Experience of researching, scientific-technical provision and exploration of unique Kovdorskiy phlogopite deposit have been exposed. Based on the conclusion of researching, possibility of usage the heavy-duty systems of underground development, taking into account specific properties of phlogopite crystales has been reasoned. Prognosis evaluation of optional versions of further development of deposit has been given and opening-underground method providing phlogopite protection from hydration has been recommended.