Magnetic susceptibility anisotropy as a predictive exploration tool of metasomatic iron oxide deposits: Example from the Panj-Kuh iron ore body, NE Iran

Abstract

The Panj-Kuh iron deposit, hosted by Late Eocene igneous rocks in north of central Iran, is associated with a syenitic intrusive body. Magnetic fabric investigations indicate that the Panj-Kuh pluton was emplaced within a sinistral shear zone. Corridors of sheared and metasomatized rocks show that the shear zone was active after emplacement of the pluton and in the final steps of crystallization. The sheared rocks allowed for circulation of hydrothermal fluids during cooling of the magma and caused the pervasive Na–Ca alteration. The ferrimagnetic phase in fresh syenite is mostly euhedral or subhedral magnetite, and is observed adjacent to, or as inclusion in biotite and pyroxene. Anisotropy of magnetic susceptibility (AMS) in the fresh syenite is due to preferred shape orientation of magnetite grains. In Na–Ca altered syenites, magnetite has reduced or has been totally absorbed. The alteration results in decreased bulk susceptibilities (Km) and higher anisotropy degrees (P%), accompanied by changes in the shape of magnetic susceptibility anisotropy ellipsoid. This study of the Panj-Kuh pluton suggests that AMS can be used for mapping lithology and hydrothermal alteration types. Since various types of hydrothermal alteration create, transform or destroy ferrimagnetic minerals in a mineralized system, susceptibility measurements aid mapping of igneous rock type, alteration zoning and mineralization. Integrating petrography, magnetic mineralogy, bulk magnetic susceptibility and preferred orientations of principal axes of the AMS in fresh and altered rocks can be used to generate predictive magnetic exploration models for metasomatic iron oxide deposits.