Palaeoproterozoic to Mesoproterozoic Geology of North Queensland IW Withnall1, NL Neumann2 & A Lambeck2 1 Geological Survey of Queensland, Department of Employment, Economic Development and Innovation 2 Geoscience Australia The Palaeoproterozoic to Mesoproterozoic rocks of north Queensland that crop out in the Georgetown, Yambo and Coen Inliers (Figure 1) are the most easterly rocks of this age in Australia. They are important to an understanding of the evolution of the continent and possible configurations of Rodinia. Most models for the evolution of the North Australian Craton assume Georgetown and the other inliers to be a part of it, although usually have given little thought to how they might fit in the model.

Tholeiitic intrusion-hosted nickel sulphide deposits are highly sort exploration targets due to their potential size and co-products platinum-group elements and copper. The Norilsk-Talnakh (Russia), Voisey's Bay (Canada) and Jinchuan (China) deposits are world class examples. Although Australia holds the largest economic resources of nickel in the world, its nickel resources are mainly sourced from komatiitic-hosted and lateritic deposits. Known resources of tholeiitic intrusion-hosted nickel sulphides are relatively small, with Nebo-Babel and Nova-Bollinger in Western Australia the most significant examples. Given the abundance of tholeiitic igneous rocks in Australia, this important deposit type seems to be under-represented when compared to other continents with similar geology. To support the discovery of world class nickel sulphide deposits in Australia, Geoscience Australia has recently undertaken a continental-scale GIS-based prospectivity analysis for tholeiitic intrusion-hosted deposits across Australia. This analysis exploits a suite of new relevant digital datasets recently released by Geoscience Australia. For example, the analysis utilises the Australian Mafic-Ultramafic Magmatic Events GIS Dataset which places mafic and ultramafic rocks across Australia into 74 coeval magmatic events based on geochronological data. Whole rock geochemistry of mafic and ultramafic rocks has been used to differentiate between magma series and discriminate between different magmatic events and units within those events. Other new datasets include crustal domain boundaries derived from both deep crustal seismic data and neodymium depleted mantle model age data as well as a coverage of the minimum thickness of mafic rocks in the crust derived from the Australian Seismogenic Reference Earth Model. This continental-scale GIS-based nickel sulphide prospectivity analysis uses a mineral systems approach to map the four essential components of ore-forming mineral systems; (1) sources of ore constituents, (2) crustal and mantle lithospheric architecture, (3) energy sources or drivers of the ore-forming system, and (4) gradients in ore depositional physico-chemical parameters. These four components are combined into a prospectivity map using weights-of-evidence GIS-based techniques, with the most prospective areas across the continent occurring where all components are present. The mineral systems approach allows for the identification of a much larger footprint than the deposit itself, and can be applied to greenfield and/or undercover areas. The results highlight areas that contain known tholeiitic intrusion-hosted nickel sulphide deposits, such as the Musgrave and Pilbara Provinces, as well as regions that do not contain any known deposits, such as the southern margin of the Arunta Province in the Northern Territory, the Mount Isa Province in Queensland and the Paterson Province in Western Australia.

This report presents the results of a geodynamic synthesis of South Australia, focusing predominantly on the Archean to Mesoproterozoic of the Gawler Craton and Curnamona Province in terms of geodynamic setting, architecture, and age, using results of a geological synthesis, seismic interpretation, sequence stratigraphy, geochronology and geochemistry. This was undertaken with the dual aims: 1. To better understand the tectonic and geodynamic setting of the Gawler Craton and Curnamona Province 2. To accompany the interpretation of recently-acquired seismic reflection transects (see related product below), and to highlight new geochemical and geochronological data collected from South Australia.

Over the last decade there have been significant advances in our understanding of the: stratigraphy; magmatism; deformation; metamorphism; and timing of mineralisation, in the Eastern Goldfields Superterrane (EGST) of Yilgarn Craton, WA. The integration of these disciplines has enabled a holistic review of the tectonic history of the EGST, thereby providing a para-autochthonous geodynamic context for its mineralisation. A significant advance has been the recognition of a ~2.81 Ga rifting event off the eastern margin of the Youanmi Terrane which set up the north-northwest trending architecture of the EGST, as expressed in the Nd TDM map. Rifting was followed by the establishment of a convergent margin characterised by a west dipping subduction zone to the east of the EGST. Subduction resulted in the deposition of the 2.715-2.67 Ga volcanic stratigraphy and the emplacement of voluminous TTG magmatism, which resulted in magmatic thickening of the crust. Volcanism was terminated by a ~5 Ma pulse of east-northeast contraction which triggering lithospheric and lower crustal delamination associated with mid-orogenic extension. The lack of ultra-high pressure metamorphism and the presence of high geothermal gradients preclude this event from recording a continent-continent collision. Mid-orogenic extension initiated at 2.665 Ga resulted in the introduction of metasomatised mantle melts (Mafic-granites and Syenites), deposition of late-stage siliciclastic basins (which record anticlockwise PTt paths) and the start of significant economic gold mineralisation in the EGST. The delamination associated with this event resulted in significant heat input into the base of the crust, which eventually led to the emplacement of Low-Ca (crustal melt) granites and cratonisation of the EGST.

Interpretation of the 2006 deep seismic reflection data across the western Lachlan Orogen of southeast Australia have provided important insights into crustal-scale fluid pathways and possible source rocks in the Victorian orogenic gold province. The seismic profiles span three of the most productive structural zones in Victoria: the Stawell, Bendigo and Melbourne zones. Variations in the age and style of gold deposits across the structural zones are reflected by changes in crustal structure and composition, as revealed by the seismic data.

New geochronological data combined with existing data suggest that the Neoproterozoic period in Australia was reasonably well mineralised, with two major periods of mineralisation: (1) 850-800 Ma sediment-hosted Cu, unconformity U, and diamond deposits, and (2) 650-630 Ma epigenetic Au-Cu deposits. The early period appears to be associated with extension related to initiation of Rodinia break-up, whereas the geodynamic setting of the latter, more restricted, event is unclear.

The western two-thirds of Australia is underlain by Precambrian rocks that are divisible into three Archean to Paleoproterozoic cratons, the West Australian, North Australian and South Australian cratons, separated by Paleoproterozoic to Mesoproterozoic orogens. The temporal and spatial record of Proterozoic rock units and orogenic events documents accretion and assembly of Precambrian, proto-Australia. The Archean Yilgarn and Pilbara cratons were assembled into the West Australian Craton along the Capricorn Orogen during the late Paleoproterozoic (2000 Ma) Glenburgh Orogeny, which then combined with the North Australian Craton along the Rudall Orogen during the 1800-1765 Ma, Yapungku Orogeny. Prior to about 1500 Ma the North and South Australian cratons show a similar geological history and are herein assumed to have evolved as a single entity, termed the North-South Australian Craton. It was bounded throughout most of the late Paleoproterozoic to earliest Mesoproterozoic by subduction zones along its south western and north eastern margins such that much of the craton occupied an upper plate, back arc basin environment. After ~1500 Ma the craton differentiated into the North Australian and South Australian cratons through rotation and lateral translation of the latter, resulting in convergence and collisional suturing with the West Australian craton along the 1345-1140 Ma Albany-Fraser Orogen. The Pinjarra Orogen developed along the margin of the West Australian Craton and records late Mesoproterozoic to Neoproterozoic strike-slip juxtaposition of India within an assembling Gondwana. The Neoproterozoic record of the Terra Australis Orogen, which extends along the eastern side of Precambrian Australia, records rifting and continental breakup within the supercontinent of Rodinia. Australian Proterozoic rocks host significant mineral resources, including world class banded iron-formations in the West Australian craton (Hamersley), and iron oxide copper gold deposits (Olympic Dam), Pb-Zn-Ag systems (Mount Isa and Broken Hill) and uranium deposits in the North-South Australian Craton.

A ~400 km long deep crustal reflection seismic survey was carried out in Central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and Western Lachlan orogens and is shown to extend down to Moho. The Avoca Fault, which is the boundary between the Stawell and Bendigo zones, is a west-dipping listric fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo structural zones can be broadly divided into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly-folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwana margin and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone has not been subducted but is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that the 'thin-skinned' structural model appears to be valid for much of the Melbourne Zone whereas the Stawell and Bendigo zones have a 'thick-skinned' structural style. Internal faults in the Stawell and Bendigo zones are mostly west-dipping listric faults, which extend from the surface, with listric geometries, to near the base of the crust. The Heathcote Fault Zone, which forms the boundary between the Bendigo and Melbourne zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the younger Melbourne Zone. The deep seismic reflection data for the Melbourne have revealed a multilayered crustal structure that supports the Selwyn Block model.

As part of initiatives by the Australian and Queensland Governments, four new seismic reflection lines and three corresponding magnetotelluric lines were acquired in 2007 over the Mt Isa, Georgetown and Charters Towers regions. These data, combined with existing multidisciplinary data, have provided new insights into the 3D architecture, geodynamics and economic potential of the North Queensland region.

Extended abstract published as part of a GA Record