Phase transformation of weakly magnetic iron minerals into a strongly magnetic magnetite is a promising technique for iron ore beneficiation with the further magnetic separation. Thermal decomposition of siderite leads to both magnetite and carbon monoxide formation. Carbon monoxide, as a reductant, sequentially triggers the transformation of hematite into magnetite. Therefore, this method of magnetite formation does not require any additional reducing agent. In the present paper, the source materials were hematite and siderite ores, which were mixed in different mass ratios. Phase transformation of ores' mixture was performed at various temperatures and for various time intervals. It was shown within the experiments that siderite and hematite transformed to magnetite. The starting temperature of magnetite formation was about 400 degrees C. X-ray diffraction analysis confirmed the formation of the spinel structure that corresponds to magnetite. The saturation magnetization of transformed samples increased immensely up to 70 Am-2/kg for the sample in 1:1 ratio of hematite to siderite. Optimal conditions for the conversion included heating the mixture at 600 degrees C for 12 min. Under these conditions, siderite and hematite completely transformed to magnetite, since solely magnetite was detected in transformed samples. Simultaneous roasting of hematite and siderite ores is a simple and an economically efficient technique for magnetic powder preparation.