Summary:
The Cloudbreak, Christmas Creek and Kutayi deposits lie within the Chichester Ranges, in northern Western Australia. Iron mineralisation is hosted by the Nammuldi Member which is the lowest member of the late Archaean aged Marra Mamba Iron Formation (MMIF). The Nammuldi Member is characterised by extensive, thick and podded iron rich bands, separated by equally extensive units of siliceous and carbonate rich chert and shale. The Nammuldi Member in the Chichester Range is interpreted to be up to 60 metres in true thickness. Underlying the Nammuldi Member rocks are black shales and volcanic rocks belonging to the Jeerinah Formation. Extended periods of tectonic activity have variably folded and faulted these rocks, together with weak metamorphism. Subsequent erosion and hardcapping or lateritic processes have altered these rocks, and present outcrop of Nammuldi Member represents a ridge of lowlying hills (relief up to 30 metres) throughout the prospect areas. These ridges are recognised as the Chichester Ranges.
Drilling within the prospects has proved that the Nammuldi target horizon extends below cover away from the hills. In these regions (recognised mineralisation has been intersected more than 6 kilometres from the outcrop) the target iron formation can be overlain by Tertiary age colluvium and alluvium (younger than 65 Million years). This colluvium can contain both cemented and un- cemented detrital products of iron enriched material, BIF, chert and shale within a matrix of finer grained sediments (including clays). Percolation of groundwater through the weathering profiles has resulted in precipitation of both calcrete and ferricrete creating resistant horizons within the extensive regolith. More proximal to the Fortescue Marsh to the south, the Tertiary sediments become finer grained and more clay dominant, with some recognised calcareous zones.
The structural geology of the area is predominantly concealed with limited exposure in outcrop. However, small scale faulting and folding (metre offsets) are observed in some outcrops, and larger-scale faults are interpreted from aero-magnetics and regional mapping, plus drilling results. In places faults may be the conduit for the mineralisation (hypogene model).
Iron mineralisation characteristically comprises hematite, goethite and ocherous goethite, with variable degrees of alteration between these minerals. The main gangue minerals are kaolinite, quartz and gibbsite, with minor amounts of carbonates, either calcite or dolomite.
Iron is enriched in the parent BIF (iron layers banded with cherts and lesser carbonates) by processes of supergene and/or hypogene enrichment. In both processes, the original iron, which is present as magnetite bands within the BIF, is oxidised to hematite and goethite. Contemporaneous with the iron enrichment, the original gangue minerals are partially to fully leached out or may be replaced by iron minerals. These processes increase the iron content of the BIF depending upon the degree of enrichment. A volume loss of up to 35 per cent can occur with enrichment due to loss of gangue minerals. Microplaty hematite (MplH) is recognised in varying degrees throughout Fortescue’s Chichester Range deposits. This is interpreted to occur due to hypogene enrichment of the MMIF in proximity to tectonic structures (faults or tight folds), which have allowed upward fluid flow, and low-grade metamorphism of the parent rock, resulting in extensive hematite mineralisation.
The majority of the iron mineralisation at the Chichester deposits, is interpreted to be martite-goethite resulting from supergene enrichment of a magnetite-rich BIF (oxidised to martite) parent rock.
Hardcapping (ferricrete development) of portions of the mineralisation has been identified in mapping and drilling. This process, which occurred during latter stages of geological development (Tertiary), has changed the physical and geochemical properties of the upper portions of the mineralisation (up to 10mthickness). Hardcapped material, which can be quite vuggy, typically has a higher density, being pervasively cemented by goethite and commonly has vitreous goethite included in the matrix. An associated increase in gangue content may be seen in hardcap due to the near surface processes of ferricretisation.
The majority of the iron mineralisation is hosted by the Nammuldi Member which is the lowest member of the late Archaean aged Marra Mamba Iron Formation (MMIF). The NammuldiMember is characterised by extensive, thick and podded iron rich bands, separated by equally extensive units of siliceous and carbonate rich chert and shale. The Nammuldi Member in the Chichester Range is interpreted to be up to 60m in truethickness. Underlying the Nammuldi Member rocks are black shales and volcanic rocksbelonging to the Jeerinah Formation. Limited iron mineralisation also occurs in the overlying CID and Tertiary alluvial material.