Mineralogy as a proxy to characterise geochemical dispersion processes: A study from the Eromanga Basin over the Prominent Hill IOCG deposit, South Australia

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. [Display omitted] •Coppe