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Contrasting styles of high-grade iron mineralisation at Weld Range, Yilgarn Craton, Western Australia

Weld Range, Yilgarn Craton, Western Australia

This two-year study on the genesis of BIF-hosted iron ore deposits in the Weld Range district was completed in mid-2011. Sponsored by Sinosteel Midwest Corporation Ltd., the goal of the project was to understand the main controls on the genesis of the ore bodies and to improve the exploration strategy for the district.  

The Weld Range greenstone belt, in the Murchison Domain of the Yilgarn Craton, hosts two Archean, high-grade magnetite±hematite±goethite deposits, Madoonga (68 Mt resource at 57.7 wt.% Fe) and Beebyn (62 Mt resource at 59.6 wt.% Fe, ASX announcement 2008). Five main styles of high-grade Fe mineralisation at Weld Range were recognised and each display characteristic grades, tonnage, and contaminant levels, which affect exploration strategies and beneficiation methods.

(1) Residual ore” formed as a result of two generations of hypogene alteration of BIF. The first alteration phase replaced silica-rich bands with siderite and/or Fe-rich dolomite; the second phase involved the removal of carbonate gangue minerals and the concentration of residual Fe oxide-rich bands in the BIF via volume reduction. The product is a high-grade (>50 % Fe), high-tonnage Fe ore body, with minor contaminants. (2) Magnetite-bearing shear and fault zones cut BIF along lithological contacts. These zones of secondary magnetite formed as a result of the addition of hypogene magnetite via the circulation of Fe-rich hydrothermal fluids through BIF. This style of mineralisation produces narrow (low-tonnage), high-grade magnetite-rich ore zones with high levels of contaminants (e.g. SiO2, Al2O3) due to the difficulty in mining these zones. (3) Specular hematite±quartz-bearing shear and fault zones cut residual ore bodies and magnetite-bearing structures. They share similar characteristics to magnetite-bearing structures. (4) Goethite-hematite supergene ore zones are controlled by brittle faults that cut BIF. These faults promoted the flow of supergene fluids through BIF and resulted in large ore zones with variable Fe grades and contaminants. (5) Detrital deposits comprise transported BIF fragments and are intensely goethite-hematite supergene-altered. These ore bodies are locally extensive, with moderate Fe grades.

In addition to mapping and detailed examination of diamond drill core, ASTER data was acquired for the Weld Range district as a pilot project to test the usefulness of ASTER geoscience products for the identification of iron oxide minerals and hypogene alteration mineral assemblages in neighbouring mafic igneous country rocks. The results were positive in that the ASTER products proved useful for the delineation of outcrop, high-grade iron ore bodies, and hypogene alteration minerals in mafic rocks (e.g. Fe-rich chlorite and Fe-rich talc).

The Weld Range study provides insights into multiple processes of iron enrichment in BIF, leading towards the formation of hybrid deposits that host more than one style of mineralisation. These deposits are heterogeneous in terms of their physical and chemical characteristics (ore grades, contamination minerals, density, magnetic susceptibility), A district-wide exploration strategy needs to first evaluate the types of Fe ore present in the district, rank these types in order of economic importance, and to then systematically explore for the ore type of greatest interest."

RELATED PUBLICATIONS

Iron Research Bibliography

Duuring, P., and Hagemann, S. G., 2010, High-grade iron mineralisation at the Beebyn deposit, Weld Range, Western Australia: 5th International Archean Symposium: Abstracts, Perth, Western Australia, 2010, p. 291-293.

Duuring, P., and Hagemann, S. G., 2011, Contrasting styles of high-grade iron mineralisation at Weld Range, Western Australia, Iron Ore 2011: Perth, The Australasian Institute of Mining and Metallurgy, p. 87-92.

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