Mineralization at the Highland Valley Cu-Mo Deposit (NTS 092I), Southwestern British Columbia

Transcription

1 40 Ar/ 39 Ar and Re-O Isotopic Ages for Hydrothermal Alteration and Related Mineralization at the Highland Valley Cu-Mo Deposit (NTS 092I), Southwestern British Columbia by C.H. Ash, P.H. Reynolds 1, R.A. Creaser 2 and M.G. Mihalynuk KEYWORDS: Quesnellia, High land Val ley de posit, Cu-Mo por phyry, geo chron ol ogy, 40 Ar/ 39 Ar, Re-Os INTRODUCTION The High land Val ley Cu (Mo, Ag, Au) de posit is lo - cated roughly 200 km north east of Van cou ver and 75 km south west of Kamloops, in south ern Brit ish Co lum bia (Fig 1). It is the larg est op er at ing base metal mine in Can - ada. Pro duc tion be gan in 1962 from sev eral sep a rate mines, which were amal gam ated in 1986 to form High land Val ley Cop per, a sub sid iary of Teck Cominco Lim ited. Re corded metal pro duc tion from the High land Val ley de posit to 2005 ex ceeds 3.33 bil lion ki lo grams of cop per (av er age grade of 0.43%), 44 mil lion ki lo grams of mo lyb de num (av er age grade of 0.03%), 1 bil lion grams of silver and 7 million grams of gold. The age of min er al iza tion at High land Val ley has been in ferred, on the ba sis of its syngenetic por phyry char ac ter, to be more or less co eval with the Late Tri as sic mag matic his tory of the host Guichon Creek batholith (Osatenko and Jones, 1976; Casselman et al., 1995). Ura nium-lead zir con dat ing of the batholith sug gests an age within the range 213 to 207 Ma (210 ±3 Ma; Mortimer et al., 1990; Fig 1). How - ever, con ven tional K-Ar iso to pic anal y sis of seri cite from the ore min eral as sem blage has yielded ages that are sig nif i - cantly youn ger than the crys tal li za tion age of the in tru sion. For ex am ple, a 202 ±8 Ma (re cal cu lated with mod ern de cay con stants from data re ported in Jones, 1975; cf. Breitsprecher and Mortensen, 2004) and 198 ±12 Ma (re - cal cu lated with mod ern de cay con stants from data re ported in Blanchflower, 1971; cf. Breitsprecher and Mortensen, 2004), and ±4 Ma (re cal cu lated with mod ern de cay con stants from data re ported in Wanless et al., 1973; cf. Breitsprecher and Mortensen, 2004). At the lower limit of er ror, all three of these ages are youn ger than the 200 Ma base of the Ju ras sic ep och. At their up per limit of er ror, the first two anal y ses are both 210 Ma (Late Tri as sic), equiv a - lent to the best crys tal li za tion age for the batholith 1 De part ment of Earth Sci ences, Dalhousie Uni ver sity, Hal i fax, NS B3H 3J5 2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3 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 website at cat_fldwk.htm (Mortimer et al., 1990). The Wanless et al. (1973) date remains Early Jurassic at its upper limit of error (195.3 Ma). Ore min eral as sem blages were resampled with the aim of sub ject ing them to more pre cise geo chron ol ogi cal dat ing tech niques. Sam ples were col lected by the first au thor dur - ing a three-day field visit to the Val ley pit in Seri cite and bi o tite were col lected for 40 Ar/ 39 Ar age de ter mi na tion, and mo lyb de nite was col lected for ap pli ca tion of the Re-Os chro nom e ter. If suc cess ful, the new dat ing would elim i nate the am bi gu ity raised by the over lap ping mag matic and hy - dro ther mal ages from the de posit that ex isted at the time. This field ex am i na tion was in tended as re con nais sance for a de tailed map ping pro ject of the High land Val ley de posit. Un for tu nately, the pro ject did not pro ceed be yond this pre - lim i nary stage and geological relationships between the units dated were not established. SAMPLE DESCRIPTIONS Three dis tinct min eral as sem blages were col lected from the 1025 m level of the Val ley pit (Fig 2). Sam ple CAS99-HV1 is mo lyb de nite from a mas sive, 8 to 10 cm wide, steeply dip ping vein on the south east ern cor ner of the pit along the bench wall. The mo lyb de nite vein cut dark to light grey bi o tite-quartz diorite in which quartz is coarse grained rel a tive to the me dium-grained ma trix feld spar and bi o tite. This unit is rep re sen ta tive of the Bethsaida phase of the Guichon Creek batholith (McMillan, 1978). Sam ple CAS99-HV2 con sists of me dium-grained seri - cite from Cu-Mo-min er al ized quartz stockwork col lected from a well-washed por tion of the min ing bench. Seri cite con sti tutes up to sev eral per cent of the dark grey quartz vein ma te rial, but is mostly con cen trated as en ve lopes along the vein mar gins. Quartz veins at this lo cal ity range from 2 to 10 mm thick and form a stockwork that makes up be tween 5 and 15% of the bench ex po sures. The plutonic hostrock in this area is min er al og i cally and tex tur ally con - sis tent with that host ing the mas sive molybdenite vein described above. The third dated sam ple is of bi o tite col lected from a tex tur ally dis tinc tive plutonic phase oc cur ring near the cen - tre of the pit. In con trast to the typ i cal Bethsaida phase, this rock is much more leucocratic. It has a coarse to very coarse grained, white, quartzofeldspathic ma trix with very coarse (2 4 cm), euhedral, black bi o tite books. It con tains no ob - serv able sul phide min er al iza tion. Con tacts with the Bethsaida phase were not established. Geo log i cal Field work 2006, Pa per

2 Fig ure 1. Lo ca tion and gen er al ized ge ol ogy of the Guichon batholith. Also shown is the dis tri bu tion of geo chron ol ogi cal data from the batholith and the lo ca tion of the High land Val ley mine, the col lec tion site of the sam ples an a lyzed for this study. Ma jor units and geo log i - cal con tacts are from the com pi la tion of Massey et al. (2005); geo chron ol ogi cal data are from the com pi la tion of Breitsprecher and Mortensen (2004; un re li able data omit ted). 20 British Columbia Geological Survey

3 Fig ure 2. Val ley pit, in di cat ing lo ca tions of the in di vid ual sam ples dated. Geo log i cal Field work 2006, Pa per

4 TABLE 1. RE-OS ISOTOPIC DATA FOR AGES FOR MOLYBDENITE, VALLEY PIT, HIGHLAND VALLEY CU-MO DEPOSIT. TABLE AR- 39 AR ANALYTICAL DATA FOR MICAS, VALLEY PIT, HIGHLAND VALLEY CU-MO DEPOSIT. 22 British Columbia Geological Survey

5 RE-OS DATING OF MOLYBDENITE Two sep a rate frac tions of the Val ley pit mo lyb de nite sam ple (CAS99-HV1a and b) were an a lyzed at the Uni ver - sity of Al berta Ra dio genic Iso tope Fa cil ity in Ed mon ton, un der the su per vi sion of R.A. Creaser. Meth ods used for mo lyb de nite anal y sis are de scribed in de tail by Selby and Creaser (2001a). The 187 Re and 187 Os con cen tra tions in mo - lyb de nite were de ter mined by iso tope di lu tion mass spec - trom e try us ing Carius-tube, sol vent ex trac tion, an ion chro - ma tog ra phy and neg a tive ther mal ion iza tion mass spec trom e try tech niques. Iso to pic anal y sis was made us ing a Micromass Sec tor 54 mass spec trom e ter by Far a day col - lec tor. To tal pro ce dural blanks for Re and Os are less than 15 and 2 pg (<25 fg 187 Os), re spec tively. These pro ce dural blanks are in sig nif i cant in com par i son to the Re and Os con cen tra tions in mo lyb de nite. The Chi nese mo lyb de nite pow der HLP-5 (Markey et al., 1998), which is used as an in-house con trol sam ple by AIRIE, Col o rado State Uni - ver sity, is also rou tinely an a lyzed at the Uni ver sity of Al - berta. This con trol sam ple yielded an av er age Re-Os date of ±0.5 Ma (±0.45% at the 2σ level, n = 17). This Re- Os age is iden ti cal to a ±1 Ma (±0.4 % at the 1σ level, n = 19) age de ter mined by al ka line fu sion (Markey et al., 1998) and two Carius-tube ages of ±0.7 and ±0.8 Ma (H. Stein, pers comm, 2000). Re-Os Results The an a lyt i cal data and cal cu lated ages for both the ini - tial and re peat anal y ses of mo lyb de nite sam ple CAS99- HV1 (a and b) are shown in Ta ble 1. Sam ple CAS99-HV1a yielded an ini tial cal cu lated age of ±1.5 Ma. This is slightly older than the re peat anal y sis age of ±1.5 Ma, but is equiv a lent within the stated limits of error. 40AR-39AR DATING OF MICAS Two dif fer ent mica sam ples, one of seri cite (CAS99- HV2) from Cu-Mo-bear ing quartz stockwork and a sec ond of coarse, euhedral mag matic bi o tite (CAS99-HV3), were an a lyzed at Dalhousie Uni ver sity by the con ven tional 40 Ar/ 39 Ar step-heat ing method un der the su per vi sion of P.H. Reynolds. In di vid ual grains were hand-picked from each of the sam ples. These grains were wrapped in alu mi num foil and in ter spersed with three to five aliquots of the flux mon i tor (the hornblende stan dard MMhb-1, with an as sumed age of 520 ±2 Ma; Sam son and Al ex an der, 1987). The en tire pack age was ir ra di ated in the McMaster Uni ver sity nuclear reactor. An in ter nal tan ta lum re sis tance fur nace of the dou ble-vac uum type was used to carry out the stepheat ing. All iso to pic anal y ses were made in a VG 3600 mass spec trom e ter. 40Ar/39Ar Results The re sults of the seri cite (CAS99-HV2) and mag matic bi o tite (CAS99-HV3) anal y ses are given in Ta bles 2a and b. The seri cite sam ple pro duced a uni form re lease spec trum, as shown in Fig ure 3a. A pla teau age of 204 ±2 Ma is de fined by 10 steps rep - re sent ing ap prox i mately 90% of the to tal gas re - leased (age un cer tainty given is at the 95% con fi - Age (Ma) AGE (Ma) (a) AGE (Ma) (B) Ma dence limit and in cludes the un cer tainty in the irradiation parameter, J). The spec tral plot for the bi o tite sam ple is shown in Fig - ure 3b. A pla teau age of 207 ±2 Ma is de fined by eight steps rep re sent ing ap prox i mately 90% of the to tal gas re leased (age un cer tainty given is at the 95% con fi dence limit and in cludes the un cer tainty in the ir ra di a tion parameter, J). DISCUSSION CAS99-HV2 Sericite % 39 Ar Released CAS99-HV3 Biotite Ma % 39 Ar Released Fig ure Ar/ 39 Ar age spec tra plots for a) hy dro ther mal seri cite from Cu-Mo-min er al ized quartz stockwork (CAS99-HV1), and b) very coarse grained euhedral bi o tite from leucocratic quartz diorite of the Guichon Creek batholith (CAS99-HV2). Rhe nium-os mium dat ing lab o ra to ries con tinue to dem on strate the re mark able ro bust ness of the Re-Os geochronometer. Not only can it with stand post-ore hy dro - ther mal meta mor phism (Selby and Creaser, 2001b), but also granulite-fa cies tem per a tures (Bingen and Stein, 2002). There fore, the Re-Os iso to pic age de ter mined from the mo lyb de nite veins should re cord their age of for ma tion, not the age of some later dis tur bance. The age for mo lyb de - num min er al iza tion at the High land Val ley de posit is in dis - tin guish able from the mag matic crys tal li za tion age based Early Jurassic Late Triassic CAS97-HV1b 204±2 CAS97-HV1b 207±2 Ar-Ar sericite K-Ar sericite Ar-Ar biotite Re-Os molybdenite U-Pb zircon CAS97-HV1b 205.8±1.5 CAS97-HV1a 206.7± ±3 Guichon Batholith (Jones, 1975) 202±4 (Mortimer et al., 1975) Fig ure 4. Sum mary of se lected iso to pic ages re ported for the High land Val - ley Cu-Mo de posit. Er ror bars in di cate 2σ un cer tainty. Geo log i cal Field work 2006, Pa per

6 upon the U-Pb zir con data of Mortimer et al. (1990; 210 ±3 Ma). Both are Late Tri as sic, with er ror en ve lopes over lap - ping be tween 207 and Ma (Fig 4). How ever, at the outer er ror lim its, mo lyb de nite min er al iza tion could be as much as 8.7 m.y. youn ger than the crys tal li za tion age. The age at which the crys tal lized body cooled through the bi o - tite clo sure tem per a ture (~350 C for a rap idly cool ing body; Har ri son et al., 1985) is re corded by the mag matic bi - o tite sam ple as 207 ±2 Ma. Er ror lim its per mit a range of in - ter pre ta tions of the time re quired to cool through the bi o tite clo sure tem per a ture: from in stan ta neous to ap prox i mately 8 m.y. Clearly, more pre cise geochronometric anal y ses are re quired be fore a de fin i tive his tory of mag matic crys tal li - za tion and min er al iza tion can be con structed. In par tic u lar, a pre cise crys tal li za tion age for the Bethsaida phase, which hosts the High land Val ley de posit, needs to be de ter mined. It is the youn gest of four ma jor in tru sive pulses, whereas the crys tal li za tion age de ter mi na tion of Mortimer et al. (1990) is from a sample of the Highland Valley phase, the second oldest major phase within the Guichon Creek batholith. When con sid ered in iso la tion, these new age data con - strain the tim ing of min er al iza tion within the Val ley pit to within 3 m.y., be tween 205 and 207 Ma (age range in - creases to 6.2 m.y. at the outer lim its of the er ror en ve lopes). The cal cu lated ages for each of the newly dated min er als ap pear to fol low a se quence that is con sis tent with the ob - served tex tural and cross cut ting re la tion ships. Mag matic bi o tite gives the old est cal cu lated cool ing age (207 ±2 Ma). Mo lyb de nite from veins that cross cut the plutonic host is next in the se quence (206.7 ±1.5 Ma, ±1.5 Ma). Seri - cite is in ferred to have formed lat est in the mag matic his tory of the batholith, and re turned the youngest age determination (204 ±2 Ma). Min er al iza tion at the High land Val ley mine, herein dated as be tween ca. 204 and ca. 207 Ma, can now be con - sid ered as nearly con tem po ra ne ous with the ca. 204 Ma min er al iza tion in the Iron Mask batholith (60 km to the east), as re ported by Lo gan et al. (2007). ACKNOWLEDGMENTS The au thors ex press their ap pre ci a tion to both Alan Gal ley (Geo log i cal Sur vey of Can ada, Ot tawa) and Bill McMillan (pre vi ously with Brit ish Co lum bia Geo log i cal Sur vey, now re tired) for their en thu si as tic in sights dur ing the three-day field visit to the High land Val ley mine. Loren Bond, with High land Val ley Cop per at the time of the field visit, is gra ciously ac knowl edged for pro vid ing a tour of the Val ley pit that en abled di rect ob ser va tion and sam pling of ore-grade ma te rial for iso to pic dat ing. Com ple tion of this pa per would not have been pos si ble with out the ef forts of Ron Graden with High land Val ley Cop per in providing digital pit maps and related information. REFERENCES Bingen, B. and Stein, H.J. (2002): Mo lyb de nite Re-Os dat ing of bi o tite de hy dra tion melt ing: the Rogaland granu lites, S Nor - way; 12th An nual V.M. Goldschimdt Conference, Davos, Swit zer land, page A78. Blanchflower, J.D. (1971): Iso to pic dat ing of cop per min er al iza - tion at Alwin and Val ley prop er ties, High land Val ley, Brit ish Co lum bia; un pub lished BSc. the sis, Uni ver sity of Brit ish Co lum bia, Van cou ver, BC. Breitsprecher, K. and Mortensen, J.K. (2004): BC Age 2004A-1: a database of isotopic age determinations for rock units from British Columbia; BC Min is try of En ergy, Mines and Pe tro - leum Re sources, Open File , URL < ing/geolsurv/pub li ca tions/ OpenFiles/OF /toc.htm> [December 2006]. Casselman, M.J., McMillan, W.J. and Newman, K.M. (1995): High land Val ley por phyry cop per de posit near Kamloops, Brit ish Co lum bia: a re view and up date with em pha sis on the Val ley de posit; in Porphyry Deposits of the Northwestern Cor dil lera of North Amer ica, Canadian Institute of Mining, Metallurgy and Petroleum, Spe cial Vol ume 46, pages Har ri son, T.M., Duncan, I. and McDougall, I. (1985): Dif fu sion of 40 Ar in bi o tite: tem per a ture, pres sure and compositional ef - fects; Geochimica et Cosmochimica Acta, vol ume 49, pages Jones, M.B. (1975): Hydrothermal alteration and mineralization of the Val ley Cop per de posit, High land Val ley, BC; un pub - lished Ph.D. the sis, Oregon State University, Corvallis, Or e - gon. Lo gan, J.M., Mihalynuk M.G., Ullrich, T and Fried man, R.M. (2007): U-Pb ages of in tru sive rocks and 40 Ar/ 39 Ar pla teau ages of cop per-gold-sil ver min er al iza tion as so ci ated with al ka line in tru sive cen tres at Mount Polley and the Iron Mask batholith, south ern and cen tral Brit ish Co lum bia; in Geo log - ical Fieldwork, 2006, BC Min is try of En ergy, Mines and Pe - tro leum Re sources, Pa per and Geoscience BC, Re port , pages Markey, R.J., Stein, H.J. and Mor gan, J.W. (1998): Highly pre cise Re-Os dat ing of mo lyb de nite us ing al ka line fu sion and NTIMS; Talanta, vol ume 45, pages Massey, N.W.D., Mac In tyre, D.G., Desjardins, P.J. and Cooney, R.T. (2005): Dig i tal ge ol ogy map of Brit ish Co lum bia: whole prov ince; BC Min is try En ergy, Mines and Pe tro leum Re sources, GeoFile , URL < ing/geolsurv/pub li ca tions/ GeoFiles/Gf2005-1/toc.htm> [June 2006]. McMillan, W.J. (1978): Ge ol ogy of the Guichon Creek Batholith (NTS 92I); BC Min is try of En ergy Mines and Pe tro leum Re - sources, notes and leg end to ac com pany Pre lim i nary Map 30, 17 pages. Mortimer, N., Van Der Heyden, P. Armstrong, R.L. and Harakal, J. (1990): U-Pb and K-Ar dates re lated to tim ing of magmatism and de for ma tion in the Cache Creek Terrane and Quesnellia, southern British Columbia; Canadian Journal of Earth Sci ences, vol ume 27, pages 117 to 123. Osatenko, M.J. and Jones, M.B. 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(1996): Re-Os iso - tope con straints on the age of Group IIA, IIIA, IVA, and IVB iron me te or ites; Sci ence, vol ume 271, pages Wanless, R.K., Stevens, R.D., Lachance, G.R. and Delabio, R.N. (1973): Age determinations and geological studies, Ar-Ar and K-Ar iso to pic ages, Re port 11; Geo log i cal Sur vey of Canada, Paper British Columbia Geological Survey