Recycling of Proterozoic crust in Pleistocene juvenile magma and rapid formation of the Ok Tedi porphyry Cu–Au deposit, Papua New Guinea M. van Dongen a, ⁎, R.F. Weinberg a , A.G. Tomkins a , R.A. Armstrong b , J.D. Woodhead c a School of Geosciences, PO BOX 28E, Monash University, 3800 VIC, Australia b PRISE, Research School of Earth Sciences, Australian National University, Acton, 0200 ACT, Australia c School of Earth Sciences, University of Melbourne, 3010 VIC, Australia abstract article info Article history: Received 20 January 2009 Accepted 5 September 2009 Available online 24 September 2009 Keywords: Magma mixing/mingling Pliocene U–Pb zircon dating ε hafnium Oxygen isotopes Asthenospheric mantle We present an investigation of the combined U–Pb, O and Hf isotope composition of zircons from a giant porphyry copper–gold deposit, hosted in a shoshonitic intermediate intrusive complex of the Ok Tedi area in Papua New Guinea. This area is part of a Late Miocene–Pliocene collisional fold-and-thrust belt related to island arc accretion to the Australian plate. Cathodoluminescence and transmitted light imaging reveal distinct zircon textures such as spongy rims and inherited zircon cores. Spongy textures, interpreted to result from corrosion of the surface by hydrothermal fluids, do not seem to affect the U–Pb, O and Hf isotope composition. Calculated SHRIMP U–Pb ages for the rims are 1.1–1.4 Ma whereas the inherited component is ~1.8 Ga. Our age results combined with existing K–Ar results, constrain the formation of the Ok Tedi deposit to < 0.5 Myr. Oxygen isotope composition (δ 18 O), measured by SHRIMP, is ~6.5‰ for Pleistocene zircons but extend to values of ~ 8.3‰ or more for Proterozoic zircon cores. Likewise, corrected Hf isotope ratios from LA-ICP-MS analyses are centred on 0.2825 (ε Hf(t) =-6.5 ± 2) for Pleistocene zircons, compared to ~0.2815 (ε Hf(t) =+5 to -3) for Proterozoic components. The Pleistocene zircon isotope signature is best explained by assimilation of Proterozoic crustal source material into asthenospheric mantle-derived magma similar to that of the Pliocene Porgera Au-only deposit in the same orogen. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Our current thinking on how porphyry copper deposits form and evolve is firmly based on the orthomagmatic model (Sillitoe, 1972; Richards, 2003). In this model, hydrous intermediate to felsic magmas are emplaced in the upper 5 km of the crust, from which hydrother- mal fluids exsolve and deposit copper and gold through fluid–rock interaction within the intrusion and the surrounding rocks. Often, these systems are characterised by multiple intrusive pulses, which can all be associated with fluid exsolution; a key to forming a hydrothermal ore deposit. However, there are numerous felsic hydrous magmas that have not formed deposits and there are many factors that contribute to the formation of an economic deposit. In this paper, we investigate the duration of the process of ore deposition by constraining the timing of magmatism and we examine the origin of the magmas involved. The interpretation of radiometric ages of porphyry systems is problematic, since thermal pulses associated with multiple intrusions have the potential to reset grains (Seedorff et al., 2005), especially those of closure temperatures that are lower than felsic magmatic crystallisation temperatures, such as used in the Ar–Ar and K– Ar method. We circumvent potential radiometric resetting and hydrothermal alteration problems by investigating the age and provenance of zircons, which are notably robust (Watson and Cherniak, 1997; Cherniak and Watson, 2001; Cherniak and Watson, 2003; Valley, 2003). We investigate zircon U–Pb isotope systematics, oxygen isotope and Hf isotope composition by SHRIMP and LA-ICP-MS analysis on samples from the Ok Tedi Cu–Au deposit. Our results show that the Ok Tedi magmatic rocks are extremely young and comprised of juvenile magmas and recycled material with a Proterozoic crustal signature. 2. Regional geology and samples In this section, the geological background of this study is described, followed by a description of the sample set and their geological context. 2.1. Geological background Collision of an island arc with the northern margin of the Australian craton, which forms the southern part of Papua New Guinea, resulted in the Papua New Guinean orogen, developed since the Early Miocene (Hill et al., 2002). The southern part of this east-west trending orogenic belt consists of a south-verging fold- and thrust-belt. The Ok Tedi Intrusive Complex is part of a discontinuous Pleistocene to recent magmatic belt of Lithos 114 (2010) 282–292 ⁎ Corresponding author. Tel.: +61 7 3365 8248; fax: +61 7 3365 1277. E-mail address: m.vandongen@uq.edu.au (M. van Dongen). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.09.003 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos