Voisey's Bay Research Articles

The Nain-Churchill Boundary and the Nain Plutonic Suite: A Regional Perspective on the Geologic Setting of the Voisey's Bay Ni-Cu-Co Deposit

The Nain-Churchill Boundary and the Nain Plutonic Suite: A Regional Perspective on the Geologic Setting of the Voisey's Bay Ni-Cu-Co Deposit

The Composition of Mineralization at the Voisey's Bay Ni-Cu Sulfide Deposit, with Special Reference to Platinum-Group Elements

The Composition of Mineralization at the Voisey's Bay Ni-Cu Sulfide Deposit, with Special Reference to Platinum-Group Elements

Contrasting Petrological and Geochemical Relationships in the Voisey's Bay and Mushuau Intrusions, Labrador, Canada: Implications for Ore Genesis

Contrasting Petrological and Geochemical Relationships in the Voisey's Bay and Mushuau Intrusions, Labrador, Canada: Implications for Ore Genesis

The Evolution of Troctolitic and High Al Basaltic Magmas in Proterozoic Anorthosite Plutonic Suites and Implications for the Voisey's Bay Massive Ni-Cu Sulfide Deposit

The Evolution of Troctolitic and High Al Basaltic Magmas in Proterozoic Anorthosite Plutonic Suites and Implications for the Voisey's Bay Massive Ni-Cu Sulfide Deposit

Low mantle heat flow at the edge of the North American Continent, Voisey Bay, Labrador

Heat flow measurements in 4 deep drillholes near Voisey Bay, Labrador, have yielded the lowest value ever reported in the Canadian Shield, 22 mW m−2. This very reliable estimate is also one of the lowest continental heat flow values world wide. It requires the crust to be very poor in radioelements in this part of the Archean Nain Province. It also strongly supports the view that mantle heat flow is low (<15 mW m−2) throughout the Canadian Shield, with no trend of increasing mantle heat flow near the edges of the continent. It also raises questions about the controlling mechanism for rifting and the opening of the Labrador Sea at 100 Ma.

Geochronology of the Voisey's Bay intrusion, Labrador, Canada, by precise U–Pb dating of coexisting baddeleyite, zircon, and apatite

The emplacement history of the Voisey's Bay troctolite intrusion, that hosts the major Ni–Cu–Co sulfide deposit of the same name, has been studied using precise U–Pb geochronology of baddeleyite, zircon and apatite. The baddeleyite U–Pb ages of multiple drill core samples of troctolite and gabbro indicate that all of the mafic rocks studied from different components of the Voisey's Bay intrusion: Eastern Deeps, Discovery Hill Zone and Reid Brook Zone, and from the adjacent Red Dog area, were emplaced at 1332.7±1.0 Ma. On the basis of combined geological and geochronological evidence, it is suggested that the Voisey's Bay Ni–Cu–Co deposit was formed during the same period. The zircons coexisting with the ca. 1333 Ma baddeleyite show a diversity of ages. The zircons from normal troctolite and some of the olivine gabbro samples are coeval with the baddeleyite, while zircon from the varied textured troctolite and feeder olivine gabbro are much younger at 1305.0±0.8 Ma. The identical ages of the younger zircon population and the Voisey's Bay syenite that cuts the mafic rocks indicate a link between zircon growth in the mafic rocks and contact metamorphism, related to the emplacement of the syenite. Various mechanisms of zircon growth were probably involved, including reaction of the 1333 Ma baddeleyite with a silica-enriched fluid with formation of a secondary polycrystalline zircon, and zircon crystallization from syenite micro-veins in the mafic rocks. The mean 207 Pb/ 206 Pb age of 1303.5±2.6 Ma of the Voisey's Bay apatites is similar to the age of the younger zircon population. The apatite age may either be a result of resetting the U–Pb system in response to the syenite intrusion, or may reflect the closure of the system during regional cooling and cessation of fluid circulation. The presence of xenocrystic zircon in a Discovery Hill Zone feeder olivine gabbro indicates that the Voisey's Bay magmas were contaminated with 1.90 Ga crustal rocks.

Sulfur and oxygen isotopic evidence of country rock contamination in the Voisey's Bay Ni–Cu–Co deposit, Labrador, Canada

The emplacement of basaltic magma into sulfide-bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation-induced changes in magma composition. Pelitic country rocks of the Proterozoic Tasiuyak Gneiss in the area of the Voisey's Bay Ni–Cu–Co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the Voisey's Bay intrusion. Sulfur isotopic compositions of the Tasiuyak Gneiss range from −0.9 to −17.0‰, values typical for sulfides produced via bacterial sulfate reduction in an open marine environment. Archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally-derived sulfur. Sulfur isotopic compositions of the sulfide minerals from the Voisey's Bay deposit show consistent variations, both spatially and with rock types. Disseminated and massive sulfides show a decrease in δ 34 S to the west, with values typically between 0 and −2‰ in the Eastern Deeps, Ovoid, and Discovery Hill zone, and between −2 and −4‰ in the Reid Brook zone. δ 34 S values of the Mushua intrusion to the north and the Normal Troctolite in the Eastern Deeps are more positive, ranging between −0.5 and 1.8‰. This range is taken to represent the isotopic composition of primary mantle-derived sulfur in the area because the Mushua intrusion and Normal Troctolite show the least geochemical evidence for contamination by country rocks. Sulfur isotopic data from the Reid Brook zone are consistent with up to a 50% sulfur contribution from the Tasiuyak Gneiss. Correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low-sulfide enderbitic and quartzofeldspathic gneisses. Oxygen isotopic values of gneiss fragments in the Basal Breccia Sequence and Feeder Breccia suggest that the assimilation process involved a greater proportion of high− 18 O contaminant to the west. δ 18 O values of the Tasiuyak Gneiss (5.9 to 14.0‰), enderbitic gneiss (6.4 to 8.7‰), and Archean quartzofeldspathic gneiss (9.5 to 9.7‰) are consistent with an increased proportion of Tasiuyak Gneiss contaminant to the west. Isotopic data strongly indicate that sulfur from the Tasiuyak Gneiss has been involved in ore deposition at the Voisey's Bay deposit. However, sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization, and that assimilation of both Proterozoic and Archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic, physical mechanisms.

Re–Os isotopic systematics of the Voisey's Bay Ni–Cu–Co magmatic ore system, Labrador, Canada

Re and Os concentrations and Os isotopic compositions have been obtained for massive, matrix, and disseminated sulphide ores from three environments within the Voisey's Bay intrusion (the `Ovoid', Eastern Deeps, and Discovery Hill Zone) in order to assess the role of crustal contamination in the genesis of this large Cu–Ni–Co deposit. These samples have high Re concentrations (148 to 288 ppb, in 100% sulphide) for their common Os concentrations (4.8 to 24 ppb, in 100% sulphide), yielding high Re/Os ratios (12 to 33). These data confirm that the magma parental to the Voisey's Bay ore system was broadly basaltic in major element chemistry rather than picritic, consistent with the low Ni/Cu ratio of the ores (∼1.5). Re–Os isotopic data exhibit a limited spread in 187 Re/ 188 Os (57 to 157) and define an imprecise 1323±135 Ma `model 3' isochron, likely the result of small R-factor variations within the ore system. The Re–Os isochron age is within error of 1334 Ma U–Pb ages obtained for baddeleyite from the ore-bearing troctolites, demonstrating that whole rock Re–Os isotopic systematics have remained closed since crystallisation. The initial Os isotopic composition of the isochron ( γ Os=1040±200) implies significant magma interactions with radiogenic Os that most likely resides in the Nain-Churchill Province crust. These data are, therefore, consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components, with further addition of base and precious metals as a function of R-factor. Analyses of sulphide separates from the Proterozoic Tasiuyak (Churchill) and Archaean Nain gneisses confirm that both units contained significant Os (8.6 ppb and 0.38 ppb, respectively) that was very radiogenic at 1334 Ma ( γ Os=1908 and 5202, respectively), yielding crustal residence T CHUR model ages of 2200 to 2400 Ma. However, these model ages may have been affected by Re and/or Os mobility during the 1.85 Ga Torngat and 1.1 Ga Grenville orogeny. Re–Os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the Nain Plutonic Suite (NPS) with either Tasiuyak (2% contamination) or Nain (16% contamination) gneiss, followed by an R-factor process (200 to 1000) which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active/replenished Voisey's Bay intrusion magma chamber. However, the dominance of plagioclase-rich (high Al 2O 3) magmas in the mafic members of the NPS (troctolites and anorthosites) may indicate that other (deeper) forms of lithospheric interaction, potentially involving mafic lower crust, may be crucial to understanding this style of magmatic sulphide ore system.

Re–Os isotopic constraints on the genesis of the Sally Malay Ni–Cu–Co deposit, East Kimberley, Western Australia

Sulphide mineralisation and a cyclic suite of unmineralised troctolite and peridotite from the Sally Malay intrusion, East Kimberley, Western Australia, a possible Voisey's Bay analogue, were analysed for their trace element and platinum group element (PGE) abundances, Sm–Nd and Re–Os isotopic compositions. Massive sulphides have low common Os concentrations and high Re/Os ratios, similar to massive sulphide ores from the Duluth Complex and Voisey's Bay, and are consistent with low R factors (∼200) ( R factor=effective mass of silicate melt with which a given mass of sulphide melt has equilibrated). All Re–Os isotopic data from massive sulphides and unmineralised sections from the Sally Malay intrusion do not form a single isochron, possibly the result of variations in the R factor within the ore system. However, samples with only disseminated or matrix sulphides yield a Model 3 isochron of 1893±57 Ma, within error of the magmatic 1844±3 Ma U-Pb zircon age, suggesting that these samples have remained isotopically closed. Massive sulphide ores yield exceptionally radiogenic initial Os isotopic compositions ( γ Os=+950 to +1300), whereas disseminated sulphide mineralisation and unmineralised troctolite and peridotite yield lower γ Os values (+60 to +470). The Os isotopic data suggest that the Sally Malay intrusion and associated Ni–Cu–Co mineralisation were derived from crustally contaminated mantle melts. The Re–Os isotopic data support a two-stage model for the development of the Sally Malay intrusion ores. The first stage required crustal contamination of a basaltic melt at depth which promoted sulphide-saturation and the formation of an immiscible sulphide melt phase. The second stage involved transport of the sulphides in the host basaltic magma, resulting in a low R factor and a low metal tenor. Passive deposition of the magma produced a basal sulphide layer with a very radiogenic crustal Re–Os isotopic composition. The variable initial Os isotopic composition is likely to reflect a variable R factor, which renders isochron systematics difficult to interpret.

Geology and petrology of the Voisey's Bay intrusion: reaction of olivine with sulfide and silicate liquids

The Voisey's Bay Ni–Cu–Co sulfide deposit is located in Eastern Labrador, and transects the 1.85 Ga collisional boundary between interbanded garnet–sillimanite and quartzo-feldspathic paragneisses (Tasuiyak gneisses) of the Proterozoic Churchill Province to the west and granitic, intermediate and mafic orthogneisses of the Archean Nain Province to the east. The deposit is associated with an upper troctolitic chamber (the Eastern Deeps chamber) which is connected to a lower troctolitic chamber (the Reid Brook chamber) by a gabbroic/troctolitic feeder sheet approximately 1 km in vertical extent. Rock sequences that have been defined to date include Leuco-troctolite (LUT) in the Reid Brook chamber, Feeder Breccia (FB) in the Reid Brook Zone, Feeder Olivine Gabbro (FOG), Leopard Troctolite (LT) and Basal Breccia Sequence (BBS) in the feeder sheet, and Olivine Gabbro (OG), Normal Troctolite (NT) and Varied-textured Troctolite (VT) in the Eastern Deeps chamber. Mineralisation has been found in the feeder sheet, as massive stringers in gneisses flanking the feeder, in the `Ovoid', a 600×300×110 m 3 basin of massive sulfide at surface, and as a zone of massive sulfide that has developed close to the entry of the feeder to the Eastern Deeps chamber. The mineralisation comprises pyrrhotite (both hexagonal and troilite in varying proportions), pentlandite, chalcopyrite, magnetite and minor cubanite. An important component of all mineralized zones is a breccia of gneissic fragments in a troctolitic to noritic matrix referred to as BBS or FB. Considerable reaction has occurred between the troctolite magma and the gneiss inclusions. Olivines in the LUT, OG and FOG have low Ni contents relative to their forsterite (Fo) contents. Olivines in the NT and VT have higher Ni contents but show a great scatter in both their Ni and Fo contents. The Ni content of olivine in the LT increases systematically with decreasing Fo content. All of these variations are explained in terms of fractional crystallisation of magma, sulfide segregation, trapped liquid shift and re-equilibration between sulfide and olivine. Our current understanding of the development of the deposit is that an initial pulse of magma rose to a lower chamber, fractionated to form mafic-ultramafic cumulates and at the same time became sulfide-saturated as a result of interacting with the enclosing, sulfide- and graphite-bearing Tasiuyak gneisses. The magma lost much of its Ni and Cu as it fractionated olivine and segregated sulfides. A new wave of fresh magma entered the lower chamber, forcing the chalcophile-depleted magma, along with some of its contained sulfide, up the feeder to spread out in the upper chamber. Remnants of this chalcophile-depleted magma are preserved as olivine gabbro which forms upper layers in the upper chamber, and which in places has frozen to the walls of the feeder. Sulfides that were swept up in this event were forced out into fractures in the feeder walls, are preserved within thickened zones within the feeder, and were deposited as the magma flow decreased at the entry of the feeder to the upper chamber. Continued influx of the later, undepleted magma resulted in sulfides being picked up, transported farther up the system, upgraded during transportation and then redeposited.

World-class Ni-Cu-PGE deposits: key factors in their genesis

Magmatic Ni-Cu sulfide deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma, and the partitioning of chalcophile elements into these from the silicate melt. Sulfide saturation of a magma is not enough in itself to produce an ore deposit. The appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals, and then is concentrated in a restricted locality so that the resulting concentration is of ore grade. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 × 2 × 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, trans-crustal fault and immediately below the centre of a 3.5 km-thick volcanic basin. Studies of the overlying basalts have shown that lavas forming a 500 m-thick sequence within these have lost 75% of their Cu and Ni and more than 90% of their PGE. Overlying basalts show a gradual recovery in their chalcophile element concentrations to reach “normal” values 500 m above the top of the highly depleted zone. The ore-bearing Noril'sk-type intrusions correlate with those basalts above the depleted zone that contain “normal” levels of chalcophile elements. The high proportion of sulfide (2–10 wt.%) associated with the Noril'sk-type intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100–400 m around the bodies), and the heavy sulfur isotopic composition of the ores (+8–+12 ∂34S) are explicable if the ore-bearing bodies are exit conduits from high level intrusions, along which magma has flowed en route to extrude at surface. The first magma to enter these intrusions reacted with much evaporitic sulfur, at a low “R” value and thus gave rise to sulfides with low metal tenors. Successive flow of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows above the highly depleted flows. The Voisey's Bay deposit lies partly within a 30–100 m-thick sheet of troctolite, interpreted as a feeder for the 1.334 Ga Voisey's Bay intrusion, and partly at the base of this intrusion, where the feeder adjoins it. Studies of olivine compositions indicate that an early pulse of magma through the feeder and into the intrusion was Ni depleted but that subsequent pulses were much less depleted. Trace element, Re-Os and S and O isotope data, and mineralogical studies indicate that the magma pulses interacted with country gneiss, probably principally in a deeper level intrusion, extracting SiO2, Na2O, K2O and possibly sulfur form the gneiss, which accounts for the magma becoming sulfide saturated. The Jinchuan deposit of north central China occurs within a 6 km-long dyke-like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt magma. Flow of magma into the intrusion has resulted in olivine and sulfide being retained where the keel was widening out into the intrusion. The West Australian komatiite-related deposits occur in thermal erosional troughs which have developed due to the channelisation of magma flow and the resulting thermal erosion of underlying sediments and basalt by the hot komatiite magma. The sediments are sulfide-rich, and may have contributed substantially to the sulfide of the ores. The mineralisation in the Duluth complex occurs in troctolitic intrusions along the western margin of the complex as a result of magma interacting with and extracting sulfur from the underlying graphite- and sulfide-bearing sediments. No magma flow channels have been identified so far, and the lack of magma flow subsequent to the development of sulfide immiscibility is regarded as the reason why these deposits are not of economic grade. When most major Ni-Cu sulfide deposits are compared, they prove to have a number of features in common; olivine-rich magma, proximity to a major crustal fault, sulfide-bearing country rocks, chalcophile element depletion in related intrusive or extrusive rocks, field and/or geochemical evidence of interaction between the magma and the country rocks, and the presence of or proximity to a magma conduit. The features are thought to explain the three key requirements (sulfide immiscibilty, adequate mixing between sulfides and magma, and localisation of the sulfides) discussed and have important implications with respect to exploration.

Environmental and Native Title Issues at Voisey's Bay, Newfoundland

Mineral Resources EngineeringVol. 07, No. 04, pp. 325-331 (1998) No AccessEnvironmental and Native Title Issues at Voisey's Bay, NewfoundlandDavid Macdougall and David MccarthyDavid MacdougallMcInnes Cooper & Robertson, Scotia Centre, Suite 602, 235 Water Street, P.O. Box 547, St. John's, Newfoundland, Canada A1C 5K8, Canada Search for more papers by this author and David MccarthyMcInnes Cooper & Robertson, Scotia Centre, Suite 602, 235 Water Street, P.O. Box 547, St. John's, Newfoundland, Canada A1C 5K8, Canada Search for more papers by this author https://doi.org/10.1142/S0950609898000341Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail FiguresReferencesRelatedDetails Recommended Vol. 07, No. 04 Metrics History PDF download

Application of the Re‐Os isotopic system to the study of Precambrian magmatic sulfide deposits of Western Australia

Mafic and ultramafic rocks that host magmatic Fe‐Ni‐Cu‐platinum‐group element (PGE) sulfide deposits are an important feature of the Archaean Yilgarn and Pilbara Cratons of Western Australia. We have applied the Re‐Os isotopic geochronometer and tracer to four komatiite‐hosted Ni sulfide deposits, one Merensky Reef‐style Cu‐Ni‐PGE disseminated sulfide accumulation in a mafic layered intrusion, and a Cu‐Ni sulfide‐rich siliceous high‐MgO basalt dyke swarm in order to constrain better the genesis of these magmatic ore systems. Significant post‐crystallisation Re and/or Os mobility is observed in the isotopic data for samples from the 3.3 Ga Ruth Well prospect (Pilbara Craton), for cumulates from the 2.9 Ga Munni Munni Complex (Pilbara Craton), for some massive sulfide ore samples from the 2.7 Ga Kambalda deposit (Yilgarn Craton), and for some disseminated ore samples from the 2.7 Ga Perseverance deposit (Yilgarn Craton). We attribute this open‐system behaviour and disturbance of the Re‐Os system to hydrothermal alteration, sulfide recrystallisation during deformation and metamorphism, and/or metasomatism associated with mesothermal lode‐gold mineralisation. However, less tectonised and metamorphosed ores and a host komatiite lava from Kambalda as well as olivine‐sulfide cumulates from Perseverance and Mt Keith yield a magmatic Re‐Os isochron age of 2706 ± 36 Ma, in good agreement with U‐Pb zircon ages of 2709–2702 Ma for metasediments from Kambalda, demonstrating that the Re‐Os isotopic systematics in these samples have remained closed for 2.7 Ga. This isochron can also be used as a tracer of the magmatic processes that brought the host komatiites to sulfide saturation. Assimilation of sulfidic crustal materials via thermal erosion has been proposed as a mechanism by which the Archaean komatiites at Kambalda, Perseverance and Mt Keith achieved sulfide saturation. However, the isochron for the Yilgarn ores yields a precise chondritic initial γ?? value of ‐0.1 ± 0.3 (per cent deviation in 187Os/188Os from chondritic mantle of the same age), whereas a proposed sulfidic sedimentary contaminant from Kambalda yields an extremely radiogenic initial γ?? value of +900. These Os isotopic data in combination with trace element and Nd isotopic data demonstrate that the major ore‐forming komatiites in Western Australia were derived from chemically depleted mantle without significant contamination (< 3%) by crust either prior to eruption or during turbulent flow at the surface. Thus, ground melting and crustal assimilation may not be as important as previously thought in the genesis of these deposits. However, Re‐Os isotopic data for a 2.4 Ga siliceous high‐MgO basalt dyke at Cowarna Rocks (Yilgarn Craton) yield a radiogenic initial Os isotopic composition (γ?? = + 250), suggesting that sulfide saturation may have been linked to crustal assimilation. Thus, the timing and mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localisation of economic concentrations of magmatic sulfide liquids. This geodynamic concept has important implications for the discovery of other styles of magmatic ore deposits, such as Voisey's Bay (Labrador, Canada)‐style sulfide mineralisation, in Australia.

Key factors in the genesis of Noril'sk, Sudbury, Jinchuan, Voisey's Bay and other world‐class Ni‐Cu‐PGE deposits: Implications for exploration

Magmatic Ni‐Cu sulfide deposits form as the result of the segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma and the partitioning of chalcophile elements into these from the silicate melt. Sulfide saturation of a magma is not enough in itself to produce a massive concentration of magmatic sulfide. The appropriate physical environment is required to concentrate sulfides from a large mass of magma into one place. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 X 2 X 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, transcrustal fault and immediately below the centre of a 3.5 km‐thick volcanic basin. An anticlinal axis that transects the axis of the basin at a high angle has brought these intrusions to surface to give rise to the two major ore junctions, Noril'sk and Talnakh. Studies of the overlying basalts have shown that basalts forming a 500 m‐thick sequence have lost 75% of their Cu and Ni and more than 90% of their PGE. Basalts above this show a gradual recovery in their chalcophile element concentrations to reach ‘normal’ values 500 m above the highly depleted zone. Two groups of mineralised bodies have been identified as correlative with these basalts: the poorly mineralised Lower Talnakh‐type bodies,which resemble the highly depleted basalts; and the ore‐bearing Noril'sk‐type intrusions which correlate with the overlying, essentially undepleted basalts. The high proportion of sulfide (2–10 wt%) associated with the Noril'sk‐type intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100–400 m around the bodies), and the heavy sulfur isotopic composition of the ores (+ 8 to +12 834S) are explicable if the ore‐bearing bodies are exit conduits from high‐level intrusions, along which magma has flowed en route to surface. The Lower Talnakh bodies are interpreted as intrusions along which magma flow stopped earlier than along those of the Noril'sk type. The first magma to enter the high‐level intrusion reacted with much evaporitic sulfur, at a low ‘R’ value and thus gave rise to sulfides with low metal tenors. Successive pulses of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows. The flow direction along the conduits is shown by the direction in which the tenor of disseminated sulfides decreases. Sulfides have settled from the moving magma to form separate injections of liquid sulfide, up to 3.5 X 1.5 X 0.05 km in size. Recent reflection seismic studies at Sudbury have shown the Sudbury Igneous complex to have been much more extensive than originally supposed. Nd and Sr isotopic studies on rocks of the Sudbury Igneous Complex and Re‐Os studies on the ores have indicated the incorporation of much country rock gneiss in the complex. Debate has centred around whether the Sudbury Igneous Complex is entirely an impact melt or the consequence of the mixing of primary magma with 50 or more wt% impact melt; the most recent evidence favours the latter hypothesis. The Jinchuan deposit of north‐central China occurs within a 6 km‐long dyke‐like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt. Flow of magma into the intrusion has resulted in olivine and sulfide being retained within this keel. The Voisey's Bay deposit lies partly within a 30–100 m‐thick sheet of troctolite, which is interpreted as a flat‐lying part of a feeder for an adjacent intrusion, and partly at the base of the intrusion, where the feeder adjoins it. Ore types range from disseminated sulfides in troctolite, which increase downward, grading into massive ore. The latter is underlain by a breccia composed of fragments of gneiss, unmineralised troctolite and peridotite in a troctolitic matrix that, in places, contains appreciable sulfide. When most major Ni‐Cu sulfide deposits, including those at Kambalda, Western Australia, are viewed in the light of studies at Noril'sk, Sudbury, Jinchuan and Voisey's Bay, three factors become apparent: (i) the concentration of sulfides in channels or conduits through which much magma has flowed (feeder conduits for intrusions are much more prospective targets for exploration than the base of the intrusions themselves); (ii) the interaction of the source magma with country rocks, either leading to the incorporation of sulfur, or the felsification of the magma in question; and (iii) fractional crystallisation of sulfide liquid giving rise to Cu‐rich ores which may be far removed from the ‘source’ ore.