type section

Kambalda type komatiitic nickel ore deposits

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.

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 lava chamber.

Genetic model

The genetic model of Kambalda-type NiS ore deposits is similar to the broader category of magmatic Ni-Cu-PGE ore deposits.

  • Source: Highly magnesian ultramafic melts which become sulfur saturated during ascent, or upon contact with the surface
  • Transport: Typically, the transport of the komatiitic melts in the Kambalda type NiS ore deposits is considered to represent a mantle plume tail impinging on a continental rift or margin. The nickel is transported within the highly ultramafic high-temperature melt.
  • Trap: Kambalda-type NiS ore is considered to be trapped at the surface by the following physical and chemical processes;
    • The sulfur capacity of mafic silicate melts increases with decreasing pressure, meaning that the pressure decrease during the ascent of a komatiitic melt will drive the melt to sulfur undersaturation (Mavrogenes and O'Neill, 1999), irrespective of the sulfur content of the melt at source. Thus, sulfur-saturation of komatiites requires changes in the composition of the melt either by assimilation of siliceous substrate material, diluting the FeO content of the melt and thus reducing sulfur solubility, or by simple assimilation of sulfurous material. Sulfidic sediments typically form part of the substrate sedimentary sequence in deposit districts. However, recent research into sulfur isotopic composition of komatiitic sulfides has revealed 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. Indeed, the sulfur isotope signature of the samples investigated was found to be more similar to that of hydrothermal exhalative massive sulfides hosted by the footwall felsic volcanics associated with komatiite sequences (Fiorentini et al, 2006).
    • 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 channel by proposed thermal erosion of the substrate by the komatiite lava
    • Nickel tenor is enhanced by the flushing of voluminous komatiitic melt across the sulfide accumulation. The tenor is enhanced because nickel, copper and platinum-group metals are chalcophile and will preferentially partition from the silicate melt into the sulfide melt

Morphology

The morphology of Kambalda-type Ni-Cu-PHE 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 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 (eg; 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 40-60m 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 (eg; Long-Victor Shoot, Kambalda) and may be present above remnant sediments and intermingled with remnant sediments (eg; 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 (eg, 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 greenschist facies or amphibolite facies tends to revert igneous olivine to metamorphic olivine, serpentinite or talc carbonated ultramafic schists.

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.

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, anthophyllite, 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.

Supergene modification

Kambalda style komatiitic nickel mineralisation was initially discovered by gossan 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 minseralisation 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.

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, 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.

General Morphological Phenomena

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 should deposit themselves in parallel systems is unknown.

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 and dunite hosted low-grade disseminated nickel sulfide deposits such as Perseverance, Mt 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 1km 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 Mt 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 Mt 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 600m 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.

Example ore deposits

Definitive Kambalda-type

  • Kambalda Dome, Western Australia
  • Otter-Juan, Lunnon, Coronet, Long, Victor, Loreto, Carneliya orebodies
  • Widgiemooltha Dome, Western Australia
    • Miitel, Mariners, Redross, Wannaway mines
  • Flying Fox, Forrestania Greenstone Belt, Western Australia
  • Black Swan, Eastern Goldfields, Western Australia

Probable Kambalda-type

  • Maggie Hays and Emily Ann, Lake Johnstone Greenstone Belt, Western Australia
  • Waterloo Nickel Deposit, Agnew-Wiluna Greenstone Belt, Western Australia

See also

References

  • Gresham, J.J., and Loftus-Hills, G.D., 1981, The geology of the Kambalda nickel field, Western Australia, Economic Geology, v. 76, p. 1373-1416.
  • W.E. Stone, M. Heydart and Z. Seat, 2004. Nickel tenor variations between Archaean komatiite-associated nickel sulphide deposits, Kambalda ore field, Western Australia: the metamorphic modification model revisited. Mineralogy and Petrology, (2004).
  • Hess, P. C. (1989), Origins of Igneous Rocks, President and Fellows of Harvard College (pp. 276-285), ISBN 0-674-64481-6.
  • Lesher, C.M., Arndt, N.T., and Groves, D.I., 1984, Genesis of komatiite-associated nickel sulphide deposits at Kambalda, Western Australia: A distal volcanic model, in Buchanan, D.L., and Jones, M.J. (Editors), Sulphide Deposits in Mafic and Ultramafic Rocks, Institution of Mining and Metallurgy, London, p. 70-80.
  • 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. ISBN 0-909869-55-3
  • Blatt, Harvey and Robert Tracy (1996), Petrology, 2nd ed., Freeman (pp. 196-7), ISBN 0-7167-2438-3.
  • S. A. Svetov, A. I. Svetova, and H. Huhma, 1999, Geochemistry of the Komatiite–Tholeiite Rock Association in the Vedlozero–Segozero Archean Greenstone Belt, Central Karelia, Geochemistry International, Vol. 39, Suppl. 1, 2001, pp. S24–S38. PDF accessed 7-25-2005
  • Vernon R.H., 2004, A Practical Guide to Rock Microstructure, (pp. 43-69, 150-152) Cambridge University Press. ISBN 0-521-81443-X
  • M.L. Fiorentini, A. Bekker, D. Rumble, M.E. Barley and S.W. Beresford (2006) 'Multiple S isotope study indicates footwall hydrothermal exhalative massive sulfides were the major sulfur source for Archean komatiite-hosted magmatic nickel-sulfides from Western Australia and Canada.' Geochimica et Cosmochimica Acta, Volume 70, Issue 18, Supplement 1, August-September 2006, Page A174
  • J A. Mavrogenes and H. St. C. O’Neill (1999) 'The relative effects of pressure, temperature and oxygen fugacity on the solubility of sulfide in mafic magmas.' Geochimica et Cosmochimica Acta, Volume 63, Issues 7-8, April 1999, Pages 1173-1180

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