Thermal History Analysis of Selected Chilean, Indonesian and Iranian Porphyry Cu-Mo-Au Deposits
by
Brent I.A. McInnes, Noreen J. Evans, CSIRO Exploration & Mining, Perth, WA, Australia, Frank Q. Fu, University of Sydney, NSW Australia, Steve Garwin, Geoinformatics Exploration Australia, Perth, WA, Elena Belousova, W.L. Griffin, Macquarie University, NSW, Australia, Alfredo Bertens, Codelco Chile, Djadjang Sukarna, Sam Permanadewi, Geological Research & Development Centre, Bandung, Indonesia, Ross L. Andrew, Rio Tinto Exploration Pty. Limited, Melbourne, Vic., Australia, and Katja Deckart, Departamento de Geología, Universidad de Chile, Santiago, Chile.
in - Porter, T.M. (Ed), 2005 - Super Porphyry Copper & Gold Deposits - A Global Perspective; PGC Publishing, Adelaide, v. 1, pp 27-42.
ABSTRACT
This paper presents U-Pb-He triple-dating age determinations for several porphyry Cu±Mo±Au deposits in Chile, Indonesia and Iran in an effort to determine their thermal histories and to explore the effects of cooling/exhumation rates on ore formation and preservation processes. 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 0.1 million years (m.y.), however modeling results for the Grasberg, Batu Hijau and El Teniente super porphyry deposits suggest formation periods on the order of 0.01 m.y. Emplacement depths on intrusions associated with porphyry mineralization range from 800 to 5500 m from the palaeosurface, with Grasberg and Rio Blanco being the shallowest and deepest super porphyry deposits studied respectively. 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/m.y. 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.
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