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

  • Max R. Verdugo-IhlEmail author
  • Cristiana L. Ciobanu
  • Nigel J. Cook
  • Kathy J. Ehrig
  • Liam Courtney-Davies


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.


IOCG Olympic Dam Fe-metasomatism Silician magnetite HFSE-bearing hematite 



We extend thanks to the staff of Adelaide Microscopy, particularly Sarah Gilbert and Benjamin Wade, who assisted with LA-ICP-MS and EPMA, respectively. Constructive comments from Peter Pollard, an anonymous reviewer, Associate Editor David Huston and Editor-in-Chief Bernd Lehmann are appreciated, and helped improve the manuscript.

Funding information

This work is a contribution to the ‘FOX’ project (Trace elements in iron oxides: deportment, distribution and application in ore genesis, geochronology, exploration and mineral processing), supported by BHP Olympic Dam and the South Australian Government Mining and Petroleum Services Centre of Excellence. N.J.C. acknowledges support from the ARC Research Hub for Australian Copper-Uranium (Grant IH130200033).

Supplementary material

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.School of Chemical EngineeringThe University of AdelaideAdelaideAustralia
  2. 2.BHP Olympic DamAdelaideAustralia

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