Characterization of ASTER spectral bands for mapping of alteration zones of volcanogenic massive sulphide deposits

Ore Geology Reviews 88 (2017) 317–335

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Review

Characterization of ASTER spectral bands for mapping of alteration zones of volcanogenic massive sulphide deposits Sankaran Rajendran a,⇑, Sobhi Nasir b a b

Department of Earth Sciences, Sultan Qaboos University, Al-Khod, 123 Muscat, Oman Earth Science Research Centre, Sultan Qaboos University, Al-Khod, 123 Muscat, Oman

a r t i c l e

i n f o

Article history: Received 22 September 2016 Received in revised form 15 March 2017 Accepted 20 April 2017 Available online 27 April 2017 Keywords: Spectral signatures ASTER Mapping Alteration zones Volcanogenic massive sulphide deposits Sultanate of Oman

a b s t r a c t In this study, the oxidized and hydrothermally altered zones, including propylitic, argillic and phyllic zones associated with volcanogenic massive sulphide deposits in the Sohar-Shinas region of the Sultanate of Oman are studied based on absorption characters of spectral bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). In this work, we have used selected imaging processing methods, including decorrelation stretching, band ratios, linear spectral unmixing (LSU) and Mixture Tuned Matched Filtering (MTMF), as well as petrological and geochemical characteristics of the alteration zones to map and understand the distribution of these alteration zones. Results indicate that 1) the decorrelation of ASTER spectral bands 3, 6, and 8 discriminate extrusive basalts, and the host lithology of altered zones; 2) the application of ASTER band ratios (5/3 + 1/2) (oxidized zone), (4 + 6)/5 (argillic zone) and (5 + 7)/6 (phyllic zone) delineate alteration zones spatially associated with mineralized zones, 3) the utilization of ASTER indices for OH bearing altered minerals (e.g. kaolin and alunite indices) confirmed the presence of alteration minerals in the mineralized zone, 4) alteration end members derived using the spectral angle mapper (SAM) method in combination with linear spectral unmixing (LSU) can be used to map the distribution of the weathering and alteration zones, 5) short wavelength infrared (SWIR) measurements on field samples were used to confirm spectral measurements from ASTER data, and 6) microscopic study and XRD analysis of field samples confirmed the presence of hematite and goethite in the oxidized zone; chlorite, epidote and calcite in the propylitic zone; kaolin and chlorite in the argillic zone; sericite and illite in the phyllic zone, the major minerals responsible to absorptions of ASTER bands 3, 8, 5 and 6 respectively. This study demonstrated the sensor capability of ASTER and the potential use of the image processing methods, and documented the spectral absorptions values of the altered minerals and the field and mineralogical characters of alteration zones of the volcanogenic massive sulphide deposits to map similar deposits elsewhere. Ó 2017 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geology of the study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Data and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spectral absorption of minerals of alteration zones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ASTER image processing and analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Mapping of extrusives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Mapping of mineralized zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Mapping of altered minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Mapping of alteration zones of VMS deposit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laboratory studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Petrography of alteration zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (S. Rajendran). http://dx.doi.org/10.1016/j.oregeorev.2017.04.016 0169-1368/Ó 2017 Elsevier B.V. All rights reserved.

318 319 319 320 321 321 323 324 325 328 328

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Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

1. Introduction Applications of remotely sensed satellite data are widely used and unique in mapping of different lithologies, mineral resources, ore deposits and minerals of alteration zone in many types of deposits, such as porphyry copper deposits (Abuzied et al., 2016; Alimohammadi et al., 2015; Pour and Hashim, 2015, 2012; Hosseinjani Zadeh et al., 2014a,b; Rajendran and Nasir, 2014; Rajendran et al., 2013; Shahriari et al., 2013; Honarmand et al., 2012; Hosseinjani and Tangestani, 2011). Although there have been a number of remote sensing studies that have mapped alteration zones around sulphide deposits (Alimohammadi et al., 2015; Pour and Hashim, 2015, 2012; Hosseinjani Zadeh et al., 2014a,b; Shahriari et al., 2013; Honarmand et al., 2012; Hosseinjani and Tangestani, 2011; John et al., 2010), very few have linked the remote sensing studies to the mineralogical and geochemical characteristics of the altered mineral assemblages being mapped.

Volcanogenic massive sulphide deposits in Al-Batina coast of the Sultanate of Oman are Cyprus-type deposits and occur as circular mounds of massive pyritic copper-rich ore with gold bearing gossans in the volcanic extrusives of ophiolite sequence (Hayman et al., 2015; Galley et al., 2007; Galley and Koski, 1999; Taylor et al., 1995). The deposits are associated with hydrothermal alteration assemblages including propylitic, argillic and phyllic. Weathering has overprinted the primary alteration assemblages to produce oxidized zones with extensive iron oxide/hydroxide minerals (gossans) and clays at the surface. Mapping of such altered zones, especially identification of the ore-proximal zones are important in the exploration of volcanogenic massive sulphide deposits (VMS), and the alteration zones containing iron oxide, carbonate, clay and hydroxyl-bearing minerals with characteristic spectral absorption properties in the visible and near infrared (VNIR) through the short wavelength infrared (SWIR) regions of the electromagnetic spectrum.

LEGEND Post-Nappe Autochthonous Units Qtgz: Recent alluvial fans and wadi alluvium Qgy: Sub-recent alluvial fans: Lower most terrace deposits Qgx: Ancient alluvial fans: Lower terrace deposits Qgw: Middle terrace deposits Qgv: Upper terrace deposits

• •

• •

•

• • •

•

• •

• • • Geotimes

• • •

•

•

•

Samail Nappe Units Sp: Olistoliths of serpentinite SiO: Olistoliths derived from the Sid’r formation MbO: Olistoliths derived from Matbat formation KwO: Olistoliths derived from Kawr group UmO: Olistoliths derived from Umar group Zb: Conglomerate Sh: Metalliferous sediments and chert SE2: Middle extrusives, basaltic to andesitic pillow and massive lava SA2: Dacitic felsitic rocks SU2: Metalliferous sediments and pelagic sediments SE1: Lower extrusives, basaltic pillow and massive basalt SU1: Metalliferous sediments SD: Sheeted dykes; >90% doleritic and basaltic dykes HG: Gabbro and hornblende gabbro CPG: Accumulate interlayered peridotite and gabbro CD: Accumulate dunite TD: Dunite in tectonic THS: Serpentinized harzburgite with minor dunite P’: Peridotite G': Gabbro Gh': Hornblende gabbro Dr': Diorite Td’: Trondhjemite D’: Late dolerite dykes • Cu, Au and Ag occurrences

Fig. 1. Geology of study area shows the occurrence of Cu, Au and Ag mainly in the lower extrusive (Ministry of Petroleum and Minerals, 1987).

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Many studies have identified hydrothermal alteration minerals and mapped alteration zones using satellite data from ASTER, Landsat 8, ETM+ and TM, and Hyperion sensors combined with image processing methods such as bands ratio, decorrelation stretching, principal component analysis (PCA), spectral angle mapper (SAM), linear spectral unmixing (LSU), spectral feature fitting (SFF), mixture tuned matched filtering (MTMF) etc. (Pour and Hashim, 2015, 2012; Hosseinjani Zadeh et al., 2014a,b; Rajendran and Nasir, 2014; Rajendran et al., 2013; Shahriari et al., 2013; Honarmand et al., 2012; Hosseinjani and Tangestani, 2011; John et al., 2010; Ranjbar et al., 2004, 2011; Tangestani and Moore, 2002; Tangestani et al., 2008). However, many of these studies have not integrated absorption characteristics of spectral bands with the mineralogical characteristics of the mapped alteration assemblages. Thus, demonstration of spectral absorption characters of specific minerals of the alteration zones in the SWIR spectral region, and evaluating the ability of spectral bands of sensor through field and petrological studies are important. This study maps alteration zones associated with VMS deposit in the Al-Batina coast using ASTER spectral bands and established image processing methods, and then characterizes the spectral response to the alteration zones using spectral measurements of surface samples and the mineralogical characteristics of the alteration assemblages. 2. Geology of the study area

to 2.43 lm at a spatial resolution of 30 m and the TIR subsystem has five recording channels covering the 8.12–11.65 lm wavelength region with a spatial resolution of 90 m. In the present study, the ASTER Level 1B image data (AST_L1B_0030418200607 0327_20110110220944_8936) acquired on April 18, 2006 is used. The supplied data is in terms of scaled radiance at-sensor data with radiometric and geometric corrections applied. It is georeferenced in the UTM projection and for the WGS-84 ellipsoid. A subset of nine VNIR and SWIR bands, among the 14 ASTER bands, covering the area of interest were processed and studied using Envi 5.2 and Arc GIS 10.4 software. Field investigations were carried out in the study area and 75 representative samples of the hydrothermally altered zones were collected. The samples were used for spectroscopic, microscopic, XRD and geochemical studies. In this study, SWIR spectroscopic data (reflectance spectra) were measured with a spectral resolution of seven nm using a portable PIMA spectrometer in the range of 1300–2500 nm wavelengths. This allowed precise interpretation of the absorption position. The data collected were analysed by automated mineral identification available in PIMA View v3.1 software, using the minerals library spectra of USGS, ASTER and JPL. The PIMA data were used to characterize the hydrothermal alteration zones of the study area. The minerals of host rock and altered zones of the VMS deposits were studied using transmitted and reflected light optical microscope and XRD analysis at the Department of Earth Sciences and the Central Analytical and Applied Research Unit (CAARU) of the Sultan Qaboos University.

The study area is located between the east longitude 56°220 –56°290 and north latitude 24°150 –24°230 , and includes the site of the ‘Geotimes’ pillow lavas in Wadi Jizzi. There are several copper mines still working this area. The occurrence and distribution of ophiolite sequence, especially the extrusive basalts (host rock of sulphide deposits), and Quaternary deposits of the area are given in Fig. 1. The Lower and Middle extrusives (SE1 and SE2, Middle Cretaceous age) and sheeted dykes (SD) are the major rock types of ophiolite sequence in the study area. The lower extrusive consists of basaltic pillows and has composition close to mid-ocean-ridge basalts (Godard et al., 2003; Einaudi et al., 2000; Ernewein et al., 1988). The middle extrusive is separated from the lower extrusive by a more-or-less continuous layer of pelagic sediments and has a mostly tholeiitic affinity with low Ti and low incompatible trace element contents (Beurrier et al., 1989; Ernewein et al., 1988; Lippard et al., 1986). The PostNappe Allochthonous Units of Late Tertiary to Quaternary age are found mostly parallel to the Wadis in the area. The occurrences of Cu, Au and Ag in the extrusive rocks and the distributions of other rock types are shown in Fig. 1. 3. Data and methods The ASTER sensor has sufficient spectral resolution in the SWIR bands to map hydrothermal alteration zones and associated massive sulphide deposits. The performance characteristics, data products, and applications of ASTER data and recently developed image processing methods applied for mapping of altered zones associated with porphyry copper and epithermal gold deposits were reviewed by Pour and Hashim (2012), Rajendran (2016), Kumar et al. (2015), Rajendran and Nasir (2015) and Rowan et al. (2003). The ASTER sensor consists of three separate instrument subsystems: 1) visible and near infrared (VNIR), 2) short wavelength infrared (SWIR) and 3) thermal infrared (TIR). The VNIR subsystem has three recording channels between 0.52 and 0.86 lm with an additional backward-looking band for stereo construction of Digital Elevation Models (DEMs). It has a spatial resolution of up to 15 m. The SWIR subsystem has six recording channels from 1.6

Calcite

Epidote Muscovite Line of Mg-OH absorption

Illite Kaolinite Line of OH and H2O absorption

Chlorite Line of Al-OH absorption

Line of iron absorption

Alunite Limonite Jarosite Goethite Hematite

1 2

3

ASTER bands

Line of carbonate absorption 5-9 4

Fig. 2. Laboratory spectra of minerals of hydrothermal alteration zones stacked from the USGS Spectral Library for minerals.

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4. Spectral absorption of minerals of alteration zones Mapping of hydrothermal alteration zones using satellite data requires an understanding of spectral absorption of hydrothermal alteration minerals occurring in the zones of the specific wavelength region. Several published works show that the hematite and goethite, which are the major minerals in the oxidized zone, have broad spectral absorptions between 9.9 and 1.1 lm due to occurrence of ferrous oxide; epidote, chlorite, and calcite minerals in the propylitic zone, and exhibit absorption features at 2.35 lm due to Mg-OH and CO3. Kaolin and alunite minerals from the argillic zone show narrower Al-OH absorptions at 2.17 lm, while illite and sericite minerals from the phyllic zone exhibit intense Al-OH absorption at 2.20 lm (e.g., Pour and Hashim, 2015; Hosseinjani Zadeh et al., 2014a,b; Mars and Rowan, 2006; Rowan et al., 2006; Dalton et al., 2004; Clark et al., 1990; Hunt, 1977; Hunt and Salisbury, 1974). The absorption characters of these minerals are shown in Fig. 2. The spectral bands of satellite target specific shortwave length infrared region can be used for the identification of these minerals (Gabr et al., 2010; Bedini et al., 2009; Tangestani et al., 2008; Moore et al., 2008; Di Tommaso and Rubinstein, 2007; Mars and Rowan, 2006; Kruse et al., 2003; Tangestani and Moore, 2002). Measured PIMA spectra of selected samples from the study area are given in Fig. 3. The absorptions positions values (ranges) are provided in the Table 1. The spectral plot of goethite (Fig. 3a) rich samples of the oxidized zone shows wide shallow(s) absorptions between 1438 to 1462 nm and deep (d) absorptions between 1940 to 1951 nm and 2471 to 2479 nm. The shallow absorptions near 1400 nm are due to the presence of OH and the absorptions

(a)

SGS1C

OH

near 1900 and 2470 nm are due to presence of Fe+2 and Fe-OH, respectively. The absorption near 2470 nm is characterizing the spectral band 9 of ASTER. The spectral plot of calcite (Fig. 3b) from the propylitic zone shows sharp shallow absorptions between 1411 and 1427 nm, two small absorptions near 1717–1742 nm and 2149–2170 nm, and a broad and deep absorptions ranges (widths) between 1912–1998 to 1933–1993 nm, 2337 to 2342 nm and 2490 to 2496 nm. The absorptions near 1400 nm are due to presence of H2O (sample moisture according to Abrams et al., 1988), and the absorptions near 1900, 2150, 2330 and 2490 nm are due to presence of CO3 (Abrams et al., 1988; Gaffey, 1986). The deep absorption near 2330 nm coincides with the spectral band 8 of ASTER. The spectral plot of samples of the propylitic zone contains chlorite and epidote (Fig. 3c) shows shallow deep absorptions between 1538 to 1551 nm, 1829 to 1833 nm and 2463 to 2483 nm, wide absorptions from 1910 to 1950, and sharp deep absorptions from 2251 to 2252 nm and 2334 to 2347 nm. The absorptions near 1550 nm is due to presence of OH, the absorption near 1900 nm is due to presence of H2O, and the absorptions near 2250 and 2340 nm are due to presence of FeMg-OH (Clark et al., 1993). The absorptions near 2250 and 2340 nm coincide with the spectral bands 7 and 8 of ASTER respectively. The spectral plot of samples of the propylitic zone that contain epidote and calcite (Fig. 3d) shows shallow deep absorptions from 1543 to 1552 nm, 1826 to 1831 nm, and 2452 to 2461 nm, and wide absorptions width ranges from 1905 to 1950, and sharp deep absorptions from 2251 to 2254 nm and 2338 to 2343 nm. The absorptions near 1550 nm is due to presence of OH, the absorption near 1920 nm is due to presence of H2O, and the absorptions near 2250 and 2335 nm are due to presence of Fe-OH and CO3. The

H2O

CL3

CO3

(b)

CO3 CL12

CL6

Fe+2

CL12a

GS1 CL5

GS1D CL7

Fe-OH GS1A GTH1

CL9 Plot of goethite minerals rich samples of oxidised zone.

GTH3

SGS1, Epi 83%+Chl 17%

GTH7

Plot of calcite minerals (100%) separated from the samples of propylitic zone.

GS2, Epi 56%+Cal 44%

(c)

GS1C, Epi 73%+Chl 36% R3A, Epi 78%+Chl 22%

GSB2, Epi 80%+Chl 20%

Fe-OH

(d)

H2O SGS2, Epi59%+Cal41%

OH ?

ECH12, Epi 74%+Chl 26%

ECL6, Epi 61%+Cal 39%

GS1B, Epi 53%+Cal 47%

Fe-OH CO3

SGS4B, Epi55%+Cal46% ECL7, Epi 63%+Cal 37%

ECH7, Epi 68%+Chl 32% ECH9, Epi 72%+Chl 28%

OH

H2O ECL9, Epi 66%+Cal 34%

Plot of chlorite and epidote minerals rich samples of propylitic zone.

Plot of epidote and carbonate minerals rich samples of propylitic zone.

Fig. 3. Spectral plots of samples of the mineralised zones viz. (a) oxidized; (b), (c) and (d) propylitic; (e) argillic and (f) and (g) phyllic zones, measured in the laboratory using PIMA spectrometer in 7 nm spectral resolution.

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KAO9

(e)

KAO3 KAO5

SG2C, Ill 22%+Chl47%

KAO6

H2O

Al-OH

(f)

OH KAO15

SH2, Ill 30%+Chl26%

KAO7

Al-OH Mg-OH H2O SH2B, Ill 25%+Chl38%

OH

SGS3A, Ill 26%+Chl32%

Plot of kaolin minerals separated from samples of argillic zone.

OH

KAO12

H2O

MS3

H2, Ill 24%+Chl47%

Plot of illite and chlorite minerals rich samples of phyllic zone.

(g)

MS5 MS6

MS7

Al-OH MS9 Al-OH

MS14

MS12

Plot of sericite (muscovite) minerals rich samples of phyllic zone.

Fig. 3 (continued)

absorptions near 2250 and 2335 nm coincide with the spectral bands 7 and 8 of ASTER respectively. The spectral plot of kaolin minerals (Fig. 3e) from the argillic zone shows shallow wide absorptions in width ranges from 1816 to 1842 nm and 1819 to 1844 nm, and shallow absorptions between 2383 to 2386 nm and 2487 to 2492 nm, and sharp deep absorptions ranges at 1390–1413 nm (two sharp absorption features), 1914–1918 nm, 2160–2166 nm and 2207–2208 nm. The absorptions near 1400 nm are due to presence of OH, the absorptions near 1915 nm are due to presence of H2O contents, and the absorptions near 2165 and 2207 nm are due to presence of Al-OH. The absorptions near 2165 and 2207 nm coincide with the spectral bands 5 and 6 of ASTER respectively. The spectral plot of samples of the phyllic zone consists of illite and chlorite minerals (Fig. 3f, the percentage of minerals given in the plot and rests are anhydrite) showed shallow deep absorptions at 1405–1412 nm, 2201–2203 nm and 2346–2355 nm, and deep absorption at 1911– 1918 nm. The absorptions near 1410 nm are due to presence of OH and the absorptions near 1915 nm are due to presence of H2O, whereas, the absorptions around 2201 nm and 2350 nm are due to presence of Al-OH and Mg-OH. The absorption features near 2210 and 2350 nm correspond to the spectral bands 6 and 8 of ASTER (Mars and Rowan, 2006; Clark et al., 1990). The spectral plot of sericite (muscovite) (Fig. 3g: samples consist of pure light green books of muscovite) from the phyllic zone shows shallow deep absorptions at 2108–2125 nm and 2344–2353 nm and deep absorptions at 1405–1410 nm, 2194–2201 nm and 2434–2445 nm, and wide deep absorptions at 1900–1963 nm. The absorptions near 1407 nm are due to presence of OH, the absorptions near 1910 are due to presence of H2O, the absorptions near 2200, 2350, 2440 nm are due to presence of Al-OH and

Mg-OH. The absorptions near 2200, 2350 and 2440 nm coincide with spectral bands 6, 8 and 9 of ASTER respectively (Mars and Rowan, 2006; Clark et al., 1990). In summary, chlorite, epidote, and calcite which are typically found in the propylitic alteration zone are characterized by an overlapping absorption features near 2.35 lm; kaolin, which is a common mineral in the argillic zone, has an absorption feature near 2.17 lm; and illite and sericite (muscovite) typically present in the phyllic alteration zone have an absorption feature near 2.20 lm (Mars and Rowan, 2006; Clark et al., 1993). 5. ASTER image processing and analysis Mapping of hydrothermally altered rocks using ASTER data and different image processing methods are reviewed by Pour and Hashim (2012) and the recent studies include the works of Pour and Hashim (2015), Rajendran and Nasir (2014) and Rajendran et al. (2013). In this study, the nine VNIR-SWIR spectral bands of ASTER and image processing methods including decorrelation stretching, band ratios, ASTER indices, linear spectral unmixing (LSU) and mixture tuned matched filtering (MTMF) are used 1) to discriminate host rock of the VMS deposits, 2) to delineate the hydrothermal altered and mineralized zones and 3) to detect and map the hydrothermal alteration assemblages around the deposits. 5.1. Mapping of extrusives Lithologies of arid region are mapped by the decorrelation stretch image processing method (Rajendran et al., 2012; Rowan et al., 2006; Philip et al., 2003; Matthews and Jones, 1992;

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Table 1 Spectral absorption values of minerals of the oxidized, propylitic, argillic and phyllic alteration zones measured in the laboratory using PIMA spectrometer in 7 nm spectral resolution. The letters ‘s’ and ‘d” with the values represent shallow and deep absorptions respectively. Zone

Sample Nos.

Rocks

Minerals of the rock

Spectral absorption is SWIR region

Oxidized

SGS1C GS1 GS1A GS1D GTH1 GTH3 GTH7

Goethite Hematite Rock

Goethite 80% (separated and measured)

1447.6s, 1444.9s, 1438.1s, 1450.3s, 1444.9s, 1462.5s, 1443.6s,

1940.6d, 1947.4d, 1943.3d, 1948.8d, 1942.0d, 1951.8d, 1940.6d,

2470.2d 2472.9d 2479.7d 2475.6d 2471.6d 2478.3d 2473.3d

Prophylitic

CL3 CL5 CL6 CL7 CL9 CL12 CL12a

Chlorite Epidote carbonate rock

Calcite 100% (collected and measured)

1413.0s, 1416.2s, 1416.2s, 1411.5s, 1421.6s, 1427.3s, 1414.6s,

1727.9s, 1731.5s, 1735.1s, 1742.2s, 1726.2s, 1726.2s, 1717.2s,

1914.3d-1995.5d, 1914.3d-1995.5d, 1912.7d-1998.7d, 1922.3d-1993.3d, 1922.3d-1995.5d, 1925.5d-1995.5d, 1933.4d-1993.9d,

Prophylitic

SGS1 GSB2 GS1C R3A ECH7 ECH9 ECH12

Chlorite Epidote rock

Chlorite, Epidote

1544.2s, 1547.2s, 1545.7s, 1551.8s, 1538.2s, 1539.7s, 1545.7s,

1833.7s, 1832.2s, 1829.1s, 1829.1s, 1829.1s, 1832.2s, 1832.2s,

1910.6–1952.8s, 1915.1–1948.2s, 1913.6–1948.2s, 1909.0–1946.7s, 1909.0–1949.7s, 1910.6–1948.2s, 1910.6–1946.7s,

2252.8d, 2252.8d, 2252.8d, 2252.8d, 2251.3d, 2252.8d, 2252.3d,

2338.7d, 2346.2d, 2346.2d, 2344.7d, 2334.2d, 2346.2d, 2347.7d,

2483.4s 2475.9s 2477.4s 2483.4s 2463.8s 2463.8s 2465.3s

Prophylitic

SGS2 GS2 GS1B SGS4B ECL6 ECL7 ECL9

Epidote Carbonate rock

Epidote, Calcite

1545.4s, 1543.7s, 1552.1s, 1548.7s, 1547.1s, 1550.4s, 1543.7s,

1831.1s, 1827.7s, 1827.7s, 1827.7s, 1829.4s, 1831.1s, 1826.1s,

1913.4–1955.5s, 1913.4–1950.4s, 1913.4–1945.4s, 1906.7–1948.7s, 1915.1–1947.1s, 1911.8–1958.8s, 1913.4–1952.1s,

2252.9d, 2252.9d, 2252.9d, 2251.3d, 2552.9d, 2254.6d, 2252.9d,

2340.3d, 2342.0d, 2340.3d, 2338.7d, 2342.0d, 2343.7d, 2338.7d,

2458.0s 2459.7s 2461.3s 2429.4s 2452.9s 2458.0s 2459.7s

Argillic

KAO3

Kaolinite (!)

Kaolin100% (collected and measured)

1390.8–1413.2d, 1818.7s, 2383.3s, 2492.9s 1396.7–1413.2d, 1816.3s, 2385.7s, 2487.0s 1396.7–1410.8d, 1816.3s, 2383.3s, 2488.2s 1394.3–1413.2d, 1817.5s, 2385.7s, 2487.0s 1393.1–1412.0d, 1817.5s, 2386.8s, 2488.2s 1395.5–1413.2d, 1819.8s, 2384.5s, 2487.0s 1395.5–1412.0d, 1819.8s, 2383.3s, 2490.2s

KAO5 KAO9 KAO6 KAO7 KAO15 KAO12

Phyllic

SG2C SH2 H2 SGS3A SH2B

Illite chlorite rock

Illite, Chlorite

1408.8s, 1408.0s, 1405.0s, 1412.5s, 1408.8s,

1911.3d, 1918.8d, 1911.3d, 1918.8d, 1918.8d,

MS3 MS5 MS7 MS6 MS9 MS14 MS12

Sericite, chlorite Muscovite rock

Sericite (100%) (collected and measured)

1410.6d, 1410.6d, 1405.6d, 1408.9d, 1410.6d, 1408.9d, 1408.9d,

1912.4–1963.5d, 1902.5–1960.2d, 1910.7–1961.9d, 1910.7–1960.2d, 1905.8–1953.6d, 1900.8–1961.9d, 1905.8–1963.5d,

Abrams et al., 1988; Rothery, 1987; Gillespie et al., 1986). Recently, the method was used by Rajendran and Nasir (2015) to map the metamorphic zonation in the As Sifah region of the Sultanate of Oman. They used the ASTER-SWIR spectral bands 5, 6 and 8 to discriminate the metamorphic rock types. In this study, the method and the ASTER VNIR-SWIR spectral bands 3, 6 and 8 are used to map the host rock extrusive basalts, which are highly weathered, and the alteration zones of VMS deposits of the study area. In this study, ASTER band 3 was chosen for basalt mapping since the band is characteristic to absorption of the weathered surfaces of basalts and goethite minerals of oxidized alteration zone. Band 6 was

2162.6s, 2170.6s, 2156.2s, 2157.8s, 2151.5s, 2149.6s, 2151.5s,

2342.4d, 2337.7d, 2340.8d, 2339.3d, 2337.7d, 2337.7d, 2339.3d,

2490.5d 2495.2d 2496.8d 2495.2d 2492.2d 2493.6d 2492.0d

1843.4s, 1911.8d, 2161.7d, 2208.8d, 1844.6s, 1918.9d, 2160.5d, 2208.8d, 1844.6s, 1916.5d, 2162.9d, 2207.7d, 1843.4s, 1915.3d, 2160.5d, 2207.7d, 1843.4s, 1915.3d, 2162.9d, 2207.7d, 1843.4s, 1915.3d, 2162.9d, 2207.7d, 1842.2s, 1914.1d, 2166.4d, 2207.7d,

2201.2d, 2201.3d, 2198.9d, 2201.6d, 2203.1d,

2355.6s 2348.1s 2350.0s 2355.6s 2346.3s

2123.7s, 2196.3d, 2117.1s, 2201.2d, 2117.1s, 2194.6d, 2110.5s, 2194.6d, 2122.0s, 2201.2d, 2108.8s, 2199.6d, 2125.3s, 2199.6d,

2351.4s, 2445.5d 2349.8s, 2443.9d 2353.1s, 2438.9d 2346.5s, 2443.9d 2344.8s, 2434.0d 2348.1s, 2435.6d 2351.4s, 2434.0d

selected to provide information of sericite (muscovite) and illite from the phyllic zone and band 8 was chosen to map chlorite, epidote and carbonate minerals of propylitic zone. A RGB image of decorrelated bands 3, 6 and 8 of the study area is given in Fig. 4. The image discriminates the occurrence and distribution of extrusives (Lower: SE1 and Middle: SE2) in pinkish red. Sheeted dykes (SD) appear in purple and the occurrences of Quaternary deposits show shades of pale blue to light green. The other rock types can be interpreted with geology of the area (Fig. 1). In the field, the Lower extrusive basalts are massive and pillowed (Fig. 5a–c). The Middle extrusives are highly weathered and friable (Fig. 5d) on

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SE1

the surface. These are intruded by sheeted dykes (Fig. 5e). Epidotebearing alteration assemblages are commonly observed in this basalt (Fig. 5f). The occurrence of Quaternary deposits over the middle extrusive basalt was observed at several places in wadi sections (Fig. 5g, h). The occurrences of Upper extrusive (Fig. 5i), as flat terrain in the study area, are observed before reaching the Middle extrusives during the field work.

SE1

SE1 SE1

SE1 SE2

SE2

5.2. Mapping of mineralized zones SE1

SE1

SE2 SE1 SE2

• Cu SE2 SE2

SE1

SE2 SE2 SE1

SE1

SE2

SE1 SE2 SE2 SE2 SE1

SE2 SE2 SE1

SE1

Fig. 4. RGB image of decorrelated ASTER spectral bands 3, 6 and 8 shows the extrusives (SE1 and SE2 delineated) of the study area (Image is linear stretched with Red: 0.2–4.4; Green: 1.8–5.8; Blue: 57.5–61.7). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

To delineate the sulphide bearing mineralized zones of VMS deposits occurring in the extrusives of the study area, the ASTER band ratios (5/3 + 1/2), (4 + 6)/5 and (5 + 7)/6 were used to characteristize to the oxidized, argillic, propylitic and phyllic zones (Hewson et al., 2005; Rowan and Mars, 2003). The resulting RGB image of the band ratios is given in Fig. 6. The Lower extrusives (SE1) appeared in pinkish purple-orange whereas the Middle extrusive (SE2) exhibited mostly as dark blue. Although this image differentiates the extrusives (SE1 and SE2), such discrimination is not provided in the decorrelation method. Copper, Au and Ag occurrences are mostly hosted the Lower extrusive. The sheeted dykes (SD) appear in pinkish yellow-green and the Quaternary deposits appear in shades of yellowish light green on the image. In the field, the Lower extrusives are more massive than the Middle extrusives. At surface, the mineralized zones in Lower extrusives are oxidized and are characterized by hydrothermal chloritization, carbonation, argillitization, sericitization and silicification. The altered rocks are highly fractured and the alteration zones are concentrically zoned away from sulphide deposits, from phyllic to argillic and then to propylitic. Oxidation of extrusive rocks gives a characteristic reddish brown or yellowish color, which can indicate the presence of former pyrite and chalcopyrite. Reddish brown exposures, rich in brownish hematite and

(a)

Hills of Massive Pillow Basalts

(b)

Cooling of basalts

(b)

inner fine grain minerals

Massive Pillow exhibits concentric layer outer coarser grain minerals Geotimes

(d)

(e)

Basalts with sheeted dyke

(f)

Middle extrusive Basalts epitotized

(g)

(h)

(i)

Quaternary deposits over extrusive Extrusive

Upper extrusive

Wadi deposits

Fig. 5. Field photographs show the occurrences of (a), (b) and (c) the Lower extrusive basalts (pillow and massive basalts), (d), (e), and (f) the Middle extrusives (basaltic to andesitic pillow lava), (g) and (h) the Quaternary deposits over the Middle extrusives and (i) the Upper extrusives in the study area.

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SE1 SE1

SE1

• Cu

SE1 SE2

orange goethite minerals, form in the oxidized zone along the surface of Lower extrusive (Fig. 7a to c). Propylitic zones are darker in color and found below the oxidized zone. These zones are rich in light chlorite, green epidote and white calcite. The presence of kaolin-quartz-rich argillic assemblages, and sericite/ illite-pyrite-rich phyllic zone (mostly powdery, flaky and relatively fine grained) were verified in the field. These zones occur as white–grey and yellowish exposures in eroded areas associated with wadis (Fig. 7d to i). The white–grey argillic alteration zone is developed mainly between the phyllic and propylitic alterations and the argillic alteration can be identified by presence of abundant clay minerals. The presence of argillic zones below the oxidized zone is uncommon relative to the presence of thick phyllic zones; the former is only rarely observed along wadi sections. The phyllic hydrothermal alteration zone is yellowish due to the presence of sericite and quartz. The white argillic alteration and the green propylitic alteration zones bound it. The entire alteration zones sequence viz. oxidized, propylitic, argillic and phyllic zones are observed at the vertical section of old mine cuttings and some barren exposures in the field. Field samples of all zones (including least-altered extrusive rocks) were collected for laboratory studies.

SE1 • Cu • Cu • Cu • Cu SE2 • Cu SE1

• Cu SE1 • Cu • Cu SE2

• Cu

SE1

SE2

• Cu SE2 SE2 • Cu SE2

SE•1Cu SE2 SE1

SE1 SE2• Ag, Au • Cu SE1

SE2

• Cu

SE1 • Cu

• Cu

SE2

5.3. Mapping of altered minerals

SE2 SE2 SE2 SE1

• Cu

SE1

Fig. 6. RGB image of ASTER band ratios (5/3 + 1/2), (4 + 6)/5 and (5 + 7)/6 shows the mineralized zones (spotted Cu occurrences) of the study area (Image is linear stretched with Red: 1.1–1.3; Green: 3.6–4.3; Blue: 1.8–2.0). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

To confirm the discrimination of mineralized and altered zones (Fig. 6), ASTER mineralogical indices used by earlier researchers were used (Yamaguchi and Naito, 2003; Ninomiya, 2003, 2004). Yamaguchi and Naito (2003) proposed several spectral indices for lithological discrimination and mapping of rock types exposed on the surface using ASTER shortwave length infrared (SWIR) bands. These included the Alunite Index, Kaolinite Index, Calcite Index, and Montmorillonite Index using linear combinations of reflectance values in each of the six

(a)

(b)

(c) Gossans on the surface

Gossans on the surface Gossans on the surface

(d)

(e)

(f)

Phyllic zone exposed along a wadi section

(g)

Argillic and Phyllic zones exposed along a wadi section

(h)

Phyllic zone exposed along a wadi section

(i)

Gossans along the wadi section

Fig. 7. Field photographs show occurrences of (a), (b) and (c) the gossans exposed on the surface (reddish brown), and (d), (e), (f), (g), (h) and (i) the gossans exposed along the wadi sections (reddish to yellowish) in the study area. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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SWIR bands. Ninomiya (2003, 2004) defined mineralogical indices for ASTER VNIR and SWIR bands by considering the spectral absorption features of different minerals and rocks in the ASTER spectral channels and provided indices as i) OH bearing altered minerals Index (OH): (band7/ band6)  (band4/ band6), ii) Kaolinite Index (KLI): (band4/band5)  (band8/ band6), iii) Alunite Index (ALI): (band7/ band5)  (band7/ band8), and iv) Calcite Index (CLI): (band6/band8)  (band9/ band8). In 2007, Zhang et al. applied PCA transformation to the mineralogical indices, including the OH bearing altered mineral Index (OHI), Kaolinite Index (KLI), Alunite Index (ALI), and Calcite Index (CLI) for delineating alteration zones and showed accurate spectral information for mapping of the vegetation, minerals, and lithological mapping. Among the indices, the OH bearing altered minerals, kaolin and calcite minerals indices of Ninomiya (2003, 2004) are used in this study to show occurrence of alterations in the zone and the resulting RGB image is given in Fig. 8. The indices image shows the occurrence of altered minerals as greenish yellow to yellow mainly over the extrusives and support the discrimination of band ratios image (Fig. 6). The band ratios image (Fig. 6) clearly discriminated the extrusive rocks and the minerals indices image (Fig. 8) showed the occurrence and distribution of altered minerals of the rocks which is depend mainly on the intensity of alterations occurred in the region. The region of Lower extrusive appear as a mixture of green and purple, and the Middle extrusive as a mixture of green and yellow (Fig. 8).

5.4. Mapping of alteration zones of VMS deposit Fig. 8. RGB image of ASTER indices (R: OH bearing altered minerals, G: kaolinite B: alunite minerals indices) shows the occurrence and spatial distribution of altered minerals in the study area (Red square is an area chosen for detailed study; the image is linear stretched with Red: 2.5–3.1; Green: 1.6–2.0; Blue: 1.0–1.4). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The spectrum of each pixel of ASTER data consists of numerous spectral bands that can be compared with known endmembers derived from image, field and laboratory. There are several studies proved image classification using endmembers (Hosseinjani Zadeh et al., 2014a,b; Pour and Hashim, 2015). In

Fig. 9. Shows (a) distribution of endmembers in the nD-Visualizer and (b) No. of pixels of the endmembers extracted from the SAM analysis.

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this study, to detect and map the different altered zones of a VMS deposit (red box in Fig. 8) in parts of the study area, the endmembers derived from images (in VNIR-SWIR regions) were used in the subpixel linear spectral unmixing (LSU) and Mixture Tuned Matched Filtering (MTMF) algorithms. The details of algorithms and uses to porphyry copper deposits are demonstrated by Abuzied et al. (2016), Resmini et al. (1997), Boardman (1998), Boardman et al. (1995), Research Systems Inc (2001), Kruse et al. (2003), Bishop et al. (2011), Hosseinjani Zadeh (2008), Hosseinjani and Tangestani (2011), Hosseinjani Zadeh

et al. (2013, 2014b). In this study, we used image endmembers, since the library and field spectra were not acquired under the same conditions of the satellite image and the image endmembers are directly associated with surface components detectable in the scene. However, the absorption characters of altered minerals of the USGS Spectral Library for minerals (Fig. 2), and the spectra measured over the field samples using PIMA spectrometer (Fig. 3) are considered for using such endmembers in this study. Here, the endmembers were extracted through SAM method (Rajendran and Nasir, 2014, 2015; Gabr et al., 2010)

n-D Class Mean #10 n-D Class Mean #9

Spectra represent the unaltered and resistant minerals

n-D Class Mean #8 n-D Class Mean #7 Mg-OH absorption Al-OH absorption

n-D Class Mean #6 n-D Class Mean #5 n-D Class Mean #4

n-D Class Mean #3 n-D Class Mean #2 Fe+2 iron absorption OH absorption

Spectra represent the poorly altered silicate minerals of Phyllic zone

n-D Class Mean #1

Spectra represent the altered silicate minerals of Argillic zone

Spectra represent the carbonate minerals of Prophylitic zone Spectrum represents the iron minerals of Oxidized supergene zone

Fig. 10. Shows endmember spectra (Class Means) for the different altered zones of the study area.

(b)

(a)

Gossan

Old mine

0

500

Fig. 11. (a) ASTER RGB (3, 2, 1) image shows the occurrence of gossan (yellow squared, the area in Fig. 16) and old mine, and (b) the distribution of pixels of the oxidized (red), propylitic (green), argillaceous (cyan) and phyllic (pink) zones derived based on SAM endmembers (1, 3, 5 and 7) over MNF image (band 2) of the gossanized area. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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since the endmembers of the method showed more reliable results among the methods studied (Rowan and Mars, 2003; Rowan et al., 2005). The SAM method (Rajendran and Nasir, 2014, 2015; Gabr et al., 2010) was performed to the studied area and has provided spectra (n-D class Mean) based on the endmembers derived. Fig. 9a and b shows distribution of endmembers in the nD-Visualizer and number of pixels of the endmembers extracted from the SAM analysis. Fig. 10 shows the endmember spectra characteristic to different altered zones of the study area. Spectra can be broadly grouped into five groups based on the iron, Al-OH, Mg-OH and carbonate minerals absorptions of the area: i) n-D class Mean #1-spectrum represents the iron minerals of oxidized zone, ii) n-D class Mean #2 and n-D class Mean #3 – spectra represent the carbonate minerals of propylitic zone, iii) n-D class Mean #4 and n-D class Mean #5 – spectra represent the altered silicate minerals of argillic zone, iv) n-D class Mean #6 to class Mean #8 – spectra represent the poorly altered silicate minerals of phyllic zone and v) n-D class Mean #9 and n-D class Mean #10 – spectra represent the unaltered and resistant minerals (Fig. 10). The spectra representing the oxidized zone show strong absorptions in bands 3 and 5 due to presence of ferrous iron and OH. The propylitic zone consists of chlorite, epidote and carbonate, which exhibit strong absorptions in bands 3 and 5 due to presence of iron and Al-OH and shallow

327

absorptions towards band 8 due to presence of carbonate. The spectra of argillic zone provide absorptions in the bands 3, 5 and 7 due to the presence of iron, Al-OH and Mg-OH in the major minerals kaolin and chlorite. The spectra of phyllic zone show absorptions in the bands 3, 5 and 7 due to presence of iron, Al-OH and Mg-OH in illite, chlorite and muscovite. The absorptions in bands 3, 5 and 7 of spectra of N-D class #9 and #10 may be related to presence of the resistant minerals such as quartz in the zone (Rajendran et al., 2013; Rajendran and Nasir, 2014, 2015). Fig. 11 is a RGB (3, 2, 1) image of the chosen area (Fig. 11a) that shows the occurrence of gossanous exposure and old mine (filled by water and mine wastes dumped around the mine) and distribution pixels of the oxidized (red), propylitic (green), argillic (cyan) and phyllic (pink) zones, derived based on SAM endmembers (1, 3, 5 and 7), over Minimum Noise Fraction (MNF) image (band 2) of the gossanized area (Fig. 11b). In this study, we used these endmember spectra in the subpixel linear spectral unmixing (LSU) method (Abuzied et al., 2016) to show the different alteration zones. The method determines relative abundance of materials that can be depicted in imagery based on the materials’ spectral characteristics. Here, the reflectance at each pixel of the image is assumed to be a linear combination of the reflectance of each material (or endmember) present within the pixel. The resulting images, which were

Fig. 12. Linear Spectral Unmixing images derived based on SAM endmembers (1, 3 and 5) show the occurrence and distribution of bright pixels represent (a) oxidized (red), (b) propylitic (green) and (c) argillic (cyan) zones (in the zoomed (4x) images corresponds to red squares). Yellow square represents the old mine area which are able to show the presence of propylitic and argillaceous (mixed with phyllic) zones materials. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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supported the results that were derived based on the band ratios and ASTER indices.

6. Laboratory studies To validate the above image interpretations and spectral absorptions of minerals of the different alteration zones, the texture and mineral assemblages of the alteration zones and the extrusives were studied using transmitted and reflected microscopy and XRD analysis.

6.1. Petrography of alteration zones

Fig. 13. Linear Spectral Unmixing image derived based on SAM endmember 1 shows the occurrence and distribution of bright pixels which represent oxidized zone that are exposed along wadi and old mine area near to the copper (Cu) occurrences in the study area.

derived based on SAM endmembers 1, 3 and 5 are shown in Fig. 12. It shows the occurrence and distribution of bright pixels of different altered zones clearly in the gossanous area (Fig. 12a, delineated in red), propylitic zone (Fig. 12b, delineated in green) and argillic zone (Fig. 12c, delineated in cyan). The distribution of these zones is well shown in the zoomed (lower; 4x) images, which correspond to red squares in the upper images. The method also shows the presence of propylitic and argillic zones material (mixed with phyllic) in the old mine area (yellow box). The SAM end member 1 image of the entire area shows the location of oxidized zones as bright pixels, many of which are near Cu occurrences (Fig. 13). To show the ability of subpixel methods, we used the SAM end members 1, 3 and 5 in the Mixture Tuned Matched Filtering (MTMF) method (Hosseinjani Zadeh et al., 2014b). The resulting images are given in Fig. 14, which shows the distribution of propylitic assemblages (Fig. 14b: delineated as green in the zoomed (4x) image, which correspond to the red box on main image). The method showed the presence of propylitic assemblages in the old mine area but failed to show the oxidized and argillic zones. Field study of area confirmed the occurrence of a gossanous zone about 650 m in length (Fig. 15a) and presence of oxidized, propylitic, argillic and phyllic zones which are clearly exposed on the surface. Our study clearly identified the presence of a goethite rich oxidized zone (Fig. 15b, c), epidotized and carbonate rich propylitic zone (Fig. 15d to g), and illite, chlorite and muscovite rich phyllic zone (Fig. 15h and i) occurred with mineralization (Fig. 15j). The LSU method is capable to differentiate and show the oxidized, propylitic and argillaceous zones. The detection of minerals and alteration zones based on the SAM endmembers

Microscopic study of samples from the alteration zones showed presence of major altered minerals including goethite, hematite, chlorite, epidote, calcite, kaolin, sericite and muscovite due to weathering and hydrothermal alterations. The mineralized samples showed presence of sulphide minerals such as chalcopyrite, pyrite and sphalerite. Under the microscope, the samples of Lower extrusive basalts (host rock of the alteration zones) showed the occurrence of coarse to medium grained phenocrysts plagioclase laths in a glassy matrix (Fig. 16a). The laths are anhedral to subhedral accompanied by biotite, brown hornblende and feldspar. The glassy groundmass is grey to dark, rich in plagioclase microlaths, quartz and feldspar fragments (Fig. 16b). The accessory minerals include mainly magnetite and apatite. A few large olivine phenocrysts (zoned – reaction rims of orthopyroxene) in the groundmass of feldspathic glass are present in the basalt (Fig. 16c). The basalts exhibit amygdaloidal texture filled by alnacime and natrolite (Fig. 16d, e). Under the reflected microscope, the samples of the oxidized zone are rich in goethite and showed the presence of sphalerite, chalcopyrite and pyrite. The pyrite shell consists of pyrite and chalcopyrite with abundant goethite, and specular hematite, which typically replaces magnetite. Sphalerite and galena occur as trace minerals in this alteration zone. Presence of concentric layers of Fe-hydroxide (Fig. 16f), and sphalerite and pyrite (Fig. 16g) are observed. The samples from the propylitic alteration zone clearly showed the presence of chlorite and epidote. The altered basalts of this zone are characterized by the presence of relict of medium-grained plagioclase, hornblende, chlorite and epidote. The green chlorite-epidote alteration is well developed in the samples. Chlorite is weakly pleochroic from pale green to green and replaces hornblende locally. The aggregates and single grain of fine-grained epidote partially replace plagioclase. The matrix consists of grey laths of the plagioclase, green chlorite and epidote and grey carbonate grains (Fig. 16h, i). Fine sericite is observed in fractures of the plagioclase. A few samples showed hematite with very fine grained pyrite and magnetite. Overall, this alteration zone is characterized by quartz + chlorite ± epidote ± calcite. The minor minerals associated with the alteration are sericite, pyrite and hematite. The samples of argillic zones are abundant in quartz. The feldspars are moderately altered to clays. The alteration is manifested by advanced replacement of plagioclase and mafic phases by clay minerals. Sericite and illite are observed in this zone. Accessories are hematite and quartz. The altered rocks are soft and white colored. The study of samples of the phyllic zone showed the replacement of lath-shaped feldspar and locally chlorite by sericite and quartz (Fig. 16j, k). The zone consists mainly of sericite, dispersed fine quartz and quartz veins. They are associated with minor chlorite, pyrite, chalcopyrite, and iron-oxide minerals (Fig. 16k). The occurrences of pyrite in veins are surrounded by chalcopyrite. Quartz in the matrix is mainly secondary and formed during the

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Fig. 14. Mixture Tuned Matched Filtering images derived based on SAM endmembers (1, 3 and 5) show the occurrence and distribution of bright pixels of propylitic zone ((b) green in the zoomed (4) images corresponds to red squares). Yellow square represents the old mine area which is also able to show the presence of propylitic zone materials. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

breakdown of feldspar to muscovite and quartz. Silicification is common in the phyllic zone. The reflected microscopic study of samples collected from the mineralised zone of the mine cutting showed the abundant occurrences of pyrites, chalcopyrites and sphalerites in fine to very coarse grains (Fig. 16l, m and n). Euhedral pyrite and chalcopyrite are common in the samples (Fig. 16o). Table 2 shows the minerals assemblage of the different alteration zones studied under the microscope. The XRD analyses of the samples confirmed further the occurrences of quartz, plagioclase, clinopyroxene, olivine and chlorite minerals in the basalts (Fig. 17a, b and c), the quartz, hematite, goethite, calcite, fischesserite and chalcopyrite in the oxidized zone (Fig. 17d, e, f and g), the quartz, plagioclase, hornblende, muscovite, chlorite, epidote, calcite in the propylitic zone (Fig. 17 h, i), and the quartz, chlorite, illite, muscovite (sericite), albite, calcite, sphalerite, chalcopyrite and pyrite in the phyllic zone (Fig. 17j, k, l and m).

7. Discussion and conclusions In this study, the laboratory measurement of spectra of samples from the different alteration zones in the SWIR region showed unique absorption features of goethite, calcite, chlorite, epidote,

illite and muscovite and within oxidized zones and in propylitic, argillic and phyllic alteration zones. The study shows that 1) the goethite-rich samples form oxidized zones have specific absorptions near 1400, 1900 and 2470 nm due to presence of OH, Fe+2 and Fe-OH, 2) chlorite in association with epidote and calcite from samples in the propylitic zone showed absorptions near 1550 nm and 1900–1920 nm due to presence of OH and H2O respectively, and the absorptions near 2250 and 2335–2340 nm are due to presence of Fe-Mg-OH and OH, 3) kaolin-rich samples from the argillic alteration zone showed absorption near 1400 nm and 1915 nm due to presence of OH and H2O respectively, and the absorptions near 2165 and 2207 nm are due to presence of Al-OH, and 4) sericiteand illite-rich samples from the phyllic zone showed absorptions near 1410 nm and 1915 nm due to presence of OH and H2O, and the absorptions around 2201, 2350 and 2440 nm are due to presence of Al-OH and Mg-OH. All the measured spectra could be compared and confirmed with microscopic studies and XRD analyses. Further, the spectroscopic study leads to select the right ASTER spectral bands (3, 6 and 8) to discriminate the extrusives (host rock of the altered zones) by well-known decorrelation image processing method and to choose the band ratios (5/3 + 1/2), (4 + 6)/5 and (5 + 7)/6 to show the mineralized zones. In this study, the chosen methods discriminated the extrusives and mineralized zones and confirmed the results using the ASTER mineral indices viz. the OH bearing altered minerals, kaolin and calcite minerals indices.

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(a)

Gossans near Wadi Bani Umar

(b) Oxidised zone

(c)

(d)

Goethites alteration in Oxidised zone Epidotised alteration in Propylitic zone

(e)

(f)

(g) Argillic alteration below Prophylitic zone

Calcite alteration in Propylitic zone

Calcite in Propylitic zone

(h)

(i) Kaolinalteration in Phyllic zone

(j) Potassic alteration

Phyllic alteration

Fig. 15. Field photographs show occurrences of (a) gossans, (b) oxidized zone, (c) goethite alteration in oxidized zone, (d) chlorite and epidote alteration in propylitic zone, (e) and (f) calcite mineralization in propylitic zone, (g) argillic alteration below propylitic zone, (h) phyllic alteration, (i) kaolin in phyllic zone and (j) copper mineralization in the phyllic zone in the gossanous zone (a).

The ASTER images in the SWIR wavelength region are useful for displaying the intensities of OH, Al-OH, Mg-OH and CO3 absorptions while the Fe-OH and Mg-OH are distinguishable based on the SWIR band. Moreover, the detection of the alteration minerals in different alteration zones of the investigated area were studied using the end members derived from the SAM and subpixel LSU methods, which clearly showed the occurrence of alteration minerals and the oxidized, propylitic and argillic zones. All the results and image interpretations were verified and validated in the field. The field study showed the occurrence and distribution pattern of the alteration zones, the most important characteristics of VMS deposit as, 1) the oxidized zone is reddish brown or yellowish rich in oxide and hydroxide iron minerals, 2) the propylitic zone occur under the oxidized zone appear in dark green due to presence of

hydrothermally altered minerals namely chlorite, epidote and calcite, 3) the argillic zone is white rich in quartz, kaolin and clay developed mainly between the phyllic and propylitic alterations, 4) the phyllic zone is thick, yellowish, mostly powdery or flaky and relatively fine grained, and consists of illite and sericite minerals surrounded by the white argillic alteration and the green propylitic alteration zones and 5) the potassic zone rich in biotite and muscovite. The samples collected from the field were studied under the microscope and XRD analyses, which confirmed the presence of these minerals within the alteration zones and supported the image interpretations and field verifications. Thus, in this study, the hydrothermal alteration zones such as oxidized, propylitic, argillic and phyllic zones associated with VMS deposits have been discriminated by characterizing the

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(a)

Plagioclase laths in glassy matrix

(d)

Amygdaloidal texture

(b)

Clinopyroxenes in basalt

(c) Alteration zoning in olivine

(e)

Amygdaloidal texture filled with analcime (black) and natrolite laths (in white).

Goethite

(f)

Concentric layers of Fehydroxide occurrences in Goethite

Sphalerites

(g)

(h)

Calcite

Chlorite

(i)

Chlorite Analcime Epidotitsation in Propylitic zone

Epidote Pyrites

(j)

Chalcopyrites

Sericite

(k)

(l) Chalcopyrites

Sericite

Sphalerites

Pyrites Chlorite

(m)

(n)

Chalcopyrite

(o)

Chalcopyrites

Pyrites

Pyrites

Pyrite Chalcopyrites Fig. 16. Microphotographs show (a), (b) and (c) the minerals and (d) and (e) the amygdaloidal texture of the Lower extrusive basalts; (f) the presence of concentric layers of Fe-hydroxide; and (g) the sphalerite and pyrite in goethite rich samples of oxidized zone; (h) and (i) the presence of chlorite, epidote and calcite minerals in the propylitic zone; (j) and (k) the presence of sericite, chlorite and pyrite in the phyllic zone; and (l), (m), (n) and (o) the ore minerals of mineralized zone. (All microphotographs are under nicols crossed except the (d) which is in parallel nicols; the (f), (g), (l), (m), (n) and (o) are ore microphotographs). Table 2 The minerals assemblage of the alteration zones of the study area. Alteration Zones

Oxidized Zone Propylitic zone Argillic zone Phyllic zone

Minerals Quartz

Hematite

Goethite

— — — —

—

—

Chlorite

Epidote

Calcite

Kaolin

Illite

Sericite

Muscovite

—

—

— ---

—

— —

—

—

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Fig. 17. Results of XRD analyses show minerals of (a), (b) and (c) the basalts; (d), (e), (f) and (g) the oxidized zone; (h) and (i) the propylitic zone; and (j), (k), (l) and (m) the phyllic zone.

spectral absorption features of the most dominant altered minerals of the zones and choosing suitable ASTER spectral bands and image processing methods. The study documented that spectroscopic data of altered minerals of the altered zones collected in SWIR region at 7 nm which can importantly be useful in remote sensing

data for exploration and better mapping of altered minerals and alteration zones. The results derived from subpixel LSU and MTMF algorithms showed that the LSU is robust and reliable for detecting the altered minerals and the hydrothermal alteration zones in a local scale. All the results of image analyses and interpretations

S. Rajendran, S. Nasir / Ore Geology Reviews 88 (2017) 317–335

333

Fig. 17 (continued)

were verified and validated through field studies. The petrographic studies and XRD analyses of samples showed the presence of alteration minerals within the alteration zones and confirmed further the image interpretations and field verifications. The results obtained from PIMA measurement, XRD analyses, and petrographic studies are correlatable with image interpretations. This integrated study demonstrated the capability of ASTER spectral bands for mapping of the hydrothermal alteration zones. The discrimination of hydrothermal alteration zones reduced the time and cost required for field evaluation and provided a reliable, simple and user-friendly approach for exploration geologists to locate hydrothermal alteration zones associated with a VMS deposit.

providing the ASTER data. This study is supported by the Sultan Qaboos internal grant IG/SCI/ETHS/14/02. The authors are thankful to Mr. Saif Amer Al-Maamari (CAARU, SQU) who carried out the analysis of XRD, and Mr. Hamdan Saif Al-Zidi and Mr. Bader Al Waili (Department of Earth Sciences, SQU) who prepared micro thin sections to this study. The authors are also thankful to Dr. Salah Al-Khirbash, Mr. Khalid Al-Syiabi, Ahmed Majed Said AlDhuli, and Mr. Hazaa Saif Al-Ali for their help during the fieldwork. The authors are very much thankful to Dr. David Huston (the Associated Editor), the anonymous reviewer and Dr. Franco Pirajno (the Editor) of the journal for their valuable reviews and providing constructive comments and suggestions that have helped to present the work lucidly.

Acknowledgements The authors are thankful to NASA Land Processes Distributed Active Archive Center User Services, USGS Earth Resources Observation and Science (EROS) Center (https://LPDAAC.usgs.gov) for

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