Physical property measurements provide a critical link between geological observations and geophysical measurements and modelling. To enhance the reliability of gravity and magnetic modelling in the Yilgarn Craton's Agnew-Wiluna greenstone belt, mass and magnetic properties were analysed on 157 new rock samples and combined with an existing corporate database of field measurements. The new samples include sulphide ore, serpentinised and olivine-bearing ultramafic host rocks, granitoid, and felsic and mafic volcanic and volcaniclastic country rock. Synthesis of the data provides a useful resource for future geophysical modelling in the region. Several rock types in the region have sufficiently distinct physical properties that a discriminant diagram is proposed to facilitate a basic classification of rock types based on physical properties. However the accumulation of emplacement, metamorphic, hydrothermal and structural processes has complicated the physical properties of the rocks by imposing duplicate and sometimes opposing physical property trends. The data confirms that massive sulphide and ultramafic rocks have the most distinctive mass and magnetic properties but with variability imposed by their complex history. Sulphide content imposes the strongest control on densities, but can only be identified when comprising > 10 vol. % of the rock. The pyrrhotite-rich Ni-sulphide assemblages generally have similar magnetic properties to the host ultramafic rocks, but can have much lower susceptibilities where the thermal history of the rocks has favoured development of hexagonal pyrrhotite over monoclinic pyrrhotite. In ultramafic rocks that contain < 10 vol. % sulphides, density and susceptibility are primarily controlled by serpentinisation, with olivine breaking down to serpentine and magnetite in the presence of water.

No abstract available

Chemical alteration to certain end-member minerals, such as magnetite, pyrrhotite and pyrite, can produce density and magnetic susceptibility contrasts. These contrasts can be detected using gravity and magnetic surveys. Interpretation of alteration is made possibly by inverting the geophysical data (calculating subsurface properties from the survey results) and combining these inverse results with 3D geological mapping. An application of this method to

Regional-scale constrained potential field inversions can be used to infer rock types, alteration, and structure. This is particularly valuable when basement is obscured by younger cover. The methods outlined in this study have been applied to a 150 km ? 150 km region around the giant Olympic Dam copper-uranium-gold deposit, where abundant haematite, sulphide, and magnetite alteration produces a strong potential field response despite thick cover. The results are used to develop the first 3D map of magnetite and haematite/sulphide alteration for the Olympic Cu-Au province, and shows that the alteration around known Cu-Au mineral occurrences can be detected using coarse regional-scale inversions. The provision of a reference model in the inversion formulation permits geological observations to be introduced into the inversion process, and to be used to guide the inversion towards more geologically reasonable outcomes. This allows hypotheses regarding 3D geological architecture to be tested rigorously for compatibility with potential field data. An iterative procedure of inversion followed by updating of the reference model allows 3D maps of alteration and structure to be created that are consistent with both the known geology and observed potential field data.

No abstract available

Geophysical inversions provide a mechanism to calculate subsurface chemical alteration in terms of alteration minerals. In the Cobar region, NSW, Australia, the base metal deposits show significant geophysical contrasts to their host rocks. These contrasts can be inverted to provide measures of the causative chemical alteration, allowing targeting for mineralisation under cover.

The Broken Hill Exploration Initiative (BHEI) started in 1994 and is a joint effort between the New South Wales Department of Primary Industries - Mineral Resources, the South Australian Department of Primary Industry and Resources and the Commonwealth Government through Geoscience Australia. The aim of the BHEI is to provide a new generation of geoscientific data for the Curnamona Province, particularly the Broken Hill-Olary region, as a basis for more effective mineral exploration by industry. This initiative aims to provide the best possible knowledge and information-base for mineral and petroleum exploration investment in western New South Wales and eastern South Australia. The region will benefit from the application of new technologies and exploration methodologies to enhance knowledge of the geological controls of mineral deposit systems. BHEI conferences are held on a regular basis to highlight the geoscientific advances made during the life of the initiative. The contents of this Record are the extended abstracts of oral and poster papers presented at the BHEI conference that was held in Broken Hill on 26-28 September 2006.

A metamorphic database covering the entire eastern Yilgarn Craton has been compiled from pre-existing mapping, 14,500 sites with qualitative metamorphic information, and 470 new key sites with detailed quantitative metamorphic data including P, T, temperature/depth ratio and P-T paths. The derived temporal and spatial patterns contrast with previous tectonic models and invariant crustal depth with the single prograde metamorphic event of the long-standing metamorphic paradigm. In particular, there are large variations in peak metamorphic crustal depths (12 to 31 km), and five metamorphic periods can now be recognised. &#149; Ma: Very localised, low-P granulite of high temperature/depth ratio (>50ºC/km). &#149; M1: High-P (8.7kb), low temperature/depth ratio (<20ºC/km) assemblages localised to major shear zones with clockwise isothermal decompression P-T paths. &#149; M2: Regional matrix parageneses with T ranging 300-550ºC across greenstone belts and elevated temperature/depth ratio of 30-40ºC/km throughout. Tight clockwise paths evolved through maximum prograde pressures of 6 kb and peak metamorphic pressures of 3.5-5.0 kb. &#149; M3a: An extension related thermal pulse localised on the Ockerburry Fault and post-volcanic late basins. Anticlockwise paths to peak conditions of 500-580ºC and 4.0 kb, define moderately high temperature/depth ratio of 40-50ºC/km. &#149; M3b: Multiple localised hydrothermal alteration events during a period of exhumation from 4 kb to 1 kb. Metamorphic patterns during each event have been temporally and spatially integrated with the new deformation framework (Blewett & Czarnota, 2007c) by a process of metamorphic domain analysis and using metamorphic field gradients. The continual evolution with time of fundamental metamorphic parameters throughout the entire history have been constructed as evolution curves and integrated with the deformation, magmatic, stratigraphic and mineralization history. <p>Related material<a href="https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=69771">East Yilgarn Craton Metamorphism and Strain</a> - Map.</p>

Globally, iron-oxide Cu-Au deposits are a prime exploration target given the super-giant status of Olympic Dam, the type example. The most commonly identified primary ingredients for these deposit types are granites, mafic igneous rocks and oxidised host sequences (e.g., Hitzman 2000). The deposits are also characterised by sodic-potassic and potassic alteration which can be significant on a regional scale. Deposits in the Tennant Creek, Cloncurry and Olympic Dam regions are the three main examples of iron-oxide Cu-Au deposits in Australia: all are Proterozoic. A continent scale-spatial analysis of their regional setting reveals some common elements and suggests that these deposit types are unlikely to be found in the Archaean or the Phanerozoic of Australia. In all three cases, intrusive rocks occur in proximity, but whether the metals come direct from these or are leached from the country rock is unimportant to this analysis. The nearby granitic rocks are all fractionated, and show evidence of release of late stage magmatic fluids. Chemically these granites are a very specific type: all strongly oxidised, metaluminous, high temperature, high Ca granites that formed as a result of well above average geothermal gradients from crustal sources. These granites contain anomalously high concentrations of K, Th and U. Such granites are not found in the Archaean and rare equivalent Phanerozoic granitic types are of an order of magnitude smaller in size. This analysis shows that the known Australian deposits occur in regions of present-day anomalously high heat flow. Heat production is enhanced by high concentrations of K, Th and U in the nearby granites, particularly in the Olympic Dam and Cloncurry areas. Such elevated heat production may have resulted in anomalously long-lived hydrothermal cells and enhanced the metal leaching process. The large areal extent of the alteration zones in these two areas may also be a result of this fundamental crustal anomaly. Mafic igneous rocks are common in the vicinity of known Australia deposits, and might be another essential ingredient of an efficient Fe-Ox Cu-Au mineral system. They are regarded as an additional source of Cu for leaching. The regions with known deposits are dominated by oxidised rock packages containing hematite?magnetite. These oxidised packages are regarded as essential to maintaining the high redox state of the metal-bearing fluids. Elsewhere where similar granites intrude reduced packages, particularly those bearing graphite, Cu deposits are rare. Where oxidised host packages are not intruded by granites these deposit types do not occur. Equivalent oxidised host packages intruded by granites are not common in the Australian Archaean or Phanerozoic.

Economic geologists and remote sensing specialists have long been interested in hydrothermal systems for their valuable ore deposits. Alteration mineral assemblages caused by hydrothermal activity typically display certain spectral characteristics due to vibration of the hydroxyl (OH-) anion in the near infrared. This feature can be exploited by satellites using imaging infrared spectrometers. Hyperspectral equipment such as the Australian built HyMap airborne system and the PIMA II field spectrometer collect detailed spectral information, often in contiguous wavelengths, which can be analysed and interpreted to make highly detailed mineralogical and alteration pattern maps . Two Precambrian hydrothermal systems, well suited to testing remote sensing of hydrothermal systems are described here, the first in the northern Flinders Ranges in South Australia, comprising one of Earth?s largest hydrothermal deposits and, the second in the North Pole Dome in the Pilbara region of north western Australia, host to many famous stromatolitic horizons.