Mount Keith Open Pit, Mount Keith, Wiluna Shire, Western Australia, Australiai

Regional Level Types
Mount Keith Open Pit	Pit
Mount Keith	Village
Wiluna Shire	Shire
Western Australia	State
Australia	Country

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Mount Keith is one of the world’s largest low grade open pit nickel mines, with total resources of 477 Mt (cut off of 0.4% Ni).

Located 90 km NNE of Leinster.

Considered the largest open cut nickel mine in the world. 85 kilometres south of Wiluna. Discovered in 1968 by local pastoralist Jim Jones, who convinced a friend with a drilling rig to drill one hole in some gossanous material, discovering disseminated Ni sulphides at a depth of 70 metres. The deposit was subsequently explored by the Metals Exploration/Freeport of Australia joint venture, with ore production starting in 1994 by Western Mining Corporation. The mine was acquired by BHP Billiton in 2005 when it took over Western Mining Corporation. The pit is 2.4 kilometres long by 1.4 kilometres wide by 300 metres deep. The deposit is mineralogically diverse due to hydrothermal alteration overprinting.

The location is of interest to rare species collectors. Woodallite was discovered here in 2001 by Ben Grguric et. al., and is found only at Mt Keith and the nearby West Jordan nickel prospect. At Mt Keith it occurs as whorls and clusters of minute platelets up to 6mm across in lizardite-brucite altered dunite, also with associated species chromite, iowaite, pentlandite, magnetite, and tochilinite. It forms as the result of hydrothermal alteration of primary magmatic chromite by Cl rich solutions at temperatures less than 320 degrees. Mt Keith is also the type locality for Mountkeithite discovered in 1981, and found at only four localities worldwide, including the nearby West Jordan nickel prospect. The mine contains many species. A very limited number and variety are in circulation. Violarite/pyrite specimens have been known to degenerate over the years.

The deposit is located in a greenstone belt containing lower interbedded volcanics, pyroclastics, shale and chert. It is overlain by an upper sequence of pillowed basalt, a thick suite of volcaniclastic rocks, a zone of komatiite and an upper series of thin komatiite flows, layered gabbro and high magnesium and tholeiitic basalt.

The mine is contained within the Mount Keith ultramafic complex, a thick komatiitic dunite body as subvolcanic sills emplaced within or below an extrusive komatiite pile. The dunite bodies are not an integral or neccessary part of the komatiite flow field. Nickel mineralisation is within a zone of orthocumulate rich ultramafic body called MKD5. The orebody strikes for 2 kilometres, to at least 500 metres deep.

The MKD5 area shows a basal olivine orthocumulate, then an un-mineralised adcumulate grading to a layered olivine-sulphide adcumulate-orthocumulate containing the orebody, and finally orthocumulate gabbroic ultramafic rocks.

No relict olivine remains, and it has been completely serpentinized. The overlying mafic/ultramafic rocks have been altered to an albite-actinolite-epidote-chlorite assemblage. It has also been strongly structurally controlled altered with intense carbonate-talc on the outer margins of MKD5, and along faults. The hanging wall and footwall show strongly sheared ultramafic rocks.

The primary sulphide zone contains lobate aggregates 1-2mm across, mainly pentlandite with magnetite. 20% of the orebody is pentlandite-millerite and heazlewoodite and magnetite. The Ni-Fe sulphides show oxidation/weathering
where pyrrhotite has been replaced by marcasite, pentlandite replaced by violarite, violarite by millerite and finally the decomposition of the sulphides. The serpentine host rock is bleached in this zone and with high weathering the serpentine is replaced by clay minerals, with alot of quartz and dolomite. Overlying this is a ferruginous zone dominated by goethite, clay minerals and the absence of serpentine. The lower part of this zone also contains manganese oxides. Overlying this is a hardpan clay zone of transported detritus.

Allophane is found as waxy, robin-egg blue coloured, as a weathering product in the oxide zone, as joint fillings, associated with pyrite in the Hanging Wall chert.

In August 2001, a handful of 'gemmy' emerald green Ni brucite was collected by geotechnical engineer Tim Summons at the North Knob section of Stage E pit. The crystals are hexagonal prisms to 15 mms as clusters, and also as fracture fill, associated with magnesite on a serpentine and pyroaurite matrix. A few are in private collections. Brucite is also seen as silky white fibrous material.

Dolomite with trace Ni as fracture coatings and wall linings, yellow-green to oil-green in colour. Some are yellow-green dolomite crystals liberally scattered on bluish-grey botryoidal chalcedony. Also quartz-carbonate-chlorite veins containing colourless sugary dolomite rhombs to 3 mms, encrusting quartz crystals in small vughs.

Abundant graphite sometimes confused with tochilinite, as very coarse lustrous aggregate in black shale units of the Hanging Wall, sometimes associated with pyrite balls. On the 349 and 379 benches of Stage D pit, mine staff collected many pyrite ball specimens, some subsequently mounted on wooden bases. These are similar to the pyrite balls listed under the Millstream Oasis Mindat locality. The Mount Keith specimens are generally yet to be seen publically. Also euhedral pyrite cubes to 10 mms lining walls of iowaite and pyroaurite filled fractures and euhedral 5 mm crystals in weathered serpentine matrix, and massive pyrite.

Clear colourless selenite slabs were collected by mine surveyors from the southern part of the Stage E pit cutback, but are yet to be seen publically.
Heazlewoodite is common in the northern part of the deposit as blebs, and coarse intergrowths with pentlandite, millerite, or godlevskite. Abundant white, cream, or Ni influenced pale green, chalky nodules and segregations of magnesite on its own, or intergrown with talc and/or antigorite.

Common magnetite as thin veinlets and segregations. In one part of the Stage D pits the rocks were covered with millions of glittering octahedral, cube, or combinations of both magnetite microcrystals, along joint planes, associated with sharp pyrite cubes, acicular clear magnesite microcrystals, and scattered pink chromian mountkeithite patches.

Millerite is common in what mine workers call the 'Millerite Domain', associated with pentlandite, cobaltian violarite, and goldlevskite. Pentlandite is the most important nickel ore at the mine as disseminated blebs, and rarely massive, and as a matrix nickel sulphide component.

Quartz in the oxide, and to a lesser extent supergene zones, as blue or grey chalcedony, sometimes coated with druses of minute quartz crystals, and variously coloured prase, poor quality chrysoprase, moss agate, moss opalite, honey opal, cherts, and poor quality coarsely banded pale agate.

Violarite as lath-like exsolution lamellae in pentlandite, sometimes assocated with small round bravoitic pyrite crystals. Unusually iowaite and pyroaurite is abundant at MKD5. These are found as vein fillings associated with cubic pyrite crystals in lizardite-brucite altered dunite. Pyroaurite is white to pale blue, and opaque, while iowaite is a deep sea green colour, translucent, turning to silky white on exposure to air.

Attractive slabbed specimens have been seen of stichtite, woodallite, and/or mountkeithite, associated with pentlandite veinlets and iowaite, in a lizardite-brucite altered dunite, serpentinite, or peridotite matrix.

A refined internal stratigraphy of the Mount Keith ultramafic unit is from stratigraphic bottom to top comprises basal olivine orthocumulate (unit 101); thick, weakly mineralized extreme olivine adcumulate (unit 102);significantly sulfide mineralized olivine adcumulate (104); discontinuous olivine pegmatoid (unit 103), which crosscuts units 102 and 104; olivine mesocumulate (unit 105A); fractionated pyroxenite and gabbro (105B); olivine mesoadcumulate (unit 106);and olivine orthocumulate (unit 107). The western margin of the Mount Keith ultramafic unit, which represents the stratigraphic top, is faulted, with units 106 and 107 missing in the southern part. Drill hole MKD153 transects the Mount Keith ultramafic unit at the thickest part of the MKD5 orebody, where the section only comprises units 101, 102, 104, and 105A. All units are metamorphosed to mid-greenschist facies, completely serpentinized, and only locally minor talc-carbonate-chlorite altered; as
textural preservation of original olivine cumulates is generally excellent, “meta” is implied and omitted from unit and lithological names. All rocks are original olivine-sulfide liquid-chromite adcumulates to mesocumulates, consisting predominantly of altered olivine with varying amounts of sulfide blebs, chromite, and alteration products of very minor intercumulus material,
except for more evolved parts, where former pyroxene and locally plagioclase were dominant.

Commodity List

This is a list of exploitable or exploited mineral commodities recorded at this locality.

Mineral List

85 valid minerals. 2 (TL) - type locality of valid minerals.

Rock Types Recorded