Phlogopite was found in small amounts in the contact-metamorphic aureole of the dunite-harzburgite complex at Yanomine. Phlogopite occurs intimately associated with hydrous silicate minerals such as serpentine and chlorite in the olivine-orthopyroxene metamorphic zone. The mineral also occurs as inclusions together with serpentine in chromian spinel grains of chromitite bands in dunite. The former interstitial phlogopites, characterized by lower TiO₂ and Na₂O contents compared with the latter included phlogopites, are suggested to have formed by a metasomatic reaction with fluids infiltrated into the rocks through cracks from the intruded granitic body in the stage later than the peak of the contact metamorphism. Prolonged infiltration of fluids changed some of the phlogopites to phlogopite hydrates. The latter included phlogopites probably crystallized from an incompatible elements-enriched hydrous melt generated by mantle-melt interaction, and were later hydrated during serpentinization.