The plagiogneisses productive for mica-bearing pegmatites compose significant areas in the Yonsky mica-bearing district, but the areas with concentration of industrial mica-bearing veins are small. In general, detailed surface prospecting was carried out in such areas, and the possibilities of finding mica-bearing bodies were almost exhausted. It became necessary to switch to the study of deposits at depth. Since 1955, in the Yonsky district for the search of blind mica-bearing veins, the North-West Geological Department began the systematic use of core drilling.