Porphyry copper deposits and related rocks of Mongolia have not been generally well dated. A few papers report radiometric ages but do not provide raw data or references and most papers that discuss ore deposits report the age at the period level only. As part of a larger study examining several deposits and prospects in Mongolia, we present new 40 Ar/ 39 Ar data for three localities. Porphyry copper deposits and host rocks at Erdenet and Bayan Uul, 250 km northwest and 180 km southwest of Ulaan Baatar, respectively, yield Early Jurassic (207.4 + or - 2.5 Ma) and Late Triassic (220-223 Ma) ages. Samples from Tsagaan Suvarga in southern Mongolia yield a Devonian (364.9 + or - 3.5 Ma) age from the ore zone and a Carboniferous (313.0 + or - 2.9 Ma) age from a porphyry dike that crosscuts the hydrothermal alteration. These ages help constrain the timing of metallogenic and tectonic events: the Devonian and Carboniferous ages record part of the development of magmatic arcs in southern Mongolia and the Late Triassic ages may be evidence for the closing of the Mongol-Okhotsk ocean.