Regional Mapping of Phyllic- and Argillic-Altered Rocks in the Zagros
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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. For permission to copy, contact [email protected] 161 © 2006 Geological Society of America 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 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 sub- duction zone (Regard et al., 2004). The north- west 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.