Geodynamic setting of giant Au-rich magmatic-hydrothermal metal deposits in Papua New Guinea: Integrating geology, isotope geochemistry and seismic tomography

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author van Dongen, Michiel۱; Armstrong, Richard۲; Spakman, Wim۳; Tomkins, Andrew۱; Weinberg, Roberto۱
Holding Date 11 October 2008

The geodynamic setting during the generation of magmatic-hydrothermal metal deposits can be enigmatic. The current paradigm, largely based on Trans-American deposits, is that they form in subduction zones that are characterised by long periods of steady-state subduction, alternating with periods with a lull in volcanic activity that sometimes coincide with the timing of ore deposit formation. A common interpretation is that these lulls represent shallowing of the subducting lithospheric plate, possibly followed by break-off or tearing of this slab, allowing hot mantle material to flow to shallow levels. This provides the latent heat necessary to melt source rocks and drive the hydrothermal fluid flow necessary for giant ore deposit formation. Therefore, the geodynamic setting ultimately controls ore deposit formation. We take a multi disciplinary approach in combining conventional geological data with geochemical and geophysical datasets to investigate the geodynamic setting of the fold-and-thrust belt that runs east west along the southern margin of the orogen crossing Papua New Guinea and Irian Jaya. Deformation and uplift in the belt has taken place since the late Miocene and is ongoing. Discontinuous, small volume intrusive complexes of K-rich to shoshonitic composition of the same age are scattered throughout the belt.
The belt hosts two of the world’s largest porphyry deposits within these intrusive complexes: Grasberg and Ok Tedi, and another intrusive complex hosts the giant magmatic-hydrothermal Porgera Au deposit. The intrusive complexes have respective ages of ~3 Ma, ~1 Ma, and 7 Ma. Since these deposits are characterised by intense alteration, the interpretation of their geochemistry is ambiguous. However, a feature that also applies to the unmineralised intrusive complexes, their high K content, is likely to represent either small degrees of partial melting of mantle lithosphere or lithospheric delamination. Lithospheric delamination has previously been proposed in the formation of Grasberg. In this study, oxygen and hafnium isotope analyses of zircons from Ok Tedi are used to further constrain this model and test its applicability. Zircon oxygen isotope results range from 5.0 to 10.9 (±0.5, 1 s.d.) indicating mantle-derived magma contamination and/or hydrothermal alteration. Because of Ok Tedi’s young age, seismic tomography sections, which image the present seismic velocity structure below Papua New Guinea, can be used to constrain the geodynamic setting of this mineral deposit belt. Previously published low-resolution sections show no evidence for lithosphere having been subducted southward for at least 20 Myr, which is supported by the lack of a continuous high volume magmatic belt, characteristic of active subduction. How important then is subduction for the ore formation process? Newly constructed tomographic sections and new zircon isotope results will be discussed in this context.