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Geophysical exploration for iron ore in the Middleback Ranges, South Australia

Iron has been mined in the Middleback Ranges-Katunga Hills in the northeast Eyre Peninsula since the turn of the last century. The iron ore mineralisation was formed primarily by supergene enrichment of Palaeoproterozoic iron formation and most deposits are located close to outcropping iron formation. Faults and fold axes probably acted as fluid conduits localising and focusing the fluids required for the ore-forming processes. Basic and ultrabasic intrusives probably acted as impervious barriers, further localising these fluids. The iron formations in the Middleback Ranges are both dense and highly magnetic when compared with other lithologies in the area and this has enabled gravity and magnetic data to be used successfully to map structure and stratigraphy. The iron ores are in general denser, but less magnetic, than the iron formation, although there is overlap in their range of physical properties. Most iron-ore deposits give rise to positive gravity anomalies, but the recognition of these anomalies is hindered by the presence of larger anomalies due to the iron formations. Also, similar anomalies may be caused by basic intrusives. There is no recognisable response from the ores in magnetic data. Trial electromagnetic surveys defined anomalies in some areas of mineralisation. However, petrophysical measurements show the ores are not conductive and these responses are probably caused by saline groundwater and clay minerals.