Bay Intrusion Research Articles

Petrography, chemical composition, and age constraints of mafic intrusions from the Mesoproterozoic Soisson Intrusive Suite in the southeastern Churchill Province (Canada)

Several kilometre-scale gabbroic intrusions with well-preserved cumulate textures occur over a ∼180 km northwest–southeast strike length area in the Core Zone, within the southeastern Churchill Province. These intrusions (1312–1311 Ma) are devoid of structural and metamorphic overprint and are mainly composed of olivine gabbro. Their overlapping petrographic, geochemical, and mineralogical characteristics suggest that all of these gabbroic intrusions are co-genetic and form the Soisson Intrusive Suite. The Soisson intrusions formed from basaltic parental magmas with low MgO (∼6–7 wt.%) and high Fe–Ti contents. Overall, they are similarly fractionated at the regional scale and incorporated various amounts of upper crustal material, as indicated by field (e.g., xenoliths, hybrid zones) and geochemical (e.g., high HILE/MILE, depletion in Nb±Ta) evidences. The Soisson Intrusive Suite has petrological, geochemical, and mineral chemistry similarities with the troctolite and olivine gabbro cumulates of the Voisey’s Bay and Mushuau intrusions from the ca. 1363 to 1289 Ma Nain Plutonic Suite (NPS), and appears to be contemporaneous with its younger gabbroic–troctolitic intrusion phase, suggesting that the Soisson Intrusive Suite and the gabbroic–troctolitic intrusions of the NPS were formed by similar petrological processes. Furthermore, like many gabbroic–troctolitic intrusions of the NPS (e.g., Voisey’s Bay intrusion), the Soisson intrusions contain Ni–Cu–Co magmatic sulfide mineralization. Among the Soisson intrusions, the Papavoine intrusion located in the southern part of this intrusive suite may be the most prospective intrusion as a Ni, Cu, and Co exploration target. Overall, the Soisson Intrusive Suite’s similarity with the NPS raises questions about their potential genetic link and magmatic connectivity at depth.

Synchronous Emplacement of the Anorthosite Xenolith‐Bearing Beaver River Diabase and One of the Largest Lava Flows on Earth

AbstractNew geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U‐Pb zircon date of 1091.83 0.21 Ma (2) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross‐cutting Silver Bay intrusion (1091.61 0.14 Ma; 2), these data tightly bracket the age of the Beaver River diabase to be 1091.7 0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59 0.27 Ma; 2)—which is further supported by indistinguishable tilt‐corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism.

Rift-related magmatism on magma-poor margins: Structural and potential-field analyses of the Mesozoic Notre Dame Bay intrusions, Newfoundland, Canada and their link to North Atlantic Opening

The magma-poor Newfoundland margin formed following lithospheric stretching, rifting and breakup resulting in the separation of the Grand Banks from Iberia, and northern Newfoundland from Ireland. Mesozoic igneous rocks contemporaneous with rifting have been documented on- and offshore Newfoundland, with later offshore magmatism occurring post-breakup. The Notre Dame Bay Magmatic Province (NDBMP) is a Jurassic-Cretaceous, potentially rift-related occurrence of igneous rocks onshore Newfoundland that includes the gabbroic Budgell Harbour Stock (BHS) and marginal lamprophyre dykes, in addition to the nearby, potentially related, Dildo Pond Intrusion (DPI) and Leading Tickles Stock (LTS). Although the BHS is penetrated by wells, it is poorly exposed at surface, and the deeper structure remained unknown. Here, results of field-based mapping combined with inversion of Full Tensor Gradiometry (FTG) and aeromagnetic data covering the BHS are presented. Although the FTG data are of a higher resolution than the magnetic data, comparable results were obtained from these independent datasets. We found that the dykes form clusters at the ends of density and susceptibility anomalies, interpreted to be lobe-like magmatic conduits. Furthermore, potential-field analysis has revealed the irregular geometry and southward-dipping nature of the BHS, in addition to the presence of multiple, near surface anomalies that may correspond to dyke clusters or other igneous bodies. Structural analysis indicates that emplacement of the dykes was controlled by pre-existing geological structures and that they have probably been deformed post-intrusion, potentially via the reactivation of pre-rift faults. The potential-field investigations and observations of dyke margin lineations from field-based structural analysis indicate that multiple dyke sources and bodies similar to the BHS may also exist. Finally, this study demonstrates that magma-poor margins, such as the Newfoundland margin, may host significant volumes of extension-related magmatic rocks, challenging the concept of a distinction between magma-rich and magma-poor rifting.

On the emplacement of the Voisey's Bay intrusion (Labrador, Canada)

The Mesoproterozoic Voisey’s Bay intrusion is a small (∼10 km3) mafic intrusive complex forming part of the 20,000 km2 Nain plutonic suite of Labrador (Canada) and hosting a world-class orthomagmatic Ni-Cu sulfide ore deposit. An extensive database of drill-core and down-hole televiewer data provides a wealth of subsurface three-dimensional information on the intrusion. The intrusion consists of the upper Eastern Deeps Chamber and lower Western Deeps Chamber, connected via a complex network of dikes. The Eastern Deeps Chamber can be divided into six units that are distinguished based on geometrical styles, dominant lithologies, and the occurrence of bodies of wall rock partly separating units. Unit I, occurring at the highest levels of the chamber, is tabular and consists dominantly of olivine gabbro with minor normal-textured (orthocumulate) troctolite. Units at midlevels of the Eastern Deeps Chamber (IIA, IIB, IIC) are sill-shaped bodies consisting of normal troctolite (i.e., orthocumulate, exhibiting tiling of plagioclase laths) with local variably textured troctolite (i.e., mesocumulate, locally pegmatitic, locally dynamically recrystallized). Units from the lowermost level (IIIA, IIIB) are irregularly shaped and consist of variable troctolite, breccias containing centimeter-sized xenoliths of host-rock paragneiss, and strongly sulfide-mineralized rocks. At least three different feeder dikes deliver magmas associated with unit IIIB. The overall dike system has an upward-curving geometry such that the dike dips shallow upward and as they approach the Eastern Deeps Chamber. This is analogous to fault geometries produced during caldera collapse or cauldron subsidence. Evolving emplacement styles reflect internal igneous processes and intermittent normal-sense activity along pre- to synemplacement brittle wall-rock structures. High-temperature normal-sense shear zones are prominent within the chamber within 100 m of its basal contact and are interpreted to have formed in response to late-stage normal faulting of the chamber floor. The geometry and internal structure of the Eastern Deeps Chamber are indicative of top-down incremental emplacement associated with floor subsidence. Our findings provide a kinematic and dynamic framework for petrogenetic models of Voisey’s Bay and other intrusion-hosted Ni-Cu sulfide deposits.

Multi-element variations in olivine as geochemical signatures of Ni-Cu sulfide mineralization in mafic magma systems—examples from Voisey’s Bay and Pants Lake intrusions, Labrador, Canada

Olivine from the olivine gabbro to troctolite intrusions at Voisey’s Bay and at Pants Lake, Labrador, was analyzed for multiple elements (Ca, Sc, Mg, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, and Zr) with secondary ion mass spectrometry (SIMS). Both intrusions have similar lithologies and petrographic characteristics and are approximately coeval (1.34 and 1.32 Ga, respectively) members of the Mesoproterozoic Nain Plutonic Suite. The Voisey’s Bay intrusion hosts a producing economic Ni-Cu sulfide deposit, whereas the Pants Lake intrusion displays evidence of Ni-Cu sulfide mineralization, but, to date, a viable ore deposit has not been discovered. Olivine from both barren and mineralized lithologies was analyzed to assess the potential of the olivine composition for providing a record of silicate melt evolution and sulfide saturation related to formation of a massive sulfide deposit. Two detailed transects were sampled, using five diamond drill holes that laterally approach the basal massive sulfide in the Eastern Deeps portion of the Voisey’s Bay intrusion from the barren central part. Olivine displays distinct trace element distributions that vary coherently with host lithology and proximity to sulfide mineralization. In particular, olivine shows an increase in Fe (∼Fo80 to ∼Fo60), Mn (∼2500 to 5000 ppm), and Zn (∼280 to 700 ppm), generally coupled with a decrease in Ni (∼1600 to ∼900 ppm), Co (∼270 to ∼190 ppm), and Cr (∼110 to 45 ppm), from barren troctolite (normal troctolite—NT) and weakly mineralized troctolite (variable-textured troctolite—VTT) towards the heavily mineralized, brecciated basal succession (basal breccia sequence—BBS). The enrichment in Fe-Mn-Zn is most pronounced in samples that laterally approach, but do not directly intersect, the massive sulfide deposit at the base of the intrusion, particularly in samples from the lower variable-textured troctolite and the basal breccia. Olivine from gabbro lithologies within the basal gabbro (BG) and upper gabbro (UG) of the Pants Lake intrusion is fairly homogeneous (∼Fo60, ∼4300 ppm Mn, ∼460 ppm Zn, ∼340 ppm Ni, ∼50 ppm Cr), with a more evolved composition (∼2100 ppm Ca, ∼20 ppm Sc, ∼300 ppm Ti, ∼4 ppm Y) than the average olivine of the Voisey’s Bay intrusion (∼570 ppm Ca, ∼6 ppm Sc, ∼65 ppm Ti, ∼0.3 ppm Y). Pants Lake olivine also commonly lacks the mutually Mn-Zn-rich signature of olivine from the basal breccia at Voisey’s Bay that is characteristic of contamination of the parent magma by country rock gneiss, and there reflects a close proximity to massive sulfides. In conjunction with petrographic observation and the stratigraphic context, a multiple-element (V-Cr-Mn-Fe-Co-Ni-Zn) regression can be applied to calculate a lateral proximity of olivine to massive sulfide mineralization in the Eastern Deeps part of the Voisey’s Bay intrusion, over a distance exceeding 150 m. The compositional variations in olivine from the economic Voisey’s Bay intrusion (bimodal and primitive) and the mainly barren Pants Lake intrusion (homogeneous and evolved) provide potential as a regional-scale mineralogical fertility indicator for mafic intrusions with comparable types of sulfide mineralization.

Wall-Rock Structural Controls on the Genesis of the Voisey's Bay Intrusion and its Ni-Cu-Co Magmatic Sulfide Mineralization (Labrador, Canada)

The Voisey’s Bay intrusion of the Mesoproterozoic Nain Plutonic Suite hosts a world-class magmatic Ni-Cu-Co sulfide deposit. Here we document the brittle and ductile structure of gneisses surrounding the intrusion using field data, borehole intersections, and subsurface structural data obtained from acoustic and optical televiewer surveys. We elucidate the role that wall-rock structure played on controlling the geometry of the intrusion and the formation of structural traps that host mineralization. The Voisey’s Bay intrusion consists at first order of two km-scale magma chambers, interpreted to represent an upper and lower chamber of the intrusion, that are connected by a system of dikes. Three types of gneiss, associated with the Paleoproterozoic Torngat orogen, form the wall rocks of the intrusion: Archean orthogneiss, Paleoproterozoic Tasiuyak paragneiss, and Paleoproterozoic enderbitic orthogneiss. Three phases of ductile deformation (D1–D3), predating intrusion emplacement, are recorded in the wall-rock gneisses. D1 produced the dominant S1 fabric, subparallel to original bedding (S) in the Tasiuyak gneiss. D2 shortening caused shallowly plunging, upright to moderately inclined, F2 folds in the enderbitic gneiss, the prototype of which was likely emplaced toward the end of D1. D3 ductile deformation resulted in steeply dipping shear zones, and flat lying to shallowly dipping C3 shear planes. At least two later phases of brittle deformation (D4 and D5) are present. D4 is a preemplacement event characterized by fractures with orientations similar to D3 shear zones and shear planes. D5 faults and joints formed during syn- to postemplacement brittle deformation likely associated with E-W brittle sinistral transtension associated with the Mesoproterozoic Gardar-Voisey’s Bay fault zone. The geometry of the Voisey’s Bay intrusion and its mineralization was controlled by pre- to synemplacement brittle structures and preemplacement ductile structures, which acted as passive wall-rock anisotropies. Mineralization hosted within Tasiuyak gneiss is controlled by the intersection of a dike with S1/S, D3 shear zones and D4 fractures. Where the intrusion occurs within enderbitic gneiss, mineralization locally coincides with shallow fabrics associated with F2 fold closures. The margins of the Voisey’s Bay dikes are controlled by steeply and shallowly dipping D4 fractures. Changes in dike thickness, which control the locations of mineralization, can be explained by thermomechanical erosion of the walls of the dike, which occurs by thermal expansion and contraction of wall rock due to repeated magma pulses, and the mechanical spalling of this wall rock along anisotropies. The strong brittle structural control on emplacement, along with previous geothermobarometry, is consistent with a mid-crustal (9–11 km) depth of emplacement. Lithosphere-scale E-W–trending D5 faults may have acted as conduits for the crustal-scale transport of primitive Voisey’s Bay magmas from their source, rather than an older orogenic suture zone.

Trace Element Variations in Olivine from the Eastern Deeps Intrusion at Voisey's Bay, Labrador, as a Monitor of Assimilation and Sulfide Saturation Processes

Ni-Cu-Co sulfide mineralization in the Mesoproterozoic Voisey’s Bay intrusion is spatially and genetically associated with magmatic breccia zones within an extensive dike system and, in particular, with the entry point of this system into a larger intrusion (Eastern Deeps intrusion). The massive sulfide- and breccia-bearing inner basal margin of this upper intrusion is enveloped by weakly mineralized variable-textured troctolite that progressively decreases in sulfide content, contains fewer paragneiss fragments, and eventually grades into a largely sulfide-barren, homogeneous plagioclase and olivine (meso)cumulate, termed normal troctolite, toward the top of the intrusion. Olivine from the troctolite lithologies, the mineralized basal breccia sequence, and the uppermost olivine gabbro in the Eastern Deeps intrusion was analyzed with secondary ion mass spectrometry (SIMS). We present multitrace element data (including petrogenetically important elements V, Cr, Mn, Co, Ni, Zn, Sr, and Y), which indicate that four petrological processes important to ore formation are recorded by the compositional diversity of olivine. The heterogeneous Ni-Co-Fo contents of olivine in the Eastern Deeps intrusion (Co/Ni ratios in olivine gabbro: 0.18–1.1, normal troctolite: 0.07–0.26, variable-textured troctolite: 0.11–0.29, basal breccia sequence: 0.20–0.34) are due to variable modification of original compositions by reequilibration with trapped silicate and sulfide liquids. The abrupt changes of Fe (Fo79–Fo66) and Ni (~2,500 to ~80 ppm) contents and some trace elements (up to 1.5x more Sr, up to 3x more Y than average normal troctolite olivine) in olivine from intervals in the barren upper normal troctolite indicate that olivine crystallized from separate pulses of magma with variable degrees of differentiation. The severe Ni depletion of one troctolite melt batch (~10 ppm Ni vs. 290 ppm Ni for average normal troctolite magma) is likely a result of sulfide saturation and segregation, either in a deeper staging chamber, or in the conduit, prior or contemporary to olivine precipitation and emplacement. The gradational increase of Fe (from Fo80 to Fo62), Mn (from ~4,000 to ~10,000 ppm), and Zn (from ~400 to ~620 ppm) in olivine from the weakly mineralized and contaminated lower part of the variable-textured troctolite downward into the massive sulfide-bearing and strongly contaminated basal breccia sequence is consistent with olivine fractionation from an increasingly country rock-contaminated, sulfide-saturated, silicate magma. Olivine here crystallized prior to or contemporaneously with the segregation of an immiscible sulfide liquid. Olivine grains that are enclosed by sulfide display homogeneous forsterite contents (~Fo62) with pronounced intragrain trace element zonation (Mn>Ni>Co>Zn). Diffusion profiles (Mn-Ni) indicate a relatively short immersion time (<<1,000 years) of these olivine grains in sulfide liquid, which likely resulted in the partial retention of Ni in the olivine. Olivine in the vicinity of massive sulfides in the Eastern Deeps intrusion (from variable-textured troctolite to basal breccia sequence) has a distinct minor and trace element signature (V-Cr-Mn-Co-Ni-Zn) compared to olivine from the barren troctolite and olivine gabbro. A multiple regression analysis using these data yields an index that describes the relative vertical proximity of olivine to massive sulfides. In conjunction with the stratigraphic context of the host lithology, this information could potentially be used as a vector toward massive sulfide mineralization in the Eastern Deeps intrusion and also support Ni exploration in other olivine-bearing mafic intrusions with similar style of sulfide mineralization.

Structural controls on the primary distribution of mafic–ultramafic intrusions containing Ni–Cu–Co–(PGE) sulfide mineralization in the roots of large igneous provinces

Deposits of Ni–Cu–Co–(PGE) sulfide often occur in association with small differentiated intrusions that reside within local transtensional spaces in strike-slip fault zones. These faults often develop in response to incipient rifting of the crust and the development of large igneous provinces due to far-field stresses generated by plume-induced continental drift. We review the geology of a number of large and small nickel sulfide deposits and the associated intrusions, and show that the geometry of the host intrusion and localization of the mineral zones can be classified into three main groups. Further, we show that the morphology of each is controlled by space created in response to deformation on structures.One group of intrusions has the plan shape of an asymmetric rhomboid with the long axis sub-parallel to a fault zone, and contacts which have often been structurally modified during and/or after emplacement of the magma. The typical cross section is a downward-closing cone shape with curved walls and often a dyke-like keel at the base. This morphology is found in the Ovoid and Discovery Hill Zones of the Voisey's Bay Deposit (Canada), the Jinchuan, Huangshan, Huangshandong, Hongqiling, Limahe, Qingquanshan, and Jingbulake (Qingbulake) Intrusions in China, and the Eagle and Eagle's Nest deposits in the USA and Canada, respectively.A second group of deposits is associated with conduits within dyke and sheet-like intrusions; these deposits are often associated with discontinuities in the dyke which were created in response to structural controls during emplacement. Examples include the Discovery Hill Deposit and the Reid Brook Zone of the Voisey's Bay Intrusion, where there are plunging domains of thicker dyke which control the mineralization inside the dyke, and thin discontinuous segments of the dyke which are associated with structurally controlled mineralization in the surrounding country rock gneisses. The Oktyabrysk, Taimyrsk, Komsomolsk, and Gluboky Deposits in the Noril'sk Region of Russia are localized at the base of thicker parts of the Kharaelakh Intrusion which appear to be a conduit that follows synformal features in the country rocks located west of the Noril'sk–Kharaelakh Fault. Other examples of dyke-like bodies with both variation in width and the development of discontinuities are the Copper Cliff and Worthington Offset Dykes which radiate away from the Sudbury Igneous Complex (Canada). The distribution of ore bodies in these Sudbury Offset Dykes is principally controlled by variations in the thickness of the dyke, interpreted to reflect the presence of conduits within the dyke.A third group of mineralized intrusions located within structural corridors have the geometry of oblate tubes; examples include Kalatongke in China, Northeastern Talnakh and Noril'sk 1 in Russia, Babel–Nebo in Australia, and Nkomati in South Africa. Sometimes these oblate tube-like intrusions form in bridging structures between larger intrusions hosted in the more significant structures. Examples include the Tamarack Intrusion in Minnesota, USA, and the Current Lake Complex in Ontario, Canada, both of which contain magmatic Ni–Cu sulfide mineralization.In all of these deposits, the intrusions appear to be open system magma pathways, and so the term “chonolith” can be applied to describe them as a group. All of these intrusions are characterized by a high ratio of sulfide/silicate; there are 1–3 orders of magnitude more sulfide in the intrusion than the magma contained in the intrusion is capable of dissolving. The formation of these deposits is considered to have taken place in open system magma conduits. It is possible that the metal tenor of the sulfides were upgraded by equilibration of successive batches of silicate magma passing through the conduit, and equilibrating with a stationary pool of magmatic sulfide. At Voisey's Bay there appears little doubt that the sulfides were injected through a conduit dyke into higher level magma chambers. A similar model has been proposed for the formation of the deposits at Jinchuan and Noril'sk–Kharaelakh. Economically significant nickel sulfide deposits that tend to be high in Ni tenor, are often related to the late injection of magma that form distinct parts of the intrusion, and the localization of mineralization tends to be related to changes in the geometry of the magma chamber. Strongly deformed and metamorphosed komatiite-associated deposits (e.g. Pechenga, Thompson, and the Yilgarn komatiite associations) appear to be the remains of open system magma conduits which are now represented by segmented and boudinaged ultramafic bodies as a result of more than 4 phases of post-emplacement deformation.LIP activity at craton margins has long been recognized as a key control on the genesis of magmatic sulfide deposits; we show that the principal regional controls of strike-slip tectonics underpin the local geometry of the intrusions, and we provide an explanation for why so many of the global nickel sulfide ore deposits are associated with intrusions that share common morphologies and characteristics. This model provides a framework for more detailed structural investigations of nickel sulfide deposits, and it is a predictive framework for mineral exploration.

S saturation history of Nain Plutonic Suite mafic intrusions: origin of the Voisey’s Bay Ni–Cu–Co sulfide deposit, Labrador, Canada

The Voisey’s Bay deposit is hosted in a 1.34-Ga intrusion composed of troctolite, olivine gabbro, and ferrogabbro. The sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites, and also occurs as veins along structures in adjacent paragneiss. A dyke is connected to the base of the north wall of the Eastern Deeps Intrusion, and the entry point of this dyke into the chamber is the locus of the Eastern Deeps nickel sulfide deposit. A detailed exploration in the area between the Eastern Deeps and the Ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke. The Eastern Deeps Deposit is surrounded by a halo of moderately to weakly mineralized variable-textured troctolite (VTT) that reaches a maximum thickness above the axis of the Eastern Deeps Deposit along the northern wall of the Eastern Deeps Intrusion. The massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the VTT, and there is a marked break in Ni tenor of sulfide between the two. Sulfides hosted in the dyke tend to have low metal tenors ([Ni]100 = 2.5–3.5%), sulfides in Eastern Deeps massive and breccia ores have intermediate Ni tenors ([Ni]100 = 3.5–4%), and disseminated sulfides in overlying rocks have high Ni tenors ([Ni]100 = 4–8%). Four principal processes control the compositions of the Voisey’s Bay sulfides. Coarse-grained loop-textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the Pd/Ir ratio which is due to mineralogical variations where pentlandite is enriched in Pd and Ir is dispersed throughout the mineral assemblage. A decrease in Ir and Rh from the margin of the Ovoid toward cubanite-rich parts at the central part of the Ovoid is consistent with fractionation of the sulfide from the margins toward the center of the Ovoid. The Ovoid ores have higher Ni and Pd tenor than the Eastern Deeps massive sulfides; this is consistent with both a higher R factor and greater degree of silicate parental magma evolution in the Ovoid than the Eastern Deeps. The disseminated sulfides surrounding the Eastern Deeps deposit have some of the highest Ni and Pd tenors at Voisey’s Bay, which are indicative of not only more primitive magmas but also higher R factors than the Ovoid or the Eastern Deeps. VTT and normal-textured troctolite of the Eastern Deeps that contain trace sulfide have 0.1–3 ppb Pt and 0.1–3 ppb Pd, whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of Pt and Pd. Troctolites and olivine gabbros from other parts of the Voisey’s Bay Intrusion and other Nain Plutonic Suite Intrusions, including the Kiglapait, Newark Bay, Barth Island, Mushua, and Nain Bay South Intrusion, also have low platinum group element abundances. Although it is possible that this is a signature of a widespread sulfide saturation event that pre-dated ore formation at Voisey’s Bay, it is more likely that platinum group element (PGE) depletion is a product of the source melting process where low degrees of melting resulted in the retention of PGE in the mantle source. If so, this indicates that PGE depletion should be used with caution as an exploration tool in the Nain Plutonic Suite.

Oxygen isotope mapping of the Archean Sturgeon Lake caldera complex and VMS-related hydrothermal system, Northwestern Ontario, Canada

The hydrothermal and magmatic evolution of the Sturgeon Lake caldera complex is graphically documented by a regional-scale (525 km2) analysis of oxygen isotopes. Spatial variations in whole-rock oxygen isotope compositions provide a thermal map of the cumulative effects of multiple stages of hydrothermal metasomatism before, during, and after volcanogenic massive sulfide (VMS) mineralization. There is a progressive, upward increase in δ18O from less than 2‰ to greater than 15‰ through a 5-km-thick section above the Biedelman Bay subvolcanic intrusive complex. This isotopic trend makes it clear that at least the earlier phases of this intrusive complex were coeval with the overlying VMS-hosting cauldron succession and provided thermal energy to drive a convective hydrothermal circulation system. The sharp contrast in δ18O values between late stage phases of the Biedelman Bay intrusion and immediate hanging wall strata indicates that the main phase of VMS-related hydrothermal activity took place before late-stage resurgence in the cauldron-related magmatic activity. Mineralogical and isotopic evidence indicates the presence of both syn- and postmineralization hydrothermal activity defined by the presence of widespread semiconformable and more restricted discordant alteration zones that affect the pre- and syncauldron strata. The semiconformable alteration zones formed during early stages of hydrothermal circulation and are defined by widespread silicification and carbonatization in association with relatively high δ18O values. The discordant alteration assemblages, containing Al-silicate minerals with chloritoid and/or Fe-rich carbonate or chlorite, centered on synvolcanic faults represent restricted zones of both seawater inflow and hydrothermal fluid upflow. A rapid increase in δ18O values (∼7–9‰) over a short distance (<200 m) suggests marked cooling of hydrothermal fluid from ∼350°C to less than 130°C either just before or during discharge onto the seafloor. Late emplacement of diorite sills and a dacite dome disrupted the isotopic imprint of cauldron-stage hydrothermal activity. The abrupt lowering of δ18O values at the transition from explosive to passive volcanism (andesite flows and dacite domes) indicates postcauldron emergence. Subsequently, thrust faulting disrupted the older synvolcanic hydrothermal isotopic patterns.

Remagnetization (∼1100 Ma) of Voosges lake tuff (1542 Ma) and other mid-proterozoic rocks, Nipigon graben, northern Ontario

Recently discovered horizons of non-deformed felsic welded crystal tuff in the Proterozoic rift system of northern Ontario yield a geologically stable paleomagnetic vector. Using alternating field (AF), thermal (TD) and low-temperature demagnetization (LTD) techniques, a single stable characteristic remanence (ChRM) vector is found in all specimens (declination/inclination = 115°/−73.5°; α 95 = 2.1°, n = 58). It is overprinted by a Bruhnes’ epoch VRM. The tuffs paleopole position coincides with that of ubiquitous sub-horizontal diabase sheets which intruded at ∼1110 Ma during Late Proterozoic Keewanawan Volcanism. The latter produced numerous volcanic and intrusive rocks along the margins of the Lake Superior paleo-rift system, during the interval 1087–1120 Ma, but the sills are most extensive. Although the Voosges Lake Tuff shares the same paleopole location as the diabase sheets (1109 Ma), the nearby Seagull pluton (1112 Ma) and the Keweenawan volcanic rocks, its primary zircons yielded a Mesoproterozoic crystallization age of 1542 ± 3 Ma. This is similar to the nearby English Bay felsic intrusions dated at 1537 − 2 + 10 Ma [Davis, D.W., Sutcliffe, R.H., 1985. U-Pb ages from the Nipigon “Plate” (sic) and northern Lake Superior. Geol. Soc. Am. Bull. 96, 1572–1579]. Whereas the paleomagnetic magnetic signal appears primary in the 1542 Ma Voosges Tuff, it was completely reset by the Late Proterozoic Keewanawan paleofield (∼1100 Ma). The Tuff units are separated from the diabase sills by stratigraphic thicknesses of ≤50 m and show no traditional petrographic evidence of annealing or thermal metamorphism. Since it is improbable that the 1542 Ma tuff returned to the same paleolatitude at 1109 Ma, we postulate that metasomatic processes caused a ∼1109 Ma remagnetization. A similar event could explain unusual ChRMs reported for certain other pre-Keweenawan Proterozoic sites with adjacent Keewanawan diabase sheets. For example, some paleopoles reported for the Gunflint Formation (1878 Ma) and Sibley Formation (>1339 Ma, perhaps ∼1537 Ma by association with the English Bay intrusions) might have been acquired due to secondary remagnetization.

Controls on the Fo and Ni Contents of Olivine in Sulfide-bearing Mafic/Ultramafic Intrusions: Principles, Modeling, and Examples from Voisey's Bay

Both Ni and Mg are compatible in mafic minerals that form early during the fractional crystallization of mafic/ultramafic magma; thus, both decrease in abundance in the silicate magma, and hence in later-forming silicates as fractionation proceeds. The concentration of Ni in silicates such as olivine and the MgO/FeO ratio of the silicates are related to values in the magma from which the olivines are crystallized by coefficients, which have been experimentally determined and therefore can be used to infer information about the magma. If the magma is saturated in sulfide so that sulfide droplets are removed along with mafic silicates during fractionation, additional Ni will be removed in comparison with the sulfide-absent situation. This will be reflected in a more rapid decrease of Ni with Fo than if sulfides were not separated. Variations of Ni with Fo are examined in the light of model curves for the Voisey's Bay Intrusion that hosts a world-class Ni-Cu-Co sulfide deposit in Labrador, Canada. In the past, it has been a practice to compare the Ni and Fo contents of olivines from a given intrusion with the field determined by Simkin and Smith (1970) for a wide variety of igneous olivines to identify those that are Ni-depleted. The objective is that these are presumed to have come from sulfide-saturated, and therefore, economically-interesting magma. This study shows that this simple comparison can lead to errors. It is important to compare natural data with model curves that have been generated for, and reflect the cumulus mineralogy of each intrusion in question. Using this approach, the natural data can be closely duplicated by model curves, which, in some cases, place additional constraints on possible petrologic interpretations. For example, at Voisey's Bay, a period of sulfide-unsaturated fractionation can be shown to have been succeeded by the removal of a sulfide liquid plus silicate minerals, followed by a period of silicate crystallization.

A synopsis of mineral deposits in the Archean and Proterozoic rocks of the Lake Nipigon Region, Thunder Bay District, Ontario

A variety of metallic and non-metallic mineral deposit types occur within Archean and Proterozoic rocks in the area encompassing the Lake Nipigon Region Geoscience Initiative. Archean deposit types include Algoma-type banded iron formation-hosted iron (e.g., Lake Nipigon iron range); volcanogenic massive sulphide copper–zinc (e.g., Onaman–Tashota belt); ultramafic intrusion-hosted chromium (e.g., Puddy–Chrome lakes); mafic to ultramafic intrusion-hosted copper–nickel – platinum group element (PGE) (e.g., Lac des Iles); and pegmatite-hosted deposits of rare metals (Li, Ta, Be), uranium, and molybdenum (e.g., Georgia Lake field, Black Sturgeon Lake, and Anderson Lake, respectively). Mesothermal lode gold deposits are prominent in the Beardmore–Geraldton camp. Superior-type iron formation occurs in Paleoproterozoic Gunflint Formation. "Red-bed" copper occurs in Mesoproterozoic Midcontinent Rift-related Osler Group volcanic and interflow sedimentary rocks. Native copper and copper sulphides occur in Mesoproterozoic Sibley Group sedimentary rocks, adjacent to ultramafic intrusions. These mafic to ultramafic intrusions, associated with Midcontinent Rift magmatism, host copper–nickel–PGE deposits (e.g., Seagull, Great Lakes Nickel). Silver-bearing veins occur in Paleoproterozoic Animikie Group sedimentary rocks in proximity to Midcontinent Rift-related mafic intrusions (e.g., Silver Islet, Silver Mountain). Lead–zinc–barite veins, uranium-bearing veins, and amethyst vein and replacement-type deposits may be cogenetic and formed at or near the unconformity between Sibley Group basal sandstone and underlying Archean granitic basement (e.g., Dorion, Black Sturgeon Lake, McTavish Township). The hydrothermal systems that produced all of these veins were probably driven by heat associated with Midcontinent rifting. Many occur in structures related to rift-bounding faults. Iron oxide – copper–gold deposits may occur near the English Bay intrusion.

Voisey’s Bay – Geology, Geochemistry and Genesis

The ~ 1333 Ma Voisey’s Bay Intrusion represents one of the earliest magmatic events recorded in the development of the Nain Plutonic Suite. The intrusion was likely emplaced at a depth of ~ 14 km.Sulphide saturation occurred due to the assimilation of bulk Churchill gneiss in a series of sub-chambers below the current level of erosion. The current economic ore zones formed by the accumulation of magmatic sulphides in physical traps from a pulsed series of sulphide-laden magmas derived from the sub-chambers.The initial melt had an MgO content of > 8 wt % and a geochemical and isotopic composition similar to some Proterozoic flood basalts.

Oxygen isotopic studies of the interaction between xenoliths and mafic magma, Voisey’s Bay Intrusion, Labrador, Canada

Sulfide mineralization in the Voisey’s Bay Intrusion, Labrador, Canada, is closely associated with country rock xenoliths that have extensively reacted with basaltic magma. In order to better understand the processes that control the assimilation of country rocks by mafic magma, a detailed study of oxygen isotope systematics related to magma-country rock interaction in the Voisey’s Bay area was undertaken. Protracted interaction of the xenoliths with magma produced refractory mineral assemblages in the xenoliths (2–10 cm in diameter) composed of Ca-rich plagioclase, corundum, hercynite, and minor magnetite. Overgrowth rims of plagioclase and biotite that surround most xenoliths separate the restites from the enclosing igneous matrix. The δ 18O values of minerals from regionally metamorphosed pelitic and quartzofeldspathic protoliths are: plagioclase (8.7–12.3‰), orthoclase (9.5–9.8‰), biotite (5.2–8.7‰), garnet (8.3–10.8‰), pyroxene (8.0–10.1‰), and quartz (9.6–14.0). The δ 18O values of minerals from the hornfels in the contact aureole of the intrusion are consistent with modeling which indicates that as a result of essentially closed system contact metamorphism oxygen isotope values should differ only slightly from those of the protoliths. Hercynite, plagioclase, and corundum separates from the xenoliths have δ 18O values that vary from 2.9‰ to 10.5‰, 5.6‰ to 10.9‰, and 2.0‰ to 6.8‰, respectively. Although a siliceous 18O-enriched melt has been lost from the xenoliths, corundum, and feldspar δ 18O values are significantly lower than expected through melt loss alone. The relatively low δ 18O values of minerals from the xenoliths may be a function of incomplete isotopic exchange with surrounding mafic magma which had a δ 18O value of ∼5.5‰ to 6.0‰. The high- 18O melt that was released from the xenoliths is partially recorded in the plagioclase overgrowth on the margin of the xenoliths ( δ 18O values from 6.2‰ to 10.7‰), and in hercynite that replaced corundum. However, mass balance calculations indicate that a portion of the partial melt must have been transferred to magma that was moving through the conduit system. δ 18O and δD values of biotite surrounding the plagioclase overgrowth range from 5.0‰ to 6.2‰ and −58‰ to −80‰, respectively. These data suggest that the outermost rim associated with many xenoliths has closely approached isotopic equilibrium with uncontaminated mafic magma. The current gabbroic to troctolitic matrix of the xenoliths shows no evidence for contamination by the high- 18O partial melt from the xenoliths. The feldspar and biotite overgrowths on the xenoliths that formed after the motion of the xenoliths relative to the magma had stopped prevented further isotopic exchange between the xenoliths and final magma. The minerals within the xenoliths are not in oxygen isotopic equilibrium with each other, due in part to rapid thermal equilibration, partial melting, and partial exchange with flow through magma.

Oxygen isotopic disequilibrium in plagioclase–corundum–hercynite xenoliths from the Voisey's Bay Intrusion, Labrador, Canada

Xenoliths of metamorphic country rocks are locally abundant within breccia sequences in gabbroic to troctolitic rocks of the Voisey's Bay Intrusion, Labrador, Canada. Thermochemical interaction with mafic magma has produced restite assemblages in the xenoliths that are composed of Ca-rich plagioclase, corundum, hercynite and minor magnetite. Hercynite was produced as a result of the breakdown of garnet and pyroxene that were originally present in the metamorphic rocks, as well as by the replacement of corundum, which was itself a product of feldspar degradation during xenolith–magma interaction. Hercynite that replaces pyroxene and garnet is granular to bulbous, whereas that which replaces corundum is acicular to skeletal. Ion microprobe analyses indicate that oxygen isotopic equilibrium was neither established during retrograde cooling of the xenolith assemblage, nor during the replacement processes which led to both types of pseudomorphous hercynite. δ 18O values of hercynite that replaces garnet and pyroxene range from 5 to 11.5‰, and in part reflect the elevated δ 18O values of the protolith minerals. Hercynite that replaces acicular corundum is characterized by δ 18O values between 2.5 and 7.6‰. Oxygen isotopic equilibration with mantle-derived magma having a δ 18O value of ∼ 5.5‰ should have produced values of hercynite near 2.5‰. Values of Δ (plagioclase–corundum) range between 0.2 and 6.1‰, whereas D (hercynite–corundum) values, which are expected to be near 0, may be as large as 9.6‰ at the sub-millimeter scale. Oxygen isotopic disequilibrium may have resulted in the xenoliths due to rapid withdrawal of partial melt, and to the crystallization of plagioclase and biotite bands around the xenoliths which prevented isotopic communication of xenoliths with enclosing mafic magma. Although diffusive transport of oxygen in the xenoliths should have been a viable mechanism for isotopic exchange, the lack of isotopic equilibration during cooling and the preservation of steep isotopic gradients in composite hercynite–corundum grains suggest that reaction and crystallization were necessary to promote oxygen isotopic exchange among minerals in the xenoliths contained within the Voisey's Bay Intrusion.

Petrologic evolution of gneissic xenoliths in the Voisey's Bay Intrusion, Labrador, Canada: Mineralogy, reactions, partial melting, and mechanisms of mass transfer

Nickel‐copper‐cobalt sulfide ores in the Voisey's Bay Intrusion are closely associated with troctolitic to gabbroic rocks that contain abundant country rock xenoliths. Potential sources of the xenoliths include pelitic paragneiss, enderbitic orthogneiss, and mafic to quartzofeldspathic gneisses that form immediate country rocks to different parts of the intrusion. Regardless of location, all xenoliths have a similar refractory mineral assemblage composed of hercynite, magnetite, Ca‐rich plagioclase, and corundum. The refractory mineral assemblage formed via partial melting immediately after xenoliths were engulfed by magma. Rapid thermal equilibration allowed the xenoliths to survive prolonged interaction with magma. Corundum was formed by the incongruent melting of Na‐rich plagioclase in pelitic and quartzofeldspathic gneisses. Corundum and Ca‐rich plagioclase assemblages are aluminous; their origin involved either multistage melting of protoliths where the production of a granitic minimum melt was followed by the liberation of a more silica‐rich and Al‐poor melt or one‐stage disequilibrium melting. Density differences between the xenoliths, restite assemblages, and enclosing mafic magma facilitated the separation of partial melt from restite. No evidence for the melts in the form of channels or interstitial glass is observed in the restite. Flow of mafic magma in the conduit system is thought to have dispersed the Si‐ and alkali‐rich melts derived from the xenoliths. The only record of the xenolith‐derived melts is in the form of concentric rims of plagioclase and biotite which crystallized from a hybrid melt in boundary layers around most xenoliths. Hercynite in the restitic assemblage was produced either by partial melting involving Fe‐ and Mg‐bearing minerals such as garnet and pyroxene or by replacement of corundum. FeO and MgO that were excluded from the boundary layer diffused inward and reacted with corundum to form hercynite. The thickness of the hercynite bands suggests formation times between 3,000 and 23,000 years. Where the insulating rims of plagioclase and biotite were not present, diffusion of FeO and MgO from the surrounding crystal mush continued, resulting in near complete conversion of corundum to hercynite. The highly refractory mineral assemblages which characterize xenoliths present in the Voisey's Bay Intrusion provide evidence for a complex history of magma–country rock interaction. The transfer of xenomelts and sulfur to flowing magma may have been essential for the formation of the magmatic sulfide ores.

Nickeliferous gabbroic intrusions of the Pants Lake area, Labrador, Canada: Implications for the development of magmatic sulfides in mafic systems

The Pants Lake intrusions are located some 80 kilometers south of the world-class Voisey's Bay Ni-Cu-Co sulfide deposit in Labrador, Canada. They are dominated by olivine gabbro, with subordinate troctolite, melagabbro, peridotite and leucogabbro. They form several sheet-like bodies that were emplaced into metasedimentary gneisses that locally contain sulfides. The largest of these, termed the North and South intrusions, have been dated by other workers at 1322 ′ 2 Ma and 1337 ′ 2 Ma respectively, indicating that they formed in two discrete events. The North intrusion is more varied petrologically, and its three units show ambiguous contact relationships implying that they partly coexisted as liquids. Disseminated sulfide mineralization is widespread near the basal contacts of both intrusions, but massive sulfide mineralization is rare. In the older South intrusion, sulfides are hosted by melagabbro and peridotite of cumulate origin, and probably represent a gravitational accumulation. In the younger North intrusion, they are hosted by a complex mineralized sequence, interpreted to represent two or more influxes of magma charged with sulfides and reacted country-rock fragments. The older South intrusion is geochemically distinct from the younger North intrusion, and more closely resembles the Voisey's Bay Intrusion. Mineralized rocks within both Pants Lake intrusions are geochemically similar to the associated unmineralized rocks, and must be closer related to them. The mafic rocks almost all have low Ni contents and low Cu/Zr ratios that imply extraction of metals by sulfide liquids. This pervasive depletion signature, coupled with consistent Ni/Cu and Ni/Co, and clustering of sulfide metal contents, suggests that sulfide liquids were developed on an intrusion-wide scale, probably at depth, rather than by local processes. Sulfides at Pants Lake typically contain less than 2 percent Ni and 2 percent Cu. These values agree with predictions from the low Ni contents of silicate rocks, and imply a low mass ratio (R factor) of silicate magma to sulfide liquid (R 1000). The pervasive Ni and Cu depletion contrasts with the more localized metal depletion seen at Voisey's Bay. This may indicate that the Pants Lake sulfide liquids were not as effectively upgraded by later batches of undepleted magma. From the perspective of metallogenesis, data from the Pants Lake intrusions support several key concepts proposed m models for the formation of the Voisey's Bay deposit, suggesting that these may apply more widely as controls on the formation of magmatic sulfide deposits in broadly gabbroic magma systems.

Paragneiss Assimilation in the Genesis of Magmatic Ni-Cu-Co Sulfide Mineralization at Voisey's Bay, Labrador: 34S, 13C, and Se/S Evidence

The Voisey’s Bay Ni-Cu-Co deposit is located in ca 1.33 Ga troctolitic rocks of the Nain Plutonic Suite, northern Labrador. It is widely regarded as a prime example of sulfide mineralization associated with a magmatic conduit system. Previous geological and geochemical studies have indicated that the Voisey’s Bay intrusion was contaminated by interaction with both sulfide-poor Archean gneisses and with Paleoproterozoic sulfide-bearing Tasiuyak metasedimentary gneiss. Limited sulfur isotope data previously indicated that only one of the four ore zones at Voisey’s Bay is characterized by δ 34S values outside the range considered normal for uncontaminated, mantle-derived mafic magmas (0 ± 2‰). New S isotope data presented here extends the previously reported range in δ 34S values for the Tasiuyak gneiss from –17.0 to +18.3 per mil, with an average of –1.4 per mil. Assimilation of this sulfur, followed by isotope homogenization in the magmatic system, is consistent with the isotope data from the ore zones, and in line with the wealth of evidence that suggests that interaction with Tasiuyak gneiss was an essential ingredient for ore formation in all the zones. Se/S values of sulfide minerals in the ore zones at Voisey’s Bay are also in the same range as those of whole-rock samples from the Tasiuyak gneiss. δ 13C values of both mineralized and nonmineralized parts of the Voisey’s Bay intrusion are anomalous (–18.3 to –24.8‰), and similar to values of graphite in the Tasiuyak gneiss. Both S and C contamination of the Voisey’s Bay intrusion via diffusive transfer in a fluid phase derived from inclusions and wall rocks are considered unlikely because the area had been subjected to granulite-grade metamorphism prior to emplacement of the Nain Plutonic Suite. Either fractional melting or total fusion of xenoliths is considered a more likely mechanism for S addition to the mafic magma via pyrrhotite melting and for C addition in the form of CO2 and CH4 produced by reaction of graphite with O- and H-bearing melt species.

WHY ARE THERE NO MAJOR Ni Cu SULFIDE DEPOSITS IN LARGE LAYERED MAFIC-ULTRAMAFIC INTRUSIONS?

Large, layered intrusions throughout the world are characterized by a remarkable paucity in economically important magmatic Ni–Cu sulfide deposits. The Sudbury intrusion in Canada is a notable exception, but sulfide ores there were formed by a unique combination of processes involving crustal melting in response to a meteorite impact. Economically important magmatic Ni–Cu sulfide deposits tend to occur in magma conduit systems. Such an environment has been documented at Noril’sk, Jinchuan, and the recently discovered Uitkomst deposit of South Africa and the Voisey’s Bay intrusion of Labrador, Canada, which are associated with the Bushveld Complex and the Nain Plutonic Suite, respectively. The enhanced ore potential of conduit systems is attributed to their specific flow environment and to the exploitation of the conduits by multiple flows of magma. Sulfides that were entrained and transported by ascending silicate magma were concentrated in the widened parts of the conduits and, at Voisey’s Bay, near the exit of the conduit into a larger chamber, owing to a decrease in the flow velocity of the magma. Further, the precipitated sulfides were upgraded by means of reaction with continued surges of undepleted magma using the same conduit. Thus, exploration for magmatic Ni–Cu sulfide ores should be redirected from layered intrusions to their associated conduit systems.