The Late Archaean granite-greenstone terranes of the Yilgarn Craton WA are the host to Australia's premier world-class orogenic Au deposits. The Eastern Goldfields Superterrane (EGST) within this region hosts the majority of Au deposits and is therefore data rich, with high-resolution gravity, magnetics, geological and geochemical maps and databases, augmented with a number of high-quality deep seismic reflection profiles, teleseismic and magnetotelluric studies. Research within Geoscience Australia and the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) have integrated these data into a set of new 3D maps at a range of scales. Integration of these data in 3D space has enabled greater understanding of the giant Au mineral systems. At the craton scale a tomographic survey was deployed to investigate whether anomalous crustal and/or upper mantle velocities exist beneath the highly mineralised EGST compared to "normal" velocities beneath barren regions in the Yilgarn Craton as a whole. The resulting velocity data were spatially represented in 3D using Gocad, enabling spatial links to be drawn between dramatic velocity variations in the tomographic data and zones of mineralisation in the upper crust. At the crustal/upper mantle scale, 2D seismic reflection lines throughout the EGST have imaged crustal penetrating shear zones that are often spatially coincident with mineralised corridors. Multiple 2D seismic reflection profiles, combined with geological and potential field data, have enabled the sub-surface geometries of faults to be constructed in three dimensions, providing insight into potential fluid pathways. A 130 km long magnetotellurics survey located along the EGF01 seismic line was also used to explore the relationship between seismic architecture and the crustal/upper mantle-scale conductivity distribution. At the camp-scale, 3D technologies have greatly improved our ability to compare complex relationships between spatially referenced 2D and 3D data, leading to improved targeting. The construction and display of the 3D distribution of lithological units and complex fault arrays, as surfaces and solid volumes, has enabled the 3D geology of a region to be statistically analysed in a quantitative way that is not possible using conventional techniques. Mine- to camp-scale 3D faults, lithologies and alteration assemblages are used to define preferential zones of fluid flow and Au deposition. This process involves detailed structural analysis of mineralisation at mine-scale to determine stress vectors during mineralisation, combined with numerical modelling of preferentially orientated structures. Favourably orientated faults calibrated with known mineralisation and alteration assemblages provide a framework to statistically analyse the relationship between structural architecture, alteration signatures and mineral deposition. 3D maps provide a tool to analyse data at a range of scales and better integrate a wide range of datasets. These technologies enable spatial and temporal relationships to be compared in a more quantitative way and also provide a 3D interpretation of geology that can be statistically analysed for predictive mineral discovery.

The concentration of sulfur in fluids associated orogenic gold deposits is important for understanding the transport of gold species but still remains poorly defined. In this study Raman spectroscopy has been used to obtain the concentrations and fugacities of H2S in individual fluid inclusions from the Archean, sub-amphibolite facies Missouri gold deposit in Western Australia. Type III, vapor-rich (XH2O < 0.4) fluid inclusions from the Missouri gold deposit were shown to contain highly variable CO2/CH4 ratios (0.01 to 1.9) and H2S fugacities of up to 1.6 MPa. The calculated values of H2S are consistent with the location of fluids within the general pyrite stability field next to the pyrite-pyrrhotite reaction boundary. This is in accord with the quartz(-plagioclase)-biotite-carbonate-pyrite alteration assemblage that occurs adjacent to the quartz veins at the Missouri deposit. Geochemical modelling was used to investigate how the concentration of H2S was affected by the fCO2/fCH4 ratio of a fluid in equilibrium with the pyrite-pyrrhotite alteration assemblage. Overall, the highest H2S concentrations are predicted for 'reduced-fluids' near the CH4 apex of the H2O-CH4-CO2 ternary diagram. Comparisons with the measured H2S concentrations show that the CH4-CO2¬-Pyrite-Troilite equilibrium could be the controlling factor on the H2S concentration of the fluid; other processes could have affected some inclusions which contain higher than predicted H2S concentrations. These are mostly restricted to the population of water-poor (XH2O < 0.1), essentially CH4-CO2 inclusions.

The poster/panel summarises result of preliminary fluid flow and chemical modelling in the Frome Embayment

A compilation of abstracts of talks and posters presented at the Broken Hill Exploration Initiative (BHEI) 2003 conference at Broken Hill, 7-9 July 2003.

Legacy product - no abstract available

This study reports results of mass transfer calculations using chemical modelling software (HCh) to determine chemical parameters that may have a significant effect on turbidite-hosted gold deposition in Phanerozoic metamorphic terranes. The geochemical models herein consider an 18 component system (Al-As-Au-C-Ca-Cl-Cu-Fe-H-K-Mg-N-Na-O-Pb-S-Sb-Si) and thermodynamic data to simulate a number of geochemical processes including fluid-rock interaction, gas partitioning and mineral precipitation in veins. Each modelling run consists of four parts, namely (1) the minerals predicted to precipitate in the vein, (2) the composition of the fluids in the vein, (3) the predicted alteration assemblage of the host-rocks due to fluid-rock interaction and (4) the composition of the fluids during fluid-rock interaction. Results of the modelling are in good agreement with observed mineral assemblages in variably-endowed orogenic gold provinces (central Victoria and NE Tasmania, Australia; Buller Terrane, New Zealand; Meguma Terrane, Canada; Sierra de Rinconada, Argentina) and illustrate that gold can be precipitated efficiently and over a wide temperature range (350 - 200 C in this study) from low-salinity, mixed aqueous-carbonic fluids containing up to 0.1m CO2. The modelling shows that the absence of certain physicochemical processes (e.g., boiling) or fluid constituents, such as low total sulphur or lack of CO2 may inhibit gold transport and precipitation in some environments. All the modelling runs, except the one involving a low total sulphur fluid, predict the precipitation of sulphides in the host-rocks due to desulphidation processes. However, the highest gold grades are predicted to occur in the vein mainly from partitioning of H2S into the vapour phase during phase separation. This indicates that the efficiency of gold mineralisation is dependant on the concentration of CO2 and other gases in the fluid because of their effect on immiscibility (in both closed and open systems) and the efficient transport and precipitation of gold. We also investigated what effect the composition of a range of source rocks (i.e., granite, turbidites, greenstones, auriferous exhalative interflow sediments) have on gold solubility, as this relates to masses of gold that are accessible by leaching fluids and hence to ore transport and formation. Our thermodynamic calculations suggest that the chemical composition of the hypothetical source rock has only a small influence on the solubility mainly influenced by the initial concentration of gold in the rock.

Submitted November 2000

A.J.Stewart and S.Liu mapped the outcrop geology on Leonora 3140 1:100 000 map sheet in the Eastern Goldfields, Western Australia, in 1998-99 as part of the National Geoscience Mapping Accord (NGMA). Briefly, mapping consists of geological boundaries/units, faults, fractures, folds, veins, dykes, joints, linears, marker beds, trends, structural measurements, and mines/mineral deposits.

Abstracts and proceedings from the Kalgoorlie 2007 conference (24-27 September 2007).

A facsimile copy of the aerial geological and geophysical survey of Northern Australia AGGSNA Reports 1 to 26, 28, 46 to 48, 51 to 59 for Western Australia Pilbara Goldfield, consisting of scanned documents and maps.