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

The 2007 North Queensland seismic survey provided a new geodynamic framework and province architecture map for the North Queensland region. Coupled with this, companion geophysical studies provided new understandings of the subsurface of the region. A major focus of the geophysical investigations was the use of potential field inversions. These inversions allow for the mapping of units undercover, predict the extension of geometries away from seismic lines, and also provide a measure of alteration. The North Queensland region also allowed for the testing of both qualitative and quantitative methods to map alteration using geophysical inversions.

No abstract available

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.

South Australia's Gawler Craton is known for its high exploration potential for iron oxide copper gold (IOCG) deposits. In addition to the giant Olympic Dam deposit, relatively recent discoveries at Prominent Hill and Carapateena and a large number of smaller prospects confirm the attractiveness of the Mesoproterozoic rocks near the eastern margin of the Craton. The challenge facing explorers is the thick and extensive sedimentary and volcanic cover that overlies those prospective basement rocks. The only way to image buried rocks is by integrated analysis of remotely measured geophysical data with geological knowledge. Deep reflection seismic data provides critical information on unit depths, thickness and geometries. Interpreted profiles along the 03GA-OD1 and transverse 03GA-OD2 reflection seismic lines centred on the Olympic Dam deposit provide the best available information on the crustal-scale 3D geometries in that area. These relationships are extended throughout a 600 km east-west by 510 km north-south subset of the eastern Gawler Craton, to a depth of 25 km below surface, using geologically-constrained 3D inversion of public domain gravity and magnetic data. Including geological constraints is critical to ensure that the 3D property models recovered using the inversions are consistent with all available geophysical and geological data. Geological constraints are developed from surface mapping, seismic profile interpretations on the Olympic Dam lines as well as the 08GA-C01 and 03GA-CU1 lines in the Curnamona Craton, and 2D potential field modelling. Where knowledge of the cover rocks exists, it is included as a constraint to enhance the resolution of features at depth.

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 Tarcoola Goldfield in central South Australia is hosted by the Palaeoproterozoic Paxton Granite and Tarcoola Formation sediments. Both of these units are intruded by narrow dykes of the Lady Jane Diorite. Crosscutting relationships and Ar-Ar dating of sericite and hornblende demonstrate that veining, alteration and mineralization occurred synchronously with intrusion of the diorite dykes at ~1580 Ma. The Lady Jane Diorite is considered to be part of the Gawler Range?Hiltaba Volcano-Plutonic event. Mineralization at the Perseverance deposit in the Tarcoola Goldfield is hosted by both granite and the lower parts of the Tarcoola Formation. Alteration intensity and mineralization are related to fracture density, suggesting the structural regime was an important control on ore location and deposition. However, at the Tarcoola Blocks mine, mineralization is hosted entirely in the Tarcoola Formation, and very high gold grades are recorded in carbonaceous horizons, suggesting chemical processes in ore deposition. Primary fluid inclusions from mineralized quartz veins are dominated by two types, a two-phase aqueous type and a three-phase H2O-CO2(l)-CO2(v)?CH4 type. Both inclusion types have salinities of <10 wt% eq. NaCl, with evidence of other salts being present. Both types have Th between ~380?C and 150?C. At the Perseverance deposit, both inclusion types occur together in single quartz grains as primary inclusions, and are interpreted to have formed from a single evolving fluid, indicating immiscibility. Pb isotopes of galena from gold-bearing mineralization are in equilibrium with K-feldspar and whole-rock samples of the Paxton Granite. This is consistent with a local source for Pb and possibly other ore components, but not all potential reservoirs were able to be tested. Variably altered and gold mineralized samples that had Paxton Granite precursors show a loss of Nd and a trend from ?Nd(1580 Ma) values of about -6.2 in slightly altered samples towards more positive ?Nd values of about -3.3 in high grade gold samples. Of the local lithologies, the Lady Jane Diorite has ?Nd(1580 Ma) values of about +0.15 and is the most probable source of the more primitive Nd isotopic components in mineralized samples. Whether the Lady Jane Diorite is a cause of mineralization at Tarcoola, or both it and the mineralization are symptoms of the same tectonothermal event at ~1580 Ma can not be resolved with the available data. The demonstrated age relationship between the diorites and mineralisation at Tarcoola, the Nd data at Tarcoola, and the presence of similar dioritic dykes in all of the known gold prospects in the Central Gawler Gold Province are strongly suggestive that such dyking was an essential ingredient in gold mineralization throughout this province.

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.

No abstract available

The Cobar project of the pmd*CRC utilised potential field inversions to delineate zones of alteration within the Cobar region, NSW. These zones of alteration and correlation with mineralisation is consistent with the mineral system analysis of the region, performed by the T11 Cobar project.