Voisey's Bay Research Articles

Three-dimensional airborne electromagnetic forward modeling based on multiscale hexahedral finite-element method

Slow forward modeling is the main factor that restricts the practical use of three-dimensional (3D) inversion and interpretation of airborne electromagnetic (AEM) data. To improve the modeling efficiency in 3D AEM, we propose a new multiscale finite-element (MsFE) method based on unstructured hexahedral meshes. Compared with the traditional 3D AEM forward modeling, the main advantage of our newly developed method is that it can simulate complex underground structures in the earth quickly. Since we can fit the earth's topography or the anomalous bodies underground using a small number of hexahedral grids, we can quickly model them using MsFE. The main idea of the MsFE forward modeling method is to construct an interpolation operator between a coarse and a dense mesh and use the interpolation operator to map the conventional FE coefficient matrix to the MsFE coefficient matrix and thus reduce the number of unknowns in the modeling process. This will vastly reduce the scale of the linear equations system. We validate our method by simulating a typical mountain peak model and demonstrate its effectiveness by simulating numerous synthetic models and a model from Voisey Bay's Ovoid sulfide deposit, Canada.

A finite‐element time‐domain forward‐modelling algorithm for transient electromagnetics excited by grounded‐wire sources

ABSTRACTA three‐dimensional finite‐element time‐domain forward‐modelling algorithm is developed to simulate transient electromagnetics excited by grounded‐wire sources. The main advantage of this finite‐element time‐domain algorithm is that full transmitting‐current waveforms and complex‐shaped sources resulting from topography can be directly dealt with in this algorithm. The models used to test this algorithm include a homogeneous half‐space model, a stratified‐medium model, the model of a complex conductor at a vertical contact and the Ovoid Zone massive sulfide deposit at Voisey's Bay, Canada. The homogeneous half‐space model is used to determine the truncation boundary for a computational domain, and to compare with the electromagnetic responses excited by step‐off, step‐on and direct current waveforms. For the stratified‐medium model, results demonstrate that full transmitting waveforms have strong effects on the observed electromagnetic responses. The model of a complex conductor at a vertical contact is designed for the grounded electrical source airborne transient electromagnetic method and is also used to examine the effectiveness of the broadside and inline configurations for such a vertical, thin plate embedded in the subsurface. The area of the Ovoid Zone massive sulfide deposit possesses non‐negligible topography, the effects of which on the shapes of the grounded‐wire sources must be taken into account when implementing the finite‐element time‐domain solver. The results show that both the broadside and inline electromagnetic responses are strongly affected by the massive conductive ore body.

Investigation of two pack ice besetting events on the Umiak I and development of a probabilistic prediction model

Pack ice besetting events are defined as incidents in which a typical ice-class vessel encounters dynamic sea ice conditions which are beyond the ship's icebreaking and maneuverability capacities, and the vessel becomes stuck in the pack ice until such conditions subside. Besetting events can cause severe disruption to a vessel's transit schedule, leading to increased operational downtime, and ship crew and fuel costs. Hence, predicting the probability of occurrence of these events can help vessel operators plan in advance accordingly, possibly rerouting the vessel, and saving operational costs. In this paper, two pack ice besetting events experienced by the Umiak I, an icebreaking bulk carrier owned and operated by Fednav Limited which runs nickel ore from the mine at Voisey's Bay, Labrador to Québec City, are analyzed in terms of the wind, surface ocean current, and sea ice conditions which caused them. The particular metocean and pack ice conditions associated with the occurrence of these events are used to develop a framework for a model to predict the future occurrence of such events for the Umiak I and quantify probabilities of besetting event occurrence linked to specific sets of coincident metocean and ice conditions. The model will serve as a tool for predicting besetting event probabilities during Umiak I navigations, as well as in hindcast shipping feasibility studies, to determine the likelihood of besetting for vessels with Umiak I capabilities in regions of new operations. The factors considered in this study are the vessel distance to the nearest coastline, regional wind and surface current speed, direction relative to the nearest coastline, divergence, and vorticity. The ice factors considered are the total concentration, mean thickness, and floe size (this represents a mean floe diameter). The datasets analyzed in this paper suggest the primary causes of the two besetting events experienced by the Umiak I in 2012 and 2013 were the relatively large ice floes encountered by the vessel (the majority of the floes during the besetting events were greater than 6 km in diameter), in combination with a convergent regional wind field and onshore winds and currents.

Metals and society: an introduction to economic geology

1. Introduction to economic geology.- 2. Classification, distribution and uses of ores and ore deposits.- 3. Magmatic ore deposits.- 4. Hydrothermal deposits.- 5. Deposits formed by sedimentary and surficial processes.- 6. Economic geology in the future. Chap. 1: Introduction to Economic Geology What is economic geology? Peak copper and related issues What is an ore, and what is an ore deposit? Factors that influence whether a deposit can be mined. Chap. 2: Classification, Distribution and Uses of Ores and Ore Deposits Classifications of Ores Classifications of Ore Deposits Global Distribution of Ore Deposits Global Production, Consumption and Trade in Mineral Resources Chap. 3: Magmatic ore deposits Introduction Chromite deposits of the Bushveld Complex Magnetite and Platinum Group Element deposits of the Bushveld Complex Magmatic sulfide deposits Kambalda nickel sulfide deposits Noril'sk-Talnakh nickel sulfide deposits Other Ni sulfide deposits -- Sudbury, Voisey's Bay, Jinchuan Other magmatic ore deposits Chap. 4: Hydrothermal Deposits Introduction Key factors in the formation of a hydrothermal ore deposit Examples of hydrothermal deposits and ore-forming processes Volcanogenic massive sulfide (VMS) deposits Porphyry deposits Sedimentary exhalative (SEDEX) deposits Mississippi Valley type (MVT) deposits Other hydrothermal deposits Uranium deposits Iron-oxide copper gold (IOCG) deposits. Gold deposits Chap. 5: Deposits formed by Sedimentary and Surficial Processes Introduction Placers Gold placers Zr and Ti minerals in Beach sands Alluvial diamonds Other placers: tin, platinum, thorium-uranium Sedimentary deposits Types and Characteristics of Iron Deposits Other sedimentary deposits: Mn, phosphate, nitrates, salt Laterites Bauxite Ni laterites Supergene alteration Chap. 6: Economic geology in the future Introduction Rare Earth Elements Lithium Mining and Mineral Exploration in the 21st century

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.

The Role of Paragneiss Assimilation in the Origin of the Voisey's Bay Ni-Cu Sulfide Deposit, Labrador: Multiple S and Fe Isotope Evidence

Isotopic and geochemical studies conducted on the Voisey's Bay deposit, Labrador, Canada, suggest crustal contamination of the primary magma as a trigger for sulfur saturation and formation of the deposit. The use of multiple S isotopes has allowed for the identification of a bacterial sulfate reduction biosignature in the Tasiuyak gneiss in the footwall to the Voisey's Bay deposit. This putative biosignature is preserved in the deposit even at high silicate magma/sulfide melt ratios (R-factor) and links the S present in the Voisey's Bay deposit to the Tasiuyak gneiss. Iron isotopes in the Voisey's Bay deposit have been reset to magmatic values at R-factors > ≈100, but S isotope data can be used to model higher R-factors. A contamination model results in calculated R-factors of 433 ± 177. The multiple S isotope data are a new proxy to directly link S from the deposit to crustal S sources even in deposits with high R-factors where the equilibration with large amounts of silicate magma can make interpreting a link between the deposit and the sulfur source difficult

Using stochastic crosshole seismic velocity tomography and Bayesian simulation to estimate Ni grades: Case study from Voisey's Bay, Canada

Modeling of grades is a key step and the major source of error in appraisal stage of mining projects. We used a geostatistical approach to explicitly integrate seismic travel time data, as well as acoustic and core logging data into the estimation of nickel grades in the Voisey's Bay deposit. Firstly, the crosshole seismic travel times are inverted using a stochastic tomographic algorithm. This algorithm allows for the inclusion of acoustic log data and seismic covariance into the inverse problem, leading to high-resolution velocity tomographic images of the orebody. Secondly, grade realizations are generated using a Bayesian sequential Gaussian simulation algorithm, which integrates the ore grades measured on the core logs and the previously inverted tomographic data. The application of the presented method to the Voisey's Bay deposit yields an improved knowledge of the geology setting and generates grade models with realistic spatial variability compared to conventional methods.

Inversion of first-arrival seismic traveltimes without rays, implemented on unstructured grids

SUMMARY We develop a method for inverting first-arrival seismic traveltimes without using ray tracing in the forward solution, nor to calculate sensitivity information. We consider unstructured 2-D triangular and 3-D tetrahedral grids for discretizing the subsurface velocity distribution. Unstructured grids provide some computational advantages when dealing with complicated shapes that are difficult to represent with rectilinear grids. However, we stress that the key details of our inversion approach that avoid ray tracing are equally relevant to rectilinear grids. The forward problem is solved using the Fast Marching Method, an efficient numerical algorithm that can be used for propagating first-arrival seismic wave fronts through a velocity distribution. Our minimum structure inversion algorithm uses sensitivity information calculated directly during the forward solution. This calculation relies on explicit symbolic differentiation of the forward modelling equations and, as such, our inversion approach does not require ray tracing and uses sensitivity information that is numerically consistent with the forward solution. Our method is applied to 2-D and 3-D geologically realistic synthetic scenarios based on the Voisey's Bay massive sulfide deposit in Labrador, Canada.

Computing first-arrival seismic traveltimes on unstructured 3-D tetrahedral grids using the Fast Marching Method

SUMMARY The Fast Marching Method is an efficient numerical algorithm for propagating interfaces such as first-arrival seismic wave fronts travelling through a velocity distribution. Fast Marching solutions have been developed for use on rectilinear grids in 2-D and 3-D. We are interested in unstructured grids as they provide some computational advantages when dealing with complicated shapes that are difficult to represent with rectilinear grids. Fast Marching solutions have also been developed for unstructured 2-D triangular grids but this has yet to be extended to unstructured 3-D tetrahedral grids. In this paper, we extend the Fast Marching Method to unstructured 3-D tetrahedral grids using a derivation that follows the 2-D case. The resulting equations are discussed in intuitive terms and an error analysis is performed. Our method is applied to a simple synthetic example and to a more complicated model based on the Voisey's Bay massive sulphide deposit in Labrador, Canada.

Inco Ltd.: Market Value, Fair Value, and Management Discretion

ABSTRACTWe examine management discretion to decide when and how much to write down an asset, in a unique case where a tracking stock provides an observable market value for the asset. We find that, despite market evidence that Inco Ltd.'s financial statements substantially overvalued the Voisey's Bay nickel mine throughout 1997 to 2000, management chose not to write down the mine until 2002. Inco management used an independent fairness opinion to justify its December 2000 redemption of the tracking stock at 25% of its initial value, indicating almost surely that Inco management was aware of the generally accepted accounting principles (GAAP) impairment. This case illustrates that GAAP's reliance on undiscounted cash flows for impairment decisions allows huge unrecorded disparities between book and market value. The management discretion exercised in this case provides a concrete example of the subjectivity inherent in fair valuation.

Inco Ltd.: Market Value, Fair Value, and Management Discretion*

This study examines management discretion in determining when and how much to write down impaired assets under generally accepted accounting principles (GAAP). Current U.S. GAAP identify an asset as impaired when its recorded value exceeds the amount at which a willing buyer and seller would transact, but allows a write-down only when the recorded value exceeds the sum of undiscounted cash flow. We examine a unique case where we can observe a separate market value for the impaired long-term asset for the majority of the period from its purchase to its write-down. Inco, a multinational nickel mining company, wrote off a substantial portion of the value of its project at Voisey's Bay (VB), Labrador, Canada in 2002. At acquisition in 1996, the company had issued a tracking stock tied only to the value of VB, which it retired in 2000. We use the tracking stock value and cash flow estimates to construct a valuation model, which we then employ to value VB in the 2000-2002 period. We find that (1) relative to market value, the assets were substantially overvalued on the financial statements for most of the period 1996-2000, yet no revaluation occurred until 2002; (2) US GAAP should have prohibited a write-down, despite substantial overvaluation, even in 2002; and (3) the 2002 write-down amount is inconsistent with parameters the market had been using to value VB. Our study raises questions regarding the discretion inherent in the timing and valuation of long-term asset impairment charges, the accounting standards that govern these valuations, and enforcement of these standards.

Geologically constrained gravity inversion for the Voisey's Bay ovoid deposit

Constructing a general, three-dimensional model of the subsurface density distribution by minimum-structure, or Occam-style, inversion of gravity data has been practical now for several years. The usual approach is to seek a model that is simple in the sense of having the least amount of spatial variation while still reproducing the observations (hence the description “minimum-structure”). This has the advantage of generating models with few, if any, fictitious features. However, often the models produced bear only a gross, diffuse resemblance to geology. It has been recognized since the inception of the minimum-structure inversion technique that geological information can be incorporated by seeking a model that is simple in the sense of being as close as possible to a reference model while still fitting the observations. This is rarely done, often because insufficient geological information is available, or because inversion of geophysical data is no longer thought to be useful once a target has been fou...

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.

Applications of Stable and Radiogenic Isotopes to Magmatic Cu-Ni-PGE Deposits: Examples and Cautions

Measurements of S, O, and radiogenic isotope ratios are all potentially powerful tracers of magma interaction with country rocks and the importance of assimilation processes in the genesis of magmatic Ni-Cu-PGE deposits. Sulfur isotope measurements of deposits such as those in the 1.1 Ga Duluth Complex, the Permo-Triassic intrusions of the Noril'sk area, and the 1.4 Ga Kabanga intrusions provide evidence for the derivation of S from both sulfide-and sulfate-bearing country rocks. The 1.3 Ga Voisey's Bay deposit provides an example where δ 34S values of the ores commonly fall within the accepted mantle range of 0 ± 2%, but detailed studies of Proterozoic metasedimentary country rocks show that their weighted average δ 34S value is also within this range. A thorough knowledge of the isotopic compositions of potential contaminants is essential for a proper evaluation of the role of country rock derived S in ore formation. When O and radiogenic isotopic measurements are employed as tracers to evaluate magma-country rock interaction, it is essential to provide evidence that open system processes have not perturbed the isotopic systematics. Low-temperature hydrothermal processes can mask evidence of high-temperature processes in the oxygen isotope system and in radiogenic systems such as Re/Os and Pb where involved elements may be mobile under hydrothermal conditions, or host phases may close to exchange and uptake at different temperatures. Careful petrographic observation and analyses of individual minerals may be required before the models involving the contamination of magmas by country rocks can be meaningfully applied.

Expanding the cobalt recovery circuit at the Thompson nickel refinery

The cobalt recovery circuit at the Inco Thompson nickel refinery needed to be expanded to allow the treatment of concentrate from the recently commissioned Voisey's Bay project. Due to higher cobalt grades than historically processed, the capacity of the cobalt circuit needed to be increased from 400 tonnes to 1,000 tonnes of cobalt peryear. This paper describes the process used at Thompson and the philosophy employed to determine what additional equipment was needed to de-bottleneck the plant, allowing the additional throughput.

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.

Oxidation of pentlandite and pyrrhotite surfaces at pH 9.3: Part 2. Effect of xanthates and dissolved oxygen

Using X-ray photoelectron spectroscopy, the oxidation/alteration of massive coexisting pentlandite (Pn) and pyrrhotite (Po) from Voisey's Bay has been studied in aqueous solution at pH 9.3, with the addition of the xanthate collector potassium amyl xanthate (KAX) at varying solution O2 concentrations. As in the accompanying XPS study, the oxidation/alteration of the two mineral surfaces is monitored by the Fe 2p, Ni 2p, S 2p, and O 1s XPS spectra. The major surface product on all surfaces is FeOOH. However, the amount of FeOOH depends greatly on the presence of KAX, the presence of ground Pn/ Po of high surface area, and the O2 solution concentration. At air-saturated O2 solution concentrations of 8.9 ppm, the amount of FeOOH on both surfaces decreases in the presence of KAX compared to the situation without KAX. In the presence of the Pn/Po powder, the oxidation of Pn decreases still more, suggesting that the powder scavenges O2, and that the oxidation of Pn is more sensitive to O2 concentration than the oxidation of Po. When the O2 concentration is decreased drastically to <0.01 ppm (by Ar purging) in the presence of KAX, both minerals are relatively free of FeOOH: indeed, both minerals show less FeOOH than the unreacted sample, which implies that the mineral surfaces are being .cleaned. under these conditions. The degree of oxidation of both surfaces increases with increase in the O2 solution concentration; but at about 0.03 ppm O2 the pyrrhotite surface is heavily oxidized, while the pentlandite surface remains largely unoxidized, and less oxidized than the initial unreacted sample. This result strongly indicates that the flotation separation of Pn and Po could be enhanced at low O2 concentrations.