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Microstructures in a banded iron formation (Gua mine, India)

Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur-721302, India

^* Author for correspondence: mamtani{at}gg.iitkgp.ernet.in

This paper provides a detailed documentation of microstructures developed in the banded iron formation (BIF) of Gua mine, located in the Bonai Synclinorium (eastern India), where the rocks have been subjected to three deformations (D₁ to D₃). Folded iron ores, quartz strain fringes around rigid core objects and folded iron ore layers, and refracted quartz veins are described from samples taken from D₂ folds in the banded iron formation. Orientations of microstructures are compared with mesoscopic structures to interpret the generations of ore minerals, planar structures and the time relationship between deformation and development of different microstructures. The mechanism of D₂ folding is worked out and its bearing on microstructure development is discussed. The D₂ folds are inferred to have developed by a combination of tangential longitudinal strain in the competent layer, flexural flow in the incompetent layers and flexural slip at the interface between layers of differing competence. Homogeneous flattening strain superposed the earlier strain, which led to modification of the folds in the competent layer from class 1B to 1C. This strain is quantified and is found to be higher in the limb than the hinge of a fold. Diffusive mass transfer by solution and bulging dynamic recrystallization in quartz are inferred as the dominant deformation processes during folding. Moreover, based on comparison with published deformation microstructure maps, the microstructures of the present study are estimated to have developed between 300 and 350 °C temperatures at a strain rate of 10^–14–10^–12 s^–1, which are geologically realistic conditions for naturally deformed rocks.

Key Words: microstructure • banded iron formations • quartz • pressure solution • recrystallization • strain • folding