Wall rock alteration, Atud gold mine, Eastern Desert ...rjstern/egypt/PDFs/CE...

Pergamon Journal of African Earth Sciences, Vol. 28, No. 3. pp. 527.551, 1999

PII:SO899-5362(99)00031-7 0 1999 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0699.5362/99 $- see front matter

Wall rock alteration, Atud gold mine, Eastern Desert, Egypt: processes and P-T-XC02 conditions of metasomatism

HASSAN 2. HARRAZ Department of Geology, Faculty of Science, Tanta University, Tanta, Egypt

ABSTRACT-The Atud gold mine, central Eastern Desert of Egypt, is located in an intrusive metagabbro-diorite complex that abutts the conglomerate-greywacke-slate series of the Pan- African Belt in Egypt. Gold-bearing quartz veins occur as fracture filling in the Neoproterozoic dioritic rocks and along their contacts with the metagabbro. Gold mineralisation is associated with discrete metasomatic alteration zones around shear zones and quartz-carbonate vein arrays. Textures indicate that the zoned wall rock alteration and associated Au mineralisation postdate regional metamorphism. Alteration zones delimited by mapping alteration isograds encompass individual veins and merge to form the mine-scale alteration. The alteration zones seem to have been formed through metasomatism of CO,, H,O, K and lesser Na, as determined by mass balance calculations on whole rock chemical data. Alteration minerals are dependent on the original rock composition, with actinolite and epidote characteristic of the unaltered host rock. Boundaries between the individual zones can be mapped as alteration isograds. In order of decreasing alteration or increasing distance from a vein or shear zone, these isograds in the diorite separate the zones: (I) chlorite and calcite; (2) albite and ankerite; and (3) albite, muscovite and kaolinite. Zone 1 contains evidence of the transition from titanite to rutile and calcite. The formation of albite is due to CO, metasomatism in an Al-rich host rock; its coexistence with muscovite is significant as it defines the K+/Na+ ratio of the hydrothermal fluid associated with the Au mineralisation. Using K+/Na+, Na+/H+ and CO,, the path of alteration was modelled corresponding to observations of texture and paragenesis. Thermodynamic calculations also suggest that the isograds represent the degree of carbonate, potassic and sodic metasomatism by reaction of the Au-bearing hydrothermal fluid with the unaltered metamorphic rocks, and display the importance of original rock composition on the mineralogy of the alteration zones. @ 1999 Elsevier Science Limited. All rights reserved.

RESUME-La mine d’or d’Atud, dans le centre du desert oriental d’Egypte, est sit&e dans un complexe de metagabbros-diorites buttant sur la serie conglomerats-graywackes-schistes de la chains pan-africaine en Egypte. Les filons de quartz mineralises en or remplissent des fractures dans les diorites neoproterozoiques ainsi que le long de leurs contacts avec les metagabbros. La mineralisation aurifere est associee a des zones d’alteration metasomatique disc&es autour d’accidents cisaillants et d’un ensemble de veines a quartz-carbonate. Les textures indiquent que I’alteration zonee des roches encaissantes et la mineralisation aurifere associee sont posterieures au metamorphisme regional. Les zones delimitees par la cartographic des isogrades d’alteration entourent les filons individuels et convergent vers la zone d’alteration correspondant a la mine. Les zones d’alteration semblent s’btre formees lors d’un metasomatisme du CO,, H,O, K et, dans une moindre mesure, Na ainsi que I’ont montre des calculs de bilan de masse bases sur des don&es chimiques sur roches totales. Les mineraux d’alteration dependent de la composition de la roche originelle, I’actinolite et I’epidote &ant caracteristiques de la roche non-alteree. Les limites entre les zones individuelles peuvent Btre cartographiees en tant qu’isogrades d’alteration. Ces isogrades dans la diorite &parent

email: hharraz@decl .tanta.eun.eg

Journal of African Earth Sciences 527

H. 2. HARRAZ

les zones suivantes (en alteration decroissante ou en s’eloignant de la veine ou du cisaillement): (1) chlorite et calcite; (2) albite et ankerite; (3) albite, muscovite et kaolinite. La zone 1 montre une transition de la titanite vers le rutile et la calcite. La formation de I’albite est due a un metasomatisme en CO, dans des roches-hates riche en aluminium; sa coexistence avec la muscovite permet de definir le rapport K+/Na+ du fluide hydrothermal associe a la mineralisation en or. En utilisant les rapports K+/Na+, Na+/H+ et le CO,, le cheminement de l’alteration a Bte mod&i& et corresponds aux observations de textures et de parageneses. Les calculs thermodynamiques suggerent Bgalement que les isogrades representent le degre de metasomatisme carbonate, potassique et sodique par la reaction du fluide hydrothermal aurifere avec les roches metamorphiques non alterees. Ils demontrent egalement I’importance de la composition originelle des roches sur la mineralogie des zones d’alteration. @ 1999 Elsevier Science Limited. All rights reserved.

(Received 818197: revised version received 2316198: accepted 1 O/7/98)

INTRODUCTION

The vein type Au deposits in eastern Egypt (more than 80 occurrences) have been the topic of much research (e.g. Garson and Shalaby, 1976; El Gaby et a/., 1988; Pohl, 1988). Gold deposits closely associated with granitoid rocks of post-tectonic events (El Gaby et al., 1988) are reported at several localities in the central block of eastern Egypt. Gold-bearing quartz veins in this granitoid terrain have been the predominant source of Au in the Eastern Desert of Egypt (e.g. El Sid, Atalla, El Sukari, Umm Rus and Atud).

The majority of these deposits occur as Au- bearing quartz veins of polymetallic character (El Bouseily et al., 1985; Hilmy and Osman, 1989; Harraz et a/., 19921, with their distribution largely controlled by two prominent fracture systems which dominate whole of the Eastern Desert (Sabet and Bondonsosov, 1984; Harraz and Ashmawy, 1994). Because of the resemblance in their structural setting and mineralogical compositions, they are interpreted collectively as products of hydrothermal activity (Garson and Shalaby, 1976) induced either by metamorphic or cooling effects (Pohl, 1988) of Lower Palaeozoic magmatism or Early Cambrian subduction-related talc-alkaline magmatic rocks (El Gaby et al., 1988). Close to the mineralised quartz veins, the wall rocks are characterised by silicification, carbonatisation, sericitisation and Na metasomatism (Sabet et a/., 1976a). The intensity of such wall rock alteration differs from one deposit to another.

The wall rock alteration around the hydrothermal Au-bearing quartz veins and lodes of the Eastern Desert show distinct mineralogical changes suggestive of metasomatic alteration !Osman and Dardir, 1989; Harraz, 1991; Harraz and El Dahhar, 1994). The metasomatic styles were strongly influenced by variations in the carbon dioxide and alkali contents of the mineralising fluid (Harraz and El Dahhar, 1994; Harraz et a/., 1992). Generally,

528 Journal of African Earth Sciences

the different types of alteration have been described chemically in some deposits in terms of gains and losses of components (Gresens, 1967; Thompson, 1986) or metasomatic zonation (Korzhinskii, 1959; Frantz and Mao, 19761, and interpreted using isobaric univariant reactions (Greenwood, 1975).

The main objective of the present study was to re-investigate the wall rock alteration at the Atud gold mine area in order to determine its nature and extent. Thus, this paper focuses on the distribution of alteration minerals and describes discrete metasomatic alteration zones in the dioritic rocks hosting the quartz veins at the Atud mine (Fig. 1). Temperature and pressure conditions of the metasomatic fluid have been independently calculated using calcite-dolomite geothermometry and amphibole geobarometry. This study describes the mineralogical and chemical characteristics of this alteration and aims to verify the physicochemical conditions prevailing during the Au- related hydrothermal alteration.

LOCATION AND HISTORY

The Atud area is located in the central Eastern Desert of Egypt at latitude 25OO’lO”N and longitude 34O24’10”E. The mine is approximately 58 km west of Mersa Alam on the Red Sea coast and ca 5 km south of the Idfu-Mersa Alam paved road (Fig. 1 I. It is one of several Au mines in the Eastern Desert of Egypt which was initially excavated during the Pharaonic times. The Atud gold mine is considered to be in the class of mesothermal vein type Au, where the mineralised quartz veins are hosted mainly in dioritic rocks (Pohl, 1988). The Atud Au deposit resembles those of Archaean Au deposits and the majority of Alaska-Juneau Au belts, as well as other mesothermal vein systems quoted in the literature (Roberts, 1988; Colvine et a/., 1988; Colvine,

Wall rock alteration: processes and P-T-X,,2 conditions of metasomatism

Lrt+! Granite -,,--1dfwMersa Alam a Gabbro m Granodiorite

)_LThlUS1

I

ES: etagabbro-diorite complex

erpentinite and their derivatives m Metavolcanic m Metaconglomerate-metagreywacke C_‘Metasediment

I , . . . , (, I

34’ I 24’ .MO 132

road

Figure 1. General geological map around the Gabal A tud area in the south Eastern Desert of Egypt. The rectangle outlines the map area show. in Fig. 2. This map is modified after Akaad and Essway (19651 and Saber et al. (197661.

1989; Groves and Foster, 1993). At Atud, the main mining activity and mineralised quartz veins are confined to the eastern and southern slopes of the Gabal Atud (Fig. 2A). The Au-bearing veins are abundant in main three localities, referred to as the Main Atud, Eastern Atud-I and Eastern Atud- II (Fig. 2A). The first is the most important, being confined to the eastern footslopes of Gabal Atud. Between 1953 and 1969, three expeditions by EGSMA (Egyptian Geologic Survey and Mining Authorities) performed underground prospecting work in the Main Atud area. Drifting was done on three levels along strike of the main lode (north- northwest - south-southeast) for a total length of 690 m. These levels were connected by three inclined shafts down the dip of the lode for a total length of 230 m (Fig. 3). Other small shafts and pits were made at Eastern Atud-I and Eastern Atud-Il. The depth of excavation varied between 20 and 78 m and the analysed Au content varied from <O.l to 31 g t-l. The principal lode was

found to contain 19,000 tons of Au ore with a Au content of 16.28 g t-l, which corresponds to 348 kg of pure Au. In addition, 1600 tons of dump with 12.4 g t-’ Au are present in the area (Hussein, 1990). Previous studies (Amin et a/., 1953; El Ramly and Akaad, 1960; Akaad and Essawy, 1964, 1965; Awad and Fasfous, 1981; Bishady et a/., 1983; Gabbra, 1986; Abu El Ela, 1990; Nakhla et a/., 1993) have described some of the geological setting, structure, mineralogy and physical characteristics of the mineralised quartz veins.

GEOLOGICAL SETTING

The Atud gold mine area (Fig. 1) covers ca 18 km2 and comprises various igneous and metamorphic rocks from the basement complex of Precambrian age. It is dominated by a metagabbro-diorite complex, together with subordinate serpentinite-talc- carbonate, metasedimentary and metavolcanic rocks.


H. Z. HARRAZ

X X X :. I_ .Gabal Atudr I- L; X X $L. A 908m’.L L: ‘X

X x f \x

LX ,,X .Y. LL L xf X x x‘

L L L :‘,X<, i’

&\\ Wadi . 30’1. (B) 20

E zabbro EZlQuartz veins mnodiorite m Shaft or Entrance

m Diorite m Metagabbro

yVThrust fault

E3S P er entinite and their derivatives a Metabasalt B Mctasediment

Figure 2. (A) Detailed geological map of the A tud gold mine district (modified from Gabbra, 1986). (8) Rose frequency diagram of field measured quartz veins in the Atud gold mine (n = 192).

The metasediments comprise a distinctive succession of elastic immature turbidite, with a repeated alternation of metaconglomerates and metagreywackes, together with subordinate metamudstones (El Ramly and Akaad, 1960; Akaad and Essawy, 1965). The metasediments occupying the small area on the southern slope of Gabal Atud are usually interposed with serpentinite-talc-carbonate rock. However, the metasediments form a platform overlain by the metagabbro-diorite complex and serpentinites-talc- carbonates (Fig. 1). Akaad and Essawy (1964) showed that the metagabbro-diorite complex abutted the metasediments and serpentinites, separated by a thrust plane dipping moderately to the north and somewhat to the west along the eastern boundary of the Gabal Atud. The thrust

is underlain by abundant slivers of highly sheared talc-carbonates (Fig. I).

The metagabbro-diorite complex occupies the main part of Gabal Atud in the middle of the mapped area (Fig. 2A). The complex was intruded into metasediments and serpentinites and was laterally intruded by gabbroic rocks. The central part of Gabal Atud is formed of mainly gabbroic rocks (i.e. olivine-pyroxene gabbro, pyroxene gabbro and hornblende gabbro). A too thin, unmappable, transitional hybrid zone, characterised by coarse-grained hornblende crystals, is well- developed between the dioritic-gabbroic boundaries. In the northeastern part of Gabal Atud the dioritic rocks may grade into granodiorite with a transitional facies present. Pohl (1988) considered the diorite-granodiorite rocks of the Atud area as


Wall rock alteration: processes and P-T-X,,, conditions of metasomatism

Main Northern

Level 165M

r30 m

?? 1,2,3 . . . . . . Number and sites of samples -J Drift

15 t-

25 50 m

fi

:I M

Figure 3. Longitudinal cross section of the Atud gold mine showing the locations of the analysed samples collected from the different mining workings: level 42M, level 72M and level 165M. The traced levels of mining activity are after the Gold Mine Authorities.

belonging to a differentiated Neoproterozoic mafic intrusion in the northeastern sector of the Pan- African Belt.

as fine isolated round grains or replacing the borders of pyrrhotite grains.

Dioritic rocks Metagabbroic rocks The metagabbroic rocks are generally medium- to coarse-grained and of greenish-grey colour. They are composed of varying amounts of andesine, titanaugite, hornblende and tremolite- actinolite, together with scarce amounts of olivine. Quartz, magnetite and/or titanomagnetite, ilmenite, calcite and titanite are accessory minerals. Ophitic to subophitic textures are the most common of those observed. The hornblende displays both a primary crystalline nature (relatively abundant with brownish-green colour and strongly pleochroic: X = greenish-brown, Y = green and 2 = dark green) and secondary brownish-green hornblende formed by deuteric alteration of titanaugite. This secondary variety occurs as long fibrous crystals, which may form aggregates or irregular crystals poikilitically enclosing andesine crystals.

The dioritic rocks are medium- to coarse-grained and of greyish-black colour. They consist of variable amounts of oligoclase and hornblende with subordinate amounts of biotite and quartz. Ilmenite, titanite, apatite, pyrrhotite, pyrite and chalcopyrite occur as accessory minerals. The hornblende is essentially a green type, which is partially altered to a brownish-green variety surrounded by chlorite. Pyrrhotite occurs as fine crystals, sometimes associating with chlorite. Chalcopyrite is found either as fine isolated crystals or at the borders of the pyrrhotite. Pyrite is present as minute dispersed grains associated with pyrrhotite and chalcopyrite.

QUARTZ VEINS Geology and distribution

The opaque mineralogy revealed that Fe-Ti oxides The area of mineralisation is ca 9 km*, localised are common phases, represented by homogeneous at the eastern and southeastern slopes of Gabal Fe-rich ilmenite and magnetite. Subordinate Atud. The mineralisation in the Atud gold mine amounts of sulphide minerals comprise pyrrhotite area is of a disseminated type and is localised in, and pyrite, together with scarce pentlandite and and related to, hydrothermal veins that occupy chalcopyrite. The pyrrhotite crystals are replaced pre-existing fractures (open-space filling type). The by goethite along their cleavage and locally area includes many mineralised and unmineralised spotted by very fine magnetite and marcasite. quartz veins. The mineralised quartz veins cut Pentlandite commonly forms a flame texture in mainly across the dioritic rocks and may extend pyrrhotite and is locally detected in the peripheral to the metagabbro (Fig. 2A, Table I). Intense wall zones of its grains. Chalcopyrite is found either rock alteration is clearly observed at the vein

Journal of African Earth Sciences 53 1

H. Z. HARRAZ

Table 1. Average chemical compositions of the country rocks, Atud gold mine

Metagabbro Diorite Range Mean S.D. Range Mean S.D.

(n=9) (n = 12) Si02 43.94 46.90 45.48 1.03 50.45 52.22 51.33 0.56

TiOz

A1203

Fe203

Fe0 MnO

MgD CaO Na,O

‘(20

p2°5

S

co2

H20

0.22 0.61 0.36 0.14

12.32 13.47 12.97 0.40

2.69 3.64 3.02 0.34

9.90 12.20 11.23 0.76 0.12 0.24 0.17 0.05

11.45 13.07 12.26 0.52 7.06 7.89 7.44 0.27 1.29 1.93 i .5a 0.21

0.04 0.12 0.08 0.03

0.03 0.06 0.04 0.01

0.00 0.01 0.00 0.00 0.05 0.12 0.07 0.03

4.05 5.40 4.77 0.48

0.48

15.15

3.52

6.10 0.12 3.66 7.28 2.06

0.13

0.06

0.00 0.05

3.29

1.20

17.20

6.08

a.20 0.20 5.50 a.72 2.62

0.40

0.10

0.01 0.37

4.36

0.86

16.22

4.94

7.36 0.15 4.17 7.96 2.38

0.26

0.08

0.00 0.13

3.89

0.24

0.74

0.92

0.73 0.03 0.68 0.49 0.21

0.09

0.02

0.00 0.12

0.38

Total 99. ia 99.81 99.48 99.37 99.96 99.72 Fe0 */Fe0 * + MgO

0.52 0.55 0.53 0.01 0.69 0.77 0.74 0.03

FeO* = Fe0 + 0.8998Fe,O,. S.D.: Standard Deviation.

margins. Contacts between veins and wall rocks are commonly sharp and occasionally outlined by carbonate, chlorite and iron oxide minerals. The mineralised quartz veins are surrounded by an alteration halo (3.5 x 2.8 m) of dominantly altered carbonate rocks with a sequence of alteration zones outlining the structurally controlled veins.

A rose diagram of 192 measured quartz-filled joints (Fig. 26) shows the following trends (in decreasing order of abundance): (I) northwest (N30°-40OW); (21 northeast (N35O-55OE); and (3) north-northeast (N5°-150E).

Main lode The main quartz lode on the eastern slopes of Gabal Atud (Main Atud) is conformable to the north-northwest - south-southeast fracture trend, while the main quartz vein lodes on the southeastern slope of Gabal Atud (Eastern Atud- I and Eastern Atud-II) are confined to a northeast- southwest fracture trend. The main Atud is the largest lode that comprises six veins, occurring within a shear zone extending north-northwest for ca 305 m across the dioritic rocks (Fig. 2A). Other quartz veins extend throughout the different underground levels (Fig. 3), invading a shear zone


filled by pockets of quartz, carbonate and chlorite. In the first level, the main quartz vein is composed of bluish or greyish quartz, which is frequently associated with variable amounts of milky quartz. By comparison on the second and third levels, the main quartz veins showed a relatively smaller amount of bluish or greyish quartz associated with the predominant milky white quartz.

The main lode consists mainly of massive milky white to bluish-grey quartz with or without carbonate (calcite and ankerite), chlorite and sulphide minerals. Locally, the quartz veins exhibit vuggy and banding features, and in most cases they are sheared and brecciated. The main lode is exploited on three levels along its strike (north- northwest - south-southeast) for a total length of 690 m, and by three inclined shafts for a total length of 230 m down dip (4OOW). The mineralised quartz veins have a general northwest- southeast direction (dip 40°-43OSW) and northeast-southwest (dip 14O-53ONW or SE). The dip of these quartz veins changes from 29OSE to 74OSE with depth. Individual quartz veins vary from a few centimetres up to 2 m wide, and < 1 .O m to more than 100 m long. The large veins trend N25O-35OW and extend discontinuously up to

Wall rock alteration: processes and P-T-X,_,2 conditions of metasomatism

270 m along the strike and from 78 to 165 m along the dip (40° WI, The vein is partially worked out up to 42 m deep, with an average thickness of 0.7 m. They usually pinch, swell, bifurcate into small veins, veinlets and stringers (off-shoots) and join other veins, given rise to a network pattern ca 5 m width.

Ore mineralogy The mineralised quartz veins are formed mainly of brecciated and fractured milky white to bluish- grey quartz with or without carbonate, chlorite, sericite, albite and sulphide minerals. The quartz is formed of coarse to medium interlocking crystals with sutured borders and wavy extinction. Calcite and ankerite, as well as long acicular crystals of tremolite, are observed both in the vein material and/or forming independent small veinlets. A late calcite (drusy form) replaces ankerite in veins close to or at the contact between the quartz vein and host dioritic rocks.

The sulphide minerals are relatively minor components (<2 wt%) of the quartz veins but are more abundant in the shear zone. Pyrite is the dominant sulphide phase forming 1.5 wt% of the vein material. Pyrite occurs as medium- to fine- grained subhedral to euhedral crystals, frequently with cubic form (up to 1 cm diameter). Occasionally they show skeletal forms and are commonly associated with carbonate, forming a granular texture. Late poikilitic pyrite contains inclusions of quartz, muscovite and ankerite in the vein. Other sulphide species present in minor amounts include pyrrhotite, chalcopyrite and sphalerite, together with one or more minerals of ilmenite, titanite, rutile, magnetite, goethite and malachite. The sulphide species occur as medium- to fine-grained crystals forming interlocking aggregates or disseminations in the hydrothermal quartz veins and particularly abundant at the contacts between the quartz veins and the wall rocks. Gold occurs both as inclusions in pyrite and also postdates the pyrite precipitation in fractures. Gold is also seen along partially healed fractures that were formed later than the pyrite precipitation.

The main auriferous quartz vein consists essentially of commonly fractured bluish-grey quartz that crystallised at an earlier stage, and milky white quartz formed at a later stage, although probably overlapping in their deposition (Nakhla et a/., 1993). The older generation comprises mainly bluish, coarse- to medium- grained and brecciated quartz containing an appreciable amount of native Au, pyrrhotite, pyrite,

ilmenite and magnetite. The younger generation quartz is dominantly composed of milky white, fine-grained and compact crystals containing a minor amount of sulphide minerals. This milky white quartz is devoid of Au and characterised by many minute veinlets filled with iron oxides in the form of reddish hematite and brownish limonite.

WALL ROCK ALTERATION

Rocks in the Atud area reached a peak of metamorphism within the lower greenschist-facies (Akaad and Essawy, 1965). Gold mineralisation is associated with extensive wall rock alteration, as represented by saussuritisation and kaoliniti- sation of the feldspars, as well as by chloritisation and carbonatisation of the mafic phases (Awad and Fasfous, 1981; Bishady et al., 1983). The intensity of this alteration envelope correlates positively with the thickness of the mineralised veins and veinlets, but rarely exceeds 30 cm, which may reflect different degrees of progressive reaction. A series of mappable alteration zones around the auriferous quartz veins have consistently developed in the dioritic host rock at Atud mine (Fig. 4). The mineralogical composition and intensity of these alteration zones are dependent to some degree on the host rock composition (Table 1). The mineralogy and textures of these zones are described below. Figure 4 shows the mineralogy and paragenetic relationships for the dioritic rocks.

Prealteration mineral assemblages The diorite consists of an assemblage of albite (An ,,*_ _; 15-50%), chlorite and/or actinolite (25%), a fine polycrystalline mesostasis of quartz (lo-20%), chlorite and epidote (25%), and ilmeno- magnetite and titanite (I O-l 5%, Fig. 4D). Albite occurs as 0.5-2.0 mm laths and has sharp contacts with all minerals except quartz, where granophyric intergrowths locally occur. Fine granular epidote occupies 30-90% of the albite grains. Coarse epidote (0.2 mm) is associated with chlorite. Quartz occurs as an interstitial constituent between plagioclase and displays four main forms:

il individual and clusters of subrounded grains (0.4 mm across);

iii fine crystalline mosaic; iii.) a granophyric intergrowth with albite; and iv) clusters of fine granular grains associated

with chlorite. Chlorite is commonly associated with actinolite

and has a purplish-brown birefringence. These chlorite grains are characterised by randomly


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ISOGRAD

Metamorphic assemblage

1 . . .._.... 1.,,.

Zone 1 la , , . . ,‘.“. 2.. L. ..-.

Zone 2

3 . . . . . . . . .

Zone 3

Zone 2

‘Zone 1

(0 Main Shaft Samde

I 1 0 0 0SRGQNBl nn.

Level

i 42M

J Level

165M

(JN Me!amorphic Zone Isograd ZOllC Zone Main Late

nssemblagc I Ia 2 3 win vein

Hornblende ---_- Actinolite Biotite Quartz Chlorite - __ .__- -

Epidote _F -_- -_.

Albite _---

Albite (h) Titnnitc Rlltik Rutile (h) Calcite --

.

Ankcritc Ksolinite Muscovite a__ - - - -__- Ilmenite -_-

Ilmeoitc - - __ mngncite

Magncrite --w-w Tourmnlinc Goetbite ---- - __ _ Pyrite --__--- - -.-- Fyrrbotitc -- Chalcopyrite Spbolerite Gold

-Time+

H. 2. HARRAZ

8

Fe

6

Si

Figure 5. Metamorphic and alteration chlorite species in the Atud gold mine (classification according to Hey, 1954). 0: metamorphic assemblage; A: metasomatic alteration; ??: late veinlets.

orientated fractures. The composition of this chlorite lies near the ripidolite-pycnochlorite boundary (Fig. 5). Magnetite grains have an average diameter of 2 mm (maximum 2.4 mm) with exsolution lamellae of titanite. Hematite occurs along narrow zones between exsolution lamellae and at their intersection points. The modal content of magnetite, hematite and/or titanite varies between 10 and 15%.

Alteration assemblages and zoning Evidence for secondary reactions is plentiful. Most of the altered samples are characterised by:

17 plagioclase feldspars highly altered to sericitic mica and kaolinite, as well as containing dispersed grains of saussurite (zoisite, clinozoisite) and calcite;

ii) thin rims of albite on feldspar grains; iiJ pyroxenes altered to chlorite and calcite; iv) hornblende altered to tremolite-actinolite

series, calcite and chlorite. Tremolite-actinolite associations occur in aggregates with chlorite and calcite as an alteration product of hornblende and pyroxene. Moreover, patches of tremolite-actinolite intergrowth with hornblende are not uncommon;

vl calcite, as well as long acicular crystals of tremolite are observed both in the quartz vein material and/or forming independent small veinlets;

vi) ilmenite is present in small amounts, usually surrounded by a rim of secondary titanite, which


occasionally forms with ilmenite a pseudo- micrographic texture. Few patches of leucoxene- rutile could be observed; and

vii) pyrrhotite and pyrite are partially altered to marcasite and magnetite.

Extreme oxidative alteration may also form colloform intergrowths of goethite and limonite, particularly where the quartz veins inject into the metagabbro.

Alteration zones within the dioritic rock are classified according to diagnostic mineral assemblages into:

i) Zone 1 : chlorite + calcite; ii) Zone 2: ankerite + albite; and iii) Zone 3: albite + muscovite + kaolinite. Silicate-carbonate mineral associations and

textures in the dioritic rocks form the basis of defining the sequence of these zones (Fig. 4). Zone boundaries are marked by the disappearance of one alteration or metamorphic mineral and the appearance of new alteration minerals (Table 2). The various alteration zones are described separately for the diorite.

Zone 1: chlorite + calcite One typical characteristic of zone 1 is the randomly orientated euhedral plates of chlorite intergrown with calcite and quartz. These minerals form similar size patches (2 mm) replacing actinolite and indicating that alteration postdates metamorphism. There is also an increase in the modal amount of secondary quartz with increasing alteration (Fig. 4). The Fe/(Fe + Mg) ratio increases from metamorphic chlorite to altered chlorite with an increase in abundance of calcite (Fig. 5). The compositional difference between the “metamorphic” and “alteration” chlorite species in diorite reflects the availability of Fe from breakdown of the metamorphic minerals (epidote, actinolite, biotite and ilmenite-magnetite) to form the alteration minerals (chlorite, secondary magnetite and, at more intense stages of alteration, calcite and ankerite) in the diorite.

Epidote and ilmenite-magnetite are much less abundant in the diorite. Fine epidote, originally formed during regional metamorphism by saussuritisation of plagioclase feldspar, is partially to totally replaced by calcite in the zone 1 rocks. Inclusions of epidote in albite are replaced by calcite and very fine-grained mica. Secondary magnetite and hematite are found in zone 1 in association with the destruction of titanite and formation of rutile, leucoxene, calcite and quartz (sample 26, Fig. 40.

Wall rock alteration: processes and P-T-X,,2 conditions of metasomatism

Table 2. Representative analyses and mineralogy of alteration zones on diorite rocks, Atud gold mine, Eastern Desert, Egypt

Sample no. 26 25 36 38 40 28 Zone 1 1-2 2 3 3 LAD+ Si02

Ti02

A1203

Fe203

Fe0 MnO

MgD CaO Na,O

K2O

p205

co2

S

H20

52.68

1.26

13.50

3.85

7.85 0.16 2.37 6.71

4.00

0.47

0.18

4.13

0.00

3.50

48.97

0.32

15.95

3.10

7.10 0.15 2.36 7.64

2.56

0.39

0.10

8.30

0.01

2.80

48.10 42.25 41.20 51.30

1.18 0.52 0.28 0.75

14.09 14.40 16.50 15.84

1.90 1.60 1.10 4.95

8.90 8.70 8.30 7.15 0.17 0.15 0.05 0.10 5.40 6.20 2.42 5.50 4.75 7.57 8.70 7.58

3.60 2.80 4.90 2.20

0.34 1.85 1.25 0.23

0.12 0.09 0.16 0.06

8.90 10.80 12.65 0.00

0.26 0.37 2.15 0.10

2.30 2.70 0.00 4.12

Total 99.94 99.75 100.00 100.00 99.66 99.88 Modal Estimates

MINERALOGY 25 Chlorite

Albite Quartz Epidote Rutile * Calcite Ankerite Muscovite Kaolinite Magnetite Pyrite

22 20 33

1.9 2.0 5.7 3.4 tr tr

3.5

8.4 27 29

7.1 tr 15 38 20 22

16 20 10

2.0 25

tr

4.5 5.0

15 3.7 6.1

2.2

6.1 33

11 18

3.0 Density Volume factor

2.84 2.90 2.70 2.82 2.84 2.95

1.17 1.18 1.16 1.18 1.20

+ : Least altered diorite; *: Rutile and leucoxene.

Zone 2: ankerite + albite Ankerite and albite occur in alteration zone 2, while calcite is absent. In the transition zone between 1 and 2, partial replacement textures are shown by ankerite rims on calcite that partially surround the coarse alteration chlorite blades, and by abundant muscovite inclusions in metamorphic albite. The chlorite, calcite and quartz patches in zone 1 are transformed dominantly to albite and ankerite in zone 2. In the presence of ankerite,

chlorite has a higher Fe/(Fe + Mg) ratio than chlorite associated with calcite in zone 1.

Zone 3: albite + muscovite + kaolinite In the strong alteration zone adjacent to the quartz veins, muscovite and hydrothermal albite are characteristic minerals. Adjacent to the Au-bearing quartz veins, zones of intense sericitisation and albitisation are well-developed. The width of these alteration salvages is proportional to the width of


Tabl

e 3.

M

iner

al

anal

yses

, ac

tivity

m

odel

s an

d ca

lcul

atio

ns,

Atud

go

ld

min

e

Epid

ote

Albi

te

Chlo

rite

Mus

covi

te

Rang

e M

ean

SD

Rang

e M

ean

SD

Rang

e M

ean

SD

Rang

e M

ean

SD

(n=8

) (n

=9)

(n =

13)

(r-

r=61

Si

Oz

37.9

2 40

.37

39.4

8 0.

73

66.0

0 68

.50

67.7

4 0.

68

21.8

0 23

.80

22.7

0 0.

54

45.1

0 48

.30

46.5

4 0.

94

Al2

03

21.5

9 23

.90

22.6

9 0.

80

18.5

0 19

.50

19.1

9 0.

26

16.2

0 21

.o

o 19

.50

1.40

28

.20

33.5

6 30

.77

1.49

Ti02

0.

02

0.10

0.

05

0.02

0.

00

0.00

0.

00

0.00

0.

10

0.30

0.

22

0.06

0.

06

0.40

0.

19

0.10

Fe0

8.05

12

.47

9.94

1.

30

0.04

0.

30

0.09

0.

08

33.6

0 45

.20

38.2

6 2.

53

0.70

3.

50

1.98

0.

80

MnO

0.

04

0.10

0.

06

0.02

0.

00

0.01

0.

00

0.00

0.

05

0.12

0.

07

0.02

0.

00

0.09

0.

02

0.03

MgO

0.

03

0.09

0.

05

0.02

0.

00

0.10

0.

04

0.04

5.

60

11.2

0 8.

03

1.94

0.

10

0.95

0.

64

0.24

CaO

22.4

9 25

.14

24.0

0 0.

89

0.02

0.

40

0.11

0.

11

0.00

0.

10

0.02

0.

04

0.00

0.

20

0.08

0.

06

Na20

0.

00

0.02

0.

00

0.01

11

.20

11.5

4 11

.38

0.12

0.

05

0.24

0.

14

0.05

0.

20

1.20

0.

51

0.26

K,O

0.

00

0.00

0.

00

0.00

0.

08

0.67

0.

25

0.18

0.

00

0.10

0.

04

0.04

9.

36

10.5

0 9.

98

0.39

a-

Tota

l Fo

rmul

a ba

sis

96.2

8 98

.80

88.9

7 90

.70

25

0 6.

530

6.44

4 4.

723

4.35

5 0.

012

0.00

7 1.

717

1.35

2 0.

014

0.00

9 0.

022

0.01

3 4.

424

4.19

7 0.

006

0.00

1

80

Si

6.26

6 Al

3.

933

Ti

0.00

2 Fe

1.

102

Mn

0.00

5 M

g 0.

007

Ca

3.92

3 Na

0.

000

0.08

2.

998

3.00

1 3.

000

0.00

4.

923

0.16

0.

989

1.00

9 1

.ooo

0.

01

4.31

2 0.

00

0.00

0 0.

000

0.00

0 0.

00

0.01

7 0.

18

0.00

1 0.

011

0.00

4 0.

00

6.08

4 0.

00

0.00

0 0.

000

0.00

0 0.

00

0.00

9 0.

00

0.00

0 0.

007

0.00

2 0.

00

1.90

1 0.

18

0.00

1 0.

019

0.00

5 0.

01

0.00

0 0.

00

0.95

2 0.

990

0.97

5 0.

01

0.02

2

28

0 5.

172

5.03

8 5.

599

5.09

3 0.

050

0.03

6 8.

521

7.08

5 0.

022

0.01

3 3.

550

2.66

2 0.

024

0.00

4 0.

101

0.06

1

0.08

0.

38

0.01

0.

57

0.00

0.

59

0.01

0.

02

6.38

6 4.

594

0.00

6 0.

083

0.00

0 0.

021

0.00

0 0.

054

22

0 6.

837

6.56

3 5.

503

5.10

2 0.

042

0.02

0 0.

412

0.23

3 0.

011

0.00

2 0.

205

0.13

3 0.

031

0.01

2 0.

315

0.13

7

0.14

;

0.13

5

0.01

0.

09

: 0.

00

0.05

0.

01

0.07

K

0.00

0 0.

000

0.00

0 0.

00

0.00

5 0.

038

0.01

4 0.

01

0.00

0 0.

029

0.01

1 0.

01

1.63

8 1.

882

1.79

6 0.

08

Activ

ity

0.65

4 0.

736

0.69

9 0.

03

0.95

2 0.

990

0.97

5 0.

01

0.00

1 0.

006

0.00

3 0.

01

0.81

9 0.

941

0.89

8 0.

03

Activ

ity

mod

el

and

calc

ulat

ions

Ep

idot

e Al

bite

Ch

lorit

e M

usco

vite

Ac

tivity

of

cl

inoz

oisi

te

in e

pido

te

a Na

AISi

,O,

Activ

ity

of

clin

ochl

ore

in c

hlor

ite

a KA

12(A

ISi3

0,0)

(OH)

2

a Ca

2A13

Si30

,2(O

H)

= (N

a)

a M

g,AI

Sr3A

101r

, =

(K)

= 0.

699

kO.0

3 =

0.97

5 kO

.01

1Ma2

+/51

5 &I

L&

= 0.

898

kO.0

3

516

1 I6

= 0.

003

+0.0

1

Act

ivit

ies

wer

e ca

lcul

ated

as

sum

ing

idea

l ac

tivi

ty

mod

els

(Ski

ppen

an

d C

arm

icha

el,

1977

). S

D:

Sta

ndar

d D

evia

tion

.

Wall rock alteration: processes and P-T-Xco2 conditions of metasomatism

Table 4. Compositions and inferred equilibration temperatures of calcite-ankerite pairs, Atud gold mine

Sample no T(°C) XCa cc xMg cc XFe cc xMn cc XCa ank xMg ank XFe ank XMn ank

Zone 1 25 --- 0.963 0.019 0.017 0.001 --- --- --- ---

26 --- 0.993 0.003 0.004 0.000 --- --- --- ---

Zone 2 36 432 0.941 0.034 0.021 0.003 0.577 0.336 0.080 0.007

37 427 0.938 0.038 0.024 0.000 0.486 0.395 0.111 0.008

42 416 0.939 0.036 0.023 0.002 0.501 0.383 0.107 0.009

31 404 0.941 0.035 0.020 0.004 0.491 0.397 0.109 0.003

Zone 3 38 341 0.958 0.025 0.013 0.003 0.498 0.408 0.090 0.004

39 --- 0.978 0.009 0.01 2 0.001 --- --- --- --- 40 402 0.937 0.037 0.018 0.008 0.447 0.424 0.125 0.004

95 ___ 0.988 0.006 0.005 0.002 --- --- --- ---

97 297 0.968 0.019 0.010 0.003 0.498 0.412 0.083 0.007

X Fe CC: mole fraction of Fe in calcite; XMg snl: mole fraction of Mg in ankerite. Temperatures are calculated according to equations from Powell er al. (1984).

the vein, but rarely exceeds 30 cm. The rare assemblage of muscovite and ankerite without kaolinite occurs adjacent to the veins (Fig. 4). Muscovite defines a foliation that cross-cuts the chlorite and ankerite. The composition of muscovite from zone 3 is remarkably similar, with Na/(Na + K)-0.1 , Si-3.15, (Fe + Mg + Mm-O.3 and Ti-0.02 atoms per 12 oxygen atoms (Table 3). Measured Na/Na + K is similar for muscovite coexisting with albite and muscovite in rocks without albite.

Hydrothermal albite forms clear, twinned grains within and adjacent to the veins and is distinct from the metamorphic albite in the wall rock, which is filled with white mica inclusions. Pyrite encloses ankerite and muscovite, and commonly encloses rutile and leucoxene aggregates. Pyrite and tourmaline occur adjacent to the quartz veins. Gold is associated with pyrite in the wall rocks adjacent to the vein margins.

DISTRIBUTION OF ALTERATION ZONES

The alteration was mapped in order to partly assess the field observations (see Figs 10 and 1 1). Mineral incompatibilities in adjacent zones are expressed in the form of a reaction equation representing an alteration isograd (Carmichael, 1970; Clark et al., 1986). The major changes in mineral assemblages with progressive alteration (Tables 3 to 5) are represented by chemical reactions deduced from the textures and mineral associations in the diorite because of the spatial relationship of the Au mineralisation with that rock type. Carbon dioxide and K are present in trace amounts in the unaltered rocks (Table 2) and, therefore, must have been added to the altered diorites to form carbonate and muscovite (see below).

A set of reactions balanced with mineral analyses (Tables 3 to 5) from the diorite and consistent with the observed textures is as follows:

0.5Ca,(Mg,Fe,Ti),Si,O,,(OH), + Ca,AI,Si,O,,(OH) f 2H,O + 2.94C0, + 10.04Fe,O,) * actinolite epidote aq. fluid magnetite

0.76(Mg,Fe),AI,Si,O,,(OH), + 2.94CaC0, + 4.18Si0, + (0.04CaTiSi0, + 0.22NaAISi,O, + 0.30,) chlorite calcite quartz titanite albite fluid(l)

CaTiSiO, + CO, * CaCO, + TiO, + SiO, (la) titanite calcite rutile quartz

0.76(Mg,Fe),AI,Si,O,,(OH), + 3.8CaC0, + 2.3Si0, + 3.8C0, + 4.60, + 1.52Na’ = chlorite calcite quartz fluid

3.8CaMg(CO,), + 1.52NaAISi,O, + 1.28Fe,O, + 3H,O + (0.2Ti0,) ankerite albite magnetite rutile(2)


Tabl

e 5.

M

ean

com

posi

tion

of

amph

ibol

e,

Atud

go

ld

min

e

Type

Le

ast

alte

red

rock

Sa

mpl

e no

. 3

4 14

R 14

c 21

R 21

C 27

28

Si

02

51.8

9 52

.52

50.4

7 52

.40

51.0

9 52

.05

51.4

6 50

.66

Ti02

40

3

Fe2C

3 Fe

0 M

nO

MgO

Ca

O Na

20

K20

Tota

l

0.24

0.

75

0.40

0.

13

0.12

0.

27

0.20

0.

22

5.90

6.

68

5.68

6.

19

6.19

6.

38

6.13

7.

27

1.12

0.

00

3.06

1.

75

1.21

1.

25

1.38

3.

61

10.6

0 13

.31

11.2

8 9.

54

10.0

1 9.

25

9.77

12

.69

0.23

0.

29

0.20

0.

16

0.14

0.

20

0.26

0.

21

15.1

7 11

.51

13.7

0 13

.60

14.3

1 13

.28

14.6

7 10

.10

12.1

3 12

.03

12.3

8 12

.50

12.2

9 12

.20

12.8

1 11

.38

0.12

0.

67

1.29

1.

10

1.78

1.

58

0.32

1.

49

0.05

0.

13

0.05

0.

15

0.25

0.

06

0.16

0.

17

97.4

5 97

.89

98.5

1 97

.52

97.3

9 96

.52

97.1

6 97

.80

Stru

ctur

al

form

ulas

ba

sed

on

a ca

tioni

c ch

art

Si

7.39

5 7.

625

7.39

5 7.

661

7.47

6 7.

687

7.45

8 7.

600

AI(W

) 0.

605

0.37

5 0.

605

0.33

9 0.

524

0.31

3 0.

542

0.40

0

(T)

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

AI(W

) 0.

385

0.76

6 0.

375

0.72

5 0.

542

0.79

6 0.

503

0.88

7 Ti

0.

026

0.08

2 0.

044

0.01

4 0.

013

0.03

0 0.

021

0.02

5

Fe3+

0.

060

0.00

0 0.

168

0.09

6 0.

066

0.06

9 0.

075

0.20

6

Fe2+

1.

259

1.61

0 1.

377

1.16

3 1.

221

1.13

8 1.

180

1.59

4

Mn2

+ 0.

028

0.03

6 0.

025

0.02

0 0.

017

0.02

5 0.

032

0.02

7

Mg

3.24

3 2.

507

3.01

1

2.98

2 3.

141

2.94

2 3.

189

2.26

1 (M

l -M

3)

5.00

1 5.

001

5.00

0 5.

000

5.00

0 5.

000

5.00

0 5.

000

Ca

1.86

9 1.

851

1.93

9 1.

930

1.90

8 1.

895

1.98

5 1.

805

NaB

0.03

3 0.

149

0.06

1 0.

070

0.09

2 0.

105

0.01

5 0.

148

(M4)

1.

902

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

000

1.95

3 Na

A 0.

000

0.03

7 0.

303

0.23

7 0.

407

0.33

8 0.

074

0.25

7 K

0.00

9 0.

024

0.00

9 0.

028

0.04

6 0.

011

0.03

0 0.

029

(A)

0.00

9 0.

061

0.31

2 0.

265

0.45

3 0.

349

0.10

4 0.

286

Mg/

Mg

+ Fe

2+

0.72

0 0.

609

0.68

6 0.

719

0.72

0 0.

721

0.73

0 0.

587

activ

ity

0.05

0.

03

0.05

0.

05

0.04

0.

01

0.05

0.

01

T (‘C

l 55

9 49

8 55

9 48

7 53

9 47

8 54

4 50

6

Met

asom

atic

ac

tinol

ite

(zon

e I)

6 12

c 12

R 25

26

48

98

53

.05

51.2

0 52

.90

51.7

3 51

.46

52.6

3 51

.94

0.11

0.

22

0.10

0.

23

0.12

0.

12

0.18

5.

39

5.19

5.

56

5.59

5.

42

5.25

5.

26

2.47

3.

96

1.74

1.

50

3.71

1.

39

1.52

11

.06

12.9

9 9.

28

11.4

5 11

.52

11.5

0 11

.40

0.21

0.

20

0.18

0.

20

0.26

0.

14

0.12

12

.00

10.9

7 14

.37

13.5

7 12

.43

13.0

1 12

.70

12.3

5 12

.41

12.5

7 12

.65

11.2

6 12

.75

12.5

0 0.

25

0.31

0.

33

0.41

0.

80

0.17

1.

53

0.07

0.

20

0.13

0.

05

0.13

0.

12

0.07

96

.96

97.6

5 97

.16

97.3

8 97

.11

97.0

8 97

.22

e of

46

ex

clud

ina

(OH)

7.

855

7.70

3 7.

674

7.54

7 7.

621

7.71

9 7.

718

X 0.

145

0.29

7 0.

326

0.45

3 0.

379

0.28

1 0.

282

9

8.00

0 8.

000

8.00

0 8.

000

8.00

0 8.

000

8.00

0 g

0.79

4 0.

622

0.62

3 0.

506

0.56

5 0.

625

0.63

8 a

0.01

2 0.

025

0.01

1 0.

025

0.01

3 0.

013

0.02

0 0.

137

0.22

3 0.

095

0.08

2 0.

206

0.07

7 0.

084

1.36

6 1.

629

1.12

2 1.

392

1.42

2 1.

406

1.41

2 0.

026

0.02

5 0.

022

0.02

5 0.

033

0.01

7 0.

015

2.66

5 2.

476

3.12

7 2.

970

2.76

1 2.

862

2.83

1 5.

000

5.00

0 5.

000

5.00

0 5.

000

5.00

0 5.

000

1.93

1 1.

979

1.93

9 1.

969

1.78

5 1.

984

1.95

5 0.

069

0.02

1 0.

061

0.03

1 0.

215

0.01

6 0.

045

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0 0.

002

0.06

8 0.

031

0.08

5 0.

014

0.03

2 0.

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Wall rock alteration: processes and P-T-X,,I conditions of me tasoma tism

5NaAISi,O, + 1.3K’ + 2.5H,O + [O. 1 (Mg,Fe),AI,Si,O,,(OH), + 0.02Ti0, + 0.1 CO,]* albite fluid chlorite rutile fluid

1 .3KAI,Si30,,(0H), + O.GAI,Si,O,(OH), + lOSi0, + 5Na+ + 0.50, -t [O.lCaMg(CO,),l muscovite kaolinite

The components in parentheses are present in minor amounts. Reaction (1) corresponds to the formation of zone 1 rocks from the metamorphic assemblage. Reaction (1 a) occurs within zone 1, reaction (2) reflects the transition from zone 1 to 2 rocks, and reaction (3) marks the boundary between zones 2 and 3 (Fig. 4). Each reaction represents an alteration isograd designated by the more intensely altered assemblage on the right side of the reaction equation. Occurrences of reactant and product mineral assemblages in the Atud mine are located on Fig. 4.

lsograd (1 I: chlorite and calcite Chlorite and calcite represent the most widespread alteration type. The metamorphic assemblage of actinolite + epidote occurs in the upper level (Level 42M) of the mine workings. On the lower level (Level 165M), all samples collected were hydrothermally altered. On the surface west of the mine site the dioritic rock contains actinolite and epidote, whereas zone 1 altered rocks are uncommon. Secondary magnetite is commonly associated with zone 1 alteration.

lsograd (1 al: rutile and calcite The metamorphic assemblage with titanite is present with chlorite + calcite + quartz 2 m away from the main vein (Fig. 4). In altered rocks, rutile is present with quartz and carbonate (calcite or ankerite) in the absence of titanite and the trace of the isograd gives a crude outline of the Au-bearing structure. The assemblage is closer to the auriferous structures than the chlorite and calcite isograd, although the positions of isograds (I) and (I a) are often indistinguishable. However, the decomposition of titanite to calcite + quartz + rutile (or other TiO, oxides) is dependant on the partial pressure of CO, and occurs at temperatures ranging from 400 to 500°C (Schuiling and Vink, 1967).

lsograd (2): albite and ankerite This alteration isograd is situated between 2.5 and 0.6 m from the vein, closer than isograd (1 a) but is difficult to distinguish mesoscopically without a carbonate stain. Identification of this isograd is an additional step towards understanding the formation of the metasomatic zones, the

quartz fluid - ankerite(3)

effects of the hydrothermal fluid on the wall rock, and the Au mineralising event.

lsograd (3): albite and muscovite In intensely altered diorite, muscovite replaces chlorite. This assemblage is found within 0.5 m of the shear zone and within fragments of diorite wall rock within the vein. Pyrite is associated with this alteration zone and no oxide minerals remain. The assemblage ankerite + sericite (muscovite and/or kaolinite) defines the envelope of intense alteration around the vein sites. Metasomatic albite occurs at the interface between the vein and wall rock.

CHEMICAL CHANGES DURING HYDROTHERMAL ALTERATION

Volume changes and isochemical components Preservation of primary textures in the outer metasomatic zone suggests that volume changes were minimal. Using the composition-volume diagrams of Gresens (I 9671, where the lines of several elements intersect near the isochemical axis, it may be inferred that these elements were neither added to nor removed from the system, but acted as conserved markers and can therefore be used to indicate the volume change of the metasomatic process (Fig. 6). In the absence of clustering of the lines of several elements, a particular element such as TiO, can be assumed to have acted as an conserved marker, thus providing a basis for computing the volume change. To avoid any erroneous results due to retrograde alteration, the mesoscopically sheared samples were rejected. Indications are that the apparent volume changes for all traverses were between 1 .I 6 and 1.20 (i.e. an overall volume increase of 16-20%).

By considering each pair of adjoining metasomatic zones as a simple thermodynamic system, the components to which each system was closed during migration of the zone boundary can be identified by their isochemical behaviour at the inferred volume factor using the composition-volume diagram (Fig. 6) (Clark et a/., 1989). In this manner, components to which each metasomatic zone was apparently closed or open


H. 2. HARRAZ

Volume factor

Volume factor

Figure 6. Composition-volume diagrams (method of Gresens, 1967) for alteration zones in the Atud Diorite. The analyses of altered samples are compared with the analyses from the least altered sample (281. Arrow indicates an estimate of volume factor for the altered samples.

were identified and are shown for the Atud diorite in Fig. 7. The number of components to which the metasomatic zones were open increases towards the vein and there is a corresponding decrease in the number of minerals in each zone. This is consistent with Korzhinskiis (1965, 1970) theory of metasomatic zoning, whereby a progressive decrease in the number of stable minerals in metasomatic zones approaching veins can be explained as due to a progressive increase in the number of mobile components as a vein is approached. Moreover, this indicates that, in the

Atud Au lode, deposits involve a decrease of volume in the outer alteration zones and an increase of volume adjacent to the veins (see Kerrich. 19831.

Alteration chemistry in terms of mineralogical changes Gains and losses of the major element oxides were computed by comparing altered diorite from each zone with a least altered diorite rock (Fig. 8, Table 2). Petrographic evidence suggests that metasomatism postdates metamorphism and


Wall rock alteration: processes and P-T-X,,I conditions of metasomatism

INFERRED

DIFFUSION/

INFILTRATION

OF

COMPONENTS

-b CO,H,(

-b KtO NW

-+ Ah01

b Fe0 MgO

bS

b&lo

Figure 7. Alteration zones in the Atud Diorite showing the minerals stable in each zone, the components inferred to be isochemical from the composition- volume diagrams, and the inferred range of diffusion and/or infiltration of non-isochemical components.

_ ._ e

* B $ lo2 s - 10

1

-10

K,O

MIO NezO

lo2

a zone 1 (26)

m Zone2(36)

a Wall rock fragment

in vein (40)

a Isograd la (2~5)

m ane3(38)

figure 8. Gains and losses in major oxides for alteration zones in the Atud Diorite progressing from left to right. Note the change in scale.

therefore the metamorphic assemblage represents and MgO. TiO, is lost from rocks that originally the starting composition. A wall rock fragment contained titanomagnetite and titanite where within a shear type vein was also compared with these minerals are altered to carbonate and rulite. the least altered diorite (Fig. 8, Table 2). MgO shows a slight addition above the

Relatively immobile elements in the alteration background 20% in zone 3 rocks, where activities zones, other than AI,O, and TiO,, are SiO,, MnO of CO, are high enough to stabilise the ankerite-


H. Z. HARRAZ

chlorite and ankerite-albite assemblages. AI,O, is significantly added in zone 3 rocks where an albite- muscovite-kaolinite assemblage is present (Fig. 8). In the diorite silica shows variation, indicating that the increase in modal quartz in the altered samples is a consequence of the destruction of silicates involving a release of silica to form quartz, or the introduction of SiO, gives a + 20% volume change (see Groves and Foster, 1993). Thus, the increase in quartz content in the veins and in the inner alteration zones is not a result of lateral silicification but is a reflection of the destabilisation of layer silicates.

The progressive changes indicated by mass balance calculations show systematic increase in CO,, K,O, and S and a change from ferric to ferrous in the oxidation state of Fe towards the veins. Sodium is mobile and shows variable patterns in the diorite.

The formation of zone 1 resulted from the addition of CO, and H,O from the fluid and redistribution of Ca from calcic amphibole, epidote and titanite to form calcite. The reduction of Fe from ferric to ferrous is reflected in the destruction of epidote, actinolite and titanite. The gains in K in zones 1 and 2 are either due to a low mole fraction of this element or due to scarce sericite in the least altered rock.

In zone 2 the chemical changes involved an increase in CO, and a decrease in ferric Fe. Sample 25 (Table 2) represents isograd (1 a) coexisting with chlorite, calcite and ankerite, and shows no massive additions of Na,O (Fig. 8). Although there is a 20% mass gain of Na,O above background in zone 2 (sample 36) containing albite, the high modal abundance of albite in those samples do not have an addition of Na, but presumably some zone 2 albite is inherited from the least altered diorite and zone 1. Therefore, sufficient Na and Al was present in the diorite in the form of chlorite and metamorphic albite to form the hydrothermal albite. Therefore, albite was formed from the metamo:phic assemblage by an increase in the mole fraction of CO, in the fluid. This is evident from the consistent distribution of alteration zones and the sequence of formation of calcite, ankerite and albite. Secondary magnetite and calcite were partially to totally replaced by ankerite, which also reflects the change in oxidation state of Fe. A small S addition indicates a change from Fe3+ to Fez+ and is also reflected in the presence of trace amounts of pyrite in the assemblage with albite and ankerite.

Zone 3 is likely to have formed by further addition of CO, and a large influx of K,O. A major


gain of K exists between zones 2 and 3, which is interpreted as a chemical front. The latter is restricted to less than 1 m from the Au-bearing structure and is reflected in the mineralogy by the presence of muscovite. The addition of S is reflected in pyrite with a constant total Fe content.

Mineral assemblages of wall rock fragments within the vein (sample 40, Table 2) indicate that CO,, K+, Na+ and S were added. Sodium is fixed in hydrothermal albite, CO, in ankerite, K’ in muscovite and S in pyrite. Chlorite is absent. Magnesium released from chlorite during progressive alteration became fixed in ankerite.

CALCITE-DOLOMITE GEOTHERMOMETRY

Calcite-ankerite from alteration zones were used to estimate temperatures during metasomatism (Table 4). Although calcite composition is close to CaCO,, calcite coexisting with ankerite (e.g. zone 3) contains more Mg i-Fe+Mn (=0.032- 0.063 atoms per 3 oxygen atoms) than calcite in rocks without ankerite (Fe + Mg + Mn = 0.013- 0.022 atoms) (Table 4). Almost all analysed calcite in zone 1 is > 96 mol.% CaCO,. Ankerite is nearly a Ca(Fe,Mg)(CO,), solid solution with Fe/(Fe + Mg) ( =O. 168-0.228, Table 4). In general, Fe/(Fe + Mg) of ankerite increases with decreasing distance from the quartz veins or shear zone. Equilibrium temperatures were calculated from the average of equilibrium below:

CaMg(COJ (ankerite) * CaMg(C0,) (calcite) (4) CaFe(C0,) (ankerite) + CaFe(C0,) (calcite) (5)

using the equilibrium relations derived by Powell et a/. (1984) and Anovitz and Essene (1987). Variations in compositions of calcite in equilibrium with ankerite and the inferred temperatures of equilibrium are presented in Table 4. Temperatures of 388 +40°C for metasomatic wall rock alteration are indicated.

AMPHIBOLE-PLAGIOCLASE GEOTHERMOBAROMETRY

The compositions of amphiboles in the metasomatic alteration zones were compared with metamorphic amphiboles to investigate possible pressure changes since regional metamorphism. Brown (1977) showed that the Na content of calcic amphibole in equilibrium with the assemblage chlorite-albite-magnetite-epidote is related to pressure. He derived an empirical relationship between the crossite content of the

Wall rock alteration: processes and P-T-XcO? conditions of metasomatism

a) (Na + K)A <OS0 Ti <O.SO

&O -$-*LOO 7.75 7.50 7.25 Si

Tremolite

0.90

OBO- Actindite

0.70- P

n f3 AA *:

0.60- “4 Actinolitir

hornbiencie

A A A Metamorphic assemblage A* * Metasomatic alteration

b)

0 0.3 .-_ t 4 kh -

m

0 0.2 0.L 0.6 08

Al on tetrahedral site

Figure 9. (A) Classification of the calcic amphiboles of the present country rocks (after Leake, 19 78). (8) Amphibole compositions at the A tud gold mine. Empirical isobars from Brown (19 7 7) are shown.

amphiboles and pressure. On the other hand, Blundy and Holland (1990) showed that the Al content of amphibole is strongly temperature dependant and a reliable thermobarometer based on the AI’” content of the amphiboles coexisting with plagioclase in silica saturated rocks.

The analyses of calcic amphibole from metasomatic and metamorphic zones pointed well to the following. Typically, the amphibole primarily deviates from the general formula Ca,(Fe,Mg)$i,O,,(OH), solid solution by incorporation of -1.2 Al atoms per 24 oxygen atoms (Table 5). Individual amphibole compositions, however, are variable in the metamorphic zone, with Fe/(Fe+Mg) in the range 0.26-0.44, Al = 0.81-l .97 and Na = 0.03- 0.50 atoms per 24 oxygen atoms. Amphiboles from the metasomatic zone contain distinctly lower Na and Al (Table 5). The metamorphic amphiboles range in chemical composition between actinolite and actionlitic-hornblende,

while the metasomatic amphibole shows a limited range of chemical compositions within the actinolite field (Fig. 9A). Some grains of metasomatic amphibole show zoning from Fe-rich actinolites (centres) to more Mg-rich actinolite rims. The relatively constant composition of metamorphic amphiboles suggest that they crystallised under relatively constant pressure and temperature conditions.

According to Brown (19771, the crossite component of the amphiboles (represented by the Na content in the M4 site) has been plotted against the tetrahedral Al (Fig. 9B). The low sodium content of the amphiboles formed during metamorphism in equilibrium with epidote-albite- magnetite-chlorite-quartz is consistent with P,,,

+ pco, values in the range 2 to 3 kbar. The metasomatic actinolites are characterised by <0.04 mole fraction Na on the M4 site and have less tetrahedral Al than the metamorphic


H. 2. HARRAZ

480

‘WV

460

380

340

P=2kb

m Regional metamorphism

f?.!.y, , ,ygy , 1

b 0.1 0.2 0:3 0.4

WCO,)

Figure 10. T-XcoI diagram for total pressure fP,= P,, + P,,/ of 2 kbar showing isobaric equilibrium curves for reactions among the minerals clinozosite ICzl, tremolite (Tr), chlorite (Chll, albite IAlbl, dolomite (Doll, calcite (Ccl, magnetite (Mtl, titanite (Tnt), rutile (RN and quartz (Qtzl in the system CaO-Na,O-MgO-TiO,-Al,O,-SiO,-CO,- H,O (after Ames et al., 19911. Bold numbers represent isograds II), llal and 12). See text for discussion. Calculated using GEO-CALC (Berman et al., 19871.

actinolites. These relationships suggest that metasomatic actinolites crystallised under lower pressure conditions than the metamorphic actinolites. According to Blundy and Holland (19901, the data of Table 5 reveal that the metamorphic dioritic rocks were crystallised at a pressure range >2 kbar and a temperature range of 478 -559OC, while the metasomatic alteration (zone 1) was formed at a pressure of 1 kbar and a temperature range of 336-426OC. Obviously, the calculated temperature for metasomatic alteration (zone 1) using coexisting amphibole- plagioclase pairs (Blundy and Holland, 1990) are in good agreement with the estimated temperature based on calcite-dolomite pairs (Table 4).

ALTERATION MODEL

Korzhinskii (1959) and Thompson (1959) introduced the concept of “local equilibrium” to geology, whereby large systems not in equilibrium owing to a gradient in intensive parameters can

be approximated by a series of smaller systems in equilibrium. Application of this concept to the metasomatic alteration zones at the Atud mine was necessary to examine the evolution of the fluid during progressive alteration, to determine the constraints placed on temperature and pressure by the alteration assemblage, and to assess the effects of the chemical and mineralogical composition of the parent rocks on the alteration mineral assemblages.

i) The Fe/(Fe + Mg) ratios in the chlorites increase toward the Au-bearing structures in the diorite, with sulphidation causing synchronous iron sulphide and Au deposition.

iii Textural evidence suggests that S was added from the fluid and reacted with the Fe in the oxides to precipitate pyrite, a common attribute of Au- bearing systems (Phillips and Groves, 1983). This process occurs more efficiently in diorite than in the surrounding metagabbro rocks due to the higher modal amounts of magnetite and ilmeno- magnetite in the metamorphic assemblage. The


Wall rock alteration: processes and P-T-X,_,2 conditions of metasomatism

i 2 i

log a(K'IH+)

fi’t = Zkb T = 370°C at constnnt.fl,*, aw

Al2 - Na, Paragonltc,

Epidote

All + Qt& +Alb

Figure 11. Activity diagram for the Atud Diorite rocks at 2 kbar and 370°C, in the modelsystem K+-H’-CaO-Na,O-MgO- TiO,-AI,03-SO,-CO,-H,O. The arrow marks the modelalteration path and can be placed within the boxes outlined by the alteration isograds. Minerals projected through albite and quartz from the diorite. Calculated using PTA (Brown et al., 19891. Abbreviations: Act: actinolite; Alb: albite; Cc: calcite; Ank: ankerite; Chl: chlorite; Mus: muscovite; Kaol: kaolinite; Ep: epidote; Qtz: quartz; Py: pyrite; Mt: magnetite: Tnt: titanite; Rt: rutile.

Fe-rich diorite is a favourable host rock for Au deposition.

iii) Pyrite contains inclusions of muscovite and ankerite and is intimately associated with Au deposition; therefore, Au mineralisation occurred late in the alteration event.

The calcite-ankerite association suggests progressive carbonation, which is best depicted on a temperature-composition diagram. A T-XcO, diagram for the magnesium end-member system for the diorite in the Atud mine was constructed using GEO-CALC (Berman et a/., 1987; Fig. IO).

Chemical compositions of the analysed actinolite, chlorite, calcite, ankerite, epidote, albite and muscovite are obtained from the same specimen and represent averages of a number of microprobe analyses (Tables 3 to 5). Activities were calculated assuming ideal activity models (Tables 3 to 5). Non-ideal mixing of H,O and CO, was also assumed. Pressure changes from 1 to 3 kbar shift the calculated invariant points to higher temperatures and lower CO,/H,O ratios by -20°C

kbar-‘, but the topology remains consistent. The total pressure was fixed at 2 kbar.

The sequence of isograd reactions with increasing Xco2 on the model is the same as that observed in nature adjacent to the Au-bearing veins in the Atud mine (Figs 4 and IO). During isothermal, isobaric CO, metasomatism of carbonate-free diorite, calcite forms at lower values

of XWZ in the fluid than dolomite. At higher Xco2, the model in Fig. 10 predicts

epidote (clinozoisite) formation in rocks lacking albite, according to the reaction:

1 Scalcite + 3chlorite + 1 1 CO, = 2clinozoisite + 3quartz f 15dolomite + 1 1 H,O (6)

In nature this has not been reported, although Ames et al. (I 991) suggested that in the Na-free system AI,O,-CaO-(Mg,Fe)O, a restricted Xcol space for epidote and dolomite exists. In the diorite at the Atud mine, albite is present in all reactions. Mineral assemblages alone do not constrain the


H. 2. HARRAZ

mole fraction of CO,, but the maximum temperature is constrained at a given pressure.

The transition from zone 1 to 2 is represented by the Mg end-member reaction:

3calcite + tremolite + 7C0, * 8quartz + 5dolomite + H,O (7)

Other volumetrically and mineralogically important mobile components are K and S. In order to model the four reactions, an fc,, versus s~,~+,~+) diagram was computed, assuming an isothermal (370°C), isobaric (2 kbar) process (Fig. 11). The values of

log fm and log alK+,,,+) were obtained by solving the equivalent reaction as the isograd reactions (l-3) for the Mg end-members using the program PTA (Brown et a/., 1989). Activity models from Tables 3, 4 and 5 were used. The balanced mineral reactions from the microprobe analysis were substituted in and the slopes were obtained from the balanced reactions. A univariant boundary from the system Fe-O-S was superimposed based on mineral associations in the diorite.

The progression of alteration zones 1, 1 a and 2 may be explained by an increase in fco2, with the model alteration path showing only a slight increase in a,K+,H+,. Zone 3 developed from a further increase in CO, and an increase in alK+,“+) in the rock. The alteration path can lie anywhere in the shaded fields (Fig. 111, with the invariant point crossed containing mineral assemblages from zones 1, 2 and 3. This point defines a minimum log a ,K+,H+) for the fluid at 3.8. The composition of the fluid evolved through interaction with the diorite wall rock, producing systematic mineralogical zoning adjacent to the fluid channels.

The importance of bulk composition on the alteration assemblages in dioritic rock is illustrated in the ACM insets with each field (Fig. 1 1). This bulk composition of the diorite is Al-rich, accounting for the formation of albite at isograd (2). Albite also formed by reaction (8) in the diorite at the Atud gold mine, but only in trace amounts. The Fe in the chlorites increases toward the Au- bearing structures in the diorite (Fig. 5) because there is abundant epidote available in relatively unaltered domains within the strongly altered rocks to supply Fe to the S-bearing fluid. In contrast, in the Golden Mile dolerite (Phillips and Brown, 1987; Neal1 and Phillips, 1987) and El Sukari and Umm Rus gold mines in the Eastern Desert of Egypt (Harraz, 1991; Harraz and El Dahhar, 1994) the

15 1 I

5 ‘O- albite

g 5--.

E

paragonhe /

O- kaolinite

-5

i -10 -5 0 5 10 15

log a(K+/H+)

Figure 12. Phase relations in the ideal system Na-K-C-O-H-Al, portraying the relatively restricted space for muscovite and albite coexisting during pyrite and Au deposition at the Atud mine and restricting the log a(Na’/K+I of the mineralising fluid. Calculated using PTA (Brown et al., 1989). P = 2 kb, T =370°C.

Fe content of the chlorite decreases toward the Au-bearing structures.

In the most intensely altered diorite fragments within the vein, Na metasomatism plays an important role in the formation of metasomatic albite, since all of the metasomatic zones and the metamorphic assemblage contain albite and the fluid is interpreted to be in equilibrium with albite, thereby allowing hydrothermal albite to form by an increase in X,,* only in an albite-bearing rock. The progression in alteration zones with albite, albite and muscovite, and then muscovite and kaolinite is illustrated in Fig. 12, calculated from PTA (Brown et al., 1989). The point where albite and muscovite coexists defines the log a,K+,H+j ratio of 4 at 370°C and 2 kbar in the ideal system Na- K-C-O-H-Al. The upper limit of acK+,H+j is constrained by the absence of K-feldspar at which the aNa+ and a,, are equal. The dominance of the muscovite-ankerite-albite assemblage suggests that Na and K both played important roles in the formation and stability of the alteration assemblages.

CONCLUSIONS

In the Atud mine, the Fe-rich composition of the diorite is favourable for Au deposition. The

0.5Ca,AJ,Si,O,,(OH) +0.5(Mg,Fe),AI,SjO,,(OH), + 5Si0, + CO, + 2.5Na+ f 3O,-L.5NaAlSi,O, + CaCO, + Fe,O, + 5H,O epidote chlorite quartz fluid albite calcite magnetite (8)


Wall rock alteration: processes and P-T-X,,? conditions of metasomatism

precipitation and character of hydrothermal alteration minerals are dependent on the host rock composition. Fluid-rock interaction changes the local chemical composition of the fluid phase to conditions suitable for Au deposition. Changes in the oxide assemblages from hematite and magnetite to pyrite suggest that the fluid had a reducing effect on the wall rock.

The abundance of host diorite rock with high Fe/(Fe+Mg) ratios, combined with the strong association of Au with iron sulphides and the destruction of chlorites and ilmenite or magnetite, suggest that one of the important Au depositing mechanisms was sulphidation of the wall rocks with concomitant destabilisation of the Au-bearing complexes.

The textures and bulk rock chemistry indicate that volume changes during metasomatism were less than 20%. The rock forming components show a variability in mobile elements. The outer metasomatic zones are marked by changes in H,O, CO,, K,O and FeO, whereas inner alteration zones are characterised by variations in components such as AI,O,, MgO, S, Na,O and CaO.

The formation of albite was due to CO, metasomatism of an Al-rich rock, in which fluid was in equilibrium with albite (reaction 2). The formation of the assemblage containing albite and ankerite was an essential step towards achieving equilibrium between the Au-bearing fluid phase and the wall rock.

Mapping of alteration isograds based on reactions involving CO,, H,O, or K+/H+ indicates that the intensity of alteration increases towards the Au-bearing structures, which acted as fluid conduits. CO, and K+ metasomatism are more widespread than the addition of S or Na; the latter two are reflected in pyrite and albite at the vein contact.

Assuming that local equilibrium conditions exist at the alteration zone boundaries, mineral compositions from microprobe data have been used to model equilibria in the system SiO,-AI,O,- Na,O-CaO-MgO-TiO,-H,O-CO,. The mineral equilibria and amphibole geobarometry suggest that pressure during metasomatism was 1 kbar. A temperature estimate of 336-426OC (average 389OC) for metasomatism from calcite-dolomite geothermometry is similar to the estimated temperature of 410 + 40°C for deposit-scale carbonatisation (Clark et a/., 1986) and significantly less than 478-559OC estimated for regional metamorphic greenschist-facies. Textures indicate that the zoned wall rock alteration and associated Au mineralisation postdate regional metamorphism.

ACKNOWLEDGEMENTS

The author is grateful to Prof. H. Pichler, Mineralogical Institute, Tubingen, Germany for helping and financially supporting the XRF and microprobe analyses. The help and advice of Prof. S. M. Aly, Tanta University, during the mineralogical and petrographic studies are deeply acknowledged. Thanks go to Prof. El Dahhar, Alexandria University, for reading and improving an earlier version of this manuscript. Editorial handling - G. W. McNeil1

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the island arc metasediments? Bulletin Faculty Science, Assiut University 19(1-F), 123-l 55.

Akaad, M.K., Essway, M.A., 1964. Geology and structure of the area east of Gabal Atud, Eastern Desert of Egypt. Bulletin Sciences Technical, Assiut University 7, 63-82.

Akaad, M.K., Essway, M.A., 1965. Petrography, origin and sedimentation of the Atud Formation and its bearing on the early part of the geological history of the basement complex of the Eastern Desert of Egypt. Bulletin Sciences Technical, Assiut University 8, 55-74.

Ames, D.E., Franklin, J.M., Froese, E., 1991. Zonation of hydrothermal alteration at the San Antonio gold mine, Bissett, Manitoba, Canada. Economic Geology 86, 600-619.

Amin, M.S., Sabet, A.H., Mansour, A.O.S., 1953. Geology of Atud district. Geological Survey, Egypt, 79p.

Anovitz, L.M., Essene, E.J., 1987. Phase equilibria in the system CaCO,-MgCO,-FeCO,. Journal Petrology 28, 389-414.

Awad, N.T.. Fasfous, B.R., 1981. Petrography of Atud gold- bearing quartz veins and associated rocks. Bulletin Nuclear Research Central Egypt 6, 628-640.

Berman, R.G., Brown, T.H., Perkins, E.H., 1987. GEO-CALC: Software for calculation and display of pressure-temperature- composition phase diagrams. American Mineralogist 72, 861-862.

Bishady, A.M., Refaat, A.M., Kabesh, M.L., Abdallah, Z.M., 1983. Petrography and geochemistry of Gabal Atud gabbros, south Eastern Desert, Egypt. Annals Geological Survey Egypt 13, 157-168.

Blundy, J.D., Holland, T.J.B., 1990. Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer. Contributions Mineralogy Petrology 104, 208-224.

Brown, E.H., 1977. The crossite content of Ca-amphibole as a guide to pressure of metamorphism. Journal Petrology 18, 53-72.

Brown, T.H., Berman, R.G., Perkins, E.H., 1989. PTA-SYSTEM: A Geo-Calc software for calculation and display of activity- temperature-pressure phase diagrams. American Mineralogist 74, 485-487.

Carmichael, D.M., 1970. Intersecting isograds in the Whetstone Lake area, Ontario. Journal Petrology 11, 147-181.

Clark, M.E., Archibald, N.J., Hodgson, C.J., 1986. The structural and metamorphic setting of the Victory gold mine, Kambalda, Western Australia. In: Macdonald, A.J. (Ed.), Gold ‘86. Konsult International Inc., Willowdalda, Ontario, pp. 253-254.

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Wall rock alteration: processes and P-T-Xco2 conditions of metasomatism

APPENDIX Sampling and analytical techniques

To approach the problem, surface and underground exposures were mapped to delineate rock types and mesoscopic alteration assemblages in the country rocks and alteration zones, followed by petrographic investigation, chemical analysis, and microprobe analysis of minerals. A total of 250 quartz vein, fresh and altered dioritic rock samples were collected to cover a wide spectrum of surface and subsurface features at the Atud gold mine (Fig. 3). Each sample was collected as a chip-sample over an area of about 1 .O m*. Some selected samples were split, crushed and ground to pass through a 150 mesh. Chemical analyses and specific gravity determinations were performed on samples from all alteration zones.

Major elements were determined by X-ray fluorescence spectrometer (Philip PW-1400) using fused pellets made from rock powder mixed (1: 9) with Li,B,O, at the laboratories of the Mineralogical Institute, Tubingen, Germany. Ferrous oxide was determined volumetrically as described in Graff (1983). Based on replicate analyses, the precision at the 95% confidence level was less than l-2%. Averages of fresh and altered country

rock samples are shown in Tables 1 and 2. Coexisting silicate and carbonate minerals were

analysed using an ARL-SEMQ microprobe using a series of natural and synthetic standards at the laboratories of the Mineralogical Institute, Tubingen, Germany. The analyses were made on grains in freshly prepared thin-polished sections, with a carbon-coated surface. An accelerating voltage of 15 kV and a current of 20-75 nA with a live time of 100 seconds were the standard operating conditions. Data were automatically reduced using ZAF4 Fls and Link analytical systems software. In per cent relative to reported values, two standard deviations equal 2% or less for oxides present in amounts >8 wt%, and 4% or less for oxides present in amounts from <8 wt%. Compositions of epidote, albite, chlorite, muscovite, carbonate and amphiboles were determined and the results are given in Tables 3, 4 and 5. Analyses in Tables 3, 4 and 5 typically represent the average of three individual “spot” analyses of 2-3 grains of each type of mineral in one thin-polished section. Mineral analyses may be converted to be conventional from of oxide wt% (with all Fe as FeO) using the cation proportions and the sum of oxides listed for each analysis.


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