Regional scale demarcation of alteration zone using ASTER imageries in South Purulia Shear Zone, East India: Implication for mineral exploration in vegetated regions

Regional scale demarcation of alterations is an important preliminary step in mineral exploration. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imageries here have been used for alteration mapping and iron-rich (e.g., Gossans) area demarcation in a highly vegetated and relatively unexplored part of Eastern India, namely, South Purulia Shear Zone (SPSZ). The area is surrounded by mineral-rich Singhbhum Shear Zone (SSZ), but the mineral potentiality of SPSZ has not yet been proven. Alteration mineral assemblages are comprise of iron-rich alteration (gossan), clay alteration with varying amounts of iron content, sericitization, ferruginization and chloritization. In this paper, we discuss a few advantages and limitations of commonly used satellite image processing techniques for mineral mapping. We integrate the outputs of band ratio, Principal Component Analysis (PCA), PCA-loading, and thresholding; termed as Directed PCA (DPCA) for delineating mineral potential areas. Our approach mapped the alteration zones with overall 74% accuracy. Our study leads to demarcation of 28% alteration area within a 10 km wide and 60 km long stretch of SPSZ. Two areas, namely, Lawa (degrees 23 1' 39.31 '' N, 86 degrees 4' 2.55 '' E) and Sindurpur (23 degrees 4' 38.30 '' N, 86 degrees 5' 3.99 '' E) were identified as mineral potential zone. Samples from those areas were studied using XRF, microscope and SEM with the conclusive presence of mineralization. Lawa is potential for hydrothermal polymetallic sulfides along with gold mineralization. Sindurpur is potential for secondary iron and manganese mineralization. The study shows that DPCA is a useful tool for ASTER data processing for narrowing target area in preliminary stages of exploration in vegetated zones.