Kambalda type komatiitic nickel ore deposits
Encyclopedia
Kambalda 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
.
Kambalda-type ore deposits are distinctive in that the deposition of nickel sulfides occurs within the lava flow channel upon the palaeosurface. This is distinct from other komatiite and ultramafic associated NiS ore deposits, where nickel sulfide accumulates within the lava conduit or upon the floor or within a subvolcanic
lava chamber.
Recent research on the S isotopic compositions of komatiitic sulfides (Bekker et al., 2009) indicates that they lack the non-mass dependent isotope fractionation typical of sulfides formed at the surface during the Archaean, as would be expected if much of the sulfur was sourced from the sedimentary substrate, confirming that the S was derived 'upstream' in the system, not from the local country rocks.
The lava channel is typically recognised within a komatiite sequence by;
The ore zone typically consists, from the base upwards, of a zone of massive sulfides, matrix sulfides, net-textured ore, disseminated and cloud sulfide.
Massive nickeliferous sulfide is composed of greater than 95% sulfide occasionally with exotic enclaves of olivine, metasedimentary or melted material derived from the footwall to the lava flow. The massive sulfide ideally sits upon a footwall of basalt or felsic volcanic rock, which the massive sulfide may intrude into vertically. This forms a carrot-structured ore, interpreted to represent either thermal erosion of the underlying substrate by the ultra-high temperature komatiite lava, or physical remobilisation during deformation.
The massive sulfide is in some cases overlain by a zone of matrix sulfide. The ideal Kambalda type-section lacks matrix sulfides, which is interpreted to be because of either physical remobilisation, or because matrix ore will only form in quiescent magma conditions, and thus does not form in active channel zones except, perhaps, late in the eruption. However, most other komatiitic nickel ore sections contain matrix to net-textured ore.
Matrix sulfide ore, in high-grade metamorphic areas, is characterised by jackstraw texture, composed of bladed to acicular metamorphic olivine which resembles spinifex textured olivines, within a matrix of nickel sulfide. This texture is formed by metamorphism of the ore, which is interpreted to have been composed of olivine crystals floating in massive sulfide.
Net-textured ore is rarely observed, being the ideal condition of sulfide-silicate immiscibility. This texture is difficult to prove from the majority of komatiite mineralisation profiles, but is known from the Jinchuan intrusive, China, where nickel sulfide forms a network textured groundmass liquid in which olivine floats. Most net-textured ores in komatiites are considered metamorphic overprints.
Disseminated sulfide zones occasionally overly the matrix sulfide zone, grading upwards into barren ultramafic olivine adcumulate. These zones are rarely economic to mine in the majority of komatiites, except when close to surface.
The massive sulfide sits within the B3 flow horizon of a typical komatiite
lava flow system.
In most cases, for instance at the type-locality Kambalda Dome, the contact ore sits upon the footwall basalt, and is flanked by sulfidic and graphitic sediment with which it can be structurally comingled or grades laterally into (e.g.; Wannaway). However, it is not unknown for basal contact ore to be developed on a basement of felsic volcanics, as at Emily Ann and Maggie Hays, or sedimentary formations thick enough to resist the thermal erosion of the main lava channel, an example being in the region of the Blair nickel deposit, on the Pioneer Dome.
Other ore types are known, which do not sit on the basal contact.
Several key features of the metamorphic history affect the present-day morphology and mineralogy of the ore environments;
facies or amphibolite
facies tends to revert igneous olivine to metamorphic olivine, serpentinite
or talc carbonate
d ultramafic schist
s.
In the ore environment, the metamorphism tends to remobilise the nickel sulfide which, during peak metamorphism, has the yield strength and behaviour of toothpaste as conceptualised by workers within the field. The massive sulfides tend to move tens to hundreds of meters away from their original depositional position into fold
hinges, footwall sediments, faults or become caught up within asymmetric shear zones.
While sulfide minerals do not change their mineralogy during metamorphism as silicates do, the yield strength of the nickel sulfide pentlandite
, and copper sulfide chalcopyrite
is less than that of pyrrhotite
and pyrite
, resulting in a potential to segregate the sulfides mechanically throughout a shear zone.
, talc
or chlorite
. Pyroxene tends to retrogress to actinolite
-cummingtonite
or chlorite
. Chromite
may hydrothermally alter to stichtite
, and pentlandite
may retrogress into millerite
or heazlewoodite
.
searching in ~1965, which discovered the Long, Victor, Otter-Juan and other shoots within the Kambalda Dome. The Redross, Widgie Townsite, Mariners, Wannaway, Dordie North and Miitel nickel gossans were identified generally at or around the time of drilling of the Widgiemoltha area beginning in 1985, and continuing till today.
Gossans of nickel mineralisation, especially massive sulfides, are dominated in the arid Yilgarn Craton
by boxworks of goethite, hematite, maghemite and ocher clays. Non-sulfide nickel minerals are typically soluble, and preserved rarely at surface as carbonates, although often can be preserved as nickel arsenates (nickeline) within gossans. Within subtropical and Arctic regions, it is unlikely gossans would be preserved or, if they are, would not contain carbonate minerals.
Minerals such as gaspeite
, hellyerite
, otwayite
, widgiemoolthalite and related hydrous nickel carbonates are diagnostic of nickel gossans, but are exceedingly rare. More usually, malachite
, azurite
, chalcocite
and cobalt compounds are more persistent in boxworks and may provide diagnostic information.
Nickel mineralisation in the regolith
, in the upper saprolite
typically exists as goethite, hematite, limonite and is often associated with polydymite
and violarite
, nickel sulfides which are of supergene
association. Within the lower saprolite, violarite is transitional with unaltered pentlandite-pyrite-pyrrhotite ore.
Geochemically
, the Kambalda Ratio Ni:Cr/Cu:Zn identifies areas of enriched Ni, Cu and depleted Cr and Zn. Cr is associated with fractionated, low-MgO rocks and Zn is a typical sediment contaminant. If the ratio is at around unity or greater than 1, the komatiite flow is considered fertile. Other geochemical trends sought include high MgO contents to identify the area with highest cumulate olivine contents; identifying low-Zn flows; tracking Al content to identify contaminated lavas and, chiefly, identifying anomalously enriched Ni (direct detection). In many areas, economic deposits are identified within a halo of lower grade mineralisation, with a 1% or 2% Ni in hole value contoured.
Geophysically
, nickel sulfides are considered effective superconductors in a geologic context. They are explored for using electromagnetic exploration techniques which measure the current and magnetic fields generated in buried and concealed mineralisation. Mapping of regional magnetic response and gravity is also of use in defining the komatiite sequences, though of little use in directly detecting the mineralisation itself.
Stratigraphic analysis of an area seeks to identify thickening basal lava flows, trough morphologies, or areas with a known sediment-free window on the basal contact. Likewise, identifying areas where cumulate and channelised flow dominates over apparent flanking thin flow stratigraphy, dominated by multiple thin lava horizons defined by recurrence of A-zone spinifex textured rocks, is effective at regionally vectoring in toward areas with the highest magma thoughput. Finally, regionally it is common for komatiite sequences to be drilled in areas of high magnetic anomalism based on the inferred likelihood that increased magnetic response correlates with the thickest cumulate piles.
Ore shoots continue, in essential parallelism, for several kilometres down plunge; furthermore in some ore trends at Widgiemooltha, ore trends and thickened basal flow channels are mirrored by low-tenor and low-grade 'flanking channels'. These flanking channels mimic the sinuous meandering ore shoots. Why extremely hot and superfluid komatiitic lavas and nickel sulfides would deposit themselves in parallel systems can only be described by Horst-Graben type faulting which is commonly seen at rift zones.
This is especially true of the peridotite
and dunite
hosted low-grade disseminated Ni-Cu-(PGE) deposits such as Perseverance, Mount Keith
MKD5, Yakabindie and Honeymoon Well, which occupy peridotite bodies which are at least 300m and up to 1200m thickness (or more).
The major difficulty in identifying adcumulate peridotite piles in excess of 1 km as being entirely volcanic is the difficulty in envisaging a komatiitic eruptive event which is prolonged enough to persist long enough to build up via accumulation such a thickness of olivine-only material. It is considered equally plausible that such large dunite-peridotite bodies represent lave channels or sills through which, perhaps, great volumes of lava flowed enroute to the surface.
This is exemplified by the Mount Keith MKD5 orebody, near Leinster, Western Australia, which has recently been reclassified according to a subvolcanic intrusive model. Extremely thick olivine adcumulate piles were interpreted as representing a 'mega' flow channel facies, and it was only upon mining into a low-strain margin of the body at Mount Keith that an intact intrusive-type contact was discovered.
Similar thick adcumulate bodies of komatiitic affinity which have sheared or faulted-off contacts could also represent intrusive bodies. For example the Maggie Hays and Emily Ann ore deposits, in the Lake Johnston Greenstone Belt, Western Australia, are highly structurally remobilised (up to 600 m into felsic footwall rocks) but are hosted in folded podiform adcumulate to mesocumulate bodies which lack typical spinfex flow-top facies and exhibit an orthocumulate margin. This may represent a sill
or lopolith
form of intrusion, not a channelised flow, but structural modification of the contacts precludes a definitive conclusion.
Intrusive equivalents
Probable Kambalda-type
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...
-copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
deposit in which the physical processes 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...
volcanology serve to enrich, concentrate and deposit nickel-bearing sulfide within the lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flow environment of an erupting komatiite volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
.
Classification
The classification of the type of ore environment sets these apart from other similar nickel sulfide ore deposits, which share many of the same source and transport criteria for nickel mineralization, according to the trap mechanism.Kambalda-type ore deposits are distinctive in that the deposition of nickel sulfides occurs within the lava flow channel upon the palaeosurface. This is distinct from other komatiite and ultramafic associated NiS ore deposits, where nickel sulfide accumulates within the lava conduit or upon the floor or within a 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...
lava chamber.
Genetic model
The genetic model of Kambalda-type Ni-Cu-(PGE) ore deposits is similar that of many other magmatic Ni-Cu-PGE ore deposits:- Metal Source: komatiitic magma, which has been generated by high-degree partial melting of the mantleMantleA mantle is an ecclesiastical garment in the form of a very full cape which extends to the floor, joined at the neck, that is worn over the outer garments....
and which was strongly undersaturated in sulfide in the source (Wendland, 1982; see also Mavrogenes and O'Neill, 1999) - Sulfur Source: S-rich country rocks (sulfidic sediments and volcanic rocks), from which the sulfide is melted by the high-temperature komatiite magma
- Dynamic System: Ni-Cu-Co-PGE are chalcophile and will preferentially partition from the silicate melt into the sulfide melt. The metal tenors (abundances in 100% sulfide) are enhanced by the flushing of voluminous komatiitic melt across the sulfide accumulation.
- Physical Trap: depressions in the footwall rocks, which may represent volcanic topographic irregularities modified by thermomechanical erosion. Sulfides within the komatiite lava flows are denser than the silicate melt and tend to pool within topographic lows, which may be enhanced in the lava channelLava channelA lava channel is a stream of fluid lava contained within marginal zones of static lava or levees. The initial channel may not contain levees per se, until the parental flow solidifies over what develops into the channel and creates simple levees. This initial levee allows for the building of a...
by proposed thermal erosion of the substrate by the komatiite lava.
Recent research on the S isotopic compositions of komatiitic sulfides (Bekker et al., 2009) indicates that they lack the non-mass dependent isotope fractionation typical of sulfides formed at the surface during the Archaean, as would be expected if much of the sulfur was sourced from the sedimentary substrate, confirming that the S was derived 'upstream' in the system, not from the local country rocks.
Morphology
The morphology of Kambalda-type Ni-Cu-PGE deposits is distinctive because the nickel sulfide can be shown to be associated with the floor of a komatiite lava flow, concentrated within a zone of highest flow in the lava channel facies.The lava channel is typically recognised within a komatiite sequence by;
- Thickening of the basal flow of the komatiite sequence
- Increased MgO, Ni, Cu, and concomitant decrease in Zn, Cr, Fe, Ti as compared to 'flanking flows'
- A 'sediment free window' where sediment has been scoured or melted from the basal or footwall contact of the komatiite with the underlying substrate
- A trough morphology, which is recognisable by a reentrant flat and steep-sided embayment in the footwall underlying thickest cumulate piles
The ore zone typically consists, from the base upwards, of a zone of massive sulfides, matrix sulfides, net-textured ore, disseminated and cloud sulfide.
Massive nickeliferous sulfide is composed of greater than 95% sulfide occasionally with exotic enclaves of olivine, metasedimentary or melted material derived from the footwall to the lava flow. The massive sulfide ideally sits upon a footwall of basalt or felsic volcanic rock, which the massive sulfide may intrude into vertically. This forms a carrot-structured ore, interpreted to represent either thermal erosion of the underlying substrate by the ultra-high temperature komatiite lava, or physical remobilisation during deformation.
The massive sulfide is in some cases overlain by a zone of matrix sulfide. The ideal Kambalda type-section lacks matrix sulfides, which is interpreted to be because of either physical remobilisation, or because matrix ore will only form in quiescent magma conditions, and thus does not form in active channel zones except, perhaps, late in the eruption. However, most other komatiitic nickel ore sections contain matrix to net-textured ore.
Matrix sulfide ore, in high-grade metamorphic areas, is characterised by jackstraw texture, composed of bladed to acicular metamorphic olivine which resembles spinifex textured olivines, within a matrix of nickel sulfide. This texture is formed by metamorphism of the ore, which is interpreted to have been composed of olivine crystals floating in massive sulfide.
Net-textured ore is rarely observed, being the ideal condition of sulfide-silicate immiscibility. This texture is difficult to prove from the majority of komatiite mineralisation profiles, but is known from the Jinchuan intrusive, China, where nickel sulfide forms a network textured groundmass liquid in which olivine floats. Most net-textured ores in komatiites are considered metamorphic overprints.
Disseminated sulfide zones occasionally overly the matrix sulfide zone, grading upwards into barren ultramafic olivine adcumulate. These zones are rarely economic to mine in the majority of komatiites, except when close to surface.
The massive sulfide sits within the B3 flow horizon of a typical 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...
lava flow system.
Ore positions
The typical position of massive sulfide ore in a komatiitic nickel sulfide deposit, and in shoots and trends within a mineralised belt, is for the sulfide to occupy the disconformity between the komatiitic lava and its underlying substrate. This is known as contact ore.In most cases, for instance at the type-locality Kambalda Dome, the contact ore sits upon the footwall basalt, and is flanked by sulfidic and graphitic sediment with which it can be structurally comingled or grades laterally into (e.g.; Wannaway). However, it is not unknown for basal contact ore to be developed on a basement of felsic volcanics, as at Emily Ann and Maggie Hays, or sedimentary formations thick enough to resist the thermal erosion of the main lava channel, an example being in the region of the Blair nickel deposit, on the Pioneer Dome.
Other ore types are known, which do not sit on the basal contact.
- Interformational sulfides; So-called serp-serp ore which is developed off a thrust pinchout, or via remobilisation of massive sulfide along a shear surface or thrust which drags ore up off the contact into the serpentinitised komatiite. Serp-serp ore may, in some cases, be similar to interspinifex ore, the diagnostic spinifex textures often absent due to thermal erosion or metamorphic overprint, and can only be determined as such by comparison of chemistry of the ultramafics above and below.
- Basalt-basalt pinchout, or pinchout or Bas-bas ore, is developed during deformation by remobilisation of massive sulfide into the footwall via attenuation of the trough and structural re-closing. Bas-bas ore can be found up to 40–60 m into the footwall leading from a trough position.
- Interspinifex ore, developed on the upper contact of the basal flow and on the basal contact of a fertile second flow. In some cases, liquid sulfide from the second flow is seen intermingled intimately with spinifex-textured ultramafic flow tops of the basal flow (e.g.; Long-Victor Shoot, Kambalda) and may be present above remnant sediments and intermingled with remnant sediments (e.g.; Hilditch Prospect, Wannaway, Bradley Prospect, Location 1 and likely others).
- Remobilised ore. In rare cases, ore may be remobilised into a bas-bas or serp-serp position geometrically variant to the stratigraphy. Such examples include Waterloo-Amorac, Emily Ann, Wannaway and potentially other small pods of remobilised and structurally complicated sulfides (e.g., Wedgetail, in the Honeymoon Well complex). In most cases, sulfides move less than 100m, although in the case of Emily Ann, over 600m of displacement is known.
Metamorphic overprint
Metamorphism is nearly ubiquitous within Archaean komatiites. The type locality for Kambalda-type Ni-Cu-PGE deposits has suffered several metamorphic events which have altered the mineralogy, textures and morphology of the komatiite-hosted ore.Several key features of the metamorphic history affect the present-day morphology and mineralogy of the ore environments;
Prograde metamorphism
Prograde metamorphism to either greenschistGreenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
facies or 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 tends to revert igneous olivine to metamorphic olivine, 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...
or talc carbonate
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...
d ultramafic schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
s.
In the ore environment, the metamorphism tends to remobilise the nickel sulfide which, during peak metamorphism, has the yield strength and behaviour of toothpaste as conceptualised by workers within the field. The massive sulfides tend to move tens to hundreds of meters away from their original depositional position into 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...
hinges, footwall sediments, faults or become caught up within asymmetric shear zones.
While sulfide minerals do not change their mineralogy during metamorphism as silicates do, the yield strength of the nickel sulfide 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...
, and copper sulfide chalcopyrite
Chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...
is less than that of 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...
and 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...
, resulting in a potential to segregate the sulfides mechanically throughout a shear zone.
Retrograde metamorphism
Ultramafic mineralogy is especially susceptible to retrograde metamorphism, especially when water is present. Few komatiite sequences display even pristine metamorphic assembages, with most metamorphic olivine replaced by serpentine, anthophylliteAnthophyllite
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...
, 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...
or chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
. Pyroxene tends to retrogress to actinolite
Actinolite
Actinolite is an amphibole silicate mineral with the chemical formula .-Etymology:The name actinolite is derived from the Greek word aktis , meaning "beam" or "ray", because of the mineral's fibrous nature...
-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...
or chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
. Chromite
Chromite
Chromite is an iron chromium oxide: FeCr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite ; substitution of aluminium occurs leading to hercynite .-Occurrence:Chromite is found in...
may hydrothermally alter to stichtite
Stichtite
Stichtite is a mineral, a carbonate of chromium and magnesium; formula Mg6Cr2CO316·4H2O. Its colour ranges from pink through lilac to a rich purple colour. It is formed as an alteration product from chromium containing serpentine....
, and 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...
may retrogress into millerite
Millerite
Millerite is a nickel sulfide mineral, NiS. It is brassy in colour and has an acicular habit, often forming radiating masses and furry aggregates...
or heazlewoodite
Heazlewoodite
Heazlewoodite, Ni3S2, is a rare sulfur-poor nickel sulfide mineral found in serpentinitized dunite. It occurs as disseminations and masses of opaque, metallic light bronze to brassy yellow grains which crystallize in the trigonal crystal system. It has a hardness of 4, a specific gravity of 5.82,...
.
Supergene modification
Kambalda style komatiitic nickel mineralisation was initially discovered by gossanGossan
Gossan is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the classic gossan or iron cap all that remains is iron oxides and quartz often in the form of boxworks, quartz lined cavities retaining the shape of the dissolved...
searching in ~1965, which discovered the Long, Victor, Otter-Juan and other shoots within the Kambalda Dome. The Redross, Widgie Townsite, Mariners, Wannaway, Dordie North and Miitel nickel gossans were identified generally at or around the time of drilling of the Widgiemoltha area beginning in 1985, and continuing till today.
Gossans of nickel mineralisation, especially massive sulfides, are dominated in the arid Yilgarn Craton
Yilgarn craton
The 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...
by boxworks of goethite, hematite, maghemite and ocher clays. Non-sulfide nickel minerals are typically soluble, and preserved rarely at surface as carbonates, although often can be preserved as nickel arsenates (nickeline) within gossans. Within subtropical and Arctic regions, it is unlikely gossans would be preserved or, if they are, would not contain carbonate minerals.
Minerals such as gaspeite
Gaspeite
Gaspeite is an extremely rare nickel carbonate mineral named for the place it was first described, in the Gaspé Peninsula, Canada.Gaspeite's formula is CO3 and it is a bright green mineral. It forms massive to reniform pappillary aggregates in fractures, bottryoidal concretions in laterite or...
, hellyerite
Hellyerite
Hellyerite, NiCO3·6, is an hydrated nickel carbonate mineral. It is light blue to bright green in colour, has a hardness of 2.5, a vitreous luster, a white streak and crystallises in the monoclinic system...
, otwayite
Otwayite
Otwayite, Ni2CO32, is a hydrated nickel carbonate mineral. Otwayite is green, with a hardness of 4, a specific gravity of 3.4, and crystallises in the orthorhombic system.- Occurrence :...
, widgiemoolthalite and related hydrous nickel carbonates are diagnostic of nickel gossans, but are exceedingly rare. More usually, malachite
Malachite
Malachite is a copper carbonate mineral, with the formula Cu2CO32. This green-colored mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses. Individual crystals are rare but do occur as slender to acicular prisms...
, azurite
Azurite
Azurite is a soft, deep blue copper mineral produced by weathering of copper ore deposits. It is also known as Chessylite after the type locality at Chessy-les-Mines near Lyon, France...
, chalcocite
Chalcocite
Chalcocite, copper sulfide , is an important copper ore mineral. It is opaque, being colored dark-gray to black with a metallic luster. It has a hardness of 2½ - 3. It is a sulfide with an orthorhombic crystal system....
and cobalt compounds are more persistent in boxworks and may provide diagnostic information.
Nickel mineralisation in the regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
, in the upper saprolite
Saprolite
Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds...
typically exists as goethite, hematite, limonite and is often associated with polydymite
Polydymite
Polydymite, Ni2+Ni23+S4, is a supergene thiospinel sulfide mineral associated with the weathering of primary pentlandite nickel sulfide.Polydymite crystallises in the isometric system, with a hardness of 4.5 to 5.5 and a specific gravity of about 4, is dark violet gray to copper-red, often with...
and violarite
Violarite
Violarite is a supergene sulfide mineral associated with the weathering and oxidation of primary pentlandite nickel sulfide ore minerals....
, nickel sulfides which are of supergene
Supergene (geology)
In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...
association. Within the lower saprolite, violarite is transitional with unaltered pentlandite-pyrite-pyrrhotite ore.
Exploration for Kambalda Ni-Cu-PGE ores
Exploration for Kambalda-style nickel ores focuses on identifying prospective elements of komatiite sequences via geochemistry, geophysical prospecting methods and stratigraphic analysis.Geochemically
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...
, the Kambalda Ratio Ni:Cr/Cu:Zn identifies areas of enriched Ni, Cu and depleted Cr and Zn. Cr is associated with fractionated, low-MgO rocks and Zn is a typical sediment contaminant. If the ratio is at around unity or greater than 1, the komatiite flow is considered fertile. Other geochemical trends sought include high MgO contents to identify the area with highest cumulate olivine contents; identifying low-Zn flows; tracking Al content to identify contaminated lavas and, chiefly, identifying anomalously enriched Ni (direct detection). In many areas, economic deposits are identified within a halo of lower grade mineralisation, with a 1% or 2% Ni in hole value contoured.
Geophysically
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...
, nickel sulfides are considered effective superconductors in a geologic context. They are explored for using electromagnetic exploration techniques which measure the current and magnetic fields generated in buried and concealed mineralisation. Mapping of regional magnetic response and gravity is also of use in defining the komatiite sequences, though of little use in directly detecting the mineralisation itself.
Stratigraphic analysis of an area seeks to identify thickening basal lava flows, trough morphologies, or areas with a known sediment-free window on the basal contact. Likewise, identifying areas where cumulate and channelised flow dominates over apparent flanking thin flow stratigraphy, dominated by multiple thin lava horizons defined by recurrence of A-zone spinifex textured rocks, is effective at regionally vectoring in toward areas with the highest magma thoughput. Finally, regionally it is common for komatiite sequences to be drilled in areas of high magnetic anomalism based on the inferred likelihood that increased magnetic response correlates with the thickest cumulate piles.
Parallel ore trends
One notable phenomena in and around the domes which host the majority of the komatiitic nickel ore deposits in Australia is the high degree of parallelism of the ore shoots, especially at the Kambalda Dome and Widgiemooltha Dome.Ore shoots continue, in essential parallelism, for several kilometres down plunge; furthermore in some ore trends at Widgiemooltha, ore trends and thickened basal flow channels are mirrored by low-tenor and low-grade 'flanking channels'. These flanking channels mimic the sinuous meandering ore shoots. Why extremely hot and superfluid komatiitic lavas and nickel sulfides would deposit themselves in parallel systems can only be described by Horst-Graben type faulting which is commonly seen at rift zones.
Subvolcanic feeder vs. mega-channels
One of the major problems in classifying and identifying komatiite-hosted NiS ore deposits as Kambalda type is the structural complication and overprint of metamorphism upon the volcanic morphology and textures of the ore deposit.This is especially true of the peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
and dunite
Dunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
hosted low-grade disseminated Ni-Cu-(PGE) deposits such as Perseverance, Mount Keith
Mount Keith
Mount Keith is a thirteener on the crest of California's Sierra Nevada, between Mount Bradley to the north, and Junction Peak to the southwest. Its north and west facing slopes feed the Kings River watershed by way of Bubbs Creek, and its east and south slopes feed the Owens River via Shepherd Creek...
MKD5, Yakabindie and Honeymoon Well, which occupy peridotite bodies which are at least 300m and up to 1200m thickness (or more).
The major difficulty in identifying adcumulate peridotite piles in excess of 1 km as being entirely volcanic is the difficulty in envisaging a komatiitic eruptive event which is prolonged enough to persist long enough to build up via accumulation such a thickness of olivine-only material. It is considered equally plausible that such large dunite-peridotite bodies represent lave channels or sills through which, perhaps, great volumes of lava flowed enroute to the surface.
This is exemplified by the Mount Keith MKD5 orebody, near Leinster, Western Australia, which has recently been reclassified according to a subvolcanic intrusive model. Extremely thick olivine adcumulate piles were interpreted as representing a 'mega' flow channel facies, and it was only upon mining into a low-strain margin of the body at Mount Keith that an intact intrusive-type contact was discovered.
Similar thick adcumulate bodies of komatiitic affinity which have sheared or faulted-off contacts could also represent intrusive bodies. For example the Maggie Hays and Emily Ann ore deposits, in the Lake Johnston Greenstone Belt, Western Australia, are highly structurally remobilised (up to 600 m into felsic footwall rocks) but are hosted in folded podiform adcumulate to mesocumulate bodies which lack typical spinfex flow-top facies and exhibit an orthocumulate margin. This may represent a sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
or 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...
form of intrusion, not a channelised flow, but structural modification of the contacts precludes a definitive conclusion.
Example ore deposits
Definitive Kambalda-type- Kambalda-St Ives-Tramways district, Western Australia (including Durkin, Otter-Juan, Coronet, Long, Victor, Loreto, Hunt, Fisher, Lunnon, Foster, Lanfranci, and Edwin shoots)
- Carnilya Hill deposit, Western Australia
- Widgiemooltha Dome, Western Australia (including Miitel, Mariners, Redross, and Wannaway deposits)
- Forrestania belt, Western Australia (including Cosmic Boy, Flying Fox, and Liquid Acrobat deposits)
- Silver Swan deposit, Western Australia
- Raglan district, New Quebec (including Cross Lake, Zone 2-3, Katinniq, Zone 5-8, Zone 13-14, West Boundary, Boundary, and Donaldson deposits)
Intrusive equivalents
- Thompson Nickel Belt, Manitoba (including Birchtree, Pipe, and Thompson deposits)
Probable Kambalda-type
- Maggie Hays and Emily AnnEmily Ann and Maggie Hays nickel minesThe 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....
, Lake Johnstone Greenstone Belt, Western Australia - Waterloo Nickel Deposit, Agnew-Wiluna Greenstone Belt, Western Australia
See also
- KomatiiteKomatiiteKomatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
- BasaltBasaltBasalt 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...
- 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...
- Rock microstructureRock microstructureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
- List of rock textures
- List of rock types
- Igneous rocks
- Definition of ultramafic rocks
- Cumulate rocks