New logical operator algorithms for mapping of hydrothermally altered rocks using ASTER data: A case study from central Turkey

Abstract

This study uses logical operator algorithms along with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) spectral reflectance and emissivity data to map hydrothermally altered igneous rocks associated with mineralization around the towns of Hamurcu and Başdere in central Anatolia, Turkey. In addition to band ratios used in previous studies, the logical operator algorithms developed in this work perform multiple mineral indices and directed principal components (DPC) analysis, thereby providing better constraints on the spatial extent of hydrothermally altered rocks around the Hamurcu and Başdere areas. We designed the unique mineral indices and DPC analysis based on the effective use of ASTER bands corresponding to diagnostic spectral absorption features of alteration minerals. The calculation of threshold values of logical operator algorithms is based on a two-stepped statistical approach. The novel logical operator algorithms successfully mapped hydrothermal alteration zones in the study area that include mineral assemblages, such as kaolinite, alunite, sericite, smectite, and ferric iron oxides together with various forms of silica. The Hamurcu area contains altered rocks that display a characteristic alteration pattern of a high-sulfidation epithermal system. On the other hand, alteration minerals are primarily located along the faults in the southern part of the Başdere area. The results of field investigations in conjunction with laboratory spectral reflectance measurements, microscopic examination of thin sections and X-ray diffraction analysis of the samples are quite consistent with ASTER mineral maps derived from the logical operator algorithms from this work. As a result, the new logical operator algorithms implemented to ASTER data of the study area prove to be a powerful tool for generating preliminary, quick, and reliable alteration mineral maps at low cost in similar terrains of different geographic locations.