TSK 11 Göttingen 2006 Gessner et al.
 Strain Localisation, Fractur-
 ing and Hydrothermal Miner-
 alisation: Numerical Models
 of the Mount Isa Copper De-
 posit, Australia Vortrag
 Klaus Gessner1,2 P.A. Jones3
 A.S. Wilde4
 There is substantial need in mineral
 exploration to understand the struc-
 tural controls on ore deposition for these
 types of deposits in order to predict
 the localities of new ones. Applica-
 tion of basic principles of rock mechan-
 ics, and numerical simulations of defor-
 mation and fluid flow processes provide
 fundamental insights to Proterozoic hy-
 drothermal mineralization at Mount Isa,
 Australia. The rheology of layered
 meta-sedimentary rocks, and the ori-
 entation and position of these layered
 rocks relative to major fault systems
 were the key controls on ore deposition.
 Rock deformation is a crucial require-
 ment for creating fluid pathways and
 depositional sites in post-metamorphic
 hydrothermal ore systems, since meta-
 morphism creates a largely imperme-
 able wall rock. Compositional layering
 in the host rock partitioned mechani-
 cal behaviour and strain, leading to se-
 lective permeability generation and fo-
 cussing of fluid flow during separate hy-
 drothermal events. Differences in phys-
 ical property values between shale and
 siltstone lead to a significant variation in
 1 Computational Geoscience, CSIRO Explo-
 ration and Mining, PO Box 1120, Bentley WA
 6102, Australia 2 Earth Systems Modelling,
 School of Earth and Geographical Sciences,
 The University of Western Australia, Craw-
 ley WA 6009, Australia 3 School of Earth
 Sciences, James Cook University Townsville
 QLD 4811, Australia 4 School of Earth Sci-
 ences, Monash University Clayton, VIC 3400,
 Australia
 deformation behaviour and changes in
 deformation related permeability. From
 field and mine observations it is rea-
 sonable to assume that the shale lay-
 ers had less cohesion and a lower fric-
 tion angle, but higher tensile strength
 than the siltstone layers. A layering of
 these rock types is thus likely to lead to
 a partitioning effect known as shear-lag,
 which occurs when ongoing stretching
 in the weak phase of a two-layer com-
 posite reduces s3, resulting in a pref-
 erential partitioning of tensile stresses
 into the strong phase. This rheologi-
 cal contrast may be the reason why a
 number of large copper and lead-zinc-
 silver deposits in the Mount Isa area
 are hosted by a single rock unit, the
 Urquhart shale. According to our model
 fine-grained carbonaceous shale layers
 preferentially failed by plastic shearing,
 whereas meta-siltstones remained elas-
 tic or failed in tension, depending on
 the magnitude of deformation and the
 pore fluid pressure. If lead-zinc silver
 mineralization is assumed to have oc-
 curred early orogenic or late syngenetic
 in consolidated and lithified sedimen-
 tary rocks, the pelitic layers of the
 Urquhart shale would have deformed
 plastically, becoming more permeable
 than elastically deforming siltstones. A
 second hydrothermal event occurred af-
 ter the metamorphic peak of the (ca.
 1590–1550Ma) Isan orogeny. Numerical
 simulations suggest that during this late
 orogenic event the orientation of layer-
 ing and the proximity to major fault sys-
 tems controlled fracturing and perme-
 ability increase in the Urquhart shale.
 Deformation patterns similar to the ob-
 served extent of dilation and brecciation
 occur in the case of E–W shortening and
 top-to-E simple shearing. These geome-
 tries correspond to the second and third
 1
Gessner et al. TSK 11 Göttingen 2006
 deformation events recognized at Mount
 Isa, during which meta-siltstone layers
 in the Urquhart shale failed in tension
 and massive silicification occurred. The
 fractured siltstones provided pathways
 for an upward flowing, over-pressured
 basement fluid, from which quartz was
 deposited during cooling. An oblique
 strike-slip strain geometry correspond-
 ing to the fourth major deformation
 event, localized strain in steeply-dipping
 pre-existing fault zones, and again, in
 the mechanically anisotropic Urquhart
 shale. The reactivation of steep struc-
 tures provided access to surface de-
 rived fluids during a third hydrother-
 mal event, causing the precipitation of
 dolomite followed by chalcopyrite ore.
 The change from regional contraction
 and simple-shear to strike-slip changed
 the hydraulic architecture significantly,
 favouring upward flow, lithostatic fluid
 pressures and hydro-fracturing in the
 former case, and access to surface-
 derived fluids in the latter. These
 findings support a sequential model for
 copper mineralization at Mount Isa,
 where alteration from a reduced base-
 ment fluid preceded introduction of a
 surface-derived oxidized metal-bearing
 brine.
 2