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Location: 167 km SE from Darwin, Northern Territory, Australia
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Most iron ore production has occurred from the Frances Creek deposits near Pine Creek, which are hydrothermally-enriched hematite deposits within the Pine Creek Orogen. The Frances Creek tenement group provides a cross section of the Early Proterozoic sedimentary stratigraphy of the Pine Creek Geosyncline. The eastern most tenements cover sedimentary rocks of the Namoona and Mt Partridge Groups; the central tenements cover sedimentary rocks of the South Alligator and Mt Partridge Groups, including the ironprospective Lower Wildman Siltstone, whilst the western tenements cover sediments of the Finnis River and South Alligator Groups. The sediments are complexly folded in a NNW trend. Conformable sills of Early Proterozoic Zamu dolerite are folded with the sediments. Cretaceous quartz-pebble conglomeritic sandstone forms remnant plateaus over the central tenement area.The South Alligator Group covers the western tenements, hosting the iron-rich Koolpin Formation. The Koolpin Formation is represented by a series of narrow mineralized brecciated siltstone units located in a north-south trending valley located about 2km due west of the present mine workings at Frances Creek. The mineralisation is primarily iron with secondary manganese. The mineralisation can be recognized in the majority of outcrops by a conspicuous dark knobbly siltstone breccia. Occasionally, the iron is a replacement mineral in non-brecciated siltstone. Outcrops containing the manganese mineralisation are less continuous and are more likely to be displaced by faulting.The Koolpin Formation unconformably overlies the Wildman Siltstone and is conformably overlain by the Gerowie Tuff. In places, sills of Zamu Dolerite have intruded along the upper and lower contacts and within the sequence.The Frances Creek Iron deposits are hosted by the lower Wildman Siltstone, which is predominantly composed of Lower Proterozoic carbonaceous shales and siltstone. The iron mineralisation on a broad scale is stratiform as it follows the trace of a regional NNW trending shallowly plunging non-cylindrical anti-form and its subordinate parasitic folds. The iron deposits generally have moderate to steep dips on the fold limbs and appear to attain best grades and thicknesses within smaller parasitic drag folds, flexures and associated fold/fault breccias. The major folds reportedly formed as a result of ENE-WSW shortening during regional deformation event D3 (NTGS, 1993).However, the iron mineralisation itself appears to post-date the D3 folding event. Undeformed breccia textures and textures indicative of high level open-space deposition (euhedral hematite and quartz, crystal lined voids, colloform banding) are ubiquitous within the deposits. The ore bodies were probably formed by low temperature hydrothermal (probably supergene) hematite (+-euhedral quartz+-kaolin) deposition within pre-existing breccias, which were formed by both high level folding in the siltstone host and within breccias possibly formed by the dissolution collapse and replacement of specific carbonate and/or sulphide beds within the Wildman Siltstone. Dolomitic carbonate and major cavities intersected in drilling directly below the Helene 6/7 and Helene 11 deposits support the role of carbonates in breccia formation (these may also host economic manganese mineralization). The fold breccias are frequently associated with F3 axial planar faults and folds or Post-D3 faults. Evidence of deformation subsequent to the formation of the iron mineralization is scarce and is restricted to brittle faulting and jointing.Dykes of Early Proterozoic Zamu dolerite are intimately associated with the iron deposits. They appear to predate iron deposition, and are mostly conformable sills that have undergone the same folding and brecciation events as the host sediments. The dolerites may also in part be replaced by hematite. The apparent close relationship of dolerites and iron mineralisation is probably due to increased brecciation around the margins of the dolerites due to pre-existing weaknesses caused by their intrusion, associated hornfelsing of sediments and the resulting rheological contrasts between dolerite and the host meta-sediments. There is no evidence to suggest that the dolerites were a source of the hydrothermal iron bearing fluids. None of the weathered dolerites seen at Frances Creek appear depleted in iron.Bleaching of siltstones in the hanging wall sequence has been postulated as an indicator of hydrothermal fluid flow. However, drill core frequently shows no or little bleaching of the carbonaceous shale footwall even where extensive areas of hematite breccia are present. Bleaching of the hanging wall is therefore more likely to be due to weathering. Typically, the footwall contact is a sharply defined redox boundary between the fully oxidised hanging wall and the relatively unaltered, weakly sulphidic carbonaceous shale footwall, with the iron ore bodies possibly formed in a redox front.Distribution of goethite and phosphorus within the deposits is not well understood. Goethite probably formed due to late hydrological processes specific to each deposit. At Helene 5, goethite-phosphorus is restricted to a discrete zone at depth and is not a continuous feature over the deposit, and may be due to late faulting. At Thelma Rosemary, a zone of stratiform >0.5% P iron ore within the orebody may be either fault related or may reflect a natural sedimentary variation in the protolith.
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