Wawa Subprovince Research Articles

Genesis of the Island Gold Deposit, Ontario, Canada: Implications for Gold Mineralization in the Wawa Subprovince of the Superior Province

Abstract The Island Gold deposit represents an uncommon example of known economic mineralization in the Wawa subprovince, which has been tectonically correlated with the neighboring Abitibi subprovince that hosts worldclass orogenic gold deposits. The Island Gold deposit is hosted in dacite, gabbro, and tonalite-trondhjemite, and accompanied D2 deformation. The main ore zone dips steeply toward the south and consists of early shear-hosted laminated quartz veins and late extensional veinlets; both vein sets host gold mineralization. A set of shallowly dipping extensional quartz veins in the Goudreau zone located north of the main ore zones also host economic gold mineralization. Multiple sulfur isotope analysis of pyrite associated with gold-bearing alteration envelopes in the main ore zone indicates no involvement of sulfur affected by mass-independent fractionation, which rules out sulfur (± gold) sourced from nearby banded iron formation or metasedimentary material. However, a single analysis of pyrite from an auriferous Goudreau zone vein indicates the involvement of sulfur that underwent mass-independent fractionation, which suggests a different fluid source. Zircon U-Pb geochronology of pre- and postmineralization rock samples at Island Gold restricts the timing of mineralization to between ca. 2724 and 2672 Ma. Including previous results of detrital zircon geochronology, gold mineralization occurred between ca. 2680 and 2672 Ma. This age range is similar to the timing of gold mineralization at the Hemlo deposit in the Wawa subprovince but is slightly older than the bulk of orogenic gold mineralization in the neighboring southern part of the Abitibi subprovince. Multiple sulfur isotopes indicate that gold mineralization at Island Gold results from fluids of igneous affinity associated with second-generation transpressional deformation. The ore-controlling structures were developed in bends in regional shear zones focused around relatively competent premineralization plutons.

A trace metal, stable isotope (H, O, S), and geochronological (U-Pb titanite) characterization of hybridized gold orebodies in the Missanabie-Renabie district, Wawa subprovince (Canada)

Mineralized shear zones in the Archean Missanabie-Renabie gold district (~ 1.1 Moz Au; Wawa, Ontario, Canada) locally define composite orebodies that record three hydrothermal events: (1) a pre-orogenic Au1 event (pre-D1 and pre-prograde-metamorphic); (2) a syn-orogenic, post-peak-metamorphic Retrograde event (syn-D3); and (3) a late syn-to post-orogenic Au2 event (late syn- to post-D4). Genetic considerations indicate the orebodies are hybrids with early intrusion-related (Au1) and later orogenic (Retrograde + Au2) events. Pearson product-moment correlation coefficients (log10) of whole-rock and LA-ICP-MS pyrite trace metal datasets distinguish Au1 from Au2 mineralization by Au-Ag, Au-Bi, and Au-Te correlations > 0.7 (p < 0.05) in the former, irrespective of sample medium and analytical method. An Au-Mo correlation in whole rock data (0.58–0.76; p < 0.05) further distinguishes Au1 from Au2 and supports an independently inferred intrusion-related origin for Au1. Sulfur isotope data is similar for both Au1 and Au2 pyrite with average δ34S values of − 5.5‰ ± 0.2‰ (1σ) and − 3.5‰ ± 0.3‰ (1σ) and average Δ33S values of 0.4‰ ± 0.1‰ (1σ) and − 0.3‰ ± 0.2‰ (1σ), respectively. SIMS δ18Oquartz values for the Au1, Retrograde, and Au2 events largely overlap and, like δ18Ocarbonate values of previous studies, tend to be lower than values typical of Archean gold deposits. The results of this study suggest that correlation coefficients in trace metal datasets are useful in discriminating and characterizing different gold events. Caution is emphasized with the use of S- and O-isotope datasets for these purposes. The presence of low δ18O values in vein quartz and carbonate is best explained by an 18O-depleted fluid formed during the Retrograde hydrothermal event. The latter is inferred at 2580 ± 21 Ma based on U-Pb geochronlogy of hydrothermal titanite, and relates to deformation and metamorphism in the nearby, amphibolite- to granulite-grade Kapuskasing metamorphic belt. Geochronological and geochemical evidence suggest that the 18O-depleted fluid may have formed via the devolatilization of biotite-bearing granitoids during deep-crustal metamorphism.

Petrogenesis and geodynamic setting of the Neoarchaean Haines Gabbroic Complex and Shebandowan greenstone belt, southwestern Superior Province, Ontario, Canada

The 2722 ± 2 Ma Archaean megacrystic anorthosite-bearing Haines Gabbroic Complex in the Wawa subprovince of the western Superior Province formed contemporaneously with the spatially associated Greenwater Assemblage of the Shebandowan greenstone belt, Ontario, Canada. Both field and petrographic observations indicate that the Haines Gabbroic Complex and the spatially and temporally associated ultramafic to intermediate volcanic and intrusive rocks of the Greenwater and Burchell assemblages experienced extensive metamorphic recrystallization and alteration as a consequence of greenschist-facies metamorphism. These metamorphic processes resulted in pervasive replacement of igneous minerals by metamorphic counterparts. Metamorphic alteration also mobilised LILE and transition metals and slightly reset the U-Th-Pb and Sm-Nd isotope systems. Consequently, the Haines Gabbroic Complex and Greenwater Assemblage have respective Sm-Nd errorchron ages of 2765 +110/‐280 Ma and 3108 +120/‐380 Ma.Major and trace element compositions suggest that the least altered anorthosites, leucogabbros and gabbros of the Haines Gabbroic Complex are not cogenetic with the Greenwater and Burchell assemblages and have distinct parental magmas and depleted mantle sources. The calc-alkaline andesites of the Burchell Assemblage, the tholeiitic basalts and gabbros of the Greenwater and Burchell assemblages, the transitional gabbros of the Greenwater and Burchell assemblages and the picrites, serpentinites and peridotites of the Greenwater Assemblage are all petrogenetically distinct and have different parental magmas and depleted mantle sources. The pyroxenites are genetically distinct from the other Greenwater Assemblage lithologies, the Burchell Assemblage and the Haines Gabbroic Complex. The Haines Gabbroic Complex crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas that fractionated from more primitive, hydrous boninitic parental magmas.The initial εNd values of the Haines Gabbroic Complex (εNd = +3.0 to +4.1) and the Greenwater (εNd = +1.8 to +4.1) and Burchell (εNd = +2.0 to +3.7) assemblages are indicative of variably depleted mantle sources. The major and trace element characteristics of the Haines Gabbroic Complex and the Greenwater and Burchell assemblages and geological relationships indicate that they formed in a Japan-style mature intra-oceanic continental arc with a large volume of TTGs. Intra-oceanic arc rifting was most likely triggered by slab rollback, followed by mantle plume emplacement. On the basis of rock association and geochemical data, we suggest that the Haines Gabbroic Complex and the Greenwater and Burchell assemblages represent a dismembered subduction-related Archaean ophiolite that marks a Neoarchaean suture zone between the Wawa and Wabigoon subprovinces.

Contrasting Fluid Chemistries, Alteration Characteristics, and Metamorphic Timing Relationships Recorded in Hybridized Orebodies of the Missanabie-Renabie Gold District, Archean Wawa Subprovince, Ontario, Canada

Contrasting Fluid Chemistries, Alteration Characteristics, and Metamorphic Timing Relationships Recorded in Hybridized Orebodies of the Missanabie-Renabie Gold District, Archean Wawa Subprovince, Ontario, Canada

Geodynamic setting, crustal architecture, and VMS metallogeny of ca. 2720 Ma greenstone belt assemblages of the northern Wawa subprovince, Superior Province

The greenstone belts along the northern margin of the Wawa subprovince of the Superior Province (Vermilion, Shebandowan, Winston Lake, Manitouwadge) formed at ca. 2720 Ma and have been interpreted to be representative of a rifted-arc to back-arc tectonic setting. Despite a common inferred tectonic setting and broad similarities, these greenstone belts have a significantly different metallogeny as evidenced by different endowments in volcanogenic massive sulphide (VMS), magmatic sulphide, and orogenic gold deposits. In this paper, we examine differences in geodynamic setting and crustal architecture as they pertain to the metallogeny of each greenstone belt by characterizing the regional-scale trace-element and isotopic (Nd and Pb) geochemistry of each belt. The trace-element geochemistry of the Vermilion greenstone belt (VGB) shows evidence for a transition from arc-like to back-arc mafic rocks in the Soudan belt to plume-driven rifted arcs in the ultramafic-bearing Newton belt. The Shebandowan greenstone belt (SGB) has a significant proportion of calc-alkalic, arc-like basalts, intermediate lithofacies, and high-Mg andesites, which are characteristic of low-angle, “hot” subduction. Extensional settings within the SGB are plume-driven and associated with komatiitic ultramafic and mid-ocean ridge basalt (MORB)-like basalts. The Winston Lake greenstone belt (WGB) is characterized by a transition from calc-alkalic, arc-like basalts to back-arc basalts upward in the strata and is capped by alkalic ocean-island basalt (OIB)-like basalts. This association is consistent with plume-driven rifting of a mature arc setting. Each of the VGB, SGB, and WGB show some isotopic evidence for the interaction with a juvenile or slightly older differentiated crust. The Manitouwadge greenstone belt (MGB) is characterized by isotopically juvenile, bimodal, tholeiitic to transitional volcanic lithofacies in a back-arc setting. The MGB is the most isotopically juvenile belt and is also the most productive in terms of VMS mineralization. The Zn-rich VMS mineralization within the WGB suggests a relatively lower-temperature hydrothermal system, possibly within a relatively shallow-water environment. The Zn-dominated and locally Au-enriched VMS mineralization, as well as mafic lithofacies and alteration assemblages, are characteristic of relatively shallower-water deposition in the VGB and SGB, and indicate that the ideal VMS-forming tectonic condition may have been compromised by a shallower-water depositional setting. However, the thickened arc crust and compressional tectonics of the SGB suprasubduction zone during hot subduction may have provided a crustal setting more favourable for the magmatic Ni–Cu sulphide and relative gold endowment of this belt.

Constrains on vorticity and non-coaxial shear direction in Neoarchean L-S tectonites, an example from northern Minnesota, USA

We present a detailed kinematic study of seven Neoarchean L-S tectonite samples in order to constrain vorticity and non-coaxial shear direction relative to foliation and elongation lineation. Samples are L-S tectonites from the Wawa Subprovince of the Archean Superior Province, more specifically the Vermilion District of NE Minnesota, a NE-trending belt of greenschist grade supracrustal rocks and granitoid bodies. Supracrustal rocks host multiple L-S tectonite packages with a well-developed sub-vertical metamorphic foliation and elongation lineation; elongation lineation generally plunges steeply to gently, although zones of shallow plunge occur locally. The Wawa Subprovince is widely interpreted as a transpressional plate margin with shear zones recording unidirectional dextral strike-slip, an interpretation held up as fundamental evidence for Archean plate-tectonic processes. However, vorticity and shear direction within Vermilion District L-S tectonites remain unconstrained. We compare data from thin-sections, X-ray computed tomography, and quartz crystallographic fabric analysis to monoclinic shear models to constrain vorticity and better understand the geometric relationships between vorticity, non-coaxial shear direction, foliation, and elongation lineation. Kinematic indicators in thin-section and image slices from X-ray computed tomography consistently record asymmetric microstructural fabrics in foliation-normal/lineation-parallel planes, whereas planes normal to foliation and elongation lineation display dominantly symmetric microstructural fabrics. Mantled porphyroclast 3D-shapes and star-volume distribution analyses indicate that porphyroclast short-axes are normal to foliation and long-axes parallel elongation lineation. Quartz crystallographic preferred orientation data show a-axes maxima sub-parallel to foliation-normal/lineation-parallel planes. Kinematic data consistently show a vorticity vector within the foliation plane and normal to elongation lineation; thus non-coaxial shear direction is sub-parallel to elongation lineation. Data are inconsistent with shear models in which non-coaxial shear direction is normal to lineation, or in which the vorticity vector is normal to foliation. Rather, kinematic data indicate that tectonites record non-coaxial shear broadly parallel to elongation lineation regardless of the geographic orientation of lineation.

Geodynamic Reconstruction of the Winston Lake Greenstone Belt and VMS Deposits: New Trace Element Geochemistry and U-Pb Geochronology

The ca. 2720 Ma Winston Lake greenstone belt of the Wawa-Abitibi terrane, Superior province, is located on the northern margin of the Wawa subprovince and has important regional tectonostratigraphic implications for the development of the Neoarchean Superior province. The metallogenic importance of the belt is evident from the presence of the Winston Lake, Pick Lake, and Zenith volcanogenic massive sulfide (VMS) deposits. However, the chemostratigraphic evolution has not been fully examined at the greenstone belt scale, nor have the VMS deposits been examined from the perspective of the belt-scale petrogenesis and volcanic reconstruction. This study (1) reviews the geology and VMS mineralization in the Winston Lake greenstone belt, and (2) reconstructs its geodynamic setting, and its magmatic and metallogenetic history using new geological, trace element, and Nd isotope data constrained by new U-Pb geochronology. The belt is subdivided into two main lithotectonic assemblages: the Winston Lake Assemblage composed of tholeiitic to calc-alkalic bimodal volcanic lithofacies, and the Big Duck Lake Assemblage composed of alkalic to tholeiitic mafic flows and sills. The contact between the Winston Lake and Big Duck Lake Assemblages is intruded by the differentiated Zenith gabbro, which hosts the 0.16 Mt Zenith VMS orebody as a series of xenoliths. The variably altered felsic lithofacies of the Winston Lake Assemblage host the 3.1 Mt Winston Lake Main and 1.3 Mt Pick Lake Zn-rich VMS deposits. The felsic volcanic lithofacies of the Winston Lake Assemblage are FIII-type rhyolites and have a bimodal distribution of Zr/Ti indicating two distinct magmas formed by different temperatures of partial melting. Mafic lithofacies of the Winston Lake Assemblage are typically calc-alkalic to transitional between calc-alkalic and tholeiitic in composition. They have arc to back-arc trace element contents with distinctive negative Nb and Ti anomalies, enrichment in the light rare earth elements (LREE), and higher Th/Nb ratios than the mafic rocks from the Big Duck Lake Assemblage. Mafic strata from the Big Duck Lake Assemblage have trace element characteristics similar to mature back-arc tholeiitic basalts, and OIB-like or oceanic plateau alkalic basalts with high Nb/Th and Ti/V ratios. The chemostratigraphy of the Winston Lake greenstone belt is consistent with a geodynamic model where arc rifting is followed by back-arc development. Nd isotope data indicate that strata from the Winston Lake and Big Duck Lake Assemblages are juvenile and show no evidence of contamination by significantly older crust. Therefore, crustal contamination signatures in the mafic flows from the Winston Lake Assemblage must have formed by contamination from crust that is of similar age and supports a comagmatic relationship between the mafic and felsic strata. The mafic flows of the Big Duck Lake Assemblage represent a mature back-arc setting with evidence for enriched plume-generated basalts in the upper mafic unit. The Winston VMS deposit formed during the early rifting of the arc and the timing is tightly constrained by the hydrothermally altered 2721.2 ± 0.9 Ma VMS-hosting felsic strata and younger, postmineralization, 2719 ± 4 Ma Zenith gabbro. The Zn-dominated VMS mineralization suggests that temperature of the hydrothermal fluid during discharge was relatively cool (<300°C) and did not maintain sufficiently high temperatures to precipitate significant amounts of copper relative to zinc. This is likely due to adiabatic cooling of the fluid because of a low-pressure (<0.5 kb) sea-floor environment and/or permeable volcaniclastic strata. A U-Pb age of emplacement of 2692.1 ± 0.7 Ma from the syndeformation quartz-feldspar porphyry in the Big Duck Lake Assemblage indicate it is comagmatic with syndeformation magmatic events typical of other greenstone belts along the northern margin of the Wawa-Abitibi terrane. This event is interpreted to represent the timing of terrane accretion of the Wawa-Abitibi terrane to the Superior province.

New U–Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province

Timiskaming-type assemblages in the Wawa–Abitibi terrane of the Superior Craton and other Archean cratons worldwide are important because of their spatial and temporal association with world-class orogenic gold mineralization, especially in the Kirkland Lake area of Ontario. These mainly locally derived sedimentary assemblages are deposited in transpressional, pull-apart basins that developed during the final stages of terrane accretion, and are commonly associated with calc-alkalic to alkalic volcanic and plutonic rocks. The Shebandowan and Vermilion greenstone belts (SGB, VGB) in the Wawa subprovince have significantly less economic base and precious metal mineralization than the greenstone belts of the Abitibi subprovince and the timing of deformation and magmatism in the SGB and VGB is less well constrained. This study presents new U–Pb geochronology from the SGB and VGB that demonstrating that the sedimentary and volcanic rocks of the Lake Vermilion Formation in the VGB are coeval with similar rocks in the SGB and their interpreted depositional environment are consistent with these units being Timiskaming-type assemblages. Analyses of magmatic zircons in a dacitic tuff breccia from the Gafvert Lake sequence, the volcanic member of the Lake Vermilion Formation, provided a high precision U–Pb age of 2689.7±0.8Ma using thermal ionization mass spectrometry. Detrital zircon geochronology using LA-ICP-MS confirms that the source of the detritus in the Lake Vermilion Formation was locally derived from the Gafvert Lake metavolcanic lithofacies. A similar temporal pattern was observed in many of the Timiskaming-type sedimentary rocks that were sampled in the SGB. The locally derived, alluvial–fluvial to shallow marine depositional environment in long, narrow basins with rapid lateral facies changes described in the SGB and VGB are consistent with deposition in structurally controlled, pull-apart basins. However, one sample from the SGB had detrital zircons with ages that were more akin to the age range found to the north in the Wabigoon subprovince, which is suggestive of trans-terrane transport of detritus in a foreland-type basin. Geochemically, the Timiskaming-type volcanic rocks in the VGB and SGB are less enriched in incompatible elements and are comparatively less alkalic than the volcanic rocks in the Timiskaming assemblage of Kirkland Lake. This may indicate that the structures that formed the basins within the SGB and VGB in the Wawa were not as deeply penetrating as those in the Abitibi, which may explain the paucity of economic gold mineralization associated with the former. The Timiskaming-type deposits of the northern Wawa–Abitibi terrane are 20Ma older than the Timiskaming assemblage in the Abitibi subprovince, and co-depositional with the flysch-like deposits of the Porcupine assemblage in the southern part of that terrane.

The Characteristics and Origin of the Big Lake Mafic-Ultramafic-Hosted Volcanogenic Massive Sulfide Occurrence, Marathon, Ontario, Canada

The Big Lake volcanogenic massive sulfide (VMS) occurrence, located in the Schreiber-Hemlo belt in the Wawa subprovince of the Superior province, is hosted in an Archean mafic-ultramafic metavolcanic sequence lacking felsic volcanic or volcaniclastic rocks. Discovered in 2006, the occurrence is a subeconomic assemblage of pyrrhotite, chalcopyrite, sphalerite, cobaltite, and galena currently defined over an area of ~0.5 × 0.5 km in plan view. The mineralized zone is less than 10 m thick, with the highest grade intersection containing 4.0 m of 7.5% Cu, 2.2% Zn, 138.0 gpt Ag, and 9.2 gpt Au. Mineralization is hosted in metavolcanic rocks of basaltic to komatiitic basalt compositions. The sulfides and spatially associated hydrothermal alteration (regionally metamorphosed to chlorite, actinolite, hornblende, biotite-phlogopite, and anthophyllite) are located 5 to 50 m structurally below a thick ultramafic cumulate complex. The host sequence to the occurrence is overturned, with VMS sulfides stratigraphically overlying the ultramafic cumulates. Rocks stratigraphically below the VMS mineralization are transitional to alkaline basalts and komatiitic basalts, with convex-upward LREE and fractionated HREE when normalized to primitive mantle. Basalts stratigraphically overlying the VMS mineralization have flat normalized REE patterns (La/Ybcn = 1–2) typical of tholeiitic plateau basalts found elsewhere in the Wawa belt. The lithostratigraphic assemblage at Big Lake is consistent with an oceanic plateau setting and is interpreted to have formed by eruption of plateau basalts and thick ultramafic flows from a mantle plume. Unlike VMS systems that formed above subvolcanic intrusions >0.5 km below sea floor, the Big Lake VMS occurrence may have formed as a result of cooling of the stratigraphically underlying ultramafic cumulates and demonstrates that similar styles of ponded flow-driven VMS systems, albeit small ones, may be preserved elsewhere in the Superior province or in other Archean cratons.

Mantle plume – volcanic arc interaction: consequences for magmatism, metallogeny, and cratonization in the Abitibi and Wawa subprovinces, CanadaThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

The Abitibi and Wawa subprovinces of the southern Superior Province differ in terms of the extent of pre-existing 2750 Ma sialic crust and relationships between mantle plume type (tholeiitic basalt – komatiite) and arc type (tholeiite to calc-alkaline basalt – andesite – dacite – rhyolite) volcanic successions but evolved in close proximity to each other. Isotopic data, evidence from the Kapuskasing uplift, continuation of major structures associated with large gold deposits from the Abitibi into the Wawa subprovince and the related occurrence of diamonds in lamprophyric rocks in both subprovinces point to a common evolution prior to and during orogeny. Differences preserved in supracrustal sequences of the two subprovinces suggest that the main petrogenetic controls on orogenic gold deposits and lamprophyre-hosted diamond deposits lay in the lower crust and upper mantle. Similar processes must also have been active where gold and diamonds are associated on other Archean cratons, such as the Slave and possibly the Kaapvaal craton. Based on evidence preserved in the Abitibi–Wawa orogen, rapid crustal growth at ∼2.7 Ga was linked to the interplay between plate tectonics and mantle plumes. Key indicators in the model developed for the Abitibi–Wawa arc are the co-existence of plume-related rock types, modern-style adakites, major gold deposits, and lamprophyre-hosted diamond occurrences, at least in cases where shoshonitic host magmas can ascend rapidly through the crust. All of these indicators are now identified on the Kaapvaal craton by 3.1 Ga and many recur together in Paleoproterzoic and younger terranes, suggesting a common mechanism for rapid crustal growth through much of Earth’s history. The variety of granitoid types found within the Abitibi–Wawa orogen demonstrates that local tectonic factors, rather than a hotter average upper mantle, were important in controlling the type of magmas formed. Based on the geodynamic history of the subprovince, mantle plume interaction with an existing volcanic arc and the subduction of oceanic plateau crust played an important role in the formation of magmas similar to Cenozoic adakites. Flat subduction beneath a mantle wedge was probably responsible for the generation of the adakites and also promoted diamond stability at shallow depths while enhancing the reservoirs for subsequent orogenic gold deposits. The history of magmatism and mineralization in the Abitibi and Wawa subprovinces precludes an early or gradual development of a cratonic keel, which instead must have coupled with crust during the latest stages of orogeny.

Re–Os depositional age for Archean carbonaceous slates from the southwestern Superior Province: Challenges and insights

Rhenium–osmium (Re–Os) dating of carbonaceous rocks using H 2SO 4-CrO 3 dissolution provides a powerful new geochronologic tool for Archean terranes. Here we document the oldest Re–Os depositional ages yet reported for carbonaceous sedimentary rocks. Carbonaceous slates are intercalated with graywackes and mafic tuffs of the Joy Lake sequence in the western Wawa subprovince of the Superior Province. Well-developed D 2 cleavage is sub-parallel to bedding. Pyrite occurs in nodules, disseminations, and bedding-parallel stringers. Samples from two drill holes (DH26503 and DH26506) yield isochron ages of 2695 ± 14 Ma and 2684 ± 16 Ma and initial 187Os/ 188Os ratios (Os i) of 0.15 ± 0.16 and − 0.29 ± 0.20, respectively. The ages, identical within uncertainty, provide the first temporal constraints for the westernmost Wawa subprovince greenstone sequence. The ages are consistent with cessation of mafic/ultramafic volcanism before ca. 2695 Ma in the western Wawa subprovince and D 2 deformation before 2685 Ma farther east. The Joy Lake sequence is likely the temporal equivalent of turbiditic graywackes deposited further east, both sourced from arc volcanoes during collision of the Wawa and Wabigoon arcs along the southern margin of the Superior Province. A chondritic Os i (0.109 at 2.69 Ga) is expected for Archean shales, but the negative Os i for DH26506 requires explanation. Re, which is significantly more mobile than Os, may have been released from intercalated high-Re/Os mafic tuffs by local oxidation during recent or Cretaceous exposure to meteoric waters. The adjacent carbonaceous rocks are a ready sink for mobile Re. A small Re addition can shift isochron data points to higher 187Re/ 188Os ratios with minimal slope change. This increases scatter and uncertainty for the Os i, shifts the isochron to the right, and produces a spuriously low Os i intercept. If Re addition is recent, the age is not significantly affected. Notably, the slates in DH26506 which yield the negative Os i are intercalated with relatively permeable and reactive lithic wackes and mafic tuffs, whereas the slates in DH26503 are in contact with less permeable massive intrusive rocks. This process can be detected only if the true Os i is approximately chondritic, and is more likely where reactive mafic rocks are prevalent—both the case for Archean greenstone belts.

Plume and arc magmatism in the Abitibi subprovince: Implications for the origin of Archean continental lithospheric mantle

Based on eclogitic mantle xenoliths that appear to be the residue of slab melts, it is commonly argued that the formation of distinctive lithospheric keels beneath Archean cratons occurred by the tectonic accretion of oceanic crust. These models are complicated by evident buoyancy of the keels, the varied compositions of the mantle xenolith suite as a whole, and a compositional constraint that a large portion of the keels originated in mantle plume melting events. Resolution of these issues is complicated by the long development history of most cratons (100’s of m.y.) and the multiple cycles of magmatism they represent. In contrast, the single 50 m.y. period of arc volcanism in the Abitibi subprovince, and the immediate onset of orogeny and craton stabilisation, avoids many of these uncertainties. Chemical and isotopic data demonstrate that syn-volcanic to syn-tectonic adakitic Abitibi granitoids were derived via slab melting. Deep seismic reflection profiles image singular slab-like features along the Moho. These are interpreted as the remnants of oceanic slabs emplaced and torn during orogeny. Nonetheless, their presence directly contradicts subcretion models of keel formation because they occupy regions that should correspond to the top of an earlier-formed subcreted keel. Post-adakite shoshonitic granitoids and lamprophyres also attest to the presence of a metasomatised mantle wedge throughout the entire period of Abitibi arc magmatism. Diamond-bearing lamprophyres in the Abitibi and adjacent Wawa subprovince are compositionally equivalent to Paleoproterozoic diamondiferous rocks and characteristic of adakite-generating shallow subduction rather than evidence of pre-existing sub-cratonic keels. Abitibi mantle plume- and subduction-related volcanism also demonstrates the continuous presence of shallow asthenospheric mantle beneath the belt over a 50 m.y. period. Komatiites and then boninites erupted along the central part of the belt, halfway through its evolution, and rifted arc-style volcanism dominated the belt within a few m.y. of orogeny. None of these events are consistent with the development of a progressively thickened continental lithospheric keel. The geodynamic evolution of the Abitibi belt demonstrates a fact recognised in some xenolith studies: the characteristic association of deep keels and Archean cratons was not developed gradually during greenstone belt volcanism. Rather, it was initiated by the coupling of shallow imbricated plume- and arc-related crust with the buoyant residue of mantle plume melting that rose from depths of 80 km to >400 km. Subduction was active along the southern and eastern margin of the Superior Province craton before and after ascent of the Abitibi plume. Renewal of subduction in the Abitibi, as marked by the occurrence of the ∼2716 Ma boninitic volcanic suite, delayed the ascent of the deep keel and its coupling to the composite crust. Final ascent of the plume residue occurred following orogeny and slab loss and may have included entrainment of fragmented slab residue into the rising diapir. The coupling of shallow crust and deep keel occurred soon after and the keel’s presence beneath the subprovince is recognised by Phanerozoic diamond deposits, xenolith chemistry and geophysical data.

Pericontinental Crustal Growth of the Southwestern Abitibi Subprovince, Canada--U-Pb, Hf, and Nd Isotope Evidence

The Abitibi subprovince is widely regarded as a classic example of Neoarchean continental crust formed in an ensimatic setting. However, sialic crust as old as 2.93 Ga occurs in the adjacent Wawa subprovince, and the two subprovinces are thought to have formed a contiguous block since the initiation of Abitibi magmatic activity at 2.75 Ga. A detailed geochemical, U-Pb geochronological, and Nd-Hf isotope study of plutonic and volcanic rocks from the southwestern Abitibi provides evidence for the involvement of older crust during magma genesis near the Abitibi-Wawa boundary (Kapuskasing structural zone). Inherited zircons dated at ca. 2.85 Ga occur both in 2747 Ma tonalite and 2700 Ma granodiorite of the Rice Lake batholith, part of the larger Kenogamissi plutonic complex. Igneous zircons from this batholith display large intrasample variations in 176 Hf/ 177 Hf, which we attribute to mixing of magmas derived from mantle ± juvenile crust and older crustal sources. Nd isotope data for felsic volcanic and plutonic rocks in this region indicate a dominantly juvenile source; however, two older felsic volcanic units (2739 and 2729 Ma) show Nd isotope evidence of significant crustal contamination. Post-tectonic plutonic units that were emplaced at 2676 and 2662 Ma also contain inherited zircons, but none are older than 2.76 Ga. In contrast, tonalitic units dated at 2744 and 2715 Ma from the Round Lake plutonic complex, located 150 km east of the Kapuskasing structural zone, lack zircon inheritance and display less intrasample Hf isotope variability in zircons. Our data suggest that the influence of older sialic crust during Abitibi volcanic and plutonic activity is restricted both in space and time. Similar evidence for an older crustal influence occurs elsewhere in the southwestern Abitibi near the Kapuskasing structural zone. The data collectively suggest that an approximately 75-km-wide belt of Abitibi crust adjacent to this structure was underlain by older Wawa crust from 2.75 Ga onward. This crustal geometry is consistent with development of the Abitibi sub-province at the edge of a Wawa protocraton, perhaps as the result of rifting of this older continental block.

Geochemical systematics of 2.7 Ga Kinojevis Group (Abitibi), and Manitouwadge and Winston Lake (Wawa) Fe-rich basalt–rhyolite associations: Backarc rift oceanic crust?

We present new geochemical data for Neoarchean Kinojevis and Manitouwadge–Winston Lake volcanic rocks from the formerly contiguous Abitibi and Wawa subprovinces. The 2702–2701 Ma Kinojevis Group (KG), Abitibi greenstone belt, is dominated by Fe- to Mg-rich basalt flows (Mg-number = 30–56) having a tholeiitic fractionation trend, and prominent magnetic striping. REE patterns are near-flat at 4 to 15 times chondrite. Normalized La/Sm and Gd/Yb ratios span 0.70 to 1.17 and 0.96 to 1.34 respectively, the latter indicative of a variable involvement of garnet in the residue. Basalts do not plot on mixing lines with average Archean upper crustal composition, and accordingly were likely erupted in an oceanic setting. Tholeiitic high-Si rhyolites also feature near-flat REE patterns, with pronounced negative Eu anomalies, consistent with extreme fractionation products of basaltic melts. Niobium contents and Zr/Nb ratios (1.7–6.4 ppm; 10–24) of basalts are mostly intermediate between respective values for Archean primitive oceanic arc tholeiites (PAT), and ocean plateau tholeiitic basalts (OPB: 2–9 ppm; 12–20) associated with komatiites, likely representing ocean plateaus erupted from mantle plumes. Similarly, Nb/Th (4–8, versus 8 for primitive mantle) ratios in KG basalts are intermediate between ranges for Archean PAT (2–6) and OPB (8–20). Compositionally, these basalts are commensurate with recent backarc basalts (BAB) that are intermediate between mid-ocean ridge basalts (MORB) and PAT: they are interpreted as oceanic crust in a backarc rift, from mildly hydrous melting of depleted upper mantle asthenosphere. Kinojevis basalts plot mostly on the MORB–ocean island basalt (OIB) array in log Nb/Y versus Zr/Y coordinates, whereas structurally overlying Blake River basalts cross that array from depleted plume asthenosphere to adakite “end-members”. Consequently, their asthenosphere sources and geodynamic settings may have differed. Tholeiitic basalts and FIII-type rhyolites in the 2722–2680 Ma Manitouwadge and Winston greenstone belts, Wawa subprovince, are compositionally akin to their Kinojevis counterparts. Basalts and rhyolites in both belts are spatially and temporarily associated with volcanogenic massive sulphide deposits (VMS), indicative of the potential of the KG for similar deposits. The KG is interpreted as intra-oceanic backarc crust. Similarly, the Wawa volcanic and metallogenic associations are interpreted to have been deposited in a backarc setting, to the north-northwest of the Schreiber-Hemlo subduction–accretion complex.

Neoarchean trans-subprovince sediment transport in southwestern Superior Province: sedimentological, geochemical, and geochronological evidence

At ~2700 Ma volcanism spawned by northward-directed subduction under Wabigoon subprovince, Canadian Shield, fed sediment into an adjacent fore-arc basin – trench system. Volcanically active island arcs to the south (Wawa subprovince) were transported northward during this interval and at ~2690 Ma collided with the accretionary complex (Quetico subprovince). Metamorphosed sedimentary units intercalated with the ocean floor and island-arc volcanic rocks of northern Wawa subprovince provided the opportunity to amass data useful in extending the paleogeographic interpretation of the area. This study examines the McKellar Harbour Formation, a several kilometre thick assemblage of graded metasandstone beds outcropping to the north of Lake Superior. Its sedimentology, geochemistry, and detrital U–Pb zircon geochronology were investigated in conjunction with the same attributes of possible correlative units in the region. Similarities between the McKellar Harbour Formation and sedimentary rocks composing the fore-arc basin and Quetico trench to the north indicate that by 2696 Ma the trench had been overwhelmed by sediment, to the point where turbidity currents were overflowing onto the abyssal ocean plain, building a prograding submarine ramp–fan that was to become the McKellar Harbour Formation. The turbiditic ramp–fan assemblage comprising most of this extensive depositional system crosses much of southern Superior Province, highlighting the interrelated genesis of rocks formed during amalgamation of the individual subprovinces.

Structure of the Archean English River subprovince: implications for the tectonic evolution of the western Superior Province, Canada

The English River subprovince is one of two metasediment-dominated terranes in the western Superior Province. It has been interpreted as an accretionary complex, a foreland, or a fore-arc basin that developed and was subsequently deformed between the metavolcanic-rich Uchi subprovince and the orthogneiss- and metaplutonic-dominated Winnipeg River subprovince during a prolonged transpressive orogeny. To test these hypotheses, we combined a satellite image, aeromagnetic image, and Lithoprobe reflection seismic profile interpretation with detailed structural mapping to better characterize the internal geometry and significance of structural features in the western part of the subprovince in Ontario. Northward-directed subduction and collision of the Winnipeg River subprovince with the Uchi subprovince at ca. &gt;2713–2698 Ma can account for the deposition of the sedimentary rocks, initial metamorphism, and the main phase of deformation in the subprovince, whereas the subduction of Wabigoon crust generated extensive tonalite magmatism in the Winnipeg River and English River subprovinces during the same period. A period of extension, after the docking of the Winnipeg River and Wabigoon subprovinces at ca. 2698 Ma, punctuated the compressive phases of the orogeny and was responsible for high-grade metamorphism, upward bending of the Moho, and localized deposition of late, coarse, alluvial–fluvial metasedimentary rocks. Renewed compression caused by the docking of the Wawa subprovince at ca. 2689–2684 Ma is likely responsible for a largely unrecognized regional upright folding and faulting event that controls the dominant structural geometry of the subprovince. Late in its tectonic evolution, strain was partitioned into dextral deformation that was strongly domainal and limited to the subprovince margins.

Evolution of Palaeoproterozoic mafic intrusions located within the thermal aureole of the Sudbury Igneous Complex, Canada: Isotopic, geochronological and geochemical evidence

Impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth. Age-dating of impact events is critical in correlating cause and effects for these catastrophic processes. The Falconbridge and Drury Township (Twp) intrusions were emplaced at the contact between Neoarchaean basement and Palaeoproterozoic volcanosedimentary rocks, and also lie at the southeast and southwest edges of the Sudbury Igneous Complex (SIC), within its thermal contact aureole. The Falconbridge Twp intrusion is dated at 2441 ± 3 Ma by U-Pb zircon, with evidence of Archaean inheritance from its host granitoids. Granitoids from the southernmost Abitibi Subprovince are dated here between 2670 ± 11 Ma for an undeformed Algoman granite, and 2696 ± 18 Ma for a foliated granitoid, consistent with existing data from the Abitibi Greenstone Belt and from the Wawa Subprovince. Major and trace element geochemical evidence, common-Pb isotopic compositions, and εNd2440 values between 0 and −1 are all consistent with a Palaeoproterozoic origin for the Falconbridge Twp intrusion, and support inclusion in the East Bull Lake-type suite of leucogabbroic plutons and sills. In contrast, the Drury Twp intrusion gives a U-Pb zircon age of 1859 ± 13 Ma, coincident with the date of SIC-emplacement. While the major and trace element compositions are comparable to the Falconbridge data, the Drury displays significant heterogeneity in εNd2440, with values ranging from +3.7 to −0.1, and contains more radiogenic Pb isotopic compositions. Field, geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the SIC itself, and indicates that the Drury too is a Palaeoproterozoic intrusion. This requires that apparently unshocked, undeformed magmatic-looking zircon has been grown or reset in a postmagmatic setting. This has significant implications for the identification of mantle-derived magmas and crustal remelts associated with large impact craters. A resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen-complexing is proposed, inducing rapid, postshock lead loss and subsequent annealing.

Archean calc-alkaline lamprophyres of Wawa, Ontario, Canada: Unconventional diamondiferous volcaniclastic rocks

Unusual diamondiferous rocks are found in the Wawa subprovince of the Southern Superior Craton. They are dated at 2.67–2.7 Ga and comprise part of a calc-alkaline volcanic sequence of the Michipicoten Greenstone Belt. Detailed mapping of an ∼40 km 2 area showed that the rocks are metamorphosed polymict volcaniclastic breccia (PVB) and lamprophyre. The breccia occurs as thick, 60–110 m conformable beds traceable in intermittent outcrops along strike for more than 4 km, whereas younger lamprophyre occurs as 0.5–3 m dykes. Magmatic predecessors for the metavolcanic rocks were determined on the basis of detailed mineralogical and petrographic observations, and are found to be calc-alkaline lamprophyres. The only preserved magmatic phenocryst phase is coarse, oscillatory-zoned amphibole of edenitic and pargasitic compositions. The parent magmas are similar in bulk composition to that of Abitibi lamprophyres and other Archean calc-alkaline lamprophyres, and may have thus also contained phenocrystal clinopyroxene and phlogopite. The Wawa lamprophyric magmas formed contemporaneously with felsic to mafic volcanic rocks and late orogenic intrusives of cycle 3 of the Michipicoten Greenstone Belt. They were emplaced episodically in local extensional areas, in an active Archean subduction zone. The breccia formed as a volcaniclastic deposit and contains fragments of pyroclastic lapilli and juvenile material. Stratigraphy, a wide range in clast lithologies, poor sorting and paucity of sedimentary structures suggests the breccia formed in a debris flow. The Wawa diamondiferous rocks may be ancient equivalents of modern lamprophyric cinder cones and demonstrably associated epiclastic deposits.

A structural reappraisal of the Beardmore–Geraldton Belt at the southern boundary of the Wabigoon subprovince, Ontario, and implications for gold mineralization

The Beardmore–Geraldton Belt occurs along the southern margin of the Archean Wabigoon subprovince, Superior Province, Ontario. The belt consists of shear-bounded interleaved metasedimentary and metavolcanic units. The units were imbricated from 2696 to 2691 Ma during D1 thrusting and accretion of the Wabigoon, Quetico, and Wawa subprovinces. Post-accretion D2 deformation produced regional F2 folds that transposed lithological units parallel to the axial plane S2 cleavage of the folds. During D3 deformation, the folds were overprinted by a regional S3 cleavage oriented anticlockwise of F2 axial planes, and lithological contacts and S2 cleavage were reactivated as planes of shear within dextral regional shear zones that generally conform to the trend of the belt. D3 is a regional dextral transpression event that also affected the Quetico and Wawa subprovinces, south of the Beardmore–Geraldton Belt. Gold mineralization at the Leitch and MacLeod-Cockshutt mines, the two richest past-producing gold mines in the Beardmore–Geraldton Belt, is associated with D3 shear zones and folds, overprinting regional F2 folds. The plunge of the ore zones is parallel to F3 fold axes and to the intersection of D3 shear zones with F2 and F3 folds.

Autochthonous development of Superior Province greenstone belts?

Models for Superior Province greenstone belt development have changed over the last 30 years from ensimatic, generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm. Models intermediate between these end-members such as autochthonous terrane models have not been prominent. Within the Superior Province, there is a secular variation from early ca. 3 Ga platformal assemblages with overlying rift-related komatiite–tholeiite units representing breakup of ca. 3 Ga continental fragments to late (ca. 2.7 Ga) pull-apart basins. Autochthonous terranes should demonstrate some sort of logical order such as that described above, whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order. In the face of this situation, this paper reviews evidence from throughout the Superior Province that suggests many, but not all greenstone assemblages developed autochthonously or parautochthonously. The earliest greenstone assemblages in the Superior Province are generally quartzite and carbonate-bearing platform sequences in the North Caribou terrane and the Marmion terrane deposited unconformably upon granitoid or greenstone basement. The evidence for autochthonous/parautochthonous development includes the presence of unconformities between Keewatin type greenstone units such as: (1) basal unconformities between ca. 3 Ga granitoid or volcanic basement and slightly younger platformal assemblages; (2) between ca. 2.9 Ga platformal assemblages and younger platformal and Andean arc assemblages. Stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern, central and southern Superior Province are considered suggestive of orderly, autochthonous progression from platforms through rifting of continental fragments and late assembly during the Kenoran orogeny. Petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the Superior Province suggests the presence of older basement, implying autochthonous or parautochthonous development of many assemblages. Younger greenstone assemblages in the Abitibi–Wawa subprovince and the North Caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages. This pattern has been interpreted to represent autochthonous greenstone development. The current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect; few actual unconformities between Keewatin type greenstone assemblages have been recognized in the Superior Province. To aid in future recognition of unconformities in Keewatin type greenstone assemblages, examples of unconformities of varying degrees of subtlety in Archean sequences are reviewed. Archean unconformities ranging from sharp angular discordances in the northern Superior Province to subtle stratigraphic patterns, to weathering-related geochemical mobilization along inter-formational contacts are described. These styles of unconformity in Archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between Keewatin type assemblages in the Superior Province generally and the Abitibi–Wawa subprovince specifically. The most important disconformity suggested in the Abitibi–Wawa subprovince is between the ca. 2726 Ma Deloro and the ca. 2708 Ma Tisdale assemblages. The presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an ‘autochthonous’ origin for many Superior Province greenstone belts. This ‘autochthonous’ origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place. That is, development of ca. 3 Ga platform sequences in selected locales followed by in place evolution of a succession of plume, oceanic and arc assemblages culminating in late pull-apart basin sequences. Implications of the autochthonous model for greenstone belts of the Superior Province, particularly the Abitibi–Wawa subprovince, include the future ability to recognize unconformity bounded sequences, potentially improving regional scale correlation.