In New Guinea, the upper crust is rich in Late Miocene and Pliocene copper-gold deposits, yet the host intrusives are mainly in the New Guinea Fold Belt and are of mantle origin and not directly subduction-related. Structural, thermochronological and geodynamic analyses of the Grasberg, Porgera, Ok Tedi and Frieda River deposits show that the richest deposits occur along the eastern edge of the intersections between long-lived crustal transfers perpendicular to strike and strike-parallel crustal extensional faults that were strongly inverted during Late Miocene -Pliocene orogenesis. The deposits are all associated with north-northeast-trending transfers, parallel to the aeromagnetic grain in basement, across which the continent-ocean suture shows >50 km horizontal separation, as identified by the southern limit of the central New Guinea ophiolites. In the fold belt, the transfers coincide with the termination of regional anticlines or uplifts that are 150-200 km long and 30-60 km wide. Balanced sections reveal that the southern limit of these regional anticlines is commonly fault-bound and coincides with major facies and thickness changes, indicating long-lived, crustal extensional faults that were inverted. Fission track and 40 Ar/ 39 Ar cooling ages show that mineralisation occurred during inversion of these faults and, hence, correlates with propagation of orogenesis from northeast to southwest. It is proposed that the pre-compression New Guinea margin comprised step-like promontories and embayments delineated by long-lived crustal fracture zones, as on Australia's North West Shelf. During Late Miocene -Pliocene compression the crust was thickened, accompanying melting of the underlying mantle, and the crustal fracture zones were reactivated as transfers. Where the transfers intersected crustal extensional faults that were being inverted, local zones of dilation occurred, allowing emplacement of mantle magmas and associated mineralisation. When the deformation propagated southwards, so did the crustal thickening and the reactivation of major faults, allowing emplacement of younger magmas and mineralisation.
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 collisionrelated, 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 fluidflow -thermal-chemical modelling uses a scenario of fluid mixing to calculate metal precipitation distribution and magnitude around an individual intrusive complex. Modelling highlights the interdependence 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.
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