Implementation of reflection spectroscopy based new ASTER indices and principal components to delineate chromitite and associated ultramafic–mafic complex in parts of Dharwar Craton, India

Abstract

We analyzed reflectance spectra of chromitite samples and its associated rocks using quantitative mineralogical data to identify the diagnostic absorption features of these rocks. Further, we derived few new indices to delineate chromitite and associated ultramafites in satellite image acquired by Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) sensor. The spectral indices were derived based on the ASTER sensor convolved laboratory derived reflectance spectra of these rocks. Analysis of ASTER convolved laboratory spectra of these rocks revealed that chromitite had higher reflectance in band 2 of ASTER data in comparison to band 1 and band 3. Mineralogical analysis of chromitite spectra was used as reference to understand the mineralogical importance of this signature. On the other hand, reflectance of metaultramafite was relatively higher in band 3 in comparison to band 1 of ASTER sensor. Based on above spectral contrast, two new indices termed as ASTER based Chromitite Index ((band 2*band 2)/(band 1+band 3)) for chromitite and ASTER based Ultramafic Index ((band 1/band 3)) for meta-ultramafite were proposed. We utilized these indices with other popular ASTER indices for gneiss ((band 5+band 7)/band 6)) and metabasalt (band 6/band 8) and serpentine rich rock s ((band 7+band 9)/band 8) to spatially delineate chromitite – ultramafic–mafic rock assemblage of the study area. In this regard, few index derived false color composite (I-FCC) were derived. Results of index based delineation were validated with reference geological map and field data on locations of rock exposures. We also validated two new indices (ACI,AUI) based on spectral parameters in addition to geological validation of these indices and its FCC images. Further, we also made eigenvector based analysis of principal components (PC) derived from aforementioned indices to identify fewer PC’s than the number of indices used to spatially map the rocks of the study area. We found PC-2 was suitable for delineating meta-ultramafite from chromitite; PC-4 was suitable for delineating metabasalt and PC-5 for gneiss.