Evolution and dynamics of fault controlled copper mineralization in the Mount Gordon fault zone, Mt Isa, Queensland, Australia

Nortje, Gustav Stefan (2009) Evolution and dynamics of fault controlled copper mineralization in the Mount Gordon fault zone, Mt Isa, Queensland, Australia. PhD thesis, James Cook University.

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Fluid flow leading to mineralization can occur on both newly formed faults, and on faults that are reactivated subsequent to their initial formation. Conventional models of fault reactivation propose that, under high pore pressures, misoriented faults may reactivate due to low fault cohesion. Timing and orientation data for a mineralized Paleo- to Mesoproterozoic terrain indicate that multiple successive new orientations of predominantly strike slip faults developed (between 1590 and ~ 1500 Ma), requiring that faults either had high cohesion and/or that pore pressures were not high enough to induce reshear. Evidence for high pore fluid pressures on the young fault set include jigsaw-fit dilatant breccias, hypogene copper mineralization in veins and breccia infill, and subvertical tensile quartz veins aligned sub-parallel to σ₁.

The generation of the new faults in previously faulted rock masses indicates that the older faults developed sufficient cohesion that the younger deformation did not affect the older faults. Many older faults display prominent quartz blows but also locally are marked by recessive valleys with some exposure of fault gouge or clay-filled breccia. The assumption that old faults consistently have little or no cohesion appears to be incorrect in this terrain. Furthermore, faults with high cohesion may have acted as barriers and compartments, so that intersections between them and newly formed faults host mineralization, not because of reactivation, but because of interaction between new faults and cohesive materials defined either by fault precipitates or rock juxtaposition.

The Mount Gordon Fault Zone (MGFZ) comprises a c.a. 150 km long zone of anastomosing mainly NNE-trending major faults, but in two notable locations (Mammoth and Investigator fault zones), curvilinear steep east-trending faults intersect the main fault zone. Exploration models for the region have focussed primarily on direct Mount Isa (black shale) analogues rather than integrated structural targeting. The most brecciated and mineralized segments in the Mt Gordon copper field occur along the Mammoth Fault where it bends from an ENE trend northwards to connect to the ENE trending Mammoth Extended Fault. Using veins, slickenlines, offsets and orientations of fault segments and applying fault slip analysis techniques the most likely far field stresses at the time of strike slip faulting and mineralization were aligned with σ₁ approximately 110° termed here D₄.

To test the veracity of this result, different tools were applied to the dataset to assess correlation between areas of known mineralization and predicted zones of mineralization. Three-dimensional fault slip tendency analyses indicate that for an applied stress field with different stress ratios slip along favourably oriented faults will occur. Prospectivity analyses indicate that fault intersections are important even when fault orientation, fault connectivity and proximity to source are included or removed from the analysis. Favourable targets are where optimally oriented faults intersect misoriented faults because these intersections create zones of high pore fluid pressure resulting in brecciation consequent on multiple rupture events. Discrete element modelling at known deposits and prospects indicate a good correlation between predicted and known mineralized zones, although pressure/stress gradients appear to be more important than actual low values of differential and mean stress, possibly reflecting the need for a fault capable of transmitting copper-bearing fluid (e.g. during extensional shear) coupled with a pressure drop at a site of tension to induce precipitation. Fault intersections are likely to be zones of enhanced permeability provided the right pressure – temperature – chemical conditions are met for mineralization to occur. The results from these analyses can be utilised in exploration to further enhance the chances of a significant discovery within the region. Together, these results and observations provide new, simple tools to stimulate copper exploration in fault hosted terrains.

Item ID: 31509
Item Type: Thesis (PhD)
Keywords: copper; exploration; faulting; fault cohesion; reactivation; Mt Isa; Mt Gordon
Date Deposited: 06 May 2014 03:02
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 100%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 100%
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