Mt Windarra, South Windarra, Cerberus

Western Australia, WA, Australia

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The Mount Windarra, South Windarra and Cerberus Archaean komatiite hosted nickel deposits are located ~25 km north-west of Laverton and 260 km NNE of Kalgoorlie in the Margaret sector of the Laverton greenstone belt, part of the greater Norseman-Wiluna greenstone belt within the Archaean Yilgarn Craton of Western Australia (#Location: Mount Windarra 28° 29' 10"S, 122° 14' 17'E; South Windarra 28° 36' 46"S, 122° 14' 29'E).

The greenstone belt in the Margaret sector is bounded by the Laverton and Celia tectonic lineaments and comprises three major cycles of ultramafic to mafic volcanics separated by thin heterogeneous units of one or more of banded iron formation, chert and carbonaceous shale.   The third cycle is overlain by conglomerate along the western margin of the Laverton tectonic lineament.   These greenstones are bounded by a variety of granitoids with intrusive contacts.

The Windarra BIF is the lowest defined marker in the first cycle, and is overlain by thick ultramafic sequence which host both the Mount Windarra and South Windarra deposits which are 17 km apart, and represent the base of this cycle.   Mount Windarra lies on the steeply dipping east limb of the broad regional scale Margaret Anticline, while South Windarra is on the shallowly dipping nose of the same structure. A granitic intrusive containing xenoliths of mafic and ultramafic rocks occupies the core of the Margaret anticline, with the structure interpreted to probably be related to the diapiric ascent of batholithic granitoids.

The host succession comprises:
i). Main banded iron formation - 80 to 150 m thick, which constitutes the base of the exposed stratigraphy, and the immediate footwall to mineralisation at South Windarra. It is composed of a thick sequence of siliceous and banded chemical sediments, containing variable amounts of quartz, grunerite, actinolite, magnetite, albite, chlorite, pyrrhotite, pyrite and garnet, typically silicate facies at the base, changing to sulphide facies towards the top. Local quartz grit and conglomerate sub-units are found between Mt Windarra and South Windarra;
ii). Corridor ultramafic - 3 to 45 m thick, is an unmineralised ultramafic unit, occurring within ,and a short distance south of, the Mount Windarra mine. It consists of a metamorphosed talc-chlorite-dolomite assemblage that lacks obvious igneous textures, although the magnesium content decreases towards its upper (eastern) chilled margin;
iii). Inter BIF - a 2 to 15 m thick unit of banded sulphidic chert (quartz-pyrite) and feldspathic quartzite, which forms the footwall to the nickel mineralisation at Mt Windarra. Sections of the unit are structurally thinned or thickened in the hinge zone of drag folds. This unit is distinguishable from the Windarra BIF by the absence of grunerite. Slump and scour structures, and graded bedding have been observed in some of the finer grained banded units;
iv). Windarra Ultramafic Sequence which is a 100 to 300 m thick pile of differentiated ultramafic flows, decreasing in Mg upwards, before being overlain by high Mg basalt and then by tholeiitic basalt. Nickel mineralisation is associated with the multiple 10 to 45 m thick olivine peridotite flows at the base of this pile localised near the Mount Windarra and South Windarra deposits. Along strike, away from the deposits, they become metamorphosed picritic flows with local thin peridotite flows at or close to the basal contact;
v). Hanging wall Basalt, a thick sequence of dominantly tholeiitic basalts (Hallberg, 1985), represented in the mine area by strongly foliated amphibolite. The contact between the ultramafic and mafic sequences is gradational, containing units of high Mg basalt interfingered with picritic and tholeiitic flows; and
vi). intrusive dolerites, generally intruded subvertically into tensional openings or parallel to fold axial surfaces and numerous irregular quartz feldspar porphyries.

The untextured, basal olivine peridotite flows of the Windarra Ultramafic Sequence are capped by thin picritic flow tops that are only 1 to 2 m thick, although 5 to 10 m thick peridotitic units higher in the sequence have distinctive spinifex textured picritic flow tops. Complete flow sequences have been observed in the upper part of the Mt Windarra ultramafic sequence, comprising flow top breccia, random spinifex, sheaf spinifex, skeletal and then basal cumulate textured zones; confirming an east facing direction for the flow sequence (J. Dwyer, unpublished data, 1977). At Mt Windarra the basal contact is occupied by two or three overlapping olivine peridotite flows.

The ore shoots at Mount Windarra, based on a 1% Ni cut-off, are ribbon-like, up to 20 m thick, with strike lengths of 50 to 350 m and down dip extents of up to 900 m. Eight distinct, steeply dipping shoots are recognised: A, A Hanging Wall, B, C, D, E, F and G Shoot. D Shoot the largest, along with A and C Shoots account for 80% of the ore.

The initial simple geometry at Mount Windarra has been made more complex by the development of the steeply plunging dextral drag folds in the Inter BIF. The A and B shoots are hosted by one (or possibly two) of the basal ultramafic flows, and are in turn on-lapped by the margin of the flow hosting the E, C, D and G shoots and A hanging wall mineralisation. The E, C, D and G shoots are separated by the hinge zone of drag folds in the underlying Inter BIF. The F shoot lies east of D shoot, and may occur at the base of a stratigraphically higher flow, though it is associated with a discontinuous BIF and may be a structural repetition of the E-G system (J. Dwyer, unpublished data, 1977).

In the primary ore Ni is restricted to the massive to disseminated sulphide zones at the base of the olivine peridotite ultramafic flows. Disseminated ore dominates. Non-massive sulphide mineralisation has been divided into three different textural types:
i). Matrix ore, which has a sulphide supported olivine cumulate texture, and occurs at the base of an olivine peridotite flow or in a transitional zone with underlying massive sulphide ore. It is rarely more than 2 m wide and generally contains between 25 and 40% sulphide;
ii). Blebby disseminated sulphide is present in several areas of the mine, generally overlying massive sulphide and/or matrix ore, containing between 20 and 30% sulphide;
iii). Finely disseminated sulphide mineralisation, containing between 5 and 25% sulphide, is the most common style of disseminated ore and overlies the other textural variations.

Where massive sulphide ores at the base of an ultramafic flow have been physically mobilised along the contact with the Inter BIF, brecciated BIF hosted ores are formed. The best example of this ore type occurs at Mt Windarra, where the barren upper picritic portion of A shoot flow overlaps the peridotitic base, and lies in contact with the Inter BIF. This type of mineralisation is always found near primary massive sulphides, localised at the base of individual peridotitic flow units. Stringers of mobilised massive sulphide are also found in several areas where minor shear zones and/or tight infolds follow the contact of the Inter BIF and overlying massive sulphide mineralisation.

The Ni tenor of the sulphides is 8 to 16% Ni, and is higher in the disseminated ore than in the massive sulphides. Pyrrhotite, pentlandite, pyrite and chalcopyrite are the most common primary sulphides, in decreasing order of abundance. The pyrrhotite to pentlandite ratio varies from 1:1 in disseminated ore, to up to 8:1 in the matrix hosted ore. The average Ni:Cu ratio is 9:1 for most ore types, though may be as low as 4:1 in the copper rich basal matrix hosted ore and remobilised massive sulphide stringers. The nickel tenor of sulphides in the ultramafic rocks is normally between 8 and 16% Ni, and invariably higher in the disseminated ores than in the massive sulphides. The massive ore in A and B shoots rarely exceeds 8% Ni, whereas in the E-C D-G and F shoots it may be as high as 12%. The brecciated BIF hosted and remobilised stringer sulphide ores are generally of a lower and more variable tenor, typically assaying between 2 and 8% Ni (Poseidon Nickel release, April, 2012).

At South Windarra, the ore shoots were ~20 m thick, 1300 m long, and extended to around 300 m down dip.

Supergene ore which extends from the base of oxidation at 40 m to 80 m at Mount Windarra is composed of pyrite and violarite, with enhanced Ni grades, and is also the dominant ore type in the top half of the South Windarra pit. Violarite and pentlandite co-exist in a transition zone, to a vertical depth of up to 180 m.

The Cerberus deposit is 4.5 km NE of Windarra South and 10.5 km south of the Mt Windarra mine. It comprises two lenses, the Main and Hanging wall zones, and irregular remobilised footwall mineralisation, which together define a zone >1100 m long, 450 m wide, which dips 30°E and plunges 45°NE (Poseidon Nickel release, April, 2012).

The three nickel sulphide zones at Cerberus comprise (Poseidon Nickel release, April, 2012):
i). Main Zone - generally composed of massive to stringer sulphide nickel with some disseminated sulphides that occur towards the base of the komatiite lava channel within the Windarra Ultramafic unit. The sulphides have characteristic features of sulphide remobilisation and upgrading resulting from structural overprinting;
ii). Hanging-wall Zone - which occurs as disseminated sulphides, ~2 to 6 m above the Main Zone within the lava channel, and is more "poddy" and lower grade;
iii). Remobilised Footwall Ore, comprising structurally remobilised and upgraded, erratically distributed nickel sulphides which have been injected into the footwall, typically with grades of between 3.5% and 7% Ni.

Both lenses are hosted within the Windarra Ultramafic unit, close to, but above the basal contact with the underlying Corridor Ultramafic unit. The mineralisation is blind, covered by transported material. Nickel grades and widths are greatest towards the centre of the komatiite lava channel, and typically become thinner and lower in grade towards the edges of mineralised zone.

Total production at Mount Windarra from 1974 to 1978 and 1981 to 1989 was - 4.723 Mt @ 1.69% Ni.
South Windarra open pit production totalled - 2.109 Mt @ 1.42% Ni and 0.278 Mt @ 1.22% Ni in separate campaigns.
The combined production from both was - 7.191 Mt @ 1.59% Ni, for 84 630 tonnes of contained Ni.
The remaining reserves in 1989 were - 0.8 Mt @ 1.3% Ni at a 1% Ni cut-off.

The remaining resources in October, 2009, within the A, A hanging wall, B, C-D-G, G upper and F shoots were (Poseidon Nickel release, April, 2012):
    Indicated + Inferred resource - 3.865 Mt @ 1.61% Ni;
The remaining resources in April, 2012, within the South Windarra deposit were (Poseidon Nickel release, April, 2012):
    Indicated + Inferred resource - 0.90 Mt @ 1.14% Ni;
The resources in April 2012 at Cerberus were (Poseidon Nickel release, April, 2012):
    Indicated resource - 4.55 Mt @ 1.51% Ni;

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 23/6/2013
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.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Reddell C T, Schmulian M L  1990 - Windarra Nickel deposits, Laverton: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 561-566
Roberts J B  1975 - Windarra nickel deposits: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 129-143

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