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Spectroscopy of the mineralized tonalite–diorite intrusions, Bulghah gold mine area, Saudi Arabia: Effects of opaques and alteration products on Fieldspec data
Ore Geology Reviews 44 (2012) 148–157
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Spectroscopy of the mineralized tonalite–diorite intrusions, Bulghah gold mine area, Saudi Arabia: Effects of opaques and alteration products on Fieldspec data Ahmed Madani ⁎, Hesham Harbi Department of Mineral Resources and Rocks, Faculty of Earth Sciences, King Abdulaziz University, P.O. BOX 80206, Jeddah, 21589, Saudi Arabia
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Article history: Received 28 November 2010 Received in revised form 14 September 2011 Accepted 15 September 2011 Available online 21 September 2011 Keywords: FieldSpec data Bulghah gold mine Mineralized tonalite–diorite intrusions Band ratios MNF technique
a b s t r a c t This paper aims to reveal the spectral characteristics of the mineralized tonalite–diorite intrusions exposed at Bulghah gold mine area, Saudi Arabia, using FieldSpec spectroradiometer and Landsat ETM+ data. Gold mineralization at Bulghah gold mine is hosted mainly by Syn- to Late-orogenic tonalite–diorite intrusions aligned in N–S direction and is associated mainly with cataclastic zones and quartz veins. Based on field, petrography and FieldSpec data two main tonalite–diorite groups namely A and B are recognized. Group “A”, recorded at the mine area, is characterized by low flat spectral profiles with overall low reflectance values (~ 10%). These low values are attributed to the presence of high content of trans-opaque phases (hematite and goethite) as indicated by petrographic study. Spectral profiles of group “B”, recorded at Bulghah North area, show high reflectance values (~40%) with three main absorption features around the 1.45 μm, 2.20 μm and 2.35 μm wavelength regions. These absorption features are attributed mainly to the presence of clay minerals, sericite, chlorite and carbonate alteration products. To discriminate the above mentioned tonalite–diorite groups, band ratios and Minimum Noise Fraction (MNF) techniques are conducted. Landsat false color composite band ratio image (7/4:R, 4/1:G and 4/5:B) discriminates easily the bluish brick red tonalite–diorite intrusions from the surrounding rock units but failed to distinguish the above mentioned two tonalite–diorite groups. On MNF3 image, the two tonalite–diorite groups can be distinguished easily in which group “A” has white color whereas group “B” has gray color. The present study proved the usefulness of FieldSpec spectral profiles and the processed Landsat ETM+ data for discriminating and delineating the mineralized tonalite–diorite intrusions exposed at the study area. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Arabian Shield can be divided into five terrains separated by four ophiolite-bearing suture zones (Al Shanti, 2009). Three of these terrains (Asir, Hijaz and Midyan) are island arc terrains formed in the western part of the shield. The other two terrains (Afif and Ar Rayn) are of continental affinity and are found further east (Al Shanti and Mitchell, 1976; Bakor, 1973; Bakor et al., 1976; Camp, 1984; Frisch and Al-Shanti, 1977; Gass, 1977, 1981; Greenwood et al., 1976). The Bulghah gold mine occurs at the boundary between the Hulayfah and Afif terrains (Lat. 24° 59′ N, Long. 41° 36′ E; Fig. 1), 70 km SW of Sukhaybarat gold mine. The Afif terrain contains younger post-orogenic granites (640–580 Ma), in addition to intermediate and felsic volcanic successions and molasse sediments (660–650 Ma) unconformably deposited over originally crystalline basement (Delfour,
⁎ Corresponding author. E-mail address: [email protected] (A. Madani). 0169-1368/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2011.09.013
1981; Stoeser et al., 1984). The Bulghah area hosts mainly syn- to latetectonic gold-bearing tonalite–diorite intrusions, Hulayfah volcanics (750–695 Ma), Murdama group and post-tectonic alkali granite–syenite intrusions. The plutonic intrusions can be divided into pre-orogenic, syn-orogenic and post-orogenic intrusions. The second and third types of plutonic intrusions are recorded at the study area. Gold deposits at Bulghah area are considered to be mesothermal gold deposits, a major type of gold mineralization in the Arabian Shield, and particularly abundant in the western part of the Afif terrane. The present study was conducted to reveal the spectral signatures of the syn- to late-orogenic tonalite–diorite intrusions exposed in the study area (the Bulghah gold mine and Bulghah North open pit area) in relation to their mineralogy, and to apply the acquired knowledge to discriminate and delineate the tonalite–diorite intrusions using band ratios and MNF techniques. Several authors have studied the reflectance spectra of minerals and rocks, e.g., Hunt and Salisbury (1970), Nash and Conel (1974), Hunt (1977), Hapke (1981), Clark (1999), Small et al. (2009) and Spinetti et al. (2009). The reflectance spectra of rocks give different absorption features based on mineralogical composition. Generally, basic rocks such as basalts show low reflectance profiles with obscured absorption features.
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Fig. 1. False color composite SPOT image shows the location of the study area. Upper left figure shows the main terrains of the Arabian Shield.
Whereas the spectral profiles of felsic rocks such as granitoids show different absorption features around wavelength regions of 1.8 to 2.50 μm based on their alteration products. Use of the portable FieldSpec spectroradiometer (ASD, 2007) is advantageous over the satellite measurements because of the continuous nature of its data as opposed to that of the Landsat data that is acquired over limited and discrete wavelength regions. Moreover, the Landsat data are modulated by the atmospheric contributions, whereas the FieldSpec spectra are not. 2. General geology and petrography of tonalite–diorite intrusions The tonalite–diorite intrusions in the Bulghah area are part of syn- to late-orogenic (670–610 Ma) plutons which intruded the Hulayfah/Siham and Murdamah/Bani Ghayy Groups (Harbi, 2004). Fig. 2 is a geological map of the study area modified from Nuqrah and Al Hissu Quadrangles (sheets 25E, GM-28 and 24E, GM-58). The Hulayfah Group represents the oldest rock unit (750–695 Ma; Agar et al., 1992) exposed at the study area and consists of an older Afna Formation and a younger Nuqrah Formation. Andesitic volcanic and volcaniclastic rocks commonly occur in the western and eastern parts of the area. The andesitic rocks include aphanitic and porphyritic andesites in addition to andesitic lapilli tuffs and agglomerates. These rocks are slightly foliated and show a greenschist facies
mineral assemblage of epidote, actinolite and chlorite in addition to altered plagioclase. Lenticular intercalations of gray marble and chert are abundant within the metasedimentary rocks of the Afna Formation. The Nuqrah Formation is mostly represented by acidic volcaniclastic rocks including agglomerate, fine laminated tuffs and intercalations of chert and/or cherty tuffs in addition to discontinuous marble bands. The volcaniclastic rocks show strong sericite, chlorite and pyrite alteration which may be related to sulfide mineralization within these rocks related to the volcanic activity during deposition. Discontinuous small lenses of gossans and jasperoidal gossans are mostly exposed in the western part of the study area. The Hulayfah volcanics are unconformably overlained by Murdama Group rocks (655–630 Ma; Johnson, 2005) which are exposed at the eastern part of the study area and are represented by sandstone, conglomerate, rhyolite, andesite and marble. Two main varieties of intrusive rocks belonging to Idah (620– 615 Ma) and Abanat (585–570 Ma) complexes intruded the stratified rock units. Previous studies (e.g., Eberte and Sahl, 1999), considered the diorite–tonalite intrusions at Bulghah area to be similar to the intrusions hosting gold mineralization at Sukhaybarat area and assigned to Idah complex. Recent U–Pb Zircon SHRIMP dating (Harbi and McNaughton, in preparation) of two samples of diorite– tonalite intrusions at Bulghah North and Bulgah mine area yielded ages of 678 ± 16 Ma and 667 ± 6 Ma, respectively. This indicates
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Fig. 2. Part of the geological map of the study area (1:250,000) shows the distribution of the different rock units cover the study area.
that the diorite–tonalite intrusions at Bulghah area are older than rocks of the Idah complex. These intrusive rocks which host gold mineralization at Bulghah area occupy the central part of the study area and occur as an elongated NNE to N–S trending body, 1 km wide and 5 km long. Several NW–SE trending andesite dykes, varying widths (5 to 12 m), are observed to cut the mineralized intrusive rocks. Two groups of tonalite–diorite intrusions in the study area are recognized during the present study. The first group, “A”, exposed at Bulghah gold mine, is characterized by dark color with low quartz content. In contrast, the second group, “B”, exposed at open pit site (Bulghah North), is more altered compared to group “A” and contains much more quartz. The second variety is observed in the northern part of the study area and is represented by alkali granite–syenite. Twenty three samples were collected from Bulghah gold mine and Bulghah North areas for spectral measurements and petrographic study. Microscopic investigation revealed that both tonalite–diorite groups are similar in primary mineral composition. Some mineralogical differences are present which influence the FieldSpec spectral profiles. Under the microscope, the tonalite–diorite intrusions are medium- to coarse-grained, composed mainly of plagioclase, clinopyroxene, hornblende and biotite as primary minerals (Fig. 3a); quartz and needles of apatite are accessories. Clay, sericite, chlorite and Fe-oxides are the main alteration products. The main mineralogical differences between the two tonalite–diorite groups are: 1) high percentage of opaque and trans-opaque phases (mainly goethite) characterizing group “A”; 2) presence of carbonate rhombs, titanite, clay minerals and chlorite characterizing group “B” (Fig. 3b).
3. Gold mineralization in the Bulghah area Gold mineralization at the study area is represented in the Bulghah gold mine and Bulghah North deposit. The Bulghah gold mine has produced gold since 2002. Gold production in 2006 totaled 3.865 t at an average ore grade of 0.6 g/t Au (SRK, 2007). Bulghah comprises an open-pit mine which mines low grade ore (b1.0 g/t Au) for processing at Bulghah heap leach processing facility. Extensive exploration activities by Ma'aden Co. in 2005 led to the discovery of the Bulghah North deposit which is currently under exploration and evaluation. Gold mineralization at the study area is mainly associated with: 1) cataclastic zones, microfractures and quartz (± carbonate) veinlets; 2) brown areas of iron oxide alterations resulting from the weathering of sulphide minerals that potentially host gold deposits; 3) contact between tonalite–diorite intrusions and Hulayfah volcanics, especially along the western fault contact. Barnicoat et al. (1998) studied several trenches along the western margin of the tonalite–diorite intrusions. They revealed presence of a narrow high grade mineralization zone (~ 2 g/t Au) traced for several 100 s of meters to the west of the intrusions. A conjugate fault system is observed in the Bulghah gold mine dissecting the tonalite–diorite intrusion. They have NE–SW and NW–SE directions, nearly vertical and about 20 cm wide. They are characterized by presence of brownish hematitic-clay material along the fault plane. Extensional and shear joint systems were recorded in the mine area. The study area was affected by three main deformational episodes (Barnicoat et al., 1998). Initial east–west crustal extension (phase I) was followed at a later time by east–west crustal compression (phase II). The final deformation
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array overlaid with an order separation filter. Each detector is geometrically positioned to receive light within a narrow (1.4 nm) range. The SWIR scanning spectrometers have one detector for SWIR1 (1000–1830 nm) and another for SWIR2 (1830–2500 nm). Each SWIR spectrometer consists of a concave halographic grating and a single thermo-electronically cooled Indium Gallium Arsenide (InGaAs) detector. The gratings are mounted around a common shaft that oscillates back and forth through a 15° swing. As the grating moves, it exposes the SWIR1 and SWIR2 detectors to different wavelengths of optical energy. The VNIR, SWIR1 and SWIR2 detectors convert incident photons into electrons. This photo current is continually converted to a voltage and is then periodically digitized by 16-bit analog-to-digital (A/D) converter. In the present study, the spectral data collection took place under suitable weather conditions (sunny, cloud-free days). Optimization and dark current collection are obtained before spectral data collection. Optimization adjusts the sensitivity of instrument detectors according to specific illumination conditions at the time of data measurements. Data measurements should be resampled as “RTRTRTRT” format in which “R” refers to reference spectra on a white panel whereas “T” refers to the measured rock sample. Fig. 4 shows the FieldSpec spectral profiles for the tonalite–diorite rock samples as well as other rock units exposed at the study areas (gossan, marble, volcanics and alkali granites). 4.2. Landsat ETM+ reflectance calculation
Fig. 3. a) Biotite (Bi), plagioclase (Pl), sericite (Sr) and quartz (Qz) in tonalite–diorite intrusions, group “A”. Note the Fe-hydroxides and chlorite alteration of biotite crystals, C.N. b) Chlorite (Ch) and Fe-oxides alteration of biotite (Bi), group “B”, C.N.
Reflectance calculations for the Landsat ETM + image subset for the study area are carried out using ENVI 4.5 software. The aim of ETM + data reflectance calculations is to convert the DN values of Landsat ETM + image subset to reflectance data used later for comparison with the ASD FieldSpec reflectance profiles. To achieve this aim, two main equations were applied (Curran, 1985; Rees, 1990): 1) Calculation of the spectral radiance (Lλ) using the equation: Lλ ¼ Lmin ðλÞ þ ð Lmax ðλÞ−Lmin ðλÞ=QCALmaxÞ QCAL
episode (phase III) was characterized by north–south crustal extension. Mineralization in the Bulghah gold mine was the result of structurally focused hydrothermal fluid, occurring emplacement during regionally extensive crustal extensional deformation and associated cataclastic fault development (Barnicoat et al., 1998). They are formed in interaplate settings and related to: a) major secondary Najd fault system, and b) granitic intrusions (Agar, 1992). They are typically associated with sulphide grains (arsenopyrite, pyrite and/or pyrrhotite) according to Harbi (2004) along veins and fractures.
where: Lmin (λ) = spectral radiance value at QCAL = 0, Lmax (λ) = spectral radiance value at QCAL = QCALmax, QCALmax = range of rescaled radiance value in digital number, QCAL = calibrated quantified scaled radiance values in digital number. Radiance is the most precise remote sensing radiometric measurement. It is the radiant flux per unit solid angle leaving an extended source in a given direction per unit of projected source area in that direction (Jensen, 2000). It is measured in Watt/m2/stredian (Wm − 2/sr − 1). Band math function in ENVI 4.5 package is used to convert the DN voltage image into radiance image. 2) Spectral radiance image was converted to reflectance values using solar irradiance and sun angle elevation values as follows:
4. Methodology 2
ρp ¼ π Lλ d =Esun ðλÞ cos θs 4.1. Field spectral data collection Throughout this study the spectral data for the mineralized tonalite–diorite rock samples collected from Bulghah gold mine and Bulghah North areas are obtained using a portable FieldSpec spectroradiometer. The FieldSpec is specifically designed for field environment to acquire visible near-infrared (VNIR) and shortwave infrared (SWIR) spectra (Analytical Spectral Devices, 2007). It is configured to have three separate halographic diffraction gratings with three separate detectors (VNIR, SWIR1 and SWIR2). The visible near-infrared (VNIR: 350–1000 nm) wavelength portion of the spectrum is measured by a 512 channel silicon photodiode
where, ρp = spectral reflectance, Lλ = spectral radiance at sensor (in mWcm − 2 ster − 1 μm − 1), d = earth–sun distance in astronomical units, Esun(λ) = mean solar exoatmospheric irradiance, θs = solar zenith angle and π = 3.14. The values of the resultant reflectance image seldom fall outside 1. Fig. 4 shows the Landsat spectral profiles for the tonalite–diorite groups as well as other rock units exposed at the study areas. Landsat spectral profile of tonalite–diorite intrusions (group A) shows direct increase in reflectance values from band 1 toward bands 3 and 5 and decrease in reflectance values in band 7. Whereas Landsat spectral profile of tonalite–diorite intrusions
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Fig. 4. FieldSpec and Landsat spectral profiles for: a) tonalite–diorite intrusion “group A”; b) tonalite–diorite intrusion “group B”; c) gossan; d) marble; e) Hulayfah volcanics; and f) Alkali granites.
(group B) shows gradual increase in reflectance values from band 1 toward bands 2, 3 and 5 and then decrease in band 7. Both groups can be distinguished spectrally using band 2 in which group “B” has relatively low reflectance value around band 2 compared to group “A”. The FieldSpec spectral profiles of tonalite–diorite intrusions (groups A and B) can also be easily distinguished. Low, nearly flat spectral profiles (~10%) in VNIR and SWIR wavelength regions characterize the tonalite–diorite intrusions of group “A” (Fig. 4a). Small absorption
feature is observed near 2.35 μm wavelength region. Moderate pronounced spectral reflectance values (~40%) characterize the tonalite–diorite intrusions (group B) at Bulghah North area (Fig. 4b). Three main absorption features around 1.45, 2.20 and 2.35 μm wavelength regions recognize the spectral profile of group “B”. Table 1 shows the spectral characteristics and related mineralogy of tonalite–diorite intrusions exposed in the study area. FieldSpec spectral profiles of other rock units (gossan, marble, volcanics and alkali granites) exposed around the study areas are also presented (Fig. 4c–f). Spectral profile of gossan which is mainly
Gradual increase in reflectance values from band 1 toward bands 2, 3 and 5 and then decrease in band 7. Group B has low reflectance values around band 2 compared to group A. 1—Moderate spectral reflectance values reaching 40% around band 5 region. 2—Three main absorption features around 1.45, 2.20 and 2.35 μm wavelength regions. Tonalite–diorite at Bulghah North (group B)
Direct increase in reflectance values from band 1 toward band 3 and 5 then the reflectance values decrease in band 7. 1—Low spectral reflectance values reaching 10% within VNIR and SWIR regions. 2—Very little absorption feature is observed near 2.35 μm wavelength region. Tonalite–diorite at Bulghah gold mine (group A)
Landsat reflectance profiles ASD FieldSpec measurements
Spectral properties Rock units
Table 1 Spectral characteristics and related mineralogy for tonalite–diorite intrusion groups exposed at the study areas.
Related mineralogical composition
– Medium to coarse grained. – Composed mainly of altered plagioclase, clinopyrxene, hornblend and biotite as essential minerals. – Quartz and needle apatite as accessories. – Fe-hydroxides and oxides are frequently recorded (high percentage (10%). – Serricite, chlorite and Fe-oxides are main lteration products. – Medium to coarse grained – Composed mainly of highly altered plagioclase, clinopyrxene, hornblend and biotite as essential minerals. – Quartz and sphene as accessories. – Serricite, clay, carbonate rhombs and Fe-oxides are main alteration products.
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composed of trans-opaque minerals (goethite and hematite) shows the following characteristic features: 1—low reflectance values (~10%) in VNIR wavelength regions; 2—abrupt increase in reflectance values around band 5 wavelength region (~70%); 3—gradual decrease in reflectance values toward band 7 (~40%); 4—small open absorption feature near 0.85 μm which is probably due to Fe-content. Landsat spectral profile of gossan resembles the FieldSpec spectral profiles in which it shows increase in reflectance values around bands 1 and 5 and then decrease in reflectance values in band 7. In addition to the broad absorption feature near 2.2 μm wavelength region, marble FieldSpec spectral profile also shows an open absorption feature around 0.90 μm wavelength region. The reflectance values increased from 10% in VNIR to 40% around band 5 wavelength region. Landsat spectral profile of marble shows the following spectral features: 1— increase reflectance values from band 1 through band 4; 2—nearly fixed reflectance values in band 5 wavelength region; and 3— gradual decrease in reflectance values in band 7. The FieldSpec spectral profile of Hulayfah volcanics shows a general low reflectance values throughout the VNIR and SWIR wavelength regions. Two small absorption features near 2.25 μm and 2.35 μm are recorded. A gradual increase in reflectance values from VNIR to SWIR regions characterizes the FieldSpec spectral profile for the alkali granite. A small absorption feature is recorded around the 2.40 μm wavelength region. 4.3. Band ratio technique Band ratio images can be simply generated by dividing the reflectance value of each pixel in one band by the reflectance value of the same pixel in another band (Drury, 1993). Ratios seldom fall outside the range of 0.25–4.0. The ratio values are rescaled to 0–255 to display the ratio images taking advantage of the existing full dynamic range. The six, non-thermal Landsat ETM + images that cover the study area, are used to generate the ratio images. Visual inspection of the generated band ratio images revealed that 7/4, 4/1 and 4/5 band ratio images are the most informative ratios for lithologic discrimination. The information contained in the above three band ratio images were integrated into one false color composite ratio image (7/4:R; 4/1:G and 4/5:B; Fig. 5). This image discriminates easily the tonalite–diorite intrusions in bluish red color. But unfortunately it failed to discriminate the two tonalite–diorite groups (A and B) exposed in the study area. Gossan, marble, Hulayfah volcanics and alkali granites have orange, blue, red and sky blue image signatures, respectively, on the false color composite ratio image in Fig. 5. To discriminate between the tonalite–diorite groups MNF transformation technique was conducted. 4.4. Discrimination of tonalite–diorite groups using the MNF transformation technique The Minimum Noise Fraction (MNF) transformation technique is used to determine the inherent dimensionality of image data, to segregate noise in the data, and to reduce the computational requirements for subsequent processing (Boardman and Kruse, 1994). It creates a set of components that contain weighted information about the variance across all bands in the raw data set. The MNF algorithm consists of two consecutive data reduction operations (Green et al., 1988). The first transformation, based on an estimated noise covariance matrix, decorrelates and rescales the noise in the data. The second step is a standard principal component transformation of the noise-whitened data. In the present study, the MNF technique is applied on the non-thermal 6 ETM+ bands whereas the ETM + thermal band is excluded from this process. The eigenvalues for the 6 output MNF eigenimages are shown in Fig. 6a. Examining the eigenvalues revealed that, the first three MNF bands (1, 2 and 3) have the highest values while the remaining bands have low values. These
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Fig. 5. False color composite ratios image (7/4:R; 4/1:G and 4/5:B) for the study area. Note closed vector refers to the location of Bulghah gold mine.
Fig. 6. MNF technique result. a) MNF eigenvalues plot of the ETM+ data. b) Scatter plot MNF bands 1 and 2 shows the tonalite–diorite cluster in the middle of the plot.
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Fig. 7. MNF eigenimages for the study area. Note MNF3 image (c) easily discriminates the two tonalite–diorite groups. The gossan has a white signature on MNF 2 image (b).
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4/5:B) discriminates easily the tonalite–diorite intrusions by bluish brick red color. This image failed to discriminate the two tonalite–diorite groups (A and B) exposed in the study areas. Gossan, marble ridges, Hulayfah volcanics and alkali granites have orange, blue, red and sky blue image signatures respectively on the above false color composite ratios image. The Minimum Noise Fraction (MNF) transformation technique is the second alternative technique used for discrimination of the tonalite–diorite groups. MNF3 image discriminates easily the two tonalite–diorite groups, in which group “A” has white color whereas group “B” has gray color. 6. Conclusions
Fig. 8. Compiled FieldSpec spectral profiles tonalite–diorite intrusions (groups A and B) exposed at Bulghah gold mine and Bulghah North areas.
bands contain most of the information compared with the rest MNF images. Fig. 7a–f shows the MNF eigenimages for bands 1 through 6. The MNF3 image (Fig. 7c) discriminates easily the two tonalite–diorite groups, in which group A has white color whereas group B has gray color. Also you can pick easily the gossan from MNF2 (Fig. 7b) by its white color. The scatterplot of the pixels of MNF bands 1 and 2 enabled to locate, identify, and cluster the tonalite–diorite groups in the middle of the plot (Fig. 6b).
The application of the FieldSpec measurements and the processed Landsat ETM + data clearly identifies and discriminates between the two groups of tonalite–diorite intrusions as well as other rock units exposed at the study areas. The low general reflectance values of tonalite–diorite intrusions of group “A” are mainly due to the presence of high percentage of opaque and trans-opaque phases. The three main absorption features observed in tonalite–diorite intrusions of group “B” reflectance profiles are probably due to sericite, clay, chlorite and carbonate alteration products. The band ratio technique failed to discriminate the two groups of tonalite–diorite intrusions which are discriminated easily on MNF 3 image in which group “A” has white color whereas group “B” has gray color. The present study presents a new technique that successfully discriminates, delineates and groups the mineralized tonalite–diorite intrusions exposed at the study area. The new discrimination and delineation will greatly help the exploration in the region. Acknowledgments
5. Discussion Field visits to the Bulghah gold mine and Bulghah North areas revealed the presence of two main tonalite–diorite groups (A and B). Samples from these areas were collected for FieldSpec measurements and petrographic study. Spectral data for tonalite–diorite groups were obtained using the FieldSpec measurements and Landsat ETM + reflectance data. Petrographic investigation revealed the similarity in the mineralogical composition between the tonalite–diorite groups (plagioclase, clinopyroxene, hornblende and biotite as main minerals whereas quartz, sphene and needle apatite are accessories) with some mineralogical differences affecting the spectral profiles of both groups. Sericite, clay, chlorite and Fe-hydroxides are main alteration products. The main mineralogical differences between the two groups are: 1) high percentage (up to 10%) of opaque and transopaque phases such as goethite characterizing group “A”; 2) the presence of carbonate rhombs and titanite characterizing group B; 3) sericite, clay and chlorite alteration products characterizing both groups with different degrees. The opaque minerals greatly affect the FieldSpec spectral profile of group A (Fig. 8) in which they are: 1) lowering the overall reflectance values; 2) obscure the absorption features that characterized the spectral profile due to alteration products. The FieldSpec spectral profiles of tonalite–diorite intrusion of group B (Fig. 8) are characterized by three main absorption features around the 1.45, 2.20 and 2.35 μm wavelength regions. They are mainly attributed to the presence of chlorite, sericite and clay alteration products as well as carbonate rhombs. Using observations from the spectral profiles, we assessed the validity of band ratios for discriminating the mineralized tonalite–diorite groups and developed alternative discriminating methodology (MNF) that utilized Landsat ETM + data over the study area. The six non-thermal Landsat ETM+ reflectance images which cover the study areas are used to generate the ratio images. 7/4, 4/1 and 4/5 band ratio images are found visually the most informative ratios for lithologic discrimination. The false color composite ratio image (7/4:R; 4/1:G and
The authors are gratefully thankful to Maa'den Company for facilitating the visit to open pit and obtaining samples from Bulghah open pit mine. The company is also acknowledged for providing accommodation for the authors in their mining camps during field work. References Agar, R.A., 1992. The tectono-metallogenic evolution of the Arabian shield. Precambrian Res. 58, 169–194. Agar, E.W., Stacey, J.S., Whitehouse, M.J., 1992. Evolution of the Southern Afif terrane: a geochronologic study: Saudi Arabian Directorate General of Mineral Resources Open-File Report DMMR-OF-10-15. 41 pp. Al Shanti, A., 2009. Geology of the Arabian Shield of Saudi Arabia. Jeddah, Scientific Research Centre. KAU, Saudi Arabia. 190 pp. Al Shanti, A.M.S., Mitchell, A.H.G., 1976. Late Precambrian subduction and collision in the Al Amar-Idsas region, Arabian Shield, Kingdom of Saudi Arabia. Tectonophysics 30, 41–47. ASD, 2007. FieldSpec® 3 User Manual. ASD inc., USA. Bakor, A.R., 1973. Jabal Al Wask: Precambrian basic and ultrabasic igneous complex in Northern Hijaz of Saudi Arabia. Unpublished Ph.D. Thesis, University of Leeds, U.K. Bakor, A.R., Gass, I.G., Neary, C., 1976. Jabal Al Wask, Northwest Saudi Arabia, an Eocambrian back-arc ophiolite. Earth Planet. Sci. Lett. 30, 1–9. Barnicoat, A.C., Freeman, S.R., Henderson, I.H.C., Phillips, G.M., 1998. Structural controls on gold mineralization in the Bulgah prospect. Rock Deformation Research – Leeds University, Report 03. 101 pp. Boardman, J.W., Kruse, F.A., 1994. Automated spectral analysis: A geologic example using AVIRIS data, north Grapevine Mountains, Nevada: in Proceedings. Tenth Thematic Conference on Geologic Remote Sensing, Environmental Research Institute of Michigan, Ann Arbor, MI. p. I-407-I-418. Camp, V.E., 1984. Island-arcs and their role in the evolution of the Western Arabian Shield. Geol. Soc. Am. Bull. 95, 913–921. Clark, R.N., 1999. Spectroscopy of rocks and minerals and principles of spectroscopy. In: Rencz, A.N. (Ed.), Manual of Remote Sensing. John Wiley & Sons, New York, pp. 3–58. Curran, P.J., 1985. Principles of Remote Sensing. Longman Scientific & Technical, England. Delfour, J., 1981. Geologic, tectonic and metallogenic evolution of the northern part of the Arabian Shield (Kingdom of Saudi Arabia). Bulletin du bureau de Researches et Minieres (deuxieme serie).; (1–2) Section II, pp. 1–19. Drury, S., 1993. Image Interpretation in Geology, 2nd edition. Chapman and Hall, London.
A. Madani, H. Harbi / Ore Geology Reviews 44 (2012) 148–157 Eberte, J.M., Sahl, M., 1999. Sukhaybarat gold deposit and mine. IUSGS/UNESCO Deposit Modeling Workshop. Field trip guide book. Jeddah, Kingdom of Saudi Arabia. 24 pp. Frisch, W., Al-Shanti, A.M.S., 1977. Ophiolite belts and the collision of island arcs in the Arabian Shield. Tectonophysics 43, 292–306. Gass, I.G., 1977. The evolution of the Pan-African crystalline basement in NE Africa and Arabia. J. Geol. Soc. 134, 129–138. Gass, I.G., 1981. Pan-African (Upper Proterozoic) plate tectonics of the Arabian-Nubian Shield. In: Kröner, A. (Ed.), Precambrian Plate Tectonics. Elsevier, Amsterdam, pp. 387–405. Green, A., Berman, M., Switzer, B., Craig, M.D., 1988. A transformation for ordering multispectral data in terms of image quality with implications for noise removal. IEEE Trans. Geosci. Remote. Sens. 26 (1), 65–74. Greenwood, W.R., Hadley, D.G., Anderson, R.F., Fleck, R.J., Schmidt, D.L., 1976. Late Proterozoic cratonization in southwestern Saudi Arabia. R. Soc. Lond. Philos. Trans. A 280, 517–527. Hapke, B., 1981. Bidirectional reflectance spectroscopy.1. Theory. J. Geophys. Res. 86, 3039–3054. Harbi, H.M., 2004. Genesis of gold mineralization at Zalm area, Central Saudi Arabia. K.S.A. 6th International Conference on Geochemistry, Alexandria University, Egypt, pp. 143–160. Harbi, H.M., McNaughton, N., in preparation. Geochronological investigation of goldhosted intrusive rocks within the Afif Terrane, Arabian Shield, Saudi Arabia. Hunt, G.R., 1977. Spectral signatures of particulate minerals, in the visible and near infrared. Geophysics 42, 501–513.
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Hunt, G.R., Salisbury, J.W., 1970. Visible and near infrared spectra of minerals and rocks. I. Silicate minerals. Mod. Geol. 1, 283–300. Jensen, J.R., 2000. Remote sensing of the environment, an earth resource perspective. Prentice Hall Series in Geographic Information Science. 544 pp. Johnson, P.R., 2005. Proterozoic geology of Western: Saudi Arabia, Northeastern sheet; notes on Proterozoic stratigraphy. Saudi Geological Survey Open-File Report SGS-OF2005-2. 39 pp. Nash, E.B., Conel, J.E., 1974. Spectral reflectance systematics for mixtures of powdered hypersthene, labradorite and ilmenite. J. Geophys. Res. 79, 1615–1621. Rees, W.G., 1990. Physical Principles of Remote Sensing. Cambridge University Press. Small, C., Steckler, M., Seeber, L., Akhter, S., Goodbred, S., Mia, B., Imam, B., 2009. Spectroscopy of sediments in the Ganges–Brahmaputra delta: spectral effects of moisture, grain size and lithology. Remote. Sens. Environ. 113, 342–361. Spinetti, C., Mazzarini, F., Casacchia, R., Colini, L., Neri, M., Behncke, B., Salvatori, R., Fabrizia, B., Pareschi, M., 2009. Spectral properties of volcanic materials from hyperspectral field and satellite data compared with LiDAR data at Mt. Etna. Int. J. Appl. Earth Obs. Geoinf. 11, 142–155. SRK Consulting (UK) Ltd., 2007. An Independent Mineral Experts' Report on the Gold Mining and Exploration Assets of Saudi Arabian Mining Company (Ma'aden). Stoeser, D.B., Stacey, J.S., Greenwood, W.R., Fischer, L.B., 1984. U/Pb zircon geochronology of the southern portion of the Nabitah mobile belt and Pan-African continental collision in the Saudi Arabian Shield, Deputy Ministry for Mineral Resources Technical Record USGS-TR-04-05.
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Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev
Spectroscopy of the mineralized tonalite–diorite intrusions, Bulghah gold mine area, Saudi Arabia: Effects of opaques and alteration products on Fieldspec data Ahmed Madani ⁎, Hesham Harbi Department of Mineral Resources and Rocks, Faculty of Earth Sciences, King Abdulaziz University, P.O. BOX 80206, Jeddah, 21589, Saudi Arabia
a r t i c l e
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Article history: Received 28 November 2010 Received in revised form 14 September 2011 Accepted 15 September 2011 Available online 21 September 2011 Keywords: FieldSpec data Bulghah gold mine Mineralized tonalite–diorite intrusions Band ratios MNF technique
a b s t r a c t This paper aims to reveal the spectral characteristics of the mineralized tonalite–diorite intrusions exposed at Bulghah gold mine area, Saudi Arabia, using FieldSpec spectroradiometer and Landsat ETM+ data. Gold mineralization at Bulghah gold mine is hosted mainly by Syn- to Late-orogenic tonalite–diorite intrusions aligned in N–S direction and is associated mainly with cataclastic zones and quartz veins. Based on field, petrography and FieldSpec data two main tonalite–diorite groups namely A and B are recognized. Group “A”, recorded at the mine area, is characterized by low flat spectral profiles with overall low reflectance values (~ 10%). These low values are attributed to the presence of high content of trans-opaque phases (hematite and goethite) as indicated by petrographic study. Spectral profiles of group “B”, recorded at Bulghah North area, show high reflectance values (~40%) with three main absorption features around the 1.45 μm, 2.20 μm and 2.35 μm wavelength regions. These absorption features are attributed mainly to the presence of clay minerals, sericite, chlorite and carbonate alteration products. To discriminate the above mentioned tonalite–diorite groups, band ratios and Minimum Noise Fraction (MNF) techniques are conducted. Landsat false color composite band ratio image (7/4:R, 4/1:G and 4/5:B) discriminates easily the bluish brick red tonalite–diorite intrusions from the surrounding rock units but failed to distinguish the above mentioned two tonalite–diorite groups. On MNF3 image, the two tonalite–diorite groups can be distinguished easily in which group “A” has white color whereas group “B” has gray color. The present study proved the usefulness of FieldSpec spectral profiles and the processed Landsat ETM+ data for discriminating and delineating the mineralized tonalite–diorite intrusions exposed at the study area. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Arabian Shield can be divided into five terrains separated by four ophiolite-bearing suture zones (Al Shanti, 2009). Three of these terrains (Asir, Hijaz and Midyan) are island arc terrains formed in the western part of the shield. The other two terrains (Afif and Ar Rayn) are of continental affinity and are found further east (Al Shanti and Mitchell, 1976; Bakor, 1973; Bakor et al., 1976; Camp, 1984; Frisch and Al-Shanti, 1977; Gass, 1977, 1981; Greenwood et al., 1976). The Bulghah gold mine occurs at the boundary between the Hulayfah and Afif terrains (Lat. 24° 59′ N, Long. 41° 36′ E; Fig. 1), 70 km SW of Sukhaybarat gold mine. The Afif terrain contains younger post-orogenic granites (640–580 Ma), in addition to intermediate and felsic volcanic successions and molasse sediments (660–650 Ma) unconformably deposited over originally crystalline basement (Delfour,
⁎ Corresponding author. E-mail address: [email protected] (A. Madani). 0169-1368/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2011.09.013
1981; Stoeser et al., 1984). The Bulghah area hosts mainly syn- to latetectonic gold-bearing tonalite–diorite intrusions, Hulayfah volcanics (750–695 Ma), Murdama group and post-tectonic alkali granite–syenite intrusions. The plutonic intrusions can be divided into pre-orogenic, syn-orogenic and post-orogenic intrusions. The second and third types of plutonic intrusions are recorded at the study area. Gold deposits at Bulghah area are considered to be mesothermal gold deposits, a major type of gold mineralization in the Arabian Shield, and particularly abundant in the western part of the Afif terrane. The present study was conducted to reveal the spectral signatures of the syn- to late-orogenic tonalite–diorite intrusions exposed in the study area (the Bulghah gold mine and Bulghah North open pit area) in relation to their mineralogy, and to apply the acquired knowledge to discriminate and delineate the tonalite–diorite intrusions using band ratios and MNF techniques. Several authors have studied the reflectance spectra of minerals and rocks, e.g., Hunt and Salisbury (1970), Nash and Conel (1974), Hunt (1977), Hapke (1981), Clark (1999), Small et al. (2009) and Spinetti et al. (2009). The reflectance spectra of rocks give different absorption features based on mineralogical composition. Generally, basic rocks such as basalts show low reflectance profiles with obscured absorption features.
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Fig. 1. False color composite SPOT image shows the location of the study area. Upper left figure shows the main terrains of the Arabian Shield.
Whereas the spectral profiles of felsic rocks such as granitoids show different absorption features around wavelength regions of 1.8 to 2.50 μm based on their alteration products. Use of the portable FieldSpec spectroradiometer (ASD, 2007) is advantageous over the satellite measurements because of the continuous nature of its data as opposed to that of the Landsat data that is acquired over limited and discrete wavelength regions. Moreover, the Landsat data are modulated by the atmospheric contributions, whereas the FieldSpec spectra are not. 2. General geology and petrography of tonalite–diorite intrusions The tonalite–diorite intrusions in the Bulghah area are part of syn- to late-orogenic (670–610 Ma) plutons which intruded the Hulayfah/Siham and Murdamah/Bani Ghayy Groups (Harbi, 2004). Fig. 2 is a geological map of the study area modified from Nuqrah and Al Hissu Quadrangles (sheets 25E, GM-28 and 24E, GM-58). The Hulayfah Group represents the oldest rock unit (750–695 Ma; Agar et al., 1992) exposed at the study area and consists of an older Afna Formation and a younger Nuqrah Formation. Andesitic volcanic and volcaniclastic rocks commonly occur in the western and eastern parts of the area. The andesitic rocks include aphanitic and porphyritic andesites in addition to andesitic lapilli tuffs and agglomerates. These rocks are slightly foliated and show a greenschist facies
mineral assemblage of epidote, actinolite and chlorite in addition to altered plagioclase. Lenticular intercalations of gray marble and chert are abundant within the metasedimentary rocks of the Afna Formation. The Nuqrah Formation is mostly represented by acidic volcaniclastic rocks including agglomerate, fine laminated tuffs and intercalations of chert and/or cherty tuffs in addition to discontinuous marble bands. The volcaniclastic rocks show strong sericite, chlorite and pyrite alteration which may be related to sulfide mineralization within these rocks related to the volcanic activity during deposition. Discontinuous small lenses of gossans and jasperoidal gossans are mostly exposed in the western part of the study area. The Hulayfah volcanics are unconformably overlained by Murdama Group rocks (655–630 Ma; Johnson, 2005) which are exposed at the eastern part of the study area and are represented by sandstone, conglomerate, rhyolite, andesite and marble. Two main varieties of intrusive rocks belonging to Idah (620– 615 Ma) and Abanat (585–570 Ma) complexes intruded the stratified rock units. Previous studies (e.g., Eberte and Sahl, 1999), considered the diorite–tonalite intrusions at Bulghah area to be similar to the intrusions hosting gold mineralization at Sukhaybarat area and assigned to Idah complex. Recent U–Pb Zircon SHRIMP dating (Harbi and McNaughton, in preparation) of two samples of diorite– tonalite intrusions at Bulghah North and Bulgah mine area yielded ages of 678 ± 16 Ma and 667 ± 6 Ma, respectively. This indicates
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Fig. 2. Part of the geological map of the study area (1:250,000) shows the distribution of the different rock units cover the study area.
that the diorite–tonalite intrusions at Bulghah area are older than rocks of the Idah complex. These intrusive rocks which host gold mineralization at Bulghah area occupy the central part of the study area and occur as an elongated NNE to N–S trending body, 1 km wide and 5 km long. Several NW–SE trending andesite dykes, varying widths (5 to 12 m), are observed to cut the mineralized intrusive rocks. Two groups of tonalite–diorite intrusions in the study area are recognized during the present study. The first group, “A”, exposed at Bulghah gold mine, is characterized by dark color with low quartz content. In contrast, the second group, “B”, exposed at open pit site (Bulghah North), is more altered compared to group “A” and contains much more quartz. The second variety is observed in the northern part of the study area and is represented by alkali granite–syenite. Twenty three samples were collected from Bulghah gold mine and Bulghah North areas for spectral measurements and petrographic study. Microscopic investigation revealed that both tonalite–diorite groups are similar in primary mineral composition. Some mineralogical differences are present which influence the FieldSpec spectral profiles. Under the microscope, the tonalite–diorite intrusions are medium- to coarse-grained, composed mainly of plagioclase, clinopyroxene, hornblende and biotite as primary minerals (Fig. 3a); quartz and needles of apatite are accessories. Clay, sericite, chlorite and Fe-oxides are the main alteration products. The main mineralogical differences between the two tonalite–diorite groups are: 1) high percentage of opaque and trans-opaque phases (mainly goethite) characterizing group “A”; 2) presence of carbonate rhombs, titanite, clay minerals and chlorite characterizing group “B” (Fig. 3b).
3. Gold mineralization in the Bulghah area Gold mineralization at the study area is represented in the Bulghah gold mine and Bulghah North deposit. The Bulghah gold mine has produced gold since 2002. Gold production in 2006 totaled 3.865 t at an average ore grade of 0.6 g/t Au (SRK, 2007). Bulghah comprises an open-pit mine which mines low grade ore (b1.0 g/t Au) for processing at Bulghah heap leach processing facility. Extensive exploration activities by Ma'aden Co. in 2005 led to the discovery of the Bulghah North deposit which is currently under exploration and evaluation. Gold mineralization at the study area is mainly associated with: 1) cataclastic zones, microfractures and quartz (± carbonate) veinlets; 2) brown areas of iron oxide alterations resulting from the weathering of sulphide minerals that potentially host gold deposits; 3) contact between tonalite–diorite intrusions and Hulayfah volcanics, especially along the western fault contact. Barnicoat et al. (1998) studied several trenches along the western margin of the tonalite–diorite intrusions. They revealed presence of a narrow high grade mineralization zone (~ 2 g/t Au) traced for several 100 s of meters to the west of the intrusions. A conjugate fault system is observed in the Bulghah gold mine dissecting the tonalite–diorite intrusion. They have NE–SW and NW–SE directions, nearly vertical and about 20 cm wide. They are characterized by presence of brownish hematitic-clay material along the fault plane. Extensional and shear joint systems were recorded in the mine area. The study area was affected by three main deformational episodes (Barnicoat et al., 1998). Initial east–west crustal extension (phase I) was followed at a later time by east–west crustal compression (phase II). The final deformation
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array overlaid with an order separation filter. Each detector is geometrically positioned to receive light within a narrow (1.4 nm) range. The SWIR scanning spectrometers have one detector for SWIR1 (1000–1830 nm) and another for SWIR2 (1830–2500 nm). Each SWIR spectrometer consists of a concave halographic grating and a single thermo-electronically cooled Indium Gallium Arsenide (InGaAs) detector. The gratings are mounted around a common shaft that oscillates back and forth through a 15° swing. As the grating moves, it exposes the SWIR1 and SWIR2 detectors to different wavelengths of optical energy. The VNIR, SWIR1 and SWIR2 detectors convert incident photons into electrons. This photo current is continually converted to a voltage and is then periodically digitized by 16-bit analog-to-digital (A/D) converter. In the present study, the spectral data collection took place under suitable weather conditions (sunny, cloud-free days). Optimization and dark current collection are obtained before spectral data collection. Optimization adjusts the sensitivity of instrument detectors according to specific illumination conditions at the time of data measurements. Data measurements should be resampled as “RTRTRTRT” format in which “R” refers to reference spectra on a white panel whereas “T” refers to the measured rock sample. Fig. 4 shows the FieldSpec spectral profiles for the tonalite–diorite rock samples as well as other rock units exposed at the study areas (gossan, marble, volcanics and alkali granites). 4.2. Landsat ETM+ reflectance calculation
Fig. 3. a) Biotite (Bi), plagioclase (Pl), sericite (Sr) and quartz (Qz) in tonalite–diorite intrusions, group “A”. Note the Fe-hydroxides and chlorite alteration of biotite crystals, C.N. b) Chlorite (Ch) and Fe-oxides alteration of biotite (Bi), group “B”, C.N.
Reflectance calculations for the Landsat ETM + image subset for the study area are carried out using ENVI 4.5 software. The aim of ETM + data reflectance calculations is to convert the DN values of Landsat ETM + image subset to reflectance data used later for comparison with the ASD FieldSpec reflectance profiles. To achieve this aim, two main equations were applied (Curran, 1985; Rees, 1990): 1) Calculation of the spectral radiance (Lλ) using the equation: Lλ ¼ Lmin ðλÞ þ ð Lmax ðλÞ−Lmin ðλÞ=QCALmaxÞ QCAL
episode (phase III) was characterized by north–south crustal extension. Mineralization in the Bulghah gold mine was the result of structurally focused hydrothermal fluid, occurring emplacement during regionally extensive crustal extensional deformation and associated cataclastic fault development (Barnicoat et al., 1998). They are formed in interaplate settings and related to: a) major secondary Najd fault system, and b) granitic intrusions (Agar, 1992). They are typically associated with sulphide grains (arsenopyrite, pyrite and/or pyrrhotite) according to Harbi (2004) along veins and fractures.
where: Lmin (λ) = spectral radiance value at QCAL = 0, Lmax (λ) = spectral radiance value at QCAL = QCALmax, QCALmax = range of rescaled radiance value in digital number, QCAL = calibrated quantified scaled radiance values in digital number. Radiance is the most precise remote sensing radiometric measurement. It is the radiant flux per unit solid angle leaving an extended source in a given direction per unit of projected source area in that direction (Jensen, 2000). It is measured in Watt/m2/stredian (Wm − 2/sr − 1). Band math function in ENVI 4.5 package is used to convert the DN voltage image into radiance image. 2) Spectral radiance image was converted to reflectance values using solar irradiance and sun angle elevation values as follows:
4. Methodology 2
ρp ¼ π Lλ d =Esun ðλÞ cos θs 4.1. Field spectral data collection Throughout this study the spectral data for the mineralized tonalite–diorite rock samples collected from Bulghah gold mine and Bulghah North areas are obtained using a portable FieldSpec spectroradiometer. The FieldSpec is specifically designed for field environment to acquire visible near-infrared (VNIR) and shortwave infrared (SWIR) spectra (Analytical Spectral Devices, 2007). It is configured to have three separate halographic diffraction gratings with three separate detectors (VNIR, SWIR1 and SWIR2). The visible near-infrared (VNIR: 350–1000 nm) wavelength portion of the spectrum is measured by a 512 channel silicon photodiode
where, ρp = spectral reflectance, Lλ = spectral radiance at sensor (in mWcm − 2 ster − 1 μm − 1), d = earth–sun distance in astronomical units, Esun(λ) = mean solar exoatmospheric irradiance, θs = solar zenith angle and π = 3.14. The values of the resultant reflectance image seldom fall outside 1. Fig. 4 shows the Landsat spectral profiles for the tonalite–diorite groups as well as other rock units exposed at the study areas. Landsat spectral profile of tonalite–diorite intrusions (group A) shows direct increase in reflectance values from band 1 toward bands 3 and 5 and decrease in reflectance values in band 7. Whereas Landsat spectral profile of tonalite–diorite intrusions
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Fig. 4. FieldSpec and Landsat spectral profiles for: a) tonalite–diorite intrusion “group A”; b) tonalite–diorite intrusion “group B”; c) gossan; d) marble; e) Hulayfah volcanics; and f) Alkali granites.
(group B) shows gradual increase in reflectance values from band 1 toward bands 2, 3 and 5 and then decrease in band 7. Both groups can be distinguished spectrally using band 2 in which group “B” has relatively low reflectance value around band 2 compared to group “A”. The FieldSpec spectral profiles of tonalite–diorite intrusions (groups A and B) can also be easily distinguished. Low, nearly flat spectral profiles (~10%) in VNIR and SWIR wavelength regions characterize the tonalite–diorite intrusions of group “A” (Fig. 4a). Small absorption
feature is observed near 2.35 μm wavelength region. Moderate pronounced spectral reflectance values (~40%) characterize the tonalite–diorite intrusions (group B) at Bulghah North area (Fig. 4b). Three main absorption features around 1.45, 2.20 and 2.35 μm wavelength regions recognize the spectral profile of group “B”. Table 1 shows the spectral characteristics and related mineralogy of tonalite–diorite intrusions exposed in the study area. FieldSpec spectral profiles of other rock units (gossan, marble, volcanics and alkali granites) exposed around the study areas are also presented (Fig. 4c–f). Spectral profile of gossan which is mainly
Gradual increase in reflectance values from band 1 toward bands 2, 3 and 5 and then decrease in band 7. Group B has low reflectance values around band 2 compared to group A. 1—Moderate spectral reflectance values reaching 40% around band 5 region. 2—Three main absorption features around 1.45, 2.20 and 2.35 μm wavelength regions. Tonalite–diorite at Bulghah North (group B)
Direct increase in reflectance values from band 1 toward band 3 and 5 then the reflectance values decrease in band 7. 1—Low spectral reflectance values reaching 10% within VNIR and SWIR regions. 2—Very little absorption feature is observed near 2.35 μm wavelength region. Tonalite–diorite at Bulghah gold mine (group A)
Landsat reflectance profiles ASD FieldSpec measurements
Spectral properties Rock units
Table 1 Spectral characteristics and related mineralogy for tonalite–diorite intrusion groups exposed at the study areas.
Related mineralogical composition
– Medium to coarse grained. – Composed mainly of altered plagioclase, clinopyrxene, hornblend and biotite as essential minerals. – Quartz and needle apatite as accessories. – Fe-hydroxides and oxides are frequently recorded (high percentage (10%). – Serricite, chlorite and Fe-oxides are main lteration products. – Medium to coarse grained – Composed mainly of highly altered plagioclase, clinopyrxene, hornblend and biotite as essential minerals. – Quartz and sphene as accessories. – Serricite, clay, carbonate rhombs and Fe-oxides are main alteration products.
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composed of trans-opaque minerals (goethite and hematite) shows the following characteristic features: 1—low reflectance values (~10%) in VNIR wavelength regions; 2—abrupt increase in reflectance values around band 5 wavelength region (~70%); 3—gradual decrease in reflectance values toward band 7 (~40%); 4—small open absorption feature near 0.85 μm which is probably due to Fe-content. Landsat spectral profile of gossan resembles the FieldSpec spectral profiles in which it shows increase in reflectance values around bands 1 and 5 and then decrease in reflectance values in band 7. In addition to the broad absorption feature near 2.2 μm wavelength region, marble FieldSpec spectral profile also shows an open absorption feature around 0.90 μm wavelength region. The reflectance values increased from 10% in VNIR to 40% around band 5 wavelength region. Landsat spectral profile of marble shows the following spectral features: 1— increase reflectance values from band 1 through band 4; 2—nearly fixed reflectance values in band 5 wavelength region; and 3— gradual decrease in reflectance values in band 7. The FieldSpec spectral profile of Hulayfah volcanics shows a general low reflectance values throughout the VNIR and SWIR wavelength regions. Two small absorption features near 2.25 μm and 2.35 μm are recorded. A gradual increase in reflectance values from VNIR to SWIR regions characterizes the FieldSpec spectral profile for the alkali granite. A small absorption feature is recorded around the 2.40 μm wavelength region. 4.3. Band ratio technique Band ratio images can be simply generated by dividing the reflectance value of each pixel in one band by the reflectance value of the same pixel in another band (Drury, 1993). Ratios seldom fall outside the range of 0.25–4.0. The ratio values are rescaled to 0–255 to display the ratio images taking advantage of the existing full dynamic range. The six, non-thermal Landsat ETM + images that cover the study area, are used to generate the ratio images. Visual inspection of the generated band ratio images revealed that 7/4, 4/1 and 4/5 band ratio images are the most informative ratios for lithologic discrimination. The information contained in the above three band ratio images were integrated into one false color composite ratio image (7/4:R; 4/1:G and 4/5:B; Fig. 5). This image discriminates easily the tonalite–diorite intrusions in bluish red color. But unfortunately it failed to discriminate the two tonalite–diorite groups (A and B) exposed in the study area. Gossan, marble, Hulayfah volcanics and alkali granites have orange, blue, red and sky blue image signatures, respectively, on the false color composite ratio image in Fig. 5. To discriminate between the tonalite–diorite groups MNF transformation technique was conducted. 4.4. Discrimination of tonalite–diorite groups using the MNF transformation technique The Minimum Noise Fraction (MNF) transformation technique is used to determine the inherent dimensionality of image data, to segregate noise in the data, and to reduce the computational requirements for subsequent processing (Boardman and Kruse, 1994). It creates a set of components that contain weighted information about the variance across all bands in the raw data set. The MNF algorithm consists of two consecutive data reduction operations (Green et al., 1988). The first transformation, based on an estimated noise covariance matrix, decorrelates and rescales the noise in the data. The second step is a standard principal component transformation of the noise-whitened data. In the present study, the MNF technique is applied on the non-thermal 6 ETM+ bands whereas the ETM + thermal band is excluded from this process. The eigenvalues for the 6 output MNF eigenimages are shown in Fig. 6a. Examining the eigenvalues revealed that, the first three MNF bands (1, 2 and 3) have the highest values while the remaining bands have low values. These
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Fig. 5. False color composite ratios image (7/4:R; 4/1:G and 4/5:B) for the study area. Note closed vector refers to the location of Bulghah gold mine.
Fig. 6. MNF technique result. a) MNF eigenvalues plot of the ETM+ data. b) Scatter plot MNF bands 1 and 2 shows the tonalite–diorite cluster in the middle of the plot.
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Fig. 7. MNF eigenimages for the study area. Note MNF3 image (c) easily discriminates the two tonalite–diorite groups. The gossan has a white signature on MNF 2 image (b).
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4/5:B) discriminates easily the tonalite–diorite intrusions by bluish brick red color. This image failed to discriminate the two tonalite–diorite groups (A and B) exposed in the study areas. Gossan, marble ridges, Hulayfah volcanics and alkali granites have orange, blue, red and sky blue image signatures respectively on the above false color composite ratios image. The Minimum Noise Fraction (MNF) transformation technique is the second alternative technique used for discrimination of the tonalite–diorite groups. MNF3 image discriminates easily the two tonalite–diorite groups, in which group “A” has white color whereas group “B” has gray color. 6. Conclusions
Fig. 8. Compiled FieldSpec spectral profiles tonalite–diorite intrusions (groups A and B) exposed at Bulghah gold mine and Bulghah North areas.
bands contain most of the information compared with the rest MNF images. Fig. 7a–f shows the MNF eigenimages for bands 1 through 6. The MNF3 image (Fig. 7c) discriminates easily the two tonalite–diorite groups, in which group A has white color whereas group B has gray color. Also you can pick easily the gossan from MNF2 (Fig. 7b) by its white color. The scatterplot of the pixels of MNF bands 1 and 2 enabled to locate, identify, and cluster the tonalite–diorite groups in the middle of the plot (Fig. 6b).
The application of the FieldSpec measurements and the processed Landsat ETM + data clearly identifies and discriminates between the two groups of tonalite–diorite intrusions as well as other rock units exposed at the study areas. The low general reflectance values of tonalite–diorite intrusions of group “A” are mainly due to the presence of high percentage of opaque and trans-opaque phases. The three main absorption features observed in tonalite–diorite intrusions of group “B” reflectance profiles are probably due to sericite, clay, chlorite and carbonate alteration products. The band ratio technique failed to discriminate the two groups of tonalite–diorite intrusions which are discriminated easily on MNF 3 image in which group “A” has white color whereas group “B” has gray color. The present study presents a new technique that successfully discriminates, delineates and groups the mineralized tonalite–diorite intrusions exposed at the study area. The new discrimination and delineation will greatly help the exploration in the region. Acknowledgments
5. Discussion Field visits to the Bulghah gold mine and Bulghah North areas revealed the presence of two main tonalite–diorite groups (A and B). Samples from these areas were collected for FieldSpec measurements and petrographic study. Spectral data for tonalite–diorite groups were obtained using the FieldSpec measurements and Landsat ETM + reflectance data. Petrographic investigation revealed the similarity in the mineralogical composition between the tonalite–diorite groups (plagioclase, clinopyroxene, hornblende and biotite as main minerals whereas quartz, sphene and needle apatite are accessories) with some mineralogical differences affecting the spectral profiles of both groups. Sericite, clay, chlorite and Fe-hydroxides are main alteration products. The main mineralogical differences between the two groups are: 1) high percentage (up to 10%) of opaque and transopaque phases such as goethite characterizing group “A”; 2) the presence of carbonate rhombs and titanite characterizing group B; 3) sericite, clay and chlorite alteration products characterizing both groups with different degrees. The opaque minerals greatly affect the FieldSpec spectral profile of group A (Fig. 8) in which they are: 1) lowering the overall reflectance values; 2) obscure the absorption features that characterized the spectral profile due to alteration products. The FieldSpec spectral profiles of tonalite–diorite intrusion of group B (Fig. 8) are characterized by three main absorption features around the 1.45, 2.20 and 2.35 μm wavelength regions. They are mainly attributed to the presence of chlorite, sericite and clay alteration products as well as carbonate rhombs. Using observations from the spectral profiles, we assessed the validity of band ratios for discriminating the mineralized tonalite–diorite groups and developed alternative discriminating methodology (MNF) that utilized Landsat ETM + data over the study area. The six non-thermal Landsat ETM+ reflectance images which cover the study areas are used to generate the ratio images. 7/4, 4/1 and 4/5 band ratio images are found visually the most informative ratios for lithologic discrimination. The false color composite ratio image (7/4:R; 4/1:G and
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