Structural, frequency and magnetic properties of magnetite ores from the Kovdor deposit, Northwest Russia, have been investigated. Scanning electron microscopy, X-ray energy dispersion microanalysis, X-ray diffraction, and Curie temperature measurements all point to non-stoichiometric magnetite as a major (ca 48 wt%) magnetic constituent of the ore. Magnetic granulometry of the samples has been analyzed using the model of interacting ferrimagnetic particles. The results of determining superparamagnetic particles on the basis of theoretical modeling are in good agreement with the experimental data obtained using the frequency-dependent susceptibility and granulometry methods. It is shown that even relatively minor concentrations of superparamagnetic grains modify the electromagnetic response of the samples, and can be detected in their impulse transient characteristics.