Large-Scale Structure and Geochronology of Porphyry and Epithermal Deposits Along the Northern Collisional Margin of the Australian Continental Lithosphere

Abstract

In this thesis I combine 40Ar/39Ar geochronology and tectonics to investigate porphyry and epithermal deposits along the northern collisional margin of the Australian continental lithosphere using case studies on the islands of New Guinea and Sulawesi. The tectonic evolution of these islands reflects the effects of progressive northward movement after rifting off from Gondwana, with this movement involving interaction with multiple subduction zones and several accretion events. The island of New Guinea contains a number of world-class porphyry deposits forming the New Guinea copper-gold belt, including the world’s largest gold producer Grasberg. In comparision, the island of Sulawesi hosts multiple small and medium sized deposits but no major porphyries like New Guinea. The work reported here established the age of the hydrothermal system at the Yandera porphyry, located in the northern highlands of Papua New Guinea. The 3 Ma age obtained is younger than lithologies observed in drill core, or at the surface, suggesting that mineralisation in Yandera is genetically unrelated to the Yandera Porphyry suite of intrusives, making Yandera a detached/wallrock porphyry. Argon geochronology using step heating diffusion experiments was shown capable of identifying the age of hydrothermal overprinting, and providing an age for mineralisation U-Pb geochronology from an earlier study showed the porphyry suite to be 6.3-7.1 Ma. The age of ductile fabrics, interpreted to be related to an extensional shear zone, and metamorphism in the Bundi Fault Zone, north of the Yandera deposit, was established to be 8-13 Ma, which is coeval with the emplacement of the Bismarck Intrusive Complex and smaller satellite intrusives. These ages were much older than the mineralisation at Yandera. Thus the surface geology, except for younger fauts, was largely irrelevant to the formation of the deposit, except in respect to ground preparation. Geochronology of the Tombulilato, Tapadaa and Gunnung Pani deposits on the North Arm of Sulawesi also produced very young ages ranging from 5.5-1.8 Ma. Similar to Yandera,the Tapadaa porphyry deposit was also found to be hosted in an igneous intrusion that was older than the age of mineralisation, placing Tapadaa in a similar time period to the nearby epithermal deposits in Tombulilato. This thesis also reports the analysis of earthquake hypocenters and seismic tomography datasets, used to interpret the geometry and structure of subducted lithosphere beneath the areas of interest. The Yandera deposit and deposits in the North Arm of Sulawesi were found to be located above portions of subducted lithosphere adjacent to slab tears. Deposits were also found to be associated with structures and processes that should allow the fast ascent of magmas through the crust such as major strike-slip faults and uplift. Formation of the deposits of Tombulilato and Tapadaa was interpreted to occur as a result of north-directed subduction and rollback of the Molucca Sea Slab. Formation of the Yandera deposit was interpreted to occur due to tearing of subducted lithosphere as the result of oblique collision with the Finisterre terrane. The formation of these deposits was proposed to coincide with initiation of movement on nearby major strike-slip faults. (U-Th)/He geochronology showed that the Yandera deposit formed during a period of rapid denudation/exhumation of the New Guinea Highlands. Analysis of published ages conducted in this thesis for deposits in the Southwest Pacific indicates that the temporal distribution of these deposits is not random, suggesting region-scale controls on the formation of such deposits. This implies large scale tectonic and/or mantle processes play a part in the formation of such deposits. Possible geodynamic scenarios are discussed in the final chapter. Show more