Mt Keith

Western Australia, WA, Australia

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The Mount Keith orebody is one of the worlds largest low grade, economically mined, disseminated nickel sulphide deposits. It is located some 425 km north of Kalgoorlie in the Archaean Yilgarn Craton of Western Australia and lies within the Agnew-Wiluna segment of the regional, composite Archaean Norseman-Wiluna greenstone belt (#Location: 27° 13' 47"S, 120° 32' 32"E).

The Agnew-Wiluna greenstone belt is part of a near-continuous string of greenstone lenses that represents the northernmost extension of the Kalgoorlie terrane that hosts the Kambalda Ni deposits (Cassidy, 2002). To the north, the exposed belt is terminated by the overlying Proterozoic Earaheedy Group.

The Agnew-Wiluna belt is composed of an ~2.7 Ga sequence of basaltic and felsic volcanic and volcaniclastic rocks, sulphidic carbonaceous shales and multiple, a number of separate (at least two, locally three) units of laterally variable komatiites, including cumulates, thin spinifex-textured bands, and komatiitic basalts. There is a gradual increase in metamorphic grade from prehnite-pumpelleyite facies in the north to amphibolite facies in the south (Binns et al., 1976; Archibald et al., 1978).

The rocks of the Agnew-Wiluna greenstone belt have been subjected to polyphase deformation (Duuring et al., 2007, 2010). To the south, there are regional, open to tight, upright folds that have hinge areas intruded by syntectonic granitoids. In contrast, to the north, between Perseverance and Wiluna, the belt is characterised by NNW-trending, steeply dipping, attenuated and dismembered supracrustal strata that are now structurally juxtaposed against granitoid rocks and granitic gneiss terranes (Fiorentini et al., 2012).

The greenstone belt has been divided into four structural domains, the: i). Wiluna; ii). Yakabindie-Mount Goode; iii). Agnew-Lawlers; and iv). Mount Clifford (McCluskey, 1996). These domains are separated by north-trending faults and shear zones. Older, pre-2.7 Ga greenstone successions are locally preserved in the Wiluna Domain and Yakabindie-Mount Goode Domain, and may represent basement to the mafic-ultramafic sequence (Cassidy, 2002).

The pre-2.7 Ga greenstones of the Wiluna Domain, the northern-most division of the Agnew-Wiluna greenstone belt, is overlain by a sequence of tholeiitic and komatiitic basalts and sedimentary rocks, intruded by dolerite and porphyry sills and dykes. These are, in turn, overlain by the Mount Keith Dacite sequence of intermediate to felsic volcanic rocks, and intruded by ultramafic sills which represent the Mount Keith ultramafic unit off Kent and Hagemann (1996). The Wiluna Domain is bounded to the west by a north-trending fault, which separates it from, and forms the eastern margin of the Yakabindie-Mount Goode, which is truncated to the west by granitoid intrusive rocks.

The Yakabindie-Mount Goode Domain comprises of the 2735±3 Ma (Black et al., 2002) Kathleen Valley Gabbro and Mount Goode tholeiitic basalts, which young to the south (Jagodzinski et al., 1999). The Yakabindie Domain greenstone sequence is thought to be unique in the region and does not appear to have an equivalent in other sequences of the belt (Fiorentini et al., 2012).

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. Rosengren et al. (2005) has confirmed that the three ultramafic units do not represent faulted repetitions of the same horizon, but three distinct komatiite units. The Mount Keith and Cliffs ultramafic units have very different volcanological and geochemical features (Fiorentini et al., 2012). The three ultramafic units are separated by a variably deformed sequence of felsic and mafic rocks, ranging in composition from dacite to Mg-rich tholeiitic basalt (Rosengren, 2004).

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 orthocumulate-textured units (Fiorentini et al., 2010). It contains >95% modal cumulus olivine and widespread hydrous magmatic mineral phases such as amphibole in orthocumulate- and mesocumulate-textured rocks, which contain ~40 to 50 wt.% MgO and <3 wt.% TiO2. 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. Basal massive nickel sulphide mineralisation (e.g., Sarah's Find) occurs in the thinner meso- and orthocumulate-textured units, whereas stratabound disseminated nickel sulphide mineralisation (e.g., MKD5 Ni Deposit) is hosted in the adcumulate-textured pods. 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).
  The Mount Keith ultramafic unit komatiites and Mount Keith Dacite exhibit spatial associations (Rosengren et al., 2005, 2008). Specifically, ultramafic pods with a complex cumulate-rich internal structure, are associated with the coherent felsic volcanic centres, while the more evolved and differentiated komatiites are spatially associated with felsic volcaniclastic sequences (Fiorentini et al., 2007a). This is interpreted to reflect the mode of emplacement of the magmatic system, whereby komatiitic and felsic volcanic rocks may have used the same structural pathways (Fiorentini et al., 2007).
  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 hanging wall 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 mineralization at Cliffs, 11 Mile Well and Sinclair.

Mt Keith is developed near the centre of the narrowest part of the Agnew-Wiluna greenstone belt, which ranges from 5 to 25 km in width. The komatiitic interval at and near Mt Keith is up to 2500 m thick while the individual komatiite bands can be correlated beyond the immediate mine area for more than 100 km along strike.

The three west facing komatiite units, the Mount Keith, Cliffs and Monument ultramafic units are identified at Mt Keith. Nickel mineralisation is hosted by a thickened zone within the regionally extensive, orthocumulate-rich Mount Keith ultramafic unit (or Eastern Ultramafic Body), known locally as the MKD5. The MKD5 has a maximum thickness of 650 m and has been subdivided into three lithologically distinct zones (Rosengren et al., 2007):

i). The Main Adcumulate Domain (MAD), situated in the lower portion of the complex (to the east), which contains the bulk of disseminated nickel sulphide and is dominated by coarse adcumulate olivine textures; it is composed of the following:
  unit 101 - basal orthocumulate, which has rhythmic layering, is composed of olivine with minor stitchtite and sulphide, and has been subjected to intense talc-carbonate alteration;
  unit 102 - which has a thickness of 40 to 330 m, is an extreme olivine adcumulate, composed of coarse olivine, with a very low content of intercumulus material, with sulphide and stitchtite;
  unit 103 - is dominated by very coarse olivine, set in a fine-grained groundmass dominantly comprised of brown bastite (after pyroxene) and occurs as a set of numerous veins or dykelets that crosscuts both units 102 and 104.
  unit 104 - a coarse-grained adcumulate with well-developed interstitial pentlandite/pyrrhotite-rich sulphide mineralisation that represents the bulk of the Mt Keith orebody. The unit is dominated by two ovoid-shaped thickened zones, the larger of which attains a maximum thickness of >300 m, and minimum of <50 m. The second ovoid thickens to ~150 m and thins to ~40 m. Textural layering on a scale of centimetres to metres is defined by olivine crystal shapes and sulphide abundance, although the dominant textural form is equant 8 to 10 mm olivine with 4 to 6% interstitial sulphide and stitchtite. The upper boundary is a sharp transition from olivine adcumulate to the overlying mesocumulate.
ii). The Upper Fractionated Zone (UPZ), dominated by mesocumulate-to-orthocumulate peridotite with domains containing oikocrystic pyroxenite and gabbroic lenses. This zone corresponds to:
  unit 105 - comprises three subdivisions, representing a continuous transition from tightly packed olivine mesocumulate (unit 105A), through to loosely packed, fractionated olivine orthocumulates with significant quantities of oikocrystic pyroxene and gabbroic segregations (unit 105B), and back to olivine mesocumulate (unit 105C). Unit 105 locally contains significant amounts of intercumulus stichtite (after chromite), with Cr concentrations of up to 6000 ppm. The entire unit varies from 40 to 130 m in thickness. The upper margin varies along the strike length of the deposit, forming the upper most sheared contact with the Cliffs ultramafic unit in the southern portion of the deposit.
iii). The Western Mineralised Zone (WMZ) - an aerially restricted unit comprising texturally and chemically distinct olivine cumulate rocks, comprising:
  unit 106 - located in the centre of the WMZ, dominated by adcumulate textured dunite, with interstitial nickel sulphide mineralisation that has significantly lower tenor (i.e., S/Ni ratios >3) than mineralised zones in units 102 and 104. Sulphides and stichtite are interstitial to olivine crystals. Sulphide assemblages are dominated by pyrrhotite and pentlandite, with partial to complete hypogene replacement of pyrrhotite by a lamellar intergrowth of pyrite-marcasite-magnetite. Sulphide grains are relatively coarse, locally up to 5 mm. Individual mesocumulate layers are relatively thin (4 to 5 m).
  unit 107 - an olivine orthocumulate with lesser mesocumulate, and localised upper 2 to 20 m thick pyroxenite margins and gabbroic segregations. This is the uppermost cumulate stratigraphic subdivision of the Mount Keith ultramafic unit.

Both the hanging wall and footwall of the MKD5 unit have been strongly sheared. Truncation of some of the uppermost internal stratigraphic units by the hanging wall contact indicates that the unit has undergone structural modification since its emplacement and suggests that a proportion of the upper section has been removed by faulting. Vertical and lateral textural transitions within the internal stratigraphy suggest the MAD and UPZ were essentially emplaced from a continuous magma flow, with the MAD representing the period of highest magma flux and the major period of emplacement. In contrast, the WMZ is interpreted to represent a later pulse of ultramafic magma, emplaced stratigraphically above the main Mount Keith ultramafic unit.

Ultramafic rocks of the MKD5 have been completely serpentinised and the overlying mafic and volcaniclastic rocks have been altered to albite-actinolite-epidote-chlorite assemblages. No relict olivine remains. In addition there is a strong structurally controlled alteration, with intense carbonate±talc on the outer margins of the MKD5, and along faults. This carbonate alteration decreases away from the fractures & shears. The highest grades are in the least carbonated rocks. The predominant nickel sulphide is pentlandite accompanied by pyrrhotite ±magnetite, while about 20% of the orebody is pentlandite-millerite ±heazlewoodite ±magnetite found in the eastern or stratigraphically lower sections (Hopf and Head, 1998).

The primary nickel bearing sulphides occur as blebs to lobate aggregates situated at the triple-point junctions of former cumulus olivine grains, generally 40 µm to 5 mm (average 0.5 mm) across, locally up to 6 mm. These blebs consist of aggregates of Ni-Fe sulphide grains surrounded by a corona of magnetite, the outer margins of which are in most cases partially replaced by iowaite, chlorian pyroaurite (±tochilinite), or ferroan magnesite. A coarse network of magnetite "crossbars" is typically found within the blebs. Sulphide abundances in ore-grade rock are usually in the 1 to 5% range (Grguric, 2002, 2003; Rosengren et al., 2007).

The sulphides were originally deposited as single-phase monosulphide solid solution (MSS). Modifications of the sulphide phase relations were subsequently driven by i). cooling and exsolution, which resulted in the breakdown of MSS into pentlandite and pyrrhotite, and ii). hydrothermal alteration under varying oxygen and sulphide fugacity conditions, to modify the sulphide/gangue phase relations. These modifications commonly involve partial or complete conversion of the Fe component of the assemblage to a magnetite or ferroan magnesite corona, resulting in enrichment in Ni, and leading to the stabilisation of nonmagmatic phases such as millerite and heazlewoodite (Grguric, 2002, 2003; ).

Nickel mineralisation extends for over 2 km along strike and continues steeply down dip to a depth of at least 500 m. Ore grade mineralisation is developed over 250 to 300 m of the 650 m thickness of the MKD5 unit.

At the end of 2004 the total reserve + resource was 647 Mt @ 0.52% Ni. Of this the economic reserve was 260 Mt @ 0.52% Ni. In 2004 the operation treated 11.13 Mt of ore at a head grade of 0.57% Ni to produce 43 076 tonnes of Ni. The deposit is exploited by open cut mining.

Published JORC compliant ore reserves and mineral resources at 30 June 2012 (BHP Billiton Annual Report 2012) were:
    Measured resource - 175 Mt @ 0.55% Ni;
    Indicated resource - 100 Mt @ 0.48% Ni;
    Inferred resource - 32 Mt @ 0.48% Ni;
    TOTAL resource - 313 Mt @ 0.52% Ni;
    Proved reserve - 91 Mt @ 0.57% Ni;
    Probable reserve - 8 Mt @ 0.50% Ni;
    TOTAL reserve - 99 Mt @ 0.56% Ni;

The mine is owned and operated by BHP Billiton - 100%.

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 3/7/2013
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.

Mt Keith

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Barnes S J,  2007 - Cotectic Precipitation of Olivine and Sulfide Liquid from Komatiite Magma and the Origin of Komatiite-Hosted Disseminated Nickel Sulfide Mineralization at Mount Keith and Yakabindie, Western Australia: in    Econ. Geol.   v102 pp 299-304
Begg, G.C., Hronsky, J.A.M., Arndt, N.T., Griffin, W.L., O’Reilly, S.Y. and Hayward, N.,  2010 - Lithospheric, Cratonic, and Geodynamic Setting of Ni-Cu-PGE Sulfide Deposits: in    Econ. Geol.   v.105 pp. 1057–1070
Burt D R L, Sheppy N R  1975 - Mount Keith Nickel Sulphide deposit: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 159-168
Fiorentini M L, Rosengren N, Beresford S W, Grguric B and Barley M E,  2007 - Controls on the emplacement and genesis of the MKD5 and Sarah’s Find Ni-Cu-PGE deposits, Mount Keith, Agnew-Wiluna Greenstone Belt, Western Australia : in    Mineralium Deposita   v42 pp 847-877
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
Gole M J, Robertson J and Barnes S J,  2013 - Extrusive Origin and Structural Modification of the Komatiitic Mount Keith Ultramafic Unit: in    Econ. Geol.   v.108 pp. 1731-1752
Hills R E T, Barnes S J, Gole M J, Dowling S E  1990 - Komatiites in the Agnew-Wiluna Greenstone Belt (Extracts for OzTour 99 Compilation): in   Physical Volcanology of Komatiites, A Field Guide to the Komatiites of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Block, Western Australia GSA (WA Division), Perth   Excursion Guide Book No. 1 pp 36-42, 67-76
Hopf S, Head D L  1998 - Mount Keith Nickel 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 307-314
Le Vaillant, M., Saleem, A., Barnes, S.J., Fiorentini, M.L., Miller, J., Beresford, S. and Perring, C.,  2016 - Hydrothermal remobilisation around a deformed and remobilised komatiite-hosted Ni-Cu-(PGE) deposit, Sarahs Find, Agnew Wiluna greenstone belt, Yilgarn Craton, Western Australia: in    Mineralium Deposita   v.51, pp. 369-388
Libby J W, Stockman P R, Langworthy P J  1997 - Nickel Mineralisation and Geology in the Perseverance-Mount Keith Segment of the Agnew-Wiluna Greenstone Belt: in    AGSO Record 1997/41    pp 97-101
Naldrett A J  1999 - World Class Ni-Cu-PGE Deposits: Key Factors in their Genesis: in    Mineralium Deposita   v34 pp 227-240
Rosengren N M, Beresford S W, Grguric B A and Cas R A F  2005 - An Intrusive Origin for the Komatiitic Dunite-Hosted Mount Keith Disseminated Nickel Sulphide Deposit, Western Australia: in    Econ. Geol.   v100 pp 149-156
Rosengren N M, Cas R A F, Beresford S W and Palich B M,  2008 - Reconstruction of an extensive Archaean dacitic submarine volcanic complex associated with the komatiite-hosted Mt Keith nickel deposit, Agnew-Wiluna Greenstone Belt, Yilgarn Craton, Western Australia: in    Precambrian Research   v161 pp 34-52
Rosengren N M, Grguric B A, Beresford S W, Fiorentini M L and Cas R A F,  2007 - Internal stratigraphic architecture of the komatiitic dunite-hosted MKD5 disseminated nickel sulfide deposit, Mount Keith Domain, Agnew-Wiluna Greenstone Belt, Western Australia : in    Mineralium Deposita   v42 pp 821-845

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