The plagiogneisses productive for mica‑bearing pegmatites occupy significant areas in the Yonsky mica‑bearing district, but the areas with concentrations of industrial mica‑bearing veins are small. For the most part, detailed surface prospecting has been carried out in such areas, and the potential for finding mica‑bearing bodies has proved almost exhausted. A need arose to move to the study of deposits at depth. From 1955 onward, in the Yonsky district, for the purpose of searching for blind mica‑bearing veins, the North‑West Geological Administration began the systematic use of core drilling.