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Testing the relationship between the Llewellyn fault, gold mineralization, and Eocene volcanism in northwest British Columbia: A preliminary report

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  • Ministry of Energy and Mines and Petroleum Resources, Government of British Columbia, Victoria, BC
Technical Report

Testing the relationship between the Llewellyn fault, gold mineralization, and Eocene volcanism in northwest British Columbia: A preliminary report

Abstract and Figures

The Llewellyn fault represents a significant geological feature in northwest British Columbia. The fault is at least 100 km long, northwest striking, and steeply dipping. An early ductile history is preserved as foliations, lineations, and local folds in the wallrocks. The fault has a later brittle history, preserved as gouge and multiple fracture sets. Dextral offset related to the brittle deformation overprints earlier ductile fabrics. The quartz-carbonate vein-hosted, past-producing Engineer gold deposit (epithermal, low-sulphidation type) is related to this late brittle deformation. Available geochronological data indicate that the deposit formed in the Eocene (ca. 55-50 Ma). Along the likely extension of the Llewellyn fault to the north in the Yukon (Tally-Ho shear zone) are the Mount Skukum gold deposit and a series of gold deposits at Montana Mountain, also of the low-sulphidation epithermal-type. Both the host volcanic rocks at Mount Skukum (Sloko Group) and gold mineralization are coeval (ca. 55 Ma). Because Mount Skukum gold mineralization is directly related to Eocene volcanism, and Engineer gold mineralization is both spatially and temporally coincident with Eocene magmatism, preliminary comparisons suggest a three-part relationship between large-scale structure, gold mineralization, and Eocene magmatism in northwest British Columbia and southwest Yukon.
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Testing the relationship between the Llewellyn fault, gold
mineralization, and Eocene volcanism in northwest
British Columbia: A preliminary report
Luke Ootes1, a, Jessica M. Elliott2, and Stephen M. Rowins1, 2
1 British Columbia Geological Survey, Ministry of Energy and Mines, Victoria, BC, V8W 9N3
2 School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, V8P 5C2
a corresponding author: Luke.Ootes@gov.bc.ca
Recommended citation: Ootes, L., Elliott, J.M., and Rowins, S.M., 2017. Testing the relationship between the Llewellyn fault, gold
mineralization, and Eocene volcanism in northwest British Columbia: A preliminary report. In: Geological Fieldwork 2016, British Columbia
Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1, pp. 49-59.
Abstract
The Llewellyn fault represents a signi cant geological feature in northwest British Columbia. The fault is at least 100 km long, northwest
striking, and steeply dipping. An early ductile history is preserved as foliations, lineations, and local folds in the wallrocks. The fault has a later
brittle history, preserved as gouge and multiple fracture sets. Dextral offset related to the brittle deformation overprints earlier ductile fabrics. The
quartz-carbonate vein-hosted, past-producing Engineer gold deposit (epithermal, low-sulphidation type) is related to this late brittle deformation.
Available geochronological data indicate that the deposit formed in the Eocene (ca. 55-50 Ma). Along the likely extension of the Llewellyn fault
to the north in the Yukon (Tally-Ho shear zone) are the Mount Skukum gold deposit and a series of gold deposits at Montana Mountain, also of
the low-sulphidation epithermal-type. Both the host volcanic rocks at Mount Skukum (Sloko Group) and gold mineralization are coeval (ca. 55
Ma). Because Mount Skukum gold mineralization is directly related to Eocene volcanism, and Engineer gold mineralization is both spatially and
temporally coincident with Eocene magmatism, preliminary comparisons suggest a three-part relationship between large-scale structure, gold
mineralization, and Eocene magmatism in northwest British Columbia and southwest Yukon.
Keywords: Llewellyn fault, gold mineralization, Eocene magmatism, Mount Skukum Mine, Engineer Mine
1. Introduction
More than 100 km long, the Llewellyn fault is a major, steeply
dipping, northwest-striking structure in northwest British
Columbia (Figs. 1, 2; Mihalynuk et al., 1994; 1999). Spatially
related to the fault are quartz-carbonate vein-hosted gold-
silver and base-metal prospects, including the past-producing
Engineer mine (Fig. 2; Mauthner et al., 1996; Millonig et al.,
2015). Also spatially related are Eocene volcano-plutonic
centres (Fig. 2; Mihalynuk et al., 1999). The Llewellyn fault
appears to continue northwest into Yukon (Tally-Ho shear
zone, Fig. 2; Doherty and Hart, 1988; Hart and Pelletier, 1989)
as indicated by similarities in vein-hosted precious and base-
metal deposits and spatially related Eocene volcanic complexes
(Doherty and Hart, 1988; Love, 1989; Love et al., 1998).
The spatial relationship between the Llewelyn fault and
vein-hosted mineralization supports a structural link, and
geochronological data indicate a temporal link between gold
mineralization (Engineer and Mount Skukum mines) and
Eocene volcanism (Love et al., 1998; Mihalynuk et al., 1999;
Millonig et al., 2015). Many of the gold deposits are epithermal
(Nesbitt et al., 1986; Walton, 1986; Love, 1989; Mauthner et
al., 1996; Mihalynuk et al., 1999; Love et al., 1998; Millonig
et al., 2015), although some, like those at Montana Mountain
(Fig. 2) are considered mesothermal (Hart and Pelletier, 1989).
The apparent relationship between structure, mineralization,
and magmatism points to the intrusion-related, epizonal, end-
member of the orogenic gold deposit model because such
deposits are typically related to rst-order crustal breaks and
synchronous magmatism (e.g., Goldfarb et al., 2005). To gain
a better understanding of the spatial and temporal relationships
between the Llewellyn fault, gold mineralization, and Eocene
magmatism and the possible connections to the orogenic-style
of gold mineralization, we conducted three weeks of eld
reconnaissance along the trace of the northern segment of
the fault in British Columbia. This work included inspecting
spatially related gold prospects, past-producing deposits and,
where possible, Eocene volcanic rocks in British Columbia
and Yukon. This report summarizes this reconnaissance and
proposes future avenues of research.
2. Geologic setting
We examined the Llewellyn fault between Engineer Mine
and Bennett Lake near the British Columbia-Yukon border
and investigated rock units adjacent to the Klondike Highway
between the communities of Carcross and Fraser (Fig. 2).
Excursions were also made to the past-producing Mount
Skukum and Montana Mountain gold-silver mines in the
southern Yukon (Roots, 1981; Hart and Pelletier, 1989; Love,
1989). Foot traverses were set out by truck along maintained
roads, ATV’s on trails (Bennett Plateau and Mount Skukum),
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Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Fig. 1. Geological terrane map of British Columbia and neighbouring jurisdictions. The study area in northwest BC is highlighted by a black
outline. Modi ed after Nelson et al. (2013).
and helicopter in more remote areas.
The part of the study area in British Columbia was previously
mapped at 1:50,000 scale and incorporated into a regional
geological map at 1:100,000 scale (Mihalynuk et al., 1999).
Geological features in the area of Mount Skukum, Yukon, were
mapped at 1:50,000 scale by Doherty and Hart (1988); detailed
deposit-scale studies were by McDonald (1987), McDonald
and Godwin (1986), Pride (1986), and Love (1989; 1990a, b).
Roots (1981, 1982) and Hart and Pelletier (1989) described
geology of the Montana Mountain area, Yukon, including the
past-producing Venus gold-silver and other mines.
The Llewellyn fault is a northwest-striking, steeply-dipping
strike-slip deformation zone that displays an early ductile
history overprinted by brittle fabrics (Fig. 3). The fault extends
from the Tulsequah area in the south (where the fault crosses
into Alaska), to Bennett Lake, British Columbia, and likely
continues northward into Yukon as the Tally-Ho shear zone
(Doherty and Hart, 1988; Hart and Pelletier, 1989; Mihalynuk
et al., 1999; Tizzard et al., 2009). South of Atlin Lake, the
Llewellyn fault may merge with the southeast striking King
Salmon thrust fault (e.g., Mihalynuk et al., 1999). Between
Tagish and Bennett lakes (see gure 2-1 in Mihalynuk et
al., 1999) the Llewellyn fault marks the eastern extent of the
Wann River gneiss, Florence River Metamorphic suite, and the
Boundary Ranges metamorphic suite, rocks all considered to be
Triassic or older. East of the Llewellyn fault are Triassic-Jurassic
rocks of the Stuhini and Laberge groups. Triassic-Jurassic,
Early Cretaceous, and Late Cretaceous-Tertiary plutons and
minor volcanic complexes are on both sides of the Llewellyn
fault, although predominantly to the west (Mihalynuk et al.,
1999). Eocene volcano-plutonic centres (Ypresian ca. 55 Ma;
Table 1) are preserved adjacent to the trace of Llewellyn fault
along its strike length; the volcanic rocks have been assigned to
the Sloko Group (Fig. 2; Mihalynuk et al., 1999). In the study
area, these magmatic centres generally cap older units, forming
steep, high-elevation terrain. Examples of these centres include
50
Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Ootes, Elliott, and Rowins
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134°30'0"W
134°30'0"W
135°0'0"W
135°0'0"W
135°30'0"W
135°30'0"W
60°15'0"N 60°15'0"N
60°0'0"N 60°0'0"N
59°45'0"N 59°45'0"N
59°30'0"N 59°30'0"N
.
02010
Kilometers
Yuk on
British Columbia
Alaska
British
Columbia
TagishLake
TutshiLake
Tagish Lake
BennettLake
Klondike
Highway
Carcross
Fraser
Mount
Skukum
MoonLake
Teepee
Engineer
Ben-My-Chree
Gridiron
RacineLake
Llewellyn fault
Tally-Hoshearzone
b
nm
nm
b
b
b
b
b
Montana
Mountain
b
b
Fault or shear zone
Intrusive rocks:Early and Middle Eocene
Volcanic rocks (Sloko Group): Early Eocene
Gold-bearing mineral occurrence (BC)
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Mineral occurrence (Yukon)
Atlin Provincial Park
Highway or road
b
Location visited (discussed in text)
Past-producing mine (discussed in text)
LEGEND
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Bennett
Plateau
Mafic dike
Fig. 2. Simpli ed geology (Eocene rock units only) near the Llewellyn fault, northwest British Columbia (Doherty and Hart, 1988; Mihalynuk
et al., 1999).
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Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Ootes, Elliott, and Rowins
Location Interpretation Age (Ma) Mineral Method Source
Engineer Mountain rhyolite - SPS ca. 54 zircon U-Pb - age only presented Mihalynuk et al. 1999
Atlin Mountain west ignimbrite - SG ca. 54 zircon U-Pb - age only presented Mihalynuk et al. 1999
Mount Switzer rhyolite - SG ca. 55 zircon U-Pb - age only presented Mihalynuk et al. 1999
quartz monzonite - SPS ca. 56 zircon U-Pb - age only presented Mihalynuk et al. 1999
diorite - SPS ca. 56 zircon U-Pb - age only presented Mihalynuk et al. 1999
West of Mount
Switzer granite - CPC ca. 55 biotite K-Ar - age only presented Mihalynuk et al. 1999
Teepee Peak granodiorite-tonalite - SPS ca. 55 biotite K-Ar - age only presented Mihalynuk et al. 1999
rhyolite flow - SG ca. 56 zircon U-Pb - age only presented Mihalynuk et al. 1999
Skelly Lake massive rhyolite - SG 58.5 ±1.5 zircon
U-Pb (TIMS) lower
intercept Mihalynuk et al. 1999
middle ridge rhyolite 124.9 ±1.5 zircon U-Pb (TIMS) upper
intercept
Mihalynuk et al. 2003
Montana Mountain* Late Cretaceous? N/A Roots 1981
Mid Cretaceous ca. 95 to 85 Hart 1995
Mount Skukum* syn-ore intermediate dike - SG 55.7 ±0.3 zircon U-Pb (TIMS) Concordia Love et al. 1998
pre-ore rhyolite dike - SG 56.3 ±0.4 zircon U-Pb (TIMS) Concordia Love et al. 1998
Diorite dykes along
Klondike Highway Eocene - CPC 51 ±1
biotite &
whole rock K-Ar Symons et al. 2000
Mineralization
Engineer mineralization 49 ±0.5
V-bearing
muscovite 40Ar/39Ar step heating F. Devine, pers. comm. 2016
Mount Skukum* ore-related alteration 54.1 ±0.3 adularia 40Ar/39Ar step heating Love et al. 1999
pre-ore alteration 55.7 ±0.3 alunite 40Ar/39Ar step heating Love et al. 1999
Montana Mountain* post-Cretaceous N/A Hart and Pelletier 1989
*Yukon
CPC - Coast Plutonic Complex
SG - Sloko Group
SPS - Sloko plutonic suite
N/A – no radiometric age available. Relative age from field
interpretations
Table 1. Temporally relevant magmatism and gold mineralization near the Llewellyn fault, northwest BC and southwest Yukon.
Engineer Mountain and Teepee Peak in the study area (Fig.
2; Mihalynuk et al., 1999), and the Bennett Lake and Mount
Skukum volcanic complexes in the Yukon (Fig. 2; Doherty and
Hart, 1988; Morris and Creaser, 2003).
The Llewellyn fault displays early high-strain fabrics that are
overprinted by later brittle features (Mihalynuk et al., 1999;
Tizzard et al., 2009). Early ductile deformation is marked by a
belt of rocks with a strong northwest striking, steeply dipping
penetrative foliation (Fig. 3a), rare lineations, and local minor
folds. Similar, possibly correlative fabrics are in the Tally-Ho
shear to the north (Doherty and Hart, 1988; Hart and Pelletier,
1989; Mihalynuk et al., 1999; Tizzard et al., 2009). Tizzard et
al. (2009) interpreted that ductile movement along the Tally-
Ho shear zone took place between ca. 208 and 173 Ma. The
timing is based on U-Pb zircon ages of a leucogabbro that
transitions into mylonite in the hangingwall of the shear zone,
and a post-deformation megacrystic granite that crosscuts the
leucogabbro and other mylonites (Tizzard et al., 2009). These
ages support timing arguments presented by Mihalynuk et al.
(1999). Although the Llewellyn fault and Tally-Ho shear zone
are along strike of one another and share early penetrative
deformation, they differ. First, the early fabrics in the Llewellyn
fault are consistently subvertical (Fig. 3; Mihalynuk et al.,
1999), whereas the Tally-Ho shear zone is folded (Tizzard et
al., 2009). Second, the Llewelyn fault is a low-grade strike-slip
shear zone (Mihalynuk et al., 1999), and the Tally-Ho shear
zone is considered a west-over-east thrust fault with mylonite
development (Tizzard et al., 2009).
The Llewellyn fault is mainly de ned by a corridor of brittle
features (Fig. 2), such as fault gouge and vein- lled fractures
that overprint the older ductile deformation fabrics (Fig. 4).
The brittle fabrics are thought to coincide with up to 2 km of
dextral offset along the fault (Mihalynuk et al., 1999). Brittle
overprinting of the early ductile fabrics is also documented
along the Tally-Ho shear zone (Tizzard et al., 2009). The
precise timing of brittle deformation remains uncertain, but
components of it are likely Eocene (Table 1; Mihalynuk et al.,
1999; Tizzard et al., 2009).
3. Mineralization
More than 50 mineral occurrences spatially related to the
Llewellyn fault have been documented in the study area (Fig.
2; for details see Table 14-1 and Figure 14-2 in Mihalynuk et
al., 1999). Many of these deposits and prospects have been
known for more than 100 years, following discoveries during
the Klondike gold rush of the late 1890s. Not surprisingly,
these occurrences are in clusters near road and rail routes and
water access sites at Bennett and Tutshi lakes in the northwest,
and Tagish Lake in the southeast.
Three past-producing vein-hosted precious and base-metal
deposits are in the British Columbia part of the study area.
The Engineer Mine was the most productive gold deposit;
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Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Ootes, Elliott, and Rowins
production at Gridiron and Ben-My-Chree was relatively
minor (Fig. 2). Gold has been mined intermittently at the
Engineer Mine for more than 90 years. BCGold Corp. reported
a NI 43-101-compliant Inferred Mineral Resource of about
41,000 tonnes grading 19.0 g/t of total contained gold (5 g/t
cut-off grade; Dominy and Platten, 2011). Sur cial and
underground maps of the Engineer deposit are presented by
Millonig et al. (2015).
The Engineer mine consists of several gold-bearing quartz-
carbonate veins. Because of historical mining, very little of
the original surface exposures remain (Fig. 5a) and veins and
mineralization are generally only visible in sample dumps
(underground workings are currently inaccessible). The mine
workings have collapsed and these provide the orientation of
past ore-hosting veins (Fig. 5a). The structures hosting ore at
the Engineer mine are related to splays of the Llewellyn fault,
10 km south of a 20° bend (Fig. 2; Mihalynuk et al., 1999;
Millonig et al., 2015). Ore-related alteration includes a mica
that is colloquially referred to as roscoellite, but is a different
vanadium-bearing muscovite (L. Millonig et al., 2015; pers.
comm. 2016). A sample of this mineral has yielded an 40Ar/39Ar
step-heating age of ca. 49 Ma (F. Devine, pers. comm. 2016).
This age supports a temporal relationship between structure,
gold mineralization, and possibly Sloko Group magmatism at
Engineer, although Sloko Group volcanic rocks are slightly
older (ca. 55 Ma; Table 1).
In addition to the past-producers, many vein-hosted precious
and base-metal mineralized prospects and skarn-like precious
metal prospects appear spatially related to the Llewellyn fault.
Collectively referred to as the Golden Eagle project, many
of these prospects are currently held by Troymet Exploration
Corp. One example is the the Skarn Zone gold prospect at
Bennett Plateau (Fig. 2), that is in a zone of deformed quartz
and amphibolite veins and intruded by granitic porphyry dikes
(Fig. 3b).
Similar to British Columbia, many mineral occurrences are
adjacent to the Llewellyn fault in the Yukon. We visited three
past-producing vein-hosted precious and base-metal mines in
the Yukon including Mount Skukum (e.g., Love, 1989), and
two on Montana Mountain (Venus and Arctic Caribou mines;
e.g., Roots, 1981). At Mount Skukum, only minor parts of the
mineralized structures are preserved at the surface, but the
collapsed underground workings provide a visual aid to the
historic location and orientation of the ore-hosting veins (Fig.
5b; Love, 1989, 1990b; Love et al., 1998). The Venus mine
is the most signi cant past-producer at Montana Mountain,
but mineralization is rarely exposed on surface and therefore
sample dumps provide the best material to study (Fig. 5c).
At Arctic Caribou, bedrock is not exposed and only scattered
waste-rock is available along a dilapidated rail-track leading
from an abandoned mine portal.
4. Ongoing and future work
In addition to regional structural analysis, we are conducting
geochronologic studies along the length of the Llewellyn fault.
The purpose of the geochronology is to address the timing of
fault movement and to assess the signi cance of ma c dikes
(ca. 51 Ma; Symons et al., 2000), some of which may be
lamprophyres.
4.1. Structural analysis
Key questions arising from our reconnaissance are: what is
the structural and/or genetic relationship between the Llewellyn
fault and gold mineralization, and how do the structural
relations compare between deposits?
The Engineer and Mount Skukum mines are well mapped
(Love, 1989) and, although the apparent timing of gold
mineralization is similar (Eocene; Table 1), these deposits have
never been directly compared. Love (1990b) used the vein
distribution at Mount Skukum to present a kinematic solution
Fig. 3. Early ductile features near the Llewellyn fault trace. a)
Well-developed cleavage in shale of the Stuhini Group (Triassic;
Mihalynuk et al., 1999). View is southeast, scribe for scale (circled).
b) Ductile deformation fabrics in well-foliated amphibolite gneiss of
the Stuhini Formation or Laberge Group (Mihalynuk, et al., 1999).
Amphibole+quartz veins are attened, folded and boudinaged. In
the foreground is a felsic hornblende-plagioclase porphyry dike that
was injected along foliation in the host rocks. We collected a sample
for U-Pb geochronology. From the Skarn Zone mineral prospect on
Bennett Plateau, east of Bennett Lake; view is to the east.
53
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Ootes, Elliott, and Rowins
Fig. 4. Late brittle features along the Llewellyn fault. a) Fault gouge with brittle deformation features (e.g., fractures) preserved in granodiorite
wall-rock (Mesozoic?). Location is west of Moon Lake and view is to the northwest. Inset is a view of the wall of the fault. b) Road-cut exposure
of the Llewellyn fault preserving brittle deformation features, including cracks and quartz vein- lled fractures in granodiorite (Mesozoic?).
Outcrop is adjacent to the Klondike Highway west of Tutshi Lake.
54
Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Ootes, Elliott, and Rowins
for the deposit (see upper left inset, Fig. 6). Millonig et al.,
2015 mapped the vein distribution at Engineer Mine (see lower
right inset, Fig. 6). Based on these summaries, the veins at both
the Mount Skukum and Engineer mines appear to have formed
under north-northeast directed compression with subsequent
development of strike-slip faults and related fault structures
(Fig. 6). Some veins could have formed along Riedel shears
(R and R’); others may be sigmoidal veins formed normal
to the least principal stress direction (σ3) in the σ1-σ2 plane
(Fig. 6). The kinematics depicted in Figure 6 are idealized,
but anisotropy created during ductile deformation could have
promoted non-idealized geometries during later brittle faulting.
Further structural analysis is required to more accurately relate
vein formation to the evolution of regional structures.
4.2. Geochronology
We collected a systematic suite of samples in an attempt to
better document the timing of movement(s) along the Llewellyn
fault, magmatism, and mineralization; results are pending.
4.2.1. Skarn Zone prospect
We collected a sample of the hornblende-plagioclase
porphyry dike that injected parallel to ductile fabrics (Fig. 3b)
at the Skarn Zone prospect for U-Pb zircon geochronology. The
Skarn Zone contains low-grade gold mineralization in veins of
amphibole and quartz that are now strongly deformed. The
dike is undeformed and strikes parallel to the main wallrock
foliation. The crystallization age of this dike will provide both
a minimum age for the ductile deformation in this area and this
style of gold mineralization.
4.2.2. Granite pluton east of Bennett Lake
A granitic pluton east of Bennett Lake at the British Columbia-
Yukon border appears to crosscut the Llewellyn fault (Fig. 6),
Fig. 5. Surface remains of past-producing gold mines. a) Collapsed
and overgrown mine workings at Engineer Mine. The 1.5 metre-wide,
030°-striking crevasse (collapsed workings, not a trench) represents
the width and strike of mined out hydrothermal veins. b) Mostly mined
out, 3-metre-wide, 040°-striking quartz vein, Cirque Zone, Mount
Skukum. The pit in the foreground is above collapsed underground
workings. All bedrock exposures hosting the hydrothermal veins are
volcanic rocks of the Sloko Group (ca. 55 Ma). c) Waste-rock-pile
at the past-producing Venus Mine, Yukon, adjacent to the Klondike
Highway.
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Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Ootes, Elliott, and Rowins
03015
Kilometres
.
^
^
^
^
Tally-Ho shear zone
Llewellyn fault
YUKON
BC
BC
ALASKA
Mount
Skukum
Mount
Skukum
Engineer
fault cross-cut
by pluton?
Double
decker
s1
Engineer vein
Shear-A
(Eocene)
s3
sigmoidal
veins
200 m
R
R’
Engineer
Lake
zone
Brandy
zone Cirque
zone
200 m
Mount Skukum
s1
s3
sigmoidal
veins R
R’
Gold deposit
Coast Plutonic Complex
Sloko Group volcanic and sedimentary rocks and
associated plutonic rocks
Surface trace of fault
(arrows indicate latest slip direction)
^
^
Surface trace of quartz-carbonate vein
(arrows indicate slip direction)
Shear zone
Inset map legend (Engineer and Mount Skukum)
Map legend
Fig. 6. Map of Llewellyn fault - Tally-Ho shear zone, Coast Plutonic Complex and Sloko Group volcano-plutonic complexes (Eocene), and
locations of past-producing Mount Skukum and Engineer vein-hosted gold mines. The location where a granitic pluton has been mapped
crosscutting the Llewellyn fault is also located. Geology is simpi ed after Doherty and Hart (1988) and Mihalynuk et al. (1999). Inset maps
show surface traces of gold-bearing quartz-carbonate veins at Engineer and Mount Skukum. Principal stress directions are based on major faults
and demonstrate veins are likely related to associated fracture sets (R and R’ and sigmoidal veins). Engineer is simpli ed after Millonig et al.
(2015) and Mount Skukum is modi ed after Love (1990b). Both demonstrate veining at each of the deposits is related to north-northeast directed
compression.
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Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Ootes, Elliott, and Rowins
as mapped by Mihalynuk et al. (1999). The granite post-dates
the early ductile deformation but is cut by a fracture set parallel
to the strike of the fault indicating it may be older than the late
brittle deformation. A U-Pb zircon crystallization age of this
granite will help provide a limit to the timing of both ductile
and brittle strain.
4.2.3. Engineer Mine
A sample of monzodiorite was collected from drill core at
the Engineer Mine. This granodiorite is part of a suite that
cuts the early ductile fabrics at Engineer, but is offset by
discrete brittle faults and veins. A U-Pb zircon crystallization
age for this monzodiorite would yield a minimum age for the
ductile deformation and a maximum age for the gold-related
hydrothermal system.
4.2.4. Granodiorites near Llewellyn fault, west of Moon
Lake
We collected samples of undeformed granodiorite adjacent
to the Llewellyn fault and granodiorite that underwent brittle
deformation in Llewellyn fault (Fig. 4a). These samples will
be used for lower temperature thermochronology including
40Ar/39Ar (hornblende, biotite, and muscovite) U- Pb (monazite
and titanite) and U/Th-He (zircon and apatite). Results of such
analyses will help provide a thermal history of movement along
the Llewellyn fault.
4.2.5. Venus and Arctic Caribou mines, Yukon
Specimens collected from sample dumps at the Venus and
Arctic Caribou mines at Montana Mountain contain arsenopyrite
that may be amenable to Re-Os isochron geochronology (e.g.,
Morelli et al., 2007). These ages could accurately date the time
of mineralization. Previous constraints indicate the timing of
mineralization at Montana Mountain is younger than Paleocene
(Table 1; Hart and Pelletier, 1989).
4.3. Establishing the signi cance of 51 Ma (?) ma c dikes
anking the Llewellyn fault
A series of ma c (gabbro/diabase) dikes cut Cretaceous and
younger plutons of the Coast Plutonic Complex (Figs. 2, 7;
Mihalynuk et al., 1999; Symons et al., 2000). At least some
of which were emplaced at ca. 51 Ma, as determined by K-Ar
whole rock and biotite dating (Table 1; Symons et al., 2000).
We recognized similar, but previously undocumented ma c
dikes, anking the Llewellyn fault. Possibly lamprophyres,
these dikes might be related to early Eocene gold mineralization
along the fault (Table 1). The dikes all have near-vertical dips
but have three distinct strikes (Fig. 2), north (010°, n=3),
northeast (040° to 055°, n=6), and east-southeast (110°, n=3).
The indirect relationship between ma c magmatism,
particularly lamprophyric magmatism, and gold mineralization
throughout geologic time has been recognized globally (e.g.,
Rock and Groves, 1988; Wyman and Kerrich, 1988). Testing
this relationship in northwest British Columbia, in conjunction
with the relationship between Eocene volcano-plutonism and
the deep structure of the Llewellyn fault, could increase its
attractiveness for mineral exploration.
5. Discussion
The Llewellyn fault represents a long-lived, structure of
signi cant strike length along which numerous gold occurrences
are distributed. Similar structures in other parts of the North
American Cordillera (e.g., Bohlke and Kistler, 1986; Nesbitt
et al., 1989) are much better mineralized, as are the structural
‘breaks’ in the Archean Abitibi greenstone belt of the Superior
Province in central Canada (e.g., Dubé and Gosselin, 2007). The
apparent difference in degrees of gold mineralization between
super cially similar structures raises the important question of
whether the Llewellyn fault is actually well mineralized but
underexplored, or whether it has simply failed to produce and
preserve large gold deposits; did late motion along the faults
lead to exposure and erosion? Dating gold mineralization and
associated igneous rocks, further structural analysis of the
deposits and their host rocks, and uid inclusion and isotopic
studies of ore uids will help characterize the physicochemical
conditions of ore formation and identify the key parameters
controlling the location of gold deposits along the Llewellyn
Fig. 7. Vertically dipping, 010°-striking ma c dike (ca. 51 Ma)
that crosscuts Cretaceous-Paleogene granite of the Coast Plutonic
Complex (beige). Adjacent the Klondike Highway near Fraser, BC.
57
Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1
Ootes, Elliott, and Rowins
fault.
Whether or not there are more economic deposits to be
discovered remains to be demonstrated. Unfortunately, much of
the Llewellyn fault in the study area lacks regional geophysical
coverage and remains a blank-spot on Canadian geophysical
maps. Historically such geophysical mapping has generated
new exploration interest in remote areas of Canada. The area
also lacks signi cant publicly available high-resolution digital
imagery. The 50+ mineral prospects and past-producing gold
deposits in a relatively infrastructure-poor region indicates
potential for future mineral exploration. Geophysical and
remote sensing mapping may help attract mineral exploration
and investment to the region.
The timing of gold mineralization and the relationship to
Eocene magmatism adjacent to the Llewellyn fault is replicated
by some gold occurrences in central British Columbia (e.g.,
Blackdome mine; Bordet et al., 2011; 2014). Further de nition
of the structural and temporal relationship between the
Llewellyn fault, gold mineralization, and Eocene magmatism
(Fig. 6) will permit comparison between these districts and will
help establish if, collectively, these areas represent an Eocene
orogenic gold belt.
Acknowledgments
The project is a partnership of the British Columbia
Geological Survey with the Geological Survey of Canada’s
Targeted Geoscience Initiative (TGI) and we bene ted from
eld visits and discussions with P. Mercier-Langevin and
Sébastien Castonguay (GSC Quebec), Adrian Hickin (BCGS),
and F. Devine (Merlin Geoscience). Helicopter support was
provided by Discovery Helicopters, Atlin, BC. The Yukon
Geological Survey kindly provided use of an ATV. BCGold
Corp., Troymet Exploration Corp., and Eagle Plains Resources
shared knowledge and allowed access to their properties.
Constructive reviews by S. Castonguay and F. Devine helped
improve the manuscript.
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... The chosen fi eld laboratory extends from the Tagish Lake area of northwest British Columbia northward to the Wheaton River area in southern Yukon (Fig. 1). This area is the locus of a series of vein-hosted gold prospects and deposits, including past-producing mines (e.g., Engineer, Mount Skukum) that are spatially related to the Llewellyn fault and Tally-Ho shear zone (e.g., Hart and Radloff, 1990;Mihalynuk et al., 1999;Tizzard et al., 2009;Ootes et al., 2017). The area also encompasses multiple lithotectonic suites that are affected by the faults (Fig. 2). ...
... Reconnaissance fi eldwork, data compilation, and preliminary reports (Castonguay et al., 2017;Ootes et al., 2017) have underlined that the epithermal gold mineralization at the Engineer and Mount Skukum deposits coincide both spatially and temporally with Eocene magmatism (also see Love et al., 1998;Millonig et al., 2017). These fi rst-order observations led to the suggestion of a relationship between large-scale deformation zones, gold mineralization, and Eocene magmatism in southwest Yukon and northwest British Columbia. ...
... There, vanadian illite, produced by fl uid-rock interaction during epithermal vein formation, has been dated by 40 Ar-39 Ar at ca. 50 Ma (Millonig et al., 2017). As the epithermal veins at Engineer are related to brittle deformation along the Llewellyn fault, this age approximates the timing of brittle strike-slip movement (e.g., Ootes et al., 2016;Millonig et al., 2017). This timing coincides with Cordillera-wide dextral strike-slip faulting (e.g., Gabrielse et al., 2006). ...
Technical Report
Full-text available
The Llewellyn fault represents a significant geological feature in northwest British Columbia. The fault is a southeast-striking, steeply dipping brittle dextral strike-slip structure that overprints 'early' ductile deformation, which is preserved as foliations, lineations, and folds in the host rocks. The Tally-Ho shear zone, Yukon, shares similar early ductile deformation and is overprinted by the Llewellyn fault. In general, the deformation corridor demarcates the eastern limit of metamorphic suites (Triassic and older rocks of the Nisling terrane and Boundary Ranges metamorphic suite) and the western limit of the younger Stuhini (Triassic) and Laberge (Jurassic) groups. Previous work and this study demonstrate that brittle strike-slip deformation along the Llewellyn fault occurred between ca. 56 and 50 Ma. New fi eld observations indicate the early ductile deformation is represented by one foliation (S main) along the Llewellyn fault and Tally-Ho shear zone corridor. Two granodiorite intrusions crosscut the early deformation features, and new U-Pb zircon chemical abrasion ID-TIMS results indicate they crystallized at ca. 75 Ma. In the Tally-Ho shear zone, the S main is parallel to a foliation in an adjacent granodiorite, mapped as part of the Whitehorse plutonic complex (ca. 120 Ma). In British Columbia, a deformed rhyolite along the Llewellyn fault yielded a preliminary ca. 120 Ma age. Based on these results, we infer that the early ductile fabrics formed before ca. 75 Ma and, potentially, after ca. 120 Ma. A goal of this study was to establish if the early ductile and late brittle structures represent a crustal-scale, ductile-brittle deformation continuum. If so, could various gold mineralization styles (epithermal, mesothermal, intrusion-related) along the structural corridor be related in time and be part of an orogenic gold mineralizing system? This study demonstrates that, although the early ductile and late brittle deformation share the same space, they developed at least ca. 20 Ma apart and are not part of a structural continuum. This result indicates the various styles of gold mineralization developed during temporally distinct tectonic events.
... Reconnaissance work (Castonguay et al., 2017;Ootes et al., 2017) has underlined that gold mineralization at the Engineer and Mount Skukum deposits is both spatially and temporally coincident with Eocene magmatism. These first order observations gave rise to a three-part model between largescale structures, gold mineralization and Eocene magmatism. ...
... This early phase is associated with early-to syn-main foliation quartz-amphibole veins hosted predominantly in metamorphic rocks, such as those at Bennett Plateau (e.g. Skarn Zone; Ootes et al., 2017), and the occurrences to the west of the Llewellyn Fault on the south shore of Tagish Lake (Wann River area). ...
Technical Report
Several gold deposits and prospects in the Canadian Cordillera of British Columbia and Yukon are associated, at least spatially, to major fault zones. The Llewellyn fault and Tally Ho shear zone region of northwestern British Columbia and southern Yukon is the locus of a series of vein-hosted gold prospects and deposits, including the past-producing Engineer and Mount Skukum mines. Many of these gold occurrences have epithermal polymetallic signatures, although some are considered mesothermal-orogenic, as such related to first-order crustal breaks and synchronous magmatism. However, the temporal and genetic relationships between the orogenic- and epithermal-style systems along the Llewellyn fault and Tally Ho shear zone remain to be clearly established. Reconnaissance work has underlined a three-part relationship between these large-scale structures, gold mineralization and Eocene magmatic complexes. Characterization of Llewellyn fault zone in the Bennett Plateau and Tagish Lake area of British Columbia, and of the Tally Ho shear zone in the Tally Ho Mountain, Gold Hill and Mount Hodnett areas of Yukon, and of their spatially associated gold mineralization, aims to test this model and ultimately draw comparisons with the much older, structurally controlled gold deposits of the Archean and Paleoproterozoic. See also companion BCGS report: Testing the relationship between the Llewellyn fault, Tally-Ho shear zone, and gold mineralization in northwest British Columbia: DOI.org/10.13140/RG.2.2.23409.53604
... 72-67 Ma Carmacks Group volcanic rocks along the fault trace to the northwest (Colpron et al., 2016). Paleogene slip likely occurred throughout the northwest-southeast-trending dextral strikeslip fault corridor, coeval with the Tintina and Denali faults (Lanphere, 1978;Hayward, 2015;Sánchez et al., 2014), and also coincides with fault-related magmatism and mineralization in northern British Columbia (Ootes et al., 2017) and in the Canadian Rocky Mountains (Pană and van de Pluijm, 2015), suggesting the existence of a regionally extensive fault-controlled fluidflow event. Subsequent slip at ca. 50 Ma (MC and HC gouge) and ca. ...
... 72-67 Ma Carmacks Group volcanic rocks along the fault trace to the northwest (Colpron et al., 2016). Paleogene slip likely occurred throughout the northwest-southeast-trending dextral strikeslip fault corridor, coeval with the Tintina and Denali faults (Lanphere, 1978;Hayward, 2015;Sánchez et al., 2014), and also coincides with fault-related magmatism and mineralization in northern British Columbia (Ootes et al., 2017) and in the Canadian Rocky Mountains (Pană and van de Pluijm, 2015), suggesting the existence of a regionally extensive fault-controlled fluidflow event. Subsequent slip at ca. 50 Ma (MC and HC gouge) and ca. ...
Article
Full-text available
The timing of slip on brittle faults in Earth’s upper crust is difficult to constrain, and direct radiometric dating of fault-generated materials is the most explicit approach. Here we make a direct comparison between K-Ar dating of fault gouge clay (authigenic illite) and U-Pb dating of carbonate slickenfibers and veins from the same fault. We have dated fault generated materials from the Big Creek fault, a northwest-striking, dextral strike-slip fault system in Yukon Territory, Canadian Cordillera. Both methods yielded dates at ca. 73 Ma and ca. 60–57 Ma, representing at least two periods of fault slip that form part of a complex fault and fluid-flow history. The Cretaceous result lies within previous indirect estimates for major slip on the fault. The Paleocene–Eocene result coincides with the estimated timing of slip of the nearby Tintina and Denali faults, which are crustal-scale, northwest-striking dextral faults, indicating Big Creek fault reactivation during regional faulting. The coincidence of periods of carbonate-crystallizing fracturing and fluid flow with intervals of seismic, gouge-generating slip supports the fault valve model, where fault strength is mediated by fluid pressures, and fluid emplacement requires seismic pumping in otherwise impermeable aseismic fault zones. The reproducibility of slip periods for distinct fault-generated materials using different decay systems indicates that these methods provide complimentary results and can be reliably applied to date brittle fault slip, opening new opportunities for investigating fault conditions with associated mineralizing fluid events.
... Immediately following, at ~177 Ma (Currie, 1994), metamorphism peaked at upper amphibolite facies within the metamorphic rocks west of the Llewellyn fault, suggesting tight interdependency. Sinistral offset related to ductile deformation within the Llewellyn fault zone (older and deeper deformation, which is overprinted by young and shallower, brittle fabrics, Ootes et al., 2017) is suggested by a new, preliminary U-Pb crystallization age of ~185 Ma (Friedman, unpublished) from a 2.5 km-long lens of protomylonitic granodiorite along the Llewellyn fault (Fig. 2) where it crosses southern Tutshi Lake . It is possible that this body intruded into the fault near its present location and intrusive contacts were obscured by later deformation, but we suggest that it is a part of the Hale Mountain granodiorite border that was offset sinistrally, by about 70 km. ...
Article
Full-text available
In: Geological Fieldwork 2016, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2017-1, pp. 153-178. Stikine terrane is one of the largest crustal blocks in the Cordillera, measuring more than 1100 km long and 250 km wide. In northern British Columbia it is well known for its large porphyry Cu-Au ±Mo-Ag deposits. Stikine terrane tapers northward, replaced by a broadening wedge of oceanic crustal rocks of the Cache Creek terrane, interpreted to have overthrust Stikine terrane reducing its exposed width to ~10 km at 60°N. This overthrust region is where Triassic-Jurassic magmatic belts with known porphyry deposits disappear, and it is mostly underlain by Triassic-Jurassic arc-derived clastic rocks of the Whitehorse trough. Overthrusting has traditionally been attributed to the no rthwest-trending King Salmon fault, which carries conspicuous Late Norian Sinwa Formation limestone in its hanging wall. On many terrane maps, the Sinwa Formation marks the western margin of the oceanic Cache Creek terrane. However, clast provenance, biochronology, conodont fossil fauna, and sedimentary facies carried by the King Salmon fault are inconsistent with this interpretation. Instead, these data suggest that rocks in the hangingwall of the King Salmon fault were deposited in the Triassic forearc of the Stikine terrane, isolated from the subducting Cache Creek oceanic lithosphere by an intervening trench. Thus, the King Salmon fault is not a terrane boundary, and although regionally important, it is but one of several faults that carry Sinwa Formation limestone. Complicating this simple tectonic picture are detrital zircons from one sample collected in the footwall of the King Salmon fault. They form a nearly unimodal population with a main peak at 242 Ma, an age unknown in Stikinia but common within volcanic and plutonic rocks of the Kutcho-Sitlika-Venables arc, which have historically been included in the Cache Creek terrane. If this provenance link is correct, it supports the Kutcho-Sitlika-Venables arc as a separate terrane, distinct from the Cache Creek, and juxtaposed with the Stikine forearc before the Bajocian (~173 Ma) juxtaposition of Cache Creek terrane. King Salmon and adjacent fault panels carry steep northeast plunging folds having southeast-dipping axial surfaces, consistent with a top to the north component of motion (or sinistral if originally steep) that may be related to a phase of deformation during latest Triassic Kutcho-Sitlika-Venables arc collision. Keywords: Sinwa Formation, Stuhini Group, Lewes River Group, Whitehorse Trough, Cache Creek terrane, Stikine terrane, Kutcho-Sitlika-Venables arc, Whitehorse trough fold and thrust belt
... The Llewellyn Fault gold project is a collaboration between the GSC and BCGS, under the TGI-5 program (Ootes et al., 2017). The Llewellyn fault is a southeast-striking, steeply dipping brittle dextral strike-slip structure that overprints 'early' ductile deformation (Fig. 9). ...
... Preliminary structural and lithological data were collected from gold deposits in order to determine whether gold mineralization events could be attributed to long-lived deformation occurring along the entire length of the Llewellyn fault. Preliminary conclusions suggest a genetic relationship exists between gold mineralization, Eocene magmatism, and structures associated with the Llewellyn fault (Ootes et al., 2017, this volume). ...
Chapter
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The Turtle Lake map area straddles the boundary between exotic, oceanic crustal and mantle rocks of the Cache Creek terrane, and Laberge Group (Early Jurassic) Whitehorse trough forearc strata atop the Stikine terrane. Exposed in the Turtle Lake area are extensive platformal carbonate rocks of the Horsefeed Formation, a regional hallmark of the Cache Creek terrane, that were deposited over at least 25 m.y. Juxtaposition of Stikine and Cache Creek terranes was accommodated by collapse of the Whitehorse trough in mid-Middle Jurassic (starting ~174 Ma) and creation of a fold and thrust fault belt. This belt was cut by the Fourth of July batholith (~172 Ma) and lamprophyre dikes, emplaced and cooled by ~162 Ma, and followed by a magmatic lull between ~165-130 Ma. In the Turtle Lake area, we fi nd a single granitic dike that crystallized in this time interval, at ~145 Ma. By 125 Ma, the Coast Belt arc had ignited, as recorded by voluminous volcanic and intrusive rocks in the west, and persisted until ~50 Ma. In the Turtle Lake area, volcanism accompanied uplift by ~110 Ma, as indicated by a unimodal detrital zircon population in karst deposits. The youngest known representative intrusions are ~56 Ma quartz diorite stocks. One of these stocks cuts the faulted contact between Whitehorse trough strata and harzburgite mantle tectonite. An analogous geological setting hosts epithermal gold-silver vein mineralization at the Engineer Mine, ~30 km to the south-southwest. The youngest rocks affected by crustal scale faulting and linked, mainly south-side-down, extensional faults are the ~80 Ma Windy-Table suite volcanic strata. We have yet to properly document the extensional faulting episode, but future work aimed at doing so will have important implications for regional tectonic reconstruction, and evaluation of mineral potential.
Technical Report
Full-text available
The Turtle Lake map area straddles the boundary between exotic, oceanic crustal and mantle rocks of the Cache Creek terrane, and Laberge Group (Early Jurassic) Whitehorse trough forearc strata atop the Stikine terrane. Exposed in the Turtle Lake area are extensive platformal carbonate rocks of the Horsefeed Formation, a regional hallmark of the Cache Creek terrane, that were deposited over at least 25 m.y. Juxtaposition of Stikine and Cache Creek terranes was accommodated by collapse of the Whitehorse trough in mid-Middle Jurassic (starting ~174 Ma) and creation of a fold and thrust fault belt. This belt was cut by the Fourth of July batholith (~172 Ma) and lamprophyre dikes, emplaced and cooled by ~162 Ma, and followed by a magmatic lull between ~165-130 Ma. In the Turtle Lake area, we fi nd a single granitic dike that crystallized in this time interval, at ~145 Ma. By 125 Ma, the Coast Belt arc had ignited, as recorded by voluminous volcanic and intrusive rocks in the west, and persisted until ~50 Ma. In the Turtle Lake area, volcanism accompanied uplift by ~110 Ma, as indicated by a unimodal detrital zircon population in karst deposits. The youngest known representative intrusions are ~56 Ma quartz diorite stocks. One of these stocks cuts the faulted contact between Whitehorse trough strata and harzburgite mantle tectonite. An analogous geological setting hosts epithermal gold-silver vein mineralization at the Engineer Mine, ~30 km to the south-southwest. The youngest rocks affected by crustal scale faulting and linked, mainly south-side-down, extensional faults are the ~80 Ma Windy-Table suite volcanic strata. We have yet to properly document the extensional faulting episode, but future work aimed at doing so will have important implications for regional tectonic reconstruction, and evaluation of mineral potential.
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