Australian Journal of Earth Sciences (2002) 49, 753–771 Copper–gold mineralisation in New Guinea: numerical modelling of collision, fluid flow and intrusion-related hydrothermal systems P. A. GOW, 1 * P. UPTON, 1† C. ZHAO 1 AND K. C. HILL 2‡ 1 Australian Geodynamics Cooperative Research Centre, CSIRO Exploration and Mining, PO Box 437, Nedlands, WA 6009, Australia. 2 Australian Geodynamics Cooperative Research Centre, Earth Sciences, La Trobe University, Vic. 3083, Australia. Two- and three-dimensional numerical modelling techniques, constrained by key geodynamic data, provide insights into the controls on development of porphyry-related Cu–Au mineralisation in the Tertiary collision zone of New Guinea. Modelling shows that the creation of local dilation to facilitate magma emplacement can be caused by reactivation of arc-normal transfer faults, where they cut the weakened fold belt. Additionally, dilation occurs where fluid overpressuring is caused by collision- related, south-directed fluid flow being localised into the more permeable units of the Mesozoic passive-margin sedimentary succession. Rapid uplift and erosion, which may be a mechanism for magmatic fluid release in these systems, is shown to be greatest in the west of West Papua, where the stronger Australian crust acts as a buttress. Within the Papuan Fold Belt, uplift is greatest near the margins, where the weaker fold belt abuts the stronger crust and/or major faults have been reactivated. Increased orographically induced precipitation and erosion exposes the lower parts of the stratigraphy within or on the margins of these uplifted zones. On a smaller scale, 2-D coupled fluid- flow – thermal–chemical modelling uses a scenario of fluid mixing to calculate metal precipitation distribution and magnitude around an individual intrusive complex. Modelling highlights the inter- dependence of the spatial permeability structure, the regional temperature gradient, and the geometry of the convection cells and how this impacts on the distribution of metal precipitation. KEY WORDS: collisional zone, New Guinea, numerical modelling, porphyry-copper deposits. INTRODUCTION The northern boundary of the Australian Plate forms the collisional margin in mainland New Guinea and is richly endowed with Late Tertiary porphyry-related Cu–Au deposits (Figure 1). The deposits formed during the Tertiary collision of the Australian and Pacific Plates, although the tectonic setting here is much more complex than most porphyry-rich areas because subduction was highly oblique, with the presence of intervening micro- plates strongly perturbing the stress field. The key deposits (Grasberg, Ok Tedi and Porgera) formed late in this history and record ages of between 6 Ma and 1 Ma (see References in Hill et al. 2002). Although the Tertiary geology and geodynamic history of New Guinea is complex, the margin is beginning to be well understood as a result of both research and resource exploration work carried out over the last 20 years (Hill et al. 2002). Despite ground access problems, the structural architecture is well delineated, and the geodynamic history and tectonic plate configuration and movement vectors are becoming well understood. Additionally, the collection of apatite fission track analysis data from the Papuan Fold Belt places good constraints on the chron- ology of uplift and deformation. The chronology of emplacement of intrusives, particularly those related to the ore deposits, is reasonably well understood following the large volumes of K–Ar work by Page (1976), the work of McDowell et al. (1996) in the Ertsberg district of West Papua, and Richards and McDougall (1990) at the Porgera deposit. Details of the geodynamic history and the regional geology of New Guinea are presented in a companion paper (Hill et al. 2002). The aim of this work is to take the archi- tecture and the dynamic tectonic history developed by Hill et al. (2002), particularly with regard to the timing and localisation of magmatic activity, and attempt to under- stand the interaction between the structural architecture and tectonic history via quantitative numerical modelling. In both this terrane and other continental arcs containing porphyry-related Cu ± Au deposits (e.g. central-northern Chile), it is vital to understand the processes and controls on magma emplacement in broadly compressional settings. In New Guinea, the magmas emplaced in the comparatively lesser mineralised and possibly locally extensional Early Miocene event are large batholithic complexes, whereas the magmas hosting the key deposits *Corresponding author and present address: MIM Exploration Pty Ltd, Locked Mailbag 100, Mt Isa, Qld 4825, Australia (pagow@mim.com.au). † Present address: Department of Geological Sciences, University of Maine, Orono, ME 04469, USA. ‡ Present address: 3D-GEO, Earth Sciences, University of Melbourne, Vic. 3010, Australia.