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

抄録

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.