Abstract Economically valuable magmatic Ni-Cu sulfide deposits in China include the large (515 million tonnes [Mt] of sulfide ore) Jinchuan deposit and 12 smaller ones (3–100 Mt), including Xiarihamu (100 Mt), Huangshanxi (80 Mt), and Poyi (40 Mt). These deposits occur in two principal tectonic environments: within continental plates and at convergent plate margins. The former group can be further divided into those that are hosted in the feeders of a flood basalt province, such as Limahe, and those that occur in rifted continental margins, such as Jinchuan. The latter group can be further divided into those that formed during active subduction, such as Xiarihamu, and those that formed shortly after subduction (20–40 m.y. later), such as Poyi. Despite different geodynamic settings, the Chinese magmatic Ni-Cu sulfide deposits are all characterized by low tenors of platinum group elements (PGEs), indicating PGE depletions in the parental magmas. The small Jinbaoshan deposit (15 Mt, 3 ppm Pt + Pd) is the only significant magmatic PGE deposit in China. It is hosted in a small sill-like ultramafic intrusion associated with the Permian Emeishan flood basalts in southwestern China. The Chinese magmatic Ni-Cu and Pt-Pd sulfide deposits collectively show a positive correlation between Ni/Cu ratios in sulfide ores and forsterite contents in olivine, indicating that fractional crystallization is an important control on Ni/Cu in the parental magmas. The Os isotope data for these deposits support the premise that addition of external sulfur is essential for the generation of ore-quality magmatic sulfide deposits. More detailed geochronological studies are necessary to detect intrusion targets that may be temporally related to other known ore-bearing intrusions in a given area or region. The recent discovery of the subduction-related Xiarihamu Ni-Cu sulfide deposit is consistent with the idea that convergent plate tectonic settings in the world have potential for world-class magmatic Ni-Cu sulfide deposits. We suggest that the Paleozoic Kunlun orogenic belt in the northern part of the Tibet-Qinghai plateau, where the Xiarihamu deposit is situated, is a new frontier for Ni-Cu exploration in China. Furthermore, the search for economic magmatic Ni-Cu and Pt-Pd sulfide deposits associated with the subvolcanic intrusions of the Permian Emeishan flood basalt province in southwestern China should continue.

Abstract 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 SiO 2 -rich magma and a later Al 2 O 3 -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 ores, which are located in magma conduits, through interaction with fresh, sulfide-unsaturated magma passing along the conduits.

Abstract The Kalgoorlie terrane of the eastern Yilgarn craton is the third largest repository of sulfide nickel ore in the world. The Agnew-Wiluna belt, at the northern end of the Kalgoorlie terrane, contains the bulk of the nickel resource within the province, including the world's two largest known nickel sulfide deposits associated with Archean komatiites, the giant Mount Keith and Perseverance deposits. Both deposits are hosted by lenticular bodies of highly magnesian olivine adcumulates, developed as pods within planar sequences of olivine mesocumulate and orthocumulate rocks. The Perseverance deposit and the satellite Rocky's Reward and Harmony deposits are highly deformed, having been subjected to an early episode of isoclinal folding and associated shearing, resulting in significant mobilization of primary magmatic sulfide ores into axial planar shear zones and subsequently refolding. The bulk of the Perseverance orebody comprises basal accumulation of matrix ores, occupying an arcuate channel feature, with an extensive asymmetric halo of disseminated sulfides. Host rocks display a complex metamorphic history involving multiple episodes of hydration, carbonation, dehydration, decarbonation, and retrograde alteration. The Perseverance Ultramafic Complex is interpreted as a high-flux, flow-through conduit, formed by evolving magmas that became progressively hotter, more primitive, and less Ni depleted with time. There is a pervasive signature of country-rock contamination throughout the complex. The complex is interpreted as either a feeder pathway to a major flow field or a as subvolcanic intrusive conduit; these alternatives are not resolvable given the tectonic overprint. The giant Mount Keith deposit occurs within an extremely olivine rich cumulate unit broadly similar to that at Perseverance but without evidence for flanking flows. On the basis of the presence of apparently crosscutting apophyses in the roof of this unit, and a general absence of spinifex textures, the Mount Keith ultramafic unit is interpreted as an intrusive subvolcanic conduit or chonolith. The degree of penetrative deformation is much less than at Perseverance, but shearing is still evident along contacts. Mineralization is exclusively centrally disposed and disseminated in character and has variable tenors (compositions of the pure sulfide component) spanning the typical range seen in the Kambalda dome deposits. Sulfide mineralogy has been variably modified during hydration and local carbonation of the host rocks, particularly through oxidation of pyrrhotite to magnetite. The mineralogy reflects lower metamorphic grade than at Perseverance and lacks metamorphic olivine. Host-rock geochemistry is broadly similar to Perseverance, although sulfide tenors are considerably higher. Ore formation is attributed to mechanical transport and deposition of sulfide droplets, combined with in situ olivine and sulfide liquid accumulation. Both deposits were emplaced into or onto a felsic volcanic country-rock sequence, from which sulfur has been derived by assimilation, probably during emplacement at the present crustal level. Both are related to strongly focussed flow of komatiite magma and contain components of very primitive melts probably derived directly from the mantle plume source with limited interaction with crustal material. Sulfur assimilation, transport and deposition took place within long-lived feeder conduits that remained as open systems through most of their lifespan. The presence of these high-flux conduits within the Agnew-Wiluna komatiite sequence is attributed to unusually prolonged, high-volume eruptions, emplaced at exceptionally high rates. Deep-seated mantle tapping structures at the edge of an older Archean cratonic block may be the critical link between this style of mineralization and other large magmatic Ni-Cu deposits in younger geologic provinces.

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 The Ni-Cu-PGE ores in the 1.9 Ga Thompson nickel belt represent one of the worlds largest accumulations of mineralization associated with komatiites. Mineralization occurs as type I basal stratiform disseminated/net-textured/massive sulfides, type II internal strata-bound disseminated sulfides hosted by komatiitic dunite intrusions, type IVa Ni-rich sulfides, type IVb hydrothermal, and type V tectonically displaced breccia sulfides hosted by adjacent Pipe Formation sulfide facies iron formations, and metapelites. Although most of the ores exhibit a strong tectonometamorphic overprint, relict igneous textures in type II ores, the basal stratigraphic positions of type I ores, and the high Ni/Cu, low Pd/Ir ratios, and high S/Se ratios of type I and II ores indicate that they are derived by interaction of komatiitic magmas with sulfides incorporated from the enclosing iron formations at relatively low magma/sulfide ratios (R factors). The restrictive spatial association with type I ores, their high Ni-Pd-Cu, intermediate Co-Ru-Rh-Ir, and very low Cr tenors, and similarities to mineralization of this type in less deformed and metamorphosed areas suggest that type IVa ores formed via diffusion of metals into the metasedimentary rocks at the magmatic stage. Many ores are depleted in Pt > Cu > Au, which is interpreted to reflect preferential mobilization of these elements into wall rocks, most likely as bisulfide complexes, during polyphase deformation and middle-upper amphibolite facies metamorphism.

Abstract The Paleoproterozoic, synvolcanic Ni-Cu sulfide deposits at Pechenga are hosted by conformable, sill-like ferropicritic differentiated intrusions, injected into carbonaceous and sulfidic graywackes and shales of the Productive Formation. Due to the presence of abundant sulfides in the country rocks, assimilation of S-rich sedimentary material has commonly been attributed as the most significant factor that triggered sulfide immiscibility and led to the formation of the Pechenga ores. Several Ni-Cu sulfide prospects are associated with a ferropicritic dike system that transects the thick pillow lava succession of the Kolosjoki Volcanic Formation underlying the mentioned sedimentary unit, showing that the magma was saturated in sulfide prior to reaching the stratigraphic level where pyritic black shales occur. Our new rhenium-osmium isotope data from the Pahtajärvi prospect (yOs in the range of +52 to +69) reveal that a significant component of radiogenic Os was present in the magma. This together with new S isotope data is compatible with the Pahtajärvi ultramafic dike acting as a feeder conduit to ore-producing magma chambers in the upper part of the Productive Formation. Our results and other evidence, indicating potential nonradiogenic osmium of seawater in the basin where sediments of the Productive Formation were deposited, requires that the current model invoking country rocks as the main source of sulfur and radiogenic osmium in the Ni-Cu deposits needs to be reevaluated. Exogenic sulfur from Archean supracrustal rocks is not supported by the absence of mass-independent fractionation of sulfur isotopes in the Pahtajärvi sulfides.