The Emily Ann and Maggie Hays

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

deposits are situated 117 km west of the town of

Norseman, Western Australia Norseman is a town located in the Goldfields-Esperance region of Western Australia along the Coolgardie-Esperance Highway, east of Perth and above sea level. It is also the starting point of the Eyre Highway, and the last major town in Weste ...

, within the Lake Johnston Greenstone Belt.Hill R.E.T, Barnes S.J., Gole M.J., and Dowling S.E., 1990. ''Physical volcanology of komatiites; A field guide to the komatiites of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Block, Western Australia.'',

Geological Society of Australia The Geological Society of Australia (GSA) was established as a non-profit organisation in 1952 to promote, advance and support earth sciences in Australia. The founding Chairperson was Edwin Sherbon Hills. William Rowan Browne was a founder of th ...

The mines had been operational since 2001''The Australian Mines Handbook: 2003–2004 Edition'', page: 28 when Nornickel suspended mining at Emily Ann and Maggie Hays in early 2009 because of drastically falling nickel prices.Norilsk Nickel Australia
Nornickel, accessed: 10 September 2009 Poseidon Nickel acquired the Lake Johnston Nickel Project from Norlisk in 2014.Poseidon Nickel Announcement
accessed: 27 January 2022

Discovery

The Maggie Hays deposit was developed in 1996 by LionOre. It was discovered by prospecting in the 1970s by Anaconda Co. and Union Miniere, who had first located a nickel geochemical anomaly and drilled the disseminated halo, but missed the lucrative high-grade massive sulfide mineralisation by as little as three metres. The recognition of the Maggie Hays orebody by LionOre geologists in the late 1990s was based upon electromagnetic

geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' some ...

surveys and deep diamond drilling of conductive anomalies. LionOre geologists credited the discovery to recognition of the electromagnetic response and drilling of the anomaly, however it is widely recognised that the initial discovery was based upon literature research and the fact that in the late 1970s and 1980s Union Miniere/Anaconda relinquished the tenements in a period of unfavourably low nickel prices. Reassessment of the geophysical signature at Maggie Hays indicates that the hangingwall

banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...

is as conductive as the massive sulfides, and that on this basis, as well as the steep orientation of the massive nickel sulfides, Maggie Hays is essentially blind to discovery. The Emily Ann orebody is situated approximately 1200 m north of the Maggie Hays orebody and was drilled first in 1998 after a prolonged and saturated effort to electromagnetically prospect the entire prospective belt. The Emily Ann orebody was unequivocally discovered by geophysical surveying, the result of a flatter orientation of the orebody, the fact it is hosted within conductively dead felsic

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

, and the depth of the upper parts of the orebody reaching to within 200m of the surface. The Emily Ann discovery was a technical triumph, because it is a mechanically displaced recumbent fold of sheared massive sulfide hosted several hundred metres off the original ultramafic-felsic contact in a position not generally expected to host nickel sulfides.

Regional geology

The Maggie Hays and Emily Ann orebodies are hosted within a komatiitic belt of rocks within the Archaean c. 2.85 Ga Lake Johnston Greenstone Belt (LJGB). There are three

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

horizons recognised within the LJGB stratigraphy: the Eastern, Central, and Western ultramafic. The entirety of the belt's nickel endowment is hosted within the Central Ultramafic Unit, known as the "CUU". However, disseminated and low tenor nickel mineralisation is known from the other ultramafic units, especially the Western Ultramafic Unit. The general stratigraphy of the belt is, from base upwards, a thick sequence of felsic ortho

composed of fragmental to glomerocrystic

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

gneiss, known as the footwall felsic sequence; the ultramafic units of komatiite affinity, "overlain" by

grunerite Grunerite is a mineral of the amphibole group of minerals with formula Fe7 Si8 O22( OH)2. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclini ...

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

- amphibole

of the Honman Formation, tholeiitic

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

and meta

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

rocks. Regionally, several subvolcanic

lopolith A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body. The term was first defin ...

layered intrusion A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around to over and several hundred metres to ...

s have been identified from mapping and drilling. These are interpreted to represent the feeder conduits to extrusive ultramafic and mafic igneous rocks stratigraphically higher in the belt. Examples include the Medcalf Ultramafic Intrusion, a 3.5 km long, ~1 km thick pile of

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

ic to pyroxenitic cumulates which contain subeconomic stratiform vanadiferous

deposits. The extrusive and intrusive igneous rocks are underlain and intruded by a series of I-type

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

intrusions and granite domes of c. 2.65 Ga age. A late dyke swarm of Proterozoic age intrudes the belt, most notably the Jimberlana Dyke which attains a thickness of some 600m and transects the Emily Ann and Maggie Hays orebodies. The Lake Johnston Greenstone Belt is

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

to amphibolite facies, and in parts is extremely highly strained. Structurally the belt is composed of overturned west-vergent isoclinal fold trains, separated by thrusts. Deformation intensity is heterogeneous, ranging from weak

foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...

overprints to extreme 10:1 or 20:1 l-tectonite

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

development.

Mining

Production at Emily Ann begun in late 2001, with the mine being owned by LionOre. Maggie Hays, 3 km south of Emily Ann, at that time, was owned partly by LionOre and

BHP BHP Group Limited (formerly known as BHP Billiton) is an Australian multinational mining, metals, natural gas petroleum public company that is headquartered in Melbourne, Victoria, Australia. The Broken Hill Proprietary Company was founded ...

. Mining at the deposit was started after the conclusion of the ramping up of Emily Ann. LionOre was taken over by Nornickel in June 2007 and, with it, all its Australian operations.Norilsk snaps up LionOre Mining

Canadian Broadcasting Corporation The Canadian Broadcasting Corporation (french: Société Radio-Canada), branded as CBC/Radio-Canada, is a Canadian public broadcaster for both radio and television. It is a federal Crown corporation that receives funding from the government. ...

29 June 2007 Norilsk suspended all Australian nickel operations, consisting of Emily Ann, Maggie Hays, the Black Swan nickel mine, the Cawse nickel mine and the Waterloo nickel mine in early 2009 because of drastically falling nickel prices.

Maggie Hays mine geology

The Maggie Hays orebody sits upon the upper surface of an overturned sequence of komatiitic ultramafic cumulates dipping west at between 65 and 75 degrees. The ultramafic body is in the form of a "keel", with a southward plunging folded surface pinching out the northern end of the resource, where it enters a heterogeneously sheared structure. The ultramafic sequence is folded at its down-dip termination, where it enters a shear, with remobilised sulfides forming a discontinuous remobilised stringer ore zone of low grade. A proposed "shelf fault" terminates the down dip portion of the ultramafic in the south of the Maggie Hays portion of the CUU, however it is equally likely that a porpoising hinge to the syncline could equally well explain the apparent truncation of the depth of the orebody in the south. The ultramafic unit is a massive

serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''se ...

, likely of mesocumulate to adcumulate composition. Peak metamorphic assemblages were in the amphibolite facies, with development of metamorphic

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

anthophyllite Anthophyllite is an orthorhombic amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of antho ...

- pyroxene assemblages. Retrograde metamorphism has occurred, leaving a serpentine-anthophyllite/

cummingtonite Cummingtonite ( ) is a metamorphic amphibole with the chemical composition , magnesium iron silicate hydroxide. Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common ...

talc Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent a ...

assemblage, with some areas of talc-carbonation. The internal structure of the ultamafic includes some interesting features, notably a so-called "chill-zone" of assumed orthocumulate composition, which occupies the marginal facies of the ultramafic unit. This chill zone may represent a chill zone which infers an intrusive origin for the ultramafic, or it may represent a zone of

metasomatism Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

and/or contamination. The orebody itself is composed of two to six metres of massive nickeliferous sulfides, usually banded and foliated

pentlandite Pentlandite is an iron–nickel sulfide with the chemical formula . Pentlandite has a narrow variation range in Ni:Fe but it is usually described as having a Ni:Fe of 1:1. It also contains minor cobalt, usually at low levels as a fraction of wei ...

pyrrhotite Pyrrhotite is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.2). It is a nonstoichiometric variant of FeS, the mineral known as troilite. Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it i ...

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

. Dodecahedral pyrite crystals to 20 cm are formed within the massive sulfide zones. The massive zone is overlain by a matrix ore zone composed of the above sulfide assemblage and coarse jackstraw textured bladed olivine, now retrogressed to black serpentinite. This is in turn overlain by stringy-beef textured recrystallised disseminated ore zones containing retrogressed metamorphic olivine and distinctive bladed anthophyllite. The structural overprint of the ultramafics and orebody by deformation during prograde metamorphism is a matter of debate, however the ductile nature of the deformation has affected the ultramafic CUU heterogenously and contrasts with the felsic footwall. The felsic footwall is subject to a pronounced stretching lineation which increases in intensity to the north. The lineation orientation is a uniform 65 degrees toward 120 degrees. Approaching the CUU body, the lineation breaks down into a zone of heavy boudinage and oblique shear with a pronounced C-S shear

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not th ...

, especially in the remobilised massive sulfides and at the leading edge of the keel structure. This is interpreted to occur due to competency contrast between the felsic footwall and the ultramafic unit.

Emily Ann mine geology

The Emily Ann orebody is a folded and highly mylonitised shear-hosted nickel sulfide and ultramafic unit, in the form of an open overturned synclinal structure plunging shallowly north-east. This synclinal structure has a lower limb dipping 30 degrees east, and an upper limb dipping between 40 and 60 degrees east. The Emily Ann sulfides are hosted within a discontinuously boudinaged ultramafic unit sandwiched within the felsic footwall units, a position considered to be the product of inclusion within a shear. The sulfides are concentrated within boudin necks between ultramafic boudins and lozenges. The morphology and position of the Emily Ann sulfides and ultramafics are unique and somewhat controversial. At its simplest, magmatic nickel sulfides are not usually found displaced so far into the footwall as at Emily Ann, where the material has moved up to 600m off the basal contact of the ultramafic unit. Models used to explain the shape and form of the orebody include: * Attenuation of thickness during ductile deformation and movement into the footwall along a shear * Folding of the resultant shear-hosted nickel sulfide and boudinage of the ultramafic unit during shearing

References

Bibliography

External links

* {{Official website, https://poseidon-nickel.com.au/
MINEDEX website: Lake Johnston / Poseidon
Database of the Department of Mines, Industry Regulation and Safety Underground mines in Australia Nickel mines in Western Australia Lake Johnston, Western Australia 2001 establishments in Australia 2009 disestablishments in Australia