الفهرس 
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Abstract
This work is concerned with studying the Permo-Triassic volcanic rocks, altered granitic rocks,
post granitic dykes and veins exposed in Nusab El Balgum area, South Western Desert, Egypt. This
included field observations, petrographic, mineralogical, geochemical and radiometric studies in
order to determining the rare metals mineralization, tectonic settings and different types of
alterations.
Nusab El Balgum area covers about 40Km2, located at the
southwestern Desert of Egypt, about 600 km south west of Aswan City near Kurkur and Dungul Oases.
It is occurred at the intersection of Lat.
23o15/ to 23o 20/ N and long 29o 15/ to 29o20/ E.
1-Geologic setting and petrography
Nusab El Balgum forms an elongated body trending in the NNE- SSW and composed of Permo-Triassic
volcanic rocks and surrounded by the peneplained surface of older sandstones. Due to the strong
effect of weathering and denudation, Nusab El Balgum is represented by a number of isolated low
hillocks (showing blocky shape) separated by numerous fault planes in different directions that are
covered by windblown sand .
The area consists mainly of sandstone (Devonian), volcanic rocks
(Permo-Triassic), altered granitic rocks, post granitic dykes and veins.
I-Sandstone: It is the oldest rock unit in the study area (Paleozoic age) and form few isolated
outcrops rise about 5 m above ground level occurring in the low-lying parts of the area.
II-Conglomerate: It separates the extrusive rocks (volcanics) from the underlying sedimentary rocks
(sandstone). The thickness of this unit ranges between 1m to 4m. The age of this unit is not
clearly defined because there were no fossils collected from its lithology.
The microscopic investigation revealed that the conglomerate is composed of quartz grains of size
up to 7.0mm associating angular quartz grains of sand size (<2.0) hold by a matrix of amorphous
silica. Zircon and monazite represent the accessory minerals.
III-Volcanic and Volcaniclastic rocks: The volcanic rocks form abundant hills of different
elevation. The prominent highest is that of Gabel Nusab El Balgum with an elevation of 366m
(a.s.l) .The volcanic rocks are also present in the form of numerous plugs with dykes and veins.
They dip gently to steeply towards the northwest and are surrounded by low land of altered granitic
rocks from the south direction. These volcanic rocks together with their associated altered
granitic rocks form triangular body(1.5 km in length and 650 m in width) with its apex looking
NNE, while its base trend in the E-W direction. This body is about.
Microscopic investigation for volcanic rocks revealed that, they range in composition from acidic
to intermediate and could be classified Petrographically, into rhyolite , dacite, in addition to
tuffs that characterized by light weight and brittle appearance.
Volcanic rocks
1-Dacites: It is an intermediate volcanic rock composed mainly of plagioclase, potash feldspar and
quartz. The studied dacites could be categorized microscopically according to their textures into
equigranular dacite and porphyritic dacite.
2-Rhyolite: The studied rhyolite represents the acidic member
of the volcanic rocks; it is composed mainly of potash feldspar and quartz with few crystals
of plagioclase. Zircon is the sole accessory mineral .
Volcaniclastic rocks
A)Crystal tuff: The porphyroclasts are individual crystals of quartz and potash feldspars(perthite,
orthoclase perthite and sanadine), and characterized by presence of radioelements bearing accessory
minerals such as monazite and zircon in addition to radioactive minerals that represented mainly by
autunite included in the porphyroclasts of quartz.
B)Banded crystal tuff: Megascopically, it is characterized by obvious banding. The porphyroclasts
are mainly quartz ranging in size from cryptocrystalline to very fine crystals.
C)Lithic-crystal tuff :This type of tuff is characterized by presence of lithic fragments in
addition to the crystals of quartz and feldspars( orthoclase perthite, perthite and zoned
plagioclase). While lithic fragments are mainly trachyte and dacite.
D- Fine grained tuff: It composed of amorphous silicic grains dominated by fissures filled by
iron oxides and cavities filled with detrital grains of quartz or other minerals.
IV-Altered granitic rocks: The altered granitic rocks are exposed at southern part of the
mapped area and form irregular low lying surface, usually rising less than 3m above ground level.
Some of these rocks occur as small isolated bodies and intruded the volcanic rocks with sharp
contact where some xenoliths of latter engulfed in the granitic rocks. Hematization,
kaolinization and silicification are the main
different wall rock alteration features developed within Nusab El Balgum granite.
Hydrothermal solutions also plays an important role in enriching the rock by uranium and formation
of secondary uranium minerals such as autunite which occurs in the form of rose-like crystals
associating the iron oxides that stained the amorphous silica. The altered granitic rocks are
divided into two types, silicified and kaolinized granite.
A-Silicified granite: The silicified granite shows abnormal radioactivity more than kaolinized
granite. The silicified is located in the south and southeastern part of the study area, forming
nearly rounded shape of low lying surface relative to volcanic rocks with 550m width. The rock is
highly altered and cuts by numerous acidic dykes in NE, NW and E-W direction.
Petrographically, this rock is characterized by complete absence of plagioclase where the potash
feldspars (perthite representing about
67.6%) and quartz (32.4%) are the main constituents. Mafic minerals are mainly alkaline pyroxenes
that intensively altered or metasomatized by meteoric or hydrothermal solutions.
B-Kaolinized granite is located in the northern part of the study area with sharp contact with the
volcanic, having elongated shape with
2km in length and 150m in width. A different types of alteration were affected on this granite,
especially kaolinization, silicification and hematization process. These rocks are medium-grained
and characterized principally by pink color referring to young-aged granites. Some of the picked
samples are altered and grade in color to light pink, reddish pink and grayish pink according to
the type and degree of alterations. Others are characterized by surface cavities formed by removal
of the weak minerals that produced by the epigenetic processes.
Petrographically, the feldspars are completely altered to sericite
and kaolin. Zircon represents the main accessory mineral occurring as aggregates of immature
crystals showing its characteristic interference colors and high relief.
V-Dykes and veins: Several dykes of different trends are injected and affected on all rocks in
Nusab El Balgum rocks, especially volcanic rocks following all faults traversing the area and
generally striking in N-S, E- W, NE, ENE and WNW.
The post granite dykes mainly acidic in composition are common in the volcanic rocks, silicified
granite and kaolinized granites in decreasing order.
These dykes range in length from few meters to few kilometers, while their width ranges from 0.1m
to 3m. They range in composition from porphyritic dacite, spherultic rhyolite through granopheric
dyke to granite porphyry.
Few quartz veins are recorded in the study area cutting the volcanics, they are massive and of
reddish color due to strong impregnation by iron rich solutions. Their width range from 0.5 to 2 m,
and extend for few hundred meters trending N 400 E-S 400 W and N 200
E-S 200 W.
Dykes: The studied dykes could be classified microscopically to granitic dyke (slower cooling
giving fine grains), rhyolitic dyke (rapid cooling giving very fine crystals), dacitic dyke
(variable cooling and variable grain size) and fault breccia.
Veins: They represent the monomineralic rocks invading the granite and could be classified in this
study to quartz vein (very fine quartz) and silica vein (amorphous silica). They attain their
importance
due to their high radioactivity and presence of some radioactive minerals
captured from the enclosing granite.
2-Mineralogical results revealed that:-
I-Conglomerate: contains REE silicate, monazite (as rare earth minerals), columbite
(as niobate- tantalite minerals) and base Metals (pyrite, nickel and barite). The accessory
minerals are represented by (zircon, apatite, garnet, titanite, rutile and ilmenite).
II-Volcanic rocks and associated dykes: contain REE silicate, cerite (as rare earth minerals);
fergusonite and columbite are niobate- tantalite minerals and base Metals (gold, galena, nickel,
atacamite). The accessory minerals are mainly zircon, fluorite, xenotime, apatite and garnet.
III-Altered granitic rocks: contains radioactive mineral (thorite, uranophane and autunite).
Bastnaesite, parisite, allanite, monazite and REE silicate are the Rare Earth Minerals.
Fergusonite, columbite and ilmenorutile (as niobate- tantalite minerals). Gold, nickel, pyrite,
galena, sphalerite, barite and atacamite are the base metals. Accessory minerals are
represented by magmatic zircon, hydrothermal zircon, xenotime, apatite, ilmenite and hematite.
Petrographical and mineralogical studies applied on altered granites samples of the study area
showed that the main mineralization types are Zr, Nb, Y, REEs and Th ones. The content of ore
minerals is directly proportional to the degree of alteration, so it increases by alteration
increase. Generally, the principal minerals according to their abundances are: thorite (ThSiO4) ,
zircon (ZrSiO4),bastnaesite((Ce, La, Nd)(CO3)F),
parisite (Ca (Ce, La, Nd)2(CO3)3F2) and monazite [(Ce, La, Th, Nd, Y)
PO4].
3-Geochemically
I-Conglomerate: The studied samples are enriched in SiO2, Al2O3, CaO and FeO but slightly depleted
in other elements. from the tectonic setting point of view, the conglomerate displays the passive
margin environments and deposited in semi-arid to semi-humid climatic conditions.
II-Volcanic rocks and associated dykes: The investigated volcanic rocks are predominantly
comenditic- pantelleritic in composition. Nearly all samples are generally not affected by
alteration. SiO2 shows negative relation with Al2O3, FeO, CaO, Na2O, L.O.I, Ba , Rb and U, and
positive relation with TiO2,V,Cr,Ni, Y, Zr, Nb and Th. SiO2 have negative relation with Y, Zr, Nb,
Th, U and positive relation with Cr, Ni, in the associated dykes.
The high degree of Zr and Y enrichment in the studied volcanic rocks is a feature of within plate
felsic igneous rocks (Bowden and Turner, 1974). In general, all volcanic rocks show moderate
enrichment of most High field strength elements (HFSE) elements as Y, Th, U, Zr
,Nb and depletion in Low field strength elements (LFSE) as K, Rb, Ba and Sr.
The relation between the Ba/Nb ratio and SiO2 (wt. %) content shows that the studied volcanic
rocks plot near the fractional crystallization with alkali feldspar trend. This indicates that the
high Ba/Nb ratio in the most primitive rocks is a characteristic feature inherited from
their source. Accordingly, the examined volcanic rocks and dykes with an average Rb/Sr ratio (2.3&2
respectively) suggests
highly differentiated magma of crustal derivation. The plotting of the
analyzed volcanic samples show continuous spectra of values fall along the same trend suggesting
that the volcanic sequence under study was derived from the same magma source by fractionation
processes with low TiO2 contents.
The studied samples might be a result of a continuous
magmatism and that they were derived from the same magma source by fractionation processes. The
high TiO2 magma is being the early fractionated, while the low TiO2 magma is produced from the more
fractionated varieties. The studied volcanic rocks imply a significant degree of fractional
crystallization along trend BC.
This leads to the conclusion that the magma did not incorporate significant amount of crust that in
turn does not affect the gross ultimate composition of the rocks.
III-Altered granitic rocks:
The granitic rocks in Nusab El Balgum area are invaded by strong acidic hydrothermal solutions
which are responsible for kaolinization, silicification and sericitization; these solutions are
enriched in Fe2+. with the progress of alteration processes and the increase in PH values and in a
more alkaline medium, hematite precipitates
The studied altered granites have been subjected to various degrees of alteration processes which
confirmed with enrichment in SiO2, FeO, MgO, CaO, Al2O3, L.O.I, Nb, Y and Zr, and depleted in total
alkalis content.
The chemical analyses show that the studied silicified and kaolinized granites are enriched in Zr
(range from 2848 to 22400ppm and from 2520 to 6476 ppm), Y (range from 134 to 9028 ppm and from
1237 to 2853 ppm), Nb (from 51 to 3945 and from 450 to 1129 ppm), Th
(from 5 to 954 and from 37 to 96 ppm) and REEs (range from 134 to
39113 and from 34 to752 ppm) respectively. The silicified granite has, in average, higher amounts
of rare metals than the kaolinized granite.
CIA values in the silicified granite is relatively less than that of kaolinized granite (78 for
silicified and 83 for kaolinized granite). High CIA values reflect extensive loss of labile
elements (Na+, Ca2+, K+) in
solution relative to immobile Al3+ and hence warm and humid climate.
The silicified granite of Nusab El Balgum are enriched in many large ion lithophile elements (LILE)
e.g. Ba, Rb, and high field strength elements (HFSE) e.g. Zr, Y, Nb, Th and REE that reflecting
their alkaline habit. Also, one can easily observe the depletion in Co, Cr, Cd, Ag, Be, Sb, Cs and
V.
All studied samples fall in A2 field or crustal A- type and are enriched in many contents of rare
metals (e.g. A u , A g , G a , M o , W , Y, Th, U, Zr, Nb and REE).
Rare metals
The mineralogical studies confirm the presence of three mineralization stages, the earliest
mineralization stage (high temperature stage) which is characterized by the formation of native Au
associated with primary sulfide minerals such as pyrite, galena, sphalerite and native Ni. The
presence of second stage (lower-temperature) is characterized by the formation of secondary
minerals; barite and atacamite. Third stage is related to carbonate facies formed after the oxide
(supergene alteration).
There is an agreement in the nature of hydrothermal solutions carrying for rare metals in volcanic
rocks and altered granites ,through the positive relation between Au and FeO, As, Ag, Zn, Pb, Y,
Th, Mo, Ga, and negative relation with SiO2, Al2O3 and Ni. from the above
paragraphs, the ferrugenation processes are associated with the enrichments of rare metals in the
study area, and the source of gold is related to hydrothermal origin not to mafic rocks.
Six stages of alteration assemblages have been identified (Fig.6.1) based on field and microscopic
investigations(sericitization, silicification, carbonization, hematization chloritization and
kaolinization).Activities of the tectonic movements created channel ways which allows hydrothermal
fluids to infiltrate wall rocks, leading to diffusive alteration processes on the two types of
granites. Sericitization followed and overprinted K- feldspar alteration. Subsequently
silicification followed by carbonization has occurred. Gold mineralization was closely related to
silicification alteration.
The recorded gold contents in Nusab El Balgum area, considered as first record. The silicified
granite samples have higher content of gold (ranges from 9 to 0.35 g/t) than either the kaolinized
granite samples (0.3 g/t),or volcanic rocks (ranges from 0.3 to 0.4 g/t) and finally
post granitic dykes (0.36 and 0.5 g/t).
The occurrence of gold mainly depends on temperature, pressure, pH values, Cl¯ concentration, and
fugacity of H2S in a hydrothermal system. Gold mostly occurs as AuCl2 in a system with temperature
higher than 4000C (Gammons and Williams-Jones,1997), and with decreasing temperature being the
primary mechanism causing gold deposition. Au(HS)¯2is the dominant phase at lower temperature, and
the maximum solubility exists in the vicinity of the H2S-HS¯-SO42-equilibrium point; then, with
the decline of oxygen fugacity, the Au-S complex breaks
down, leading to gold precipitation (Cooke and Simmons, 2000; Robb,2005):-
2Au (HS) ¯+H O→2Au + 4HS¯
¯
+2H+
2-
+ 0.5O2
+
2Au (HS) 2
+8H2O→2Au + 4SO4
¯
+4H
+ 8H2
FeCO3+Au (HS) 2 ↔FeS2pyrite + CO2 + H2O+ Au
The rare metals enrichment in the study area are formed due to the subsequent processes; including
the ascending hydrothermal solution (alkaline and acidic hydrothermal solutions), with further
contribution of the descending acidic meteoric water; supergene enrichment processes. The alkaline
hydrothermal solution in silicified granite was under PH more than 7and T. (3000C to 3500C) where
FeOt (Av.6.5 in silicified and
4.3 in kaolinized) total. The acidic solution was under PH ranges from 2
to 3 with low temperature varying between 2000C to 2500C (Helgeson,
1969).
Precipitation of hematite is due to extensive alteration of pyrite and probably decreased the pH of
the alkaline solution and rising acidic fluids. The presence of clay minerals (related to
alteration of feldspars) indicates a high temperature environment (higher than 200ºC). The
mixing of volatile fluids with meteoric water and fluid- wall rock interaction result in changes in
pH and oxygen activity and deposition of hypogene rare metals (nickel, gold, galena and sphalerite)
and supergene ones(atacamite, barite and secondary uranium) in fractures filling of altered
granites.
The sudden change in the pH and temperature of the fluids will lead to destabilization of rare
metal complexes favoring their deposition (Alexandrov et al., 1985).The rare metals mineralization
are accumulate in the residual melt of the late fractionate (Bright, 1974) especially upward in the
magmatic system (Smith, 1979). During the emplacement of the post granitic dykes, the rare metals
were precipitated from saline
and reduced fluids as sulfides due to cooling, fluid mixing and wall rock
reaction.
Depending on Oxygen fugacity (ƒO2) in the final magmatic fractionation, uranium tend to form the
relatively soluble uranyle ion (UO2)+2, hence, hydrothermal processes during final stages of
crystallization of the granitic magma are important in controlling the later redistribution of
uranium within the rocks (O’Connor et al., 1982).
The origin of uranium appears to be closely associated with the rare metals mineralization and may
be reflects readily their intimate coherence.
Fig.(6.1). Paragenetic sequence for the minerals in the altered granites, Nusab El Balgum area.
Mineral
Qz
Stage K-feldspar alteration
Sericitization Silicification Hematization Kaolinization Carbonitization
K-feldspar Albite Sericite Kaolin Pyrite Sphalerite Galena Atacamite Barite Zircon Thorite
Uranophane Native Ni Gold
REEs in altered granites
Generally there is enrichment in ∑REEs in the study silicified from 134 to 39113 ppm and 34 to 752
ppm in kaolinized granites compared with total Clarke contents (∑REECl = 189.8 ppm, ∑LREECl =
158.4 ppm; ∑HREECl = 31.4 ppm, after Ovchinnikov, (1990).
In silicified granite SiO2 shows negative relation with all elements, but FeO have
positive relation with all elements and strong relation with HREEs. Zr- Y- Nb-Th have very strong
positive relation with each other and moderatly with REEs.
In kaolinized granite Al2O3 have positive relation with Y, Zr, Nb
, Th and very strong negative with REEs. The high Al2O3 content is due to the presence of kaolin
mineral as alteration product. Y-Nb-Zr-Th have very strong positive relation with each other and
negative with REEs.
This relation is in aggregement with the mineralogical and petrographical studies, the presences of
fergusonite, zircon, thorite and rare earth minerals is more abundant in silicified granite, and
the rock is affected with hydrothermal solution rich in Fe ,HFSE and REES.
The calculated values of the tetrad effect range from 0.8 to 1.2 in silicified granite
and from 0.89 to 1.25 in kaolinized granite.
The Chondrite-normalized REE patterns of the silicified and kaolinized granite show M-type tetrad
effect similar to that quoted by Masuda et al., (1987).The Y/Ho ratios in the silicified and
kaolinized granite of the study area (10 to 2332 &159 to 1209) respectively are higher than the
chondritic ratio (28 after (cf. Anders and Grevesse,
1989).
The rare earths of altered granites show M-type tetrad effect and indicate higher LREEs, negative
Ce anomaly (due to the alteration fluids) and negative Eu anomaly.
Accessory minerals of magmatic origin may be partially to entirely
altered and replaced by other minerals. Each accessory mineral has specific behavior under given
physical and chemical conditions, and will transform into specific alteration products (Michel
Cathelineau, 1988), as shown by figure (6.2). In this case, changes in the trace element content
of the rock are described by the mass balance of the chemical reactions of solubilization or
alteration of the early minerals. The most representative reactions for carbonates and phosphates
are given in the following paragraphs. Possible dissolution and alteration reactions of apatite,
monazite and xenotime are illustrated.
(REE, Y)PO4 + H2O ↔ HPO42-+ (REE,Y)3+ + OH-
Monazite or xenotime
2(REE, Y) PO4+ Ca2+ + 2F-+ 3HCO3- ↔ Ca (REE, Y) 2(CO3)3F2
+2HPO42- +H+
Parisite or Y-parisite
Ca5 (PO4)3(OH) +3H2O↔5Ca2++3HPO42-+4OH-
Apatite
Rare earth elements were often accepted as rather immobile elements, but some studies have shown
that they can be mobilized by hydrothermal fluids circulation (Alderton et al., 1980; Michard and
Albarede, 1986). Zr and Th may be mobile especially in high temperature hydrothermal
environments with strong complexing agents such as fluorine, sulphide and others (Keppler, 1993).
The REE are commonly insensitive to hydrothermal processes (Winchester and Floyd, 1977; Floyd
and Winchester, 1978).
REE mobility is favored by: low pH, high water rock ratios and
abundant complexing ions (CO32-, F-, Cl- , PO43- , and SO42-) in the hydrothermal solutions
(Michard, 1989; Lottermoser, 1990, 1992).
The REE in Nusab El Balgum altered granites were brought by
hydrothermal fluids into rocks during alteration and may be precipitated in carbonate minerals such
as bastnaesite (LREE) (CO3) F and parisite Ca (LREE) 2(CO3)3F2 .
REEs in volcanics
The chondrite–normalized rare earth element of the volcanics and associated dykes are characterized
by (1) enrichment in light rare earth element (LREE) (Av.1293 &1459ppm) for volcanic rocks and
associated dykes respectively (2) depletion in heavy rare earth element (HREE) (Av.190&208), and
(3) negative Eu- anomaly. The average of total rare earth elemental concentration of volcanic rocks
and associated dykes are
1483&1667 ppm respectively, with Eu-anomaly (Eu/Eu* =0.2&0.18), and Ce anomaly (Ce/Ce*=0.87
&1.1).The calculated values of the tetrad effect range from 0.9 to 1.1 in volcanic rocks and from
1.03 to 1.5 in dykes.
While conglomerate has ∑REEs (333-1360 ppm). The rock have negative Ce* and Eu and no tetrad is
visible.