|
Paul Henson, Geoscience Australia (Australia)
Richard Blewett, Geoscience Australia (Australia)
Terry Brennan, Geoscience Australia (Australia)
John Walshe, CSIRO (Australia)
|
|
The Archaean granite-greenstone terranes of the Yilgarn Craton host Australia's premier Au deposits. The region is data rich, with high-resolution gravity, magnetics, geological and geochemical maps and databases, augmented with a number of high-quality deep seismic reflection profiles, teleseismic and magnetotelluric studies. Geoscience Australia and the pmd*CRC have integrated these data into a series of new 3D maps at a range of scales, enabling a greater understanding of the giant Au mineral systems.
At the craton scale a tomographic survey was deployed to investigate whether anomalous crustal and/or upper mantle velocities exist beneath the highly mineralised regions compared to 'normal' velocities beneath barren regions in the Yilgarn Craton as a whole. The resulting velocity data were spatially represented in 3D using Gocad, enabling spatial links to be drawn between dramatic velocity variations in the tomographic data and zones of mineralisation in the upper crust.
At the crustal/upper mantle scale, 2D seismic reflection lines throughout the EGST have imaged crustal penetrating shear zones that are often spatially coincident with mineralised corridors. Multiple 2D seismic reflection profiles, combined with geological and potential field data, have enabled the sub-surface geometries of faults to be constructed in three dimensions, providing insight into potential fluid pathways. A 130 km long magnetotellurics survey located along the EGF01 seismic line was also used to explore the relationship between seismic architecture and the crustal/upper mantle-scale conductivity distribution.
At the camp-scale, 3D technologies have greatly improved our ability to compare complex relationships between spatially referenced 2D and 3D data, leading to improved targeting. The construction and display of the 3D distribution of lithological units and complex fault arrays, as surfaces and solid volumes, has enabled the 3D geology of a region to be statistically analysed in a quantitative way that is not possible using conventional techniques.
Mine- to camp-scale 3D faults, lithologies and alteration assemblages are used to define preferential zones of fluid flow and Au deposition. This process involves detailed structural analysis of mineralisation at mine-scale to determine stress vectors during mineralisation, combined with numerical modelling of preferentially orientated structures. Favourably orientated faults calibrated with known mineralisation and alteration assemblages provide a framework to statistically analyse the relationship between structural architecture, alteration signatures and mineral deposition.
3D maps provide a tool to analyse data at a range of scales and better integrate a wide range of datasets. These technologies enable spatial and temporal relationships to be compared in a more quantitative way and also provide a 3D interpretation of geology that can be statistically analysed for predictive mineral discovery.
|
