Origin and serpentinization of ultramafic rocks in dismembered ophiolite north of Trembleur Lake, central British Columbia

Abstract

Serpentinization of ultramafic rocks can produce alteration minerals such as brucite (Mg(OH) 2), which has the potential to sequester carbon dioxide, and awaruite (Ni3Fe), a potential source of nickel. The Trembleur ultramafite is part of a dismembered ophiolite in central British Columbia. Field and petrographic data indicate that it is heterogeneous both in protolith and alteration. The protolith consists mainly of harzburgite, lherzolite, dunite, and lesser pyroxenite. Dunite, more abundant than previously recognized, could be a replacement of harzburgite (±lherzolite). All ultramafic rocks in the Decar area, north of Trembleur Lake, are altered to some extent (mainly partially to pervasively serpentinized) and locally contain higher temperature metamorphic assemblages. Carbonate alteration postdated serpentinization, degrading minerals such as brucite and awaruite to locally produce ophicarbonates, soapstone, and listwanite. The primary olivine-pyroxene ratio of the protolith may have controlled fluid pathways and thus the extent of serpentinization and the abundance, distribution, and grain size of brucite and awaruite, which has implications for carbon sequestration and nickel potential in the Decar area.

Full text

Origin and serpentinization of ultramafi c rocks in
dismembered ophiolite north of Trembleur Lake,
central British Columbia
Katrin Steinthorsdottir1, a, Jamie Cutts1, Greg Dipple1, Dejan Milidragovic2, and
Frances Jones1
1 Bradshaw Research Initiative for Minerals and Mining, Department of Earth, Ocean and Atmospheric Sciences,
University of British Columbia, Vancouver, BC, V6T 1Z4
2 British Columbia Geological Survey, Ministry of Energy, Mines and Petroleum Resources, Victoria, BC, V8W 9N3
a corresponding author: ksteinth@eoas.ubc.ca
Recommended citation: Steinthorsdottir, K., Cutts, J., Dipple, G., Milidragovic, D., and Jones, F., 2020. Origin and serpentinization of
ultramafi c rocks in dismembered ophiolite north of Trembleur Lake, central British Columbia. In: Geological Fieldwork 2019, British
Columbia Ministry of Energy, Mines and Petroleum Resources, British Columbia Geological Survey Paper 2020-01, pp. 49-58.
Abstract
Serpentinization of ultramafi c rocks can produce alteration minerals such as brucite (Mg(OH)2), which has the potential to sequester carbon
dioxide, and awaruite (Ni3Fe), a potential source of nickel. The Trembleur ultramafi te is part of a dismembered ophiolite in central British
Columbia. Field and petrographic data indicate that it is heterogeneous both in protolith and alteration. The protolith consists mainly of
harzburgite, lherzolite, dunite, and lesser pyroxenite. Dunite, more abundant than previously recognized, could be a replacement of harzburgite
(±lherzolite). All ultramafi c rocks in the Decar area, north of Trembleur Lake, are altered to some extent (mainly partially to pervasively
serpentinized) and locally contain higher temperature metamorphic assemblages. Carbonate alteration post-dated serpentinization, degrading
minerals such as brucite and awaruite to locally produce ophicarbonates, soapstone, and listwanite. The primary olivine-pyroxene ratio of the
protolith may have controlled fl uid pathways and thus the extent of serpentinization and the abundance, distribution, and grain size of brucite
and awaruite, which has implications for carbon sequestration and nickel potential in the Decar area.
Keywords: Trembleur ultramafi te, Decar, Baptiste deposit, serpentinite, listwanite, CO2 sequestration, brucite, awaruite, dunite, harzburgite,
lherzolite, pyroxenite
1. Introduction
Alteration of ophiolitic rocks is common and includes
hydration (e.g., serpentinization) and carbonation (e.g.,
formation of listwanite). Serpentinization produces minerals
such as serpentine, magnetite, brucite (Mg(OH)2), and awaruite
(Ni3Fe; O’Hanley, 1996). Brucite, along with serpentine, can
react with and sequester atmospheric CO2 in carbonate minerals
(Power et al., 2013) and awaruite forms with serpentinization
in reducing environments from primary olivine or sulphides
and is a potential source of nickel (Eckstrand, 1975; Britten,
2017). Listwanite is formed by the dehydration and carbonate
alteration of these hydrated minerals and is a natural analogue of
CO2 sequestration through mineral carbonation (Hansen et al.,
2005). Characterizing the chemical, mineralogical, and textural
variability of a protolith is key to constraining its variable
alteration to serpentinite and listwanite (Hall and Zhao, 1995;
Hansen et al., 2005; Milidragovic and Grundy, 2019).
The Trembleur ultramafi te is in the southern segment of the
Cache Creek terrane, which extends through British Columbia
into southern Yukon (Fig. 1; e.g., Monger and Gibson, 2019).
The Trembleur ultramafi te in the Decar area form part of
a dismembered supra-subduction zone ophiolite and is
variably serpentinized and carbonate-altered (Britten, 2017;
Milidragovic and Grundy, 2019). The ultramafi c protoliths are
heterogeneous and consists of diverse peridotites and lesser
pyroxenites. The serpentinized ultramafi c rocks in the Decar
area contain brucite, of potential environmental value (e.g.,
Vanderzee et al., 2019) and awaruite (Ni3Fe), of potential
economic value (Britten, 2017). This contribution summarizes
the fi eld and petrographic results of a study designed to evaluate
the protoliths and alteration of the Trembleur ultramafi te in
order to constrain the controls on the formation, distribution,
and abundance of brucite and awaruite and to identify the
extent of alteration in the fi eld.
2. Geological setting
The Decar area is underlain by rocks of the Cache Creek
terrane, a tectonostratigraphic unit that contains Late Devonian
to Middle Jurassic oceanic rocks and extends from southern
British Columbia to Yukon (Fig. 1a; Cordey et al., 1991;
Golding et al., 2016). These oceanic rocks were deformed
and metamorphosed by ca. 172 Ma, following collision with
the Stikine terrane on its western fl ank (Mihalynuk et al.,
1994; Struik et al., 2001; Mihalynuk et al., 2004; Monger
and Gibson, 2019). Exposed in the area are: greenschist- to
amphibolite-facies ultramafi c to intermediate igneous rocks
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Fig. 1. a) Geological map of the area north of Trembleur Lake. b) Geological map of the Decar area with sample locations and mapped awaruite grain size. Modifi ed from Britten (2017)
and Milidragovic (2019).
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and metasedimentary rocks of the Rubyrock igneous complex
(early Permian to Late Triassic); altered ultramafi c rocks of the
Trembleur ultramafi te (early Permian to Late Triassic); volcanic,
siliciclastic, and carbonate rocks of the Sowchea succession
(Upper Pennsylvanian to Lower Jurassic); and sedimentary
and volcanic rocks of the Sitlika assemblage (Upper Triassic to
Lower Jurassic; Fig. 1a; for details see Milidragovic et al., 2018;
Milidragovic, 2019). The rocks are exposed in fault-bounded,
northwest-trending belts that are dismembered by cross faults.
The Sowchea succession and parts of the Sitlika assemblage
are interpreted to record volcanism and sedimentation in
an intraplate oceanic setting (Schiarizza and Massey, 2010;
Milidragovic and Grundy, 2019). In contrast, the Rubyrock
igneous complex and Trembleur ultramafi te are interpreted
to represent the crustal and mantle sections of a dismembered
supra-subduction zone ophiolite (Schiarizza and MacIntyre,
1999; Struik et al., 2001; Schiarizza and Massey, 2010; Britten
2017; Milidragovic and Grundy, 2019). In this contribution, we
focus on the Trembleur ultramafi te.
The least serpentinized and carbonate-altered rocks of
the Trembleur ultramafi te outcrop in areas of relatively high
elevation on Mount Sidney Williams (also known as Mount
Sidney, Tselk’un or Red Rock) and an unnamed ridge ca. 3 km
ENE of Mount Sidney Williams (Fig. 1b). In contrast, the
most altered rocks occupy areas of low elevation. Highly
serpentinized areas host awaruite, which is coarse grained at
Baptiste and other places in the area (shown as irregular red
polygons in Fig. 1b; Britten, 2017). Variable abundances of
brucite have been documented in the highly serpentinized area
in the Baptiste area (Vanderzee et al., 2019), but abundances
are unknown in other parts of the Trembleur ultramafi te.
Ophicarbonate (serpentine-magnesite), soapstone (talc-
magnesite), and listwanite (magnesite-quartz) assemblages are
mainly in the southeastern and eastern part of the Trembleur
ultramafi te (Milidragovic et al., 2018).
3. Methods
A total of 89 hand samples of ultramafi c rocks were selected
for this study (Fig. 1b). Of these, 55 are surface samples collected
during the 2019 fi eld season, 15 are surface samples collected
during the 2017 fi eld season (Milidragovic and Grundy, 2019),
and 19 are cores provided by FPX Nickel Corp. from drilling
in nine holes at the Baptiste deposit. Hand samples were
selected to represent the full range of alteration and textures of
the serpentinized or carbonate-altered Trembleur ultramafi te,
whereas drill core samples were chosen at various depths (12
to 182 m), to include different rock types, bulk compositions,
and mineral content based on Vanderzee et al. (2019).
4. Trembleur ultramafi te protolith
The ultramafi c rocks at Decar have all undergone some
degree of serpentinization or carbonate alteration. Harzburgite
(olivine and orthopyroxene-rich, <5% clinopyroxene) is the
predominant rock type (Grundy, 2018; Milidragovic and
Grundy, 2019), with lesser lherzolite (5-15% clinopyroxene),
and dunite (>90% olivine). Relatively unaltered pyroxenite
(>60% pyroxene) dikes and veins are a minor component.
These rock types are distinguished in the fi eld by the colour
and texture of their weathered surfaces. Weathered surfaces of
dunitic rocks are typically smooth and dun compared to more
pyroxene-rich rocks, which weather rough and darker brown.
The ultramafi c rocks are not obviously strained, but shear zones
and related folds are locally developed (Figs. 2a, b).
4.1. Harzburgite and lherzolite
Pyroxene-bearing peridotite, spanning the compositional
spectrum between harzburgite and clinopyroxene-poor
lherzolite, are the most common rocks at Decar. It is diffi cult
to differentiate between the two rock types in the fi eld because
most are altered to some extent. Harzburgite (±lherzolite) is
composed of olivine (≥60%), orthopyroxene (20-40%), and
primary spinel. Orthopyroxene-poor harzburgite (ca. 20%
pyroxene), which grades into dunite, is less common and
occurs in layers or as pods. Orthopyroxene grains range
from <0.5 to1.5 cm and are commonly altered to bastite. The
colour of bastite varies in hand specimen from dark grey-
green to light grey to white, and in thin section from brown to
grey with increasing degree of serpentinization (Figs. 3c, d).
Milidragovic et al. (2018) described relatively unaltered spinel
harzburgite with coarse orthopyroxene, olivine, and rare fi ner
grained clinopyroxene (Fig. 3c).
Lherzolite is distinguished from harzburgite by more abundant
(5-13%) clinopyroxene that occurs as <0.5-1 mm-sized
subhedral to euhedral prismatic grains (Fig. 3e). Clinopyroxene
occurs as individual grains or as aggregates surrounded by
orthopyroxene and/or olivine. Some clinopyroxene grains
show lamellae of secondary spinel along cleavage planes. In
both lherzolite and harzburgite samples, primary spinel grains
vary from <0.3 to 2 mm, range from red to black, and typically
show a vermicular or irregular habit, although some are equant.
Chlorite and serpentine locally form haloes around spinel, thus
spinel could be the source of aluminum for chlorite.
4.2. Dunite
Dunite comprises ca. 15% of all ultramafi c rocks at Mt. Sidney
and on the unnamed ridge to the east, where it is more abundant
than was previously noted by Milidragovic and Grundy
(2019). The dunite consists mostly of 1-6 mm equigranular
olivine (Fig. 3a) and some samples contain rare brown or
grey, fi ne-grained (<4 mm) bastite after orthopyroxene or
clinopyroxene. Fine-grained (<1 mm) unaltered orthopyroxene
and clinopyroxene grains are locally (<5%) preserved. Dunite
typically occurs within harzburgite (±lherzolite) although it
also forms massive uniform outcrops >20 m across. The contact
between dunite and harzburgite is typically sharp although
rare gradational contacts were also observed. Where dunite is
hosted in harzburgite (±lherzolite) it forms sets of parallel to
randomly oriented veins or dikes (0.5-50 cm wide), lone dikes,
irregular- to lenticular-shaped pods or lenses <1 m wide, or as
parallel, discontinuous layers 1-50 cm thick (Fig. 2; Fig. 4a).
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Fig. 2. a) Dunite with thin pyroxenite veins and spinel (Spl) grains. b) Centimetre-scale shear zone cutting harzburgite (±lherzolite) and dunite
layers. c) Dunite with spinel (Spl) as discontinuous layers and irregular pods in harzburgite (±lherzolite). d) Coarse spinel (Spl) grains with
apparent long axes concordant to the walls of a dunite dike in harzburgite (±lherzolite). e) Pyroxenite vein concordant with dunite dike in
harzburgite (±lherzolite). f) Pyroxenite dike crosscutting dunite and harzburgite (±lherzolite) layers.
Primary spinel is common in dunite and occurs as disseminated
grains or multi-crystal aggregates. In plane-polarized light,
spinel grains are red to black. Disseminated spinel grains range
from <0.5 mm to 0.5 cm and characteristically have an equant
habit (Fig. 3a). Apparent long axes of coarse disseminated
spinel grains are typically aligned with the walls of the host
dunite dikes. Spinel aggregates, which form 0.5-5 cm wide
veins, are also aligned with host dunite dikes or layers (Fig. 2d).
Less commonly, thin spinel veinlets are at an angle to dunite
dikes and pyroxenite veins and pinch out in peridotite host rock
(Fig. 2a).
4.3. Pyroxenite
Pyroxenite typically forms 0.5-5 cm wide veins in dunite
layers (Figs. 2a, e) and ca. 10 cm-wide dikes that crosscut
dunite and harzburgite (Fig. 2f). The modal abundance
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Fig. 3. Partial and pervasive serpentinites. a) Partially serpentinized dunite with olivine (Ol), serpentine (Srp) veins and primary spinel (Spl).
b) Pervasively serpentinized dunite with olivine (Ol), serpentine (Srp) veins, mesh serpentine texture (Mesh Srp), and brucite (Brc) ±serpentine
vein. c) Partially serpentinized harzburgite (±lherzolite) with bastite grains, olivine (Ol) and serpentine (Srp) veins. d) Pervasively serpentinized
harzburgite with bastite, relict olivine (Ol), interlocking serpentine texture (Interl. Srp.) and serpentine (Srp) veins. e) Pervasively serpentinized
lherzolite with clinopyroxene (Cpx), orthopyroxene (Opx) bastite, relict olivine, and mesh serpentine texture (Mesh Srp). f) Pervasive serpentinite
with a coarse serpentine vein and interlocking serpentine texture (Interl. Srp.) overprinted by interpenetrating serpentine (Interp. Srp.) texture
and secondary spinel (Spl). All images are in cross-polarized light.
of orthopyroxene and clinopyroxene in the pyroxenites is
variable. In orthopyroxene-rich pyroxenite, the orthopyroxene
grains are 4-6 mm with fi ner grained clinopyroxene, olivine,
and red to black spinel of holly leaf texture. In clinopyroxene-
rich pyroxenite, the clinopyroxene grains are >2 mm with fi ner
olivine grains.
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Fig. 4. Partial and pervasive serpentinites, slightly carbonated serpentinites, and listwanite. a) Dunite dike in harzburgite (±lherzolite) with
serpentine (Srp) veins. b) Pervasive serpentinite (Srp) lens in a scaly serpentinite (Srp) matrix. c) Pervasive serpentinite with polygonal pattern
of serpentine (Srp) veins. d) Slightly carbonated pervasive serpentinite with serpentine (Srp) veins, interlocking serpentine texture (Interl. Srp.),
carbonate grains (Cb), and secondary spinel (Spl) (crossed-polarized light). e) Nearly fully carbonated serpentinite showing relict serpentinite
(Srp) texture. f) Listwanite with a quartz (Qtz) vein bounded by fuchsite.
5. Ser pentinized Trembleur ultramafi te
All the rocks we observed in the fi eld and collected for this
study are serpentinized and/or carbonated to some extent.
For ease of discussion, we subdivide the altered rocks into
partial serpentinite, pervasive serpentinite, ophicarbonates
and soapstone-listwanite. Partial serpentinite is typically grey,
preserves obvious primary textures, and contains ca. 30-70 vol%
relict minerals (olivine and pyroxenes). In contrast, pervasive
serpentinite is light to dark green, contains ca. 0-30 vol%
relict minerals, and commonly displays a foliation. The
serpentinization process results in volume increases (up to
40%: Komor et al., 1985) and produces alteration minerals
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(e.g., olivine + H2O goes to serpentine group minerals +
magnetite + awaruite + brucite; Johannes, 1968; Britten, 2017).
Carbonatization results in breakdown of the serpentinization
minerals (e.g., brucite + CO2 goes to magnesite + H2O; Hansen
et al., 2005). Increasing the intensity of CO2alteration results in
a progressive decrease in the volume of the relict serpentinite
enclaves from ophicarbonates to soapstone to complete
conversion to listwanite. Chemical reactions that change
volume (density) and abundance of magnetite can be used as
fi rst-order proxies for the degree and type of alteration (Komor
et al., 1985; Toft et al., 1990; Hansen et al., 2005; Cutts et al.,
in press).
5.1. Serpentinization and protolith variability
In partially serpentinized rocks, the serpentinization textures,
alteration minerals, and extent of alteration appear to vary
between protoliths. In dunite-rich outcrops, serpentine veins
are generally subparallel to dunite dikes or layers, rather than
penetrating into the surrounding harzburgite or lherzolite
(Fig. 4a). In dunite, serpentine veins crosscut primary olivine
grains and these veins are progressively thicker and more
abundant with increasing degrees of serpentinization (compare
Figs. 3a and 3b). In dunite, the pyroxene, if present, is typically
strongly altered to bastite.
In contrast, although harzburgite and lherzolite contain
serpentine veins (Fig. 3c), with further alteration serpentine
principally occurs in the groundmass after olivine and as bastite
alteration of pyroxene (Figs. 3d, e). With increasing degree of
alteration, bastite colour ranges from beige to dark brown to
grey or light yellow, and grain shapes range from subhedral
to anhedral (Figs. 3c-e). Altered bastite grains commonly
contain secondary magnetite and crosscutting serpentine
veins. In partial and pervasive serpentinites, olivine is the least
preserved, followed by orthopyroxene and then clinopyroxene
(Fig. 3d). Orthopyroxene grains or their bastite equivalents are
consistently more altered than clinopyroxene, which typically
retains a subhedral shape and high birefringence (Fig. 3e).
Olivine-poor pyroxenite-rich dikes are relatively unaltered
and the few observed olivine grains, which are interlocked
with pyroxene, appear to be the most altered minerals in the
assemblage.
5.2. Serpentinite and serpentine texture
Typically, the fresh surface of serpentinite is light to dark
grey-green, rarely vibrant green and weathers grey-brown-
green to off-white. Pervasive serpentinites locally contain
lenses of relatively competent serpentinite in an anastomosing
scaly serpentinite matrix and contain serpentine veins parallel
to one another (Fig. 4b). Locally, pervasive serpentinites are
brecciated, whereas others display polygonal patterns of
uniformly spaced serpentine veins (Fig. 4c).
The degree of serpentinization is most effectively determined
in thin-section by comparing modal abundances of relict
primary versus alteration minerals (Fig. 3), identifying
alteration minerals, and distinguishing between serpentine
textures. In thin section, Decar serpentinites displays mesh
(Figs. 3b, e), hourglass, and ribbon textures, which are typical
of the lizardite polymorph. Interlocking (Figs. 3d, e; Fig. 4d)
and interpenetrating (Fig. 3e) serpentine textures are also
present and likely refl ect the antigorite polymorph (Wicks
and Whittaker, 1977). Many samples contain a mixture of
overprinting textures, indicating multi-stage serpentinization.
Figure 3f illustrates an example where an interpenetrating
texture overprints interlocking texture and serpentine veins.
Serpentine veins occur in samples of all protoliths and extents
of serpentinization, but they vary in abundance and can be
oriented either parallel or oblique to one another. The veins are
white-grey-green and weather to grey-white (Fig. 4c), vary in
width from <20 μm to >4 cm, and commonly have a massive,
wavy, or interlocking texture and rarely, serpentine selvages
(Figs. 3; Figs. 4a-d). Serpentine veins wider than 1 cm are
commonly fi brous, and likely composed of chrysotile.
In addition to serpentine and relict primary minerals,
serpentinites commonly contain the secondary minerals
magnetite (Fig. 3f), brucite (Fig. 3b), awaruite, chlorite, talc,
and tremolite, and locally, metamorphic olivine and diopside
(Britten, 2017; Milidragovic and Grundy, 2019). Brucite is
in both partially and pervasively serpentinized rocks, derived
from dunite, harzburgite, and lherzolite protoliths. Brucite
most commonly occurs in mesh serpentine or adjacent to
relict olivine as discrete grains (<150 μm), aggregates, or as
thin veins typically spatially associated with serpentine and/
or magnetite (Fig. 3b). In partially serpentinized samples from
all protoliths, fi ne- to coarse- grained magnetite, sulphides, and
awaruite are commonly in or close to serpentine veins, whereas
in highly serpentinized samples, these minerals are typically
in the serpentine groundmass and/or metamorphic olivine.
Awaruite grains vary in size (<10 to 800 μm across), shape,
and association; they occur as monomineralic grains, locally
rimmed by secondary magnetite, or as polymineralic grains
intergrown with magnetite and sulphides (e.g., pentlandite,
heazlewoodite; Britten, 2017; Milidragovic and Grundy, 2019).
Chlorite and talc are common in pyroxene-rich rocks.
6. Carbonate alteration and listwanite
The degree of carbonate alteration and dehydration
of ultramafi c rocks is highly variable. The CO2-bearing
assemblages form a spectrum consisting of serpentine-
magnesite (ophicarbonate), talc-magnesite (soapstone),
and quartz-magnesite±fuchsite (listwanite), in order of
increasing carbonation. The colour of the rocks, their magnetic
susceptibility, and specifi c gravity vary by the extent to which
they have been carbonated (Figs. 4d-f; Cutts et al., in press).
Ophicarbonates are common throughout the Decar area and
are nearly indistinguishable from serpentinites in the fi eld
but can be identifi ed in thin section and by bulk chemistry.
Carbonate minerals occur in partially to highly serpentinized
samples or even in samples containing metamorphic olivine.
Carbonate veins overprint serpentine veins (±magnetite) and
serpentine in groundmass (Fig. 4d), indicating that carbonate
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alteration post-dated serpentinization. In moderately carbonate-
altered samples (Fig. 4e), relict serpentine typically exhibits
an interlocking or interpenetrating texture, likely antigorite.
Brucite grains occur in weakly carbonated samples but are
absent in more carbonated samples. Awaruite, sulphides, and
spinel persist except in the most strongly carbonated rocks.
In fully carbonated rocks, relict serpentinite is rare, although
locally it may occur in small patches, both with or without
spinel. Fuchsite is most abundant adjacent to <10 cm thick
quartz veins and diminishes in abundance away from the veins
(Fig. 4f).
7. Timing of serpentinization and carbonate alteration with
respect to dikes in the Trembleur ultramafi te
Several generations of non-ultramafi c dikes occur in the
Decar area. In the Baptiste area, at least some serpentinization
and carbonation was after emplacement of a dike that is altered
to a rodingite (a non-ultramafi c rock that is altered by reduced
and alkaline fl uids during serpentinization; Barnes et al., 1972;
Bach and Klein, 2009; Britten, 2017) and an altered dike of
probable Rubyrock igneous complex affi nity. A third, relatively
fresh fi ne-grained intermediate dike apparently post-dates
serpentinization and is likely Eocene.
8. Discussion and implications
8.1. Origin of dunite
The origin of peridotites in the Trembleur ultramafi te has
been discussed by Britten (2017), Milidragovic and Grundy
(2019) and Grundy (2018). The predominantly harzburgitic
mineralogy, with subordinate depleted lherzolite, is consistent
with a refractory origin for the Decar peridotites resulting
from high degrees of partial melting of a fertile precursor
(Britten, 2017; Milidragovic and Grundy, 2019). The
harzburgite (±lherzolite) is more depleted (≤2 wt.% Al2O3),
than the moderately depleted lherzolitic upper mantle (DMM1;
2.38 wt.% Al2O3 and ca. 8% clinopyroxene at 1 GPa: Workman
and Hart, 2005). Below we consider the origin of the dunite.
Based on fi eldwork, the dunite comprises ~15% of the
total ultramafi c rocks. In contrast, using drill core from the
Baptiste deposit, Britten (2017) estimated that dunite makes up
only ca. 5% (407 out of 7739 splits) of the peridotite volume
at Decar. Britten (2017) may have underestimated the true
abundance of dunite because the drill core from Baptiste is
highly serpentinized and data were at times collected from
splits greater than 5 m. Dunite observed in the fi eld is typically
restricted to <0.5 m thick dikes or layers. Consequently,
these small dunite bodies could be diluted and misidentifi ed
as olivine-rich harzburgite in examination of drill core bulk
chemical data (Britten, 2017).
Three origins of dunite have been proposed: 1) as residue
from extensive partial melting of fertile peridotite in the
mantle (melt fraction >35%; e.g., Takahashi et al., 1993); 2)
as a cumulate due to fractionation of olivine from mafi c melt
or liquids; 3) as a replacement of pyroxene-rich harzburgite or
lherzolite by a magnesian±Cr rich magma, commonly at mantle-
crust transition zones in ophiolites (Quick, 1981; Nicolas
and Prinzhofer, 1983; Kelemen, 1990). At some localities,
multiple origins of dunite have been suggested (e.g., the Trinity
peridotite; Quick, 1981), but more commonly, one process is
predominant. The relationship of the dunite shape, size, and
contacts with the host rock (sharp or diffuse) is important in
understanding the origin of dunite (Kelemen, 1990; Kubo,
2002; Morgan and Liang, 2003). However, if highly strained the
shape of a dunite body (e.g., lenticular lenses) may no longer
refl ect the original magmatic process (Nicolas and Prinzhofer,
1983). Residue dunite is best distinguished from other origins
by bulk-rock and olivine compositions (Kelemen, 1990; Su
et al., 2016). Cumulate dunite typically shows systematic
layering, with dunite and chromitite at the base, and troctolitic
and/or wehrlite, pyroxenite, and gabbro at the top (Nicolas and
Prinzhofer, 1983). Replacement dunite is typically irregularly
shaped and displays evidence of volume increase, it may
contain traces of clinopyroxene (Kelemen, 1990), and spinel
should show a more equant habit relative to that in host rocks
(Nicolas and Prinzhofer, 1983; Arai, 1994; Dandar et al., 2019).
The discordant and irregular shaped dunite layers and pods
described above resemble replacement dunites described by
Kelemen and Ghiorso (1986) and Kelemen (1990). Field and
thin section observations suggest that spinel is equant and more
abundant in dunite compared to the mostly irregular, vermicular,
and commonly fi ne-grained primary spinel in the surrounding
harzburgite. A replacement dunite mechanism would also be
consistent with the fi ne-grained clinopyroxene found in dunite
resulting from the consumption of orthopyroxene (Kelemen,
1990). These observations suggest that the dunite at Decar, or at
least at Mt. Sidney Williams and the unnamed ridge, may have
formed by replacement. Further geochemical characterization
is underway to determine the origin of dunite at this locality.
8.2. Serpentinization processes
Field and petrographic observations can place constraints on
the timing of serpentinization and the source of hydrothermal
fl uids. Thin-section and outcrop-scale observations indicate
that fl uid infi ltration during serpentinization of dunite differs
from that of harzburgite and lherzolite. Movement of H2O-rich
fl uids through dunite was apparently along fractures related
to veins compared to less focussed fl ow in harzburgite and
lherzolite (Figs. 3a-d). The grain size and modal abundance of
primary minerals (i.e., olivine, pyroxenes, spinel) along with
the chemistry and pH of fl uids can play an important role in the
serpentinization rate and variability (Barnes et al., 1972; Lafay
et al., 2012). The overall rate of serpentinization is typically
fastest for olivine and slowest for clinopyroxene (Coleman and
Keith, 1970; Moody, 1976; Wicks and Whittaker, 1977; Komor
et al., 1985); however, this general rule can vary as a function
of the composition of the serpentinizing fl uid. Olivine may be
more resistant to serpentinization than orthopyroxene during
interactions with high Mg2+-rich fl uids such as during seawater-
peridotite interaction. In contrast, orthopyroxene tends to be
more resistant than olivine during interactions with fl uids that
Geological Fieldwork 2019, British Columbia Ministry of Energy, Mines and Petroleum Resources, British Columbia Geological Survey Paper 2020-01
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Steinthorsdottir, Cutts, Dipple, Milidragovic, and Jones

are Si(OH)4-rich; such fl uids are generated when seawater
reacts with crustal rocks before reaching peridotite (Peacock,
1987; O’Hanley, 1996). Observations of partially serpentinized
harzburgite and lherzolite show that olivine grains are the most
altered, followed by orthopyroxene; clinopyroxene is relatively
unaltered (Fig. 3e). The extent of alteration of these primary
minerals could indicate that most of the serpentinization did
not occur at the seafl oor, which is consistent with isotope data
indicating a meteoric fl uid source (Britten, 2017).
Protolith composition can exert a primary control on
alteration mineral assemblages, along with fl uid composition,
temperature, and pressure. For example, temperature, oxidation
of Fe or high SiO2 can favour formation of serpentine or talc
instead of brucite (O’Hanley, 1996; Evans et al., 2013; Sciortino
et al., 2015). Harzburgite seems to have a more extensive
formation of secondary magnetite relative to dunite which can
also be seen in a general higher magnetic susceptibility (Toft et
al., 1990; Cutts et al., in press). Possible olivine compositional
differences in dunite versus harzburgite could be the cause of
variability in awaruite grain size or/and abundance. Future
work will explore the formation and stability controls of brucite
and awaruite in more detail.
9. Summary
The protoliths of altered rocks in the Trembleur ultramafi te
are heterogeneous and dunite appears to be more abundant
than previously thought. Most of the Trembleur ultramafi te
is partially to pervasively serpentinized and the primary
olivine-pyroxene ratio of the protoliths seems to have exerted
a primary control on the types and abundances of alteration
minerals, such as brucite and awaruite, and the style of
infi ltration of serpentinizing fl uids. Carbonate alteration post-
dated serpentinization, and consumed brucite and awaruite.
The heterogeneity in the Trembleur ultramafi te protolith and
the distribution and extent of alteration thus has implications
for the abundance and distribution of brucite and awaruite and
therefore on the carbon sequestration and nickel potential of
the area.
Acknowledgments
We are grateful for both the Tl’azt’en Nation and Binche
Whut’en First Nation for their help and knowledge of the
Decar area. Special thanks to Edgar John and Marian Duncan,
First Nation representatives, for their help in the fi eld. FPX
Nickel Corp. and geologists Peter Bradshaw and Trevor Rabb
are thanked for their support, discussions, and knowledge of
the Decar area. We also thank Alex Zagorevski for a critical
review. This research is funded by Natural Resources Canada
Clean Growth Program and FPX Nickel Corp.
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