We use passive seismic and gravity data to characterize the crustal structure and the crust-mantle boundary of the east Albany-Fraser Orogen in Western Australia, a Proterozoic orogen that reworked the southern and southeastern margin of the Archean Yilgarn Craton. The crustal thickness pattern retrieved from receiver functions shows a belt of substantially thickened crust that follows the trend of the orogen, but narrows to the southwest. Common conversion point profiles show a clear transition from a wide, symmetric Moho trough in the northeast to a one-sided, northwestern Moho dip in the southwest, where the Moho appears to underthrust the craton towards its interior. This change appears to coincide with the inferred trace of the Ida Fault, a major terrane boundary within the Yilgarn Craton. Forward modelling of gravity anomaly data using the retrieved Moho geometry as a geometric constraint shows that a conspicuous, elongated gravity low on the northwestern side of the eastern Albany-Fraser Orogen is almost certainly caused by thickened Archean crust. To obtain a model that resembles the regional gravity pattern the following assumptions are necessary: high-density rocks occur in the upper crust of the Fraser Zone, at depth inside the Moho trough and in parts of the eastern Nornalup Zone east of the Moho trough. Although our gravity models do not constrain at which crustal level these high-density rocks occur, active deep seismic surveys suggest that large extents of the east Albany-Fraser Orogen's lower crust include a Mesoproterozoic magmatic underplate known as the Gunnadorrah Seismic Province. The simplest interpretation of the imaged crustal structure is that the Gunnadorrah Seismic Province is underthrust beneath the Yilgarn Craton, likely as a consequence of crustal shortening during accretion further east. The imaged geometry overall appears to show a wedge of Archean lower crust that was driven between the exhumed Fraser Zone and the Gunnadorrah Seismic Province, effectively splitting the Paleoto Mesoproterozoic crust of the east Albany-Fraser Orogen. The vertical splitting of Proterozoic crust by a cratonic crustal wedge, comparable to what we image in this study, may be a process that contributed to forming many craton margins around the world.

The amphibolite-facies, Au-mineralized mafic rocks at the Plutonic Gold Mine are intruded by a suite of dolerite dykes of unknown age. The zones between these intrusive units often host significant Au mineralization. It is unclear whether this enrichment in Au mineralization is a function of the intrusion of the dolerites themselves or the influence of pre-existing structures (e.g. faults or shears). Geochemical characterization of the different microcrystalline dolerite units is important to the understanding of the structural architecture of the deposit and to the possible relationship of the dolerites to Au mineralization. The collection of a large geochemical dataset ( n = 497) from the dolerite dykes from across the deposit using portable X-ray fluorescence technology allows us to break them into four distinct geochemical groupings. Thus we can define their geometries with greater confidence than was possible using lithology alone. Traverses across individual dolerite dykes indicate that the chill margins are the most geochemically homogenous and most likely to represent the chemistry of the source magma. Plots of Ti v. Zr combined with principal component analysis (PCA) define four geochemically distinct suites of dolerites. By applying this understanding to dolerites in a small area of the deposit, a new interpretation was generated whereby significant amounts of rock that were previously modelled as being dolerite were reclassified as potential host-rock, thus increasing the potential for Au in this area.

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SUMMARYThis paper highlights the complimentary potential field studies that have been done in parallel to the interpretation of the 13GA-EG1 Eucla-Gawler deep crustal reflection seismic line. Gravity and magnetic images have been interpreted and potential field data has been modelled using edge detection, forward modelling and inversions to pick out the main domains and structures. Seismic, MT and drill core analysis has been progressing in parallel to the potential field investigations. The different approach taken here was to allow more freedom and independence in the interpretations originating from the potential field studies, rather than constraining them with a predefined architecture from the seismic interpretation. Initial results show gravity and magnetic worms correlating with interpreted structures and domain boundaries. Inversions show the 3D distribution of magnetic susceptibility and densities associated with major features such as the Mundrabilla Shear Zone and folded feature seen in the Nawa Domain. This paper summarises the main findings from the potential field studies, which, in conjunction with the parallel studies, allows for a more robust understanding of the crustal architecture and assessment of the mineral potential of the region.

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