CENTRAL GAWLER CRATON - TOWARDS AN INTEGRATED STRATEGY FOR MINERAL EXPLORATION

BACKGROUND The central Gawler Craton in South Australia is a semi arid region of Archaean to Proterozoic basement regarded as having high mineral prospectivity but largely covered by thin sediment cover (mostly 100m) preserved in crystalline basement rocks. CRC LEME activities in the region commenced in 1996 following successful application by companies of regolith carbonate “calcrete” sampling to define areas of anomalous Au – a technique developed from CSIRO studies supported through AMIRA, and extended and promoted by LEME (Lintern, 1997; Lintern, 2002). The Challenger Au Mine (~500,000 ozs Au at average grade 4g/t) was an outcome of this exploration approach (Edgecombe, 1997; Poustie & Abbot, 2006). Investigations were initially on Au prospects defined by calcrete sampling, mostly in Archaean terrains. Regolith characterisation and mapping plus geochemical characterisation in 3D were the principle methods employed to provide recommendations on a preferred geochemical sampling strategy and analytical requirements to explore for Au mineralisation in the region (Lintern, 2004). Prospect scale airborne EM and hydrogeochemical sampling were trialled with some useful results, however the general applicability and cost benefit of these techniques remain to be fully assessed (Lane & Worrall, 2002; Worrall & Gray, 2004; Gray & Pirlo, 2005). The involvement of Primary Industries and Resources, South Australia (PIRSA) as a core participant in a renewed LEME bid in 2001 introduced a broader range of activities, with a focus in LEME on regional data compilations, emphasis on Archaean nickel prospectivity, Mesoproterozoic Au systems, and landscape evolution; the latter largely developed from studies of the sediment cover. Investigations of Tertiary palaeodrainage network and sediment fill were extended to include coastal barrier systems and the potential for mineral accumulation in the transported regolith (Hou & Alley, 2003). This work led to a revised stratigraphy and an evolutionary model for the extensive barrier dunes marginal to the Eucla Basin, originally recognised by Benbow (1990) and in part outlined by present day Ooldea and Barton sand ranges (Hou et al., 2003a; Hou et al., 2003b). Speculation by Hou et al. (2003c) of substantial heavy mineral accumulations in the barrier system, backed by models of possible trap sites based on the revised stratigraphy and dune development, was proved essentially correct with discovery by Iluka Resources, during October to December 2004, of world class resources of zircon at Jacinth and Ambrosia prospects (Hou & Warland, 2005). The total resource at Jacinth and Ambrosia is estimated at 9.2 million tonnes in situ heavy minerals of average grade 4.6% and an average zircon assemblage of 48% (Iluka Resources Ltd., 2006). These were followed by further new discoveries by Iluka at Tripitaka in 2005, and Gullivers in 2006. The in-ground value of resources identified in these 4 prospects is in excess of A$4 billion at current zircon prices (Folwell, 2005; Yates, 2006). Recent LEME activities in the central Gawler include mapping dispersion of Au in sand dune cover, examining the role of vegetation in metal cycling, biomediation of Au in carbonate, clarifying the effect of fluvial deposits on geochemical dispersion, spectral mapping of alteration minerals, both hydrothermal and weathering, in areas of Au mineralisation, and investigating new methods of mapping palaeochannel distribution to the level of detail required for preliminary exploration targeting. The central Gawler Craton was selected by Geoscience Australia (GA), in concert with LEME, for a pilot study of techniques for a regional baseline soil geochemical and groundwater sampling program that ultimately will provide a continent-scale perspective of element distribution to assist mineral exploration and environmental studies. New ideas and approaches continue to evolve from collaboration and interaction with exploration companies and other research groups active in the region, in particular PIRSA Gawler team, GA Gawler mineral promotion project team, and pmd * CRC. New techniques recently applied to Au prospects in the Gawler Craton include spectral core logging using the HyLogger™ system, developed by CSIRO Mineral Mapping Technologies Group, and the CHIM electro-geochemical prospecting technique as modified by Professor Luo at Guilin University of Technology, China. Some recent results are outlined below with comment on how these might impact on mineral exploration strategies in the central Gawler Craton.