The Guzelyayla porphyry copper mineralization area that lies between latitudes 40 degrees and 42 degrees N in northeastern Turkey constitutes the part of Balkans-Caucasus-Iran metallogenic porphyry copper mineralization belt. Mineralization is related to a rock sequence ranging from calc-alkaline volcanic porphyry dacite to granodioritic rocks. Electron microprobe analyses of biotite indicate that igneous and alteration biotite define different composition spectrum. three alteration zones respectively potassic, phyllic and propylitic were developed from inward to outward. Potassic alteration temperatures determined for the Guzelyayla porphyry copper mineralization area lie in the range of 360 degrees to 410 degrees C.