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 40Ar/39Ar biotite, illite, and alunite dates (at 2 sigma 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 plus or minus 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 plus or minus 0.5 Ma (plateau age) and 32.6 plus or minus 0.3 Ma (plateau age), respectively. An Re-Os age of 33.3 plus or minus 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 plus or minus 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 40Ar/39Ar plateau age of igneous biotite for a monzonite intrusion that hosts copper mineralization is 35.2 plus or minus 0.3 Ma. The monzonite is intruded by postmineralization porphyry dikes of similar composition, from which igneous biotite yielded a 40Ar/39Ar age of 34.7 plus or minus 0.3 Ma. Igneous biotite in the Rosario Porphyry cooled through its closure temperature during and after the formation of illite, implying that porphyry-style ore and alteration minerals in the Rosario deposit had formed by 34.3 Ma. The age of alunite at Rosario and La Grande indicate that a second discrete episode of hydrothermal activity was superimposed, 1.8 plus or minus 0.4 m.y. later, onto the earlier-formed porphyry Cu system. Hydrothermal activity at Ujina is constrained by the 40Ar/39Ar ages of igneous biotite in the premineralization and postmineralization intrusions and occurred during a minimum interval of 0.5 plus or minus 0.4 m.y. The biotite granite at La Profunda, 1.5 km east of Ujina, has a igneous biotite age of 81.2 plus or minus 2.9 Ma, indicating that this intrusion is unrelated to the mineralized Eocene-Oligocene porphyry intrusions at Ujina, Rosario, and Quebrada Blanca. The 40Ar/39Ar data for igneous biotite in the Rosario and Ujina porphyries are cooling ages after multiple hydrothermal events. In most cases, these are minimum hydrothermal ages (i.e., ages of the last high-temperature hydrothermal event to have affected the samples), not magmatic ages. However, the biotite age for the Inca Porphyry is possibly only slightly younger than the intrusion age because biotite has not been affected by later hydrothermal alteration