HIGH-PRECISION AR-40 AR-39 DATING OF 2 CONSECUTIVE HYDROTHERMAL EVENTS IN THE CHUQUICAMATA PORPHYRY COPPER SYSTEM, CHILE/

Ar-40/Ar-39 data have been used to resolve the relatively high tempera ture (> 350 degrees C) potassic alteration event at Chuquicamata from the lower temperature (< 350 degrees C) quartz-sericite alteration pha se. Four K-feldspars from the potassic zone yielded plateau/large doma in ages in the range 32.9-33.8 Ma (mean = 33.4 +/- 0.3 Ma). The fact t hat apparent ages of co-existing biotites span a similar range suggest s that there was very rapid cooling through feldspar and biotite closu re temperatures, especially if these minerals closed at substantially different temperatures. Multi-domain diffusion modelling of the feldsp ar data suggests that at this time cooling was rapid (ca. 60-80 degree s/My) through closure temperatures of 330-440 degrees C. As these clos ure temperatures are substantially higher than the ones normally assum ed for biotite (i.e. about 300 degrees C), they and/or the calculated cooling rates may be unreliable. Four sericites from the quartz-serici te 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-feIdspars in the quartz-seri cite 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 increas es. In the more remote areas, effects of the later event appear constr ained to narrow, fault-controlled alteration zones. The data are consi stent 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 Symposi um 'Copper deposits of the Andes, New Developments'. Concepcion Chile, October 17-18, 1994. Proceedings 2, 1647-1651.] wherein potassic alte ration by late magmatic fluids occurred at depth, prior to rapid exhum ation of the system. Quartz-sericite alteration, perhaps caused by a n ew pulse of porphyry intrusion at depth, occurred some 2-3 My later pr incipally in the core of the deposit, although alteration did propagat e outwards through the action of hydrothermal fluids passing through a brittle, relatively shallow fracture system. (C) 1998 Elsevier Scienc e B.V. All rights reserved.