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The Cliffs nickel deposit, is an underground, narrow veined nickel deposit, located ~700 km NE of Perth ~4 km south of Mt Keith and 60 km NNW of Leinster in the Archaean Yilgarn craton of Western Australia (#Location: 27° 18' 27"S, 120° 32' 36"E).
The deposit lies within the more than 240 km long, NNW-SSE trending Agnew-Wiluna greenstone belt which also hosts other significant nickel sulphice deposits such as Mount
Keith, Perseverance, Yakabindie, Honeymoon Well, Waterloo, Cosmos-Alec Mairs, Sinclair, Tapinos, Prospero, Rocky's Reward-Harmony and 11 Mile Well.
See the Mt Keith Record for details of the regional setting and characteristics of the Agnew-Wiluna greenstone belt.
The stratigraphic relationships between the various units of the Agnew-Wiluna greenstone belt are apparently best preserved at Mount Keith (Dowling and Hill, 1990, 1992; Hill et al., 1990; Fiorentini et al., 2007; Rosengren et al., 2005, 2007), where three ultramafic units have been delineated: i). the Mount Keith; ii). the Cliffs; iii). the Monument ultramafic units. All three face west, with steep to subvertical dips, although local shallow dipping, east-facing sections of the Monument ultramafic unit have been attributed to isoclinal south-plunging synclinal structures. The Mount Keith and Cliffs ultramafic units are mineralised, whilst the Monument ultramafic unit, which only comprises thin (<1 m thick) flow units, has to date found to be barren and unprospective.
The overall sequence within the Agnew-Wiluna greenstone belt is a follows, from the base (Fiorentini et al., 2012):
McFarlanes Basalt - a laterally extensive, up to 1000 m thick, frequently pillowed, basalt unit.
Mount Keith Dacite - a dacitic volcanic and locally volcaniclastic unit, which is also laterally extensive, but is only found along the axis of the greenstone belt, being notably absent from the Agnew-Lawlers and Mount Clifford domains. Although dacite, with a tonalite-trondhjemite-dacite (TTD) geochemical affinity is the volumetrically dominant lithology, compositions range from andesite to rhyolite. The Mount Keith Dacite occurs both above and below the Mount Keith ultramafic unit.
Mount Keith ultramafic unit - composed of numerous >500 m thick adcumulate-textured pods or lenses, flanked by laterally extensive meso- and ortho-cumulate-textured units (Fiorentini et al., 2010). In marked contrast to the Cliffs ultramafic unit, no spinifex or other textures indicating extrusive emplacement have been recognised over the hundreds of kilometres strike length of komatiites. Disseminated nickel sulphides occur interstitially to former olivine crystals (Barnes et al., 2008), concentrated in lensoidal zones interpreted to represent channels in shallow sills, such as the MKD5 nickel deposit at Mount Keith (Grguric et al., 2006).
At Mount Keith, ages of 2713±6 and 2706±6 Ma have been determined (SHRIMP U-Pb on magmatic zircon and titanite grains) from Mount Keith Dacite in the footwall and hangingwall respectively, of the Mount Keith ultramafic unit, whilst numerous detrital and xenocrystic zircon grains yield ages of from ~2730 to ~2740 Ma (Fiorentini
et al., 2005). This has been taken to indicate that this ultramafic unit was emplaced into the felsic volcanic unit and volcaniclastic sequence as a large sill (Rosengren
et al., 2005).
Centenary Bore Basalt - comprises a high Mg/high Fe basalt succession of tholeiitic affinity, with intercalated carbonaceous shales and pyritic cherts, and stratigraphically overlies the Mount Keith Dacite (Dowling and Hill, 1990; Beresford et al., 2004). This unit, which separates the Mount Keith Dacite and Cliffs ultramafic unit, locally pinches out, or reaches a thickness of up to ~800 m at other locations (Rosengren et al., 2007; Fiorentini et al., 2007).
Cliffs ultramafic unit - which is locally >150 m thick, comprises a sequence of differentiated olivine spinifex-textured flows and trends NNW, consistent with the regional trend of the Agnew-Wiluna greenstone belt. The basal member is the thickest and contains basal massive nickel sulphide mineralisation at Cliffs, 11 Mile Well and Sinclair. The Cliffs ultramafic unit is interpreted to represent a broadly comagmatic equivalent of the Mount Keith ultramafic unit, which is taken to be a subvolcanic sill that intruded and partially assimilated rocks of the Mount Keith Dacite.
The Cliffs ultramafic unit is the hosts to mineralisation at Cliffs. It has a variable thickness and comprises two main lithofacies along its strike extent: i). spinifex-bearing thin flows, which are barren and abundant higher in the unit, and ii). thicker ortho- to mesocumulate-dominant units, such as the mineralised, up to ~200 m thick peridotite unit (as at the Cliffs deposit) that occurs sporadically along the basal portion of the Cliffs ultramafic unit, generally beneath a thin, spinifex-rich upper komatiites member (Dowling and Hill, 1992). Massive nickel sulphide mineralisation occurs at the base of this unit.
The mineralised peridotite unit at the Cliffs has been subjected to significant deformation, with shearing frequently obscuring stratigraphy.
In contrast to the Mount Keith ultramafic unit, the Cliffs ultramafic unit is generally vesicular (Arndt et al., 2008). Segregation vesicles are present within the upper harrisitic portions of the B-zone of thick (>30 m) members, as well as segregation cylinders and sulphide filled vesicles or amygdales. Rare carbonate filled amygdales have been observed in thinner (<30 m) texturally differentiated spinifex-bearing flows of the Cliffs ultramafic unit. The varying content of volatile-bearing mineral phases in the Mount Keith ultramafic unit and Cliffs ultramafic unit may reflect by the degree of volatile exsolution/degassing of the intrusive and extrusive depositional settings respectively (Fiorentini et al. 2012).
The deposit is mined underground, occurring as steeply dipping, massive textured nickel-sulphide mineralisation.
Production commenced in 2008 with pre-mining resources (BHP Billiton Annual Report 2007) of:
Indicated resource - 1.3 Mt @ 4.8% Ni;
Inferred resource - 1.2 Mt @ 3.7% Ni;
TOTAL resource - 2.5 Mt @ 4.3% Ni.
Remaining JORC compliant ore reserves and mineral resources at 30 June 2012 (BHP Billiton Annual Report 2012) were:
Measured resource - 0.5 Mt @ 4.7% Ni;
Indicated resource - 1.2 Mt @ 4.1% Ni;
Inferred resource - 4.5 Mt @ 1.9% Ni;
TOTAL resource - 6.2 Mt @ 2.6% Ni;
Proved reserve - 0.5 Mt @ 3.2% Ni;
Probable reserve - 1.0 Mt @ 3.1% Ni;
TOTAL reserve - 1.5 Mt @ 3.1% Ni;
Nickel is extracted from underground and trucked to the surface where it is then loaded onto road trains and taken to BHP Billiton's Leinster Nickel Operation 60 km to the SSE for processing.
The most recent source geological information used to prepare this summary was dated: 2012.
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 to this deposit in the PGC Literature Collection:
Fiorentini M, Beresford S, Barley M, Duuring P, Bekker A, Rosengren N, Cas R and Hronsky J, 2012 - District to Camp Controls on the Genesis of Komatiite-Hosted Nickel Sulfide Deposits, Agnew-Wiluna Greenstone Belt, Western Australia: Insights from the Multiple Sulfur Isotopes : in Econ. Geol. v.107 pp. 781-796|
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