Magmatic Sulfide-Rich Nickel-Copper Deposits Related to 

Picrite and (or) Tholeiitic Basalt Dike-Sill Complexes: 

A Preliminary Deposit Model

By Klaus J. Schulz, Val W. Chandler, Suzanne W. Nicholson, Nadine Piatak, Robert R. Seal, II, 

Laurel G. Woodruff, and Michael L. Zientek

Open-File Report 2010–1179

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Schulz, K.J., Chandler, V.W., Nicholson, S.W., Piatak, Nadine, Seall, II, R.R., Woodruff, L.G., and Zientek, M.L., 2010, 

Mamatic sulfide-rich nickel-copper deposits related to picrite and (or) tholeiitic basalt dike-sill complexes—A 

preliminary deposit model: U.S. Geological Survey Open-File Report 2010–1179, 25 p. (available at 

http://pubs.usgs.gov/of/2010/1179/).

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Introduction.................................................................................................................................................................... 1

Deposit Type and Related Deposits .............................................................................................................................. 3

Regional Environment ................................................................................................................................................... 3

Physical Description of Deposit ..................................................................................................................................... 5

Geophysical Characteristics .......................................................................................................................................... 6

Hypogene and Supergene Ore Characteristics ............................................................................................................. 7

Hypogene and Supergene Gangue Characteristics....................................................................................................... 8

Weathering - Supergene Processes.............................................................................................................................. 9

Geochemical Characteristics of Ores ...........................................................................................................................10

Petrology of Related Igneous Rocks.............................................................................................................................13

Petrology of Related Sedimentary and Metamorphic Country Rocks ...........................................................................14

Theory of Deposit Formation ........................................................................................................................................16

Exploration and Resource Assessment Guides............................................................................................................17

Regional geological guides .......................................................................................................................................17

Local geological guides.............................................................................................................................................17

Exploration methods .................................................................................................................................................18

Geoenvironmental Features .........................................................................................................................................18

References ...................................................................................................................................................................20

Figures

1. Location of some of the world’s nickel mining camps and types of deposits................................................... 2

2. Comparison of sulfur isotope variations in magmatic Ni-Cu±PGE deposits.................................................. 12

Tables

1. Form, distribution, textures and timing of segregation of Ni-Cu±PGE sulfide mineralization. ......................... 6

2. Sulfide geochemistry of magmatic Ni-Cu (±PGE) deposits (modified from Naldrett, 2004). ......................... 11

3. Comparison of exposed sedimentary and metamorphic country rocks associated with magmatic 

Ni-Cu ±PGE sulfide deposits. ....................................................................................................................... 14
iv

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

cubic kilometer (km3) 0.2399 cubic mile (mi3) 

Mass

gram (g)

milligram (mg)

0.03527

3.527 x 10-5

ounce, avoirdupois (oz)

ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

______________________________________________________________________
v

Symbols and Abbreviations ~ approximately > greater than ≥ greater than or equal to < less than

≤ less than or equal to

g/L gram per liter

μg/L microgram per liter

mg/L milligram per liter

mg/kg milligrams per kilogram % percent ‰ permille εNd epsilon neodymium δ180 delta O-18 δ34S delta S-34 γOs gamma Os

Al aluminum

Au gold

Ba barium

Bi bismuth

Cd cadmium

Co cobalt

Cr chromium

Cu copper

Fe iron

Ga gigayears

Ir iridium K potassium

La lanthanum

LIP large igneous provinces

MgO magnesium oxide

MORB Mid-Ocean Ridge Basalt

Myr million years

Nb niobium

Nd neodymium

Ni nickel

NNP net-neutralizing potential

O oxygen

Os osmium P phosphorus

Pb lead

Pd palladium

Pt platinum

PGE platinum-group elements

ppb parts per billion

Re rhenium

REE rare earth elements

Rh rhodium

Ru ruthenium S sulfur

Sm samarium

Sr strontium

Th thorium

Ti titanium V vanadium

Zn zinc

1

Magmatic Sulfide-Rich Nickel-Copper Deposits

Related to Picrite and (or) Tholeiitic Basalt Dike-Sill 

Complexes: A Preliminary Deposit Model

By Klaus J. Schulz,

1 Val W. Chandler,2 Suzanne W. Nicholson,1 Nadine Piatak,1 Robert R. Seal, II,1

Laurel G. Woodruff,

3 and Michael L. Zientek4

Introduction

Magmatic sulfide deposits containing nickel (Ni) and copper (Cu), with or without (±)

platinum-group elements (PGEs), account for approximately 60 percent of the world’s Ni 

production and are active exploration targets in the United States and elsewhere. On the basis of

their principal metal production, magmatic sulfide deposits in mafic rocks can be divided into 

two major types (Naldrett, 2004): those that are sulfide-rich, typically with 10 to 90 percent 

sulfide minerals, and have economic value primarily because of their Ni and Cu contents; and 

those that are sulfide-poor, typically with 0.5 to 5 percent sulfide minerals, and are exploited 

principally for PGE. Because the purpose of this deposit model is to facilitate the assessment for 

undiscovered, potentially economic magmatic Ni-Cu±PGE sulfide deposits in the United States, 

it addresses only those deposits of economic significance that are likely to occur in the United 

States on the basis of known geology. Thus, this model focuses on deposits hosted by small- to 

medium-sized mafic and (or) ultramafic dikes and sills that are related to picrite and tholeiitic 

basalt magmatic systems generally emplaced in continental settings as a component of large 

igneous provinces (LIPs). World-class examples (those containing greater than 1 million tons Ni) 

of this deposit type include deposits at Noril’sk-Talnakh (Russia), Jinchuan (China), Pechenga 

(Russia), Voisey’s Bay (Canada), and Kabanga (Tanzania) (fig. 1). In the United States, this 

deposit type is represented by the Eagle deposit in northern Michigan, currently under 

development by Kennecott Minerals. Descriptions of other magmatic Ni-Cu±PGE sulfide 

deposit types are given in Naldrett (2004), Barnes and Lightfoot (2005), and Eckstrand and 

Hulbert (2007).

________________________

1 U.S. Geological Survey, 954 National Center, Reston, VA 20192

2 Minnesota Geological Survey, 2642 University Ave. W., St. Paul, MN 55114

3 U.S. Geological Survey, 2280 Woodale Dr., Mounds View, MN 55112

4 U.S. Geological Survey, West 904 Riverside Ave., Spokane, WA 99201 
2

Figure 1. Location of some of the worlds major nickel mining camps and types of deposits.

The deposits in the Sudbury district of Ontario, Canada, are not included here because 

they are associated with a unique meteorite impact-related melt sheet (see Naldrett, 2004, and 

Ames and Farrow, 2007, for reviews of Sudbury deposits). There is no evidence for similar 

sulfide deposits being associated with any other known impact structure elsewhere in the world. 

Also not included are the deposits associated with komatiitic lava flows and sills (see Lesher and 

Keays, 2002, for a review of komatiite-associated deposits). Although these deposits are an 

important source for Ni, particularly in Australia, Canada, and Zimbabwe, komatiite deposits are 

rare in the United States and have not proven to be prospective for significant Ni sulfide

mineralization.1

This deposit model is intended to provide the basis for assessing the occurrence of 

undiscovered, potentially economic magmatic Ni-Cu±PGE sulfide deposits in the United States 

as part of the next National Mineral Resource Assessment by the U.S. Geological Survey 

(USGS). The strategy employed by the USGS in quantitative mineral resource assessments 

(Singer, 1993; 2007) includes: (1) delineating tracts permissive 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 the 

Ni-Cu sulfide deposits, particularly features such as host lithology, tectonic setting, structure, 

ore-gangue-alteration mineralogy, geochemical and geophysical signatures, theory of deposit 

formation, and geoenvironmental characteristics.

 1 Although this report does not focus on the magmatic Ni-Cu sulfide deposits at Sudbury or those related to 

komatiites, these 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 occasionally made where appropriate.
3

Deposit Type and Related Deposits

The deposits described in this model are referred to here as magmatic sulfide-rich 

Ni-Cu±PGE deposits related to picritic or tholeiitic basalt dike-sill complexes. The name 

emphasizes the relationship of these Ni-Cu deposits to picrite and tholeiitic basalt magmas and to 

mostly small- to medium-sized dikes and sills in comparison to the generally much larger layered 

mafic-ultramafic intrusive complexes that typically host sulfide-poor PGE-enriched deposits. A 

variety of other names has 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. In addition, deposits have been named after giant deposits of that type, such as Noril’sk 

type or Voisey’s Bay type.

Magmatic sulfide deposits form when mantle-derived, sulfur undersaturated picritic or 

tholeiitic basalt magmas become sulfide saturated, commonly following interaction with crustal 

rocks. Sulfur saturation results in the formation of an immiscible sulfide liquid; the sulfide liquid 

tends to segregate into physical depressions or other areas in the lower parts of dike- and (or)

sill-like intrusions because of changes in magma flow dynamics. Economic deposits develop 

almost exclusively within dynamic magmatic systems that experience repeated surges of magma. 

Such dynamic systems promote the interaction of a sulfide liquid with a sufficiently large 

amount of silicate magma to concentrate chalcophile elements to economic levels. The principal 

sulfide minerals generally consist of intergrown pyrrhotite, pentlandite, and chalcopyrite. Cobalt 

(Co) is found in pentlandite, and PGEs are generally present as small grains of PGE-bearing 

sulfides, arsenides, atimonides, bismuthinides, and tellurides. Most deposits also contain 1- to 

15-percent magnetite.

The tectonic setting of these magmatic sulfide deposits is principally flood basaltdominated LIPs, which are generally attributed to the upwelling and melting of buoyant mantle 

as a plume beneath crustal lithosphere (Condie, 2001). The sulfide deposits and their host 

intrusions range in age from Archean to Middle Jurassic. They are generally found close to 

deeply penetrating structures that allow for the efficient transport of sulfur-undersaturated mantle 

magma to relatively shallow crustal depths. Although sulfur-bearing crustal rocks such as black 

shales, evaporates, or paragneisses are proximal to many deposits, they may not be the primary 

source of sulfur (Hoatson and others, 2006). Deposits are hosted by a wide range of olivinebearing mantle-derived rocks; these rocks and example locations include ferropicrite (Pechenga, 

Russia), tholeiitic picrite (Noril’sk-Talnakh, Russia), and high aluminum basalt (Voisey’s Bay, 

Canada). No known economic deposits are associated with mid-ocean ridge basalts (MORB), 

ophiolites, or alkaline rocks.

Magmatic Ni-Cu±PGE sulfide deposits can occur with a number of other magmatic ore 

deposits. These include low-sulfide, contact-type, and reef-type PGE deposits, chromite deposits, 

and Fe-Ti-V deposits. In addition, secondary processes can produce associated asbestos, 

soapstone, and native copper deposits.

Regional Environment

Magmatic sulfide-rich Ni-Cu±PGE deposits typically occur in continental geologic 

settings where a large volume of mostly tholeiitic mafic-ultramafic magma has been emplaced in 

the crust and erupted on the surface by processes not associated with “normal” spreading ridge or 
4

subduction environments. These intraplate LIPs (and example locations) include continental 

flood basalts (Deccan, Karoo, Siberian Traps, Columbia River) and volcanic rifted margins 

(North Atlantic Igneous Province), along with their plumbing system components (dikes, sills,

and layered intrusions) (Coffin and Eldholm, 1994). The LIPs also are represented by oceanic 

plateaus (Ontong-Java) and ocean basin flood basalts (the Caribbean Flood Basalts) (Coffin and 

Eldholm, 1994). Many LIPs have areal extents of greater than 1 million km2 and volumes greater 

than 1 million km3 (Courtillot and Renne, 2003). The largest continental LIP, the Siberian Traps, 

which hosts the Noril’sk-Talnakh deposits, is estimated to have a minimum size of 4 million km3 

(Ivanov, 2007). Although magmatic activity in LIPs can have lifespans of 50 million years (Myr)

or more, the largest proportion ( greater than 75 percent) of the total igneous volume is generally 

emplaced during a short duration pulse(s) of approximately 1 to 5 Myr (Courtillot and Renne, 

2003; Bryan and Ernst, 2008). The emplacement of such large volumes of mafic and ultramafic 

magma is attributed to the rise and impingement of mantle plumes on continental and oceanic 

lithospheric plates (Condie, 2001; Pirajno, 2007).

Plume-related magmatism and LIPs extend from the present to at least 2.7 gigayears (Ga)

(Ernst, 2007). Mesozoic-Cenozoic continental LIPs are typically the 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 are commonly more deeply eroded, with only flood basalt 

remnants preserved but with their plumbing system exposed as giant dike swarms, sill 

complexes, and layered intrusions. Recognition of LIPs in the Archean is more 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 areas. 

Economic magmatic Ni-Cu±PGE sulfide deposits in LIP settings range in age from 

Archean to Middle Jurassic and are almost exclusively associated with dynamic (replenished) 

magmatic systems, including periodically replenished subvolcanic feeder sills, feeder dikes, and 

volcanic vents (Lesher, 2004). Less dynamic (unreplenished) systems such as sheeted sills and 

larger layered intrusions are normally 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 is probably a reflection 

of the more deeply eroded nature of older LIPs, which exposes more of the plumbing system 

present below the extensive and barren flood basalt sections that are still preserved in younger 

LIPs.

Extension, rifting, and development or reactivation of deeply penetrating faults typically 

accompany development of LIPs and play an important role in providing zones of weakness and 

dilation along which magma may ascend into the crust (Pirajno, 2007). These magma-focusing 

structures facilitate development of dynamic subvolcanic feeder systems that typically host 

sulfide-rich Ni-Cu±PGE deposits (Arndt, 2005). The resulting intrusions are generally small to 

medium sized (less than 3-km thick and hundreds of meters to tens of kilometers long) and range 

from oblate circular through sheet shaped to rod shaped.

The formation of Ni-Cu±PGE sulfide minerals is a direct result of the evolution of the 

dynamic magmatic system that hosts the deposits. However, country rocks, including basement 

metamorphic rocks and upper crustal sedimentary rocks, play two important roles in the 

formation of these deposits. First, crustal structure helps control the formation of magma 

chambers, defining sites where ascending magmas can be trapped at density discontinuities in the 

crust (Arndt, 2005). Second, a common trigger for sulfur saturation and segregation of an 
5

immiscible sulfide liquid is interaction between the sulfur undersaturated magma and crustal 

rocks. This interaction either changes the temperature and composition of the magma so as to 

decrease the sulfur solubility and (or) adds sulfur to the magma. Horizontal flow through upper 

crustal dike-sill complexes can augment the period of interaction between magma and wallrocks, thereby increasing the extent of contamination (Arndt, 2005). Although sulfide- and (or) 

sulfate-bearing rocks are commonly present in the country rocks near intrusions, the rocks that 

the magma(s) interacted with during their ascent through the crust appear to be more important 

than the country rocks at the final level of emplacement (Hoatson and others, 2006).

Physical Description of Deposit

Magmatic sulfide-rich Ni-Cu±PGE deposits typically occur in clusters, and many 

deposits contain multiple mineralized zones. Sulfide ores occur within host intrusions and 

country rocks in a variety of forms (table 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 mobilization (Lesher, 2004). In general, 

three distinct types of Ni-Cu±PGE sulfide mineralization occur in stratiform or stratabound 

deposits:

1. Massive sulfide (greater than about 66 percent sulfide minerals) and matrix or nettextured (about 33 to 66 percent sulfide minerals) ores tend to form dikes and flat-lying 

sheets and lenses at the bottom of intrusions, in some cases also protruding downward 

into the footwall rocks. At Noril’sk, massive ores can extend for several kilometers along 

the long axis of the host intrusion with thicknesses as much as 50 m and widths as much 

as 1 km (Diakov and others, 2002). Massive ores typically occur in physical depressions 

or where changes in the geometry or topography of the footwall occur. The thickness of 

the massive ores correlates with the thickness of the intrusion, generally pinching out 

where the intrusion thickness decreases (Diakov and others, 2002). Massive ores can 

show distinct sulfide mineral zonation, ranging from pyrrhotite-dominated, chalcopyritepentlandite assemblages in the outermost and lower parts, through progressively more 

Cu-rich zones, to mainly Cu-rich sulfides that are commonly enriched in PGE in the 

middle or upper parts. This zonation of sulfides is attributed to in situ fractionation of the 

original sulfide liquid (Naldrett, 2004). 

2. Breccia ores can form semi-conformable, sheet-like zones along both the lower and the 

upper contacts of the intrusions and may enclose massive ore. The breccias comprise 

fragments of both the intrusion and the wall-rocks in a matrix of mainly massive sulfide. 

Sulfide stringers and disseminations may accompany the breccias.

3. Disseminated sulfides (about 1 to 33 percent sulfides) form lenticular to tabular layers 

within the middle and lower parts of intrusions. In most cases, they occur as irregular 

1-mm- to 1-cm-diameter patches interstitial to the silicate and oxide minerals. However, 

at some localities (for example, Noril’sk-Talnakh, Pechenga, and Insizwa in South 

Africa), 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 (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, 
6

disseminated ore is nearly continuous along the strike axis of the intrusion (Diakov and 

others, 2002).

Table 1. Form, distribution, textures, and timing of segregation of types of Ni-Cu±PGE sulfide ores

(from Lesher, 2004).

Ore Type Form/Distribution Texture Timing

I Stratiform or stratabound, at or 

near the base of host unit

Massive, net-textured, 

disseminated

Early magmatic segregation

II 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

III Stratiform or stratabound in 

country rocks adjacent to Type 

I ores

d. Mineralized 

metasedimentary rocks

e. Interbreccia, 

interpillow, veins in 

footwall rocks

Early magmatic and (or) 

metamorphic diffusion

Early magmatic percolation

IV 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. Metamorphichydrothermal

V Massive to semi-massive 

within shears and fault zones 

mainly associated with Type I 

ores

Normally foliated, normally 

inclusion-bearing

Tectonically mobilized

Geophysical Characteristics

Magmatic sulfide-rich Ni-Cu±PGE deposits hosted in picritic and tholeiitic dikes and 

sills present some unique and challenging targets for geophysical exploration. Because these 

sulfide deposits involve some combination of pyrrhotite, pentlandite, and chalcopyrite, they 

typically are characterized by highly anomalous physical properties, including density, 

magnetization, electrical conductivity, electrical chargeability, and acoustic velocity (King, 

2007). As a result, geophysical methods can be very useful in detecting and characterizing such 

deposits (Ford and others, 2007). Unfortunately, the intrusive host rocks, as well as the wallrocks themselves, can also have anomalous physical properties, and their geophysical signatures 

can obscure those of the Ni-Cu sulfide minerals. Furthermore, the geophysical signatures of the 

sulfide deposits, as well as the host intrusions, may vary widely, owing to a variety of geometric 

and geologic factors, and a geophysical method that works effectively in one region may not 

work at all in another (King, 2007). Finally, the Ni-Cu±PGE sulfide deposits, as well as their 

host intrusions, are small, commonly only tens or hundreds of meters in length, and thus 

represent the proverbial “needle in a haystack” for any regional-scale exploration program. 

Overall, a successful geophysical program for these types of deposits requires a creative and 

highly flexible strategy.

In most situations, the aeromagnetic survey method is the most effective tool for 

reconnaissance-scale exploration of dike- and sill-hosted Ni-Cu±PGE sulfide deposits. In 

addition to the relatively low cost, the aeromagnetic survey method is particularly effective in 
7

detecting the pronounced anomalies that commonly arise from either the host intrusions or the 

pyrrhotite component of the sulfide deposits. Wherever they are available, regional surveys 

(400-m line spacing or wider) are used for broad reconnaissance, after which detailed surveying, 

with line spacing on the order of 100 m or less, are flown in selected target areas. Ground-based 

magnetic surveying may subsequently follow up on specific targets. In spite of the general utility 

of magnetic methods for these types of deposits, not all applications are successful. At Voisey’s 

Bay, for example, neither the pyrrhotite-dominated sulfide assemblage, nor the host intrusions 

are magnetic, and the observed magnetic signature primarily reflects pre-intrusion features in the 

wall rock (King, 2007). In these situations, other geophysical methods, such as electromagnetic 

(EM) surveying, may have to be used.

These sulfide-rich deposits are usually excellent conductors of electricity and are, 

therefore, detectable by EM surveying (King, 2007). Similar to magnetic surveying, most EM 

methods can be readily applied from airborne platforms, allowing rapid and efficient acquisition 

over large areas. However, most airborne EM methods are considerably more expensive than 

aeromagnetic surveying, and they are consequently applied during late- or post-reconnaissance 

stages of exploration, once several targets or target areas have been identified based on other 

geophysical, geochemical, or geologic data. Following airborne applications, a variety of 

ground-based EM methods may be applied to selected targets. The interpreter must always be 

mindful that these sulfide deposits are usually small and their EM signature can be easily 

obscured by a variety of factors, including depth of burial, conductive overburden, and the 

presence of graphitic conductors in the wall rock. 

In addition to ground-based magnetic and EM surveying, other ground-based geophysical 

methods have been applied to Ni-Cu±PGE sulfide deposits, usually to specific targets. Electrodebased methods, including resistivity, induced polarization, and spontaneous potential, have been 

used in some areas to help locate and characterize sulfide deposits (Guo and Dentith, 1997; King, 

2007). Both the host intrusions and the sulfide bodies usually have much higher densities than 

those of the surrounding rocks, and the resulting gravity anomalies are useful in interpreting the 

subsurface geometries of these bodies (Ford and others, 2007; King, 2007). Seismic reflection 

profiling has recently emerged as a potentially powerful tool for magmatic sulfide exploration. 

For example, profiling at Sudbury has clearly demonstrated that the igneous host rocks, as well 

as the sulfide masses, produce detectable reflection and diffraction signatures (Salisbury and 

Snyder, 2007). Although far too expensive to use as a reconnaissance tool, 3-D seismic reflection 

profiling may serve as a useful guide for test drilling once a highly favorable target has been 

located.

In summary, geophysical methods have proven useful for exploration of dike- and sillhosted sulfide-rich Ni-Cu±PGE deposits, and their use is likely to continue and improve. 

Particularly promising frontiers may exist for 3-D seismic profiling of specific targets and for 

airborne gravity surveying. The latter method has been used rather sparingly to date (King, 

2007), but improvements in technology and lower costs are likely to continue, resulting in much

broader usage.

Hypogene and Supergene Ore Characteristics

The hypogene ore minerals for magmatic Ni-Cu±PGE sulfide deposits are lower

temperature assemblages of high-temperature 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 
8

nonmagmatic hydrothermal fluids. The dominant minerals in these deposits are mainly variants 

of Fe-, Cu-, and Ni-bearing sulfide minerals, typically pyrrhotite, pentlandite, and chalcopyrite, 

with or without lesser cubanite and troilite; a number of other minerals are found in trace 

amounts. The occurrence of platinum group minerals (PGMs) in these deposits is of critical 

economic interest because PGE, typically present only in trace amounts, may make a deposit 

economic.

The mineralogy and distribution of elements and minerals within major deposits have 

distinct zoning patterns that reflect the fractionation path of an immiscible sulfide liquid. Many 

deposits have basal zones of massive sulfide interpreted to have crystallized from the base 

upward, with a fractional, residual liquid enriched in Cu, Au, Pd, and Pt becoming concentrated 

near the top. At the Oktyabr’sky deposit at Noril’sk, lower and marginal ores are mainly Fe-OsIr-Ru-Rh-rich pyrrhotite, whereas the central overlying ores are Cu-Pd-Pt-Au-rich chalcopyrite 

and mooihoekite (Distler, 1994; Barnes and Lightfoot, 2005; Gorbachev, 2006).

Sulfides in magmatic deposits may be divided into disseminated sulfides (sulfides not 

connected to one another), matrix- or net-textured sulfides (sulfides in an interconnected matrix 

with silicate minerals), and massive sulfide (concentration of sulfide exceeding 66 percent of the 

rock volume). Sulfides can occur as coarse crystals as much as 2 cm in diameter or as fine 

exsolution lamellae. Where present, PGMs are typically small euhedral crystals occurring at the 

boundary of Ni-Cu sulfide minerals.

The most complete information on supergene development for magmatic Ni-Cu±PGE 

deposits is in the voluminous literature describing the numerous Ni deposits of Western 

Australia. Although this specific type of komatiite-related Ni mineralization is not considered for

this report, commonalities of hypogene ores among magmatic-hosted Ni-Cu±PGE deposits and 

similar ultramafic-hosted Ni deposits suggest that the supergene mineral reactions described for 

the deposits in Western Australia would be typical of supergene reactions for many magmatic 

Ni-Cu±PGE sulfide deposits. In general, whereas supergene enrichment is recognized in many 

weathered profiles developed on Ni deposits, such enrichment does not have a significant role in 

upgrading the metal tenor of magmatic Ni-Cu±PGE deposits.

Consistent mineral assemblages are reported for most magmatic Ni deposits that have 

undergone supergene alteration. In deeply weathered terranes, supergene sulfide minerals have 

lost sulfur and base metals, and the new mineralogy includes bravorite, violarite, pyrite, millerite, 

marcasite, and native copper or copper-oxides (Barnes and Lightfoot, 2005). 

When exposed to supergene alteration, most Ni-Cu±PGE sulfides tend to develop distinct 

mineral zones that reflect increasing degrees of oxidation towards the surface. The first 

indication of the initiation of sulfide oxidation of primary pyrrhotite-pentlandite is the 

appearance of violarite-bravoite/pyrite-marcasite. With decreasing depth, conversion of 

pyrrhotite-pentlandite continues, although primary chalcopyrite and pyrite persist. Above the 

water table, sulfide minerals are replaced by a weathered gossan of hydrated ferric oxides, 

various carbonate, and silica minerals. Gossans developed from the weathering of magmatic 

Ni-Cu±PGE hypogene minerals typically will retain immobile PGE.

Hypogene and Supergene Gangue Characteristics

Gangue mineralogy for magmatic Ni-Cu±PGE deposits can be divided into three 

categories: (1) magmatic silicates and oxide minerals of the ore-hosting mafic to ultramafic 

intrusions; (2) metamorphic minerals resulting from contact and post-depositional regional 
9

metamorphic events; and (3) minerals in partially ingested xenoliths and hornfelsed country 

rock.

Magmatic gangue minerals for most deposits are the minerals of the host mafic and 

ultramafic silicate rocks. Typical minerals are olivine, orthopyroxene, clinopyroxene, 

plagioclase, magnetite, ilmenite, and Cr-spinel. Gangue minerals at deposits that have undergone 

magmatic and post-depositional alteration typically reflect the original mineralogy. 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 in contact with 

mineralized host rocks. Xenoliths may be incorporated into magma by thermal erosion of conduit 

walls. At Voisey’s Bay, 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 plagioclase broke down to 

produce corundum (Li and Naldrett, 2000). At Noril’sk, metamorphism of sedimentary country 

rock, which includes dolomite, argillite, evaporates, and terrigenous coal measures, involved

prograde development of hornfels and marble and retrograde development of calc-silicates/skarn 

(Likhachev, 1994). Evidence of reaction with country rock, particularly one with potentially 

readily available sulfur (pyrite), is a favorable indicator when evaluating an intrusion as a 

potential exploration target (Naldrett, 2004). 

Supergene gangue characteristics are complex because of the variable rock types that host 

magmatic sulfide ore bodies. The nature of gangue minerals enclosing sulfide minerals can exert 

a strong influence on weathering reactions, solution pH, and resulting precipitates. Weathering of 

gangue minerals such as feldspars, amphiboles, and pyroxenes releases alkali metals, as well as 

aluminum. In solutions with very low pH, relatively insoluble sulfate minerals, such as alunite 

[ 3 4 ( ) ( ) 2 6 KA SO OH 1 ] and jarosite [ ( ) ( ) 3

3 4 6 2 KFe OH SO + ] precipitate (Thornber, 1985; Burns, 

1988). At deposits where country rocks have a buffering effect on pH, acid produced by 

oxidation is neutralized by interaction with nearby silicate and carbonate minerals. At higher pH, 

ferric ions precipitate as poorly crystalline iron hydroxides, such as goethite; secondary 

carbonates may form; silica will reprecipitate as quartz, jasper, or opal; and if sufficient 

aluminum is present, clay minerals or aluminum hydroxides are formed. 

Weathering – Supergene Processes

Sulfide minerals are unstable in the presence of oxidized aqueous solutions in the 

unsaturated vadose zone located between the land surface and the groundwater table. In this 

zone, chemical weathering reactions convert primary sulfide minerals to secondary sulfide, 

sulfate, and oxide mineral assemblages. Below the groundwater table, deep weathering of 

primary magmatic Ni-Cu-sulfide minerals is initiated by dissolved ferric iron (Fe3+) ions in 

percolating groundwater (Burns, 1988). The role for dissolved oxygen is to produce Fe3+ in 

the near surface environment and it is not essential for the formation of secondary sulfides in 

Ni-Cu±PGE deposits, such as violarite, pyrite, and covellite.

Most sulfide minerals are coherent electronic conductors, and thus sulfide weathering 

proceeds by electrochemical reactions involving coupled half-cell reactions. Oxidation involves 

the loss of electrons. In weathering reactions, sulfide minerals act as an anode and electrons are 

conducted away from a sulfide through the groundwater electrolyte to cathode reaction sites. 

In ore-bodies, these reactions may act on the scale of an individual mineral (for example, 

disseminated sulfides) or over hundreds of meters in massive ore (Thornber, 1975). The general 
10

weathering processes involve conversion of magmatic massive sulfide, made up of pyrrhotite 

and pentlandite with minor chalcopyrite and pyrite, to violarite and secondary pyrite, with further 

conversion of all sulfide minerals to gossanous materials above the water table. 

Magmatic sulfide ore-bodies may act as giant corrosion cells, with electrochemical 

processes driving metal diffusion (Thornber, 1975). By this process the oxidative power of 

atmospheric oxygen is the driving force for the weathering of sulfides in the absence of bacterial 

activity. The rate at which atmospheric oxygen dissolves in groundwater and is transported to the 

reactive sulfide, where it is reduced, may be the rate-limiting step in the weathering process. 

Groundwater movement will result in some transport of oxygen, but close to the sulfide surface,

diffusion will be the rate-limiting step. 

Although this basic model remains valid, recent experimental work (Xia and others, 

2009) demonstrates that the replacement of pentlandite by violarite, the principal reaction in 

supergene alteration of nickel sulfide ore, is an example of a pseudomorphic coupled dissolutionreprecipitation reaction. Thus, the replacement of pentlandite proceeds by dissolution (reaction 

1) followed by violarite precipitation (reaction 2), rather than by solid state diffusion.

( ) ( )

2 2

9 8 2 aq 2 2 aq Fe Ni S 18H 0.5O ( 9 )Ni xFe 8H S H O + + +

× −× + + = −× + + + (1)

pentlandite

( ) ( )

2 2

2 aq 2 aq y 3y 4 2 2 yFe (6 2 y )Ni O 8H S 2Fe Ni S 12H 2H O + + + +− + + = + + − (2)

violarite 

Xia and others (2009) argued that the slowest among the three critical steps in this 

process may be transport of solute to and from the reaction front, pentlandite dissolution, or 

violarite precipitation. Kinetic results from Xia and others (2009) demonstrate that, in mildly 

acidic conditions (1< pH <6), the rate-limiting step is pentlandite dissolution. However, under 

strongly acidic conditions (pH = 1), the rate-limiting step is violarite precipitation. 

The ultimate development and preservation of a supergene enrichment/weathering zone 

above an ore-body are achieved only through an intricate combination of highly favorable 

climate, geology, topography, and preservation factors. For example, the convoluted geological 

and climatic paths that resulted in formation and exposure of Archean sulfide mineralization in 

Western Australia involved a long history of uplift, glaciation, peneplanation, and erosion 

combined with swings in climate from cold to warm, and from arid to humid, ending with a final 

fortuitous configuration that resulted in the recognition of more than 20 major nickel sulfide 

deposits in the Yilgarn Shield, virtually all discovered by “gossan hunting” (Smith, 1984).

Geochemical Characteristics of Ores

Metal contents and ratios such as Ni/Cu, Ni/Co, and PGE/Ni+Cu can be used to 

distinguish dike/sill tholeiitic magmatic sulfide deposits from other types of Ni-Cu±PGE 

magmatic sulfide deposits (table 2). Tholeiitic dike/sill-related deposits typically have somewhat 

lower Ni/Cu ratios than reef-type and komatiitic-hosted deposits and much lower PGE/Ni+Cu 

ratios than reef-type deposits. 

Without the influence of external factors, sulfide minerals typically will not form early in 

the magmatic crystallization process. Modeling suggests that large volumes of sulfide melt, 
11

typically rich in Ni, Cu, and Co but relatively poor in PGE, can be produced by assimilation of 

sulfide-rich country rock (Mungall, 2007). If sulfur saturation of the magma is reached early 

before much olivine crystallization has occurred, then Ni will not yet have been removed by 

olivine and thus the resulting sulfide mineral assemblage will have a high Ni/Cu ratio. If sulfide 

saturation occurs after significant olivine crystallization, then the resulting sulfide assemblage 

will have a much lower Ni/Cu ratio (Mungall, 2007). These ratios can be used to evaluate the 

degree to which magmas have previously segregated sulfides and (or) been affected by crustal 

contamination (Naldrett and others, 1992). 

Table 2. Sulfide geochemistry of magmatic Ni-Cu (±PGE) deposits (from Naldrett, 2004).

[---, data not available]

Deposits Ni/Cu Ni/Co Pd/Pt Pd/Ir (Pt+Pd)/(Ni+Cu)

Tholeiitic sill/dike deposits

Eagle (United States) 1.23 35.7 0.64 - 0.2

Insizwa (Karoo) (South Africa) 0.91 - 2.40 18.12 0.73

Jinchuan (China) 1.76 56.0 1.00 14.37 0.04 to 0.45

Kabanga (Tanzania) 5 to 10 10 to 15 0.3 to 2.0 2 to 20 0.06 to 0.4 (+1.6) 

Noril'sk–Talnakh (Russia) 0.58 58.0 3.43 217.34 1.1 to 4.8

Pechenga (Russia) 1.86 26.0 1.33 9.7 0.05 to 0.26

Voisey's Bay (Canada) 1.87 18.0 1.29 59.98 0.05 

Reef-type deposits

Duluth (United States) 0.33 10.50 3.35 184.17 2.9 to 4.8

Platreef (South Africa) 1.27 - 1.5 88 to 166 2.1 to 4.4

Merensky reef (South Africa) 2.6 - 0.54 14.69 17.4 (+112.6)

J-M reef (United States) 2.03 - 3.47 901.7 353.99 

Komatiitic Ni-Cu deposits

Kambalda (Australia) 13.5 0.07 1.39 8.87 0.19 to 0.26 

Meteoritic impact-related deposits

Sudbury (Canada) 1.11 32.00 1.26 30.65 0.06 to 1.37

Sulfur and oxygen isotopes can be used to assess the origin of sulfur and metal-carrying 

fluids, whereas other isotopic systems (Re-Os, Sm-Nd, Pb) can be used to evaluate the source of 

the metals and degree of assimilation of country rocks. Stable isotope data and resulting 

interpretations of these data are very variable between deposits. Xenoliths of country rocks were 

entrained in magmas at the Voisey’s Bay, Kabanga, and some of the Noril’sk deposits. In most 

cases, xenoliths and their host rocks have sulfur and oxygen isotope values that demonstrate 

significant country rock sulfur contributions (Grinenko, 1985; Ripley and others, 1999, 2002, 

2003; Mariga and others, 2006) (fig. 2). In contrast, 18 δ O values of silicate minerals at the 

Uitkomst deposit in South Africa are consistent with magmatic values, but heavy 34 δ S values for 

sulfide minerals from the same horizons indicate that sulfur was added to magmas as a 

hydrothermal fluid rather than through bulk assimilation of country rocks (Sarkar and others, 

2008). Likewise, at the Jinchuan deposit, hydrothermal fluids, likely derived from evolved 

seawater or a metamorphic fluid, appear to have altered the 18 δ O values in silicates in the early 

stages of alteration (Ripley and others, 2005). In contrast, sulfides have 34 δ S values near 0‰, 

which is consistent with magmatic values, or possibly unexposed country rocks with mantle-like 
12

values (Ripley and others, 2005). At the Nebo-Babel deposit in Western Australia and at some of 

the Noril’sk deposits, sulfur saturation is inferred to have occurred due to assimilation of 

orthogneisses at depth and not as a result of the addition of external sulfur (Seat and others, 

2009; Keays and Lightfoot, 2010).

Figure 2. Comparison of sulfur isotope variations in magmatic Ni-Cu±PGE deposits (modified from Maier 

and others, 2010). Sources for data are: Eagle (Ding and others, 2009), Insizwa (Lightfoot and others, 

1984), Jinchuan (Ripley and others, 1999), Kabanga (Maier and others, 2010), Nebo-Babel (Seat and 

others, 2009), Noril’sk (Grinenko, 1985; Li and others, 2003; Ripley and others, 2003), Pechenga (Abzalov 

and Both, 1997; Barnes and others, 2001), Uitkomst (Li and others, 2002), and Voisey’s Bay (Ripley and 

others, 1999, 2002). The shaded area indicates the spread of sulfur isotope values typical of mantlederived sulfide minerals.

The Re-Os isotopic system is an ideal tracer for determining the origin of metals and the 

extent of crustal contamination in magmatic Ni-Cu±PGE deposits because the large partitioning 

coefficients for these chalcophile elements mean that the metals are strongly concentrated in 

immiscible sulfide liquids compared to silicate melts (Lambert and others, 1998; 1999a, 1999b, 

2000). The Re-Os isotopes, when coupled with other isotopic systems such as Sm-Nd and Pb, 

can constrain the magmatic and assimilation/fractionation histories of magmas (Lightfoot and 

others, 1993; Horan and others, 1995).
13

At Voisey’s Bay, very high initial Υ Os values for the ores suggest a significant 

contribution from old adjacent gneisses (Lambert and others, 1999b). Complex magmatic 

histories can be defined using Nd, Os, and Sr isotopic analyses of sulfide ores. For example, at 

Noril’sk, Arndt and others (2003) found isotopic evidence of strong contamination of some 

magmas at depth, but the major ore bodies appear to be derived from the interaction of 

chemically distinct, less contaminated mafic magmas that reached sulfur saturation when in 

contact with crustal anhydrite. Post-ore metamorphism commonly affects the Os, Pb, and Sr

isotopic systems, preferentially causing disruptions in isotopic signatures of disseminated ores 

rather than massive ores; for example, Jinchuan (Yang and others, 2008).

Petrology of Related Igneous Rocks

Magmatic sulfide-rich Ni-Cu±PGE deposits are consistently found with small- to 

medium-sized differentiated mafic and (or) ultramafic dikes, sills, chonoliths (tens to a few 

hundreds of meters in thickness, ≤ 100- to ≥ 2,000-m-wide, and hundreds of meters to several 

kilometers in length), and plug-like intrusions (generally <10 km in diameter). The dike- and silllike 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 considerable distances—for example, as much as 400 m at Noril’sk. 

Intrusions generally appear to have formed as the plumbing system to the emplacement of mafic 

magmas in LIPs.

Intrusions hosting sulfide-rich Ni-Cu±PGE deposits are related to a range of generally 

olivine-bearing, mantle-derived magmas including ferropicrite (Pechnega; Eagle), tholeiitic 

picrite (Noril’sk-Talnakh), high-aluminum basalt (Voisey’s Bay), and siliceous high-magnesium 

basalt (Uitkomst). These diverse magma types produce a variety of igneous lithologies 

depending on composition, depth of emplacement, and the rate of cooling, ranging from finegrained basalt and diabase to medium- and coarse-grained gabbro, pegmatite, and breccia. 

Cumulate rocks are common in some intrusions and are generally composed of varying 

proportions of olivine, orthopyroxene, clinopyroxene, and plagioclase. Chromite, iron-titanium 

oxides, and apatite may be present as accessory minerals and biotite may occur near interstitial 

sulfide minerals. Sulfide-hosting intrusions are typically texturally and lithologically 

heterogeneous, exhibiting changes in texture and mineralogy on a variety of scales, from 

centimeters to tens of meters. Textures indicative of chilling and unidirectional growth may be 

present. Typically, the sequence of layers, if present, and changes in mineral composition can be 

used to define igneous stratigraphy within an intrusion and may provide evidence for recharge 

and open system behavior.

Magmas parental to intrusions that host magmatic sulfide-rich Ni-Cu±PGE deposits, 

although diverse in major element composition, are generally characterized as olivine-bearing, 

high-degree mantle partial melts, which are undersaturated in sulfur and relatively enriched in 

Ni, Cu, and PGE (Keays, 1995; Arndt and others, 2005). In addition, the silicate melts are 

enriched in most of the strongly incompatible elements, such as K, P, Ba, Sr, Pb, Th, Nb, and 

light REE (Zhang and others, 2008). They also tend to have relatively high Os contents 

(≥ 0.03 to 10 ppb), and low Re/Os (< 10) and display trends of Sr-Nd-Pb isotopes that suggest 

interaction between plume-related magmas and ancient cratonic subcontinental lithospheric 

mantle (Zhang and others, 2008). Evidence for local crustal contamination can include high 

Th/Nb and La/Nb ratios, crustal δ34S, high 87Sr/86Sr, strongly negative εNd, and elevated Pb 
14

isotope ratios. Depletion in chalcophile elements (for example, high Cu/Pd, low PGE contents) 

characterizes silicate magmas that have experienced sulfur saturation and separation of an 

immiscible sulfide liquid.

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 3). 

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 rock 

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.

Table 3. Comparison of exposed sedimentary and metamorphic country rocks associated with magmatic 

Ni-Cu±PGE sulfide deposits.

Deposit/Age Related Sedimentary Rocks Related Metamorphic Rocks Metamorphic Grade

Insizwa 

(South Africa)

(183 Ma)

Black shales, calcareous 

nodules Hornfels (0.5 m thick) Contact metamorphism

Wellgreen 

(Canada)

(232 Ma)

Carbonates, volcaniclastics, 

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)

(1,109 Ma)

Quartzites, cherts, 

graywackes, slates Gneisses, metasediments Regional greenschist facies

Kabanga

(Tanzania)

(1,275 Ma)

Sulfide-bearing pelitic rocks 

(footwall); quartzites, 

immature clastic rocks, 

shales (hanging wall)

Muscovite-stauralite-biotite ± 

garnet schists

Contact metamorphism; 

country rocks are midamphibolite 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 amphibolites 

facies
15

Table 3. Comparison of exposed sedimentary and metamorphic country rocks associated with magmatic 

Ni-Cu±PGE sulfide deposits.—Continued

Uitkomst 

(South Africa)

(1,977 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

In a closed system, fractional crystallization of magma with low sulfur contents will 

produce only a small volume of sulfide minerals late in a crystallization sequence. To produce 

sufficient sulfide to form a deposit, an external mechanism is needed to induce sulfur saturation 

early in the crystallization 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 or by selective uptake by magma of 

externally derived sulfur as a vapor or fluid phase from surrounding country rocks; or (2) 

assimilation of felsic wall-rocks during magma ascent, thus increasing the silica content of the 

magma and thereby reducing sulfur solubility and inducing sulfur saturation (Lightfoot and 

others, 1984; Hoatson and others, 2006). For some deposits, both mechanisms may have played a 

role. Li and others (2009) 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. 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 the likelihood of sulfur saturation (Arndt, 2005).

The effect of metamorphism and deformation on magmatic sulfide deposits can be 

manifested in several ways. First, metamorphism of the country rocks due to emplacement of 

intrusions may result in the development of skarns and contact aureoles (for example, Noril’sk, 

Uitkomst). Also, post-emplacement cataclastic metamorphism may remobilize, and possibly 

disperse, sulfide ores, as in the development of matrix-brecciated sulfides at Pechanga (Naldrett, 

2004) or the remobilization of sulfides along shear zones as at the Kabanga deposit (Maier and 

others, 2010). Finally, deformation may alter the orientation of a deposit.

The physical nature of country rocks may also influence intrusion emplacement styles 

(Naldrett, 2004). Continental crust is typically 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). For example, differences in metamorphic grade between 

packages of rocks may provide a structural contrast or weakness that can allow for emplacement 

of an intrusion along those structural breaks. One example would be emplacement of intrusions 

at Jinchuan along a marble/gneiss boundary. In addition, 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). 
16

Theory of Deposit Formation

The formation of magmatic Ni-Cu±PGE sulfide deposits is a consequence of normal 

crystallization of mafic and ultramafic magmas. However, although separation of an immiscible 

sulfide liquid is a normal product in the crystallization of most mafic-ultramafic systems, usually 

only a small quantity of sulfide is found. This is because of limitations on the abundance of 

sulfur in most magmas and because separation of a sulfide liquid usually is late in the 

crystallization history when silicate minerals are abundant. The typical result is a small amount 

of disseminated sulfide minerals among silicate minerals, generally insufficient in quantity and 

metal tenor to be economic. 

There are four critical aspects to the formation of magmatic Ni-Cu±PGE sulfide deposits 

(Naldrett, 2004):

1. Generation of a metal-bearing mafic or ultramafic parental magma through partial 

melting in hotter-than-normal parts of the mantle. These magmas generally represent 

high degrees of partial melting (> 25%), have relatively high MgO contents, and are 

sulfur-undersaturated during their rise from the mantle and emplacement into the crust 

(Keays, 1995). Such magmas are a common component in LIPs where picrites and highMgO basalts are generally present particularly early in the magmatic history.

2. The magma interacts with its crustal wall-rocks during ascent and staging in subcrustal 

magma chambers. Such interaction can form hybrid or contaminated magma and 

commonly results in incorporation of crustal sulfur. Both processes can result in driving 

the magma to sulfur saturation and the generation or segregation of an immiscible 

sulfide liquid (Li and others, 2001). For the efficient accumulation of massive sulfide, it 

is important that sulfide saturation and sulfide liquid segregation occur early in the 

crystallization sequence before abundant silicate minerals crystallize and trap the 

sulfides. In addition, early sulfide saturation is necessary to form typical nickel sulfide 

ore. This is 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 (Naldrett, 2004).

3. The resulting immiscible sulfide liquid has an opportunity to interact dynamically with a 

much 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 (Mungall, 2005). The Ni, Cu, 

and PGE contents of many ores are much 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, sulfide liquid must have interacted 

with, and extracted ore metals from, a much larger volume of silicate magma. Such 

upgrading is enhanced in magma conduit systems where sulfide can interact with new 

pulses of undepleted magma (Maier and others, 2001). However, exactly how sulfide 

liquid and new pulses of magma interact is not clear and may vary from deposit to 

deposit.

4. Finally, metal-rich sulfide liquid becomes concentrated in a restricted locality in a 

quantity sufficient to form an economic deposit (Maier and others, 2001). Structural 

traps, where the flow velocity of magma is reduced, appear to be important sites for the 

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 
17

sides (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 the preferential 

concentration of sulfide droplets (Voisey’s Bay; Li and others, 2000).

Exploration and Resource Assessment Guides

Regional geological guides

1. An LIP with evidence of primitive Mg-rich melts and large volumes of tholeiitic 

magmatic rocks occurring on or near the edges of ancient cratons.

2. Province boundaries, rifts, and deeply penetrating faults that can allow for efficient 

transport of magma through the crust.

3. Small- to medium-sized differentiated mafic and (or) ultramafic dikes, sills, chonoliths 

and plug-like intrusions. Deposits are generally not hosted in thick, large-layered 

intrusions.

4. Sulfur-bearing (sulfide and (or) sulfate) crustal rocks.

Local geological guides

1. Clusters of small- to medium-sized, generally mafic-dominated tholeiitic intrusions with 

ultramafic rocks, if present, generally concentrated in the lower parts of the intrusion. 

However, intrusions related to picrites may be wholly or dominantly ultramafic.

2. Massive or layered intrusions with variably textured rocks and (or) magmatic breccias. 

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) 

subophitic-textured rocks.

3. Laterally extensive contact metamorphism and (or) metasomatism of country rocks near 

intrusions.

4. Local evidence of crustal contamination and (or) chalcophile element depletion in 

intrusions and (or) related volcanic rocks. Widespread evidence of contamination and 

(or) chalcophile element depletion may reflect source characteristics or magma 

contamination in the lower crust.

5. 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 

(slow, fast, or turbulent flow) and conduit geometry (changes from narrow vertical 

conduits to subhorizontal broad magma chamber; physical traps) are important factors 

for the precipitation and accumulation of massive sulfide.

6. Evidence for a dynamic, open, periodically replenished magmatic system, such as 

magmatic breccias, reversals in fractionation indicators, and changes in chalcophile 

element contents.

7. 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 
18

or in feeder conduits. Therefore, determination of the base of a given intrusion is an 

important criterion in exploration targeting, particularly in deformed terranes.

8. 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 surrounding sulfide-bearing intrusions should also be 

investigated in strongly deformed terranes.

Exploration methods

1. Exploration using geophysics must be tailored to a study area on the basis of factors 

such as the maximum depth of detection and physical properties of the overburden, 

intrusions, host lithologies, and mineralization. Regional aerial magnetic and gravity 

surveys are used to identify prospective areas of LIPs showing the regional 

characteristics noted above and to determine the extent, geometry, and basal contacts of 

intrusions. At district to local scales, ground magnetics are used to delineate lithologic 

layering and identify small-scale structural embayments and depressions along the basal 

contact of intrusions. Airborne and ground electromagnetics and induced polarization 

are used to identify and delineate conductive sulfides.

2. Stream sediment and soil geochemistry can be useful if intrusions are exposed or are in 

subcrop. Lithogeochemical study of prospective intrusions for S, Cu, Ni, PGE contents, 

as well as Cu/Pd ratio and Mg-number helps to determine stratigraphic variations and 

the occurrence of sulfur-saturation events. 

3. Composition of olivine (Ni content versus Mg-number) can be used as an indicator of 

nickel depletion in magmas from which the olivine has crystallized.

4. Evidence for crustal contamination and (or) sulfur addition can be determined from S, 

O, Sr, Nd, Pb, and Os isotopes; high Th/Nb, and La/Nb ratios, and enriched light REE 

and alkali contents; hybrid rocks; and xenoliths of country rock and (or) xenocrysts.

5. 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.

6. Closely spaced drilling, particularly of basal contacts and feeder conduits, as well as 

down-hole geophysics, such as time-domain electromagnetics, mise-a-la-masse 

methods, and radio imaging methods can help direct exploration toward the most 

mineralized parts of the intrusion (Diavok and others, 2002).

Geoenvironmental Features

The geoenvironmental features of magmatic Ni-Cu±PGE sulfide deposits arise from the 

mineralogy of the ore deposits and their mine wastes, the mining and ore-processing methods 

used to exploit them, and the local hydrologic and climatic characteristics of individual deposits. 

Common gangue minerals associated with mine waste from magmatic Ni-Cu deposits include

olivine, orthopyroxene, clinopyroxene, plagioclase, magnetite, ilmenite, quartz, chlorite, and 

minor amounts of calcite or dolomite. Common sulfide minerals include pyrrhotite, pyrite, 

chalcopyrite, and pentlandite. 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 of carbonate minerals in their 
19

mineralization, alteration, and host rock assemblages. Acid generated by the oxidative 

weathering of pyrrhotite can attack associated ore sulfide minerals and gangue silicate minerals, 

releasing a variety of potentially toxic elements, particularly Fe, Cu, Ni, Co, and Mn, and to a 

lesser extent Al, Cd, Zn, Cr, and Pb, to solution or to secondary solid phases (McGregor and 

others, 1998; Johnson and others, 2000).

In terms of pre-mining geochemical baselines, magmatic Ni-Cu±PGE sulfide deposits are 

geochemical anomalies that can express themselves in groundwater, surface water, and soil. 

Near the buried Eagle deposit, northern Michigan, maximum concentrations of Fe (190 μg/L), Ni 

(59 μg/L), and Zn (88 μg/L) in groundwater near the deposit reflect the presence of the ore body, 

whereas all concentrations of Cu (<5 μg/L) and Co (<10 μg/L) are below detection limits. In 

contrast, the concentrations of Cu, Ni, and other trace elements in surface waters are 

indistinguishable from background due to the buried nature of the deposit (Kennecott Eagle 

Minerals, 2006). The concentrations of various metals, such as Cu and Ni, in soils are also 

indistinguishable from regional background. In contrast, the geochemical signatures in deflation 

lag and soil samples in the vicinity of the outcropping Babel and Nebo deposits, central 

Australia, have anomalous concentrations of Ni, Cu, Pt, Pd, and Au (Baker and Waugh, 2005).

The dominance of pyrrhotite in mine tailings results in waste material with a high acidgenerating 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 timeframes from olivine 

and pyroxene. Massive sulfide ore from the Eagle deposit is typified by a mean acidic netneutralizing potential (NNP) of -1111 kilograms (kg) CaCO3 per tonne (t) of waste; the mean 

NNP of semi-massive sulfide ore at Eagle is -378 kg CaCO3/t (Kennecott Eagle Minerals, 2006). 

The NNP of mineralogically similar tailings from Sudbury is similar (McGregor and others, 

1998; Johnson and others, 2000).

Mine drainage from abandoned and active mines shows a strong link with the underlying 

geology of the mineral deposits. Johnson and others (2000) documented the generation of low 

pH (as low as 3.0) waters with dissolved concentrations of as much as 9.8 g/L Fe, 24 g/L 

SO4,1,130 mg/L Al, and 698 mg/L Ni in groundwater in the tailings pile at the Nickel Rim mine, 

Sudbury. As much as 3.5 mg/L Cu and 2.5 mg/L Co were also measured. Similarly, for 

groundwater and surface water in the vicinity of the Hitura mine, western 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.

The geochemistry of soils near developed magmatic Ni-Cu±PGE sulfide deposits can 

reflect the combined influences of mining and smelting activities. Surrounding the extensive 

mining district at Sudbury, 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 (Dudka and others, 1995). Likewise, residential soils in the 

Noril’sk mining district have significant maximum concentrations 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).

Ecosystem risks associated with magmatic Ni-Cu±PGE sulfide deposits center around 

acid-mine drainage and the associated leaching of metals. Metal concentrations described above 

all exceed aquatic ecosystem guidelines. Human-health risks include drinking water and soils. 

The maximum concentrations in surface water and groundwater for Ni, Cu, Pb, and Zn all 
20

exceed drinking water guidelines. Likewise, the maximum concentrations of Cu and Ni exceed 

residential soil guidelines.

Climate can play an important role in determining the behavior of mine waste associated 

with magmatic Ni-Cu sulfide deposits. Meldrum and others (2001) found that permafrost 

conditions, 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.

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