Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization and metallogenetic aspects

Journal of African Earth Sciences, Vol. 17, No. 4, pp. 525-539, 1993 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0899-5362/93 $6.00 + 0.00

Pergamon

0899-5362(94)E0009-4

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization and metallogenetic aspects E H. MOHAMED GeologyDepartment, Alexandria University,Alexandria, Egypt (First received 30th December, 1992;revised versionreceived 12th November, 1993) Abstract - Three "younger granite" plutons from the Eastern Desert of Egypt are studied: petrographic and geochemical characteristics of the barren pink granites at Wadi Sikait and Wadi Nugrus are similar, of alkaline, mildly peraluminous nature and are enriched in LIL-elements and LREE with moderate negative Eu anomalies. In.contrast, the Sn-Ta-W-bearingalbite granite ofAbu Dabbab is alkaline,peraluminousmuscovite granite; its chemical specialization is manifested by the pronounced enrichment in Ta, Sn, W, F, Rb and Li coupled with marked depletion in Ca, Ti, Mg, Sr and Ba. Elemental ratios (e.g., K/Rb, Rb/Sr, Ba/Rb) discriminate the albite granite and the pink granites into "mineralized and barren granites", respectively. The albite granite is derived from Na-rich magma of within-plate characteristics. Fluorine was an important eomplexing anion during magmatic evolution history. The albite granite is emplaced at shallow depth (< 100 MPa) and at the intersection of structural weaknesses. The pink granites might have a crustal and/or LILelement enriched mantle sources, in which the subduction-relatedfingerprints are partly obliterated. For both types, reactivation of regional structures played a significant role in magma generation. Acid metasomatism is mainly manifested by the development of thin greisen veins along fracture systems in the albite granite. The chemistry of greisenization using mass balance approach reveals that the process is accompanied by dramatic increase in SiO2, Fe203, MnO, F, Sn and Li as well as significant loss in NazO, K20, Ba, Nb and Zn. The process causes a significant increase in volume (30 %). Changes in chemical components are consistent with the observed mineralogical changes. Microprobe results reveal that the wolframite crystals are typically huebnerite with Fe-rich cores and Mn-rich rims. Compositional variations in wolframite crystals are attributed to the physicochemical conditions (pH, T, etc.) and chemistry of the ore-bearing fluids.

INTRODUCTION The post-tectonic y o u n g e r granite plutons (El Ramly and Akaad, 1960) are widely distributed all over the Nubian Shield. They are emplaced within a time s p a n from 620 to 530 Ma (Hassan and H a s h a d , 1990). They are epizonal and unfoliated p l u t o n s t h a t have sharp c o n t a c t s with their country rocks. They commonly comprise small (1-10 km in diameter), equidimensional to ovoid bodies of pink to red granitic rocks. The majority of the plutons are confined to regional structural weaknesses. These granitic rocks display alkaline to peralkaline affinity and range from s u b s o l v u s to hypersolvus types, although transitional varieties (transsolvus) do occur (Greenberg, 1981). These alkaline granites were emplaced during the later stages of the Pan-African acidic m a g m a t i s m which is dominated by calc-alkaline plutonic-volcanic association. These alkaline rocks have b e e n considered either as the fractionated e n d - m e m b e r s of calc-alkaline m a g m a s (Rogers and Greenberg, 1981) or as the initial expression of within-plate magmatism (Harris and Marriner, 1980). Among the y o u n g e r granite group some are associated with

r a r e - m e t a l m i n e r a l ~ a t i o n . These mineralized plutons are extremely differentiated albite-rich granites which were previously termed apogranites (Sabet e t al., 1976; Hussein, 1990). S u c h albite-rich granitoids m a y be either derived from crystallization ofNa-richmagma (Asran, 1985; Pollard, 1989; Schwartz, 1992) or interaction of Na-bearing fluids with consolidated granites (Beus, 1970; Sabet e t al., 1976). The rare metal-bearing apogranite of Abu Rusheid, E a s t e r n Desert of Egypt is considered as a typical example of a metasomatic origin (Mohamed, 1989). Two different granitic types are investigated. The first type includes the pink granites 5fWadi Sikait and Gabal Nugrus plutons, while the other type is r e p r e s e n t e d by the rare m e t a l - b e a r i n g albite granite of Abu Dabbab. The three plutons are closely associated with major structural trends (Fig. 1A). The aim of the p a p e r is to compare the geochemical characteristics of the two granitic types to a s s e s s the metallogenic a s p e c t s of Abu D a b b a b albite granite. Further, the petrogenesis of the two different granitic types and their relation to the evolution of the Egyptian shield during the Pan-African time will be discussed.

525

526

F. H. MOHAMED ted p l u t o n (~.10 k m in diameter) at t h e u p p e r p a r t o f W a d i Nugrus. The N u g r u s a n d the Sikait granites are aligned along a m a j o r low a n g l e - t h r u s t n a m e l y "Nugrus t h r u s t " , w h i c h f o r m s t h e roof t h r u s t o f W a d i Sikait d u p l e x (Greiling e t a l , , 1987).

GEOLOGIC SETTING

The t h r e e granitic p l u t o n s are e m p l a c e d w i t h i n a s e q u e n c e of ophiolitic m e l a n g e (Fig. 1B. C) a n d p o s t d a t e d t h e s u t u r i n g a n d the earlier t r a n s i t i o n of t h e Pan-African mobile belt of NE Africa f r o m oceanic to c o n t i n e n t a l c r u s t in Late P r e c a m b r i a n time (Gass, 1981), The p i n k g r a n i t e s form bodies w i t h s h a r p intruslve c o n t a c t s , a small p l u t o n (=. I k m in diameter) @ is located at the u p p e r r e a c h e s o f W a d i Sikait a n d c o n s i d e r e d a s a c o n t i n u a t i o n of t h e larger elonga-

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Fig. I. Maps showing: (A) Locations of the investigated granites and the distribution of tectonic zones, modified from Krs eta/., 1973 and Garson and Krs, 1976. (B) Geologic map ofAbu Dabbab deposit, modified from Sabet et al., 1976. (C) Geologic map ofWadi Nugrus, modified after photogeol, maps of SE Desert, Egypt, 1- I00000, 1967, El Shazly and Hassan, 1972, Geologic map of Aswan quadrangle, 1-500000, 1978, and Greiling et aL, 1987.

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization ... In the field, the t h r u s t a p p e a r s as major s h e a r zone and w a s considered to represent the b o u n d a r y b e t w e e n the central and the s o u t h E a s t e r n Desert (Stern and Hedge, 1985). The time of extensive t h r u s t i n g in the region w a s b r a c k e t e d b e t w e e n the e m p l a c e m e n t of the older granitoids and the intrusion of the y o u n g e r granites (Kroener eta/., 1987). The albite granite ofAbu D a b b a b is an elongated stock-like b o d y a b o u t 0.06 k m 2 in area and 100130 m in height (Fig. 1B). It is located along a fault ofthe N 60° E trending deep-seated fracture system (Krs et a/., 1973) and near its intersection with a NW trending s h e a r zone (Garson and Krs, 1976), (Fig. 1A). The granite m a s s is cut across b y series ofcassiterite-wolframite greisen and quartz-topazfluorite veins (Fig. 1B). The m a s s owes an intrusive contact, with a chilled margin of a b o u t 0.5 m wide (Asran, 1985) and emplaced within a metavolcanosedimentary sequence. The Abu D a b b a b albite granite is considered as one of the biggest raremetal (Ta-Sn-Nb) deposits in the Eastern Desert of Egypt (Sabet eta/., 1976).

PETROGRAPHY The albite granite is fine- to medium-grained leucogranite with massive felted appearance, and impregnated with Mn-minerals. Laths of albite (average 0.5 m m long) form the g r o u n d m a s s and are seen to enclose p h e n o c r y s t s of granular quartz (up to 2.5 m m across). Frequently, albite laths are oriented concentrically along the growth surfaces of the quartz p h e n o c r y s t s exhibiting a "snow ball" texture (Pollard, 1989). Microcline is not a b u n d a n t and s e e n as anhedral crystals (up to 1 m m across) filling the interstices between the quartz phenocrysts and albite laths. Cassiterite forms disseminated idiomorphic crystals up to 0.2 m m in length. Topaz c o n s t i t u t e s coarser prismatic crystals up to 0.4 mm. Fluorite occurs as small anhedral crystals (0.05-1 m m in size) t h a t occasionally attain larger sizes up to 3 mm. The N u g r u s and Sikait granites are mediumgrained with a pink colour. They are porphyritic and c o m p o s e d chiefly of quartz and feldspars with few biotite. Mesoperthites are a b u n d a n t as phenocrysts (up to 4 m m across) enclosed in the fine to medium-grained matrix. G r a n u l a r quartz inclusions are a b u n d a n t in the p o t a s h feldspar phenocrysts forming granophyric texture. Plagioclase (albite-oligoclase) forms laths varied in length from 0.5 m m to 2 mm. Quartz is r e p r e s e n t e d by granular crystals with a grain size of 0.2 up to 2 m m t h a t u s u a l l y exhibit u n d u l a t o r y extinction. Biotite occurs as brown shreddy flakes (up to 1 m m in length) which are partly chloritized. Muscovite is s u b o r d i n a t e and is represented by thin flakes derived mainly from the alteration of plagi0clase.

527

Titanite forms elongated lenses (0. I mm) along the cleavage of the altered biotite. Other accessories include zircon, fluorite and iron oxides. A rare variety of the granite differs from the biotite granite in its mineralogical composition (sample no. N9, N l l ) . It is c o m p o s e d chiefly of p h e n o c r y s t s of plagioclase (up to 4 mm) with a b u n d a n t flakes of muscovite. The latter occurs interstitially and as replacement of plagioclase. Moreover, muscovite rather replaces all biotite.

ANALYTICAL PRO CEDURES Chemical analyses were carried out at the Institute of Mineralogy and Petrology, University of Cologne and at the Special R e s e a r c h Project Arid Areas, "SFB 69", Technical University of Berlin, F. R. Germany. Twenty-one rock samples and three heavy mineral concentrates were analyzed using different analytical procedures. Major, except Mg, Ti, and trace elements (Nb, Zr, Rb) were determined by using XRFS. After HF/HC104 dissolution, Mg, Ti, Li, Pb, Zn and Cu were m e a s u r e d byAAS, while Ba, Sr and Y were determined by ICP-AES. Hf, Ta, Th and W were m e a s u r e d by INAA for some representative samples. Fluorine w a s determined using a specific ion electrode. The REE, after their separation by anion exchange technique, were determined by ICP-AES. Analytical precision w a s controlled by natural standards. The precision is generally lower t h a n 2 % for major elements and within 5 % - 10 % for trace elements.

GEOCHEMISTRY The chemical compositions for the pink granites of Nugrus and Sikait and the albite granite of Abu D a b b a b are shown in Table 1.

Major-Element G e o c h e m i s t r y The pink granites and the albite granite are highly siliceous with SiO 2 values ranged from 73.4 to 78.5 %. Normatively, they are classified as granites (Fig. 2), however few a n a l y s e s straddle the granite b o u n d a r y to the trondhjemite field, which reflects the a b u n d a n c e ofsodic plagioclase in these samples. The p r o n o u n c e d depletion in Ca for the albite granite is manifested by the distribution of its plots close to the Ab-Or side. The pink granites are mildly p e r a l u m i n o u s with average A/CNK ratio, (A1203/CaO + Na20 + K20 mol.), of 1.12, whilst the albite granite is notably p e r a l u m i n o u s with average ratio of 1.22. The plumasitic chemical signature of b o t h the pink granites and the albite granite are reflected b y the consistent a p p e a r a n c e of c o r u n d u m in the CIPW

528

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Fig. 2. O ' C o n n o r Ab-An-Or d i a g r a m showing the c o m p o s i t i o n s of the Abu Dabbab albite granite (&) a n d the p i n k granites of N u g r u s (@) and Sikait (o).

Fig. 3. Characteristic mineral diagram, after Debon and Le Fort, 1986. The two p a r a m e t e r s (A & B) are in grama t o m s x 103 in 100 g of rock. Rock s y m b o l s as Fig. 2.

n o r m s and the plotting of all analyses over the Xaxis of the characteristic mineral diagram (Fig. 3). The analyses of the albite granite are plotted in field I reflecting a muscovite predominance over biotite while, the plots of the pink granites are mainly r e p r e s e n t e d in field II indicating a biotite predominance. This is in good accordance with the petrographic observations. The alkalinity ratio of Wright (1969) exhibits the alkaline affinity for both the pink a n d albite granites. The Nugrus and Sikait pink granites are quite similar to t h a t of low-calcium granite, (Turekian andWedepohl, 1961), (Table 1). O n t h e other hand, the albite granite exhibits lower average values of Ca. Mg and Ti oxides relative to low-calcium granite, which reflects its chemical specialization (Tischendorf, 1977).

1986). A m a r k e d e n r i c h m e n t in Sn and Ta typified the Abu Dabbab granite, while the Abu Rusheid apogranite is notably enriched in Nb and Zr with respect to the former. Like the major oxide compositions of the pink granites, their average trace element compositions are closely similar to low-calcium granite. Therefore, the albite granite is dissimilar to "normal" granitic rocks, to which the pink granites compare well, but it is r a t h e r similar to chemically specialized granites. This chemical dissimilarity between the studied albite granite and the pink granites is again illustrated by the different diagrams (Fig. 4AC), which discriminate between mineralized and barren granites according to certain values proposed by m a n y workers in this field (Table 2). Figure 4 clearly classifies the albite granite as mineralized granite, while the pink granites are b a r r e n ones. The diagram also exhibits the close similarity in the chemical compositions for both Abu Dabbab granite and Abu Rusheid apogranite, as all analyses of the Abu Dabbab ab granite are plotted within the Abu Rusheid Field.

Trace-Element Geochemistry The pink granites are enriched in LIL-elements (Ba, Sr, Rb), on the contrary, the albite granites show a p r o n o u n c e d e n r i c h m e n t in HFS elements like Ta, Sn, Nb and W (Table 1). The trace element compositions of the Abu Dabbab granite exhibit a m a r k e d e n r i c h m e n t in granitophile e l e m e n t s especially Sn, Ta, Rb, Li, F and W, This again reflects the chemical specialization of the albite granite. Although. there are some differences in the rare-metal (Sn, Ta, Nb, Zr) concentration levels, the average composition of the Abu Dabbab granite c o m p a r e s broadly with those of the Abu Rusheid r a r e - m e t a l bearing apogranite from s o u t h e r n E a s t e r n Desert (Mohamed, 1989) and the specialized granites from the Arabian Shield (Ramsay,

REE Geochemistry Chondrite-normalized REE patterns (Fig. 5) for the albite granite (AD 17) exhibit an e n r i c h m e n t in HREE with extreme negative Eu anomaly and a slight e n h a n c e m e n t in LREE. La/Yb ratio is less t h a n 1 indicating a reverse fractionation of the REE (~'LREE < I~HREE). The pattern of a greisen sample (AD 14) exhibits a higher REE a b u n d a n c e especially in HREE with a less p r o n o u n c e d negative Eu anomaly relative to the original rock.

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization ...

529

Table 1. Major-oxide and trace-element compositions of the Nugrus and Sikait pink granites and the Abu Dabbab albite granite. Nugrus pink granite N2

$11ple

HI0

Sikait pink granite

NI3

NI7

.19

N9

.11

~6

75.07

73.88

number i

Nlllior

oxtdll

SiO 2

Ti02 11203

Fe203t

szz2

~,lS

~17

~2~

sR23

75.55

(vtZ) 76.05

7 4 . ~,7

73.99

74.12

75.46

78.26

78.51

75.97

75.28

75.1e

0.08

0.20

0.15

0.12

0.19

0.11

0.15

0.11

0.08

0.12

0.12

0.14

0.14

12.9(3

13.12

13.28

14.43

13.90

13.73

13.50

13.85

12.28

13.10

13.52

13.74

13.67

1.2B

1.79

1.41

1.32

1.91

1.75

1.60

0.92

0.37

1 .Or

1.90

1.4B

1.25

0.04

0.06

0.04

0.0~

0.0~

O.Ot

0.03

0.02

0.03

0.03

0.03

0.03

0.03

NIlO

0.09

0.27

0.35

0.11

0.21

0.36

0.20

0.02

0.03

0.02

0.06

0.18

0.13

CaK)

0.52

0.80

0.85

0.5t

0..~1

1.46

0.30

0.36

0.61

0.82

0.35

0.72

0.36

Ila20

3..~5

3.20

3.46

3.B8

3.~

3.76

4.2;)

4.25

4.20

4.21

4.33

4.14

4.16

1120

4.82

5.30

4.62

5.25

5.05

2.47

1.78

4.71

3.39

4.13

4.36

4.33

4.37

P205

0.02

0.04

0.02

0.02

0.02

0.05

0.02

0.01

0.01

~1

0.01

0.03

0.01

LDI

0.72

0.84

'1.09

0.71

0.97

1.93

0.95

0.4,8

0.58

0.91

0.34

0.72

0..55

I(:~1.06

1(0.08

1(0.32

1(0.41

1(0.42

101.16

IOI.QIB

918.59

100.09

1(0.3~

99.30

1(X).69

1(30.22

318

751

~

423

310

695

438

1190

277

477

439

377

471

Ce

62

86

68

79

81

60

79

B4

42

61

50

58

67

C. F

4

6

5

12

12 _

15

10

9

6

36

9

4

3

_

37

Total eie~mt

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311

Lit

33

33

40

63

46

30

39

46

17

24

30

33

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8

8

8

2

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16

10

2

2

2

4

8

6

lib

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16

25

49

40

211

30

3

21

8

4

5

14

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fld

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90

ad

Ild

M

IXi

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i~I

nd

nd

rid

Ib

131

113

IOD

134

142

68

37

95

79

99

109

109

lob

,fm

nd

M

II

~1

,~

I~1

~i

~l

INI

~1

nd

nd

nd

Sr

43 4

132

126

78

123 _

183 _

121

:~5

73

53

66

66

83

TI Q Th"

17

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V*

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-

_

Y

2B

16

26

36

30

7

16

10

3

11

20

15

18

Zn

37

46

40

106

66

40

33

38

20

42

37

28

28

135 199 ei,mm~t,, ( p r o )

135

175

189

152

169

20b

137

168

178

173

167

2r lare-~rth LA

58

28

¢,e

108

5*

Pr Nd

9.2

6.3

45

27

5.8

3,5

0.5

0.4

Gd

6.6

3.8

Dy Ho

3.3 1.i

3.8 0.8

£r Yb

2.7 1.6

16 2.~

1~

0.2

G..

¢'~

Fe~0^: total Fe oxide; nd not detected; ~J

- no data; * determined

by INAA

MINERALIZATION ALTERATION

The p i n k g r a n i t e s (N 10, S R 21) s h o w fractiona t e d p a t t e r n s with m o d e r a t e l y negative Eu anomalies. The p a t t e r n s have negative s l o p e s with L a / Y b ratio m u c h g r e a t e r t h a n unity. C o m p a r e d to the albite granite, the p i n k g r a n i t e s are LREEenriched and HREE-depleted.

AND

PROCESSES

The r a r e - m e t a l potential of t h e A b u D a b b a b granite is o b v i o u s l y m a n i f e s t e d b y its m a j o r a n d trace e l e m e n t c o m p o s i t i o n s . It is c h a r a c t e r i z e d b y high r a r e - m e t a l c o n c e n t r a t i o n s ofTa, Sn, F, W, Rb a n d Li c o u p l e d with very low v a l u e s of Mg, Ca, Ti, Ba a n d Sr. Ratios of g e o c h e m i c a l l y c o h e r e n t a n d

530

F.H. MOtIAMEI)

Table i. (continued).

Abu Dabbab.

Abu Dabbab

albite granite AD1

• nmmmr P~r

greisen veins

ADO

JtJ)l 1

ADI 3

JUDI7

M)3

~

in7

AOi &

t.q

DS

r e c t u m (wt%)

StO 2

7/*.61

73. 61

73.41

75.46

73.63

83.67

88.9S

81.16

70.25

74.89

75.41

1"102

0.02

O. 02

0.02

0.01

0.03

0.02

0.01

0.03

O. 13

0.01

0.10

0.20

~U.203

15.21

IS.gO

13.75

15.11

10.26

9.91

13.21

18.72

13.46

12.82

13.60

Fe203c

0.23

0.20

0.18

O.17

0.23

0.73

0.82

0.86

1.48

1.43

0.77

2.03

HmO

0.35

0.06

0.10

0.10

0.23

O.M

0.22

0.25

2.61

0.09

O.ot

0.05

14.~

74.22

0.07

0.05

O.OS

0.03

O.Ot

0.05

0.12

0.10

0.09

0.03

0.09

0.27

CaO

0.16

0.14

0.1"2

0.19

0.20

0.12

0.12

0.10

0.38

0.16

0.62

0.71

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4.93

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5.60

1.82

0.56

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4.19

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3.55

3.67

2.82

4.05

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1.29

1.28

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0.39

0.37

0.98

0.97

1.11

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99.74

99.12

99.30

100.77

99.32

99.64

99.76

99.45

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211

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4.7

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1.8 0.7

- -

Dy

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Ho

0.7

Er

1.3

Tb

4.1

18.8

1~

0.6

4.0

AR : average composition of Abu Rusheid rare metal-bearing PS : average composition LCG : average composition

2.2 40

of plumasitic

specialized

of low calcium granite

apogranite

(Mohamed,

1989);

granite from Arabian Shield (Ramsay,

(Tureklan and Wedepohl,

1961).

!986);

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization ...

500-

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Fig. 4. Variation in K/Rb-Rb (A), Ba-Rb (B) and Rb-Sr (C). Stippled area is the field ofAbu Rusheid apogranite (Mohamed, 1989). Rock symbols as in Fig. 2. /

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a n t i p a t h e t i c e l e m e n t s c a n be u s e d as indicators for the r a r e - m e t a l mineralized g r a n i t e s (Fig. 4), s u c h as very low K / R b (<100) a n d B a / R b (<0.5) as well as the high R b / S r (> 10). S u c h c h e m i c a l specialization for the albite g r a n i t e s u p p o r t s t h e earlier finding (Sabet eta/., 1976), w h i c h c o n s i d e r e d the Abu D a b b a b granite as one of t h e m o s t i m p o r t a n t Ta-Sn target in the c e n t r a l E a s t e r n Desert of Egypt. The m i n e r a l i z a t i o n o c c u r s in the g r a n i t e as fineg r a i n e d d i s s e m i n a t e d t a n t a l i t e (Kamel a n d El Tabbal, 1980), Ta-rich cassiterite, topaz a n d fluorite. Post m a g m a t i c a l t e r a t i o n s are r e p r e s e n t e d by series of f r a c t u r e - c o n t r o l l e d greisen a n d q u a r t z veins, in w h i c h the m o s t i m p o r t a n t ore m i n e r a l s 300C are cassiterite a n d woKramite.

I

"-I

I

~

AD 14

100

_

ILl

/-

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AD I"t

I,,-

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QD

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La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

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Yb

Lu

Fig. 5. Chondrite normalized rare-earth patterns for Abu Dabbab albite granite (AD 17), greisen vein (AD 14) and the pink granite of Nugrus (N 10) and Sikait (SR 2 I).

532

F.H. MOr~MED Table2. Recommendedvalues ~rdiscrimination between b a ~ e n - a n d r a r e me~l-bearinggranites. Element

Value characteristic for

or ratio

ore bearing granites

Reference

Rb

~ 500 ppm

Tischendorf,

Li

~ I00 ppm

Beus & Sitnin,

K/Rb

~I00

Tischendorf,

Mg/Li

~

Beus & Sitnin,

Ba/Rb

~

Rb/Sr

~

Acid Metasomatism

30 0.5 i0

(Greisenization)

The g r e i s e n o c c u r s as t h i n veins (10-50 cm) on t h e SE side of t h e g r a n i t e m a s s (Fig. 1B). The veins are p r e d o m i n a n t l y c o m p o s e d of q u a r t z a n d mica. They derived from the granite by H ÷ion m e t a s o m a tism, w h i c h destabilized earlier m i n e r a l s m a i n l y albite a n d p e r t h i t e a n d r e s u l t s in c h a n g i n g the c o m p o s i t i o n of mica. The new g e n e r a t i o n of minerals as revealed p e t r o g r a p h i c a l l y are quartz, Limica, topaz, fluorite a n d ore m i n e r a l s , chiefly cassiterite a n d wolframite. Q u a r t z forms large a n h e d r a l c r y s t a l s (=.4 ram) e m b e d d e d in s u b h e d r a l aggregates of m i c a a n d t o p a z with grain size from 0.02 to 1.4 m m . Occasionally topaz f o r m s excellent s t u b b y idiomorphic crystals. Fluorite is represented by small a n h e d r a l rare crystals. The above m e n t i o n e d mineralogical c h a n g e s m u s t have b e e n c o n c o m i t a n t with s i m u l t a n e o u s chemical c h a n g e s . G a i n s a n d losses of chemical elem e n t s d u r i n g greisenization can be evaluated u s i n g t h e c o m p o s i t i o n a l - v o l u m e d i a g r a m s of G r e s e n s ' (1967). He derived e q u a t i o n to express c h a n g e s of c h e m i c a l c o m p o n e n t s b e t w e e n original (albite granite) and metasomatically altered rocks (greisens). The e q u a t i o n is a s follows: AX = a [fv {gJg^) C B - C AI where, A X : loss or gain (in grams) of e l e m e n t n d u r i n g a l t e r a t i o n of rock A to B a : t h e initial q u a n t i t y , c o m m o n l y d e s i g n a t e d as 100 g so t h a t AXn b e c o m e s weight p e r c e n t fv : is the v o l u m e factor, ratio b e t w e e n the final a n d initial v o l u m e s of the rock m a s s g^.B : specific gravities of the fresh a n d altered rock respectively. CnA,B : weight f r a c t i o n s of c o m p o n e n t n in the p a r e n t rock a n d p r o d u c t rock respectively. To solve the e q u a t i o n the value of v o l u m e factor m u s t be k n o w n . C o m p o s i t i o n - v o l u m e (C-V) dia-

Olade, Ramsay,

1977 1968

1977 1968

1980 1986

g r a m s c a n be u s e d to d e d u c e a possible (fv) value. The (C-V) d i a g r a m s (Fig. 6) plot g a i n s a n d losses as a f u n c t i o n of t h e v o l u m e factor. The lines of elem e n t s t h a t r e m a i n e d immobile d u r i n g the alteration process will cross t h e zero m a s s e x c h a n g e axis (AXn = 0) at a p p r o x i m a t e l y the s a m e point. This allows a prediction of the fv value. The alteration of the albite g r a n i t e to the greisen is s h o w n in the c o m p o s i t i o n - v o l u m e d i a g r a m of the pair AID 17 -~ AD 3 (Fig. 6A). The c u r v e s for CaO, TiO2and Al,,Oa cross the z e r o - e x c h a n g e axis at a fv value b e t w e e n a b o u t 1.3 a n d 1.35. Hence, f f a v a l u e offv = 1.3 is a s s u m e d , there would be little gain or loss (relative immobility) of t h e s e t h r e e elements. This v o l u m e factor implies t h a t t h e greisenization is a c c o m p a n i e d by a v o l u m e i n c r e a s e of a b o u t 30 %. This is c o n s i s t e n t w i t h the field observation t h a t m a n i f e s t e d by a b u n d a n t e x t e n s i o n a l f r a c t u r e s a c c o m p a n y i n g the greisenization process. The steep slope of SiO 2 line i n d i c a t e s a d r a m a t i c increase of Si. Fluorine, Fe2Oa, MnO a n d MgO are a d d e d in lesser a m o u n t s t h a n silica, w h e r e a s K.20 a n d especially Na20 are removed in large a m o u n t s . The g r e i s e n ~ a t i o n e q u a t i o n t h a t is derived for fv = 1.3is 100 g of AD 17 + 4 1 . 7 g S i O 2 + O.79 Fe203 + 0.72 MnO + 0.03 Mg0 --, 140 g ofAD 3 + 3.0 g Na20 + 0.93 g K20 + 0.06 g A1203 + 0.03 CaO + 0.002 g TiO 2 The trace e l e m e n t d a t a (Fig. 6B) reveal a m a r k e d extent. Zinc is relatively immobile, while Nb a n d Ba are a p p a r e n t l y removed from the s y s t e m . These c h e m i c a l c h a n g e s are c o n s i s t e n t with petrographic observations. The significant d e c r e a s e s in Na20, K~O a n d Ba as well as the m a r k e d concomit a n t i n c r e a s e s in SiO 2, F, Li a n d Rb explained the d e s t r u c t i o n of albite a n d K-feldspar a n d the generation of quartz, Li-mica a n d topaz d u r i n g greisenization. Tin, Fe a n d Mn are g a i n e d by the introd u c t i o n of ore m i n e r a l s like cassiterite a n d wolframite. A l u m i n a liberated by the d e s t r u c t i o n of felds p a r s w a s fL,ced as newly f o r m e d m i c a a n d topaz.

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization ...

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Fig. 6. Compositional-volume diagram for major oxides (A) and trace elements (B) for the sample pair AD 17--~ AD 3 from Abu Dabbab albite granite. The mobility of Zr is revealed by t h e s e c o n d a r y g r o w t h of p y r a m i d a l faces of zircon c r y s t a l s at the e x p e n s e of t h e p r i s m a t i c ones.

Wolframite Compositions The wolframite in the greisen veins o c c u r s in l a m e l l a r form as b l a d e s up to several c e n t i m e t e r s long. Nine spot m i c r o p r o b e a n a l y s e s of six woKramite c r y s t a l s are given in Table 3. The c r y s t a l s were a n a l y z e d for Fe, Mn a n d W, whilst no o t h e r elem e n t s were f o u n d in detectable a m o u n t s . All the a n a l y s e s have m o l e c u l a r ratio of h u e b n e r i t e molecule (MnWO 4) exceeding 80 %, w h i c h indicates t h a t t h e s a m p l e s are typically h u e b n e r i t e . The individual a n a l y s e s exhibit a n a r r o w range of h u e b n e r i t e mole. % from 83 to 96. There is a c o n s i s t e n t t r e n d b e t w e e n individual c r y s t a l s w i t h r i m s h u e b n e r i t e - r i c h relative to cores. M a n y l i t e r a t u r e b e a r a t t e n t i o n to the u s e of h u e b n e r i t e / f e r b e r i t e ratio as a g e o t h e r m o m e t e r . However, t h e r e is a great d e b a t e c o n c e r n i n g t h i s subject. Most of t h e E u r o p e a n a u t h o r s (Oelsner, 1944; B o l d u a n , 1954) s u g g e s t e d a n increase of the H / F ratio with i n c r e a s i n g t e m p e r a t u r e of formation. The R u s s i a n w o r k e r s (Ganeev a n d S e c h i n a , 1960) proved the exact opposite of this concept. T h e y c o n c l u d e d t h a t t h e wolframite c r y s t a l s have Fe-rich core relative to the rims. Takla (1976) a n d

Takla e t al. (1977) in their s t u d y on E g y p t i a n wolframites are in general a g r e e m e n t with the E u r o p e a n opinion. These conflicting views lead m a n y w o r k e r s to disregard the H / F ratio ofwolframite as t e m p e r a t u r e d e p e n d e n t , b u t r a t h e r a function of the F e / M n ratio of the m i n e r a l i z i n g fluids ( G r o v e s a n d B a k e r , 1972: A m o s s e , 1978; Campbell a n d Petersen, 1988; Polya, 1988). Therefore, the c h e m i s t r y ( F e / M n ratio) of the h y d r o u s s y s t e m t h a t i n t r o d u c e d the wolframite in greisen veins could be a potential factor leading to c h e m i c a l z o n a t i o n of crystals. However, o t h e r factors s u c h as fluorine-ion c o m p l e x i n g could not be neglected. On the granitic m a g m a scale, Bailey (1977) a r g u e d t h a t fluorine could be a viable m e c h a n i s m for Fe a n d Mn s e p a r a t i o n by preferential complexing of Mn a n d its later precipitation. The s a m e m e c h a n i s m for F e / M n f r a c t i o n a t i o n is recorded in a m i n e r a l solid s o l u t i o n (Columbitetantalite) t h a t w a s i n t r o d u c e d as well in later s t a g e s of granite evolution (~ern~ e t al., 1986). T h u s , it s e e m s likely t h a t Fe-ion complexing of Mn h a d b e e n a significant factor t h a t controlled the c o m p o s i t i o n a l v a r i a t i o n (chemical zoning) in Abu D a b b a b woKramites. This m a y be s u p p o r t e d by the significant c o n c e n t r a t i o n s of Fe-rich m i n e r a l s like topaz a n d m i c a s in the g r e i s e n veins t h a t h o s t e d t h e wolframite. Moreover, the h u e b n e r i t e c r y s t a l s have Mn-rich r i m s (later formed) relative to core

534

F.H. MortaMED

Table 3.AnatysesofwolKamitecrystalsfromthegreisenveeinscutting theAbu Dabbabalbitegranite, Eastern DesertofEgypt. Chemical Composition ADP

-~mple

Crystal I

Crystal 2

AD49 Crystal I rim

core

AD14

Cxysr.al I

Crystal 2

Crystal 3

rim

core

rim

core

WO 3

75.34

75.79

75.04

75.51

74.66

75.15

75.39

75.16

75.44

MnO

19.55

19.38

20.13

20.03

20.72

22.05

19.79

20.41

19.15

FeO

5.11

4.83

4.83

4.47

4.62

2.80

4.83

4.43

5.41

Molecular Norms

H~bnerite

79.48

80.25

80.85

81.95

81.96

88.86

80.59

82.36

78.20

Ferberite

14.25

15.78

ii.37

12.58

8.41

3.79

13.36

10.43

16.06

Excess FeO

6.27

3.97

7.78

5.47

9.63

7.25

6.05

7.21

5.74

Molecular Ratio H~bnerite

84.80

83.57

87.67

86.69

90.69

95.91

85.78

88.76

82.96

Ferberite

15.20

16.43

12.33

13.31

9.31

4.09

14.22

11.24

17.03

5.60

5.09

6.51

9.74

23.45

6.03

7.90

4.87

H/F Ratio 7.il

t h a t are Fe-rich. The c o m p o s i t i o n a l v a r i a t i o n s of w o l f r a m i t e s c o u l d be a s well a f u n c t i o n d e p e n d e n t of the p h y s i c o c h e m i c a l c o n d i t i o n s (pH, T, etc.) of t h e greisenizing fluids. The H ÷ ions of the acidic h y d r o t h e r m a l s o l u t i o n s are c o n s u m e d b y the different greisenization reactions. This r e s u l t s in t h e g r a d u a l i n c r e a s e of t h e pH of s u c h s o l u t i o n s (Schwartz a n d Surj ono, 1990). Moreover, the experimental i n v e s t i g a t i o n s of G u n d l a c h a n d Thorm a n n (1960) s h o w t h a t t h e s e q u e n c e of t u n g s t a t e precipitation b y n e u t r a l i z a t i o n of silicotungstic acid a n d a s pH i n c r e a s e s is from ferberite (pH = 5.9) to h u e b n e r i t e (pH = 6.7) at 25°C. This t e m p e r a t u r e is indeed m u c h lower t h a n the t e m p e r a t u r e of greisenization process. However, the r e s u l t s of G u n d l a c h a n d T h o r m a n n (op. cit.) c o u l d be u s e d tentatively to explain the c h e m i c a l zonation in wolframite c r y s t a l s from A b u D a b b a b .

c o m p o n e n t in the granite genesis. Moreover, the p a t t e r n s have very low c o n c e n t r a t i o n s of e l e m e n t s characteristic to continental crustal s o u r c e s (Brown et al., op. cit.) like Th, K, La, Ce, S m a n d Tb. This reflects the significant role of alkaline-mantle source in the granite evolution. C o m p a r e d to t h e albite granite, the pink granite (Fig. 8) h a s higher concent r a t i o n s of Ba, Th, K, La0 Ce, S m a n d Tb a s well a s lower a b u n d a n c e of Ta a n d Nb. T h u s , t h e c r u s t a l involvement a n d / o r LIL-element e n r i c h e d m a n t l e ( s u b d u c t i o n zone) c o m p o n e n t s are m o r e signific a n t in the p i n k granite generation. The p a t t e r n of the p i n k granite r e s e m b l e s t h o s e of g r a n i t e s from m a t u r e arc (Brown et al., op. cit.) with m a r k e d negative a n o m a l i e s in Sr, P a n d Ti. The c o n t r a s t i n g tectonic setting, p e t r o g r a p h y a n d g e o c h e m i c a l s i g n a t u r e s of the albite g r a n i t e s a n d the pink g r a n i t e s (Figs 4-9) n e c e s s i t a t e two different m o d e l s for generation.

GENETIC MODEL Albite G r a n i t e

The A b u D a b b a b albite granite is d i s c r i m i n a t e d a s a w i t h i n - p l a t e granite (Fig. 7). O n the contrary, t h e p i n k g r a n i t e s are LIL-enriched a n d affiliated to t h e p o s t - t e c t o n i c C a l e d o n i a n I-type granite according to t h e classification s c h e m e of Pitcher (1987). Their p r e s e n t a t i o n in m o r e t h a n one field (Fig. 7) reflects the post-collision setting, w h i c h c o u l d n o t be d i s c r i m i n a t e d in this d i a g r a m (Pearce eta/., 1984). P r i m o r d i a l - m a n t l e n o r m a l i z e d p a t t e r n s for the albite granite (Fig. 8) exhibit high a b u n d a n c e of Ta a n d Nb, which, a c c o r d i n g to B r o w n et al. (1984) is indication for i n c o r p o r a t i o n of within-plate m a n t l e

Albite-rich g r a n i t e s a s s o c i a t e d with r a r e - m e t a l mineralization m a y have either a m e t a s o m a t i c or a m a g m a t i c origin. S a b e t etal. (1976) a n d Kamel a n d E1 T a b b a l (1980) are inclined to a m e t a s o m a t i c origin f o r A b u D a b b a b granite a n d related mineralization. However, its m a g m a t i c origin is s u p p o r t e d b y field, p e t r o g r a p h i c a n d g e o c h e m i c a l f e a t u r e s like: The s h a r p intrusive c o n t a c t of t h e granite with the c o u n t r y r o c k s a n d the a n g u l a r b r e c c i a t e d i n c l u s i o n s of the latter within t h e granite along t h e contact.

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization ...

535

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Fig. 7. Rb versus (Nb + Y) plot from Pearce e t al. (1984). The inset shows the Rb vs (Ta + Yb) plot. Stippled area is the field ofAbu Rusheid apogranite (Mohamed, 1989). Rock symbols as in Fig. 2.

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Fig. 8. Primordial mantle-normalized patterns for Abu Dabbab albite granite and Nugrus pink granite, normalized values after Wood, (1979). No relics of the original or p a r e n t granite t h a t s h o u l d be m e t a s o m a t i c a l l y altered to the rarem e t a l b e a r i n g granite. The c h a r a c t e r i s t i c "snow ball"-quartz t e x t u r e (Pollard, 1989) of the granite, w h i c h indicates t h e s i m u l t a n e o u s c r y s t a l l ~ a t i o n of q u a r t z a n d albite from t h e melt. The relative c h e m i c a l h o m o g e n e i t y w i t h i n the g r a n i t e a n d t h e r e g u l a r REE p a t t e r n .

The albite granite c o n s i s t s m a i n l y of q u a r t z a n d f e l d s p a r a n d the plagioclase is chiefly albite. Therefore, the s y s t e m Qz-Ab-Or-H20 is a good a p p r o x i m a t i o n of the albite g r a n i t e s y s t e m . CIPW n o r m a t i v e c o m p o s i t i o n s for the g r a n i t e are plotted in the Qz-Ab-Or triangle (Fig. 9), t o g e t h e r with the m i n i m u m c o m p o s i t i o n s for 0.5 to 5 kb w a t e r p r e s s u r e (Winkler e t al., 1975) a n d the m i n i m u m c o m p o s i t i o n s for 1 kb total fluid p r e s s u r e in the

536

F. H. MOHAMED

p r e s e n c e of 1, 2 a n d 4 wt. % F (Manning, 1981). One effect of the addition of F to t h e haplogranitic s y s t e m is to shift the m i n i m u m t o w a r d s the albiterich c o m p o s i t i o n s (Manning, op. cit.). The enrichm e n t of t h e granite in H F S e l e m e n t s a n d F reflects its crystallization from dry-F-rich m a g m a relative to the p i n k granites. Therefore, the albite granite is e x p e c t e d to e m p l a c e at shallower d e p t h s (lower h y d r o u s p r e s s u r e ) t h a n the p i n k granites, b u t plots of the albite granite are shifted t o w a r d s higher h y d r o u s total p r e s s u r e (or t o w a r d s Ab apex), (Fig. 9). This c o u l d be a t t r i b u t e d to t h e significant role of fluorine in shifting the A b u D a b b a b granite m i n i m u m - m e l t - c o m p o s i t i o n s t o w a r d s the albite apex. Most of t h e d a t a p o i n t s of the albite granite plot close to t h e m i n i m a at 1 kb for F c o n t e n t s of b e t w e e n 0 . 7 to a b o u t 1%. A l t h o u g h the F c o n t e n t s of t h e granite are lower t h a n t h e s e v a l u e s , the p r e s e n c e o f F - e n r i c h e d greisen veins (Table 1) m o s t p r o b a b l y indicate late- to p o s t - m a g m a t i c loss ofF. Therefore, the granite c o u l d have crystallized at total p r e s s u r e of a b o u t 1 kb. The significant role of fluorine in the albite granite g e n e s i s is clearly m a n i f e s t e d by: Its trace e l e m e n t c h e m i c a l signature. It h a s high a b u n d a n c e of F u p to 5 8 4 7 ppm. Moreover, its REE p a t t e r n s h o w s H R E E - e n r i c h m e n t (Fig. 5) a n d the granite c o n t a i n s a n o m a l o u s c o n c e n t r a t i o n s of H F S e l e m e n t s like Ta, Sn, Nb a n d W. The role of F to form high c o o r d i n a t i o n c o m p l e x e s with HREE (Mineyev01963) a n d o t h e r H F S e l e m e n t s (Collins et al., 1982) is well d o c u m e n t e d . This r e s u l t s in the e n r i c h m e n t of t h e s e e l e m e n t s d u r i n g progressive crystallization. The greisen veins c r o s s c u t t i n g the granite a n d

w h i c h have very high c o n c e n t r a t i o n s of F u p to 2.3 % (Table 1) a n d a b u n d a n t F - e n r i c h e d m i n e r a l s like topaz a n d fluorite. In addition, t h e greisen is c h a r a c t e r i z e d b y higher a b u n d a n c e of H R E E t h a n t h e granite (Fig. 5). This reflects the existence of f l u o r o - c o m p l e x e s of H R E E a n d their later destabilization b y h y d r o l y s i s (Moeller, 1972) during the late s t a g e s o f m a g m a t i c crystallization a n d greisen formation. The p r o n o u n c e d negative Eu a n o m a l i e s exhibited b y the granite c o m p a r a b l e with t h e w e a k ones of the a s s o c i a t e d g r e i s e n v e i n s (Fig. 5). This reflects the Eu fractionation b y the action of halogen-rich volatile p h a s e s (Flynn a n d B u r n h a m , 1978; Hildreth, 1981) t h a t lead to deplete Eu in silicic m a g m a s a n d its e n r i c h m e n t in t h e crystallization p r o d u c t s like pegmatitic v e i n s (Fowler a n d Doig, 1983}. The albite granite exhibits c h a r a c t e r i s t i c s of Atype m a g m a with a significant c o n t r i b u t i o n from within-plate m a n t l e s o u r c e (Figs 7, 8). A-type m a g m a t i s m s in Africa are of two time-tectonic a s s o c i a t i o n s (Bowden, 1985). One a s s o c i a t i o n linked tectonically with progressive uplift longt e r m d o m i n g a n d s u b s e q u e n t rifting, h a v i n g periodic or c o n t i n u o u s m a g m a t i s m , e.g., the A-type y o u n g e r granite of Nigeria. The o t h e r association developed in v a r i o u s mobile z o n e s during the late p h a s e s of the Pan-African orogeny. The A b u D a b b a b granite is a n e x a m p l e of this s e c o n d association. Its e m p l a c e m e n t is controlled b y m a j o r s t r u c t u r a l trends, w h i c h initiated d u r i n g late Proterozoic- early Paleozoic t i m e s (Garson a n d Krs, 1976). The granite is aligned along the W. A l h a m d s h e a r zone of NW t r e n d a n d at its i n t e r s e c t i o n with

Q

Q O o

/ I

Ab

Or

Fig. 9. Compositions of the Abu Dabbab albite granite and the pink granites of Nugrus and Sikait plotted in the quartz-albite-orthoclase "granite" system. Rhombs represent minimum melt compositions at PH2o = Pctota~=0.5, 1 . 2 . 4 , 5 kb (Whinkler et al., 1975). Asterisks indicate minimum melt compositions at 1 kb with excess H20 and 1, 2 and 4 % F (Manning, 1981).

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization ... a m a j o r d e e p - s e a t e d fault ofN 60°E t r e n d (Fig. 1A). S o m e r a r e - m e t a l b e a r i n g g r a n i t e s in t h e E a s t e r n D e s e r t have similar setting. This p e c u l i a r s t r u c t u ral setting m a y explain the derivation of A-type m a g m a in the N u b i a n Shield. The reactivation of such major structural weaknesses could cause a d i a b a t i c d e c o m p r e s s i o n a n d melting at t h e lower c r u s t - u p p e r m a n t l e interface {Greenberg, 1981; Harris e t a / . , 1986). The involvement of regions from t h e m a n t l e , w h i c h have not b e e n h y d r a t e d b y s u b d u c t i o n p r o c e s s e s d u r i n g melting, could yield m a g m a of w i t h i n - p l a t e c h a r a c t e r s . Pink

Granites

The m a j o r i t y of t h e p o i n t s for t h e p i n k granites are plotted slightly below the m i n i m a t r e n d a n d t o w a r d s slightly Or-enriched, Qz-depleted c o m p o sitions, c o r r e s p o n d i n g to c r y s t a l l ~ a t i o n in the P(H20) r a n g e f r o m 0 . 5 to a b o u t 2 kb (Fig, 9). However, few a n a l y s e s plot in a t r e n d t o w a r d s the Qz-Ab j o i n t h a t m o s t p r o b a b l y reflects the role of h y d r o t h e r m a l alteration p r o c e s s e s (e.g., sericitization). The p i n k g r a n i t e s are located within a m a j o r s h e a r zone n a m e l y "Nugrus shear" (Figs 1A, C) a n d along a low-angle t h r u s t (Greiling et al,, 1987). Following t h r u s t i n g a n d c r u s t a l thickening, melting c o u l d i n d u c e at lower c r u s t d u e to t h e r m a l relaxation a n d at u p p e r m a n t l e d u e to a d i a b a t i c d e c o m p r e s s i o n t h a t a c c o m p a n i e s uplift (Harris et al., 1986). The selective e n r i c h m e n t of the p i n k g r a n i t e s in the LIL-elements c o u p l e d with the low a b u n d a n c e of H F S e l e m e n t s implies a s o u r c e in the LIL-element fluxed m a n t l e wedge overlying s u b d u c t i o n zone. However, s u b d u c t i o n n e e d n o t be active at t h e time of melting, b u t the m a n t l e m u s t still be h y d r a t e d from a p r e c e d i n g period of s u b d u c t i o n (Harris, 1985). A m a n t l e s o u r c e region m a y b e s u p p o r t e d b y low initial Sr87/SrS6 ratios ( 0 . 7 0 2 - 0 . 7 0 3 ) for t h e E g y p t i a n y o u n g e r p i n k g r a n i t e s (Fullagar, 1980; Greenberg, 1981). The siliceous n a t u r e of the p i n k granite a n d the enrichm e n t in K, Rb, Ba a n d LREE (Fig. 6) does not p r e c l u d e a lower c r u s t a l involvement in the melt. The similarity b e t w e e n the p a t t e r n s of the pink granites a n d t h o s e of m a t u r e arc granitoids (Browen e t a / . , 1984) c o u p l e d with the e n h a n c e d levels of Nb, to be t r a n s i t i o n a l b e t w e e n volcanic arc a n d within-plate m a g m a s (Pearce a n d Gale, 1977) m a y reflect c h a n g e s within the h y d r a t e d - m a n t l e s o u r c e d u r i n g t h e t i m e - s p a n b e t w e e n the e m p l a c e m e n t of the s u b d u c t i o n - r e l a t e d older granitoids (710-610 Ma) a n d the p o s t - o r o g e n i c p i n k g r a n i t e s (600-540 Ma). S u c h c h a n g e s of m a n t l e s o u r c e could r e s u l t from the i m p a r t r e m o v a l of the s u b d u c t i o n - r e l a t e d g e o c h e m i c a l fingerprints d u e to m a n t l e convection o r d e e p e r m a n t l e regions are being involved (Harris, 1985).

537

CONCLUSION

The m a j o r a n d trace e l e m e n t c o m p o s i t i o n s clearly classified the albite granite a s a mineralized granite, while t h e p i n k g r a n i t e s are b a r r e n . S o m e e l e m e n t s a n d e l e m e n t a l ratios are good i n d i c a t o r s for rare-metal b e a r i n g potential like: the high levels of Ta, Sn, Nb, W, F, Rb, Li c o m p e n s a t e d b y extremely low levels of Ca, Ti, Mg a n d c o u p l e d with low ratios of K / R b , Mg/Li, B a / R b a n d high v a l u e s of R b / S r ratio. Fluorine w a s an i m p o r t a n t c o m p l e x i n g a n i o n in t h e g e n e s i s of t h e albite granite a n d the a s s o c i a t e d r a r e - m e t a l m i n e r a l ~ a t i o n . This is e v i d e n c e d b y (i) high c o n c e n t r a t i o n s of F a n d H F S elements, (ii) strong negative Eu a n o m a l i e s c o m p e n s a t e d b y H R E E e n r i c h m e n t , (iii) shifting of the granite m i n i m u m melt c o m p o s i t i o n s t o w a r d s t h e albite granite reflect a significant c o n t r i b u t i o n from a n alkali m a n t l e source. Therefore, the geochemical c h a r a c t e r i s t i c s of the p i n k g r a n i t e s a n d the albite granite m o s t p r o b a b l y reflect g r a d u a l c h a n g e s in m a n t l e s o u r c e regions with time from s u b d u c t i o n enriched- to within p l a t e - m a n t l e s o u r c e s .

Acknowledgements

- I gratefully acknowledge all colleagues from the University of Cologne and Technical University of Berlin who provided help with the analytical work. Thanks to Prof. Y. M. Anwar, Alex. Univ. and Dr. G. Matheis, Tu Berlin, for critically reading the manuscript. REFERENCES

Amosse, J. 1978. Variations in wolframite composition according to temperature, at Borralha, Portugal and Enguyales, France. Econ. Geol. 73, 1170-1175. Asran, M. H. A. 1985. Geolo/=~-,petrography and geochemistry of the apogranites at Nuweibei and Abu Dabbab areas, E. D., Egypt. M. Sc. thesis, Sohag, Assiut Univ., 149 p. Bailey, J. C. 1977. Fluorine in granitic rocks and melts: A review. Chem. Geol. 19, 1-42. Beus, A. A. 1970. Metasomatic zoning in deposits of rare elements of the albitite formation. In: Problems of Hydrothermal OreDepositiorc (Edited by Pouba, Z. and Stemprok, M.), E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, 325-329. Beus, A. A. and Sitnin, A. A. 1968. Geochemical specialization of magmatic complexes as criteria for the exploration of hidden complexes. Proc. XXIII Intern. Geol. Congr. Prague 6, 101-105. Bolduan, H. 1954. Genetische Untersuchung der wolfrmnittagertaette (Vogtland) unter besonderer Beruecksichtigung der Verteilung des H / F Koeffizienten und der Spurenelemente Niob and Tantal im Wolframit. Freiberg. Forschungsh. C 10, 46-61. Econ. Geol. 32, No. 224. Bowden, P. 1985. The geochemistry and mineralization of alkaline ring complexes in Africa (a review). J. Aft. EarthSct 3, 17-39.

538

F. H. MOIIAMED

Brown, G. C., Thorpe, R. S. and Webb, P. C. 1984. The geochemical characteristics of granltoids in contrasting arcs a n d c o m m e n t s on m a g m a sources, d. Geol. Soc. London, 1 4 1 , 4 1 3 - 4 2 6 . Campbell, A. a n d Petersen, U. 1988. Chemical zoning in wolframite from S a n Cristobal, Peru. Mineral. Deposita, 23, 132-137. ~ern~, P, Goad, B. E., Hawthorne, F. C. and C h a p m a n , R. 1986. Fractionation trends of Nb- and Ta-bearing oxide minerals in the Greer Lake pegmatitic granite and its pegmatite aureole, S o u t h e a s t e r n Manitoba, Amer. Miner. 7 1 , 5 0 1 - 5 1 7 . Collins, W. J., Beams, S. D., White, A. J. R. and Chappell, B.W. 1982. Nature a n d origin of A-type granites with particular reference to S o u t h e a s t e r n Australia. Contrib. Mineral Petrol. 80, 189-200. Debon, F. a n d Le Fort, P. 1986. Une classification c h i m l c o m i m i n 6 r a l o g i q u e des r o c h e s p l u t o n l q u e s c o m m u n e s et de leurs associations. M6thode et applications. French translation f r o m Ciencias d a Terra (Brazil). E1Ramly, M. F. a n d Akaad, M. K. 1960. The b a s e m e n t complex in the CED of Egypt between Lat. 24 ° 36' and 25 ° 40'. Geol. Surv. Egypt, paper No. 8, 33 p. E1Shazly, E. M. a n d Hassan, M. A. 1972. Geology and radioactive mineralization at Wadi Sikait-Wadi EIGemal area, E a s t e r n Desert, Egypt. J. Geol. 16, 201234. Flynn, R. T. a n d B u r n h a m , C. W. 1978. An experimental determination of rare earth partition coefficients between a chloride containing vapor phase and silicate melts. Geochim. Cosmochim. Acta 42, 685-701. Fourcade, S. a n d Allegre, C. J. 1981. Trace element behaviour in granite genesis: a case study, the calcalkaline plutonic a s s o c i a t i o n from the Qu6rigut Complex (Pyr6n6es, France). Contr. Miner. Petrol. 76, 177-195. Fowler, A. D. and Doig, R. 1983. The significance of Eu anomalies in the REE spectra of granites and pegmatites, Mr. Laurier, Quebec. Geochim. Cosrnochirrc Actor 47, 1131-1137. Fullagar, P. D. 1960. Pan-African age granites of North E a s t e r n Africa: New or reworked sialic materials ? In: Salem, M. J. a n d Busrewil, M. T. (eds.), Geology of Libya. Second s y m p o s i u m on the geology of Libya 3, Academic Press, New York, 105 i- 1058. Ganeev, I. G. a n d Sechina, N. P. 1960. Geochemical peculiarities ofwolframites. Geochem. 6, 617-623. Garson, M. a n d Krs, M. 1976. Geophysical and geological evidence of the relationship of Red Sea transverse tectonics to ancient fractures. Geol. Soc. Amer. Bull. 87, 169-181. Gass, I. G. 198 I. Pan-African (upper Proterozoic) plate tectonics of the Arabian-Nubian Shield. In: Kroener, A. (ecl), Precambrian Plate Tectonics, Elsevier, Amsterdam, 387-405. Geological m a p of Aswan quadrangle, scale 1:500000. 1978. Geol. Surv. Egypt, Cairo. Greenberg, J. K. 1981. Characteristics and origin of Egyptian y o u n g e r granites: s u m m a r y . Geol. Soc. Amer. Bull. I. 92, 224-232.

Greiling, R. O., Kroener, A., El Ramly, M. F. a n d R a s h w a n , A. A . 1987. S t r u c t u r a l r e l a t i o n s h i p s between the s o u t h e r n and central parts of the Eastern Desert of Egypt: Details of a fold and t h r u s t belt. In: El Gaby, S. a n d Greiling, R, (eds.), The Poxt-Afficox~ beltof NE-Africa a n d adjacent areas-Tectonic evolution and Economic aspects o f a late Proterozoic orogen. Vieweg, Wiesbaden, 212-145. Gresens, R. L. 1967. Composition-volume relationships of m e t a s o m a t i s m . Chem. GeoL 2, 47-65. Groves, D. I. and Baker, W. E. 1972. The regional variation in composition ofwolframites from Tasmania. Econ. Geol. 67, 362-368. Gundlach, H. and T h o r m a n n , W. 1960. Versuch einer D e u t u n g der E n t s t e h u n g yon Wolfram-und Zinnlagerstaetten. Zeilschr. Deutschen Geol. Gesell. 112, 135. Harris, N. B. W. 1985. Alkaline complexes from the Arabian Shield. J. Afr. Earth ScL 3, 83-88. Harris, N. B. W. and Marriner, G. F. 1980. Geochemistry and petrogenesis ofa peralkaline granite complex from the Midian Mountains, Saudi Arabia. Lithos. 1 3 , 3 2 5 337. Harris, N. B. W., Pearce, J. A. and Tindle, A. G. 1986. Geochemical characteristics of collision-zone m a g m a tism. In: Coward, M. P. and Ries, A. C. (eds.), Special publication of the Geological Society London, 67-81. Hassan, M. and Hashad., A. H. 1990. Precambrian of Egypt. In: Said, R. (ed.), The Geology o f Egypt. Balkema, Rotterdam, Brookfield, 201-245. Hildreth, E. W. 1981. Granitoids in silicic m a g m a chambers : implications for lithostatic m a g m a t i s m . J. Geophys. Res. 86, B11, 10153-10192. Hussein, A. A. 1990. Mineral deposits. In: Said, R. (ed.). The Geology of Egypt. Balkema, Rotterdam, Brookfield, 511-566. Kamel, O. A. and E1Tabbal, H. K. 1980. Petrology and mineralogy of Nuweibi and Abu D a b b a b rare metal apogranites, Eastern Desert, Egypt. Proc. Geodynamic Evolution of the Afro-Arabic Rift S y s t e m , Atti. Cony. Lincei. 47, 685-705, Roma. Kroener, A., Greiling, R., Reischmann, T., Hussein, I. M., Stern, R. J., Duerr, S., Krueger, J. and Zimmer, M. 1987. Pan-African crustal evolution in the Nubian segment of n o r t h e a s t Africa. In: Kroener, A. (ed.), Proterozoic Lithospheric Evolution. Am. Geophys. Union, Geodynamics Series 17, Washington, 235-257. Krs, M., Soliman, A. A. H. and Amin, A. H. 1973. Geophysical p h e n o m e n a over deep-seated tectonic zones in s o u t h e r n part of E a s t e r n Desert of Egypt. Ann. Geol. Surv. Egypt 3, 125-138. Manning, D. A. C. 1981. The effect of fluorine on liquid us p h a s e relationships in the s y s t e m Qz-Ab-Or with excess water at ikb. Conirib. Mineral. Petrol 76, 206215. Mineyev, D. A. 1963. Geochemical differentiation of the rare earths. Geokhimiya 12, 1082-1 I00. Mohamed, F. H. 1989. Geochemical prospecting in Nugrus area, S o u t h e r n E a s t e r n Desert, Egypt. Ph.D thesis, Alex. Univ., 201 p. Moeller, T. 1972. Complexes of the lanthanides. In: Emdens, H. J. and Braynall, K. W. (eds.), MTPlnternalionaI R e v i e w o f S c i e n c e lnorganic ChemistryT, Butterworth Univ. Press London, 275-298.

Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization ... Oelsner, O. 1944. U p p e r Erzgebirgische Wolframite Ber. Freib. Geol. Ges. 20, 44-49. Olade, M. A. 1980. G e o c h e m i c a l characteristics of tinb e a r i n g a n d t i n - b a r r e n granites, n o r t h e r n Nigeria. Econ. Geo/. 7B, 71-82. Pearce, J. A. a n d Gale, G. H. 1977. Identification of oredeposition e n v i r o n m e n t from t r a c e - e l e m e n t geochem i s t r y of a s s o c i a t e d igneous h o s t rocks. In: Volcanic Processes in Ore Genesis. Spec. Publ. Inst. Min. Metall. a n d Geol. Soc. London, 7, 14-24. Pearce, J. A., Harris, N. B. W. a n d Tindle, A. G. 1984. T r a c e e l e m e n t discrimination d i a g r a m s for the tectonic i n t e r p r e t a t i o n of granitic rocks. J. Petrol. 25, 956983. Photogeological m a p of S o u t h e a s t e r n Desert, Egypt. Scale 1:100000. 1967. H u n t i n g Geology a n d Geophysics Ltd. Pitcher, W. S. 1987. Granites a n d yet m o r e granites forty y e a r s on. Geologische R u n d s c h a u 76, 51-79. Pollard, P . J . 1989. Geologic characteristics a n d genetic p r o b l e m s a s s o c i a t e d with the d e v e l o p m e n t of granitehosted deposits of t a n t a l u m a n d niobium. In: Moeller, P., Cerny, P. a n d S a u p e , F. (eds.) Lanthanides, Tantalum a n d Niobium: Mineralogy, geochemistry, characteristics o f primary ore deposits, prospecting, processing a n d applications, Springer-Verlag, Berlin, 240-256. Polya, D.A. 1988. Compositional Variation in wolframites from the B a r r o c a G r a n d e mine, Portugal: evidence for fault-controlled ore formation, Min. Mag. 5 2 , 4 9 7 503. R a m s a y , C. R. 1986. Specialized felsic plutonic rocks of the A r a b i a n Shield a n d their p r e c u r s o r s . J. Aft. Earth ScL 4, 153-168. Rogers, J. J. W. a n d Greenberg, J. K. 1981. Trace e l e m e n t s in c o n t i n e n t a l - m a r g i n m a g m a t i s m : Part 3. Alkali granites a n d their relationship to cratonization. Geol. Soc. Arr~ Bull. pt. 1, 92, 6-9.

539

Sabet, A. H., Tosgoev, V. I3., Sarin, L.P., Azzazi, S. A., E1 Bedewi, M.A. a n d Ghobrial, G. A. 1976. T i n - t a n t a l u m deposits ofAbu Dabbab. Ann. Geol. Surv. Egypt 6, 93117. Schwartz, M. O. 1992. Geochemical criteria for distinguishing m a g m a t i c a n d m e t a s o m a t i c albite-enrichm e n t in granitoids- e x a m p l e s from the Ta-Li granite Yichun [China} and the Sn-W deposit Tikus (Indonesia). Mineral. Deposita 27, 101-108. Schwartz, M. O. and Surjono. 1990. Greisenization and a l b i t i z a t i o n a t the T l k u s t i n - t u n g s t e n d e p o s i t , 13elitung, Indonesia. Econ. Geol. 8S, 691-713. Stern, R. J. a n d Hedge, C. E. 1985. Geochronologic and isotopic c o n s t r a i n s t s on late P r e c a m b r i a n crustal evolution in the E a s t e r n Desert of Egypt. Amer. J. Sci. 28S, 97- 127. Takla, M. A. 1976. Elecron m i c r o p r o b e of zoned Wolframite from Elba, Egypt, N. Jb. Miner. Mt~ 1 1 , 4 7 7 - 4 8 3 . Takla, M. A., Sharkawi, M. A. a n d Fawzi, E. 1977. The g e o c h e m i s t r y of Egyptian wolframites. Chem. Erde, 36, 10-19. Tischendorf, G. 1977. G e o c h e m i c a l a n d petrographic characteristics of silicic m a g m a t i c rocks associated with r a r e - m e t a l mineralization. In: S t e m p r o k , M., Burnol, L. a n d Tischendorf, G. (eds.) MetaUization Associated with Acid Magmatism 2, Usterdni Ustav Geologicky, Prague, 41-98. Turekian, K. K. a n d Wedepohl, K. H. 1961. Distribution of the e l e m e n t s in s o m e m a j o r units of the earth's crust. Geol. Soc. Amer. Bull- 72, 175- 192. Winkler, H. G. F., Boese, M. a n d Marcopoulos, T. 1975. Low t e m p e r a t u r e granitic melts. N. Jb. Miner. Mtu 6, 245-268. Wood, D. A. 1979. Avariably-veined s u b - o c e a n i c u p p e r mantle: genetic significance for mid-ocean ridge basalts from geochemical evidence. Geology 7, 499-503. Wright, J. 13. 1969. A simple alkalinity ratio a n d its application to q u e s t i o n s of n o n - o r o g e n i c granite genesis. Geol. Mag. 106, 370-384.