Foliation boudinage structures at the Mount Isa Cu Deposit, Australia

Williams, Benjamin J. 2023. Foliation boudinage structures at the Mount Isa Cu Deposit, Australia. PhD Thesis, Cardiff University. Item availability restricted.

Preview	PDF - Accepted Post-Print Version Download (102MB) | Preview
	Archive (ZIP) (Appendices) - Supplemental Material Download (24MB)
	PDF (Cardiff University Electronic Publication Form) - Other Restricted to Repository staff only Download (91kB)

Abstract

Copper (Cu) is increasingly important as the global economy becomes more reliant on renewable energy sources. The Mount Isa Inlier, NW Queensland, is one of Australia’s most important metallogenic provinces, hosting several deposits of Cu, Pb, Zn, Ag and U, including the famous world-class Mount Isa deposit. The Mount Isa Cu-Pb-Zn-Ag deposit is the second largest producer of Cu in Australia, yet the formation, geometry and timing of the mineralisation is still debated. One reason for this lack of consensus is that Mount Isa has undergone a long and complex deformation history with north-south shortening (D1), east-west shortening (D2), top to the east shear (D3), ENE-WSW shortening (D4a), WNW-ESE shortening (D4b), and east-west shortening (D5a and D5b). Recent studies at Mount Isa have suggested that Cu mineralisation initiated in D4a and proceeded through to the main Cu mineralisation in D4b, where the orebodies have a strong structural control on their geometries. Foliation boudinage structures, which form in anisotropic and homogeneous rocks, have been identified in the Mount Isa Cu orebodies. Confusions in terminology that exist in the foliation boudinage literature have required a detailed review of the literature leading to a synthesis of current ideas. The formation and development of foliation boudinage structures have previously been modelled in 2-dimensions through analogue and computer techniques. However, no previous studies have investigated the formation and evolution of natural foliation boudinage structures in 3-dimensions. This study combines structural observations, kinematic analysis, petrography, and 3-dimensional modelling of foliation boudinage structures using computerised tomography (CT) and serial sectioning, to constrain their timing and importance in the mineralisation at Mount Isa and to infer the processes involved in their evolution. Small scale foliation boudinage structures were sampled in drill core proximal to the Mount Isa Cu deposit and their orientations were recorded. Most drillholes analysed at Mount Isa were unoriented, however, foliation boudinage structures have been reoriented using new methods developed in this study. Desurveying methods were first compared to find the most precise method for plotting the drillhole pathway using examples from Mount Isa. New computer code was developed to determine the orientations and positions of drillholes along their entire length using the existing desurveying methods. The drillhole survey measurements given by the computer codes allow the original orientations of the foliation boudinage structures to be determined. Foliation boudinage structures occur most commonly in the unaltered Urquhart Shale where the anisotropy and homogeneity provided by the shale layering is intact. Detailed petrographic analysis of the foliation boudinage structures has identified that an initial rim of quartz and dolomite was followed by infill and replacement by later sulphides. The dominant sulphides in the foliation boudinage structures are pyrrhotite and minor chalcopyrite, both associated with the Cu mineralisation at Mount Isa. The paragenesis of the foliation boudinage structures is consistent with the established paragenesis of the main Cu mineralisation. The foliation boudinage structures plunge gently to the north and south as a result of approximately layer-normal shortening and layer-parallel extension of the steeply west-dipping Urquhart Shale. North-plunging structures formed by ENE-WSW dextral-reverse shortening during the D4a deformation event and the south-plunging structures formed by WNW-ESE sinistral-reverse shortening during the D4b deformation event. Electron Backscatter Diffraction (EBSD), combined with 2-dimensional and 3-dimensional strain analyses show that the ore-related sulphide mineral infills of foliation boudinage structures at Mount Isa have undergone significant intracrystalline deformation. Pyrrhotite grains show well-developed grain shape and crystallographic preferred orientations, with c-axes normal to the shale layers. The strain ellipsoid shows a change from plane to flattening strain, indicating a progressive deformation during foliation boudinage. Previous studies have suggested that the foliation boudinage structures remain as fluid filled voids throughout their development. However, strain of the pyrrhotite infill, which has a low strength under the conditions of deformation, indicate that the voids were mostly filled during the final stages of foliation boudinage. 3-dimensional modelling of foliation boudinage structures suggest a model of propagation along their long-axes, with the oldest sections, greatest displacements, and most complex mineral phase boundaries in the central part of the structure, in a similar fashion to faults. Stages identified in the evolution of foliation boudinage structures at Mount Isa, are: 1) dolomite replacement of unaltered shale, 2) fracture and cleavage development, 3) exploitation of existing discontinuities by the developing structures during shortening, 4) silica-dolomite infill and 5) final infill with ore related sulphides. As the foliation boudinage structures at Mount Isa exploit existing discontinuities, fluid pressures during foliation boudinage are likely to be lower than during other types of foliation boudinage developed within undeformed host rocks. This study, along with recent studies at Mount Isa, contrast to previous ideas about the Cu orebodies, and enable an improved explanation for their geometries, kinematics, and timing. This study supports the most recent ideas that the structural controls on the Mount Isa Cu orebodies resulted from epigenetic mineralisation at the time of ENE-WSW and WNW-ESE shortening of the Urquhart Shale during D4 deformation. This relatively late timing of foliation boudinage and Cu mineralisation in the deformation sequence is in contrast to the syngenetic and early timing of Cu mineralisation proposed by some previous authors. The strain and kinematics expected in both D4a and D4b are recorded in the formation and evolution of foliation boudinage structures at Mount Isa. The drill core scale structures proximal to the Cu orebodies may be used as a tool for vectoring towards mineralisation at the Mount Isa deposit. Foliation boudinage structures likely impart a structural control on the Cu mineralisation at the scale of the orebodies and may control the geometries of other deposits in similar rock types.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Funders:	NERC Grant NE/L002434/1
Date of First Compliant Deposit:	25 May 2023
Last Modified:	26 May 2023 09:08
URI:	https://orca.cardiff.ac.uk/id/eprint/159984

Actions (repository staff only)

Edit Item