JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 5
PAGES 1021^1039
2007
doi:10.1093/petrology/egm009
Crustal Contamination of Picritic Magmas
During Transport Through Dikes: the Expo
Intrusive Suite, Cape Smith Fold Belt,
New Quebec
JAMES E. MUNGALL*
DEPARTMENT OF GEOLOGY, UNIVERSITY OF TORONTO, 22 RUSSELL STREET, TORONTO, ON, M5S 3B1, CANADA
RECEIVED SEPTEMBER 29, 2005; ACCEPTED FEBRUARY 21, 2007
ADVANCE ACCESS PUBLICATION APRIL 25, 2007
The Proterozoic Expo Intrusive Suite comprises a series of mafic to
ultramafic intrusions crosscutting the Povungnituk Group of the
Cape Smith Fold Belt in New Quebec. The intrusions are mainly in
the form of blade-shaped dikes that penetrate a sediment-rich horizon
in the middle of the Beauparlant Formation and terminate downward against massive basalts of the lower Beauparlant Formation.
Significant accumulations of magmatic sulfide occur at the basal
terminations of the dikes. At stratigraphic levels above the
Beauparlant Formation the intrusions appear as broad dikes or sills
within the Nuvilik Formation, below the mineralized lava flows and
subvolcanic intrusions of the Raglan Formation.The Expo Intrusive
Suite and the mineralized bodies of the Raglan Formation are
probably coeval and comagmatic with the overlying Chukotat
Group. Post-emplacement folding has exposed the Expo Intrusive
Suite over about 5 km of structural relief, revealing the basal sulfide
concentrations where dike segments terminate on the flanks of
anticlines. The parent magma as preserved in chilled margins and
narrow dikes was a picrite containing 17 wt % MgO (i.e. komatiitic basalt) and slightly depleted in Th, U and Nb relative to
middle and heavy rare earth elements. The compositions of ultramafic cumulate rocks within the intrusions are strongly enriched inTh,
U and Nb relative to heavy rare earth elements, reflecting assimilation of the enclosing basalts and metasediments. Modeling of the
assimilation process suggests that the picritic magma was capable
of assimilating masses of basalt or sediment up to 50% of the
original mass of magma. Assimilation of 10% of a mixture of
basalt and sediment caused the magma to become sulfide-saturated,
and was accompanied by the crystallization of masses of ultramafic
cumulates approximately equal to the mass of rock assimilated. The
presence of dikes whose chilled margins resemble uncontaminated
*Corresponding author. Telephone: 1416 978 2975. Fax: 1416 978 3938.
E-mail: [email protected]
primary magmas but that contain abundant cumulates recording
wholesale assimilation of host-rocks indicates that the process
of assimilation and fractional crystallization required to produce
continental tholeiites from picritic parent magmas may not require
the presence of long-lived magma chambers, but can occur during
transport along dikes and reaction with wall-rocks.
KEY WORDS: komatiite; Expo Intrusive Suite; assimilation; fractional
crystallization; sulfide mineralization
I N T RO D U C T I O N
The processes by which primitive high-MgO basalts
undergo assimilation of crustal rocks and concomitant
fractional crystallization (i.e. AFC) are generally inferred
from studies of the compositions of lavas (e.g. Wooden
et al., 1993; Lightfoot & Hawkesworth, 1997), and sometimes also from studies of shallow subvolcanic intrusive
rocks (Arndt et al, 2003). The AFC process is commonly
assumed to occur in a mid-crustal staging chamber, which
is then tapped to feed eruption of lava flows (e.g. O’Hara &
Mathews, 1981), although in many cases the inferred AFC
chamber is not actually observed. In this study the structure and petrology of a series of upper crustal intrusions
collectively termed the Expo Intrusive Suite (after the
Expo^Ungava Ni^Cu^Pt^Pd^Au deposit at their centre)
are interpreted to represent a complex system of upper
crustal conduits in which AFC processes took place.
These observations provide important constraints on the
processes that modify the compositions of mafic magmas
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JOURNAL OF PETROLOGY
VOLUME 48
as they might migrate through the crust, including those
that lead to the generation of economically significant concentrations of magmatic sulfide.
R E G I O N A L G E O L O GY
The Expo Intrusive Suite was emplaced into
Paleoproterozoic supracrustal rocks of the Cape Smith
Fold Belt of the Trans-Hudson Orogen (Fig. 1) in northern
Quebec at 18827 13 Ma (Randall, 2005). The following
interpretation of the regional geological history is based
on the results of recent mapping by the author and previously published reviews of available geochronological
and structural data (Picard et al., 1990; Lucas & St-Onge,
1992; St-Onge et al., 1992; Picard, 1995; Randall, 2005).
The Cape Smith Fold Belt can be subdivided into
allochthonous northern and parautochthonous southern
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domains. These two domains had contrasting metamorphic and tectonic histories, until they were amalgamated during the climax of the Trans-Hudson Orogeny at
c. 1838 Ma. The southern domain, comprising the
Povungnituk and Chukotat Groups, appears to represent
a long-lived passive continental margin that laps onto the
Archean Superior Province which existed for over 150 Myr.
The northern domain, comprising the Spartan, Parent,
and Watts Groups, has been interpreted as a series of fragments of an island arc and its underlying oceanic basement
(Scott et al., 1989, 1992) that were accreted during terminal
collision of the Superior Province with the Archean Rae
Province to the north. The Narsajuaq arc to the north is
interpreted to represent the plutonic roots of the arc and
post-collisional felsic intrusions.
The geology of the studied portion of the Southern
domain is shown in Fig. 2. This map was compiled at a
Fig. 1. Location map. The map shows a portion of northern Quebec, including the Cape Smith Fold Belt and the northern tip of the Labrador
Trough. The approximate positions of the mineralized Raglan Formation and the Expo Intrusive Suite are shown.
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MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
1023
Fig. 2. Geological map of the central part of the Expo Intrusive Suite. The area shown includes all of the deposits in advanced stages of exploration, but the Expo Suite is known to extend
at least another 50 km westward and 20 km eastward.
JOURNAL OF PETROLOGY
VOLUME 48
scale of 1:20 000 using derivative products from a highresolution airborne geophysical survey (Aerotem total
magnetic field, conductance; July 2003), a 1:5000 scale
map of the Expo deposit area by the author, and numerous
traverses conducted by the author in 2002, 2003 and 2004
while mapping onto 1:20 000 scale air photographs or
Ikonos satellite images, using global positioning satellite
(GPS) data for navigation. Features of interest such as
dikes or sills of the Expo Intrusive Suite were followed
along their entire lengths, and all areas of outcrop in
these areas were walked and prospected by the author.
Major formational conductors were drawn from the
Aerotem survey results and verified in many instances in
the field as graphitic argillites commonly associated with
carbonate-, silicate- or sulfide-facies iron formations.
Using the formational conductors as key marker horizons,
the regional stratigraphy was divided into several units
that can be correlated with the recognized stratigraphic
column for the region (Fig. 3).
At the base of the succession, the Dumas Formation
(also referred to separately by some as the Lamarche
Group) of the Povungnituk Group represents a clastic
wedge with a maximum thickness of 3400 m. The Dumas
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Formation is an autochthonous to parautochthonous
assemblage of ferruginous sandstone, conglomerate,
quartzite and iron formation, overlain by progressively
finer-grained wackes and rare carbonate metasediments,
which unconformably overlies the Superior Province
basement. Portions of the unconformable contact with
Archean basement rocks are faulted, presumably as a
result of thrust imbrication during the closure of the
ocean basin during the Trans-Hudson Orogeny.
The Dumas Formation is overlain by the predominantly
basaltic Beauparlant Formation and the sedimentary
Nuvilik Formation. The Beauparlant Formation comprises
2^3 km of tholeiitic basalts with ocean island basalt
(OIB) affinity (Modeland et al., 2003) intercalated with
volcaniclastic and locally graphitic siltsone horizons and
minor carbonate metasediments. A mafic sill in the upper
Dumas Formation, dated at 2038 þ 4/2 Ma (Machado
et al., 1993), is considered to be comagmatic with the volcanic rocks of the Beauparlant Fm. A diorite intrusion crosscutting pillow basalt near the top of the Beauparlant Fm
has been dated at 1991 2 Ma (Machado et al. 1993). The
volcanic activity that produced the Beauparlant
Formation is, therefore, constrained to have occurred
Fig. 3. Stratigraphic column. Ages are from the following sources: (1, 2) Machado et al. (1993); (3) Parrish (1989); (4) Randall (2005); (5) Parrish
(1989); (6) Wodicka et al. (2002); (7) St Onge et al. (1992). The column is drawn to illustrate the coeval and probable consanguinous nature of the
Raglan Formation and the Expo Intrusive Suite, as discussed later in the text. Approximate extents of dated intrusive rock suites are indicated
with shaded symbols. The Expo and Raglan suites are coloured darker grey.
1024
MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
between 2038 and 1991Ma. The presence of a clastic sedimentary sequence 43 km thick beneath the lavas of the
Povungituk Group indicates that subsidence occurred
before volcanic activity commenced, allowing this to be
classified as a non-volcanic rifted margin (McKenzie &
Bickle, 1988; White & McKenzie, 1989) rather than a
volcanic rifted margin as has previously been suggested
(Hynes & Francis, 1982; Francis et al., 1983). The
Beauparlant Fm is overlain by a succession of deep-water
sediments of the Nuvilik Fm, including greywackes,
cherts, and graphitic argillites and rare pebbly quartz
arenites. Elsewhere in the Cape Smith belt, the Nuvilik
Formation overlies alkaline volcanic rocks of the Cecilia
Formation, which have been dated at 19586 þ 31/27 Ma
(Parrish, 1989).
The Beauparlant Fm is interpreted here to be a single
sequence 3 km in true original thickness, and to comprise
three distinct members that are preserved in their
original stratigraphic sequence everywhere in the map
area. This stratigraphic column can also be applied to the
Povungnituk Fm for distances of more than 30 km east and
west of the map area. The lower Beauparlant Fm consists
of sheets and pillowed flows of basalt. In much of the map
area this unit is nearly flat-lying and consequently has little
topographic expression. The middle Beauparlant Fm is
distinguished by the presence of regionally extensive formational conductors associated with distinctive magnetic
highs at its lower and upper contacts with the lower
and upper Beauparlant Fm. Outcrop is scarce within the
area of the middle Beauparlant Fm, and it was mapped
as metasediment in the past. Where it is encountered in
outcrops along glacial outwash channels and in diamond
drill cores it is found to be largely composed of basaltic
sheet flows and pillowed basalts with abundant but
volumetrically subsidiary intercalated metasediments.
The metasediments include carbonates, carbonatecemented sandstones, siltstones, graphitic argillites, silicate
iron formation, chert, and turbiditic beds of quartz arenite.
There is no indication anywhere that that the middle
Beauparlant Fm is in fault contact with the underlying
lower Beauparlant Fm. The magnetic and conductive
metasedimentary horizon used to separate the lower and
middle Beauparlant Fm is continuous throughout the map
area, and where it is observed in outcrop it shows no sign of
the intense deformation expected along a crustal-scale
shear zone.
The upper Beauparlant Fm consists of sheet and pillowed basalt flows with only very minor quantities of
graphitic interflow sediment. Much of what has been
mapped in the past as gabbro sills within the Beauparlant
Fm is actually a series of thick basaltic lava flows with
vesicular chilled flow tops and highly distinctive columnar
joints produced during cooling at the sea floor. As a
result of the scarcity of interflow sediments, the upper
Beauparlant Fm forms highly resistant ridges throughout
the map area.
The Nuvilik Fm is poorly exposed, occurring in a basinal structure in the north of map area. The predominant
lithology in the Nuvilik Fm is laminated to thin-bedded
siltstone. Other units identified are minor carbonates, conglomeratic quartzite and two horizons of graphitic argillite
that form strong conductors easily traced in the airborne
electromagnetic survey data. Where it could be observed,
the contact between the uppermost pillow basalts of the
Beauparlant Fm and the base of the Nuvilik Fm was invariably seen to be conformable. There is no evidence for a
thrust fault at or near the base of what is here classified as
the Nuvilik Fm, and no reason to believe that it is tectonically transported metasediment of the Dumas Formation
as has been proposed by Lucas & St-Onge (1992).
The Nuvilik Formation is overlain north of the map area
by the predominantly volcanic deep-water assemblage of
the Chukotat Group. The Chukotat Group is about 5 km
thick, and comprises several cyclic volcanic sequences
with compositions ranging from komatiitic basalt with
MgO contents between 19 and 11%, through pyroxenephyric tholeiitic basalts, to plagioclase-phyric basalts that
show some compositional resemblance to normal midocean ridge basalt (N-MORB). The volcanic rocks of the
Chukotat Group are intercalated with fine-grained siliciclastic sediments including graphitic argillites. The uppermost Nuvilik Formation and the lower Chukotat Group
contain ultramafic bodies of the Raglan Formation, which
host the Raglan Ni^Cu^PGE (platinum group element)
deposits and are generally interpreted to represent feeders
and incised lava channels coeval with the Chukotat Group
(Giovenazzo et al., 1989; Le¤vesque & Lesher, 2003).
The contact between the upper Nuvilik Formation and
the base of the Raglan formation is locally faulted but
generally conformable, and has been interpreted as being
essentially in place (C. M. Lesher, personal communication, 2005). Intrusive rocks near the top of the Nuvilik
Formation, considered to be comagmatic with the
Chukotat Group, have been dated at 1918 9 Ma by
Parrish (1989). However, the Chukotat suite is intruded
near its top by a gabbroic sill dated at 1870 Ma (St-Onge
et al., 1992) and a gabbro intrusion belonging to the
Raglan Formation has been dated at 1887 þ39/11Ma
(Wodicka et al., 2002). It has been argued that the
Raglan intrusions served as feeders for the komatiitic
basalts of the Chukotat Group, and that the ultramafic
bodies of the Raglan Formation might be largely extrusive
(e.g. Barnes et al., 1982; Be¤dard et al., 1984; Barnes &
Barnes, 1990).
The strata of the Povungnituk Group were intruded
at 18827 13 Ma (Randall, 2005) by a series of mafic to
ultramafic bodies of the Expo Intrusive Suite, which are
described in detail below. The Raglan Formation,
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JOURNAL OF PETROLOGY
VOLUME 48
Chukotat Group, and Expo Intrusive Suite are all of similar age and might be considered a single magmatic suite.
Continental collision between the Rae and Superior
Provinces occurred at c. 1838 Ma (Lucas & St Onge, 1992;
St Onge & Lucas, 1992; Randall, 2005), causing the
development first of 1^2 km wavelength tight to isoclinal
upright folds with WSW^ENE-trending axes. The early
folds were refolded into 10 km wavelength open folds
along NW^SE-trending axes. The earlier folding event
was largely confined to the area north of the outcrop area
of the Expo Intrusive Suite, whereas the final folding event
affected both the Expo Suite and its host supracrustal
rocks, leading to gentle warping of the dike-like bodies of
the Expo Suite. Local folding of the dikes around vertical
axes is well developed in the area immediately to the east
of the Expo^Ungava deposit, in the centre of the synclinorium that surrounds it (Fig. 2).
The map in Fig. 2 is somewhat different from what has
been reported previously (e.g. St Onge & Lucas 1993),
possibly because of the greatly increased resolution at
which mapping was done in the present study, aided by
the availability of high-quality and high-resolution airborne geophysical data. The principal difference between
the present stratigraphic interpretation and those previously published is that there appears to be no evidence
whatsoever in the field for the existence of the thrust
faults proposed by St Onge and coworkers to cause structural repetitions of the Beauparlant Fm basaltic pile.
Indeed, it would be exceedingly difficult to justify the
need for a thrust fault anywhere in the map area on
purely structural grounds. A key consideration in any
attempt to place thrust faults with the area of Fig. 2 is that
horizons along which the faults have been proposed, corresponding to the tops of the Lower and Upper members of
the Beauparlant Formation as recognized here, are crosscut in several places by dikes of the Expo Intrusive Suite
that are demonstrably not offset in any way. It is impossible
to reconcile this observation with the proposition that
faulting along these interfaces occurred during continental
collision several tens of million years after the emplacement of the Expo Suite at 18827 Ma. There is no evidence
for the existence of any significant thrust faults anywhere
in the current map area, and as there may not be any
such faults at the base of the Chukotat Group, the entire
sequence from the base of the Beauparlant Formation to
the top of the Chukotat Group may actually be preserved
in place.
There are several places in the map area where metabasalts and metasediments of the Beauparlant Formation are
intensely deformed along structural corridors up to 100 m
wide, and in early stages of mapping these were interpreted as major thrust faults. When mapping was complete, these areas were found to correspond to the hinge
zones of isoclinal folds. A good example occurs in the fold
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just north of the Hilltop^TK^Mesamax NW segment of
the Expo Intrusive Suite. The intense deformation associated with this synclinal structure does not extend past
the closure of the fold in which it occurs, and it is therefore
not considered to be a fault of regional significance.
A shear zone such as this, if it was encountered during
a north^south mapping traverse, would be incorrectly
identified by any geologist as a major fault; it can only be
correctly interpreted in the context of detailed mapping
supported by high-quality geophysical data.
E X P O I N T RU S I V E S U I T E
Localities
The Expo Intrusive Suite has been the target of exploration for Ni^Cu^Co^PGE mineralization since 1957, and
has seen intense activity since 2001. The locations of several
mineral occurrences or deposits are shown in Fig. 2. The
Expo^Ungava deposit contains net-textured to massive
magmatic sulfide hosted by peridotite in a large troughlike body that cuts metasediments of the Nuvilik
Formation. The Mesamax NW, TK, Tootoo, Mequillon,
and Vaillant deposits comprise net-textured to massive sulfides hosted by pyroxenite and peridotite at the bases of
shallowly plunging trough-shaped bodies at the terminations of dikes. Other localities mentioned in the text and
illustrated in Fig. 2 are the Hilltop dike, a relatively
narrow apophysis of the main Expo dike system between
TK and Expo^Ungava, and the Snow Owl pipe, which is
an ovoid outcrop of dunitic serpentinite SE of the main
Expo^Ungava deposit area.
Lithologies
In the following discussion, the metamorphosed igneous
rocks of the Expo Intrusive Suite are referred to using
names that would be appropriate terms for their protoliths.
The addition of the prefix ‘meta’ would make the discussion awkward, but its use is implied throughout whenever
reference is made to the intrusive rocks in their original
states. The application of protolith names to metamorphosed ultramafic rocks becomes especially problematic
for the term pyroxenite, as discussed below; the use of the
field term pyroxenite for amphibolites, however, is so
deeply entrenched in the local terminology that it is difficult to avoid.
Gabbronorite
Much of the Expo Suite is composed of dikes of mediumgrained olivine melagabbronorite spanning widths ranging
from several meters up to several hundred meters. Locally
this rock is relatively unaltered, with white saussuritized
feldspar laths surrounded by partially uralitized orthopyroxene and clinopyroxene. Where the lower greenschistfacies metamorphic effects are more pervasive, the
gabbronorite consists of tremolite after clinopyroxene,
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MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
talc, anthophyllite and serpentine after orthopyroxene and
olivine, and chlorite, clinozoisite or a fine-grained nearly
opaque mixture of albite, hydrogrossular, zoisite and sericite replacing plagioclase. Minor chromite appears as
submillimeter-sized euhedra with magnetite overgrowths,
commonly within chlorite^serpentine pseudomorphs of
olivine. Minor amounts of apatite, magnetite and mica
are observed in pockets of late-crystallized melt. There is
widespread development of late, poikilitic primary magmatic hornblende, which gives the rock numerous brown
blotches that are easy to see in drill core and lend it the
field name hornblende-gabbro. At chilled margins the gabbronorite fines down to an aphanitic brownish-black rock
resembling basalt but showing a darker colour than the
basalts of the Beauparlant Fm. In a few rare examples
where the gabbronorite forms dikes tens of meters wide,
these show columnar jointing perpendicular to their contacts. In the interiors of very wide dikes (hundreds of
meters wide) the gabbronorite can be very coarse-grained,
with pyroxene oikocrysts up to 2 cm across
(commonly pseudomorphed by tremolite) enclosing
serpentinized olivine chadocrysts.
Within the thickest part of the intrusive complex near
the Expo^Ungava deposit, the gabbronorite is locally differentiated to form a leucogabbro that commonly shows
variable texture from mafic pegmatite to fine-grained
gabbro over distances of several centimeters. The pegmatitic patches may contain blebby sulfide and have been found
in thin section to contain rare zircon euhedra [dated at
18827 13 Ma by Randall (2005)].
Pyroxenite
In the study area, any rock predominantly composed of
tremolite^actinolite has traditionally been called pyroxenite in the field. The term ‘pyroxenite’ is used to describe
fine-grained rocks resembling quenched liquids and also
to describe adcumulate rocks originally composed almost
entirely of olivine and pyroxene with minor interstitial
melt or hornblende and plagioclase; that is, olivine melagabbronorites, olivine websterites, and websterites all tend
to be grouped together. On freshly broken surfaces these
rocks are generally light green and show many reflective
cleavage surfaces from the abundant tremolite crystals.
Primary microtextures are effectively erased by the uralitization of the primary pyroxenes and the growth of extensive fibrous epitaxial tremolite that replaces intercumulus
material. Primary plagioclase contents are easy to underestimate because of pervasive replacement of pyroxene
and plagioclase by tremolite and chlorite.
Peridotite
Peridotite is completely serpentinized, to form a massive,
highly resistant rock that weathers deep maroon to
brown. The metamorphosed peridotite is composed of fine
intergrowths of serpentine, talc, anthophyllite and chlorite
forming pseudomorphs after olivine and orthopyroxene,
dotted with grey tremolite pseudomorphs after clinopyroxene. Poikilitic primary hornblende, very fine-grained
chlorite, abundant chromite, and traces of sulfides are
commonly present in the intercumulus spaces of what generally appears to be an orthocumulate to heteradcumulate
texture. Oikocrysts of fresh clinopyroxene are occasionally
present. The peridotites are generally strongly magnetic
because of the presence of abundant microscopic magnetite crystals formed during serpentinization of iron-bearing
olivine. In some places the peridotites have been extensively altered to a talc- and carbonate-rich assemblage,
that weathers to a rusty brick-brown gravelly surface and
has very low magnetic susceptibility. As a result, there are
locally very complex patterns of magnetic susceptibility in
areas that are entirely underlain by peridotite, making
field checking essential for the correct delineation of ultramafic bodies using geophysical responses.
Dunite
The interiors of the ultramafic bodies of the Expo Suite
commonly consist of a serpentine^chlorite^magnetite rock
that is classified as dunite. Dunite weathers to a yellowishgreen or brown patina and lacks the hackly surface produced by resistant tremolite crystals in the peridotites. On
fresh surfaces it is fine-grained and deep green to black,
with a flinty, almost conchoidal fracture. Relict primary
orthocumulate texture is commonly visible in thin section,
as a result of the presence of minor quantities of primary
intercumulus hornblende and intercumulus chlorite possibly replacing plagioclase. Dunite has a high magnetic susceptibility, and forms prominent highs in the total
magnetic field.
Distribution
Map scale
The entire map area is traversed by numerous mafic and
ultramafic intrusions of the Expo Suite (Fig. 2), most of
which are sharply discordant, more or less tabular bodies
and are therefore classified as dikes. Locally these bodies
are highly irregular in form or appear to be more concordant in nature (e.g. at the Expo^Ungava deposit), and in at
least one locality (Snow Owl occurrence) they are ovoid in
plan view, entirely surrounded by metasedimentary hostrocks, and are therefore interpreted to represent pipe-like
conduits. All known base- and precious-metal sulfide
mineralization in the region is associated with these intrusions, usually along their margins or internal autointrusive
contacts. The majority of Expo Suite intrusions have trends
approximately ENE^WSW, apparently cutting across the
folded Povungnituk Group. However, the Expo dike in the
area of the Expo^Ungava deposit itself appears to have
been folded about vertical axes during NE^SW-directed
compression.
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JOURNAL OF PETROLOGY
VOLUME 48
The schematic cross-section in Fig. 2b shows the likely
subsurface configuration of the Expo Suite, based on interpretation of the 5 km structural relief afforded by the
presence of the late NW^SE-trending folds. The reasoning
behind this interpretation is given at more length in the
discussion below.
Contact relations
The Expo Suite forms a complex of cross-cutting gabbronoritic dikes, ultramafic dikes, dunite pipes, and sill-like
peridotite massifs within the Nuvilik Fm in the area surrounding the Expo^Ungava deposit (Fig. 2). The magma
was, therefore, able to intrude the metasedimentary rocks
easily in a variety of orientations. A large sill-like peridotite body extends southeastward from the Cominga
area along the base of the Nuvilik Fm, whereas a pair of
sub-parallel dikes of gabbronorite and ultramafic rock
hundreds of meters wide trends east^west across the
entire doubly plunging synclinorium occupied by the
Nuvilik Fm. The Snow Owl dunite pipe appears to be
the surface expression of a steeply inclined or vertical
cylindrical mass of dunite. A very deep vertical extent is
implied by a broad, high-amplitude magnetic anomaly
associated with this relatively narrow structure.
The trough-like form of the mineralized peridotite body
at Expo^Ungava is illustrated in Fig. 4. Here the Expo
Suite forms at least two and possibly more stacked troughlike intrusions that are roughly concordant with the enclosing sediments. Immediately outside the Expo^Ungava
deposit there is no known base to the intrusions, which
may truly be dikes with kilometer-scale vertical extents;
the mineralized sills hosting the deposit therefore
appear to be narrow horizontal offshoots from a generally
dike-like ultramafic intrusion. On the other hand, the
Fig. 4. Cutaway view from the NE of a portion of the mineralized
peridotitic trough structure containing the Expo^Ungava deposit.
This trough forms the northern branch of the large peridotitic massif
at the point indicated ‘Expo Ungava’ in Fig. 2. The host metasediments dip gently to the SW in the deposit area. The presence of two
distinct ultramafic bodies separated from the one that crops out by
partial or complete screens of metasediment should be noted.
The structure remains untested at depth.
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mineralized troughs could be bounded by steeply dipping
reverse faults along which the centre of the synclinorium
has been elevated as a result of tectonic extrusion during
NE^SW-directed compression. In this case the Expo^
Ungava deposit might be expected to continue at greater
depth along the bases of the ultramafic bodies to its east
and west.
Where the Expo Suite intrudes the basalts of the upper
Beauparlant Fm, the contacts between the gabbronorite
dikes and the enclosing basalts are invariably sharp and
planar, and where both sides of the intrusion can be
observed they are parallel. In these examples there is a
chilled margin on the gabbronorite, and the basalt shows
textural evidence for metamorphism to the pyroxene hornfels facies over short distances (51m).
Where the Expo Suite intrudes the basalt and metasediments of the middle Beauparlant Fm, the wall-rocks are
generally strongly metamorphosed. The host-rocks commonly show signs of incipient melting, such as the presence
of pockets of pegmatite. In these examples the intrusive
rock is a coarse-grained pyroxenite that has evidently not
been chilled against the enclosing wall-rocks, implying
that the magma was passing by the contact at a sufficient
rate to heat it to temperatures well above the solidus temperature of wall-rock. Contacts with carbonate sediments
are occupied by calc-silicate hornfels zones. Where the
intrusions encountered beds of quartzite in the predominantly semipelitic siltstone strata they were less able to
assimilate it because of its refractory, monomineralic
nature; in these situations, intrusive breccias were formed
or individual beds of quartzite were left intact in their original attitudes but surrounded by ultramafic rock.
Where the Expo Suite intrudes basalt of the lower
Beauparlant Fm at the Tootoo, Mesamax, TK, and
Mequillon deposits it forms dikes whose trough-like basal
terminations are lined with gabbronorite that locally
shows a gradational contact with the enclosing basalt. The
exact point of contact is commonly difficult or impossible
to determine, because the wall-rock basalt has been heated
and metamorphosed to a coarse-grained pyroxene hornfels
or sanidinite-facies assemblage closely resembling the
medium-grained gabbro of the intrusive body. This hybrid
gabbroic marginal facies of the dikes grades into pyroxenite followed inward toward the core of the dike by peridotite and dunite. In some instances dunite clasts tens of
centimeters across are suspended within the peridotite
matrix. The general form of these structures is illustrated
in Fig. 5, which shows a perspective cutaway view of
the mineralized ultramafic dike associated with the
Mequillon deposit; this is representative of the Mesamax,
TK, Tootoo, Mequillon and Vaillant deposits.
It is noteworthy that the lithology and contact relations
of the Expo Suite are very strongly controlled by the
nature of the enclosing rocks (Fig. 2). As individual dike
1028
MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
Appendix at http://petrology.oxfordjournals.org). In
subsequent figures these data are compared with the compositions of basalts and ultramafic rocks of the Raglan
Formation (Burnham et al., 1999). Major element concentrations were determined by XRF on fused beads; selected
minor element concentrations were determined by XRFon
pressed powder pellets, and the remainder of the trace element concentrations were determined by solution ICP-MS
after complete sample digestion in closed Teflon beakers.
Data for major elements are reported after recalculation
to anhydrous equivalent compositions. Detection limits
for each element in the whole-rock analyses are shown in
the Electronic Appendix at the top of each column.
Classification of Expo Intrusive Suite
Fig. 5. Cutaway perspective view of the mineralized dike at the
Meqillon deposit. The concentric zonation of the ultramafic body
from a narrow gabbronoritic contact facies inward from the vertical
walls through pyroxenite to peridotite should be noted. Net-textured
and massive sulfide are concentrated at the base of the trough, where
peridotite is in direct contact with pyroxene hornfels-facies
metabasalts.
segments are traced across the region, their lithologies
vary systematically with the stratigraphic level at which
they are observed. The intrusions are exclusively gabbroic
within the upper Beauparlant Fm, but are predominantly
peridotite sheathed by pyroxenite within the middle
Beauparlant Fm. The dikes are zoned from pyroxenite to
peridotite or dunite, occasionally with a discontinuous
sheath of gabbro in the lowermost Beauparlant Fm, and
never pass below the base of the lower member of the
Beauparlant Fm. In the Nuvilik Fm the intrusions are
predominantly composed of peridotite and dunite with
subsidiary gabbronorite. Some explanations of these
distinctive contact relations are offered in a subsequent
section discussing the controls on assimilation of
host-rocks by the Expo magma.
L I T H O G E O C H E M I S T RY
Data collection
A total of 118 rock samples from both outcrops and diamond drill cores from the map area were analyzed by
X-ray fluorescence spectroscopy (XRF) and solution
inductively coupled mass spectrometry (solution ICP-MS)
for a suite of major and trace elements at the Geoscience
Laboratory of the Ontario Geological Survey. Samples
came from the narrow dike in the Hilltop area, and from
diamond drill holes in the Mesamax NW, Tootoo, Expo
and TK areas (Fig. 2). Representative data are shown in
Table 1 (the complete dataset is available as an Electronic
As a starting point, it is interesting to consider the compositions of the melagabbronorite and chilled margins or
fine-grained dike rocks assigned to the Expo Intrusive
Suite in Table 1. The compositions of all of these rocks are
similar, and in view of the very fine-grained nature of some
of them they can probably all be considered to represent
the compositions of liquids that have not accumulated
any olivine. Taking the Hilltop dike, for example,
the average of about 17 wt% MgO, 11% Al2O3 and only
07 wt% TiO2 allows them to be classified as picrites at
the boundary between basaltic komatiite and komatiite, of
the Al-undepleted variety. Considering the ubiquitous
association of the melagabbronorite with ultramafic members of the Expo Suite, it thus appears likely that the entire
suite is derived from a komatiitic parental liquid.
Comparison of the Expo Suite with
other magma types
In Fig. 6 the trace element concentrations in a variety of
rocks interpreted to represent the parental magmatic
liquids of the Expo and other suites are shown normalized
to concentrations in the primitive mantle (McDonough &
Sun,1995). The samples of the Beauparlant Fm in this study
are identical to those of Francis et al. (1983), which were
sampled in the Kenty Lake area west of the map in Fig. 2.
The similarity between the olivine-phyric basalts of the
Chukotat Group (Burnham et al., 1999) and the proposed
primitive magma of the Expo Suite is striking, as also is
the difference between the Expo rocks and the enclosing
basalts of the Beauparlant Fm (Mesamax basalt). The
Expo Suite primitive magma (chilled margin) and the olivine-phyric basalts of the Chukotat Group are compositionally almost indistinguishable. The main features of the
Expo Suite trace element signature are a flat heavy rare
earth element (HREE) pattern and slight depletion in the
light REE (LREE), a depletion that becomes increasingly
evident in the concentrations of the large ion lithophile
elements (LILE) U, Th and Nb. On the other hand, the
basalts of the Beauparlant Fm, both from the present
study and from published data, are characterized by
1029
JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 5
MAY 2007
Table 1: Compositions of members of the Expo Intrusive Suite and its host-rocks
Hilltop Dike
Element chill
Mesamax
core
hblgb
Replacement body
dyke
gb
px
pe
if
ss
ba
dun
px
gnrep
gnfr
cpx
opx
wt %
SiO2
4569
4382
4491
4703
4625
4001
4333
4831
5563
4779
4765
5287
5578
TiO2
064
068
064
069
069
050
026
014
085
205
019
025
018
034
030
003
Al2O3
962
1001
1136
1060
1088
754
516
49
1542
1383
400
969
1189
1636
213
178
1278
1286
1172
1379
134
2040
3329
1055
1477
1494
1071
956
991
716
817
Fe2O3*
169
382
3956
442
MnO
022
019
017
022
018
021
015
032
013
022
017
018
019
015
022
019
MgO
1602
1690
1491
1133
1228
1615
2448
18
597
610
2977
2231
1324
899
1635
1895
CaO
944
1000
956
870
1089
799
142
345
392
1205
231
1075
1854
1330
2132
1344
Na2O
050
042
092
219
144
024
004
019
576
205
007
012
019
163
029
K2O
095
011
086
080
011
005
195
008
026
002
n.d.
n.d.
018
—
—
P2O5
004
011
004
004
005
003
001
053
023
020
001
n.d.
n.d.
002
—
—
LOI
355
409
364
353
277
413
665
523
289
176
923
587
129
161
—
—
9946
9914
9873
9891
9895
9726
9838
9891
9945
9928
Total
n.d.
1001
1014
1011
1001
10063
028
9861
ppm
Cr
1264
1212
1039
836
832
1152
3270
506
1081
2018
866
566
343
1277
947
Cu
30
386
133
216
157
3696
955
134
70
480
1090
243
4192
1359
—
—
Ni
584
589
509
329
327
3536
2771
63
36
57
1548
782
1001
933
—
—
Sc
46
46
43
52
58
41
26
19
20
53
32
51
46
51
—
—
V
289
271
271
363
323
207
261
67
100
347
122
192
160
196
—
—
Zn
113
87
144
73
103
140
98
65
258
54
—
—
—
—
97
76
95
78
Y
1406
1636
1327
137
146
Zr
351
366
413
391
377
Nb
17
18
19
23
Th
016
015
023
U
0047
0045
Hf
10
La
Ce
991
335
328
2139
1933
303
72
421
1399
1504
104
114
129
124
—
—
24
15
06
53
124
155
06
04
03
06
—
—
017
028
022
016
437
1018
144
017
n.d.
n.d.
011
—
—
007
0054
008
0071
0043
1186
1684
0333
0026
0013
0012
0031
—
—
11
11
11
11
09
02
11
35
39
03
04
04
04
—
—
192
218
252
237
364
126
118
1529
3498
1461
087
050
056
122
—
—
511
588
629
604
830
351
252
2605
7360
3467
200
132
129
288
—
—
Pr
0813
0942
0939
0946
1183
0553
0343
339
0295
0223
0255
0449
—
—
Nd
417
512
471
490
564
284
166
1364
3466
2165
136
120
110
227
—
—
Sm
143
177
151
158
168
105
039
277
607
512
044
044
039
074
—
—
Eu
0509
0625
0628
0572
0686
0314
0097
031
1389
1718
0077
0121
0255
0372
—
—
Gd
2040
2541
2105
2141
2317
1507
0499
3051
4879
5644
0591
068
0647
1134
—
—
Tb
0349
0459
0371
0373
0397
0271
0083
0462
0667
0857
0099
0119
0124
0204
—
—
Dy
2397
3000
2402
2498
2612
1804
0564
2980
3636
5057
0654
0814
0827
1367
—
—
Ho
0537
0641
0523
0533
0583
0391
0125
0649
0728
1031
0146
0178
0183
0300
—
—
Er
1543
1873
1534
1552
1697
1108
0387
1956
1997
2897
0458
0530
0556
0886
—
—
Tm
0225
0265
0221
0229
0248
0166
0056
0288
0272
0412
0066
0078
0083
0131
—
—
Yb
146
166
140
151
160
110
036
174
170
260
043
051
051
084
—
—
Lu
0217
0242
0216
0228
0246
0162
0051
0247
0238
038
0066
0074
0078
0127
—
—
9277
2561
4753
347
462
477
767
Fe2O3*, total iron calculated as Fe2O3. n.d., below detection limit; —, not determined. Hilltop dike: chill, chilled margin
within 10 cm of contact; core, coarse melagabbronorite 8 m from contact. Mesamax: hblgb, hornblende gabbro; px,
pyroxenite; pe, unmineralized peridotite; if, silicate-facies iron formation; ss, carbonaceous siltstone; ba, interior of basalt
sheet flow (Beauparlant Fm). Replacement body: dun, dunitic orthocumulate; px, pyroxenite replacing gabbronorite;
gnrep, gabbronorite showing incipient replacement; gnfr, unaltered and relatively fresh gabbronorite; cpx, Ca-rich
clinopyroxene; opx, inverted Ca-poor clinopyroxene, now orthopyroxene.
1030
MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
Fig. 6. Trace element abundances in magmatic liquids. The ranges of compositions of several chilled margins of the Expo Intrusive Suite at the
Mesamax NW deposit are indicated by the shaded grey area, compared with the average composition of an isolated dike of melagabbronorite at
the Hilltop occurrence. Also shown are a representative olivine-phyric basalt from the Chukotat suite (Burnham et al., 1999) and the average
composition of basalt of the middle member of the Beauparlant Fm from the Mesamax area. The compositions of ocean island basalts (OIB),
normal mid-ocean ridge basalts (N-MORB) and enriched mid-ocean ridge basalts (E-MORB) are shown for comparison. Whereas the
Beauparlant Fm shows characteristics intermediate between OIB and E-MORB, the Expo and Chukotat magmas show a moderate depletion
in Th, U, and Nb, somewhat less than that shown by N-MORB. The compositions of the Expo and Chukotat suites are almost identical.
strongly LILE-enriched patterns transitional between
enriched MORB (E-MORB) and OIB and conspicuously
lacking the negative Nb anomalies that are characteristic
of continental tholeiites (see Modeland et al., 2003).
DISCUSSION
Evolution of the Expo Intrusive Suite
The principal controls on the evolution of the Expo Suite
are likely to have been the removal or accumulation of
crystals, and the assimilation of host-rocks. Figure 7 shows
cation proportions of Al and Mg, showing the compositions of members of the Expo Suite compared with those
of basalts of the Chukotat and Povungnituk Groups, ultramafic rocks from the Raglan Formation (Katinniq area)
and sediments from the vicinity of the Mesamax NW
deposit. The cation plot is used to simplify the projection
of stoichiometric mineral compositions. Also shown are
the compositions of olivine calculated to be in equilibrium
with the average Hilltop dike magma (Roedder & Emslie,
1970), and the compositions of pyroxenes from the gabbronorite near the Expo^Ungava deposit. Although norm
calculations (not shown) indicate the presence of several
per cent of plagioclase in most of the ultramafic rocks,
much of this normative plagioclase may be represented by
the minor hornblende observed in the mode. The average
Hilltop dike plots in the center of the small scatter of
chilled margin compositions, slightly more Mg-rich than
the olivine-phyric Chukotat basalt (Burnham et al., 1999).
The ultramafic rocks from the Expo suite fall along a
Fig. 7. Cation plot of Al vs Mg: squares, liquid compositions;
diamonds, cumulate rocks; circles, mineral compositions; crosses,
lavas; triangles, metasediments. Compositions of ultramafic rocks
from the Raglan Formation (Katinniq; Burnham et al., 1999) are
indicated by a shaded grey area. Cumulate rocks are scattered
toward the olivine composition calculated after Roedder & Emslie
(1970) and clinopyroxenes, consistent with the presence of a
peridotitic cumulus assemblage and a considerable amount of trapped
liquid in most of the cumulate rocks. The similarity of the Chukotat
and Expo primitive magmas, evolved liquids, and cumulates should
be noted. Hybridized liquids scatter toward the compositions of some
of the metasediments at low Al and low Mg contents.
trend between the chilled margins and the olivine composition, indicating a major role for olivine accumulation in
the genesis of these rocks. Fine-grained gabbroic rocks
from the Expo Suite trend towards basaltic compositions
1031
JOURNAL OF PETROLOGY
VOLUME 48
Fig. 8. Primitive mantle normalized trace element abundances in
Beauparlant Formation host-rocks of the Mesamax NW deposit.
The mafic tuff and the basalt share the OIB^E-MORB character of
the Beauparlant Fm. The siltstone, the black^grey schist and the iron
formation all show steep LILE-enriched, and Nb-depleted patterns
typical of their presumed continental provenances.
similar to the pyroxene-phyric basalts of the Chukotat
Group. The trend shown by hybrid (i.e. strongly contaminated) melts is off toward the metasediments, below the
main trend.
Assimilation of host-rocks
Assimilation of host-rocks by the parental magma is probably a necessary precondition for the formation of base
metal sulfide deposits, because a primitive mantle-derived
magma will be far from saturation with a sulfide liquid
until it has undergone a substantial amount of cooling,
contamination, and crystallization (e.g. Wendlandt, 1982;
Mavrogenes & O’Neill, 1999). Apart from the textural evidence discussed above for assimilation of sediments, there
is also strong evidence for this process in the compositions
of the ultramafic rocks of the Expo Intrusive Suite.
To assess possible contaminants several examples of host
lithologies were sampled from Mesamax NW (Fig. 8).
The host-rocks comprise basalt, mafic tuff, iron
formation and clastic sediments (siltstones) of the
middle Beauparlant Fm. All of these rocks share steep
LILE-enriched trace element patterns that would lead to
distinctive changes in trace element ratios if they were
assimilated by the Expo Suite magma with its nearly flat
to slightly LILE-depleted character.
Numerical models of magmatic evolution
Assimilation^fractional crystallization
The evolution of the Expo Intrusive Suite during assimilation of host-rocks and concomitant crystallization has been
modeled using the thermodynamic modelling software
NUMBER 5
MAY 2007
PELE (Boudreau, 1999). The PELE program uses the
algorithms and thermodynamic parameters incorporated
in MELTS (Ghiorso & Sack, 1995) in a Windows-based
utility to calculate the compositions of minerals and melts
given a bulk composition, temperature and pressure.
PELE was used to calculate a liquid line of descent for
the parental magma of the Expo Intrusive Suite, and this
matches the predictions of MELTS very well. On the
other hand, whereas MELTS does not conveniently treat
isenthalpic assimilation into a magma, the same problem
has been incorporated very well into PELE. The consequences of the primitive Expo magma assimilating two
possible contaminants (Table 2) were evaluated; one is the
average Mesamax basalt, the other is a typical graphitic
schist from the Mesamax area (i.e. middle Beauparlant
metasediment). PELE was run starting with 100 g of the
Expo magma at its liquidus temperature, adding 5 g increments of either assimilant as solid minerals at 3008C, and
recalculating the temperature and phase equilbria in the
bulk system at each step. The model explicitly accounts
for the heats of fusion of minerals during their dissolution
or precipitation reactions with the melt.
The metasedimentary assimilant was modelled as an
assemblage of 64.38% plagioclase (An29), 31.95% cummingtonite (grunerite50), 1.28% biotite, 1.39% titanite,
0.74% magnetite, and 0.25% pyrrhotite. It was assumed
to have been fully consolidated at the time of intrusion of
the Expo Suite, which occurred at least 100 Myr after the
deposition of the Beauparlant Fm (see preceding discussion
of geological history). The basalt was modeled as an assemblage of 41.5% plagioclase, 36.99% cummingtonite, 9.26%
clinopyroxene, 5.33% biotite, 0.34% apatite, 4.76% magnetite, and 1.83% titanite.
As can be seen from the results in Table 2, the magma is
capable of assimilating at least 50% of its own mass of
either basalt or semipelite. Numerous other runs using a
variety of assimilants produced similar results. As the process of incorporation of cold host-rock continues, the
magma cools considerably, but it actually increases in
volume; that is, the mass of ultramafic cumulate produced
to compensate for the heat of fusion of the metasediments
or basalt is slightly less than the mass of assimilated material. This surprising result is a consequence of the fact that
the heat of fusion of the metasediment is compensated for
by the heat of crystallization of the cumulates, and the
depression in temperature of the mixture is compensated
for by the radical decrease in liquidus temperature accompanying the removal of magnesian olivine and the addition
of fluxing components such as silica, alkalis and water. The
key reason why komatiitic magmas have such a large capacity to assimilate their host-rocks is that they are not multiply saturated, and the olivine liquidus surface descends
steeply to lower temperatures without producing large
masses of olivine crystals.
1032
MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
Table 2: Model results for assimilation of basalt and metasediment by EIS magma
Average Beauparlant Fm basalt from Mesamax NW, added at 3008C, 1 kbar
wt added (g):
bas
0
5
10
15
20
25
30
35
40
45
50
T (8C):
300
1391
1353
1317
1282
1249
1217
1188
1164
1148
1136
1124
Mass of cumulus phases (g)
Ol
–
0
4
9
13
17
20
24
27
31
33
36
Opx
–
0
0
0
0
0
0
0
0
0
0
0
Cpx
–
0
0
0
0
0
0
0
0
02
4
8
Plag
–
0
0
0
0
0
0
0
2
8
13
17
Sulfide
–
0
0
0
0
0
0
0
0
0
0
0
474
478
Melt compositions (wt %)
SiO2
4938
4661
4700
4819
4858
4896
4938
4992
5018
TiO2
074
043
047
050
053
056
059
062
066
073
081
090
Al2O3
1304
1065
1123
1179
1232
1282
1328
1371
1372
1298
1287
1278
Fe2O3*
1633
5048
1369
1369
1410
1436
1452
1457
1450
1434
1443
1500
1572
MgO
735
1798
1608
1435
128
1141
1019
912
832
782
730
678
CaO
808
1064
1098
1129
1158
1185
1209
1230
1241
1232
1169
1107
Na2O
189
044
054
063
071
080
088
095
101
105
111
117
K2O
004
012
015
017
020
022
024
027
029
033
038
044
Metasediment similar to black-grey schist from Mesamax NW, added at 3008C, 1 kbar
wt added (g):
sed
0
5
10
15
20
25
30
35
40
45
T (8C):
300
1391
1350
1314
1279
1247
1215
1187
1161
1147
1132
Mass of cumulus phases (g)
Ol
–
0
47
89
128
166
201
233
263
295
325
Opx
–
0
0
0
0
0
0
0
0
0
0
Cpx
–
0
0
0
0
0
0
0
0
0
Plag
–
0
0
0
0
0
0
0
0
78
Sulfide
–
0
0
001
006
012
018
023
029
032
036
5369
0
145
Melt compositions (wt %)
SiO2
5758
4654
4735
4815
4893
4969
5042
5113
5184
5276
TiO2
057
043
046
048
050
052
054
056
058
064
070
Al2O3
1573
1065
1137
1207
1272
1332
1388
1439
1472
1399
1328
Fe2O3*
1251
1201
1368
1368
1358
1339
1311
1275
1233
1198
1228
MgO
521
1789
1586
1400
1234
1086
955
844
752
698
647
CaO
404
1064
1078
1090
1098
1105
1108
1109
1103
1072
1042
Na2O
545
044
071
097
123
148
172
195
216
236
255
K2O
013
012
013
013
014
014
015
015
015
017
018
The modeling results indicate that the interaction of
large volumes of Expo magma with the Povungnituk Fm
could lead to the removal of the host sediments by assimilation, simultaneously with the deposition of a similar
mass of ultramafic cumulates in the space left by the
melted sediments. One can envision this at the kilometer
scale as a replacement process, but the model says nothing
about the physical mechanism by which this might be
accomplished. Despite the uncertainty regarding the
details of the process, the presence of the contaminated
ultramafic cumulates of the Expo Intrusive Suite indicates
that this has indeed taken place in some way. These arguments indicate that we should not expect to see evidence of
widespread deformation around voluminous ultramafic
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JOURNAL OF PETROLOGY
VOLUME 48
intrusions, as a result of wall-rock assimilation during their
emplacement, and they obviate the common ‘room problem’ faced in an interpretation of their origins.
The modal proportions of cumulus phases resulting from
the addition of a semipelitic assimilant at various proportions of assimilant to initial liquid are shown in Fig. 9.
Over a wide range of amounts of assimilation, the cumulate rock would be a dunitic orthocumulate. Choices of
other assimilants produced similar results, but with different cotectic phases at very large amounts of assimilation
(e.g. wehrlite, troctolite, harzburgite, etc.).
The PELE output was used to predict the compositions
of contaminated liquids and coexisting cumulus phases.
A model cumulate rock composition was calculated by
Fig. 9. Modal proportions of cumulus phases produced during assimilation of metasediment by EIS magma. The proportion of sulfide
liquid has been magnified 10 times to make it visible.
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MAY 2007
combining the solids with a small amount of liquid to
represent a trapped liquid fraction of 20%. The results of
the model after assimilation of 10 g of semipelite per 100 g
of initial liquid are shown in Fig. 10, compared with the
compositions of peridotite at the Mesamax NW deposit,
the putative liquid starting composition based on the ultramafic dike at Hilltop, and the composition of a contaminated pyroxene-phyric basalt from the Chukotat Group
(O. M. Burnham, personal communication, 2006).
The correspondence between the model compositions
and the actual compositions of ultramafic cumulates and
apparently comagmatic basaltic liquids lends very strong
support to the hypothesis that the ultramafic rocks of
the Expo Suite formed as a result of assimilation of their
host-rocks by a primitive komatiitic basalt magma. The
end result was the eruption of a strongly cooled and contaminated mafic liquid resembling continental flood
basalt, and the formation of ultramafic rock with fairly
strongly contaminated trace element signatures, as evidenced by the ubiquity of these characteristics in the present sample suite.
Figure 11 shows the ratio La/SmN vs Th/NbN for samples
including the cumulate rocks and liquids of the
Expo Intrusive Suite, lavas and sediments of the
Beauparlant Formation, compared with the range of compositions of ultramafic rocks from the Raglan Formation
(Burnham et al., 1999). The model compositions resulting
from assimilation of basalt and semipelite are shown with
tick marks indicating the amount of assimilant. The element ratios of liquids and their cumulates are indistinguishable on this diagram because of the very small
partition coefficients of all four elements plotted. The ratio
La/SmN is a measure of the steepness of the slope of the
Fig. 10. Primitive mantle normalized trace element abundances to illustrate the modeled assimilation^fractional crystallization process. In the
model, 100 g of the primitive magma (Hilltop dike) assimilates 10 g of the semipelite (black^grey schist) to produce an olivine cumulate (modeled here with 20% trapped liquid; compared with a peridotite sample from Mesamax NW) and a residual liquid (compared here with a
contaminated pyroxene-phyric basalt from the Chukotat Group; O. M. Burnham, personal communication, 2006).
1034
MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
Fig. 11. Variations in La/Sm and Th/Nb (normalized to primitive
mantle) in rocks of the Expo Intrusive Suite and host-rocks.
The primitive magma (Hilltop dike) has ratios intermediate between
primitive mantle (i.e. values of one) and modern N-MORB
(McDonough & Sun, 1995), reflecting a moderately depleted mantle
source. The basalts and mafic tuffs of the Beauparlant Formation
resemble E-MORB or OIB (McDonough & Sun, 1995). The compositions of ultramafic cumulate rocks and of lavas of the Chukotat Group
(Burnham et al., 1999) follow a trend from the primitive magma
toward the clastic sediments. The results of the model of assimilation
and crystallization are shown as two trends, one resulting from assimilation of indicated percentages of a semipelitic metasediment,
the other from assimilation of indicated percentages of basalt or tuff
of the Beauparlant Formation. Both the Chukotat lava and the Expo
Suite cumulates show compositions that could have resulted from
assimilation of about 10% of some combination of metasediment
and basalt of the Beauparlant Formation. A few cumulate rocks have
compositions similar to the hybrid magmas, reflecting very large
degrees of assimilation in the range of 50% sediment and basalt.
primitive mantle normalized LREE pattern, which is generally high in continental crust but equal to about one in
primitive tholeiites and komatiites (compare with Figs 6,
8 and 10). The Beauparlant Fm basalts have significantly
higher La/SmN than the Expo Suite and Chukotat basalts,
in keeping with their classification as having affinities to
ocean island basalts. The ratio Th/NbN is influenced by
the assimilation of Th-rich sedimentary rocks of the
upper continental crust. Whereas Th/NbN is similar for
the Beauparlant Fm, Chukotat Group and Expo magma,
the ultramafic rocks of the Expo Intrusive Suite show a
broad scatter toward the relatively high values shown by
the metasediments hosting the Mesamax NW deposit.
The compositions of the ultramafic cumulates of the Expo
Intrusive Suite do not coincide exactly with either trend,
but could reflect combined assimilation of a mixture of up
to 10% of semipelite and 10% basalt, dominated in most
cases by sediment.
The primitive Expo magma is assumed to have been
sulfide-undersaturated when it left its mantle source
region. This assumption is based upon the large degree of
partial melting required to generate a komatiitic basalt
and upon the observation that the magma was enriched
in the highly chalcophile elements such as Pt (Table 1).
Given these constraints, there cannot have been more
than about 900 ppm S in the initial melt (e.g. Mavrogenes
& O’Neill, 1999). The assimilation model shows that digestion of basalt would not trigger sulfide saturation in the
contaminated magma unless at least 1000 ppm of S was
present initially. On the other hand, assimilation of the
metasediment, itself containing 0.6% FeS, caused the
model magma to reach sulfide saturation after it had
assimilated about 10% of its weight in host-rock (Fig. 9).
The physical controls on the assimilation of hostrocks by flowing magma were explored quantitatively
by Huppert & Sparks (1989) and Williams et al. (2001).
What follows is a qualitative discussion of the fundamental
controls on the process in the study area. It might appear
surprising that basalt could have been assimilated as easily
as sediment by the Expo magma, in light of the very different contact relations observed where the Expo Intrusive
Suite cuts these different lithologies. The reason for such
different behaviour probably lies in the phase relations of
the two possible contaminants. Although both could be
assimilated in amounts as great as 50 wt % of the invading melt, this would occur only if the assimilant and the
magma could be thoroughly mixed mechanically before
reaching thermal equilibrium. In real systems, mixing
can occur only if the physical states of the magma and the
assimilant permit it. When magma at 14008C comes into
contact with sediment or basalt at 3008C, the temperature
at the contact initially will be 8508C. This temperature is
well below the solidus of komatiitic basalt, so that in both
cases the magma will form an aphanitic chilled margin
against its host-rock. The contact temperature is also
below the solidus of metabasalt, so that, where the wallrocks are basaltic, the contact will begin in a solid state
and remain so indefinitely. In a closed system, as cooling
of the intrusion continues, the cooled and crystallized
margin will propagate inward and the heated thermal aureole will propagate outwards, but no part of the host-rock
(basaltic) will ever be at its melting point. Intrusion of the
Expo magma into basalt host-rocks will therefore not
result in assimilation of the basalt unless some other process (e.g. stoping) intervenes to mechanically mix the
basalt into the magma, or unless the magma flows continuously past the contact, heating it sufficiently to induce
melting. On the other hand, the contact temperature of
8508C is above the likely solidus temperature of the metasediments. The contact zone would therefore be occupied
by a thin crust of chilled magma in a vertical orientation
with partially melted sediment on one side and stillmolten intrusive rock on the other. This arrangement is
mechanically unstable on a vertical dike wall and would
break up the chilled crust, allowing the partially molten
metasediment to mingle with the magma and therefore
promote efficient assimilation of the host-rocks.
Abundant fine-grained pyroxenite clasts form a locally
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JOURNAL OF PETROLOGY
VOLUME 48
clast-supported breccia with a peridotitic matrix in the
lower portions of the Mequillon and Tootoo intrusions
and could be interpreted as remnants of the foundered
chilled margin in such a scenario. In summary, it is
entirely consistent that the magma formed sharp chilled
contacts with the host basalts rather than assimilating
them, but was able to digest large quantities of the
metasediments.
In either case, if the magma within the dike continues to
flow, the temperature at the contact will increase and the
chilled margin will tend to be resorbed. At this point the
wall-rocks will be in contact with hot magma and assimilation (thermal erosion) will ensue regardless of the composition of the host-rock.
The preceding discussion of assimilation, fractional
crystallization, and sulfide segregation leads to some interesting conclusions. The first is that a komatiitic basalt
intruded into dikes and sills in the upper crust can assimilate up to 50% of its mass of host-rock, although in many
cases it may only reach values nearer to 15%. The second is
the observation that whereas the Expo magma reached the
level of the Beauparlant Fm in a pristine, uncontaminated
state as preserved in the chilled margins, it erupted at the
surface (Chukotat Group) with a highly contaminated
composition. In other words, the assimilation^fractional
crystallization process happened at the present level of
exposure within the dikes and sills that fed the overlying
flows of the Chukotat Group, at a depth of less than 5 km
below the paleosurface, rather than deep in the crust in
some large but unexposed ‘staging’ magma chamber.
Wherever ultramafic cumulate rocks are exposed in dikes
or sills that show compositional evidence for contamination, a similar process might be inferred to have occurred.
GEOLOGICA L MODEL
Structure and emplacement
The Expo Suite shows a distinct intrusive style in each of
the three members of the Beauparlant Fm and in the
Nuvilik Fm. Intrusions are essentially absent from the
lower Beauparlant Fm, are broad and tend to show ultramafic character in the middle member, and are gabbroic in
the upper member. Within the Nuvilik Fm, the intrusions
are almost exclusively peridotitic and tend to form sills as
well as dikes, whereas all of the intrusions hosted by the
Beauparlant Fm appear to be dikes.
The preferred explanation for these observations is illustrated in the inset block diagram in the upper part of
Fig. 2. The interpreted longitudinal section in Fig. 2 was
drawn schematically along vertical panels following the
trace of the intrusions through the mineralized Vaillant
Lake, Mequillon, Cominga, Expo, Hilltop, TK and
Mesamax NW portions of the system. Because the line of
section was chosen to follow the axes of the dikes, they all
appear in the longitudinal section as continuous bands;
NUMBER 5
MAY 2007
however, they extend into or out of the page only a few
tens or hundreds of meters. The Expo Intrusive Suite
appears to have been emplaced into a nearly flat-lying
package of Povungnituk Group basalts and was subsequently deformed during the Trans-Hudson Orogeny
along with the host basalts into the prominent NW^SE
fold structures that dominate the present map pattern.
The emplacement of the Expo Intrusive Suite was guided
by the mechanical response of the layered rocks it
intruded and by the buoyancy contrast between magma
and host-rock, such that it was unable to penetrate far
into the lower Beauparlant, but formed initially vertical,
blade-shaped dikes through the middle and upper
Beauparlant members and up into the overlying Nuvilik
Fm. Where these dikes were surrounded by basalt of the
upper Beauparlant Fm, they were unable to melt their
host-rocks significantly and they therefore tended to freeze
in place. Where the dikes passed through the volcanic^
sedimentary pile of the middle Beauparlant Fm, they
were able to melt the metasedimentary horizons and then
pluck and assimilate basaltic components of the wall-rocks,
opening themselves out into broad, long-lived magma
conduits. Solidification of the upper portions of the
gabbroic dikes in the upper Beauparlant sealed the midBeauparlant dike segments off from the surface and aided
their development as long-lived conduits for lateral magma
transport over distances of tens of kilometers. Lateral
transport permitted the thermal erosion of the wall-rock
sediment, and, to a more limited extent, of basalt within
the middle Beauparlant Fm. Assimilation of wall-rock
metasediment and basalt in the middle Beauparlant Fm
caused the magma to become sulfide-saturated, a process
that is recorded by ubiquitous minor sulfide mineralization
along the contacts. As the sulfide melt generated in this setting settled, it sank to the bottom of the dikes and collected
along the length of their basal terminations.
Where the Expo Suite magma broke through the upper
Beauparlant Fm into the Nuvilik Fm, it either formed sills
along the top of the contact, as in the Cominga area, or
continued upward as dikes or cylindrical conduits like
that at Snow Owl. Because of the ease of assimilation of
the semipelitic host-rocks of the Nuvilik Fm, the intrusions
developed into a complex of cross-cutting dikes and sills
filled almost entirely with ultramafic rocks that effectively
replace at constant volume the sediments they intruded.
The present distribution of the Expo Intrusive Suite in
outcrop is dictated by the interplay between the original
stratigraphic control on magma migration pathways and
the subsequent deformation. Where the dikes cross the
hinges of major anticlines, there are gaps in their outcrop
because these fold hinges expose lower Beauparlant
Formation or even Dumas Formation, below the lowest
extent of the dikes. Where the intrusions cross the hinges
of major synclines, the basal portions are deeply buried
1036
MUNGALL
CRUSTAL CONTAMINATION OF PICRITIC MAGMAS
Fig. 12. Cutaway view illustrating the form of the mineralized dikes of the Expo Suite within the Beauparlant Fm. Sulfide melt precipitated at
sites of assimilation of metasediment and basalt along the vertical walls of the dikes when they were emplaced as sword-shaped intrusions
floored within the lower Beauparlant Formation (a), and collected in the trough-like basal terminations of the dikes. Later folding (b) and
subsequent erosion (c) have left canoe-shaped remnants of the dikes exposed. Mineralization now crops out at the tips of the canoe-shaped
dike segments, but is commonly present intermittently along the entire strike length at the base.
several kilometers below the present erosional surface and
all that can be seen at surface are the gabbroic upper portions of the dike. The preservation of a typical exposure of
a dike at the current erosional level in a syncline is illustrated in Fig. 12, which shows the configuration at the
time of emplacement, after folding, and finally after erosion. Sulfide deposits appear at the surface in those places
where dike segments terminate at the current erosional
surface, but the sulfide bodies appear to be continuous
down plunge along the entire lengths of dikes (Fig. 5).
The Expo^Ungava deposit and its surrounding ultramafic intrusions are exposed in the center of a 10 km scale
doubly plunging synclinorium, or basin, exposing the
Nuvilik Fm. These represent the highest structural levels
preserved at present in the Expo Intrusive Suite, with the
basal portions at present probably 4^5 km below the surface. The focusing of the basin structure on the Expo area
may be a consequence of the presence of the dense
ultramafic rocks themselves, as this large mass could have
loaded the Beauparlant Fm and nucleated a synform during
lateral compression. On the other hand, the type of intrusion
visible in the Expo^Ungava deposit area may originally
have been present along the entire length of the dike system
and the segment found at Expo^Ungava may have been
preserved purely by chance during regional folding.
Relationship between Expo, Chukotat
and Raglan suites
The Expo magma shows remarkable similarities both to
the olivine-phyric basalts of the Chukotat Group and to
the putative initial liquid of the Raglan suite, which generated the important Ni^Cu^PGE deposits of the Raglan
Mine. It is generally assumed that the Raglan suite constitutes a series of shallow subvolcanic intrusions and associated channelized lava flows related to the emplacement
of the Chukotat Group volcanic rocks (Be¤dard et al., 1984;
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JOURNAL OF PETROLOGY
VOLUME 48
Barnes & Barnes, 1990; Burnham et al, 1999), and furthermore that the Expo Intrusive Suite formed contemporaneously as the upper-crustal feeder system to the Chukotat
Group (e.g. Giovenazzo et al., 1989). The available radiometric ages and compositions are consistent with this interpretation. Large igneous provinces involving voluminous
picritic magmas such as those of the Expo and Raglan
suites probably undergo an episode of peak magma production lasting about 1^3 Myr, and effusion is generally
entirely completed within a span of about 10 Myr (e.g.
Courtillot & Renne, 2003). The estimated 30 Myr time
span between the emplacement of the Expo and Raglan
suites and the 1918 Ma age of a gabbroic intrusion below
the Chukotat Group is too long to allow an interpretation
that all three are comagmatic; hence, the earlier intrusion
may not bear any relationship to the main magmatic event
that produced the Expo, Raglan, and Chukotat suites at
c. 1887^1870 Ma.
CONC LUSIONS
The parent magma of the Paleoproterozoic Expo Intrusive
Suite was a komatiitic basalt with a weakly LILE-depleted
composition. Neither the basalts of the Povungnituk
Group nor the intrusions of the Expo Intrusive Suite were
emplaced in a rifting environment. The Povungnituk
Group was emplaced onto a long-lived passive continental
margin, followed at least 108 Myr later by the simultaneous
intrusion of the sills and dikes that form the Expo Suite
and eruption of the lavas of the Raglan Formation. A very
strong control on the intrusive style and differentiation of
the EIS was exerted by its country rocks. Where the intrusions were emplaced into metasediments, they formed
large, sill-like and relatively irregular ultramafic cumulate
bodies accommodated by assimilation of the wall-rocks.
Where the intrusions were emplaced into predominantly
basaltic host-rocks they were confined by their relatively
infusible hosts and formed sharp-walled dikes. Assimilation of metasediments in the middle member of the
Beauparlant Fm caused the magma to reach sulfide saturation. Sulfide melt percolated down the dike walls and accumulated in the basal terminations of the blade-shaped
dikes, where they taper out within the lowest basalts of
the Beauparlant Fm.
The very strong crustal contamination signature
acquired by the magmas residual to the formation of the
ultramafic cumulates gave them a classical continental
tholeiite trace element signature, which is recorded at present in the compositions of some of the overlying basalts of
the Chukotat Group. The mechanism and structural aspect
of the assimilation^fractional crystallization scenario proposed here is different from the mid-crustal staging chamber commonly proposed as the site of AFC processes in
other suites currently represented only as basalts and shallow intrusions, such as the Noril’sk^Talnakh intrusions.
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MAY 2007
Many other continental or continental margin suites may
have been emplaced through similar conduit systems, producing similar dike-like or sill-like bodies, which were the
sites of extensive interaction with the continental crust. A
very similar mineralized body is being mined at Jinchuan,
China (Chai & Naldrett, 1992), and might have formed in
much the same manner.
AC K N O W L E D G E M E N T S
This work has been made possible entirely through the
generous support of Canadian Royalties Inc., who are
exploring the Expo Intrusive Suite for Ni^Cu^PGE mineralization. Helicopter support, camp accommodation, and
support for lithogeochemical analysis and petrography
has been unstintingly offered over four field seasons. CRI
geologists Todd Keast, Bruce Durham, and William
Randall have offered invaluable advice and discussion of
the ideas presented here. The present manuscript has benefited enormously from careful and thoughtful reviews by
Peter Lightfoot, Mike Lesher, and associate editor Nick
Arndt.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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