2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 2
Presentation Time: 1:45 PM


MCINNES, Brent I.A.1, EVANS, Noreen J.1, FU, Frank Q.2, GARWIN, Steve3, BERTENS, Alfredo4 and GRIFFIN, William L.5, (1)CSIRO Exploration and Mining, PO Box 1130, Bentley, WA 6151, Australia, (2)School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia, (3)Geoinformatics Exploration Australia, West Perth, 6005, Australia, (4)GCEX, CODELCO-Chile, Santiago, 1270, Chile, (5)GEMOC Key Centre, Macquarie University, NSW 2109, Australia, Brent.McInnes@csiro.au

Despite the fact that porphyry deposits have been exploited for 100 years (open mining at Bingham began in 1905), a number of time- and temperature-related variables involved in their genesis remain poorly understood, such as: (1) the longevity of the ore precipitation event during the thermal decline of magmatic-hydrothermal system, (2) the depth of emplacement, (3) the preservation potential of hypogene ores during orogenic uplift and exhumation, and (4) the formation potential of supergene ores from eroded hypogene precursors. We report on a comparative study of thermal, exhumation and preservation histories of porphyry Cu-Mo-Au deposits of the Andean, Tethyan and IndoPacific magmatic arcs using zircon and apatite U-Pb-He triple-dating methods. Inverse thermal modeling of measured time-temperature history data from these deposits was conducted to quantitatively constrain the depth of emplacement, duration of ore deposition, exposure ages and cooling/exhumation rates. The duration of hypogene ore formation for the deposits studied generally occurs within timeframes of 105 years, however modeling results for the Grasberg, Batu Hijau and El Teniente super porphyry deposits suggest formation periods on the order of 104 years. Emplacement depths of intrusions associated with porphyry mineralization range from 800m to 5500 m from the paleosurface, with Grasberg and Rio Blanco being the shallowest and deepest super porphyry deposits studied. The thermochronology data indicates a positive correlation between metal grade and cooling rate during hypogene ore formation, but further investigation is warranted. Exhumation rates varying from 0.3 to 1.1 km/my have implications for the preservation potential of hypogene ore deposits, with superporphyry deposits like Sar Cheshmeh potentially losing 3.5 Mt of copper to erosion over the last 5 million years. The potential for supergene ore formation under such conditions is high, as is the potential for the formation of proximal Exotica-type deposits.