Eskay Rift Project (NTS 103O, P, 104A, B, G, H),Northwestern British Columbia

By D.J. Alldrick

KEYWORDS: re gional map ping, Eskay mine, Iskut, Eskayrift, lithogeochemistry, Hazelton Group, Bow ser Ba sin,eco nomic ge ol ogy.

INTRODUCTION

Lo cated in north west ern Brit ish Co lum bia, the EskayRift is the geo log i cal set ting for the world’s rich estvolcanogenic mas sive sul phide de posit: the Eskay Creekgold-sil ver mine (Fig. 1). The rift struc ture was first rec og -nized by Geo log i cal Sur vey of Can ada ge ol o gists (An der -son and Thorkelson, 1990), and they ex tended the struc turesouth ward in 1998 (Evenchick et al., 1998; Evenchick andMcNicoll, 2002).

The BC Geo log i cal Sur vey be gan de tailed map ping ofthe north ern half of the rift in 2003, com bined withlithogeochemical stud ies at Dalhousie Uni ver sity. A se riesof re ports (Alldrick et al., 2005a, b; Barresi et al., 2005;Barresi and Dostal, 2005; Alldrick et al., 2004a, b) and col -oured maps (Alldrick et al., 2006a, b; Alldrick et al., 2005a, b; Alldrick et al., 2004a, b) pres ent the field and lab o ra torywork completed to date.

REGIONAL GEOLOGY

The Eskay Rift is a fault-bounded ba sin host ing thickac cu mu la tions of bi modal ba salt and rhy o lite flows, within ter ca lated sed i men tary rocks. Rift strata re cord the fi nalerup tive events of the Early to Mid dle Ju ras sic HazeltonGroup, an is land-arc com plex that ex tends from the Yu konto Wash ing ton state. The rift hosts 60 VMS de pos its andpros pects, in clud ing two new VMS pros pects discovered in outcrop in 2004.

2005 FIELD PROGRAM

This year’s field pro gram con tin ued strati graphic in -ves ti ga tions within the rift (Mount Shir ley, Sixpack Range,Todagin Pla teau, Klastline Pla teau, Mount Madge, Arc ticGrid); re con nais sance in ves ti ga tion of Up per Hazeltonstrata be yond the Eskay Rift, where rift-in di ca tor fea tureshad been noted (Treaty Gla cier, Tim Wil liams Gla cier,Oweegee Dome, Bait Range, Mount Dilworth); and ex am i -na tion of pros pects and gos sans (Ta ble Moun tain, Smitty,Knipple Gla cier, Tim Wil liams Gla cier, Bait North). Sevensites of eco nomic in ter est de serve fol low-up: a py ritic

siltstone mem ber within the Eskay-age ba salt in the cen tralBait Range; the east ern strati graphic ex ten sions of theRAM pros pect; the Timbit gos san east of Cas tle Rock; a re -cently ex posed dacite cryptodome within py ritic SalmonRiver siltstone on Mount Dilworth; py ritic car bon atelaminae with sand stone and siltstone mem bers be tween ba -salt flows north east of the toe of Tim Wil liams gla cier; theZinc Moss – Dome – Com pass – Griz trend of pros pects onTa ble Moun tain; and py ritic rhyolite with minorintercalated basalt on northwestern Oweegee Dome.

Ar eas iden ti fied for in ves ti ga tions next sea son in cludethe fol low ing:

• Up per Hazelton stra tig ra phy north and east ofSmithers

• An un mapped area with polymetallic pros pectsnorth east of the Red Chris prop erty and west of theKlappan River

• Rhy o lite and pil low ba salt within the Georgie Riverpen dant

• Py ritic dacite in the up per Kitsault River val ley• The set ting of the VMS pros pects at Sen eca, west of

Har ri son Lake• Eskay-equiv a lent sed i men tary strata out lined by

Souther on the east ern flank of Mount Edziza• The Eskay mine – Mount Madge cor ri dor• The area sur round ing the Dome pros pect on Ta ble

Moun tain

SUMMARY

The Eskay Rift is pre served as a nar row, dis con tin u -ous, 250 km long belt ex tend ing from Klastline Pla teau inthe north to Anyox in the south (Fig. 1; Alldrick, 2000).There are 25 in di vid ual seg ments of these rift-fill-se quence rocks pre served. De tailed map ping along the north ern halfof the rift shows that most seg ments have gen er ally sim i larlithological char ac ter is tics, but pro por tions of key rocktypes within and be tween rift seg ments vary widely. Somerift-fill se quences were clearly de pos ited in iso la tion fromthose of ad ja cent rift seg ments, sug gest ing that they oc cu -pied nearby but un con nected bas ins. Paleowater depth var -ied be tween these dif fer ent rift seg ments, rang ing fromsubaerial, to shal low to mod er ate wa ter depth (0–500 m), to deep-wa ter ocean floor set tings (>1000 m). As so ci atedVMS deposits reflect these varying water depths.

Exhalitive min eral de pos its and pros pects are knownin three rift seg ments: Eskay Creek, Anyox and Ta bleMoun tain. The Eskay Creek and Anyox rift seg ments hostpro duc ing and past-pro duc ing mines; the sev eral exhalitive pros pects in the Ta ble Moun tain rift seg ment were re centlydis cov ered, have lim ited out crop ex po sure and have so far

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

This publication is also available, free of charge, as colourdigital files in Adobe Acrobat PDF format from the BC Ministryof Energy, Mines and Petroleum Resources internet website athttp://www.em.gov.bc.ca/Mining/Geolsurv/Publications/catalog/catfldwk.htm

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

Fig ure 1. Dis tri bu tion of Eskay Rift strata in north west ern Brit ish Co lum bia.

yielded subeconomic as says. These exhalitive de pos its andpros pects de vel oped at a range of wa ter depths, ac count ingfor pro nounced dif fer ences in min er al ogy and metal as so ci -a tions, and re quir ing dif fer ent ex plo ra tion strat e gies. TheGeorgie River and Down pour Creek rift seg ments have had lim ited ex plo ra tion work to date and de serve fur ther in ves -ti ga tion due to their fa vour able lithological as so ci a tionsand prom i nent fel sic vol ca nic cen tres. The five small, re -mote rift seg ments be tween Frank Mackie Icefield and theGranduc mine were last mapped and explored in the mid-1960s.

The Eskay Rift has been re garded as the fi nal, brief(176–174 Ma) ep i sode of Hazelton Arc his tory, but newmap ping and dates from the Sixpack Range sug gest that theon set of rift ing may have started in early Hazelton time(191 Ma). In this par tic u lar rift seg ment, rift-fill strata restdi rectly on meta mor phosed Pa leo zoic substrate.

The Eskay Rift re cords a ma jor Early to Mid dle Ju ras -sic crustal break that ex tended from a subaerial set tingwithin is land-arc rocks at its north ern end to a near-con ti -nent, mid-ocean-ridge set ting at its south ern end. Anal o -gous set tings are pre served in Tri as sic arc rocks of thesouth east ern Alaska pan han dle and along the east-trendingarm of the Great African Rift.

REFERENCES

Alldrick, D.J. (2000): Ge ol ogy of the Anyox Min ing camp; BCMin is try of En ergy, Mines and Pe tro leum Re sources, OpenFile Map 2000-28, Scale 1:20 000.

Alldrick, D.J., Nel son, J.L., Barresi, T., Stew art, M.L. andSimpson, K.A. (2006a): Ge ol ogy of up per Iskut River area,north west ern Brit ish Co lum bia; BC Min is try of En ergy andMines, Open File Map 2006-2, Scale 1:100 000.

Alldrick, D.J., Nel son, J.L. and Barresi, T. (2006b): Ge ol ogy of the Vol cano Creek – More Creek area, north west ern Brit ish Co -lum bia; BC Min is try of En ergy and Mines, Open File Map2006-6, Scale 1:50 000.

Alldrick, D.J., Nel son, J.L. and Barresi, T. (2005a): Ge ol ogy of the More Creek – Palmiere Creek area, north west ern Brit ishCo lum bia; BC Min is try of En ergy, Mines and Pe tro leum Re -sources, Open File Map 2005-5, scale 1:50 000.

Alldrick, D.J., Nel son, J.L. and Barresi, T. (2005b): Track ing theEskay Rift through north ern Brit ish Co lum bia: ge ol ogy andmin eral oc cur rences of the up per Iskut River area; in Geo -

log i cal Field work 2004, BC Min is try of En ergy, Mines andPe tro leum Re sources, Pa per 2005-1, pages 1–30.

Alldrick, D.J., Nel son, J.L., Barresi, T., Stew art, M.L. andSimpson, K.A. (2006b): Ge ol ogy of up per Iskut River area,north west ern Brit ish Co lum bia; BC Min is try of En ergy,Mines and Pe tro leum Re sources, Open File Map 2006-2,scale 1:100 000.

Alldrick, D.J., Stew art, M.L., Nel son, J.L. and Simpson, K.A.(2004a): Ge ol ogy and min eral oc cur rences of the up perIskut River area: track ing the Eskay Rift through north ernBrit ish Co lum bia; in Geo log i cal Field work 2003, BC Min is -try of En ergy, Mines and Pe tro leum Re sources, Pa per 2004-1, pages 1–18.

Alldrick, D.J., Stew art, M.L., Nel son, J.L. and Simpson, K.A.(2004b): Ge ol ogy of the More Creek – Kinaskan Lake area,north west ern Brit ish Co lum bia; BC Min is try of En ergy,Mines and Pe tro leum Re sources, Open File Map 2004-2,scale 1:50 000.

An der son, R.G. and Thorkelson, D.J. (1990): Me so zoic stra tig ra -phy and set ting for some min eral de pos its in Iskut River map area, north west ern Brit ish Co lum bia; in Cur rent Re search,Part E, Geo log i cal Sur vey of Can ada, Pa per 90-1E, pages131–139.

Barresi, T. and Dostal, J. (2005): Geo chem is try and pe trog ra phy of Up per Hazelton Group volcanics: VHMS fa vour able stra -tig ra phy in the Iskut River and Tele graph Creek map ar eas,north west ern Brit ish Co lum bia; in Geo log i cal Field work2004, BC Min is try of En ergy, Mines and Pe tro leum Re -sources, Pa per 2005-1, pages 39–48.

Barresi, T., Nel son, J.L., Alldrick, D.J. and Dostal, J. (2005): Pil -low Ba salt Ridge fa cies: de tailed map ping of Eskay Creekequiv a lent stra tig ra phy in north west ern Brit ish Co lum bia;in Geo log i cal Field work 2004, BC Min is try of En ergy,Mines and Pe tro leum Re sources, Pa per 2005-1, pages 31–38.

Evenchick C.A. and McNicoll, V.J. (2002): Stra tig ra phy, struc tureand geo chron ol ogy of the Anyox pen dant, north west Brit ishCo lum bia, and im pli ca tions for min eral ex plo ra tion; Ca na -dian Jour nal of Earth Sci ences, Vol ume 39, pages 1313–1332.

Evenchick C.A., McNicoll V.J., Holm K., Alldrick D.J. andSnyder L.D. (1998): Ge ol ogy of the Ob ser va tory In let area;Geo log i cal Sur vey of Can ada, Open File Map 3454, scale1:50 000.

Souther, J.G. (1992): The Late Ce no zoic Mount Edziza Vol ca nicCom plex, Brit ish Co lum bia. Geo log i cal Sur vey of Can ada,Mem oir 420, 319 pages.

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