well as other large IOCG deposits (i.e. Prominent Hill and. Carrapateena). The region is ... Laz Katona, Martin Fairclough, Steven Hill. The Geological Survey of ...
Integrated 3D systems maps for iron copper gold Integrated 3Dmineral Mineral Systems Maps for oxide Iron Oxide Copper Gold (IOCG) Deposits, Eastern Gawler Craton, (IOCG) Deposits, Eastern Gawler Craton, South Australia South Australia Simon van der Wielen, Adrian Fabris, John Keeling, Alan Mauger, Georgina Gordon, Tim Keeping, Phillip Heath, Gary Reed, Laz Katona, Martin Fairclough, Steven Hill
The Geological Survey of South Australia, Department of Manufacturing, Innovation, Trade, Resources and Energy.
Simon van der Wielen, Adrian Fabris, John Keeling, Alan Mauger, Georgina Gordon, Tim Keeping, Phillip Heath, Gary DavidReed, Giles Laz Katona, Martin Fairclough, Steven Hill The Geological Survey Deep of South Australia, Department of Manufacturing, Innovation, Trade, Resources and Energy. The University of Adelaide, Exploration Technologies Cooperative Research Centre. David Giles Scott Halley The University Adelaide, Deep Exploration Technologies Cooperative Research Centre. Mineral Mappingof Propriety Limited Scott Halley Mineral Mapping Propriety Limited Abstract. The eastern Gawler Craton hosts the giant Olympic Dam iron oxide copper gold (IOCG) deposit as well as other large IOCG deposits (i.e. Prominent Hill and Carrapateena). The region is covered by a thick sequence of Mesoproterozoic to Cenozoic sedimentary and volcanic rocks making the region difficult to explore using conventional methods. The project utilises high quality geoscientific datasets that have been systematically acquired by the Geological Survey of South Australia over the eastern Gawler Craton. Modern IOCG mineral system models are combined with the latest three dimensional geological mapping and data integration techniques to produce 3D mineral system maps. These maps provide a powerful new predictive tool to explore for IOCG-style mineralisation undercover.
system formation. It is generally regarded, however, that multiple fluids and metal sources play an important role in the formation of economic mineralisation (Williams et al. 2010).
Keywords. IOCG mineral systems, 3D modelling, data integration, geophysics, geochemistry, spectroscopy, HyLogger™, gOcad™, Gawler Craton.
1 Introduction The Gawler IOCG belt is located along the eastern margin of the Gawler Craton and covering approximately 126,500 km2 (Fig. 1). The belt hosts the giant Olympic Dam deposit, several large deposits and numerous prospects. Despite the proven mineral endowment the region remains largely under explored due to a thick sequence of Mesoproterozoic to Cenozoic sedimentary and volcanic rocks. This makes exploration using conventional methods costly and restrictive. To assist IOCG exploration within the Gawler Craton a collaborative research project has been set up between the Geological Survey of South Australia (GSSA), University of Adelaide and the Deep Exploration Technologies Cooperative Research Centre. The project uses mineral system concepts (Wyborn et al. 1994) along with the latest data integration techniques to produce integrated IOCG mineral system maps from the vast trove of open file geoscientific data that has been systematically acquired by the GSSA.
2 IOCG Mineral System Model There is still conjecture over processes of IOCG mineral
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Figure 1. A locality map showing an outline of Gawler Craton, eastern Gawler IOCG belt and IOCG deposits in relation to the regional and Emmie Bluff study areas.
The following features are important in the formation of IOCG mineral systems within the Gawler Craton: 1. Melting of metasomatised mantle resulted in the emplacement of mafic intrusions along the eastern margin of the Gawler Craton. This also provided a source of metals and ligands and an energy source to drive hydrothermal fluids (Hayward and Skirrow 2010). 2. Meteoric water percolated through a rift package
MINERAL DEPOSIT RESEARCH FOR A HIGH-TECH WORLD ▪ 12th SGA Biennial Meeting 2013. Proceedings, Volume 1
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containing thick sequence of evaporates producing dense oxide saline brine. The brine stripped out metals from the underlying rocks forming the characteristic distal Na-Ca alteration (e.g. albitisation) that is widespread in the belt (Bastrokov et al. 2007). The mechanism for metal deposition for IOCG systems within the Gawler Craton is still unclear and may vary between deposits. The consensus is that metal deposition will occur at a change in redox conditions, either by fluid mixing or fluid wall rock interaction (Williams et al. 2010). Temperature, pH and ligand activity may play an important role in particular deposits (Bastrokov et al. 2007).
IOCG mineral systems have a characteristic pattern of zoned alteration that extends significant distances from the deposit (Fig. 2). K-feldspar-sericite forms the distal alteration assemblage and becomes progressively more intense towards the centre of the system. The deepest parts of the system have magnetite-biotite alteration assemblage. Higher in the system magnetitechlorite alteration assemblage becomes dominate followed by hematite-sericite alteration that forms at the highest levels of the system (Fig. 2). Chemical modelling by Bastrokov et al. (2007) has shown that high grade copper and gold mineralisation tends to occur in regions where hematite has replaced earlier magnetite alteration.
3 Workflow A workflow to map IOCG mineral systems in 3D has been developed: 1. Construction of a 3D geological map. This is used as a container to integrate geophysics, geochemistry and spectral data, and to query and visualise results. 2. Mapping hematite and magnetite in 3D using potential field inversions. 3. Mapping chlorite, sericite, K-feldspar, albite in 3D with geochemistry and spectral data. 4 Construction of 3D geological map Three dimensional geological surfaces were built in gOcad™ using solid geology, drill hole stratigraphic markers, and seismic data (Fig. 3a). The geological surfaces were converted into geological block model (Fig. 3b) so that other proprieties such as geochemistry, spectral, potential field and petrophysical data can be integrated with geology, queried and visualised.
Figure 3. The Emmie Bluff 3D geological map showing geological surfaces and major faults (a); and Emmie Bluff 3D geological voxet (b).
Figure 2. A conceptual mineral system model for granite hosted IOCG deposits (a); and an alteration model for Emmie Bluff IOCG deposit.
5 Mapping the distribution of magnetite and hematite with potential field inversions In simplest terms alteration changes the mineral composition of the host rock. These changes in mineral composition in turn modify the physical and/or chemical properties of a rock. For example, hematite alteration 3D-modelling of ore deposits
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will increase the iron content of a rock and increase the density. Where the physical properties of the un-altered host lithology and the alteration minerals are known, it may be possible to predict the type and amount of alteration present by measuring the petrophysical properties of a rock. Using the principles outlined above Chopping and van der Wielen (2009) developed a technique using potential field inversions in conjunction with petrophysical data and a geological block model to map oxide and sulphide minerals in 3D. This technique was used to map out the distribution of magnetite and hematite alteration associated with IOCG mineral systems in 3D (Fig. 4).
Figure 4. The Emmie Bluff 3D model showing mapped distribution of hematite and magnetite that was derived from potential field inversions (a); magnetic susceptibility inversion values draped onto the alteration shell (b); and density inversion values draped onto the alteration shell (c).
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6 Mapping the distribution of sericite, albite, Kfeldspar and chlorite alteration using geochemistry and HyLogger™ data The GSSA is custodian of exploration drill core that has become public domain in South Australia. Drill holes that have intercepted basement within eastern Gawler IOCG province are being systematically scanned with a visible to thermal infra-red wavelength spectrometer HyLogger™ and basement intercepts are sampled every 10 metres and analysed for a suite of 66 elements (Fabris et al. 2012).
Figure 5. Geochemistry data for Emmie Bluff region displayed on a feldspar Na-K general element diagram (a); geochemical samples displayed in 3D and characterised using the feldspar Na-K general element diagram (b); a 3D model of alteration derived from gridded geochemistry data (c).
MINERAL DEPOSIT RESEARCH FOR A HIGH-TECH WORLD ▪ 12th SGA Biennial Meeting 2013. Proceedings, Volume 1
As with potential field data, geochemistry can be used to predict the amount and type of alteration present. Drill core geochemistry can be plotted on a feldspar Na-K general element ratio diagram to discriminate sodium and potassium alteration from unaltered host lithology (Fig. 5a). Albitisation will increase the Na/Al ratio when compared to an un-altered host lithology whereas chlorite, sericite and K-feldspar alteration will decrease Na/Al ratio (Fig. 5a). Chlorite, sericite and K-feldspar alteration can be distinguished from each other by variation of the K/Al ratio (e.g. chlorite low K/Al ratio, sericite moderate K/Al ratio and K-feldspar alteration high K/Al ratio). This can be taken a step further and gridded geochemical data can be classified and plotted in 3D space (Fig. 5).
Sericite alteration is made up of fine-grain white mica, usually muscovite (K2Al4(Si6Al2)O20(OH)4) or phengite (K2(Al,Mg,Fe)4(Si6+x,Al2-x)O20(OH)4). Muscovite and phengite are indistinguishable in hand specimen but can be readily identified with HyLogger™. Muscovite has an AlOH absorption minimum at wavelength ~2,206 nm whereas phengite has an absorption minimum at longer wavelengths (e.g. 2,218 nm). Variation in sericite mineral composition can be used as a proxy for fluid chemistry. Assuming the temperature is the same muscovite will form under acidic conditions whereas phengite will form under neutral conditions. This is important as metals will likely to be precipitated at sites where there is a change in fluid chemistry. By plotting the wavelength of the AlOH absorption minimum (Fig. 6) it is possible to directly map the spatial distribution of muscovite (e.g. green and blue regions in Fig. 6c) and phengite (e.g. orange and red regions in Fig. 6c) and identify likely regions where chemical conditions favoured metal deposition.
Acknowledgements The research is coordinated and supported through the Deep Exploration Technology Cooperative Research Centre and is a collaborative project between Geological Survey of South Australia and the University of Adelaide. GSSA is thanked for providing indirect financial support to the primary author to attend the twelfth SGA Biennial Meeting.
References
Figure 6. HyLogger™ data displayed in 3D and coloured by wavelength of the AlOH absorption feature (a); a 3D model of sericite and sericite-chlorite alteration derived from gridded geochem data (b); wavelength of the a AlOH absorption feature draped onto the sericite alteration shell (c).
Bastrokov EN, Skirrow RG, Davidson GJ (2007) Fluid Evolution and Origins of Iron Oxide Cu-Au Prospects in the Olympic Dam District, Gawler Craton, South Australia. Economic Geology 102:1415-1440 Chopping R, van der Wielen SE, (2009) Querying potential field inversions for signatures of chemical alteration: an example from Cobar, NSW. ASEG Extended Abstracts 2009, pp 1-8 Fabris A, van der Wielen SE, Mauger A, Halley S, Keeping T, Keeling J (2012) Geochemical trends in IOCG alteration – new data from the Gawler Craton. In: Baker T (eds) Unlocking SA’s mineral wealth, technical forum, 2 May 2012, extended abstracts, pp 7-8 Hayward N, Skirrow RG (2010) Geodynamic Setting and Controls on Iron Oxide Cu-Au (±U) Ore in the Gawler Craton, South Australia. In: Porter TM (eds) Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, PGC Publishing, Adelaide, 3:105-131 Williams PJ, Kendrick MA, Xavier RP, (2010), Sources of ore fluid components in IOCG deposits. In: Porter TM, (eds) Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide, 3:107-116 Wyborn LAI, Heinrich CA, Jaques AL (1994) Australian Proterozoic Mineral Systems: Essential Ingredients and Mappable Criteria. The AusIMM Annual Conference, pp 109115
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