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

¹ U.S. Geological Survey, National Center, Mail Stop 954, Reston, Virginia 20192, USA

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.

Keywords: ASTER • remote sensing • porphyry copper • hydrothermal alteration • Iran

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