Year

2014

Degree Name

Bachelor of Science (Hons.)

Department

School of Earth and Environmental Sciences

Abstract

Zircons are best known as effective U-Pb geochronometers and more recently for their application to igneous and metamorphic inclusion suite petrology. A wide range of phosphate, oxide and silicate mineral inclusions in zircon have been studied but for reasons unknown, sulfide inclusions have been scarcely documented. This thesis explores the capacity of sulfide inclusions in zircon to preserve the metallogenic evolution of copper porphyry deposits, applied to porphyries in the Macquarie Arc, New South Wales. Zircons were extracted from the copper porphyries of Cadia Hill (Cadia), Endeavour 26 (Northparkes) and Copper Hill and observed by optical and electron microscopy techniques..

Globular inclusions predominantly of chalcopyrite-like compositions were found in zircon from both Cadia and Northparkes. These are interpreted to have been magmatic sulfide melts which became attached to zircon via surface tension and encapsulated as zircon continued to grow. U-Pb zircon age determinations of analysis sites interior and exterior to magmatic sulfide inclusions were within error of the mean ages of their respective zircon populations. A history of subsequent hydrothermal fluid evolution is preserved in zircon as a sequence of sulfides and sulfates, hydrothermally precipitated along fractures. No sulfide inclusions were identified in zircon from Copper Hill and it may be that sulfide saturation did not occur, or did so to a lesser degree than at Cadia and Northparkes.

Sulfide saturation is interpreted to have occurred as one or more temporally discrete events at Cadia and Northparkes. The observed magmatic sulfides are associated with copper, gold, silver and cadmium which is considered evidence of the strong chalcophile partitioning behaviour of sulfide melt phases. Magmatic sulfide melt globules may have heightened the availability of copper and other metals to hydrothermal fluids. A lack of sulfide saturation at Copper Hill may have limited the metal-budget and ore-forming potential of hydrothermal fluids.

Detrital sulfides are readily oxidised and destroyed yet may be preserved when included within stable minerals such as zircon. High temperature sulfide inclusions were found in detrital zircons downstream of Cadia Hill. These are interpreted to have been eroded from intra-plate volcanic rocks unrelated to any porphyry system. Sulfide inclusions in detrital zircon may be a useful greenfields exploration proxy of copper porphyry deposits, however further research is required to define discriminating features which reveal their geological setting of origin.

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