Boundary Project: Reassessment of Paleozoic Rock Units of the Greenwood Area (NTS 82E/02), Southern British Columbia

Transcription

1 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 United States bor der. These oc cur within the Quesnel Terrane, which is dom i nated by such Pa leo zoic mafic vol ca nic and pelitic 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 ary Dis 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 200, field - work was con cen trated in the Green wood Rock Creek area (NTS 082E/02). In fu ture years, field work will prog - ress into the Osoyoos (NTS 082E/03) and Keremeos (NTS 082E/04) map sheets. The Green wood map sheet strad dles High way 3 be - tween Osoyoos and Grand Forks. Five weeks of field work in 200 were re stricted to ar eas im me di ately around and east of Green wood, and to the area be tween Mid way and Rock Creek (see Fig. 7 for lo ca tions). This in cludes most of the area mapped by Fyles (1989, 1990), upon which this study 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 wood sheet (Massey, work in prog ress); col lect ing geo chem i cal sam ples from vol ca nic rocks 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 for paleontological 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 WORK The 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 This publication is also available, free of charge, as colour digital files in Adobe Acrobat PDF format from the BC Ministry of Energy, Mines and Petroleum Resources internet website at catfldwk.htm 1860s and lode de pos its in the 1880s. The first geo log i cal re port was that of Bauerman (188), pre pared as part of the Bound ary Com mis sion Ex pe di tion of Re - gional map ping has been un der taken by Brock (1902, 1903, 190a, b), Daly (1912), Lit tle (197, 1983), Mon ger (1968), Church (1986), Templeman-Kluit (1989a, b) and Fyles (1990). De tailed map ping around in di vid ual mines and min ing camps in cludes work by LeRoy (1912, 1913), McNaughton (194), Ser a phim (196), Carswell (197), Church (1970) and Nixon (2002). PALEOZOIC STRATIGRAPHY OF THE GREENWOOD 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. Called the Knob Hill Group by pre vi ous work ers, the name is changed here to Knob Hill Com plex, in line with mod ern ac 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 and Salvador, 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 rupted ophiolite (Lit tle, 1983; Dostal et al., 2001). It was in cluded in the Okanagan subterrane of Quesnellia by Mon ger et al. (1991), along with other as sem blages of oce anic af fin ity such as the Kobau and Old Tom Shoe maker Groups. The strati 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 of metres, whereas Fyles (1990, 199) es ti mated the complex to be more than 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 by Lit 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 rock age of 28 ±10 Ma (Perm ian) for uralitized gab bro from the Win ni peg mine. How ever, the re li abil ity of such K-Ar whole 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 com plex awaits fur ther paleontological and geochronological determinations. Seven map units are rec og nized within the Knob Hill Com 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 ite pass north ward, and prob a bly up ward, into greenstone, Geo log i cal Field work 200, Pa per

2 119 30' E ' E ' E Tertiary Penticton Group Paleozoic Sequences Penticton? Detachment Fault Normal Fault Thrust Fault? 49 30' N 49 30' N Beaverdell Fault 0 00' N 0 00' N Metamorphic Core Complexes? Keremeos?Oliver Greenwood? 0 Grand Forks? Osoyoos 10 km USA ' E ' E ' E 49 00' N 49 00' N? Figure 1. Distribution of Paleozoic Quesnellian rock suites in the Boundary District (south-central part of NTS 082E), amended from Massey et al. (200). mixed greenstone and chert, and finally chert and argillite. In the Greenwood area, Fyles (1990) suggested that the lower part of the Knob Hill Complex consisted of a highly variable mix of chert, argillite, conglomerate and breccia interfingering with greenstone flows. The upper part of the complex is mainly chert and greenstone. Due to the limited fieldwork undertaken in the area by the author, the following descriptions of the units draw heavily upon previous observations by Church (1986) and Fyles (1990). GREENSTONE UNIT The basaltic greenstone is mostly flows with minor breccia, agglomerate and tuffaceous sandstone. Chert, cherty argillite, jasper and rare limestone may occur between flows. Most flows are grey to green, massive and aphanitic. Chlorite, calcite and epidote are common alteration minerals in veins, fractures and within the rock. Pillow structures are rare, though they have been inferred by previous mappers (Fyles, 1990). Geochemically, the flows of the greenstone unit are oceanic basalt (Dostal et al., 2001) and may have formed at a mid-ocean ridge or in a back-arc basin. Medium-grained diabase to microgabbro bodies also occur within the greenstone unit. In general, these are featureless rock units in outcrop. However, outcrops along the power line adjacent to the Winnipeg mine show well-developed chilled margins, suggesting that they may be part of an 100 unmapped sheeted dike complex (Fig. 2a). Dikes are also seen intruding the nearby gabbro (Fig. 2c). CHERT-ARGILLITE UNIT Sedimentary rocks in the Knob Hill Complex are typically fine grained, mainly grey to white chert interbedded with grey to black argillite and minor greenstone. The chert is highly fractured and jointed, tends to be massive to thickly bedded and only rarely shows ribbon structures. Radiolarians are reported in thin section (Church, 1986) but are rarely seen in hand specimen. Variable recrystallization o f t h e c h e r t p r o d u c e s a f in e t o me d i u m- g r a in e d saccharoidal texture. Argillite is rarely bedded and may show gradational contacts with chert horizons. Thin white crystalline limestone forms occasional outcrops. MIXED GREENSTONE AND CHERT UNIT In the western part of the map sheet, between upper Nichol son Creek and Ket tle River, a belt of mixed greenstone and chert in roughly equal proportions forms a transition between the greenstone unit to the south and the chert-argillite unit to the north (Fig. 3). Rock types are similar to those found in the other two units. CHERT BRECCIA Two areas of buff to cream and grey chert breccia and pebble conglomerate were outlined by Fyles (1990). They contain angular to subrounded chert clasts similar to the British Columbia Geological Survey

3 (a) (b) (c) (d) Fig ure 2: Diabase dikes in the Knob Hill Com plex: a) chilled con tact be tween diabase dikes (0NMA02-08; UTM Zone 11, N, 38389E, NAD83); b) gab bro screen be tween two diabase dikes (0NMA02-09; UTM Zone 11, N, 3837E, NAD83); c) diabase in trud ing gab bro body (0NMA02-09; UTM Zone 11, N, 3837E, NAD83); d) diabase in clu sion within gab bro (0NMA02-10; UTM Zone 11, N, 38333E, NAD83). chert of the chert-argillite unit. They lack the va ri ety of other clast types, such as greenstone, ser pen tin ite, jas per, and lime stone, found in the Tri as sic Brook lyn For ma tion sharpstone con glom er ate. Black siltstone and chert sand - stone oc cur as interbedded lenses. Sim i lar chert brec cia also 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 probably correlative. 2001) and are prob a bly comagmatic with the greenstone flows. SERPENTINITE Ser pen tin ite oc curs dom i nantly as fairly con tin u ous sheets along faults, or as ir reg u lar len tic u lar bod ies that are com monly spa tially as so ci ated with gab bro bod ies. The 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 with mod ern strati graphic no men cla ture. The gab bro is com posed of white plagioclase and 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 sive and 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 liths are in cluded in the gabbro (Fig. 2d). Green wood Gab bro sam ples are geochemically com pa ra ble to oce anic plutonic rocks (Dostal et al., Fig ure 3. Interbedded chert (c) and greenstone flows (b) of the Knob Hill Com plex, east side of the Ket tle River val ley (0NMA23-08; UTM Zone 11, N, 37292E, NAD83). Geo log i cal Field work 200, Pa per

4 (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 (0NMA21-02; UTM Zone 11, 43661N, 33922E, NAD83); b) very coarse peg ma titic gab bro (0NMA02-03; UTM Zone 11, N, 38180E, NAD83); c) col our and grain-size lay er ing in gab bro (0NMA02-11; UTM Zone 11, N, 38264E, NAD83); d) plagiogranite dike in vari ably tex tured gab bro (0NMA02-11; UTM Zone 11, N, 38264E, NAD83). ser pen tin ite is usu ally dark green to black and schis tose to blocky, with greasy frac ture coat ings. Fo li ated ser pen tin ite may 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, though coarse rel ict pyroxene is seen in roadcuts along High way 3 about 1. km south of the Phoe nix Ski Hill turn off. Talc and talc-car bon ate are common in sheared margins of serpentinite bodies. Listwanite is com mon in the west ern part of the map sheet along the Ket tle River val ley north of Rock Creek. This is a pale buff-white, crys tal line, car bon ate-rich rock that weath ers to an or ange-brown col our. There may be a soapy feel on some sur faces. Si li ceous patches and veins weather pos i tively and of ten show an ap ple green col our due to chlorite or mariposite(?). Late quartz veins con tain mi nor sulphide minerals. METAMORPHIC UNIT De formed and higher grade meta mor phosed rocks as - cribed to the Knob Hill Com plex (Fig. ) oc cur in three main 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 to chert-argillite and greenstone se quences found to the north of 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 the map sheet, be tween Bound ary Falls and Grand Forks, as a thrust slice be tween the Mount Wright and No 7 faults (see Fig. 7). This belt in cludes chlorite and chlorite-am phi bole schists, quartz-bi o tite schist and phyllite, quartz-bi o tite gneiss, quartz ite, chert, cal car e ous phyllite and crys tal line lime stone, mi nor greenstone and metagabbro. The third belt of the meta mor phic unit ex tends from Mount Roderick Dhu west to Wind fall Creek. Meta mor - phic grade in creases northwestward, from the greenstone and metachert found south of Jewel Lake into am phi bo lite, metagabbro, quartz ite, micaceous quartz ite, quartz-bi o tite schist, bi o tite-mus co vite schist, quartz-bi o tite and quartzfeld 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 their lithological sim i lar i ties to other mapped units in the Knob Hill. How ever, the con trast in struc tural styles to the undeformed lower grade Knob Hill rocks is still enig matic and unexplained. Attwood Formation Rocks of the Att wood For ma tion are more re stricted in the Green wood area than the Knob Hill Com plex, the main out crop ar eas be ing around the north ern slopes of Mount Att wood, the slopes of Rusty Moun tain and Mount Mc - Laren, and in the ex treme south west ern cor ner of the map 102 British Columbia Geological Survey

5 (a) (b) (c) (d) Fig ure. Rocks of the meta mor phic unit of the Knob Hill Com plex: a) bi o tite-quartz-feld spar gneiss, Wind fall Creek (0NMA20-02; UTM Zone 11, N, E, NAD83); b) metaquartzite (metachert?), High way 3, Eholt Creek val ley (0NMA26-12; UTM Zone 11, N, E, NAD83); c) fo li ated mar ble, High way 3, Bound ary Falls (0NMA0-01; UTM Zone 11, 4326N, E, NAD83); d) quartz-bi o tite gneiss, Mount Roderick Dhu (0NMA09-22; UTM Zone 11, 40126N, 3824E, 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 group level 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 argillitesiltstone member, a lime stone member and pos si ble mi nor vol 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 age Road (Or chard, 1997). No strati graphic con tact is found with the Knob Hill Com plex and it is un clear what their orig i nal spa tial or tem po ral re la tion ships were. Mon ger et al. (1991) placed the Att wood, along with the Knob Hill, in the Okanagan subterrane of Quesnellia. How ever, Nel son et al. (199) sug gested that the clastic rocks of the Att wood show 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-grained clastic sed i men tary rocks, mostly black to grey argillite, lighter grey siltstone, dark grey to black chert and cherty argillite, and grey phyllite and slate. Lami na tions, thin bed - ding and graded beds are well de vel oped in places, but may be ob scured by slaty cleav age and frac tures (Fig. 6). Mi nor sand stone, chert con glom er ate and grey lime stone also oc - cur as interbeds. Fine-grained dark grey siltstone or quartz wacke, with mi 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 found around the Morn ing Star de posit in Wash ing ton, where they are cor re lated with the Lower Ju ras sic Archibald For - ma tion of the Rossland Group (Caron, pers. comm., 200). 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 or lami na tions. Thin, pale green to white chert beds and black argillite oc cur as in ter ca la tions. Lit tle (1983, page 8) also re corded the pres ence of chert gran ule lime stone. Fossiliferous beds, oc cur ring at var i ous lo cal i ties, have Geo log i cal Field work 200, Pa per

6 (a) (b) (c) (d) Fig ure 6. Att wood For ma tion rock types: a) slaty argillite-siltstone interbeds, Skomac mine (0NMA18-14; UTM Zone 11, 43668N, 37126E, NAD83); b) graded sand stone-siltstone beds, Myers Creek area (0NMA 24-11; UTM Zone 11, N, 34727E, NAD83); c) lime stone bed above argillite, Haul age Road (0NMA 03-10; UTM Zone 11, N, E, NAD83); d) chert brec cia, Skomac mine (0NMA 18-10; UTM Zone 11, 43660N, 37038E, 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 ic rocks 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 of Little (1983). How ever, a sliver of vol ca nic rocks, ap par ently in the im me di ate footwall of the Lind Creek fault (see Fig. 7 for lo 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 tially in dis tin guish able from the Knob Hill greenstone. Con tacts with 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 the Knob Hill greenstone. Or ange to buff-col oured mus co vite schist and buff silty tuff are interbedded with grey argillite and cherty siltstone within the Att wood For ma tion in the Myers Creek area, 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 are not seen else where in the map area. STRUCTURAL RELATIONSHIP OF PALEOZOIC 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 has dis 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 of the Green wood map sheet. De tailed de scrip tions of the thrust sheets have been pro vided by Fyles (1990) and only a brief sum mary is given here. The sheets are in for mally num bered from south to north, struc tur ally from bottom to top. In the east ern part of the map area, sheet 1 strad dles the Can ada United States bor der, be ing bounded to the north by the No 7 fault and to the south by the White Moun tain fault (Stoffel et al., 1991). It com prises prob a ble Att wood For 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 of Att wood For ma tion rocks, bounded to the north by the Myers 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 104 British Columbia Geological Survey

7 119 00' ' 49 1' 49 00' Rock Creek X 1 2 MCF Fault Paleozoic Sequences Grand Forks Gneiss Complex Normal Fault 3 Detachment Fault Thrust Fault Pre-Jurassic Thrust Sheet? Midway X Greenwood U S A X 2? MWF No 7 F 1 4 LCF MAF NMWF 0 10 km 3 2 LCF 4 TMF EMF LCF GRF Grand Forks X 49 1' 49 00' ' ' 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 ) that con tain the Pa leo zoic se 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. mapped thrusts also oc cur in ter nal to this sheet. The sheet com 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 Myers Creek fault, are here cor re lated with sheet 2. The hangingwall thrust is not ex posed but is pos tu lated based on the con trast in meta mor phism and de for ma tion with the gab bro, greenstone and chert to the north of Rock Creek. Sheets 3 and 4 only oc cur east of Green wood. Sheet 3 com prises mostly Brook lyn For ma tion strata un con form - able on Att wood For ma tion, bounded by the North Mount Wright and Mount Att wood faults. Sheet 4 sim i larly con - sists of Att wood and Brook lyn For ma tions, but is bro ken into sev eral frag ments by north erly trending Ter tiary faults. The footwall thrust is the Mount Wright fault in the west and the Mount Att wood and Ea gle Moun tain faults to the east. The Lind Creek fault is the hangingwall thrust. Sheet con tains the bulk of the Knob Hill Com plex rocks in the map area, with un con form ably over ly ing Brook 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 of this thrust sheet, al though sep a rated from the east ern area by 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, though it may belong to sheet 1. Chert-argillite unit out crops along the north ern side of High way 3 in the Eholt Creek val ley are much higher in meta mor phic grade and more de formed than Knob Hill units 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 ture along the Eholt Creek val ley, per haps an other thrust. How - ever, the Knob Hill units south east of Jewel Lake are lower grade and ap pear to show a more grad ual tran si tion north - ward into the higher grade meta mor phic rocks of Mount Roderick Dhu and Wind fall Creek. With out more con vinc - ing ev i dence, all these units are con sid ered to probably be part of thrust sheet. The age of the thrust struc tures is still un cer tain. Clasts of 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 zoic strata by the Mid dle Tri as sic. How ever, the in volve ment of Brook lyn For ma tion strata in the thrust sheets shows that the 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 mum age for thrusting of Ma (Lambert, 1989). Church (1992) re ported a dis cor dant U-Pb zir con age of ±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 ing that thrust ing took place prior to Ju ras sic time. The Chesaw fault in Wash ing ton is in ter preted to be the con tin u a tion of the Myers Creek fault (Cheney et al., 1994). It is ap par ently Geo log i cal Field work 200, Pa per

8 cut by the Loomis pluton, which has yielded a dis cor dant K-Ar hornblende age of 194 ±6 Ma (Rinehart and Fox, 1972; Stoffel, 1990). How ever, the Chesaw fault also cuts the Ellemehan For ma tion, which has been cor re lated with the Sinemurian to Pliensbachian Elise For ma tion of Brit ish Co 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 eleven Cu-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 7 fault zone. Min er al iza tion stretches in a lin ear belt from the No 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 in this 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 ment were pro vided by Church (1970, 1986) and Ser a phim et al. (199). The No 7 fault is marked by a thick ser pen tin ite sheet along its length from McCarren Creek to the United States bor 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 body known as the Lexington por phyry (Church, 1992). The quartz-feld spar por phyry body in the footwall of the No 7 fault near the con flu ence of McCarren and Gidon creeks has been cor re lated with the Lexington quartz por phyry. The quartz-feld spar por phyry is, how ever, marked by large pink K-feld spar pheno crysts that are ap par ently ab sent from the type Lexington por phyry, which raises doubts about their cor re la tion. Ura nium-lead zir con geochronometry and pet ro chem is try are be ing conducted to help resolve this issue. Sev eral au thors have mapped ar eas of andesitic to dacitic volcaniclastic rocks around some of the mines in the Lexington 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., 200). Dacitic volcaniclastic rocks that host min er - al iza tion are also re ported by com pany ge ol o gists in drillcore from the area (But ler, 1997; Cow ley, 2004). Ser a - phim et al. (199), how ever, as serted that these units at Lexington are not orig i nal vol ca nic rocks but the re sult of cataclasis of the por phyry within the fault zone, al though they do al low for the pres ence of xe no liths and screens of old vol ca nic rocks in the in tru sion. On go ing pet ro log i cal and pet ro chem i cal studies are directed at this problem. The rec og ni tion of these units as true vol ca nic rocks could have sig nif i cant im pli ca tions, not only for re gional cor 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 ily rec og nized in the bound ary re gion gen er ally. Pos si ble finegrained buff tuff was found in the Att wood For ma tion in the Myers Creek area (see above), but no coarser lapilli tuff was found. If the volcaniclastic rocks are co eval with the 200 Ma Lexington por phyry, it may sug gest that they are cor re la tive with the Lower Ju ras sic Elise For ma tion of the Rossland 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 sic vol ca nic rocks over ly ing the stratabound Lamefoot Cu-Au de posit have yielded an Ar/Ar pla teau age of 19 Ma on seri 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 ing Star (Caron, pers. comm., 200). How ever, in the Rossland area 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 (Höy and An drew, 1989; Andrew et al., 1990). ACKNOWLEDGMENTS The au thor is much in debted to Jim Fyles and Neil Church for all their in valu able help and ad vice dur ing the plan 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 ing Corp. for his co-op er a tion and en thu si as tic as sis tance in sam 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 on an early ver sion of this paper by JoAnne Nelson were much appreciated. REFERENCES An drew, K.P.E., Höy, T and Drobe, J. (1990): Stra tig ra phy and tec - tonic set ting of the Archibald and Elise for ma tions, Rossland Group, Bea ver Creek area, south east ern Brit ish Columbia (82F4E); in Geological Fieldwork 1989, BC Min - is try of En ergy, Mines and Pe tro leum Re sources, Pa per , pages Bauerman, H. (188): Ge ol ogy of the coun try near the Forty-ninth Par al lel of North Lat i tude, west of the Rocky Moun tains; Geological Survey of Canada, Re port of Prog ress, , pages B 42B. Brock, R.W. (1902): The Bound ary Creek dis trict, Brit ish Co lum - bia; Geological Survey of Canada, Sum mary Re port 1901, Vol ume XIV, pages 1A 69A. Brock, R.W. 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