leucogranite and G3: riebeckite & graphic granites ± syenite, albitite) with ultramafic and mafic intrusions. Gneisses, amphibolites ± high-grade schist and.
Proceeding of the 8th Conf. Geolog of Sinai for Development Ismailia, 2007, pp.
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT. El-Arabi H. Shendi 1, Ismail, A. M. 2and Tamer E. Attia 1 Department of Geology, Faculty of science,Suez Canal University, Ismaila.1 Egyptian General for Mineral Resource Authority (EGMRA).2 ABSTRACT The present study gives the light to the processing and interpretation of potential gravity and magnetic methods in order to detect the promising sites for expected mineral occurrences in the basement rocks in south Sinai. The two promising sites were selected by the point view of geophysical potential field methods and the nearly likeness between geological environments of the already known sites of mineralization and the new ones. Each site was profiled and interpreted to the nearly approximated subsurface feature model and the geometrical parameters of these two bodies such as depth, thickness, width, dipping, density contrast, and magnetic susceptibility contrast could be delineated.
1-Introduction:
2-Geology of area:
The area of study lies in the south
In the south Sinai, a complex of
of Sinai Peninsula between the Gulf of
igneous
Suez and Gulf of Aqaba. It bounded by
primarily Pre-Cambrian age, the evolution
latitudes 27 35´ to 29 30 N and
of
longitudes 33 18´ to 34 38´ E. It covers
interpreted on the basis of the classical
an area of approximately about 23,000 km2
geosynclinal orogenic cycle model. All its
(Fig 1).
rock units were formed during successive
these
and
metamorphic
rocks
was
rocks
of
conventionally
The main target of this study is
stages of its evolution i.e., early-, syn-, and
trying to use the available geological
late-orogenic and epiorogenic stage. (El
gravity and magnetic data of the basement
Ramly & Akaad, 1960, Sabet, 1972, El
rocks of the south Sinai in order to identify
Shazly, 1964, Akaad & Noweir, 1980).
the lithological and structural features,
(Hume, 1934, Issawi & jux, 1982)
which may control the occurrences of
reported the igneous rocks of Phanerozoic
mineralization (metallic minerals).
age are recognized within the basement
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
complex as ring complexes, dykes and sills
2-2Tectonics and structural setting:
as well as volcanic flows and pyroclastics.
The Sinai Peninsula is wedged between the African and Arabian plates the boundaries of which are defined by the Gulf of Suez and Gulf of Aqaba-Dead sea rift system. In the south Sinai, exposed PreCambrian igneous and metamorphic rocks from the Arabo-Nubian shield. Field and petrographic evidence indicate that the shield consists of a series of island arcs which where cratonized during the late Proterozoic-early Paleozoic (1200 to500 my B.c.) Pan-African orogeny. (Gass, 1981). Their pale-surface of the shield dips
Fig.1: Base map of the study area south Sinai, Egypt
gently northward with the consequent 2-1General stratigraphy:
thickening of the overlying sediments,
A complete section of Phanerozoic
ranging from Cambrian to Recent. From
rocks is exposed in the area non-
the south, where the strata are practically
conformably overlying the basement rocks.
horizontal, the structure of the sedimentary
The latter are composed mostly of granites
cover
(G1:
granodiorite
complex towards the north. The surface
muscovite,
tectonic trends as traced from the tectonic
leucogranite and G3: riebeckite & graphic
map of north and central Sinai (after Neev
granites ± syenite, albitite) with ultramafic
1975, and Agah, 1981), (Fig.2).
diorite,
association,
and
mafic
G2:
tonalite, biotite,
intrusions.
Gneisses,
amphibolites ± high-grade schist
becomes
progressively
more
The predominant directions of the fault
and
systems within Egypt based on the analysis
Dokhan volcanics are common within the
of the linear features presented on the
basement sequence. (Geological Atlas of
magnetic map of Egypt are recognized by
Sinai, 2004).
(Ahmed and Hassaneen, 1985) as follow:
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
The basement rocks are the oldest rocks in Egypt; they are expected to record all tectonic events affecting Egypt, since the early Pre-Cambrian to the present. The surface tectonic trends show a large number of tectonic elements of varying length and trends in areas where basement rocks crop out and in areas of relatively thin sedimentary cover. (Said, 1990).
3-Potential field methods: The main goal of studying gravity and magnetic data is to provide better understanding
of
the
formation
and
location of mineralization in the basement of south Sinai. This because, the gravity and magnetic fields are potential fields and thus obeys Laplace’s equations. Both methods are based on measurements on or Fig.2: Structural map of the study area of south Sinai, modified after (Neev 1975, and Agah, 1981). parallel anticlinal trends, which
near the surface of the total earth fields. The fundamental problem then is to extract
The N-S direction is predominant.
the component of the total fields due to
The E-W direction (Mediterranean Sea
sources with in the depth and extent of
trend).
interest.
The NE-SW direction is represented by
The magnetic anomalies caused by
five directions 10, 25, 45, 50 and
subsurface magnetic susceptibility contrast
70 from the north (ENE is the Syrian
are difficult to determined because there
arc trend and NNE is the Gulf of
are more than one factor controlling the
Aqaba trend).
construction of the magnetic anomalies
The NW-SE direction is represented by
such as direction of the main earth’s
three directions 15, 25 and 40 from the north (Gulf of Suez trend).
magnetic field in addition to the inclination and the declination of the causative body with respect to the direction of magnetic
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
north, in contrary, the gravity anomalies
29° 30´
North
caused by subsurface density contrast are easier to locate because the shape resulting from given gravity anomalies, are the function
of
the
subsurface
29°
density
distribution only. Consequently, gravity anomalies, unlike magnetic anomalies.
28° 30´
3-1Potential fields maps: 3-1-1-Bouguer gravity map: The Bouguer gravity map of the
28°
studied area is complied with scale of 1:500,000 and 1 mgal. contour interval, 27 30
(Fig.3). (Ismail, et.al., 2001).
33º
3-1-2-Magnetic map:
34º 30´
34º
33º 30´
35º
Fig.4: Total magnetic map of the study area of south
The total magnetic anomaly map of
Sinai, modified after (Ismail, et.al., 2001).
the studied area is compiled with a scale of 1:500,000 and 5 nT contour interval, The Fast Fourier Transform (FFT)
(Fig. 4) (Ismail, et.al.,2001).
was applied on the gravity and magnetic 29° 30´
data for calculating the energy spectrum curve. To estimate the depths of residual (shallow sources) and regional (deep
29°
28° 30´
sources).
Deep gravity sources Deep gravity sources
Shallow Shallow gravity gravity sources sources
Noise
28°
27 30 33º
33˚ 30´
34º
34º 30´
35º
Fig.3: Bouguer gravity map of the study area of south Sinai, modified after(Ismail, et.al., 2001).
Fig.5: Redially averaged power spectrum curve of the Bouguer gravity map.
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
(Fig.9) respectively, where the residual gravity and residual magnetic (Fig.8) & (Fig.10), respectively, are computed. magnetic DeepDeep gravity sources
sources
Shallow magnetic sources
29° 30´
North
Shallow gravity sources Noise
29°
28° 30´
Fig.6: Redially averaged power spectrum curve of the Total magnetic map. 28°
On (Fig.5) two mean average levels at depths 5.57 and 18.25 Km were revealed for the shallow and deep-seated gravity sources respectively. Whereas, (Fig.6) the
27 30 33˚
33º 30´
34º 30´
34º
35º
Fig.7: Regional gravity map of the study aea south
two mean average levels at depths 6.25 and
Sinai.
19 Km were revealed for the shallow and deep-seated gravity sources respectively.
29° 30´
North
3-3- Residual-Regional separation: 29°
Usually in gravity and magnetic surveying, the local anomalies are of prim interest and first step in interpretation is the
A1 28° 30´
B2
removal of the regional field to isolate the A2
residual anomalies. The technique, originally described
B1
by (Griffin,
1949),
is the way of
28°
calculating the regional field and give satisfactory results. The regional gravity and regional magnetic fields (Fig.7) &
27 30 33º
33º 30´
34º
34º 30´
35º
Fig.8: Residual gravity map of the study aea south Sinai.
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
29° 30´
North
3-4-Sites
of
major
structures
may
forming minerals: The main target in using residual
29°
potential field data that the assumption of shallow structures which depicts low amplitude and high frequency anomaly. The location map of different already
28° 30´
known mineralization was constructed in (Fig.11), illustrates the localities of copper, iron and manganese oxides, pyrite, silver,
28°
and gold as away to know the qualified lithological and structural environments
27 30 33º
34º 30´
34º
33º 30´
35º
Fig.9: Regional magnetic map of the study aea south
that formed these types of mineralization. Two Sites are determined (A1) in the northern west part of the study area and (B1) in the eastern part of the study area in
Sinai.
south Sinai, (Fig.8 & Fig.10). 29° 30´
North
29° 30´
North
29° 29° B2
A1 28° 30´
A2
28° 30´
B1
28° 28°
27 30 33º
33º 30´
34º
34º 30´
Fig.10: Residual magnetic map of the study aea south Sinai.
27 30
35º 33º
33º 30´
34º
34º 30´
35º
Iron and manganese oxides Copper Pyrite Silver & Gold
Fig.11: Previous locations of mineralization ocurrences in south Sina.
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
The residual gravity anomaly in the site (A1) is nearly elongated, striking NWSE and most of different mineralizations collected previously are disseminated from inside to outside of this closure. The
site
(B1)
had
perfectly
elongated anomaly, striking NNE-SSW and
had
the
pervious
mineralization
accumulated on the outer border of this closer. However, the residual magnetic anomalies in the sites (A1 & B1)
Fig.12: Rose digram showing the total average tectonicnic trends in the site (A1).
considered another image that ensures the existence of (previous mineralization) the gravity
anomalies
shifts
average trends arranged accordingly to the
according to the difference in the physical
priority as follow N10E to N20E and N40E
property and polarity which these two
to N50E (Fig.13), and these all average
potential methods measure. The residual
trends mentioned above are nearly the
magnetic
same trends of the both residual gravity
anomaly
with
in
the
some
In the site (B1), there are two main
two
sites
composed of two closers the northern
anomalies
and
the
residual
magnetic
closer is low relatively to the southern one,
combined anomalies. So the previous
depicts a shallow structure striking NW-SE in the site (A1) and a shallow structure striking NNE-SSW in the site (B1).
According to the structural print in these two sites, the site (A1) had been affected
by
many
structural
trends
illustrated from the tectonic map of north and central Sinai (after Neev 1975, and gah, 1981) the main trend is the averaged value from N30W to N40W (Fig.12).
Fig.13: Rose digram showing the total average tectonicnic trends in the site (B1).
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
mineralization occurrences are strongly expected to be controlled by structure.
Two sites (A2 & B2), (Fig.8 & Fig.10) were expected to be the sites of mineral occurrences because of their lithological and structural compatibility with the previous mineral occurrences sites (A1 & B1).
The
same
lithology
is
extended from the site (A1) to site (A2) and also from the site (B1) to the site (B2).
Fig.14: Rose digram showing the total average tectonicnic trends in the site (A2).
The residual gravity anomalies in the site (A1) are nearly elongated, striking
and N40W to N50W respectively (Fig. 14),
NW-SE
same
according to their priorities, and this trend
direction, similarly to the anomaly in the
is nearly the same average trend of
site (A1).
anomaly forming the site (A1), so the
The site (B2) have perfectly elongated
mineralization may be expected to occur
anomaly, striking NNE-SSW direction,
and control by structure.
direction
giving
the
giving the same concept forming the anomaly in the Site (A2). Besides to that, the residual magnetic view in the Sites (A2 & B2) considered also another image qualifying the existence (expected mineral occurrences) of the gravity anomalies with some shifts according to the difference in the physical property and polarity which these two potential methods measure. According to the structural print in these two expected sites, the site (A2) has been affected by many structural trends, the main two average trends are arranged as follow N-S to N10W, N30W to N40W
Fig.15: Rose digram showing the total average tectonicnic trends in the site (B2).
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
The site (B2) has been affected by many
The density contrast between the
structural trends, there are two main
causative body and its surroundings in the
average trends arranged as follow N10E to
four sites were (0.304, 0.313, 0.231 &
N20E and N30E to N40E respectively
0.270g/cm3) respectively. The dip angles
(Fig.15), according to their priorities, and
of the causative body in the four profiles of
these average trends are nearly the same
sites (A1, B1, A2 & B2) were (20º,----, 18
trends of anomaly forming the site (A2), so
º, 156º) respectively. The depth to the top
the mineralization also may be expected to
of the causative body in four profiles
occur and control by structure.
within the sites (A1, B1, A2 & B2) were (1458, 1432, 1883 &1237m) respectively,
3-5-Results of the interpreted potential fields profiles:
table (1).
3-5-1-Residual Gravity profiles of the site (A1, B1, A2 & B2):
3-5-2-Residual Magnetic profiles of the site (A1, B1, A2 & B2):
Four profiles were selected for
Four profiles were selected for
interpretation, one profile for each residual
interpretation, one profile for each residual
gravity anomaly forming the sites (A1, B1,
magnetic anomaly forming the sites (A1,
A2 & B2). The foure profiles are modeled
B1, A2 & B2). The foure profiles are
in the Gravmod of the Geosoft, 1994.
modeled in the Magmod of the Geosoft,
The
residual
gravity
anomalies
1994.
located in sites (A1, B1, A2 & B2)
The residual magnetic anomalies
elongated in the direction of (N60W, N8E,
located in sites (A1, B1, A2 & B2) had two
N45W & N20E) had values ranging from
combined anomalies stroked the direction
(1.5-12, 0-6.5, 0-8 & 0.5-7mgal) were
of (N42W, N14E, N48W & N21E) have
stroked by four profiles in the direction of
values ranging from (-70 to 80, -290 to
(N50E,
N75W)
110, -30 to 80 & -160 to 70nT) were
approximation
stroked by four profiles in the direction of
N80W,
respectively.
The
N53E best
&
fitting of four profiles in the sites (A1, B1,
(N56E,
N77W,
A2 & B2) were done with the (Tabular2
respectively.
model, Wedge model, Tabular2 model,
fitting of four profiles in the sites (A1, B1,
Tabular2 model) respectively (Fig.16 (a),
A2 & B2) were done with the (Tabular2
(b), (c) & (d)).
model, Step model, Tabular2 model,
The
N23W best
&
N69W)
approximation
Tabular2 model) respectively (Fig.17 (a),
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
(b), (c) & (d)). (Fig.20, 21, 22 & 23). The
2-There is a compatibility agreement
magnetic susceptibility contrast between
between the residual gravity anomalies and
the causative body and its surroundings in
residual magnetic anomalies in the new
the four sites were (854x10-6, 216x10-5,
sites (A2: in the south western Sinai &B2:
-6
899x10
-5
& 105x10
emu) respectively,
table (2).
in the south eastern Sinai). 3-The major subsurface causative body
The dip angles of the causative
modeled in the site (A2) has the depth to
bodies in the four profiles of sites (A1, B1,
the top (1432, 1439m) in the gravity and
A2 & B2) were (19, 167º, 20º, 170º)
magnetic modeling respectively. The major
respectively. The depth to the top of the
subsurface causative body modeled in the
causative body in four profiles within the
site (B2) has the depth to the top (1237,
sites (A1, B1, A2 & B2) were (1467, 1439,
1243m) in the gravity and magnetic
1894 & 1243m) respectively, table (2).
modeling respectively.
4- Conclusion:
4-Ferther detailed geophysical prospecting
1-The structural environments are nearly
work is recommended in the sites (A2 &
the same between the already known sites
B2)
of mineralization (A1 & B1) and between
mineralization zones.
the
new
sites
expected
for
to
locate
the
more
potential
mineral
occurrences (A2 & B2), concluded that the mineral occurrences may be controlled by the structural. Table (1): Geometrical prameters of the four residual gravity profiles in gravmode modelling.
Table (2): Geometrical prameters of the four residual magnetic profiles in magmode modelling.
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
Site (A1) residual gravity profile Tabular 2 model
(a) Site (A2) residual gravity profile Tabular 2 model
Site (B1) residual gravity profile Wedge model
(b) Site (B2) residual gravity profile Tabular 2 model
156
(d) Fig.16: The four modeled residual gravity profiles: (a) Residual gravity profile in the site (A1), Tabular 2 model. (b) Residual gravity profile in the site (B1), Wedge model. (c) Residual gravity profile in the site (A2), Tabular 2 model. (d) Residual gravity profile in the site (B2), Tabular 2 model.
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
Site (A1) residual magnetic profile Tabular 2 model
(a) Site (A2) residual magnetic profile Tabular 2 model
(c)
Site (B1) residual magnetic profile Step model
(b)
Site (B2) residual magnetic profile Tabular 2 model
(d)
Fig.17: The four modeled residual magnetic profiles: (a) Residual magnetic profile in the site (A1), Tabular 2 model. (b) Residual magnetic profile in the site (B1), Step model. (c) Residual magnetic profile in the site (A2), Tabular 2 model. (d) Residual magnetic profile in the site (B2), Tabular 2 model.
PROCESSING AND INTERPRETATION OF GRAVITY AND MAGNETIC DATA TO LOCATE NEW SITES OF EXCPECTED MINERAL OCCURRENCES IN THE BASEMENT OF SOUTH SINAI, EGYPT.
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