S. Chellaiah
D-95 BIT Mesra(PO)
Ranchi-835215, INDIA
Email: [email protected]
Introduction
Mapping of lithology and alteration zones in inaccessible mountain and forest terrain has always posed a challenge. There always existed disputes on the accuracy of lithological boundaries and structural details in these maps. Vast area to be surveyed and its inaccessibility, forbids physical investigation of every outcrop.At this juncture ,the potential of remote sensing is appreciatable.The greatest advantage of satellite remote sensing is the synoptic view it provides,which gives a regional and integrated perspective of and interrelations between various land features.The availability of multispectral and high resolution data and the advanced capabilities of digital image processing techniques in generating enhanced and interpretable image have further enlarged the potential of remote sensing in delineating the lithological contacts and geological structure in great details and with better accuracy.
In the regions where bed rock is exposed multispectral remote sensing is useful for mapping lithology and alteration zones. Subsequent Studies ,in South-West USA and elsewhere (Prost 1980, Rowan and Khle 1982 , Podwysocki et al 1983,Abrams, 1984 ) have confirmed that areas of hydrothermal alterations may be distinguishable using the ratio of TM band 5 and 7. Also it is evident that the spectral characteristics of TM bands are suitable for mapping ultramafic and associated lithology.
Study Area
The Singhbhum and Chhotanagpur regions represent the part of Protero-Epiproterozoic ultramafic complex,(Jain ,R.S and Nautyal). Distinctive iron oxide and clay mineral alterations associated with these rocks are exposed. These have distinctive surface expressions, even though some areas are under extensive forest and soil covers.
The study attempts to identify and map ultramafic and associated lithology.
The Seka and its adjacent areas are geologically a volcanic area, situated in the South East of Ranchi.The Geological processes produced large areas of hydrothermal alterations, characterised by quartz veins and amphibolites.
The study attempts to use Landsat TM data to identify map ultramafic and associated lithology.
Tectonic Setting
The transition from an essentially mantle dominated volcano-tectonic region in the Archaean to a dominant ensiatic sedimentary environment place in parts of the globe in the Proterozoic . Economically important deposits in the Proterozoic fold belt of the Singhbhum Group and Chhotanagpur Gneisses are expected to be confined within the structrully favourable locations.Tamar-Porapahar Shear zone might have acted as the first-order structure , allowing the deposit occurrence.
Geology
The Tamar-Porapahar shear zone is situated within Singhbhum Group of rocks . The area constitutes quartz-mica schist, cherty quartzite and chlorite schist which are intruded by quartz veins and amphibolites. The Geological milieu is associated with sulphide- facies banded iron formations,which is subjected to high grade metamorphism and intense deformations.
Digital Image Analysis
As image processing is highly scene dependent the methodology was experimented in the study area.Landsat TM images were used to map alteration zones and associated lithology in the ultramafics complex .The Landsat TM, IRS 1C images were used to help understand their lithological and structural controls.
Land sat TM Images
We processed Land sat TM data to generate three colour composites for the Seka alteration zones .
Normal colour images using band 7as red , 5 as green and 4 as blues.(Fig1a).
TM image 7,5,4as RGB. (Fig1b ).
TM ratio image using band ratios 5/7as red , 4/5 as green, and 3/1 as blue.( Fig2).
Landsat TM band ratios 5/7and 3/1 are used because clay and iron minerals have reflectance and absorptions features in these bands . Landsat TM band -ratios 4/5 is used because it emphasizes silicate minerals compared to FeO-rich minerals .(Abrams et al 1983).
The 7,5,4 and 7,4,2 Land sat TM/ IRS 1C images provide a synoptic overview of the study area that is useful for mapping regional morphological features such as rivers , bedrock outcrops and soils.Moreover these images are useful in identifying linear structure faults and fractures.
The image (Fig2) is used to discriminate different rock types in the study area.This image is useful in identifying alteration zones.These alteration zones are labelled 1,2,3,4,5 appear red , suggesting the domination of clay minerals.
Result and Conclusions
Landsat TM data have been used to map the lithology associated with banded iron formations because of their characteristic spectral properties . Clay minerals such as Sericite have reflectance maxima within Landsat TM band 5 (1.55-1.75micrometer) and reflectance minima within Landsat TM band 7 (2.08to 2.35 micrometer). Hunt and Ashley, 1979).Iorn minerals such as limonite have reflectance maximum within the TM band 3 (0.63 to 0.69 micrometer) and reflectance minima within the Landsat TM band1(0.45-0.52) (Hunt et al 1971). So this TM ratio image reveals Ultramafic and its associated lithology in the Tarmar- Porapahar shear zone.
This study proves that ultramafic lithology associated with sulphide-facies banded iron formations can be mapped effectively.
Acknowledgement
We thank our Vice-Chancellor Emeritus and Vice -Chancellor , for encouraging to take up such research work. Our Departmental Colloquies ,Research Fellows , Students for their help and support through out this study.
I also like to thank the ISRO , Chairman , NRSA Director,NDC Head, NDC Staffs especially, Mr. D.Chandrasekharan, Dr. (Mrs).Aparna, Mr. Subbunagulu, Ms. P.S.Sujatha, Mrs .Malani and other Staffs for their help in procurement and initial image processing .I am also indebted to Mr. Ganesh Kumar, Mr. Krishnaiah,and Mr.Meganath ,Mr.Shastri and other staffs and friends of QC , NRSA, Hyderabad. Also I like to express my thanks to Mr. John during my stay at NRSA. I also like to express my due regards for Staffs and Scientists , IRS, Anna university, Chennai, for their valuable comments and help.
References
- Abrams,(1984),Landsat 4,ThematicMapper and ThematicMapper Simulator data for a Porphyry Copper Deposit.Photogrammetric Engineering and Remote Sensing,50,1171-1173
- Prost,G, 1980, Alteration Mapping with Airborne multispectral scanners. Economic Geology, 75,894-906.
- Rowan, L.C and Kahle, A.B., (1982) Evaluations of 0.46- 2.36 micrometer. multispectral scanners images of the East Tintic mining district , Utah, for mapping hydrothermally altered rocks. Economic Geology,77, 441-452.
- Podwysocki,M.H., Segal,D.B. and Abrams, M.J. (1983), Use of multispectral scanner images for assessment of hydrothermal alteration in the Marysvale,Utah, mining area. Economic Geology. 78, 675-687.Photogrammetric Engineering and Remote Sensing , 50, 1171-1173.
- Jain, R.S., and Nautyal, Prospective diamondiferous Areas, Record, Vol. 108. Geological Survey of India.
- Hunt,G.R and R.P.Ashley,(1979).Spectra of altered rocks in the visible and near infra red, Economic Geology, 74: 1613-1629.
