Robert Kerrich

P—T—t—deformation—fluid characteristics of lode gold deposits: evidence from alteration systematics

Structurally hosted lode gold-bearing quartz vein systems in metamorphic terranes possess many characteristics in common, spatially and through time; they constitute a single class of epigenetic precious metal deposit, formed during accretionary tectonics or delamination. The ore and alteration paragenesis encode numerous intensive and extensive variables that constrain the pressure—temperature—time—deformation—fluid (P—T—t—d—f) evolution of the host terrane and hence the origin of the deposits. The majority of lode gold deposits formed proximal to regional translithospheric terrane—boundary structures that acted as vertically extensive hydrothermal plumbing systems; the structures record variably thrust, and transpressional—transtensional displacements. Major mining camps are sited near deflections, strike slip or thrust duplexes, or dilational jogs on the major structures. In detail most deposits are sited in second or third order splays, or fault intersections, that define domains of low mean stress and correspondingly high fluid fluxes. Accordingly, the mineralization and associated alteration is most intense in these flanking domains. The largest lode gold mining camps are in terranes at greenschist facies; they possess greenschist facies hydrothermal alteration assemblages developed in cyclic ductile to brittle deformation that reflects interseismic—coseismic cycles. Interseismic episodes involve the development of ductile S—C shear zone fabrics that lead to strain softening. Pressure solution and dislocation glide microstructures signify low differential stress, slow strain rates of ≤ 10−13 s−1, relatively high confining stress, and suprahydrostatic fluid pressures. Seismic episodes are induced by buildup of fluid pressures to supralithostatic levels that induce hydraulic fracturing with enhanced hydraulic conductivity, accompanied by massive fluid flow that in turn generates mineralized quartz veins. Hydrothermal cementing of ductile fabrics creates ‘hardening’, lowers hydraulic conductivity, and hence promotes fault valve behaviour. Repeated interseismic (fault valve closed), coseismic (valve open) cycles results in banded and/or progressively deformed veins. Alteration during both interseismic and coseismic episodes typically involves the hydrolysis of metamorphic feldspars and Fe, Mg, Ca-silicates to a muscovite/paragonite—chlorite ± albite/K-feldspars assemblage; carbonization of the metamorphic minerals to Ca, Fe, Mg-carbonates; and sulphidation of Fe-silicates and oxides to sulphides. Geochemically this is expressed as additions of K, Rb, Ba, Cs, and the volatiles H2O, CO2, CH4, H2S in envelopes of meter to kilometer scale. K/Rb and K/Ba ratios are close to average crustal values, potentially ruling out late stage magmatic fluids where K/Rb and K/Ba are respectively lower and higher than crustal values. Smaller deposits are present in subgreenschist, and amphibolite to granulite facies terranes. The former are characterized by subgreenschist facies alteration assemblages, vein stockworks, brittle fracturing and cataclastic microstructures, whereas the latter feature amphibolite to granulite facies alteration assemblages, ductile shear zones, ductilely deformed veins, and microstructures indicative of dislocation climb during interseismic episodes. Hence the lode gold deposits constitute a crustal continuum of deposits from subgreenschist to granulite facies, that all formed synkinematically in broad thermal and rheological equilibrium with their host terranes. These characteristics, combined with the low variance of alteration assemblages in the higher temperature deposits, rules out those being metamorphosed counterparts of greenschist facies deposits. Deposits at all grades have a comparable metal inventory with high concentrations of Au and Ag, where Au/Ag averages 5, with enrichments of a suite of rare metals and semi-metals (As, Sb, ± Se, Te, Bi, W, Mo and B), but low enrichments of the base (Cu, Pb, Zn, Cd) and other transition (Cr, Ni, Co, V, PGE, Sc) metals relative to average crust. The hydrothermal ore-forming fluids were dilute, aqueous, carbonic fluids, with salinities generally ≤ 3 wt.% NaCl equivalent, and X(CO2 ± CH4) 10–24 wt.%. They possess low Cl but relatively high S, possibly reflecting the fact that metamorphic fluids are generated in crust with ∼ 200 ppm Cl, but ∼ 1 wt.%S. Primary fluid inclusions are: (1) H2OCO2, (2) CO2-rich with variable CH4 and small amounts of H2O, and (3) 2-phase H2O (liquid-vapor) inclusions. Inclusion types 2 and 3 represent immiscibility of the type 1 original ore fluid. Immiscibility was triggered by fluid pressure drop during the coseismic events and possibly by shock nucleation, leading to highly variably homogenization temperatures in an isothermal system. A thermodynamic evaluation of alteration assemblages constrains the ore fluid pH to 5–6; redox controlled by the HSO4/H2S and CO2/CH4 buffers; and XCO2 that varies. The higher temperature deposits formed under marginally more oxidizing conditions. Stable isotope systematics of the ore and gangue minerals yields temperatures of 200–420°C, consistent with the crustal spectrum of the deposits, very high fluid rock ratios, and disequilibrium of the externally derived ore fluids with wall rocks. The ore fluid δD and δ18O overlap the metamorphic and magmatic ranges, but the total dataset for all deposits is consistent only with dominantly metamorphic fluids. Carbon isotope compositions of carbonates span −11 to +2% and show provinciality: this is consistent with variable proportions of reduced C (low δ13C) and oxidized C (higher δ13C) in the source regions contributing CO2 and CH4 to the ore fluids. In some instances, C appears to have been derived dominantly from proximal to the deposits, as in the Meguma terrane (δ13C ∼ − 22%). Sulphur isotope compositions range from 0 to +9‰, and are consistent with magmatic S, dissolution or desulphidation of magmatic sulphides, or average crustal sulphides. 34S-depleted sulphides occur in ore bodies such as Hemlo where fluid immiscibility led to loss of H2S and consequent fluid oxidation. Gold is probably transported as an Au(HS)−2 complex. Relatively high S but low Cl in the hydrothermal fluid may explain the high Au slow base metal characteristic of the deposits. Gold precipitated in ore bodies due to loss of S from the ore fluid by sulphidation of wall rock, or immiscibility of H2S; and by oxidation or reduction of the fluid, or by chemisorption, or some combination of these processes. Most lode gold deposits have been brittly reactivated during uplift of host terranes, with secondary brines or meteoric water advecting through the structures. These secondary fluids may remobilize gold, generate retrograde stable isotope shifts, reset mineral geochronometers, and leave trails of secondary fluid inclusions. Data on disturbed minerals has led to invalid models for lode gold deposits. The sum of alteration data leads to a model for lode gold deposits involving a clockwise P—T—t evolution and synkinematic and synmetamorphic mineralization of the ‘deep later’ type. During terrane accretion oceanic crust and sediments are subcreted beneath the terrane boundary. Thermal equilibration generates metamorphic fluids that advect up the terrane structure, at lithostatic fluid pressure, into the seismogenic zone where the majority of deposits form. Thus many lode gold deposits are on intrinsic part of the development of subduction—accretion complexes of the high-T, low-P type.

Geochemistry of fine grained clastic sediments in the Archean Abitibi greenstone belt, Canada : Implications for provenance and tectonic setting

Abstract Archean sandstone and shale samples from turbiditic (TF) and alluvial facies (AF) in the Southern Volcanic Zone of the Abitibi greenstone belt, Canada, have been analyzed for major and trace elements. The sandstones from TF are mainly greywackes, whereas those from AF are lithic arenite. SiO 2 ranges from 51 to 68%. Geochemically, the clastic sediments cluster into mafic element (MEP) and low mafic element (LMEP) populations. The MEP sediments, which include turbiditic facies sediment samples from the Larder Lake Group in the Kirkland Lake area, are characterized by elevated abundances of MgO (>5%), Ni (>200 ppm), Cr (>400 ppm), Sc (>22 ppm), and FeO (>7.5%) abundances; lower contents of incompatible elements, such as Th, U, Ta, Hf, Zr, and total REEs ( N ratios are ≤4.5, with Eu/Eu* ≥ 1. The LMEP sediments (which include samples from the Beatty, Dome, and Three Nations Lake formations in Timmins, the Kewagama Group, Noranda area, and Timiskaming Group in the Kirkland Lake area), however, show reverse abundance distributions. The geochemical gaps shown by the major and trace elements define two populations of sediments, suggesting contrasting provenances for them: the MEP sediments require a dominantly mafic and ultramafic (90%) source with 10% felsic contribution, whereas the LMEP sediments require a mixture of felsic (75%) and mafic (25%) rocks. This compositional distribution of the sediments is closely related to their tectonic environments. The MEP was formed in basins proximal to or within an active oceanic rift, consequently having ultramafic and mafic rocks as the major source. For the LMEP, calc-alkaline volcanic sequences formed in an island arc-like setting were the major source. These two contrasting volcanic-plutonic terranes were tectonically juxtaposed during late Archean collisional tectonics.

Geochemistry of Neoarchean (ca. 2.55-2.50 Ga) volcanic and ophiolitic rocks in the Wutaishan greenstone belt, central orogenic belt, North China craton : Implications for geodynamic setting and continental growth

Geological investigation of the Neoarchean (2.55–2.50 Ga) Wutaishan greenstone belt in the central orogenic belt of the North China craton has provided new information on the geodynamic origin of this belt and its mineral deposits. Structural, geochronological, and geochemical characteristics of the Wutaishan greenstone belt suggest that it formed in a forearc tectonic environment at ca. 2.55 Ga and accreted to the Eastern continental block at ca. 2.50 Ga. A ridge subduction model is proposed to explain several unique geological features of the Wutaishan greenstone belt, such as the generation of dunites and chromitite-hosting harzburgites with U-shaped rare earth element (REE) patterns, formation of volcanogenic massive sulphides (VMS) and banded iron formations (BIF), extrusion of mafic to felsic volcanic rocks, and intrusion of tonalite-trondhjemite-granodiorite plutons (TTG). Anomalously high geothermal gradients in the subarc mantle-wedge beneath the Wutaishan forearc may have increased its buoyancy, resulting in its accretion to the continental crust. We propose that ridge subduction also played an important role in the growth of Archean continental crust. In this model, the origin of Archean TTG is genetically linked to eclogites through partial melting of accreted and/or underplated oceanic plateaus and normal oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of an anomalously hot asthenospheric mantle window resulting from ridge subduction. TTG suites intruding Archean accretionary complexes formed the nuclei of intra-oceanic island arcs; subsequent juxtaposition of these arcs resulted in the lateral growth of Archean continental crust.

Boninite series: low Ti-tholeiite associations from the 2.7 Ga Abitibi greenstone belt

Abstract Boninite series volcanic flows, interfingered with komatiites and tholeiitic basalts, occur at several localities in the ∼2.7 Ga Abitibi greenstone belt. Flows from Whitney Township, Ontario, have a compositional range of SiO2 44–60 wt%, MgO 24–7.4, Mg# 83–69, and Ni 930–200 ppm. Low TiO2 (0.14–0.31 wt%) but high Al2O3 (13–25 wt%) contents yield variably high Al2O3/TiO2 ratios of 48–100. These boninite series volcanics are characterized by fractionated HREE where Gd/Ybn 0.3–0.7; positive normalized Zr(Hf)/MREE anomalies, and Zr/Hf > 36; generally negative normalized Nb anomalies; and LREE depletion to enrichment (La/Smn 0.72–1.4). Flows with similar compositional affinities occur in the neighbouring Kidd Volcanic Complex and Tisdale volcanic group. Alteration, and/or contamination by continental crust can be ruled out as the cause of the distinctive and coherent compositions. If the areally extensive komatiite–tholeiite association represents an ocean plateau derived from a mantle plume and the boninite series formed in a convergent margin, then the interfingering of komatiite and boninite series flows may represent interaction of a plume with a subduction zone.

Assembly of Archean cratonic mantle lithosphere and crust: plume–arc interaction in the Abitibi–Wawa subduction–accretion complex

Abstract Recent thermodynamic models suggest that direct interaction between mantle plumes and island arcs will enhance long-term arc buoyancy and contribute disproportionately to the crustal record. However, crustal growth models have also proposed a range of differing mechanisms for Archean crust formation that emphasize specific rock types, such as tonalite–trondhjemite–granodiorite (TTG) plutons or high magnesian andesites. Studies in the Abitibi–Wawa subprovince, allow these proposals to be evaluated in the context of the worlds largest greenstone belt. Crustal growth in the southern Superior Province was the product of subduction–accretion tectonics enhanced and modified by mantle plume processes, particularly mantle plume–island arc interaction. High Archean geothermal gradients promoted volumetrically minor slab melts from the earliest phases of the Abitibi–Wawa arc, resulting in the adakite–high magnesium andesite–Niobium–enriched basalt association. However, recent flat subduction models for the formation of adakites also provide important insights into the generation of syn-tectonic Archean TTG batholiths that were probably derived from subducted, rather than accreted, oceanic crust. The distribution of Niobium enriched basalts (NEB) in the southern Superior Province may reflect plume controlled shallow subduction beneath the Abitibi belt that limited melting depths within a restricted asthensopheric mantle tongue. A well-constrained tectonic history, the mantle source requirements of successively-formed components of the Abitibi–Wawa crust, and detailed seismic interpretations of crustal architecture all preclude the presence of an autochthonous mantle lithospheric root beneath the Abitibi–Wawa arc. Instead, the late diapiric ascent of buoyant refractory plume residue and subducted oceanic crust resulted in the coupling of the mantle root and arc crust 10s of million years following batholith emplacement. High-velocity material identified at the base of the crust and centered beneath the Abitibi–Pontiac suture zone is probably Archean aged rather than Proterozoic. It corresponds to minor melts generated during ascent of the plume residue diapir and underplated prior to and during formation of the >200 km thick Abitibi–Wawa continental mantle lithosphere root.

Controls on platinum-group elemental distributions of podiform chromitites : A case study of high-Cr and high-Al chromitites from Chinese orogenic belts

Abstract A study of podiform chromite deposits from the Asiatic Orogenic Belt and the Qilian-Qiangling-Kunlun-Himalaya Tectonic Domain provides new insights into the geochemistry of the PGEs in podiform chromite deposits and the genesis of the deposits themselves. The bulk of deposits, which occur in mantle peridotites of ophiolites, have typical ophiolitic PGE patterns that are depleted in Pt and Pd relative to the average upper mantle and have negatively sloping distributions on mantle-normalized diagrams. Type I (high-Cr) chromitites have higher Os, Ir, Ru, and Rh contents than Type II (high-Al) chromitites, although both have similar Pd and Pt. Most of the Type I and II chromite deposits have lower Pd and Pt contents than the upper mantle peridotites in which they occur. Podiform chromitites are essentially products of melt/rock interaction in the upper mantle; their Cr and PGEs were contributed by not only the invading magmas but also by the upper mantle host; the chromite deposits are, in part, metasomatic replacement bodies. The Type I (high-Cr) chromitite PGE patterns were produced by interaction between S-undersaturated boninitic magmas and depleted harzburgites, whereas the Type II (high-Al) chromitite PGE patterns were formed by interaction between initially S-saturated tholeiitic magmas and depleted harzburgites. The low to very low Pd and Pt contents of both Type I and Type II chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides, and hence Pd and Pt, prior to formation of the chromite deposits; in addition, no or little Pd and Pt were deposited by the invading magma which either remained S-undersaturated (boninite) or became (MORB) S-undersaturated due to interaction with the S-depleted harzburgitic mantle. It is suggested that the very low Ir, Os, and Ru contents of boninites in general might be due to loss of Ir during the formation of podiform chromitites. It is suggested that podiform chromitites with Type I PGE patterns were formed in an island arc environment, whereas those with Type II PGE patterns were formed in a back-arc setting.

HFSE/REE fractionations recorded in three komatiite-basalt sequences, Archean Abitibi greenstone belt : Implications for multiple plume sources and depths

Abstract Komatiite-tholeiite sequences (2710–2725 Ma) in the Archean Abitibi greenstone belt show systematic differences of rare earth element (REE) patterns and high field strength element (HFSE)/ REE fractionations between volcanic centres. Type 1 komatiites are Al undepleted, and have flat REE patterns, with mantle normalized Nb, Zr, Hf/REE ≈ 1. Type 2 komatiites are relatively MgO rich (16–24 wt%), Al undepleted, with LREE depletion and positive normalized Nb, Zr, Hf/REE fractionations. Type 3 komatiites are Al depleted, Fe rich, HREE depleted, and feature negative normalized Hf, Zr/ REE fractionations. The three types of komatiites may have formed in plumes originating at progressively greater depths: type 1 in undepleted mantle at 700 km, that subsequently migrated to shallower depth and melted at Magnesium tholeiites associated with type 1 and 3 komatiites are likely the fractionation products of komatiites at low and high pressures, respectively, whereas type 2 Mg tholeiites originated in an undepleted source relative to the coexisting komatiites with “perovskite signature.” Early greenstone belt evolution was characterized by numerous oceanic plateaus developed from multiple plumes originating at different depths from heterogeneous mantle; the plateaus may have been preferentially obducted.

Geodynamic setting of mesothermal gold deposits: An association with accretionary tectonic regimes

Mesothermal gold provinces of Phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs. A recurring sequence of transpressive deformation, uplift, late kinematic mineralization, and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust. Mesothermal gold camps in the Superior province are spatially associated with large-scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes: these boundary structures are characterized by the sequence of significant horizontal shortening, uplift, late-kinematic mineralization, and shoshonitic lamprophyres and therefore may have the same geodynamic significance as Phanerozoic counterparts. In this model, thermal re-equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime, over time scales of 10 to 40 m.y., is a necessary precursor to gold mineralization. Hydrothermal fluids are released along boundary faults and their splays during uplift: the uniform temperature, low salinity and mole% CO2 signify uniform source conditions, whereas the variable O, C, Sr, and Pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits. Ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction-related crustal underplating and deep, late metamorphism, rather than magmatic or metamorphic events in the supracrustal rocks. Secular variations in the generation of Archean, Proterozoic, and Phanerozoic mesothermal Au provinces reflect the timing of collisional orogenies within terranes of these eras.

The late Archean Schreiber-Hemlo and White River-Dayohessarah greenstone belts, Superior Province: collages of oceanic plateaus, oceanic arcs, and subduction-accretion complexes

The late Archean (ca. 2.80–2.68 Ga) Schreiber–Hemlo and White River–Dayohessarah greenstone belts of the Superior Province, Canada, are supracrustal lithotectonic assemblages of ultramafic to tholeiitic basalt ocean plateau sequences, and tholeiitic to calc-alkaline volcanic arc sequences, and siliciclastic turbidites, collectively intruded by arc granitoids. The belts have undergone three major phases of deformation; two probably prior to, and one during the assembly of the southern Superior Province. Imbricated lithotectonic assemblages are often disrupted by syn-accretion strike-slip faults, suggesting that strike-slip faulting was an important aspect of greenstone belt evolution. Field relations, structural characteristics, and high-precision ICP–MS trace-element data obtained for representative lithologies of the Schreiber–Hemlo and White River–Dayohessarah greenstone belts suggest that they represent collages of oceanic plateaus, juvenile oceanic island arcs, in subduction–accretion complexes. Stratigraphic relationships, structural, and geochemical data from these Archean greenstone belts are consistent with a geodynamic evolution commencing with the initiation of a subduction zone at the margins of an oceanic plateau, similar to the modern Caribbean oceanic plateau and surrounding subduction–accretion complexes. All supracrustal assemblages include both ocean plateau and island-arc geochemical characteristics. The structural and geochemical characteristics of vertically and laterally dismembered supracrustal units of the Schreiber–Hemlo and White River–Dayohessarah greenstone belts cannot be explained either by a simple tectonic juxtaposition of lithotectonic assemblages with stratified volcanic and sedimentary units, or cyclic mafic to felsic bimodal volcanism models. A combination of out-of-sequence thrusting, and orogen-parallel strike-slip faulting of accreted ocean plateaus, oceanic arcs, and trench turbidites can account for the geological and geochemical characteristics of these greenstone belts. Following accretion, all supracrustal assemblages were multiply intruded by syn- to post-tectonic high-Al, and high-La/Ybn slab-derived trondhjemite–tonalite–granodiorite (TTG) plutons. The amalgamation processes of these lithotectonic assemblages are comparable to those of Phanerozoic subduction–accretion complexes, such as the Circum-Pacific, the western North American Cordilleran, and the Altaid orogenic belts, suggesting that subduction–accretion processes significantly contributed to the growth of the continental crust in the late Archean. The absence of blueschist and eclogite facies metamorphic rocks in Archean subduction–accretion complexes may be attributed to elevated thermal gradients and shallow-angle subduction. The melting of a hotter Archean mantle at ridges and in plumes would generate relatively small, hot, and hence shallowly subducting oceanic plates, promoting high-temperature metamorphism, migmatization, and slab melting. Larger, colder, Phanerozoic plates typically subduct at a steeper angle, generating high-pressure low-temperature conditions for blueschists and eclogites in the subduction zones, and low-La/Ybn granitoids from slab dehydration, and wedge melting. Metasedimentary subprovinces in the Superior Province, such as the Quetico and English River Subprovinces, have formerly been interpreted as accretionary complexes, outboard of the greenstone belt magmatic arcs. Here the greenstone–granitoid subprovinces are interpreted as collages of subduction–accretion complexes, island arcs and oceanic plateaus amalgamated at convergent plate margins, and the neighbouring metasedimentary subprovinces as foreland basins.

Geochemical characteristics of aluminum depleted and undepleted komatiites and HREE-enriched low-Ti tholeiites, western Abitibi greenstone belt: A heterogeneous mantle plume-convergent margin environment

A compositionally diverse suite of komatiites, komatiitic basalts, and basalts coexist in the Tisdale volcanic assemblage of the late-Archean (∼2.7 Ga) Abitibi greenstone belt. The komatiites are characterized by a spectrum of REE patterns, from low total REE contents (9 ppm) and pronounced convex-up patterns to greater total REE (18 ppm) and approximately flat-distributions. Thorium and niobium are codepleted with LREE. Komatiites with the most convex-up patterns have low Al2O3 (4.7 wt%) contents and Al2O3/TiO2(12) ratios; they are interpreted to be the Al-depleted variety of komatiite derived from a depleted mantle source. Those komatiites and komatiitic basalts with flatter REE patterns are characterized by greater Al2O3 (7.0 wt%) and near chondritic Al2O3/TiO2 (20) ratios; they are interpreted to be Al-undepleted komatiites generated from trace element undepleted mantle. For the komatiites and komatiitic basalts collectively, ratios are negatively correlated with , but positively with MgO and Ni. The spectrum of patterns is interpreted as mixing between Al, HREE, Y-depleted, and Sc-depleted komatiites and Al-undepleted komatiites in a heterogeneous mantle plume. Auminum-depleted komatiites are characterized by negative Zr and Hf anomalies, consistent with majorite garnet-liquid Ds for HFSE and REEs, signifying melt segregation at depths of >400 km. Tisdale Al-undepleted komatiites and komatiitic basalts have small negative to zero Zr(Hf)/MREE fractionation, signifying melt segregation in or above the garnet stability field. Collectively, the komatiites have correlations of and Hf/Hf∗ with , and hence the Zr(Hf)/MREE fractionations are unlikely to have stemmed from alteration or crustal contamination. Two types of basalts are present. Type I basalts are Mg-tholeiites with near flat REE and primitive mantle normalized patterns, compositionally similar to abundant Mg-tholeiites associated with both Al-undepleted and Al-depleted komatiites in the Abitibi belt. They have absolute concentrations and ratios of most moderately and highly compatible elements comparable to N- MORB (Zr ∼79 vs. 74, Y ∼30 vs. 28, and ), but are relatively less depleted in highly incompatible elements and lack positive Nb or P anomalies. Type II basalts are relatively aluminous (Al2O3 ∼ 16 wt%), with high Al2O3/TiO2 (24–28) ratios. They are characterized by low Th, Nb, and LREE contents at eight to ten times chondrite, with slightly convex-up LREE patterns (), but strongly fractionated and enriched HREEs, Y, and Sc, where and consistently positive Zr(Hf)/MREEs anomalies. These basalts are tentatively interpreted as low-Ti tholeiites formed in a convergent margin setting with second stage melting, induced by fluids and melts enriched in incompatible elements and Zr(Hf) relative to MREEs, of a mantle source depleted during first stage melting. They are analogous to the Phanerozoic low-Ti tholeiite—boninite association. Accordingly the Tisdale volcanic sequence records a plume-convergent margin interaction. New analyses of Al-undepleted komatiites from the classical locality at Pyke Hill in Munro Township confirm the presence of small positive anomalies of P, Zr, and Hf, with ratios generally < 36. These signatures are similar in spinifex and cumulate zones signifying that they are unlikely to have resulted from alteration. The data were generated by INAA and ICP-MS using both HFHNO3 dissolution and Na2O2 sinter. The lack of LREE enrichment with negative Nb, Ta, P, and Ti anomalies in any of the Tisdale or Munro komatiites confirms an intraoceanic setting for the volcanic stage of the Western Abitibi greenstone belt.