A
BI
V
O
EY
BR
I
COLU
M
SH
GE
TI
LO
GICAL SUR
British Columbia Geological Survey
Geological Fieldwork 1984
GEOLOGY OF THE BROOKS PENINSULA, VANCOUVER ISLAND
(9 2 w 4 1
INTRODUCTION
A brief reconnaissance of the bedrock geology of the Brooks Peninsula,
8 t o August. 13,1984..
In
n o r t h e r n Vancouver I s l a n d , was made fromAugust
a d d i t i o n , a streamsedimentgeochemicalsurveyofmostoft.hedrainages
on t h e P e n i n s u l a was completed.
This work forms t h e b a s i s o f
a mineral
potential evaluation study
of t h e a r e a u n d e r t a k e n
as p a r t of t h e
M i n i s t r y ' s commitment t o r e s o u r c e e v a l u a t i o n r e q u i r e d f o r
].and u s e
planning.
The MinistryofLands,ParksandHousinghaveproposed
two a r e a s on t h e
n o r t h s i d e of t h e P e n i n s u l a t o t a l l i n g
3 833 h e c t a r e s a s E c o l o g i c a l
Reserves(Fig.55).
They wish t o p r o t e c t a r e a s
of b i o l o g i c a li m p o r t a n c e ,
b e l i e v e d t o havebeen
g l a c i a lr e f u g i a .I ft h er e s e r v e s
are approved,,the
a r e a s would be a l i e n a t e d from mineral exploration and development.
The a r e a is i s o l a t e d and remote with
a c c e s s l i m i t e d t o b o a t or a i r c r a f t .
Thetwo-man p a r t y was p o s i t i o n e d a t Columbia Coveby t h e P r o v i n c i a l
Museum's 5 0 - f o o t r e s e a r c h v e s s e l , t h e
Nesika, i n c o n j u n c t i o n w i t h a
four-man p a r t y of p a l e o c o l o g i s t sa n db i o l o g i s t s .
The s u r v e yu t i l i z e d a
z o d i a cb o a ta n dl i m i t e dh e l i c o p t e rt r a v e r s e s .
It was o n l y p a r t i a l l y
completed because poor weather forecasts caused the premature departure
of t h e h e l i c o p t e r .
PREVIOUS WORK AND REGIONAL GEOLOGY
Muller published the first description
of t h e g e o l o g y of t h e Brooks
P e n i n s u l a( M u l l e r ,
e t d l . , 1 9 7 4 ) . H e i d e n t i f i e df o u r major
geologicalelements(Fig.55);fromnortheast
t o s o u t h w e s tt h e s ea r e :
(1)
The Westcoast F a u l t . A major f a u l tt h a ts e p a r a t e st h eP e n i n s u l a
fromVancouverIsland.
The a d j a c e n t Vancouver I s l a n d a r e a i s
u n d e r l a i n by a s e q u e n c e o f T r i a s s i c a n d J u r a s s i c v o l c a n i c a n d
sedimentaryrocksoftheVancouverand
BonanzaGroupsand
granit.ic
rocksoftheIslandIntrusions.
(2)
The ?iestcoast Complex. A variouslydeformedandmetamorphosed
m i x t u r eo fg a b b r o i c ,d i o r i t i c ,a n dg r a n i t i cr o c k s
whichunderlie!s
95 p e r c e n t of t h e P e n i n s u l a .
(3)
The Cape
Cook
F a u l t . A m a j o rn o r t h w e s t - t r e n d i n gs t r u c t u r et h a t
juxtaposes the Westcoast
Complex a n d t h e o u t b o a r d P a c i f i c
Rim
Complex.
161
162
(4)
The P a c i f i c R i m Co,nple.x. A melange u n i t t h a t i s c o n f i n e dt ot h e
o u t e r m o s t p a r t of t h e P e n i n s u l a , b u t
is more e x t e n s i v e t h a n shown on
M u l l e r ' s map ( 1 9 8 3 ) .
A f i f t h e l e m e n tc o n s i s t i n g
o u t on thesouthwestedge
map (1983).
of Tertiaryconglomeratesandsandstonescrops
of t h e P e n i n s u l a h u t
i s n o t shown on M u l l e r ' s
An a r e a on t h e s o u t h e a s t c o r n e r
of t h e P e n i n s u l a shown t o b e u n d e r l a i n
Bonanza v o l c a n i c r o c k s by Muller(1983) i s h e r e i n t e r p r e t e d t o h e
underl a i n by the Westcoast Complex.
GENERAL
GEOLOGY
WESTCOAST COMPLEX ( U N I T S 1 and 2 )
W e s t c o a s t C r y s t a l l i n e Complex i s a termproposed by Muller ,and Carson
( 1 9 6 9 ) f o r a complex of a m p h i b o l i t e ,b a s i cm i q m a t i t e ,
and g n e i s s i c q u a r t z
d i o r i t e andgabbro t h a t o u t c r o p s on t h e west c o a s t a n d i n i n l e t s
i n the
et d l . (1974)subsequentlyapplit?dthe
Albernimap-area.Muller,
name t o s i m i l a r r o c k s t h a t o u t c r o p
on t h e Brooks P e n i n s u l a .
Two main
map u n i t s ( u n i t s 1 and 2 ) wererecognized
i n t h e complex i n t h i s s t u d y .
Unit 1
- Gabbro,
Metagabbro,
Mylonitic
Schists,
and Mafic Dykes
U n i t 1 c o n s i s t s of t h en o n - g r a n i t i cr o c k s
of t h eW e s t c o a s t Complex.
It
c o m p r i s e st h r e es u b - u n i t s :l a
an e x t e n s i v e u n i t of v a r i a b l y deformed
andmetamorphosed
gabbro,lb
m y l o n i t i cs c h i s t s ,a n d
IC
maficdykes.
-
Suh-unitla
-
-
-
Gabbro,Metagabbro,AmphiboliteGneiss,andMiqmatite
Undeformed t o weaklydeformedgabbro
c o n s t i t u t e s less t h a n 'IO p e r cent of
t h i ss u h - u n i t .
It i s exposedmostly
on t h es o u t h e a s ts i d e
of t h e
P e n i n s u l a , away from t h e Westcoast f a u l t . A crude mineraloqical banding
d e f i n e d by c o n c e n t r a t i o n of maficminerals on a 3 t o 5 - c e n t i m e t r e s c a l e
is d e v e l o p e dl o c a l l y on t h en o r t hs h o r e
of N a s p a r t iI n l e t .
The banding
i s discontinuousover 2 metres and may h e t h e r e s u l t
of magmaticflowage
d i f f e r e n t i a t i o n ,n o tg r a v i t ys e t t l i n g .
In most p l a c e s t h e u n i t c o n s i s t s
of f o l i a t e d g a b b r o s a n d a m p h i b o l i t e s .
Tn zonesofhigh
strain they are converted to
banded a m p h i b o l i t e g n e i s s ,
such as s o u t ho fG u i l l i a m sI s l a n d .M i g m a t i t ea n da g m a t i t ea r el o c a l l y
developedadjacent t o g r a n i t i c i n t r u s i o n s , f o r
example, s0uV.h ofCape
Cook Lagoon.
Sub-unitla
i s everywhereintruded by g r a n i t o i d dykesandpegmatitesand,
i n a t l e a s t two l o c a l i t i e s , by g r a n i t o i d i n t r u s i o n s ( u n i t
2 ) up t o 4
k i l o m e t r e sa c r o s s .F o l i a t e di n t r u s i o nb r e c c i aw i t ha l i g n e dz e n o l i t h so f
gabbro i s exposed a t t h e n o r t h e r n c o n t a c t
oE t h e Columbia Cove g r a n i t , e .
163
Sub-unit lb
- Mylonitic
Schist
A narrow unit (approximately350 metres) of banded mylonite outcrops at
the southern entrance to Columbia Cove. The mylonite varies in
composition from granitic to calc-silicate. Thin amphibolite bands are
also present. Isolated, lensoid granitic fragments upto 50 centimetres
across occur surrounded in calc-silicate schists. These superficially
resemble a metaconglomerate but are more likely highly attenuated and
flattened intrusion breccia.
The northern contact of the mylonitic schists is not exposed.
The
IC) that
southern contact is obscured by a swarm of mafic dykes (sub-unit
cut the schists.
Sub-unit IC
-
Dyke Complex
narrow unit of fine-grained and porphyritic mafic dykes outcrops
on the
south shore of Jacobsen Point. The dykes are undeformed to mildly
deformed in contrast to the schists that they intrude. They trend
160 degrees and are dark green, grey to black. Rare screens of
medium-grained gabbro are present.
A
which
The mafic dykes are intruded by a Tertiary(?) dyke 5),
(unit
provides an upper age limit for them.
Unit 2
- Granitoid Intrusions
Numerous granitic dykes, pegmatites, and two mappable granitoid
intrusions cut sub-units la and lb. The intrusions are informally
referred to as the Columbia Cove granite and the Cape Cook
granite.
Lagoon
The Columbia Cove granite is medium and coarse-grained,
hornblende-biotite granite. It is massivein thecentre but towardthe
intrusion
eastern contact with the gabbros a 25-metre-wide wideofzone
breccia with a strong schistosity is developed.
The Cape Cook Lagoon granite is a deformed hornblende-biotite granite.
It contains numerous aligned and flattened inclusions and screens of
gabbro, diorite, and pyroxenite, indicating systectonic intrusion.
Locally, granitoid dykes are more deformed than the gabbro host rocks:
they were probably intruded into active shear zones.
PACIFIC RIM COMPLEX (UNIT3 )
Pacific Rim Complex toa
Muller, et d., (1974) assigned the name
highly disturbed and faulted sequence of
argillite, greywacke, sandstone,
164
and q u a r t z i t e e x p o s e d n e a r
Cape Cook.The
u n i t i s i n f a c t i). melange
containingexoticblocksor'knockers'in
a m a t r i x ofdeformed
b l a c k .and
g r e e ns h a l e .
Some of t h ek n o c k e r sa r e
s o l a r g e ( u p t o 50 metres) t h a t
theycanbedistinguished
on a 1 :50 000-scale map.The
complex i s muoh
widerthan
shown by Muller(1983).
I t formsmostof
thesouthwest COaSt
of t h e P e n i n s u l a , and t h e numerous r o c k s , r e e f s ,
and s h o a l s t h a t e x t e n d
up t o 1 k i l o m e t r eo f f s h o r e .
Many of t h e s ef e a t u r e sa r ei s o l a t e d ,
resistantknockers,standing
above t h e e r o d e d s h a l e matrix.
A t e c t o n i c slice orblockcontaining
beddedribboncherts(sub-unit3a)
i s well exposed on t h e c o a s t a b o u t
1 k i l o m e t r en o r t h e a s to f
Cape Cook.
The r i b b o nc h e r t so c c u r
i n a sequence ofgreywacke,
c h e r t ,b r e c c i a ,
and
l e s s e rb l a c ks h a l e
melange.
N o t o p s were determined.
The c h e r t sa r e up
and a r e t i g h t l y andcomplexlyfolded.Individual
t o 2 0 m e t r e st h i c k
r i b b o n s a r e commonly 3 t o 5 c e n t i m e t r e s t h i c k : t h e y a r e s e p a r a t e d
by t h i n
laminaeofblack
argillite.
The c h e r t s a r e l i g h t g r e e n t o q r e y
and
weather white.
by b l a c k
This sequence i s s t r u c t u a l l y o v e r l a i n t o t h e n o r t h e a s t
b r e c c i a t e d a r g i l l i t e s ( 7 ) which l i e a d j a c e n t t o t h e Cape Cook f a u l t .
The g r e y w a c k e s ( s u b - u n i t 3 c ) . w h i c h o u t c r o p a d j a c e n t t o t h e r i b b o n c h e r t s
and a s k n o c k e r s t h r o u g h o u t t h e
melange, a r e f i n e g r a i n e d , d a r k g r e e n t o
g r e y ,m a s s i v e ,a n di n d i s t i n c t l y
bedded.Small
r i p - u pc l a s t : :
of b l a c k
s h a l ea r e common. Greywackes a l s ou n d e r l i eS o l a n d e rI s l a n d
(C. Yorath,
personalcommunication), which l i e s 2 kilometressouthwest of Cape Cook.
Knockers c o n s i s t i n g ofbeddedandgradedcobbleconglomerateand
sandstone(sub-unit3b)occur
i n t h e melange 2 k i l o m e t r e s s o u t h o f
Cape
Cook. The c l a s t s a r e subroundedand
c o n s i s to fs a n d s t o n e ,( c h e r t ,b l a c k
a r g i l l i t e ,q u a r t z i t e ,g a b b r o ,
and g r a n i t e . One l a r g ek n o c k e r 5 metres
a c r o s s c o n t a i n s a sequence of beds,eachup
t o 1 metre t h i c k , of
sandstone,conglomerate,andblackshale.
Blocks of m a f i c p i l l o w l a v a ( s u b - u n i t 3 d ) o c c u r
i n t h e melange on t h e
shore n o r t h of Banks R e e f .
The p i l l o w sc o n t a i nc a r b o n a t ei n t e r p i l l o w
material.
The melange a l s o c o n t a i n s a t h i n wedge of t h i n l y l a m i n a t e d , d a r k g r e y ,
fine-grained,calcarcoussiltstonetowardthesoutherncontactwith
the
Cape Cook f a u l t .
The s i l t s t o n e s a r e e x t e n s i v e l y b r e c c i a t e d c l o s e t o t h e
fault.
RHYOLITE PORPHYRY AND DYKES ( U N I T 4 )
Two small, p o s t - t e c t o n i c r h y o l i t e p o r p h y r y i n t r u s i o n s c u t t h e W e s t c o a s t
Complex on t h en o r t h w e s tc o a s t
of t h eP e n i n s u l a .
The t o p of a
porphyriticrhyoliteintrusion,
5 metresacrosscuts
deformt?d gabbro
s o u t h of t h eW e s t c o a s tf a u l t .
The r h y o l i t e i s compositiona:llybanded
p a r a l l e l t o t h e domed c o n t a c t . The r h y o l i t e c o n t a i n s phenocrystsof
q u a r t z ,K - f e l d s p a r ,a n db i o t i t e :
it weatherswhite.
165
A 2 m e t r e - w i d e ,f l o w - b a n d e dp o r p h y r i t i cr h y o l i t e
dyke c u t s m i g m a t i t i c
g n e i s s e s on a s m a l li s l a n ds o u t h w e s t
of Cape Cook Lagoon.
It is
c o m p o s i t i o n a l l ys i m i l a rt ot h er h y o l i t ei n t r u s i o n .
The r h y o l i t e s p o s t d a t e m a j o r d e f o r m a t i o n a n d f a u l t i n g i n t h e a r e a
b e l i e v e d t o b e of T e r t i a r y a g e .
and a r e
INTERMEDIATE DYKES (UNIT 5 )
A number o f n o r t h e a s t - t r e n d i n g , i n t e r m e d i a t e d y k e s c u t b o t h t h e W e s t c o a s t
C r y s t a l l i n e Complex a n d t h e P a c i f i c
R i m Complex. Thesedykes are
u n a f f e c t e d by t h e i n t e n s e c a t a c l a s i s a s s o c i a t e d w i t h t h e
Cape Cook f a u l t
and a r e assumed t o be T e r t i a r y i n age. The dykes a r ed a r kg r e y ,f i n e
grained to porphyritic; locally they contain
rounded q u a r t z p h e n o c r y s t s
o r amygdules.
BOULDER CONGLOMERATE, SANDSTONE, AND MINOR BASALT ( U N I T 6 )
A t h i n sequence of c o b b l e t o b o u l d e r c o n g l o m e r a t e t h a t p a s s e s
upward i n t o
g r i t s andsandstonesuriconformablyoverliescataclasticgabbroofthe
W e s t c o a s t C r y s t a l l i n e Complex i n t h e immediate v i c i n i t y of t h e Quineex
IndianReserve(locally
known as S h e l t e r S h e d s ) .
The conglomerates are exposed
a t low t i d e l e v e l i n a narrowgraben
(approximately 25 m e t r e s w i d e ) t h a t d e f i n e s t h e
Quineex c a n o e r u n a s well
as on i s l a n d s andheadlands 1 k i l o m e t r e n o r t h e a s t and 0.5 k i l o m e t r e
southwest. The u n i t i s undeformedandbeds
d i p 2 0 d e g r e e s oceanward.
Only about 50 metres of s t r a t a is exposed.
The unconformity surface i s exposed i n many p l a c e s a n d h a s
a r e l i e f up t o
1 metre.Boulders
of g a b b r ob r e c c i au pt o
1 metre a c r o s so c c u r on t h e
unconformitysurface.
The conglomeratecontainsroundedcobblesand
b o u l d e r s of f o l i a t e dg a b b r o ,g r e e n s c h i s t ,g r a n i t e ,a n dc o b b l e sa n d
p e b b l e s ofrounded
quartz.
A f i n e - g r a i n e d ,d a r k
brown b a s a l t f l o w o r s i l l i s i n t e r l a y e r e d w i t h t h e
basalconglomerate i n t h e Quineexgraben.
The b a s a l t i s exposed a t low
t i d e and i s a t l e a s t 2 metres t h i c k : t h e t o p
i s unexposedunderwater.
The b a s a l t t o n g u e s i n t o t h e u n d e r l y i n g c o n g l o m e r a t e s a n d i s o l a t e d
conglomerate fragments are caught
up i n t h e b a s a l t .
The conglomerate i s t e n t a t i v e l y c o r r e l a t e d w i t h t h e E o c e n e / O l i g o c e n e
EscalanteFormation,
which o c c u p i e s a s i m i l a r l i t h o s t r a t i g r a p h i c p o s i t i o n
i n t h e Nootka Sound map a r e a 7 5 k i l o m e t r e s to t h e s o u t h (Muller, 1981).
These outcrops on Brooks Peninsula are the
most n o r t h e r l y known onshore
exposures of t h i s f o r m a t i o n .
166
STRUCTURE
The Cape Cook f a u l t i s t h e major s t r u c t u r e i n t h ep e n i n s u l a .
It
s e p a r a t e s t h e P a c i f i c R i m Complex and t h e W e s t c o a s t C r y s t a l l . i n e COmpll?X
and t r e n d s n o r t h w e s t e r l y a c r o s s t h e s o u t h w e s t t i p
of t h eP e n i n s u l a .
'fie
f a u l t i s exposednorthwestof
Amos Creekwhere
it i s markedby
tectonically interleaved thin slices
of l i m e s t o n e a n d c h e r t ,
which a r e up
t o 50 c e n t i m e t r e st h i c k ,w i t hb r e c c i a t e dg a b b r o s .
A t t h i s locality,
sedimentary rocks of t h e P a c i f i c R i m Complex d i p s t e e p l y t o t h e n o r t h w e s t
under t h e Westcoast Complex and the gabbros of the Westcoast.
Complex a r e
c l o s e l yb r e c c i a t e d .E x t e n s i v eb r e c c i a t i o no ft h eg a b b r o sc c t n t i n u e sf o r
up t o 3 k i l o m e t r e s away from t h e f a u l t .
The b r e c c i a s c o n s i s : t o f a n g u l a r
fragments of g a b b r o ,g e n e r a l l y
from 3 t o 10 c e n t i m e t r e s a c r o s s , i n
a
comminuted groundmass:no
v e i n i n g was noted.This
cataclasis i s
i n t e r p r e t e d t o havebeencaused
by u n d e r t h r u s . t i n g of t h e P a c i f i c R i m
Complex under the Westcoast
Complex.
The N e s t c o a s t f a u l t a l s o t r e n d s n o r t h w e s t a n d
i s exposed a t t h e e n t r a n c e
t o Johnson Lagoon.
It i s markedby
a 7001netre-widezone
of m y l o n i t i z e d
I s l a n dI n t r u s i o ng r a n i t e .
The m y l o n i t ed i p sm o d e r a t e l yt ot h en o r t h w e s t .
Brecciated and foliated gabbros and foliated greenschists
of t h e
Westcoast Complex a r ej u x t a p o s e da g a i n s tt h em y l o n i t e .
The g r e e n s c h i s t s
and t h e g r a n i t i c m y l o n i t e a r e o p e n l y f o l d e d a b o u t n o r t h w e s t - t r e n d i n g
axes.
The Westcoast f a u l t was n o t o b s e r v e d on t h e n o r t h c o a s t where i t
juxtaposes Westcoast Complex a n d m a f i c v o l c a n i c r o c k s
of t h e T r i a s s i c
KarmutsenFormation.
MINERALIZATION
Mylonites associated with the Westcoast fault at Johnson
Lagoon c o n t a i n
t h i n s t r i n g e r s of p y r i t e .
Assays d i d n o t r e v e a l anyanomalous
g o l do r
silver values.
Large,rusty,pyrite-bearingbouldersoffine-grainedgranite
observedon t h e h e a d l a n d 1.5 kilometres southwest of Johnson
were
Lagoon.
Disseminated pyrite occurs
i n volcanic rocks of the
KarmutsenFormation
1.6 k i l o m e t r e s n o r t h e a s t of t h e Westcoast f a u l t a t Brooks Bay.
Coarse and fine-grained placer gold has been reported
from n13ar t h e
j u n c t i o n of Amos andGoldCreeks
(MINFILF 92L/248).Thisarea
was worked
i n t h e 1 9 1 0 ' s b u t t h e r e i s n or e c o r d of p r o d u c t i o n . The asst?ssment
r e p o r t (Neave, 1 9 1 3 ) d e s c r i b i n g t h e work is u n f o r t u n a t e l ym i s s i n g from
theMinistry'sfiles.
The s o u r c eo ft h ep l a c e rg o l dh a sn o tb e e n
determined.
167
MINERAL
POTENTIAL
The metallic mineral potential of the Westcoast Complex and the Pacific
no evidence of
Rim Complex appears to be low. The Cape Cook fault shows
associated hydrothermal activity. The Westcoast fault contains some
pyrite and probably deserves closer attention
to evaluate its potential.
The small Tertiary rhyolite stock and dykes mapped
on the north shoreof
the Peninsula line up with a belt of Tertiary volcanic rocks that trend
northeast across Vancouver Island. This trend was named the Brooks
Peninsula Fault Zone by Muller,
et al. ( 1 9 7 4 ) . A warm spring,
reported to exist at the source of a small stream that exits at the
northeast corner of Drift Whale Bay, also lies on this trend. These data
indicate probable Tertiary igneous activity
in this area.
Tertiary volcanic and intrusive rocks are favourable metallotects
elsewhere on Vancouver Island (Carson,1969; Muller, e t a . l . , 1 9 7 4 ) .
Probably the greatest mineral potential of the Brooks Peninsula lies in
discovering mineralized Tertiary igneous rocks in the unexplored interior
of the Peninsula.
LAND USE -DATION
A land
use recommendation will not be made until the geochemical
is data
on the
available. Any persons knowing of additional mineral occurrences
Peninsula are requested
to make this information available to the
Ministry.
ACKNOWLEDGMENTS
Mr. Don Travers was a competent and cheerful field assistant, and a good
boatman. Mr. Ross Brand, skipper of theNesika, and his crew are thanked
for their generous hospitality and transportation. Dr. Richard Hebda
kindly invited us to join the expedition. He and his colleagues were a
constant source of encouragement and good humour
in the seemingly never
ending rains. Dr. Bill McMillan kindly reviewed the manuscript and
suggested many improvements.
REFERENCES
B.C.
M i n i s t r y of Energy,Mines
6 Pet. Res.:
MINFILE, File 92L,
1983.
Carson, D.J.T. ( 1 9 6 9 ) : Tertiary Mineral Depositsof Vancouver Island,
Bull., Vol. 72, pp. 116-125.
British Columbia, C.I.M.,
Muller, J. E. ( 1 9 8 1 ) : Geology, Nootka Sound, British Columbia,
Gpol.
.Sum., Canada, Map 1537A.
168
.......... (1983):
Geology, Alert Bay-Cape Scott, British Columbia,
Geol. Surv., C a n a d a , Map 1552~1(1969): Geology and Mineral Depositsof
Muller, J. E. and Carson, D.J.T.
Alberni Map Area, British Columbia (92F),
Geol. Surv., C a n a d a ,
Paper 68-50.
Muller, J. E., Northcote, K. E., and Carlisle, D. (1974): Geology and
Mineral Depositsof Alert-Cape Scott Map Area, British Columbia,
Geol. Sum., C a n a d a , Paper 74-8.
Neave, H. E. (1913): Auriferous Gravels
Brooks Peninsula, B.C.
Ministry of E n e r y g , Mines & Pet. Res., private Assessment
Report.
-
169
170