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The Early Cretaceous interbedded sandstone, siltstone and mudstone of the Cadna-owie Formation and the reduced mudstone of the Bulldog Shale overlying the Prominent Hill IOCG deposit in South Australia (total tonnage: 140 Mt. @ 1.2% Cu and 0.5 g/t Au; OZ Minerals Limited 2017) are shown to preserve Cu concentrations up to 5043 ppm Cu. These cover sequence materials are approximately 100 m thick over the basement rocks that host the deposit. This study investigates the source of elevated Cu in the cover material by assessing the Cu deportment in these formations and focuses on understanding element migration processes resulting in development of elevated Cu concentrations. Petrographic and QEMSCAN® analysis show that Cu resides within sulphides including high-temperature Cu-sulphides (e.g. bornite) in the Cadna-owie Formation. The Cu-sulphides are associated with Fe-oxides (magnetite) and minerals including pyroxene, amphibole, garnet, chlorite and plagioclase that are interpreted to be detrital and are likely to be of metamorphic origin. Based on these observations, lateral physical processes driven by sedimentary dynamics are interpreted to be the main processes responsible for elevated Cu concentrations in the Cadna-owie Formation in the Prominent Hill area. Chemical migration processes likely had only localised influence and resulted in minor Cu redistribution. QEMSCAN® data collected on samples from the weathering profile of the Bulldog Shale shows that weathering processes (e.g. pyrite oxidation, serpentinisation and feldspar hydrolysis) mostly affect the upper Bulldog Shale and that Eh-pH conditions gradually change with depth from the oxidised and acidic upper Bulldog Shale to the reduced and slightly acidic lower Bulldog Shale. Copper speciation and Eh-pH conditions are used to interpret downward chemical migration of Cu due to weathering as the main process for development of elevated Cu concentrations within the weathered Bulldog Shale in the Prominent Hill area. Upward chemical and lateral physical migration processes may have influenced Cu concentrations within the unweathered Bulldog Shale, however the degree of influence is yet to be determined. Overall this study shows that elevated Cu concentrations in the Early Cretaceous cover sequences overlying the Prominent Hill deposit can be linked back to mineralisation itself and that processes resulting in elevated Cu concentrations include chemical and physical migration. Refereed/Peer-reviewed

Cover sequence materials of the Bulldog Shale and underlying Cadna-owie Formation within the Eromanga Basin (South Australia) that overlie the Prominent Hill iron oxide-copper-gold (IOCG) deposit are evaluated for their potential to host a geochemical expression of the underlying mineral system and thereby as geochemical exploration sampling media. The Bulldog Shale is clay-rich and has elevated concentrations of trace elements, however, its major and trace element (Cu, Mo, Ni, Pb, U, Zn and REE) chemical characteristics are significantly influenced by intense surface weathering to a depth of ∼50 m from the landsurface. The Cadna-owie Formation is sand-dominated with lesser clay-rich horizons and has elevated concentrations of multiple trace elements (Cu, U, Mo, Ni and REE). A positive correlation between trace and Al2O3 suggests these elements are concentrated within clay-rich horizons. A comparison with background element concentrations within the Cadna-owie Formation in the broader Eromanga Basin, however, shows introduction of additional elements via upward element migration from the underlying mineralised basement rocks and/or downward element migration from the Bulldog Shale. Once weathering influences and natural background concentrations are accounted for, elevated Cu concentrations in the Bulldog Shale and trace elements in the Cadna-owie Formation appear to be spatially associated with the Prominent Hill deposit and surrounding prospects and can then be useful sampling media for mineral exploration in the Prominent Hill region.