ResearchOnline@JCU This file is part of the following reference: Keys, Damien Leslie (2008) Mechanical localisation of copper deposits in the Mt Isa Inlier, North Queensland. PhD thesis, James Cook University. Access to this file is available from: http://eprints.jcu.edu.au/29144/ The author has certified to JCU that they have made a reasonable effort to gain permission and acknowledge the owner of any third party copyright material included in this document. If you believe that this is not the case, please contact [email protected] and quote http://eprints.jcu.edu.au/29144/ Mechanical localisation of copper deposits in the Mt Isa Inlier, North Queensland Thesis submitted by Damien Leslie Keys BSc (Hons) (University of Melbourne) in February 2008 for the degree of Doctor of Philosophy in the School of Earth and Environmental Sciences at James Cook University of North Queensland Damien L Keys i STATEMENT OF ACCESS I, the undersigned, author of this work, understand that James Cook University will make this thesis available for use within the University Library and, via the Australian Digital Theses network, for use elsewhere. I understand that, as an unpublished work, a thesis has significant protection under the Copyright Act and; I wish this work to be embargoed until 1st March 2009. Signature Date Damien L Keys ii STATEMENT OF SOURCES I declare that this thesis is my own work and has not been submitted in any form for another degree or diploma at any university or other institution of tertiary education. Information derived from the published or unpublished work of others has been acknowledged in the text and a list of references is given. Signature Date Damien L Keys iii STATEMENT OF CONTRIBUTIONS General contributions towards this study have included: Australian Postgraduate Award (APA) scholarship pmd*CRC top-up scholarship pmd*CRC educational and training fund Damien L Keys iv AKNOWLEDGMENTS I would like to thank my principal supervisor and mentor, Nick Oliver for his patience, guidance and sense of humour. There have definitely been some funny times, particularly in the field. This project was initiated by Nick and Xstrata Exploration geologists Dugi Wilson and Paul Gow. Several Xstrata Copper geologists are thanked for facilitating underground visits, accommodation and discussion. They include Ian Holland, Trev Shaw, Nath Bullock, Frank Santaguida, Tim Raggett, and Dwayne Povey and Karl Malitz (Ernest Henry). A substantial amount of time was spent at the Aditya Birla Mt Gordon Mine and geologists Steve Oxenburgh, Martin Bennett, Craig Irvine and Simon Tear are all thanked for their support and comments. Copperco have also contributed largely to this project. Many thanks to Paul Hogan, Ken Maiden and Rosie Da Costa for their input. Thanks also to the Copperco fieldies who did a fantastic job of fishing a dinged up Hilux out of Borstad Creek. Finally thanks to Matrix Metals (Greenmount) for access to the prospect and data. Bob Dennis, Tony Alston and Bryan Frew are thanked for their thoughts and comments on the findings of this study. I have also been fortunate enough to complete this PhD as part of the pmd*CRC I7 project, lead by Barry Murphy. Thanks to the pmd*CRC and in particular Barry, Bob Haydon, Russell Korsh and Andy Barnicoat for encouraging student participation and providing funding to attend research meetings. I thank the other CRC researchers and contributors for their input into the PhD at various project delivery meetings. Many thanks to James Cleverly and John Walshe for challenging me to think in terms of isotopes, fluids sources and pathways and of course the “black juice”. Unfortunately the thesis doesn’t contain the quite the geochemical bent you may have hoped for but all is not lost! John Miller is also thanked for his contribution to the Mt Isa genetic story. His thorough mapping and analysis paved the way for the modelling conducted at Mt Isa in this study. Thanks go to John McLellan, Tom Blenkinsop and Mike Rubenach for fielding modelling, structural and geochemical questions at a moments notice. I’m sure without your assistance this project would have taken considerably longer to complete and I am extremely grateful. Damien L Keys v Office mates Gustav Nortje, Jim Austin and Kris Butera have ensured that there was never a dull moment. Thanks for being the sounding boards for some of the lesser ideas that have arisen, for your help in the field (at times) and for your friendship. To the EGRU PhD students (Dave, Louise, Julie, Arianne and Mark) of the last three years thanks also. Last but most certainly not least, I’d like to thank my beautiful wife. Although not at all geologically inclined, she has never ceased to be totally supportive and caring, and was an amazing help with the drafting and checking references. Damien L Keys vi ABSTRACT Copper mineralisation in the Mt Isa Inlier is typically associated with brittle deformation that has taken place during the ESE-WNW oriented D4 event of the Isan Orogeny. Distinct element modelling of deposit-scale fault arrays has provided an extraordinary tool in the elucidation of far-field stresses and prime conditions for rock failure associated with mineralisation. Not only does the deposit-scale modelling results match well with regional-scale models, they provide a future way in which prospectivity analysis can be improved. With only one known exception (Osborne) the bulk of the copper mineralisation in the Mt Isa Inlier is aged between 1530 Ma and 1505 Ma. The only age constraints on the D4 deformation event are from the Snake Creek Anticline (foliation development associated with the intrusion of the Saxby Granite) and from the dating of the specific mineral deposits e.g. Ernest Henry. The Snake Creek age of D4 is 1527±4 Ma and age dates from mineralised structures within the individual deposits vary from 1530 Ma to 1505 Ma. The typically brittle D4 deformation event occurs late in the Isan Orogeny and is only seen to have a ductile character in deformation around the intruding granites of the Williams and Naraku batholiths. In these locations D4 can be seen expressed as a ductile foliation and in limited sites is associated with porphyroblast growth. The D4 deformation event, when associated with economic copper grades, is typically brittle and as such requires specific mechanical conditions. Most importantly, it appears that temperature was a primary control, such that if temperatures exceeded the onset of quartz creep (350 ºC), brittle failure did not occur and conditions for mineralisation were inappropriate. Ar-Ar dating of minerals appears to be a useful way of discriminating between rocks that are likely to fail via brittle modes versus brittle-ductile or ductile conditions. Consequently it is apparent that most of the copper deposits are found in areas where biotite Ar-Ar ages indicate the rocks had cooled sufficiently to behave brittley at around 1530 Ma. Conversely, rocks adjacent to granites or otherwise showing protracted cooling histories (e.g. Ar-Ar in hornblende younger than 1520 Ma) did not cool to brittle conditions quickly enough to allow the coincidence of brittle deformation and copper mineralisation. Damien L Keys vii The host rocks to mineralisation are typically at depths of 5 to 8 km at the time of mineralisation. In a hot orogen, this implies that these rocks are nearly at the brittle-ductile transition themselves. Earthquake nucleation is known to occur at depth near this transition. Sibson (1989) proposed that rupture nucleation and the direction of earthquake rupture propagation could be a useful tool in prediction of dilatant sites and hence prospective sites for Mt Isa style copper mineralisation. An example of how this technique could be used was conducted on the Mt Gordon Fault Zone. The D4 mineralisation event also coincides with cratonisation of the Mt Isa Inlier and a large change in the apparent polar wander path. This may have been triggered by collision of Baltica with the Laurentian supercontinent, causing a switch in convergence along the Laurentia-NAC boundary and providing the right conditions for low strain strike slip deformation and the ingress of copper-bearing fluids into the exhuming Isan orogenic belt. Damien L Keys viii 1 INTRODUCTION................................................................................................1 PART 1 – THESIS OVERVIEW AND REGIONAL GEOLOGY .........................1 1.1 Project Overview...........................................................................................1 1.1.1 Thesis Aims ............................................................................................1 1.1.2 Research Methodology...........................................................................2 1.2 Overview of Mt Isa Geology .........................................................................3 1.2.1 Sedimentology........................................................................................3 1.2.2 Magmatism.............................................................................................5 1.2.3 Metamorphism and Metasomatism........................................................7 1.2.4 Structure...............................................................................................12 PART 2 – ANALYTICAL METHODS ...................................................................14 1.3 Structural Analytical Techniques.................................................................14 1.3.1 Overview ..............................................................................................14