To explain the formation of magnetic minerals under hydrothermal conditions, a series of laboratory experiments were conducted. The main changed parameters were the rock composition, salinity of the solution, and the temperature inside equipment. For the initial experiment, samples of olivinites, gabbronorites with magnesian and ferrugious olivine, dolerites and diorites were used. The experiments were carried out in autoclaves made of titanium alloy with oriented rock samples in a special furnace shielded from an external magnetic field with a controlled internal magnetic field. Before the experiment all samples were completely demagnetized. The experiments with duration of 15–100 days were carried out at 300–450 °C and up to 1000 bar using different composition of solutions. After a series of the experiment, samples underwent a full processing cycle, including studies of magnetic mineralogy and stepwise alternating field (AF) demagnetizations. The petromagnetic investigation showed that single domain magnetite was formed during the hydrothermal transformation. In this case, the magnetization direction corresponds to the internal magnetic field. Two mechanisms of single-domain magnetite formation are possible. The first mechanism is associated with formation of serpentine along cracks in olivine and pyroxene crystals. In the mafic rocks, a second mechanism realized as transformation of ilmenite into titanite due to calcium and silicon removed from the rock leads to the formation of hydrothermal single-domain magnetite. However, serpentinization may be completed only partially. The resulting magnetic minerals can maintain a new magnetization direction at the time of a thermal event and be used for paleomagnetic studies.