The Mount Whaleback orebody is located near the town of Newman and is 370 km SSW of its export port at Port Hedland. It is the largest known high grade hematite deposit in the Hamersley province and the largest known single, continuous iron ore deposit in Australia.
It had an original measured resource of 1700 Mt of low phosphorous, hard, micro-platy hematite ore. Prior to mining the deposit extended from 240 m above, to more than 300 m below plain level, with much of the reserve now below the present water table. It is 5.5 km long, while the final pit will be 1.5 km wide.
The main Mt Whaleback and associated satellite deposits are hosted by both the Marra Mamba and Brockman Iron Formations, within a faulted outlier of Hamersley Group rocks. These rocks are part of the Archaean to Palaeoproterozoic volcanic and sedimentary sequence of the Mount Bruce Supergroup, which spans a time interval of over 400 m.y., from >2.77 to near 2.35 Ga. The ~2500 m thick Hamersley Group contains three main iron formation units, the basal Marra Mamba, medial Brockman and upper Weeli Wolli. It is underlain by the mafic and bimodal volcanic rocks and clastic sediments of the up to 5000 m thick Fortescue Group, and overlain by the basal Boolgeeda Iron Formation and coarsening upwards sequence of clastic sedimentary rocks that make up the 3000 to 5000 m thick Turee Creek Group.
The Marra Mamba Iron Formation is exposed to the south of the Brockman ore. It is composed of three units, commencing with the basal 135 m thick Nammuldi Member, comprising cherty BIF interbedded with thin shales; the overlying 35 m thick MacLeod Member, made up of BIF, chert and shale interbeds; and the uppermost Mount Newman Member, 60 m of interbedded BIF with carbonate and shale. High-grade martite-goethite ore (~45 to 50 m thick), is primarily found in the upper Mount Newman Member, with some enrichment in the upper 10 to 15 m of the MacLeod Member in some areas. Ore extends for 200 to 400 m down dip from the outcrop to depths of 100 to 150 m, locally to 300 m. The footwall contact with un-enriched BIF is sharp, although the transition along strike is over an interval of >100 m. An 18 m thick hard cap covers the elevated outcrops. The Marra Mamba ore occurs over a broad area within 2 km to the south and SE of the main Mount Whaleback Brockman ore deposit. It is developed over a strike length of >3 km within a shallowly WNW-plunging synform with shallow limbs (Orebody 29), and and antiform to the west, with a steep northern (Orebody 30) and shallow southern limb (Orebody 35).
The Marra Mamba Iron Formation is followed by the Wittenoom Dolomite, West Angelas Member shale, BIF and chert, and the Mt Sylvia Shale.
The Brockman Iron Formation overlies the next unit, the McRae Shale, the uppermost member of which - the Colonial Chert - also contains some ore enrichment. The Brockman commences with the Dales Gorge Member, the main horizon containing extensive high grade hematite ore at Mt Whaleback. It is an alternating assemblage of 17 BIF and 16 shale macro-bands. The shale macro-bands are each 0.1 to 2 m thick, and the BIF bands 1 to 7 m. In the mine, this member is ~65 m thick, but is up to 135 m thick where not enriched.
The Dales Gorge Member is overlain by the Whaleback Shale Member, which forms the hanging wall to the main ore horizon. It is locally split into three zones, namely the 6 to 8 m thick basal shale, the 2 to 7.5 m central chert and the 12 to 25 m thick upper shale. The basal shale contains five alternating shale and cherty BIF macro-bands, the lower of which is commonly enriched and included in the orebody. The central chert is not usually enriched, but is strongly contorted, while the upper shale contains numerous chert bands and has a gradational contact with the overlying Joffre Member.
The Joffre Member is approximately 240 m thick and comprises mainly BIF with only minor thin shale interbeds, with regular macro-banding being absent. It is the host for some hematite enrichment, and at depth this ore is indistinguishable from that of the Dales Gorge Member. In general the ores in this member are more goethitic and softer.
The deposit is structurally complex. The Brockman orebody occurs on the limbs of two major, westerly plunging, overturned synclines, the South and East synclines, separated by an overturned anticline with a shallowly south-dipping axial normal faiult. A major, steeply south-dipping normal fault zone with a 900 m throw, the Whaleback Fault terminates the orebody to the north, while to the east it is limited by a major reversal of plunge on the East syncline. In the northern and western sections the ore is concealed by generally barren BIF of the Joffre Member. The Marra Mamba Orebody 29 syncline is up-plunge from the South Syncline, where the axis plunges at 10 to 20°NW.
The Marra Mamba martite-goethite ore is essentially friable martite and ochreous yellow goethite.
Physically the Brockman hematite ore at Mt Whaleback is hard, blue, fine grained and weakly banded (reflecting the original BIF meso-banding) with a porosity of approximately 20%. At greater depths it is less porous where hematite has recrystallised into pore spaces, while some of the banded ore is also brecciated. The hematite ore contains minor primary magnetite and martite octahedra. Chemically it contains approximately 69% Fe, with shale interbeds also being enriched. The contacts between high grade hematite and barren BIF is gradational over 15 to 30 m of medium to low grade, friable, siliceous hematite.
Massive, occasionally banded, non-porous, goethitic hematite occurs near the surface resulting from hydration of the hematite ore, and can extend to as deep as 30 m below the present topography. In addition, large areas of non-ore BIF have also been enriched in the same manner as a 2 to 30 m skin below the present surface, particularly over the Joffre Member.
The information in this summary is mainly sourced from Harmsworth et al., 1990.
Proven + probable reserves at Mt Whaleback and its satellites in June 2000 were:
1.23 Gt @ ~64% Fe, 0.053% P, 4.3% SiO2, 1.7% Al2O3.
Resources and reserves at Mt Whaleback and its satellites at 30 June 2012 (BHP Billiton, 2012) were:
Measured + indicated + inferred Brockman resources - 3.152 Gt @ 60.6% Fe, 0.12% P, 5.0% SiO2, 2.6% Al2O3, 4.9% LOI;
Measured + indicated + inferred Mara Mamba resources - 1.257 Gt @ 59.4% Fe, 0.07% P, 4.2% SiO2, 2.5% Al2O3, 7.3% LOI;
Proved + probable Brockman reserves - 1.133 Gt @ 62.9% Fe, 0.10% P, 4.1% SiO2, 2.0% Al2O3, 3.2% LOI;
Proved + probable Mara Mamba reserves - 78 Mt @ 61.6% Fe, 0.06% P, 3.1% SiO2, 1.8% Al2O3, 6.4% LOI.
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2012)