LASER PROBE AR-40 AR-39 AGES OF EARLY-STAGE AND LATE-STAGE ALTERATIONASSEMBLAGES, ROSARIO PORPHYRY COPPER-MOLYBDENUN DEPOSIT, COLLAHUASI DISTRICT, I-REGION, CHILE/

Sensibly identical laser probe Ar-40/Ar-39 dates of 32.9 +/- 0.5 (2 si gma) and 32.9 +/- 0.6 Ma are determined for, respectively, early-stage hydrothermal biotite and late-stage hydrothermal muscovite from the g iant Rosario porphyry copper deposit in the Collahuasi district of nor thern 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 th erefrom, obviating the requirement fur mineral separation and permitti ng direct petrographic control of the analytical procedure. The age da ta, although not unambiguous, imply tl lat this extremely large hydrot hermal center was emplaced over a short interval. Hydrothermal activit y in this central Andean transect migrated eastward over a period of c a. 3 to 4 m.y., from Copaquire, through Quebrada Blanca to Rosario, th e intensity af mineralization increasing with time. Rosario, Chuquicam ata, and La Escondida, the largest of the ''West Fissure'' (Domeyko fa ult system) array, were apparently emplaced coevally, in the terminal stage of the late Eocene-early Oligocene metallogenic episode.