The Nuqrah belt lies within the Hulayfah group of volcanic and volcano-sedimentary
rocks. This group comprises two formations, the lower or Afna formation
and the upper or Nuqrah formation. The Afna formation is mostly andesitic,
where basaltic flows exist containing the agglomeration of mafic tuff
and breccia. Units of rhyo-andesitic quartz-crystal tuffs are found near
the top of Afna formation. The upper, or Nuqrah formation, is a sequence
of alternating volcanic and sedimentary rocks in which three subunits
||The lower subunit begins with an alternating sequence of andesitic
crystal tuff and rhyolitic crystal tuff. This is followed by massive
felsic tuff, containing lithic fragments of rhyolite and subordinate
andesite, intercalated with rhyolite flows
||A middle subunit composed of lenticular beds of graphitic tuff,
calc-dolomitic marble and subordinate jasper with local concentrations
of base metal sulfides. Above this basal complex, the dominant facies
is bedded crystal and lapilli tuffs and individual flows of Na-rhyolite
||The upper subunit is mainly composed of volcaniclastic rocks beginning
with a thick conglomerate containing cobbles and of rhyolite and granophyre.
This grades upward to alternating beds of conglomerate and carbonate-cemented
felsic tuffite. Syntectonic intrusions include layered gabbro, diorite
and migmatite. Delfour (1977) considered all of these to be essentially
contemporaneous with Hulayfah group volcanism. Late- and posttectonic
intrusions include calc-alkaline, alkalic and peralkalic granites.
In the immediate Nuqrah district, Delfour (1977) noted large variations
in the rock assemblages of the lower and middle subunits of the Nuqrah
formation, which he attributed partly to deformation but mostly to the
variability of the original paleovolcanic environment. The succession
begins with about 2000 m of andesite, basaltic flows and related pyroclastics
of the Afna formation. These are succeeded by the Nuqrah formation, the
lower part of which is a rhyolitic and/or lapilli tuff grading laterally
to andesitic and basaltic tuff and breccia intercalated with rhyolite
andesite and basaltic flows. The top of this unit is composed of bedded
rhyolitic tuff, graphitic shale, marble, jasper and local lenses of base
metal sulfides. Above this is a succession of alternating Na-rhyolite
flows, rhyolitic tuff, graphitic tuff and carbonate-rich tuff and cherty
tuffite. In this area, Nuqrah formation rocks are intruded by porphyritic
rhyolite, small stocks and sills of gabbro and diabase all of which were
regarded by Delfour (1977) as being subvolcanic.
Location of Study Area
The study area, Nuqrah district, is located in the North-Eastern part
of the Arabian Peninsula (Fig. 1). The geographic coordinates
range from 40 to 42° East longitude and 25 to 26° North latitude.
The Nuqrah village is situated along a road linking the cities of Buraydah
and Medina. The village is about 230 km South West of the city of Buraydah
and 300 km North-East of Medina.
Location of study area, (GPS-acquired geographic
coordinates: 25°36.218` N, 41°30.875` E)
MATERIALS AND METHODS
A combination of geological, landcover and ASTER satellite datasets has
been studied to accomplish the present research. The Geo-science datasets
included the geo-referenced thematic layers of lithology, geomorphology
and structural geology.
In addition to visual interpretation, digital processing of multi-spectral
imagery datasets has been carried out to classify features of geological
significance in the study area. Image processing operations are algorithms
that map input images into output images. The mapping of hydrothermal
alteration has been an important outcome of the present research effort
carried out with the application of various image processing operations.
The addition of two shot-wave infrared bands in Thematic Mapper Sensor
of the LANDSAT System, many clay minerals have been found to have their
more diagnostic absorption features (Hunt and Salisbury, 1970).
Since 2000, Advanced Space-borne Thermal Emission and Reflection Radiometer
(ASTER) multispectral data have been used in mineralogical and lithological
studies (Rowan and Mars, 2001) and (Ninomiya, 2004). ASTER is a cooperative
effort between NASA and Japan`s Ministry of Economic Trade and Industry
(METI). The instrument was launched on board NASA`s TERRA spacecraft in
December 1999. ASTER is an advanced multispectral imager which covers
a wide spectral region of the electromagnetic spectrum from the Visible
near Infrared (VNIR) to the Thermal Infrared (TIR).
Terra has an orbital path similar to Landsat 7`s. Five instruments on
the spacecraft, including ASTER, can be combined to monitor all earth
systems (Abrams et al., 2002) and generate data in 60x60 km scenes.
ASTER data are used for a range of applications, including land-use studies,
mapping, water resources, coastal resources, environmental monitoring,
generation of Digital Elevation Models (DEMs) and mapping alteration patterns
or specific mineral assemblages known to be associated with mineral systems.
ASTER consists of three separate instrument subsystems (Table
||Visible and Near Infrared (VNIR)
||Shortwave Infrared (SWIR)
||Thermal Infrared (TIR)
The remote sensing operations are applied to the satellite imagery
to digitally enhance the feature details and render it more interpretable
in comparison to its raw state. As any image involves radiometric errors
as well as geometric errors, these errors should be corrected. Radiometric
correction is to avoid radiometric errors or distortions, while geometric
correction is to remove geometric distortion. When the emitted or reflected
electro-magnetic energy is observed by a sensor on board an aircraft or
spacecraft, the observed energy does not coincide with the energy emitted
or reflected from the same object observed from a short distance. This
is due to the sun`s azimuth and elevation, atmospheric conditions such
as fog or aerosols, sensor`s response etc. which influence the observed
energy. Therefore, in order to obtain the real irradiance or reflectance,
those radiometric distortions must be corrected.
||ASTER statistics along with 14 bands for data acquisition
At first, radiometric correction was applied to remove the atmospheric
effects. The correction was achieved by subtracting the minimum values,
corresponding to atmospheric haze, from each band.
To eliminate influence of water in the imagery, water bodies were masked.
The procedure involved band ratioing, where a ratio of band 4 to 2 was
computed and a threshold value was selected for masking the water features.
It was found that all pixel values below 0.6 represented water features.
The ratioed-image was displayed containing only the values greater than
0.6. The image represented all landcover features except the water feature.
That is how water features were masked out of the imagery dataset.
Image Analysis Through Digital Processing Techniques
An image of Aster for area of interest was processed for subsequent
analysis. Analysis of Aster data for lithological discrimination is based
on relationship between the spectral absorptance or emittance and the
mineral composition of rock units under investigation. Various image enhancement
and spatial filtering techniques were applied to the ASTER Imagery for
making the features visually prominent.
Processing of Imagery Dataset Comprising SWIR Bands
ASTER Imagery Dataset containing Short Wave Infra Red (SWIR) bands
were processed and analyzed for the investigation of Hydrothermal Alteration
and Gossan. The application of digital processing techniques on color
composite, developed from the combination of bands 4, 6 and 9, revealed
the existence of Gossan in the areas of hydrothermal alteration. The Gossan
appeared in red color while other areas of hydrothermal alteration appeared
in a range of colors from Verde reddish to light green. Figure
2 showed hydrothermal alteration zones and the Gossan in the processed
ASTER Imagery. It can be asserted by saying that all the SWIR bands can
be exploited for the investigation of the Gossan and the hydrothermal
alteration zones because data is captured at these bands in optical spectroscopy.
SWIR color composite (bands 469-RGB) in 3-A and field
photograph in 3-B
||Color composite of ratio images 1/2, 4/5 and 7/5 (RGB)
Prepared from ASTER, showing Gossan in bright red, while areas of
Hydrothermal alteration from pink to red
Image Rationing Technique
For identification and classification of Gossan and hydrothermal alteration
zones, ratio images were prepared for the study area. The ratio images
were created by dividing the Digital Number (DN) in one band by the corresponding
DN in another band for each pixel, stretching the result value and plotting
the new values as an image. The method is used by (Cappaccioni et al.,
2003) to extract spectral information from multispectral imagery. Colore
Composite of ratio images 1/2, 4/5 and 7/5 (RGB) that revealed Gossans
in red while showing the hydrothermal alteration zones from pink to red
(Fig. 3). This band combination is recommended because
many other rocks also have the resembling appearance.
RESULTS AND DISCUSSION
This study demonstrates the application of image processing techniques,
coupled with allied geo-spatial datasets, for the exploration, identification
and classification of Iron-rich Cap or Gossan and associated Hydrothermal
alteration zone, which forms clay Kaolinite, Montmorillonite, calcite
and chlorite-rich zones at Nuqrah area. The hydrothermal alteration which
usually consists of acid and some of the riolite and covers massive sulphide
deposits, is considered a good source of the raw material.
ASTER data processing shows good correlation with field data for the
appraisal of ore deposits.The analysis of ASTER`s SWIR datasets has been
found suitable for the detection and mapping of hydrothermal alteration
zones and Gossan.The research effort has produced fairly reliable and
accurate results and can therefore, be recommended for mineral exploration.