O A

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Journal of Applied Sciences Research, 9(11): 5878-5903, 2013
ISSN 1819-544X
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLES
Gold-Bearing Sulphides Associated With The Granitic Wall-Rock Alterations At The
Fawakhir Area, Central Eastern Desert, Egypt.
1
Said H. Abd El Rahim, 1El Said R. El Nashar, 2Ali F. Osman and 1Nahla I. Abd El Ghaffar
1
2
Geological Sciences Department, National Research Centre, Cairo, Egypt.
Geology Department, Faculty of Science, Ain Shams University, Cairo, Egypt.
ABSTRACT
El Sid and Fawakhir gold mines represent the largest mesothermal vein-type gold occurrence in the Eastern
Desert of Egypt. The gold mines area is geologically dominated by a granitoid pluton which intrudes the older
serpentinites and metabasalts. The gold mineralization occurs in two distinct zones (4 km apart) at the NW
(Fawakhir occurrence) and SW (El Sid deposit) contacts of the Fawakhir granitoid pluton with the ultramaficmafic rocks. Apart from the gold-bearing quartz veins loads, the wall-rock alteration zones around the
mineralized quartz veins contain minute gold grains in intimate association with the sulphides. The host
granitoid rocks around the gold-mineralized quartz veins ( the granitic hanging and footwalls of the main ore
bodies ) at the El Sid and Fawakhir gold mines exhibit different gradual wall-rock alteration zones, starting from
the inner (phyllic alteration) to the outer (propylitic alteration) and silicification (quartz veinlets and
stockworks). These wall-rock alterations are attributed to the gold mineralizing hydrothermal solutions which
percolated and ascended along the deep-seated major facture systems (closely associated with the intersections
of the ENE strike-slip faults and the NNE, NNW normal faults). The phyllic and propylitic wall-rock alterations
are affected and superimposed by a later supergene argillic alteration due to the interaction of meteoric water.
Petrography and opaque mineralogy of these wall-rock alteration zones are investigated in conjunction with
SEM-EDAX analyses. In general, the reported gold is intimately associated with the sulphides, particularly
pyrite and arsenopyrite. The sulphides are mainly represented by pyrite, arsenopyrite, sphalerite, galena,
chalcopyrite and minor amounts of bornite and pyrrhotite, in a decreasing order of abundance. They occur as
disseminated and micro-veinlets forms. Gold is following and filling the available micro-fractures in pyrite and
arsenopyrite. In the phyllic wall-rock alteration zone, the recorded Au content ranges from 92.77 wt. % to 8.12
wt. % and the associating Ag content varies from 5.00 wt. % to 7.90 wt. %. In the propylitic wall-rock alteration
zone, the reported Au content ranges from 80.39 wt. % to 38.84 wt. % and the associating Ag content varies
from 4.87 wt. % to 14.36 wt. %. Regarding the recorded silicification, the reported Au content attains about
73.99 wt. % and the associated Ag content is about 9.55 wt. %. The mineralogical composition, ore textures, and
paragenetic sequence of the gold-bearing sulphides of the granitic wall-rock alterations at the El Sid and
Fawakhir gold mines may be the same as, and comparable with that of the main gold mineralized quartz veins,
suggesting their contemporaneous nature.
Key words: Gold-bearing sulphides, Granitic wall-rock alterations, El Sid and Fawakhir gold mines.
Introduction
The hydrothermal rock-alteration zones in and around the magmatic plutons and the other magmatic bodies
could generally serve as a guide in prospecting for different ore deposits (e.g., Lowell and Guilbert, 1970; Searle
et al., 1976; Beane and Titley, 1981; Jensen and Bateman, 1981; Coveney, 1981; Rasmy et al., 1983; Osman
and Dardir, 1986, 1989; Hassen, 1987; El Ghawaby et al., 1989; Harraz, 1990; Abd El Rahim, 1994; El Dahhar,
1995; Niazy et al.,1995; Kamel et al.,1996; Harraz,1999; Abd El Rahim, 2002; Takla et al., 2003, 2004;
Kreuzer, 2006; Yoo et al., 2010).
In the Eastern Desert of Egypt, more than 95 localities of ancient gold mines have been reported (El Ramly
et al., 1970). Among these localities, the El Sid and Fawakhir gold mines are distinguished as they represent the
largest mesothermal vein-type gold occurrences in the Eastern Desert of Egypt (Neubauer, 1962; Sabet and
Bondonosov, 1984; Hussein, 1990; Harraz et al., 1992). The gold- mineralized, quartz-veins are hosted in the
granodiorite-granitic rocks and had been extensively exploited since the Pharaonic times until 1958. As an
example, El Sid gold mine (260 00’ 17” N and 330 35’ 42” E) contributed about 45% of the total quantity of the
Egyptian gold production in twenties century (Hussein, 1990). The extracted gold from the mineralized quartzcalcite veins may reach up to 29.7 g/t; the silver is also present in grades up to 22.9 g/t (Wassef et al., 1970).
Corresponding Author: Said H. Abd El Rahim, Geological Sciences Department, National Research Centre, Cairo, Egypt.
E-mail address: [email protected]
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Generally, the gold contents decrease with depth (Sabet et al., 1976), whereas the gold was exploited mainly
from quartz-veins down to a depth of about 160m. Yet, the potentialities of the deposits have not been
exhausted.
Generally speaking, the gold had been exploited only from the major gold-bearing quartz-veins and some
vienlets cutting the hanging and foot walls of the main ore bodies. Hence, the major gold- mineralized quartzveins at the Fawakhir area ( El Sid and Fawakhir gold mines ) have attracted the attention of many workers (e.g.,
Hume, 1937; Barakat and El Shazly, 1956; Neubauer, 1962; Kochin and Bassyuni, 1968; El Ramly et al., 1970;
Wassef et al., 1970; Sabet et al., 1976; Fakhry and Eid, 1980; Kamel et al., 1980; Sabet and Bondonosov, 1984;
Harraz, 1985, 1995, 2000; El Bouseily et al., 1985a, 1985b, 1986, 1987, 1998; El Bouseily, 1987; Hussein,
1990; Hussein and El Sharkawi, 1990; Harraz et al., 1992; Harraz and Ashmawy, 1994; El Dahhar, 1995;
Loizenbauer and Neumayr, 1996; El Shemi, 2005).
However, almost all of these previous works largely neglected the importance of the wall-rock alteration
zones around the gold-bearing quartz-veins of Fawakhir area (Fig. 1) as possible gold resources. Hence, they
received very little attention, compared to the main gold-bearing quartz veins. In the present work, we shed light
on the wall- rock alteration zones at the Fawakhir area (El Sid and Fawakhir gold mines). Petrography and ore
microscopy in conjunction with SEM and EDAX analyses of representative samples from these alteration zones
were accomplished to elucidate their potentiality as possible gold resources.
Fig. 1. Landsat image showing the location of the Fawakhir area on the Quseir-Qift highway.
Geologic setting:
The Fawakhir area (121 km2) lies in the Central Eastern Desert of Egypt between latitudes 25°57' 24˝- 26°
01' 44˝ N and longitudes 33° 35’ 52˝- 33°39' 20˝ E (Fig. 2). The area covered by an ophiolitic sequence
(serpentinites, metagabbro and metabasalt), topped by Hammamat sediments, granodiorite, monzogranite, and
Dokhan volcanics. The Fawakhir granitoids (granodiorite and monzogranite) pluton (25km2) forms an
ellipsoidal intrusive body which extends N - S parallel to the elongation direction of the enveloping rocks. The
granitoid pluton intrudes the ophiolitic sequence, mostly (serpentinites and metabasalt). The gold mineralization
occurs in two distinct zones (4km apart) at the NW (Fawakhir occurrence) and SW (El Sid deposit) contacts of
the Fawakhir granitoid pluton with the ultramafic-mafic rocks.
The area was deformed by three major trends of faults (Fig. 3). The ENE trend is younger than the other
two NNE, NNW trends. The gold-mineralized quartz veins at the El Sid-Fawakhir gold mines are closely
associated with the intersections of the ENE strike-slipe faults (right-lateral type) and the NNE, NNW normal
faults. The main quartz vein lode is conformable with the ENE faults. The ENE and NNE trending quartz-veins
are mineralized, whereas those with the NNW trend are barren. The individual quartz veins vary from few
centimeters to 1.5m wide and from 0.2m to 7.5m long. They are hosted mainly in the granitoid rocks of the
Fawakhir pluton and their adjacent ultramafic-mafic rock association. They usually bifurcated into smaller
veins, veinlets and stringers (offshoots), and join other veins giving rise to a network pattern of about 100m
wide. The large veins trend mostly ENE-WSW and extend discontinuously for about 900-1100m along their
strike and for some 150-455m along their dips (400-450SE). El Sid gold mine encompasses the largest auriferous
quartz vein lode. It is cropping out entirely at the contact between the granitoid rocks and the metagabbro
complex and occasionally extends into the serpentinite rocks through a thick zone of graphite schist (15m).
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Fig. 2: Geologic map of Fawakhir area (modified after Fowler, 2001).
Fig. 3: Structural map of the El Sid-Fawakhir area after Harraz and Ashmawy (1994).
Field observations of the granitic wall rock alterations:
At El Sid and Fawakhir gold mines, the fresh granitoids hosting the gold-bearing quartz-veins exhibit
conspicuous gradual wall-rock alteration zones (Figs. 4A-4D) around these mineralized quartz lodes (the
granitic hanging and footwalls of the main ore bodies). Systematically, the inner zone exhibits chloritization and
sericitization zone (phyllic alteration); this is followed outward by saussuritization or epidotization associating
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calcite zone (propylitic alteration) and silicification signature (Fig. D) in the form of quartz veinlets, quartz
stockworks and masses of augmented quartz. The silicification is confined to the gold mineralizing
hydrothermal alterations; it is most probably the result of SiO2 leaching during alteration by the hydrothermal
solutions or could be the product of another phase of silica-rich hydrothermal solution. Similar feature was
described by Jensen and Bateman (1981).
The gold mineralization is mainly structurally controlled (Fig.3) by the deep-seated major fracture systems
(closely associated with the intersections of the ENE strike-slip faults with the NNE and NNW normal faults)
which have affected and deformed the Fawakhir area (Harraz and Ashmawy, 1994; Loizenbauer and Neumayer,
1996). The described hydrothermal wall-rock alterations are contemporaneous with the gold mineralization at
the Fawakhir area. These wall-rock alterations reach their maximum extent at the gold mines sector, and mainly
affect the granitoid rocks.
The described wall-rock alterations (phyllic and propylitic) were superimposed and affected by a later
argillic alteration most probably related to the interaction of meteoric water (Lowell and Guilbert, 1970). The
argillic rock is a clay-like material with a powder-like appearance. This supergene alteration is also evident by
the enrichments of grey to pale blue or brown smithosonite (ZnCO3); deep yellow, orange, dark red zincite
(ZnO); zincite and smithosonite represent an alteration products of sphalerite (Phillips and Griffen, 1986) as
well as yellowish and brownish or brick-red colours mainly of iron hydroxides as alteration products after the
primary sulphide minerals.
Fig. 4: Field observations of the granitic wall rock alterations:A) a distant view of the fresh host granitoids (G)
at El Sid-Fawakhir gold mines. Note, tailings (T) at the front and the old small church for the British
gold miners, B) a close up view showing gradational contact between the wall-rock alterations (W.R)
and the fresh granitoids (G) at the mine adit. Note, the intensive alteration is aligned along a major fault,
C) a close up view showing the wall-rock alterations stained with brownish iron oxides at the mine adit.
Note, the intensive alteration is structurally controlled by a major fault, D) a close up view from the
inside of the mine adit (Level I) showing thin quartz veinlets and stockworks (silicification) intersecting
the phyllic and propylitic wall-rock alterations which are stained with brownish iron oxides, greenish
malachite and orange red cinnabar.
Materials and Methods
About 65 representative rock-samples were collected from the different levels (I, II, III) of the mining
activity of El Sid Gold Mine (Fig.5) as documented by the Gold Mining Authorities. The collected samples
represent the phyllic and propylitic wall-rock alterations as well as the silicification (quartz veinlets, quartz
stockworks and masses of augmented quartz) which are mainly affecting the granitoid rocks at the Fawakhir
area. Sampling through the level (IV) was hindered by the presence of the underground water. These levels had
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been dug during the British mining activity (through the British occupation of Egypt) to exploit the gold. As
seen in Figure 5, the gold mineralization occurs at the contact of the granitoids with the serpentinite.
Fig. 5: Longitudinal cross-section (underground map) in the El Sid Gold Mine showing the locations of the
collected samples from the different levels ( I, II, III, IV ) of the mining activity.
At least two to three polished surfaces and one thin section were prepared for each sample. The polished
surfaces and thin sections were studied in detail using Reflected Light Ore Microscope and Transmitted Light
Polarizing Microscope, respectively for the determination of their mineralogical composition, ore textures, and
paragenetic sequence. Besides, the ore microscopic identification of the gold-bearing sulphides is confirmed by
Scanning Electron Microscope (SEM) and EDAX analyses of some polished surfaces of the selected ore
samples. SEM and EDAX analyses were carried out by using SEM model Philips XL30 attached with EDAX
unit 30, with accelerating voltage 30KV, magnification up to 400000 X and resolution for W. (3.5 nm).
Results and Discussion
The identification of the alteration zones according to their mineral assemblages is given following the
description of Guilbert (1970) & Bean and Titley (1981). The phyllic alteration is characterized by the
assemblage: quartz-sericite- pyrite, while the propylitic alteration zone exhibits the assemblage of quartzchlorite-epidote-carbonate-pyrite and the argillic alteration zone is characterized by the assemblage of quartzkaolinite-montimorillonite-dickite-illite. In general, the phyllic alteration zone is the richest in gold content,
compared to the other zones. The sulphides are mainly represented by pyrite, arsenopyrite, sphalerite, galena,
chalcopyrite and minor amounts of bornite and pyrrhotite, in a decreasing order of abundance. Gold is recorded
as minute grains and speckles, filling the micro-fractures in the pyrite and arsenopyrite.
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Phyllic wall-rock alteration zone:
Microscopically, the main mineralogical composition of this zone are sericite, intensively sericitized
orthoclase and oligoclase, quartz and opaques (Figs.6A, 6B and 6C). Chlorite and highly chloritized biotite and
hornblende are subordinate in abundance, being associated uncommonly with epidote. Quartz exhibits irregular
crystals and wavy extinction due to strain effects and commonly forms augmented clots or patches.
Sericite is the most common mineral within the phyllic wall-rock alteration zone at expense of orthoclase
and oligoclase crystals (Figs.6A, 6B and 6C). Sericite is in all probability a late hydrothermal mineral which
occurs as minute shreds. Opaques are mainly pyrite that commonly forms pseudomorphs after a basal section of
hornblende (Figs.6B and 6C); the pyrite was then corroded and embayed by quartz and sericite.
Chlorite is a pale green pennine type which exhibits abnormal Berlin blue interference colors. It occurs as
different pseudomorphs after biotite and hornblende. The chloritization process starts and guided by the mineral
cleavage, with liberation of iron oxides (Fig.6A).
Opaques are mainly represented by pyrite and gold. The pyrite is the most abundant mineral in the phyllic
wall rock alteration zone. Pyrite is characterized by yellowish white colour, weak to distinct anisotropism, and
displays no internal reflections. Pyrite occurs in two generations: a) as disseminated hypidiomorphic
equigranular fine crystal aggregates (Figs.6D and 6E) and b) as subidiomorphic relatively coarse isolated
crystals pseudomorphs after a basal section of hornblende (Figs.6B, 6C and 6F). Also, hypidiomorphic pyrite
crystal aggregates are recorded either filling the micro-veinlets, or in interstitial spaces between other minerals
(Figs.6D and 6E). Pyrite was later on corroded and embayed by the gangue minerals (Figs.6B, 6C and 6F)
which are mainly represented by quartz and sericite.
Fig. 6: Petrography and opaques of the phyllic alteration zone: A) Highly sericitized orthoclase and chloritized
biotite associating quartz and pyrite. C.N., 40X, B) Assemblage of quartz and sericite associating a
pyrite pseudomorph after a basal section of hornblende. C.N., 40X, C) A subhedral pyrite crystal
embayed by sericite and very few epidote. C.N., 40X, D) Micro-veinlet filled by hypidiomorphic crystal
aggregates of pyrite. RPL, 50X, E) Disseminated hypidiomorphic crystal aggregates of pyrite. RPL,
50X, F) A disseminated subidiomorphic isolated pyrite crystal pseudomorph after a basal section of
hornblende. RPL, 100X.
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The SEM-EDAX analyses of pyrite (Figs.7.1 and 7.2) of the phyllic-rock alteration show that the S content
up to 52.37 wt.% and Fe content up to 47.63 wt% (Fig. 7.1). The pyrite in Figure 7.2 exhibits a relative less
contents of S (up to 51.60 wt. %) and Fe (up to 40.28 wt. %), owing to the presence of Au content (up to 8.12
wt. %).
Fig. 7.1: SEM photo of a disseminated subidiomorphic isolated pyrite (py) crystal pseudomorph after a basal
section of hornblende and its EDAX analysis. The phyllic alteration zone.
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Fig. 7.2: SEM-EDAX analysis of gold (Au). Note, gold irregular grains corrode pyrite (Py) crystals along their
fractured margins. The phyllic alteration zone.
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Fig. 7.3: SEM-EDAX analysis of gold (Au). Note, gold forms very fine microveinlet filling the microfracture in
Pyrite (Py). The phyllic alteration zone.
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Fig. 7.4: SEM-EDAX analysis of gold (Au). Note, gold occurs as irregular grain which replaces pyrite (Py)
along its fractured margins. The phyllic alteration zone.
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Gold is detected and observed by the SEM-EDAX analyses of pyrite (Figs. 7.2, 7.3 and 7.4) of the phyllic
wall-rock alteration. Fig.7.2 shows the composition of the analyzed pyrite crystal: the Au content is about 8.12
wt. %, S content (up to 51.60 wt. %) and Fe content (up to 40.28 wt. %). In Figure 7.3, the Au content in the
analyzed minute gold shred is significantly higher (up to 88.33 wt. %), Ag content is about 7.90 wt. % and Fe
content is around 3.78 wt. %. In Figure 7.4, the analyzed gold minute grain exhibits the highest Au content (up
to 92.77 wt. %) with Ag content of about 5 wt. % and Fe content of about 2.23 wt. %. In general, the analyses
indicate that the Au contents (8.12 wt. %, 88.33 wt. %, and 92.77 wt.%); the Ag contents (up to 5 wt. % and
7.90 wt. %) are not enough to form the mineral electrum (Au, Ag) in which the Ag-content is higher than 25%
(Uytenbogaardt and Burke, 1971). Besides, gold always forms unlimited solid solutions with silver; the mix
crystals containing (30-45 wt. % Ag) are called electrum (Ramdohr, 1969). S content (up to 51.60%) and Fe
contents (up to 2.23%, 3.78%, and 40.28%) are attributed to the pyrite constituent. Gold occurs as irregular
grains which corrodes and replaces pyrite crystals along their fractured margins (Figs.7.2 and 7.4). Gold also
forms fine microveinlets filling the microfractures in pyrite (Fig.7.3). It is most probably due to the high
electrochemical potentials of the pyrite and arsenopyrite (Sakharova and Lobacheva, 1967) and trapping of Aubearing solutions in the fractures of the pyrite and arsenopyrite.
Propylitic wall-rock alteration zone:
Microscopically, the major mineral constituents of this zone are epidote, peidmontite, carbonate, chlorite,
quartz and opaques (mainly sulphides). Relics of kaolinitized, sericitized and saussuritized feldspars are also
observed (Figs. 8A-F). Quartz occurs as irregular crystals and exhibits wavy extinction due to strain effects. The
mineral exhibits a pre-existing myrmekitic texture (Fig. 8B). It also forms augmented clots or patches (Figs. 8D,
8E and 8F).
Epidote represents a very common alteration and saussuritization products at expense of the pre-existing
plagioclase and mafic minerals (biotite and hornblende). The mineral occurs as colourless to yellowish green
granular to columnar aggregates (Figs.8A, 8B, 8D and 8E). It exhibits yellow, blue, green, orange, and violet of
the second order interference colours.
Peidmontite (Mn - epidote variety) is also observed (Figs. 8C and 8D). It occurs as hydrothermal product
(Basta et al., 1979) after the plagioclase and mafic minerals. The mineral has a columnar habit and exhibits
characteristic vivid axial colours (yellow, orange, red, violet); it is pleochroic with the following absorption
formula; X=yellow to orange; Y=amethyst to violet; Z=carmine to deep red. It usually appears masked between
crossed nicols. Opaques are mainly sulphides, which are corroded and embayed by peidmontite (Fig.8C).
Carbonates are mainly represented by calcite which is a very common alteration and saussuritization
products of plagioclase. Calcite forms colourless hypidiomorphic fine to coarse aggregates. It exhibits pearl grey
or bright interference colours of fourth order (Figs.8A, 8D, 8E and 8F). A micro-veinlet filled by calcite and
pennine type chlorite that intersects the intensively saussuritized plagioclase is also observed (Fig.8A). Chlorite
is a pale green pennine type. It occurs as different pseudomorphs after biotite and hornblende. The mineral is
filling micro-veinlets, mainly associating calcite as well as epidote (Fig.8A).
Opaques are mainly represented by pyrite, arsenopyrite, sphalerite, galena, chalcopyrite, pyrrhotite and
gold.
Pyrite is the most abundant mineral in the propylitic wall-rock alteration zone. It is observed as
hypidiomorphic equigranular crystals filling the micro-veinlets (Fig.9A) or as disseminated subidiomorphic
crystals which are corroded by arsenopyrite (Fig. 9B). Commonly, the hypidiomorphic equigranular pyrite
crystal aggregates are corroded by arsenopyrite and sphalerite (Fig. 9C) and are replaced by the gangue minerals
(Fig.9B and 9C) which are represented by quartz, chlorite, epidote, peidmontite, and calcite.
Arsenopyrite is a common and abundant mineral in the propylitic wall-rock alteration zone. The
arsenopyrite is characterized by white colour with a faint creamy or pinkish tint, weak, but noticeable
bireflectance, strong anisotropism with blue, green and reddish brown-yellow colours (nicols not completely
crossed), and displays no internal reflections. It occurs as disseminated idiomorphic rhomb and rectangular
crystals (Fig. 9B and 9D) and hypidiomorphic equigranular crystal aggregates which corrode and replace pyrite
(Fig. 9B and 9C). Arsenopyrite is corroded and replaced by sphalerite and chalcopyrite (Fig.9B, 9C and 9E).
The mineral arsenopyrite also exhibits core replacement by chalcopyrite (Fig.9B) due to the zonal texture of
arsenopyrite (Uytenbogaardt and Burke, 1971). The arsenopyrite is later corroded and replaced by the gangue
minerals (Fig. 9B, 9C and 9E) which are represented by quartz, chlorite, epidote, peidmontite, and calcite.
Sphalerite is also a common mineral in the propylitic wall-rock alteration zone. It exhibits grey colour
(Fig.9B and 9C). The mineral is characterized by its isotropism and visible yellowish white internal reflections
(Uytenbogaardt and Burke, 1971). The sphalerite occurs as disseminated hypidiomorphic crystal aggregates
which corrode pyrite and arsenopyrite (Fig.9B and 9C). The mineral sphalerite was later on corroded and
replaced by the gangue minerals which are represented by quartz, chlorite, epidote, peidmontite and calcite.
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Fig. 8: Petrography of the propylitic alteration: A) Assemblage of intensively saussuritized plagioclase, pennine
type chlorite, quartz and opaques. A micro-veinlet is filled by carbonate and pennine type chlorite. C.N.,
40X, B) Quartz exhibits a pre-existing myrmekitic texture and associating carbonate, epidote as well as
opaques. C.N., 40X, C) Opaques corroded and embayed by enrichment of epidote (peidmontite). C.N.,
40X, D) Association of epidote, carbonate, augmented patches of quartz and opaques. C.N., 40X, E)
Augmented clots of quartz associating carbonate, epidote and opaques. C.N., 40X, F) Carbonate
associating epidote, augmented patches of quartz and opaques. C.N., 40X.
Galena is a common mineral in the propylitic wall-rock alteration zone and occurs as micro-veinlets filling
the micro-fractures of pyrite. Sometimes, it corrodes and replaces the pyrite along its fractured margins.
Chalcopyrite occurs as minor amounts in the propylitic wall-rock alteration zone. It forms brassy yellow
medium to fine hypidiomorphic crystals. Chalcopyrite displays weak bireflectance, and weak to distinct
anisotropism with grey-blue and greenish yellow colours. It corrodes and replaces arsenopyrite (Fig. 9B and
9E). The arsenopyrite exhibits core replacement by chalcopyrite (Fig. 9B) due to the zonal texture of
arsenopyrite (Uytenbogaardt and Burke, 1971).
Pyrrhotite occurs also asminor amounts in the propylitic wall-rock alteration zone. It forms massive,
irregular crystal aggregates (Fig. 9F) which are interlocked with pyrite implying a probable contemporaneous
paragensis. Pyrrhotite is characterized by its cream colour with a faint pinkish brown tint, and very distinct
bireflectance. It is also characterized by very strong anisotropism with the following polarization colours
(yellow-grey, greenish grey or greyish blue), and displays no internal reflections. It occurs as disseminated
hypidiomorphic crystal aggregates which are later corroded by the gangue minerals.
The SEM-EDAX analyses of pyrite (Figs. not shown here) of the propylitic wall-rock alteration zone show
that the S content is varying from 52.93 wt. % to 53.60 wt. % and the Fe content is ranging from 46.40 wt. % to
47.07 wt. %. The SEM-EDAX analyses of arsenopyrite (Figs.10 and 11) of the propylitic wall-rock alteration
zone exhibit the following contents: S is varying from 22.06 wt. % to 23.66 wt. %, Fe is ranging from 35.34 wt.
% to 36.01 wt. % and the As content from 40.33 wt. % to 42.61wt. %). The SEM-EDAX analysis of sphalerite
(Fig. not shown here) of the propylitic wall-rock alteration zone shows that Zn content is about 58.75%, S
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content is about 34.20 wt. % and Fe content ~ 7.05 wt. %. However, a part of Zn content is practically
substituted by Fe (Ramdohr, 1969). The SEM-EDAX analyses of galena, chalcopyrite, and pyrrhotite are not
given here and are available on request. The studied galena of the propylitic wall-rock alteration zone exhibits
about 89.44 wt. % Pb and about 10.56 wt. % S. The chalcopyrite contains about 37.71 wt. % S, 31.93 wt. % Fe
and 30.35 wt. % Cu. The pyrrhotite is characterized by 39.13 wt. % S and up to 60.87 wt. % Fe.
Fig. 9: Opaque mineralogy of the propylitic alteration: A) A micro-veinlet filled by hypidiomorphic pyrite
crystals. RPL, 100X, B) A disseminated subidiomorphic pyrite (Py) corroded by arsenopyrite (Aspy).
Arsenopyrite rhombs is corroded by sphalerite (gray) and chalcopyrite (yellow). RPL, 200X, C)
Disseminated hypidiomorphic aggregates of pyrite (Py) corroded by arsenopyrite (Aspy). Pyrite and
arsenopyrite are later corroded by sphalerite (grey). RPL, 400 X, D) Disseminated arsenopyrite exhibits
idiomorphic rectangular and rhomb crystals. RPL, 100X, E) Disseminated subidiomorphic and
hypidiomorphic arsenopyrite crystal aggregates. Arsenopyrite is later corroded by chalcopyrite (yellow).
RPL, 200X, F) Disseminated hypidiomorphic pyrrhotite crystal aggregates which are later corroded by
the gangue minerals. RPL, 100X.
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Fig. 10: SEM-EDAX analysis of arsenopyrite (Aspy). Note, arsenopyrite corrodes and replaces pyrite (Py). The
propylitic alteration zone.
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Fig. 11: SEM-EDAX analysis of arsenopyrite. Note, arsenopyrite forms disseminated idiomorphic rectangular
and rhomb crystals. The propylitic alteration zone.
Gold is detected and confirmed by the SEM-EDAX analyses pyrite and arsenopyrite (Figs.12, 13, 14) in
the propylitic wall-rock alteration zone. Figure 12 shows that the analyzed contains up to 38.84 wt. % Au, 4.87
wt. % Ag, up to 35.95 wt. % S and about 20.35 wt. % Fe. Regarding the analyzed gold grain in Figure 13, the
Au content is high (up to 80.39 wt. % Au) and the Fe content is about 19.61 wt. %. In Figure 14, the gold (up to
68.14 wt. % Au), contains appreciable Ag amount (up to 14.36 wt. %), in addition to about 12.63 wt. % S and
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up to 4.88 wt. % Fe. Again, the given Au (38.84 wt. %, 68.14 wt. %, and 80.39 wt. %) and Ag contents (4.87wt.
% and 14.36%) are not enough to form the mineral electrum "Au, Ag" (Ramdohr, 1969; Uytenbogaardt and
Burke, 1971). Gold is generally observed along the micro-fractures of pyrite (Fig. 12, 13) or at the contacts
between pyrite and arsenopyrite (Fig. 14). Such an intimate association of gold and both pyrite and
aresenopyrite is attributed most probably to the high electrochemical potentials of the pyrite and arsenopyrite
(Sakharova and Lobacheva, 1967) and trapping of Au-bearing solutions in the fractures of the pyrite and
arsenopyrite.
Fig. 12: SEM-EDAX analysis of gold (Au). Note, gold forms micro-veinlet filling the micro-fracture of pyrite
(Py). The propylitic alteration zone.
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Au
Fig. 13: SEM-EDAX analysis of gold (Au). Note, gold and galena (Gn) form micro-veinlets filling the microfractures of pyrite (Py). The propylitic alteration zone.
Silicification:
The silicification or the silicified parts are mainly represented by thin quartz veinlets forming a network of
quartz veinlets (quartz stockworks) and masses of augmented quartz intersecting the wall-rock alteration zones.
It is one of the most common types of alteration, and it occurs in many different styles. One of the most
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common styles is called silica flooding which forms replacement of the rock with microcrystalline quartz
(chalcedony). Another common style of silicification is the formation of closed-spaced fractures in a network of
veinlets or stockworks which are filled with quartz. Silica flooding and/or stockworks are commonly present in
the wall-rock along the margins of gold-bearing quartz-veins.
Fig. 14: SEM-EDAX analysis of gold (Au). Note, gold forms micro-veinlet at the contact between pyrite (Py)
and arsenopyrite (Aspy). The propylitic alteration zone.
The silicification is commonly observed at the gold mines sectors in the Fawakhir area, and it mainly
affects the granite and granodiorite rocks. Megascopically, the mineralized quartz-veinlets and stockworks are
very hard, but highly fractured. They consist mainly of medium to fine grained quartz with opaques (mainly
sulphides) filling the fracture and also disseminated within the massive quartz. They appear smoky in colour.
Microscopically, they are holocrystalline with granular texture. They consist of essential quartz grains with
subordinate opaques and very few calcite crystals. Quartz occurs in two different generations (Figs.15A and
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15B): the first one forms coarse subidiomorphic to hypidiomorphic granular crystals, while the second
generation occurs as hypidiomorphic fine-grained quartz or microcrystalline aggregates of quartz. Opaques are
corroded and embayed by quartz aggregates (Fig.15B).
Opaques are mainly represented by pyrite, arsenopyrite, sphalerite, galena, chalcopyrite and gold. Pyrite is
the most common and abundant mineral in the quartz veinlets and stockworks. It occurs as disseminated
hypidiomorphic crystals which are corroded and replaced by arsenopyrite (Fig.15C) and idiomorphic as well as
hypidiomorphic crystal aggregates which are corroded and replaced by sphalerite (Fig.15D). Pyrite was later on
corroded by the gangue mineral quartz.
Arsenopyrite is a common and abundant mineral in the quartz veinlets and stockworks. It occurs as
disseminated hypidiomorphic crystal aggregates which corrodes and replaces pyrite (Fig.15C) and idiomorphic
rhomb and rectangular crystal aggregates (Fig.15E) as well as disseminated subidiomorphic crystal aggregates
which are corroded and replaced by chalcopyrite (Fig.15F). Arsenopyrite was later on corroded by the gangue
mineral quartz. Sphalerite is also a common mineral in the quartz veinlets and stockworks. The sphalerite occurs
as disseminated hypidiomorphic crystal aggregates which corrodes and replaces pyrite (Fig.15D). The mineral
sphalerite was later on corroded and replaced by the gangue mineral quartz.
Fig. 15: Petrography and opaques of the silicification (quartz veinlets and stockworks): A) Two quartz
generations associating opaques and very few calcite. C.N., 40X. B) Opaques are corroded and
embayed by both generations of quartz crystal aggregates. C.N., 40X. C) Disseminated
hypidiomorphic pyrite (Py) corroded by arsenopyrite (Aspy). RPL, 100X. D) Disseminated
idiomorphic pyrite cubes and hypidiomorphic crystal aggregates corroded by sphalerite (grey). RPL,
100X. E) Disseminated idiomorphic arsenopyrite crystal aggregates. RPL, 100X. F) Disseminated
subidiomorphic arsenopyrite crystal aggregates corroded by chalcopyrite (yellow). RPL, 400X.
Galena is also among the common minerals in the quartz veinlets and stockworks. It occurs as microveinlets filling the micro-fractures of pyrite and as irregular grains which corrode and replace pyrite along its
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fractured margins. Chalcopyrite occurs as minor constituent in the quartz veinlets and stockworks. It forms
brassy yellow medium to fine hypidiomorphic crystals. It corrodes and replaces arsenopyrite (Fig.15F).
Fig. 16: SEM-EDAX analysis of arsenopyrite (Aspy). Note, gold (Au) forms micro-veinlets filling the microfractures of the arsenopyrite. Quartz stockworks.
The SEM-EDAX analysis of pyrite (not shown here and is available on request) of the quartz veinlets and
stockworks shows that the S content is about 53.75 wt. % and the Fe content is about 46.25 wt. %. The SEM-
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EDAX analysis of arsenopyrite (Fig.16) of the quartz veinlets and stockworks is characterized by As content of
42.42 wt. %, up to 33.78 wt. % S and about 34.80 wt. % F.
Gold forms micro-veinlets filling the micro-fractures of the arsenopyrite (Fig.16).The SEM-EDAX analysis of
sphalerite (Fig.17) of the quartz veinlets and stockworks is characterized by Zn content of about 61.17 wt. %, of
about 31.86 wt. % S and about 6.97 wt. % Fe. The presence of iron (up to 6.97 wt. %) is most probably due to
the rarely occurrence of sphalerite as pure ZnS, whereas a part of the Zn is practically always isomorphously
substituted by Fe (Ramdohr, 1969). The sphalerite and pyrite are later corroded and replaced by galena (appears
white colour). The chemical composition of galena is illustrated in Figure 18, where the Pb content is about
88.69 wt. % and S content attains 11.31 wt. %. It is interesting to note that galena (appears white colour)
corrodes and replaces pyrite along its fractured margins (Fig. 18). Galena forms micro-veinlets filling the microfractures of the pyrite.
Fig. 17: SEM-EDAX analysis of sphalerite (Sp). Note, sphalerite corrodes pyrite (Py). The sphalerite and pyrite
are later corroded and replaced by galena (white). Quartz stockworks.
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Fig. 18: SEM-EDAX analysis of galena (white). Note, galena replaces pyrite (Py) and forms micro-veinlets
filling the micro-fractures of the pyrite. Quartz stockworks.
Gold is detected under ore microscope and confirmed by the SEM-EDAX analysis of arsenopyrit (Fig.19),
where the analyzed gold shred exhibits up to 73.99 wt. % Au and about 9.55 wt. % silver. Also it contains up to
9.00 wt. % S. 3.98 % as and about 3.48 wt. % F. The analysis indicates that the Ag content (up to 9.55%) are not
enough to form the mineral electrum "Au, Ag" (Ramdohr, 1969; Uytenbogaardt and Burke, 1971). S content (up
to 9.00%), Fe content (up to 3.48%) and As content (up to 3.98%) are attributed to the arsenopyrite constituents.
Gold forms micro-veinlets filling the micro-fractures of the arsenopyrite. Gold corrodes and replaces the
arsenopyrite crystals along their fractured margins as seen in Figure16).
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Fig. 19: SEM-EDAX analysis of gold (Au). Note, gold forms micro-veinlets filling the micro-fractures of the
arsenopyrite (Aspy). Gold corrodes and replaces the arsenopyrite crystals along their fractured margins.
Quartz stockworks.
Conclusion:
At Fawakhir area in the Central Eastern Desert of Egypt, the gold mineralization occurs in two distinct
zones (4 km apart) at the NW (Fawakhir occurrence) and SW (El Sid deposit) contacts of the Fawakhir granitoid
pluton with the ultramafic-mafic rocks. Apart from the gold-bearing quartz veinslodes, the wall-rock alteration
zones around the mineralized quartz veins contain minute gold grains in intimate association with the sulphides.
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The host granitoid rocks around the gold - mineralized quartz veins (the granitic hanging and footwalls of the
main ore bodies ) at the El Sid and Fawakhir gold mines exhibit different gradual wall-rock alteration zones,
starting from the inner (phyllic alteration) to the outer (propylitic alteration) and silicification (quartz veinlets
and stockworks). These wall-rock alterations, attributed to the gold mineralizing hydrothermal solutions which
percolated and ascended along the deep-seated major facture systems (closely associated with the intersections
of the ENE strike-slip faults and the NNE, NNW normal faults). The phyllic and propylitic wall-rock alterations
are affected and superimposed by a later supergene argillic alteration due to the interaction of meteoric water.
The detailed petrographical and opaque mineralogical investigations which are confirmed by SEM-EDAX
analyses of polished surfaces indicate that the gold is intimately associated with the sulphids. The latters are
mainly represented by pyrite, arsenopyrite, sphalerite, galena, chalcopyrite and minor amounts of bornite and
pyrrhotite, in a decreasing order of abundance. They occur as disseminated and micro-veinlets forms. Gold is
following and filling the available micro-fractures in pyrite and arsenopyrite, most probably due to their high
electrochemical potentials and trapping of Au-bearing solutions in their fractures. In the phyllic wall-rock
alteration zone, the recorded Au content ranges from 92.77 wt. % to 8.12 wt. % and the associating Ag content
varies from 5.00 wt. % to 7.90 wt. % ). In the propylitic wall-rock alteration zone, the reported Au content
ranges from 80.39 wt. % to 38.84 wt. % and the associating Ag content varies from 4.87 wt. % to 14.36 wt. %.
Regarding the recorded silicification, the reported Au content attains about 73.99 wt. % and the associated Ag
content is about 9.55 wt. %. Generally, in all recorded wall-rock alteration zones, the Ag contents are not
enough to form the mineral electrum (Au, Ag), in which the Ag content is higher than 25 wt. %.
The mineralogical composition, ore textures, and paragenetic sequence of the gold-bearing sulphides of the
granitic wall-rock alterations at the El Sid and Fawakhir gold mines may be the same as, and comparable with
that of the main gold mineralized quartz veins, suggesting their contemporaneous nature. It is recommended to
assess and exploit the gold from the granitic wall-rock alteration zones at the El Sid and Fawakhir gold mines as
well as the similar localities in the Eastern Desert of Egypt.
Acknowledgments
The authors would like to thank Prof. Dr. Mohamed Wahbi Ali-Bik at the Geological Sciences Department,
National Research Centre, Cairo, Egypt, for his critical and helpful comments.
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