Occurrence model for magmatic sulfide-rich nickel-copper-(platinum-group element) deposits related to mafic and ultramafic dike-sill complexes
<p>Magmatic sulfide deposits containing nickel (Ni) and copper (Cu), with or without (±) platinum-group elements (PGE), account for approximately 60…
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U.S. Department of the Interior U.S. Geological Survey Scientific Investigations Report 2010–5070–I Occurrence Model for Magmatic Sulfide-Rich NickelCopper-(Platinum-Group Element) Deposits Related to Mafic and Ultramafic Dike-Sill Complexes
COVER. Ferrar dikes and sills intruding Beacon sandstones, Finger Mountain, Dry Valleys, Antarctica. Photograph by Dean Peterson, Duluth Metals Limited.
Occurrence Model for Magmatic SulfideRich Nickel-Copper-(Platinum-Group Element) Deposits Related to Mafic and Ultramafic Dike-Sill Complexes By Klaus J. Schulz, Laurel G. Woodruff, Suzanne W. Nicholson, Robert R. Seal II, Nadine M. Piatak, Val W. Chandler, and John L. Mars
Chapter I of Mineral Deposit Models for Resource Assessment Scientific Investigations Report 2010–5070–I U.S. Department of the Interior U.S. Geological Survey
U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette Kimball, Acting Director U.S. Geological Survey, Reston, Virginia: 2014 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit ://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit ://www.usgs.gov/pubprod To order this and other USGS information products, visit ://store.usgs.gov Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Schulz, K.J., Woodruff, L.G., Nicholson, S.W., Seal, R.R., II, Piatak, N.M., Chandler, V.W., and Mars, J.L., 2014, Occurrence model for magmatic sulfide-rich nickel-copper-(platinum-group element) deposits related to mafic and ultramafic dike-sill complexes: U.S. Geological Survey Scientific Investigations Report 2010–5070–I, 80 p., ://dx.doi.org/10.3133/sir20105070I. ISSN 2328-0328 (online)
Contents 1. Introduction Overview 1 Scope 1 Purpose 3 2. Deposit Type and Associated Deposits Name and Synonyms 3 Brief Deposit Description 3 Associated Deposit Types 4 Primary and Byproduct Commodities 4 Example Deposits 4 3. Historical Evolution of Descriptive and Genetic Knowledge and Concepts 4. Regional Environment Geotectonic Environment 10 Temporal (Secular) Relations 11 Duration of Magmatic-Hydrothermal System and/or Mineralizing Processes 11 Relations to Structures 11 Relations to Igneous Rocks 12 Relations to Sedimentary Rocks 12 Relations to Metamorphic Rocks 12 5. Physical Description of Deposits Size and Shape of Deposits 13 Size of Mineralizing System Relative to Extent of Economically Mineralized Rock 15 Host Rocks 16 Structural Setting(s) and Controls 16 6. Geophysical Signatures Introduction 16 Magnetic Signature 17 Gravity Signature 17 Electrical Signature 20 Electromagnetic (EM) Signature 21 Seismic Signature 23 Spectral Characteristics 24 7. Hypogene and Supergene Ore Characteristics Hypogene Ore Characteristics 31 Mineralogy and Mineral Assemblages 31 Paragenesis 32 Zoning Patterns 34 Textures and Structures 35 Grain Sizes 36 Supergene Ore Characteristics 37 8. Hypogene Gangue Characteristics Mineralogy and Mineral Assemblages 37 Paragenesis 37
9. Geochemical Characteristics of Ores Trace Elements and Element Associations 38 Metal Normalizations and Ratios 38 Stable Isotopes 41 Sulfur Isotopes 41 Oxygen Isotopes 42 Radiogenic Isotopes 44 Rhenium-Osmium Isotopes 44 Rubidium-Strontium, Samarium-Neodymium, and Lead Isotopes 45 10. Petrology of Associated Igneous Rocks Rock Names 46 Forms of Igneous Rocks and Rock Associations 46 Mineralogy 46 Textures, Structures, and Grain Size 48 Petrochemistry 49 Trace-Element Geochemistry 52 Isotope Geochemistry 52 11. Petrology of Related Sedimentary and Metamorphic Country Rocks 12. Theory of Deposit Formation 13. Exploration and Resource Assessment Guidelines Geological Guides for Regional Mineral Exploration and Resource Assessment 57 Geological Guides for Local Mineral Exploration and Resource Assessment 57 Exploration Methods 58 14. Geoenvironmental Features Weathering Processes 58 Sulfide-Oxidation, Acid-Generation, and Acid-Neutralization Processes 59 Metal Cycling Associated with Efflorescent Sulfate Salts 60 Secondary Precipitation of Hydroxides and Hydroxysulfates 60 Pre-Mining Baseline Signatures in Soil, Sediment, and Water 61 Past and Future Mining Methods and Ore Treatment 61 Volume of Mine Waste and Tailings 62 Mine Waste Characteristics 62 Mineralogy 62 Acid-Base Accounting 62 Element Mobility Related to Mining in Groundwater and Surface Water 63 Pit Lakes 63 Ecosystem Issues 63 Human Health Issues 64 Climate Effects on Geoenvironmental Signatures 64 15. Knowledge Gaps and Future Research Directions References Cited 66
Figures
1–1. Locations of some of the world’s major nickel and platinum-group element
(PGE) mining camps and types of deposits 2
1–2. Histogram comparing the total nickel metal resources for the fifteen largest
deposits worldwide 2
2–1. Nickel grade versus total ore tonnage for some nickel sulfide and PGE deposits
of the world 5
3–1. Simplified geologic map of the Sudbury, Canada, structure showing the location
of major Cu-Ni sulfide deposits 8
3–2. Schematic diagram illustrating the continued flow of magma through an idealized magma conduit 9
4–1. Schematic section through Earth’s core and mantle showing the relations among
hot plumes, spreading centers, and subduction zones 10
5–1. Schematic cross section of a typical Noril’sk (Russia) ore-bearing intrusion
showing internal structure, lithologies, and major ore types 13
5–2. Typical shapes of ore bodies at Noril’sk, Russia 14
5–3. Graph showing the variation in cross-sectional area and distribution of
disseminated and massive ore along the Talnakh intrusion, Noril’sk, Russia 15
5–4. Schematic diagram showing the extent of contact metamorphism and
metasomatism surrounding typical ore-bearing Noril’sk-type (Russia) intrusions 16
6-1. Magnetic anomaly maps of prospects in the James Bay Lowlands, Ontario,
Canada 18
6–2. Aeromagnetic data collected over the Ovoid and Eastern Deeps deposits in
the Voisey’s Bay area, Canada 19
6–3. Detailed Bouguer anomaly map of the Ovoid deposit at Voisey’s Bay, Canada 20
6–4. Voisey’s Bay, Canada, sulfide deposits 22
6–5. Velocity values of various rock types plotted against their density with lines of
constant acoustic impedance overlain within field and Nafe-Drake curve for
common rocks at a standard confining pressure of 200 MPa 24
6–6. Two-dimensional reflection section across the Sudbury Basin, Ontario, Canada,
showing reflectors associated with deep structure and lithologic contacts 25
6–7. Three-dimensional seismic image of the Sudbury area, Ontario, Canada, showing surface grid, horizon slice corresponding to the footwall complex, and time slice
corresponding approximately to 2,000-m depth 25
6–8. Vertical slice of a three-dimensional seismic reflection survey of the Faroe-
Shetland basin, northeastern Atlantic Ocean margin 26
6–9. Ridged sill in the Faroe-Shetland Basin, northeastern Atlantic Ocean margin 26 6–10. Vertical seismic profiling survey 27 6–11. Schematic illustration of cross-hole seismic methods 27 6–12. Visible-near infrared to short-wave infrared (VNIR–SWIR) reflectance sample
library spectra 27
6–13. Visible-near infrared to short-wave infrared (VNIR–SWIR) reflectance sample
library spectral pairs of actinolite, epidote, serpentine, and chlorite (lower
spectrum) and the same mineral spectra resampled to Advanced Spaceborne
Thermal Emission and Reflection Radiometer (ASTER) bandpasses (upper
spectrum) 29 6–14. Thermal infrared (TIR) emissivity sample spectral pairs of mafic and ultramafic
rocks and the rock spectra resampled to Advanced Spaceborne Thermal Emission
and Reflection Radiometer (ASTER) bandpasses 29 6–15. Thermal infrared (TIR) emissivity sample library spectral pairs of biotite, augite,
serpentine, diopside, hornblende, phlogopite, and olivine (lower spectrum) and
the same mineral spectra resampled to Advanced Spaceborne Thermal Emission
and Reflection Radiometer (ASTER) bandpasses (upper spectrum) 30 6–16. Thermal infrared (TIR) emissivity sample spectral pairs of mafic and ultramafic rocks and the rock spectra resampled to Advanced Spaceborne Thermal Emission
and Reflection Radiometer (ASTER) bandpasses (upper spectrum) 30
7–1. Idealized fractionation path of an immiscible high-temperature sulfide liquid 33
7–2. Plan map of mineral-assemblage zonation in massive sulfide ores in the
Oktyabr’sky mine area, Kharayelakh intrusion, Noril’sk region, Russia 35
9–1. Platinum-group and related elements for magmatic sulfides from tholeiitic sill/dike Ni-Cu deposits, normalized to primitive mantle, using normalization factors 39
9–2. Cu/Pd versus Pd plots 40
9–3. Pd/Ir versus Ni/Cu and NiPd versus Cu/Ir plots 42
9–4. Comparison of sulfur isotope variations (delta 34 S, 34S) in magmatic Ni-Cu±PGE
deposits 43
9–5. Gamma osmium (gOs) versus age in billions of years for some sulfide-rich Ni-Cu
deposits 45 10–1. The International Union of Geological Sciences (IUGS) classification and
nomenclature of gabbroic and ultramafic rocks based on the proportions
of olivine, orthopyroxene, clinopyroxene, and plagioclase 47 10–2. Volcanic rock stratigraphy of the Noril’sk region, Russia 51 10–3. Primitive mantle-normalized trace-element patterns for volcanic and intrusive
rocks 52 10–4. Primitive mantle-normalized chalcophile element patterns for average
continental flood basalt (CFB), ocean island basalt (OIB), and mid-ocean
ridge basalt (MORB) 53 12–1. Stages in the formation of magmatic sulfide-rich Ni-Cu±PGE deposits 56 14–1. Acid-base accounting characteristics of magmatic Ni-Cu deposits 63 Tables
2–1. Examples of Ni-Cu±PGE deposits hosted by mafic and/or ultramafic dike-sill complexes 6
5–1. Form, distribution, textures and timing of segregation of Ni-Cu±PGE sulfide
mineralization (after Lesher, 2004) 13
6–1. Copper-nickel sulfide ore mineral and host-rock magnetic susceptibilitie in SI
units (dimensionless proportionality) × 10–3 19
6–2. Density values for sulfides and mafic-ultramafic host rocks 20
7–1. Major and minor minerals, with chemical formula, typically found in magmatic Ni-Cu±PGE sulfide deposits 31
7–2. Platinum-group element (PGE) minerals reported for Ni-Cu-sulfide deposits from Noril’sk, Russia 32
9–1. Ratios of metals calculated as concentration of metal in 100-percent sulfides for mag matic Ni-Cu±PGE deposits 41
9–2. Characteristics of the five chemical components identified in the Norilsk-Talnakh intrusions, Russia, calculated using an emplacement age of 251 Ma 45 10–1. Lithology, mineralogy, and texture of rocks that host sulfide-rich Ni-Cu±PGE mineralization at Voisey’s Bay, Canada and at Noril’sk, Russia 48 10–2. Compositions of inferred parental magmas to selected worldwide sulfide-rich Ni-Cu±PGE deposits 50 10–3. Lithological and geochemical characteristics of volcanic series at Noril’sk, Russia 51 11–1. Comparison of exposed sedimentary and metamorphic country rocks associated with magmatic Ni-Cu±PGE sulfide deposits 54 14–1. Environmental guidelines relevant to mineral deposits with respect to potential
risks to humans and aquatic ecosystems from mine waste and mine drainage 65
SI to Inch/Pound Multiply By To obtain Length centimeter (cm) inch (in.) millimeter (mm inch (in.) meter (m) foot (ft) kilometer (km) mile (mi) Area square kilometer (km2) acre square kilometer (km2) square mile (mi2) Volume liter (L) ounce, fluid (oz) liter (L) pint (pt) liter (L) quart (qt) liter (L) gallon (gal) cubic kilometer (km3) cubic mile (mi3) Mass gram (g) ounce, avoirdupois (oz) milligram (mg) 3.527 x 10-5 ounce, avoirdupois (oz) kilogram (kg) pound avoirdupois (lb) Conversion Factors
g/L gram per liter µg/L microgram per liter mg/L milligram per liter mg/kg milligrams per kilogram % percent ‰ per mil eNd epsilon neodymium eSr epsilon strontium δ180 delta O-18 δ34S delta S-34 γOs gamma osmium Al aluminum Ag silver As arsenic Au gold Ba barium Bi bismuth Cd cadmium Co cobalt Cr chromium Cu copper Fe iron Ga Giga-annum; age in billions of years Hg mercury Ir iridium IPGE iridium-group PGEs (Os-Ir-Ru) K potassium km kilometers La lanthanum LIP large igneous province m meters Mg magnesium MgO magnesium oxide MORB mid-ocean ridge basalt Ma Mega-annum; age in millions of years Mt million metric tons My million years Mkm2 million square kilometers Mkm3 million cubic kilometers NAP net acidizing potential Nb niobium Nd neodymium Ni nickel Abbreviations Used in This Report
NNP net-neutralizing potential O oxygen Os osmium P phosphorus Pb lead Pd palladium PGE platinum-group elements PPGE platinum-group PGEs (Rh-Pt-Pd) Pt platinum ppb parts per billion Re rhenium REE rare earth elements Rh rhodium Ru ruthenium S sulfur Sb antimony Se selenium Sm samarium Sr strontium t metric tons Ta tantalum Te tellurium Th thorium Ti titanium vanadium Yb ytterbium Zn zinc
1. Introduction Overview Magmatic sulfide deposits containing nickel (Ni) and copper (Cu), with or without (±) platinum-group elements (PGE), account for approximately 60 percent of the world’s nickel production (Naldrett, 2004). Most of the remainder of the Ni production is derived from lateritic deposits, which form by weathering of ultramafic rocks in humid tropical conditions (Elias, 2002; Freyssinet and others, 2005). Magmatic Ni-Cu±PGE sulfide deposits are spatially and genetically related to bodies of mafic and/or ultramafic rocks. The sulfide deposits form when the mantle-derived mafic and/or ultramafic magmas become sulfide-saturated and segregate immiscible sulfide liquid, commonly following interaction with continental crustal rocks (Arndt and others, 2005). Deposits of magmatic Ni-Cu sulfides occur with mafic and/ or ultramafic bodies emplaced in diverse geologic settings. They range in age from Archean to Tertiary, but the largest number of deposits are Archean and Paleoproterozoic (Eckstrand and Hulbert, 2007). Although deposits occur on most continents, ore deposits (deposits of sufficient size and grade to be economic to mine) are relatively rare; major deposits are present in Russia, China, Australia, Canada, and southern Africa (fig. 1–1). NickelCu sulfide ore deposits can occur as single or multiple sulfide lenses within mafic and/or ultramafic bodies with clusters of such deposits comprising a district or mining camp. Typically, deposits contain ore grades of between 0.5 and 3 percent Ni and between 0.2 and 2 percent Cu (Eckstrand and Hulbert, 2007). Tonnages of individual deposits range from a few tens of thousands to tens of millions of metric tons (Mt) bulk ore (Barnes and Lightfoot, 2005). Two giant Ni-Cu districts, with ≥10 Mt Ni, dominate world Ni sulfide resources and produc tion (fig. 1–2). These are the Sudbury district, Ontario, Canada, where sulfide ore deposits are at the lower margins of a mete orite impact-generated igneous complex and contain 19.8 Mt Ni; and the Noril’sk-Talnakh district, Siberia, Russia, where the ore deposits are in subvolcanic mafic intrusions related to flood basalts and contain 23.1 Mt Ni (Eskstrand and Hulbert, 2007). In the United States, the Duluth Complex in Minnesota, comprised of a group of mafic intrusions related to the 1.1 Ga Midcontinent Rift system, represents a major Ni resource of 8 Mt Ni, but deposits generally exhibit low grades (0.2 percent Ni, 0.66 percent Cu) and remain in the process of being proven economic. The sulfides in magmatic Ni-Cu deposits generally constitute a small volume of the host rock(s) and tend to be concentrated in the lower parts of the mafic and/or ultramafic bodies, often in physical depressions or areas marking changes in the geometry of the footwall topography (Barnes and Lightfoot, 2005). In most deposits, the sulfide mineralization can be divided into disseminated, matrix or net, and massive sulfide, depending on a combination of the sulfide content of the rock and the silicate texture. The major Ni-Cu sulfide mineral ogy typically consists of an intergrowth of pyrrhotite (Fe7S8), pentlandite ([Fe, Ni]9S8), and chalcopyrite (FeCuS2). Cobalt, PGE, and gold (Au) are extracted from most magmatic Ni-Cu ores as byproducts, although such elements can have a signifi cant impact on the economics in some deposits, such as the Noril’sk-Talnakh deposits, which produce much of the world’s palladium. In addition, deposits may contain between 1 and 15 percent magnetite associated with the sulfides (Barnes and Lightfoot, 2005). Scope A number of schemes have been used to subdivide mag matic Ni-Cu±PGE sulfide deposits, with most based on sulfide compositions, tectonic settings, and petrologic characteristics of Occurrence Model for Magmatic Sulfide-Rich NickelCopper-(Platinum-Group Element) Deposits Related to Mafic and Ultramafic Dike-Sill Complexes 1 U.S. Geological Survey, Reston, Virginia 2 U.S. Geological Survey, Mounds View, Minnesota 3 Minnesota Geological Survey, St. Paul, Minnesota By Klaus J. Schulz,1 Laurel G. Woodruff,2 Suzanne W. Nicholson,1 Robert R. Seal II,1 Nadine M. Piatak,1 Val W. Chandler,3 and John L. Mars1
2 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits the mafic and/or ultramafic rocks that host the deposits (Page, 1986a–f; Naldrett, 1989, 1997, 2004; Eckstrand, 1995; Lesher and Keays, 2002; Lesher, 2004; Barnes and Lightfoot, 2005; Hoatson and others, 2006). Based on their principal metal production, however, magmatic sulfide deposits are best divided into two major types (Naldrett, 2004). The first are sulfide-rich, typically with 10 to 90 percent sulfide minerals, and which have economic value primarily because of their Ni and Cu content. Second are those that are sulfide-poor, typically with 0.5 to 5 percent sulfide minerals, and exploited principally for PGE. The sulfide-rich Ni-Cu±PGE deposits can be further subdivided into two compositional groups on the basis of their Ni/Cu ratios (Barnes and Lightfoot, 2005). Deposits in the first subgroup have Ni/Cu ratios in the range of 0.8 to 2.5 and Ni concentrations (concentrations calculated in 100 percent sulfides) ranging from 1 to 6 percent (Barnes and Lightfoot, 2005). With the exception of the deposits in the Sudbury district, Canada, which are related to a meteorite-impactgenerated melt sheet, these deposits are associated with small- to medium-sized differentiated mafic and/or ultramafic dikes and sills related mostly to tholeiitic basalt magmas emplaced in continental settings. In addition, most of the major Ni deposits in this subgroup, including the three largest producing Ni-Cu sulfide camps after Sudbury (Noril’sk-Talnakh, Russia; Jinchuan, China; and Voisey’s Bay, Canada; Eckstrand and Hulbert, 2007) are related to dynamic magmatic systems and occur in intrusions that served as magma conduits (Naldrett, Basalt-related dike-sill-hosted Basal contact and “reef” PGE-enriched low-sulfide Komatiite-associated Astrobleme-associated Other EXPLANATION THOMPSON (BOWDEN) DULUTH EAGLE SUDBURY VOISEY'S BAY CAPE SMITH (RAGLAN) NORIL'SK-TALNAKH JINCHUAN KABANGA ZIMBABWEAN DEPOSITS BUSHVELD (PLATREEF & MERENSKY) SELEBIPIKWE WESTERN AUSTRALIAN (MT KEITH) PECHENGA O'TOOLE AGUABLANCA WELLGREEN NEBO-BABEL ABITIBI (DUMONT) GREAT DIKE STILLWATER UITKOMST INSIZWA EMEISHAN (BAIMAZHAI) HITURA Figure 1–1. Locations of some of the world’s major nickel and platinum-group element (PGE) mining camps and types of deposits. Figure 1–2. Histogram comparing the total nickel metal resources for the fifteen largest deposits worldwide. Numbers are in millions of metric tons. Figure modified from Hoatson and others (2006). NORIL'SK (23.1) SUDBURY (19.8) DULUTH (8.0) PLATREEF (6.5) MERENSKY REEF (6.3) JINCHUAN (5.5) GREAT DIKE (5.4) PECHENGA (4.0) THOMPSON (3.5) VOISEY'S BAY (2.2) Nickel metal resources, in million metric tons Komatiite-associated Ni-Cu deposits Astrobleme-associated Ni-Cu deposits Basalt-related dike-sill-hosted Ni-Cu deposits Basal contact and “reef” low-sulfide deposits DUMONT (0.7) RAGLAND (0.7) SELEBI-PIKWE (0.5) KABANGA (1.4) WESTERN AUSTRALIA 15 LARGEST (11.6) EXPLANATION
Brief Deposit Description 3 1999; Maier and others, 2001). Deposits in the second sub group have Ni/Cu ratios greater than 3 and Ni concentrations (in 100 percent sulfides) generally in the range of 6 to 18 per cent (Barnes and Lightfoot, 2005). Those deposits mostly are hosted by komatiitic lava flows, dikes and sills (for example, Mt. Keith, Australia; Raglan and Dumont, Canada). The deposits in the Pechenga Camp in the Kola Peninsula, Russia, however, are hosted by ferropicrite-related intrusions (Brügmann and others, 2000; Barnes and others, 2001). The sulfide-poor deposits generally occur as sparsely dispersed sulfide minerals in basal units or stratabound layers or reefs in very large to medium-sized, typically layered mafic/ ultramafic intrusions (for example, Platreef and Merensky Reef, Bushveld Complex, South Africa; J-M Reef, Stillwater Complex, Montana, United States; Duluth Complex, Minne sota, United States). These low-sulfide PGE-enriched deposits are not included in this deposit model. Purpose This deposit model is intended primarily to provide the basis for assessing the occurrence of undiscovered, potentially economic magmatic sulfide-rich Ni-Cu±PGE deposits in the United States. The strategy employed by the U.S. Geological Survey (USGS) in quantitative mineral-resource assessments (Singer, 1993, 2007) consists of (1) delineating tracts permis sive for the occurrence of undiscovered magmatic Ni- and Cu-bearing sulfide deposits, (2) estimating the amount of metal and some ore characteristics by means of appropriately selected grade and tonnage models, and (3) estimating the number of undiscovered deposits by deposit type. Thus, it is critical to the assessment methodology to have accurate and reliable data on the major characteristics of magmatic sulfiderich Ni-Cu±PGE deposits, particularly host lithology, tectonic setting, structure, age, ore-gangue-alteration mineralogy, geochemical and geophysical signatures, theory of deposit formation, and geoenvironmental features. Because the purpose of this model is to facilitate the assessment for undiscovered, magmatic sulfide-rich Ni-Cu±PGE deposits in the United States, this model addresses only those sulfide-rich deposits of economic sig nificance likely to occur in the United States based on known geology. These primarily are deposits hosted by small- to medium-sized mafic and/or ultramafic dikes and sills related to picrite and tholeiitic basalt magmatic systems generally emplaced in large igneous provinces (LIPs). The deposits in the Sudbury district are not included because they are associ ated with a unique meteorite impact-related melt sheet (Keays and Lightfoot, 2004; Naldrett, 2004; Ames and Farrow, 2007). There is no evidence for similar sulfide deposits being associ ated with any other impact structure elsewhere in the world. Also not included are the deposits associated with komati ite lava flows, dikes, and sills (Lesher and Keays, 2002). Although these deposits are an important source for nickel, particularly in Australia, Canada, and Zimbabwe, komatiites are rare in the United States and have not proven to be pro spective for significant nickel sulfide mineralization. Although this report does not focus on the magmatic Ni-Cu sulfide deposits at Sudbury or those related to komati ites, those sulfide deposits have been studied extensively and share many similarities with the deposits related to picrite and tholeiitic basalt dike-sill complexes. Therefore, reference to the Sudbury and komatiite-related deposits is made where appropriate. 2. Deposit Type and Associated Deposits Name and Synonyms The deposits described in this model are referred to here as magmatic sulfide-rich Ni-Cu±PGE deposits related to mafic and/or ultramafic dike-sill complexes. The name emphasizes the relation of these Ni-Cu sulfide-rich deposits to mafic and ultramafic rocks derived from picrite and tholeiitic basalt magmas and to mostly small- to medium-sized dikes and sills as opposed to the generally much larger layered maficultramafic intrusive complexes that typically host sulfide-poor PGE-enriched deposits (for example, the Stillwater Complex, Montana). A variety of other names have been applied to these deposits in the literature that generally emphasize aspects of mineralization style, tectonic setting, and/or lithology of the host rocks. Names include tholeiitic basal segregation type, gabbroid-associated layered intrusive type, mafic-ultramafic intrusion-hosted type, flood-basalt-related type, and feeder/ conduit-type deposits (Barnes and Lightfoot, 2005; Eckstrand and Hulbert, 2007). In addition, deposits have been named after giant deposits of that respective type, such as Noril’sktype and Voisey’s Bay-type deposits (Page, 1986c; Schulz and others, 1998). Brief Deposit Description These magmatic sulfide deposits are significant produc ers of Ni and Cu resources, primarily from deposits in Russia (Noril’sk-Talnakh, Pechenga), Canada (Voisey’s Bay), and China (Jinchuan). In addition, the Noril’sk-Talnakh deposits are the world’s largest producer of palladium. These sulfide deposits form when mantle-derived, sulfur-undersaturated picrite or tholeiitic basalt magma becomes sulfide-saturated, commonly following interaction with continental crustal rocks. Sulfur saturation results in formation of an immiscible sulfide liquid. The sulfide liquid tends to segregate into physi cal depressions or other areas in the lower parts of dike- and/ or sill-like intrusions because of changes in the magma flow dynamics. Economic deposits appear to be associated almost
4 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits exclusively with dynamic magmatic systems that experience repeated surges of magma. Such dynamic systems appear to promote the interaction of sulfide liquid with a sufficiently large amount of silicate magma to concentrate chalcophile elements to economic levels. The tectonic setting of these magmatic sulfide deposits principally is flood-basalt-dominated LIPs, generally attrib uted to the upwelling and melting of a buoyant mantle plume beneath crustal lithosphere (Condie, 2001). The sulfide deposits and their host intrusions range in age from Archean to at least Middle Jurassic. They generally are found close to deeply penetrating faults that allow for the efficient transport of sulfur-undersaturated magma from the mantle to relatively shallow crustal depths. Sulfur-bearing crustal rocks such as black shales, evaporates, or paragneisses are proximal to many deposits and a potential source of sulfur. Deposits are hosted by a wide range of typically olivine-bearing mantle-derived rocks related to ferropicrite (Pechenga), tholeiitic picrite (Noril’sk-Talnakh), and high-aluminum basalt (Voisey’s Bay). No known economic deposits are associated with mid-ocean ridge basalts (MORB), ophiolites, or alkaline rocks. The principal sulfide minerals in most sulfide-rich Ni-Cu±PGE deposits consist of intergrown pyrrhotite, pent landite, and chalcopyrite. Cobalt (Co) is found in pentlandite, and PGE generally appear as small grains of PGE-bearing sulfides, arsenides, antimonides, bismithides, and tellurides. Deposits also locally contain 1 to 15 percent magnetite (Barnes and Lightfoot, 2005). The largest concentrations of sulfide minerals tend to occur either at the base of the host intrusion or in the immediate footwall. The sulfides generally are present as disseminated, matrix or net, and massive sulfide, based on a combination of the sulfide content and silicate texture of the rock. In most cases the massive and matrix ore is zoned, with Cu-rich zones relatively enriched in gold (Au), palladium (Pd), and platinum (Pt). Those zones, as footwall dikes and veins, either overlie or are separated from Cu-poor zones relatively enriched in osmium (Os), iridium (Ir), ruthe nium (Ru), and rhodium (Rh). The compositional zonation is attributed to fractionation of monosulfide solid solution (MSS) from a sulfide liquid (Naldrett, 2004). Associated Deposit Types Magmatic sulfide-rich Ni-Cu±PGE deposits can occur with numerous other magmatic ore deposits. These include low-sulfide, contact-type and reef-type PGE deposits, chro mite deposits, and Fe-Ti-V deposits. In addition, secondary processes can affect the sulfide deposits, producing asbestos, soapstone, Ni-laterite, and native-copper deposits. Primary and Byproduct Commodities In sulfide-rich Ni-Cu±PGE deposits, Ni constitutes the main economic commodity, with grades typically about 0.5 to 3 percent. The range of Ni grades and tonnages for some known deposits are compared with other types of Ni deposits in figure 2–1. Copper may be either a coproduct or byproduct, and Co, PGE, and Au typically are byproducts. The atypical enrichment of PGE in the Noril’sk-Talnakh deposits results in PGE as a significant coproduct. Other commodities recovered, in some deposits, include silver (Ag), sulfur (S), selenium (Se), and tellurium (Te). In addition, sulfide minerals may contain anomalous arsenic (As), bismuth (Bi), mercury (Hg), lead (Pb), antimony (Sb), and zinc (Zn), which can contribute to environmental contamination. Example Deposits There are relatively few sulfide-rich Ni-Cu±PGE deposits related to mafic and/or ultramafic dike-sill complexes cur rently recognized around the world. Selected representatives are presented in table 2–1, along with some of their general characteristics. The Eagle deposit in northern Michigan, discovered in 2002 and currently under development by Kennecott Minerals Company, is the only economic deposit of this type currently known in the United States.
Example Deposits 5 Figure 2–1. Nickel grade versus total ore tonnage for some nickel sulfide and PGE deposits of the world. Diagonal lines indicate the contained nickel metal amount in tons. World-class deposits (greater than 1 million metric tons [Mt] Ni metal) are shown by large symbols. Field in orange shows range for komatiite-hosted Ni sulfide deposits; world-class deposits are labeled. 1,000 10,000 10,000,000 tons Ni 1,000,000 tons Ni 100,000 tons Ni 10,000 tons Ni 1,000 tons Ni Tonnage, in million metric tons Nickel grade, in percent NORIL'SK SUDBURY DULUTH VOISEY'S BAY PECHENGA JINCHUAN PLATREEF GREAT DIKE MERENSKY REEF SELEBI-PIKWE World-class nickel deposits World-class nickel deposits Komatiite-hosted deposits EAGLE Basalt-related dike-sill-hosted deposits Basal contact and "reef" low-sulfide deposits Astrobleme-associated deposits KABANGA EXPLANATION KAMBALDA THOMPSON PERSEVERANCE HONEYMOON WELL YAKABINDIE MT KEITH Komatiite-related deposits WELLGREEN NEBO-BABEL UITKMOST
6 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Table 2–1. Examples of Ni-Cu±PGE deposits hosted by mafic and/or ultramafic dike-sill complexes. [Ma, million years; Mt, million metric tons; t, metric ton; wt, weight; %, percent] Deposit Location Discovered Age (Ma) Size (Mt) Ni (wt. %) Cu (wt. %) Contained Ni (t) Reference Noril’sk (N) Talnakh (T) Siberian Platform, Russia ~1926 (N) 1960 (T) 251±0.3 23,174,610 Naldrett, 2004 Jinchuan Sino-Korean Craton, China 831.8±0.6 5,459,000 Zhang and others, 2010 Chai and Naldrett, 1992 Pechenga Baltic Shield, Kola Peninsula, Russia 1977±55 4,000,200 Barnes and others, 2001 Hanski and others, 1990 Voisey’s Bay Torngat orogen, Labrador, Canada 1333±1 2,173,530 Naldrett, 2004 Amelin and others, 1999 Kabanga Kibaran Orogenic Belt, Tanzania 1,524,000 Maier and others, 2010 Duchesne and others, 2004 Wellgreen Quill Creek ultramafic complex, Alaska 1,199,000 Schmidt and Rogers, 2007 Nebo-Babel Giles complex, Musgrave province, Australia 1078±3 990,000 Hoatson and others, 2006 Seat and others, 2007 and 2009 Uitkomst Bushveld complex, South Africa 913,000 Maier and others, 2004 Insizwa Mount Ayliff complex, South Africa Maier and others, 2002 Eagle Baraga basin, Michigan, United States 1107.2±5.7 131,019 Ding and others, 2010
3. Historical Evolution of Descriptive and Genetic Knowledge and Concepts 7 3. Historical Evolution of Descriptive and Genetic Knowledge and Concepts The history of understanding of magmatic Ni-Cu±PGE sulfide deposits has been reviewed most recently by Papunen (1991) and Naldrett (2005) and is briefly summarized here. Unlike other commodities such as iron and copper, which have long been known and extracted, the element Ni was only first identified in 1751 by the Swedish chemist Cronstedt, who separated the new metal from cobalt ores from the Los mine in Sweden (Howard-White, 1963). The earliest mining of Ni did not begin until 1848, shortly after a new Fe-Ni sulfide mineral, pentlandite, was discovered at the old Espendalen copper mine in Norway (Boyd and Nixon, 1985). Nickel production from the Espendalen deposit was soon followed by the discovery and mining of several other sulfide deposits in southern Norway, and by 1876, Norwegian production peaked at 360 metric tons (t) per year (Howard-White, 1963). Norway’s position as the world’s leading nickel producer was superseded in the 1880s by production of nickel from laterite deposits in New Caledonia. With discovery of the giant Sudbury Ni-Cu sulfide deposits in1883, however, Canada soon became the world’s leading nickel producer, dominating the market through much of the twentieth century. Today, Russia has overtaken Canada as the world’s leading nickel producer (Kuck, 2010). Concepts of the origin of magmatic Ni-Cu±PGE sul fide deposits have been influenced strongly by studies of the Sudbury deposits (fig. 3–1). These concepts fall mostly into two opposing interpretations, one emphasizing a magmatic origin and the other a hydrothermal origin. As early as 1891, Bell (1891a) proposed that the Sudbury ores were the result of the separation and settling of a dense sulfide liquid during differentiation of the magma that gave rise to the Sudbury Igneous Complex (SIC). This conclusion was supported by Barlow (1906), who recognized (1) two principal units in the SIC, a lower noritic layer and an overlying granophyric layer; and (2) the occurrence of the ore bodies in embayments along the lower contact of the norite. Coleman (1905, 1913) subsequently showed that SIC extended around the whole of the recognized Sudbury basin (Bell, 1891b) and argued that the SIC had been emplaced as a sill along the contact between overlying breccias and metasedimentary units and older basement rocks. Further, he argued that the restriction of the sulfide deposits to the outer (that is, lower) contact of the SIC, the intimate mixture of sulfide and norite in all proportions, the lack of hydrothermal alteration, and the occurrence of the largest concentrations of sulfides in embayments along the footwall contact strongly supported the gravitationally induced settling of sulfide liquid from the SIC magma. The magmatic interpretation was challenged by others studying the Sudbury ores. For example, Dickson (1903) sug gested that the ores had been deposited by hot aqueous fluids. A hydrothermal origin also was supported by Knight (1917), based on his observation of the almost universal occurrence of the ores in breccias and of the partial replacement of breccia fragments by sulfides. A subsequent extensive microscopic study of the Sudbury ores by Wandke and Hoffman (1924) also led them to conclude that the deposits were hydrothermal in origin. As noted by Naldrett (2005), arguments about the mag matic versus hydrothermal origin of the Sudbury ores persisted through most of the first half of the twentieth century, with those mapping and studying the SIC favoring a magmatic origin and those most familiar with the mines favoring a hydro thermal origin. However, with the publication in 1962 of J.E. Hawley’s comprehensive study of the Sudbury ores (Hawley, 1962), a strong case was made for the magmatic ori gin of the sulfide ores with only relatively minor superimposed secondary features that include crystallization of secondary minerals, localized alteration, late hydrothermal mineralization, and local remobilization of ores by both granitic and basaltic intrusions. Since Hawley’s work, the magmatic origin of the Sudbury Ni-Cu sulfide deposits has been generally accepted, although a hydrothermal origin for some footwall Cu- and PGE-enriched deposits is still debated (Naldrett, 2004, 2005). General acceptance of a magmatic origin for Ni-Cu±PGE sulfide ores at Sudbury led to the model that magmatic sulfide ore bodies form along the base of large, layered mafic intru sions owing to gravitational settling of a dense sulfide liquid segregated from the parental magmas upon emplacement. This, in turn, led to exploration for magmatic sulfide deposits along the basal contacts of many large, layered mafic-ultramafic intrusions, which included the Bushveld (South Africa), Duluth (United States), Great Dike (Zimbabwe), Kiglapait (Canada), Muskox (Canada), Skaergaard (Greenland), and Stillwater (United States) intrusions. Although low-grade, mostly dis seminated Ni-Cu±PGE sulfide mineralization was discovered along the basal contacts of several of these intrusions (for example, Bushveld, Duluth, Great Dike, Muskox, Stillwater), no economic Ni-Cu±PGE sulfide deposits were discovered. The apparent lack of success in the discovery of major Ni-Cu±PGE sulfide deposits using the Sudbury model is now generally attributed to the ore-forming processes at Sudbury having been in a large part unique. This uniqueness derives from the now widely accepted interpretation that the Sudbury structure is an astrobleme, as first proposed by Dietz (1964). Therefore, the Sudbury model is not applicable to other layered intrusions formed by normal terrestrial igneous processes. In addition, recent geochemical and isotope studies (Walker and others, 1991; Mungall and others, 2004), as well as thermal modeling (Grieve, 1994), strongly suggest that the entire SIC is the product of an impact-generated melt sheet and that the sulfide ores were derived from in situ melting of pre-existing sulfides in crustal target rocks. Thus, the sulfide deposits likely formed by gravitational settling of a dense immiscible sulfide liquid that segregated from the parental impact melt. There is no evidence of a mantle-derived magma component in the SIC or sulfide deposits (Walker and others, 1991). Although large, layered mafic-ultramafic intrusions have not proven prospective for economic sulfide-rich Ni-Cu±PGE deposits, important economic deposits of variable sizes have
8 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Sudbury GARSON FROOD MURRAY CREIGHTON LEVACK Granophyre Augite-norite Sublayer and footwall dikes Volcanics, sediments and basic intrusions Chelmsford Formation Onwatin Formation Onaping Formation Gneiss and granite Faults Copper-nickel deposits EXPLANATION 10 KILOMETERS 10 MILES N Figure 3–1. Simplified geologic map of the Sudbury, Canada, structure showing the location of major Cu-Ni sulfide deposits. Figure modified from Souch and others (1969).
3. Historical Evolution of Descriptive and Genetic Knowledge and Concepts 9 been found in smaller intrusions, including those in Pechenga (Kola Peninsula, Russia) in 1921, Noril’sk (Siberia, Russia) in 1926, Jinchuan (Gansu Province, China) in 1958, Talnakh (Siberia, Russia) in 1960, Kabanga (Tanzania) in 1976, Uit komst (South Africa) in the early 1990s, Voisey’s Bay (Labra dor, Canada) in 1993, and Eagle (Michigan, United States) in 2002. In addition, important Ni sulfide deposits were identified in the Eastern Goldfields region of the Yilgarn Archean craton, Western Australia (Woodall and Travis, 1969) in 1966, in association with sequences of komatiite lavas. These deposits occur mostly at or near the base of the ultramafic sequences, generally within trough-like structures along the contact with underlying metabasalts. Studies of the Ni-Cu±PGE sulfide deposits associated with the smaller mafic-ultramafic intrusions and komatiites since the mid-1960s, along with experimental studies of sulfide and sulfide-silicate systems and improve ments in analytical methods, have led to important advances in the understanding of the genesis of these types of sulfide deposits (see Naldrett, 2005). In particular, studies of the Noril’sk-Talnakh and Voisey’s Bay deposits in the 1990s led to the recognition of the importance of sulfur-understaturated mafic-ultramafic magmas, crustal contamination, and dynamic magmatic systems in the formation of economic sulfide-rich Ni-Cu±PGE deposits (Naldrett and others, 1995; Naldrett, 1999; Li and others, 2001). As a result, the current model for the formation of these deposits emphasizes magma-conduit systems as the most promising targets because conduits provide an ideal environment for crustal contamination and segregation of any immiscible sulfide liquid entrained in ascending maficultramafic magmas (Maier and others, 2001). Changes in flow dynamics along conduit systems can lead to the deposition of sulfide as massive or highly concentrated bodies of sulfide ore (fig. 3–2). In addition, the sulfide bodies may react with new surges of magma moving along the same conduit, resulting in an increase of the Ni, Cu, and PGE tenor of the ore. Figure 3–2. Schematic diagram illustrating the continued flow of magma through an idealized magma conduit. A, An initial surge of sulfur-saturated magma carries entrained sulfide droplets, which may be deposited along the footwall in widened parts of the conduit to form “proto-ore.” In places, sulfide melt may be injected into the footwall. B, Continued surges of undepleted magma may stir up previously deposited sulfide melt, upgrading the sulfide in Ni, Cu, and PGE content and reprecipitating it downstream in the conduit. Proto-ores injected into the footwall may remain shielded from later magma surges and from upgrading in metal content. Figure modified from Maier and others (2001). A B Initial surge of magma Entrained sulfide droplets Initial accumulation of sulfide ("proto-ore"); some possibly injected into the floor New surge of magma Proto-ore shielded from reaction with later magma surges Remobilization, upgrading, and redeposition of sulfides by new magma surge
10 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits 4. Regional Environment Geotectonic Environment The bulk of the world’s large magmatic sulfide-rich Ni-Cu±PGE deposits appear to be located near the margins of cratonic blocks, where large volumes of dominantly tholeiitic mafic and lesser ultramafic magma have been emplaced in the crust and erupted on the surface by processes not associated with “normal” spreading ridge or subduction environments (fig. 4–1; Begg and others, 2010). These large igneous prov inces (LIPs) are defined (Bryan and Ernst, 2008, p. 177) as: “magmatic provinces with areal extents greater than 0.1 million km2, igneous volumes greater than 0.1 million km3, and maximum lifespans of approximately 50 My that have intraplate tectonic settings or geochemical affinities and are characterized by igneous pulse(s) of short duration (approxi mately 1–5 My), during which a large proportion (greater than 75 percent) of the total igneous volume has been emplaced.” Examples include continental flood basalts (Deccan, Karoo, Siberian Traps, Columbia River) and volcanic rifted margins (North Atlantic Igneous Province), as well as their plumbing system components (dikes, sills and layered intru sions) (Coffin and Eldholm, 1994). The LIPs in oceanic settings are represented by oceanic plateaus (Ontong-Java) and ocean-basin flood basalts (the Caribbean Flood Basalts) (Coffin and Eldholm, 1994). In addition, giant dike (and sill) swarms (McKenzie dike swarm) and some large layered intrusions (Bushveld) are interpreted as eroded remnants of LIPs (Ernst, 2007). Although LIPs mainly are mafic igneous provinces with generally subordinate ultramafic components, felsic and intermediate igneous rocks produced by fractional crystallization or partial melting of crustal rocks commonly are present and can be locally abundant in continental LIPs. A few continental LIPs are comprised mainly of felsic volcanic rocks, including the Sierra Madre Occidental in Mexico, the Chon Aike province in southern Argentina, and the Gawler province in Australia (Bryan, 2007). The LIPs can have areal extents greater than one mil lion km2 and volumes greater than one million km3 (Courtillot and Renne, 2003). The largest continental LIP, the Siberian Traps in Russia, which hosts the Noril’sk-Talnakh deposits, is estimated to have a size of four million km3 (Ivanov, 2007). However, because the amount of intrusive and underplated rock is difficult to estimate but probably equals or exceeds the volcanic component, size estimates for LIPs are minimum values. Emplacement of such large volumes of mafic magma generally is attributed to the rise and impingement of hot mantle plumes on continental and oceanic lithospheric plates (Condie, 2001; Pirajno, 2007). Plume-related magmatism and LIPs extend from at least 2.7 Ga to the present (Ernst, 2007). Mesozoic-Cenozoic continental LIPs are best preserved, with extensive flood basalt sections as much as several kilometers thick that consist of large tabular lava flow units typically without significant interlayered sedimentary rocks. In contrast, older Precambrian to Paleozoic LIPs commonly have no or only limited remnants of preserved flood basalts, but the plumbing system is exposed as giant dike swarms, sill complexes, and large layered Figure 4–1. Schematic section through Earth’s core and mantle showing the relations among hot plumes, spreading centers, and subduction zones. Large igneous provinces (LIPs) are characterized by large volumes of flood basalts and form above hot plumes that rise from a compositionally enriched thermal boundary layer above the core. Mid-ocean ridge basalts (MORBs) erupt at spreading centers, and island-arc basalts occur along subduction zones. Figure modified from Campbell (2001). Spreading center Mid-oceanridge basalt Flood basalt Island-arc basalt Subduction zone Lithosphere Hot plume Enriched mantle Upper mantle mantle Lower 660 km discontinuity Large igneous province (LIP)
Relations to Structures 11 intrusions. Recognition of LIPs in the Archean is uncertain because most volcanic rocks and associated intrusions occur in deformed and fault-segmented greenstone belts (de Wit and Ashwal, 1997) and generally cannot be traced over LIP-scale extents. Some greenstone belts, however, consist of tholeiite and komatiite volcanic rock sequences that may be remnants of Archean LIPs originally formed as oceanic plateaus or on continental platforms (Ernst, 2007). Continental LIPs characteristically are associated with the breakup or attempted breakup of continents (Condie, 2001; Ernst, 2007; Begg and others, 2010). As a result, pre-Mesozoic LIPs may be intensely fragmented by continental breakup, with parts of a single LIP being dispersed on different crustal blocks. This fragmentation and dispersal, as well as erosion removing some or all of the flood basalt cover, can result in magmatic units too small in size and volume to qualify as a LIP. Therefore, it is important to determine if particular dike swarms, sill complexes, layered intrusions and/or flood basalt remnants are components of a fragmented LIP. Establishing such a relation, however, can be difficult, particularly for older Paleozoic and Precambrian terranes for which plate reconstructions are less robust and constrained. A combination of dike trends, geochemistry, age measurements, and paleo magnetic determinations can be used to help reconstruct LIP fragments (Ernst and Buchan, 2001; Ernst, 2007; Bryan and Ernst, 2008). Temporal (Secular) Relations Economic magmatic sulfide-rich Ni-Cu±PGE deposits in LIP settings range in age from Archean to Middle Juras sic and are almost exclusively associated with (replenished) dynamic magmatic systems, including periodically replen ished subvolcanic feeder sills, feeder dikes, and volcanic vents (Lesher, 2004). Less dynamic (unreplenished) systems such as sheeted sills and larger layered intrusions normally are barren of sulfide-rich Ni-Cu±PGE mineralization unless they are a lateral component in a dynamic magma system. The apparent restriction of economic sulfide-rich Ni-Cu±PGE deposits to Jurassic and older magmatic systems probably is a reflection of the more deeply eroded nature of older LIPs, thereby exposing more of the plumbing system below extensive and barren flood basalt sections. Currently, there are too few known deposits to determine if there is any significant secular variation in their occurrence (Naldrett, 2010). Studies of LIPs through time suggest, however, that there are few significant gaps in the post-2.6-Ga record, with an emplacement rate of about one LIP every 20 million years (Ernst, 2007). It does appear that more voluminous and/or more focused magmatic events produced larger and/or more abundant ore deposits than less voluminous and/or less focused magmatic events (Lesher, 2004). Duration of Magmatic-Hydrothermal System and/or Mineralizing Processes Few studies have been undertaken to determine the dura tion of magmatic systems that formed individual sulfide-rich Ni-Cu±PGE deposits. However, from studies of mafic magma systems, and thermal modeling of emplacement and cooling of intrusions, it appears that magma residence in mafic systems is approximately a few thousand years at most and generally only years to hundreds of years (Reid, 2003). For example, steady-state modeling of the most active conduit system beneath the summit of Kilauea Volcano in Hawaii suggests a basaltic magma residence time of 30 to 40 years during the late twentieth century (Pietruszka and Garcia, 1999). Geo physical estimates of magma residence time beneath Kilauea, based on volumes of magma reservoirs, range from about one hundred years to several thousand years (Reid, 2003). The largest estimate likely reflects the entire interconnected melt volume beneath Kilauea whereas the shorter duration likely represents only the liquid part of the reservoir that contributes directly to lava output. Relations to Structures Extension, rifting, and development or reactivation of deeply penetrating faults typically accompany develop ment of LIPs and play an important role in providing zones of weakness and dilation along which magma may ascend into the crust (Pirajno, 2007; Begg and others, 2010). These magma-focusing structures facilitate development of dynamic subvolcanic feeder systems that typically host sulfide-rich Ni-Cu±PGE deposits (Arndt, 2005). At Noril’sk, many of the ore deposits are aligned along the major NNE-trending Noril’sk-Kharaelakh fault, which may have controlled the emplacement of the ore-forming magmas. In addition, the main ore deposits within the Noril’sk region are confined to volcanic-plutonic depressions or troughs that range from 50 to 150 km long and from 40 to 90 km wide (Diakov and others, 2002). These depressions contain thick Paleozoic sedimentary deposits, including evaporites and coal-bearing formations that host a number of mineralized intrusions, overlain by 2- to 4-km-thick sequences of barren Siberian flood basalts. Although Ni-Cu sulfide deposits commonly are spatially asso ciated with rifted continental margins (Begg and others, 2010), the deposits may not be related directly to the rifting event that formed the margin. Rather, they may form later during deflection of a mantle plume by thick continental lithospheric root, causing lateral spreading and rise of the plume under the thinned lithosphere present along continental margins.
12 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Relations to Igneous Rocks The formation of sulfide-rich Ni-Cu±PGE deposits is a direct result of evolution of mantle-derived mafic and ultra mafic magmas emplaced in dynamic, flow-through, replen ished magmatic systems (Naldrett and others, 1995; Maier and others, 2001; Barnes and Lightfoot, 2005). Relatively high degrees of partial melting of the mantle result in parental magmas enriched in Ni and PGE (Keays, 1995). These paren tal magmas form a variety of olivine-bearing mantle-derived rocks related to ferropicrite (Pechenga, Russia; Eagle, United States), tholeiitic picrite (Noril’sk-Talnakh, Russia), and highAl basalt (Voisey’s Bay, Canada). Although the metal con tents of the primary magmas can influence ore compositions, factors such as temperature, viscosity, volatile content, and mode of emplacement are the more important controls on ore genesis (Arndt and others, 2005). Because the parent mafic and ultramafic magmas typically are sulfur-undersaturated on their ascent from the mantle (Keays, 1995), ore formation generally involves the incorporation of sulfur from sulfur-rich crustal rocks. Sulfide deposits generally are hosted by dike- or sill-like intrusions emplaced in the upper crust as feeders to other intrusions and/or surface lava flows. The intrusions gener ally are small- to medium-sized differentiated mafic and/ or ultramafic dikes, sills, chonoliths (tens to a few hundreds of meters in thickness, about 100 to 2,000 meters wide, and hundreds of meters to several kilometers in length), and pluglike intrusions (generally <10 km in diameter), and range from oblate circular through sheet-shaped to rod-shaped (Lesher, 2004). The intrusions consist of a variety of igne ous lithologies depending on magma composition, depth of emplacement, and the rate of cooling. Those lithologies range from fine-grained basalt and diabase to medium- and coarsegrained cumulate rocks, gabbro, pegmatite, and breccia. The largest sulfide deposits typically are concentrated along the base of the intrusions in locations where changes in footwall geometry affected magma flow dynamics (Barnes and Light foot, 2005). Relations to Sedimentary Rocks Country rocks, including basement metamorphic rocks and upper crustal sedimentary rocks, play two important roles in the formation of these deposits. First, crustal struc ture helps control the formation of magma chambers, defin ing sites where ascending magmas can be trapped at density discontinuities in the crust (Arndt, 2005). When ascending magma reaches horizontally bedded sedimentary strata, magma can spread out along bedding planes, and horizontal intrusions or sills typically form. If magma intrudes along successively higher bedding planes, a series of upwardstepping sills will form complexes of stacked, saucer-shaped intrusions (Chevallier and Woodford, 1999). Second, a common trigger for sulfur saturation and segregation of an immiscible sulfide liquid is interaction between the sulfurundersaturated magma and crustal rocks, which may be sulfur-bearing. This interaction can change the temperature and composition of the magma by incorporation of country rock, which may decrease the sulfur solubility of the magma, and/or add sulfur to the magma; either process potentially can result in sulfur saturation in the magma. Horizontal flow through upper crustal dike-sill complexes can augment the period of interaction between magma and wall-rocks, thereby increasing the extent of contamination (Arndt, 2005). Although sulfide- and/or sulfate-bearing sedimentary rocks commonly are present in the country rocks near intrusions, the rocks that the magma(s) interacted with during ascent through the crust probably are more important for achieving sulfur saturation than are the country rocks at the final level of emplacement (Hoatson and others, 2006). Relations to Metamorphic Rocks Although metamorphism does not contribute directly to the formation of magmatic sulfide-rich Ni-Cu±PGE deposits, metamorphism can manifest with deposits in several ways. First, metamorphism of the country rocks due to emplace ment of intrusions can result in the development of skarns and contact aureoles, such as at Noril’sk and Uitkomst. Also, post-emplacement cataclastic metamorphism may remobilize, and possibly disperse, sulfide ores, as in the development of matrix-brecciated sulfides at Pechenga (Naldrett, 2004) or the remobilization of sulfides along shear zones as at the Kabanga deposit (Maier and others, 2010). In addition, meta morphism can result in significant upgrading of the tenor of disseminated sulfide minerals if olivine is destroyed, thereby releasing Ni that will be taken up by sulfides. Also, sulfides may be recrystallized, resulting in coarser sulfide textures that can make the mineralization more amenable to mineral processing. Because massive sulfides are incompetent and concentrate stresses during deformation, massive sulfide may be mobilized and displaced away from host intrusions and concentrated into pressure shadows in deformed terranes. 5. Physical Description of Deposits Magmatic sulfide-rich Ni-Cu±PGE deposits typically occur in clusters, and many deposits contain multiple min eralized zones. Sulfide ores occur within host intrusions and country rocks in a variety of forms (table 5–1), depending on the timing of sulfide segregation relative to the emplacement and crystallization history of the host rock and the degree of subsequent magmatic, hydrothermal, and/or tectonic mobili zation (Lesher, 2004).
Size and Shape of Deposits 13 Size and Shape of Deposits The size and shape of deposits and ore zones depend on the form and orientation of the host unit, generally an intru sive body and adjacent footwall rocks, on any ore-localizing feature (for example, footwall embayments), and on the nature and distribution of the sulfides (table 5–1). Host intru sions typically range from tens to a few hundreds of meters in thickness, about 100 to 2,000 meters wide, and hundreds of meters to several kilometers in length, to plug-like intru sions generally <10 km in diameter. As a result, deposits and ore zones form a continuum of shapes and sizes from large oblate spherical masses hundreds of meters across (Voisey’s Bay Ovoid deposit) to meter- to tens of meters-sized pods, rods, and stringers (Noril’sk). Massive sulfide (greater than about 66 percent sulfide minerals) and matrix or net-textured (about 33 to 66 percent sulfide minerals) ores tend to form dikes and flat-lying sheets, as well as lenses at the bottom of intrusions, which, in some cases, also protrude downward into the footwall rocks (fig. 5–1). At Noril’sk, massive ores extend for several kilometers along the long axis of the host intrusion, with thicknesses of as much as 50 m and widths of as much as 1 km (fig. 5–2; Diakov and others, 2002). Massive ores typi cally occur in physical depressions or where changes in the geometry or topography of the footwall occur. The thickness of the massive ores often correlates with the thickness of the intrusion, generally pinching out where the intrusion thick ness decreases (fig. 5–3; Diakov and others, 2002). Massive ores can show distinct sulfide mineral zonation, ranging from pyrrhotite-dominated, chalcopyrite-pentlandite assemblages in the outermost and lower parts, through progressively more Cu-rich zones, to mainly Cu-rich sulfides, commonly enriched Table 5–1. Form, distribution, textures, and timing of segregation of Ni-Cu±PGE sulfide mineralization (after Lesher, 2004). Type Form/distribution Texture Timing Stratiform or stratabound, at or near the base of host unit Massive, net-textured, disseminated Early magmatic segregation Stratabound to podiform, internal to host unit a. Massive, net-textured, or heavy disseminated b. Blebby disseminated c. Fine disseminated Early magmatic segregation Early or intermediate magmatic segregation Late magmatic segregation Stratiform or stratabound in country rocks adjacent to Type I ores a. Mineralized metasedimentary rocks b. Interbreccia, interpillow, veins in footwall rocks Early magmatic and/or metamorphic diffusion Early magmatic percolation Veins in country rocks associated with Type I ores Massive or semi-massive veins, often with only very narrow alteration selvages a. Magmatic-hydrothermal b. Metamorphic-hydrothermal Massive to semi-massive within shears and fault zones mainly associated with Type I ores Normally foliated, normally inclusion-bearing Tectonically mobilized Weak sulfide mineralization Peripheral sills Main body Leucogabbro and variably textured gabbro Upper copper ore Sparse sulfides Disseminated ore Variably textured gabbro Massive ore Copper ore Contact and lower olivine gabbro Olivine melagabbro Olivine gabbro Olivine-bearing gabbro Figure 5–1. Schematic cross section of a typical Noril’sk (Russia) ore-bearing intrusion showing internal structure, lithologies, and major ore types. Figure modified after Naldrett (2004).
14 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits in PGE in the middle or upper parts. That zonation of sulfides is attributed to in situ fractionation of the original sulfide liquid (Naldrett, 2004). In contrast to massive ores, breccia ores generally form semi-conformable, sheet-like zones along both the lower and upper contacts of the intrusions and may enclose massive ore (Voisey’s Bay). The breccias comprise fragments of both the intrusion and wallrocks in a matrix mainly of massive sulfide. Sulfide stringers and disseminations may accompany the breccias. Disseminated sulfides (about 1 to 33 percent sulfides) form lenticular to tabular layers generally within the middle and lower parts of intrusions (fig. 5–1). In most cases, they occur as irregular 1-mm- to 1-cm-diameter patches interstitial to silicate and oxide minerals. However, at some localities, such as Noril’sk-Talnakh and Pechenga, disseminated sulfides can occur as 1- to 10-cm-sized spheres or globules of chalcopyrite, pentlandite, and pyrrhotite (dispersed through the host rock) and can be zoned in the manner of the massive ores, with a pyrrhotite-rich base and chalcopyrite-rich top. In the Talnakh intrusion, the thickness of the disseminated ore zone tends to follow the outline of the footwall of the intrusion. However, whereas massive ore pinches out along strike, disseminated ore is nearly continuous along the strike axis of the intrusion (fig. 5–3; Diakov and others, 2002). Figure 5–2. Typical shapes of ore bodies at Noril’sk, Russia. Figure modified from Diakov and others (2002). Orebodies in plan view Axis of maximum thickness of orebody Fault Talnakh orebodies Kharaelakh orebodies Noril’sk 1 orebodies EXPLANATION 1 MILE 2 KILOMETERS
Size of Mineralizing System Relative to Extent of Economically Mineralized Rock 15 Size of Mineralizing System Relative to Extent of Economically Mineralized Rock Unlike mineral deposits formed by hydrothermal pro cesses such as porphyry copper and volcanogenic massive sulfide deposits, which generally affect areas much larger than the resulting mineral deposit, magmatic sulfide-rich Ni-Cu±PGE deposits have a relatively small “footprint” out side the deposit. However, although the individual intrusions that typically host these deposits are generally relatively small, they are often parts of larger and complex igneous systems. For example, the Pechenga greenstone belt is part of a larger, discontinuously developed Paleoproterozoic magmatic prov ince that stretches over a distance of 1,000 km in the Kola Peninsula region of Russia. At Pechenga, there are at least 226 differentiated mafic-ultramafic intrusions present within graphitic pyrite-bearing sedimentary rocks underlying the volcanic rock sequence. Of those intrusions, 25 contain Ni-Cu±PGE sulfide ores, 68 contain Ni-Cu±PGE sulfide occurrences, and 113 are viewed as “barren” (Barnes and Lightfoot, 2005). Similarly, at Noril’sk there are a large num ber of intrusive complexes occupying various positions within the stratigraphic sequence of sedimentary rocks and lavas; however, only a very few of the intrusions host Ni-Cu±PGE sulfide ores (Diakov and others, 2002). Most intrusions are elongated and irregular in shape in plan view, U-shaped in cross section, and extend from a few kilometers to few tens of kilometers in length. Within the mineralized intrusions, mas sive sulfides generally occur within swells near pinch-outs; swells are typically spaced from 500 to 2,000 m apart and average 1,000 to 1,500 m in length (Diakov and others, 2002). Many of the intrusive magmatic systems occur at sub volcanic levels in the crust, and contact alteration and meta morphism of the host rock commonly occur adjacent to both mineralized and unmineralized intrusions. It was noted early during exploration in the Noril’sk area, however, that unminer alized intrusions generally are surrounded by relatively thin, low-temperature hornfels aureoles, whereas ore-bearing intrusions have higher temperature hornfels aureoles 1.5 to 2.5 times as thick as the intrusion itself (fig. 5–4; Diakov and others, 2002). In addition, the aureoles above the intrusions are normally four to five times as thick as the zone below and are thicker at the terminations of intrusions (30 to 400 m) relative to their rearward counterpart. The presence of these large aureoles created by high heat flow adjacent to orebearing intrusions is taken as evidence that the intrusions were open conduits for magma ascending towards the surface (Naldrett and Li, 2009). 18,000 16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 North South Cross-sectional area Disseminated ore Massive ore Distance, in meters Thickness, in meters Figure 5–3. Graph showing the variation in cross-sectional area and distribution of disseminated and massive ore along the Talnakh intrusion, Noril’sk, Russia. Figure modified from Diakov and others (2002).
16 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Host Rocks Magmatic sulfide-rich Ni-Cu±PGE deposits are hosted by a variety of mafic and/or ultramafic, variably textured, com monly layered intrusive rocks, as well as by adjacent sedi mentary or metamorphic country rocks (Naldrett, 2004). At Noril’sk, where three main types of intrusions have been distinguished (undifferentiated, differentiated, and transi tional), only the fully differentiated intrusions host sulfide ores (Kunilov, 1994). The Talnakh intrusion consists of 3 to 10 m of massive sulfide at the base of a variably textured (local term, taxitic; Zientek and others, 1994) olivine gabbro (fig. 5–1). The variably textured olivine gabbro is overlain by olivine melagabbro (local term, picritic gabbrodolerite; Zientek and others, 1994) which, with decreasing olivine con tent, is overlain by biotite-bearing olivine gabbro to gabbro, followed by local domains of pegmatite and leucogabbro at the upper contact (fig. 5–1). The Eagle deposit in Michigan also is associated with olivine-rich rocks that include feldspathic peridotite, melatroctolite and olivine melagabbro (Ding and others, 2010). In contrast, intrusive rocks at Voisey’s Bay consist of troctolite, anorthosite, diorite, and granite; massive and matrix ores are associated with uniform- and variedtextured troctolites (Li and others, 2000). Structural Setting(s) and Controls As discussed above, extension, rifting, and development or reactivation of deeply penetrating faults typically accom pany development of LIPs, and these structures may play an important role in focusing magma ascent and facilitate the development of dynamic subvolcanic feeder systems (Arndt, 2005). At Noril’sk, sulfide-bearing intrusions are confined to volcanic-plutonic depressions (rift basins) and aligned along major crustal-penetrating faults (Diakov and others, 2002). However, a direct connection to major faults is less evident at some other deposits, such as Voisey’s Bay. 6. Geophysical Signatures Introduction Magmatic sulfide-rich Ni-Cu±PGE deposits, generally hosted by relatively small picrite or tholeiitic basalt dike-sill complexes, present some unique challenges to geophysical exploration. Because these types of deposits involve some combination of pyrrhotite, pentlandite, and chalcopyrite (Ford and others, 2007), they typically are associated with highly anomalous physical properties, including density, magneti zation, electrical conductivity, electrical chargeability, and acoustic velocity. As a result, geophysical methods can be used effectively to detect and characterize such deposits. Unfortunately, the mafic-ultramafic host rocks, as well as the enclosing wall rocks, also can have a variety of anoma lous physical properties, thereby complicating geophysical recognition of sulfide deposits. Furthermore, the geophysi cal signatures of the sulfide deposits, as well as of the host intrusions, may vary widely due to a variety of geometric and geologic factors. Thus, a geophysical method that works effectively in one region may not work at all in another (King, 2007). Finally, Ni-Cu±PGE sulfide deposits, as well as their host intrusions, are small, commonly only tens or hundreds of meters in width, and they present a “needle in a haystack” scenario for any regional-scale geophysical program. Overall, a successful geophysical program for these types of deposits requires a creative and highly flexible strategy. This section provides an overview of the major geo physical signatures that generally are known to be associated with magmatic Ni-Cu±PGE sulfide deposits associated with mafic-ultramafic dike-sill complexes. This is not meant to Pyroxene hornfels Amphibole hornfels Muscovite hornfels Cu ore Massive Ni-Cu ore Disseminated sulfides Mineralization Metamorphic aureole Intrusion EXPLANATION 375 FEET
200 METERS Figure 5–4. Schematic diagram showing the extent of contact metamorphism and metasomatism surrounding typical ore-bearing Noril’sk-type (Russia) intrusions. Figure modified from Naldrett and Li (2009).
Gravity Signature 17 be a comprehensive review of all the geophysical methods that have been used on these deposits, in part because many pertinent case-histories are still unavailable because they are in proprietary company reports. In addition, this section also reviews which geophysical methods have been most effective so far, and which appear most promising for future assess ments and exploration programs. Magnetic Signature In the magnetic method, highly precise measurements of Earth’s magnetic field are used to investigate the subsurface. Once the acquired magnetic data have been corrected for temporal variations and for the geomagnetic reference field (core-derived component), the remaining anomalies reflect upper crustal geology (fig. 6–1). Most magnetic anomalies reflect variations in the quantity and physical nature of the ferromagnetic iron oxide mineral magnetite, a common accessory in many igneous and metamorphic rocks, and these anomalies typically have amplitudes ranging from a few nanoTeslas to several thousand nanoTeslas. In comparison, Earth’s background geomagnetic field for much of the United States is approximately 55,000 nanoTeslas. The magnetic method is one of the most applied techniques in mineral exploration, mainly because of its sensitivity to lithologic and structural variations in igneous and metamorphic terranes that commonly host mineral deposits. In addition, the method can be readily applied using aircraft for aeromagnetic survey ing, thereby allowing large areas to be covered rapidly and efficiently. The magnetic method is particularly well-suited for reconnaissance-scale investigations for mafic-ultramafic dike-sill complexes that might host Ni-Cu±PGE sulfide deposits (Ford and others, 2007). The intrusions that host these deposits, as well as enclosing wall rocks, typically con tain varying amounts of magnetite, and the magnetic method thereby is useful in recognizing intrusions and enclosing geologic frameworks where conditions for sulfide deposi tion may have been favorable. In addition, pyrrhotite can be strongly magnetic under certain physiochemical conditions (table 6–1), and because of its near-ubiquity in magmatic Ni-Cu±PGE deposits, magnetic surveying might, in some cases, directly detect sulfide bodies. Not surprisingly, magnetic surveying has been used extensively in the search for these deposits in Canada (Ford and others, 2007; King, 2007; Balch and others, 2010), and it has apparently also played a signifi cant role in the discovery of the Eagle and Tamarack deposits in the United States (Ware and others, 2008; Boerboom, 2009; Ding and others, 2010). Because these deposits and their host intrusions are small in size, magnetic surveying must be con ducted with very high-resolution specifications. If available, “regional” surveys with a line spacing of 200 to 400 m may be used to target more-detailed surveys that use line spacings of 100 m or less. An example of this magnetic survey approach is illus trated for the McFaulds Lake prospects in the Hudson Bay lowlands of Canada (fig. 6–1). The regional survey, which was originally flown as part of a diamond exploration pro gram, revealed a semicircular ring of magnetic highs that correspond to a series of mafic to ultramafic igneous rocks ringing a granodioritic pluton within the Sachigo greenstone belt of the Archean Superior Province (fig. 6–1A). This ring of intrusions, also known as the “Ring of Fire,” has become best known, perhaps, because of the recent discovery of large chromite deposits (Mandel, 2009; Mungall, 2009; Smyk, 2010), but the exploration program here also has revealed two significant Ni-Cu±PGE sulfide deposits, the Eagle’s Nest and the AT-12 (Balch and others, 2010; Mungall and others, 2010). The regional aeromagnetic survey (fig. 6–1A) was useful in delineating mafic to ultramafic intrusions that warranted further investigation, but it failed to resolve either of the sulfide deposits. A higher resolution total field and gradiometer survey, with 100-m line spacing and 50-m elevation, was part of a follow-up investigation. It revealed the 100- by 300-m footprint of pyrrhotite in both sulfide deposits (fig. 6–1B and 1C). In some situations ground-based magnetic methods also may be used effectively, once a favorable target has been established by either aeromagnetic data or geologic mapping (Guo and Dentith, 1997), to further delineate sulfide mineralization. The magnetic survey method has been particularly helpful at the Noril’sk deposit, where deep burial beneath conductive sedimentary rocks can thwart electromagnetic (EM) and other geophysical applications (Diakov and others, 2002). Although the Ni-Cu±PGE deposits lack a distinctive geophysical signature, magnetic data provide clues regard ing the geologic framework associated with the deposits. For example, magnetic data have been used to recognize indi vidual flows and sills near the surface, buried intrusions, and faults that may have focused magma flow. Unfortunately, not all Ni-Cu±PGE sulfide deposits and their host rocks provide magnetic signatures. For example, for the Voisey’s Bay deposits, high-resolution aeromagnetic data from the area typically show little or no signature that can be linked to the deposits or to the host intrusions, but instead reflect the structure of the gneissic wall rocks (fig. 6–2; King, 2007). This is because pyrrhotite both in the ores and the host intrusions in this region are nonmagnetic and correspondingly provide no magnetic signature (Balch, 1999). Under such con ditions, airborne EM or some other geophysical method may have to be attempted at the reconnaissance stage to identify intrusions and sulfide mineralization. Gravity Signature The gravity method uses very small variations (anoma lies) in Earth’s gravity field to investigate subsurface geology. The geologic signature, most commonly expressed as Bouguer anomaly data, is isolated by several corrections
18 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits A B AT-12 ENZ AT-12 ENZ Figure 6-1. Magnetic anomaly maps of prospects in the James Bay Lowlands, Ontario, Canada. A, Total magnetic intensity (TMI) map from a regional-scale aeromagnetic survey in northern Ontario. Line spacing is 200 m. The 60-km-wide, semicircular ring of anomalies in the north-central part of the image roughly outlines the so-called Ring of Fire exploration area. The white box delineates the area for maps B and C. B, High-resolution TMI map of the McFaulds Lake area based on a fixed-wing survey with lines spaced 100 m apart and flown 50 m above terrain. Sites ENZ and AT-12 refer to the Eagle’s Nest (zone) and to the AT-12 Ni-Cu±PGE sulfide deposits, respectively. C, Horizontal gradiometer data from same survey as map B. Labels ENZ and AT-12 refer to the Eagle’s Nest (zone) and to the AT-12 Ni-Cu±PGE sulfide deposits, respectively. Map A is from Mungall and others (2010), and maps B and C are from Balch and others (2010).
Gravity Signature 19 related to variations in time, latitude, elevation, and topogra phy. Bouguer anomalies typically range from less than one milligal to several tens of milligals. In comparison, the total gravity field of the Earth is approximately 980,000 milligals. Similar to magnetic data, gravity anomaly data can be effec tive for investigating the geology of the igneous and metamor phic terranes where mineral deposits may occur. Bouguer gravity anomalies ultimately reflect subsurface variations in density, which in turn are related to mineral composition and porosity. Densities of earth materials typi cally range from about 2.00 g/cm3 for unconsolidated materi als to about 2.95 g/cm3 for mafic igneous rocks (table 6–2). Ni-Cu±PGE sulfide deposits can have host rock densities exceeding 3.00 g/cm3 and ore densities exceeding 4.00 g/ cm3 (table 6–2). Thus, even relatively small mineral deposits Figure 6–2. Aeromagnetic data collected over the Ovoid and Eastern Deeps deposits in the Voisey’s Bay area, Canada. Figure modified from King (2007) and Balch (1999). Table 6–1. Copper-nickel sulfide ore mineral and hostrock magnetic susceptibilities in SI units (dimensionless proportionality) × 10–3, modified from King (2007). Rock type Range Average Sulfides/oxides Pyrrhotite 1–6,000 1,500 Pyrrhotite (mono) Pyrrhotite (hex) Pentlandite
Chalcopyrite Magnetite 1,200–19,200 6,000 Host rocks Felsic Igneous 0–80 Mafic Igneous 0.5–97 Ultramafic Rocks (Peridotite) 90–200 Ultramafic Rocks (Serpentinite) Moderately high Moderately high
20 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits are capable of producing anomalies of as much as several milligals (Ford and others, 2007). Model studies of gravity data have been used effectively to investigate the subsurface extent of the Jinchuan deposit (Guo and Dentith, 1997) and the Ovoid deposit in the Voisey’s Bay district (Ash and oth ers, 2006; Ford and others, 2007), although such an approach requires careful separation of the deposit-related anomalies from those arising from other sources. The 4-milligal residual gravity anomaly that is associated with the Ovoid deposit at Voisey’s Bay is shown in figure 6–3. Until relatively recently, gravity surveying for min eral exploration relied on ground-based acquisition, thereby increasing the time and cost of acquiring data. Consequently the use of gravity methods for Ni-Cu±PGE sulfide explora tion mainly has been restricted either to detailed investigations of specific targets (see above) or to regional-scale investiga tion of geology and crustal structure using coarsely sampled, regional data sets (Rempel, 1994; Diakov and others, 2002). Ongoing advances in airborne gravity and gravity gradiometry, however, are likely to allow greater usage of high-resolution gravity data in the reconnaissance for favorable intrusions, such as is currently implemented with aeromagnetic surveying (King, 2007). Balch and others (2010) reported that the relatively small 0.25-milligal signature associated with the Eagle’s Nest deposit in the Hudson’s Bay lowlands was near the resolution limits of the currently implemented Bell gradiometer system. Electrical Signature Most electrical geophysical methods involve measure ment of electrical potential at the surface, typically by using a pair of electrodes, to investigate geology at depth. Electrical methods are based on electrical resistivity, induced polar ization, or spontaneous potential. Electrical resistivity and induced polarization methods use an artificial current that is introduced via a separate electrode pair, whereas spontaneous potential methods rely on small, passive currents that already exist in the subsurface. In all three approaches, standard electrode arrays are used, and by systematically varying the position and spacing of electrodes, the variation of electrical properties at depth, and by inference geology, can be investigated. Because all three methods rely on electrodes, ground-based deployment—with the related costs—is necessary. Consequently, use of electrical methods in explor ing for Ni-Cu±PGE sulfide deposits typically is focused on specific targets or target areas that have been recognized by earlier geological, geophysical, or geochemical studies. In electrical resistivity (ER) methods, the subsurface structure is interpreted in terms of resistivity. Resistivity, which is the inverse of conductivity, is an intrinsic property of a material that expresses the degree to which it resists electrical current, and in electrical exploration it is expressed in ohm-meters (ohm-m). Resistivity for most earth materi als is a function of the quantity and quality of water that is present interstitially, and it generally ranges from a few tens of ohm-meters for clays and shales to ≥1,000 ohm-meters for fresh crystalline rocks (Sharma, 1997). Sulfide minerals have extremely low resistivities (much less than 1 ohm-meters), but their tendency to occur within discrete, three-dimensional volumes makes them difficult to image properly with standard resistivity methods, which are best suited for imaging broad, layer-like features with relatively high resistivities (King, 2007). The ER methods still could be used to investigate thickness and composition of overburden, which may have implications for other geophysical applications (Ford and others, 2007). Resistivity surveying over the Jinchuan deposit showed detectable resistivity responses over some ore bodies, although this effect can be obscured seriously by conductive alluvium (Guo and Dentith, 1997). Induced polarization (IP) methods use the same elec trode configurations as ER methods, and they are commonly conducted together (Sharma, 1997). The IP effect is a charge Figure 6–3. Detailed Bouguer anomaly map of the Ovoid deposit at Voisey’s Bay, Canada. Image from www.geo.pubc. ca/ubcgif/ and modified from Ford and others (2007). mGal Northing (m) 1,500 1,000 1,000 1,500 Easting (m) Gal Table 6–2. Density values for sulfides and mafic-ultramafic host rocks from King (2007). [Abbreviation: g/cm3, grams per cubic centimeter] Mineral/rock type Range (g/cm3) Average (g/cm3) Sulfides Pyrrhotite 4.5–4.8 Pentlandite Chalcopyrite 4.1–4.3 Host rocks Felsic igneous rocks 2.33–3.11 Mafic igneous rocks 2.09–3.17 Ultramafic rocks (Peridotite) 2.78–3.37
Electromagnetic (EM) Signature 21 buildup in the subsurface that dissipates over a short time after the current has been shut off. The effect is caused by interfa cial interactions between electrolytic groundwater and highly conductive grains of sulfide or clay particles. The IP effect typically is expressed as chargeability, which is based on the decay of residual potential once the current has been shut off (time-domain IP) or as percent frequency effect (PFE), which is based on apparent resisitivity observed at two discrete current frequencies (frequency-domain IP). Because the IP effect is proportional to interactive area, disseminated sulfide deposits are more likely to produce a pronounced IP effect than massive sulfide deposits, although disseminated haloes that commonly surround massive deposits can produce a significant IP response. IP/ER surveying has been applied with some success to the Ovoid deposit at Voisey’s Bay (Balch, 1999) and to ore bodies at the Jinchuan deposit (Guo and Dentith, 1997), although application in the latter case was limited in some areas by conductive overburden. Unfor tunately, IP methods typically cannot discriminate between barren disseminated sulfides, perhaps with just pyrite or pyrrhotite, from those sulfides enriched in Ni-Cu or PGE (Balch, 2005). Spontaneous potential (SP) methods are based on passive monitoring of ambient currents in the shallow subsurface. The SP currents can be caused by changes in potentials related to movement of ion-bearing groundwater (streaming or electro kinetic potential), differences in ionic concentration within groundwater (diffusion potential), or by a variety of electro chemical reactions in the vicinity of a conductive body (mineral or electrode potential). Most SP anomalies are several tens to hundreds of millivolts, although anomalies as large as 1,000 millivolts can occur. The SP surveying is among the oldest and simplest electrical methods in exploration geo physics, but its shallow depth of penetration precludes its use in most current mineral exploration programs, and the SP method’s primary use today is for engineering and environ mental investigations (Sharma, 1997). Guo and Dentith (1997) reported a large SP anomaly over an ore body at the Jinchuan deposit, with secondary anomalies apparently reflect ing weathered ultramafic rocks and carbon-bearing marble in the wall rock. Electromagnetic (EM) Signature Electromagnetic (EM) methods are used extensively for sulfide exploration (Ford and others, 2007). This preference is mainly because EM methods are particularly sensitive to the high conductivities of massive sulfide bodies and because most methods do not require the use of electrodes, readily accommodating airborne applications. Typically, conductivi ties of sulfides range between 103 and106 Siemens/meter, which is usually one order of magnitude above that of the enclosing host rocks (King, 2007). Most EM surveys utilize artificially induced currents produced by fluctuating magnetic fields from a transmitter coil or antenna, but some economic applications utilize naturally induced currents including audiofrequency magnetotelluric (AFMAG) methods. Regardless of their source, magnetic field fluctuations induce currents in subsurface conductors, and those currents produce secondary magnetic field fluctuations that can be picked up by a receiver coil or antenna. The EM methods span a wide spectrum of frequencies, extending from the subhertz range for magneto telluric investigations, up to the megahertz range for geo-radar applications; most EM methods used in sulfide exploration, however, operate with frequencies between 1 and 1,000 hertz. Exploration by EM is divided into two approaches: fre quency domain, where the EM response is observed at varying frequencies, or time domain, where the decay of induced currents is observed following a pulsed EM signal. Most exploration EM methods are used to investigate targets within 300 m of the surface, although the Titan 24 System devel oped by Quantec Geoscience combines AFMAG methods with resistivity and IP surveying to investigate targets as deep as 1,500 m (Ford and others, 2007). Similarly, ZTEM is a telluric-based airborne method that is capable of detect ing conductivity variations well beyond the practical limits of current exploration (Balch and others, 2010). Initially, EM methods were used as simple anomaly detectors that were focused on locating massive sulfide occurrences, but modern methods can allow conductivity estimates of target bodies, as well as resistivity (inverse of conductivity) estimates, of wall rock and overburden (King, 2007; Ford and others, 2007). A recent improvement on time-domain methods has been the implementation of the so-called B-field, which is the integra tion of the decaying magnetic signal observed at the receiver. This derived parameter greatly enhances conductance resolu tion (Balch and others, 2010). Sulfide minerals associated with sulfide-rich Ni-Cu±PGE deposits are highly conductive, and massive to semi-massive occurrences present highly favorable EM targets (King, 2007). The EM methods have been particularly helpful with the Voisey’s Bay deposits and a variety of frequency- and time-domain methods have been successfully applied there (Balch, 1999; King, 2007; Ford and others, 2007). The results of AFMAG and helicopter-borne electromagnetic surveys of some of the Voisey’s Bay deposits are shown in figure 6–4. In spite of these successes, pitfalls in the application to EM methods include the interference of conductive graphite layers in the wall rock, as well as the masking effect of conductive overburden. With regard to the latter limitation, conduc tive sedimentary rocks in the Noril’sk region of Russia have largely negated the use of airborne and ground-based EM methods, although borehole based applications have met with some success (Diakov and others, 2002). Balch and others (2010) have recently reported on the highly effective application of helicopter-borne, time domain EM methods (HTEM) on the Eagle’s Nest and AT-12 Ni-Cu±PGE deposits in the McFaulds Lake area of Ontario. Early exploration in the McFaulds Lake area relied on the more traditional approach of a fixed-wing (airplane), time
22 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits A B Figure 6–4. A, Location and extent of Voisey’s Bay, Canada, sulfide deposits. B, Apparent resistivities calculated from an audio-frequency magnetotelluric (AFMAG) survey over the Eastern Deeps, Voisey’s Bay, massive sulfide body (after Balch, 1999). C, Low apparent resistivity associated with the Voisey’s Bay sulfide deposits, as calculated from a helicopter-borne electromagnetic (EM) survey. Image from Ford and others (2007).
Seismic Signature 23 domain EM system (Geo-TEM), followed by ground-based methods that typically consisted of horizontal Loop EM (HLEM) and magnetic surveying. The HTEM methods, how ever, were undergoing major improvements during the time of the McFaulds Lake exploration, and significant advances were made with regard to horizontal resolution, depth of penetration, and resolution of conductivities. The explora tion in the McFaulds Lake area consequently evolved into in-field interpretation of line EM data, with almost immediate recommendations regarding further acquisition, such as infill lines on interesting targets. This technology, in turn, led to a drilling program that was largely directed by HTEM data, with essential elimination of the expensive ground-based programs, and to a situation where prioritization of targets was more of a problem than detection of targets. On the basis of their McFaulds Lake experience, Balch and others (2010) predict that currently existing HTEM technology will revolutionize the exploration for magmatic Ni-Cu±PGE sulfide deposits. Seismic Signature The geologic settings associated with most magmatic sulfide deposits have historically not been considered to be favorable for most seismic methods. The igneous and metamorphic terranes that host such deposits were generally perceived to have structures that were too complex and impedance contrasts that were too low to be properly imaged by seismic methods. Consequently, most seismic studies in such settings were generally restricted to deep-crustal refrac tion and reflection studies that had little direct significance to mineral deposits. One possible exception is the deep seismic imaging reported for the Noril’sk region by Rempel (1994). In this study, explosion and earthquake sources were used to image crustal layering and apparent offsets in the Moho. Some of these offsets were attributed to deep crustal-penetrating faults that, according to Rempel (1994), might have served as conduits for ore-forming magmas. Recent seismic-reflection-profiling studies, such as those coordinated by the Canadian Lithoprobe Project (Clowes, 2010), have greatly dispelled some of the old prejudices regarding seismic applications for mineral deposits in igne ous and metamorphic terranes. Recent investigations have revealed that sulfide minerals and their host rocks can have large contrasts in acoustic impedance (fig. 6–5), and that many massive sulfide occurrences should be associated with detectable reflection or diffraction signatures, provided that the seismic data are properly acquired and processed (Salis bury and Snyder, 2007). Two-dimensional seismic reflection data (data acquired along a profile) from the Sudbury basin effectively imaged several geologic horizons of the basin, including the base of the norite where many of the primary sulfide ore bodies occur (Milkereit and others, 1992, 1994, 1996). The asymmetry of the Sudbury basin, as well as the offset of the basal sequence along numerous faults, was also evident from these data (fig. 6–6). Three-dimensional seismic data (data acquired over an areal grid) from the Sudbury basin (fig. 6–7) reveal structural details regarding the footwall of the norite (green) and show a prominent diffraction pattern caused by a massive sulfide deposit (Salisbury and Snyder, 2007). The seismic technology developed for the Sudbury basin should be readily applicable to mafic-ultramafic dike- and sill-hosted Ni-Cu±PGE sulfide deposits. Although not focused on mineral deposits, two recent three-dimensional seismic investigations of Cretaceous– Paleocene intrusive sill complexes in the Faroe-Shetland Basin of the northeastern Atlantic Ocean have findings that may be significant to sulfide assessments and exploration. Hansen and others (2004) describe individual sills that occur as trough- or saucer-like bodies, 2 to 8 km in diameter and with a relief of several hundred meters. The seismic data reveal that the sills are interconnected by a variety of junctions that yield impor tant clues on the emplacement history of the sill complex (fig. 6–8). Three–dimensional seismic imaging of a sill by Trude and others (2004) revealed a complex of ridges along the top of one sill that appear to indicate magma flow. These ridges, with wavelengths of 220 to 350 meters and amplitudes of 25 to 50 m, were likely created by viscous drag near the top of the sill and appear to radiate from distinct feeder zones (fig. 6–9). Sill geometry, as well as magma flow, is impor tant to the development of dike- and sill-hosted Ni-Cu±PGE sulfide deposits, particularly near feeder zones. The seismic studies in the Faroe-Shetland Basin indicate that threedimensional seismic imaging may be a powerful tool for recognizing intrusion geometry and identifying magma feeder systems. Another seismic method that may assist in exploration for dike- and sill-hosted Ni-Cu±PGE sulfide deposits is vertical seismic profiling (VSP). The VSP methods are particularly useful for imaging steeply dipping structures (>60°) not prop erly imaged by two- or three-dimensional reflection profiling (Salisbury and Snyder, 2007). For example, VSP could be used to image a vertical feeder beneath a sill. In this method, a receiver (seismometer) is lowered to the bottom of a drill hole and then raised in stages as shots are fired at the surface (fig. 6–10). If the positions of the shots and the drill hole geometry are precisely known, then velocities can be calcu lated from the shot receiver distances and the geometry of steep reflectors can be imaged through processing. Cross-hole seismic methods are essentially a variation of VSP where one has an array of holes. By systematically interchanging source and receiver positions at various positions in the holes (fig. 6–11), the three-dimensional velocity structure between holes can be imaged (King, 2007). Such a procedure can be used to detect hidden sulfide deposits that might lie between test holes. In spite of their great potential, seismic reflection surveys are very expensive, and their selection must be weighed very seriously. Salisbury and Snyder (2007) estimated that two dimensional surveying would cost about $6,000/line km, whereas three dimensional surveying would cost about
24 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits $60,000/km2 on land. Assuming a suitable drill hole already exists, VSP surveying to a depth of 500 m would cost about $30,000. These costs are large for many mineral exploration budgets, and they assure that if seismic methods are used, they will most likely be attempted at a very advanced stage of a prospect exploration. For example, two-dimensional data might be best used in the final stages of reconnaissance, three-dimensional data might be best used during discovery and delineation, and VSP might be best used in delineation only. As pointed out by Salisbury and Snyder (2007), “it is important to match the method to the intent of the survey and the type of deposit sought.” If the expense can be justified, then no other geophysical method can offer the depth of penetration and degree of resolution provided by seismic reflection methods. Spectral Characteristics Spectral absorption features associated with mafic and ultramafic rocks in the Visible-Near Infrared to Short Wave Infrared (0.5–2.5 micrometers; VNIR–SWIR) range typically are subdued by the low reflectance characteristics of mafic and ultramafic minerals, including magnetite, biotite, and horn blende that have Fe2+ spectral absorption features in the 0.5- to 1.65-micrometer region (fig. 6–12A; Rowan and others, 2005). Olivine, an exception among ultramafic VNIR–SWIR lowreflectance minerals, has a broad spectral absorption feature cen tered at 1.05 micrometers (fig. 6–12A). Additional SWIR Fe(Mg) OH spectral absorption features in phlogopite, hornblende, and biotite are centered at 2.31 to 2.33 micrometers (fig. 6–12B). Figure 6–5. Velocity values of various rock types plotted against their density with lines of constant acoustic impedance (Z) overlain within field and Nafe-Drake curve (gray) for common rocks at a standard confining pressure of 200 MPa (from Salisbury and others, 2003). Also shown are values for pyrite (Py), pentlandite (Pn), pyrrhotite (Po), chalcopyrite (Ccp), sphalerite (Sp), hematite (Hem), magnetite (Mgt), gangue (g), and fields for host rock–ore mixtures. A reflection coefficient (R) of 0.06 is sufficient to give a strong reflection. Galena (Gn, off scale) has a velocity of 3.7 km/s and a density of 7.5 g/cm3. Mgt Kimberlites Sediments Eclogite Peridotite Pyroxenite Metapelite Amphibolite Gabbro Mafic granulite Anorthosite Metagabbro (greensch. fac.) Int. granulite Qtz. mica schist Felsic granulite Felsic gneiss (amphib.) Granite Granodiorite Serpentinite Quartzite Py+ Density, in grams per cubic centimeter Velocity, p, in kilometers per second 200 MPa Mixed sulfides Mgt Hem Gn Po Pn Py + g Sp Ccp Po + g Z 20 R 0.06 Ccp/Sp + g
Spectral Characteristics 25 Depth, in kilometers Time, in seconds South Range Sudbury Basin North Range Levack Gneiss Complex Chelmsford Onwatin Onaping Granophyre Norite Sublayer Borehole control 1 MILE 3 KILOMETERS S N EXPLANATION Figure 6–6. Two-dimensional reflection section across the Sudbury Basin, Ontario, Canada, showing reflectors associated with deep structure and lithologic contacts. Data from Milkereit and others (1992); image modified from Salisbury and others (2007). Figure 6–7. Three-dimensional seismic image of the Sudbury area, Ontario, Canada, showing surface grid (horizontal plane), horizon slice (green) corresponding to the footwall complex, and time slice corresponding approximately to 2,000-m depth. Red spot represents known position of the Trillabelle sulfide deposit. Note diffraction in time slice corresponding to sulfide deposit. Data from Milkereit and others (1997); image modified from Salisbury and others (2007).
t Depth, in kilometers N
26 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits A B Magma flow direction Ridge feature Sill boundary Feeder zones Survey boundary 4 KILOMETERS 2 MILE N Figure 6–9. A, Horizon slice image of the top surface of a ridged sill in the Faroe-Shetland Basin, northeastern Atlantic Ocean margin. Schlumberger GeoViz software with false illumination from southeast. B, Simplified interpretation of the ridge features with proposed magma flow directions and magma feeder zones. Figure modified from Trude and others (2004). Two-way time, in seconds 1 MILE 2 KILOMETERS Two-way time, in seconds Figure 6–8. Vertical slice of a three-dimensional seismic reflection survey of the Faroe-Shetland basin, northeastern Atlantic Ocean margin. Individual sills are delineated by strong, concaveupwards reflections along and below 3 seconds. Note apparent interconnections between individual sills. Figure modified from Hansen and others (2004).
Spectral Characteristics 27 A B Shot Time Borehole Fault Receiver Position Distance Direct Reflected Figure 6–10. A, Fundamental layout for a vertical seismic profiling (VSP) survey. B, Distance-versus-time plot showing transit times for direct and reflected arrivals from a steeply dipping reflector. Figure modified from Salisbury and others (2007). Figure 6–11. Schematic illustration of cross-hole seismic methods (red marks rock body). Modified from King (2007). Figure 6–12. A, Visible-near infrared to short-wave infrared (VNIR–SWIR) reflectance sample library spectra of olivine, biotite, hornblende, and magnetite. Biotite, magnetite, and hornblende have low spectral reflectance when compared to olivine. Wavelength, micrometers Reflectance Olivine Hornblende Biotite Magnetite Spectral Library Plots A Transmitters Receivers
28 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Mafic and ultramafic host rocks for many deposits may be metamorphosed to greenschist facies. Serpentinization of mafic and ultramafic rocks forms serpentine, actinolite, epidote, and chlorite, which have prominent Fe(Mg)OH 2.31- to 2.33-micrometer absorption features (fig. 6–13). These minerals also have broad Fe2+ spectral absorption features in the 1.65- to 0.5-micrometer region (fig. 6–13). The feldspathic peridotite and altered gabbro metamorphosed to greenschist facies at the Eagle deposit exhibit low VNIR–SWIR reflec tance and contain weak 2.31- to 2.33-micrometer spectral absorption features (fig. 6–14). Fifteen- and 30-m Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) VNIR–SWIR data also have been used to map the 2.31- to 2.33-micrometer spectral absorption features and low reflectance of mafic and ultramafic rocks in Mordor, Australia (Rowan and others, 2005). Minerals associated with mafic and ultramafic rocks tend to have diagnostic spectral absorption features in the Thermal Infrared (TIR) 8.0- to 12.0-micrometer region (fig. 6–15). Fundamental vibrations of the Si-O bond in silicate minerals cause diagnostic emissive spectral absorption features from 9.75 to 11 micrometers in mafic and ultramafic minerals, such as olivine (10.2 micrometers), phlogopite (9.75 micrometers), hornblende (10.05 micrometers), diopside (10.25 and 10.8 micrometers), augite (10.25 and 10.9 micrometers), and biotite (9.8 micrometers) (fig. 6–15). Serpentinized mafic and ultramafic rocks also exhibit TIR spectral absorption features in the 9.75- to 11-micrometer region (fig. 6–15). Thus, mafic and ultramafic rocks, such as feldspathic peridotite and altered gabbro from the Eagle deposit and syenite, pyroxenite, and shonkinite rocks from Mordor in Australia, typically exhibit high emissivity in the 8- to 9-micrometer region and low emis sivity in the 9- to 12-micrometer region (fig. 6–16; Rowan and others, 2005). Mafic and ultramafic rocks at Mordor also have been mapped successfully using ASTER TIR data (90-m reso lution; Rowan and others, 2005). The ASTER 90-m resolution TIR data lack a spectral band positioned at 10 micrometers but still have sufficient spectral resolution to resolve diagnostic spectral absorption features of mafic-ultramafic mineral groups and rocks (figs. 6–15 and 6–16; Rowan and others, 2005).
Wavelength, micrometers (ASTER Bands) 5 6 7 8 9 Reflectance (offset for clarity) Olivine Phlogopite Hornblende Biotite 10% Reflectance B Figure 6–12. B, VNIR–SWIR reflectance sample library spectral pairs of biotite, hornblende, olivine, and phlogopite (lower spectrum) and the same mineral spectra resampled to Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) bandpasses (upper spectrum). The sample spectra biotite, hornblende, and phlogopite have 2.33- to 2.31-micrometer spectral absorption features illustrated in the ASTER spectra. Olivine has a 1.05-micrometer absorption feature expressed as low band-3 reflectance in the resampled ASTER spectrum. Spectra are offset for clarity of spectral shapes. The ASTERband center positions are illustrated at top of graph.
Spectral Characteristics 29 Wavelength, micrometers Actinolite Epidote Chlorite (ASTER Bands) Reflectance (offset for clarity) 10% Reflectance Serpentine Figure 6–13. Visible-near infrared to short-wave infrared (VNIR–SWIR) reflectance sample library spectral pairs of actinolite, epidote, serpentine, and chlorite (lower spectrum) and the same mineral spectra resampled to Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) bandpasses (upper spectrum). The sample spectra have 2.33- to 2.31-micrometer spectral absorption features illustrated in the ASTER spectra. The spectra also exhibit broad Fe2+ absorption in the 0.5- to 1.65-micrometer region. Spectra are offset for clarity of spectral shapes. The ASTER-band center positions are illustrated at top of graph. Wavelength, micrometers (ASTER Bands) Reflectance (offset for clarity) 10% Reflectance Felspathic peridotite sample 1 Felspathic peridotite sample 2 Altered gabbro sample 3 Figure 6–14. Thermal infrared (TIR) emissivity sample spectral pairs of mafic and ultramafic rocks and the rock spectra resampled to Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) bandpasses (upper spectrum). The sample spectra exhibit high emissivity in the 8- to 9-micrometer region. Spectra are offset for clarity of spectral shapes. The ASTER band center positions are illustrated at top of graph.
30 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits 5% Emissivity Wavelength, micrometers Emissivity (offset for clarity) Felspathic peridotite sample 1 Felspathic peridotite sample 2 Altered gabbro sample 3 (ASTER Bands) Figure 6–16. Thermal infrared (TIR) emissivity sample spectral pairs of mafic and ultramafic rocks and the rock spectra resampled to Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) bandpasses (upper spectrum). The sample spectra exhibit high emissivity in the 8–9 micrometer region. Spectra are offset for clarity of spectral shapes. ASTER band center positions illustrated on top of graph. Figure 6–15. Thermal infrared (TIR) emissivity sample library spectral pairs of biotite, augite, serpentine, diopside, hornblende, phlogopite, and olivine (lower spectrum) and the same mineral spectra resampled to Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) bandpasses (upper spectrum). Sample spectra primarily have spectral absorption features in the 9- to 12-micrometer region illustrated in the ASTER spectra. Spectra are offset for clarity of spectral shapes. The ASTER band center positions are illustrated at top of graph. Olivine Phlogopite Hornblende Diopside Augite Biotite 5% Emissivity Emissivity (offset for clarity) Wavelength, micrometers (ASTER Bands)
Hypogene Ore Characteristics 31 7. Hypogene and Supergene Ore Characteristics Hypogene Ore Characteristics Mineralogy and Mineral Assemblages The hypogene ore minerals for magmatic Ni-Cu±PGE sulfide deposits are low-temperature assemblages of hightemperature sulfide liquids that have undergone fractionation, physical redistribution and separation, recrystallization with cooling, possible reheating and remelting by recurring igneous injections, and potential remobilization by both magmatic and nonmagmatic hydrothermal fluids. The dominant sulfide min erals in magmatic Ni-Cu±PGE deposits mainly are variants of Fe-, Cu-, and Ni-bearing sulfides, which typically are pyrrho tite, pentlandite, chalcopyrite, and, less consistently, cubanite and troilite (table 7–1). The mineral assemblage of an individual ore body is dependent on many factors, including the composition of the silicate magma and contaminants, temperature of forma tion, chalcophile element concentrations, sulfur and oxygen fugacities, and the cooling history of the original immiscible high-temperature sulfide liquid. Within a district, different ore bodies may have different mineral assemblages depending on varying combinations of the factors listed above. For example, in the Ovoid deposit of the Voisey’s Bay district, a high metal/ sulfur ratio of the high-temperature sulfide liquid resulted in formation of abundant cubanite, rather than chalcopyrite, and troilite, rather than pyrrhotite. In contrast, at the Eastern Deeps deposit, cubanite is rare and troilite is absent, implying that the sulfide liquid had a much lower metal/sulfur ratio (Naldrett and others, 2000). Similar variations in primary sulfide min eralogy are observed among the different deposits at Noril’sk (Naldrett, 2004). The occurrence of platinum group minerals (PGMs) in magmatic Ni-Cu±PGE deposits is of critical economic interest as PGE, typically present only in trace amounts, may ultimately make a Ni-Cu±PGE deposit economically viable. Noril’sk stands out among other major magmatic Ni-Cu±PGE sulfide deposits because of exceptionally high concentrations of PGE, possibly due to upgrading by influxes of new magma interacting with pre-existing sulfides (Naldrett, 2004). How ever, there are differences in PGE contents among Noril’sk deposits. Sulfides from the Talnakh area are one order of mag nitude richer in PGE than sulfides from the Oktyabr’sky area (Barnes and others, 1997). At Noril’sk the majority of PGMs occur as coexisting minerals in disseminated ore, rather than in massive ore (Distler, 1994). The list of PGMs described for Noril’sk is given in table 7–2. Table 7–1. Major and minor minerals, with chemical formula, typically found in magmatic Ni-Cu±PGE sulfide deposits. Major minerals Formula Monoclinic pyrrhotite Fe9S10 Hexagonal pyrrhotite Fe11S12 Troilite FeS Chalcopyrite CuFeS2 Talnakhite Cu9(Fe,Ni) 8S16 Mooihoekite Cu9Fe9S16 Putoranite Cu9(Fe,Ni)9S16 Pentlandite (Fe,Ni)9S8 Cubanite Fe2CuS3 Magnetite Fe3O4 Minor/secondary minerals Formula Bornite Cu5FeS4 Bravoite (Fe,Ni)S2 Digenite Cu9S5 Cassiterite SnO2 Chalcocite Cu2S Covellite CuS Galena PbS Gersdorffite NiAsS Haycockite Cu4Fe5S8 Ilmenite FeTiO3 Linnaeite Co3S4 Mackinawite (Fe,Ni)S0.9 Marcasite FeS2 Maucherite Ni11As8 Melonite NiTe2 Millerite NiS Native bismuth Bi Native copper Cu Native gold Au Native platinum Pt Native silver Ag Niccolite NiAs Parkerite Ni2Bi2S2 Pyrite FeS2 Sphalerite ZnS Tsumonite AgTe Violarite Ni3S4
32 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Paragenesis Paragenesis is the sequence of mineral deposition in an ore body. Determining the paragenesis of magmatic Ni-Cu±PGE sulfide deposits is complicated by the recrystal lization of early formed sulfides and the potential for separa tion of solids and liquids during cooling and fractionation of the sulfide liquid. The concentrations of metals and sulfur in a silicate magma and the timing of the development of an immiscible sulfide liquid from its silicate host influence the partitioning of chalcophile elements into the immiscible sulfide liquid. At high temperatures, Ni, Cu, Co, Ag, Au, and PGE strongly partition into a sulfide liquid from sili cate magma. Copper and Ni have relatively high sulfide/ silicate liquid partition coefficients (D 102 to 103; Peach and others, 1990; Peach and Mathez, 1993), whereas PGE have extremely high partition coefficients (D 103 to 104; Peach and others, 1990; Fleet and others, 1999). The ultimate concentrations of Ni, Cu, and PGE in a sulfide liquid are con trolled not only by their relative partition coefficients, but also by the R-factor, the relative mass ratio between silicate liquid and sulfide liquid (Campbell and Naldrett, 1979). Immiscible sulfide liquids fractionate as they cool, and the fractionation path has a direct influence on sulfide mineralogy and metal contents. A schematic of an idealized fractionation path for a cooling sulfide liquid that has segre gated from its host silicate magma is presented in figure 7–1. Chalcophile elements concentrate into a high-temperature, homogeneous sulfide liquid (fig. 7–1A). With initial cool ing, an Fe-rich monosulfide solid solution (MSS) crystallizes (Naldrett and others, 1967; Kullerud and others, 1969) (fig. 7–1B). Iron, Os, Ir, Ru, and Rh partition into MSS, whereas Cu, Co, Au, Ag, Pd, and Pt remain in the co-existing sulfide liquid (Barnes and others, 1997; Mungall and others, 2005). Any Bi, Te, As, and Sb present in the silicate magma will accumulate in the Cu-rich liquid (Howell and McDonald, 2010). The partitioning of Ni between MSS and the Cu-rich sulfide liquid is complex and dependent on temperature, as well as sulfur and oxygen fugacities (Mungall and oth ers, 2005). Nickel is moderately incompatible with MSS at temperatures >1,150 °C, whereas at temperatures <1,100 °C, Ni becomes moderately compatible with MSS (Barnes and Lightfoot, 2005; Barnes and others, 2006). Experiments by Mungall and others (2005) found that though the partition coefficient between Ni and MSS was variable, Ni ultimately was incompatible with MSS at all experimental conditions, although less incompatible than Cu. Cobalt can behave simi larly to Ni, with a slight preference for the Cu-rich liquid at high temperatures and a slight preference for Fe-rich sulfides at low temperatures (Fleet and others, 1993, Barnes and oth ers, 1997). With continued cooling, the Cu-rich sulfide liquid crystallizes to intermediate solid solution (ISS) (fig. 7–1C). Palladium, Pt, and Au do not partition into ISS, but rather concentrate into a residual sulfide liquid. At temperatures below 600 °C, MSS undergoes subsolidus conversion to pyr rhotite and pentlandite (Naldrett and others, 1967), and ISS Table 7–2. Platinum-group element (PGE) minerals reported for Ni-Cu-sulfide deposits from Noril’sk, Russia (Distler, 1994). PGE minerals Formula Atokite Pd2Sn Braggite (Pt,Pd,Ni)S Cabriite Pd2SnCu Cooperite PtS Froodite PdBi2 Geversite PtSb2 Hessite Ag2Te Hollingworthite (Rh,Pt,Ru,Ir)AsS Insizwaite PtB2 Isoferroplatinum Pt3Fe Isomertieite Pd11Sb2As2 Kharayelakhnite (Cu,Pt,Fe,Pb,Ni)9S8 Kotulskite PdTe Laurite RuS2 Majakite PdNiAs Maslovite PtBiTe Menshikovite Pd3Ni2As3 Merenskyite Pd(Te,Bi)2 Mertieite II (Pd,Pt)8(Sb,As)3 Michenerite PdBiTe Moncheite PtT2 Niggliite PtSn Palarstanide Pd5(As,Sn)2 Palladoarsenide Pd2As Paolovite Pd2Sn Plumbopalladinite Pd3Pb2 Polarite Pd(Bi,Pb) Rustenburgite Pt3Sn Sobolevskite PdBi Sopcheite Ag4Pd3Te4 Sperrylite PtAs2 Stannopalladinite (Pd,Pt)5Sn2Cu Stibiopalladinite Pd5Sb2 Stillwaterite Pd8As3 Sudburyite PdSb Taimyrite (Pd,Pt)9Sn4Cu3 Telargpalite (Pd,Ag)3T Tetraferroplatinum PtFe Tulameenite Pt2FeCu Urvantsevite Pd(Bi,Pb)2 Vincentite Pd(As,Te) Vysotskite (Pd,Ni)S Zvyagintsevite Pd3Pb
Hypogene Ore Characteristics 33 converts to pyrrhotite and chalcopyrite, with lesser cubanite, mooihoekite, and talnakhite (Cabri, 1973; Craig and Scott, 1974; Naldrett, 1989) (fig. 7–1D). The behavior of PGE in the fractionation of a sulfide liquid can be dependent on the presence or absence of semi metals such as Bi, Te, Sn, and As. Because of strong bonding with these semimetals, Pd and Pt concentrated in residual liquids with Bi, Te, Sn, and As will form late-stage PGM bismuthide, telluride, antimonide, and arsenide minerals (Hemley and others, 2007). In the Noril’sk deposits, Genkin and Evstigneeva (1986) suggested that Pt, Pd, Sn, Te, Pb, As, Sb, and Bi were concentrated in a Cu-rich liquid that accumu lated in pockets along the upper contacts of the ore bodies or filled veins crosscutting the massive base-metal sulfides. In the absence of these semimetals, Pt and Pd will enter the lattice of high-temperature sulfides and subsequently exsolve as lamel lae in pentlandite (Barnes and others, 2006). In sulfide droplets from the Medveszky Creek mine at Noril’sk, which mainly preserves a closed sulfide fractionation sequence, Barnes and others (2006) concluded that Pd was mostly concentrated into pentlandite; Os, Ir, Re, and Rh were concentrated into both pyrrhotite and pentlandite that exsolved from MSS; and Au and Pt were retained in the final fractionated liquid to form discrete grains among ISS minerals. The sulfide liquid fractionation path in figure 7–1 represents the ideal path that may not reflect the nonideal physical world. Analyses of MSS cumulates often have much higher concentrations of incompatible elements than would be expected by sulfide liquid fractionation (Mungall and oth ers, 2005). Mungall and others (2005) suggested that devia tions from an ideal sulfide fractionation path may best be explained by adopting a model of equilibrium crystallization for sulfide melt. Mungall (2007) has constructed an empirical thermodynamic model for magmatic sulfide crystallization supporting the conclusion that Ni is incompatible for all sul fide magma compositions until very high Ni contents and very low temperatures are reached. Mungall (2007) recommends that all sulfides in a magmatic system be considered as cumu lates rather than products derived from residual liquids. Thus, it is likely that the pattern of element distribution between a sulfide liquid and silicate magma falls along a continuum between fractional crystallization and equilibrium crystalliza tion of the sulfide liquids, constrained by always unique physi cal parameters and events. A complication in parsing element partitioning behavior among sulfide species based on observations of sulfide miner als in silicate rocks is the fact that the mineral paragenesis and suggested element distributions (fig. 7–1) may be radically modified by later events. During final cooling, many elements may diffuse into different sulfide phases, which complicate the simple zoning patterns (fig. 7–1E). For example, at Voisey’s Bay, high-temperature hexagonal pyrrhotite recrystallized into monoclinic pyrrhotite, and troilite exsolved from sulfur-poor hexagonal pyrrhotite (Naldrett and others, 2000). In a dynamic magmatic environment, there is potential for modification and redistribution of fractionating sulfide liquids and crystallized sulfides. At the Eastern Deeps deposit in Voisey’s Bay, massive sulfide is rich in Os, Ir, Ru, and Rh, but depleted in Pt, Pd, Au, and Cu. This indicates that during fractionation, the bulk of the Cu-rich sulfide liquid was dis placed from crystallizing MSS (Naldrett and others, 2000). At the Noril’sk–1 deposit, massive Ni-rich ore and related Cu-rich ore were mined, but an upper Cu-rich ore, typically found at other deposits in the district, was missing, although PGE-rich low-sulfide mineralization was well-developed (Naldrett, 2004). Figure 7–1. Idealized fractionation path of an immiscible high-temperature sulfide liquid: A, chalcophile elements strongly partition in a high-temperature, immiscible sulfide liquid; B, as the sulfide liquid cools, Fe-rich monosulfide solid solution (MSS) crystallizes; C, with continued cooling to about 900 °C, the Cu-rich liquid crystallizes to intermediate solid solution (ISS); D, below 600 °C, ISS begins to exsolve to chalcopyrite (cp) and pyrrhotite (po), and MSS begins to exsolve to pyrrhotite and pentlandite (pn); E, below 250 °C, Cu-rich minerals include chalcopyrite, cubanite (cub), talnakhite (tal) and moohoekite (moo). The size of an element’s name in the illustration represents its relative abundance. Figure modified after Prichard and others (2004), Barnes and others (2006), and Howell and McDonald (2010). liquid Fe, Cu, Ni PGEs, Au +/- Bi, As, Te, Sb liquid Ni, Cu, Pt, Pd, Au, +/- Bi, As, Te, Sb Fe-rich mss Fe, Os, Ir, Ru, Rh liquid Pt, Pd, Au, +/- Bi, As, Te, Sb iss Cu, Ni mss Fe, Ni Pd, Os, Ir, Ru, Rh T 1200°C T 1100°C T 900°C liquid Pt, Pd, AU, Bi, AS, Te, Sb iss subsolidus conversion to cp + po mss subsolidus conversion to po + pn T 600°C PGMs, Au, bismuthides tellurides, arsenides, antimonides cp + po +/- cub, tal, moo po + pn +/- Pd-rich pn, IPGEs in ss T 250°C A B D E
34 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Early formed sulfides collected in some type of conduit or other trap may be modified by interaction with new, hot silicate magma (fig. 3–2B). New magma pulses may partly disaggregate and remelt previously formed sulfides or upgrade metal tenor through interaction of early formed sulfide with later batches of metal-bearing silicate magma (Kerr and Leitch, 2005). Li and others (2009) suggested that an influx of new, fertile magma interacting with sulfide was responsible for upgrading of sulfide tenor, particularly for PGE, in the Talnakh deposit, Noril’sk. In the Jinchuan deposit, dissemi nated sulfides that did not form a continuous network, particu larly sulfides trapped near the margins of the ore bodies, have lower Ni contents and form an envelope surrounding higher grade ore. This suggests that metal contents of early formed sulfides in the central parts of ore bodies were upgraded by interaction with influxes of fresh magma (Chai and Naldrett, 1992). Kerr and Leitch (2005) also suggested that dissolution of early formed sulfides could proceed to completion, with sufficient new magma input; in other words, all sulfide liquid could be dissolved and metals reincorporated into the silicate magma. They pointed out that this scenario has important implications for mineral assessment because metal-distribution patterns in such a situation would not be informative of miner alization potential. Because a late-stage Cu-rich liquid does not crystal lize until its temperature is <850 °C, but an enclosing silicate matrix becomes mostly solid by 1,000 °C, there can be a stark contrast in the behavior of a sulfide liquid and its brittle sili cate matrix under deformation stress. Barnes and others (1997) proposed that on-going magma movement in an intrusive system has the potential to create fractures and to open spaces in both host rocks and country rocks such that late-stage Curich sulfide liquids may be injected into interstitial and fracture spaces, effectively removing late liquids from proximity to crystallized MSS. The migration of residual Cu-rich liquids may be enhanced because, with increasing Cu and decreasing sulfur contents, those residual liquids develop strong wetting abilities against silicates (Naldrett, 1969). The capacity for redistribution of late-stage Cu-rich sulfides is an explanation for the commonly reported occurrence of fractures and brec cias filled with Cu-rich sulfides at many deposits, including “Upper Copper Ore” at the Kharaelakh intrusion at Noril’sk (Naldrett and others, 1994). At Pechenga, it was suggested that chalcopyrite-rich veins formed when a Cu-rich residual liquid was squeezed into surrounding sediments (Brügman and others, 2000). Also, at the Eagle deposit in Michigan, Cu-rich veins with variable mineralogy cut the surrounding metasedi mentary country rocks (Ding and others, 2010). Zoning Patterns The fractionation path in figure 7–1 implies a closed system and is in part modeled on the mineralogy and chem istry of confined sulfur droplets that typically have a pyrrho tite bottom and a chalcopyrite top, as described for Noril’sk (Barnes and others, 2006), Waterfall Gorge at Insizwa, South Africa (Lightfoot and others, 1984), and the Crystal Lake intrusion in Ontario, Canada (Cogulu, 1993). In general, the mineralogy and distribution of elements and minerals within crystallized sulfide globules mimics the mineralogy of larger deposits. For example, the Ovoid deposit at Voisey’s Bay has a large basal zone of massive sulfide that was interpreted to have crystallized from the base upward, with a fractionated, residual liquid enriched in Cu, Au, Pd, and Pt concentrated near the top (Naldrett and others, 2000). Chai and Naldrett (1992) described similar mineralogical zoning for the Jinchuan deposit. In the western ore bodies at Jinchuan, Ni, Cu, and PGE contents increase from the margins to the center of the bodies. Two types of mineral zoning, complex and simple, are recognized in massive ores in the Talnakh area of Noril’sk. Simple zoning mimics the fractionation sequence described above for a sulfide liquid, with gradual changes in the oremineral assemblage due to changes in the proportions of pyrrhotite and chalcopyrite (Torgashin, 1994). At Talnakh, ores change from pyrrhotite to pyrrhotite+chalcopyrite to chalcopyrite+pyrrhotite to chalcopyrite, with pentlandite present with all assemblages (Torgashin, 1994). Massive ores that occur in intrusive rocks display complex zoning, with a roughly spherical pattern of varying mineralogy that defines zones based on Cu content, as shown by the Oktyabr’sky massive sulfide body (fig. 7–2). Pentlandite occurs throughout the ore body. Ores with the lowest Cu content surround ores with increasing Cu contents, with maximum Cu content in the center of the massive sulfide body (fig. 7–2; Distler, 1994; Torgashin, 1994). Platinum-group element concentrations at Oktyabr’sky have a similar zoning pattern. Lower-position and marginal ores mainly are Fe-Os-Ir-Ru-Rh-rich pyrrhotite ores, whereas the central overlying ores are Cu-Pd-Pt-Aurich chalcopyrite and mooihoekite (Distler, 1994; Barnes and Lightfoot, 2005; Gorbachev, 2006). Platinum and Pd in the central area are found as PGMs, rather than in solid solution (Distler, 1994). This zoning pattern generally is attributed to fractionation of an immiscible sulfide liquid (Torgashin, 1994; Zientek and others, 1994; Gorbechev, 2006). The sulfide ores are composed of pyrrhotite and pentlandite, typically containing several weight percent Ni and weight percent Cu, and represent a primary magmatic cumulate of MSS with minimal trapped intercumulus sulfide liquid. The sulfide rocks, composed of pyrrhotite, pentlandite, and chalcopyrite, but typically contain ing weight percent Ni and as much as 35 weight percent Cu, represent fractionated sulfide liquids removed from the MSS cumulates. In deposits that have undergone post-depositional metamorphism and deformation, however, such as Pechenga (Distler and others, 1990) or Baimazhai in China (Zhang and others, 2006), the distribution pattern for sulfide miner als and metals may be significantly different from patterns expected for mainly unmetamorphosed magmatic deposits. Zhang and others (2006) suggested that the occurrence of
Hypogene Ore Characteristics 35 “alteration-modified” sulfides, enriched in Cu, Au, and Pd, with hydrous silicates, is the result of the pervasive hydrother mal alteration of the sulfide body at the Baimazhai deposit. Textures and Structures For the most part, sulfides in magmatic deposits world wide may be divided into disseminated sulfides, matrix- or net-textured sulfides, and massive sulfide, based on the rela tive percentages of sulfides and silicates and the rock texture. Sulfide mineralization also can occur with tectonic and mag matic breccias and in veins cutting earlier ore or country rock. Where sulfides are not connected to one another, the texture is considered to be disseminated. Disseminated sulfides occupy from 1 to about 33 percent of a rock and typically fill inter stitial space between silicate minerals. Disseminated sulfides commonly are 1-mm to 10-mm irregular sulfide patches inter stitial to silicate and oxide minerals. At some deposits, such as Medvezky Creek, Noril’sk, Russia, (Barnes and others, 2006), disseminated sulfides also occur as small (1 to 10 cm) sulfide droplets crystallized into rounded forms of globular sulfide that may have a chalcopyrite (Cu-rich) top and a pyrrhotite (Fe-rich) bottom, mimicking liquid sulfide fractionation. Net-textured or matrix-textured sulfides are intercon nected to form a matrix with silicate minerals and occupy about 33 to 66 percent of the space. A variant of net texture is “leopard texture,” a term used to describe sulfide with black Figure 7–2. Plan map of mineral-assemblage zonation in massive sulfide ores in the Oktyabr’sky mine area, Kharayelakh intrusion, Noril’sk region, Russia. Figure modified after Torgashin (1994). Mooihoekite Cubanite-Mooihoekite Mooihoekite-Cubanite Cubanite-Chalcopyrite Pyrrhotite-Chalcopyrite Chalcopyrite-Pyrrhotite-Cubanite Chalcopyrite-Pyrrhotite Pyrrhotite Outline of massive ores Fault EXPLANATION 750 FEET
300 Meters N
36 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits spots of augite and olivine oikocrysts in a yellow sulfide matrix common at the Eastern Deeps deposit at Voisey’s Bay, Labrador (Evans-Lamswood and others, 2000). When sulfide exceeds 66 percent of the rock volume, it is considered semi-massive to massive. Massive sulfides typically can be divided further into contact sulfides, which occur along or near the lower contact between mafic rocks and country rocks, or vein sulfides, which occur in offshoots from the main massive sulfide body (Barnes and others, 1997). At Voisey’s Bay, mineralization occurs as semi-massive to massive sulfide, leopard-textured sulfides, magmatic brec cias with medium- to coarse-grained blotchy sulfides, and locally disseminated sulfides (Evans-Lamswood and others, 2000; Naldrett and others, 2000). In a conduit-style deposit such as Voisey’s Bay, magma dynamics and conduit geometry, rather than simple gravitational settling, controlled ore textures (Evans-Lamswood and others, 2000). Sulfides entrained in magma moving through a conduit are concentrated preferen tially in traps where physical irregularities in dike morphol ogy, such as swelling and pinching or splaying, favor precipi tation, capture, and preservation of sulfides due to resultant changes in magma velocity and viscosity (Evans-Lamswood and others, 2000). The Reid Brook mineralized zone is a near-vertical, thickened part of the Voisey’s Bay feeder conduit with mineralized leopard-texture sulfides enclosed by mineralized breccias and transected by steep massive sulfide veins. The Ovoid deposit is a flat-lying concave-shaped lens of massive sulfide enclosed in mineralized leopard-textured troctolite and breccias in a widened part of the feeder conduit. The Eastern Deeps, located where the feeder conduit widened into a larger magma chamber, exhibits a massive sulfide lens enclosed in a complex mineralized sheath of leopard-textured troctolite and breccia (Evans-Lamswood and others, 2000). These ore types represent sulfide-enriched traps in the feeder conduit: where the through-going flow of magma physically changed, and suspended droplets of immiscible sulfide liquid settled gravitationally out of the moving magma, thus produc ing sulfide accumulations that crystallized to form massive Ni-Cu±PGE sulfide. At the Baimazhai magmatic Ni-Cu±PGE deposit, China, massive sulfide forms an inner core surrounded by orthopy roxenite, websterite, and gabbro (Wang and Zhou, 2006). Massive sulfide ores are composed mainly of pyrrhotite, with lesser pentlandite and chalcopyrite. The massive ores generally have sharp contacts with net-textured ores that fill interstitial voids in orthopyroxene cumulates (Wang and Zhou, 2006). Coarse-grained pyrrhotite commonly is associated with pentlandite that occurs as rims surrounding pyrrhotite, as oriented lamellae, or as blades along fractures and grain boundaries (Wang and Zhou, 2006). Chalcopyrite forms vari ably sized anhedral grains and typically occurs as aggregates in interstices between pentlandite, pyrrhotite, and magnetite grains. Wang and Zhou (2006) determined the order of crys tallization for ore minerals at Baimazhai as magnetite, then pyrrhotite+pentlandite, and finally, chalcopyrite. Massive sulfide at Uitkomst, South Africa, occurs as a series of lenticular bodies, which originally may have been a single body prior to tectonism, located in the immediate footwall of a tabular intrusion (Maier and others, 2004). In contrast, in massive ore at Jinchuan, China, pyrrhotite and pentlandite generally form a banded texture with dissemi nated chalcopyrite and magnetite, which Chai and Naldrett (1992) took as evidence that the massive sulfide was affected by stress and that sulfide melt flowed during crystallization. At Pechenga, Russia, ore types are classified as magmatictextured and include disseminated sulfides in mafic-ultramafic intrusions and brecciated and veined ore in fault zones (Distler and others, 1990). At a number of deposits, such as Jinchuan, Voisey’s Bay, and Noril’sk, massive ore or ore breccias locally cut host rocks or are found within underlying country rock. Those occur rences indicate that the ores may have been injected at or near the base of intrusions as separate pulses of massive sulfide liquid or sulfide-rich breccias (Arndt and others, 2005). The movement of dense sulfide liquids invoked for this mode of occurrence suggests mobilization during structural readjust ments or deformation along pre-existing structures. Grain Sizes Grain size varies across deposits. In massive sulfide from Voisey’s Bay, pyrrhotite occurs as very coarse crystals, exceeding 10 cm in diameter in some samples. Pentlandite is characterized by coarse grains 1 to 2 cm in diameter, except at the Eastern Deeps where coarse pentlandite is rare, and pentlandite more typically forms rims surrounding pyrrhotite crystals or occurs as lamellae within pyrrhotite (Naldrett and others, 2000). Troilite, where present, occurs as fine exsolution lamellae in pyrrhotite, and cubanite occurs as discrete grains and exsolution lamellae in chalcopyrite (Naldrett and others, 2000). At Noril’sk, pyrrhotite grain size ranges from very fine 1 mm) to very coarse (>10 cm in so-called “pegmatoid ore”; Stekhin, 1994). That coarse pyrrhotite always contains flame pentlandite lamellae. Where present, PGEs typically form small euhedral crystals (0.005 to 0.2 mm) at the boundaries of Ni-Cu sulfides (Distler and others, 1990; Chai and Naldrett, 1992). At the Eagle deposit, Michigan, pentlandite occurs both as early formed euhedral crystals and as late-stage exsolution lamel lae and flame structures in pyrrhotite (Kooistra and Chesner, 2010). Chalcopyrite is massive or occurs as exsolution lamellae in generally massive pyrrhotite (Kooistra and Chesner, 2010). Pyrrhotite at Jinchuan forms anhedral to subhedral 0.1- to 3-mm crystals; pentlandite commonly forms subhedral to euhedral 0.1- to 2-mm crystals enclosed by anhedral pyrrhotite (Chai and Naldrett, 1992). Exsolution flames of pentlandite in pyrrhotite are rare at Jinchuan, in contrast to textures at Noril’sk, where flame pentlandite is very common (Chai and Naldrett, 1992).
Paragenesis 37 Supergene Ore Characteristics Supergene processes can affect all types of mineral deposits, and supergene enrichment can provide an important upgrade in the metal tenor, especially for some iron, nickel, gold, and copper deposits. Such processes are not generally significant, however, either for development or further enrich ment of mafic- and ultramafic-hosted Ni-Cu±PGE sulfide deposits. The gossans that can develop from weathering of sulfides can be important guides to mineralization. Gossans represent advanced chemical weathering in which sulfide minerals are extensively oxidized and leached of most met als, leaving behind hydrated iron oxides and, rarely, sulfates. Gossans can be very large and may be targeted remotely using various satellite and airborne multispectral and hyperspectral data because iron oxides and hydrous iron oxides possess distinctive spectral signatures in the VISNIR range (Hunt and Ashley, 1979). The gossan zone at Voisey’s Bay is very large and easily can be seen from the air. It had been mapped as a “pyritic gossan” by government geologists before two prospectors looking for diamond indicators found chalcopyrite stringers in freshly broken gossan. A subsequent geochemical prospecting program in the area eventually resulted in recogni tion of disseminated Ni-Cu-Co mineralization on Discovery Hill, leading to the initial diamond-drill discovery holes. Following the Voisey’s Bay discovery, exploration activity in northern Labrador rapidly expanded, and gossans were a significant target that focused geophysical exploration (Kerr and Smith, 1997). 8. Hypogene Gangue Characteristics Mineralogy and Mineral Assemblages Gangue mineralogy for magmatic Ni-Cu±PGE deposits can be divided into three categories: (1) magmatic silicates and oxides of the mineralized host mafic-ultramafic intrusion, (2) minerals in partially ingested xenoliths and hornfelsed country rock, and (3) metamorphic minerals from magmatic hydrothermal and post-deposition metamorphic events. Mag matic gangue minerals for most deposits are the minerals of the host mafic and ultramafic silicate rocks. Typical minerals primarily are olivine, orthopyroxene, clinopyroxene, plagio clase, magnetite, ilmenite, and Cr-spinel. At Pechenga, miner als in the ultramafic intrusions also include magmatic kaersu tite [NaCa2(Mg4Ti)Si6Al2O23(OH)2] and Ti-bearing phlogopite [KMg3(Si3Al)O10(F,OH)2] (Hanski and Smolkin, 1995). Minerals formed by reactions with country rock, either by contact metamorphism or by ingestion of xenoliths, are much more complex because of the wide range of lithologies enclos ing mineralized host rocks. Xenoliths may be incorporated into magma by thermal erosion of conduit walls. At Voisey’s Bay, contaminating country rock generally is orthogneiss or paragneiss. Near-total fusion of gneissic xenoliths in magma produced immiscible aluminous liquids, garnet oxidized to form hercynite and magnetite, hypersthenes and potassium feldspar reacted together to also produce hercynite, and pla gioclase broke down to produce corundum (Li and Naldrett, 2000). At Noril’sk, metamorphism of sedimentary country rock, which there includes dolomite, argillite, evaporates, and terrigenous coal measures, involved prograde develop ment of hornfels and marble and retrograde development of calc-silicates/skarn (Likhachev, 1994). Anhydrite occurs in evaporitic xenoliths, and in some intrusions may be a trace magmatic mineral (Li and others, 2009). Evidence of reaction with country rock, particularly country rock with potentially readily available sulfur, such as in pyrite, is a favorable indicator when evaluating an intru sion as a potential deposit target (Naldrett, 2004). The extent of wallrock alteration may be an indication of the duration of the thermal pulse related to ore deposition. At Noril’sk, the hornfels facies of the hanging wall is about 250 m wide and is about 100 m wide in the footwall (Likhachev, 1984). Gangue minerals at deposits that have undergone mag matic and post-depositional alteration typically reflect the original mineralogy. At Uitkomst, cumulate rocks were affected by widespread serpentinization of olivine, uralitiza tion of pyroxene, and saussuritization of plagioclase (de Waal and others, 2001; Sarkar and others, 2008). In addition, there was intense talc-carbonate alteration in the lower ultramafic units enclosed by dolomitic country rock, which may have strongly influenced the alteration mineralogy (de Waal and others, 2001). At Pechenga, host peridotites variably are altered to serpentinites and talc-chlorite-carbonate-serpentine, pyroxenites are altered to chlorite-actinolite, and gabbros are strongly saussuritized (Naldrett, 2004). At Noril’sk, meta morphism of the intrusive rocks is characterized by deuteric alteration of original host mineralogy, with development of biotite, amphibole, serpentine, talc, chlorite, iddingsite, bolingite, sericite, prehnite, pumpellyite, potassium feldspar, and quartz (Likhachev, 1994). The Baimazhai Ni-Cu±PGE deposit in China was subjected to pervasive hydrothermal alteration some time after formation (Zhang and others, 2006). At Baimazhai, the host mafic and ultramafic rocks were altered to mineral assemblages containing varying proportions of tremolite, actinolite, epidote, biotite, chlorite, quartz, talc, serpentine, clinochlore, prehnite, and carbonate (Zang and others, 2006). Paragenesis The paragenetic sequence for gangue magmatic silicates and oxides of the mineralized host mafic to ultramafic intru sions provides important information about the potential for formation of an economic sulfide deposit. Mantle-derived mafic magmas crystallize minerals, such as olivine, pyrox ene, plagioclase, and oxides, according to their solidus
38 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits temperatures, which in turn depend on the temperature, pres sure, and oxygen fugacity of the magma. The following parage netic sequence was modeled by Li and others (2001a) from the composition of the chilled margin of the Bushveld Complex and is applicable to many intrusions that host Ni-Cu±PGE deposits. At 100 bars total pressure and a QFM-1 (quartz-fay alite-magnetite) buffer, olivine crystallizes between 1,358 and 1,312 °C, followed by orthopyroxene between 1,312 and 1,301 °C, then by orthopyroxene plus spinel between 1,301 and 1,166 °C; plagioclase begins to crystallize at 1,166 °C; and clinopy roxene begins to crystallize at 1,158 °C (Li and others, 2001a). Variations in this paragenetic sequence can have an impact on sulfur solubility and thus on the timing of sulfur saturation and development of an immiscible sulfide liquid (Li and others, 2001b). The timing of silicate crystallization can dictate sulfide texture. Early crystallizing silicates, for example, may include sulfide blebs as inclusions. In disseminated and matrix-textured sulfide ores, sulfide-silicate contacts commonly suggest that an early immiscible liquid sulfide was interstitial to silicate cumulus phases. The relative timing of olivine crystalliza tion and sulfide segregation can have a profound influence on Ni content, with important implications for sulfide tenor. In a mantle-derived melt, Ni partitions to both olivine and sul fide liquid. If a sulfide liquid forms prior to crystallization of olivine, then Ni is depleted in the silicate magma, but there is no change in the FeO/MgO ratio of the magma. Olivine that crystallizes after sulfide separation will be depleted in Ni, with no change in its major element chemistry. In contrast, early crystallization of olivine will deplete magma in Ni and increase its FeO/MgO ratio. Thus, olivine chemistry can provide clues to a mafic body’s silicate/sulfide crystallization sequence and provide an assessment/exploration tool. Li and others (2001b) also pointed out that pervasive Ni depletion by early crystal lization of olivine prior to sulfide immiscibility may result in development of Ni-depleted sulfide of questionable economic value. An example of this effect may be the Katahdin pyrrhotite body, hosted within a gabbro stock in northern Maine (Miller, 1945). The huge, but very low-grade, sulfide mineralization occurs primarily as interstitial sulfide between silicate minerals and is composed almost entirely of pyrrhotite. The sulfides at Katahdin have very low Ni and Cu contents relative to typical magmatic sulfides related to mafic intrusions (Thompson and Barnes, 1984). 9. Geochemical Characteristics of Ores Trace Elements and Element Associations Nickel and copper found in sulfide ores in magmatic sulfide-rich Ni-Cu±PGE deposits related to mafic and/or ultra mafic dike-sill complexes commonly are associated with Co (typically present at the 0.1- to 0.3-percent level) and Pt, Pd, and Au (which may be present to as much as the ppm level). Other elements present in trace concentrations may include Ag, As, Bi, S, Sb, Se, Zn, Os, Ir, Ru, Rh, and Te (Barnes and Lightfoot, 2005). Trace element associations can be used for exploration, however, concealed Ni-Cu deposits buried more than a few meters are difficult to identify from geochemical analyses of soil, lake-sediment, till, or water samples alone. Multi-element soil or sediment sampling has been successful in identify ing very shallow (<5 m) Ni-Cu mineralization. For example, the Nebo and Babel deposits in arid western Australia ini tially were discovered by broad surface sampling of soil and lag deposits (Baker and Waugh, 2005) in an area previously targeted through satellite and geophysical imagery. Anomalous concentrations of both Ni (about 550 ppm) and Cu (as much as 950 ppm), coupled with moderately elevated Pt (as much as 27 ppb), led to discovery of subcrop mineralization (Baker and Waugh, 2005). In the arid region around the Karatungk deposit in China, rock and soil analyses found Cu, Ni, and Co anoma lies around exposed intrusions with Ni-Cu mineralization but not over concealed bodies. Anomalous concentrations of As, B, Ba, and Mo were present, however, over what proved to be mineralized Ni-Cu bodies concealed at depths >100 m (Yinggui and others, 1995). Soil sampling at the exposed Discovery Hill deposit at Voisey’s Bay showed high concentra tions of Pd (as much as 824 ppb) in a halo that extends several hundred meters away from the deposit (McConnell, 2003). Although soil geochemistry over buried mineralization at Voisey’s Bay does not show geochemical anomalies, lake-sed iment geochemical surveys in areas near known or suspected Ni-Cu mineralization do show elevated Pt (as much as 56 ppb) and Pd (as much as 42 ppb) (McConnell, 2003). In Minnesota, surface waters interacting with shallow Ni-Cu mineralization cause dissolution of sulfides, resulting in mobilization of Ni, Cu, and SO4, which can be important pathfinders for Ni-Cu mineralization in a cool, humid climate (Miller and others, 1992). In a similar climate, the Ni-Cu mineralization associated with the Thompson Nickel Belt (komatiite-hosted magmatic Ni-Cu deposits with similar sulfide mineralogy) has been shown to be identifiable in the <0.063-mm sediment fraction separated from glacial till. Those analyses show elevated Ni (as much as 3,760 ppm), Cu (to 215 ppm), Cr (to 209 ppm), PGE (Pt, as much as 13 ppb; Pd, to 98 ppb), Au (as much as 27 ppb), as well as elevated Co, As, Cd, Sb, Bi, Se, Te and S in the ppb range (McClenaghan and others, 2011). The presence of these elements, in conjunction with a Pd/Pt ratio >3, suggests the presence of magmatic Ni-Cu mineralization (McClenaghan and others, 2011). Metal Normalizations and Ratios The metal contents of sulfides can be used to evaluate the origin and evolution of the magmatic system and whether there is potential for economic accumulations of
Metal Normalizations and Ratios 39 sulfides. Barnes and others (1988) and Barnes and Lightfoot (2005) showed two normalization methods for interpreting Ni-Cu±PGE data: (1) normalizing whole-rock and sulfide data to primitive mantle and (2) normalizing sulfide data to 100 percent sulfide. By normalizing metal contents of silicate rocks to primi tive mantle, it is possible to distinguish among the patterns of several mantle-derived magma types (for example, komatiites, ocean floor basalts, high-MgO basalts, and ophiolitic chro mitites). Likewise, by normalizing the metal contents of sul fides to primitive mantle, it is possible to use the shape of the normalized pattern to identify those sulfides that have sepa rated from magmas that previously experienced some sulfide segregation. When a sulfide liquid separates from a silicate liquid, the resulting fractionated silicate liquid is depleted in PGE compared to Ni and Cu because of the very strong affin ity for PGE in sulfide versus silicate melts. Any subsequent crystallization of silicates or sulfides from the fractionated melt would continue to show the relative depletion of PGE (a trough-shaped plotted pattern). For example, sulfides from the Jinchuan and Noril’sk deposits show relatively smooth normalized patterns (fig. 9–1), suggesting segregation from a source that had not seen significant prior sulfide segregation. However, sulfides from Voisey’s Bay show significant deple tion in PGE compared to Ni and Cu (Barnes and Lightfoot, 2005), suggesting that at least some of the magma had previ ously undergone sulfide segregation that scavenged the PGE. Normalization of metal values to 100 percent sulfide is used in order to compare rocks with different sulfide contents (Barnes and Lightfoot, 2005). This assumes that all chalco phile elements are hosted only by sulfides and that there is no loss or gain of sulfur due to metamorphism or alteration. This normalization can be used to highlight variations among disseminated, matrix, and massive sulfide ores. For instance, disseminated sulfides are commonly richer in Pt, Pd, and Au than average matrix and massive sulfides (fig. 9–2), whereas vein sulfides commonly are enriched in Cu, Pd, and Pt (Barnes and Lightfoot, 2005). Because Ni in mafic/ultramafic rocks containing low modal percentages of sulfide may largely be present in olivine, and rocks with low sulfide content are more vulnerable to sulfur redistribution during weathering and metamorphism, the composition of sulfides should not be recalculated for rocks containing modal percent sulfides. For rocks where the bulk of the sulfides are present as pyr rhotite, pentlandite, and chalcopyrite, the concentration of an element in the sulfide may be calculated element in the sulfide may be calculated as:
C(100% sulf) Cwr*100/(2.527*S (1)
+ 0.3408*Cu + 0.4715*Ni), where C(100% sulf) represents concentration of an element in 100 percent sulfides; Cwr indicates concentration of the ele ment in the whole-rock analysis; and S, Cu, and Ni indicate concentrations of the respective elements in the whole-rock data, in weight percent (Barnes and Lightfoot, 2005). Metal ratios provide another means to compare trace elements in sulfides without necessarily having to recalculate the data to 100 percent sulfide or to normalize the data as in figure 9–1 (Barnes and others, 1988). For example, Se/S ratios in sulfide-bearing rocks have been used to evaluate whether the rocks have sustained a loss of sulfur due to alteration or metamorphism (Maier and others, 1998) and as an approxi mate guide to the presence of sedimentary contaminants to the magmas (for example, Ripley, 1990). The Se/S × 106 ratio of the mantle is about 230 to 350 (Eckstrand and Hulbert, 1987). For sulfide-bearing rocks with high Se/S × 106 ratios (>1,000), metamorphism may have led to sulfur loss (Maier and Barnes, 1999). Low Se/S × 106 ratios (<250) suggest that sedimentary rocks may have contributed sulfur to the magma and to the resulting sulfide liquid (Ripley, 1990). However, use of Se/S ratios is complicated by other processes that may cause changes in either Se or S concentrations (for example, the R factor, alteration or metamorphism, and variations in initial concentrations of S and Se in sulfides and crustal rocks), and thus, this ratio should be used with caution (Queffurus and Barnes, 2010). For assessment and exploration purposes, metal ratios such as Cu/Pd in both whole-rock silicate and sulfide analyses also can be used as a general guide to evaluate whether sulfides have already segregated from the melt (Barnes and others, 1993; Maier and others, 1998). When a sulfide liquid sepa rates from a silicate melt, the Cu/Pd ratio of the sulfide liquid is much lower than the Cu/Pd ratio of the silicate melt due to the very high partition coefficient for Pd in sulfide liquid (fig. 9–2A) (Maier and others, 1998). The fractionated silicate melt will have high Cu/Pd as will any rocks that crystallize from Figure 9–1. Platinum-group and related elements for magmatic sulfides from tholeiitic sill/dike Ni-Cu deposits, normalized to primitive mantle, using normalization factors reported in Barnes and Lightfoot (2005). Disseminated ores (open symbols) tend to be slightly more platinum-group-element-rich than massive ores (solid symbols). Data from Naldrett (2004). [Symbols: Voisey’s Bay, Ontario, Canada, circles; Jinchuan, China, triangles; Insizwa, South Africa, star; and Noril’sk, Russia, squares] Ni Os Ir Ru Rh Pt Pd Au Cu Rock/Primitive mantle
40 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits that previously fractionated melt. A primitive mantle magma without prior sulfide segregation should have a Cu/Pd value of about 6,500 (Barnes and others, 1988); thus, rocks with Cu/ Pd <6,500 should contain Pd-rich sulfides, whereas silicate rocks with whole-rock values of Cu/Pd >6,500 are likely to have previously segregated sulfides (Maier and others, 1998). As an example, massive sulfides associated with the Uitkomst complex have low Cu/Pd ratios (about 2,700) when compared to disseminated sulfides in the overlying gabbro (Cu/Pd about 8,450; Maier and others, 1998). The low ratios in the massive sulfides suggest crystallization from a magma that had not lost PGEs through prior sulfide segregation, whereas the dissemi nated sulfides likely crystallized from another magma from which sulfides had previously fractionated. Practically speaking, this suggests that when rock units have very high Cu/Pd ratios, they have experienced sulfide segregation and sulfide deposits, if present, would occur in the rock units stratigraphically below the PGE-depleted units (Barnes and Lightfoot, 2005). The Cu/Pd ratio of the sulfide phases also can be used as a graphical guide (fig. 9–2B) to estimate the R value (the weight ratio of silicate liquid to sulfide liquid; Barnes and others, 1993; Barnes and Lightfoot, 2005), which can be an important factor in determining the potential for economic accumulations of sul fides. Three factors control the metal content in a sulfide liquid (assuming a closed system): the concentration of the metal in the silicate liquid, the partition coefficient between the silicate and associated sulfide liquids, and the volume of silicate magma from which the sulfide scavenges the metal—the R value (Campbell and Naldrett, 1979; Barnes and Lightfoot, 2005). In basaltic systems (such as those that give rise to conduit-hosted Ni-Cu deposits), maximum enrichment of Ni occurs at R values of 1,000–3,000, whereas maximum enrichment of PGEs occurs at R >10,000 (Barnes and Lightfoot, 2005). For example, Barnes and Lightfoot (2005) evaluated the Cu/Pd ratio for the Noril’sk and Talnakh intrusions to estimate the R values required to produce the observed sulfide ores (fig. 9–2B). For the Noril’sk intrusions, the Cu/Pd ratios of disseminated ores suggest R values of about 10,000, whereas for the Talnakh intrusions, R values appear lower (about 1,000). Such high R values imply a very high volume of magma flow, enhancing PGE enrichment, which is indeed characteristic of the Noril’sk sulfide ores. Other metal ratios such as Pd/Ir, Ni/Pd, Cu/Ir, Ni/Co, and PGE/Ni+Cu can be used along with Ni/Cu to examine the effects of sulfide segregation (Barnes and others, 1988; Barnes and Lightfoot, 2005). For instance, the Pd/Ir versus Ni/Cu diagram can be used to distinguish among magma suites, and within suites, to distinguish between massive and disseminated sulfides (fig. 9–3A). Barnes and Lightfoot (2005) used this dia gram to show that for a given deposit, the disseminated sulfides (which tend to be Cu-rich) generally have higher Pd/Ir and lower Ni/Cu ratios than associated massive sulfides (which tend to be Fe-rich). The Pd/Ir ratio has also been used to distinguish magmatic from hydrothermal sulfides (Keays and others, 1982). Although there are exceptions, in general a Pd/Ir ratio >100 sug gests a hydrothermal origin, whereas a Pd/Ir ratio <100 suggests a magmatic origin (Maier and others, 1998). In a Ni/Pd versus Cu/Ir plot (fig. 9–3B), Barnes and Lightfoot (2005) demonstrated, using examples from Voisey’s Bay, that once a sulfide liquid segregates from a silicate Figure 9–2. A, Cu/Pd versus Pd for sulfides from tholeiitic Ni-Cu deposits (solid symbols) compared with sulfides from reef deposits (open squares), komatiite (asterisk) and impact-related deposits (crosses), from Barnes and others (1993). This emphasizes that the reef deposits typically are enriched in PGE, whereas the tholeiitic sill/dike deposits are enriched in Cu (and Ni). Data from Naldrett (2004). B, Cu/Pd versus Pd for the Noril’sk, Russia, magmatic system. This plot from Barnes and Lightfoot (2005) shows tie lines between silicate liquid compositions represented by volcanic flows and sulfides in equilibrium with those liquids at several different R factors. The dots represent the composition of a rock that contains mixtures of 1, 10, or 100 percent sulfides. Sulfides in the Noril’sk intrusions appear to result from R values an order of magnitude larger than the apparent R values that produced sulfides in the Talnakh intrusions. Cu/Pd Palladium, in parts per billion Depeleted Enriched Pechenga Jinchuan Voisey’s Bay Noril’sk Kambalda Sudbury Platreef Merensky Reef J-M reef Mantle A Cu/Pd Palladium, in parts per billion Depleted Enriched Noril’sk Mantle R=100 R=1000 R=10000 1% 10% 100% 1% 10% 100% 1% 10% 100% Addition of sulfide liquid 2% crystal fractionation with removal of sulfide liquid in cotectic portions Talknakh sulfides Noril’sk sulfides Silicate rocks within intrusions B
Stable Isotopes 41 liquid, the remaining silicate liquid (and any sulfides that subsequently segregate from this remaining silicate liquid) will be significantly depleted in PGE compared to Ni and Cu and will have very high Ni/Ir and Cu/Ir ratios. Thus, where sulfides are substantially displaced towards higher Ni/Pd and Cu/Ir and away from primary melt fields, as at Voisey’s Bay, it is possible to infer that the sulfides were segregated from a magma from which an earlier episode of sulfide segregation had already occurred, leaving behind a depleted silicate liquid. This suggests that there may be PGE-rich sulfides present at depth. Where the sulfides plot within the fields of primary melts, it is possible to infer that the sulfides were segregated from a fertile mantle source, as suggested for sulfides at Noril’sk (Barnes and Lightfoot, 2005). Stable Isotopes Sulfur Isotopes For many economic magmatic Ni-Cu±PGE deposits, the large quantities of sulfur required to reach sulfur satura tion and development of an immiscible sulfide liquid are assimilated from external sources (Barnes and others, 1997). As a result, sulfur isotopes of the sulfide ore minerals com monly reflect the highly variable sulfur sources of the country rocks that the ultramafic to mafic magmas intrude. The wide variation in sulfur isotope values reported for a number of magmatic Ni-Cu±PGE deposits is shown in figure 9–4. Deviations of δ34S values from mantle-like values of 0±2 per mil provide the strongest evidence by which to evaluate the importance of externally derived S in mafic magmatic systems (Ripley and Li, 2003). At Voisey’s Bay, δ34S val ues for massive and brecciated sulfides range from –4.0 to +1.8 per mil (Ripley and others, 1999), but there is a distinct zonation of values. In the western area (Reid Brook deposit), δ34S ranges from –4.1 to –2.3 per mil whereas in the eastern part (Eastern Deeps, Ovoid, Mini-Ovoid, and Discovery Hill deposits), δ34S ranges from –2.4 to 0 per mil, with slightly more positive values (–0.5 to +1.8 per mil) found in unminer alized troctolite at Eastern Deeps (Ripley and others, 1999). These data indicate that much of the assimilated sulfur in the Reid Brook deposit was derived from the Tasiuyak Gneiss country rock, with δ34S values ranging from –17 to 18.3 per mil (Ripley and others, 2002). The eastern deposits were emplaced in low-sulfur orthogneiss that has δ34S values rang ing from 4.6 to +3.3 per mil (Ripley and others, 1999). In the Pechenga area, two populations of sulfur isotopes among the deposits suggest two different sources of sulfur (Abzalov and Both, 1997). Deposits in the western part of the Pechenga Complex (Kaula, Kotselvaara, and Kamikivi) have δ34S values close to mantle values, ranging from –5 to +2 per mil; deposits in the larger and generally more productive intru sions in the eastern part of the complex (Kierdzhipori and Pilgujarvi) have δ34S values that range from +2 to +6 per mil. Table 9–1. Ratios of metals calculated as concentration of metal in 100 percent sulfides for magmatic Ni-Cu±PGE deposits (from Naldrett, 2004). [Symbol: dash, data not available] Ni/Cu Ni/Co Pd/Pt Pd/Ir (Pt+Pd) (Ni+Cu) Tholeiitic sill/dike deposits Eagle (Michigan)a — Jinchuan (China) 0.04 to 0.45 Kabanga (Tanzania)b 5 to 10 10 to 15 0.3 to 2.0 2 to 20 0.06 to 0.4 (+1.6) Nebo-Babel (Australia)c — Noril’sk-Talnakh (Russia) 1.1 to 4.8 Pechenga (Russia) 0.05 to 0.26 Voisey’s Bay (Canada) Wellgreen (Alaska)d — Insizwa (South Africa) — Komatiitic Ni-Cu deposits Kambalda (Australia) 0.19 to 0.26 Contact and reef-type deposits Duluth (Minnesota) 2.9 to 4.8 J-M reef (Montana) — Merensky reef (South Africa) — 17.4 (+112.6) Platreef (South Africa) — 88 to 166 2.1 to 4.4 Meteoritic impact-related deposits Sudbury (Canada) 0.06 to 1.3 aUnpublished data (USGS) bData from Macheyeki (2011) and Maier and Barnes (2010) cData from Godel and others (2011) dData from Schmidt and Rogers (2007)
42 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Sedimentary rocks in the region have early diagenetic pyrite, with δ34S values ranging from –4.5 to +2 per mil, and later sulfides with values from +8 to +20.9 per mil. Barnes and others (2001) suggested the western deposits assimilated sulfur from unconsolidated sediments, whereas sulfur in the eastern deposits reflects the heavier sulfur of the later sulfides. At the Uitkomst Complex, sulfide-bearing units have δ34S ranging from –7.1 to +2.6 per mil, whereas values for sulfide-poor units range from –0.9 to +2.6 per mil, suggesting assimilation of sulfur from a source with negative sulfur isotope values (Li and others, 2002; Sarkar and others, 2008). There is good agree ment among the sulfur isotope values for sulfides, intrusions, and country rock at the Kabanga deposit, indicating relatively proximal derivation of most sulfides (Maier and others, 2010). Several deposits have δ34S values similar to the 0±2 per mil range expected for mantle-derived δ34S (fig. 9–4). Primary sulfides at Jinchuan have δ34S values close to mantle values (mostly about –2 to +2 per mil), but secondary pyrite in veins and stringers, and as rare replacement of pyrrhotite, has very negative values (ranging from –27 to –7 per mil), attributed to secondary oxidation of primary sulfides during a hydrother mal/metamorphic event (Ripley and others, 2005). Sulfides from the Waterfall Gorge deposit at Insizwa have δ34S values close to 0 per mil, but Lightfoot and others (1984) suggest that this does not rule out a crustal source of sulfur. The NeboBabel deposit in Western Australia has a very small range of δ34S values (0 to +0.8 per mil), which Seat and others (2009) invoke as evidence that crustal sulfur addition was not a factor in the genesis of that deposit, a potentially important observation for the generally accepted genesis for this type of magmatic Ni-Cu±PGE deposit. Oxygen Isotopes Partly ingested xenoliths at Voisey’s Bay provide evi dence that silicate rocks were incorporated into the magma during emplacement, and oxygen isotopes provide some information on the processes involved. Oxygen isotope values of minerals in xenoliths record a geochemical history that includes rapid thermal equilibration with magma and produc tion of a refractory mineral assemblage (Ripley and others, 2000; Mariga and others, 2006). High 18O partial melts were generated from quartz- and feldspar-bearing protoliths, and a part of that melt was dispersed in magma that passed through the conduit system (Mariga and others, 2006). Leucogabbronite units at the Nebo-Babel deposit locally also contain country rock orthogneisses that were partly to fully resorbed (Seat and others, 2007). Seat and others (2009) state that oxy gen isotope results indicate contamination of the Nebo-Babel intrusion was from orthogneissic country rock rather than from deeper pelitic rocks. Oxygen isotope data for ore-bearing intrusions from the Kabanga area also suggest incorporation of country rock (Maier and others, 2010). Typical δ18O values for rocks derived from the mantle are about +5.5 to +6.0 per mil (Ripley, 1999). Olivine, plagioclase, and pyroxene at Kabanga have δ180 values that range from +5.2 to +7.8 per mil, gen erally heavier than what would be expected for magmatic Pd/Ir Ni/Cu A Mantle High-MgO basalts Flood basalts Cu-rich sulfide veins Layered intrusions PGE reefs Ophiolites Figure 9–3. A, Pd/Ir versus Ni/Cu plot showing that for a given deposit, disseminated sulfides (open symbols) generally have higher Pd/ Ir and lower Ni/Cu ratios than associated massive sulfides (solid symbols) (after Barnes and others, 1993; Barnes and Lightfoot, 2005). B, Ni/Pd versus Cu/Ir plot showing sulfides that segregated from magmas that have been contaminated by substantial amounts of crust (for example, Jinchuan, China, and Voisey’s Bay, Canada), and have likely experienced an early episode of sulfide segregation, have higher Ni/Pd and Cu/Ir ratios than uncontaminated (PGE-enriched) magmas that still retain their mantle signature (for example, Noril’sk, Russia, and Insizwa, South Africa). Fields after Barnes (1990). Data from Naldrett (2004). [Symbols: Voisey’s Bay, circles; Pechenga, Russia, diamond; Jinchuan, triangles; Insizwa, star; and Noril’sk, squares] Mantle Komatiites MORB PGE reefs Cu-rich sulfide veins Chromitites Layered intrusions Ophiolites Flood basalts HighMgO basalts Ni/Pd Cu/Ir B
Stable Isotopes 43 values. Oxygen isotope values for pelitic metasedimentary country rocks that host the intrusions at Kabanga range between +8.9 and +12.6 per mil. Differences in δ18O values between harzburgites and gabbronorites suggest that parental gabbroic magma assimilated approximately 20 percent of the country rock and the parental harzburgite magma assimilated approximately 10 percent of the country rock (Maier and others, 2010). The model proposed by Maier and others (2010) suggests that fine-grained pyroxenites and gabbronorites formed by bulk assimilation of country rock by convecting picritic magmas within large staging chambers, followed by intrusion of contaminated magma into the shallow crust. Heavy δ34S values for sulfides in the Kabanga area are similar to δ34S values for the pelitic metasedimentary rocks, support ing wholesale incorporation of country rock. In contrast, at the Uitkomst Complex, δ18O values of olivine and pyroxene from the sulfide-bearing harzburgite horizon have a very small range (from +5.2 to +5.9 per mil), which is consistent with magmatic values rather than assimilation of high 18O country rock. Heavy δ34S values for the same horizons suggest that sulfur was added to magmas as a hydrothermal fluid, rather than with assimilated country rock (Sarkar and others, 2008). Sulfide-poor intrusions at Uitkomst have oxygen and sulfur isotope values consistent with magmatic values (Sarkar and others, 2008). At the Jinchuan deposit, δ18O values for olivine (+3.5 to +5.9 per mil), plagioclase (+1.1 to +3.1 per mil), serpentine (+2.1 to +4.3 per mil), and amphibole (+2.4 to +4.1 per mil) are all lower than magmatic values and thus record a nonmagmatic process. Ripley and others (2005) concluded that these oxygen isotope data, along with δD data, suggest extensive interaction with evolved meteoric water or seawater, Figure 9–4. Comparison of sulfur isotope variations (delta 34 S, d34S) in magmatic Ni-Cu±PGE deposits (figure modified from Maier and others, 2010). Sources for data include Eagle, United States (Ding and others, 2009); Insizwa, South Africa (Lightfoot and others, 1984); Jinchuan, China (Ripley and others, 2005); Kabanga, Tanzania (Maier and others, 2010); NeboBabel, Australia (Seat and others, 2009); Noril’sk, Russia (Grinenko, 1985; Ripley and others, 2003; Li and others, 2003); Pechenga, Russia (Abzalov and Both, 1997; Barnes and others, 2001); Uitkomst, South Africa (Li and others, 2002); and Voisey’s Bay, Canada (Ripley and others, 1999). The gray, shaded area indicates the range of sulfur isotope values typical for mantlederived sulfides (0 ± 2 per mil). Aquablanca Eagle Archean tonolite Proterozoic metasediments sulfides Insizwa Jinchuan Kabanga sulfides Main Intrusion Block 1 North Intrusion Upper Intrusion MNB Luoma Nebo-Babel Noril’sk evaporites Noril’sk I, Northwestern, Main Talnakh sulfide-poor mineralization Pechenga sulfide-rich shales Kaula, Kotselvaara, Kammikivi Kierzhipor, Pilgujarvi Uitkomst upper gabbronorite pyroxenite main harzburgite cumulate harzburgite lower harzburgite basal gabbronorite Voisey’s Bay Tasiuyak Gneiss Eastern Deeps, Ovoid, Discovery Hill Reid Brook +10 +20 +30 δ34Sulfur
44 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits consistent with large-scale hydrothermal convection and a rift-related origin for the magmatism. Post-magmatic hydro thermal alteration is an unlikely contributor based on the water-poor nature of the metamorphic rocks and the large volume of water necessary to account for observed alteration and isotope characteristics (Ripley and others, 2005). Radiogenic Isotopes Radiogenic isotope systems can play an important role in identifying intrusions that have experienced sulfide seg regation and those that have not. Because Re and Os behave very differently in response to geologic processes (Lambert and others, 1998, 1999a), Re and Os can be used as tracers in determining the origin of the metals in the ores and the extent of crustal contamination. However, caution is necessary when interpreting Re-Os isotope characteristics of sulfides in dynamic magmatic systems because significant variation in the R factor (that is, the mass of silicate magma that has equili brated with a given mass of sulfide magma) can mask strongly the effects of crustal contamination when compared to those crustal effects seen in other isotopic systems (for example, S, Sm-Nd; Lambert and others, 1998). Whereas Re and Os parti tion strongly into metals and sulfides, the lithophile elements involved in other isotopic systems such as Sm-Nd, Rb-Sr, and U-Th-Pb partition strongly into silicate minerals. Whole-rock neodymium, strontium, and Pb isotope analyses of intrusions that host sulfides are used to evaluate the magmatic sources and possible crustal contaminants of the intrusions and, where possible, the age of magmas. Whole-rock Sr and Pb isotope analyses also may be used to evaluate the effect of hydrother mal mobilization of elements (Lightfoot and others, 1993; Arndt and others, 2003). Rhenium-Osmium Isotopes Early models for the Noril’sk deposits by Naldrett and others (1992, 1995) and Lightfoot and Hawkesworth (1997) postulated that magmas staged at intermediate levels in the crust, and the resulting contamination by granitoid rocks led to sulfide saturation and the segregation of massive sulfide. How ever, Arndt and others (2003) used the Re-Os isotopic compo sitions of silicate rocks to suggest another model, one in which the massive sulfides are derived from a second, less contami nated magma. At Noril’sk there are three main ore-bearing (or Noril’sk-style) intrusions: (1) Noril’sk I, (2) Talnakh, and (3) Kharaelakh, each of which hosts Ni-Cu-PGE massive sulfides. Silicate rock samples from these ore-bearing intru sions exhibit gOs (gamma osmium, relative Os deviation from the chondritic upper mantle value) ranging from +4.0 to +10.4 (Horan and others, 1995; Arndt and others, 2003), and their ores exhibit a range in gOs from +1 to +13 (Walker and others, 1994), similar to mantle compositions. These data indicate that the metals were derived from a primitive mantle magma that experienced little crustal contamination (Arndt and others, 2003). Other intrusions in the area host disseminated sul fides (Lower Talnakh-style) or are unmineralized. Rocks from those intrusions typically have gOs ranging from +10.2 to +71.0 (Arndt and others, 2003; Walker and others, 1994), which Arndt and others (2003) attributed to contamination of a primitive magma by anhydrite-rich evaporitic sediments in the upper crust (table 9–2). Thus, at Noril’sk, Arndt and others (2003) found isotopic evidence of strong crustal contamina tion of some magmas at intermediate crustal levels (as shown by the elevated S and Re-Os isotopic data), leading to early sulfide segregation in some units. However, the major ore bodies appear to be derived from magmas that did not reach sulfur saturation at depth and thus still retained their chalco phile elements until the magmas assimilated differing amounts of anhydrite-rich sediments at shallow crustal levels, resulting in sulfide saturation and ore deposition in the Noril’sk-I and Kharaelakh deposits (Arndt and others, 2003; Li and others, 2009). The apparent discrepancy between the Re-Os isotopic results and those of other isotopic systems (for example, S) may reflect the tendency for Re-Os isotopic signatures to mask input from crustal contamination when the system is very dynamic (R factor about 40,000) and the sulfide melt interacts with an enormous volume of mantle-derived magma (Lambert and others, 1998). In other sulfide-rich Ni-Cu±PGE deposits such as Wellgreen (250 Ma; Marcantonio and others, 1994) and Eagle (1.1 Ga; Ding and others, 2008), the gOs values of massive sulfides and their host rocks are about 0 but can be as great as +80. Because typical mantle gOs values are near 0, this sug gests that the primary magmas from which the ores segregated were unlikely to have had much interaction with continental lithosphere or crust. However, in order to explain the higher gOs values of some ores at Wellgreen, Marcantonio and others (1994) invoked the addition of radiogenic Os from surround ing country rocks, via a hydrothermal fluid during emplace ment of the intrusion, as a mechanism for altering primary isotopic values. In the case of the Eagle deposit, Ding and others (2008) suggested that the Os isotopic variation might be related to late-stage hydrothermal processes accompanying widespread serpentinization. At the 1.33-Ga Voisey’s Bay deposit, the three main areas of mineralization (Ovoid, Eastern Deeps, and Discovery Hill) all exhibit very high initial gOs values (+898 to +1,127) for pyrrhotite and chalcopyrite analyzed from massive, dis seminated, and matrix ores. These data suggest that a primary basaltic magma strongly interacted with adjacent gneisses (fig. 9–5; Lambert and others, 1999b, 2000). However, these variable, but commonly high initial gOs values also suggest a very dynamic magma system through which large volumes of geochemically distinct magmas passed; calculated R values are relatively low, ranging from 50 to 500 (Lambert and oth ers, 2000). Using Re-Os isotopic characteristics, Lambert and others (2000) corroborate the proposal of Li and Naldrett (1999) that the Voisey’s Bay deposit represents a magmatic system that encompasses two magma chambers about one
Radiogenic Isotopes 45 kilometer apart and connected by a feeder zone. At least two stages of crustal contamination also occurred. Some sulfides and a related troctolite yield an isochron age as young as about 1,000 Ma, suggesting that a later hydrothermal event may have at least partially reset the Re-Os isotope systematics of the deposit and some of the surrounding country rocks (Lambert and others, 1999b). Rubidium-Strontium, Samarium-Neodymium, and Lead Isotopes At Noril’sk, four groups of intrusions are recognized, each with their own isotopic characteristics (table 9–2) (see Arndt and others, 2003; Wooden and others, 1992). The orebearing intrusions (Noril’sk I, Talnakh, and Kharaelakh) have the following isotopic signatures: eNd (epsilon neodymium, relative Nd deviation from the chondritic upper mantle value) –0.8 to +1.4, 87Sr/86Sr 0.707 to 0.709, and 206Pb/204Pb 17.5 to 18.175, 207Pb/204Pb 15.43 to 15.56, and 208Pb/204Pb 37.16 to 37.98. Noril’sk II is mineralized but does not contain massive sulfides and has the following isotopic signatures: eNd +0.5 to +0.8, 87Sr/86Sr 0.705, and 206Pb/204Pb 18.03 to 18.06, 207Pb/204Pb 15.4 to 15.48 and 208Pb/204Pb 37.82 to 37.86. Unmineralized intrusions (lower Talnakh-type) show significantly more negative eNd (–4.8 to –5.4), higher 87Sr/86Sr (>0.707) and somewhat higher 206Pb/204Pb (18.04 to 18.33), and similar 207Pb/204Pb (15.47 to 15.49) and 208Pb/204Pb (37.68 to 37.95) than the ore-bearing intrusions (Arndt and others, 2003). The youngest intrusions in the area show distinctly dif ferent isotopic signatures (table 9–2). Arndt and others (2003) propose a model in which the mantle-derived magma that fed the Lower Talnakh-type intrusions interacted with continental, likely granitoid crust, causing sulfur saturation and the early segregation of sulfides. This in turn led to contaminated but depleted magmas forming some of the unmineralized Lower Talnakh-type intrusions and volcanic suites. The contamina tion by crustal rocks substantially altered the Nd, Sr, and Os isotopic characteristics of the resulting magma. The negative eNd (–5) and high 87Sr/86Sr (>0.707) are consistent with crustal contamination of mantle-derived magma. In contrast to these magmas, Arndt and others (2003) suggested that different magmas fed the ore-bearing intrusions (Noril’sk, Talnakh and Kharaelakh) and underwent less crustal contamination in intermediate-level magma chambers, and thereby, no sulfide segregation at mid-crust levels. The chondritic eNd value (about 0) is consistent with the Nd isotopic signature of a primitive mantle-derived magma, but the high initial 87Sr/86Sr (>0.707) suggests that additional Sr may have been added after emplacement (Naldrett, 2004). These chalcophile-enriched magmas ultimately interacted at higher crustal levels with anhydrite, which was probably the catalyst for sulfide segrega tion in those magmas. The Nd and Sr isotopic systems were Figure 9–5. Gamma osmium (gOs) versus age in billions of years for some sulfide-rich Ni-Cu deposits. Modern ocean-island basalts (OIB) show nearly chondritic gOs (about 0). Most of the deposits show a Re-Os signature dominated by the mantle. In comparison, Sudbury, Canada (an impact-related deposit) has a high Re-Os signature dominated by crust. The deposits at Pechenga, Russia, and especially at Voisey’s Bay, Canada, also show isotopic compositions dominantly derived from continental crust. Data sources include OIB (Shirey and Walker, 1998); Noril’sk, Russia (Horan and others, 1995); Wellgreen, Alaska (Marcantonio and others, 1994); Jinchuan, China (Yang and others, 2008); Eagle, Michigan (Ding and others, 2008); Voisey’s Bay (Lambert and others, 1999b); Sudbury (Walker and others, 1991); and Pechenga (Walker and others, 1997). [Abbreviation: gOs, gamma osmium, relative Os deviation from the chondritic upper mantle value] Table 9–2. Characteristics of the five chemical components identified in the Norilsk-Talnakh intrusions, Russia, calculated using an emplacement age of 251 Ma (data from Arndt and others, 2003). [Abbreviations: eNd, epsilon neodymium; 87Sr/86Sr, strontium 87/86 ratio; gOs, gamma osmium; d34S, delta sulfur 34; and 206Pb/204Pb, lead 206/204 ratio] Component eNd 87Sr/86Sr gOs d34S 206Pb/204Pb Mantle +4 +6 Ore-bearing Noril’sk-type intrusion 0 to +2 +6 +7 to +13 17.66 to 18.12 Noril’sk II +0.6 +6 18.04 to 18.08 Lower Talnakh-type intrusion –5 +10 to +71 18.04 to 18.34 Younger intrusions –12 to +3 0.704 to 0.707 +5 to +68 +16 to +20 Contaminant A (evaporates) –7 +16 to +20 Contaminant B (granitoids) +5 +1 1,000 1,200 γOs Age, in billion years SUDBURY PECHENGA OIB EAGLE VOISEY’S BAY JINCHUAN WELLGREEN NORIL’SK
46 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits significantly altered by this interaction, but the Os isotopes were not (Arndt and others, 2003). This complex history of multiple mantle-derived magmas interacting with various crustal contaminants is reflected in the isotopic compositions of the sulfides ores and the particular host rocks (table 9–2) (for example, Wooden and others, 1993). Amelin and others (2000) evaluated Nd, Sr, and Pb isotopic signatures of the Voisey’s Bay intrusion and a rela tively unmineralized intrusion, the Mushuau intrusion, both part of the Nain Plutonic Suite. The Voisey’s Bay intrusion is characterized by initial isotopic compositions most similar to mantle values, suggesting derivation from a mantle magma that underwent a small amount of crustal interaction at midcrustal levels (magma defined by eNd –1 to –2, 87Sr/86Sr 0.7034 to 0.7038, and 206Pb/204Pb 15.34 to 15.54, 207Pb/204Pb 15.10 to 15.18 and 208Pb/204Pb 35.24 to 35.56). The amount of mid-level contamination was probably not sufficient to cause sulfide segregation at that point, and sulfide saturation was probably not reached until the magma was contaminated by the Tasiuyak gneisses at a higher level, resulting in ore deposition. Nearby intrusions, such as Mushuau, contain only minor sulfide mineralization and have the following isotopic signatures: eNd –3 to –10, 87Sr/86Sr 0.7034 to 0.7052, and 206Pb/204Pb 14.21 to 14.55, 207Pb/204Pb 14.63 to 14.77, and 208Pb/204Pb 34.36 to 34.65 (Amelin and others, 2000). The enriched Nd (more negative) and elevated Sr signatures are consistent with substantial contamination by Archean gneisses (likely 15–35 percent), which likely caused the magma to reach early sulfur saturation at mid-crustal levels and resulted in residual magmas that were depleted in Ni-Cu-PGE upon reaching the upper crust (Amelin and others, 2000). From an assessment/exploration perspective, mantle-derived magmas that experience only limited crustal contamination in mid-crust would be less likely to experience early sulfur saturation and would be more likely to be prospective for economic concen trations of sulfides when emplaced at shallow depths. Likewise, at the Insizwa deposit, the basal picrite, basal gabbro, and Central Zone gabbros each have different initial 87Sr/86Sr and initial 143Nd/144Nd values, which indicate that they each represent different batches of magma not directly related to one another (Lightfoot and others, 1984). At Insizwa, only the basal gabbro and picritic units are mineralized and are characterized by eNd –2.4 to –4.8 and 87Sr/86Sr 0.7071 to 0.7086, suggesting some contamination, likely at depth. Overlying unmineralized granophyres likely are crustal melts (eNd about 5.4 and 87Sr/86Sr >0.710) (Lightfoot and others, 1984). The effect of hydrothermal alteration on intrusions also can be evaluated using radiogenic isotopes. Marcantonio and others (1994) combined Sr, Nd, and Os isotopes to conclude that hydrothermal alteration played an important role in the Wellgreen deposit, but whether such alteration occurred during or at some time after the deposit formed is uncertain. Although both the Sr and Os isotope systematics appear to have been disturbed significantly, there is no correlation between Sr and Os with Nd isotope ratios, reflecting the relative immobility of Nd during low-grade metamorphism. Post-ore metamor phism also has affected the Os, Pb, and Sr isotopic systems at the Jinchuan deposit, preferentially causing disruptions in the isotopic signatures of disseminated ores rather than of the massive ores (Yang and others, 2008). 10. Petrology of Associated Igneous Rocks Rock Names Magmatic sulfide-rich Ni-Cu±PGE deposits are associ ated spatially with and hosted by a wide variety of mafic and ultramafic igneous rock types generally classified on the basis of varying amounts mostly of olivine, pyroxene, and plagio clase, as well as their textural relations (fig. 10–1). Common rock types include peridotite, pyroxenite, olivine gabbro, troctolite, gabbro, leucogabbro, and picrite (table 10–1). How ever, the use of rock names is complicated by a number of problems including changes in rock nomenclature over time, inconsistent use of rock names (gabbro, gabbronorite, and monzogabbro, for example), variable bases for naming rocks (cumulus minerals versus modal mineralogy), and use of local terminology (for example, Russian taxitic gabbro and picritic gabbrodolerite). Forms of Igneous Rocks and Rock Associations Magmatic sulfide-rich Ni-Cu±PGE deposits gener ally are found in association with small- to medium-sized mafic and/or ultramafic dikes, sills, chonoliths (tens to a few hundreds of meters thick, tens to a few thousand meters wide, and hundreds of meters to several kilometers in length), and plug-like intrusions (generally <10-km diameter). Hypabyssal dike- and sill-like intrusions may show significant changes in dip and width along their strike length and may be surrounded by intense metamorphic and metasomatic aureoles that can extend outwards into the country rocks for as much as 400 m. The intrusions generally appear to have formed as the plumb ing system to large-scale emplacements of mafic magmas in LIPs (Ernst and Buchan, 2001). Mineralogy The mafic and ultramafic igneous rocks spatially associ ated with sulfide-rich Ni-Cu±PGE deposits dominantly are composed of varying amounts of olivine, orthopyroxene, clinopyroxene, and plagioclase (table 10–1). Common primary
Mineralogy 47 Figure 10–1. The International Union of Geological Sciences (IUGS) classification and nomenclature of gabbroic and ultramafic rocks based on the proportions of olivine, orthopyroxene, clinopyroxene, and plagioclase (from Le Maitre, 1989). Fields in A show rock types that compose the Noril’sk-type (Russia) ore-bearing intrusions. Modified from Czamanske and others (1995). Ol dunite harzburgite olivine orthopyroxenite Opx orthopyroxenite websterite olivine websterite clinopyroxenite Cpx olivine clinopyroxenite wehrlite lherzolite peridotite pyroxenite Pl Px Ol plagioclase-bearing ultramafic rocks anorthosite gabbronorite troctolite olivine gabbro(norite) (mela-) (leuco-) leucocratic gabbro olivinebearing gabbro variable-textured (taxitic) gabbro contact gabbro olivine gabbro olivine melagabbro Cpx Pl Pl Opx norite gabbro gabbronorite plagioclase-bearing pyroxenites B A clinopyroxene norite orthopyroxene gabbro Cpx= clinopyroxene, Ol olivine, Opx orthopyroxene, Pl plagioclase, Px pyroxene(s)
48 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits accessory phases include chromite, ilmenite, titanomagnetite, apatite, and sulfides. Hornblende, biotite, and/or phlogopite are common accessory minerals in some rocks, particularly near interstitial sulfide minerals. Zircon and/or baddeleyite may be present as accessory phases, particularly in coarser grained mafic rocks, and are useful for age determination (Heaman, 2009). Mafic minerals generally have relatively high MgO contents but often show zoning to more Fe-rich com positions. Plagioclase generally is relatively anorthitic with zoning to more albitic compositions. For both mafic miner als and plagioclase, compositional zoning may be varied and complex. With metamorphism and alteration, olivine gener ally is replaced by tremolite and/or serpentine (± magnetite); pyroxenes by talc, amphibole, and chlorite; and plagioclase by saussurite, epidote, and chlorite. Because olivine generally is the first phase to crystallize from mafic and ultramafic magmas, its composition can be particularly useful in estimating liquid compositions at various stages during crystallization of mafic/ultramafic intrusions (Naldrett, 2004). In particular, the forsterite (Fo) content of olivine can indicate the FeO/MgO ratio of the magma from which it crystallized (Roeder and Emslie, 1970), while anoma lously low Ni content can be an indication of prior sulfide saturation and fractionation of a sulfide liquid (Naldrett, 2004). Textures, Structures, and Grain Size Textures of the igneous rocks spatially associated with sulfide-rich Ni-Cu±PGE deposits are widely variable, ranging from aphanitic to diabasic, porphyritic, ophitic, skeletalacicular, coarse-grained equigranular, and pegmatitic (table 10–1). Many such rocks, however, are characterized by hypidiomorphic-granular textures. In many cases, the rocks are composed of accumulations of minerals and show cumu late textures (Irvine, 1982). Grain size varies from aphanitic to coarse to pegmatitic, although many rocks are fine to medium grained. Grain size, mineralogy, and crystal form can vary at all scales, from centimeters to tens of meters, resulting in varied-textured (taxitic) rocks, such as at Noril’sk and NeboBabel. Textures indicative of chilling, unidirectional growth, and magma flow also may be present. Magmatic Table 10–1. Lithology, mineralogy, and texture of rocks that host sulfide-rich Ni-Cu±PGE mineralization at Voisey’s Bay, Canada (Li and others, 2000) and at Noril’sk, Russia (Czamanske and others, 1995). [Abbreviations: cum, cumulate; intercum, intercumulate; Ap, apatite; Bt, biotite; Cpx, clinopyroxene; Gn/incl, gneiss inclusions; Hb, hornblende; Hy, hyper sthene; Mt, magnetite; Ol, olivine; Opx, orthopyroxene; Pl, plagioclase; %, percent] Intrusion Intrusive phase Lithodemic unit Minearology Texture Voisey’s Bay Phase I Chilled margin gabbro cum Pl (<10%) Ophitic intercum Pl, Cpx, Opx, Hb, Bt, Mt Feeder olivine gabbro cum Pl (<10%), Ol (<10%) Subophitic intercum Pl, Ol, Hy, Hb, Bt, Mt, Ap Ferrodiorite cum Pl (<5%) Massive intercum Pl, Cpx, Hb, Bt, Mt, Ap Leucotroctolite cum Pl (60–70%), Ol (15–25%) Massive intercum Pl overgrowth, <15% Cpx+Bt Intermediate Olivine gabbro cum Pl (60–70%), Ol (5–10%) Lamination Basal breccia Gn/incl (>25%), cum Pl (<10%), Ol (<10%) Breccia intercum Opx, Pl, Hb, Bt, Mt, sulfides Feeder breccia Gn/incl (>25%), cum Pl (<10%), Ol (<10%) Breccia intercum Opx, Pl, Hb, Bt, Mt, sulfides Phase II Variable troctolite cum Pl (50%), Ol (20–30%), Gn/incl (5–15%) Variable intercum Cpx, Hb, Bt, Mt, sulfides Leopard troctolite cum Pl (40–50%), Ol (20–40%) Leopard intercum Cpx, Bt, sulfides Normal troctolite cum Pl (40–60%), Ol (20–40%) Lamination intercum Pl overgrowth, <15% Cpx+Bt+Mt Ore-Bearing Noril’sk Contact Pl + Cpx + Ol (10–17%) Fine grained Variable olivine gabbro Pl + Ol (7–18%) + Cpx + Mt + Bt + sulfide Variable Olivine melagabbro Ol (40–80%) + Pl + Cpx + Bt + sulfide Fine to medium grained Olivine gabbro Pl + Cpx + Ol (10–27%) + Bt + Mt Subophitic, fine to medium grained Olivine-bearing gabbro Pl + Cpx + Ol (3–7%) + Mt Fine to medium grained, trachytic Leucogabbro Pl + Cpx + Ol (0–3%) + Mt + sulfide Coarse grained
Petrochemistry 49 breccias locally are present, particularly at the base and some times at the upper contact of intrusions, such as at Noril’sk and Voisey’s Bay. Breccias may contain fragments of country rock and “exotic” lithologies, such as the ultramafic rocks not present in the host intrusion at Voisey’s Bay, and commonly are mineralized. Texture and grain size are products of a number of factors that include depth of emplacement, magma composition, cooling rate, volatile content, magma mixing, contamination, and subsequent metamorphism. Host intrusions commonly show some form of miner alogical and/or compositional layering, although layering is generally not as well developed nor of the same type as in larger layered mafic intrusions like the Stillwater or Bushveld complexes. In undeformed terranes, igneous layers generally are subhorizontal, but they also can be parallel to the margins of dikes and to the roof of intrusions. The presence in some intrusions of abrupt transitions between rock types, variations in major and trace element concentrations, reversals in (zoned) mineral compositions, and the discontinuous distribution of lithologies indicate that multiple, distinct magma pulses were emplaced into these intrusions (for example, Noril’sk, Czamanske and others, 1995; Voisey’s Bay, Li and others, 2000; Uitkomst, de Waal and others, 2001; Eagle, Ding and others, 2010; Kabanga, Maier and others, 2010). These intru sions cannot be viewed as the product of in situ fractional crystallization and differentiation of a single crystal-free magma nor of the single input of a crystal- and sulfide-bearing magma. Because of the complexities that can result from mul tiple injections of magma in conduit systems, the oldest layers may not be at the base nor the youngest near the top of the intrusions. Petrochemistry The igneous rocks spatially associated with sulfide-rich Ni-Cu±PGE deposits range in composition from ultramafic to mafic to highly evolved (granophyric). Commonly the rocks have cumulate textures and do not represent magmatic liquid compositions. Most sulfide deposits and their host intrusions, however, appear to have been the product of primitive mantlederived magmas including ferropicrite (Pechenga, Eagle), tholeiitic picrite (Noril’sk-Talnakh), high-Mg basalt (Jinchuan), and siliceous high-Mg basalt (Uitkomst, Kabanga) (table 10–2). Possible exceptions include the Voisey’s Bay deposits, which appear to be related to a distinct high-Al-type basaltic magma (Scoates and Mitchell, 2000), although Li and others (2000) concluded that the geochemistry of the troctolitic intrusions at Voisey’s Bay is consistent with deriva tion from picritic basalts similar in composition to those from Noril’sk and West Greenland. There are no known deposits associated with mid-ocean ridge basalts (MORB) or alkaline rocks (Barnes and Lightfoot, 2005). Subduction-related igneous rocks also typically do not host sulfide-rich Ni-Cu±PGE deposits, although some Ni sulfide occurrences in the Svecofennian terrane of Finland and the Aguablanca Ni-Cu sulfide deposit in Spain are inferred to have developed in Andean-type continental magmatic arcs from calc-alkaline magmas (Peltonen, 2004; Piña and others, 2010). A large body of literature exists describing the geochem istry and petrogenesis of LIP-related igneous rocks and those hosting sulfide-rich Ni-Cu±PGE deposits (Mahoney and Coffin, 1997; Ernst and Buchan, 2001; Foulger and Jurdy, 2007; Zhang and others, 2008). Picrites, primitive and nearprimitive high-Mg tholeiites (those with Mg numbers [Mg/ (Mg+Fe)] ≥0.65 and/or MgO weight percent) occur as a minor component of most LIP sequences, generally at the base. However, the picrites and tholeiites appear more abun dant in those LIPs that host sulfide-rich Ni-Cu±PGE deposits (Zhang and others, 2008). In general, the sequences that host major sulfide deposits are more depleted in basaltic compo nents such as Al2O3 and Na2O than those that are barren. One of the best-studied volcanic-intrusive sequences associated with sulfide-rich Ni-Cu±PGE deposits is the Sibe rian Traps region in the Noril’sk area of Russia. Eleven volca nic suites have been recognized in 3,500 m of vertical thick ness of extrusive rocks (Naldrett and others, 1992) (fig. 10–2). The lower part of the sequence (Lower Series or assemblage) consists of mostly alkaline volcanic rocks with compositions ranging from ferropicrite to basalt and basanite to trachybasalt. The middle of the sequence (Middle Series) consists of tholei itic basalts and picrites of the Tuklonsky Suite and the crustalcontaminated basalts of the lower part of the Nadezhdinsky Suite. The upper part of the sequence (Upper Series) consists of a compositionally relatively uniform series of moderately contaminated and evolved tholeiitic basalts. The lithologic and geochemical characteristics of the Noril’sk volcanic suites are summarized in table 10–3. The intrusions of the Noril’sk region are mostly comag matic with the volcanic rocks and have been classified into three general groups on the basis of field relations, age, petrog raphy, geochemistry, and isotopic composition, with those synchronous with the main phase of tholeiitic volcanism dis tinguished from those that intruded earlier or later (Diakov and others, 2002). The pre-tholeiite intrusions, such as the Ergal akh complex and North Kharaelakh intrusion, are linked to the Lower Series alkaline volcanism, whereas those emplaced after flood-basalt volcanism include both mafic intrusions, such as Oganer and Daldykan, and more-felsic intrusions. All Ni-Cu±PGE sulfide ore deposits are related to the syn-tholeiite intrusive group that is further subdivided into (1) differentiated mafic-ultramafic ore-bearing or Noril’sk-type intrusions; (2) weakly mineralized, differentiated, mafic-ultramafic Lower Talnakh-type intrusions; and (3) a variety of other intrusions ranging from homogeneous to differentiated, mostly barren or with weak, disseminated mineralization (Diakov and others, 2002). The Noril’sk ore-bearing intrusions have petrologic and geochemical characteristics similar to the MorongovskyMokulaevsky volcanic suites (Fedorenko and others, 1996). These magmas are interpreted to have formed through crustal contamination of a tholeiitic picrite magma (see table 10–2)
50 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Table 10–2. Compositions of inferred parental magmas to selected worldwide sulfide-rich Ni-Cu±PGE deposits. Sources are Pechenga, Russia, Lammas intrusion, Hanski and Smolkin (1995); Noril’sk, Russia, Tukllonsky (Tk) picrite, Naldrett (2004); Jinchuan, China, Chai and Naldrett (1992); Uitkomst, South Africa, Barnes and Maier (2002); Voisey’s Bay, Canada, Scoates and Mitchell (2000). [Abbreviations: 87Sr/86Sri, initial strontium 87/86 ratio; 206Pb/204Pb, lead 206/204 ratio; εNd, epsilon neodymium, relative Nd isotope-ratio deviation from the chondritic uniform reservoir value; γOs, gamma osmium, relative Os deviation from the chondritic upper mantle value; %, percent; ppm, parts per million; ppb, parts per billion; FeOt, total iron as ferrous (+2) iron oxide] Pechenga Noril’sk Jinchuan Uitkomst Voisey’s Bay Ferropicrite (Lammas) Tholeiitic picrite (Tk) High-Mg basalt High-Si-Mg basalt High-Al basalt SiO2 wt.% TiO2 Al2O3 FeOt MnO –– MgO CaO Na2O K2O P2O5 –– Rb ppm Sr –– Ba –– Th –– U –– –– Sc Cr 1,638 –– Y –– –– Ta –– –– Nb –– –– –– –– Zr Hf –– –– –– La Ce Nd Sm Eu Gd –– –– –– Tb –– Yb Lu Ni Co –– –– –– Cu –– Os ppb –– –– –– Ir –– –– –– Ru –– –– Rh –– –– –– Pt –– –– Pd –– –– Au –– –– –– –– 87Sr/86Sri –– –– 0.7032–0.7057 ~0.7033 206Pb/204Pb –– –– –– –– εNd +1.4 –2.03 –8 –4.59 to –6.31 γOs +6.0 +4.68 –– –– ––
Petrochemistry 51 like that found in the Tuklonsky Suite (Wooden and others, 1993; Lightfoot and others, 1994; Naldrett, 2004). In contrast, the slightly older Lower Talnakh-type intrusions share many geochemical features with the volcanic rocks of the Nadezhin sky Suite (Arndt, 2005). In contrast to the picrite-related ore-bearing intrusions at Noril’sk, the Voisey’s Bay Ni-Cu±PGE deposits are associ ated with troctolitic rocks related to a high-Al-type basaltic magma (Scoates and Mitchell, 2000). This is reflected in the abundance of high-Al compositions controlled by proportions of cumulate plagioclase, olivine, and trapped silicate liquid (Li and others, 2000). Until the discovery of the Voisey’s Bay deposits in 1993, such troctolitic-anorthositic intrusions were not considered permissive for the occurrence of significant Ni-Cu±PGE sulfide mineralization because the rocks generally lack major accumulations of ferromagnesian silicate minerals (olivine, pyroxene), have intermediate mineral compositions (An40–60; Fo40–60), and have relatively low Ni and Cr abundances. Figure 10–2. Volcanic rock stratigraphy of the Noril’sk region, Russia. Figure modified from Czamanske and others (1995). Note that volcanic-unit abbreviations are appended to the unit names in the stratigraphic sequence depiction (colored column). Table 10–3. Lithological and geochemical characteristics of volcanic series at Noril’sk, Russia. Modified from Arndt (2005). [Abbreviations: %, percent; εNd, epsilon neodymium, relative Nd isotope-ratio deviation from the chondritic uniform reservoir value] Suites Rock types Geochemical characteristics Origin Upper Series Samoedsky, Kumginsky, Kharaelakhsky, Mokulaevsky, Upper Morongovsky (Mr2) Tholeiitic basalt of remarkably uniform composition Evolved, moderately siliceous basalt (SiO2 48.4–50%; MgO 6.3–8.1%). Moder ate enrichment of incompatible trace elements; negative Nb-Ta anomalies; εNd 0 Moderate crustal contamina tion of high-degree mantle melts Middle Series Lower Morongovsky (Mr1), Nadezhdinsky Lavas and tuffs of tholeiitic basalt Siliceous basalt (SiO2 52–55%; MgO 6.3–8.1%). Moderate to strong enrichment of incompatible trace elements; large negative Nb-Ta anomalies; εNd 0 to –11 Highly contaminated tholeiitic magma Tuklonsky Basalt and picrite Basalt (SiO2 49–50 %; MgO 8–9%); picrite (SiO2 47–49%; MgO 10–17%). Moderate enrichment of incompatible trace elements; negative Nb-Ta anomalies; εNd –1 to –4.5 Moderately contaminated, more primitive magmas Lower Series Gudchikhinsky Lavas and tuffs of alkali picrite and basalt Basalt (SiO2 49–52%; MgO =5.5–9%); picrite (SiO2 46–51%; MgO 10–21%). Moderate enrichment of incompatible trace elements; no Nb-Ta anomalies; εNd –2 to +4.6 Relatively uncontaminated moderate-degree mantle melts Syverminsky, Ivakinsky Lavas and tuffs of alkali trachybasalt and basalt Variable compositions (SiO2 47–55%; MgO 2.8–7.5%). Variable enrichment of incompatible trace elements; variable Nb-Ta anomalies; εNd 0 to –4 Relatively uncontaminated low-degree mantle melts Volcanic Series 3,000 METERS 2,000 1,000 Upper Permian Lower Triassic Basalt Assemblage Lower Middle Upper Tungusskaya series Ivakinsky (Iv) Syverminsky (Sv) Gudchichinsky (Gd) Khakanchansky (Hk) Tuklonsky (Tk) Nadezhdinsky (Nd1, Nd2) Morongovsky (Mr1) Morongovsky (Mr2) Mokulaevsky (Mk) Kharaelakhsky (Hr) Kumginsky (Km) Samoedsky (Sm) Uncontaminated alkali picrite, basalt, trachybasalt Highly crustally contaminated tholeiitic picrite and basalt Evolved, moderately crustally contaminated tholeiitic basalt Tuff
52 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Trace-Element Geochemistry Magmas parental to the intrusions that host magmatic sulfide-rich Ni-Cu±PGE sulfide deposits are themselves relatively enriched in Ni, Cu, and PGE (Keays, 1995; Arndt and others, 2005). In addition, the magmas are variably enriched in most of the strongly incompatible elements such as K, P, Ba, Sr, Pb, Th, and light REE (fig. 10–3; Zhang and others, 2008). In contrast, abundances of these same elements in primitive basalts from sequences without Ni-Cu±PGE sulfide deposits generally are lower (Zhang and others, 2008). Rare earth element patterns are variable but typically contain enriched light REE ([La/Yb]n—chondrite-normalized ratio— as much as 60), with depleted to flat middle and heavy REE. Low-Yb samples also are depleted in Al2O3 and have high CaO/Al2O3 ratios, all features reflecting the role of residual garnet in the source region. Depletion of high-field-strength elements, such as Nb, Ta, and Ti, is common, and the majority of samples have Ba/Nb (as large as 300) and La/Nb (as large as 18.5) ratios much greater than ratios of primitive mantle (9 and 0.9, respectively) and ocean floor basalts (Zhang and others, 2008). Primitive continental flood basalts tend to have higher PGE abundances than MORB but similar abundances to average ocean island basalts (fig. 10–4; Crocket, 2002). Continental flood basalts typically show systematically greater PGE abundances than their oceanic counterparts, both show ing a broad trend of decreasing Pt and Pd with decreasing Mg number, with a proportion of samples exhibiting strong PGE depletion indicative of sulfide fractionation (Fiorentini and others, 2010). Zhang and others (2008) suggest that LIPs host ing Ni-Cu±PGE sulfide deposits (fertile) tend to have enriched PGE contents (for example, Os ≥ 0.03 to 10 ppb) and low Re/ Os (typically 0) relative to barren LIPs. In contrast, Fiorentini and others (2010) found no PGE-based evidence to support anomalous chalcophile-element enrichment in LIPs that host Ni-Cu±PGE sulfide deposits. Fiorentini and others (2010) sug gest that the higher average Os contents of ore-associated LIP magmas are a consequence of greater abundance of primitive high-MgO magmas, which typically are characterized by higher Os contents owing to the strongly compatible behavior of Os. All LIP-related samples (fig. 10–4) tend to show fractionation between PPGE (Pt, Pd, and Rh) and IPGE (Os, Ir, and Ru), with supra-chondritic Pd/Ir (ratios ≥2.6). Isotope Geochemistry The LIP basalts show a large range of Sr and Nd isotopic ratios with εSr(i) values (epsilon strontium, the relative stron tium isotope-ratio deviation from a primitive (initial) mantle 87Sr/86Sr value) between –18 and +180 and εNd(i) values (initial epsilon neodymium, the relative Nd isotope-ratio deviation from the value of the chondritic uniform Nd reservoir) between +9 and –16 (Zhang and others, 2008). In general, LIP basalts lack the depleted MORB-type Sr-Nd isotopic signatures, and many samples also plot far outside the field for oceanic island basalts (see figs. 5A and B in Zhang and others, 2008). With respect to Pb isotopes, most samples plot above the present-day Northern Hemisphere Reference Line (NHRL; Hart, 1984) on both 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, whereas 206Pb/204Pb ratios range from 15.4 to 22.7, with most samples cluster ing between 17.4 and 19.2 (see figs. 5C and D in Zhang and Figure 10–3. Primitive mantle-normalized trace-element patterns for volcanic rocks of A, the Noril’sk region, Russia, and B, intrusive rocks of the Voisey’s Bay district, Canada. Also included in B is a high-Al basalt proposed as a parental composition for the Voisey’s Bay troctolites (see table 11–2 in Scoates and Mitchell, 2000). Data from Naldrett (2004); Voisey’s Bay data from Li and others (2000). Primitive mantle values from Sun and McDonough (1989). [Stratigraphic abbreviations in A, same as figure 10–2 for volcanic-series units: Sm, Samoedsky; Km, Kumginsky; Hr, Kharaelakhsky; Mk, Mokulaevsky; Mr, Morongovsky; Nd, Nadezhdinsky; Tk, Tuklonsky; Gd, Gudchichinsky; Sv, Syverminsky; and Iv, Ivakinsky.] 1,000 Cs Rb Ba Th U Ta K La Ce Pb Pr Sr P Nd Zr SmEu Ti Dy Y Yb Lu Rock/Primitive Mantle A Noril’sk Iv, Sv Gd Tk Nd Mr Mk Hr, Km, Sm EXPLANATION 1,000 Cs Rb Ba Th U Ta K La Ce Pb Pr Sr P Nd Zr SmEu Ti Dy Y Yb Lu Rock/Primitive Mantle B Voisey’s Bay Normal/varied-textured troctolite Conduit rocks Melatroctolite Mineralized melatroctolite High-Al basalt EXPLANATION
11. Petrology of Related Sedimentary and Metamorphic Country Rocks 53 others, 2008). The 187Re/188Os ratios of LIP basalts also show a large range of 0.04 to 1,800, with a time-integrated 187Os/188Os variation of 0.124 to 5.2. Positive correlations between 187Re/188Os and 187Os/188Os for basalts from some LIP prov inces, however, produce “isochron” ages in close agreement with the age of magmatism determined by other methods and initial 187Os/188Os ratios close to chondritic, lower than ratios observed in most ocean island basalts and MORB (γOs(i) ≥2.4; Shirey and Walker, 1998, where γOs is gamma osmium, rela tive Os deviation from the chondritic upper mantle value) and continental crustal rocks. One distinctive isotopic feature of LIP basalts is that, despite considerable scatter in the data, correlations between Sr-Nd-Pb ratios for individual provinces appear to converge toward a common end-member composition, FOZO (focal zone; Zindler and Hart, 1986). In addition, neither continental nor oceanic LIP basalts show the HIMU-type (high μ-mantle; μ 238U/204Pb) isotopic signatures common in oceanic island basalts (Zhang and others, 2008). In detail, basalts from individual LIPs define distinct Sr-Nd-Pb isotopic trends with varying slopes extending from a depleted (FOZO-like) source to varying enriched fields. 11. Petrology of Related Sedimentary and Metamorphic Country Rocks Magmatic sulfide-rich Ni-Cu±PGE deposits range in age from Archean to Mesozoic and have been emplaced into a broad spectrum of country rock and basement terranes (table 11–1). However, as noted by Hoatson and others (2006), the presence of Ni-Cu±PGE mineralization shows no obvious correlation with the composition or metamorphic grade of basement lithologies. Thus, the importance of the country rocks that host magmatic Ni-Cu±PGE deposits lies not in their chemical composition but in whether the rocks may contribute to sulfur saturation of magma and to the physical controls that country rocks may exert on intrusion dynamics. In a closed system, fractional crystallization of magma with low sulfur content will produce only a small volume of sulfide minerals late in the crystallization sequence. To produce sufficient sulfide to form a deposit, an external mechanism gen erally is needed to induce sulfur saturation early in the crystal lization process (Lightfoot and others, 1984; Arndt, 2005). Two possible external mechanisms have been identified: (1) addition of sulfur to a magma either by bulk assimilation of sulfide- or sulfate-bearing country rocks (for example, Noril’sk: Naldrett, 1997; Voisey’s Bay: Ripley and others, 2002) or by selective uptake by magma of externally derived sulfur as a vapor or fluid phase from surrounding country rocks (Ripley, 1981); and (2) assimilation of felsic wall rocks during magmatic ascent, thus increasing the silica content of the magma and thereby reducing sulfur solubility and inducing sulfur saturation (for example, Duluth Complex: Li and Naldrett, 1993; Nebo-Babel: Hoatson and others, 2006; Seat and others, 2009). For some deposits, both mechanisms may have played a role. Li and others (2009) have suggested that addition of external sulfur, as well as addition of siliceous material to drive sulfur saturation, were crucial for the development of the sulfide deposits at Noril’sk. The sulfur content of crustal reservoirs plays an impor tant role in determining whether a mafic-ultramafic magmatic system will produce sulfide mineralization. Keays and Lightfoot (2010) compared the unmineralized Deccan flood basalt system to the Siberian flood basalts which host the Noril’sk deposits and determined that simply having significant crustal contami nation of the magmatic system was not sufficient to produce large Ni-Cu±PGE sulfide deposits. Although a wide variety of sedimentary and metamorphic rock types may interact with a S-undersaturated magma (table 11–1), unless a threshold level of sulfur is present within the crustal contaminant, the mag matic system likely will not reach early sulfur saturation and produce significant mineralization (Keays and Lightfoot, 2010). Where a magmatic system may be only slightly undersaturated with respect to sulfur, however, additional silica contributed by crustal contamination may be enough to trigger sulfide satura tion, regardless of the sulfur content of the contaminant (Seat and others, 2009). The effect of metamorphism and deformation on mag matic sulfide deposits can be manifested in several ways. First, metamorphism of the country rocks due to emplacement of intrusions may result in development of skarns and contact aureoles, such as at Noril’sk and Uitkomst. Post-emplacement cataclastic metamorphism may remobilize, and possibly dis perse, sulfide ores, as in the development of matrix-brecciated sulfides at Pechenga (Naldrett, 2004) or the remobilization of sulfides along shear zones as at the Kabanga deposit (Maier and Figure 10–4. Primitive mantle-normalized chalcophile element patterns for average continental flood basalt (CFB), ocean island basalt (OIB), and mid-ocean ridge basalt (MORB). Also shown is the field for Noril’sk basalts and picrites. Basalt data from Crocket (2002); values for primitive mantle from Barnes and Lightfoot (2005). Ni Os Ir Ru Rh Pt Pd Au Cu Rock/Primitive mantle MORB OIB CFB Noril’sk
54 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits others, 2010). High-grade metamorphism can be accompanied by recrystallization and an increase in grain size in both the host intrusion and the related sulfide zones. Equigranular granoblas tic textures may be present in the intrusion, and a coarsening in the grain size of the sulfides can have a positive effect with respect to grinding requirements during mineral processing. The intensity and style of deformation may have a significant impact on the morphology of sulfide lenses that may be thick ened in pressure shadows or conversely may be dismembered and/or attenuated. The physical nature of country rocks also may influence intrusion emplacement styles (Naldrett, 2004). Continental crust typically is stratified with a lower layer of dense granulitefacies rocks, an intermediate layer of lower density granitoid and metamorphic rocks, and, in many cases, an upper layer of still less-dense sedimentary rocks. This crustal structure helps control the formation of magma chambers as ascending magma tends to become trapped at each density discontinuity (Arndt, 2005). When magma reaches horizontally bedded sedimentary strata in the upper crust, horizontal intrusions and sills tend to form, such as at Noril’sk (Naldrett, 2004), Kabanga (Maier and others, 2010), Uitkomst (Li and others, 2002), and Wellgreen (Marcantonio and others, 1994). Horizontal flow through upper crustal sill complexes can augment the period of interaction between magma and wall rocks, thereby increasing the extent of contamination and likelihood of sulfur saturation (Arndt, 2005). Differences in metamorphic grade between packages of rocks also may provide a structural contrast or weakness that can allow emplacement of an intrusion along such structural breaks. The deposit at Jinchuan is an example of an intrusion that likely was emplaced along a disconformity between marbles and felsic gneisses (Lehmann and others, 2007). 12. Theory of Deposit Formation The formation of magmatic sulfide-rich Ni-Cu±PGE deposits is a consequence of the crystallization of mafic and ultramafic magmas. Separation of an immiscible sulfide liquid is a normal product in the crystallization of most mafic and ultramafic systems. Typically only a small quantity of sulfide is formed relatively late in the crystallization history because of limitations on abundance of sulfur in most magmas and because sulfur saturation and separation of a sulfide liquid generally occur late in the crystallization history when silicate Table 11–1. Comparison of exposed sedimentary and metamorphic country rocks associated with magmatic Ni-Cu±PGE sulfide deposits. [Ma, Mega-annum] Deposit/age Ma Related sedimentary rocks Related metamorphic rocks Metamorphic grade Wellgreen (Canada) (232 Ma) Carbonates, volcaniclastic and clastic sedimentary rocks None Unmetamorphosed Noril’sk-Talnakh (Russia) (251 Ma) Dolomites, limestones, argillites, evaporites, coal seams Hornfels (100–400 m thick) Contact metamorphism Jinchuan (China) (827 Ma) Shales, limestones Marbles, gneisses, schists Upper amphibolite facies (basement rocks), greenschist facies (cover rocks) Nebo-Babel (Australia) (1,078 Ma) None Orthogneisses Amphibolite to granulite facies Eagle (United States) (1,107 Ma) Quartzites, cherts, graywackes, slates Gneisses, metasedimentary rocks Regional greenschist facies Kabanga (Tanzania) (1,275 Ma) Sulfide-bearing pelitic rocks (footwall); quartzites, immature clastic rocks, shales (hanging wall) Muscovite-staurolite-biotite ± garnet schists Contact metamorphism; country rocks are mid-amphibolite facies Voisey's Bay (Canada) (1,334 Ma) None Orthogneisses, paragneisses Amphibolite-granulite facies Pechenga (Russia) (1,977 Ma) Sandstones, siltstones, tuffs Phyllites Greenschist to amphibolite facies Uitkomst (South Africa) (2,054 Ma) Shales, quartzites, dolomites, chertconglomerates Archean granitic gneisses, quartzites Contact metamorphism; skarn near dolomites Radio Hill (Australia) (2,892 Ma) None Metavolcanic rocks, gneisses, migmatites Variable, mostly high
12. Theory of Deposit Formation 55 minerals are abundant. The common result is a small amount of disseminated sulfide minerals among more abundant silicate minerals; the sulfide minerals typically are insufficient both in quantity and metal tenor to be economic. In addition, because olivine commonly is the first phase to crystallize from most mafic and ultramafic magmas upon cooling, the late-forming disseminated sulfides exhibit relatively low Ni contents and Ni/Cu ratios because olivine removes Ni from the silicate melt as it crystallizes (Barnes and Lightfoot, 2005). Therefore, for mation of sulfide-rich Ni-Cu±PGE deposits requires that sulfur saturation occurs relatively early in the crystallization history of the magmas. For this to occur typically requires incorpora tion of crustal sulfur (Li and others, 2001b, 2009). Formation of magmatic sulfide-rich Ni-Cu±PGE depos its can be viewed as the outcome of several stages in the evolution of mafic-ultramafic magmatic systems (fig. 12–1; Naldrett, 2010). In the first stage, a metal-bearing primitive mafic or ultramafic parental magma is formed through partial melting in hotter-than-normal parts of the mantle. That magma typically represents the product of relatively high degrees of partial melting (≥20 to 25 percent), has large MgO content (>12 weight percent), is relatively enriched in Ni and PGE, and is sulfur-undersaturated during ascent from the mantle and emplacement into the crust (Keays, 1995; Naldrett, 2010). Such primitive magmas are a common component in most LIPs and are particularly prevalent early in a LIP’s magmatic history. Naldrett (2010) modeled the melting of a representative mantle composition at low (15 kb) pressure. For that mantle composition, the modeling showed that by 11 percent melting, all sulfide present in the source had dissolved into the silicate liquid. Therefore, as the percentage of mantle melting increases above 11 percent, Pt, Pd, and Cu, originally contained in mantle sulfide minerals, are diluted in the melt and decrease in concentration, whereas the Ni content, origi nally contained both in mantle sulfide and in olivine, continues to increase in the melt. Further, although the melt may be sulfur-saturated at the source, because of the negative effect of pressure on sulfur solubility (Wendlandt, 1982) as the magma rises, it will become undersaturated in sulfide upon emplace ment into the crust. The degree of sulfide undersaturation of the magma as it enters the crust and begins to cool, however, has an important influence on capacity of the magma to form an economic sulfide deposit. If the magma composition is close to sulfide saturation upon emplacement, then sulfide liquid may segregate deep in the crust, leaving ore metals at depth and the remaining magma depleted in chalcophile metals. Alternatively, strongly sulfide-undersaturated magmas may rise to shallower levels in the crust but will require access to large external amounts of sulfur to achieve sulfide satura tion (Arndt and others, 2005). Sulfide-undersaturated magmas may interact with crustal wall rocks during ascent and staging in subcrustal magma chambers. Such interactions can form hybrid or contaminated magmas with higher silica contents and also may result in incorporation of crustal sulfur. Both processes can drive a magma to sulfur saturation and cause the formation of an immiscible sulfide liquid (Li and others, 2001b). The common derivation of sulfur from country rock is supported by the sulfur isotope composition of ores from a number of sulfiderich Ni-Cu±PGE deposits, including those at Noril’sk, Pech enga, Voisey’s Bay, and Eagle. For the efficient accumulation of massive sulfide, it is important that sulfide saturation and resultant sulfide liquid segregation occur early in the crystal lization sequence, before abundant silicate minerals crystallize and trap the sulfides. In addition, early sulfide saturation is necessary because significant olivine crystallization prior to formation of an immiscible sulfide liquid will deplete a silicate liquid in Ni, thus limiting the quantity of Ni available to a sulfide liquid (Barnes and Lightfoot, 2005; Naldrett, 2010). Once a sulfide liquid has formed, chalcophile metals are concentrated. Campbell and Naldrett (1979) have shown that, in a closed system, the concentration of a metal in a sulfide liquid (Cs) is a function of the concentration of the metal in the initial silicate liquid (CL), the partition coefficient between the sulfide and silicate liquids (D), and the ratio of the mass of silicate magma to the mass of sulfide that reached equilibrium together, expressed as R:
Cs CLD(R + 1)/(R + D) (1) The effect of R is most pronounced for PGE, such that at low R values, in the range of 100 to 2,000, the Ni and Cu contents of the sulfides may be in the range typical of most Ni-Cu-sulfide ores, but Pt and Pd contents will be relatively low. In contrast, when R is in the range of 10,000 to 100,000, Ni and Cu contents will not be much higher than contents at lower R, but Pt and Pd contents will be much higher and in the range of concentrations found in some Noril’sk deposits and in “reefs” in layered intrusions, such as at the Merensky Reef in the Bushveld Complex, South Africa. Because of their very high partition coefficients, PGE are extremely sensitive to removal of sulfides from silicate melt. Thus, whereas a small amount of early crystallization of olivine or sulfides will not lower the Ni content of the magma significantly, removal of even small amounts of sulfide will sharply deplete the PGE content and mildly deplete Cu (Barnes and Lightfoot, 2005). Lightfoot and Keays (2005) attribute early separation of a sulfide liquid for the exception ally low PGE abundances observed in basaltic lavas overlying the Noril’sk deposits. In addition, any sulfide liquid that forms subsequently to an initial sulfide loss in such magmas will be depleted in PGE relative to Ni and Cu contents. This may explain why some deposits with comparable Ni contents have very different PGE contents, such as Noril’sk with high PGE versus Voisey’s Bay with low PGE (Barnes and Lightfoot, 2005). The resulting immiscible sulfide liquid may interact dynamically with a larger mass of silicate magma, thereby increasing the tenors of the ore metals, particularly for the strongly chalcophile elements that have very large partition coefficients between sulfide liquid and silicate magmas
56 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits (Mungall, 2005). The Ni, Cu, and PGE contents of many ores are higher than would be expected of sulfide that had separated from the relatively small quantity of magma represented by their host intrusions (Campbell and Naldrett, 1979). Therefore, the sulfide liquid must have interacted with, and extracted ore metals from, a larger volume of silicate magma. Such upgrad ing is enhanced in magma conduit systems where previously formed sulfide can interact with new pulses of undepleted magma (Maier and others, 2001). Exactly how sulfide liquid and new pulses of magma interact is not clear, however, and may vary from deposit to deposit. Kerr and Leitch (2005) noted that new pulses of undepleted magma likely would be undersaturated in sulfide and, as a result, could be expected not only to contribute chalcophile elements to the sulfides but also to dissolve and remove sulfide. Because PGE, Ni, and Cu are much more chalcophile in nature than Fe, the bulk of the dissolved sulfide would be FeS, with the more chalcophile metals remaining with the original sulfide. Modeling of such upgrading with accompanying dissolution shows a much steeper increase in the metal content of the sulfide (high R factors) than would result from simple batch equilibration (Kerr and Leitch, 2005; Naldrett, 2010). Based on estimates of the diffusivities of the elements concerned, it appears that simple passage of undepleted magma over a pool of massive sulfide would not result in significant interaction and upgrad ing of metal tenors (Mungall, 2002). Thus, upgrading of metal tenors likely requires the intimate mixing of silicate magma and sulfide liquid on a droplet scale. To form an economic Ni-Cu±PGE sulfide deposit, a sufficient quantity of metal-rich sulfide liquid must become concentrated in a restricted locality (Maier and others, 2001). Because the densities of sulfide liquids (about 4 to 5 g/cm3) are greater than that of silicate magmas within which they develop (about 2.8 to 3.4 g/cm3; table 6–2), sulfide droplets will tend to settle in a silicate melt unless impeded by such factors as the presence of phenocrysts, melt viscosity, surface tension, or melt transport velocity (Mungall and Su, 2005; Chung and Mungall, 2009). Because of density differences, sulfides can be transported vertically in a silicate melt if a magma’s upward velocity is greater than the sulfide settling rate; modeling suggests that mafic magma moving upwards at 0.1 m/s could effectively transport 1-cm-diameter sulfide droplets (Lesher and Groves, 1986; de Bremond d’Ars and others, 2001). Further, the sulfide droplets are unlikely to coalesce during Figure 12–1. Stages in the formation of magmatic sulfide-rich Ni-Cu±PGE deposits (modified from Naldrett, 2010). Mantle Crust 1. Birth of Source Partial melting in mantle; magma ascends through mantle into crust 2. Fertilization of Source Magma forms sills in the crust and undergoes a period of cooling, crystallization, and interaction with crust; sulfide liquid forms 3. Delivery Magma (with immiscible sulfide) rises higher through the crust 4. Growth Continued ascent of magma, emplacement, and concentration of sulfide liquid 5. Nourishment Sulfide metal tenor enriched through interaction with new pulses of magma 6. Full Maturity Magma extrudes at surface or forms sills in the crust; sulfide liquid concentrates and crystallizes Fault
Geological Guides for Local Mineral Exploration and Resource Assessment 57 transport because they will be armored from each other by a coating of silicate melt (de Bremond d’Ars and others, 2001). Any decrease in upward velocity, however, could cause the sulfide droplets to settle and pond. Therefore, structural traps, where the flow velocity of magma is reduced, are important sites for collection of sulfides. This includes embayments in the footwall, where dense sulfide liquid can migrate downward along the axis of the embayment and down the flanking sides (as at Noril’sk-Talnakh; Diakov and others, 2002), and where a narrow feeder conduit enters a larger intrusion, resulting in a decrease in flow velocity and preferential concentration of sulfide droplets (for example, Voisey’s Bay; Li and oth ers, 2000). If deformation occurs before the sulfide liquid has completely solidified, then residual sulfide liquid may migrate or be injected into dilatant structures in the footwall (Barnes and Lightfoot, 2005). In most cases, sulfide liquids have lower solidus tempera tures than silicate melts and can fractionate as crystallization proceeds with decreasing temperature (Naldrett, 2004). On cooling, the first phase generally to crystallize from a sulfide liquid is monosulfide solid solution (MSS, the Ni-bearing, high-temperature equivalent of pyrrhotite). Experimental studies of the partitioning of chalcophile metals between MSS and sulfide liquid (Mungall and others, 2005, and references therein) and observations on natural ores both show that fractionation of a sulfide liquid results in enrichment of Cu, Pt, Pd, and Au in residual liquid. If the resulting Cu-enriched liquid escapes from the crystallizing MSS, then it can form veins in dilatant structures in the footwall, such as at Noril’sk and Eagle. 13. Exploration and Resource Assessment Guidelines A variety of geological, geochemical, and geophysical guides can be used in mineral exploration and resource assess ment for magmatic sulfide-rich Ni-Cu±PGE deposits. In addi tion, some guides apply more at the regional scale and others at a local scale. A number of regional and local guides are outlined below along with typically used exploration methods. Geological Guides for Regional Mineral Exploration and Resource Assessment A Large Igneous Province (LIP) with evidence of primitive Mg-rich melts and large volumes of sulfurundersaturated tholeiitic magmatic rocks occurring on or near the edges of ancient cratons. Cratonic margins, rifts, and deeply penetrating faults that can allow for efficient transport of magma through the crust. Presence of small- to medium-sized differentiated mafic and/or ultramafic dikes, sills, chonoliths, and plug-like intrusions. Deposits generally are not hosted in large, thick, layered intrusions. Presence of sulfur-bearing (sulfide and/or sulfate) crustal rocks. Geological Guides for Local Mineral Exploration and Resource Assessment Clusters of small to medium-sized, generally maficdominated tholeiitic intrusions with ultramafic rocks, if present, typically concentrated at or near the base. Intrusions related to picrites, however, may be wholly or dominantly ultramafic, whereas intrusions related to high-Al basalts will tend to be dominantly plagioclase-rich troctolites and anorthosites. Massive or layered intrusions with variably textured rocks and/or magmatic breccias along the margins. These features may be evidence for multiple injec tions of magma. Intrusions can exhibit changes in texture and mineralogy on a variety of scales (a few centimeters to tens of meters) with cumulus-, equigranular-, intergranular-, and/or subophitictextured rocks. Laterally extensive contact metamorphism, par ticularly above intrusions, and/or metasomatism of country rocks near intrusions. Local geochemical evidence of crustal contami nation and/or chalcophile-element depletion in intrusions and/or related volcanic rocks. Widespread evidence of contamination and/or chalcophileelement depletion may reflect source characteristics or prior magma contamination and sulfide frac tionation in the lower crust, thus suggesting a less favorable environment for the presence of a major Ni-Cu±PGE sulfide deposit at shallow depths. Significant changes in dip and width along the strike of an intrusion indicating sites of possible changes in magma flow dynamics. Changes in the dynamics of magma flow (whether slow, fast, or turbulent flow) and conduit geometry (changes from narrow vertical conduits to broad, subhorizontal magma chamber; physical traps) are important factors for the precipi tation and accumulation of massive sulfide.
58 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Evidence for a dynamic, open, periodically replen ished magmatic system, such as sharp and crosscutting lithologic contacts, magmatic breccias, reversals in fractionation indicators, and changes in chalcophile-element contents within intrusions and/ or associated volcanic rocks. Presence of massive, matrix, and disseminated Fe-Ni-Cu-Co-bearing sulfide minerals in the lower parts of intrusions and Cu-Pd-Pt-Au-enriched sulfide minerals either stratigraphically above Cu-poor sulfides or as footwall dikes and veins that provide evidence of remobilized and fractionated sulfide liquid. Massive sulfides typically are confined to structural embayments and depressions along the basal contacts of intrusions or in feeder conduits. Therefore, determination of the base of a given intrusion is an important criterion in exploration targeting, particularly in deformed terranes. Because massive sulfides are incompetent and concentrate stress during deformation, they may be displaced significant distances (≥1 km) from their host intrusions. Therefore, country rocks surround ing sulfide-bearing intrusions also should be investi gated in strongly deformed terranes. Exploration Methods 1. Exploration using geophysics needs to be tailored to a study area based on factors such as the max imum depth of detection and physical properties of the overburden, intrusions, host lithologies, and mineralization. Regional aerial magnetic and gravity surveys are used to identify pro spective areas of LIPs showing the regional characteristics noted above, and to determine the extent, geometry, and basal contacts of intru sions. At district to local scales, ground magnet ics are used to delineate lithologic layering and to identify small-scale structural embayments and depressions along the basal contact of intrusions. Airborne and ground electromagnet ics and induced polarization are used to identify and delineate conductive sulfides. 2. Stream-sediment and soil geochemistry for Ni, Cu, Cr, and PGE may be useful if intrusions are exposed or are in subcrop. Gossans, if present, are generally strongly leached of Ni and Cu but may have elevated PGE contents. 3. Lithogeochemical study of prospective intrusions for S, Cu, Ni, PGE contents, as well as for Cu/ Pd ratio and Mg-number helps to determine stratigraphic variations and the occurrence of sulfur-saturation events. 4. Composition of olivine (Ni content versus Mgnumber) can be used as an indicator of nickel depletion in magmas from which the olivine has crystallized. 5. Evidence for crustal contamination and/or sulfur addition can be determined from S, O, Sr, Nd, Pb, and Os isotopes; from high Th/Nb and La/ Nb ratios and enriched light REE and alkali contents; from hybrid rocks; and from xenoliths of country rock and/or xenocrysts. 6. Evidence of depleted chalcophile elements, partic ularly PGE, such as shown by high Cu/Pd ratios (>10,000) and Pt and Pd contents near or below detection levels (<1 ppb). On a local scale, such values indicate the presence of sulfides strati graphically below the depleted horizon, or up the magmatic flow direction from it, and provide a vector for focusing exploration. 7. Unusual gossan chemistry and mineralogy, such as high Bi, Pt, Pd, and Ir or the presence of chalcopyrite stringers and Ni-carbonates, can distinguish unmineralized pyritic gossans from gossans developed on Ni-Cu±PGE deposits. 8. Close-spaced drilling, particularly of basal con tacts and feeder conduits, as well as downhole geophysics, such as time-domain electromagnet ics and radio-imaging methods, can help direct exploration toward the most mineralized parts of an intrusion. 14. Geoenvironmental Features Weathering Processes Modern weathering processes operating on mine wastes from sulfide deposits result in acid generation and mobility of metals. Acid-mine drainage can be one of the most significant issues related to sulfide-rich Ni-Cu deposits because of an abundance of pyrrhotite and a general lack of any significant neutralizing potential or alkalinity. The geochemistry of acidmine drainage was reviewed by Nordstrom and Alpers (1999), and additional aspects of the weathering of a variety of ore and gangue minerals were discussed by Plumlee (1999). Foose and others (1995) reviewed the geoenvironmental characteristics of magmatic sulfide deposits. Whereas the magmatic Ni-Cu±PGE deposits related to the Sudbury structure in Ontario represent a unique geologic phenomenon and therefore do not fall within this strict deposit classification, the overall geological and
Sulfide-Oxidation, Acid-Generation, and Acid-Neutralization Processes 59 geochemical characteristics of the Sudbury ores nevertheless generally conform to other magmatic Ni-Cu±PGE sulfide deposits. Therefore, environmental geochemical case stud ies of the Sudbury district should have broader relevance to other, similar sulfide deposits. Likewise, komatiite-hosted Ni deposits also represent a subclass of magmatic Ni-Cu sulfide deposits that is distinct from basaltic dike- and sill-related deposits but may offer some general environmental insights which may be useful. Geochemical aspects of the formation of acid-mine drain age and its burden of metals and other elements of concern can be divided into three broad topics: (1) sulfide-oxidation, acid-generation, and acid-neutralization processes; (2) metal cycling associated with efflorescent secondary sulfate salts; and (3) secondary precipitation of hydroxides and hydroxysul fates and their sorption of metals. Sulfide-Oxidation, Acid-Generation, and Acid-Neutralization Processes The abundance of pyrrhotite in magmatic Ni-Cu±PGE sulfide deposits dominates most aspects of the environmen tal behavior of these deposits and their mine wastes. Acid generated by oxidative weathering of pyrrhotite can aggres sively attack other ore and gangue minerals, thereby liberat ing a variety of potentially toxic elements, including metals from sulfides and Al from silicates. Metal-laden acid-sulfate waters can adversely affect surrounding surface water and groundwater. Within the hydrologic system of mine workings or mine wastes, minerals and other compounds, such as lime used in flotation circuits, can neutralize acid generated by the oxidative weathering of sulfide minerals. Thus, the chemistry of drainage from a mine site is the result of the competing processes of acid generation and acid neutralization. The oxidation of pyrrhotite and other sulfide minerals proceeds with either dissolved oxygen (O2) or dissolved ferric iron (Fe3+) as the oxidizing agent. Dissolved oxygen is the most important oxidant at pH values above ≈4, whereas ferric iron dominates below pH values ≈4 (Williamson and others, 2006). The aqueous oxidation of pyrrhotite by dissolved oxy gen is described by Reaction 1:
Fe1–xS + (2–x/2) O2 + x H2O (1)
→ (1–x) Fe2+ + SO4 2– + 2x H+ where x ranges from 0.000 to 0.125. Pyrrhotite oxidation by ferric iron is described by Reaction 2:
Fe1–xS + (8–2x) Fe3+ + 4 H2O (2)
→ (9–3x) Fe2+ + SO4 2– + 8 H+ Reactions 1 and 2 actually represent the mass action of numerous intermediate reactions. In the oxidative weathering of pyrrhotite, a common initial reaction is the oxidation of pyrrhotite to either pyrite or marcasite as described by Reaction 3:
2Fe1–xS + (1/2–x) O2 + (2–4x) H+ (3)
→ FeS2 + (1–2x) Fe2+ + (1–2x) H2O Textural evidence of marcasite replacement of pyrrhotite is common in pyrrhotitic mine wastes (Jambor, 1994, 2003; Hammarstrom and others, 2001). Although pyrite is not ubiquitous in mine waste from magmatic Ni-Cu±PGE deposits, its oxidation reactions are pertinent to understanding the behavior of wastes from this deposit type. The aqueous oxidation of pyrite by dissolved oxygen is described by Reaction 4:
FeS2 + 7/2 O2 + H2O (4)
→ Fe2+ + 2 SO4 2– + 2 H+ The aqueous oxidation of pyrite by ferric iron is described by Reaction 5:
FeS2 + 14 Fe3+ + 8 H2O (5)
→ 15 Fe2+ + 2 SO4 2– + 16 H+ For reactions 2 and 5 where ferric iron is the oxidant, ferrous iron (Fe2+) must be oxidized to ferric iron as described by Reaction 6:
Fe2+ + ¼ O2 + H+ → Fe3+ + ½ H2O (6) The rate of the oxidation of ferrous iron to ferric iron is strongly dependent on pH, and can be greatly enhanced by the iron-oxidizing bacterium Acidithiobacillus ferrooxidans. Singer and Stumm (1970) observed that A. ferrooxidans increased the rate of oxidation of ferrous iron to ferric iron by a factor of 100,000 compared to the abiotic rate. In the reactions for pyrrhotite (Reactions 1 and 2) and pyrite (Reac tions 4 and 5), additional acid is generated by the oxidation and hydrolysis of the aqueous ferrous iron as described by Reaction 7:
Fe2+ + ¼ O2 + 5/2 H2O → Fe(OH)3 + 2 H+ (7) This also produces the orange and brown iron-oxide precipitates that typify acid-mine drainage. Weathering of chalcopyrite and pentlandite also con tributes metals, sulfate, and acidity to waters. The oxidative weathering of chalcopyrite by dissolved oxygen, which does not generate acid, can be described by Reaction 8:
CuFeS2 + 4 O2 → Cu2+ + Fe2+ + 2 SO4 2– (8) However, the continued oxidation and hydrolysis of ferrous iron, as described by Reaction 7, will generate acid. The oxidative weathering of chalcopyrite by ferric iron can be described by Reaction 9:
60 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits
CuFeS2 + 16 Fe3+ + 8 H2O → Cu2+ (9)
+ 17 Fe2+ + 2 SO4 2– + 16 H+ As stated previously, continued oxidation of ferrous iron will generate additional acid. Likewise, the oxidative weathering of pentlandite, approximated by the formula (Fe0.5Ni0.5)9S8, by dissolved oxygen can be described by Reaction 10:
(Fe0.5Ni0.5)9S8 + 15.5 O2 + H2O → 4.5 Fe2+ (10)
+ 4.5 Ni2+ + 8 SO4 2– + 2 H+ The oxidative weathering of pentlandite by ferric iron can be described by Reaction 11:
(Fe0.5Ni0.5)9S8 + 66 Fe3+ + 32 H2O → 70.5 Fe2+ (11)
+ 4.5 Ni2+ + 8 SO4 2– + 64 H+ The continued oxidation and hydrolysis of ferrous iron produced by Reactions 10 and 11 will generate additional acid. Pyrrhotite and other sulfides, such as chalcopyrite, can also undergo nonoxidative dissolution under anoxic conditions when exposed to acid, as described by the respective Reac tions 12 and 13:
Fe1–xS + (2–2x) H+ + x H2 → (1–x) Fe2+ + H2S (12)
CuFeS2 + 4 H+→ Cu2+ + Fe2+ + 2 H2S (13) These reactions effectively decouple iron and sulfur oxidation and both reactions consume acid. Gangue minerals in host rocks and surrounding litholo gies can react and consume some of the acid generated by the oxidation of sulfide minerals. Silicate minerals commonly found with magmatic Ni-Cu±PGE sulfide deposits, such as olivine, orthopyroxene, and plagioclase, can neutralize minor amounts of acid generated by the oxidation of sulfide minerals as described by the respective idealized Reactions 14–16:
Mg2SiO4 + 4 H+ → 2 Mg2+ + H4SiO4(aq) (14)
MgSiO3 + 2 H+ + H2O → Mg2+ + H4SiO4(aq) (15)
CaAl2Si2O8 + 2 H+ + H2O (16)
→ Ca2+ + Al2Si2O5(OH)4(s) Solid solution of ferrous iron for magnesium in olivine and orthopyroxene can partially counteract the acid neutraliza tion potential of Reactions 14 and 15 because of the continued oxidation and hydrolysis of ferrous iron (Reaction 7). Olivine is one of the most reactive silicate minerals with respect to acid neutralization (Jambor and others, 2002, 2007). Carbon ate minerals, such as calcite, may also be present in country rocks surrounding magmatic sulfide deposits. Calcite can consume acid as described by Reaction 17:
CaCO3 + H+ → Ca2+ + HCO3 – (17) The influence of minor amounts of calcite is most prominent in the anoxic portions of tailings piles where it can effectively neutralize acid (Blowes and others, 2003). Metal Cycling Associated with Efflorescent Sulfate Salts Evaporative concentration of sulfate-rich mine drainage can produce a series of highly soluble secondary sulfate salts. Evaporative processes can operate during hot, arid conditions, within mine workings or other sheltered areas, or in tailings piles beneath snow packs. Common second ary sulfate salts in mining environments associated with magmatic Ni-Cu sulfide deposits include melanterite [Fe2+(SO4)·7(H2O)], rozenite [Fe2+(SO4)·4(H2O)], halotrichite [Fe2+Al2(SO4)4·22(H2O)], alunogen [Fe2+Al2(SO4)4·22(H2O)], copiapite [Fe2+Fe3+ 4(SO4)6(OH)2·20(H2O)], morenosite [Ni(SO4)·7(H2O)], and chalcanthite [Cu(SO4)·5(H2O)] (Jam bor, 1994; Jambor and others, 2000; Hammarstrom and others, 2001). Gypsum [Ca(SO4)·2(H2O)] is another common second ary sulfate, which can contribute dissolved solids to drainage, but does not store acidity or metals. Metal-sulfate salts offer a means of temporarily sequestering acidity and metals for later dissolution during rain events or snowmelt (Jambor and others 2000; Hammarstrom and others, 2001; Jambor, 2003). The effects of salt dissolution events can be dramatic, cycling through a watershed in a matter of hours. Secondary Precipitation of Hydroxides and Hydroxysulfates The oxidation of dissolved ferrous iron and neutraliza tion of mine drainage produces a wide variety of secondary Fe or Al hydroxides and hydroxysulfates significantly less soluble than efflorescent sulfate salts. These phases range from nearly amorphous compounds to others that are well crystal lized. Important Fe minerals include ferrihydrite [nominally Fe5HO8·4H2O], schwertmannite [Fe8O8(SO4)(OH)6], jarosite [KFe3(SO4)2(OH)6], and goethite [α-FeO(OH)]. Important Al phases include amorphous Al hydroxide [Al(OH)3], gibbsite [γ-Al(OH)3], and basaluminite [Al4(SO4)(OH)10·4H2O]. In mine-drainage environments, neutralization and hydrolysis are the main processes leading to the precipitation of the aluminum phases, whereas oxidation is additionally important for the precipitation of the iron phases. Jarosite tends to form in low-pH (1.5 to 3.0), high-sulfate (>3,000 mg/L) environ ments, schwertmannite in moderately acidic (pH 3.0 to 4.0),
Past and Future Mining Methods and Ore Treatment 61 moderate-sulfate (1,000 to 3,000 mg/L) environments, and ferrihydrite in near-neutral (pH >5.0) environments (Bigham, 1994; Bigham and Nordstrom 2000; Stoffregen and others, 2000). Aluminum-bearing phases commonly precipitate at pH values above 4.5 (Nordstrom and Alpers, 1999). An important aspect of the secondary iron hydroxides is their ability to sorb significant quantities of trace metals and remove them from solution. Sorption behavior is pH-dependent. Metal cations, such as Ni2+, Cu2+, Zn2+, and Cd2+, generally sorb to a greater extent with increasing pH (Smith, 1999). Thus, secondary ferric hydroxides and hydroxysulfates can effectively remove metals from solution. In the Sudbury Camp, Durocher and Schindler (2011) identified iron phases associated with acidic historical tail ings ponds and a natural lake surrounded by waste rock. They found pH values ranging from 2.5 to 3.2 in the tailings ponds, and a pH of 6.2 in the lake. Total dissolved base metals (Cu + Co + Ni + Pb + Zn) in these water bodies ranged from 10.9 to 43.5 mg/L in the tailings ponds and 3.4 mg/L in the lake. Under these conditions, the secondary iron mineralogy was dominated by goethite, ferrihydrite, schwertmannite, and jarosite. Pre-Mining Baseline Signatures in Soil, Sediment, and Water Mine permitting and remediation require an estimate of pre-mining natural background conditions, particularly in regulated media such as groundwater, surface water, soil, and sediment, to serve as a goal for post-mining reclamation. Detailed baseline geochemical characterization prior to the onset of mining is essential. However, when baseline charac terization was not done prior to mining, as is the case for many abandoned mines, a variety of methods have been used to estimate pre-mining backgrounds (Runnells and others, 1992, 1998; Alpers and others, 1999; Alpers and Nordstrom 2000). In terms of pre-mining geochemical baselines, geochemi cal anomalies from magmatic Ni-Cu±PGE sulfide deposits can express themselves in groundwater, surface waters, and soils. The elemental suite and magnitude of these anomalies depend upon a number of factors, including extent of exposed outcrop, thickness and nature of overburden, climate, and topography. For example, the geochemical signatures in deflation lag and soil samples in the vicinity of the outcropping Babel and Nebo Ni-Cu±PGE sulfide deposits in central Australia have elevated and anomalous concentrations of Ni, Cu, Pt, Pd, and Au (Baker and Waugh, 2005). Pre-mining geochemical data for groundwater and sur face water near the Eagle deposit in northern Michigan and the Raglan deposit in northern Quebec were acquired during their respective permitting processes. The ultramafic intrusion hosting the Eagle deposit is buried beneath glacial outwash and till. Groundwater near the deposit has maximum con centrations of Fe (190 μg/L), Ni (59 μg/L), and Zn (88 μg/L) that may reflect the influence of mineralized rock; maximum concentrations of Cu (<5 μg/L) and Co (<10 μg/L) are below detection limits. Concentrations of Cu, Ni, and other trace elements in nearby surface waters are indistinguishable from concentrations in regional surface-water samples (Kennecott Eagle Minerals, 2006). Baseline characterization of surface waters near the Raglan Ni-Cu deposit in northern Quebec has been conducted by Nicholson and others (2003). Nickel concentrations in the Deception River prior to mining reached a maximum of 11 µg/L. Other surface-water samples near the deposit reached maximum Ni concentrations of 14.5 µg/L and sulfate concentrations of 5.25 mg/L. Past and Future Mining Methods and Ore Treatment Mining methods and ore-beneficiation techniques signifi cantly influence the potential environmental impacts of mas sive sulfide deposits. Both open-pit and underground methods have been used to mine magmatic Ni-Cu±PGE sulfide deposits in historical and modern operations. The hydrologic differ ences between underground and open-pit mines are significant, particularly at abandoned mines. Evaporative concentration is prominent in open pits, particularly pits in semiarid to arid settings. Mineral processing causes a number of physical and chemical changes to the ore during the production of metal concentrates. Most magmatic Ni-Cu±PGE sulfide deposits contain an excess of iron-sulfide minerals, such as pyrrhotite, relative to valuable base-metal sulfide minerals. The nature of ore processing and the method of disposal of the sulfide tailings and waste rocks are critical parameters that influence the scope of environmental impacts associated with min ing magmatic Ni-Cu±PGE sulfide deposits. Some modern mines discharge fine-grained sulfide-rich tailings into tail ings ponds underlain by impermeable linings, but historical tailings impoundments lack impermeable barriers at the base. Thus, many historical mining operations discharged tailings in a manner that has resulted in significant contamination of surface water and shallow groundwater. Base-metal sulfide minerals, such as chalcopyrite and pentlandite, are typically separated by froth flotation. Some surfactants used in the process are toxic, but most are recycled and only relatively minor amounts are discharged to tailings facilities. The flotation properties of various sulfide minerals are affected by pH. Thus, base addition, typically in the form of lime [CaO] or sodium carbonate [Na2CO3], is a common practice to produce various sulfide-mineral concentrates; other additives to flotation circuits include potassium amyl xanthate, alcohols, ethers, pine oil, sodium cyanide [NaCN], and cupric sulfate [CuSO4·nH2O], all of which affect the flotation proper ties of various minerals (Biswas and Davenport, 1976). Most of these chemicals leave the sites as the tailings piles dewater. However, some may remain and continue to influence drain age chemistry.
62 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits The fine grain size, the typically large size of tailings piles, and the addition of a variety of chemicals establish dis tinct geochemical environments in tailings piles. The fine grain size enhances the reactivity of sulfide and gangue minerals by increasing surface area but also facilitates the formation of hardpan layers that can act as semipermeable to impermeable barriers to oxygen diffusion, thus limiting sulfide oxidation (Blowes and others, 1991, 2003). Numerous studies of tailings from a variety of mineral deposit types indicate that the pH of pore waters in the unsaturated and saturated zones of tailings piles is generally buffered by a predictable series of solid phases. Commonly, pore waters show a step-decrease in pH from 6.5–7.5, to 4.8–6.3, to 4.0–4.3, and finally to <3.5, which corresponds to buffering by calcite, siderite, Al(OH)3, and Fe(OH)3, respectively (Blowes and Ptacek, 1994; Jurjovec and others, 2002; Blowes and others, 2003). Thus, despite being a minor component of many of these mineralized systems, carbonate minerals exert an important control on the geochem istry of anoxic pore waters in tailing piles. Smelter slag is another important type of mine waste, and the reactivity of slags is significant (Parsons and others, 2001; Piatak and others, 2004; Piatak and Seal, 2010). Leaching studies demonstrated that the suite of metals in leachates var ies according to the compositional character of the ore (Piatak and others, 2004). Airborne transport of contaminants has historically been a significant issue surrounding smelters lacking modern mitigation approaches such as scrubbers. The geochemistry of soils near magmatic Ni-Cu±PGE sulfide mining operations can reflect the combined influences of mining and smelting activities. In the Sudbury mining and smelting region, elevated concentrations of Cd 0.06 to 10.1 mg/kg), Co (0.9 to 113.3 mg/kg), Cu (11.4 to 1,891 mg/kg), Ni (5.3 to 2,149 mg/ kg), and Zn (1.5 to 336 mg/kg) have been documented in soils, although modern methods have greatly reduced air emissions (Dudka and others, 1995). Residential soils in the Noril’sk mining district of Siberia have significant maximum concen trations of Co (1,229 mg/kg), Cu (16,000 mg/kg), Ni (2,915 mg/kg), and Zn (172.4 mg/kg) (Yakovlev and others, 2008). Volume of Mine Waste and Tailings Magmatic Ni-Cu±PGE sulfide deposits historically range in size from hundreds of thousands of tonnes to tens of millions of tonnes of sulfide ore. The largest district, Noril’sk-Talnakh, contains 1,903 million tonnes of ore, with Ni grades between 0.7 and 3 percent and Cu grades between 0.2 and 2 percent (Eckstrand and Hulbert, 2007). Because of the rela tively low ore grades, the tonnage of tailings is similar to the tonnage of ore. However, the amount of waste rock will vary mostly based on the mining method. Open-pit mines may need to strip significant amounts of subeconomic, but potentially problematic, waste rock, whereas the amount of waste rock generated by underground mines is typically less. Mine Waste Characteristics Mineralogy The environmental characteristics of these deposits are dominated by their significant acid-generating potential due to the abundance of pyrrhotite, and their limited acid-neutralizing potential due to the general absence or trace abundance of carbonate minerals in their mineralization, alteration, and host rock assemblages. Common sulfide minerals include pyrrhotite, pyrite, chalcopyrite, and pentlandite. Common gangue minerals include olivine, feldspar, pyroxene, quartz, chlorite, serpentine, and minor amounts of calcite or dolomite. Secondary phases include gypsum, magnetite, goethite, jarosite, native sulfur, covellite, marcasite, siderite, and clay (McGregor and others, 1998; Johnson and others, 2000).
Acid-Base Accounting A series of static-test methods have been developed to predict the acid-generating potential of mine wastes as a tool to assist in waste disposal. These tests are known as acid-base accounting or ABA (Sobek and others, 1978; White and oth ers, 1999), which is discussed in detail by Jambor (2003). The primary and secondary mineralogy of ores, their solid mine wastes, and associated rock types can affect ABA calculations. Acid-base accounting is based on the stoichiometric reaction:
FeS2 + 2CaCO3 + 3.75 O2 + 1.5H2O (18)
→ Fe(OH)3 + 2SO4 2– + 2Ca2+ + 2CO2(g) which is simply the sum of Reactions 4 and 17 above to elimi nate H+ as a constituent. Reaction 18 describes acid generation through the oxidation of pyrite and subsequent neutraliza tion by calcite (Sobek and others, 1978). For pyrrhotite, the dominant mineral of magmatic Ni-Cu±PGE sulfide deposits, acid-base accounting can be approximated by the simplified stoichiometric reaction:
FeS + CaCO3 + 2.25 O2 + 1.5 H2O (19)
→ Fe(OH)3 + SO4 2– + Ca2+ + CO2(g) The net result of the proportion of CaCO3 per unit of total S is the same as in Reaction 18, but the total S per unit of solid will be lower because pyrrhotite has approximately half of the S of pyrite. Secondary metal-sulfate salts that commonly accumu late as intermediate products of sulfide oxidation also will contribute acidity (Alpers and others, 1994; Cravotta, 1994; Hammarstrom and others, 2001). For example, melanterite [FeSO4·7H2O], rozenite [FeSO4·4H2O], copiapite [Fe2+(Fe + 4 ) (SO4)6 (OH)2·20H2O], and halotrichite [Fe2+Al2(SO4)4·22H2O], are common and highly soluble; less soluble sulfate miner als, such as jarosite [KFe3+(OH)6(SO4)2] and schwertmannite
Ecosystem Issues 63 [Fe8O8(OH)6(SO4)·nH2O], also are common in mining environments. Secondary metal-sulfate salts, such as chal canthite [CuSO4·5H2O], bonattite [CuSO4·3H2O], moreno site [NiSO4·7H2O], retgersite [NiSO4·6H2O], and alunogen [Al2(SO4)3·17H2O] may be important and act as highly soluble secondary sources of Ni, Cu, and Al. Modifications to the original ABA procedures attempt to accommodate these problems (White and others, 1999). The dominance of pyrrhotite in mine tailings results in waste material with a high acid-generating potential; the minor presence of carbonate minerals in the ore assemblage and the lower reactivity of neutralizing minerals such as olivine or pyroxene offer minimal short-term acid neutralizing potential although some may be realized over longer time frames from olivine and pyroxene. However, much of the host rock surrounding these deposits can have very low acidgenerating potential. Despite genetic differences, the acid-base accounting characteristics of all types of magmatic Ni-Cu deposits share many features (fig. 14–1). Massive sulfide ore from the Eagle deposit is typified by a mean acidic net-neutralizing potential (NNP) of –1,111 kg CaCO3 per tonne of waste; the mean NNP of semimassive sulfide ore at Eagle is –378 kg CaCO3/t (Ken necott Eagle Minerals, 2006). The NNP of mineralogically similar tailings from Sudbury ores is similar (McGregor and others, 1998; Johnson and others, 2000). In contrast, NNP values from waste rock at the Raglan mine, Quebec, which is a komatiite-associated deposit, ranged from –106 to 62 kg CaCO3/t; however, the samples with the highest (most alkaline) NNP values had total sulfur concentrations less than 0.5 weight percent (Rinker and others, 2003). Element Mobility Related to Mining in Groundwater and Surface Water The quality of mine drainage is controlled by the geo logical characteristics of the mineral deposit modified by the combined effects of the mineralogy, mining and ore-beneficia tion methods, the hydrologic setting of the mine workings and waste piles, and climate. Groundwater and surface-water data associated with magmatic Ni-Cu±PGE sulfide deposits and mines are limited. Johnson and others (2000) documented the generation of low pH (as low as 3) waters with high maximum dissolved concentrations of Fe (9.8 g/L), sulfate (24 g/L), Al (1,130 mg/L), and Ni (698 mg/L) in groundwater in the tailings pile at the Nickel Rim mine in the Sudbury district. Copper (3.5 mg/L) and Co (2.5 mg/L) also were important constituents. Similarly, for groundwater and surface-water in the vicinity of the Hitura Ni mine, western central Finland, Heikkinen and others (2002) found variable pH waters (2.8 to 8.7) with a range of dissolved Fe (<0.02 to 330 mg/L), Al (<0.01 to 20.4 mg/L), Mn (<0.01 to 19.5 mg/L), Cd (<0 .2 to 1,120 μg/L), Co (<0.03 to 2,610 mg/L), Cu 0.01 to 7.51 mg/L), Ni (<0.02 to 2,860 mg/L), Pb (<0.05 to 0.11 mg/L), Zn (<0.05 to 760 mg/L), and sulfate (<1 to 17,250 mg/L) concentrations. Pit Lakes Pit lakes are lakes formed as surface water and ground water fill abandoned open pit mines. Studies of pit lakes associated with magmatic Ni-Cu sulfide deposits have not been undertaken. Ecosystem Issues The oxidative weathering of pyrrhotite is described above by Reactions 1 to 3. The low pH values generated by the oxidation of pyrrhotite enhance the solubility of base metals such as Cu, Ni, Zn, Cd, and Co, and the dissolution of silicate-gangue minerals, thus liberating Al and Mn. Most metals have greater solubility at lower pH values; however, Al and ferric iron have solubility minimums at circumneutral pH values, with greater solubility at both lower and higher pH. Once liberated, the metals and acidity can affect downstream Figure 14–1. Acid-base accounting characteristics of magmatic Ni-Cu deposits. Basalt-related dike/sill-hosted deposits are shown in red (Kennecott Eagle Minerals, 2006); basal contact PGEenriched, low-sulfide deposits are shown in dark blue (Lapakko and Antonson, 2006; Lapakko and others, 2003); astroblemeassociated deposits are shown in light blue (Tran and others, 2003); and komatiite-associated deposits are shown in yellow (Lupankwa and others, 2006; Lei and Watkins, 2005; Rinker and others, 2003). Samples with a net-neutralizing potential/acidneutralizing potential ratio (NP:AP) greater than 2 generally are considered nonacid generating (NAG), and those with ratios less than 1 are considered probable acid generating (PAG). Those samples with ratios between 1 and 2 are uncertain. Raglan Sudbury Black Swan Duluth Trojan Eagle
1,000 1,200 Net-neutralizing potential, in kilograms calcium carbonate per metric ton NP:AP 2 NP:AP 1 EXPLANATION Acid neutralizing potential, in kilograms calcium carbonate per metric ton
64 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits aquatic ecosystems. Metal contamination can also be dispersed downstream by the erosion and transport of tailings, which subsequently release metals to the water column. The toxicity of Ag, Cd, Cr, Cu, Ni, Pb, and Zn to aquatic ecosystems is dependent on water hardness; toxicity limits of these metals are higher at higher hardness values (U.S. Envi ronmental Protection Agency, 2006). Hardness is primarily a measure of the concentrations of Ca and Mg. The concentra tion of hardness is expressed in terms of an equivalent concen tration of CaCO3, typically in mg/L. The U.S. Environmental Protection Agency has presented hardness-dependent expres sions for both acute (one-hour exposure) and chronic (four-day exposure) toxicity, and limits independent of hardness for cyanide, Al, As, Sb, Fe, Hg, Se, and Tl (U.S. Environmental Protection Agency, 2006; table 14–1). The metal concentra tions described in the sections above locally exceed aquatic ecosystem guidelines. Human Health Issues Human-health impacts of magmatic Ni-Cu±PGE sulfide mining activities generally result from ingestion or inhalation of metals. Copper is an essential micronutrient for humans, whereas Ni is not. Human health effects of excessive Ni exposure can include contact dermatitis, lung fibrosis, cardio vascular and kidney disease, and cancer (nasal and lung); these effects are most notable among Ni refinery workers (Denkhaus and Salnikow, 2002; Cempel and Nikel, 2006). Environmental guidelines protecting human health from drinking water and residential and industrial soils are sum marized in table 14-1 for Ni and Cu, and a number of other contaminants commonly associated with mine sites in general. The U.S. Environmental Protection Agency has a drinking water standard for Cu (1,300 µg/L) but lacks one for Ni (U.S. Environmental Protection Agency, 2009). The World Health Organization has drinking water standards for both Cu (2,000 µg/L) and Ni (70 µg/L) (World Health Organization, 2008). Human-health vectors for metals include both drinking water and soils. The maximum concentrations in surface water and groundwater for Ni, Cu, Pb, and Zn all exceed drinking water guidelines. Likewise, the maximum concentrations of Cu and Ni exceed residential soil guidelines. Climate Effects on Geoenvironmental Signatures Climate plays a particularly important role in the poten tial environmental impact from mines that exploit magmatic Ni-Cu±PGE sulfide deposits. However, its effect is diffi cult to quantify systematically because insufficient data are available for this deposit type in a wide spectrum of climatic settings. Nevertheless, temperature and humidity are the prime variables which control evaporation. Evaporation can be expected to limit the amount of water in semiarid to arid climates. Evaporation can concentrate solutes in all climates. Acidity and total metal concentrations in mine drainage in arid envi ronments are typically several orders of magnitude greater than in more temperate climates because of the concentrating effects of the evaporation of mine effluent and the resulting “storage” of metals and acidity in highly soluble metal-sulfate mineral salts. Minimal surface-water flow in those areas inhibits generation of significant volumes of highly acidic, metal-enriched drainage. Concentrated release of these stored contaminants to local watersheds may be initiated by precipi tation following a dry spell. In wet climates, high water tables may reduce exposure of abandoned ore bodies to oxidation and may continually flush existing tailings and mine dumps. Although metal-laden acidic mine water does form, it may be diluted to benign metal concentrations within several hundred meters of mixing with a higher order (larger flow) stream. Meldrum and others (2001) found that permafrost conditions,
15. Knowledge Gaps and Future Research Directions 65 such as those found in Arctic Canada, can greatly reduce the oxidation rate of sulfide-rich mine wastes and the associated generation of acid drainage. 15. Knowledge Gaps and Future Research Directions Much progress has been made in understanding the major factors controlling the formation of sulfide-rich Ni-Cu±PGE deposits and several recent publications provide useful over views of recent developments (Naldrett, 2004; Barnes and Lightfoot, 2005; Eckstrand and Hulbert, 2007; Li and Ripley, 2009, 2011). However, important questions remain and require further study including the following. What is the most important factor in triggering sulfide saturation in a given magma? A number of factors can lead to saturation of magmas in an Fe sulfide liquid including (Li and Ripley, 2005): (1) a rise in pressure, (2) a fall in temperature, (3) a change in magma composition (in particular a decrease in the Fe content or an increase in SiO2, Na2O + K2O, or MgO), (4) an increase in ƒO2 (fugacity of oxygen), and (5) a decrease in ƒS, (fugacity of sulphur). For the formation of sulfide-rich Ni-Cu±PGE deposits, the mechanisms commonly proposed to achieve saturation are assimilation of crustal rocks by the primary magma thereby changing the magma composition, or directly adding sulfur from sulfur-rich crustal rocks (Nal drett, 1966; Grinenko, 1985; Thériault and Barnes, 1998, Ripley and Li, 2003; Li and others, 2009). It remains unclear, however, which of these two mechanisms is the more important in producing sulfide saturation and ore formation. Recently, Keays and Lightfoot (2010) have suggested that crustal contamination without sulfur addi tion is insufficient to achieve sulfide saturation and the formation of magmatic Ni-Cu sulfide deposits. Although there is considerable isotopic evidence support ing the addition of crustal sulfur in some deposits, such as Noril’sk, Pechenga, and Voisey’s Bay, the mechanism by which sulfur is transferred from crustal rocks to the magma is still unclear. Initially, when the link between crustal sulfur addition and sulfide segregation was first proposed, it was assumed that the sulfur was volatilized from sulfur-rich country rocks (Naldrett, 1966). Subse quently, several ways were recognized as possible mecha nisms for transferring sulfur from crustal rocks including (Arndt and others, 2005) (1) complete dissolution of all available sulfur by a hot and sulfide-understaturated magma; (2) wholesale melting, incongruent melting, or devolatilization from wall rocks, with or without other volatile and low-melting components, extracting some or all available sulfur; (3) extraction of sulfide from wall rocks to form blobs of “xenomelt” that mix with but never dissolve in a hot, already sulfide-saturated silicate melt; or (4) transfer of sulfur to the magma as a gas (Grinenko, 1985). One or more of these processes may have oper ated to varying degrees in different deposits (Lesher and others, 2001). Table 14–1. Environmental guidelines relevant to mineral deposits with respect to potential risks to humans and aquatic ecosystems from mine waste and mine drainage. Modified from U.S. Environmental Protection Agency (2006, 2009), World Health Organization (2008). [Abbreviations: USEPA, U.S. Environmental Protection Agency; WHO, World Health Organization; mg/kg, milligrams per kilogram; µg/L, micrograms per liter; mg/L, milligrams per liter] Element: Human Health Aquatic Ecosystem Media: Residential soil Industrial soil Drinking water Drinking water Acute toxicity Chronic toxicity Units: mg/kg mg/kg μg/L μg/L μg/L mg/L Source: USEPA USEPA USEPA WHO USEPA USEPA Al 77,000 990,000 As Cd
0.25* Cu 3,100 41,000 1,300 2,000 13* 11* CrTotal Cr(III) 120,000 1,500,000** 570* 16* Cr(VI) Fe 55,000 720,000 1,000 Hg Mn 1,800 23,000 Mo 5,100 Ni 1,600 20,000 470* 52* Pb 65* 2.5* Se 5,100 U 3,100 Zn 23,000 310,000 5,000 120* 120* *Hardness-dependent water-quality standards; value is based on a hardness of 100 mg/L CaCO3. **Values in excess of 1,000,000 mg/kg for some contaminants are used by U.S. Environmental Protection Agency for risk-screening purposes.
66 Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits Although there is evidence that metal tenors of many ores are higher than would be expected of sulfide that had segregated from the quantity of magma represented by related small host intrusions (Campbell and Neldrett, 1979), exactly how magma interacts with sulfide is not clear and may vary from deposit to deposit. For example, in order to achieve large metal enrichments observed in the Noril’sk disseminated sulfides, large R factors (1,000 to 10,000) are required (Barnes and Lightfoot, 2005), implying an extremely dynamic, open, magmatic system. Droplets of sulfide have a large surface area-to-volume ratio that promotes scavenging of chalcophile elements as droplets are transported by flowing magma. Once sulfide droplets are deposited in a layer (which would have a low surface area-to-volume ratio), however, the droplets no longer would extract chalcophile elements efficiently from overlying silicate melt. This suggests that achieving high magma-to-sulfide ratios (high R factors) probably requires continued suspension and mixing of sulfide droplets with silicate melt. Alternatively, Kerr and Leitch (2005) suggest that the most effective way to significantly increase metal tenors is by resorption of sulfide liquid by new injections of sulfide-undersaturated silicate magma. Massive or brecciated ores at several sulfide deposits, including Noril’sk, Jinchuan, Voisey’s Bay, and Eagle, appear locally to transgress their host rocks and to have Cu/Pd ratios and PGE abundances different from associ ated disseminated mineralization. Those ores perhaps were injected as separate pulses of massive sulfide liquid or sulfide-rich breccia, generally along the base of host intrusions. It remains unclear, however, how a dense, low-viscosity sulfide liquid can be extracted from a site of initial accumulation, transported upward through much less-dense crustal rock, and subsequently injected along the base of the host intrusion. The transgressive relation may have resulted from mobilization of still-molten sulfide liquid during structural readjustments around the host intrusion; this alone, however, would not account for compositional differences from associated disseminated mineralization. Are parental magmas to sulfide-rich Ni-Cu±PGE deposits typically enriched in ore metals, or do ore-forming pro cesses depend more on the magmatic history of mantlederived magmas, particularly the manner in which they interact with crustal rocks during their passage to the surface? Some studies concluded that the basaltic sequences that host major Ni-Cu±PGE sulfide deposits are enriched in chalcophile elements relative to barren sequences (Zhang and others, 2008). Other studies, however, contend that there is no difference in the chalcophile-element content between ore-hosting and barren sequences (Fiorentini and others, 2010). Komati itic, tholeiitic, and alkaline mafic and ultramafic magmas all have sufficiently high metal contents to form deposits, yet only komatiites and tholeiitic picrites appear to host significant Ni-Cu±PGE sulfide deposits (Arndt and others, 2005). Arndt and others (2005) have suggested that the absence of deposits in alkaline sequences reflects the rapid ascent of alkaline magmas through the crust because of their volatile-rich, low-density character. References Cited Abzalov, M.Z., and Both, R.A., 1997, The Pechenga Ni-Cu deposits, Russia—Data on PGE and Au distribution and sulphur-isotope compositions: Mineralogy and Petrology, v. 61, p. 119–143. 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Schulz and others—Occurrence Model for Magmatic Sulfide-Rich Nickel-Copper Deposits—Scientific Investigataions Report 2010–5070–I ISSN 2328-0328 (online) ://dx.doi.org/10.3133/sir20102070I