Fluid inclusions in the mesozonal gold deposit at Atud mine, Eastern Desert, Egypt

Fluid inclusions in the mesozonal gold deposit at Atud mine,Eastern Desert, Egypt

Hassan Z. Harraz

Department of Geology, Faculty of Science, Tanta University, Tanta, Egypt

Accepted 24 October 2002

Abstract

Gold mineralization at the Atud mine occurs as fracture-filling auriferous quartz veins hosted in Neoproterozoic dioritic rocks

and along their contact with metagabbro. Gold mineralization is associated with metasomatic alteration zones around shear zones

and quartz–carbonate vein arrays. The mineralized veins consist of quartz, carbonate and albite gangue enclosing minor amounts of

pyrrhotite, arsenopyrite, pyrite and sphalerite. Trace amounts of galena, chalcopyrite, magnetite and rutile are also present. Para-

genetically, the mineralization is divided, with decreasing temperature, into three stages, namely: (1) Early (Au-poor, pyrrhotite–

arsenopyrite–pyrite–quartz vein); (2) Main (Au-rich, sphalerite–arsenopyrite–pyrite–galena� chalcopyrite–quartz vein); and (3)Late (quartz–carbonate–pyrite� galena).Gold (15.6–36.2 at.% Ag) is present mainly as discrete grains of native gold (<5–20 lm in diameter). Free gold appears mainly as

inclusions in quartz and as microscopic inclusions (<5 lm) in arsenopyrite and pyrite. Gold also occurs in fractures and grainboundaries of pyrite, arsenopyrite and base metal sulphides. These occurrences of gold indicate that several influxes of gold and/or

stages of remobilization took place.

Based on temperatures inferred from arsenopyrite compositions by electron microprobe, the estimated temperatures for Early

and Main mineralized stages reach 340–430, and 273–368 �C, respectively. The sulphur activity (atm) of ore formation at the Atuddeposit was estimated for each stage as 10�6:5–10�9:6, and 10�8–10�12:2, respectively.

Fluid inclusions in quartz intimately associated with the mineralization are dominated by aqueous H2O–CO2þNaCl types. Inmost cases, these fluid inclusions co-exist in individual samples and show various CO2 phase volume proportions at 40 �C. Co-existing H2O-rich liquid and CO2-rich vapour fluid inclusions homogenized into liquid and carbonic vapour phases, respectively,

over the same temperature range (270–490 �C). The petrographic observations and microthermometric data suggest that fluid in-clusions in quartz intimately associated with the early and main mineralizing events were trapped during phase separation of an

originally homogenous H2O–CO2 liquid, with low salinity (2.8–8.2 wt.% NaCl equiv.) and high density of 0.8–0.9 g/cc (corre-

sponding to 15–76 mol% CO2). These data are consistent with transportion of gold as a bisulphide complex. Gold deposition

occurred over a temperature range of 270–430 �C and pressures of 160–272 MPa (�6–11 km depths), likely due to decreases insulphur activity (10�6:5–10�12:2) accompanying fluid unmixing. Gold deposition in the Atud diorites was related to fluid phase

separation, sulphidization and carbonatization of host dioritic rocks during hydrothermal alteration and mineralization. The di-

orites are considered to have acted as preferential sites for fluid flow and ore precipitation due to their brittle nature during regional

deformation.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Fluid inclusions; Microthermometry; Mineralogy of quartz veins; Atud deposit; Gold-Egypt

1. Introduction

In the Arabo–Nubian Shield, gold mineralization is

widely associated with post-tectonic granitoid rocks (El-

Gaby et al., 1988; Pohl, 1988). The majority of these

deposits occur as Au-bearing quartz veins with a poly-

metallic sulphide assemblage, exhibiting pinch-swell veintextures and showing evidence of multiple stages of

mineralization (Hilmy and Osman, 1989; Hussein, 1990;

Harraz and El-Dahhar, 1993). P–T conditions at or

below greenschist metamorphic boundary favoured es-

tablishment of brittle–ductile and brittle structures in

which gold deposits were selectively sited (El-Gaby

et al., 1988). The source of the ore-bearing fluids is

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related to Late Precambrian subduction and associatedcalc-alkaline magmatism (El-Gaby et al., 1988). The

mineralized solution was induced either by metamor-

phism or cooling magmas (Pohl, 1988). Leaching and

remobilization from a hidden source are advocated by

Hilmy and Osman (1989); Takla et al. (1990); Harraz

and El-Dahhar (1993).

Atud area is located in the central Eastern Desert of

Egypt at the intersection of latitude 25�0001000N andlongitude 34�2401000E, �58 km west of Mersa Alam onthe Red Sea coast and �5 km south of the Idfu–MersaAlam paved road (Fig. 1). The Atud gold mine is con-

sidered to be a mesothermal vein type gold deposit

which occurs in quartz veins hosted mainly in Neopro-

terozoic dioritic rocks at Gabal Atud (Pohl, 1988). The

deposit is spatially and genetically associated with a

metagabbro–diorite complex emplaced at shallow levelsin serpentinite and metasedimentary rocks (Gabra,

1986; Harraz, 1999). This mine was first excavated

during Pharaonic times but no ore has been produced

since then. Between 1953 and 1969, Egyptian Geologic

Survey and Mining Authorities (EGSMA) performed

underground prospecting work in the Main Atud site

through three expeditions (Fig. 1). Drifting was carried

out on three levels along strike of the main lode (NNW–SSE) for a total length of 690 m. These levels were

connected by inclined shafts down the dip of the lode for

a total length of 230 m (Fig. 2). Other small shafts and

some pits were made at East Atud-I and East Atud-II

(Fig. 1). The depth of the excavations varied between

<20 and 78 m, and the Au content ranged from <0.1 to31 g/t. The principal lode contains 1900 t of gold ore

grading 16.28 g/t gold. In addition, 1600 t of dump with

12.4 g/t gold are stock-piled in the area (Hussein, 1990,

p. 547).

The present work addresses the geothermometry,

mineralogy and paragenetic relations of the Au-bearingquartz veins at the Atud mine, and discusses the com-

position, nature and origin of mineralizing fluids.

Fig. 1. Detailed geological map of the Atud gold mine district (after

Harraz, 1999).

Fig. 2. Longitudinal section of the Atud gold mine, showing locations of the analyzed samples collected from the different levels of the mining

workings: Level 42M, Level 72M and Level 165M. The traced levels of mining activity are after the Gold Mine Authorities. Traced quartz vein stages

and alteration zones are after Harraz (1999).

348 H.Z. Harraz / Journal of African Earth Sciences 35 (2002) 347–363

2. Geologic setting

The Atud gold mine area covers �18 km2 and isgeologically dominated by a metagabbro–diorite com-

plex with relics of serpentinite–talc-carbonate, meta-

sediment and metavolcanic rocks (Fig. 1). The

metagabbro–diorite complex occupies the main part of

the Gabal Atud in the center of the mapped area. This

complex was emplaced into metasediments and serpen-tinites and later intruded by gabbroic rocks. In the

northeastern part, the dioritic rocks may grade into

granodiorite with transitional phases. The metasedi-

ments occupy a small area on the southern slope of

Gabal Atud and are usually mixed with serpentinite–

talc-carbonate. Akaad and Essawy (1964) noted that the

metagabbro–diorite complex shouldered the metasedi-

ments and serpentinites by a thrust plane dippingmoderately to the north and the west along the north-

eastern boundary of the Gabal Atud. The thrust con-

tains abundant planes of highly sheared talc-carbonates

(Fig. 1).

Joints are conspicuous at the Atud gold mine area

and trend in four directions: NNW–SSE, NW–SE, NE–

SW and NNE–SSW. The cross cutting relationship of

the joint systems show that the older joint system istrending N2–32�W and dipping 25�NE–44�SW while theyounger is trending N10–52�E and steeply dipping 37–47�NW. The NNW–SSE fracture systems extend be-yond the margins of Gabal Atud. Therefore, it is not

directly related to volume shrinkage due to crystalliza-

tion of gabbroic magma as suggested by Phillips (1972),

but must be related to re-activation of older regional

thrust structures that affected the whole of the EasternDesert during Red Sea opening (Sabet and Bondonosov,

1984; El-Gaby et al., 1988; Meshref, 1990). However,

the dominant NNW–SSE strike-slip zones are oriented

subparallel to the dominant strike-slip fault along the

Idfu–Mersa Alam Road (El-Gaby et al., 1988) as well as

to the Najd fault system. The younger NE and NNE

joint trends are tension fractures associated with com-

pressive stress during the intrusion of the youngergabbroic rocks.

3. Quartz veins

3.1. Geology and distribution

The area of mineralization is �9 km2, localized at theeastern and southeastern slopes of Gabal Atud. The

area includes many mineralized and unmineralized

quartz veins. The mineralized quartz veins cut mainly

across the dioritic rocks and may extend to the meta-gabbro (Fig. 1). The main quartz vein lode in the eastern

slopes of Gabal Atud (Main Atud) is conformable to the

NNW fracture systems, while the main quartz vein lodes

in the southeast (Atud East-I and Atud East-II) areconfined to NE fractures. The Main Atud is the largest

set that comprises four significant veins; namely: Main,

Eastern, Western and Southern veins; occurring within a

shear zone extending NNW and extending for �305 macross the dioritic rocks (Fig. 1). Other quartz veins

extend throughout different underground levels (Fig. 2),

invading a shear zone filled by pockets of quartz, car-

bonate and chlorite. At the first level (Level 42M), themain quartz vein is comprised mainly of bluish or

greyish quartz frequently associated with variable

amounts of milky quartz. At the second and third levels

(Level 72M and Level 165M, respectively), the main

quartz veins are predominantly milky quartz showing

relatively smaller amounts of bluish or greyish quartz.

The main sauriferous quartz veins are commonly frac-

tured and consist of brecciated bluish-grey to grey orgreyish- to bluish-white quartz, that crystallized in an

earlier and main stage, respectively. Late stage milky

quartz veins also occur. These varieties of quartz crys-

tallized successively with probable overlapping depo-

sition (Nakhla et al., 1993; Arslan and Harraz, 2001).

Mineralization in the Atud gold mine is disseminated

in nature and is localized in. and relateo to, hydrother-

mal veins that occupy pre-existing fractures (open-spacefilling). Fractures are filled with quartz with or without

carbonate, chlorite, albite, sericite, kaolinite, and sul-

phide minerals. Gold mineralization in association with

metasomatic alteration zones is observed at vein mar-

gins around shear zones and in quartz–carbonate vein

arrays (Harraz, 1999). Intense wall rock alterations are

also observed at the margins of mineralized quartz

tension gashes (Awad and Fasfous, 1981; Harraz, 1999).Contacts between veins and wall rocks are commonly

sharp and occasionally outlined by carbonate, chlorite

and iron oxide mineral selvages. Alteration zones asso-

ciated with structurally controlled veins in dioritic rock

were classified by Harraz (1999) according to their

diagnostic mineral assemblages into: Zone 1: chlorite–

carbonate; Zone 2: ankerite–albite; and Zone 3: albite–

sericite–kaolinite (Fig. 2). Zones 2 and 3 coinciderespectively with the Au-bearing quartz veins of both

early- and main-quartz vein generations, and are

sometimes surrounded and/or gradually merging into

zone 1. The latter coincides with late carbonate–quartz

veins (Fig. 2). However, the intensity of wall rock al-

teration is variable throughout the mine.

The main quartz vein lode is trending NNW and

dipping 40�W. Other mineralized quartz veins have ageneral NW (with dip 40–43�SW) andNE trend (with dip14–53�NW). Individual quartz veins vary from a few

centimetres up to 2 m wide, and<1.0 m to more than 100m long. The large veins trend N25–35�W and extend

discontinuously up to 270 m along strike and from 78 to

165 m down dip (40�SW). The veins were partiallyworked out to 42m deep, with average thickness of 0.7 m.

H.Z. Harraz / Journal of African Earth Sciences 35 (2002) 347–363 349

They usually pinch, swell, bifurcate into small veins,veinlets and stringers (off-shoots) and join other veins,

giving rise to an anastomosing network �5 m in width.

3.2. Quartz vein types and paragenetic sequences

Sulphide minerals are mainly pyrite together with

arsenopyrite, sphalerite and pyrrhotite. Other subordi-

nate species include chalcopyrite, galena and gold. Very

rarely pentlandite, graphite, marcasite, rutile, ilmenite,

magnetite, hematite, titanite, covellite, malachite, go-

ethite and limonite are present. The sulphide species

occur as medium- to fine-grained interlocking aggre-

gates or disseminations in the quartz veins and along thecontacts between the quartz veins and wall rocks. Limo-

nite and goethite represent the final weathering products

of pyrite and arsenopyrite. Weathering of the sulphides

resulted in the formation of cavities.

Three distinct stages of mineralization can be distin-

guished on the basis textural relationships of the various

minerals. A generalized paragenetic sequence of the

constituents in the mineralized veins is shown in Fig. 3.For simplicity, the mineralized veins can be divided into

three stages: (1) Early mineralized quartz veins (EQV),

(2) Main mineralized quartz veins (MQV), and (3) Late

quartz–carbonate veins (LQV).

3.2.1. Early mineralized quartz veins

These are represented by bluish- to dark-grey col-

oured quartz veins and range in morphology from large

tabular veins to quartz cemented breccia. The EQV in-

clude: (1) laminated quartz veins, (2) zones of subpar-

allel quartz stringers, and (3) pervasive silica flooding

with sulphide impregnation zones. The EQV range inthickness from a few centimeters up to 2.0 m (average

0.3 m). They are characterized by medium to coarse

grained (5–10 mm in size), highly brecciated and frac-

tured, subhedral to euhedral quartz crystals with vari-

able amounts of ankerite, albite and chlorite; and rarely

titanite, ilmenite, magnetite, and hematite. The quartz

commonly shows variable degrees of strain (undulose

extinction, granulated grain boundaries, subgrain de-velopment and formation of boehm lamellae, with par-

allel planes of inclusions). Wall rock laminae (a few

millimeters to >0.5 m in thickness) are present in manyveins running nearly parallel to the vein walls. In some

places, the wall rocks are extensively fragmented and

recemented by quartz. Ankerite, albite and probable

chlorite form a characteristic green envelope (up to 30

cm) around large quartz veins near to their surfaceoutcrops.

The sulphide minerals consist mainly of pyrrhotite,

arsenopyrite and pyrite together with scarce amounts of

pentlandite, chalcopyrite and sphalerite. Pyrrhotite, ar-

senopyrite and pyrite are typically disseminated as id-

iomorphic crystals throughout the margins of the quartz

veins and ankerite–albite alteration assemblages. Pyr-

rhotite crystals (1–6 mm across) are locally spotted byfine magnetite. Pyrite occurs as fine euhedral crystals

(0.5–3 mm diameter) and is frequently found intergrown

with pyrrhotite, or occurs as fine aggregates within

magnetite. Arsenopyrite assumes an euhedral or sub-

hedral form up to 4 mm in diameter (31.38–32.65 at.%

As, n ¼ 10; Fig. 4A). In most cases, pyrite partially orcompletely replaces pyrrhotite and arsenopyrite.

Sphalerite occurs as fine grams (<0.2 mm) along cleav-

Fig. 3. Generalized paragenetic sequence of minerals from veins of the Atud gold mine. Bars indicate the paragenesis derived from vein relationships

and mineralography during this study. The temperature scale is based on fluid inclusion analysis.


age and grain boundaries of pyrrhotite. Pentlandite

commonly forms a flame texture in the peripheral zones

of pyrrhotite. Chalcopyrite is found either as fine iso-

lated round grains or replacing the borders of pyrrho-

tite. Magnetite forms irregular grains, some of which are

partially replaced by pyrite. Gold occurs as microscopic

inclusions (<5 lm) in arsenopyrite and pyrite, and isfound along partially healed fractures that were formed

later than pyrite. EMP analyses of gold grains yield Ag

values ranging from 22.78 to 36.23 at.% (Table 1).

Fig. 4. (A) Enlarged isoscales quadrangle in the Fe–As–S triangle, showing the variation in As/S atomic ratios of arsenopyrite from the Atud Au

mine; (B) frequency histogram of FeS content of sphalerite from main mineralized quartz vein stage of the Atud mine, in relation to mineral as-

semblages; (C) bipyramidal arsenopyrite (AsPy) crystals as inclusion in pyrite (Py). Rutile crystals (Ru) occur as inclusions in arsenopyrite. Gold (Au)

associated with arsenopyrite occurs as inclusions in coarsegrained pyrite; and (D) gold peripheral to arsenopyrite and filling fractures in coarse-

grained pyrite.

Table 1

Electron microprobe analyses of gold grains from Atud gold mine, Eastern Desert, Egypt

Sample no. Associated sulphide % Au % Ag Total Au/Ag

EQV22 Arsenopyrite 63.77 36.23 100.00 1.76

EQV22 Pyrite 66.60 33.39 99.99 1.99

EQV18 Pyrite 77.20 22.78 99.98 3.39



MQV67 Pyrite 83.22 16.78 100.00 4.96

MQV67 Pyrite 84.35 15.63 99.98 5.40

MQV31 Pyrite 82.03 17.97 100.00 4.56

MQV31 Pyrite 82.86 17.12 99.98 4.84


3.2.2. Main mineralized quartz veins

These are represented by bluish- to greyish-white and

brecciated quartz. They are closely associated with al-

bite–sericite–kaolinite alteration zones that have local

carbonates and graphite. They often crosscut EQV but

sometimes grade into LQV. These veins range in thick-

ness from 0.3 to 10 m (average of 0.75 m). They fre-

quently occur in parallel swarms of tiny veinlets with a

veinlet spacing of 5–20 mm through the hosting diorite.Many of the MQV occur in obvious extensional sites

where hydraulic fractures (i.e. tensional vein sets), lam-

inated crack-seal veins with a ribbon texture and hy-

draulic breccias are developed. However, the tension

gashes locally develop a sigmoidal form within a thin

(<0.2 m) zone near the large quartz vein where goodprimary/pseudoprimary fluid inclusions can be ob-

tained. Quartz occurs as coarse to medium grained,euhedral to subhedral crystals, traversed by a network

of fine cracks commonly filled with interlocking irregu-

lar grains of milky quartz. Some quartz grains have been

strained and recrystallized to form mosaics of fine

(0.01 mm) polygonal interlocking grains with graphite,ankerite, albite, rutile and sulphide minerals as well as

fragments of wall rocks.

The MQV have Au-contents ranging from 3.61 to

15.35 ppm (see, Table 2). This stage is characterized by

an abundance of sphalerite, arsenopyrite, pyrite, galena,

and ankerite; variable amounts of chalcopyrite, mag-

netite, rutile and graphite; and trace amounts of pyr-

rhotite, goethite and chlorite. Sphalerite grains areusually blocky with a reddish-yellow colour. The com-

mon minerals coexisting with sphalerite are pyrite, ar-

senopyrite and galena. Sphalerite grains are commonly

free of obvious exsolved inclusions or lamellae of chal-

copyrite and pyrrhotite. FeS contents of sphalerite

(10.38–19.38 mol% FeS, n ¼ 45) appear to vary sys-tematically with ore mineral assemblages (Fig. 4B).

Sphalerite coexisting with pyrite contains 14.81–19.38mol% FeS (n ¼ 25) and late sphalerite intergrown withgold and arsenopyrite displays the lowest FeS contents

(10.38–14.65 mol%: n ¼ 20). Pyrite occurs in fine tocoarse (1 mm to >1 cm across) subhedral to near eu-

Table 2

Description of fluid inclusions from Atud gold mine and the nature of the trapped fluids

Stage Early mineralized quartz vein Main mineralized quartz vein Late quartz–carbonate vein

Aua (ppm) 0.60 to 2.60 3.61 to 15.35 <0.30 to 0.50

Aga (ppm <0.30 to 0.46 0.32 to 2.50 <0.30

Fluid inclusion:

Type a; P c; S b/a; P/PS c/d; S a; P d; S

Size (lm) 3 to 10 <1 to 5 <3 to 15 <1 to 5 5 to 18 <1 to 12

Volume percent-

ages of aqueous

liquid

20 to 50 (40) 65 to 85 (75) 10 to 70 (50) 70 to 95 (82) 65 to 95 (80) 85 to 95 (90)

Th CO2 L–V (�C) 9.6 to 17.3 (12.9) 5. 6 to 9.8 (7.6) 17.0 to 21.5 (20.5)

Tm ice (�C) )5.3 to )2.6()3.6)

)3.2 to )1.8()2.1)

)2.8 to )1.8()2.3)

)3.1 to )1.7()2.5)

)1.6 to )0.5()1.0)

)1.5 to )0.8()0.9)

TClath (�C) 5.6 to 7.6 (7.0) 7.1 to 8.5 (8.0) 7.6 to 8.5 (8.3) 7.1 to 8.5 (7.8) 8.5 to 9.4 (9.0)

Tm CO2 (�C) )57.7 to )56.2()56.6)

)61.2 to )57.2 )55.9 to )54.6()55.8)

Th (�C) 372 to 490b (415) 182 to 256 (200) 269 to 398b (310) 152 to 251 (185) 170 to 265 (225) 122 to 198 (145)

Homogenization

phase

Aqueous, Liquid

(CO2)

Liquid Liquid (aqueous) Liquid Aqueous Liquid

Wt.%NaCl equiv. 4.3 to 8.2 (5.7) 3.0 to 4.9 (4.0) 2.8 to 4.6 (3.9) 2.9 to 4.9 (4.2) 1.1 to 2.6 (2.0) 1.3 to 2.5 (1.8)

XNaCl 0.64 to 1.93 (1.20) 0.23 to 1.26 (0.72) 0.32 to 0.91 (0.57)

XH2O 42.24 to 70.81

(63.80)

23.64 to 83.55

(71.59)

83.68 to 95.22

(90.11)

XCO2 27.26 to 57.12

(35.00)

15.19 to 76.13

(27.69)

3.87 to 16.00

(9.32)

dCO2 0.80 to 0.86 (0.84) 0.86 to 0.89 (0.88) 0.76 to 0.80 (0.78)

dH2O 1.06 to 1.08 (1.07) 1.08 to 1.09 (1.08) 1.09

dt 0.88 to 0.96 (0.93) 0.88 to 1.02 (0.98) 0.98 to 1.07 (1.03)

Abbreviations for fluid inclusion description (at room temperature)––a: Liquid-H2OþCO2-rich liquid; b: Liquid-H2OþCO2-rich liquid�CO2-richvapour; c: H2O-rich liquidþCO2-Hydrate; d: H2O-rich liquidþ vapour; P: Primary fluid inclusions; S: secondary fluid inclusions; PS: Pseudo-secondary fluid inclusions. Th CO2 L–V: homogenization temperature with CO2 phase; Tm ice: final melting of ice; Tm CO2 : initial melting of CO2phase; TClath: final melting of clathrate; Th: total homogenization of inclusion contents; wt.% NaCl equiv. salinity; XNaCl: mole fraction of NaCl;XCO2: mole fraction of CO2; dCO2: density of CO2; dH2O: density of H2O; dt: total density.aAfter Harraz and El-Makky (1999a,b).b Including decrepitation temperatures.


hedral crystals, thai frequently have cubic forms. Oc-casionally pyrite and arsenopyrite form interlocking

aggregates with pyrrhotite, or disseminations in albite

and quartz, particularly at the contacts between quartz

veins and wall rocks where graphite and rutile are

abundant. Arsenopyrite occurs as bipyramidal crystals

(29.56–31.19 at.% As, n ¼ 12), commonly associatedwith pyrrhotite and replaced by pyrite (Fig. 4C). Rutile

crystals occur as inclusions in arsenopyrite (Fig. 4C).Galena occurs as large anhedral grains or aggregates

commonly associated with sphalerite and pyrite. Gold

occurs as discrete grains of native gold (5–20 lm in di-ameter), and as inclusions and filling fractures in pyrite,

arsenopyrite and base metal sulphides. Gold is also seen

along partially healed fractures and marginal to arsen-

opyrite crystals, that were formed early than pyrite

precipitation (Fig. 4D). EMP analyses of gold grainsindicate that gold contains 15.63–17.97 at.% silver

(Table 1). These occurrences of gold indicate that sev-

eral influxes of gold and/or stages of remoblization took

place.

3.2.3. Late quartz–carbonate veins

These occur as relatively thin quartz veins post-dat-

ing all other types. The LQV range in width from 15 to

45 cm and frequently fill minute cracks in the earlier

vein varieties. They are comprised mainly of vuggy

milky quartz, calcite and pyrite. Quartz is often present

as euhedral coarse grains or euhedral terminated prisms(up to 3 cm in length) found in vugs projecting inward

from the vein walls. Quartz is also enclosed by coarse

calcite crystals forming a poikilitic texture. Calcite

generally constitutes �8 vol.% of the vein materials andoccurs in two distinctive forms: (1) white to transparent,

well-developed rhombohedrals crystals and (2) pearly

scalenohedral crystals. White transparent calcite is

found rarely as epitaxial overgrowths on the quartz invugs, while the pearly scalenohedral calcite crystals are

usually coated by malachite and goethite. A late drusy

calcite coats and/or replaces quartz in close vicinity

to the contact between the LQV and host dioritic

rocks.

Trace amounts of goethite, galena, covellite, and

malachite are also detected in the LQV. Pyrite is found

either as fine to very fine isolated round cubic crystals orreplacing the borders of galena grains. Pyrite is partially

altered to marcasite and magnetite. The LQV are also

affected by weathering and have many minute veinlets

filled by iron oxides in the form of reddish hematite and

brownish limonite. Weathering also forms colloform

intergrowths of goethite and limonite, particularly in the

upper portion of the mine.

These three vein types are also distinguished by thenature of their associated alteration assemblages. The

oldest are characterized by ankerite–albite and graphite

envelopes, the second by albite–sericite–kaolinite and

graphite envelopes, and the youngest by chlorite–car-bonate wall rock alteration (Fig. 3).

3.3. Temperature and sulphur fugacity of mineralized

quartz veins

The variation in chemical composition of EQV and

MQV auriferous fluids can be traced by examining

available mineral assemblages. Ranges of temperature

and fugacity of sulphur (fS2) were estimated from phaserelations and mineral compositions in the system Fe–

As–S (Kretschmar and Scott, 1976), and Fe–Zn–S

(Barton and Skinner, 1979) as shown in Fig. 5.At the Atud Au deposit, arsenopyrite is associated

with various Fe-sulphide minerals, corresponding to

EQV and MQV stages, and displays a wide range of As/

S ratios (Fig. 4A). The arsenopyrite may be employed as

a geothermometer through determination of As:S ratios,

provided an independent estimate can be made of sul-

phur activity during ore formation (Kretschmar and

Scott, 1976). As shown in Fig. 4A, all chemical com-positions of the Atud arsenopyrites display S-excess and

As-deficiency. EQV arsenopyrite (31.4–32.7 at.% As) is

associated closely with pyrite and/or pyrrhotite, indi-

cating approximate depositional temperatures of 340–

430 �C and log fS2 values of )6.5 to )9.6 atom (Fig. 5).The condition of formation for the MQV stage may be

estimated from the chemical compositions of sphalerite

and arsenopyrite coexisting with pyrite. The As content

Fig. 5. Sulphur fugacity–temperature diagram showing the ranges

of ore depositional conditions indicated by mineral assemblages in

EQV and MQV stages Atud Au mine. Curves (a) and (b) represent

the reactions of pyriteþAs$ arsenopyrite and pyriteþ liquid$arsenopyrite, respective�, (from Kretschmar and Scott, 1976). Com-

positional isopleths of sphalerite (XFes) are calculated from Barton andSkinner (1979). Po pyrrhotite; Py pyrite; XFes mole fraction of FeS insphalerite. Hatched area EQV stage; Cross-hatched area MQV stage.


of arsenopyrite and FeS mol% of sphalerite of the MQVvary from 29.6 to 31.2 and 10.4 to 19.4, respectively. The

ore minerals of MQV are estimated to have been formed

at temperatures of 273–368 �C and log fS2 of )8.0 to)12.2 atom (Fig. 5).The estimated temperature range for the mineralized

assemblages (273–430 �C) are consistent with meso-thermal conditions of gold deposition at Atud mine, and

is comparable to other gold deposits in central EasternDesert of Egypt. There appear to have been decreases in

temperature and sulphur fugacity with increasing time,

from EQV to MQV stages. Decrease of sulphur fuga-

city, through sulphide precipitation and/or H2S loss,

may favour gold deposition through destabilization of

gold bisulphide complexes such as Au(HS)2 (Seward,

1984; Drammond and Ohmoto, 1985; Bowers, 1991).

Thus, the decreases in fugacity of sulphur and temper-ature of ore fluids were likely causes of gold deposition

at Atud gold deposit.

4. Fluid inclusion studies

4.1. Sample selection and analytical techniques

Fluid inclusions (278 primary and 190 secondary)

were examined in 29 quartz samples from the EQV

MQV and LQV stages. Locations of the selected sam-

ples are shown in Fig. 2. Inclusions were examined in

quartz along vein margins associated with symmetrically

developed selvages of internally projecting, calcite, an-

kerite and albite crystal intergrowths with minor pyrite

and gold. This selection ensured that the inclusions weretaken from the same type of material.

Fluid inclusions were examined in thin (<0.30 mmthick) doubly polished slices. Microthermometric mea-

surements were performed using the Linkam THMSG-

600 programmed heating–freezing stage and employing

standard procedures (Shepherd et al., 1985). Stage cali-

bration was carried out at )56.6, 10.0, 30.8 and 294 �Cusing synthetic H2O–CO2 fluid inclusion standards.Measurements below 31.1 �C (the critical point of CO2)are accurate to within �0.2 �C and measurements abovethis temperature to within �1 �C. Initial melting of CO2phase (Tm CO2), final melting of ice (Tm ice), final melting

of clathrate (TClath), homogenization within CO2 phase(Th CO2 L–V) and total homogenization of inclusioncontents (Th) were measured. The warming rate wasmaintained at about 1 �C/min. The cooling rate is dif-ficult to control but it generally falls between 5 and 15

�C/min. Super-cooling was necessary for freezing ofboth CO2-hydrate and ice. Homogenization temspera-

tures were determined by observing the temperature at

which the boundary between liquid CO2 and H2O dis-

appears. A heating rate of 5 �C/min was used during the

initial stages of each heating run and reduced to 2 �C/min close to the homogenization temperature.

Salinities were calculated using the final melting

temperatures of CO2-clathrate (Collins, 1979) and ice

(Roedder, 1963) and expressed as wt.% NaCl equiv.

(Table 2). The homogenization temperature of the CO2-

rich phase in inclusions (Th CO2 L–V), together withvolume data on phase ratios (calculated from relative

areas), are used in calculating the molar ratios of CO2and H2O and the fluid density (Table 2). Disk copies of

the entire data file in the PC-Dos spreadsheet are

available from the author.

4.2. Nature of the fluid inclusions

Within all studied quartz samples, only two major

types of primary and secondary fluid inclusions are

recognized, namely: liquid-rich and vapour rich inclu-

sions. None of the studied inclusions contain any

daughter or accidentally trapped solid phases. Quartz

contain a high ratio of primary to secondary inclusionsthat are predominantly liquid-rich, representing �30 to70% of all detected inclusions at room temperatures.

Collectively the two types exhibit a relatively wide range

of liquid/vapour volume ratios indicating heterogeneity

of the trapped liquid–vapour mixtures. Most of the

quartz grains have suffered from some degree of strain

and recrystallization, showing several generations of

secondary fluid inclusion trails. However, workableprimary inclusions were located in all quartz vein stages

after careful petrographic examination. Quartz near vein

margins was found to be considerably less strained than

quartz in vein interiors.

The investigated primary fluid inclusions are com-

posed of H2O and CO2 phases in variable proportions,

and apparently represent mixtures of two end members

(Table 2). Generally, the fluid inclusions varied in sizefrom <3 to 18 lm, in largest dimension and are usuallyirregular to oblate spheroids with rare perfect negative

crystals. Most of these inclusions are located in un-

strained material, isolated by distances >5 times the in-clusion diameter, and randomly distributed within the

thickness of the section. These characteristics indicate a

primary and/or pseudosecondary origin of these inclusion

types according to the criteria defined by Roedder (1984).At 40 �C, all primary fluid inclusions observed in the

Atud vein materials consist of two (liquid-H2Oþ liquid-CO2) or three (liquid-H2OþCO2-rich liquidþCO2-richvapour) phases (Table 2). With cooling down to )10 �C,all of the liquid CO2-bearing two phase inclusions form

a CO2-rich vapour. According to relative abundance of

CO2-rich phase compared to the aqueous phase, at 40

�C, the liquid-H2Oþ liquid-CO2 inclusions can be fur-ther classified into CO2-rich and H2O-rich subtypes,

which suggest phase separation (confirmed below). The

volumetric percentages of CO2-component (LþV), at


40 �C, are 50–70%, 40–90% and 20–35% of the inclu-

sions from quartz intimately associated with the EQV,MQV and LQV stages, respectively (Table 2, Fig. 6A).

Secondary inclusions trails are abundant in most

materials with densities dependent upon degree of strain

followed by recrystallization. These fluid inclusions

consist of two phases (H2O-rich liquidþ vapour) atroom temperature. In quartz from EQV and MQV

stages the secondary aqueous inclusions form clathrates

during cooling, while in quartz from LQV stagesecondary aqueous inclusions do not form a clathrate.

The secondary inclusions have very variable shapes from

small (<1 to 5 lm) oblate spheroidal forms arrangedwith long axes parallel to the controlling fracture planes,

to large (>10 lm) more irregular dendritic forms.

They often display microthermometric properties which

are distinct from those of the apparent primary inclu-sions with which they are spatially associated. During

heating runs, it was noted that decrepitation and homo-

genization (vapour to liquid) take place at much lower

temperatures than the primary inclusions (Fig. 7). These

observations suggest that secondaries were the result of

later processes after the main mineralization event.

4.3. Microthermometry

The microthermometric data, summarized in Table 2and Figs. 6 and 7, reveal wide and different ranges of

homogenization temperature of primary fluid inclusions

in quartz intimately associated with the mineralization

Fig. 6. Histograms representing the microthermometric data of the primary and pseudosecondary fluid inclusions from the three paragenetic stages

at Atud gold mine. (A) volume% CO2 at 40 �C; (B) initial melting temperature of CO2 (Tm CO2 ); (C) homogenization temperature within the CO2(Th CO2 L–V); (D) salinity (wt.% NaCl equiv.); (E) final melting temperature of clathrate CO2 (TClath); and (F) total homogenization temperature ofinclusion contents (Th).


event. However, by analyzing peaks within histograms

of fluid inclusion data (Fig. 6), it is possible to decipher

individual hydrothermal events which are related to

specific mineral assemblages. Plotting of salinity (wt.%

NaCl equiv.) versus Th (Fig. 7) also allows discrimina-tion between the three paragenetic stages of the studied

mineralization. The EQV stage is characterized by fluid

inclusions of elevated salinity (4.3–8.2 wt.% NaClequiv.) and Th (372–490 �C). The MQV stage is charac-terized by moderate salinity (2.8–4.6 wt.% NaCl equiv.)

and Th (269–398 �C). The LQV stage shows low salinity(1.1–2.6 wt.% NaCl equiv.) and Th (170–265 �C). Al-though these data define three separate fields for each

type of quartz vein, the diagram suggests a trend from

EQV stage to LQV stage, through MQV stage. These

may reflect a continuum of three hydrothermal stagesduring the formation of the quartz in the Atud gold

mine rather than one single event (Roedder, 1984;

Nesbitt, 1993), which will be discussed below.

4.4. Inclusions in mineralized quartz vein stages

The Tm CO2 of primary and pseudosecondary fluid

inclusions in the quartz intimately associated with the

EQV stage are clustered between )57.7 and )56.2 �C(centered near )56.64 �C, Fig. 6B). Clearly, the liquid-CO2 component is reasonably pure, since the mean

value of )56.64 �C is close to the invariant melting

temperature of pure CO2 at )56.6 �C (Fig. 6B). In

some cases, the fluid inclusions revealed Tm CO2 between)57.7 and )57.0 �C, which indicates variations in thecompositions of the mineralized fluid.

The MQV stage is characterized by CO2-rich inclu-

sions, probably with other miscible gaseous components

such as CH4, N2 or H2S (Burruss, 1981; Read and

Meinert, 1986) as indicated by a Tm CO2 , clustered

around )58.5 and )57.2 �C (Fig. 6B) and lower thanpure CO2 ()56.6 �C). This amount of freezing pointdepression is consistent with 3–10 mol% CH4 or 15–25

mol% N2 within the CO2 phase (Swanenberg, 1980).

Moreover, a few inclusions in some quartz samples in-

timately associated with MQV, exhibited Tm CO2 as low

as )61.2 �C (Table 2, Fig. 6B), suggesting much higherconcentration of CH4, and/or have lower molar volume

of CO2-rich phase or some other component (Burruss,

1981; Heyen et al., 1982). The identification andquantification of these components through microther-

mometry were not possible because of the very small

quantities involved, and the data were therefore inter-

preted using the chemical system H2O–CO2–NaCl. A

common feature for Au-deposits, in which CH4 is in-

ferred in fluid inclusions, is the presence of ‘‘graphitic

material’’ (Ho et al., 1985). The occurrence of graphitic

materials at the Atud gold mine may be attributed tothe alteration of carbonate minerals by fluid-wall rock

reactions (Harraz, 1999). This is in agreement with the

conditions encountered in the studied samples from

level 42M (sample nos. 30, 41, 42, 49 and 50) and

level 165M (sample nos. 34 and 56; Fig. 2). Conse-

quently, the possible presence of CH4 indicates that

reduction might have played a significant role in Au

deposition (Bottrell et al., 1988; Naden and Shepherd,1989), since CH4 may not have been a component of the

original fluids. It worthy to note that CO2–H2O–CH4bearing hydrothermal fluids are a common feature

of mesothermal and other Au-bearing fluid (Mumin

et al., 1996). They (op. cit) demonstrated that a CO2–

H2O–CH4 primary gold-bearing fluids would be

produced by metamorphism of marine sedimentary

rocks.The homogenization of CO2-rich liquid–vapour

phases (always to a liquid) within the mineralized veins

(Fig. 6C) was measured in the absence of ice and

clathrate compounds. Fluid inclusions from the EQV

stage show a wide range of Th CO2 L–V (þ9.6� and þ17.3�C), while those in the MQV stage range between þ5.6and þ9.8 �C (Fig. 6C). Estimated densities of CO2 in-clusions in the mineralized veins reveal a narrow rangeof 0.80 to 0.89 g/cc and CO2 concentration ranges be-

tween 15 and 76 mol%. The MQV stage is characterized

by higher CO2-densities of 0.86–0.89 g/cc with 15.2–76.1

mol% CO2 than that of the EQV stage (0.80–0.86 g/cc

and 27.3–57.1 mol% CO2, Table 2).

TClath values of fluid inclusions in quartz from mine-ralized veins were below the þ10 �C quadruple point

Fig. 7. Total homogenization temperature (Th) versus salinity (wt.%NaCl equiv.) plots of the primary and secondary fluid inclusions from

the three paragenetic stages at Atud gold mine. Fluid unmixing has

produced the observed temperature–salinity trends of primary inclu-

sions, whereas secondary inclusions recorded later incursion of more

dilute waters. Solid symbols denote primary fluid inclusions; Open

symbols denote secondary fluid inclusions.


(Burruss, 1981), indicating a range from þ5.6 to þ8.5 �C(Fig. 6E). The TClath of CO2-rich inclusions in the EQVranges from þ5.6 to þ7.6 �C, while those in MQV showa range between þ7.6 and þ8.5 �C (Fig. 6E). However,most of Tm ice from the MQV exhibit a higher cluster

center ()2.3 �C) than the EQV stage ()3.6 �C). Aqueousphase salinities determined from clathrate melting

points for inclusions containing relatively pure CO2(CO2 m.pt. >56.6 �C) are relatively low with a totalrange from 4.3–8.2 wt.% NaCl equiv. (Fig. 6D). The

TClath values indicate a salinity of 4.3–8.2 wt.% NaCl

equiv., corresponding to the presence of 0.64–1.93 mol%

NaCl in the EQV, as well as 2.8–4.6 wt.% NaCl equiv.,

corresponding to the presence of 0.23–1.26 mol% NaCl

in the MQV (Table 2).

CO2-rich inclusions show a complex behaviour dur-

ing both heating and freezing experiments. This can beattributed to elevation of the internal pressure of the

fluid inclusion containing CO2 and/or CH4 during

heating, and/or to the development of fissures already in

existence (Burruss, 1981; Roedder, 1984). Although

most of fluid inclusions in the quartz from the mine-

ralized vein stages decrepitated prior to total homoge-

nization, 222 total homogenization (Th) were measured.H2O–CO2 inclusions in any one sample show similarranges of homogenization temperatures (269–490 �C)but to liquid H2O and the CO2 to vapour (Fig. 6F).

Hence these inclusions may be interpreted as repre-

senting two immiscible coexisting fluids (Ramboz et al.,

1982), and/or that phase separation accompanied mine-

ralization. Total homogenization occurred at tempera-

tures of 372 to 490 �C (cluster near 415 �C) for EQVinclusions and 269 to 398 �C (cluster near 310 �C) forMQV inclusions (Table 2 and Fig. 6F).

4.5. Inclusions in the late quartz–carbonate vein stage

The TClath in the fluid inclusions from the LQV stagerange from þ8.5 to þ9.4 �C (cluster near þ9.0 �C;Fig. 6E). These correspond to a maximum salinity

ranging between 1.1 and 2.6 wt.% NaCl equiv. (around

a modal value of 2.0 wt.% equiv.) in aqueous phase

(Fig. 6D). Tm CO2 of fluid inclusions in quartz from the

LQV stage range from )55.9 to )54.6 �C (cluster near)55.8 �C) and falls near the triple point of pure CO2.The lack of CH4 in the fluid inclusions from this stage

reflects a high oxidation state. The Th CO2 L–V fall

within the range þ17.0 to þ21.5 �C (clustering near

þ20.5 �C; Fig. 6C). The estimated CO2-densities ofthese inclusions reveal a narrow range from 0.76 to 0.80

g/cc, and CO2 concentrations from 3.9 to 16.0 mol%

(Table 2). Th of fluid inclusions in quartz from the LQVstage, which ranges from 170 to 265 �C (cluster near225 �C; Fig. 6F) are low relative to the other examinedmaterials.

4.6. Secondary fluid inclusions

The detected secondary fluid inclusions invariably

exhibit homogenization temperatures over a wide range

from 152 to 256 �C, with salinities from 2.9 to 4.9 wt.%NaCl equiv. (Fig. 7), indicating a decay of the temper-

ature regime subsequent to the mineralization event.

The first melting of ice in secondary aqueous inclusions

in quartz from the mineralized vein stages was recog-nized near )21 �C, although it was so difficult to observethat only a few measurements were recorded. These

measurements indicate TClath for some secondary aque-ous inclusions in quartz from mineralized vein stages are

þ7.1 to þ8.6 �C, corresponding to salinities of 2.9–4.9wt.% NaCl equiv. (Table 2). In fact, the Th and salinityrange for secondary fluid inclusions in the EQV and

MQV stages is nearly the same. The majority of thesecondary fluid inclusions in EQV stage range from 182

to 256 �C (cluster near 200 �C) with a salinity range from3.0 to 4.9 wt.% NaCl equiv. (centered near 4 wt.% NaCl

equiv.), while those from the MQV stage range from 152

to 251 �C (cluster near 185 �C) with salinity range from2.9 to 4.9 wt.% NaCl equiv. (centered near 4.2 wt.%

NaCl equiv.).

Secondary fluid inclusions in quartz from the LQVstage do not nucleate a recognizable clathrate phase,

indicating minor amounts (<2.7 wt.%) of CO2 (Heden-quist and Henley, 1985). These inclusions homogenized

at temperatures from 122 to 198 �C (centered near 145�C). Salinities of these secondary inclusions range from1.3 to 2.3 wt.% NaCl equiv. (cluster near 1.8 wt.% NaCl

equiv.; Fig. 7). The marked decrease in Th and salinitysuggests that the LQV stage fluids may have been cooledand diluted by meteoric water.

4.7. Fluid immiscibility

Fracturing was present throughout all stages of

mineralization at Atud gold mine. Many of the miner-

alized quartz veins occur in dilational shear zones in

obvious extensional sites where tension gashes, lami-

nated crack-seal veins with a ribbon texture and hy-

draulic breccias are developed Immiscibility in an ore

fluid can result from pressure release during tensional

fracturing, decreasing hydrostatic pressure during as-cent, and decrease in temperature.

As described previously, fluids trapped as primary

fluid inclusions in mineralized veins are either two phase

(two liquids) or three phase (two liquidsþ vapour), areH2O–CO2 bearing, and show quite variable vapour- to

liquid-ratios indicating heterogeneity of the trapped

liquid–vapour mixtures. These inclusions however,

show a wide range of CO2:H2O ratios, and apparentlyrepresent mixtures of two end members (Table 2)

possibly due to either unmixing of the original fluid

through a pressure decrease or CO2 phase separation


(Mumin et al., 1996). Both H2O-rich liquid and CO2-rich liquid inclusion phases are observed in quartz inti-

mately associated with the EQV and MQV stages.

Where the two inclusion types occur together, they tend

to homogenize at similar temperatures to liquid and

the CO2 to vapour (Fig. 6F). Hence these inclusions

are interpreted to represent two immiscible coexisting

fluids or phase separation accompaning mineralization

(Ramboz et al., 1982). On the other hand, primary fluid(liquid-H2OþCO2-rich liquid) inclusions show higherTh than the secondary aqueous ones. Moreover, areas ofprimary inclusions tend to show either variable

H2O:CO2 ratios even in adjacent inclusions, or a rela-

tively consistent H2O:CO2 ratios corresponding to �60and 50 vol.% CO2 in EQV and MQV, respectively.

These observations indicate that H2O–CO2 phase sepa-

ration was sporadic during deposition of mineralizedquartz veins.

The relationship between homogenization tempera-

ture and salinity of the quartz vein fluids (Fig. 7) indi-

cates a complex history of unmixing (CO2 effervescence)

induced by pressure decrease over the temperature range

of �490–270 �C. Moreover, homogenization tempera-tures of primary and pseudosecondary fluid inclusions

decrease with time from EQV to LQV stages throughthe MQV stage (Fig. 7). The lower temperatures and

especially the lower salinities of secondary inclusions in

the mineralized quartz veins are similar to those of

primary inclusions in the LQV stage of mineralization

(Table 2, Fig. 7); representing the waning stage of quartz

mineralization. Secondary H2O-richþ vapour fluids canbe produced by unmixing of a relatively primary low-

salinity H2O–CO2 fluid, because nearly all of the salt willbe fractionated into the residual H2O-rich liquid phase

rather than into the CO2-rich vapour phase (Bowers and

Helgeson, 1983a,b; So et al., 1995). Therefore, fluid

unmixing may have accompanied the observed tempe-

rature–salinity trends.

As described earlier, the liquid-H2Oþ liquid-CO2inclusions in the quartz intimately associated with the

EQV stage have relatively uniform CO2 phase ratios(50–70 vol.%; corresponding to XCO2 of about 0.35).Homogenization temperatures (n ¼ 108) for these in-clusions are high (372–490 �C). These inclusions mayrepresent the parent homogeneous fluids from which the

aqueous and CO2-rich fluids were derived through

progressive fluid unmixing.

4.8. Pressure considerations

Fluid inclusion data provide information only on the

minimum pressure and temperature during trapping,

except for the case of simultaneous trapping of immis-cible boiling fluid (Roedder and Bodnar, 1980). There-

fore, estimating entrapment pressures of fluid inclusions

using evidence of fluid immiscibility is a delicate matter

(Roedder and Bodnar, 1980; Ramboz et al., 1982). Thehomogenization temperatures of immiscible, H2O-rich

and CO2-rich (in EQV and MQV stages) inclusions

correspond to the minimum entrapment temperatures of

these fluids. The presence of a homogeneous parent fluid

(H2O–CO2) indicates that P–T conditions were above

the two-phase region (Fig. 8) for at least short periods of

time during fluid entrapment. In the calculation, the

dissolved salt was neglected because it was always <1mol% of the bulk composition (see Table 2). The highest

possible P–T conditions during mineralization can be

obtained by crossing isochores for a CO2-rich fluid of

medium to high density.

For the system H2Oþ 30 mol% CO2 (Grawford,

1981), the solvus curve is intersected by isochores of

0.80–0.89 g/cc (the calculated range of CO2 fluid density

and homogenization temperatures range from 270 to490 �C for EQV and MQV inclusions; Table 2) at

pressures of �125–185 MPa (Fig. 8). The estimatedpressures correspond to minimum depths of �5.0–7.4km, assuming a purely lithostatic pressure regime.

4.9. P–T conditions during quartz vein stages

The best estimates for mineralization temperatures

are derived from fluid inclusion homogenization tem-

peratures. Since there is evidence that H2O–CO2 phase

separation accompanied mineralization, trapping is

thought to have occurred along an immiscibility surfacein the H2O–CO2-dissolved salt system. Hence, homo-

genization temperatures (Fig. 6F) are equal to trapping

temperatures, with some perturbations associted with

the formation of mixed inclusions with intermediate

compositions and cooling. Knowing the bulk composi-

tion of the parent fluid, the conditions of fluid entrap-

ment during the mineralized stages at the Atud gold

mine can be estimated. The highest possible P–T con-ditions during mineralization are defined by the iso-

chores for CO2-rich inclusions in quartz intimately

associated with the mineralization event (Fig. 8). Iso-

chores for liquid-H2O, liquid-CO2 and liquid-H2Oþliquid-CO2 inclusions in mineralized quartz veins were

calculated based on their densities as well as the known

volumetric properties of H2O–CO2 system (Bowers

and Helgeson, 1983a,b). They imply homogenizationtemperatures in the range of 490–269 �C with CO2-

densities from 0.80 to 0.89 g/cc (�15 to 76 mol% CO2).Dissolved salt was neglected because it was always <1mol% of the bulk composition (Table 2). These ranges

are bound by the upper thermal limit for the assemblage

arsenopyriteþ pyrite at 491 �C (Kretschmar and Scott,1976), and critical curve for H2Oþ 30 mol% CO2 at 270�C (Grawford, 1981). Hence, if it is assumed that thesefluids were trapped on the solvus for the H2O–CO2system containing <2 mol% NaCl with temperatures of270–490 �C, then the maximum and minimum trapping


would occur at pressures between 160 and 272 MPa

(�6.4–11.0 km depths) and 125–185 MPa (�5–7.4 kmdepths), respectively (Fig. 8). These possible P–T con-

ditions are defined by the intersection of the 0.86 g/cc

CO2-density isochore with both the critical curve for

H2Oþ 30 mol% CO2 at 270 �C (Grawford, 1981) and theupper thermal limit for the assemblage arsenopyriteþpyrite at 491 �C (Kretschmar and Scott, 1976). The esti-mated pressure will be even greater if more volatile

species are considered, such as CH4 and N2 (Hollister

and Burruss, 1976).

If the coexistence of H2O–CO2 and H2O inclusions in

the LQV stage is interpreted in terms of simultaneous

trapping at the time of an event of fluid unmixing, dif-

ferent P–T conditions show a point of intersection at 265�C and 125 MPa, which lies close to the critical curve forH2Oþ 30 mol% CO2 at 270 �C and 115 MPa (Fig. 8).These estimates are in accordance with the total homo-

genization temperatures of the H2O–CO2 inclusions. At

these conditions the total pressure was probably equal

to the hydrostatic pressure (i.e., close to hydrostatic fluid

pressure conditions).

Using the estimated fluid density and final Th (Table2; Fig. 8), the confining pressures and depths of the

different stages are found to be as follows:

ii(i) Early mineralized quartz vein stage: pressure (125–

160 MPa) and depth (5.0–6.4 km).

i(ii) Main mineralized quartz vein stage: pressure (160–

185 MPa) and depth (6.4–7.4 km).

(iii) Late quartz–carbonate vein stage: pressure (110–125

MPa) and depth (4.4–5.0 km).

The fluid inclusions show a more or less continuous

evolution in composition and density. The interpreted

formation depth of mineralization at Atud is consistent

Fig. 8. The ranges of calculated isochores for the fluid inclusion in quartz from the three vein stages represented in a pressures–temperatures dia-gram.

Conditions of mineralization were probably confined to the rectangular box. The highest P–T conditions (indicated as black dot) were obtained by

cross-cutting isochores of 0.86 g/cc CO2-rich inclusions and pyrite–arsenopyrite temperature (Kretschmar and Scott, 1976). The lower P–T conditions

(indicated as open dot) were obtained by the 0.86 g/cc CO2-isochore and critical curve for H2Oþ 30 mol% CO2, at 270 �C (Grawford, 1981). The P–Tcondition of LQV can be obtained by the isochore of H2O-density. The lower pressure limit of the box, 160 MPa, is defined by the confining pressures

necessary to prevent boiling of an aqueous fluid containing 30 mol% CO2, which occurs in some fluid inclusions. The upper pressure boundary, 272

MPa, is fixed by the intersection of the 0.86 g/cc CO2-isochore with the 491 �C of pyriteþ arsenic$ arsenopyrite (from Kretschmar and Scott, 1976).

Lower temperature limits are defined by homogenization temperatures of the fluids; upper temperature limits are from the intersections of fluid in-

clusion isochores with the 272 MPa upper limit for pressure. Depths indicated are based upon assumed lithostatic conditions.


with the mesozonal class of orogenic Au-deposits (6–12km) proposed by Groves et al. (1998).

5. Discussion

5.1. Nature and origin of ore fluids

The EQV stage has Th CO2 L–V (þ9.6 to þ17.3 �C)markedly higher than the MQV (þ5.6 to þ9.8 �C). Thisanomalous behaviour might be attributed to the pres-

ence of additional gases and/or higher salinities. In addi-tion, the Tm CO2 measurements reveal that the deposition

of the gold was largely affected by wall: rock interaction

as manifested by the inferred occurrence of CH4 in some

samples collected from the graphite materials.

Previous studies of Au deposits limit the fluid source

to three main possibilities: (i) a meteoric/volcanic source

such as epithermal Au deposits, (ii) a hypabyssal mag-

matic source, and (iii) a metamorphic source. Accordingto the geologic setting of the Atud mineralized quartz

veins, the following may be concluded: (i) The large

quartz veins invade dioritic rocks and occasionally ex-

tend into adjacent metagabbroic rocks through a shear

zone. The metagabbro–diorite complex shoulder the

metasediments and serpentinites along a thrust plane

dipping moderately to the north and the west along the

north-eastern boundary of Gabal Atud. The thrustcontains abundant planes of highly sheared talc-car-

bonates, (ii) The country rocks of the Atud mining

district were metamorphosed under low greenschist

fades conditions. Typical mineral assemblages are seri-

cite–quartz–carbonate–chlorite–epidote–graphite–rutile

in the metasediments (Abu El-Ela, 1990; Harraz, 1999),

and (iii) The Au-bearing quartz veins were structurally

controlled by the older faulting system (north-north-west) that formed during the Pan-African tectonic event

and affected the whole of the Eastern Desert, with pos-

sibly a younger tectonic event with a NE trend (El-Gaby

et al., 1988; Meshref, 1990).

During these deformational events, the diorite host

rocks may have acted as preferential sites for fluid flow

and ore precipitation due to their brittle nature during

regional deformation and dilation of the Atud structuralbelts. Many of the mineralized quartz veins occur

in obvious extensionai sites where tension gashes,

laminated crack-seal veins with a ribbon texture and

hydraulic breccias are developed. Moreover, the mic-

rothermometric investigation emphasizes that there

were a continuum of mineralization events. The range of

salinity from 2.8 to 8.2 wt.% NaCl equiv. and Th from269 to 490 �C of the H2O–CO2-rich inclusions (Fig. 6,Table 2), do not support a direct magmatic source of the

ore fluids. It is more likely that the ore-bearing fluids

were of metamorphic origin and obtained their metal

contents from the leaching of country rocks duringwater/rock interactions.

The Atud Au deposits resemble Archaean lode Au

deposits; most of which formed under mesozonal con-

ditions between 300 and 475 �C at 150–300 MPa (Gebre-Mariam et al., 1995; Groves et al., 1998). Like most of

the Archaean Au deposits, the Atud deposit exhibits a

pinch and swell vein texture and evidence of multiple

mineralization events (Sibson et al., 1988). The preferredmodel for the formation of the Atud deposit is a brittle

shear zone in the earth�s crust that formed a hydro-thermal fluid conduit along vein failure surfaces (Sibson,

1990; Cole et al., 2000). The fluid is thought to have

originated through devolatization of the metasediment

and serpentinite–talc-carbonate rocks at depth and mi-

grated upward through the structural conduit during

dilation of the Atud structural belts. Hence, the dioritesmay have acted as preferential sites for fluid flow and

ore precipitation due to their brittle nature during re-

gional deformation (Yao et al., 1999). Therefore, the

source of H2O–CO2-rich fluids could be the metasedi-

ment-serpentinite rocks at depth mixed with meteoric

fluids existing at shallow crustal levels during uplift

processes. Fluid pressure drop post-dating fracture

failure at the end of the EQV stage could contribute toAu precipitation. A drop in fluid pressure is likely to

induce fluid immiscibility in a complex H2O–CO2–NaCl

fluid which further aids precipitation of the vein min-

erals (Drammond and Ohmoto, 1985; Spooner et al.,

1987). Immiscibility in an ore fluid can also result from

pressure release during tensional fracturing, decreasing

hydrostatic pressure during ascent, and decrease in

temperature.

5.2. Transport and deposition of gold

Gold deposition in the Atud area was related to fluidphase separation, sulphidization and carbonatization of

host dioritic rocks during hydrothermal alteration and

mineralization. During phase separation, volatile species

selectively partition into a secondary gas phase accord-

ing to their gas/liquid partition coefficients (H2 >CH4 > CO2 > H2S > SO2).Thiswouldaffect the residualore fluid by-causing rapid oxidations, loss of metal-

complexing ligands (mainly HS), decrease in tempera-ture, and pH fluctuations (Drammond and Ohmoto,

1985; Spooner et al., 1987). Hence, the solubility of

metals in the residual ore fluids would readily decrease

by several orders of magnitude, causing precipitation of

sulphide and gold (Drammond and Ohmoto, 1985;

Spooner et al., 1987; Seward, 1989). Iron for sulphide

minerals in the Atud ores was derived from dissolution

of Fe-bearing carbonates, wall rock Fe oxides andsilicates (Seward, 1984). Au is thought to initially have

deposited in solid solution and as inclusions in pyrite,

arsenopyrite and base metal sulphides (c.f. Mumin et al.,


1994). The formation of sulphides played a significantrole in concentrating the gold, a common feature of Au-

bearing systems (Phillips and Groves, 1983).

Fluid inclusion studies of gold mineralization indicate

that ore fluids containing CO2-rich and H2O-rich im-

miscible phases play an integral part in Au deposition in

many mesothermal vein systems (Goldfarb et al., 1989).

In light of the experimental system of the H2O–CO2–

NaCl (Bowers and Helgeson, 1983a,b), the data of thepresent study indicate that the two volatile species re-

mained unmixed at 125 MPa at temperatures above 270

�C and salinities <2 mol% NaCl when the CO2 exceed15 mol% (Naden and Shepherd, 1989). Because of the

relatively high solubility of CO2 at elevated tempera-

tures (270–490 �C) and pressures (125–185 MPa), theextent of this reaction required much greater immisci-

bility in the Atud Au-deposit. Rapid CO2–H2O phaseseparation occurred episodically throughout the evolu-

tion of the hydrothermal system.

The conditions of deposition at the Atud gold deposit

are comparable with many other mesothermal vein

systems quoted in the literature (Groves and Foster,

1993; Nesbitt, 1993; Groves et al., 1998; Yao et al.,

1999). The loss of CO2 and other volatiles from the fluid

phase during phase separation and/or boiling will causean increase of the pH, lower the fO2, decrease the activityof the bisulphide complex (Drammond and Ohmoto,

1985; Seward, 1984). At the same time total S is de-

creasing due to volatilization of H2S and precipitation of

sulphides. The loss of total S from the fluid can cause

Au-saturation and precipitation. These would effectively

destabilize the gold bisulphide complex and lead to de-

position of sulphides and gold as well as the silicate andcarbonate gangue at relatively high crystal levels and

low ambient pressure and temperature.

6. Conclusions

The Atud gold deposit formed in at least two suc-

cessive stages. The early one started with the formation

of coarse grained pyrite with pyrrhotite and arsenopy-

rite from H2O–CO2-rich solutions (27–57 mol% CO2and densities 0.80–0.86 g/cc), with low salinity (5.7 wt.%

equiv.) and high temperature (an average homogeniza-tion temperature of �415 �C). This was followed by themain stage after fracturing the Au-bearing quartz veins

and precipitating an assemblage of fine grained sphale-

rite–arsenopyrite–pyrite–galenaþ chalcopyrite. The lat-ter formed from mineralizing solutions characterized by

H2O–CO2-rich fluids (15–76 mol% CO2), high CO2density (0.86–0.89 g/cc), low salinity (3.9 wt.% equiv.)

and moderate temperature (an average homogenizationtemperature of �310 �C).Gold deposition occurred at temperature above 270

�C and pressures of at least 125 MPa (>5 km depths),

and is largely related to fluid-wall rock interaction. Goldwas repeatedly deposited during these two mineraliza-

tion stages, being more enriched in the early time of the

main mineralization stage. This may explain why gold is

mainly detected along microfractures of pyrite and ar-

senopyrite as well as along partially healed fractures that

were formed later than pyrite precipitation.

Dissolution and transportation of the Au might have

been achieved through complexing a bisulphide, whichis comparable with many mesozonal lode-gold deposits

all over the world. Gold deposition in the Atud area was

related to fluid phase separation, sulphidization and

carbonatization of the host dioritic rocks during hy-

drothermal alteration and mineralization. The diorites

are considered to have acted as preferential sites for

fluid flow and ore precipitation due to their brittle na-

ture during regional deformation. The loss of CO2 andother volatiles from the fluid phase during phase sepa-

ration will cause an increase of the pH, lower the fO2,decrease the activity of the bisulphide complex. At the

same time total S is decreasing due to volatilization of

H2S and precipitation of sulphides. The loss of total S

from the fluid can cause Au-saturation and precipita-

tion. These would effectively destabilize the gold bisul-

phide complex and lead to deposition of sulphides andgold as well as the silicate and carbonate gangue at

relatively high crystal levels and low ambient pressure

and temperature.

Acknowledgements

The author is grateful to Dr. M.F. El-Sharkawy(Tanta University) who arranged for the microprobe

analyses at the Camborne School of Mines, Exeter

University, UK. W. Pohl and A.M. Mumin AES re-

viewers are thanked for their constructive comments on

the initial manuscript.

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Fluid inclusions in the mesozonal gold deposit at Atud mine, Eastern Desert, Egypt