Composition-Volume Changes during Metasomatic Alteration ... · rocks, but volume changes are hard to assess in rocks with fine grained me-sostases (Lesher et al. 1986). In view of

Composition-Volume Changes during Metasomatic Alteration of... 177JKAU: Earth Sci., vol. 11, pp. 177-206 (1420 A.H. / 1999 A.D.)

177

Composition-Volume Changes during Metasomatic Alterationof Pan African Andesitic Volcanic Rocks at Gabal El Dokhan

Area, Northeastern Desert, Egypt.

EZZ EL DIN A. KHALAF

Geology Department, Faculty of Science, Cairo University, Giza, Egypt

Received: 7/12/1998 Revised: 9/4/2000 Accepted: 26/6/2000

ABSTRACT. Metasomatic alterations of the Late Proterozoic andesitesat Gabal El Dokhan area display obvious change in mineralogical andchemical composition, if compared with the least altered andesiticrock types (assumed protoliths). These altered andesitic rocks havebeen metamorphosed to lower-middle greenschist facies typified bybiotite-actinolite-chlorite-albite-epidote-quartz-opaque minerals/spheneassemblages. Five alteration facies have been recognized: 1) Bio-titization-silicification, 2) Sericitization-silicification, 3) Albitization-silicification, 4) Epidotization-silicification, and 5) Chloritization.

The biotitized-silicified rock types are enriched in K-Rb-Ba-FTSMand depleted in LREE relative to the least altered rock. The se-ricitized-silicified rock types are significantly enriched in K-Rb-Baand depleted in FTSM and LREE. The albitized-silicified rock typesare strongly enriched in Na and depleted in LILE and LREE relativeto the least altered rock types. The epidotized-silicified rock types areenriched in Ca-Cu-Pb and depleted in LILE and LREE. The chlo-ritized rock types exhibit relative enrichment in Mg, FTSM, Zn-As-Pband depletion in LILE and LREE.

The behavior of the Ti, Al, Fet, P, and most HFS elements duringbiotitization, sericitization, albitization, epidotization and chlorit-ization suggests that they were inert during metasomatic modificationof the andesites. Mass balance calculations suggest that volumechange was restricted between zero and up to + 20% for the alteredupper andesites and �25 and up to +25% for the altered lower an-desites during the different types of metasomatic facies. Elementalmobilities are interpreted in terms of the controlling mineral phasesand the possible nature of the various metasomatising fluids.

Ezz El Din A. Khalaf178

Introduction

During hydrothermal system, weathering, metasomatism and metamorphism,emplacement and crystallization of magmas, chemical components are re-distributed among fluid and solid phases. Metasomatism has played an im-portant role, resulting in considerable elements redistribution and re-equilibration of the major rock-forming minerals. The actual gains and lossesthat take place in metasomatic alterations cannot be obtained without knowl-edge of the relationship between composition and volume changes that ac-company the process. The importance of composition-volume relationships hasbeen recognized by Poldervaart (1953), Turner and Verhoogen (1960), and Bo-golepov (1963). In the study of metasomatically altered rocks, the immediatequestions concern the nature of the original rock and gains and losses of com-ponents necessary to produce the altered rock. By careful consideration of thepetrology and field setting of an area, one may be able to determine a "least- al-tered equivalent". This is probably the major step in unraveling the alterationprocess. Assuming that this has been done, one has to consider next the basisfor determining the relative gains and losses that gave rise to the altered rock. Insome cases the assumption of constant volume seems to work, in some, constantalumina, and in other cases, other components seem to have been relatively im-mobile (Grant, 1986). The procedure used in this paper demonstrates how gainsand losses may be calculated using the actual chemical analysis.

Gabal El Dokhan area has been the subject of several studies since early his-toric Roman time. Several authors described the Dokhan volcanics as post tec-tonic, almost unmetamorphosed volcanic rocks, represented mainly by an-desites, porphyrites, rhyodacites, pyroclastics and Imperial porphyries (Akaadand Noweir, 1969; and El-Ramly, 1972). Others suggested that Dokhan volcan-ics are propylitized by post magmatic regional and thermal metasomatic pro-cesses (Gobrial and Lotfi, 1967, Basta et al. 1979; and Khalaf, 1994).

The studies of metasomatic alterations are important because they provide in-formation concerning the physical and chemical properties of the ore-formingfluids, and because they characterize the ore-related alteration signatures whichmay be useful in mineral exploration. Despite the mineralogical and geo-chemical changes that accompany such metasomatic alteration, many inter-pretations of chemical fluxes have assumed that rock volume is conserved (Riv-erin and Hodgson, 1980, and Gibson et al. 1983). Most arguments forconservation of volume are based on good textural preservation in the alteredrocks, but volume changes are hard to assess in rocks with fine grained me-sostases (Lesher et al. 1986). In view of the significant mineralogical changesobserved in the metasomatic altered volcanics, the assumptions of constant vol-ume metasomatism might not be necessarily valid.

Composition-Volume Changes during Metasomatic Alteration of... 179

The principal objectives of this study are: i) to assess the nature of chemicalmobility during the hydrothermal alteration of Dokhan volcanic rocks; ii) toplace constraints on volume changes during such alteration; iii) to identifythose elements, which remain immobile during such metasomatism. These com-putations have the advantage over binary variation diagrams in that they are ca-pable of comparing individual analyses in a metasomatic environment with acommon parent. They also reveal the interrelationships between and withingroups of elements and their relative mobilities.

Geological Setting

The Dokhan area lies to the northwest of Hurghada on the Red Sea coast. It isdelimited by latitudes 27º17′34″ - 27º19′59″ N and longitudes 33º12′30″ -33º15′00″ E. The studied area is mainly covered by volcanic sequences, Ham-mamat facies sediments, intrusive rocks and younger rock dykes (Fig.1). Thevolcanic rocks are unconformably overlain by Hammamat facies sediments es-pecially at the southern side along Gabal Umm Tawat. These volcano-sedimentary rocks are intruded by different suites of intrusive rocks includinggabbroic and granitic rocks besides bimodal mafic-felsic rock dykes and ptyg-matic folded quartz veins.

The studied volcanics at Gabal El Dokhan are part of Late Proterozoic vol-canic series of Arabo-Nubian massif which is known in the Northern EasternDesert as Dokhan volcanics according to their type locality Gabal El Dokhan(1661 m above sea level). The Dokhan volcanics represent an important strat-igraphic unit within the Egyptian basement complex. These volcanics are es-sentially composed of andesites and andesitic pyroclastics with subordinate ba-saltic andesites conformably overlain by eruptions of rhyolites and ash flowtuffs. The andesites are massive, unfoliated and well exposed along a largenumber of wadies draining Gabal El Dokhan. Interbedded with the andesites arebanded tuffs, volcanic breccias and agglomerates.

Detailed field mapping of the Dokhan volcanics revealed that the andesitesand associated pyroclastics can be divided into two different volcanic se-quences, termed lower (or older) and upper (or younger) andesitic rock se-quences (Khalaf, 1994). The lower (old) eruptions are mainly composed of am-phibole-free andesites and pyroclastics as well as minor basaltic andesites,which are exposed along wadies Sidri, Umm Lang and El-Atrash. The upper(young) eruptions are essentially composed of amphibole-rich andesites and py-roclastics including the famous Imperial porphyries exposed only along wadiMaamal, where they cut across the early andesitic sequences in the form of vol-canic plugs and sheets. Occasionally, these Dokhan volcanics are affected byshear zones, resulting in schistose pencil structure trending generally N 25º W


FIG. 1. Simplified geological map of the study area.


and dipping 57º SE. Greenschist metamorphism has modified the mineralogyand texture of these volcanics especially along shear and fault zones. The highlysheared and altered rock types have been metamorphosed to lower-middlegreenschist facies typified by chlorite-actinolite-albite-epidote-quartz-opaqueminerals/sphene assemblages, even though the preservation of volcanic texturesand lack of any penetrative fabric indicate that the degree of deformation isminimal. The nature and degree of alteration varies from one rock group to an-other and may be overprinted by granitic contact metamorphic events. Alongthe granitic contacts, these andesites are granulated, hybridized, mylonitized,thermally metamorphosed, with hornfelsic, decussate and granoblastic fabricand display intensive silicification, biotitization, sericitization and pigmentation,with red feldspar phenocrysts dispersed in aphanitic groundmass. Generally, thepropylitic alteration (epidote, actinolite, chlorite, and quartz) is the most wide-spread facies through the entire volcanic sequences, while the phyllic (sericite,quartz) and potassic alterations (biotite, orthoclase, sericite, and quartz) aredominant and localized along the contacts of granitic rocks and quartz veins. Si-licification typifies the Dokhan volcanics on a regional scale, related spatiallyand temporarily to the intrusion of granitic rocks and hydrothermal solutions. Itmanifests itself as irregular white patches and as halos mantling amygdales andfractures. This alteration is pervasive and characteristically patchy; the end re-sult is a heterogeneous-looking rocks, with dark green areas of weakly silicifiedandesites (weathered brown) adjacent to and gradational with irregular area ofwhite, strongly silicified andesite. Hydrothermal fluids associated with volcanicexhalation and granitic rocks were channeled and discharged by volcanic faultsthat breached the volcanics and produced different types of alterations. All al-teration types are found within the volcanic sequences. The highly meta-somatised rock types cannot be recognized in the field as the altered rocks lookentirely fresh and undistinguishable from the original ones. The alteration, how-ever, affects the mineralogy of the rocks as seen under the microscope and ismost conspicuous in chemical composition.

Petrography

Based on the field and petrographic criteria, two andesitic rock types havebeen recognized. Younger amphibole-rich andesites cut older amphibole-freeandesites and associated pyroclastics. The petrographic characteristics of theleast-altered andesites from the two volcanic sequences and corresponding met-asomatised rock types are described below.

Least-Altered Andesites

The least-altered lower andesites consist primarily of glomeroporhyritic pla-gioclase feldspar (An37), clinopyroxene, opaque minerals, and accessory apatite


with secondary quartz, sericite, and epidote set in a fine grained groundmassdisplaying trachytic and hyalopilitic textures. Plagioclase feldspars (46% bymode) occur as zoned and twinned subhedral crystals. They suffer slight kao-linization and sericitization. Clinopyroxene (13% by mode) displays pale green,faint pleochroism and slight alteration to uralitic hornblende, accompanied byopaque minerals especially along cleavage and crystal borders.

The least-altered upper andesites encompass essentially zoned plagioclasephenocrysts (An31), green and brown hornblende, brown apatite with quartz,Mn-epidote (piemontite), opaque minerals and sphene as xenocrysts embeddedin a fine grained groundmass exhibiting subfluidal and felty texture. Plagioclasecrystals (30% by mode) form cumuloporphyritic clots and subophitic texturewith hornblende crystals. Oscillatory zoning is a common optical feature withinthe plagioclase crystals. The plagioclase phenocrysts suffer a slight degree of al-teration through sericitization. Hornblende glomeroporphyritic crystals (18% bymode) occur as euhedral to subhedral fresh crystals of green and brown types.They display twinning and strong pleochroism. Microprobe analysis shows thatthe most dominant hornblende crystal types are magnesio hornblende (greencolor) which grade to magnesio hastingsitic hornblende (brown color) especial-ly along crystal borders. From the above description, it is clear that the least-altered lower and upper andesites are fresh rocks and are petrographically char-acterized by: i) the absence of schistose texture; ii) the absence of veins; iii) theclean and fresh appearance of phenocrysts; iv) the paucity of opaque minerals inthe groundmass.

Biotitized-Silicified Andesites

The biotitized-silicified andesites have coarse porphyritic texture with dom-inant plagioclase, actinolite and brown biotite as phenocrysts. Opaque minerals,sphene, quartz and apatite form xenocrysts. All the phenocrysts are set in quart-zofeldspathic groundmass. The plagioclase forms glomeroporphyritic clots andsuffers slight to complete alteration through kaolinization, sericitization and epi-dotization. Plagioclase phenocrysts have dusty and inclusion-rich core, mainlyof opaque minerals, glass and quartz displaying sieve texture. Occasionally, pla-gioclases are mantled by orthoclase (Fig.2A). Shreddy biotite occurs as an-hedral xenocrysts, displaying strong pleochroism and replaces the groundmass(Fig.2B). The groundmass consists essentially of feldspar microlites, re-crystallized quartz granoblasts, actinolite, biotite, opaque minerals, sphene andapatite with granoblastic texture. In strongly silicified andesitic rock types,quartz xenocrysts concentrate preferentially in the empty spaces as pockets,amygdales and linings of vesicles.


FIG. 2

. Ph

otom

icro

grap

hs s

how

ing

petr

ogra

phic

cha

ract

eris

tics

of th

e al

tere

d an

desi

tes.

A

� S

ieve

-tex

ture

d pl

agio

clas

e (P

l) a

re m

antle

d by

ort

hocl

ase

(Or)

of

non-

unif

orm

wid

th.

B �

Rec

ryst

alliz

ed b

iotit

e fl

akes

(B

t) o

ccup

y th

e sp

aces

bet

wee

n pl

agio

clas

e cr

ysta

ls (

Pl).

C �

Cor

rode

d pl

agio

clas

e (P

l) e

nclo

se p

oiki

litic

xen

omor

phic

qua

rtz

incl

usio

ns g

ivin

g a

spon

ge-l

ike

appe

aran

ce.

Not

e th

e ab

unda

nce

ofgr

ound

mas

s in

for

eign

qua

rtz

(Qz)

cry

stal

s (s

ilici

fica

tion)

. D

� C

ompl

etel

y re

sorb

ed h

ornb

lend

e ph

enoc

ryst

s (H

b) w

ith ty

pica

l ser

iate

text

ure

embe

dded

in q

uart

z-ep

idot

e ri

ch g

roun

dmas

s.


Sericitized-Silicified Andesites

The sericitized-silicified lower andesites consist essentially of completely se-ricitized and kaolinitized plagioclase with seriate porphyritic texture. Quartzxenocrysts and opaque minerals form the secondary minerals. Apatite is themost common accessory phase. The groundmass encompasses the same phasesas the phenocrysts and occasionally interstitial glass. The sericitized-silicifiedupper andesites contain sericitized plagioclase glomeroporphyritic clots andchloritized hornblende with secondary quartz, Mn-epidote (piemontite) andopaque minerals. All the phenocrysts are set in a fine-grained groundmass high-ly enriched in sericite and quartz granoblastic.

Albitized-Silicified Andesites

The albitized-silicified lower andesites are porphyritic and contain primaryplagioclase, chloritized clinopyroxene with secondary quartz, epidote andopaque minerals. They are set in a devitrified fine-grained groundmass. The pla-gioclase (An15) suffers different degrees of kaolinization, sericitization and epi-dotization. Occasionally, epidotization is complete, so that epidote (mainly zoi-site) replaces most of the plagioclase crystals especially at the crystal cores.Plagioclases encloses granoblastic quartz and opaque minerals as poikilitic in-clusions giving spongy like appearance (Fig.2C). The groundmass is crypto tomicrocrystalline and contains quartz xenocrysts, sodic plagioclase laths, opaqueminerals, epidote and recrystallized biotite flakes. The albitized-silicified upperandesites consist essentially of epidotized plagioclase glomeroporphyritic clotsand highly resorbed hornblende with well developed opacite mantles (Fig. 2D).Quartz, piemontite, opaque minerals and sphene form secondary minerals. Thegroundmass is tuffaceous in nature and contains albitized plagioclase microlites,quartz-epidote filled amygdales and resorbed hornblende with felted to micro-litic texture.

Epidotized-Actinolitized Andesites

The epidotized-actinolitized lower andesites contain completely epidotizedplagioclase clots and actinolite with secondary quartz, chlorite, carbonate andopaque minerals set in quartzofeldspathic fine grained groundmass with amyg-daloidal and granoblastic texture. The epidotized-silicified upper andesites en-compass epidotized plagioclase, actinolite with secondary quartz, opaque miner-als and sphene. The groundmass consists of albitic plagioclase microlites,epidote, quartz and carbonate-chlorite filled amygdales with felted to fluidaltexture.


Chloritized Andesites

The chloritized andesites have a texture ranging from almost aphyric tostrongly porphyritic with dominant phenocrysts of epidotized plagioclase andchloritized clinopyroxene. Quartz, epidote, sphene and chlorite form the sec-ondary minerals. All the phenocrysts are set in a devitrified glassy groundmasswith hyaloplitic texture.

The petrographic investigations therefore indicate that chloritization is an ear-ly alteration that was operative in the lower andesites prior to extrusion of theupper andesites. Sericitization and albitization postdates the chloritization. Silic-ification is a later alteration that affects all the stratigraphic units and is super-imposed on previously mentioned alterations.

Geochemistry

Major and trace elements of the rock samples were determined by X-ray fluo-rescence (XRF) at the Institute of Mineralogy and Petrology, Bonn University,Germany, on a Phillips 1480 automated X-ray fluorescence spectrometer. Theprogram used for the calculation of element concentration was Oxiquant distrib-uted by Phillips for their XRF spectrometry. The chemical analyses are sub-jected to certain errors. The major element determinations have certain errors upto ± 2 rel.-%; those of trace elements have up to ± 8 rel.-% or even ± 12 rel.-%in the case of REE.

For clarity and brevity of description, several of the elements analyzed in thisstudy are grouped according to their geochemical characteristics namely; large-ion lithophile elements (LILE: K, Rb, Ba, Sr), first transition series metals(FTSM: Mn, Fe, Zn, Sc, Ti, Cr, Ni), high field strength elements (HFSE: Y, Zr,Nb) and light rare earth elements (LREE: La, Ce, Nd, Sm). The significance ofthese grouping and the geochemical characters of the altered rocks relative tothe least altered rock types will be discussed further below.

The whole rock geochemical data for the 20 analyzed altered samples in addi-tion to the least altered fresh rock samples (samples SIII.1 and MA.14) from thetwo Dokhan volcanic rock sequence are given in Table (1). The rock sampleswere collected from the various stratigraphic units and the alteration facies. Thesample suite comprises biotitized-silicified andesites (S.26, SVI.1, and SVI.3),sericitized-silicified andesites (S.46 and SIII.3), albitized-silicified andesites(S.47, SII.5, and SD.1), epidotized and chloritized andesites (SIV.5 and SF.1 re-spectively) from the altered lower andesites. Besides, sericitized andesites(MA.16, MIII.4, and MV.2), albitized andesites (MV.12, MIV.8, and MIV.11)and epidotized andesites (MA.11, MV.4, MI.6, and MA.42) have been collectedfrom the altered upper andesites. Sample locations are shown in Fig. (1). An-


Sam

ple

SIII

. 1S.

26

SVI.

1SV

I. 3

S. 4

6SI

II.3

S. 4

7SI

I. 5

SD. 1

SIV

. 15

SF. 1

fac

ies

LA. A

ndB

t Qz

And

Bt Q

z A

ndB

t Qz

And

Ser Q

z A

ndSe

r Q A

ndA

B Q

z A

ndA

b Q

z A

ndA

b Q

z A

ndEp

Act

And

Chl

And

SiO

2 (%)

59.2

8

66.2

6

64.5

7

64.3

9

60.5

9

64.3

6

58.5

1

61.4

559

.03

61

.94

62

.28

Ti

O2

1.00

0.86

0.79

0.83

0.99

0.89

1.08

0.8

20.

930.

990.

88A

l 2O

318

.10

15

.72

15

.40

16

.13

18

.00

16

.33

18

.63

17

.87

16

.89

14

.79

14

.95

Fe

2O3*

5.57

4.46

4.93

4.49

5.00

4.86

5.63

5.07

5.34

5.42

5.58

MnO

0.05

0.08

0.06

0.04

0.06

0.05

0.08

0.06

0.09

0.11

0.10

MgO

1.56

1.13

2.50

1.46

2.02

1.00

2.40

2.02

3.89

3.15

3.31

CaO

4.04

3.21

3.17

3.16

3.82

3.59

3.26

3.09

4.08

6.64

4.16

Na 2

O4.

384.

273.

523.

883.

823.

585.

515.

066.

444.

204.

16K

2O2.

301.

802.

443.

253.

183.

321.

472.

260.

680.

691.

88P 2

O5

0.31

0.26

0.26

0.25

0.31

0.29

0.36

0.31

0.28

0.29

0.23

L.O

.I**

1.80

1.40

1.19

1.10

1.86

1.2

6

1.90

1.38

1.34

0.87

2.12

Tota

l98

.39

99

.45

98

.83

98

.98

99

.65

99

.53

98

.83

99

.39

98

.99

99

.09

99

.65

Sc (p

pm)

4 4

4 6

8 3

75

1112

11V

9680

8199

113

84

107

97

111

10

9

106

C

r19

112

86

9023

1922

2213

6

131

13

7

Ni

1842

7044

1435

1816

3367

67C

un.

d.n.

d. 9

n.d.

76n.

d.10

14n.

d.n.

d.n.

d.Zn

5669

5835

3156

9373

8576

128

G

a21

19n.

d.n.

d.n.

d.n.

d.21

1920

2122

As

4656

5563

3168

2128

n.d.

3774

Pb15

1216

1915

912

1111

1629

Rb

6138

5163

4864

2132

139

54Sr

862

89

4

894

58

5

975

79

6

1026

86

9

622

77

6

601

B

a67

4

899

80

4

739

10

43

94

4

610

594

15

6

223

53

7

Y11

2210

1113

1116

1515

1418

Zr15

9

244

128

14

2

161

13

9

171

157

16

7

170

209

TA

BL

E 1

.Who

le r

ock

chem

ical

com

posi

tions

for

the

hydr

othe

rmal

ly a

ltere

d an

desi

tes.


Sam

ple

SIII

. 1S.

26

SVI.

1SV

I. 3

S. 4

6SI

II.3

S. 4

7SI

I. 5

SD. 1

SIV

. 15

SF. 1

fac

ies

LA. A

ndB

t Qz

And

Bt Q

z A

ndB

t Qz

And

Ser Q

z A

ndSe

r Q A

ndA

B Q

z A

ndA

b Q

z A

ndA

b Q

z A

ndEp

Act

And

Chl

And

Nb

7 8

4 5

7 7

6 5

7 9

10La

4744

3247

3444

2645

3834

39C

e64

5339

2436

5250

3649

4662

Nd

3539

2321

2820

2032

2829

31

S.G

.***

2.73

2.71

2.68

2.73

2.78

2.71

2.78

2.76

2.71

2.75

2.67

Ti/Z

r 3

8 2

1 3

7 3

5 3

7 3

8 3

8 3

1 3

3 3

5 2

5A

l/Zr

602

341

637

601

592

622

577

602

535

460

379

P/Zr

8.50

4.70

8.90

7.70

8.40

9.20

9.20

8.60

7.30

7.40

4.80

Rb/

Zr0.

380.

160.

400.

440.

300.

460.

120.

200.

080.

050.

26Sr

/Zr

5.40

3.70

7.00

4.10

6.10

5.70

6.00

5.50

3.70

4.60

2.90

Ba/

Zr4.

203.

706.

305.

206.

506.

803.

603.

800.

931.

302.

60Y

/Zr

0.07

0.09

0.08

0.08

0.08

0.08

0.09

0.10

0.09

0.08

0.09

Nb/

Zr0.

040.

030.

030.

040.

040.

050.

040.

030.

040.

050.

05La

/Zr

0.30

0.18

0.25

0.33

0.21

0.32

0.15

0.29

0.23

0.20

0.19

Ce/

Zr0.

400.

220.

300.

170.

220.

370.

290.

230.

290.

270.

30N

d/Zr

0.22

0.16

0.18

0.15

0.17

0.14

0.12

0.20

0.17

0.17

0.15

TA

BL

E 1

. (C

ontin

ued)


Sam

ple

MA

. 14

MA

. 16

MII

I. 4

MV

. 2M

V. 1

2M

IV. 8

MIV

. 11

MA

. 11

MV

. 4M

I. 6

MA

. 42

faci

esLA

. And

Ser Q

z A

ndSe

r Qz

And

Ser Q

z A

ndA

b Q

z A

ndA

b Q

z A

ndA

b Q

z A

ndEp

Qz

And

Ep Q

z A

ndEp

Qz

And

Ep Q

z A

nd

SiO

2 (%)

60.3

9

64.0

559

.11

63

.65

61

.93

65

.16

65

.16

63

.69

65.5

3

65.5

1

64.1

3 Ti

O2

0.97

0.91

0.99

0.84

0.90

0.86

0.85

0.87

0.81

0.79

0.89

Al 2

O3

15.5

3

14.7

1

15.3

6

14.2

6

15.1

4

14.8

8

14.3

8

14.7

1 14

.27

15

.00

14

.47

Fe

2O3*

5.63

5.05

5.59

4.68

5.06

4.69

4.76

4.99

4.77

4.11

4.92

MnO

0.09

0.07

0.08

0.09

0.07

0.06

0.07

0.07

0.05

0.06

0.09

MgO

3.71

3.08

3.63

2.96

2.28

2.31

2.76

2.39

1.90

2.09

2.43

CaO

5.31

2.43

4.29

3.62

2.95

2.56

3.29

3.74

3.05

4.24

5.27

Na 2

O3.

203.

743.

883.

824.

905.

124.

124.

094.

394.

113.

19K

2O2.

713.

012.

392.

402.

892.

182.

352.

501.

562.

112.

18P 2

O5

0.28

0.27

0.28

0.26

0.27

0.25

0.25

0.25

0.23

0.23

0.27

L.O

.I**

1.59

1.67

2.79

2.10

1.92

1.02

1.04

1.02

1.39

1.08

1.25

Tota

l99

.63

99

.26

98

.68

98

.97

98

.58

99

.34

99

.24

98

.57

98

.29

99

.55

99

.34

Sc (p

pm)

94

10 4

7

7 3

7 3

3 3

V11

5

8410

9

89

3263

8110

5

7487

61C

r15

8

109

17

2

125

10

6

102

94

129

10

9

6987

Ni

3834

4831

25

4342

3426

2123

Cu

1063

n.d.

n.d.

8n.

d. 4

1210

16

n.

d.n.

d.Zn

8679

8076

7165

6374

6657

71G

a21

1421

1514

1718

1711

2020

As

2060

7525

3561

5626

n.d.

6414

2

Pb6

718

n.d.

11 5

1212

n.d.

1410

Rb

4448

n.d.

4744

3541

3829

4143

Sr10

75

54

8

835

93

5

747

56

2

683

62

1

607

71

3

479

B

a67

4

1204

1172

1066

905

664

67

2

777

53

8

553

54

2

Y15

1717

16 1

516

1614

1418

18Zr

177

16

1

171

155

17

014

8

155

16

0

149

15

9

164

TA

BL

E 1

. (C

ontin

ued)


Sam

ple

MA

. 14

MA

. 16

MII

I. 4

MV

. 2M

V. 1

2M

IV. 8

MIV

. 11

MA

. 11

MV

. 4M

I. 6

MA

. 42

faci

esLA

. And

Ser Q

z A

ndSe

r Qz

And

Ser Q

z A

ndA

b Q

z A

ndA

b Q

z A

ndA

b Q

z A

ndEp

Qz

And

Ep Q

z A

ndEp

Qz

And

Ep Q

z A

nd

Nb

8 9

9 7

8

6 9

8 9

10 8

La39

4235

4053

4031

4450

3948

Ce

4366

3737

5639

5733

4257

60N

d27

3625

2029

3730

2030

3124

S.G

.***

2.73

2.65

2.71

2.71

2.67

2.70

2.70

2.68

2.65

2.70

2.72

Ti/Z

r 3

3 3

4 3

5 3

2 3

2 3

5 3

3 3

1 3

3 3

0 3

3A

l/Zr

464

484

475

487

471

532

491

487

507

499

467

P/Zr

6.90

7.30

7.10

7.30

6.90

7.40

7.00

6.80

6.70

6.30

7.20

Rb/

Zr0.

250.

300.

350.

300.

260.

240.

260.

240.

190.

260.

26Sr

/Zr

6.00

3.40

4.90

6.00

4.40

3.80

4.40

3.90

4.10

4.50

2.90

Ba/

Zr3.

807.

506.

906.

905.

304.

504.

304.

903.

603.

503.

30Y

/Zr

0.08

0.11

0.10

0.10

0.09

0.11

0.10

0.09

0.09

0.09

0.11

Nb/

Zr0.

050.

060.

050.

050.

050.

040.

060.

050.

060.

060.

05La

/Zr

0.22

0.26

0.20

0.26

0.31

0.27

0.20

0.28

0.34

0.25

0.29

Ce/

Zr0.

240.

410.

220.

240.

330.

260.

370.

210.

280.

360.

37N

d/Zr

0.15

0.22

0.15

0.13

0.17

0.25

0.19

0.13

0.20

0.19

0.15

TA

BL

E 1

. (C

ontin

ued)

LA

. And

= L

east

alte

red

ande

site

(as

sum

ed p

roto

lith)

Bt Q

z A

nd =

Bio

tite

quar

tz a

ndes

iteSe

r Q

z A

nd =

Ser

icite

qua

rtz

ande

site

Ab

Qz

And

= A

lbite

qua

rtz

ande

site

Ep

Qz

And

= E

pido

te q

uart

z an

desi

teE

p A

ct A

nd =

Epi

dote

act

inol

ite a

ndes

iteC

hl A

nd =

Chl

orite

and

esite

*

= T

otal

iron

as

Fe2O

3**

=

% w

t. lo

ss o

n ig

nitio

n**

* =

Cal

cula

ted

spec

ific

gra

vity

n.d.

= n

ot d

eter

min

ed


desite as used here is describes the rocks with intermediate silica content withinthe range 58.5-66% SiO2Æ

The geochemical characteristics of the least-altered andesites (assumed proto-liths) and the major alteration facies at Gabal El Dokhan (Fig.3) are describedbelow. Alteration must also be evaluated before the petrogenesis and tectonicsetting of ancient volcanic rocks can be interpreted (Condie, 1981; and Luddenet al. 1982). The superposition of some alteration types (e.g., sericitization onsilicification) hampers resolution of the precise alteration signatures of some fa-cies. Many of the rock samples, however, are dominated by one type of altera-tion, so they are discussed below in terms of that alteration.

Least-Altered Andesites

The least-altered rocks of Dokhan andesites (SIII.1, and MA.14) and theirequivalent rock types in the studied area are classified as calc-alkaline volcanicson the basis of major element geochemistry (Irvine and Baragar,1971). TheDokhan andesites show pronounced enrichment in LILE and LREE and slightdepletion in HFSE. The upper andesites are characterized by strong FTSM en-richment relative to the lower andesites. On the other hand, the chemistry of thelower andesites displays relative enrichment in total alkalies and LILE if com-pared with the upper andesites. The lower and upper andesites correspond toAndean type subduction-related magmatism; i.e. diagnostic ratios for arc lavassuch as high LILE/LREE, LILE/HFSE and LREE/HFSE characterize the entirelava suites. Chondrite-normalized REE abundance for andesitic volcanics showLREE enrichment and low HREE pattern concentration with less negative Euanomaly. Khalaf (1994) concluded that polybaric fractional crystallization withassimilation and /or magma mixing models can explain the evolution of theDokhan volcanics at their type locality, Gabal El Dokhan.

Biotitization-Silicification

This type of metasomatism is widespread particularly in andesitic rock types,which lie near the intrusive granites. The typical reaction includes replacementof feldspar by biotite/or sericite and sometimes plagioclase by potash feldspar.The presence of pseudomorphous replacement of the phenocrysts by shreddy bi-otite is diagnostic of potassium metasomatism. Biotite-quartz alterations of thelower andesites are characterized by LILE (K-Rb-Ba) and FTSM (Cr-Ni) en-richment and depletion in LREE, relative to the least-altered andesite (SIII.1).FTSM elements are accommodated in biotite crystals, while K-Rb-Ba elementsare concentrated in biotite, sericite and orthoclase. The relative Ca and P de-pletion probably reflects the breakdown of apatite. The remaining elements ex-hibit minor dispersion, but appear to have remained relatively immobile.


FIG. 3

. D

iagr

ams

show

ing

calc

ulat

ed e

nric

hmen

t dep

letio

n of

ele

men

ts in

alte

red

ande

site

s re

lativ

e to

the

leas

t-al

tere

d (s

olid

bar

s in

A-H

) an

desi

tes.

The

rel

ativ

e en

rich

men

t de

plet

ion,

-lo

g K

v re

pres

ents

the

den

sity

wei

ghte

d ra

tio o

f th

e ab

unda

nces

of

the

com

pone

nts

in t

he i

nfer

red

prot

o-lit

h an

d al

tere

d ro

ck

(Eqn

.3

in

text

).

Not

e th

at

the

actu

al

degr

ee

of

enri

chm

ent

depl

etio

n de

pend

s on

th

e tr

ue

valu

e of

K

v.


Sericitization-Silicification

Sericitization in the lower and upper volcanic sequences exhibit K-Rb-Ba en-richment and slight depletion in Ca, Sr and LREE especially in the altered lowerandesites, relative to the least-altered andesitic rocks (SIII.1 and MA.14 re-spectively). The large alkali elements are concentrated in sericite during thebreakdown of feldspar, whereas divalent Ca and Sr are excluded. The sericitizedandesites are enriched in Si relative to the fresh andesites, reflecting silic-ification (albitization?). The upper sericitized andesites show pronounced Cu-Pb enrichment, relative to the least-altered andesite (MA.14). The remainingelements exhibit minor deviation and behave as immobile elements.

Albitization-Silicification

The albite-quartz alteration is characterized by Na enrichment and slight de-pletion in LILE (K-Rb-Sr-Ba) relative to the least-altered andesites. During al-bitization of feldspar, Na is retained in albite, but LIL elements are excludedduring devitrification of volcanic glass and its alteration products which containthese elements. In many instances, devitrification of acidic glasses, particularlywhen induced by hydrothermal solutions, leads to major changes in the totalrock composition, affecting the glass as well as the crystalline phases of therock. The general trend is a metasomatic replacement of Na by K, an addition ofSi and H2O, and more rarely, of small amount of Al as well as an increase in the

Fe3+/Fe2+ ratio (Ewart, 1971). The lower albitized andesites exhibit enrichmentin FTSM elements which are accommodated in the chloritized clinopyroxene.With the exception of minor P, the remaining HFSE are immobile. Those liber-ated during the breakdown of the igneous assemblages (and/or lower grade pre-cursors) are accommodated in the alteration assemblages.

Epidotization-Chloritization

The epidote-chlorite-quartz alteration of the two andesitic sequences is char-acterized by mild Ca enrichment and strong depletion in LILE (K-Rb-Sr-Ba) rel-ative to the least altered andesites. The lower epidotized andesites exhibit strongenrichment in FTSM elements (Mn-Mg-Sc-Cr-Ni) which are accommodated inchlorite, while Ca is accommodated in epidote. The LIL elements are expelled,probably reflecting the breakdown of volcanic glass and its alteration products.The remaining elements show minor dispersion behaving as immobile elements.

Chloritization

The chlorite alteration is characterized by strong enrichment in FTSM ele-ments and slight depletion in P, LILE and LREE, relative to the least altered an-desite (SIII.1). FTSM enrichment reflects complete replacement of pyroxene bychlorite and epidote which accommodates these elements.


Composition-Volume Computations

The observed major and trace element concentrations in the alteration prod-ucts are mainly controlled by chemical properties of the precipitates. Many geo-chemical studies have demonstrated the mobility of major and trace elementsduring hydrothermal alteration. Such processes are controlled by pressure, tem-perature, fluid composition, waterrock ratio and time (Lesher et al. 1986). Sev-eral geochemical studies (Condie et al. 1977) have demonstrated the mobility ofthe large ion lithophile elements (LILE). In contrast, HFSE are potentially muchless mobile. On the other hand, Ludden et al. (1982) and Nystrom (1984) dem-onstrated the potential mobility of the light rare earth elements (LREE) duringextreme epidotization and carbontization. P and U+6 are also potentially mobileÆThey have high ionic potentials and may be soluble as complex anions in thehydrothermal fluids (Mason, 1966). The first transition series metals (FTSM: Scthrough Cr, Mn through Zn) have different crystallographic site preference en-ergies and may exhibit several valencies, so that their geochemical behavior issomewhat less predictable. Their mobility is largely dependent on the stabilityof the mafic silicate phases which contain them.

To evaluate major and trace element mobilities in the studied altered volcanicrocks and place-constraints on volume change during such alteration, composi-tion-volume calculations are performed following the methods of Gresens(1967) and Avison (1985). Gresens (1967) derived equations, which were latermodified by Babcock (1973), to express the chemical relationship between thewhole-rock composition of metasomatic rocks and a parental or initial rockcomposition (assumed protolith), quantifying the actual exchange of materialbetween parent and product rocks. Gresen's basic argument is that some com-ponents are likely to have been immobile in the alteration process, and that ifthese components can be identified, they can be used to establish any volumechange which has taken place. Gains or losses of other components can then becalculated assuming that the volume change is a factor common to the behaviorof all components. Gresens general equation can be expressed as follows:

a {KV (PB/PA) Xn

B � XnA } � ∆Xn = 0 1

Where KV is the ratio between rock A volume to that of rock B; PA and PB are therock densities, Xn

A and XnB are the weight fractions of element n, and ∆Xn is the

absolute mass flux of n between rocks B and A. At constant volume (KV = 1) :

∆Xn = { XnB ( PB/PA ) - Xn

A } 2

and for constant mass (∆Xn = 0) :

KV = { XnA PA/Xn

B PB } 3


Avison (1985) suggested that the numerical magnitude of ∆Xn is not directlycomparable with that for other rock components. Such relationships can be clar-ified if the values of ∆Xn are normalized relative to the original parent rock con-tent of each component n, so that:

∆Fn = ∆Xn / XnA 4

Using the known compositions and densities of the rocks (Table 1), solutionsto Eqn. (1) are linear functions of ∆Xnand KV. Graphical analysis (Fig. 3 in Gre-sens, 1967) of composition-volume relationships between the least-altered an-desite (assumed protolith) and the altered andesites and associated andesiticrocks indicates that the HREE and most HFS elements have been relatively im-mobile. The linear solutions to Eqn. (1) intersect in clusters corresponding tonegligible mass changes within a very limited range of volume factors. Thus,the composition-volume relationships can be mathematically calculated (Table2) and graphically illustrated on simple enrichment-depletion diagrams fol-lowing the method of Lesher et al. (1986). In this method, the degree of enrich-ment/depletion is represented by KV for ∆Xn = 0 (Fig. 3). Propagation of sam-pling and analytical uncertainties in the calculations result in dispersion in KVfor most elements. The assessment of the composition-volume change can bedetected if certain elements have a constancy values of KV and ∆Fn which canbe interpreted that those elements have remained relatively immobile (Gresens,1967 and Campbell et al. 1984). The value of the composition-volume com-putations is revealed from Fig. (3) and Table (2) using equations (3) and (4)when performed on the bulk analysis of the least-altered andesites and differentalteration facies of propylitic andesites. The indicated volume factors (log KV)for altered rock types range from 1.00 to 1.39 for SIII.3 and SVI.3, through ±0.7 for SD.1 and SIV.15 and, to �1.39 for SF.1 (Fig.3 A, B, C, D, E and Table2). These correspond to volume increase up to 10 and 25% in sericitized and bi-otitized-silicified andesites respectively, conservation of volume in albitized-epidotized-silicified andesites and volume decrease up to 25% in chloritized an-desite. Concerning the upper andesites, the indicated volume factors extendfrom 1.00 through 1.17 to 1.30 for MA.16, MIV.8 and MA.4 (Fig.3 F, G, H andTable 2). These reflect volume increase of up to 10, 15 and 20% in sericitized,albitized and epidotized-silicified andesites respectively. The similarity of somemajor and HFS element ratios in the entire suites and the similarity of HFS ele-ment abundance in the least-altered andesites (SIII. and MA.14) and the mostaltered rock samples confirm that the use of SIII.1 and MA.14 as the least al-tered rock types is justified. Thus, elements which preserve interelement ratioshave probably remained relatively immobile.

Composition-Volume Changes during Metasomatic Alteration of... 195T

AB

LE

2.

Cal

cula

ted

valu

es o

f K

v an

d ∆F

n fo

r hy

drot

herm

ally

alte

red

ande

site

s.

S. 24

7SI

I. 5

SD. 1

SIV.

15SF

. 1

Ab Q

z A

ndAb

Qz

And

Ab Q

z A

ndEp

Act

And

Chl

And

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

1

.00� 0

.01�

4.00

0.04

0.0

0 0

.00� 4

.00 0

.04�

5.00

0.05

� 7

.00 0

.08 2

1.00

� 0.18

7.0

0� 0

.07 1

.00� 0

.01

14.00

� 0

.12�

3.00

0.03

1.

00� 0

.01

7.00

� 0.07

22.0

0� 0

.18� 2

1.00

0.1

7�

1.00

0.01

10.0

0� 0

.09

4.00

� 0.04

3.00

� 0.03

0.

00 0

.00�

38.00

0.60

� 17.0

0 0

.20� 4

4.00

0.80

� 55.0

0

1.20

0� 5

0.00

1.00

� 35

.00 0

.54� 2

3.00

0.29

� 60.0

0 1

.49� 5

0.00

1.02

� 53.0

0 1

.12

24.0

0� 0

.19 3

1.00

� 0.24

� 1.

00 0

.01� 3

9.00

0.64

� 3.

00 0

.03�

21.00

0.26

� 13.0

0 0

.16� 3

2.00

0.47

4.0

0� 0

.04

5.00

� 0.05

56

.00� 0

.36

2.00

� 0.02

238

� 0.70

233

� 0.70

22.0

0� 0

.18�

14.00

0.06

0.

00 0

.00 1

1.00

� 0.10

� 7.00

0.06

� 35.0

0 0

.26�

43.00

0.75

� 20.0

0 0

.25� 6

4.00

1.75

� 67.0

0 2

.00� 6

4.00

1.75

� 10

.00 0

.11�

1.00

0.01

� 14.0

0 0

.16� 1

2.00

0.14

� 8.

00

0.10

� 14

.00 0

.16� 1

4.00

0.16

� 86.0

0 6

.16� 8

5.00

5.89

� 86.0

0 6

.21

0.0

0 0

.00 1

3.00

� 0.11

� 45.0

0 0

.83� 7

3.00

2.72

� 73.0

0 2

.72

0.0

0 0

.00

0.00

0.00

0.

00 0

.00

0.00

0.00

0.0

0 0

.00�

40.00

0.66

� 23.0

0 0

.30� 3

4.00

0.52

� 26.0

0 0

.36� 5

6.00

1.29

13

.00� 0

.14 1

1.00

� 0.10

12.0

0� 0

.11 1

3.00

� 0.14

15.0

0� 0

.1411

9.0� 5

4.0 6

4.00

� 0.39

0.

00 0

.00 2

4.00

� 0.20

�38.0

0 0

.61

25.00

� 0.20

36.0

0� 0

.27 3

6.00

� 0.27

� 6.00

0.07

� 48.0

0 0

.9319

0� 0

.66 91

.0� 0

.4836

9� 0

.79 5

76� 0

.85 1

3.00

� 0.11

� 16

.00 0

.19�

1.00

0.01

39.0

0� 0

.28 1

1.00

� 0.10

43.0

0� 0

.30 1

0.00

� 0.09

13.0

0� 0

.1233

2� 0

.77 2

02� 0

.67 2

6.00

� 0.20

� 31

.00 0

.45� 2

7.00

0.36

� 27.0

0 0

.36� 2

1.00

0.27

� 39.0

0

.64�

7.00

0.0

8

1.00

� 0.01

5.

00� 0

.05� 6

.00 0

.07� 2

4.00

0.31

17

.00� 0

.14 4

0.00

� 0.28

0.0

0 0

.00� 2

2.00

0.29

� 30.0

0 0

.43

81.00

� 0.45

4.

00� 0

.04 2

4.00

� 0.19

38.0

0� 0

.28 2

1.00

� 0.17

28

.00� 0

.22 7

8.00

� 0.44

31.0

0� 0

.23 3

9.00

� 0.28

3

.000

� 0.03

75

.00� 0

.43 9

.00� 0

.09 2

5.00

� 0.20

21.0

0� 0

.17 1

3.00

� 0.11

� 5Ze

ro+

5� 5

� 25

S.

26SV

I. 1

SVI.

3S.

46SI

II. 3

Bt Q

z A

nd

Bt Q

z A

ndBt

Qz

And

Ser O

z A

ndSe

r Oz

And

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

SiO 2 (%

)� 1

1.0

0.12

� 8.0

0 0

.09�

8.00

0.09

� 2.

00 0

.02�

8.00

0.09

TiO 2

16.0

0� 0

.14 2

7.00

� 0.21

20.0

0� 0

.17

1.00

� 0.01

12.0

0� 0

.11Al

2O 3 1

5.00

� 0.13

18.0

0� 0

.15 1

2.00

� 0.11

1.

00� 0

.01 1

1.00

� 0.10

Fe2O 3

25.0

0� 0

.20 1

3.00

� 0.11

24.0

0� 0

.19 1

1.00

� 0.10

15.0

0� 0

.13M

nO� 3

8.00

0.60

� 17.0

0 0

.20 2

5.00

� 0.20

� 17.0

0 0

.20

0.00

0.00

MgO

38.0

0� 0

.28�3

8.00

0.60

7.

00� 0

.06� 2

3.00

0.29

56.0

0� 0

.36Ca

O 2

6.00

� 0.21

27.0

0� 0

.22 2

8.00

� 0.22

6.

00� 0

.05 1

3.00

� 0.11

Na2O

3.

00� 0

.03 2

4.00

� 0.20

13.0

0� 0

.11 1

5.00

� 0.13

22.0

0� 0

.18K 2O

28.0

0� 0

.22 �

6.00

0.06

� 29.0

0 0

.41� 2

8.00

0.38

� 31.0

0 0

.44P 2O 5

19.0

0� 0

.16 1

9.00

� 0.16

24.0

0� 0

.19

0.00

0.00

7.

00� 0

.06Sc

(ppm

)

0.00

0.00

0.0

0 0

.00� 3

3.00

0.50

� 50.0

0 1

.00 3

3.00

� 0.30

V 2

0.00

� 0.17

19.0

0� 0

.16� 3

.00 0

.03� 1

5.00

0.18

14.0

0� 0

.13Cr

� 83.0

0 5

.00� 7

8.00

3.53

� 79.0

0 3

.74� 1

7.00

0.21

0.

00

0.00

Ni� 5

7.00

1.33

� 74.0

0 2

.89� 5

9.00

1.44

29.0

0� 0

.22� 4

9.00

0.94

Cu

0.00

0.00

0.

00 0

.00

0.00

0.00

0.

00 0

.00

0.00

0.0

0Zn

� 19.0

0 0

.23�

3.00

0.04

60

.00� 0

.38� 2

6.00

0.36

0.

00 0

.00Ga

11

.00� 0

.10

0.00

0.00

0.

00 0

.00

0.00

0.00

0.

00 0

.00As

� 18.0

0 0

.22� 1

6.00

0.20

� 27.0

0 0

.37 4

8.00

� 0.33

� 32.0

0 0

.49Pb

25

.00� 0

.20�

6.00

0.07

� 6.

00 0

.07

0.00

0.00

67.0

0� 0

.40Rb

61

.00� 0

.38 2

0.00

� 0.16

� 3.

00 0

.03 2

7.00

� 0.21

� 5.

00 0

.05Sr

� 4.

00 0

.04� 4

.000.0

4 4

7.00

� 0.32

� 12.0

0 0

.13

8.00

� 0.08

Ba� 2

5.00

0.33

� 16.0

00.1

9�

9.00

0.10

� 35

.0

0.55

� 29.0

0.40

Y� 5

0.00

1.00

10.0

0� 0

.09

0.00

0.00

� 15.0

0 0

.18

0.00

0.00

Zr

25.00

� 0.53

24.0

0� 0

.19 1

2.00

� 0.11

1.

00 0

.0114

.00� 0

.13Nb

� 13.0

0 0

.14 7

5.00

� 0.43

40.0

0� 0

.29 n

.d. n

.d. n

.d.n.d

.La

7.

00� 0

.06 4

7.00

� 0.32

0.

00 0

.00

38.00

� 0.28

7.

00� 0

.06Ce

21

.00� 0

.17 6

4.00

� 0.39

16

7� 0

.63 7

8.00

� 0.44

23.0

0� 0

.19Nd

� 10.0

0 0

.11 5

2.00

� 0.35

67.0

0� 0

.40 2

5.00

� 0.20

75.0

0� 0

.43Vo

lum

e +

20+

20+2

0Ze

ro +

10 ch

ange

Sam

ple

facie

s


MIV

. 11

MA.

11M

V. 4

MI.

6M

A. 42

Ab Q

z A

ndEp

Qz

And

Ep Q

z A

ndEp

Qz

And

Ep Q

zAn

d

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

� 7

.00 0

.08�

5.00

0.05

� 8.

00 0

.09�

7.00

0.08

� 6.

00 0

.06 1

4.00

� 0.12

11.0

0� 0

.10 2

0.00

� 0.16

23.0

0� 0

.19

9.00

� 0.08

8.

00� 0

.07� 0

.06� 0

.05

9.00

� 0.08

4.

00� 0

.03

7.00

� 0.07

18.0

0� 0

.15 1

3.00

� 0.11

18.0

0� 0

.15 3

7.00

� 0.27

14.0

0� 0

.13 2

9.00

� 0.22

29.0

0� 0

.22 8

0.00

� 0.44

50.0

0� 0

.33

0.00

0.00

34.0

0� 0

.26 5

5.00

� 0.36

95.0

0� 0

.49 7

8.00

� 0.44

53.0

0� 0

.35 6

1.00

� 0.38

42.0

0� 0

.30 7

4.00

� 0.43

25.0

0� 0

.20

1.00

� 0.01

� 22.0

0 0

.29 2

2.00

0.28

� 27.0

0 0

.37� 2

2.00

0.28

0.

00 0

.00 1

5.00

� 0.13

8.

00� 0

.08 7

4.00

� 0.42

28.0

0� 0

.22 2

4.00

� 0.20

12.0

0� 0

.11 1

2.00

� 0.11

22.0

0� 0

.18 2

2.00

� 0.18

4.

00� 0

.0420

0� 0

.67 2

9.00

� 0.22

200

� 0.67

200

� 0.67

200

� 0.67

42.0

0� 0

.30 1

0.00

� 0.09

55.0

0� 0

.36 3

2.00

� 0.24

89.0

0� 0

.47 6

8.00

� 0.41

22.0

0� 0

.18 4

5.00

� 0.31

129

� 0.56

82.0

0� 0

.45� 1

0.00

0.11

12.0

0� 0

.11 3

1.00

� 0.32

81.0

0� 0

.45 6

5.00

� 0.39

150

� 0.60

17.0

0� 0

.20� 9

9.00

100

0.

00 0

.00

0.00

0.00

37.0

0� 0

.27 1

6.00

� 0.14

� 77.0

0 0

.23 5

1.00

� 0.16

21.0

0� 0

.17

0.00

0.00

0.

00 0

.00

0.00

0.00

0.

00 0

.00

0.00

0.00

� 68.0

0 2

.00� 0

.46 1

.00

0.00

0.00

� 69.0

0 2

.00� 8

6.00

6.00

� 13.0

0 0

.14� 5

0.00

1.00

� 50.0

0 1

.00

0.00

0.00

� 57.0

0 1

.50

7.00

� 0.07

16.0

0� 0

.14 5

2.00

� 0.34

7.

00� 0

.07

2.00

� 0.02

57.0

0� 0

.36 7

3.00

� 0.42

77.0

0� 0

.44 5

1.00

� 0.34

124

� 0.55

0.0

0 0

.00� 1

3.00

0.15

25.0

0� 0

.20 2

2.00

� 0.20

24.0

0� 0

.20�

6.00

0.07

7.

00� 0

.07

7.00

� 0.07

0.

00 0

.00� 1

7.00

0.20

14� 0

.12 1

1.00

� 0.10

19.0

0� 0

.16 1

1.00

� 0.10

8.

00� 0

.07n.d

.n.d

.n.d

.n.d

.n.d

.n.d

.n.d

.n.d

.n.d

.n.d

. 2

6.00

� 0.21

� 11.0

0 0

.13� 2

2.00

0.28

0.

00 0

.00 8

1.00

� 0.23

� 25.0

0 0

.33 3

0.00

� 0.23

2.

00� 0

.02� 2

5.00

0.14

72.0

0� 0

.40� 1

0.00

0.11

35.0

0� 0

.26� 1

0.00

0.11

� 13.0

0 0

.11 1

3.00

� 0.11

+ 15

+ 10

+ 20

+ 10

+ 10

M

A. 16

MIII

. 4M

V. 2

MV.

12M

IV. 8

Se

r Oz

And

Ser O

z A

ndSe

r Oz

And

Ab O

z A

ndAb

Oz

And

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

K v

∆Fn

SiO 2 (%

)�

6.00

0.06

2.

00� 0

.02�

5.00

0.05

� 2.0

0 0

.03�

7.00

0.08

TiO 2

7.

00� 0

.06�

2.00

0.02

15.0

0� 0

.13

8.00

� 0.07

13.0

0� 0

.11

Al2O 3

6.

00� 0

.05

1.00

� 0.01

9.

00� 0

.08

3.00

� 0.03

4.

00� 0

.04

Fe2O 3

11.0

� 0

.10

1.00

� 0.01

20.0

0� 0

.17 1

1.00

� 0.10

20.0

0� 0

.17

MnO

29.0

0� 0

.22 1

3.00

� 0.33

0.

00 0

.00 2

9.00

� 0.22

50.0

0� 0

.33

MgO

20.0

0� 0

.17

2.00

� 0.02

25

.00� 0

.20 6

3.00

� 0.39

61.0

0� 0

.38

CaO

119

� 0.54

24.0

0� 0

.19 4

7.00

� 0.32

80.0

0� 0

.44 1

07� 0

.52

Na2O

� 14.0

0 0

.17� 1

8.00

0.34

� 16.0

0 0

.19� 6

5.00

0.53

� 38.0

0

0.60

K 2O

� 10.0

0 0

.11 1

3.00

� 0.12

13

.00� 0

.11�

6.00

0.07

24.0

0� 0

.20

P 2O 5

4.00

� 0.03

0.

00 0

.00

8.00

� 0.07

4.

00� 0

.04 1

2.00

� 0.11

Sc

(ppm

) 1

25� 0

.56� 1

0.00

0.11

125

� 0.56

29.0

0� 0

.22 2

9.00

� 0.22

V

37.0

0� 0

.27

6.00

� 0.05

29.0

0� 0

.2325

9� 0

.72 8

3.00

� 0.45

Cr

45.0

0� 0

.31�

8.00

0.09

26.0

0� 0

.21 4

9.00

� 0.33

55.0

0� 0

.35

Ni 1

2.00

� 0.11

� 21.0

0 0

.26 2

3.00

� 0.18

52.0

0� 0

.34� 1

2.00

0.13

Cu� 8

4.00

5.00

0.

00 0

.00

0.00

0.0

0 2

5.00

� 0.20

0.

00 0

.00

Zn

9.00

� 0.08

8.

00� 0

.07 1

3.00

� 0.07

27.0

0� 0

.17 3

2.00

� 0.2

4

Ga

0.00

0.00

0.

00 0

.00

0.00

0.00

0.

00 0

.00

0.00

0.0

0

As�

0.66

2.00

� 0.61

3.00

� 0.

45 1

.00� 4

3.00

0.75

� 67.0

02.0

0Pb

� 14.0

0 0

.17� 6

7.00

2.00

0.

00 0

.00� 4

5.00

0.83

� 13.0

0 0

.83

Rb�

8.00

0.09

00.0

0 0

.00�

6.00

0.07

0.

00 0

.00 2

6.00

� 0.20

Sr

96.0

0� 0

.49 2

9.00

� 0.22

15.0

0� 0

.13 4

4.00

� 0.31

91.0

0 0

.48

Ba� 4

4.00

0.79

� 42.0

0 0

.74� 3

6.00

0.58

� 26.0

0 0

.34

2.00

� 0.01

Y

� 12.0

0 0

.13� 1

2.00

0.13

� 6.

00 0

.07

0.00

0.00

� 6.00

0.07

Zr 1

0.00

� 0.09

4.

00� 0

.03 1

4.00

� 0.12

4.

00� 0

.04 2

0.00

� 0.16

Nb

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

La�

7.00

0.08

11.0

0� 0

.10 �

3.00

0.03

� 26.0

0 0

.36�

3.00

0.02

Ce

� 35.0

0 0

.53 1

6.00

� 0.14

1

6.00

� 0.14

� 23.0

0 0

.30 1

0.00

� 0.0

9

Nd� 2

5.00

0.33

8.

00� 0

.07

35.0

0� 0

.26�

7.00

0.07

� 27.0

0 0

.37

Volu

me

+ 10

Zero

+ 15

+ 5

+ 15

chan

ge

Sam

ple

facie

s

TA

BL

E 2

. (C

ontin

ued)


Discussion

The geological interpretations of composition and volume changes in thestudied altered volcanic rocks are impeded by several uncertainties. First, thepresent metamorphic mineral assemblages are not necessarily those of the orig-inal alteration assemblage. Second, the hydrothermal alteration probably oc-curred under varying physiochemical conditions during the dynamic evolutionof the volcanic pile and the geothermal system, resulting in continuous and su-perimposed alteration. Third, the REE behavior in hydrothermal system is verycomplex, and there appear to be no simple rule applicable to the wide range ofgeological hydrothermal situations (Lesher et al. 1986 and Terakado and Fu-jitani, 1998). In spite of these limitations, the geological results can be madethrough comparison with the experimental studies and with mineral as-semblages predicted by equilibrium thermodynamic calculations. The chlorite -actinolite - albite - epidote - quartz - opaque minerals/sphene mineral as-semblages of the altered volcanic rocks are typical of low grade, regionally met-amorphosed mafic volcanic rocks (Jolly, 1980; and Gelinas et al. 1982). Similarmineral assemblages are forming in the basaltic and andesitic reservoir rocks inIcelandic high temperature (more than 200ºC) geothermal systems (Tomassonand Kristmannsdottir, 1972; and Kristmannsdottir, 1975).

Silicification

Silicification is a pronounced metasomatic alteration facies in the andesiticvolcanics and results from the addition of silica. The concentration of dissolvedSiO2 equals or exceeds quartz saturation in most experimental studies at 150 to500ºC (Mottl, 1983). Experimental studies of silica solubility in water (Ken-nedy, 1950) indicate that quartz solubility is strongly dependent on temperatureat temperatures above the water-vapor critical point (374ºC, 218 bars). Only attemperatures less than about 350ºC and at higher temperatures with pressuresgreater than 900 bars, quartz solubility decreases markedly with decreasing tem-perature. Accordingly, the silicification probably resulted from cooling of hy-drothermal fluids that are initially less than 350ºC. With time and progressivesilicification; permeability within the volcanic rocks would decrease (Facca andTonai, 1967).

Sericitization/potassium metasomatism

Sericitization/potassium metasomatism is characterized by replacement ofsodic and calcic phases by potassic phases. Exchange of K for Na and Ca maybe accompanied by silicification, addition of Cu and hydration. All the authorsattribute the chemical changes to potassium metasomatism by hydrothermal so-lution or vapors (Simons, 1962 and Macdonald, 1975). These observations


agree with the experimental results. Truesdell (1966) showed by the membraneelectrode method that the ion exchange between silicic glasses and dilute aque-ous solutions is highly selective with a selectivity sequence 2H+ > 2K+ > 2Na+ >Ca2+ > Mg2+. If the hydrothermal solutions are acidic, alkalis are leached andthe altered nature of the rocks is, in most cases, readily recognized by the pres-ence of clay minerals, zeolites, or alunite and the dissolution of feldspar (Agronand Bentor, 1981). Alteration by alkaline solutions, however, frequently leavesno obvious indications and the rocks are entirely fresh looking as are those ofGabal El Dokhan area. Na mobility appears to be dependent primarily on the ef-fective water/rock ratio, while, experimental studies indicate that K mobility isdependent primarily on temperature. K is leached from basalt at high tem-perature (more than 150ºC) (Bischoff and Dickson, 1975; Mottl and Holland,1978 and Menzies and Seyfried, 1979) but removed from solution at lower tem-perature (Seyfried and Bischoff, 1979). This suggests that sericitization/potassium metasomatism of the altered volcanics is a late stage and probablyoccurred at water/rock ratios more than 10 and lower temperature (less than150ºC) during waning the hydrothermal activity.

Albitization

Albitization usually accompanies epidotization or chloritization in the volcan-ic sequences. Characteristic mineralogical changes in highly altered rocks in-clude replacement of calcic plagioclase by sodic plagioclase and mafic mineralsby chlorite and actinolitic amphibole, followed by prominent epidote. Na andNa-Ca metasomatism are characterized by exchange of Na for Ca or K and sec-ondarily by Ca for Fe and Mg. They grade into calcic alteration in some rockvarieties. Na-exchange at high temperatures (more than 150ºC) is influencedprimarily by the effective water/rock mass ratio (Seyfried and Bischoff, 1979).Na is leached from basalt and enriched in solution at water/rock ratios morethan 10, whereas Na is removed from water and fixed in basalt (Mottl and Hol-land, 1978) at water/rock ratios less than 5. Rosenbauer et al. (1988) also sug-gest that excess silica, high Na/Ca ratio and low Mg content in the fluid are re-quired to form these types during deep circulation of seawater. In any case, theformation of albite and epidote-rich alteration assemblages in volcanic se-quences requires an evolved Mg-depleted fluid. This is probably water, mod-ified by reaction with mafic or felsic volcanics lower in the stratigraphic se-quence, or by mixing with magmatic fluids from subvolcanic magma chamber(Campbell et al. 1981). The appearance of epidote and disappearance of albiteis interpreted to reflect decreasing Na/Ca ratio and increasing temperature andpressure of the hydrothermal fluid.


Chloritization

Magnesium metasomatism results from exchange of Mg for Fe and Ca ac-companied by hydration. Experimental and theoretical studies by Mottl (1983)and Reed (1983) indicate that chloritization may occur at all water/rock ratios,and that chlorite-quartz assemblages are dominant at high water/rock ratios(more than 50). Chloritization is also characterized by LREE depletion. LREEfractionation has been reported in chloritized felsic volcanics by Baker and De-groot (1983) and Campbell et al. (1984). The LREE fractionation accom-panying chloritization probably reflects the breakdown of LREE-enriched ac-cessory phases (Clark, 1984). The sympathetic depletion of P-LREE in the chlo-ritized andesitic rocks suggests that breakdown of apatite is responsible for themobility of these elements.

Conclusion

In conclusion, the classification of metasomatic rocks in Table (3) has beenprepared together with a qualitative assessment of the probable nature of themetasomatising fluid by employing the calculated data Table (2) KV and ∆Fn

values along with petrographic descriptions.

The present study revealed the relative mobilities of many of the major andtrace elements (summarized in Table 4) during metasomatism and low-greenschist metamorphism together with their probable controlling phases. It isclear from Table (4) that Ti, Al, Fe, P, Zr and Nb coupled with P/Zr, Ti/Zr andAl/Zr display minor dispersion and behave as immobile elements. The im-mobile characters of these elements are well illustrated through their nearly con-stant values of KV and ∆Fn . On the other hand, the Mg, FTSM, LILE (K-Rb-Ba-Sr-Ca) form a predictable group of elements which along with LREE andoccasionally Y exhibit an organized pattern of mobility. This study has illustrat-ed the potential of composition-volume computations and their superiority overconventional binary element plots, in characterizing metasomatism at a semi-quantitative level.

Summary

Five alteration facies at Gabal El Dokhan area have been recognized based onthe mineralogy and geochemistry, namely biotitization-silicification, sericit-ization-silicification, albitization-silicification, epidotization-silicification andchloritization. Some of the chemical modifications are very similar to those ob-served in hydrothermal altered volcanic rocks elsewhere in the world, or pro-duced in experimental studies and predicted by theoretical models.


Relative to the least altered andesite (assumed protolith), the biotitized-silicified andesites are enriched in K-Rb-Ba-FTSM and depleted in LREE. Theelements like Ti, Al, P and HFSE particularly Zr are immobile and retain theiroriginal abundances which indicate volume increase up to 25 percent. The se-ricitized-silicified andesites are strongly enriched in K-Rb-Ba and slightly de-pleted in Ca-Sr-FTSM, whereas the albitized-silicified andesites are enriched inNa and depleted in LILE and LREE. The immobile elements which have con-stants values of KV and ∆ Fn indicate that volume change shows minor vari-ations (zero/+15 percent) during sericitization, but it exhibits moderate dis-persion (� 5/+15 percent) during albitization. Concerning the altered upperandesites, the immobile elements show volume increase up to 15 percent duringboth the sericitization and albitization. The epidotized lower andesite is en-riched in Ca-FTSM and depleted in LILE and LREE accompanied minor vol-ume loss up to 5 percent, but the epidotized-silicified upper andesites are en-riched in Na-Ca-Cu-Pb and depleted in P-FTSM and LILE accompaniedvolume increase up to 20 percent. The chloritized lower andesite is strongly en-riched in Mg-FTSM and depleted in K-Rb-Ba and LREE with volume decreaseup to 25 percent.

Silicification is a widespread alteration facies that characterizes lower andupper andesites and probably resulted from cooling of hydrothermal fluids atwater/rock ratios < 50 and lower temperatures (250-350ºC). Sericitization prob-ably occurred at water/rock ratio more than 10 and lower temperatures (lessthan 150ºC) during waning hydrothermal activity, whereas albitization occurredas the result of excess silica, high Na/Ca and low Mg content. Appearance ofepidote and disappearance of albite reflects decreasing Na/Ca ratio, and in-creasing temperature and pressure of the hydrothermal fluid. Chloritization isprimarily a product of water-rock interaction at high water/rock ratios (probablymore than 50).

Acknowledgement

The author thanks Prof. Andersen and Prof. Rollinson for their valuable sug-gestions and careful reading of the manuscript. Also, he gratefully acknowledgesProf. El-Bouseily for his advices and constructive criticism of this work.

References

Agron, N. and Bentor, Y.K. (1981) The volcanic massif of BiQ'at Hayareah (Sinai-Negev): Acase of potassium metasomatism. J. Geol. 89: 479-494 Æ

Akaad, M.K. and Noweir, A.M. (1969) Lithostratigraphy of the Hammamat-Umm Seleimat dis-trict, Eastern Desert, Egypt. Nature 503: 284-285.

Avison, M. (1985) Metasomatism in the lough Guitane volcanic complex, south west Ireland- anapplication of composition-volume computation. Chem.Geol. 48: 79-92.


Babcock, R. S. (1973) Computational models of metasomatic processes. Lithos 6: 279-290.Baker, J. H. and Degroot, P. A. (1983) Proterozoic seawater-felsic volcanics interaction. West

Bernslagen, Sweden. Evidence for high REE mobility and implications for 1.8G seawatercompositions. Contrib. Mineral. Petrol. 82: 119-130.

Basta, E.Z., Kamel, O.A. and Awadallah, M.F. (1979) Petrography of the Gabal El Dokhan vol-canics, Eastern Desert, Egypt. Annals Geol. Surv. of Egypt 22: 145-171.

Bischoff, J. L. and Dickson, F.W. (1975) Seawater-basalt interaction at 200ºC and 500 bars: Im-plications for origin of sea-floor heavy metal deposits and regulation of seawater chemistry.Earth Planetary Science Letters 25: 385-379.

Bogolepov, V. G.(1963) The recomputation of the chemical analyses of rocks in studying meta-somatic processes. International Geol. Review 1962, 5(15): 1585-1592.

Campbell, I. H., Franklin, J. M., Gorton, M.P., Hart, T.R. and Scott, S. D. (1981) The role ofsubvolcanic sills in the generation of massive sulfide deposits. Econ.Geol. 76: 2248-2253.

Campbell, I.H., Lesher, C.M., Coad, P., Franklin, J.M., Gorton, M.P. and Thurston, P.C.(1984) Rare-earth element mobility in alteration pipes below massive Cu-Zn sulfide de-posits. Chem. Geol. 45: 181-202.

Clark, A.M. (1984) Mineralogy of the rare earth elements. In: Rare Earth Geochemistry (Editedby Henderson) Chapt. 2: 33-61.

Condie, K.C., Viljoen, M.J. and Kable, E. J. D. (1977) Effects of alteration on element dis-tributions in Archean tholeiites from the Barberton greenstone belts, south Africa.Contrib. Mineral. Petrol. 64: 75-89.

Condie, K.C. (1981) Archean Greenstone Belts. 434P. Elsevier, Amsterdam. El-Ramly, M.F. (1972) A new geological map of the basement rocks in the Eastern and South-

western part of Egypt, scale 1: 1.000.000. Annals Geol. Surv. of Egypt 11: 1-18.Ewart, A. (1971) Chemical changes accompanying spherulitic crystallization in rhyolitic lavas,

central volcanic region, New Zealand. Mineral. Mag. 38: 424-434. Facca, G. and Tonai, F. (1967) The self-sealing geothermal field. Bull. of Volcanol. 30: 271-273.Gelinas, L., Mellinger, M. and Trudel, P. (1982) Archean mafic metavolcanics from Rouyn-

Noranda district, Abitibi; Greenstone Belt, Quebec. 1. Mobility of the major elements.Canad. J. of Earth Science 19: 2258-2275.

Ghobrial, M.G. and Lotfi, M. (1967) The geology of Gabal Gattar and Gabal El Dokhan area.Annals Geol. Surv. of Egypt 40: 26p.

Gibson, H.L., Watkinson, D. H. and Comba, C.D.A. (1983) Silicification. Hydrothermal altera-tion in an Archean geothermal system within the Amulet "Rhyolite Formation" Noranda,Quebec. Econ. Geol. 78: 954-971.

Grant, J. A. (1986) The Isocon Diagram. A simple solution to Gresen's Equation for meta-somatic alteration. Econ. Geol. 81: 1976-1982Æ

Gresens, R. L. (1967) Composition-volume relationships of metasomatism.Chem.Geol. 2: 47-65.Irvine, T. N. and Baragar, W. R. A. (1971) A guide to the chemical classification of the com-

mon volcanic rocks. Canad. J. of Earth Science 8: 523-548.Jolly, W. T. (1980) Development and degradation of Archean lavas, Abitibi area, Canada in light

of major element geochemistry. J. of Petrol. 21: 323-363.Kennedy, G. C. (1950) A portion of the system silica-water. Econ. Geol. 45: 629-653.Khalaf, E.D.H. (1994) Petrography, Geochemistry and Petrogenesis of volcanics and associated

rocks of Gabal EL Dokhan area, Northeastern Desert, Egypt. Ph.D. dissertation 172 p. Cai-ro University.

Kristmannsdottir, H. (1975) Hydrothermal alteration of basaltic rocks in Iceland geothermal ar-eas. Second U.N. symposium on the development and utilization of geothermal resources,proceedings, pp. 441-445.


Lesher, C.M., Gibson, H.L. and Campbell, I.H. (1986) Composition-volume changes duringhydrothermal alteration of andesites at Buttercup Hill, Noranda District, Quebec. Geo-chimica et Cosmochimica Act. 50: 2693-2705.

Ludden, J. N., Gelinas, L. and Trudel, P. (1982) Archean metavolcanics from the Rouyn-Noranda district, Abitibi Greenstone Belt. 2. Mobility of trace elements and petrogeneticconstraints. Canad. J. of Earth Science Letters 19: 2276-2287.

Macdonald, R. (1975) Nomenclature and petrochemistry of the peralkaline oversaturated ex-trusive rocks. Bull. of Volcanol. 38: 498-516.

Mason, B. (1966) Principles of Geochemistry. 329 p. Wiley, New York.Menzies, M. and Seyfried, W. E. IR. (1979) Basalt-seawater interaction: Trace element and

strontium isotopic variations in experimentally altered glassy basalt. Earth Planetary Sci-ence Letters 44: 463-472.

Mottl, M.J. (1983) Metabasalts, axial hot springs and the structure of hydrothermal system atmid-ocean ridge. Geological Society of American Bull. 94: 161-180.

Mottl, M. J. and Holland, H.D. (1978) Chemical exchange during hydrothermal alteration of ba-salt by seawater- I. Experimental results for major and minor components of seawater. Geo-chimica et Cosmochim Acta 42: 1103-1115.

Nystrom, J.O. (1984) Rare earth element mobility in vesicular lava during low-grade meta-morphism. Contrib. Mineral. Petrol. 8 : 328-331

Poldervaart, A. (1953) Petrological calculations in metasomatic processes. American J. of Sci-ence 251: 481-504.

Riverin, G. and Hodgson, C.J. (1980) Wall rock alteration at the Millenbach Cu-ZnMine,Noranda, Quebec. Econ. Geol. 75: 424-444.

Rosenbauer, R.J., Bichoff, J. L. and Zierenberg, R.A. (1988) The laboratory albitization ofmid-ocean ridge basaltÆ J. of Geol. 96: 237-244.

Seyfried, W. E. IR. and Bischoff, J. L. (1979) Low temperature basalt alteration by seawater :An experimental study at 70ºC and 150ºC. Geochimica et Cosmochim Acta 43: 1937-1947.

Simons, F. S. (1962) Devitrification dikes and giant spheralites from Klondyke, Arizona. Amer.Mineral. 47: 871-885.

Terakado, Y. and Fujitani, T. (1998) Behaviour of the rare earth elements and other trace ele-ments during interactions between acidic hydrothermal solutions and silicic volcanic rocks,Southwestern Japan. Geochimica et Cosmochim Acta 62: 1903-1917.

Tomasson, J. and Kristmannsdottir, H. (1972) High temperature alteration minerals and ther-mal brines, Reykjanes, Iceland. Contrib. Mineral. Petrol. 36: 123-134.

Truesdell, A. H. (1966) Ion exchange constants of natural glasses by the electrode method. Amer.Mineral. 51: 110-122.

Turner, F.J. and Verhoogen, J. (1960) Igneous and metamorphic petrology. 694P. McGrawhill,New York, N.Y.


TA

BL

E 3

. C

lass

ific

atio

n of

met

asom

ic fa

cies

in a

ltere

d an

desi

tic ro

ck ty

pes.

M

etas

omat

ism

Vol

ume

chan

geEl

emen

ts e

nric

hed

Elem

ente

s de

plet

edM

etas

omat

isin

g

M

iner

alog

ical

cha

nges

flui

d

Low

erU

pper

ande

site

san

desi

tes

1

) B

iotit

izat

ion

+ 20

to +

25%

�K

-Rb

Ba-

FTSM

LREE

K- r

ich

Bio

tite

and

orth

ocla

se re

plac

epl

agio

clas

e ph

enoc

ryst

s.

2

) Se

riciti

zatio

nZe

ro to

+ 1

0%Ze

ro to

+ 1

5%K

-Rb-

Ba

Mg-

FTS

M- C

a- S

rK

- ric

hA

ltera

tion

of fe

ldsp

ar to

ser

icite

.

3

) A

lbiti

zatio

n�

5 to

+ 5

%+

5 to

+ 1

5%N

aLI

LE-L

REE

Na-

rich

Alte

ratio

n of

cal

cic

plag

iocl

ase

toal

bite

.

4

) Ep

idot

izat

ion

� 5%

+ 10

to +

20%

Ca-

Cu-

PbLI

LE-L

REE

Ca-

rich

Alte

ratio

n of

pla

gioc

lase

and

maf

-ic

min

eral

s to

epi

dote

.

5

) C

hlor

itiza

tion

� 25

%�

Mg-

FTS

M-Z

n-A

s-Pb

LILE

-LR

EEFe

- Mg

rich-

Alte

ratio

n of

pyr

oxen

e to

chl

orite

alka

li po

or

Ezz El Din A. Khalaf204T

AB

LE 4

. Su

mm

ary

of b

ehav

iour

of

maj

or a

nd tr

ace

elem

ents

.

Ele

men

tM

obili

tyC

ontr

ollin

g ph

ases

Cor

rela

tion

with

Com

men

ts

SiH

igh

Qua

rtz

Neg

ativ

e w

ith T

i and

Al

Qua

rtz

xeno

crys

ts c

omm

on, S

iO2

fixe

d in

alu

min

osili

cate

not

mob

ile

Ti

Imm

obile

Sphe

ne -

opaq

ueO

ther

imm

obile

ele

men

ts

�

Al

Low

Alu

min

osili

cate

feld

spar

Oth

er im

mob

ile e

lem

ents

�

FeT

Low

M

afic

pne

nocr

ysts

- op

aque

Oth

er im

mob

ile e

lem

ents

�

Mn

Hig

hM

afic

phe

nocr

ysts

-chl

orite

Mg

�

Mg

Ver

y hi

ghM

afic

phe

nocr

ysts

-chl

orite

Mn

Enr

iche

d in

chl

oriti

zed

rock

s

Ca

Ver

y hi

ghM

afic

-pla

gioc

lase

phe

nocr

ysts

Mn-

Mg-

SrA

lbite

alm

ost f

ree

from

Ca

Na

Ver

y hi

ghA

lbite

�

�

KV

ery

high

Pota

ssic

pha

ses

(ser

icite

-bio

tite)

K-R

-Ba

�

PL

owA

patit

eO

ther

imm

obile

ele

men

tsA

patit

e pr

esen

t alm

ost i

n al

l roc

ks

Ga

Low

Alu

min

osili

cate

Al

�

Cu-

Zn-

As-

Hig

hM

etal

s

�A

ltere

d ro

cks

have

hig

h ch

alco

phile

con

tent

s

Cr-

Ni

Mod

erat

eM

afic

phe

nocr

ysts

-chl

orite

Mn-

Mg

�

Rb

Ver

y hi

ghSe

rici

te a

nd b

iotit

eK

-Ba

�

Ba

Ver

y hi

ghSe

rici

te-b

iotit

e-al

umin

osili

cate

?K

-Ba

Hig

h va

riat

ion

amon

g al

tere

d ro

cks

SrV

ery

high

Plag

iocl

ase

phen

ocry

sts

Ca

Sr e

xpel

led

from

pla

gioc

lase

dur

ing

albi

tizat

ion

YM

oder

ate

Acc

esso

ry p

hase

sL

RE

E

�

Zr

Imm

obile

Zir

con

Oth

er im

mob

ile e

lem

ents

Fres

h un

alte

red

zirc

on in

alte

red

rock

s

Nb

Ver

low

?T

i-ri

ch m

iner

als

Oth

er im

mob

ile e

lem

ents

KV

var

iatio

n pr

obab

ly d

ue to

ana

lytic

al im

prec

isio

n

LR

EE

Mod

erat

eA

cces

sory

pha

ses

Y

�


X�e�b�_« �u�$ �u%Ë dOG� �ö� w�O�d��«Ë wL�(« dOG��«dB� ,�WO�dA�« ¡«d�B�« �UL� ,�ÊU�b�« q�� WIDM� w� WO�U�d��«

nK� rOJ(« b�� s�b�« e��d�UI�« WF�U� ,�ÂuKF�« WOK� ,�UO�u�uO'« r��

WO�dF�« dB� W�dNL��−��e��O'«

VI?� W�U?N� w?� ÊU?�b�« q�?� W?IDM� U?O?�U?�d� qL?A� Æ�hK�?�?�*« U??��«e�b�_« ,�W??O?�?��U?��« U??��«e�b?�_« s� �«u�√ vK� p�Ë�Ëd??O�Ëd??��«w� ��u�u*« W�d�B�« �«u�_« r�√ U��«e�b�_« q�9 YO� ,� U�O�u�«d�«ËW?O??�«d?�Ëd??�?�?�«Ë W?OKI??(« U?�«�b�« X?�?{Ë√ b?� Ë Æ�W??�«�b�« W??IDM�W1b?I�« U��«e�b�_« U?L�Ë U��«e�b�_« s� ÊU?�u� �UM� ÊQ� WO?zUOL?OJ�«ËWO?MG�«® W�uKF�« W��b?(« U��«e�b�_«Ë ©�u?�O?H�ô« Ê�U?F� W1b?�® WOKH?��«WM�� v�≈ X�u% b� U��«e�b�_« w� ÊU�uM�« Ê«c�Ë Æ�©�u�OH�ô« Ê�UF0− X�ôuMO??�?�ô« − XO?�u?O?��« Ê�U??F0 e?O?9 YO?� ¡«d??C?)« X�?O??A�«Ê√ YO???� Æ�5H???�ô« − e��«uJ�« − Ëb???O�ù« − XO???��_« − X��u?KJ�«

Æ�Èd�√ v�≈ W�uL�� s� W�ËUH�� u��« W��Ë WFO��:��«u�√ W�L� v�≈ W�u��*«Ë �dOG�*« UM��« rO�I� sJ�√ b�Ë

,�e��«u?� − X�U?�Ëd?O��« WM?��−≤ ,�e��«u?� − XO�u?O?��« WM�?��−±WM��−µ ,�e��«u?� − ËbO�ù« WM��−¥ ,�e��«u� − XO?��_« WM��−≥

Æ�X��uKJ�«��UM�« W?O{�_« d?$UMF�« w� ��U�e�U� e��«u?� − XO�uO?��« WM�� e?OL?��W��UI*U� ��UM�« W?O{�_« d$UMF?�« w� U�dI�Ë W?O�UI��ô« W?O�Ë_« d$UMF�«ËeO?L�� e��«u� − X�U�Ëd?O��« WM�� U?�√ Æ�«dOG� q�_« X�e�b�_« �u?�B�Âu��u?B�«Ë ÂuO?�M�U*« w� U�dI?�Ë WO?{�_« W�uKI�« d?$UMF�« w� ��U�e�U�uKF� e??O?L?�� e��«u??� − XO?��_ « WM�?� U??�√ Æ��UM�« W?O?{�_« d??$UMF�«Ë Âu�b??O�Ëd?�« − Âu??O??�U�u??��«® W�uK?I�« d??$UMF�« w� U?�d??I??�Ë Âu��u??B�«


e��«u� − Ëb?O�ù« WM�?� eOL?�� Æ��UM�« WO?{�_« d$UMF�«Ë ©Âu?��U��«Ëd?$UMF�« w� U�d?I?�Ë W?O�U?I?��ô« W?O�Ë_« d$U?MF�«Ë Âu?O��U?J�« w� ��U�e�U�w� ��U�e�U� eOL�� X��uKJ�« WM�?� U�√ Æ��UM�« WO{�_« d$UMF�«Ë W�uKI�«d$UMF�« w� U�d?I�Ë �U$d�«Ë W?O�UI��ô« W?O�Ë_« d$UMF�«Ë − Âu?O�M�U*«

Æ��UM�« WO{�_« d$UMF�«Ë ÂuOA�d��ô«Ë �uH�uH�«Ë ,�WO{�_« W�uKI�«b�b(«Ë Âu?OM*_«Ë ÂuO�U�O?��« d$UM� Ê√ b�Ë W?�«�b�« Ác� �ö� s�Ë�ö?� s�Ë Æ�d?OG?��« U?OKL?� �ö� UÎ�U?�� d?�?�√ ÂuO?M�d�e�«Ë �uH?�u?H�«ËdH?$ 5� ÕË«d� b� wL?�(« dO?G��« Ê√ b�Ë W�e?�*« WK�J�« U�U�?� ¡«d�≈ U��e�b�_« �u�B� %≤µ v�≈ ≤µ− �W�uKF�« U��e�b�_« �u�B� %≤∞Ë

Æ��dOG�*« UM��K� WHK�<« �«u�_« �ö� WOKH��«

Composition-Volume Changes during Metasomatic Alteration ... · rocks, but volume changes are hard to assess in rocks with fine grained me-sostases (Lesher et al. 1986). In view of

Documents