Potassium-argon age determinations on 19 samples of igneous rocks from the Park City mining district, Utah, and on 7 samples of hydrothermal alteration minerals associated with ore deposits in the district, indicate that a varied sequence of intrusion, volcanism, and mineralization took place within a period of a few million years in earliest Oligocene, and perhaps latest Eocene, time. Ages determined on biotite from igneous rocks suggest that igneous activity occurred 36 to 31 m.y. ago, whereas ages determined on hornblende suggest a somewhat longer period between 41 and 31 m.y. ago. Ages of hydrothermal minerals are 36 to 33 m.y. and thus are indistinguishable from rock ages.