Geological mapping and spectral based classification of basement rocks using remote sensing data analysis: The Korbiai-Gerf nappe complex, South Eastern Desert, Egypt

Accepted Manuscript Geological mapping and spectral based classification of basement rocks using remote sensing data analysis: the Korbiai-Gerf nappe complex, South Eastern Desert, Egypt Safaa M. Hassan, Mohamed F. Sadek PII:

S1464-343X(17)30291-1

DOI:

10.1016/j.jafrearsci.2017.07.006

Reference:

AES 2958

To appear in:

Journal of African Earth Sciences

Please cite this article as: Safaa M. Hassan, Mohamed F. Sadek, Geological mapping and spectral based classification of basement rocks using remote sensing data analysis: the Korbiai-Gerf nappe complex, South Eastern Desert, Egypt, Journal of African Earth Sciences (2017), doi: 10.1016/ j.jafrearsci.2017.07.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Geological mapping and spectral based classification of basement rocks using remote

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sensing data analysis: the Korbiai-Gerf nappe complex, South Eastern Desert, Egypt

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Safaa M. Hassan and Mohamed F. Sadek

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[email protected]

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National Authority for Remote Sensing and Space Sciences, 23 Joseph Tito Street, El-Nozha

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El-Gedida, P.O. Box: 1564 Alf -Maskan, Cairo, Egypt.

ABSTRACT

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The Pan-African Neoproterozoic Korbiai-Gerf nappe complex in the extreme South Eastern

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Desert of Egypt comprises dismembered ophiolite assemblages tectonically thrusted over pelite-

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psammopelite, quartzo-feldspathic gneiss and island-arc schistose metavolcanics. The whole

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sequence is intruded by syn-late to post tectonic mafic and felsic intrusions.

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The enhanced Landsat-8 band ratio (bands 6/2, 6/7 and 6/5×4/5) and Advanced Spaceborne

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Thermal Emission and Reflection Radiometer (ASTER) Principal Component (PC2, PC6, and

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PC5) successfully discriminated most of the exposed lithological units and produced a detailed

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geological map. Granitoids, psammopelite-pelite, gneiss and serpentinite-talc carbonate rocks

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have been discriminated using ASTER kaolinite, clay, sericite-muscovite and calcite-carbonate

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indices respectively.

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Three spectral based classification algorithms have been compared using Landsat-8 and the

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Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) datasets to

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obtain the best lithological classification for the exposed basement rock units. Results from the

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present study revealed that, Support Vector Machine (SVM) classifier algorithm provided the

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best lithological classification accuracy (97.72%) using the combination of 9 ASTER bands and

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20 ASTER derivative images.

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The results of the present study concluded that, the integrated data of ASTER and Landsat-8

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enhanced images are effective in the discrimination and classification of the basement rock units

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exposed at Korbiai-Gerf nappe complex and can be applied in similar areas in the Arabian-

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Nubian Shield.

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Keywords: Korbiai-Gerf, Eastern Desert of Egypt, ASTER, Landsat-8, Support vector machine

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(SVM) classifier.

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1. INTRODUCTION

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The Korbiai-Gerf area is located in the extreme southern part of the Eastern Desert of Egypt.

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It is situated between latitudes 22º 23´and 22º 53´ N and longitudes 34º 55´and 35º 25´E (Fig.1).

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This Nappe is the largest ophiolitic metamorphosed ultramafic mass in the Eastern Desert of

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Egypt (Shaddad, 1982; Nasr et al., 1998) extending over an area of about 570 km2.

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The Korbiai-Gerf ophiolitic mass was previously described as an intrusive complex (e.g.

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EGSMA. 1981; Shaddad, 1982). However, most workers have regarded it as an ophiolitic terrain

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(e.g. Kroner et al., 1987; Bennett and Mosley, 1987; Stern et al., 1989; Kroner et al., 1992;

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O'Conner et al., 1993; Greiling et al., 1994; Sadek et al., 1996 and 1997; Abdel Magid et al.,

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1996; Nasr et al., 1998; Tolba, 2000; EGSMA, 2002; Nasr and Beniamin, 2002; Abdel-Karim et

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al., 2001; Abdel Gawad, 2002; Sadek, 2005, Gahlan and Arai, 2009; Abdel Aal et al., 2016).

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This Nappe constitutes the highest mountainous peaks extending with N-S general trend for

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some 50-km forming a roughly circular mass at its northern part. These peaks are namely from

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north to south; Gabal Mineiga, Gabal Korbiai, Gabal Gerf, and Gabal Maqur, followed

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southwards by the peaks of Madarai, Abu Hireig and Abu Hodeid which form a longitudinal

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connected mass extending for about 30 km . They decrease in their width from about 25 km in

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the northern part to about 2 km at the southern end part. On the other hand, the mountains of

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Korbiai and Gerf at the northern part of this complex form a roughly circular huge mass about 25

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km in diameter. The peak of Gabal Gerf is the highest (1419 m a.s.l.) whereas the name Gerf

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nappe complex is mostly given to the whole ophiolitic mass.

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One of the main applications of satellite images is creating maps of ground features through

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assigning image pixels to distinguishable real world classes using image classification automated

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processes. The different classification techniques are significant in determining the quality of the

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classification results using remote sensing data (James and Daniel, 2002; Lu and Weng, 2007;

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Perumal and Bhaskaran, 2011; Salati et al., 2011; Li et al., 2011; Yu et al., 2012; Mondal et al.,

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2012 and Hassan et al., 2014). Yu et al., (2012) implemented a spatial image processing method

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for lithological classification using SVM algorithm which is applied to an automated lithological

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classification using ASTER-DEM imagery to get the best lithological classification -. Many

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studies have been carried out for lithological discrimination and geological mapping using

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Landsat-8 and ASTER remote sensing data (e.g. Vaughan et al., 2005; Ninomiya et al., 2005;

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Qiu et al., 2006; Liu et al., 2007; Gabr et al., 2010; Amer et al., 2010; Aboelkhair et al., 2010;

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Pour and Hashim, 2011; Madani and Emam, 2011; Rajendran et al., 2013; Hassan and Ramadan,

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2014; Hassan et al., 2014; Sadek et al., 2015; Gabr et al., 2015).

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The first aim of this study is to present a geological study on the Korbiai-Gerf area based on

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the analysis of remote sensing data, previous geological mapping, petrographical studies and

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field observation. The second goal is to propose a new classification method (i.e. Spectral Angle

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Mapper (SAM), Spectral Information Divergence (SID) and Support Vector Machine (SVM)

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with Radial Base Function (RBF) using the integrated data of Landsat-8 and ASTER satellite

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images to enhance lithological discrimination of various rock types and units in Korbiai-Gerf

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area.

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2. MATERIALS AND METHODS

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2.1. Remote sensing data

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ASTER is an imaging tool on board the Terra satellite, launched in December 1999 as a part

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of NASA's Earth Observing System program. ASTER data covers a wide spectral range with 14

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bands of narrow band widths cover the ranges from the visible to the thermal infrared regions.

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Landsat-8 data have higher radiometric resolutions (16 bits) and lower spectral resolution

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compared with ASTER data.

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The ASTER shortwave infrared (SWIR) channels increase the accuracy of the spectral

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identification of minerals and rock units of the Earth surface (e.g. Crósta et al., 2003; Ninomiya

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et al., 2005 and 2006; Gad and Kusky, 2006 and 2007; Hassan et al., 2014; Gabr, et. al., 2015;

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Sadek et al., 2015). Landsat-8 data with high radiometric resolution is an effective tool for

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detailed geological mapping (Hassan and Ramadan, 2014; Sadek, et al., 2015).

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In the present study, both Landsat-8 and ASTER L1B images, acquired in August, 2016 and

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January 2003 respectively, have been used. Nine ASTER VNIR and SWIR spectral bands have

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been stacked and processed using ERDAS Imagine 2015 and ENVI 5.3. The geological map of

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the study area and layout of the processed images has been produced using the ArcGIS 10.4

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Software package.

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2.1.1. Band ratio

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This technique is applied by dividing the Digital Number (DN) values of one band by the

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corresponding DN values of another band and displaying the new DN values as a grey scale

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image that provides relative band intensities (Sabins, 1997 and 1999). The proposed ASTER and

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Landsat-8 (VNIR and SWIR) band ratio images were used to enhance the boundaries between

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the exposed lithological units in the study area. In addition, different ASTER indices including

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kaolinite, clay and sericite-muscovite and calcite-carbonate indices have been used to identify

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the widely-exposed rock units based on their enrichment with essential and secondary mineral

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constituents.

2.1.2. Principal Component Analysis (PCA)

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Principal Component Analysis (PCA) as one of the spectral enhancement techniques has

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been used for lithological discrimination. It is also used to improve the classification results in

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the present proposed method. On the other hand, Independent Components (IC) analysis is a type

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of spectral un-mixing method that does not require knowledge of targeted surface materials

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(Gómez et al., 2007). The (IC) analysis derives a new dataset containing new bands which

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comprise a linear combination of the input bands.

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2.1.3. Spectral classifiers

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Spectral based classification methods were developed for use on both hyperspectral and

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multispectral remote sensing data, often with enhanced results that can easily be compared to

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spectral properties of materials. In the present study, SAM, SID and SVM supervised

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classification techniques have been used to detect lithological units based on their spectral

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properties. Information about the rock units in the study area have been collected from different

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sources including field observation and the published geological maps. This information has

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been used to train the satellite image classifier and applied to satellite images (i.e. ASTER and

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Lansat-8) using the ENVI version 5.3 Software to extract the different rock types in the whole

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image.

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The SAM is a supervised classification method that permits rapid mapping by calculating the

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spectral similarity between the image spectrum to reference reflectance spectra (Schwarz and

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Staenz, 2001). The reference spectra used by SAM has been extracted from the image as area of

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interest (ROI average) using the field observation data. The spectral similarity between the

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spectra is determined by calculating the angle between the spectra and converting them to

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vectors in a space with dimensionality equal to the number of the spectral bands.

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Spectral Information Divergence (SID) is a famous spectral classification technique that uses a

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divergence measure for matching pixels to reference spectra. The smaller the divergence, the

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more likely the pixels are similar.

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The supervised SVM classifier is known for its classification accuracy output (Burges, 1998;

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Hassan, et al., 2015), due to the fact that it is built on a rigorous mathematical model (Perumal

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and Bhaskaran, 2011). The Radial Basis Function (RBF) is one of the non-linear SVM classifier,

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which uses the penalty factor C to adjust the penalty of the classifier and γ (Gaussian kernel's)

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parameters to further optimize the classification output (Ding, 2011). The larger C, the higher

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outcome accuracy in the training phase. The γ parameter, has a greater effect on the classification

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process than the C parameter (Linden et al., 2010). In the present study, the optimal values of C

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and γ are 100 and100. These values were applied on ASTER-DEM (slope, curvature)

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+PCA+ICA as well as Landsat-8 (VNIR+SWIR)+DEM+PCA-ICA data layers.

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In the current study, a full comparison of the different remote sensing data inputs of Landsat-8

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and ASTER using SAM, SID and SVM classification techniques is obtained to identify the most

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discriminatory data layers for lithological classification at Korbiai-Gerf area and the results are

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summarized in a flowchart (Fig. 2). The accuracy assessment of SAMs, SIDs and SVMs

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classification is evaluated using various combinations of data inputs in order to get the best

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lithological discrimination for the widely exposed rock units.

2.2. Field investigation

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The interpreted information from the processed remote sensing data was verified through the

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field observation whereby the exposed lithological units have been identified and their

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boundaries have been confirmed. Representative samples from the different exposed rock units

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were collected for the petrographic study.

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3. GEOLOGICAL SETTING

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The Gabal Korbiai-Gerf area consists of a sequence of late Neoproterozoic Precambrian

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basement rocks, which comprises quartzo-feldspathic gneiss, psamopelite-pelite association,

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ophiolitic assemblages and intrusives (Fig. 1). The ophiolitic rocks comprise serpentinite - talc

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carbonate rocks, metagabbro and locally pillowed basic-intermediate. metavolcanics. The

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ophiolitic assemblages are eastwards thrusted over the metasediments, gneisses and calc-alkaline

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island-arc schistose metavolcanics and their related meta-volcaniclastics and tuffs. The whole

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sequence is intruded by syn-to late tectonic gabbroic and granitoid intrusions as well as dykes

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and veins. The Precambrian rocks are intruded by post tectonic syenite and quartz syenite which

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are related to the Mesozoic Late Cretaceous Nugrub El-Fuqani alkaline ring complex (El Ramly

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and Hussein, 1983; Sadek et al., 1996; Tolba, 2000; EGSMA, 2002).

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3. 1. Gneisses and pelites-psammopelites

N-S trending sequence of quartzo-feldspathic gneiss and psammopelite-pelites are exposed

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at the southern eastern sector of the mapped area forming the northern part of the Hamisana

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shear zone (e.g. Stern, 1989; Miller and Dixon, 1992; de Wall et al., 2001; Ali-Bik et al., 2014).

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These rocks are intruded by syn- to late-tectonic granitoids.

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3.1.1. Quartzo-feldspathic gneisses

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These rocks were suggested to be of igneous origin (Bennet and Mosley, 1987; Greiling et

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al., 1994). They are folded and dissected by dyke swarms (Miller and Dixon, 1992; Stern et al.,

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1989). The quartzo-feldspathic gneisses show gneissic texture marked by the presence of

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alternating felsic quartzo-feldspathic bands with mafic bands consisting mainly of biotite,

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muscovite and hornblende (Fig. 3a). Locally, these gneisses are migmatized and show ptygmatic

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folding, pegmatitic intrusios and quartz veins. Dark bands of hornblende gneiss form

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intercalations within the biotite gneiss and psammopelitic-pelitic rocks.

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Petrographically, the quartzo-feldspathic gneisses are subdivided into biotite gneiss and

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hornblende gneiss (Ali-Bik et al., 2014). Biotite gneiss is composed mainly of quartz,

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plagioclase, microcline, hornblende and biotite. Sphene, epidote, chlorite, sericite, garnet and

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opaques are accessories. The felsic and mafic constituents are arranged in parallel alignment

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giving gneissic texture (Fig. 3b). Hornblende gneisses are medium-grained and foliated rocks.

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They consist mainly of hornblende, few biotite and plagioclase, minor quartz (up to 10% mode),

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opaques, apatite and sphene.

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3.1.2. Psammopelite - pelite association

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An association of sedimentary origin consisting mainly of laminated pelite and psammopelite

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is exposed forming a thin strip east of the Madarai-Abu Hodeid ophiolitic mass. The pelitic and

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psammopelitic rocks are intercalated with dark hornblende schist bands of variable widths.

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Along the zone of thrust contact with the serpentinites, the metapelite rocks are highly deformed

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and tectonized. Sheared elongated serpentinite - talc carbonate pods and slices are tectonically

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incorporated within the pelitic and psammopelitic rocks.

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Microscopically, psammopelitic rock shows alternating layers of mica and quartz with

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plagioclase. Biotite and/or muscovite aggregates are predominant forming the micaceous layers.

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Cordierite is formed at the expense of biotite, and it is replaced by fine aggregates of muscovite

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during the final retrogressive stage (Ali-Bik et al., 2014).

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3.2. Ophiolitic Assemblages

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The serpentinite-talc-carbonate rocks, metagabbro and the pillowed metavolcanics represent

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the dismembered ophiolitic assemblages in the Gerf nappe complex (Fig. 1). At the southern

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part, these rocks are thrusted over the surrounding gneisses and meta-psammoplites and

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metapelite- association, while they are thrusted over the calc-alkaline island-arc schistose

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metavolcanics at the northern part (Fig. 3c). The thrust contacts of the serpentinite rocks with the

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surrounding metavolcanics in the central parts are irregular due to the irregular boundaries of

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these metavolcanics. The ophiolitic assemblages are intruded by syn-late tectonic granitoids,

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gabbro-diorite, late tectonic gabbro and monzogranite.

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3.2.1. Serpentinite-talc carbonate rocks The serpentinite-talc carbonate rocks forming the Korbiai-Gerf mass are slightly massive at

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the northern part, while at the eastern and western and southern parts they are highly sheared and

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altered and dissected by magnesite veins (Fig. 3d). At Gabal Korab Kansi and southwards along

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Wadi Shinai area, the serpentinite rocks are highly altered and deformed, where the talc

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carbonate and ankerite rocks are predominant. Korbiai old gold mine is located within the

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alteration shear zone in the metavolcanics along the tectonic contact zone with the ophiolitic

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serpentinites.

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Petrographically, the serpentinite talc-carbonate rocks are considered to have been derived

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from peridotite-dunite (Tolba, 2000 and Abdel Gawad, 2002) and they are grouped into massive

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serpentinites, altered sheared serpentinite and talc-carbonate rocks. The massive serpentinites are

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the predominant rock types which composed mainly of relics of olivine, antigorite, and

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chrysotile, replaced by magnesite, carbonates, talc and magnetite giving mesh structure (Fig. 3e).

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In the sheared serpentinite variety, the serpentine minerals are replaced by carbonate and talc

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with secondary iron oxides.

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3.2.2. Metagabbro

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These rocks occur as small outcrops and scattered mega sheared pods within the serpentinite

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rocks at the eastern part of the Gerf ophiolitic mass. They show tectonic contact with the

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ophiolitic metavolcanics. The metagabbroic rocks are heterogeneous, sheared and locally

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layered. Microscopically, the metagabbroic rocks consist essentially of altered labradorite, augite

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and hornblende. Quartz, epidote and opaques are accessories. Tremolite, epidote and chlorite are

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the main alteration secondary minerals. Ophitic and subophitic textures are common.

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3.2.3. Basic-intermediate metavolcanics A Circular exposure of ophiolitic basic to intermediate metavolcanics (pillowed in parts) is

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structurally emplaced within the central part of Korbiai-Gerf serpentinite body. The

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metavolcanics are slightly massive, fine-grained and vesicular. Petrographically, they are basaltic

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to basaltic andesite in composition and they consist mainly of epidotized plagioclase,

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hornblende, augite, chlorite and few quartz porphyroclasts in groundmass of the same

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composition.

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3.3. Island-arc basic-intermediate metavolcanics and related tuffs

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A sequence of schistose metavolcanics mainly basaltic andesite in composition locally

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associated with schistose acidic metavolcanics and their related meta-volcaniclastics are exposed

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at the northern eastern, western and southern parts of Korbiai-Gerf ophiolitic mass (Fig. 1). At

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the western side of Gabal Gerf and northeast of Korbiai old gold mine, these metavolcanics are

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highly foliated and sheared (Figs. 3f and 4a) and transformed into chlorite actinolite schist at the

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eastern and western parts of the Korbiai ophiolitic mass. Along the sheared contact zones, the

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metavolcanics enclose irregular serpentinite - talc carbonate slices. NE of Gabal Korbiai, a

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trench has recently been dug within the sheared metavolcanics to collect gold-bearing smoky

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quartz veins (Fig. 4b).

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According to the mineral assemblages, the exposed meta-volcaniclastic meta-tuffs are

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classified into andesite to dacite tuffs which can be classified into andesitic meta-tuffs and lapilli

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meta-tuffs. Lapilli meta-tuffs comprise lithic and crystal tuffs. Petrographically, the metavolcanic

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varieties are grouped into metabasalts, meta-andesites and epidote-tremolite-actinolite schist

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(Sadek, 2005 and Abdel Gawad, 2002).

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Metabasalt consists mainly of altered pyroxene, hornblende and plagioclase with subordinate

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amounts of actinolite and chlorite. The rock shows subophitic and diabasic textures. Meta-

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andesites consist mainly of phenocrysts of plagioclase, hornblende and quartz embedded in a

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fine-grained groundmass of plagioclase, calcite, zoisite, chlorite and iron oxides are the main

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accessories. Porphyroblastic and porphyritic textures are observed. Epidote-tremolite-actinolite

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schist consists mainly of altered plagioclase and pseudomorphs actinolite, tremolite pheoncrysts

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embedded in fine groundmass of the same composition in addition to epidote, chlorite and iron

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oxides. Schistose and porphyritic textures are common.

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Tuffs consist mainly of quartz and plagioclase phenocrysts in fine-grained groundmass (Fig.

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4c). The metamorphic mineral assemblages in these rocks suggest the lower to upper greenschist

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amphibolite facies metamorphism.

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3.4. Magmatic assemblages

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3.4.1. Syn tectonic gabbro-diorite

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This rock association crops out as few outcrops in the study area as intrusive in serpentinites

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and the schistose metavolcanics particularly at the northern eastern part of Gabal Korbiai.

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Petrographically, gabbro-diorite rocks are medium-coarse grained and range in composition from

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hornblende gabbro to diorite. They consist of plagioclase, hornblende, minor biotite together

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with accessory opaques. Chlorite and epidote are the common secondary minerals (Fig. 4d).

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3.4.2. Syn-tectonic tonalite-granodiorite

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These rocks range in composition from tonalite to granodiorite. They are grouped into two

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varieties including; 1) the foliated rocks which are exposed at the eastern part of Korbiai-Gerf

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ophiolitic mass, and 2) slightly massive variety outcropping at the northern western and western

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parts of the mapped area dissected by E-W trending basic dyke swarms (Fig. 1). In general,

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these rocks are jointed, xenolithic and show exfoliation. They intrude the surrounding ophiolitic

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serpentinites, the schistose metavolcanics and gabbro-diorite. Microscopically, the tonalite-

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granodiorite rocks consist mainly of variable amounts of plagioclase (about 40-50%), quartz

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(about 30%), few K-feldspars (less than 10%) and biotite ± hornblende (about 10%), zircon,

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apatite and iron oxides are accessories. Epidote, sericite and chlorite are secondary constituents.

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Granular, myrmekitic and perthitic textures are common.

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The late tectonic gabbroic intrusion is located at the southern border of Gabal Korab Kansi at

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the western side of the mapped area where they have intruded the serpentinite-talc-carbonate

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rocks and schistose metavolcanics (Fig. 1). This intrusion was previously mapped as ophiolitic

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gabbro (Zimmer et al., 1995) and as a late tectonic mafic intrusion (e.g. O'Conner et al., 1993;

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Abdel Magid et al., 1996; Sadek et al., 1997; Nasr et al., 2000; Nasr and Beniamin, 2002; Sadek,

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2005) and as ultramafic-mafic intrusion (Abdel Gawad, 2002).

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This intrusion is very similar to the Abu Fas layered ultramafic-mafic intrusion which is

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located southward at Wadi Allaqi and has been described by Sadek (1995) and Sadek and El

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Ramly (1996). Three discontinuous titano-magnetite layers with NNW-SSE trend and steep to

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vertical dips are present interlayered with olivine gabbro, they vary in width from 5 to 10 m and

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extend for 2 km. The general trend of the titano-magnetite layers is concordant with the layering

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trend of the host olivine gabbro.

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Petrographically, the intrusion is made up of a crude primary large scale layered assemblage

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of olivine-pyroxene gabbro and pyroxene-hornblende gabbro. Olivine pyroxene gabbro consists

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essentially of labradorite, augite, hypersthene and olivine showing ophitic texture (Fig. 4e).

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Magnetite is the main accessory mineral while chlorite, epidote, tremolite-actinolite and

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iddingsite and secondary magnetite are the main secondary minerals. Pyroxene-hornblende

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gabbro consists mainly of augite, labradorite and hornblende. Iron oxides are found as

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accessories. Chlorite, epidote, saussurite and actinolite-tremolite are present as secondary

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minerals. Ophitic and subophitic textures are observed in all varieties.

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3.4.4. Late tectonic monzogranite

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At the southern eastern side of the mapped area, low relief outcrops of monzogranite are

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exposed intruding the quartz feldspathic gneiss and serpentinites (Fig. 1). Petrographically, they

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are medium to coarse-grained consisting mainly of orthoclase, microcline, quartz, subordinate

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albite-oligoclase, biotite, muscovite and accessory iron oxides. Sericite, epidote and chlorite are

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present as secondary minerals. Microperthitic and myrmekitic textures are common (Fig. 4f).

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3.5. Dykes and veins

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All of the exposed metamorphic and magmatic rocks in the study area are traversed by

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numerous basic and acidic including granitic dykes of varying trends and widths. Northwest of

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Gabal Korbiai and south of Gabal Korab Kansi, E-W trending swarms of basic-intermediate in

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composition dykes dissect the syn-tectonic tonalite-granodiorites. Quartz, magnesite and

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pegmatite veins of variable trends traverse the serpentinite talc-carbonate rocks. Gold was mined

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during the Roman times from the smoky quartz veins dissecting the schistose metavolcanics and

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gabbro particularly at the tectonic thrust and alteration zones around Gabal Korbiai and Gabal

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Madarai.

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3.6. Post tectonic Syenite-quartz syenite

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Small outcrop of post tectonic syenite and quartz syenite is exposed at the northern western

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corner of the mapped area. It is the southern extension of Gabal Nugrub Al- Fuqani ring complex

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which consists mainly of syenite, alkaline granite and syenogranite. The ring complex is related

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to Mesozoic Lower Cretaceous in age (El Ramly and Hussein, 1983). These rocks are medium to

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coarse-grained with grey and pinkish varieties. Petrographically, the syenites are composed

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mainly of orthoclase, perthitic orthoclase, aegrine, biotite and nepheline. The accessory minerals

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are apatite, sphene and iron oxides. The secondary alteration products are epidote, sericite and

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chlorite. Rare quartz crystals are present in the quartz syenite variety. Perthitic and

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hypidiomorphic textures are observed.

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4. General tectonic and structural settings The Korbiai-Gerf Nappe is justaposed along N-S trending west-verging thrust zone against

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a wide belt of foliated metasediments and metavolcanics. The available evidence does not

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support the status of Gerf nappe as a suture zone but as allochthonous ophiolitic fragments which

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can be traced southward to the Hamisana shear zone.

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The Korbiai-Gerf mafic and ultramafic rocks represent a slice of Neoproterozoic oceanic

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lithosphere, with an age of 750 Ma (Kröner et al., 1992 and Zimmer et al., 1995). The study area

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is affected by the East African Orogeny (Stern, 1994), which evolved during the Pan-African

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tectono-thermal events affected most of the Arabian-Nubian Shield (ANS) in the Neoproterozoic

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from 950 to 450 Ma (Engel et al., 1980; Vail, 1988).

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Both primary and secondary structural features have been recorded in the Korbiai-Gerf area.

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Lamination in the meta-volcanisedimentary sequence and layering in the late tectonic gabbro

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represent the main recorded primary structural elements in the study area, while thrust, foliations,

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folds and faults are the main secondary structural features.

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Lamination and bedding in the meta-volcaniclastics display NW-SE and NNE-SSW trends

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which are mostly concordant with the foloiation trend S1. Primary NNW-SSE layering with sub-

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vertical and steep dips to E are developed in the Korab-Kansi late tectonic gabbro.

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The ophiolitic assemblages have been tectonically emplaced within the surrounding

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metasediments and metavolcanics showing variable thrust plane trends including N-S, E-W and

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NW-SE trends with the surrounding island arc-metavolcanics, the dip angles vary from 30o-50o

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towards W, N, and NE.

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The thrust sheets within the Gerf ophiolitic mass can be interpreted as minor thrusts below the

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major Gref thrust plane. The gneisses and pelite-psammopelites show general N-S and NNE-

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SSW foliation trends with dip angle about 30o to W and SSW directions. The foliation in the

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schistose metavolcanics displays N-S, NW-SE and E-W variable trends. The dip angles vary

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from 40o to 60o toward E, NE and S directions.

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NW-SE and E-W trending strike slip and normal faults are observed along the ophiolitic masses.

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Crenulation and overturned open and tight folds are recorded within the metasediments and the

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metavolcano-sedimentry rocks. These folds show NW-SE general trend of axial planes with

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moderate plunge angles toward NW. Stretching mineral and slickenside lineation together with

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the recorded other kinematic indicators suggest type nappe transport to the NW and subsequently

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redirected to the west (O’Conner, 1993; Nasr et al., Nasr et al., 1996).

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5. RESULTS AND DISCUSSIONS

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5.1.

Geological mapping

Based on the integrated remote sensing data, field investigation and previous geological

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mapping (e.g. Abdel Magid et al., 1996; EGSMA, 2002; Tolba, 2000; Abdel Gawad, 2002;

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Sadek, 2005; Gahlan and Arai, 2009), an enhanced lithological -map for the study area has

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been produced. The principal component analysis of Landsat-8 image (PC4, PC5 and PC2) in

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RGB (Fig. 5a) discriminates Gerf ophiolitic basic metavolcanics with yellow color and the

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tonalite-granodiorite at the northeast of Gabal Korbiai with red color as well as the

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psammopelite-pelite rocks east Gabal Madarai with bright magenta color. On the other hand,

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the island-arc basic intermediate metavolcanics exposed at the northern eastern part of Gabal

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Korbiai has been emphasized with bright green color. The gabbro-diorite is poorly emphasized

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in this Landsat-8 PC image.

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The slightly massive serpentinite of Korbiai- Gerf mass have been discriminated with very

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dark blue color while the yellowish green color successfully emphasized the highly deformed

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serpentinites (mainly talc-carbonate) exposed at Wadi Shinai, Gabal Madarai at the southern end

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of the ophiolitic mass and Gabal Korab Kansi at the western part of the study area. These two

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serpentinite varieties are poorly discriminated on the ASTER principal component image (PC2,

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PC6 and PC5) in RGB (Fig. 5b), while the gabbro-diorite rocks are emphasized with bright red

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color and the Korab Kansi layered gabbro exhibits bright cyan colour.

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Landsat-8 band ratio image (bands 6/2, 6/7 and 6/5×4/5 on RGB) (Fig. 5c) discriminates the

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lithological units in the study area with accurate tracing of their contacts in the produced

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geological map (Fig. 1). The serpentinite rock varieties and ophiolitic basic metavolcanics have

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been emphasized by light red and cyan colors respectively. The island-arc assemblage of basic

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to intermediate metavolcanics were discriminated with blue color, while the layered gabbro and

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serpentinite rocks were poorly discriminated on this band ratio image.

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4.2. Lithological Classification The lithological classification for the exposed basement rock units in the study area has

384

been carried out using classified Landsat-8 and ASTER images, SID, SAM and SVM algorithms

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Flowchart (Fig. 2), which have been applied on Landsat-8 and ASTER satellite data of the study

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area and the results are shown in figures 6 to 11. The location of training sets representing the

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different lithological units have been selected based on the distributions of the different rock

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units in the study area (Fig. 1).

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Independent Component (IC) and Principal Component (PC) derivative images have been

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extracted using both Landsat-8 and ASTER data in order to get more spectral derivative bands,

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which can be later stacked with the raw spectral bands (Flowchart Fig. 2). Curvature and

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topographic slope have been extracted from ASTER-DEM (resampled to 15x15 m per pixel) to

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improve the lithological information and increase overall accuracy of the classification results.

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The SAM, SID and SVM algorithms have been used to classify lithological units extracted from

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all the pixel locations using Landsat-8 and ASTER dataset images as well as its derivative

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images (Flowchart Fig. 2). Figures 6, 7 and 8 show the results of SID, SAM and SVM classifier

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algorithms applied in the Korbiai-Gerf area using different Landsat-8 dataset. As shown in

398

Figure (8), the best lithological discrimination of the exposed rock units has been obtained using

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SVM classifier applied on 20 bands of Landsat-8 data sets (3 bands SWIR+3 bands VNIR+2

400

bands DEM (curvature and slope) + 6 bands IC bands + 6 bands PC). The overall accuracy of

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SAM, SID and SVM against changing parameters gamma and penalty have been calculated and

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listed in Table 1 for all lithological classes in the study area.

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The results of applying these classifiers lead to the conclusion that, the overall accuracies of SAM,

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(SWIR+VNIR+ICA+PCA+DEM) datasets are 40.32%, 41.56% and 95.43% respectively

406

revealing the best overall accuracy of the SVM classifier algorithm for the lithological

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classification (Table 1). Figures 9, 10 and 11 show the results of SAM, SID and SVM classifier

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algorithms applied in the study area using different ASTER datasets (Fig. 2). The combination of

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29 bands of ASTER bands (9 bands SWIR and VNIR + 2 bands DEM + 9 bands IC and 9 bands

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PC) produces a classification with an overall accuracy 97.72% using SVM algorithm, while both

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SID and SAM algorithms show overall accuracy 62.72% and 71.52% respectively (Table 2).

and

SVM

classifier

algorithms

derived

from

Landsat-8

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SID

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As shown in Figure 11 the optimum discrimination for the exposed rock units in the study area

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(SWIR+VNIR+DEM+IC+PC), while the rock units are poorly differentiated using both SAM

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and SID classifier techniques respectively applied on different ASTER datasets (Figs. 9 and 10).

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Table 3 shows the average of producer’s and user’s accuracy calculated for each lithological unit

417

using different ASTER datasets and their calculated surface areas in square kilometers. The

418

results indicated that the ASTER (SWIR+VNIR+DEM+ IC+PC) data sets improve the

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discrimination accuracy of all the exposed lithological units from 93.8% to 99.8%.

been

provided

using

SVM

classifier

applied

on

ASTER

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has

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4.3. Rock forming minerals discrimination

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Various ASTER band ratio images (ASTER indices) have been used to identify the exposed

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rock units based on their enrichment with some essential and secondary minerals. ASTER band

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ratio of kaolinite index (bands 4/5×8/6) image shows a bright tone emphasizing the granitoids

425

and gneisses (Fig. 12a). Calcite index (bands 6/8×9/8) image of ASTER VNIR and SWIR bands

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discriminated the serpentinite talc-carbonate rocks (in calcite-rich) with bright tone (Fig. 12b).

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The ASTER band ratio (bands 5×7/6×6) image shows the bright tone emphasizing the rocks

428

enriched in secondary clay minerals such as the granitoids at Gabal Gerf and psammopelite-

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pelite rocks exposed at the eastern side of Gabal Madarai (Fig. 12c). Sericite, muscovite and illite

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ASTER index (bands 5+7/6) image is effective in discrimination of the quartzo-feldspathic

431

gneiss (Fig. 12d).

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ASTER alunite-kaolinite-pyrophylite index (bands 5×7/6×6) shows a bright tone

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emphasizing the exposed serpentinite and gabbro (Fig. 13a), while ASTER carbonate/chlorite

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index (bands 9+7/8 band ratio image) displays bright tone differentiating the talc-carbonate and

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ophiolitic basic metavolcanic rocks (Fig. 13b).

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5. CONCLUSIONS

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Gabal Korbiai-Gerf basement complex in the extreme South Eastern Desert of Egypt mainly

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forms a huge ophiolitic Nappe composed of serpentinite talc-carbonate rocks and locally

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pillowed basic schistose metavolcanics. These rock units are tectonically thrusted over the island

441

arc pelite-psammopelite, quartzo-feldspathic gneiss and metavolcanics. The whole sequence is

442

intruded by syn-to late-tectonic gabbroic and granitoid intrusions.

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sequences and intrusions are pierced by Mesozoic alkaline ring complexes composed mainly of

444

syenite. In the present study, the integrated data of remote sensing, and previously published

445

geological mapping are used to discriminate and classify the lithological units and prepare a

446

detailed geological map of the study area. The obtained results have been verified by field

447

observation and petrographical investigation. Results indicated that, the processed remotely

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The overall mentioned

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sensed data of ASTER (PC2, PC6, PC5) and Landsat-8 (PC4, PC5, PC2) principal component

449

images (have successfully discriminated most of the widely-exposed basement rock units in the

450

Korbiai-Gerf area. In addition, Landsat-8 band ratio images (bands 6/2, 6/7) and (bands 6/5×4/5)

451

differentiated these lithological units with enhanced lithological boundaries.

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Moreover, various classifier algorithms including SAMs, SIDs and SVMs have been applied

453

to classify the basement rocks in the study area. These classifiers were tested using several

454

datasets to select the optimal inputs that deliver the best classification accuracy for the most

455

accurate lithological classification. Results indicated that the best overall accuracy percentage of

456

the applied classifiers is 97.72% and that the accurate classified image is given using the

457

classifier algorithm of SVM applied on the ASTER (SWIR+VNIR+ICA+PCA+DEM) datasets.

458

This accuracy is higher than the same accuracy derived from Landsat-8 (95.43%), which can be

459

attributed to the higher spectral resolution of the ASTER spectral bands. The total surface areas

460

of the exposed rock units have been calculated using this SVM ASTER classified image.

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The various ASTER band ratio images representing different indices successfully

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emphasized the exposures of different rock units based on their enrichment in some essential and

463

secondary minerals. Kaolinite, clay and sericite-muscovite indices images discriminated

464

granitoids, psammopelite-pelite and quartzo-feldspathic gneiss. Serpentinite talc-carbonate rocks

465

are clearly differentiated on ASTER calcite and carbonate indices images.

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Results from the present study clearly indicate that the basement rock units exposed in the

467

Korbiai-Gerf area are clearly represented in the detailed geological map produced from the

468

analyzed remote sensing data. The lithological boundaries obtained are well emphasized and

469

show a good resemblance to those of the previously published geological maps of the study area.

470

The SVM classifier algorithm, which is used for the first time in classification of the lithological

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units exposed in the Eastern Desert of Egypt at Korbiai-Gerf area, could be applied in similar

472

areas in the Arabian -Nubian -Shield.

473

ACKNOWLEDGEMENTS

475

The authors would like to offer their thanks and gratitude to the National Authority for Remote

476

Sensing and Space Sciences (NARSS) staff and colleagues for their kind support throughout this

477

study. The authors also wish to acknowledge the anonymous reviewers particularly Prof. Abdel-

478

Rahman Fowler for their comments and suggestions, which greatly aided revision of this paper.

479

Thanks also to Dr. Safwat Gabr for his valuable discussions.

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ACCEPTED MANUSCRIPT Table 1: Overall classification accuracy calculated for various combinations of data sets using SVM, SAM, SID using LANDSAT-8 data. Support Vector Spectral Angle Spectral Information Machine (SVM) Mapper (SAM) Divergence (SID) Accuracy

6 Landsat-8VNIR-SWIR

89.64%

51.66%

14 stack (6 Landsat-8VNIR-SWIR + 6PC+c+s) bands

95.37%

40.32%

14stack (6 Landsat-8VNIR-SWIR +6 IC+c+s) bands

95.17%

51.66%

20 stack bands (6 Landsat-8VNIR-SWIR +6 IC+ 6PC+c+s)

95.43%

40.32%

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36.58%

41.19%

51.86%

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97.26%

44.97%

20 ASTER ( SWIR-VNIR+ 97.67% PCA+c+s) bands

42.29%

20 ASTER ( SWIR-VNIR+ 97.57% ICA+c+s) bands

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62.72%

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Table 3: Average (Producer’s and User’s Accuracy) calculated for the lithological units using SVM classifier applied on different ASTER datasets. 20 ASTER (SWIRVNIR+ PCA+c+s) bands

20 ASTER (SWIRVNIR+ ICA+c+s) bands

29 ASTER (9SWIRVNIR+9I CA+9PC A+c+s) bands

Surface area (Km2) calculated using ASTER SVM classifier applied on 29 ASTER layer

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Class Name / lithological unit

92.65

93.63

93.57

99.65

584.22

Syenite - quartz syenite

90.40

91.50

91.50

99.80

1.20

Late-tectonic monzogranite-alkali feldspar granite

98.37

Late tectonic hornblende pyroxene gabbro, olivine gabbro and gabbro norite

91.53

Syn tectonic tonalite – granodiorite locally quartz diorite

96.83

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98.14

99.35

57.92

93.53

93.54

98.69

140.44

96.905

96.735

97.20

421.21

92.35

92.33

92.82

98.92

167.29

91.84

91.64

91.38

93.82

115.33

90.77

91.43

92.27

98.48

250.59

Ophiolitic basic-intermediate metavolcanics

91.13

91.95

92.70

94.05

55.32

Ophiolitic metagabbro

87.79

89.62

89.73

89.71

59.98

Ophiolitic Serpentinite talccarbonate

92.91

91.17

93.14

99.18

473.38

Psammopelite-pelite association

91.39

93.17

93.02

95.29

128.25

Quartzo-feldspathic gneiss

91.62

92.85

93.77

99.81

216.79

Syn tectonic foliated tonalitegranodiorite

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Calc-alkaline island-arc schistose metavolcanics

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Fig. 1: Detailed geological map of Gabal Korbiai-Gerf area, based on the integrated remote sensing data, field investigations, petrographical studies and the previous geological mapping.

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Fig. 2: Flowchart summarizing the classification process.

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Fig. 3: (a) Alternating mafic and felsic bands in quartzo-feldspathic gneiss. (b) Oriented biotite (Bi) and quartz (Qz) in quartzo-feldspathic gneiss. (c) Gabal Korbiai ophiolitic serpentinites (sp) thrusted over the calc-alkaline schistose metavolcanics (mv). (d) Serpentinite rocks dissected by magnesite veins, Gabal Korab Kansi. (e) Relics of olivine (Ol) replaced by antigorite (Tg) and talc showing mesh structure in serpentinite rocks. (f) Highly foliated basic-intermediate metavolcanics west of Gabal Gerf.

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Fig. 4: (a) Strongly deformed and sheared intermediate metavolcanics, NE of Gabal Korbiai. (b) Recent trench dug to collect the gold-bearing quartz veins within the sheared metavolcanics, NE of Gabal Korbiai. (c) Quartz (Qz) and plagioclase (Pl) phenocrysts in fine-grained groundmass, lithic tuffs. (d) Plagioclase and hornblende (Hb) in gabbro-diorite. (e) Olivine (Ol) and labradorite (Lb) with ophiticsubophitic texture in olivine gabbro. (f) Myrmekitic texture in monzogranite.

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Fig.5: (a) Landsat-8 (PC4, PC5 and PC2), (b) ASTER (PC2, PC6 and PC5) images on RGB, (c) Landsat-8 band ratio image (b6/b2, b6/b7 and b6/b5xb4/b5 on RGB) discriminating the different lithological units. Serpentinite (Sr), Ophiolitic basic-intermediate metavolcanics (Vb), Tonalitegranodiorite (Gd), Foliated tonalite-granodiorite (Gf), Gneiss (Gn), Gabbro norite (Gb) and Psammopelite (Ps), Gabbro diorite (Gbd).

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Fig. 6: Lithological classification of the exposed rock units using SID classifier applied on Landsat-8 data (black color represents unclassified pixels). (a) Landsat-8 (6 SWIR-VNIR bands), (b) Landsat-8 (6 SWIR-VNIR bands +6 PC bands + curvature + slope). (c) Landsat-8 (6 SWIRVNIR bands +6 IC bands + curvature +slope). (d) Landsat-8 (20 (6 SWIR-VNIR+6PC+6IC+ curvature +slope).

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Fig. 7: Lithological classification of the exposed rock units using SAM classifier applied on Landsat-8 data (black color represents unclassified pixels). (a) Landsat-8 (6 SWIR-VNIR bands), (b) Landsat-8 (6 SWIR-VNIR bands +6 PC bands + curvature + slope). (c) Landsat-8 (6 SWIRVNIR bands +6 IC bands + curvature +slope). (d) Landsat-8 (20 (6 SWIR-VNIR+6PC+6IC+ curvature +slope).

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Fig. 8: Lithological classification of the exposed rock units using SVM classifier applied on Landsat-8 data (black color represents unclassified pixels). (a) Landsat-8 (6 SWIR-VNIR bands), (b) Landsat-8 (6 SWIR-VNIR bands + 6 PC bands + curvature + slope). (c) Landsat-8 (6 SWIRVNIR bands + 6 IC bands + curvature + slope). (d) Landsat-8 (20 (6 SWIR-VNIR+ 6PC + 6IC + curvature + slope).

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Fig. 9: Lithological classification of the exposed rock units using SAM classifier applied on ASTER data (black color represents unclassified pixels). (a) ASTER (9 SWIR-VNIR bands), (B) ASTER 20 (9 SWIR-VNIR bands + 9 PC + curvature + slope), (C) ASTER 20 (9 SWIR-VNIR + 9 IC bands + curvature + slope) bands), (d) ASTER (29 (9SWIR-VNIR + 9PC + 9IC + curvature + slope) bands).

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Fig. 10: Lithological classification of the exposed rock units using SID classifier applied on ASTER data . (a) ASTER (9 SWIR-VNIR bands), (B) ASTER 20 (9 SWIR-VNIR + 9 PC+ curvature + slope) bands), (C) ASTER 20 (9 SWIR-VNIR + 9 IC bands + curvature + slope) bands), (d) ASTER (29 (9SWIR-VNIR + 9PC + 9IC bands + curvature + slope).

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Fig. 11: Lithological classification of the exposed rock units using SVM classifier applied on ASTER data. (a) ASTER (9 SWIR-VNIR bands), (B) ASTER 20 (9 SWIR-VNIR + 9 PC+ c + s) bands), (C) ASTER 20 (9 SWIR-VNIR +9 IC bands + curvature + slope) bands), (d) ASTER (29 (9SWIR-VNIR + 9PC + 9IC + curvature + slope) bands).

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Fig. 12: Grey scale ASTER band ratio images representing different mineral indices, (a) Kaolinite index b4/b5xb8/b6. (b) Calcite index (b6/b8xb9/b8). (c) Clay index (b5xb7/b6xb6). (d) sericite/muscovite/elite index (b5 + b7/b6).

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Fig. 13: Grey scale ASTER mineral indices images. (a) Alunite-Kaolinite- pyrophyllite index (b5xb7/b6xb6). (b) Carbonate/chlorite index (b9 + b7/b8).

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Highlights:

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• The Korbiai-Gerf area comprises Precambrian ophiolitic and island-arc assemblages. • The ophiolitc assemblages are thrusted over pelites-psammopelites and schistose metavolcanics. • The intrusives include tonalite-granodiorite, gabbro-diorite, gabbro-norite, monzogranites and syenite-quartz syenite. • The integrated Landsat-8 and ASTER data successfully discriminated the exposed rock units. • The SAM, SID and SVM classification algorithms were used for the first time to classify the basement rocks in the Eastern Desert of Egypt. • The produced geological map comprises well discriminated lithological units. The surface areas of the exposed rock units have been calculated.