Microstructural observations of fracture-filling goethite vein along the Kerajang Fault Zone in the Rengali Province of eastern India

Microstructural observations and chemical composition analyses of a fracture-filling goethite vein in quartzite bands adjacent to a terrain-boundary fault zone were carried out, primarily using electron microscopy, to determine its formation process. Two domains A and B were identified, based on microstructural and chemical characteristics. The domain A formed a layered structure characterized by goethite grains with higher Al contents, smaller grain size (several hundred nanometers to micrometers in size), with development of the strong shape-and lattice-preferred orientations (SPO and LPO) of [020] and [110] along the wall rock contact, whereas the inner region had lower Al contents with larger grain size (several micrometers in size). The domain B exhibited concentric zoning characterized by variation in chemical composition, grain-size grading approximately ten to several tens of nanometers in size, a change in the porosity, and the alignment of goethite [110] perpendicular to the zoning plane. The grain size distribution and development of SPO and LPO in domain A can be explained by the inhibition of crystal growth (due to the incorporation of Al3+ instead of Fe3+) and geometrical selection, respectively. Two possible formation processes for domain B can be proposed based on the analogy of chalcedony; precipitation of goethite colloidal particles with electrophoretic force or heterogeneous nucleation from the Fe-rich supersaturated fluid and subsequent crystal growth. The study results suggest that the goethite vein was formed by multiple stages of Fe-rich fluid infiltration, which may have been derived from the Banded Iron Formation in the Singhbhum cratonic crust related to the activation of the Kerajang Fault Zone.