Abstract <p>The article presents the results of studies of the composition, morphology, and relationships of rock-forming minerals of phoscorites—magnetite, apatite, biotite, and carbonates—which made it possible to determine the conditions of their formation. The morphology and relationships of minerals were studied on the core material of boreholes 801 (177 running meters) and 59 (150 running meters) by optical and electron scanning microscopy. The chemical composition was determined by microprobe analysis for 100 magnetite–ilmenite pairs, 132 biotite grains, and 163 apatite grains. For magnetite with structures of solid-phase decomposition of ilmenite and biotite, the formation temperature of 850–530 and 750–550°C, respectively, was obtained using geothermometers. Based on the studied characteristics, conclusions were drawn about the participation of magmatic, metasomatic, and hydrothermal stages in the formation and transformation of the Tomtor massif phoscorites. Within the massif of ultramafic alkaline rocks and carbonatites, an ore occurrence of apatite–magnetite ores, Onkuchakh, was discovered, with iron resources of about 1 bln t and P₂O₅ resources of about 500 mln t. Apatite–magnetite ores (phoscorites) of the Tomtor massif form vein and lens-shaped bodies, dipping at an angle of 75°–80° in the direction of the carbonatite core and in contact with rocks of the silicate complex (foidolites, alkaline syenites) in the immediate vicinity of the carbonatites.</p>