Geological Fieldwork 2010, Paper 2011-1

Geology and Mineral Potential of the Southern Alexander Terrane and western Coast Plutonic Complex near Klemtu,

Northwestern British Columbia

by J.L. Nelson, L.J. Diakow, S. Karl1, J.B. Mahoney2, G.E. Gehrels3, M. Pecha3

and C. van Staal4

KEYWORDS: Alexander terrane, Grenville Channel fault, Coast Mountains, Coast Plutonic Complex

INTRODUCTION Although the northern coastal area of British

Columbia contains a significant number of tracts with high mineral potential assessments (Categories 1 and 2 out of 10; BC Mineral Potential Assessment Program; Kilby, 1995; see also Mineral Resources Assessment – Mineral Potential, on Mapplace.ca), mineral exploration has been at low levels, as indicated by the small number of assessment reports and recorded mineral showings. Just as it has received comparatively little exploration interest, this area also has not seen systematic public geological mapping since the original Geological Survey of Canada work in the 1960s (Roddick, 1970; Hutchison, 1982).

This report covers the second of a planned 3-year program study of the bedrock geology and mineral potential of the British Columbia North Coast area (Figure 1). The North Coast bedrock mapping and mineral deposit study is part of a cooperative, Natural Resources Canada (NRCan)-led endeavour, the Edges Multiple Metals – NW Canadian Cordillera (British Columbia and Yukon) Project. The Edges Project aims to increase our understanding of the far travelled lithotectonic terranes that make up the outer, accreted margin of the Canadian Cordillera and assess their metallic mineral potential (for a detailed project description see http://gsc.nrcan.gc.ca/ gem/min/edges_e.php). Edges is a contribution to the GEM Program (Geo-mapping for Energy and Mineral), a federal program that was initiated in 2008, to enhance public geoscience knowledge of northern Canada in order to stimulate economic activity in the energy and mineral sectors. The Edges project is a collaboration between the 1 United States Geological Survey, Anchorage, AK 2 University of Wisconsin at Eau Claire, WI 3 University of Arizona, Tucson, AZ 4 Geological Survey of Canada, Vancouver, BC This publication is also available, free of charge, as colour digital files in Adobe Acrobat® PDF format from the BC Ministry of Forests, Mines and Lands website at http://www.empr.gov.bc.ca/Mining/Geoscience/PublicationsCatalogue/Fieldwork.

Geological Survey of Canada, British Columbia Geological Survey, and Yukon Geological Survey and involves the United States Geological Survey and Canadian and American academic contributors.

The northern coastal area of British Columbia is underlain in part by rocks of the southern Alexander terrane, a large composite crustal fragment that underlies most of southeastern Alaska and extends farther north into part of the St. Elias Range on the Yukon-Alaskan border, (Figure 1; Wheeler et al., 1991). The Alexander terrane as a whole has attracted considerable exploration interest because of the volcanogenic massive sulphide deposits that it hosts, including Niblack and others on southern Prince of Wales Island, just north of the British Columbia-Alaska border, as well as a trend of Triassic deposits, notably Windy Craggy and the Greens Creek mine (Figure 1). In 2009, the first year of the North Coast project, geological mapping began on and near Porcher Island, at the northern end of Alexander terrane rocks along the north coast, in order to take advantage of proximity to the much better known stratigraphy in southeastern Alaska, and to nearby volcanogenic mineral deposits. Mapping in 2010 focused on the southern end of the terrane near Klemtu. In 2011, sparse exposures of pre-plutonic stratified rocks in the intervening region will be documented.

PREVIOUS WORK The northern coastal region of British Columbia was

first mapped systematically as part of Geological Survey of Canada regional coverage of the entire Coast Mountains batholith and enclosed metamorphic rocks. The Porcher Island – Grenville Channel area was covered as part of the Prince Rupert – Skeena sheet (Hutchison, 1982) and the Douglas Channel – Hecate Strait sheet (Roddick, 1970), and the area around Klemtu as part of the Laredo Sound sheet (Baer, 1973), all at a scale of 1:250 000. The focus of these studies was on the plutonic rather than supracrustal rocks; in addition, modern tools for the analysis of metamorphosed volcanic and sedimentary sequences, including uranium-lead geochronology and trace element geochemistry, were not available at that time. Recent geological work in the northern coastal region of British Columbia has focused on understanding the structural and igneous history of the

Geological Fieldwork 2010, Paper 2011-1 73

Fi20

gure 1. Location009 and 2010 fie

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74 British Columbia Geological Survey

Coast Mountains orogen. Within the North Coast project area, Porcher Island and Grenville Channel have been visited by researchers in the course of much broader structural studies and plutonic syntheses (Chardon et al., 1999; Chardon, 2003; Butler et al., 2006; Gehrels et al., 2009). Detailed geology, in particular the pre-batholithic stratified rocks of the Alexander terrane, have not been thoroughly investigated. The sole exception to this is the ongoing, mostly unpublished, regional geologic work of George Gehrels, part of which is summarized in Gehrels (2001) and Gehrels and Boghossian (2000).

REGIONAL GEOLOGICAL SETTING The Alexander terrane (Wheeler et al., 1991), which

forms the principal focus of this project, is flanked by variably metamorphosed and deformed metasedimentary-metavolcanic rock units that comprise the Banks Island assemblage to the west, and the Gravina belt and the Yukon-Tanana terrane to the east (Figure 2). With the exception of the Banks Island assemblage, which has only been recognized along the outer coast of northern British Columbia, these terranes continue northward into adjacent portions of southeastern Alaska, where equivalents have been described by Gehrels and Saleeby (1987; 1996), Rubin and Saleeby (1992), Saleeby (2000), and Gehrels (2001).

Alexander terrane The Alexander terrane in southeastern Alaska and

northern coastal British Columbia consists of a broad range of volcanic, sedimentary, and plutonic rocks and their metamorphic equivalents that are primarily of early Paleozoic age, overlain in places by thin younger sequences (Figure 3). In southeastern Alaska, these rocks have undergone limited post-Paleozoic metamorphism, deformation, and plutonism. Farther to the southeast, in northern coastal British Columbia, Cretaceous plutons become more widespread and the degree of Mesozoic deformation and metamorphism increases. In spite of these younger overprinting events, it is possible to correlate geologic units of southeastern Alaska with those of northwestern coastal British Columbia, and as such we use the nomenclature established in southeastern Alaska wherever possible. The following unit descriptions are taken from the well-preserved portion of the Alexander terrane in southern southeastern Alaska as decribed in Eberlein et al. (1983), Gehrels and Saleeby (1987) and Gehrels et al. (1996).

The oldest rocks recognized in the Alexander terrane consist of Late Proterozoic to Cambrian metavolcanic and metasedimenary assemblages of the Wales Group (Figure 3; Gehrels and Saleeby, 1987). Metavolcanic components range from mafic to felsic in composition, with lithic units ranging from metres to hundreds of metres in thickness. Relict textures indicate that protoliths of these rocks were pillowed flows, flow breccias, tuffaceous breccias, and tuffs. Metasedimentary units, similar in abundance to the

metavolcanic rocks, consist of volcanic clast-rich metagreywacke, pelitc phyllite or schist, and marble. These assemblages are intruded by bodies of complexly interlayered gabbro, diorite, tonalite, and granodiorite, with layering commonly on a metre to decimetre scale. All rocks of the Wales Group have a strong foliation and lineation that are deformed by outcrop-scale open folds. Metamorphism ranges from greenschist facies (actinolite-chlorite-epidote assemblages) to amphibolite facies (amphibole-biotite-muscovite and rare garnet).

Rocks of the Wales Group in southeastern Alaska are overlain by a less deformed, Early Ordovician to Late Silurian suite of greenschist or lower metamorphic grade volcanic and sedimentary rocks referred to as the Descon Formation. Protoliths of these rocks are similar to those in the Wales Group. Dioritic to granitic plutons that are coeval (and probably cogenetic) with volcanic rocks of the Descon Formation are widespread.

Lower Paleozoic strata are overlain unconformably by a variety of Devonian strata that commonly include a basal clastic sequence (conglomerates and sandstones, including redbeds) of the Karheen Formation, mafic volcanic rocks of the Coronados Volcanics and St. Joseph Islands Volcanics, and limestones of the Wadlegh Formation (Eberlein and Churkin, 1970). The basal conglomerate is interpreted to represent a major phase of uplift and erosion, the Klakas orogeny, as it overlies and contains clasts of a wide variety of older rocks (Gehrels and Saleeby, 1987). Younger, local unconformities are represented by conglomerates in the Late Devonian Port Refugio Formation. The Port Refugio Formation also includes fossiliferous and locally dolomitic limestone, radiolarian chert, mafic and felsic volcanic rocks, and volcaniclastic turbidites. The variability of facies and the presence of locally-derived conglomerates and bimodal volcanic sequences in these formations suggest that they were deposited in rift basins.

Younger strata in the Alexander terrane include fine to medium grained clastic rocks, carbonate, minor basalt of Carboniferous and Permian age, and Triassic basal conglomerate overlain by bimodal volcanic rocks, carbonate, and volcaniclastic strata. The Upper Jurassic to Upper Cretaceous Gravina belt, described separately below, overlies the Alexander terrane.

VMS potential of the Alexander terrane The Niblack prospect on southern Prince of Wales

Island (Figure 1) is a copper-zinc-gold-silver-rich Kuroko-type volcanogenic massive sulphide deposit, with 2.6 million tonnes of indicated mineral resource grading 1.18 per cent copper, 2.33 grams per tonne gold, 2.19 per cent zinc and 33.18 grams per tonne silver; and 1.7 million tonnes of inferred mineral resource grading 1.55 per cent copper, 2.08 grams per tonne gold, 3.17 per cent zinc and 32.56 grams per tonne silver, as of July 2009 (http://www.heatherdaleresources.com/hdr/Projects.asp).


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The age of the Banks Island assemblage is constrained by the following relationships:

1) detrital zircons recovered from two quartzites are as young as ~415 Ma (Silurian-Devonian boundary; G. Gehrels, unpublished data),

2) an orthogneiss on Aristazabal Island that has undergone the regional deformation and amphibolite-facies metamorphism along with the adjacent marble and metabasite has yielded a U-Pb age of 357 Ma (Early Mississippian), and

3) plutons of Late Jurassic age are emplaced into these rocks (Gehrels et al., 2009) and at least locally intrude across the regional foliation and folds.

These constraints suggest that at least some portions of the Banks Island assemblage accumulated during mid-Paleozoic time.

Yukon-Tanana terrane

East of the Alexander terrane are metavolcanic and metasedimentary rocks of the Yukon-Tanana terrane that underlie the western margin of the Coast Mountains, along the length of southeastern Alaska and northern coastal British Columbia (Figure 2). In general, these rocks form a panel that dips eastward and youngs westward, suggesting an overall inverted stratigraphy. Using the nomenclature defined in southeastern Alaska (Gehrels et al., 1992), the Yukon-Tanana terrane includes the following units (Figure 3):

1) Tracy Arm assemblage: This package contains marbles, quartzites, pelitic schists, and orthogneisses, which are commonly high in metamorphic grade and migmatitic. The age of this unit is constrained as Devonian or older based on ages from the overlying Endicott Arm assemblage.

2) Endicott Arm assemblage: This unit has a distinctive basal conglomerate containing clasts derived from the Tracy Arm assemblage. Overlying strata include greenschist to amphibolite facies felsic to mafic metavolcanic rocks, pelitic schists, and minor marble. Available faunal and U-Pb geochronologic constraints suggest that most strata are Devonian-Mississippian in age.

3) Port Houghton assemblage: These strata gradationally overlie the Endicott Arm assemblage and consist of greenschist to amphibolite facies metaturbidites, pelitic schist, and metabasalt. Available faunal constraints suggest that most strata are late Paleozoic in age.

In northwestern British Columbia, the Ecstall belt (Alldrick, 2001; Alldrick et al., 2001; see also Gareau and Woodsworth, 2000) with its enclosed Devonian volcanogenic deposits, is also assigned to the Yukon-

Tanana terrane. The host units are equivalent to the middle, Endicott Arm assemblage of southeastern Alaska.

Gravina belt

Rocks of the Alexander terrane are overlain by Upper Jurassic to Upper Cretaceous (Oxfordian to Cenomanian) turbidites and subordinate mafic volcanic rocks of the Gravina belt. These rocks can be traced, generally along the inboard margin of the Alexander terrane, for the length of southeastern Alaska (Berg et al., 1972) and into northern coastal British Columbia (Figure 2). On Tongass Island in southeastern Alaska, and on the mainland east of Port Simpson (Lax Kw’alaams) rocks assigned to the Gravina belt also overlie a sequence of metavolcanic and metasedimentary rocks that have been assigned to the Yukon-Tanana terrane (Gehrels, 2001).

Plutons of the western Coast Plutonic Complex

Tonalitic to granodioritic plutons of the Coast Plutonic Complex, or Coast Mountains batholith, occur as isolated bodies in northern and western portions of northern coastal British Columbia, and increase in extent southeastward to form huge continuous bodies of plutonic rock (Gehrels et al., 2009; Figure 2). Compositionally, most are tonalite and granodiorite, with subordinate diorite and minor gabbro and leucogranodiorite. A large majority of plutons have hornblende abundances exceeding those of biotite, are rich in titanite, and some plutonic suites contain euhedral epidote that is interpreted to be magmatic in origin.

According to a recent comprehensive geochronological summary (Gehrels et al., 2009), plutonic U-Pb ages record a history of eastward migration of emplacement across the Coast Mountains. The westernmost plutons are 160-140 Ma (Late Jurassic) tonalites and granodiorites. Early Cretaceous (120-100 Ma) tonalites and granodiorites occur directly east of the Late Jurassic bodies. A nearly continuous band of 100-85 Ma plutons (e.g. Ecstall pluton of Hutchison, 1982) underlies the western margin of the Coast Mountains, succeeded eastward by mainly tonalitic sills of ca. 70-60 Ma (latest Cretaceous-earliest Tertiary) age, and the central and eastern portions of the Coast Mountains are underlain by huge 60-50 Ma (Eocene) granodiorite bodies.

The emplacement depth of plutons also increases eastward across the Coast Mountains as shown by hornblende barometric studies conducted by Butler et al. (2001). This work suggests that westernmost Late Jurassic bodies were emplaced at depths of ~15 km, whereas Early Cretaceous plutons were slightly deeper, ~20 km, and farther east, mid-Cretaceous plutons of the Ecstall belt were emplaced at significantly greater depths, perhaps 25-30 km. This increase in depth of emplacement correlates well with the eastward increase in metamorphic grade.


SUMMARY OF PORCHER ISLAND – NORTHERN GRENVILLE CHANNEL GEOLOGY

The 2009 map area, comprising the vicinity of Porcher Island, northwestern Pitt Island and Grenville Channel, is underlain by metamorphosed supracrustal and plutonic rocks, intruded by late synkinematic Cretaceous plutons and cut by an array of northwest-striking sinistral faults that divide the geology of Porcher Island into a series of panels (Nelson et al., 2010a, b). Some faults mark major lithologic breaks, whereas others repeat similar sequences. The Grenville Channel fault (GCF) is the master fault; from it the Salt Lagoon and Useless fault splays cross Porcher Island. The northern continuation of GCF in Telegraph Passage may be an older, dextral structure (J. Angen, personal communication, 2010).

Detailed field mapping, supported by U-Pb geochronology (Gehrels and Boghossian, 2000; Butler et al., 2006; J.B. Mahoney, unpublished data, 2010; G. Gehrels, unpublished data, 2010) has allowed positive identification of many units in the mapped area, and tentative assignment of others. Because they are based on more complete geochronological data, unit ages and assignments shown on the open file map of the area (Nelson et al., 2010b) supercede those in Nelson et al. (2010a). Most important, field identification in 2009 of possible Wales Group equivalents on Porcher Island (Nelson et al., 2010a) was refuted by subsequent Ordovician U-Pb ages. The main metavolcanic sequence, which comprises most of the Alexander terrane in this area, is correlated with the Ordovician Descon Formation (s.l.) of southeast Alaska. In particular, it resembles the rhyolite-bearing Moira Sound unit, which hosts the Niblack volcanogenic massive sulphide deposit. Clastic rocks on Kennedy Island and near Baron Point on the mainland are correlated with the Early Devonian Karheen Formation.

Pre-Cretaceous plutonic bodies include the Ordovician McMicking and Hunt Inlet plutons, the Early Mississippian (?) Swede Point pluton, and a Devonian pluton in Porcher Inlet. Southwest of the metamorphosed supracrustal units, two metamorphosed igneous complexes, the Ogden Channel and Billy Bay complexes, are recognised. The Billy Bay complex is an intrusive equivalent of the Descon volcanic sequence. At this point, no conclusive age determination on the Ogden Channel complex has been made.

LOCAL GEOLOGY – 2010 MAPPING NEAR KLEMTU

The 2010 map area is located 100 km southeast of the 2009 map area, in eastern Laredo Sound (103A) map sheet. It extends from Return Channel in the south to Graham Reach, on the eastern side of Princess Royal Island, in the north (Figure 1). As shown in Figure 5, most

of this area is underlain by large plutons, with deformed and metamorphosed older stratified rocks of the Alexander terrane exposed in narrow pendants between them. The southern projection of the Grenville Channel fault passes through Klemtu Pass at Klemtu, then crosses Finlayson Channel and continues south through Jackson and Oscar passages and into southern Mathieson Channel.

Because of difficult access to the island interiors and extensive forest cover, most of the observations that form the basis of our mapping were made along shorelines. These were supplemented with logging road traverses, helicopter spot checking and limited traverses, together with image analysis of 5-metre resolution SPOT-5 satellite data captured between 2004 and 2006.

The geology in Figure 5 is based on a 1:50 000-scale open file map in preparation that will be available in early 2011 (Nelson et al., 2011).

Stratified Units

Mathieson Channel Formation

Strong lithologic similarities between layered metasedimentary and metavolcanic pendants in the Alexander terrane scattered throughout the 2010 map area have led to their inclusion within a single map unit, herein named the Mathieson Channel Formation. Because the original depositional relationships and stratigraphic continuity of units is disrupted by strike-slip faults, several generations of intrusive rocks and repeated by isoclinal folding, no simple stratigraphic section can be constructed, and true thicknesses of the stratigraphic layers are uncertain due to folding and structural repetition. However, overall stratigraphy of the Mathieson Channel Formation is based on consistent internal lithologic features and contact relationships that are documented in the layered units throughout the area. Figure 6 shows an interpretive stratigraphic section of the formation, measured along the eastern shoreline of Pooley Island. It includes the following provisional stratigraphic members:

1) Clastic-carbonate member Calcareous siliciclastic rocks and calcarenite make up the most widespread and abundant map unit.

2) Marble member Clastic-poor carbonates grade into the main clastic-carbonate member. They form mappable bodies in the Graham Reach area.

3) Conglomerate-greywacke member Coarse clastic units are locally important on eastern Pooley Island where they are interlayered with the clastic-carbonate member. They also occur at a few other sites in the area.

4) Andesite-gabbro member Andesite sills and flows (?) are restricted to part of the eastern shore of Graham Reach.


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olcanic and/or matrix

stward multiple

layers. merate assage

en the finer-

bonate ers are anitoid trusive ck bed nd in ze and s and posits. energy hieson upon


Figure 8a. Cconglomerat5832156N).

Figure 8b. Cgreen mafic

Figure 8c. clasts in car5842872N).

it, and thus

Conglomerate-grte with erosi

Conglomerate-grvolcanic clasts (

Conglomerate-rbonate matrix, G

s does not repre

reywacke membeve base (10

reywacke membe(10SK04-2; 5531

-greywacke meGriffin Passage (1

esent a basal co

er. East Pooley IJN11-04; 5521

er. Granitoid and01E, 5833771N

mber. Conglom10JN18-01; 5447

onglomerate.

sland 198E,

d dark ).

merate 781E,

Mar

the beddgraddisctranconsduriweafresh103ABay

And

shorappeandetextupseucoarThissupr

Vol

carbalonsoutin Gthe defodistiscorothemetaThecom

basaChaChawhitbandand brecdark

contmemof tuffarich likelderivsout

rble member

Pure or nearlyMathieson C

ded marbles, dationally intcontinuous mansect Jackson asiderably in ting deformatiather buff yellh surfaces OnA 007), was p

y pulp mill, 11

desite-gabbr

Dark green mre of Grahamear to be metamesitic compoures. Localudofragmental rse grained gabs unit may be eracrustal succe

canic memb

Mappable bodbonate pods, ong Spiller Chth end of Sarah

Green Inlet. ThGrenville Cha

ormed exposuinguishable (Fria in carbonaterwise mafic abasalts also cse minerals su

mpositions. Small occurr

alt at a few lannel exposureannel and into te weathered, ding (Figure 9b

in one outcccias occur, coker matrix.

Along Bullotacts betweenmbers of the Mthe clastic m

faceous layers;h in orthoclase ly that the abived from rhyothern end of Bu

r

y pure marble Channel Form

thickest alonto more clasarble units alsoand Oscar passthickness, dueon that prod

low-tan with wne of them, at previously minekm to the north

ro member

meta-igneous rom Reach southmorphosed diksition with lly there

textures, andbbro clasts in aentirely intrusivession.

ber

dies of metabaoccur in southwannel, in Math Island, in nohere are also mannel fault an

ures, pillows Figure 9a), as te matrix. Mic

(hornblendeontain biotite a

uggest unusual

rences of metlocalities, at bes, and along S

the south end coherent rhyob). At the nortcrop along Sonsisting of an

ock Channel, n the volcaniMathieson Chanmember there; and some si

compared to bundant potasolitic sources, ullock Channe

forms mappabmation. Intervang Graham Rstic-rich variao occur alongsages. Marble

e in part to pduced tight fwhite coarselyQuarry Point

ed for use at thh.

ocks occur aloh of Green I

kes and/or sills fine to coaare fragm

d some intrusiva highly deformve and thus no

asalt, in placeswestern Bullocthieson Channrthern Graham

metabasic mylond its splays. Iand pillow bwell as layers

croscopically, se-calcic plagiand/or potassiully potassium-r

arhyolite accoboth ends of tSpiller Channe

of Bullock Cholite shows deth end of BulloSpiller Channengular white fr

there are ic and clastinnel Formation

e contains finiltstones are asiltstones else

ssium feldsparas may be thel. This volcani

ble layers in als of pure Reach, pass ants. Thin,

g faults that layers vary

plastic flow folds. They y crystalline

(MINFILE he Swanson

ong the east Inlet. Some of

arse-grained mental or ves contain med matrix.

ot part of the

s containing ck Channel, nel, on the

m Reach and onites along In the least

breccias are s of angular some of the ioclase-rich) um feldspar. rich, alkalic

ompany the the Bullock l. In Return hannel, off-licate flow-

ock Channel el, rhyolite

fragments in

gradational ic-carbonate n. The base ne grained

anomalously where. It is r is locally

e case at the ic to fine


Fiprm

Fi01

vonoinFonosuse

ocCgeplfo

DDP

mliman

gure 9a. Volcanreservation of etamorphism (10

gure 9b. Volcan1; 564086E, 5796

olcaniclastic tortheastward. On the more wesormation. For ortheast-facingucceeded by clection.

The rhyolitccupy a low sthannel Formaeochronology tlace a lowerormation.

DEPOSITIODETRITAL ZPOSSIBLE

The Mathiemarine conditio

mestone. Its and gradational

nic member. Pilloprotolith feat

0SK02-8; 560835

nic member. Flow6866N).

to clastic traOverall, volcansterly exposurethis reason, w

g sequence, wlastic and carbo

e flow in Retutratigraphic poation and wasto determine itr stratigraphic

ONAL ENVIZIRCON CCORRELA

eson Channel Fons, as show

along-strike hocontact relati

ow basalt, Spilletures in amp5E, 5814316N).

w-banded metarh

nsition appeanic rocks are mes of the Mathiwe tentatively with a volcanonate member

urn Channel isosition within t sampled for ts eruptive agec age constr

RONMENTHARACTE

ATIONS Formation was

wn by the pmogeneity andionships betwe

r Channel; note hibolite grade

hyolite (10JN12-

ars to young most abundant ieson Channel regard it as a nic-rich base s in the upper

s presumed to the Mathieson

U-Pb zircon which would raint on the

T, AGE, ER AND

s deposited in prevalence of d interlayered een all of the

members Depths compensasequence depositionarea on Pforms lesarenaceoulocal occand moreindicate pbasin-maragainst dethe inferrof small-sorigin forsubtropiclocally denvisageddepositedwith back

In soresemblesIsland in2010a) wKarheen Formationsandstoneon PrinceGehrels abiostratigDevonianSilurian asubaerial and thick1970; Ggeochronca. 420-4from Latesouthern abundant of unknow

Like from conis dominagrained compositibiotite. Hboulder-rcharacteriMathiesonlocal conflow depKarheen carbonateare so cChannel F

suggest deposdid not ge

ation depth. Poare also a

n. ConglomeraPooley Island ss than 5 perus clastics incurrences of coe widespread periodic mass frgin faulting. eposition in a red general shascale bimodal r the basin. Oal, arid settin

discharged higd. Sandy and d by tidal currekground carbonome regards, ts the sandstonn northern Grwhich, in turnFormation of n was named e, siltstone, she of Wales Island Saleeby,

graphy indicaten (Pragian) iand older rockto shallow m

kens to the sGehrels and Snology from th450 Ma domine Ordovician aAlexander teMiddle Proter

wn cratonic dethe Karheen

nglomerate in tated by shallow

to porphyritions are simiHowever, the rich facies anize the Karheen Channel For

nglomerates in posits, some wFormation doe

e or the monotocommon in thFormation is f

sition within a enerally exceossible redbed an indication ates are most aand nearby; o

r cent of an onterlayered wioarse, poorly but thin inte

flow depositioTheir restrictelarge-scale sub

allow depth of volcanism is c

Overall, a rifteng, into whichgh loads of silty fractions

ents or storm snate depositionthe Mathieson ne-conglomerarenville Channn, has been southeastern Afor a successi

hale and minorland (Eberlein 1987). Conode the formatiin age. The ks, and is inte

marine clastic wsoutheast (EbeSaleeby, 198he Karheen Fo

nant populationand Silurian p

errane, and a rozoic to Late

erivation (GehrFormation, ththe Mathiesonwly emplaced tic) granitoidilarly arkosic

great thicknnd large-scaleen Formation armation. Insteathe latter are

with erosive es not containonously thin-bhis unit. Ovefiner grained a

single marine eed the carbprotoliths with

of shallowabundant in thoverall conglomoverall sequenith carbonatessorted conglom

ervals of greyn, perhaps rela

ed occurrence bmarine fan, awater. The pre

consistent withed, epeiric sea

h ephemeral stcoarse sedim may have be

surges that alten.

Channel Formate unit on Kenel (Nelson ecorrelated wit

Alaska. The Kaion of conglomr limestone exand Churkin,

dont and brachon is middle formation ov

erpreted as parwedge that coerlein and Ch7). Detrital ormation inclu

n, apparently dplutonic rocks

diverse, mucArchean popu

rels et al., 1996he cobble popun Channel Form

(unfoliated, mds, and san

with dissemness and extee crossbeddingare not present ad, the fairly minterpreted as bases. Furthe

n regular intervbedded siltstonrall, the Mathand more calc

basin. bonate hin the

w-water he type merate nce of s. The merate

ywacke ated to argues

as does esence h a rift a in a treams

ment is een re-ernated

mation ennedy et al., th the arheen

merate, xposed

1970, hiopod

Early verlies rt of a

oarsens hurkin, zircon

udes a derived

of the h less ulation 6). ulation mation

medium dstone

minated ent of g that in the

minor, mass-

er, the vals of es that hieson areous


than the Ksmaller sca

At preanalysed frMathiesona unimodBoghossianis somewhpopulation(Gehrels aDevonian the signatuthan the mFormation

GivenChannel Fmarine barepresent awhich theAnother posoutheast Formation,consists osiltstone anflows, bothrhyolite.

Jorkins An un

on the hesouthern Sbecause it unlike theChannel Fpoorly socentimetre quartzite, diopside-ridiopsidic calong the polymicticsiltstone crounded (Fthe aspect its continumixing ooccurrencedegree of mbasins, forBritish Coexposure septum ocalcsilicateplagioclaseMathieson

Microby coarse g

Karheen Formale uplift in a leesent, a singlerom the congloChannel Form

dal peak at n, 2000; Gehrehat younger t

ns in the Karheeand Boghossiamaximum ageure reflects a mainly Ordov.

n the probable Formation and sin bounded ia separate basie Karheen Foossible correlaAlaska is the, defined by Eof greywackend shale, limeh pillowed and

Point conglnusual quartziteadland 2 kmSwindle Island

represents a pe plutonic-volcFormation. At orted and im

to decimetalong with a ich, carbonate-calcsilicate ma

shoreline, thc with increasclasts, and beFigure 10b). Tof a nearly sination to the no

of lithologic e of conglomematurity resemr instance alonolumbia (Alldto the south

of tonalite ane-metasiltstonee (±orthoclasChannel Formscopically, thegrained, interlo

mation, featureess active basin detrital zircon

omerate-sandstmation on Poole

about 420 Mels, unpublishethan typical Oen Formation oan, 2000) ande for the unit. plutonic sourc

vician volcanis

Devonian ageits mode of o

in part by locin analogous tormation probative in the Ale Late DevonEberlein and Ce, conglomerastone, basalt id brecciated, a

lomerate te-cobble cong

northeast of d. It is describpericontinental canic sources its southernm

mmature, constre-size fragmfew clasts of

-altered metavoatrix (Figure

he conglomerasing quantitiesetter sorted;

The most southngle-sourced f

orth indicates fatypes. Textu

erate and the amble base-of-scng the Eskay rdrick et al., of the congl

nd diorite, ie unit that resese) arenitic mation. e quartzite claocking quartz, b

s that may ren. n sample has tone member oey Island ; it shMa (Gehrels ed data, 2010). Ordovician-Siluof southeast Ald gives an ea

Like the Karhce terrane yousm of the De

e of the Mathiorigin in a shalcal scarps, it cto the rift basinably accumullexander terrannian Port RefChurkin (1970ate, thinly-bein the form of and tuff and m

glomerate outcf Jorkins Poinbed separately

source complof the Mathi

most exposure, isting of ang

ments of imf metasiltstoneolcanics in a d10a) Farther nate becomes ms of volcanic cobbles are mherly exposurefanglomerate, warther transporurally, the abrupt variatiocarp deposits inrift in northwe

2005). The lomerate, acrois a thin-layembles the qusiltstones in

asts are dominbut they also

eflect

been of the hows

and This

urian laska rliest heen, unger escon

ieson llow, could ns in lated. ne of fugio 0). It edded f lava minor

crops nt on

here letely ieson it is

gular mpure e and dirty, north more

and more e has while rt and local

on in n rift

estern next

oss a yered uartz-

the

nated

Figucong(10J

Figuwith 01a;

contdiop

varyand hornstronMatorthof sCha

congbetw2600Gehof tsignAmeperi1998

ure 10a. Jorkinsglomerate of mN02-01; 535273

ure 10b. Jorkins more clast roun

535255E, 58110

tain minor qpside, titanite, h

Siltstone clasying proportio

orthoclase, nblende, titanitng resemblancthieson Chanhoclase in themsomewhat restrannel Formation

A detrital zircglomerate shoween about 900 Ma; no Pal

hrels, unpublishthe Jorkins Ponatures of boterica (Gehrelsicratonic Yuko8; Nelson and

s Point conglomeostly quartzite

3E, 5810889N).

Point conglomernding and sortin050N).

quantities of hornblende andsts are finelyns of fine gra

and mafic te and diopsid

ce to siltstones nnel Formatim is particularlricted occurrenn. con sample ofows a very b00 and 2000 Mleozoic grains hed data, 2010oint quartzites th autochthons et al., 1995on-Tanana terrd Gehrels, 200

erate. Chaotic, clasts in calcs

rate. Polymictic cng. FOV 2 metr

plagioclase, d tiny zircon g

y laminated aained quartz, p

laminations e dominate. Ththat are widespon. The prly noteworthy, nce within the

f quartzite clasbroad PrecamMa, with less

were identifi0). The craton

contrasts marnous northwes5) or the morane (Gehrels 07). It suggest

poorly sorted ilicate matrix

conglomerate res. (10JN02-

orthoclase, rains.

and contain plagioclase,

in which hese have a pread in the resence of in that it is

e Mathieson

sts from the mbrian peak

er peaks to ied (George al signature rkedly with stern North ore inboard,

and Kapp, ts an exotic


origin. However, the resemblance of siltstone clasts to the Mathieson Channel Formation may provide a sedimentological link between the two units. In this case, pericontinental rocks possibly formed part of the basement to the Mathieson Channel basin, and the Jorkins Point conglomerate could have formed along a western scarp. Gehrels et al. (1996) argued that the Silurian-Devonian Klakas orogeny involved interaction between the Alexander terrane and an outboard (present coordinates) continental fragment, such as is represented by the Banks Island assemblage (Gehrels and Boghossian, 2000; Figure 2, this paper). The Jorkins Point conglomerate may represent a “missing link” between the two. Ongoing detrital zircon studies (J.B. Mahoney, 2010-11) may shed light on this important potential correlation.

Intrusive units As shown on Figure 2, over 90 per cent of the project

area is underlain by Cretaceous intrusive rocks of the western Coast Plutonic Complex. In the original mapping of the Laredo Sound area (Baer, 1973), the granitoids were given unit assignments based on composition and degree of foliation. This study benefited from the use of uranium-lead zircon dates from representative sites in the area (Gehrels et al., 2009), as well as an enhanced appreciation for styles of deformation of the plutonic bodies. This has resulted in significant changes in the location of plutonic contacts and interpreted contact relationships.

Uranium-lead ages reported by Gehrels et al. (2009) were important in the definition of three main plutonic suites in the study area (Figure 5). The oldest plutonic suite is based on an age of ca. 123 Ma from a site north of the east end of Jackson Passage (Figure 5), obtained from a penetratively deformed, amphibolite grade diorite-pyroxenite-gabbro complex. In our mapping, this metaplutonic complex corresponds to most of Baer’s unit 2 and some of his unit 3; several small bodies of trondhjemite, shown on his map as unit 5, are also part of the complex. The second plutonic suite ranges in age from ca. 104 to ca. 94 Ma and is dominated by large homogenous tonalite bodies that comprise much of Sarah and Roderick islands, adjacent to Finlayson Channel. These plutons range from comparatively leucocratic and unfoliated cores (Baer’s unit 5) to darker and more foliated margins (Baer’s unit 3). The youngest plutonic suite, designated as unit 14a in Baer (1973), have several ages of approximately 82 Ma. One small intrusion was mapped near James Bay, along the western shoreline of Mathieson Channel. This body continues southeast across the channel where a coeval age indicates that it also includes a body previously mapped as Baer’s unit 5. These significant changes emphasise the importance of new detailed mapping in concert with U-Pb dating as necessary to the understanding of the Coast Plutonic Complex.

Early Cretaceous (ca. 123 Ma) mafic intrusive complex

A deformed and metamorphosed, generally mafic plutonic suite outcrops extensively along both sides of Mathieson Channel. Similar bodies also occur northwest of Klemtu and north of Green Inlet. Diorite is the dominant phase, although variants from ultramafite to trondhjemite are present. Where large enough, the gabbro-ultramafite and trondhjemite bodies have been mapped separately from the main, undivided, dominantly dioritic bodies. It should be noted, however, that this suite shows strong variability even at outcrop scale (Figure 11a).

Diorites, which are most abundant throughout these complexes, are typically penetratively foliated to protomylonitic with asymmetric fabrics (Figure 11b). The foliation involves fine grained metamorphic hornblende, accompanied by quartz, calcic plagioclase, and titanite. Primary igneous minerals survive as pseudomorphs and porphyroclasts. Gabbro bodies contain areas of ultramafic cumulates, for instance near the eastern entrance of Jackson Passage. Ultramafites are metamorphosed to coarse grained tremolite/actinolite-clinochlore-biotite assemblages, in one case with bright green picotite (?) grains. Trondhjemites south of southern Mathieson Channel are highly foliated to protomylonitic, with a strong biotite fabric (Figure 11c).

No relationships with angular discordance were observed between phases of this complex and the Mathieson Channel Formation. Foliation involving amphibolite-facies assemblages is strongly developed in both, parallel to their contacts.

A U-Pb age of 123.3 ± 1.4 Ma was reported by Gehrels et al. (2009) from this suite, at a location 1.5 km north of the eastern end of Jackson Passage. The sample is from a medium-grained diorite with moderate foliation, which grades compositionally into gabbro and tonalite. The latest phase at this outcrop is a highly foliated trondhjemite/pegmatite dike (Figure 11a).

An unusual body of partly protomylonitic granodiorite in Neekas Inlet is tentatively assigned to this suite, based on its occurrences as a northwest-aligned sliver, and its degree of deformation. In it, coarse igneous microcline grew in equilibrium with plagioclase, and large brown allanite grains are rimmed with epidote. Subsolidus mylonitic fabrics include wispy biotite trains partly overgrown by late muscovite and chlorite, quartz ribboning and extensive development of trains of small neoblasts. Well-formed epidote grains grow across biotite. This body will form part of a U-Pb geochronological study by M. Pecha, aimed at constraining ages of deformation (2010-11).


Figure 11aJackson Paparallel folimetres.

Figure 11bmetadiorite

Figure 11ctrondhjemite

. Ca. 123 Ma inassage; outcrop-ation in crossc

b. Ca. 123 M(10JN20-02; 549

c. Ca. 123 Mae (10JN25-06; 55

ntrusive complex-scale intrusive cutting trondhjem

Ma intrusive co9391E, 5816174N

a intrusive com51404E, 5808794

x. U-Pb site nocomplex. Note

mite dike. FOV

omplex. Well-foN).

mplex. Protomyl4N).

rth of wall-

V 1.5

oliated

lonitic

Midqua

comFinldomdesirelatvaricomplutgreyubiqmodencl

graindominterfoliaof mTheybut pthanmingrainstraiin sh

sevesingend

Figutonaencla

d-Cretaceouartz dioriteBodies of t

mpositions undlayson Channe

minates the Lignated these rtively minor iability are rec

mbined (Figuretons include unyish shades wquitous mafic derate strain mlaves (FiguresInternally, th

ined, equigranminated by zonrstial quartz. ation. Titanite mafic enclavey typically maparticularly in

n 40 per centnerals, they areined texture, in intensity, thhape. Tonalite in M

eral U-Pb agesgle age of 94.3

of the tonalite

ure 12a. Mid-Crelite from the miaves.

us (ca. 94-1

tonalite transitderlie most ofel, forming partaredo Sound rocks as units

internal comcognized and he 5). Distinguniform off-wh

with increasingenclaves, and

manifested by al12a, b).

he large tonanular and haved plagioclase

Aligned biis sparse but

es distinguisheake up less thaborder phases. Composed oe amphiboliteand variably

he enclaves var

Meyers Channs averaging ab

3 ± 1.4 Ma wa body on Sarah

etaceous (94-10id-Cretaceous s

05 Ma) tona

tional to quaf the islands t of a batholithmap area. B

s 3 and 5; howmpositional anhence these unuishing featurehite color that g mafic minerd comparativeligned mafic m

alite bodies ave consistent , euhedral horniotite aggregaubiquitous. Th

es these tonaan 3 per cent os they may conof plagioclase s with fine- tstrained. Dep

ry from ovoid t

nel, west of Kbout 104 ± 1.5s obtained at th Island (Gehre

05 Ma) tonalite ssuite displays elo

alite and

artz diorite adjacent to

hic mass that Baer (1973) wever, only nd textural

nits are now es of these approaches

ral content, ely weak to minerals and

are medium mineralogy

nblende and ates define he presence lite bodies. of the rock, stitute more

and mafic to medium-pending on to lenticular

Klemtu has 5 Ma, and a the northern els, 2009).

suite. Typical ongate mafic


Fifopoide53

FiCrfom

mexanshfeobththdilomcofotofa

gure 12b. Mid-Cliated tonalite

orphyroblasts inentical to igneo34258E, 5820134

gure 12c. Mid-retaceous tonaliliation, cuts anetadiorite (10JN2

Contacts bemafic complex xposed on shornd in southernharp breaks, prew localities bserved, in whhe transpositionhe amphibolite ikes intrude rococality tonalite

metadiorite andomplex. In sooliated, mafic onalite pluton acies metadiori

Cretaceous (94-1with mafic e

n the enclavesous plagioclase4N).

-Cretaceous (94ite with enclaved surrounds ca25-02; 552821E,

etween tonaliteand Mathieso

relines of Sarahn Mathieson Cresumed to be clear crosscut

hich phases of n foliation in tfabric in the ccks of the oldeengulfs round

d pyroxenite atouthern Math

enclave-rich contains largete (Figure 12c)

105 Ma) tonaliteenclaves – nos, which are e in the tonalit

4-105 Ma) tonaes, showing moa. 123 Ma amp, 5814165N).

e and rocks fron Channel Fh Island in Tol

Channel. Moststeep faults; h

tting relationsthe tonalite su

the metamorphca. 123 Ma pluer mafic compled, milled bloct its contact whieson Channe

border phase inclusions of).

e suite. Detail of ote plagioclase

compositionally te (10JN21-01;

alite suite. Mid-derate igneous phibolite facies

rom the older Formation are lmie Channel, t contacts are however, in aships can beuite cut across hic rocks, andutons. Tonalite lex and at one cks of foliated

with the mafic el, a weakly e of a large f amphibolite-

Late CrA to

Mathiesonstudy areaa larger southeastarea. Apogeneral nare sharpChannel Near the Bay, angundeformare whitehigher esupportin

The tthose comafic enfeatures dunlike patonalite igrains of to 10-15 lesser hozoned plpristine an

The tonalitic eastern eChannel, An equivnorth from(1973) ubecause absence oother unit

Whitcross-cuttrelated toPegmatitishow thaintrusion.

STRUCMETAM

Strucarea are MathiesonCretaceoudeformatipolyphaseand penePlanar fabsteeply tthroughou

retaceous (conalite stock n Channel repa. This small bpluton, unit

tward across thophyses of the

northwest regiop and intrusivFormation andmargin of the

gular inclusionmed tonalite mae; and bold celevation into ng only sparse ttonalite body

omprising the nclaves are nodue to regiona

arts of the oldeis homogeneo

f equigranular pper cent com

ornblende). Exlagioclase by

and unstrained. timing for emintrusions is i

extension of thwhich is 81.7

valent U-Pb dam a circular pl

unit 5. Elsewhof distinguish

of supporting gt 5 tonalitic plute pegmatite ating all othero the mid- andic boudins in tat deformatio.

CTURAL GMORPHISMctural styles an

specific to n Channel Forus intrusive suiion at amphiboe folding withetrative foliatiobrics in the mto the east, ut the western

ca. 80 Ma) tnorth of Ja

presents the yobody comprises

14a, that Bhe channel ane pluton cut oonal structural ve into rocksd Early Cretacpluton on the

ns with internaatrix. Shorelinecliffs with spa clusters of tree cover. near James Bamid-Cretaceo

ot present anal metamorphier suite. Internous and consiplagioclase an

mbined mafic mxcept for incip

fine opaquePrismatic titan

mplacement oinferred from he body that 7 ± 1.2 Ma (Gate was also obluton originallyhere in the sthing lithologi

geochronologyutons to the miand aplite dikr rock units. d Late Cretacethe Mathieson

on continued

EOLOGY AM nd metamorphithe major rormation and thite are characteolite grade: thihin the layeredons in the pl

map area strike an orientationCoast Mounta

tonalite ames Bay, woungest pluton s the western laer (1973) ex

nd beyond the obliquely acrofabric, and co

s of the Mathceous plutonicsouth side of

al foliation oce exposures typarse jointing p

steep-sided

ay overall reseous suite; hownd the pluton ism or deform

nally, the Jameists of mediumd quartz, in ad

minerals (biotipient replacems, all mineranite is abundanf these undefU-Pb dating

straddles MathGehrels et al.,

btained immedy assigned to Btudy area, howic features a, we have reasid-Cretaceous ses occur regioThey are pro

eous intrusive n Channel Form

during Creta

AND

ic grades in thck units. Bote older (ca. 12erized by peneis is characterizd metamorphiclutons (Figure

northwest andn that is co

ains (Rusmore e

western in the

lobe of xtends study

oss the ontacts hieson

c suite. James

ccur in pically pass at

knobs

embles wever,

lacks mation, es Bay m-size ddition te and

ment of als are nt. formed at the hieson 2009). diately Baer’s wever,

and in signed suite. onally, obably suites. mation aceous

he map th the

23 Ma) etrative zed by

c rocks 13a). d deep mmon et al.,


Figure 13. northeasternLinear minostructures (m

a) Cartoon Cretn Laredo Sound r structures (F1 amineral, stretchin

taceous structurmap area; b) Po

and F2 rootless ang and mylonitic

al history of easoles to planar stand isoclines - Llineations) in Cre

stern Laredo Sotructures (beddin1 and L2 minera

etaceous intrusio

ound map area.ng, transpositional lineations) in Mons.

b-d) Stereo plo fabrics) in Math

Mathieson Chann

ots of minor struhieson Channel nel Formation; d)

uctures in the Formation; c) ) Linear minor


20an20in

pealGsokiafplwwCno

M

CtoprreanopmprfoprFoispo14reinFlindomridmacex

co(Findoshadshfe13

sethIs

005; Crawfordnd along nort010a). Within ncreases toward

Plutons ofenetratively delong major

Grenville Channouth-central painematic indffecting all rolutons and als

were emplaced with minor de

hannel fault orthern Swindl

Mathieson ChRegional

hannel Formato the northeastrevalent throueflect lithologicnd sandstone open folds w

metavolcanic reservation of olds. Because roportion of mormation, smoclines are olyphase foldin4a). F1 minor efold the transnvolve both lay1 and F2, whicneations, varyowndip. Steep

more common dges southea

metasiltstones ccommodated bxtent (Figure 1

Sinistral asonglomerate cFigure 14c). Ron many cases. Sominantly puhortening andddition, a sinhown by the eatures develop3b).

Open to cloeen west of Grahe conglomerasland. F3 fold a

d et al., 2000) athern Grenvillthe map area, ds the northeasf the mid-C

eformed, excepfault strandsnel fault passeart of the mapdicators showock units inclso younger lealong fault st

extral displacearray in soutle Island.

hannel Formafabric orientation strike nort (Figure 13b)

ughout the unc control; the

on eastern Poolwith an axiasections devoriginal textuof its thinly

marble layers mall-scale isoc

well-developeng during progfolds are intraposition fabric

yering and fabrch are generally in plunge f

plunges in ththan in the n

ast of Greenattest to theby this unit, pa4b). symmetries arclasts and bouootless isoclineStrain in these ure shear, d overall sonistral reverse

predominantlyped on northea

osed F3 folds aham Reach, in

ate-sandstone maxes trend eithe

as well as on Pe Channel (Nthe intensity o

st. Cretaceous supt for local mys. The northes through the area (Figure

w sinistral luding the m

eucocratic diketrands. North-sements join tthern Tolmie

ation ations in therthwesterly, an). Transpositio

nit. The excepthick-bedded

ley Island are dal planar cloid of carbures and few y layered natu

in the Mathieclinal folds ed within i

gressive deformafolial and do c, whereas F2 ric. Isoclinal foy coaxial - andfrom gentle te southeasternnortheast (Figun Inlet, sheae large amouarticularly in it

re observed iudins throughes show sinistrrocks appearswith northeuthwesterly vcomponent o

y southeast-pluast-dipping lay

on hundred mn Griffin Passamember on eaer north or nort

Porcher Island Nelson et al., of deformation

uite are not lonitic fabrics hwest-striking e western and 5). Measured displacement, id-Cretaceous e phases that striking faults the Grenville Passage and

e Mathieson nd dip steeply on fabrics are ptions to this conglomerate deformed into leavage, and bonate show outcrop-scale

ure and high eson Channel and rootless it, recording

mation (Figurenot appear to folds clearly

old axes - both d also mineral to steep and

n quadrant are ure 13c). On

ath folds in unt of strain s northeastern

in “tails” on hout the area ral asymmetry s to have been east-southwest vergence. Inof motion is unging linearyering (Figure

metre scale are age, and affect astern Pooley thwest. These

Figure 14Formation.isoclinal F(10JN11-0

Figure 14Formation.Inlet (10JN

Figure 14Formation.conglomer

folds appinsufficie

4a. Deformation. Thin siltstoneF2 minor folds 5; 551556E, 583

4b. Deformation. Sheath folds i

N07-02; 546439E

4c. Deformation. Sinistrally arate clasts (10JN

ear to be localent lithologic

nal features ine layers in mar

refolding the 30550N).

nal features inin siltstone-calcs

E, 5865759N).

nal features inasymmetric pre

N17-03; 554736E

l accommodativariation o

n Mathieson Crble, showing ttransposition f

n Mathieson Csilicate, east of

n Mathieson Cessure shadow

E, 5830695N).

ion features. Thr sedimentary

Channel tight to foliation

Channel

Green

Channel ws on

here is y top


indicators establish tthrust repe

Deformoccurred indicated bof trains accompaniplagioclaseMetasiltstopolygonal orthoclase,Strong bilayering dethe F1 isoIndividual laminationlayering. Tfabric, witcleavage.

One mfrom Hiekprobably muscovite compositiomost comChannel Fthe siltstoconglomersubordinatdepending and epidotand/or phcalcsilicateprobable sc

EARLY – META

The odistinguishdeformatiocompositiothrough dcommon clinopyroxpleochroic are overgrhornblendematrix. Ulof clinoctrondhjemimetamorph

Widesand mylonemplaced sinistral dtrondhjemi

in the Matthe degree oftition that it mmation in theduring amphiby assemblage

of well-orienied by polye; quartz, dones comprise

quartz, plag, alternating wiiotite alignmeefines the transoclines formed

biotite platelens are also wellTight to isoclth some regro

metatuff from Bkish Narrows cof contact mare very rare

on of the silicimmon calcsilicFormation, occuones and marates and sane amphibole–ton rock com

te. A few of thlogopite ande sample fromcapolite.

CRETACEAMORPHIS

ldest of the thhed by its don. Phases in on, ranging fiorites to tronphase. In the

xene is pseudhornblende, a

rown by metame along with seltramafites are chlore, biotiteites and the Nhic biotite fabrspread developnitic fabrics in

synkinematicdeformation. ite phases are s

thieson Channf large-scale fay have experi

e Mathieson Cibolite-facies mes in the metabnted subidiobygonal aggrediopside, biot

laminations agioclase, and ith biotite or dent parallel sposition fabricd during the trets within the l aligned parallinal F2 folds owth of bioti

Bullock Channcontain post-ki

metamorphic oe, attesting toiclastic protolicate mineral urring as discr

arbles, and fodstones. It isremolite/actino

mposition, as wthe purer marb/or forsterite.

m eastern Pool

EOUS PLUTM AND DE

hree Cretaceoudegree of mthe suite are

from gabbros ndhjemites. De gabbro anddomorphed band original igmorphic aggreecondary calcic

converted to e, and amphNeekas granodics.

pment of subson this suite indcally during Contacts betwsteep, northwe

nel Formationfolding or intienced. Channel Formmetamorphismbasalts that co

blastic hornbleegates of ctite and titaand thin layer

in some cdiopside-rich la

to compositc. It is assumedransposition equartzofeldsp

lel to compositrefold the bi

te in axial p

el and a greywinematic cordiorigin. Garnet o the alumina-ith. Diopside iin the Mathirete laminationorming matrixs accompaniedolite to hornblewell as clinozobles contain g. One sandstley Island con

TONIC SUIEFORMATIus plutonic suitmetamorphism

highly variaband ultrama

iorite is the d diorite, oriby Mg-rich, pgneous hornbleegates, with dac plagioclase inspectacular sp

hibole. Both diorite show st

lidus, metamordicates that it ongoing reg

ween diorite st-striking zon

n to ernal

ation m, as onsist ende, calcic anite. rs of cases ayers. ional

d that vent.

pathic ional iotite lanar

wacke erite,

and -poor s the ieson ns in x to d by ende, oisite arnet tone-

ntains

ITE ON tes is

and le in

afites, most ginal pale-

endes arker n the prays

the trong

rphic was

ional and

nes of

sheaJackfabrmylevid(Figsuchporp15).linea

THMIDSH

as aGrenIslanstronPrinsoutareaPrinCoufaultsuitegradthe acconearitselCretprotneobgradof isuggduri

Figumeta(10J

aring. At the cakson Passage, ric in metadiorionitic fabric

dence that the gure 11a). Outsh as asymmetrphyroclasts are Both steeply aations are seen

E GRENVID-CRETAC

HEARING Regionally, th

a prominent tonville Channelnd, where a smng northweste

ncess Royal Itheast across La. There, it encess Royal Isugar Bay and colt cuts across e. On Swindle dual transition west, to a zon

ompanied laterer to the Grelf, the mafictaceous sutomylonitizatioblasts, and sudual transitionsigneous fabricsgest that thesing motion on t

ure 15. Detail, agabbro, southeN25-01; 552934

a. 123 Ma U-Pa leucotrondh

ite, but itself dparallel to itcooling felsic

side of discreteric pressure shae developed inand gently plunn.

ILLE CHANCEOUS SIN

he Grenville Copographic linel, south througmall offset or erly topographIsland (inset, Laredo Inlet aemerges on tsland in severaontinues to theplutons of theIsland and norcan be traced f

ne in which coe stages of menville Channc enclave-richuite has on, with quarubsolidus biots from unfoliats, to narrow mse plutons mathe fault.

sinistral sensern Mathieson C

4E, 5814518N).

Pb sample localhjemite dike

displays a strons walls, whic

c body localize fault zones, sadows around

n the metadiornging streaky t

NNEL FAUNISTRAL

hannel fault caear feature fro

gh the channel ndeflection plachic linear thaFigure 1). It

and into the pthe southeasteal strands unde southeast (Fige ca. 105-94 Mrth towards Cofrom unfoliatedonsiderable fla

magmatic recrynel fault. Alonh tonalite of

been affertz ribboning,tite recrystallizted cores into bmylonites alonay have been

e of shear in hannel north of

lity north of cuts across

ngly lineated ch provides ed shearing

shear fabrics plagioclase

rites (Figure to mylonitic

LT AND

an be traced om northern north of Gil ces it in the at transects t continues

present map ern end of er and near gure 5). The Ma tonalite

ougar Bay, a d tonalite to attening has ystallization ng the fault f the mid-ected by , trains of zation. The broad zones ng the fault n emplaced

ca. 123 Ma Neekas Inlet


JatoCpranplshplasroofinCththofacandemdunoof(Freinfatethsin

costinthdiFaw

Fifasin53

From Cougackson and Osonalite and grahannel fault. Trotomylonitizend quartz ribblutons, they whown by stablelagioclase ovesymmetric sheaotated porphyrf mylonitic lndicating nearhannel fault (F

his is the presenhe complex zonf Swindle Islccommodationnd north alonextral displacem

most prominentue north alonorthward projeffset the sinisFigure 5). Althepresent a sentimate associaault array and erminate againshey form nortngle system.

A presumeorresponds wittrain zones than southern Sarahis structure onisplaying porparther south al

where several pr

gure 16. Deformult zone: mylonnistral rotation 34542E, 5822824

gar Bay througscar passages, anodiorite occuThese late bodd, with pervas

boning (Figurewere deformede biotite, chlorierprinted by ar indicators suroclasts characlineations arerly pure tranFigure 13d). Once of more stne of intersectinand. These sstructures. O

ng Tolmie Chment offset thet of these is ag the east sidection does n

stral fault thathough it is poseparate, later ation with the

the possibilitst sinistral faultherly-oriented

ed splay of thth the northweat are developeah Island. At tn the island, thphyroclastic telong this structrobable hypaby

mation adjacent titized granodiori

of plagioclas4N).

gh Klemtu Padikes of plagi

upy strands of dies are themsesive porphyrocle 16). Unlike d in greenschite and epidoteheavy saussur

uch as pressurecterize these roe generally vnslation on tOne significantteeply plungingng faults at the

steep faults mn northern Swhannel, faultse main sinistra

a mylonite zonde of Tolmie not appear to t cuts across ssible that the

transcurrent e main Grenvty that some lts both favour

conjugate str

he Grenville Cst alignment o

ed within variothe northern exhe tonalite is prxture with felture is an impoyssal intrusive

to and within Grte, Klemtu sheae porphyroclas

ss and across ioclase-phyric the Grenville

elves strongly lastic textures the 123 Ma

hist facies, as e, and igneous rite. Sinistral

e shadows and ocks. Plunges very shallow, the Grenville t exception to g lineations in e northern end may represent windle Island, with minor

al strands. The ne that strikes

Channel. Its significantly

Sarah Island dextral faults event, their

ville Channel of them may r the idea that ructures in a

Channel fault of spaced high ous rock units xposed end of rotomylonitic, ldspar augen.ortant locality phases,

renville Channel ar zone. Shows st (10JN21-05;

includingporphyry,a high strweakly sintrusive Pecha (20Sarah IslaChannel metabasamarble laeast off thGrenvillePoint souparallelinmetabasazircon damotion on

STRUCThe

regional mdeformatinew, impnorthwestdocumenta late stagChardon, but no mIn this aredeeply bushared s(Figure 1Formationgrade dysinistral slineationsmap area,features th

By 1exhumed assemblagactivity dreverse (cthe mid-regime oSinistral zone, the

The plutonismabove forLate CretMountain10 millioplate vectThe sinistevent. Thorthogonathen dex

g potassium fe, are in paralle

rain zone that instrained tonaliphase is part o010-11). The sand is marked Formation, e

alts are at ampayers that are crhe main body. e Channel faultuth of Klemtu.ng a steep pealts. Sampling ate is intenden Grenville Ch

CTURAL Hintense dev

metamorphic ion in the ca.portant constrtern British ted that ca. 11ge in the sinist2003; Bulter

maximum age foea, Early Cretauried Alexandsinistral-revers13a). In commn, this intrusivynamothermal sense of shears. Earlier faults, but they are n

that we observe05 Ma, the noto the point of

ges. At this tidelivered largecompressionalCretaceous pl

of nearly pureshear progressGrenville Cha

tectonic cym, and sinistralr the western Ctaceous contrans to the east (Ron years. Thetors and largertral componenhe Late Cretacal shortening t

xtral-normal b

eldspar-rich apel and vertical n turn is crosscite. Uranium-lof a geochronosouthern extenby metabasite

exposed at Pphibolite graderosscut by tonaTonalite dikest strand strikin Here tonalite enetrative foliof these tonal

ed to partly channel fault.

ISTORY velopment of fabrics and sin123 Ma plutoaint on timinColumbia, se0-90 Ma plutotral shearing (Cet al., 2006; N

for this event haceous plutonsder terrane supe deformation

mon with the ve suite exper

metamorphisr associated ws and shear zonot required toe. rth coastal regf quenching thime a renewe

e bodies of ton) strain compolutonic suite we strike-slip msively focussedannel fault. ycle of buril-orthogonal c

Coast Mountainactional event Rusmore et al.,se two eventsr scale interpla

nt is unique to tceous event wthat evolved toby Eocene tim

plite and plagijuxtaposition

cut by comparalead dating ofological study nsion of the fae from the MathPering Point. e and alternatealite dikes projs also occupy ang northerly at dikes form boiation developite dikes for a

constrain timin

f amphibolite-nistral-oblique

onic suite provng of this eveeveral studies ons were emplaChardon et al.,Nelson et al., 2has been establs were emplacepracrustal rockn and exhumMathieson Ch

rienced amphibsm and distr

with steeply pluones may exist o explain the te

gion was upliftehe amphibolite-d pulse of intnalitic magma onent dwindlewas emplaced

motion (Figured into a single

ial, metamorpontraction desns predates thein the central , 2005) by mors reflect changate relative mothe Early Creta

was characterizo dextral-reverme. Also, the

ioclase within atively f each by M.

ault on hieson These

e with jecting

another t Keen oudins ped in a U-Pb ng for

-facies e shear vides a ent. In

have aced at 1999;

2010a), lished. ed into ks and mation hannel bolite-ributed unging in the

ectonic

ed and -facies trusive

. The d, and

d in a e 13a). e fault

phism, scribed e main Coast

re than ges in otions. aceous zed by se and

e Late


Cretaceouscrustal thictectonic reand both coupling band subduccase the deof the arc and invoemplacemeyear interv

AU MINCENTRA

The Son northerpresent mamine with its genesimetallogenfollowing along withvon Einsiethe MINFI

Surf Iquartz veinstructures, Channel faand Pugleyfault, whicorder faultfrom the mmines are lof the fauseveral sheThe hostrotonalite boMortensenQuartz vetypically hchlorite, scoarse quachalcopyrimolybdeni

Produintermitten1946. In tothe two mgrams perMINFILE)Resources downdip ex

The hsimilaritiesunmetamoamphibolitCretaceous

s event involckening and segimes involv

probably probetween the octing plates in eformation was(which migrateolved interplent of volumin

val of Early and

ERAL POTAL COASTurf Inlet mine rn Princess Roap area (inset, considerable ris represents ny of the centaccount is bash detailed geoedel (2001), anILE database. Inlet is a classn system that

along a secoault. There are y mines, both ch underlies Bet that controls tmain fault, culocated within

ult trajectory. Hear zones withock, a deformdy, has yielded

n and R. Frieeins occupy lhave alterationsericite, carbonartz contains te, with lesser ite (Figure 18).ction from

ntly by severaotal, nearly 1 m

mines averagingr tonne silver ). The proper

Ltd. Economxtensions of th

host pluton to s to mid-Cretrphosed, so te-facies eves plutons. Igneo

lved much gubsequent upled southwest-obably formed

overriding Northe Pacific oc

s concentrateded east betweelay betweennous magmas d Late Cretaceo

TENTIAL OT

(MINFILE 10oyal Island 45Figure 1). A p

remaining undean importan

tral British Cosed on a brief ological mappind descriptions

sic mesothermaformed in a se

ond-order splaytwo separate located within

ear Lake (Figuthe orebodies d

urving to the nthe north strikHere, the faul

h average dips med and protod a U-Pb zircondman in Gehate-stage, brit

n selvages comnate and epidvery coarse cchalcocite, bo the mine w

al companies bmillion tonnes wg 13.0 grams p

and 0.31 perrty is currentlmic potential he mined ore zothe Surf Inlet aceous pluton

emplacementnt that affous foliation m

greater degreelift. However, -vergent structd in responsrth American cean realm. In d in the axial reen 110 and 80 n tectonics

over a 40 mious time.

OF THE

03H 027) is loc5 km north ofpast producing erground resount aspect of olumbia coast.field visit in 2ing available s of the proper

al (orogenic) geries of third oy of the Grenorebodies, the

n 2 km of the ure 17). The sediverges northnortheast. The

king, middle selt zone consisof 45° to the womylonitic dion age of 106 M

hrels et al., 20ttle structures mposed of shedote. In the vclots of pyriteornite, covellite

was carried between 1917was produced per tonne goldr cent copper y held by Ruis recognized

ones. mine bears st

ns elsewhere. t post-dated

fected the omanifested by

es of both

tures, e to plate each

egion Ma), and

illion

cated f the gold

urces, f the

The 2010, from

rty in

gold-order nville

Surf main

econd ward

e two ction ts of west. orite-

Ma (J. 009).

and eared veins, and e and

out and from

d, 6.8 (BC

upert d for

trong It is

the older

FiguPink

Figu(507

aligncuttiplutoccucontsettion nal., orderepequar

ure 17. Detailed k = Cretaceous g

ure 18. Surf In7636E, 5881725N

nment of coaring plates is iton west of Kurred at low ttents. This deing to the Surfnorthwestern P2010a). Surf Per shears relateeated associatirtz veins with

map of Surf Inleranitoids; purple

nlet ore, from N).

se biotites withidentical to teKlemtu. The temperatures,

eposit is similaf Point (Edye

Porcher Island (Point is also loed to the Grenion of signifi

h structures re

et mine and its s= metamorphic

creekbed in ta

h subordinate,extures in the

protomylonitiwith high H2Oar in type andPass) gold-qu

(inset, Figure 1ocated on seconnville Channe

ficant mesotheelated to the l

surroundings. rocks.

allings dump

late, cross-ca 105 Ma c overprint O and CO2 d structural

uartz system 1; Nelson et nd and third l fault. The

ermal gold-later, brittle


phases of sinistral motion on the Grenville Channel and related faults demonstrates an important mid-Cretaceous metallotect in coastal British Columbia.

DISCUSSION

The southern Alexander Terrane in coastal British Columbia

In 2009 and 2010, detailed mapping was completed for the Alexander terrane near both ends of its extent south of Alaska. The two areas are underlain by stratigraphically different units. At the northern end of the Alexander terrane, Ordovician volcanogenic and related plutonic rocks are widespread on Porcher Island and northern Pitt Island. By contrast, the southern end of the Alexander terrane consists of a siliclastic, carbonate and bimodal volcanic unit named herein the Mathieson Channel Formation and speculated to be of Devonian age. Broadly similar clastic rocks are limited to a few locales in the north. Regionally, the oldest exposed Alexander terrane rocks progress in age from Precambrian-Cambrian Wales Group on southern Prince of Wales Island, through Ordovician rocks along the northern Inside Passage, to possible Devonian clastic rocks in the far south, in the Laredo Sound map area.

Volcanogenic base metal mineral potential in the Alexander terrane is primarily associated with Ordovician arc sequences, notably the Moira Sound unit of Karl et al. (2009), which hosts the Niblack deposit on southern Prince of Wales Island. Thus the Mathieson Channel Formation in the current map area is considered unprospective for syngenetic volcanogenic deposits. The bimodal volcanic rocks in it erupted into a shallow basin, probably under oxidizing conditions as indicated by the possible presence of redbeds.

The Mathieson Channel Formation and the quartzite-siltstone-volcanic conglomerate at Jorkins Point are significant indicators of the tectonic history of the Alexander terrane. They reflect patterns of tectonics and sedimentation in the aftermath of the earliest Devonian Klakas orogeny. The Mathieson Channel Formation, with its dominant fine-grained siliciclastics and carbonates, its narrow intervals of bimodal and potentially alkalic volcanics, and its sporadic local influxes of very coarse clastic debris, is inferred to represent a rift basin fill.

In the Alexander terrane of southeastern Alaska, the slightly older Silurian Heceta conglomerates have been speculated to be equivalent to late Caledonian rift basin sequences (Soja and Krutikov, 2008). A similar correlation will be possible for the Mathieson Channel Formation, if suspected Devonian ages are confirmed by detrital U-Pb geochronology of possible rift-related clastic rocks.

The Jorkins Point conglomerate may have been deposited in or near the Mathieson Channel basin. The presence of Mathieson-like siltstone clasts supports this idea; although its depositional age is not yet known. The

prominence of quartzite clasts in Jorkins Point conglomerate certainly demonstrates recycling of continentally derived debris, possibly due to arc-continent sliver amalgamation events such as the Early Devonian Klakas orogeny.

Early to mid-Cretaceous sinistral tectonics, the Grenville Channel fault, and mesothermal gold mineralization

In 2010 mapping, we have recognized the southern Grenville Channel fault near Klemtu as a locus of mid-Cretaceous sinistral shearing, and also documented a somewhat older Early Cretaceous event of distributed shear during amphibolite facies metamorphism. Magmatic ages constrain the timing of the distributed sinistral shear event between 123 and 105 Ma, the age of the synkinematic versus postkinematic plutons. These observations provide an important dimension to the mid-Cretaceous sinistral shear history of the northern Grenville Channel fault and its splays. Intense fabric development in the ca. 123 Ma plutonic suite indicates that this deformation is entirely Early Cretaceous, rather than a continuation of the mid-Jurassic accretion kinematics of Alexander and Yukon-Tanana terranes.

The Early to mid-Cretaceous sinistral-oblique shear system, as recorded in exposed rocks, evolved from deep-crustal, sinistral-reverse motion, to upper crustal, nearly pure transcurrent motion on the Grenville Channel fault, in which the latest synkinematic phases record greenschist facies metamorphism at high fluid pressures as shown by the presence of abundant chlorite, sericite and carbonate. Tectonically, regimes of partitioned transcurrent motion during exhumation from amphibolite to greenschist facies are recognized worldwide as highly favourable to the emplacement of mesothermal (orogenic) gold-quartz vein systems. Therefore it is no surprise that two significant ex-producing gold mines of this type lie proximal to the Grenville Channel fault, controlled by second and third order fault arrays. Another promising gold-quartz vein system, Yellow Giant (MINFILE 103G 021, 24, 25, 26, 30), is located on Banks Island. It is currently held by Imperial Metals, who hope to begin an exploration program there in 2011 (Jim Miller-Tait, personal communication, November 2010).

FUTURE RESEARCH DIRECTIONS AND MAPPING PLANS

The third and final field season of the North Coast Project is planned for 2011. It will have the following key goals:

1) Revisit and resample for geochronology parts of the areas previously mapped, in which present structural, lithologic and geochronometric data indicate unsolved problems. Examples of this include a) the Devonian pluton in Porcher Inlet, which cuts an older orthogneiss complex that so far has only yielded an apparent Jurassic U-Pb


Laserchron age displaying complex systematics (J.B. Mahoney, unpublished data, 2010); b) the Swede Point pluton on Porcher Island, which may be Mississippian or, alternatively, Mesozoic with inherited cores and c) Kumealon Inlet, where a structural contact between Alexander terrane and Yukon-Tanana terrane has been inferred (Nelson et al., 2010b), but detrital zircon data are conflicting, and where U-Pb dating of the metavolcanic rocks has yet to be done.

2) Collaborate with Joel Angen, who is studying the fault systems on Porcher Island and along northern Grenville Channel as an M.Sc. thesis at the University of Waterloo. His work is sponsored by the Geological Survey of Canada as a contribution to the Edges project.

3) Visit key mineral occurrences in the area, particularly the Yellow Giant mesothermal Au deposit and the Pitt volcanogenic prospect, in order to place them in regional context.

4) Complete field investigation of the Alexander terrane between Grenville Channel and northern Princess Royal Island, targeting a full update of this part of the BC Geological Map in 2012.

SUMMARY AND CONCLUSIONS In this second year of operation, the North Coast

project can list the following accomplishments: Completion of geological map coverage of an area of

30 by 50 km, covering the channels and islands between Return Channel and northern Princess Royal Island, in eastern Laredo Sound map area (103A). This map will be released as a British Columbia Geological Survey open file in early 2011 (Nelson et al., 2011).

Rock units of the Alexander terrane of southeastern Alaska can be traced into northwestern British Columbia (Porcher and Pitt islands), including those that are known to host Ordovician volcanogenic massive sulphide mineralization. Farther south, in the current map area, the Alexander terrane is represented by younger, probably Devonian, clastic-carbonate strata.

The Grenville Channel fault is a mid-Cretaceous sinistral fault of regional extent (>150 km). It and its splays form the tectonic framework for mesothermal gold-quartz vein systems including Surf Point on Porcher Island, Surf Inlet on Princess Royal Island, and probably Yellow Giant on Banks Island. This is an important new metallotect on the coast of British Columbia.

ACKNOWLEDGMENTS We once again acknowledge the key role of our

captain, Don Willson and his boat, the Phoebe, in providing a convenient logistical base of operations and safe coordination of our daily zodiac traverses. The senior author (Nelson) wishes to thank all the members of the

project team for their strong contributions to our task, deterred by neither metamorphism nor voluminous intrusion nor polyphase deformation from reconstructing original depositional environments and establishing the regional framework of faults. We were also fortunate to have two excellent, hard-working student assistants from the University of Wisconsin, Brennan Kadulski and Julia Potter. The North Coast Project is part of Edges (Multiple Metals – NW Canadian Cordillera (Yukon, British Columbia); financial and logistical support from the Geological Survey of Canada made it all possible. Moreover, the conduct of this project within the larger framework of Edges, with its focus on understanding the exotic terranes, their accretion and interaction with the continent margin, gives it more meaning than if it stood alone. Fil Ferri’s careful review improved the accuracy and clarity of the manuscript.

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