August 2012 LIP of the Month

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The East Yilgarn LIP: ore-forming plume volcanism in the late Archaean

Steve Barnes
CSIRO Earth Sciences and Resource Engineering, Perth, Western Australia; Steve.Barnes@csiro.au

Introduction

The Eastern Goldfields Superterrane in the Archaean Yilgarn Craton is an archetypal granite-greenstone terrane that hosts vast reserves of nickel and gold in a volcanic assemblage dominated by mafic and ultramafic volcanic rocks. The western half of the Superterrane (Figure 1) is defined by a distinctive volcanic stratigraphy, mostly  formed in a restricted age span between 2715 and 2670 Ma, with the bulk of the constituent basalts and komatiites formed between 2690 and 2705 Ma[1]. This sequence is likely to have been several km thick (actual thicknesses are largely unknown owing to intense tectonism), and extended over an area of at least 100,000 square km (Blewett and Czarnota, 2007; Czarnota et al., 2010; Kositcin et al., 2007).

The ~2700 ma East Yilgarn komatiite-basalt association has all of the essential hallmarks of a Large Igneous Province: huge volumes of anomalously hot, mantle derived lavas, erupted over a vast area in a restricted time period. It’s a particularly interesting LIP, not only because it hosts two of the world’s most valuable ore deposit provinces, but also because it is a key piece of evidence in one of the most fundamental ongoing  controversies in geology: when did plate tectonics start on Planet Earth?


Figure 1 : Maps of the East Yilgarn Large Igneous province, showing major terranes and greenstone domains (coloured areas). Dashed black lines represent western and eastern boundaries of the predominantly 2690-2705 Ma komatiite-basalt sequences that define the LIP. Left: symbols indicate MgO contents of komatiites, median plus one standard deviation for each locality; right, symbols indicate basalt occurrences showing geochemical type. Komatiite outcrop areas in purple, post-2670 granitoid plutons in white.

The East Yilgarn LIP (EYLIP) comprises three major groupings of volcanic rocks: komatiites, basalts, and a “felsic” suite of volcanic and volcaniclastic andesites, dacites and rhyodacites. The entire sequence was intruded over the subsequent 30 Ma by a vast suite of intermediate to felsic plutons, culminating in a craton wide “granite bloom” between 2680 and 2640 Ma (Champion and Sheraton, 1997).

Komatiites

Komatiites are a widespread component of the volcanic assemblage, and extend across the entire mapped area of the Kalgoorlie and Kurnalpi terranes that form the western half of the Eastern Goldfields Superterrane. They are almost always closely associated with basalts, both tholeiitic and komatiitic. They are exclusively of Al-undepleted “Munro Type” character, implying derivation from mantle plumes at depths starting around 150-200 km (Arndt et al., 2008). Komatiite sequences contain a wide variety of lithologies, defined largely by abundance and crystal habit of olivine, interpreted by Hill et al. (1995) as the result of crystallisation in extensive, long-lived, channel-fed lava flow fields on a scale of hundreds of km (Figure 2).


Figure 2 : Komatiite flow field model after Hill et al 1995 showing characteristic rock types and the inferred pre-deformation arrangement of component volcanic facies

The feeder channels to these flow fields are marked by extremely olivine rich cumulates, in some cases pure adcumulates, containing olivines up to Fo94.5 in composition. They represent some of the hottest magmas ever erupted on the planet, with MgO contents up to 32% and eruption temperatures estimated as high as 1600oC. These cumulate-dominated channels are the host rocks to some of the world’s largest nickel sulphide ore deposits including Perseverance and Mt Keith (Barnes, 2006; Barnes et al., 2011). Smaller but extremely rich nickel ore deposits are found in the less extremely olivine enriched lava tubes that fed the lava flows of the Kambalda Dome  Detailed studies of these deposits led to the currently dominant paradigm for the origin of this class of nickel sulphide ores, by thermo-mechanical erosion and assimilation of sulphide-rich sediments in the immediate substrate to the flowing lava (Lesher et al., 1984). The combination of turbulent flow and Ni-rich ultramafic magmas gave rise to the highest grade Ni-sulphide deposits on Earth.

The distribution of MgO contents of komatiite across the EYLIP gives a revealing insight into the tectonics. A belt of highly magnesian, cumulate-dominated komatiites extends along the Kalgoorlie Terrane, from Widgiemooltha in the south to Wiluna in the north, over a strike length of more than 500 km (Figure 1), and hosts almost the entire tonnage of known nickel ores in the LIP. This belt is interpreted to represent the locus of maximum eruption rate of the hottest komatiite magmas (Barnes and Fiorentini, 2012). Further to the east across the Kurnalpi Terrane, komatiites are widespread, but are marked by more evolved, lower MgO compositions, and a prevalence of compound spinifex-textured flows interpreted as the distal flanks of the major flow fields. The cumulate-rich Kalgoorlie Terrane komatiites are overlain by similar spinifex-rich rocks, implying that the rate of eruption declined with time, such that less voluminous, episodically emplaced flow facies were erupted closer to the vents (Hill et al., 1995).

Basalts

Basalts are the most voluminous component of the EYLIP, and come in four basic flavours (Barnes et al., 2012): komatiitic basalts, mildly depleted low-Th tholeiites (LTT), strong enriched siliceous high-Mg basalts (SHMB), and a group of tholeiites intermediate in chemistry between LTT and SHMB. All of these groups can be found across the LIP, commonly in close association with the komatiites, but the LTT group is by far the most voluminous and in most cases the oldest unit. A more complex subdivision has been proposed by Said et al (2012), but on essentially similar lines. The distribution of these basalt types is shown in the right panel of Figure 1.

The LTT suite shows a remarkable similarity in its chemistry to basalts from Phanerozoic oceanic plateaus, and also to LIP basalts associated with early stages of continental rifting such as the Tertiary flood basalts of East Greenland (Barnes et al 2012).  The SHMB suite has been widely interpreted over many years as the product of extensive crustal contamination of komatiite magmas; this accounts for the similarity with komatiites in contamination-insensitive element ratios such as Al/Ti, and also the apparent restriction of this magma type to the Archaean (Barnes et al 2012).  The intermediate tholeiite suite is relatively diverse and probably is not really a coherent suite at all, but may reflect variable degrees of crustal contamination of both komatiites and LTT parents. An alternative view is taken Said et al. (2012), who prefer a model of derivation of the intermediate Th basalts and the SHMB suite from unusually enriched “streaks” in a source plume, with additional inputs from metasomatised mantle lithosphere.

The felsic-intermediate volcanic suite

Felsic to intermediate volcanic and volcaniclastic rocks, mostly of dacite to rhyodacite composition with minor andesites, are widespread across the EYLIP, in a variety of stratigraphic relationships to the komatiites. The thickest and most extensive unit, the Black Flag Beds, comprises volcaniclastic rocks and epiclastic sediments, and overlies the komatiite-basalt sequence in the southern part of the Kalgoorlie Terrane. Also within the Kalgoorlie Terrane, to the east of Kalgoorlie in the south (Figure 1) and in the Agnew-Wiluna Belt in the north, dacite flows and tuffs are intimately intercalated with komatiites. In the Kanowna-Black Swan region (Figure 1) the two magma types appear to have been erupted simultaneously, forming complex mingling textures (Trofimovs et al., 2004) and invasive flow lobes of komatiite injected into soft or partially molten dacite (Figure 3), in a spectacular example of bimodal komatiite-dacite volcanism.


Figure 3 : Komatiite- TTG Dacite mingling during bimodal volcanism in the Boorara Domain (~70 km NE of Kalgoorlie).  A: stratigraphic reconstruction of komatiite-dacite units based on close space-diamond drilling at the Black Swan locality, after (Hill et al., 2004). Ooc=olivine orthocumulate, Omc = olivine mesocumulate. B:  lake-bed outcrop showing irregular lenses of komatiite (K) containing internal flow-lobe features, probably emplaced  as invasive, erosive flow units into partially consolidate dacite tuff (D). Gordon Serdar Lake, ~60 km NE of Kalgoorlie. C: Outcrop-scale magma mingling between komatiite (green due to weathering) and dacite, showing peperite-like textures due to infiltration of komatiite into fractures in unconsolidated dacite country rock. Alunite Pit locality near Kanowna, 50km NE of Kalgoorlie.

Chemically, the felsic rocks of the Black Flag beds, the Black Swan area and the Agnew-Wiluna Belt have the distinctive characteristics of the TTG (tonalite-trondjhemite-granodiorite) suite, with moderately enriched incompatible element abundances, negative Nb anomalies and mildly depleted HREE relative to MREE. These features are interpreted as the result of melting of mafic lower crust leaving garnet-bearing residues (Champion and Sheraton, 1997). In contrast, a restricted area of the Kurnalpi terrane around Welcome Well (Figure 1) contains a package of andesites with major and trace element compositions very similar to those of Phanerozoic island arc andesites. These rocks have been taken as the main line of evidence for the existence of island arc volcanism in the Kurnalpi Terrane (Barley et al., 2008; Barley et al., 2006), and have led to the formulation of a comprehensive arc accretion model for the Superterrane (Blewett and Czarnota, 2007; Czarnota et al., 2010). The question of how to reconcile these observations with evidence for terrane-wide eruption of plume-head basalts of roughly the same age is one of many unresolved questions in east Yilgarn geology. Did plumes and arcs co-exist in the Archaean, as has been suggested for the Abitibi province in Canada (Wyman et al., 2002)? Or do the apparently arc-like rocks of the Kurnalpi terrane have a different origin, with the resemblance to arc magmas being coincidental?

Putting it together: a model for the evolution of the East Yilgarn LIP

The Kalgoorlie Terrane, with its endowment of major nickel sulphide and lode Au deposits, occupies a linear belt, coinciding with the location of the most olivine-rich, highest-flux komatiite flows. Lower MgO komatiite assemblages and plume-head tholeiites extend further to the east, creating an asymmetric profile. Phanerozoic LIPs tend to show a more concentric distributions of lithologies with the highest temperature melts (picrites, rather than komatiites) concentrated close to the plume axis (Campbell, 2007). Why is the “hot centre” of the EYLIP, the Kalgoorlie Terrane,  a linear belt and not a bulls-eye?

The answer to this questions may lie in the interaction between a major mantle starting plume (Griffiths and Campbell, 1990) and pre-existing lithospheric architecture. The linear Kalgoorlie Terrane  belt of high-flux komatiites and its associated ores is spatially related to the edge of the Youanmi Terrane ‘archon,’ a block of older cratonised crust delineated by the isotopic signal of source rocks to the granitoids (Champion and Cassidy, 2007) (Figure 4). This craton-margin relationship, common to many of the world’s major nickel sulfide provinces, is attributed by Begg et al. (2010) to emplacement of a mantle plume under the archon, and consequent localisation of flow of the plume head to give rise to maximum melting beneath the thinner lithosphere around the edges.


Figure 4 : -Sm model ages of late low-Ca granites, after Champion and Cassidy (2007) as a proxy for the age of the lower crust at the time of formation of the EYLIP. Nickel deposits in red, gold deposits in yellow. KT = Kalgoorlie Terrane.

Distribution of mafic and ultramafic mafic magmatism across the superterrane at ca 2705 Ma can be explained by emplacement of a major driving plume under the ‘lid’ of the Youanmi Craton (Figure 5). The base of thickened, buoyant lithosphere under the archon diverted flow-lines within the plume towards the archon margin. Arrival of the plume head, possibly under an original suture or zone of weakness at the archon margin, produced extension and continental rifting. Voluminous eruption of plume-tail komatiite was concentrated and focused along the eastern margin of the Youanmi craton in what became the Kalgoorlie Terrane, while plume-head basalts were erupted over a much wider area. The patchy distribution of komatiites, generally lacking in thick cumulate- filled conduit facies, across the eastern portions of the Eastern Goldfields Superterrane can be accounted for by generally eastward flow of extensive komatiite flow fields sourced from eruption sites in the Kalgoorlie Terrane.


Figure 5 : Plume  impingement model, drawn approximately to the same vertical and horizontal scale,  after Barnes et al. (2012) for origin of the EYLIP by interaction of a starting plume with the margin of the Youanmi cratonic nucleus.

This model is broadly along the lines of that proposed by Campbell & Hill (1988), with additional constraints from the Nd model age data set. It does not entirely preclude the possibility of arc development and subsequent accretion outboard from the archon margin. However, the uniform plume character of the mafic–ultramafic association, and the scarcity of demonstrably arc-related mafic magmatism, argue strongly that the entire mafic component of the Eastern Goldfields Superterrane is unrelated to subduction processes. This provides an important constraint for holistic models of Eastern Goldfields Superterrane evolution.

What’s next?

Detailed stratigraphic and geochemical studies of the EYLIP are ongoing. A current focus is on improving our understanding of the sequence stratigraphy of the volcanic pile in different domains of the LIP, and especially on understanding the geochemistry of the felsic volcanic component and how it relates to the komatiites and basalts. The prime goal is to understand the extraordinary cataclysmic events that led to widespread granite-greenstone formation in the late Archaean, and to the world-class ore deposits found therein. The EYLIP is in many ways a typical LIP, but it formed at a highly atypical phase of the history of the planet and it has much to tell us about the geodynamics of the late Archean.

Acknowledgments

The data and ideas presented here are the result of more than thirty years research, primarily focussed on the nickel deposits of the EYLIP. Significant contributions have been made by my former CSIRO colleagues Rob Hill, Martin Gole, Sarah Dowling and Caroline Perring, and by many exploration and mining companies that have provided financial support and access to samples. The ideas presented here owe a great deal to discussions with Graham Begg, Marco Fiorentini, Steve Beresford, Charlotte Hall, Mark Pawley, Martin Van Kranendonk and Ian Campbell. This contribution is an output of the Minerals Down Under National Research Flagship.

August 2012.

References

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Barley, M., Krapez, S., and Kositcin, N., 2008, Physical volcanology and geochemistry of a late Archaean volcanic arc; Kurnalpi and Gindalbie terranes, Eastern Goldfields Superterrane, Western Australia: Precambrian Research, v. 161, p. 53-76.

Barley, M. E., Brown, S. J. A., and Krapez, B., 2006, Felsic volcanism in the eastern Yilgarn Craton, Western Australia: evolution of a Late Archean convergent margin: Geochimicia Cosmochimica Acta, v. 70, p. A35.

Barnes, S. J., 2006, Nickel sulfide deposits of the Yilgarn Craton: geology, geochemistry and geophysics applied to exploration SEG Special Publication, Volume 13: Littleton, Co, Society of Economic Geologists.

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Barnes, S. J., Fiorentini, M. L., Duuring, P., Grguric, B. A., and Perring, C. S., 2011, The Perseverance and Mount Keith Ni deposits of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia: Reviews in Economic Geology, v. 17, p. 51-88.

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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., eds., Sulphide deposits in mafic and ultramafic rocks: London, Institute of Mining and Metallurgy, p. 70-80.

Said, N., Kerrich, R., Cassidy, K., and Champion, D. C., 2012, Characteristics and geodynamic setting of the 2.7 Ga Yilgarn heterogeneous plume and its interaction with continental lithosphere: evidence from komatiitic basalt and basalt geochemistry of the Eastern Goldfields Superterrane: Australian Journal of Earth Sciences, v. 59, p. 737-764.

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[1] The Kalgoorlie Terrane is the currently favoured term for what was formerly known as the Norseman-Wiluna Greenstone Belt.