MULTISPECTRAL REMOTE SENSING BASED ALTERATION AND LITHOLOGY MAPPING IN KHETRI COPPER BELT, NORTHWEST INDIA

The surface expressions of subsurface ore deposits are manifested as rock alteration of hypogene or supergene origin, which aid in their remote detection and mapping. Most alteration minerals have distinct spectral features in the visible to near-infrared (VNIR; 0.4-1.0 μm) and shortwave-infrared (SWIR; 1.5-2.5 μm) wavelength regions. Important rock forming silicate minerals have diagnostic spectral features in the thermal infrared (TIR, 7-15 μm) wavelength region.

We use atmospherically and topographically corrected Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) 3-band VNIR and 6-band SWIR data of part of the Khetri Copper Belt (KCB) in northwest India to map major rock alterations associated with mineralization.

KCB is a structurally complex and regionally metamorphosed NNE-SSW striking linear polymetallic-sulfide mineralized belt consisting primarily of early Proterozoic metasedimentary rock units, and late Proterozoic basic and acidic intrusives. The older psammitic Alwar formation (mainly pure and impure quartzites, and micaceous quartzites), and younger pelitic Ajabgarh formation (mainly phyllites, schists, impure marbles and calc-silicate units) comprise the main lithostratigraphic units of KCB, along with many small stocks, sills and dikes of granite, dolerite and amphibolite. The study area is marked by a prominent NNE-SSW striking cataclastic/shear zone along which numerous felsic (granitic and pegmatitic) intrusives are emplaced. Small, but significant sulfide mineralization close to the surface in a few places and in vicinity of these intrusives is observed. Primary rock alteration in the form of sericitization, chloritization and carbonatization is widespread all along the shear zone. Secondary alteration due to weathering is common and is manifested as small gossans and limonitic surface coatings.

Indices developed for 5-band ASTER TIR surface radiance data are used for lithologic mapping of three main rock categories: quartzose, mafic-rich and carbonates. Surface emissivity data, generated using the TES technique, are used for quantitative silica abundance estimation, using Metal Mining Agency of Japan’s K-value technique. The results vindicate the utility of limited spectral resolution ASTER multispectral data in mineral exploration.