Kambalda type komatiitic nickel ore deposits

Fe Ni Cu PGE ore deposit

Kambalda type komatiitic nickel ore deposits are a class of magmatic iron nickel copper platinum group element ore deposit in which the physical processes of komatiite volcanology serve to deposit, concentrate and enrich a Fe Ni Cu (PGE) sulfide melt within the lava flow environment of an erupting komatiite volcano.

The classification of the type of ore environment sets these apart from other magmatic Ni Cu PGE ore deposits, which share many of the same genetic (formational) controls. Kambalda type ore deposits are distinctive in that the deposition of an immiscible Fe Ni Cu sulfide melt occurs within a lava flow channel upon the palaeosurface. This is distinct from other magmatic Ni Cu PGE deposits, where Fe Ni Cu sulfide melt accumulates within a subvolcanic feeder dike, sill, or magma chamber.

Komatiite associated Ni Cu PGE deposits can form in a wide range of volcanic environments and overlie a wide range of footwall rocks, including basalts (e. g., Kambalda, Western Australia), andesites (e. g., Alexo, Ontario), dacites (e. g., Bannockburn, Ontario; Silver Swan, Western Australia), rhyolites (e. g., Dee's Flow, Ontario), sulfide facies iron formations (e. g., Windarra, Western Australia), and sulfidic semi pelites (e. g., Raglan, Quebec).

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.

Ultramafic mineralogy is especially susceptible to retrograde metamorphism, especially when water is present. Few komatiite sequences display even pristine metamorphic assemblages, 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.

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 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.

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 throughput. 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.

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.

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 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 en route 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 spinifex 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.