Reflectance spectroscopy and ASTER based mapping of rock-phosphate in parts of Paleoproterozoic sequences of Aravalli group of rocks, Rajasthan, India

This paper applies reflectance spectroscopy and Advanced Space borne thermal emission reflection radiometer (ASTER) data processing to delineate dolomite-hosted rock phosphate around Udaipur City, Western India, using visible-near infrared (VNIR) and short wave infrared (SWIR) bands of the ASTER sensor. A number of ASTER image spectra and field spectra (resampled to ASTER bands) of pure dolomite and rock phosphate are compared and found to be consistent in terms of the presence of absorption minima at 2.3 mu m for dolomite and the absence of the same feature in the spectra of the rock-phosphate. Before attempting spectral mapping, ASTER SWIR band data are clipped using the spatial extent of host rock; which has been prepared from the reference geological map and consequently updated using VNIR-SWIR derived image enhanced products at selected part of the study areas. Two sub-pixel spectral methods namely adaptive coherence estimator (ACE) and matched filtering (MF) methods are applied to ASTER SWIR bands to delineate rock-phosphate-rich zones within dolomite. The output images of the above algorithms are integrated in a principal component (PC) image to delineate phosphate pixels. A principal component; which has similar sign of Eigen vector loading for ACE and MF abundance image is used to delineate rock phosphate within dolomite. The results are validated in the field using a qualitative colorimetric method and also by comparing image spectra of spectrally anomalous zone with the ASTER resampled laboratory spectra of rock phosphate samples. A number of sporadic but important patches of phosphate rich zones in addition to the main surface exposures of the Jhamar Kotra Mine are confirmed in the field. The proposed method is a novel application of spectral mapping of phosphate deposits in dolomitic and carbonatic rocks and present approach of ASTER data processing can be used to delineate phosphate rich zones occurring within similar geological provinces in any parts of the world.