The wide-angle reflection seismic survey coincident with regional transect through Northern Yilgarn focused on the Leonora-Laverton Tectonic Zone, Western Australia was carried out to supplement deep seismic reflection studies. The major objectives were to collect high-density refraction information for offsets up to 60 km, to carry out a comparative study of near-vertical and wide-angle recordings of vibroseis energy at various offsets within the Leonora-Laverton tectonic zone and to obtain velocity information for upper crust. The survey deployed 120 short period recorders with spacing of 500 m. Acquisition parameters used for wide-angle reflection experiment were selected to fit into conventional reflection survey. The same vibrations were recorded in both surveys simultaneously. The major challenge in processing the Vibroseis data is to manage the huge volume of data. The processing of data includes several steps: sorting into receiver and source gathers, cross-correlation with reference sweeps and summing original seismic traces to form single source point traces, producing seismograms from individual traces and finally creating seismic record section from separate seismograms. The major step in processing and interpretation of data is to analyse recorded wave fields on the basis of seismological criteria prior to seismic velocity modelling. Seismic velocity models developed by using forward and inverse travel modelling software will supplement geological interpretations for this complex region and allow an estimation of its crustal composition.

No abstract available

The Stuart Shelf overlies the eastern portion of the Gawler Craton. This part of the Gawler Craton is South Australia's major mineral province and contains the world-class Olympic Dam Cu-U-Au deposit and the recent Cu and Au discovery at Prominent Hill. The Stuart Shelf is several kilometres thick in places. As such, little is known of the crustal structure of the basement, its crustal evolution or its tectono-stratigraphic relationship to adjacent areas, for example the Curnamona Province in the east. There has been much effort applied to advancing our understanding of basement, mainly through the use of potential field data and deep drilling programmes; though drilling has proved very costly and very hit and miss. The Stuart Shelf area needs new data and methods to bring our knowledge of it to the next level of understanding. At a Gawler Craton seismic planning workshop held in July 2001, stakeholders from industry, government, and university stakeholders identified several criteria fundamental to undertaking any seismic survey within the Gawler Craton. These were - Location of seismic traverse across a known mineral system in order to improve understanding and enhance knowledge of the region's mineral systems. Access to surface and/or drill hole geological knowledge to link geology data with the seismic interpretation. Good coverage of potential field data, and Potential for the seismic data to stimulate area selection and exploration in the survey region.

The Palaeoproterozoic to Mesoproterozoic (<1850-<1490 Ma) southern McArthur Basin, Northern Territory, Australia, contains an unmetamorphosed, relatively undeformed succession of carbonate, siliciclastic and volcanic rocks that host the McArthur River (HYC) Zn-Pb-Ag deposit. Seismic reflection data obtained across this basin have the potential to revolutionise our understanding of the crustal architecture in which this deposit formed. These data were collected in late 2002 as part of a study to examine the fundamental basin architecture of the southern McArthur Basin, particularly the Batten Fault Zone, and the nature of the underlying basement. Geoscience Australia, the Northern Territory Geological Survey and the Predictive Mineral Discovery Cooperative Research Centre combined to acquire an east-west deep seismic reflection profile (line 02GA-BT1) approximately 110 km long, commencing 15 km west of Borroloola, and extending westwards along the Borroloola-Roper Bar road to the Bauhinia Downs region (Fig. 1). A short 17 km north-south cross line (02GA-BT2) was also acquired in collaboration with AngloAmerican. The seismic data were acquired through the Australian National Seismic Imaging Resource (ANSIR).

Seismic reflection, seismic refraction and portable broadband data collected within Western Australia's Yilgarn Craton, in particular the Eastern Goldfields Province, are providing detailed images of several of its highly mineralized terranes as well as new insights into the crustal architecture of the region. When the results from these seismic techniques are integrated, the results are providing a better understanding of the structure of the crust and lithosphere beneath the Yilgarn Carton, from the surface to depths in excess of 300 km.

Chemical modeling of gold mineralisation in the Lachlan Fold Belt shows that gold can be precipitated over a wide temperature range (from 320 to 200 ?C in this study) from CO2-bearing, low salinity, aqueous fluid flowing upwards through faults in turbiditic sequences. In agreement with field observations, the veins are predicted to be mostly quartz (> 93 vol.%) with minor amounts of pyrite, arsenopyrite and muscovite (sericite) precipitating above 230 ?C. The predicted alteration assemblage contains pyrite, arsenopyrite, calcite, muscovite (sericite), chlorite and feldspar. Varying some of the chemical characteristics of the initial fluid has resulted in the following changes to the model: Preventing the fluid from boiling stops gold precipitating below 310 ?C but has little effect on the vein mineralogy or the mineralogy of the surrounding alteration assemblage. Removing CO2 from the fluid also prevents gold precipitation in the veins below 300 ?C. The modeling also generates an alteration assemblage with a number of Ca-rich minerals as less calcium carbonate exists in this system. Removing sulfur species from the initial fluid decreases the amount of gold precipitated by more than a factor of ten, which is to be expected if sulfur ligands are the main species for gold transport. However, the vein assemblage and the lack of sulfide minerals in the surrounding alteration assemblage also suggest that sulfur species are important in this mineral system. Increasing the initial oxidation state (?O2) of the fluid inhibits gold precipitation in the veins above 260 ?C and leads to a high proportion of dolomite in the surrounding alteration assemblage. On the other hand, decreasing the initial oxidation state of the fluid lead to gold precipitation over a range of temperatures below 310 ?C but predicts that mainly graphite ? quartz precipitates in the veins and that the surrounding alteration assemblage is dominated by feldspar proximal to the veins. This style of mineralogy is not commonly observed in gold deposits in the Lachlan Fold Belt. Increasing the initial pH of the fluid inhibits the amount of minerals that precipitate in the veins, which are dominated by calcite at high temperatures and graphite at low temperatures and corresponding minor amounts of gold. The proximal alteration assemblage is dominated by K-feldspar with amphibole, biotite and epidote. This mineral assemblage is not commonly observed in these deposits. Decreasing the initial pH of the fluid allows gold to precipitate below 280 ?C but generates a proximal alteration assemblage dominated by pyrophyllite, which again is not commonly observed. The results are in agreement with the widely accepted premise that gold is transported as bisulfide complexes and that the ore-bearing fluid is typically a low-salinity, mixed aqueous-carbonic fluid with low-moderate CO2 contents (Ridley and Diamond, 2000). However, the modeling has shown that the absence of certain physico-chemical processes or fluid constituents, such as boiling or lack of CO2 may inhibit gold precipitation in some environments. Large fluctuations in ?O2 or pH will also significantly change the vein and alteration mineralogy and generally reduce the amount of gold that is precipitated. This suggests that these fluids remain rock buffered during their journey from the source to the trap site.

No abstract available

Models for the crustal evolution of the Yilgarn Craton have changed in the last 25 years from generally autochthonous greenstone development on sialic crust (Gee et al. 1981, Groves & Batt 1984) to alloch-thonous models that highlight the importance of accretionary tectonics (Myers 1995). Recent models highlight the importance of mantle plumes and long-lived convergent margins for both Au and Ni (Barley et al. 1998). The role of sialic crust in the development of the abundant mineral systems in the Yilgarn, and Archaean cratons in general, however, remains problematic. Felsic rocks from across the Yilgarn Craton are used as crustal probes, with their geochronology, zircon inheritance and Nd isotopic character used to constrain the age and extent of basement terranes. The studies reveal a collage of crustal fragments and implicate both autochthonous and allochthonous crustal development, with increasing importance of accretionary tectonics, particularly after 2.8 Ga. The crustal evolution places significant constraints on the development of metallogenic associations.

Until recently, the development of better constrained models for gold mineralisation in the western Lachlan Fold Belt has been hindered by the paucity of data on the chemistry of the ore-bearing fluids, the temperatures and pressures at the time of mineralisation, and the extent and nature of wallrock alteration. Newly available data, however, has allowed us to use thermodynamic mass transfer modelling to test some of the models proposed for gold mineralisation in the western Lachlan Fold Belt and to investigate whether the results of the modelling correlate with the styles of mineralisation and alteration assemblages observed in these turbidite-hosted gold deposits. The first model (cf. Keays, 1987) invokes initial seawater alteration of tholeiitic rocks, followed by leaching of gold from these rocks by a metamorphic fluid. This fluid then ascends into the overlying turbidite succession where fault-valve behaviour leads to phase separation and progressive water-rock interaction. Phase separation results in gold precipitation but the associated mineral assemblage contains significant amounts of feldspar, epidote and prehnite, which is not in accord with observed vein and alteration mineralogies. The third model (cf. Gray et al., 1991; and Cox et al.,1995) proposes that the ore-bearing fluids originated from metamorphic devolatilisation process occurring in deep level crustal rocks. This fluid then ascends to upper crustal levels as before. Phase separation leads to a relatively large decrease in the activity of sulfur in the fluid, resulting in gold precipitation and the precipitation of quartz, muscovite, arsenopyrite with chlorite, feldspar and pyrite forming at lower water-rock ratios. Therefore, this model is the one that best reproduces the vein and alteration assemblages commonly observed in the western Lachlan Fold Belt.

Introduction Developing predictive numerical models of hydrogeochemical systems requires an understanding of the physical and chemical processes affecting the composition of the water. Physical processes like mixing and evaporation can be reasonably well defined using the chemical data but redox sensitive chemical processes are more difficult to quantify. Applying the isotope chemistry of dissolved sulfate to characterise and even quantify these redox processes enhances the capabilities of numerical modelling, in particular those associated with acid mine drainage, acid sulfate soils and sulfide mineral exploration. This work describes how the stable isotopes of sulfur and oxygen in sulfate can be used to better characterise geochemical processees and thereby improve reactive transport models. Discussion Groundwater, pore water and surface water from a number of areas in Australia have been used to determine the sources of sulfur in acid sulfate susceptable systems. Several trends become apparent, sulfate reduction, and sulfide oxidation commonly dominate the chemical processes controlling sulfur in a groundwater system. Bacterial sulfate reduction (BSR) can be recognised by the affect on the 34S and 18O of sulfate. Both ratios increase as the lighter isotope is removed through dissimilatory bacterial reduction, leaving behind the heavier isotopes. Oxidation of sulfides occurs through 2 processes, one involving molecular oxygen (O2) and the other involving oxidised iron (Fe3+). The different pathways result in considerable differences in the oxygen isotopic composition of the product sulfate. Surface water and some groundwater from the Loveday basin in SA show evidence of evaporation and BSR while the near surface pore waters, although similarly evaporated, contain sulfate that predominantly originates from sulfide oxidation. Sulfate in groundwater from several other regions has stable isotopic compositions that indicate sulfide oxidation involving either the O2 or the Fe3+ pathways. The implications of are that the sulfate history can be understood through isotopic analysis and that this can be used in geochemical models to trace