UNDERSTANDING MOLYBDENITE RE-OS AGES FROM THE EL TENIENTE PORPHYRY COPPER DEPOSIT, CHILE

Re-Os dating of molybdenite has made great contributions to ore deposit chronology, but the behavior of the molybdenite Re-Os system is not well understood. A resetting mechanism for Re-Os in molybdenite is unknown, but chemical alteration could produce geologically meaningless ages. In this paper, we compare Re-Os ages of molybdenite-bearing veins in the giant (>75 Mt Cu) El Teniente porphyry Cu-Mo deposit with U-Pb and 40Ar/39Ar ages of mineralized felsic intrusions and alteration minerals to better understand the behavior of the Re-Os system. Three distinct mineralized intrusive units yielded SHRIMP U-Pb zircon crystallization ages of 5.58 ± 0.21 Ma for eastern quartz diorite/tonalite stocks, 5.28 ± 0.10 Ma for a dacite porphyry intrusion, and 4.82 ± 0.09 Ma for the central latite ring dykes. Five molybdenite- and chalcopyrite-bearing veinlets yielded molybdenite Re-Os ages of 5.60 ± 0.02, 4.87 ± 0.03, and 4.42 ± 0.02 Ma, suggesting three mineralization events, two of which coincide with intrusive events dated by U-Pb geochronology. 40Ar/39Ar dating of biotite, sericite and altered whole rock at El Teniente produced 33 plateau ages ranging from 4.81 ± 0.05 to 4.37 ± 0.05 Ma. The 40Ar/39Ar ages do not correspond to specific rock types within the deposit, implying resetting of the K-Ar system by subsequent hydrothermal alteration. The older Re-Os molybdenite ages, and their correlation with ages obtained by other isotopic systems, suggest that the Re-Os system was closed during multiple superposed hydrothermal events at El Teniente, in turn supporting the interpretation of Re-Os molybdenite ages as preserved mineralization ages. Therefore the robustness of the molybdenite Re-Os ages within porphyry copper deposits is strengthened by this systematic geochronological work in the world’s largest porphyry copper deposit.