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.
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 (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.
Several key features of the metamorphic history affect the present-day morphology and mineralogy of the ore environments;
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.
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.
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 should deposit themselves in parallel systems is unknown.
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.
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