Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 4. Platinum group element distribution in the ores,and genetic implications

The Black Swan komatiite sequence, in the Eastern Goldfields province of the Archaean Yilgarn Craton in Western Australia, is a body of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. The massive sulfide orebodies of the Black Swan Succession are pervasively depleted in all platinum group elements (PGEs), particularly Pt and Pd, despite very high Ni contents. This depletion cannot be explained by R-factor variations, which would also require relatively low Ni tenors. The PGE depletion could be explained in part if the ores are enriched in a monosulfide solid solution (MSS) cumulate component, but requires some additional fractional segregation of sulfide melt upstream from the site of deposition. The Silver Swan orebody shows a remarkably consistent vertical zonation in PGE contents, particularly in Ir, Ru, Rh, Os, which increase systematically from very low levels at the stratigraphic base of the sulfide body to maxima corresponding roughly with the top of a lower layer of the orebody rich in silicate inclusions. Platinum shows the opposite trend, but is somewhat modified by remobilisation during talc carbonate alteration. A similar pattern is also observed in the adjacent White Swan orebody. This zonation is interpreted and modelled as the result of fractional crystallisation of MSS from the molten sulfide pool. The strong IPGE depletion towards the base of the orebody may be a consequence of sulfide liquid crystallisation in an inverted thermal gradient, between a thin rapidly cooling upper rind of komatiite lava and a hot substrate.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: Peter Lightfoot     

Komatiites and nickel sulphide orebodies of the Black Swan area,Yilgarn Craton,Western Australia. 1. Petrology and volcanology of host rocks

The Black Swan Succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km NNE of Kalgoorlie, within the 2.7-Ga Eastern Goldfields greenstone province of the Yilgarn Craton. The komatiite stratigraphy comprises a steep dipping, east facing package about 700 m in maximum thickness and about 2.5 km in strike length (Fig. 1), which hosts a number of economically exploitable Ni sulphide orebodies including the Silver Swan massive ore shoot (approximately half a million tonnes at about 10.5% Ni). The sequence can be subdivided into a Lower Felsic Unit, comprising coherent and autobrecciated facies of multiple dacite lava flows; an upper Eastern and lower Western Ultramafic Unit, each showing marked lateral facies variation, and an Upper Felsic Unit coeval with the Eastern Ultramafic Unit. The komatiite sequence has been metamorphosed at sub-greenschist facies in the presence of high proportions of CO2-rich fluid, giving rise to pervasive talc–carbonate and talc–carbonate–quartz assemblages, with extensive preservation of pseudomorphed igneous textures. Cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession. The degree of penetrative deformation is generally very low, and original stratigraphic relationships are largely intact in much of the sequence. The Eastern Ultramafic Unit and Western Ultramafic Unit are interpreted as components of a single large komatiite flow field, representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows. The Western Ultramafic Unit which hosts the bulk of the high-grade massive and disseminated ores is a sequence dominated by coarse-grained olivine cumulates, 2 km wide and up to 500 m thick, with major magma pathways represented by thick, homogenous olivine mesocumulate piles at its northern and southern ends: respectively 400 and 200 m thick. The sequence between the two major pathways consists of olivine orthocumulates (oOC) with minor spinifex-textured intervals. The Unit is capped by a persistent spinifex-textured crust less than 1 m thick, and is locally vesicular. The Eastern Ultramafic Unit contains the Black Swan Cumulate Zone, a 500-m thick sequence of very coarse-grained hopper-textured, locally vesicular oOC containing disseminated sulphides in its lower 200 m. The zone is flanked to the north and south by complexly interdigitated sequence of highly irregular, spinifex-capped, olivine cumulate-rich flow lobes between 1 and 100 m thick, and dacitic lavas and tuffs. The complexity of the 3-D spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite, combined with thermal or thermomechanical erosion. The Eastern and Western Units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava, which developed localised thermo-mechanical erosion channels in the dacitic substrate. Komatiite and dacite eruption was synchronous, giving rise to complex interdigitation and extensive contamination and hybridisation.Editorial handling: Peter Lightfoot     

Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 2: Geology and genesis of the orebodies

The Black Swan Ultramafic Succession hosts a number of magmatic Fe–Ni–Cu–PGE sulfide ore shoots, ranging from high grade massive ore to low grade disseminated sulfides. Of these, the most economically significant is the Silver Swan massive sulfide orebody, associated with the basal contact of the succession. The deposit varies in thickness between 5 and 20 m, reaches a N–S strike length of 75 m, extends for at least 1.2 km of vertical plunge and is open at depth. Overlying matrix (net-textured) ore is rare. Inclusions of dacite are abundant within the lower 5 m of the massive sulfide. They range from angular fragments through smooth sinuous and plumose morphologies to fine lace-like intergrowths with the sulfide matrix, and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts. Dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures, some well above their liquidus temperature. The composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30% komatiite. The consistent thickness of the inclusion-bearing basal layer within the massive sulphide is interpreted as the extent of 3-D physical connectivity between the inclusions and a partially molten underlying hybrid layer. Primary contacts between the Silver Swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse-grained chevron-textured chromites with skeletal textures. Compositions of these chromites match those from Kambalda, Perseverance and other localities, and are inconsistent with a metamorphic origin. They are interpreted as markers of primary magmatic contacts. The combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates. Taken together with observations on the internal fractionation of platinum group elements within the massive ores, these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube. The floor of the tube had been previously heated by passage of large volumes of lava, such that it had reached its melting range. The felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma. This constitutes strong evidence for the operation of thermo-mechanical erosion during ore emplacement. The disseminated Cygnet and Black Swan orebodies show a number of distinctive features. Cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip-up, transport and redeposition of sulfides from a pre-existing massive sulfide orebody, of which Black Duck may be a remnant. The Black Swan orebody, by contrast, does not show xenolithic features, but is characterised by an association of sulfide blebs with segregation vesicles, and by unusually coarse-grained olivine. The Black Swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals.Editorial handling: Peter Lightfoot     

Chromite in komatiites: 3D morphologies with implications for crystallization mechanisms

High-resolution X-ray computed tomography has been carried out on a suite of komatiite samples representing a range of volcanic facies, chromite contents and degrees of alteration and metamorphism, to reveal the wide range of sizes, shapes and degrees of clustering that chromite grains display as a function of cooling history. Dendrites are spectacularly skeletal chromite grains formed during very rapid crystallization of supercooled melt in spinifex zones close to flow tops. At slower cooling rates in the interiors of thick flows, chromite forms predominantly euhedral grains. Large clusters (up to a dozen of grains) are characteristic of liquidus chromite, whereas fine dustings of mostly individual ~20-μm grains form by in situ crystallization from trapped intercumulus liquid. Chromite in coarse-grained olivine cumulates from komatiitic dunite bodies occurs in two forms: as clusters or chains of euhedral crystals, developing into “chicken-wire” texture where chromite is present in supra-cotectic proportions; and as strongly dendritic, semi-poikilitic grains. These dendritic grains are likely to have formed by rapid crescumulate growth from magma that was close to its liquidus temperature but supersaturated with chromite. In some cases, this process seems to have been favoured by nucleation of chromite on the margins of sulphide liquid blebs. This texture is a good evidence for the predominantly cumulus origin of oikocrysts and in situ origin of heteradcumulate textures. Our 3D textural analysis confirms that the morphology of chromite crystals is a distinctive indicator of crystallization environment even in highly altered rocks.     

Komatiite hosted Hunters Road nickel deposit,central Zimbabwe: physical volcanology and sulfide genesis

The nickel sulfide bearing Main Flow at Hunters Road is a thick komatiite unit situated at the base of a well developed lava channel complex overlying a prominent banded iron formation that caps a thick sequence of felsic lavas and volcaniclastic rocks intruded by a probably comagmatic feeder sill. The 300–350 m thick inner flank comprises a 200–250 m thick central olivine meso to adcumulate, relatively narrow lower and upper ortho to mesocumulates and a 2–10 m thick olivine spinifex zone at the top. Approximately 700 m towards the 90 m thick outer flank, the spinifex zone is up to 30 m thick, the central meso to adcumulate lenses out between the upper and lower mesocumulates, and the lower orthocumulate is more rich in clinopyroxene. In places, the flanks are directly overlain by rubbly hyaloclastite. The less well preserved, 500 m thick central axis coincides with a floor rock embayment of demonstrably primary origin, which is 200 m deep and at least 800 m wide. Nickel sulfide mineralisation is disseminated, texturally cocumulus to olivine and confined to the meso to adcumulate, the highest nickel grades being located towards the top. Large floor rock xenoliths occur in the mineralised meso to adcumulate of the inner flank within reaction envelopes of barren, pyroxene bearing, olivine ortho to mesocumulate. Formation of the embayment and xenoliths (clear evidence of large scale thermomechanical floor rock erosion) and of the olivine sulfide meso to adcumulate (the sulfur probably derived by the assimilation of sulfidic wall rocks adjacent to the feeder sill) is attributed to prolonged focused flow of hot turbulent lava close to the vent. The Main Flow is interpreted as the product of a thick channelised sheet flow comprising: (i) a turbulent lava river, open along the central axis and partly tubed over along the inner flanks; and (ii) a tubed over levée facies along the outer flanks accommodating overflow from the central axis by inflationary growth under laminar flow conditions.     

Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine     

Primary stratigraphic controls on ore mineral assemblages in the Wannaway komatiite-hosted nickel-sulfide deposit,Kambalda camp,Western Australia

The 18 m-long UWA-04-02 drillcore from the Fe-Ni-Cu-PGE Wannaway deposit in the Widiemooltha Dome district (Eastern Goldfields, Western Australia) intersects the whole sequence of a komatiite-hosted layer of metal-rich sulfide magma. In spite of regional deformation and amphibolite facies metamorphism the sequence in the drillcore still preserves some of the original, magmatic textures and assemblages and these were examined in a great detail. The magmatic orebody typically consists of basal massive sulfides grading to net-textured (matrix) and disseminated sulfide mineralization upward into the komatiite host. The ore zone is underlined by sulfide-rich black shale passing to basalts. Country rock xenoliths are locally enclosed in the massive sulfides. Portions of the drillcore untouched by penetrative deformation and with minimal imprint by late-stage serpentinization allow the construction of a fairly complex framework where mineral assemblages and mineral chemistry of sulfides, spinels and silicates vary systematically with stratigraphy and may reflect original conditions of ore deposition. The general ore assemblage is dominated by Fe-sulfide and pentlandite, with minor sphalerite and chalcopyrite, spinels (Zn-rich chromite, Ti-magnetite), alabandite (MnS), accessory PGE-rich sulfarsenides and tellurides and rare molybdenite. Monoclinic and high-S hexagonal pyrrhotite and fresh Zn-Mn-rich chromite characterize the basal massive facies, whereas the matrix ore facies is marked by magnetite, sphalerite and a gradually S-depleted and reduced assemblage now represented by troilite exsolving low-S hexagonal pyrrhotite and alabandite. Compositional modifications of the Fe-sulfides across the whole orebody and occurrence of alabandite testify to progressive sulfur loss concomitant with the establishment of low fO₂ conditions over several meters upsequence in the matrix ore facies. PGE-rich sulfarsenides disseminated across the whole mineralized sequence display igneous textures and PGE fractionation trends. The composition of olivine intergrown with matrix sulfides and in the serpentinized hangingwall komatiite deviates from the typical unmetamorphosed komatiite-related, highly-forsteritic type. However the Fe, Mn and Zn contents of olivine crystals decrease systematically and gradually with distance from mineralization towards the hangingwall komatiite. Contamination may be an alternative to metamorphic recrystallization of olivine as the cause of these trends. The role of contamination is also shown by the trends of whole-rock data from the mineralized sequence across the entire drillcore. Textures and mineral chemistry of minerals from the different rock facies in the drillcore are evaluated in terms of metamorphic effects, although the remarkable relationship observed between stratigraphy and several major and accessory phases over metric distances is suggestive of alternative options including primary processes involving the komatiitic lava flow in its interaction both with the black shale substrate and with the sulfide melt ponding at its base.     

Spinifex and swirling olivines in a komatiite lava lake,Munro Township,Canada

Spinifex veins, and veins filled with swirling, tabular olivine grains, intrude the upper part of a 120 m thick lava lake in Munro Township, Ontario, Canada. These veins are 20–200 cm wide, tens of metres long and are oriented roughly parallel to the upper surface of the lava lake. The spinifex texture differs from that found in simple layered komatiite flows: rather than increasing downwards, the size of skeletal olivine grains is greatest in the centres of the veins, or varies irregularly. Upper contacts meander and veinlets penetrate overlying rock; lower contacts are transitional. The other type of vein contains 50–70% of ∼5 mm long hopper olivine grains whose orientation varies widely to produce a swirling, contorted fabric. Both types of vein are believed to have formed when lava from the centre of the lake flowed into fractures in the solidified crust.The lava lake has a dunitic lower portion and an olivine porphyry upper portion. The composition of chill samples, the compositions of olivine phenocrysts, and variations in major element abundances throughout the unit are used to show that the lava originally was komatiite with about 22% MgO. The lava lake is overlain by mafic scorias and breccias with moderate vesicularity and welding structures suggesting eruption in shallow water.     

The petrogenesis of the Lac Guyer komatiites and basalts and the nature of the komatiite-komatiitic basalt compositional gap

A sequence of ultramafic rocks in the Lac Guyer Archean greenstone belt exhibit brecciated flow tops, pillow structures, and spinifex textures testifying to their volcanic origin. Massive, spinifex-textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite, with MgO ranging from 19–25 wt.%. Associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with MgO contents ranging from 16 to 19 wt.% MgO and are best termed pyroxenitic komatiites. Unlike other komatiitic occurrences, the peridotitic and pyroxenitic komatiites at Lac Guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt.% MgO. The contrasting behaviour of highly compatible elements, such as Ni and Cr, versus incompatible elements, such as Zr, indicate that this compositional spectrum was produced by a variation in the extent of partial melting (10–40%) of a garnet lherzolite source in the Archean mantle. The pyroxenitic komatiites represent liquids produced during lower (10–20%) degrees of melting during which garnet remained in the mantle residue. However, a change in slope in the distribution of Zr vs. Y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites. The Lac Guyer volcanic rocks display a population minimum at 15 wt.% MgO separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation. The population minimum near 18 wt.% MgO which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin.     

The Perseverance Ultramafic Complex, Western Australia: The Product of a Komatiite Lava River

The Perseverance ultramafic complex is a body of olivine-richkomatiitic rocks spatially associated with the Agnew nickeldeposit, in the Agnew-Wiluna greenstone belt of the ArchaeanYilgarn Block in Western Australia. The complex consists ofa central lenticular body, up to 700 m thick, of olivine adcumulates,flanked by laterally extensive sheet-like bodies of olivineorthocumulates and spinifextextured komatiite flows. Rocks progressivelyfurther away from the central lens have chemical compositionsreflecting higher original proportions of komatiite liquid tocumulus olivine. Parent liquids had MgO contents between 25and 32% MgO, approximately chondritic Al/Ti ratios and HREEpatterns, and moderate depletion in LREE. Olivines within the adcumulate lens show a progressive increasein forsterite content from Fo₉₃ at the bottom to Fo_94?5, atthe top. Calculated original olivine compositions in the flankingrocks are similar to those at the base of the central lens.Original olivine nickel contents show a symmetrical variationfrom maximum values of 3500 ppm at the top of the central lens,through minimum values of 1000 ppm at the base and margins ofthe central lens to intermediate values in the distal rocks.The complex as a whole shows evidence for nickel depletion relativeto other komatiite suites. These observations are explained in terms of prolonged eruptionand flow of komatiitic lava down a major flow channel or lavariver. Adcumulates crystallized on the floor and sides of thecentral channel, which was formed at an early stage by thermalerosion of floor rocks. Episodic overflow of the central channelproduced distal ‘flood plain’ rocks consisting ofolivine orthocumulates and layered flows. Lavas became moremagnesian and nickel-rich with time, giving rise to the observedspatial variation in primary olivine composition. Nickel depletionof the earliest lavas is attributed to pre-eruption segregationof large volumes of immiscible Fe-Ni-sulfide, which were concentratedto form the underlying Agnew nickel deposit.     

A comparative study of olivine and clinopyroxene spinifex flows from Alexo,abitibi greenstone Belt,Ontario, Canada

A petrological and geochemical study of an olivine and of a clinipyroxene spinifex textured flow, from Alexo, indicates that the initial liquid in both flows probably came from the same mantle melting event and that the source was incompatible element depleted. The starting liquid of the clinopyroxene flow had experienced more olivine fractionation (10%) prior to its emplacement at Alexo, than the initial liquid of the olivine spinifex flow. The development of each of the textural and compositional zones in the flows can be modelled by means of crystal fractionation. In the case of the clinopyroxene flow the B-zone is formed by the fractionation of olivine, low-Ca pyroxene and chromite. An unusual feature of the Alexo clinopyroxene flow is presence of a peridotitic komatiite above the pyroxene cumulate layer, where a basaltic komatiite would usually be present. The presence of the peridotitic komatiite suggests an influx of new magma and hence a dynamic model for the flow. The composition of the clinopyroxene spinifex zone represents a mixture of clinopyroxene plus liquid, rather than simply a frozen liquid. This could happen if the clinopyroxene needles grew stalactitelike from the chilled upper surface of the flow into a flowing basaltic liquid. In the olivine spinifex flow the zones can be modelled as frozen liquids in the A₂-zone, as initial liquid which has fractionated 30% olivine in the A₃-zone and as liquid plus 50% olivine in the B-zone. But, if the clinopyroxene spinifex developed by stalactite growth of clinopyroxene needles into the a flowing liquid, the possibility that the olivine spinifex represent fractionated liquid plus stalactite olivines arises.     

Low- and high-alumina komatiites from a Late Archaean sequence,Newton Township,Ontario

Late Archaean komatiitic lavas from Newton Township, Ontario, consist of 6 chemically distinct magma types: 3 komatiites and 3 komatiitic basalts. The succession is unusual in containing both Al- and HREE-depleted komatiites and Al- and HREE-undepleted komatiites. The two types form distinct stratigraphic units separated by komatiitic basalts. Two komatiite types are strongly LREE depleted, whilst the third and the associated komatiitic basalts range from mildly depleted to enriched. Of the six magma types, only the two strongly LREE depleted komatiites represent primary mantle melts. The other komatiite type and the komatiitic basalts were derived from the primary komatiite magmas by combinations of olivine (+chromite) fractionation, assimilation of continental crust, and magma mixing. The two primary magmas may have been derived from similar sources, their contrasting chemistry being due to differing degrees of garnet segregation during melting. A generally applicable conclusion is that a wide range of komatiitic magma types can be generated from a relatively homogeneous depleted mantle, under conditions likely to prevail during the eruption of late Archean greenstone belt sequences.     

Crystallochemistry and origin of pyroxenes in komatiites

We present a detailed mineralogical and major- and trace-element study of pyroxenes in two Archean komatiitic flows in Alexo, Canada. The pyroxenes in spinifex-textured lavas commonly are zoned with cores of magnesian pigeonite and rims of augite. Concentrations of incompatible trace elements are low in pigeonite and jump to higher values in the augite mantles, a variation that can be modelled using accepted partition coefficients and assuming crystallization from komatiitic liquids. Crystallization sequences are very different in different parts of both flows. In the flow top, the sequence is olivine followed by augite: deeper in the spinifex sequence, pigeonite crystallizes after olivine, followed by augite; in lower cumulates, orthopyroxene or augite accompany olivine. In spinifex lavas, pigeonite crystallizes sooner than would be predicted on the basis of equilibrium phase relations. We propose that contrasting crystallization sequences depend on the position in the flow and on the conditions of crystal growth. In the flowtop, rapid cooling causes quench crystallization. Deeper in the spinifex layer, constrained growth in a thermal gradient, perhaps augmented by Soret differentiation, accounts for the early crystallization of pigeonite. The cumulus minerals represent a near-equilibrium assemblage. Augites in Al-undepleted Archean komatiites in various localities in Canada and Zimbabwe have high moderate to high Wo contents but their Mg# (Mg/(Mg + Fe) are lower than in augites in komatiites from Barberton, South Africa. We attribute the combination of high Wo and high Mg# in Barberton rocks to the unusually high CaO/Al₂O₃ of these Al-depleted komatiites.     

Partial melting and recrystallization of Archeaan komatiites by residual heat from rapidly accumulated flows

Spinifex-textured komatiites at Honeymoon Well, Western Australia, show evidence of partial melting and recrystallization of original igneous textures. Their textures and mineral compositions differ markedly from those typical of komatiites. Spinifex olivine plates are bent and broken, while interstitial space between spinifex and cumulus olivine is occupied by polygonal aggregates of clinopyroxene, orthopyroxene, minor olivine and plagioclase. Similar granular pyroxene-plagioclase aggregates occur as diffuse veins cutting spinifex zones and cumulate zones of the flows and, in places, form the matrix to a breccia containing corroded fragments of spinifex rock. Thermometry based on the two pyroxene assemblages yields temperatures of 1055° to 1141° C, just below the low-pressure komatiite solidus. Mineral compositions are different from those of typical komatiites: clinopyroxenes are Al-poor and Cr-rich, olivines are unusually iron-rich and depleted in Cr and Ca, and the low-Ca pyroxene is bronzite rather than the more typical pigeonite. We interpret these observations as the results of thermal metamorphism, partial remelting and subsequent slow crystallization of originally normal spinifex-textured komatiite flows. The rocks in question occupy a 40–70 m interval sandwiched between two olivine-rich units: an underlying 90 m-thick olivine adcumulate layer, forming part of the cumulate zone of a basal 160 m-thick flow, and an overlying 1 km-thick extrusive body composed mostly of olivine mesocumulate and adcumulate and capped in turn by spinifex-textured flows. Thermal modelling shows that a sinusoidal temperature profile of cool flow tops and hot flow centres would exist within this sequence shortly after eruption. Conductive thermal relaxation of this profile could reheat spinifex zones to the extent of inducing partial melting and textural reconstitution. Such reheating is largely dependent on the time interval between the emplacement of successive flows. Calculations suggest that at Honeymoon Well the emplacement interval must have been of the order of 10 years or less. Textural reconstitution may have contributed to the development of the thick orthocumulate sequences characteristic of komatiites in the Agnew-Wiluna belt. Present address: Geochemex Australia, P.O. Box 281, West Perth, 6005, Western Australia     

Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Munro Township, in the Archean Abitibi greenstone belt of northeastOntario, contains volcanic and hypabyssal rocks of two magmaseries: (1) an Fe-rich tholeiitic series of basaltic to daciticlava flows (3–10 m thick), layered peridotite-pyroxenite-gabbroflows (120 m thick), and layered sills (700 m thick); (2) anultramafic-mafic komatiitic series, comprising discrete lavaflows of peridotitic to andesitic composition (1–17 mthick), layered peridotite-gabbro flows (120 m thick), and layeredsills (500 m thick). The komatiitie lavas form a successionabout 1000 m thick that is both underlain and overlain by thickersuccessions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic,and basaltic komatiites. The most ultramafic are peridotiticcumulates rich in forsteritic olivine (Fo_89–94), at thebases of flows and sills. Many less mafic peridotitic komatiites(MgO: 20–30 per cent), which typically form the upperparts of flows and the marginal parts of small intrusions, exhibitspinifex textures indicative of their formation from ultrabasicliquids. Pyroxenitic komatiites (MgO: 12–20 per cent)also may contain olivine, but are dominated by clinopyroxene,usually in spinifex textures. Basaltic komatiites (MgO <12per cent) are composed mainly of clino-pyroxene and plagioclaseor devitrified glass, rarely in spinifex texture and more commonlyin equigranular textures. Peridotitic komatiite with roughly30 per cent MgO appears to represent a parental liquid fromwhich the more ultramafic komatiites formed by accumulationof olivine, and the less mafic types were derived by fractionationof olivine, joined and finally succeeded in later stages byclinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristicsof the komatiites from the Barberton Mountain Land, South Africa(Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al₂O₃ratios that distinguish the Barberton rocks. The Munro komatiitesare identical in this respect to ultramafic volcanic rocks inAustralia, Canada, Rhodesia, and India. It is proposed thatthe definition of the term komatiite be broadened so that itincludes all members of this ultramafic-mafic rock series, notonly those from Barberton Mountain Land. The proposed criteriaare: (1) highly ultramafic compositions in noncumulate lavas;(2) unusual volcanic structures such as spinifex texture andpolyhedral jointing; (3) low Fe/Mg ratios at given Al₂O₃ valuesor high CaO/Al₂O₃ ratios; low TiO₂ at given SiO₂; and high MgO,NiO, and Cr₂O₃.     

The Archean komatiite-hosted, PGE-bearing Ni–Cu sulfide deposit at Vaara, eastern Finland: evidence for assimilation of external sulfur and post-depositional desulfurization

Archean komatiites host important resources of Ni, Cu, Co, and PGE, particularly in Western Australia and Canada. In Finland, several small, low-grade sulfide deposits have been found in komatiites, including the ca. 2.8 Ga Vaara deposit in the Archean Suomussalmi greenstone belt. It occurs in the central part of the serpentinized olivine cumulate zone of a komatiitic extrusive body and is composed of disseminated interstitial sulfides consisting of pyrite, pentlandite, millerite, violarite, and chalcopyrite accompanied by abundant magnetite. Although currently subeconomic, the mineralization is interesting due to the very high chalcophile element contents of the sulfide fraction (38 wt% Ni, 3.4 wt% Cu, 0.7 wt% Co, 22.4 ppm Pd, and 9.5 ppm Pt). The sulfides occur in relatively Cr-poor olivine cumulates suggesting involvement of a chromite-undersaturated magma. The parental magma was an Al-undepleted komatiite with an estimated MgO content of at least 24 wt%. In contrast to the common komatiite types in the eastern Finland greenstone belts, the Vaara rocks are moderately enriched in LREE relative to MREE, suggesting that crustal contamination played an important role in the genesis of the Vaara deposit. Multiple sulfur isotope data reveal considerable mass-independent sulfur isotope fractionation both in country rock sedimentary sulfides (Δ³³S ranges from ?0.50 to +2.37?‰) and in the Vaara mineralization (Δ³³S ranges from +0.53 to +0.66?‰), which provides strong evidence for incorporation of crustal sulfur. Extensive replacement of interstitial sulfides by magnetite and the presence of millerite- and violarite-bearing, pyrrhotite-free sulfide assemblages indicate significant post-magmatic, low-temperature hydrothermal oxidation of the primary magmatic pyrrhotite-pentlandite-chalcopyrite assemblages and associated sulfur loss that led to a significant upgrading of the original metal tenors of the Vaara deposit.     

Petrology of a 2.41 Ga remarkably fresh komatiitic basalt lava lake in Lion Hills,central Vetreny Belt,Baltic Shield

 In the central Vetreny Belt, southeastern Baltic Shield, an areally extensive 110 m deep lava lake is exposed consisting of remarkably fresh differentiated komatiitic basalt. During eruption, the liquid had a temperature of 1380–1400 °C and contained ∼15% MgO. The lava ponded in a large topographic depression soon after eruption. The differentiation of the lava lake was controlled by settling of transported olivine and chromite phenocrysts and caused the origin of prominent internal layering. The last portions of the trapped liquid crystallized at temperatures of 1250– 1070 °C. A Sm-Nd isochron of 2410±34 Ma for whole rock samples, olivine, augite and pigeonite separates from the lava lake provides a reliable estimate for the time of formation of the uppermost sequences in the Vetreny Belt. This age is in good agreement with the Sm-Nd and Pb-Pb isochron ages of 2449±35 and 2424±178 Ma for the volcanic rocks from the same stratigraphic level in the northwestern Vetreny Belt. Modeling of Nd-isotopes and major and trace elements shows that the komatiitic basalts at Lion Hills may have had a komatiite parent depleted in highly incompatible elements. It can be shown that this initial liquid was contaminated by 7–9% of Archaean upper crustal material from the adjacent Vodla and Belomorian Blocks en route to the surface thus acquiring the observed geochemical and isotope signatures including relative enrichment in Zr, Ba, and LREE, negative Nb- and Ti-anomalies and ɛNd(T) of −1. Received: 8 December 1995/Accepted: 26 March 1996     

Spinifex-textured komatiites in the south border of the Carajas ridge,Selva Greenstone belt,Carajás Province,Brazil

Spinifex-textured komatiites in the Selva greenstone belt are the first unequivocal examples of komatiites in the Transition Subdomain of the Carajás Mineral Province. Outcrops of spinifex-textured komatiites, located ∼1.5 km to the south of the Carajás ridge, were discovered during regional exploration for Ni–Cu–(PGE) sulfide deposits by VALE. They are associated with a 3.8 km long unit consisting of variable types of ultramafic rocks (talc schist, serpentinite and spinifex-textured komatiite). This ultramafic unit follows the steep dipping NW–SE trending Selva greenstone belt composed mainly by quartz-chlorite schists (interpreted as metasediments) and chlorite-actinolite schists (interpreted as metabasalts). Greenschist facies metamorphic parageneses characterize all rock types in the Selva greenstone belt.The komatiitic rocks in the Selva belt comprise a sequence of flows consisting of an upper spinifex-textured layer and a lower olivine cumulate layer. Although the spinifex and cumulus textures are well preserved in the field, the primary mineralogy of the komatiites has been completely replaced by greenschist facies metamorphic minerals. Platy olivine spinifex texture, consisting of an array of roughly parallel olivine plates, and random spinifex texture, consisting of randomly oriented olivine plates, are the most common primary volcanic textures in komatiites in the Selva greenstone belt. Platy and random spinifex texture is defined by former plates of olivine replaced by serpentine with minor actinolite, chlorite and magnetite, alternating with former matrix replaced by abundant actinolite and minor chlorite, talc, serpentine, and magnetite. The domains between olivine plates in both platy and random spinifex-textured rocks contain irregular arrays of fine-grained parallel crystals, representing primary fine-grained “quench” clinopyroxene crystals replaced by actinolite.Spinifex-textured komatiites have MgO contents bracket between 22.8 and 26.9 wt.%, and cumulate textured komatiites have MgO contents up to 40.6 wt.%. When plotted vs MgO contents, most major and minor elements fall on well-defined linear trends indicating control by olivine fractionation or accumulation. Komatiites from the Selva and Seringa (located in the Rio Maria Domain) belts are Al-undepleted with Al₂O₃/TiO₂ ratios close to 20. Results for CaO, Na₂O, and REE suggest that these elements were mobile and their abundances have been modified during metasomatic alteration. REE contents in some samples are very high (up to 40 times primitive mantle values) and REE patterns vary from flat (La/Yb_MN ∼ 1) to highly enriched in LREE (La/Yb_MN up to ∼ 10). The REE mobility may be related to hydrothermal alteration associated to Cu–Au mineralization in the region.The identification of spinifex-textured komatiites close to the Carajás Basin suggests the continuation of 3.0–2.9 Ga greenstone belts of the Rio Maria Domain within the Transition Subdomain, and enlarges the area with potential to host komatiite-associated Ni–Cu–PGE deposits.     

Introduction to nickel sulfide orebodies and komatiites of the Black Swan area,Yilgarn Craton,Western Australia

The Black Swan district, 70 km north east of Kalgoorlie in the Archaean Yilgarn Craton of Western Australia, hosts massive and disseminated nickel sulfide orebodies associated with komatiites. The host rocks and ores preserve some remarkable primary igneous features, which provide important clues as to the origin of komatiite-hosted nickel ores. The series of papers that follow report an extremely detailed study of the petrology, volcanology and geochemistry of these unusually well-preserved orebodies and their host rocks.Editorial handling: Peter Lightfoot     

The Ruiga intrusion: A typical example of a shallow-facies paleoproterozoic peridotite-gabbro-komatiite-basaltic association of the Vetreny Belt,Southeastern Fennoscandia

The Ruiga differentiated mafic-ultramafic intrusion in the northwestern part of the Vetreny Belt paleorift was described for the first time based on geological, petrological, geochronological, and geochemical data. The massif (20 km² in exposed area) is a typical example of shallow-facies peridotite-gabbro-komatiite-basalt associations and consists of three zones up to 810 m in total thickness (from bottom to top): melanogab-bronorite, peridotite, and gabbro. In spite of pervasive greenschist metamorphism, the rocks contain locally preserved primary minerals: olivine (Fo _75–86), bronzite, augite of variable composition, labradorite, and Cr-spinels. A mineral Sm-Nd isochron on olivine melanogabbronorite from the Ruiga Massif defines an age of 2.39 ± 0.05 Ga, while komatiitic basalts of the Vetreny Belt Formation were dated at 2.40–2.41 Ga (Puchtel et al., 1997). The rocks of the Ruiga intrusion and lava flows of Mt. Golets have similar major, rare-earth, and trace element composition, which suggests their derivation from a single deep-seated source. Their parent magma was presumably a high-Mg komatiitic basalt. In transitional crustal chambers, its composition was modified by olivine-controlled fractionation and crustal contamination, with the most contaminated first portions of the ejected melt. In terms of geology and geochemistry, the considered magmatic rocks of the Vetreny Belt are comparable with the Raglan Ni-PGE komatiite gabbro-peridotite complex in Canada (Naldrett, 2003).