Alimohammadi2015.pdf

Ore Geology Reviews 70 (2015) 290–304

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Application of ASTER data for exploration of porphyry copper deposits: A case study of Daraloo–Sarmeshk area, southern part of the Kerman copper belt, Iran Masoumeh Alimohammadi a,b,⁎, Saeed Alirezaei a, Daniel J. Kontak b a b

Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada

a r t i c l e

i n f o

Article history: Received 31 December 2014 Received in revised form 25 March 2015 Accepted 5 April 2015 Available online 28 April 2015 Keywords: ASTER imagery Porphyry copper deposit Hydrothermal alteration Daraloo–Sarmeshk Urumieh–Dokhtar Iran

a b s t r a c t The Cenozoic Urumieh–Dokhtar Magmatic Belt (UDMB) of Iran is a major host to porphyry Cu ± Mo ± Au deposits (PCDs). Most known PCDs in the UDMB occur in the southern section of the belt, also known as the Kerman Copper Belt (KCB). Three major clusters of PCDs are distinguished in the KCB and include the Miduk, Sarcheshmeh and Daraloo clusters. The Daraloo and Sarmeshk deposits occur in a northwest–southeast-trending fault zone that is characterized by the presence of a narrow zone of alteration–mineralization that contains a series of Oligocene granitoids and Miocene porphyritic tonalite–granodiorite plutons that cut Eocene andesitic lava flows and pyroclastic rocks. Here we use various techniques, including different ratio images, minimum noise fraction, pixel purity index, and matched filter processing to process ASTER data (14 bands) and generate maps that portray the distribution of hydrothermal minerals (e.g., sericite, kaolinite, chlorite, epidote and carbonate) related to PCD alteration zones. In order to validate the ASTER data, follow-up ground proofing and related mineralogical work was done which, in all cases, proved to be positive. The results of this work have identified the regional distribution of hypogene alteration zones (i.e., phyllic, argillic, propylitic and silicic), in addition to areas of secondary Fe-oxide formation, which are coincident with known sites of PCDs. The regional distribution and extent of the alteration zones identified also highlighted the role of regional structures in focusing the mineralizing/altering fluids. These results demonstrate very convincingly that ASTER imagery that uses the appropriate techniques is reliable and robust in mapping out the extent of hydrothermal alteration and lithological units, and can be used for targeting hydrothermal ore deposits, particularly porphyry copper deposits where the alteration footprint is sizeable. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Porphyry copper deposits (PCDs) presently provide most of the world's Cu, Mo, and Re, about 20% of the world's Au, and minor amounts of other metals such as Ag, Pd, Te, Se, Bi, Zn, and Pb (Sillitoe, 2010). These deposits typically develop at shallow crustal depths (b 2–6 km from surface) and are associated with extensive hydrothermal alteration, typically zoned from an inner potassic assemblage, dominated by biotite and K-feldspar, which grades outward and upward into phyllic, argillic, and propylitic zones, respectively (e.g., Lowell and Guilbert, 1970; Mars, 2010, 2014; Mars and Rowan, 2006). The phyllic zone typically contains sericite and pyrite-rich rocks, the argillic zone consists of alunitic and kaolinitic rich-rocks, and the outer propylitic zone consists of a variable mineralogy of chlorite–epidote and calcite (e.g., Abrams and Brown, 1984; Hunt and Ashley, 1979; Lowell and ⁎ Corresponding author at: Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada. E-mail address: [email protected] (M. Alimohammadi).

http://dx.doi.org/10.1016/j.oregeorev.2015.04.010 0169-1368/© 2015 Elsevier B.V. All rights reserved.

Guilbert, 1970; Rowan and Mars, 2003; Seederoff et al., 2005; Spatz and Wilson, 1995). In addition, silicic alteration distinguished by the replacement of earlier alteration products and also primary igneous minerals by silica, and also by silica lithocaps and quartz veins, has been reported from the upper parts of many porphyry systems (e.g., Alimohammadi and Alirezaei, 2012; Ninomiya, 2003; Sillitoe, 1995; Titley, 1972; Tommaso and Rubinstein, 2007). The potassic and phyllic alteration zones are closely associated with economic sulfide mineralization and, therefore, are considered as prime targets for PCD exploration. In particular, phyllic alteration can cover a large area and has served as an efficient tool in regional exploration for PCDs. This alteration typically overprints earlier potassic assemblages due to ingress of acidic to near-neutral fluids and is associated with the destruction of original magmatic minerals (e.g., plagioclase, K-feldspar), as well as secondary hydrothermal biotite and K-feldspar (e.g., Dilles and Einaudi, 1992; Reed, 1997; Singer et al., 2008; Sillitoe, 2010; Titley, 1972). The hypogene ore in these systems is characterized by the common occurrence of pyrite (b1–10%) and chalcopyrite (b 1– 3%) with subordinate bornite and molybdenite. Exhumation of PCDs

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results in their differential exposure at surface and more importantly also enhances oxidation of sulfides with the development of Fe oxides/hydroxides, jarosite (a hydrous K–Al sulfate) and production of acid alteration of earlier mineral assemblages. During this exhumation, Cu-bearing minerals can, at least partly, be leached and the Cu mobilized and re-precipitated to produce a supergene enriched blanket below the contemporaneous water table, and this leads to upgrading of the original hypogene ore. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is an advanced multispectral satellite imaging system that has created new opportunities for the mapping of geological structures and detecting certain alteration minerals or assemblages (e.g., Cudahy et al., 2008; Hewson et al., 2005; Mars and Rowan, 2006; Rowan and Mars, 2003; Rowan et al., 2003). ASTER is a cooperative effort between NASA and Japan's Ministry of Economic Trade and Industry (METI). The instrument was launched on board NASA's TERRA spacecraft in December 1999 and consists of three separate subsystems with a total of 14 bands: (1) the visible near infrared (VNIR) subsystem obtains optical images of three bands (0.52 to 0.86 μm), with a spatial resolution of 15 m; (2) the shortwave infrared (SWIR) subsystem scans optical images of six bands (1.60 to 2.43 μm), with a spatial resolution of 30 m; and (3) the thermal infrared (TIR) subsystem obtains optical images of five bands (8.12 to 11.65 μm) with a spatial resolution of 90 m (Fujisada, 1995). ASTER also has a backward-looking VNIR telescope with a resolution of 15 m which means that stereoscopic VNIR

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images can be acquired at 15 m resolution. The swath width is 60 km, but off-nadir pointing capability extends the total cross-track viewing of ASTER to 232 km (Fujisada, 1995). The Cenozoic Urumieh–Dokhtar Magmatic Belt (UDMB) of Iran is an important metallogenic terrain for hosting porphyry Cu ± Mo ± Au deposits (Fig. 1; e.g., Alirezaei and Hassanpour, 2011; McInnes et al., 2005; Richards et al., 2012). The UDMB is a relatively narrow (50–80 km), linear belt dominated by calc-alkaline intrusive and extrusive rocks, and associated pyroclastic materials. The evolution of the UDMB is associated with the successive stages of closure of the Tethyan Ocean, including subduction during the Cretaceous–Oligocene, and continent–continent collision in the late Paleogene–Neogene (e.g., Agard et al., 2005; Allen et al., 2004; Berberian et al., 1982; Dercourt et al., 1986; McClay et al., 2004; Mohajjel et al., 2003; Ricou, 1994). Most of the known PCD systems are located in the southern section of the UDMB, also known as the Dehaj–Sardoieh belt or Kerman copper belt (Fig. 1). The National Iranian Copper Industries Company (NICICO) has conducted extensive exploration for PCDs in the UDMB, as well as other Cenozoic magmatic assemblages in Iran, since 2005. Processing satellite images has proven to be an effective tool in regional exploration for PCDs, including a variety of settings in Iran (e.g., Mars and Rowan, 2006; Mars, 2010; Pour et al., 2011; Ranjbar et al., 2004; Tangestani et al., 2008), examples of which include the following: (1) Ranjbar et al. (2004) identified iron oxides/hydroxides and hydroxyl-bearing minerals associated with hydrothermal and supergene alterations in PCD systems in the southern part

Fig. 1. (a) Major structural and geological subdivisions of Iran, after Stocklin (1968) and Nabavi (1976), which show the location of the Kerman Copper Belt (KCB) in the Urumieh–Dokhtar Magmatic Belt; (b) simplified litho-structural map of the KCB and location of three major porphyry Cu deposit clusters (Miduk, Sarcheshmeh and Daraloo) that are discussed in the text. Compiled from Dimitrijevic, 1973; Saric and Mijalkovic, 1973; Walker, 2006; Shafiei et al., 2009.

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of the UDMB with the use of ETM+ data; (2) Mars and Rowan (2006) developed logical operator algorithms based on ASTER defined band ratios for regional mapping of phyllic- and argillic-altered rocks in the Zargros magmatic zone; (3) Tangestani et al. (2008) evaluated ASTER Level-1B ‘radianceat-sensor’ and surface reflectance (AST-07) data for alteration zone enhancement related to PCD formation in northern Shahr-e-Babak; (4) Mars (2010) distinguished the sericitic, argillic and propylitic alteration zones associated with the Sarcheshmeh deposit in the Kerman copper belt, by using logical operator algorithms; and (5) Pour et al. (2011) discriminated hydrothermal alteration zones associated with Miduk and Sarcheshmeh PCDs in the southern UDMB using ASTER Level-1B ‘radiance-at-sensor’ data. The aim of this paper is to evaluate the accuracy of ASTER images, using data recorded in the spring and autumn of 2001, for targeting the alteration zones of PCD systems in an extensive area of the southern part of the Kerman copper belt where previous work was focused on a more regional scale (Mars and Rowan, 2006). A detailed analysis of the spectral mapping method is used to detect various alteration minerals associated with Cu mineralization in the Daraloo and Sarmeshk deposits. Upon correcting for the influence of topography and atmospheric conditions, the ASTER performance is evaluated as a means for both identification and discrimination of lithologies and also for determining their distributions (i.e., as a mapping instrument). Finally, we also compare the signature of the satellite data with that derived from laboratory-based measurements, and then apply the appropriate image processing techniques. The processing of these data enabled us to generate an alteration map that highlights a large area of hydrothermal alteration that has a high potential for PCD mineralization in the southern part of the Kerman copper belt, Iran. The results of this study form part of a broader study of the nature of the porphyry mineralization in the area that includes whole-rock chemistry, detailed petrological studies of the alteration and mineralization, geochronology, fluid inclusion studies, and stable and radiogenic isotope analyses in order to characterize the source and nature of the magmas and fluids responsible for the alteration and mineralization in the Daraloo and Sarmeshk deposits. The results of this work are currently being prepared for publication, but have been presented in preliminary form (Alimohammadi et al., 2014a,b). 2. Geological background 2.1. Regional geology of the Kerman copper belt Arc volcanism in the KCB started in Middle Eocene to form the Bahr– Aseman complex, and continued into the Upper Eocene to form the Razak complex (Dimitrijevic, 1973; Hassanzadeh, 1993). The volcanic complexes consist of calc-alkaline basaltic to rhyolitic lava flows and pyroclastic materials that are interbedded with mostly marine sedimentary rocks. These Eocene volcanic-sedimentary successions were intruded by voluminous Late Eocene–Oligocene plutonic bodies, some of batholithic size (Fig. 1). The intrusions range in composition from granites to diorites and locally gabbros, and are commonly characterized by having granular textures (Dimitrijevic, 1973; Ghorashizadeh, 1978; McInnes et al., 2003; Shafiei et al., 2009). In addition, these intrusions have a chemistry that indicates a high- to medium-K calc-alkaline affinity (Atapour, 2007; Shafiei et al., 2009). These intrusions are known as the Jebal-Barez type granitoids (Dimitrijevic, 1973), named after a mountain range in the southern Kerman province where they outcrop extensively, and to date no major metallic mineralization has been reported to occur within these intrusive bodies. Volcanic activity continued into the Middle Oligocene and generated the Hezar calc-alkaline volcanic complex (Dimitrijevic, 1973; Hassanzadeh, 1993) that is overlain unconformably by Upper Oligocene to Middle Miocene molassetype sediments and carbonate rocks (Dimitrijevic, 1973). The Middle and Upper Miocene was a time of magmatic reactivation in the KCB region, with numerous shallow intrusive bodies cutting the

older volcanic and plutonic rocks (Ghorashizadeh, 1978; Hassanzadeh, 1993; McInnes et al., 2005). These intrusions range in composition from diorites and quartz-diorites to granodiorites with the quartz-diorites dominating (Alirezaei and Hassanpour, 2011). The intrusions, known as the Kuh-Panj type granitoids after the Kuh-Panj Mountain southeast of Sarcheshmeh (Dimitrijevic, 1973), are associated with major porphyrystyle mineralization in the KCB (Fig. 1). These intrusions have a high-K calc-alkaline chemistry and display features typical of I-type magmas (e.g., Atapour, 2007). In addition, many of the porphyritic intrusions have chemical attributes, such as high Sr/Y and La/Yb ratios, that suggest an adakitic affinity (e.g., Alirezaei and Mohammadzadeh, 2009; Shafiei, 2008). This shallow-level intrusive activity continued into the Late Miocene and Pliocene in a post-collisional tectonic setting (Atapour, 2007; Shafiei et al., 2009) with the generation of many sub-volcanic intrusions (Dehaj type), two isolated stratovolcanoes (Masahim type) and several cones (Aj type) (Fig. 1). These rocks vary in composition from dacite to rhyolite and locally andesite (Dimitrijevic, 1973; Hassanzadeh, 1993). The youngest manifestation (Plio-Quaternary) of magmatic activity in the KCB is represented by lamprophyres, olivine-alkali basalts and foidites (Atapour, 2007; Dimitrijevic, 1973; Hassanzadeh, 1993).

2.2. Nature of alteration and mineralization of the Kerman porphyry copper deposits As in many other PCD provinces worldwide, the deposits within the KCB occur in clusters (Fig. 1) and three major clusters have been distinguished that include, from northwest to southeast, the following: (1) the Miduk cluster which is represented by the Miduk and several smaller deposits (e.g., Abdar, Serenu, Sara, Chahfiruzeh, Iju, Kader, and God-e-Kolvari,); (2) the Sarcheshmeh cluster which consists of the Sarcheshmeh deposit at its center that is surrounded by several smaller deposits (e.g., Darrehzar, Kuh-Panj, Sarkuh, Nowchun, Seridune, and Baghkhoshk); and (3) the Daraloo cluster that consists of Daraloo and several smaller deposits (e.g., Sarmeshk, Hanza, Peynegin, Lalehzar, Goruh and Chahartaq). In the Kerman copper belt, the largest known deposit is Sarcheshmeh (~ 1200 Mt of ore at 0.7% Cu and 0.03% Mo) where its host rock has been dated at 13.6 ± 0.1 Ma based on zircon U–Pb dating (McInnes et al., 2003). At the smaller Miduk deposit (~170 Mt of ore at 0.85% Cu, 0.006% Mo), the host rock has been dated at 12.5 ± 0.1 Ma (zircon U–Pb, McInnes et al., 2003). For the Daraloo deposit (80 Mt of ore at 0.5% Cu and N 0.003% Mo), a similar Miocene age of ca. 12 Ma has been obtained, but in this case by dating hydrothermal mica using 40 Ar/39Ar method (unpublished data of M. Alimohammadi), which provides the time of cooling of the mica below its blocking temperature, and given the high-level nature of PCDs is considered a good estimate of the time of hydrothermal activity. The density of porphyry Cu mineralization within the KCB increases from southeast to northwest with the majority of the deposits being located in elevated, thickened arc crust (45–50 km) (Shafiei et al., 2009). No volcanic rocks have been identified that are contemporaneous with the mineralized porphyry intrusions (Waterman and Hamilton, 1975; Hassanzadeh, 1993). Country rocks in most deposits are considered to be Eocene volcanic–pyroclastic materials with local intercalations of sandstones, shales and limestones (e.g., Atapour, 2007; Dimitrijevic, 1973; Shafiei, 2008). Most of the PCDs in the KCB are associated with well-developed potassic, sericitic, silicic, propylitic, and locally argillic alteration zones. The hypogene mineralization occurs as quartz-sulfide stockworks, as well as disseminated sulfides, in both the Miocene porphyritic stocks and the Eocene volcanic rocks. Common hypogene minerals are chalcopyrite and pyrite, with subordinate molybdenite and bornite. Most known deposits are deficient in molybdenum (i.e., b 100 ppm). The deposits have been variably affected by supergene oxidation and secondary Cu-

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enrichment, which is best developed only in the larger PCDs in the Kerman copper belt, such as Sarcheshmeh, Miduk, and Darrehzar. 3. Methods 3.1. ASTER data normalization Two ASTER level 1B scenes cover the study area, the AST_L1B 0111070701030111221288 acquired on 07/11/2001, and the AST_L1B 0108030704250108120031 acquired on 03/08/2001. In both scenes, pre-processing procedures are essential to obtain spatially and radiometrically corrected images in order to analyze and compare spectral data. For geometrically correcting remote sensing images, each raw image must be separately converted to an ortho-image. A 3D model geometric correction has been applied by generating a Digital Elevation Model (DEM) based on digital topographic data and using a group of Ground Control Points (GCPs) in the orthorectification of ASTER images. Radiometric correction is also essential for ensuring high-quality information from remote sensors. The ASTER SWIR data may be affected by the ‘crosstalk’ instrument problem, which is an offset or additive error in radiance due to the leakage of photons from one detector element to another. This cross detector leakage is most pronounced from band 4 to bands 5 and 9, but it also affects all SWIR bands (Iwasaki et al., 2001; Rowan and Mars, 2003). For very dark pixels adjacent to bright pixels, the crosstalk effect will approach 100% of the input radiance signal (Hewson et al., 2005). A spatial software correction for crosstalk has been developed by Iwasaki et al. (2001) and has since been incorporated by the Japanese ASTER Ground Data System (GDS) as a part of its L1B Pre-processing. In order to maximize the dynamic range of the SWIR and VNIR data and process the L1B data into calibrated radiance at the sensor (W/ m2/sr/μm), a set of gains (unit conversion coefficients) were applied after the crosstalk correction (Abrams et al., 2002). The calibration to spectral radiance units of the ASTER data was then obtained using the equation (Hewson et al., 2005): Radiance ¼ ðDN–1Þ  Gain:

ð1Þ

After calibrating the ASTER SWIR and VNIR data to radiance, they were converted to reflectance data in order to reduce the atmospheric effects. The path radiance effect was minimized by using the Dark Subtract method and the solar radiation response, and atmospheric absorption features were minimized by applying the IAR Reflectance calibration (Internal Average Relative Reflectance) or Log Residual calibration methods that are useful and also applied to derive aerosol scattering effects (Abrams et al., 2002). ASTER TIR bands were corrected for the atmospheric effects after orthorectification, using the ENVI utility named Thermal Atmospherical Correction, which is based on a Normalized Pixel Regression method (Bertoldi et al., 2011; Scheidt et al., 2008). The TIR data were calibrated to emissivity by using Emissivity Normalization Envi programs (Abrams et al., 2002). After radiometric correction, a mask based on the Normalized Difference Vegetation Index (NDVI) was applied to the data in order to mask out vegetation spectral adsorption features that may interfere with mapping spectral absorption features of minerals associated with PCDs (Mars and Rowan, 2006). For the ASTER sensor, NDVI is defined as follows: NDVI ¼ ðband 3−band 2Þ=ðband 3 þ band 2Þ:

ð2Þ

Negative NDVI values indicate cloud and snow/ice pixels, since these are more reflective in the VIS region than in the NIR one. Values of NDVI greater than 0.2–0.3 are evidence of vegetation and NDVI values less than 0.2–0.3 include rocks and soil pixels, as well as snow/ice and cloud pixels (Bertoldi et al., 2011; Fung and Siu, 2000). In the study

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area, values greater than 0.97 represent vegetated pixels in the autumn image, and values greater than 1.16 are vegetated pixels in the spring image. After pre-processing procedures, 3 VNIR bands were combined with 6 SWIR bands which resulted in a SWIR image that has the same dimensions as the VNIR image and helped forming a nine band image data set VNIR + SWIR in 15 m spatial resolution. This method of processing allows construction of useful ratio images for qualitative analysis and quick detection of alteration minerals with high accuracy geolocation (cf., Fujisada et al., 2001). In support of the above study, and to characterize the mineralogy of various alteration assemblages in the porphyry copper deposits, a large set of X-ray diffraction (XRD) analyses were carried out in collaboration with the National Iranian Copper Industries Company (NICICO). The results are summarized as follows: 1) For the phyllic alteration, the analyses detected muscovite, illite, and quartz as major phases, and kaolinite, albite, chlorite, goethite and jarosite as minor phases; 2) In argillic alteration, the analyses detected as major phases kaolinite, quartz and albite along with minor amounts of muscovite, illite, montmorillonite and goethite; 3) In propylitic alteration zones, the analyses indicated the occurrence of chlorite, epidote, clinozoisite and quartz as major phases, with minor calcite and muscovite; 4) For sodic–calcic alteration, the analyses indicated actinolite, tremolite, quartz and albite with minor epidote; 5) For potassic alteration, biotite and orthoclase are the characteristic phases, associated with quartz; and 6) for the silicic alteration, the major phases are quartz, muscovite and illite. 3.2. Spectral properties To find suitable analogue data for detecting the target minerals, we analyzed the laboratory spectral signatures of minerals from the ASTER Spectral Library which includes contributions from the Jet Propulsion Laboratory (JPL), Johns Hopkins University (JHU) and the United States Geological Survey (USGS). We also selected the laboratory spectral features from the USGS Mineral Spectral Library (Clark et al., 1993) which is loaded in Envi ver. 4.2 software. In the visible through shortwave infrared (VNIR-SWIR) range, the Fe-oxide/hydroxide phases along with phyllic, argillic and propylitic alteration types were characterized using the following features: (1) The phyllic-altered rocks typically contain sericite, which exhibits a prominent absorption feature at 2.20 μm (6th ASTER band, due to AL–O–H absorption, Fig. 2a), and a less intense absorption feature at 2.33 μm (8th ASTER band), due to Fe–, Mg–O–H absorption; (2) The argillic-altered rocks are typified by kaolinite and alunite, and these minerals exhibit AL–O–H absorption features at 2.20 and 2.17 μm (5th ASTER band), respectively (Abrams and Brown, 1984; Hunt and Ashley, 1979; Rowan et al., 2003; Spatz and Wilson, 1995); (3) Propylitic-altered rocks typically contain varying amounts of chlorite, epidote and carbonates, commonly calcite, which exhibit an absorption feature at 2.33 μm due to Fe– O–H, Mg–O–H and CO3 vibrational bonds (Rowan and Mars, 2003); and (4) The Fe-oxide/hydroxide phases present in PCD systems, which are due to secondary processes, typically include limonite, goethite, hematite, and jarosite. Goethite, hematite, and limonite have strong Fe3+ absorption features at 0.97–0.83 (Fig. 2b) and 0.48 μm, whereas jarosite has Fe–O–H absorption features at 0.94 and 2.27 μm (Hunt, 1977). In order to allow a comparison between laboratory signatures and ASTER spectra, the laboratory spectra were re-sampled to the 9 VNIR + SWIR ASTER band passes. As shown in Fig. 2, the key spectral absorption features of the high resolution laboratory signatures (dashed lines) are still present at the lower ASTER resolution (solid lines), except for some of the Fe-oxides/hydroxides. Thus, these ASTER mineral spectra can be distinguished in spite of their much lower spectral resolution for the more important and dominant alteration phases and predicting mineral suites (Rowan et al., 2003) which is summarized in Fig. 2: (1) ASTER band 1 and band 3 absorption features detect mainly Feoxides/hydroxides; (2) band 5 and band 6 absorptions detect Al–O–H

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Fig. 2. Laboratory spectra of common hydrothermal alteration minerals (Clark et al., 1993) showing spectra at full resolutions (420 bands, dashed lines) and ASTER re-sampled to the 9 VNIR + SWIR bands (solid lines). The vertical arrows indicate wavelength location of important absorption features in the adjacent spectrum, as discussed in the text. ASTER band wavelength position is shown at the top of the box diagrams. (a) The laboratory spectra which are used in this study are grouped according to alteration assemblages and include the following: (1) muscovite, which is typical in phyllic alteration, with a 2.20 μm absorption feature (6th ASTER band); (2) kaolinite and alunite, which are typical constituents in argillic alteration, exhibit Al–OH absorption features at 2.20 and 2.17 μm (5th ASTER band), respectively; and (3) epidote, chlorite and calcite are associated with propylitic alteration and display absorption peaks at 2.31–2.33 μm absorption features (8th ASTER band); (b) laboratory spectra of limonite, jarosite, hematite, and goethite. Hematite, limonite and goethite have strong Fe3+ absorption features at 0.97–0.83 and 0.48 μm, whereas jarosite has Fe–O–H absorption features at 0.94 and 2.27 μm.

caused by alunite, clay minerals and muscovite/sericite; (3) band 7 detects Fe–O–H caused mainly by jarosite and/or Fe-muscovite; and (4) ASTER band 8 detects Mg–O–H caused mainly by chlorite–epidote and/or carbonates (CO3) (Tommaso and Rubinstein, 2007). Based on these unique absorption features for each mineral, many useful approaches have been applied for discrimination and mapping of the hydrothermal alteration zones in the Daraloo cluster including Daraloo, Sarmeshk and some smaller deposits (e.g., Hanza, Peynegin, Lalehzar and Chahartaq; Fig. 3) which have been under exploration by National Iranian Copper Industries Company (NICICO) since 2010. 4. Results and discussion 4.1. ASTER false color composite and ratio images ASTER False color composite 468 (RGB) images typically show argillic- and phyllic-altered rocks as red tones, and propylitic-altered rocks as green tones due to Al–O–H (centered at ASTER band 6) and Fe–, Mg–O–H (centered at ASTER band 8) absorption features, respectively (e.g., Mars, 2010; Tommaso and Rubinstein, 2007). The false color composition RGB: 468 for the study area shows the alteration halo enhanced in two different color zones, the phyllic- and argillicaltered rocks with light red to pink color highlighting the PCDs which have been mapped in the field, and the propylitic-altered rocks with green color surrounding the PCDs (Fig. 4). Ratio images designed to display the spectral contrast of specific absorption features, have been used extensively in geologic remote sensing (e.g., Cudahy et al., 2008; Rowan et al., 1977, 2006; Tommaso and

Rubinstein, 2007). Image spectral reflectance of alunite, muscovite and kaoline, as well as jarosite, shows absorption features in bands 5, 6 (Al–O–H absorption, Fig. 2a), and 7 (Fe–O–H absorption), respectively; therefore band ratio transformation RGB: 4/5, 4/6, and 4/7 (Fig. 5) highlights the jarositic phyllic- and argillic-altered rocks in white color and the areas underlain by PCD mineralization within the white regions. The band ratio transformation RGB: 4/6, 5/8, and 3/4 (Fig. 6) is also useful for discriminating among different lithologies present. In this false color view, three main units can be discriminated in the study area: (1) a red zone, having high 4/6 band ratio values, which indicates the presence of muscovite and clay minerals and coincides with outcrops of intrusions of both Middle–Late Miocene (Kuh-Panj type) and Late Eocene–Oligocene (Jebal-Barez type) age, as well as felsic–intermediate volcanic and pyroclastic rocks (compare with Fig. 3); (2) a green zone, having high 5/8 band ratio values, which reflects the presence of regionally extensive secondary chlorite, epidote and calcite phases. This zone corresponds mainly with propylitic alteration of the Eocene–Oligocene volcanic-sedimentary rocks (Bahr-Asman, Razak and Hezar complexes) that surround the intrusive bodies; and (3) a blue zone, having high 3/4 band ratio values, which shows unaltered rocks that include volcanic and pyroclastic rocks. This band ratio diagram also highlights the observation that extensive propylitic alteration zones form well-defined regions around a core area that includes the phyllic and argillic alteration zones and hosts the porphyry deposits, as verified by field investigations. In addition, particularly in the southeast, the areas of phyllic and argillic alteration define northwesttrending corridors that likely reflect a structural control for the hydrothermal fluids that caused this alteration.

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Fig. 3. Simplified geological map of the southern part of the Kerman copper belt (redrawn from Saric and Mijalkovic, 1973) superposed on a shaded relief map of the area. The location of Daraloo cluster of porphyry copper deposits is shown by the white outline and includes the Daraloo, Sarmeshk, Hanza, Peynegin, Lalehzar and Chahartaq. which have been under exploration by National Iranian Copper Industries Company since 2010.

Relative Band Depth (RBD) images are useful for displaying the intensities of Al–O–H, Fe–, Mg–O–H, and CO3 absorption (Crowley et al., 1989; Rowan and Mars, 2003). The RBD is defined by the ratio between the sum of the bands at the shoulders of a defined absorption peak and the band closest to the peak itself (Crowley et al., 1989; Mars and Rowan, 2006). The following RBD images are calculated: (1) RBD4 [(band 3 + band 5)/(band 4) ∗ 2], used for detecting volcanic and sedimentary rocks. The laboratory spectral signatures of some volcanic and sedimentary rocks (andesite, siltstone and sandstone) from the ASTER Spectral Library, Johns Hopkins University (JHU), show absorption features in band 4. This laboratory spectral signatures (not shown in the figures) have been used to calculate this RBD and found to be consistent with the presence of volcanic and sedimentary rocks in the area; (2) RBD5 [(band 4 + band 6)/(band 5) ∗ 2] and RBD6 [(band 4 + band 7)/(band 6) ∗ 2] for detecting Al–O–H absorption in muscovite and clay minerals; however, the RBD6 is better for detecting muscovite; and (3) the RBD8 [(band 7 + band 9)/(band 8) ∗ 2] is focused on Fe–, Mg–O–H and CO3 absorption and is used for delineating chlorite, epidote and carbonates. In the study area, the RGB false color composite of RBD 6, RBD 8 and RBD 4 confirmed the result of band ratio transformation 4/6, 5/8, and 3/4, as is shown in Fig. 6, and highlights both the areas of phyllic and propylitic alterations, and unaltered volcanic and pyroclastic rocks. In the RGB false color composite of RBD 5, RBD 6 and RBD 4 (Fig. 7), four main units could be discriminated: (1) a yellow zone, which includes high RBD 5 and RBD 6 values, which indicates the

presence of muscovite and clay minerals and that is consistent with the alteration of intrusive rocks (Kuh-Panj and Jebal-Barez type granitoids), as well as volcanic and pyroclastic rocks. Note that porphyry type mineralization underlies this area; (2) a light blue zone with high RBD 4 values, which is coincident with outcrops of unaltered volcanic and pyroclastic rocks; (3) a dark blue color, which indicates the presence of mainly chlorite, epidote, and minor calcite, and reflects the propylitic alteration of volcanic and pyroclastic rocks. Similar to yellow zone, these dark blue areas also define northwest-trending corridors; and (4) and a dark green zone that is mainly related to the presence of carbonates (CO3) in the propylitically altered rocks, and which is confined to areas underlain by sedimentary rocks (Oligocene, Qom-Chahar Gonbad Formations in Fig. 3). 4.2. Application of spectral mapping method in the Daraloo–Sarmeshk deposits 4.2.1. An overview of deposit scale geology, alteration and mineralization The Daraloo and Sarmeshk copper deposits are situated in a northwest-trending fault zone, 10 km long and 0.5–1 km wide, with two deposits located at the ends of this corridor (Fig. 8a). The area is characterized by a series of porphyritic tonalite–granodiorite plutons of inferred Miocene age based on regional correlations with units towards the northwest (e.g., Ghorashizadeh, 1978; Hassanzadeh, 1993; McInnes et al., 2003; Mirnejad et al., 2013; Shahabpour and Kramers, 1987); this is also substantiated by our recent 40Ar/39Ar dating of

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Fig. 4. ASTER false color composition RGB 468 shown on the Daraloo cluster area. In this image, rocks with phyllic (serecitic) and argillic alteration are enhanced with red color, and green color shows rocks with propylitic alteration. The areas with significant porphyry copper mineralization are highlighted by the white ellipses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

hydrothermal mica phases (unpublished data of M. Alimohammadi). These intrusions cut Eocene andesitic volcanic and pyroclastic rocks. All these units are intruded by post-Miocene diabasic, andesitic and rhyodacitic dykes (Fig. 8b, c). The ore deposit at Daraloo defines an elongated body of mineralized rocks of approximately 0.7 km2 that occurs within a 1.2 km long northwest–southeast trending corridor. Alteration assemblages in Daraloo are comparable to those in porphyry Cu ± Mo deposits and are well

developed, with both sericitic and silicic alterations predominantly observed in outcrops. Hypogene sulfide mineralization occurs in two styles: quartz-sulfide stockworks and disseminated mineralization, and both of these occur in the porphyritic intrusive rocks and the adjacent volcanic and pyroclastic rocks within the deposit area. Hypogene mineralization is characterized by the presence of abundant pyrite and magnetite, with minor chalcopyrite, and trace bornite and molybdenite.

Fig. 5. ASTER band ratio values 4/5, 4/6, and 4/7 shown on the Daraloo cluster area. The areas underlain by white show a response of band 5 and band 6 (Al–OH absorption) and band 7 (Fe– OH absorption) which highlights jarositic phyllic and argillic alterations associated with porphyry copper mineralization. The areas outlined by the white ellipses are the porphyry copper deposit areas shown in Fig. 4.

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Fig. 6. ASTER band ratio values 4/6, 5/8, and 3/4 shown on the Daraloo cluster area. In this view of false colors, three main units can be discriminated: (1) a red zone with high 4/6 band ratio values, indicating the presence of muscovite and clay minerals; (2) a green zone with high 5/8 band ratio values, indicating the presence of chlorite–epidote and calcite; and (3) a blue zone with high 3/4 band ratio values, showing unaltered basement rocks. The areas outlined by the white ellipses are the porphyry copper deposit areas shown in Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Mineralization in the Sarmeshk deposit can be subdivided into two types based on the nature of the associated alteration. In the western part of the deposit, the features are more similar to those observed at Daraloo, with the Cu mineralization closely associated with areas of intensively developed sericite and silica alteration of the host pyroclastic

rocks. The mineralized area, which has an average grade of 0.3% Cu, occurs in a wedge-shaped zone approximately 0.3 km2 that is up to 500 m wide. In contrast to the above style of mineralization, in the eastern part of the Sarmeshk deposit, the mineralization, with an average grade of 0.2% Cu or less, is coincident with sodic–calcic and lesser potassic

Fig. 7. The RGB false color composite of RBD5, RBD6 and RBD4 shown on the Daraloo cluster area. The false colors show consistent results for lithologic discrimination, and four main units are distinguished: (1) a yellow zone with high RBD 5 and RBD 6 values, indicating the presence of muscovite and clay minerals; (2) a blue zone with high RBD 4 values, showing unaltered volcanic-sedimentary rocks; (3) a dark blue color indicating the presence of mainly chlorite–epidote; and (4) a dark green color mainly related to the presence of carbonates (CO3) in propylitic alteration and in the sedimentary rocks. Note that the results are in good agreement with the geological map in Fig. 3. The areas outlined by the white ellipses are the porphyry copper deposit areas shown in Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. False color ASTER image and geological maps for the Daraloo–Sarmeshk area which is characterized by a series of plutonic and subvolcanic bodies of felsic-intermediate compositions that intrude Eocene–Oligocene lava flows and pyroclastic rocks. (a) A 3D surface image view of a band combination RGB: 468 showing the Daraloo–Sarmeshk area along a northwesttrending fault zone. The light red to pinkish areas consist of phyllic- and argillic-altered rock whereas the light to dark green areas consist of propylitic rock; (b and c) Geological-alteration maps from field mapping of the Daraloo and Sarmeshk deposit areas, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) After National Iranian Copper Industries Company (NICICO), 2010.

alteration, as observed in outcrops. The host rocks are mostly thermally metamorphosed and silicified andesitic lavas and pyroclastic rocks which occur from surface to shallow depths (100–150 m), based on drill hole data. In the two mineralized zones, drilling indicates that stockwork type mineralization is not well developed and, instead, mineralization occurs mainly as disseminations and sparse veinlets. Propylitic alteration, developed mostly in the host volcanic rocks, is distinguished by the presence of chlorite, epidote and carbonates. Argillic alteration, which appears to be mainly of supergene origin, has affected both the Daraloo and Sarmeshk deposits (Fig. 8b, c). Examples of phyllic, argillic, and propylitic alterations exposed in outcrops, hand samples, and thin sections are shown in Fig. 11. Supergene enrichment is poorly developed at Sarmeshk; however an enriched blanket, 5 to 50 m thick, is developed at Daraloo. This lack of supergene enrichment at Sarmeshk might be attributed to less efficient leaching due to intense silicic alteration, and to lower copper assays in the hypogene mineralization. 4.2.2. ASTER VNIR and SWIR data analysis of porphyry style alteration and secondary Fe oxides The VNIR + SWIR data were analyzed by using a matched filtering procedure (Harsanyi et al., 1994; Farrand and Harsanyi, 1997). This procedure minimizes the response of the background materials by projecting each pixel vector onto a subspace, which is orthogonal to the background spectra, and then maximizes the response of the end

members of interest by comparing the residual pixels to each of the reference spectra (Rowan et al., 2006). For the study area, ASTER bands 1 through 9 were used for all the end members because of the importance of ferric and ferrous iron absorption, and reference spectra were selected from the ASTER image by using the Pixel Purity Index (PPI), which was preceded by minimum-noise transformation processing (c.f. Boardman et al., 1995; Green et al., 1988). The reference spectra were compared to ASTER resampled spectra from the USGS spectral library (Fig. 9) for identification of phyllic, argillic and propylitic mineral groups, and Fe-oxides/ hydroxides. The SWIR wavelength region of the reference spectra for rocks with phyllic, argillic and propylitic alteration is also shown in Fig. 9. The overall shapes of the ASTER image spectra in the SWIR wavelength region are similar to the ASTER laboratory spectra; there are, however, some differences in the absorption features in the VNIR wavelength region that show the presence of Fe-oxides/hydroxides associated with phyllic- and argillic-altered rocks. The hydrothermal alteration maps of the Daraloo–Sarmeshk area, compiled from matched filtering (Fig. 10), show that argillic (kaolinite) and phyllic (muscovite/illite) mapped rocks are associated with the PCDs and mainly reflect alteration of intrusive rocks as well as volcanic and pyroclastic rocks. Chloriteepidote and minor calcite are related to propylitic alteration of the volcanic–pyroclastic rocks in the area. The numbers in Fig. 10 indicate the location of field sites where rock sampling was done in order to verify

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Fig. 9. A summary diagram of the ASTER image spectra that have been used as references in the matched-filter processing of VNIR + SWIR data to map phyllic, argillic and propylitic mineral groups and Fe-oxides/hydroxides. For each hydrothermal alteration mineral shown, the reference spectra (dashed lines) were compared to ASTER re-sampled spectra from the USGS spectral library (solid lines), and for all minerals (except for jarosite and goethite) the SWIR wavelength region of the reference spectra is also shown. The short vertical arrows indicate some important ASTER band wavelengths in the related spectrum that are used to help distinguish mineral phases present. The ASTER band wavelength positions are also shown at the top of the box diagram.

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the physical alteration types inferred from ASTER imagery. In Fig. 11 the observations of the various alteration types are summarized, and samples collected from the study area verify the matched filter hydrothermal alteration map. Ratio images and matched filter processing were employed to delineate the Fe-oxides/hydroxide phases in the Daraloo–Sarmeshk region (Fig. 12). The ASTER re-sampled reflectance spectra display an intense ferric iron absorption feature in ASTER bands 1 and 2; therefore the ratio band 2/band 1 is useful for expressing the reflectance decrease in band 1 relative to band 2 for ferric-iron absorption. Ferrous-iron absorption may be discernable in some ASTER reflectance spectra due to its low reflectance in the VNIR bands coupled with either nearly constant values in the SWIR bands or a steep rise towards bands 4 and 5. However, the lack of a band pass in the 1.00 μm region in the ASTER instrument limits the capability to detect ferrous iron absorption (Rowan and Mars, 2003). Fe-oxides/hydroxide phases in the Daraloo–Sarmeshk region include jarosite, goethite, and minor hematite and limonite. As shown in the reference spectra of jarosite in Fig. 9, the weak absorption feature in band 6 may be due to low concentrations of muscovite/illite or kaolinite in the Fe-oxides/hydroxide phases. As for the other ASTER maps, field checks were used to verify the alteration types and these sites are shown in Figs. 12 and 13, the latter being photos of the field samples. In summary, the data obtained by ASTER imagery and the conclusions regarding the types and distribution of alteration present in the study area are in good agreement with the geological–alteration maps (compare to Fig. 8b, c). As noted already, the alteration maps derived from the ASTER data were controlled by field observations (Figs. 11, 13), petrographic studies, and XRD analyses for different alteration assemblages in the Daraloo–Sarmeshk area, and the results were found to be in good agreement. 4.2.3. ASTER TIR data analysis 4.2.3.1. Spectral emittance feature analysis for silica. Feldspar and quartz generally do not show any absorption features in VNIR + SWIR regions, with the exception of a small absorption peak observed at 2.22 μm, which is attributed to vibration modes from isolated OH point defects of Si–OH groups (Bertoldi et al., 2011; Cordier and Doukhan, 1991).

Fig. 10. Mineral map showing the results of matched filtering method using ASTER SWIR data superposed on the ASTER PCA1 gray-tone image of the Daraloo–Sarmeshk area; phyllic (red), argillic (blue) and propylitic (green). 1–3: location of sampled field sites.

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Fig. 11. (a) Regional view of phyllic alteration in volcanic and pyroclasic rocks from the western part of the Sarmeshk area (site 1, Fig. 10); (b–c–d) Close-up view of outcrop and handspecimens showing phyllic alteration at site 1; (e) Photomicrograph of highly sericitized and silicified sample from site 1. The silica occurs as fine-grained quartz as well as quartz veinlets, whereas the sericite occurs in patches; (f, g) Regional view of argillic alteration in volcanic rocks of the Daraloo area showing intense bleaching (site 2 in Fig. 10); (h to j) Close-up view and hand-specimens of argillic alteration in site 2; (k) Regional view of propylitic alteration in andesitic–basaltic country rocks (site 3, Fig. 10); (l to o) Close-up view, hand-specimens and photomicrograph showing the propylitic alteration at site 3 in Fig. 10.

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Fig. 12. Mineral map showing the results of matched filtering method using ASTER VNIR data (Fe-oxides/hydroxides, in brown) superposed on an ASTER PCA1 gray-tone image. The Sarmeshk area is shown by white box, and the numbers 4–5 are the location of sampled field sites to verify the ASTER data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Laboratory spectral emissivity features for silica are shown in Fig. 14a, where the sample spectrum shows the stretching vibrations of the Si– O bonds in the thermal infrared region, with doublet emissivity

absorption at 8.29 μm (10th ASTER band) and 9.07 μm (12th ASTER band), and a small peak at 8.63 μm (11th ASTER band). Samples were measured with a Micro-FTIR spectrometer using a gold plate, as

Fig. 13. (a) Regional view of Fe-oxides/hydroxides distribution in volcanic rocks (sites 4 and 5, Fig. 12) in contact with a porphyry stock in center; (b–c) Close-up views of supergene alteration from site 4 in Fig. 12; (d) Hand-specimens of Fe-oxide/hydroxide-bearing volcanic country rocks from site 5 in Fig. 12.

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Fig. 14. (a) Emissivity spectra for quartz at Micro-FTIR spectrometer resolution (Tommaso and Rubinstein, 2007); (b) Re-sampled spectrum to the 5 ASTER TIR band passes; (c) Pixel spectral plot extracted from the emissivity image in the study area.

referenced in the Infiernillo porphyry deposit, Argentina (Tommaso and Rubinstein, 2007), and then re-sampled to ASTER TIR band resolution (Fig. 14b). Hydrothermally altered silica-rich rocks in the Daraloo–Sarmeshk area, were mapped in the 5 thermal-infrared (TIR) bands by using the Quartz Index (Ninomiya, 2003) that is given as Qi = (b11 ∗ b11)/ (b10 ∗ b12); this factor, Qi, is expected to be high for quartz and low for K-feldspar (inverse Qi) (Ninomiya et al., 2005). Some desired and almost pure pixels for quartz (Fig. 14c) were extracted from the Qi-image and subsequently used as reference in the matched filter processing of TIR data to map the distribution of silicified rocks in the study area (Fig. 15). Hydrothermally altered silica-rich rocks associated with PCDs consist primarily of quartz veins, silica lithocaps, or silicified materials (e.g., Sillitoe, 1995, 2010; Titley, 1972). In the Daraloo–Sarmeshk area, quartz-bearing rocks include both intrusive bodies and silicified rocks. Outcrops with potassic and sodic–calcic alterations in the study area are both sparse and small in size and could not therefore be distinguished in the TIR pixel resolution; however, all the intrusive rocks and the area of sodic–calcic and potassic alteration have been silicified at surface exposures and shallow depths (down to 100–150 m), based on drill hole data. The ASTER TIR band passes were useful in detecting

the silicic alteration. The areas of silica enrichment follow the patterns of other alteration types noted above, and define a northwesttrending linear structure that is spatially coincident with the presence of mineralized centers. The minerals mapped remotely are consistent with the dominant mineralogy, as determined by petrographic studies and XRD analyses, which confirms that quartz is the dominant mineral. Examples of rocks and prepared thin sections of material for selected sites in areas of silica alteration are shown in Fig. 15 with evidence of the alteration highlighted in Fig. 16. It is further noted that similar silicic alteration occurs in the Raziabad, Zavork, and Kerver PCDs in the southern section of the Kerman copper belt (Alirezaei and Hassanpour, 2011; Alimohammadi and Alirezaei, 2013). 5. Conclusions The use and application of VINIR + SWIR ASTER data for mapping the regional extent of hydrothermal alteration in the southern part of the Kerman copper belt of Iran, including a more detailed analysis of the highly prospective Daraloo–Sarmeshk porphyry district, are accurate and helpful in detecting and mapping out extensive zones of phyllic, argillic and propylitic alteration, and also Fe-oxides/hydroxides. In addition, TIR

Fig. 15. Mineral map showing the results of matched filtering method using ASTER TIR data to detect silicified rocks in Daraloo–Sarmeshk area which is overlain on an ASTER PCA1 graytone image. The numbers shown (6–7–8) refer to site locations sampled within the silicified alteration zones.

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Fig. 16. Outcrop and hand specimens along with the correlated thin section photomicrographs showing examples of silica alteration in the study area (see Fig. 15 for location of the sites); (a, b) Outcrop and photomicrograph of highly silicified rocks in Daraloo (site 6). Note that the silicification occurs as both fine-grained quartz as well as quartz veinlets; (c, d) Hand specimen of silicified pyroclastic rock in Sarmeshk (site 7) with accompanying thin section photomicrograph; (e, f) Outcrop of a highly silicified volcanic rock adjacent to the porphyry intrusion in eastern part of the Sarmeshk area (site 8) with accompanying thin section photomicrograph.

emissivity analysis is useful for distinguishing and mapping out silicic alteration, with the most promising results obtained by the matched-filter processing technique. For all alteration types noted, follow-up field observation and sampling validated the inferred alteration types based on ASTER data and actual alteration zones present. The results of this study indicate the presence of extensive phyllic and silicic alteration zones, with more restricted areas of kaolinite (argillic), that are surrounded by broader zones of propylitic alteration. Thus, the alteration zonation mapped with the use of ASTER data coincides well with classical models portraying the zonal distribution of alteration types associated with porphyry deposits (i.e., Lowell and Guilbert, 1970). The distribution of the alteration patterns highlights the role of regional structures in localizing fluid flow, an integral part of porphyry deposit formation (e.g., Richards, 2003). Furthermore, the analysis of ASTER TIR data and follow-up field observations also show that silicic alteration, which is not generally discussed as a common alteration feature in porphyry systems (e.g., Seederoff et al., 2005; Sillitoe, 2010) tends to be associated with the porphyry type mineralization in the well mineralized Daraloo– Sarmeshk area. In summary, the results of this study confirm that ASTER images can be a useful and powerful tool in the initial steps of exploration for those deposit types where large alteration zones are an integral part of the deposit model because these data provide highly accurate and reliable information about the distribution of alteration minerals. When integrated with field studies, the use of ASTER data provides an effective means to delineate areas that are most favorable for mineralization.

Acknowledgments We are sincerely grateful to the National Iranian Copper Industries Company (NICICO), in particular the Research and Development Department, for providing access to the area and logistical support. We thank Dr. Jeffrey L. Mauk, the Associate Editor of Ore Geology Reviews who handled this paper, Dr. John C. Mars and an anonymous reviewer for their detailed and insightful comments which helped to clarify several aspects which resulted in substantial improvement and clarity of the paper.

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