Mechanical localisation of copper deposits in the Mt Isa Inlier, North Queensland

Keys, Damien Leslie (2008) Mechanical localisation of copper deposits in the Mt Isa Inlier, North Queensland. PhD thesis, James Cook University.

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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.

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.

Item ID: 29144
Item Type: Thesis (PhD)
Keywords: mineralisation; Mt Isa Inlier; North Queensland; Mt Gordon copper mines; Mt Isa copper mines; Mt Kelly copper mine; Ernest Henry copper-gold mine; Greenmount copper-gold deposit; structural controls; mechanical factors; structural framework; copper mineralisation; geodynamic history; stratographic sequence
Date Deposited: 05 Sep 2013 22:20
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 33%
04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 34%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 33%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 50%
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