Anomalous magnetic properties of Paleoproterozoic ferruginous quartzites of the Kursk block (Voronezh crystalline massif)

Abstract

Introduction: paleoproterozoic ferruginous-siliceous formations play a key role in the formation of the anomalous magnetic field of the Kursk block of the Voronezh crystal massif (VCM). The purpose of this investigation is to study the petromagnetic characteristics of ferruginous quartzites of the Kursk block of the VCM, focused on solving the problems of constructing geologically meaningful magnetic models of the lithosphere of the region.

Methodology: core samples of ferruginous quartzites from a well on the flank of the Stoilenskoye deposit of the KMA were studied. Measurements of the value of magnetic susceptibility, its anisotropy and temperature dependence were carried out. The value of natural residual magnetization and parameters of hysteresis loops of rock samples was determined. A petrographic description of the mineral composition of the ore component of the rock is carried out for samples with the most contrasting petromagnetic properties.

Discussion of the results: the magnetic susceptibility of the samples ranges from 0.32 to 1.45 SI with an average of 0.87 SI and anisotropy from 24 to 136%. Natural residual magnetization is characterized by abnormally high values of up to 2645 A/m with an average value of 660 A/m. Koenigsberger ratio for the studied samples varies from 8 to 34. The value of saturation magnetization ranges from 1.30 to 12.34 mAm². The coercive force varies from 2.52 to 21.01 mT, and the residual coercive force from 10.73 to 62.36 mT. Thermomagnetic determinations indicate the predominance of the ferromagnetic phase with a Curie point of about 568°C in the studied samples.

Conclusions: the obtained estimates of magnetic susceptibility show that when modeling ore objects composed of ferruginous quartzites, it is necessary to take into account the effect of demagnetization on the formation of an external magnetic field. The estimation of the anomalous value of residual magnetization can act as an upper limit on the desired solutions to the inverse magnetic problem, which takes into account inductive/residual magnetization and the demagnetization effect. The presence of single-domain magnetite grains and a high value of coercive force contribute to the conservation of the vector of natural residual magnetization of rocks of the ferruginous-siliceous formation of the Kursk block.