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3659
Journal of Applied Sciences Research, 9(6): 3659-3673, 2013
ISSN 1819-544X
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLES
Contribution to the Geochemistry, Composition and Origin of the Dolostones of Um
Gheig Formation, Middle Miocene, Red Sea Coast, Egypt
Esmat A. Abou El-Anwar and Hamed S. Mekky
Geological Sci. Dep. National Research Center, Cairo.
ABSTRACT
The dolostones of Um Gheig Formation, Middle Miocene consist mainly of shallow, subtidal dolostones. It
has been possibly originated due to two different fluids which are comparable to those in the dolostones of Um
Bogma Formation. The main fluid source is related to meteoric water. The second fluid is originated from
deeper hydrothermal or basin sources related to the Middle Miocene saliferous rocks (Abu Dabbab Formation)
and ascended through faults. The dolomite of Um Gheig Formation is composed of fine to coarse, subhedral
rhombs with polymodal distribution. They are formed initially by syn-depositional, subtidal dolomitization in
evaporative or modified marine water, as indicated by the close association of supratidal gypsum deposits with
subtidal dolomite. The petrographic investigation of the studied dolostones reveals that their original textural
and compositional characteristics were diagenitically modified by neomorphism, dolomitization, dissolution,
compaction, and fracture - cavity fillings. XRD revealed that the studied carbonate samples consist mainly of
dolomite and minor calcite. The dolomite of the Um Gheig Formation is nearly stoichiometric and ordered,
indicated that dolomite crystals were deposited in mixing zone. In addition, it is formed in the near-surface
oxidizing environment. Geochemically, the studied dolomites (Um Gheig Formation) are characterized by the
enrichment of Mn, Ba, Pb, Zn and As which derived from hydrothermal solution. The Sr distribution coefficient
of the studied dolostones indicated that they are formed in temperature <150 °C. The dolostones of Um Gheig
Formation is significantly affected by chemical weathering and anaerobic bacteria. The CF (Contamination
factor) and Igeo (Geo-accumulation index) have indicated that the Um Gheig dolomite rocks are polluted
especially for As, Rb, Ba as well as Pb, Zr, Mn, Sr and Zn, which similar to those of Um Bogma Formation. The
formation of the dolostones of Um Gheig Formation is similar to dolomites occurred in the Gebel Hadifa,
Tunisia, Adriatic off shore and Po Plain (Venetian Southern Alps), Italy, Maestrat Basin, Spain, Bayan Obo,
China, southern Laurentia, Canada and Um Bogma Formation, Egypt.
Key words: Egypt, Um Gheig, Dolostones, Genesis, Geochemistry, Contamination.
Introduction
The chemical and mineralogical compositions of marine sediments provide important information about
their origins and can be therefore used to infer paleoclimate changes, weathering trends, provenance of the
sediment, depositional environment and sources of pollution in the region. Chemical reactions between
magnesium-bearing solutions and calcium carbonate sediments form dolomites via dolomitization.
Dolomitization is a significant diagenetic process that influences porosity development. Hydrothermal dolomites
are formed under burial conditions from high salinity fluids at temperatures higher than the ambient temperature
of the host formation (Davies and Smith, 2006 and Azomani, et al., 2013). Hydrothermal fluids are played an
important role in this process and the resulting dolomites exhibit a major control on the distribution of porosity
and tectonically activity (Azmy, et al., 2008, Azmy and Conliffe, 2010 and Conliffe, et al., 2010 and 2012).
The geology and the stratigraphy of Um Gheig area have been studied by some authors, among them, ELShazly and Hassan, (1962), Samuel and Saleeb-Roufaiel, (1977), Abu-Khadrah and Abdel–Wahab, (1984),
Said, (1990) and Felesteen, et al., (1994). As far as the authors are aware, the published data on the
geochemistry and pollution of the dolomite rocks are certainly lacking.
The main aim of the present paper is to give detailed petrography, diagenesis and geochemical studies for
15 samples. As well as, to discuss some aspects concerning the environmental conditions and pollution process
during the deposition of the Middle Miocene (Um Gheig Formation) in its type locality at the Um Gheig area .
The studied massive dolomite of Um Gheig Formation is located in between Longitudes 34o 28' 56" and 34o 29'
16" E and Latitudes 25o 42' 12" and 25o 42' 83" N, (Figs. 1 and 2).
Corresponding Author: Dr. Esmat Ahmed Abou El-Anwar, Assentant Professor, Geological Sci. Dep.
National Research Center, Cairo.
E-mail: [email protected]
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J. Appl. Sci. Res., 9(6): 3659-3673, 2013
Fig. 1: Location map of the studied area.
Fig. 2: Schematic section of the studied area.
Geology:
Um Gheig mine is located in the Red Sea coast at Longitude 34o 28' 57" E and Latitude 25o 42' 53" N. Um
Gheig Formation (Middle Miocene) conformably overlies Abu Dabbab Formation and unconformably underlies
Samh Formation; it reaches to 37m (EL-Shazly and Hassan, 1962). This formation forms the basal bed of Samh
Formation (Abu- Khadrah and Abdel-Wahab, 1984). It is grey, very hard and compacted dolostones. Said,
(1990), mentioned that the formation is rich in crinoids, oncoids, algae and bioclasts. Gypsum and anhydrite are
the evaporate minerals encountered in the studied rocks. Anhydrite and gypsum are precipitated from later
diagenetic solutions, epically as fracture fillings. Felesteen, et al., (1994) mentioned that dolostones of the Um
Gheig formation indicating a gradual high stand of the sea level. Also, they mentioned that the top of it is
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distinct by an unconformity surface indicating sudden lowering of the sea level. Carbonate rocks of Um Gheig
Formation termed “oil-tainted limestone” by Beadnell, (1924) and “oil-rocks” by Felesteen, et al., (1994), due to
the presence of reflection of organic matter. Stratigraphy, Felesteen, et al., (1994), mentioned that the
assemblage of Borelis melo Zone is identified in the dolostones of Um Gheig Formation. Consequently, they
assign a Middle Miocene (Late Langhian) age to Um Gheig Formation. The Um Gheig Province is intersected
by three major sets of faults with trends ~N160°E (Clysmic faults) parallel to the Red sea trend, N70°E
(perpendicular to the Clysmic trend) and N100°E (Dawi direction, perpendicular to the Aqaba trend), El-Gaby,
et al., (1984).
Petrography and Diagenesis:
The dolostones in the studied area are grey massive rocks. The crystal size of Um Gheig Formation
dolostones ranges from 15 to 300 µm which indicate early to late diagenetic dolomite. The non–planar
boundaries are generally irregular to curved and embedded in micrite dolostones matrix (Plate I: A and B). In
addition, some patches of iron oxy-hydroxides scattered in the matrix (Plate I: C). Planar-s rhombs are
characterized in the investigated samples and ranging in size from 15 to 100 µm (Plate I: C and D). The
microfabric and composition of the micrite are controlled by the original mineralogy of the precursor sediments;
a sit is in shallow-marine micrites (Lasemi and Sandberg, 1993).
The solutions caused the dolomitization are under-saturation lead to the formation of planar boundaries and
generally small crystal size. Whatever, at super-saturation non-planar interfaces formed and the crystal size is
generally larger, (Sibley and Gregg, 1987). At more advanced stage the fossils are totally replaced by fine
crystalline dolomite as mimic replacement, they are ranging in size from 20 to 75 µm (Plate I: E). There is a
correlation between the rate of dolomitization and the fabric of the dolomite; allochems that are rapidly
dolomitized to cryptocrystalline mosaics display mimic replacement. Conversely, slowly dolomitized microcrystalline allochems display non mimic replacement fabrics. This indicated that the original dolostone was
porous and the diagenetic solution percolating through pores caused the recrystallization and rapidly
dolomitization processes, which is in agreement with Conliffe, et al., (2012). Consequently, all rock components
and matrix are completely replacement by dolomite as shown in (Plate I: E). It is composed of strongly
interlocking fine to coarse dolomite subhedral crystals with anhedral polymodal distribution (Plate I: F). Some
vugs and molds are filled with limpid dolomite crystals that exhibit subhedral planar form and matrix are stained
by iron oxy-hydroxides (Plate II: A and B). The Um Gheig Formation seems to be significantly affected by
chemical weathering. The pseudomorophosed framboidal pyrite scattered in dolomitic matrix, which is
attributed to a deposition in anoxic water but under shallow water conditions (Plate II: A and Plate I: F), which
is in agreement with Fleurance, et al., (2013). Such framboidal pyrite indicates possible role of anaerobic
bacteria during the early diageneting stages, which is in agreement with (El-Kammar and El-Kammar, 1996,
Abou El Anwar, 2006 and 2012 and Fleurance, et al., 2013).
Carbonate rocks are particularly susceptible to diagenesis partly because minerals are more soluble in water
and so are subjected to dissolution and re-precipitation. The main diagenetic features encountered in the studied
area are neomorphism, fracture fillings, compaction, dissolution and dolomitization.
1- Neomorphism:
The studied rhombs up to 300 µm which indicate that late diagenetic dolomite, which is in agreement with
Conliffe, et al., (2012). Generally thought that micrite can eventally undergo transformation to microspar though
aggrading neomorphism. The term ‘‘neomorphism’’ was proposed by Folk (1965) for diagenetic processes in
which original crystals are consumed, and where their place is simultaneously occupied by new crystals of the
same mineral or by a polymorph. Depending on the origin and carbonate mineralogy the carbonate sediments
undergo different path ways under the influence of meteoric water (Tucker and Wright, 1990 and Braithwaite,
2005). Neomorphic dolomites are resulting from the recrystallization of the pre-existing dolomite (Gregg and
Sibley, 1984). Dolomites that are poorly ordered and non-stoichiometric, such as modern, fine-grained
dolomites formed at or near the surface, or from evaporitic brines, tend to re-crystallized with time and during
burial (Machel, 2004). Consequently, the studied neomorphic dolomites are subjected to an increasing number
of non-planar crystal boundaries, which is an indicator of neomorphism (recrystallization) from the pre-existing
dolomite during burial.
Xenotopic dolomites (anhedral, irregular or curved crystals) formed by dolomitization of limestone or/and
recrystallization of dolomite at > 50 C o after burial (Gregg and Sibley, 1984). Whatever, idiotopic (euhedral to
subhedral crystals) produced near surface at < 50 Cº. Both types of dolomite may be encountered in the same
sample indicating to two phases or generations of neomorphic dolomite which means that neomorphism took
place in more than one stage. The earlier phase is generally xenotopic, while the later one is idiotopic (Plate II:
C, D, E and F).
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Plate I: A) Dolostones of Um Ghieg Formation showing manily non-planar boundaries, dissolution and vug
filled by dolomite crystals, S. No. 5. B) Microsparite dolomite crystales embeded in micrite dolostone
matrix, S. No. 9. C) Dolostones of Um Ghieg showing patches of iron oxy-hydroxides scattered in
micrite dolostone, S. No. 9. D) Xenotopic neomorphic dolomite crystals with non-planar boundaries,
S. No. 10 . E) Dolostones of Um Ghieg showing fossils are totally replaced by fine to medium
crystalline dolomite as mimic replacement, fracture filled with iron oxy-hydroxides and neomorphism,
S. No. 9. F) Some vugs and molds are filled with velar limpid dolomite crystals. Note: Framboidal
pyrite scattered in matrix, S. No. 9.
2- Fracture and cavity fillings:
The fracture and cavity in the studied dolostones are file with iron oxides, anhydrite and/or fibrous gypsum.
Some fracture filled with dolosparite, which represented late diagenetic stage, which is in agreement with
Conliffe, et al., 2012 (Plate I: F., Platte II: A and B and Plate III: A and B). The fractures within the studied
dolomite probably resulted from mechanical compaction (resulting of the tectonic activity in the Um Gheig
Province), during which they were filled by sparitic dolomite, iron oxides or evaporates.
3- Compaction:
Compaction involves reorientation and breakage of some delicate fractures (Plate III: C and D). Much
compaction, both chemical and mechanical, began under an overburden of tens to a few hundred meters. It is
produced at early stages of diagenesis, (Adams and Mackenze, 1998). Consequently, during the progressive
burial, physical compaction and, more importantly, chemical compaction (pressure solution) change the fabric
of the studied rocks through dissolution / re-precipitation (cementation / neomorphism) and dolomitization,
which is in agreement with Wanless, (1983). Mechanical compaction is frequent diagenetic processes that
existed in this environment. Burial compaction led to common fracturing, while chemical compaction is a result
of diagenetic dissolution at high pressure (Plate III: E and F).
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Plate II: A) Sample shows neomorphism, dissolution and frampoidal pyrite in dolomitic matrix, S. No.9. B)
Some vugs and fracture filled with dolosparite, which represented late diagenetic stag, S. No.10. C)
Xenotopic neomorphic dolomite crystals with non-planar boundaries, S. No. 2. D) Non-planar
neomorphic dolomite crystals and patches of iron oxy-hydroxides, S. No. 5. E) Recrystallization of
the pre-existing dolomite and dissolution, S. No., 8. F) Neomorphism process showing two
generations of dolomite crystals and framboidal pyrite scattered in dolomitic matrix, S. No. 10.
4- Dissolution:
Dissolution plays a moderate role in the diagenesis of the carbonate sediments of Um Gheig Formation.
Petrographic examination revealed that some fossils have been affected by dissolution under the action of
meteoric water in the meteoric phreatic zone and filled by microsparite and/or sparry dolomite crystals ((Plate
III: E and F). However, some moulds remain free which suggests subaerial diagenesis, probably in the vadoes
zone. Micritic cements were also leached during early diagenetic meteoric diagenesis. This enhanced the preexisting intra- and intergranular porosity. The studied dolomite may form by there placement (dolomitization) of
micrite matrix and of allochems, or alternatively it may occur as intra-particle and inter-particle cements.
Dissolution takes place particularly in the near-surface meteoric condition (Tucker and Wright, 1990). So,
replacement involves the dissolution of a carbonate precursor and the ensuing precipitation of dolomite in the
resulting pore or fractures in the near surface meteoric environment.
5- Dolomitization:
Dolomite is kinetically inhibited from direct precipitation in modern seawater, except via microbial
mediation in shallow, anoxic environments (Vasconcelos and McKenzie, 1997) or in evaporation-sabkah
environments (Tucker and Wright, 1990). The dolomite rhombs are planar-s and non–planar boundaries in
texture with crystal size ranging from 15 to 300 µm (Plate I: A, B, C and D). Many ancient dolomites are nearer
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to the ideal stoichiometry due to an excess of calcium that substitutes for magnesium in the crystal structure
(Lumsdem and Chimahusky, 1980). There is a correlation between the rate of dolomitization and the fabric of
the dolomite; allochems that are rapidly dolomitize to cryptocrystalline mosaics display mimic replacement
(Plate I: E). The investigated dolomite rhombs are fine to coarse crystalline. The authors suggested that the
dolomitization of the studied rocks is most probably post depositional, is due to an arid climate, which promotes
a high Mg/Ca ratio, and consequently, an abundance of Mg2+ ions in the fluids.
Plate III: A) Cavity files with two types of iron oxides, S.No.2. B) The studied dolostones shows cavity files
gypsum as well as the dissolution effect, S. No. 10. C) Dolostones of Um Ghieg showing with
compacted of delicate fractures, S. No. 9. D) Mechanical compaction at early stages of diagenesis, S.
No. 9. E) Dolostones of Um Ghieg showing showing intergranular porosity, S. No. 10. F)
Dolostones of Um Ghieg showing with intra-intergranular porosity, S. No. 10.
Petrographic examination identified at least three generations of dolomites in the Um Gheig Formation,
which are: (1) an early replacement sub-hedral micritic dolomite 15-30 µm, (2) neomorphic sub-hedral dolomite
(50-100 µm) and (3) non–planar, irregular to curved rhombs (100-300 µm). The near-micritic grain size,
suggests an early precipitation of dolomite at low temperatures of near-surface conditions from solutions likely
formed by mixing of sea and meteoric waters, which is in agreement with Azomani, et al., 2013. Coarse
dolomite rhombs are resulting of neomorphism processes at late diagenetic stage.
Anhydrite and gypsum are predicated from natural water concentrated by evaporation. The studied section
is conformably overlies Abu Dabbab Formation (Evaporite Formation). On evaporation of seawater, the first
minerals to precipitate after carbonate are the calcium sulphates. The hydrous form, gypsum occurs only near
the earth surface, where anhydrite is formed at the surface and also replaces gypsum at depth (Adams and
Mackenzie, 1998). So, fine-grained dolomites associated with evaporates are often near-stoichiometric.
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The Mineralogy:
Mineralogically, selected 5 samples were investigated by the X- ray technique at the Metallurgical Center
for Research and Development Institute (Tebbin, Egypt) using a Phillips X-ray diffraction model with Nifiltered Cu-K α radiation. XRD revealed that the studied dolostones of the Middle Miocene of Um Gheig are
consisting mainly of dolomite and subordinate amount of calcite. Also, X-ray diffraction also, reveals that the
dolomite of the Um Gheig area is nearly stoichiometric and ordered. However, the CaCO3 mole % calculated
from the equation of Lumsden (1979) is ranging from 47.38 to 49.78 % with averaging 48.45 %. Therefore, the
studied samples are nearly stoichiometric dolomite. Dolomite stoichiometry indicates formation from waters
with a high Mg/Ca ratio and normal seawater-like fluids under near surface conditions (Kaczmarek and Sibley,
2011 and Zhao and Jones 2012). The crystalline dolomites are more stoichiometric than the dolomicritic
(Lumsden and Chimahusky, 1980).
Geochemistry:
Six samples were selected to determine the chemical composition by using Axios Sequential WD_XRD
Spectrometer, Analytical 2005 in National Research Center laboratories. Table (1) summarizes the distribution
of major and trace elements of Um Gheig dolomites. Normative calcite content varies from 3.77 to 17.14 %,
averaging 10.34 %, while the normative dolomite content ranges from 80.26 to 93.91 %, averaging 87.77 %. . In
this regard, a high degree of stoichiometry as well as ordering of a dolomite is indicators of recrystallization
(Kupecz, et al., 1993).
Table 1: Chemical analysis data of major oxides (%) and trace elements (ppm) in the studied dolostones.
Elements
S. No.
Min.
1
3
5
7
9
10
SiO2
0.3
0.25
0.29
0.45
0.79
0.57
0.25
Al2O3
0.11
0.1
0.11
0.12
0.12
0.19
0.1
Fe2O3
0.12
0.37
0.64
0.99
1.74
0.41
0.12
MnO
0.42
0.53
0.66
0.75
2.2
0.51
0.42
ZnO
0
0.01
0
0
0.01
0
0
MgO
20.43
19.84
17.55
18.35
18.43
20.53
17.55
CaO
30.21
32.15
34.02
33.1
34.75
30.75
30.21
Na2O
0.15
0.21
0.26
0.28
0.29
0.34
0.15
K2O
0.1
0.12
0.12
0.13
0.13
0.22
0.1
SO3
0.18
0.2
0.23
0.25
0.29
0.06
0.06
P2O5
0.03
0.03
0.05
0.08
0.1
0.08
0.03
Cl
0.02
0.01
0
0.01
0.02
0.01
0
L.O.I.
47.93
46.18
46.07
45.49
41.13
46.33
41.13
Nor.Calc.
3.18
8.11
17.14
13.5
16.24
3.89
3.18
Nor.Dolo.
93.45
90.74
80.26
83.94
84.31
93.91
80.26
Sr ppm
150
510
930
1460
2114
4482
150
As ppm
0
100
0
80
152
0
0
Pb ppm
0
200
0
300
186
0
0
Rb ppm
150
164
183
200
183
275
150
Ba ppm
0
0
0
0
0
1523
0
Zr ppm
0
0
0
0
0
666
0
Mg/Ca
0.57
0.521
0.45
0.47
0.45
0.56
0.45
Sr/Ca
0.0005
0.0016
0.0027
0.0044
0.0061
0.0146
0.0005
L.O.I. = Loss of ignition Nor. Dolo. = Normative dolomite
Nor. Calc. = Normative calcite
Max.
Average
0.79
0.19
1.74
2.2
0.01
20.53
34.75
0.34
0.22
0.29
0.1
0.02
47.93
17.14
93.91
4482
152
300
275
1523
666
0.57
0.0146
0.44
0.13
0.71
0.85
0.003
19.18
32.5
0.26
0.14
0.2
0.06
0.01
45.52
10.34
87.77
1608
55
114
193
254
111
0.5
0.005
SiO2 content in the studied samples ranges from 0.25 to 0.79 % with an average of 0.44 %. The carbonate
rocks of the studied succession record a low content of Al2O3 and K2O (average 0.13 and 0.14 %, respectively).
The low content of both Al2O3 and K2O can mainly be attributed to the very low clay content, which is not
detected even by X-ray diffraction analysis. Na2O contents of the studied samples ranges from 0.15 to 0.34 %,
averaging 0.26 %. Positive correlation between Na2O and both K2O and Al2O3 (r= 0.80 and 0.72, respectively)
as well as negative correlation with Cl (r= - 0.3) indicated that Na was related with clay minerals and not to
halite. SO3 contents of the investigated samples vary from 0.06 to 0.29 %, averaging 0.2 %. Positive correlation
between SO3 and both CaO and Fe (r= 0.80 and 0.69, respectively). Consequently, the SO3 content can be
attributed to the presence of the evaporate minerals (gypsum) and pyrite encountered in the studied dolostones.
Fe2O3 contents of the studied dolomite rocks ranges from 0.12 to 1.74 %, averaging 0.71 %. The low values
for the Um Gheig dolomite indicated that the formation of dolomite in the near-surface oxidizing environment
(Choquette and James, 1990). The positive correlation between Fe2O3 and Sr indicates that iron oxy-hydroxides
may be scavenge and uptake Sr, which is an essential function of many microbes (Fig. 3), which is in agreement
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with Abou El Anwar, 2005 and 2007, Shata and Mira, 2010, Nutman, et al., 2010, Parry, 2011, Tribovillard, et
al, 2012, Danise, et al., 2012, Gao, et al, 2012 and Fleurance, et al., 2013.
The Sr concentration in modern supratidal dolomites is typically around 600 ppm. High Sr concentrations
(approaching 1000 ppm) occur in dolomites precipitated from intensively evaporated waters and from
dolomitizing fluids with high Sr/Ca ratios (Morrow, 1990). Sr contents of the investigated dolomite rocks ranges
from 150 to 4482 ppm, averaging 1608 ppm. High content of Sr could be derived from evaporated water and
hydrothermal fluids. This average is very higher than that recorded modern supratidal and the evaporated
dolomite. In general, fine-grained early formed dolomites have higher Sr contents than later-diagenetic coarsely
crystal in dolomites (Tucker and Wright, 1990). Empirical determination of the Sr distribution coefficient in
dolomite yields 0.015 to 0.06 (Budd, 1997, Vahrenkamp and Stewart, 1990 and Banner, 1995). These values are
less than the Sr/Ca ratio of the present day seawater (0.0086) according to Drever, (1988). For the Sr distribution
coefficient in dolomite yield a value of 0.023 to 0.048, formed in condition higher than 150°C (Katz and
Matthews, 1977). The Sr/Ca for the Um Gheig dolomitize fluid ranges from 0.0005 to 0.0146 with an average of
0.005 (Table 1). Consequently, the dolomitization of Um Gheig area formed in temperature < 150°C similar to
Um Bogma dolostones (Abou El-Anwar, 2013, under published).
Fig. 3: Relationship between Fe2O3 % and Sr ppm.
The recorded value of Ba in the studied dolomite is 1523 ppm in sample number 10 which possibly related
to the epigenetic processes. This plane favored circulation of hydrothermal fluids and gave rise to many of the
epigenetic barite which is with agreement to Dill, (1988).
The concentrations of heavy metals in sediments reflect both logical mineralogy and the origin and nature
of sediments (Alagarsamy, 2006 and Anu, et al., 2009). The average of Mn, Ba, Pb, Zn and As content of the
studied dolomites are 4200, 254,114, 111, and 55 ppm, respectively. These values are higher than averages of
carbonates (1100, 10, 9, 20 and 1 ppm, respectively) given by Turekian and Wedepohl, (1961). The enrichment
assemblage of Mn, Ba, Pb, Zn and As could be derived from hydrothermal (Hewett, et al., 1963), Michard, 1989
and Nicholson, 1992) and may be ascended through faults similar to dolomites occur in the Gebel Hadifa,
Tunisia (Al-Aasm and Abdallah, 2006) and Adriatic offshore and Po Plain (Venetian Southern Alps), Italy
(Ronchia, et al., 2012), Bayan Obo, China (Lai and Yang, 2013), Maestrat Basin, Spain (Martín-Martín, et al.,
2013), southern Laurentia, Canada (Azomani, et al., 2013) and Um Bogma Formation (Abou El-Anwar, 2013
under published).
The heavy metals detected in the studied carbonate rocks Um Gheig area (Fe and Mn. Zn and As,) are
mainly originated from supergene and hypogene alteration (Dill, 2010). So, dolomites in the studied area, may
possibly subject to chemical weathering which is in agreement with Yang, et al., 2008, Dill, 2010 and Abou ElAnwar, 2010, 2011 and 2013, under published).
Paleo-environmental condition:
Iron (Fe) and Manganese (Mn) concentrations are indicators of environmental conditions during
dolomitization. The general increase in the mean values of Fe and Mn reflects a decrease in oxidizing conditions
from the earliest dolomite generation to the latest generation (Azomani, et al., 2013). The low values of Fe and
Mn (0.71 % and 0.85 %, respectively) content of dolomite on the Um Gheig Formation suggests oxidizing
conditions consistent with burial settings. The high Mg/Ca ratio (= 0.5, Table 1) of the studied dolomites in
potential source region reflects high content of dolomite, a mineral with a stoichiometric Mg/Ca ratio of 1:1.
This potential source region might ultimately be derived from the marine strata in arid condition (Li et al., 2007
and Li, et al., 2013).
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Model of Dolomites formation:
The major and trace element geochemical characteristics (Table 1) show that mineralized dolomites have
distinctively different characteristics compared to sedimentary carbonates and some characteristics similar to
carbonatites, (e.g., high MnO and Sr) contents. Sedimentary carbonates are concentrated near the origin in the
SrO-MnO diagram compared to the carboantite (Yang and Le Bas, 2004). The mineralized dolomites of Um
Gheig Formation fall into the fractional crystallization trend of carbonatite magma. A plausible explanation is
that mineralization of the dolomites of Um Gheig area might have been resulted from hydrothermal
metasomatism (Fig. 4).
Fig. 4: Diagram of SrO v.s. MnO (after Yang, et al., 1998 and Yang and Le Bas., 2004). Field 1 falls into the
area of sedimentary carbonate rocks, fields 2 and 3 are falling into the area of igneous carbonatite. The
dashed lines show the trend of fractional crystallization of carbonatite magma, based on the study of
Clarke, et al. (1994).
Based on the discussion above, we propose that mineralized dolomites resulted from hydrothermal, have
distinctively different characteristics compared to sedimentary carbonates, and some characteristics similar to
carbonatites, (e.g., high MnO and Sr) contents. The average of Mn and Sr are 4200 and 1608 ppm in the studied
dolostones. These values are very higher than the average of marine carbonates (1100 and 610 ppm,
respectively) given by Turekian and Wedepohl (1961). Consequently, metasomatism of the sedimentary
carbonate rocks of Um Gheig Formation occurred by carbonatitic magma and/or associated fluids through
faults, which is in agreement with Lai and Yang, (2013). The hydrothermal fluids played an important role in
this process and the resulting dolomites exhibit a major control on the distribution of faults in the Um Gheig
carbonates. Hydrothermal dolomites are formed under burial conditions from high salinity fluids at temperatures
higher than the ambient temperature of the host formation (Davies and Smith, 2006 and Azomani, et al., 2013).
Temperature of dolomitization must be at least 5 Cº greater than the maximum burial temperature of the host
formation for the resultant dolomites to be classified as hydrothermal in origin (Davies and Smith, 2006).
Therefore, the dolostones of Um Gheig Formation formed at high salinity and temperature and could be
classified as hydrothermal in origin, which is in agreement with Davies and Smith, 2006 and Azomani, et al.,
2013.
Consequently, the dolomitization of the host rock is spatially associated with the basement faults, and thus
is fault controlled. Hydrothermal dolomitizaton indirectly documents a fault activity in the studied area. As the
rifting further developed, sedimentary rocks in the Um Gheig Formation underwent thermal metasomatism
along faults, which is in agreement with Al-Aasm and Abdallah, 2006, Nader, et al., 2006, Ronchia, et al., 2012,
Lai and Yang, 2013, Martín-Martín, et al., 2013, Azomani, et al., 2013 and Abou El-Anwar, 2013 (under
published).
Pollution and contamination:
Metal concentration is controlled by varied parameters such as nature of substrate, dissolution and
precipitation of metals, the pollution sites etc. Sediments have the ability to evidence the history and indicate the
degree of contamination and pollution (Anu, 2009 and Nobi, et al., 2010). Contamination factor (CF) is the ratio
between the sediment element concentration at a given site and the background value of the metal (Turekian and
Wedepohl, 1961). CF is considered to be an effective tool in monitoring the pollution over a period of time.
3668
J. Appl. Sci. Res., 9(6): 3659-3673, 2013
CF = Measured concentration of the element / Background value of the element
If CF is >1 for a particular metal, it means that the sediment is contaminated by that element and if CF is
<1, then there is no metal enrichment by natural or anthropogenic inputs (Raj and Jayaprakash, 2007).
The present study shows that only Al, K, Na, Si, S and P had no significant enrichment, whereas the other
entire element remained within the high contaminated to uncontaminated state (Table 2). Thus, with respect to
the background concentration values, the present study found that the dolomites of the Um Gheig area are seen
to be highly polluted with As followed by Rb, Ba, Pb, Zr, Mn, Na, Sr, Zn and Fe (Table 2 and Fig. 5).
Table 2: The contamination factors of the studied dolostones and those of Um Bogma Formation (after Abou El-Anwar, 2013, under publ.).
Element
The studied area (Um Gheig Formation)
Um Bogma Formation
1
3
5
7
9
10
Min
Max
Average
Min
Max
Average
Si
0.03
0.03
0.03
0.03
0.03
0.04
0.03
0.04
0.03
0.65
12.87
2.29
Al
0.33
0.29
0.33
0.5
0.88
0.64
0.29
0.88
0.5
0.31
2.5
0.9
Fe
0.24
0.79
1.36
2.1
3.7
0.88
0.24
3.7
1.51
1.73
82.79
35.92
Mn
3
3.73
3.64
5.27
15.46
3.64
3
15.46
5.79
0.07
10.49
2.55
Mg
2.62
2.55
2.32
2.56
2.37
2.63
2.32
2.63
2.51
0.05
2.57
2.11
Ca
0.72
0.76
0.8
0.78
0.82
0.73
0.72
0.82
0.77
0.13
0.9
0.65
Na
2.75
4
5.25
5.25
5.5
6.25
2.75
6.25
4.83
ND
1.75
0.73
K
0.3
0.37
0.37
0.41
0.4
0.67
0.3
0.67
0.42
0.15
1.59
0.65
S
0.58
0.82
0.758
0.836
1
0.33
0.33
1
0.72
0.08
2.5
0.49
P
0.25
0.25
0.5
1
1.1
1
0.25
1.1
0.68
0.33
0.98
0.38
Cl
1.33
0.67
0
0.67
1.33
0.67
0
1.33
0.78
0.67
10.67
2.67
Sr
0.25
0.84
1.53
2.93
3.47
7.35
0.25
7.35
2.73
ND
7.37
1.58
As
0
100
0
80
1542
0
0
1542
287
ND
577
184
Pb
0
22.22
0
33.33
20.67
0
0
33.33
12.7
ND
11.11
3.6
Rb
50
54.67
61
66.67
61
99.67
50
99.67
65.5
ND
35
18.77
Ba
0
0
0
0
0
152.3
0
152.3
25.38
ND
21
80
Zr
0
0
0
0
0
35.05
0
35.05
5.84
ND
29.11
7.05
Ni
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
8.75
2.5
Co
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
5400
2270
Cu
ND
ND
ND
ND
ND
ND
ND
ND
ND
10.5
584
92.25
Zn
0
5
0
0
5
0
0
5
1.67
0.45
20
80
W
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
241
55
Y
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
3.5
1.84
ND = Not detrained
Consequently, CF of the studied heavy metals indicated that the studied area subjected to pollution. Abou
El Anwar (2013, under published) concluded that the dolomites of the Um Bogma area are seen to be highly
polluted with Co followed by As, Cu, Zn, Ba, W, Fe, Rb, Zr, Pb, Mn, Si, Ni, Mg, Y, Sr and Ti (Table 2).
Consequently, the studied dolostones of the Um Gheig Formation are highly polluted in As, Rb, Pb, Mn, Mg
and Sr than those in Um Bogma Formation, whatever, dolostones of the Um Bogma Formation are highly
polluted in Co, Ba, Zn, Fe and Zr than those of the Um Gheig Formation (Table 2 and Fig. 6).
Fig. 5: The contamination factor of the studied dolostones.
3669
J. Appl. Sci. Res., 9(6): 3659-3673, 2013
Fig. 6: The contamination factor of the studied dolostones and those of Um Bogma Formation.
The geo-accumulation index (Igeo) is a quantitative measure of the metal pollution in aquatic sediments
(Ranjan, et al., 2008).
Igeo = log2Cn / 1.5Bn
Where Cn is the measured content of element, and Bn is the background value of the same element
(Turekian and Wedepohl, 1961). The contamination level may be classified in a scale ranging from 1 to 6 as
follows: Igeo 0 - ¼ unpolluted, Igeo < 1 ¼ unpolluted to moderately polluted, Igeo < 2 ¼ moderately polluted,
Igeo < 3 ¼ moderately to strongly, Igeo < 4 ¼ strongly polluted, Igeo < 5 ¼ strongly to very strongly polluted,
Igeo > 5¼ very strongly polluted (Loska, et al., 1997 and Macias, et al., 2006).
In the present study, (Igeo) was used to understand the heavy elements reported for marine sediments were
taken as the background values. Present results of geoaccumulation index (Table 3 and Fig. 7) reveal that
sediments of the Um Gheig are remaining unpolluted to very strong pollute. Whatever, Fe, Mn and Sr
unpolluted to moderately polluted, Zr is moderately polluted, Zn and Pb are moderately to strongly polluted as
well as As, Rb and Ba are classified as very strongly polluted (Loska, et al. 1997) and Macias, et al. (2006),
Table (3) and (Fig. 7). Abou El Anwar (2013, underpublished) mentioned that the geoaccumulation index of
dolostones of Um Bogma Formation reveals that those are highly polluted with Co, followed by As, Cu and W.
So, the dolostones of the Um Gheig are highly polluted in Rb, Ba and Pb than those of Um Bogma Formation
(Table 3 and Fig. 8).
Table 3: The geo-accumulation index (Igeo) of the studied dolostones and those of
under publ.).
Element
The studied area (Um Gheig Formation)
1
3
5
7
9
10
Min
Si
0.005
0.005
0.005
0.005
0.005
0.008
0.005
Al
0.15
0.13
0.15
0.23
0.39
0.29
0.13
Fe
0.048
0.158
0.274
0.42
0.742
0.176
0.048
Mn
0.6
0.748
0.93
1.058
3.1
0.93
0.6
Zn
0
7.53
0
0
7.53
0
0
Na
0.552
0.803
1.053
1.053
1.104
1.254
0.552
K
0.059
0.074
0.074
0.082
0.802
0.133
0.059
S
0.117
0.134
0.151
0.167
0.201
0.067
0.067
P
0.05
0.05
0.1
0.2
0.22
0.2
0.05
Cl
0.268
0.134
0
0.134
0.268
0.134
0
Sr
0.05
0.168
0.306
0.481
0.695
1.475
0.05
As
0
20.07
0
16.054
30.5
0
0
Pb
0
4.46
0
6.689
4.148
0
0
Rb
10.03
10.97
12.24
13.38
12.24
18.4
10.03
Ba
0
4.014
0
0
0
30.45
0
Zr
0
0.528
0
0
0
7.04
0
Ni
ND
ND
ND
ND
ND
ND
ND
Co
ND
ND
ND
ND
ND
ND
ND
Cu
ND
ND
ND
ND
ND
ND
ND
Zn
ND
ND
ND
ND
ND
ND
ND
W
ND
ND
ND
ND
ND
ND
ND
Y
ND
ND
ND
ND
ND
ND
ND
ND = Not determined
Um Bogma Formation (after Abou El-Anwar, 2013,
Max
0.008
0.39
0.742
3.1
7.53
1.254
0.802
0.201
0.22
0.268
1.475
30.5
6.689
18.4
30.45
7.04
ND
ND
ND
ND
ND
ND
Average
0.006
2.23
0.303
1.23
2.51
0.97
0.204
0.14
0.137
0.156
0.529
11.04
2.55
12.88
5.744
1.27
ND
ND
ND
ND
ND
ND
Min
0.14
0.07
0.34
0.01
0.09
ND
0.03
0.02
0.04
0.14
ND
ND
ND
ND
ND
ND
ND
ND
2.26
0.09
ND
ND
Um Bogma
Max
Average
2.58
0.52
0.62
0.3
10.08
1.77
2.11
0.52
4.01
1.61
0.35
0.11
0.32
0.13
12
1.29
0.2
0.08
2.14
0.54
1.48
0.32
323
59.43
2.23
0.82
7.02
3.77
4.21
1.61
5.84
1.52
1.76
0.5
1084
454
117
18.5
4.01
1.61
80.61
18.5
0.7
0.37
3670
J. Appl. Sci. Res., 9(6): 3659-3673, 2013
Fig. 7: The geo-accumulation index (Igeo) of the studied dolostones. Um Gheig.
Fig. 8: The geo-accumulation index (Igeo) of the studied dolostones and Um Bogma Formation (after Abou ElAnwar, 2013, under publ.).
Finally, the contamination factor and geoaccumulation indexs reveal that dolostones of the Um Gheig are
strong polluting in Rb and Pb than dolostones of Um Bogma Formation. Whatever, dolostones of Um Bogma
Formation are very strong polluting in Co, As, S and Fe than those of Um Gheig area. Consequently, the CF and
Igeo indicated that the Um Gheig dolomite rocks are highly polluted by elements such as As, Rb, Ba, Pb, Zr,
Mn, Sr and Fe.
Conclusions:
Um Gheig dolostones are mainly crystalline, consisting of non-planer and subhedral rhombs. Diagentically,
the studied carbonate rocks are subjected to neomorphism, compaction, dissolution, fracture fillings and
dolomitization. Dissolution that resulted in the formation of vugy and moldic pores likely reflects vadose zone
conditions. Post-depositional compaction is well illustrated in the form of grain to grain sutured contacts.
Mineralogically, XRD revealed that the studied samples consist mainly of dolomite and subordinate calcite.
The dolostones of the Um Gheig Formation is nearly stoichiometric and ordered. Geochemically, the studied
dolomites of Gheig Formation are characterized by elements derived from hydrothermal solution. Dolostones of
Um Gheig area formed at temperature <150°C. Trace element chemistry indicates that the dolomitize fluids may
be originated from deeper hydrothermal solution ascended through faults. The heavy elements detected in the
studied carbonate rocks Um Gheig area are mainly originated from supergen and hypogen alteration and may
possibly be subject to chemical weathering. The CF (Contamination factor) and Igeo (Geo-accumulation index)
have indicated that the Um Gheig dolomite rocks are subjected to highly polluted in As, Rb, Zn, Ba and Pb.
The petrographical, mineralogical and geochemical studies of the studied Um Gheig dolostones indicated
that their origin can be explained by two hypotheses. The first is dolomitization of partially stabilized
magnesium calcite sediments. The second hypothesis invokes two dolomitization events-an earlier, pervasive
dolomitization by marine or hyper-saline water, followed by recrystallization in meteoric or hydrothermal fluids.
Both hypotheses are consistent with the geologic setting of these sediments. Consequently, the dolomitization
the studied Um Gheig dolostone is formed by marine or hyper-saline water, followed by recrystallization in
meteoric or hydrothermal fluids.
The dolomitization of the host rock is spatially associated with the basement faults, and thus is fault
controlled. As the rifting further developed, sedimentary rocks in the Um Gheig Formation underwent thermal
3671
J. Appl. Sci. Res., 9(6): 3659-3673, 2013
metasomatism along faults (which is in agreement with Al-Aasm and Abdallah, 2006, Ronchia, et al., 2012, Lai
and Yang, 2013, Martín-Martín, 2013, Azomani, et al., 2013 and Abou El-Anwar, 2013).
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