Komatiite-hosted nickel sulphide deposits, Australia

Sarah E. Dowling & Robin E.T. Hill, CSIRO Exploration and Mining, Private Bag, PO Wembley, WA 6014, Australia

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EXPLORATION MODEL

Examples	Type 1	Type 2
	Silver Swan, Nepean, Scotia, Windarra, Kambalda, Maggie Hays, Digger Rocks, Cosmic Boy, Perseverance, Ruth Well* *Pilbara Craton	Black Swan, Honeymoon Well*, Yakabindie, Mount Keith *includes type 1 deposits, Gole et al.(1996)
Target (Barnes et al. 1994)	Basal accumulation of massive and matrix nickel sulphide ore.	Central accumulation of disseminated nickel sulphide ore.
Grade	Variable, 1.5-20% Ni, massive ore 2-20% Ni, matrix ore av. 2.5% Ni, minor disseminated ore < 1% Ni.	Relatively constant low-grade 0.6-1.5% Ni, may be layered.
Tonnage	0.05-50 Mt	5-300 Mt
Metal ratio	Bulk Ni:Cu 7-19	Bulk Ni:Cu >19
Metal credits	Cu, Ag, Au, PGM, Co	Cu, As, Co

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Mining and treatment

Type 1 deposits

• Underground and open-pit mining.
• Beneficiation involves 3-stage crushing, followed by conventional (ball and rod milling) and autogenous grinding, sizing using screens and cyclones, cell flotation, drum magnetic separation, thickening, flash/spray drying to produce a dry nickel concentrate to refinery with a concentrate grade of 12% Ni, 1% Cu.
• Sherritt-Gordon hydrometallurgical refining process. Oxidation-hydrolysis using a continuous ammoniacal pressure leach, producing nickel briquettes, copper and ammonium sulphate.
• Flash furnace smelting and simultaneous self-roasting of sulphide ore concentrate, producing nickel matte and Fe silicate slag and precious metal collection.
• BioNIC process piloted by Gencor Ltd at Maggie Hays involves bacterial oxidation of nickel sulphide ore with concentrate grades up to 14% Ni. A ferro-nickel product of variable Fe:Ni can be produced to meet the requirements of stainless steel manufacturers.
• BioNIC process avoids smelting and refining steps and operating and capital costs are one-third of traditional metallurgy.

Type 2 deposits

• Open-pit mining.
• Conventional drill, blast, load and haul system.
• Primary gyratory crusher, two-stage grinding and ball milling, flotation feed desliming and conditioning, flotation and concentrate washing and dewatering. Concentrate produced as moist filter cake. Head grade at Mt Keith 20% Ni.
• Refining and smelting as above.
• Activox (Yakabindie) process developed by Dominion during evaluation of the Yakabindie deposit involves fine grinding and low-pressure leach to produce a ferro-nickel oxide product.

Economics of mining depend on suppression of Ni arsenides, Mg silicates, Ni chlorides, and Ni sulphates during metallurgy.

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Regional geological criteria

• Age of extrusion ca. 2.7-3.0 Ga within Archaean greenstone belts in Yilgarn and Pilbara Cratons, Western Australia (Wang et al. 1996, Claoué-Long et al. 1988).
• Regionally extensive komatiite sequences which contain thick olivine cumulate flow units.
• Stratigraphic succession characterised by coeval komatiite/tholeiite and komatiite/felsic-intermediate volcanism.
• Presence of sulphidic flows and/or sulphidic sediments as substrates to komatiite units.

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Local geological criteria

• Komatiite flow unit containing identifiable lava pathways now occupied by olivine-rich cumulates.

Type 1 deposits

• Presence of sulphidic substrate to komatiite unit (felsic intermediate volcanics, chemical-exhalative sediments).
• Transgressive embayment features at base of preferred lava pathway-evidence for substrate erosion and/or sulphide trap-site.
• Preferred lava pathway occupied by olivine ortho-mesocumulate flanked by episodically emplaced flow units.
• Podiform, ribbon-like shoots, or second-order channels in broad-shallow embayments.
• Thickness 5-50 m, width 50-300 m, down-plunge extent up to 2 km.
• Chemical evidence of substrate erosion and immediate host-rock contamination.

Type 2 deposits

• Large, lenticular zones of thickening with transgressive basal contacts, up to 800 m thick, 1-3 km wide, down-plunge extent >1 km, occupied by olivine meso-adcumulate.

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Mineralisation features

• Primary sulphide and oxide minerals: pyrrhotite, pentlandite, pyrite, chalcopyrite, ferrian chromite, magnetite.
• Massive sulphides contain spinel-rich zones, commonly at orebody contacts, pyrite-rich bands, and variable grain sizes and textures influenced by thermal and structural history.
• Primary matrix and disseminated sulphide assemblages (types 1, 2) modified by subsolidus re-equilibration with silicates and by reaction with metamorphic fluids:
  • talc-carbonate-hosted assemblages include pyrite, millerite, vaesite, bravoite, polydymite, arsenic-bearing minerals (e.g. gersdorffite); opaque oxides are magnetite and hematite;
  • serpentinite-hosted assemblages include nickel-rich pentlandite, heazlewoodite, millerite, godlevskite, pyrite, magnetite.
• Supergene alteration profiles:
  • oxide (goethite, carbonate) zone above water table (up to 50-60 m depth);
  • violarite-pyrite zone (80-180 m depth);
  • transition zone to primary ore (80-400 m depth).

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Deposit geochemical criteria

• MgO (anhydrous) in komatiite host rocks; lavas >18 wt% MgO, host olivine cumulates; 30-50 wt% in type 1 deposits, 48-53wt% in type 2 deposits, reflecting variation in olivine content.
• Type 2 deposit geochemistry reflects presence of oMc-oAC (MgO+FeO/SiO₂ molecular ratio = 1.95-2.00).
• Chemical variation in host rocks is due to progressive fractional crystallisation of olivine+chromite+clinopyroxene+plagioclase from primitive mantle melts.
• Chemical analyses can be used to identify komatiitic host rocks where metamorphism has reconstituted igneous textures and mineralogy, e.g. Mg#, M²⁺/Si, (Barnes et al. 1988), MgO-CaO-Al₂O₃ variation diagram.
• Substrate/crustal assimilation + contamination of komatiites by thermal erosion detected by elevated LREE, Nd, Zr, Y, Ti, Al, Fe (Lesher & Arndt 1995, Barnes et al. 1995, Perring et al. 1996).
• PGE contents elevated in sulphides (500-3000 ppb total PGE in massive sulphides), also weakly anomalous in Se, As (Keays 1982).
• Scavenging of chalcophile elements, including PGE, by sulphide during ascent or after eruption produces Ni depletion in parent lava (Barnes et al. 1995).
• S isotopes match those in substrate sediments (Lesher 1989).
• Os¹⁸⁷/Os¹⁸⁶ ratios in ores show chondritic initial ratios. Os isotopic evidence suggests that S is derived from mantle source, not from sedimentary substrate (Foster et al. 1996).

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Figure 1. Interpreted geological cross section through the Mt Keith Ultramafic Complex (Dowling & Hill 1993). Select for a larger version of image [118k]
Figure 2. Interpreted distribution of Ni grade across MKD5 orebody, section 31525N. Select for a larger version of image [80k]

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Surficial geochemical criteria

• Most deposits buried beneath laterite, alluvial, colluvial or lacustrine sediments.
• Gossans of massive sulphide, olivine-sulphide cumulate rare, distinguishable by coincident high Ni, Cu, PGE.
• Essential to use regolith mapping to interpret supergene geochemistry sampled by RAB drilling.
• Multi-element scan ICPMS/AAS common analytical methods.
• Ni, Cr, Co, Mn soil/RAB/outcrop geochemical anomalies indicate olivine cumulates.
• Ni, Cu, PGE, Cr coincident soil/RAB/outcrop geochemical anomalies indicate sulphide mineralisation.
• Cu, As, Zn soil/RAB/outcrop geochemical anomalies indicate presence of sulphidic interflow/footwall sediment.
• Mobile metal ion (MMI) technique useful in defining discrete soil geochemistry anomalies over massive NiS orebodies (Mann et al. 1993).

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Geophysical criteria

Magnetics

• Massive sulphide orebodies are magnetic (monoclinic pyrrhotite, magnetite).
• Magnetic susceptibility of komatiite host determined by alteration history:
  • serpentinisation increases susceptibility. Talc-carbonate alteration produces variable to low susceptibilities. Olivine-dominated lithologies generally produce higher susceptibilities.
• Regional aerial magnetic surveys used to locate and define stratigraphic trends involving komatiite sequences:
  • low-level closely spaced aerial magnetic data used to define small-scale prospective volcanic features, such as lava pathways, to resolve structure and detect basal sulphide accumulations.
• Ground magnetic data used for delineating lithological contacts in non-outcropping terrain.

Electromagnetics

• Sulphide ores are poor to good electrical conductors. Deposits are targeted and delineated using surface and down-hole EM techniques. Useful summaries of techniques in McCall et al. 1995, Rekola & Hattula 1995.

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Comments on genesis

Type 1 deposits— recent models (Lesher et al. 1984, Lesher 1989, Lesher & Campbell 1993, Barnes et al. 1994, Perring et al. 1995) involve thermal and/or physical erosion of sulphidic substrate at the base of preferred lava pathways. Droplets of immiscible sulphide liquid or a discrete, dense, basal flow of sulphide are carried along by, and scavenge Ni, Cu and PGE from the komatiite lava and are ultimately trapped on the basal contact by a change in flow rheology, change in slope or direction of preferred lava pathway.

Type 2 deposits—recognised as extrusive in origin (Hill et al.1989) and crystallise within large lensoid erosional lava pathways. Consistency of the proportion of sulphide and the Ni grade is a function of segregation of cotectic proportions of sulphide liquid (MSS) and olivine from a lava precisely at sulphur saturation (Duke 1986).

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Figure 3. Upper - Schematic lateral section through a regional inflationary komatiite flow field developing via sustained eruption of lava, portraying possible relationships between various volcanic facies, and depicting those eruptive environments (with examples) conducive to the formation of types 1 and 2 Ni deposits (after Hill et al. 1995).
Lower - Vertical sections through the regional komatiite flow field, illustrating the various volcanic facies depicted in the upper figure, showing lithological associations and the environments of accumulation of types 1 and 2 Ni deposits. Select for larger versions of images [119k,48k]