New strato-tectonic model and geochemical tool for revitalised IOCG targeting

Anderson, John1

1Austrike Resources Pty Ltd, Glenalta, Australia

The combination of a new strato-tectonic model for the Olympic Metallogenic Event (OME) and zircon-based geochemical tool is a significant step-change for ore vectoring for a spectrum of IOCG and coeval deposit styles in the Gawler Craton.

Minerals exploration by Aberfoyle Resources, MIM Exploration and Investigator Resources resulted in the recognition of the Nankivel advanced argillic cap in the nineties and resulting discovery of the 42Moz Paris silver deposit in 2011. Paris is interpreted as an intermediate sulphidation epithermal associated with Nankivel and Helen copper gold silver magnetite skarn, all within a 100km2 sericite pyrite lithocap. The mineralisation is hosted in part by 1620Ma subduction monzodiorites with the Paris-Nankivel mineralisation dated within 2Ma of the 1590Ma Olympic Dam mineralisation.

The revised model proposes the Paris-Nankivel epithermal-porphyry belt is formed at the same time as Olympic Dam on the margins of a super caldera filled with upper Gawler Range Volcanics (GRV). Prior subduction tectonics produced precursor epithermal-porphyry conditions on the southern shoulder of the caldera, whereas IOCGs formed on the northern and eastern margins with haematite- or magnetite-dominated systems respectively forming on the shoulder or in hotter more reduced conditions under the GRV blanket within the caldera.

The epithermal/porphyry and IOCG belts are both fluorine-anomalous and connected by a conductive magneto-telluric (MT) corridor possibly representing a fossil transfer fault and metal source along the mantle interface.

Variations of a mid-GRV stratigraphic marker of the caldera collapse and OME are interpreted as the Bitalli Rhyolite at Paris, Nankivel palaeosurface and as lateral ferruginous sediments and volcanogenic conglomerate facies that collapsed into the IOCG systems preserved under the Stuart Shelf. Drill logs indicate the overlying Pandurra Formation is transitional in places with the mid-GRV marker, so the cover role of that unit needs reassessment with implications for deposit preservation and exploration.

The Zircon Alteration Index (ZAI = 40 – Zr/Hf) is a robust search tool that is universally applicable to the OME spectrum of deposit styles and hosts. Compared with other pathfinder applications, ZAI is a simpler, more reliable and far-seeing proximity tool requiring less assay samples in less drill holes for future targeting. The exclusive association of Hf with Zr in zircon enables wholerock analysis of the Zr/Hf ratio to measure the amount of hydrothermal overprinting of the inherited volcanic or detrital zircon in a host rock and hence proximity to a mineral target. Downhole variations of ZAI and comparisons with lithologies enables lateral or vertical target vectors to be often added to the proximity measure.

The ZAI tool and strato-tectonic vectoring have been validated for the Stuart Shelf IOCG systems with a comprehensive study of 35 holes with the requisite Zr and Hf analyses in varying proximities to known IOCG deposits. Preliminary target ranges are assigned to the ZAI values. Combined with the approach of structural and MT targeting, ZAI identified ten drilled prospects recommended for reconsideration by the tenement holders. The mineral systems targeting approach is also recommended to select areas elsewhere in the Gawler Craton for ZAI target analysis.


Geologist by training and mineral explorer seeking research developments with applications to discovery. Formerly Aberfoyle, MIM Exploration and founding Managing Director of Investigator Resources responsible for several mineral discoveries and subsequent mines. Current Chair of Advisory Board for the Institute of Mineral and Energy Resources at the University of Adelaide.

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