Integration of ASTER imagery and field data for chromite exploration in the Eastern Khoy Ophiolite Complex, NW Iran

Modern remote sensing imagery is the only technique with large spatial coverage to explore new chromite occurrences and associated platinum group elements (PGEs) in the ophiolitic complex on a regional scale. Consequently, innovative and cost-effective techniques are critical to identify potential areas of chromite mineralization in ophiolitic complexes. In this study, the identification of zones of podiform chromite mineralization is investigated using ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite data in the Eastern Khoy ophiolite complex in northwestern Iran. Different image processing techniques such as Band Ratio (BR), Principal Components Analysis (PCA), Spectral Angle Mapper (SAM), Minimum Noise Fraction (MNF), Optimum Index Factor (OIF) and Decorrelation Stretch (DS) were applied to the ASTER visible and near infrared (VNIR) and shortwave infrared (SWIR) spectral bands for lithological and alteration mapping in the study area. The band ratios of (4/1, 4/5, 4/7), (5/3, 5/1, 7/5) and (4/7, 4/1, 4/3) were developed to distinguish different lithological units such as ultramafic rocks, meta-gabbro, amphibolite, and metagranite in the ophiolitic complex. The principal components (PC3, PC4, PC1), (PC4, PC3, PC2), and (PC4, PC2, PC1) images in the red-green-blue (RGB) color composites were also adapted to accurately map and demonstrate the boundaries of the main lithological units. The SAM method was used to identify alteration minerals in the ultramafic units, in particular to map serpentinized harzburgite, which may indicate areas of high potential for chromite mineralization in the study area. Furthermore, the MNF and DS methods have illustrated the ability of the ASTER data to provide information to identify chromite host rocks, particularly serpentinized harzburgite, which is crucial for chromite exploration in the study area. Typically, extensive fieldwork and laboratory analysis confirmed the results of the ASTER image processing data. By incorporating data derived from the ASTER and field investigations in this study, the majority of lithological units in the Eastern Khoy ophiolite complex were successfully mapped and areas of high potential for chromite mineralization and exploration were delineated. The results show that the integration of ASTER image processing data and comprehensive field surveys could be a broadly pertinent and cost-effective approach for exploration of chromite and associated PGEs in the ophiolitic complexes worldwide.