tracking yukon-tanana terrane vms host stratigraphy and...

Tracking Yukon-Tanana Terrane VMS Host Stratigraphy andIntrusion-Related Gold in the Southern Sylvester Allochthon

(Beale Lake Map Area, 104I/14N)

By JoAnne Nelson

KEYWORDS: Mississippian, Sylvester Allochthon, Dorseyassemblage, Yukon-Tanana Terrane, northern BritishColumbia, volcanogenic massive sulphides, gold veins.

INTRODUCTION

This report describes a new mapping project in theCassiar Mountains, located 70 kilometres northeast ofDease Lake in northern British Columbia. The area is attrac-tive for two reasons. First, it contains rocks that are likelycorrelative with those of the Yukon-Tanana Terrane, a tractwith known volcanogenic massive sulphide potential. Sec-ond, combined geological and regional geochemical datasuggest possible intrusion-related gold mineralizationwithin it.

The Beale Lake area, bordering the northeast side of themid-Cretaceous Cassiar Batholith, is largely underlain bythe Sylvester Allochthon, a stack of thrust sheets of oceanicto pericratonic arc affinity that overlies the para-autochthonous Cassiar Terrane (Figure 1).

The structurally highest sheet is a highly deformedpericratonic assemblage (Gabrielse, 1998). Like the Yu-kon-Tanana Terrane, it contains early Mississippian intru-sions. The YTT in the Finlayson Lake belt in southern Yu-kon hosts several significant volcanogenic massivesulphide deposits associated with early Mississippian vol-canic suites. Over the past 4 years, the Ancient Pacific Mar-gin Natmap project has traced pericratonic Yukon-TananaTerrane stratigraphy continuously southwards from theFinlayson belt into the Jennings River map area of northernB.C., where equivalent strata, including Mississippian fel-sic volcanic rocks and exhalative units, occur within the BigSalmon Complex and Dorsey assemblage (Mihalynuk et al.1998, 2000, Nelson 1999, 2000). It has been suggested thatthe pericratonic assemblage in the Sylvester Allochthon is adirect southeastward extension of the Dorsey assemblage,interrupted by the Cassiar batholith (Harms, 2000; Nelson,2001). The present project is in hot pursuit of these crucialearly Mississippian volcanogenic hosts.

The second rationale for this study sprang from identi-fication of strong gold and related anomalies (Bi, As, Hg) inthe Cry Lake Regional Geochemical Survey (RGS) release(Jackaman, 1996), a suite of elements commonly associatedwith intrusion-related gold mineralization. Sets ofgold-bearing veins are known in the area, notably the Niziand Beale Lake properties; but RGS anomalies occur out-

side areas of documented mineralization as well. The Niziveins in the northeastern corner of the Beale Lake map areahave been interpreted as Eocene in age and epithermal in or-igin (Plint et al., 1997), whereas the Beale Lake veins showat least in part a bismuth-tungsten association that may placethem at a deeper level in a intrusion-related system (Durfeldand Fleming , 2001) . The reg iona l ly ex tens ivemid-Cretaceous Cassiar batholith underlies most of south-ern Beale Lake map area, and the Eocene Major Hart plutonis located 20 kilometres to the southeast (Figure 2).

Field mapping in 2001 completed the northern half ofthe Beale Lake map sheet, 104I/14. The adjacent map area,104I/15, will be covered in future work. Results of explora-tion significance are: recognition of bipartite Dorsey as-semblage in the uppermost, pericratonic unit of theSylvester Allochthon, with felsic volcanic rocks in its upperpart; and discovery of several new mineral showings, in-cluding veins that enlarge the potential area of interest forintrusion-related gold significantly beyond that previouslyreported.

REGIONAL GEOLOGY

The northern Cry Lake map area is partly underlain bythe Sylvester Allochthon, which outcrops between theCassiar Batholith to the southwest, and subadjacent Paleo-zoic continental shelf strata of the Cassiar Terrane to thenortheast and south (Figure 2, from Gabrielse, 1998). Previ-ous mapping in the McDame map area to the north showedthat the allochthon comprises three distinctive, stacked tec-tonic elements (Nelson, 1993). Structurally lowest are latePaleozoic oceanic rocks of the Slide Mountain Terrane,overlain (depositionally?) by Middle to Late Triassic sedi-mentary strata. Above a thrust fault lie late Paleozoic islandarc rocks assigned to the Harper Ranch subterrane ofQuesnellia. Early Mississippian and older pericratonic andarc rocks occur at the structurally highest level. Thispericratonic unit was first recognized by Gabrielse andHarms (1989). Harms (1990) and Harms et al. (1993) re-ferred to it as the Rapid River tectonite. In northern CryLake area, Gabrielse (1998) distinguished the pericratonicunit, which he called the siliceous tectonite unit, from therest of the allochthon. He also defined two largemid-Permian plutons. His work has provided an excellentframework for further subdivision.

Geological Fieldwork 2001, Paper 2002-1 41

ldegroot

2001

42 British Columbia Geological Survey

Wolf

Ala

ska

Yuko

nA

lask

aYu

kon

YukonYukon

B.C.B.C.

YukonYukon

B.C.B.C.

TintinaFault

TintinaFault

Position of B.C.-Yukon border

after restoration of 450 km

dextral motion on Tintina Fault

WhitehorseWhitehorse

Dease Lake

SylvesterAllochthonSylvesterAllochthon

St. CyrklippeSt. Cyrklippe

Stewart LakeklippeStewart Lakeklippe

Campbell Rangebelt

Fyre Lake

Wolverine

Wolf

Kudz Ze Kayah

Finlayson Lakebelt

Dorsey AssemblageDorsey Assemblage

DorseyAssemblageDorseyAssemblage

CassiarBatholith

DawsonDawson

60o

14

1o

TeslinZone

TeslinZone

Dorsey assemblage, units in Campbell Range and Teslintectonic zone, Stewart Lake klippe

Sheared and high-P-T metamorphosed units; EarlyMississippian to Permian cooling ages: St. Cyr/Quiet Lake klippen;

Neoproterozoic(?) to Paleozoic pericratonic terranesYukon-Tanana in part, Teslin Zone, Big Salmon

Slide Mountain terrane: Late Paleozoic oceanicterrane with faint continental affinity(minor siliciclastic sediments)

Late Paleozoic arc and arc-related sequences (limestones,marine clastics) = Harper Ranch subterrane,Campbell Range belt, Klinkit, Swift River

Triassic-Early Jurassic granitoids

ALLOCHTHONOUS PERICRATONIC TERRANES

AUTOCHTHONOUS AND PARA-AUTOCHTHONOUS(NORTH AMERICAN CONTINENTAL MARGIN)

Mainly shelf facies

Selwyn Basin, Kechika Trough (basinal facies)

VMS deposit

Cretaceous granites

Quesnellia, Stikinia

Cache Creek Terrane (exotica)

BEALE LAKEMAP AREA

100 km

Figure 1. Regional geological setting of the Yukon-Tanana and surrounding terranes, Yukon and northern British Columbia.

LOCAL GEOLOGY

The Sylvester Allochthon in northern Beale Lake maparea comprises a set of stacked thrust slices folded into anupright syncline-anticline pair (Figure 3, 4).

Its general structure mirrors that seen farther north be-tween the Dease River and the Yukon border. The structur-ally lowest slivers, truncated to the southwest by the CassiarBatholith, are ultramafite, gabbro and basalt; they probablyform a southern strike extension of Slide Mountain Terraneexposures along the Stewart-Cassiar Highway near Cassiar.They also correspond to Package I of Harms (1986). Abovethem, across a highly sheared contact, lies the Four Mile as-semblage, a set of thrust slices of more diverse character, in-cluding widespread felsic to intermediate volcanic rocks ofprobable arc affinity. The volcanic rocks are intruded by themid-Permian Meek and Nizi plutons. These rocks show asimilar tectonic environment to Harper Ranch subterranepanels exposed farther north, for instance the Huntergroupvolcanics and an unnamed Permian pluton in the headwatersof Big Creek near the Yukon border (Nelson and Bradford,1993).

The Beale Mountain Thrust, exposed for many kilo-metres on the western face of Beale Mountain above theFour Mile River valley, spectacularly marks the base of thedeformed and metamorphosed pericratonic allochthon (Fig-ure 6a). It juxtaposes amphibolite and metasedimentaryrocks, which were ductilely deformed in early Mississip-pian time, above a virtually undeformed Pennsylva-nian-Permian volcanic/plutonic footwall. Above the thrust,the deformed rocks can be divided into two units, which cor-respond to the lower and upper Dorsey assemblage in thesouthern Yukon and northern British Columbia west of theCassiar Batholith (Roots and Heaman, 2001; Nelson, 1999,2000). The lower unit consists of strongly deformed, garnetgrade basinal sedimentary rocks - metachert, meta-argillite,pelite and quartzite - with large lenses of amphibolite andmetagabbro. The well-foliated but less tectonized upper unitcontains phyllite, quartzite, metachert and most signifi-cantly, quartz-feldspar porphyry rhyolite and dacite flowsand tuffs.

The youngest rocks in the area are the Zinc Lake volca-nic/intrusive suite, which outcrops near the informallynamed Zinc Lake on the Nizi property in the northeasterncorner of the map area. Fine grained, undeformed andesite,


Beale Lk.

Cry Lake

Majo

r Hart Rive

r

Rap

id

River

128 15o '

58 45o ' 58 45o '

129 00o ' 128 30o '129 15o '

59 00o '59 00o '

104I/14 104I/15 104I/16

Sail (Westmin, 1996)Qss; massive sulfide

Nizi: Eocene epithermalAu-Ag-Pb-Zn veins

Beale LakeAu-Ag veins

Limit of detailedmapping 2001

Intrusive units

EoceneMajor Hart pluton

Eocene? Nizi volcanics

mid-Cretaceous graniteCassiar Batholith

Permian intrusions

Devonian-Mississippianintrusions

Ultramafic-metabasitecomplexes

Slide Mountain Terrane

Harper Ranch subterrane(Package II of Harms, 1986)

Ancestral North America(Cassiar Terrane)

Dorsey assemblage

5 km

Figure 2. Geological setting of northern Cry Lake map area, prior to this project. Geology from Gabrielse (1998), Harms (1986), and Plintet al. (1997).


POST-ACCRETIONARYUNITS

YUKON-TANANA TERRANE

HARPER RANCH SUBTERRANE

Four Mile assemblage

SLIDE MOUNTAIN TERRANE

EOCENEMID-CRETACEOUS

Zinc Lakeintrusions

Zinc Lakevolcanics

Cassiar Batholith gossans

Swift River succession

Foliated diorite sill; basalt

Upper Dorsey assemblage

Layered gabbro sill

Lower Dorsey assemblage

Ram Creek assemblage

Mid-PermianMeek pluton (granite)

Mid-PermianNizi pluton (gabbro-diorite)

Pennsylvanian-Permianmainly felsic volcanics

Pennsylvanian-Permiangreen chert, tuff, limestone

Siltstone, argillite,felsic tuff

Basalt, andesite flows

Foliated intermediatevolcaniclastics

showings

Deformed tonalite

Black phyllite, siltstone

Utik Mtn.intrusive complex

Basalt, gabbro, ultramafiteBEALE MTN. THRUST

THRUST

THRUST

58 48’O

58 54’O

59 00’O

12

90

0’

O

12

91

2’

O

12

93

0’

O

12

92

4’

O

TetipistikwanMtn.

UtikMtn.

ColdrockRidge

BEALEMTN.

MEEKPLUTON

BealeLake

BealeLk.

MeekLake Nizi

Gunsight

No Fishvein

Yursovein

Perm

Corydalis

BealeMtn.Thrust

KeithLuckyLuke

Toboggan

Least one

Griz

Four

Mile

Riv

er

NiziCr.


A

A’

B’

B

BealeLk.

MeekLake Nizi

Gunsight

No Fishvein

Yursovein

Perm

Corydalis

BealeMtn.Thrust

KeithLuckyLuke

BealeLake

Toboggan

Least one

Griz

Four

Mile

Riv

er

NiziCr.


A

A’

B’

B

Kilometres

0 42

MEEKPLUTON

NIZIPLUTONNIZIPLUTONNIZIPLUTONNIZIPLUTON

F PPPP

BEALEMTN.

MeekCr.

ColdrockRidge

UtikMtn.

TetipistikwanMtn.

Figure 3. Geology of northern Beale Lake map area (104I/14), based on field mapping 2001. Some topographic names from Kaska eldersof Dease River Band (personal communication, 2001).

dacite and rhyolite flows and pyroclastic deposits are in-truded by rhyolite and quartz-feldspar megacrystic dikes.This suite hosts the Nizi vein system. Galena lead signaturesfrom the veins resemble other epigenetic, Creta-ceous-Tertiary deposits of the Cassiar Terrane; the nearestcorrelative of the felsic intrusions is the Eocene Major Hartpluton. These observations led Plint et al. (1997) to assignan Eocene age to the suite. They termed it the Nizi volcanicsequence. The name Zinc Lake suite is proposed for the vol-canic and associated intrusive rocks, to avoid confusionwith the nearby Permian Nizi pluton.

SYLVESTER ALLOCHTHON

Rocks of the Sylvester allochthon are described in as-cending structural order, i.e. Slide Mountain Terrane,Harper Ranch subterrane, and Yukon-Tanana Terrane(Dorsey assemblage). This order also corresponds to theirpre-accretionary paleogeographic positions vis-à-vis NorthAmerica, with a late Paleozoic Slide Mountain ocean themost inboard, a Yukon-Tanana pericratonic fragment mostallochthonous, and a late Paleozoic Harper Ranch arc lo-cated between.

Slide Mountain Terrane

Ultramafic rocks, gabbro, basalt and minor argilliteoutcrop in a narrow northwesterly-trending strip located im-mediately northeast of the Cassiar Batholith. They lie struc-turally at the lowest exposed level in the SylvesterAllochthon in the Beale Lake map area (Figure 3). Afew en-claves of texturally intact, serpentinized harzburgitetectonite hint at the ophiolitic affinity of this assemblage.Most rocks are highly sheared. The assemblage has beenmetamorphosed in greenschist facies with actinolite stablethroughout. Most of the ultramafic bodies have been

strongly altered to quartz-carbonate (-mariposite). No agecontrol is available locally; however Slide Mountain ba-salt-sedimentary and ultramafite- gabbro panels nearCassiar have been dated as Mississippian throughmid-Permian by conodonts and U-Pb methods (Nelson andBradford, 1993).

Four Mile Assemblage(Harper Ranch Subterrane)

This group of thrust slices, informally termed the FourMile assemblage, is bounded below by the Slide Mountainultramafite/gabbro/basalt panel, and above by the BealeMountain Thrust (Figure 3). The lowest slice, outcroppingnear Meek Lake in the northwestern part of the map area,consists of the Utik Mountain intrusive complex, a variablemafic complex with metasedimentary and metavolcanicscreens. The main slice extends from south of Beale Lake,though the Meek Creek and Four Mile River drainages, intoNizi Creek to the northeast. It is structurally repeated on asecondary fault below the Beale Mountain Thrust on southBeale Mountain. Within these two repeated and relatedslices, undeformed volcanic rocks of varied compositionsdirectly overlie a basement of polydeformed black phyllite,siltstone and argillite, which is cut by a tectonizedtonalite/granodiorite pluton in exposures south of BealeLake. The age of the undeformed volcanic rocks is con-strained by a single Pennsylvanian-Permian conodont col-lection from a locality 6 kilometres east of Meek Lake(Gabrielse, 1998; see Figure 3). The mid-Permian Meek andNizi plutons both show unequivocal intrusive contactsagainst volcanic country rocks in the main slice. The highestslice, directly below the Beale Mountain Thrust, consists ofhighly sheared, greenschist-grade metaplutonic and meta-volcanic rocks that range from quartz-feldspar porphyries togabbro. It is correlated with the Ram Creek assemblage, a


Kilometres

0 1 2

AA’

B

B’

F3

F3 F2

F2

Four MileRiver

BealeLake

Figure 4. Cross sections of the Sylvester allochthon in the Beale Lake map area. See Figure 3 for geological legend and location of lines.

strongly deformed, greenschist-grade assemblage withMississippian plutonic and volcanic protoliths, which di-rectly underlies the Dorsey assemblage west of the Cassiarbatholith (Harms and Stevens, 1996; Nelson 2000).

The Utik Mountain intrusive complex is restricted to asingle thrust slice, and does not resemble any other unit inthe assemblage. Its component intrusions vary considerablyin texture and composition, from clinopyroxene andhornblende gabbros cut by actinolite pegmatites, to finegrained hornblende-phyric basalt dikes with chilled mar-gins. It contains rafts of chert and thin-bedded tuff andargillite, and at least one enclave of quartz-sericite schist,deformed and metamorphosed before intrusion. The vari-ability of this complex, along with the prevalence ofphenocrystic hornblende, suggests arc rather than oceanicaffinity.

The black phyllite/siltstone/argillite unit forms thebases of both the main thrust sheet and of the structurally re-peated sliver on south Beale Mountain. It is dominantlyblack and very fine grained. Laminated and cross-laminatedsiltstone beds are seen in places. This unit is characteristi-cally foliated and crenulated, with several generations offolding evident. Post-kinematic andalusite, probably re-lated to the Cassiar Batholith, is common in exposures northof Beale Lake. Diopside porphyroblasts occur in more cal-careous lithologies south of Beale Lake. South of BealeLake, this unit exhibits interfingering intrusive contactswith a coarse grained tonalite/granodiorite pluton. Texturesin the pluton range from pristine, with only moderate re-placement of original hornblende by biotite, to laminatedmylonite unrecognizable as to protolith. This unit resemblespolydeformed carbonaceous Devonian-Mississippianmetasedimentary units, such as the autochthonous EarnGroup and the Nasina Series of the Yukon-Tanana Terrane.It is overlain on abrupt, unsheared contacts in twowell-exposed localities by undeformed volcanic and associ-ated sedimentary strata.

The undeformed sequence is subdividable into severalmap units. The most widespread unit, which extends fromsouth Beale Mountain to the northern border of the maparea, is dominated by felsic, nearly aphanitic rhyolite anddacite flows and related pyroclastic deposits. The felsic unitreaches thicknesses of more than 2 kilometres. In outcropthe flow rocks are pale green, aphanitic, and commonlyseamed with reticulating networks of fine chlorite veinlets.Lapilli tuffs, composed of matrix-supported angular felsicfragments, form pods and crude beds interspersed within theflows. Both Kspar-rich (rhyolitic) and Kspar-absent(dacitic) compositions have been confirmed by staining. Inthe northern part of the area, the felsic flow/tuff unit makesup most of the upper volcanic portion of the main thrustslice, above 100-300 metres of tuff, argillite and limestone.Excellent exposures of this steeply northeast-dipping se-quence are seen on Coldrock Ridge between Meek Creekand the Fourmile River. Gabrielse (1998) reports recoveryof Pennsylvanian-Permian conodonts from an outcrop thatlies between the Meek pluton and a northwestern finger ofthe Nizi pluton (Figure 3). The locality is a breccia that con-tains limestone and felsic volcanic clasts in a pale green

tuffaceous matrix (H. Gabrielse, pers. comm. 2001), under-lain by sea-green, ribbon-bedded chert. Southward acrossthe Four Mile River, the monotonous felsic unit interfingerswith hornblende-plagioclase phyric andesites, well-beddedwaterlain tuffs, and basalt, giving way along strike to otherunits. Farther to the southeast, two units predominate: aright side-up, northeast-facing volcanic-sedimentary unit,and an underlying basalt/basaltic andesite flow unit. Thevolcanic-sedimentary unit includes dark brown-greyargillite, siltstone, limestone, and tuff. The lapilli tuffs con-sist of white ash fragments in a brownish-grey,silt-mudstone matrix; they are in direct, depositional contactwith dark green, nearly aphanitic basalt to andesite flows ofthe underlying unit. This relationship exhibits thecompositional heterogeneity of the volcanic rocks in theFour Mile assemblage. They are thought to be of volcanicarc affinity, based on their basalt to rhyolite compositions,the predominance of felsic compositions, features likecoarse lapilli tuffs and large phenocrysts, interfingeringw i t h c l a s t i c s e d i m e n t s , a n d t h e i r d e f o r m e dmetasedimentary basement.

The Meek and Nizi plutons are two compositionallydistinct bodies arranged along regional trend in the north-eastern part of the map area (Figure 3). They are mostly sep-arated by a mildly foliated volcanic/volcaniclastic inlier,which consists of green porphyritic andesite to dacite flowsand lapilli tuff. A finger of the Nizi pluton, extending alongthe western margin of the Meek pluton, is intruded by gra-nitic and quartz-feldspar-phyric rhyolite dikes from it (Fig-ure 5a).

Apophyses of both plutons intrude the felsic volcanicunit (Figure 5b). The southern boundary of the Nizi plutonbelow the deformed pericratonic rocks is not an intrusivecontact, as reported in Gabrielse (1998), but a thrust faultwith shear zones developed in both hanging and footwall(Figure 5c). Therefore the revised map interpretation is thatboth plutons lie in the footwall of the Beale MountainThrust, with the geologically younger Meek pluton intrud-ing the Nizi pluton and both intruding rocks as young asPennsylvanian-Permian.

The Nizi pluton is a very heterogeneous, multiphasebody dominated by gabbro and diorite, with minor felsictonalite and clinopyroxenite. Like compositions, textures inits component phases are strongly variable; hornblende inparticular ranges from equant and poikilitic, replacingaugite, to acicular. By contrast, the Meek pluton is a homo-geneous granite. Typically coarse grained and equigranular,it consists of convex quartz intergrown with stubbyplagioclase and orthoclase and lesser biotite. Neither plutonis strongly deformed, although zones of weak cataclasis af-fect them. Compositional layering and weak foliation areobserved in the Nizi pluton, and its upper part adjacent to theBeale Mountain Thrust is highly sheared. The Meek pluton

has been dated at 270±4 Ma by U-Pb methods on zircon,from a locality 1.5 kilometres west of the Four Mile Riverand .5 kilometres north of the Beale Lake map area. Thesame sample site yielded a 262±2 Ma U/Pb age on titanite,and a 266±4 Ma K/Ar age on biotite (Gabrielse et al., 1993).A K/Ar hornblende analysis from the Nizi pluton gave an


age of 262±8 Ma (Hunt and Roddick, 1988). Therefore, al-though the Meek granite is geologically younger than theheterogeneous and more mafic Nizi pluton, in terms ofavailable isotopic ages they are indistinguishable.

Dorsey Assemblage

The Dorsey assemblage is a layered sequence of meta-morphic rocks intruded by deformed early Mississippianplutons, which lies structurally above the undeformed andweakly metamorphosed late Paleozoic rocks of the FourMile assemblage. It occupies the core of a syncline thattrends from Beale Mountain southeastwards across BealeLake, and on the northeastern flank of the anticline that ex-poses the footwall Nizi pluton (Figure 3). It comprises twounits, the upper and lower Dorsey assemblages, which are incontact across a layering-parallel, transitional zone. In spiteof considerable penetrative deformation, and shearing par-ticularly in the lower unit, these two units constitute an orig-inal and still readily interpretable stratigraphy. The term“tectonite” should be applied to these rocks with some cau-

tion, as they constitute neither broken formation nor tec-tonic melange.

Previous work suggested that the Permian Nizi plutonwas emplaced into the metamorphic unit, constituting acommon element with coeval plutons in the underlying pan-els (Gabrielse et al., 1993). This study has documented amajor fault, the Beale Mountain Thrust, between them. Theexistence of a Permian intrusive suite within thepericratonic assemblage, similar to the Ram Stock in thelower Dorsey Terrane near Swift River, Yukon (Stevens andHarms, 1995) and small pegmatites farther south in BritishColumbia (Nelson, 1999), has thus lost a major support.There is now only one known Permian intrusion within it, asmall body between the Dease and Four Mile rivers insouthern McDame map area (sample 89-SY-72A; Gabrielseet al., 1993). However, a large, heterogeneous, unfoliated,diorite to tonalite body cuts the lower Dorsey assemblagenortheast of Nizi Creek; part of it is exposed in the northeast-ern corner of Figure 3. In composition and texture it stronglyresembles the nearby Nizi pluton, and could be its displacedroof. It has been sampled for U/Pb dating. In addition,sparse, greenish-black amphibolite rafts in the Nizi pluton


A.B.

C.

Lower Dorseyassemblage

Ram Creek assemblage

Nizi pluton

Figure 5. Key field relationships of the mid-Permian plutons. A) Dike of granite, texturally similar to, and probably an offshoot of MeekPluton, cutting gabbro in an apophyse of the Nizi pluton. B) Nizi pluton diorite cutting undeformed felsic volcanic country rock of FourMile assemblage. C) Upper thrust contact of the Nizi pluton below the Ram Creek and lower Dorsey assemblages.

indicate that it penetrated a basement not unlike the amphi-bolite-bearing lower Dorsey assemblage.

Lower Dorsey Assemblage

The lower Dorsey assemblage is an interlayered se-quence of siliceous metasedimentary strata, amphiboliteand metagabbro. The metasedimentary rocks, now finegrained quartzites, quartz-biotite-muscovite(-garnet-plagioclase) schists, marble and calc-silicates, probably hadmostly chert, argillite and impure limestone protoliths. Truepelites are rare; most micaceous rocks are dirty quartzites.Coarse-grained, pure, thickly layered orthoquartzites arealso rare; the best example occurs near the exposed top ofthe unit on the west-facing spur north of Beale Lake. The in-terpreted metacherts form grey, ribbon-layered sequences.They tend to be highly pyritic. Some contain abundant gar-nets which may indicate manganese-rich compositions, andminor quartz-sericite schist suggests alteration.

Kyanite occurs in two samples from the lower Dorseyassemblage in the Sylvester Allochthon. In both cases it isrelict, partly resorbed, and overgrown by muscovite. In oneof these, from the west side of Beale Mountain, laterfibrolitic sillimanite grows in the predominant fabric. Thisparagenetic sequence of kyanite to sillimanite mirrors alu-minosilicate growth recorded in the lower Dorsey assem-blage west of the Cassiar Batholith (Nelson, 2000). Harmset al. (1993) report amphibolite-grade assemblages of gar-net-muscovite-zoisite and biotite-garnet-staurolite frompelites elsewhere in the deformed pericratonic assemblage.

Amphibolite occurs on all scales, from fine layers alter-nating with metachert and siliceous pelite to moun-tain-scale, lensoid bodies. Mineralogy varies from simplehornblende-plagioclase-quartz-titanite to garnet- and gar-net-diopside-calcite-bearing varieties. The latter have a cal-careous component in excess of that for normal basalticrocks. It could be due to admixture with carbonate sedi-ments, or seafloor alteration. In general these rocks showthoroughly metamorphic fabrics. Very fine, continuous ti-tanite laminae could possibly reflect tuffaceous parentage.One example of plagioclase-quartz-garnet-fil ledamygdules(?) was noted.

Metamorphic fabrics in the amphibolites range fromextremely foliated to nearly isotropic. The most foliated va-rieties consist of ribbons of very well aligned hornblendeprisms, alternating with more felsic quartz-plagioclase-bearing laminae. Fine titanite trains parallel the layering.More equant and randomly oriented hornblende overgrowsthese textures, suggesting that peak metamorphism suc-ceeded peak deformation. It is possible that the extremelyfoliated amphibolites inherited their fabrics from earlierphyllonites developed at lower metamorphic grade (J. Ryanpers. comm. 2001).

The metagabbros form a distinct, geologically youngersuite than the amphibolites. Although foliated and sheared,they show clear relict plutonic textures, and never containgarnet. They may be cogenetic with a large layered gabbrobody that intrudes the lower Dorsey assemblage south ofBeale Lake. This body has strongly foliated and shearedmargins. Apophyses of it show cross-cutting relationships

with surrounding schists and amphibolites. Its unfoliatedcore is cumulate layered, involving sets of coarse andfine-grained gabbro, gabbro/leucogabbro, and gab-bro/hornblendite rhythmites.

The lower Dorsey assemblage represents a pericratonicbasin in which both intrusive and extrusive mafic materialaccumulated, possibly an intracontinental rift. Petrochemi-cal analysis of amphibolites from west of the CassiarBatholith shows a progression from N-MORB to E-MORBto within-plate compositions (Nelson, 2001). Themetagabbros may belong to the syntectonic Mississippianintrusive suite identified by Gabrielse et al. (1998).

Upper Dorsey Assemblage

The upper unit of the Dorsey assemblage contrastsstrongly with the lower unit. On both sides of Beale Lake,amphibol i tes abrupt ly disappear upsect ion, andwell-foliated quartz-feldspar phyric metavolcanic rocks be-come important. They are accompanied by dark grey, grey,tan and green phyllite, quartzite with remnant quartz grains,and metachert. The identification of abundant felsic meta-volcanic and epiclastic material in the upper part of thepericratonic unit is an important step in regional geologicaland metallogenetic correlations, since such rocks character-ize both the metal-rich Finlayson Lake belt and, more lo-cally, the upper Dorsey assemblage near Swift River (Rootsand Heaman, 2001). U-Pb analyses are pending.

The felsic rocks include metamorphosed flows inwhich sparse quartz and feldspar porphyroclasts occur infinely (flow?-)laminated, dense, very fine grainedquartz-plagioclase-Kspar matrix. Tuffaceous sequencesshow interbedding of phyllite, quartz- and feldspar-eyephyllite, and coarse volcaniclastics with abundant largequartz, plagioclase and Kspar augen in quartzo- feldspathicmatrix. These are not mylonites; the textural variety is due tooriginal interbedding of volcanic and clastic material, not tostrain recrystallization. No papery, rusty quartz-sericiteschists indicative of early hydrothermal alteration were seenin this area, although minor quartz-muscovite schists arepresent.

South of Beale Lake, large parts of the upper Dorsey as-semblage are dominated by green and grey phyllite andquartzite. Sequences like this, which outcrop near ObliqueCreek in southern Jennings River map area, were assignedto the Swift River succession (Nelson 1999). However, inthe Beale Lake area the phyllite and quartzite clearlyinterfinger with quartz-feldspar phyric metatuffs. Perhapsthe Oblique Creek metasedimentary sequences are a varianton the upper Dorsey assemblage, rather than the structurallyoverlying Swift River succession.

Mafic sills - well foliated metagabbros and in a fewcases hornblendites - intrude the upper Dorsey assemblage.These bodies may belong to the same generation as the gab-bros in the lower Dorsey assemblage.

Although well foliated, the upper Dorsey assemblageexhibits less obvious penetrative shearing than the lowerDorsey. Its metamorphic grade is apparently lower.Phyllites predominate over schists, original quartzite tex-


tures are preserved, and pale green, actinolitic hornblende isdeveloped instead of deep green hornblende in them e t a g a b b r o s . M i n e r a l o g i c a l l y h o w e v e r , t h emetasedimentary rocks contain similar assemblages ofmuscovite, biotite and less common garnet.

Swift River Succession

A small area of dark-coloured chert, argillite, phylliteand white quartzite occurs in the synclinal core, structurallyabove the felsic metatuffs north of Beale Lake. These rocksare tentatively assigned to the Swift River succession, byanalogy with sequences west of the Cassiar Batholith (Nel-son, 1999, 2000).

POST-ACCRETIONARY UNITS

Cassiar Batholith

A portion of the regional, northwesterly-trendingCassiar Batholith (Figure 1) occupies the southern part ofthe map area. This body consists mainly of coarse grainedgranite and granodiorite, with pegmatitic and aplitic phasesalso present. One K/Ar biotite age of 107 Ma is availablefrom this map area (GSC K/Ar 4063). This is typical of K/Arages for the batholith (Gabrielse, 1998, Table 8). Farthernorth, the eastern side of the batholith is cut by younger,Late Cretaceous stocks, which are asssociated with por-phyry-skarn-manto mineralization (Panteleyev, 1980; Nel-son and Bradford, 1993). In the Beale Lake area, distinct,cross-cutting bodies have not been identified, perhaps be-cause the detailed mapping project terminated near thebatholith margin. However, potentially younger, relativelyfine-grained, quartz-feldspar megacrystic dikes lie alongfractures in the country rocks east of the batholith; they maybelong to the Late Cretaceous suite.

Zinc Lake Volcanics and Intrusions

Undeformed volcanic and volcaniclastic rocks and fel-sic intrusions outcrop in a northwesterly-striking belt in thenortheastern corner of the map area, extending onto adja-cent 104I/15 where Zinc Lake itself lies. These rocks weredescribed in detail by Plint et al. (1997). Their contacts withthe surrounding Dorsey assemblage and its intrusive bodiesare unconformable or cross-cutting, and may in part be con-trolled by synvolcanic faults. Their overall outcrop patternis that of a northwesterly-elongate graben. The felsic intru-sions, which range from fine grained, glassy, flow-bandedrhyolite to quartz-feldspar megacrystic granite, concentratealong the northeastern margin of the complex. Volcanic andvolcaniclastic rocks range from andesites and andesitelapilli tuffs with tiny hornblende phenocrysts throughdacites to flow-banded, aphanitic rhyolites. Two bulbousrhyolite bodies, possibly flow domes, were mapped on theridge above Zinc Lake. The highest gold values on the prop-erty occur in this area (Plint et al., 1997).

STRUCTURE

The Sylvester Allochthon is built of stacked, internallyimbricated thrust packages, separated by major thrust faults,

such as the Beale Mountain Thrust (Figure 6a).

Attitudes of these thrust faults, and of the units betweenthem, have been affected by post-emplacement folding.Units and structures on the southwestern side of theallochthon, near the Cassiar Batholith, dip steeply northeast(Figure 4). This is a common feature along the length of thebatholith, defining the western side of the “McDamesynclinorium” of Gabrielse (1963). Forceful intrusion com-bined with crustal compression apparently caused the lay-ered rocks to arch up over the eastern side of the batholith.Farther northeast, major contacts within the allochthon arewarped into a series of upright, open, regional folds; for in-stance the syncline between Beale Mountain and BealeLake, and the anticline that exposes the Nizi pluton (Figure3). Prior to this (mid-Cretaceous?) folding event theallochthons probably lay above the Cassiar Terrane onnearly flat contacts.

Development of penetrative structures within the FourMile assemblage was minimal, except for the black phylliteunit and Ram Creek assemblage. Strong shear fabrics aredeveloped in the late Paleozoic volcanic rocks only withintens of metres of the Beale Mountain Thrust. By contrast,the Ram Creek assemblage is foliated, sheared, andmylonitized throughout. This is partly due to its position inthe immediate footwall of the Beale Mountain Thrust; how-ever, its degree of deformation vastly exceeds that devel-oped in the Nizi pluton or the late Paleozoic volcanic rocks,even where they lie adjacent to the Beale Mountain Thrust.A twofold deformation history is suggested. A plagioclaseporphyry dike, which cuts across structures in the RamCreek assemblage but does not penetrate its external con-tacts, was collected for U/Pb dating.

The Dorsey assemblage is characterized by at least twoepisodes of penetrative deformation and shearing. One epi-sode occurred in the early Mississippian, as shown by latesynkinematic plutons of that age (Gabrielse et al., 1993). Asecond, later shearing event accompanied its emplacementon top of the Four Mile assemblage as the hanging wall ofthe Beale Mountain Thrust (Figure 6a). This event musthave postdated the youngest footwall rocks, themid-Permian Meek and Nizi plutons. Distinct sets of fabricsand strain gradients can be related to these two events.

Ductile strain diminishes gradually upwards within

the entire tectonite. The lower Dorsey assemblage con-

tains sheath folds and very strong quartz stretching (E1)

and mineral streaking (M1) lineations, associated with

shear bands and c-s fabrics in appropriate lithologies.

These features decrease to uncommon occurrence in the

upper Dorsey assemblage. Rootlessly folded small

leucosomes, a common feature of lower Dorsey amphi-

bolites, do not appear in the upper Dorsey assemblage.

The E1 stretching and M1 streaking lineations show

a pronounced northeasterly maximum at 58/18, and a

lesser southwesterly one (Figure 6c). These represent

measurements on the two limbs of the upright, F3


syncline, which also folded transposed layering and

schistosity (S1) in the Dorsey assemblage (Figure 6b).

Shear bands and asymmetric porphyryoclasts both in out-

crop and thin section indicate tops to the northeast. Tenta-

tively, this early tectonic event is assigned to the early

Mississippian, probably immediately after deposition of

the upper Dorsey assemblage. A pegmatite has been col-

lected for U/Pb dating. Although it is cut by prominent

mylonite zones, it locally crosscuts foliation in the amphi-

bolite, and does not exhibit the stretching lineation that is

so well-developed in its country rocks. These features fix

its emplacement as late in the ductile shearing event.

Close-spaced shear zones are developed in rocks tens

of metres from the Beale Mountain Thrust, including the

lower Dorsey assemblage, the Ram Creek assemblage

and the otherwise undeformed Four Mile assemblage.

Shear bands indicate top-to-the-northeast motion on

these subsidiary shears, and thus by implication on the

main thrust fault. Interestingly, the post mid-Permian em-

placement of the Dorsey assemblage on top of the

subadjacent late Paleozoic arc assemblage followed the

same sense as its much earlier internal deformation. Ei-

ther the same large-scale crustal architecture controlled

both, or crustal geometries shaped during the early Mis-

sissippian event influenced later accretion.

MINERAL OCCURRENCES ANDMINERAL POTENTIAL

The Beale Lake map area is host to a diverse set of min-eral occurrences and exploration possibilities, many of themdocumented here for the first time. Types of deposits repre-sented include the following (Table 1):

1. Intrusion-related gold-silver-polymetallic veins.

2. Epithermal gold-silver-polymetallic veins.

3. Porphyry-style copper mineralization associated withzones of intrusive breccias and silicification in the Nizipluton.

In addition, geological evidence supports the possibleoccurrence of the following deposit types in the area:

1. Syngenetic massive sulphide occurrences associatedwith pyrite-garnet-bearing (exhalative?) metachert inlower Dorsey assemblage.

2. Syngenetic massive sulphide occurrences associatedwith felsic volcanics and pyrite-garnet-bearing (exhala-tive?) metachert in upper Dorsey assemblage.

4. Mesothermal gold-quartz veins associated withquartz-carbonate-mariposite alteration in Slide Moun-tain ultramafic rocks.


NEqual Area

(Schmidt)

Axial N = 133

E

+2S

+4S

+6S

+8S

+10S

+12S

S1 E1 + M1

NEqual Area

(Schmidt)

Axial N = 52

E

+2S

+4S

+6S

+8S

+10S

+12S

+14S

F3 F3

C.B.Figure 6. Structures in and around the Dorsey assemblage. A) The Beale Mtn. Thrust on Beale Mountain,. Note the gently dipping base ofthe upper pericratonic allochthon above rocks of the Harper Ranch subterrane. B) Attitudes of S1 foliation and transposed layering in theDorsey assemblage. C) Early quartz stretching (E1) and mineral alignment (M1) in the Dorsey assemblage.

Exhalites in Lower Dorsey Assemblage

Very rusty, pyritic metachert layers occur at two locali-ties roughly at the same structural (and thus stratigraphic?)level in the lower Dorsey assemblage (Figure 3, Table 1).The layers are continuous over hundreds of metres to a kilo-metre of strike length, and are overall 5-10 metres thick.They contain two compositional varieties: dark grey, py-rite-rich chert with trace to abundant garnet, and white py-ritic chert with minor, discontinuous quartz-sericite schist.Their mineralogy is consistent with that described fromcoticules, or metamorphosed Fe-Mn-silica exhalites. Geo-chemically, they are characteristically anomalous in manga-nese and barium (Table 2).

A sample from one very small occurrence, 01JN16-3,from near the top of the lower Dorsey assemblage 4 kilo-metres north of Beale Lake, contains over 17,000 ppm bar-ium.

Potential for Syngenetic Sulphide Occurrencesin Upper Dorsey Assemblage

The upper Dorsey assemblage near Beale Lake con-tains significant accumulations of felsic volcanic andvolcaniclastic products. Exposures of blue-grey, yel-low-stained phyllite similar to the “gunsteel” slates of theautochthonous Earn Group are associated with the felsicrocks. This suite is a probable equivalent of the early Missis-sippian upper Dorsey assemblage in the Swift River area,which in turn has been correlated with the felsic volcanichosts of syngenetic massive sulfide deposits in theFinlayson Lake belt (Roots and Heaman, 2001). Thus, ageologically favorable environment for such deposits alsoexists within the upper Dorsey assemblage in the SylvesterAllochthon.

A10 metre-thick section of rusty, pyritic chert, overlainby garnetiferous quartz-muscovite schist with unusuallyabundant tourmaline and apatite, is exposed on thewest-facing spur north of Beale Lake. It is probably of sili-


TABLE 1MINERAL OCCURRENCES, SHOWINGS AND ALTERATION ZONES, BEALE LAKE MAP AREA

UTM

east

UTM

north

MINFILE

number Deposit type Capsule description

Nizi A, B Zone 498637 6538060 104I032Polymetallic veins

Ag-Pb-Zn±Au.

samples assayed up to 12.0 g/tonne gold and up to

3428 g/tonne silver. These base metal zones are

generally <20 centimetres wide and traceable for tens

of metres.

Nizi H Zone 499240 6537900 104I032Polymetallic veins

Ag-Pb-Zn±Au.

Vein samples assayed up to 2.33 g/tonne gold and

627.43 g/tonne silver, 18.3% zinc and 7.% lead.

Nizi Surprise vein 499380 6537663 104I032Polymetallic veins

Ag-Pb-Zn±Au.

Chip/channel samples assayed up to 27.09 g/tonne

gold and 1220.58 g/tonne silver over 2 metres.

Gunsight 496483 6534714 104I041Polymetallic veins

Ag-Pb-Zn±Au.

quartz vein with argentiferous galena, pyrite and

sphalerite. One 20-centimetre sample contained .09

g/tonne Au, 110 g/tonne Ag, 8.36% Pb and 3.09% Zn.

Beale upper vein 494235 6529734 104I098Polymetallic veins

Ag-Pb-Zn±Au.quartz vein + Au, Ag

Beale lower vein 494146 6530533 104I098Polymetallic veins

Ag-Pb-Zn±Au.quartz-arsemopyrite-galena vein+Au, Ag

Yurso vein 498116 6525956Polymetallic veins

Ag-Pb-Zn±Au.

grab sample: Au (to 1250 ppb), Ag (greater than

measurable using ICPMS technique), Bi (to 413 ppm),

Pb (to 23,000 ppm), Sb (to 8000 ppm), anomalous Se,

Te, Cu and Zn.

No Fish vein 497824 6529441Polymetallic veins

Ag-Pb-Zn±Au.

pyritiferous quartz vein with anomalous values of Cu,

Te, Se, Ag, Mn

Perm 490989 6536006 Porphyry Cusilicified intrusion breccia with disseminated pyrite,

trace chalcopyrite

Corydalis 492624 6535272 Porphyry Cusilicified intrusion breccia with disseminated pyrite,

trace chalcopyrite

Keith 489414 6531816 Siliceous exhalite chert + pyrite, garnet

Lucky Luke 491403 6529879 Siliceous exhalite chert + pyrite, garnet

Toboggan 479062 6536436.6Listwanite

alteration

Fe-Mg carbonate-altered ultramafite, anomalous As,

Sb, Mn

Least one 482128 6533259Listwanite

alteration


Sb, Mn

Griz 483689 6530499Listwanite

alteration


Sb, Mn


TA

BL

E2

LIT

HO

GE

OC

HE

MIC

AL

DA

TA

,2001

SA

MP

LE

S,B

EA

LE

LA

KE

MA

PA

RE

A(1

04I/

14N

)

ceous exhalative origin. Like the pyritic-garnetiferouscherts of the lower Dorsey assemblage, it contains anoma-lous manganese and barium (sample 01LL7-7a, Table 2).

Porphyry-Style Mineralization in the NiziPluton

Prominent gossans, including the Perm and Corydalisshowings, occur within the Nizi pluton. They arecross-cutting, linear, probably fracture-controlled zones,with west-northwesterly trends. The Perm is a prominentgossan on the bare west face of Tetipistikwan Mountainabove the Four Mile River, roughly 50 metres across and600 metres in strike length. The Corydalis gossan crosses aspur ridge of the mountain in the headwaters of Nizi Creek.Somewhat less intense than the Perm, it is 200 metres acrossand 600 metres in strike length. Within the gossans,plagioclase-porphyry dikes and zones of finely comminutedintrusive breccia cut the main-phase tonalites, diorites andgabbros of the pluton. The intrusive breccias incorporate an-gular clasts of their country rocks. They characteristicallycontain rounded, milled single-crystal plagioclase andhornblende fragments in a matrix of dust-sized rock debriscemented by secondary silica. Many are laced with finequartz veinlets. Disseminated pyrite and lesser chalcopyriteoccur throughout the matrix, as well as in fractures in thesurrounding country rock. Samples from these gossans con-tain 1100-1400 ppm Cu, with anomalous Ag, Zn and Hg(Table 2).

These gossans are not described in assessment reports,although very old claim posts lie on them. The posts maydate from early exploration of the Kirk or Four Mile prop-erty (MINFILE 104P 027; Gabrielse, 1963, p. 113). TheFour Mile property, a shear-hosted copper-silver prospect,is located in McDame map area 10 kilometres to the north inthe Four Mile River valley. It was staked for the first time byBeale Carlick, the Kaska trapper and prospector after whomBeale Lake and Beale Mountain were named.

Quartz-Carbonate-Mariposite Alteration Zonesand Mesothermal Gold Vein Potential

Ultramafic bodies in the belt of Slide Mountain rocksalong the eastern margin of the Cassiar Batholith are exten-sively altered to orange-weathering iron-magnesium car-bonate, veined with quartz and carrying significantmariposite in some areas. This type of alteration is associ-ated with the gold-quartz veins of the Cassiar camp(Pantaleyev and Diakow, 1982, Nelson and Bradford,1993). Three alteration zones are exposed on ridges, sepa-rated by overburden-covered valleys. Overall the systemextends over 8 kilometres along strike. Our samples weremildly anomalous in As, Sb, and Mn, as well as theultramafite-related suite Ni-Co-Cr.

Polymetallic Veins

Polymetallic veins are the best-known deposit type inthe Beale Lake map area. The Nizi property has been the tar-get of several advanced exploration programs, includingdrilling. The current Beale Lake claims (previously held as

the Keel and the Flag claims) and the RN claims have beensystematically prospected and geochemically sampled.This report and the companion geological map (Nelson andLepage, 2002) document and precisely locate the knownshowings, as well as previously unreported veins discov-ered in the course of field work this year.

Nizi Epithermal Vein System (MINFILE 104I-032 )

Mineralization on the Nizi property is a vein-stockworksystem with associated hydrothermal brecciation, cospatialand probably cogenetic with the relatively young Zinc Lakevolcanic/intrusive suite (“Nizi volcanics” of Plint et al.,1997). Known veins occur over a northwesterly-elongatearea 2 kilometres long by 1 kilometre wide.Two distinctmineralization styles are present: sulphide-poor,gold-silver-quartz veins and stockworks associated withpervasive silicification, and sulphide-rich iron carbon-ate-sphalerite-galena veins associated with pervasive car-bonate alteration. Six main mineralized areas have been out-lined, the Zinc Lake Zone, Discovery Vein/Surprise Vein,Grizzly Ridge Vein, H Zone, Gully A Zone and B Zone,mainly through the exploration and drilling programs ofGold Giant Minerals Incorporated in 1987-1992 (Cavey andChapman, 1992; McIntosh and Scott, 1991). In 1996,Madrona Mining drilled six holes, five in the Discov-ery/Surprise vein area, and one to test the southeastern ex-tension of the Zinc Lake Zone (Plint et al., 1997). The corefrom this project is stored at the east end of Beale Lake.

The Discovery/Surprise Vein area represents the bestexploration target identified to date on the Nizi Property. Itis characterized by multistage, microcrystal l inequartz-carbon-sulphide-barite stockworks with veryfine-grained pyrite, galena, sphalerite, chalcopyrite,tetrahedrite and acanthite. Assay values up to 41.0 g/t Auwere obtained from these veins, with typical channel sam-ples returning 1.5 to 30 g/tonne Au and 190 to 1200 g/tonneAg over 1-2 metres (Bond, 1993).

The Gul ly A and B zones a re i ron ca rbon-a t e - m i c r o c r y s t a l l i n e q u a r t z - r h o d o c h r o s i t e -sphalerite-galena-pyrite veins; the H Zone, banded carbon-ate-quartz-sphalerite-galena-pyrite veins. Both are con-trolled by north-striking subvertical fractures and/or shears.A 1.8 metre chip sample of H Zone sphalerite-galena- py-rite-carbonate-rhyolite breccia returned values of 2.26 g/tAu, 278.1 g/t Ag; best grab sample assays from the GullyZone are 11.38 g/t Au and 22.4 g/t Ag (Bond 1993).

Plint et al. (1997) show that the galena lead isotopic sig-nature of the Nizi plots above, and near the very young endof the shale curve defined by Godwin and Sinclair (1982). Itis similar to lead from veins and skarns near the SeagullBatholith, the Cassiar gold-quartz veins, Midway, ButlerMountain and other Cretaceous-Early Tertiary epigeneticdeposits in the Cassiar area (Bradford, 1988). The coinci-dent centering of the alteration halo and the gold-quartzveins around the rhyolites near Zinc Lake strongly favorscogenesis between Nizi volcanic activity and the epithermalevent. Gabrielse (1994) assigned the kaolinizedquartz-orthoclase porphyry rhyolite on the Nizi property tothe Eocene suite, by correlation with the Major Hart pluton


(Figure 2). The entire Zinc Lake volcanic/intrusive se-quence is tentatively assigned to the Eocene. It is thus possi-ble that igneous/hydrothermal systems ranging from intru-sion-related to epithermal may extend from the Major Hartpluton as far north as the Four Mile River.

Beale Lake Intrusion-Related Vein System(MINFILE 104I-098)

Gold and silver-bearing quartz-sulphide veins werefirst discovered on the ridge north of Beale Lake in 1982-3by the Cassiar Joint Venture, who conducted grid soil sam-pling and prospecting (Fleming, 1983). In 1996, followingthe Cry Lake regional geochemical data release (Jackaman,1996), the ground was restaked in 1996 by Westmin Re-sources Ltd., and contour soil sampling and limited pros-pecting carried out (Jones, 1997), mainly aimed atvolcanogenic massive sulphide-style targets. The currentproperty owners have collected new, and reinterpreted ex-isting, soil geochemical data; and conducted further pros-pecting (Durfeld and Fleming, 2001). Their work identifiesa set of veins with overall dimensions of 1 by 2 kilometres,which comprises two compositionally and spatially distincts t y l e s o f g o l d m i n e r a l i z a t i o n . To t h e w e s t ,quartz-arsenopyrite- pyrite-scheelite veins are associatedwith Au-As-W-Bi anomalies. To the east, quartz-base metalsulphide veins are associated with anomalous values of Au,As, Pb, and local Ag, Zn, Sb, Cu and Bi. They suggest azoned, intrusion-related gold system.

The Beale Lake veins are hosted by metamorphosedsedimentary and volcaniclastic rocks of the upper Dorseyassemblage. Our traverses in 2001 relocated some of thesampled veins, including a prominent trench in silicifiedvein breccia from which Durfeld and Fleming (2001) report2 7 a n d 4 1 g / t o n n e A u i n g r a b s a m p l e s , a n d aquartz-arsenopyrite-galena vein from which they report 2.5g/tonne Au, 5.5 g/tonne Ag. Repeat grab samples from theseveins returned geochemically highly anomalous Au and Ag(Au to 2716 ppb by fire assay) (Table 2). Although in the al-pine, much of the property is covered by overburden. Miner-alized quartz vein float throughout the area hints at addi-tional, unexposed veins.

Gunsight (MINFILE 104I-041)

The Gunsight showing was discovered during prelimi-nary exploration in 1992, on claims adjoining the Nizi prop-erty to the south (RN Group; Termuende, 1993). It is awest-northwesterly-striking quartz vein with argentiferousgalena, pyrite and sphalerite. One 20-centimetre samplecontained .09 g/tonne Au, 110 g/tonne Ag, 8.36% Pb and3.09% Zn. The vein is associated with brecciatedhornblende granodiorite of the Permian Nizi pluton. Thisvein may relate to the Nizi system, to the Beale Lake veins,or to the porphyry-style occurrences near the headwaters ofNizi Creek. It was not visited this season.

New Discoveries

Two polymetallic veins were encountered during 2001traverses southeast of Beale Lake, on ground not covered byassessment reports. One, the No Fish vein, outcrops in a wa-

terfall in a north-draining creek. It is 2-8 centimetres wideand strikes west-northwest. The other, the Yurso vein, out-crops in the pass at the head of the creek, where it has beentraced as subcrop over 1000 metres along a northwesterlystrike (Nelson and Lepage, 2002). The No Fish vein con-tains pyrite, and a grey metallic mineral. It is anomalous inCu, Se, Te, Ag, Mn and Co (Table 2). The Yurso vein con-tains arsenopyrite, pyrite, stibnite and tetrahedrite. Analy-ses show it to be highly anomalous in a broad suite of metals,including gold (to 1250 ppb), silver (to 1824 g/t), bismuth(to 413 ppm), lead (to 23,000 ppm), antimony (to 8000ppm), as well as Se, Te, Cu and Zn. Heavily oxidized veinfragments of secondary base metal sulphates and carbon-ates, such as anglesite and smithsonite, are abundant in floatdownslope from the projected vein trace. The largest piecesof vein material are 20-30 centimetres in their shortest di-mension (Figure 7).

The fracture that hosts the Yurso vein is also occupiedby granite porphyry dikes that contain round quartz andorthoclase megacrysts. They may be offshoots from thenearby Cassiar Batholith, or younger, Late Creta-ceous-Eocene intrusions. Some show strong argillic alter-ation, which also affects rocks of the upper Dorsey assem-blage around the vein.

These vein occurrences are on strike with, and 2 to 5kilometres southeast of the Beale Lake vein set. They sug-gest that it may extend through the low-lying countryaround Beale Lake and, potentially, onto 104I/15. The gran-ite porphyry dikes that accompany them lend strength to theinferred connection between late plutonic activity and goldmineralization.

CONCLUSIONS

This investigation of the Beale Lake map area has pro-duced some significant changes to our understanding of thesouthern Sylvester Allochthon. Most important, it hasfirmly established and made specific the correlation of theuppermost, deformed pericratonic allochthon with theDorsey assemblage west of the Cassiar Batholith. This rela-tionship was previously published only in abstracts, as asuggestion based on perceived general similarities. It is nowclear that the same two twofold unit subdivision can be


Figure 7. Typical chunk of the Yurso vein, rusty-stained quartzwith pyrite, arsenopyrite, tetrahedrite and stibnite.

equally applied to the Dorsey assemblage in its type area,and to the pericratonic assemblage in the SylvesterAllochthon. The lower unit, with its extensive cliff-formingamphibolites, is recognizable in localities from the SwiftRiver valley in southern Yukon (Stevens and Harms, 1985)to the west face of Beale Mountain. The upper unit, with itsfelsic volcanic component, is probably equivalent to Missis-sippian felsic units in the Yukon-Tanana Terrane, includingthose known to host volcanogenic massive sulphide depos-its.

Structurally below the Dorsey assemblage, theSylvester Allochthon can be divided into two distinct as-semblages, a lower one of oceanic and a middle one of is-land arc affinity. This accords with the gross structure of theallochthon between the Dease River and the Yukon border.The mid-Permian Meek and Nizi plutons form parts of a sin-gle complex that intrudes the middle, arc-related, assem-blage. Referred to here as the Four Mile assemblage, itshares a broad age and affinity with Division III, the middleunit of the allochthon farther north. However, in detail thesetwo units differ significantly enough to warrant separateunit names. As exposed in the Huntergroup Range and onJuniper Mountain near Cassiar, Division III is dominated byPennsylvanian-Permian augite-phyric basaltic andesitesand limestone-limestone breccia-chert sequences (Nelsonand Bradford, 1993). In contrast, the most extensive unit inthe Four Mile assemblage is a complex of nearly aphaniticdacite to rhyolite flows. The deformed phyllite is alsounique to this area. This diversity is typical of island arc se-quences, which can comprise a wide variety of volcanicsources and evanescent sedimentary environments.

Polymetallic, precious metal-bearing veins are wide-spread in the eastern part of the map area, including the epi-thermal Nizi vein system associated with young volcanicrocks in the north, and the deeper, intrusion-related BealeLake system. The latter is now shown by new discoveriesthis summer to extend at least 5 kilometres south of BealeLake. These results, combined with the pattern of Au andAg and As, Sb, Bi and Hg in stream sediments that extendsfrom Beale Lake area to the eastern border of Cry Lake maparea (Jackaman 1996), suggests that the potential for furtherintrusion-related gold discoveries is alive and well in theCassiars.

ACKNOWLEDGEMENTS

The study area lies within the traditional territory of theKaska Dene. We acknowledge the warm welcome of theDease River Band, in particular Mr. Frank Dennis, BandChief, and Mr. Walter Carlick, Band Manager. Two mem-bers of the band, Damian Leonard and Christian Carlick,participated in the field work. Their enthusiasm and interestin mineral exploration in the Beale Lake area were much ap-preciated.

This project would never have happened without theastute observations of Hu Gabrielse and Tekla Harms. LucLepage helped make the map happen, from negotiatinghairy gullies, to discovering new showings and geologicalgems, to alligator wrestling with printer drivers in the

dreaded Blanshard Bayou. Thanks to Dave Lefebure for agood job of textual tuning. Aesthetics by Janet Holland.

REFERENCES

Bond, W.D. (1993): Geological, geochemical and diamond drillreport on the Nizi mineral claims, Gold Fields CanadianMining Ltd. British Columbia Ministry of Energy, Minesand Petroleum Resources, Assessment Report 22840.

Bradford, J.A. (1988): Geology and genesis of the Midway sil-ver-lead-zinc deposit, north-central British Columbia, un-published M.Sc. thesis, The University of British Columbia,280 pages.

Cavey, G. and Chapman, J. (1992): Report on the Nizi Project forGold Giant Minerals Inc., Liard Mining Division, BritishColumbia, NTS 104I/14,15 and 104P2,3. Private Report.

Durfeld, R.M. and Fleming, D.B. (2001): Summary report on theBeale Lake gold property; Private Report.

Fleming, D.B. (1983): Beale Lake property, 1983 geochemical as-sessment report, British Columbia Ministry of Energy,Mines and Petroleum Resources, Assessment Report 12181.

Gabrielse, H. (1963): McDame Map Area, Cassiar District, BritishColumbia, Geological Survey of Canada, Memoir 319, 138pages.

Gabrielse, H. (1994): Geology of Cry Lake (104I) and Dease Lake(104J/E) Map Areas, North Central British Columbia; Geo-logical Survey of Canada, Open File Map 2779.

Gabrielse, H. (1998): Geology of Cry Lake and Dease Lake mapareas, north-central British Columbia, Geological Survey ofCanada, Bulletin 504, 147 pages.

Gabrielse, H., and Harms, T.A., (1989). Permian and Devonianplutonic rocks in the Sylvester allochthon, Cry Lake andMcDame map areas, northern British Columbia; in CurrentResearch Part E, Geological Survey of Canada, Paper89-1E, pages 1-4.

Gabrielse, H., Mortensen, J.K., Parrish, R.R., Harms, T.A., Nel-son, J.L., and van der Heyden, P. (1993): Late PaleozoicPlutons in the Sylvester Allochthon, Northern British Co-lumbia; in Radiogenic Age and Isotopic Studies, Report 7,Geological Survey of Canada, Paper 93-1, pages 107-118.

Godwin, C.J. and Sinclair, A.J. (1982): Average lead isotopegrowth curves for shale-hosted zinc-lead deposits, CanadianCordillera: Economic Geology, Volume 7, pages 675-690.

Harms, T.A. (1986): Structural and tectonic analysis of theSylvester Allochthon, SW McDame map area, northernBritish Columbia: Implications for paleogeography and ac-cretion. Unpublished Ph. D. thesis, University of Arizona,80 pages.

Harms, T.A. (2000): Proposed terrane affinities of pericratonic as-semblages in northern British Columbia and southern Yu-kon, Canada; Geological Society of America, Abstracts withPrograms, Volume 32, page A-17.

Harms, T.A. and Stevens, R.A. (1996): Assemblage analysis of theDorsey Terrane; Slave-Northern Cordillera LithosphericEvolution (SNORCLE) and Cordilleran Tectonics Work-shop, Report of the 1996, Combined Meeting; pages199-201.

Hunt, P.A. and Roddick, J.C. (1988): A compilation of K-Ar ages,Report 18, in Radiogenic Age and Isotopic Studies: Report2, Geological Survey of Canada, Paper 88-2, pages 127-153.

Jackaman, W. (1996): British Columbia Regional GeochemicalSurvey: NTS 104/I - Cry Lake; Stream Sediment and WaterGeochemical Data; B.C. RGS 44; British Columbia Minis-try of Employment and Investment Open File; B.C. RGS 44,52 pages.

Jones, M.I. (1997): Flag property, 1996 assessment report, Flag 1to 4 claims; Geological mapping and soil sampling surveys,


Liard Mining Division, NTS map area 104I/14, Latitude58

o54’00"N, Longitude 129

o09’00"W, British Columbia

Ministry of Energy and Mines Assessment Report 25044.

McIntosh, R. and Scott, G. (1991): Report on the Cry Lake Projectfor Omega Gold Corporation, Liard Mining Division, Brit-ish Columbia, NTS 104I; Internal Company Report.

Mihalynuk, M. Nelson, J.L. and Friedman, R.M. (1998): Regionalgeology and mineralization of the Big Salmon Complex(104N NE and 104O NW), in Geological Fieldwork 1997,British Columbia Ministry of Employment and Investment,Geological Survey Branch, Paper 1998-1, pages 6-1 to 6-20.

Mihalynuk, M.G., Nelson, J., Roots, C.F. and Friedman, R.M.(2000): Ancient Pacific Margin part III: Regional geologyand mineralization of the Big Salmon Complex (104N/9,10& 104O/12,13,14W); in Geological Fieldwork 1999, BritishColumbia Ministry of Energy and Mines, Geological SurveyBranch, Paper 2000-1, pages 27-46.

Nelson, J.L. (1993): The Sylvester Allochthon: Upper Paleozoicmarginal basin and island-arc terranes in northern BritishColumbia; Canadian Journal of Earth Sciences, Volume 30,pages 631-643.

Nelson, J. (2001): Cross sections of the allochthonous units west ofthe Cassiar Batholith and the Sylvester Allochthon: Pancaketectonic stratigraphy above the Cassiar Platform; inSlave-Northern Cordillera Lithospheric Evolution(SNORCLE) and Cordilleran Tectonics Workshop, Reportof the 2001 Combined Meeting; pages 76-77.

Nelson, J. (1999): Devono-Mississippian VMS Project: Con-tinuing studies in the Dorsey Terrane, northern British Co-lumbia; in Geological Fieldwork 1998, British ColumbiaMinistry of Energy and Mines, Geological Survey Branch,Paper 1999-1, pages 143-155.

Nelson, J.L. (2000): Ancient Pacific Margins Part VI: Still headingsouth: Potential VMS hosts in the eastern Dorsey Terrane,Jennings River(104O/1; 7,8,9,10); in Geological Fieldwork,1999, British Columbia Ministry of Employment and Invest-

ment, Geological Survey Branch, Paper 2000-1, pages107-126.

Nelson, J. and Bradford, J.A. (1993): Geology of the Mid-way-Cassiar area, northern British Columbia; BritishColumiba Ministry of Energy, Mines and Petroleum Re-sources, Bulletin 83, 94 pages.

Nelson, J., and Lepage, L.D. (2002): Geology and mineral occur-rences, Beale Lake map area (NTS 104I/14N) . British Co-lumbia Ministry of Energy and Mines,Open File Map2002-6, 1:50,000 scale.

Pantaleyev, A. (1980): Cassiar Map-Area. British Columbia Min-istry of Energy, Mines and Petroleum Resources, GeologicalFieldwork 1979, Paper 1980-1, pages 51-60.

Pantaleyev, A., and Diakow, L.J. (1982): Cassiar Gold Deposits,McDame Map Area (104P/4,5), British Columbia Ministryof Energy, Mines and Petroleum Resources, GeologicalFieldwork 1981, Paper 1982-1 pages 156-161.

Plint, H., McMillan, W.J. , Nelson, J. and Pantaleyev, A. (1997):The Nizi property: a classic (Eocene?) epithermal gold sys-tem in far northern British Columbia; ) in Geological Field-work, 1997, British Columbia Ministry of Employment andInvestment, Geological Survey Branch, Paper 1998-17,pages 17-1 to 17-13.

Roots, C.F. and Heaman, L.M. (2001): Mississippian U-Pb datesfrom Dorsey terrane assemblages in the upper Swift Riverarea, southern Yukon; in Current Research, Geological Sur-vey of Canada, Paper 2001-A1, 9 pages.

Stevens, R.A. and Harms, T.A. (1995): Investigations in theDorsey Terrane, Part I: Stratigraphy, structure and metamor-phism in the Dorsey Range, southern Yukon Territory andnorthern British Columbia; in Current Research, GeologicalSurvey of Canada, Paper 1995A, pages 117-127.

Termuende, T. (1993): Assessment Report for the RN claim group,Liard Mining Division, NTS 104I/14E, 104I/15W, Latitude59

o00’N, Longitude 129

o15’W, British Columbia Ministry

of Energy and Mines, Assessment Report 22946.


tracking yukon-tanana terrane vms host stratigraphy and...

Documents