Regional Mapping of Phyllic- and Argillic-Altered Rocks in the Zagros

Home , Dehaj, Geologic map, Rayen

Regional mapping of phyllic- and argillic-altered rocks in the Zagros magmatic arc, Iran, using Advanced Spaceborne Thermal Emission and Reﬂ ection Radiometer (ASTER) data and logical operator algorithms

John C. Mars* Lawrence C. Rowan U.S. Geological Survey, National Center, Mail Stop 954, Reston, Virginia 20192, USA

ABSTRACT west of the Zagros-Makran transform zone swath width is 60 km, but off-nadir pointing in an eroded, exhumed, and dormant part of capability extends the total cross-track viewing A method for regional mapping of phyl- the magmatic arc, whereas only 11 potential of ASTER to 232 km (Fujisada, 1995). lic and argillic hydrothermally altered rocks porphyry copper deposits were mapped to The purpose of this study was to (1) develop a using Advanced Spaceborne Thermal Emis- the southeast of the transform, in the volca- systematic, effi cient method to map argillic- and sion and Refl ection Radiometer (ASTER) nically active part of the magmatic arc. The phyllic-altered rocks at a regional scale using data was developed and tested at the Cuprite, Zagros-Makran transform zone, which sepa- ASTER data and to (2) evaluate the usefulness Nevada, calibration and validation site, and rates the volcanically dormant and active of regional hydrothermal alteration maps in rela- then extensively used in the Zagros magmatic parts of the Zagros magmatic arc, exhibits tion to mineral assessments, regional structures, arc in Iran, which consists of the High Zagros extensive linear patterns of phyllic-altered and tectonic processes.1 The Zagros magmatic and Jebal Barez Mountains, and the Bazman rocks that indicate the potential for polyme- arc in Iran, which consists of the High Zagros volcanic area. Logical operator algorithms tallic-epithermal vein deposits. and Jebal Barez Mountains, and the Bazman vol- were developed to perform multiple band canic area, was selected for evaluation because ratio and threshold value calculations, which Keywords: ASTER, remote sensing, porphy- of the extensive exposures of major porphyry can be applied to a scene using a single algo- ry copper, hydrothermal alteration, Iran. copper deposits, which include the Sar Chesh- rithm, thus eliminating separate production meh and Meiduk mines (Fig. 3; Plates 1 and and application of vegetation and dark pixel INTRODUCTION 2; Tangestani and Moore, 2002; Hassanzadeh, masks. Argillic and phyllic band-ratio logical 1993; Taghizadeh and Mallakpour, 1976). In operators use band ratios that defi ne the 2.17 Porphyry copper deposits are typically char- addition, previous hydrothermal alteration map- µm and 2.20 µm absorption features to map acterized by zoned assemblages of hydrothermal ping of the Sar Cheshmeh and Meiduk mines kaolinite and alunite, which are typical in alteration minerals (Lowell and Guilbert, 1970; using ASTER and Landsat Thematic Mapper argillic-altered rocks, and muscovite, which is Fig. 1). These minerals exhibit spectral absorp- (TM) data provides validation for the regional a common mineral in phyllic-altered rocks. tion features in the visible near-infrared (VNIR) mapping algorithms tested in this study (Ranj- Regional mapping of the Zagros magmatic through the short-wave infrared (SWIR; 0.4–2.5 bar et al., 2004; Tangestani and Moore, 2002). arc using the logical operators illustrates µm; Fig. 2) and/or the thermal-infrared (TIR; The study area receives ~140 mm of annual distinctive patterns of argillic and phyllic 8.0–14.0 µm) wavelength regions (Abrams et al., precipitation and, thus, has excellent bedrock rocks that can be associated with regional 1983; Spatz and Wilson, 1995). Multispectral exposure with minimal vegetation (http://www. structural features and tectonic processes, images with suffi cient spectral and spatial reso- uk.ac.ir/Visitors/About_Kerman.jsp). and that can be used in regional mineral lution to delineate spectral absorption features This paper describes (1) the relationship assessments. Semicircular patterns, 1–5 km can be used to identify and remotely map these between VNIR and SWIR spectral refl ectance in diameter, of mapped phyllic- and argil- altered rock zones in well-exposed terranes. and alteration mineral assemblages associ- lic-altered rocks are typically associated with The Advanced Spaceborne Thermal Emis- ated with porphyry copper deposits, (2) data Eocene to Miocene intrusive igneous rocks, sion and Refl ectance Radiometer (ASTER) processing for regional mapping of hydrother- some of which host known porphyry copper measures refl ected radiation in three bands in mally altered rocks, (3) accurate geometric reg- deposits, such as at Meiduk and Sar Chesh- the 0.52–0.86 µm wavelength region (VNIR); istration of the ASTER results, (4) the logical meh. Linear phyllic-altered rock patterns six bands in the 1.6–2.43 µm wavelength operators used for regional argillic and phyllic associated with extensive faults and fractures region (SWIR); and fi ve bands of emitted radi- indicate potential epithermal or polymetal- ation in the 8.125–11.65 µm wavelength region 1GSA Data Repository item 2006116, the ArcView lic vein deposits. On the basis of argillic and (TIR) with 15 m, 30 m, and 90 m resolution, shape fi les for the argillic and phyllic alteration units phyllic alteration patterns, ~50 potential por- respectively (Table 1; Fujisada, 1995). ASTER in Iran, projected in geographic latitude and longi- phyry copper deposits were mapped north- also has a backward-looking VNIR telescope tude, is available online at www.geosociety.org/pubs/ ft2006.htm, or on request from editing@geosociety. with 15 m resolution. Thus, stereoscopic VNIR org or Documents Secretary, GSA, P.O. Box 9140, *E-mail: [email protected] images can be acquired at 15 m resolution. The Boulder, CO 80301-9140, USA.

Geosphere; May 2006; v. 2; no. 3; p. 161–186; doi: 10.1130/GES00044.1; 22 ﬁ gures, 2 tables, 2 plates, Data Repository item 2006116.

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Explanation: Hydrothermal Alteration Zones, Minerals, mapping, (5) the relationships between regional Kaol - Kaolinite Chl - Chlorite and Ores in a Porphyry Copper Deposit Alun - Alunite alteration types and patterns to geologic setting, Epi - Epidote cp - Copper and (6) the comparison of the alteration map- Carb - Carbonate gal - Galena Q - Quartz ping results to the locations of copper mines sl - Sulfide Ser - Sericite and copper prospects. Successful application Au - Gold K-feld - Potassium of this remote-sensing method will augment Ag - Silver Feldspar mb - Molybdenite more conventional mineral resource appraisal Bi - Biotite mag - Magnetite methods in this area and other similar sparsely Anh - Anhydrite Pyrite Shell py - Pyrite Argillic Peripheral vegetated well-exposed regions and contribute cp-gal-sl-Au-Ag py 10% to our understanding of relationships between Q-Kaol-Alun cp .01-3% Propylitic Chl hydrothermal alteration and regional-scale tec- Chl-Epi-Carb tonic processes. Phyllic Q Geology of the Zagros Magmatic Arc Ore Ser Low- Low- Shell This study defi nes the Zagros magmatic arc py Pyrite Grade py 1% as the northwest-trending mountain belt in cen- Potassic Shell Q-K-feld Core cp 1-3% tral Iran, including the High Zagros Mountains, py 2% cp-py -Bi +- anh mb .003% the Jebal Barez Mountains, and the Bazman vol- mb canic area north of the Makran subduction zone Q (Fig. 3; Plate 1; Walker and Jackson, 2002). The Ser High Zagros Mountains make up the north- Chl mag mag mag western part of the Zagros magmatic arc and K-feld py py py consist of the Urumieh-Dokhtar magmatic arc assemblage, which is classifi ed as an Andean ABChl-Ser-Epi-mag (Modified from Lowell and Guilbert, 1970) magmatic arc (Plate 1; Alavi, 1980; Berberian et al., 1982). The northwestern part of the Zagros Figure 1. Illustrated deposit model of a porphyry copper deposit (modifi ed from Lowell and magmatic arc is the product of Tethys oceanic Guilbert, 1970). (A) Schematic cross section of hydrothermal alteration minerals and types, plate subduction under the Iranian microplate which include propylitic, phyllic, argillic, and potassic alteration. (B) Schematic cross sec- followed by continent-to-continent collision of tion of ores associated with each alteration type. 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 Spectral Library Plots subduction that continues along the Makran subduction zone (Regard et al., 2004). The northwest collision and southeast subduction zones Epidote of the Zagros magmatic arc are separated by the Zagros-Makran transfer zone and Sabzevaran and Gowk strike-slip fault systems, which are Figure 2. Laboratory spectra of epidote, part of the Jebal Barez Mountains (Fig. 3). The Calcite calcite, muscovite, kaolinite, chlorite, and Zagros-Makran transfer zone, and Sabzevaran alunite, which are common hydrothermal and Gowk strike-slip fault systems are part of alteration minerals (Clark et al., 1993b). a convergent transform, which is referred to in Muscovite Alunite and kaolinite have Al-O-H absorp- this study as the Zagros-Makran transform zone tion features at 2.17 and 2.20 µm. Mus- (Fig. 3; Plate 1; Regard et al., 2004; Walker and Jackson, 2002). Kaolinite covite has a prominent Al-O-H 2.20 µm absorption feature and a secondary 2.35 The High Zagros Mountains are a volca- µm absorption feature. Chlorite and epi- nic succession of Eocene calc-alkaline basal- dote have an Fe-Mg-O-H 2.32 µm absorp- tic andesites and Oligocene shoshonitic rocks 2+ intruded by Neogene quartz diorites, quartz Reflectance (Offset For Clarity) Reflectance (Offset For tion feature and a broad Fe feature from Chlorite 1.65 to 0.6 µm. Calcite has a prominent monzonites, and granodiorites that contain 2.33 µm CO absorption feature. mined deposits of copper (Huber, 1969a; Has- 3 sanzadeh, 1993). Additional plutonic rocks Alunite include granite and gabbro, and volcanic rocks 20% Reflectance include basalt, andesite, and dacite, which were erupted as lava fl ows, ignimbrites, and pyroclastic fl ows (Huber, 1969a; Hassanzadeh, 0.5 1.0 1.5 2.0 2.5 1993). The majority of volcanism occurred Wavelength (μm) from Eocene to Miocene time (Huber, 1969a;

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Dimitrijevic, 1973; Hajian, 1977; Amidi, 1984). TABLE 1. ASTER BASELINE PERFORMANCE REQUIREMENTS Extensive mineralization occurred from Mio- Spectrometer Band Bandpass Radiometric Absolute Spatial Signal quantization cene to Pliocene time and produced porphyry number (spectral range) resolution accuracy* resolution levels† and vein mineralization. Large porphyry cop- (µm) (σ) (m) (bits) per mines in the region include Sar Cheshmeh, VNIR 1 0.52–0.60 NEΔρ ≤ 0.5% ≤+4% 15 8 in a quartz monzonite pluton, and Meiduk, in 2 0.63–0.69 NEΔρ ≤ 0.5% ≤+4% 15 8 Δρ ≤ ≤ a quartz diorite pluton (Hassanzadeh, 1993). 3N 0.78–0.86 NE 0.5% +4% 15 8 3B 0.78–0.86 NEΔρ ≤ 0.5% ≤+4% 15 8 Crosscutting relationships between plutons and SWIR 4 1.600–1.700 NEΔρ ≤ 0.5% ≤+4% 30 8 strata and age dating of volcanic rocks indicate 5 2.145–2.185 NEΔρ ≤ 1.3% ≤+4% 30 8 that the magmatic arc has been active from the 6 2.185–2.225 NEΔρ ≤ 1.3% ≤+4% 30 8 Late Jurassic to present (Nabavi, 1972). 7 2.235–2.285 NEΔρ ≤ 1.3% ≤+4% 30 8 The Zagros-Makran transform zone contains 8 2.295–2.365 NEΔρ ≤ 1.0% ≤+4% 30 8 Δρ ≤ ≤ highly faulted Eocene and Oligocene rhyolites, 9 2.360–2.430 NE 1.3% +% 30 8 TIR 10 8.125–8.475 NEΔΤ ≤ 0.3% ≤+4% 90 12 dacites, and andesites, which were primarily 11 8.475–8.825 NEΔΤ ≤ 0.3% ≤3k (200–240) 90 12 erupted as tuffs (Huber, 1969b; Grabeljsek et 12 8.925–9.275 NEΔΤ ≤ 0.3% ≤2k (240–270) 90 12 al., 1972). Intrusive rocks of the Zagros-Makran 13 10.25–10.95 NEΔΤ ≤ 0.3% ≤1k (270–340) 90 12 transform zone consist primarily of Eocene dio- 14 10.95–11.65 NEΔΤ ≤ 0.3% ≤2k (340–370) 90 12 rite, granodiorite, and granite. Copper deposits Stereo base-to-height ratio 0.6 (along-track) in the Zagros-Makran transform zone are typi- Swath width (km) 60 Total coverage in cross-track direction by pointing (km) 232 (VNIR = ±24°, SWIR AND TIR 8.55° cally associated with veins in faulted andesites, Direction by pointing) granodiorites, and diorites or are associated with Mission life (yr) 5 dikes that consist of granite-porphyry, dacite, MTF at Nyquist frequency§ 0.25 (cross-track), 0.20 (along-track) and albite-trachyte microdiorite (Grabeljsek et Band-to-band registration‡ Intra-telescope: 0.2 pixels Inter-telescope: 0.3 pixels of coarser band al., 1972; Valeh, 1972). Peak data rate (Mpbs) 89.2 Volcanic rocks southeast of the Zagros-Makran Mass (kg) 406 transform zone in the Bazman volcanic area con- Peak power (W) 726 sist primarily of Quaternary andesites and basalts Note: Performance parameters for the Advanced Spaceborne Thermal Emission and Reﬂ ection Radiometer (Huber, 1969b). Less-extensive Paleogene vol- (ASTER) (Fujisada, 1995). Noise equivalent delta reﬂ ectance (NEΔρ) refers to absolute radiometric accuracy canic rocks are dispersed around the periphery in the visible near-infrared (VNIR) and 1.6–2.43 µm wavelength short-wave infrared region (SWIR) bands, and noise equivalent delta temperature (NEΔΤ) refers to absolute temperature accuracy in the 8.125–11.65 µm of the Quaternary intermediate volcanic rocks. wavelength thermal-infrared region (TIR) bands. Some of the Paleogene volcanic rocks are highly *Absolute radiometric accuracy—conformity of radiance measurement to value at standard target. faulted and contain copper deposits (Huber, †Signal quantization level—digitization level of measured radiance. §Modulation transfer function (MTF) of Nyquist frequency—function that measures the spatial frequency 1969b; Taghizadeh and Mallakpour, 1976). modulation response at one-half the sampling rate. ‡Band-to-band registration—accuracy with which pixels from corresponding x and y positions at different Previous Satellite Remote-Sensing Studies wavelengths correspond.

Remote-sensing data analysis has been applied to mineral resource studies since multispectral imagery became available soon after ing bands centered at 1.65 and 2.20 µm, and the materials, particularly hydrothermally altered World War II, but the digital multispectral capa- spatial resolution was improved to 30 m. Selec- rocks, but important ambiguities were recog- bility to investigate large inaccessible areas tion of the 2.20 µm band (TM band 7) was based nized because of the breadth of TM band 7. For became feasible when the fi rst Earth Resources on laboratory and fi eld spectral refl ectance example, carbonate rocks that contain calcite Technology Satellite (ERTS-1) was launched on measurements that showed that many clay, car- and dolomite (2.33 and 2.32 µm absorption fea- 3 July 1972, which was later renamed Landsat. bonate, sulfate, and hydrous minerals exhibit tures, respectively) and hydrothermally altered Numerous investigators have evaluated Land- spectral absorption features in this wavelength rocks that contain minerals such as alunite and sat Multispectral Scanner (MSS) data in order region due to molecular vibrational processes kaolinite (2.17 and 2.2 µm absorption features) to delineate geomorphological expressions of (Fig. 2; Hunt and Salisbury, 1970). An airborne are commonly diffi cult to distinguish spectrally intrusive bodies and regional structural features experiment conducted in the Cuprite, Nevada, in TM images, because both have absorption with which porphyry copper deposits might be area demonstrated that these absorption features features located in TM band 7 (Fig. 4A). associated (Raines, 1978; Rowan and Wetlaufer, could be used for mapping extensively well- The spectral bands of the ASTER SWIR sub- 1981; Abrams and Brown, 1984; Abrams et al., exposed hydrothermally altered rocks, which system were designed to measure refl ected solar 1983). Anomalously limonitic rocks, which can are bleached, opalized rocks containing several radiation in one band centered at 1.65 µm, and be a potential indicator of hydrothermal altera- spectrally distinctive argillic and advanced argil- fi ve bands in the 2.10–2.45 µm region in order tion, can be mapped at 79 m spatial resolution in lic alteration minerals (Abrams et al., 1977). to distinguish Al-OH, Fe,Mg-OH, H-O-H, and

the MSS bands, but no other mineralogical infor- Simple ratio images portrayed the altered rocks CO3 absorption features (Fig. 4B; Table 1). Sev- mation can be extracted spectrally from MSS based mainly on the intense absorption features eral investigators have documented identiﬁ ca- images (Fig. 4A; Rowan et al., 1974; Schmidt, in the 2.2 µm region related to these alteration tion of speciﬁ c minerals, such as calcite, dolo- 1976; Krohn et al., 1978; Raines, 1978). minerals. This spectral band (TM band 7) and mite, and muscovite, as well as mineral groups, The Landsat Thematic Mapper (TM) extended the band located at 1.65 µm (TM band 5) greatly through analysis of ASTER data (Rowan and the spectral range into the SWIR region by add- enhanced the capability to discriminate surface Mars, 2003; Rowan et al., 2003).

Geosphere, May 2006 163

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45°E 60°E

IRAN 30°N 35°N Figure 3. Location map of the Tertiary volcanic Outline Of and igneous intrusive rocks of the Zagros mag- ASTER Coverage matic arc and outline of Advanced Spaceborne Thermal Emission and Reﬂ ectance Radiometer Zagros-Makran (ASTER) scenes used to map hydrothermally Transform Zone altered rocks. The Sabzevaran and Gowk strike- slip fault systems and the Makran transfer zone are deﬁ ned as the Zagros-Makran transform zone

30°N 35°N (yellow dashed lines), which divides the active southeastern part of the magmatic arc from the dormant northwestern part of the arc (Regard et al., 2004; Walker and Jackson, 2002). Scale 200 km

45°E 60°E Tertiary Volcanic And Intrusive Rocks Of The Zagros Magmatic Arc (Huber, 1969a & b) Zagros-Makran Transfer Zone (Regard et al., 2004) Sabzevaran and Gowk Strike-Slip Fault Systems (Walker and Jackson, 2002) Boundary of Zagros-Makran Transform Zone

Plate 1. Orthorectifi ed Landsat Thematic Mapper (TM) band 7 of Plate 2. Orthorectifi ed Landsat Thematic Mapper (TM) band 7 the Zagros magmatic arc, Iran, with phyllic and argillic alteration of the Zagros magmatic arc, Iran, with alteration, mines, occur- units compiled from Advanced Spaceborne Thermal Emission and rences, fi gure locations, and Advanced Spaceborne Thermal Emis- Refl ectance Radiometer (ASTER) data. Numbers indicate poten- sion and Refl ectance Radiometer (ASTER) and geologic map cov- tial porphyry copper deposits determined from alteration spec- erage. If you are viewing the PDF, or if you are reading this offl ine, tral units (*mine at location). If you are viewing the PDF, or if please visit http://dx.doi.org/10.1130/GES00044.PL2 or the full- you are reading this offl ine, please visit http://dx.doi.org/10.1130/ text article on www.gsajournals.org to view the full-size plate. GES00044.PL1 or the full-text article on www.gsajournals.org to view the full-size plate.

164 Geosphere, May 2006

A B Laboratory Spectra Resampled To ASTER Bandpasses Limonite 167892 3 (ASTER Bands) 4 5 LAB Limonite AST LS

MSS Muscovite LAB Calcite

AST Kaolinite

MSS LS LAB Alunite Kaolinite

AST

Epidote MSS LS LAB Reflectance (Offset for Clarity) Reflectance (Offset for Reflectance (Offset For Clarity) Reflectance (Offset For Calcite Alunite AST

Chlorite

MSS LS 10% Reflectance 20% Reflectance

0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 μ Wavelength (μm) Wavelength ( m) Figure 4. (A) Laboratory spectra of limonite, calcite, kaolinite, and alunite resampled to Landsat Multispectral Scanner (MSS), Thematic Map- per (TM), and Advanced Spaceborne Thermal Emission and Reﬂ ectance Radiometer (ASTER) bandpasses. (B) Laboratory spectra of limonite, muscovite, kaolinite, alunite, epidote, calcite, and chlorite resampled to ASTER bandpasses. Spectra include limonite with a broad 0.66–1.165 µm absorption feature; muscovite, typical in phyllic alteration, with a 2.20 µm absorption feature; kaolinite and alunite, which are common in argillic alteration, have 2.165 and 2.20 µm absorption features; and epidote, calcite, and chlorite, which are typically associated with propylitic alteration and display 2.32, 2.33, and 2.32 µm absorption features, respectively. Epidote and chlorite have a broad Fe2+ absorption feature that affects ASTER bands 2, 3, and 4 (0.66–1.65 µm). The numbers across the top of the graph indicate the ASTER band center positions (Clark et al., 1993b).

Remote-Sensing Characteristics of Porphyry- to country rock compositional differences, but Phyllic alteration spectral characteristics Copper Deposits and ASTER Data epidote, chlorite, and carbonate minerals are include illite/muscovite refl ectance spectra that common constituents (Fig. 1). Titley (1972) exhibit an intense Al-OH absorption feature, In the idealized porphyry copper deposit noted that both country rock and intrusive rock which is typically centered at 2.20 µm (ASTER model, a core of quartz and potassium-bearing can host copper mineralization, and both can band 6), and a less intense feature near 2.38 minerals, mostly K-feldspar and biotite, is sur- be hydrothermally altered. Additional factors µm (ASTER band 8) (Fig. 4B). Substitution of rounded by multiple zones of alteration minerals that affect the portrayal of these mineral assem- Fe2+ for Al causes the minimum to shift to lon- (Fig. 1; Lowell and Guilbert, 1970). The hydrous blages in images are vegetation cover, topogra- ger wavelengths, which can be detected using zones are characterized by mineral assemblages, phy, amount of exposure, structural confi gura- ASTER data (Rowan and Mars, 2003). Intense which contain at least one mineral that exhib- tion, and anthropogenic activities. This analysis Fe3+ absorption suppresses refl ectance in the its diagnostic spectral absorption features. The of ASTER data concentrates on the phyllic and VNIR wavelength region in minerals such as broad phyllic zone, which is commonly limo- argillic alteration zones because of the variabil- limonite (Fig. 4B). nitic due to oxidation of pyrite, is characterized ity of the propylitic mineral assemblage and the Argillized rocks, including rocks classi- by illite/muscovite (sericite), and the narrower generally limited exposure of the core alteration fi ed as advanced argillic, also display an Al- argillic zone can be indexed by kaolinite and zone. The general spectral refl ectance of the pro- OH absorption feature near 2.20 µm, but both alunite (Fig. 1) (Abrams and Brown, 1984; Spatz pylitic assemblage is described here, however, kaolinite and alunite exhibit signifi cantly differ- and Wilson, 1995). The mineral assemblage of because of the spectral contrast of propylitic ent spectral shapes compared to muscovite/illite the outer propylitic zone is more variable due with phyllic and argillic assemblages. (Fig. 4B) (Hunt, 1977; Hunt and Ashley, 1979;

Geosphere, May 2006 165

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0 100 km

28° 28° 30° 30° 32°N 32°N Figure 5. Index map of Advanced Spaceborne Thermal Emission and Reﬂ Advanced Spaceborne 5. Index map of Figure AST_07 reﬂ 07, which indicate that the type of scene is an label are at the end of scene label to distinguish between scenes taken on same d Some labels have a letter month, and day. year,

166 Geosphere, May 2006

Rowan et al., 2003). Kaolinite displays a sec- to 600 m in high-relief terrain. Therefore, each samples, and from calibrated hyperspectral data ondary feature or shoulder at 2.17 µm, whereas 9-band image was geometrically registered to indicate anomalously low refl ectance for ASTER alunite exhibits a minimum at 2.17 µm (ASTER an orthorectifi ed Landsat TM mosaic (NASA, band 5 (Fig. 6). If band 5 is too low, the spectra band 5), instead of 2.20 µm, and a minor fea- 2003). The Landsat mosaic data have a spatial are more similar in shape to argillic minerals, such ture at 2.20 µm (Fig. 4B); both minerals also resolution of 28 m and spatial accuracy of ±50 m. as alunite, and the mapping algorithms will map exhibit secondary absorption features near 2.38 Although this registration procedure corrected too much argillic-altered rock and not enough µm. These spectral differences are detectable in for the off-nadir viewing offset, the images were phyllic-altered rock (Figs. 6 and 7). Hyperion ASTER data because the SWIR spectral band- not corrected for terrain displacement. Using a data were resampled to ASTER bandpasses and passes of the instrument were tailored for this second-order polynomial warp registration algo- used to correct ASTER band 5. Hyperion, a purpose (Rowan et al., 2003) (Table 1). rithm and at least nine ground control points for hyperspectral instrument fl own on board the EO1 Propylitic mineral-assemblage refl ectance each scene produced root mean square (RMS) satellite platform, has 196 spectral bands in the spectra are characterized by Fe,Mg-OH absorp- errors of <2.0 (60 m). The mosaic of Landsat 4 0.45–2.4 µm region (Kruse et al., 2003). Hyper-

tion features and CO3 features caused by molec- and 5 data was also used as a base map to illus- ion data can be properly calibrated to refl ectance ular vibrations in epidote and chlorite and in trate regional ASTER alteration units for the using the additional Hyperion atmospheric bands carbonate minerals, respectively (Spatz and Iranian study area (Plate 1). Argillic and phyllic not available in ASTER data. Wilson, 1995) (Fig. 4B). These absorption fea- alteration units were converted to vector data, Hyperion radiance data were calibrated to tures are situated in the 2.35 µm region (ASTER exported from the ASTER scenes, and illus- refl ectance data using ACORN atmospheric band 8). In addition, Fe2+ absorption features are trated on band 7 of the Landsat mosaic (Plate 1). removal software. The Hyperion refl ectance displayed by epidote and chlorite in the VNIR The Landsat mosaic and ASTER alteration units data were resampled to ASTER VNIR-SWIR wavelength region (Fig. 4B). These spectral were spatially resampled to 60 m resolution to bandpasses and georegistered to the orthorec- refl ectance features contrast strongly with those conserve computer fi le space. tifi ed Landsat TM imagery (NASA, 2003). of the phyllic and argillic assemblages. Average spectra were extracted from areas of The spectral refl ectance characteristics of Correction of ASTER Data using Hyperion overlap for the ASTER and Hyperion scenes. phyllic, argillic, and propylitic rocks provide a Hyperspectral Data A scalar correction, consisting of the ASTER basis for distinguishing hydrothermally altered resampled Hyperion band ratio 5/6 divided by rocks associated with porphyry copper deposits Comparisons of AST_07 ASTER refl ectance the ASTER band ratio 5/6, was applied to all from most country rock lithologies, but ambigu- data to spectra taken in situ, from laboratory ASTER band 5 data. ities may result where the country rock contains the same minerals that are typical of the altered rocks. For example, limonitic quartz-muscovite Playa Reflectance Spectra from Cuprite, Nevada schist could be confused with phyllic-altered 167892 3 (ASTER Bands) 4 5 rocks, and kaolinitic weathering surfaces might resemble argillized rocks spectrally. Thus, con- In Situ Field Spectrum sideration of the country rock compositional Resampled To ASTER range and spectral refl ectance is an important Bandpasses aspect of the analysis.

METHODS

Processing of ASTER Data AVIRIS Spectrum Resampled To ASTER Bandpasses The ASTER spectral refl ectance data ana- lyzed for mapping hydrothermally altered rocks were produced at the EROS Data Center, Sioux Falls, South Dakota, from level 1b data. The Reflectance (Offset For Clarity) Reflectance (Offset EROS Data Center AST_07 product consists of AST_07 EDC Product the 9 ASTER VNIR-SWIR channels calibrated to refl ectance using atmospherically corrected radiance at the surface (Thome et al., 1999). The 10% Reflectance 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- 0.5 1.0 1.5 2.0 2.5 resolution SWIR data were expanded by a factor Wavelength (μm) of two to correspond to the VNIR spatial dimen- sions, and then the 6 SWIR and 3 VNIR bands Figure 6. Spectra of playa from Cuprite, Nevada. Airborne Visible Infrared Imaging Spec- were combined to form 9-band data sets. trometer (AVIRIS) and in situ fi eld spectra illustrate a slight 2.20 µm absorption feature. The 62 individual ASTER scenes used in The AST_07 spectrum of the same playa illustrates that band 5 (red arrows) is 10–15% this study were recorded at four different view- lower than the AVIRIS or in situ fi eld spectra in relation to band 6. The AST_07 spectrum ing angles ranging from nadir to 8.2 degrees erroneously has a similar shape to alunite spectra illustrated in Figure 10. The numbers off nadir, which causes misregistration of up across the top of the graph indicate the ASTER band center positions.

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A total of eight Hyperion scenes were used to correct the 62 ASTER scenes that covered Library Spectra Resampled the study area. Difference ratios for the eight to ASTER Bandpasses Hyperion scenes ranged from 5.8% to 10.3%. An average of the scalar correction value (7.9%) 167892 3 (ASTER Bands) 4 5 was used to correct the ASTER data for the study area. Muscovite Figure 7. Laboratory spectra of muscovite, kaolinite, ASTER Band Ratios and Relative Band and alunite resampled to Depth Ratios Advanced Spaceborne Ther- mal Emission and Refl ec- Ratio images, which display the spectral con- tance Radiometer (ASTER) trast of specifi c absorption features, have been bandpasses. The spectra used extensively in geologic remote sensing illustrate the positions and (Rowan et al., 1974, 1977). Relative absorption intensities of absorption band depth (RBD; Crowley et al., 1989) images Kaolinite features in the 2.0–2.5 µm are an especially useful three-point ratio for- region used to defi ne ratios mulation for displaying Al-O-H, Fe,Mg-O-H, in the argillic and phyllic and CO3 absorption intensities (Figs. 4B and mapping algorithms. The 8). For each absorption feature, the numerator muscovite spectrum dis- is the sum of the bands representing the shoul- Alunite plays a 2.20 µm absorption ders, and the denominator is the band located feature, whereas kaolinite nearest the absorption feature minimum (Fig. 8; Reflectance (Offset For Clarity) Reflectance (Offset For and alunite exhibit 2.17 and Crowley et al., 1989). RBD5 {[(ASTER band 4 2.20 µm absorption features. + ASTER band 6)/ ASTER band 5] and RBD6 The numbers across the top [(ASTER band 4 + ASTER band 7)/ ASTER of the graph indicate the band 6]} images have been used in previous ASTER band center posi- studies to delineate argillic and phyllic mineral tions (Clark et al., 1993b). assemblages using ASTER SWIR data (Rowan 20% Reflectance and Mars, 2003). The presence of vegetation in ASTER pixels impacts the usefulness of these RBD5 and 0.5 1.0 1.5 2.0 2.5 RBD6 images for mapping argillic and phyllic μ Wavelength ( m) 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, True Absorption Relative Absorption a digital mask was produced from the band 3/ band 2 ratio and applied to these RBD images Band Depth Band Depth to delete pixels containing green, photosyntheti- cally active chlorophyll (Rowan et al., 2005). This mask, however, did not eliminate dry veg- A C etation lacking chlorophyll absorption. (A + C) A RBD = Intense Fe3+ absorption is displayed in band B 2/band 1 ratio images as high digital number (DN) values, because intense absorption in the B ultraviolet and in the band 1 region causes rela- tively low band 1 refl ectance (Rowan and Mars, Radiance

Reflectance C 2003). Inclusion of vegetation causes lower B ratio values, as chlorophyll absorption decreases the band 2 reﬂ ectance. Dry vegetation has low chlorophyll absorption, which has less effect on the band 2/band 1 ratio. λ λ Logical Operators Figure 8. Relative band depth (RBD) ratio schematic (modiﬁ ed from Crowley et al., 1989). For each pixel the logical operator algorithm performs a series of band ratios. Each logical

168 Geosphere, May 2006

operator determines a true or false value for Laboratory Spectra, each ratio by comparing the band ratio to a Dry Sagebrush Leaves (Brown) predetermined range of threshold values. All of the ratios in the algorithm have to be true in 167892 3 (ASTER Bands) 4 5 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 spec- troscopically assessed to determine the range of ratios and band DN values for constrain- ing the logical operator algorithms. Due to the amount of noise in the ASTER data (Table 1), all ASTER alteration units were median ﬁ ltered using a 3 × 3 matrix. Logical operators were used in conjunction Reflectance (Offset For Clarity) Reflectance (Offset For with band ratios in order to streamline regional 10% Reflectance 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 0.5 1.0 1.5 2.0 2.5 ASTER scenes, it was not practical to manu- Wavelength (μm) ally threshold each band ratio image (Fig. 3). In addition, multiple ratios and band thresholds Figure 9. Laboratory and Advanced Spaceborne Thermal Emission and Reﬂ ectance Radi- can be applied to a scene using one algorithm, ometer (ASTER) spectra of dry sagebrush. The arrows indicate locations of cellulose absorp- thus, eliminating the separate production and tion features. Prominent 2.165 (ASTER band 5) and 2.33 (ASTER band 8) µm absorption application of vegetation and dark pixel masks features are documented in the ASTER spectrum. The numbers across the top of the graph (Fig. 10). indicate the ASTER band center positions.

Interactive Data Language (IDL) LOGICAL OPERATORS (A) Logical operator to map argillic alteration. Ratio to map 2.165 μm feature Mask vegetation Mask dark pixels associated with argillic alteration Figure 10. (A) The logical operator algorithm that maps argil- ((float(b3)/b2) le 1.35) and (b4 gt 260) and ((float(b4)/b5) gt 1.25) lic-altered rocks using band ratios 4/5, 5/6, and 7/6, which and ((float(b5)/b6) le 1.05) and ((float(b7)/b6) ge 1.03) defi ne the 2.17 µm absorption feature. (B) The logical opera- Ratio to delineate argillic from phyllic alteration Ratio to map the 2.200 μm feature tor algorithm that maps phyl- associated with argillic alteration lic-altered rocks using band (B) Logical operator to map phyllic alteration. ratios 4/6, 5/6, and 7/6, which Ratio to map 2.200 μm feature defi ne the 2.20 µm absorption Mask vegetation Mask dark pixels associated with phyllic alteration feature. Pixels with green vegetation and low refl ectance (dark pixels) are masked in the argil- ((float(b3)/b2) le 1.35) and (b4 gt 260) and ((float(b4)/b6) gt 1.25) lic and phyllic logical operator and ((float(b5)/b6) gt 1.05) and ((float(b7)/b6) ge 1.03) algorithms using a band ratio of 3/2 and band 4 threshold, respectively. Ratio to delineate argillic from phyllic alteration Ratio to map the 2.200 μm feature associated with phyllic alteration Explanation of ENVI operators: float—convert to floating point le—less than or equal to gt—greater than ge—greater than or equal to

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Argillic Band Ratio Logical Operator Algorithm A ASTER Image Spectrum The fi rst part of the argillic band ratio logical 167892 3 (ASTER Bands) 4 5 operator (ABRLO) algorithm performs a band 3000 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 2500 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 2000 absorption features (Fig. 9). The ABRLO algorithm performs a threshold Digital Number (Reflectance) of band 4 to mask out pixels with low refl ectance. 1500 Pixels with low refl ectance contain abnormally high band 5 and band 9 values, which may be due to energy leakage from the band 4 detec- 1.0 1.5 2.0 tor into adjacent band 5 and band 9 detectors, Wavelength (μm) which is referred to as “crosstalk” (Iwasaki et B ASTER Image Spectrum al., 2002; Rowan and Mars, 2003). Pixels with 167892 3 (ASTER Bands) 4 5 low refl ectance that are affected by “crosstalk” Shaded in this study typically include shadows, and Area 1600 mafi c and ultramafi c rocks. Abnormally high band 5 and 9 values due to “crosstalk” produce 1500 anomalous band 6 and 8 absorption features 1400 (Fig. 11). Spectral analyses of ASTER band 4 pixels with DN values less than 260 were deter- 1300 mined to have inaccurate spectral signatures due N 1200 to “crosstalk” and were thus excluded using a Digital Number (Reflectance) digital mask. 1100 Spectral analysis of ASTER image spectra and resampled ASTER laboratory spectra 0 100 km 1000 showed that band ratios 4/5, 5/6, and 6/7 were 1.0 1.5 2.0 needed to map the 2.17 and 2.2 absorption fea- μ ASTER False-Color Composite Wavelength ( m) tures, thereby delineating argillic- and phyllic- R=3, G=2, B=1 of a Granite altered rocks (Figs. 7 and 10A). Band ratios 4/5 Outcrop C ASTER Image Spectrum 167892 3 (ASTER Bands) 4 5 and 5/6 map the 2.165 and 2.2 absorption fea- 3000 tures, 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 argil- 2500 lic-altered rock; thus, the 5/6 band ratio delin- eates argillic from phyllic rocks by classing ratio values of 1.05 and less as argillic altera- 2000 tion (Figs. 7 and 10A). ASTER spectra of argillic-altered rocks also illustrate that band 4 is at

Digital Number (Reflectance) least 25% greater than band 5, and band 7 is 3% 1500 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 1.0 1.5 2.0 0.03, respectively, to classify a pixel as argillic Wavelength (μm) alteration (Fig. 10A). Figure 11. An Advanced Spaceborne Thermal Emission and Refl ectance Radiometer Phyllic Band Ratio Logical Operator (ASTER) image of a granite outcrop and refl ectance spectra from three locations. Spectra Algorithm A and C were taken from sun-illuminated areas and illustrate a slight 2.20 µm absorption The phyllic band ratio logical operator feature typical of spectra typical for muscovite-bearing granite. Spectrum B, taken from (PBRLO) algorithm is almost identical to the an area that consists of granite, however, is shaded. This results in anomalously high band ABRLO algorithm (Fig. 10B). The PBRLO 5 and band 9 refl ectance values and produces incorrect and prominent 2.20 and 2.33 µm algorithm uses the same methods to mask green absorption features. vegetation and pixels with low refl ectance. ASTER spectra of phyllic-altered rocks show that band 5 is at least 5% greater than band 6,

170 Geosphere, May 2006

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 sand- stones contain large amounts of detrital clays such as montmorillonite, illite, and kaolinite. In US 95 order to mask out detrital clays, an igneous rock N 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 01 km rock units. The vectors were then converted to a mask, which was applied to the ASTER alteration data, thus, conﬁ ning alteration mapping to 37°30'N areas underlain by igneous rocks. Silicified Rocks Validation of Alteration Mapping Algorithms at the Cuprite, Nevada, Calibration and Validation Test Site Opalized Rocks

117°14'35"W Cuprite, Nevada, was selected as a site to test Argillized Rocks Nevada the accuracy of the ABRLO and PBRLO algorithms (Fig. 10). Previous geologic studies at Cuprite, Nevada, have mapped extensive argil- Unaltered Rocks lic, opalized, and silicifi ed hydrothermal alteration zones (Fig. 12; Ashley and Abrams, 1980; Figure 12. Generalized map showing the distribution of silicifi ed (red map unit), opalized Swayze, 1997). Well-exposed, muscovite-rich (blue map unit), and argillized (yellow map unit) rocks at Cuprite, Nevada (modifi ed from Cambrian phyllitic siltstone bounds the western Ashley and Abrams, 1980); inset map shows location of area in southern Nevada. 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- method involved a visual comparison of mineral erals with Al-OH absorption features (Clark et rich mineralogy found in hydrothermally altered maps from previous remote-sensing studies to al., 1993a; Clark and Swayze, 1996). In the sec- phyllic rocks. Calibration and validation stud- argillic and phyllic maps produced from the ond validation method, the ASTER-simulated ies of hyperspectral and multispectral imaging ABRLO and PBRLO algorithms (Rowan et al., data were coregistered to the ASTER data set detectors at Cuprite, Nevada, include ASTER, 2003; Clark et al., 1993a; Clark and Swayze, in order to quantitatively assess alteration-map- the Airborne Visible Infrared Imaging Spec- 1996). The second method quantitatively com- ping accuracy. trometer (AVIRIS), and Hyperion (Rowan et al., pared ASTER-derived argillic and phyllic maps The fi rst validation method results show that 2003; Clark et al., 1993a; Kruse et al., 2003). to similar maps compiled from an ASTER- argillic and phyllic patterns are very similar Two methods were used to test the accu- simulated AVIRIS data set that was resampled to maps from previous studies; however, the racy of the ABRLO and PBRLO algorithms to ASTER bandpasses (Fig. 14). AVIRIS is an ABRLO and PBRLO algorithms tend to map at Cuprite, Nevada. In both methods AST_07 airborne hyperspectral sensor with 224 spectral up to ~30% more altered rock (Rowan et al., ASTER refl ectance data from Cuprite, Nevada, bands in the 0.45–2.4 µm region (Green et al., 2003; Clark et al., 1993a; Clark and Swayze, were used, and a Hyperion scene of the same 1998). Previous studies at Cuprite, Nevada, have 1996). The overestimation of altered rocks area was used to adjust ASTER band 5. The fi rst illustrated that AVIRIS can accurately map min- refl ects the inclusion of mineral mixtures in the

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ABRLO and PBRLO algorithms, whereas mapping algorithms in previous studies focused on distinguishing individual minerals. The overestimation of altered rocks also refl ects 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 N data show similar patterns of argillic and phyllic U S rocks produced from the ABRLO and PBRLO 95 algorithms (Fig. 14). Ninety-fi ve percent of the argillic rocks mapped in the ASTER data set correlate to the simulated-ASTER data set. How- 01 km ever, the ABRLO algorithm mapped additional areas of argillic-altered rocks in the ASTER 37°30'N data, totaling 47% more than the argillic-altered rocks mapped in the ASTER-simulated data. The Qal average argillic spectrum from the ASTER data Quaternary has a shape similar to kaolinite and is classifi ed Qp as argillic (Figs. 4B, 10, and 15). Comparison of the simulated-ASTER and average ASTER 117°14'35"W Unconformity Nevada argillic spectra indicates that relative to band 6, band 5 in the ASTER data is ~3% higher than Tb2 band 5 in the simulated-ASTER data (Fig. 15). Contact The 3% variation falls within the noise param- Tsf eters of the ASTER SWIR detector (Table 1). Tertiary Seventy-two percent of the phyllic rocks Tb1 Fault mapped in the ASTER data set correlate to the simulated-ASTER data set. An additional 56% Tf Ts Down- of the phyllic pixels in the ASTER data set are Thrown not mapped as phyllic rocks in the ASTER sim- Unconformity Block ulated data. The average phyllic spectrum from the ASTER data has a shape similar to musco- Ce vite and is classifi ed as phyllic (Figs. 4B, 10, and 15). Comparison of the simulated-ASTER Cms Cambrian and average ASTER phyllic spectra indicates that relative to band 6, band 5 in the ASTER Ch 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 Figure 13. Generalized geologic map of the Cuprite mining district, Nevada. Qal—sand, SWIR detector (Table 1). As mentioned in the gravel, and boulders; Qp—playa deposits; Tb2—olivine basalt; Tsf—sodic ash-fl ow tuff; fi rst validation method, variations between the Tb1—porphyritic olivine basalt; Ts—crystal-rich rhyolite and latite tuff, conglomerate, and simulated-ASTER and ASTER phyllic and sandstone; Tf—quartz latitic felsite; Ce—limestone and chert; Cms—limestone and limey argillic data sets are due to the lower signal-to- siltstone; Ch—phyllitic siltstone and minor sandy limestone (modifi ed from Ashley and noise in the ASTER system than the AVIRIS Abrams, 1980; Swayze, 1997); inset map shows location of area in southern Nevada. system and the larger pixel size of the ASTER

172 Geosphere, May 2006

ASTER Band 3 Images Of Cuprite, Nevada

117°14W 117°12 117°10 117°14W 117°12 117°10 112 37°34N 37°34N 73 37°30 37°32 73 37°30 37°32 37°32 37°32

US 95 US

117°14 117°12 117°14 117°12 ASTER Argillic Unit (AVIRIS) ASTER Simulated Argillic Unit

117°14W 117°12 117°10 117°14W 117°12 117°10 3 4 37°34N 37°34N 73 37°30 37°32 73 37°30 37°32 37°32 37°32

US US 95

117°14 117°12 117°14 117°12 ASTER Phyllic Unit N (AVIRIS) ASTER Simulated Phyllic Unit

1 km Figure 14. Maps of argillic and phyllic rocks at Cuprite, Nevada, using logical operator algorithms: (1) Advanced Spaceborne Thermal Emis- sion and Reﬂ ectance Radiometer (ASTER) argillic alteration, (2) ASTER-simulated (AVIRIS) argillic alteration, (3) ASTER phyllic rocks, and (4) ASTER-simulated (AVIRIS) phyllic rocks. Phyllic and argillic units are superimposed on ASTER and ASTER-simulated band 3 images.

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167892 3 (ASTER Bands) 4 5 the central part of the Zagros Magmatic Arc Avg. Spectrum Argillic, ASTER 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 Avg. Spectrum Phyllic, ASTER volcanoes suggest a high potential for copper- porphyry deposits based on similarities to cur- rent hydrothermal alteration models and previ- Avg. ous prospects and studies (Lowell and Guilbert, Spectrum 1970; Ranjbar et al., 2004; Tangestani and Argillic, AVIRIS Moore, 2002). In addition, concentrations of Resampled argillic- and phyllic-altered rocks derived from ASTER data exhibit semicircular patterns for two large copper producing mines in the north- Avg. central part of the study area (Sar Cheshmeh and Spectrum Phyllic, AVIRIS Meiduk), which are classifi ed as porphyry cop- Resampled per deposits (Figs. 17B and 18B; Plate 1; Has- sanzadeh, 1993; Ranjbar et al., 2004; Tangestani Reflectance (Offset For Clarity) Reflectance (Offset For and Moore, 2002). Alteration maps produced for 10% Reflectance 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- 0.5 1.0 1.5 2.0 2.5 altered rocks form elongate patterns in the cores Wavelength (μm) of plunging folds (Fig. 18; Plate 1). The cores of the folds contain more argillic- than phyl- Figure 15. Average spectra of argillic and phyllic spectral units for Advanced Spaceborne lic-altered rocks. Although the alteration is not Thermal Emission and Refl ectance Radiometer (ASTER) and ASTER-simulated (AVIRIS associated with exposed altered intrusive bod- resampled to ASTER bandpasses) data. ies, the shape and limited lateral extent of the alteration and confi ning structure suggest local- ized, intense, fracturing fl uid fl ow along fold axes and the potential for mineralization. data, at 30 m compared to 18 m pixels of the fracture and fault zone, and suggest that the type Eocene-Oligocene volcanic rocks consisting AVIRIS data (Yamaguchi et al., 2001; Green et of alteration is associated with polymetallic primarily of tuff extend throughout the Zagros- al., 1998; Clark and Swayze, 1996). and epithermal vein deposits (Cox and Singer, Makran transform zone in the south-central part 1986). of the Zagros magmatic arc (Fig. 19). Eocene HYDROTHERMAL ALTERATION MAP Geologic maps indicate extensive Eocene to granodiorite that intrudes Eocene volcanic OF ZAGROS MAGMATIC ARC Miocene diorite, quartz diorite, quartz monzo- rocks is common in the southwestern part of nite, and granodiorite porphyry intrusive rocks the transform zone (Fig. 19). All of the volcanic Distribution of Hydrothermally Altered throughout the central part of the Zagros mag- and intrusive rocks contain extensive northwest- Rocks in the High Zagros Mountains matic arc (Huber, 1969a). Most of the intrusive trending faults (Fig. 19; Regard et al., 2004). rocks in the central part of the Zagros magmatic Phyllic alteration dominates the area and tends Mapped alteration patterns in the Zagros arc are 1–2 km in diameter, and country rocks to form linear patterns associated with mapped magmatic arc are categorized as semicircular are primarily Eocene and Oligocene volca- faults and linear features seen in ASTER and and linear patterns (Plate 1). Most of the hydro- nic rocks that contain extensive volcanic tuffs TM imagery (Fig. 19; Plate 1). Only a small part thermally altered rocks consist of phyllic-altered (Figs. 17 and 18). Hydrothermal alteration of the alteration is associated with the Eocene rocks with minor amounts of argillic-altered units in the central part of the Zagros magmatic granodiorite and granite intrusive rocks. Domi- rocks (Plate 1). The geology in the northwestern arc typically form semicircular to circular pat- nant phyllic linear alteration patterns along faults part of the study area is dominated by Eocene terns, 1–5 km in diameter and are associated and fractures suggest the potential for polyme- volcanic rocks that contain northwest- and west- with hydrothermally altered Eocene to Mio- tallic vein– and epithermal vein–style mineral- trending faults (Fig. 16A). Hydrothermal altera- cene igneous intrusive bodies (Figs. 17 and 18; ization and are supported by mineral occurrence tion patterns in this part of the study area are lin- Plate 1). The semicircular patterns of hydro- data taken from previous geologic mapping in ear, consist almost exclusively of phyllic-altered thermally altered rocks are also centered on the Makran-Zagros transform zone, which has rocks, and tend to follow linear topographical partially eroded volcanoes when superimposed documented mineralization along veins and and structural features (Fig. 16B; Plate 1). The on ASTER and TM imagery and geologic maps dikes (Grabeljsek et al., 1972; Valeh, 1972). linear phyllic alteration patterns in the north- (Fig. 17; Plate 1). Although most of the mapped The southeastern part of the Zagros mag- western part of the study area are similar to pat- altered rocks classify as phyllic-altered, there matic arc adjacent to the Zagros-Makran trans- terns produced by alteration along an extensive are substantially more argillic-altered rocks in form zone consists primarily of Eocene grano-

174 Geosphere, May 2006

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Meiduk

0 10 km

Explanation Fault Eocene Granodiorite and Monzonite Eocene-Oligocene Tuff Volcano Crater Eocene-Oligocene Volcanic Rock Quaternary Undifferentiated Eocene-Oligocene Shale and Siltstone Quaternary Tuff and Agglomerate Paleogene Tuff and Agglomerate Pliocene Conglomerate Lower Eocene Tuff and Volcanic Rock Neogene Redbeds Paleogene Flysch Neogene Conglomerate Paleogene Conglomerate Neogene Volcanic Rock and Tuff Cretaceous Melange Neogene Diorite, Quartz Diorite, Cretaceous Limestone and Granodiorite Porphyry Cretaceous Flysch

176 Geosphere, May 2006

Meiduk

0 10 km

Explanation Argillic Alteration Phyllic Alteration Outline Of Igneous Rock Units Fault From Geologic Map

Figure 17 (on this and previous page). (A) Geologic map and (B) Landsat Thematic Mapper (TM) band 7 image with mapped argillic and phyllic alteration of the area around the Meiduk copper mine, Iran, in the central part of the study area (modiﬁ ed from Huber, 1969a). Location of ﬁ gure is shown on Plate 2.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/2/3/161/3335512/i1553-040X-2-3-161.pdf by guest on 27 September 2021 ASTER REGIONAL ALTERATION MAPPING METHOD, ZAGROS MAGMATIC ARC, IRAN 0 5 km e Zagros-Makran transform zone, in the e Zagros-Makran Argillic Alteration Alteration Explanation Argillic Phyllic Units Outline Of Igneous Rock Geologic Map From Fault B is shown on Plate 2. gure 0 5 km Paleogene Tuff Paleogene Eocene-Oligocene Tuff Eocene-Oligocene Rock Volcanic Cretaceous Limestone Redbeds Triassic ed from Huber, 1969b). Location of ﬁ Huber, ed from Explanation Fault Quaternary Undiff. Neogene Redbeds and Conglomerate Eocene Granodiorite and Granite A south-central part of the study area (modiﬁ south-central part of the study area Figure 19. (A) Geologic map and (B) Landsat Thematic Mapper (TM) band 7 image with mapped argillic and phyllic alteration of th Thematic Mapper 19. (A) Geologic map and (B) Landsat Figure

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/2/3/161/3335512/i1553-040X-2-3-161.pdf by guest on 27 September 2021 J.C. MARS and L.C. ROWAN 0 5km e area southeast of the Zagros-Makran southeast of the Zagros-Makran e area is shown on Plate 2. gure Argillic Alteration Explanation Alteration Argillic Phyllic Units Outline Of Igneous Rock Geologic Map From Fault ed from Huber, 1969b). Location of ﬁ Huber, ed from Eocene-Oligocene Tuff Eocene-Oligocene Rock Volcanic Rock Eocene Volcanic Eocene Granodiorite and Granite and Tuff Paleogene Rock Volcanic 0 5km Explanation Fault Quaternary Undifferentiated QuaternaryRock Volcanic Oligocene Limestone Neogene Redbeds and Conglomerate AB Figure 20. (A) Geologic map and (B) Landsat Thematic Mapper (TM) band 7 image with mapped argillic and phyllic alteration of th Thematic Mapper 20. (A) Geologic map and (B) Landsat Figure (modiﬁ transform zone, in the south-central part of study area

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diorite and granite that have intruded Eocene associated with altered rocks, which is a key and band thresholds to mask green vegetation and Oligocene volcanic rocks (Fig. 20). The factor in mineral assessments. Two sets of cop- and pixels affected by “crosstalk” and map northwest-trending faults are not as extensive as per mines and occurrences data were plotted on altered rocks. The logical operators use ratios of compared to the adjacent transform zone. In the an alteration map (Plate 2). One of the data sets, ASTER bands 4, 5, 6, and 7 to resolve absorp- area adjacent to the transform zone, up to 30% taken from a 1:2,500,000-scale mineral distribution features in the 2.17–2.20 µm region. Stan- of the Eocene granodiorites and granites contain tion map of Iran (Taghizadeh and Mallakpour, dard ratio values are determined from ASTER phyllic alteration with minor amounts of argillic 1976), illustrates good association of mines image data and spectral libraries. Because the alteration (Fig. 20; Plate 1). Some of the Eocene and occurrences with ASTER-mapped hydro- logical operators use standard ratio values for tuffs and other volcanic rocks also exhibit exten- thermal alteration (Plate 2). The location of the multiple scenes, Hyperion data are used to cor- sive phyllic alteration (Fig. 20; Plate 1). The large mines and occurrences, however, is not accurate rect the ASTER band 5 anomaly. percentage of ASTER-mapped phyllic altera- enough for statistical analysis of documented The calibration and logical operator mapping tion in the area adjacent to the transform zone mineralization and hydrothermal alteration. technique was tested at Cuprite, Nevada, using may be due to the presence of muscovite-rich The second data set of copper mines and mineral maps from previous studies, an AVIRIS granodiorites and granites. However, ASTER- occurrences was taken from a set of 1:100,000- data set resampled to ASTER bandpasses, and mapped phyllic alteration extends across geo- scale geologic maps, which cover the cen- the Hyperion-corrected ASTER AST_07 refl ec- logic units and includes some volcanic tuffs tral part of the study area (Dimitrijevic, 1971; tance data set. The argillic and phyllic spectral (Fig. 20). In addition, argillic-altered rocks can Dimitrijevic et al., 1971a, 1971b, 1971c, 1972a, units produced by the ABRLO and PBRLO only be associated with hydrothermal alteration, 1972b; Djokovic et al., 1972a, 1972b, 1972c, algorithms were similar in shape to phyllic and thus, the distribution of ASTER-mapped argillic 1973; Grabeljsek et al., 1972; Mijalkovic et al., argillic mineralogy in maps from previous stud- alteration in the area adjacent to the transform 1972; Srdic et al., 1972a, 1972b; Timotijevec et ies. Ninety-fi ve percent of the argillic-altered zone indicates that at least 50% of the ASTER- al., 1972; Valeh, 1972, 1973a, 1973b; Plate 2). rocks mapped in the simulated-ASTER data set mapped phyllic alteration is the result of hydro- In order to statistically analyze the relation- were mapped in the AST_07 refl ectance data thermal alteration (Fig. 20; Plate 1). ship between documented mineralization and set. Seventy-two percent of the phyllic-altered The southeastern part of the Zagros mag- hydrothermal alteration, 60 m and 1 km zones rocks mapped in the simulated-ASTER data set matic arc consists primarily of Quaternary around each location were assessed. There are were mapped in the AST_07 refl ectance data andesite and includes at least one active com- 10 copper mines and 50 copper occurrences set. Spectroscopic examination of erroneously posite volcano (Figs. 21A and 21B; Plate 1). (sites) located on the 1:100,000-scale geologic mapped argillic (47%) and phyllic (56%) areas Less extensive basalt fl ows, Paleogene inter- maps (Plate 1; Table 2). Forty-four of the mines indicated that the error was most likely due to mediate volcanic rocks and tuffs, and Eocene and occurrences have hydrothermal alteration noise and spatial resolution variations between granites and granodiorites make up the rest of within a 1 km radius of each site, and 15 of the AVIRIS and ASTER instruments. the southeastern part of the magmatic arc. Older the sites have alteration within a 60 m radius To test the new mapping method on a Paleogene rocks, such as Eocene granites and of each site. The average percentage of hydro- regional scale, an argillic and phyllic alteration granodiorites, tend to be extensively faulted. thermal alteration within the 1 km radius of map was compiled for the High Zagros Moun- Mapped alteration is mainly in the older faulted each site is 16%. More detailed percentages of tains, Iran. Argillic and phyllic spectral units Paleogene rocks, and in particular, the Eocene alteration within a 1 km radius of each mine mapped using ABRLO and PBRLO algorithms granites, granodiorites, and Paleogene tuffs or deposit site show 16 sites with no alteration indicated distinctive patterns of alteration that (Figs. 21A and 21B; Plate 1). Mapped altera- within a 1 km radius, 14 sites with <0.01–1% could be associated to faulting and geologic tion in the granites and granodiorites tends to be alteration within a 1 km radius, 10 sites with 1– units. In the northwestern part of the study phyllic and forms linear patterns associated with 10% alteration within a 1 km radius, 5 sites with area, primarily phyllic-altered rocks form lin- fractures and faulting, indicating potential for 10–20% alteration within a 1 km radius, 7 sites ear patterns and are associated with faults and polymetallic or epithermal vein mineralization with 30–40% alteration within a 1 km radius, other linear features. The central part of the (Fig. 21; Cox and Singer, 1986). Mapped altera- and 8 sites with 40–100% alteration within a study area contains numerous 1–5 km semicir- tion in the Paleogene tuffs consists of semicir- 1 km radius (Table 2; Fig. 22). Visual assess- cular patterns of mapped phyllic- and argillic- cular patterns of phyllic and argillic alteration ment of 15 m VNIR ASTER data indicates that altered rocks that are associated with Eocene to and may be associated with porphyry copper 12 of the sites with altered rocks within a 1 km Miocene intrusive igneous rocks. The Eocene deposits (Fig. 21; Lowell and Guilbert, 1970). radius show some type of disturbance, such as a to Miocene altered intrusive rocks host mined The only laterally extensive alteration associ- road, pit, or dump, within ~150 m of the site. Of porphyry copper deposits, such as Meiduk and ated with the Quaternary andesites is situated the 16 sites that did not map altered rocks within Sar Cheshmeh. The Zagros-Makran transform on the slopes of the composite volcano Bazman a 1 km radius, 11 were outside of the igneous zone contains primarily phyllic-altered rocks (Figs. 21A and 21B). Most of the alteration on rock mask, and the other 5 sites showed no dis- that form linear patterns and are associated the volcano maps as argillic. turbance from visual assessment using ASTER with extensive faulting. The southwest area 15 m false-color composite images. adjacent to the transform zone contains later- Comparison of Hydrothermal Alteration ally extensive phyllic-altered rocks that are Spectral Units and Mapped Mines and SUMMARY AND CONCLUSIONS associated with granodiorites and granites. The Occurrences active part of the Zagros magmatic arc contains Logical operators effectively map argillic- mostly andesitic and basaltic rocks that are not Regional argillic and phyllic alteration maps and phyllic-altered rocks, provided ASTER altered except for the summit of a recent com- and existing economic geology data for the data are correctly calibrated using Hyperion or posite volcano, which consists of mostly argil- study area provide important information to other data, such as in situ or laboratory spectra. lic-altered rocks. There are some Paleogene help determine the percentage of mineralization Logical operators apply multiple band ratios tuffs and intrusive rocks in the southeastern

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A 0 10 km

Bazman ο 28 N

59ο E 59ο 30’E 60ο E

Explanation

Fault Oligocene Conglomerate Volcano Crater Eocene Intermediate Volcanic Rock Quaternary Undifferentiated Eocene Granite and Granodiorite Quaternary Tuff and Agglomerate Paleogene Tuff and Volcanic Rock Quaternary Intermediate Paleogene Flysch Volcanic Rock Cretaceous Melange Pleistocene Tuff And Volcanic Ash Cretaceous Limestone Pliocene Conglomerate Jurassic Diorite and Granodiorite Miocene Redbeds Paleozoic Rock Lower Miocene Shale

182 Geosphere, May 2006

B 0 10 km

Bazman ο 28 N

59ο E 59ο 30’E 60ο E

Explanation Argillic Alteration Phyllic Alteration Outline Of Igneous Rock Units Fault From Geologic Map

Figure 21 (on this and previous page). (A) Geologic map and (B) Landsat Thematic Mapper (TM) band 7 image with mapped argillic and phyllic alteration of the southeastern part of the Zagros magmatic arc (modiﬁ ed from Huber, 1969b). Location of ﬁ gure is shown on Plate 2.

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part of the study area that contain argillic- and TABLE 2. KNOWN DEPOSITS AND PERCENTAGE OF ASSOCIATED SURFICIAL PHYLLIC- AND phyllic-altered rocks. ARGILLIC-ALTERED ROCKS Regional hydrothermal alteration maps pro- Deposit or mine Latitude Longitude Percent alteration in 1 Alteration in 60 m Site in igneous vide important data for regional mineral assess- (°N) (°E) km radius buffer zone radius of site? masked zone? ments. The argillic- and phyllic-altered rock (%) maps of the Zagros magmatic arc defi ne 61 Deposit 30.6242 54.7711 99.91 Yes Yes potential copper-porphyry deposits and areas Deposit 30.5432 54.9521 35.01 Yes Yes of potential epithermal and polymetallic vein Deposit 30.5958 54.9827 8.82 Yes Yes Deposit 30.5864 55.0622 0.66 No Yes deposits (Plate 1). Most of the potential por- Deposit 30.6137 55.1244 0.00 No No phyry copper deposits are in the High Zagros Deposit 30.4449 55.1378 47.48 Yes Yes Mountains, whereas most of the potential Mine 30.4242 55.1685 94.36 Yes Yes Deposit 30.3198 55.3227 58.88 No Yes polymetallic and epithermal vein deposits are Deposit 30.4714 55.5679 0.00 No No in the Makran transform zone (Plate 1). This Deposit 30.3299 55.6333 0.00 No Yes data could be used in mineral assessments to Mine 30.3170 55.6411 0.00 No No Deposit 30.3119 55.6523 0.00 No No more accurately defi ne mineral deposit tracts Deposit 30.2149 55.7027 12.67 No Yes and number of deposits. In addition, statistical Deposit 30.1532 55.8389 0.86 No Yes data on percentage of area of altered rocks and Deposit 29.9692 55.8043 0.00 No Yes Mine 29.9493 55.8729 93.07 Yes Yes known mineralized deposits could be used to Deposit 29.9456 55.8688 95.06 Yes Yes more accurately determine grade and tonnage Deposit 29.8976 55.9055 0.00 No Yes in quantitative mineral assessments (Plate 2; Deposit 29.8847 55.9094 35.78 Yes Yes Table 2). Deposit 30.0031 55.9918 13.37 Yes Yes Mine 29.9876 56.0449 5.31 No Yes Regional mapping of argillic- and phyllic- Mine 29.8708 56.0656 47.28 Yes Yes altered rocks provides a better understanding Deposit 29.8294 55.9929 0.08 No Yes of tectonic infl uence on hydrothermal altera- Deposit 29.8238 56.1201 0.00 No No Deposit 29.9875 56.1566 0.34 No Yes tion. On the basis of ASTER-mapped altera- Deposit 30.0187 56.1871 0.07 No Yes tion patterns, the central part of the study area Deposit 29.9371 56.2195 0.00 No Yes contains at least 50 localities with potential Deposit 29.6304 56.1055 0.53 No Yes Deposit 29.6471 56.1549 34.16 Yes Yes porphyry copper deposits (Figs. 17 and 18; Mine 29.5865 56.1852 32.26 No Yes Plate 1, 1–50 localities) and is associated with Deposit 29.6032 56.2123 31.59 No Yes the older “closed part” of the Zagros magmatic Mine 29.5661 56.2493 35.47 No Yes arc, where large inactive volcanic complexes Mine 29.5921 56.2933 7.67 No Yes Deposit 29.6293 56.3066 3.88 No Yes have been unroofed by erosion. Linear-shaped Deposit 29.7303 56.5755 0.00 No No ASTER-mapped alteration patterns in the Deposit 29.7320 56.5833 0.59 No Yes active Zagros-Makran transform zone suggest Deposit 29.7132 56.6111 0.00 No No Deposit 29.7165 56.6773 0.72 No Yes the potential for epithermal and polymetal- Deposit 29.6864 56.6600 0.00 No Yes lic vein deposits. The linear-shaped alteration Deposit 29.6593 56.8500 0.41 No Yes patterns are associated with extensive fractures Deposit 29.6643 56.8506 0.20 No Yes Deposit 30.0314 56.7712 0.00 No No and faults, which are common in transform Mine 29.3439 56.7724 0.00 No No zones (Plate 1; Fig. 19). To the southeast of Deposit 29.4224 56.9294 17.18 Yes Yes the Zagros-Makran transform zone along the Deposit 29.4166 57.1005 0.00 No Yes Deposit 29.3760 57.2101 18.68 No Yes active part of the magmatic arc, ~11 localities Deposit 29.2182 57.1990 1.90 No No with potential porphyry copper deposits can be Deposit 29.2126 57.2607 35.97 Yes Yes defi ned on the basis of ASTER-mapped altera- Deposit 29.1304 57.3439 0.00 No Yes tion patterns (Plate 1, 51–61 localities). The Mine 29.0555 57.3084 3.16 No Yes Deposit 29.0648 57.4867 0.03 No Yes area along the southeast margin of the Zagros- Deposit 29.2883 57.4757 11.91 No Yes Makran transform zone, which contains later- Deposit 29.3324 57.4333 0.77 No Yes ally extensive altered rocks, may be the result Deposit 29.4691 57.4417 6.21 Yes Yes Deposit 28.9760 57.6887 1.69 No Yes of deep erosion caused by prolonged thermal Deposit 28.9594 57.6743 1.09 No Yes heating and uplift along the convergent trans- Deposit 28.7800 57.8033 0.52 No Yes form (Fig. 20; Plate 1). Farther to the southeast, Deposit 28.7593 57.8600 1.05 No Yes Deposit 28.5643 58.0018 0.26 No No the intermediate Quaternary volcanic rocks of Deposit 29.6125 56.4888 69.66 Yes Yes the active part of the magmatic arc cover most 16.11 15 49 of the older Eocene rocks, which are more (average) (of 60 total) (of 60 total) likely to contain porphyry copper, epither- Note: List of copper mines and occurrences in the study area from 1:100,000-scale maps with mal, and polymetallic vein deposits (Fig. 21; information on proximity to altered rocks (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; Plate 1). Thus, on the basis of regional altera- Srdic et al., 1972a, 1972b; Timotijevec et al., 1972; Valeh, 1972, 1973a, 1973b). Data include latitude and tion patterns and structures in the Zagros mag- longitude, percent alteration within a 1 km radius of each site, the alteration within a 60 m radius of each matic arc, tectonic processes control the degree site, and if the site is within the igneous rock mask. of exhumation of altered rocks, and hydrothermal deposit types and distribution.

184 Geosphere, May 2006

Histogram of Percent Alteration Within 1 km of Copper Deposit or Mine

16 16

14 14

10 Figure 22. Histogram of per- 10 cent alteration within a 1 km radius of 60 mine and occur-

Frequency 8 7 rence sites in the central part of the study area. 6 5 4 4

2 2 11 0 00 0 0.0% 1.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0%

Percent Alteration Within 1 km of Deposit or Mine (non-linear axis)

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