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Boundary Project:Reassessment of Paleozoic Rock Units of the Greenwood Area

(NTS 82E/02), Southern British Columbia

By N.W.D. Massey

KEYWORDS: Green wood, Quesnellia, Knob Hill com -plex, Att wood For ma tion, Lexington por phyry

INTRODUCTION

The Bound ary Pro ject has been ini ti ated with the pur -pose of better char ac ter iz ing the lithological and geo chem i -cal vari a tions within and be tween the var i ous Pa leo zoic se -quences in the south ern Okanagan re gion along the UnitedStates bor der. These oc cur within the Quesnel Terrane,which is dom i nated by such Pa leo zoic mafic vol ca nic andpelitic rocks, un con form ably over lain by Tri as sic and Ju -ras sic vol ca nic and sed i men tary rocks. In the Bound aryDis trict, such se quences in clude the Knob Hill Com plex,Att wood For ma tion, Kobau Group and An ar chist Schist.These are ex posed be tween down-faulted blocks of Ter -tiary vol ca nic and sed i men tary cover pre served in struc -tural keels be tween gneissic domes (Fig. 1). In 2005, field -work was con cen trated in the Green wood – Rock Creekarea (NTS 082E/02). In fu ture years, field work will prog -ress into the Osoyoos (NTS 082E/03) and Keremeos (NTS082E/04) map sheets.

The Green wood map sheet strad dles High way 3 be -tween Osoyoos and Grand Forks. Five weeks of field workin 2005 were re stricted to ar eas im me di ately around andeast of Green wood, and to the area be tween Mid way andRock Creek (see Fig. 7 for lo ca tions). This in cludes most ofthe area mapped by Fyles (1989, 1990), upon which thisstudy builds. Field work was focused on,

• field check ing pre vi ous map ping in or der to pro -duce a new com pi la tion map for the Green woodsheet (Massey, work in prog ress);

• col lect ing geo chem i cal sam ples from vol ca nicrocks in the Knob Hill Com plex and Att wood For -ma tion;

• iden ti fy ing and col lect ing suit able ma te ri als forpaleontological or geo chron ol ogi cal age de ter mi na -tions of the Pa leo zoic sequences; and

• in ves ti gat ing the re ported oc cur rence of fel sic vol -ca nic rocks within the ‘Lexington por phyry’.

PREVIOUS WORKThe Green wood – Rock Creek area has a min ing his -

tory dat ing from the first dis cov er ies of placer gold in the

1860s and lode de pos its in the 1880s. The first geo log i calre port was that of Bauerman (1885), pre pared as part of theBound ary Com mis sion Ex pe di tion of 1859–1861. Re -gional map ping has been un der taken by Brock (1902,1903, 1905a, b), Daly (1912), Lit tle (1957, 1983), Mon ger(1968), Church (1986), Templeman-Kluit (1989a, b) andFyles (1990). De tailed map ping around in di vid ual minesand min ing camps in cludes work by LeRoy (1912, 1913),McNaughton (1945), Ser a phim (1956), Carswell (1957),Church (1970) and Nixon (2002).

PALEOZOIC STRATIGRAPHY OF THEGREENWOOD – ROCK CREEK AREA

Knob Hill Complex

The Knob Hill Com plex dom i nates the Quesnellia Pa -leo zoic rocks ex posed in the Green wood map sheet. Calledthe ‘Knob Hill Group’ by pre vi ous work ers, the name ischanged here to Knob Hill Com plex, in line with mod ernac cepted strati graphic no men cla ture, to re flect the in clu -sion of in tru sive units with the lay ered rocks (Murphy andSalvador, 1999).

The com plex is com posed of chert, argillite, ba salt,gab bro and ser pen tin ite that to gether con sti tute a dis ruptedophiolite (Lit tle, 1983; Dostal et al., 2001). It was in cludedin the Okanagan subterrane of Quesnellia by Mon ger et al.(1991), along with other as sem blages of oce anic af fin itysuch as the Kobau and Old Tom – Shoe maker Groups. Thestrati graphic thick ness of the Knob Hill Com plex is un cer -tain and dif fi cult to es ti mate due to struc tural dis rup tion.Lit tle (1983) sug gested “a min i mum of a few thou sands ofmetres”, whereas Fyles (1990, 1995) es ti mated thecomplex to be more than 5 km thick.

The age of the Knob Hill Com plex is still poorly de ter -mined. A Car bon if er ous to Perm ian age was as signed byLit tle based on a sin gle macrofossil lo cal ity (Lit tle, 1983,page 12, lo cal ity F7). How ever, this same lime stone bed has yielded cono donts of Late De vo nian (Frasnian) age (Or -chard, 1993). Church (1986) re ported a K-Ar whole rockage of 258 ±10 Ma (Perm ian) for uralitized gab bro from theWin ni peg mine. How ever, the re li abil ity of such K-Arwhole rock ages is very sus pect, and it is doubt ful that this is re cord ing a crys tal li za tion age. Re fine ment of the age of the c o m p l e x a w a i t s f u r t h e r p a l e o n t o l o g i c a l a n dgeochronological determinations.

Seven map units are rec og nized within the Knob HillCom plex in the Green wood map sheet. Strati graphic re la -tion ships be tween map units are not al ways clear. How ever, in the area north of Rock Creek, gab bro and ser pen tin itepass north ward, and prob a bly up ward, into greenstone,

Geo log i cal Field work 2005, Pa per 2006-1 99

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mixed greenstone and chert, and fi nally chert and argillite.In the Green wood area, Fyles (1990) sug gested that thelower part of the Knob Hill Com plex con sisted of a highlyvari able mix of chert, argillite, con glom er ate and brec ciainterfingering with greenstone flows. The up per part of thecom plex is mainly chert and greenstone. Due to the lim itedfield work un der taken in the area by the au thor, the fol low -ing de scrip tions of the units draw heavily upon pre vi ousob ser va tions by Church (1986) and Fyles (1990).

GREENSTONE UNIT

The ba saltic greenstone is mostly flows with mi norbrec cia, ag glom er ate and tuffaceous sand stone. Chert,cherty argillite, jas per and rare lime stone may oc cur be -tween flows. Most flows are grey to green, mas sive andapha ni tic. Chlorite, cal cite and epidote are com mon al ter -ation min er als in veins, frac tures and within the rock. Pil -low struc tures are rare, though they have been in ferred bypre vi ous map pers (Fyles, 1990). Geochemically, the flowsof the greenstone unit are oce anic ba salt (Dostal et al.,2001) and may have formed at a mid-ocean ridge or in aback-arc basin.

Me dium-grained diabase to microgabbro bod ies alsooc cur within the greenstone unit. In gen eral, these are fea -ture less rock units in out crop. How ever, out crops along thepower line ad ja cent to the Win ni peg mine show well-de vel -oped chilled mar gins, sug gest ing that they may be part of an

un mapped sheeted dike com plex (Fig. 2a). Dikes are alsoseen in trud ing the nearby gabbro (Fig. 2c).

CHERT-ARGILLITE UNIT

Sed i men tary rocks in the Knob Hill Com plex are typ i -cally fine grained, mainly grey to white chert interbeddedwith grey to black argillite and mi nor greenstone. The chertis highly frac tured and jointed, tends to be mas sive tothickly bed ded and only rarely shows rib bon struc tures.Radio lar ians are re ported in thin sec tion (Church, 1986) but are rarely seen in hand spec i men. Vari able recrystallizationof the cher t pro duces a f ine to me dium-grainedsaccharoidal tex ture. Argillite is rarely bed ded and mayshow gradational con tacts with chert ho ri zons. Thin whitecrys tal line lime stone forms occasional outcrops.

MIXED GREENSTONE AND CHERT UNIT

In the west ern part of the map sheet, be tween up perNichol son Creek and Ket tle River, a belt of mixedgreenstone and chert in roughly equal pro por tions forms atran si tion be tween the greenstone unit to the south and thechert-argillite unit to the north (Fig. 3). Rock types are sim i -lar to those found in the other two units.

CHERT BRECCIA

Two ar eas of buff to cream and grey chert brec cia andpeb ble con glom er ate were out lined by Fyles (1990). Theycon tain an gu lar to subrounded chert clasts sim i lar to the

100 Brit ish Co lum bia Geo log i cal Sur vey

U S A

?

?

?

?

?

?

?Greenwood

Grand Forks

Beaverdell

Osoyoos

Penticton

Keremeos

Oliver

Tertiary Penticton Group

Paleozoic Sequences

Metamorphic Core Complexes

Fault

Normal Fault

Detachment Fault

Thrust Fault

118°30' E119°00' E119°30' E

118°30' E119°00' E119°30' E4

9°0

0' N

49

°30

' N5

0°0

0' N

49

°00

' N4

9°3

0' N

50

°00

' N

0 km10

Fig ure 1. Dis tri bu tion of Pa leo zoic Quesnellian rock suites in the Bound ary Dis trict (south-cen tral part of NTS 082E), amended fromMassey et al. (2005).

chert of the chert-argillite unit. They lack the va ri ety ofother clast types, such as greenstone, ser pen tin ite, jas per,and lime stone, found in the Tri as sic Brook lyn For ma tionsharpstone con glom er ate. Black siltstone and chert sand -stone oc cur as interbedded lenses. Sim i lar chert brec ciaalso forms a prom i nent hill just south of Nor we gian Creek,im me di ately west of the Sappho prop erty, and is probablycorrelative.

GREENWOOD GABBRO

This unit was orig i nally in for mally called the‘Old Diorite’ by Church (1986) and re named the‘Green wood Gab bro’ by Dostal et al. (2001). The lat -ter term is pre ferred here as be ing in agree ment withmod ern strati graphic no men cla ture.

The gab bro is com posed of white plagioclaseand green to black pyroxene that is ex ten sively re -placed by hornblende. Chlorite is com mon on frac -tures and in shears. The gab bro is gen er ally mas siveand char ac ter is ti cally vari able and patchy in tex ture,with coarse-grained gab bro phases grad ing into finer microgabbro or even coarser peg ma titic phases(Fig. 4). No chilled con tacts are seen be tween phases. Light-col oured fel sic veins (plagiogranite?) criss -cross some out crops. Oc ca sional diabase xe no lithsare in cluded in the gabbro (Fig. 2d).

Green wood Gab bro sam ples are geochemicallycom pa ra ble to oce anic plutonic rocks (Dostal et al.,

2001) and are prob a bly comagmatic with the greenstoneflows.

SERPENTINITE

Ser pen tin ite oc curs dom i nantly as fairly con tin u oussheets along faults, or as ir reg u lar len tic u lar bod ies that arecom monly spa tially as so ci ated with gab bro bod ies. The


(a) (b)

(c) (d)

Fig ure 2: Diabase dikes in the Knob Hill Com plex: a) chilled con tact be tween diabase dikes (05NMA02-08; UTM Zone 11, 5436632N,385389E, NAD83); b) gab bro screen be tween two diabase dikes (05NMA02-09; UTM Zone 11, 5436682N, 385375E, NAD83); c) diabasein trud ing gab bro body (05NMA02-09; UTM Zone 11, 5436682N, 385375E, NAD83); d) diabase in clu sion within gab bro (05NMA02-10;UTM Zone 11, 5436766N, 385333E, NAD83).

Fig ure 3. Interbedded chert (c) and greenstone flows (b) of the Knob HillCom plex, east side of the Ket tle River val ley (05NMA23-08; UTM Zone 11,5443408N, 357292E, NAD83).

ser pen tin ite is usu ally dark green to black and schis tose toblocky, with greasy frac ture coat ings. Fo li ated ser pen tin itemay con tain boudins of blocky ser pen tin ite. Orig i nal min -er al ogy is al most to tally re placed by antigorite, thoughcoarse rel ict pyroxene is seen in roadcuts along High way 3about 1.5 km south of the Phoe nix Ski Hill turn off. Talc andtalc-car bon ate are common in sheared margins ofserpentinite bodies.

Listwanite is com mon in the west ern part of the mapsheet along the Ket tle River val ley north of Rock Creek.This is a pale buff-white, crys tal line, car bon ate-rich rockthat weath ers to an or ange-brown col our. There may be asoapy feel on some sur faces. Si li ceous patches and veinsweather pos i tively and of ten show an ap ple green col ourdue to chlorite or mariposite(?). Late quartz veins con tainmi nor sulphide minerals.

METAMORPHIC UNIT

De formed and higher grade meta mor phosed rocks as -cribed to the Knob Hill Com plex (Fig. 5) oc cur in threemain belts. South of Rock Creek, the rocks com prise chert,quartz ite, micaceous quartz ite, greenstone, chlorite schist,lime stone and do lo mite. These are lithologically sim i lar tochert-argillite and greenstone se quences found to the northof Rock Creek, al though their struc tural re la tion ship is un -cer tain due to lack of outcrop.

A sec ond belt out crops in the south east ern part of themap sheet, be tween Bound ary Falls and Grand Forks, as a

thrust slice be tween the Mount Wright and No 7 faults (seeFig. 7). This belt in cludes chlorite and chlorite-am phi boleschists, quartz-bi o tite schist and phyllite, quartz-bi o titegneiss, quartz ite, chert, cal car e ous phyllite and crys tal linelime stone, mi nor greenstone and metagabbro.

The third belt of the meta mor phic unit ex tends fromMount Roderick Dhu west to Wind fall Creek. Meta mor -phic grade in creases northwestward, from the greenstoneand metachert found south of Jewel Lake into am phi bo lite,metagabbro, quartz ite, micaceous quartz ite, quartz-bi o titeschist, bi o tite-mus co vite schist, quartz-bi o tite and quartz-feld spar-bi o tite gneiss.

Lit tle (1983) re ferred to the lat ter two belts as a meta -mor phic as sem blage of pre-Car bon if er ous (pos si ble Pro -tero zoic?) age. Church (1986), Fyles (1990) and this au thor in clude them in the Knob Hill Com plex be cause of theirlithological sim i lar i ties to other mapped units in the KnobHill. How ever, the con trast in struc tural styles to theundeformed lower grade Knob Hill rocks is still enig maticand unexplained.

Attwood Formation

Rocks of the Att wood For ma tion are more re stricted inthe Green wood area than the Knob Hill Com plex, the mainout crop ar eas be ing around the north ern slopes of MountAtt wood, the slopes of Rusty Moun tain and Mount Mc -Laren, and in the ex treme south west ern cor ner of the map


(a) (b)

(c) (d)

Fig ure 4. Green wood Gab bro: a) typ i cal vari ably tex tured gab bro, with white fel sic vein net work (05NMA21-02; UTM Zone 11, 5436561N,353922E, NAD83); b) very coarse peg ma titic gab bro (05NMA02-03; UTM Zone 11, 5436880N, 385180E, NAD83); c) col our and grain-sizelay er ing in gab bro (05NMA02-11; UTM Zone 11, 5436808N, 385264E, NAD83); d) plagiogranite dike in vari ably tex tured gab bro(05NMA02-11; UTM Zone 11, 5436808N, 385264E, NAD83).

sheet. The term Att wood ‘Se ries’ was in tro duced by Daly(1912) and re fined by Lit tle (1983) to Att wood For ma tion.Al though the sta tus of the Att wood was raised to the grouplevel by Church (1986) and Fyles (1990), Lit tle’s def i ni tion is preferred here.

The Att wood For ma tion com prises an argillite-siltstone mem ber, a lime stone mem ber and pos si ble mi norvol ca nic rocks. In all out crop ar eas, the rocks are folded and the in ter nal stra tig ra phy of the for ma tion is un known.Strati graphic thick ness is also un known, though Lit tle(1983) es ti mated it to be at least 400 m, with ap par ent thick -ness greater than 1200 m.

Fos sil col lec tions from the Att wood For ma tion lime -stone have yielded macrofossils and cono donts of Car bon -if er ous to Perm ian age (Lit tle, 1983; Church, 1986;Templeman-Kluit, 1989a). A Mis sis sip pian age is sug -gested for cono donts from a lime stone along the Haul ageRoad (Or chard, 1997). No strati graphic con tact is foundwith the Knob Hill Com plex and it is un clear what theirorig i nal spa tial or tem po ral re la tion ships were. Mon ger etal. (1991) placed the Att wood, along with the Knob Hill, inthe Okanagan subterrane of Quesnellia. How ever, Nel sonet al. (1995) sug gested that the clastic rocks of the Att woodshow a greater af fin ity to the arc-de rived clastic sed i men -tary rocks of the Harper Ranch subterrane.

ARGILLITE-SILTSTONE

The argillite-siltstone unit con sists of fine-grainedclastic sed i men tary rocks, mostly black to grey argillite,lighter grey siltstone, dark grey to black chert and chertyargillite, and grey phyllite and slate. Lami na tions, thin bed -ding and graded beds are well de vel oped in places, but maybe ob scured by slaty cleav age and frac tures (Fig. 6). Mi norsand stone, chert con glom er ate and grey lime stone also oc -cur as interbeds.

Fine-grained dark grey siltstone or quartz wacke, withmi nor peb ble con glom er ate, crop out around Mount Mc -Laren. This unit was as sumed to be Ju ras sic by Lit tle(1983), but in cluded in the Att wood For ma tion by Church(1986) and Fyles (1990). Sim i lar argillite, in a sim i lar struc -tural po si tion in the footwall of the No 7 fault, is foundaround the Morn ing Star de posit in Wash ing ton, wherethey are cor re lated with the Lower Ju ras sic Archibald For -ma tion of the Rossland Group (Caron, pers. comm., 2005).

LIMESTONE

Lime stone is white to grey, fine to coarse-grained,vari ably recrystallized to saccharoidal mar ble. The lime -stone is gen er ally mas sive, but may show faint streak ing orlami na tions. Thin, pale green to white chert beds and blackargillite oc cur as in ter ca la tions. Lit tle (1983, page 8) alsore corded the pres ence of chert gran ule lime stone.Fossiliferous beds, oc cur ring at var i ous lo cal i ties, have


(a) (b)

(c) (d)

Fig ure 5. Rocks of the meta mor phic unit of the Knob Hill Com plex: a) bi o tite-quartz-feld spar gneiss, Wind fall Creek (05NMA20-02; UTMZone 11, 5430392N, 372191E, NAD83); b) metaquartzite (metachert?), High way 3, Eholt Creek val ley (05NMA26-12; UTM Zone 11,5443713N, 383281E, NAD83); c) fo li ated mar ble, High way 3, Bound ary Falls (05NMA05-01; UTM Zone 11, 5435265N, 376209E, NAD83);d) quartz-bi o tite gneiss, Mount Roderick Dhu (05NMA09-22; UTM Zone 11, 5450126N, 382554E, NAD83).

yielded cor als, brachi o pods, bryo zoans and crinoids(Little, 1983).

VOLCANIC UNITS

A large pro por tion of the greenstone and meta vol can icrocks in the Green wood area were as signed by Church(1986) to the Att wood Group [sic], though he later re as -signed some to the Knob Hill (Dostal et al., 2001). Fyles(1990), how ever, rec og nized some of the re main ing ‘Att -wood’ meta vol can ic rocks as ac tu ally be long ing to the Tri -as sic Brook lyn For ma tion, and in cluded the rest of the ba -salt in the Knob Hill, fol low ing the ear lier suggestion ofLittle (1983).

How ever, a sliver of vol ca nic rocks, ap par ently in theim me di ate footwall of the Lind Creek fault (see Fig. 7 forlo ca tion of faults), may be in the Att wood For ma tion(Fyles, 1990). Out crops in this unit along the Haul age Road are fine-grained, mas sive apha ni tic greenstone es sen tiallyin dis tin guish able from the Knob Hill greenstone. Con tactswith lime stone and argillite to the south were not found, and the sta tus of this greenstone is pres ently un clear. Pet ro -chem is try may help re veal whether or not it is akin to theKnob Hill greenstone.

Or ange to buff-col oured mus co vite schist and buffsilty tuff are interbedded with grey argillite and chertysiltstone within the Att wood For ma tion in the Myers Creekarea, in the south west cor ner of the map sheet. These are be -

lieved to be fel sic tuff ho ri zons, although sim i lar rocks arenot seen else where in the map area.

STRUCTURAL RELATIONSHIP OFPALEOZOIC UNITS

The Pa leo zoic rocks of the Green wood area are pre -served in a se ries of five north ward-dip ping thrust sheets(Fig. 7). Many of the bound ing thrusts are marked by ser -pen tin ite lay ers or pods. Ter tiary extensional fault ing hasdis rupted and mod i fied the thrust sheets and makes cor re la -tion un cer tain be tween the west ern and east ern halves ofthe Green wood map sheet. De tailed de scrip tions of thethrust sheets have been pro vided by Fyles (1990) and only a brief sum mary is given here. The sheets are in for mallynum bered from south to north, struc tur ally from bottom totop.

In the east ern part of the map area, sheet 1 strad dles theCan ada – United States bor der, be ing bounded to the northby the No 7 fault and to the south by the White Moun tainfault (Stoffel et al., 1991). It com prises prob a ble Att woodFor ma tion sed i men tary rocks over lain by Tri as sic Brook -lyn For ma tion vol ca nic rocks. In the west, a sim i lar sheet ofAtt wood For ma tion rocks, bounded to the north by theMyers Creek fault, is here cor re lated with thrust sheet 1.

Sheet 2 is bounded by the No 7 fault as footwall and the North Mount Wright fault as hangingwall. Sev eral other


(a) (b)

(c) (d)

Fig ure 6. Att wood For ma tion rock types: a) slaty argillite-siltstone interbeds, Skomac mine (05NMA18-14; UTM Zone 11, 5435668N,375126E, NAD83); b) graded sand stone-siltstone beds, Myers Creek area (05NMA 24-11; UTM Zone 11, 5430982N, 354727E, NAD83); c)lime stone bed above argillite, Haul age Road (05NMA 03-10; UTM Zone 11, 5434200N, 384961E, NAD83); d) chert brec cia, Skomac mine(05NMA 18-10; UTM Zone 11, 5435660N, 375038E, NAD83).

mapped thrusts also oc cur in ter nal to this sheet. The sheetcom prises Knob Hill Com plex meta mor phic rocks with mi -nor Brook lyn For ma tion. The Knob Hill Com plex meta -mor phic rocks south of Rock Creek, and above the MyersCreek fault, are here cor re lated with sheet 2. Thehangingwall thrust is not ex posed but is pos tu lated basedon the con trast in meta mor phism and de for ma tion with thegab bro, greenstone and chert to the north of Rock Creek.

Sheets 3 and 4 only oc cur east of Green wood. Sheet 3com prises mostly Brook lyn For ma tion strata un con form -able on Att wood For ma tion, bounded by the North MountWright and Mount Att wood faults. Sheet 4 sim i larly con -sists of Att wood and Brook lyn For ma tions, but is bro keninto sev eral frag ments by north erly trending Ter tiary faults. The footwall thrust is the Mount Wright fault in the westand the Mount Att wood and Ea gle Moun tain faults to theeast. The Lind Creek fault is the hangingwall thrust.

Sheet 5 con tains the bulk of the Knob Hill Com plexrocks in the map area, with un con form ably over ly ingBrook lyn For ma tion. Knob Hill Com plex out crops north of the Ket tle River in the west ern half of the map are part ofthis thrust sheet, al though sep a rated from the east ern areaby Ter tiary faults. The inlier of dom i nantly Brook lyn For -ma tion rocks north of West Mid way is also ten ta tively in -cluded in sheet 5, though it may belong to sheet 1.

Chert-argillite unit out crops along the north ern side ofHigh way 3 in the Eholt Creek val ley are much higher in

meta mor phic grade and more de formed than Knob Hillunits to the south on Mon te zuma Ridge, and are com pa ra -ble to the rocks of the meta mor phic unit. Such a sharp con -trast of meta mor phic grade may sug gest a ma jor struc turealong the Eholt Creek val ley, per haps an other thrust. How -ever, the Knob Hill units south east of Jewel Lake are lowergrade and ap pear to show a more grad ual tran si tion north -ward into the higher grade meta mor phic rocks of MountRoderick Dhu and Wind fall Creek. With out more con vinc -ing ev i dence, all these units are con sid ered to probably bepart of thrust sheet 5.

The age of the thrust struc tures is still un cer tain. Clastsof nearly all Pa leo zoic rock types, in clud ing ser pen tin ite,are found in the Ladinian Brook lyn For ma tion con glom er -ate, in di cat ing up lift and jux ta po si tion of the Pa leo zoicstrata by the Mid dle Tri as sic. How ever, the in volve ment ofBrook lyn For ma tion strata in the thrust sheets shows thatthe main thrust ing event oc curred, or con tin ued, into post-Ladinian time. Diorite and granodiorite stocks of the Nel -son suite in trude the thrust sheets, pro vid ing a min i mumage for thrusting of 164–169 Ma (Lambert, 1989).

Church (1992) re ported a dis cor dant U-Pb zir con ageof 199.4 ±1.4 Ma from the Lexington por phyry. This is pos -si bly in tru sive into the No 7 fault (see be low), sug gest ingthat thrust ing took place prior to Ju ras sic time. The Chesawfault in Wash ing ton is in ter preted to be the con tin u a tion ofthe Myers Creek fault (Cheney et al., 1994). It is ap par ently


No 7 F

LCF

LCF

EMFMAF

LCF

MCF

MW

F

GR

F

TMF

NM

WF

U S A

Greenwood

Grand Forks

Midway

Rock Creek

X

X

X

X

Grand Forks Gneiss Complex

Paleozoic Sequences

Fault

Normal Fault

Detachment Fault

Thrust Fault

3Pre-Jurassic Thrust Sheet

33

22

22

11

4444

55

55

55

55

55

22

11

?5?5

55

?5?5

118°30'119°00'

118°30'119°00'

49

°15

'4

9°0

0'

49

°15

'4

9°0

0'

0 10 km

Fig ure 7. Main struc tural el e ments of the Green wood area, out lin ing the five ma jor thrust sheets (num bered 1–5) that con tain the Pa leo zoicse quences (mod i fied from Fyles, 1990). Ab bre vi a tions: GRF, Granby River fault; LCF, Lind Creek fault; MAF, Mount Att wood fault; MCF,Myers Creek fault; MWF, Mount Wright fault; NMWF, North Mount Wright fault; No 7 F, No 7 fault; TMF, Thim ble Moun tain fault.

cut by the Loomis pluton, which has yielded a dis cor dantK-Ar hornblende age of 194 ±6 Ma (Rinehart and Fox,1972; Stoffel, 1990). How ever, the Chesaw fault also cutsthe Ellemehan For ma tion, which has been cor re lated withthe Sinemurian to Pliensbachian Elise For ma tion of Brit ishCo lum bia (Read and Okulitch, 1977; Stoffel et al., 1991),though lacking paleontological support.

LEXINGTON PORPHYRY REVISITED

The Lexington min ing camp com prises up to elevenCu-Au por phyry de pos its in Brit ish Co lum bia and Wash -ing ton, hosted by the Lexington por phyry within the No 7fault zone. Min er al iza tion stretches in a lin ear belt from theNo 7 mine in the north west to the Lone Star mine, in Wash -ing ton, in the south east. Ex plo ra tion and de vel op ment inthis camp started in 1890 and con tin ues to this day. Im por -tant re views of min er al iza tion and his tor i cal de vel op mentwere pro vided by Church (1970, 1986) and Ser a phim et al.(1995).

The No 7 fault is marked by a thick ser pen tin ite sheetalong its length from McCarren Creek to the United Statesbor der. Along Goosmus Creek, the ser pen tin ite is ap par -ently split in two by the in tru sion of a quartz por phyry bodyknown as the Lexington por phyry (Church, 1992). Thequartz-feld spar por phyry body in the footwall of the No 7fault near the con flu ence of McCarren and Gidon creekshas been cor re lated with the Lexington quartz por phyry.The quartz-feld spar por phyry is, how ever, marked by largepink K-feld spar pheno crysts that are ap par ently ab sentfrom the type Lexington por phyry, which raises doubtsa b o u t t h e i r c o r r e l a t i o n . U r a n i u m- l e a d z i r c o ngeochronometry and pet ro chem is try are be ing conductedto help resolve this issue.

Sev eral au thors have mapped ar eas of andesitic todacitic volcaniclastic rocks around some of the mines in theLexington camp (e.g., Church 1986, Fig. 9) and equiv a lents in Wash ing ton (e.g., Morn ing Star de posit; Caron, pers.comm., 2005). Dacitic volcaniclastic rocks that host min er -al iza tion are also re ported by com pany ge ol o gists indrillcore from the area (But ler, 1997; Cow ley, 2004). Ser a -phim et al. (1995), how ever, as serted that these units atLexington are not orig i nal vol ca nic rocks but the re sult ofcataclasis of the por phyry within the fault zone, al thoughthey do al low for “the pres ence of xe no liths and screens ofold vol ca nic rocks in the in tru sion.” On go ing pet ro log i caland pet ro chem i cal studies are directed at this problem.

The rec og ni tion of these units as true vol ca nic rockscould have sig nif i cant im pli ca tions, not only for re gionalcor re la tion and tec tonic his tory but also for the pos si ble po -ten tial for volcanogenic mas sive sul phide (VMS) de pos its.Sim i lar fel sic volcaniclastic rocks are not found in the im -me di ate map area and cor re la tive equiv a lents are not eas ilyrec og nized in the bound ary re gion gen er ally. Pos si ble fine-grained buff tuff was found in the Att wood For ma tion in the Myers Creek area (see above), but no coarser lapilli tuffwas found. If the volcaniclastic rocks are co eval with the200 Ma Lexington por phyry, it may sug gest that they arecor re la tive with the Lower Ju ras sic Elise For ma tion of theRossland Group. Church (1992) draws a com par i son be -tween the Lexington por phyry and fel sic in tru sions of sim i -lar age in the Nel son area (Dunne and Höy, 1992). Fel sicvol ca nic rocks over ly ing the stratabound Lamefoot Cu-Aude posit have yielded an Ar/Ar pla teau age of 195 Ma onseri cite (Ras mus sen, 2000) and have been cor re lated with

the Elise For ma tion, as have fel sic rocks at the Morn ingStar (Caron, pers. comm., 2005). How ever, in the Rosslandarea it self, the Elise For ma tion com prises mafic to andesitic vol ca nic rocks and lacks any sig nif i cant fel sic rocks (Höyand An drew, 1989; Andrew et al., 1990).

ACKNOWLEDGMENTS

The au thor is much in debted to Jim Fyles and NeilChurch for all their in valu able help and ad vice dur ing theplan ning stages of this pro ject and since. The col lab o ra tion, sup port and ready wel come of the Bound ary Dis trict ex plo -ra tion com mu nity were ex cep tional and, in par tic u lar,many thanks are ex tended to Bruce Laird of Merit Min ingCorp. for his co-op er a tion and en thu si as tic as sis tance insam pling the Lexington rocks. The au thor also ac knowl -edges the in dis pens able as sis tance of Ashley Dittmar dur -ing field work and Sean Mullen in the of fice. Com ments onan early ver sion of this paper by JoAnne Nelson were muchappreciated.

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