An Analysis of Airborne GaSamma Ray Spectrometric Data of ... · Early gamma ray spectrometric surveys were used almost exclusively for the direct detection of U and Th orebodies

JAKU: Earth Sci., Vol. 23, No. 2, pp: 19-42 (2012 A.D. / 1433 A.H.)

DOI: 10.4197 / Ear. 23-2.2

19

An Analysis of Airborne GaSamma Ray Spectrometric

Data of Gabal Umm Naggat Granitic Pluton, Central

Eastern Desert, Egypt

Sami Hamed Abd El Nabi

Geophysics Department, Faculty of Science, Ain Shams University,

Abbassia 11566, Cairo, Egypt

E-mail address: [email protected]

Received: 26/09 /2011 Accepted: 04/06/2012

Abstract. The present work deals with the analysis of the airborne

gamma ray spectrometric data of Gabal Umm Naggat granites, Central

Eastern Desert, Egypt. It shows distinct spectral pattern of the

radioelement zoning which is related to the associated mineralogical

variation of such granitic pluton. The pluton is divided into three

spectral domains: northern, middle and southern. The northern domain

(A) is characterized by high radioelements contents and high ratios. It

is characterized by high Zr, Nb, Y, Th, U and HREE in the most

albitized granite varieties. The middle domain (B) is K, Th rich

reflects the alkali feldspar rich granite. The southern domain (C) is

characterized by high three radioelements contents, but lower than the

northern one. Such zonation is evidenced by the results of

geochemical analysis of previous works.

To assess the potentiality of the gamma ray spectrometric

signatures of areas of change associated with hydrothermal Nb-Ta-Sn

occurrences, two approaches such as residual anomaly method on the

basis of lithologic normalization (Th-normalized approach) and the F-

parameter of Effimov approach are applied. Low anomalous

potassium (Kd) that connected with high anomalous uranium variation

(Ud), and high F-parameter anomaly strip highlight areas of change

associated with hydrothermal Nb-Ta-Sn occurrences.

Keywords: Spectral domains, Th-normalized approach, Umm Naggat

granites.

20 Sami Hamed Abd El Nabi

Introduction

Early gamma ray spectrometric surveys were used almost exclusively for

the direct detection of U and Th orebodies (back to the years following

World War II). However, with the development of spectrometers in the

1960s, the scope of the gamma ray method was expanded to include

base-metal mineral exploration, geological and environmental mapping.

In base-metal mineral exploration the radioelement data can be used for

the indirect detection for hydrothermal alteration of host rock that can

give measurable gamma spectrometric haloes aiding to such metal

deposits. Porphyry Cu-Au deposits (Pires 1995), Pb and Zn deposits

(Gnojek and Prichystal, 1985), potash prospecting in Qinghai, Xinjiang,

China, (Zhang et al., 1998) and gold in quartz veins (Hoover & Pierce

1990; and Kuosmanen, 1994). These may be detected through potassium

alteration, and gold through Au-U associations. The interpretation of K

alteration from radioelement data has led to several mineral discoveries

(Dickson and Scott 1997; and Shives, et al. 1997& 2000). Further

examples are given in other IAEA publications. In addition, it was used

for many oil and gas exploration (Saunders & Terry 1985; and Saunders

et al., 1987). However, an understanding of how radiometric surveys can

be applied to exploration problems requires us to consider the geological

sources of radioactivity.

Over the last ten years, several geological/geochemical studies have

been undertaken that seek to identify the rare-metal (Li–F) tantalum–tin-

tungsten granite deposits in Umm Naggat granites. The present work

highlights the role of these results which in turn would assist in analysis

of the airborne radiometric signatures of this highly prospective region.

Such radiometric signatures are used to provide further insight in the

origin of the radioelements zoning pattern of Umm Naggat granites and

to assess the potential of the gamma ray spectrometric method

characterization of areas of change associated with hydrothermal Nb-Ta-

Sn occurrences. This is considered as a step to maximize the potential of

the radiometric approach for mineral exploration and to establish a

radiometric database reference for geophysical exploration in the Eastern

Desert region of Egypt.

An Analysis of γ-ray spectrometry of G. Um Naggat granitic pluton… 21

Airborne γ-Ray Spectrometric Survey

Airborne γ-ray spectrometric data of the G. Umm Naggat area,

Central Eastern Desert (CED), Egypt, were obtained as a part of an

airborne spectrometric and magnetic surveying programme carried out in

1984, by Aero-Service Division, Western Geophysical Company of

America USA. Such surveys were conducted along parallel flight lines

that were oriented in a NE-SW direction, at 1.0 km spacing. Meanwhile,

the tie lines were flown perpendicularly in a NW-SE direction, at 10 km

intervals. Multichannel radiospectrometric measurements were made at

93 m intervals at a nominal sensor altitude of 120 m terrain clearance. A

high-sensitivity 256-channel airborne gamma-ray spectrometer (50 l NaI

“Tl” crystals) mounted in a tail-stinger configuration, was the primary

sensor elements in the Aero-Service CODAS/AGRS 3000F computer-

based digital data acquisition system (Aero-Service, 1984). The obtained

airborne radiometric survey data were reduced, compiled and finally

presented in the form of contour and composite profile maps at a scale of

1:50,000.

Geological Setting of G. Umm Naggat Area

The Gabal Umm Naggat area lies in the central part of the Egyptian

Eastern Desert, particularly extends from 25o

25' 00

'' to 25

o 32

' 30

'' N

latitudes and 34o 07

' 30

'' to 34

o 17

' 30

'' E longitudes (Fig. 1).

Fig. 1. Location map of G. Umm Naggat area, central Eastern Desert, Egypt.


The regional geological setting of such an area is depicted in Fig. 2.

Exposed Precambrian igneous and metamorphosed rocks constitute most

of the terrain in the area. The G. Umm Naggat granite pluton was

emplaced at shallow crustal levels in a Pan-African island arc assemblage

of calc-alkaline metavolcano-sedimentary association and metagabbro-

diorite (Hassanen et al., 2008). In the country rocks and in the Um

Naggat massif, as well, Cretaceous-Paleogene dykes of quartz

porphyries, felsite porphyries, trachytes, dolerites and basalts are widely

distributed. In the central and southern parts of the massif, the granite is

cut by trachyte necks associated with relics of trachyte tuffs, tuff and lava

breccias. Many of them are located along submeridional faults and in the

feathering disturbances (Sabet, 1961).

Fig. 2. Geological map of G. Umm Naggat area, central Eastern Desert, Egypt (After Sabet

et al., 1973).

The internal structure of the massif is complex and considered as an

asymmetrical zones of gradual transions for granite facies. Such facies

are successively replaced by one another from the northern contact deep

into the massif. The most albitized varieties are situated in the immediate

vicinity of the northern contact and compose the apical parts of the dome

shaped projections. Then follows silicified and less albitized granite

which is replaced by the brick red microclinized varieties. The central

and southern parts of the massif are mainly composed of unaltered

coarse-grained amphibole granite which may be considered as the


original parent granite (Sabet, 1976a). The Umm Naggat granite is

massive coarse-grained (3-4 mm) with a hypidiomorphic granoblastic

texture, rarely cataclastic. It is composed of quartz (50%) and microcline

(45%) with subordinate amount of amphibole (5%) and biotite in the

altered albitized varieties (5%).

The detection of rare metal mineralization in apogranites (alkali

feldspar granites which have suffered post magmatic, metasomatic

alteration called apogranites in the sense of Beus et al. (1962) of the

albitite type in the Central Eastern Desert of Egypt was discovered in

1970 by a group of Soviet and Egyptian geologists (e.g. Baburin et al.,

1971; Sabet et al., 1976a, 1976b, 1976c). These geologists have

attributed the formation of rare metal mineralization to the metasomatic

alteration processes affecting the host granites. However, a zone about

600m, wide of metasomatically altered granite was traced through a

distance of about 3 Km along the northern endocontact of Umm Naggat

pluton.

In late 2001, Gippsland Limited identified rare-metal (Li–F)

tantalum–tin- tungsten granite deposits in the Umm Naggat granite

massive, Central Eastern Desert, Egypt with Ta2O5 reserves 25 Mt at

0.0151% (Fetherston, 2004).

Hassanen et al., (2008, pers. Commun.) concluded that the Um

Naggat granitic pluton consists of three petrographic types: i) A

subsolvus monzo-and syenogranite, ii) a hypersolvus alkali feldspar

granite and iii) a subordinate roof facies of albite granite that host

greisens and Nb-Ta-Sn mineralization. The least evolved monzogranite is

characterized by high K/Rb ratio (880 - 400), moderately fractionated

REE pattern (Lan/Ybn = 13 - 3) and negative Eu anomalies (Eu/Eu*=0.3

- 0.6). The alkali feldspar granite has lower K/Rb ratio (350-50) and less

fractionated REE pattern (Lan/Ybn = 3) with strong negative Eu

anomalies (Eu/Eu*=0.1 - 0.3). The albite granite is characterized by very

low K/Rb ratio (35 - 15) with less fractionated REE patterns (Lan/Ybn <

1). They added that Fluid fractionation was a major differentiation

process during the terminal stages of evolution of the pluton. It plays an

imprtant role in the genesis of the albite granite which is characterized by

low K/Rb, Eu/Eu* high Zr, Nb, Y, Th, U and HREE. The parent magma

of this granite pluton was formed by fractional crystallization of mantle-


derived mafic magma in a post-orogenic extension-related tectonic

enviroment.

The northern part of Umm Naggat granite massif has suffered

extensive post-magmatic metasomatic reworking which results into the

development of (Zr, Hf, Nb, Ta, U, Th, F)- and albite-enriched and

greisenized apogranite body of 600 m wide, and more than 3 km in the

strike length (Fig. 2). Albitization produced an enrichment in Zr (av.

2384 ppm), Hf (61), Nb (419), and U (43). Localization of disseminated

mineralization of Zr and Nb is registered in the apical parts of the

albitized and greisenized apogranite body (EL–Afandy et al., 2000;

Abdalla et al., 2009).

Interpretation of Gamma Ray Spectrometry

Unlike the other airborne geophysical methods, there are no

mathematical models required to calculate the theoretical radiometric

response of a specific source. Interpretation of radiometric data is,

therefore, more similar to interpreting the results of a conventional

geological survey. It is usually necessary to correlate the results of

geological and/or geochemical sampling with, for example, the colour

patterns in a radiometric ternary map to achieve a full understanding of

the implications of the map. However, an understanding of how

radiometric surveys can be applied to exploration problems requires us to

consider the geological sources of radioactivity.

The Umm Naggat airborne gamma-ray spectrometric survey dataset

are typically displayed as images. Individual radioelements are displayed

as pseudocoloured images (Fig. 4, 5 and 6) or combined as false colour

composites in ternary radioelement image (Fig. 7). Figures (4, 5 and 6)

can be contrast ratioed (e.g. K/eTh, eU/K and eU/eTh) to highlight subtle

variations in the data (Fig. 8, 9 and 10).

Low values of total gamma radiation count (Fig. 3) are associated

with outcrops of the basic metavolcanic rocks occupied the northeastern

and southwestern corners of the mapped area. Areas with high values can

be related to the presence of granitic rocks of the Umm Naggat pluton.

Intermediate to high values in the areas of east, south and west around

the Umm Naggat granitic pluton are associated with outcrops of the

Epidiorite Complex.


Fig. 3. Total count contour map, G. Umm Naggat area, central Eastern Desert, Egypt.

Equivalent Th, equivalent U and K % coloured maps (Fig. 4, 5 and

6) show a positive correlation between low values and spatial positions of

the basic metavolcanic rocks. High values in these maps represent

granitic rocks of the Umm Naggat area.

Fig. 4. Equivalent thorium contour map, G. Umm Naggat area, central Eastern Desert,

Egypt.


Fig. 5. Equivalent turanium contour map, G. Umm Naggat area, central Eastern Desert,

Egypt.

Fig. 6. Potassium percent contour map, G. Umm Naggat area, central Eastern Desert,

Egypt.


The ternary (three-component) radioactive element map is an

effective method of displaying variations in total radioactivity and in the

relative abundances of the three radioactive elements. Areas of the image

with the same colour will have similar ratios of K, eU, eTh, and the

intensity of that colour is a measure of the total radioactivity. This allows

the map to represent the radioactive element distribution better than any

of the other single variable maps. The ternary map is often easier to work

with to get an overview of the distribution of radioactivity; however, it

does not replace the more detailed, quantitative information available on

the other 7 maps (total radioactivity, K, eU, eTh, eU/eTh, eU/K and

eTh/K) (Broome et al., 1987). Ternary radioelement maps of airborne

gamma-ray spectrometric surveys acquired over exposed granitoid

intrusions commonly show a zoning in radioelement concentrations

(Ford and O'Reilly, 1985; Broome et al., 1987; Harris et al., 1990;

Goossens, 1992; Dickson and Scott, 1997). These zonations have been

interpreted as reflecting fractionation in combination with segregation of

intercumulus melt (Goossens, 1992), magmatic differentiation, sensu lato

(Charbonneau, 1991; Wellman, 1998), or hydrothermal enrichment and

alteration (Charbonneau, 1991). An understanding of the processes that

give rise to radioelement zoning and the associated mineralogical

variation in granites has important implications in the search for

mineralization of U, Sn, W, Mo, Cu-Au and rare earth elements (Ford

and O'Reilly, 1985; Goossens, 1992; and Dickson and Scott, 1997).

The ternary radioelement map of the Umm Naggat granite (Fig. 7),

with %K modulating green-magenta, ppm eTh modulating red-cyan, and

ppm eU modulating yellow-blue, shows three spectral domains (A, B and

C). However, it shows an inward transition from blue and magenta hues

in the northern marginal zone (domain A) to reddish and brownish hues

in the middle (domain B), then blue and magenta hues in the southern

marginal zone of the pluton (domain C). Table (1) shows the three

spectral domains signatures of the Umm Naggat granitic pluton. Also, the

zoning is mainly reflected in the thorium channel (Fig. 4). Such zonation

is evidenced by the results of geochemical analysis carried out by

Hassanen et al. (2008) which will be discussed in section 5. White and

black hues in the image correspond to high or low abundances,

respectively, of all three of the radioelements. Mixture of K, eU and eTh

appear as magenta-blue and cyan hues.


Fig. 7. Ternary radioelement map, G. Umm Naggat area, central Eastern Desert, Egypt.

Table 1. Spectral domains of the Umm Naggat granitic pluton as assigned in Fig. 7.

Domain Ternary map

hue

Radioelements Remarks

Northern

domain

(A)

Strong blue-

magenta and

white coloured

hues

-High eU, eTh.

-Low K relative to eU &

eTh.

-High eU/K, eTh/K and

relatively high eU/eTh

ratios.

Strong hues appear primarily

associated with albite granite

characterized by high Zr, Nb, Y,

Th, U, HREE and very low K/Rb

ratio (35-15) Hassanen et al.,

(2008).

Middle

Domain

(B)

Reddish and

brownish hues

-High K, relatively high

eTh, eU.

-High eU/eTh, low eU/K,

eTh/K ratios.

K, Th spectral signatures appear

associated with alkali-feldspar

granite has lower K/Rb ratio

(350-50) & trachyte dykes and

necks, Hassanen et al., (2008).

Southern

domain

(C)

Blue and

magenta hues

-High eTh, K.

-relatively high eU.

- Low eU/eTh, eTh/K

ratios.

Relatively strong hues appear

primarily associated with

monzogranite has high K/Rb

ratio (880-400) & trachyte necks

Hassanen et al., (2008).

In addition, ternary radioelement map (Fig.7), provides additional

information where low Th values are associated with the basic

metavolcanic complexes (Fig. 4) also appear as low values in the ternary


map (dark colours). The northern domain A of the granitic pluton appears

in light blue tones and white coloured areas, indicating a relative

predominance of U content; although it shows high absolute values for

the three channels. Such enrichment of the three elements can be

interpreted as evidence for hydrothermal alteration, where a change in

chemical conditions led to enrichment in these radiometric elements. It is

also possible to detect areas of predominant Th (red tones in domain B),

which indicates an enrichment of this element in the alkaline feldspar

granite which is cut by trachyte dykes and necks. Domain B is prevailed

in the central parts of the granitic pluton. Then light blue tones appear

again in the southern domain C of the granitic pluton which is associated

with trachyte necks (Fig. 7).

It can be noticed from the airborne gamma-ray spectrometric survey

dataset (Fig. 3, 4, 5, 6 & 7) that, the Umm Naggat younger granite is

more radioactive than their country rocks. This may be due to its mineral

content; where it is characterized by high Zr, Nb, Y, Th, U and HREE,

particularly the northern part of the granite massif (EL–Afandy et al.,

2000; Hassanen et al., 2008; and Abdalla et al., 2009). Hussein and

Sayyah (1992) recorded that, generally younger granites in several

localities of Eastern Desert are more radioactive than their country rocks

(5 to 15 times) due to their relative high uranium and thorium contents.

In addition to the interpretation of ternary radioelement, ratio images

are useful for identifying the overall radioelement variations. In general

the ratio maps indicate a relative enrichment of one or more elements. In

turn, enrichment of potassium and (or) uranium can be indicative of

alteration processes that are associated with various types of

mineralization. Enrichment of thorium can be indicative of the presence

of the heavy mineral monazite, which may contain rare earth elements. In

particular, the eTh/K ratio can be a sensitive indicator of K alteration

associated with mineralization and can be used as a direct indicator of

mineralization (Broome et al., 1987). The ratio maps of the study area

(Fig. 8, 9 & 10) depict the highest eTh/K, eU/K and eU/eTh ratio values

that reach 10.4, 5.5 and 0.7, respectively, over the northern portion of the

Umm Naggat granite. Such high ratio values are considered as indicator

of K alteration and/or metasomatism associated with Nb-Ta-Sn

mineralization at the northern margin of such granitic pluton.


One of the modern applications of gamma ray spectrometry is the

possibility of identifying areas of hydrothermal alteration and

establishing its relationship with processes forming base metal

mineralization (Cu-Pb-Zn), and gold and silver in various geological

environments (e.g. Shives et al. 2000). The research has a

methodological and focuses on a series of tools used in mineral

exploration, with emphasis on gamma ray spectrometry, with the main

purpose of testing them in the study of Nb-Ta-Sn mineralization and

determination of areas favorable to its occurrence in the Umm Naggat

granite pluton. To assess the potential of the gamma ray spectrometric

method characterization of areas of change associated with hydrothermal

Nb-Ta-Sn occurrences and propose new targets exploration, two

approaches such as residual anomaly method on the basis of lithologic

normalization (Th-normalized approach) and the F-parameter of Efimov

approach are applied.

Fig. 8. eTh/k% ratio contour map, G. Umm Naggat area, central Eastern Desert, Egypt.


Fig. 9. eU/k% ratio contour map, G. Umm Naggat area, central Eastern Desert, Egypt.

Fig. 10. eU/eTh ratio contour map, G. Umm Naggat area, central Eastern Desert, Egypt.


The Th-Normalized Approach (Saunders et al., 1987)

Geochemically, the variations in concentration of various elements

reflect factors. The surface lithology is the main factor influencing the

variation of the content of the rocks radioelements. So it is necessary to

suppress the effects caused by variations lithology and soil types and

environmental conditions before evaluating any other side effects (Foote

1969). Several procedures have been proposed in order to remove the

lithological and environmental effects (Saunders et al. 1978, 1987, 1993,

1994, and Galbraith & Saunders 1983). Most of these studies aimed at

identifying accumulations of oil and no identification of deposits vent

from the analysis of gamma-spectrometric data. Normalizing data for

thorium would eliminate the primary effects of all undesirable variables.

The basic premise of this approach is that thorium, which is relatively

stable in the near surface of the Earth and a sensitive indicator of

lithology, can well reflect the original distribution of rock and soil.

However, in normal rocks, correlations exist between Th, U and K (linear

or logarithmic), while geological and geochemical activities (such as the

formation of oil and gas reservoirs, hydrothermal alteration associated

with mineralization, etc.) can cause local changes in the contents of K

and U, the content of Th remains relatively stable.

According to Saunders et al. (1987), who proposed an 'ideal' content

of K and U can be calculated from that of Th and used as the background

value for various rocks in an area. By subtracting the 'ideal' contents of U

and K from the contents determined from the survey, a residual content

of U and K can be obtained which can be considered as having the

interference of removed lithology. They applied normalization of

potassium and uranium by thorium, in prospecting for oil and gas fields

in east Texas and Australia (Saunders et al. 1993, 1994) and

normalization calculated as follows:

Ki = (mean Ks / mean Ths) x Ths; Kd = (Ks - Ki) / Ki

Ui = (mean Us / mean Ths) x Ths; Ud = (Us - Ui) / Ui,

Where, Ki and Ui indicate the optimal values determined from the report,

Kd and Ud are the deviations from the ideals and values represent the Ks

and Us original data.

Pires (1995) first used this approach in mineral prospecting,

identifying areas of hydrothermal change successfully in Crixás Guarinos


in the state of Goias, Brazil through anomalous potassium (Kd). The

awareness of the value of such normalization approach in mineral

exploration is rapidly increasing Ferreira et al. (1998), Blum (1999),

Carvalho (1999), Biondi et al. (2001), Cainzos (2001), Fornazzari et al.

(2001), among others.

Figures (6 & 11) show the airborne gamma-ray spectrometry K

content and residual Kd % content contour maps over the Umm Naggat

granitic pluton. From the maps it can be seen that the mineralized zone is

within the area of low K. But from the original K contour map, it is very

difficult to mark the range of the mineralized zone which is shown very

clearly on the residual K map obtained from Th-normalization. Besides,

Kd % map of the study area (Fig. 11) highlights the anomalously low

adjusted potassium anomalies over areas favorable to Nb-Ta-Sn

mineralization in the northern part, as well as areas of trachyte necks in

the southern part of the Umm Naggat granite pluton. Through the weak

anomalous potassium (Kd), (Fig. 11) that connected with anomalous

uranium variation (Ud), (Fig. 12) the Umm Naggat mineralization can be

highlighted.

Fig. 11. Deviations from the K ideals (Kd%) contour map, G. Umm Naggat area, central

Eastern Desert, Egypt.


Fig. 12. Deviations from the U ideals (Ud%) contour map, G. Umm Naggat area, central

Eastern Desert, Egypt.

The F-Parameter of Efimov Approach

Another indicator factor, the F-parameter of Efimov (1978), is used

in conjunction with the normalization approach to delineate the Nb-Ta-

Sn mineralized alteration zone of Umm Naggat granite. The expression

for the F-parameter is K* U/Th or K/(Th/U) or U/(Th/K), which can be

thought of as the ratio of potassium abundance and Th/U or as the ratio

of uranium abundance and Th/K. The F-parameter is an important

alteration index of rocks. Usually, for unaltered rocks, the F-parameter

value is no more than 1.2-1.3, but for altered rocks, it can be 2-5 and

sometimes above 10 (Gnojek and Prichystal, 1985). The F-parameter has

been very useful for locating potassic alteration zones related to gold,

copper and polymetallic mineralization in China (Zhang et al. 1998), to

new Zn mineralization in Czechoslovakia (Gnojek and Prichystal, 1985),

to Cu, Ni mineralization in Abu Swayel, Egypt (Abd El Nabi 1993). Over

the Umm Naggat granitic pluton there exists an obvious F-parameter

anomaly strip having values more than 1.6 (Fig. 13), which shows the

potassic alteration metasomatism associated with ore zone mineralization

along the northern endocontact of Umm Naggat pluton and the dykes of


quartz porphyries, felsites porphyries, trachytes, dolerites and basalts and

trachyte necks which are widely distributed in the central parts of the

massif.

Fig. 13. F-parameter of Efimov contour map, G. Umm Naggat area, central Eastern Desert,

Egypt.

Statistical Aspect

Three histograms for each Umm Naggat granitic pluton portion

show the concentration of eU, eTh, and K % (Fig. 14.a, .b, & .c).

Correlation the similar histograms in the three portions reveal a

significant difference between their radiometric properties. However,

Fig. (14.a, .b, & .c) and Table 2 show that the concentrations differ

considerably in the three portions for examples the mean K concentration

of 2.49 %, 2.90 %, 2.66 % for the northern, central and southern portions

of the granite respectively. The depletion of K content in the northern

portion with respect to the other granite portions may be due to the

increase of Na in the apogranite (albitized variety) and the presence of

trachyte necks associated with relics of trachyte tuffs, tuff and lava

breccias in the central and southern parts of the massif (Sabet, 1961). In

contrary, eU and eTh contents are higher in the northern portion; 10.29

ppm, 20.65 ppm than both the central and the southern portions. The

central portion has eU content of equal 5.47 ppm, eTh content of equal

11.90 ppm and the southern portion has the lowest eU and eTh contents;


3.65 ppm and 10.72 ppm respectively. The highest eU and eTh contents

in the northen portion are due to its mineral content; where it is

characterized by high Zr, Nb, Y, Th, U and HREE in the most albitized

varieties, particularly in the vicinity of the northern endocontact (EL–

Afandy et al., 2000; Hassanen et al., 2008; Abdalla et al., 2009). As well

as, the highest values of the three ratios eU/eTh, eU/K and eTh/K of the

northern portion (Table 2) indicate to areas of favorable to the

metasomatic processes associated with Nb-Ta-Sn mineralization

occurrence in the Umm Naggat granite pluton.

Fig. 14. F-Histogram for radioelements of G. Umm Naggat granite pluton.

Table 2. Summary statistics of eU, eTh, K for: a) Northern-, b) Central- and c) Southern

Umm Naggat Pluton.

Pluton area

No. of

obser-

vations

eU

ppm

eTh

ppm K %

eU/eT

h

eU/

K eTh/K

Northern Umm

Naggat Pluton n=102

10.29 20.65 2.49 0.49 4.42 8.59 Average

05.39 07.76 0.42 0.12 2.88 3.69 Standard

Deviation

Central Umm

Naggat Pluton n=194

5.47 11.90 2.90 0.46 1.89 4.10 Average

2.24 4.16 0.39 0.09 0.75 1.32 Standard

Deviation

Southern Umm

Naggat Pluton n=49

3.65 10.72 2.66 0.35 1.36 4.00 Average

1.16 3.78 0.36 0.07 0.33 1.19 Standard

Deviation

Additional insight into the relationships between the K, eTh and eU

airborne concentrations and the associated spectral trends may be


obtained by analysing their interrelationships in K-eTh-eU space

(Kuosmanen, 1994; and Wellman, 1998). Bivariate diagram eTh-K plot

(Fig. 15) shows that K values of central portion have a higher

concentration than that of the northern and southern portions. Also, it

shows that the northern portion has a higher eTh concentration than the

central and southern portions of the Umm Naggat granite. According to

Muecke and Chatterjee (1983), who reported that bodies which show a

thorium enrichment trend with increasing differentiation appear to be

more favourable exploration target than spatially associated bodies in

which thorium is depleted. As a consequence, the northern apogranite is

considered more favourable for exploration than the central and southern

portions of the Umm Naggat granites.

Fig. 15. Bivariate diagram eTh Vs. K plot of G. Umm Naggat granitic pluton.

Geochemical Aspect

Spatial interpretation of the airborne gamma ray spectrometric data

requires correlation with additional data, such as geology, geochemistry

and other airborne or ground geophysical data. From geochemical point

of view, rare metal granites have been identified as those with high

concentrations of normally dispersed elements such as F, Li, Rb, Cs, Sn,

Ta, Nb, Zr, and REE (Tauson, 1974; and Kovalenko, 1978). To

distinguish the granitoids with mineralization potential from barren ones,

geochemical indicators have been used, such as Rb, Li, F, Zr, Ba and Sr

proposed by Dall Agnol et al., (1994); Scheepers, (2000). The available

geochemical results carried out by Hassanen et al. 2008; Abdalla et al.

2009 are used to provide further insight in the origin of the zoning pattern


which is assigned by gamma spectrometric maps (Fig. 3-7). Besides, the

northern Umm Naggat albite granite has suffered extensive post-

magmatic metasomatic reworking that lead to development of Zr, Hf, Nb,

Y, Ta, U, Th, F (EL–Afandy et al., 2000). Hassanen et al. (2008);

Abdalla et al. (2009) reported that it is characterized by very low K/Rb,

Eu/Eu, high Zr, Nb, Y, Th, U and HREE. In addition, Hassanen et al.

(2008, pers. Commun.) reported that the southern monzogranite part of

the Umm Naggat pluton is characterized by high K/Rb ratio (880-400),

the central alkali feldspar granite has lower K/Rb ratio (350-50) and the

northern albite granite is characterized by very low K/Rb ratio (35-15).

Consequently, low values for the ratios, K/Rb correspond to the fertile

granites of Um Naggat granite pluton such as the subordinate roof facies

of albite granite that host greisens and Nb-Ta-Sn mineralization at the

northern margin of such granitic pluton.

Conclusion

Gamma-ray spectrometry data acquired over the G. Umm Naggat

granitic pluton show a conspicuous and complex radioelement zoning, in

particular eTh, which in turn reflects mineralogical associations in the

pluton. The pluton is divided into three northern, middle and southern

spectral domains. The geochemical results of previous works are used to

provide further insight in the origin of this zoning pattern. The integrated

analysis of the gamma-ray spectrometry and geochemical data provides a

tool to map the regional extent of the compositional patterns within the

pluton, and provides insight into the most probable differences in

mineralogy associated with the radioelement zoning.

Normalization approach in conjunction with the F-parameter

approach are used to assess the potential of the gamma ray spectrometric

method characterization of areas of change associated with hydrothermal

Nb-Ta-Sn occurrences. Low anomalous potassium (Kd) connected with

high anomalous uranium variation (Ud), and high F-parameter anomaly

strip highlight such areas. This is considered as a step to maximize the

potential of the radiometric approach for mineral exploration.

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