Komatiite-Associated Ni-Cu-(PGE) Deposits, Abitibi Greenstone Belt, Superior Province, Canada
M. G. Houlé, C. M. Lesher, 2011. "Komatiite-Associated Ni-Cu-(PGE) Deposits, Abitibi Greenstone Belt, Superior Province, Canada", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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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.
Figures & Tables
Magmatic sulfide deposits fall into two major groups when considered on the basis of the value of their contained metals, one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the PGE are the most important. The first group includes komatiite- (both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to Ural-Alaskan–type intrusions. PGE deposits are mostly related to large intrusions comprising both an early MgO- and SiO2-rich magma and a later Al2O3-rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical validity of this observation.
A number of key events mark the development of a magmatic sulfide deposit, partial melting of the mantle, ascent into the crust, development of sulfide immisciblity as a result of crustal interaction, ascent of magma + sulfides to higher crustal levels, concentration of the sulfides, their enrichment through interaction with fresh magma (not always the case), cooling and crystallization. Factors governing this development include (1) the solubility of sulfur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallization, (2) the partitioning of chalcophile metals between sulfide and silicate liquids, and how the results of this vary during mantle melting and subsequent crystallization and sulfide immiscibility (degree of melting and crystallization, R factor and subsequent enrichment), (3) how effectively the sulfides become concentrated and the factors controlling this, and (4) processes that occur during the cooling of the sulfide liquid that govern aspects of exploration and mineral beneficiation. These topics are discussed first in general terms and then with specific reference to deposits at Noril’sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modeling is consistent with the very low PGE concentrations in this deposit being the result of some sulfide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the