A synopsis of the channel iron deposits of the Hamersley Province, Western Australia

Abstract

The channel iron deposit type or CID is one of the two major iron ore types mined in the Hamersley Province. Channel iron deposits provided around 40% of the total of 394 Mt of iron ore mined from the Hamersley Province in 2009, and the current CID resource is around 7 billion tonnes. Channel iron deposits occupy numerous meandering palaeochannels in a mature surface that includes Precambrian rocks and ferruginous Palaeogene valley fill. These palaeochannels are generally less than a kilometre but can range to several kilometres in width and from one metre to more than 100 m thick. There are two currently mined CID areas: the first in the western Hamersley Province in the Robe palaeochannel, and the second in the eastern Yandi palaeochannel. These two major CID channels extend over 100 to 150 km, with the Robe system up to 5 km wide. The granular ore facies typically contains ooids and lesser pisoids with hematite nuclei and goethite cortices, with abundant goethitised wood/charcoal fragments and goethitic peloids, minor clay, and generally minimal porous goethitic matrix. Post deposition weathering is common and produced secondary facies. Ooids and pisoids were mostly derived from the stripping of a well-vegetated, deep ferruginous surface. The peloids were derived intra-formationally from both fragmentation and reworking of desiccated goethite-rich mud. Minute wood/charcoal fragments in the soil were initially replaced by goethite and then dehydrated to hematite, forming nuclei for many ooids and pisoids. In addition, abundant, generally small (<10 mm) fragments of wood/charcoal were replaced in situ by goethite within the consolidating CID. This profusion of fossil wood, both as ooids and pisoids nuclei and as discrete fragments, as well as the local presence of kenomagnetite suggests major episodic wild fires in heavily vegetated catchments. The goethitic matrix was the result of chemically precipitated iron hydroxyoxides, resulting from leaching of iron-rich soils in an organic environment.