Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

K. Naha ¹, A. K. Chaudhuri ¹, P. Mukherji ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, IN

Abstract

Large scale mapping of the metasedimentary bands bordered by and partially engulfed in migmatites of the Banded Gneissic Complex in the 'hammer-head syncline' between Sangat and Kelwa, along with tectonic analysis in all scales, has led to the recognition of structures of three phases. Very tight to isoclinal folds on westery axial trend have been coaxially folded, with the formation of reclined, inclined and locally upright folds. Overprinting of NNE-trending upright folds of varying tightness on these homoaxial isoclinal folds with diverse axial surfaces has resulted in an intricate foldgeometry which is brought out most dramatically by the marble band. Superposition of gentle, upright folding with NNE trend on inclined or upright isoclinal folds with westerly trend has led to broad, arcuate map patterns of the metasedimentary bands. Mirror image type interference patterns have developed where reclined isoclinal folds have been involved in upright folding on NNE trend. Overprinting of tight to isoclinal upright folds of NNE trend on reclined isoclinal folds plunging westward has resulted in book-shaped patterns with double closures. Marble bands detached completely from the main band in the present topographic surface are also due to fold-interference. The apparently erratic variation in the thickness of the metasedimentary bands is the result of tight folding of different generations, with the limbs of the folds coalesced.

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A. K. Chatterjee ¹, P. Mukherji ¹

Affiliations
1 National Mineral Development Corporation, Pioneer House, Somajiguda, Hyderabad 500004, IN

Abstract

The paper describes the localisation of iron ore bodies in the Malangtoli group forming the southeastern end of the Bonai Synclinorium comprising Singhbhum-Keonjhar-Sundergarh group of iron formations. Three fold systems affect the area. Barring a few instances which are due to facies changes, the overall pattern of Occurrence of iron ore bodies is fairly conformable with the tectonic set up and the behaviour of such ore bodies are continuous with the structural pattern. The distribution of iron ore body/bodies in space is/are controlled by the superposed fold systems, the ore body reaching maximum depths in structural basins and/or canoes and minimum depths (occasionally base rock outcropping) in structural highs, i.e., domes and inverted canoes. Certain conclusions arising out of this are analysed.

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