Defining early stages of IOCG systems: evidence from iron oxides in the outer shell of the Olympic Dam deposit, South Australia

Abstract

The IOCG deposit at Olympic Dam (South Australia) is hosted within the Roxby Downs Granite, which displays a weakly mineralised contact to the orebody (hereafter ‘outer shell’). In a mineralogical-geochemical characterisation of Fe-oxides from the outer shell, we show silician magnetite (Si-magnetite) and HFSE-bearing hematite define the early stages of alkali-calcic alteration. This association forms in the presence of hydrothermal K-feldspar and calc-silicates via overprinting of magmatic magnetite and ilmenite breakdown. Geochemical modelling, at ≥ 400 °C, shows such reactions occur at pH-fO2 conditions coinciding with shifts from K-feldspar to sericite, and ilmenite to rutile stability. The subsequent Si-magnetite+siderite association forms down-T in the absence of K-feldspar. Transition from granular to bladed morphologies in Si-magnetite is part of a series of Fe-oxide interconversions, followed by formation of zoned, U-W-Sn-Mo-bearing hematite. Enrichment in REE, Y and U in Si-magnetite and the prevalence of U-W-Sn-Mo-bearing hematite support a granite-derived fluid. Combined, petrographic and geochemical evidence show a transition among Fe-oxides from the outer shell to the orebody attributable to the evolution of the same fluid. Unusual massive magnetite intervals and Fe-oxide nodules in granite are considered due to either the presence of inherited lithologies, metasomatic products, or the result of magnetite-rich, crystal mush forming in the melt. We propose a model, corroborated by recently published data including high-precision U-Pb dating of magmatic zircon and hydrothermal hematite, in which an ‘outer shell’ is initiated at the 6–8 km depth of granite emplacement during volatile release from fluids ponding at intrusion margins. Granite cupola collapse at shallower levels (2–3 km?) follows via uplift along faults, facilitating intense brecciation and ore formation.