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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.
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Origin and serpentinization of ultrama 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
ultrama 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 ultrama 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 ultrama 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 ultrama 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 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 ultrama 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 ultrama 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 ultrama 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 ultrama c protoliths are
heterogeneous and consists of diverse peridotites and lesser
pyroxenites. The serpentinized ultrama 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 eld and petrographic results of a study designed to evaluate
the protoliths and alteration of the Trembleur ultrama 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 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 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 ultrama 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. Modi ed from Britten (2017)
and Milidragovic (2019).
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Steinthorsdottir, Cutts, Dipple, Milidragovic, and Jones
and metasedimentary rocks of the Rubyrock igneous complex
(early Permian to Late Triassic); altered ultrama c rocks of the
Trembleur ultrama 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 ultrama 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 ultrama te.
The least serpentinized and carbonate-altered rocks of
the Trembleur ultrama 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 ultrama te.
Ophicarbonate (serpentine-magnesite), soapstone (talc-
magnesite), and listwanite (magnesite-quartz) assemblages are
mainly in the southeastern and eastern part of the Trembleur
ultrama te (Milidragovic et al., 2018).
3. Methods
A total of 89 hand samples of ultrama c rocks were selected
for this study (Fig. 1b). Of these, 55 are surface samples collected
during the 2019 eld season, 15 are surface samples collected
during the 2017 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 ultrama 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 ultrama te protolith
The ultrama 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 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 ultrama 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 dif cult
to differentiate between the two rock types in the 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 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 ultrama 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, 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 ner grained clinopyroxene, olivine,
and red to black spinel of holly leaf texture. In clinopyroxene-
rich pyroxenite, the clinopyroxene grains are >2 mm with 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 ultrama te
All the rocks we observed in the 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
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 re 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 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, 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 ultrama 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 speci 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 eld
but can be identi 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 ultrama te
Several generations of non-ultrama 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-ultrama c rock that is altered by reduced
and alkaline uids during serpentinization; Barnes et al., 1972;
Bach and Klein, 2009; Britten, 2017) and an altered dike of
probable Rubyrock igneous complex af nity. A third, relatively
fresh 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 ultrama 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 eldwork, the dunite comprises ~15% of the
total ultrama 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 eld is typically
restricted to <0.5 m thick dikes or layers. Consequently,
these small dunite bodies could be diluted and misidenti 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 ma 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
re 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 ne-grained primary spinel in the surrounding
harzburgite. A replacement dunite mechanism would also be
consistent with the 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
uids. Thin-section and outcrop-scale observations indicate
that uid in ltration during serpentinization of dunite differs
from that of harzburgite and lherzolite. Movement of H2O-rich
uids through dunite was apparently along fractures related
to veins compared to less focussed 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 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 uid. Olivine may be
more resistant to serpentinization than orthopyroxene during
interactions with high Mg2+-rich uids such as during seawater-
peridotite interaction. In contrast, orthopyroxene tends to be
more resistant than olivine during interactions with uids that
Geological Fieldwork 2019, British Columbia Ministry of Energy, Mines and Petroleum Resources, British Columbia Geological Survey Paper 2020-01
56
Steinthorsdottir, Cutts, Dipple, Milidragovic, and Jones
are Si(OH)4-rich; such 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 sea oor, which is consistent with isotope data
indicating a meteoric uid source (Britten, 2017).
Protolith composition can exert a primary control on
alteration mineral assemblages, along with 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 ultrama te
are heterogeneous and dunite appears to be more abundant
than previously thought. Most of the Trembleur ultrama 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
in ltration of serpentinizing uids. Carbonate alteration post-
dated serpentinization, and consumed brucite and awaruite.
The heterogeneity in the Trembleur ultrama 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 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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Steinthorsdottir, Cutts, Dipple, Milidragovic, and Jones
... The Decar Nickel District was selected as a case study site to further test volume estimation from magnetic inversion, and to explore the effect of remanence on inversion results. The site benefits from having extensive drilling, recent mapping (Milidragovic, 2019), detailed lithological/geochemical data (e.g., Britten, 2017;Milidragovic and Grundy, 2019;Steinthorsdottir et al., 2020), and detailed aeromagnetic survey coverage. ...
... The ultramafic rock package occurs as an oblong body elongated in the NW-SE direction that parallels the regional structural grain (Fig. 8). The rocks are variably serpentinized-although most are highly-serpentinized-and a belt of carbonated rocks (ophi-carbonate, soapstone, and listwanite) is inferred to occur along the central axis of the ultramafic body Milidragovic, 2019;Milidragovic and Grundy, 2019;Steinthorsdottir et al., 2020). Serpentinite density values directly correlate with degree of serpentinization and the samples show the full range of magnetic susceptibilities, although typically they are relatively high (>30 SI x 10 -3 ). ...
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... The 441 samples in this study were collected from the western Canadian Cordillera by various research groups (e.g., Hansen et al., 2004Hansen et al., , 2005McGoldrick et al., 2017McGoldrick et al., , 2018Milidragovic & Grundy, 2019;Steinthorsdottir et al., 2020;Zagorevski et al., 2018 and references therein) from the traditional territories of the Taku River Klingit, Kaska Dena, Tāłtān Konelīne, Carcross/Tagish, Teslin Tlingit, Tl'azt'en, Binche Whut'en, Yekooche, and Takla First Nations. All localities comprise rocks that are part of the Atlin (equivalent to the undivided Cache Creek in southern B.C.) terrane (Figure 1), which represents Middle Permian to Middle Triassic discontinuous, dismembered ophiolitic massifs that may have formed either as ocean core complexes or in supra-subduction zone settings . ...
... All localities comprise rocks that are part of the Atlin (equivalent to the undivided Cache Creek in southern B.C.) terrane (Figure 1), which represents Middle Permian to Middle Triassic discontinuous, dismembered ophiolitic massifs that may have formed either as ocean core complexes or in supra-subduction zone settings . The samples were not subjected to greater than greenschist-facies conditions post-emplacement Milidragovic & Grundy, 2019;Steinthorsdottir et al., 2020;Zagorevski et al., 2018) and, thus, should preserve their serpentinization-and carbonation-related physical properties (Shive et al., 1988). The northern segment is subdivided into the ophiolitic Atlin terrane and sedimentary overlap assemblages of the Cache Creek complex, whereas the southern segment remains undivided . ...
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Orogenic dunites are common in high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belts. This paper reviews the origins of orogenic dunites from over 40 localities around the world and identifies three major types: residual dunite, replacive dunite and cumulative dunite. These orogenic dunites, which were generated from different formation processes, are expected to have diverse compositional characteristics and geodynamic histories. Residual dunites are the products of high degrees of partial melting (25–60%) of the primitive mantle and exhibit high Mg# values (commonly > 92) and low FeOT values (< 8 wt.%) in bulk-rock compositions, which are similar to ancient cratonic peridotites. These dunites contain olivines with high Fo values [atomic 100 ∗ Mg/(Mg + Fe)] (mostly 92.0–93.2) and spinels with high Cr# values [atomic 100 ∗ Cr/(Cr + Al)] (50–90) and low TiO2 concentrations (< 0.2 wt.%). The Re–Os isotopic data indicate that these dunites were mainly derived from the Archean–Paleoproterozoic subcontinental lithospheric mantle (SCLM). In contrast, replacive dunites were formed from the reaction of ambient pyroxene-rich peridotite with silica-undersaturated melt. The typical texture of the melt–rock reaction results from the dissolution of orthopyroxene associated with the precipitation of olivine neoblasts. Compared with the residual dunites, replacive dunites generally have lower SiO2 and higher FeOT concentrations (> 8 wt.%) for a given MgO concentration in the bulk compositions. These patterns obviously deviate from the partial melting curves. Olivines from replacive dunites show relatively low Fo values (mostly 90.4–92.4) with variable NiO contents (0.18–0.53 wt.%), and spinels yield a positive correlation between the Cr# (< 65) and TiO2 contents (up to 0.6 wt.%). The osmium isotopic compositions of the replacive dunites, ranging from subchondritic to suprachondritic, are higher than those of the residual dunites and yield young or future rhenium-depletion ages (TRD). Additionally, the replacive dunites display variable platinum group element (PGE) abundances ranging from slight enrichment to extreme depletion relative to the ambient pyroxene-rich peridotites. The cumulative dunites were formed via crystal cumulation from mantle magmas and only occupy a small proportion of the orogenic dunites. These dunites generally exhibit a large range in bulk compositions that overlap the values of residual and/or replacive dunites due to the diverse compositions of the original magma, various crystallization environments, and late-stage crustal metasomatism. Nevertheless, the olivines in the cumulative dunites have the lowest Fo values (mostly 88.5–90.9) among the orogenic dunites and feature a rapid decrease in the NiO content with decreasing Fo values. Moreover, the spinels have remarkably lower Mg# values and higher TiO2 contents than those from residual and replacive dunites, making them a useful mineral for distinguishing cumulative dunite from the other two types.
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A possible Mesozoic-Cenozoic trajectory for the North American craton is outlined from latitude changes of the craton derived from a revised apparent polar wander path, and from westward movement of the craton based on the assumption that Africa has been the least mobile continent geographically since the latest Paleozoic. During each of five time intervals that span 220 million years, the craton trajectory had a different vector. Each vector appears to be reflected in the Canadian Cordillera by the dominant style and orientation of structures formed during that interval. For much of the past ~ 220 million years, the ~meridionally-oriented western margin of the Pangea-Laurasia-North America Plate has been the site of arc magmatism where weak arc/back arc lithosphere, sandwiched between strong ocean-floor and craton lithospheres at times coupled across convergent or transform plate boundaries, focused and recorded strain. When the craton apparently moved due westward, between ~180–160 Ma and ~120–60 Ma, the dominant structures formed then record orogen-normal compression and were accompanied by orogeny. Structures formed during the earlier episode are mainly in the eastern and interior Cordillera and their formation shortly followed or coincided with accretion of most terranes to the craton margin. In the later episode, compressional structures span the entire Cordillera, which emerged as a tectonic and physiographic entity. Before and between these intervals, when the craton apparently moved mostly northwestward, mainly geological and paleomagnetic considerations indicate some terranes moved southward (sinistrally) relative to the craton. After ~60 Ma, southwestward movement of the craton was concurrent with large northward (dextral) strike-slip faults that disrupted the newly-established ancestral Cordillera. The coincidence between the age of structures that record dominant orogen-normal compression at times when the craton apparently moved due westward, and orogen-parallel displacements when the craton had either northward or southward components of motion, suggests the craton, acting as a “Continental Bulldozer” was the primary driver of Cordilleran deformation and orogenesis.
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Upper Paleozoic to Lower Jurassic oceanic rocks of the Cache Creek Terrane near Fort St. James, in central British Columbia, form a stack of thrust sheets cut by steeply dipping strike-slip faults. Paleontologically dated upper Paleozoic strata include bioclastic shallow-water limestone and ribbon chert. Isotopically dated Permian rocks consist of tonalite sills and stocks and rhyolite flows intercalated with basalt flows. Paleontologically dated lower Mesozoic rocks include greywacke, sandstone, siltstone, argillite, ribbon chert, conglomerate, limestone, and basalt tuff. Trembleur Ultramafite unit of the Cache Creek Complex, in places part of an ophiolite suite, forms thrust sheets and klippen that overlie lower Mesozoic sedimentary rocks. Sedimentological, lithochemical, paleontological, petrological, and textural comparisons with other areas and established models demonstrate that Cache Creek Terrane is an accretionary complex, a structurally stacked assemblage of rocks that originated in diverse and disparate oceanic paleoenvironments. These environments include spreading ridge, oceanic plateau, atoll, trench fill, and possibly arc. Internal imbrication of the terrane is as young as Early Jurassic, as determined from fossil evidence, and the minimum age of obduction of the thrust stack westward onto Stikine Terrane is Middle Jurassic, as determined from dating of a crosscutting pluton. Triassic blueschist and eclogite of Cache Creek Terrane are interpreted to have been primarily uplifted to upper crustal levels during Triassic subduction. Cache Creek Terrane, as a remnant of that subduction process, and caught in the collision between Stikine and Quesnel terranes, marks the position of a lithosphere-scale suture zone, the Pinchi Suture.
Article
Eight occurrences and one large resource of broadly disseminated awaruite mineralization discovered since 2008 are all hosted in serpentinized ultramafic units of the Cache Creek complex in northern British Columbia and southern Yukon. The most significant discovery that has been delineated to date is the Baptiste deposit at the Decar property, with an indicated resource of 1.160 billion tons (Bt) at 0.124% magnetically recovered nickel and an inferred resource of 0.87 Bt at 0.125% magnetically recovered nickel. It is the only known deposit consisting solely of awaruite to generate a positive preliminary economic analysis. The disseminated awaruite (Ni3Fe) within mineralized zones in the Decar property has compositions that average 76.9% Ni, 21.6% Fe, 0.6% Co, and 0.8% Cu. This awaruite has very high magnetic susceptibility (higher than magnetite) and a high specific gravity of 8.2. These properties make awaruite amenable to magnetic and gravity separation that yields a high-quality product containing minimal sulfur. Alloy grains range in size from <50 to >400 μm, whereas awaruite <10 μm cannot be recovered easily using mechanical processes. Peridotites containing 10 to 30% medium to coarse orthopyroxene and minor clinopyroxene are the main host of significant awaruite mineralization at the Decar and Mich properties. The homogeneous and coarser (<50->200 μm) awaruite mineralization is associated with antigorite-lizardite-magnetite assemblages that formed at temperatures >∼300°C in the Decar and Mich properties, contrasting with the fine-grained sized (2-20 μm) awaruite-bearing assemblages associated with lower temperature (<300°C), lizardite-chrysotile-magnetite ± antigorite assemblages in the Wale, Orca, and Letain occurrences. Metamorphic olivine and diopside noted locally in the antigorite-dominant assemblages indicate temperatures reached above 450°C and precipitated more abundant coarser awaruite grains in the Decar and Mich properties. Two populations of sparse fine-and coarse-grained awaruite mineralization within the Baptiste deposit of the Decar property suggest that many of the early fine-grained awaruite was dissolved and moved short distances (<50 μm->2 mm) to precipitate coarser grains as temperatures increased from 300° to 450°C. The Cache Creek terrane accreted to ancestral North America sometime after the lower Mesozoic, generating later north-northwest shear and regional metamorphic events that included serpentinization, the process that generated the awaruite mineralization in this region. Depleted upper mantle material, predominantly harzburgite but also lherzolite and lesser amounts of dunite, within the Cache Creek ophiolite sequence typically contains 0.22 to 0.30% Ni that initially resided primarily in olivine. In alpine or suture settings, microfracturing and foliations are generally subparallel to northwest shears or faults, and other fracture systems that provided high permeability to focus continental or meteoric waters. Regional metamorphism, shearing, and exothermic serpentinization generated a source of heat to produce the pervasive awaruite-bearing antigorite-lizardite-magnetite assemblages. The ferrous component in magmatic olivines maintained low fO2 values during mineralizing and serpentinization events and was buffered by relict olivines in mineralized zones. This condition caused oxidation of iron and the precipitation of magnetite in addition to generating a high H2-rich metamorphic fluid, with Ni and Fe ions reduced, mobilized, and stabilized as awaruite during serpentinization. Serpentinization of orthopyroxene increased aSiO2, reduced the production of brucite, and possibly aided the growth of coarsegrained awaruite. Peridotites in abyssal, subduction, and mantle settings are not likely to have generated the coarse-grained awaruite mineralization caused by serpentinization in these environments; however, peridotites from any of these settings could be a potential host following accretion and shearing in an alpine or suture setting assuming attendant higher temperature serpentinization and magnetite-awaruite mineralization.
Article
Given quantitative information on mantle melting, conditions of melting of various basalt magmas and the nature of their source materials are discussed. The conclusions are consistent with the hypothesis that typical mid oceanic ridge basalts represent low pressure, low temperature partial melting products of mantle peridotite. Island arc picritic tholeiites may also be regarded as partial melts of a peridotitic source, at 1-2 GPa pressures and TP ranging from 1400 to 1500°C. However, proposed primary magmas for Hawaiian tholeiite are difficult to produce by partial melting of typical mantle peridotite at any depth under anhydrous conditions. -from Authors
Article
Serpentinized dunites and harzburgites from the Burro Mountain peridotite show no change in the ratio of iron and magnesia to silica when compared with the same ratio for the unserpentinized equivalents. The mineral assemblage resulting from serpentinization consists of lizardite-chrysotile, brucite, and magnetite and is determined by the original bulk composition of the peridotite. The chemical and mineralogical data indicate that serpentinization proceeded under isochemical conditions except for the introduction of water into the peridotite. Expansion accompanies serpentinization because the serpentine products occupy a greater volume than the peridotite protolith. Tectonic emplacement of the Burro Mountain peridotite was facilitated by serpentinization and the attendant expansion.
Article
The Dumont sill in the Abitibi region of Quebec is a zoned intrusive body which is differentiated into a lower ultramafic zone, composed of peridotite and dunite, overlain by a mafic zone, with clinopyroxenite, gabbro, and quartz gabbro subzones. Although the dunite contains several layers enriched in primary Ni sulfide minerals, which add considerable value to the deposit, 47% of the resource consists of awaruite- and heazlewooditebearing serpentinized dunites which formerly contained Ni only in silicate minerals. Serpentinization occurred isochemically with respect to major components, apart from the addition of H2O and minor loss of CaO. Serpentinization occurred under conditions of very low water-rock ratio, resulting in buffering of fluid to extremely low activities of silica and H2O and allowing the widespread replacement of primary pentlandite by awaruite and heazlewoodite due to accompanying reductions in fO2 and fS2. In layers containing primary accumulations of magmatic sulfide, serpentinization has resulted in the remobilization of nickel from olivine to enrich cumulus sulfides. In layers lacking primary sulfide accumulations, serpentinization was accompanied by the formation of awaruite by reduction of Ni originally hosted by olivine. Early stages of serpentinization were marked by the generation of large volumes of Fe-rich serpentine containing abundant ferric iron as well as lesser amounts of brucite rich in ferrous iron. Later stages of serpentinization caused the replacement of early iron-rich serpentine and brucite by more magnesian serpentine and brucite and the formation of abundant magnetite. Assemblages that are completely serpentinized contain lower nickel in silicates, higher nickel tenor sulfides, and more modal awaruite compared to those that are weakly and partially serpentinized, which are characterized by higher nickel in silicates and lower Ni tenor sulfides. Although awaruite first appears early in the serpentinization process, the highest modal abundances of economically extractable heazlewoodite, awaruite, and Ni-rich pentlandite are observed in rocks that have been completely transformed to the Mg serpentine facies. Regardless of the final opaque mineral host for Ni, the key to removal of Ni from silicate minerals and its sequestration in phases amenable to beneficiation is the early formation of awaruite; otherwise, Ni partitions readily into serpentine and remains inaccessible to mineral extraction. Exploration for similar deposits should be focused on dunite adcumulates formed from unfractionated komatiites, which have undergone complete serpentinization under conditions of diffusion-controlled low water-rock ratio, as evidenced by the absence of wholesale deformation or veining.