SCIENTIFIC COMMUNICATIONS LASER PROBE 4øAF/39Ar GES OF EARLY- AND LATE-STAGE ALTERATION ASSEMBLAGES, ROSARIO

Sensibly identical laser probe 4øAr?9Ar dates of 32.9 _ 0.5 (20') and 32.9 _ 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. i cm 2, 1- to 0.5-ram-thick wafers of altered rock or on smaller chips broken therefrom, obviating the requirement for mineral separation a d permitting direct petrographic control of the analytical procedure. The age data, although not unambiguous, imply that this extremely arge hydrothermal center was eraplaced over a short interval. Hydrothermal ctivity in this central Andean transect migrated eastward over a period of ca. 3 to 4 m.y., from Copaquire, through Quebrada Bianca 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, inthe terminal stage of the late Eocene-early Oligocene metallogenic episode