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Geochemistry, Petrogenesis, and Metallogenesis of Komatiites in the Abitibi Greenstone Belt, Canada

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Geochemistry, Petrogenesis, and Metallogenesis of Komatiites in the Abitibi Greenstone Belt, Canada Geochemistry, Petrogenesis, and Metallogenesis of Komatiites in the Abitibi Greenstone Belt, Canada R.A. Sproule1, C.M. Lesher1, M.G.Houlé1, R.R. Keays2, J. A. Ayer3 and P.C. Thurston1 1Mineral Exploration Research Centre (Laurentian University, Willet Green Miller Centre, Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 5B6) 2Department of Earth Sciences, VIEPS, Monash University, Clayton, VIC 3168 Australia 3Ontario Geological Survey (Willet Green Miller Centre, Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 5B6) e-mail: [email protected] The ~2.7 Ga Abitibi greenstone belt is one not necessarily those that are preferentially of the youngest parts of the Archean Superior mineralized (Muir, 1979; Baird, 1999), because the Province. Komatiites in the Abitibi greenstone belt differences in Cr/Mg and Cr/Ni simply reflect occur in four lithotectonic assemblages with well- differences in the degree of replenishment and defined ages based on high precision U-Pb single fractionation between channel-flow and sheet-flow zircon geochronology: the 2750-2735 Ma Pacaud facies (Lesher and Arndt, 1995; Lesher and Stone, assemblage (PCA), the 2725-2720 Ma Stoughton- 1996; Lesher et al., 2001). However it does appear Roquemaure assemblage (SRA), the 2718-2710 Ma that Cr contents may be used to distinguish lava Kidd-Munro assemblage (KMA) and the 2710-2703 channels, lava channel facies of channelized sheet Ma Tisdale assemblage (TSA) (Ayer et al., in press; flows, and magma conduits (olivine cumulates) from Sproule et al., in press). We have assembled a sheet flows and sheet sills (olivine + chromite database of >2500 samples from this study, cumulates) in komatiites (e.g., Abitibi, Western government data, company data, and the literature to Australia, Zimbabwe), but they cannot distinguish examine the geochemistry, petrogenesis, and facies in komatiitic basalts (e.g., Cape Smith, metallogenesis of komatiitic rocks in the Abitibi probably Thompson: see discussion by Lesher and greenstone belt. The database has been filtered to Stone, 1996). remove altered samples, yielding a total of 1905 least- Based on the above criteria there are altered samples, all of which have been analyzed for important differences between the komatiites in each major and minor elements and base metals, ~200 of of the komatiite-bearing assemblages of the Abitibi which have been analyzed for lithophile trace greenstone belt (see Fig. 1 and also Houlé et al., elements, and ~60 of which have been analyzed for 2001): PGEs. Pacaud assemblage komatiites formed from The major element compositions of Abitibi komatiitic basalt to low-Mg komatiitic magmas. They greenstone belt komatiites vary as a function of the form predominantly spinifex-textured and pillowed composition of the parental magma and the degree of komatiites; rare cumulate komatiites are komatiitic olivine accumulation. As discussed by Lesher and peridotites with orthocumulate textures. There are Stone (1996), Baird (1999), and Barnes and Brand few mesocumulate and no adcumulate komatiites. (1999), one of the most useful ways to distinguish Stoughton-Roquemaure assemblage between cumulate rocks derived from komatiite komatiites formed from komatiitic basalt and low to magmas and komatiitic basalts is the abundance of Cr high-Mg komatiite magmas. All of the high-Mg at a given MgO content (Fig. 1). Magmas with komatiitic rocks occur in the Reaume-McCart trend >~20% MgO are undersaturated in chromite and (in the north-west portion of the Abitibi greenstone accumulate only olivine, whereas magmas with belt) and are likely intrusive. The remainder of the <~20% MgO are chromite-saturated and accumulate komatiites form spinifex-textured and pillowed cotectic proportions (~50:1) of olivine and chromite komatiites. Rare cumulate zones are komatiitic (Murck and Campbell, 1986). Thus, cumulates peridotites with ortho- to mesocumulate textures, derived from komatiitic magmas have relatively low excluding rare komatiitic dunites with meso- Cr contents, those derived from komatiitic basaltic adcumulate textures in the Reaume-McCart trend. magmas have relatively high Cr contents, and those Kidd-Munro assemblage komatiites formed derived from magmas that became saturated in from low-Mg and high-Mg komatiite and lesser chromite during crystallization have intermediate Cr komatiitic basalt magmas. They include spinifex- contents (Lesher and Stone, 1996). textured, massive (cumulate > aphyric), pillowed The komatiites in the Abitibi greenstone belt (rarer), and volcanoclastic (rare) facies and with the highest Mg or Ni and lowest Cr contents are commonly contain komatiitic peridotites and dunites Thus, it seems probable that the komatiites in the with ortho-, meso-, and adcumulate textures. Abitibi greenstone belt were generated from more Tisdale assemblage komatiites formed from than one plume. komatiitic basalt and low-Mg and high-Mg komatiitic The Abitibi greenstone belt komatiites magmas. They include spinifex-textured, massive exhibit stratigraphic and temporal variations in (cumulate > aphyric), pillowed (rarer), and geochemistry. Komatiites in the Pacaud assemblage volcanoclastic (rare) facies and commonly contain are Ti-depleted komatiites (TDK) with high komatiitic peridotites and dunites with ortho-, meso-, Al2O3/TiO2 (25-35) and low [Gd/Yb]CN (~0.6) (CN: and adcumulate textures. normalization values from McDonough and Sun, Thus, the Kidd-Munro and Tisdale 1995), suggesting a garnet-rich source or formation assemblages contain a greater abundance of meso- to via dynamic melting similar to Gorgona komatiites adcumulate komatiitic peridotites and dunites (Arndt et al., 1997). Komatiites in the Stoughton- compared to the Pacaud and Stoughton-Roquemaure Roquemaure assemblage are largely Al-depleted to assemblages (Fig. 1), which appear to represent lava Ti-enriched komatiites (ADK–TEK) with lesser Al- channels or lava channel facies of channelized sheet undepleted komatiites (AUK) with variable flows. Komatiite-hosted Ni-Cu-(PGE) deposits have Al2O3/TiO2 (6-25) and [Gd/Yb]CN >1, suggesting only been identified within the Kidd-Munro some garnet fractionation during melting. Komatiites assemblage (e.g., Alexo, Dundonald, Marbridge, in the Kidd-Munro assemblage are dominated by Dumont) and Tisdale assemblage (e.g., Sothman, AUK, with lesser ADK–TEK, suggesting a Texmont, Redstone, Hart, Langmuir). This is combination of relatively shallow (garnet-absent) and consistent with the interpretation that komatiite- relatively deep (garnet-rich) sources. Komatiites in associated Ni-Cu-(PGE) deposits form primarily the Tisdale assemblage are exclusively AUK, within lava channel facies or channelized sheet flow suggesting a relatively shallow (garnet-poor) source. facies, not within sheet flow or lava lobe facies The temporal evolution in komatiite composition is (Lesher, 1989). consistent with a decreasing influence of garnet in the The high MgO contents of the komatiites source region of the plume(s) with time. This may and komatiitic basalts in the Abitibi greenstone belt represent successively shallower depths of melting suggest that they formed in a mantle plume system. due to factors including plume(s) ascent or a decrease The duration of komatiite magmatism in the Abitibi in the thickness of the lithosphere during plume greenstone belt spanned ~47 My, comparable to ascent, perhaps related to lithospheric thinning by modern, long-lived hotspots, but was more protracted rising plume(s) (Sproule et al., in press). than the 1-2 My duration of modern plume systems. 10000 TSA 9000 KMA OCac SRA 8000 PCA FC Komatiitic basalt liquids (MgO < ~20 wt%) 7000 AFC Olivine OCmc 6000 5000 COoc Cr (ppm) Cr OCoc 4000 3000 O(C)mc 2000 Ooc 1000 Komatiite liquids (MgO > ~20 wt%) Omc Oac 0 10 15 20 25 30 35 40 45 50 55 MgO Figure 1. Cr (ppm) versus MgO (wt%) for komatiitic rocks from the Abitibi greenstone belt. AFC and FC modelling parameters from Lesher and Arndt (1995). O = olivine; C = Chromite; oc = orthocumulate; mc = mesocumulate; ac = adcumulate. Rocks which form from liquids with MgO < ~20 wt% crystallize olivine+chromite (predominantly komatiitic basalt liquids), whereas cumulates which form from liquids with MgO >~20 wt% crystallize olivine only (komatiites senso stricto). 1.00E+06 Kidd Munro +Ol 1.00E+05 Pacaud Cu-richCu-rich Stoughton Roquemaure +Cr 1.00E+04 sulfidesulfide Tisdale veinsveins 1.00E+03 1.00E+02 FloodFlood Pd/Ir BasaltsBasalts 1.00E+01 MORB 1.00E+00 Mantle 1.00E-01 High-Mg Basalts Komatiites 1.00E-02 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 Ni/Cu Figure 2. Pd/Ir versus Ni/Cu for unmineralized komatiitic rocks from the Abitibi greenstone belt. Comparsion fields from Barnes et al. (1988). Ol = Olivine; Cr = Chromite. [Nb/Th]MN and [Th/Sm]MN in the Kidd- contamination during ascent through the upper crust, Munro and Tisdale assemblages range between which would result in contamination over a broader depleted upper mantle values and values similar to stratigraphic interval (Lesher & Arndt, 1995; Lesher upper Archean continental crust indicating up to et al., 2001). Thus, contamination most likely ~35% contamination (Sproule et al., in press). The occurred during emplacement. eruption of Kidd-Munro and Tisdale assemblage Sixty-one analyses of PGE in unmineralized komatiites through sialic crust is also supported by komatiites from each of the assemblages, including the presence of inherited zircons ranging in age up to samples from areas/assemblages that contain Ni-Cu- 2725 Ma in associated felsic
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