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Abstract

A method for regional mapping of phyllic and argillic hydrothermally altered rocks using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data was developed and tested at the Cuprite, Nevada, calibration and validation site, and then extensively used in the Zagros magmatic arc in Iran, which consists of the High Zagros and Jebal Barez Mountains, and the Bazman volcanic area. Logical operator algorithms were developed to perform multiple band ratio and threshold value calculations, which can be applied to a scene using a single algorithm, thus eliminating separate production and application of vegetation and dark pixel masks. Argillic and phyllic band-ratio logical operators use band ratios that define the 2.17 µm and 2.20 µm absorption features to map kaolinite and alunite, which are typical in argillic-altered rocks, and muscovite, which is a common mineral in phyllic-altered rocks.

Regional mapping of the Zagros magmatic arc using the logical operators illustrates distinctive patterns of argillic and phyllic rocks that can be associated with regional structural features and tectonic processes, and that can be used in regional mineral assessments. Semicircular patterns, 1–5 km in diameter, of mapped phyllic- and argillic-altered rocks are typically associated with Eocene to Miocene intrusive igneous rocks, some of which host known porphyry copper deposits, such as at Meiduk and Sar Cheshmeh. Linear phyllic-altered rock patterns associated with extensive faults and fractures indicate potential epithermal or polymetallic vein deposits. On the basis of argillic and phyllic alteration patterns, ∼50 potential porphyry copper deposits were mapped northwest of the Zagros-Makran transform zone in an eroded, exhumed, and dormant part of the magmatic arc, whereas only 11 potential porphyry copper deposits were mapped to the southeast of the transform, in the volcanically active part of the magmatic arc. The Zagros-Makran transform zone, which separates the volcanically dormant and active parts of the Zagros magmatic arc, exhibits extensive linear patterns of phyllic-altered rocks that indicate the potential for polymetallic-epithermal vein deposits.

INTRODUCTION

Porphyry copper deposits are typically characterized by zoned assemblages of hydrothermal alteration minerals (Lowell and Guilbert, 1970; Fig. 1). These minerals exhibit spectral absorption features in the visible near-infrared (VNIR) through the short-wave infrared (SWIR; 0.4–2.5 µm; Fig. 2) and/or the thermal-infrared (TIR; 8.0–14.0 µm) wavelength regions (Abrams et al., 1983; Spatz and Wilson, 1995). Multispectral images with sufficient spectral and spatial resolution to delineate spectral absorption features can be used to identify and remotely map these altered rock zones in well-exposed terranes.

The Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) measures reflected radiation in three bands in the 0.52–0.86 µm wavelength region (VNIR); six bands in the 1.6–2.43 µm wavelength region (SWIR); and five bands of emitted radiation in the 8.125–11.65 µm wavelength region (TIR) with 15 m, 30 m, and 90 m resolution, respectively (01Table 1; Fujisada, 1995). ASTER also has a backward-looking VNIR telescope with 15 m resolution. Thus, stereoscopic VNIR 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 purpose of this study was to (1) develop a systematic, efficient method to map argillic- and phyllic-altered rocks at a regional scale using ASTER data and to (2) evaluate the usefulness of regional hydrothermal alteration maps in relation to mineral assessments, regional structures, and tectonic processes.1 The Zagros magmatic arc in Iran, which consists of the High Zagros and Jebal Barez Mountains, and the Bazman volcanic area, was selected for evaluation because of the extensive exposures of major porphyry copper deposits, which include the Sar Cheshmeh and Meiduk mines (Fig. 3; Plates 1 and 2; Tangestani and Moore, 2002; Hassanzadeh, 1993; Taghizadeh and Mallakpour, 1976). In addition, previous hydrothermal alteration mapping of the Sar Cheshmeh and Meiduk mines using ASTER and Landsat Thematic Mapper (TM) data provides validation for the regional mapping algorithms tested in this study (Ranjbar et al., 2004; Tangestani and Moore, 2002). The study area receives ∼140 mm of annual precipitation and, thus, has excellent bedrock exposure with minimal vegetation (http://www.uk.ac.ir/Visitors/About_Kerman.jsp).

This paper describes (1) the relationship between VNIR and SWIR spectral reflectance and alteration mineral assemblages associated with porphyry copper deposits, (2) data processing for regional mapping of hydrothermally altered rocks, (3) accurate geometric registration of the ASTER results, (4) the logical operators used for regional argillic and phyllic mapping, (5) the relationships between regional alteration types and patterns to geologic setting, and (6) the comparison of the alteration mapping results to the locations of copper mines and copper prospects. Successful application of this remote-sensing method will augment more conventional mineral resource appraisal methods in this area and other similar sparsely vegetated well-exposed regions and contribute to our understanding of relationships between hydrothermal alteration and regional-scale tectonic processes.

Geology of the Zagros Magmatic Arc

This study defines the Zagros magmatic arc as the northwest-trending mountain belt in central Iran, including the High Zagros Mountains, the Jebal Barez Mountains, and the Bazman volcanic area north of the Makran subduction zone (Fig. 3; Plate 1; Walker and Jackson, 2002). The High Zagros Mountains make up the northwestern part of the Zagros magmatic arc and consist of the Urumieh-Dokhtar magmatic arc assemblage, which is classified as an Andean magmatic arc (Plate 1; Alavi, 1980; Berberian et al., 1982). The northwestern part of the Zagros magmatic arc is the product of Tethys oceanic plate subduction under the Iranian microplate followed by continent-to-continent collision of the Arabian and Eurasian plates (Regard et al., 2004). Quaternary volcanic rocks of the Bazman volcanic area along the southeastern part of the Zagros magmatic arc are the product of active subduction that continues along the Makran subduction zone (Regard et al., 2004). The northwest collision and southeast subduction zones of the Zagros magmatic arc are separated by the Zagros-Makran transfer zone and Sabzevaran and Gowk strike-slip fault systems, which are part of the Jebal Barez Mountains (Fig. 3). The Zagros-Makran transfer zone, and Sabzevaran and Gowk strike-slip fault systems are part of a convergent transform, which is referred to in this study as the Zagros-Makran transform zone (Fig. 3; Plate 1; Regard et al., 2004; Walker and Jackson, 2002).

The High Zagros Mountains are a volcanic succession of Eocene calcalkaline basaltic andesites and Oligocene shoshonitic rocks intruded by Neogene quartz diorites, quartz monzonites, and granodiorites that contain mined deposits of copper (Huber, 1969a; Hassanzadeh, 1993). Additional plutonic rocks include granite and gabbro, and volcanic rocks include basalt, andesite, and dacite, which were erupted as lava flows, ignimbrites, and pyroclastic flows (Huber, 1969a; Hassanzadeh, 1993). The majority of volcanism occurred from Eocene to Miocene time (Huber, 1969a; Dimitrijevic, 1973; Hajian, 1977; Amidi, 1984). Extensive mineralization occurred from Miocene to Pliocene time and produced porphyry and vein mineralization. Large porphyry copper mines in the region include Sar Cheshmeh, in a quartz monzonite pluton, and Meiduk, in a quartz diorite pluton (Hassanzadeh, 1993). Crosscutting relationships between plutons and strata and age dating of volcanic rocks indicate that the magmatic arc has been active from the Late Jurassic to present (Nabavi, 1972).

The Zagros-Makran transform zone contains highly faulted Eocene and Oligocene rhyolites, dacites, and andesites, which were primarily erupted as tuffs (Huber, 1969b; Grabeljsek et al., 1972). Intrusive rocks of the Zagros-Makran transform zone consist primarily of Eocene diorite, granodiorite, and granite. Copper deposits in the Zagros-Makran transform zone are typically associated with veins in faulted andesites, granodiorites, and diorites or are associated with dikes that consist of granite-porphyry, dacite, and albite-trachyte microdiorite (Grabeljsek et al., 1972; Valeh, 1972).

Volcanic rocks southeast of the Zagros-Makran transform zone in the Bazman volcanic area consist primarily of Quaternary andesites and basalts (Huber, 1969b). Less-extensive Paleogene volcanic rocks are dispersed around the periphery of the Quaternary intermediate volcanic rocks. Some of the Paleogene volcanic rocks are highly faulted and contain copper deposits (Huber, 1969b; Taghizadeh and Mallakpour, 1976).

Previous Satellite Remote-Sensing Studies

Remote-sensing data analysis has been applied to mineral resource studies since multispectral imagery became available soon after World War II, but the digital multispectral capability to investigate large inaccessible areas became feasible when the first Earth Resources Technology Satellite (ERTS-1) was launched on 3 July 1972, which was later renamed Landsat. Numerous investigators have evaluated Land-sat Multispectral Scanner (MSS) data in order to delineate geomorphological expressions of intrusive bodies and regional structural features with which porphyry copper deposits might be associated (Raines, 1978; Rowan and Wetlaufer, 1981; Abrams and Brown, 1984; Abrams et al., 1983). Anomalously limonitic rocks, which can be a potential indicator of hydrothermal alteration, can be mapped at 79 m spatial resolution in the MSS bands, but no other mineralogical information can be extracted spectrally from MSS images (Fig. 4A; Rowan et al., 1974; Schmidt, 1976; Krohn et al., 1978; Raines, 1978).

The Landsat Thematic Mapper (TM) extended the spectral range into the SWIR region by adding bands centered at 1.65 and 2.20 µm, and the spatial resolution was improved to 30 m. Selection of the 2.20 µm band (TM band 7) was based on laboratory and field spectral reflectance measurements that showed that many clay, carbonate, sulfate, and hydrous minerals exhibit spectral absorption features in this wavelength region due to molecular vibrational processes (Fig. 2; Hunt and Salisbury, 1970). An airborne experiment conducted in the Cuprite, Nevada, area demonstrated that these absorption features could be used for mapping extensively well-exposed hydrothermally altered rocks, which are bleached, opalized rocks containing several spectrally distinctive argillic and advanced argillic alteration minerals (Abrams et al., 1977). Simple ratio images portrayed the altered rocks based mainly on the intense absorption features in the 2.2 µm region related to these alteration minerals. This spectral band (TM band 7) and the band located at 1.65 µm (TM band 5) greatly enhanced the capability to discriminate surface materials, particularly hydrothermally altered rocks, but important ambiguities were recognized because of the breadth of TM band 7. For example, carbonate rocks that contain calcite and dolomite (2.33 and 2.32 µm absorption features, respectively) and hydrothermally altered rocks that contain minerals such as alunite and kaolinite (2.17 and 2.2 µm absorption features) are commonly difficult to distinguish spectrally in TM images, because both have absorption features located in TM band 7 (Fig. 4A).

The spectral bands of the ASTER SWIR subsystem were designed to measure reflected solar radiation in one band centered at 1.65 µm, and five bands in the 2.10–2.45 µm region in order to distinguish Al-OH, Fe,Mg-OH, H-O-H, and CO3 absorption features (Fig. 4B; 01Table 1). Several investigators have documented identification of specific minerals, such as calcite, dolomite, and muscovite, as well as mineral groups, through analysis of ASTER data (Rowan and Mars, 2003; Rowan et al., 2003).

Remote-Sensing Characteristics of Porphyry-Copper Deposits and ASTER Data

In the idealized porphyry copper deposit model, a core of quartz and potassium-bearing minerals, mostly K-feldspar and biotite, is surrounded by multiple zones of alteration minerals (Fig. 1; Lowell and Guilbert, 1970). The hydrous zones are characterized by mineral assemblages, which contain at least one mineral that exhibits diagnostic spectral absorption features. The broad phyllic zone, which is commonly limonitic due to oxidation of pyrite, is characterized by illite/muscovite (sericite), and the narrower argillic zone can be indexed by kaolinite and alunite (Fig. 1) (Abrams and Brown, 1984; Spatz and Wilson, 1995). The mineral assemblage of the outer propylitic zone is more variable due to country rock compositional differences, but epidote, chlorite, and carbonate minerals are common constituents (Fig. 1). Titley (1972) noted that both country rock and intrusive rock can host copper mineralization, and both can be hydrothermally altered. Additional factors that affect the portrayal of these mineral assemblages in images are vegetation cover, topography, amount of exposure, structural configuration, and anthropogenic activities. This analysis of ASTER data concentrates on the phyllic and argillic alteration zones because of the variability of the propylitic mineral assemblage and the generally limited exposure of the core alteration zone. The general spectral reflectance of the propylitic assemblage is described here, however, because of the spectral contrast of propylitic with phyllic and argillic assemblages.

Phyllic alteration spectral characteristics include illite/muscovite reflectance spectra that exhibit an intense Al-OH absorption feature, which is typically centered at 2.20 µm (ASTER band 6), and a less intense feature near 2.38 µm (ASTER band 8) (Fig. 4B). Substitution of Fe2+ for Al causes the minimum to shift to longer wavelengths, which can be detected using ASTER data (Rowan and Mars, 2003). Intense Fe3+ absorption suppresses reflectance in the VNIR wavelength region in minerals such as limonite (Fig. 4B).

Argillized rocks, including rocks classified as advanced argillic, also display an Al-OH absorption feature near 2.20 µm, but both kaolinite and alunite exhibit significantly different spectral shapes compared to muscovite/illite (Fig. 4B) (Hunt, 1977; Hunt and Ashley, 1979; Rowan et al., 2003). Kaolinite displays a secondary feature or shoulder at 2.17 µm, whereas alunite exhibits a minimum at 2.17 µm (ASTER band 5), instead of 2.20 µm, and a minor feature at 2.20 µm (Fig. 4B); both minerals also exhibit secondary absorption features near 2.38 µm. These spectral differences are detectable in ASTER data because the SWIR spectral bandpasses of the instrument were tailored for this purpose (Rowan et al., 2003) 01(Table 1).

Propylitic mineral-assemblage reflectance spectra are characterized by Fe,Mg-OH absorp tion features andCO3 features caused by molecular vibrations in epidote and chlorite and in carbonate minerals, respectively (Spatz and Wilson, 1995) (Fig. 4B). These absorption features are situated in the 2.35 µm region (ASTER band 8). In addition, Fe2+ absorption features are displayed by epidote and chlorite in the VNIR wavelength region (Fig. 4B). These spectral reflectance features contrast strongly with those of the phyllic and argillic assemblages.

The spectral reflectance characteristics of phyllic, argillic, and propylitic rocks provide a basis for distinguishing hydrothermally altered rocks associated with porphyry copper deposits from most country rock lithologies, but ambiguities may result where the country rock contains the same minerals that are typical of the altered rocks. For example, limonitic quartz-muscovite schist could be confused with phyllic-altered rocks, and kaolinitic weathering surfaces might resemble argillized rocks spectrally. Thus, consideration of the country rock compositional range and spectral reflectance is an important aspect of the analysis.

METHODS

Processing of ASTER Data

The ASTER spectral reflectance data analyzed for mapping hydrothermally altered rocks were produced at the EROS Data Center, Sioux Falls, South Dakota, from level 1b data. The EROS Data Center AST_07 product consists of the 9 ASTER VNIR-SWIR channels calibrated to reflectance using atmospherically corrected radiance at the surface (Thome et al., 1999). The extent of the image coverage of the High Zagros Mountains study area is complete, except for a few narrow gaps (Fig. 5; Plate 1). The 30-m-resolution SWIR data were expanded by a factor of two to correspond to the VNIR spatial dimensions, and then the 6 SWIR and 3 VNIR bands were combined to form 9-band data sets.

The 62 individual ASTER scenes used in this study were recorded at four different viewing angles ranging from nadir to 8.2 degrees off nadir, which causes misregistration of up to 600 m in high-relief terrain. Therefore, each 9-band image was geometrically registered to an orthorectified Landsat TM mosaic (NASA, 2003). The Landsat mosaic data have a spatial resolution of 28 m and spatial accuracy of ±50 m. Although this registration procedure corrected for the off-nadir viewing offset, the images were not corrected for terrain displacement. Using a second-order polynomial warp registration algorithm and at least nine ground control points for each scene produced root mean square (RMS) errors of <2.0 (60 m). The mosaic of Landsat 4 and 5 data was also used as a base map to illustrate regional ASTER alteration units for the Iranian study area (Plate 1). Argillic and phyllic alteration units were converted to vector data, exported from the ASTER scenes, and illustrated on band 7 of the Landsat mosaic (Plate 1). The Landsat mosaic and ASTER alteration units were spatially resampled to 60 m resolution to conserve computer file space.

Correction of ASTER Data using Hyperion Hyperspectral Data

Comparisons of AST_07 ASTER reflectance data to spectra taken in situ, from laboratory samples, and from calibrated hyperspectral data indicate anomalously low reflectance for ASTER band 5 (Fig. 6). If band 5 is too low, the spectra are more similar in shape to argillic minerals, such as alunite, and the mapping algorithms will map too much argillic-altered rock and not enough phyllic-altered rock (Figs. 6 and 7). Hyperion data were resampled to ASTER bandpasses and used to correct ASTER band 5. Hyperion, a hyperspectral instrument flown on board the EO1 satellite platform, has 196 spectral bands in the 0.45–2.4 µm region (Kruse et al., 2003). Hyper-ion data can be properly calibrated to reflectance using the additional Hyperion atmospheric bands not available in ASTER data.

Hyperion radiance data were calibrated to reflectance data using ACORN atmospheric removal software. The Hyperion reflectance data were resampled to ASTER VNIR-SWIR bandpasses and georegistered to the orthorectified Landsat TM imagery (NASA, 2003). Average spectra were extracted from areas of overlap for the ASTER and Hyperion scenes. A scalar correction, consisting of the ASTER resampled Hyperion band ratio 5/6 divided by the ASTER band ratio 5/6, was applied to all ASTER band 5 data.

A total of eight Hyperion scenes were used to correct the 62 ASTER scenes that covered the study area. Difference ratios for the eight Hyperion scenes ranged from 5.8% to 10.3%. An average of the scalar correction value (7.9%) was used to correct the ASTER data for the study area.

ASTER Band Ratios and Relative Band Depth Ratios

Ratio images, which display the spectral contrast of specific absorption features, have been used extensively in geologic remote sensing (Rowan et al., 1974, 1977). Relative absorption band depth (RBD; Crowley et al., 1989) images are an especially useful three-point ratio formulation for displaying Al-O-H, Fe,Mg-O-H, and CO3 absorption intensities (Figs. 4B and 8). For each absorption feature, the numerator is the sum of the bands representing the shoulders, and the denominator is the band located nearest the absorption feature minimum (Fig. 8; Crowley et al., 1989). RBD5 {[(ASTER band 4 + ASTER band 6)/ASTER band 5] and RBD6 [(ASTER band 4 + ASTER band 7)/ASTER band 6]} images have been used in previous studies to delineate argillic and phyllic mineral assemblages using ASTER SWIR data (Rowan and Mars, 2003).

The presence of vegetation in ASTER pixels impacts the usefulness of these RBD5 and RBD6 images for mapping argillic and phyllic rocks, because some organic-compound (typically cellulose) absorption features centered near 2.10 and 2.30 µm are near the wavelength of some of the main Al-OH and Fe,Mg-OH absorption features (arrows—dry sagebrush leaves, Figs. 4B and 9). In previous studies, a digital mask was produced from the band 3/band 2 ratio and applied to these RBD images to delete pixels containing green, photosynthetically active chlorophyll (Rowan et al., 2005). This mask, however, did not eliminate dry vegetation lacking chlorophyll absorption.

Intense Fe3+ absorption is displayed in band 2/band 1 ratio images as high digital number (DN) values, because intense absorption in the ultraviolet and in the band 1 region causes relatively low band 1 reflectance (Rowan and Mars, 2003). Inclusion of vegetation causes lower ratio values, as chlorophyll absorption decreases the band 2 reflectance. Dry vegetation has low chlorophyll absorption, which has less effect on the band 2/band 1 ratio.

Logical Operators

For each pixel the logical operator algorithm performs a series of band ratios. Each logical operator determines a true or false value for each ratio by comparing the band ratio to a predetermined range of threshold values. All of the ratios in the algorithm have to be true in order for a value of 1 to be assigned to the byte image, otherwise a 0 value is produced. Thus, a byte image consisting of zeros and ones is produced with each algorithm. Four ASTER scenes from the study area in Iran, a calibration site in Cuprite, Nevada, and laboratory spectra were resampled to ASTER bandpasses and spectroscopically assessed to determine the range of ratios and band DN values for constraining the logical operator algorithms. Due to the amount of noise in the ASTER data 01(Table 1), all ASTER alteration units were median filtered using a 3 × 3 matrix.

Logical operators were used in conjunction with band ratios in order to streamline regional mapping and consistently threshold band ratios used to map altered rocks for the entire study area. In a study area that covers more than 60 ASTER scenes, it was not practical to manually threshold each band ratio image (Fig. 3). In addition, multiple ratios and band thresholds can be applied to a scene using one algorithm, thus, eliminating the separate production and application of vegetation and dark pixel masks (Fig. 10).

Argillic Band Ratio Logical Operator Algorithm

The first part of the argillic band ratio logical operator (ABRLO) algorithm performs a band 3/2 ratio to mask out green vegetation (Fig. 10A). A spectral analysis of image and library spectra suggests that band 3/2 ratio threshold values of 1.35 and less typically constitute areas that lack green vegetation. The ratio does not mask out dead vegetation, which has 2.17 and 2.33 µm absorption features (Fig. 9).

The ABRLO algorithm performs a threshold of band 4 to mask out pixels with low reflectance. Pixels with low reflectance contain abnormally high band 5 and band 9 values, which may be due to energy leakage from the band 4 detector into adjacent band 5 and band 9 detectors, which is referred to as “crosstalk” (Iwasaki et al., 2002; Rowan and Mars, 2003). Pixels with low reflectance that are affected by “crosstalk” in this study typically include shadows, and mafic and ultramafic rocks. Abnormally high band 5 and 9 values due to “crosstalk” produce anomalous band 6 and 8 absorption features (Fig. 11). Spectral analyses of ASTER band 4 pixels with DN values less than 260 were determined to have inaccurate spectral signatures due to “crosstalk” and were thus excluded using a digital mask.

Spectral analysis of ASTER image spectra and resampled ASTER laboratory spectra showed that band ratios 4/5, 5/6, and 6/7 were needed to map the 2.17 and 2.2 absorption features, thereby delineating argillic- and phyllic-altered rocks (Figs. 7 and 10A). Band ratios 4/5 and 5/6 map the 2.165 and 2.2 absorption features, respectively. Spectral analysis of ASTER spectra indicates that band 5 must be at least 5% lower than band 6 in order to map as an argillic-altered rock; thus, the 5/6 band ratio delineates argillic from phyllic rocks by classing ratio values of 1.05 and less as argillic alteration (Figs. 7 and 10A). ASTER spectra of argillic-altered rocks also illustrate that band 4 is at least 25% greater than band 5, and band 7 is 3% greater than band 6. Thus, values in the ABRLO algorithm for band ratios 4/5 and 7/6 must be greater than 1.25, and greater than or equal to 0.03, respectively, to classify a pixel as argillic alteration (Fig. 10A).

Phyllic Band Ratio Logical Operator Algorithm

The phyllic band ratio logical operator (PBRLO) algorithm is almost identical to the ABRLO algorithm (Fig. 10B). The PBRLO algorithm uses the same methods to mask green vegetation and pixels with low reflectance. ASTER spectra of phyllic-altered rocks show that band 5 is at least 5% greater than band 6, which is expressed in the PBRLO algorithm as classifying all 5/6 band ratio values greater than 1.05 as phyllic-altered rocks (Figs. 7 and 10B). ASTER spectra also indicate that band 6 is at least 25% lower than band 4, and band 7 is at least 3% greater than band 6 (Figs. 7 and 10B). Thus, values in the PBRLO algorithm for band ratios 4/6 and 7/6 must be greater than 1.25, and greater than or equal to 0.03, respectively, in order to classify a pixel as phyllic alteration (Figs. 7 and 10B).

Masking Detrital Clays in Sedimentary Rocks

Detrital clays in sedimentary rocks can be erroneously mapped as hydrothermal alteration clay minerals. Many sedimentary rocks such as mudstone, shale, claystone, and litharenite sandstones contain large amounts of detrital clays such as montmorillonite, illite, and kaolinite. In order to mask out detrital clays, an igneous rock mask was produced using two 1:1,000,000-scale Iranian geologic maps (Huber, 1969a, 1969b). Each map was digitized, georegistered, and polygon vectors were drawn around igneous rock units. The vectors were then converted to a mask, which was applied to the ASTER alteration data, thus, confining alteration mapping to areas underlain by igneous rocks.

Validation of Alteration Mapping Algorithms at the Cuprite, Nevada, Calibration and Validation Test Site

Cuprite, Nevada, was selected as a site to test the accuracy of the ABRLO and PBRLO algorithms (Fig. 10). Previous geologic studies at Cuprite, Nevada, have mapped extensive argillic, opalized, and silicified hydrothermal alteration zones (Fig. 12; Ashley and Abrams, 1980; Swayze, 1997). Well-exposed, muscovite-rich Cambrian phyllitic siltstone bounds the western part of the hydrothermal alteration units (unit Ch; Fig. 13). Although the phyllitic siltstones at Cuprite, Nevada, are not a product of hydrothermal alteration, they contain the same muscovite-rich mineralogy found in hydrothermally altered phyllic rocks. Calibration and validation studies of hyperspectral and multispectral imaging detectors at Cuprite, Nevada, include ASTER, the Airborne Visible Infrared Imaging Spectrometer (AVIRIS), and Hyperion (Rowan et al., 2003; Clark et al., 1993a; Kruse et al., 2003).

Two methods were used to test the accuracy of the ABRLO and PBRLO algorithms at Cuprite, Nevada. In both methods AST_07 ASTER reflectance data from Cuprite, Nevada, were used, and a Hyperion scene of the same area was used to adjust ASTER band 5. The first method involved a visual comparison of mineral maps from previous remote-sensing studies to argillic and phyllic maps produced from the ABRLO and PBRLO algorithms (Rowan et al., 2003; Clark et al., 1993a; Clark and Swayze, 1996). The second method quantitatively compared ASTER-derived argillic and phyllic maps to similar maps compiled from an ASTER-simulated AVIRIS data set that was resampled to ASTER bandpasses (Fig. 14). AVIRIS is an airborne hyperspectral sensor with 224 spectral bands in the 0.45–2.4 µm region (Green et al., 1998). Previous studies at Cuprite, Nevada, have illustrated that AVIRIS can accurately map minerals with Al-OH absorption features (Clark et al., 1993a; Clark and Swayze, 1996). In the second validation method, the ASTER-simulated data were coregistered to the ASTER data set in order to quantitatively assess alteration-mapping accuracy.

The first validation method results show that argillic and phyllic patterns are very similar to maps from previous studies; however, the ABRLO and PBRLO algorithms tend to map up to ∼30% more altered rock (Rowan et al., 2003; Clark et al., 1993a; Clark and Swayze, 1996). The overestimation of altered rocks reflects the inclusion of mineral mixtures in the ABRLO and PBRLO algorithms, whereas mapping algorithms in previous studies focused on distinguishing individual minerals. The overestimation of altered rocks also reflects the more noisy characteristics and larger spatial resolution (30 m) of ASTER data compared to high signal-to-noise 18-m-spatial resolution AVIRIS data (Yamaguchi et al., 2001; Green et al., 1998; Clark and Swayze, 1996). To further assess the accuracy of the algorithms, areas where phyllic and argillic rocks were mapped in the ASTER data, but not mapped in previous studies, were spectrally assessed for absorption features, and an average spectrum was produced for each alteration unit. The spectra of the suspect areas mapped as altered rocks in the ASTER data contain 2.20 and 2.165 µm absorption features, and average spectra for each alteration unit are similar to muscovite and kaolinite-alunite mixed spectra (Fig. 15).

In the second validation method, mineral maps of the ASTER data and simulated-ASTER data show similar patterns of argillic and phyllic rocks produced from the ABRLO and PBRLO algorithms (Fig. 14). Ninety-five percent of the argillic rocks mapped in the ASTER data set correlate to the simulated-ASTER data set. However, the ABRLO algorithm mapped additional areas of argillic-altered rocks in the ASTER data, totaling 47% more than the argillic-altered rocks mapped in the ASTER-simulated data. The average argillic spectrum from the ASTER data has a shape similar to kaolinite and is classified as argillic (Figs. 4B,10, and 15). Comparison of the simulated-ASTER and average ASTER argillic spectra indicates that relative to band 6, band 5 in the ASTER data is ∼3% higher than band 5 in the simulated-ASTER data (Fig. 15). The 3% variation falls within the noise parameters of the ASTER SWIR detector 01(Table 1).

Seventy-two percent of the phyllic rocks mapped in the ASTER data set correlate to the simulated-ASTER data set. An additional 56% of the phyllic pixels in the ASTER data set are not mapped as phyllic rocks in the ASTER simulated data. The average phyllic spectrum from the ASTER data has a shape similar to muscovite and is classified as phyllic (Figs. 4B,10, and 15). Comparison of the simulated-ASTER and average ASTER phyllic spectra indicates that relative to band 6, band 5 in the ASTER data is ∼2% lower than band 5 in the simulated-ASTER data (Fig. 15). The 2% variation falls within the noise parameters of the ASTER SWIR detector 01(Table 1). As mentioned in the first validation method, variations between the simulated-ASTER and ASTER phyllic and argillic data sets are due to the lower signal-to-noise in the ASTER system than the AVIRIS system and the larger pixel size of the ASTER data, at 30 m compared to 18 m pixels of the AVIRIS data (Yamaguchi et al., 2001; Green et al., 1998; Clark and Swayze, 1996).

HYDROTHERMAL ALTERATION MAP OF ZAGROS MAGMATIC ARC

Distribution of Hydrothermally Altered Rocks in the High Zagros Mountains

Mapped alteration patterns in the Zagros magmatic arc are categorized as semicircular and linear patterns (Plate 1). Most of the hydrothermally altered rocks consist of phyllic-altered rocks with minor amounts of argillic-altered rocks (Plate 1). The geology in the northwestern part of the study area is dominated by Eocene volcanic rocks that contain northwest- and west-trending faults (Fig. 16A). Hydrothermal alteration patterns in this part of the study area are linear, consist almost exclusively of phyllic-altered rocks, and tend to follow linear topographical and structural features (Fig. 16B; Plate 1). The linear phyllic alteration patterns in the northwestern part of the study area are similar to patterns produced by alteration along an extensive fracture and fault zone, and suggest that the type of alteration is associated with polymetallic and epithermal vein deposits (Cox and Singer, 1986).

Geologic maps indicate extensive Eocene to Miocene diorite, quartz diorite, quartz monzonite, and granodiorite porphyry intrusive rocks throughout the central part of the Zagros magmatic arc (Huber, 1969a). Most of the intrusive rocks in the central part of the Zagros magmatic arc are 1–2 km in diameter, and country rocks are primarily Eocene and Oligocene volcanic rocks that contain extensive volcanic tuffs (Figs. 17 and 18). Hydrothermal alteration units in the central part of the Zagros magmatic arc typically form semicircular to circular patterns, 1–5 km in diameter and are associated with hydrothermally altered Eocene to Miocene igneous intrusive bodies (Figs. 17 and 18; Plate 1). The semicircular patterns of hydrothermally altered rocks are also centered on partially eroded volcanoes when superimposed on ASTER and TM imagery and geologic maps (Fig. 17; Plate 1). Although most of the mapped altered rocks classify as phyllic-altered, there are substantially more argillic-altered rocks in the central part of the Zagros Magmatic Arc than mapped in the northwestern part of the study area (Figs. 16,17B, and 18B; Plate 1). Semicircular alteration patterns, the presence of phyllic and argillic rocks, and association with altered intrusive bodies and partially eroded volcanoes suggest a high potential for copper-porphyry deposits based on similarities to current hydrothermal alteration models and previous prospects and studies (Lowell and Guilbert, 1970; Ranjbar et al., 2004; Tangestani and Moore, 2002). In addition, concentrations of argillic- and phyllic-altered rocks derived from ASTER data exhibit semicircular patterns for two large copper producing mines in the north-central part of the study area (Sar Cheshmeh and Meiduk), which are classified as porphyry copper deposits (Figs. 17B and 18B; Plate 1; Hassanzadeh, 1993; Ranjbar et al., 2004; Tangestani and Moore, 2002). Alteration maps produced for the Sar Cheshmeh and Meiduk areas in previous studies show good agreement with the ASTER phyllic and argillic alteration maps (Ranjbar et al., 2004; Tangestani and Moore, 2002).

Some of the mapped phyllic- and argillic-altered rocks form elongate patterns in the cores of plunging folds (Fig. 18; Plate 1). The cores of the folds contain more argillic- than phyllic-altered rocks. Although the alteration is not associated with exposed altered intrusive bodies, the shape and limited lateral extent of the alteration and confining structure suggest localized, intense, fracturing fluid flow along fold axes and the potential for mineralization.

Eocene-Oligocene volcanic rocks consisting primarily of tuff extend throughout the Zagros-Makran transform zone in the south-central part of the Zagros magmatic arc (Fig. 19). Eocene granodiorite that intrudes Eocene volcanic rocks is common in the southwestern part of the transform zone (Fig. 19). All of the volcanic and intrusive rocks contain extensive northwest-trending faults (Fig. 19; Regard et al., 2004). Phyllic alteration dominates the area and tends to form linear patterns associated with mapped faults and linear features seen in ASTER and TM imagery (Fig. 19; Plate 1). Only a small part of the alteration is associated with the Eocene granodiorite and granite intrusive rocks. Dominant phyllic linear alteration patterns along faults and fractures suggest the potential for polymetallic vein– and epithermal vein–style mineralization and are supported by mineral occurrence data taken from previous geologic mapping in the Makran-Zagros transform zone, which has documented mineralization along veins and dikes (Grabeljsek et al., 1972; Valeh, 1972).

The southeastern part of the Zagros magmatic arc adjacent to the Zagros-Makran transform zone consists primarily of Eocene granodiorite and granite that have intruded Eocene and Oligocene volcanic rocks (Fig. 20). The northwest-trending faults are not as extensive as compared to the adjacent transform zone. In the area adjacent to the transform zone, up to 30% of the Eocene granodiorites and granites contain phyllic alteration with minor amounts of argillic alteration (Fig. 20; Plate 1). Some of the Eocene tuffs and other volcanic rocks also exhibit extensivephyllicalteration(Fig. 20;Plate 1).Thelarge percentage of ASTER-mapped phyllic alteration in the area adjacent to the transform zone may be due to the presence of muscovite-rich granodiorites and granites. However, ASTER-mapped phyllic alteration extends across geologic units and includes some volcanic tuffs (Fig. 20). In addition, argillic-altered rocks can only be associated with hydrothermal alteration, thus, the distribution of ASTER-mapped argillic alteration in the area adjacent to the transform zone indicates that at least 50% of the ASTER-mapped phyllic alteration is the result of hydrothermal alteration (Fig. 20; Plate 1).

The southeastern part of the Zagros magmatic arc consists primarily of Quaternary andesite and includes at least one active composite volcano (Figs. 21A and 21B; Plate 1). Less extensive basalt flows, Paleogene intermediate volcanic rocks and tuffs, and Eocene granites and granodiorites make up the rest of the southeastern part of the magmatic arc. Older Paleogene rocks, such as Eocene granites and granodiorites, tend to be extensively faulted. Mapped alteration is mainly in the older faulted Paleogene rocks, and in particular, the Eocene granites, granodiorites, and Paleogene tuffs (Figs. 21A and 21B; Plate 1). Mapped alteration in the granites and granodiorites tends to be phyllic and forms linear patterns associated with fractures and faulting, indicating potential for polymetallic or epithermal vein mineralization (Fig. 21; Cox and Singer, 1986). Mapped alteration in the Paleogene tuffs consists of semicircular patterns of phyllic and argillic alteration and may be associated with porphyry copper deposits (Fig. 21; Lowell and Guilbert, 1970). The only laterally extensive alteration associated with the Quaternary andesites is situated on the slopes of the composite volcano Bazman (Figs. 21A and 21B). Most of the alteration on the volcano maps as argillic.

Comparison of Hydrothermal Alteration Spectral Units and Mapped Mines and Occurrences

Regional argillic and phyllic alteration maps and existing economic geology data for the study area provide important information to help determine the percentage of mineralization associated with altered rocks, which is a key factor in mineral assessments. Two sets of copper mines and occurrences data were plotted on an alteration map (Plate 2). One of the data sets, taken from a 1:2,500,000-scale mineral distribution map of Iran (Taghizadeh and Mallakpour, 1976), illustrates good association of mines and occurrences with ASTER-mapped hydrothermal alteration (Plate 2). The location of the mines and occurrences, however, is not accurate enough for statistical analysis of documented mineralization and hydrothermal alteration.

The second data set of copper mines and occurrences was taken from a set of 1:100,000-scale geologic maps, which cover the central part of the study area (Dimitrijevic, 1971; Dimitrijevic et al., 1971a, 1971b, 1971c, 1972a, 1972b; Djokovic et al., 1972a, 1972b, 1972c, 1973; Grabeljsek et al., 1972; Mijalkovic et al., 1972; Srdic et al., 1972a, 1972b; Timotijevec et al., 1972; Valeh, 1972, 1973a, 1973b; Plate 2). In order to statistically analyze the relationship between documented mineralization and hydrothermal alteration, 60 m and 1 km zones around each location were assessed. There are 10 copper mines and 50 copper occurrences (sites) located on the 1:100,000-scale geologic maps (Plate 1; 02Table 2). Forty-four of the mines and occurrences have hydrothermal alteration within a 1 km radius of each site, and 15 of the sites have alteration within a 60 m radius of each site. The average percentage of hydrothermal alteration within the 1 km radius of each site is 16%. More detailed percentages of alteration within a 1 km radius of each mine or deposit site show 16 sites with no alteration within a 1 km radius, 14 sites with <0.01–1% alteration within a 1 km radius, 10 sites with 1–10% alteration within a 1 km radius, 5 sites with 10–20% alteration within a 1 km radius, 7 sites with 30–40% alteration within a 1 km radius, and 8 sites with 40–100% alteration within a 1 km radius (02Table 2; Fig. 22). Visual assessment of 15 m VNIR ASTER data indicates that 12 of the sites with altered rocks within a 1 km radius show some type of disturbance, such as a road, pit, or dump, within ∼150 m of the site. Of the 16 sites that did not map altered rocks within a 1 km radius, 11 were outside of the igneous rock mask, and the other 5 sites showed no disturbance from visual assessment using ASTER 15 m false-color composite images.

SUMMARY AND CONCLUSIONS

Logical operators effectively map argillic- and phyllic-altered rocks, provided ASTER data are correctly calibrated using Hyperion or other data, such as in situ or laboratory spectra. Logical operators apply multiple band ratios and band thresholds to mask green vegetation and pixels affected by “crosstalk” and map altered rocks. The logical operators use ratios of ASTER bands 4, 5, 6, and 7 to resolve absorption features in the 2.17–2.20 µm region. Standard ratio values are determined from ASTER image data and spectral libraries. Because the logical operators use standard ratio values for multiple scenes, Hyperion data are used to correct the ASTER band 5 anomaly.

The calibration and logical operator mapping technique was tested at Cuprite, Nevada, using mineral maps from previous studies, an AVIRIS data set resampled to ASTER bandpasses, and the Hyperion-corrected ASTER AST_07 reflectance data set. The argillic and phyllic spectral units produced by the ABRLO and PBRLO algorithms were similar in shape to phyllic and argillic mineralogy in maps from previous studies. Ninety-five percent of the argillic-altered rocks mapped in the simulated-ASTER data set were mapped in the AST_07 reflectance data set. Seventy-two percent of the phyllic-altered rocks mapped in the simulated-ASTER data set were mapped in the AST_07 reflectance data set. Spectroscopic examination of erroneously mapped argillic (47%) and phyllic (56%) areas indicated that the error was most likely due to noise and spatial resolution variations between the AVIRIS and ASTER instruments.

To test the new mapping method on a regional scale, an argillic and phyllic alteration map was compiled for the High Zagros Mountains, Iran. Argillic and phyllic spectral units mapped using ABRLO and PBRLO algorithms indicated distinctive patterns of alteration that could be associated to faulting and geologic units. In the northwestern part of the study area, primarily phyllic-altered rocks form linear patterns and are associated with faults and other linear features. The central part of the study area contains numerous 1–5 km semicircular patterns of mapped phyllic- and argillic-altered rocks that are associated with Eocene to Miocene intrusive igneous rocks. The Eocene to Miocene altered intrusive rocks host mined porphyry copper deposits, such as Meiduk and Sar Cheshmeh. The Zagros-Makran transform zone contains primarily phyllic-altered rocks that form linear patterns and are associated with extensive faulting. The southwest area adjacent to the transform zone contains laterally extensive phyllic-altered rocks that are associated with granodiorites and granites. The active part of the Zagros magmatic arc contains mostly andesitic and basaltic rocks that are not altered except for the summit of a recent composite volcano, which consists of mostly argillic-altered rocks. There are some Paleogene tuffs and intrusive rocks in the southeastern part of the study area that contain argillic- and phyllic-altered rocks.

Regional hydrothermal alteration maps provide important data for regional mineral assessments. The argillic- and phyllic-altered rock maps of the Zagros magmatic arc define 61 potential copper-porphyry deposits and areas of potential epithermal and polymetallic vein deposits (Plate 1). Most of the potential porphyry copper deposits are in the High Zagros Mountains, whereas most of the potential polymetallic and epithermal vein deposits are in the Makran transform zone (Plate 1). This data could be used in mineral assessments to more accurately define mineral deposit tracts and number of deposits. In addition, statistical data on percentage of area of altered rocks and known mineralized deposits could be used to more accurately determine grade and tonnage in quantitative mineral assessments (Plate 2; 02Table 2).

Regional mapping of argillic- and phyllic-altered rocks provides a better understanding of tectonic influence on hydrothermal alteration. On the basis of ASTER-mapped alteration patterns, the central part of the study area contains at least 50 localities with potential porphyry copper deposits (Figs. 17 and 18; Plate 1, 1–50 localities) and is associated with the older “closed part” of the Zagros magmatic arc, where large inactive volcanic complexes have been unroofed by erosion. Linear-shaped ASTER-mapped alteration patterns in the active Zagros-Makran transform zone suggest the potential for epithermal and polymetallic vein deposits. The linear-shaped alteration patterns are associated with extensive fractures and faults, which are common in transform zones (Plate 1; Fig. 19). To the southeast of the Zagros-Makran transform zone along the active part of the magmatic arc, ∼11 localities with potential porphyry copper deposits can be defined on the basis of ASTER-mapped alteration patterns (Plate 1, 51–61 localities). The area along the southeast margin of the Zagros-Makran transform zone, which contains laterally extensive altered rocks, may be the result of deep erosion caused by prolonged thermal heating and uplift along the convergent transform (Fig. 20; Plate 1). Farther to the southeast, the intermediate Quaternary volcanic rocks of the active part of the magmatic arc cover most of the older Eocene rocks, which are more likely to contain porphyry copper, epithermal, and polymetallic vein deposits (Fig. 21; Plate 1). Thus, on the basis of regional alteration patterns and structures in the Zagros magmatic arc, tectonic processes control the degree of exhumation of altered rocks, and hydrothermal deposit types and distribution.

GSA Data Repository item 2006116, the ArcView shape files for the argillic and phyllic alteration units in Iran, projected in geographic latitude and longitude, is available online at www.geosociety.org/pubs/ft2006.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

We thank Fred Kruse for providing us with the Hyperion data set from Cuprite, Nevada, and technical reviewers Bernard Hubbard, James Crowley, Jeff Doebrich, Alan Gillespie, and Michael Abrams for their helpful suggestions.