South Flank - Grand Central, Highway, Vista Oriental

Western Australia, WA, Australia

Main commodities: Fe
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The South Flank iron ore deposits Grand Central, Highway and Vista Oriental are located parallel to and within 5 to 10 km to the south of Mining Area C. They are also 90 km NW of Mount Whaleback and immediately west of the Hope Downs mine in the Hamersley Basin, overlying the southern margin of the Pilbara Craton of Western Australia (#Location: 22° 59' 22"N, 118° 55' 8"E).

South Flank is a martite-goethite iron deposit cluster hosted within the Neoarchaean to early Palaeoproterozoic 2.8 to 2.3 Ga sedimentary rocks of the Mount Bruce Supergroup in the Hamersley Basin Iron Province. See the Hamersley Basin Iron Province record for the regional setting and stratigraphy.

Whilst Mining Area C exploits Marra Mamba and Brockman ore on the northern limb of the the regional-scale, east-west trending Weeli Wolli Anticline, South Flank contains Marra Mamba Ore on the southern limb of the same structure. This anticline is doubly plunging and has an 'M' shape in cross section. Second-order, mesoscale folds, which predate the Weeli Wolli Anticline, have sinuous hinge lines, both in the horizontal and in the vertical plane, and are uniformly north vergent. The interference of the two fold generations resulted in a complex outcrop pattern that produced a number of smaller domes superimposed on the broader Weeli Wolli Anticline. Both fold generations have been interpreted to be linked to progressive deformation during the 2215 to 2145 Ma Ophthalmian Orogeny (Kepert, 2001).

The outcropping limbs of the Weeli Wolli Anticline are predominantly composed of the Mount Newman, MacLeod, and Nammuldi Members of the Hamersley Group Marra Mamba Iron Formation, whilst outcrops of the underlying Fortescue Group Jeerinah Formation are restricted to the anticlinal hinge zones. The MacLeod and Nammuldi Members contain few iron oxide micro- and mesobands (Kepert, 2001). Fresh, unmineralised Nammuldi Member, which is ~80 m thick and overlies the Jeerinah Formation, comprises a lower carbonate-dominated sequence overlain by chert-rich carbonate and banded iron formation (BIF), with minor interbedded tuffaceous shales. The overlying MacLeod Member, which is ~75 m thick, is characterised by prominent podded units and a thick central tuffaceous bed that has been largely converted to stilpnomelane. The podded units are largely composed of chert, which is partly a silicified carbonate, and chert-rich BIF. This member is composed of close-spaced Fe oxide-chert ±minnesotaite and chert-silicate ±carbonate mesobands with eight shale macrobands composed dominantly of either stilpnomelane or carbonate. The bulk of the mineralisation at both the North and South Flank deposits is restricted to the ~65 m thick Mount Newman Member, which is subdivided into three units, informally known as, from bottom to top, N1, N2 and N3. The N2 unit is more shale-rich, with ~24% shale macrobands, whilst N1 and N3 contain <5% (Kepert, 2001; Perring, Crowe and Hronsky 2020).

Iron mineralisation at South Flank is hosted by the Marra Mamba Iron Formation and occurs as a series of strata-bound tabular bodies that together make up the 3 deposits that are continuous over a strike length of ~25 km. Individual ore zones can extend to depths of 300 m, ~820 m down-dip and are up to 150 m in thickness. Martite-goethite and ochreous goethite ore is predominantly hosted within the N2 and N3 subunits of the Mount Newman Member. It is best developed in east-west trending, upright to north-vergent asymmetric synclines and associated low-angle reverse faults, that together have led to substantial thickening of host rocks. The primary textures of the banded iron formation are largely preserved within ore zones and influence the location and grade of iron mineralisation. Recent extensive ferricrete ('hardcap') has overprinted both unmineralised iron formations and ore zones (Knight et al., 2018).

The distribution of mineralisation on both the north and south limbs of the Weeli Wolli Anticline is a factor of depth and host unit. High-grade mineralisation containing >57 wt.% Fe persists to a maximum vertical depth of ~290 m and down-dip extent of ~650 m. However, in mesoscale synclines, the entire syncline is typically mineralised to ~150 m vertical depth from the current topographic surface and ~3 km down the plunge of the keel as measured from where it is exposed at surface. Beyond these limits, the limbs may be partially mineralised, but the keel is usually barren (Knight et al., 2018)..

Low-grade mineralisation with >48 wt.%Fe, can persist to ~300 m vertical depth and ~820-m down-dip. The most extensive down-dip mineralisation occurs in the tabular body of ore in the Vista Oriental deposit, the eastern most of the three that make up the South Flank. This orebody thins with depth, with thin, low-grade mineralisation continuing beyond the economic extents of the deposit, marking the N3 contact with the overlying West Angelas 1 unit. Despite the considerable depth extent of martite-goethite mineralisation, the bulk of ore is located within 200 m of the current land surface. For both the North and South Flank combined, sampling of bedrock by Perring, Crowe and Hronsky (2020) found mineralisation containing >48 wt.% Fe is distributed: 0 to 100 m = 43%, 100 to 200 m = 38%, and 200 to 300 m = 18%. In addition, within the high-grade >57 wt.% Fe ore shell, the 90th percentile value for the depth below current topographic surface is 116 m. They also found there is a tendency for mineralisation to become discordant to bedding where it is shallow, i.e., <50 m deep, predominantly restricted to the 'hard-cap' zone of ferruginous duricrust that is associated with younger lateritic weathering.

Low levels of P, Al2O3 and volatiles co-vary with Fe, whilst SiO2 is strongly negatively correlated with Fe, reflecting the transition from 30 to 35 wt.% Fe iron formation to 50 to 65 wt.% Fe iron ore.

The BHP Western Australia Iron Ore Assett South Flank mine utilises already existing infrastructure at Mining Area C, which it has an expanded crushing and screening plant, and is linked by an overland conveyor. Construction commenced in July 2018 and the first production was in May 2021 at a planned rate of ~80 Mtpa.

BHP does not now report Mineral Resource and Ore Reserve estimates for individual deposit, only for ore types across its operations. However, an increase of reported Ore Reserves for the Marra Mamba type ore between 2017 and 2018 was stated to be due to changes in prices, depletion and the declaration of Ore Reserves for the Grand Central, Highway and Vista Oriental deposits in the South Flank Operation. This difference in tonnage and grade provides an estimate of the minimum reserve for the the deposit, and amounted to the addition of:
  960 Mt @ 62% Fe, 0.0525% P, 2.9% SiO
2, 1.55% Al2O3.

Knight et al. (2018) quote a resource for South Flank of ~1.8 Gt.

The most recent source geological information used to prepare this summary was dated: 2020.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Knight, J., Perring, C., Stephens, D. and Crowe, M.,  2018 - Discovery, Geologic Setting, and Controls on Iron Mineralization, South Flank, Western Australia: in Arribas, A., Jeffrey, R and Mauk, L., (Eds.), 2018 Metals, Minerals and Society, SEG Special Publication, No. 21,   Chapter 10, pp. 321-346. doi.org/10.5382/SP.21.14
Perring, C., Crowe, M. and Hronsky, J.,  2020 - A New Fluid-Flow Model for the Genesis of Banded Iron Formation-Hosted Martite-Goethite Mineralization, with Special Reference to the North and South Flank Deposits of the Hamersley Province, Western Australia: in    Econ. Geol.   v.115, pp. 627-659.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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