Mineralogy and geochemistry of Gabal El-lneigi Granite and ...rjstern/egypt/PDFs/CE...

Journal of African Earth Sciences, Vol. 32, No. 1, pp. 29-45. 2001 0 2001 Elsevier Science Ltd

Pll:SO699-6362(00)00021-6 All rights reserved. Printed in Great Britain

0899-5362/01 s- see front matter

Mineralogy and geochemistry of Gabal El-lneigi Granite and

associated fluorite veins, Central Eastern Desert, Egypt: application of fluid inclusions to fluorite genesis

LA. SALEM’, A.A. ABDEL-MONEUM2, A.G. SHAZLY’and N.H. EL-SHIBINY3 ‘Geology Department, Faculty of Science, Tanta University, Tanta, Egypt

*Nuclear Materials Authority, Cairo, Egypt 3National History Department, Faculty of Education (Kafer El-Sheikh),

Tanta University, Tanta, Egypt

ABSTRACT-Geological, mineralogical, geochemical and fluid inclusion studies were carried out on both the granitic rocks at the Gabal El-lneigi Pluton and associated fluorite veins in order to examine their genetic relations. Gabal El-lneigi rocks range from adamellite to granite composition. They orginated from metaluminus talc-alkaline magma having strong alkaline tendencies. They have similar characteristics to l-type granites and were probably generated within an extensional environment due to crustal relaxation during a post-collision episode ( < 600 Ma). Studies of fluid inclusions from vein fluoriie and quartz show that they are aqueous with phases (L + VI and that secondary inclusions predominate. The fluoriie mineralisation probably took place at temperatures of > 250°C; the fluid salinities ranged up to 21.4 equiv. wt% NaCI. The quartz veins were formed at lower temperatures (- 1 20°C) and fluid salinites ranging up to 10.36 equiv. wt% NaCI. Rare earth element abundances in fluorite are variable and the relation between Tb/Ca versus Tbl La confirms a hydrothermal origin for fluorite. The negative Ce anamolies indicate high 0 fugacities at the source of the hydrothermal fluids. The negative Eu anomalies suggest equilibration of the hydrothermal fluids with the host granites. o 2001 Elsevier Science Limited. All rights reserved.

RESUME-Une etude geologique, mineralogique, geochimique et des inclusions fluides a 6th entreprise a la fois sur les granites du pluton de Gabal El-lneigi et sur les filons de fluorite associes dans le but de comprendre leurs relations genetiques. Le pluton de’ Gabal El-lneigi comprend des monzogranites et des syenogranites. Ces derniers proviennent d’un magma calco-alcalin metalumineux a forte affinite alcaline. Ils possedent les caracteristiques des granites de type I et ont probablement 6te engendres dans un contexte extensif du a la relaxation crustale lors d’un episode post-collisionnel ( < 600 Ma). L’etude des inclusions fluides des filons de fluorite et de quartz indique qu’elles sont aqueuses avec les phases (L + V). Les inclusions secondaires dominent. La mineralisation en fluorite s’est probablement produite a une temperature > 250°C. la salinite des fluides atteignant des valeurs de 21.4 equiv. %poids NaCI. Les filons de quartz se sont form& a des temperatures plus basses de I’ordre de 1 20°C, la salinite des fluides atteignant des valeurs de 10.36 bquiv. %poids NaCI. Les concentrations en terres rares dans la fluorite sont variables et la relation entre les rapports Tb/ Ca et Tb/La confirme I’origine hydrothermale de ce mineral. Les anomalies negatives en Ce indiquent des fugacites en 0 Blevees pour la source des fluides hydrothermaux. Les anomalies negatives en Eu suggerent une reequilibration des fluides hydrothermaux avec le granite h&e. @ 2001 Elsevier Science Limited. All rights reserved.

(Received 16/l O/98: accepted 9/9/00)

Journal of African Earth Sciences 29

LA. SALEM et al.

INTRODUCTION The Egyptian Basement Complex of the Eastern Desert is dominated by granitoid rocks, which belong to two main phases of the Pan-African Orogeny. The older group (685-610 Ma: Stern and Hedge, 1985) comprises syn- to late tectonic granitoids forming batholithic masses that exhibit wide compositional variations (trondhjemites to granodiorites with minor granites). The younger group (600-540 Ma: Stern and Hedge, op. ~a.1 comprises post-tectonic pluton to stock-sized granitic bodies, generally rich in K-feldspars and sometimes associated with rare metal mineralisation.

The Gabal El-lneigi Pluton (571 Ma: El-Manharawy, 1977) is located in the Central Eastern Desert of Egypt at some 80 km from Mersa Alam along the road to Idfu, which runs about 10 km to the south of the pluton. It is bounded by latitudes 25O12’20” and 25”14’50”N and longitudes 34OO4’23” and 34O 10’OO”E (Fig. 1). It covers an area of - 23 km2, com- prising numerous isolated hillocks of which Gabal El- lneigi is a cone-like body representing the highest peak (985 m) in the area.

Field studies revealed that Gabal El-lneigi Pluton has the following geological features:

i) The contacts of the Gabal El-lneigi Pluton with the metagabbros exposed at the southweastern part

of the pluton are sharp and without thermal effects. In the western part, the pluton shows an intrusive relationship with the older granites. No xenoliths from the country rock envelopes are observed within El- lneigi Granites.

iiJ The Gabal El-lneigi Pluton is dissected by joints generally trending in the N20°W and N70“E directions.

iiil The Gabal El-lneigi Pluton is traversed by few post-granite dykes. These are generally basic in composition and possess a general N200W trend. The contacts between these dykes and the granites are generally sharp. They have a uniform thickness of usually between 3 and 5 m and extend in length for more than 100 m.

iv) Quartz veins are developed within the north and southeastern parts of the pluton. They range from 0.5 to 5.0 m in thickness and are more than 150 m long. The general trend of these veins is N80°W, and they are mostly vertical. Some of the quartz veins show appreciable wall rock alteration; while further away from the contacts, weakly altered zones pass gradually into fresh granite. The alteration zones vary in thickness from 0.3 to 0.6 m. Some of the quartz veins contain fluorite and sulphide mineralisation.

v) The fluorite deposits of the Gabal El-lneigi Pluton consist mainly of fluorite-quartz veins; however, rare pure fluorite veins cutting the granitic mass are also

Legend 25”12’17”

lIZI Wadi deposits

El

- a) Observed

Younger granites __--- bl Approximate Faults

El

..,._-.-. c) inferred

Older granites

El F Dvkes

Metagabbros + Drainage lines

Ei Serpentinites A Triangulation points

EQI Metavolcanics 01 2 3 km 1 - I

figure 1. Geological map (Scale 1:40 0001 of El-lneigi Huton, Eastern Desert, Egypt (after El-Shibiny, 1995).

30 Journal of African Eerth Sciences

Mineralogy and geochemistry of Gabal El-lneigi Granite and associated fluorite veins

observed. These veins represent fillings of open, large and minute fissures developed mainly along fault planes. The fluorite veins occur mainly in the southern and western sectors of the pluton. The fluorite veins are regular but vary in thickness from 0.5 to 5.0 m and in length from 20 to more than 300 m. They trend mainly N70“E and dip 88’to the northwest or south- east. Vertical veins are also observed. Several authors have studied the fluorite veins at the Gabal El-lneigi Pluton (e.g. Moustafa et al., 1954; Moharram et al., 1970; Heikal, 1973; Fasfous and Awad, 1985; Yonan, 1990; El-Shibiny, 1995).

This paper studies the geological, mineralogical and geochemical characteristics, as well as fluid inclusions (temperature and salinity), of the fluorite veins associated with the granitic rocks in an attempt to reveal their genesis. To achieve the aim of the study, about 100 samples, collected from the granites and associated fluorite veins, were subjected to petrographical and ore mineralogical studies, and X-ray diffraction analysis. Major element analyses were carried out at the laboratory of the Nuclear Material Authority, Cairo, using conventional wet chemistry techniques (Shapiro and Brannock, 1962). The estimated procedure error is cr = f 5%. Thorium, U and trace and rare earth elements (REE) were analysed using X-ray spectrographic and neutron activation methods at the Geology and Prospecting Institute, Moscow. The estimated error for these counting techniques is o = f 10%. Fluid inclusions were studied at the Virginia Polytechnic Institute, USA in two representative samples of both fluorite and quartz veins, using a special freezing and heating stage mounted on a microscope.

PETROGRAPHY OF THE GRANITIC ROCKS

Fresh samples from the Gabal El-lnegi Pluton are generally equigranular, leucocratic, pale pink to red in colour and medium- to fine-grained with a hypidiomorphic texture. Microscopically, they contain quartz, alkali feldspar and plagioclase with subordinate amounts

of biotite. Fluorite, magnetite, ilmenite, goethite and apatite are the most common accessories.

The quartz is mainly interstitial showing undualtory extinction due to stress. Occasionally, it is inter- grown with K-feldspar, forming a graphic texture. The alkali feldspars are mainly microcline perthite with faint cross-hatched twinning. The plagioclase is oligoclase with An = 16-l 9. The albite twinning shows some bending and gliding due to deformation. Biotite occurs as brown plates partially altered to chlorite. Magnetite is the dominant opaque mineral (70-800/b) occurring as separated euhedral crystals or disseminated in biotite.

Eight fresh samples, representing the Gaba! El-lneigi Pluton were subjected to modal analyses and plotted in a quartz-alkali feldspar-plagioclase (QAP) ternary diagram (Streckeisen, 1976). The data are given in Table 1 and plotted in Fig. 2, which shows that Gabal

Figure 2. QAP ternary diagram (after Streckeisen, 1976). la: quartzolite; lb: quartz-rich granitoids; 2: alkali-feldspar

granite; 3a: syenogranite; 36: monzogranite; 4: granodiorite; 5: tonalite; 6: alkali feldspar syenite; 6’: quartz alkali feldspar s yenite; 7: s yenite; 7 l : quartz s yenite; 8: monzonite; 8’: quartz monzonite; 9: monzodiorite; 9,: auartz monzodiorite; 10: diorite; 10’: quartz diorite.. ’

Table 1. Modal composition (vol.%) of representative samples from the El-lneigi Granites

Serial No.

1 2 3 4

5

6

7

Sample No. 114 126 134 135 201 205 233 234

02 K-feldspar

42.96 40.35 29.98 32.89 42.92. 20.25 42.37 26.15 34.02 31.74 54.53 20.19 50.50 25.53 38.60 28.94

Plagioclase

15.60

Mafics

31.36

30.60

27.48

24.26

25.29

21.71

-

1.84 -

1.15

2.48 -

1.52

Opaques

1.09 4.08 6.23 2.86 7.49

0.74 25.73 - 1 7.33

Total

100.00 100.15 100.00 100.01

99.99 100.01 100.00 100.00

Journal of African Earth Sciences 3 1

I.A. SALEM et al.

El-lneigi granitic rocks plot in the monzogranite field. The scatter of the data points reflects the high variability of the quartz contents, which vary from 30% to about 55%. The K-feldspar/total feldspar

ratios vary from 0.40 to 0.57, except for one sample with a ratio of 0.72. These ratios indicate adamellite

composition with one sample having typical granite

composition.

GEOCHEMISTRY OF THE GRANITIC ROCKS

Specialised granites are those having spatial and

genetic association with mineralisation of rare metals

and are distinguished from normal granites by high

enrichment of large ion lithophile (LIL) elements

(Tischendorf, 1977). The geochemistry of the major

oxides of the fresh granitic rocks from the Gabal El- lneigi Pluton associated with fluorite mineralisation

will be discussed here, in order to determine what

type of specialised granites they represent. The eight samples representing the fresh granitic

rocks of Gabal El-lneigi Pluton were subjected to

geochemical analyses. The major and trace elements,

CIPW norms, Niggli values and some geochemical

parameters for the analysed samples are given in

Table 2. The average chemical composition of the fresh

granitic rocks from the Gabal El-lneigi Pluton (Table

3) is compared with that of some published Egyptian

granites (El-Gaby, 1975; Rogers and Greenberg,

1983). Also, Table 3 shows the average chemical compositions of the four types of specialised granites recognised in the Arabian Shield by Ramsay (19861,

as well as the average chemical composition of the

specialised granites of the world (Tischendorf , 1977).

The Gabal El-lneigi Granites are comparable with

the younger granites of El-Gaby (1975) but have lower AI,O, and higher P,O, contents. In addition, they are

comparable to G, granites (Rogers and Greenberg,

1983) but have relatively higher P,O, contents.

Compared with the specialised granites, they are

transitional between the average agpaitic and plumasitic specialised granites of the Arabian Shield

(Ramsay, 19861, but they have higher CaO, MgO,

Na,O and P,O,, and lower K,O. They also are similar

to the world average of specialised granites

(Tischendorf, 19771, but they have lower AI,O, and Fe0 and higher CaO and Na,O contents.

Magma type The total alkalis content fNa,O + K,O) of the samples

are plotted against SiO, contents as shown in Fig. 3. The data points plot in the alkali sub-field with the

exception of one sample in the high alumina sub-field of the sub-alkaline field. The alkalinity ratios (Wright,

32 Journal of African Earth Sciences

1969) of the samples, when plotted against SiO, contents as shown in Fig. 4, indicate their alkaline nature.

The log (K,O/MgO) ratios of the samples are plotted against SiO, in Fig. 5. This relationship has been used to distinguish between the talc-alkaline and alkali granite fractionation trends (Greenberg, 1981; Rogers

and Greenberg, 1981). The majority of the samples

fall within the alkali granite field, indicating the effects of specialisation processes (Abou Seda, 1989).

The Gabal El-lneigi Granites are of soda-potash composition as shown in Fig. 6 and generally are alkali-

rich (i.e. they have Niggli values mainly of alk > 2/3 al

as indicated by Fig. 7).

On the basis of molecular proportions of A&O,-CaO

and Na,O +K,O in the samples (Fig. 81, El-lneigi

Granites range from metaluminous to peralkaline in composition, with respect to alumina saturation.

An-Ab-Or and Qz-Ab-Or normative ternary diagrams

(Figs 9 and IO) illustrate some possible conditions of crystallisation for the studied granitic rocks. It is clear

that the analysed samples plot in the granite field and

are clustered about the isobaric univariant curve,

indicating that crystal-liquid equilibrium was the

dominant mechanism involved in the genesis of these granites (James and Hamilton, 1972). Also, the

investigated granites have compositions close to the

minimum melting point at low to moderate P,,,,

indicating that these rocks originated by partial melting

within the crust (Presenall and Bateman, 1973). Such

conditions may be created within a thickened crust and may be due to its relaxation at the end of plate

collision (late tectonic) or an l-type magma tectonic setting.

Tectonic setting Chappel and White (1974) used the Al,03/Na,0- + K,O + CaO molecular ratio against SiO, wt% var-

iation diagram (Fig. 11) to distinguish between the I-

(igneous) type (subduction related) and the S-(sedi- mentary) type (collision-related) granitic rocks. All the

data points of the studied granites plot within the I-

type field. The chemistry of the Gabal El-lneigi Granites

point to a similarity with the l-type granites. The

following chemical parameters are used to establish

this resemblance, based on the study given by Chappel

and White (op. cit.): il SiO, content is dominantly in the range of 65-7 1

wt%; iii relatively high content of Na,O (more than 3.2

wt%); and Zi) their molar ratio AI,O,/Na,O+K,O+CaO is

<l.l. Data, provided by El-Gaby et al. (19881, argued that

the younger granites of the Eastern Desert of Egypt are of l-type.


Table 2. Major element analysis of El-lneigi fresh granitoids

Sample No./% 114 126 134 135 139 201 205 234 Average

SiO2 73.60 73.30 73.30 74.60 73.62 72.76 73.10 71.48 72.22

TiOl n.d n.d n.d. n.d. 0.12 0.23 0.18 n.d. 0.07

AlzO, 12.46 11.78 11.60 11.46 12.20 11.65 11.50 12.78 11.93

Fez03 0.14 0.96 0.90 1.03 1.02 1.30 0.61 1.07 0.88

Fe0 0.24 0.40 0.28 0.16 0.16 0.28 0.36 0.60 0.31

MnO 0.05 0.05 0.04 0.03 0.05 0.06 0.03 0.03 0.04

MgQ 0.20 0.80 0.40 0.40 0.10 0.70 0.10 0.60 0.41

CaO 0.70 1.18 0.84 1.12 0.84 1.12 0.84 0.84 0.94

Na20 4.40 5.00 4.59 4.25 4.38 4.05 5.06 5.06 4.59

K20 3.89 4.03 4.16 3.49 4.16 3.89 4.97 4.03 4.08

p205 0.78 0.13 1.14 0.99 0.96 0.80 1.14 0.89 0.85

LOI 0.78 0.77 0.76 0.72 0.62 1 .oo 0.80 0.56 0.75

Total

CIPW norms

QZ

Or

Ab

An

AC

ns

di

hv

C

mc

hm

il

ap

97.24 98.40 98.01 98.25 98.22 97.84 98.69 97.94 98.07

33.87 28.16 31.33 36.86 32.87 34.00 30.83 26.34 31.78

23.85 24.41 25.30 21.16 25.19

38.55 38.97 37.43 36.83 37.90

23.76

35.35

0.89

30.03 24.48

32.05 43.91

2.17

0.25

4.19

0.93 0.86

1.04

0.21

2.84

2.17

1.80

1.80

1.47

0.25

0.02 0.02 0.02

0.29 2.56

1.03 0.26 1.81

0.70 0.49 0.46

0.60 0.16 0.45

0.64 0.93 1.04

0.02 0.35 0.45

2.22 2.15 1.80

0.68 1.80

0.08

1.59

0.35 0.02

2.54 2.00

24.77

37.62

0.11

0.85

0.53

0.52

1.11

0.35

0.41

0.33

0.16

1.70

bv 1.77 0.22

Total 100.24 99.99 100.53 100.06 100.30 100.01 100.51 100.22 100.50

si 475.83 409.49 447.96 471.77 458.38 434.02 438.60 394.60 441.30

al 47.39 38.71 41.70 42.63 44.68 40.88 40.59 41.50 42.26

frill 4.18 12.82 9.42 9.68 6.79 13.77 5.60 12.30 9.32

c 4.85 7.06 5.50 7.59 5.60 7.16 5.40 4.97 6.02

alk 43.58 41.40 43.37 40.10 42.92 38.19 48.41 41.33 42.4C

ti 0.05 0.04 0.05 0.05 0.84 1.03 0.81 0.04 0.3E

P 2.13 0.31 2.94 2.65 2.53 2.02 2.89 2.08 21.94

k 0.37 0.35 0.37 0.35 0.39 0.39 0.39 0.34 0.37

m9 0.46 0.52 0.39 0.39 0.14 0.45 0.16 0.40 0.36

f mc

Elemental ratios

Mol.K20/Na20

Alkalinty ratio

Felsic/mafic

Agpaitic coef.

Differentiation In.

Rl

R2

Solidification In.

9.03 19.89 14.92 17.27 12.39 20.93 11.00 17.27 15.34

0.58 0.53 0.59 0.54 0.63 0.63 0.65 0.53 0.59

4.40 5.60 5.74 4.20 4.80 4.29 9.68 5.01 5.47

1.28 3.34 2.42 2.71 2.12 3.40 1 91 3.11 2.54

0.92 1.07 1.04 0.94 0.96 0.93 1.19 0.99 1 .Ol

96.27 91.54 94.05 94.85 95.97 93.10 92.91 94.72 94.16

2193.10 1855.70 2022.30 2361.90 2099.90 2225.02 1633.60 1731.30 2015.00

341.36 407.01 346.96 373.90 342.19 395.94 327.40 380.52 364.40

11.12 8.57 9.46 10.45 10.18 9.39 8.90 8.53 9.58

LOI: Loss on ignition; n.d.: not determined; R, =4Si-11 (Na + K)-2(Fe +Ti); R, =6Ca + 2Mg +Al; Differentiation In. = (Y,SiO, + K,O)-

(CaO + MgO); Alkalinity ratio =(AI,O,+ CaO) + 2(Na,O)/(AI,O, + CaO)-Z(Na,O)


Table 3. Comparison of average chemical composition of El-lneigi fresh granites with Egyptian and world granitic rocks

1 2 3 4 5 6 7 6 73.22 75.04 75.5 74.18 75.29 71.29 63.45 73.38

0.06 0.22 0.10 0.32 0.10 0.36 0.71 0.16 11.93 13.03 12.5 11.10 12.82 14.29 17.16 13.97 0.88 1.07 1.02 2.86 0.77 1.36 2.14 0.80 0.31 0.61 1.21 0.59 1 .Ol 1.23 1 .I0 0.41 0.26 0.03 0.08 0.09 0.50 0.87 0.47 0.94 0.75 0.53 0.50 0.62 1.65 1.99 0.75 4.59 3.87 4.00 4.25 4.19 4.04 5.52 3.20 4.058 4.43 4.70 4.61 4.51 4.29 5.10 4.69 0.85 0.04 0.04 0.02 0.11 0.15 -

1: El-lneigi Granite; 2: Younger Granite (El-Gaby, 1975); 3: Younger Granite, Gl (Rogers and Greenberg, 1983); 4: average Agpaitic specialised granites, Arabian Shield (Ramsay, 1986); 5: average Plumasitic specialised granites, Arabian Shield (Ramsay, op. cit.); 6: average Calcic specialised granites (Ramsay, op. cit.); 7: average speciaiised syenitoids, Arabian Shield (Ramsay, op. cit.); 8: average specialised granites of the world (Tischendorf, 1977).

LA. SALEM et al.

tholeiitic

0 I I I 1 50 60 ‘O 80

SiO, (%I

Figure 3. Na,O + K,O versus SiO, binary diagram (after Irvine

and Baragar, 1971; Kuno, 19681.

40 1 I 1

0 1 2 3 4 5 6 7 8910

Alkalinity ratio

Figure 4. Alkalinity, variation diagram (fields are after Wright, 1969J.

70 -

,_.’

a ‘;; ,.:

./ 0” f 60 -

Cl / _“&

_.. ..I

alkaline,.,:.” _/ ,.:

,:’ ,.’ ,,... ..

50 - /” /’

../ / / /.-- . . . .

c.... .* , I -2 -1

Log,o~~*OIMgo~ l

2

Figure 5. SiO, versus Loglo(K,O/MgOJ diagram (after Rogers

and Greenberg, 19BlJ.

A K,O versus SiO, diagram (Fig. 121, after Coleman and Peterman (19751, shows that the El-lneigi Granites fall in the continental granophyre field, indicating that some crustal material was probably involved in their magma generation.

Batchelor and Bowden (I 985) distinguished between the erogenic and non-erogenic plutons using R, versus R, parameters of de La-Roche eta/. (I 980) (Fig. 13). All the data points of El-lneigi Granites lie on the line between the late erogenic and anorogenic fields, except two points in the syn-collisional field. This is consistent with Stoeser and Elliott (I 9801, who suggested that alkaline granites make up the majority of the post-collision suite, whereas talc-alkaline granites are predominant in the syn-collisional suite in the Arabian Shield.



Ab 50 All

Figure 6. The Or-Ab-An diagram for the studied fresh granitoids (after Shand, 192 71.

alk = al

A /

Relatively alkali-poor alk< l/2 al

0 10 20 30 al

40 50 60

Fipre 7. Alk-al diagram for the studied fresh granitoids (after Niggli, 1954).

Peraluminous

Metaluminous

CaO

Figure 8. Al,O,-CaO- fNa,O+ K,Ol mol. % plot for studied fresh granitoids (fields are after Shand, 7927).

Ab 30 50 Or I Figure 9. Normative Or-Ab-An proportions for the studied fresh granitoids (after O’Conner, 1965; Barker, 19791. The solid line represents the two feldspar boundary curves for the quartz-saturated ternary feldspar system at 1000 bars water vapour pressure (James and Hamilton, 19721.

Ab 50 Or

Figure 10. Normative Qz-Ab-Or of the studied fresh granitoids (fields are after Luth et al., 1964).

Trace and rare earth elements The eight samples were also analysed for Rb, Sr, U and Th (Table 4). The rocks have high concentrations of the incompatible elements Rb, U and Th, and are depleted in Sr. The K/Rb ratio averages 166, which is below the crustal average of 230 (Taylor, 1965). Also, the Rb/Sr ratio averages 60, which indicates that the granite originated from a highly evolved liquid and was emplaced at a shallow level. Hanson (1978) showed that high Rb/Sr ratios indicate the presence of plagioclase and, to a lesser extent, K-feldspar in the residue. This is possibly due to feldspars, which either were

Journal of A frlcan Earth Sciences 35

LA. SALEM et al.

8 1.2- not involved in the partial melting process or were

‘is separated earlier from the melt by fractional crystal-

E l.l- s-type lisation in the magma chamber before emplacement.

Two representative samples were selected from

s l.O- El-lneigi Granites for REE analysis (Table 5). The total

A A

REE range between 77.5 and 109.9 ppm (averaging m” 0.9 - z ’ AL 94 ppm), which indicates that these rocks are depleted

0’ 0.8.

b . I-type in REE compared to normal granites averages of 250-

9” . 270 ppm (Herrmann, 1970). The REE contents have been

3 0.7. 1 normalised to the average chondrites (Nakamura,

1 a 60 65 70 75 80 1974) and are plotted in Fig. 14, which shows nearly

SiO, (%I symmetric V-shaped curves slightly elevated on the right, due to higher HREE abundances and showing a

Figure 11. Aluminosity index versus SiO, for the studied very noticeable negative Eu anomaly. The positive Ce fresh granitoids. Boundary line separates l-type and S-type anomalies indicate low 0 fugacity at the source of granites (after Chappel and White, 19741. the magma (Constantopoulos, 1988). The negative

Eu anomalies are probably due to the separation of plagioclase from the melt by fractional crystallisation

8-

0 -j--F+

where they are left with the residue (Emmermann ef

4- Continental granochyre al., 1973; Clarke, 1992). Also, Cullers and Graf

c ,- /IC (1984) suggested that alkali feldspar-rich granitic

2- /*- rocks, with moderate to large negative Eu anomalies, Continental theolitic / Conthental require abundant residual feldspar in the source and

l- basalt’->

‘;; - C,/ varied small amounts of minerals like garnet,

b 0

/’

/_~~~nd;jemite

..,’ amphibole or pyroxene to account for the varied LREE/

# 2” . Subalkalin%%~~?ic I’ Oceanic

‘1 HREE ratios and total REE contents. They suggested

\

basalt and gabPro [

that alkali feldspars and muscovite significantly plagiogranite I

0.1 - 1 k-1’ ‘N\,,

I’

contribute to the melt to provide the large K content

Me in the granitic melt, and the plagioclase should be in

\-.._-@ @-- the residue. Such source materials could be meta-

/M---Y greywackes, meta-pilites or siliceous granulites.

( Curn~~te gabbro \ 5-w-H

3.01 THE FLUORITE DEPOSIT I 1

40 50 60 70 80 The mode of occurrence of the fluorite bearing veins,

SiO, (o/o) their size, attitude and field relations with the host granite were described above. In hand specimens,

Figure 12. K,O versus SiO, diagram (after Coleman and the fluorite displays a range of colours including green, Peterman, 1975). violet, grey and sometimes an almost white variety

2000 c

Late erogenic

--c-&-AL.& Anorogenic

0 Post->rogenic 1

0 1000 2000 3000 4000

R

Figure 13. Plots of the studied fresh granitoids on the diagram of major granitoid associations (after Ba tchelor and Bo wden, 1985).



Table 4. Concentration of some selected radioactive elements and elemental ratios in the El-lneigi fresh granites

Sample K% U Th No. ppm ppm 114 3.23 27 29 123 - 21 36 126 3.35 32 39 134 3.46 16 38 135 2.89 33 31 139 3.46 16 15 201 3.23 18 42 205 - 11 23 234 3.35 23 36

Rb

PP 31: 310 272 166 155 159 177 196 232

Sr ppm

5 7

10 5

16 5

45 2 1

Th-U K-Rb Rb-Sr

1.07 102.22 63.2 1.71 - 44.29 1.22 123.16 27.2 2.38 208.43 33.2 2.94 186.45 9.69 0.93 217.61 31.8 2.33 182.49 3.93 2.09 - 98 1.56 144.39 232

Table 5. REE analysis (ppm) of the El-lneigi fresh granites

La Ce Nd Sm Eu Tb Yb Lu Sample 9.2 58 5 7.8 0.2 2.8 23.3 3.61

No. 9.7 27 14 6.5 0.3 1.8 15.9 2.26

. 234

0 A”. fresh

1-hi-f-l I I , I I I I ! , 1 I

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 14. Chondrite-normalised REE patterns for El-lneigi fresh granitic rocks.

with green, yellow and brown patches. The fluorite crystals with their cubic habit are coarse, well formed and the finer ones are subordinate. Galena is usually associated with the fluorite veins. It occurs as well- defined cubic crystals of variable sizes.

The granite wall rock exhibits hydrothermal alteration. At the contact between the veins and the host rock, the highly altered granite has a friable appearance with a pale yellow colour and lacks its original texture. These highly altered zones grade into a weakly altered, yellowish-brown granite, which still preserves its original texture. The thickness of these alteration zones range from 0.3 to 0.6 m. The contact of the fluorite veins with the granitic wall rock are sharp and commonly brecciated. Red colouration is a

common feature observed in the altered rocks. Kaoli- nisation is well developed on the walls of the fluorite veins.

MICROSCOPIC INVESTIGATION

Polarised and reflected light microscopy revealed that the vein samples consist mainly of fluorite, quartz and microcrystalline silica, together with variable amounts of galena, chalcopyrite, sphalerite and covellite. Relics of altered host granite are also observed within the vein material. Fluorite occurs as coarse massive aggregates. The individual crystals are euhedral to subhedral with square outlines due to cubic habit. The crystals are up to 5 mm in length, and some show two sets of cleavage. Some of these large coarse fluorite crystals are cracked into finer, cubic crystals exhibiting cubic outlines and ranging up to 5 mm in length. They commonly occur as euhedral crystals amongst the quartz matrix. Usually the fluorite crystals are mantled and corroded by quartz. Quartz occurs in various forms, as coarse euhedral to subhedral and sometimes as six-sided crystals bordering and corroding the fluorite crystals, and show strong undulose extinction. Veinlets of recrystallised clear quartz are recorded cutting and protruding into the fluorite. Rounded to irregular aggregates of quartz, forming a mosaic texture, are usually observed filling the cavities between the coarser quartz.

A study of the polished sections revealed that the main opaque minerals in the fluorite veins (about 4%) is galena, chalcopyrite, sphalerite with minor


LA. SALEM et al.

amounts of covellite as an alteration product of chalcopyrite.

Galena is the most common opaque mineral. It occurs as subhedral to anhedral masses (0.3-5.0 mm in diameter) characterised by visible triangular pits. It is generally white in reflected light with high reflectivity and is isotropic. Galena is occasionally replaced by chalcopyrite. Chalcopyrite occurs in considerable amounts as fine to medium anhedral grains (0.1-0.4 mm in diameter) associated with galena. It is bright yellow in colour with weak anisotropism. The chalcopyrite is occasionally altered, particularly along its edges, to covellite. Sphalerite occurs in small amounts as fine anhedral to irregular masses (up to 0.2 mm in diameter) occasionally associated with chalcopyrite and sometimes replaces it. Sphalerite is grey in reflected light with moderate reflectivity. Covellite occurs as anhedral masses (O.l- 0.4 mm in diameter) with a characteristic deep blue colour and strong anisotropism. It is formed due to the alteration of chalcopyrite.

The paragenetic sequence can be presented as follows (beginning with the oldest): fluorite-quartz, microcrystalline silica, galena, chalcopyrite, sphalerite and covellite. However, in rare cases, the formation of quartz began before that of fluorite and continued after the formation of fluorite.

GEOCHEMICAL CHARACTERISTICS OF FLUORITE

Eleven representative bulk samples of the fluorite veins were analysed for major, trace and rare earth elements by a neutron activation technique. The concentrations of 19 elements are shown in Tables 6 and 7.

Major elements

The fluorite-rich samples are characterised by variable CaO content ranging from 21 to 70%. Deer et a/. (1962) gave CaO content of several fluorite types of

the world, ranging from 71.39 to 71.76%. The lower CaO values found in this study reflect the incorporation of other mineral phases within the bulk samples, especially quartz and galena (e.g. sample no. 217, which contains 21% CaO and 15% Pb). The fluorite samples exhibit very low concentrations of Na,O, ranging from 0.003 to O.lOO%, with an average of 0.030%.

Trace elements The fluorite samples of El-lneigi are characterised by the enrichment of Pb, which ranges from 20 to 150,000 ppm, with an average of 34,500 ppm. Table 6 shows that the grey variety exhibits the highest Pb content (average of 78,000 ppm), whereas the white and violet varieties have the same content (15,000 ppm). The green variety contains the lowest average Pb value (2300 ppm). The high Pb content in the Gabal El-lneigi grey fluorite is attributed to the presence of galena. Copper ranges from 1 to 3000 ppm, with an average of 470 ppm. The grey variety contains the highest average Cu content (1600 ppm), while the green variety contains the lowest average value (2 ppm). The highest Cu content in the grey variety is attributed to the presence of chalcopyrite. Molyb- denum is not detected in the green variety, but it is recorded in the other varieties and ranges from 0.3 to 7.0 ppm with an average of 1 .O ppm. Tin is not recorded in the white and green varieties but is recorded only in the grey and violet varieties and ranges from 3 to 30 ppm with an average of 2 ppm. Chromium is recorded in all varieties and ranges from 1 .O ppm in the violet and green varieties to 200 ppm in the white and grey varieties with a total average of 90 ppm. Gallium is not recorded in the white, violet and green varieties but is recorded in all analysed samples of the grey variety, ranging from 1 .O to 10.0 ppm, with an average of 4 ppm. Lithium is recorded in some samples of the white and grey varieties but

Table 6. Major (wt%) and trace elements (ppm) of El-lneigi fluorite veins

Colour Sample CaO NazO Pb CU MO Cr Sn Ga Li Bi Y No.

White 156 56 0.05 30000 5 2 100 - - 200 - 2000 181 50 0.07 20 5 2 200 - - - - 50

Grey 190 56 0.011 2000 150 0.5 150 3 3 - 5 10 214 28 0.10 80000 1000 7 200 5 1 200 50 50 215 56 0.005 80000 1000 0.5 10030 1 -50 300 217 21 0.14 150000 3000 3 250 10 10 200 50 300

Violet 222 70 0.005 30000 l--7-_- 3000 223 70 0.014 1000 10 0.3 1-- -- 3000

Green 226 70 0.01 2000 1 - 1-- -- 3000 229 70 0.004 30200 1 - 3-- -- 3000 231 70 0.01 2000 5 - 3-- -- 3000



Table 7. REE analysis (ppm) of the El-lneigi fluorite veins

Colour Sample La Ce Sm Eu Tb Yb Lu Ta TblLa

No. White 156 24.4 4.8 3.4 0.9 0.4’ 1.54 0.1 0.2 0.02

181 46.8 0.4 11.2 0.2 0.1 0.37 0.04 0.1 0.002 Grey 190 36.6 - 6.4 - - 2.16 - - -

214 74.6 5.7 3.4 0.8 0.8 0.5 0.4 5.6 0.01 215 19.8 5.9 1.9 0.5 0.4 2.02 0.2 0.6 0.02 217 93.6 0.7 4.3 0.07 0.08 1.03 0.07 0.7 0.01

Violet 222 18 3.6 2.9 0.8 1.5 1.13 1 0.13 0.083 223 107 13.1 3.3 4.1 3.3 8.75 0.4 0.27 0.03 226 89.2 11 5.1 2.6 2.1 8.77 1.4 0.2 0.023

Green 229 2.6 3.8 2.4 0.2 0.47 2.76 0.72 0.1 0.18 231 72.4 10.5 5.2 1.4 2.8 13.3 2.4 0.2 0.038

is not recorded in the violet and green varieties. Bismuth is recorded only in the grey variety, ranging from 0.5 to 50.0 ppm, with a total average of 14 ppm. Yttrium is recorded in all varieties, ranging from 10 ppm in the grey variety to 3000 ppm in the green one, with an average of 1600 ppm.

Rare earth elements According to Constantopoulos (1988) and as a result of the different stabilities of the REE complexes, early formed fluorite is La-rich and Tb-poor (lower Tb/La ratio). As crystallisation proceeds, the fluorine concentration of the fluid is rapidly depleted, which leads to the decomposition of TbF*+ and the other rare earth-fluoro complexes. Since much of the La has already been taken up, late-stage fluorite will be relatively Tb-rich and the fluorite will have a higher Tb/La ratio. The fluorite varieties show considerable variation in REE contents ranging from 13.05 ppm in the green variety to 140.22 ppm in the violet variety (Table 7). Fleischer (I 969) reported that fluorite from hydrothermal deposits exhibits considerable variation in REE composition, even among samples from the same deposit.

The values of Tb/La ratios (Table 7) indicate that the grey variety has the lowest Tb/La (0.008) ratio, which is indicative of earlier crystallisation. The highest Tb/La ratio in the green variety indicates that it crystallised from more evolved fluids (late stages of the evolution of the hydrothermal fluids) (Constantopoulos, 19881.

The Tb/Ca ratio has also been used as an envi- ronmental index, since REE contents were found to vary with Ca concentrations (Schneider et al., 1975; Moller et al., 1976; Moller and Morteani, 1983). In addition, it has been demonstrated that the magnitude of the Tb/Ca ratio relative to the lb/La ratio in a fluorite could be used as a criterion for the genesis of this mineral (Jacob, 1974). In this study, all of the fluorite

samples plot within and/or close to the hydrothermal field (Fig. 15).

Chondrite-normalised plots (Fig. 16) reveal per- sistent negativr, Ce anomalies, indicating high 0 fugacities and resultant oxidation of Ce+3 to Ce+4 in the source of the hydrothermal fluids (Constan- topoulos, 1988). The fluorite samples show variable Eu anomalies, indicating heterogeneity in the Eu composition of the source fluids. The observed Eu anomalies cannot be explained in terms of oxidation- reduction conditions, but this implies that the hydrothermal fluids equilibrated with the granitic rocks of the Gabal El-lneigi Pluton, which exhibits significant negative Eu anomalies as described above.

Uranium and Th in fluorite

The U and Th contents in the fluorite veins have been determined in the 11 samples by means of instru- mental X-ray spectrographic analysis (ARF-6) using a Mb tube. Table 8 gives the contents of U and Th in El-lneigi fluorite veins. The estimated error of this technique is + 10%.

Uranium contents in the fluorite veins range from 9 to 50 ppm with an average of 29 ppm. The green variety is characterised by higher U contents with an average of 43 ppm. The violet variety has the lowest U content with an average of 17 ppm. The white and grey varieties have similar average contents of 25 and 27 ppm, respectively.

The Th contents range from 9 to 194 ppm with an average of 116 ppm. Table 8 shows that the grey variety contains the highest average amount of Th, 229 ppm. In the green variety, the Th contents vary between 24 and 74 ppm with an average of 43 ppm, whereas in the white variety, it ranges from 29 to 96 ppm with an average of 63 ppm. The lowest content of Th is observed in the violet variety, which ranges from 9 to 65 ppm, with an


I.A. SALEM et al.

I / flourite

,001 .Ol Tb/La

.l

Figure 15. The Tb/Ca versus Tb/La relationship in fluorite (field is after Jacob, 19741.

0 156

. 181

0 100

190

. 214

. 215

. 217

10

1

0.1 ’ I I I I / I,,, La Ce PC Nd Sm Eu Gd Tb Oy Ho Er Tm Yb Lu

000

-3 ) b

lLaCe Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0 222

. 223

o 226

,229

. 231

Figure 16. Chondrite-normalised REE pattern for the different varieties of El-lneigi fluorite veins (Nakamura, 19741: (al white and grey varieties; and Ibl green varieties.

average of 37 ppm. It is to be noted that the green and violet varieties, which are the purest fluorites, contain the lowest average U and Th contents, which might suggest that the excess U and Th in the white and grey varieties are associated with other minerals.

Table 8. Concentration of U and Th (ppm) in El-lneigi fluorite veins

FLUID INCLUSIONS OF FLUORITE VEINS

Fluid inclusion petrography and microthermometry were carried out on one representative sample (no. 190) of the fluorite veins associated with galena.

The fluid inclusion population is dominated by aqueous two phase (L + VI inclusions at room temperature. They exhibit a high degree of fill (0.7-0.9). The inclusions range from 30 to 50 mm in size, are lensoidal in shape and are clearly secondary in origin. They occur along annealed fracture planes of different orientations.

Heating and freezing measurements All fluid inclusions homogenise to the liquid phase between 80 and 281 “C, but the majority homogenises in the range of 80-I 50°C with an average of 125°C (Table 9). The histogram (Fig. 17) suggests that the two fluid inclusions, which homogenised at 236 and 281 “C, might represent relics of the primary temperatures at which the fluorite veins were originally formed, but the rest of the secondary fluid inclusion homogenisation temperatures probably represent annealing or recrystallisation temperatures. The temperature of first ice melting is observed close to -20.5’C, indicative of solutions in the H,O-NaCI system (Potter er a/., 1978). The temperatures of the last ice melting of these fluid inclusions range from 0.0 to -18.6OC.

The above range of final melting temperatures of ice (TJ in the aqueous inclusions corresponds to fluid salinities ranging from 0.0 to 21.40 equiv. wt% NaCl (Table 9). This range of salinities can be interpreted to represent the mixing of two fluid components meteoric water and a more saline fluid (Kyle, 1990). The density of solution can be obtained by plotting salinity versus homogenisation temperature (Fig. 18). It reveals that the fluorite may have been deposited from a solution with a density ranging from I .Ol to 1.059 cm-3.

40 Journal of African Earrh Sciences

(sample no. 190)

F.I. Th (“c) Tm Salinity Vapour F.I. Th (“c) Tm (“Cl Salinity Vapour

type (“C) Equiv. wt% volume type Equiv. volume NaCl % wt% %

NaCl

1 142.0 -12.0 15.96 15 1 102.3 -9.8 13.72 10

1 156.4 -8.9 12.73 10 1 94.4 -5.8 8.95 10

1 142.0 -9.5 13.40 10 1 49.4 -8.3 12.05 10

1 187.6 -8.1 11.81 25 1 101.0 -8.3 12.05 20

1 135.0 -6.8 10.24 15 1 111.7 -8.3 12.05 25

1 138.0 -12.5 16.23 30 1 102.3 0.0 0.00 30

1 144.2 -10.4 14.36 30 1 106.7 -0.01 0.18 25

1 129.4 -10.4 14.36 15 1 122.7 -11.0 14.97 30

1 129.4 -10.4 14.36 15 1 98.4 10.9 14.87 30

1 148.0 -15.5 19.05 10 1 176.0 -10.6 14.57 30

1 236.6 -15.2 18.80 20 1 163.0 -10.6 14.57 20

1 281.0 -3.2 5.26 20 1 193.2 -10.2 14.15 15

1 115.0 -8.0 11.70 20 1 167.0 -10.0 13.94 10 1 78.8 -10.5 14.46 10 1 140.8 -10.2 14.15 30

1 85.0 -8.2 11.93 25 1 173.0 -9.6 13.51 30 1 96.2 0.0 0.00 15 1 162.0 -3.1 5.11 25

1 149.0 -1.4 2.41 15 1 80.6 -16.9 20.15 20

1 80.6 -17.6 20.67 30 1 99.0 -18.6 21.40 15 1 105.0 0.0 0.00 15 1 99.0 -18.6 21.40 10 1 123.0 0.0 0.00 20 1 111.0 -8.3 12.05 10

F.I.: Fluid inclusion; T, (T): the fluid inclusion homogenisation temperature in "C; T,,, (“C): the final melting point of the ice in "C.

Mineralogy andgeochemistry of Gabal El-lneigi Granite and associated fluorite veins

Table 9. Homogenisation, melting temperatures and salinity of fluid inclusions in El-lneigi fluorite veins

10-l

70 110 150 190 230 270 310

T, (“C)

Figure 17. Histogram of final homogenisation temperatures

determined in El-lneigi fluorite veins.

FLUID INCLUSIONS OF EL-INEIGI QUARTZ VEINS

One fluid inclusion type has been observed in El-lneigi quartz veins. Microscopically, they consist of regular- shaped fluid inclusions ranging from 3 to 20 mm in size. They are aqueous two phase (L +V) inclusions exhibiting a low degree of fill ranging from 0.5 to 0.15 of the total volume. These fluid inclusions are secondary in origin and occur as trails associated with fractures of different

orientations.

30.00

2

u, z 20.00

5

2 >

‘E ‘-

cz 10.00

0.00

100.00 200.00 300.00

T, PC)

1

Halit saturation /

Figure 18. Salinities plotted against homogenisation tem-

peratures for fluorite. lsodensity lines have been drawn at

intervals of 0.05 g cm-3 (data from Haas, 1976; Konnerup-

Madson, 1979).

Journal of African Earth Sciences 4 1

/.A. SALEM et al.

Heating and freezing measurements The results of the fluid inclusion homogenisation temperature measurements are given in Table 10. They range from 90 to 128OC and are represented in the histogram (Fig. 19). All these inclusions homogenise to the liquid phase.

The final melting point of the ice ranges from -0.5 to -6.9C, corresponding to salinities ranging from 0.88 to 10.36 equiv. wt% of NaCl (Table 10). The density of the hydrothermal solution may be obtained by

15

1

01 80 1

T, V.-Z)

-

Figure 19. Histogram of final homogenisation temperatures determined in El-lneigi quartz veins.

plotting salinity versus homogenisation temperature (Fig. 20). It is indicated that the quartz was deposited from a solution with density around 1 .OO g cm-3.

SUMMARY AND DISCUSSION

The rocks of the Gabal El-lneigi Pluton have an adamel- litic to granitic composition. They originated from metaluminus talc-alkaline magma with strong alkaline tendencies. The pluton has chemical characteristics similar to l-type granites. The granitic magma was probably generated in an extensional environment due to crustal relaxation after a collisional episode ( < 600 Ma). A crust thickened by collision would generate magmas with an alkaline nature Batchlor and Bowden, 1985).

Gabal El-lneigi granitic rocks have low total REE (averaging 94 ppm) compared with normal granite values of 250-270 ppm. They are characterised by pronounced negative Eu anomalies and positive Ce ones. The depletion of Eu is probably due to the re- moval of plagioclase from the source (Clarke, 19921, or it could be later removed by fractional crystallisation in the magma chamber before emplacement (Emmer- man et al., 1973). The negative Eu anomaly can also be achieved by low temperatures and a small degree of partial melting mechanism, which is consistent with the extensional tectonic regime suggested for the formation of this granite. The positive Ce anomalies indicate low 0 fugacites during magma crystallisation.

The fluorite-bearing veins occupy faults and fractures. Quartz is the main gangue mineral associated with fluorite. The other minerals include galena, chalcopyrite, sphalerite and covellite.

Table 10. Homogenisation, melting temperature and salinity of the fluid inclusions of El-lneigi quartz veins

Th (“c) Tm V’C) Th (“cl F.I. Salinity Vapour F.I. Tm (‘0 Salinity Vapoul

type Equiv. volume type Equiv. volume wt% % wt% % NaCl NaCl

1 123.0 -2.5 4.18 10 1. 93.8 -1.7 2.90 10 1 115.0 -2.1 3.55 10 1 117.0 -2.9 4.80 10 1 90.0 -4.6 7.31 10 1 117.0 -4.3 6.88 10 1 90.4 -3.8 6.16 10 1 102.0 -2.8 4.65 15 1 99.6 -2.5 4.18 15 1 101.1 -0.5 0.88 10 1 121.0 -4.0 6.45 10 1 101.0 -1 .o 1.74 IO 1 121.0 -5.4 8.41 15 1 113.9 -2.5 4.18 10

1 117.0 -6.6 10.36 20 1 117.1 -1.8 3.06 15 1 115.4 -5.8 8.96 10 1 111.2 -2.0 3.39 15 1 104.8 -4.7 7.45 10 1 100.0 -2.2 3.71 15 1 115.0 -4.7 7.45 10 1 105.0 -1.1 1.91 10 1 106.0 -4.5 7.17 15 1 90.0 -1.7 2.90 15 1 103.0 -3.8 6.16 IO 1 93.2 -2.0 3.39 10 1 103.0 -3.8 6.16 10 1 128.0 -2.5 4.18 10

F.I.: Fluid inclusion; T, PC): the fluid inclusion homogenisation temperature in T; T,(T): the final melting point of the ice in “C.



& l

I I I I I 1oo.cKl 200.00 300.00

T, (“C)

Figure 20. Salinities plotted against homogenisation temperatures for El-lneigi quartz veins. lsodensity lines have been drawn at intervals of 0.05 g cm-3 (data from Haas, 1976; Konnerup-Madsen, 1979).

Several attempts have been made to correlate the blue, green, yellow, violet and other various colours of fluorite to its trace element contents, especially to certain individual REE and radioactive elements like U (Schneider et a/., 1975).

Regarding the trace elements, the grey fluorite exhibits the highest content of Pb (average 78,000 ppm) and Cu (average 1600 ppm), and this is attributed to the presence of galena and chalcopyrite minerals.

REE contents are variable and the highest concentration (140 ppm) is found in green fluorite. The Tb/Ca ratio confirms a hydrothermal origin for fluorite. The Tb/La ratio suggests that the grey fluorite was deposited early and the green fluorite was deposited later from more evolved fluids. The negative Ce anomalies indicate high 0 fugacities at the source of the hydrothermal fluids. The negative Eu anomalies can be due to equilibration of the fluids with the host granites, which has pronounced negative Eu anomalies through water-rock interaction (i.e. an inherited feature from the source rock).

One main aqueous two phase (L + V) type inclusion was observed and studied petrographically and microthermometrically in fluorite and quartz veins. The inclusions are secondary, being arranged along

fractures of different orientations. They exhibit a high degree of fill (0.7-0.9) in fluorite but have a low degree of fill (0.05-o. 15) in quartz. The fluid inclusions of the fluorite veins have homogenisation temperatures ranging from 80 to 15O“C, except two inclusions which gave 236 and 281°C. These last two might represent relict priman/ inclusions indicating that fluorite was formed at temperatures > 250°C. The melting temperature of ice in fluorite indicates salinities of up to 21.4 equiv. wt% NaCI. However, the fluid inclusions from the quartz veins exhibit lower homogenisation temperatures ranging from 90 to 128°C and the ice melting temperatures indicative of lower salinities (0.88-10.36 equiv. wt% NaCI). The petrographic relations indicate later crystallisation and formation of the quartz.

El-Shatoury et a/. (1974) and El-Shatoury (1979) reported three types of fluid inclusions in the quartz crystals of the granitic rocks of Gabal El-lneigi. These are liquid-rich inclusions (L + VI with a high degree of fill, gas-rich inclusions (V + L) with a low degree of fill and mono-phase liquid inclusions probably due to the necking of the former two types. These inclusions range in size from a few microns to 10 pm and are distributed randomly or along certain planes in the quartz crystal. They are mostly regular in shape with a few irregular ones and sometimes show the pheno- menon of necking down. The coexistence of liquid- rich and gas-rich types of inclusions in the same quartz crystal suggests trapping of liquid and gas from a heterogeneous mixture under boiling conditions, which would be caused by a steep pressure drop during the emplacement of the granite at shallow depths. The low pressure also allowed the separation of the gaseous from the liquid phase.

From the above discussion, it is clear that both the fluorite and quartz veins are characterised by larger- size fluid inclusions compared with the much smaller ones in the hosting granite. The presence of only liquid- rich inclusions with high degrees of fill and high salinities in contrast to the only gas-rich inclusions with low degrees of fill and low salinities in the quartz veins suggests the formation of both the fluorite and quartz in a highly fractured zone, where boiling was effective and complete separation of the vapour liquid phase took place. In addition, it might suggest the possible formation of fluorite veins at deeper levels relative to quartz veins. The low salinities in the gas- rich inclusions of the quartz suggest the possible di- lution of the hydrothermal fluids by meteoric or connate waters. This is consistent with the lower homogenisation temperatures observed for inclusions in the quartz veins.

In conclusion, the fluorite-quartz veins of El-lneigi Granites were formed as fillings of cracks and fractures


I.A. SALEM et al.

in the pluton during cooling, followed by the injection of magmatic fluids rich in fluorine at moderate temperatures and salinity from a crystallising granitic melt into the fractured zones. At the late stage of

magmatic fluids, quartz veins of lower temperatures

and salinities were formed. Editorial handling - P. Bo wden

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