Spectral signatures and spectral mixture modeling as a tool for targeting aluminous laterite and bauxite ore deposits, Koraput, India I. C. Das Scientist, Geosciences Group, National Remote Sensing Agency, Balanagar, Hyderabad-500 037, India. Tel: 91- 40 - 3884225(o), Fax: 91- 40 - 3884258 E-mail: icdas@rediffmail.com Introduction: The Eastern Ghats mobile belt is one of the oldest groups of rocks (average age of about 2900 million years) of Indian Peninsula. The rocks of this belt are mainly consisting of Charnockites, Khondalites, Granites, Granodiorites and unclassified Granulites. The belt shows a fairly consistent trend (NE-SW) for over 1000 kms from Parkasam district of Andhra Pradesh, to the south-eastern edge of the Talchir coalfield of Orissa. The Khondalites in the area generally form high linear ridges while the charnockites occur in the form of low and domal shaped hills. In contrast to the rugged topography presented by these rocks the laterite/bauxite cappings are characterised by wide and flat to gently sloping plateau-like tops and are marked by conspicuous vertical scarp faces around the peripheries. This is because of the presence of hard and compact laterite on the top of the hills with very thin or no soil covers. The hilltops are mostly barren with no major trees growing over it. Grass and dwarf date palm trees grow abundantly over these hills. The hills are well dissected and give rise to mesa like landforms. Laterite floats are also very common on the hilltops (Photo #1). 
Imaging space-borne Satellite sensors acquire information about the earth materials in different ranges of electromagnetic spectrum (spectral resolution) with certain amount of energy coming out of a specific area (spatial resolution) in a particular direction. In nature, there are n-numbers of materials randomly distributed over the earth’s surface. Spectral signature of any material depends upon its composition and molecular structure. Hence spectral signatures are unique to each material. As remote sensing deals with spectral reflectance of the objects in a spatial domain (ground resolution or IFOV) there may be the chance that reflectance from the closer objects are mixed. Depending upon the spatial and spectral resolution of the imaging sensors the spectral information is modified in the images. Again geological Remote Sensing in most of the cases is based on indirect evidences, as natural rock exposures are scanty and patchy in occurrences. Therefore spectral unmixing is required to separate target material from the background by estimating approximate sub-pixel abundance in the images. Study area: The present study area, an Archaean metamorphic terrain is situated in the eastern part of India. It is located east of Koraput town in the southern part of Orissa province with latitude 18°45' N to 19° 05' N and longitude 82° 50' E to 83° 05' E (figure 1). The extent of the area is about 900 square kilometers. The area is well connected in all modes of transport with the city of Vishakhapatnam situated southeast of the study area. 
Geology: The Eastern Ghats mobile belt is one of the oldest Pre-Cambrian geosynclinal orogenic belts of Indian Shield. The major rock types of this belt are Khondalite, Charnockite and Granite gneiss. Khondalite is the oldest rock of the Eastern Ghat Super group mainly composed of quartz, feldspar, garnet, sillimanite and minor amounts of graphite at places. The Charnockite, another major group of rock is mostly composed of quartz, feldspar and hypersthene and shows intrusive relationship towards the Khondalite. Other type of rock that is found in this region is granite gneiss, which is mostly quartzo-feldspathic in composition with some amount of pyroxene and hornblende. The aluminous laterite and bauxite deposits of this area constitute a part of East Coast Bauxite deposits developed by residual weathering of Eastern Ghats Group of rocks (mainly Khondalite and Charnockite) in a series of plateaus and hill ranges spread over the states of Orissa and Andhra Pradesh (Rao and Ramam, 1979). Laterite and bauxite ore deposits occur as blanket cover over highly dissected plateaus. At Panchpatmali hill, the Bauxite occurs as a blanket cover over Khondalite in a plateau of about 15 square kilometers. The average thickness of the bauxite deposits in panchpatmali hill is around 14 meters. Laterites are highly porous in nature and are formed as capping over the bauxite deposits and their thickness varies from 3 to 5 meters. The hills containing laterite and bauxite deposits are normally devoid of vegetation with thin or no soil cover. A particular plant species, Phoenix acaulis (dwarf date) grows abundantly over the plateau. This plant species is not recorded in the low altitude areas surrounding Panchpatmali, Kodingamali and other bauxite bearing hills. This plant may be a geobotanical indicator of laterite/bauxite deposits of this region (Photo # 2). 
Mineralogical study carried out by X-ray diffraction, thermal analysis, electron microscopy and heavy mineral analysis of bauxites derived from different parent rocks reveal gibbsite as the main aluminous mineral. Other associated minerals include hematite, goethite, kaolinite and anatase etc. The bauxites are very hard and massive on the top and moderately hard and spongy below. The pale buff to creamy white colored bauxite is mostly of high grade variety and is recorded mostly on the upper part of the bauxite zone and pale pink to reddish brown bauxite with a clayey appearance, is found to occur towards the lower part of the bauxite zone. The porosity and moisture contents of bauxite are highly variable and specific gravity ranges between 2.1 and 2.5.
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