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S. N. Sarkar ¹, K. V. V. S. Gautam ¹, Sajalendu Roy ²

Affiliations
1 Indian School of Mines, Dhanbad 826004, IN
2 CMPDI, Ranchi, IN

Abstract

Systematic analysis of structural geometry of a part of the Sausar Group rocks in Chikla-Sitekesa area throws new light on the tectonic pattern and evolution of the area. The country rocks mainly include Tirodi gneiss, Sitasaongi, Lohangi, Mansar, Chorbaoli and Junewani Formations and three ore-horizons of the Sausar Group in which four types of s-planes (S₁-S₄) and six types of linear structures (L₁-L₆) have been recognised.

For tectonic analysis the area has been subdivided into three sectors and 15 subareas and selective and synoptic π and lineation diagrams of megafabric elements (1500 s-planes and 516 lineations) have been critically studied along with the structural maps. The results indicate that the rocks of the area exhibit triclinic symmetry in megascopic, mesoscopic and macroscopic scale with reference to all structural elements, though there are domains of monoclinic symmetry with reference to one or more fabric elements.

Kinematic analysis indicates the generation of isoclinal (B₁) folds and ischolar_mainless hinges with axial planes mostly dipping SSE due to rotational stress (P₁) along N168°N348° with anticlockwise rotation sense when looking ENE. Considerable variation in the orientation of the first fold axes may be due to inhomogeneity of rock pile, irregular distribution of stresses and different degrees and phases of deformation. Subhorizontal stress (P₂) acting along N10°-N190° with deformation plane dipping steeply westward is responsible for the generation of east to southeast plunging L₅ puckers and minor (B₂) folds along with steeply dipping S₃ planes on S₂, Stress (P₃) acting along N146°-N326° seems to have generated widely spaced kink bands and gentle superimposed L₆ folds on S₂ in Sector-I and Sector-III and macroscopic scale anti formal B₃ fold (Alesur: antiform) in Sector-II. The rocks represent B₁ Λ B₂Λ B₃ tectonites formed during successive deformation phases (F₁-F₃) of the Sausar orogenic cycle.

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Deepak C. Srivastava ¹, S. N. Sarkar ²

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee, U. P., IN
2 Indian School of Mines, Dhanbad, Bihar, IN

Abstract

Systematic structural analysis of the Archaean Iron Ore Group rocks around Barda and Ulugutu throws new light on the tectonic pattern and structural evolution of the area. The polyphase tectonics (D₁-D₃) is evident by the style and over-printing relationship between three types of s-planes (S_O-S₂), folds (F₁-F₃), and related lineations. Folds (F₁) showing complex geometries, related lineations showing variable plunges and slaty cleavages (S₁) have developed synchronously during the earliest tectonic phase (D₁). The concomitant evolution of a new set of structures and deformation of preexisting D₁ structures took place during the second phase (D₂). The numerous smallscale culminations - depressions of F₂ folds are attributed to the variable orientations of s-surfaces prior to F₂ folding. Critical field evidences suggest the evolution of both F₁ and F₂ folds through similar deformational plans including buckling and post-buckle flattening. The concentric warps (F₃) are envisaged to have developed under flexural conditions during D₃ phase in local domains.

En-echelon arrangement of elliptical interference structures are ascribed to the nonorthogonal axial surfaces of F₁ and F₂ folds, and axial migrations during F₂ folding. The co-existence of type-1 and type-2 patterns is due to F₂-flexure of alternating tight and open folds (F₁) within the same layer. That the bedding (s_O) has been successively subjected to layer parallel compression is implied by buckling nature of repeated folding. Therefore, finite strain-ellipse of the whole area corresponds to constrictional field (1>λ₁>λ₂)·

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S. N. Sarkar ¹, J. R. Trivedi ², K. Gopalan ²

Affiliations
1 Indian School of Mines, Dhanbad 826 004, IN
2 Physical Research Laboratory, Ahmedabad 380 009, IN

Abstract

Six whole rock samples of the Tirodi gneiss belonging to the basal part of the Sausar Group rocks in the Dongri Buzurg area yield a good Rb-Sr isochron corresponding to an age of 1525±70 Ma and an initial Sr ratio of 0.7148±0.0033. Mineral isachrons for two of these rocks give concordant but younger ages at 860 Ma in good agreement with the previously reported K/Ar ages on minerals separated from the Sausar Group rocks. The 1525 Ma event is interpreted to mark the main phase of regional metamorphism of the Sausar Group of rocks including the granitization of the basal Tirodi unit. The deposition of the sediments of the Sausar Group and their basement must therefore be even older. The good agreement of the RblSr and K/Ar mineral ages at about 860 Ma indicates the latest thermal overprint on the Sausar - rocks, The regional metamorphism of the Tirodi gneiss at about 1500 Ma ago is also reflected in the Rb/Sr and K/Ar mineral ages of the Amgaon gneiss and Dongargarh granite to the east of the Tirodi gneiss.

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A. Roy ¹, A. K. Saha ², S. N. Sarkar ³

Affiliations
1 Geological Survey of India, ER, 12 Russel St., Calcutta-700071, IN
2 Department of Geology, Presidency College, Calcutta-700073, IN
3 95A, Ballygunge Gardens, Calcutta-700019, IN

Abstract

Correspondence analysis, using the concept of joint probability distribution of two random variables, individual samples and chemical variables, was applied to two sets of multi-metallic data from Mosaboni copper mines (18th, 21st & 25th revels) and Sargipalli lead-zinc mine falling within the Gangpur-Singhbhum metallo-tectonic belt. The analysis results factor loadings for individual samples and variables, which when plotted as points on two-dimensional coordinate (factorial) axes with the same origin recognize distinct geochemical domains characterised by natural groupings or clusters of samples and variables. The simultaneous variation of trace elements in these two data sets with sample locations (background/anomalous) seems to reflect overall trends of geochemical evolution of base metal mineralisation in the region.

Keywords

Statistical Studies, Economic Geology, Copper, Lead-Zinc, Sargipalli, Mosaboni, Bihar, Orissa.

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