Using trace element chemistry of magnetite as an indicator mineral at the Starra iron-oxide copper gold deposits, Northwest Queensland

Iron Oxide-Copper-Gold (IOCG) deposits are hydrothermal ore deposits that occur worldwide and globally account for significant amounts of copper, gold, and uranium. They are defined by large scale potassic, sodic and iron alteration that often extend several kilometres from the mineralised centre, making it difficult to differentiate between fertile and barren alteration systems. In recent years, it has been suggested that trace element chemistry of alteration minerals may be used to discriminate fertile from barren hydrothermal systems. In this aims to test the discrimination potential of magnetite at the Starra Au-Cu system which is hosted in the Eastern Fold Belt of the Mount Isa Inlier in Northwest Queensland (Australia). Five deposits occur along a circa 6 kilometres long interval. The mineralisation is spatially associated with magnetite-hematite dominated ironstones along the Starra shear, which are focused at the contact between the Answer Slate to the west and the Staveley Formation to the east. Magnetite is the dominant iron oxide mineral in the ironstones and define a strong magnetic anomaly that can be detected for more than 20 kilometres. Incorporation of trace elements in magnetite is influenced by physicochemical parameters at time of precipitation such as temperature and redox, making it ideal to record process-based information. In addition, the extensive nature of the ironstones at Starra makes it an ideal location to study the trace and major elements variations in magnetite with increasing distance to the ore bodies.

At Starra, mineralisation zones are associated with variable magnetite overprinted by hematite. New laser ablation ICP-MS of magnetites from distal, proximal, and mineralised setting reveals that magnetite spatially associated with the mineralisation contain lower V content compared to distal magnetite, suggesting higher fO2 conditions in mineralised areas. Hematite within in ironstones replaced the pre-existing sedimentary lithology retains the original texture as shown by different crystal orientation and distinct trace element concentrations in hematite. Scheelite inclusions in magnetite support reduction of hematite to magnetite due to relative incompatibility of W in magnetite compared to hematite. Together, these indicate a complex evolution of prevailing redox conditions during formation of the IOCG system.

Our findings support previous formation models that suggest the main controlling factor on the Cu-Au precipitation in IOCG systems is the oxygen fugacity fO₂ of the hydrothermal fluid. At Starra, previous genetic model suggested that oxidized fluids interacted with magnetite and were reduced, leading to reduction of sulphates to bisulphides and precipitating Cu.