Magnetic anomaly of the Jõhvi iron ore, northeastern Estonia, controlled by subvertical remanent magnetization

The Jõhvi magnetic anomaly is situated within the Jõhvi structural zone that is part of the Bergslagen–Livonia microcontinent. Drilling in the 1930s and 1960s has revealed a complex of magnetite ore alternating with granites, pegmatites and gneisses. The study presents the results of ground magnetic mapping, measurements of drill core (Jõhvi I and II) petrophysical properties (density, magnetic susceptibility, intensity and inclination of the natural remanent magnetization), modelling of the anomalous magnetic field in the Jõhvi area and frequency domain electromagnetic (FrEM) measurements. The magnetic anomaly is composed of three major peaks named western, eastern and northern anomalies. The maximum amplitude of the western total field anomaly is 19 290 nT, of the eastern anomaly 15 880 nT and of the northern anomaly 8080 nT. The 3-dimensional model along five profiles extends from the basement surface to a depth of 1000 m. The direction of strong remanent magnetization coincides with the dip of the iron ore formation. The strong remanence hints at the significant presence of small (<1 μm) magnetite grains in the Jõhvi ore, which needs future (magneto)mineralogical investigation. Results of the FrEM coincide with our earlier knowledge of the electrical resistivity of the crystalline basement and Phanerozoic cover. The method, however, did not provide much information on the ores due to the screening effect of the ~200 m thick pile of low-resistivity Cambrian and Ediacaran clays and sandstones on top of the Palaeoproterozoic basement.