Brockman #2, Brockman #4, Nammuldi, Silvergrass
Western Australia, WA, Australia
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The Brockman operation, comprises the Brockman #2 and adjacent Nammuldi (#Location: 22° 24' 44"S, 117° 20' 14"E) and Silvergrass (#Location: 22° 18' 16"S, 117° 18' 32"E) deposits, which are located 54 km north-west of Mt Tom Price, and Brockman #4 (#Location: 22° 35' 54"S, 117° 13' 16"E) that is 20 km to the SW, and ~55 km WNW of Mt Tom Price. These mines are ~250 km SSW of their Indian Ocean export port of Dampier.
The initial operation commenced in 1992, mining high grade Brockman detrital iron ores adjacent to the Brockman #2 deposit and was completed in 1998. Operations then shifted to the Brockman #2 bedded iron deposit which is hosted by the Dales Gorge Member of the Brockman Iron Formation. Brockman #4 has subsequently been opened with ore developed in both the Dales Gorge and Joffre members, with high grade haematite and low goethite contents.
See the Hamersley Basin Iron Province
and Mt Tom Price records for the regional setting and stratigraphy.
The Brockman #4 deposit, which is generally 0.9 to 1.4 km, and locally up to 3 km wide, has been divided into Eastern, Central and Western deposits, occurring almost continuously over a 14 km long, east-west trending ridge of Brockman Iron Formation. This ridge rises to over 150 m above the adjacent valley floors, and is flanked to the north by the up to 8 km wide valley of the west flowing Boolgeeda Creek. To the south, the ridge is flanked by a narrow valley, the South Strike Valley, which separates the outcropping Brockman Iron Formation from a parallel ridge of Marra Mamba Iron Formation. The main Brockman #4 ridge is on the southern limb of the east-west trending Brockman Syncline. Both limbs strike east-west, cut by several NW-SE trending faults and dykes which truncate the orebody in several places, notably, between the western and central deposits where preferential erosion in the fault zone results in a saddle in the Brockman ridge. There is apparent vertical displacement across these cross-cutting faults in the South Strike Valley, locally bringing the Wittenoom Formation to the surface in the floor of the valley.
All Hamersley Group units are encountered in the Brockman Syncline, although in the Boolgeeda and South Strike valleys, basement subcrop is masked by a variable thickness of Tertiary valley fill and lateritic sediments. The Hamersley Group, which may be up to 2500 m thick, comprises, from the base:
Marra Mamba Iron Formation, (230 m thick)
Nammuldi Member - cherty BIF with occasional shale bands,
MacLeod Member - BIF with extensive interbedded shales and "podded" BIF horizons,
Mount Newman Member - BIF with thin shale bands,
Wittenoom Formation, (max. 700 m thick)
West Angelas Member - manganese-rich shale with minor BIF and chert bands,
Paraburdoo Member - dolomite, some of which is karstic,
Bee Gorge Member - calcareous shale and dolomite,
Mt Sylvia Formation (30 m thick) - mudstone, siltstone, chert, dolomite and BIF bands,
Mt McRae Shale (~50 m thick) - graphitic (2-8 wt.% TOC), pyritic (1-10 wt.% S) and chloritic shales interbedded with BIF,
Brockman Iron Formation, (620 m thick)
Dales Gorge Member - interbedded BIF and shale,
Whaleback Shale Member - interbedded shale, chert and BIF,
Joffre Member - BIF with minor shale bands,
Yandicoogina Shale Member - shale and BIF,
Weeli Wolli Formation (~600 m thick) - BIF (commonly jaspilitic), mudstone and siltstone with common interlayered metadoleritic sills,
Woongarra Volcanics (~800 m thick) - rhyolite, rhyodacite and rhyolitic lavas and volcaniclastic breccia, pyroclastic rocks and BIF,
Boolgeeda Iron Formation (450 m thick) - fine-grained, laminated, dark grey-brown to black flaggy BIF, minor chert, jaspilite, shale.
In the Brockman #4 pits, the mineralised zone cuts through low-permeability units of unenriched BIF of the Brockman Iron Formation and overlying Weeli Wolli Formation. The footwall follows the low-permeability Mount McCrae Shale down-dip. The mineralisation is hosted by the Dales Gorge and Joffre members, and is principally microplaty hematite with very little goethite.
The Brockman Syncline South Ridge Marra Mamba martite-goethite deposits are located close to the Brockman #4 mine on the southern limb of the Brockman Syncline, distributed over a strike length of ~45 km. The mineralisation is hosted in the Newman Member of the Marra Mamba Iron Formation and as detritals. Mineralisation consists of a series of lenses that partially outcrop at surface and dip at 20 to 50°N.
The Brockman #2 (in the Dales Gorge Member of the Brockman Iron Formation) and adjacent, but underlying Nammuldi (in the Marra Mamba iron formation) deposits, are located on the northern limb of the Brockman Syncline, 10 km west of the closure and ~25 km NE of Brockman #4.
The Silvergrass deposit (in the Marra Mamba iron formation) is located on the northern limb of the adjoining anticline immediately north of the Brockman Syncline, and ~8 km NNW of Nammuldi.
The total Brockman #2 reserve in 2001 was stated at 30 Mt @ 62.86% Fe, 2.59% Si, 1.86% Al, 0.128% P, 0.037% Mn, 5.08% LOI.
The Nammuldi Marra Mamba and detrital resource, immediately to the north of Brockman #2, originally had a developed, pre-mining resource of 330 Mt @ 62.5% Fe.
Remaining JORC compliant ore reserves and mineral resources at December 31 2011 and adjacent years (in brackets), after Rio Tinto 2011 and 2012 Annual Reports,were:
Brockman #2, Brockman Iron Formation ore:
Proven + probable reserves - 35 Mt @ 62.3% Fe; (44 Mt @ 62.0% Fe, 31 Dec., 2012; 12 Mt @ 62.6% Fe, at 31 Dec., 2010);
Measured + indicated + inferred resources - 20 Mt @ 62.3% Fe; (Not reported 31 Dec., 2012; 40 Mt @ 62.6% Fe at 31 Dec., 2010);
Brockman #2 and Nummaldi detrital ore:
Indicated + inferred resources - 26 Mt @ 60.2% Fe; (Not reported 31 Dec., 2012; 77 Mt @ 60.6% Fe at 31 Dec., 2010)
Nammuldi Marra Mamba Iron Fomation ore:
Proved + probable reserves - 179 Mt @ 62.5% Fe; (166 Mt @ 62.6% Fe, 31 Dec., 2012; 16 Mt @ 61.3% Fe, at 31 Dec., 2010)
Measured + indicated + inferred resources - 83 Mt @ 62.2% Fe; (Not reported, 31 Dec., 2012; 274 Mt @ 62.7% Fe, at 31 Dec., 2010)
Brockman #4, High Grade Brockman Iron Formation ore:
Proved + probable reserves - 585 Mt @ 62.0% Fe; (561 Mt @ 62.0% Fe, 31 Dec., 2012; 603 Mt @ 62.0% Fe, at 31 Dec., 2010)
Measured + indicated + inferred resources - 64 Mt @ 62.2% Fe; (Not reported 31 Dec., 2012; 54 Mt @ 62.1% Fe at 31 Dec., 2010)
Brockman #4, Low Grade Brockman Iron Formation ore November, 2010 (Rio Tinto release, 26 November, 2010):
Measured + indicated resources - 40 Mt @ 58.9% Fe;
At November, 2010, the Brockman Syncline South Ridge Marra Mamba resources (Rio Tinto release, 26 November, 2010) were:
Inferred resources - 190 Mt @ 61.5% Fe
The figures above represent a total resource of >1.2 Gt @ ~62% Fe.
JORC compliant ore reserves (no resources quoted) at Silvergrass, which commenced mining in 2015, were (Rio Tinto Annual report, 2015):
Proved reserve - 114 Mt @ 62.6% Fe;
Probable reserve - 44 Mt @ 59.5% Fe;
Proved + probable reserves - 157 Mt @ 61.7% Fe.
Note: Reserves are additional to resources.
The most recent source geological information used to prepare this summary was dated: 2010.
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.
Harmsworth R A, Kneeshaw M, Morris R C, Robinson C J, Shrivastava P K 1990 - BIF Derived Iron Ores of the Hamersley Province: in Hughes FE (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne v1 pp 617-642|
McKenna D M, Harmsworth R A 1998 - Brockman No.2 detritals (B2D) iron ore deposit: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 375-380|
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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