Final Reports (Stage One Projects)

Project H4: 40Ar/39Ar geochronology and halogen/noble gas geochemistry of mineralising systems.

Introduction

The principal objective of the H4 Project was to evaluate existing techniques and develop new enabling technologies in geochronology and isotope geoscience to significantly improve our ability to constrain the timing of ore formation and, therefore, expand predictive capabilities.

A fundamental goal was the evaluation of existing dating methods and interpretations, particularly with respect to gold deposits in low grade terrains, and to investigate potential new dating methods such as 40Ar/39Ar laser probe analyses of mica-bearing pyrite linked to gold mineralisation. A second major goal of the H4 project was to initiate development of halogen/noble gas geochemistry technology and expertise within the pmd *CRC to enable this unique geochemical method to be applied to ore deposits in the flagship terrains in the second stage programme of pmd *CRC.

High Impact Results
Summary research Outcomes

The key achievements of the H4 Project have been the establishment of 40Ar/39Ar methods to reliably date mica-bearing pyrite grains and the development of instrumentation, expertise and analytical methods for halogen/noble gas analyses of minerals related to mineralisation systems.

Chapter one of the Final Report documents 40Ar/39Ar age data from the boundary between the Delamerian and Lachlan Orogens. This study showed a revised orogenic boundary in the Stawell Zone in western Victoria. The revised orogenic boundary has implications for the location of 440 Ma orogenic gold deposits in south and southeast Australia, as it implies that 440 Ma gold mineralisation may not be restricted to the Lachlan fold-belt.

Chapter 2 discusses the causes behind the discordance associated with 40Ar/39Ar dating of fine-grained mica.  In this study, the 40Ar/39Ar technique was applied to alteration within samples from various phases of alteration and gold mineralisation that are structurally well constrained within the Stawell gold deposit. The study highlighted the danger in relying too much on 40Ar/39Ar ages from wall rock sericite adjacent to gold lodes, even within structurally well-constrained systems such as Stawell.  It is concluded that the way forward in dating fine-grained micas is via the mica-in-pyrite method discussed in chapter 3.

In chapter 3, we test the hypothesis that pyrite is capable of armouring potassium-bearing mineral inclusions from alteration-induced argon loss, thus providing improved 40Ar/39Ar age results. It concluded that pyrite acts as a partially closed system for argon, but that the 40Ar/39Ar 'pyrite' dating method has the potential to 'see through' later thermal/alteration events and resolve controversial aspects of ore deposit geochronology.

Chapter 4 describes the noble gas and halogen degassing behaviour of quartz. In this study, samples were selected from the Eloise and Osborne, Iron Oxide Copper Gold (IOCG) ore deposits, and the Railway Fault, 13 km south of the Mt Isa Mine, in the Mt Isa Inlier. Quartz was found to exhibit a bimodal degassing profile with mechanical decrepitation of fluid inclusions accounting for the release of gas at temperatures of <700 (C. Changes in the Br/Cl, I/Cl, Ar/Cl and 40Ar/36Ar composition of gas released at different temperatures up to 700 ºC can be related to the decrepitation of different types of fluid inclusion.

The second mode of quartz degassing occurs between 1200 and 1450 (C and releases a greater volume of gas than the first mode. Several lines of evidence indicate that the gas released at high temperature is also from the fluid inclusion reservoir. However, its release may be triggered by a metastable phase transition of quartz (~1200 ºC) and caution is required in interpretation of the fluid compositions obtained at these temperatures.

Chapter 5 involves a study of the argon isotope systematics of quartz vein samples related to the Mt Isa copper mineralisation, and the Osborne plus Eloise iron-oxide-copper-gold (IOCG) deposits. The goal of this study was to ascertain the potential for 40Ar/39Ar dating of fluid inclusions in quartz. Results obtained demonstrate that, when fluid inclusions contain captive-mica impurities, excess 40ArE is strongly correlated with fluid inclusion Cl while radiogenic 40ArR is strongly correlated with K. 

The data define a plane in 3D 40Ar-K-Cl-36Ar space, which enables the determination of a robust 3D isochron.  However, we conclude that in this case the 1027 Ma age obtained for Mt Isa is related to thermal cooling and in these circumstances quartz ages do not always represent the age of quartz formation.  In contrast, fluid inclusions from Osborne and Eloise are K-rich but do not contain captive-mica. In these cases multiple stepped heating experiments constrained the maximum formation ages to a precision of 5% for Osborne and 15% for Eloise. 

The data indicate that quartz is retentive to Ar over billions of years.  However, where captive mica is present 40Ar-loss can occur from within the sub-reservoir into the surrounding fluid inclusion. The presence of mica impurities can be identified from sample degassing profiles and mean K/Cl values
of >1.

List of Partners/Sponsors

The team gratefully acknowledges the important contributions of sponsors:

* Please note that references and appendices are not published in this Final Report Summary. These can be found in the Final Report and available via the links below.

Full Report PDF32.6Mb

Appendices

  • Posters

  • Presentations

     

    For Further Information Contact

    Project Leader
    Dr David Phillips
    School of Earth Sciences
    The University of Melbourne
    Tel: +61 3 8344 7455
    Email: dphillip@unimelb.edu.au

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    Monash University University of Western Australia University of Melbourne James Cook University Geoscience Australia Victorian Institute of Earth and Planetary Sciences CSIRO Centre for Exploration Targeting AMIRA