High-precision 40Ar/39Ar dating of two consecutive hydrothermal events in the Chuquicamata porphyry copper system, Chile

40Ar/39Ar data have been used to resolve the relatively high temperature (>350°C) potassic alteration event at Chuquicamata from the lower temperature (<350°C) quartz–sericite alteration phase. Four K-feldspars from the potassic zone yielded plateau/large domain ages in the range 32.9–33.8 Ma (mean=33.4±0.3 Ma). The fact that apparent ages of co-existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures, especially if these minerals closed at substantially different temperatures. Multi-domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid (ca. 60–80°/My) through closure temperatures of 330–440°C. As these closure temperatures are substantially higher than the ones normally assumed for biotite (i.e. about 300°C), they and/or the calculated cooling rates may be unreliable. Four sericites from the quartz–sericite zone yielded variably discordant age spectra. The least discordant of these has a plateau at 31.1±0.3 Ma, and this is interpreted as the time of this alteration event. Some K-feldspars in the quartz–sericite zone were completely overprinted at this later time, and most of the other ones studied show partial overprinting. Overprinting appears to decrease as distance from the central quartz–sericite zone increases. In the more remote areas, effects of the later event appear constrained to narrow, fault-controlled alteration zones. The data are consistent with a conceptual model proposed by Zentilli et al. [Zentilli, M., Leiva, G., Rojas, J., Graves, M.C., 1994a. The Chuquicamata system revisited. (Extended Abstract), Society of Economic Geologists Symposium `Copper deposits of the Andes, New Developmentsʹ. Concepcion Chile, October 17–18, 1994. Proceedings 2, 1647–1651.] wherein potassic alteration by late magmatic fluids occurred at depth, prior to rapid exhumation of the system. Quartz–sericite alteration, perhaps caused by a new pulse of porphyry intrusion at depth, occurred some 2–3 My later principally in the core of the deposit, although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle, relatively shallow fracture system.