Extended abstract version of short abstract accepted for conference presentation GEOCAT# 73701

Mineral deposits are a product of the coincidence of favourable geological conditions within a given spatial and temporal setting. Collectively, these key geological elements may be considered as aspects of a mineral system. Mineral system-based investigations of the potential for a range of uranium systems have recently been undertaken in northern Queensland, east-central South Australia and the southern Northern Territory. Building on the methodology employed in northern Queensland, the mineral system assessment in South Australia and the Northern Territory consists of four key system components: (1) sources of metals and fluids, (2) drivers of fluid flow, (3) fluid pathways and architecture, and (4) depositional sites and mechanisms. Favourable geological criteria are developed from these four components, which are in turn translated into mappable geological proxies. Thus, the mineral systems framework drives the collection and synthesis of geoscientific data. This approach minimises the influence of localised geological controls, which may only be significant at the mine scale, and allows the system to be mapped on a broad scale, maximising the 'footprint' of mineralisation. Locations of known mineralisation are not considered in the assessment but are used to verify results. By employing a systems-based approach, the potential of relatively unexplored areas may be assessed objectively, transparently and systematically. Significantly, the approach used here is able to predict the potential for unrecognised mineralisation beneath cover. The assessments undertaken for uranium potential successfully reproduce the location of known uranium deposits and, importantly, delineate several areas where uranium mineralisation is currently unknown.

The global ocean absorbs 30% of anthropogenic CO2 emissions each year, which changes the seawater chemistry. The absorbed CO2 lowers the pH of seawater and thus causes ocean acidification. The pH of the global ocean has decreased by approximately 0.1 pH units since the Industrial Revolution, decreasing the concentration of carbonate ions. This has been shown to reduce the rate of biological carbonate production and to increase the solubility of carbonate minerals. As more CO2 is emitted and absorbed by the oceans, it is expected that there will be continuing reduction in carbonate production coupled with dissolution of carbonate sediments. This study was undertaken as part of a program to collect baseline data from Australia's seabed environments and to assess the likely impacts of ocean acidification on continental shelf sediments. Over 250 samples from four continental shelf areas of northern Australia (Capricorn Reef, Great Barrier Reef Lagoon, Torres Strait, Joseph Bonaparte Gulf) were analysed to characterise the surface sediment mineral and geochemical composition. Of particular importance was the quantification of carbonate minerals (calcite, aragonite, high-magnesium calcite) and the magnesium content in high-magnesium calcite. The latter determines the solubility of high-magnesium calcite, which is most soluble of all common carbonate minerals. The thermodynamic stability of carbonate minerals as referred to the state of saturation was calculated using the current and predicted equatorial ocean water composition [1]. Northern Australian continental shelf sediments are largely dominated by carbonate. High-magnesium calcite had the highest abundance of all carbonate minerals followed by aragonite in all areas. The average mol% MgCO3 in high-magnesium calcite varied from 13.6 to 15.5 mol% for the different areas, which is in agreement with the global average magnesium concentration in high-magnesium calcite in tropical and subtropical regions [2].

Benthic habitats on the continental shelf are strongly influenced by exposure to the effects of surface ocean waves, and tidal, wind and density driven ocean currents. These processes combine to induce a combined flow bed shear stress upon the seabed which can mobilise sediments or directly influence organisms disturbing the benthic environment. Output from a suite of numerical models predicting these oceanic processes have been utilised to compute the combined flow bed shear stresses over the entire Australian continental shelf for an 8-year period (March 1997- February 2005 inclusive). To quantify the relative influence of extreme or catastrophic combined flow bed shear stress events and more frequent events of smaller magnitude, three methods of classifying the oceanographic levels of exposure are presented: 1. A spectral regionalisation method, 2. A method based on the shape of the probability distribution function, and 3. A method which assesses the balance between the amount of work a stress does on the seabed, and the frequency with which it occurs. Significant relationships occur between the three regionalisation maps indicating seabed exposure to oceanographic processes and physical sediment properties (mean grain size and bulk carbonate content), and water depth, particularly when distinction is made between regions dominated by high-frequency (diurnal or semi-diurnal) events and low-frequency (synoptic or annual) events. It is concluded that both magnitude and frequency of combined-flow bed shear stresses must be considered when characterising the benthic environment. The regionalisation outputs of the Australian continental shelf presented in this study are expected to be of benefit to quantifying exposure of seabed habitats on the continental shelf to oceanographic processes in future habitat classification schemes for marine planning and policy procedures.

A depth to magnetic basement map has been produced for the Gawler-Curnamona region of South Australia. The map combines depth to magnetic source estimates with outcrop, drill hole and seismic data. The spectral domain method of analysing the slope of straight line segments in the power spectrum was used to produce the majority of the magnetic source depth estimates. The spectral domain method was incorporated into a semi-automated in-house software package to rapidly produce the regional scale map. The reliability of the depth to magnetic basement map is heavily dependent on the reliability of the depth to magnetic source estimation methods. There are a number of factors that can lead to errors, such as data quality and wrongly assigning magnetic sources to the cover or basement. The spectral domain method tends to slightly over estimate depths, however the average absolute errors are less than %30 when compared to known depths which is considered reasonable for the production of this type of regional scale map. The map delineates large areas of prospective Gawler Craton and Curnamona Province basement beneath less than 300 m of cover material, providing a useful tool for the mineral explorer. The map also delineates large areas under thick sequences of sediments, greater than 1000 m, which may prove of interest for the hydrocarbon explorer or act as a thermal blanket for the geothermal explorer.

High quality refraction and wide-angle reflection seismic data recorded by ocean-bottom seismographs (OBSs) deployed by the Australian Geological Survey Organisation along the 700 km long transect in the Carnarvon Basin effectively supplement results obtained by means of the conventional reflection technology. Velocity information can now be derived from both CDP (nearvertical reflection) and OBS (refraction/wide-angle reflection) data. Generally, CDP-derived average velocities are lower than OBS-derived velocities and this deviation increases with depth: from ~0.1 km/s at 8 s two way time (TWT) to 0.8-1.6 km/s at 16 s TWT. If the CDP-derived velocities are used to depth convert reflection data, then depth to these TWTs would be underestimated by 0.4 to 6.4-12.8 km respectively. Some local anomalies (at ~6s TWT CDP-derived velocities may be more than 0.1 km/s higher than the OBS-derived velocities) distort this general trend. These would result in ~0.3 km local overestimates of the depth equivalent of 6s TWT. Co-analysis of the interval velocity field reconstructed from the travel time-based interpretation of the OBS data and the conventional reflection image of the crust in some cases shows their poor correlation.

Geoscience Australia has collaboratively developed a number of open source software models and tools to estimate hazard, impact and risk to communities for a range of natural hazards to support disaster risk reduction in Australia and the region. These models and tools include: - ANUGA - a collaboration between the Australian National University (ANU) and GA to develop hydrodynamic software; - EQRM - earthquake risk model; - TCRM - tropical cyclone risk model; - PythonFall3D - python wrapper for an existing volcanic ash model; - TsuDAT - a collaboration between GA, the Australia-Indonesia Facility for Disaster Reduction, the ANU National Computing Infrastructure (NCI), OpenGeo and the World Bank to develop the tsunami data access tool; - RICS - rapid inventory collection system; - FiDAT - field data analysis tool. This presentation will discuss the drivers for developing these models in open source software and the benefits to the end-users in the emergency management and planning community as well as the broader research community. Key challenges in the risk modelling process will be discussed and how these may be addressed through the enhancement of existing models and the development of new models and workflows. Example challenges include image analysis for the development of building information from high resolution photography, project of future communities and reducing uncertainty in the frequency of natural hazard events. These challenges can be progressed through collaboration in the mathematical community.

Regional geology and prospectivity of the Aileron Province in the Alcoota 1:250 000 mapsheet area

The subsidence histories of most, but not all, basins can be elegantly explained by extension of the lithosphere followed by thermal rethickening of the lithospheric mantle to its pre-rift thickness. Although this model underpins most basin analysis, it is unclear whether subsidence of rift basins developed over thick lithosphere follows the same trend. Here the subsidence history of the Caning rift basin of Western Australia is modelled which putatively overlies lithosphere - 180 km thick, imaged using shear wave tomography. The entire subsidence history of the, < 300 km wide and <6 km thick, western Canning Basin is adequately explained by Ordovician rifting of ~120 km thick lithosphere followed by post-rift thermal subsidence as described by the established model. In contrast, the < 150 km wide and 15 km thick Fitzroy Trough of the eastern Canning Basin, reveals an almost continuous phase of normal faulting between Ordovician and Carboniferous Periods followed by negligible post-rift thermal subsidence which cannot be accounted for by the established model. This difference in basin architecture is attributed to rifting of thick lithosphere constrained by the presence of diamond bearing lamproites intruded into the basin depocentre at ~20 Ma. In order to account for the observed subsidence, at standard crustal densities, the lithospheric mantle is required to be depleted by 50-70 kg m-3. The actual depletion of the lowermost lithospheric mantle was assessed by modelling REE concentrations of the ~20 Ma lamproites along with other ultrapotassic rocks from the Kimberley, Yilgarn and Pilbara blocks which reveal a depletion of 40-70 kg m-3. Together these results suggest that thinning of thick lithosphere to thicknesses > 120 km is thermally stable and is not accompanied by post-rift thermal subsidence driven by thermal rethickening of the lithospheric mantle. The discrepancy between estimates of lithospheric thickness derived from subsidence data in the Western Canning and that derived from shear wave tomography suggests that the latter technique cannot resolve lithospheric thickness variations on < 300 km half wavelengths.

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