Rb-Sr Geochronology of the Lodge Bay Granite, a ca. 3.0 Ga Basement in the Beaverlodge Area, Saskatchewan

Michael Bikerman 1, Keith Belf and John Blenkinsop2

Bickerman, M., Bell, K. and Btenki~sop, J. (1990): Geochronology of the Lodge Bay Granite, a ca. 3.0 Ga basement in the Beave~lodge ~ea, Saskatchewan; m Summary of Investigations 1990, Saskatchewan Geological Survey· Saskatchewan Energy and Mmes, Miscellaneous Report 90-4. '

As part of a program of remapping of the geology of the Beaverlodge area north of Lake Athabasca, geochronological determinations were carried out on a number of plutons. This paper reports on the unusual results of Rb-Sr analyses on the Lodge Bay Granite.

The Lodge Bay Granite, generally a uniform massive leucogranite, was originally mapped by Christie (1953) as a plutonic intrusion emplaced within low grade metasediments of the 'T azin Series' of unknown Precambrian age. These supracrustals were sub­sequently identified as part of the Murmac Bay Group (e.g: Tremblay, 1972; Macdonald and Slimmon, 1984) -a mixed assemblage of quartzites, conglomerates, car­bonates, pelites and pillowed basaltic flows and epiclas­tics - which covers an area of about 600 km2 along the north sho!e of Lake Athabasca. In the area of the Lodge Bay Granite, Murmac Bay rocks have attained lower am­phibolite facies grade and generally retain original dep~sitional structures such as cross bedding. The granite and some of the contiguous supracrustals con­tain thick gabbroic sheets, which appear to be sills on the map; Christie (1953), and Sibbald and Lewry (1980) regarded these as intruded by the granite.

Macdonald (1984) in company with Roscoe (pers. comm.) noted an apparent unconformity in the Elliot Bay locality, at the south end of the granite, between granite and overlying granite/ quartz pebble con­glomerate of the Murmac Bay Group. In 1985 an ex­posure of a suspected deformed unconformable junc­tion between quartzite and granite was identified on the northeastern flank of the Lodge Bay Granite by Van Schmus et al. (1986), who collected samples for U-Pb zircon determination from which they derived an im­precise U-Pb upper intercept of 3072±41 Ma. Samples for Rb-Sr determinations had been collected previously from the Lodge Bay Granite by the present authors which also indicated a comparable age, but the results were not published earlier due an apparent discrepancy in the initial ratios.

A few kilometres east of the lodge Bay Granite small granitic bodies known as the 'Mine Granites' have been interpreted to be derived by feldspar metasomatism of pre-existing supracrustal rocks of the Murmac Bay Group (Sibbald, 1984; Sibbald and Jiricka, 1985). Still farther east are the larger Mackintosh Bay and Cameron Island plutons. Bell et al. (in press) using Rb-Sr deter­minations and incorporating U-Pb zircon results from

some of these granites (Van Schmus et al., 1986} present a model in which an intrusive event at ca. 2350 ~a has been succeeded by the feldspar metasomatis-1ng event at ca. 2000 Ma

In this paper we present corroborative evidence that the lodge Bay Granite is significantly older than any of these granites, and that it is part of the basement under­lying the Murmac B~y Group, and may have been part of the crust from which the later granites were derived.

1. Procedures Whole rock samples of the Lodge Bay Granite were pul­verized to less than 100 mesh in a Beuhler mill. Aliquots were pressed into 32 mm boric acid-backed briquets for X-ray fluorescence analysis for Rb and Sr. The XRF analyses were carried out using a modified version of the Norrish and Chappell (1967) method wtth a U.S. Geological Survey standard G-2 run before and after each sample. Baseline counts were made before and after each peak count, and each was counted for 40 seconds. Each sample was run twice and the average values are reported. Precision of the Rb/ Sr ratios is about one percent.

All isotopic analyses were carried out following chemical preparation and ion-exchange chromatography on chlorides using single Ta filaments for Sr and double Re filaments for Rb on a Finigan-MAT 261 multicoflector ~lid·i~urce mass spectrometer. On this machine 85Rb,

Sr, Sr and 88gr are measured simultaneously. Inter­nal precision for Sr isotopic ratios is considered to be 5 parts in 70,000. Isotope dilution mass-spectrometric checks on two samples show variances in Rb and Sr ~?ntents friSm XRF determinations as shown in Table 1.

Rb and Sr spikes were used.

The isochron plot (Figure 1) shows the measured points and the calculated values of the date and intercept using York's (1969) method. The Steiger and Jager (1977) Rb half life value of 48.8 x 109 yrs. was used. Table 2 shows the XRF chemical analyses for four samples from the Lodge Bay Granite.

Thirt~en XRF determinations for Rb and Sr, and two duplicates run by isotope dilution (marked ID in Table 1) and their correspon~i~~ Sr jsotor.e ~nalyses give a date of 2839±67 Ma, an 1nrt1al 8 Sr;8 Sr intercept ratio of

(I) Oepmtmeot of Geology and Planetary Science, Unmrsity of Pittsburg, Pittsburg, PennsylYanla (2) Ottawa-Carleton Geosclence Centre. Department of Earth Sciences, Carlelon University, Ottawa. Ontario

Saskatchewan Goological Survey 143

Table 1 - Lodge Bay Rubidium And Strontium Isotopic Data

Sample Rb ppm Sr ppm Rb/ Sr 87Rb;88Sr 87Sr,t88Sr

SK-1 124 114 1.09 3.175 0.82449 SK·2 108 111 0.98 2.861 0.81041 SK-3 120 129 0.94 2.768 0.81125 SK-4 63 134 0.47 1.368 0.75026 SK-710 73 124 0.59 1.721 0.76208 SK-711 of 132 0.51 1.467 0.75061 SK-745 123 76 1.63 4.790 0.89084 SK-746 111 71 1.57 4.616 0.87739 SK-747 97 36 2.72 8.078 0.98146 SK-748 103 40 2.55 7.591 1.00892 SK-749 150 41 3.63 10.960 1.15338 SK-750 99 64 1.55 4.557 0.88266 SK·751 91 74 1.23 3 .608 0.84590

SK-71110 61 .27 125.1 0.49 1.423 0.75058 SK-74910 132.7 37.19 3.57 10.772 1.14894

ID-Isotope dilution Rb and Sr contents; rest by XRF

0.69276±0.00239 and an MSWD of 0.726. Eliminating the point for SK-746 which lies well off the line changes the values slightly to 2896±73 Ma, 0.69134±.00253 and 0.154.

2. Discussion Since the points are well spread out along the isochron, the differences between the variations described above is small, and no matter how the data in Table 1 is inter· prated, it seems that a date of ca. 2900 Ma and an inter­cept significantly less than 0.700 is obtained. The date is significantly older than any other of the plutons in the

Table 2 - Lodge Bay Granite Chemical Analyses - XRF

Major elements in weight %

OXIDE SK-711 SK-745 SK-746 SK-749

Sl02 72.32 75.12 76.66 75.19 Al203 14.28 13.87 13.12 14.09 Fe203 1.14 0.53 0.39 0.5 MgO 1.44 0.5 0.35 0.28 eao 0.89 0.39 0.28 0.37 Na20 3.9 3.11 3.86 4.2 K20 3.87 5.41 4.26 4.59 Ti02 0.11 0.04 0.04 0.02 P205 0.03 0.022 0 0.01 MnO 0.09 0.01 O.ot 0.02 TOTAL 98.07 99.002 98.97 99.27

Trace elements in ppm

Ba 596 441 168 181 Cr 21 13 11 17 ZI 74 22 21 16 Sr 124 66 30 33 Rb 61 111 88 135 y 3 3 5 6 Nb 1 2 2 6 Zn 20 7 8 15 v 2 2 0 0

region. The intercept seems too low as it is lower than the meteorite values of BABI (0.69898), Allende (0.69877) and ADOR (0.69883) (Wasserburg et al., 1977; Birck and Allegre, 1978; Wasserburg, 1987), the 3.8 Ga Am'rlsoq Gneiss value of 0.6973±0.0006 (Baadsgaard et al. 1976) or the 0.698±0.014 of the 1.18 Ga TugtutOq central complex of south Greenland (van Breeman and Upton, 1972). If the meteorite values set

the actual constraints on ter­

1.25r---------------- restrial initial Sr ratios - i.e. if all ter­restrial rocks must have some ex­cess of 87Sr above meteoritic values - then our determination can be considered to reflect 1.15 -

... 1.05 -en

«>

"" -.... ,_en

0.95 -

/ ""

DATE, 2839 ± 67/

o.ss .- / + XRF

0.75 L Y / 0.69276 :.t: 0.00239

• ID

0.65 ' 1 1

0 2 4 6 8 10 12 87 86

Rb/ Sr Figure 1 - Rb-Sr lsochron of the Lodge Bay Granite. The dat& of all points (including two duplicates run by isotope dilution - marl<t>d 10 here and in Table 1) is 2839±67 Ma, with an initial 875,tMsr interct1pt ratio of 0.69276±0.00239 and an MSWD of 0.726. Eliminat­ing the point of SK-746 gives 2896±73 Ma, 0.69134±0.00253 and 0.154.

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. ·----------- ----~-··- .,.. .. ·-···---·~···--··---- ~· ~

some geological event(s) which rotated the isochron .

It is proposed that the Lodge Bay Granite is a parent to the Cameron Island and Mackintosh Bay granites and their metasomatic progenies the Mine Granites (Bell et al., in press). The Cameron Island and Mackintosh Bay granites give we/I-constrained isochrons at ca. 2010 to 2020 Ma, with high initial

87 Sr /

6Sr ratios.

However, since the average Rb/ Sr value for the Lodge Bay Granite is 1.77, it is too deficient in Rb to serve as the source for metasomatism of the Mine Granites.

The date of ca. 2.9 Ga for the Lodge Bay Granite gives a lower bracket for the age of the overly­ing Murmac Bay Group. An upper

Summary of Investigations 1990

bracket appears to be set by the ca. 2.35 Ga U-Pb zir­con date (Van Schmus et al., 1986) of the intruding Mackintosh Bay and Cameron Island plutons. One may speculate that the Murmac Bay Group correlates with the Prince Albert Group (Schau, 1977) and other quartzite-carbonate-volcanic assemblages now known in the Canadian Shield which have been dated within this interval.

3. References Baadsgaard, H., Lambert A. St.J. and Krupicka, J. (1976):

Mineral isotopic relationships in the polymetamorphic Amitsoq gneisses, Gothaab District, West Greenland; Geochim. Cosmochim. Acta, v-40, p513-527.

Belt, K. , Bik11rman, M., Blenkinsop, J., Sibbald, T.1.1. and Macdonald, R. ~n press): Rb-Sr Geochronology of the auriferous North Shore Plutons, Lake Athabasca, Sas­katchewan; Can. J. Earth Sci.

Birck, J.L and Allegre, C.J. (1978): Chronology and chemical history of th~arent body of basaltic achondrites studied by the 87Rb· Sr method; Earth Planet. Sci. Lett. , v39, p37-51 .

Christie, A.M. (1953): Goldfields-Martin Lake map-area, Sas­katchewan; G&ol. Surv. Can., Mem. 269, 126p.

Macdonald, R. {1984): Bedrock compilation, greater Beaver­lodge area (NTS-74N-6 to -11) in Summary of Investiga­tions 1984, Sask. Geol. Surv., Misc. Rep. 84·4, p42-44.

Macdonald, R. and Slimmon, W.L. (1984): Bedrock geology ol the greater Beaverlodge area, NTS 74N-6 to ·11; Sask. Energy Mines, Map 24~A, scale 1:100,000.

Norrish, K. and Chappell, B.W. (1967): X-Ray fluorescence spectrography; in Zussman, J. (ed .), Physical Methods in Determinative Mineralogy, Academic Press, New York, p161-214.

Schau, M. (1977): Komatiites and quartzites in the Alchean Prince Albert Group; in Barager, W.R.A, Coleman, LC. and Hull, M.J. (eds.), Volcanic regimes in Canada, Geol. Assoc. Can., Spec. Pap. 16, p341-354

Saskatchewan Geological Su,vey

Sibbald, T.1.1. (1984): Gold metallogenic studies, Goldfields area; in Summary of Investigations 1984, Sask. Geol. Surv., Misc. Rep. 84-4, p116-121 .

Sibbald, T.1.1. and Jiricka, D.E. (1985): Geology of the gold deposits, Goldfield&, Saskatchewan; in Gold in the Western Shield (Program and Abstracts); CIM Geology Division Gold Symposium and Field Tours, Saskatoon, Sept. 7-12, p33-35.

Sibbald, T.1.1, and Lewry, J.F. (1980): Uranium me1allogenic studies: Lodge Bay area, Lake Athabaska; in Summary of Investigations 1980, Sask. Geol. Surv., Misc. Rep. 80-4, p44-48.

Steiger, R. and Jager, E. (1977): Subcommission on geochronology: Convention on the use of decay con­stants in geo- and cosmochronology; Earth Planet. Sci . Let.I, v36, p359-362.

Tremblay, LP. (1972): Geology of the Beaverlodge mining area, Saskatchewan; Geol. Surv. Can., Mem. 367, 165p.

van Breeman, 0. and Upton, B.G.J . (1972): Age of some Gar­dar lntrusi,..e Complexes, South Greenland; Geol. Soc. !vrl. Bull., v83, p3381·3390.

Van Schmus, W.R., Persons, S.S., Macdonald, R. and Sibbald, T.1.1. (1986): Preliminary results from U-Pb zircon geochronology of the Uranium City Region; in Summary of Investigations 1986, Sask. Geol. Surv., Misc. Rep. 86-4, p108-111 .

Wasserburg, G.J. (1967): Isotopic abundances: inferences on solar system and planetary evolution; Earth Planet. Sci. Lett., v66, p129-173.

Wasserburg, G.J., Tera, F., Papanastassiou, D.A. and Hunecke, J.C. (1977): Isotopic and chemical investiga­tions on Angra dos Reis; Earth Planet. Sci. Lett., v35, p294-316.

York, D. (1969): Least-squares fitting of a straight line with cor­related errors; Earth Planet. Sci . Lett., vs, p320-324.

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