Channel Iron Deposits In Ore Deposits
The channel iron deposits (CID) are iron-rich fluvial sedimentary deposits of possible Miocene age occupying meandering palaeochannels in the early to mid – tertiary Hamersley paleosurface of western Australia.
The deposits are anomalously high in iron for detrital material and exclude detrital iron deposits typified by scree of hematitic banded iron formation and accumulations of currently forming maghemite pisolite alluvials.
CIDs are a major source of cheap, high grade iron ore, exploited primarily in the pilbara and muvchison regions of Western Australia. (Ramanaidou et al., 2003).
The morphology shows that channel iron deposits are typically partly eroded and currently are from between less than 1m to 100m thick, with preserved channel widths of between 100m and greater than 5km. Mineralized channels are up to 150 kilometers in length, but not all of the preserved length of the CID is of ore grade. The channel iron system typically form within a depression on the tertiary “Hamersley surface and form several pods downstream on the paleo-drainage”.
The channels shows typically fluvial sedimentary morphology, with channel scours truncating or lncising the channel iron deposits and rare examples of graded bedding, and so on.
Individual ore deposits are subsets of a larger sub-economic mineralized system, which varies laterally and along the paleo-drainage. The deposits form lensoidal accumulations with interbeds of clays, gravel and siliceous detrital materials.
No clear geochronological data exist for CIDs, as no radioisotope methods are applicable to directly date CID. Palynological data do exist but cannot constrain ages sufficiently beyond centering on the middle Miocene.
The formation mechanism proves that the source of iron for the CIDs is believed to be a Miocene aged iron –rich soils which developed upon a palaeosurface (since eroded), which developed in the early Miocene during hot humid conditions. The erosion of this ferritic palaeosurface in the mid Miocene transported of the iron-rich soils into the palaeo-drainage system, where the iron become consolidated within the existing river courses.
The river beds were at the time rich-humic swamps with thick vegetation, and accumulation of peat or thick detrital vegetation. Most CIDs are under laid by organic rich clays and or Miocene aged lignite.
The iron becomes fixed in place in the existing humic material via replacement with goethite (Morris and Ramanaidou 2007).