Genesis history of iron ore from Mesoarchean BIF at the Wodgina mine, Western Australia

ABSTRACT

Several iron-ore deposits hosted within Mesoarchean banded iron formations (BIFs) are mined throughout the North Pilbara Craton, Western Australia. Among these, significant goethite±martite deposits (total resources >50 Mt at 55.8 wt% Fe) are distributed in the Wodgina district within 2 km of the world-class pegmatite-hosted, tantalum Wodgina deposits. In this study, we investigate the dominant controls on iron mineralisation at Wodgina and test the potential role of felsic magma-derived fluids in early alteration and upgrade of nearby BIF units. Camp-scale distribution and geochemistry of iron ore at Wodgina argue against any significant influence of identified felsic intrusions in the upgrade of BIF. Whereas, the formation of BIF-hosted goethite±martite iron ore at Wodgina involves: (i) early (ca 2950 Ma) metamorphism of BIF causing camp-scale recrystallisation of pre-existing iron oxides to form euhedral magnetite, with local enrichment to sub-economic grades (∼40 wt% Fe) within or proximal to metre-wide, bedding-parallel shear zones, and (ii) later supergene lateritic enrichment of the magnetite-bearing BIF and shear zones, forming near-surface goethite±martite ore. The supergene alteration sequence includes: (i) downward progression of the oxidation front and replacement of magnetite by martite, (ii) local development of silcrete at ∼40 m below the modern surface caused by the lowering of the water-table, (iii) intensive replacement of quartz by goethite, resulting in the goethite±martite ore bodies at Wodgina, and (iv) late formation of ferricrete and ochreous goethite. Goethitisation most likely took place within the hot and very wet climate that prevailed from the Paleocene to the mid-Eocene. Goethite precipitation was accompanied by the incorporation of trace elements P, Zn, As, Ni and Co, which were likely derived from supergene fluid interaction with nearby shales. Enrichment of these elements in goethite-rich ore indicates that they are potentially useful pathfinder elements for concealed ore bodies covered by trace element-depleted pedogenic silcrete and siliciclastic rocks located throughout the Wodgina mine.

KEYWORDS:

• Iron ore
• BIF
• Pilbara
• Cleaverville Formation
• Wodgina
• Western Australia
• supergene
• goethite

Acknowledgements

This work has been funded and logistically supported by the Mineral Research Institute of Western Australia (MRIWA), Atlas Iron Ltd and the Geological Survey of Western Australia, as part of the MRIWA M426 project. We are grateful to Atlas Iron Ltd for support and access to mines and drill cores. In particular, we acknowledge Pip Darvall, Neil Hannaway and Paul Howard for their involvement. Arthur Hickman is thanked for sharing his passion and extensive knowledge of the Pilbara Craton. Bradley MacDonald and Noreen Evans from Curtin University are thanked for their technical support during LA-ICP-MS analyses. Stéphane Roudaut provided helpful assistance during sample processing. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. PD acknowledges funding from the Geological Survey of Western Australia's Exploration Incentive Scheme (a Royalties for Regions initiative).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Minerals and Energy Research Institute of Western Australia [grant number M426].