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.

Abstract The Jinchuan Ni-Cu-(PGE) deposit is the largest single magmatic Ni deposit in the world. It is hosted by a small ultramafic intrusion in the Longshoushan terrane located in the western part of the North China craton. Phase equilibrium analysis using available whole-rock and mineral chemical data confirms that the parental magma of the intrusion is of high Mg basaltic composition, but the primary magma may have contained up to 18.5 wt percent MgO. The involvement of a long-term enriched subcontinental lithosphere mantle (SCLM) source is inferred from high, negative ε Nd values (–6 to –12). Small amounts (mostly 5–15%) of crustal contamination are suggested by Sr-Nd isotopes. Positive γ Os values (20–150) are consistent with selective assimilation of crustal sulfide, but evidence from sulfur isotopes is inconclusive. The δ 34 S values of most samples (∼80%) from the Jinchuan deposit vary between –2 and +2 per mil, which is within the range that is characteristic of mantle-derived sulfur. Analytic modeling suggests that both fractional crystallization and crustal contamination played a role in triggering sulfide saturation in the Jinchuan magmatic system. Lower olivine/sulfide ratios in the orebodies than the cotectic ratio suggest that material sorting by flow differentiation and gravitational settling was important during ore formation. A low amount of trapped silicate liquid in the intrusion (<30%) can be best explained by loss of liquid to the peripheral sills or dikes of the Jinchuan magma plumbing system. Lower PGE tenors in the Jinchuan deposit relative to those of sulfide liquids expected to segregate from PGE undepleted high Mg basaltic magma are consistent with previous sulfide segregation at depth. Recent U-Pb zircon-baddeleyite dating has shown a crystallization age of ∼830 Ma for the Jinchuan intrusion. The new age indicates that the emplacement of the Jinchuan intrusion was contemporaneous with the initial stage of Rodinia breakup. Some researchers have suggested that the breakup of the Rodinia supercontinent was triggered by a hypothetical super mantle plume located beneath the Yangtze craton and that the Jinchuan intrusion and country rocks are from the Yangtze craton. However, regional stratigraphic correlation indicates that the Longshoushan terrane was part of the North China craton prior to Rodinia breakup. This suggests that the Jinchuan mafic-ultramafic magmatism took place in the North China craton, not in the Yangtze craton, and that regional exploration for the Jinchuan-type deposits should focus on the western part of the North China craton instead of the Yangtze craton.

Abstract Mafic to ultramafic intrusive systems necessarily contain "conduits" through which magma has passed enroute to shallow levels of the crust. It is now commonly accepted that conduit systems are key sites for the accumulation of immiscible sulfide liquid and the generation of Ni-Cu-(PGE) deposits. Deposits such as those of Voiseys Bay in Labrador and Eagle in northern Michigan illustrate that sulfide deposits may form in both near vertical and horizontal portions of conduit systems. Although the Voisey Bay deposit is considerably larger than that associated with the Eagle intrusion, there are important similarities that emphasize the role of magma conduits in localizing sulfide mineralization. in addition to the similarities related to the physical collection of dense sulfide liquid, both deposits occur in provinces where anorthosites and early-stage picritc magmatism (Nain Plutonic Suite and the Midcontinent Rift system) are key elements of geochemical development. Geochemical evidence for the contamination of mafic magma by country rocks exists at both Voiseys Bay and Eagle. Conduit systems such as those of Voiseys Bay and Eagle provide an environment for enhanced magma-country rock interaction that may trigger the attainment of sulfide saturation of passing magma and initiate the ore-forming process.

Abstract The deposits of the Norilsk-Talnakh region in northern Russian are some of the richest in the world and the type example of a magmatic sulfide deposit associated with a large igneous province. The ores—massive and disseminated, Cu-, Ni-, and PGE-rich sulfides—are hosted in small mafic intrusions that intrude the sedimentary sequence that immediately underlies the Siberian flood volcanic sequence. In general the origin of the deposits is relatively well understood. Magma, probably from a large mantle plume, interacted with crustal rocks during its passage to the surface. The assimilation of evaporites led to the addition of sulfur and to the segregation of ore metal-rich sulfides that accumulated in the lower parts of shallow-level intrusions. Aspects of the model that require clarification are (1) the role of lithosphere architecture in the localization of the ore deposits, (2) the exact relationship between the assimilation of crustal rocks and the segregation of sulfide, and (3) the mechanism of transport and accumulation of the sulfides.

Abstract The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with current measured and indicated mineral resources of 37.2 million metric tons (Mt) at 2.63 percent Ni and inferred mineral resources of 21 Mt at 2.6 percent Ni (Dec. 2010, Xstrata.com). The sulfides occur in sill-like and chonolithic ultramafic-mafic intrusions that form part of the approximately 500-km long, 1.4-Ga Kabanga-Musongati-Kapalagulu mafic-ultramafic igneous belt, within the Karagwe-Ankole belt in northwestern Tanzania and adjacent Burundi. The intrusions are up to ∼1 km thick and 4 km long and crystallized from several pulses of compositionally distinct magma emplaced into sulfide-bearing pelitic schists. The first magma pulse consisted of siliceous high magnesium basalt with approximately 13 percent MgO. It formed a network of fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg no. opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotope signatures (δ 18 O 6–8), consistent with ∼20 percent contamination of primitive picrite with the sulfidic schists. Subsequent magma pulses were more magnesian, containing approximately 14 to 15 percent MgO, and less contaminated (e.g., δ 18 O 5.1–6.6). They injected into the earlier sills, forming medium-grained harzburgites and orthopyroxenites (Fo 83–89 , Mg no. Opx 86–89 ), and magmatic breccias consisting of gabbronorite-orthopyroxenite fragments within an olivine-rich matrix. The Kabanga intrusions contain abundant disseminated sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of the Kabanga North and Kabanga Main intrusions, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. Postemplacement tilting of the intrusions caused solid-state mobilization of ductile sulfides into shear zones, notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach a thickness of >10 m and can extend for several 10s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits. Compared to other sulfide ores that segregated from magnesian basalts (e.g., Jinchuan, Pechenga, Raglan), most Kabanga sulfides have low Ni (<1–3%), Cu (∼0.1–0.4%), and PGE contents (<<1 ppm), and high Ni/Cu (5–15) ratios. Higher metal contents (∼5% Ni, 0.8% Cu, 10 ppm PGE) are found in only one unit from Kabanga North. The observed metal contents are consistent with segregation of magmatic sulfides from fertile to strongly metal-depleted magmas, at intermediate to very low mass ratios of silicate to sulfide liquid (R factors) of approximately 10 to 400. The sulfides have heavy S isotope signatures (δ 34 S wr = 10–24) that broadly overlap with those of the country-rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe-Ankolean sedimentary sequence. Based on the relatively homogeneous distribution of disseminated sulfides in many of the intrusive rocks we propose that the magmas reached sulfide saturation prior to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The early magmas assimilated smaller quantities of country rocks but, in addition, sulfur was selectively assimilated, either by means of a volatile phase or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The unusually large degree of crustal contamination and the low R factors render Kabanga an end member in the spectrum of magmatic Ni sulfide ores.

Abstract The total grams of platinum group elements (PGE) in the Critical zone of the Bushveld when projected to the original horizontal and expressed as g/m 2 is relatively uniform for the Critical zone of the Bushveld Complex as a whole, varying from 157 to 171 g/m 2 in the western Bushveld and from 105 to 116 in the eastern Bushveld. However, in the interval from the top of the UG-2 to the top of the Merensky Reef, the northwestern Bushveld is twice as rich as the southwestern Bushveld, which is again significantly richer than the eastern Bushveld. This latter trend is also paralleled by the PGE "tenor factor" of the sulfides in samples close to the upper chromite seam of the Merensky Reef. These observations support the conclusions of Eales et al. ( 1988 , 1990 ), Scoon and Teigler ( 1994 ), and Maier and Teigler ( 1995 ) that a feeder for the Bushveld existed in the northwest during Merensky Reef time. Variation in the Cu/Pd ratio of sulfide-poor rocks suggests that the more mafic sequences in the interval between the top of the UG-2 and the Merensky Reef in the southwest Bushveld are due to influxes of an unusually PGE rich mafic magma across cumulates crystallizing from a magma with both plagioclase and orthopyroxene on the liquidus. The Merensky and Bastard reefs in this area are the culmination of these mafic influxes and their mixing with resident magma, albeit influxes that in these cases developed immiscible sulfide. The decrease in PGE tenor and increase in Cu/Pd ratio from near the upper chromite seam of the Merensky Reef upward into the overlying Merensky pyroxenite is consistent with the fractional segregation of sulfide from a PGE-enriched magma. A model is proposed for the emplacement of the Merensky Reef whereby influxes of magma spread away from the centers, mixing with resident magma, segregating sulfide, and becoming progressively depleted in PGE as they progressed. In general, the average PGE content of massive chromitites increases upward from the LG-1 to the UG-2/3. The LG-1 to LG-4 chromitites differ from those higher in the stratigraphy in having a much lower (Pt + Pd)/(Ru + Ir + Os) ratio. It has been suggested that the latter developed along with an immiscible magmatic sulfide liquid, while the former lacked this sulfide liquid. All chromitites are characterized by very high PGE/S ratios, and, in the case of those that contained original sulfide liquid sulfide, this is attributed to the original sulfide having been destroyed due to Fe entering the structure of cooling chromite, the f S2 rising, and the sulfur eventually being lost to the environment. Along with the sulfur, Cu and some of the Ni and Pd have also been lost, but not Pt, Rh, Ru, Ir, or Os. Ideas on the formation of Bushveld chromitites fall into two main groupings, those that maintain that they formed in situ ("onstage") and those that maintain that the chromite crystallized elsewhere ("offstage") and was introduced as a slurry. The problem that has puzzled researchers is what causes chromite to separate from a magma alone, without accompanying silicates. Observations reported here have shown that there are similar systematic variations in PGE profiles in the MG-3 and MG-4 chromitites in areas that are separated by 270 to 300 km and a progressive upward increase in V content in the UG-2 at Waterval shaft, all of which could not have resulted from introduction of the chromite in these units as a slurry; such observations argue strongly for an on-stage origin. An investigation has been made of spinel and orthopyroxene in a magma with the average composition of Critical zone magma, as a function of temperature, pressure, Cr 2 O 3 content, and H 2 O content and mixing with both fractionated mafic magma and felsic melt. Chromitites can form in situ if spinel crystallization precedes orthopyroxene on cooling. Accepting the usual maximum f o2 for the Bushveld as <QFM, MELTS modeling shows that this will only be the case if the Cr 2 O 3 content is >0.20 wt percent. Increasing total pressure has no effect on the f O2 at which spinel precedes orthopyroxene, indicating that increase in pressure will not precipitate a chromitite. The application of MELTS to mixtures of primary with 10 and 20 percent fractionated magma, and with admixtures of 20 percent average continental upper crust slightly increases the f O2 at which spinel precedes orthopyroxene, indicating that these mechanisms will not cause spinel to appear on the liquidus before orthopyroxene. It has been found that if the mixed magmas both have olivine (rather than orthopyroxene) along with spinel on the liquidus, a small amount of spinel will crystallize alone before it is joined by olivine; however, the modeling shows that those who have extended this concept to orthopyroxene—rather than olivine-rich magmas—are incorrect. The same is true of adding H 2 O. Calculations show that the limited amount of chromite that could form from a magma containing 0.25 percent Cr 2 O 3 at or below QFM means that a 70-cm-thick massive chromitite would have to seperate from a column of magma ≥1,000 m, supporting Eales's ( 2000 ) conclusion that the chromite in the Critical zone must have come from a volume of magma equal to many times the existing volume of cumulates. The whereabouts of the missing magma (the Critical zone cumulates are, on average, less than 2,000 m thick) is unknown, but it is suggested that part has exited the complex up its walls, leaving the Platreef and Sheba's Ridge deposits as witnesses to its passing—the pudding basin model. While contamination of Critical zone magma with a felsic melt will not promote chromite crystallization, it will also not prevent it at a given f O2 . Contamination with a melt of average upper crustal composition will, however, promote sulfide immiscibility, and it is proposed that this is the reason why some chromitites have developed with immiscible sulfide and are therefore rich in Pt and Pd, and others have not.

Abstract The Platreef is one of the largest and most valuable Ni-Cu-PGE orebodies on Earth. It is located at the base of the northern limb of the 2.06 Ga Bushveld Complex and stratigraphic relationships with other limbs of the complex and stratiform orebodies such as the Merensky Reef and UG2 chromitite are not clear. The Bushveld Complex intruded along the axis of the >2.9 Ga Thabazimbi-Murchison lineament and this may have acted as a barrier between the northern limb and the rest of the complex for some or all of the intrusion history. Research since the turn of the millenium has demonstrated that the Platreef represents a sill or complex of sills intruded into basement granite-gneiss and sediments of the Transvaal Supergroup. Different sills display variable lithologic units, thicknesses, bulk chemical signatures, and mineralization arising from different inputs of magma and the effects of local wall-rock contamination. Chilling and injection of Main zone gabbronorites took place into already solidified and deformed Platreef, indicating a major break in time between these events. Aspects of mineral chemistry and bulk geochemistry and Nd and Os isotopes in the Platreef overlap completely with the Merensky Reef but not the Upper Critical zone. Conventional and mass independent S isotopes suggest a mantle source of S that was overprinted by addition of local crustal S where the Platreef intruded pyrite-rich shales. Assimilation and introduction of external S is viewed as an ore-modifying process, not as the primary trigger for mineralization. The genesis of the Platreef is more likely to have involved introduction of PGE-rich sulfide droplets with the intruding Platreef magma. These sulfides may have been derived from the same magma(s) that formed the Merensky Reef and which injected up and out along the intrusion walls as the chamber expanded. Alternatively, the sulfides may have formed in pre-Platreef staging chambers where they were upgraded by repeated interactions with batches of Lower zone magma before being expelled as a crystal-liquid-sulfide mush by an early injection of Main zone magma, prior to the formation of the bulk of the Main zone which crystallized above (and partially eroded) the solidified Platreef.

Abstract The Great Dyke of Zimbabwe hosts the world's second largest reserve of platinum-group elements (PGE). Economic PGE mineralization is restricted to sulfide disseminations mainly in pyroxenites of the P 1 layer, the Main sulfide zone, which is currently under extensive exploration. The producing Ngezi and Mimosa mines are the lowest cash-cost platinum mines in the world. The Main sulfide zone has a fine structure made up by a number of successive, geochemically distinct layers and typical vertical element distribution patterns characterized by a general upward zoning sequence in the order → Pd Pt → base metal sulfides. With some overlap, a number of sublayers can be distinguished in the PGE subzone and in the base metal sulfides subzone of the Main sulfide zone. These layers and the element decoupling patterns are regarded to represent first-order, primary magmatic features of sulfide accumulation and concomitant scavenging of PGE in relationship to their different (and probably variable) sulfide/silicate partition coefficients. The PGE are bimodally distributed in the Main sulfide zone: Large proportions of Pd and Rh are hosted in pentlandite, whereas Pt is dominantly present in the form of discrete platinum-group minerals (PGM). The distribution patterns of the various PGM within the Main sulfide zone suggest that a large fraction of the PGE, primarily concentrated in sulfide at magmatic conditions, was redistributed following the crystallization of sulfides in the subsolidus stage. PGE expelled from the annealing sulfides formed PGM with reactant partners like As, Te, and Bi, whose proportions and availabilities differ regionally. It is assumed that these reactions took place under largely isochemical conditions; however, chemical gradients within the Main sulfide zone and magmatic-hydrothermal fluids may have supported the small-scale redistribution of the PGE and the reactions that formed the PGM. The current work on the Great Dyke emphasizes the role of sulfide generation and accumulation on PGE concentration which take place in the course of the magmatic evolution of layered intrusions. Sulfur saturation leading to sulfide segregation appears to be the most important factor in the primary magmatic concentration of the PGE. The enrichment of the economically most important Pd group PGE (PPGE) i.e., Pt, Pd, and Rh, is sulfide controlled. The geochemical offset patterns are regarded to reflect a first-order, dominantely magmatic control of the mineralization as these patterns are observed persistently over wide areas of the Great Dyke. In contrast, the different sulfide and PGM assemblages are viewed to represent second-order reaction products that came into existence downtemperature during annealing of the mineral assemblages. The variability of PGM assemblages appears to be controlled mostly by the presence of semimetals.

Abstract The Lac des Iles Complex contains the Roby, Twilight, and High-grade zones, which make up Canada's only primary platinum-group element (PGE) ore deposit with a grade of ∼3 ppm Pd + Pt. The ores have remarkably high Pd/Pt ratios, averaging 7 in the Roby and Twilight zones and even higher, 14, in the High-grade zone. In contrast most PGE-dominated deposits have Pd/Pt ratios of 0.5 to 3. The Lac des Iles ore zones occur within a small (∼2 ×y 3.5 km) concentrically zoned mafic intrusion and are approximately 0.5 km wide by 1 km long at surface. There are three major rock types present, gabbronorite, metagabbronorite, and chlorite-actinolite schist. The Roby and Twilight zones consist of magmatic breccia of gabbronorite or metagabbronorite, which contains pegmatoidal and varitextured patches. The gabbronorites have adcumulate textures and consist of deformed plagioclase and orthopyroxene with minor interstitial clinopyroxene, biotite, and hornblende. In the metagabbronorites the pyroxenes and hornblende have been replaced by actinolite and the plagioclase has been partly altered to sericite. In the most altered metagabbronorites the plagioclase has been replaced by chlorite. The High-grade zone occurs between the breccia of the Roby zone and the homogeneous East Gabbro. The main rock type is actinolite-chlorite ± talc schist. Three sulfide assemblages are present: (1) pyrrhotite, pentlandite, chalcopyrite ± pyrite; (2) pentlandite, chalcopyrite, and pyrite; and (3) chalcopyrite, pyrite, and millerite. Assemblage (1) is present in all rock types and shows equilibrium textures in the fresh rocks. Assemblages (2) and (3) are present only in the metagabbronorite and chlorite-actinolite schist and show disequilibrium textures. Pentlandite is an important host for Pd in assemblages (1) and (2), the other important hosts for Pd in these assemblages are Pd tellurides. In assemblage (3) the Pd is found in a wide variety of platinum-group minerals (PGM); Pd tellurides, Pd sulfides, Pd antinomides, and Pd arsenides. The PGM in assemblages (1) and (2) are found in association with the sulfides, while in assemblage (3) they are found as isolated grains. Whole-rock geochemistry shows that the most of the rocks no matter what their texture or degree of alteration have similar compositions. Most compositions fall on plagioclase-orthopyroxene tie lines. Mantle-normalized patterns show that the rare earth element (REE) and high field strength element (HFSE) concentrations are low (0.8–2 times mantle) and similar. In this flat pattern there are positive Sr, Eu, Pb, and Sc anomalies. These observations are consistent with the rocks being plagioclase-orthopyroxene adcumulates. A small group of metagabbronorite and schist samples show MgO, CaO, and Cr enrichment, indicating the presence of some cumulate olivine, clinopyroxene, and/or chromite. The rocks no matter what their texture or degree of alteration have similar incompatible element ratios, indicating that they all are comagmatic. The low normative clinopyroxene concentrations and the low HFSE content of these rocks suggest that there is very little trapped liquid component present. This observation appears to be in contradiction to the field appearance of the magmatic breccia which indicates the matrix represents a frozen magma. We suggest that the magma chamber was being deformed at the time of intrusion and the fractionated liquid was squeezed out of both the matrix and fragments during this process. The formation of pegmatite and varitextured rocks could have occurred when the magma became fluid saturated and this fluid infiltrated the partially consolidated gabbronorite causing recrystallization. The composition of the varitextured and pegmatiodal rocks is similar to that of the other rocks and thus the fluid did not appreciably change the composition of the recrystallized rocks. Processes that have been considered for forming the ores include: collection by a sulfide liquid from a silicate magma; zone refining of the sulfides during repeated injections of magmas; and collection of the metals by deuteric or hydrothermal fluids. For samples from the Twilight and Roby zones there is a strong correlation between S, Cu, and PGE, indicating that sulfide minerals control the PGE and thus collection by a sulfide liquid could have occurred. However the high Pd/Pt ratio of the ores suggests that the sulfide liquid did not segregate from a primary mafic magma. Possibly, there was a feeder chamber to the Lac des Iles intrusion. The magma in the feeder system became saturated in sulfide liquid and this collected and crystallized in a structural trap between chambers. A fresh injection of S-undersaturated magma from the lower chamber partially melted the sulfides, enriching the magma in S, Cu, Pd, and Au. The Pd-enriched magma was then injected into the Lac des Iles chamber, mixed with the partially consolidated resident magma, and Pd-rich sulfides segregated from it. In the High-grade zone there appears to have been an additionally low temperature process that added Pd, Au, As, and Sb to these rocks. Possibly these elements were added by a fluid that exsolved from the magma in underlying magma chamber and which scavenged the elements from the sulfides formed at depth. The fluid was focussed in the shear zone between the East Gabbro and the Roby zone because most of the Lac des Iles intrusion had solidified at this point.

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

Abstract Exploration for world-class Ni-Cu-(PGE) deposits in mafic and/or ultramafic igneous rocks has focused on extensional environments where high degrees of mantle melting have occurred in association with mantle plumes. Where continental rifting has been involved, the interaction between large volumes of mafic magma and crustal rocks in either intrusive or extrusive settings may have resulted in contamination that triggered sulfide saturation or melting of sulfides within country rocks. Staging chambers and conduits in the subvolcanic environment and embayments associated with channels in the volcanic environment are localities where immiscible sulfide liquid may accumulate. The large-tonnage, high-grade deposits in conduit and magma chamber environments, such as those at Noril’sk, Siberia, remain high priorities for greenfields exploration, and it is now clear that intrusions with even small footprints may be important exploration targets. Examples of small footprint deposits include the large-tonnage ore systems at Voisey’s Bay in the Nain plutonic suite, Labrador, and the low-tonnage, high-grade mineralization at the Eagle deposit in the Keweenawan of northern Michigan. The high-grade mineralization in small deposits is particularly attractive as incremental feed if smelters are located nearby and transportation routes are available. Low-tonnage, high-grade deposits can also be mined using underground methods, and having lesser environmental impact and remediation is typically more straightforward. Although convergent margin environments have not been universally viewed as viable target areas for magmatic sulfide-rich Ni-Cu-(PGE) deposits, suprasubduction zone environments have high degrees of mantle melting, and they provide locations for crust-magma interaction and conduit geometries where sulfides may collect. Deposits such as Kalatongke in China, Aquablanca in Spain, and the Turnagain and Duke Island Ural-Alaskan intrusions illustrate that convergent margins should not be dismissed as targets for magmatic Ni-Cu-(PGE) ores. New advances in hydrometallurgical techniques, particularly pressure leach methods, are making the extraction of Cu, Ni, and PGEs from large-tonnage but low-grade deposits economically promising. The large disseminated sulfide-rich Ni-Cu-(PGE) resources of the Duluth Complex are an example where advances in process technology may permit future development of low-grade occurrences that have traditionally been considered to be of marginal economic value.