Abstract

Zircon U-Pb ages measured in situ by excimer laser ablation–inductively coupled plasma–mass spectrometry (ELA-ICP-MS) and verified by sensitive high-resolution ion microprobe (SHRIMP) on ore-bearing felsic porphyries from the Chuquicamata porphyry copper-molybdenum deposit, northern Chile, identify two discrete igneous events. The volumetrically dominant East porphyry has an age of 34.6 ± 0.2 Ma, whereas the Bench and West porphyries yield ages of 33.3 ± 0.3 Ma and 33.5 ± 0.2 Ma, respectively. The age of the East porphyry is indistinguishable from a Re-Os age for early molybdenite mineralization (35 Ma) and the oldest reported 40Ar-39Ar ages for hydrothermal alteration, confirming a genetic link with mineralization. Previous geological studies and 40Ar-39Ar and Re-Os geochronology identify two main hydrothermal events: high-temperature potassic alteration with chalcopyrite at 33.4 ± 0.3 Ma followed by lower temperature quartz- sericite alteration with pyrite at 31.1 ± 0.3 Ma. The ages of the West and Bench porphyries match the ages for potassic alteration. Younger quartz-sericite alteration may reflect an additional fourth intrusion concealed at depth. The anomalously large size of Chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic-hydrothermal systems.

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