Sensibly identical laser probe 40 Ar/ 39 Ar dates of 32.9 + or - 0.5 (2Sigma ) and 32.9 + or - 0.6 Ma are determined for, respectively, early-stage hydrothermal biotite and late-stage hydrothermal muscovite from the giant Rosario porphyry copper deposit in the Collahuasi district of northern Chile. The age determinations were carried out on ca. 1 cm 2 , 1- to 0.5-mm-thick wafers of altered rock or on smaller chips broken therefrom, obviating the requirement for mineral separation and permitting direct petrographic control of the analytical procedure. The age data, although not unambiguous, imply that this extremely large hydrothermal center was emplaced over a short interval. Hydrothermal activity in this central Andean transect migrated eastward over a period of ca. 3 to 4 m.y., from Copaquire, through Quebrada Blanca to Rosario, the intensity of mineralization increasing with time. Rosario, Chuquicamata, and La Escondida, the largest of the "West Fissure" (Domeyko fault system) array, were apparently emplaced coevally, in the terminal stage of the late Eocene-early Oligocene metallogenic episode.