Komatiite, Archean volcanic rock, Ontario

Komatiite from the Timmins district, northeast Ontario,

A souvenir from the Hot, Young Earth

Figs. 1-2: Photographs of rusty exterior and wet, sawn interior of a classic komatiite specimen. The bluish-black hue is typical of many peridotites, and so the similarity is not surprising in this, a volcanic equivalent of ultramafic intrusive rocks. The grass-like, lined and ridged pattern on the weathered exterior, the pattern in the sawn face, is known as spinifex texture, named after a bladed grass from southern Africa. This is a dense rock, rich in iron and magnesium.

The sawn piece, sample 125, is circa 13x10x18 cm in size, weighs 1879 grams, and is quite strongly magnetic. The magnetic susceptibility was measured on different faces as 20.3 to 36.4 x10^-3 SI units (n=4).

"Rock of the Month # 264, posted for June 2023" ---

History, Setting, Tectonics

This is a classic igneous rock type, to close out the 22^nd year of "Rocks of the Month". Komatiite is a familiar type to petrologists in some parts of the world (e.g., parts of South Africa and Zimbabwe, India and Western Australia, Brazil and Canada), but quite exotic elsewhere, especially in regions with young bedrock. That is because, like banded iron formations, komatiites are largely a facet of Precambrian geology. Indeed, though rare examples may be found in the past 200 million years (Gorgona Island, Colombia: Herzberg, 1992; Grove and Parman, 2004), they are really a signature of the processes of the Archean, when the Earth was a younger, hotter, indeed a smouldering planet compared to the present. This may be hard to believe, given the hundreds of active and dormant volcanoes found along hot spots, mid-ocean ridges and subduction zones around the globe ***.

Nevertheless, more than 2.5 billion years ago, the Earth's internal radioactive heat sources were ticking over, undergoing radioactive decay, at the same rates, but with more "fuel" to burn, than today. Komatiites, variable in age from Onverwacht to Timmins to Gorgona Island, are thought to have formed by variable degrees of high-pressure melting of mantle peridotites at depths of 130-330 km (Herzberg, 1992). The tectonic situation of the extra-hot komatiite magmas was a puzzle. Campbell et al. (1989) deduced that basalts and komatiites, as at Kambalda in Western Australia, could both be produced by a thermal plume in a warmer Archean mantle. Komatiites could form in the high-temperature axis of the plume, and basalts in the cooler head. Komatiite melts were at very high temperatures, at least 1400-1500°C, and (upon eruption, before rapid cooling) could have melted many of the rocks over which they flowed. This "thermal erosion" could cut channels into the substrate, a possible locus for related sulphide ores (see on).

The term komatiite was defined by Viljoen and Viljoen (1969) for ultramafic units described in the Komati River valley of the Barberton greenstone belt in eastern South Africa. Magnesian volcanics in the greenstone belt were found to comprise a suite of extensive ultramafic lava flows, thin units with fine-grained chilled margins and distinctive spiky crystals (later termed spinifex texture) in the upper parts of the units. The existence of ultramafic feeder dykes offered further proof of the presence of an ultramafic magma type in this Archean setting. The Barberton rocks are very old, circa 3540 Ma (that is, 3.54 billion years, when Earth was just 22% of its current age).

*** The Global Volcanism Project (as of 03 June 2023) list 1,324 volcanoes worldwide that are or have been active in the Holocene period, roughly the past 10,000 years. While komatiite is extremely rare in the Phanerozoic (the age of diverse life, circa the past 600 Ma), island arcs, as in the Bonin Islands and Papua New Guinea, display modern volcanics with related characters, known as boninites (Cameron et al., 1979).

Regional Geology

The figured sample is from Munro township, in the Archean Abitibi greenstone belt near Timmins, Ontario. The Abitibi greenstone belt is some 500 x 150 miles (800x240 km) in size, and as such is the largest single continuous belt in the Canadian shield (Goodwin and Ridler, 1970; Jackson and Fyon, 1991). Low-medium greenschist facies metamorphism prevails. The volcanic stratigraphy has been studied intensively (see, e.g., various field guides, such as Pyke et al., 1978; Houle et al., 2010). Ultramafic flows akin to those in the Barberton Mountain Land were found in the Timmins district (e.g., Arndt and Naldrett, 1987). In Munro township, Dee's Flow (one of several named flows in the area: Fred's Flow, Theo's Flow...) is the only recognized komatiite lava channel, and hosts Fe-Ni-Cu sulphide mineralization (Davis et al., 1993). This flow is 20 m thick at the flanks, but 50-100 m thick in the central part. Satterly (1952) described the township geology. The Archean bedrock includes sediments and basaltic volcanics, ultramafic rocks, felsic porphyries and other intrusives, with appreciable carbonatization. Historical gold mines in the township include the Croesus mine, that yielded some of the richest gold ore ever won from a Canadian deposit. In later years, the ultramafic rocks and their nickel potential attracted more attention. A crystallization date for the Munro township komatiites was later estimated at 2726 ± 93 Ma (Walker et al., 1988: when Earth was now circa 40% of its current age).

Fig. 3 : A frosted window pane (?). Looking through the covered thin section of sample 413, collected by the writer at Pyke's Hill in Munro township in May 1984. The area of the sawn sample mounted on the glass slide is 3.5x2.1 cm. Skeletal olivine crystals lie in subparallel sheaves, interstices infilled by what seems to be glass. Under the microscope, the glass is seen to be partially devitrified, bearing curved bundles of minute crystallites.

Chemistry, Mineralogy and Textures

Komatiites crystallize from high-temperature magmas with 18-32% MgO (Le Maitre et al., 1989, p.81; Dostal, 2008). They can be subdivided into basaltic komatiite and the more magnesian peridotitic komatiite. Olivine, pyroxenes and chromite are typical komatiite primary minerals (later alteration and weathering generate serpentine, iron oxyhydroxides and other secondary minerals). The silicate crystals are typically skeletal and bladed, set in abundant glass, testifying to rapid quenching upon eruption at the surface. Olivine is especially prone to forming skeletal crystals, that can reach surprisingly large dimensions in komatiites (Cox et al., 1979, p.190). For the purposes of geochemical exploration in glaciated terranes, chromite, being a refractory and mechanically robust spinel, can be a good indicator mineral, flagging the existence of komatiites buried under glacial till (Averill, 2011).

The origins of the characteristic spinifex textures in komatiites were investigated by Shore et al. (1999). In some flows, sheaves of olivine crystals may grow to exceed 1 m in length, even in flows <10 m thick. The Pyke's Hill ("Pike hill") locality in Munro township is cited in this study of crystal growth in lava flows. Arndt (1986) described the 16-metre-thick Alexo flow, with a spinifex -textured upper layer and an olivine-rich lower layer. The spinifex olivines grew downward from the roof of the flow, other olivine initially crystallized in the basal portion, and further crystallization occurred within the body of the flow until the crystals exceeded about 50 volume percent, whereon convection was inhibited and final solidification took place. The striking textures are seen in related variants of olivine-rich intrusive rocks, such as harrisites, in which coarse branching olivines grow perpendicular to the layering (see, e.g., Donaldson, 1974, 1982; Le Maitre et al., 1987, p.72; and in Ontario: Houle et al., 2010).

Despite being defined as recently as 1969, a robust literature soon sprang up to describe komatiites and associated mineral deposits: there are over 900 papers in the MINLIB bibliography, a healthy 1% of the library (as a rule of thumb, I consider a topic with >300 articles to be mature!).

Figs. 4-6 : Three photomicrographs in transmitted light. Left: olivine crystals in glassy matrix (nominal magnification 50X, long axis FOV 1.7 mm, PPL, TL). Centre and right: close-up showing olivine crystals, and curving crystallites in the quenched interstitial glass (nominal magnification 100X, long axis field of view 0.9 mm, XP, TL, right-hand image with sensitive tint plate).

Associated Mineralization

In an early account of the Alexo nickel deposit in Dundonald Township, Uglow (1911) reported samples averaging an impressive 7.08% Ni, with a paragenesis (sequence of mineral deposition) of early magnetite, then pyrrhotite, then pentlandite and minor chalcopyrite. In an interesting though genetically misleading comparison, Uglow observed the abundance of pyrrhotite and likened it to sulphidic augite porphyry of the Rossland mining camp in southeast British Columbia, volcanic-hosted gold mineralization rich in sulphides, associated with felsic intrusives such as quartz monzonite. The hand-specimen observation of abundant pyrrhotite was accurate, though the mechanisms of ore formation were very different.

Komatiite-hosted magmatic nickel sulphide deposits are a well-characterized and widely exploited suite of nickel deposits. They may also contain quantities of copper, cobalt, platinum group elements and other components, some of which may be economically recoverable. Although less well-known than the famed gold deposits of the Timmins-Porcupine mining camp, deposits such as Alexo, Dundonald and Langmuir are important examples of magmatic sulphide ores (Green, 1978; Fyon and Green, 1990). These nickel deposits have been widely studied by a few groups of scientists and students, such as those of Tony Naldrett at the University of Toronto (Naldrett, 1966), Sarah-Jane Barnes (University of Quebec, Chicoutimi) and Steve Barnes (CSIRO, Perth, Australia). For detailed reviews of this deposit type, in the context of important Australian examples, see Barnes (2006, 2007). Komatiite terminology and the analysis of geochemical data to exploration in Western Australia was also reviewed by Mamuse et al. (2010).

The Alexo komatiite contained a classic magmatic suite of massive, net-textured and disseminated sulphides. The sulphide ore textures of pyrrhotite and pentlandite were described by Naldrett et al. (1967). The sulphide-rich rocks display a wide range of Pd/Ir ratios (palladium is relatively more mobile, less refractory, than iridium). The platinum-group element chemistry can be explained in terms of the crystallization of an early sulphide phase (mss, monosulphide solid solution) from a sulphide liquid (Barnes and Naldrett, 1986, 1987). Further work on the trace element geochemistry, of volcanic rocks and ore, was conducted by Puchtel et al. (2004).

Acknowledgements: A definite University of Toronto connection here, evident in the references. Sample 125 (Figs. 1-2) was collected by Richard Bedell, circa 1980, and donated to (curated by, lost and found by) the writer. Sample 125 resurfaced in 2023, with help from Colin Bray.

References

Arndt,NT (1986) Differentiation of komatiite flows. J.Petrol. 27, 279-301.

Arndt,NT and Naldrett,AJ (1987) Komatiites in Munro township, Ontario. In "Centennial Field Guide Volume 5", Northeastern Section of the Geological Society of America (Roy,DC editor), Geol.Soc.Amer., 481pp., trip 69, 317-322.

Averill,SA (2011) Viable indicator minerals in surficial sediments for two major base metal deposit types: Ni-Cu-PGE and porphyry Cu. Geochemistry: Exploration, Environment, Analysis 11, 279-291.

Barnes,S-J and Naldrett,AJ (1986) Variations in platinum group element concentrations in the Alexo mine komatiite, Abitibi greenstone belt, northern Ontario. Geol.Mag. 123, 515-524.

Barnes,S-J and Naldrett,AJ (1987) Fractionation of the platinum-group elements and gold in some komatiites of the Abitibi greenstone belt, northern Ontario. Econ.Geol. 82, 165-183.

Barnes,SJ (2006) Komatiite-hosted nickel sulfide deposits: geology, geochemistry, and genesis. In "Nickel Deposits of the Yilgarn Craton: Geology, Geochemistry, and Geophysics Applied to Exploration" (Barnes,SJ editor), SEG Spec.Publ. 13, 210pp., 119-138.

Barnes,SJ (2007) Cotectic precipitation of olivine and sulfide liquid from komatiite magma and the origin of komatiite-hosted disseminated nickel sulfide mineralization at Mount Keith and Yakabindie, Western Australia. Econ.Geol. 102, 299-304.

Cameron,WE, Nisbet,EG and Dietrich,VJ (1979) Boninites, komatiites and ophiolitic basalts. Nature 280, 550-553, 16 August.

Campbell,IH, Griffiths,RW and Hill,RI (1989) Melting in an Archaean mantle plume: heads it's basalts, tails it's komatiites. Nature 339, 697-699, 29 June.

Cox,KG, Bell,JD and Pankhurst,RJ (1979) The Interpretation of Igneous Rocks. George Allen and Unwin, 450pp.

Davis,P, Lesher,CM, McLaughlin,AD and Clemmer,S (1993) Komatiite lava channelization, thermal erosion of andesite, and localization of Archean Fe-Ni-Cu sulfide mineralization, Munro township, Ontario. Geol.Soc.Amer. Abs.w.Progs. 25 no.6, 489pp., 400, Boston.

Donaldson,CH (1974) Olivine crystal types in harrisitic rocks of the Rhum pluton and in Archean spinifex rocks. Bull.Geol.Soc.Amer. 85, 1721-1726.

Donaldson,CH (1982) Spinifex-textured komatiites: a review of textures, compositions and layering. In "Komatiites" (Arndt,NT and Nisbet,EG editors), George Allen and Unwin, 526pp., 213-244.

Dostal,J (2008) Igneous rock associations 10. Komatiites. Geoscience Canada 35, 21-31.

Fyon,JA and Green,AH (editors) (1990) Geology and Ore Deposits of the Timmins District, Ontario. Field Trip Guidebook 6, 8th IAGOD Symposium, Ottawa (also GSC Open File Report 2161), 156pp.

Goodwin,AM and Ridler,RH (1970) The Abitibi orogenic belt. GSC Pap. 70-40, 1-30.

Green,AH (1978) Evolution of Fe-Ni sulfide ores associated with Archean ultramafic komatiites, Langmuir township, Ontario. PhD Thesis, University of Toronto, 355pp.

Grove,TL and Parman,SW (2004) Thermal evolution of the Earth as recorded by komatiites. Earth Planet.Sci.Letts. 219, 173-187.

Herzberg,C (1992) Depth and degree of melting of komatiites. J.Geophys.Res. 97 no.B4, 4521-4540.

Houle,MG, Lesher,CM, Prefontaine,S, Ayer,JA, Berger,BR, Taranovic,V, Davis,PC and Atkinson,B (2010) Stratigraphy and physical volcanology of komatiites and associated Ni-Cu-(PGE) mineralization in the western Abitibi greenstone belt, Timmins area, Ontario: a field trip for the 11th International Platinum Symposium. 11th International Platinum Symposium, Sudbury, field trip guide. B1. OGS OFR 6255, 99pp.

Jackson,SL and Fyon,JA (1991) The western Abitibi subprovince in Ontario. In `Geology of Ontario' (Thurston,PC, Williams,HR, Sutcliffe,RH and Stott,GM editors), OGS Spec.Vol. 4, part 1, 709pp., 404-482.

Le Maitre,RW, Bateman,P, Dudek,A, Keller,J, Lameyre,J, Le Bas,MJ, Sabine,PA, Schmid,R, Sorensen,H, Streckeisen,A, Woolley,AR and Zanettin,B (1989) A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications Ltd, Oxford, 193pp.

Mamuse,A, Porwal,A, Kreuzer,O and Beresford,S (2010) Spatial statistical analysis of the distribution of komatiite-hosted nickel sulfide deposits in the Kalgoorlie terrane, Western Australia: clustered or not? Econ.Geol. 105, 229-242.

Naldrett,AJ (1966) The role of sulphurization in the genesis of iron-nickel sulphide deposits of the Porcupine district, Ontario. CIM Bull. 59, 147-155.

Naldrett,AJ, Craig,JR and Kullerud,G (1967) The central portion of the Fe-Ni-S system and its bearing on pentlandite exsolution in iron-nickel sulfide ores. Econ.Geol. 62, 826-847.

Puchtel,IS, Humayun,M, Campbell,AJ, Sproule,RA and Lesher,CM (2004) Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada. Geochim. Cosmochim. Acta 68, 1361-1383.

Pyke,DR, MacVeigh,JG and Middleton,RS (1978) Volcanic stratigraphy and geochemistry in the Timmins mining area. In GAC/MAC Field Trip Guidebook, Toronto meeting, pp.160-184.

Shore,M and Fowler,AD (1999) The origin of spinifex texture in komatiites. Nature 397, 691-694, 25 February.

Satterly,J (1952) Geology of Munro Township. Ontario Dept. of Mines Ann.Rep. 60 part 8, 60pp

Uglow,WL (1911) The Alexo nickel deposit. Ontario Bureau of Mines Ann.Rep. 20 part 2, 34-39.

Viljoen,MJ and Viljoen,RP (1969) The geology and geochemistry of the lower ultramafic unit of the Onverwacht Group and a proposed new class of igneous rocks. Geol.Soc.S.Africa Spec.Publ. 2, 55-86.

Walker,RJ, Shirey,SB and Stecher,O (1988) Comparative Re-Os, Sm-Nd and Rb-Sr isotope and trace element systematics for Archean komatiite flows from Munro township, Abitibi Belt, Ontario. Earth Planet.Sci.Letts. 87, 1-12.

Graham Wilson, posted 02-07 June 2023

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