Abstract attached

Abstract The Palaeoproterozoic, from 2100 to 1800 Ma, is recognised as the third largest period of orogenic gold mineralization. In contrast to earlier Archean orogenic gold episodes which occur predominantly in greenstone terranes, supracrustal sedimentary rocks became increasingly important as hosts in the Palaeoproterozoic. Unusually iron-rich 1840 Ma marine mudstones in the Tanami region host one world class gold deposit and many other gold deposits. Fluid-rock modelling at 350°C suggest a strong correlation between gold grade and these iron-rich, fine-grained sedimentary rocks and suggest that gold may precipitate in the iron-rich sediments in the first stage of mineralization, before remobilization of the gold further enhances the grade of the deposit. New regional stratigraphic correlations for similar iron-rich rocks to those in the Tanami region are suggested with ~1860 Ma gold-bearing stratigraphy in the Pine Creek region and potentially with ~1860 Ma host rocks in the Tennant region. These Northern Australian Palaeoproterozoic iron-rich sedimentary rocks could be linked globally to similar aged iron-rich and gold-bearing sedimentary rocks in Homestake, U.S., Ghana, West Africa and elsewhere. From about 2400 to 1800 Ma the Palaeoproterozoic is also marked by the occurrence of mainly Superior-style BIF's, which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans. The high iron concentrations noted in pre-1800 Ma marine sediments in Northern Australia could also be related to this same process and help explain the anomalous concentration of orogenic Au deposits from 2100 to 1800 Ma.

The Palaeoproterozoic Westmoreland region is located 1250 km southeast of Darwin. The Westmoreland region is flanked on the southeast by the Palaeoproterozoic Mt Isa Inlier and the Neoproterozoic South Nicholson Basin and in the northwest it is overlapped by Mesoproterozoic sediments of the McArthur Basin. The northern and southern ends of the McArthur basin share many geologic attributes including correlative stratigraphic rock types, which suggests that there is potential for unconformity-related uranium deposits in the southern McArthur basin and adjacent Westmoreland region. In fact, over fifty occurrences of uranium (some with minor gold) and copper mineralisation have been recorded in the Westmoreland region. Fluid inclusion studies have been carried out on selected uranium and copper prospects on the Northern Territory side of the Westmoreland region. Four types of inclusions have been observed, (Type A) Vapour-rich inclusions containing 30 - 100 vol.% vapour. Varying amounts of CO2 ± N2 ± CH4 have been detected in these inclusions, (Type B) Liquid-rich inclusions with up to 30 vol.% vapour, (Type C) Liquid-only inclusions, and (Type D) Three-phase (vapour + liquid + solid) liquid-rich inclusions containing a small daughter crystal. Type A, vapour-rich inclusions and some Type B, liquid-rich inclusions homogenised over the range 171 to 385 °C and are thought to be related to early metamorphic events. Other Type B and Type D inclusions typically homogenised between 100 and 240 °C with a mode around 120 °C, while the presence of liquid-only inclusions suggests trapping at temperatures below 50 °C. Eutectic melting temperatures indicate the presence of CaCl2 in the fluids but final melting temperatures show the presence of both high and low salinity brines. This suggests mixing between saline basinal fluids and low salinity meteoric fluids that continued down to temperatures below 50 °C.

The distribution of chemical elements at the Earth's surface is complex and reflects the geochemistry and mineralogy of the original substrate modified by environmental factors that include physical, chemical and biological processes over time. Geochemical data typically is illustrated in the form of horizontal maps or vertical cross-sections, where the composition of regolith, bedrock or any other material is represented. These are primarily point observations that frequently are interpolated to produce rasters of element distributions. Here we propose the application of environmental or covariate regression modelling to predict and better understand the controls on major and trace element geochemistry within the regolith. Available environmental covariate datasets (raster or vector) representing factors influencing regolith composition are intersected with the geochemical point data in a spatial statistical correlation model to develop a system of multiple linear correlations. The spatial resolution of the environmental covariates, which typically is much finer (e.g. ~90 m pixel) than that of geochemical surveys (e.g. 1 sample per 10 to 10,000 km2), carries over to the predictions. Therefore the derived predictive models of element concentrations take the form of continuous geochemical landscape representations that are much more informative than geostatistical interpolations. Environmental correlation is applied to the Sir Samuel 1:250 000 scale map sheet in Western Australia to produce distribution models of individual elements describing the geochemical composition of the regolith and exposed bedrock. As an example we model the distribution of two elements chromium and sodium. We show that the environmental correlation approach generates high resolution predictive maps that are statistically more accurate and effective than the popular ordinary kriging and inverse distance weighting interpolation methods. Furthermore, insights can be gained into the landscape processes controlling element concentration, distribution and mobility from analysis of the covariates used in the model. This modelling approach can be extended to groups of elements (indices), element ratios, isotopes or mineralogy over a range of scales and in a variety of environments.

Geophysical responses, such as gravity anomalies, arise from variations in physical properties, such as density, in the subsurface. These physical properties are predominantly controlled by mineralogy. Chemical alteration varies the mineralogy of a rock, potentially producing a geophysical response due to the alteration. Physical property models can be calculated for numerical simulations of chemical alteration, such as reactive transport simulations; these physical properties allow the geophysical signatures of alteration to be calculated.

An abstract outlining capabilities of GA's FreeGs system for geochemical modelling for 1st Russian-Swiss Seminar on "Methods for modelling of geochemical processes: algorithms, data prediction, experimental validation, and relevant applications"

Fluid inclusion studies have been carried out on quartz veining from Jackson's Pit and Eva uranium mines and the Dianne and St Barb copper prospects in the Westmoreland region. Four types of inclusions have been observed. Type A, vapour-rich inclusions, contain 30 - 100 vol.% vapour with varying amounts of CO2 ± N2 ± CH4. Type B, liquid-rich inclusions, contain up to 30 vol.% vapour. Type C inclusions are liquid-only. Type D, three-phase (vapour + liquid + solid) liquid-rich inclusions, contain a small daughter crystal. Type A, vapour-rich inclusions and some Type B, liquid-rich inclusions homogenised over the range 171 to 385°C. Other Type B and Type D inclusions typically homogenised between 100 and 240°C with a mode around 120°C, while the presence of liquid-only inclusions suggests trapping at temperatures below 50°C. This may indicate three phases of fluid flow in the region with progressively cooling fluids. Eutectic melting temperatures as low as -79.8ºC in Type B and C inclusions suggest the presence of CaCl2 and other salts in the fluids. Final ice meeting temperatures for Type B and C inclusions fall into two groups. The first group has final melting temperatures below -10ºC while the second group shows final meeting above -10ºC and more typically close to 0ºC indicating the presence of low salinity fluids. This suggests mixing between saline basinal fluids and low salinity meteoric fluids that continued down to temperatures below 50°C.

Several scenarios of an original 3D model based on the petroleum systems model of Fuji et al. (APPEA 2004) were simulated using the PetroMod 3D V.10 modeling software. In general the results of the modelling study presented here confirms the modelling results of Fuji et al. (2004) with respect to the timing of generation in the different sub-basins as well as present day maturity. The main differences between the work of Fuji et al. (2004) and the work presented here are based on the use of PhaseKinetic models for the individual source rock formations and the ensuing compositional predictions of the fluids in different fields. Source rock transformation ratios as well as the bulk generation rates indicate that the source rocks are presently still generating. The Central Swan Graben area is presently more mature than the other kitchen area of the Vulcan Sub-basin, the Cartier Trough. The locations of predicted accumulations coincide with the locations of most of the proven fields. In many cases accumulation sizes and predicted column heights are large, mainly due to the fact that the resolution of the numerical model is low which leaves rather large volumes of the cells to be filled. Modelling results predict a series of accumulations at locations which have, as yet, not been tested. However, most of them depend on fault closure, thus increasing exploration risk. The main risks as observed from this modelling exercise are: 1) source rock presence and definition, 2) definition of the traps, 3) resolution of the input model, 4) cap rock properties, which are still largely unconstrained. The different scenarios modelled show distinct variations with respect to predicted petroleum distribution as well as the physical properties of the accumulated fluids.

Globally supracrustal sedimentary rocks are known to preferentially precipitate gold between 2400 Ma and 1800 Ma (Goldfarb et al. 2001). The Palaeoproterozoic Tanami and Pine Creek regions of Northern Australia host one world-class gold deposit and many other gold deposits in anomalously iron-rich marine mudstones (Figure 1). New fluid-rock modelling at temperatures between 275 - 350C suggest a strong correlation between gold grade and these Palaeoproterozoic iron-rich, fine-grained sedimentary rocks.

High-CO2 gas fields serve as important analogues for understanding various processes related to CO2 injection and storage. The chemical signatures, both within the fluids and the solid phases, are especially useful for elucidating preferred gas migration pathways and also for assessing the relative importance of mineral dissolution and/or solution trapping efficiency. In this paper, we present a high resolution study focused on the Gorgon gas field and associated Rankin trend gases on Australia's Northwest Shelf of Australia. The gas data we present here display correlate-able trends for mole %-CO2 and %C CO2 both areally and vertically. Generally, CO2 % decreases and becomes depleted in %C (lighter) upsection and towards the north; a trend which also holds true for the greater Rankin trend gases in general. The strong spatial variation of CO2 content and %C and the interrelationship between the two suggests that processes were active to alter the two in tandem. We propose that these variations were driven by the precipitation of a carbonate phase, namely siderite, which is observed as a common late stage mineral. This conclusion is based on Rayleigh distillation modeling together with bulk rock isotopic analyses of core, which confirms that CO2 in gases are genetically related to the late stage carbonate cements. The results from this study have important implications for carbon storage operations and suggest that significant CO2 may be reacted out a gas plume over short migration distances.