Abstract The Abitibi greenstone belt is part of the Abitibi-Wawa terrane, one of the world's largest, best-exposed, and most richly mineralized Archean greenstone belts, containing world-class orogenic lode Au deposits (e.g., Timmins, Kirkland Lake, Val d'Or), world-class Cu-Zn VMS deposits (e.g., Kidd Creek, Noranda, La Ronde Bousquet), and significant Ni-Cu-(PGE) mineralization (e.g., Dumont, Shebandowan). It is one of the places where skeletal olivine "chicken-track" (now known as spinifex) texture was first described, and where the first Ni-Cu-(PGE) deposits (Alexo, Shebandowan) associated with what are now known to be komatiites were discovered. The Abitibi greenstone belt has a long history of exploration and mining of Ni-Cu-(PGE), with several periods of extensive exploration and discovery, including a major renewal in the past decade. Komatiites occur sporadically throughout the Superior province of the Canadian Shield, but appear to be most abundant in the ~2.7 Ga Abitibi greenstone belt, which contains the classic exposures at Alexo (Dundonald township), Pyke Hill (Munro township), and Spinifex Ridge (La Motte township). Komatiites typically represent only 2 to 10 percent of the volcanic rocks in the Abitibi greenstone belt, and have been identified thus far within three end-member lithostratigraphic associations: (1) bimodal komatiite-komatiitic basalt sequences, (2) bimodal komatiite-basalt sequences, and (3) bimodal komatiite-rhyolite-dacite-andesite sequences. High-precision U-Pb TIMS zircon geochronology indicates that komatiites occur mainly within four major volcanic episodes (2760–2735, 2723–2720, 2720–2710, and 2710–2704 Ma), but the two youngest host almost the entire Ni-Cu-(PGE) endowment of the belt. Although the komatiite-associated Ni-Cu-(PGE) mineralization in the Cape Smith belt in New Quebec, Thompson nickel belt in Manitoba, Wiluna-Norseman belt in Western Australia, and the Zimbabwe craton appears to occur at fairly specific stratigraphic levels, mineralization in the Abitibi greenstone belt occurs at multiple levels of single komatiitic volcanic-subvolcanic edifices. Although most of the komatiites in the Abitibi greenstone belt have been previously considered to be extrusive, increasing numbers of units have been shown to be intrusive and it now appears that komatiite-associated Ni-Cu-(PGE) mineralization occurs within a spectrum of environments ranging from intrusive (e.g., Dumont, Sothman) through subvolcanic (e.g., Dundonald South, McWatters) to extrusive (e.g., Alexo, Hart, Langmuir, Redstone). Komatiite-associated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt, regardless of volcanic setting, are similar to other deposits of this type in that most contain type I basal stratiform, type II internal disseminated, and less common type IV sedimenthosted mineralization; most are hosted by relatively undifferentiated olivine mesocumulate cumulate units that normally have very distinctive geophysical-geochemical signatures and that have been interpreted as lava channels, subvolcanic sills, or feeder dikes; most are associated with S-rich country rocks; most are localized in foot-wall embayments; and most exhibit evidence of magma-wall rock interaction (e.g., xenoliths, geochemical contamination) during emplacement, consistent with them having formed in dynamic systems. However, the deposits in the Abitibi greenstone belt differ from other deposits of this type in commonly occurring at multiple stratigraphic levels, and several occur within highly differentiated komatiitic units (Dumont, Dundeal) and one (Bannockburn C zone) is hosted by heterolithic breccias. Geochemical studies indicate that regardless of age or petrogenetic affinity (Al undepleted vs. Al depleted vs. Ti enriched vs. Fe rich), almost all of the parental magmas were undersaturated in sulfide prior to emplacement and therefore represent favorable magma sources for Ni-Cu-(PGE) mineralization. Volcanological studies indicate that the physical volcanology—in particular, the degree of lava-magma channelization—one of the most critical factors in ore genesis. The smaller sizes of the deposits in the Abitibi greenstone belt compared to Western Australia, Thompson, or Raglan is attributed to a more juvenile tectonic setting and lower density of continental crust. The more complex volcanic-subvolcanic architecture within the Abitibi reflects the variability of the near-surface rocks within each volcanic episode and makes it more difficult to predict the location of mineralized lava channels and channelized sheet flows and sills within different komatiitic-bearing successions. However, targeting Ni-Cu-(PGE) mineralization within those environments still relies on identifying areas of high magmatic flux within deformed and metamorphosed greenstone belts, requiring an under-standing of the physical volcanology of magma-lava pathways and their geophysical-geochemical signatures. One of the most important implications, however, is that contrary to previous interpretations, Ni-Cu-(PGE) mineralization is not restricted to specific stratigraphic contacts, but may occur in any environment throughout the stratigraphy where lava pathways have had access to external S. Increased understanding of the volcanology and stratigraphy of komatiites coupled with recent discoveries (e.g., Bannockburn C zone, Langmuir W4) highlight the potential of finding new Ni-Cu-(PGE) deposits associated with komatiites in both less-explored and also more-explored camps within the Abitibi-Wawa terrane. Furthermore, the recognition of similar subvolcanic-volcanic architectures within other komatiite-bearing greenstone belts of the Canadian Shield points to the need to assess their economic potential in the light of this new knowledge gained about the komatiites in the Abitibi greenstone belt.

Abstract One hundred and five lead isotope analyses have been obtained for 47 volcanogenic massive sulfide deposits and occurrences within the Abitibi and Wawa subprovinces, largely from minor amounts of galena that form an integral part of the deposits. When 52 nonanomalous compositions for the best dated of these deposits are recalculated to a uniform age of 2716 Ma, they define the Abitibi-Wawa linear array at a slope of 1.034 ± 0.062. Two-stage calculation yields an age of 4490 ± 35 Ma for the Pb isotope sources responsible for this linear array if they each subsequently evolved in a single-stage manner from a source that was homogeneous prior to about 4490 Ma. If the assumption of single-stage evolution is valid, 4490 ± 35 Ma could represent the time of a major mantle differentiation event. The data indicate that the lead in deposits for a distance of at least 710 km from Matagami, Quebec, to the Winston Lake deposit in the Terrace Bay-Schreiber area of Lake Superior, Ontario, was derived from equivalent sources. This evidence helps confirm that the Abitibi and Wawa subprovinces, contiguous except for the Kapuskasing tectonic zone, had, at least in some respects, a common evolution history. Further study will be required to show that the Shebandowan subprovince west of Lake Superior, considered an extension of the Wawa subprovince, shared this isotopic history. The Pb isotope data for deposits in the Wabigoon and Uchi subprovinces of western Superior province, which lie to the north of the Wawa-Shebandowan belts, indicate strikingly different isotopic sources that were more isotopically evolved at a given age. The use of data for older deposits elsewhere in the world permits initial isotopic parameters (A o = 9.431, B o = 10.495, C o = 29.681) to be established for the Abitibi-Wawa model at 4490 Ma. In contrast to the Wabigoon-Uchi model (formerly referred to as the “western Superior model”), the Abitibi-Wawa model is not widely applicable elsewhere in the world. However, it does yield congruent model ages for some volcanogenic massive sulfide deposits in the Yilgarn craton, Western Australia, which shows that it has validity beyond the Abitibi-Wawa greenstone belts. Analyses for deposits of the Manitouwadge district, Ontario, and for an Archean deposit within the Grenville province east of Val d’Or, Quebec, illustrate the minor influence of high-grade, upper amphi-bolite to granulite metamorphism on Pb isotope compositions. Anomalous compositions were obtained for galenas from veins and for most low lead sulfides, such as pyrite and sphalerite. These indicate that a Grenvillian remobilization or isotopic resetting occurred throughout the Abitibi subprovince, although the character of such an event in the Matagami district, 190 km from the Grenville front, is very obscure. Other older events are also indicated by the data.