40Ar/39Ar and Re-Os Geochronology of Porphyry Copper-Molybdenum Deposits and Related Copper-Silver Veins in the Collahuasi District, Northern Chile.

The Rosario and Ujina porphyry Cu-Mo deposits, together with porphyry copper mineralization at the nearby Quebrada Blanca deposit, constitute the third-largest concentration of copper mineralization associated with the Domeyko fault system in northern Chile. At Rosario, fault-hosted Cu-Ag-rich massive sulfide veins are associated with pyrophyllite-alunite-quartz altered rocks. The Rosario massive sulfide veins cut biotitealbite- magnetite, K-feldspar and illite-chlorite altered rocks associated with porphyry-style copper-molybdenum mineralization. Similar massive sulfide veins occur in the La Grande area, 1 to 2 km south of Rosario. Copper ore at Ujina is associated with a K-feldspar-biotite-altered quartz monzonite intrusion. The K-feldspar-biotite assemblage has been overprinted by white mica-chlorite alteration. New geochronologic data presented in this paper constrain the ages of hydrothermal activity in the Rosario and Ujina deposits. The ⁴Ar/³⁹Ar biotite, illite, and alunite dates (at 2σ error), and a Re- Os molybdenite date (at 0.5% error) are reported for porphyritic rocks and hydrothermal alteration from the Rosario and Ujina mineralized centers. A weighted mean ⁴⁰Ar/³⁹Ar plateau age of 34.4 ± 0.3 Ma is obtained for igneous biotite in a monzonite porphyry that hosts copper mineralization at the Rosario deposit. Illite and hypogene alunite from separate overprinting alteration events yielded ⁴⁰Ar/³⁹Ar ages of 34.5 ± 0.5 Ma (plateau age) and 32.6 ± 0.3 Ma (plateau age), respectively. An Re-Os age of 33.3 ± 0.2 Ma for molybdenite at Rosario is slightly younger than the ⁴⁰Ar/³⁹Ar age of illite, but older than the alunite. A weighted mean plateau age of 32.7 ± 1.6 Ma for hypogene alunite from the La Grande Cu-Ag-(Au) vein south of Rosario is indistinguishable from the age of Rosario alunite. At Ujina, the weighted mean... [ABSTRACT FROM AUTHOR]