Lead Isotope Evidence on the Genesis of the Silver-Arsenide Vein Deposits of the Cobalt and Great Bear Lake Areas, Canada

Econom•: (7eoloœy Vol. 69. 1974. pp. 777-791 Lead Isotope Evidence on the Genesis of the Silver- Arsenide Vein Deposits of the Cobalt and Great Bear Lake Areas, Canada RALPH THORPE Abstract Silver veins of Proterozoic age in the Cobalt area, Ontario, are associated with the thick Nipissing diabase sheet, and those at Great Bear Lake, N.W.T., are also, in part, associated with a diabase sheet. Host rocks in the Cobalt area are flat-lying Aphebian sedimentary rocks, steeply dipping Archean volcanic and associated sedimentary rocks, and diabase, while host rocks in the Great Bear Lake area are intermediate and felsic 'volcanic rocks and tuffs of the Echo Bay group. Model-lead ages for ordinary galenas are about 1,630 and 2,280 m.y. for the Great Bear Lake and Cobalt veins, respectively, whereas the pitchblende U-Pb age for the Great Bear Lake veins is about 1,445 m.y., and the apparent Rb-Sr age for the Cobalt veins is about 2,160 m.y. An Rb-Sr isochron age of 1,425 ----- 48 m.y. has been obtained for the diabase sill in the Port Radium area, Great Bear Lake. In the Cobalt area essentially identical lead isotope compositions were obtained for interflow base metal mineralization in Archean volcanic rocks, galena along bedding in sedimentary rocks of the Cobalt group, and some late-stage sulfide veins. One interflo•v sample with a more primitive composition has a model age of at least 2,805 m.y. Mineralization, including deposition of most of the lead in Archean interflow beds, is interpreted to have taken place at about 2,160 m.y. and to be genetically related to the Nipissing diabase. In addition, two anomalous lead lines are defined, one of which sug- gests an age of 3,260 ----_ 100 m.y. for Archean volcanic rocks. A shallow line, represented in only late-stage sulfide veins, is difficult to interpret but could be due to the late addition of a radiogenic component during Paleozoic or younger events not normally considered as causing mineralization, or even from ground water. Leads from the Great Bear Lake area could represent a single anomalous lead line with a slope of 0.1085 ___ 0.0070, and thus have a maximum possible age of 1,130 m.y. Geological events younger than this are known in the Bear Province, but not in the vicinity of the veins, and the same problems of interpretation exist as for the Cobalt area. The source of lead is not defined in either area. However, a source with homogeneous lead is required and the lack of extensive wall-rock alteration suggests that the lead was not leached from the country rocks. The diabase is tentatively favored as the source, but a connate brine or other source is possible. Introduction Lv.^x> isotope compositions have been determined for galenas from silver veins in the Great Bear Lake area, Northwest Territories, and in the Cobalt area, Ontario. Silver veins in the two areas are similar in their composition, mineralogical zoning, Proterozoic age, and, at least in part, in their association with diabase sills. The veins in the Great Bear Lake area differ in that they contain pitchblende and more abundant native bismuth and copper and apparently formed at a lower temperature. Preliminary data for the Great Bear Lake area showed a great difference between the model-lead age for galena and the U-Pb age for pitchblende (Jory, 1964) from the same veins. At first the model-lead age was accepted and the difference in ages attributed to updating of the pitchblende (Thorpe, 1971). More high-precision lead isotope analyses were obtained subsequently on anomalous leads to resolve this problem and obtain evidence on the genesis of the veins. Kanasewich and Farquhar (1965) presented lead isotope analyses for eleven specimens from the im- mediate Cobalt area. These data outlined a general pattern confirmed by the present study. However, Kanasewich and Farquhar also included analyses from gold and massive sulfide deposits from the Val d'Or, Noranda, Kirkland Lake, Matachewan, and Timmins areas on their plots. An origin by mixing of two ordinary leads was suggested for galenas with 777 778 RALPH THORPE compositions lying between an Archcan cluster and a Cobalt cluster. The authors did not, however, present convincing geological explanations for either the development and preservation of these ordinary leads or the proposed mixing mechanism. These lead isotope data show that the silver veins in the Great Bear Lake and Cobalt areas contain both ordinary and anomalous leads, and it was thought that (1) the ordinary leads could help de- fine the age of the veins and thus restrict the possible interpretations for the anomalous leads, (2) ano- malous lead from veins of Precambrian age might be easier to interpret than those from younger veins, and (3) the close genetic relationship postulated be- tween the veins and the Nipissing diabase sheet in the Cobalt area suggested a possible mantle origin of the Cobalt lead. Geologic Setting Great Bear Lake The silver-arsenide veins of the Great Bear Lake area (Fig. 1) occur within the Bear structural prov- o 5 i I I MILES = e/••' • 118"30' FiG. 1. I.ocation map for properties in the Great Bear Lake area. ince (Stockwell, 1961), a distinct metallogenic prov- ince characterized by deposits of uranium, copper, and the silver-arsenic-cobalt-nickel-bismuth assem- blage, and by the absence of significant gold mineral- ization. The Echo Bay group of volcanic and sedi- mentary rocks of Aphebian age are the host rocks to the silver veins. They were intruded by granitic magma during the Hudsoriian orogeny approximately 1,800 m.y. ago (Stockwell, 1973). The Echo Bay group consists predominantly of intermediate and felsic lava flows and volcaniclastic rocks. The Echo Bay group in the Port Radium area has been subdivided into a lower subgroup of tuffaceous and sedimentary rocks and an upper sub- group of andesitic volcanic rocks (Robinson, 1971; Robinson and Morton, 1972). Volcanic rocks of the Echo Bay group belong to an alkali-rich calc-alkaline suite (Badham, 1973). In the Camsell River area a sequence of tuffs with alternating red and green beds is overlain by a se- quence of waterlaid volcaniclastic sedimentary de- posits. At the Terra mine a copper-rich sulfide body of possible syngenetic origin occurs at the contact between these sequences. Cobalt area The Cobalt area lies within the Superior structural province in which the last major orogenic event, the Kenoran orogeny, ended about 2,560 m.y. ago (Stockwell, 1973). The main Cobalt area near Lake Timiskaming lies about 18 to 20 miles from the Grenville Front, but the South Lorrain district with similar mineralization is near the Grenville-Superior boundary. The Gowganda area lies about 55 miles northwest of Cobalt. The following account of the geology is summarized from a number of papers, primarily by Petruk (1971a, b, and c) and Jambor (1971a, b, and c). Archcan rocks (>2,560 m.y.; Stockwell, 1973) in the Cobalt area consist predominantly of massive, locally pillowed, intermediate to mafic volcanic rocks with minor intercalated cherty sedimentary and pyro- clastic rocks. Some of the cherty rocks are min- eralized with sulfides. The volcanic sequence is overlain by Archcan sedimentary deposits, principally conglomerate, graywacke, and slate. These Archcan rocks were highly folded, probably during the Keno- ran orogeny, and underwent low-grade meta- morphism to the greenschist facies. Also during the Kenoran orogeny granitic plutons intruded the strati.- fled Archcan rocks followed by intrusion of lampro- phyre and diabase dikes. In the lower Aphebian (HuronJan) sediments of the Cobalt group were deposited with marked angu- lar unconformity on the Archcan rocks. Early sedi- mentation was confined to basins, troughs, and valleys SILVER-ARSENIDE VEIN DEPOSITS 779 but later spread to blanket all of the Archean rocks. Near Cobalt the Gowganda formation forms the lower part of the Cobalt group and is subdivided into the older Coleman member, predominantly con- glomerate with some deposits of argillite, quartzite, and arkose, and the younger Firstbrook member of well-bedded graywacke and argillite. The Gowganda formation is believed to be of glacial origin. For the most part the attitudes of Cobalt group sedimentary rocks have been very little changed by the later events. Thick sheets of Nipissing diabase intrude the Co- balt group and older rocks. Surfaces of the sheets are undulating, forming a series of basins and domes. The diabase was emplaced without regard for the Archean-Proterozoic unconformity; in places a sheet may lie entirely within the Cobalt group, whereas elsewhere it may lie entirely within the Archcan basement. In the Cobalt area the combined thickness of diabase sheets ranges from about 1,1t30 feet to less than 200 feet. A few dikes of quartz diabase, and rarely olivine diabase, were emplaced after the Nipiss- ing diabase. The Nipissing diabase has not been folded. Comparison of Mineral Deposits Vein deposits Veins in both areas consist of native silver and Co-Ni-Fe arsenides, with variable amounts of as- sociated sulfides, in a gangue that is predominantly carbonate. Veins in the Great Bear Lake area con- tain, in addition, important native bismuth and pitch- blende. Copper is also abundant in the Great Bear Lake veins and constitutes an important by-product. Veins in the Cobalt-Gowganda area occur within 700 feet above or below the contact of the thick Nipissing diabase sheets (Petruk, 1971a). In almost all cases where veins occur within the dia- base they are either within the upper or lower parts of a sheet and only rarely are they within the central part. Commercial deposits are related to basin and dome configurations assumed by the diabase sheet during intrusion (Petruk, 1971a). In the Great Bear Lake area the Eldorado and Echo Bay deposits at Port Radium (Fig. 1) are in close proximity to a diabase sill about 150 feet thick. However, in the Camsell River area, about 30 miles to the south, the veins of the Terra, Norex, and Silver Bay properties are not related to such diabase sills. Petruk (1971b) has found that large veins in the Cobalt area commonly show a distinct zoning of the arsenide minerals relative to the center of the diabase sheet. Ni-As and Ni-Co-As assemblages are present within or near the diabase, succeeded outward by Co-As and Co-Fe-As assemblages, and finally by the Fe-As assemblage. However, in some veins varia- tions and reversals of this zoning pattern are en- countered. Small veins are generally not zoned, and unzoned veins in most cases consist of the Co- Fe-As assemblage. In layered veins where several assemblages are present, the sequence of deposition parallels the zoning, with nickel arsenides deposited first. The same sequence of arsenide deposition and zoning has been documented by Shegelski (1973) for the Terra mine in the Great Bear Lake area. The temperature of vein formation in the Great Bear Lake area appears to have been much lower than in the Cobalt area. The arsenide stage of min- eralization in the Cobalt area was deposited in the range of 500 ø to 600øC (Petruk, 1971c), whereas the nickel-arsenide phase of mineralization in the Terra mine took place at 180 ø to 370øC (Shelgelski, 1973), and a maximum temperature of about 200øC has been suggested by Robinson and Ohmoto (1973) for mineralization at the Echo Bay mine. In the Cobalt area the veins are largely restricted to relatively small fractures, and it is unusual for veins to be located along major faults (Jambor, 1971c). In the Port Radium area, Great Bear Lake, the silver-arsenide veins are in major northeast- striking faults and their subsidiary breaks. The vein systems at Port Radium are thus quite persistent along strike, although the ore shoots are more re- stricted features within the veins. However, in the Camsell River area the silver-arsenide veins do not occupy a single system of fractures, and it seems likely that there the vein fractures are controlled locally (Shegelski, 1973). Other deposits The present lead isotope study extends in both areas to mineralization other than the silver-arsenide veins. A conformable, copper-rich, pyritic layer at the Terra mine, which is crosscut by the silver- arsenide veins, contains minor lead and zinc, lies in calcareous tuffs adjacent to beds of chert, and is underlain by a thin unit of siliceous hematitic iron- formation (Shegelski, 1973). Because this associa- tion is suggestive of a volcanic exhalative origin, the isotopic composition of galena from the pyritic layer was investigated. A sample was also analyzed from a galena vein (not silver-bearing) on Clut Island, about six miles east-southeast of the Norex property. Two types of mineralization in the Cobalt area were studied in addition to the silver-arsenide veins. Galena, apparently syngenetic (Petruk, 1971c), is present locally in association with pyrrhotite, chal- copyrite, and sphalerite in interflow sedimentary beds in the Archean volcanic sequence. Within the Silver- fields mine (Fig. 2) thin layers of sphalerite and/or galena are present along bedding planes in the Co- 780 RALPH THORPE miles •,/2-85e I •'obalt •hl-I,•/ ß VEIN 65 •'••DEER 47o22 ' - T LODE ß 79040 ' Fro. 2. Location map'for samples from the Cobalt area. balt group graywackes not far above Archean base- ment rocks. Geochronology Great Bear Lake--country rocks The majority of the K-Ar dates obtained by the Geochronology Laboratory of the Geological Survey of Canada for rocks in the Bear Province fall in the range 1,700 to 1;870 m.y. These dates largely represent plutonic rocks intruded during the Hud- sonjan orogeny and updated rocks of greater true age. In the Port Radium area Jory (1964) obtained a discordia chord, U-P.b zircon age of 1,820 ñ 30 m.y. for granodiorite exposed within the Eldorado mine and on Granite Island. Robinson (1971) obtained a whole-rock Rb-Sr isochron age of 1,770 ñ 30 m.y. (at one standard deviation with X87Rb -- 1.39 x 10-XX/yr.) for the volcanic rocks of the Echo Bay group. The isochron is strongly controlled by a single sample and, if this sample is ignored, Robin- son (1971, p. 51) noted that the isochron for the other six data points with more unfavorable ratios is 1,810 ----- 90 m.y., a result that is not statistically dis- tinguishable from the 1,770 ñ 30 m.y. date. K-Ar geochronology by Robinson (1971) gave an isochron at about 1,650 m.y. for the granite, granodiorite, and volcanic rocks of the upper Echo Bay group. Robin- son and Morton (1972) suggested that the difference in Rb-Sr and K-Ar ages could be due to the 8•Rb decay constant chosen and the analytical errors in the K-Ar ages. However, they also noted that com- plete loss of argon from hornblende and biotite under the conditions of low zeolite facies of regional meta- morphism might have been possible over a long period of time with highly active pore fluids present. Robinson (1971) also reported a K-Ar age of 1,420 ñ 60 m.y. for magnetite-actinolite veins at Port Radium, and he concluded that these probably had a close genetic relationship to a diabase sheet. This 150-foot thick diabase sheet apparently cuts all but the latest stages of vein mineralization at Port Radium (Jory, 1964). Wanless et al. (1970) have reported a K-Ar age of 1,400-----75 m.y. for a biotite-hornblende mixture from a diabase sheet on Hogarth Island, about 16 miles northeast of Port Radium. Because of 'the possible close age relationship between the mineral- ization and diabase sheets in the area, the sheet at Port Radium has been dated by the Geological Sur- vey of Canada using the Rb-Sr method. Five of the six analyzed samples fall on an isochron at 1,425 -+- 48 m.y. (at 2tr; XS*Rb = 1.39 x 104X/yr.). One sample from the chilled contact gave a K-Ar age of 1,222-----48 m.y., but this was, unfortunately, the sample that did not plot on the Rb-Sr isochron. Great Bear Lake---veins The U-Pb study of the Port Radium pitchblende by Jory (1964) is the most concerted attempt to date directly the vein mineralization. Pitchblende in the Eldorado mine forms an integral part of the epigene- tic vein deposition of arsenides, native silver, and associated minerals. Jory's data for pitchblende were obtained on seven samples of various sizes taken from three specimens from the Eldorado mine. He concluded that pitchblende in carbonate gangue was deposited at 1,445 ñ 20 m.y.. However, this age was based heavily on a single sample that could have undergone redeposition, so he could not prove con- clusively a single age of uranium mineralization (Jory, 1964, p. 173). Thorpe (1971) pointed out, however, that if one combines the results of earlier isotopic analyses (Cumming et al., 1955) on radio- genic lead from pitchblende at Port Radium with Jory's data, his age for the pitchblende mineralization is confirmed. Cobalt Area--country rocks Neither the Archean volcanic nor sedimentary rocks in the area have been dated, but they were folded during the Kenoran orogeny, which ended about 2,560 m.y. ago (Stockwell, 1973). Ages for granitic plutons that were emplaced into Archean SILVER-.4RSENIDE VEIN DEPOSITS 'l'am, F- 1. Results of Lead Isotope Analyses and Absolute Values for Standard Samples 781 Standard Lab. (Reference) •ø4Pb% •ø*Pb% •ø?Pb% •ospb% T 1003 Stacey et al. (1969, absolute) 1.4686 23.516 22.613 52.403 T 1003 Geol. Survey of Canada (1971) 1.468 4- 0.005 23.488 4- 0.035 22.619 4- 0.009 52.425 4- 0.049 T 1003 Geol. Survey of Canada (1972) 1.483 4- 0.003 23.599 4- 0.019 22.625 4- 0.007 52.292 4- 0.026 NBS 981 Teledyne Isotopes (1973) 1.43 4- 0.005 24.14 4- 0.012 22.09 4- 0.009 52.34 4- 0.009 NBS 981 Catanzaro et al. (1968, absolute) 1.4255 24.144 22.083 52.347 rocks, probably during the Kenoran orogeny, in- clude a K-Ar age of 2,525----. 72 m.y. (Jambor, 1971a) for a pluton at South Lorrain. Ages reported for the Round Lake batholith between Cobalt and Kirkland Lake (located 55 miles north-northwest of Cobalt) are a K-Ar biotite age of 2,605 m.y. (Lowden et al., 1963), an Rb-Sr biotite age of 2,570 m.y. (Aldrich and Wetherill, 1960), and an Rb-Sr isochron age of 2,390-----80 m.y. (Purdy and York, 1968; XS*Rb - 1.39 X 10-1X/yr.). An Rb-Sr isochron age of 2,288-----87 m.y. has been reported by Fair- bairn et al. (1969) for the Gowganda formation in the Gowganda area. This could be a true age for rocks of the Cobalt group but is close to an Rb-Sr metamorphic age of 2,230 m.y. for a post-Archean and pre-Cobalt group rock (Aldrich and Wetherill, 1960). The Nipissing diabase sheet at Gowganda has been dated by an Rb-Sr isochron by Fairbairn et al. (1969) at 2,162-----27 m.y. Jambor (1971a) has reported .K-Ar ages of 1,465 ----- 168 m.y. for a post-Nipissing quartz diabase dike and 1,400 -+ 116 m.y. for actino- lite in a fault cutting the silver-arsenide veins. Methods of Analysis All specimens except for TQ70-256 (matildite) consisted of galena. Lead isotope analyses were done at the Geological Survey of Canada using PbS with boric acid on a single prebaked rhenium filament and a solid-source mass spectrometer. Galena samples were dissovled in perchloric acid, which was then diluted and a portion removed for precipitation of PbS using H2S gas. One galena, sample TQ70-307, did not give a stable analysis when processed in this manner, and stability was not achieved until the third attempt when the solution was purified by passage through an anion-exchange column. The matilidite specimen required special treatment because of the bismuth interference at mass 209 with the lead spectrum. A few grains were dissolved in 8N hydrochloric acid, and the solution was passed through an anion-exchange column using hydro- chloric acid at various normalities to selectively re- move the lead from the bismuth. The absence of a 209 peak during the analysis of the very small PbS precipitate that was obtained attested to the com- ple,te separation of the lead from the bisnmth. Analyses were made on a ten-inch radius of curva- ture mass spectrometer in 1971 and a fifteen-inch instrument in 1972. Peaks were switched magnetic- ally in both instruments, and the ion currents were amplified by electron multipliers for detection by vibrating reed electrometers and measurement by integrating digital voltmeters. The analyses by the University of British Columbia were by the double- spike technique and a solid source was employed. Results of analyses on standards are listed in Table 1. These values were the result of eight mea- surements on the T1003 (Broken Hill No. 1) stan- dard by the Geological Survey of Canada in 1971 and 11 measurements in 1972. Three measurements on the NBS 981 standard were made by Teledyne Iso- topes in 1973. The Geological Survey of Canada (1972) results presented in this report were normal- ized to absolute values using factors derived from this comparison. Normalization was not required for the Geological Survey of Canada (1971) analyses nor for those by Teledyne Isotopes, because the results for standards in these cases agree with the absolute values within the stated precision. The error limits, at two standard deviations as based on multiple measurements of the standard, to be assigned to individual lead isotope analyses are given in Table 2. Quoted errors or ranges for model- lead ages and ages obtained by two-stage calculations are 2• values and are based on analytical errors alone without consideration of uncertainties in decay con- stants. The error limits for the University of British Columbia double-spike analyses are estimates provided by that institution. The true error limits to be assigned to the 1973 analyses by Teledyne Iso- topes are probably somewhat greater than indicated in the table, because only three analyses of the standard were made. Results and Interpretation For the calculation of reference model curves and for two-stage calculations according to equations by Kanasewich (1968), the decay constants and pri- mordial lead composition that have been used are those used by Stacey et al. (1969). x x x(•"U) = 1.537 x 10-X'/yr., X(mU) = 9.722 X 10'•/yr., X(•Th) =4.99 X 10-=/yr., ao--9.346, ho--- 10.218, co=29.7, t,,-- 4.55 b.y. 782 RALPH THORPE TABLI• 2. Error Limits (Two Standard Deviations) Applicable to Lead Isotope Analyses Lab. (date) •ø6Pb/"ø4Pb 2ø7pb/•ø•Pb •ø8Pb/"ø4Pb •ø7pb/•ø6Pb •'øsPb/"ø•Pb Geol. Survey of Canada (1971) Geol. Survey of Canada (1972) Univ. British Columbia (1972) Teledyne Isotopes (1973) 0.60% 0.90% 1.30% 0.35% 0.70% 0.46% 0.70% 0.88% 0.24% 0.43% 0.1% 0.15% 0.2% 0.71% 0.71% 0.70% Great Bear Lake The results of the lead isotope analyses, together with the lead isotope compositions reported by Jory (1964), are presented in Table 3 and Figure 3. An analysis by Teledyne Isotopes in 1971 for galena from the Silver Bay property must be disregarded as .the ratios for samples analyzed at this time are much below later repeat analyses. The properties sampled are shown on Figure 1. Of the 24 analyses listed in Table 3 for the Great Bear Lake area, eleven are considered to represent ordinary or single-stage lead because they satisfy all of the criteria suggested by Kanasewich (1968), with the exception of correspondence of model-lead age to ages obtained by other methods. Three of the ordinary lead analyses from Jory (1964) and five from this study together defined one ordinary lead composition, with some spread along a 2ø4Pb error line. This ordinary lead closely fits the model curve of Stacey et al. (1969) and has a model age of about 1,630 m.y. Two samples (three analyses) of ordinary lead, one from a galena vein on Clut Island and the other from the conformable sulfide zone at the Terra mine, have an older model age of about 1,780 m.y. The 1,630 -+ 40 m.y. model age of galenas from the silver-arsenide veins is much greater than the 1,445 ---+ 20 m.y. age for pitchblende in the veins and 1,425 ---+ 48 m.y. and 1,420-+ 60 m.y. ages for a diabase sill and magnetite-actinolite veins, respectively, which may be genetically related to the mineralization. The two samples with a model age of about 1,780 m.y. are not from the silver-arsenide veins and could represent an older mineralization. Their model age is near that for the Hudsonian orogeny and is a reasonable age for the mineralization. However, if this model age is adjusted as required for ordinary leads from the silver veins, a true age of about 1,605 m.y. is obtained, which does not correspond to any known geological event in the area. Eleven anomalous leads include samples from the Terra, Echo Bay, and Eldorado mines. Some of the samples are highly anomalous (Fig. 3). From the plot, however, it is not certain whether the two most anomalous samples, 27 and 260, represent a normal T•,BI,•. 3. Lead Isotope Compositions of Samples from the Great Bear Lake Area Property or Sample no. Lab. (Ref.) location 206/204 207/204 208/204 207/206 208/206 PR 26 Jory (1964) Eldorado 15.82 15.25 35.41 .9634 2.237 PR 88 Jory (1964) Eldorado 15.88 15.30 35.46 .9634 2.232 S 207 Jory (1964) Glacier Bay 15.90 15.34 35.63 .9652 2.242 PR 41 Jory (1964) Eldorado 22.79 16.07 41.97 .7052 1.842 PR 42 Jory (196•,) Eldorado 17.58 15.58 35.54 .8865 2.020 PR 87 Jory (1964) Eldorado 22.48 16.02 41.17 .7122 1.832 PR 90 Jory (196•,) Eldorado 22.83 16.09 42.18 .7047 1.848 PR 91 Jory (1964) Eldorado 22.43 16.07 42.12 .7163 1.876 PR 3 jory (196•) Eldorado 99.95 23.54 35.70 .2356 0.357 TQ 65-14 G.S.C. (1072) Echo Bay 20.35 15.85 39.74 .7700 1.953 TQ 66-19 G.S.C. (1972) Echo Bay 21.87 15.99 40.61 .7312 1.857 TQ 69-27 G.S.C. (1072) Terra 27.88 16.77 48.63 .6015 1.744 TQ 60-30 Isotopes (1971) Silver Bay 15.69 15.02 34.58 .9574 2.205 TQ 69-34 G.S.C. (1971) Norex 16.01 15.41 35.80 .9626 2.236 TQ 70-151 G.S.C. (1972) Echo Bay 21.85 16.04 39.78 .7340 1.821 TQ 70-256 G.S.C. (1972) Terra 17.39 15.52 37.00 .8926 2.128 TQ 70-260 G.S.C. (1971) Terra 27.17 16.53 47.78 .6085 1.759 TQ 70-276 G.S.C. (1971) Norex 15.90 15.37 35.63 .9664 2.241 TQ 70-278 G.S.C. (1971) Norex 15.92 15.37 35.61 .9652 2.236 TQ 70-279 G.S.C. (1971) Norex 16.01 15.48 35.98 .9672 2.248 TQ 70-361 G.S.C. (1971) Echo Bay 15.96 15.41 35.74 .9653 2.240 TQ 71-299 G.S.C. (1972) Terra 16.22 15.37 35.81 .9476 2.207 TQ 71-305 G.S.C. (1972) Clut Island 15.63 15.29 35.23 .9787 2.254 TQ 71-305 U.B.C. (1972) Clut Island 15.66 15.33 35.29 .9794 2.254 TQ 71-306 G.S.C. (1972) Terra 15.70 15.35 35.35 .9777 2.252 SILVER-ARSENIDE VEIN DEPOSITS 783 16.8 16.4 16.0 2o?pb 2o4pb 15.6 15.2 ß Jory 1964 o This study i i i i 16 18 •o o I I I I I I I I I 20 22 24 26 28 '06 pb/204pb FtG. 3. Plot of lead isotope data for the Great Bear Lake veins. The line through the anomalous galenas is a least squares line with a slope of about 0.112. deviation about a single anomalous lead line. Ac- cordingly, the York (1966) procedure was used to calculate a best fit, least squares line for the data obtained in this study, excluding samples 27, TQ69- 30, and those giving a model age of about 1,780 m.y. This program takes into account the error limits ap- plicable to the individual lead isotope compositions. This calculation gave a slope of 0.1034 --+ 0.0066 for the anomalous lead line. With sample 27 sub- stituted for 260 the calculation yielded a slope of 0.117-+ 0.0075, a result that is not statistically dis- tinct. A single anomalous lead line with a slope of 0.1085--+ 0.0070, calculated including both samples 27 and 260, is thus the best interpretation of the data. If a two-stage evolution is assumed, this line indi- cates a maximum time of 1,050 --+ 80 m.y. for forma- tion of the anomalous galenas and a maximum source age of 1,805 4- 125 m.y. Pitchblende veins in the Hottah Lake area, about 40 miles south-southwest of the Camsell River area, with a lead isochron age of 790 to 980 m.y. and FranklinJan diabase dikes with K-Ar ages of 600 to 800 m.y., both generally in northeast-striking struc- tures, represent the only events known to be younger than 1,130 m.y. in the Bear Province. The anomalous galenas might have been formed during the former event, although evidence of this event is not known in the Camsell River and Port Radium areas. Al- ternatively the anomalous galenas may have been formed at about 685 m.y., the mean age of Franklin- ian diabases, from a source 1,370 4- 140 m.y. old. These two-stage calculations are not completely satisfactory because neither pitchblende mineraliza- tion nor diabase intrusions of late Proterozoic age are known in the Port Radium or Camsell River areas. It should be noted that apices of small porphyry bodies are encountered in workings of the Echo Bay mine. Although it has been assumed that these are related to volcanic activity older than the vein min- eralization, this may not be the case. Two specimens analyzed by Jory (1964) were found to contain a radiogenic mixture. He found sample PR3 (tetrahedrite-chalcopyrite) to contain some uranium but to have a very low lead content. These facts suggest that the uranogenic lead has been generated in situ. If so, an age of 1,580 to 1,630 m.y. for the source uranium mineral can be calculated using Jory's sample S207 for the composition of the ordinary lead component. This age is difficult to explain in relation to an apparent true age for the mineralization of about 1,450 m.y. As Jory's sample PR42 contains only a small uranogenic component, calculations are subject to too much error for any firm conclusions. Cobalt area Seventeen samples have been analyzed from veins of the Cobalt-Gowganda region (Table 4 and Figs. 784 RALPH THORPE TAm. E 4. Lead Isotope Compositions of Cobalt Area Samples Sample no. Property (location) Type of mineralization 206/20 •. 207/204 208/204 207/206 208/206 Lab. (date) SP 1689 Silverfields mine Archean interflow 13.37 14.45 33.11 1.081 2.477 G.S.C. (1972) SP 1696 Old Shaft No. 1 Archean interflow 14.69 15.08 34.17 1.027 2.326 Isotopes (1973) (McKiuley Darragh) SP 1701 Vein 65 Archean interflow 14.74 15.12 34.30 1.026 2.327 G.S.C. (1972) TQ 71-64 Agnico No. 96 mine Archean interflow 15.28 15.31 34.81 1.002 2.279 G.S.C. (1972) TQ 71-72 Hudson Bay-Nipissing Archean interflow 14.90 15.21 34.68 1.021 2.327 G.S.C. (1972) TQ 72-86 Silverfields mine Archean interflow 14.78 15.16 34.42 1.026 2.330 Isotopes (1973) TQ 72-87 Deer Horn mine Archean interflow 14.79 15.16 34.41 1.025 2.326 Isotopes (1973) SP 1704 Silverfields mine Layers in Cobalt seds. 14.79 15.14 34.40 1.024 2.327 G.S.C. (1972) SP 1709 Silverfields mine Layers in Cobalt seds. 14.74 15.06 34.15 1.022 2.318 G.S.C. (1972) SP 1709 Silverfields mine Layers in Cobalt seds. 14.74 15.08 '34.21 1.023 2.321 G.S.C. (1972) TQ 72-91 Silverfields mine Layers in Cobalt seds. 14.80 15.16 34.40 1.024 2.324 Isotopes (1973) TQ 71-307 Silverfields mine Late sulfide vein 17.59 15.49 37.40 0.8807 2.126 U.B.C. (1972) TQ 71-307 Silverfields mine Late sulfide vein 17.63 15.54 37.56 0.8816 2.131 G.S.C. (1972) TQ 72-79 Deer Horn mine Late sulfide vein 14.88 15.26 34.64 1.026 2.329 Isotopes (1973) T• 72-81 Cobalt Lode mine Late sulfide vein 15.04 15.20 34.55 1.011 2.297 Isotopes (1973) TQ 72-82 Silverfields mine Late sulfide vein 18.76 15.69 38.62 0.8362 2.058 Isotopes (1973) TQ 72-84 Silverfields mine Late sulfide vein 18.57 15.60 38.35 0.8401 2.065 Isotopes (1973) TQ 71-85 S.W. of McKenzie fault Veinlets in graywacke 15.03 15.17 34.49 1.010 2.295 G.S.C. (1972) TQ 71-77 Siscoe Metals, Gowganda Veinlet in diabase 14.90 15.21 34.60 1.021 2.321 G.S.C. (1972) 2, 4). Unfortunately, galena does not accompany the arsenide assemblage of the main stage of silver- vein mineralization. Five samples represent a late sulfide assemblage that commonly consists of galena, sphalerite, marcasite, stephanite, and acanthite. Seven samples are of interflow mineralization as- sociated with cherty and graphitic sedimentary bands in the Archean volcanic sequence. Three samples are from thin sphalerite-galena layers in the basal sedi- mentary beds of the Cobalt group. Another sample, 71--85, consists of,Archean graywacke from south- west of the McKenzie fault. Galena and minor chalcopyrite occur on schistosity planes parallel to the fault and in small cross fractures. The remain- ing sample, 71-77, is from a galena-bearing veinlet in a narrow diabase seam that intruded graphitic Archean sedimentary rocks at Gowganda. A 2ø6Pb/ 2ø4Pb-2ø7Pb/•ø4Pb plot of the data is presented (Fig. 4). The most unexpected result of this lead isotope study is that galenas in Archean interflow beds do not all have Archeart model ages. Only one sample, 1689, of interflow mineralization has an Archean model age of 2,840-----35 m.y. If sample 1689 had not been analyzed, it might have been concluded that the Archean rocks had unusually high tz values for part of their evolutionary history (tz = the 2ø4Pb ratio extrapolated to the present) and that in this case model-lead ages of about 2,280 m.y. repre- sented a true age (syngenetic sulfide deposition) of greater than 2,560 m.y. It seems probable, therefore, that most of the inter flow mineralization has been introduced later into these beds. One group of samples is distributed approximately along a •ø•Pb error line and could be interpreted as having a model age of about 2,280 m.y. An ano- malous lead line with a slope of about 0.446 appears to be defined by samples 1709, 1689, 64, 81, and 85. Three samples from the late sulfide assemblage are highly anomalous. Two samples analyzed by Kanase- wich and Fraquhar (1965) have similar anomalous compositions. On a •'ø7Pb/•'ø*Pb-•øsPb/2ø*Pb plot (Fig. 5) of the lead isotope data, the scatter within the main cluster is greatly reduced, but samples 64, 81, and 85 are still anomalous. The main cluster of leads is tentatively interpreted as being genetically related to the Nipissing diabase, because the lead isotope compositions are closely simi- lar in spite of very different host rocks. Samples 72 and 77 may belong to the main cluster. Samples 64, 81, and 85 appear to be collinear with some of the data points in the main cluster on the •'ø7Pb/2øePb- •øsPb/•øePb plot (Fig. 5), although these samples do not form a good linear array on a standard plot (Fig. 4). If the line is valid, it may have been pro- duced by a mixing process with lead from the Archean country rocks, probably similar in composi- tion to sample 64, being mixed with lead derived from the Nipissing diabase. However, it is probably better to interpret samples 64, 81, and 85 to be collinear with sample 1689 (Fig. 5) and to define an anomalous lead line with a slope of 0.466--+---0.020 (Fig. 4). Although this line is interpreted as due to mixing, two-stage calculations could be valid if leads are being derived from the same Archean rocks in both cases. The same argument can be made follow- ing the interpretation that a mixing line exists be- tween sample 64 and the main duster, but the 0.466 slope in this case might only represent a maximum value. Two-stage calculations for a line with a slope of 0.446--+---0.020 indicate a maximum age of min- eralization and a minimum source age of 2,780 +-- 58 m.y. With an age of mineralization at 2,162 SILVER-ARSENIDE VEIN DEPOSITS 785 16.0 15.5 2o7pb 2o4pb 15.0 14.5 late sulfide veins o interflow [] other I I I I I I I I I • I I 13 14 15 16 17 18 19 2o6pb/2O4pb Fro. 4. Plot of lead isotope data for deposits in the Cobalt area. Note that only one sample of inter- flow mineralization has a primitive lead isotope composition and that four samples from the late sulfide assemblage are highly radiogenic. 27 m.y. (Rb-Sr age for the Nipissing diabase) the calculated source age is 3,260---+ 100 m.y. This age agrees closely with that suggested by Kanasewich and Farquhar (1965) for Archean rocks in the Cobalt- Noranda region. Because applicability of two-stage calculations is doubtful, this age of 3,260---+ 100 m.y. for the Archean rocks is also questionable and direct dating of the rocks should be attempted. If leads of the main cluster are genetically related to the Nipissing diabase, they can be viewed as essentially ordinary, and the best date for their formation is the 2,162 ---+ 27 m.y. Rb-Sr isochron age for the diabase. The anomalous lead line represented by the late- stage sulfide assemblage is not v, ell defined. In fact, if a single anomalous lead line is represented, it can only be concluded at present that a slope between 0.111 and 0.140 is probable. If two-stage calcula- tions are appropriate, a maximum age of mineraliza- tion of 1,360 m.y. and a maximum source age of 2,260 m.y. are indicated. Formation of the ano- malous galena during post-Silurian movements on the Lake Timiskaming system of rift faults (Lovell and Caine, 1970) would require, by two-stage calcu- lations, an undocumented homogenizing event at about 2,000 m.y. Because a local country-rock source for the anomalous lead component seems likely and the latest known event which could have caused widespread isotopic homogenization (the metamor- phic-mineralization episode related to emplacement of the diabase sheet) was at about 2,160 m.y.; a maxi- mum age for the late-stage sulfide veins is apparently 220 m.y. This date, as for the Great Bear Lake anomalous lead line, is not very satisfactory because none of the younger geological events in the area are considered capable of causing deposition of the late- stage sulfides. The anomalous lead line, however, may be the result of a mixing process, rather than due to evolution systems with variable and sometimes high t• values, and thus its slope may have no age significance. Discussion Lead isotope interpretation Kanasewich (1968) has listed the principal cri- teria for recognition of ordinary or single-stage leads. These criteria deal with the allowable variability in isotopic composition of the group of leads being con- sidered, the close agreement of the t• value and Th/U ratio for the source of the leads to a model curve generated from large conformable sulfide ore bodies, and the fact that model-lead ages should agree rea- sonably well with other age determinations. The Great Bear Lake ordinary leads fit the above criteria with the exception of model age agreement. The model-lead ages of about 1,630 and 2,280 m.y. for ordinary leads from the Great Bear Lake veins and from the Cobalt area deposits, respectively, ap- pear reasonable in relation to known ages of the 786 RALPH THORPE 2.28 2.30 2oSpb 2o6pb 2.32 2.34 MAIN CLUSTER / ?•o 64 / i I i 1.04 1.03 1.02 1.01 1.00 207pb/206pb Fro. 5. •nb/anPb-•øsPb/anPb plot of the data from the central part of Figure 4. Ratios are plotted in reversed manner so that a superimposed model curve would have the same form as in a •Pb/•Pb-•Pbflø'Pb plot. The arrow points toward the composition of sample SP1689. Symbols for deposit types are the same as for Figure 4. country rocks. However, the true age of the Great Bear Lake veins is well established at 1,450 m.y by Pb-Pb and U-Pb study of pitchblende. Galenas from the Cobalt-Gowganda region may have formed at about 2,160 m.y., although the age is not so well established as in the case of the Great Bear Lake veins. The disagreement of model ages in these cases raises the question of reliability of model ages in general. From data presently available it would seem that model ages, even for stratiform massive sulfide deposits, are only indicative in a general way of true age. More determinations of lead isotope compositions of young massive sulfide deposits of known age and more attempts to document accurately ages of host rocks of Precambrian massive sulfide deposits are required in order to define the error limits for model-lead ages. It may well be found that these error limits are as great as q- 200 m.y. The reasons for such broad error limits are viewed as entirely geological. Two extreme hypothetical cases based on rock lead data can be used to illus- trate the problem of model age interpretation. Lead isotopic study of the Amitsoq gneisses, Greenland, by Black et al. (1971) indicated that they were highly depleted in uranium, and very little radio- genic lead has thus accumulated in these rocks since 3,620-3,750 m.y. ago. If these rocks were homogen- ized today by magmatic melting and some of the lead extracted to form a stratiform massive sulfide de- posit, this lead could have a model age of 3,400 to 3,500 m.y. On the other.hand, such a deposit formed in association with some modern oceanic volcanics, which contain lead of very radiogenic and variable composition, could have a model age several hundred million years in the future. From these examples, although admittedly extreme, both leads which are unradiogenic and "anomalous" or radiogenic with regard to their true age are to be anticipated. Thorpe and Sangster (1973) have reported a range of apparent • values from 8.63 to about 9.30 for some Canadian massive sulfide deposits. On this basis they suggested that if large conformable mas- sive sulfide deposits are accepted as of single-stage origin, a family of valid growth curves must be as- sumed. These curves would represent variable • values in the source regions in which the lead has evolved, whether these be mantle or crustal. Ac- cording to present plate tectonic theory the genera- tion of the calc-alkaline suite of volcanic rocks, with which many conformable massive sulfide deposits are associated, is related to the subduction and consump- tion of oceanic crust, with contamination possible either by crustal material above the descending plate or by variable amounts of relatively highly radio- genic pelagic sediments from the thin veneer capping the oceanic plate. This implies that the model of a purely mantle origin is not tenable, as suggested by Richards (1971). A single mantle curve could exist but is not applicable to the strict interpretation of ages of most ore deposits. Plotting of data (from a paper in preparation by S. M. Roscoe, J. Franklin, W. D. Loveridge, and D. F. Sangster) for many Canadian massive sulfide deposits on a plot of 2ø7Pb/ 2øøPb versus 2øsPb/•ø6Pb suggests the possibility of separate short two-stage lines for many of the min- ing districts. If this is the case, it could be a reflec- tion of the contamination process suggested above and could explain the scatter in apparent • values. If a true mantle curve exists and if it can be defined, it would serve as a base from which to evaluate this contamination or crustal contribution. Very low slopes obtained for anomalous lead lines in the Great Bear Lake and Cobalt areas cause two- stage calculations to yield young apparent ages for the anomalous galenas. These young ages are not entirely satisfactory for galenas that appear to have formed during a late stage of the main silver-arsenide mineralization, especially considering the absence of young, documented, geological events that could have formed the anomalous leads. Maximum calculated source ages are 1,805 q- 125 m.y. for the radiogenic component in the Great Bear Lake galenas and 2,260 m.y. in the case of Cobalt. The Great Bear Lake age could represent the true age of the country rocks (> 1,790 m.y. by available dating) or isotopic homogenization of these rocks during the Hudsonian SILVER-ARSENIDE VEIN DEPOSITS 787 orogeny, whereas the Cobalt age could be the age of deposition of the Cobalt group sediments, of a low-grade regional metamorphism, or possibly a true U-Pb age for the Nipissing diabase sheet. This method of interpretation of the anomalous galenas requires recent addition of a radiogenic com- ponent, possibly by ground water, to previously formed galenas. Continuous addition of a radiogenic component from the country rocks is an alternate theoretical possibility but is rejected because many galenas have not had a radiogenic component added. Tugarinov et al. (1964) have advanced such an explanation for a suite of anomalous leads in galena veinlets in iron deposits in the region of the Kursk magnetic anomaly, Soviet Union, and Thorpe (1971) also considered the mechanism for the Great Lake galena's. Boyle (1959) concluded that much greater mobility should be anticipated for the radiogenic lead in rocks, relative to the initial common lead com- ponent (largely in feldspars), during all later dis- turbances including surficial weathering. Formation of the anomalous galenas by addition of a purely radiogenic component requires that this component account for nearly 28 percent and 11 to 12 percent of the total lead in the most radiogenic Great Bear Lake and Cobalt samples, respectively. Because a radiogenic component will likely contain some common lead, these must be regarded as mini- mum percentages. While there are difficulties in interpretation of lead isotope compositions insofar as age of the deposits is concerned, it would seem that these compositions can be of great help in establishing the genetic processes. Genesis of the veins Great Bear Lake area: Many of the galenas from the silver-arsenide veins in the Great Bear Lake area consist of ordinary lead from mines in both the Port Radium and Camsell River areas. The source of this homogenous lead component is difficult to iden- tify. It is tempting to call on the diabase sheet in the case of the Eldorado and Echo Bay mines, be- cause the age of the sheet agrees closely with the time of mineralization. 'However, in the Camsell River area such a diabase sheet' is not evident and it becomes necessary to postulate that a sheet has been removed from above the veins by erosion or will be encountered at greater depth. An alternative source might be the ore-forming brines which Robinson and Ohmoto (1973) suggest could have been of c0nnate, meteoric, or sea-water, but not magmatic, origin. If so, it seems necessary to conclude that the brines did not react with the country rocks at the time of ordinary lead deposition, because it is con- sidered that the brines would pick up variable amounts of radiogenic lead during such reaction. On the other hand extensive alteration of the wall rocks, which has not occurred in the case of these veins, could possibly result in derivation of lead with homogeneous composition from the country rocks. In the Port Radium area the large granitic bodies are much older (Jory, 1964) than the vein mineral- ization and there can thus be no direct genetic rela- tionship. However, in the case of the Terra mine in the Camsell River area, Shegelski (1973) has con- cluded, on the basis of proximity and of the zoning within the silver veins, that an adjacent syenite body was responsible for the mineralization. He con- sidered this syenite to be part of the Great Bear batholith. Because, in view of the very close simi- larity in mineralogy and in lead isotope composition, different ages for the deposits in the Camsell River area and in the Port Radium area are extremely un- likely, it would seem that either the syenite body at the Terra mine is much younger than granitic rocks of the Great Bear batholith or that the conclusion of a genetic relationship between the syenite and the veins is incorrect. Dating of the syenite body should be undertaken to resolve this problem. One suggestion made by Robinson and Ohmoto (1973) was that sea-water could have become a con- centrated brine due to boiling caused by shallow in- trusions of quartz-diabase. Other features of the fluids are considered to have been controlled by re- action of the concentrated brines with the tuffs through which they migrated. Robinson and Ohmoto (1973) also considered that the variation in lead isotope composition of galena (some highly ano- malous) might indicate derivation of the ore metals by leaching of the tuffaceous country rocks. If this is so, it would seem that the depositing fluids must have differed in character from those which de- posited the ordinary galenas. The anomalous galenas by two-stage calculations indicate a late mineralization event which has not yet been substantiated geologically. If a simple two- stage model is rejected for the anomalous lead and if they are assigned to a late phase of the mineralization event at about 1,450 m.y., then it seems necessary to account for these leads by a mixing process. A radiogenic component could possibly have accumu- lated in the host tuffaceous rocks until 1,450 m.y. from their deposition at 1,800 to 2,000 m.y. In order to explain the low slope of the mixing line it would appear that the overall t• value for these source rocks, or the v value that would represent their evolution prior to their deposition, must be lower than the t• value for the source of the ordinary lead. Cobalt area: Kanasewich and Farquhar (1965) suggested that some galenas in the Cobalt-Noranda region represent a mixture of two ordinary leads 788 RALPH THORPE having ages of 3,250 and 2,300 m.y. They considered the simplest interpretation to be that sulfides from the first mineralization were deposited in regional faults and that lead was remobilized from this source during the second mineralization by solutions moving along the same channelways. The mixing line is open to question because it has been generated by plotting data from many different types of deposits from a very large area of differing host-rock ages and lithology. It seems likely that a number of the intermediate data points could represent essentially single-stage evolution. For the Cobalt area, assum- ing that mixing occurred, the mechanism of Kanase- wich and Farquhar (1965) is not applicable because neither of the mineralizations took place in regional faults. This lead isotope study of galenas from the Co- balt-Gowganda region failed to yield distinct isotopic compositions for the three main types of mineraliza- tion. In spite of the fact that Archean interflow mineralization at Cobalt has been interpreted by Petruk (1971c) and others to be syngenetic, galena from these interflow beds only rarely has a composi- tion that could clearly be interpreted as syngenetic. Other galena in these beds may have formed during the second main mineralization, that associated with the Nipissing diabase. The source of the lead is not conclusively indicated, but it may have been derived from the Archeart country rocks or from the Nipissing diabase. The above conclusion is significant in that it im- plies that other occurrences of minor base metal sulfide mineralization associated with volcanic rocks, for which the interpretation of a syngenetic origin is in vogue, may be epigenetic. The lead in galena grains distributed along the bedding in basal sediments of the Cobalt group does not differ appreciably in isotopic composition from that present in mineralized Archean interflow beds. It is possible that Archeart interflow galena, deposited or homogenized duri.ng an earlier event such as the Kenoran orogeny, was deposited as a detrital com- ponent in the Cobalt group sediments or that the lead was leached from Archean source rocks (or sedimentary detritus from these rocks) during a period of mineralization related to the Nipissing diabase. However, the nearly identical isotope com- positions suggest rather that the lead has been con- tributed by the Nipissing diabase in both cases. Late sulfide veins contain galena that is in part similar in isotopic composition to galena from the other two types of mineralization. However, some of the galenas are much more radiogenic. Two- stage calculations indicate mineralization at some time more recent than 1,360 m.y., which does not fit well with the known geology, although quartz dia- base dikes and some faults may be younger than this (Jambor, 1971a). Formation of these anomalous leads by a mixing process is considered likely. How- ever, this requires addition of a homogeneous radio- genie lead component from an external source as yet unidentified to the available ore or rock lead in the Archean and Cobalt group rocks. The timing of this phase of mineralization is completely unsettled, although it is known that this sulfide assemblage is later than the main arsenide veins, because it is com- mon in offsetting cross veins. The above discussion suggests that the Nipissing diabase was the source of the lead for most of the mineralization in Archean interflow bands, for galena "beds" in basal sediments of the Cobalt group, and for those galenas with similar isotopic compositions in late sulfide veins. This source was also favored by Jambor (1971c) for the metals in silver-arsenide veins of the main stage when his study of the dia- base (Jambor, 1971b) indicated that the appropriate trace elements could have become concentrated in a volatile-rich phase when the diabase crystallized. Boyle and Dass (1971), however, suggested that the ore and gangue elements might have had their source in Keewatin interflow sedimentary beds, which are commonly mineralized. Previous studies (Petruk, 1968, 1971b; Scott 1972) have established the dia- base sheets as the probable source of the heat for the hydrothermal system that formed the silver- arsenide veins. Conclusions 1. Leads in the Great Bear Lake silver veins that are apparently of single-stage origin have a model lead age of 1,633 +-- 34 m.y., whereas the U-Pb pitch- blende age is 1,445-+20 m.y. For this case of epigenetic vein mineralization the model lead age, by the model of Stacey et al. (1969), must be re- jected. 2. Interpretation of lead isotope data suggests that the Archeart volcanics in the Cobalt area are 3,260 +-- 100 m.y. old. 3. A sheet or sill of diabase (Western Channel diabase of Irving et al., 1972) at Port Radium, Great Bear Lake, has been dated by an Rb-Sr iso- chron (X87Rb = 1.39 x 10-•/yr.) at 1,425 +- 48 m.y. (error limit at 2•). 4. Indicated ages of mineralization in the Great Bear Lake area are as follows: (a) Mineralization older than the silver-arsenide veins has a modeMead age of about 1,780 m.y. (b) A previously reported U-Pb age of 1,445 -+ 20 m.y. is accepted for the main silver-arsenide stage. (c) If two-stage calculations are appropriate, anomalous galenas in the silver veins have a maximum age of 1,050 + 80 m.y. or have had a radiogenic component added since that date. 5. Indicated ages of mineralization in the Cobalt area are as follows: (a) Sulfides in Archean inter- SILVER-ARSENIDE VEIN DEPOSITS 789 flow sedimentary rocks only rarely (one sample of seven) have a model age that is Archean, 2,805 m.y. or greater, but appear to have generally formed at the time of a much younger mineralization, probably that associated with the Nipissing diabase at about 2,160 m.y. (b) Galena-sphalerite laminae in Cobalt group sedimentary rocks and the main stage of silver- arsenide vein formation probably also formed in as- sociation with the diabase. (c) Two-stage calcula- tions suggest that anomalous galenas in a late-stage sulfide assemblage have a maximum age of 1,360 m.y. or that a radiogenic component has been added to these galenas since that date. 6. Genesis of the ordinary galenas in the Great Bear Lake silver-arsenide veins is considered to be related to diabase sheets; the diabase itself could have served as a homogeneous source of lead. The mode of genesis of anomalous leads in the veins is obscure. 7. In the Cobalt area lead isotope evidence favors epigenetic deposition for most of the Archean inter- flow mineralization. The lead was probably derived from the Nipissing diabase. Similarly, lead for galena-sphalerite layers in basal sedimentary rocks of the Cobalt group was probably contributed by the diabase. The galena in some late-stage sulfide veins may have formed in the same manner, but a genetic process has not been established for the more ano- malous samples of this type. Interpretation of highly anomalous galenas from the Great Bear Lake and Cobalt areas is difficult, because two-stage calculations yield mineralization ages that seem to be unreasonably young in relation to the known geology. Nevertheless, several pos- sible mechanisms, which will require further investi- gation, have been discussed. This study helps point up other problems for further investigation. Archean (Keewatin) vol- canic rocks in the Cobalt area and syenite at the Terra mine and porphyritic hypabyssal intrusions at the Echo Bay mine in the Great Bear Lake area should, if possible, be dated directly. More precise analyses of the anomalous leads belonging to the late sulfide assemblage in the Cobalt area are required in order to define better the slope of the anomalous lead line. Rock lead isotopic study of the Nipissing dia- base, including its granophyric phase, in the Cobalt area and of the host tuffaceous rocks in the Great Bear Lake area might indicate whether these rocks could have supplied highly radiogenic lead at the time of vein formation. These studies should help refine and confirm, or disprove, some of the sugges- tions made in this study as to ages of mineralization stages and genetic processes. Many more precise lead isotope analyses of Canadian massive sulfide deposits should be made in order to confirm the suggested presence of short anomalous lead lines, possibly indicating two-stage evolution of the deposits. Also needed are more isotopic analyses for young massive sulfide deposits of known age and more dating studies of host rocks to Precambrian deposits of this type. These studies should help establish (1) the meaning and usefulness of a model curve and (2) the error limits to be as- signed to model-lead ages, and whether these vary along the length of the model curve. Acknowledgments Lead isotope analyses carried out at the Geological Survey of Canada were by courtesy of R. K. Wanless and W. D. Loveridge of the Geochronology Labora- tory. The writer is also indebted to D. F. Sangster of the Geological Survey who collected five specimens from the Cobalt area and supplied corresponding lead isotope analyses. Six samples from this area were kindly provided by W. Petruk of the Mines Branch and one by J. L. Jambor of the Geological Survey. An earlier draft of this paper was read by D. F. Sangster, J. L. Jambor, and W. D. Loveridge and their suggestions have been most helpful. In par- ticular, the aid of W. D. Loveridge in preparation of the section on Methods of Analysis and in inter- pretation of the Cobalt area analyses is acknowledged. GEOLOGICAL SURVEY OF CANADA 601 BOOTH STREET 0•rAWA K1A OES, C^•rAD^ REFERENCES Aldrich, L. T., and Wetherill, G. W., 1960, Rb-Sr and K-Ar ages of rocks in Ontario and northern Minnesota: Jour. Geophys. Research, v. 65, p. 337-340. Badham, J.P. N., 1973, Calc-alkaline volcanism and plu- tonism from the Great Bear Batholith, N.W.T.: Canadian Jour. Earth Sci., v. 10, p. 1319-1328. Black, L. P., Gale, N.H., Moorbath, S., Pankhurst, R. J., and McGregor, V. R., 1971, Isotopic dating of very early Precambrian amphibolite facies gneisses from the Godthaab district, West Greenland: Earth Planet. Sci. 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K., 1972, Palco- magnetism of the Western Channel diabase and associated rocks, Northwest Territories: Canadian Jour. Earth Sci., v. 9, p. 960-971. 790 RALPH THORPE Jambor, J. L., 1971a, General geology, in Berry, L. G., ed., The silver-arsenide deposits of the Cobalt-Gowganda re- gion, Ontario: Canadian Mineralogist, v. 11, p. 12-33. 1971b, Distribution of some minor elements in the Nipissing diabase, in The silver-arsenide deposits of the Cobalt-Gowganda region, Ontario: Canadian Mineralogist, v. 11, v. 320-357. 1971c, Origin of the silver veins of the Cobalt- Gowganda region, in The Silver-arsenide deposits of the Cobalt-Gowganda region, Ontario: Canadian Mineralogist, v. 11, p. 402-413. Jory, L. T., 1964, Mineralogical and isotopic relations in the Port Radium pitchblende deposit, Great Bear Lake, Can- ada: Unpub. Ph.D. thesis, Calif. Inst. Technology, 275 p. Kanasewich, E. R., 1968, The interpretation of lead isotopes and their geological significance; in Hamilton, E. I., and Farquhar, R. M., eds., Radiometric dating for geologists: New York, Wiley Interscience Publishers, 506 p. and Farquhar, R. M., 1965, Lead isotope ratios from the Cobalt-Noranda area, Canada: Canadian Jour. Earth Sci., v. 2, p. 361-384. Lovell, H. L., and Caine, T. W., 1970, Lake Timiskaming rift valley: Ontario Dept. Mines, Misc. Paper 39, 16 p. Lowdon, J. A., Stockwell, C. H., Tipper, H. W., and Wanless, R. K., 1963, Age determinations and geological studies: Canada Geol. Survey Paper 62-17, 140 p. Petruk, W., 1968, Mineralogy and origin of the Silverfields silver deposit in the Cobalt area, Ontario: EcoN. GEo•.., v. 63, p. 512-531. 1971a, General characteristics of the deposits, in The silver-arsenide deposits of the Cobalt-Gowganda region, Ontario: Canadian Mineralogist, v. 11, p. 76-107. 1971b, Mineralogical characteristics of the deposits and textures of the ore minerals, in The silver-arsenide deposits of the Cobalt-Gowganda region, Ontario: Canadian Miner- alogist. v. 11, p. 108-139. 1971c, Depositional history of the ore minerals, in The silver-arsenide deposits of the Cobalt-Gowganda region, Ontario: Canadian Mineralogist, v. 11, p. 396-401. Purdy, J. W., and York, D., 1968, Rb-Sr whole-rock and K-At mineral ages of rocks from the Superior Province near Kirkland Lake, northeastern Ontario, Canada: Cana- dian Jour. Earth Sci., v. 5, p. 699-705. Richards, J. R., 1971, Major lead orebodies--mantle origin?: EcoN. GEo•.., v. 66, p. 425-434. Robinson, B. W., 1971, Studies on the Echo Bay silver de- posit, Northwest Territories, Canada: Unpub. Ph.D. thesis, Univ. 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H., 1961, Structural provinces, orogenies, and time classification of rocks of the Canadian Precambrian Shield: in Lowdon, J. A., ed., Age determinations by the Geological Survey of Canada, Rept. 2. Isotopic ages, Can- ada Geol. Survey Paper 61-17, p. 108-127. - 1973, Revised Precambrian time scale for the Canadian Shield: Canada Geol. Survey Paper 72-52, 4 p. Thorpe, R. I., 1971, Lead isotopic evidence on age of mineralization, Great Bear Lake, District of Mackenzie, in Blackadar, R. G., ed., Report of activities: Canada Geol. Survey Paper 71-1, pt. B., p. 72-75. -- and Sangster, D. F., 1973, An integrated model for lead isotopic evolution for samples from the Canadian Shield: Discussion: Canadian Jour. Earth Sci., v. 10, p. 1693-1696. Tugarinov, A. I., Bibikova, E. V., and Zykov, S. I., 1964, Age of rocks of the Kursk magnetic anomaly: Geochem- istry Internat., no. 5, p. 945-950. Wanless, R. K., Stevens, R. D., Lachance, G. R., and Delabio, R. N., 1970, Age determinations and geological studies: Canada Geol. Survey Paper 69-2A, p. 45. York, D., 1966, Least-squares fitting of a straight line: Canadian Jour. Physics, v. 44, p. 1079-1086. APPENDIX Sample Descriptions TQ 65-14 Echo Bay mine, 102 zone, upper adit level. Coarse-grained galena, medium- grained sphalerite, and a little coarse chalcopyrite and argentite, with white to gray to brown carbonate, from a 4- inch vein adjacent to a one-inch width of arsenide-bearin• tan to brown dolo- mite. TQ 66--19 Echo Bay mine, 202 stope, No. 1 adit level. Coarse-grained galena deposited on fine-grained chalcopyrite and over- lain by crystalline calcite that has grown in a rug. The chalcopyrite was deposited on fragments of vein dolo- mite. TQ 69-27 Terra mine, Camsell River area. Sam- ple from midway down the decline. Coarse-grained galena with abun- dant sphalerite and some marca- site and chalcopyrite. In general galena and a white carbonate have been deposited last. The sphalerite shows depositional banding by color variation and thin marcasite and chal- copyrite interlayers. Early carbonate is flesh colored. Silver Bay property. Coarse-grained galena, with minor associated blebs of niccolite, and quartz surround areas of white to pinkish carbonate which show some banding parallel to their margins. Norex property, Camsell River area, south drill site on the 1TLDO claim group. Echo Bay mine, 205 subdrift. A thin film of galena surrounds fragments of fine-grained calcite or dolomite. Fine- to coarse-grained pyrite deposited on the galena; calcite forms the final ce- ment. Terra mine, surface dump. Large euhedral crystals of matildite in a matrix of white calcite. Terra mine, 208 stope. Seam of galena beside a chalcopyrite-bearing calcite ve•rl. TQ 69-30 TQ 69-34 TQ 70-151 TQ 70-256 TQ 70-260 SILVER-ARSENIDE VEIN DEPOSITS 791 TQ 70-276 TQ 70-278 TQ 70-279 TQ 70-361 TQ 71-299 TQ 71-305 TQ 71-306 SP 1689 SP 1696 SP 1701 SP 1704 SP 1709 TQ 71-64 TQ 71-72 Norex property. Galena disseminated in silicified and mineralized wall rock beside the eastward extension of the main vein. Norex property. Galena in quartz veins cutting quartz-feldspar porphyry near the No. 2 vein. Norex property, main vein. Coarse galena in carbonate. Echo Bay mine. Fine-grained inter- growth of galena and arsenide min- erals in tetrahedrite. Terra mine. Roughly layered chal- copyrite-rich massive sulfide ore from the conformable sulfide zone. The specimen contains some pyrite, sphale- rite, and galena. Clut Island, Camsell River area. Coarse galena forming the matrix to bright green fragments of altered rock which rarely contain some pink feld- spar. Terra mine. Patches of coarse- grained galena forming a discontin- uous layer in a specimen consisting predominantly of a pyrite-marcasite intergrowth and some quartz gangue. Silverfields mine, Cobalt area, 5 level. Sulfide minerals in Archean interflow sedimentary rocks. Old Shaft No. 1 location, McKinley- Darragh property, south of Cobalt Lake. Sample from a surface exposure consisting of massive galena in Ar- chean interflow beds. Vein 65, northeast of Cobalt Lake. Archean interflow mineralization. Silverfield mine, 5 level. Sulfide min- erals in Cobalt group (Huronion) sedimentary rocks. Silverfields mine, 5 level. Sulfide minerals in Cobalt group (Huronion) sedimentary rocks. Agnico No. 96 mine, 3rd level, 22-2 drift. Galena associated with abun- dant sphalerite and a little pyrrhotite in the altered chloritic matrix to finely fragmented black chert. The galena is concentrated, in particular, along a slip along one side of the specimen. Hudson Bay--Nipissing property, Shaft 64 area. Galena is from a galena- and sphalerite-rich band less than 1/2-inch wide along the schisto- TQ 71-77 TQ 71-85 TQ 71-307 TQ 72-79 TQ 72-81 TQ 72-82 TQ 72-84 TO 72-86 TQ 72---87 TQ 72-91 sity of sheared Keewatin interflow sedimentary rocks. Sphalerite and chalcopyrite (plus carbonate) form separate veinlets along fine fractures in adjacent massive black chert or cherty sediment. Siscoe Metals, Gowganda district. Galena with calcite in a veinlet 1-mm wide in fine-grained diabase which forms a narrow seam in a graphitic Archean interflow sedimentary band. West side of Crosswise Lake. Thin galena coatings, with a little pyrite and chalcopyrite, on fracture surfaces in slightly sheared Archean gray- wacke of the Timiskaming group. Silverfields mine, 3 level. Coarse- grained sphalerite, galena, stephanite, and acanthite, with late clear crystal- line .calcite, veining and replacing white to pink opaque carbonate. The four-inch vein is an apparent con- tinuation of a silver-rich arsenide vein, but according to Petruk (pers. commun.) this type of mineralization is definitely later than the arsenide- native silver mineralization. Deer Horn mine, 1,035 level. Pocket of sulfide minerals in calcite 500 feet from the main vein. This pocket con- tained high-grade silver and was located in the Archean about 500 feet below the Nipissing diabase sheet. Late sulfide assemblage. Cobalt Lode mine, 3 level, 317 vein. Fairly massive galena from within the Christopher fault in the Archean above the diabase sheet. Late sulfide assemblage. Silverfields mine, 4 level, no. 5 vein. Galena and sphalerite in a late vein transecting the cobalt-arsenide assem- blage. Late sulfide assemblage. Silverfields mine, 4 level, no. 2 vein. Galena with calcite, sphalerite, and ruby silver. Late sulfide assemblage. Silverfields mine, 5 level, no. 1 vein. Galena from interflow mineralization in the Archean below the diabase sheet. Deer Horn mine, 1,140 level. Inter- flow mineralization in Archean rocks. Silverfields mine. Galena scattered along certain bedding planes in basal sediments of the Cobalt group.