Mr Oleg Koudashev

PhD research title: Large-Scale Structure and Geochronology of Porphyry and Epithermal Deposits Along the Northern Collisional Margin of Australia

My PhD focussed on ⁴⁰Ar/³⁹Ar geochronology and tectonics of porphyry and epithermal deposits. My study sites were located in Papua New Guinea and Sulawesi, Indonesia, which are both some of the most tectonically active areas of the world and rich in mineral deposits. Below is an outline of several key results from my thesis.

Dating mineralisation at the Yandera porphyry

K-feldspar apparent age spectrum from the Bismarck Granodiorite with propylitic alteration seen in the microphotograph as chlorite veins with epidote selvages

The age of the hydrothermal system at the Yandera porphyry, located in the northern highlands of Papua New Guinea, was established to be 3 Ma. This is younger than lithologies observed in drill core or at the surface aged 7.1-6.3 Ma, suggesting that mineralisation in Yandera is not genetically related to the Yandera Porphyies suit of intrusives. This makes Yandera a detached/wallrock porphyry. Conducting argon geochronology via diffusion experiments was shown to pick up the age of hydrothermal overprinting and provide an age for mineralisation which was not picked up by earlier U-Pb geochronology. (U-Th)/He geochronology shows that mineralisation at Yandera occurred in the latest stages in the exhumation of the northern New Guinea Highlands.

Exhumation of the northern New Guinea Highlands

Apparent age spectra of plagioclase feldspar from deformed metadiorite and white mica from deformed metapelite from the lower plate of the extensional shear zone to the north of the Bismarck Granodiorite

A sliver of metamorphic rocks along the northeast margin of the Bismarck Granodiorite have been mapped as the Early Cretaceous Goroka Formation located to the east. My research shows that both the metamorphic assemblage and ⁴⁰Ar/³⁹Ar geochronology suggest that these are the result of contact metamorphism. They are juxtaposed against unmetamorphosed Eocene Asai Beds by an extensional shear zone with the hot granodiorite and contact metamorphics in the lower plate and the cold Asai beds in the upper plate.
⁴⁰Ar/³⁹Ar geochronology constrains this shear zone to about 12-8 Ma, the same timing as metamorphism and intrusion of the granodiorite, showing that the granodiorite was being emplaced in the lower plate of an extensional system. Low temperature (U-Th)/He geochronology shows the late stages of exhumation of the northern New Guinea Highlands occurred between 5.9-2.9 Ma cooling at an estimated rate of 30-35°C/Ma.

Tectonic Setting of Porphyry deposits

The geometry of subducted lithosphere beneath Yandera (red dot) viewed from top and east

Interpretation and visualisation of the distribution of earthquake hypocenters shows that the Yandera deposit and deposits in the North Arm of Sulawesi are located above portions of subducted lithosphere adjacent to tears. This supports earlier work in the Structure and Tectonics Group by Gordon Lister and Tomas O’Kane showing the importance of tears and slab structures for controlling the location of porphyry deposits. 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.