Untitled Document
Earthquakes, aftershocks and gold deposits
Stephen Cox
Mesothermal lode gold systems are a major global gold resource. This deposit type is particularly well-developed in Archaean greenstone sequences such as the Yilgarn Craton in Western Australia, the Superior Province in Canada, and in Zimbabwe. They also occur in some Phanerozoic terrains, such as the Bendigo zone of the Lachlan Fold Belt in Australia, the Meguma region of the Appalachian fold belt, and in Mesozoic to Tertiary circum-Pacific orogens. In these terrains, the deposits tend to form at depths of 8 km to 20 km in post-peak metamorphic regimes in fold and thrust belts.
Mesothermal gold deposits are typically located in low displacement faults and shear zones. Gold deposition within these structures requires large fluid fluxes to be loclised within them while they were deforming, typically at depths within the continental seismogenic regime, or just below the seismic-aseismic transition.
The large-scale factors controlling localisation of fluid flow to restricted parts of crustal-scale shear systems are not well understood. Two intriguing aspects of the development of mesothermal gold systems are:
- they tend to develop predominantly in low displacement faults and shear zones adjacent to to much larger shear zones, rather than in the major, crustal-scale shears;
- mesothermal gold deposits tend to form in clusters of deposits in localised domains along small segments of volumetrically more extensive shear systems.
By analogy with the distribution of slip in modern seismogenic systems, the low displacement faults which host mesothermal gold deposits are interpreted as aftershock structures whose development is related to stress changes driven by repeated large slip events on the nearby, crustal-scale shear systems. Local permeability enhancement in low displacement aftershock networks seems to play a key role in controlling the architecture of fluid flow and related gold deposition in crustal-scale hydrothermal systems.
Analysis of aftershock distributions in modern seismogenic systems indicate that aftershocks cluster in restricted regions around mainshock ruptures, and that this distribution is influenced by mainshock rupture geometry, slip directions and magnitude, and the orientations of the regional stress field. 3D finite element modelling (Coulomb stress transfer modelling, or STM) of stress changes associated with large earthquake rupture events indicate that aftershocks occur preferentially in domains where stress changes due to the mainshock have moved the stress state closer to failure.
ST modelling is being tested in the WA goldfields for its potential as a tool to predict the distribution of low displacement faults and shear zones, and associated domains of high fluid flux in ancient, gold-hosting shear systems. A more robust understanding of the factors which may lead to localisation of fluid flow and fluid-rock reaction within crustal-scale shear networks has application in the development of more advanced area selection strategies in exploration programs targetting mesothermal gold systems. A basic understanding of these processes will also find application for understanding the evolution of other epigenetic mineralisation styles whose development is also influenced by growth of fault and fracture systems (eg. some epithermal systems, magmatic-related systems, and Carlin systems).
The distribution of gold deposits near Kambalda, in Western Australia, is being used to examine potential relationships between slip on the crustal-scale Boulder-Lefroy fault system and the localisation of gold deposits in the St Ives goldfield. The distribution of aftershock domains is particularly sensitive to the location of rupture arrest sites on mainshock structures. Static stress changes have been modelled on the basis that mainshock arrest was repeatedly localised on a kilometre-scale contractional jog on the Playa Fault the St Ives area. The modelling indicates that strike-slip, reverse and normal faults located immediately north-west of the Victory jog are all brought closer to failure by sinistral mainshock on the Boulder-Lefroy-Playa system. A remarkable correlation exists between the modelled domain of increased Coulomb faulure stress and the distribution of gold deposits in the St Ives goldfield (Figure 1). Proximity to failure is significantly increased for distances up to 10 km north-west of the Victory jog.
Significantly, STM indicates that deposits hosted by low displacement "aftershock" faults can form in domains of Coulomb stress increase which are up to 15 km away from the structures which localise mainshock arrest. So stress transfer modelling indicates that exploration should not just focus on the immediate vicinity of jogs or bends on large displacement structures.
With the support of an ARC-Linkage grant, and in collaboration with a number of minerals industry partners, including The Australian Minerals Industry Research Association (AMIRA), the project will be expanded in 2002-2003 to undertake further detailed case studies elsewhere in the Yilgarn Craton in WA and in the Abitibi Belt in Canada.
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Figure 1. a. Map illustrating part of the area covered by stress transfer modelling in the St Ives goldfield, WA. Position of the modelled slip patch on the Boulder-Lefroy-Playa fault (BLPF) is indicated, together with locations of gold deposits and prospects. The modelled sinistral slip patch on the BLPF has a northern termination at the Victory complex, an imbricate thrust array in a contractional jog on the BLPF. b. Distribution of modelled Coulomb stress changes for strike-slip aftershock structures in the St Ives Goldfield. There is a strong correlation between gold occurrence and lobes of increase of Coulomb failure stress.
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