Emily Ann and Maggie Hays nickel mines
Encyclopedia
The Emily Ann and Maggie Hays nickel deposits are situated approximately 150 km west of the town of Norseman, Western Australia
, within the Lake Johnston Greenstone Belt.
Having been operational since 2001, Norilsk suspended mining at Emily Ann and Maggie Hays in early 2009 because of drastically falling nickel prices.
) and Union Miniere
, who had first discovered a nickel
geochemical anomaly
and drilled the disseminated halo, but missed the lucrative high-grade massive sulfide mineralisation by as little as 3 metres.
The recognition of the Maggie Hays orebody by LionOre geologists in the late 1990s was based upon electromagnetic geophysical
surveys and deep diamond drilling of conductive anomalies. LionOre geologists credit 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 unfavorably low nickel prices.
Reassessment of the geophysical signature at Maggie Hays indicates that the hangingwall banded iron formation
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 1200m north of the Maggie Hays orebody and was drilled first in 1998 after a prolonged and saturated effort to elecromagnetically 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
, 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 conact in a position not generally expected to host nickel sulfides.
belt of rocks within the Archaean c. 2.85 Ga Lake Johnston Greenstone Belt
(LJGB). There are three ultramafic 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 (WUU).
The general stratigraphy of the belt is, from base upwards, a thick sequence of felsic
orthogneiss
composed of fragmental to glomerocrystic feldspar
gneiss, known as the footwall felsic sequence; the ultramafic units of komatiite
affinity, 'overlain' by grunerite
-magnetite
-quartz
-amphibole
banded iron formation
of the Honman Formation, tholeiitic basalt
and metasedimentary rocks.
Regionally, several subvolcanic
lopolith
ic layered intrusions 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
ic to pyroxenitic
cumulates which contain subeconomic stratiform vanadiferous magnetite
deposits.
The extrusive and intrusive igneous rocks are underlain and intruded by a series of I-type granite
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
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
overprints to extreme 10:1 or 20:1 l-tectonite
mylonite
development.
. Mining at the deposit was started after the conclusion of the ramping up of Emily Ann.
LionOre was taken over by MMC Norilsk Nickel
in June 2007 and, with it, all its Australian operations.
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.
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
, likely of mesocumulate to adcumulate composition. Peak metamorphic assemblages were in the amphibolite
facies, with development of metamorphic olivine
-anthophyllite
-pyroxene
assemblages. Retrograde metamorphism has occurred, leaving a serpentine-anthophyllite/cummingtonite
-talc
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
and/or contamination.
The orebody itself is composed of 2 to 6 metres of massive nickeliferous sulfides, usually banded and foliated pentlandite
-pyrrhotite
-pyrite
. 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
, 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.
The Emily Ann sulfides are hosted within a discontinuously boudinage
d 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;
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
, within the Lake Johnston Greenstone Belt.
Having been operational since 2001, Norilsk suspended mining at Emily Ann and Maggie Hays in early 2009 because of drastically falling nickel prices.
Discovery
The Maggie Hays deposit was first officially discovered in 1996 by LionOre Australia, although it had essentially been found by prospecting in the 1970s by Anaconda Mines (now Minara ResourcesMinara Resources
Minara Resources Ltd is one of the major mining companies of Australia, specializing in the exploration of cobalt and nickel. Based in Perth, Minara Resources was founded in 1994 as the successor to Anaconda Nickel Ltd. which was founded by Fortescue Metals Group chief Andrew Forrest; it...
) and Union Miniere
Union Minière du Haut Katanga
The Union Minière du Haut Katanga was a Belgian mining company, once operating in Katanga, in what is now the Democratic Republic of the Congo...
, who had first discovered a nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
geochemical anomaly
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
and drilled the disseminated halo, but missed the lucrative high-grade massive sulfide mineralisation by as little as 3 metres.
The recognition of the Maggie Hays orebody by LionOre geologists in the late 1990s was based upon electromagnetic geophysical
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
surveys and deep diamond drilling of conductive anomalies. LionOre geologists credit 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 unfavorably low nickel prices.
Reassessment of the geophysical signature at Maggie Hays indicates that the hangingwall banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
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 1200m north of the Maggie Hays orebody and was drilled first in 1998 after a prolonged and saturated effort to elecromagnetically 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
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
, 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
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
of sheared massive sulfide hosted several hundred metres off the original ultramafic-felsic conact in a position not generally expected to host nickel sulfides.
Regional geology
The Maggie Hays and Emily Ann orebodies are hosted within a komatiiticKomatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
belt of rocks within the Archaean c. 2.85 Ga Lake Johnston Greenstone Belt
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
(LJGB). There are three ultramafic horizons recognised within the LJGB stratigraphy
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
; 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 (WUU).
The general stratigraphy of the belt is, from base upwards, a thick sequence of felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
orthogneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
composed of fragmental to glomerocrystic feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
gneiss, known as the footwall felsic sequence; the ultramafic units of komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
affinity, 'overlain' by grunerite
Grunerite
Grunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O222. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The luster is glassy to pearly with colors...
-magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
-quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
-amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
of the Honman Formation, tholeiitic basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
and metasedimentary rocks.
Regionally, several subvolcanic
Subvolcanic rock
A subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates at medium to shallow depths within the crust and contain intermediate grain size and often porphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocks include diabase and...
lopolith
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...
ic layered intrusions 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
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
ic to pyroxenitic
Pyroxenite
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...
cumulates which contain subeconomic stratiform vanadiferous magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
deposits.
The extrusive and intrusive igneous rocks are underlain and intruded by a series of I-type granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
intrusions and granite domes of c. 2.65 Ga age. A late dyke
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
swarm of Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
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
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
to amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
facies, and in parts is extremely highly strained. Structurally the belt is composed of overturned west-vergent isoclinal fold
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
trains, separated by thrusts. Deformation intensity is heterogeneous, ranging from weak foliation
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
overprints to extreme 10:1 or 20:1 l-tectonite
Tectonite
Tectonites are metamorphic or tectonically deformed rocks whose fabric reflects the history of their deformation, or rocks with fabric that clearly displays coordinated geometric features that indicate continuous solid flow during formation. Planar foliation results from a parallel orientation of...
mylonite
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
development.
Mining
Production at Emily Ann begun in late 2001, with the mine being owned by LionOre Mining International. Maggie Hays, 3 km south of Emily Ann, at that time, was owned partly by LionOre and BHP BillitonBHP Billiton
BHP Billiton is a global mining, oil and gas company headquartered in Melbourne, Australia and with a major management office in London, United Kingdom...
. Mining at the deposit was started after the conclusion of the ramping up of Emily Ann.
LionOre was taken over by MMC Norilsk Nickel
MMC Norilsk Nickel
MMC Norilsk Nickel is a nickel and palladium mining and smelting company. Its largest operations are located in the Norilsk–Talnakh area, in northern Russia. MMC stands for "Mining and Metallurgical Company"....
in June 2007 and, with it, all its Australian operations.
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 dippingStrike and dip
Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented...
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
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
could equally well explain the apparent truncation of the depth of the orebody in the south.
The ultramafic unit is a massive serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
, likely of mesocumulate to adcumulate composition. Peak metamorphic assemblages were in the amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
facies, with development of metamorphic olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
-anthophyllite
Anthophyllite
Anthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
-pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
assemblages. Retrograde metamorphism has occurred, leaving a serpentine-anthophyllite/cummingtonite
Cummingtonite
Cummingtonite is a metamorphic amphibole with the chemical composition 7Si8O222, magnesium iron silicate hydroxide.Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being...
-talc
Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
assemblage, with some areas of talc-carbonation
Talc carbonate
Talc carbonate is a geologic term for a suite of rock and mineral compositions found in metamorphic ultramafic rocks.The term refers to the two most common end-member minerals found within ultramafic rocks which have undergone talc-carbonation or carbonation reactions, talc and the carbonate...
.
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
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
and/or contamination.
The orebody itself is composed of 2 to 6 metres of massive nickeliferous sulfides, usually banded and foliated pentlandite
Pentlandite
Pentlandite is an iron-nickel sulfide, 9S8. Pentlandite usually has a Ni:Fe ratio of close to 1:1. It also contains minor cobalt.Pentlandite forms isometric crystals, but is normally found in massive granular aggregates. It is brittle with a hardness of 3.5 - 4 and specific gravity of 4.6 - 5.0 and...
-pyrrhotite
Pyrrhotite
Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: FeS . The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic...
-pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
. 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
Lineation
In Western handwriting, the base line, the x-height or corpus size, the height of the ascenders and the bottom line of the descenders make up four horizontal lines which represent the lineation of handwriting...
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
Boudinage
thumb|Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as a bed of sandstone, is stretched and deformed amidst less competent surroundings...
and oblique shear with a pronounced C-S shear fabric
Fabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...
, 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 boudinage
Boudinage
thumb|Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as a bed of sandstone, is stretched and deformed amidst less competent surroundings...
d 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
See also
- Kambalda type komatiitic nickel ore depositsKambalda type komatiitic nickel ore depositsKambalda type nickel ore deposits are a class of magmatic nickel-copper ore deposit in which the physical processes of komatiite volcanology serve to enrich, concentrate and deposit nickel-bearing sulfide within the lava flow environment of an erupting komatiite volcano.-Classification:The...
- NickelNickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
- KomatiiteKomatiiteKomatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
- Yilgarn cratonYilgarn cratonThe Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
Sources
- The Australian Mines Handbook: 2003-2004 Edition, Louthean Media Pty Ltd, Editor: Ross Louthean