In addition to typical seafloor VHMS deposits, the ~3240 Ma Panorama district contains contemporaneous greisen- and vein-hosted Mo-Cu-Zn-Sn occurrences that hosted by the Strelley granite complex, which drove VHMS circulation. High-temperature alteration zones in volcanic rocks underlying the VHMS deposits are dominated by quartz-chlorite±albite assemblages, with lesser low-temperature quartz-sericite±K-feldspar assemblages, typical of VHMS hydrothermal systems. Alteration assemblages associated with granite-hosted greisens and veins, which do not extend into the overlying volcanc pile, include quartz-topaz-muscovite-fluorite and quartz-muscovite(sericite)-chlorite-ankerite. Fluid inclusion and stable isotope data suggest that the greisens formed from high temperature (~590C), high salinity (38-56 wt % NaCl equiv) fluids with high densities (>1.3 g/cm3) and high -18O (9.3±0.6-), which are compatible with magmatic fluids evolved from the Strelley granite complex. Fluids in the volcanic pile (including the VHMS ore-forming fluids) were of lower temperature (90-270C), lower salinity (5.0-11.2 wt % NaCl equiv), with lower densities (0.88-1.01 g/cm3) and lower -18O (-0.8±2.6), compatible with evolved Paleoarchean seawater. Fluids that formed the quartz-chalcopyrite-sphalerite-cassiterite veins, which are present within the upper granite complex, were intermediate in temperature and isotopic composition (T = 240-315C; -18O = 4.3±1.5-) and are interpreted to indicate mixing between the two end-member fluids. Evidence of mixing between evolved seawater and magmatic-hydrothermal fluid in the granite complex, along with a lack of evidence for a magmatic component in fluids from the volcanic pile, suggest partitioning of magmatic-hydrothermal from evolved seawater hydrothermal systems in the Panorama VHMS system, interpreted as a consequence swamping of the system by evolved seawater or density contrasts.

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.

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 Kangaroo Caves zinc-copper deposit in the Archaean Panorama District in the northern Pilbara Craton, Western Australia contains an Indicated and Inferred Mineral Resource of 6.3 million tonnes at 3.3% zinc and 0.5% copper. The Kangaroo Caves area is characterised by predominantly tholeiitic volcanic rocks of the Kangaroo Caves Formation, which is overlain by turbiditic sedimentary and volcanic rocks of the Soanesville Group. Zinc-copper mineralisation is hosted mainly by the regionally extensive Marker Chert, the uppermost unit of the Kangaroo Caves Formation, and structurally controlled by D1 synvolcanic faults. The upper area of the deposit is characterised by quartz-sphalerite ± pyrite ± baryte ± chalcopyrite, whereas the lower area contains mainly chlorite-pyrite-quartz-carbonate-sericite ± chalcopyrite ± sphalerite. Laser ablation inductively coupled plasma mass spectrometry analyses show that cobalt-nickel ratios in pyrite are significantly greater in the upper, zinc-rich area (median copper/nickel = 0.4) of the deposit than the lower, zinc-poor area (median copper/nickel = 5). Structural analysis of the Kangaroo Caves area combined with Leapfrog modelling of ore and trace element distribution shows that the deposit is predominantly an elongate sheet of zinc mineralisation (-1%), which plunges ~30° to the northeast and is approximately 1000 metres in length. The morphology of the Kangaroo Caves deposit was retained from its original formation, despite rotation during the D2 event. Variations in hydrothermal alteration assemblages, including the copper and nickel contents of pyrite within the deposit and underlying dacite, are interpreted to be the result of variations in the influx and mixing of seawater with upwelling volcanogenic fluids during zinc-copper mineralization. At the Kangaroo Caves area the cobalt-nickel ratio of pyrite can be used as an exploration vector towards high-grade zinc-copper mineralization.

Gold deposits in the Archaean Eastern Goldfields Province in Western Australia were deposited in greenstone supracrustal rocks by fluids migrating up crustal scale fault zones. Regional ENE-WSW D2 shortening of the supracrustal rocks was detached from lower crustal shortening at a regional sub-horizontal detachment surface which transects stratigraphy below the base of the greenstones. Major gold deposits lie close to D3 strike slip faults that extend through the detachment surface and into the middle to lower crust. The detachment originally formed at a depth near the plastic-viscous transition. In orogenic systems the plastic-viscous transition correlates with a low permeability pressure seal separating essentially lithostatic fluid pressures in the upper crust from supralithostatic fluid pressures in the lower crust. This situation arises from collapse in permeability below the plastic-viscous transition because fluid pressures cannot match the mean stress in the rock. If the low permeability pressure seal is subsequently broken by a through-going fault, fluids below the seal would flow into the upper crust. Large, deeply penetrating faults are therefore ideal for focussing fluid flow into the upper crust. Dilatant deformation associated with sliding on faults or the development of shear zones above the seal will lead to tensile failure and fluid-filled extension fractures. In compressional orogens, the extensional fractures would be sub-horizontal, have poor vertical connectivity for fluid movement and could behave as fluids reservoirs. Seismic bright spots at 15-25 km depth in Tibet, Japan and the western United States have been described as examples of present day water or magma concentrations within orogens. The likely drop in rock strength associated with overpressured fluid-rich zones would make this region just above the plastic-viscous transition an ideal depth range to nucleate a regional detachment surface in a deforming crust.