Summary:
Eastern Pilbara Region – Deposits in the Newman Area
WAIO’s Newman tenure extends approximately 60km E-W and 15km N-S and is located close to the eastern end of the Hamersley Province near the town of Newman. This area hosts the world-class Mount Whaleback BKM deposit which was the first major iron ore mine for BHP and has been in production since 1969. The Eastern Ridge, Western Ridge and Shovelanna deposits located in the Newman area are also currently under active mining and feed into the Newman processing hub.
The outcrop in the Newman area is dominated by iron formations, with the BKM IF forming prominent ranges of hills and the MM IF having a more subdued topographic expression. The intervening Wittenoom Formation is typically deeply eroded and overlain by a mix of Mesozoic to Cenozoic sedimentary rocks.
The BKM IF crops out more or less continuously, with a west northwesterly strike, over the entire 60km length of WAIO tenure (the Ophthalmia Range). Apparent sinistral offset on a subvertical, NNE-trending fault (called Fortescue River Fault) divides the range into two geologically coherent blocks. At the western end of the range, late normal movement on WNW-trending, moderately S-dipping faults (e.g., Homestead and Pika Faults) has resulted in duplication of the BKM IF and MM IF within the Ophthalmia Range. A further belt of BKM IF and MM IF stratigraphy strikes northwest through the town of Newman, and BKM IF dominates the remaining hilly areas, known as the Western Ridge and the Eastern Ridge. This entire block has been downthrown, relative to the Ophthalmia Range stratigraphy, by late normal movement on the NE-trending, moderately SE-dipping Whaleback Fault.
While the fault architecture controls the distribution of BIFs, the outcrop pattern is dominated by regional-scale, north-verging to recumbent folds that plunge gently to the west northwest. These are superimposed on an earlier generation of meso-scale folds, also consistently north-verging and WNW-plunging, that are particularly clear in outcrop in the Eastern Ridge area. The youngest generation of folds are upright, open folds with axes that trend NW to NE.
Mount Whaleback – The Mount Whaleback deposit is in production, and is located approximately 5km west of Newman. This is the only deposit in the WAIO portfolio to be dominated by the hypogene martite-microplaty hematite (M-mplH) style of mineralisation and as a result the resource is particularly high-grade and ‘clean’. Mineralisation is hosted by a doubly-plunging pair of synclines of BKM IF and extends for approximately 5.5km E-W, 1.7km N-S and to a depth of 470m. The BIF has been down-faulted against the Jeerinah Formation by late normal movement on the NEtrending Whaleback Fault. Low-angle faults, such as the Central Fault and Eastern Footwall Fault, appear to have acted as local feeder conduits for the hydrothermal fluids. The upper surface of the hypogene mineralisation is subhorizontal and transgressive to the stratigraphy (Dales Gorge and Joffre Members, Whaleback Shale). The Mount McRae Shale forms the stratigraphic base of the orebody. A thin blanket of M-G mineralisation was originally present and mantled the top of Mount Whaleback, but this has long since been mined out. Overall, this deposit is high-grade and the mineralisation has a natural cut-off of 50% Fe.
Western Ridge – The Western Ridge deposit is under development as a sustaining mine. The Hamersley Group rocks of the Whaleback-Western Ridge belt extend for 17km to the southwest of Newman. The outcrop pattern is dominated by two regional-scale synclinal keels of BKM IF. The synclines plunge gently to the west northwest and are truncated against the Whaleback Fault. The MM IF crops out to the southeast of the BKM IF and, in addition to the regional-scale folds, a number of N-verging, recumbent meso-scale folds are evident from the outcrop pattern and from drilling.
Mineralisation (excluding Mount Whaleback) is semi-continuous in both BKM and MM IF, with individual orebodies having the following range of dimensions: 1.5-9.5km in strike length, 500-1000m in width and extending to depths of 100-400m. Some of these orebodies contain cores of hypogene M-mplH mineralisation which have been overprinted by the supergene ore-forming event. Steeply-dipping faults, including the Whaleback Fault, appear to have acted as fluid conduits for the hypogene ore fluids. The other orebodies in this group are all supergene martite-goethite (M-G) types (both BKM and MM). The better thicknesses of supergene mineralisation are localised in the hinge zones or short limbs (occasionally thrustthickened) of asymmetric, N-verging, meso-scale folds. Mineralisation also occurs in the synclinal keels of the later regional-scale folds and the limbs of these folds, where they have a moderate dip. The natural cut-off grade that separates unmineralised BIF from mineralisation in this area is 48% Fe.
Eastern Ridge – The Eastern Ridge deposit is in production. It is located to the northeast of Newman and the stratigraphy is duplicated by late normal movement on the moderately S-dipping Homestead Fault.
A regional-scale overturned syncline dominates the structure to the south of the Homestead Fault. To the north of the Homestead Fault, mineralisation occurs within the steeply N-dipping limb of a regional anticline. Some of the better thicknesses of mineralisation throughout this area are associated with an earlier generation of meso-scale folds, clearly visible in outcrop, that plunge gently to the west northwest and verge towards the north.
Mineralisation occurs in both the BKM IF and the MM IF. It is semi-continuous in both BKM and MM IF, with individual orebodies having the following range of dimensions: 4-10km in strike length, 200-700m in width and extending to depths of 100-400m. The majority of the mineralisation in this area is of the M-G type but there are small localised patches of hypogene M-mplH mineralisation in the west and more extensive M-mplH mineralisation in places (some of it clearly associated with the steeply-dipping, NE-trending Central Fault). The natural cut-off grade that separates unmineralised BIF from mineralisation in this area is 48% Fe.
Shovelanna – The Shovelanna deposit is in production. It is located about 40km east of Newman. Mineralisation occurs in the Dales Gorge and Joffre Members of the BKM IF and is semi-continuous along strike with the following dimensions: 6km in strike length, 200-800m in width and extending to depths of 100-200m. The majority of the mineralisation is M-G type ore, with occasional patches of M-mplH.
Mineral Deposit Types and Mineralisation Styles
Fresh iron formations of the Hamersley Group have subtle but consistent differences in mineralogy and chemical composition and these differences are carried through into the respective BIF-hosted Fe ores. For this reason, bedrock deposits and the associated mineralisation are classified as being of Brockman (BKM) or Marra Mamba (MM) types.
In addition to these bedrock deposit types, two types of detrital mineralisation are also found in the Hamersley Province. These are the pisolitic channel iron deposits (CID) and a variety of iron-rich detrital materials collectively referred to as detrital iron deposits (DID).
A brief description of these deposit / ore types is provided below.
Bedded ore types classified as per host Archaean or Proterozoic iron formation, which are Brockman and Marra Mamba; also present is iron-rich detrital material.