occur as domal protrusions or cupolas with gentle outward dipping contacts which permit the retention of post-magmatic fluids and hence increased the metasomatic processes, especially the greisenisation;

iii/ post-magmatic metasomatic albite-enrichment and extensive endogreisenisation of the granite massifs along fracture zones and peripheral parts of the stocks may distinguish the metasomatically Be-bearing granitoids from the magmatically formed ones; and

iv) the metasomatically-specialised granitoids are commonly characterised by the development of radiometric anomalies (e.g. Abdalla et a/., 1996).

CONCLUSIONS

Two paragenetic types of beryl mineralisation occur in the Precambrian rocks of Egypt: (1) emerald-schist; and (2) beryl-specialised granitoid associations. Geological and geochemical features of the emerald deposits substantiate the role of syn-tectonically emplaced leucogranites as a source for the Be solutions. Infiltration of such solutions through the nearby permeable sheared schist sequence and the intercalated serpentinite bands will cause pervasive metasomatism of the serpentinites and schists into phlogopite-rich rocks and subsequent localisation of emer,alds.

Beryl associated with granitoids occurs in pegmatite veins, greisen bodies and cassiterite quartz veins, of which the greisens are more likely to have the highest potential. The greisenisation of the country rocks (i.e. exogreisens) is a prominent phenomenon associated with the magmatically-specialised granitoids (Li-albite granites), whereas the greisenisation of the granite massif itself (i.e. endogreisens) is commonly encountered in the metasomaticaly-specialised granitoids (apogranites).

It is suggested that the diverse chemistry of the Be-bearing fluids (carbonatef + CH,)-chloride- based for emerald and Na-fluoride-based for beryl) play the leading role in the formation of the different paragenetic types of emerald-schist and beryl- schist granitoid exogreisens.


ACKNOWLEDGMENTS

The chemical analyses and fluid inclusion investigations have been performed during the leave of the authors to the Hokkaido (H.M.A.) and Tohoku (F.H.M.) Universities, Japan. The authors are indebted to Dr H. Matsueda and Prof. S. Kanisawa for their generous facilities given during the laboratory work. The Division of XRF and XRD of the Nuclear Materials Authority, Egypt is also acknowledged for their assistance in analysing some additional samples. The earlier manuscript benefitted greatly from suggestions by the reviewers. Editorial Handling - J. R. Baldwin & D. C. Turner

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kiiash JournalofAfrican Earth Sciences. Vol. …rjstern/egypt/PDFs/SE...RESUME-L’ btude mineralogique, chimique et d’ inclusions fluides met en evidence deux environnements favorables