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High-Mg Lavas in the Karmutsen Flood Basalts,
Northern Vancouver Island (NTS 092L): Stratigraphic Setting
and Metallogenic Significance
G.T. Nixon, J. Larocque1, A. Pals1, J. Styan1, A.R. Greene2 and J.S. Scoates2
KEYWORDS: Van cou ver Is land, NTS 092L, Wrangellia,
Van cou ver Group, Karmutsen For ma tion, stra tig ra phy,
vol ca nol ogy, metallogeny, flood ba salt, high-Mg ba salt,
picrite, Ni-Cu-PGE
INTRODUCTION
The oc cur rence of high-Mg lavas in the Tri as sic
Karmutsen flood bas alts of the Wrangellia Terrane on
north ern Van cou ver Is land has re cently been doc u mented
by Greene et al. (2006). They iden ti fied a num ber of sep a -
rate oc cur rences of high-Mg or picritic pil low lavas and
subvolcanic dikes, and des ig nated the ex cel lent ex po sures
of pil lowed flows at Keogh Lake as the type lo cal ity. Al -
though all cur rently known picritic ba salt flows clearly
formed in the sub ma rine en vi ron ment, their pre cise strati -
graphic po si tion within the Karmutsen Formation has
remained uncertain.
The ex pan sion of new log ging road sys tems on north -
ern Van cou ver Is land over the last de cade or so has pro -
vided ac cess to more re mote ar eas un der lain by Karmutsen
vol ca nic rocks. It is now pos si ble to map dis tinct strati -
graphic sub units within the Karmutsen For ma tion, sim i lar
to those es tab lished by ear lier work ers else where on Van -
cou ver is land. This re port, there fore, de scribes the in ter nal
stra tig ra phy of the Karmutsen For ma tion, as re vealed by
re cent map ping, and at tempts to place known oc cur rences
of high-Mg ba salt in the context of this new stratigraphic
framework.
The ex is tence of high-Mg bas alts in the Karmutsen
suc ces sion has im por tant ram i fi ca tions for the min eral po -
ten tial of this Tri as sic flood ba salt prov ince. North ern Van -
cou ver Is land is well known for a num ber of sig nif i cant
metal pros pects and de pos its, in clud ing in tru sion-re lated
Cu-Au-Ag(-Mo) por phyry (e.g., Hushamu, MINFILE
92L 240; the for mer Is land Cop per mine, MINFILE
92L 158; MINFILE, 2007); base and pre cious-metal
skarns (e.g., Merry Widow, MINFILE 92L 044); and epi -
ther mal pre cious-metal en vi ron ments (e.g., Mount
McIntosh – Hushamu, MINFILE 92L 240). All such styles
of min er al iza tion have strong ge netic links to su pra-
subduction zone met al lo gen ic events as so ci ated with the
Late Tri as sic to Early Ju ras sic Bo nanza mag matic arc
(Nixon and Orr, 2007). How ever, ad di tional ex plo ra tion
op por tu ni ties ex ist in the flood ba salt en vi ron ment, which
is known to host world-class mag matic Ni-Cu-PGE
deposits associated with high-Mg basalts or their intrusive
counterparts.
REGIONAL SETTING
The ge ol ogy of north ern Van cou ver Is land has been
pub lished in a se ries of 1:50 000 scale maps (Nixon et al.,
2006a–d), and re vi sions to the Early Me so zoic stra tig ra phy
were made re cently by Nixon and Orr (2007). A gen er al -
ized ge ol ogy map and strati graphic col umn for north ern
Van cou ver Is land are pre sented in Figures 1 and 2.
Van cou ver Is land be longs to the Wrangellia
tectonostratigraphic terrane (Jones et al., 1977) of Late Pa -
leo zoic to Early Me so zoic rocks, which ex tends north -
wards through the Queen Char lotte Is lands into south ern
Alaska (Wheeler and McFeely, 1991). Wrangellia was
amal gam ated with the Al ex an der Terrane in the Alaska
pan han dle to form the In su lar Superterrane as early as the
Late Car bon if er ous (Gardner et al., 1988), and was
accreted to in board ter ranes of the Coast and Intermontane
belts as late as the mid-Cre ta ceous (Mon ger et al., 1982) or
as early as the Mid dle Ju ras sic (van der Heyden, 1991;
Mon ger and Journeay, 1994). At the lat i tude of north ern
Van cou ver Is land, Wrangellia is in truded to the east by
granitoid rocks of the Coast Plutonic Com plex and fault
bounded to the west by the Pa cific Rim Terrane and meta -
mor phosed and in tru sive rocks of the Westcoast Crys tal line
Com plex (Wheeler and McFeely, 1991). De vo nian to Early
Perm ian is land-arc vol ca nic, volcaniclastic and sed i men -
tary rocks of the Sicker and Buttle Lake groups (Massey,
1995a–c), which form the base ment to Wrangellia, are ex -
posed on south ern and cen tral Van cou ver Is land, and the
over ly ing Mid dle Tri as sic shale (‘Daonella beds’) at the
base of the Karmutsen is well ex posed in the Schoen Lake
area, some 30 km southeast of the area shown in Figure 1.
The stra tig ra phy of north ern Van cou ver Is land is
founded upon the Tri as sic tri par tite se quence of Karmutsen
flood ba salt, Quatsino lime stone and Par son Bay mixed
car bon ate-clastic-vol ca nic suc ces sion, which is di ag nos tic
of Wrangellia (Jones et al., 1977). The oc cur rence of is -
land-arc vol ca nic and volcaniclastic strata in the Par son
Bay For ma tion led Nixon and Orr (2007) to take the Par son
Bay For ma tion out of the Van cou ver Group and place it in
the Late Tri as sic–Mid dle Ju ras sic Bo nanza Group. The lat -
ter group of vol ca nic and sed i men tary rocks, to gether with
co eval granitoid in tru sions of the Is land Plutonic Suite,
Geo log i cal Field work 2007, Pa per 2008-1 175
1 Uni ver sity of Vic to ria, Vic to ria, BC
2 University of British Columbia, Vancouver, BC
This publication is also available, free of charge, as colour
digital files in Adobe Acrobat® PDF format from the BC
Ministry of Energy, Mines and Petroleum Resources website at
http://www.em.gov.bc.ca/Mining/Geolsurv/Publications/catalog/
cat_fldwk.htm
con sti tute the main phase of magmatism of the Bo nanza is -
land arc (Northcote and Muller, 1972; DeBari et al., 1999).
A ma jor contractional event is marked by an an gu lar
un con formity un der ly ing Ju ras sic–Cre ta ceous clastic se -
quences de pos ited on the eroded sur face of the Bo nanza
Group. This ep i sode of de for ma tion is con strained by strata
of Late Ju ras sic age (Oxfordian–Tithonian) that lo cally un -
der lie more wide spread Cre ta ceous sed i men tary rocks on
north ern Van cou ver Is land and in the Queen Char lotte Is -
lands (Gamba, 1993; Haggart, 1993; Haggart and Carter,
1993).
176 Brit ish Co lum bia Geo log i cal Sur vey
Fig ure 1. Re gional ge ol ogy of north ern Van cou ver Is land (com piled by Massey et al., 2005). Ab bre vi a tions for se lected geo graphic lo cal i -
ties: AL, Al ice Lake; BC, Bea ver Cove; BL, Bo nanza Lake; KL, Keogh Lake; ML, Maynard Lake; NL, Nimpkish Lake; RB, Robson Bight; SL,
Sara Lake; VL, Vic to ria Lake.
The his tory of fault ing on north ern Van cou ver Is land is
com plex and em bod ies Cre ta ceous transpression and Ter -
tiary ex ten sion. The pres ent crustal ar chi tec ture ex hib its a
dom i nant north west erly-trending struc tural grain man i -
fested by the dis tri bu tion of ma jor lithostratigraphic units
and granitoid plutons (Fig 1). Nu mer ous fault-bounded
blocks of homoclinal, Early Me so zoic strata gen er ally dip
to the south west and west (Mul ler et al., 1974). Jura-Cre ta -
ceous clastic strata are pre served as dis pa rate fault-
bounded rem nants of for merly more ex ten sive Cre ta ceous
bas ins (Mul ler et al., 1974; Jeletzky, 1976; Haggart, 1993).
The rel a tively low re lief and high heat flow of north ern
Geo log i cal Field work 2007, Pa per 2008-1 177
Fig ure 2. Early Me so zoic stra tig ra phy of north ern Van cou ver Is land, us ing the no men cla ture for Tri as sic–Ju ras sic lithostratigraphic units
pro posed by Nixon and Orr (2007). The geo log i cal time scale is that of Gradstein et al. (2004) ex cept for the Carnian–Norian stage bound -
ary, which is taken from Furin et al. (2006).
Van cou ver Is land re flect tectonism as so ci ated with the de -
vel op ment of the Queen Char lotte Ba sin, a Ter tiary
transtensional prov ince re lated to oblique con ver gence of
the Pa cific and Juan de Fuca plates with the North Amer i -
can Plate (Riddihough and Hyndman, 1991; Lewis et al.,
1997). The dis tri bu tion of Ter tiary vol ca nic cen tres ap pears
to be strongly in flu enced by high-an gle faults. The north -
east erly-trending Brooks Pen in sula fault zone ap pears to
co in cide with the south ern limit of Neo gene vol ca nism in
the re gion and de lin eate the south ern bound ary of the Ter -
tiary extensional regime in the Queen Charlotte Basin
(Armstrong et al., 1985; Lewis et al., 1997).
WRANGELLIAN FLOOD BASALTS
The geo log i cal set ting and evo lu tion of the Tri as sic
flood bas alts in the Wrangellia Terrane of north ern Van cou -
ver Is land was re cently re viewed by Greene et al. (2006).
The weight of cur rently avail able geo log i cal ev i dence in di -
cates that the vo lu mi nous (>106 km3) Wrangellian flood
bas alts rep re sent an accreted oce anic pla teau formed by the
rise and de mise of a man tle plume (Rich ards et al., 1991).
From fauna and lithostratigraphic cor re la tions, Carlisle and
Suzuki (1974) con cluded that the Karmutsen bas alts range
from Mid dle to Late Tri as sic (Ladinian–Norian) in age and
were emplaced within ap prox i mately 2.5 to 3.5 Ma. More
re cent U-Pb and geo chron ol ogi cal stud ies have so far failed
to ac cu rately re solve the age and lon gev ity of Karmutsen
vol ca nism, which is placed at ca. 217 to 233 Ma (see
Greene et al., 2006). De spite their age and low (prehnite-
pumpellyite) grade of ‘burial’ meta mor phism (see Green -
wood et al., 1991 and ref er ences therein), volcanological
and petrographic fea tures of the basalts are typically well
preserved, as shown below.
KARMUTSEN FORMATION
The early work of Gun ning (1932) es tab lished the
‘Karmutsen volcanics’, named for the type area in the
Karmutsen Range just west of Nimpkish Lake, as the dom i -
nant mem ber of the Van cou ver Group. Sub se quently, this
unit was el e vated to for mal lithostratigraphic sta tus by
Suther land Brown (1968) and Mul ler and Car son (1969).
The Karmutsen suc ces sion in cen tral and north ern Van cou -
ver Is land was later sub di vided by D. Carlisle and co work -
ers (Carlisle, 1963, 1972; Carlisle and Suzuki, 1974; Mul -
ler et al., 1974) into three dis tinct and mappable vol ca nic
units: 1) closely packed pil low lavas in the lower part of the
suc ces sion (2900 ±150 m); 2) pil low brec cia and ‘aquagene
tuff’ in the mid dle (610–1070 m); and 3) mas sive flows at
the top (2600 ±150 m). Intervolcanic sed i men tary lenses,
prin ci pally micritic to bioclastic lime stone and black si li -
ceous shale, oc cur near the top and, less com monly, near the
base of the mas sive flow unit, and are lo cally as so ci ated
with pil low lavas, pil low brec cias and finer grained, bed -
ded volcaniclastic de pos its. The base of the Karmutsen suc -
ces sion over lies a thick (760–920 m) se quence of black si li -
ceous to cal car e ous shale (‘Daonella beds’) of Mid dle
Tri as sic age, in truded by abun dant mafic sills that are con -
sid ered comagmatic with Karmutsen vol ca nic rocks
(Carlisle, 1972; Carlisle and Suzuki, 1974; Mul ler et al.,
1974). The shale unit is ex posed south of the cur rent area of
in ter est near Schoen Lake (see Greene et al., 2006). Ac -
cord ing to pre vi ous work, es ti mates for the cu mu la tive
thick ness of the Karmutsen For ma tion ex ceed 6000 m.
Yor a t h et al. (1999) pro posed that the ex cel lent exposures
along Buttle Lake on central Vancouver Island represent
the most complete section.
Re cent map ping of the Karmutsen For ma tion on north -
ern Van cou ver Is land has re sulted in a tri par tite di vi sion of
the vol ca nic suc ces sion (Fig 3–5), anal o gous to the sub -
units orig i nally es tab lished by Carlisle (1963, 1972): 1) a
pil low lava unit at the base, dom i nated by closely packed
pil lowed flows with mi nor in ter ca lated sheet flows; 2) an
over ly ing hyaloclastite unit char ac ter ized by pil low-frag -
ment brec cias and fine-grained hyaloclastite de pos its, and
in ter ca lated lo cally with pil lowed flows through out the
suc ces sion; and 3) an up per mas sive flow unit dom i nated
by subaerial lavas but in clud ing mi nor lime stone, fine-
grained siliciclastic sed i men tary rocks, pil low lavas and
volcaniclastic de pos its near the top and base. The ge netic
term ‘hyaloclastite’ has been adopted to de note vol ca nic
rocks formed by quench-frag men ta tion and autobrec -
ciation dur ing in ter ac tion with wa ter, rather than the clas sic
term ‘aquagene tuff’, as orig i nally em ployed by Carlisle
(1963), which may con note an or i gin via ex plo sive frag -
men ta tion and de po si tion di rectly by pyroclastic pro cesses
(Cas and Wright, 1987). Al though cer tain ba saltic shard
morphologies de scribed by Carlisle (1963) may have a
pyroclastic or i gin, gen er ated, for ex am ple, by steam ex plo -
sions where lava flows en tered the ocean, there is abun dant
ev i dence to con clude that the over whelm ing ma jor ity of
volcaniclastic prod ucts in the Karmutsen suc ces sion are re -
lated to the em place ment and granulation of pillowed
flows, as well as resedimentation of hyaloclastites in the
submarine environment.
Pillow Lava Unit
Pil low lavas in the basal part of the Karmutsen For ma -
tion are ex posed on the coast north of Robson Bight, and in
roadcuts and quar ries along log ging roads stretch ing west
from the Karmutsen Range to Sara Lake, just west of Twin
Peaks (Fig 3). The base of the pil low lava unit is typ i cally
cut off by fault ing and is not ex posed in the map area. The
top of the unit ap pears broadly con form able with over ly ing
beds of hyaloclastite, and map pat terns north west of
Maynard Lake in di cate interdigitation of pil lows and the
basal part of the hyaloclastite unit. The min i mum thick ness
of the pil low lava se quence, as es ti mated in a coastal sec -
tion be tween Bea ver Cove and Robson Bight, is ap prox i -
mately 3000 m, as sum ing no significant repetition by
faults.
Dark grey to grey-green pil lows gen er ally ex hibit
nearly equidimensional to lobate forms up to about 1.5 m in
length and 1 m across, and typ i cally are closely packed with
very dark grey to black, well-chilled, chlorite-rich selvages
(Fig 6A). The ma jor ity of the pil low se quence is apha ni tic,
nonamygdaloidal and strongly to non mag netic. How ever,
to wards the top of the suc ces sion, pil low lavas may carry
plagioclase pheno crysts and ex hibit amyg da loid al tex tures.
In di vid ual pil lows may dis play pro nounced ra dial joint ing,
but this is not a com mon fea ture. In ter stices are com monly
de void of clastic ma te rial, or may host subequant to rect an -
gu lar or dis tinctly elon gate, curvilinear shards that rep re -
sent the spalled rims of pil lows (Fig 6B). This ma te rial is
com monly partly al tered to chlorite, epidote, quartz and
car bon ate, for which the lo cal term ‘dallasite’ has been
coined. Com plete re place ment or in fill ing of in ter stices and
ir reg u lar frac tures by quartz, chlorite, epidote, cal cite, po -
tas sium feld spar and ze o lite usu ally aids in iden ti fy ing pil -
178 Brit ish Co lum bia Geo log i cal Sur vey
low morphologies in blasted out crops. Rarely, large pil lows
in cross-sec tion ex hibit quartz-filled ledges with flat floors
and con vex roofs, in ter preted to rep re sent cav i ties evac u -
ated by lava dur ing em place ment (i.e., small lava tubes).
These drain age chan nels pro vide valu able struc tural in di -
ca tors for way-up, flow ori en ta tion and flow con tacts (bed -
ding). Such fea tures may oc cur as stacked lava tubes within
a single pillow, and have been described previously (e.g.,
Yorath et al., 1999, Fig 46).
Pock ets of pil low brec cia and rare, lam i nated to thinly
bed ded hyaloclastite sand stone and mas sive sheet flows are
en coun tered lo cally through out the suc ces sion. The sheet
flows may be more com mon than ap pre ci ated due to the
scar city of well-ex posed con tacts and sim i lar ity of their
tex tures to subvolcanic dikes and sills. The best ex po sures
of sheet flows oc cur in quar ries and high roadcuts (Fig 6C).
The con vo lute na ture of their lower con tact, which is typ i -
cally draped over un der ly ing pil lows, serves to dis tin guish
the sheet flows from subvolcanic in tru sions (Fig 6D). Al -
though most sheet flows lack in ter nal struc tures, some ex -
hibit a crude curvilinear joint ing ori ented at a high an gle to
their con tacts. The oc cur rence of sheet flows deep within
the pil low ba salt suc ces sion, and their gen er ally
nonamygdaloidal na ture, in di cates that they are truly sub -
ma rine in or i gin rather than the re sult of sea-level os cil la -
tion be tween sub ma rine and subaerial con di tions. Most ap -
pear to have lensoid ge om e try and likely re flect a local
increase in the flow rate relative to the rate of emplacement
of pillows.
In thin sec tion, ba saltic pil low lavas are weakly to
nonamygdaloidal, and aphyric or por phy ritic with small
amounts of plagioclase and/or ol iv ine pheno crysts.
Plagioclase-bear ing and/or amyg da loid al lavas be gin to ap -
pear near the top of the pil low lava suc ces sion. Euhedral to
subhedral plagioclase and ol iv ine pheno crysts (<2 mm)
may form glomerophyric clots, and groundmass plagio -
clase gen er ally forms acicular crys tals. Ol iv ine is com -
pletely re placed by fine-grained inter growths of ser pen -
tine, chlorite, car bon ate, opaque ox ides and/or quartz. In
aphyric pil low lavas, clinopyroxene usu ally forms ra di at -
ing den dritic to sheaf-like crys tals with a vario lit ic tex ture,
or inter gra nu lar microphenocrysts. In por phy ritic lavas,
clinopyroxene is typ i cally some what coarser grained and
exhibits a prismatic or ophitic habit.
Hyaloclastite Unit
The hyaloclastite unit crops out on the coast be tween
Bea ver Cove and Robson Bight, and can be traced west -
ward through well-for ested ground ex tend ing from the
west flank of the Karmutsen Range to Sara Lake (Fig 3).
Where ad e quate struc tural con trol is pres ent, the up per and
lower con tacts of this unit ap pear con form able with over ly -
ing flow and un der ly ing pil low se quences. The thick ness of
this unit var ies con sid er ably across the map area. It at tains
an es ti mated max i mum thick ness of ap prox i mately 1550
±200 m in a coastal sec tion north of Robson Bight, and may
be less than 40 m thick in the Maynard Lake area.
Ex cel lent ex po sures of the hyaloclastite unit are found
along the coast south of Bea ver Cove (Fig 3). Mas sive to
thickly bed ded vol ca nic brec cia is the dom i nant li thol ogy,
interbedded with sub or di nate, well-bed ded ba saltic sand -
stone and lesser amounts of pil low lava. The re la tion ships
be tween all three rock types and their in her ent tex tural
char ac ter is tics are well il lus trated in the coastal sec tion. At
one lo cal ity, for ex am ple, dark green ish grey, closely
packed pil low lavas are suc ceeded up ward by sand stones
and vol ca nic brec cias, and tex tures and a wealth of sed i -
men tary struc tures elu ci date the or i gin and mode of de po si -
tion of the volcaniclastic rocks (Fig 7). The low er most pil -
lows (<1 m in length) are apha ni tic and nonamygdaloidal,
and have lo cally trapped lam i nated and de formed sed i ment
in their in ter stices dur ing em place ment (Fig 7A). The pil -
lows are over lain by pale buff to dark grey-green weath er -
ing, thinly lam i nated to me dium-bed ded, me dium to
coarse-grained sand stone com posed pre dom i nantly of an -
gu lar to subangular ba saltic shards of hyaloclastite or i gin.
The thicker beds of hyaloclastite sand stone may en close
an gu lar to subrounded clasts of ba salt (<8 cm across), some
of which pre serve chilled pil low rinds, whereas lam i nated
ho ri zons may dis play crossbedding and spec tac u lar slump
folds and fluidization struc tures (Fig 7B, C). At the lo cal ity
il lus trated, slumped beds lie di rectly be neath a very thick
bed of dark grey-green hyaloclastite brec cia con tain ing
sparse pil low frag ments and rare whole pil lows, up to 1.5 m
in length, set in a finely comminuted sand-size ma trix of
gran u lated hyaloclastite ma te rial. The clasts are gen er ally
ma trix sup ported and poorly sorted. Many clasts are not ob -
vi ously de rived from pil low rims and prob a bly rep re sent
frag mented pil low cores. Else where, pil low-frag ment
brec cias are more eas ily rec og nized due to the abun dance
of clasts of bro ken and whole pil lows (Fig 7D). The lower
con tact of the un der ly ing dis turbed-bed ding ho ri zon
(Fig 7B) is marked by a fairly sharp dis con ti nu ity, whereas
the up per con tact is tran si tional into the ma trix of the brec -
cia bed. Graded bed ding is ev i dent in some of the thin sand -
stone beds, and crude nor mal grad ing may be rarely rec og -
nized in some of the brec cia lay ers. From these well-
pre served sed i men tary fea tures, it is clear that the coarse
vol ca nic brec cias were emplaced via de bris flows car ry ing
un con sol i dated hyaloclastite material downslope, thereby
causing rapid loading and dewatering, and localized
detachment, of semiconsolidated hyaloclastite sandstone
deposited previously by turbidity currents.
In thin sec tion, the hyaloclastite sand stone and the
brec cia ma tri ces are rich in dark brown, an gu lar to
subangular, subequant to highly elon gate shards of
palagonitized and devitrified, pseudo-iso tro pic ba saltic
glass, rep re sent ing the resedimented quench-frag men ta -
tion prod ucts of pil low lava (Fig 8). Finely comminuted
ma tri ces are gen er ally per va sively al tered to a fine-grained
mix ture of chlorite, quartz and prehnite, along with mi nor
epidote, car bon ate and iron ox ide. The rims of shards and
clasts are com monly bleached and/or marked by con cen tra -
tions of finely crys tal line opaque ma te rial, and some frag -
ments pre serve whole or par tial ves i cles usu ally infilled by
sec ond ary min er als. Bro ken plagioclase crys tals oc cur in
some ma tri ces, and larger clasts may en close euhedral to
subhedral plagioclase phenocrysts (Fig 8B).
Massive Flow Unit
A thick se quence of subaerial flows caps the lava pile
and forms the most ae ri ally ex ten sive map unit in the
Karmutsen For ma tion of north ern Van cou ver Is land. The
lower con tact with the hyaloclastite unit is es sen tially con -
form able (lo cal, irresolvable disconformities prob a bly ex -
ist), and flows gen er ally rest on pil low-frag ment brec cia or
pil low lava lo cally pres ent at the top of the hyaloclastite
suc ces sion. At some lo cal i ties, pil low-like morphologies
scat tered within more mas sive flows may mark a tran si tion
Geo log i cal Field work 2007, Pa per 2008-1 179
180 Brit ish Co lum bia Geo log i cal Sur vey
Fig ure 3. Gen er al ized ge ol ogy of the Late Tri as sic Karmutsen For ma tion in the Port Al ice – Port McNeill – Robson Bight area, show ing the lo ca tion of ol iv ine-bear ing and high-Mg bas alts in re -
la tion to the dis tri bu tion of mas sive flow, hyaloclastite and pil low lava se quences. Other map units are un dif fer en ti ated (grey), ex cept for the Ter tiary Alert Bay vol ca nic rocks and in tru sions of
the Is land Plutonic Suite (see Fig 1).
from sub ma rine to subaerial ef fu sive vol ca nism. At the up -
per con tact, Quatsino lime stone rests di rectly on the flows,
or is sep a rated from them by a thin (<1 m) layer of cal car e -
ous ba saltic sand stone and siltstone rep re sent ing the wa ter-
laid weath er ing prod ucts of the lava shield. The up per most
part of the flow se quence has a dis tinc tive
stra tig ra phy that in cludes thin (typ i cally
<6 m thick and rarely ex ceed ing 15 m)
pock ets and lenses of pale grey weath er -
ing, micritic to bioclastic, and rarely
oolitic, intervolcanic lime ston e, sim i lar to
the Quatsino For ma tion, and rare shale
and siltstone. Lo cally, intra-Karmutsen
lime stone is as so ci ated with thin se -
quences of pil low lava and hyaloclastite
de pos its, which may also oc cur in ter ca -
lated be tween the flows. Dis tinc tive
megacrystic lavas, charged with blocky to
pris matic, euhedral to subhedral
plagioclase crys tals reach ing 1 to 2 cm in
length, are re stricted to this part of the
sec tion (Fig 5). The min i mum thick ness
of the mas sive flow unit, as es ti mated
from its up per contact with Quatsino
limestone to the granitoid intrusion west
of Beaver Cove, is approximately
1500 m.
The dark grey to grey-green or dusky
red flows are gen er ally apha ni tic to fine
grained, es pe cially in flow in te ri ors, or
plagioclase phyric, and have strongly
amyg da loid al to nonamygdaloidal tex -
tures and mod er ate to strong mag netic re -
sponse. Plagioclase pheno crysts (typ i -
cally <4 mm) are more com mon in the
up per part of the flow se quence.
Plagioclase megacrysts (usu ally 1–2 cm)
oc cur in bas alts near the top of the unit,
gen er ally be low intra-Karmutsen lime -
stone, and are evenly dis trib uted through -
out the flow (10–20 vol% crys tals) or lo -
cally con cen trated in zones with up to
40 to 50 vol% crys tals. The megacrysts
are gen er ally ar ranged hap haz ardly, pre -
sum ably due to the high viscosity of these
crystal-choked layers.
The mor phol ogy of the mas sive flow
unit is com monly ren dered as ledges pro -
trud ing from steep hill sides. The unit is
com posed pre dom i nantly of sim ple
flows, gen er ally rang ing from about 2 to
6 m thick, al though some flows are thin -
ner (<0.5 m) and oth ers ap pear to ex ceed
15 m in thick ness. Rarely, ropy and
smooth ‘pahoehoe’ lava crusts and lobes
in com pound flows are well pre served
(Fig 9A–C). The con tacts be tween in di -
vid ual flows are typ i cally sharp and pla -
nar to curvilinear, and are in dis tinct in
many out crops. De fin i tive fea tures mark -
ing flow con tacts in clude amyg da loid al
flow tops over lain by dense, com pact
zones at the base of the over rid ing flow
that lo cally ex hibit hackly joint pat terns
and/or zones of pipe ves i cles com monly
filled with quartz and zeolites (Fig 9D).
Con tacts may ex hibit no tice ably dif fer ent de grees of ox i da -
tion, as re flected by hematitic al ter ation, but flow brec cia
and paleosol de vel op ment are en tirely ab sent. As noted by
Greene et al. (2006), co lum nar joint ing, com mon in many
con ti nen tal flood ba salt prov inces, is com pletely lack ing in
Geo log i cal Field work 2007, Pa per 2008-1 181
Fig ure 4. Leg end for the ge ol ogy map of the Karmutsen For ma tion (Fig 3, op po site).
Fig ure 5. Stra tig ra phy of the Karmutsen For ma tion, show ing the tri par tite sub di vi sion
into basal pil low lavas, over ly ing hyaloclastite de pos its and youn ger mas sive flow units.
Note that known oc cur rences of high-Mg ba salt ap pear to lie near the top of the pil low
lava se quence.
Karmutsen flows. Crude co lum nar joint ing, how ever, has
been ob served in cer tain sills. By far the most com mon in -
ter nal fab ric is a flow lam i na tion marked by amyg dule con -
cen tra tions rang ing from sev eral centi metres to (rarely)
0.5 m thick and com monly ar ranged in par al lel lay ers
within the out crop. This amyg da loid al lay er ing in vari ably
shares the same ori en ta tion as flow con tacts, where pres ent,
and may be used as a valu able struc tural in di ca tor in the ab -
sence of bed ding in for ma tion (Fig 3). Nested drain age
chan nels filled with drusy quartz crys tals, iden ti cal to those
de scribed pre vi ously in the basal pil low lava unit, are
sparsely distributed yet provide good substitutes for
bedding and clear evidence of local flow orientation (Fig
9E, F).
In thin sec tion, the por phy ritic flows con tain pheno -
crysts of plagioclase, or both ol iv ine and plagioclase, set in
a fine-grained groundmass con tain ing clinopyroxene,
plagioclase and opaque ox ides. Subhedral to anhedral ol iv -
ine crys tals (<1.5 mm) are found in lavas with as lit tle as
6 wt% MgO (see be low). Megacrystic flows con tain euhe -
dral laths and blocky crys tals of calcic plagioclase up to
2 cm in length and ex hibit ei ther a se rial size gra da tion to -
wards the groundmass or hiatal tex tures. Subhedral ol iv ine
com monly forms glomerophyric inter growths with, or in -
clu sions within, plagioclase pheno crysts. Clinopyroxene
may be pres ent as inter gra nu lar grains, par tic u larly in
aphyric flows, or dis play subophitic to ophitic tex tures,
most prev a lent in plagioclase-phyric lavas, es pe cially the
cen tres of thick flows. Sec ond ary al ter ation as sem blages
and amyg dule in fill ings in clude quartz, epidote, chlorite,
ze o lite, po tas sium feld spar and car bon ate, and rarely iron
oxides, chalcopyrite and native copper.
High-Mg Lavas
The high-Mg lavas are ar bi trarily de fined as hav ing
MgO >10 wt% and, in the geo chem i cal clas si fi ca tion de -
scribed be low, are pre dom i nantly magnesian ba salt and
picrite. The prin ci pal out crops are at Keogh Lake, the type
lo cal ity, and close to Sara Lake in the west and Maynard
Lake in the east (Greene et al., 2006). New ex po sures of
high-Mg lavas iden ti fied in this study are sit u ated on the
east ern side of the Maynard fault (Fig 3). To date, prac ti -
cally all high-Mg lavas are re stricted to the basal pil low ba -
salt unit, where they form both pil lowed and sheet flows. A
subvolcanic dike and pil low-frag ment brec cia de scribed by
Greene et al. (2006, Fig 7, 8) like wise have high-Mg com -
po si tions. The strati graphic po si tion of these high-Mg
rocks, as de ter mined in this study, lies close to the top of the
basal pil low ba salt se quence and within the base of the
overlying hyaloclastite unit (Fig 3, 5).
All high-Mg rocks con tain vari able amounts of ol iv ine
pheno crysts, rarely vis i ble in out crop due to com plete re -
place ment by sec ond ary min eral as sem blages (see Greene
et al., 2006, Fig 6, 7). Ol iv ine pheno crysts, typ i cally ac -
com pa nied by plagioclase, also oc cur in rocks with
182 Brit ish Co lum bia Geo log i cal Sur vey
Fig ure 6. Karmutsen pil low ba salt: A) closely packed pil lows in ter ca lated with pil low brec cias in ferred to lie near the top of the mas sive
subaerial flow unit (lo cal ity 06GNX3-3-1); B) elon gate to curvilinear glassy shards formed from quench-frag mented rinds of tholeiitic pil lows
in the basal pil low ba salt unit, devitrified and al tered to chlorite, quartz, epidote and ze o lite (‘dallasite’); C) lo cal ized mas sive sheet flow in -
ter ca lated with high-Mg pil low lavas near the top of the pil lowed unit (lo cal ity 06GNX34-4-1); D) de tail of base of sheet flow (in set in C)
draped over pil lows.
Geo log i cal Field work 2007, Pa per 2008-1 183
Fig ure 7. Volcanological and sed i men tary fea tures of the hyaloclastite unit: A) intraformational pil low lava (P) with trapped interpillow
hyaloclastite sed i ment (I), over lain by thinly bed ded hyaloclastite sand stone ex hib it ing remobilized bed ding near the con tact with over ly ing
hyaloclastite brec cia (Hbr) rich in bro ken pil low frag ments (lo cal ity 07GNX10-5-1); B) slumped and ro tated hyaloclastite beds be neath pil -
low-brec cia de bris flow (area shown in A); C) well-de vel oped flame struc tures (F) and load casts (L) in beds be low a de bris flow that also ex -
hibit crossbeds (X) and con tain dis persed ba saltic clasts (C; in set shown in A); D) pil low-frag ment brec cia ex hib it ing dis persed whole pil -
lows and a rusty, py ritic hyaloclastic ma trix (lo cal ity 06GNX34-6-1).
Fig ure 8. Pho to mi cro graphs of hyaloclastite tex tures: A) dark brown, palagonitized shards of devitrified, pseudo-iso tro pic ba saltic glass
and more finely comminuted ma te rial in lam i nated hyaloclastite sand stone, par tially re placed and ce mented by prehnite (white); B) small
an gu lar clast of palagonitized ve sic u lar ba salt (lower left) with a par tially em bed ded plagioclase phenocryst (Pl) en closed in hyaloclastite
sand stone. Both pho to mi cro graphs are of sam ple 07GNX11-4-1 in plane-po lar ized trans mit ted light.
<10 wt% MgO, which ap pear more wide spread (Fig 3).
High-Mg lavas are dif fi cult to dis tin guish from tholeiitic
ba salt in the field, and pos i tive iden ti fi ca tion re quires geo -
chem i cal anal y sis and petrographic ob ser va tion. The most
di ag nos tic phys i cal prop erty is their gen er ally non mag netic
char ac ter and extremely low magnetic susceptibility
readings.
The dark grey to green ish grey high-Mg pil low lavas
are typ i cally closely packed with vir tu ally no in ter sti tial
hyaloclastite, and are spa tially as so ci ated with less
184 Brit ish Co lum bia Geo log i cal Sur vey
Fig ure 9. Volcanological fea tures of subaerial Karmutsen bas alts: A) cross-sec tion through thin ‘pahoehoe’ flow lobes with smooth un du la -
tory con tacts dip ping gently to wards cam era (lo cal ity 06GNX11-5-1); B) close-up of curvilinear, ropy (pahoehoe) lava crust pro trud ing be -
low thin over ly ing flow (area shown in A); C) smooth pahoehoe flow lobe in the same quarry; D) sharp con tact be tween mas sive flows, show -
ing amyg dule-rich flow top in sharp con tact with the dense base of the over ly ing flow, which ex hib its a well-de vel oped zone of pipe ves i cles
de formed in the di rec tion of flow (left to right; lo cal ity 07GNX25-5-1); E) mas sive flow unit, ex hib it ing mul ti ple lava drain age ledges (lave
tubes) infilled with quartz (lo cal ity 07GNX13-5-1); F) close-up of vug gy quartz ledges (area shown in E).
magnesian tholeiitic ba salt. The Keogh Lake picritic ba salt
forms both pil lowed flows and mi nor sheet flows (Fig 10;
cf. Greene et al., 2006, Fig 6). The sam ple of high-Mg pil -
low brec cia taken close to the base of the hyaloclastite unit
co mes from a dark grey-green, mas sive layer of cha ot i cally
dis trib uted, an gu lar to subrounded, ma trix-sup ported clasts
(<35 cm), some of which ex hibit chilled pil low mar gins, set
in a chloritized and car bon ate-al tered clastic matrix.
Petro graphi cally, the high-Mg pil low lavas con tain
abun dant ol iv ine, typ i cally 10 to 25 vol% but reach ing
40 vol% in some sam ples. Euhedral to subhedral ol iv ine
crys tals (1–4 mm) are the sole phenocryst phase in high-Mg
lavas with >12 wt% MgO (i.e., picrite, ac cord ing to the re -
vised IUGS clas si fi ca tion dis cussed be low). The mor phol -
ogy and abun dance of ol iv ine pheno crysts is sim i lar in both
high-Mg sheet flows as so ci ated with the pil low lavas
(Fig 10) and the rare dikes that fed these lavas. The ol iv ine
in these rocks is pre dom i nantly re placed by talc, in con trast
to the less magnesian ol iv ine-bear ing ba salt, where re -
place ment prod ucts gen er ally in clude ser pen tine, chlorite,
car bon ate and quartz. Subequant to lath-shaped pheno -
crysts (<4 mm) of calcic plagioclase gen er ally co ex ist with
ol iv ine in the lat ter group of lavas. Where ol iv ine and
plagioclase form glomerophyric inter growths, ol iv ine is
the sub or di nate phase. Clinopyroxene in the high-Mg lavas
is typ i cally intergrown with calcic plagioclase to form
subophitic to well-de vel oped variolitic textures (illustrated
by Greene et al., 2006, Fig 5).
GEOCHEMICAL CLASSIFICATION
Whole-rock anal y ses of Karmutsen ba salt from north -
ern Van cou ver Is land are plot ted in Fig ure 11. All an a lyzed
sam ples were ex am ined petro graphi cally to de ter mine the
de gree of al ter ation, based on the abun dance of amygdules
infilled by sec ond ary min er als, no ta bly quartz, chlorite,
car bon ate, epidote, seri cite, po tas sium feld spar, prehnite,
zeolites and, rarely, pumpellyite; and the de gree of re place -
ment of calcic plagioclase, mainly by seri cite and clay min -
er als but also in volv ing par tial re place ment by chlorite, car -
bon ate and epidote. Ac cord ingly, the geo chem i cal di a -
grams show only the least al tered rocks, ex cept for Fig -
ure 11D, where al tered and amyg da loid al samples (>10 vol
% amygdules) are represented.
A to tal al ka lis ver sus sil ica (TAS) plot shows the
subalkaline na ture of the suite (Fig 11A). The lavas fall
within the ba salt fie ld in the TAS clas si fi ca tion (LeMaitre et
al., 1989) and ex hibit a large range of to tal al kali con tent
(approx. 0.5–5.0 wt%) rel a tive to their rather lim ited vari a -
tion in sil ica (approx. 50 ±2.5 wt%). The AFM di a gram
(Fig 11B) shows that the subalkaline bas alts be long to a
tholeiitic lin eage with mod er ate iron enrichment.
Both least-al tered and al tered bas alts are plot ted in the
IUGS clas si fi ca tion scheme for high-Mg lavas (Fig 11C, D,
re spec tively; Le Bas, 2000). Ol iv ine-bear ing ba salt dis -
plays a con sid er able range of MgO abun dances (approx.
5.5–19 wt%), ex tend ing from ‘nor mal’ ba salt MgO con cen -
tra tions through picrite to komatiitic com po si tions. The
sam pling is suf fi cient to il lus trate a con tin uum of MgO
abun dances in these ol iv ine-bear ing lavas. The picrite in
this clas si fi ca tion scheme is de fined by MgO >12 wt%,
SiO2 <52 wt% and Na2O + K2O <3 wt%. Komatiite has
MgO >18 wt%, SiO2 <52 wt%, to tal al ka lis <2 wt% and
TiO2 <1 wt% (and see Greene et al., 2006, Fig 2C). Lavas
that fall within the picrobasalt field in the high-Mg clas si fi -
ca tion are, in fact, sim ply ba salt in the TAS clas si fi ca tion
(Fig 11A) be cause, in or der to qual ify as ‘picrobasalt’, sil -
ica in these rocks must lie within the range 41 to 45 wt%
SiO2 (Le Bas, 2000), which is not the case. Ol iv ine-free ba -
salt falls near the lower limit of MgO con cen tra tions
(<7.5 wt%), but there is con sid er able compositional over -
lap be tween the ol iv ine-free and ol iv ine-bear ing groups.
Mag ne sium-poor lavas in the latter group are characterized
by minor to trace amounts of olivine (Fig 11E).
The amyg da loid al and al tered rocks plot ted in Fig -
ure 11D show a dis tinct bias to wards higher av er age al kali
con tents in the Mg-poor part of the compositional spec -
trum. The com po si tions of the least al tered lavas like wise
ex tend to sim i lar high al kali abun dances (Fig 11C). We
con clude that this be hav iour re flects vary ing de grees of al -
kali metasomatism un suc cess fully fil tered by petrographic
criteria alone.
Greene et al. (2006) ob served that the Keogh Lake
picritic ba salt has the high est MgO abun dances pres ently
known within the Wrangellian flood ba salt prov ince, and
that its prim i tive na ture re flects par tial melt ing gen er ated
by the as cent of a man tle plume be neath the oce anic pla -
teau. It must be noted, how ever, that Karmutsen lavas with
the high est MgO con tents do not nec es sar ily rep re sent the
com po si tions of such par tial melts. For ex am ple, it is clear
from the strong pos i tive cor re la tion be tween MgO and
modal ol iv ine (Fig 11E) that crys tal sort ing has played a
role in de ter min ing the MgO con tent of these lavas. The
high modal abun dance of ol iv ine in the most magnesian
lavas may be rec on ciled by crys tal ac cu mu la tion, whereas
com po si tions de pleted in MgO likely re flect crys tal frac -
tion ation of a more magnesian pa ren tal magma. It is im por -
tant to note that rocks fall ing within the ‘komatiite’ field in
the geo chem i cal clas si fi ca tion di a gram con tain ap prox i -
mately 40 vol% poly he dral ol iv ine pheno crysts that are
mor pho log i cally iden ti cal to those ob served in the picrite
and more magnesian ba salt. These tex tural at trib utes, to -
gether with the com plete lack of spini fex tex tures and close
as so ci a tion with picritic ba salt, are con sis tent with a cu mu -
late or i gin for these MgO-rich com po si tions (cf. Kerr and
Geo log i cal Field work 2007, Pa per 2008-1 185
Fig ure 10. Pho to mi cro graph of high-Mg ba salt in a mas sive sheet
flow at the type lo cal ity at Keogh Lake. Euhedral to subhedral ol iv -
ine pheno crysts are set in a groundmass of calcic plagioclase,
clinopyroxene and gran u lar opaque ox ides. The ol iv ine is com -
pletely re placed by talc±chlorite and fine-grained mag ne tite. Sam -
ple 07GNX37-1-1 in plane-po lar ized trans mit ted light.
186 Brit ish Co lum bia Geo log i cal Sur vey
Fig ure 11. Whole-rock geo chem i cal plots of Karmutsen bas alts: A) to tal al ka lis vs SiO2 plot, show ing the discriminant for al ka line and
subalkaline rock se ries (Irvine and Baragar, 1971) and the IUGS clas si fi ca tion of LeMaitre et al. (1989); B) to tal al ka lis – to tal Fe as
FeO (FeOt) – MgO (AFM) plot, show ing the discriminant be tween tholeiitic and calcalkaline rock se ries (Irvine and Baragar, 1971); C)
and D) to tal al ka lis vs MgO plots, show ing the IUGS re clas si fi ca tion for high-Mg and picritic vol ca nic rocks (Le Bas, 2000); ‘least al -
tered’ sam ples are shown in C, whereas al tered and amyg da loid al sam ples are plot ted in D; E) MgO vs modal ol iv ine (vol%) plot for
se lected ba salt sam ples. All anal y ses are re cal cu lated to 100 wt% an hy drous with to tal iron as FeO.
Arndt, 2001). Thus, the most prim i tive melts in the
Karmutsen For ma tion may have picritic rather than
komatiitic com po si tions, which has relevance to potential
models for Ni-Cu-PGE mineralization in the Wrangellian
flood basalt province, as discussed below.
EVOLUTION OF THE OCEANIC
PLATEAU
The stra tig ra phy of the Karmutsen For ma tion re flects
the evo lu tion of the Wrangellia oce anic pla teau, a re cord of
events not un like mod ern hot-spot vol ca noes such as Ha -
waii. The enor mous out pour ings of mainly aphyric pil low
lava and lo cal ized sheet flows, rep re sented by the basal pil -
low lava unit, mark the ini tial phase of sub ma rine ef fu sive
ac tiv ity, which con structed a seamount(s) ris ing from the
deep ocean floor. The amyg da loid al na ture of the up per -
most pil low lavas and brec cias in the over ly ing
hyaloclastite unit in di cate that, as the vol ca nic ed i fice grew
and reached rel a tively shal low depths (<500 m), mag mas
were able to exsolve small amounts of volatiles. The erup -
tion and em place ment of high-Mg lavas oc curred at the
tran si tion from pillow lava emplacement to hyaloclastite
deposition.
The thick se quence of volcaniclastic rocks de pos ited at
the tran si tion from sub ma rine to subaerial vol ca nism likely
re flects a va ri ety of frag men ta tion pro cesses, in clud ing
ther mal con trac tion of cool ing pil lows and in ter nal ex pan -
sion of grow ing pil low tubes, wave ac tion, mi nor ex plo sive
ac tiv ity trig gered by vol a tile re lease and sea wa ter-magma
in ter ac tion, and earth quake-in duced and grav i ta tional col -
lapse of oversteepened pil low ram parts. Many pil low-frag -
ment brec cias and sand stone ho ri zons are rich in
hyaloclastite ma te rial, pre sum ably gen er ated in shal low
wa ter and re de pos ited downslope by debris flows and
turbidity currents, respectively.
As erup tions breached sea level, out pour ings of highly
amyg da loid al, fluid ba saltic lava be gan to con struct a
gently slop ing shield vol cano. The plagioclase-phyric and
megacrystic flows were emplaced late in the evo lu tion of
the shield. As vol ca nism waned due to de cay of the man tle
plume, ther mal con trac tion of the litho sphere led to sub -
mer gence of the shield. Lo cal ized de po si tion of
intervolcanic lime stone, pil low lava and hyaloclastite de -
pos its in ter ca lated with subaerial flows at the top of the
Karmutsen suc ces sion re cords the fi nal stages of ef fu sive
ac tiv ity as sub mer gence com menced. The pres ence of thin,
dis con tin u ous beds of ba saltic sand stone and ab sence of a
well-de vel oped regolith or coarse con glom er atic de pos its
re flect the lack of deeply in cised to pog ra phy and rapid sub -
mer gence. Ces sa tion of vol ca nic ac tiv ity led to the
deposition of platform carbonate, represented by Quatsino
limestone.
POTENTIAL FOR NI-CU-PGE
MINERALIZATION
Flood ba salt vol ca nism is as so ci ated with some im por -
tant mag matic ore de pos its, one pre mier ex am ple be ing the
Ni-Cu-PGE sul phide min er al iza tion of the Noril’sk-
Talnakh re gion of Si be ria (Fig 12). Eco nomic con cen tra -
tions of ore met als (pro duc tion + re serves) at Noril’sk to tal
some 555 mil lion tonnes of 2.7% Ni, 3.9% Cu, 3 g/t Pt and
12 g/t Pd (Lightfoot and Hawkesworth, 1997). Naldrett
(2004) noted that the Noril’sk ores ex ceed all other de pos -
its, both Ni-Cu and PGE de pos its, in the value of in situ
met als per tonne. The sulphides are hosted by comagmatic
in tru sions that have been in ter preted as con duits for part of
a thick (~3.5 km) se quence of con ti nen tal flood bas alts
erupted at the Perm ian–Tri as sic bound ary. Like the
Wrangellian flood bas alts, the enor mous vol ume of Si be -
rian trap bas alts (>2 x 106 km3; Fedorenko, 1994) erupted
over such a short time in ter val (250 ±1 Ma; Sharma, 1997)
that their con se quently rapid erup tion rate and lack of de -
fin i tive ev i dence for em place ment dur ing a ma jor rift ing
event have been used to sup port a plume ini ti a tion model
for their or i gin (cf. Greene et al., 2006). As pects of the ge ol -
ogy and gen e sis of the Noril’sk de pos its and their related
flood basalts, therefore, have metallogenic significance
here.
Geo chem i cal stud ies of the host in tru sions and flood
bas alts at Noril’sk have in di cated that, in a gen eral sense
(and al beit con tro ver sial), the pro cesses that formed the ore
de pos its are to some de gree re flected in the lavas (cf. Arndt
et al., 2003; and see Naldrett, 2004 for a sum mary of per ti -
nent con tro ver sies). The mag matic sulphides are hosted by
gab bro-dolerite (9–16 wt% MgO) and ol iv ine-rich picrite
(18–29 wt% MgO) in tru sions emplaced in Pa leo zoic sed i -
Geo log i cal Field work 2007, Pa per 2008-1 187
Fig ure 12. Sche matic geo log i cal cross-sec tion of the Noril’sk re gion (af ter Naldrett, 2004). Ver ti cal scale is greatly
ex ag ger ated rel a tive to hor i zon tal scale.
men tary rocks that in clude sul phate-rich evaporites and
coal mea sures. In or der to sat isfy the geo chem i cal data,
most work ers agree that the com plex ar chi tec ture of con -
duits pres ently plugged by these in tru sions were the sites of
dy namic, open mag matic sys tems feed ing the vol ca nic pile.
Hot, prim i tive high-Mg (picritic) lavas form a small pro -
por tion of the over ly ing flood-ba salt stra tig ra phy in the
Noril’sk re gion (<1% of the strati graphic thick ness, ac -
cord ing to Fedorenko, 1994), and are con sid ered to rep re -
sent pa ren tal mag mas for the frac tion ated tholeiite. The
pres ence of high-Mg lavas is none the less im por tant, since
these mag mas in her ently con tain el e vated abun dances of
Ni, and po ten tially Cu and PGE, and are the most likely
can di dates to be sul phur undersaturated and there fore ca pa -
ble of precipitating economic concentrations of metals in
magmatic sulphides (Keays, 1995).
Some of the im por tant evo lu tion ary as pects of the
Noril’sk-Talnakh sys tem are shown sche mat i cally in Fig -
ure 13. Lightfoot and Hawkesworth (1997) ob served that
the chem i cal stra tig ra phy of the flood ba salt pile re cords a
strong de ple tion of chalcophile el e ments, as mon i tored by
Cu, over ap prox i mately 200 m of stra tig ra phy, fol lowed by
a grad ual up ward re cov ery in the tenor of Cu, Ni and PGE
in the suc ceed ing lavas over a 700 m strati graphic in ter val.
They em pha sized that the lavas that show the stron gest de -
ple tions of chalcophile el e ments, re flect ing equil i bra tion
with sul phide ores at depth, are also those that have ex pe ri -
enced the most crustal con tam i na tion by lower to
midcrustal rocks. They ar gued that the in crease in sil ica in
con tam i nated mag mas was pri mar ily re spon si ble for driv -
ing mag mas to wards sul phide ore for ma tion in shal lower
crustal magma cham bers, and not the ad di tion of crust-de -
rived S from evaporite-rich sed i ments. Since the sul phide
ores con tain an un usu ally high tenor of met als, the im mis ci -
ble sul phide drop lets must have scav enged met als from
very large vol umes of ba saltic magma mi grat ing through
the con duit sys tem, leav ing the erupted lavas sym pa thet i -
cally de pleted in these met als (the ‘R fac tor’ of Camp bell
and Naldrett, 1979). The grad ual re cov ery in metal abun -
dances ex hib ited by lavas higher in the strati graphic sec tion
is con sid ered to re flect pro gres sive iso la tion of sulphides
from dy namic in ter ac tion with fresh in puts of magma, pos -
si bly by grav i ta tional set tling of heavy sul phide drop lets in
hy dro dy namic traps so as to form the more massive
orebodies where Cu-rich ores subsequently fractionated
188 Brit ish Co lum bia Geo log i cal Sur vey
Fig ure 13. Sche matic model for the evo lu tion of the Noril’sk sys tem (af ter Lightfoot and Hawkesworth, 1997). Stage 1: picritic
and tholeiitic lavas of the Tuklonsky For ma tion (Tk) were erupted through con duits sit u ated east of Noril’sk. Stage 2: Erup tion
of Nd1 lavas at Noril’sk gen er ated by con tam i na tion and crys tal frac tion ation of prim i tive Tk mag mas with granodiorite at
depth fol lowed by as sim i la tion of evaporitic sed i ments at shal low lev els in the con duit sys tem; the el e vated sil ica and sul phur
con tents of con tam i nated mag mas lead to pre cip i ta tion of im mis ci ble sulphides that ponded and re acted with fresh magma
batches of Nd1 (Nadezhdinsky lavas) pass ing through the con duits. Stage 3: Con tin ued through put of Nd1 magma up graded
the Ni, Cu and PGE tenor of ponded sul phide liq uids, caus ing the metal de ple tion ob served in the lavas; as sub se quent Nd2,
Nd3 and Mr (Morongovsky) mag mas as cended through the con duits, the sulphides be came pro gres sively iso lated from the
mag mas and the de gree of metal de ple tion de clined, pro duc ing the ob served up ward in crease of chalcophile el e ment abun -
dances with strati graphic height. Stage 4: The Noril’sk sys tem shut down as magmatism mi grated north east; sul phide liq uids
frac tion ated to form the Cu-rich ores.
(Czamanske et al., 1992; Lightfoot and Hawkesworth,
1997).
Ex plo ra tion strat e gies for Ni-Cu-PGE de pos its as so ci -
ated with the Wrangellian flood bas alts should find cer tain
as pects of the Noril’sk model in trigu ing. The oc cur rence of
high-Mg ba saltic lavas in the Karmutsen For ma tion of
north ern Van cou ver Is land dem on strates that this part of
the flood ba salt prov ince re ceived a sup ply of hot prim i tive
magma, pre sum ably S-undersaturated and there fore ca pa -
ble of form ing mag matic sul phide ores. The lat eral ex tent
of these high-Mg lavas is pres ently un known, and no sys -
tem atic geo chem i cal stud ies of the flood ba salt stra tig ra phy
have been un der taken with a view to pros pect ing for lavas
with anomalously low chalcophile-element abundances.
Subvolcanic plumb ing sys tems are ex posed at the base
of the Karmutsen For ma tion as dikes and sills in the Mid dle
Tri as sic ‘sed i ment-sill’ unit (Mul ler et al., 1974, ‘Daonella
beds’ of Fig 2). These py ritic shale and siltstone beds, and
the older Pa leo zoic base ment rocks they over lie, are po ten -
tial sources of the si li ceous and sul phur-bear ing con tam i -
nants ap par ently re quired to in duce mag matic sul phide seg -
re ga tion in prim i tive melts. As doc u mented by Greene et al.
(2006), small con cen tra tions of dis sem i nated sulphides
have been ob served at the con tact of some subvolcanic
Karmutsen sills; and some chem i cal sub types of tholeiitic
ba salt with sig nif i cant quan ti ties of PGE ap pear to have
erupted close to sul phur sat u ra tion (J.S. Scoates, un pub -
lished data). These ob ser va tions, to gether with the lim ited
amount of geo chem i cal data cur rently avail able for the Tri -
as sic flood bas alts and their in tru sive counterparts, should
be particularly encouraging for mineral exploration.
ACKNOWLEDGMENTS
We wish to ex press our sin cere thanks to Mar ga ret
Hanuse in Port McNeill and Dave Ross in the com mu nity of
Holberg for gra ciously pro vid ing shel ter from the storm;
Kim Pilotte for cheer fully deal ing with truck and boat lo -
gis tics; and Brian Grant for a field visit. We also thank
Kathryn Gillis, Di rec tor of the School of Earth and Ocean
Sci ences at the Uni ver sity of Vic to ria, for pro vid ing ac cess
to point-count ing equip ment; and Tania Demchuk and
Brian Grant for editorial logistics.
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